3D micro and nanostructuring of an epoxy based resist by electron beam lithography

G. Rius, J. Bausells, C. Martín, A. Llobera and F. Pérez-Murano Institut de Microelectrònica de Barcelona, IMB-CNM-CSIC, Barcelona, SPAIN Abstract We present the results of producing three dimensional micro- and nanostructures on an epoxy based resist. Epoxy based resists are very interesting in microsystems technology for their good mechanical properties, that allow to produce high aspect ratio microstructures. We have optimized the definition of free standing structures by either using electron beam lithography alone or combining electron beam lithography and UV optical lithography. To tune the energy and dose of the electron beam exposure properly, Monte Carlo simulations are used.

Keywords: negative resist, nanofabrication, electron beam lithography

1. Introduction

Fabrication of polymeric micro- and nanostructures is of great interest for Microsystems Technology. Polymers are materials which are simple to process, compatible with many technologies, easy to modify in order to include specific functions (like, for example, sensing of specific bioentities). Polymers are also widely used in the area of microfludics, and in consequence they find large applications for developing lab-on-a-chip systems.

On the other hand, polymers are the base material for many resists used in lithography. Photoresists are widely used in microelectronics and microsystems to transfer patterns by UV lithography. Smaller patterns can be obtained using electron beam lithography (EBL), where an electron beam sensitive resist is locally exposed to high energy beams that either induce the cross-linking of the polymer chains (negative resists) or its breaking in smaller molecules (positive resists).

Although lithography is usually employed to define two dimensional patterns on surfaces, here we present results on the direct structuring of polymers using electron beam lithography. We show how free standing structures can be realized by tuning the dose and energy of the electrons. Then, specific zones of the polymers are cross-linked as a consequence of the spatial distribution of absorbed energy in the resist. The result is a three-dimensional polymeric structure after development.

2. Experimental Experiments are performed using an epoxy based resist [1]. Similar to SU8, it is a negative-tone chemically amplified photoresist negative resist based on EPON SU8 resin. It is photosensitized by the addition of a triaryl sulfonium salt, e.g. Cyracure UVI from Union Carbide, which is also called Photo Acid Generator (PAG). SU8 is widely used for the fabrication of microstructures [2] The resist is deposited at 2500 rpm spin speed for 45 s, which provides a resist layer of ~5 µm. The pre- and post- exposure processing conditions are identical and fixed. They consist of baking from 65 to 95 ºC for 20 minutes, on air. Synthesized to be exposed under

UV light, the performance for electron irradiation is established experimentally, but the assessment of electron scattering simulations helps to determine the trajectories of incoming irradiation. Indeed, scattering processes studied by Monte Carlo simulations for the SU8 resist suggest that beam energy strongly determines the electron penetration depth in the polymer. 3. Results

Based on simulations in 5 µm thick layer of SU8 to determine the electron penetration depth as a function of beam energy, it is found that beam energies >18 keV can be used to fully expose the thick layer (Figure 1).

Fig. 1 Tilted SEM image of a line produced by exposing a 5 µm width line at 20 keV (left). The resulting line profile in depth is in agreement with the beam spread within the resist shown in electron trajectories simulations (right). Low forward scattering is achieved just for 400 nm in depth.

© 2008 Cardiff University, Cardiff, UK. Published by Whittles Publishing Ltd. All rights reserved.S. Dimov and W. Menz (Eds.)Multi-Material Micro Manufacture

The simulations also indicate how the required

dose as a function of energy has to be tuned. Beam energies of 18 and 20 keV are tested and both achieve to fully expose the whole resist thickness if an appropriate dose is used. But 20 keV is more suitable since it guarantees a stronger adhesion.

The feature shape distortion is remarkable. Squared patterns become nearly round due to the deviations that electrons suffer along the thick resist. Figure 1, left, shows the result of exposing a line of a 5 µm in width. In the top, electrons are little deviated from the incoming axis only for a thickness of about 400 nm in agreement with the simulations (Figure 1, right).

From these results, it is clear that design and exposure conditions are crucial. They determine and limit the pattern that can be obtained. However, it is possible to take advantage of the strong energy dependence of electron penetration depth in order to fabricate free standing structures. The evaluation of shape dependence on energy is determined by exposing narrow lines (that is, lines defined as Single Pixel Lines, SPL, in the design, i.e. zero linewidth) at different beam energies (3, 5, 10, 15 and 20 keV) with wide transversal lines exposed at 20 keV to hold them (defined as 5µm linewidth in the design). As an example, anchored lines performed at 5, 10 and 15 keV are shown in Figure 2 (top). Not only penetration depth is proportional to the beam energy, but also a strong influence on linewidth is found. At low energies, 3-5 keV, the results of line exposure are more similar to strips than to lines, whereas beyond 15 keV, lines are narrower, but also deeper.

Fig. 2 (Top) Profile SEM images of lines defined at different beam energies. At higher beam energies the penetration range is higher and linewidth increases with dose (from left to right). (Bottom) Lines produced at 20keV show good correlation with electron trajectories simulations, where line depth is proportional to exposure dose.

