Process issues with Mo∕a-Si:H Schottky diode and thin film transistors integration for direct x-ray detection

Abstract

This article presents an alternate strategy to reduce 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. However, that approach resulted in narrow process latitude and compromised x-ray sensitivity. Alternatively, the mechanical stress in the Mo can be reduced by reducing and/or avoiding the extrinsic stress exerted on the 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 x-ray sensitivity has been measured at different x-ray spectra in the range of 40–100kVp. The pixel shows good linearity with x-ray dosage and high detection efficiency at low x-ray tube voltages.