Patent Description:
Radiation detectors may be devices used to measure the flux, spatial distribution, spectrum or other properties of radiations.

Radiation detectors may be used for many applications. One important application is imaging. Radiation imaging is a radiography technique and can be used to reveal the internal structure of a non-uniformly composed and opaque object such as the human body.

Early radiation detectors for imaging include photographic plates and photographic films. A photographic plate may be a glass plate with a coating of light-sensitive emulsion. Although photographic plates were replaced by photographic films, they may still be used in special situations due to the superior quality they offer and their extreme stability. A photographic film may be a plastic film (e.g., a strip or sheet) with a coating of light-sensitive emulsion.

In the <NUM>, photostimulable phosphor plates (PSP plates) became available. A PSP plate may contain a phosphor material with color centers in its lattice. When the PSP plate is exposed to radiation, electrons excited by radiation are trapped in the color centers until they are stimulated by a laser beam scanning over the plate surface. As the plate is scanned by laser, trapped excited electrons give off light, which is collected by a photomultiplier tube. The collected light is converted into a digital image. In contrast to photographic plates and photographic films, PSP plates can be reused.

Another kind of radiation detectors are radiation image intensifiers. Components of a radiation image intensifier are usually sealed in a vacuum. In contrast to photographic plates, photographic films, and PSP plates, radiation image intensifiers may produce real-time images, i.e., do not require post-exposure processing to produce images. radiation first hits an input phosphor (e.g., cesium iodide) and is converted to visible light. The visible light then hits a photocathode (e.g., a thin metal layer containing cesium and antimony compounds) and causes emission of electrons. The number of emitted electrons is proportional to the intensity of the incident radiation. The emitted electrons are projected, through electron optics, onto an output phosphor and cause the output phosphor to produce a visible-light image.

Scintillators operate somewhat similarly to radiation image intensifiers in that scintillators (e.g., sodium iodide) absorb radiation and emit visible light, which can then be detected by a suitable image sensor for visible light. In scintillators, the visible light spreads and scatters in all directions and thus reduces spatial resolution. Reducing the scintillator thickness helps to improve the spatial resolution but also reduces absorption of radiation. A scintillator thus has to strike a compromise between absorption efficiency and resolution.

Semiconductor radiation detectors largely overcome this problem by direct conversion of radiation into electric signals. A semiconductor radiation detector may include a semiconductor layer that absorbs radiation in wavelengths of interest. When a radiation particle is absorbed in the semiconductor layer, multiple charge carriers (e.g., electrons and holes) are generated and swept under an electric field towards electrical contacts on the semiconductor layer. <CIT> discloses a method of fabricating a modular sensor assembly comprising a sensor array coupled to an electronics array in a stacked configuration. The sensor array comprises a plurality of sensor modules, each comprising a plurality of sensor sub-arrays. The electronics array comprises a plurality of integrated circuit modules, each comprising a plurality of integrated circuit chips.

According to the invention, there is provided a method comprising the following consecutive steps: attaching a wafer to a substrate, wherein the substrate comprises discrete electrodes, wherein the wafer comprises a radiation absorption layer and a plurality of electrical contacts, wherein each of the electrical contacts is connected to at least one of the discrete electrodes;identifying a defective area of the wafer;replacing a portion of the wafer with at least one chip configured to absorb radiation, wherein the portion comprises the defective area. The invention is further defined by the appended claims.

<FIG> schematically shows a cross-sectional view of a radiation detector <NUM>, according to an embodiment. The radiation detector <NUM> may include a radiation absorption layer <NUM> and an electronics layer <NUM> (e.g., an ASIC) for processing or analyzing electrical signals incident radiation generates in the radiation absorption layer <NUM>. In an embodiment, the radiation detector <NUM> does not comprise a scintillator. The radiation absorption layer <NUM> may comprise a semiconductor material such as CdZnTe (CZT). The semiconductor may have a high mass attenuation coefficient for the radiation energy of interest.

