Microelectronic Engineering 61–62 (2002) 423–428www.elsevier.com/ locate /mee

Chemical nano-patterning using hot embossing lithography*H. Schift , L.J. Heyderman, C. Padeste, J. Gobrecht

Laboratory for Micro- and Nanotechnology, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland

Abstract

We demonstrate the patterning of silanes on silicon substrates using hot embossing and lift-off. Periodicstructures with feature sizes , 500 nm have been replicated. We are expecting this method to be easilyapplicable for a range of different chemicals and for structure sizes down to 100 nm. 2002 Elsevier ScienceB.V. All rights reserved.

Keywords: Hot embossing; Nanoimprint lithography; Chemical contrast; Replication

1. Introduction

Nanoimprint technologies offer a variety of surface patterning possibilities and have beendeveloped during the last few years to an impressive maturity. Hot embossing lithography (HEL) hasproven its potential for structuring resists with high aspect ratios by thermoplastic molding and is animportant fabrication process for topographic nanostructures down to a lateral resolution of some tensof nm [1,2]. The process is parallel and can be carried out with high throughput at a low cost.

During HEL embossing step a thickness profile is created in a resist similar to standard lithographicmethods. The topographical contrast can be used for lift-off processes or electroplating, and also foretch processes where the resist mask is competitively etched with the underlying substrate. Diverseoptical, electronic and mechanical devices can be fabricated this way. New application fields arise iflocal modification of surface chemistry can be achieved.

In our contribution we show how chemical contrast can be obtained using hot embossinglithography and lift-off (Fig. 1). First a thin thermoplastic film is coated onto a substrate andembossed. Then resist windows are opened by an oxygen plasma flash to remove the residual layer atthe bottom of the embossed resist. A chemical coating is then applied from the gas phase or by

*Corresponding author. Tel.: 1 41-56-310-2839; fax: 1 41-56-310-2646.E-mail address: helmut.schift@psi.ch (H. Schift).

0167-9317/02/$ – see front matter 2002 Elsevier Science B.V. All rights reserved.PI I : S0167-9317( 02 )00513-0

424 H. Schift et al. / Microelectronic Engineering 61 –62 (2002) 423 –428

Fig. 1. Process scheme for achieving local chemical contrast using hot embossing lithography and lift-off.

immersion in a solution. The resist is dissolved away in acetone. As a result a chemical surfacemodification restricted to the areas of the resist openings is obtained.

2. Chemical surface patterning

Chemical surface coatings allow control over wettability, adhesion, chemical reactivity, electricalconduction, and mass transport to the underlying substrate. Silanes are often used for this task(silanisation), because they are covalently binding onto the surface. Self-assembled monolayers(SAMs) can be used as a resist-forming ink. Also functional biomolecules can be coated on artificialsurfaces to be used for biosensors, for cell studies and tissue engineering applications. Forbiomolecular immobilization the head-group of a silane needs to be equipped with a chemicalfunctionality which enables to link biomolecules such as antibodies or enzymes. The surface coatingscan be applied by casting, immersion, vapor deposition, spraying, etc. For many practical applications,patterned chemical surface coatings are needed.

In recent years several techniques have been developed to generate local chemical patterns on solidsubstrates [3]. Using micro-contact printing (‘soft lithography’), molecules are transferred like an inkby a soft stamp onto a substrate by printing [4]. The pattern definition is given by the surface relief ofthe patterned stamp and the amount of ink applied to the stamp. The printing force and the printingduration can be varied in order to get a closed monolayer of molecules which can serve as a resist.The printing process can be carried out without the need of expensive equipment. However, problemsmay arise when a precise alignment is required. Also, care has to be taken for the mechanical stabilityof the stamp and the diffusion behavior of the ink molecules during the printing process [5]. Althoughsoft stamps provide conformal contact over rough surfaces, non-flat structures with steep sidewalls are

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difficult to coat. For example, structures with both a topographical and chemical contrast may bedifficult to obtain. Standard photolithographic techniques and HEL using thick resists provide moreflexibility.

3. Experimental

Silicon masters were fabricated using optical lithography and a Leica Lion LV1 e-beam writer withsubsequent RIE of silicon dioxide. Gratings with a length of several 100 mm, covering an area ofsome square mm and with periods of down to 1 mm were written. Periodic line and dot patterns werealso produced as test structures with feature sizes down to 50 nm.

