Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L27/00—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
  • H01L27/14—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation
  • H01L27/144—Devices controlled by radiation
  • H01L27/146—Imager structures
  • H01L27/14665—Imagers using a photoconductor layer
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L27/00—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
  • H01L27/14—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation
  • H01L27/144—Devices controlled by radiation
  • H01L27/146—Imager structures
  • H01L27/14665—Imagers using a photoconductor layer
  • H01L27/14676—X-ray, gamma-ray or corpuscular radiation imagers
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01T—MEASUREMENT OF NUCLEAR OR X-RADIATION
  • G01T1/00—Measuring X-radiation, gamma radiation, corpuscular radiation, or cosmic radiation
  • G01T1/16—Measuring radiation intensity
  • G01T1/24—Measuring radiation intensity with semiconductor detectors
  • G01T1/246—Measuring radiation intensity with semiconductor detectors utilizing latent read-out, e.g. charge stored and read-out later
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L27/00—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
  • H01L27/14—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation
  • H01L27/144—Devices controlled by radiation
  • H01L27/146—Imager structures
  • H01L27/14601—Structural or functional details thereof
  • H01L27/14603—Special geometry or disposition of pixel-elements, address-lines or gate-electrodes
  • H01L27/14607—Geometry of the photosensitive area
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
  • H04N25/00—Circuitry of solid-state image sensors [SSIS]; Control thereof

Definitions

• This invention relates to systems for producing images, and especially those systems which use photoconductive materials to absorb radiation to create a latent image, followed by a selective detection of the latent image in the photoconductive material.
• Imaging system employs photoconductive materials to absorb incident radiation representative of an image of an object. Suitable photoconductive materials will absorb the radiation and produce electron-hole pairs (charge carriers) which may be separated from each other by an electric field applied across the photoconductor, creating a latent image at the surface of the photoconductor (which is typically a thin planar layer).
• a narrow beam of scanning radiation substantially completes discharge of the photoconductor, by creating motion of a second set of charge carriers.
• the distribution of these second charge carriers in the plane of the photoconductor is affected by the distribution of the first charge carriers, i.e., by the latent image.
• the motion of the second charge carriers is detected and digitized in an appropriate circuit, and thus the latent image is captured in digital form.
• the photoconductor is part of a multilayer structure comprising two electrodes, between which are the photoconductive layer and an insulating layer.
• a high voltage power supply maintains electric fields in the structure during exposures to the incident radiation and the scanning radiation (although not necessarily the same field strength is present during each exposure).
• An example of this type of system is taught in U.S. Patent 4,176,275 (Korn et al.).
• Application of the electric field across the photoconductive layer can be assisted by establishing a prior (reverse) field across the insulating layer, as taught in U.S. Patent 4,539,591 (Zermeno et al.).
• a second and closely related approach known as the air-gap photoinduced discharge (PID) method, employs air as the insulating layer, and requires that a uniform separation be maintained between the two electrodes, typically by high-precision mechanical or piezoelectric devices.
• a corona charges the surface of the photoconductor prior to exposure to radiation, producing an electric field in the material.
• the incident radiation partially discharges the surface to produce a latent image, and the read-out signal is induced by the charge motion under the influence of the residual electric field in response to the scanning radiation.
• U.S. Patent 4,961,209 (Rowlands et al.) employs a transparent sensor electrode positioned over the photoconductive layer, and a pulsed laser which scans the photoconductive layer through the transparent sensor electrode.
• a pulsed laser which scans the photoconductive layer through the transparent sensor electrode.
• Imaging stack i.e., the electrodes, insulator, photoconductive material, etc.
• fabrication of the imaging stack requires applying layers of material to each other, typically by constructing two sub-stacks, and then applying them to each other.
• These procedures can introduce non-uniformities into the thicknesses of the imaging stack.
• reflection and scattering of incident radiation can occur at the interfaces between layers, reducing image quality. This problem, and the attempted solutions to it, are compounded by the non-uniformities in thicknesses.
• the invention is a system for producing an image of radiation incident upon an imaging device.
• the imaging device comprises a first conductive layer, an insulative material, a photoconductive insulative layer, an electrically blocking layer, and a second conductive layer (comprising a segmented array of conductive electrodes), in that order.
• the system further includes means for creating an electric field between the first and second conductive layers such that electron-hole pairs are created by the absorption of incident radiation, then separated to create a current, resulting in the formation of a latent electrostatic image at the interface between the insulative material and the photoconductive insulative layers.
