| dc.description.abstract | Breast cancer is the most common cause of cancer-related death in women in the world. However, early and reliable detection of breast cancer can reduce its high fatality rate. Diagnosis hinges on medical X-ray imaging techniques like screen-film mammography and digital mammography, which are burdened by false positives, increased radiation exposure, and patient discomfort due to breast compression. Conversely, three-dimensional (3-D) imaging modalities, like magnetic resonance imaging and digital breast tomosynthesis, aim to reduce false positives but have several limitations, including high cost, patient discomfort, and difficulties imaging dense breast tissue.
Alternatively, dedicated breast computed tomography (DBCT) with a single-photon-counting (SPC) detector offers 3-D imaging without breast compression and lower radiation exposure, but faces challenges related to the fabrication of the X-ray sensor. Commonly-used X-ray-sensor materials for SPC detectors are silicon (Si), cadmium zinc telluride (CZT), and cadmium telluride (CdTe). However, Si has low detection efficiency and polycrystalline CZT and CdTe face yield issues due to the bump-bonding process needed for integration with a complementary metal-oxide-semiconductor (CMOS) readout integrated circuit (ROIC).
An alternative to these materials for monolithic X-ray SPC imagers is amorphous selenium (a-Se), a well-established X-ray-sensitive material. While the use of an a-Se sensor is more cost-effective for large-area deposition, the major drawbacks of a-Se are its limited temporal and energy resolution, preventing its use in detectors for DBCT.
In this thesis, we tackle the intrinsic limitations of a-Se and demonstrate, for the first time, the potential of a monolithic a-Se/CMOS X-ray imager to meet the demanding DBCT requirements. First, we demonstrate single-photon counting with an a-Se sensor, monolithically integrated on a CMOS ROIC, for the first time. We report the electrical characterization of a previously-designed CMOS SPC imager (Chip 1) and outline the limitations in achieving the electrical performance necessary for photon counting. Subsequently, we present circuit simulations and measurements that identify these limitations and suggest methods for overcoming the associated electrical barriers. Additionally, we discuss the technical challenges hindering the integration of a-Se on Chip 1 and propose our solution to address these issues.
Next, we present the first measured transient X-ray response from a monolithic a-Se/CMOS imager and the first measured pulse-height spectroscopy results using our designed CMOS ROIC (Chip 2). We also experimentally demonstrate the small-pixel effect (SPE) with an a-Se/CMOS detector for the first time and show its potential to dramatically improve energy resolution and temporal response. Additionally, with device-level simulations, we analyze the impact of carrier movement in an a-Se sensor on SPE.
We then demonstrate, for the first time, photon-counting results using new a-Se/CMOS pixel (Chip 3), which is able to meet the stringent DBCT requirements. To achieve this, we design and verify a novel large-area scalable 92~ $\times$ 92~$\mu$m$^2$ pixel having a sub-pixel SPE-enhancement technique. We also present a novel area-efficient foreground calibration circuit for SPC pixels, which employs a new area-efficient current-steering calibration digital-to-analog converter.
Finally, we demonstrate a photon-counting pixel (Chip 4) that features a new adaptive common-mode leakage-compensation circuit with offset correction to further improve the maximum pixel count rate for medical imaging. | en |
