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
  • H01L27/14676—X-ray, gamma-ray or corpuscular radiation imagers

Definitions

• This patent specification relates to radiation sensors in general and more particularly to a radiation detection panel comprising a plurality of radiation sensors useful for imaging x-ray exposures. More specifically, this patent specification is directed to a panel that has a layer with an imaging characteristic that can be changed as needed in clinical use, after manufacture of the panel. The relevant characteristic can be the electrical conductivity of the layer.
• Radiation sensors that convert incident radiation directly to an electrical charge related to the incident radiation are known.
• such sensors comprise a layered structure that includes a bottom and a middle conductive electrode separated by a dielectric to form a charge storing capacitor.
• a radiation detection layer which may be a photoconductive layer, is placed over one of the electrodes. Over the photoconductive layer there is a "dielectric" layer, and then a top electrode.
• Charge blocking layers are often provided between the conductive electrodes and the photoconductive layer, and there can be other elements as well.
• a charging voltage is applied between the top electrode and the bottom capacitor plate.
• Read-out of the stored charge is usually done by addressing the middle electrode and transferring the capacitor charge to a charge-measuring device such as a charge-integrating amplifier.
• a plurality of such sensors can be formed as an array of rows and columns of a radiation detection panel.
• This process can image the radiation incident on the panel after it has passed through a subject illuminated by the radiation.
• the radiation is X-ray radiation and the subject is a patient the resulting image is a radiogram, captured as a plurality of charges.
• This radiogram can be displayed on a Cathode ray tube or other device for viewing.
• the charge stored in the capacitor is read-out using a switch that connects, upon command, the middle electrode to the input of a charge measuring device.
• Such switch is usually an FET transistor created integrally with the sensor, having its source electrode connected directly to the middle electrode of the sensor. Both the drain electrode and the gate are accessible from outside the sensor. The drain is connected to a charge integrator. An electrical signal applied to the gate switches the transistor to a conductive state and permits the charge to be transferred from the capacitor to the integrator for detection.
• Exposure is used in this specification to designate the product of the intensity of the incident radiation and the time during which the radiation impinges on the sensor.
• Exposure is used in this specification to designate the product of the intensity of the incident radiation and the time during which the radiation impinges on the sensor.
• One solution to overexposure is to provide a layer that has a sufficiently high electrical resistance to create a secondary field limiting the excessive build-up of charges and voltages.
• the secondary field needs to be cleared before the next x-ray exposure. In radiography, where the time between exposures is relatively long, this can be a part of an extra, post-exposure step in which the electrical charge pattern creating the secondary field is allowed to dissipate.
• a protective layer between the radiation detection layer and the top electrode that can reduce this risk by presenting a barrier to the migrating charges, which begin accumulating at the interface between the radiation detection layer and such a protective layer. These accumulated charges set up a secondary field opposing the applied charging field, thus inhibiting further charge migration and providing a limit to the rising voltage at the middle electrode.
• a flat panel x-ray detector having a control layer that can have its electrical conductivity (the reciprocal of resistivity) changed significantly in response to an externally supplied control such as selective exposure to energy such as light, e.g., infrared light.
• a detector can comprise: a) a charge storage capacitor; b) a first photoconductive radiation-sensitive layer over said charge storage capacitor; c) a control layer over the radiation sensitive layer, where the control has an electrical conductivity v controllable over a range, e.g., by the application of energy from an external source.
• the parameters of the energy (e.g., light) that controls the electrical conductivity of the control layer can depend upon the expected x-ray exposure and the imaging mode of the panel (e.g, static radiography vs. fluoroscopy).
• the light frequency preferably is in the range from visible to infrared.
• the control layer may be an organic photoconductor (OPC) in which the control properties, e.g., the electrical conductivity y, can be modified using light modulation.
• OPC organic photoconductor
• the detector panel uses an array of a multiplicity of radiation sensors, each of said sensors including: a) a charge storage capacitor; b) a radiation sensitive layer over said charge storage capacitor; c) an externally controlled control layer over said radiation sensitive layer, and d) a top conductive layer over said control layer.
