Fabrication of high-aspect-ratio microstructures using SU8 photoresist

Abstract

SU8 photoresists are used widely in the development and fabrication of the high-aspect-ratio microstructure. However, SU8 process is difficult to repeat when the film thickness and aspect ratio are increasing. In this paper, the soft bake is investigated. The retained solvent level in SU8 film is studied by using the theory of free volume and diffusion process. The result shows that the proper and uniform solvent level in the photoresist film is crucial to achieve good SU8 microstructures in the ultra-thick film process. The microstructure with 5 lm linewidth and aspect ratios larger than 50 was obtained. IntroductionHigh aspect ratio microstructures are needed in many advance applications. A variety of thick photoresists were applied and optimized for microfabricating of high aspect ratio MEMS (Micro Electro-Mechanical System). The SU8 has been proven the suitable material for microfabrication combining the processing of ultra-thick photoresist layers and an outstanding UV-based lithographic performance. However, the quality of microstructures is sensitive to the variation of process parameters in the thick film process [1-3]. Typical aspect ratio would be 20, and 25 for good process. Although the microstructure with aspect ratio larger than 50 has been obtained, the repeatability may be a problem.Processing of SU8 is a challenging task, especially in the ultra-thick photoresist film. Every process step influences strongly the results obtained and has to be adapted individually to the desired application. Therefore, the parameter values described in the literature vary in a wide range according to various structural features and it is difficult to decide which parameters are most crucial to achieve good SU8 microstructures. Although the previous research [4] showed that the soft bake plays the key role in the SU8 process, the details about this process step haven't given. In this paper, the soft bake is investigated. The retained solvent level in SU8 film is studied by using the theory of free volume and diffusion process. The result shows that the proper and uniform solvent level is crucial to achieve good SU-8 microstructures in the thick film process. ExperimentalThe SU8-2075 photoresist was chosen in this paper because it allows a wide coverage of thickness. The SU8 was statically dispensed onto a 2 inch cleaned silicon wafer and spread at 500 rpm for 5 s. The different rotation speed was applied and held for 30 s to attain the target film thickness (100, 200, 300, 400 micron). The weight of silicon wafer and SU8 film were measured by using DTG160 balance (Shanghai balance factory). The wafer was heated on the hotplate at 95°C for predetermined time. The weight was measured after the wafer has been cooled to room temperature. Then, the SU8 film was exposed in contact mode using a narrowband mercury source, followed by a PEB (post exposure bake) at 95°C. The mask with different linewidth structures was applied to fabricate the microstructures with different aspect ratio. Finally, the microstructures were deve...

A SU-8 dish for cell irradiation

“…This negative photo resist has been widely used to fabricate high aspect ratio microstructures in addition to biological applications [4]. These applications include optical waveguides for on-line cell counting and sorting [5], bio-fluidic dermal patches to measure the concentration of bio-molecules [6], micro-channels for thermal flow-sensors [7] and the fabrication of SU-8 pallets to sort monolayer cultures for posterior cell assays [8].…”

Optimized surface silylation of chemically amplified epoxidized photoresists for micromachining applications

“…The maximum film thickness achievable with SU8 is about 2 mm, whereas the maximum aspect ratio is in excess of 50 for 365-nm lithography. 5 In this study, we covered the following film thicknesses: 0.5, 3, 15, and 100 lm; that is, the thickness was varied by more than two orders of magnitude.…”

“…Epoxy functionalized polymers are used extensively in microlithography as components of negative tone materials in a number of applications, including traditional electron-beam (e-beam) resists, 1 high-resolution chemically amplified resists for e-beam 2 or optical lithography, 3,4 and micromachining (e.g., SU8 resists 5 ). Silylation is defined as the replacement of the active hydrogen of a hydroxyl group (AOH), an amino group (ANH 2 ), or a sulfide group (ASH) by a silicon-bearing group.…”

Ultrashort Pulsed Lasers – Efficient Tools for Materials Micro-Processing

“…However, a fraction of solvent always remains in the volume of the material, and increasing the Hard Backing temperature causes a gradient of the solvent concentration and distortion of the microstructure. Low bake temperature for longer time can provide a constant evaporation rate, and finally low and uniform concentration of the remaining solvent in the resulting solid film resin (Liu et al, 2005). Other materials such as Ormocers and Ormosil can be directly processed without any intermediate thermal treatment.…”