Topmetal-L: A Low Noise Charge-Sensitive Pixel Sensor for POLAR2/LPD

TL;DR

The study targets a high-throughput, low-noise X-ray polarimeter for POLAR-2/LPD by introducing Topmetal-L, a large-area charge-sensitive pixel sensor fabricated in 130 nm CMOS with 356×512 pixels at 45 μm pitch and a 26×26 μm^2 collecting window. A single analog output channel paired with sentinel readout enables rapid, region-focused data acquisition while preserving track integrity for gas-based photoelectron polarimetry. Key results show ENC ≈ 22.8 e^−, gain ≈ 76.04 μV/e^−, and an active area of 3.69 cm^2 with 720 mW total power, along with system-level validation achieving a residual modulation of 0.26% ± 0.45% at 5.90 keV and a modulation factor of 66.67% ± 0.45% at 8.05 keV, plus a linear count-rate up to 15 k counts cm^−2 s^−1. The sentinel readout reduces full-frame time to 730 μs, enabling large-area, high-frame-rate operation suitable for wide-field GRB polarization surveys and offering a versatile platform for gas-based X-ray imaging applications.

Abstract

POLAR-2 is a next-generation space astronomy platform led by China, with its core scientific objective focused on high-precision polarization measurements of gamma-ray bursts. As one of its key payloads, the Low-energy Polarization Detector (LPD) is designed to perform wide-field surveys to capture X-ray polarization information from gamma-ray bursts in the 2$\sim$10 keV energy range. This paper presents Topmetal-L, a dedicated charge-sensitive pixel sensor developed for the LPD prototype upgrade. Fabricated in a 130 nm CMOS process in 2024, the chip integrates a 356 $\times$ 512 pixel array with a pixel pitch of 45 $μ$m. Each pixel incorporates a 26 $\times$ 26 $μ$m$^2$ charge-collecting window and is capable of simultaneously outputting both energy and position information of deposited charges. Topmetal-L has been systematically optimized for power consumption, noise performance, and readout efficiency. It exhibits an input dynamic range of 0$\sim$4 ke$^{-}$, a typical charge-to-voltage conversion gain of 76.04 $μ$V/e$^{-}$, an average equivalent noise charge of approximately 22.8 e$^{-}$, a sensitive area exceeding 3.69 cm$^2$, and a total power consumption of 720 mW per chip. To meet the requirements of large-area, high-frame-rate readout for gas-based polarization detectors, a sentinel readout scheme is proposed, reducing the full-frame readout time to 730 $μ$s. A prototype Topmetal-L-based gas polarization detection system was evaluated across key energies: it exhibited a residual modulation of 0.26% $\pm$ 0.45% at 5.90 keV, a modulation factor of 66.67% $\pm$ 0.45% for a linearly polarized 8.05 keV source, and a count rate saturated at 15 k counts$\cdot$cm$^{-2}$$\cdot$s$^{-1}$ when tested at 5.40 keV.

Figures (17)

• Figure 1: Schematic diagram of the GMPD detection principle.
• Figure 2: Optical photograph of Topmetal-L, showing the overall appearance and a detailed view of the pixel array.
• Figure 3: Overall architecture of the Topmetal-L.
• Figure 4: (a) Layout of a single pixel cell in the Topmetal-L; (b) Top-metal structure.
• Figure 5: Architecture of the analog channel in Topmetal-L.