Oxygen Plasma Surface Activation of Polynorbornene For

OXYGEN PLASMA SURFACE ACTIVATION OF POLYNORBORNENE FOR

BONDING TO GLASS WITH APPLICATIONS TO MICROFLUIDIC SYSTEMS

RUSSELL LYNN SMITH

Submitted in partial fulfillment of the requirements

For the degree of Master of Science

Thesis Advisor: Professor Christian A. Zorman

Department of Electrical Engineering and Computer Science

CASE WESTERN RESERVE UNIVERSITY

May, 2011

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List of Tables ...... 4

List of Figures ...... 6

Acknowledgments...... 9

1. Introduction ...... 11

1.1 Microfluidics ...... 11

1.1.1 Applications ...... 11

1.1.2 Materials ...... 12

1.1.3 Fabrication Techniques ...... 13

1.1.4 PDMS ...... 14

1.2 Avatrel™ 2585P Polynorbornene ...... 15

1.2.1 Material Properties ...... 16

1.2.2 Conventional Uses ...... 18

1.2.3 Usage of Photopatterned Avatrel™ in MEMS and Microfluidics ...... 18

1.3 Advantages of Avatrel™ 2585P PNB for Microfluidics ...... 19

1.4 Thesis Investigation...... 20

2. Experimental Methods ...... 21

2.1 Oxygen Plasma Surface Activation and Contact Angle Measurement ...... 21

2.1.1 March Instruments PX-250™ Oxygen Plasma System ...... 21

2.1.2 Contact Angle Goniometer ...... 25

2.1.3 Sample Preparation ...... 41

2.1.4 Design of Experiments ...... 42

2.1.5 Time Lapse Testing...... 44

2.2 Bond Strength Testing ...... 45

2.2.1 Method ...... 45

2.2.2 Instruments and Techniques ...... 46

2.2.3 Sample Preparation ...... 50

2.2.4 Bonding ...... 50

2.2.5 Post-bonding Hot Plate Treatment ...... 50

2.2.6 Blister Test ...... 52

2.2.7 Experimental Design ...... 53

3. Data and Analysis ...... 55

3.1 Oxygen Plasma Surface Activation and Contact Angle Measurement ...... 55

3.1.1 Preliminary Testing: Moderate Power and Time ...... 55

3.1.2 Preliminary Testing: Higher Power and Time ...... 58

3.1.3 Primary Testing ...... 61

3.1.4 Time Lapse Testing...... 66

3.2 Bond Strength Testing ...... 67

3.2.1 Preliminary Testing ...... 68

3.2.2 Blister Test ...... 68

3.2.3 Bond Strength Analysis ...... 70

4. Conclusion and Future Work ...... 75

4.1 Conclusion ...... 75

4.2 Future Work ...... 76

Appendix A: Correlation of March Instruments PX-250™ Plasma Pressure with Oxygen

Flow Rate ...... 78

Appendix B: Drop Mass Measurements for Calculation of Drop Volume for EFD™

Dispenser Tip ...... 79

Appendix C: Diameters of Electrochemically-Drilled Holes ...... 80

Appendix D: Preliminary Contact Angle Testing ...... 81

Appendix E: Primary Contact Angle Testing ...... 82

Appendix F: Contact Angle Goniometer Software ...... 83

contact_angle_multi.m ...... 83

contact_angle.m ...... 84

Appendix G: Contact Angle Goniometer Standard Operating Procedures ...... 89

References ...... 96

List of Tables

Table 1-1: Material properties of Avatrel™ 2585P polynorbornene [10] ...... 17

Table 2-1: High contact angle repeatability testing on eight samples of untreated 50 µm

PNB yielding a standard deviation of 1.00° for the manual measurement method ...... 36

Table 2-2: Low contact angle repeatability testing on eight samples of plasma-treated 10

µm PNB yielding a calculation of standard deviation of 0.48° for the manual measurement method and demonstrating that manual measurement is not significantly affected by operator mindset...... 38

Table 2-3: Contact angle of glass coverslip cleaned for 20 minutes in piranha solution of

3:2 98% H2SO4 : 30% H2O2 by elapsed time since cleaning. Contact angle is significantly reduced compared to untreated nominal value of 49.5° and returns gradually over a period of days...... 48

Table 2-4: Contact angle of glass coverslip cleaned for 50 minutes in piranha solution of

2:1 98% H2SO4 : 30% H2O2 by elapsed time since cleaning. Contact angle is further reduced compared to shorter piranha treatment in Table 2-3...... 48

Table 3-1: Prediction model report for primary testing over range of time 10 – 240s, power 8 – 100W, and flow 8 – 100%. Report shows strong sensitivity of contact angle to variations in both power and time and a relatively low sensitivity to flow...... 62

Table 3-2: Contact angle of PNB and PDMS samples at increasing intervals following plasma treatment. Data shows hydrophobic recovery of PNB occurs more slowly than recovery of PDMS...... 67

Table 3-3: Blister test results for tests conducted immediately after bonding. All samples held to 60 psi...... 69

Table 3-4: Blister test results for tests conducted one week (samples 1, 3, and 7) or two weeks (samples 2, 4, 5, 6, and 8) after bonding...... 70

List of Figures

Figure 1-1: Cross-section of Sekar et al. device structure. Silicon microchannels are sealed by a layer of Avatrel™ which is patterned to provide access to vias [25]...... 19

Figure 2-1: March Instruments PX-250™ oxygen plasma system with ENI AGC-3B-

01™ power supply ...... 22

Figure 2-2: March Instruments PX-250™ plasma system oxygen flow rate setting and resulting plasma pressure. Graph suggests a linear relationship...... 24

Figure 2-3: Contact angle of hydrophobic (left) and hydrophilic (right) surfaces ...... 25

Figure 2-4: Contact angle goniometer hardware ...... 29

Figure 2-5: Completed automatic contact angle measurement showing x‟s at detected three-phase points and tangent lines used for calculation of the angle...... 33

Figure 2-6: Two screen captures of manual measurement of left angle performed on same image of a sample with high contact angle. Angle measurement of capture on left is

75.6°; measurement of capture on right is 72.8°...... 39

Figure 2-7: Two screen captures of manual measurement of left angle performed on same image of a sample with low contact angle. Angle measurement of capture on left is 4.1°; measurement of capture on right is 4.8°...... 40

Figure 2-8: Electrochemical drilling setup ...... 47

Figure 2-9: Blister test apparatus ...... 52

Figure 2-10: Cross section of bonded sample affixed to vacuum chuck ...... 53

Figure 3-1: Jackknife plot for preliminary testing over range of time 0 – 60s, power 5 –

75W, and flow 5 – 100%. Predicted angle does not closely match measured angle indicating that the prediction model is not useful for this data set...... 56

Figure 3-2: Marginal model plots for preliminary testing over range of time 0 – 60s, power 5 – 75W, and flow 5 – 100%. Since the jackknife plot does not indicate that the prediction model is useful for this data set, the fitted curves should not be relied upon. . 57

Figure 3-3: Jackknife plot for preliminary testing over range of power 20 – 100W and time 20 – 240s with flow held constant at 100%. Predicted angle matches measured angle reasonably closely indicating that the prediction model may be useful...... 59

Figure 3-4: Marginal model plots for preliminary testing over range of power 20 – 100W and time 20 – 240s with flow held constant at 100%. Since the jackknife plot indicates that the prediction model is useful for this data set, the curve fits may be used cautiously.

...... 60

Figure 3-5: Jackknife plot for primary testing over range of time 10 – 240s, power 8 –

100W, and flow 8 – 100%. Points cluster around a line of unity slope through the origin indicating that the data fits the prediction model...... 62

Figure 3-6: Marginal model plots for primary testing over range of time 10 – 240s, power

8 – 100W, and flow 8 – 100%. Since the jackknife plot indicates that the prediction model is useful for this data set, the fitted curves may be interpreted as representing trends in the data...... 63

Figure 3-7: Surface profiler plots produced from prediction model fit to data from primary testing over range of time 10 – 240s, power 8 – 100W, and flow 8 – 100%. Plot shows contact angle versus time and power for flow of 100%. The intersection in the grid is drawn at the point selected as the optimal plasma treatment (time 150 seconds and power 50W)...... 64

Figure 3-8: Stress chart produced by FEA simulation. Blue indicates compressive stress; red indicates tensile stress. Compressive stress is present at the location of the applied force and tensile stresses result from reactions to the compressive stress...... 71

Figure 3-9: Displacement chart produced by FEA simulation. Red indicates large displacement; green indicates moderate displacement; blue indicates small displacement.

Displacement occurs at the location of the applied force and is greater in PNB than in silicon...... 71

Figure 3-10: Schematic representation of blister test ...... 72

Acknowledgments

Many thanks…

… to my advisor, Prof. Zorman, for his guidance, creative ideas, patience,

enthusiasm for research, and indispensible advice

… to Prof. Feng and Prof. Merat for serving on my committee

… to Chris Roberts for lively discussions, for keeping me going when progress

was slow, and for knowing every paper and technique I needed before I knew I

needed it

… to Allison Hess for fabrication of test samples, expertise, and being a great

person with whom to share a lab

… to Andrew Barnes and Jeremy Dunning for expertise and being great people

with whom to share a lab

… to Maria D‟Agostino for initial development of the contact angle goniometer

… to Shubin Yu of the CWRU Electronic Design Center for expertise and sample

dicing

… and to my parents for always supporting me.

Oxygen Plasma Surface Activation of Polynorbornene for

Bonding to Glass with Applications to Microfluidic Systems

Abstract

RUSSELL LYNN SMITH

This work investigates the surface activation in oxygen plasma of the polymer material polynorbornene (PNB) for bonding to glass. Like the material PDMS which is frequently utilized in microfluidic systems, PNB may be rendered hydrophilic for bonding by treatment in oxygen plasma. For rapid prototyping of microfluidic devices, PNB has the advantage over conventionally-processed PDMS of being directly photodefinable. An investigation of PNB contact angle following oxygen plasma treatment was conducted to determine the optimal plasma power, pressure, and duration for surface activation.

Development of a contact angle goniometer is discussed. Hydrophobic recovery of oxygen-plasma treated PNB is also investigated and compared to PDMS. Bonding of

PNB to piranha-cleaned glass is demonstrated and bond strength is evaluated by blister test.

1. Introduction

1.1 Microfluidics

The field of microfluidics refers to the branch of microsystems dealing with manipulation of small volumes of fluids and gases in channels and reservoirs with small features sizes, often on the order of 10 – 100 µm or smaller. Microfluidic systems present many possibilities for chemical, biological, and medical applications.

Miniaturization of fluidic test devices offer many potential benefits: more rapid analysis, increased separation efficiency, increased portability, and decreased cost of manufacture, use, and disposal. Utilizing smaller volumes of test samples likewise has many benefits: ability to test when larger samples are unavailable, reduced consumption of reagents, and reduced production of potentially harmful byproducts. Microfluidic devices may also be used for some types of testing which are not possible with larger scale devices. For example, microfluidic devices can approximate the size and flow conditions of human capillaries.

1.1.1 Applications

Microfluidic devices have a wide range of applications in chemical, biological, and medical fields [1]: high density genomic sequencing, DNA fingerprinting, combinatorial analysis, forensics, gene expression assays, early detection and identification of pathogens and toxins, rapid analysis of blood and bodily fluids, point of care diagnostics based on immunological or enzymatic assays, electrochemical detection, cell counting, combinatorial synthesis and assaying for drugs, toxicological assays, analysis of environmental conditions, devices for in vivo drug delivery, in vivo

11 monitoring for disease conditions, combinatorial chemical synthesis, purification of biological samples for analysis, polymerase chain reaction, studies on fluid flow in small channels, studies on diffusion, enzyme substrates for studies of chemical reactions, development of machines that mimic biological functions, and detection of single molecules.

1.1.2 Materials

The first microfluidic devices were developed in the 1970s. Early microfluidic device research utilized fabrication techniques derived from microelectronics. Devices were designed using glass and silicon as materials. Photolithograpy combined with dry and wet etching processes were used for device fabrication because the processes were already highly developed. However, silicon and glass have disadvantages: etching is time-consuming, bonding requires high voltages or temperatures, silicon is opaque in the visible spectrum, and glass is difficult to etch to produce vertical sidewalls.

More recent research has focused on the development of additional materials, especially polymers, for microfluidic systems. Polymers have a number of advantages: materials are generally inexpensive, devices may be formed by molding or embossing rather than etching, and devices can be sealed thermally or with adhesives. Polymers used in microfluidics may be grouped into three main categories: thermoset materials, thermoplastic materials, and elastomers [2].

