Passive Pixel Sensor Research Articles

26‐1: Embedded a‐Si Photo‐Transistor Sensors Integration in Remote Optical Touch‐input Panel Using Four‐Mask Process Architecture Technology

An optical type touch‐input flat panel is designed. The driving system is based on the passive pixel sensor (PPS) array with amorphous silicon (a‐Si) TFT technology. The embedded sensing structure of the optical sensor is using the a‐Si TFT phototransistor and four‐mask process architecture technology. This paper presents a primary optical pixel sensor circuit that utilizes hydrogenated amorphous silicon photo‐transistor sensor. The a‐Si TFT phototransistor sensor is high reliability by using Multi‐N+ layer in the a‐Si TFT structure.

Radiation tolerant, thin, passive CMOS sensors read out with the RD53A chip

The radiation hardness of passive CMOS pixel sensors fabricated in 150nm LFoundry technology is investigated. CMOS process lines are especially of interest for large-scale silicon detectors as they offer high production throughput at comparatively low cost. Moreover, several features like poly-silicon resistors, MIM-capacitors and several metal layers are available which can help enhance the sensor design. The performance of a 100µm thin passive CMOS sensor with a pixel pitch of 50µm at different irradiation levels, 5 × 1015neqcm−2 and 1 × 1016neqcm−2, is presented. The sensor was bump-bonded and read out using the RD53A readout chip. After the highest fluence a hit-detection efficiency larger than 99% is measured for minimum ionising particles. The measured equivalent noise charge is comparable to conventional planar pixel sensors. Passive CMOS sensors are thus an attractive option for silicon detectors operating in radiation harsh environments like the upgrades for the LHC experiments.

A CMOS Image Sensor with a Novel Passive Pixel Array and High Precision Current Amplifier for a Compact Digital X-ray Detector

An active pixel sensor (APS) in a digital X-ray detector is the dominant circuitry for a CMOS image sensor (CIS) despite its lower fill factor (FF) compared to that of a passive pixel sensor (PPS). Although the PPS provides higher FF, its overall signal-to-noise ratio (SNR) is lower than that of the APS. The required high resolution and small focal plane can be achieved by reducing the number of transistors and contacts per pixel. We proposed a novel passive pixel array and a high precision current amplifier to improve the dynamic range (DR) without minimizing the sensitivity for diagnostic compact digital X-ray detector applications. The PPS can be an alternative to improve the FF. However, size reduction of the feedback capacitor causes degradation of SNR performance. This paper proposes a novel PPS based on readout and amplification circuits with a high precision current amplifier to minimize performance degradation. The expected result was attained with a 0.35-μm CMOS process parameter with power supply voltage of 3.3 V. The proposed PPS has a saturation signal of 1.5 V, dynamic range of 63.5 dB, and total power consumption of 13.47 mW. Therefore, the proposed PPS readout circuit improves the dynamic range without sacrificing the sensitivity.

Characterization of small-pixel passive CMOS sensors in 150 nm LFoundry technology using the RD53A readout chip

Passive CMOS pixel sensors in 150nm CMOS technology offered by LFoundry were designed and assembled into hybrid pixel modules. Advantages of commercial CMOS processes are high throughput at comparatively low costs which makes them attractive for the usage of large-area detectors. Further benefits originate from multiple metal layers, metal–insulator–metal capacitors, and polysilicon layers which can be used to enhance the sensor design. Thinned sensors were bump-bonded to the RD53A readout chip and characterized in laboratory environment and with a minimum ionizing 2.5GeV electron beam. Their performance in terms of noise and hit-detection efficiency equals that of conventional planar pixel sensors.

56.2: Embedded Fingerprint/Image Sensor Array Panel using a‐Si TFT photo‐sensor and Four‐Mask Halftone Process Technology

Fingerprint recognition is becoming a familiar technology. To further foster its use, it is important to make thinner sensors, adequate for embedding in various devices including display devices. An Optical type fingerprint scanning/touch‐input flat panel is designed. The driving system is based on the passive pixel sensor (PPS) array with amorphous silicon (a‐Si) TFT technology. The embedded sensing structure of the optical sensor is using the a‐Si TFT phototransistor and four‐mask process architecture technology. The pixel density is over 500 PPI and the a‐Si TFT phototransistor sensor is high reliability by using Multi‐N+ layer in the a‐Si TFT structure.

GaN-Based Ultraviolet Passive Pixel Sensor on Silicon (111) Substrate.

