Photosensitive poly(dimethylsiloxane) materials for microfluidic applications

Microelectronic Engineering 84 (2007) 1104–1108 www.elsevier.com/locate/mee

Photosensitive poly(dimethylsiloxane) materials for microfluidic applications Katerina Tsougeni *, Angeliki Tserepi, Evangelos Gogolides

*

Institute of Microelectronics, NCSR ‘‘Demokritos’’, 153 10, Aghia Paraskevi, Attiki, Greece Available online 25 January 2007

Abstract Poly(dimethylsiloxane) (PDMS) is used as a thermally crosslinked material in microfluidics and Bio-MEMS. Recently photo-patternable materials show increasing interest, as the demand for easy alignment arises for multilayered structures. We present a photopatterning process for PDMS in microfluidics, for two main uses: (a) as a thin (approximately 10 lm) structural layer, and (b) as a very thin (approximately 1 lm) hard mask for oxygen plasma etching of microfluidic polymeric substrates down to several tens of microns. We study the deep-UV and I-line photocrosslinking properties of siloxane copolymers containing vinyl-methyl-siloxane groups as polymerizable units. These materials are sensitive to DUV and can be sensitized to 300–400 nm using free radical initiators. We prove that even thermally curable PDMS (Sylgard 184, base) can become photosensitive in DUV, although its practical use is limited to very thin films, due to its small molecular weight.  2007 Elsevier B.V. All rights reserved. Keywords: Poly(dimethylsiloxane); Deep-UV lithography; Photoinitiators; UV curing; Microfluidics

1. Introduction Silicones are attractive for lithographic applications in multilayer resist schemes. The first use of organosilicon polymers and more specifically of PDMS as thin top imaging resists in bilayer systems was reported by the group of Hatzakis at IBM [1–5]. These bilayer techniques can offer increased resolution and sensitivity, by imaging only a thin radiation sensitive layer, and transferring this pattern to an underlying planarizing polymer layer. While PDMS vinyl copolymers are sensitive to DUV, thin films (<1 lm) of PDMS alone are 193 nm, VUV, and e-beam sensitive and may also be used as resist in bilayer lithography [6,7]. It would thus be useful to develop photopatternable PDMS materials/processes for thicker films sensitive at 254 nm, as well for 300–400 nm radiation, and use these materials in the fabrication of MEMS and microfluidic devices as *

Corresponding authors. Tel.: +30 210 6503237. E-mail addresses: [email protected] (K. Tsougeni), [email protected] (E. Gogolides). 0167-9317/$ - see front matter  2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2007.01.011

an alternative to thermally cured PDMS since several labs own DUV or I-line mask aligners. Ideally, one would like to use the same PDMS material for both thermal crosslinking (thermally crosslinkable PDMS contain vinyl groups) and photocuring. Recently several companies have also commercialized photosensitive silicon containing polymers: WL-5351 and WL-5150 silicones by Dow Corning for electronic packaging applications [8], Cyclotene photosensitive resins from Dow Chemical, (derived from Benzocyclobutene (BCB) monomers) as dielectrics [9], and inorganic–organic hybrid polymers (ORMOCER) [10] for optical applications developed at Fraunhofer Institute and commercialized by Micro Resist. These new materials are however, of different chemical nature and softness compared to the vinyl copolymers or pure PDMS polymers used in lithography thus far. We present a photopatterning process for PDMS in microfluidics, for two main uses: (a) as a thin (10 lm) structural layer, and (b) as a thin (1 lm) hard mask for oxygen plasma etching of polymeric substrates down to several tens of microns. We study the DUV and UV

K. Tsougeni et al. / Microelectronic Engineering 84 (2007) 1104–1108

(300–400 nm) photocrosslinking properties of siloxane copolymers. Several PDMS copolymers with vinylmethyl-siloxane groups as polymerizable units were selected. These copolymers can be sensitized to the 300– 400 nm using free radical initiators.

CH3CH2 CH3CH2 N

1105 O

CH2CH3

C

N CH2CH3

(a) Photoinitiator A O

2. Experimental In this work, two PDMS copolymers, one from ABCR [11] VDT 954, and one from United Chemical Technologies [12] PS 264, were used, as well as the base SYLGARD 184 from Dow Corning; All are shown in Fig. 1. To induce the crosslinking of the material in 300–400 nm region, a suitable free radical photoinitiator has to be introduced into the material. We used three free radical photoinitiators, which are shown in Fig. 2. The optical density of PDMS films with and without photoinitiator was determined with a UV–Vis Lambda 40 Spectrophotometer. The photosensitive PDMS materials were dissolved in methyl isobutyl ketone (MIBK) or in toluene and the films were spin-coated either on Si or on PMMA substrates, (after priming with HMDS solution in toluene), and then baked at 50 C for 1 min. The films were then exposed to DUV light broadband 254 nm (usually without photoinitiator) or to UV light broadband 365 nm (always with photoinitiator). After the UV exposure, the samples were

