A counting pixel chip and sensor system for X-ray imaging

Abstract

I<caults ohtaiiied with ii (photon) counting pixel irnagilig the thresholds adjust in cvery pixel leading to negligible tlircsliold variations, and tlic usc of ii linear fccdhack shift rcgistcr used a s a couiitcr 121 and resulting in a very compact chip layout.In this paper wc reporl on results iiow obtained with a cornplctc chip-sciisor photon detection system asseinblcd using the bump antl Ilip-chip tccliiiology at IZM Berlin 13 1, This tcchnology secins now to be generally rnastcrcd also for pitch valucs as low as 50 ptu~The sensor is a p+ in n sensor oL' 300 pin lliickncss wilh a 433.4 pin x 50 pm pixel size. 'The pixel geometry was clioscri to benclit from existing sensors already available at CPPM. Thc rcctaugular geoinelry choscn for high energy particle physics applications clearly is not optimal for X-ray imaging.'This dclicit will he changed in the next chip-sensor gcneratioii. The MPEC chip was also dcrivcd from chip dcvclopincnts within ATLAS 141. The analog part, for instance, is adopted froin Uie Bier&Pastis chip 151.Tlic linearity of the responsc function is the main at1v;intagc of counting systems coinparcd to integrating systems like e.g. X-ray film bascd methods, providing, at least priucipally, iin uiilirnitcd dynamic range. Linearity measurcinents as well as first irnagcs ohtaiued willi tlic present cliipsilicon scnsor system including studies of the spatial rcsolution arc described in the following sections. Figure 1: M P K pixel block diagram. 'I'lie pixel control logic is not shown.The charge sensitive mplilicr 161 is coiislructcd using ti folded cascodc stage with a 1.7 1F feedback capacitor auid an adaptive DC feeclhack scheme. It provides high detector leakage curreiit tolerancc (up to 100 nA) antl a high stability of tlic feedback loop. The fccdhack circuit discharges the fecdback capacitor will1 a nearly constillit current.The discriminator is AC-conplctl to the amplifier to decouple tlic threshold from DC voltage shifts of the aniplilicr. It has two thrcsliold controlling inputs. 01ic of tlicm is used to set the global tlircsliold, Uie otlicr is used to store a correction voltage on a capacitor ill cvcry pixel. This is achieved hy selecting Uic pixel with the coutrol shift register aiid can bc done during data acquisition. The range of possihlc Uircshold scttiugs reaches from 500 c to about 15000 e and llic cell to cell threshold variations can bc adjusted to stay below 35 e. The couiiter is realized as a linear feedback sliifi rcgistcr 121. It consists of a 15 hit shift rcgistcr with a single XNOR fecdback, produciug periodically ZN-I difkreiit states. Each state is directly rclatcd to Uic nuinbcr of clock pulses and can ' This work has I~ecii supported by (lie Ministerinin fiir Wissenscliaft und Vorscliung des 1 .antles Nortlrliciii-Wcs(~~ileii under conlracl iio.1V A5106 011 9X.

Trends in pixel detectors: tracking and imaging

“…9. The MPEC chip features 32 × 32 pixels (200×200µm 2 ), double threshold operation, 18-bit counting at ∼1 MHz per pixel as well as low noise values (∼120e with CdTe sensor) and threshold dispersion (21e after tuning) [30], [27]. A technical issue here is the bumping of individual die CdTe sensors which has been solved using Au-stud bumping with In-filling [31].…”

Pixel detectors for tracking and their spin-off in imaging applications

“…9(b). The MPEC chip features 32×32 pixels (200×200µm 2 ), double threshold operation, 18-bit counting at ∼1 MHz per pixel as well as low noise values (∼120e with CdTe sensor) and threshold dispersion (21e after tuning) [26,24]. A technical issue here is the bumping of individual die CdTe sensors which has been solved using Au-stud bumping with In-filling [27].…”

Single photon counting X-ray imaging with Si and CdTe single chip pixel detectors and multichip pixel modules

“…As Si is a well understood semiconductor and homogeneous large Si wafers are easily available, Si is the preferred sensor material for applications where a high flux of ionizing radiation is used, e.g. material science or protein crystallography [3]- [8].…”