ASIC Scoping Study Final Report

A three layer Compton telescope for dose monitoring in hadron therapy is under development at IFIC - Valencia. It will consist of three detector layers and each layer will be made of a continuous LaBr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> crystal coupled to four Silicon Photomultiplier (SiPM) arrays. A prototype made of two detector layers has already been assembled and tested. The detector layers consist of a 32×36×5 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> and 32×36×10 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> LaBr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> crystal, each coupled to four SiPM arrays. The front-end electronics of each detector is based on the VATA64HDR16 application specific integrated circuit (ASIC). The gamma rays coming from a <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">22</sup> Na source have been detected with the two detector layers working in time coincidence, allowing energy and position estimation.

We have designed FlexToT, a flexible application specific integrated circuit (ASIC) using time-over-threshold (ToT) techniques for the readout of silicon photomultiplier (SiPM) arrays in positron emission tomography (PET) detectors. The FlexToT ASIC accommodates 16 independent channels in a small die size (2.93 mm by 2.54 mm) with low power consumption (10 mW per channel), providing output signals whose duration is proportional to the current amplitude at its inputs. The threshold, gain and other parameters can be independently set for each channel in a flexible configuration that can be tailored to different applications and devices. Herein we present the results on the evaluation of the FlexToT ASIC operating as front-end electronics for SiPM arrays S11828-3344M from Hamamatsu (Japan) coupled to LYSO:Ce scintillators in a simple PET tomograph demonstrator. Each detector block is composed of a LYSO:Ce matrix coupled to 4 SiPM arrays in a 2×2 mosaic, totaling 64 pixels (8×8) which are read out by 4 FlexToT ASICs. Using this demonstrator we have obtained PET images of <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">22</sup> Na point sources with spatial resolutions better than 2 mm full width at half maximum (FWHM), validating our design of the FlexToT ASIC on the readout of SiPM matrices and scintillators in PET applications.

Pixel radiation detectors for X-rays and gammarays are commonly built by hybridization of sensor and readout application-specific integrated circuit (ASIC) chips using flip-chip bonding. A typical example is pixelated high-Z semiconductor sensor material (e.g. CdTe) bonded to a matching pixel array on an ASIC to form a hybrid module. Conventionally, the I/O connections of the ASIC for such hybrids are wire bonded to a Printed Circuit Board (PCB) which subsequently is connected to a data acquisition system. The wire bond pads and PCB connections require a considerable amount of the module's footprint and usually protrude at one side of the detector module by approximately 1.5mm. The disadvantage of this practice is that those modules are only suitable for close proximity tiling modules along three sides of the detector (3-side buttable) to form larger sensor areas. In recent years Through-Silicon Via (TSV) technology was introduced to overcome this disadvantage which permits the fabrication of potentially 4-side buttable modules. Commonly a TSV-last process step in the ASIC production is integrated. However, ASICs with TSV are expensive and the TSV-last process step is prone to failure of the TSV metallization. The consequences of the latter are missing I/O connections. Alternatively, 4-side buttable hybrid detector modules can be fabricated using interposers as pitch convertors. Here the sensor has a larger pixel-pitch than the ASIC and therefore the sensor covers a larger area than the ASIC. A Re-Distribution-Layer (RDL) on the interposer facilitates connections to the I/O of the ASIC. Therefore, ASIC and all other wire bonded connections can be placed under the sensor with all four sides of the sensor being the boundary of the full-sized module. This was demonstrated in the past with a conversion from 250um-pitch on the ASIC to a 500um-pitch on the sensor using a layered PCB. In order to achieve detectors with smaller sensor pixel pitch, the layout of pitch re-routing on the interposer has to be considerably reduced in size. As a consequence, the fabrication of such interposers becomes more difficult. This paper studies desirable designs and interposer materials and compares these with the limitations of current fine-pitch interposer fabrication methods and design rules for a pitch convertor interposer. In addition, the paper will also present necessary changes such as indium deposition on individual sensor die instead of wafer-level fabrication that have to be taken into consideration for sensor/ASIC hybridization of fine-pitch radiation detectors.