S

(b) Photoinitiator B O OCH3

OCH3

(c) Photoinitiator C Fig. 2. Photoinitiators used in this work: (a) 4, 4’-bis(diethylamino)benzophenone, 99+% (A(365) = 1 for concentration 0.0008%, in acetonitrile), (b) thioxathen-9-one, 98% (A(365) = 0.15 for concentration 0.0005%, in acetonitrile) and (c) Igracure 651 (2,2-dimethoxy-2-phenyl acetophenone), (A(254) = 0.5 for concentration 0.001% in acetonitrile), used by Shaw et al. [5] for DUV. A is used for thin films 1 lm, B for thicker films 10 lm, C is used for DUV exposures.

thermally treated on a hotplate at 120 C for 5 min, to induce crosslinking of the film. Subsequently, the layers were developed for approximately 3 min using a mixture of methyl isobutyl ketone and 2-propanol (1:1). The thickness of the films was measured by a M2000 Woolam spectroscopic ellipsometer (245–1000 nm, 470 wavelengths). 3. Results and Discussion Fig. 3 shows the absorption spectrum of PDMS films (PS 264) without and with 4% photoinitiator A (see Fig. 2a). As can be seen, the PDMS material without photoinitiator only absorbs in the DUV region, which is attributed to phenyl groups of PS 264. Therefore, it is not necessary to add photoinitiator for DUV, although

1.0

Optical Absorbance

0.8

Fig. 1. PDMS materials used in this work: (a) vinyl-containing poly(dimethylsiloxane) [(VDT 954), vinylmethylsiloxane-dimethylsiloxane-trimethylsiloxy terminated copolymer, 300,000–500,000 cST (vinyl amount 11.0– 13.0%)], (b) PS 264, Poly(dimethylsiloxane) – (5%) – (diphenylsiloxane) (0.1–0.3%) (methylvinylsiloxane) copolymer, Mn = 450,000 and Mw = 990,000, Gum, and (c) vinyl terminated poly(dimethylsiloxane) (SYLGARD 184 Base, MW 60,000).

365 nm

0.6

With Photoinitiator 0.4

0.2 254 nm

No Photoinitiator

0.0 200

300

400

500

600

Wavelength (nm) Fig. 3. Optical absorption of 1.2 lm thick PDMS (PS 264 copolymer) without and with 4% photoinitiator A (Fig. 2a). [a = 0.026 lm 1 at 248 nm and a = 0.38 lm 1 at 365 nm, respectively].

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K. Tsougeni et al. / Microelectronic Engineering 84 (2007) 1104–1108 1.2

a Normalized Film Thickness Remaining

Optical Absorbance

deep-UV

Without bake

1.0 0.8

o

50 C

0.6 0.4

o

70 C

0.2

o

100 C o 120 C

0.0 200

300

400

500

PS 264

1.00

VDT 954 0.95 332 mJ / cm

2

1.2 μ m thick

2

2060 mJ/cm 1110 mJ/cm

0.90 0.85

0.18 μm thick 0.80

Wavelength (nm)

8 μm thick

SYLGARD,base

0.75 100

600

2

1000

Incident Exposure Dose (mJ/cm2)

such an addition improves photo-speed by 5· [5]. Shaw et al. [5] showed that the sensitivity increased from 100 mJ/cm2 to 20 mJ/cm2 for 180 nm film, with the addition of Igracure 651 (see Fig. 2c). For exposures in 300– 400 nm range, the addition of 4 wt% of the photoinitiator A (see Fig. 2a) increases the absorption of a 1200 nm thick film and permits curing. Fig. 4 shows the spectrum of VDT 954 with the addition of 9% of similar photoinitiator A, which again shows that the material absorbs at 365 nm. In addition, Fig. 4 presents the optical absorbance of PDMS (VDT 954) in different post-apply bake temperatures. As can be seen, the photoinitiator escapes after baking the films in temperatures in the range of 40–120 C. We bake our films at 50 C for 1 min and we estimate that starting from 9% photoinitiator A, a film approximately of 5.4% photoinitiator A remains after baking. Contrast curves for DUV exposures are shown in Fig. 5a. As can be seen in Fig. 5a, 332 mJ/cm2 exposure dose would be required to crosslink the material PS 264 of film thickness 1.2 lm in DUV without the addition

b Normalized Film Thickness Remaining

Fig. 4. Optical absorption of 1.2 lm thick PDMS (VDT 954 copolymer) with different Post Apply Bakes, and 9% photoinitiator A (Fig. 2a). Some photoinitiator escapes the film even at bake temperatures as low as 50 C.

