Development of Novel SU-8 Based Nanoimprint Lithography

Development ofnovel SU-8 based nanoimprint lithography

I Shen-Qi XieI, Bing-Rui LUI, ling WanI, Rong YangI, Yifang Chenz, Xin-Ping QUI, and Ran Liu * IState key lab ofAsic and system, Department ofMicroelectronics, Fudan University, Shanghai 200433, China ZRutherford Appleton Laboratory, Chilton, Didcot, Oxon, OXII OQX, UK *Emai1: [email protected]

Abstract well with the simulation results [3, 4]. The tri-Iayer NIL process was first developed to duplicate imprint We present two kinds of novel nanoimprint templates which are very expensive and time-consuming lithography techniques based on SU-8 photoresist with through normal EBL and RIE processes. Additionally, single layer and tri-Iayer approaches. The imprint the tri-Iayer technique also shows flexibility to transfer templates with high aspect ratio were first fabricated by nanopatterns onto different substrates with the ability to electron beam lithography (EBL) and reactive ion etch control critical dimension (CD). Due to the high optical (RIE), and then duplicated by the SU-8/SiOz/PMMA transmittance of SU-8 in the visible range, we further tri-Iayer technique we developed. Imprint properties applied the developed techniques to fabricate gratings of such as pressure and temperature are determined to high groove density (5000 lines/mm) in relatively large fabricate different nanostructures with various imprint area (IOmm* IOmm) and planar chiral photonic depth. The developed processes, which provide us the meta-materials. And based on the crosslink reaction of capability to fabricate both imprinted nanostructures on SU-8 after UV exposure, it also provides us an excellent resist and transferred nanopatterns on substrate with high method to fabricate nanofluidics channels by bonding volume and low cost, are further applied in producing two layers ofhalfcured SU-8 together. high density gratings, planar chiral photonic meta materials, and nanofluidics channels. 2. Experimental details

1. Introduction 2.1 Fabrication oftemplates Fabrication of templates is one of the most critical After the invention of nanoimprint lithography steps in the NIL technique. The process started with (NIL) by Stephen Y. Chou in 1995 [1], this technique electron beam lithography (EBL) with a high resolution aroused great interests both in academia and industry due vector beam writer (VB6-HR) from Vistech pIc to define to its high resolution and low-cost yet high-volume high density grating patterns in a PMMA (MW nano-pattering capability. The International Technology IOOK)/PMMA (MW 350K) bilayer resist on silicon Roadmap for Semiconductors has recently included NIL wafers. After metallization and lift-of process, the as a potential candidate lithography technique for future grating pattern was transferred onto a 50 nm thick Cr IC chip manufacturing. However, in the traditional film, which was subsequently used as etching mask. hot-embossing NIL there exist some limitations caused Reactive ion etch (RIE) was then carried out in fluorine by the different thermal expansion between the resist and based plasma by a Samco RIE-IONR dry etcher. the substrate, leading to possible pattern distortion [2]. Recently, the emergence of UV-curable NIL has shown great technical advantages and promises, and accelarated the further development and industrialization of this technology. In this paper we present a novel single SU-8 layer UV-NIL technique and a SU-8/Si02/PMMA tri-Iayer resist NIL technique. SU-8 was originally introduced as an epoxy resin-based negative amplified photoresist, its low glass transition temperature (55°C) and low volume shrinkage coefficient make it an ideal candidate for NIL in high resolution fabrication of nanostructures. The single layer NIL technique was systematically studied Figure 1. SEM image of fabricated 200 nm period Si under various pressures and temperatures and agreed grating templates with 310 nm height

Authorized licensed use limited to: University of Massachusetts Amherst. Downloaded on July 14,2010 at 14:37:23 UTC from IEEE Xplore. Restrictions apply. Si template ~SU-8 Pyrex

Pyrex t t t t t t uv Figure 2. SEM image of fabricated 4 J-lm period chiral structures with 830 nm height

Throughout the dry etch study, the mixture of SF6 and

CHF3 gas was used, where SF6 did the etching and CHF3 was used to prevent the sidewall from being overetched, to ensure a desired anisotropic etch. The gas flow was 5

sccm for SF6 and 100 sccm for CHF3• The pressure Figure 3. Schematic process flow of the SU-8 based during the process was fixed at 45 mTorr and the etch single layer NIL power was 50 W [5]. Fig. I and Fig. 2 show the SEM ~ images of fabricated 200 nm period Si grating template 1.11 :llIri!1;1I:;~;:III;I/ 101 ~~~] and 4 J-lm period chiral structures with aspect ratio over 3:1.

