Radiant energy – Invisible radiant energy responsive electric signalling – Semiconductor system
Reexamination Certificate
1999-07-21
2001-08-21
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Semiconductor system
C250S370050, C250S370110
Reexamination Certificate
active
06278117
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to radiation dosimetry. More generally, it relates to organic semiconductor devices that are photoconductive detectors of optical and ionizing radiation having the same response to radiation as human tissue, and to their design and fabrication.
BACKGROUND OF THE INVENTION
Currently there are many types of devices for measuring ionizing radiation including ion chambers, proportional counters, diode detectors, photographic film, and thermoluminescent devices.
Ion chambers measure the ionization in a known fixed volume of air by applying a voltage across that known volume and collecting any ion pairs that are created as ionizing radiation passes through the volume. Proportional counters work on a similar principle. Again, a known fixed volume of air or some other substance has a voltage applied across it. Unlike the ion chamber, this applied voltage is sufficiently high so that the ion pairs are substantially accelerated within the fixed volume. These accelerated ions collide with other molecules/atoms and create new ion pairs. The total number of ion pairs that will be collected now will be proportional to the number of ion pairs created by ionizing radiation as it passes through the volume.
Diode detectors are semiconductor diodes, normally made of a group IV semiconductor material. Typically the diode is back biased. Ionizing radiation passing through the depleted region of the diode will cause charge carriers to be generated there. These quickly drift though the depletion region and are collected through the bulk material making up the remainder of the diode.
Photographic film typically has a silver compound that is modified to form a latent image when it is exposed to ionizing or optical radiation. The film can then be developed. The amount of silver compound removed from the film will be a function of the amount of radiation that to which the film was exposed.
Thermoluminescent devices (TLD's) are normally made of a crystalline material. When a TLD is exposed to ionizing is radiation, there is resulting damage to the crystal structure and dislocation cites are created. These dislocations are metastable. When the TLD is heated, the crystal will change state and will radiate low energy photons. These photons are counted using some collection measurement equipment. Typically this would be a photomultiplier tube and the appropriate photon counting electronics. The number of photons counted will be a function of the collection equipment's dark current, its efficiency, and the number of dislocations created in the TLD by the ionizing radiation.
All of the detectors described above provide mechanisms for measuring ionizing radiation dose. Dose is simply the amount of energy deposited per unit mass. A problem is that the absorption cross section for any material is dependent on the radiation energy and the material's physical properties. Before any of the detectors described above are used to measure human exposure (often referred to as dose equivalent) the response of the human tissue over the expected energy range must be characterized. All subsequent measurements of dose must be converted to dose equivalent using a variety of calculation techniques that depend on such factors as the radiation type, ambient pressure and temperature. (See, e.g., Task Group 21 Report by the American Association of Physicists in Medicine or AAPM, and The Quality Factor in Radiation Protection from Report 40 of the
International Committee for Radiation Units and Measurement or ICRU) Methods are known for making tissue equivalent radiation detectors ( See, Microdosimetry and its Application to Biological Processes, Plenum Publishing Corp, 1986, Zaider, M. and Rossi, H. H.). These methods employ devices that are usually some sort of ion chamber or proportional counter filled with a gas having the same proportions of carbon, hydrogen, oxygen, and other elements as tissue.
There are many polymers or polymer composite materials that are radiation sensitive (e.g. U.S. Pat. No. 4,975,222 for “Radiation Detecting Elements and Method of Detection”, Yoshino et. al.; U.S. Pat. No. 5,100,762 for “Radiation Sensitive Polymer and Radiation Sensitive Composition Containing The Same”, Tanaka et. al.). Typically, these materials are used passively (i.e. not quantified in real time) to detect radiation. They may operate by forming fizzures that are subsequently measured or etched away photographically like film. They may be used in the processing and design of integrated circuits (e.g. U.S. Pat. No. 5,691,089 for “Integrated Circuits Formed in Radiation Sensitive Material and Method for Forming Same”, Smayling et. al; U.S. Pat. No. 5,596,199 for “Passive Solid State Microdosimeter with Electronic Readout”, McNulty et al.). These materials have also been used to produce active optical devices such as light emitting diodes and photodetectors (See U.S. Pat. No. 5,523,555 for “Photodetector Device Having a Semiconductive Conjugated Polymer”, Friend et. al).
BRIEF DESCRIPTION OF THE INVENTION
The present invention is a solid state photoconducting detector that senses ionizing radiation. Unlike current radiation detectors, this device is primarily made out of organic material that has a density very close to that of normal tissue. The invention solves the problem of making a semiconductor out of a polymer and measuring the current through the polymer without relying on long range conduction by employing microstructures. Unlike the prior art which had structures of relatively large capacitance the present invention arranges microstructures in a particular geometry that reduces the capacitance and the noise associated with large capacitance. The polymer preferably lies on the top of a double layer which reduces the capacitance of the system. Because the detector's chemical composition is similar to that of tissue, it has a tissue equivalent response to radiation. The detector's efficiency for any given radiation quality and energy can be measured. Once this is done, the detector can directly measure dose equivalence.
The invention provides a tissue equivalent solid state detector comprising a polymeric substrate having on its surface, by deposition or other means, a metallic binder layer. A metallic electrode layer contacts the metallic binder layer. An active polymeric layer is cast onto the polymeric substrate, so that the metallic electrode layer is embedded in the active polymeric layer. The metallic electrode layer has at least two interdigitated conductor lines, each leading to a wire such that there is a small capacitance between the pair of wires. In operation a source of potential is place across the wires and the resistance and/or current across the conductors is measured with an electrometer, bridge or electronic monitoring circuit.
The invention further comprises a method for manufacturing such a detector. The method uses photolithographic techniques on a polymer surface by the steps of printing interdigitated metal patterns on the substrate by a liftoff process, sputtering a binder layer and a metal electrode layer onto the substrate, peeling or lifting off the metal layers to leave the interdigitated patterns of polymer substrate, dicing the substrate so that each interdigitated pattern set becomes one die detector, bonding a die detector into a tissue equivalent case, bonding wires to permit connecting the die's bonding pads to external connections, and applying a polymer as the active region of the device. The polymer is selected from among polythiophene, polyanaline, polyphenylene, and polyphenylene vinylene polymers. Further refinements of this method are discussed in the detailed description of the invention below.
REFERENCES:
patent: 4445036 (1984-04-01), Selph
patent: 4641037 (1987-02-01), Butler et al.
patent: 4975222 (1990-12-01), Yoshino et al.
patent: 5079600 (1992-01-01), Schnur et al.
patent: 5100762 (1992-03-01), Tanaka et al.
patent: 5117114 (1992-05-01), Street et al.
patent: 5523555 (1996-06-01), Friend et al.
patent:
Gabor Otilia
Gottlieb Rackman & Reisman P.C.
Hannaher Constantine
QEL Inc.
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