MEMS Materials and Fabrication Technology on Large Areas: The Example of an X-ray Imager
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MEMS Materials and Fabrication Technology on Large Areas: The Example of an X-ray Imager J.H. Daniel1, a, B. Krusor1, R. Lau1, J.P. Lu1, Y. Wang1, M. Mulato1, R.B. Apte1, R.A. Street1, A. Goredema2, D.C. Boils-Boissier2, S.E. Silver2, P.M. Kazmaier2 1 Xerox Palo Alto Research Center, 3333 Coyote Hill Rd, Palo Alto, CA 94304 a
[email protected]
2
Xerox Research Center Canada, 2660 Speakman Dr, Mississauga, Ontario, L5K 2L1
ABSTRACT Micromachining has potential applications for large area image sensors and displays, but conventional MEMS technology, based on crystalline silicon wafers cannot be used. Instead, large area devices use deposited films on glass substrates. This presents many challenges for MEMS, both as regards materials for micro-machined structures and the integration with large area electronic devices. We are exploring the novel thick photoresist SU-8, as well as plating techniques for the fabrication of large area MEMS. As an example of its application, we have applied this MEMS technology to improve the performance of an amorphous silicon based image sensor array. SU-8 is explored as the structural material for the X-ray conversion screen and as a thick interlayer dielectric for the thin film readout electronics of the imager.
INTRODUCTION Electronics based on amorphous silicon (a-Si:H) has led to large area active matrix displays and image sensors. There are also promising opportunities for Micro-Electro-Mechanical Systems (MEMS) technology in this field and this is referred to as large area MEMS (LAMEMS). The considered processes must be scaleable and suitable for large area compatible substrates such as glass or plastics. MEMS structures may also be combined with thin film electronics. As a demonstration we have applied LAMEMS technology to the fabrication of an X-ray imager. In particular, we are studying processes and properties of the photopolymer SU-8 and plating techniques. The compatibility of SU-8 with thin film processes, adhesion issues and interconnect metallization are objectives of the presented research. The X-ray imager consists of an image sensor array, which is sensitive to visible light, and an X-ray conversion screen (figure 1). The image sensor is an array of photodiodes with pixel switches (thin film transistors, TFTs) based on amorphous silicon technology on a glass substrate [1]. X-ray imagers up to 30×40 cm2 in area are commercially available [2]. Medical X-ray imagers have a thick (200-500 µm) layer of phosphor, which converts the Xrays into visible light, placed directly on top of the photodiodes. Spatial resolution is limited because of light scattering in the phosphor. In order to obtain the full resolution of the pixel array, the phosphor layer needs to be micro-patterned into cells which collimate the generated light [3]. This cell structure is made from a 300-400 µm thick layer of the negative acting photopolymer SU-8 [4]. SU-8 has become popular for patterning high aspect-ratio MEMS structures [5]. The SU-8 cell walls have to be reflective in order to prevent
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