A numerical and experimental study on gap compensation and wavelength selection in UV-lithography of ultra-high aspect ratio SU-8 microstructures

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Abstract

This paper presents a study on UV-lithography of thick SU-8 resist using air gap compensation and optimal wavelength selection for ultra-high aspect ratio microstructures. Both numerical simulations and experiments were conducted to study effects of different lithography conditions: broadband light source with and without air gap compensation, filtered light source with glycerol liquid, and filtered light source with Cargille refractive index matching liquid. A thick PMMA sheet was used as an optical filter to eliminate most of the i-line components of a broadband light source. Using the filtered light source and gap compensation with the Cargille refractive index liquid perfectly matching that of SU-8, patterns with feature sizes of 6 μm thick, 1150 μm tall (aspect ratio of more than 190:1) and high quality sidewalls were obtained. Microstructures with height up to 2 mm and good sidewall quality were also obtained and presented. The study also proved that Cargille refractive index matching liquid is compatible with UV-lithography of SU-8 and may be used as an effective air gap compensation solution.

Introduction

UV-lithography of ultra-thick photoresist with high aspect ratio, high sidewall quality, and good dimensional control is very important for MEMS and MOEMS. Although X-ray lithography of PMMA can meet these requirements, the expensive beam lines are not readily available for many researchers. The high cost of X-ray lithography also makes it impractical for many applications. UV-lithography of SU-8 as a cheaper alternative has found many applications in MEMS and MOEMS.

SU-8 is a negative tone, epoxy-type photoresist based on EPON SU-8 epoxy resin from Shell Chemical. Due to its low optical absorption in the near-UV range, SU-8 can be lithographed in thickness of hundreds of micrometers with very high aspect ratios. For SU-8's near-UV contact printing, normally broadband near UV light between 320 nm–420 nm is used for the exposure. With well-controlled lithography conditions and pressure contact exposure or vacuum contact exposure, cross-linked polymer microstructures with high aspect ration could be obtained at the heights of more than 1000 μm [1], [2], [3], [4], [5], [6], [7]. Chang and Kim obtained 25 μm tall structures with 1 μm feature size [1]. Ling et al. obtained 360 μm tall structures with 14 μm feature sizes [2]. With the help of well-collimated proximity ultraviolet source, Dentinger et al. obtained aspect ratios exceeding 20:1 for film thickness of 200–700 μm [6]. Williams and Wang obtained 65:1 high aspect ratio structure up to 1 mm high with Quntel aligner [7].

In an ideal contact exposure, the mask and the resist surface should be brought to perfect contact without any gap. However, errors such as the surface flatness, surface roughness, etc., caused by the standard spinner and hotplate, are always inevitable. Air gaps of 10–100 μm are very common for thick resist. The thicker the resist layer is, the larger the errors may be because the high viscosity of the resist makes it very difficult to obtain uniform spin-coat. For ultra thick photoresist layer, the diffraction effects of the mask pattern may result in severe sidewall distortions. To obtain better lithography results, many researchers have used glycerol to fill air gap between mask and the resist for diffraction reduction [8]. As a diffraction reduction material, the refractive index of glycerol (synonyms: glycerin, 1,2,3-propanetriol) is around 1.470–1.475, which is lower than the refractive index of SU-8 (n = 1.668 at λ = 365 nm, n = 1.650 at λ = 405 nm, data from the SU-8 vendor: Microchem Inc.). There is obviously a significant difference between the refractive indexes. The results obtained using glycerin also showed some significant diffraction effects.

To minimize the diffraction effects and achieve the optimal lithography results, it is therefore highly desired to search for a new material that can be used for air gap compensation with perfect refractive index match. This would require an exactly same refractive index at the lithography wavelength as SU-8 resist, and most importantly, easy to remove after lithography. Because absorption spectrum of SU-8 shows much higher absorption coefficients at shorter wavelengths, lithography using broadband light source tends to result in over-exposure at the surface of the resist layer and under exposure at the bottom. Some researchers therefore used selective filtration of light source to eliminate undesirable wavelengths and to obtain better lithography results. For example, Reznikova et al. used a 100 μm thick SU-8 resist layer to filter exposure radiation around 334 nm to obtain better lithography result [9]. Lee et al. reported an interesting work using Hoya UV-34 filter to eliminate the T-top (over-exposed top part) of the resist caused by the shorter wavelength component of the light source [10].

The research reported in this paper concentrated on two important parameters of the ultra thick SU-8 lithography: Fresnel diffraction and wavelength-dependent absorption. Normally, Fresnel diffraction and photoresist's absorption cause the aerial image to degrade and the light intensity distribution to vary as it propagates in the resist. In this paper, numerical simulations for Fresnel diffraction effects with the air gap, gap compensations with glycerol, and with refractive index matching liquid are presented. With calibrated optical refractive index matching liquid to reduce the diffraction and with wavelength selection to reduce the absorption of resist at surface layer, ultra-high aspect ratio microstructures were obtained in the optimized processing conditions.

Section snippets

Refractive index matching liquid for diffraction reduction

SU-8 in general has excellent surface planarizing property. However, as the thickness of SU-8 resist increases, the non-uniformity of the resist can become a serious issue. Fabrication of ultra-high aspect ratio microstructures requires to spin-coat resist layers ranging from several hundreds to thousand micrometers. In such cases, high viscosity SU-8 resist, such as SU-850 or SU-8100, is always used. The typical flatness errors could range from 10 to 100 μm. Other factors such as unintentional

The absorption spectrum of unexposed SU-8 and using PMMA plate as optical filter

For the ultra thick photoresist, the absorption of the resist to the light source greatly affects the lithography quality. As the light beam penetrates the SU-8 resist layer from the top to the bottom, the light intensity drops gradually as the light is absorbed. As the result, the top part of SU-8 resist absorbs higher dosage than that at the bottom part, and frequently leads to over-dosage at the top and under-dosage at the bottom. This is one of the major reasons that inexperienced operator

Compensation and wavelength selection

Since SU-8 is a negative tone resist, the pattern profile is defined by the light intensity higher than the threshold energy to cure SU-8 within the targeted region. With the attenuation of intensity in SU-8 along the vertical direction (Z direction) and diffraction caused by the micropatterns, the aerial dimension of the projection image is varied as the light passes through the resist. The edges of the aerial image are defined by the edges of Fresnel diffraction pattern with energy higher

Experimental results and discussions

Three different groups of experiments were conducted: (1) broadband light source with/without gap compensation, (2) broadband light source using glycerin for gap compensation; and (3) filtered light source with PMMA sheet (i-line eliminated) and gap compensation using the Cargille refractive index matching liquid as suggested in this paper.

The processing conditions for a 1150 μm thick SU-8100 film are as follow: (1) clean Si wafer with Acetone, IPA, DI water; (2) spin-coat SU-8100 at 400 rpm; (3)

Conclusions

Both theoretical and experimental studies were conducted on UV-lithography of ultra thick SU-8 resist. Specifically, the research efforts have focused on two important processing conditions: wavelength selection and air gap compensation. It has been proved that the wavelength selection plays significant rule in obtaining high aspect ratio microstructures using UV-lithography of SU-8 while air gap compensation played a marginal one, with more effect at the top parts of the microstructures. It

Acknowledgement

This work is supported in whole or in part by the National Science Foundation under Grant number EPS-0346411, Grant Number ECS# 0104327, and the State of Louisiana Board of Regents Support Fund.

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