The H2M Monolithic Active Pixel Sensor -- characterizing non-uniform in-pixel response in a 65 nm CMOS imaging technology

TL;DR

This paper characterizes the non-uniform in-pixel response of a monolithic active pixel sensor fabricated in 65 nm CMOS technology, highlighting the effects of pixel size and layout on sensor uniformity.

Contribution

It introduces a novel monolithic pixel sensor design in 65 nm CMOS, analyzes its in-pixel response non-uniformities, and discusses their dependence on pixel layout and size.

Findings

Non-uniform in-pixel response related to n-well size and location.

Asymmetry in pixel response observed with larger pixel pitch.

Design insights for improving uniformity in monolithic sensors.

Abstract

The high energy physics community recently gained access to the TPSCo 65 nm ISC (Image Sensor CMOS), which enables a higher in-pixel logic density in monolithic active pixel sensors (MAPS) compared to processes with larger feature sizes. To explore this novel technology, the Hybrid-to-Monolithic (H2M) test chip has been designed and manufactured. The design followed a digital-on-top design workflow and ports a hybrid pixel-detector architecture, with digital pulse processing in each pixel, into a monolithic chip. The chip matrix consists of 64$\times$16 square pixels with a size of 35$\times$35 um2, and a total active area of approximately 1.25 um2. The chip has been successfully integrated into the Caribou DAQ system. It is fully functional, and the measured threshold dispersion and noise agree with the expectation from front-end simulations. However, a non-uniform in-pixel response related to the size and location of the n-wells in the analog circuitry has been observed in test beam measurements and will be discussed in this contribution. This asymmetry in the pixel response, enhanced by the 35 um pixel pitch - larger than in other prototypes - and certain features of the readout circuit, has not been observed in prototypes with smaller pixel pitches in this technology.