Thermoset materials commonly used in microfluidic systems include polyimide and the photoresist SU-8. Molecular polymer chains in these materials begin to cross-link upon exposure to high temperatures or light. This curing process is an irreversible chain reaction and once complete it is not possible to reshape the material. The glass transition

12 temperature of these materials is generally very close to the decomposition temperature.

Devices using thermoset materials may be structured by casting, lithography, or etching.

Thermoplastic materials include poly(methyl methacrylate), polycarbonate, and cycloolefin polymers and copolymers such as polynorbornene (PNB). These materials tend to have a larger separation between glass transition temperature and decomposition temperature. Near their glass transition temperature, they can be structured and processed using techniques such as injection molding and hot embossing. Devices using thermoplastic materials may also be structured by thermoforming and laser ablation.

Elastomer materials have molecular chains that are longer than other types of polymers and those chains typically form the structure of the material by physical entanglement rather than chemical interaction. Devices using elastomer materials are typically structured by casting in a mold. Due to low cost and ease of processing, the elastomeric material poly(dimethylsiloxane) (PDMS) has emerged as a first-choice material for microfluidic devices.

1.1.3 Fabrication Techniques

Techniques for fabricating microfluidic devices can be divided into two groups: structuring techniques and back-end processing techniques.

Structuring techniques include two classes: photodefinable processes and replication processes. Photodefinable processes include photolithography, stereolithography, and laser ablation. Replication processes require creation of a replication master (mold or replication tool) which is used to form the final structures.

These include hot embossing, microthermoforming, injection molding, injection

13 compression molding, and casting. Two additional types of structuring technique not belonging to either class are precision machining and plasma etching [2].

Back-end processing techniques are required to create a finished device and may account for up to 80% of the total manufacturing cost. Cutting and dicing methods which are commonly used include mechanical saw, water jet, and laser cutting. Electrode fabrication can be performed by sputtering, evaporation, or screen printing methods.

Methods of encapsulation include adhesion between untreated surfaces, adhesion between surfaces activated by exposure to plasma, application of adhesive substances, thermal pressure bonding, solvent bonding, ultrasonic welding, and laser welding [2].

1.1.4 PDMS

PDMS is a preferred material for microfluidic device structures because of the ease and speed of processing. Conventional PDMS processing consists of rapid prototyping of the device structure using computer-aided design (CAD) software, production of photomasks using transparencies and a high-resolution computer printer, photolithographic patterning of SU-8 photoresist on a silicon wafer which serves as the master, casting of PDMS prepolymer into the master to produce a negative replica of the master, curing of PDMS, and removal of the PDMS structure from the mold [1]. The

PDMS structure must then be sealed to a flat surface to complete the channels and other features. Sealing may be accomplished by reversible, conformal contact with a flat surface or by irreversible sealing following exposure to an air plasma [1].

Oxygen plasma treatment of PDMS allows irreversible bonds to be formed to

PDMS, glass, silicon, silicon dioxide, quartz, silicon nitride, polyethylene, polystyrene, and glass carbon [3]. The resulting bonds are watertight and able to contain channel

14 pressures up to 30 – 50 psi, making them suitable for the fabrication of microfluidic systems. Attempting to separate plasma-treated PDMS bonds results in failure of the bulk

PDMS before separation of the bond. One mechanism proposed for this bonding process is that silanol groups are created during plasma exposure which condense with groups on the opposing surface once the surfaces are brought into contact [1].

It has been observed that oxygen plasma treatment of PDMS also causes the surface to become hydrophilic [4]. Bhattacharya et al. found a strong correlation between the contact angle of plasma-treated PDMS and bond strength of PDMS-PDMS and

PDMS-glass bonds. This relationship was then used to determine the optimal plasma parameters for bonding [5].

The bonding process for PDMS-PDMS and PDMS-glass in use at the CWRU

Emerging Materials Development and Evaluation Laboratory consists of oxygen plasma treatment of the PDMS at 25 W for 25 seconds at 900 millitorr O2. The bond is then made by placing the surfaces into contact and the device is heated on a hot plate at 150°C for 30 minutes.

A technique for making PDMS photodefinable by mixing the prepolymer with the photoinitiator benzophenone has previously been published [6]. However, this technique seems to require careful control over solvent and photoinitiator concentrations, UV photopatterning intensity, and curing temperature. The photoinitiator also raises concerns regarding biocompatibility [7].

1.2 Avatrel™ 2585P Polynorbornene

Polynorbornene (PNB) is a polycyclic olefin polymer prepared from a norbornene monomer. Nobornene is a bicylic olefin molecule which can be functionalized by

15 derivatization or by low or high temperature Diels-Alder reactions and then polymerized by a transition metal-catalyzed process to produce PNB. By tuning the monomer functionalization, it is possible to adjust a wide range of material properties of the final polymer: glass transition temperature, coefficient of thermal expansion, optical loss, crosslink density, adhesion, refractive index, solubility, Young‟s modulus, toughness [8] and decomposition temperature [9].

Several formulations of polynorbornene designed to be suitable for particular applications are produced by Promerus™ LLC of Brecksville, Ohio. Formulations exist which are intended for use as optical polymers in flat panel display substrates and waveguides, dielectrics and encapsulants for electronic packaging, sacrificial materials for microelectromechanical systems, photoresists, and adhesives [8]. This work investigates the Avatrel™ 2585P formulation of PNB which has been developed specifically for electronic packaging.

1.2.1 Material Properties

The material properties of Avatrel™ 2585P are presented in Table 1-1. Avatrel™ is currently utilized primarily in packaging of microelectronic devices where it is used for encapsulation of devices, dielectric isolation, and stress compensation. It has also been used for encapsulation of microelectromechanical systems.

Property Test Condition Avatrel™ 2585P Tensile strength Room temp. 18 MPa Young‟s modulus Room temp. 0.8 GPa Elongation Room temp. 32% Glass transition temperature 10ºC/min 280ºC Coefficient of thermal expansion 10ºC/min 180 ppm Dielectric constant 1 MHz / Room temp. 2.42 Dissipation factor 1 MHz / Room temp. 0.007 Volume resistivity Room temp. 2 x 1015 Ω•cm Dielectric strength Room temp. 411 kV/mm Water absorption 24 hours / Room temp. 0.07%

Table 1-1: Material properties of Avatrel™ 2585P polynorbornene [10]

The Avatrel™ family exhibits a number of characteristics which make it attractive for use in microelectronic and MEMS devices [11]. Moisture absorption is very low and nearly comparable to liquid crystal polymer (LCP) (0.1 wt% compared to 0.04% for LCP and 2 – 3 wt% for polyimide). Thermal stability and adhesion to metals, silicon, and silicon dioxide are good. Avatrel™ may be patterened by RIE with soft photoresist mask or hard silicon dioxide mask. Avatrel™ films up to 100 µm thick may also be directly photopatterned (negative-tone) in UV light. Shrinkage on cure is 0.5% or less, compared to other polymer dielectrics which exhibit shrinkage of up to 50%. Avatrel™ films exhibit low stress because of the low shrinkage on cure and the moderate tensile modulus which compensates for thermal stress effects.

Preliminary biocompatibility testing of PNB has been promising. A study conducted by Keesara investigated growth of colorectal cancer cells on PNB in comparison to a control sample on polycarbonate after 24 and 48 hours. No difference in cell growth was found [12]. A study conducted by Hess investigated growth of mouse fibroblast cells on PNB in comparison to glass, silicon, and silicon dioxide controls after

1, 3, and 7 days. The percentage of total cells still alive on PNB was found to be slightly

17 lower than on glass but comparable to silicon and silicon dioxide. However, the total number of cells on PNB was significantly lower than on samples. One explanation which was suggested is that this is not due to inhibition of cell growth but rather that the extremely hydrophobic nature of PNB inhibits adhesion of cells [13].

1.2.2 Conventional Uses

Most of the published research on microfluidic devices which utilize Avatrel™ make use of it as an encapsulation material which can withstand the thermal treatment required for decomposition of a polymer sacrificial layer [14] [15] [16] [17]. For this application, the Avatrel™ is either unpatterned or patterned only to open electrical contacts.

Two publications make use of photopatterned Avatrel™ as an encapsulation material for microelectromechanical systems (MEMS) [18] [19]. Another formulation of

PNB, Unity™, decomposes at a lower temperature than Avatrel™ and is also photodefinable. Photopatterned Unity™ has been used as a sacrificial material in MEMS devices [20].

1.2.3 Usage of Photopatterned Avatrel™ in MEMS and Microfluidics

There are a few publications which describe the use of Avatrel™ for more complex device structures. Avatrel™ has been used in 3D chip interconnects, as a structural material for alignment pillars [21]. It has also been patterned for use as a structural material for neural electrode arrays [22] [13] [23] [12].

Two publications describe the use of photopatterned Avatrel™ in microfluidic devices. King et al. describe its use as a photo-definable sacrificial material for serpentine

18 microfluidic channels [24]. Avatrel™ was not utilized as a structural material in this design. Sekar et al. describe the use of Avatrel™ for sealing silicon microchannels which provide cooling in 3D integrated circuit designs (shown in Figure 1-1) [25]. The microchannels are formed by silicon etching, spin coating and polishing a sacrificial material, spin coating an Avatrel™ overcoat, and thermally decomposing the sacrificial material. Avatrel™ was photopatterned to provide access to vias but was not structured to form channels. The Avatrel™ was not adhered to the silicon by a bonding process; it was spin cast and cured.

Avatrel™ copper microchannel silicon

Figure 1-1: Cross-section of Sekar et al. device structure. Silicon microchannels are sealed by a layer of Avatrel™ which is patterned to provide access to vias [25].

1.3 Advantages of Avatrel™ 2585P PNB for Microfluidics

PDMS, as an elastomeric material, is easy to work with once cured and devices can be sealed conveniently to PDMS, glass, or silicon after oxygen plasma surface activation. However, formation of patterned structures in PDMS using conventional processing techniques first requires the creation of a mold, usually in SU-8. Techniques for making PDMS photodefinable require careful process control and create biocompatibility concerns.

SU-8 can be directly photopatterned. However, because it is a thermoset material

SU-8 is difficult to work with after it has been initially cured which makes sealing of channels problematic. One process for sealing an SU-8 device to another layer of SU-8 19 requires that the second SU-8 film be spun onto a wafer, soft-baked, brought into conformal contact with the device, then hard-baked [26]. Most SU-8 fabrication processes utilize a UV-activated adhesive for sealing.

PNB has low moisture absorption and promising preliminary biocompatibility testing which suggest that it may be useful for biological and medical devices. As a thermoplastic polymer material available in photodefinable formulations, it has the potential to fill the gap between PDMS and SU-8 and provide the benefits of both materials. Such a material would be attractive for rapid device prototyping because it would simplify the two key steps required for fabrication of a microfluidic device: formation of structures and sealing of channels.

1.4 Thesis Investigation

PNB may prove to be an attractive material for rapid microfluidic device fabrication due to its ability to be photopatterned if a convenient bonding process can be established. To the best of our knowledge, no bonding process currently exists for PNB.

In an effort to fill this gap by developing a bonding process for PNB that is analogous to the process commonly used for PDMS, this work investigates surface activation of

Avatrel™ 2585P PNB by oxygen plasma treatment for bonding to glass.

2. Experimental Methods

2.1 Oxygen Plasma Surface Activation and Contact Angle

Measurement

Oxygen plasma cleaning systems are frequently employed in MEMS fabrication processes to enable adhesion of incompatible materials. Oxygen plasma treatment results in alteration of the surface (“surface activation”) and allows a strong bond between materials that would otherwise bond poorly or would not bond at all. Oxygen plasma treatment of PDMS is a common technique to enable bonding to PDMS, glass, silicon, and other materials for sealing of microfluidic devices. PDMS bond strength is known to correlate strongly with contact angle after plasma treatment.

Oxygen plasma treatment of PNB is investigated to determine the plasma parameters which produce the minimum contact angle. Hydrophobic recovery of PNB is also investigated by making contact angle measurements at spaced intervals following plasma treatment.

2.1.1 March Instruments PX-250™ Oxygen Plasma System

The March Instruments PX-250™ oxygen plasma system is designed for cleaning and surface activation applications. The device consists of a sealed chamber containing three metal shelves connected to an RF source, an ENI AGC-3B-01TM 13.56 MHz supply with a maximum output of 300 W. The shelves are connected to the RF supply. When the

RF source is active, plasma is generated in the area between the upper shelf and lower shelf. The system is connected to a vacuum pump, a tank of oxygen regulated to 10 psi,

21 and a tank of nitrogen regulated to 56 psi. A photograph of the plasma system is included as Figure 2-1.