The fabrication of a single pixel sensor, which is a fundamental element device for the fabrication of an array-type pixel sensor, requires an integration technique of a photodetector and transistor on a wafer. In conventional GaN-based ultraviolet (UV) imaging devices, a hybrid-type integration process is typically utilized, which involves a backside substrate etching and a wafer-to-wafer bonding process. In this work, we developed a GaN-based UV passive pixel sensor (PPS) by integrating a GaN metal-semiconductor-metal (MSM) UV photodetector and a Schottky-barrier (SB) metal-oxide-semiconductor field-effect transistor (MOSFET) on an epitaxially grown GaN layer on silicon substrate. An MSM-type UV sensor had a low dark current density of 3.3 × 10−7 A/cm2 and a high UV/visible rejection ratio of 103. The GaN SB-MOSFET showed a normally-off operation and exhibited a maximum drain current of 0.5 mA/mm and a maximum transconductance of 30 μS/mm with a threshold voltage of 4.5 V. The UV PPS showed good UV response and a high dark-to-photo contrast ratio of 103 under irradiation of 365-nm UV. This integration technique will provide one possible way for a monolithic integration of the GaN-based optoelectronic devices.

Cascaded systems analysis of a-Se/a-Si and a-InGaZnO TFT passive and active pixel sensors for tomosynthesis

Medical imaging systems like full field digital mammography (FFDM) and digital breast tomosynthesis (DBT) commonly use amorphous selenium (a-Se) based passive pixel sensor (PPS) direct conversion x-ray detectors. On one hand, direct conversion detectors inherently offer better resolution characteristics in terms of a higher modulation transfer function (MTF), in comparison to the indirect CsI:Tl PPS x-ray imager. On the other hand, especially at lower doses, this superior performance of the direct imager is seldom retained in its detective quantum efficiency (DQE) curves.It is well known that a-Se PPS x-ray imagers suffer from high additive electronic noise originating from the from the amorphous silicon (a-Si) thin film transistor (TFT) array that is being used in the current back-plane technology. This degrades the noise power spectrum (NPS) and subsequently the overall DQE. To address this deficiency, we propose to replace the PPS back-plane by active pixel sensor (APS) back-plane technology, which has the potential to reduce the back-plane electronic noise by amplifying the input signal, especially at low doses. The proposed APS is based on amorphous In–Ga–Zn–O (a-IGZO) TFT technology, which can offer high mobility (5–20 cm2 V−1 s−1), low leakage current (<10−13 A) and low flicker noise (Hooge’s parameter αH ~ 1.5 10−3), leading to better imager noise performance.To test our hypothesis, we used linear cascaded systems analysis to model the imaging performance (MTF, NPS and DQE) of the PPS and APS a-Se direct imagers. This model was first validated using experimentally measured data obtained for a 85 µm pixel pitch a-Se/a-Si TFT PPS imager. Using this model, we analyzed the noise performance of the direct a-Se and indirect CsI:Tl x-ray a-IGZO APS imagers at different dose and electronic noise levels. Obtained results clearly showed that lowering back-plane electronic noise can significantly improve the performance of the a-Se/a-IGZO TFT APS imager. Our simulated results showed that a higher DQE at lower radiation doses (maximum DQE of 0.6 can be achieved at an exposure level of 1 µGy) can be achieved with the a-Se detector, thereby making this combination a promising candidate for low dose applications like DBT.

76‐4: Late-News Paper: Flat Panel Fingerprint/Touch‐input Image Sensor Using a‐Si TFT Photo‐transistor and Four‐Mask Process Architecture Technology

An Optical type fingerprint scanning/touch‐input flat panel is designed. The driving system is based on the passive pixel sensor (PPS) array with amorphous silicon (a‐Si) TFT technology. The embedded sensing structure of the optical sensor is using the a Si TFT phototransistor and four‐mask process architecture technology. The pixel density is over 500 PPI and the a‐Si TFT phototransistor sensor is high reliability by using Multi‐N+ layer in the a‐Si TFT structure.

P‐215: Organic‐Inorganic Hybrid Thin‐film Photo‐detector for Fingerprint Recognition

A near‐IR based optical type fingerprint sensor is designed and implemented. The driving system is based on the passive pixel sensor (PPS) array with Oxide (IGZO) TFT technology. The sensing element of the optical sensor is composed of the organic photosensitive diode (OPD) exhibited typical sandwich structure. Clear fingerprint images can be successfully captured by such organic‐inorganic hybrid thin‐film photo‐detector. Further, flexible fingerprint sensor is also realized based on PI substrate.

