We have developed a high resolution amorphous selenium (a-Se) direct detection imager using a large-area compatible back-end fabrication process on top of a CMOS active pixel sensor having 25 micron pixel pitch. Integration of a-Se with CMOS technology requires overcoming CMOS/a-Se interfacial strain, which initiates nucleation of crystalline selenium and results in high detector dark currents. A CMOS-compatible polyimide buffer layer was used to planarize the backplane and provide a low stress and thermally stable surface for a-Se. The buffer layer inhibits crystallization and provides detector stability that is not only a performance factor but also critical for favorable long term cost-benefit considerations in the application of CMOS digital x-ray imagers in medical practice. The detector structure is comprised of a polyimide (PI) buffer layer, the a-Se layer, and a gold (Au) top electrode. The PI layer is applied by spin-coating and is patterned using dry etching to open the backplane bond pads for wire bonding. Thermal evaporation is used to deposit the a-Se and Au layers, and the detector is operated in hole collection mode (i.e. a positive bias on the Au top electrode). High resolution a-Se diagnostic systems typically use 70 to 100 μm pixel pitch and have a pre-sampling modulation transfer function (MTF) that is significantly limited by the pixel aperture. Our results confirm that, for a densely integrated 25 μm pixel pitch CMOS array, the MTF approaches the fundamental material limit, i.e. where the MTF begins to be limited by the a-Se material properties and not the pixel aperture. Preliminary images demonstrating high spatial resolution have been obtained from a frst prototype imager.
CCD sensor capable of 100 Mfps (millions frames per second) burst rate has been designed, fabricated and tested. ILT-architecture sensor can capture 16 successive frames with 64x64 pixels and down to 10 ns time resolution. Each pixel consists of a photosite and 16 storage elements arranged in two separate CCD shift registers of 8 elements each. The shift registers connect continuously (and serially) from pixel to pixel to form a column. During burst integration, charge from the photosite is read out alternatively upward and downward into storage elements. During readout time, the photosite is reset while previously integrated charge packets are transferred into horizontal CCD registers located at opposite sides of the sensor. Lag as low as 10% at 100Mfps burst frame rate has been demonstrated. To compensate for low fill factor, microlens array is attached to the die.
We present some design details and characterization results for a VGA CMOS image sensor designed for high sped inspection applications. The sensor has 16 analog outputs, which can each operate at 50 MHz data rate, and can capture images at 1600 frames per second. The image sensor has exposure control functionality, antiblooming capability and a on-rolling shutter architecture to implement snap-shot image capture mode. The pixel architecture incorporates 5 transistors on a 15.3 micron pitch with 50 percent fill factor.
We have developed two single-chip CCD sensor architectures for high-speed, 3-channel color imaging. Both are line-scan sensors for Time Delay and Integration (TDI) imaging. One architecture achieves a sub-microsecond TDI register shift time by contacting metal to poly-Si gates through the imaging regions. The other has no metal in the imaging regions and requires a longer shift time. Both sensors are capable of 40 MHz data rate per channel. Line rates for 2048-pixel devices of 16.5 and 18.5 kHz (shift times of 7.5 and 0.7 microsecond(s) /stage) are achieved.
A femtosecond continuum, with a bandwidth of 90 nm, has been used in a pump-probe geometry to investigate nondegenerate two-photon absorption (2 PA) from 660 nm to 570 nm, with pump wavelength (lambda) equals 620 nm, in hexagonal CdS at 300 K. A dependence upon probe wavelength and relative beam polarization is observed. The polarization anisotropy has allowed us to measure the relative magnitudes of particular (chi) <SUP>(3</SUP>) tensor elements. The nondegenerate 2 PA coefficient increases by a factor of about two from 660 nm to 570 nm. The anisotropy and dispersion are discussed in terms of a two-parabolic-band model.