We present a low-power, low-noise prototype pixel readout application specific integrated circuit (ASIC) for hyperspectral enegy-resolving X-ray imaging detectors. The ASIC provides 16-by-16 channels to read out positive and negative charges from 16-by-16 heagonal silicon or CZT detector arrays, at a pitch size of $250 \mu \mathrm{m}$, to achieve good spatial resolution and the ability to record the energy of a detected photon as well as its position. The readout is done by bumpbonding the anodes to the inputs of the ASIC. Each channel of the ASIC provides low-noise charge amplification, high-order shaping with baseline stabilization, discrimination, extraction of amplitude (with neighbour channels), multiplexing, and dissipates $\sim0.6$ mW. A smart readout of the triggered pixel and its adjacent six pixels in the hexagonal configuration allows reconstruction of events with charge sharing correction, and can be used to estimate the depth of the photon interaction and to suppress background events. The target equivalent noise charge (ENC) is $\sim10$ electrons for silicon detector pixel and $\sim15$ electrons for CZT detector pixel.

We are developing an MR-compatiblepositron emission tomography (PET) detector system comprising silicon photomultipliers (SiPMs), and application specific integrated circuits (ASICs) mounted in the same printed circuit board (PCB). Since the heat generated from the ASICs can affect the performance of temperature-dependent SiPMs, a scalable PCB cooling method for stable and uniform SiPM performance was designed, developed and investigated.To cool the SiPM surface within the PCB, a thermal layer is added below the SiPM layer, which is then connected to the bottom layer through thermal vias for efficient heat conduction. Then, either a “cooling plate” or “square tubes” is attached to the bottom layers of multiple detector modules. A proof-of-concept thermal experiment was performed to test the feasibility of the PCB cooling method for SiPM-ASIC PET detector modules, then a 3D transient heat transfer simulations was performed to analyze the temperature profile on the SiPM surface of the more realistic PCB. The thermal experiments were performed using a prototype PCB with a 6 W heater strip mimicking the ASICs and 15° water flowing through the cooling structure; the plate which has more contact to the PCB resulted in 18.0° average temperature and 0.8° maximum temperature difference across the 5 cm width SiPM layer, while square tubes showed slightly worse results of 18.7° and 1.3°, respectively. In realistic thermal simulations, the square tubes with thermal vias showed deviation from the experimental results with 21° average temperature and 6.5° maximum temperature difference. In addition, the thermal gradient was significantly worse without the thermal vias. Considering the compact and simple geometry of the square tubes compared to the plate configuration, the former with thermal vias are an attractive choice to cool the SiPM-ASIC PET detector modules; ASICs can further optimize the minor non-uniform SiPM performance using digital calibrations.

Vertex detectors in high luminosity particle colliders are subject to high hit rates leading to high occupancies. The proposed Timing Vertex Detector (TVD) is a potential solution that requires a readout application specific integrated circuit (ASIC) that measures pulse arrival times with a timing resolution in the range of $100 fs$ or less. Present efforts are centered on the development of such an ASIC, called the RFpix. One of the critical requirementsis low added jitter. In this paper, we present results of the study on added jitter and jitterpropagation in inverter chains for $\mathrm{a130 nm}$ technology node. Furthermore, we proposea novel fully differential two-level delay-locked loop (DLL) circuit for the RFpix prototype with a simulated (layout parasitics included) worst case added jitter of $28 fs$.

The X-ray Astronomy and Observational Cosmology (XOC) group at Stanford University, in collaboration with the Massachusetts Institute of Technology (MIT) and MIT Lincoln Laboratory (MIT-LL), is developing next generation X-ray detector and readout technologies. Specifically, the XOC group is developing a fast, low noise readout application specific integrated circuit (ASIC) that is intended to be paired with X-ray charge-coupled device (CCD) detectors in development at MIT-LL. This readout ASIC is denoted as the MIT CCD Readout Chip or MCRC. The MCRC prototype is designed in a 350 nm technology node featuring 8 channels. Each channel is composed of two inputs. One is dedicated to read out the proven source follower-based (SF) CCD architecture with two selectable gain settings, an input referred noise of 1.63 e⁻_RMS, an input dynamic range of ±160 mV, channel to channel crosstalk less than -75 dBc, a power consumption of roughly 30 mW/channel, and a bandwidth of approximately 50 MHz, translating to an effective rise time of around 5 ns. Such a response can comfortably support readout speeds for large CCD pixel matrices in excess of 5 Mpixel/s/channel. The chip also features a second input which is an experimental drain-current readout (DR) topology to support Single electron Sensitive Read Out (SiSeRO) CCD stages currently in development at MIT-LL. The MCRC excels in speed while its input noise and bandwidth are on par with commercial discrete offerings, but with smaller power and real-estate footprints. Here we present the latest measurement results for this state-of-the-art prototype readout ASIC.