Three-dimensional cascaded system analysis of a 50 µm pixel pitch wafer-scale CMOS active pixel sensor x-ray detector for digital breast tomosynthesis

High-resolution, low-noise x-ray detectors based on the complementary metal-oxide-semiconductor (CMOS) active pixel sensor (APS) technology have been developed and proposed for digital breast tomosynthesis (DBT). In this study, we evaluated the three-dimensional (3D) imaging performance of a 50 µm pixel pitch CMOS APS x-ray detector named DynAMITe (Dynamic Range Adjustable for Medical Imaging Technology). The two-dimensional (2D) angle-dependent modulation transfer function (MTF), normalized noise power spectrum (NNPS), and detective quantum efficiency (DQE) were experimentally characterized and modeled using the cascaded system analysis at oblique incident angles up to 30°. The cascaded system model was extended to the 3D spatial frequency space in combination with the filtered back-projection (FBP) reconstruction method to calculate the 3D and in-plane MTF, NNPS and DQE parameters. The results demonstrate that the beam obliquity blurs the 2D MTF and DQE in the high spatial frequency range. However, this effect can be eliminated after FBP image reconstruction. In addition, impacts of the image acquisition geometry and detector parameters were evaluated using the 3D cascaded system analysis for DBT. The result shows that a wider projection angle range (e.g. ±30°) improves the low spatial frequency (below 5 mm−1) performance of the CMOS APS detector. In addition, to maintain a high spatial resolution for DBT, a focal spot size of smaller than 0.3 mm should be used. Theoretical analysis suggests that a pixelated scintillator in combination with the 50 µm pixel pitch CMOS APS detector could further improve the 3D image resolution. Finally, the 3D imaging performance of the CMOS APS and an indirect amorphous silicon (a-Si:H) thin-film transistor (TFT) passive pixel sensor (PPS) detector was simulated and compared.

A Numerical Study of an Amorphous Silicon Dual-Gate Photo Thin-Film Transistor for Low-Dose X-Ray Imaging

The amorphous silicon flat panel X-ray detector (FPD) has gained significant adoption in digital radiography such as chest radiography due to its large size and superior performance. As a fundamental building block of a FPD, the pixel is composed of sensor, storage and readout units to collect charges and transfer them to external electronics. In all current pixel architectures, however, these three aforementioned components are unanimously separate. It would inevitably compromise spatial resolution due to increased pixel size. Thus, we propose a compact design of a “smart” pixel to combine sensor, storage and readout units into one single dual-gate photo thin-film transistor (TFT). Simulation results indicated that an implementation of a $\pi$ -shape channel structure would enhance photo absorption and preserve switching performance as well. The sensitivity of the device is dependent on the threshold voltage shift induced by light illumination, which in turn enlarges the photocurrent. A numerical study also showed a total noise of approximately ${\sim} {\hbox{700}}$ e and a dynamic range of 120 dB can be achieved, which potentially outperforms current available amorphous silicon passive pixel sensor. The “smart” pixel therefore holds a great promise for high-resolution and low-dose X-ray imaging, which may potentially reduce the cancer risk induced by X-ray radiation, especially for pediatric patients.

Direct-Conversion X-Ray Detector with 50-µm High-Gain Pixel Amplifiers for Low-X-Ray-Dose Digital Mammography

This study proposes an active pixel sensor (APS) design for readout signal amplification to replace the traditional passive pixel sensor (PPS) design. The APS design adds only one extra thin-film transistor (TFT) amplifier, giving it the high signal sensitivity of the APS architecture and the high spatial resolution of the PPS architecture. The mobility of the amorphous silicon (a-Si) TFT is 0.56 cm2/Vs, the signal amplification of the APS circuit is more than 10×, the on/off current ratio is more than 107, and the leakage current is on the order of 1 fA. A 200-µm-thick layer of amorphous selenium (a-Se) as the X-ray photoconductor was deposited on the APS array with 160 × 160 pixels of various sizes, down to 50 µm. The 40-kVp X-ray spectra obtained with tube currents of 40 and 50 mA, respectively, were used for measuring the gray-level contrast induced by the X-rays. The experimental results show that the gray-level contrasts for the 50-µm pixels were 5.57 ± 0.89 and 13.19 ± 1.94 and those for the 70-µm pixels were 15.94 ± 2.98 and 24.19 ± 2.76 under 40-kVp X-ray exposure at 40 and 50 mA, respectively.

