Fabrication of large SU-8 mold with high aspect ratio microchannels by UV exposure dose reduction

Sensors and Actuators B 101 (2004) 175–182

Fabrication of large SU-8 mold with high aspect ratio microchannels by UV exposure dose reduction Mary B. Chan-Park b,∗ , Jun Zhang a , Yehai Yan b , C.Y. Yue b a

b

The Biological and Chemical Processing Laboratory, School of Mechanical and Production Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore Singapore-MIT Alliance, Innovations in Manufacturing Systems and Technology, School of Mechanical and Production Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore Received 30 October 2003; received in revised form 4 February 2004; accepted 26 February 2004 Available online 17 April 2004

Abstract Patterned SU-8 can be used as a master mold for soft lithography. Fabrication of SU-8 molds with high aspect ratio microchannels for this purpose is non-trivial due to the contrary imperatives of mold hardness and faithful replication of the photomask pattern. Increased UV exposure time, which improves resist hardness, is found to result in overexposure of the shadowed resist and unresolved pattern structure in the wafer center. We have found that reduction of UV exposure dose from the resist manufacturer’s recommendation of 500–600 mJ/cm2 to 350 mJ/cm2 permits the successful fabrication of dense SU-8 gratings with relatively wide (80 ␮m) SU-8 bars separated by narrow (10 ␮m) microchannels with aspect ratio of 10 over the entire 100 mm-diameter wafer. The underexposed SU-8 was rather soft but could be sufficiently hardened to attain a useable hardness (Vickers hardness (VH) number of 25) by hard baking at a relatively low temperature (95 ◦ C). A hard baking time of 20 min resulted in saturation of the hardness; further increase of hard baking time resulted in no significant increase in hardness. The hard-baked SU-8 gratings were successfully used for replication of soft silicone rubber. Without hard-baking, the silicone rubber broke cohesively within the SU-8 gratings during demolding. A method of fabricating 100 mm-diameter SU-8 mold consisting of 80 ␮m wide SU-8 bars separated by 10 ␮m narrow channels with aspect ratio of 10 for soft lithography has been demonstrated. © 2004 Elsevier B.V. All rights reserved. Keywords: High aspect ratio; UV exposure dose; SU-8; Diffraction; PDMS

1. Introduction Ultra-thick resists are used for fabrication of various high aspect ratio micro-electromechanical systems (MEMS) such as electrostatic sensors and actuators and microfluidic channels. Epon SU-8, an epoxy-based negative ultra-thick photoresist [1], has been widely used for high aspect ratio applications because of its very low absorption in the near UV range. SU-8 can also be used for the fabrication of molds for replica molding [2–7]. For replica molding of high aspect ratio microstructures, the master mold must have correspondingly high aspect ratio micron-sized cavities. In this report, we consider a simple class of microstructured SU-8 mold which has narrow, deep and long microchannels separated by wide SU-8 bars. This class of microstructured mold can cost-effectively produce moldings with narrow walls sepa-

∗ Corresponding author. Tel.: +65-6790-6064; fax: +65-6792-4062. E-mail address: [email protected] (M.B. Chan-Park).

0925-4005/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2004.02.049

rated by wide cavities. Deep and wide microchannels will result in high surface area and increased fluid throughput. Moldings of this kind will find many applications in microfluidics and chemical sensors. However, creation of high aspect ratio microchannels in SU-8 is a challenge because of the stray light beneath the dark-field area of the mask resulting from Fresnel diffraction at the edge of the dark-field [8,9] and reflection from the highly reflective silicon wafer substrate. Edge bead, resulting from spin coating, leads to an air gap between the mask and SU-8 coating on the wafer and intensifies the stray light beneath the dark-fields. SU-8 is highly sensitive to this stray light; it has eight epoxy groups per molecule resulting in relatively low gelation energy compared to the recommended UV processing energy. To date, most of the high aspect ratio SU-8 structures processed by others are sparse [10,11] i.e. not densely packed on the substrate. There is no reported work on the processing of dense SU-8 gratings with high aspect ratio microchannels (i.e. dense SU-8 gratings separated by narrow and deep channels). We are interested in the fabrication of