Figure 2-1: March Instruments PX-250™ oxygen plasma system with ENI AGC-3B-01™ power supply

To treat a sample, the operator opens the chamber door and places a sample on either the middle or lower shelf. The plasma is not active above the top shelf. For consistency, all samples treated for this work were placed on the lower shelf. The operator selects and starts a cleaning program and automatic operation begins. During

22 operation, the chamber is first pumped down by vacuum pump to a threshold of 80 millitorr. Oxygen is allowed to flow into the chamber and the system waits for pressure to stabilize as oxygen inlet flow reaches equilibrium with vacuum pump exhaust flow.

Oxygen plasma pressure correlates linearly with oxygen flow rate (see Figure 2-2 below); the flow rate can be set by the operator as a percentage of maximum flow. Once oxygen pressure has stabilized, the RF generator is activated at the programmed power setting resulting in formation of oxygen plasma between the shelves in the chamber. If reflected power is detected, an automatic tuning network in the plasma system compensates until all RF power is absorbed by the plasma. The RF generator remains active for the duration of the selected exposure time and is then deactivated by the plasma system. Oxygen flow is shut off and the chamber is depressurized by the vacuum pump until the pressure reaches a base pressure of 80 millitorr. The vacuum pump valve is closed and the nitrogen valve is opened, allowing the chamber pressure to equalize with atmospheric pressure. Once the pressure has equalized, the chamber door may be unlatched and opened and the samples are ready for testing or bonding.

Testing was conducted to determine the range of parameters over which the plasma system is capable of operating. First, power can be varied from 5 W to 300 W.

Below 5 W, the system is unable to stabilize at the setpoint; 300 W is the maximum power that can be supplied by the RF generator. However, power greater than 100 W may be expected to etch polymer materials resulting in degradation of the polymer so this was chosen as an upper limit. Second, no limits were observed with regard to exposure time: testing was successful from 0 seconds to 240 seconds. Finally, flow rate can be varied from 5 percent to 100 percent. When the oxygen flow rate is set to less than five

23 percent, the chamber pressure tends to drop slightly during the cycle until it is below the base point endpoint which triggers an alarm and terminates the program.

Testing was also conducted to determine the relationship between oxygen flow rate and plasma pressure. Figure 2-2 is a graph of plasma pressure with respect to oxygen flow rate and indicates that there is a nearly linear relationship between pressure and flow rate. Therefore, the investigation may focus on flow with the understanding that evenly spaced test points of flow will corresponds to evenly spaced test points of plasma pressure. See Appendix A for test data used to generate Figure 2-2.

1000 900 800 700 600 500 400 300

Plasma Plasma Pressure (mtorr) 200 100 0 0 20 40 60 80 100 Oxygen Flow Rate (percent)

Figure 2-2: March Instruments PX-250™ plasma system oxygen flow rate setting and resulting plasma pressure. Graph suggests a linear relationship.

A recurring error was observed when the plasma system was set for powers less than 50 W and exposure times greater than 45 seconds. This was explained by consulting the plasma system manual [27]: “Flashing POWER LED indicates „POWER‟ setpoint not achieved within 45 seconds.” At powers less than 50 W, plasma power readout on the RF generator was correct but the readout on the plasma system itself was observed to

24 indicate 1 – 2 W higher than the setpoint. Errors in the automatic tuning network were ruled out and the RF generator was swapped with an identical model with no effect. Since the error appeared to be caused by an overzealous alarm function and could not be easily resolved, testing in this range of values was conducted with the machine in manual mode in order to bypass the alarm. Manual mode requires the operator to monitor pressure and time readouts in order to open and close valves at the appropriate times but is otherwise identical to automatic operation.

2.1.2 Contact Angle Goniometer

2.1.2.1 Principles

Contact angle is a measure of the angle formed by a drop of test liquid on the surface of a material. Contact angle can be used to calculate surface tension or surface free energy of the surface. Materials with a high contact angle are said to be hydrophobic; materials with a low contact angle are said to be hydrophilic. Hydrophobic and hydrophilic contact angles are illustrated in Figure 2-3.

θ θ

Figure 2-3: Contact angle of hydrophobic (left) and hydrophilic (right) surfaces

A contact angle goniometer measures the contact angle of a material by applying a droplet of a test liquid to the surface of the material and measuring the angle formed between the liquid-vapor interface and the solid surface. Most systems consist of a stage for placement and leveling of the sample, a syringe or electrically-actuated droplet dispenser, a diffuse lighting source, and a magnifying lens or camera for observing the drop profile. In some systems, the angle is determined by visual comparison to a scale behind the droplet; others can be interfaced to a computer for automatic analysis and for capturing and saving images of tests.

In the static sessile method, contact angle is measured by measuring the angle formed by a stationary drop of test liquid on the sample surface. The dynamic sessile method can determine the advancing (maximum) and receding (minimum) contact angles of a material. The advancing contact angle is measured by adding liquid to the drop until a maximum angle is observed. The receding contact angle is measured by removing liquid from the drop until a minimum angle is observed. Deionized water (DI) is a common choice for the test liquid; use of different test liquids results in different contact angles.

Systems that use a computer to automatically calculate the drop angle may perform a mathematical fit to the observed drop profile before calculating the angle in order to compensate for deviations from an ideal drop shape due to the effects of gravity.

2.1.2.1.1 Commercially Available Options

The most widely used contact angle goniometer systems consist of an adjustable stage with hand-operated volumetric drop dispenser mounted above it, a diffused

26 backlight, and a camera interfaced to a computer for automatic contact angle calculation

[28].

Another type of goniometer in common use is similar to the type described above but does not have a camera. The operator observes the sample through a viewing lens and compares the drop profile against a protractor. This is the type of goniometer owned by the CWRU Department of Biomedical Engineering which was used as a reference for goniometer characterization.

2.1.2.1.2 Software Techniques

Goniometers which interface a camera to a computer can use software on the computer to analyze the image and automatically determine the contact angle. One method simply identifies the gas-liquid-solid three-phase points (intersection of drop profile with solid surface) and calculates the tangent angle at the drop edge. This is analogous to the traditional goniometer design where an operator observes the drop through a viewing glass and compares against a scale to determine the angle. The disadvantage of this technique is that if drop size is too large or is not consistent, gravitational effects distort the drop and the measured contact angle does not correlate directly with surface tension. This is the method of operation used by the goniometer employed in this work. Because this research is only concerned with relative contact angle between materials and not with calculation of surface tension, the effect of gravity described above is not a significant concern. Drop consistency was considered and quantified during the design of the goniometer system.

A more advanced technique is axisymmetric drop shape analysis (ADSA). ADSA typically uses geometric information from the drop profile in the captured image to solve

27 the Laplace equation of capillarity and determine contact angle [29]. One variant of

ADSA, known as ADSA-D, uses a diameter of the drop determined from a view of the drop from above to calculate contact angle [30]. This method has advantages for measuring very low contact angles which can be difficult to measure with traditional goniometers or drop profile ADSA.

2.1.2.2 Goniometer Design

The CWRU Emerging Materials Development and Evaluation Laboratory is equipped with a custom-built contact angle goniometer [31]. Improvements were made to the hardware design and measurement software in order make the system more suitable for the testing conducted in the course of this thesis research. In particular, improvements were made to allow rapid measurement of contact angles and to allow more accurate measurements of very low contact angles.

2.1.2.2.1 Goniometer Hardware

The goniometer design consists of a backlit adjustable stage with camera. Figure

2-4 shows the contact angle goniometer in its final form.

Figure 2-4: Contact angle goniometer hardware

The stage is made from acrylic, laser-cut to shape, and spray-painted black to reduce unwanted reflections and light transmission. Three bolts screw into an acrylic sub- stage and fit into notches drilled in the underside of the stage to provide support and allow it to be leveled by tilting side to side and forward to back. The sub-stage is mounted to a micrometer dial X-Y-Z positioner which can be adjusted so that the sample is oriented in the center of the camera field of view. This is mounted to the goniometer frame which is constructed from T-slot extruded aluminum railing. The frame is supported by bolts screwed into T-slot angle brackets, allowing the frame to be leveled.

A fiber-optic light source illuminates the droplet from the side of the stage opposite the camera. The light source aperture fits into an acrylic mount attached to the frame by T-slot railing to ensure the light source is positioned so that it evenly illuminates the stage. A light diffuser consisting of frosted acrylic is attached to the stage at a right angle so that background lighting within the field of view of the camera is uniform.

A 3 mL syringe fitted with an EFD™ dispensing tip (EFD ULTRA™ #5132 1/4

B #32GP 0.004 X 0.250 YELLOW) was used to apply a drop of DI water to the sample, holding the syringe as closely above the sample as possible. The syringe was positioned above the sample by hand rather than mounting it permanently to allow more rapid changing of samples. The syringe with EFD™ dispensing tip was found to produce drops of regular shape and consistent volume when filled with at least 1 mL of water and when the plunger was smoothly depressed until a drop released from the tip due to gravity.

Testing of drop volume performed by measuring the mass of ten drops with an analytical balance found the mean drop volume to be 3.94 µL with standard deviation of 0.24 µL

(see data in Appendix B). This value does not exceed the volume at which a drop loses its spherical profile, which occurs beyond 3 – 5 µL [32]. Contact angles for drops with volume greater than this may not depend solely on surface tension due to effects of gravity and other forces.

A Finnpipette II™ 2-20 µL pipetter was also evaluated. The volumetric size of the drop produced by the pipetter can be adjusted from 2.0 to 20.0 µL and the size of drops produced is very regular. However, the pipetter sometimes does not release the drop cleanly which results in an irregular drop shape and invalidates the contact angle

30 measurement. Also, when using the pipetter it is necessary to fill the pipetter tip before releasing each drop which adds time to the measurement process. Since a rapid, reliable test process was required for the testing conducted in this thesis research, the 3 mL syringe with EFD™ dispenser tip was used for all contact angle testing.

The camera is a C-mount color Tucsen TCA-1.31C™ with a resolution of

1280 x 1024 which interfaces to a computer by USB 2.0, allowing images to be captured by the Tucsen TSView™ software. The lens assembly fits into an acrylic mounting bracket which is attached to the goniometer frame by a vertical T-slot rail. The lens has rings for adjustment of focus and magnification and fits the C-mount threads of the camera. The camera resolution of 1280 x 1024 with magnification provided by the lens was found to provide a sufficiently clear image for measuring even very low contact angles.

More accurate angle measurements would require improvements to drop placement and volume control in addition to higher camera resolution or lens magnification. Worth noting when considering future improvements is that Hoorfar and

Neumann found that the addition of a Mikkle blue filter eliminated chromatic aberrations and produced a greater reduction in goniometer error than did increasing camera resolution [29].

2.1.2.2.2 Goniometer Software

The goniometer software is implemented as a Matlab™ script and was developed using Matlab™ version R2010b. The user selects one or more previously captured images to be analyzed and the software processes the images sequentially, displaying each result before moving on to the next. The Matlab™ image import routines support

31 most common image formats. Images captured for this work using the TSView™ camera software were stored in the JPEG image format. A full listing of the goniometer software is provided in Appendix F.

Automatic measurement is performed by first converting the image to grayscale.

Edge detection is then performed using the Canny method with appropriate thresholds.

The first intersection of a detected edge with a vertical centerline extending from the top of the frame is taken to be a point on the profile of the drop. The full drop profile is detected by tracing from that point along the detected edge. The leftmost point is taken to be the left drop three-phase point (the intersect of the drop profile with the sample surface) and the drop profile is traced again, resulting in drop profile coordinates ordered from left to right. The rightmost point is taken as the right drop three-phase point. The stage slope and drop width are calculated from the positions of the two three-phase points. The tangent slopes of the left and right angles are found by calculating the slopes of tangent lines drawn from the three-phase points to points on the drop profile offset horizontally by 0.05 times the drop width from the three-phase point. The angles are calculated using the following equation:

. Equation 2-1

Left, right, and average angles are displayed and annotation lines are drawn on top of the drop to indicate the detected drop profile, three-phase points, and tangent lines so the user can verify visually that the reported angle measurement is reliable. Figure 2-5 shows a completed automatic contact angle measurement.

Figure 2-5: Completed automatic contact angle measurement showing x‟s at detected three-phase points and tangent lines used for calculation of the angle.

