Space instrument programs occasionally need an estimate of how dark current distribution of a silicon CCD changes versus proton radiation exposure and temperature. The task that is the subject of this article was started by adopting a relevant gamma distribution model produced by M.S. Robbins [1] and estimating its parameters, α and β, by using data acquired by Demara [2]. The fortuitous result was that α was found to depend solely on damage displacement dose and β was found to be practically equal to the native bulk dark current of silicon. These implications were tested with information from three published articles. In comparison with the published results, the model was found to be accurate within a factor of two.
We report on the fabrication and characterization of SiO2 thin films by high-density plasma enhanced chemical vapor deposition (HD-PECVD) technique at a processing temperature lower than 400°C for gate dielectric applications in thin film transistor (TFT) devices. An inductively coupled plasma source was used to couple the rf power to the top electrode. The SiO2 thin films were fabricated on p-Si wafers using nitrogen, nitrous oxide, and silane precursors. The deposition process was optimized in terms of the effects of rf power, gas flow rates, and system pressure on deposition rate, chemical etch rate, optical properties, and electrical characteristics. The effects of the processing variables on the refractive index, Si-O bond formation, and impurity related bonds were analyzed. The electrical properties of the films were evaluated from the I-V and C-V characteristics of the MOS capacitors. The effects of the SiO2 film thickness on the electrical characteristics of MOS capacitors were also investigated in the range of 30-100 nm. The influence of the low temperature processed gate dielectric on the performance of 500 Å poly-Si TFTs was evaluated in terms of the transfer and gate leakage characteristics. The microstructural and electrical characteristics of the HD-PECVD deposited SiO2 thin films suggest their suitability for the low temperature integration of TFTs on glass or other low temperature substrates.
Incorporating low temperature poly-Si (LTPS) technology as an active matrix (AM) in flat panel displays (FPDs) has been a struggle and remains costly. The situation motivates companies to investigate alternative AM technologies. Some projects try to relieve the low temperature constraint by switching the substrate material to a metal foil. Some companies try simply to make the most out of a-Si:H technology. Another strategy is to replace silicons as the TFT channel material. Examples include CdSe and even an organic material like pentacene. When crystalline silicon is considered best, engineers consider how to replace excimer laser annealing (ELA) and its cost. A prominent example is the effort at MIT, which printed devices with a kind of 'semiconductor ink'. The best prospects for inexpensive AM backplanes may be plastic transistors and c-Si block assembly. Current leaders of AM technology, a-Si:H and ELA- LTPS, might still end up offering better overall performance, but their costs will probably be higher.
KEYWORDS: Charge-coupled devices, CCD image sensors, Image analysis, Systems modeling, Radiation effects, Signal attenuation, Temperature metrology, Detection theory, Distortion, Lead
Radiation promotes charge transfer inefficiency (CTI) in a CCD, causing focused images to become smeared. For example, such smearing will degrade the accuracy and precision of a CCD-based pointing system. A model has been created whereby CTI smearing caused by a radiation induced traps can be projected, given the spatial distribution and pixel position of the image, the large signal CTI, temperature, parallel and serial transfer frequencies. It is well known that CTI increases with signal size to some asymptotic level, and the model description offers a theory and function for this behavior. The final product indicates several complicating factors affecting CTI evaluation which could inflate results, and make precise comparisons between experiments.
A thinned CCD QE model derived by Morley Blouke provided more than an estimation of quantum efficiency. The derivation of the model also produced charge flux equations in the direction normal to the imaging surface. First, these equations were converted into velocity expressions and subsequently into velocity profiles along the normal axis. Such profiles are excellent tools for visualizing the charge collection dynamics as a function of device structure. Profile by-products include dark layer thickness and speed of response (collection). The latter characteristic essentially constrains transverse charge spread. A simple random walk model was created in order to evaluate the extent of this spread in terms of radial rms distances. This distance is a measure of point resolution and is expressed as a function of CCD design and incident wavelength. The theoretical radius of charge spread from a point source ranges from about 3 micrometers for near-IR to about 20 micrometers for near-UV light. This article also discusses justifications for the model, although the final justification must wait for funding of experimental work.
An analytical depletion-mode MOSFET (DMFET) and gate-to-channel capacitance (Cgc) models were derived from the potential and charge distribution model created by Van der Tol and Chamberlain [1]. The DMFET model predicts ''ds from the four terminal voltages structural parameters and material properties. Three parameters - Nd /tn and x - were extracted from Ids versus Vds curves by deriving conductivity and transconductance expressions from the DMFET model and knowing Na W/L and x07. The DMFET model was then utilized in a source follower configuration to establish the quiescent operating point. The FET''s quiescent point determines its gain and Cgc. Sensitivity is the product of (OVs/9Vg) . (Vg/OQn) i. e. gain and floating node capacitance. This composite model was then used to predict the effects of Rs VOD and VDD on sensitivity and linearity. Comparisons are made to SAGE CCD measurements. 1.
This paper will discuss the design and performance of a serial tap. The tap is intended to permit higher effective data rates from CCDs by extracting data from the device in parallel. The design under consideration is structured such that data may be transferred in both directions in the serial register bypassing the tap or extracted through the tap. Performance of the tap was evaluated using a specially designed test chip. Preliminary performance data are presented. 1.
This paper discusses the development of two materials as AR coatings for thinned backilluminated charge-coupled devices. The first material is the heavy metal oxide Ta205 deposited as a spin on layer using sol-gel technology. The second material is Si3N4. Both these films have the high index of refraction and low absorption coefficients needed to produce good AR coatings in the near UV down to 300 nm. The goal of the program was to produce a coating which would yield devices with quantum efficiencies of greater than 50 at 300 nm. Both these materials satisfy this goal. Data on test devices will be reported. . 1.
The development of the 20482 CCD for a second generation Space Telescope instrument has resulted devices with very few defects dark currents of less than 12 electrons/pixel/hour at 80 readout noise levels of less than 4 electrons rms and excellent charge transfer efficiency at signal levels of less than 10 electrons. A second generation of devices that capitalize on these characteristics have been produced and are currently in test. Faster frame transfer devices that preserve these characteristics have been designed that include tn-directional taps in the serial register. 1.
A third-generation SAGE instrument is about to be designed as part of the NASA Earth Observational System. Previous instruments have used individual diodes as detectors. The new instrument will use a custom design CCD to dramatically enhance the study of the gas and aerosol components of the upper atmosphere. The CCD is a 3 by 400 imaging array that has a single serial register and an exposure control drain. It will be used at the focal plane of a spectrometer covering the spectral range from 288 nm to 1.02 micron.
A third-generation SAGE instrument is about to be designed as part of the NASA Earth Observational System. Previous instruments have used individual diodes as detectors. The new instrument will use a custom design CCD to dramatically enhance the study of the gas and aerosol components of the upper atmosphere. The CCD is a 3 X 400 imaging array that has a single serial register and an exposure control drain. It will be used at the focal plane of a spectrometer covering the spectral range from 288 nm to 1.02 micrometers .
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