Methods for verification of ultra-pure water with air gap membrane distillation

Focusing on applications in the semiconductor industry

Pedram Pirouzfar

Faculty of Health, Natural and Engineering Sciences Master of Science in Energy and Environmental Engineering Degree project 30 credits Supervisor: Karin Granström Examiner: Roger Renström February 2020

Abstract

In the semiconductor industry, the purification process of the silicon wafers is of a great importance. If water of sufficient quality is not used, the silicon wafer surface runs a risk of being destroyed by particles and bacteria sticking to its surface. Semiconductors cannot be manufactured on the destroyed surfaces and to achieve the highest efficiency of the circuits, water with high purity is required for the purification process. The silicon wafers produced by the manufacturer have an oxide layer on them as a protective layer. This oxide layer needs to be cleaned off before it can be used for the manufacture of semiconductors. The oxide layer is removed by applying 5% hydrogen fluoride (HF) to the surface which is afterwards cleaned away with water. It is mainly within this part of the purification process that particles and bacteria get stuck on the surface of the silicon wafer. At present, water of poor quality is used which is unable to dilute and purify the mixture that becomes with hydrogen fluoride and the oxide layer.

As development is constantly advancing and the line width of the circuits becomes narrower and smaller, water with almost no particles is needed to clean these small areas. The particle size of the water must not exceed 20 nm in order to effectively clean the silicon wafers and preferably the particle size should not exceed 10 nm.

In the present study, an air gap membrane distillation module was investigated for the purpose of verifying the purity of the water where spherical spheres of 20 nm diameter were added into the purified water and examined in a dynamic light scattering (DLS). Because ultra-pure water (UPW) is a very aggressive water, storage is a problem. Four different container materials ability to store UPW with maintained purity were studied; white borosilicate ice cream, brown borosilicate ice cream, ethylene chlorotrifluoroethylene (ECTFE) and polyvinylidene fluoride (PVDF).

Experiments were also done to further verify the purity of the water by adding ultra-pure water on a silicon wafer and allowing it to dry to study the dry spots. The dry spots were studied in an SEM to see if the water left any particles behind on the surface. The same experiment was also done with tap water and distilled water which was dripped on a silicon wafer and dried. These dry spots were examined in a scanning electron microscope (SEM). To investigate how effectively ultra-pure water cleans a silicon wafer, an amount of 5% hydrogen fluoride on a silicon wafer was added and rinsed with ultra-pure water and tap water respectively. The same experiment was also done with tap water for comparison. These silicon wafers were studied in an SEM to see if any particles were left on its surface from the respective water. An initial methodology was also done when 5% hydrogen fluoride was diluted with ultra-pure water and tap water to compare the amount of respective water it used to dilute this acid.

In the present study, simulations were made on the air gap membrane distillation module in COMSOL where four different geometries were simulated with the aim to see how the temperature profile on the hot and cold side changed as the geometry and area of the membranes changed.

The purity of the water produced with the air gap membrane distillation were verified with DLS and the particle size did not exceed 20 nm. Further experiments showed that with UPW, there were no dry spots on the surface of the silicon wafer and no particles could be seen when the silicon wafer was examined in an SEM. When the tap water was dropped on the silicon wafer and dried, one could clearly see the drying spots. When the silicon wafer was examined in an SEM, there were many particles left on the surface. The distilled water left no drying stains on the surface but on the other hand, it was able to see particles on the surface examined when in an SEM. When 5% hydrogen fluoride had been dropped on the surface and washed away with UPW, no particles could be detected when examined in an SEM. However, particles were found when the same amount of hydrogen fluoride was rinsed off with tap water.

When 5% hydrogen fluoride was diluted to a neutral pH of 6-7, about 200 ml of UPW was used as separated from tap water where it went to the quadruple to dilute the same amount of hydrogen fluoride. This showed the purity of the ultra-pure water compared to tap water.

For the simulations it was possible to see how the temperature profile changed with the area. With a large area, the temperature profile on the hot and cold side became very poor. The temperature on the hot side dropped a lot and on the cold side it increased a lot. The largest area simulated was 255x255 mm. With a smaller area, a more even temperature profile was obtained. The area that gave the best temperature profile was 180x100 mm, which was the smallest area investigated. In contrast, the diffusion area becomes smaller as the area decreases, leading to a reduced production of ultra-pure water.

This study is close to research and is about developing new technology and modifying/improving existing technology.

Preface

I want to start this thesis by thanking all the people at Rejlers Karlstad who have taken good care of me during these months and made me feel like one of them. An extra big thank you to Björn Holmström who has taken care of me since day one and supervised me in an excellent way.

I would like to thank Harald Näslund at Nanosized who has been constantly available for my questions and concerns. And for a nice collaboration on the experiments where I brought with me very valuable knowledge. I would also like to thank all the staff at electrum laboratory in Stockholm who have been very helpful during the experiments.

For this thesis, I would like to thank Karin Granström who has supervised me and has always been available for my questions and concerns. Even Tim Andersson who helped me with questions about COMSOL. Many thanks to all the teachers at the Division of Energy and Environmental Engineering at Karlstad University for these years, you have taught me an incredible amount. I would also like to extend an extra big thank you to Roger Renström, who has been there when needed and who constantly guides his students in an exemplary way.

Thank you to all my friends I have been honoured to get to know during my studies, you have made these years unforgettable. Thank you to my family and friends, without you I would not have passed this training.

This report is a master's thesis of 30 credits and completes my studies as a Master of Science in Energy and Environmental Engineering at Karlstad University. This work has been presented orally to an audience in the subject.

Pedram Pirouzfar

Karlstad University 2020 Nomenclature list AGMD Air gap membrane distillation UPW Ultra-pure water HF Hydrofluoric acid DLS Dynamic light scattering measures the particle size of molecules smaller than a micron in diameter dispersed or dissolved in liquid SEM Scanning electron microscope MD Membrane distillation PTFE Ptfe PP Polypropylene PVDF Polyvinylidene fluoride ECTFE Ethylene trifluoroethylene DCMD Direct contact membrane distillation NP Nanoparticles ISO International Organization for Standardization SC Standard clean APM Ammonium peroxide mixture HDPE High density polyethylene TOC Total organic carbon HT Heat transfer, in COMSOL physics module spf Laminar flow, in COMSOL physics module

Mutilations Aqueous solution Liquid substance consisting of water together with substances dissolved in the water Suspended substances Measure of the combined content of inorganic and organic substances in a solution Wetting The fluid's ability to keep in contact with a solid surface Permeat Outgoing purified water Porosity The amount of cavities inside a material and the proportion of the total volume of the material consisting of cavities Colloidal A substance that is finely divided into another Glass conversion temp. Heating a material above this temperature causes the material to soften Oligotrof An organism that can live in an environment with very low levels of nutrients Sorption A physical and chemical process in which one substance becomes attached to another substance, the most common forms of sorption are adsorption, absorption and ion replacement Suspension Colloidal mixture of solid particles scattered in a liquid Auto correlation function Describes the correlation between a process at different time points Standard clean (SC) Process for removal of organic residues and films from silicon wafers Cryptic growth A balance between cells that are born and cells that die. The total number of cells remains unchanged. Line Width The width of the smallest details of a component. The line width is built on the frequency a circuit wants to achieve. Circuits that want to achieve higher frequencies are based on smaller wavelengths, which in turn provide a smaller line width

Table of Contents

1. Introduction ...... 1 1.1 Prevention of the degree project ...... 1 1.2 Goal ...... 1 1.3 Boundaries ...... 1 1.4 Audience ...... 2

2. Background ...... 2 2.1 Development of air gap membrane distillation ...... 2 2.2 General about membrane distillation ...... 4 2.2.1 Benefits of membrane distillation ...... 6 2.2.2 Heat transfer in membrane distillation ...... 7 2.2.3 Membrane membrane distillation ...... 7 2.2.4 Pros and cons of air gap membrane distillation compared to other purification methods ...... 9 2.3 Ultra-pure water ...... 10 2.3.1 Definition of nanoparticles and their impact on ultra-pure water ...... 12 2.4 Silicon wafer and semiconductor industry ...... 14 2.5 Retainer material for UPW ...... 16

3. Method ...... 17 3.1 Literature compilation ...... 17 3.2 Experiment ...... 17 3.2.1 The clean room ...... 17 3.2.2 Air gap membrane distillation module ...... 17 3.2.3 Dynamic Light Scattering for Measuring Water Purity ...... 19 3.2.4 Container material selection ...... 19 3.2.5 Drip test of tap water and UPW on silicon wafers ...... 20 3.2.6 Dilution of hydrogen fluoride with tap water and UPW respectively ...... 21 3.3 COMSOL ...... 22 3.3.1 Modelling and simulations ...... 22 3.3.2 Description of geometries ...... 23 3.3.3 Mesh ...... 25 3.3.4 Physics modules in COMSOL ...... 26 3.3.4.1 " Heat Transfer in Solids and Fluids" &" Laminar Flow" ...... 26

4. Results ...... 28 4.1 Container material selection ...... 29 4.2 Drip test of tap water and UPW on silicon wafers ...... 33 4.3 Dilution of hydrogen fluoride with tap water and UPW respectively ...... 37 4.4 Simulations in COMSOL ...... 40 4.4.1 Temperature profile on hot and cold side for 180x180 mm ...... 40 4.4.2 Temperature profile on hot and cold side for 255x255 mm ...... 41 4.4.3 Temperature profile on hot and cold side for 250x130 mm ...... 43 4.4.4 Temperature profile on hot and cold side for 180x100 mm ...... 45

5. Discussion ...... 47 5.1 Air gap membrane distillation module ...... 47 5.2 DLS and Space Balls ...... 48 5.3 Container material selection ...... 48 5.4 Drip test on silicon wafer ...... 49 5.4.1 Drop with hydrogen fluoride on silicon wafer rinsed off with UPW and plain water ...... 50 5.5 Dilution of hydrogen fluoride with tap water and UPW ...... 50 5.6 COMSOL ...... 51 5.6.1 Temperature profile for the hot and cold side of COMSOL ...... 51

6. Conclusion ...... 53

7. Future research ...... 54

References ...... 55

Literature ...... 58

Annex I ...... 59

1. Introduction

1.1 Prevention of the degree project

Rejlers Karlstad had been commissioned by Harald Näslund, Chairman of the Board of Nanosized, to manufacture an air gap membrane distillation module (AGMD) that produces ultra-pure water (UPW). Näslund had seen problems with cleaning silicon wafers used to manufacture circuits where far too dirty water is used. If not enough clean water is used for cleaning, islands are formed with dead bacteria and particles on the silicon wafer. These islands are then destroyed surfaces and cannot be manufactured circuits on. If the silicon wafers are instead cleaned with ultra-pure water, a larger surface of the disc should be used for the manufacture of circuits1.

When the AGMD module was designed and completed by Rejlers Karlstad, Nanosized needed help to conduct experiments on the produced water to verify its purity and see if it effectively cleans a silicon wafer without leaving islands with dead bacteria and Particles.

If the purity of the water can be verified, it will have a major impact in the semiconductor industry. Since the silicon wafers can be cleaned more efficiently, this leads to a more sustainable development in the industry. On the one hand, less water will be used during the purification steps of the silicon wafers, which will save the environment, while larger areas of the silicon wafers will be able to be used, reducing the production of silicon wafers. Even the efficiency of the circuits manufactured will increase because the surface will be cleaner and better circuits can then be manufactured. There is a constant development in the semiconductor industry, which also places higher demands on the purity of water used in cleaning (Nakata et al. 2017; Yonezawa et al. 2018).

This study is close to research and is about developing new technologies and modifying/improving existing technologies.

1.2 Goal

This work has three objectives. To verify the purity of ultra-pure water (UPW) produced with air gap membrane distillation (AGMD, Air Gap Membrane Distillation) by (1) space ball sample, (2) study drying stains on silicon wafers, (3) measure the volume needed to dilute a quantity of hydrofluoric acid. To assess the ability of different container materials to store UPW while maintaining purity. The fact that, by simulations in COMSOL examine temperature profiles for four geometries on the membranes.

1.3 Boundaries

This work focused on the production of UPW and verifying its purity as well as examining its various uses. The energy aspect in terms of heat loss and other aspects of the system was not

1 Harald Näslund, Chairman of the Board Nanosized. Meeting, 25 October 2019 1 taken into account, nor the economic aspects (cost of manufacturing materials and assembly) but focused on producing UPW and verifying that it is ultra-clean.

The measuring instrument used to measure the purity of the water was a Dynamic Light Scattering (DLS). No deeper research was done on this instrument as it is confidential from the inventor on how the instrument was calibrated and what settings were used for the instrument during the measurements.

The simulations in COMSOL were delimited so that only the temperature profiles (for both hot and cold sides) for each geometry were examined. No simulations were made on the outflow of the purified water on the different geometries.

1.4 Audience

This thesis is aimed primarily at Nanosized in order to get better basis for further research and development. The study will make it easier for Nanosized to continue running and developing its project and now be able to verify its theories with actual studies and experiments.

This work is also aimed at people who do research on ultra-clean water and different companies in the semiconductor industry.

