Printed and flexible hybrid electronics is an emerging technology with potential applications in smart labels, wearable electronics, soft robotics, and prosthetics. Printed solution-based materials are compatible with plastic film substrates that are flexible, soft, and stretchable, thus enabling conformal integration with non-planar objects. In addition, manufacturing by printing is scalable to large areas and is amenable to low-cost sheet-fed and roll-to-roll processes. FHE includes display and sensory components to interface with users and environments. On the system level, devices also require electronic circuits for power, memory, signal conditioning, and communications. Those electronic components can be integrated onto a flexible substrate by either assembly or printing. PARC has developed systems and processes for realizing both approaches. This talk presents fabrication methods with an emphasis on techniques recently developed for the assembly of off-the-shelf chips. A few examples of systems fabricated with this approach are also described.
Wireless sensing has broad applications in a wide variety of fields such as infrastructure monitoring, chemistry, environmental engineering and cold supply chain management. Further development of sensing systems will focus on achieving light weight, flexibility, low power consumption and low cost. Fully printed electronics provide excellent flexibility and customizability, as well as the potential for low cost and large area applications, but lack solutions for high-density, high-performance circuitry. Conventional electronics mounted on flexible printed circuit boards provide high performance but are not digitally fabricated or readily customizable. Incorporation of small silicon dies or packaged chips into a printed platform enables high performance without compromising flexibility or cost.
At PARC, we combine high functionality c-Si CMOS and digitally printed components and interconnects to create an integrated platform that can read and process multiple discrete sensors. Our approach facilitates customization to a wide variety of sensors and user interfaces suitable for a broad range of applications including remote monitoring of health, structures and environment. This talk will describe several examples of printed wireless sensing systems. The technologies required for these sensor systems are a mix of novel sensors, printing processes, conventional microchips, flexible substrates and energy harvesting power solutions.
A novel jet-printing approach to fabricate thin film transistor (TFT), active matrix backplanes for x-ray imagers is described. The technique eliminates the use of photolithography and has the potential to greatly reduce the array manufacturing cost. We show how jet-printing is used to pattern the layers of the active matrix array and also to deposit semiconductor material. The technique is applied to both amorphous silicon and polymer transistors, and small prototype arrays have been fabricated and tested, including arrays with a high fill factor amorphous silicon p-i-n photodiode layer for indirect detection x-ray imaging applications. The TFT characteristics are excellent, and acquired x-ray images will be presented and compared to those from conventional TFT arrays. The printing process has been extended to flexible substrates which are important for rugged x-ray imagers, using a low temperature amorphous silicon process to accommodate the plastic substrate. Polymer TFT arrays made with jet-printed polymer solutions have also been demonstrated and we present data from arrays, and discuss options for integrating organic photodiodes or direct detection sensors. The opportunities and challenges of using polymer semiconductors in x-ray imaging arrays, are discussed and we show that the TFT performance meets the needs of radiographic imaging, although the radiation hardness and long term degradation are not sufficiently studied.
Photoelectric X-ray polarimeters based on pixel micropattern gas detectors (MPGDs) offer order-of-magnitude improvement in sensitivity over more traditional techniques based on X-ray scattering. This new technique places some of the most interesting astronomical observations within reach of even a small, dedicated mission. The most sensitive instrument would be a photoelectric polarimeter at the focus of a very large mirror, such as the planned XEUS. Our efforts are focused on a smaller pathfinder mission, which would achieve its greatest sensitivity with large-area, low-background, collimated polarimeters. We have recently demonstrated a MPGD polarimeter using amorphous silicon thin-film transistor (TFT) readout suitable for the focal plane of an X-ray telescope. All the technologies used in the demonstration polarimeter are scalable to the areas required for a high-sensitivity collimated polarimeter.
Photoconductive polycrystalline mercuric iodide coated on amorphous silicon flat panel thin film transistor (TFT) arrays is the best candidate for direct digital X-ray detectors for radiographic and fluoroscopic applications in medical imaging.
