Significant Enhancement of the Adhesion between Metal Films and

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Significant Enhancement of the Adhesion between Metal Films and Polymer Substrates by UV-ozone Surface Modification in Nanoscale Junshan Liu, Licheng He, Liang Wang, Yuncheng Man, Luyi Huang, Zheng Xu, Dan Ge, Jingmin Li, Chong Liu, and Liding Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09930 • Publication Date (Web): 17 Oct 2016 Downloaded from http://pubs.acs.org on October 17, 2016

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ACS Applied Materials & Interfaces

Significant Enhancement of the Adhesion between Metal Films and Polymer Substrates by UV-ozone Surface Modification in Nanoscale

Junshan Liu,1, 2* Licheng He,1 Liang Wang,1 Yuncheng Man,1 Luyi Huang,3 Zheng Xu,1 Dan Ge,3 Jingmin Li,1 Chong Liu,1, 2 and Liding Wang1, 2

1

Key Laboratory for Micro/Nano Technology and System of Liaoning Province, Dalian

University of Technology, Dalian, Liaoning, 116024, China. 2

Key Laboratory for Precision and Non-Traditional Machining Technology of the Ministry of

Education, Dalian University of Technology, Dalian, Liaoning, 116024, China. 3

School of Chemical Engineering, Dalian University of Technology, Dalian, Liaoning, 116024,

China.

ACS Paragon Plus Environment

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ABSTRACT Polymer metallization is extensively used in a variety of micro and nano system technologies. However, the deposited metal film exhibits poor adhesion to polymer substrates, which may cause difficulties in many applications. In this work, ultraviolet (UV)-ozone surface modification is for the first time put forward to enhance the adhesion between metal films and polymer substrates. The adhesion of sputtered Cu films on UV-ozone modified polymethylmethacrylate (PMMA) substrates is enhanced by a factor of 6, and that of Au films is improved by a factor of 10. Moreover, metal films on the modified PMMA substrates can withstand a long-time liquid immersion. To understand the mechanism for the adhesion enhancement, the surface modification is studied with contact angle measurements, attenuated total reflection Fouriertransform infrared spectrometry (ATR-FTIR) and atomic force microscopy (AFM). Detailed characterization results indicate that the significant adhesion enhancement is attributed to the increases of both the surface wettability by generating some polar functional groups and the roughness of the surface in nanoscale. To demonstrate this novel polymer metallization method, a 6-inch PMMA chip with arrays of three-electrode electrochemical microsensors is designed and fabricated, and the microsensor exhibits excellent reproducibility, uniformity and long-term stability.

KEYWORDS: polymer metallization; polymethylmethacrylate (PMMA); surface modification; ultraviolet (UV)-ozone; electrochemical sensor


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1. Introduction Compared to inorganic materials such as silicon, polymer provides numerous advantages in terms of cost, mechanical and optical properties, and ease of fabrication. Polymer has been increasingly used in many micro and nano system technologies such as microfluidics,1 flexible and stretchable electronics,2, 3 displays,4 wearable electronics,5 and implantable devices.6 Polymer metallization can integrate various metal components on the device, which can improve the device integration and expand polymer applications, such as electrodes on a microfluidic chip for electrochemical detection or biosensing,7-9 electrodes on a triboelectric nanogenerator for generating and collecting charge,10 an antenna on a wearable device for communications,11 and heaters on a PCR device for temperature controlling.12 However, since the polymer surface is nonpolar and has a low wettability, the adhesion of deposited metal films on polymer substrates is normally poor.13, 14 This is especially evident for weakly reactive metals such as Cu, Ag, Au or Pt.15 Several methods have been explored to modify the polymer surface to enhance the adhesion, such as plasma treatment using different gases,16-18 irradiation with ion beam19 or excimer laser,20 and spin-casting organic solvents.21, 22 By these methods, some new polar groups are formed at the polymer surface, and the wettability is increased. Meanwhile, the polymer surface roughness is increased by a few of the aforementioned methods such as plasma treatment, which increases the surface area and also contributes to the improvement in the adhesion.13 Ultraviolet (UV)-ozone treatment by a low-pressure mercury lamp was initially developed to remove photoresist polymers from the substrate.23 Recently, the UV-ozone modification was used to decrease the glass transition temperature at the surface of polymer plates and achieve low temperature bonding of polymer microfluidic chips.24,