2. Background

2.1 Development of air gap membrane distillation

The module used for the experiments is a prototype that has been developed; this prototype still has some improvements and adjustments ahead of it before the final product is reached. More research, development and funding will be needed before the AGMD module is fully developed. Figure 1 shows the first prototype of the module that was developed and used; this prototype was fully manually driven. So everything had to be run by hand. Hot water and cold water were put in by hand and UPW was then received. The membranes were held together with buckles. This prototype worked to test the idea of producing UPW with AGMD. But a development was needed to make it more safe and self-sustaining.

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Figure 1. First prototype of the AGMD module

This prototype came to be developed to become more safe and precise. Figure 2 shows an improvement on the first prototype with more membranes and more proper frames. This variant is also fully manually driven, which means that the water on the hot and cold side was tucked in by hand.

Figure 2. A developed variant of the first prototype.

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Figure 3 shows the prototype used today; it is a further development of the two modules above. It is almost completely automatically driven except for the replenishment of hot water on the hot side during the process. Also this variant has many improvement opportunities but is a good and stable prototype at this stage that does its job of producing UPW.

Figure 3. The latest model of AGMD used today for experiments.

2.2 General about membrane distillation

According to Kamaz et al. (2019), membrane distillation is a growing membrane separation technique with a great potential to treat various aqueous solutions with a high content of suspended substances. The driving force of this technique is the vapour pressure difference over a porous hydrophobic gaseous membrane and it is the temperature difference over the membrane that starts the process, see Figure 4 (Lawson & Lloyd 1997; Hendren et al. 2009; Imtisal-e-Noor et al. 2019; Kamaz et al. 2019). Water vapour and other volatile substances from the feed will pass to the permeable side of the membrane while the non-volatile substances, dissolved salts and other minerals will be pushed aside. As water vapour and non-liquid water

4 pass through the membrane pores, the membrane must be sufficiently hydrophobic to suppress wetting or passage of water along with other dissolved substances and non-volatile species (Drioli et al. 2015a; Malmali et al. 2017). Only the steam molecules are transported through the porous hydrophobic membranes. The liquid lining to be treated with MD must be in direct contact with one side of the membrane and it should not penetrate the dry pores of the membranes. It is because of surface tension forces that the hydrophobic nature of the membrane prevents liquid solutions from entering its pores. As a result, a liquid/vapour interface is formed at the entrances to the membrane pores (Kimura et al. 1987; Schofield et al. 1987; Bandini et al. 1992; Banat & Simandl 1994; Lawson & Lloyd 1997b; Khayet et al. 2000).

Figure 4. Illustration of the membrane filtration of the air column. The product that is obtained is ultra- clean water. The outflow of hot water can be used as inflow in a closed system together with a heat source for heating the water.

The membrane in MD acts only as a barrier to keep the liquid/steam interface at the entrance to the pores. One of the main requirements for the MD process is that the membrane must not be sharpened, there may only be steam and non-condensable gases in its pores. The pore size of the membranes is in the size of 100 nm and 1 μm. To avoid wetting of the pores, the membrane must be hydrophobic with a high contact angle for the water and minimal size of pores. However, the flow becomes lower as the pores become smaller but this is necessary to avoid wheezing. It is enough that only a few pores are a little too large to destroy the efficiency of the process by allowing fluid to penetrate the larger pores. Therefore, it will be necessary to optimize the pore sizes to find an optimal size where the flow and efficiency are high enough without the pores becoming too large to avoid wetting (Khayet et al. 2004a; El-Bourawi et al. 2006; Alkhudhiri et al. 2012; Drioli et al. 2015a).

It is important that there is a temperature difference throughout the process, which causes a vapour pressure difference. The water molecules evaporate on the warm side, which then crosses the membrane in the steam phase to condense on the cold side. Once this process has been completed, the condensed and purified water can be extracted (Drioli. & Wu 1985; Franken et al. 1987; Khayet et al. 2004b; Alklaibi & Lior 2005; Zereshki 2012). The phase

5 separation in the MD process is based on steam-liquid equilibrium where latent evaporation heat drives the change in phase from liquid to steam (Zereshki 2012; Imtisal-e-Noor et al. 2019).

2.2.1 Benefits of membrane distillation

According to Lawson & Lloyd (1997), membrane distillation is a technology that is both cheaper and more energy efficient than the other more commercial water purification techniques such as reverse osmosis. The advantages of membrane distillation are several, according to Lawson & Lloyd (1997) and Liu & Martin (2006), one of them is that in theory, diaphragm distillation can remove ions, macromolecules, colloids, cells and other non-volatile substances completely from the water. Furthermore, the same high temperatures are not needed for the process to work, which means that energy consumption is reduced. Similarly, lower pressure is required compared to other pressure-based purification techniques, which provides benefits in robustness and energy efficiency. There will also be a reduced chemical interaction between the membrane and the process solutions. Membrane distillation also makes membrane mechanical properties less demanding and a reduced steam space compared to conventional distillation processes (Lawson & Lloyd 1997).

Previous research has shown that MD can produce high quality water using a variety of raw materials. This allows for the use of flushwater in new recycling systems. This makes membrane distillation ideal for integration with process heat recovery and/or cogeneration on site (Liu & Martin 2016).

The membrane distillation process equipment does not take up much space, which means that it does not take up unnecessary space and becomes easy to use and maintain. The water on the warm side does not need to be heated to above boiling point, but can be between 60°C and 90°C. Temperatures as low as 30°C have also been used. This, together with the relatively small equipment itself, leads, among other things, to less heat loss to the environment. This means that less energy is required to power the system and, in the unlikely way of high temperatures, this system can be connected to, for example, solar and geothermal energy, which makes the system even more cost and energy efficient. Since membrane distillation is a thermally driven process, it means that the operating pressure of the system does not have to be as high, this gives, among other things, two advantages. One is that the system will be cheaper because it does not need as expensive equipment but above all it will be a safer system (Lawson & Lloyd 1997; Liu & Martin 2006).

Another advantage of membrane distillation that Lawson & Lloyd (1997) takes up is that its membranes have a minimal role in the separation itself. In the membrane distillation process, the membrane acts solely as a support for the steam-liquid interface and there is no difference between solution components on a chemical basis. This aspect allows membrane distillation membranes to be manufactured from chemically resistant polymers such as polytetrafluoroethylene (PTFE), polypropylene (PP) and polyvinylide fluoride (PVDF). Since the pores in the membranes of an MD process are relatively large compared to reverse osmosis and ultrafiltration, the pores are not as clogged and the soil ing of the membranes in membrane 6 distillation does not become as big a problem (Lawson & Lloyd 1997). In addition, this process is relatively insensitive to fluctuations in pH and concentrations (Khayet & Matsuura 2011; Imtisal-e-Noor et al. 2019).

There are also some limitations with membrane distillation. The primary limitation is that the process solutions must be aqueous and sufficiently diluted. This is to prevent the waving of the hydrophobic microporous membrane.

2.2.2 Heat transfer in membrane distillation

During the membrane distillation process, a heat transfer process takes place, where heat is first transferred from the heated feed solution fed into the system. The heated water has a uniform temperature 푇푓. The heat transfer takes place over the thermal boundary layer to the membrane surface at a speed according to (1). At the surface of the membrane, the liquid evaporates and heat is transmitted through the membrane at a speed according to (2), where 푁 is the speed of the mass transfer and ∆퐻푣 is the evaporation heat. Heat is also passed through the membrane material and the vapour that fills the pores at a speed according to (3). The total mass transfer through the membrane can be described according to (4) (Lawson & Lloyd 1997).

푄 = ℎ푓∆푇푓 (1)

푄푣 = ℎ푣∆푇푚 = 푁∆퐻푣 (2) 푄푚 = ℎ푚∆푇푚 (3) 푄 = 푄푣 + 푄푚 (4)

2.2.3 Membrane membrane distillation

According to Lawson & Lloyd (1997), membranes with porate size from 100 Å to 1 μm can be used. There are two important aspects to consider when selecting pore size, one being that the pores must be large enough to promote the desired flow and the other is that the pores must be small enough to prevent fluid penetration through Membrane.

The performance of MD is affected by the different characteristics of the membranes. Among other things, its thickness, porosity, medium size of pores, poring distribution and its geometry. The successful outcome of the process is expected to depend on the membrane's ability to create an interface between two mediums without spreading one phase to another, as well as being able to combine a high volumetric mass transfer with a high resistance to fluid intrusion into the pores (Drioli et al. 2015). According to Drioli et al. (2015), the membranes and its physical properties are characterized by five points:

1. High input pressure for the liquid. Which is the lowest hydrostatic pressure that must be applied to the input flow before it overcomes the hydrophobic forces of the membrane and penetrates the membranes. This is a characteristic of each membrane and

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it is this property that prevents wetting of the membrane pores. To achieve high input pressure for the liquid, it is necessary to use membrane material that is highly hydrophobic with a small maximum pore size. 2. High permeability. The flow will increase as the pore size increases and with a reduced thickness of the membrane. Thus, in order to obtain a high permeability, the surface layer that controls the membrane transport must be as thin as possible while its porosity and pore size must be as large as possible. However, a conflict arises between these requirements. To achieve high mass transmission, thinner membranes are required and to get as low conductoric heat loss as possible, thicker membranes are required. 3. Lay soiling on the membranes. When porous membranes are used, the contamination of these is one of the major problems. However, processes that make use of gas-fluid contactors are less sensitive to pollutants because there is no convection flow through the membranes. On the other hand, a pre-filtering in industrial applications may require gas and liquid currents to contain highly suspended particles. These can then clog the pores because of its small hollow diameter. 4. High chemical stability. The chemical stability of membrane material has a significant effect on its long-term stability. Any reaction that occurs between the solvent and the membrane material can affect the membrane matrix and its surface structure. Liquid containing acidic gases is corrosive by nature and will thus affect the membrane material in such a way that it becomes less resistant to chemical attacks. 5. High thermal stability. During high temperatures, the membrane material cannot withstand degradation or decomposition. What affects how the nature of the membrane changes depends on the glass conversion temperature, T for amorphous polymers and melting point, Tm for crystalline polymers. Above these temperatures, the properties of polymers change drastically.

As mentioned above, the membrane thickness is not very easy to relate to. The smaller the thickness of the membranes, the higher the mass flow is obtained, but the energy losses will be greater due to heat flow flowing over the membranes (Al-Obaidani et al. 2008). To solve this problem with the membrane thickness, double and even triple membranes have been introduced (Bonyadi & Chung 2007). These membranes consist of a hydrophobic active layer and a hydrofelt support layer. The support layer contributes to thermal insulation and ensures the necessary mechanical robustness of the membrane while the active layer of the membrane retains the liquid. When selecting the thickness of the active layer, it is important to think about when too small a thickness can cause liquid to pass through the pores. A thin active layer may also be insufficient to withstand the chemical attack from the feeding side, especially during processes that take longer.

According to Laganà et al. (2000) the optimal thickness of the active layer is 30-60 μm. On the other hand, Drioli et al. (2015) believes that a broader perspective on the effect of membrane thickness on membrane distillationperformance reveals that the literature lacks clear and conclusive statements.

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Regardless of the material selection on the membrane, it must be hydrophobic to ensure that it is only steam passing and no liquid. PVDF, PP and PTFE are the most common polymeric materials used for membrane distillation. Among these, PTFE has the best hydrophobic properties, however, there is most research on PVDF membranes due to its simple machinability (Drioli et al. 2015a).

Drioli et al. (2015) address different methods to improve the hydrophobic properties of membranes. These include adding evidence at the membrane surface that is made of various low-energy fluoride polymers. Plasma modification, where plasma is defined as an ionized gas that can be used to modify surfaces of various kinds, in this case membrane surfaces. Making the surface roughness of the membranes coarser is also a method of improving its hydrophobic properties, but this can produce negative effects such as soil surface contaminants and thermal polarization.

It is not only pore size that has a crucial role in determining the flow, membrane porosity has an important role in this as well. More porous membranes provide a larger diffusion surface for the vapours while reducing the thermal conductivity of the membrane. This is because the air captured in the pores has 10 times lower conductivity than the polymer materials used.

2.2.4 Pros and cons of air gap membrane distillation compared to other purification methods

According to Liu & Martin (2006), most of today's UPW system relies on membrane techniques. Reverse osmosis and ultra/nano-filtration are two of the techniques used for the production of UPW for the semiconductor industry. These two techniques have long been the two key systems for the production of UPW and there has been a lot of research and development around these two in recent decades. Although reverse OSMOSis has been shown to be relatively cost-effective and a reasonable technique, there are several drawbacks to this technology that have been difficult to overcome. Careful treatments and aftertreatments on the water are required, high operating and maintenance requirements, high electricity consumption and low recycling rates are the problems encountered with reverse osmosis. Therefore, an alternative technology that retains the benefits of traditional membrane technology but has increased robustness, lower costs and better environmental performance is a continuing priority (Liu & Martin 2006).

According to Drioli et al. (2015), the benefits of AGMD are several, among other things, this method has a relatively high flow with low heat loss. No wetting occurs on the permeat side and the pre-ororening tendencies on the membranes are smaller Summers et al. (2012). One of the drawbacks is that the air gap used gives further resistance to the fumes that pass through the membranes. Summers et al. (2012) also believes that the module design is difficult because many variables are involved, which makes it difficult to model the technology.

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Another advantage of AGMD, compared to other configurations of membrane distillation, is its ability to condense the purified water molecules on the cold surface rather than directly into a cold surface, as is the case with direct contact membrane distillation (DCMD). In this configuration, the mass transmission ladder involves movement in the fluid supply towards the membrane surface, evaporation at the membrane interface and transport of the steam through the membranes and the air gap before condensation (Bouguecha et al. 2003; Liu & Martin 2006).

