Marek Gersbach

A 128

An imager for time-resolved optical sensing was fabricated in CMOS technology. The sensor comprises an array of 128times128 single-photon pixels, a bank of 32 time-to-digital-converters, and a 7.68 Gbps readout system. Thanks to the outstanding timing precision of single-photon avalanche diodes and the optimized measurement circuitry, a typical resolution of 97 ps was achieved within a range of 100 ns. To the best of our knowledge, this imager is the first fully integrated system for photon time-of-arrival evaluation. Applications include 3-D imaging, optical rangefinding, fast fluorescence lifetime imaging, imaging of extremely fast phenomena, and, more generally, imaging based on time-correlated single photon counting. When operated as an optical rangefinder, this design has enabled us to reconstruct 3-D scenes with milimetric precisions in extremely low signal exposure. A laser source was used to illuminate the scene up to 3.75 m with an average power of 1 mW, a field-of-view of 5deg and under 150 lux of constant background light. Accurate distance measurements were repeatedly achieved based on a short integration time of 50 ms even when signal photon count rates as low as a few hundred photons per second were available.

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We report on the first implementation of a single photon avalanche diode (SPAD) in 130 nm complementary metal-oxide-semiconductor (CMOS) technology. The SPAD is fabricated as p+/n-well junction with octagonal shape. A guard ring of p-well around the p+ anode is used to prevent premature discharge. To investigate the dynamics of the new device, both active and passive quenching methods have been used. Single photon detection is achieved by sensing the avalanche using a fast comparator. The SPAD exhibits a maximum photon detection probability of 41% and a typical dark count rate of 100 kHz at room temperature. Thanks to its timing resolution of 144 ps full-width at half-maximum (FWHM), the SPAD has several uses in disparate disciplines, including medical imaging, 3D vision, biophotonics, low-light illumination imaging, etc.

128 Single-Photon Image Sensor With Column-Level 10-Bit Time-to-Digital Converter Array

We report on the design and characterization of a novel time-resolved image sensor fabricated in a 130 nm CMOS process. Each pixel within the 3232 pixel array contains a low-noise single-photon detector and a high-precision time-to-digital converter (TDC). The 10-bit TDC exhibits a timing resolution of 119 ps with a timing uniformity across the entire array of less than 2 LSBs. The differential non-linearity (DNL) and integral non-linearity (INL) were measured at ±0.4 and ±1.2 LSBs, respectively. The pixel array was fabricated with a pitch of 50 μm in both directions and with a total TDC area of less than 2000 μm2. The target application for this sensor is time-resolved imaging, in particular fluorescence lifetime imaging microscopy and 3D imaging. The characterization shows the suitability of the proposed sensor technology for these applications.

A Single Photon Avalanche Diode Implemented in 130-nm CMOS Technology

A Time-to-Amplitude Converter (TAC) with embedded analog-to-digital conversion is implemented in a 130-nm CMOS imaging technology. The proposed module is conceived for Single-Photon Avalanche Diode imagers and can operate both as a TAC or as an analog counter, thus allowing both time-correlated or time-uncorrelated imaging operation. A single-ramp, 8-bit ADC with two memory banks to allow high-speed, time-interleaved operation is also included within each module. A 32x32-TACs array has been fabricated with a 50-µm pitch in order prove the highly parallel operation and to test uniformity and power consumption issues. The measured time resolution (LSB) is of 160 ps on a 20-ns time range with a uniformity across the array within ±2LSBs, while DNL and INL are 0.7LSB and 1.9LSB respectively. The average power consumption is below 300µW/pixel when running at 500k measurements per second.

A Time-Resolved, Low-Noise Single-Photon Image Sensor Fabricated in Deep-Submicron CMOS Technology

We report the design and characterisation of a 32×32 time to digital (TDC) converter plus single photon avalanche diode (SPAD) pixel array implemented in a 130nm imaging process. Based on a gated ring oscillator approach, the 10 bit, 50µm pitch TDC array exhibits a minimum time resolution of 50ps, with accuracy of ±0.5 LSB DNL and 2.4 LSB INL. Process, voltage and temperature compensation (PVT) is achieved by locking the array to a stable external clock. The resulting time correlated pixel array is a viable candidate for single photon counting (TCSPC) applications such as fluorescent lifetime imaging microscopy (FLIM), nuclear or 3D imaging and permits scaling to larger array formats.