The dependence of line depth with dose is shown more in detail in Figure 2 (bottom). It evidences wider and deeper free standing structures as a function of increasing exposure dose, which correlates with the predictions of Monte Carlo simulations of punctual electron irradiation in conditions equivalent to the experimental ones. Regarding the result of the exposures performed at 20 keV, the depth-dose dependence is clear (Figure 2, bottom and Figure 3, top). Increasing the internal dose factor by 1 in each line from left to right side leads to correspondingly deeper lines, in particular, lines from 0.6 µm to more than 2.5 µm in depth are found. Line exposure results are summarized in the plot of Figure 3. For 15 and 20 keV, the two curves represent the measured linewidth on top and bottom, while for 3 and 5 keV uniform linewidth is found for all the depth of the strip.

Fig. 3 (Top) Top view of lines produced at 20keV with increasing dose from left to right. Linewidth is proportional to exposure dose. (Bottom) Dependence of linewidth with dose factor shows that at higher energies forward scattering gets relevant (red and blue lines).

Fig.4 Experimental results for lines defined at 20 and 3 keV. For 20 keV, line is 200 nm in width and 750 nm in depth, whereas at 3 keV linewidth is 0.5 µm and depth is 140 nm. As can be seen, top values show approximately the expected trend of higher resolution at higher energies, but the line widening with increasing dose has a significantly different increasing rate. Also, electrons at 3 keV beam energy may be so easily stopped that the whole exposure process differs from the rest of tendencies.

Concerning to linewidth, resolution is also determined by beam energy and dose. At 20 keV, linewidth of less than 200 nm are achieved with ~ 650 nm remaining thickness (55 nm on the top of the line), whereas at 3 keV, minimum linewidth is around 365 nm, but remaining thickness does not reach 200 nm (Figure 4). Somehow, resolution is higher at low energies.

The combination of different beam energies (grey scale lithography) suggests that a great diversity of structures can be fabricated, from single or double clamped beams (Figure 5, top, and Figure 3), to channels or many other fancy configurations (Figure 5, right) that could be used for many applications, for example, microfluidic devices [3]. Microfluidic devices are obtained when combining high beam energy to pattern the channel walls and very low beam energy to define the top cover of the channel.

Taking advantage of forward scattering events and the characteristic electron stopping power in the resist, specific shapes are obtained. As an example, in Figure 5, right, bowl-shaped structures are obtained when irradiating a 0.1µm circunference at 20keV. The structure is produced by taking advantage of the profile shown in figure 2 bottom, where the top of the resist consist on a sharp definition and the bottom is widen, overlapping with the opposite spart of the circumference. Such kind of objets could be used as containers for particles and represent an additional way to prepare molds, for example, to be used in soft lithography.

Fig. 5 Free standing cantilevers or 3D structures can be defined combining the proper beam energies and design. A final approach consists on the combination of EBL with UV lithography. One of the interesting feature of the resist is that it is both electron and photon sensitive. These properties allow to produce very complex structures without a penalty on the fabrication time [4]. EBL is optimal to produce high resolution features and three dimensional structures, but it presents a very low throughput. On the other hand, UV lithography presents very high throughput. Figure 6 shows examples of free-standing structures fabricated by the combination of UV and electron beam lithography. After depositing a 5 µm thick resist layer on a silicon surface, the sample is exposed to UV radiation through an optical mask to define the holding structures. The optical dose is selected so that the full thickness of the resist is exposed. After this step, the sample is transferred to the chamber of the scanning electron microscope where it is exposed to the electron beam. The exposure consists of an array of lines that will define nanometer scale free-standing beams between the holding structures. In this case, the electron dose is chosen so that only the very top layer of the resist becomes cross-linked, ensuring a good spatial resolution. The electron beam exposure is performed blindly (i.e., without alignment with respect to the optical pre-defined features), because at this stage of the processing, these latter features do not present any contrast. After this step, the sample is developed to remove all the non exposed areas and post-baked. As it can be observed in figure 6, the resulting beams are clearly defined, their width is of few hundreds of nanometers and they are pretty straight, indicating good mechanical properties like a low internal stress. This example represents a new approach to the fabrication of nanomechanical polymeric devices.

Fig.6 Combination of EBL with UV shows that high resolution structures can be obtained. As an example, EBL is used to define high resolution features anchored in UVL defined features. Lines are defined at 5 keV (left) and 20 keV (right). 4. Conclusions We have shown that by tuning the energy and dose of the electrons, three dimensional structures can be directly fabricated on an electron sensitive epoxy based resist. In addition, since the resist is also sensitive to UV radiation, combination of optical and electron beam lithography allows to fabricate complex structures with high flexibility in the design. Application of the resulting structures in the areas of microfluidics, soft lithography and biochemical sensing are foreseen. Acknowledgements

This work was partially supported by European project Novopoly, NMP3-CT-2005-013619. We acknowledge MicroResist GmbH for supplying of the polymeric material. References [1] mr-L 5005 XP, under development by MicroResist

GmbH. http://www.microresist.de [2] K.Y. Lee, N. LaBianca, S.A. Rishton, S.

Zolgharnain, J.D. Gelorme, J. Shaw and T.H.P. Chang. Micromachining applications of a high resolution ultrathick photoresist, Journal of Vacuum Science & Technology B, 13 (6), 3012-3016 (1995).

[3] P. Mali, A. Sarkar and R. Lal, Facile fabrication of

microfluidic systems using electron beam lithography, Lab on a Chip 6 (2)310-315 (2006).

[4] V. Kudriashov, X.C. Yuan, W.C.Cheong and K.

Radhakrishnan, Greyscale structures formation in SU8 with e-beam and UV, Microelectronic Engineering 67-8, 306-311 (2003).