According to Liu & Martin (2006), the main advantages of membrane distillation that the process does not need high operating temperatures compared to traditional distillation processes, as well as its lower pressure differences compared to, for example, reverse osmosis. Due to its low operating temperatures, it is possible to utilize a variety of waste heat currents, antigen in the plant directly or via external sources such as cogeneration plants and solar energy, etc. However, there is no experience under Liu & Martin (2006) of long-term performance in this process and there are also uncertainties in terms of costs. These two points constitute an obstacle to the application membrane distillation in industry.

According to Liu & Martin (2006) and Drioli et al. (1999), the benefits of membrane distillation for the production of UPW can be summarized in seven points:

1. 100% theoretical removal of ions, macromolecules, colloids, cells and other non- volatile substances 2. Lower operating temperatures than other membrane technologies 3. Lower surgical pressure differences compared to other pressure-based membrane technologies 4. Reduced chemical interaction between membrane mechanical properties 5. Reduced steam spaces compared to conventional distillation processes 6. High resistance to pollution 7. High recovery rate, up to 77%

2.3 Ultra-pure water

Ultra-clean water is often used in the semiconductor industry where a large volume of ultra- clean water is required for the preparation of various electronic devices for computers, among other things. This requires the ultra-clean water to contain minimal total dissolved substances along with a high resistance (Bhadja et al. 2015).

According to Ruth & Berndt (2016), ultra-clean water under the supply of semiconductor devices is the most important aspect of supply volume and a number of other applications. The ultra-clean water is used as a detergent, solvent and many other purposes. It can be used for itself or as the main component in mixtures of chemicals. Since ultra-pure water is a universal chemical in the manufacturing process, its quality will determine the possibility of defect-free manufacturing in subsequent stages. Ruth & Berndt (2016) also highlights the importance of

10 removing particles less than 10 nm in order for the water to be allowed to call itself ultra-pure water.

An important aspect when measuring the purity of ultra-pure water is the prevention of pollution from the air to the purified water, as well as contact with solid surfaces when the ultra-pure water is chemically unstable and has the property of dissolving chemical substances to some extent, which pollutes the water (Melnik & Krysenko 2019). When the ultra-pure water is placed in an open vessel, it begins for several seconds to absorb the carbon dioxide of the air with the formation of carbonic acid; this means that its resistivity is reduced a lot which is not desirable. The resistivity of the ultra-pure water is greatly reduced even when stirring or spraying water, indicating a strengthening of the pollution process of products absorbed from the air. In view of the above problems, the purity of the water should be examined in closed systems to prevent pollution from the air as well as from contact surfaces (Melnik & Krysenko 2019).

Ultra-pure water is used during the rinsing steps in the semiconductor industry. It is important that there is not a single bacterial cell because the presence of bacteria and/or the presence of cell degradation can seriously damage the quality of the final product. Nitrogen is often used above the ultra-pure water instead of ordinary air to avoid carbon dioxide and oxygen being dissolved in the ultra-pure water. In order to prevent ion load on the ion replacement resins, it is important that the ultra-clean water is kept carbon-free. A decrease in oxygen concentrations also leads to a minimal (Kulakov et al. 2002).

Kulakov et al. (2002) believes that despite the aforementioned precautions, it is almost impossible to completely remove contaminated microorganisms. Pipelines, membranes, tanks and other surfaces within the UPW system provide beneficial locations for bacterial adhesion and cell growth. The conditions for life in ultra-pure water are very limited and although this water contains less than one part per billion of inorganic and organic molecules, there is a group of microorganisms that have adapted to these conditions. These are oligophens. Many of these bacteria can secrete extracellular polysaccharides, allowing adhesion to different surfaces as well as a potential resistance to disinfection. These extracellular polysaccharides act as a diffusion barrier against nutrients and cellular products, allowing nutrients to reach the bacterial cells.

Cryptic growth is a phenomenon that can occur in the cinema films of an UPW system. That's when dead cells accumulate in these biofilms and can be used as a source of coal for succesful generations of bacteria. Disruption or removal of biofilms in pipelines is seen as a challenge by UPW users (Kulakov et al. 2002).

According to Melnik & Krysenko (2019), ultra-pure water is mainly used in two categories. One is in nuclear and thermal power plants. A high degree of water purity and steam reduces the risk of corrosion on the equipment, increasing the reliability of nuclear power plants, heating plants and combined thermal power plants. The second category where ultra-clean water is widely used is in the semiconductor industry and microelectronics. In the manufacture of micro

11 and nano dimensional devices, it is important to use water that does not contain any dissolved particles, ions or other foreign particles. Should there be particles in the water where the size varies between tens of nanometers to tens of microns and these reach the microdevice, there is a high risk that the device will be knocked out altogether. When ultra-clean water is produced from natural water, it requires careful removal of all types of pollutants including suspended materials, inorganic compounds, organic compounds, dissolve gases and microorganisms.

Melnik & Krysenko (2019) also means that it is not possible to completely remove specified pollutants with only one method. As a result, different technical processes are required to obtain the ultra-clean water. A system will then be formed with stages of preliminary preparation, primary and secondary purification as well as pipeline systems.

In the first stage, which is primary purification, different methods are used depending on the quality of the initial water that is inserted into the system. Two of these methods that can be used are reverse osmosis and electrodialysis. The system of preliminary preparation (depending on the type of primary refuelling system) should ensure the safe and stable operation of these two units. After these two stages of preliminary purification and primary purification, most of the suspended substances, ions, organic compounds and gases dissolved in the water are removed.

The secondary purification step (which is the final purification step) ensures maximum reduction of organic substances and ions in the water. This step removes virtually all microparticles and bacteria. The system of pipelines for the distribution and delivery of the ultra-clean water to its consumption sites should exclude the risk of deterioration in its quality. Since water has a unique ability to dissolve almost all chemical substances to a certain extent and maintains virtually all forms of life, its purity is very unstable. There are mainly five types of pollutants that can impair the purity of the water via the pipeline system, these are suspended substances, inorganic compounds, organic molecules, dissolved gases and microorganisms. These technical processes in a system should purify all pollutants in the water and ensure that it is ultra-clean water that is obtained.

2.3.1 Definition of nanoparticles and their impact on ultra-pure water

For the manufacture of electronic devices, quality control on the ultra-clean water is an important and challenging parameter. The quality check applies to contaminants that can cause typographical errors; pollutants such as nanoparticles, metals, ionic species, organic compounds, bacteria and more. Nanoparticles are of particular interest because the size of electronic devices is constantly decreasing, which means that small particles, such as nanoparticles in the water, can cause problems for the final product (Herrling & Rychen 2017).

There are different definitions of nanoparticles that are accepted, one of those definitions is according to the International Organization for Standardization (ISO). They argue that the nanoscale is a length scale between 1 nm and 100 nm. The reason that the lower limit is set to 1 nm is to exclude single or groups of different molecules that can be up to 1 nm large from the 12 definition. ISO defines a nanoparticle as a group of nano-objects that in turn is a discrete material with one, two or three outer dimensions on the nanoscale. Nanoparticles all have three dimensions on the nanoscale. Before the term "nanoparticle" was introduced, terms such as colloid or colloidal were used as synonymous with nanoparticle. A colloid was then defined as an object with a dimension with size between 1 nm and 1000 nm. As you can see, the two definitions overlap. This definition includes all kinds of materials, such as organic and inorganic nuclear materials (Herrling & Rychen 2017).

In order to more effectively control the presence of nanoparticles, the source of these particles should be examined in more detail. According to Herrling & Rychen (2017), there are four possible sources of nanoparticles being obtained in the ultra-pure water, these are presented below and a compilation can be seen in Figure 5.

• Raw water: nanoparticles can come directly from the raw water used as feed. Larger nanoparticles can be degraded and thus transported through the system. It is assumed that these nanoparticles make up the minority of the nanoparticles present in the ultra- pure water. • System components: nanoparticles may occur when physical-chemical erosion occurs on the ultra-pure water components. These can be components such as pipe components, valves, tanks, feed, flow gauges and more. The release of nanoparticles depends on hydraulic conditions and material properties. • Processteg: nanoparticles can also come from the UPW system's process steps, such as ion exchange retits. Grinding of different reaper materials can lead to the release of nanoparticles during the rehesitchange and hydraulic transport becomes possible. • Formation: When water is purified from minerals and salts and deionized, there is a small amount of ions left in the water as well as bacteria. When this water is then beamed with UV radiation, the bacteria die and in turn flock with each other as well as the remaining ions. These flocks then by definition become nanoparticles and have thus arisen from new formation.

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Figure 5. Illustration of four simple examples of nanoparticles (NP) in ultra-pure water.

Herrling & Rychen (2017) that there are two strategies for controlling nanoparticles in ultra- pure water. One of these strategies is to limit its sources by replacing the system components or improving processes. One suggestion to improve the processes may be to remove the hydrogen peroxide molecules from UPW. This is to reduce the damage to the ion replacement resins and thus increase the resin purity. However, more research is needed to confirm this. The second strategy is to find sinks for nanoparticles in the UPW process. A potential submersion for nanoparticles may be filter modules, especially ultrafiltration modules.

2.4 Silicon wafer and semiconductor industry

The manufacture of components for the semiconductor industry is a very complex process with a very high demand for water. According to statistics, approximately 6,000 litres of water are spent on the cleaning of a disc of 200 mm (Wiesler 2003; Liu & Martin 2006). This figure is expected to increase significantly each year as the standard dimension of the disc increases from 200 mm to 300 mm. The purity of the water used during the cleaning process is becoming increasingly stringent as the manufacturing technology improves for the circuits. This problem, in particular, with environmental impact and energy costs, are important challenges that need to be met (Liu & Martin 2006).

In the manufacture of circuits, certain unintentional substances tend to stick to the surface of the device, thus reducing the replacement of the product. Among these pollutants, it is particles with the size rate about 10-1000 nm generated from the manufacturing device or from the cleaning chemicals and these particles are one of the pollutants that should be more carefully controlled. To clean silicon wafers and remove impurities, cleaning technology where water is

14 used is essential. This is because the development is constantly progressing where it strives for more efficient circuits with higher resolution patterns (Ohmi 1990; Itano et al. 1993; Nakata et al. 2017).

Metallic pollutants on the silicon disc have been one of the main sources of performance errors, this by impairing the degradation voltage and reducing the life of the components (Chung et al. 2001; Lee et al. 2018). It has been reported that the metallic pollutants on the silicon disc must not exceed a limit of 1x1010 atoms/cm2. This limit is subject to change in the future to 1x109 atoms/cm2 (Bullis 2000; Reinhardt & Kern 2018).

According to Nakata et al. (2017) and Yonezawa et al. (2018), the particles in the ultra-clean water used for purification of the silicon discs need to be significantly reduced as the line width of the semiconductors is constantly becoming tighter and smaller. Therefore, cleaner water will be needed with smaller particles to purify these devices. The purification method has already been changed from cleaning these silicon plates together to cleaning them individually. This is to prevent the contamination from one plate to another. Nakata et al. (2017) highlights another problem with UPW and particles in the semiconductor industry. With today's limitations on microfabrication, much focus has been on finding the channel material of the future with germanium-containing compounds. Particles are considered more dangerous as the line width of the circuits shrinks. Therefore, information about cleaning water that promotes or suppresses particle adhesion of the Germanium-based compound is very important (Onsia et al. 2005; Sioncke et al. 2009; Nakata et al. 2017).

Cleaning of silicon wafers is a very costly process due to the resources used and the waste management that comes with it. Cleaning is an often applied operation in a typical manufacturing process for the circuits and has a key role in determining the manufacturing yield (Ruzyllo 2010).

One of the pillars of cleaning of silicon wafers according to Ruzyllo (2010) is RCA cleaning also called "standard clean", SC. This cleaning method was developed by Werner Kern and his colleague David Puotinen where in their report entitled "Cleaning Solution Based on Hydrogen Peroxide for Use in Silicon Semiconductor Technology" they presented the cleaning method RCA. The cleaning method has two steps where the first step is to use an alkaline mixture with a high pH, and the second an acid mixture with a low pH value. This cleaning method was developed in the 70's but is valid and applicable to this day. Originally, the main focus was with this cleaning technique to cleanse away organic surface films as well as metallic pollutants on the surface. It also turned out that the first step, with the alkaline mixture, was very effective in removing particle-shaped impurities (Kern 1970; Ruzyllo 2010). One of the challenges according to Ruzyllo (2010) with the cleaning of the silicon disc is to remove pollutants without destroying the disc and its material properties. Therefore, various non-water volatile substances have started to be used in this cleaning method, including hydrogen fluoride.

In order to achieve an ultraren surface with high reproducible, Lee et al. (2018) believes that it is important to remove pollutants and prevent them from being relocated on the substrate

15 surface. Ammonium peroxide compound (APM) cleaning is very effective in removing particles from the substrate surface. However, it is very easy for metals such as iron and aluminium to get a hold of the disc again (Lee et al. 2018). To solve this problem, it is possible to use a hydrochloric acid oxide mixture (HPM) which is a cleaning solution for metals. However, some other problems may arise with HPM, such as precious metal residues or particle deposition. In order to avoid these problems, a diluted hydrogen fluoride solution has been proposed as an alternative approach to cleaning the silicon wafer. However, some metal residues such as copper and aluminum may stick to the disc when its surface becomes hydrophobic (Ryuta et al. 1992; Lee et al. 2018). In order to prevent the deposition of kopparu, it is important not to allow reduction reaction between the metal ions (Zhang et al. 2011; Lee et al. 2018). Therefore, according to Lee et al. (2018) the redox potential for copper in a hydrogen fluoride solution should be lower than for silicon, in order to avoid copper deposition.