The mercuric iodide is vacuum deposited by Physical Vapor Deposition (PVD). This coating technology is capable of being scaled up to sizes required in common medical imaging applications. Coatings were deposited on 2”×2” and 4”×4” TFT arrays for imaging performance evaluation and also on conductive-coated glass substrates for measurements of X-ray sensitivity and dark current. TFT arrays used included pixel pitch dimensions of 100, 127 and 139 microns. Coating thickness between 150 microns and 250 microns were tested with beam energy between 25 kVP and 100kVP utilizing exposure ranges typical for both fluoroscopic, and radiographic imaging.
X-ray sensitivities measured for the mercuric iodide samples and coated TFT detectors were superior to any published results for competitive materials (up to 7100 ke/mR/pixel for 100 micron pixels). It is believed that this higher sensitivity can result in fluoroscopic imaging signal levels high enough to overshadow electronic noise. Diagnostic quality of radiographic and fluoroscopic images of up to 15 pulses per second were demonstrated. Image lag characteristics appear adequate for fluoroscopic rates. Resolution tests on resolution target phantoms showed that resolution is limited to the TFT array Nyquist frequency including detectors with pixel size of 139 microns resolution ~3.6 lp/mm) and 127 microns (resolution~3.9 lp/mm). The ability to operate at low voltages (~0.5 volt/micron) gives adequate dark currents for most applications and allows low voltage electronics designs.
The x-ray response of polycrystalline HgI2 for direct detection x-ray imagers, is studied using test arrays with 512 X 512 pixels of size 100 micron. We quantify the contributions to the x-ray sensitivity from electron and hole charge collection, x-ray absorption, effective fill factor and image lag, for x-ray energies from 25-100 kVp. The data analysis compares the measured sensitivity to the theoretical limit and identifies the contributions from various loss mechanisms. The sensitivity is explained by the ionization energy of approximately 5 eV, coupled with small corrections arising from incomplete x-ray absorption, incomplete charge collection, and image lag. Hence, imagers with HgI2 approach the theoretical maximum response for semiconductor detectors, with external array sensitivity demonstrated to within 50 percent of the limit.
New results for polycrystalline HgI2 detectors are reported here. Due to its decent electrical properties and high stopping power for X-rays and gamma rays, HgI2 is a good candidate for many medical imaging applications. HgI2 were deposited by a hot wall Physical Vapor Deposition (PVD) method, and the electrical properties of the films, including X-ray response and dark current data are reported. Results of imaging capabilities and spatial resolution obtained by polycrystalline HgI2 deposited onto a 2'x2' TFT imaging array on an amorphous silicon substrate are also given. These tests were carried out at Xerox-PARC Research Center.
Polycrystalline mercuric iodide (HgI2) photoconductor material was directly deposited on flat panel amorphous silicon (a-Si) thin film transistor (TFT) pixel arrays in order to test their application as direct x-ray conversion detectors. The 4' x 4' and 2' x 2' detector plates were fabricated either by Physical Vapor Deposition (PVD) or the Screen-Print (SP) method. Although developed for medical radiological imaging, they can also be used for nondestructive test imaging. The present HgI2 arrays have 100 μm x 100 μm pixels on the 2' x 2' detector and 139μm x 139μm on the 4' x 4' imager. The initial results are very promising and show high x-ray sensitivity and low leakage current. The advantage of these detectors is that they can be directly deposited on the pixellated arrays containing the TFTs and other electronic read out circuits and can be fabricated in large sizes. These polycrystalline PVD-HgI2 thick film detectors have now been fabricated up to 1,800μm thick, which makes them also useful for higher-energy X-ray applications. Imaging results obtained by both PVD- and SP-HgI2 will be shown. The effect of the crystallite size on the imaging properties will be demonstrated and the difference in sensitivity applying positive or negative bias on the top electrode will be discussed. Comparison of x-ray sensitivity to other photoconductor materials such a-Se and PbI2 will also be presented.
