Issue 17, 2017

A user-friendly guide to the optimum ultraviolet photolithographic exposure and greyscale dose of SU-8 photoresist on common MEMS, microsystems, and microelectronics coatings and materials

Abstract

We provide here a user-friendly guide to find the optimum i-line (365 nm) photolithographic exposure dose of an arbitrary thickness of SU-8 on various substrate materials and thin film coatings used in MEMS, microsystems and microelectronics technologies: semiconductors, 2D materials (graphene and MoS2) plastics, glass, metals and ceramics. By considering the variation of the absorption coefficient of SU-8 to ultraviolet light and the effect of partial reflections during the photolithography, we develop an analytical model for the exposure of SU-8. The critical exposure dose of the SU-8 enables a calculation of the exact greyscale photolithographic exposure time of the photoresist which optimizes the fabrication of microsystems structures (microcantilevers, microbridges, microchannels…) of a desired thickness. The optimum exposure doses are presented in both graphical and tabular format to enable user-friendly information based on the desired SU-8 thickness, the desired greyscale thickness and the specific wafer or coating used for the deposition. Interestingly, in the context of grey-scale lithography the model predicts that the surface reflectivity has a major impact on the resulting membrane thickness for a fixed dose and reducing the SU-8 thickness – on a highly reflecting surface a thicker membrane is obtained, on a low reflecting surface a thinner membrane in obtained when reducing the SU-8 thickness. The result is a useful guide for designers working with SU-8 in the context of many fabrication processes, e.g. MEMS, laboratory on a chip, microfluidics, microsystems, microengineering, micromoulding, and flexible electronics etc. – where a myriad of coatings and wafers are now used.

Graphical abstract: A user-friendly guide to the optimum ultraviolet photolithographic exposure and greyscale dose of SU-8 photoresist on common MEMS, microsystems, and microelectronics coatings and materials

Article information

Article type
Paper
Submitted
02 Mar 2017
Accepted
30 Mar 2017
First published
07 Apr 2017

Anal. Methods, 2017,9, 2495-2504

A user-friendly guide to the optimum ultraviolet photolithographic exposure and greyscale dose of SU-8 photoresist on common MEMS, microsystems, and microelectronics coatings and materials

M. Gaudet and S. Arscott, Anal. Methods, 2017, 9, 2495 DOI: 10.1039/C7AY00564D

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