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In this paper, the UV-ozone 3


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modification is for the first time, to our knowledge, evaluated as an alternative method for enhancing the adhesion between metal films and polymer substrates. Polymethylmethacrylate (PMMA) is one of the most popular thermoplastic polymers in optical, biological, medical and chemical applications.14,

16, 24, 26

Here, we report an effective surface

modification of PMMA substrates by UV-ozone, which realizes significant enhancement of the adhesion between sputtered Cu and Au films and PMMA substrates. The adhesion of Cu films was enhanced by a factor of 6, and that of Au films was improved by a factor of 10. The improved adhesion is comparable to that of metal films on glass substrates and significantly surpasses that of metal films on oxygen plasma treated PMMA substrates. Moreover, metal films on UV-ozone modified PMMA substrates can withstand a long-time liquid immersion. The characterization of modified PMMA surfaces indicates that the adhesion enhancement is attributed to the increases of both the surface wettability by generating some polar oxygencontaining functional groups and the roughness of the surface in nanoscale. To demonstrate this novel polymer metallization method, a 6-inch PMMA chip with arrays of three-electrode electrochemical sensors was fabricated and tested.

2. Results and Discussion 2.1. Polymer Metallization Figure 1 shows a schematic diagram of the polymer metallization. A commonly used UV-ozone cleaner (BZD250-S, HWOTECH Co., Ltd., China) equipped with a low pressure mercury vapour grid lamp was used to modify the surface of PMMA plates (28 × 28 × 2 mm). The lamp emits UV lights at 185 nm and 254 nm wavelengths. The output power of the lamp is about 10 mW/cm2 according to the manufacturer’s specification. The lamp was warmed for 15 min. Then


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the PMMA plate was placed in the UV-ozone cleaning chamber, and treated at a distance of 7.5 cm from the lamp for 5 min under atmospheric conditions. The oxygen molecules (O2) in the air absorbed 185 nm UV light and formed ozone, and then the ozone molecules (O3) absorbed 254 nm UV light and broke into atomic oxygen (O), as shown in Figure 1a. The atomic oxygen here acted as a strong oxidizing agent. A Cu layer (100 nm) or an Au layer (100 nm) with a Ti adhesion layer (10 nm) was sputtered on the modified PMMA surface (Figure 1b). Then, a positive photoresist was patterned on the metal surface (Figure 1c). Finally, the exposed metal was chemically etched, and the residual photoresist was removed (Figure 1d). The detailed process parameters were presented in the Experimental Section.

2.2. Enhanced Adhesion Strength Referring to the method of Augustine’s group,21, 22 a tape test adhesion measurement modified from the ASTM D3359-09 method was used to measure the adhesion of metal films on PMMA substrates. The representative micrographs of PMMA plates containing an 11 × 11 square array of circular metal dots before and after the tape test were shown in Figure S1 (Supporting Information). The fraction of metal remaining on the plate was defined by dividing the number of black pixels after the test to the original number. The average fraction of metal remaining obtained from 10 plates for each type of samples was shown in Figure 2. It is clearly shown that the adhesion strength is significantly enhanced by the UV-ozone modification. The fraction of Cu remaining for the virgin PMMA plates was only 15.6%, and it was enhanced up to 94.1% for the modified ones. The adhesion of Cu films is improved by a factor of 6, which is comparable to that of Cu films on glass substrates (86.9%). Similarly, the fraction of Ti/Au remaining for the virgin plates was 9.5%, and it was improved to 96.0%. The adhesion of Ti/Au films is enhanced by a factor of 10, which significantly surpasses the adhesion for an oxygen plasma treated sample


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(26.3%).21 Besides, even without the Ti adhesion layer, the fraction of Au remaining on the UVozone modified PMMA plate was also up to 89.9%, which is comparable to the result reported by spin-casting organic solvents (89.6%).21 Moreover, no additional solvent molecules here are introduced on PMMA surfaces, which is more desirable for biological or chemical applications.

