F. Zappa

AVALANCHE PHOTODIODES AND QUENCHING CIRCUITS FOR SINGLE-PHOTON DETECTION

Avalanche photodiodes, which operate above the breakdown voltage in Geiger mode connected with avalanche-quenching circuits, can be used to detect single photons and are therefore called singlephoton avalanche diodes SPADs. Circuit configurations suitable for this operation mode are critically analyzed and their relative merits in photon counting and timing applications are assessed. Simple passive-quenching circuits (PQCs), which are useful for SPAD device testing and selection, have fairly limited application. Suitably designed active-quenching circuits (AQCs) make it possible to exploit the best performance of SPADs. Thick silicon SPADs that operate at high voltages (250-450 V) have photon detection efficiency higher than 50% from 540- to 850-nm wavelength and still ~3% at 1064 nm. Thin silicon SPADs that operate at low voltages (10-50 V) have 45% efficiency at 500 nm, declining to 10% at 830 nm and to as little as 0.1% at 1064 nm. The time resolution achieved in photon timing is 20 ps FWHM with thin SPADs; it ranges from 350 to 150 ps FWHM with thick SPADs. The achieved minimum counting dead time and maximum counting rate are 40 ns and 10 Mcps with thick silicon SPADs, 10 ns and 40 Mcps with thin SPADs. Germanium and III-V compound semiconductor SPADs extend the range of photon-counting techniques in the near-infrared region to at least 1600-nm wavelength.

Silicon planar technology for single-photon optical detectors

Design and fabrication of single photon avalanche detector (SPAD) in planar technology is reported. Device design and critical issues in the technology are discussed. Experimental test procedures are described for dark-counting rate, afterpulsing probability, photon timing resolution, and quantum detection efficiency. Low-noise detectors are obtained, with dark counting rates down to 10 c/s for devices with 10 /spl mu/m diameter, down to 1 kc/s for 50 /spl mu/m diameter. The technology is suitable for monolithic integration of SPAD detectors and associated circuits.

SPAD Smart Pixel for Time-of-Flight and Time-Correlated Single-Photon Counting Measurements

We present a smart pixel based on a single-photon avalanche diode (SPAD) for advanced time-of-flight (TOF) and time-correlated single photon counting (TCSPC) applications, fabricated in a cost-effective 0.35- m CMOS technology. The large CMOS detector (30- m active area diameter) shows very low noise (12 counts per second at room temperature at 5-V excess bias) and high efficiency in a wide wavelength range (about 50% at 410 nm and still 5% at 800 nm). The analog front-end electronics promptly senses and quenches the avalanche, thus leading to an almost negligible afterpulsing effect. The in-pixel 10-bit time-to-digital converter (TDC) provides 312-ps resolution and 320-ns full-scale range (FSR), i.e., 10-cm single-shot spatial resolution within 50-m depth range in a TOF system. The in-pixel 10-bit memory and output buffers make this smart pixel the viable building block for advanced single-photon imager arrays for 3-D depth ranging in safety and security applications and for 2-D fluorescence lifetime decays in biomedical imaging.

Time-resolved diffuse optical spectroscopy up to 1700 nm by means of a time-gated InGaAs/InP single-photon avalanche diode.

We present a new compact system for time-domain diffuse optical spectroscopy of highly scattering media operating in the wavelength range from 1100 nm to 1700 nm. So far, this technique has been exploited mostly up to 1100 nm: we extended the spectral range by means of a pulsed supercontinuum light source at a high repetition rate, a prism to spectrally disperse the radiation, and a time-gated InGaAs/InP single-photon avalanche diode working up to 1700 nm. A time-correlated single-photon counting board was used as processing electronics. The system is characterized by linear behavior up to absorption values of about 3.4 cm−1 where the relative error is 17%. A first measurement performed on lipids is presented: the absorption spectrum shows three major peaks at 1200 nm, 1400 nm, and 1700 nm.