Manual measurement mode requires the user to click on four points on the image: first the left and right three-phase points, then points along the drop profile through which the tangent lines will be drawn. Vectors are calculated from the three-phase point to the corresponding point on the drop profile. A vector from the left three-phase point to the right is calculated to find a vector parallel to the stage. The left and right contact angles are then calculated using the following equation:

. Equation 2-2 | || |

Left, right, and average angle is displayed and annotation lines are drawn on top of the drop to indicate the tangent lines chosen by the user. If the selected tangent line location does not match the drop profile exactly, the user may click again to move the tangent line and recalculate the angle.

2.1.2.2.3 Goniometer Usage

To prepare the goniometer for use, the lens cap is removed from the camera lens and the dust protector is removed from the stage. The fiber optic light source is powered on and the camera is connected to the computer. The stage is leveled using a bubble level so that the stage slopes downward toward the camera. The downward slope allows the three-phase points to be captured clearly in the image and allows a reflection of the drop on the sample surface to be captured. Goclawski and Urbaniak-Domagala have shown that the reflection of the drop can be used to calculate the contact angle [33]. This can be useful for materials with small contact angles which produce images that are difficult to analyze.

A sample is placed in the center of the stage and a drop is applied to the sample with the syringe by smoothly depressing the plunger until the dispensing tip releases a drop. The X-Y-Z positioner is adjusted so that the drop is centered in the field of view of the camera. Focus is adjusted so that the drop profile is sharply defined. The image is captured using the Tucsen TSView™ software supplied with the camera and stored in

JPEG format. This procedure is repeated for each sample being tested. Detailed standard operating procedures for the contact angle goniometer are supplied in Appendix G.

2.1.2.3 Goniometer Measurement Characterization

Testing was performed to ensure that the goniometer provides useful results for the purposes of this work. Potential concerns were addressed by showing that results were consistent with those obtained from a trusted reference goniometer, that variations in drop placement or image capture did not result in a large error in measured contact angle, that the automatic and manual analysis and angle calculation methods produced consistent results, that results were consistent for materials with both large and small contact angles, and that results from the manual method were not significantly affected by operator mindset.

2.1.2.3.1 Comparison to Reference Goniometer

As part of the senior project work under which the goniometer was developed, a comparison was performed against an operating goniometer in the CWRU Department of

Biomedical Engineering [31]. The BME goniometer is similar in design but is read by the operator by viewing the drop through a magnifying lens and comparing against a protractor. Contact angles for DI water on a silicon dioxide on silicon substrate and on a polyimide substrate were measured with both goniometers. Results were found to be acceptable.

Changes made to the goniometer during the course of this thesis work to improve its suitability for making rapid measurements of materials with low contact angles should not degrade performance under any usage condition. The comparison to the BME system can be expected to apply to the device in its current form.

2.1.2.3.2 High Angle Repeatability Testing

Measurements were made on eight samples with high contact angle to ensure that drop placement and image capture were consistent between samples of the same material.

Measurements made by automatic and manual methods were recorded to determine whether the methods of analysis produced results that were consistent.

PNB films that were 50 µm in thickness and cast on silicon substrates were used without any oxygen plasma treatment. Samples were washed in DI water and dried in compressed air before use. Samples were not reused.

Results are presented in Table 2-1. All reported angles are averages of left and right angle. Automatic measurements are consistently lower than manual measurements by a few degrees. This is likely the result of the software choosing a tangent point farther along the drop profile and could be adjusted if desired.

Sample Automatic Manual 1 68.3° 73.6° 2 68.3° 75.5° 3 70.6° 75.4° 4 64.3° 76.4° 5 69.5° 76.2° 6 71.3° 76.1° 7 76.9° 76.9° 8 72.0° 76.0°

Mean 70.15° 75.76° Std. dev. 3.63° 1.00°

Table 2-1: High contact angle repeatability testing on eight samples of untreated 50 µm PNB yielding a standard deviation of 1.00° for the manual measurement method

Based on this testing, the manual calculation method was selected for use in this work. Manual measurements were found to have a standard deviation of 1.00° which is

36 acceptable for this work. For comparison, one published study correlating oxygen-plasma treatment of PDMS to contact angle and bond strength reported contact angle measurements accurate to ±2.5° [5].

2.1.2.3.3 Low Angle Repeatability Testing

Measurements were made on eight samples that exhibited low contact angles to ensure that drop placement and image capture were consistent between samples of the same material, that oxygen plasma treatment was consistent, and that manual angle measurement was consistent between samples. Manual analysis was repeated on previously captured images to ensure that operator mindset did not affect results.

Automatic testing was not investigated for low contact angles because the automatic analysis software in its current form is not able to reliably distinguish the drop profile from the stage for surfaces with low contact angles.

PNB films that were 10 µm in thickness and cast on silicon substrates were used with oxygen plasma treatment at 25 W for 25 seconds at 100% flow. Samples were washed in DI water and dried in compressed air before plasma treatment. Each sample was plasma treated immediately before contact angle measurement and samples were not reused.

Manual analysis was first performed (Day 1) during the course of testing at the end of the day when operator fatigue might be expected to have a maximum effect on interpretation of the images. Analysis was repeated at the beginning of the next day (Day

2) using the same images as on Day 1. Mean values for both sets of measurements fell within one standard deviation of each other, indicating that no additional action was needed to account for operator mindset for the purposes of this work.

Results from this study are presented in Table 2-2. All reported angles are averages of left and right angle. Day 1 measurements yielded a standard deviation of

0.48° which is acceptable for this work. For comparison, one published study correlating oxygen-plasma treatment of PDMS to contact angle and bond strength reported contact angle measurements accurate to ±2.5° [5].

Sample Manual Manual (Day 1) (Day 2) 1 6.3° 6.3° 2 6.1° 5.8° 3 6.3° 5.9° 4 7.2° 6.5° 5 5.5° 5.9° 6 6.3° 5.6° 7 5.9° 6.2° 8 6.4° 5.6°

Mean 6.25° 5.97° Std. dev 0.48° 0.33°

Table 2-2: Low contact angle repeatability testing on eight samples of plasma-treated 10 µm PNB yielding a calculation of standard deviation of 0.48° for the manual measurement method and demonstrating that manual measurement is not significantly affected by operator mindset.

2.1.2.4 Sources of Error

Sources of error exist which may affect a single measurement without systematically affecting an entire group of tests. Operator experience and vigilance can reduce the occurrence of error due to these factors but may not eliminate it.

The manual measurement system requires the operator to visually align the tangent line to the drop profile. Small shifts in the tangent line which may be difficult to distinguish visually may still have a significant effect on the measurement of contact angle.

For samples with large contact angles (near 90°) a shift by even a single pixel may correspond to a significant fraction of a degree. Figure 2-6 demonstrates a small shift in tangent line position which results in a change in angle from 75.6° to 72.8°.

However, it is usually possible to obtain very clear images of large contact angle samples which make reliable measurement more likely. To obtain good results, the operator must carefully place the three-phase points and be consistent in how far along the drop profile the tangent point is placed. Multiple images should be captured in case one is not clear.

Figure 2-6: Two screen captures of manual measurement of left angle performed on same image of a sample with high contact angle. Angle measurement of capture on left is 75.6°; measurement of capture on right is 72.8°.

For samples with small contact angles (less than 5°) the size of the important features of the captured image may approach the limits which can be resolved with clarity as determined by the camera resolution and optics. This is a common problem when studying surfaces with very low contact angles. For example, the study of PDMS contact angle under plasma treatment conducted by Bhattacharya et al. did not quantify contact angles determined to be less than 5° [5]. Figure 2-7 demonstrates a small shift in tangent line position which results in a change in angle from 4.1° to 4.8°. Since the drop slope is

39 near horizontal at the tangent point, a shift in tangent point by a small distance does not have as large an effect. However, the measured angle is also small so even small errors can be significant. Again, it is important to capture multiple images in case the first is not clear. Operator experience in consistently determining where to mark the drop boundary when the edge that is visible in the image is not distinct is also important.

Variations in drop size and shape can also significantly affect the angle measurement, especially for materials with low contact angles. Drops that are significantly larger or smaller than other drops may be affected by gravity to a different degree yielding measurements that are not comparable to other measurements in the group. Uneven drop shapes also prevent accurate measurement and tend to occur when the drop spreads across a surface with very low contact angle. It is important for the operator to visually inspect the drop for size and shape; if the drop is not acceptable the test should be repeated.

2.1.3 Sample Preparation

Samples consisting of 10 µm-thick films of PNB were used for preliminary testing. Samples were tested 6 – 12 months after being prepared. In the CWRU

Microfabrication Laboratory, Avatrel™ 2585P was spin cast onto a 100 mm silicon wafer at 800 rpm for 10 seconds, then 2000 rpm for 30 seconds. The wafer was soft-baked on a hot plate for 5 minutes at 120° C, exposed to a UV light source fitted with a 365 nm filter, baked on a hot plate for 4 minutes at 90° C, and cured for one hour at 160°C.

Samples were separated by cleaving the wafer along crystal planes with a diamond scribe to produce samples of approximately 15 mm x 15 mm. All samples were washed in DI water and dried in compressed air before use.

The 24-point primary contact angle tests, time lapse tests, and bond strength tests used samples of 50 µm-thick PNB. The 24-point primary contact angle tests were conducted within one week of sample preparation and other tests were conducted within

8 weeks of sample preparation. Avatrel™ 2585P was spin cast onto a 100 mm silicon wafer at 800 rpm for 10 seconds, then 1000 rpm for 30 seconds. The wafer was soft- baked on a hot plate for 5 minutes at 120°C, exposed to a UV light source fitted with a

365 nm filter, baked on a hot plate for 4 minutes at 90°C, and cured for one hour at

160°C. Samples were diced by the CWRU Electronic Design Center resulting in 24 samples per wafer with a size of 15 mm x 15 mm. Bond strength testing samples used for later testing were separated by cleaving along crystal planes as described above. All samples were washed for 10 seconds in DI water and dried in compressed air before use.

Samples were handled under a HEPA-filtered flow hood to prevent contamination by dust.

2.1.4 Design of Experiments

In order to maximize the information obtained from a given number of contact angle tests, a space-filling Latin hypercube design was used for the experiment. The space-filling type of the design refers to the fact that it is intended to cover the entire range of parameter space rather than restricting the investigation to a subset such as the edge cases. Latin hypercube spacing is a compromise between the sphere packing method, which spreads the test points as far from each other as possible, and the uniform distribution method, which spreads test points evenly over the parameter space [34]. As a result, it is a good choice to maximize coverage of the parameter space while distributing points as evenly as possible across the range of each parameter.

The JMP 9TM statistical software package was used to select test points and analyze the resulting measurements. After the ranges of plasma treatment parameters to be investigated and number of test points to be run are entered, JMPTM generates a list of the test points. Once testing is performed and the results are entered, JMPTM provides tools to analyze the data as a Gaussian process.

The marginal model plots produce a 2D plot of contact angle response with respect to each plasma treatment parameter investigated. Because each plot does not incorporate information on the other parameters not used as the independent variable for that plot, the trend in individual data points is not meaningful. For example, in a plot of contact angle in response to exposure time, a data point with a relatively high contact angle may only have occurred because that test point happened to be taken at a very low power setting. However, JMP plots a curve fit created using a cubic correlation prediction model which allows for useful conclusions to be drawn from the plots.

JMP fits the data using cubic spline interpolating predictors for the correlation function [35]. This prediction model has parameters that must be solved for to fit the data. A likelihood estimation function gives an estimate of the probability of the recorded data occurring based on the prediction model. The maximum-likelihood estimate is the value of prediction model parameters that gives the highest probability of the observed data fitting that model [36]. JMP uses the prediction model corresponding to the maximum-likelihood estimate as the fit in the marginal model plots.

JMP provides several pieces of information to determine the quality of the prediction model used for the curve fit. First, the jackknife plot graphs actual value versus value predicted by the model. Ideally, this plot would be a line through the origin with slope of one indicating that the model perfectly predicts the values present in the data set.

The quantity -2*LogLikelihood is minimized by JMP to find the optimal prediction model. Since many aspects of experimental design affect the likelihood, the value of this measure that indicates a good fit varies widely for different experimental designs. As a result, it is useful in comparing experiments with similar or identical designs but there is no single threshold value indicating a good fit.

JMP also provides a main effect and total sensitivity for each parameter. The main effect is a measure of how much of the variation is explained by that parameter. The total sensitivity is a measure of how much of the variation is explained by that parameter including all second-order effects (i.e., including interactions with other parameters).