2.3. Soaking Test In many applications, metal films directly contact the liquid. Especially in biological, medical and chemical applications, sometimes they even have to be immersed in a liquid for a long time, which makes the adhesion of metal films on substrates more important. Otherwise, metal films will be easily lifted off.27 Ten UV-ozone modified PMMA plates with Cu dots were immersed into deionized water (18 MΩ·cm). Before the immersion, each plate was numbered, and weighed separately by an electronic balance with an accuracy of 0.01 mg (AG245, Mettler-Toledo International Inc., Switzerland). The plates were observed and weighed each day. After 7 days, all Cu dots on these ten plates were still attached to the PMMA surface, which further demonstrates that a strong adhesion between metal films and PMMA surfaces is obtained by the UV-ozone modification. However, the color of Cu dots was changed from bright orange to dark red, and many tiny cracks were observed in Cu dots (Figure S2a, Supporting Information). Obviously, Cu was corroded by water, and it was oxidized to Cu2O.28 In addition, the measurement of the weight of PMMA plates showed that an average of 17.66 mg water was absorbed by one PMMA plate after 7 days of the immersion. The weight of the absorbed water is about 1% of that of one PMMA plate (1.811 g). Therefore, the PMMA plate surely swelled, and it could be deduced that a tensile stress in Cu2O was induced from the swelling and the cracks were caused by the poor stretchability of Cu2O. Similarly, another ten UV-ozone modified PMMA plates with Au dots were also


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immersed into deionized water. After 30 days of the immersion, no detachment of Au dots was observed. Moreover, no color changes and cracks were detected (Figure S2b, Supporting Information), which is because Au is chemically inert and has a good stretchability. In general, the adhesion strength of metal films on the UV-ozone modified PMMA plate is strong enough to make the metal film withstand a long-time liquid immersion.

2.4. Characterization of Modified PMMA Surfaces In order to understand the mechanism for the significant enhancement of the adhesion, the effects of the UV-ozone modification on the PMMA surface were investigated by using contact angle measurements, ATR-FTIR and AFM. Contact angle The wettability of PMMA surfaces was evaluated in terms of the water contact angle. The contact angle was also used to optimize the UV-ozone modification time. Each contact angle reported here was the average of three separate drops of water on three substrates. As shown in Figure 3a, the virgin PMMA plate was nearly hydrophobic with a contact angle of 80.0 °. The contact angle dramatically decreased with the modification time, and reached a minimum value of 29.9 ° at 5 min. When the modification time was longer than 7 min, the PMMA plate became a little distorted due to the heat generated in this modification process. Therefore, an optimal modification time of 5 min was used in all subsequent experiments. The contact angle of 29.9 ° is about 8 ° smaller than that obtained from the oxygen plasma treatment (37.7 °). Moreover, the UV-ozone modification has a better stability than the oxygen plasma treatment. The hydrophilic property of the modified PMMA surface can be maintained in air for at least 14 days (Figure 3b). The contact angles were 30.1 ° after 1 day, 37.3 ° after 7 days, and 38.6 ° after 14 days. In contrast, the hydrophilicity of the PMMA surface modified by the oxygen