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dense SU-8 gratings with high aspect channels as a mold for soft lithography [12]. The supplier’s recommended UV dose is typically suitable for fabricating sparse high aspect ratio SU-8 microstructures. In our application, the channel between neighboring SU-8 bar structures is only 10 ␮m and diffraction and edge bead result in unresolved channels at the wafer center if the supplier’s recommended UV dose is applied; underexposure is needed. Decreased exposure dose reduces the energy deposited at all points in the resist, and in particular the energy deposited under the dark-field. However, underexposed SU-8 is generally too soft for handling or for use as a mold for replica molding. Further, silicone prepolymer will diffuse into insufficiently crosslinked SU-8 resulting in interlocking of the SU-8 and the cured silicone rubber so that the latter cannot be removed from the SU-8 without cohesive failure within the softer rubber. To increase the SU-8 crosslinking, SU-8 is hard-baked. The hard-bake temperature suggested by the resist supplier to reach full crosslinking is between 150 and 200 ◦ C. In our experiments, this range was found to result in cracking of the SU-8 during the hard-bake even with gradual heating up and cooling down. A lower hard-bake temperature (95 ◦ C) was investigated to avoid cracking and this was found to adequately increase crosslinking density and build sufficient strength for the underexposed structures. This study focuses on the photolithographic fabrication of high aspect ratio (10) microchannels in SU-8 over the entire surface of 100 mm wafers. The large-area pattern was fabricated using a Mask A with alternating 10 ␮m-wide dark-fields and 80 ␮m-wide light-fields (Fig. 1a). High aspect ratio PDMS structures replicated from the gratings were also demonstrated. The effects of exposure dose and

(a) Mask A

dark field light field

(b) Mask B Fig. 1. Masks used (a) whole-wafer patterning (b) for UV exposure time study.

hard-bake duration on the hardness of SU-8 were also studied in detail using a small-area Mask B (Fig. 1b).

2. Experimental 2.1. Materials and masks The photoresist and developer used were NANOTM SU-8-100 and SU-8 developer provided by Microchem Corp. (Newton, Massachusetts). The SU-8 photoresist is an octa-functional epoxy resin and the SU-8 developer is propylene glycol methyl ether acetate (PGMEA). 100 mm diameter 1 0 0 P type silicon wafers (475 ␮m±25 ␮m thick) were used as substrates. For the whole-wafer-covered SU-8 gratings, Mask A (Fig. 1a) containing repetitive 1000 ␮m-long 10 ␮m-wide dark-field lines separated by 80 ␮m-wide light field lines covering the entire 120 mm × 120 mm mask was used. For studying the effect of exposure time, Mask B (Fig. 1b) containing a single column of 20,000 ␮m-long 23 ␮m-wide dark-field lines separated by 40 ␮m-wide light field lines was used. 2.2. SU-8 Processing Silicon wafers were cleaned by immersing them in Piranha solution (15:1 (v/v) 96 wt.% H2 SO4 :H2 O2 ) at 120 ◦ C for 20 min. They were then rinsed six times in de-ionized water with each rinse lasting 3 min. The acid-cleaned wafers were then blown dry with filtered compressed nitrogen gas and then oven dried at 200 ◦ C for 30 min in a convection oven. SU-8 100 was then spin-coated onto the cleaned wafers at 3000 revolutions per minute (rpm) for 30 s (Table 1). The coated wafers were placed on a leveled surface for 15 min to allow the SU-8 to level out. Pre-exposure soft baking was done at 65 ◦ C for 10 min and 95 ◦ C for 30 min on a well-leveled hotplate. The wafers were slowly cooled to room temperature after soft baking. The SU-8 was exposed to 365 nm light (at the rate of 10 mW/cm2 ) using the J500-IR/VIS Mask Aligner from Optical Associates Inc. (Milpitas, California). The exposure times are summarized in Table 1. For the whole-wafer-covered SU-8 patterns made using Mask A, both underexposure (35 s) and normal exposure (55 s) were employed to study the effect of reducing the exposure dose. For the small-area pattern made using Mask B, the exposure time was varied from 50 to 70 s. A soft cushion was placed under the wafer during exposure to slightly bend the wafer to reduce the air gap between the resist and mask arising from edge bead. Before exposure, a release coating, specifically Loctite Frekote® 700-NC supplied by Henkel Loctite Corp. (Rocky Hill, Connecticut), was wiped on the wafer-side of the mask using tissue paper. The post exposure baking (PEB) was done using a hot plate. The PEB baking times and temperatures are also summarized in Table 1. After the samples were slowly cooled to room temperature, they were relaxed for 30 min to release