To determine the ranges of parameters worth investigating, two preliminary test runs were performed using a 6-point space-filling Latin hypercube design: one at lower parameter values and one including higher values of power and exposure time. Parameter

43 ranges were selected based on the limitations of the plasma treatment system and optimal treatment parameters for PDMS and SU-8. The minimum contact angle and strongest bond strength in PDMS occurs when treated at 20 – 30 W, 20 – 30 seconds, and 1000 millitorr (corresponding to 100% flow) [5]. The standard oxygen plasma treatment parameters for making SU-8 hydrophilic are 400 W for 4 min [37]. As described in

Section 2.1.1, the range of parameters available with the March Instruments PX-250™ plasma system is 5 – 100 W, 0 – 240 seconds, and 5 – 100% flow (corresponding to 130

– 920 millitorr).

After completion of both preliminary tests, areas of interest were identified in the marginal model plots. These areas of interest were used to guide the selection of the ranges for the primary contact angle testing.

2.1.5 Time Lapse Testing

Plasma-treated PDMS is known to recover its hydrophobic character. The increase in contact angle is rapid immediately after treatment and then slows until the angle ultimately approaches the angle of an untreated sample after a period of days to weeks. Therefore, PDMS must be bonded quickly after plasma treatment to ensure a strong bond.

Hydrophobic recovery of PDMS is advantageous for certain types of microfluidic devices which require control over the wettability of the device surfaces. Stanton developed a micro-scale pH-stat capable of transporting fluids by passive capillary action which utilizes hydrophilic glass for the wide top and bottom of the fluid channels and hydrophobic PDMS for the sidewalls [38]. This design depends on the ability to make the

PDMS hydrophilic for bonding and to then allow it to return to its nominal hydrophobic state.

Testing was performed to determine whether PNB returns to its nominal hydrophobic state. Three groups of samples were tested: two of PNB and one of PDMS.

One group of PNB was treated at the combination of plasma treatment parameters which produce the minimum contact angle. The other group of PNB was treated at lower power and exposure time to determine whether hydrophobic recovery was more rapid. A group of PDMS was included for comparison. Sample coupons consisting of 15 mm x 15 mm squares with 50 µm-thick PNB films on silicon substrates as described above for the contact angle testing were used. For the PDMS group, samples of 60 µm-thick

Sylgard-184™ PDMS on silicon were used.

All samples in a group were treated in the oxygen plasma system simultaneously and then stored in a wafer carrier to protect them from dust and air currents. At increasing intervals, one sample was removed from each group and the contact angle was recorded.

A new sample from each group was used for each test; samples were not reused.

2.2 Bond Strength Testing

Testing was conducted to demonstrate bonding of PNB to glass and to determine bond strength for samples treated in oxygen plasma at parameters producing minimum contact angle and at suboptimal treatment parameters.

2.2.1 Method

Common methods of measuring bond strength in wafer bonding and polymer film bonding include: tensile and shear load tests, the crack opening test, the blister test, and

45 the four point bending test [39]. Bond strength of PDMS has been studied by blister test

[5] [40] and by shear test [41].

The blister test was selected due to the ready availability of the necessary testing apparatus, ease of fabrication of samples, and direct applicability to microfluidics. The ability to contain fluids under pressure is a common requirement for devices incorporating fluid-carrying microchannels.

For the blister test, a sample is bonded to a substrate which has an opening passing through it. The substrate is bonded to a stage connected to a regulated compressed gas supply. Pressure is increased until the bond fails [42].

2.2.2 Instruments and Techniques

2.2.2.1 Electrochemical Drilling

Electrochemical drilling was selected to form holes in the glass cover slips.

Electrochemical drilling is a process which utilizes an electrical supply and an electrolyte solution to rapidly dissolve material from a target sample [43]. The sample is electrically connected to the anode and dissolution occurs when the cathode is brought into close proximity to the sample in the presence of the electrolyte. The electrolyte may be pumped at high pressure to prevent deposition of reaction products on the surface.

Azizi described a simple electrochemical drilling (ECD) procedure for forming holes in glass slides [44]. The slide is immersed in a 50% NaOH solution. The anode of a

40 V DC power supply is immersed in the solution and the cathode is connected to a sharp needle. Holes with diameter 200 µm can be drilled through glass slides of 150 µm in approximately 20 seconds. An illustration of ECD is shown in Figure 2-8.

glass NaOH

Figure 2-8: Electrochemical drilling setup

The etching solution consisted of 40 g NaOH pellets dissolved in 40 mL DI water in a 50 mL beaker. The solution was allowed to cool for 10 minutes to avoid cracking the glass. Stainless steel sewing needles were used for the cathode and anode. The glass coverslip was held in PTFE tweezers while the cathode needle was applied and removed.

Penetration of the coverslip occurred immediately upon application of the cathode. DC current flow was 0.4 A – 1.2 A during drilling.

Because the cathode needle was applied and removed by hand, holes varied in size and some were not perfectly round. Since analysis of bond strength depends on hole geometry and dimensions, measurements were performed on the holes of twelve samples.

Two orthogonal diameters were recorded for each sample. Mean diameter with standard deviation was 1.47 mm ± 0.20 mm in the horizontal direction and 1.53 mm ± 0.17 mm in the vertical direction. See Appendix C for data.

2.2.2.2 Piranha Clean

A common cleaning technique for glass is immersion in a piranha solution of sulfuric acid and hydrogen peroxide. Piranha cleaning has been used to prepare glass [5] 47 and PSG-, USG-, and LTO-passivated silicon [41] for bonding to PDMS. Because the piranha solution is a strong oxidizer, it is very effective at removing organic residues.

Additionally, piranha treatment of glass has been shown to render the glass surface more hydrophilic than an untreated surface. A contact angle of 8.2° has been reported after a two hour clean in 3:1 v/v 50% H2SO4 : 30% H2O2 piranha solution [45].

One mechanism which has been proposed is hydroxylation of the glass surface resulting in a higher concentration of silanol groups [46]. Testing was conducted to confirm this effect. The contact angle of an untreated glass cover slip was measured to be 49.5°.

Contact angle was significantly reduced by piranha cleaning and returned to the untreated value over a period of several days. Piranha cleaning of longer duration resulted in a lower contact angle. See results in Table 2-3 and Table 2-4.

Elapsed Time Since Clean Sample 30 minutes 24 hours 5 days 1 18.4° 20.6° 30.1° 2 31.9° 34.8° 41.6°

Table 2-3: Contact angle of glass coverslip cleaned for 20 minutes in piranha solution of 3:2 98% H2SO4 : 30% H2O2 by elapsed time since cleaning. Contact angle is significantly reduced compared to untreated nominal value of 49.5° and returns gradually over a period of days.

Elapsed Time Since Clean Sample 60 minutes 3 8.6° 4 12.0°

Table 2-4: Contact angle of glass coverslip cleaned for 50 minutes in piranha solution of 2:1 98% H2SO4 : 30% H2O2 by elapsed time since cleaning. Contact angle is further reduced compared to shorter piranha treatment in Table 2-3.

Prior to bonding and bond strength testing, glass cover slips were cleaned in 2:1 v/v 98% H2SO4 : 30% H2O2 piranha solution. 30 mL H2SO4 was added to 15 mL H2O2

48 and allowed to cool for 10 minutes to avoid cracking the glass. Cover slips were then immersed in the piranha solution for 30 minutes, rinsed with DI water, and dried in compressed air.

2.2.2.3 Epoxy

A means of securely affixing the glass slide of the sample being tested to the vacuum chuck used as the stage of the test apparatus was required. Blanchard wax and methyl-2-cyanoacrylate were investigated and found to be unable to hold the glass to the chuck tightly enough to prevent the glass from cracking under pressure.

Loctite™ 3108 UV-cured epoxy was investigated and found to be suitable. This product is designed for sealing glass to metal joints [47]. The recommended UV cure is

100 mW/cm2 for seven seconds.

A Novacure™ fiber-optic UV source was used for curing. The epoxy was cured for six minutes at 30W which corresponds to approximately 2500 mW/cm2. The aperture of the UV source was clamped to a lab stand above the target so that an area of approximately 12 cm2 was exposed. During curing, the sample was rotated several times to ensure even exposure. The UV curing dose used was much higher than the minimum recommended by the product data sheet to ensure curing through the silicon and to prevent failure due to inadequate or uneven curing.

Cured epoxy can be loosened to recover vacuum chucks by soaking in acetone overnight.

2.2.3 Sample Preparation

Glass cover slips were selected as the glass to be bonded due to their high degree of surface smoothness and convenient size. Fisherfinest™ Premium Cover Glass of size

22 mm x 22 mm was used. Cover slips were prepared by forming a hole through the center of the slide and cleaning in piranha solution, both as described above. Slips were then rinsed in DI water and dried in compressed air.

Samples consisting of 50 µm-thick PNB films on silicon substrates which were diced to 15 mm x 15 mm as described above in the contact angle testing section were used for bond strength testing. Samples for the last tests were separated by cleaving along crystal planes with a diamond scribe.

2.2.4 Bonding

To bond a test sample, a PNB-silicon sample was placed PNB side up in the

March Instruments PX-250™ oxygen plasma system and treated at the desired parameters. Upon completion of the cycle, the chamber door was opened and a piranha- cleaned glass cover slip with hole produced by ECD was placed on the shelf next to the

PNB sample. The PNB was carefully placed by hand into position on top of the cover slip. Force was applied by thumb, repeatedly rolling outward from the center of the sample for approximately ten seconds to ensure even bonding at the edges.

2.2.5 Post-bonding Hot Plate Treatment

The standard PDMS bonding process in use at CWRU calls for a 30 minute curing step at 150°C on a hot plate following plasma treatment and bonding [38]. A

50 comparison was made between PDMS and PNB to determine a suitable post-bonding thermal treatment.

PDMS has a glass transition temperature of -120°C and a decomposition temperature of 200°C [2]. PDMS cures at 65°C during initial processing.

The glass transition temperature of Avatrel™ 2585P PNB is 280°C [10]. The decomposition temperature is not provided by the manufacturer. However, the decomposition temperature of PNB can be tailored by adjusting the monomer functionalization and one PNB formulation intended for use as a sacrificial material is designed to be decomposed at 425°C [9]. Avatrel™ is designed to withstand decomposition of the Unity™ sacrificial PNB formulation and can be expected to have a relatively high decomposition temperature. After spin casting, Avatrel™ cures at 90°C for four minutes followed by 160°C for 60 minutes.

As a thermoplastic material, PNB can be expected to form a stronger bond with post-bond thermal treatment occurring at a temperature approaching the glass transition temperature. However, temperatures too high may result in loss of detail in structural features.

The range of 200 – 250°C for post-bonding thermal treatment of Avatrel™ can be expected to produce good bonds without damaging features or decomposing the material.

The low end of this range will be investigated. If bonding is unsuccessful, testing will proceed to the higher end of the range.

2.2.6 Blister Test

The blister test apparatus is shown in Figure 2-9. Compressed air is fed to a pressure regulator, valves, an analog pressure gauge, and out to a vacuum chuck which is used as the stage for the test sample.

Figure 2-9: Blister test apparatus

A cross section of the sample bonded to the vacuum chuck is shown in Figure

2-10. To affix the sample to the chuck, alignment marks are made in permanent marker on the underside of the glass and the surface of the chuck so that the hole in the glass can be lined up with the opening in the chuck. Epoxy is applied to the surface of the chuck

52 where the glass will be bonded, stippled with a razor blade, and scraped to leave a thin film. Epoxy is not applied within 3 mm of the opening in the chuck to ensure that epoxy does not seep into the test sample (inspection of debonded samples has shown that it does not). The sample is placed on the chuck, alignment marks are lined up, and the sample is pressed down to ensure good contact with the chuck and epoxy. Epoxy is applied around the edges of the glass cover slip, taking care not to deposit epoxy on the PNB-silicon sample which might prevent debonding. The epoxy is cured and the sample is ready to be tested.

Si PNB glass epoxy vacuum chuck Air (0 – 60 psi)

Figure 2-10: Cross section of bonded sample affixed to vacuum chuck

The vacuum chuck is attached to the compressed air line fitting and placed in a container of water so bond failures and leaks at the vacuum chuck gas line connector can be detected easily. A sheet of acrylic is placed above the chuck for safety.

The valves in the compressed air line are opened and the regulator is used to gradually increase pressure. If the bond fails, the valve to the chuck is shut off and pressure is recorded by reading the gauge. The maximum pressure tested is 60 psi.

