Pixel Geometry Research Articles

Three-Dimensional Electromagnetic and Electrical Simulation of HgCdTe Pixel Arrays

We have investigated the combined electromagnetic and electrical response of HgCdTe-based pixel detector arrays with different geometries. We have computed the propagation of the optical signal in the detector structure by solving Maxwell’s curl equations using a finite-difference time-domain approach. From the field distribution inside the device, we have evaluated the optical carrier generation rate. Using this information in a three-dimensional (3D) numerical model based on a drift–diffusion approach, we have computed the quantum efficiency and photoresponse of a number of pixel geometries. Specifically, we have analyzed the response of both mesa-type and planar detector arrays with and without CdZnTe substrate. Furthermore, the electromagnetic response has also been evaluated for different metal contact dimensions and configurations. It is found that, for mesa-type arrays without the substrate, significant reflection effects take place in the device that lead to resonance peaks in the photoresponse.

CZT performance for different anode pixel geometries and data corrections

We have investigated the energy resolution and absolute photopeak efficiency of CZT detectors with three anode pixel geometries after three data correction steps. Before data corrections, the energy resolution is 4–5% at 122 keV, and the efficiency is 46–70% for a 21 keV photopeak window. Charge sharing events, which make up 10–25% of all events, are a significant cause of the poor efficiency. We discovered that large gaps between anode pixels have a strong negative impact on both charge sharing and absolute photopeak efficiency. Using three-dimensional position-dependent energy corrections, we were able to compensate for some of this reduced sensitivity, increasing the efficiency by up to 35% to a final efficiency of 62–80% of incoming gamma events.

Use of Zernike polynomials for efficient estimation of orthonormal aberration coefficients over variable noncircular pupils

An efficient way of estimating orthonormal aberration coefficients on variable noncircular pupils is proposed. The method is based on the fact that all necessary pieces of information for constructing orthonormal polynomials (via the Gram-Schmidt process) can be numerically obtained during a routine least-squares fit of Zernike polynomials to wavefront data. This allows the method to use the usual Zernike polynomial fitting with an additional procedure that swiftly estimates the desired orthonormal aberration coefficients without having to use the functional forms of orthonormal polynomials. It is also shown that the method naturally accounts for the pixelation effect of pupil geometries, intrinsic to recording wavefront data on imaging sensors (e.g., CCDs), making the coefficient estimate optimal over a given pixelated pupil geometry. With these features, the method can be ideal for real-time wavefront analysis over dynamically changing pupils, such as in the Hobby-Eberly Telescope (HET), which is otherwise inefficient with analytic methods used in past studies.

Collected charge of planar silicon detectors after pion and proton irradiations up to 2.2 ×1016 neq cm−2

The planned luminosity upgrade of the Large Hadron Collider at CERN (Super-LHC) will provide a challenging environment for the tracking and vertexing detector systems. Planar, segmented silicon detectors are one of the few radiation tolerant technologies under consideration for use for the Super-LHC tracking detectors in either pixel or strip geometries. In this paper, charge collection measurements are made with planar silicon sensors with 2 different substrate materials (float zone and magnetic Czochralski) and 3 different diode configurations (p+ strip in n-bulk, n+ strip in n-bulk, and n+ strip in p-bulk).For the first time, a comparison of the charge collection of these devices will be made after irradiation up to 6 ×1014neqcm−2 with 280MeV charged pions, and up to 2.2 ×1016neqcm−2 with 26MeV protons. This study covers the expected range of final fluences for the different layers of pixel and microstrip sensors of the ATLAS and CMS experiments at the Super-LHC. These measurements have been carried out using analogue, high-speed (40MHz) electronics and a Strontium-90 beta source.

