Thin Film On ASIC Research Articles

High Spatial Resolution of Thin-Film-on-ASIC Particle Detectors

Thin-film-on-ASIC (TFA) detectors are monolithic pixel devices that consist of amorphous silicon detecting diodes directly deposited on readout electronics. This paper presents a characterization of the TFA spatial resolution using the electron-beam-induced current (EBIC) technique, in which pixel pads patterned in microstrips were swept by the beam. We measured the spatial resolution for different configurations and thicknesses of the TFA active layer with different beam energies, currents and sweep speeds. We observed that the generated electron-hole pairs are collected most efficiently when the beam is over the microstrips. This better collection efficiency gives a larger signal than off the strips, and thereby enabled us to distinguish strips as small as 0.6 μm wide which are spaced by 1.4 μm gaps. This high spatial resolution was obtained even though microvoids in the amorphous silicon layer-induced by an ASIC morphology as rough as 2 μm-were observed in the detector cross section, thus demonstrating the potential of the TFA architecture even with non-planar readout electronics.

Hydrogenated Amorphous Silicon Sensor Deposited on Integrated Circuit for Radiation Detection

Radiation detectors based on the deposition of a 10 to 30 mum thick hydrogenated amorphous silicon (a-Si:H) sensor directly on top of integrated circuits have been developed. The performance of this detector technology has been assessed for the first time in the context of particle detectors. Three different circuits were designed in a quarter micron CMOS technology for these studies. The so-called TFA (Thin-Film on ASIC) detectors obtained after deposition of a-Si:H sensors on the developed circuits are presented. High internal electric fields (104 to 105 V/cm) can be built in the a-Si:H sensor and overcome the low mobility of electrons and holes in this amorphous material. However, the deposited sensor's leakage current at such fields turns out to be an important parameter which limits the performance of a TFA detector. Its detailed study is presented as well as the detector's pixel segmentation. Signal induction by generated free carrier motion in the a-Si:H sensor has been characterized using a 660 nm pulsed laser. Results obtained with a TFA detector based on an ASIC integrating 5 ns peaking time pre-amplifiers are presented. Direct detection of 10 to 50 keV electrons and 5.9 keV X-rays with the detectors are then shown to understand the potential and the limitations of this technology for radiation detection.

TFA pixel sensor technology for vertex detectors

Pixel microvertex detectors at the SLHC and a future linear collider face very challenging issues: extreme radiation hardness, cooling design, interconnections density and fabrication cost. As an alternative approach we present a novel pixel detector based on the deposition of a Hydrogenated Amorphous Silicon (a-Si:H) film on top of a readout ASIC. The Thin-Film on ASIC (TFA) technology is inspired by an emerging microelectronic technology envisaged for visible light Active Pixel Sensor (APS) devices. We present results obtained with a-Si:H sensor films deposited on a glass substrate and on ASIC, including the radiation hardness of this material up to a fluence of 3.5×10 15 p/cm 2.

Thin Film on ASIC (TFA) - A Technology for Advanced Image Sensor Applications

AbstractThanks to its three-dimensional integration and the use of amorphous as well as crystalline silicon, the TFA technology is suitable for advanced image sensor applications. This paper describes the fundamentals of the properties: sensitivity, dark current, temporal and fixed-pattern noise of these TFA image sensors. It compares the different sensitivity definitions, especially current sensitivity and the charge conversion factor. Further, the dark current sources are pointed out, and their temperature behavior is described. By noise calculations, different pixel input stages are compared with regard to low light level detection.

Vertically integrated sensors for advanced imaging applications

A thin film on ASIC (TFA) image sensor is fabricated depositing an amorphous silicon thin-film detector onto a CMOS ASIC. With regards to advanced imaging systems, TFA provides enhanced performance and more flexibility than conventional technologies. Extensive on-chip signal processing is feasible, as well as small pixels for high resolution imagers. Two new TFA imager prototypes have recently been fabricated. High-resolution image sensor (HIRISE II) with 1024/spl times/128 pixels is an active pixel sensor suited for digital photography. Local autoadaptiver sensor (LARS II) with 368/spl times/256 pixels splits the illumination information into two signals, thereby providing a dynamic range of more than 120 dB, as required by automotive applications. Both prototypes include correlated double sampling and double delta sampling for efficient suppression of fixed pattern noise.

