High Frame-rate TCSPC-FLIM Using a Novel SPAD-based Image Sensor

Imaging techniques based on time-correlated single photon counting (TCSPC), such as fluorescence lifetime imaging microscopy (FLIM), rely on fast single-photon detectors as well as timing electronics in the form of time-to-digital or time-to-analog converters. Conventional systems rely on stand-alone or small arrays (up to 32) of detectors and external timing and memory modules. We recently developed a fully integrated image sensor containing 32x32 pixels and fabricated in a 130 nm CMOS technology. The chip produces an overall data rate of 10Gb/s in terms of time-of-arrival measurements in each pixel. As opposed to conventional single detector FLIM systems, the present system can acquire a full image, albeit at low resolution, without the need of an optical scanning system. As a consequence the complexity of the optical setup is reduced and the acquisition speed is dramatically increased. We show the potential of this new technology by presenting high time resolution (119 ps) TCSPC-FLIM images of pollen grains with acquisition times as low as 69 ms. Furthermore, the low noise (similar to 100 Hz) and high photon detection probability (up to 35%) ensure a good photon economy over the visible spectrum. We believe that this technology will open the way to fast TCSPC-FLIM recordings of transient signals in the bio- and life sciences, such as in neuron signaling.

Single-photon detector arrays for time-resolved fluorescence imaging

Marek Gersbach

Time-correlated single photon counting techniques have recently established themselves in numerous fields of research. In particular, for fluorescence lifetime imaging, time-correlated single photon counting is often considered the absolute reference in terms of accuracy and image quality. However, the lack of significant detector arrays capable of single photon sensitivity and precise photon timing with sub-nanosecond resolution has imposed the use of optical scanning to acquire fluorescence lifetime images. As a consequence, the acquisition times are often prohibitively long and the image acquisition of fast phenomena or the rendering of live and moving objects very difficult to achieve. This thesis presents a new type of detector array capable of single photon sensitivity, low noise and accurate photon timing. The core detector technology is based on single photon avalanche diode structures. Given both the complexity of achieving sub-nanosecond time resolution on many parallel pixels and the large amount of data time-resolved imagers produce, the use of advanced electronic fabrication process is inevitable. Thus the design of high performance single photon detectors in deep-submicron (130 nm) CMOS is investigated, leading to excellent photon detection probability (up to 15 – 35 %, depending on bias and wavelength), low noise (a few tens of Hertz) and a photon timing jitter of 125 ps. The fabrication of these detectors has opened the way to the integration of complex electronics on pixel level. A time discriminator implemented as a compact time-to-digital converter was designed relying on a two-level interpolator approach. A global clock provides a stable, coarse resolution while an in-pixel delay line allows for a finer time resolution of 119 ps. The combination of single photon detector and time discriminator, with the addition of some read-out and glue electronics, enabled the fabricating of a small (50 × 50 µm), scalable pixel for integration in a larger array. An array consisting of 32 × 32 pixels was implemented and thoroughly characterized. With a timing mismatch of only a few tens of picoseconds across the entire array, a low noise count and good sensitivity, the array is well suited for various time-correlated applications, and FLIM in particular. The large data rate (up to 10.24 Gb/s) of the 1024 parallel acquisition channels is handled by a motherboard with significant computing power in the form of two FPGA's. An on-board data compression scheme is implemented by creating a histogram of photon arrival times for each individual pixel within the array, allowing for an efficient data transfer out of the motherboard. Finally, the suitability and efficiency of the proposed detector array for fluorescence lifetime imaging is shown with a wide variety of measurements, from high time-resolution single detector measurements to wide-field fluorescence lifetime image acquisition. Fluorescence lifetimes were resolved with high accuracy for very short acquisition times, down to a few hundreds of milliseconds for the parallel acquisition on all pixels.

EPFL2009

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

Edoardo Charbon, Matthew Fishburn, Marek Gersbach, John Richard Walker

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 32x32 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 mu m in both directions and with a total TDC area of less than 2000 mu m(2). 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.