Charge collection efficiencies of planar silicon detectors after reactor neutron and proton doses up to [formula omitted

The planned luminosity upgrade of the large hadron collider at CERN (Super-LHC) will provide a challenging environment for the tracking and vertexing detector systems. The innermost devices at a radius about 4 cm from the interaction region will have to be able to withstand a combined charged and neutron hadron dose in the order of 10 16 1 MeV neutron equivalent particles ( n eq ) per square centimeter over the anticipated 5 year lifespan of the SLHC experiments. Planar, segmented silicon detectors with n-strip readout are one of the many radiation tolerant technologies under consideration for use for the Super-LHC tracking detectors in either pixel or strip geometries. This paper details charge collection efficiency measurements made with silicon sensors that have been irradiated to doses as high as 1.5 × 10 16 n eq cm - 2 with reactor neutrons and as high as 1.6 × 10 16 n eq cm - 2 with 26 MeV protons and 24 GeV protons. In this study, n + segmented strip readout in either n-bulk (n-in-n) or p-bulk (n-in-p) substrates are considered. Both diode configurations were processed in substrates grown with float zone (FZ) and magnetic Czochralski (MCz) techniques. For the fluences studied, all the n + strip readout technologies are still viable assuming that adequate bias voltage and cooling can be supplied and low noise, low threshold readout electronics can be designed.

C-QWIP focal plane arrays

We have been developing corrugated quantum well infrared photodetector (C-QWIP) technology for long wavelength applications. A number of large format 1024 × 1024 C-QWIP focal plane arrays (FPAs) have been demonstrated. In this paper, we will provide a detailed analysis on the FPA performance in terms of quantum efficiency η and compare it with a detector model. We found excellent agreement between theory and experiment when both the material parameters and the pixel geometry were taken into account. By changing the number of quantum wells, doping density, spectral bandwidth and pixel size, a range of η from 13% to 37% was obtained. This range of η, combined with the wide spectral width, enables C-QWIPs to be operated at a high speed. For example, model analysis shows that a C-QWIP FPA with 10.7 μm cutoff and 25 μm pitch will have a thermal sensitivity of 16 mK at 2 ms integration time with f/2 optics in the presence of 900 readout noise electrons.

Charge Collection Efficiency Measurements of Heavily Irradiated Segmented n-in-p and p-in-n Silicon Detectors for Use at the Super-LHC

The planned luminosity upgrade of the Large Hadron Collider at CERN (Super-LHC) will provide a challenging environment for the tracking and vertexing detector systems. With a proposed nearly 10-fold luminosity increase to 1035 cm2 middot s-1, the innermost devices will have to be able to withstand a hadron dose on the order of 1times1016 1 MeV neutron equivalent per square centimeter over the anticipated five year lifetime of the experiments. Planar, segmented silicon detectors with n-strip readout are one of the radiation tolerant technologies under consideration for use for the Super-LHC tracking detectors in either pixel or strip geometries. This paper details measurements made with n-strip (n-in-p) and p-strip (p-in-n) planar devices that have been irradiated to doses as high as 1times1016 neq cm-2 with reactor neutrons and fast protons. The doses under study cover the expected range of final fluences for the different tracking systems of the ATLAS and CMS experiments at the Super-LHC. The charge collection measurements have been carried out using analogue, high-speed (40 MHz) electronics and a Strontium electron source. These measurements show that p-in-n sensors are not radiation tolerant enough for the requirements of the micro-strip detectors at the SLHC; whereas, the charge collection properties exhibited by the n-in-p strip devices may be sufficient for all tracker regions (pixels and strips) of the SLHC assuming that appropriate low noise and threshold readout electronics are designed and adequate bias voltage can be routed to the sensors.