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plasma treatment can only be maintained for a very short time: the contact angle increased to 45.8 ° only after 0.5 h, and 64.2 ° after 2 h. That is, metal films should be deposited very quickly after the PMMA surface was treated by oxygen plasma. However, it typically takes a few hours to finish all the preparation work such as loading the PMMA plate and vacuuming the process chamber before depositing metal films. Therefore, it was believed that the short-lived hydrophilicity was the main reason why the oxygen plasma treatment had very limited effectiveness in increasing the metal adhesion.18, 21 Apparently, the wettability of the PMMA surface is greatly improved by the UV-ozone modification, and the increase of the wettability indeed significantly enhances the adhesion between metal films and PMMA substrates just as we expected. We suggest that the improvement of the wettability is mainly attributed to the increase of the surface polar functional groups, which will be discussed below. ATR-FTIR The chemical structure of the PMMA surface was characterized with ATR-FTIR. The spectra of PMMA surfaces were recorded from 4000 to 400 cm-1, as shown in Figure 4a. To clearly show the spectral changes before and after the UV-ozone modification, the difference spectrum (Figure 4b) was made by subtraction of the spectrum of the modified PMMA from the spectrum of the virgin PMMA. The modified PMMA had a broad absorption band at 3000-3600 cm-1 assigned to O-H stretching vibrations, which indicates that the –OH group was generated. Meanwhile, the intensities of the bands at 1050-1100 cm-1 and 1220 cm-1 were increased significantly, and these two bands were assigned to C-O stretching vibrations of alcohols and carboxylic acids respectively. Therefore, it was believed that alcohols and carboxylic acids were formed at the modified PMMA surface.


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The absorption band at 1600-1800 cm-1 was assigned to C=O stretching vibrations of ester groups (Figure 4a). After the UV-ozone modification, the intensity of the band centered at 1724 cm-1 was decreased sharply (Figure 4b). Similarly, the absorption band at 1000-1300 cm-1 was assigned to C-O-C stretching vibrations of ester groups, and the intensity of the band at 11501200 cm-1 was also decreased largely after the modification. The noticeable decreases of C=O and C-O-C bonds were due to the photolysis of the ester group and, in turn, the photolysis of the ester group resulted in the generation of the carboxylic acid.29 The band assigned to C=O stretching vibrations of the carboxylic acid was also detected at the wavenumber of 1765 cm-1. The absorption band at 2800-3000 cm-1 was assigned to C-H stretching vibrations (Figure 4a). The decrease of the band at 2900-3000 cm-1 (Figure 4b) was caused by the photooxidation of methyl and methylene groups. Accordingly, the photooxidation led to the generation of alcohols, ketones and aldehydes.29 The increases of both the band at 1710 cm-1 assigned to C=O stretching vibrations of ketones and aldehydes and the band at 2810 cm-1 assigned to C-H stretching vibrations of aldehydes confirmed the generation of ketones and aldehydes. In sum, the photolysis of ester groups and the photooxidation of methyl and methylene groups caused by the UV-ozone modification result in the generation of some polar oxygen-containing functional groups, including alcohols, carboxylic acids, ketones and aldehydes. As suggested above, it is the presence of these polar oxygen-containing functional groups that increases the wettability of the modified PMMA surface. These oxygen-containing polar functional groups at the polymer surface interact strongly with the deposited metal atoms, and some strong chemical bonds between the polymer surface and the metal film are established, which is beneficial for the adhesion enhancement.19, 30


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AFM The surface morphology of PMMA plates was observed by AFM. The surface of the virgin PMMA plate was very smooth with a surface roughness (Ra) of 1.022 nm (Figure 5a). However, the surface was significantly roughened in nanoscale by the UV-ozone modification. Numerous nanoscale hillocks and pits were formed (Figure 5b), and the surface roughness (Ra) was increased to 5.148 nm. The formation of hillocks and pits was caused by UV-ozone induced polymer chain scission and local melting on the PMMA surface.25, 31 The increased roughness in nanoscale contributes to the improvement in the adhesion of metal films on PMMA substrates in two ways. On one hand, a more effective surface area is provided at the metal-polymer interface. On the other hand, the hillocks and pits provide mechanical anchoring sites between the metal film and the PMMA substrate, and the strong mechanical interlocking effect is beneficial for improving the adhesion.19