As shown in a detailed cross-sectional view of the radiation detector <NUM> in <FIG>, according to an embodiment, the radiation absorption layer <NUM> may include one or more diodes (e.g., p-i-n or p-n) formed by a first doped region <NUM>, one or more discrete regions <NUM> of a second doped region <NUM>. The second doped region <NUM> may be separated from the first doped region <NUM> by an optional intrinsic region <NUM>. The discrete regions <NUM> are separated from one another by the first doped region <NUM> or the intrinsic region <NUM>. The first doped region <NUM> and the second doped region <NUM> have opposite types of doping (e.g., region <NUM> is p-type and region <NUM> is n-type, or region <NUM> is n-type and region <NUM> is p-type). In the example in <FIG>, each of the discrete regions <NUM> of the second doped region <NUM> forms a diode with the first doped region <NUM> and the optional intrinsic region <NUM>. Namely, in the example in <FIG>, the radiation absorption layer <NUM> comprises at least one diode having the first doped region <NUM> and a first electrical contact 119A as a shared electrode. The first doped region <NUM> and the first electrical contact 119A may also have discrete portions.

When a radiation particle hits the radiation absorption layer <NUM> including diodes, the radiation particle may be absorbed and generate one or more charge carriers by a number of mechanisms. A radiation particle may generate <NUM> to <NUM> charge carriers. The charge carriers may drift to the electrodes of one of the diodes under an electric field. The field may be an external electric field. A second electrical contact 119B may include discrete portions, each of which is in electrical contact with the discrete regions <NUM>. In an embodiment, the charge carriers may drift in directions such that the charge carriers generated by a single radiation particle are not substantially shared by two different discrete regions <NUM> ("not substantially shared" here means less than <NUM>%, less than <NUM>%, less than <NUM>%, or less than <NUM>% of these charge carriers flow to a different one of the discrete regions <NUM> than the rest of the charge carriers). Charge carriers generated by a radiation particle incident around the footprint of one of these discrete regions <NUM> are not substantially shared with another of these discrete regions <NUM>. A pixel <NUM> associated with a discrete region <NUM> may be an area around the discrete region <NUM> in which substantially all (more than <NUM>%, more than <NUM>%, more than <NUM>%, or more than <NUM>% of) charge carriers generated by a radiation particle incident therein flow to the discrete region <NUM>. Namely, less than <NUM>%, less than <NUM>%, less than <NUM>%, or less than <NUM>% of these charge carriers flow beyond the pixel.

As shown in an alternative detailed cross-sectional view of the radiation detector <NUM> in <FIG>, according to an embodiment, the radiation absorption layer <NUM> may comprise a resistor of a semiconductor material such as CdZnTe (CZT), but does not include a diode. The semiconductor may have a high mass attenuation coefficient for the radiation of interest.

When a radiation particle hits the radiation absorption layer <NUM> including a resistor but not diodes, it may be absorbed and generate one or more charge carriers by a number of mechanisms. A radiation particle may generate <NUM> to <NUM> charge carriers. The charge carriers may drift to the electrical contacts 119A and 119B under an electric field. The field may be an external electric field. The electrical contact 119B includes discrete portions. In an embodiment, the charge carriers may drift in directions such that the charge carriers generated by a single radiation particle are not substantially shared by two different discrete portions of the electrical contact 119B ("not substantially shared" here means less than <NUM>%, less than <NUM>%, less than <NUM>%, or less than <NUM>% of these charge carriers flow to a different one of the discrete portions than the rest of the charge carriers). Charge carriers generated by a radiation particle incident around the footprint of one of these discrete portions of the electrical contact 119B are not substantially shared with another of these discrete portions of the electrical contact 119B. A pixel <NUM> associated with a discrete portion of the electrical contact 119B may be an area around the discrete portion in which substantially all (more than <NUM>%, more than <NUM>%, more than <NUM>% or more than <NUM>% of) charge carriers generated by a radiation particle incident therein flow to the discrete portion of the electrical contact 119B. Namely, less than <NUM>%, less than <NUM>%, less than <NUM>%, or less than <NUM>% of these charge carriers flow beyond the pixel associated with the one discrete portion of the electrical contact 119B.

The electronics layer <NUM> may include an electronic system <NUM> suitable for processing or interpreting signals generated by radiation particles incident on the radiation absorption layer <NUM>. The electronic system <NUM> may include an analog circuitry such as a filter network, amplifiers, integrators, and comparators, or a digital circuitry such as a microprocessor, and a memory. The electronic system <NUM> may include components shared by the pixels or components dedicated to a single pixel. For example, the electronic system <NUM> may include an amplifier dedicated to each pixel and a microprocessor shared among all the pixels. The electronic system <NUM> may be electrically connected to the pixels by vias <NUM>. Space among the vias may be filled with a filler material <NUM>, which may increase the mechanical stability of the connection of the electronics layer <NUM> to the radiation absorption layer <NUM>. Other bonding techniques are possible to connect the electronic system <NUM> to the pixels without using vias.