X-ray imaging properties are reported for HgI2 and PbI2, as candidate materials for future direct detection x- ray image sensors, including the first results from screen- printed HgI2 arrays. The leakage current of PbI2 is reduced by using new deposition conditions, but is still larger than HgI2. Both HgI2 and PbI2 have high spatial resolution but new data shows that the residual image spreading of PbI2 is not due to k-edge fluorescence and its possible origin is discussed. HgI2 has substantially higher sensitivity than PbI2 at comparable bias voltages, and we discuss the various loss mechanisms. Unlike PbI2, HgI2 shows a substantial spatially non-uniform response that is believed to originate from the large grain size, which is comparable to the pixel size. We obtain zero spatial frequency DQE values of 0.7 - 0.8 with PbI(subscript 24/ under low energy exposure conditions. A model for signal generation in terms of the semiconducting properties of the materials is presented.
We report x-ray imaging results on polycrystalline HgI2 detector used for direct x-ray imaging. Due to its good electrical properties and high stopping power for x-rays and gamma rays, the material is a good candidate for many applications in medical imaging. The deposition of the HgI2 thick films is made by hot wall physical vapor deposition, (PVD) method and some of the structural features are described here. The x-ray response and some dark current data measured on some recently prepared detectors are reported. Some results obtained with poly-HgI2 thin film deposited on an amorphous-Si TFT's imaging array 4' X 4' performed at the Ginzton Technology Center is reported for the first time. Also phantom images received with a similar deposited poly-HgI2 thin film deposited on an amorphous- Si TFT's imaging array 2' X 2' performed at Xerox-PARC Research Center is also given here. The status of HgI2 technology will be discussed.
For the first time polycrystalline HgI2 photoconductor material directly evaporated on a-Si array for direct conversion of x-rays for imaging purposes, were successfully imaged at Xerox-Palo Alto Research Center. The initial results are very promising and show a high x-ray sensitivity and low leakage current. Since Ti-W alloys are used as pixel electrodes, an intermediate passivation layer must be used to prevent a chemical reaction with the detector plate. The thickness that these Polycrystalline HgI2 thick film detectors have been fabricated until now is up to 1,800 micrometers , which makes them useful also for high energy applications. The characterization of the Polycrystalline HgI2 thick films deposited with or without the passivation layers by measuring their dark currents, sensitivity to 65 and 85 kVp x-rays and residual signals after 1 minute of biasing, will be shown for several detectors. Some preliminary results will be shown for some novel screen-printed HgI2 detectors.
Measurements of polycrystalline HgI2 films on active matrix direct detection image sensors are described, for possible application to high sensitivity room temperature x- ray detection. The arrays exhibit low leakage current and very high sensitivity - roughly an order of magnitude better than has been demonstrated with other designs. The uniformity of the response varies randomly from pixel to pixel, for reasons that are not yet understood, but are probably related to the large grain size.
Amorphous silicon (a-Si:H) technology has created a successful manufacturing business for large area active matrix arrays, of which liquid crystal displays (AMLCD) are the best known, and image sensors are an emerging technology for medical x-ray imaging. The large area, flat plate, format is the key feature of the technology that sets it apart from other digital imaging approaches. The principal requirements for medical imaging are sensitivity and high dynamic range. A-Si:H detectors have already proved to perform at least as well as x-ray film for radiographic applications and comparable to image intensifiers for fluoroscopy. There are several approaches to improving the performance of the image sensors is order to achieve higher sensitivity and higher spatial resolution. This paper describes some of these approaches.