2.5. Three-electrode Electrochemical Sensors Electrochemical detection is sensitive, label-free and easy for miniaturization and integration, and has been becoming one of the most popular detection methods for microchip analyses.32 To demonstrate the PMMA metallization method reported here, a 6-inch PMMA chip with arrays of three-electrode electrochemical sensors was fabricated and tested. As shown in Figure 6, this chip includes 29 identical sensors. Each sensor consists of a gold working electrode, a gold pseudo-reference electrode and a gold counter electrode. The diameter of the working electrode is 4 mm. Because these three electrodes are made of the same material, they can be fabricated by single step sputtering, photolithography and wet etching, as depicted in Figure 1b-d, which simplifies the fabrication and lowers the cost of the sensor. The chip was cut into individual sensors by a laser cutter before use.


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Dopamine is an important neurotransmitter, and recently the electrochemical technique is widely used for dopamine detection.33 Here, dopamine (Sigma, USA) was used as a model analyte to evaluate the electrochemical performance of the sensor. Cyclic voltammetry detection for dopamine was carried out by an electrochemical system. With one sensor, two different concentrations (50 µM, 100 µM) of standard dopamine samples dissolved in 0.1 M phosphate buffered saline (PBS) solution were consecutively measured 10 times, respectively. The relative standard deviations (RSDs) of peak currents of two concentrations of dopamine were 0.90% and 1.20%, respectively, which are lower than those obtained from a polycarbonate microchip with an electroless-plated Au working electrode.34 With six random sensors, these two concentrations of dopamine were separately examined. The peak currents obtained from these six sensors were shown in Figure 7a, and RSDs of peak currents were 2.14% and 4.39%, respectively. The excellent reproducibility from single and different sensors indicates that the electrochemical sensor fabricated here has a good stability and a good uniformity on a 6-inch chip, which is essential to the high-throughput analysis and disposable usage of microchips. The soaking test discussed above showed that Au films could withstand a long-time liquid immersion. Hence, to further demonstrate the stability of the electrochemical sensor, one sensor was immersed in 0.1 M PBS solution for 6 days. On each day, the sensor was taken out for about 0.5 h to measure two concentrations of dopamine. The peak currents measured on each day were shown in Figure 7b, and RSDs of peak currents were 6.44% and 5.96%, respectively. This experiment indicates that the sensor fabricated on the UV-ozone modified PMMA plate has a great potential for a variety of long-time and real-time electrochemical analyses.


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3. Conclusions The adhesion between metal films and PMMA substrates was significantly enhanced by UVozone surface modification, and the enhancement was attributed to the increases of both the surface wettability by generating polar oxygen-containing functional groups and the roughness of the surface in nanoscale. Moreover, the modification process is quite simple by only using a commonly used UV-ozone cleaner and compatible with well-developed microfabrication techniques, which makes this novel polymer metallization method suitable for low-cost mass production of various micro and nano devices. As a demonstration, an array of three-electrode electrochemical sensors was fabricated on a 6-inch UV-ozone modified PMMA substrate, and the sensor exhibited excellent reproducibility, uniformity and long-term stability. In addition, this novel polymer metallization method should be applicable for some other polymers, such as polycarbonate, polystyrene and cyclic olefin copolymer, and will be further studied.

