Raj B. Apte

All jet-printed polymer thin-film transistor active-matrix backplanes

Thin-film transistor (TFT) backplanes fabricated by using jet printing as the only patterning method are reported. Additive and subtractive printing processes are combined to make 128×128 pixel active matrix arrays with 340μm pixel size. The semiconductor used, a regioregular polythiophene, poly[5,5′-bis(3-dodecyl-2-thienyl)-2,2′-bithiophene]; (PQT-12) is deposited by inkjet printing and exhibits average TFT mobility of 0.06cm2∕Vs, on/off ratios of 106, and minimal bias stress. The printed TFTs have high yield with a narrow performance distribution. The pixel design benefits from the registration accuracy of jet printing and it is shown that the electrical performance is suitable for addressing capacitive media displays.

Two Dimensional Amorphous Silicon Image Sensor Arrays

Large two dimensional amorphous silicon image sensor arrays offer a new approach to electronic document input and x-ray imaging. The sensor array technology is now capable of image capture at greater than 10 frames/sec and with resolution of 200–400 spi. We describe our new high resolution imaging system, comprising a page-sized sensor array with nearly 3 million pixels, and the accompanying high speed read out and processing electronics. The key technological issues of pixel resolution, sensor fill factor, leakage currents and noise are reviewed. Measurements of a new array architecture are described, in which the sensor is formed as a single continuous film on top of the matrix addressing components.

Printing Methods and Materials for Large-Area Electronic Devices

Two digital printing methods for the fabrication of active matrix thin-film transistor (AM-TFT) backplanes for displays are described. A process using printed resists layers, referred to as digital lithography, was used to fabricate arrays of hydrogenated amorphous silicon TFTs. TFTs were also fabricated using a combination of digital lithography to pattern metals and inkjet printing to pattern and deposit a polymeric semiconducting layer. The relative performance of amorphous silicon and polymer TFTs were evaluated. The utility of digital lithographic processing was demonstrated by the fabrication of prototype reflective displays using electrophoretic media.

Polycrystalline pentacene thin films for large area electronic applications

Abstract The growth of and the electronic transport in pentacene films on organic and inorganic dielectrics were studied. The morphology and structural properties of pentacene films are clearly correlated with the deposition parameters and substrate properties. To study the electronic properties we have formed inverted staggered transistors. The mobility in the transistors is correlated with the structural properties of the films and increases with crystal size. The TFTs exhibit typical mobilities of 0.4 cm 2 / V s and on/off ratios >108 on thermal oxide and smooth silicon nitride. The dielectrics influence on device performance will be discussed.

Two-dimensional amorphous silicon image sensor arrays

Abstract Large two-dimensional amorphous silicon image sensor arrays offer an advantage for high speed document scanning and medical X-ray imaging. We describe our page sized 200 spot per inch imager and the accompanying high speed readout electronics. The spatial resolution performance for white light and X-ray imaging is illustrated. We discuss how the important issues of noise and resolution depend on the properties of a-Si:H, and show how the material can be used to give improved performance of the imagers.

Amorphous silicon sensor arrays for X-ray and document imaging

Abstract Amorphous silicon image sensor arrays are being developed for X-ray imaging and document scanning. The arrays have a two-dimensional structure with each pixel containing a sensor and a TFT, both fabricated with a-Si:H by photolithography on 12 × 13″ glass substrates. The design and characteristics of a recent complete X-ray imaging system is described. Measurements show that the sensor can detect a photocurrent as low as 1 fA, with imager integration times up to 250 s. These results are largely determined by the sensor and TFT leakage currents. New information about the TFT transient off-current is obtained.

Defect Identification in Large Area Electronic Backplanes

We describe a rapid testing system for active matrix thin-film transistor (TFT) backplanes which enables the identification of many common processing defects. The technique spatially maps the charge feedthrough from TFTs in the pixel and is suited for pixels with switched-capacitor architecture.

High Resolution, High Fill Factor A-SI:H Sensor Arrays for Optical Imaging

Amorphous silicon large area sensor arrays are in production for x-ray medical imaging. The most common pixel design works very well for many applications but is limited in spatial resolution because the available sensor area (the fill factor) vanishes in small pixels. One solution is a 3-dimensional structure in which the sensor is placed above the active matrix addressing. However, such high fill factor designs have previously introduce cross talk between pixels. We present data for a design in which the a-Si:H p-i-n photodiode sensor layer has a continuous i-layer and top p + -layer, and a patterned n + -layer contact to the pixel. Arrays of 64 μm and 75μm pitch have been fabricated and are the highest resolution a-Si:H arrays reported to date. The resolution matches the pixel size, and sensitivity has been improved by the high fill factor. Comparison is made between arrays with standard TFTs and TFTs with self-aligned source and drain contacts. Data line capacitance is improved by use of the self-aligned contacts. Measurements are included on the contact to bias capacitance. The high fill factor design greatly suppresses lateral leakage currents, while retaining ease of processing. Provided illumination levels remain below saturation, the resolution matches expectation for the pixel size.

Large area amorphous silicon x-ray imagers

Abstract Large two dimensional amorphous silicon imaging arrays are of interest for electronic document input and x-ray imaging. The device is a matrix-addressed array of light detectors fabricated from hydrogenated amorphous silicon on a glass substrate. Each imaging pixel consists of a light sensor and a thin film transistor (TFT). X-ray imaging is accomplished by placing a phosphor in contact with the image sensing surface, or by direct detection with a thick photoconductor. The imager technology is now capable of 10 in. arrays with image capture at greater than 10 frames/sec and with resolution of 4–6 lp/mm. We describe our new high resolution imaging system, comprising the sensor array with an active area of approximately 8 × 10 in. having nearly 3 million pixels, and the accompanying readout electronics. Key technological issues and alternative array designs are discussed.

High performance amorphous silicon image sensor arrays

Abstract We discuss approaches to improve the performance of amorphous silicon (a-Si) X-ray imagers. Imager size has been increased to 30×40 cm and the number of pixels to >7 million. Processing improvements have nearly doubled the sensor fill factor (and hence sensitivity) and have reduced the data line capacitance which is the principal contribution to the electronic noise of the system. With new thin film transition (TFT) technology both amorphous and polysilicon TFTs made with the identical structure, using laser recrystallization are produced. The hybrid approach allows smaller leakage current a-Si:H devices to be used for the pixel TFT, and high mobility polysilicon devices for peripheral electronics. Arrays incorporating a lead iodide X-ray photoconductor have promise for higher sensitivity provided the leakage current can be reduced.