Improved Adhesion of Gold Thin Films Evaporated on Polymer Resin: Applications for Sensing Surfaces and MEMS

Behrang Moazzez,Stacey O'Brien,Erika Merschrod S.

doi:10.3390/s130607021

Behrang Moazzez, Stacey O'Brien + Show 1 more

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https://doi.org/10.3390/s130607021

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Abstract

We present and analyze a method to improve the morphology and mechanical properties of gold thin films for use in optical sensors or other settings where good adhesion of gold to a substrate is of importance and where controlled topography/roughness is key. To improve the adhesion of thermally evaporated gold thin films, we introduce a gold deposition step on SU-8 photoresist prior to UV exposure but after the pre-bake step of SU-8 processing. Shrinkage and distribution of residual stresses, which occur during cross-linking of the SU-8 polymer layer in the post-exposure baking step, are responsible for the higher adhesion of the top gold film to the post-deposition cured SU-8 sublayer. The SU-8 underlayer can also be used to tune the resulting gold film morphology. Our promoter-free protocol is easily integrated with existing sensor microfabrication processes.

Highlights

Gold is often used in microfabricated devices due to its high electrical conductivity and optical reflectivity, combined with a low Young’s modulus of 79 GPa [1]
Problems with mechanical stability can be exacerbated when one layer is deposited at elevated temperature as in the case of gold thermal evaporation, where the different thermal expansion coefficients of metal, underlayer and substrate can lead to substantial residual stress upon cooling [27]
Post-gold-deposition curing of a polymer underlayer is a reliable protocol to have a robust gold thin film satisfying the mechanical requirements for sensors

Summary

Introduction

Gold is often used in microfabricated devices due to its high electrical conductivity and optical reflectivity, combined with a low Young’s modulus of 79 GPa [1] It has been used widely in chemical and biochemical sensors as a signal transducer based on each of the abovementioned unique properties or their combinations [2]. One of the main issues in the applicability of gold thin films in microfabrication technology is their weak adhesion to inert and commonly used glass and silica substrates even using adhesion promoting processes [6,7,8] Oxidative metals such as chromium, used as an intermediate layer, can enhance gold adhesion, but because chromium diffuses to the gold surface and oxidizes, the top gold layer morphology and electrical properties are affected dramatically [6,9]. Ge et al [10] and Nordström [1] have studied the adhesion of Cu and Au, respectively, to photosensitive epoxies including SU-8

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