4. Experimental Section 4.1. Fabrication of Cu and Au Patterns PMMA plates were purchased from Goodfellow Cambridge Limited (Huntingdon, UK). All reagents were of analytical grade and purchased from Tianjin Kemiou Chemical Reagent Co., Ltd (Tianjin, China) unless otherwise specified. First, a 100 nm thick Cu layer or an Au layer (100 nm) with a Ti adhesion layer (10 nm) was sputtered on the PMMA surface at a sputtering power of 300 W. Second, a positive photoresist (BP212, Beijing Institute of Chemical Reagents, China) was patterned on the surface of metal films. The photoresist was spin-coated at 2600 rpm for 30 s, soft-baked at 60 ℃ for 1 h, exposed to UV light at a dose of 4.2 mJ/cm2 for 30 s through


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a photomask, developed in 0.5% NaOH solution, and hard-baked at 60 ℃ for 1 h. Third, the exposed copper was etched in buffered HNO3 solution, or gold in the mixture of I2, KI and H2O (1 g: 5 g: 50 mL) and titanium in buffered hydrofluoric acid. Fourth, the residual photoresist was secondly exposed to UV light for 3 min without a photomask, and removed by 0.5% NaOH solution.

4.2. Tape Pull Test The tape test adhesion measurement used in this paper was modified from the ASTM D3359-09 method. The ASTM D3359-09 method is mainly used for assessing the adhesion of coating films on a metal substrate. In the ASTM D3359-09 method, a lattice pattern is cut into the coating film by a sharp razor blade or other cutting devices and removed by a tape. However, it is very difficult to cut the metal film on a polymer substrate only to the metal-polymer interface because polymer is soft. Therefore, in this work, by referring to the method of Augustine’s group,21, 22 an array of circular metal dots made by photolithography and wet etching processes replaced the lattice pattern cut by a razor blade The PMMA plate for testing the adhesion contained an 11 × 11 square array of circular metal dots. The diameter of the dot was 1.5 mm, and the distance between two adjacent dots was 0.1 mm. A 19.1 mm wide adhesive tape (3M Scotch Magic Tape) was pressed on the surface of the plate and removed. A digital camera was used to capture the images of the plate before and after the tape test. All images were converted to grayscale and measured by using the Image J software. The fraction of metal remaining on the plate was defined by dividing the number of black pixels after the test to the number before.

4.3. Characterization of PMMA Surfaces


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The contact angle was measured by a Krüss DSA 100 Contact Angle Goniometer (Krüss GMBH, Hamburg, Germany) using the sessile drop method. The contact angle was calculated by the Drop Shape Analysis software using the tangent fit method. Each value reported here was the average of three separate drops of water on three substrates. The ATR-FTIR experiment was carried out by using a Vertex 70 FTIR with a Platinum ATR accessory equipped with a single reflection diamond crystal (Bruker, Germany), and the angle of incidence was 45 °. The spectra were recorded from 4000 to 400 cm-1 by collecting 64 scans with a 2 cm-1 resolution. The AFM experiment was performed with a NS3A-02 microscope (Veeco Co., USA), and the tapping mode was applied.

4.4. Electrochemical Detection Prior to use, the Au electrodes on the electrochemical sensor were immersed in the mixture of KOH and H2O2 (3: 1, v/v) for 1 min, flushed with deionized water (18 MΩ·cm), and dried with nitrogen. The sensor was connected with the Reference 600+ electrochemical system (Gamry Instruments, USA) by a homemade electrical connector. For each analysis run, a 50 µL dopamine solution was dropped on the electrodes by a pipette.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Representative images of 11 × 11 circular metal dot arrays on substrates, and microscopy images of metal dot arrays on the UV-ozone modified PMMA plate after the soaking test.

AUTHOR INFORMATION


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Corresponding Author *E-mail: [email protected]

Author Contributions J. Liu, L. He and L. Wang designed the experiments. J. Liu, L. He, L. Wang, Y. Man, L. Huang, Z. Xu and D. Ge performed experiments and analysis. J. Li, C. Liu and L. Wang gave some useful suggestions. J. Liu, L. He and L. Wang wrote the paper.