• the invention includes a scanner which, in a first time-ordered pattern, energizes a single spot of the imaging device at a time, each spot producing a second current, comprising mobile charge carriers, within the imaging device.
• the system further includes detection electronics attached to the conductive electrodes and sensitive to motion of the charge carriers, and the sensitivity of the electronics is timed to the members of the array in a second time-ordered pattern.
• the array of electrodes is a plurality of elongated parallel stripes, each stripe lying in a first direction
• the scanner scans the array, one member at a time, in a second direction substantially different from the first direction
• the detection electronics interprets a coincidence of the first and second patterns as a pixel of the image.
• the system has improved output signal strength and higher resolution than known systems.
• Figure 2 is a schematic representation of another embodiment of the invention.
• Figure 3 is an electrical schematic diagram of a preferred amplifier for use with the invention.
• Figure 4 is a cross-section of a portion of the embodiment of Figure 2.
• Figure 5 is a trace of an electronic signal for an amplifier circuit in accordance with the invention.
• Figure 6 is a graph of the performance of an embodiment of the invention.
• Figure 7 is a schematic diagram of electrode stripes in accordance with the invention.
• Figures 8 and 9 are graphs showing signal strength as a function of pixel number for a detector in accordance with the invention.
• FIG 10 is a graph showing signal strength as a function of electrode stripe width for a detector in accordance with the invention.
• the imaging device 10 comprises a layered stack: a first conductive layer 12, an insulative material 14, a photoconductive insulative layer 16, an electrically blocking layer 18, and a second conductive layer 20, in that order (top to bottom as shown).
• the second conductive layer 20 comprises a segmented array of conductive electrodes, 20a-20p in the sixteen channel embodiment shown (other numbers of channels are possible).
• a support for the layered stack such as a supporting transparent substrate and/or a mechanical frame of some type, would generally be used:
• the layered stack may be built up on the substrate from individual components, and then placed in the frame.
• the layered stack may not require a substrate or a mechanical frame, and thus the invention is not so limited as to require their use.
• the substrate may be any material providing mechanical support, dimensional stability, and low electrical conductivity.
• glass having a thickness of approximately 2 to 4 mm is a suitable substrate upon which the segmented array of the second conductive layer may be created by depositing a planar conductive sheet and then etching away undesired material.
• the preferred material for the photoconductive insulative layer is amorphous selenium, which may be incorporated into the layered stack in a conventional manner.
• Lead oxide, cadmium sulfide, and mercurous iodide, among other materials, are suitable, as are organic photoconductor s.
• the photoconductive insulative layer will have low conductivity in the absence of radiation so that an electric field may be maintained across it for a sufficient period of time, such as a resistivity of about 10 9 ohm-cm or greater.
• the thickness of the photoconductive insulative layer should be sufficient to allow it to absorb about 50% or more of the flux of incident radiation (described below). For amorphous selenium and diagnostic x-ray radiation, for example, this thickness is approximately 250-550 micrometers.
• the insulative material may be a fluid material (including a gaseous material such as air) at the operating temperature of the system, or a layer of material which is non-fluid at the operating temperarture of the system.
• the insulating layer is typically 100 to 300 microns thick. It may be created through the use of vapor-deposited polymeric materials such as poly-p-xylene or Union Carbide "Parylene-C," a technique that is preferred for its ability to create a uniformly thick layer, although this ability is adversely affected somewhat when the material is deposited from a point source. Alternatively, the first conductive layer may be separately deposited upon a flexible insulative material, such as an gold layer evaporated onto a polymeric film, and that product added to the stack by use of an optical adhesive, typically in a layer of 1 to 30 microns thick.
• a flexible insulative material such as an gold layer evaporated onto a polymeric film
• Voltage source 22 creates an electric field between the first and second conductive layers 12 and 20, such that electron-hole pairs created in the photoconductive insulative layer (see below) are separated in the imaging device 10 by first incident radiation 30.
• An electric field of 5-20 V/micron is typical. Field strengths in the higher end of this range improve the carrier separation efficiency of the system.
• the electrically blocking layer is chosen such that positive polarity exists on the electrode nearest the insulative layer, and the same relative polarity is used in all phases of the operation of the system, but this is also not necessary if appropriate adjustments are made.
• the first electrode may be biased negatively with respect to the second electrode while the photoconductive layer is exposed to uniform radiation, thereby creating a uniform charge density at the interface between the photoconductive and insulating layers.