The readout electronics in time-of-flight positron emission tomography (TOF-PET) detector modules conventionally contain a separate timing circuit and a separate energy measurement circuit. For channel-by-channel readouts, the front-end electronics will be unacceptably large and must rely on application specific integrated circuit (ASIC) chips. In this paper, we present a novel field programmable gate array (FPGA)-based readout scheme that unifies the time and energy measurements with only one analog amplifier per channel in addition to the FPGA. By constructing a simple and effective fast linear discharge circuit, one time-to-digital converter (TDC) per channel can implement time and energy readouts simultaneously with a short measurement dead time. To evaluate the performance of the new approach, a 16-channel PET detector module consisting of LYSO crystals coupled with a silicon photomultiplier (SiPM) array was constructed connecting to the readout electronics. Using a reference detector with a 164.7-ps time resolution to perform 22Na coincidence measurements, the average energy, and time resolution among 16 channels were measured as 12.44% and 372.6 ps, respectively. The typical measurement dead time is approximately 300 ns. The test results show the simplicity and feasibility of the proposed readout scheme, which is particularly useful for laboratory instrumentation.

Performances of two depth-encoding positron emission tomography (PET) detectors with dual-ended readout based on silicon photomultiplier (SiPM) arrays with the same pitch size but different active areas were compared. The pitch sizes of the SiPM arrays were both 4.2 mm. One detector was based on 8 × 8 arrays of MicroFJ-30035 SiPMs, and the other one was based on 8 × 8 arrays of MicroFJ-40035 SiPMs. A 20 × 20 array of 1.42 × 1.42 × 20 mm3 polished lutetium-yttrium oxyorthosilicate (LYSO) elements was used to construct the detectors. The pitch size of the LYSO array was 1.5 mm and barium sulfate (BaSO4) with a thickness of 80 μm was used as reflector. The results showed that the detector based on the MicroFJ-40035 SiPMs provided better performance due to the larger active area. At the optimal bias voltage of 29.0 V for flood histogram and a temperature of 22.8̂C, the detector based on the MicroFJ-30035 SiPMs had a flood histogram quality of 3.84 ± 0.17, energy resolution of 12.7 ± 1.9%, signal-to-noise ratio of 102.6 ± 11.6, DOI resolution of 2.22 ± 0.51 mm and timing resolution of 978.3 ± 102.3 ps, and the detector based on the MicroFJ-40035 SiPMs had a flood histogram quality of 4.40 ± 0.25, energy resolution of 10.9 ± 1.5%, signal-to-noise ratio of 119.4 ± 10.5, DOI resolution of 2.01 ± 0.35 mm and timing resolution of 815.7 ± 86.4 ps.

A fully integrated 100 kHz X-ray charge coupled device (CCD) readout application specific integrated circuit (ASIC) employing delta sigma ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\Delta \Sigma $</tex-math></inline-formula> ) digitization is presented. To achieve high linearity with small chip size and low power consumption, the correlated double sampling (CDS) is realized by the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\Sigma \Delta{\rm ADC}$</tex-math></inline-formula> instead of the analog front end (AFE) as in conventional CCD readout circuits. Besides, the proposed decimation filter features simple structure and eases the integration. The chip is fabricated in <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$0.35\ \upmu\hbox{m}$</tex-math></inline-formula> CMOS technology and the measured integral nonlinearity (INL) throughout the input dynamic range of ASIC is 0.055% with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$35.1 \pm 0.3\ \upmu\hbox{V}$</tex-math></inline-formula> input referred noise. A CCD detection system is built and tested with the sensitivity of CCD being <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$4\ \upmu\hbox{V}/\hbox{e}^-$</tex-math></inline-formula> . The integration test results show that the readout noise is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$11.8\ \hbox{e}^ {-} $</tex-math></inline-formula> at 100 kHz readout pixel rate and the achieved energy spectrum resolution is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$168\ \hbox{eV} \pm 4.7\ \hbox{eV}$</tex-math></inline-formula> (Full Width at Half Maximum: FWHM) at 5.9 keV.

