Image sensor pixel integration for large area imaging

This thesis examines the various issues underlying integration of Mo/a-Si:H image sensor with inverted-staggered thin film transistor (TFT) for large area digital x-ray imaging applications.; The thin film transistor process has been modified to include a highly doped (n+) microcrystalline silicon (μc-Si:H) film as the contact layer in the source and drain regions. The deposition temperature of the n+ (μc-Si:H) film is varied from 200°C to 400°C to observe the crystalline and amorphous phase formations in these films. The results show that the films deposited at low temperatures are more crystalline in nature than their high temperature counterparts.; A comparison of the performance of aluminium (Al)-gated thin film transistors is presented in which the process parameters are varied in terms of the sputter deposition temperature, process pressure, and power. Gate films deposited at 30°C/5mTorr/300W yield TFT characteristics with low leakage current (∼10 fA at low VDS), an ON/OFF ratio better than 108, and a mobility of 1.1 cm2/Vs. In contrast, films deposited at 150°C/10mTorr/400W, yield a significant degradation in leakage current (∼1 pA) and mobility (0.77 cm2/Vs). The degradation stems from the high surface roughness of the a-SiN:H gate insulator, and hence the TFT channel.; Two different pixel integration processes for direct x-ray detection are studied. In one process, the Schottky diode is stacked on top of the TFT. In the other process, the two devices did not overlap. The two processes are compared in terms of mask count and pixel performance. Fully-overlapped and partially-overlapped pixel structures provide high fill factor, but suffer from the stresses due to multi-layers. In addition, the TFT leakage current of a fully-overlapped pixel is very high (∼10−7 A), due to parasitic capacitance. The partially-overlapped structure also suffers from high stress, however, the leakage current is considerably lower and comparable to that of a discrete TFT. The stress is not as severe in the non-overlapping structures. Also, the leakage current is of the order of 100fA.; The high intrinsic film stress associated with the stacked pixel structures has been further investigated. It is shown that the high compressive stress of Mo film (∼109 Pa) can be reduced to ∼107 Pa by varying the deposition process parameters without seriously undermining the physical and electrical properties of the films.; A small-scale (4 rows x 5 columns) teat array has been designed and characterized. The results show that several technical challenges need to first be solved before extending the design to a larger array for x-ray image generation. The most outstanding challenge is to improve the uniformity of the Schottky diode interface.