A first image of some tiny screws were obtained for the first time with polycrystalline HgI2 acting as the photoconductor material deposited on a-Si direct conversion X- ray image sensors, produced by Xerox -- Palo Alto Research Center. The initial results are very promising and show a high X-ray sensitivity and low leakage current. The response of these detectors to a radiological X-ray generator of 65 kVp has been studied using the current integration mode. Already its sensitivity expressed in (mu) C/R*cm2, is very high, values of 20 (mu) C/R*cm2 have been measured for films of 100 - 250 microns thickness and bias of 50 - 200 volts respectively, which is superior to the published data for competing materials such as polycrystalline PbI2 and a-Se detectors. The fabrication and characterization measurements of the Polycrystalline HgI2 thick film detectors will be given. The characterization data which will be reported here consists of: (a) sensitivity, (b) dark currents, (c) stability of sensitivity dependence on the number of exposure, (d) X-ray response dependence on dose energy and (e) signal decay dependence on the number of exposures.
We report on a-Si direct detection x-ray image sensors with polycrystalline PbI2, and more recently with HgI2. The arrays have 100 micron pixel size and, we study those aspects of the detectors that mainly determine the DQE, such as sensitivity, effective fill factor, dark current noise, noise power spectrum, and x-ray absorption. Line spread function data show that in the PbI2 arrays, most of the signal in the gap between pixels is collected, which is important for high,DQE. The leakage current noise agrees with the expected shot noise value with only a small enhancement at high bias voltages. The noise power spectrum under x-ray exposure is reported and compared to the spatial resolution information. The MTF is close to the ideal sinc function, but is reduced by the contribution of K-fluorescence in the PbI2 film for which we provide new experimental evidence. The role of noise power aliasing in the DQE and the effect of slight image spreading are discussed. Initial studies of HgI2 as the photoconductor material show very promising results with high x-ray sensitivity and low leakage current.
The performance of the new generation of high fill factor two- dimensional imagers with high spatial resolution and low data line capacitance is described. These arrays have a continuous a-Si:H sensor layer deposited over the whole imager to improve sensitivity. We have studied charge collection and lateral leakage in the gap region in between two neighboring pixels. Experiments demonstrate that a 10 micrometer gap between pixels leads to an effective fill factor of approximately 92% and can be fabricated in a way to reduce the charge leakage between pixels to a very low level. We have also studied the capacitance of the data lines that can lead to increased electronic noise, degrading the imager performance. Experimental determination of the actual capacitance for different insulator materials are compared with numerical simulations, to identify the optimum structure. Based on these results, the new imager generation could be manufactured with a total parasitic capacitance of about 6 fF/pixel. Finally, we report measurements of the high fill factor imager under light and X-ray exposures.
In this paper, we discuss recent progress that has been made in the development of high resolution X-ray imaging detectors using photoconducting films of lead iodide (PbI2). PbI2 is a wide bandgap semiconductor with high X- ray stopping efficiency. We have been investigating thick films of lead iodide which can be prepared in large areas in a cost effective manner. These films can be coupled to readout technologies such as amorphous silicon flat panel arrays and vidicon tubes to produce X-ray imaging detectors for applications such as mammography, fluoroscopy, X-ray diffraction and non-destructive evaluation. Recent results obtained when these PbI2 films are coupled to 512 X 512 flat panel a-Si:H array are reported. This includes dark current, signal and resolution measurements. Properties of lead iodide films which are relevant to imager performance are also discussed.
Amorphous silicon (a-Si:H) matrix-addressed imager sensors are the leading new technology for digital medical x-ray imaging. Large-area systems are now commercially available with good resolution and large dynamic range. These systems image x-rays either by detecting light emission from a phosphor screen onto an a-Si:H photodiode, or by collecting ionization charge in a thick x-ray absorbing photoconductor with as selenium, and both approaches have been widely discussed in the literature. While these systems meet the performance needs for general radiographic imaging, further improvements in sensitivity, noise and resolution are needed to fully satisfy the requirements for fluoroscopy and mammography. The approach taken for this paper uses indirect detection, with a phosphor layer for x-ray conversion. The thin a-Si:H photodiode layer for detects the scintillation light. In contrast with the present generation of devices, which have a mesa-isolated sensor at each pixel, these imagers use a continuous sensor covering the entire front surface of the array. The p+ and i layers of a-Si:H are continuous, while the n+ contact has been patterned to isolate adjacent pixels. The continuous photodiode layer maximizes light absorption from the phosphor and provides high x-ray conversion efficiency.