2.5 Retainer material for UPW

According to Khvataeva-Domanov et al. (2017), it is recommended to use antigen surface- treated brown glass bottles or borosicate glass to collect UPW. Because ordinary glass bottles can leak out silica and alkali metals, which will cause problems. Some plastic containers, depending on its construction material, should be avoided. This is because plastic can contain antistatic agents, softeners, stabilizers or even dust particles as a result of the manufacturing packaging or storage processes, which destroy the quality of UPW. Another important aspect of the container for UPW is to try to use as small a container as possible to minimize exposure and contamination to the water from the container.

Riché et al. (2017) compared two different plastics for storing UPW. One was high density polyethylene (HDPE) and the other plastic was polypropylene (PP). The water was stored for 24 hours and the test was performed three times. What was measured was total dissolved carbon (TOC) where HDPE was better than PP to store UPW in, when the water dissolved smaller TOC in the container made by HDPE.

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3. Method

3.1 Literature compilation

A literature summary was made to research more about the subject and the problem of why ultra-pure water is needed and what it can be used for. A more specific image was given by membrane distillation as well as ultra-pure water and how it can be used for various purposes. The problem of the purification of the silicon wafer used to produce circuits was also investigated. The OneSearch, Web of Science, and Google databases were used with various specific keywords that examined the title, summary, introduction, results, and conclusion of the articles. Subsequently, a selection was made depending on the relevance of the articles to the work.

3.2 Experiment

The experiments were carried out in a clean room at the Electrum laboratory in Stockholm. The reason the experiments were carried out in a clean room was to minimize contamination from the environment to the purified water. The equipment used for all experiments was an air gap membrane distillation that had been designed by Rejlers Karlstad. Dynamic Light Scattering (DLS) was used to measure the purity of the water. Four different types of containers were used to collect the purified water. A scanning electron microscope (SEM) detected possible impurities on the silicon wafers after various drip tests with water of varying quality.

On the hot side of the AGMD, water was inlaid which had been purified via reverse osmosis, to be further purified. This water was taken from the Electrum laboratory where they have a reverse osmosis plant in the basement and every workbench in the laboratory has access to this water. All other materials were available at the Electrum laboratory.

For the simulations, the simulation program COMSOL was used where a model of the AGMD module was built in 3D. Four different sizes of the module were examined, one of which was the size used in reality. The temperature profile on the hot and cold side was studied for each size.

3.2.1 The clean room

As it was water with high purity produced, it was important to reduce all possible sources of contamination. All experiments were carried out in a clean room with protective clothing to reduce the risk of contamination from both man and the environment to the samples. The lights in the clean room were of the wavelength of yellow light because there were instruments in the clean room that were sensitive to ordinary light. This explains the yellow-tinted images that can be seen in the report.

3.2.2 Air gap membrane distillation module

The module used for the production of UPW was an AGMD with a total of 10 membranes for a higher production of ultra-pure water. The components belonging to the module were a closed expansion vessel (1), a water heater to heat the inlet water to a desired temperature (3). A pump to circulate the hot water in the closed system (4). Inlet and outlet for the cooling water (6) and (7). As well as an electrical cabinet to be able to regulate the pump flow and keep track of the 17 temperatures of the hot water and cooling water (5). Even the pressure inside and out of the system could be read via the electrical cabinet. See Figure 6 for image of module from front.

Figure 6. Visual image of the module and all its components marked with (1)-(8).

The start of the module for the production of UPW took place in a few steps. At first, the module had to be filled full of water on the hot side. The hose at the expansion vessel (1) was disconnected and water was filled via the transparent hose with funnel on (2). When water flowed out of the opened hose, the module was filled and the hose was put back. When the module was filled with water, the water heater was started to heat the water (3). After a while when it started, the pump was started to get a circulation on the water (4). The temperature was constantly read via the electrical cabinet to ensure that there was an increase in the temperature on the hot side (5). As the temperature increased, the cooling water was connected to the system via the green hose (6). The cooling water was taken directly from the tap from the laboratory and had a constant flow. The cooling water flowed out of the hose into a well (7). Once the cooling water had been switched on, UPW could be obtained (8). The module needed to run for a while to purify itself and after about 30 minutes tests could be taken on the water to verify its purity before the experiments could be started.

It required a refill of water on the hot water side as a lot of water like UPW was constantly lost. The filling was done via the funnel (2) where distilled water from the laboratory was tucked in for purification into ultra-pure water.

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3.2.3 Dynamic Light Scattering for Measuring Water Purity

Dynamic Light Scattering (DLS) is an optical technique used to assess the particle size of a suspension. The light that hits the suspension is dispersed and by placing a detector at a certain angle that records the scattered light intensity, a time series is recorded. The width of the autocorrelation function of the recorded time series is proportional to its diffusion coefficient. This in turn depends on the particle diameter leading to a quick procedure for measuring the mean of the diameter of the particles (Chicea; Berne & Pecora 2000; Chicea & Rei 2019).

Figure 7. Image of the DLS instrument where the kyvette with the test samples is inserted into the black cover.

Once the module was up and running with the purification process, water was taken directly from the module to measure its purity. Space balls of 20 ± 4 nm were dripped into the purified water and raised into a kyvett to be tucked into the DLS. Once it was verified that the water was clean enough (i.e. it gave a rash of 20 nm space bullets), the experiments were started where clean water was collected in different containers. The samples were taken from these containers and were tucked into the DLS instrument to measure the purity of the various samples. One test took about 15 minutes to run with the DLS instrument. See Figure 7 for image of the DLS instrument.

3.2.4 Container material selection

Four different materials for containers were selected for the experiments where the choice of materials was made based on the project participants' previous knowledge. The purpose of the containers was to collect the purified water and investigate how quickly the water becomes dirty in the respective material over time. Two glass materials and two plastic materials were selected for examination. See Figure 8 for the different container materials.

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Figure 8. Four different container materials used in the experiments. The different materials from the left are white borosilicate glass, brown borosilicate glass, ECTFE plastic and PVDF plastic.

The containers were placed under the mouth where UPW flowed out. When the first drop fell into the respective container, a timetag watch was started to keep track of how long the water lay in each container. The experiments were conducted over a 15 minute period where tests on the purity were performed after minutes 0, 5, 10 and 15 respectively.

3.2.5 Drip test of tap water and UPW on silicon wafers

Three different drip tests were carried out on a silicon wafer, the aim of which was to investigate what the drying spots look like on the surface of the silicon wafer and how much particles each water leaves behind. This is to further verify the purity of the ultra-pure water produced.

Ordinary tap water, UPW water and distilled water from the laboratory were dripped on to three separate silicon wafers; the silicon wafers with the water droplets had to dry overnight and the results were examined both visually and in an SEM. See Figure 9 for sem image.

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Figure 9. SEM with a silicon disc in it for examination.

After the silicon wafer was placed according to Figure 9, the door was closed and via a computer connected to SEM various smears could be studied on the surface of the silicon wafer by zooming in on the surface.

There were also two experiments where hydrogen fluoride with a concentration of 50% was dripped on the surface of a new silicon wafer and washed away with UPW. A similar experiment was conducted where 5% hydrogen fluoride was dripped on to a silicon wafer and rinsed with tap water. The purpose of this was to see if there will be any contaminants or islands with dead bacteria with the respective water. The silicon wafers were tucked into sem and examined. For these experiments, 5% hydrogen fluoride was to be used for both experiments, but in execution hydrogen fluoride with a higher concentration happened to be taken.

3.2.6 Dilution of hydrogen fluoride with tap water and UPW respectively

An initial test was carried out to investigate the dilution of hydrogen fluoride with a concentration of 5%. A small amount of hydrogen fluoride was measured in a beaker and poured into a PVDF container. The pH was measured with a pH stick for the hydrofluorocarbon acid only. Subsequently, UPW was added in batches of 50 ml to see when the hydrofluorocarbon acid is diluted to a natural pH of approximately 7.

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This was also done for tap water in order to be comparable to the results of UPW with the aim of seeing if UPW is more aggressive and better than tap water at diluting a strong acid. See Figure 10 for the amount of hydrogen fluoride measured and Figure 11 for the pH stick used to measure the pH.

Figure 10. Beaker with 5% HF. Figure 11. pH-stick used to measure the pH of the dilution of HF with UPW and tap water.

3.3 COMSOL

COMSOL is a simulation program for multiphysics where finite element method is used together with numerical solutions. For flows where a liquid is to be simulated, the program uses a fluid dynamics calculation module (CFD) where different boundary conditions are defined by the user. Boundary conditions such as inlet, outlet and wall conditions. This CFD module can in turn be connected to a heat transfer module to simulate energy flows in the system. Different boundary conditions are also defined here by users, such as temperatures and heat flows. The domains in the geometry are in turn connected to a material where all the physical properties needed are included in the program. The domains are then connected with the respective physics module to be investigated (COMSOL 2018).

The computer used for simulations was a Dell Precision 7510 with Intel CORE i7 processor.

3.3.1 Modelling and simulations

The AGMD model has been designed in COMSOL so that flow enters the hot and cold side with a speed and a temperature of the respective flow, leaving the system at the same speed. The simulations were made over a membrane, although the module actually has 10 membranes. The assumption was made that the temperature profile looks the same over all membranes because it is the same water with the same temperature that enters the respective membrane. See Figure 12 for the flow directions in the COMSOL model.

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Figure 12. Flow arrows in the model that show inflow and outflow for the hot and cold sides respectively.

3.3.2 Description of geometries

Four different geometries on the AGMD module were examined in COMSOL and compared. The input was the same through all the simulations, the only thing that changed were the geometries. The geometries examined were 180x180 mm (the geometry of the module in reality), 255x255 mm, 250x130 mm and 180x100 mm, see Figures 13 to 16.

Each geometry is built on three blocks where the two outermost blocks represent water and the block in the middle air. Figure 13 shows the first geometry examined, 180x180 mm.

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Figure 13. Square geometry with the 180x180 mm space. This size represents the AGMD module in real life. Figure 14 shows twice the area compared to the real geometry. The purpose of this geometry was to see how the temperature profile changes as the range doubles.

Figure 14. Square geometry with the 255x255 mm space.

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Figure 15 shows a rectangular geometry with approximately the same area as the module in reality. The aim was to see how the temperature profile changes with a different geometry compared to the square geometry. Figure 16 shows a rectangular geometry that was about 1.8 times smaller than the real-life space. The purpose here was to investigate how the temperature profile changes with a smaller area and a different geometry compared to the area and geometry used in reality.

Figure 15. Rectangular geometry with the Figure 16. Rectangular geometry with the 250x130 mm space. Same area as the AGMD 180x100 mm space, which was the smallest space module in reality. simulated.

3.3.3 Mesh

Since COMSOL is built on the finite element method, the geometry to be simulated must have a mesh. The mesh allows a node representation of the domain geometry, allowing the simulations to converge. The same mesh was used for all geometries which was "Finer", see Figure 17.

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Figure 17. Mesh for 180x180 mm geometry. Mesh size was chosen as "Finer" for all geometries.

3.3.4 Physics modules in COMSOL

The simulations took place in one step, consisting of two physics modules. The physics modules used were "Heat Transfer in solids and Fluids" (HT) together with "Laminar Flow"(spf). These two physics modules were connected in "Nonisothermal Flow" (nitf) to include both the heat transport part and the flow part of the module. The simulations used the results of previous parts as initial values, this to improve the probability of convergence and the time it takes to simulate the solution.

3.3.4.1 " Heat Transfer in Solids and Fluids" &" Laminar Flow"

The HT module is used to calculate heat transport in solids and fluids via conduction, convection and radiation. The Spf module is used to calculate the speed and pressure of a liquid in the laminating flow regimen.

Inside the HT and spf physics modules, both the speed and temperature of the water were defined on both the hot side and the cold side. These values were taken from reality and set in COMSOL, see Table 2 for all inputs used.

The SPF module was chosen on the assumption that both flows were laminära. This can be calculated using Reynolds' number. Then Re<2300 we have a laminar flow. On the hot side, Reynolds' number was in the transition phase between laminar flow and turbulent flow,

26 assuming that there was also a laminar flow on the hot side. Reynolds number calculated by equation (5):

휌∗푣∗퐷 푅푒 = (5) 휇 where ρ, V, D and μ are the density of the water, the speed of the water, the diameter of the pipe for the inflow and the dynamic viscosity of water. Re<2300 describes a laminar flow, 23004000 indicates that the flow is turbulent. See Table 1 for input for each physics module and Reynolds hot and cold side number.

Table 1. Input for ht and spf modules and Reynolds number for the flows on the hot and cold side.

Input for the HT module Twarm (°C) 76 Tkall (℃) 19 Input for the spf module Vwarm (m/s) 0,048 Vcold (m/s) 0,0189 3 ρwater (kg/m ) 1000 D (m) 20*10-3 -4 μwarm (Pa*s) 3,797*10 -3 μcold (Pa*s) 1,002*10 Rehot 2528 Rekall 377

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4. Results

All the data presented in the result have been produced by the student himself. Figures, images and tables were made by hand and the model for the simulations in COMSOL was made from scratch.

