Charge-cloud-based micrometer resolution in deep-silicon photon-counting CT

We are developing a monolithic deep-silicon photon-counting sensor targeting spatial resolution on the order of $1\mu m$ . This work investigates how pixel pitch, noise level, threshold number, and threshold placement affect the achievable tangential and wafer-thickness resolution to guide the CMOS electronics and sensor design.

Allpix Squared simulations were used to evaluate pixel pitches with two noise levels and two threshold-placement schemes over varying threshold numbers. Interaction position was estimated in both dimensions using lookup tables trained on the thresholded pixel output, and performance was quantified using the MTF.

Decreasing pixel pitch strongly improved tangential resolution for Compton interactions, whereas photoelectric interactions showed a weaker dependence on pixel pitch and threshold placement. Equal-counting thresholds approached the achievable resolution with fewer thresholds than equidistant thresholds. For eight equal-counting thresholds, the 10% MTF across the studied pixel sizes ranged from $\text{1284\hspace{0.17em}\hspace{0.17em}}\mathrm{lp}/\mathrm{cm}$ to $\text{2452\hspace{0.17em}\hspace{0.17em}}\mathrm{lp}/\mathrm{cm}$ in the tangential direction and from 15 to $71\mathrm{lp}/\mathrm{cm}$ in the wafer-thickness direction for Compton interactions, compared with 409 to $507\mathrm{lp}/\mathrm{cm}$ and 18 to $79\mathrm{lp}/\mathrm{cm}$ , respectively, for photoelectric interactions. RMSE analysis further showed that some interactions for the $25\mu m$ pitch reached below $2\mu m$ tangentially and a few tens of micrometers in the wafer-thickness direction.

Spatial resolution improved with a threshold number up to about 8 to 10, beyond which additional programmable thresholds gave only modest gain. The achieved tangential resolution also suggests that analyzer-free phase-contrast imaging is feasible for realistic geometries and expected interference patterns.