Medipix4 is the latest member in the Medipix/Timepix family of pixel detector chips aimed at high rate spectroscopic X-ray imaging using high-Z materials. The chip address the limitations of conventional hybrid pixel detectors for X-ray imaging. Its predecessor, Medipix3RX, covered some of those limitations and demonstrated the possibility of spectroscopic X-ray imaging at a fine pitch while keeping the spectral fidelity using a charge-sharing correction algorithm. However, its use in medical imaging, synchrotron applications, material analysis, and other applications highlighted some limitations. Indeed, the 3-side buttable architecture in Medipix3RX and other actual X-ray imaging systems introduces a dead zone in the imaging that closes the door to constructing large-area detectors. Moreover, the improvement in the dynamic energy range, the count-rate capability, and the energy resolution will benefit those applications.
This thesis describes the Medipix4 chip implementation and discusses the proposed new pulse processing electronics in the analog pixel. The readout architecture relies on single photon counting with charge sharing correction for the energy binning of incoming hits. The chip consists of 320 x 320 pixels of 75 µm x 75 µm. It can work in Fine Pitch Mode (FPM) with 75 µm pixel pitch and two threshold bins per pixel or in Spectroscopic Mode (SM) with 150 µm pitch and up to eight energy threshold bins. Unlike its predecessor, Medipix3RX, it will be possible to tile the ASIC fully in both x and y directions, permitting seamless large area coverage. The chip size is 24 mm x 24 mm and covers 99.37% active area when using TSV connections only.
The ASIC is designed in a commercial CMOS 130 nm process technology with a power supply of 1.2 V. The new analog front-end architecture improves the energy dynamic range, the count-rate capability, and the energy resolution compared with Medipix3RX while the charge sharing correction is still supported. Those improvements come at the expense of power consumption and spatial resolution. The latter should not be a problem since studies have shown that the optimal pixel pitch for CdTe or CdZnTe should be slightly larger than the Medipix3RX pixel in order to account for a larger fraction of fluorescence photons.
Each analog pixel contains a Charge Sensitive Amplifier with a DC leakage compensation network up to 50 nA. Two pulse-shaping circuits in the second stage implement the charge sharing correction mode. The new shaper amplifier has a reduced baseline drift at high flux compared to the amplifier implemented in the previous Medipix/Timepix chips. The implemented ASIC has three modes of operation: High Dynamic Range Mode (HDRM), Low Noise Mode (LNM), and Ultra-Fast Mode (UFM). In HDRM, the chip can process X-ray photons with energies up to 154 keV with a CdTe sensor, implying 40% improvement compared to Medipix3RX. In LNM, the expected energy resolution has been improved by 55 %. In UFM, the post-layout simulated count-rate capability of the front-end is 19 x 10^6 photons.mm^(-2).s^(-1) at 10% hit loss for a 150 µm pixel pitch and not affected by charge sharing effect, showing an improvement by a factor of 5. In addition, the pixel includes a digital pile-up filtering method that improves spectral fidelity at high rates.