Verification of air gap membrane distillation module

Figures 18 to 20 show the purity of the water when three different tests were carried out on the module on three different days. 20 nm space balls were dripped into the purified water before being analyzed with DLS. The text in the green fields shows the impact of the space balls, which verifies the purity of the water. The results from DLS are obtained in μm which is a factor of 1000 greater than nm. The measurements resulted in the hydrodynamic diameter being 23.08 nm, 23.87 nm and 24.46 nm respectively, which shows that DLS measured the space balls and that there were no particles larger than the space balls in the water.

The values in the gray fields in Figures 18 to 22 are variables and the basis of the algorithm the DLS instrument uses to arrive at the result. These in themselves are not relevant to the result, but only some instrument shows.

Figure 18. Purity of the newly produced water by Figure 19. Purity of the newly produced water 20 ± 4 nm space bullets, day 1. with 20 ± 4 nm space balls, day 2.

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Figure 20. Purity of the newly produced water with 20 ± 4 nm space balls, day 3.

Figure 21 shows the purity of the Electrum laboratory's distilled water, which was used as inlet water in the module to be further purified. Figure 22 shows the purity of ordinary tap water.

Figure 21. The purity of the distilled water of the Figure 22. Cleanliness of cold tap water. laboratory.

Figures 21 and 22 were used as a comparison with the purity of the AGMD, to show how much cleaner water is obtained with the AGMD module compared to ordinary tap water and distilled water.

4.1 Container material selection

Figure 23 shows the purity over time of white borosilicate glass. The purity was measured in μm and time in minutes. No clear trend could be seen other than that the water quickly became dirty when it came into contact with the glass surface. Table 2 shows the temperatures for the hot and cold side and the pressure in and out of the AGMD for each time period. The purity of the different time periods is clearly presented in Table 2.

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White borosilicate glass 60

50

40

30

Purity Purity (μm) 20

10

0 0 2 4 6 8 10 12 14 16 Time (min)

Figure 23. Graph of results for white borosilicate glass where the purity of each time period studied can be read.

Table 2. Values that could be read from the AGMD module when tests were performed on white borosilicate glass.

White borosilicate glass Time (min) 5 5 10 15 15

Tv (℃) 75,1 70,7 71 73,4 73,4 Tk (℃) 19 19,8 19,6 26 19,6 Pin (kPa) 27,2 23,8 27,2 22,3 24,5 Put (kPa) 2,2 1,3 10,3 5,3 2,7 Purity (μm) 52,453 19,61 42,29 5,897 23,95

Figure 24 shows the purity over time of brown borosilicate glass. The measuring points were far too scattered to see a clear trend in how the water gets dirty over time. What can be said is that the water quickly became dirty in this container. See Table 3 for the temperatures on the hot and cold side and the pressure in and out of the AGMD.

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Brown borosilicate glass 60

50

40

30

Purity Purity (μm) 20

10

0 0 2 4 6 8 10 12 14 16 Time (min)

Figure 24. Graph of results for brown borosilicate glass where the purity of each time period studied can be read.

Table 3. Values that could be read from the AGMD module when tests were carried out on brown borosilicate glass.

Brown borosilicate glass Time (min) 5 5 10 10 15 15

Tv (℃) 70,7 69 70,5 73 70,7 72,6 Tk (℃) 18,5 19,8 19,6 22 18,4 19,8 Pin (kPa) 25,2 36 26,3 21,5 13,4 27,5 Put (kPa) 3,9 19,1 4,8 6,2 7 0,9 Purity (μm) 0,036 19,37 53,386 34,11 4,255 50,25

Figure 25 shows the purity over time of ectfe which, unlike the containers above, is a plastic. No clear trend could be seen and the water got dirty quickly in this container. See Table 4 for the temperatures on the hot and cold side and the pressure in and out of the AGMD module when the experiments were conducted.

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ECTFE 30

25

20

15

Purity Purity (μm) 10

5

0 0 2 4 6 8 10 12 14 16 Time (min)

Figure 25. Graph of ectfe results where the purity of each time period studied can be read.

Table 4. Values that could be read from the AGMD module when testing was performed on ECTFE.

ECTFE Time (min) 5 5 10 15

Tv (℃) 72,8 74,1 70,7 71,3 Tk (℃) 18,1 20,9 19,5 18,8 Pin (kPa) 26,7 21,6 25,6 24,9 Put (kPa) 6,5 – 3 3,6 9,1 Purity (μm) 17,584 8,714 12,7 28,31

Figure 26 shows the purity over time of THE PVDF. Like the three results above, it was not possible to see a clear trend in how dirty the water became in this container. What can be said was that the water gets too dirty in this container very quickly. Table 5 shows the temperatures on the hot and cold side and the pressure in and out of the AGMD module.

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PVDF 300

250

200

150

Purity Purity (μm) 100

50

0 0 2 4 6 8 10 12 14 16 Time (min)

Figure 26. Graph of pvdf results where the purity of each time period studied can be read.

Table 5. Values that could be read from the AGMD module when testing was performed on PVDF.

PVDF Time (min) 5 10 15 15 15

Tv (℃) 73,2 69,7 70,5 75,1 72,5 Tk (℃) 18,5 19,6 18,4 19,4 18,1 Pin (kPa) 28,9 21,9 20,2 23,3 28,8 Put (kPa) 4,7 3,3 5,5 4,2 9,2 Purity (μm) 2,66 64,708 39,467 0,08598 259,69

For ECTFE and PVDF, a slightly clearer trend can be seen than the other two glass containers. However, there is a wide spread in the values even for the plastics. What can be observed for all containers is that the water gets dirty very quickly regardless of material.

4.2 Drip test of tap water and UPW on silicon wafers

Figure 27 shows tap water that has been dripped on to a silicon wafer to be dried. Figure 28 shows the dry spots on the silicon wafer, which became after ordinary tap water was evaporated in a clean room environment. The silicon wafer was studied in an SEM to see if there were particles on the silicon wafer. Figures 29 to 32 show some of the different particles found on the silicon wafer when the water evaporated.

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Figure 27. Tap water dripped on a silicon wafer. Figure 28. Drying stains from the tap water.

Figure 29. SEM image 1. Zoomed in 3200 times. Figure 30. SEM image 2. Zoomed in 230 times.

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Figure 31. SEM image 3. Zoomed in 5520 times. Figure 32. SEM image 4. In-zoomed 65100 times.

Figure 33 shows newly produced UPW dripping on a silicon wafer, and Figure 34 shows the UPW droplets evaporating in a clean room environment. With the purified water, no drying stains are seen from the water. Figure 35 is the surface where the UPW drops dried in an SEM, magnified 300 thousand times. The disk was studied in an SEM and no particles could be found from the ultra-pure water on the surface.

Figure 33. Newly produced UPW dripped on Figure 34. Drying stains from UPW. silicon wafer.

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Figure 35. SEM UPW drops dried. No particles and bacteria can be seen.

A drip test was also carried out on the laboratory's distilled water in order to compare with the tap water and UPW, see Figure 36 for the drip stains on the silicon wafer and Figure 37 for the drying stains. Figures 38 and 39 show the particles that were visible on the silicon wafer when it was placed and examined in SEM.

Figure 36. Drip stains from the laboratory's Figure 37. Drying stains from the distilled water. distilled water on silicon wafer.

Figure 38. SEM. Zoomed in 1300 times. Figure 39. SEM 2. Zoomed in 350 times.

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Figure 40 shows 5% hydrogen fluoride that has been dripped onto a silicon wafer to be rinsed off with tap water. Figure 41 shows one of the many particles the tap water left behind when rinsing.

Figure 40. Drop with hydrogen fluoride on silicon Figure 41. SEM image of hydrogen fluoride drop wafer which should then be purified with tap that has been purified off with tap water. One of water. many particles is clearly visible. Zoomed in 11,000 times.

Figure 42 shows 50% hydrogen fluoride dripping onto a silicon wafer that has since been rinsed off with UPW. The purified part of the silicon wafer was studied in SEM where it was enlarged 300,000 times to detect any particles and bacteria, see Figure 43.

Figure 42. Drop with hydrogen fluoride on silicon Figure 43. SEM image of hydrogen fluoride drop wafer which should then be purified with newly that has been purified off with UPW. No produced UPW. particles and/or bacteria were detected.

4.3 Dilution of hydrogen fluoride with tap water and UPW respectively

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An initial test was performed by diluting 5% hydrogen fluoride with tap water and UPW to see how much of each water is used to dilute hydrogen fluoride to a neutral pH of 6-7.

Figure 44 shows the pH of only 5% hydrogen fluoride. pH was about 1.

Figure 44. pH for only 5% hydrogen fluoride.

Figure 45 shows the pH of diluted hydrogen fluoride when ordinary tap water is poured in. Approximately 800 ml of tap water was used to dilute hydrogen fluoride with tap water to a pH of about 6-7. See Figure 40 for the amount of hydrogen fluoride diluted.

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Figure 45. pH of a drop of hydrogen fluoride mixed with 800 ml of tap water. See Figures 46 to 47 for the pH of hydrogen fluoride after INCRENESS OF UPW. In Figure 46, 100 ml of UPW has been poured in and the pH is about 3. In Figure 47, an additional 100 ml of UPW has been poured in (a total of 200 ml) and the pH is 6-7, which is neutral.

Figure 46. pH for 5% hydrogen fluoride when Figure 47. pH for 5% hydrogen fluoride when a 100 ml of UPW has been poured into. pH value is total of 200 ml of UPW is poured into. pH value is about 3. 6-7.

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4.4 Simulations in COMSOL

In the simulations, only the temperature profiles on the hot and cold side for each geometry were studied and compared. Depending on geometry, it took 75 minutes (for the fastest simulation) to 130 minutes (for the slowest geometry) to simulate the results.

4.4.1 Temperature profile on hot and cold side for 180x180 mm

Figure 48 shows the simulated temperature profile of the 180x180 mm geometry of the warm side. It can be seen that at the edges the temperature drops by about five Kelvin, compared to the inlet temperature of 349 Kelvin. And at the exit, the temperature has dropped by about three Kelvins.

Figure 48. Temperature profile for the warm side with geometry 180x180 mm

Figure 49 shows the temperature profile for the same geometry but for the cold side. The temperature at the edges increases by about nine Kelvin, compared to the inlet temperature of 292 Kelvin and the outlet temperature increases by about four Kelvin.

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Figure 49. Temperature profile for the cold side with geometry 180x180 mm 4.4.2 Temperature profile on hot and cold side for 255x255 mm

Figure 50 shows the simulated temperature profile of the largest geometry examined, 255x255 mm. At the edges the temperature drops by about nine Kelvin and at the exit the temperature has dropped by about five Kelvin.

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Figure 50. Temperature profile for the warm side with geometry 255x255 mm

Figure 51 shows the temperature profile for the same geometry but for the cold side. With this geometry, the temperature at the edges increased by about 16 Kelvin and at the outlet the temperature has increased by about seven 8 Kelvin.

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Figure 51. Cold side temperature profile with geometry 255x255 mm

4.4.3 Temperature profile on hot and cold side for 250x130 mm

Figure 52 shows the simulated temperature profile of the 250x130 mm geometry of the warm side. At the edges, the temperature has decreased by about four Kelvin and at the outlet it has decreased by about three Kelvin.

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Figure 52. Temperature profile for the warm side with geometry 250x130 mm

Figure 53 shows the temperature profile for the same geometry but for the cold side. The temperature on the cold side increased by about eight Kelvin at the critical areas at the edges and about 6 Kelvin at the outlet.

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Figure 53. Cold side temperature profile with geometry 250x130 mm

4.4.4 Temperature profile on hot and cold side for 180x100 mm

Figure 54 shows the temperature profile of the smallest space examined, with a rectangular geometry measuring 180x100 mm. With this area, the temperature decreases very little above the surface. Down at the edge there is a slight drop in temperature by about three Kelvin. Above the surface, the temperature is otherwise very even where it differs about one to two degrees Kelvin compared to the inlet temperature.

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Figure 54. Temperature profile for the warm side with geometry 180x100 mm

Figure 55 shows the same geometry but for the cold side. The temperature on the cold side did not increase significantly, at the critical areas (at the edges) the temperature increased by about four Kelvins.

Figure 55. Temperature profile for the cold side with geometry 180x100 mm

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5. Discussion

This work is very close to research and has dealt with research and development of new technologies and methods as well as modifications and improvements to existing ones. The work that has been done confirms the theories that exist in the field of actual experiments, this is also a very good basis for future research.

5.1 Air gap membrane distillation module

Figures 18 to 20 verify that the AGMD module produces ultra-pure water. The particles in the water do not exceed the order of 20 nm when the water is newly produced. Probably the particle size in the ultra-pure water is much less than 20 nm but since there are currently no space balls less than 20 ± 4 nm, it is difficult to verify this. What can definitely be said is that the average size of the particles in the water does not exceed 20 ± 4 nm. Figures 21 to 22 show the particle size of distilled water and tap water; it is possible to see a clear difference in the quality of the water by comparing the size of the particles detected in each water. Thus, the module works as it should and produces water that is ultra-pure.