• the electric field across the insulator is much higher than that across the photoconductive layer.
• the voltage source is adjusted so that the voltage across the insulating layer is shared with the photoconductor, eg., by using a voltage source value of zero.
• the incident radiation leaves a latent image in the imaging stack by creating charge carriers within the photoconductive insulative layer 16.
• the charge carriers separate under the influence of the electric field created by the voltage source 22.
• a field of approximately 1-5 V/micron may be left across the structure by disconnecting the voltage source 22 and relying on the relatively slow dark decay rate of the photoconductive insulative layer to maintain the field, or by using another voltage source (not shown) at that field strength, and holding it constant.
• the image of interest is in the form of a pattern of incident radiation 30, which is incident upon the imaging stack from either side.
• the incident radiation 30 is incident from the direction of the array of electrodes, but this is illustrative only.
• the array of electrodes and the electrically blocking layer 18 must be semitransparent at the wavelength of the incident radiation 30.
• a preferred embodiment of the invention is designed for use with incident radiation in the form of x-rays (wavelength 10 *7 to 10' 10 cm), for which thin metallic (e.g., aluminum) layers are sufficiently semitransparent.
• a third electric field strength is maintained across the layered stack, typically 1-5 V/micron, and optionally of reverse polarity from that used during the exposure to the image.
• Scanner 26, in a first time-ordered pattern, utilizes scanning radiation 28 to energize the imaging device 10 to produce a second current, comprising mobile charge carriers, within the imaging device 10.
• Scanning radiation 28 may have a wavelength substantially similar to that of the incident radiation, or a substantially different wavelength. Scanning radiation 28 may be ultraviolet, visible, or infrared radiation. Generally, the first time-ordered pattern will ensure that the entire surface of the layered stack holding the latent image is scanned; a preferred pattern scans the entire surface of the layered stack, since until the scan is performed the location of the image on the surface is not known. For most efficient operation at highest resolution, any point on the surface is scanned only once, and no points are missed.
• a preferred pattern is a series of parallel lines in which the scanning proceeds in the same direction in each line, allowing time for the scan to return to the other side of the stack between lines. Such a pattern can be oriented at angles up to 45° to the direction of the electrodes, but preferably is oriented perpendicular to the direction of the electrodes. Scanning radiation 28 is absorbed in the photoconductive insulative layer
• the scanner is a continuous laser and the scanning radiation is in the visible wavelength range. The wavelength is determined by the energy required to excite charge carriers in the photoconductive layer.
• a blue-green laser is appropriate for an amorphous selenium photoconductive insulative layer.
• a laser is preferred for its focusing and intensity properties, but is not preferred for its coherence.
• the use of a coherent light source with an insulating layer having a non-zero thickness can produce interference effects. These may be minimized by reducing the reflection of the scanning radiation from the surfaces of the insulating layer, such as through the use of antireflection coatings on one or both sides of the layer. Methods of accomplishing this have been taught in many sources, including U.S. Patent 4,711,838 (Grezskowiak et al.).
• the scanning radiation 28 energizes the imaging stack 10 by passing through the first conductive layer 12 and the insulative layer 14 prior to absorption.
• the conductive layer through which the scanning radiation passes must be semitransparent at the wavelength of the scanning radiation (e.g., the wavelengths on the order of several hundred nanometers that are typical of visible lasers).
• either conductive layer will be semitransparent due to a metallic construction at a small thickness (e.g, gold), or a non-metallic construction at a greater thickness (e.g., indium tin oxide 0.1 to 0.5 microns thick).
• the electrically blocking layer 18 must also be semitransparent and is typically 0.01 tb 0.1 microns thick.
• the insulating layer 14 may be transparent due to its polymeric composition (e.g, polyester). Also, if a substrate is present and radiation passes through it, it must be transparent at the wavelengths involved. As indicated in Figure 2, the array of electrodes 20 is a plurality of elongated parallel stripes. In x-ray radiation applications, a stripe width of 10-200 microns is preferred.
• the individual stripes are simply connected together in any convenient manner (not shown in Figure 2).
• the direction of the electrodes 20 must be substantially different from the direction of the scan performed by scanner 26, i.e., scanner 26 scans the array in a "vertical" direction, indicated by arrow 32, and the electrodes lie in a "horizontal” direction indicated by arrow 34.