i-line 1.00

VDT 954

1 μm thick

2

8400 mJ / cm 0.95 2

970 mJ / cm

0.90 0.85

8 μm thick

0.80 0.75 100

1000

10000

Incident Exposure Dose (mJ/cm2) Fig. 5. (a) Contrast curves of PDMS in DUV with and without photoinitiator. (i) PS 264 for 1.2 lm thickness without photoinitiator, (ii) VDT 954 for 8 lm thickness with photoinitiator C (see Fig. 2) and (iii) Sylgard 184-base for 0.18 lm thickness with photoinitiator C (see Fig. 2) and (b) contrast curves of PDMS at UV (300–400 nm radiation) with appropriate photoinitiator. (i) VDT 954 for 1.2 lm thickness with photoinitiator A (see Fig. 2) and (ii) VDT 954 for 8 lm thickness with photoinitiator B (see Fig. 2). For UV exposures at 300–400 nm, a glass slide was used to filter out radiation below 300 nm, and to make sure that crosslinking was due to 300–400 nm. Sensitivity values shown on the figure are determined at 99% of final attained thickness.

Table 1 Evaluation of the sensitivity of all three PDMS polymers (see Fig. 1) and three photoinitiators (see Fig. 2) Materials

Wavelength

Thickness (lm)

Photoinitiator (% w/w) – conditions

Sensitivity (mJ/cm2) (99% of full thickness)

PS 264 PS 264 PS 264 PS 264 PS 264 VDT 954 VDT 954 VDT 954 VDT 954 VDT 954 VDT 954 VDT 954 VDT 954 Sylgard 184-base Sylgard 184-base

DUV DUV DUV UV (300–400 nm) UV (300–400 nm) DUV DUV DUV DUV DUV UV (300–400 nm) UV (300–400 nm) UV (300–400 nm) DUV DUV

1.2 1.2 8 1.2 8 1.2 1.2 1.2 8 8 1.2 8 60 0.18 1.08

None None/bake during exposure (100 C) None A (4%) B (4%) None A (9%) C (8%) None C (2%) A (9%) B (9%) C (12%) C (10%) C (8%)

332 237 2680 1180 9000 740 371 200 4940 2060 970 740 20,600 1110 19,650

K. Tsougeni et al. / Microelectronic Engineering 84 (2007) 1104–1108

Fig. 6. Optical microscope photographs of PDMS (VDT 954) exposed in a mask aligner (300–400 nm). (a) 1.2 lm thick films on Si obtained with 970 mJ/cm2 dose, (i) 10 lm wide lines, (ii) 50 lm wide lines, (b) 8 lm thick PDMS microfluidic channel supported on PMMA substrate, 175 lm, 100 lm lines obtained with 8400 mJ/cm2, (c) a 1.2 lm thick PDMS microfluidic channel, similar to that of Fig. 7b transferred 50 lm down in PMMA using oxygen plasma etching, and (d) cross section of the PMMA microfluidic channel after etching.

of a photoinitiator. For further increase of the polymerization efficiency one may: (a) either add photoinitiator C [5] or (b) bake during exposure. The behaviour of VDT 954 of film thickness 8 lm is also shown in Fig. 5a. VDT 954 is sensitive in DUV, although it does not contain phenyl groups and has a very small absorbance a < 0.02 lm 1 in DUV. The sensitivity can be increased from 4940 mJ/cm2 without photoinitiator to 2060 mJ/cm2 with the addition of 2% photoinitiator C which absorbs in DUV (note: no PAB was used prior to exposure). In addition, Fig. 5a shows the DUV sensitivity of Sylgard 184 base photosensitized