Numerical analysis of delamination and cracking phenomena in multi-layered flexible electronics

The first part of this paper deals with the analysis of layer buckling and delamination of thin film multi-layer structures that are used in flexible display applications. To this end, 250 nm thick indium tin oxide (ITO) layers have been deposited on a 200 μ m thick high temperature aromatic polyester substrate (Arylite ™ ) with a 3 μ m silica–acrylate hybrid hard coat (HC). Typical buckle morphologies are determined from two-point bending experiments, in which buckle geometries are measured after straightening of the sample. Finite element simulations have been performed to estimate the corresponding interface properties and compressive strains in the layers, and to illustrate the effect of sample straightening on the buckle geometry. The second part considers the fracture sensitivity analysis of an active matrix Thin Film Transistor (TFT) display containing 512 × 256 pixels, with a typical pixel dimension of 160 × 160 μ m . Due to the large scale differences between the display and the TFT patterning a multi-scale modelling framework has been developed, which allows to include realistic material stacks and a detailed pixel geometry. The resulting patterning effect shows an anisotropic behaviour at both the pixel level and the global display level due to the multi-scale approach. Using the Area Release Energy (ARE) method an energy-based failure criterion has been utilised to analyse cohesive failure at several (critical) locations in the pixel geometry. Specifically, the sensitivity to tunnelling cracks in the dielectric layers is considered, which have been experimentally observed in comparable TFT samples that are subjected to tension. This local failure analysis allows for the identification of critical regions within the pixel geometry, which strongly depend on the location within a pixel.

Applicability Of An EM-CCD For Spatially Resolved TIR-FCS

Diffusion constants (DC) of surface near fluorescent particles can be measured by total internal reflection fluorescence correlation microscopy (TIR-FCS). The usage of EM-CCDs instead of photo diodes offers a high degree of parallelization and the possibility of extracting additional information by spatial cross-correlation (TIR image correlation spectroscopy, TIR-ICS).Since temporal autocorrelation functions of particle fluorescence critically depend on CCD parameters such as pixel size and geometry, binning, sampling rate, and gain, we explored systematically the performance of an EM-CCD as detector in TIR-ICS. We found that variations in the sample geometry can be well described by a structure term (ST). Whereas in axial direction the ST is described by evanescent field depth, the lateral extension of the detection volume was found to be well approximated by a Gaussian fit to the convolution of the CCD pixel geometry with the measured point-spread-function for single pixel read-out. For higher binning we empirically could show a linear relationship between the Gaussian approximation for the lateral ST and the size of the quadratic ROI on the CCD used for detection (binning), with a correction factor (slope) that is independent of the CCD chip used.To test the performance of CB TIR-ICS we measured diffusion coefficients (DC) and particle numbers (PN) of fluorescent probes of different sizes (Fluorospheres and GFP) at varying viscosities, concentrations, and sampling rates. This allowed calculating the resolution of the method expressed as the minimal relative resolvable difference in PNs or DCs. Distinguishing between different probe concentrations was possible with differences in PN of 30%. In contrast differences as low as 6% in DC could be distinguished at DC-to-sampling-frequency-ratios smaller than 0.5∗10ˆ4nmˆ2.This renders TIR-ICS suitable and ideal for measuring spatially resolved dynamics of proteins in viscous media such as in live cells.

Pixel-Structured Scintillator with Polymeric Microstructures for X-Ray Image Sensors

We introduce a pixel-structured scintillator realized on a flexible polymeric substrate and demonstrate its feasibility as an X-ray converter when it is coupled to photosensitive elements. The sample was prepared by filling Gd2O2S:Tb scintillation material into a square-pore-shape cavity array fabricated with polyethylene. For comparison, a sample with the conventional continuous geometry was also prepared. Although the pixelated geometry showed X-ray sensitivity of about 58% compared with the conventional geometry, the resolving power was improved by about 70% above a spatial frequency of 3 mm -1 . The spatial frequency at 10% of the modulation-transfer function was about 6 mm -1 .