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Table 1 Detailed SU-8 processing conditions Mask

A A B

rpma

3000 3000 3000 a

Thickness (␮m)

92 97 92

Soft bake time (min) 65 ◦ C

95 ◦ C

10 10 10

30 30 30

UV time (s)

35 55 50–70

PEB time (min)

HB time (min)

65 ◦ C

95 ◦ C

95 ◦ C

3 3 3

10 10 10

20 None None

rpm: revolutions per minute.

the residual stress. The samples were developed in SU-8 developer at room temperature. Following development, the patterned wafers were rinsed briefly with isopropyl alcohol (IPA) and then dried with a gentle stream of compressed nitrogen. For the underexposed SU-8 pattern made with Mask A, the SU-8 was also hard-baked (HB) at 95 ◦ C for 20 min (Table 1). The developed SU-8 microstructures were examined using optical profilometry (OP) and scanning electron microscopy (SEM). The SU-8 was platinum coated prior to SEM examination. SEM was done using either a JEOL JSM-5600LV or a HITACHI S3500N SEM. OP was done with a WYKO optical profilometer, using the vertical scan imaging (VSI) mode. 2.3. Measurement procedure of gelation dose of 100 µm thick SU-8 During UV exposure, small octa-functional SU-8 molecules react to become a large cross-linked network molecule. At the gelation point, the network is one large molecule and becomes insoluble in the developer. We determined the minimum UV energy required for gelation of 100 ␮m-thick SU-8 photoresist, i.e. the “threshold gelation energy,” by varying the exposure time in small increments and evaluating the solubility of the exposed resist in the developer. 100 ␮m thick SU-8 photoresist was spin-coated onto a silicon wafer and then soft baked at 65 ◦ C for 10 min and 95 ◦ C for 30 min. The SU-8 was then UV exposed through a mask with a 5 mm × 5 mm square light field opening; the mask was moved from place to place on the coated wafer to produce an array of exposures. The exposure time was varied from 0.5 to 40 s in increments of 0.5 s, i.e. the exposure dose was varied from 5 to 400 mJ/cm2 in increments of 5 mJ/cm2 . The PEB was done at 65 ◦ C for 3 min and 95 ◦ C for 10 min. The exposed samples were developed with the SU-8 developer for 20 min. The minimum UV dose required for non-solvation of the SU-8 in the developer, which was evident from visual inspection of the developed exposure array, was adopted as the “threshold gelation energy”. 2.4. Hardness test of cross-linked SU-8 A 100 ␮m thick SU-8 layer was spun onto a cleaned silicon wafer and soft baked at 65 ◦ C for 10 min and 95◦ for

30 min. Using different wafers, the UV exposure time was varied from 5 to 100 s (i.e. the exposure varied from 50 to 1000 mJ/cm2 ). Then the resist was post exposure baked at 65 ◦ C for 3 min and 95◦ for 10 min and developed. The development time was 10 min. After development, the structure was hard-baked at 95 ◦ C. The hard baking time was varied from 0 to 45 min. Vickers hardness (VH) was determined using the DMHp-2 Micro hardness tester (Matsuzawa Seiki, Tokyo, Japan). The test force was 25 g for the samples which were exposed for less than 30 s, and 50 g for samples exposed for more than 30 s. The test force was maintained for 15 s. The Vickers hardness number was computed as the test force divided by the surface area of the sloped indentation in the resist produced by the test apparatus indenter. 2.5. PDMS molding The full-wafer grating pattern made using Mask A was used as the master for PDMS molding. The PDMS microstructures were replicated using Silastic J RTV (Dow Corning). The prepolymer and curing agent were thoroughly mixed with a 10:1 weight ratio following the supplier’s instruction [13]. The prepolymer mixture was degassed at 4 Pa for 30 min to remove air bubbles and then carefully poured onto the SU-8 master. The assembly of prepolymer mixture and the master was degassed for another 30 min at 4 Pa to ensure that the prepolymer mixture fully filled the microchannels. A piece of glass was placed on top of the silicone rubber. Then, the PDMS prepolymer was cured at 65 ◦ C for 16 h. After curing, the PDMS replica was peeled from the SU-8 master.