The masters were replicated in a thin film of PMMA (molecular weight 75 kg/mol, 80–120 nm inthickness), spin-coated on an oxidized silicon substrate. The thin polymer film was embossed for 10min at a temperature of up to 270 8C, and a pressure of 50 bar. The experimental set-up and the rangeof operating parameters used for hot embossing have been described in more detail elsewhere [6].Following removal of the residual layer by O -RIE, the resulting PMMA mask height was in the2

range of 40–80 nm.Two silane coating procedures were applied. In the first procedure a substrate was coated with a

silane from the gas phase. Samples were first dried in vacuum at , 10 Torr for some minutes andthen protected by a monolayer of tridecafluoro-1,1,2,2,-tetrahydrooctyl-trichlorosilane (ABCR) bytheir exposure to the vapor of this silane at pressures , 40 Torr. This procedure had been successfullyused to produce an antisticking layer on silicone stamps [5], or for silicon hot embossing stamps. Thecoating layer has found to be very stable during hot embossing and supports cleaning usingisopropanol without deterioration of its anti-sticking properties. This procedure was now used for thelocal coating of the oxidized silicon substrate. The layer covers both the resist and the silicon oxide atthe positions of the PMMA windows. At the open silicon windows, reactive binding sites are providedby the plasma treated oxide. After lift-off in acetone in ultrasound for some minutes, a monolayer ofsilane remains only in the areas not covered by the resist.

The wetting behavior was found to be different for the regions with different surface properties.Structures with mm-dimensions can be easily visualized using an optical microscope (Figs. 2 and 3).The structures were exposed to humidity and droplets formed by condensation on the cold substrate.In the areas where the resist has been removed, large dense arrays of drops are forming. In the areas,which are covered by silane, the forming of water drops is clearly reduced. Drops can only form inareas which are large with respect to the drop size. This technique is only qualitative and useful forstructures in the micrometer range but serves as a quick test of the wetting behavior.

In a second procedure the bifunctional mercaptopropyltrimethoxy silane (MPTMS) bearing a thiolfunctionality was dissolved in ethanol. The substrate with the PMMA structures was prepared asdescribed before and immersed in the silane solution for 10 min. In this procedure the silane binds tothe silicon oxide creating a thiol terminated surface. Afterwards the PMMA was removed in acetone.The difference in surface chemistry was visualized by silver staining. For this the substrate wasimmersed in a solution of a silver ammonia complex and glucose. Because this solution isthermodynamically unstable, elemental silver was depositing, preferentially on the thiol terminatedsurface. The resulting patterns could be resolved in an optical microscope (Fig. 4).

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Fig. 2. (Above) Micrograph of an embossed resist structure (dark) on an oxidized silicon substrate (bright). (Below) Afterchemical coating and lift-off, the local difference in wetting behavior can be visualized by exposing the structure tohumidity.

4. Discussion

Chemical nanopatterning has been demonstrated by a combination of HEL, chemical surfacemodification and lift-off. More advanced test methods can be applied for the quantitative characteriza-tion of the silane patterns. For optical detection of bound molecules in a microscope fluorescent labelsare most often used. Lateral force microscopy (LFM) can be applied for the detection of localdifferences in surfaces energy.

In the future, further structuring routes can be envisioned. The wafer can be covered by a silanemonolayer before spin-coating the resist. After embossing, the silane is removed by the plasma usedfor the window opening. The now cleaned substrate can then be covered by an different silane layer,before the resist is taken away by lift-off. In similar experiments, using optical lithography, patterns oftwo proteins on one surface were created for guiding of nerve cell growth [3].

For this proposed process the following issues need to be considered. First, a silane applied beforeembossing must support the hot embossing step. Temperatures used in both prebake and hot

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Fig. 3. Optical micrograph of an extended view of the structure in Fig. 2. Large drops are mainly forming in the areas whichwere previously covered with resist during silane deposition.

Fig. 4. Optical micrograph of structure with periods down to 1 mm (horizontal lines). Silver deposited in areas not coveredwith resist during silane deposition (bright areas).

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embossing can lead to decomposition of the chemicals used. Second, the silane should not reduce theadhesion of the resist to the substrate. Finally, high resist structures may be necessary to ensure thatthe polymer mask is not etched away before completely removing the underlying silane. However, formany applications the lift-off process, where the silane is applied after embossing and windowopening, will be the procedure favored for chemical patterning.

5. Conclusion

Hot embossing lithography provides a high resolution unmatched by other imprint methods. Silanesare available with various functional head groups. These processes can be developed for the tailoredtopographical and/or chemical patterning of surfaces on the nanoscale. This is particularly needed forlarge area sensors because the same resist pattern can be used both as an etching mask and for lift-off,making it possible to fabricate metal electrodes which are separated by chemically modified areas.

Acknowledgements

¨We would like to thank D. Bachle, E. Deckardt, F. Glaus, B. Haas, T. Neiger, B. Steiger and K.Vogelsang for their help and valuable contributions.

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