Magnetic field distribution in the presence of paramagnetic plates in magnetic resonance imaging: A combined numerical and experimental study

The amount and geometric distribution of paramagnetic components in tissue is considered as the basis of T2*-weighted magnetic resonance imaging (MRI). Such techniques are routinely applied for assessment of iron in parenchymal organs such as the liver (hemosiderosis). Furthermore, susceptibility sensitive MRI is discussed as an alternative method to x-ray techniques for quantitative assessment of paramagnetic spongy bone components in patients with osteoporosis. The presented work is dedicated to systematically examining the possible influences of macroscopic arrangements of paramagnetic plates on the magnetic field. In a theoretical approach magnetic field distribution was simulated applying decomposition of the plates in single dipoles. Plate size and distances between parallel plates, as well as plate orientation with respect to the static field, were varied for these numerical simulations. Experiments on corresponding plate arrangements were carried out on a 3 T whole body MR scanner using the field-sensitive MR sequence technique for B0 field mapping. Further examinations were carried out on a bone preparation of the femur, where T2* maps were measured and analyzed on a pixel-by-pixel basis at two orientations with respect to the static field. A series of experiments were performed using isotropic and anisotropic volume elements in three-dimensional gradient echo sequences. Resulting magnetic field distributions in the experimentally recorded B0 field maps were in good agreement with the numerical simulations. Field distortions dominated in areas close to the plates and especially near the edges. Those areas showed strong local field gradients, leading to pronounced signal dephasing effects. The examination of the bone preparations revealed different T2* values for identical regions in the bone when the orientation of the bone or the pixel geometry was changed with respect to the magnetic field. Those effects amounted to nearly 70% (22.9 ms versus 13.6 ms in a region of interest in the femur) for 90 degrees rotation of the femur in the magnetic fields. The orientation of anisotropic picture elements with constant size also showed a strong influence on the derived T2* value (up to 80%, increasing with anisotropy of picture elements).

Position sensitivity of segmented planar HPGe detectors for the DESPEC project at FAIR

One of the proposed germanium detector systems for the upcoming DESPEC array at the FAIR facility consists of triple modules of electrically segmented planar high-purity germanium detectors. In this work, the position sensitivity obtained by means of pulse shape analysis (PSA) for simulated γ-ray interactions has been studied for the possible segmentation patterns for such detectors—the double-sided strip detector (DSSD) and the one-sided pixelated geometry. An over-all similar number of readout channels has been considered for each case, resulting in 8+8 strips and 16 pixels. It has been found that the higher physical granularity of the DSSD results in a significantly higher position resolution, as well as in a somewhat lower probability of merging multiple interaction points. A drawback of the DSSD geometry is the inability to assign the interactions to the correct voxels for certain complex multiple interaction patterns. This effect has been studied in detail. Finally, a comparison has been made with the existing 25-pixel planar detector at KTH [A. Khaplanov, et al., Nucl. Instr. and Meth. A 580 (2007) 1075; L. Milechina, B. Cederwall, Nucl. Instr. and Meth. A 550 (2005) 278)].

Assessment of Tight Pixel Geometry Influence on Surface Chemistry of Hg1−X Cd X Te Focal Plane Array by Time of Flight–Secondary Ion Mass Spectroscopy: Novel Analytical Methods

Factors such as lateral control of surface chemistry, reproducible etch depths and widths, and elimination of cross-contamination are vital to achieving world-class Hg1-XCdXTe focal plane infrared detector arrays. Raytheon Vision Systems (RVS) has made significant progress toward a greater understanding of these factors. The current investigation applies time of flight–secondary ion mass spectroscopy (TOF-SIMS) to assess the manner in which Hg1-XCdXTe surface chemistry is influenced by three different critical physical-chemical factors: (1) process chemistry, including baseline and four alternatives; (2) patterned photoresist format; and (3) etch geometry, including aspect ratio, trench depth, trench width, and unit cell spacing. The first of two patterned photoresist formats consisted of an array having 15-μm unit cells with 5-μm-wide and either 3.5-μm- or 6-μm-deep trenches. The second format consisted of a special diagnostic array of parallel dry-etched stripes having various permutations of trench depths (6 μm and 10 μm), trench widths (3 μm and 5 μm), and trench-to-trench separations (3 μm, 5 μm, and 20 μm). The surface chemistry results relative to etch-depth/width ratios, exposed Hg1−XCdXTe area (etch widths), and trench separation distances show that these parameters have a measurable influence on cross-contaminant abundance and type and on the relative ranking of the process cleaning efficacies. Novel analytical methods for using TOF-SIMS data to qualitatively assess cleaning efficacy, geometry-dependent surface species distributions, the polar/hydrophilic and nonpolar/hydrophobic nature of the processed surface, and the dependence of cleaning efficacy on surface chemistry are also discussed. These methods are intended for use in a variety of studies.

