Technology and the Business of Water Treatment

ULTRAPURE WATER®

Technology and Business of Water Treatment

ELECTRONICS FABRICATION WITH DISSOLVED O3: AN ENVIRONMENTALLY FRIENDLY SOLUTION

By Christiane Gottschalk, Ph.D., and Hans Sundstrom MKS Instruments Inc.

Reprinted with permission from ULTRAPURE WATER® journal, January/february, 2006 OZONE

ELECTRONICS FABRICATION WITH DISSOLVED O3: AN ENVIRONMENTALLY FRIENDLY SOLUTION

equipment configuration and on the for the measurement of both low ambi- nature of the wetted materials used within ent and high process ozone concentra- the equipment. Typically, an ozone/ tions, since other measurement tech- water system consists of: an ozone gen- niques, based on electrochemical or erator, a contactor, an ozone gas de- semiconductor principles, are less reli- struct unit, and at least one ambient able. Different concentration ranges zone (from the ozone monitor. Additional on-line ozone (low ambient levels versus high process Greek word analyzers may be required, depending concentrations) will require different ozon, neuter on the process requirements (e.g., for detector configurations (different cuvette Opresent participle of ozein, to smell) was mass balances), the influent and efflu- lengths) for effective measurement. first identified by chemist Christian Wern- ent ozone gas concentrations as well as Certain UV photometers can also be er Schönbein in 1839 as a by-product of the dissolved ozone concentrations that used for the direct measurement of the electrolysis of water. The first ozone must be measured (Figure 1). ozone dissolved in high-purity water. generator was built by Werner von Sie- mens 18 years later. Since then, ozone Safety considerations. Safety and Materials in contact with ozone. Ozone has found broad industrial application, environmental considerations are criti- is a very strong oxidant, and therefore, especially for the treatment of drinking cal in ozone generating equipment, as all component materials within a system water and wastewater. In the 1990s, the ozone has toxic properties. Effluent that come into contact with ozone must semiconductor industry found applica- gaseous ozone must be re-converted to be highly corrosion resistant. Table A

tions for ozone and ozone/water mix- oxygen before leaving the DIO3 gener- provides an overview of specific mate- tures in its process technology. Dis- ating system and this is usually done rial compatibilities for use with ozone. with an ozone-destruct unit based on Note that some purity and process re- solved ozonated water (DIO3), in particular, has been developed as an envi- either catalytic or thermal principles. An quirements may preclude the use of

ronmentally friendly alternative to sulfu- ambient ozone safety monitor for con- certain materials with DIO3, despite the ric acid and ammonia-based mixtures tinuous monitoring of a potential ozone fact that they are ozone resistant. For that have historically been used in the leak within the system should be provid- example, high quality stainless steel, traditional RCA surface cleaning pro- ed in areas where equipment operators while resistant to gaseous ozone, may cess for semiconductor device fabrica- could face possible exposure, and produce unacceptable metal contami- tion. should additionally be coupled with an nation levels when used with ozonated automatic ozone generator shut-off. water. Great care must therefore be DIO3-based surface cleaning chemistries have highly desirable characteris- Typically, UV-photometers are preferred taken in screening all wetted compo- tics that include improved performance, fewer cleaning steps, and reduced chemical consumption as well as reduced temperature and costs. This article reviews the equipment require-

ments for the generation of DIO3 and surveys the wet cleaning applications of ozonated water in semiconductor device fabrication.

Ozone Generating Equipment In order to produce ozonated water effectively, close attention must be paid to

By Christiane Gottschalk, Ph.D., and Hans Sundstrom MKS Instruments Inc.

ISSN:0747-8291. COPYRIGHT (C) Tall Oaks Pub- lishing, Inc. Reproduction in whole, or in part, including by electronic means, without permission of publisher is prohibited. Those registered with the Copyright Clearance Center (CCC) may photocopy Figure 1. Components for dissolved ozone system. this article for a flat fee per copy.