• a system including a preferred embodiment of a flat panel detector can also include a source of light radiation used to modulate the electrical properties of the control layer, so that the system includes the detector panel described above and, also: e) a source of light, with wavelengths that preferably can be in the range of 750- 1000 nanometers, used to illuminate the control layer, such as in a uniform manner across the panel.
• the source of the energy that controls the electrical conductivity of the control layer can be controlled by the system computer that also controls the readout of the flat panel detector, in order to suitably time the clearing of the secondary field to the readout of the image.
• this patent specification describes a direct x-ray imaging panel that has an imaging property (e.g., the electrical conductivity of a layer) that can be controlled by the application of energy from an external source (e.g., a light source) in a manner that makes the panel suitable for different types of x-ray imaging.
• an imaging property e.g., the electrical conductivity of a layer
• an external source e.g., a light source
• Figure 1 is a schematic elevation of internal structure of a radiation sensor in accordance with a preferred embodiment.
• Figure 2 is a plan view illustrating a portion of a detector panel comprising a plurality of sensors of the type shown in Fig. 1.
• Figure 3 illustrates a simplified electrical equivalent circuit of the sensor of Fig. 1.
• Figure 4 shows relative absorbance of light in the 400-800 nm range of materials that can be used for the photoconductor layer and for the variable conductivity control layer in a preferred embodiment.
• Figure 5 is a vertical section similar to Fig. 1 but also illustrating an applied external electromagnetic field.
• Figure 6 illustrates an operation of the sensor of Figs. 1 and 5 in the presence of radiation and an applied electric field across the device.
• Figure 7 is a view similar to Figs. 1 , 5 and 6 but also shows provisions for reducing the sensitivity of a switching transistor to infrared light.
• a sensor 10 is built on a substrate 30 that may be glass, ceramic, or other suitable insulating material providing enough mechanical strength to support the layers and associated electrical circuitry.
• An electrically conductive element 32 forms a first or bottom conductive microplate 32, and an electrically conductive element 36 forms a second conductive microplate.
• a dielectric layer 34 is placed between the two microplates to form a charge-accumulating capacitor 14.
• Conductive elements 32 and 36 can be thin layers of a conductive material such as indium-tin-oxide, or a thin layer of an electrically conductive material such as a metallic material, e.g., between 5 ⁇ A and 100A thick.
• An FET transistor 40 is also built on substrate 30.
• Such transistor preferably comprises a gate electrode 42, a semiconductor 43 which is typically ⁇ Si, a contact layer 44, typically a layer of ⁇ Si doped to n+ conductivity, a source electrode 46 and a drain electrode 48.
• Source electrode 46 is connected to the second conductive microplate 36
• the drain electrode 48 is connected to a conductor leading to a contact for connecting the FET 40 to a charge detector.
• the technology for making arrays of FET transistors connected to arrays of microplates is known. (United States Patent 5,641 ,974 issued to den Boer et. al. discusses one way of making such a transistor but there are many other ways to make such arrays that are suitable for the purpose of the detector panels discussed here.)
• a radiation detection layer which is typically a photoconductive layer 50, and which preferably exhibits very high dark resistivity (low conductivity), is over the previously deposited layers.
• the incident radiation is X-ray radiation
• the radiation detection layer is an X-ray photoconductor.
• the photoconductive layer can comprise amorphous selenium, lead iodide, lead oxide, thallium bromide, cadmium telluride, cadmium sulfide, mercuric iodide or any other suitably photoconductive material. It can comprise organic materials such as polymers, loaded with X-ray absorbing compounds exhibiting photoconductivity when the captured radiation is X-ray radiation.
• this layer is a continuous amorphous selenium layer with graded arsenic content, 200 to 1000 microns thick.
• This patent specification refers to "selenium" as the material for layer 50, but includes in that term the preferred photoconductor that comprises amorphous Se with graded As content and possibly dopants such as chlorine, as used in the current commercial product of the assignee, as well as possible other doping. See U.S. Patent 5,880,472.
