Pixel Geometry Research Articles

Pixelated CdZnTe drift detectors

A technique, the so-called Drift Strip Method (DSM), for improving the CdZnTe detector energy response to hard X-rays and gamma-rays was applied as a pixel geometry. First tests have confirmed that this detector type provides excellent energy resolution and imaging performance. We specifically report on the performance of 3 mm thick prototype CZT drift pixel detectors fabricated using material from eV-products. We discuss issues associated with detector module performance. Characterization results obtained from several prototype drift pixel detectors are presented. Results of position and energy resolution measurements, charge sharing and IV-measurements are discussed.

P‐91: TFT Substrate Birefringence Induced Asymmetrical Viewing Angle and Compensation Methods

AbstractThin‐film transistor (TFT) processes for small pixel geometries can induce optical birefringence effects and change the viewing angle from symmetrical to asymmetrical. We experimentally identified the TFT substrate as the source of the birefringence and developed four methods to compensate the added optical birefringence effects to restore the symmetrical viewing angle. We also developed two special wide viewing angle polarizers with Fuji Film wide viewing angle film, and special compensation films to compensate the TFT substrate's birefringence.

Fabrication process responsible for fundamentally improving Silicon X-ray microcalorimeter arrays

We have developed an improved microcalorimeter array that will be used on the AstroE-2 satellite mission. The 6×6 array consists of a grid of 36 suspended pixels. Each 1.5μm thick pixel has an ion-implanted thermometer, four thermal links (support beams), and four X-ray absorber support tabs. Improvements in Silicon micro-machining capabilities and the availability of custom Silicon-on-Insulator (SOI) wafers has enabled us to precisely control pixel geometry, lead widths, and develop a more compact array. Knowing the silicon thickness, we can calculate a precise implant dose for the thermometer. Using a high-temperature anneal, we can uniformly diffuse the implant throughout the depth of the top layer of the SOI wafer. Defining the length, width, and thickness of the support beams, we can control the thermal conductance of the pixel. Advancements in polymer-photo resists have enabled us to develop a new absorber support tab attachment scheme resulting in more controlled heat dissipation from the absorber to the thermometer on the pixel. An overview of fabrication improvements focusing on these topics will be discussed.

Single photon counting X-ray imaging with Si and CdTe single chip pixel detectors and multichip pixel modules

Multichip modules (MCM) have been successfully built and operated. These use 4 single-photon counting MPEC 2.3 chips that are bump bonded to 1.3 cm /spl times/ 1.3 cm CdTe or Si semiconductor pixel sensors. Single-chip detectors were built as well. The MPEC 2.3 chip provides a pixel count rate up to 1 MHz with a large dynamic range of 18 bits, 2 counters and energy windowing with continuously adjustable thresholds. Each MPEC has 32 /spl times/ 32 pixels of 200 /spl mu/m /spl times/ 200 /spl mu/m pixel size. For a MCM the 4 chips are arranged in a 2 /spl times/ 2 array which leads to a 64 /spl times/ 64 sensor pixel geometry. The MCM construction is described, and the imaging performance of the different detectors is shown. A newly developed USB readout system has been used.

Broadband and narrow band light coupling for QWIPs

In this work, we will present a detailed study on the corrugated coupling scheme, which uses optical reflection at the angled sidewalls. In particular, we have investigated, both theoretically and experimentally, corrugated quantum well infrared photodetectors (QWIPs) with different cover materials over a wide range of spectral regime. We found that the coupling is insensitive to the wavelength but depends strongly on the optical properties of the cover material. We will also present the results of C-QWIP focal plane arrays (FPA) characterized so far. An important advantage of the corrugated coupling is its scalability to small pixel size. Based on one corrugation per pixel geometry, we have fabricated a 1024 × 1024 C-QWIP FPA with 99.5% connectivity. With the corrugated coupling geometry, many application restrictions on QWIPs can be eliminated. We will further show that the lack of normal incident absorption in QWIPs can actually be an advantage. It allows one to manipulate the absorption characteristics of QWIPs through different optical coupling structures and creates new detector functionality. The present example is an infrared spectrometer. It consists of a linear array of quantum grid infrared photodetector (QGIP) elements. Each QGIP element shares the same QWIP material but has different grid geometry. The QWIP is made of broadband materials such as the binary superlattices to give an extremely broadband (∼10 μm) absorption. The structure of the grid, on the other hand, determines the specific wavelength to detect at each detector element in the array. In the QGIP spectrometer, detectors with different line widths will detect different wavelengths simultaneously, thus yielding the emission spectrum of an object. A collection of QGIP spectrometers can form an adaptive FPA.