ULTRAPURE WATER® JANUARY/FEBRUARY 2006--UP230131 33 Figure 2. Silent electric discharge. Figure 3. An example of a static mixer.

nents for material compatibility with re- discharge and molecular oxygen concentrations of 20 %weight (approxi- spect to ozone. present in the gap initiate reactions that mately 300 g/Nm³) ozone in oxygen, for For instance, silver, especially in the lead to the dissociation of the molecular DBD ozone generation systems. oxide form, cannot be used in ozone oxygen into atomic O; ozone is then Two additional methods for ozone pro- systems, since it will catalytically de- produced through recombination of duction should be mentioned. Ozone compose ozone, producing particle and these oxygen atoms with molecular ox- can be produced either through the metal contamination. If conventional ygen (Figure 2). Semiconductor appli- electrolysis of water, or by UV irradiation VCR fittings are used in equipment con- cations require a minimum oxygen puri- of an oxygen containing gas at wave- figured for ozone generation then prob- ty of 99.995% with low levels of total lengths below 200 nanometers (nm), lems will arise, since standard VCR gas- organic carbon (TOC) and humidity. which is similar to the natural process kets are silver-plated. Thus, in ozone Ozone generators based on the silent occurring in the atmospheric strato- systems, non-plated stainless steel gas- discharge principle require small quan- sphere. These methods have capacity kets must be specified in order to avoid tities of a dopant with the oxygen feed. limitations and the cost and size of the this effect. Other oxides (e.g., manga- Typically, these dopants are nitrogen generation equipment are high com-

nese [Mn] or palladium [Pd]), will also (N2) and carbon dioxide (CO2). The use pared to that used in the silent dis- lead to the catalytic decay of ozone and of noble gases as dopants, such as charge method. may be used for the efficient destruction helium (He), neon (Ne), and argon (Ar), of ozone. has also been proposed (1). The pres- Dissolved ozone. The ozone gas pro- Polyvinyl chloride (PVC) and polyvi- ence of the dopant helps to produce duced in a silent discharge can be used nylidene fluoride (PVDF), while less ex- long-term stability in the ozone-genera- directly for certain dry applications. Wet pensive than polytetrafluoroethylene tion process. The dopant concentration (aqueous) applications require an ad- (PTFE)/polyfluoroalkoxy (PFA), are also in the feed may range from 10 parts per ditional process step to transfer the less stable over time. PVC can be used million (ppm) to 10%, which is depen- ozone gas into the fluid. This can be for drain lines but it is not recommended dant upon the ozone generator design done by injection into water within an for process lines. Polypropylene (PP) and the quality of the feed gas. Some application tool, or with a separate con- and polyethylene (PE) must to be avoid- ozone generators can operate without tacting system to produce dissolved ed in all cases since they will be imme- the addition of a dopant gas, providing ozone at the necessary concentrations diately destroyed, producing danger- that the oxygen feed is not “too clean” upstream of the application tool. Inti- ous ozone leaks. (i.e., the oxygen already contains small mate contact of the ozone gas with the amounts of impurities such as nitrogen). fluids can be achieved through a variety Ozone gas generation. Ozone is an Ozone production is an endothermic of different configurations of gas diffus- unstable gas and must therefore be process (ΔH = +143 kilojoule per mole ers so long as they are constructed of generated on-site. Clean ozone gas for [kJ/mol]) (2). Also, the process is rela- materials resistant to ozone. The injec- use in semiconductor applications is tively inefficient, as only 4% to 12% of tion of ozone into solution can be ac- produced from pure oxygen and electri- the electrical energy supplied to the complished using fine porous plates cal energy using a silent electrical dis- DBD goes into the formation of ozone; immersed in the fluid baths, membranes, charge (also known as a dielectric bar- the rest of the input energy is trans- injector nozzles, static mixers, or bub- rier discharge or DBD) in an ozone gen- formed into heat (3). Since the ozone ble columns. erator. Oxygen is passed between two molecule decays quickly at elevated PTFE membrane mixers direct the liq- electrodes separated by a solid dielec- temperatures, efficient cooling is nec- uid phase through a bundle of micro- tric. An alternating high voltage field is essary for the efficient production of porous tubing that is housed in an ap- applied to the one electrode, producing ozone. This balance between energy propriate shell. The gas is passed a discharge in the gap. Impact events input and the need for low process tem- through the outer shell countercurrent between higher energy electrons in the peratures typically results in maximum to the liquid and diffuses through the