<FIG> schematically shows that the radiation detector <NUM> may have an array of pixels <NUM>. The array may be a rectangular array, a honeycomb array, a hexagonal array or any other suitable array. Each pixel <NUM> may be configured to detect a radiation particle incident thereon, measure the energy of the radiation particle, or both. For example, each pixel <NUM> may be configured to count numbers of radiation particles incident thereon whose energy falls in a plurality of bins, within a period of time. All the pixels <NUM> may be configured to count the numbers of radiation particles incident thereon within a plurality of bins of energy within the same period of time. Each pixel <NUM> may have its own analog-to-digital converter (ADC) configured to digitize an analog signal representing the energy of an incident radiation particle into a digital signal. The ADC may have a resolution of <NUM> bits or higher. Each pixel <NUM> may be configured to measure its dark current, such as before or concurrently with each radiation particle incident thereon. Each pixel <NUM> may be configured to deduct the contribution of the dark current from the energy of the radiation particle incident thereon. The pixels <NUM> may be configured to operate in parallel. For example, when one pixel <NUM> measures an incident radiation particle, another pixel <NUM> may be waiting for a radiation particle to arrive. The pixels <NUM> may be but do not have to be individually addressable.

<FIG> schematically shows that the radiation detector <NUM> may comprise a plurality of chips <NUM>. Each of the chips <NUM> may include a single pixel <NUM> configured to detect the radiation. Each of the chips <NUM> may include the radiation absorption layer <NUM>, the first electrical contact 119A, and the second electrical contact 119B. The second electrical contact 119B may be used to electrically connect to the electronics layer <NUM>. The electronics layer <NUM> comprises the substrate <NUM> having a first surface <NUM> and a second surface <NUM>. A "surface" as used herein is not necessarily exposed, but can be buried wholly or partially. The electronics layer <NUM> comprises one or more electrodes <NUM>, which may be on the first surface <NUM>. The second electrical contact 119B of each of the chips <NUM> may be electrically connected to one or more electrodes <NUM>. The electronic system <NUM> may be in or on the substrate <NUM>. The electronics layer <NUM> may comprise one or more vias <NUM> extending from the first surface <NUM> to the second surface <NUM>. The electronics layer <NUM> may comprise a redistribution layer (RDL) <NUM> on the second surface <NUM>. The RDL <NUM> may comprise one or more transmission lines <NUM>. The electronic system <NUM> may be electrically connected to the electrodes <NUM> and the transmission lines <NUM> through the vias <NUM>.

The substrate <NUM> may be a thinned substrate. For example, the substrate may have at thickness of <NUM> microns or less, <NUM> microns or less, <NUM> microns or less, <NUM> microns or less, <NUM> microns or less, or <NUM> microns or less. The substrate <NUM> may be a silicon substrate or a substrate or other suitable semiconductor or insulator. The substrate <NUM> may be produced by grinding a thicker substrate to a desired thickness.

The one or more electrodes <NUM> may be a layer of metal or doped semiconductor. For example, the electrodes <NUM> may be gold, copper, platinum, palladium, doped silicon, etc..

The vias <NUM> pass through the substrate <NUM> and electrically connect electrical components (e.g., the electrodes <NUM>) on the first surface <NUM> to electrical components (e.g., the RDL) on the second surface <NUM>. The vias <NUM> are sometimes referred to as "through-silicon vias" although they may be fabricated in substrates of materials other than silicon.

The RDL <NUM> may comprise one or more transmission lines <NUM>. The transmission lines <NUM> electrically connect electrical components (e.g., the vias <NUM>) in the substrate <NUM> to bonding pads at other locations on the substrate <NUM>. The transmission lines <NUM> may be electrically isolated from the substrate <NUM> except at certain vias <NUM> and certain bonding pads. The transmission lines <NUM> may be a material (e.g., Al) with small mass attenuation coefficient for the radiation energy of interest. The RDL <NUM> may redistribute electrical connections to more convenient locations.

