Boomerang is a 3GeV synchrotron radiation accelerator, currently being constructed in the State of Victoria, Australia. The outline design of two beamlines, suitable for the fabrication of MEMS devices using the LIGA process, is presented, along with an estimate of the exposure doses throughout the resist. The most commonly used resist is PMMA, which requires a minimum dose of about 4500 J/cm3 for accurate microstructure definition. Exposure with such a dose, in resist thicknesses of several hundred microns, can take hours. Fortunately, SU-8 resist is becoming more widely used as the minimum dosage required is about 35 J/cm3, leading to exposure times of only a few minutes. Although Boomerang will shorten exposure times due to its higher irradiance at the substrate, the full benefits may not be realizable due to excessive resist heating. Heating effects have been simulated and suggest that helium cooling will be essential if the glass transition temperature of the resist (100°C for PMMA, 50°C for SU-8) and thermal distortion of the mask are to be avoided. The parameters chosen in this study of the future performance of Boomerang have been inserted into a cost model. The model shows that Boomerang exposure can become competitive with other exposure methods, particularly where large quantities of devices with deep structures are required.
LIGA is a technology that offers significant advantages where high accuracy, high aspect ratio microstructures are required. The application of LIGA to the manufacture of real products has been delayed by technical problems that exist with the individual process steps and the limited availability of integrated facilities, enabling users to subcontract the complete manufacturing process. These problems have been dominated by the limited availability of high quality masks, long and expensive exposure at synchrotron radiation sources and the electrodeposition of thick stress-free layers. This paper describes the practical solutions developed at the Central Microstructure Facility, RAL, for the key process steps of manufacturing high precision gold-on- beryllium masks, exposure of SU-8 resist using a 2 GeV synchrotron, electrodeposition of deep ($GTR 500 mm), stress-free metal layers and resist stripping procedures fro 3 micrometers minimum features up to 500 mm deep on 4-6 inch wafers. A cost model shows that the reduction in the exposure time using SU-8 instead of PMMA resist may enable x-ray LIGA to be cost competitive with other techniques such as uv LIGA, DRIE or direct laser ablation.
A 193nm excimer laser microstepper has been developed for deep UV photolithography research at this wavelength. The system incorporates a x10, 0.5NA, 4mm field diameter, high-resolution imaging lens of either all-refractive or catadioptric design. An all-fused silica refractive lens has been used in the results reported here to carry out exposures in polymethylmethacrylate and polyvinylphenol photoresists. Well-resolved images of 0.2micrometers dense lines and spaces and 0.35micrometers diameter contact holes have been produced in PMMA and polyvinylphenol resists.
The use of excimer lasers and the development of deep UV lenses for the semiconductor industry has produced a technology capable of sub-micron feature generation. The high irradiance levels available at a working surface can give rise to various laser-induced processes that enable material to be removed or deposited precisely. One of the laser-induced processes that has received much attention over the last few years is ablation. The paper will concentrate on the relevant properties of excimer lasers, the development of deep UV lens technology and the phenomenon of ablation used for material removal. A number of examples will be given to illustrate the capability of excimer laser systems to ablate sub-micron features in polymers and other materials.
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