34 ULTRAPURE WATER® JANUARY/FEBRUARY 2006--UP230131 porous tube into the liquid. To avoid bubbles, the pressure on the gas side must always be lower than on the fluid side. Static mixers (Figure 3) consist of sev- eral mixing elements arranged in a se- ries within a pipe. Such systems are small and easy to handle. The hydrodynamic conditions provide for a uniform distribution of small bubbles of dissolved ozone over the entire cross section in which the fluid is contained. The ozone gas does not completely dissolve in the fluid in static mixers. The surplus gas appears as bubbles in the outlet of the mixer. If gas bubbles are undesirable in a particular process, a debubbler must be installed downstream of a static mixer. Bubble columns are a very simple type of contactor, but have relatively low transfer efficiency. Modifications that Figure 4. Oxide thickness as a function of ozone concentration and dip time (RC: increase gas/fluid contacting surfaces recirculation mode, OF: overflow mode), 10 Å =1 nm. are known to enhance the efficiency of ozone dissolution, but are seldom reported. Each type of contactor has unique hydrodynamic behavior and mass transfer characteristics and it is important to take these factors into consideration when evaluating experimental results and optimizing processes that require dissolved ozone. Current developments in contacting technology provide ozone concentrations of 30 ppm (parts per million) dissolved in DI water at flowrates of 30 liters per minute (L/min) and over 90 ppm at 5 L/min at ambient temperature (5).

Applications Figure 5. Stripping rates with ozonated DI water as a function of temperature (after Surface cleaning is a critical and often 9). repeated step throughout the manufac- ture process for semiconductor devic- es. Flat panel display (FPD) manufac- ture also requires critical surface cleaning, albeit with fewer cleaning steps in the process and much larger substrate size. Wet techniques offer greater effi- ciencies for surface cleaning and therefore dominate over dry cleaning. The standard RCA cleaning sequence is by far the predominant technique. In this method, the surface is first cleaned

using a sulfuric acid (H2SO4)/hydrogen

peroxide (H2O2) mixture at 120 to 150°C to remove organics, followed by treatment with dilute HF for oxide removal. The surface is then subjected to an

ammonium hydroxide (NH4OH)/H2O2 mixture to eliminate particles and a hy- Figure 6. Metal surface concentration after three types of clean (after 11). Reference: after alkaline etch, RCA clean: modified RCA > 35 min, Spin Drying HF/ drochloric acid (HCl)/H O mixture to 2 2 O3: < 15 min remove soluble metal contamination,

ULTRAPURE WATER® JANUARY/FEBRUARY 2006--UP230131 35 TABLE A Material Compatibility (*: material compatible, X: material not compatible)

Material O3 gas O3 dissolved Comment Metals metals can suffer severe corrosion Stainless steel * X Silver X X silver and other metals can destroy ozone catalytically Inorganic oxides Glass, quartz * * Alumina oxide * X Fe-, Cu, Mn- oxide X X efficient catalyst Organics Most organics are severely attacked PTFE, PFA * * PVDF, PVC X (*) PVDF/PVC are attacked in gas phase, used in drain lines PP, PE X X Kalrez, chemraz * * sealing

both at 80 to 90°C. Alternative process- self-limited oxide thickness 10 to 11Å form layer of silicon dioxide at that inter- es that can accomplish all of the func- (6). The maximum observed oxide thick- face. This can be produced by saturat- tions of the RCA cleaning sequence, ness of SC1 is around 8Å (7). The main ing the surface with oxygen and subse- while either reducing or replacing the parameters influencing oxide growth quently performing an etch-back using