3. Results and discussion The effect of UV exposure dose on microchannel width was studied using gratings created with Mask B. UV exposure time was varied from 50 to 70 s. Microchannel width was found to be significantly affected by the exposure dose. The corresponding mask dark-field width for the channel was 23 ␮m. Fig. 2 shows that higher exposure dose caused narrower micro channels. The best-fit line to the experimental microchannel top width (Wt ) shows a decrease from 20.4 to 17.8 ␮m as the exposure time (texp ) increases from 50 to 70 s. Similarly, the experimental bottom width (Wb ) shows

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M.B. Chan-Park et al. / Sensors and Actuators B 101 (2004) 175–182 experimental top width experimental bottom width designed width Linear (experimental bottom width) Linear (experimental top width) 24

Channel width (micron)

22 y = -0.1295x + 26.883 R2 = 0.9326

20 18 16 14

y = -0.1782x + 25.683 2

12

R = 0.9542

10 48

52

56

60 Exposure time (s)

64

68

72

Fig. 2. Effect of UV exposure time on channel width (using Mask B).

a corresponding decrease from 16.8 to 13.2 ␮m when exposure increases from 50 to 70 s. With an exposure time of 70 s, the actual bottom width of the channels was decreased by 9.8 ␮m resulting in an actual width of 13.2 ␮m compared to the designed width of 23 ␮m. With a smaller exposure dose of 50 s, the decrease of the bottom width was smaller, 6.2 ␮m (17.8 ␮m compared to 23 ␮m). The observed trend in Fig. 2 can be explained by considering the stray (diffracted and/or reflected) light in the “shadowed” region of the SU-8 and the low threshold gelation energy of SU-8 (30 mJ/cm2 for 100 ␮m-thick SU-8). UV light was diffracted from the edge of the dark-field line of the mask into the “shadowed” area of the photoresist. Increasing the incident exposure dose by increasing the exposure time increased the UV light exposure energy in the “shadowed” areas of the photoresist. At low levels of UV light, some photoacid is generated in the SU-8 but the epoxy reaction is limited and insufficient to cause gelation. As the UV dose increases beyond a threshold value, gelation, and crosslinking occur. With longer exposure, the regions beneath the shadowed regions receive more than the threshold gelation dose, the boundary between over- and under-gelation exposure moves further into the shadowed region and the developed microchannels become narrower. The diffraction-induced narrowing of the channel bottom is a function of the exposure time. For a fixed UV dose and resist thickness, there is a minimum dark-field width for successfully resolving the channel between the SU-8 bars. For dark-field narrower than this minimum value, V-shaped rather than rectangularor trapezoid-shaped channels are formed instead. The problem is exacerbated for whole-wafer-covered SU-8 processing since spin coating always produces an edge bead. The

edge bead leads to an air gap between the photomask and the surface of the resist, which leads to diffracted light penetrating further into the shadowed area of the SU-8. We attempted to fabricate whole-wafer-covered gratings with 80 ␮m wide SU-8 bars separated by 10 ␮m wide channels using the supplier recommended dose of 500 to 650 mJ/cm2 (50 to 65 s exposure time). The narrow micro-channel of 10 ␮m width coupled with the edge bead led to stray exposure above the threshold gelation energy in the shadowed regions. With normal exposure, most part of the wafer could be well patterned (Fig. 3a) except at the wafer center. There was a small round area with diameter of about 1.8 cm right at the wafer center where the microchannel could not be developed (Fig. 3b). The soft cushion placed beneath the wafer during UV exposure bent the wafer to reduce the air gap caused by the edge bead. However, the air gap right at the wafer center was deeper and a small air gap remained even with the soft cushion. To fabricate well-resolved SU-8 gratings using Mask A over the entire wafer, we found it necessary to reduce the UV exposure dose significantly. When UV exposure times of 25 to 40 s were used, well-resolved channels over the entire wafer surface could be successfully developed. Fig. 3c shows the 80 ␮m wide SU-8 bars fabricated using an exposure dose of 350 mJ/cm2 , much lower than the 500 to 650 mJ/cm2 dose recommended by the supplier. The gelation dose of 100 ␮m-thick SU-8 was measured to about 30 mJ/cm2 . Typically, the resin needed to be exposed beyond the gelation point so that it could withstand the development and rinse without deformation. In spite of this structural requirement for exposure beyond the minimum gelation exposure, there was still sufficient latitude

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Fig. 3. Gratings with 80 ␮m wide SU-8 bars (Mask A). (a) SU-8 pattern using supplier recommended dose—at the rim of the wafer, (b) SU-8 pattern using supplier recommended dose—at the center of the wafer, and (c) using under exposure.

