Mechanical Stress and Process Integration of Direct X-ray Detector and TFT in a-Si:H Technology
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Mechanical Stress and Process Integration of Direct X-ray Detector and TFT in a-Si:H Technology Czang-Ho Lee, Isaac Chan, and Arokia Nathan Department of Electrical and Computer Engineering, University of Waterloo, Waterloo, ON, N2L 3G1, Canada ABSTRACT This paper presents an alternate strategy to reduce the mechanical stress issues pertinent to the process integration of molybdenum/hydrogenated amorphous silicon (Mo/a-Si:H) Schottky diodes and thin film transistors (TFTs), used as X-ray sensor pixels for medical imaging. The previous approach was to minimize the intrinsic stress in the Mo layer through appropriate process conditions and film thickness, but over narrow process latitude and with a compromise in X-ray sensitivity. Alternatively, the mechanical stress in Mo can be reduced by reducing and/or avoiding the extrinsic stress exerted on Mo by the underlying films through a different masking sequence in the fabrication. This modified process allows for a more flexible design of the Mo layer for enhanced X-ray sensitivity, while maintaining the mechanical integrity of the various layers. Also, the performance of the Schottky diode is improved, in terms of its forward current. The pixel shows good linearity in the X-ray response over the range of 40 ~ 100 kVp. INTRODUCTION Hydrogenated amorphous silicon (a-Si:H) flat panel technology for diagnostic X-ray imaging is a very promising technology for the creation of digital X-ray images, and can potentially replace conventional film imaging techniques [1-4]. We have previously developed a direct X-ray detection scheme based on a Mo/a-Si:H Schottky diode structure for low-energy Xrays [5]. Here, a Mo layer acts not only as a Schottky metal but also as a direct converter, which transforms X-rays into energetic electrons by virtue of the photoelectric effect. These electrons are injected into an a-Si:H layer. They are highly energetic and undergo various scattering events leading to generation of electron-hole pairs. Owing to the small diffusing length in the a-Si:H, the effective separation of the generated electron-hole pairs is enhanced by the electric field inside the reversed-biased a-Si:H depletion region, thereby producing an output signal [6]. The process of the Mo/a-Si:H Schottky diodes is also fully compatible with that of the a-Si:H thin film transistor (TFT) which is used as a switching element for charge readout from each pixel [7]. The operating principle and photograph of a fabricated pixel are shown in Figure 1. One critical issue with the Mo/a-Si:H Schottky diode process is the mechanical stress in the Mo, both intrinsic and extrinsic, which constrains the process integration of the detector and TFT. Extensive characterization of the intrinsic stress in Mo has been reported [8, 9], in an attempt to reduce the stress through appropriate process conditions [8] and film thickness (∼ 300 nm) [9]. However, this is achievable only over a narrow process window [8] and is further compounded by a compromise in X-ray sensitivity (15 % drop from the op
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