• the directions 32 and 34 are perpendicular to each other, but other substantially different directions are possible with appropriate modifications to detection electronics 40.
• the latent image is captured by detection electronics 40, which is attached to the conductive electrodes 20, and is sensitive to motion of the charge carriers set in motion by the scanning radiation 28. Approximately 10-50 microseconds may be required for the charge carriers to reach the electrodes. For each electrode, the change in induced charge is detected and amplified to produce a signal indicative of the capture of that part of the latent image.
• the sensitivity of the detection electronics 40 is timed to the members of the array 20 in a second, time-ordered pattern.
• a single electrode is made more sensitive than adjacent electrodes by holding the adjacent electrodes at "effective" or “virtual" ground level (not necessarily absolute ground level) relative to the single electrode between them, and by triggering an integrator circuit to begin collecting charge on that electrode.
• the second time-ordered pattern would follow the "direction" of sensitivity, i.e., the location of the most sensitive electrode as a function of time would appear to move repeatedly across the layered stack in the same direction as each pass of the scanning radiation. This apparent motion would be synchronized with the scanning pattern, including pauses at each end of the stack to allow the scanner to move to the next line.
• the detection electronics 40 interprets a coincidence of the first and second patterns as a pixel of the image produced by incident radiation 30. This is contrary to the known practice of scanning a striped electrode in this type of system with a single line-shaped pattern, and coordinating the electronics to read all electrodes simultaneously in a parallel fashion, as taught in U.S. Patent 4,176,275 (Korn et al.) at column 6 lines 18-36.
• Figure 2 shows an example of how the array of striped electrodes could be attached to the detection electronics 40, although other techniques are possible. For convenience of illustration, only nine electrodes are shown in Figure 2: the first through fourth, inclusive; the Nth; and the N+ lst to N+4th, inclusive. Beginning with the first electrode, every Nth next electrode stripe is electrically tied together, i.e., the first and the N+ 1st are tied together, as are the second and the N+2nd electrodes, the third and the N+3rd electrode, etc.
• N channels may be created from M electrodes, where M is greater than N, but only N circuits are required, although of course up to M circuits could be used.
• the array of electrodes also contains a start-of-scan and an end-of-scan electrode at opposite ends of the array, each of which may have a dedicated circuit if desired. This allows the electronics to identify positively if the scanning radiation is at either of these positions in the array, and thus synchronize the detection circuitry for each scanned line.
• FIG. 3 shows an electrical diagram of the preferred circuit 50 for each of the N channels.
• the circuit 50 is comprised of three combinations 51 , 53, and 55 of circuit elements.
• the first combination 51 includes an operational amplifier 52, which can be Burr-Brown OPA637, and a feedback resistor 54, which can be 1 x 10 7 ohms, connected in parallel with a compensation capacitor 56, which can be 70 femtofarads.
• This combination of circuit elements serves as a transimpedance amplifier which results in a conversion of charge pulses into a corresponding voltage pulse.
• the second combination 53 of circuit elements acts as a low-pass filter and includes a resistor 64 and a capacitor 66. This low-pass filter should be designed so that the response rolls off at a desired frequency.
• the third combination 55 of circuit elements includes an operational amplifier 72, such as Burr-Brown OPA627, a capacitor 76, such as 0.001 microfarad, a variable input resistor 74, having a resistance of 0-20 kilo-ohms, and a remotely-controlled switch 78, such as Siliconix VN0300M N-channel enhancement FET.
• This combination of circuit elements functions as a switched integrator controlled by an external signal.
• the resistor 74 is adjusted to give the desired integrator response in volts per coulomb.
• the voltage output of the integrator can be sampled by an analog multiplexer that is controlled by an external signal.
• N 32 and th spacing between electrode center lines is 170 microns (about 5.9 electrodes per millimeter).
• the lateral displacement of the scanning radiation should then be about 5.9 lines/mm to procace about 34.6 of the preferred square pixels per squ 2 millimeter of image.
• each electrode has a width equal to 10-90% of the spacing between electrode center lines, and 50-80% is most preferred. Values above this range lead to increased capacitance between electrodes, and values below this range tend to increase electrode resistance and difficulty in fabrication.
• FIG 4 shows a cross-section of a portion of the system shown in Figure 1.
• Electrodes 20q and 20r are representative members of the array of electrodes forming the second conductive layer 20. There is a gap between electrodes 20q and 20r, and thus it appears that a portion of the image 30 would not be captured by the system.