2.2.7 Experimental Design

Preliminary tests at the plasma parameters used for PDMS were conducted using a qualitative peel test to gauge bond strength. The bond was considered inadequate if it

53 could be peeled apart by finger or razor blade. This testing was used to determine hot plate temperature and glass type and treatment needed to produce acceptable bonds. It was also used to determine whether weights or clamps were needed to apply force to the sample while on the hot plate.

Tests were conducted at the plasma treatment parameters shown by contact angle testing to produce the minimum contact angle. Tests were also conducted at lower power and time to determine whether bond strength was comparable. Both sets of samples were tested immediately after treatment and after a sixty minute delay. Two samples were tested at each of these points, resulting in eight total tests.

A few samples were retested after one week to determine whether the PNB-glass bond remains strong over time like the irreversible bonds made by PDMS.

3. Data and Analysis

3.1 Oxygen Plasma Surface Activation and Contact Angle

Measurement

Testing was conducted to determine the oxygen plasma treatment parameters for

PNB which resulted in minimum contact angle.

3.1.1 Preliminary Testing: Moderate Power and Time

A preliminary test was conducted at low to moderate powers and times and across the full range of oxygen flow rates (which correspond linearly to plasma pressure). Time ranged from 5 – 60 seconds, power ranged form 5 – 75 W, and flow ranged from 5 –

100%. The experiment was conducted using a 6-point space-filling Latin hypercube design as generated by JMP. Samples of 10 µm-thick PNB films on silicon substrates as described in Experimental Methods were used for the testing. The jackknife plot and marginal model plots of the resulting data are provided in Figure 3-1 and Figure 3-2. See data in Appendix D.

Figure 3-1: Jackknife plot for preliminary testing over range of time 0 – 60s, power 5 – 75W, and flow 5 – 100%. Predicted angle does not closely match measured angle indicating that the prediction model is not useful for this data set.

The jackknife plot in Figure 3-1 indicates that the predicted values of angle do not closely match the measured values since the points do not fall close to a line of unity slope. This is due to the data point taken at 5W plasma treatment which was not high enough power to significantly lower the contact angle of that sample. Since the number of samples is small, the model has difficulty accounting for this point and curve fits superimposed on the time and flow marginal models are not useful.

However, visual inspection of the points in the marginal model plots can be used to identify possible regions of interest for further investigation. Two regions were identified in this plot: (1) there appears to be a decrease in contact angle as power is increased in the range of 30 – 50 W; (2) there appears to be a decrease in contact angle at flow of 40%.

3.1.2 Preliminary Testing: Higher Power and Time

A second preliminary test was conducted at higher powers and times. Time ranged from 20 – 240 seconds, power ranged form 20 – 100 W. Oxygen flow rate was held at 100% since the full range of flows would be investigated in the primary contact angle testing. The experiment was conducted using a 6-point space-filling Latin hypercube design as generated by JMP. Samples of 10 µm-thick PNB films on silicon substrates as described in Experimental Methods were used for the testing. The jackknife plot and marginal model plots of the resulting data are provided in Figure 3-3 and Figure

3-4. See data in Appendix D.

The jackknife plot in Figure 3-3 indicates that the prediction model at least has some value since the points appear to be clustered around a line of unity slope. However, because of the small size of the data set, the curve fits in the marginal model plots should be used only to identify trends in data and not as predictions of expected contact angle.

Additionally, the standard deviation of measurements made with the goniometer at low contact angles was found to be 0.5°. Since the error due to the goniometer is large

60 compared to the differences in recorded data, any trends identified must be confirmed by additional testing.

By inspection of the plotted points and curve fits, three regions of interest were identified: (1) there appears to be a decrease in contact angle as power is increased in the range 30 – 60 W; (2) there appears to be an additional decrease in contact angle as power is increased in the range 70 – 100 W; (3) there appears to be a minimum contact angle with respect to time which is reached somewhere in the range of 120 – 180 seconds.

3.1.3 Primary Testing

The ranges of parameters to be investigated were selected based on the regions of interest identified in preliminary testing and the ranges supported by the plasma treatment system. Powers greater than 100 W were not investigated because they approach the powers used for plasma etching of PNB and would risk damage to the surface.

For the primary investigation, time ranged from 10 – 240 seconds, power ranged from 8 – 100 W, and flow ranged from 8 – 100%. The experiment was conducted using a

24-point space-filling Latin hypercube design as generated by JMP. Samples consisting of 50 µm-thick PNB films on silicon substrates as described in Experimental Methods were used for the testing. The jackknife plot, prediction model report, marginal model plots, and surface profiler plot of the resulting data are provided in Figure 3-5, Table 3-1,

Figure 3-6, and Figure 3-7. See data in Appendix E.

It should be noted that the primary investigation used 50 µm-thick PNB films while preliminary testing used samples of 10 µm-thick PNB films. Combined with the fact that the samples for preliminary testing were 6 – 12 months old at the time of testing, this likely explains the observed difference in contact angle values obtained in

61 preliminary and primary contact angle testing. However, the trends in contact angle with respect to plasma parameters remain consistent between preliminary and primary testing.

Figure 3-5: Jackknife plot for primary testing over range of time 10 – 240s, power 8 – 100W, and flow 8 – 100%. Points cluster around a line of unity slope through the origin indicating that the data fits the prediction model.

Parameter Theta Total Main Time Power Flow Sensitivity Effect Interaction Interaction Interaction time 0.0046483 0.6414451 0.0593947 . 0.5586778 0.0233725 power 0.0193366 0.7330749 0.0748291 . 0.0995679 flow 0.0101888 0.1514969 0.0285565 0.0233725 0.0995679 .

µ σ2 2.9811219 0.2172162

-2*LogLikelihood 26.442816 Fit using the Cubic correlation function.

Figure 3-6: Marginal model plots for primary testing over range of time 10 – 240s, power 8 – 100W, and flow 8 – 100%. Since the jackknife plot indicates that the prediction model is useful for this data set, the fitted curves may be interpreted as representing trends in the data.

The jackknife plot indicates that the prediction model has value since the points appear to be clustered around a line of unity slope. The standard deviation of measurements made with the goniometer at low contact angles was found to be 0.5°.

Since the error due to the goniometer is large compared to the differences in recorded data, some variation in the data will result from the error in the tool and should not be expected to be accounted for by the prediction model.

The prediction model report shows that the total sensitivity of contact angle to flow is 0.15, indicating that variations in flow have only a small effect on contact angle.

Flow does not show significant interaction with power or time. The marginal model indicates that there is a slight decrease in contact angle at high flows. The maximum flow setting of 100% was selected as the optimal flow setting. This corresponds to an oxygen plasma pressure of 920 millitorr.

The prediction model report shows that power and time have small main effects

(first-order effects) on contact angle. However, total sensitivity is high for both: 73.3% and 64.1%, respectively. Total sensitivity includes second-order effects of other parameters. The report indicates that both time and power have a significant effect on contact angle and that this effect depends on their interaction.

It is worth noting that 73.3% of the variation in the data is explained by the relationship to power with secondary effects of time and flow. This provides further support that the model is applicable to this data set.

The curve fits superimposed on the marginal model plots were used to determine the values of power and time producing minimal contact angles. Minimal contact angles occurred at powers of 50 W and 100 W and at a time of 150 seconds. These points correspond to regions of interest identified in the preliminary testing. Because contact angles associated with powers of 50 W and 100 W were nearly the same, the selection between them was made based on the fact that lower power plasma treatment is more attractive for microfluidic fabrication processes because there is less risk of damage to the PNB or to other materials in the device.

An exposure time of 150 seconds, a power of 50 W and a flow of 100% produced the minimal contact angle. These plasma treatment parameters were selected for further testing to determine whether PNB can be successfully bonded to glass.

3.1.4 Time Lapse Testing

Plasma-treated PDMS is known to return to its nominal, untreated contact angle over a period of several days. To determine whether PNB also undergoes hydrophobic recovery, testing was conducted to compare the contact angles of PNB and PDMS at increasing intervals following plasma treatment. Many samples were plasma treated at the same time and a fresh sample was used for each data point; samples were not reused.

Two groups of PNB samples were used. One group was treated at the plasma parameters found to produce the minimum contact angle: 150 seconds, 50 W, and 100% flow. Another group was treated at a lower power and exposure time to determine whether less vigorous plasma treatment resulted in more rapid hydrophobic recovery.

This group was treated at 20 W for 20 seconds at 100% flow which was expected to result in a contact angle that was higher but still less than 5°. The PDMS group was treated using the standard PDMS bonding recipe used at CWRU: 25 W for 25 seconds at

100% flow. As described in Experimental Methods, both PNB groups used samples of 50

µm-thick PNB films on silicon substrates and the PDMS group used 60 µm-thick PDMS films on silicon substrates.

Data is presented in Table 3-2. Goniometer repeatability testing found standard deviation of ±1.0° for a surface of angle 76° and ±0.5° for a surface of angle 6.0°. As discussed in the section on goniometer design, measurements below 5° can be expected to be less accurate due to difficulties in capturing and distinguishing the drop profile. The

66 measurement error associated with the goniometer is significant compared to the measured values and likely accounts for the fact that the data does not increase monotonically.

For comparison, the contact angle of untreated PNB was found to be 76° and the contact angle of untreated PDMS was found to be 94°.

Time After PNB at PNB at PDMS at Plasma Treatment 50 W / 150 s / 100% 20W / 20s / 100% 25W / 25 s / 100% 1 min. 2.2° 3.9° 2.1° 5 min. 2.6° 4.2° 2.7° 20 min. 3.1° 4.6° 3.8° 60 min. 2.9° 11.8° 7.5° 3 hour 3.3° 10.1° 12.3° 14 hour 7.7° 13.4° 31.5° 1 day 6.7° 14.2° 50.4° 2 day 6.9° 14.9° 50.5° 4 day 10.1° 17.0° 46.6°

Table 3-2: Contact angle of PNB and PDMS samples at increasing intervals following plasma treatment. Data shows hydrophobic recovery of PNB occurs more slowly than recovery of PDMS.

Hydrophobic recovery of PNB was observed but occurs much more slowly than the recovery of PDMS. The PNB treated at lower power and time showed more rapid recovery than the PNB treated at plasma parameters producing minimal contact angle; however, recovery was still slower than the recovery of PDMS.

3.2 Bond Strength Testing

PNB-glass bonding was demonstrated and the blister test was used to evaluate bond strength. Analysis was conducted to obtain a measure of bond strength which was not dependent on test geometry.

3.2.1 Preliminary Testing

In preliminary testing, samples were bonded and evaluated using a peel test. If a fingernail or razor blade could separate the PNB sample from the glass, bond strength was deemed insufficient. As a result of preliminary testing, piranha-cleaned glass coverslips were selected as the glass material for bonding and a post-bonding hot plate treatment at 200°C was found to be adequate. Weighting or clamping the sample was found to be unnecessary and was abandoned because of the increased risk of cracking the glass coverslip.

PNB-to-PNB bonding was tested with plasma treatment of 25 seconds, 25W, and

100% oxygen flow. A bond strong enough to pass the peel test was formed. PNB-to-PNB bonding was not pursued further in this work.

3.2.2 Blister Test

Bond strength was evaluated using the blister test described in Experimental

Methods. Testing was conducted on samples treated at the plasma parameters found to produce the minimum contact angle and at samples treated at the lower power and time used in time lapse testing. Testing was conducted on samples bonded immediately after plasma treatment and on samples which were bonded 60 minutes following plasma treatment. All samples tested held to 60 psi. Results are presented in Table 3-3.

Sample Oxygen Plasma Treatment Delay Expected Result Time Power Flow Before Contact Angle (seconds) (W) (% of Bonding at Bonding max.) 1 150 50 100% Immediate 2.2° Held to 60 psi 2 150 50 100% Immediate 2.2° Held to 60 psi 3 150 50 100% 60 minutes 2.9° Held to 60 psi 4 150 50 100% 60 minutes 2.9° Held to 60 psi 5 20 20 100% Immediate 3.9° Held to 60 psi 6 20 20 100% Immediate 3.9° Held to 60 psi 7 20 20 100% 60 minutes 11.8° Held to 60 psi 8 20 20 100% 60 minutes 11.8° Held to 60 psi

Table 3-3: Blister test results for tests conducted immediately after bonding. All samples held to 60 psi.

All samples were retested after a period of time exceeding one week. Results are presented in Table 3-4. The sample treated at 20 W for 20 seconds, which would be expected to have a higher contact angle at bonding than the other samples, was the only sample to hold to 60 psi. Because the top layer of the samples is opaque silicon, this test setup determined whether a failure occurred but did not allow the failure to be observed to determine whether it was caused by failure of the bond or of another part of the test sample such as the glass coverslip or the glass-epoxy-vacuum chuck bond. Further testing would be required before drawing conclusions regarding the strength of the bonds over time.

