Yield and performance validation of the Monolithic Stitched Sensor (MOSS), the first wafer-scale prototype for the ALICE ITS3 upgrade

TL;DR

The ITS3 upgrade targets a dramatic reduction in material budget by using wafer-scale stitched MAPS, and this study validates the first stitched prototype (MOSS) as a feasible path forward. It demonstrates high regional yield and robust in-beam performance that meet the ITS3 efficiency and fake-hit-rate requirements, with irradiation tolerance up to 4 kGy of ionising dose and 4×10^12 cm^−2 of non-ionising dose and a linear ToT response in the soft X-ray range 1.86–6.5 keV. The results establish stitching as a viable route to wafer-scale MAPS for ITS3, provide detailed performance as a function of pixel pitch, and identify remaining challenges for large-scale deployment. This work lays the groundwork for deploying stitched MAPS in ALICE ITS3 and informs design choices for future large-area detectors.

Abstract

The ALICE Inner Tracking System upgrade (ITS3) will employ stitched, wafer-scale Monolithic Active Pixel Sensors (MAPS) for the first time in high-energy physics, achieving a material budget of only 0.09$\,$%$\,$X$\mathrm{_{0}}$ per layer. Its first stitched prototype, the Monolithic Stitched Sensor (MOSS), underwent serial testing confirming sensor yield compliance with ITS3 requirements. In-beam tests show the device meets the ITS3 efficiency requirement of >$\,$99$\,$% while maintaining a fake-hit rate below 0.1$\,$hits/pixel/s, with performance sustained up to irradiation levels of 4$\,$kGy and $4\times10^{12}$1MeV$\,$n$\mathrm{_{eq}}\,$cm$\mathrm{^{-2}}$. The sensor demonstrates excellent charge-collection properties and linearity between time-over-threshold and deposited energy in the 1.8 - 6.5$\,$keV range in response to soft X-ray emissions. This article provides an overview of the validation steps and characterisation results.

Figures (6)

• Figure 1: Sensor layout, illustrating the modular architecture consisting of Left End-Cap (LEC), Right End-Cap (REC) and Repeated Sensor Units (RSUs) with Stitched Boundaries (SB) indicated. moss
• Figure 2: Detection efficiency and fake-hit rate as a function of the threshold for a non-irradiated sensor, a sensor irradiated to 10kGy ionising dose, and a sensor irradiated to 1e13 non-ionising dose. All three sensors show a threshold range above 50e^- complying with the ITS3 requirements. moss
• Figure 3: Spatial resolution and cluster size versus threshold for three pixel variants, showing the effect of pitch and gap size. At 160e^-, the 18µm pitch variant reaches about 4.5µm resolution, compared to 5.7µm for the 22.5µm pitch. Doubling the gap size improves the resolution by roughly 0.4µm. moss
• Figure 4: Yield losses per failure category, normalised per region. Excluding readout-architecture failures, a yield of about 85% is achieved. When additionally excluding powering issues not relevant to the final sensor design, the effective region yield exceeds 98%.moss
• Figure 5: Single-pixel cluster ToT spectrum in response to $^{55}$Fe X-ray emissions. The primary Mn-K$_{\alpha}$ and Mn-K$_{\beta}$ peaks are resolved, along with the secondary Si-K$_{\alpha}$ and Mn-K$_{\alpha, \beta}$ - Si-K$_{edge}$ features originating from X-ray interactions within the silicon. Fit residuals remain within 2.5 standard deviations. moss