• charge carriers collected at the edge of electrode 20q, designated as point 21 may have migrated along path 23 from the interface 24, originating in the vicinity of point 25.
• charge carriers collected at the edge of electrode 20r, designated as point 27, may have migrated along path 29 from the vicinity of point 25.
• charge carriers are collected from the entire extent of interface 24, and all charge carriers created at the interface 24 are collected by electrodes 20q and 20r. A similar result occurs for all pairs of electrodes in the array.
• the applied field strength, the thickness of the photoconductive insulative layer, and the spacing between electrodes must all be chosen such that the trajectories and velocities (toward the first conductive layer as well as transverse to that direction) of the charge carriers are optimized for complete recovery of the charge carriers at the interface.
• a single electrode it is possible, although not as preferred, for a single electrode to support multiple lines of resolution, thus more than one pixel of the electrostatic image in the vertical direction will be produced by a single stripe of the array.
• This can be accomplished by using multiple scans of an intensity-modulated laser spot having a size smaller than the width of a stripe, and scanning at higher rates than the preferred embodiment. Each scan will involve modulation of the intensity of the smaller spot over a different subportion of the stripe.
• the first and second time-ordered sequences may be resynchronized periodically (e.g., at least once per scan) during each scanned line.
• the scanned image may be processed in many ways. Typically, after scanning, the electronics processes the image signal through analog/digital circuitry. Each pixel of image is represented as a (preferably at least 12-bit) number indicating the intensity of the image. A single line of the image may be handled as a single block of data. If not done so already, interference effects due to a non-zero thickness insulating layer and a coherent light source should be removed from the image, preferably through digital image enhancement techniques. Preferably, a "windowing" technique produces an 8-bit value from the 12-bit value to enhance the contrast of the image prior to display on a monitor or hard copy device.
• the use of a scanned point beam of light and the collection of induced current using segmented, narrow electrodes located adjacent to the photoconductive insulative layer provides several independent advantages over known systems.
• the invention offers greatly improved image resolution. This advantage results from the location of the segmented electrode, rather than the segmentation per se. For example, with a selenium layer 350 microns thick, a polycarbonate insulator 175 microns thick, and using striped electrodes on 100 micron centers, when the stripes are adjacent the photoconductor they can resolve a latent image pattern with 5 cycles/mm modulation with 50% contrast.
• segmentation of the electrodes allows for use of a plurality of amplifiers to detect portions of the image from different portions of the imaging stack, which in turn allows for the entire image to be formed in less time.
• a single amplifier can wait for all the charges present on the associated electrode to reach a single electrode while the scanning radiation continues on to another region of the image.
• Korn et al. teach increased imaging rates, they use a line (rather than point) source of incident radiation, requiring that each stripe of the segmented array have a dedicated amplifier, and that all such amplifiers are active simultaneously.
• Use of a point source of scanning radiation permits several stripes to be connected together on a single amplifier, and does not require that all amplifiers operate identically in time.
• the narrow striped electrode provides a 10% to 80% increase in strength of the measured signal. This is the opposite of the prior art belief that faster image collection rates exact some degree of loss of signal detection efficiency.
• each amplifier is loaded by the capacitance of only M/N stripes rather than the capacitance of the entire imaging stack. Because amplifier noise increases with the size of the input capacitance, segmentation of the electrode reduces the magnitude of electronic noise associated with each amplifier.
• a detector was constructed as described above and shown in the Figures.
• the first electrode was made by depositing multiple groups of unconnected parallel stripes at center line distances of 170 microns on a glass substrate. Additional depositions near the ends of the stripes provided for the connection of every thirty-second stripe to a bus electrode which terminated at an edge connector pad, allowing connection of each stripe to one of a set of thirty-two amplifier circuits. Start-of-scan (SOS) and end-of-scan (EOS) stripes were added and connected to pads. An oxide blocking layer was then formed over the stripes, followed by a layer of amorphous selenium about 400 microns in thickness. A layer of insulating material, 175 micron thick .
• the insulating layer had a film of transparent conductive material on the outside surface of the polycarbonate to provide a second electrode.
• This detector was mounted in a holder which provided electrical connections and light control to protect the latent image.
• Electronics were connected to the sample holder to provide a controlled high voltage of 3000V to the second electrode, and also thirty-two amplifier circuits, each of which included a current-to-voltage gain stage and a gated integrator stage.