(500–600 mJ/cm2 ) generally resulted in VH number of about 25 (Fig. 4) and even without hard baking this hardness was generally found to be sufficient for SU-8 microstructures to be handled and used for soft lithography. Using our reduced UV exposure dose of 350 mJ/cm2 , the hardness was too low

32

28

24 Vickers hardness

to permit underexposure with respect to the manufacturer’s recommendation for the purpose of achieving good channel resolution by limiting the diffracted and reflected light under the mask dark-fields. The consequent lower cross-link density, and hence mechanical strength and hardness, were compensated by longer or higher-temperature post exposure baking. The underexposed microstructures withstood the surface tension forces of developing solvent during drying. Twenty minute hard baking at 95 ◦ C was used to harden the underexposed structures after development. Gradual cooling after PEB and hard baking were found to be essential to avoid structure deformation and distortion. Fig. 4 shows that the hardness of a 100 ␮m thick unstructured cross-linked SU-8 was greatly affected by the UV exposure dose. We first consider the effect of exposure dose on the SU-8 without post-exposure hard baking. The Vickers hardness number increased rapidly from 10.0 ± 1.1 to 20.5 ± 0.3 when the exposure dose increased from 50 to 200 mJ/cm2 . Beyond 200 mJ/cm2 , the hardness began to saturate. When the exposure dose increased from 600 to 1000 mJ/cm2 , the VH of the SU-8 without hard baking increased only slightly from 26.7 ± 0.2 to 27.4 ± 0.3. The SU-8 processed with the supplier’s recommended UV dose

20

16 Hard bake 20min @95C 12

No hard bake

8 0

200

400 600 800 Exposure dose (mJ/cm2)

1000

1200

Fig. 4. Effect of exposure dose on VH of SU-8 (H = 100 ␮m).

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Vickers hardness

27 26 25 24 23

Exposure dose 300mJ/cm2 Exposure dose 500mJ/cm2 Exposure dose 1000mJ/cm2

22 21 20 0

10

20

30

40

50

Hard bake time (min)

Fig. 5. Effect of hard-bake time on VH of SU-8 (H = 100 ␮m).

(the VH number was about 22) for SU-8 to be used as a mold. When liquid silicone prepolymer was cast on the underexposed SU-8 microstructures which were not hard-baked, the cured silicone rubber broke within the SU-8 during demolding.

Fig. 5 shows the effect of hard baking on the hardness of SU-8. Hard baking was found to enhance the crosslinking and hence the hardness, but only to a limited extent in comparison with the effect of exposure on hardness. For an UV exposure dose of 500 mJ/cm2 , the Vickers hardness number increased from 25.0 ± 0.1 to 28.2 ± 0.2 when the hard-bake time increased from 0 to 20 min. The Vickers hardness saturated in the range 28.2±0.2 to 28.4±0.2 when the hard-bake time increased from 20 to 45 min. Saturation of the hardness at bake times beyond 20 min was observed for all tested exposures. Fig. 4 shows that the increase of hardness produced by hard-baking is more pronounced for lower exposures than for higher exposures. It also shows that for the 100 ␮m-thick SU-8 mold substrates considered here, the minimum exposure is 24 s (exposure dose 240 mJ/cm2 ). The VH number after this exposure but prior to hard-bake is 20.7±0.2; this is increased to the desired VH of 25.1 ± 0.4 after hard-baking (Table 1). These tests indicate that well-resolved microstructured SU-8 gratings can be created using very low exposure doses compensated by hard baking at 95 ◦ C. The whole-wafer-covered SU-8 grating created using these reduced-exposure with compensating hard-bake techniques was tested as a master for PDMS replication. The

Fig. 6. (a) SU-8 grating: (i) SEM image; and (ii) OP profile of height across line X–X ; (b) PDMS replica: (i) SEM image and (ii) OP profile of height across line Y–Y ).