Reduction of the switching time of polymer-dispersed liquid crystal using field oriented addressing

Polymer dispersed liquid crystal (PDLC) is a promising liquid crystal for reflective displays in low power mobile applications. However, it is difficult to obtain full color PDLC displays. Color sequential PDLC is one solution but this requires short PDLC switching times. It is demonstrated that the PDLC switching times can be reduced to color sequential levels by changing the orientation of the electric field (field-oriented addressing). By choosing the right pixel geometry, this can be done at relatively low switching voltages compared to the classic addressing method which is based on changing the magnitude of the electric field.

P‐86: Grid‐Aligned Multi‐Domain Mode

AbstractA new method to obtain multi‐domain textures in homeotropically aligned nematic liquid crystal displays is proposed and the mechanisms responsible for texture formation are described. We present textures and electrooptic properties for various possible pixel geometries.

Spatial aggregation of two-dimensional objects in raster data structures

Raster data structures are characterized by pixel geometry and resolution. A method is proposed to reduce resolution to a minimum using spatial aggregation without information loss. The method is based on a matrix approach and developed for a two-dimensional spatial object. Perimeter measures and fractal dimension are shown not to be invariant to spatial aggregation.

Further investigation of the operation of Cd 1− xZn xTe pixel array detectors

In this work we study the spectral performance of Cd 1− x Zn x Te pixel array detectors. The specific effects and relative importance of various detector parameters on spectral response are determined. While bulk material uniformity is still the factor most impacting detector performance, pixel size relative to detector thickness is found to also have a large impact. The localization of the charge induction profile, or weighting potential, leads to a reduction in the importance of hole transport. Thus, pixel geometry is a key design parameter setting spectroscopic performance. Finally, the contact material choice has some effect on performance, in that reductions in leakage current obtained with blocking gold contacts can improve the energy resolution for low photon energies, although the effect is small for small-area pixels.

The swept charge device, a novel CCD-based EDX detector: first results

The first results from a novel MOS-CCD-based EDX detector are presented. The device has been fabricated on high-resistivity epitaxial silicon and uses a novel pixel geometry to simplify operation over more conventional CCDs and possesses high sensitivity, high rate and high resolution for EDX applications. The charge is continuously clocked towards a single read-out at the corner of the 1 cm 2 detector, with no spatial information retained and the device is functionally equivalent to a linear CCD in operation. Because of this sweeping action we have termed the device the “Swept Charge Detector”. The device uses a biased substrate which allows it to be operated in an extremely low-leakage current mode and results are presented at temperatures ranging from −35°C to room temperature. It can be operated in a photon-counting mode with pixel-by-pixel reset for optimum resolution or a reset-on-demand mode for optimum through-put. Its characteristics are compared to other types of silicon X-ray detectors.

Fast modelling of the optical characteristics of electroluminescent pixel structures

An analytic model of the optical behaviour of laterally light emitting thin film structures is developed. It is employed to calculate the outcoupled light of a pixel used in light emitting dot matrix displays such as laterally emitting thin film electroluminescent displays (LETFEL) with micromirrors. Consequently, one can identify the optimum pixel geometry. Here, the optical behaviour of the circular, square and hexagonal pixel geometry is modelled. The presented closed form solutions are based on a ray optics approximation whereby the absorption of the light within the light generating medium (phosphor material) and the transmission behaviour of the phosphor–air interface is taken into account, as well as the micromirror width. These solutions, however, neglect back reflected light. The effect of this neglect is investigated for square pixels by taking into account the first reflection. The model is applied to a typical LETFEL display with ZnS material doped with Mn. An optimal pixel diameter of 35 μm is estimated for that particular type of display.

Alternative area-perimeter ratios for measurement of 2D shape compactness of habitats

Area-perimeter ratios are often used to quantify 2D shape compactness. Shape compactness is of main importance to evaluate the effect of external disturbance on natural habitats. Reference values ( A min( p), A max( p), P min( a), P max( a)) for area ( A) and perimeter ( P) are defined and proven. The calculation of these reference values is based on the characteristics of the object studied and on pixel geometry. The cases for 4- and 8- connectivity are discussed. Using these references, alternative area-perimeter ratios are composed. Examples are elaborated to illustrate the use of the indices and their performance.