• Digital control circuits provided timed signals for gating the integrator stages and connecting each integrator in sequence to an analog-to-digital converter circuit.
• Additional circuitry provided SOS and EOS signals and timing circuits to synchronize the integrator gating sequence to the position of the laser spot as it scanned across the detector in a direction perpendicular to the stripes.
• the scanning spot was formed by optics to an essentially gaussian 40 microwatt intensity profile of 100 micron width from a gated 442 nanometer helium-cadmium laser source, scanned with a rotating hologon element at a velocity of 29 meters/second at the detector plane.
• the detector was translated perpendicularly to the scan direction by a motor-driven stage moving at a velocity such that successive scans were spaced at 170 micron separations.
• Control circuitry synchronized application of the readout voltage to the second electrode, stage motion, and scan rotation with the data collection signals to activate the amplifier integrator gates and collect digital pixel values from the analog-to-digital converters.
• a trace of a signal taken from the current-to-voltage stage of the eleventh amplifier circuit is shown in a graph 80 in Figure 5.
• the graph 80 shows the current pulses from charge released by the laser light as it crossed stripes 11, 43, 75, 107, etc.
• the pulses are separated by 190 microseconds as determined by the scan velocity, and are each 60 microseconds wide reflecting the transit time of carriers through the selenium. These pulses were integrated to define the pixel signal array.
• a signal trace from the twelfth amplifier circuit showed essentially similar shape except it was shifted by a 6 microsecond delay needed for the light scan to move 1 stripe.
• Another plate of similar construction designated 79O was used to capture an X-ray image.
• a reverse voltage -4 kilovolts
• a readout voltage of zero volts was connected to the second electrode and the plate scanned with a laser spot of 10 microwatts power moving across a pixel stripe in 6 microseconds. Current pulses of shape and magnitude very similar to those in Figure 5 were observed, and the image was clearly visible.
• a detector was constructed to provide comparative data with respect to other constructions.
• a glass plate substrate was pre-coated with a thin bonding layer of chromium and then vacuum-coated with a layer of aluminum to a thickness of about 0.6 microns.
• a pattern of striped electrodes was created using conventional photoresist and etching techniques. The pattern comprised several sets of stripes of differing width and spacing, each electrode being electrically accessible from the perimeter of the substrate.
• a thin layer of aluminum oxide was formed on the aluminum electrodes, and a layer of amorphous selenium was deposited to a thickness of 300 microns.
• a 175 micron thick sheet of polyester was prepared with a transparent conductive electrode of indium tin oxide (ITO) on one surface having a resistivity of 30 ohms/square.
• ITO indium tin oxide
• the untreated side of the polyester was then applied to the etched substrate with an optical adhesive.
• the sample was then mounted in a holder providing electrical connection and illumination control.
• the stripes were connected to amplifiers, each amplifier being a multistage op-amp circuit having a current-to-voltage transimpedence amplifier for the first stage, a voltage gain and output driver for the second stage, and a gated integrator for the optional third stage.
• integrators were not used, and the current signals were displayed on a multi-trace oscilloscope.
• a voltage source was connected between the ITO layer and the ground reference, thus connecting the stripes and the ITO layer through the voltage source.
• Timing circuits controlled the intensity and position, over the ITO layer side of the sample, of a spot of light from an argon-ion laser of approximately 488 nm wavelength.
• the laser light spot was focused to approximately gaussian shape and a diameter of about 95 microns, positioned with a galvanometer and minor, and modulated by an aperture and acoustic-optic element.
• the laser spot could be scanned in a direction essentially perpendicular to the stripe electrodes at speeds of 0-100 m/s, and modulated in intensity through a range of less than one percent to one-hundred percent of full power (22 microwatts) for intervals as short as 2 microseconds.
• the laser light position and intensity signals were also displayed on the oscilloscope.
• the cunent signals from the two stripes showed satisfactory time offset from each other, and the duration of the signals varied satisfactorily with the field across the selenium layer, as verified by carrier transit time calculations. This showed that the cunent from each stripe conesponded to the carriers which were released in the selenium layer adjacent the attached insulating layer in the area defined by the light spot width and stripe spacing.
• the lines 92 and 94 represent the cunent as a function of time in microamps per microsecond for two adjacent stripes 80 microns wide, separated by 20 micron gaps.
• the line 98 represents the calculated light intensity falling on the second stripe from a 95 micron diameter spot with a stripe crossing time of 3.6 microseconds.
