3D Photon-to-Digital Converter for Radiation Instrumentation: Motivation and Future Works

Abstract

Analog and digital SiPMs have revolutionized the field of radiation instrumentation by replacing both avalanche photodiodes and photomultiplier tubes in many applications. However, multiple applications require greater performance than the current SiPMs are capable of, for example timing resolution for time-of-flight positron emission tomography and time-of-flight computed tomography, and mitigation of the large output capacitance of SiPM array for large-scale time projection chambers for liquid argon and liquid xenon experiments. In this contribution, the case will be made that 3D photon-to-digital converters, also known as 3D digital SiPMs, have a potentially superior performance over analog and 2D digital SiPMs. A review of 3D photon-to-digital converters is presented along with various applications where they can make a difference, such as time-of-flight medical imaging systems and low-background experiments in noble liquids. Finally, a review of the key design choices that must be made to obtain an optimized 3D photon-to-digital converter for radiation instrumentation, more specifically the single-photon avalanche diode array, the CMOS technology, the quenching circuit, the time-to-digital converter, the digital signal processing and the system level integration, are discussed in detail.

Keywords: 3D heterogeneous integration; 3D photon-to-digital converter; SPAD array; SiPM; digital SiPM; liquid argon; liquid xenon; positron emission tomography; silicon photomultiplier; single-photon avalanche diode; time-of-flight.

Figure 1

Schematic of an analog SiPM.

Figure 2

Schematic of a PDC. A CMOS microelectronic circuit is used to quench 1-to-1 a given SPAD.

Figure 3

Cross-section of a CMOS SPAD. Shown here is a frontside illuminated p⁺ in n-well SPAD. The inset shows a uniform high electric field region under the whole sensitive area of the SPAD achieved by optimization of the guard-ring geometry and doping level.

Figure 4

Illustration of the trade-off between the SPAD and the electronic functionality for 2D PDCs sharing the same technology node compared to a 3D PDC. In (a), a 2D PDC with large SPAD (blue), but limited in-pixel electronics functionalities (yellow). In (b), a 2D PDC with small SPAD (blue), but greater in-pixel electronic functionalities (yellow). In (c), a 3D PDC with large SPAD (blue) and large area for in-pixel electronic functionality (yellow).

Figure 5

3D PDC used as a charged particle detector. (Illustration courtesy of Inge Diehl, DESY).

Figure 6

Cross-section of a 3D-PDC with SPAD on both layer for DCR mitigation using coincidence detection. (Illustration courtesy of Lucio Pancheri, University of Trento).

Figure 7

SEM image of Sherbrooke’s first 3D PDC.

Figure 8

Block diagram of the front-end quenching circuit. A monostable circuit is used to control the hold-off (for afterpulsing mitigation). Also shown, the current source which is triggered when an avalanche is detected. The analog sum of each precise current source is made by a transimpedance amplifier on the printed circuit board. As shown, the current pulse width and height can be tailored.

Figure 9

Measurement of the analog sum output of the 3D PDC. Each step represents a triggered SPAD. The current pulse width and height can be tailored.

Figure 10

Illustration of typical backside and frontside illuminated SPADs including the different locations of the absorption/drift region, the avalanche region and a simplified electric field. The blue and red sketch lines roughly represent the typical light absorption distribution for short (UV-blue) and long (red-NIR) wavelengths, respectively.

Figure 11

Photon penetration depth in silicon as a function of the wavelength (data taken from [111]). The minimum penetration of 4.2 nm is at a wavelength of 285 nm. The wavelengths for LXe (175 nm) and LAr (125 nm) are marked.

Figure 12

Simulation of the energy bands with and without delta-doping placed at the SPAD surface. Electrons generated past the delta-doping barrier drift toward the p⁺n junction. Only electrons absorbed too close to the surface are lost. Electrons in interface states are trapped at the surface, further reducing the SPAD dark noise.

Figure 13

Block diagram of a 3D PDC dedicated to PET. (1) The SPAD, QC, TDC and counter array; (2) An array readout to send the information to the digital signal processing; (3) The digital signal processing with TDC uniformity correction, timestamp sorting and filtering and a time estimator based on multiple timestamps.

Figure 14

Block diagram of a 3D PDC dedicated to LAr and LXe. (1) The SPAD and QC; (2) A flag is set as soon as a photon is detected on any SPAD; (3) The acquisition module saves the actual count of triggered SPADs; (4) The FIFO saves the number of counts as a function of time; (5) The transmitter sends the counter’s value to the acquisition system; (6) The analog current sum is proportional to the number of detected photons, as shown in Figure 9.

Figure 15

In the middle, the readout ASIC with an array of QC. On both sides, strips of various flavors of SPAD under study during fabrication process optimization.