컬럼 커패시터와 피드백 구조를 이용한 CMOS 이미지 센서의 동작 범위 확장

This paper presents a wide dynamic range complementary metal oxide semiconductor (CMOS) image sensor with column capacitor and feedback structure. The designed circuit has been fabricated by using <TEX>$0.18{\mu}m$</TEX> 1-poly 6-metal standard CMOS technology. This sensor has dual mode operation using combination of active pixel sensor (APS) and passive pixel sensor (PPS) structure. The proposed pixel operates in the APS mode for high-sensitivity in normal light intensity, while it operates in the PPS mode for low-sensitivity in high light intensity. The proposed PPS structure is consisted of a conventional PPS with column capacitor and feedback structure. The capacitance of column capacitor is changed by controlling the reference voltage using feedback structure. By using the proposed structure, it is possible to store more electric charge, which results in a wider dynamic range. The simulation and measurement results demonstrate wide dynamic range feature of the proposed PPS.

Amorphous In-Ga-Zn-O thin-film transistor active pixel sensor x-ray imager for digital breast tomosynthesis.

The breast cancer detection rate for digital breast tomosynthesis (DBT) is limited by the x-ray image quality. The limiting Nyquist frequency for current DBT systems is around 5 lp/mm, while the fine image details contained in the high spatial frequency region (>5 lp/mm) are lost. Also today the tomosynthesis patient dose is high (0.67-3.52 mGy). To address current issues, in this paper, for the first time, a high-resolution low-dose organic photodetector/amorphous In-Ga-Zn-O thin-film transistor (a-IGZO TFT) active pixel sensor (APS) x-ray imager is proposed for next generation DBT systems. The indirect x-ray detector is based on a combination of a novel low-cost organic photodiode (OPD) and a cesium iodide-based (CsI:Tl) scintillator. The proposed APS x-ray imager overcomes the difficulty of weak signal detection, when small pixel size and low exposure conditions are used, by an on-pixel signal amplification with a significant charge gain. The electrical performance of a-IGZO TFT APS pixel circuit is investigated by SPICE simulation using modified Rensselaer Polytechnic Institute amorphous silicon (a-Si:H) TFT model. Finally, the noise, detective quantum efficiency (DQE), and resolvability of the complete system are modeled using the cascaded system formalism. The result demonstrates that a large charge gain of 31-122 is achieved for the proposed high-mobility (5-20 cm2/V s) amorphous metal-oxide TFT APS. The charge gain is sufficient to eliminate the TFT thermal noise, flicker noise as well as the external readout circuit noise. Moreover, the low TFT (<10(-13) A) and OPD (<10(-8) A/cm2) leakage currents can further reduce the APS noise. Cascaded system analysis shows that the proposed APS imager with a 75 μm pixel pitch can effectively resolve the Nyquist frequency of 6.67 lp/mm, which can be further improved to ∼10 lp/mm if the pixel pitch is reduced to 50 μm. Moreover, the detector entrance exposure per projection can be reduced from 1 to 0.3 mR without a significant reduction of DQE. The signal-to-noise ratio of the a-IGZO APS imager under 0.3 mR x-ray exposure is comparable to that of a-Si:H passive pixel sensor imager under 1 mR, indicating good image quality under low dose. A threefold reduction of current tomosynthesis dose is expected if proposed technology is combined with an advanced DBT image reconstruction method. The proposed a-IGZO APS x-ray imager with a pixel pitch<75 μm is capable to achieve a high spatial frequency (>6.67 lp/mm) and a low dose (<0.4 mGy) in next generation DBT systems.

P.23: Nano‐Si Optical Pixel‐Sensor Array using TFT Technology as Image‐Scan/Fingerprint Panel

AbstractWe report the controllable UV‐Visible nano‐Si photonic sensor, band‐gap spectrum engineering, and integrated in TFT technology as Image‐scan/Fingerprint panel. Photonic Sensor using nano‐Si as sensitizer sandwiched between two electrodes structure is proposed to integrate optical input function for new application in next generation portable devices. The optical input function, used for image scanner, fingerprint recognition, and touch‐input application, are achieved by integrating these image sensor elements within each pixel. A 1‐transistor (1T) passive pixel sensor (PPS) is used to integrate a 250/500 ppi image sensor within a scan‐input panel.

An integrated organic passive pixel sensor

We demonstrate an imaging passive pixel sensor circuit consisting of a bottom-gate, top-contact pentacene organic thin-film transistor (OTFT) integrated with a top-illuminated, inverted subphthalocyanine/C 60 organic photodetector (OPD). The vacuum-deposited OTFT utilizes parylene as the gate insulator, achieving a drain current ON/OFF ratio of 10 5. The transistor hole mobility is 0.09 ± 0.02 cm 2/V s. The inverted OPD has a dark current of 20 pA at a reverse bias of 1.5 V. By integrating the two components, a 12-bit dynamic range passive pixel sensor is achieved, with an OFF current of 31 ± 5 pA and a pixel readout time of 0.4 ± 0.05 ms, limited by the discharge time of the OTFT channel. The integrated pixel has potential for use in large-scale focal plane array imagers.