Sample Oxygen Plasma Treatment Delay Expected Result Time Power Flow Before Contact Angle (seconds) (W) (% of Bonding at Bonding max.) 1 150 50 100% Immediate 2.2° Failed at 36 psi 2 150 50 100% Immediate 2.2° Failed at 36 psi 3 150 50 100% 60 minutes 2.9° Failed at 46 psi 4 150 50 100% 60 minutes 2.9° Failed at 40 psi 5 20 20 100% Immediate 3.9° Failed at 41 psi 6 20 20 100% Immediate 3.9° Failed at 43 psi 7 20 20 100% 60 minutes 11.8° Held to 60 psi 8 20 20 100% 60 minutes 11.8° Failed at 37 psi

Table 3-4: Blister test results for tests conducted one week (samples 1, 3, and 7) or two weeks (samples 2, 4, 5, 6, and 8) after bonding.

3.2.3 Bond Strength Analysis

3.2.3.1 Finite Element Analysis

Finite element analysis of the blister test was conducted using COMSOL

MultiphysicsTM. Appropriate material properties of glass, PNB, and silicon were used to configure the materials for the simulation. The simulation geometry consisted of three layers: 500 µm silicon, 50 µm PNB, and 500 µm glass with the lower side set to be immobile and a 1.5 mm diameter cylindrical hole in the center. A force equal to that exerted by a pressure of 60 psi was applied to the exposed PNB surface through the hole in the glass. The simulation verified that the combination of geometry and material properties results primarily in compressive stresses and displacement in the vertical direction prior to debonding. A stress chart and a displacement chart are presented as

Figure 3-8 and Figure 3-9.

Figure 3-9: Displacement chart produced by FEA simulation. Red indicates large displacement; green indicates moderate displacement; blue indicates small displacement. Displacement occurs at the location of the applied force and is greater in PNB than in silicon. 71

3.2.3.2 Bond Surface Energy Calculation

By referencing the literature on wafer bonding and thin film bonding, bond surface energy was selected as a measure of bond strength which does not depend on device dimensions to allow comparison to other test setups and other materials.

Figure 3-10 shows the cross section of a blister test. When a critical pressure is reached, a crack forms at the rim of the circular cavity and propagates through the bond interface in a circular shape.

Figure 3-10: Schematic representation of blister test

By setting up an energy balance between work input and the energy required to create a new surface, a relation between critical pressure and surface energy of the bond can be established. Applying linear plate theory for a circular clamped plate and solving yields the equation [48] [49]:

Equation 3-1

72 where is the surface energy, is the critical pressure, is the Poisson‟s ratio of the film, is the radius of the hole, is the thickness of the film, and is the Young‟s modulus of the film.

Studies have been conducted to approximate the material properties of a two-layer film in bulge testing [50]. The composite film can be accurately modeled by a single layer with thickness equal to both layers and an effective Young‟s modulus found by linear combination of the Young‟s modulus for each layer. Therefore, the effective thickness of the PNB-silicon layer can be found by teff = tSi + tPNB and the effective

Young‟s modulus can be found by:

( ) ( ) . Equation 3-2

The Poisson‟s ratio is nearly identical for silicon and PNB (0.30 versus 0.27) and the calculated surface energy values assuming all silicon or all PNB vary by less than 1%.

The value of silicon was used for calculation.

For a blister test performed on samples having holes of 1.5 mm diameter which held to 60 psi, the bond surface energy at the PNB-glass interface exceeds 76 µJ/m2.

Since the PNB-glass bond did not fail, a lower bound on surface energy can be set but the actual value of surface energy remains undetermined.

For comparison, surface energy of bonding for SU-8–PDMS is 0.047 J/m2, for

2 2 SiO2–SU-8–pyrex is 0.5 J/m , and for silicon–pyrex is 1.3 J/m . All are several orders of magnitude higher than the PNB-glass test, but these values were determined by testing the samples to failure, which could not be done for many of the samples in this thesis. If

73 the PNB-glass blister testing were repeated with the hole size increased to 15 mm (a factor of 10), the test would then correspond to 0.76 J/m2 if the samples could resist failure at 60 psi.

4. Conclusion and Future Work

4.1 Conclusion

Avatrel™ 2585 PNB has been shown to form strong bonds to glass after treatment in oxygen plasma. PNB surfaces are rendered hydrophilic by oxygen plasma treatment and undergo hydrophobic recovery more slowly than PDMS, although the speed of hydrophobic recovery is affected by plasma treatment power and exposure time. Plasma treatment at 50 W for 150 seconds with a plasma pressure of 920 millitorr results in the smallest contact angle and greatest degree of surface activation. Lower power and time treatments and delayed bonding were also found to produce strong bonds which could not be distinguished from bonds of samples treated at optimal plasma parameters by the testing conducted. Bond surface energy was calculated to exceed 76 µJ/m2 for samples subjected to a pressure of 60 psi through a 1.5 mm-diameter hole. Bond strength tests conducted after one week found that the samples treated at optimal plasma parameters failed at pressures less than 60 psi; one sample prepared using the low power and time settings did hold to 60 psi. More testing would be needed to confirm and explain this result.

PNB is photodefinable which makes it easier to structure than PDMS. It is a thermoplastic material and, after surface activation in oxygen plasma, it can be easily bonded to glass which makes it easier to bond than SU-8. Preliminary biocompatibility testing has been promising. Its material properties can be adjusted by altering its monomer functionalization. As a result, PNB has the potential to become a useful

75 material for rapid fabrication of microfluidic devices with advantages over currently used materials.

4.2 Future Work

Further investigation of PNB bonding is required to develop it as a structural material for microfluidic devices. Bond strength testing should be conducted in such a manner as to induce observable bond failure in order to obtain a measure of surface energy rather than a lower bound. If the blister test is used, the diameter of the opening in the cover slip should be increased by at least a factor of ten to test the PNB bonds under similar conditions to published studies for SU-8 and silicon wafer bonding. The upper substrate layer should use a transparent material (i.e., glass) or infrared imaging should be utilized to allow the locations and causes of bond failures to be determined.

Additional bond strength testing should test bond strength with respect to variations in plasma parameters and attempt to correlate contact angle to bond strength. More accurate contact angle measurements could be made with a goniometer using an

ADSA-D approach. If lower power and time plasma treatments produce stronger bonds, it may be that higher plasma doses damage the PNB surface and degrade bond quality.

The surface chemistry of PNB under plasma treatment and under bonding should be investigated. Worth noting is that piranha cleaning of glass results in a decrease in contact angle by a mechanism similar to the decrease in contact angle of PDMS in oxygen plasma; piranha cleaning the glass for bonding to PNB may aid bonding for reasons beyond the removal of organic residues.

PNB-PNB bonding should be investigated, as well as bonding to other common materials used in microfluidic devices such as silicon, silicon dioxide, and glass which

76 does not have as high a degree of surface smoothness. Post-bonding hot plate treatment temperature and clamping or weighting should be investigated and optimized.

Changes in bond strength over time should be evaluated to determine whether the bond holds initially but fails later which might indicate that PNB-glass bonding is not an irreversible bond as in the case of PDMS-glass. If this is found to be the case, it may be that the bond only holds while the PNB is hydrophilic and weakens during hydrophobic recovery.

Biocompatibility testing would be required to establish suitability for biological and medical devices.

Finally, fabrication of a process demonstrator incorporating photopatterned PNB structures and sealed by oxygen plasma assisted bonding would establish the usefulness of PNB for microfluidic systems.

Appendix A: Correlation of March Instruments PX-250™

Plasma Pressure with Oxygen Flow Rate

Oxygen Plasma Flow Rate Pressure (percent) (millitorr) 100 920 90 840 80 760 70 680 60 620 50 530 40 450 30 370 20 280 10 180 5 130 1 Triggers alarm

Appendix B: Drop Mass Measurements for Calculation of Drop Volume for EFD™ Dispenser Tip

Drop mass (mg) 3.72 4.04 4.13 4.04 4.19 4.32 3.91 3.74 3.72 3.58

Appendix C: Diameters of Electrochemically-Drilled

Holes

Sample Horizontal Vertical diameter (mm) diameter (mm) 1 1.609 1.714 2 1.549 1.632 3 1.185 1.172 4 1.623 1.473 5 1.877 1.701 6 1.472 1.562 7 1.441 1.642 8 1.467 1.288 9 1.583 1.417 10 1.319 1.582 11 1.129 1.473 12 1.421 1.718

Mean 1.473 1.531 Std. dev. 0.203 0.173

Appendix D: Preliminary Contact Angle Testing

Time Power Flow Contact (seconds) (W) (Percent of Angle maximum) 60 61 81 4.4° 5 19 24 9.5° 16 47 100 5.9° 27 75 43 5.8° 38 5 62 94.1° 49 33 5 10.7°

Results of lower power and time 6-point space-filling Latin-hypercube experimental design with ranges of time 5 – 60 sec., power 5 – 75 W, and flow 5 – 100%.

Time Power Contact (seconds) (W) Angle 20 84 6.3° 152 100 2.9° 108 52 3.6° 196 36 4.3° 240 68 3.6° 64 20 4.3°

Results of higher power and time 6-point space-filling Latin-hypercube experimental design with ranges of time 20 – 240 sec. and power 20 - 100 W. Flow constant at 100%.

Appendix E: Primary Contact Angle Testing

Time Power Flow Contact (seconds) (W) (Percent of Angle maximum) 150 56 72 2.5° 190 88 64 3.7° 160 80 100 2.9° 120 68 40 3.5° 30 84 32 2.7° 180 60 16 3.0° 70 48 56 2.9° 100 92 60 2.9° 60 40 92 3.6° 40 44 24 2.9° 170 96 28 2.7° 20 76 76 2.5° 200 12 52 2.5° 220 52 48 2.8° 140 36 44 2.6° 10 20 68 4.1° 50 8 36 3.9° 90 72 8 3.1° 80 100 96 2.6° 240 64 88 2.8° 110 24 12 3.2° 210 32 84 2.5° 130 16 80 2.6° 230 28 20 3.2°

Results of 24-point space-filling Latin-hypercube experimental design with ranges of time 10 – 240 sec., power 8 - 100 W, and flow 8 – 100%.

Appendix F: Contact Angle Goniometer Software contact_angle_multi.m function contact_angle_multi()

[filenames, pathname] = uigetfile('data\*.*', 'Select one or more Image or MATLAB data file(s)', ... 'MultiSelect', 'on');

% no file selected if isequal(filenames,0) || isequal(pathname,0) return end if iscellstr(filenames) for i = 1 : length(filenames) contact_angle( pathname, filenames{i} ) end else contact_angle( pathname, filenames ) end

83 contact_angle.m function contact_angle(pathname,filename)

%[filename, pathname] = uigetfile('*.*', 'Select an Image or MATLAB data file'); drop_gray = rgb2gray(imread(fullfile(pathname, filename))); %drop_gray = zeros(1024,1280); f = figure; imshow(drop_gray); title(filename,'Interpreter','none'); hold on;

% edge detection by Laplacian of Gauss (faster) or Canny method (better) %dropedge = edge(drop_gray,'log',0.001); %dropedge = edge(drop_gray,'canny', [0.04 0.10], 1.5); dropedge = edge(drop_gray,'canny', [0.02 0.15], 1.2);

% find where the centerline crosses a detected edge center_x = floor(size(dropedge,2) / 2); center_intersects = find( dropedge( :, center_x ) ); i = 1; while true if ( i > length(center_intersects) ) errordlg('No drop edge intersect with centerline'); return; % ignore white border introduced by Tucsen camera elseif ( center_intersects(i) < 10 || center_intersects(i) > size(drop_gray,1) - 10 ) i = i + 1; else center_y = center_intersects(i); break; end end

% trace edge starting at intersect with centerline, returns (y,x) points droptrace = bwtraceboundary( dropedge, [center_y, center_x], 'E' );

% find left endpoint as min x value [droptrace_mins, endpoint_left_idx] = min( droptrace ); 84 endpoint_left = droptrace( endpoint_left_idx(2), : );

% trace edge again, starting at left endpoint droptrace = bwtraceboundary( dropedge, endpoint_left, 'NE' );

% find right endpoint as max x value [droptrace_maxs, endpoint_right_idx] = max( droptrace ); endpoint_right = droptrace( endpoint_right_idx(2), : );

