Coating processes – Electrical product produced – Photoelectric
Reexamination Certificate
1998-11-20
2001-01-09
Beck, Shrive (Department: 2878)
Coating processes
Electrical product produced
Photoelectric
C427S065000, C427S160000, C430S085000, C430S086000, C430S095000
Reexamination Certificate
active
06171643
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an improved multilayer plate for X-ray imaging and to a method for producing such plate, used for converting X-rays into a latent electrostatic image. This latent electrostatic image can subsequently be read out by various schemes, such as by a scanning laser beam, a microcapacitor active matrix panel, or a bank of electrostatic probes.
2. Description of the Prior
It is already known to produce multilayer X-ray imaging plates which are sometimes referred to as xeroradiographic plates.
For example, U.S. Pat. No. 3,975,635 of Aug. 17, 1976 discloses a xeroradiographic plate consisting of a conductive backing having thereon a photoconductive layer of selenium and an intermediate layer of an alloy comprising about 15-45 wt % of arsenic and 55-85 wt % of selenium, which intermediate layer has a thickness of about 15-150 &mgr;m and is used to reduce the capacitance of the structure with the result that images are obtained which are capable of development at lower fields without substantial loss of resolution.
U.S. Pat. No. 4,286,033 of Aug. 25, 1981 discloses a multilayer inorganic photosensitive device which comprises a number of various layers, one of which is a hole trapping layer consisting of a halogen doped selenium arsenic alloy wherein the amount of selenium ranges from 95-99.9 wt %, the amount of arsenic ranges from 0.1 to 5 wt % and the amount of halogen is from 10-200 ppm (parts per million). This hole trapping layer has a thickness of 0.01-5 &mgr;m (microns), and is used to retain positive charges at the interface between the generating layer and the overcoating insulating layer, thereby improving image quality.
U.S. Pat. No. 4,338,387 of Jul. 6, 1982 relates to an overcoated photoreceptive device containing a layer of electron trapping material and a hole trapping layer, these layers being comprised of a halogen doped selenium arsenic alloy wherein the amount of selenium is about 95-99.9 wt %, the amount of arsenic is between 0.1-5 wt % and the amount of halogen is from 10 ppm to 200 ppm.
U.S. Pat. No. 4,770,965 of Sep. 13, 1988 discloses a selenium alloy imaging member suitable for X-ray imaging, which is characterized by providing on the Se alloy layer a thin protective organic overcoating layer having about 0.5-3 wt % of nigrosine. This is claimed to result in a greater resolution at a significantly reduced X-ray dosage. In this U.S. Pat. No. 4,770,965, the concept of using intermediate polymer adhesive primer layers between the selenium layer and the metal oxide surface is also disclosed. However, these polymer layers have high thermal expansion coefficients and are not effective in reducing the shear stress due to different thermal expansion of the various layers in the device and may result in film cracking.
In U.S. Pat. No. 4,891,290 of Jan. 2, 1990 there is disclosed a multilayer photosensitive material for electrophotography, (rather than x-ray imaging) wherein a high surface hardness is obtained by providing a surface protective layer of an arsenic-selenium alloy having a composition of approximately As
2
Se
3
. Such photosensitive material has a high printing resistance. It is also indicated that such photosensitive material may include a buffer layer comprising an arsenic-selenium alloy disposed between the surface protection layer and the charge generation layer which allows for high temperature operation. It should be noted that in electrophotography, to which this U.S. patent relates, the toner particles are mechanically cleaned between images, whereas in digital X-ray imaging there is no mechanical abrasion of the surface and thus a high surface hardness in not required.
In U.S. Pat. No. 4,990,419 of Feb. 5, 1991 assigned to Fuji Electric Co. Ltd., a multilayer electrophotographic photoreceptor is again disclosed, which comprises an As
2
Se
3
carrier transport layer, a 30 to 50 wt % Te—Se alloy carrier generation layer and an As
2
Se
3
surface protection layer as well as an outer layer of a transparent insulating material and in U.S. Pat. No. 5,021,310 of Jun. 4, 1991 also assigned to Fuji Electric Co. Ltd. a further thermal expansion relieving layer comprising arsenic and selenium is provided within the photoreceptor. It is stated in this patent that the As concentration of the thermal expansion relieving layer varied from 10 wt % to 38.7 wt % and its overall thickness was 1 &mgr;m. A surface protective layer of As
2
Se
3
containing 1000 ppm of iodine was deposited thereon to a thickness of 3 &mgr;m. Again, this patent relates to an electrophotographic photoreceptor, rather than to an X-ray imaging device.
According to U.S. Pat. No. 5,023,661 of Jun. 11, 1991, it has been determined that a fatigue artifact is caused by a defect in the xeroradiographic plate in the form of a selenium crystallite at the lower surface of the selenium layer of the plate, which allows positive charges in the form of holes, to enter the selenium layer from the aluminum base during the transfer step. These are often called “catastrophic spot producing artifacts”, and the U.S. patent provides a process for eliminating such artifacts by pre-charging the detector after a thermal relaxation step to eliminate the trapped space charge in the device.
In U.S. Pat. No. 5,320,927 of Jun. 14, 1994 the technology for manufacturing an improved selenium alloy X-ray imaging member on a transparent substrate is examined, wherein a bulk selenium arsenic material containing 0.1 to 0.6 wt % As is evaporated onto said substrate in a controlled fractionation process and the evaporation is discontinued when the weight of the selenium alloy remaining in the boat is 2-10% of the original weight. This patent also teaches the use of a selenium arsenic alloy (1-24% As) between the X-ray absorbing material and the substrate material to reduce the crystallite-induced defects. However, this patent fails to address the issue of mechanical stability of the photoreceptor as well as the space charge neutralization capability of the structure.
In U.S. Pat. No. 5,330,863 of Jul. 19, 1994 a photosensitive material for use in electric photography is disclosed wherein carrier injection preventing layers consisting of selenium/arsenic/sulphur alloy are inserted between the conductive substrate and the carrier transport layer or between the carrier generation layer and the overcoat layer, or between both. This makes the photosensitive material resistant to friction, heat, dark decay and fatigue and exhibits little deterioration under high temperature environments. This patent does not relate to X-ray imaging.
In U.S. Pat. No. 5,396,072 of Mar. 7, 1995 a fairly complex X-ray image detector is disclosed, which comprises a plurality of X-ray sensitive sensors each of which has a collecting electrode, a reference electrode and a switching element which connects the collecting electrode to an output lead; a photoconductor layer is provided between the individual collecting electrodes and a bias electrode; and each of the collecting electrodes comprises two electrically contacting electrode portions arranged and situated in a very specific manner, so that the majority of the charge carriers generated in the photoconductor flow to the collecting electrodes.
In U.S. Pat. No. 5,436,101 of Jul. 25, 1995 an X-ray photoreceptor is disclosed which has a high arsenic interstitial layer 5-40 &mgr;m in thickness sandwiched between the substrate and the selenium layer for trapping positive charges injected from the interface. This structure was designed to prevent rather than promote hole injection from the substrate material into the photoreceptor device.
It should be noted that the concept of using multilayer structures based on amorphous selenium alloys (a-Se alloys) originated in the electrophotographic or xerographic industry (see, for example, U.S. Pat. No. 3,041,166 of Jun. 26, 1962) in an effort to make the spectral response of the photoreceptor more panchromatic to compete with the lower cost organic photoreceptors. For example, alloyi
Jean Alain
Polischuk Bradley Trent
Beck Shrive
Cleveland Michael
FTNI Inc.
Primak George J.
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