a-Si:H-Silicon Hybrid Low Energy X-ray Detector

Low energy X-ray (< 20 keV) detection is a key technological requirement in applications such as protein crystallography or diffraction imaging. Silicon based optical cameras based on CCDs or CMOS imaging chips coupled to X-ray conversion scintillators have become a mainstay in the field. They are attractive because of fast readout capability and ease of integrated circuit implementation due to modern semiconductor fabrication technology. More recently, hydrogenated amorphous silicon (a-Si:H) thin film technology, that had enabled a huge influx of large area display products into the commercial display market, has been introduced to digital imaging in the form of active matrix flat panel imagers (AMFPIs). Although thin film technology can enable large area X-ray imaging at a potentially lower cost, the existing technology lacks spatial resolution requirements for higher performance crystallography and diffraction imaging applications. This work introduces a high resolution direct conversion silicon X-ray detector integrated with large area thin film silicon technology for sub-20 keV photon X-ray imagers. A prototype pixel was fabricated in-house using a fabrication facility (G2N) utilizing plasma enhanced chemical vapor deposition (PECVD), reactive ion etching (RIE), photo-lithography, and metal sputtering technologies. Unlike most active matrix display products, top-gate staggered a-Si:H thin film transistor (TFT) were implemented to take advantage of a novel thin film silicon pixel amplification device architecture. The detector performance was evaluated with an iron 55 isotope gamma ray source to mimic low energy X-ray exposure. I-V and C-V measurement techniques indicate that the hybrid pixel functions as expected and is promising for low cost, high resolution, large area X-ray imaging (< 20 keV) applications. We also performed a noise spectrum investigation to estimate the lowest detection signal level limit and proposed a model rooted in device physics for the pixel output and gain

A Coarse imaging sensor for detecting embedded signals in infrared light

Machine vision technology has become prevalent in touch technology, however, it is still limited by background noise. To reduce the background noise present in the images of interest it is important to consider the imaging device and the signal source. The architecture, size, sampling scheme, programming, and technology of the imaging device must be considered as well as the response characteristics of the signal source. Several pixel architectures are explained and implemented with discrete components. Their performance was measured through their ability to track a modulated signal source. Potentially, an imaging sensor comprised of a system designed to modulate the light to be imaged could drastically reduce background noise. Further, with a less noisy image, the processing steps required for touch event detection may be simplified

Active Pixel Sensor Architectures for High Resolution Large Area Digital Imaging

This work extends the technology of amorphous silicon (a-Si) thin film transistors (TFTs) from traditional switching applications to on-pixel signal amplification for large area digital imaging and in particular, is aimed towards enabling emerging low noise, high resolution and high frame rate medical diagnostic imaging modalities such as digital tomosynthesis. A two transistor (2T) pixel amplifier circuit based on a novel charge-gate thin film transistor (TFT) device architecture is introduced to shrink the TFT based pixel readout circuit size and complexity and thus, improve the imaging array resolution and reliability of the TFT fabrication process. The high resolution pixel amplifier results in improved electrical performance such as on-pixel amplification gain, input referred noise and faster readouts. In this research, a charge-gated TFT that operates as both a switched amplifier and driver is used to replace two transistors (the addressing switch and the amplifier transistor) of previously reported three transistor (3T) APS pixel circuits.. In addition to enabling smaller pixels, the proposed 2T pixel amplifier results in better signal-to-noise (SNR) by removing the large flicker noise source associated with the switched TFT and increased pixel transconductance gain since the large ON-state resistance of the switched TFT is removed from the source of the amplifier TFT. Alternate configurations of 2T APS architectures based on source or drain switched TFTs are also investigated, compared, and contrasted to the gate switched architecture using charge-gated TFT. A new driving scheme based on multiple row resetting is introduced which combined with the on-pixel gain of the APS, offers considerable improvements in imaging frame rates beyond those feasible for PPS based pixels. The novel developed 2T APS architectures is implemented in single pixel test structures and in 88 pixel test arrays with a pixel pitch of 100 µm. The devices were fabricated using an in-house developed top-gate TFT fabrication process. Measured characteristics of the test devices confirm the performance expectations of the 2T architecture design. Based on parameters extracted from fabricated TFTs, the input referred noise is calculated, and the instability in pixel transconductance gain over prolonged operation tine is projected for different imaging frame rates. 2T APS test arrays were packaged and integrated with an amorphous selenium (a-Se) direct x-ray detector, and the x-ray response of the a-Se detector integrated with the novel readout circuit was evaluated. The special features of the APS such as non-destructive readout and voltage programmable on-pixel gain control are verified. The research presented in this thesis extends amorphous silicon pixel amplifier technology into the area of high density pixel arrays such as large area medical X-ray imagers for digital mammography tomosynthesis. It underscores novel device and circuit design as an effective method of overcoming the inherent shortcomings of the a-Si material . Although the developed device and circuit ideas were implemented and tested using a-Si TFTs, the scope of the device and circuit designs is not limited to amorphous silicon technology and has the potential to be applied to more mainstream technologies, for example, in CMOS active pixel sensor (APS) based digital cameras

Photon Quantum Noise Limited Pixel and Array architectures in a-Si Technology for Large Area Digital Imaging Applications