usage of these highly corrosive and rate in DIO3 are the dissolved ozone dHF, or by a more easily controlled environmentally unfriendly chemicals, concentration, the process time, pH, oxide growth process that adjusts, for

are being vigorously pursued within the temperature, and the presence and type example, the exposure time in a DIO3 semiconductor industry. Any new clean- of additives coupled with equipment bath to yield the required thickness. ing processes must address the current configuration (8). challenges in sub-micron particle re- Figure 4 indicates the influence of Photoresist stripping. Photoresists can moval as well as the environmental im- dissolved ozone, dip time, and process be removed by either dry (e.g., plasma pact from the high consumption of water configuration on the thickness of an ashing) or by wet chemical processes. and chemicals and increasing costs. oxide layer grown on a silicon surface in Wet chemical processes commonly use

Ozone-based chemistries have been DIO3. The process configurations in- sulfuric acid (H2SO4), often combined

developed that can replace RCA clean- clude either recirculation mode (RC) or with H2O2, (SPM) or O3 (SOM). LCD ing sequences and fulfill the environ- overflow mode (OF). In the RC mode, panel processing differs somewhat in mental, economic and process require- ozone gas is brought into the bath by a that the photoresist is stripped using a ments of advanced device fabrication. diffuser (solid markers and lines). While mixture of organic solvents such as dim- Ozone is of high interest for semicon- processing the wafer, the gas flow is ethylsulfoxide (DMSO) and mono etha- ductor applications because of its fast stopped to avoid bubbles. In the OF nol amine (MEA). Both the sulfuric acid- reaction rate, a direct consequence of mode (open markers and dotted lines), based and organic solvent-based pro-

its high oxidation potential combined ozonated water (DIO3) is delivered con- cesses are performed at elevated

with low activation energy of its cleaning tinuously by a stand-alone DIO3 system. temperatures. reactions. The following section gives The RC and OF modes yield compara- Ozonated water processes have been some examples of the application of ble results in terms of oxide thickness. used for photoresist stripping, albeit ozone in semiconductor device fabrica- The continuous supply of ozonated DI within certain limitations. Higher strip tion. water used in OF mode was observed to rates can be achieved by increasing the result in superior oxide thickness unifor- dissolved ozone concentration as well Oxide growth. Silicon surfaces that are mities. as the process temperature. However,

treated with ozone or DIO3 solutions The thickness of the oxide grown in the instability of ozone and its decreased

exhibit self-limiting oxide growth and a DIO3 can be effectively controlled by solubility at moderately elevated tem- hydrophilic state in the resultant sur- exposure time and ozone concentra- peratures has hindered process optimi-

face. Silicon dioxide growth in DIO3 at tion. This is a desirable attribute in the zation for resist stripping. Figure 5 shows room temperature self-limits at 10 to process since the full self-limited oxide the stripping performance for I line and 11Å (Angstrom), a relatively thick layer thickness is not always advantageous DUV resist normalized at 20°C (9). A as compared with other chemical oxi- in device construction. For example, rate enhancement factor of up to 2.5 dations. For example, oxide growth with establishing high quality electrical char- was observed when going to 85°C. hydrogen peroxide self-limits at a thick- acteristics of the interface between a It is noteworthy during ozonated water ness of about 7Å. The addition of plat- high-k oxide film (higher dielectric con- stripping processes that the photoresist

inum (Pt) to H2O2 enhances the oxida- stant than silicon oxide) and a silicon is not completely degraded to CO2 and

tion characteristic through the genera- substrate requires the presence of a H2O. Low molecular weight fragments tion of radical species, increasing the well-defined, extremely thin, and uni- with high oxygen content and high sol- 36 ULTRAPURE WATER® JANUARY/FEBRUARY 2006--UP230131 ubility such as acetic acid and oxalic acid are also produced (10).