The AGMD module is still in the development phase and there are opportunities for improvement. Among other things, it is possible to improve and simplify the start-up of the module as it currently takes about an hour to start, but it is not possible to get down that time too much because the module needs to run for a while to purify itself. What can be done is to improve the heating of the water that is going on to the hot side as it is currently unstable and takes a long time to get up to temperature. Getting to a more stable temperature of the hot water is desirable to get a more stable process. Even the pressure in and out of the module can become more stable.

Once the module is up and running, there are some development opportunities linked to making the module more autonomous. Constant monitoring is needed today to keep track of the temperature on the hot side and production. At present, the module must be constantly replenished with water on the hot side, as it disappears a quantity like UPW, and this is done manually. Here the module could be developed so that the replenishment takes place more automatically where even the temperature is more automatically controlled. Since the temperature drops a few degrees when water is replenished at the hot side, it is desirable, for example, to have it set for the water to reach a stable and even temperature of 80 °C, which then happens automatically when water is replenished. The module also needs to be vented at regular intervals, which is done manually in the prototype that exists today. Today the venting takes place in such a way that a hose is disconnected from the system and water is filled on the hot side until the system is full again and all the air is gone, then the hose is plugged in again and some water has to be refilled. This makes the production of UPW a bit choppy because the temperature of the hot water changes by a few degrees Celsius when venting. It would be desirable to have an open expansion vessel that automatically vents the system without having to disconnect a hose and get the air out by pumping through water. This had improved the production of UPW and had a more stable and even process.

The emptying of the module when it has been completed for the day is also done manually and in several steps where first the cold side must be emptied and then the hot side by unplugging certain hoses and tilting the module in different ways to get out of the water. An automatic

47 emptying had simplified the handling of the module and accelerated the emptying of it and ensured that the module was actually emptied completely.

The prototype that exists today definitely produces ultra-pure water, which is the most important thing. It is also durable, meaning that it can be running many hours a day and produce UPW. The membranes hold tight and they do not break easily, the module can handle high temperatures without problems. To have the module more automatically controlled in the future is definitely desirable partly to get it more stable and partly to simplify the work of the person working with the module.

5.2 DLS and Space Balls

When UPW was produced and it was to be verified that it was ultra-pure water produced, space balls with a diameter of 20 ± 4 nm were dropped into the purified water, which was then tucked into the DLS to see if it gave a rash. This is not a recognised method in the industry to measure the purity of water, but instead the conductivity of the water is measured. In very clean water, the conductivity is 18.2 MΩ. In this case, measuring the conductivity of ultra-pure water would not have talked about how clean the water really is, because it had not been told how small/large the particles in the water are, but only something about its conductivity. Therefore, in this case, it is more appropriate to measure the particle size of the water via a DLS that can detect small particles through their random movement in the water.

To get even better track of how small particles there really are in the water, even smaller space balls can be used. At present, there are no space balls less than 20 nm but in the future it may be that space bullets with a diameter of 10 nm are produced. Then it would be advisable to use them to see if the particles are even less than 10 nm. At present, it is not possible to say whether the particles are so small, what can be said is that the particles are not larger than 20 nm.

A test with DLS took about 15 minutes to run. Doing experiments on the purified water over many time periods therefore takes a long time. For example, if one would want to test the purity of the water in a given container every 30 seconds over a five-minute period, it would take 2.5 hours in the only driving time of the DLS instrument. Ideally, it would be to have multiple DLS instruments to be able to perform multiple experiments simultaneously and be able to compare several different containers and time periods at the same time. However, this is not fully applicable in reality as a DLS instrument costs a lot and more operators will then be needed to handle the measuring instruments.

5.3 Container material selection

Figures 23 through 26 show the results of the purity of the different containers. As can be seen in the figures, it is very difficult to say anything about how quickly the water gets dirty and at what time because it happened so quickly and the results were widely distributed. Immediately at the first measuring point at five minutes, the water was too dirty in all containers. Subsequently, the results varied widely.

In Figure 23 for white borosilicate glass, the water was at its cleanest as it lay in the container for 15 minutes if compared to the other time periods. This went against the theory that the water will get dirtier over time. Similarly, in Figure 24 for brown borosilicate glass, it was possible to see how the water got dirtier over time, but at minute 15 a result was obtained that was below the previous results. When a retry was made at minute 15 for this container, the results were

48 higher. Similarly, at minute 5 for the same container, two completely different results were obtained. Similar can say about Figure 25 for ECTFE and Figure 26 for PVDF. Some results deviated far too much to be able to say anything about the results and no clear trend could be seen.

It is difficult to say what these abnormal results are due to, on the one hand, the water is very aggressive and unstable, every little detail in the handling can be crucial for purity. Also, how much of the water was in contact with each container can have been of great importance on purity. Since the containers had different geometries and sizes, it becomes difficult to compare them fairly, as the water had different interfaces with the respective containers. It may also depend on how many times a material has been used, whether the material cleanses itself over time or whether over time it becomes more worn and releases more dirt.

In order to reduce the contamination from man and environment to the samples, all experiments were carried out with protective clothing and in a clean room. However, it was not entirely certain that all particles were eliminated from man and the environment, there is a small risk that the samples could still be contaminated. However, many small factors can affect the quality of water over time. All the values obtained from the water were certainly right, but probably the experimental conditions made the values different and far too scattered to see a clear trend. This shows the incredible sensitivity and purity of the water.

More research and more time will be needed on the choice of materials for the containers. One suggestion is to test materials with the same geometry and size and perform many experiments at each time period. Since the water becomes too dirty already at 5 minutes, tests could be carried out during the time period from zero to five minutes to see at what time the water exceeds a particle size of 20 nm and thus does not give a rash on the space balls. Another suggestion is to take the same material and make the same attempt at the same time over and over again, to see if materials exposed to UPW become increasingly cleaner or release more particles.

However, the purpose of this produced water is to be used directly and on site, which could be verified with these results. The water is too aggressive and too clean to be stored and it should be used directly it is produced for best results.

5.4 Drip test on silicon wafer

The drip test on the silicon wafer confirms the purity of the water produced with AGMD, see Figure 33 for drip testing with UPW and Figure 34 for its drying stains. Figure 35 shows the drying spots in a scanning electron microscope where no particles could be found on the surface of the silicon wafer. This was compared to when tap water and distilled water were dripped onto a silicon wafer and dried. Clear differences were seen depending on the water dripping on the disc. Figure 37 shows the drying spots for distilled water and Figures 38 to 39 show when it was studied in an SEM.

With UPW produced with AGMD, no particles could be found on the silicon wafer, which is another verification of the purity of the water and perhaps even a verification that the particles present in the water are much less than 20 nm. This is because it is possible to enlarge 300 thousand times in an SEM on the surface. Had there been particles the size of 20 nm, these could have been detected which they did not. However, it was possible to see large and clear

49 particles in the tap water that got stuck on the silicon wafer. Even with the distilled water, particles could be found.

With ordinary tap water there were clear drying spots on the disc which could be seen visually. This was not the case with the laboratory's distilled water or with the ultra-clean water. However, particles were found on the silicon wafer with the laboratory's distilled water when examined in an SEM. It is difficult to say exactly where these particles come from, but it can be, for example, plastic particles that have come loose from pipes and the like. It may also depend on how long the distilled water lay in the containers before it was used, which is difficult to say because it is produced continuously in the laboratory's basement. However, this is a clear example of the importance of what material the water is stored in and that it cannot be stored for long, even if it is not ultra-pure water.

Further experiments on this could be done to measure the purity of the water, as a complement to the DLS instrument. It could be possible to store the water in different containers over different time periods and then drip onto a silicon wafer and study any particles in an SEM, to see which materials release particles to the water and after how long. Thus instead of measuring the purity of a DLS, you study the particles that dried on the surface, in this way also get reactions between the silicon disk and the water with.

5.4.1 Drop with hydrogen fluoride on silicon wafer rinsed off with UPW and plain water

In the semiconductor industry, the silicon wafers are cleaned with 5% hydrogen fluoride, which is then rinsed off with large amounts of water to purify the silicon wafer. Yet particles and other things stick from the water on the disc, causing islands with dead bacteria/particles to form. These areas are then destroyed and it is not possible to manufacture anything on them, therefore it was important to investigate whether UPW that was produced left any islands with dead particles /bacteria on the silicon wafer when it rinses off the hydrogen fluoride droplet on the silicon wafer.

Figure 41 shows the silicon wafer in an SEM when a drop of hydrogen fluoride has been rinsed off with tap water. It was possible to find islands with particles and dead bacteria on the surface of the silicon wafer. This shows how poor-quality water purifies a silicon wafer.

Figure 43 shows sem image of the same experiment but instead of rinsing with tap water, the disc was rinsed with UPW. No particles or bacteria could be detected. This means that the ultra- pure water is ideal for purifying a silicon wafer that has been treated with 5% hydrogen fluoride.

In this experiment, hydrogen fluoride was dropped only on a small part of the surface of the silicon wafer, it would be interesting to pour hydrogen fluoride on the entire surface of the disc and clean with UPW to see if any particles stick to the surface then. It would also need to do more experiments in which way it is most effective to purify the surface. By pouring UPW on to the surface and if so, what flow it is required to clean the surface properly. Or immerse the silicon wafer with hydrogen fluoride into a bath with UPW.

5.5 Dilution of hydrogen fluoride with tap water and UPW

5% hydrogen fluoride is a very strong acid with a pH value of about 1, see Figure 44. Diluting this acid requires a basic solution and the amount needed depends on how alkaline and

50 aggressive the solution is. Figure 45 shows when 5% hydrogen fluoride has been diluted with tap water, 800 ml of water was used to dilute a drop of hydrogen fluoride. Compare with UPW where only 200 ml was used to dilute the same amount of hydrogen fluoride, see Figure 47. This shows how clean the ultra-clean water is compared to ordinary water.

This test also verifies the purity of the water as it consumed 4 times more water of inferior quality to dilute the same amount of hydrogen fluoride. This experiment was an initial methodology for future research and there will need to be more experiments to develop a better methodology. A foundation is laid for the future in case similar experiments are to be carried out. What could be improved is to use better instruments to measure pH to give an exact value. It would also be interesting to test diluting hydrogen fluoride with UPW that has been stored in different containers over different periods of time. This is to get an additional method to study the ability of different container materials to store UPW while maintaining purity.

5.6 COMSOL

The module built up in COMSOL was a simplified version of it in reality. Many components that exist in reality were removed in the simulation model, however, these components had no bearing on the simulations. However, the model in COMSOL could be developed and become more similar to that in reality to get even more accurate results. The COMSOL model simulated only over one membrane while in reality it consists of 10 membranes. The assumption was made that the temperature profile looks exactly the same for all membranes. This would need to be tested to really verify this assumption.

There were no simulations on the outflow of UPW. More research would be required on the module in real life to get more accurate input to be able to compare with the simulations. Then the model in COMSOL could be built on and developed to also simulate the outflow of UPW. Then the number of membranes and different geometries could be simulated to see how the production of UPW changes with these factors.

5.6.1 Temperature profile for the hot and cold side of COMSOL

It was clear to see how the temperature profile on the hot and cold side changed depending on the geometry and size of the membranes. Figure 48 shows the temperature profile on the hot side of the geometry of the module in reality. On the hot side, the temperature profile was even; there were areas along the edges where the temperature dropped by about 5 Kelvin. If the temperature drops in some places on the surface, it will lead to a worsening diffusion in these places compared to the surfaces where the temperature is kept even. On the cold side, the temperature increased by about 9 Kelvin at the edges and 4 Kelvin at the outlet, see Figure 49. If these temperature profiles are compared to the one with a twice the area, it was clear to see how the temperature profile deteriorates with a larger area as the flow and inlet temperatures are kept constant, see Figures 50 and 51.

As the geometry changed from square to rectangular, there were no major differences in the temperature profile of the two different geometries with the same area (180x180 mm and 250x130 mm). There was a slightly lower temperature loss on the hot side for the rectangular surface but on the other hand, the temperature increased some degree more on the cold side, see Figures 52 and 53.

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The size with the clear best temperature profile on both the hot and cold sides was the smallest range (180x100 mm), see Figures 54 and 55. This confirms that the temperature profile is improved with a smaller area. However, the diffusion surface becomes smaller as the area decreases, resulting in a lower flow (Drioli et al. 2015). Thus, it is important to optimize the geometry and area in terms of both temperature profile and production of UPW. It is not possible to look solely at the temperature profile, even the production of UPW must be taken into account. This will be further research and development of the COMSOL model. However, it can be said that with the 255x255 mm area, the temperature profile becomes poor, there is a large loss in heat over the surface. If you want a larger area, the speed and temperature on the hot side should be increased to maintain a good enough diffusion over the entire surface. Otherwise, it will be unnecessary to have a large area if only part of the area is used because the remaining part has lost too much temperature to be able to give diffusion effectively.

In order to increase the production of UPW and get a better temperature profile for each geometry, several things could be tested. Among other things, to increase the speed of flow on both the hot and cold sides. Try to lower the temperature of the water entering the cold side while raising the temperature on the hot side.

It was not possible to say whether the temperature profiles improve as the geometry changes from square to rectangular. More sizes had to be compared and also tested in reality. However, it will be difficult to try different membranes because the production cost will be too high; it would require the manufacture of more membranes and a complete redesign of the entire module.