This paper reports a newly developed chip-level micro-electromechanical system (MEMS) accelerometer for tilt sensing based on differential array area-change capacitive displacement transducer and through-silicon-wafer etching process. The proposed MEMS accelerometer consists of a silicon-based acceleration-sensitive spring-mass structure, a upper glass cover plate and an universal capacitive readout application specific integrated circuit (ASIC) die, which is used to apply differential driving signals and pickup capacitance change due to area-variation under in-plane motion. The MEMS accelerometer has a linear measurement range of ±30° with a scale factor of 33.6 mV/°. The tilt measurement resolution of the MEMS accelerometer is evaluated by the modified Allan deviation with the results showing that an angle resolution of 0.003° at an integral time of 3 seconds. Compared with the commercial MEMS tilt sensors, the proposed MEMS accelerometer in this work has a larger full measurement range and a larger bandwidth but a little bit poor linearity. Hence, it is believed that the proposed chip-level MEMS accelerometer is promising to be a high-resolution tilt sensor with a lower power consumption and smaller volume.

High-resolution pixel detectors for second generation digital mammography

The Voxtel VX-798 is a prototype X-ray pixel array detector (PAD) featuring a silicon sensor photodiode array of 48 × 48 pixels, each 130 µm × 130 µm × 520 µm thick, coupled to a CMOS readout application specific integrated circuit (ASIC). The first synchrotron X-ray characterization of this detector is presented, and its ability to selectively count individual X-rays within two independent arrival time windows, a programmable energy range, and localized to a single pixel is demonstrated. During our first trial run at Argonne National Laboratory's Advance Photon Source, the detector achieved a 60 ns gating time and 700 eV full width at half-maximum energy resolution in agreement with design parameters. Each pixel of the PAD holds two independent digital counters, and the discriminator for X-ray energy features both an upper and lower threshold to window the energy of interest discarding unwanted background. This smart-pixel technology allows energy and time resolution to be set and optimized in software. It is found that the detector linearity follows an isolated dead-time model, implying that megahertz count rates should be possible in each pixel. Measurement of the line and point spread functions showed negligible spatial blurring. When combined with the timing structure of the synchrotron storage ring, it is demonstrated that the area detector can perform both picosecond time-resolved X-ray diffraction and fluorescence spectroscopy measurements.

We developed a novel high-resolution detector with intrinsic depth-of-interaction (DOI) correction for PET, based on a position-sensitive silicon photomultiplier (SiPM) array coupled to a monolithic scintillation crystal. The spatial distribution of the scintillation light measured by the SiPM array is used to estimate the entry point of the annihilation photon on the crystal front surface. This approach offers the benefit of intrinsic DOI correction. SiPM arrays furthermore enable integration of these detectors in a MRI device while offering faster response and much higher gain than APDs. Initial characterization of a detector prototype consisting of a 13.2 mm × 13.2 mm × 10 mm monolithic LYSO:Ce <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> scintillator and a 4 × 4 SiPM array shows that front surface readout (FSR) of the crystal results in significantly better performance than conventional back-surface readout (BSR). PSFs of ≪ 1.6 mm FWHM were measured at the centre of the detector (uncorrected for the ∼0.54 mm FWHM test beam diameter). Hardly any loss of spatial resolution was observed for angles of incidence of up to 45°. Energy resolutions of ∼12% were obtained. The timing resolution, measured in coincidence with a BaF <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> detector, equaled 960 ps FWHM.

In this paper, we present a single-chip 65 ×42 element ultrasonic pulse-echo fingerprint sensor with transmit (TX) beamforming based on piezoelectric micromachined ultrasonic transducers directly bonded to a CMOS readout application-specific integrated circuit (ASIC). The readout ASIC was realized in a standard 180-nm CMOS process with a 24-V high-voltage transistor option. Pulse-echo measurements are performed column-by-column in sequence using either one column or five columns to TX the ultrasonic pulse at 20 MHz. TX beamforming is used to focus the ultrasonic beam at the imaging plane where the finger is located, increasing the ultrasonic pressure and narrowing the 3-dB beamwidth to [Formula: see text], a factor of 6.4 narrower than nonbeamformed measurements. The surface of the sensor is coated with a poly-dimethylsiloxane (PDMS) layer to provide good acoustic impedance matching to skin. Scanning laser Doppler vibrometry of the PDMS surface was used to map the ultrasonic pressure field at the imaging surface, demonstrating the expected increase in pressure, and reduction in beamwidth. Imaging experiments were conducted using both PDMS phantoms and real fingerprints. The average image contrast is increased by a factor of 1.5 when beamforming is used.