Afterpulse-like noise limits dynamic range in time-gated applications of thin-junction silicon single-photon avalanche diode

We describe a source of noise in thin-junction silicon single-photon avalanche diode arising after strong illumination either during the ON (voltage above breakdown) or the OFF (voltage below breakdown) time. It increases the background noise with respect to primary dark count rate similarly to the afterpulsing process, but it is not related to a previous detector ignition. The amount of noise is linearly dependent on the power of light impinging on the detector and time constants are independent of the electric field. This phenomenon is the main limiting factor for the dynamic-range during time-gated measurements in condition of strong illumination.

CMOS Circuit Analysis with Luminescence Measurements and Simulations

Hot-carriers in MOSFETs are responsible for timedependent near-infrared emission, synchronous with the switching transitions in CMOS circuits. Fast electrical waveforms propagating through integrated circuits can be effectively measured by means of high sensitivity solid-state photodetectors with sharp time-resolution. Thanks to a time jitter of less than 30ps, we obtained an equivalent analog bandwidth of about 30GHz. We developed a photoemission model in SPICE in order to simulate the luminescence waveforms. By comparing measured and simulated optical waveforms, in-depth insight of circuit behavior is reported, leading to a powerful identification of defects and failures.

Modeling and Probing Hot-Carrier Luminescence From MOSFETs

We present an extended modeling for MOSFETs that time-dependently predicts the spontaneous photon emission due to hot carriers. Such very faint luminescence waveforms (less than one photon for every 106 switching events) can be measured by means of high-sensitivity single-photon detectors, where time jitter is lower than 30 ps. Thanks to the model, which runs in most computer-aided-design circuital simulators, it is possible to cross-check measurements with simulations in order to quickly identify either design and fabrication errors or mismatches due to parasitism, interconnections, etc. Moreover, we demonstrate that the proposed modeling is a powerful investigation tool not only for digital circuits but also for analog ICs.

On-chip probes for silicon defectivity ranking and mapping

We present process probes useful to investigate the process-dependent quality of p-n junctions in semiconductors. The probes are sensitive to the presence of thermal generation centers, which ignite macroscopic current avalanches. Since the carrier generation events are promoted by the presence of localized imperfections such as dislocations, stacking faults, etc., the avalanche ignition rate represents a suitable figure of merit for ranking the overall process cleanliness. In particular, by using these probes we report a nonuniform distribution of lattice defects within certain junctions. This phenomenon has been verified by means of standard etching and infrared optical inspection. Some technological hints are finally provided, capable of reducing the defectivity and improving the fabrication of microelectronic devices.

0.16 µm BCD single-photon avalanche diode with 30 ps timing jitter, high detection efficiency and low noise

CMOS SPADs are nowadays an established imaging technology for applications requiring single-photon sensitivity in a compact form-factor (e.g. three-dimensional LIDAR imaging and fluorescence lifetime FLIM microscopy). However, we aimed at further enhance overall SPAD performances, by exploiting smart power technologies, such as the BCD (Bipolar-CMOS-DMOS) one. We achieved the present state-of-the-art SPADs fabricated in the 0.16 μm BCD technology by STMicroelectronics, attaining >60% photon detection efficiency at 500 nm, dark count rate density < 0.2 cps/μm2, and less than 30 ps FWHM timing jitter.

Optical spectroscopy in the time-domain beyond 1.1 μm: A tool for the characterization of diffusive media

Summary form only given. Time-resolved diffuse optical spectroscopy (TRS) is a valuable technique for the characterization of a variety of diffusive media like biological tissues, fruit, wood and pharmaceutical tablets. The technique is based on the injection of a short pulse (~ps) on the surface of the sample. The time-of-flight distribution (TOF) of photons detected on a different location of the surface gives information about the absorption and scattering probabilities [1]. Due to the widespread application of the technique to biological tissues, where there is a low light attenuation in the 0.6-1.1 μm range, the range beyond 1.1 μm is relatively unexplored with time-resolved techniques, also because of the difficulty to combine mode-locked continuously tunable sources with detectors having a sensitivity down to the single-photon level. Nevertheless beyond 1.1 μm there are organic compounds that contribute to interesting spectral structures like collagen, lipids, hydroxyapatite, starch, glucose and lignin. In this work we explore the application of TRS beyond 1.1 μm to the characterization of a variety of samples.