• a control layer 52 in this example a control layer whose electrical conductivity can be controlled with an external source of energy, is placed over the photoconductive layer 50, and a conductive top electrode 20 is placed over the control layer 52.
• Top electrode 20 is preferably a layer of chromium; other conductive material such as indium-tin-oxide, aluminum, etc. may be used.
• the top electrode properties are selected so that it is substantially transparent to the radiation one wishes to detect.
• the top electrode is preferably a conductive layer which is highly penetrable by such radiation.
• control layer 52 the radiation sensor 10 preferably is substantially the same as in the flat panel detector commercially available from the assignee hereof. However, the presence and use of control layer 52 is believed to significantly improve performance and expand the fields of use of the sensor.
• Control layer 52 is made of a material that normally exhibits relatively low dark conductivity (high dark resistivity), preferably dark conductivity much lower than that of the photoconductive layer 50, but is sensitive to and responds to incoming energy such as light to increase its conductivity to a value higher than that of the photoconductive layer 50.
• control layer 52 is an organic photoconductor (OPC) layer that can be continuous and, for example and without limitation, 10 to 50 microns thick, and has a high photosensitivity to electromagnetic radiation such as light having wavelengths in the range of, for example and without limitation, 750-1000 nanometers.
• OPC organic photoconductor
• Various types OPC materials are commonly used to coat the photoconductor drums of office laser printers, for example in Hewlett Packard Laser Jet 5 printers.
• a thin layer 54 that can be either a unidirectional charge blocking layer permitting passage of one type of charge and not another, i.e., either negative charges or positive, between the second microplate and the photoconductor, or an insulating layer which permits no charge flow between the microplate and the photoconductor, is placed between the second microplate 36 and the photoconductive layer 50.
• Such unidirectional charge blocking layers are known in the art, and typically comprise a non-conductive oxide formed on the microplate surface facing the photoconductor. Commonly assigned United States Patent 6,025,599, hereby incorporated by reference, teaches the use of such a layer.
• Another unidirectional charge blocking layer 60 is optionally placed between the control layer 52 and the conductive top electrode 20
• the technology for forming the sensors preferably involves vacuum deposition of alternating layers of conductive and insulating materials, and is known in the art. See for instance "Modular Series on Solid State Devices” Volume 5 of Introduction to Microelectronics Fabrication by R.C. Jaeger, published by Addison-Wesley in 1988.
• Control layer 52 preferably is spin-coated.
• a programmable power supply 90 for applying a charging voltage to the sensor.
• the power supply 90 is connected to the top electrode 20 and the bottom microplate 32 of the storage capacitor.
• a plurality of sensors 10 can be arrayed on a supporting structure 12 to provide an imaging panel capable of capturing radiation produced images.
• Such a panel can be made as a single unit or can be a composite of a plurality of smaller panels assembled to achieve a desired size.
• U.S. Patent 5,381 ,014 issued to Lee et al. on October 8, 1996 discusses making larger panels using smaller units.
• an electrical equivalent of the multilayer structure of sensor 10 can be represented as a number of capacitors connected in series.
• the detector includes a control layer 52
• Cd is the capacitor formed by the top electrode 20 and the radiation detection layer top surface 51 and includes the control layer 52 separating the top electrode from the radiation detection layer. This capacitor dissipates its charge in a manner that depends to a significant degree on the electrical conductance of layer 52.
• layer 52 is designed to conduct electricity is an important characteristic for the operation of sensor 10.
• Cse is the capacitor formed by the top surface 51 of the radiation detection layer 50, and the insulating layer 54.
• Cin is the capacitor formed by the interface between radiation detection layer 50 and insulating layer 54.
• Cst is the storage capacitor formed by the middle and bottom microplates 36 and 32.
• the capacitor values can be optimized such that, as a non-limiting example, a 10 V/micron electric field is initially applied across the radiation detection layer 50.
• Figure 3 also shows another electrical equivalent of a sensor, in which the capacitors Cd, Cse, Cin and Cst are represented each as a pure capacitor Cd', Cse',
• Cin' and Cst' each connected in parallel with a resistance Rd, Rse, Rin and Rst.