Then there is also the aesthetic aspect, as the module looks today, it best fits with having square membranes. And with a larger area it takes up more space which can make everything a little more difficult with handling and operation. Emptying and cleaning of AGMD can also be made more difficult if the range is much larger.

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6. Conclusion

This study that has been done is very close to research and a good basis for further research in the subject. The study was about developing new technologies and methods for experiments and developing and modifying existing techniques and methods.

The AGMD module designed by Rejlers Karlstad is stable and works very well, there are also opportunities for improvement to make it more self-propelled. It is capable of running for extended periods of time and is durable, no leaks or anything else was detected. It produces ultra-pure water where the average diameter of the particles is not greater than 20 nm.

The purity of the water was first and foremost verified by space bullets in the water tested in DLS. The purity was also verified on its drying stains on a silicon wafer studied in an SEM. Ordinary water and distilled water left bacteria and particles on the silicon wafer. Since cleaning a silicon wafer is very important for the final product, it is critical that no particles are left behind after the cleaning step on the disc, which it did not do with UPW.

It is not possible to store the water in the four different container materials examined without the water getting dirty. The ultra-clean water is very aggressive, which was also seen as it required four times as much tap water than UPW to dilute 5% hydrogen fluoride. More research and experimentation on the storage of UPW is needed. More materials of similar size and geometry should be investigated. At present, this water should be used directly it is produced without storing it in any container.

The simulations in COMSOL showed that with a smaller area, a better temperature profile is obtained and vice versa with a larger area. The geometry of the membranes did not matter much for the temperature profile. It is important to optimize the temperature profile against the production of UPW and with a smaller area a smaller diffusion surface is obtained, which leads to a reduced production of ultra-pure water. The area currently used (180x180 mm) has a good temperature profile and therefore does not need to be changed to any of the other three options.

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7. Future research

There are many possible future research opportunities in this area. Developing and improving the AGMD module is one of them because it is still in the development phase. One question to investigate for future research for the AGMD module is, among other things, whether it is better to have, for example, 30 membranes in the module for an increased production of UPW or whether it is better to have three separate modules with 10 membranes each.

Producing smaller space balls and examining how small the particles really are in the water would give even better understanding of the purity of the water. This is an important study for future research to be able to be at the forefront with ultra-pure water and be able to offer a really clean water whose purity has also been verified.

Future research in this area should focus on finding materials capable of storing UPW while maintaining purity. More experiments should be conducted over more time periods where water purity is examined every 30 seconds over a given period of time, such as five, ten or 15 minutes, to find a good trend in purity over time.

For the simulations in COMSOL, future research would focus on including the production of UPW. This is to make it easier to optimize the best size and range taking into account both temperature profile and production of UPW. Then different temperatures on the hot and cold side would also be examined and the inlet speed on the hot and cold side to optimize them as well.

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References

Alkhudhiri, A., Darwish, N. & Hilal, N. (2012). Membrane distillation: A comprehensive review. Desalination, 287 2–18. doi:10.1016/j.desal.2011.08.027. Alklaibi, A. M. & Lior, N. (2005). Membrane-distillation desalination: Status and potential. Desalination, 171(2), 111–131. doi:10.1016/j.desal.2004.03.024. Al-Obaidani, S., Curcio, E., Macedonio, F., Di Profio, G., Al-Hinai, H. & Drioli, E. (2008). Potential of membrane distillation in seawater desalination: Thermal efficiency, sensitivity study and cost estimation. Journal of Membrane Science, 323(1), 85–98. doi:10.1016/j.memsci.2008.06.006. Banat, F. A. & Simandl, J. (1994). Theoretical and experimental study in membrane distillation. Desalination, 95(1), 39–52. doi:10.1016/0011-9164(94)00005-0. Bandini, S., Gostoli, C. & Sarti, G. C. (1992). Separation efficiency in vacuum membrane distillation. Journal of Membrane Science, 73(2), 217–229. doi:10.1016/0376- 7388(92)80131-3. Berne, B. J. & Pecora, R. (2000). Dynamic Light Scattering: With Applications to Chemistry, Biology, and Physics. Courier Corporation. Bhadja, V., Makwana, B. S., Maiti, S., Sharma, S. & Chatterjee, U. (2015). Comparative Efficacy Study of Different Types of Ion Exchange Membranes for Production of Ultra-pure Water via Electrodeionization. Industrial & Engineering Chemistry Research, 54(44), 10974–10982. doi:10.1021/acs.iecr.5b03043. Bonyadi, S. & Chung, T. S. (2007). Flux enhancement in membrane distillation by fabrication of dual layer hydrophilic–hydrophobic hollow fiber membranes. Journal of Membrane Science, 306(1), 134–146. doi:10.1016/j.memsci.2007.08.034. Bouguecha, S., Chouikh, R. & Dhahbi, M. (2003). Numerical study of the coupled heat and mass transfer in membrane distillation. Desalination, 152(1), 245–252. doi:10.1016/S0011-9164(02)01070-6. Bullis, W. M. (2000). Current trends in silicon defect technology. Materials Science and Engineering: B, 72(2), 93–98. doi:10.1016/S0921-5107(99)00509-7. Chicea, D (2010). Nanoparticles and nanoparticle aggregates sizing by DLS and AFM. Optoelectronics and Advanced Materials, 7. Chicea, Dan & Rei, S. M. (2019). Using Dynamic Light Scattering for Monitoring the Size of the Suspended Particles in Wastewater. Transylvanian Review of Systematical and Ecological Research, 21(3), 1–10. doi:10.2478/trser-2019-0015. Chung, H.-Y., Kim, Y.-H., Cho, H.-Y., Lee, B.-Y., Yoo, H.-D. & Lee, S.-H. (2001). Collection Efficiency of Metallic Contaminants on Si Wafer by Vapor-Phase Decomposition-Droplet Collection. Analytical Sciences, 17(5), 653–658. doi:10.2116/analsci.17.653. Drioli, E., Laganà, F., Criscuoli, A. & Barbieri, G. (1999). Integrated membrane operations in desalination processes. Desalination, 122(2), 141–145. doi:10.1016/S0011- 9164(99)00034-X. Drioli, E. & Wu, Y. (1985). Membrane distillation : An experimental study. Desalination, 53(1), 339–346. doi:10.1016/0011-9164(85)85071-2. Drioli, Enrico, Ali, A. & Macedonio, F. (2015a). Membrane distillation: Recent developments and perspectives. Desalination, 356 56–84. doi:10.1016/j.desal.2014.10.028. Drioli, Enrico, Ali, A. & Macedonio, F. (2015b). Membrane distillation: Recent developments and perspectives. Desalination, 356 56–84. doi:10.1016/j.desal.2014.10.028. El-Bourawi, M. S., Ding, Z., Ma, R. & Khayet, M. (2006). A framework for better understanding membrane distillation separation process. Journal of Membrane Science, 285(1), 4–29. doi:10.1016/j.memsci.2006.08.002. 55

Franken, A. C. M., Nolten, J. A. M., Mulder, M. H. V., Bargeman, D. & Smolders, C. A. (1987). Wetting criteria for the applicability of membrane distillation. Journal of Membrane Science, 33(3), 315–328. doi:10.1016/S0376-7388(00)80288-4. Hendren, Z. D., Brant, J. & Wiesner, M. R. (2009). Surface modification of nanostructured ceramic membranes for direct contact membrane distillation. Journal of Membrane Science, 331(1), 1–10. doi:10.1016/j.memsci.2008.11.038. Herrling, M. P. & Rychen, P. (2017). Review of nanoparticles in ultra-pure water: definitions and current metrologies for detection and control - Ultra-pure Micro. https://www.ultra-puremicro.com/articles/review-of-nanoparticles-in-ultra-pure-water- definitions-and-current-metrologies-for-detection-and-control [2019-10-10]. Imtisal-e-Noor, Coenen, J., Martin, A. & Dahl, O. (2019). Performance assessment of chemical mechanical planarization wastewater treatment in nano-electronics industries using membrane distillation. Separation and Purification Technology, 116201. doi:10.1016/j.seppur.2019.116201. Itano, M., Kern, W., Miyashita, M. & Ohmi, T. (1993). Particle removal from silicon wafer surface in wet cleaning process. IEEE Transactions on Semiconductor Manufacturing, Semiconductor Manufacturing, IEEE Transactions on, IEEE Trans. Semicond. Manufact., (3), 258. doi:10.1109/66.238174. Kamaz, M., Sengupta, A., Gutierrez, A., Chiao, Y.-H. & Wickramasinghe, R. (2019a). Surface Modification of PVDF Membranes for Treating Produced Waters by Direct Contact Membrane Distillation. International Journal of Environmental Research and Public Health, 16(5), 685. doi:10.3390/ijerph16050685. Kamaz, M., Sengupta, A., Gutierrez, A., Chiao, Y.-H. & Wickramasinghe, R. (2019b). Surface Modification of PVDF Membranes for Treating Produced Waters by Direct Contact Membrane Distillation. International Journal of Environmental Research and Public Health, 16(5), 685. doi:10.3390/ijerph16050685. Kern, W. (1970). Cleaning Solution Based on Hydrogen Peroxide for Use in Silicon Semiconductor Technology. RCA Review, 31 187–205. Khayet, M., Khulbe, K. C. & Matsuura, T. (2004a). Characterization of membranes for membrane distillation by atomic force microscopy and estimation of their water vapor transfer coefficients in vacuum membrane distillation process. Journal of Membrane Science, 238(1), 199–211. doi:10.1016/j.memsci.2004.03.036. Khayet, M., Khulbe, K. C. & Matsuura, T. (2004b). Characterization of membranes for membrane distillation by atomic force microscopy and estimation of their water vapor transfer coefficients in vacuum membrane distillation process. Journal of Membrane Science, 238(1), 199–211. doi:10.1016/j.memsci.2004.03.036. Khayet, Mohamed, Godino, P. & Mengual, J. I. (2000). Theory and experiments on sweeping gas membrane distillation. Journal of Membrane Science, 165(2), 261–272. doi:10.1016/S0376-7388(99)00236-7. Khayet, Mohamed & Matsuura, T. (2011). Membrane Distillation: Principles and Applications. Elsevier. Khvataeva-Domanov, A., Altmaier, S., Lange, M. & Mabic, S. (2017). Tips and Tricks for Handling High Purity Water in the LC-MS Laboratory. EMD MILLIPORE. Kimura, S., Nakao, S.-I. & Shimatani, S.-I. (1987). Transport phenomena in membrane distillation. Journal of Membrane Science, 33(3), 285–298. doi:10.1016/S0376- 7388(00)80286-0. Kulakov, L. A., McAlister, M. B., Ogden, K. L., Larkin, M. J. & O’Hanlon, J. F. (2002). Analysis of Bacteria Contaminating Ultra-pure Water in Industrial Systems. Applied and Environmental Microbiology, 68(4), 1548–1555. doi:10.1128/AEM.68.4.1548- 1555.2002.

56

Laganà, F., Barbieri, G. & Drioli, E. (2000). Direct contact membrane distillation: modelling and concentration experiments. Journal of Membrane Science, 166(1), 1–11. doi:10.1016/S0376-7388(99)00234-3. Lawson, K. W. & Lloyd, D. R. (1997a). Membrane distillation. Journal of Membrane Science, 124(1), 1–25. doi:10.1016/S0376-7388(96)00236-0. Lawson, K. W. & Lloyd, D. R. (1997b). Membrane distillation. Journal of Membrane Science, 124(1), 1–25. doi:10.1016/S0376-7388(96)00236-0. Lee, D.-H., Kim, H.-T., Jang, S.-H., Yi, J.-H., Choi, E.-S. & Park, J.-G. (2018). Effect of organic acids in dilute HF solutions on removal of metal contaminants on silicon wafer. Microelectronic Engineering, 198 98–102. doi:10.1016/j.mee.2018.06.012. Liu, C. & Martin, A. R. (2006). The use of membrane distillation in high-purity water production for the semiconductor industry. Ultra-pure Water, 23(3), 32–38. Liu & Martin (2016). Malmali, M., Fyfe, P., Lincicome, D., Sardari, K. & Wickramasinghe, S. R. (2017). Selecting membranes for treating hydraulic fracturing produced waters by membrane distillation. Separation Science and Technology, 52(2), 266–275. doi:10.1080/01496395.2016.1244550. Melnik, L. A. & Krysenko, D. A. (2019). Ultra-pure Water: Properties, Production, and Use. Journal of Water Chemistry and Technology, 41(3), 143–150. doi:10.3103/S1063455X19030020. Nakata, K., Fukui, T. & Nagai, T. (2017). Particle Adsorption Onto Si-Based Wafers in Ultra- pure Water; Its Mechanism and Effect of Carbon Dioxide. IEEE Transactions on Semiconductor Manufacturing, 30(4), 371–376. doi:10.1109/TSM.2017.2759323. Ohmi, T. (1990). Foresightedness in RCA cleaning concept and their present problem. I Proc. 8th Workshop Ultra Clean Technology. ss.5–15. Onsia, B., Conard, T., Gendt, S. D., Heyns, M., Hoflijk, I., Mertens, P., Meuris, M., Raskin, G., Sioncke, S., Teerlinck, I., Theuwis, A., Steenbergen, J. V. & Vinckier, C. (2005). A Study of the Influence of Typical Wet Chemical Treatments on the germanium Wafer Surface. Reinhardt, K. & Kern, W. (2018). Handbook of Silicon Wafer Cleaning Technology. William Andrew. Riché, E., Regnault, C., Gérion, B. & Bôle, J. (2017). High Purity Water: Hints and Tips. Good Practices in Using a Water Purification System and Handling High Purity Water. EMD MILLIPORE. Ruth, J. & Berndt, R. (2016). Quality control for ultrafiltration of ultra-pure water production for high end semiconductor manufacturing. I 2016 27th Annual SEMI Advanced Semiconductor Manufacturing Conference (ASMC). Presenterad vid 2016 27th Annual SEMI Advanced Semiconductor Manufacturing Conference (ASMC), ss.16–22. Ruzyllo, J. (2010). Semiconductor Cleaning Technology: Forty Years in the Making. Interface, 19(1), 44–46. doi:10.1149/2.F05101if. Ryuta, J., Yoshimi, T., Kondo, H., Okuda, H. & Shimanuki, Y. (1992). Adsorption and Desorption of Metallic Impurities on Si Wafer Surface in SC1 Solution. Japanese Journal of Applied Physics, 31(8R), 2338. doi:10.1143/JJAP.31.2338. Schofield, R. W., Fane, A. G. & Fell, C. J. D. (1987). Heat and mass transfer in membrane distillation. Journal of Membrane Science, 33(3), 299–313. doi:10.1016/S0376- 7388(00)80287-2. Sioncke, S., Brunco, D. P., Meuris, M., Uwamahoro, O., Van Steenbergen, J., Vrancken, E. & Heyns, M. M. (2009). Etch Rate Study of Germanium, GaAs and InGaAs: A Challenge in Semiconductor Processing. Solid State Phenomena. https://www.scientific.net/SSP.145-146.203 [2019-10-14].