P‐131: Three‐Dimensional Modeling of Liquid Crystal Pixels

AbstractIn this paper an approach to modeling the optics of liquid crystal (LC) pixels or sub‐pixels in a liquid crystal display (LCD) panel in three dimensions is described. A commercial finite element modeling package (ANSYS) was chosen as the primary platform for this solving the director electrostatics problem. Within ANSYS, the pixel geometry and electrical boundary conditions can be defined and the electrostatic field and the LC orientation subsequently determined. The algorithm crucially depends on a scaling of the LC material properties to lower the aspect ratio of the problem. This information is then used to calculate optical transmission through the pixel using a ray tracing algorithm.

Optimization of corrugated-QWIPs for large format, high quantum efficiency, and multi-color FPAs

Previously, we demonstrated a large format 1024×1024 corrugated quantum well infrared photodetector focal plane array (C-QWIP FPA). The FPA has a cutoff at 8.6μm and is BLIP at 76K with f/1.8 optics. The pixel had a shallow trapezoidal geometry that simplified processing but limited the quantum efficiency QE. In this paper, we will present two approaches to achieve a larger QE for the C-QWIPs. The first approach increases the size of the corrugations for more active volume and adopts a nearly triangular pixel geometry for larger light reflecting surfaces. With these improvements, QE is predicted to be about 35% for a pair of inclined sidewalls, which is more than twice the previous value. The second approach is to use Fabry–Perot resonant oscillations inside the corrugated cavities to enhance the vertical electric field strength. With this approach, a larger QE of 50% can be achieved within certain spectral regions without using either very thick active layers or anti-reflection coatings. The former approach has been adopted to produce two FPAs, and the preliminary experimental results will be discussed. In this paper, we also describe using voltage tunable detector materials to achieve multi-color capability for these FPAs.

Remote Sensing of Bridge Displacement Using Digital Image Processing Techniques

In this study, an optical method of real-time displacement measurement of such bridges was carried out by means of digital image processing techniques. A commercially available digital video camera combined with a telescopic device takes a motion picture of the target panel with known geometry, which is installed on the measurement location of a bridge. The displacement of the target is calculated based on the captured images in real-time manner using image processing techniques, which require a texture recognition algorithm, projection of the captured image, and calculation of the actual displacement using target geometry and number of pixels moved. For the purpose of verification of the presented method, a laboratory test was made using shaking table test and the measured displacement by image processing techniques was compared with the data from a contact-type sensor, a linear variable differential transformer (LVDT). The proposed method gave close results to a conventional sensor. Field tests were carried out on a bridge with steel plate girders and a bridge with steel box girders. The test results gave sufficient dynamic resolution in frequency as well as the amplitude.

