Thick-layer resists for surface micromachining

Abstract

Interest in thick-photoresist applications is steadily growing. In addition to bump fabrication and wire interconnect technology (WIT), the process of patterning thick-layer photoresists by UV lithography is specially qualified for applications in microelectromechanical systems (MEMS). Specialized equipment and new photoresists have been developed or are under development to cope with the new challenges in the field of preparing extremely thick photoresist layers, the process of patterning these thick resists, and to deal with the difficulties of the following galvanoplating step. As one of the most critical steps in thick-photoresist processing, the baking procedure was investigated. Positive tone photoresists (AZ 4562, maP 100) were processed by means of three different baking methods: air-forced oven, ramped hotplate, and IR radiation. It could be shown that IR baking is advantageous compared to the other methods with respect to process duration and energy consumption. As for edge steepness, resolution, edge loss, and surface roughness, all methods deliver nearly the same results. A minimum width of 2-3 µm for the resist bars was found to be necessary to withstand the fabrication process of lines and spaces in about 15 µm thick resists. For thicker layers, high aspect ratios of about 10 as well as steep edges of more than 88 • could be fabricated. The development of SU-8, a chemically amplified negative tone photoresist for the 300-450 nm region opened totally new dimensions for the UV depth lithography. Even under development, SU-8 delivers results otherwise only achievable by x-ray lithography. The deposition of photoresist on highly-structured surfaces demands advanced methods. Electrodeposition of resist is one solution. PEPR 2400 was used for patterning by UV light in order to generate resist patterns around a free standing silicon bar. The achieved resist patterns were moulded by using electroplating. For microsystem applications some metals and alloys were deposited. Three-dimensional micro components were fabricated as demonstrators for the new technique. Electrodeposition allows the use of materials with interesting properties which could not be provided by standard processes in microelectronics.

Surface tension-powered self-assembly of microstructures - The state-of-the-art

“…Feature heights have increased significantly with the development of new thick-layer resists [27], [28]. The main advantage of metal micromachining is its low temperature, which has allowed postprocessing of MEMS onto completed circuits [29].…”

Fabrication and use of a transient contractional flow device to quantify the sensitivity of mammalian and insect cells to hydrodynamic forces

“…Unlike traditional photoresists, which are restricted to a layer thickness on the order of 10 m, SU-8 allows one to build thick (up to 1 mm) and high-aspect-ratio structures (Leochel, 2000). SU-8 100, the most viscous one in this series of resists, was used in this study to build channels with a depth up to 650 m.…”

“…While inclined sidewalls will not undermine the performance of the microfluidic channel, their presence complicates accurate measurements of the true dimensions of the channel, in addition to complicating CFD simulations of the flow. Leochel (2000) reported that the gap between the photomask and the resist and the associated diffraction were the major causes of nonvertical sidewalls. In this study, it was also found that the slope of the sidewall was related to the postexposure baking temperature and duration.…”

Fabrication of 3D air-core MEMS inductors for very-high-frequency power conversions

“…Because of the isotropic etching profile, the pitch between nearby turns is limited. To reduce the winding pitch, alternative methods are mould-based electroplating 19,20,43 and anisotropic plasma etching of Cu [44][45][46] .…”