A-Si:H color imagers and colorimetry

Amorphous silicon (a-Si:H) based color detectors have matured to an established technology since the first color image sensor in thin film on ASIC (TFA) technology (ASIC=application specific integrated circuit) equipped with a trichomatic photo detector has been successfully demonstrated. This color sensitive device applies an optimized pi 3n diode with stacked intrinsic absorption layers that has a voltage controlled spectral response. It combines excellent color separation with a large dynamic range (>100 dB). The color recognition properties are examined by colorimetrical calculations as well as experimental analysis of color images recorded with the TFA sensor array CAESAR (color array with enhanced sensitivity and resolution) consisting of 512×104 pixels that allows detection of the three fundamental colors of the incident illumination in each sensor pixel. The experimental results show that the color rendering performance of the TFA color imager is comparable to conventional color sensor arrays using mosaic filters.

100000-pixel, 120-dB imager in TFA technology

A locally autoadaptive image sensor LARS II (Lokal-AutoadaptiveR Sensor) with 368/spl times/256 pixels was designed and fabricated in thin film on ASIC (TFA) technology. Every pixel contains an automatic shutter, made of 17 transistors and two capacitors on an area of 40/spl times/38.3 /spl mu/m/sup 2/, that adapts the integration time to the local intensity. This allows the capture and processing of scenes with extremely high dynamic range. For the amorphous detector system, local contrast and temperature stability are demonstrated to be excellent while the transient response is sufficient for imaging applications. Sample images shown in this paper verify the total dynamic range of 120 dB.

Sensitivity of CMOS based imagers and scaling perspectives

CMOS based imagers are beginning to compete against CCDs in many areas of the consumer market because of their system-on-a-chip capability. Sensitivity, however, is a main weakness of CMOS imagers and enhancements and deviations from standard CMOS processes are necessary to keep up sensitivity with downscaled process generations. In the introductory section several definitions for the sensitivity of image sensors are reviewed with regard to their potential to allow meaningful comparison of different detector structures. In the main section, the standard CMOS sensor architecture is compared to detector structures designed to improve the sensitivity, namely the photogate (PG), the pinned photodiode (PPD) and the thin film on ASIC (TFA) approach. The latter uses a vertical integration of the photodiode on top of the pixel transistors. A careful analysis of the relevant electrical, optical and technological parameters and many previously published experimental data for different imagers reveals that only the PPD and the TFA enhancements provide satisfactory sensitivity and withstand scaling down to 0.18 /spl mu/ processes. Due to the higher fill factor and the higher quantum efficiency TFA provides significantly better values than PPD. The radiometric sensitivity of a 5 /spl mu/m/spl times/5 /spl mu/m TFA pixel is found to amount to 11.9 V/(/spl mu//cm/sup 2/) for a 0.25 /spl mu/m process and 27.5 V/(/spl mu/J/cm/sup 2/) for a 0.18 /spl mu/m process.

Design and fabrication of a high-dynamic-range image sensor in TFA technology

Image sensors in thin film on ASIC (TFA) technology are composed of amorphous silicon (a-Si:H) thin-film detectors on top of crystalline ASIC's. With regard to advanced imaging systems, TFA provides enhanced performance and more flexibility than conventional technologies. Extensive on-chip signal processing is feasible, as well as small pixels for high-resolution imagers. Several prototypes of TFA sensors have been developed, optimized for considerably different applications. This paper focuses on a TFA sensor for automotive vision systems that allows each pixel to adapt its individual sensitivity to the local illumination intensity. By this means, a dynamic range of 120-dB minimum along with high local contrast is achieved. The recent second prototype array consists of 368/spl times/256 pixels with an area of 40/spl times/38 /spl mu/m/sup 2/ each. The ASIC's were fabricated in a 0.7-/spl mu/m CMOS technology, whereupon the a-Si:H thin film was deposited in a plasma-enhanced chemical vapor deposition cluster system.