% choose drop profile roi as points from left endpoint to right endpoint toptrace = droptrace( 1 : endpoint_right_idx(2), : ); annot_handles(1) = plot(toptrace(:,2),toptrace(:,1),'g','LineWidth',2); y = toptrace(:,1); x = toptrace(:,2);

% calc tangent line slopes drop_width = norm(endpoint_right - endpoint_left); tanpt_ofs = ceil(0.05 * drop_width); left_slope = ( y(1) - y(tanpt_ofs) ) / ( x(1) - x(tanpt_ofs) ); right_slope = ( y(end-tanpt_ofs) - y(end) ) / ( x(end- tanpt_ofs) - x(end) );

% calc tangent vector, calc tangent point, and draw for left and right side tvl = [left_slope 1] / norm([left_slope 1]); tpl = endpoint_left + 0.5*drop_width*tvl; %disp(endpoint_left);disp(x(1));disp(y(1)); annot_handles(2) = line( [endpoint_left(2) tpl(2)], [endpoint_left(1) tpl(1)], 'Color', [0.7 0.7 0.7] ); %tpl = [y(1) x(1)] - 0.5*drop_width*tvl; %annot_handles(1) = line( [x(1) tpl(2)], [y(1) tpl(1)], 'Color', 'b' ); %tvr = [1 right_slope] / norm([1 right_slope]); tvr = [right_slope 1] / norm([right_slope 1]); tpr = endpoint_right - 0.5*drop_width*tvr; annot_handles(3) = line( [endpoint_right(2) tpr(2)], [endpoint_right(1) tpr(1)], 'Color', [0.7 0.7 0.7] ); annot_handles(4) = plot(endpoint_left(2),endpoint_left(1), 'rx', 'MarkerSize', 8); annot_handles(5) = plot(endpoint_right(2),endpoint_right(1), 'rx', 'MarkerSize', 8);

85 stage_slope = ( y(end) - y(1) ) / ( x(end) - x(1) ); left_slope = -left_slope; right_slope = -right_slope;

% calc angles from slopes and convert to degrees angle_left = atan((left_slope- stage_slope)/(1+left_slope.*stage_slope)); angle_right = atan((right_slope- stage_slope)/(1+right_slope.*stage_slope)); angle_left = angle_left*180/pi angle_right = -angle_right*180/pi angle_avg = (angle_left + angle_right) / 2 angletext = { ['automated measurement'], ['left angle = ', sprintf('%.1f',angle_left), '\circ'], ['right angle = ', sprintf('%.1f',angle_right), '\circ'], ['avg angle = ', sprintf('%.1f',angle_avg), '\circ']}; angletext_handle = text(20,20, angletext,'BackgroundColor',[1.0 1.0 1.0],'HorizontalAlignment','left','VerticalAlignment','top' );

keydown = waitforbuttonpress; if ( keydown == 1 && int8( get(gcf,'CurrentCharacter') ) == 27 ) % stop if pressed close(f); return; end for h = annot_handles(1:end) set(h,'Visible','off'); end man_endpoints = ginput(2); p = [ ginput(1); man_endpoints; ginput(1) ]; updateManualAngle(p) centerx = mean(man_endpoints(:,1)); while true % get a point by mouse click newpt = ginput(1);

if int8( get(gcf,'CurrentCharacter') ) == 27 % stop if pressed break; end

% replace closest tangent point in x with new point if newpt(1) > centerx p(4,:) = newpt; else p(1,:) = newpt; end

% delete existing manual measurement annotations and recalculate delete(findobj('Color','b')); updateManualAngle(p); end for h = annot_handles(1:end) set(h,'Visible','on'); end while true % get a point by mouse click newpt = ginput(1); if int8( get(gcf,'CurrentCharacter') ) == 27 % stop if pressed break; end end close(f)

% Callback function that calculates the angle and updates the title. % Function receives an array containing the current x,y position of % the three vertices. function updateManualAngle(p) % create vectors from the vertices vleft = [p(1,1)-p(2,1), p(1,2)-p(2,2)]; vstage = [p(3,1)-p(2,1), p(3,2)-p(2,2)]; vright = [p(4,1)-p(3,1), p(4,2)-p(3,2)];

% find angle and convert to degrees

87 theta_left = acos(dot(vleft,vstage)/(norm(vleft)*norm(vstage))); theta_right = acos(dot(vright,vstage)/(norm(vright)*norm(vstage))); angle_left = (theta_left * (180/pi)); angle_right = 180-(theta_right * (180/pi)); angle_avg = (angle_left + angle_right) / 2; mantext = { ['manual measurement'], ['left angle = ', sprintf('%.1f',angle_left), '\circ'], ['right angle = ', sprintf('%.1f',angle_right), '\circ'], ['avg angle = ', sprintf('%.1f',angle_avg), '\circ'] }; xmax = xlim; mantext_handle = text(xmax(2)-20,20, mantext,'Color','b','BackgroundColor',[1.0 1.0 1.0],'HorizontalAlignment','right','VerticalAlignment','top '); plot(p(2:3,1),p(2:3,2), 'bo', 'MarkerSize', 4); pend = p(2,:)+100*((p(3,:)-p(2,:))/norm(p(3,:)-p(2,:))); line([p(2,1) pend(1)], [p(2,2), pend(2)], 'Color', 'b', 'LineStyle', '-'); pend = p(2,:)+100*((p(1,:)-p(2,:))/norm(p(1,:)-p(2,:))); line([p(2,1) pend(1)], [p(2,2), pend(2)], 'Color', 'b', 'LineStyle', '-'); pend = p(3,:)+100*((p(2,:)-p(3,:))/norm(p(2,:)-p(3,:))); line([p(3,1) pend(1)], [p(3,2), pend(2)], 'Color', 'b', 'LineStyle', '-'); pend = p(3,:)+100*((p(4,:)-p(3,:))/norm(p(4,:)-p(3,:))); line([p(3,1) pend(1)], [p(3,2), pend(2)], 'Color', 'b', 'LineStyle', '-');

Appendix G: Contact Angle Goniometer Standard

Operating Procedures

Goniometer Setup

To be performed before any use of the goniometer.

1. Remove lens cap from camera and dust protector weigh boat from stage. Clean dust from stage if

necessary.

2. Turn light source on and set to fourth intensity setting (180° from off position).

3. Plug camera USB cable into computer.

4. Start TSView7. Two windows will open: camera preview and image browser.

5. In the camera preview window: click the drop-down next to the Snap button. Click Config. Select

“Use File Save Config.” Check “Use Time-stamped.” Ensure file extension is “.jpg” and click OK.

Note: When the File Save Dialog is used instead of a timestamp filename, the image is not

captured until the user types in the filename.

6. In the image browser window: select the directory you want the image files to be saved to. Note

that directories created after TSView was started will not appear.

7. In the camera preview window: click the Property button, open the Image Adj. tab, and check the

V Flip box. This will flip the image so that the drop appears above the stage.

Goniometer Measurement

To be performed for each sample.

1. Center sample on stage. Use the small hole in the center as a guide. Sample should be clean or the

drop may spread out into an irregular shape which will not yield reliable contact angle

measurements.

2. Using a syringe or pipette place a drop of DI water on the sample, centered relative to the stage. If

drop is smaller than usual or spreads out into an irregular shape, it may be necessary to repeat the

measurement.

3. Use the micrometer dial on the X-Y-Z positioner to move the stage horizontally into the center of

the camera field of view. It may be difficult to identify the drop if the lens is significantly out of

focus. If that is the case, see next step to adjust focus and then adjust stage horizontal position.

4. Adjust camera focus by rotating the ring at the aperture of the lens assembly. Adjust focus such

that the image of the drop is as sharp as possible.

5. Click the Snap button to save the image. It will be saved to the selected directory immediately

using a date/time stamp for the filename.

6. Small drops evaporate rapidly and as they evaporate the drop shape changes. Work quickly! It

should be possible to apply a drop, move the stage, adjust the focus, and record the image in less

than 15 seconds.

7. Remove sample and repeat for additional samples.

8. Replace lens cap and stage dust protector. Power off light source.

Contact Angle Calculation Software analysis of goniometer images.

1. Start Matlab™ and open contact_angle_multi.m .

2. Debug -> Run or press F5 to run the script.

3. Select one or more image files. Most common image formats should be supported by the Matlab™

image import routines.

4. Automatic contact angle measurement will appear. Automatic measurement is currently useful

only for drops with sharp outlines and contact angles between approximately 20° and 90°. Verify

that the detected drop outline (green), drop endpoints (red X), and tangent lines (grey) appear

reasonable before using the automatically calculated angle.

5. Click once to activate manual contact angle calculation OR press ESC to close and proceed to the

next image.

6. In manual mode, mouse crosshairs will be displayed and automatic measurement annotations will

be hidden.

7. Click at the left drop endpoint, where the drop profile meets the stage (see Figure 1).

8. Click at the right drop endpoint (see Figure 2).

9. Click a point along the left side of the drop surface to construct the left tangent line (see Figure 3).

10. Click a point along the right side of the drop profile to construct the right tangent line (see Figure

4).

11. Endpoints, stage markers, and tangent lines will be displayed in blue and the angle calculation will

appear in the upper right corner (see Figure 5).

12. Tangent line position can now be moved by clicking again in the window. Click left of the drop

centerline to move the left tangent line and right of the drop centerline to move the right tangent

line.

13. Press Esc to exit manual measurement mode and compare automatic measurement to manual

measurement annotations.

14. Press Esc again to close the window and proceed to the next image.

Manual Contact Angle Calculation Examples

Figure 1: Left endpoint (step 7) Figure 2: Right endpoint (step 8)

Figure 3: Left tangent line point (step 9) Figure 4: Right tangent line point (step 10)

Figure 5: Completed manual measurement

Goniometer Adjustment and Maintenance Tasks which are only necessary when the goniometer is first set up or is moved.

1. Ensure lens is inserted into lens mount facing the stage.

2. Ensure Tucsen TCA-1.31C™ camera is screwed into lens.

3. Ensure camera magnification adjustment ring is adjusted to maximum magnification.

Magnification ring is on the camera end of the assembly and is NOT the ring on the aperture end

(which is the focus ring). When viewed from behind the camera it should be rotated fully

clockwise.

4. Ensure fiber optic light source is plugged in and aperture is inserted into light source mount so that

the center of the light diffuser is uniformly illuminated.

5. Ensure stage is approximately centered horizontally and vertically in the field of view of the

camera by adjusting the micrometer dials of the X-Y-Z positioner.

6. Ensure stage is level using a 2D bubble level. Rotate the bolts underneath the stage to level the

stage if adjustment is required. Stage should be level horizontally but should slope slightly

downward toward the camera. If using a General No. 847 bubble level, the center of the bubble

should fall on the outside ring.

7. Prepare a 3 mL syringe by removing the needle and attaching a dispensing tip (EFD ULTRA™

#5132-1/4-B #32GP 0.004 X 0.250 YELLOW is available in the lab) and filling with DI water.

8. If preferred, a Finnpipette II™ 2-20 µL pipetter may be used instead if set to 2 µL drop size. The

pipetter is slower than the syringe and may occasionally not release the drop cleanly requiring the

test to be repeated.

9. Install the Tucsen TCA-1.31C™ camera driver and TSView™ software from the install CD or

tucsen.com. Current latest versions are :

http://www.tucsen.com/Download/Driver/TCA-1.31C%20Driver%20Setup.exe

http://www.tucsen.com/Download/TSView/TSView7.1.1.0-eng-Setup.exe

10. Install Matlab™ R2010b and copy the contact angle measurement scripts to the computer. Run the

script contact_angle_multi.m. It opens a file selection dialog and then calls the script

contact_angle.m with each selected image file to perform the automatic and manual

measurements. contact_angle_man.m is an alternative version of contact_angle.m with all

automatic measurement code removed.

Notes

Note that this contact angle measurement is calculated by taking the tangent of the drop at the three-phase point and will not exactly correlate to surface free energy due to effects of forces such as gravity. However, this software is suitable for comparing the relative contact angle of materials and should also be approximately consistent with true contact angle when used with a drop of 2 – 3 µL volume. If true contact angle is needed (in order to calculate surface free energy), the user should investigate axisymmetric drop shape analysis techniques which determine contact angle by fitting the recorded drop shape to a modified spherical model.

There is currently no Directshow™ driver for the Tucsen TCA-1.31C™ camera which supports 64-bit

Windows™. However, there is a driver for 32-bit Windows™. The Matlab™ image acquisition toolbox

(imaqtool) requires a DirectShow™ driver. If you would like to use Matlab™ to capture images (e.g., to automate shots over a period of time) you will need to use a computer with a 32-bit version of Windows™ installed.

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