2.2 SU-8 based single layer UV-NIL The imprint process was done following the process flow as shown in Fig. 3. A 2.5 J-lm thick SU-8 was first spin-coated onto the Pyrex substrates and then soft baked at 95°C on a hotplate for 10 min to evaporate all the solvents in the SU-8 resist. The SU-8 was then heated at a certain temperature above the glass transmission temperature (Tg) in oven for 10 min. After that, a pressure was applied on the template for another 10 minutes with the heat on. Before raising the template, a UV exposure by a 365 nm UV light was undertaken for Figure 4. Schematic process flow ofthe tri-layer NIL 5 min to cure the SU-8 and tum it into a rubbery state under 200°C [6]. Templates were finally separated from the substrates to form the fully cross-linked SU-8 nanostructures.

2.3 SU-8/Si02/PMMA tri-Iayer NIL Fig. 4 schematically illustrates of the tri-layer NIL process. A 200nm thick PMMA layer was spin-coated onto the silicon substrate and then baked at I80a C for 1h in oven, followed by the deposition of 20 nm SiOz layer onto PMMA by ion beam sputtering. A 100 nm thick SU-8 resist layer was then spin -coated and soft baked on a hotplate at 100aC to remove the solvent. Imprint was carried out at 100aC which is much higher than the glass transition temperature of SU-8 and lower than that of Figure 5. SEM image ofvertical undercut after RIE PMMA with the imprint pressure of20 MPa.

Authorized licensed use limited to: University of Massachusetts Amherst. Downloaded on July 14,2010 at 14:37:23 UTC from IEEE Xplore. Restrictions apply. The RIE is a key step of the duplication process diffraction intensity profiles were found and results show

where the residual SU-8 is removed by O2 plasma. The periodic diffraction orders with varying intensity etch time was carefully controlled to remove the entire distributions [4]. bottom residual SU-8 and to minimize the dimensional

loss. The middle layer of Si02 was then etched in CHF3 3.2 Planar chiral photonic metamaterials plasm~ with the top SU-8 layer as an etching mask. Planar chiral meta-materials are a group of

Finally the bottom PMMA layer was etched by O2 artificially patterned structures with special optical

plasma masked by the middle Si02 layer and formed a properties. All the chiral elements are periodically good undercut as shown in Fig. 5. A 30 nm thick Cr was arranged. We fabricated the repeated chiral structure then deposited onto the structure, followed by lift-off in arrays with the period of41lm for infrared spectral range acetone. as shown in Fig.7, and also the structures with 150nm line width in a 600nm period for applications in the 3. Applications visible light range. Embedded arrays of nanoscale perforated chiral structures in thin films show 3.1 High density & large area gratings polarization effects in diffraction that depends on the With their unique properties, diffraction and structural elements of the array. Figure 8 shows the sub-wavelength gratings have taken an indispensable diffraction pattern defined by a beam of white light place in optical instrumentation and applications in through the 41lm period chiral arrays. Detailed optics, opto-electronics, communications, nanophotonic explanation can be found in [7]. and nanobio-science. Based on our single layer NIL Due to its unique optical activities, planar chiral technique, we further extend this NIL technique to the photonic metamaterials have found their great potentials fabrication oftransmission SU-8 gratings applicable in a of applications in optics, communications, bioscience broader wavelength range from 1 Ilm (1000 lines/mm) and nanolithography. down to 200 nm (5000 lines/mm). Fig. 6(a) shows the imprinted results of 300 nm period grating with large area (1 Omm* IOmm) and Fig. 6(b) shows the diffraction pattern under 266 nm laser. Due to the flexibility of the NIL, we can fabricate various trench depths under different imprint conditions. An unique property of the effects of trench depth on

Figure 7. SEM images of the imprinted chiral structures in SU-8. The line width of the imprinted planar chiral structure is 215 nm.