57

Summers, E. K., Arafat, H. A. & Lienhard, J. H. (2012). Energy efficiency comparison of single-stage membrane distillation (MD) desalination cycles in different configurations. Desalination, 290 54–66. doi:10.1016/j.desal.2012.01.004. Wiesler, F. (2003). How to meet today’s dissolved oxygen specifications with degasification membranes, 20 38–46. Yonezawa, S., Dobashi, K., Shigeyuki, Iwai, Ichinose, T. & Sakaguchi, T. (2018). Development of extreme microanalysis technology for metallic impurity on a silicon wafer surface. 2018 International Symposium on Semiconductor Manufacturing (ISSM), Semiconductor Manufacturing (ISSM), 2018 International Symposium on, 1. doi:10.1109/ISSM.2018.8651161. Zereshki, S. (2012). Distillation: Advances from Modeling to Applications. BoD – Books on Demand. Zhang, X., Xie, M., Yan, Y., Wu, Y. & Xu, J. (2011). Reactive Oxygen Species and p38 Mitogen-activated Protein Kinase Mediate Exercise-induced Skeletal Muscle-derived Interleukin-6 Expression. Journal of Exercise Science & Fitness, 9(2), 123–127. doi:10.1016/S1728-869X(12)60008-2.

Literature

COMSOL (2018). COMSOL Multiphysics User ́s Guide VERSION 5.4. COMSOL AB.

58

Annex I

A summary was made of the search method of the audited articles. The articles have been searched through One Search, Web of Science or Google. The keywords that were used the most and in combination with other words were: UPW, membrane distillation, semi conductor, air gap membrane distillation and silicon wafer.

Author and year article Item name Keywords Publisher was published Al-Obaidani (2008) Potential of membrane distillation Membrane Journal of Membrane in seawater desalination: Thermal distillation Science efficiency, sensitive study and cost estimation Alkhudhiri 2012 Membrane distillation: A Membrane Desalination comprehensive review distillation Alklaibi, A. M. & Membrane distillation Membrane Desalination Lior, N. (2005) desalination: Status and potential distillation Banat, Simandl Theoretical and experimental From previous Desalination (1994) study in membrane distillation references Bandini (1992) Separation efficiency in vacuum From previous Journal of Membrane membrane distillation references Science Berne, B. J. & Pecora, Dynamic Light Scattering: With “Dynamic Light Courier Corporation R. (2000) Applications to Chemistry, Scattering” Biology and Physics Bhadja, V., Makwana, Comparative Efficacy Study of Ultra-pure Industrial & B. S., Maiti, S., Different Types of Ion Exchange water, Engineering Chemistry Sharma, S. & Membranes for Production of membrane Research Chatterjee. U. (2015) Ultra-pure Water via Electrodeionization Bonyadi, Chung Flux enhancement in membrane From previous Journal of Membrane (2007) distillation by fabrication of dual reference Science layer hydrophilic- hydrophilic hollow fiber membranes Bouguecha ,S., Numerical study of the coupled Heat transfer, Desalination Chouikh, R. & heat and mass transfer in membrane Dhahbi, M. (2003) membrane distillation distillation Bullis, W. Murray Current trends in silicon defect Silicon wafer Materials Science and (2000) technology Engineering: B Chicea, D. Nanoparticles and nanoparticle From previous Optoelectronics and aggregates sizing by DLS and references Advanced Materials AFM Chicea, D., Rei, S. M. Using Dynamic Light Scattering Dynamic Light Transylvanian Review (2019) for Monitoring the Size of the Scattering, of Systematical and Suspended Particles in Wastewater Nanoparticles Ecological Research Chung, H-Y., Kim, Y- Collection Efficiency of Metallic Silicon wafer, Analytical Sciences H., Cho, H-Y., Lee, Contaminants on Si Wafer by HF B-Y., Yoo, H-D. & Vapor-Phase Decomposition- Lee, S-H. (2001) Droplet Collection Drioli, E., Laganá, F., Integrated membrane operations in Membrane Desalination Criscuoli, A. & desalination processes distillation Barbieri, G. (1999) Drioli, E. & Wu, Y. Membrane distillation: An From previous Desalination (1985) experimental study references Drioli, E., Ali, A. & Membrane distillation: Recent Membrane Desalination Macedonio, F. (2015) development and perspectives distillation

El-Bourawi, M.S., A framework for better Membrane Journal of Membrane Ding, Z., Ma, R. & understanding membrane distillation Science Khayet, M. (2006) distillation separation process Franken, A. C M., Wetting criteria for the From previous Journal of Membrane Nolten, J. A. M., applicability of membrane references Science Mulder, M. H. V., distillation Bardeman, D. & Smolders, C.A. (1987) Hendren, Z. D., Brant, Surface modification of Membrane Journal of Membrane J. & Wiesner, M.R. nanostructured ceramic distillation Science (2009) membranes for direct contact membrane distillation Herrling, M. P. & Review of nanoparticles in ultra- Ultra-pure Ultra-pure Micro Rychen, P. (2017) pure water: definitions and current water, metrologies for detection and Nanoparticles control - Ultra-pure Micro Imtisal-e-Noor., Performance assessment of Air gap Separation and Coenen, J., Martin, A. chemical mechanical planarization membrane Purification & Dahl, O. (2019) wastewater treatment in nano- distillation Technology electronics industries using membrane distillation Itano, M., Kern, W., Particle removal from silicon From previous IEEE Transactions on Miyashita, M. & wafer surface in wet cleaning references Semiconductor Ohmi, T. (1993) process Manufacturing, Semiconductor Manufacturing, IEEE Transactions on, IEEE Trans. Semicond. Manufact. Kamaz, M., Sengupta, Surface Modification of PVDF Membranes, International Journal A., Gutierrez, A., Membranes for Treating Produced PVDF, of Environmental Chiao, Y-H. & Waters by Direct Contact membrane Research and Public Wickramasinghe, R. Membrane Distillation distillation Health (2019) Kern, W. (1970) Cleaning Solution Based on Cleaning, RCA Review Hydrogen Peroxide for Use in Silicon wafer, Silicon Semiconductor HF Technology Khayet, M. & Membrane Distillation: Principles Membrane Journal of Membrane Matsuura, T. (2011) and Applications distillation, Science membranes Khayet, M., Godino, Theory and experiments on From previous Journal of Membrane P. & Mengual, J. I. sweeping gas membrane refenser Science (2000) distillation Khayet, M., Khulbe, Characterization of membranes for From previous Journal of Membrane K. C. & Matsuura, T. membrane distillation by atomic references Science (2004) force microscopy and estimation of their water vapor transfer coefficients in vacuum membrane distillation process Khvataeva-Domanov, Tips and Tricks for Handling High High purity EMD MILLIPORE A., Altmaier, S., Purity Water in the LC-MS water Lange, M. & Mabic, Laboratory S. (2017)

Kimura, S., Nakao, S- Transport phenomena in From previous Journal of Membrane I. & Shimatani, S-I. membrane distillation references Science (1987) Kulakov, L. A., Analysis of Bacteria Ultra-pure Applied and McAlister, M. B., Contaminating Ultra-pure Water water, Environmental Ogden, K. L., Larkin, in Industrial Systems Microbiology M. J. & O’Hanlon, J. F. (2002) Laganá, F., Barbieri, Direct contact membrane From previous Journal of Membrane G. & Drioli, E. (2000) distillation: modelling and references Science concentration experiments Lawson, K. W. & Membrane distillation From previous Journal of Membrane Lloyd, D. R. (1997) references Science Lee, D-H., Kim, H-T., Effect of organic acids in dilute Semiconductor, Microelectronic Jang, S-H., Yi, J-H., HF solutions on removal of metal HF Engineering Choi, E-S. & Park, J- contaminants on silicon wafer G (2018) Liu, C. & Martin, The use of membrane distillation From previous Ultra-pure Water A.R. (2006) in high-purity water production for references the semiconductor industry Malmali, M., Fyfe, P., Selecting membranes for treating Membrane Separation Science and Lincicome, D., hydraulic fracturing produced Distillation, Technology Sardari, K. & waters by membrane distillation high purity Wickramasinghe, S.R. water (2017) Melnik, L. A. & Ultra-pure Water: Properties, Ultra-pure Journal of Water Krysenko, D. A. Production, and Use water Chemistry and (2019) Technology Nakata, K., Fukui, T. Particle Adsorption Onto Si-Based Ultra-pure IEEE Transactions on & Nagai, T. (2017) Wafers in Ultra-pure Water; Its water, silicon Semiconductor Mechanism and Effect of Carbon wafer Manufacturing Dioxide Ohmi, T. (1990) Foresightedness in RCA cleaning From previous Ultra-Pure Technology concept and their present problem references Onsia,B., Conard, T., A Study of the Influence of From previous Solid State Phenomena Gendt, S. De., Heyns, Typical Wet Chemical Treatments references M., Hofliijk, I., on the germanium Wafer Surface Mertens, P., Meuris, M., Raskin, G., Sioncke, S., Teerlinck, I., Theuwis, A., Steenbergen, J. V. & Vinckier, C. (2005) Reinhardtm K, & Handbook of Silicon Wafer Silicon Wafer, Elsevier Kern, W. (2018) Cleaning Technology cleaning, Semiconductor Riché, E., Regnault, High Purity Water: Hints and High purity EMD MILLIPORE C., Gérion, B. & Bôle, Tips. Good Practices in Using a water, storage J. (2017) Water Purification System and Handling High Purity Water Ruth, J. & Berndt, R. Quality control for ultrafiltration Ultra-pure Advanced (2016) of ultra-pure water production for water, Semiconductor high end semiconductor Semiconductor, Manufacturing manufacturing particle

Ruzyllo, J. (2010) Semiconductor Cleaning Cleaning, The Electrochemical Technology: Forty Years in the Silicon wafer Society Making Ryuta, J., Yoshimi, Adsorption and Desorption of From previous Japanese Journal of T., Kondo, H., Okuda, Metallic Impurities on Si Wafer references Applied Physics H. & Shimanuki, Y. Surface in SC1 Solution (1992) Schofield, R. W. Heat and mass transfer in From previous Journal of Membrane (1987) membrane distillation references Science Sioncke, S., Brunco, Etch Rate Study of Germanium, From previous Solid State Phenomena D. P., Meuris, M., GaAs and InGaAs: A Challenge in references Uwamahoro, O., Van Semiconductor Processing Steenbergen, J., Vrancken, E. & Heyns, M. M. (2009) Summers, E. K., Energy efficiency comparison of Membrane Desalination Arafat, H. A. & single-stage membrane distillation distillation, air Lienhard, J. H. (2012) (MD) desalination cycles in gap membrane different configurations distillation Wiesler, F. (2003) How to meet today's dissolved Semiconductor ULTRA-PURE oxygen specifications with industry, WATER degasification membranes membrane, high purity water Yonezawa, S., Development of extreme Silicon wafer, 2018 International Dobashi, K., microanalysis technology for semiconductor Symposium on Shigeyuki,. Iwai,. metallic impurity on a silicon devices Semiconductor Ichinose, T. & wafer surface Manufacturing (ISSM) Sakaguchi, T. (2018) Zereshki, S. (2012) Distillation: Advances from From previous BoD - Books on Modeling to Applications references Demand Zhang, X., Xie, M., Reactive Oxygen Species and p38 From previous Journal of Exercise Yan, Y., Wu, Y. & Mitogen-activated Protein Kinase references Science & Fitness Xu, J. (2011) Mediate Exercise-induced Skeletal Muscle-derived Interleukin-6 Expression COMSOL (2018) COMSOL Multiphysics User ś COMSOL AB Guide VERSION 5.4