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (51475080, 51321004, 31300809), the National Key Research & Development Plan (2016YFC1202503), the National Key Technology R&D Program (2015BAI03B08), and the Fundamental Research Funds for the Central Universities (DUT16QY06, DUT16TD20). We thank Jinhui Song for revisions, and Riye Xue for scientific discussions.

REFERENCES (1) Malic, L.; Zhang, X.; Brassard, D.; Clime, L.; Daoud, J.; Luebbert, C.; Barrere, V.; Boutin, A.; Bidawid, S.; Farber, J.; Corneau, N.; Veres, T. Polymer-based Microfluidic Chip for Rapid and Efficient Immunomagnetic Capture and Release of Listeria Monocytogenes. Lab Chip 2015, 15 (20), 3994-4007. (2) Jang, H.; Park, Y. J.; Chen, X.; Das, T.; Kim, M. S.; Ahn, J. H. Graphene-Based Flexible and Stretchable Electronics. Adv. Mater. 2016, 28 (22), 4184-4202. (3) Rogers, J. A.; Someya, T.; Huang Y. Materials and Mechanics for Stretchable Electronics. Science 2010, 327 (5973), 1603-1607.


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(14) Kääriäinen, T. O.; Cameron, D. C.; Tanttari, M. Adhesion of Ti and TiC Coatings on PMMA Subject to Plasma Treatment: Effect of Intermediate Layers of Al2O3 and TiO2 Deposited by Atomic Layer Deposition. Plasma Process. Polym. 2009, 6 (10), 631-641. (15) Bébin, P.; Prud'homme, R. E. Comparative XPS Study of Copper, Nickel, and Aluminum Coatings on Polymer Surfaces. Chem. Mater. 2003, 15 (4), 965-973. (16) Schulz, U.; Munzert, P.; Kaiser, N. Surface Modification of PMMA by DC Glow Discharge and Microwave Plasma Treatment for the Improvement of Coating Adhesion. Surf. Coat. Tech. 2001, 142, 507511. (17) Švorčík, V.; Kotál, V.; Slepička, P.; Bláhová, O.; Šutta, P.; Hnatowicz, V. Gold Coating of Polyethylene Modified by Argon Plasma Discharge. Polym. Eng. Sci. 2006, 46 (9), 1326-1332. (18) Kim, S. M.; Kim, S. H.; Park, E. J.; Cho, D. L.; Lee, M. S. Gold Coating of a Plastic Optical Fiber Based on PMMA. In Proceedings of the 13th International Conference on Human Computer Interaction, San Diego, CA, USA, 19-24 July 2009, pp. 760-767. (19) Kupfer, H.; Wolf, G. K. Plasma and Ion Beam Assisted Metallization of Polymers and Their Application. Nucl. Instr. and Meth. in Phys. Res. B 2000, 166, 722-731. (20) Petit, S.; Laurens, P.; Amouroux, J.; Arefi-Khonsari, F. Excimer Laser Treatment of PET before Plasma Metallization. Appl. Surf. Sci. 2000, 168 (1-4), 300-303. (21) Mo, A. K.; DeVore, T. C.; Augustine, B. H.; Zungu, V. P.; Lee, L. L.; Hughes, W. C. Improving the Adhesion of Au Thin Films onto Poly(methyl methacrylate) Substrates using Spun-cast Organic Solvents. J. Vac. Sci. Technol. A 2011, 29 (3), 030601. (22) Mo, A. K.; Brown, V. L.; Rugg, B. K.; DeVore, T. C.; Meyer, H. M.; Hu, X.; Hughes, W. C.; Augustine, B. H. Understanding the Mechanism of Solvent-Mediated Adhesion of Vacuum Deposited Au and Pt Thin Films onto PMMA Substrates. Adv. Funct. Mater. 2013, 23 (11), 1431-1439. (23) Bolon, D. A.; Kunz, C. O. Ultraviolet Depolymerization of Photoresist Polymers. Polym. Eng. Sci. 1972, 12 (2), 109-111. (24) Tsao, C. W.; Hromada, L.; Liu, J.; Kumar, P.; DeVoe, D. L. Low Temperature Bonding of PMMA and COC Microfluidic Substrates using UV/ozone Surface Treatment. Lab Chip 2007, 7 (4), 499-505.