• Rd resistance
• Rde resistance
• Rin resistance
• Figure 4 illustrates the relative absorbance of an organic photoconductor (TiOPc) and amorphous selenium (a-Selenium) as a function of the wavelength of applied light. It can be seen that for light of wavelength of 750 nanometers, for example, selenium has much less relative absorbance than an organic photoconductor.
• TiOPc organic photoconductor
• a-Selenium amorphous selenium
• Selenium is essentially transparent to infrared light, and has an absorption coefficient of about 10/cm for light of wavelength around 750 nanometers, resulting in about 40% absorption in a 500- micron thick layer. Selenium is even less absorbing for longer wavelength light. See, for example, Hartke and Regensburger, Electronic States in Vitreous Selenium, Phys. Rev. A, 139, 3A, A970 (1965).
• the relatively high transparency and low photosensitivity of selenium to infrared light means that the OPC layer 52 should be affected much more significantly than the selenium layer 50.
• a method of controlling the characteristics of the control layer 52 is illustrated in Fig. 5.
• An external source of electromagnetic radiation 100 preferentially infrared light uniformly illuminating the sensor, is applied to the layer 52.
• the responses of layers 52 and 50 to this radiation 100 are shown in Figure 4, for suitable layer materials of OPC for layer 52 and selenium (with As content) for layer 50.
• a proper selection of the control layer 52 should allow controlling the time constant of this layer using the external light 100.
• the time constant for the layer 52 in the dark, ⁇ dark is given by equation (2), and the dark conductivity ⁇ dark of the layer 52 ideally is relatively low so that the ⁇ dark can be several seconds or longer.
• ight ideally is shorter, for example and without limitation, less than about 0.5 seconds, preferably less than 0.05 seconds, and most preferably less than 0.03 and even 0.01 seconds.
• the layer 52 is thin relative to the thickness of the radiation-sensitive layer 50.
• layer 52 is 10-50 microns thick (about 20 microns is currently preferred) while layer 50 is 200-1000 microns thick.. Because of the thinness of layer 52 and because it is comprised of low-Z materials, layer 52 is substantially non-responsive to x-rays. Thus its behavior can be modulated using the external light source 100.
• T By changing the electrical conductivity Y of the control layer material 52, T may be adjusted and consequently the time for the charges accumulated on the interface between the radiation detector 50 and the control layer 52 to dissipate, can also be adjusted.
• a plurality of radiation detection sensors arrayed on a support is used to create a panel to capture an image. Each of the sensors forms a PIXEL, or picture element.
• the panel is charged by applying a charging voltage between the top electrode and the bottom microplates. It is then exposed to radiation, which carries image information as a modulated intensity. The radiation impinges on the panel for a preset duration, and charges related to the radiation intensity are generated and stored in the storage capacitors.
• Appropriate signal processing (preferably of the type discussed in United States Patent 5,648,660 issued to Lee et al.) is used to recover the accumulated charges in all of the storage capacitors in the detectors in the panel and to reconstruct a visible image.
• Two of the x-ray imaging modes in medical radiology are (a) single shot imaging to produce still pictures, and (b) real-time continuous exposure imaging for real-time continuous image observation.
• the first kind is generally referred to as radiography, and involves the taking of single exposure still pictures known as radiograms. Exposures are short, typically a few milliseconds, often as few as 0.002 sec. and the intensity of the radiation is relatively high.
• the second method requires a continuing image capture and display, usually in real time, generally known as fluoroscopy. In this instance the radiation exposure is relatively long and the radiation levels low. Exposures of a few minutes are common, and the display can be a real-time display of a plurality of sequentially obtained images from the panel.
• Images may be displayed as rapidly as 30 images per second to create the visual impression of motion, or can be displayed at a lower frame/image rate.
• the individual charge storage capacitors can be read out and discharged as frequently as, for example and without limitation, every 0.020 to 0.100 sec.
• the control layer desirably has a longer time constant T associated with it.