2012

Single-photon image sensors in CMOS

Cristiano Niclass

Among advanced electronic designs, vision systems are perhaps those that have undergone the most dramatic technological revolutions of the last decades. Recently, a new family of vision systems has emerged aimed at three-dimensional (3D) imaging. Many applications exist or will soon come to existence that can use 3D image sensors and the image sensor community is responding with conventional and unconventional solutions. This thesis presents a new concept for imaging and, in particular, for 3D imaging, based on single-photon detection. In our approach, single-photon detection is performed by a device known as single-photon avalanche diode (SPAD). Large arrays of SPADs were demonstrated for the first time in this thesis and time resolutions consistently in the picosecond range were achieved. It has thus become feasible to design solid-state 3D imagers with millimeter depth perception based on the time-of-flight principle. Initially, large arrays of SPAD devices and associated circuitry have been investigated in a 0.8µm CMOS technology. Extremely low-noise detectors were implemented in this technology. In arrays of 64x48 pixels, the median dark count rate was 0.5Hz per square micrometer of active area. For these devices, the maximum photon detection probability was 26% at 550nm optical wavelength. Subsequently, a technology migration of SPADs towards deep-submicron CMOS has been successfully achieved. For the first time, SPADs in 0.35µm and 0.13µm CMOS technologies were designed and characterized. Outstanding photon detection probabilities up to 35.4% and 34.5% have been measured in devices manufactured in 0.35µm and 0.13µm CMOS, respectively. Median dark count rate has increased with technology scaling. The median dark count rate over an array of 128x128 pixels was 18 Hz/µm2 in 0.35µm CMOS, whereas it was 385Hz/µm2 in 0.13µm CMOS for a single device. With the aim of enabling high-performance system-on-a-chip implementations using single-photon detectors, appropriate front-end and ancillary circuits utilized in the detection, measurement, and storage of time-of-flight evaluations have been introduced. For the first time, passive quenching circuits using a single MOS transistor were investigated. Based on the proposed biasing regime, the single MOS quenching circuit provides well defined dead time and leads to high pixel fill factor. Active recharge is achieved in this thesis by means of a new dual-threshold quenching and recharge circuit. This circuit allows the implementation of a hold-off time using low in-pixel transistor count, thus improving afterpulsing probability with little or no penalty in fill factor. 3D image sensors based on a pulsed detection technique known as time-correlated single-photon counting (TCSPC) have been investigated. Moreover, a technique has also been investigated for range imaging using SPADs that operate with continuously modulated optical signals. This technique, known as single-photon synchronous detection (SPSD), was invented in the course of this thesis and it is introduced here for the first time. In order to demonstrate the potential of TCSPC in solid-state 3D imaging, a complete theoretical and experimental investigation was conducted. An analytical model for the evaluation of ranging performance based on TCSPC was introduced. A fully-integrated TCSPC system for single-photon time-of-flight evaluation was implemented in CMOS, for the first time. Depth maps of 3D scenes were achieved at millimeter precisions using only 1mW of laser power and an integration time of 50ms. Thanks to SPAD technology, accurate distance measurements are now possible even with extremely low photon count rates of a few thousand per second. The maximum non-linearity in distance measurement was 9 millimeters over the full measurement range. Time-varying uncertainty (1σ) at the farthest distance was 5.2 millimeters. The SPSD approach has been theoretically and experimentally investigated. The design of the first fully-parallel implementation of a single-photon image sensor in CMOS has been introduced. The sensor has enabled the acquisition of real-time 3D images, based on CMOS SPADs, for the first time. A 3D camera prototype was designed and built based on the SPSD image sensor with a field-of-view of 50°. Experimental results showed that the SPSD rangefinder is effective. Distance measurement performance was characterized with a maximum non-linearity error of 11cm within a range of a few meters. In the same range, the maximum repeatability error was 3.8cm.