Characterization of the H2M monolithic CMOS sensor

The H2M (Hybrid-to-Monolithic) is a monolithic pixel sensor manufactured in a modified 65 nm CMOS imaging process with a small collection electrode. Its design addresses the challenges of porting an existing hybrid pixel detector architecture into a monolithic chip, using a digital-on-top design methodology, and developing a compact digital cell library. Each square pixel integrates an analog front-end and digital pulse processing with an 8-bit counter within a 35 μ m pitch. This contribution presents the performance of H2M based on laboratory and test beam measurements, including a comparison with analog front-end simulations in terms of gain and noise. A particular emphasis is placed on backside thinning in order to reduce material budget, down to a total chip thickness of 21 μ m for which no degradation in MIP detection performance is observed. For all investigated samples, a MIP detection efficiency above 99% is achieved below a threshold of approximately 205 electrons. At this threshold, the fake-hit rate corresponds to a matrix occupancy of fewer than one pixel per the 500 ns frame. Measurements reveal a non-uniform in-pixel response, attributed to the formation of local potential wells in regions with low electric field. A simulation flow combining technology computer-aided design, Monte Carlo, and circuit simulations is used to investigate and describe this behavior, and is applied to develop mitigation strategies for future chip submissions with similar features.