Figure 6. (a) Imprint results of 300 nm period SU-8 Figure 8. The diffraction patterns defined by periodic gratings in large area; (b) Diffraction pattern under 266 chiral holes in SU-8 produced by a white light beam. nm laser

Authorized licensed use limited to: University of Massachusetts Amherst. Downloaded on July 14,2010 at 14:37:23 UTC from IEEE Xplore. Restrictions apply. 3.3 Fabrication ofnanotluidics Channels area of optics and bioscience, but also in other nanofabrication with high resolution at low cost. Polymer-based nanofluidic devices are becoming increasingly attractive for biological applications such as Acknowledgments single molecule detection, separation of DNA and The work was supported by National high protein molecules as well as control of biomolecules. technology program (2006AA03Z352), National Basic There is a growing interest in coupling nanofluidic Research Program of China (2006CB302703), Seed structures (e.g.nanochannels), microfluidic structures Funds for Key Science and Technology Innovation (e.g. microchambers) with conventional systems to form Projects, MOE(708020) and the "985" Micro/nano what is nowadays called a lab-on-a-chip (LOC). electronics science and Technology Innovation Platform. Based on the excellent chemical resistance ofSU-8, we developed a convenient, fast and low cost process to References produce nanofluidics channels by NIL and soft lithography. Nanochannels were produced on SU-8 by [1] S. Y. Chou, P. R. Krauss, P. J. Renstrom, Appl. UV-NIL process. SU-8 was spin-coated on POMS which Phys. Lett. 1995, 67, 3114. was first pre-treated with O2 plasma to make the surface [2] Yifang Chen, Jiarui Tao, Xingzhong Zhao, and hydrophilic, followed by pre-baking and short-term UV Zheng Cui, Journal of Microlithography, exposure to make it half cured. And then it was bonded Microfabrication, and Microsystems, vol 5(1). p.l towards another spin-coated SU-8 layer which was (2006). thinner than 100 om with our in-house designed imprint [3] Bing-Rui Lu, Shen-Qi Xie, Van Sun, Yifang Chen, tool and heated at a certain temperature above Tg in oven for 5 min and another 5 min with the heat on after Xin-Ping Qu, Ran Liu, in: International Conference applying the pressure. Then a UV exposure by the 365 on Nanoscience & Technology, Beijing, China, nm UV light was undertaken for 3min and followed by 2007. 3min post-exposure bake (PEB) to fully cure the SU-8. [4] Shen-Qi Xie, Jing Wan, Bing-Rui Lu, Van Sun, Since POMS can be easily peeled off from the cured Yifang Chen, Xin-Ping Qu, Ran Liu, SU-8, nanofluidic channels were finally created as Microelectronic Engineering 85 (2008) 914-917. shown in Fig. 9. [5] Shen-Qi Xie, Bing-Rui Lu, Van Sun, Yifang Chen, Xin-Ping Qu, Ran Liu, Abstract No. 10-068, in: International Conference on Nanoscience & Technology, Beijing, China, 2007. [6] R Feng, RJ Farris. J. Micromech. Microeng.13, 80(2003). [7] Bing-Rui Lu, Jing Wan, Shen-Qi Xie, Van Sun, Yifang Chen, Xin-Ping Qu, Ran Liu, Microelectronic Engineering 85 (2008) 866-869.

Figure 9. SU-8 nanofluidic channels with 1Jlm period

4. Conclusion

Novel SU-8 based NIL techniques with single layer and tri-Iayer processes were successfully developed. The imprint templates were fabricated under optimized conditions and successfully duplicated by the novel tri-Iayer process. We applied this technique to the fabrication of large area high density gratings and planar chiral photonic structures, and observed excellent optical properties. SU-8 nanofluidic channels were also made for further application in LOC. The developed techniques have show great potentials not only in the

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