High-speed CCD imaging system for monitoring neural activity in vivo and in vitro, using a voltage-sensitive dye

We have designed and constructed a high-speed CCD imaging system for optically detecting neural activity from preparations stained externally with a voltage-sensitive dye, and have used this system to image evoked and epileptiform neural activity in the rat somatosensory cortex. The imaging system uses a commercially available 1/3-in. CCD chip, and it can continuously capture images for more than 8 s, at 1000 frames/s, with a spatial resolution of 128×62 pixels. The spatial/temporal resolution of the CCD sensor is variable by changing the geometry of on-chip binning pixels, which can be controlled by a PC/AT computer. Dye bleaching correction was not necessary for long-term imaging of epileptiform neural events, since the sensitivity of the CCD sensor was increased by combining the signal from adjacent pixels.

Modelling of the small pixel effect in gallium arsenide X-ray imaging detectors

Abstract A Monte Carlo simulation has been carried out to investigate the small pixel effect in highly pixellated X-ray imaging detectors fabricated from semi-insulating gallium arsenide. The presence of highly non-uniform weighting fields in detectors with a small pixel geometry causes the majority of the induced signal to be generated when the moving charges are close to the pixellated contacts. The response of GaAs X-ray imaging detectors is further complicated by the presence of charge trapping, particularly of electrons. In this work detectors are modelled with a pixel pitch of 40 and 150 μm, and with thicknesses of 300 and 500 μm. Pulses induced in devices with 40 μm pixels are due almost totally to the movement of the lightly-trapped holes and can exhibit significantly higher charge collection efficiencies than detectors with large electrodes, in which electron trapping is significant. Details of the charge collection efficiencies as a function of interaction depth in the detector and of the incident photon energy are presented.

Twist number statistics as an additional measure of habitat perimeter irregularity

Individual habitats are altered by the surrounding landscape matrix. Quantitative analysis of habitat boundary is therefore necessary. A shape index is proposed based on the perimeter twist number; twists divide the patch perimeter in seperate segments. Using Principal Components Analysis, the shape index is compared with fractal dimension (per-patch calculation based on area and perimeter) and with four shape (compactness) indices based upon pixel geometry, next to area and perimeter statistics. The analysis is executed with simulated data based on percolation maps. The proposed index can be used as a complementary shape descriptor because of (i) its independence of fractal dimension, (ii) its restricted correlation with only one compactness index and (iii) its ability to identify habitats characterized by limes divergens and a high interior-edge ratio.

A reference value for the interior-to-edge ratio of isolated habitats

Isolated habitats, the consequence of the fragmentation process, are the object of external disturbance. This divides the patch area into two zones: interior and edge. The interior-to-edge ratio quantifies the potential disturbance impact. A method is presented to calculate a reference value for the interior-to-edge ratio, based upon the minimum edge for a given interior. The method is based on pixel geometry features and mathematical morphology. A corrected interior-to-edge ratio is defined using the reference value. The method is illustrated for woodlot fragments in the Belgian Kempen region.

Multi-element CdZnTe detectors for gamma ray detection and imaging

A small pixellated gamma-ray detector has been constructed from a 5 mm thick CdZnTe substrate and characterized for spectroscopic and spatial properties. Emphasis has been placed on revealing 'pixel-level' results to further understand optimum array design and operation. The anode design is (1.5/spl times/1.5) mm/sup 2/ pixels on a 1.625 mm pitch. Spectroscopic results with a /sup 57/Co source display an average energy resolution of 3.9 keV FWHM. Analysis of the electronic noise for different pixels showed good agreement with a theoretical model and similar performance between edge and interior pixels. A test of spatial response with scanned sources incident on the anode face showed good agreement with the pixel geometry and an undetectable amount of dependence on interaction depth.

Fast bistable nematic display with grey scale

We present a novel principle for a fast bistable nematic display with intrinsic grey scale. The geometry of a single pixel is the usual hybrid texture in between two conductive flat plates. The device is written by creating surface walls when an electric field is applied, above the thresholds to achieve both the planar anchoring breaking and an electro-hydrodynamic flow. Erasing is obtained by simply breaking the anchoring in absence of vortices. The distorted regions around surface walls depolarize the incident light. As the surface defect density can be modulated, a grey scale is achieved.

Improved heterodyne mixing efficiency and signal-to-noise ratio with an array of hexagonal detectors

An array of hexagonal pixel geometries is considered and its improvement in the mixing efficiency as well as the signal-to-noise ratio is compared with an equal-area array of square pixel detectors. We also consider the phase-front misalignment between the signal and the local oscillator wave fronts for both pixel geometries.

Gallium arsenide for vertex detectors

We give an overview of recent results in the development of GaAs detectors: they now have 100% charge collection efficiency with good reliability; they are bond-compatible with silicon detectors, with both strip and pixel geometry. New results on pixel detectors are reported as well as a short summary on the radiation hardness of SIU-GaAs detectors.