Conference Paper

Fabrication of Test Structures to Monitor Stress in SU-8 Films used for MEMS Applications

DOI: 10.1109/ICMTS.2010.5466870 Conference: IEEE International Conference on Microelectronic Test Structures (ICMTS 2010)

ABSTRACT

SU-8, an epoxy based negative photoresist, is widely used in the manufacture of micromechanical systems but can exhibit significant levels of stress build-up during processing. This paper describes micromechanical test structures that provide the opportunity to spatially characterise stress the in SU-8 at different stages of the process. The structures are fabricated in a thick layer of SU-8 and are subsequently released from the underlying substrate using a dry chemical vapour etch process. The initial results indicate that there is significant tensile stress in the SU-8, and that this demonstrates a radial variation along with a dependence on the process conditions.

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Available from: Jeremy Murray, Jul 19, 2015
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    ABSTRACT: SU-8 is an epoxy-based, negative photoresist that is widely used in the manufacturing of micromechanical systems. The polymer cross-linking that occurs during the photolithographic processing of SU-8 can result in high levels of stress in the patterned film. This has significant implications for the yield and reliability of SU-8 structures and needs to be understood if the material is to be integrated with other technologies. This paper describes micromechanical test structures that provide the opportunity to wafer map the stress in SU-8 at different stages of the process. The structures are fabricated in a thick layer of SU-8 and are subsequently released from the underlying substrate using a dry chemical vapor etch process. An automated optical measurement system has been built to allow rapid optical inspection of many thousands of test structures fabricated on 200mm wafers. Initial results indicate significant tensile stress in the SU-8, which demonstrates a radial variation along with a dependence on the process conditions.
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    ABSTRACT: This work presents an SU-8 surface micromachining process using amorphous silicon as a sacrificial material, which also incorporates two metal layers for electrical excitation. SU-8 is a photo-patternable polymer that is used as a structural layer for MEMS and microfluidic applications due to its mechanical properties, biocompatibility and low cost. Amorphous silicon is used as a sacrificial layer in MEMS applications because it can be deposited in large thicknesses, and can be released in a dry method using XeF2, which alleviates release-based stiction problems related to MEMS applications. In this work, an SU-8 MEMS process was developed using α-Si as a sacrificial layer. Two conductive metal electrodes were integrated in this process to allow out-of-plane electrostatic actuation for applications like MEMS switches and variable capacitors. In order to facilitate more flexibility for MEMS designers, the process can fabricate dimples that can be conductive or nonconductive. Additionally, this SU-8 process can fabricate SU-8 MEMS structures of a single layer of two different thicknesses. Process parameters were optimized for two sets of thicknesses: thin (5–10 µm) and thick (130 µm). The process was tested fabricating MEMS switches, capacitors and thermal actuators.
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