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master had microchannels measuring 1 mm long, 9.2 ␮m wide, and 92.2 ␮m deep (i.e. aspect ratio of 10) separated by 80.0 ␮m-wide SU-8 walls (Fig. 6a). Fig. 6b shows the SEM image and OP profile of the corresponding PDMS replica. The dimensions of the PDMS walls were 1 mm long, 9.2 ␮m wide, and 92.0 ␮m deep. The microchannels in the PDMS replica were 80.2 ␮m wide. The data indicate that the microstructures of the SU-8 grating were faithfully replicated; the reduced-exposure plus compensating hard-bake technique produces dense and high-aspect ratio SU-8 gratings suitable for reproduction via soft lithography. Hard baking of the SU-8 was found to be essential for increasing the hardness and rigidity of SU-8 as well as preventing the silicone rubber from adhering to the SU-8. We also tested for suitability as a mold another SU-8 grating processed under the same conditions except without the last step of hard-baking; the silicone rubber broke and remained in the mold when we tried to demold it.

4. Conclusions Increase of UV exposure time in SU-8 UV lithography has been found to result in the narrowing of dark-field-pattern structures due to the stray light and the low gelation energy of this photoresist. Reduction of UV exposure dose to 350 mJ/cm2 (30–40% below the manufacturers suggested process parameters) enables the successful fabrication of whole-wafer-covered dense SU-8 grating with relatively wide (80 ␮m) SU-8 bars separated by narrow (10 ␮m) microchannels with aspect ratio of 10. The underexposed SU-8 gratings are rather soft but can be sufficiently hardened to attain a useable Vickers hardness of 25 by hard baking at a relatively low temperature (95 ◦ C). The resist hardness is found to saturate at a hard-bake time of 20 min. We have successfully used SU-8 gratings created using these techniques for faithful replication in soft silicone rubber. We find that hard-baking is essential to the fabrication of usable molds using the under-exposure technique; without it silicone rubber replicas break cohesively within the SU-8 gratings during demolding. These procedures and processes demonstrate a method of fabricating 100 mm diameter mold consisting of 80 ␮m wide SU-8 bars separated by 10 ␮m narrow channels with aspect ratio of 10 for soft lithography.

Acknowledgements This research was supported by a Start-up grant (SUG 10/02) from the Nanyang Technological University and an A-STAR (Singapore) grant (Project No. 022 107 0004). Y.H. Yan acknowledges the financial support of the Singapore-MIT alliance Program (IMST) through a post-doctoral fellowship. J. Zhang acknowledges the financial support of Nanyang Technological University through a Research Scholarship.

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Biographies Mary B. Chan-Park received her BEng (Chem) and PhD (Polymers) from the National University of Singapore and Massachusetts Institute of Technology (MA, USA), respectively. She has previously worked as a technical manager in Triton Systems, MA, USA from 1998 to 1999 and senior technical manager at SiPix Imaging, CA, USA from 1999 to 2001. She is currently an associate professor at the Nanyang Technology University (Singapore) and director of the Biological and Chemical Processing Laboratory. She is also an associate of the Innovations in Manufacturing Science and Technology (IMST) program of the Singapore-MIT Alliance. Her research interests are polymeric micro- and nano-replication, tissue engineering, controlled drug release and polymer synthesis. Jun Zhang received his BEng in 1994 from the Beijing University of Aeronautics and Astronautics, Beijing, China. Then he earned his MEng in 2001 from the Northwestern Polytechnical University, Xi’an, China. He worked as a researcher and Engineer-in-charge at the Guizhou Aeroengine Research Institute (China) from 1994 to 2001 and then joined Nanyang Technological University (Singapore) as a PhD student in 2002. Yehai Yan received his BEng and MEng in 1996 and 1999, respectively, from the Qingdao Institute of Chemical Technology. Then, he joined the Institute of Chemistry, the Chinese Academy of Sciences, and earned

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his PhD in 2002. He is currently working as a research fellow in the IMST program of the Singapore-MIT Alliance in Nanyang Technological University (Singapore). C.Y. Yue received his BEng and PhD from Monash University (Australia). He worked as Lecturer at University of Hong Kong (Hong Kong) from

1983 to 1990. He joined Nanyang Technological University (Singapore) in 1990 where he is currently professor and dean of School of Mechanical and Production Engineering. He is also the Program Chair of IMST in the Singapore-MIT Alliance. His research interests are in polymer science and engineering, composite science and technology, adhesion, biomaterials, and polymer blends.