The Ultimate CCD for Laser Guide Star Wavefront Sensing on Extremely Large Telescopes

All of the extremely large telescopes (ELTs) will utilize sodium laser guide star (LGS) adaptive optics (AO) systems. Most of these telescopes plan to use the Shack-Hartmann approach for wavefront sensing. In these AO systems, the laser spots in subapertures at the edge of the pupil will suffer from spot elongation due to the 10 km extent of the sodium layer and the large separation from the projection laser. This spot elongation will severely degrade the performance of standard geometry wavefront sensing systems. In this paper, we present a CCD with custom pixel morphology that aligns the pixels of each subaperture with the radial extension of the LGS spot. This CCD design will give better performance than a standard geometry CCDs for continuous wave lasers. In addition, this CCD design is optimal for a pulsed sodium laser. The pixel geometry enables each subaperture to follow a laser pulse traversing the sodium layer, providing optimal sampling of a limited number of detected photons. In addition to novel pixel layout, this CCD will also incorporate experimental JFET sense amplifiers and use CMOS design approaches to simplify the routing of biases, clocks and video output. This CCD will attain photon-noise limited performance at high frame rates, and is being incorporated in the plans for the Thirty Meter Telescope (TMT).

A vision-based system for remote sensing of bridge displacement

This study proposed the vision-based system which remotely measures dynamic displacement of bridges in real-time using digital image processing techniques. This system has a number of innovative features including a high resolution in dynamic measurement, remote sensing, cost-effectiveness, real-time measurement and visualization, ease of installation and operation and no electro-magnetic interference. The digital video camera combined with a telescopic device takes a motion picture of the target installed on a measurement location. Meanwhile, the displacement of the target is calculated using an image processing technique, which requires a target recognition algorithm, projection of the captured image, and calculation of the actual displacement using target geometry and number of pixels moved. For the purpose of verification, a laboratory test using shaking table test and field application on a bridge with open-box girders were carried out. The test results gave sufficient dynamic resolution in frequency as well as the amplitude.

Photonic Metal Bandgap Structure for Intersubband Detection

We would like to present here a geometry of Quantum Well Infrared Photodetectors (QWIPs) for increasing the detectivity. It consists of a mesa (a squared etched pixel) with a metal grating on top of it. The grating concentrates the electromagnetic field in the electric area defined by proton implantation. We focus on the design of the grating based on Floquet Bloch S-Matrix calculations. For the optimisation, the relation between the electronic structure of the detector and the pixel geometry is highlighted.

洪水による都市域の浸水区域特定へのレーダーリモートセンシングデータ（ラダルサット）の適用性について

洪水災害マネジメントにおいて詳細な洪水区域図は必須条件である．冠水し易い区域の特定に基づいて詳細な洪水災害の情報を得る手法を用いれば，損失を引き起こす洪水を最小限に抑えることができる．本研究では，レーダーリモートセンシング（ラダルサット）のデータを利用してこれを評価し，バングラデシュ国のダッカ市で1998年に発生した洪水に伴う浸水区域の分布を高い精度で抽出した．適切な洪水対策を講じるために有用と考えられる最大浸水区域を正確に特定するために，洪水期間の7画像と乾期の1画像を使用した．レーダー画像をデジタル処理してピクセルデータに変換し，微小ノイズを除去するためにフィルタリング処理を施した．最大尤度判定規則を用いて画像を浸水区域と非浸水区域に区分した．レーダーデータの区分の精度は，地表データを参照して評価した．洪水の多時点解析によると，浸水区域は1998年7月（浸水率34.65％）から増加し始め，主にモンスーン性の豪雨と河川水位の早い時期の上昇により8月25日にピークを迎えた（浸水率51.29％）ことが明らかとなった．市内東部では深刻な洪水被害が発生した一方で，西部では相対的に標高が高く洪水制御設備も完成していたことから被害はなかった．画像の区分の精度からは，本研究で用いたデータ画像は十分な結果を得られたことは明らかである．最も高い精度だったのは7月31日の画像で88.57％に達し，つづいて7月7日（82.86％），9月10日（81.90％）の順となった．また，すべての画像に対して算出したKappa係数は，8月の画像をのぞいて良い一致を得た．この結果，SARデータは，ダッカ市などの急成長している都市での実時間洪水モニタリングにおいて極めて重要であり，洪水災害の情報とマネジメントのために重要な情報を与えることが可能であることが明らかとなった．