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with 10% photoinitiator C. Thus, we prove that the thermally curable PDMS can be photosensitized at DUV with appropriate photoinitiator. Sensitivity is 1110 mJ/cm2 for 0.18 lm thin films. For larger thicknesses, we need longer exposure times to crosslink the material (see Table 1), due to the very low molecular weight of Sylgard 184. For more detailed results see Table 1. Fig. 5b shows the UV (300–400 nm) sensitivity of VDT 954 sensitized with photoinitiator A for 1.2 lm thin films and with photoinitiator B for 8 lm thick films. VDT 954 has 10 times more vinyl groups compared to PS 264 and is indeed more sensitive in 300–400 nm; however the improvement compared to PS 264 is small due to the much smaller molecular weight of VDT 954 polymer. For UV exposures and better sensitivities, one would need a material such as VDT 954 (11.0–13.0% vinyl groups) but with a high molecular weight (Mw 990,000) comparable to that of PS 264. We performed detailed evaluation of the sensitivity of all three polymers (see Fig. 1) and three photoinitiators (see Fig. 2) and the results are shown in Table 1. We concluded that in principle, even a thermally curable vinyl terminated siloxane can be photosensitized at DUV, while for UV (300–400 nm) sensitization larger molecular weights and higher vinyl contents are required for practical exposure doses. We have managed to crosslink even up to 60 lm polymer albeit with long exposure times (see Table 1). Having presented the contrast curves, we move in actual patterning applications. Fig. 6a presents photos of (i) 10 lm wide lines, (ii) 50 lm wide lines on Si of PDMS (VDT 954) exposed in a mask aligner (300–400 nm) for 970 mJ/cm2 dose. These lines are 1 lm thick. Similar lines can be obtained with 10 lm thick films. In addition, we can do lithography on plastic substrates. Fig. 6b shows 175 lm and 100 lm wide lines of 8 lm thick PDMS (VDT 954) microfluidic channel supported on PMMA substrate for 8400 mJ/cm2. Such or much thinner channels (e.g., 1 lm thick) can be used as a hard mask to etch microfluidic channels in PMMA [13]. Fig. 6c presents a photo of the microfluidic channel on PMMA in detail, after etching (top down) in O2 plasma and Fig. 6d shows the cross section of the channel. 4. Conclusions In conclusion, UV and DUV sensitive poly(dimethylsiloxane) films have been prepared for photochemical patterning using commercial vinyl copolymers of PDMS, and are useful in the fabrication of MEMS and microfluidic devices. In addition, we have shown that vinyl terminated thermally curable films (such as Sylgard 184 base) can be sensitized at DUV although their sensitivity is low, due to their small molecular weight. Acceptable sensitivities for PDMS copolymer were shown for both DUV and UV exposures and film thicknesses up to 10 lm. For thicker films and for further increase of sensitivity, it is nec-

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essary to have a concentration of vinyl groups in PDMS in the range 5–13%, (as in VDT 954) and high MW (>300,000), (as in PS 264). Photopatterning of PDMS was demonstrated, and plasma etching of polymer substrates using PDMS as a hard mask was performed. Acknowledgement The authors thank N. Vourdas for the photographs of the microfluidic channel after the oxygen plasma etching. References [1] J. Shaw, M. Hatzakis, J. Paraszczak, E. Babich, Microelectron. Eng. 3 (1985) 293–304. [2] J. Shaw, M. Hatzakis, J. Paraszczak, J. Liutkus, E. Babich, Polym. Eng. Sci. 23 (1983) 18. [3] J. Paraszczak, E. Babich, R. McGouey, J. Heidenreich, M. Hatzakis, J. Shaw, Microelectron. Eng. 6 (1987) 453–460.

[4] E. Babich, J. Shaw, M. Hatzakis, J. Paraszczak, R.W. Lenz, P.R. Dvornich, Microelectron. Eng. 6 (1987) 511–518. [5] J. Shaw, E. Babich, M. Hatzakis, J. Paraszczak, Solid State Technol. (1987). [6] A. Tserepi, G. Cordoyiannis, G.P. Patsis, V. Constantoudis, E. Gogolides, E.S. Valamontes, D. Eon, M-C Peignon, G. Cartry, C. Cardinaud, G. Turban, J. Vac. Sci. Technol. B 21 (1) (2003) 174. [7] A. Tserepi, E.S. Valamontes, E. Tegou, I. Raptis, E. Gogolides, Microelectron. Eng. 57–58 (2001) 547–554. [8] Product information for WL-5351 and WL-5150, Dow Corning . [9] Product information for Cyclotene, Dow Chemical [10] Woo-Soo Kim, R. Houbertz, Tae-Ho Lee, Byeong-Soo Bae, J. Polym. Sci. 42 (2004) 1979–1986. [11] Product information for VDT 954, ABCR, Germany [12] Product information for PS 264, United Chemical Technologies [13] N. Vourdas, A. Tsougeni, A. Tserepi, A.G. Boudouvis, E. Gogolides, S. Tragoulias, T.K. Christopoulos, in: J. Mostaghimi, et al. (Eds.), Seventeenth International Symposium on Plasma Chemistry (ISPC) (full paper in CD), Toronto, Canada, August 7–11, 2005.