Use of a photoresist sacrificial layer with SU-8 electroplating mould in MEMS fabrication

Abstract

In this paper, new processes have been developed to fabricate micromechanical components and systems that utilize two different photoresists (Shipley S1813 and Hoechst AZ P4620) as sacrificial layers in conjunction with SU-8 photoresist used as an electroplating mould. The use of photoresists as sacrificial layers offers several advantages including reduction in processing steps and hence processing cost. However, the normal sacrificial layer photoresist processing cycle fails when used in conjunction with SU-8 photoresist. The latter is often used as an electroplating mould, especially for high-aspect ratio or tall microstructures in MEMS fabrication. In this paper, problems arising in the use of a photoresist sacrificial layer are discussed and new sacrificial layer processes are developed. New curing temperatures for photoresist sacrificial layers are determined, which prevent damage that otherwise occurs from the use of SU-8 as an electroplating mould. New hard baking temperatures determined are 175 °C and 200 °C for S1813 and AZ P4620 photoresist processes, respectively. A comb drive structure is utilized to demonstrate these new fabrication processes.

Multistep Synthesis on SU-8: Combining Microfabrication and Solid-Phase Chemistry on a Single Material

“…The selection of the sacrificial layer was based on good adhesion properties towards SU-8, ease of handling and simplicity of etching process. [17][18][19][20] The last variable is critical and requires that the sacrificial layer is efficiently and rapidly removed, releasing the SU-8 particles into suspension without compromising the presence of residual surface epoxy groups to be used for functionalization.…”

“…17 However, during the SU8 processing, some of the microparticles were released suggesting that the S1813 layer was insufficiently stable. Hard baking at an elevated temperature of 175°C for 1 h resulted in a significant improvement in the stability of the S1813 layer, but increased the difficulty of releasing the particles in the final lift-off phase.…”

“…Hard baking at an elevated temperature of 175°C for 1 h resulted in a significant improvement in the stability of the S1813 layer, but increased the difficulty of releasing the particles in the final lift-off phase. 17 As an alternative, the use of aluminum was investigated. 18 A thin layer was evaporated onto a substrate, followed by spin coating with Ti primer prior to SU-8 processing, which resulted in excellent adhesion of the SU-8.…”

Photon imaging using post-processed CMOS chips

“…Two noteworthy exceptions are reports on effects attributed to remaining solvents [88,89]. It was necessary to gather more information tailored for the use of SU-8 in ambients with a critical gas composition or in vacuum.…”

A programmable mask for direct write lithography.

“…After the apertures are etched the movable parts need to be freed from the sacrificial layer. To etch the sacrificial layer a dry etch is used as a wet etch may cause stiction problems [8]. The SPR 220 is etched in the RIB with an oxygen plasma and the sacrificial layer is etched inside a plasma asher at a forward power of 50 watts and pressure of 0.5 torr for approximately 8 hours as the photoresist was difficult to remove after the long hard bakes.…”