UV Imager in TFA Technology

AbstractAn image sensor with enhanced sensitivity for near ultraviolet radiation (UVA) has been fabricated in TFA (Thin Film on ASIC) technology. The device employs an amorphous silicon pin detector optimized for UV detection by carbonization and layer thickness variation. The front electrode consists of an Al grid or TCO. Measurements show a peak responsivity of 90 mAW-1 at 380 nm. The UV Imager prototype consists of 128 × 128 pixels with a size of 25 μm × 25 [tim each, fabricated in a 0.7 μm CMOS process. Global sensitivity control serves to achieve a dynamic range in excess of 80 dB. The sensor can be used in fields such as chemical, medical and astronomical applications. Furthermore, a UV monitor has been developed, suited to warn of excessive sunlight exposure, considering skin type and sun protection factor.

Noise of a-Si:H Pin Diode Pixels in Imagers at Different Operating Conditions

AbstractThe noise current spectral density of an a-Si:H pin diode can be calculated with experimentally determined flicker noise coefficients by superposition of the shot and flicker noise spectra of photocurrent and dark current. The dependence of the flicker noise current spectral density in pin diodes on the pixel area is calculated with our expansion of Hooge's law for flicker noise. We propose a new method for the calculation of dynamic range (DR) and signal-to-noise ratio (SNR) in pin diode pixels as a function of pixel area, dark current, photocurrent and the integration time of the imager. DR and SNR of the pin diodes are calculated for Thin Film on ASIC (TFA) image sensors.

Image Sensors in Tfa Technology - Status and Future Trends

ABSTRACTImage sensors in TFA (Thin Film on ASIC) technology have been successfully fabricated and tested. This paper provides a survey of TFA research results so far and outlines future perspectives. The properties of different a-Si:H b/w and color thin film detectors are evaluated, including spectral sensitivity, dark current, temperature influence and transient behavior. Furthermore several TFA prototypes and emerging concepts are presented, ranging from a simple one-transistor cell design to a locally autoadaptive sensor.

Colour sensing applications of hydrogenated amorphous silicon carbide

Abstract Varying the stoichiometric parameter x in hydrogenated amorphous silicon carbide alloys (a-Si 1 − x C x :H) allows the optical bandgap to be shifted continuously throughout the entire visible range. Throughout this range, all bandgaps are direct, i.e. efficient optical absorption takes place within thin films with thicknesses of the order of 1 μm. It is shown how the optoelectronic properties of a-Si 1 − x C x :H films and devices on glass substrates can be used in combination with crystalline silicon photosensors to construct different kinds of colour sensing devices. Considering the low-temperature-deposition potential of these alloys, our results suggest that a-Si 1 − x C x :H films are likely to play an important role within emerging thin-film-on-ASIC (TFA) technologies.

Realization of an Amorphous N-I-P-I-N Three Color Sensor

ABSTRACTUtilizing the high optical absorption of amorphous silicon in conjunction with advanced signal processing capabilities of modern ASICs offers the possibility of simply realizing smart color sensors with random pixel access and without any losses in active area due to the signal processing circuitry. Thin-film photodiodes have to be incorporated into a layer sequence of ASIC/insulator/metal/a-Si:H based multilayer/TCO for this so-called Thin Film on ASIC (TFA) concept[l]. Amorphous silicon and silicon carbide photodiodes have been studied, and discrimination of the fundamental colors (red, green and blue) has been obtained from 2-terminal n-i-p-i-n devices at small bias voltages within ±2 V. Changing of the bias voltage applied to the two back-to-back p-i-n junctions allows for adjusting the location of predominant photo-carrier collection, and therefore enables color readout according to the energy dependent absorption profile of the incident light. Film thickness and optical bandgap of the individual layers have been optimized to achieve optimum color sensitivity. Switching experiments in the voltage mode give some information about conceivable frame rates these TFA sensors could be operated at. Light-induced degradation does not hamper the performance of the sensors thanks to low light intensity, reverse bias conditions, and small i-layer thickness.