Abstract
Fully Depleted Monolithic Active Pixel Sensors (FD-MAPS) represent an appealing alternative to hybrid detectors for radiation imaging applications. We have recently demonstrated the feasibility of FD-MAPS based on a commercial 110 nm CMOS technology, adapted using high-resistivity substrates and backside post-processing. A p/n junction diode, fabricated on the detector backside using low-temperature processing steps after the completion of the front-side Back End of Line (BEOL), is reverse-biased to achieve the full depletion of the substrate and thus fast charge collection by drift. Test diodes including termination structures with different numbers of floating guard rings and different pitches were fabricated together with other Process Control Monitor structures. In this paper, we present the design of the backside diodes, together with results from the electrical characterization of the test devices, aiming to improve understanding of the strengths and limitations of the proposed approach. Characterization results obtained on several wafers demonstrate the effectiveness of the termination rings in increasing the breakdown voltage of the backside diodes and in coping with the variability of the passivation layer characteristics. A breakdown voltage exceeding 400 V in the worst case was demonstrated in devices with 30 guard rings with 6 μm pitch, thus enabling the full depletion of high-resistivity substrates with a thickness larger than or equal to 300 μm. Additionally, we show the first direct comparison for this technology of measured pixel characteristics with 3D TCAD simulations, proving a good agreement in the extracted operating voltages.
Highlights
Monolithic Active Pixel Sensors (MAPS) are considered promising candidates for the replacement of standard hybrid pixel radiation detectors in medical imaging, high-energy physics and space applications due to their advantages in terms of production costs and low material budget [1,2,3,4]
The MAPS produced for the upgrade of the ALICE Inner Tracking System at CERN represent one of the first examples of monolithic sensors applied to a large high-energy physics experiment [5]
The main charge collection mechanism is diffusion, limiting their radiation tolerance and speed. Starting from this technology, a new generation of MAPS-based detectors with charge collection dominated by drift is being developed [6,7]
Summary
Monolithic Active Pixel Sensors (MAPS) are considered promising candidates for the replacement of standard hybrid pixel radiation detectors in medical imaging, high-energy physics and space applications due to their advantages in terms of production costs and low material budget [1,2,3,4]. The MAPS produced for the upgrade of the ALICE Inner Tracking System at CERN represent one of the first examples of monolithic sensors applied to a large high-energy physics experiment [5] In these detectors, the main charge collection mechanism is diffusion, limiting their radiation tolerance and speed. Starting from this technology, a new generation of MAPS-based detectors with charge collection dominated by drift is being developed [6,7] The fabrication of these sensors relies on a modification of a standard 180nm CMOS process including a deep n-type implantation which enables fully depleting the epitaxial layer. In the first production run, both active pixel arrays with embedded electronics and passive test structures, designed to extract the device characteristic parameters and to characterize the sensor backside diodes, were included.
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