Study of the depletion depth in a frontside biased CMOS pixel sensors

Abstract

Depletion of the sensitive volume for semiconductor based detectors is a key to achieve high performance. It is for instance required for charged particle detection in highly radiative environment and for X-ray spectroscopy. PIPPER-2 is a CMOS pixel sensor featuring an architecture that allows the application of the reverse bias of the pn junction from the frontside (cathode), on the electronic side, without process modification. Biasing voltages up to 45 V have been applied to sensor prototypes fabricated on two different high resistivity substrates: a thin epitaxial layer (1 k cm) and a 40 µm thick bulk substrate (600 cm). Calculations from a simplified analytical model and 3D-TCAD simulations were conducted to predict the evolution of the depletion depth with the bias voltage. These expectations were compared to measurements of PIPPER-2 illuminated with two X-ray energies. We conclude that the frontside biasing method allows the full-depletion of the thin epitaxial layer. In contrast, depletion of the bulk substrate reaches about half-depth but X-rays are still detected over the full depth .

On the radiation tolerance of CMOS Monolithic Active Pixel Sensors with partially depleted active volume

“…With respect to MIMOSA-34, this chip was modied by extending the radius of the diode but more importantly by choosing a substantially more sizeable opening of the p-well surrounding it. As reported in [79], the chip operated at depletion voltages of up to 40 V. Based on 55 Fe amplitude spectra and TCAD simulations, it is concluded that the chip becomes fully depleted once the depletion voltage exceeds about 15 V. The simulation depth reported is ∼ 12 µm, which corresponds to about 66% of the volume of the epitaxial layer. Based on TCAD simulations, the authors argue that a depletion layer is formed in this epitaxial layer.…”

Progress on the radiation tolerance of CMOS Monolithic Active Pixel Sensors

“…expects n = 3 for an ideal point-like diode and values as high as n ≈ 6 were reported to fit measured data [103] (details see below).…”

“…The precise value of n > 2 is so far not established and the equation will in any case provide a rough approximation of the complex geometry only. One expects n = 3 for an ideal point-like diode and values as high as n ≈ 6 were reported to fit measured data [106] (details see below).…”

“…Pipper-2 [103] relies on the same Tower/Jazz 180 nm quad-well CMOS process as ALPIDE. It was developped for X-ray imaging but also tested for radiation tolerance in the context of CBM.…”