R. Lau

Image capture array with an organic light sensor

A large-area image capture device using an organic sensor is reported. The 512×512 pixel array, with 100×100 micrometer pixel size, combines amorphous silicon matrix addressing with a continuous organic sensor. The bilayer sensor comprises a tetraphenyldiamine hole transport layer on top of a benzimidazole perylene generator layer. This combination provides high sensitivity across the visible with low dark current. We present imaging properties and in particular show that the lateral charge transport between pixels is small, and that the effective fill factor is ∼90%. X-ray imaging with a phosphor converter is demonstrated.

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.

Micro-electro-mechanical system fabrication technology applied to large area x-ray image sensor arrays

Micromachining has potential applications for large area image sensors and displays, but conventional micro-electro-mechanical system (MEMS) technology based on crystalline silicon wafers cannot be used. Instead, large area devices use deposited films on glass substrates. This presents many challenges for MEMS, both with regard to materials for micromachined structures and to integration with large area electronic devices. We are exploring the novel thick photoresist SU-8 as well as plating techniques for the fabrication of large area MEMS. As an example of its application, we have employed this MEMS technology to improve the performance of an amorphous silicon based x-ray image sensor array. SU-8 is explored as the structural material for the x-ray conversion screen and as a thick interlayer dielectric for the thin film readout electronics of the imager. Plating techniques are employed to metallize the deep contact vias in SU-8. Processing challenges that are particularly important for large area fabrica...

Active matrix of amorphous silicon multijunction color sensors for document imaging

An integrated color image sensor, made entirely with amorphous silicon (a-Si:H) large-area technology, is presented. The a-Si:H based sensor is a double-junction p-i-n-i-p photodiode that discriminates two spectral bands according to the bias voltage. The active-matrix addressed array has 512×512 pixels with 75 μm pixel pitch and uses thin-film transistors as pixel switches. The array structure and the spectral response are discussed, and color images taken by the system using two bias voltages demonstrate the compatibility of color sensors with large-area active-matrix addressing techniques.

Two-color amorphous silicon image sensor

A large-area two-color image sensor array made with amorphous silicon (a-Si:H) technology is described. Mesa-isolated double-junction p-i-n-i-p a-Si:H sensors discriminate the two spectral bands—blue/green and red—according to the polarity of the applied voltage bias. The 512×512 element active-matrix array with 75 μm pixel pitch is addressed using a-Si:H thin-film switching transistors. Under steady state illumination, the array exhibits a linear response, good color separation, and good spatial response as measured by the line-spread function, so that images obtained with the array are clear and sharp. The response to transient illumination exhibits image lag and a strong dependence of the signal on integration time, both of which depend on the bias polarity. Switching the bias voltage also induces strong transient properties. These effects are attributed to the back-to-back diodes, which act as capacitative dividers in the generation and readout of the signal. The transient effects compromise the pract...

Laser processing of amorphous silicon for large-area polysilicon imagers

Pulsed excimer-laser processing of amorphous silicon on non-crystalline substrates allows for the fabrication of high-quality polysilicon materials and thin-film transistors (TFTs). Under optimized processing conditions, these polysilicon TFTs have high mobilities, sharp turn-on, low off-state leakage currents and good spatial uniformity. These improved parameters, particularly the low off-state leakage currents and good uniformity, enable, not only displays, but also the more demanding flat-panel imaging arrays to be fabricated in polysilicon, and results on an imager are presented.

Image Sensor Arrays with Organic Photoconductors

Two-dimensional image sensor arrays incorporating an organic light sensor are reported, with potential application to digital cameras and x-ray imaging. The 512×512 element array has pixel size of 100 micron, and operates with active matrix addressing using amorphous silicon thin film transistors. The array design allows the use of a continuous layer of the organic sensor material without the need for pixel-level patterning. The sensor is a bilayer comprising a thin generator layer and a thicker hole transport layer. Generator materials benzimidazole perylene (BZP) and hydroxy gallium phthalocyanine (HOGaPc) have been studied, with a tetraphenyldiamine (TPD) transport layer. Both sensors give excellent imaging properties with low leakage current, good charge collection and high spatial resolution.

Two-Dimensional Amorphous Silicon Color Sensor Array

This paper reports on the first full realization and characterization of a two-dimensional array of amorphous silicon (a-Si:H) color sensors, addressed by integrated amorphous silicon-based thin-film transistors (TFTs). The array includes 512 × 512 pixels with 75-µm pitch, or about 340 dpi. Each pixel features a color sensor realized by a p-i-n-i-p stack of doped and undoped a-Si:H layers, and the TFT. The color sensors are made of two back-to-back p-i-n diodes, which selectively sense the illumination according to the polarity of the applied bias voltage. The sensor layers are grown on top of the TFTs to improve the array fill factor. The p-in-i-p sensor stack is mesa-isolated into single sensors to reduce cross-talk. Images are acquired using two bias voltages and yield the red and blue/green components of the original with a good color separation. A color image is reconstructed using the information from the two images acquired. Aside from a color bias, which is expected for a two-color reconstruction, the imaging system works well. In particular, the array shows very low leakage currents, which enable a very large dynamic range and sensitivity. In the response of the array to a light pulse, the bottom thick diode ensures a fast drop in the signal after the flash, while the top thin diode exhibits some residual image lag.