Amorphous and Polycrystalline Photoconductors for Direct Conversion Flat Panel X-Ray Image Sensors

In the last ten to fifteen years there has been much research in using amorphous and polycrystalline semiconductors as x-ray photoconductors in various x-ray image sensor applications, most notably in flat panel x-ray imagers (FPXIs). We first outline the essential requirements for an ideal large area photoconductor for use in a FPXI, and discuss how some of the current amorphous and polycrystalline semiconductors fulfill these requirements. At present, only stabilized amorphous selenium (doped and alloyed a-Se) has been commercialized, and FPXIs based on a-Se are particularly suitable for mammography, operating at the ideal limit of high detective quantum efficiency (DQE). Further, these FPXIs can also be used in real-time, and have already been used in such applications as tomosynthesis. We discuss some of the important attributes of amorphous and polycrystalline x-ray photoconductors such as their large area deposition ability, charge collection efficiency, x-ray sensitivity, DQE, modulation transfer function (MTF) and the importance of the dark current. We show the importance of charge trapping in limiting not only the sensitivity but also the resolution of these detectors. Limitations on the maximum acceptable dark current and the corresponding charge collection efficiency jointly impose a practical constraint that many photoconductors fail to satisfy. We discuss the case of a-Se in which the dark current was brought down by three orders of magnitude by the use of special blocking layers to satisfy the dark current constraint. There are also a number of polycrystalline photoconductors, HgI2 and PbO being good examples, that show potential for commercialization in the same way that multilayer stabilized a-Se x-ray photoconductors were developed for commercial applications. We highlight the unique nature of avalanche multiplication in a-Se and how it has led to the development of the commercial HARP video-tube. An all solid state version of the HARP has been recently demonstrated with excellent avalanche gains; the latter is expected to lead to a number of novel imaging device applications that would be quantum noise limited. While passive pixel sensors use one TFT (thin film transistor) as a switch at the pixel, active pixel sensors (APSs) have two or more transistors and provide gain at the pixel level. The advantages of APS based x-ray imagers are also discussed with examples.

An a-Si Active Pixel Sensor (APS) Array for Medical X-ray Imaging

Active pixel sensor (APS) circuits are an alternate to passive pixel sensor (PPS) circuits, which, while common in CMOS technology, have yet to be incorporated into commercial amorphous silicon (a-Si) large-area imagers. A proof-of-concept 64 × 64 APS array for low-exposure medical X-ray imaging is fabricated in a-Si technology and mated with an amorphous selenium photoconductor. Modulation transfer function (MTF) response and transient response for the APS imager indicate significant charge trapping at the top insulator/a-Se interface. MTF response indicates an effective fill factor of 94.5 % for a geometric fill factor of 57% at an electric field strength of 10 V/μm. Signal-to-noise ratio (SNR) performance from the prototype imager is comparable to a state-of-the-art commercially available a-Si PPS X-ray imager for X-ray exposures down to 1.5 μR using an RQA5 standard fluoroscopic characterization beam. Pixel design and fabrication process improvements are suggested to improve the SNR performance of the APS imager below 1.5 μR.

Digital X-Ray Imaging Using Avalanche a-Se Photoconductor

This work presents the development of a novel detector comprised of an avalanche amorphous selenium (a-Se) photoconductor and an amorphous silicon (a-Si:H) passive pixel sensor for digital X-ray imaging, in particular, for low exposure imaging applications. Electrical compatibility of a high voltage (~1000 V) avalanche a-Se photoconductor with a low voltage (~25 V) a-Si:H pixel sensor is demonstrated. Single pixel readout is done as a proof of concept.

A monolithically integrated organic photodetector and thin film transistor

We demonstrate a vacuum deposited molecular organic photodetector (OPD) integrated with a top gate, bottom-contact, pentacene-based organic thin film transistor (OTFT) for use as a switchable organic passive pixel sensor in focal plane imaging arrays. The heterojunction OPD consists of a 110 Å thick film of the donor, boron subphthalocyanine chloride, and a 325 Å thick film of the acceptor, C 60. The OTFT switch allows the photodetector to be switched between an active (ON) state with a light intensity dependent photocurrent peaking at 20 nA, and an inactive (OFF) state with current <50 pA, corresponding to an 8-bit dynamic range. The integrated device has a spectral coverage from wavelengths of 400–600 nm, and a pixel switching frequency >400 kHz. The integrated structure is fabricated at room temperature, making this circuit compatible with low cost, flexible plastic substrates.