A 32x32-pixel array with in-pixel photon counting and arrival time measurement in the analog domain

We present an array of 128times128 highly miniaturized SPAD (single-photon avalanche diodes) pixels with a bank of 32 time-to-digital converters (TDCs) on chip. A decoder selects a 128-pixel row. Every group of 4 pixels in the row shares a TDC based on an event-driven mechanism. As a result, row-wise parallel acquisition is obtained with a low number of TDCs. Because of the outstanding timing precision of SPADs and an optimized TDC design, a typical resolution of 97 ps is achieved within a range of 100 ns (10 b) at a maximum rate of 10 MS/s per TDC. The TDC bank exhibits a DNL of 0.08LSB and an INL of 1.89LSB.

A 32×32 50ps resolution 10 bit time to digital converter array in 130nm CMOS for time correlated imaging

We report on the design and characterization of a 32 × 32 time-to-digital converter (TDC) array implemented in a 130 nm imaging CMOS technology. The 10-bit TDCs exhibit a timing resolution of 119 ps with a timing uniformity across the entire array of less than 2 LSBs. The differential- and integral non-linearity (DNL and INL) were measured at ± 0.4 and ±1.2 LSBs respectively. The TDC array was fabricated with a pitch of 50µm in both directions and with a total TDC area of less than 2000µm2. The characteristics of the array make it an excellent candidate for in-pixel TDC in time-resolved imagers for applications such as 3-D imaging and fluorescence lifetime imaging microscopy (FLIM).

A 128×128 Single-Photon Imager with on-Chip Column-Level 10b Time-to-Digital Converter Array Capable of 97ps Resolution

Image sensors capable of resolving the time-of-arrival (ToA) of individual photons with high resolution are needed in several applications, such as fluorescence lifetime imaging microscopy (FLIM), Förster resonance energy transfer (FRET), optical rangefinding, and positron emission tomography. In FRET, for example, typical fluorescence lifetime is of the order of 100 to 300ps, thus deep-subnanosecond resolutions are needed in the instrument response function (IRF). This in turn requires new time-resolved image sensors with better time resolution, increased throughput, and lower costs. Solid-state avalanche photodiodes operated in Geiger-mode, or single-photon avalanche diodes (SPADs), have existed for decades [1] but only recently have SPADs been integrated in CMOS. However, as array sizes have grown, the readout bottleneck has also become evident, leading to hybrid designs or more integration and more parallelism on-chip [2,3]. This trend has accelerated with the introduction of SPAD devices in deep-submicron CMOS, that have enabled the design of massively parallel arrays where the entire photon detection and ToA circuitry is integrated on-pixel [4,5].

A parallel 32×32 time-to-digital converter array fabricated in a 130 nm imaging CMOS technology

We report on a new single photon avalanche diode (SPAD) fabricated in a 130 nm CMOS imaging process. A novel circular structure combining shallow trench isolation (STI) and a passivation implant creates an effective guard ring against premature edge breakdown. Thanks to this guard ring, unprecedented levels of miniaturization may be reached at no cost of added noise, decreased sensitivity or timing resolution. The detector integrated along with quenching and read out electronics was fully characterized. Optical measurements show the effectiveness of the guard ring and the high degree of electric field planarity across the sensitive region of the detector. With a photon detection probability of up to 30% and a timing jitter of 125 ps at full-width-half-maximum this SPAD is well suited for applications such as 3-D imaging, fluorescence lifetime imaging and biophotonics.

A 160×128 single-photon image sensor with on-pixel 55ps 10b time-to-digital converter

The fluorescence decay of high-affinity nonratiometric Ca2+ indicator Oregon Green BAPTA-1 (OGB-1) is analyzed with unprecedented temporal resolution in the two-photon excitation regime. A triple exponential decay is shown to best fit the fluorescence dynamics of OGB-1. We provide a model for accurate measurements of the free Ca2+ concentration and dissociation constants of nonratiometric calcium indicators.