• the dotted line 96 represents the cunent predicted by an electrostatic model for carriers released by a gaussian scanned light spot and an initial (field-dependent) carrier transit time of 16 microseconds.
• Example 3 A detector was constructed according to these teachings, but consisting of only four interconnected electrode stripes, each sunounded by wide electrodes attached to an electrical ground as shown in Figure 7.
• the detector 100 included an active electrode stripe 102 which was 380 microns wide, an active electrode stripe 106 which was 780 microns wide, and an active electrode stripe 108 which was 80 microns wide, as shown in Figure 7. Each of the active electrode stripes 102, 106, and 108 were separated from their neighboring ground pads by gaps of 20 microns.
• the detector 100 also included a fourth active electrode stripe 104 which was comprised of eight interconnected 80 micron stripes, each separated from its neighbors by a gap of 20 microns, thereby forming, for the purposes of this example, another single 780 micron wide electrode separated from its adjacent ground pads by 20 microns on each side. This pattern was formed in thin (600 nanometer) aluminum on a glass plate.
• a blocking layer was then formed over the stripes, followed by a layer of amorphous selenium about 300 microns thick.
• a 175 micron thick polyester insulator layer was then attached to the amorphous selenium with an optical adhesive, and this layer was then coated with semi- transparent conductive indium-tin-oxide layer.
• the detector construction was then mounted in a holder that provided electrical connections and light control.
• a positive high voltage was applied to the indium-tin-oxide electrode to create an initial electric field across the selenium layer of approximately 10 volts per micron and the detector was exposed to a flux of approximately 8 milliroentgens of X-ray radiation from a tungsten anode at an accelerating voltage of 70 kV peak.
• Half of the detector was protected by a heavy lead shield during this X-ray exposure.
• electronics were connected to the sample holder to provide a controlled high voltage to the second (semitransparent) electrode sufficient to produce an electric field of about 5 V/micron in the selenium, and to provide for attachment of the interconnected electrodes to an amplifier circuit which includes a cunent-to-voltage stage and a gated integrator stage.
• Digital control circuits provided timed signals for gating the integrator stage, connecting the integrator to the analog-to-digital converter circuit and synchronizing the integrator gating sequence to the position of the laser spot as it scanned across the detector in a direction perpendicular to the stripes.
• the scanning spot was formed by optics to an essentially gaussian intensity profile of 85 micron width from an argon-ion laser beam of wavelength 488 nanometers, translated past the stripes at a rate of approximately 1 meter/second by deflection by a galvanometer minor, and modulated by an aperture and accousto-optic element.
• the detector was translated perpendicular to the scan direction by a motor-driven stage moving at a velocity that spaced successive scans at 85 micron separations. Control circuitry synchronized stage motion, laser gate opening and application of the readout voltage to the second electrode with data collection signals which were used to activate the amplifier integator gate and collect digital pixel value values from the analog-to-digital convertor.
• Figures 8 and 9 show the resulting signals as a function of position perpendicular to the stripes. Similar curves were generated by interconnecting adjacent stripes to form stripes of different widths.
• Graph 110 of Figure 8 shows the signal strength for a region of the plate that was exposed to the X-ray flux.
• the signals for the electrode stripes 102, 104, 106, and 108 are represented by peaks 112, 114, 116, and 118, respectively.
• Graph 110 shows the enhancement of the signal as the laser passes over the edge of a stripe.
• the shape of peak 116 (conesponding to electrode stripe 106) indicates that the signal strength diminishes as the laser enters the central regions of the stripe.
• peak 114 demonstrates that the segmented-but-interconnected electrode stripe 104 exhibits the same behavior as a solid electrode having the same width, e.g. electrode stripe 106. Therefore, a nanow stripe alone does not produce an enhancement of the signal. Rather, what is necessary is that the nanow stripe be read out against adjacent electrodes which are held at a fixed potential, as is the case with electrode stripe 108.
• Graph 120 of Figure 9 shows the signal strength as a function of position perpendicular to the stripes for a region of the plate that was not exposed to the X-ray flux.
• the signals for the electrode stripes 102, 104, 106, and 108 are represented by peaks 122, 124, 126, and 128, respectively.
• a comparison of the four peaks of graph 120 of Figure 9 and the four peaks of graph 110 of Figure 8 indicates that the peaks of Figure 9 are higher. This demonstrates that exposure to X-ray flux diminishes signal strength.
• the signal from each stripe reflects the information from the one or more

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