• a control layer 52 with a time constant of the order of seconds and possibly as long as, e.g., 20 seconds can be used in order to provide the needed overexposure protection. See U.S. Patent 5,319,206.
• time constants as low as a few milliseconds are desirable to assure sufficient dissipation of the accumulated charges at the interface between readouts. Time constants of 100 milliseconds or shorter may thus be called for.
• a dual-purpose panel is provided that is useful in both types of medical radiography. This can be achieved by selectively and reversibly adjusting the time constant of layer 52, without disassembly of or remaking the panel.
• layer 52 has lower conductivity and, consequently, a longer time constant, and the sensor can behave, during the short time that the panel is being exposed to x-rays and is then read out, similarly to the sensor described in U.S. Patent 5,319,206.
• the time constant of layer 52 can be long enough to provide the needed charge build-up that prevents damage to the transistor due to voltage build-up in the charge storage capacitor.
• the action of energy such electromagnetic radiation that can be infrared light
• the top electrode 20 is transparent to such light, and so the light penetrates to layer 52 and causes its conductivity to increase sufficiently so that the time constant of this layer is reduced, for example and without limitation, to approximately 50-100 milliseconds or less.
• This time constant permits the charges trapped at layer 52 to dissipate rapidly so that any remnants of such charges from previously read-out x-ray images would not have a significant effect on x-ray image read-out later during long, multi-frame fluoroscopic type imaging.
• the lower time constant of layer 52 can still be enough to protect against overvoltage.
• the control over the timing and other parameters of the electromagnetic radiation can be carried out by the same computer that controls functions such as the timing of flat panel detector readout, schematically illustrated at 56 in Fig. 5.
• a manual control can be provided, such as a manual switch to turn on an infrared light source secured to or integrated within the x-ray imaging panel.
• the externally applied energy such as infrared light can be applied continuously during fluoroscopic exposure or it can be applied intermittently, in a sequence and/or at energies calculated to achieve the desired reduction in the time constant of layer 52.
• the energy that reduces the time constant of layer 52 is kept off or at such a level as to maintain the layer 52 at lower conductivity and, therefore, with a longer time constant.
• a material useful in control layer 52 in this embodiment is an organic photoconductor such as that manufactured by AEG Elektro Stammie Gmbh, Warstein-Belecke, Germany, and discussed in the article by Deiter Adam, Hans-
• the light decay constant, ⁇ ⁇ ght (decay during stimulation with light), is 0.067 seconds.
• the dark resistivity p dark is calculated to be about 2.4e14 ⁇ -cm, and the light resistivity p, ight about 2.7e11 ⁇ -cm.
• the observed light decay constant is relatively independent of light wavelengths in the range 750-1000 nanometers. Illumination with the proper wavelength of light allows modulation of the time constant of the layer, with suitable faster and slower time constants for fluoroscopic and radiographic imaging.
• a protective shield 210 can be formed, if needed, over the amorphous semiconductor layer 43 in the sensor's FET switch to prevent damage due to light.
• the selenium photoconductor layer 50 is relatively transparent to infrared radiation, so such radiation can reach the aSi layer 43.
• This layer is sensitive to such radiation and could be damaged if left unprotected. For example, the damage could change the amount of leakage current, or cause other changes.
• layer 43 can be shielded with a light-blocking or at least significantly attenuating shield 210 extending over all or most of the material of layer 43.
• This shield can comprise a metallic material such as Al or an alloy, or some other opaque material that is sufficiently thick to essentially block the light 100.
• the second conductive microplate 36 comprises ITO, which is relatively transparent to infrared radiation, so the sensor can benefit from a light shield such as 210.
• the shield 210 may not be needed.
• a panel useful for both radiographic and fluoroscopic examinations comprising an array of a plurality of detectors can be constructed by depositing an array of a plurality of first microplates on a glass substrate and building a TFT switching transistor in a space adjacent to each of the microplates. Connecting leads, as needed, are placed between the microplates connecting the drain and gates of the TFT to connection points along the panel sides. Additional leads are placed to provide electrical access to the first microplates. A dielectric layer is placed over the plurality of first microplates, leads and TFTs, and a second plurality of microplates is deposited thereover to produce a TFT module.