A Voltage Controlled Oscillator (VCO) based pixel and array architecture is reported using amorphous silicon (a-Si) technology for large area digital imaging applications. The objectives of this research are to (a) demonstrate photon quantum noise limited pixel operation of less than 30 input referred noise electrons, (b) theoretically explore the use of the proposed VCO pixel architecture for photon quantum noise limited large area imaging applications, more specifically protein crystallography using a-Si, (c) to implement and demonstrate experimentally a quantum noise limited (VCO) pixel, a small prototype of quantum noise limited (VCO) pixelated array and a quantum noise limited (VCO) pixel integrated with direct detection selenium for energies compatible with a protein crystallography application. Electronic noise (phase noise) and metastability performance of VCO pixels in low cost, widely available a-Si technology will be theoretically calculated and measured for the first time in this research. The application of a VCO pixel architecture in thin film technologies to large area imaging modalities will be examined and a small prototype a-Si array integrated with an overlying selenium X-ray converter will be demonstrated for the first time. A-Si and poly-Si transistor technologies are traditionally considered inferior in performance to crystalline silicon, the dominant semiconductor technology today. This work v aims to extend the reach of low cost, thin film transistor a-Si technology to high performance analog applications (i.e. very low input referred noise) previously considered only the domain of crystalline silicon type semiconductor. The proposed VCO pixel architecture can enable large area arrays with quantum noise limited pixels using low cost thin film transistor technologies

Development and applications of inkjet printed conducting polymer micro-rings

A drying sessile drop moves the solute particles to the periphery where they get deposited in the form of a ring. This phenomenon is prevalent even with micro drops falling at high velocity from a piezo-actuator based inkjet printer. In polymer microelectronic field, this phenomenon is a major challenge for fabricating devices using inkjet printing. We exploited this problem and applied it for various novel applications in the field of polymer microelectronics. Various dispensing techniques and temperature variations for micro-drop printing were used for modifying the micro-drops in such a way that the periphery of the micro-ring holds most of the solute as compared to inner base layer. Reactive ion etching (RIE) was used for removing the inner base layer in order to make the micro-rings completely hollow from the center. These micro-rings were applied in the fabrication of polymer light emitting diode, humidity sensor and vertical channel field effect transistor. High resolution polymer light emitting diode array (\u3e200 pixels/inch) was fabricated by inkjet printing of micro-ring and each micro-ring acts as a single pixel. These micro-rings were applied as a platform for layer-by-layer (LbL) nano-assembly of poly-3,4-ethylenedioxythiophene:poly-styrenesulfonate (PEDOT:PSS) for the fabrication of humidity sensor. Enhanced sensitivity of the humidity sensor was obtained when the inkjet printed micro-rings are combined with LbL assembled PEDOT:PSS films. During the fabrication of vertical channel field effect transistors, inkjet printed PEDOT:PSS micro-rings were used as source and the inner spacers between the adjacent micro-rings were used to make channel. These micro-rings can also find other applications in the field of biological sciences. These micro-rings can be used as cell culture plates and as scaffolds for cell and/or tissue growth

Organic Semiconductor Detector for Large Area Digital Imaging

Organic semiconductor technology has gained attention in both the sensor and display markets due to its low cost and simple fabrication techniques. The ability to fabricate organic semiconductor devices such as photodetectors and transistors on a flexible, lightweight substrate makes them less fragile and ideal candidates for portable large-area imaging applications. The use of organic semiconductor technology in large-area medical imaging can bring about a new generation of flexible and lightweight indirect X-ray imagers. These imagers are immune to mechanical shock and should be ideal for portable intraoral X-ray radiology. In order to realize these organic flexible imagers and their use in large-area medical imaging, many challenges associated with the device performance and fabrication need to be overcome. Among these challenges, one of the greatest is to improve the dark current performance of the organic semiconductor photodetectors (key for imager performance) with a high-photo to-dark current ratio. Low dark current is needed to improve the sensitivity of the imager, whereas a large photo-to-dark current ratio reduces noise in the extracted image. Numerous techniques have been reported to improve the dark current performance in vertical organic photodetector design; however, lateral photodetectors still lack research attention. This thesis presents a lateral multilayer photodetector design and a simplified technique to improve the dark current performance of lateral organic semiconductor photodetectors. Our technique allows us to apply a large bias voltage while maintaining a low dark current, high photo-to-dark current ratio, and improves detector speed; thus, the overall sensitivity of the detector is improved. We further show the integration of an organic photodetector with an organic backplane readout circuit to form a flexible large-area imager. This imager can be used for large-area digital imaging applications such as in medical radiology.4 month

Analysis and comparison of resistive, ferroelectric and pyroelectric uncooled bolometers for electronic imaging systems

The performance parameters (responsivity (Rv). detectivity (D*), total noise and response time) of resistive, pyroelectric and ferroelectric bolometer detectors are dependent on a large number of key variables including chopping frequercy, the input impedance and voltage noise of the readout circuitry, the structure dependent parameters (particularly thermal conductance and thermal capacitance), and material properties such as dielectric constant, pyroelectric coefficient, loss tangent and thin film thickness. The interrelationship between the key variables and their influence on performance is often complex and not easily discerned for the three major types of thermal detectors: resistive, pyroelectric and ferroelectric bolometers. In this thesis research, the dependence of Rv, D* and total noise on these key parameters were analyzed and written as equations from which computer calculations could easily be made. The analyzed results were used to compare the pertbrmance of the three types of sensors for present-day structure and material characteristics and also for material characteristics and structures that night be developed in the future

Fabrication of electronic devices using high volume printing techniques

This thesis studies the possibility of using high volume offset lithographic and flexographic printing to fabricate single and multiple layer electronic devices, and discusses the performance and applications of the printed devices