Thesis (Ph.D.)--University of Waterloo (Canada), 2003
This work extends amorphous silicon (a-Si) TFT technology from traditional switching applications to on-pixel small signal amplification for diagnostic medical digital imaging applications. The developed a-Si pixel amplifier offers improved signal-to-noise ratios, lower cost, and less off-panel circuit complexity compared to its (traditional) a-Si switch counterpart. The pixel amplifier advances the state-of-the-art by offering a large area real-time imaging solution for low-noise fluoroscopic medical imaging that is not viable with current a-Si switch based pixels. More significantly, the pixel (because of its circuit gain) offers potentially reduced patient x-ray doses for other medical imaging modalities, hence improving the safety standards associated with current x-ray imaging practices
Investigations of additional noise due to the a-Si pixel amplifier indicate that this noise is minimized for small pixel capacitance implying the need for low capacitance detectors such as amorphous selenium (a-Se) photoconductors. However, in contrast with CMOS amplified pixels, the flicker noise added by the a-Si readout circuit is comparable to the traditionally prohibitive reset noise. Measurements on in-house TFTs and pixel readout circuits indicate the feasibility of amorphous silicon APS pixels for low noise, diagnostic medical imaging applications such as digital fluoroscopy
Appropriate bipolar bias voltages in the TFT ON and OFF states minimize the characteristic threshold voltage shift of the a-Si TFTs in the amplified pixel. Due to intrinsic feedback, the developed amplified pixel readout circuit has a compensatory effect on the pixel's gain. This gain compensation, coupled with small (typically <0.1%) duty cycles for diagnostic medical imaging applications leads to a <5% variation in circuit gain over a 10,000 hour array lifetime
In order to obtain high fill factor, the three TFTs in the a-Si amplified pixel are assumed embedded under the sensor unlike the conventional co-planar layout of sensor and TFTs. The major challenge with vertically stacked architectures is an increase in TFT leakage current due to the presence of an overlying sensor back electrode.An in-house fabricated vertically stacked pixel based on a double gate TFT illustrates how a low leakage current can be obtained even in the presence of a bias on an overlying sensor back electrode
Lastly, the integration of an in-house amplified pixel a-Si TFT array with a continuous layer a-Se photoconductor X-ray detector is detailed including its design, fabrication and custom test setup. Results from double sampled array operation including x-ray sensitivity, gain and noise are promising and highlight the feasibility of a-Si amplified pixel arrays coupled with a-Se x-ray detectors for diagnostic medical x-ray imaging applications
The research presented in this thesis indicates that proper circuit and pixel architecture design can overcome a-Si material shortcomings related to area, noise and metastability. The demonstrated gain, noise, and stability results indicate that a-Si on-pixel amplifiers, coupled with a low capacitance, well established x-ray detection technology such as a-Se, can meet even the stringent requirements of low noise, digital fluoroscopy (less than 1000 input referred noise electrons) without resorting to newer high x-ray absorption materials such as PbI. The results presented provide the impetus to expedite development of large area a-Si APS arrays for low noise, real-time medical imaging