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Figure 1. Schematic diagram of polymer metallization. a) Illustration of the UV-ozone surface modification. b) Sputtering Cu or Au. c) Photolithography. d) Etching Cu or Au and removing the residual photoresist.


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Figure 2. Tape test results displaying the average percent metal remaining for different sample types (28 mm × 28 mm) (One-way ANOVA analysis, NS: not significant, ***p < 0.001.). un.: metal films on unmodified PMMA substrates, mod.: metal films on modified PMMA substrates.


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Figure 3. Contact angle measurement results. a) Effect of the UV-ozone exposure time on the water contact angle on the PMMA surface. b) Variations of the contact angle of PMMA surfaces with aging time after the UV-ozone modification or the oxygen plasma treatment.


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Figure 4. ATR-FTIR results of PMMA surfaces. a) Spectra before and after the UV-ozone modification. b) Difference spectra after the modification. ν denotes the stretching vibration.


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Figure 5. AFM images of PMMA surfaces (the scan area: 2 µm × 2 µm). a) A virgin PMMA plate. b) An UV-ozone modified PMMA plate.


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Figure 6. A 6-inch PMMA chip with 29 Au-Au-Au three-electrode electrochemical microsensors.


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Figure 7. The performance of PMMA electrochemical microsensors with an Au-Au-Au three-electrode system. a) The peak current responses of 50 µM and 100 µM dopamine obtained from six sensors. b) The peak current responses of 50 µM and 100 µM dopamine obtained from one sensor immersed in 0.1 M phosphate buffered saline solution after 0-6 days.


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Figure 1. Schematic diagram of polymer metallization. a) Illustration of the UV-ozone surface modification. b) Sputtering Cu or Au. c) Photolithography. d) Etching Cu or Au and removing the residual photoresist. 63x24mm (300 x 300 DPI)


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Figure 2. Tape test results displaying the average percent metal remaining for different sample types (28 mm × 28 mm) (One-way ANOVA analysis, NS: not significant, ***p < 0.001.). un.: metal films on unmodified PMMA substrates, mod.: metal films on modified PMMA substrates. 128x102mm (300 x 300 DPI)



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Figure 3. Contact angle measurement results. a) Effect of the UV-ozone exposure time on the water contact angle on the PMMA surface. b) Variations of the contact angle of PMMA surfaces with aging time after the UV-ozone modification or the oxygen plasma treatment. 68x29mm (300 x 300 DPI)


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Figure 4. ATR-FTIR results of PMMA surfaces. a) Spectra before and after the UV-ozone modification. b) Difference spectra after the modification. ν denotes the stretching vibration. 62x24mm (300 x 300 DPI)



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Figure 5. AFM images of PMMA surfaces (the scan area: 2 µm × 2 µm). a) A virgin PMMA plate. b) An UVozone modified PMMA plate. 60x22mm (300 x 300 DPI)


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Figure 6. A 6-inch PMMA chip with 29 Au-Au-Au three-electrode electrochemical microsensors. 118x88mm (300 x 300 DPI)



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Figure 7. The performance of PMMA electrochemical microsensors with an Au-Au-Au three-electrode system. a) The peak current responses of 50 µM and 100 µM dopamine obtained from six sensors. b) The peak current responses of 50 µM and 100 µM dopamine obtained from one sensor immersed in 0.1 M phosphate buffered saline solution after 0-6 days. 65x26mm (300 x 300 DPI)


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A 6-inch PMMA chip with arrays of electrochemical microsensors 35x28mm (300 x 300 DPI)


Significant Enhancement of the Adhesion between Metal Films and

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