The x-ray imaging performance is reported using polycrystalline lead iodide as a thick semiconductor detector on an active matrix flat panel array. We have developed a test image sensor with 100 micron pixel size in a 512 X 512 format, using amorphous silicon TFTs for matrix addressing. The new 14 bit electronic system allows radiographic and fluoroscopic x-ray imaging. PbI2 has larger x-ray absorption and higher charge generation efficiency than selenium, and has the potential for higher sensitivity imaging. The films are deposited by vacuum sublimation and have been grown thicker than 100 micrometer. Measurements of the carrier transport and charge collection, together with modeling studies show how the x-ray sensitivity depends on the material properties. Imaging measurements find excellent spatial resolution and confirm models of the x-ray sensitivity. Both radiographic and fluoroscopic imaging are demonstrated. While good overall imaging is obtained, the dark leakage current and image lag need further improvement.
We describe new amorphous silicon (a-Si:H) image sensor arrays which are the highest resolution imagers so far reported. The pixel sizes of 64 micrometer (resolution 8 lp/mm) and 75 micrometer (6.7 lp/mm) are made possible using a photodiode technology that enables high sensor fill factor even in very small pixels. This approach allows the a-Si:H imagers to satisfy high resolution requirements of digital mammography. Each array contains 512 X 512 pixels with matrix addressing provided by a-Si:H thin film transistors (TFT). The high fill factor structure contains a continuous a-Si:H photodiode layer grown on top of the TFT array, with contacts to each pixel through a patterned metal/n+ layer. X-ray detection is accomplished by use of a phosphor layer superimposed on the array. The continuous photodiode layer maximizes light absorption from the phosphor and provides high x-ray conversion efficiency. Since the photodiode forms a continuous layer, crosstalk between adjacent pixels due to the lack of isolation is a particular concern, and has been extensively studied. We find that the high fill factor structure can be made such that the lateral charge leakage is minimal in the dark or under moderate illumination, although small amount of charge spreading is observed under conditions of sensor saturation. The measured MTF for optical illumination exceeds 60% at the Nyquist frequency, even for long integration times.
We report the fabrication and evaluation of a Pbl2 imager using large area amorphous silicon technology. This approach uses a thick Pbl2 x-ray photoconductor to absorb x-rays and collect ionization charge under the action of an applied field, while amorphous silicon thin film transistors (TFT) provide a matrix-addressed read out of the signal to external electronics. The x-ray sensitivity of Pbl2 is high, and mobility-lifetime product is large enough to yield a high charge collection at low applied fields. The test arrays used to evaluate Pbl2 have 256 X 256 pixels of size 200 microns. Each pixel contains an amorphous silicon switching transistor, gate and data addressing lines, a charge storage capacitor and a metal pad to contact the Pbl2 layer. Early evaluation of the image sensor indicates the promise of Pbl2 but indicates that reduction of the leakage current is important.
2D amorphous silicon arrays can be sued for medical imaging, non-destructive testing, and high-speed document scanning. We have built a 200 spi imaging system with an active area containing 2304 X 3200 pixels, the largest amorphous silicon imaging system described to data. Packaged with the array are peripheral electronics which include active matrix drivers, charge sensitive amplifiers, two 12 bit A/D converters, and control logic. Digital data travel via fiber to a frame grabber in a personal computer. Software includes gain/offset corrections, line and pixel corrections, window and level controls, and a user interface. Through a combination of layout optimization, amplifier design, and system timing, we have demonstrated a noise level of 1.5 Ke RMS and a signal to noise ratio of 1900.