• a passivation layer is created over the second microplates to act as a unidirectional charge-blocking layer and prevent direct ohmic contact between the second microplates and the photoconductive layer to be coated thereon. Finally the TFT source electrode is connected to the middle microplate.
• the TFT modules are fabricated using technology for microfabrication of the transistors and capacitors, which is known in the art. See, for example, the aforementioned United States Patent 5,641 ,974 and the references referred to therein.
• a radiation detection layer of a selenium photoconductor is next applied to the TFT module using conventional vacuum deposition techniques.
• Apparatus and techniques for vacuum deposition are known to those skilled in the art. Vacuum deposition techniques are discussed, for example, in the Handbook of Deposition Technologies for Films and Coatings, 2nd. Ed., R. F. Bunshah, Ed., Noyes Publications, Park Ridge, NJ, 1994. Physical vacuum deposition of selenium is described, for example, in U.S. Patent 2,753,278 issued to Bixby, et al. Over the selenium photoconductor is next coated an organic photoconductor layer.
• the OPC material comprises a sequence of layers, deposited using spin coating (preferred), or blade coating, dip coating, or another suitable coating technique.
• the OPC can include a blocking layer (optional for layer 52 and preferably not used), a charge-generation layer (CGL) and a charge-transport layer (CTL). While in applications such as electrophotographic drums for copiers and printers it is believed that the order of the layers on the Al drum is Al, blocking layer, CGL, and CTL, in the preferred embodiment disclosed here the order is layer 50, then CTL, then CGL, then electrode 20.
• a blocking layer 60 can be interposed between the selenium 50 and the CTL or GTL, depending on the preferred sequence. Blocking layers are known to be used to improve the uniformity of subsequent layers of the OPC coated over Al.
• a preferred hole transporter is N,N'-bis-(3- methylphenyl)-N,N'-bis-(phenyl)-benzidine (MPPB) (CAS 65181-78-4). This is mixed in equal amounts with a solvent soluble polymer.
• a preferred polymer is poly (bisphenol A carbonate).
• a preferred solvent is dioxolane. Equal parts by weight of MPPB and polymer are mixed with solvent with gentle heating (fire hazard!) to give a
• the cooled mixture is spin coated onto selenium to give a CTL approximately 5 to 50 microns thick.
• a preferred thickness is 20 microns.
• TiOPc titanyl phthalocyanine
• CAS 26201-32-1 A well studied charge generation layer that has minimal X-ray cross section yet good response to infrared is titanyl phthalocyanine (TiOPc)(CAS 26201-32-1). It is a pigment and a preferred particle size is 1 micron or less. TiOPc is suspended in a solvent soluble polymer to bind the pigment to the substrate when dried (solvent evaporated).
• a preferred polymer is polyvinylbutral.
• a more preferred polymer is poly (bisphenol A carbonate).
• a preferred solvent is tetrahydrofuran (THF).
• a more preferred solvent is dioxolane. Equal parts by weight of TiOPc and polymer are mixed with solvent with gentle heating (fire hazard!) to give a 1 to 2% solids content.
• the cooled mixture is spin coated onto the CTL to give a CGL 0.1 to 5 microns thick.
• a preferred thickness is 1 micron.
• a top conductive electrode is placed over the OPC layer by deposition of a thin layer of metal, to complete the panel.
• a blocking layer can be used as well.
• the layers of the sensor can be arranged in different order. For example, the order can be essentially reversed relative to that in Fig.1.
• Those skilled in the art having the benefit of the teachings of this patent specification can effect numerous modifications thereto.
• OPC control layer such as 52
• other materials can be used so long as their electrical properties can be reversibly adjusted in order to provide sufficient overvoltage protection in modes such as radiography and also to allow for sufficiently fast removal of residual effects of frames (images) in other modes, such as fluoroscopy.
• infrared light has been discussed, it should be clear that other forms of energy serving the purpose of reversing the appropriate properties of layer 52 can be used.

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