Metal and particle removal. Particu- late and metal contamination levels will continue to play a critical and potentially negative impact on device yields as feature sizes move into the nanometer regime. Sources of metallic contamination include metal leached from tools and tubing, as well as pre-existing contamination in chemicals and even DI water. Particle contaminations, both organic and inorganic, come from the clean room, process tools, and occa- sionally from chemical reactions. Particle and metal contaminants are currently removed by the use of a two- step cleaning process. SC1 (standard clean) cleans are used to remove particles and SC2 cleans to remove metal Figure 7. Detailed chemical cost per run for PR strip in a spray tool (after 9). contamination from wafer surfaces. Ozonated water cleaning can effectively remove both particulate and metal cess can be enhanced by the use of tation costs. contamination, and similarly requires megasonics. ● Ozone can reduce chemical and the use of a two-chemical process. The Starflinger et al. (12) reported the use high-purity water consumption. sequential use of ozonated water with of this sequence in a single wafer tool acid (e.g., HF and/or HCl) yields excel- with an improved micro roughness com- ● Low disposal costs. lent results in the removal of particulate pared to a typical RCA clean. and metal contaminants. Metals are ● Benign by-products: oxygen and oxidized by ozonated water to form metal Disinfection. The higher oxidation po- water. oxides and hydroxides. These oxides/ tential of ozone makes it a more power- hydroxides can be incorporated into the ful disinfectant than either hydrogen Figure 7 shows a cost comparison for self- layer or bond to the Si-surface. peroxide or chlorine. In comparison to chemicals (including all process gas- Acids can then act as ion exchangers to chlorine, ozone attacks a bacterium by es) between a standard SPM-SC1 clean and a DIO -SC1 photoresist stripping yield soluble and thus removable metal directly destroying the cell membrane, 3 species or, in the case of soluble oxides whereas chlorine must first diffuse process. The comparison shows that with hydrofluoric acid HF, they can sim- through the cell wall to the cytoplasm, the ozone-based process results in a ply dissolve the particle. Ozone alone where it then oxidizes the enzymes de- cost savings of 63%. Ozone-based may be sufficient to remove organic stroying the bacterium. processes thus provide better, or at particulate contaminants. Ozone effectively controls bacteria in least comparable, performance to stan- The advanced cleaning and drying storage tanks and piping. Dosage lev- dard technologies with the additional (ACD) method, developed by ASTEC els are calculated as a function of time benefits of environmentally friendly (Berg, Germany), employs a mixture of t and the dissolved ozone concentra- chemistries and substantial reduction ■ dilute HF and ozone, and combines tion c. A ct value of 1.6 to 2.0 milligrams in process costs. metal removal and drying into one pro- per liter per minute (mg/L/min) (e.g., 0.4 cess. Figure 6 shows a comparison of mg/L of ozone for 5 minutes) is consid- References metal contamination on a Si <100> sur- ered sufficient for disinfection, corre- 1. Manning, T. “Production of Ozone in an face after a single HF/O3 clean, a mod- sponding to ozone concentrations that Electrical Discharge Using Inert Gases ified RCA clean, and an alkaline etch. are much lower than for wafer cleaning. as Catalysts“, Ozone; Science and Engi- Contamination levels of 1 E9 atoms per Residual ozone is easily decomposed neering 22(1), pp. 53-64 (January/Febru- 2 ary 2000). square centimeter (atoms/cm ) or less using UV irradiation. were achieved. 2. Manning, T.; Hedden, J. ”Gas Mixtures Insoluble particles can also be re- Summary and Ozone Production in an Electrical moved from a wafer surface by the se- Ozone is a strong oxidizing agent suit- Discharge“, Ozone; Science and Engi- neering, vol. 23, pp. 95-103 (2001). quential use of ozone and dilute HF. As able for advanced wafer cleaning. previously noted, ozone creates an ox- Ozone-based processes have been 3. Ozonek, J.; Fijalkowski, S.; Pollo, I. “A ide layer that can incorporate these shown to be effective replacements for New Approach to Energy Distribution in particles. Once formed, this layer is traditional wet cleaning processes with Industrial Ozonizers”, Proceedings of the International Ozone Symposium: Appli- easily removed using dHF. Care must the following additional benefits: be taken in the choice of process for cation of Ozone in Water and Wastewater ● Ozone is generated at the point of Treatment, Bin, A.K., ed., pp. 218-227, dHF cleaning since re-deposition and Warsaw Poland (May 26-27, 1994). roughening must be avoided. The pro- use, avoiding storage and transpor-

ULTRAPURE WATER® JANUARY/FEBRUARY 2006--UP230131 37 4. “Static Mixer: A Contactor for Dissolving High-k Deposition”, Proceedings of the Phenomena, Vol. 76–77, pp. 203-206 Ozone Gas in Fluids”, MKS, Wilmington, 7th international Symposium on Ultra Clean (2001). Mass., http://www.mksinst.com/pdf/ Processing of Silicon Surfaces UCPSS, astexSMds.pdf (2004). pp. 37-38, Brussels, Belgium (Sept. 19- Author Christiane Gottschalk, Ph.D., is 22, 2004). 5. “LIQUOZON® Ozonated Water Delivery the sales & marketing manager of MKS

Systems Models 100 and XF”, MKS, Wilm- 9. Truscello, A. “DIO3 post ash cleaning Instruments, ASTeX and has worked in ington, Mass., http://www.mksinst.com/ implementation for significant cost sav- the company’s office in Berlin, Germa- pdf/ASTEXliquozonDS.pdf (2002). ings”, third annual FSI Surface Condition- ny, since 1997. Previously she was ing Symposium, Minneapolis, Minn. (July ozone product manager, technical mar- 6. Ohmi, T.; Isagawa, T.; Kogure, M.; Imao- 9-10, 2003). ka, T. “Native Oxide Growth and Organic keting and project manager. She re- Impurity Removal on Si Surface with 10.Vankerckhoven, H.; De Smedt, F.; Van ceived her Ph.D. from the Technical Ozone-Injected Ultrapure Water”, Jour- Herp, B.; Claes, M.; De Gendt, S.; Heyns, University of Berlin where she studied nal of Electrochemical Society, vol. 140, M.M.; Vinckier, C. “Determination of Pho- environmental engineering with a focus pp. 304-310 (1993). toresist Degradation Products in O3/DI Processing”, Solid State Phenomena, vol. on drinking water research. 7. Claes, M.; Röhr, E.; Conrad, T.; De Smedt, 76-77, pp. 207-210 (2001). Coauthor Hans Sundstrom is ozone F.; De Gendt, S., Storm, W.; Bauer, T.; product marketing manager with MKS Mertens, P.; Heyns, M. M. “Surface Char- 11.Gottschalk, C.; Schweckendiek, J. “Us- and is based out of Wilmington, Mass. acterization after Different Wet Chemical ing Dissolved Ozone in Semiconductor Cleans”, Solid State Phenomena, vol. 76– Cleaning Applications”, Micro, pp. 81- This paper was presented at ULTRAPURE WATER 77, pp. 67-70 (2001). 85, (March, 2004). Portland 2004, which was conducted in Portland, 8. Onsia, B.; Caymax, M.; Conard, T.; De 12. Starflinger, W.; Sellmer, R; Detterbeck, Ore., Oct. 26-27, 2004. Gendt, S.; Delabie, A.; Green, M.” On the S.; Lechner, A.; Leberzammer J.; Kruwi- Application of a Thin Ozone-based Wet nus, H.-J. “New Single Wafer double- Key words: OZONE, SEMICONDUC- Chemical Oxide as an Interface for ALD Sided Spin Cleaning Method“, Solid State TORS

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