<FIG> further schematically shows bonding between the chip <NUM> and the electronics layer <NUM> at the electrical contact 119B and the electrodes <NUM>. The bonding may be by a suitable technique such as direct bonding or flip chip bonding. Direct bonding is a wafer bonding process without any additional intermediate layers (e.g., solder bumps). The bonding process is based on chemical bonds between two surfaces. Direct bonding may be at elevated temperature but not necessarily so. Flip chip bonding uses solder bumps <NUM> deposited onto contact pads (e.g., the electrical contact 119B of the radiation absorption layer <NUM> of the chip <NUM> or the electrodes <NUM>). Either the radiation absorption layer <NUM> or the electronics layer <NUM> is flipped over and the electrical contact 119B of the radiation absorption layer <NUM> are aligned to the electrodes <NUM>. The solder bumps <NUM> may be melted to solder the electrical contact 119B and the electrodes <NUM> together. Any void space among the solder bumps <NUM> may be filled with an insulating material.

<FIG> schematically show a process of making the radiation detector <NUM>, according to an embodiment not making part of the present invention.

<FIG> schematically shows that multiple chips <NUM> are obtained. Each of the chips comprises the radiation absorption layer <NUM> and the electrical contacts 119A and 119B. The electrical contacts 119B are not visible in this view. The electrical contacts 119A are omitted from the view. The chips <NUM> may be obtained by dicing a wafer with multiple dies. Each of the chips <NUM> may be tested to verify characteristics. For example, each chip may be subject to electrical tests or optical inspections.

<FIG> schematically shows that the electrical contacts 119B of the chips <NUM> are aligned to the electrodes <NUM> of the electronics layer <NUM>. In this view, the electrical contacts 119B are not visible because they face the electronics layer <NUM>.

<FIG> schematically shows that the chips <NUM> are bonded to the electronics layer <NUM> using a suitable bonding method. The electrical contacts 119B are now electrically connected to the electrodes <NUM>. After bonding, the radiation detector <NUM> may be tested, for example, for electrical characteristics and functionalities. Defective chips (e.g., those crosshatched chips in <FIG>) may be identified based on testing results among the chips <NUM> bonded to the electronics layer <NUM>, and removed from the electronics layer <NUM>. A variety of processes may be used to remove the defective chips. For example, elevated temperature may be applied to the area containing the defective chips to melt the solder bumps <NUM> in the area. As shown in <FIG>, functional chips <NUM> may be inserted and bonded to the electronics layer <NUM> to replace the removed defective chips.

<FIG> schematically shows that the gaps between chips <NUM> are filled with an electrically insulating material <NUM>. Examples of the electrically insulating material <NUM> may comprise oxide, nitride, glass, polymer, epoxy, or a combination thereof.

<FIG> schematically shows that the electrical contacts 119A of the plurality of chip <NUM> are electrically connected with an electrically conductive surface of a carrier substrate <NUM>. For example, the electrically conductive surface of the carrier substrate <NUM> may be a layer of metal deposited over the chips.

<FIG> schematically shows that the radiation detector <NUM> may comprise a wafer with multiple pixels <NUM> configured to detect the radiation, according to the invention. The wafer comprises the radiation absorption layer <NUM>, the first electrical contact 119A, and the second electrical contacts 119B. The second electrical contacts 119B may be used to electrically connect to the electronics layer <NUM>. The electronics layer <NUM> comprises the substrate <NUM> having the first surface <NUM> and the second surface <NUM>. The electronics layer <NUM> comprises one or more electrodes <NUM>, which may be on the first surface <NUM>. The second electrical contacts 119B may be electrically connected to one or more electrodes <NUM>. The electronic system <NUM> may be in or on the substrate <NUM>. The electronics layer <NUM> may comprise one or more vias <NUM> extending from the first surface <NUM> to the second surface <NUM>. The electronics layer <NUM> may comprise a redistribution layer (RDL) <NUM> on the second surface <NUM>. The RDL <NUM> may comprise one or more transmission lines <NUM>. The electronic system <NUM> may be electrically connected to the electrodes <NUM> and the transmission lines <NUM> through the vias <NUM>.

<FIG> further schematically shows bonding between the radiation absorption layer <NUM> of the wafer and the electronics layer <NUM> at the electrical contacts 119B and the electrodes <NUM>. The bonding may be by a suitable technique such as direct bonding or flip chip bonding. Direct bonding is a wafer bonding process without any additional intermediate layers (e.g., solder bumps). The bonding process is based on chemical bonds between two surfaces. Direct bonding may be at elevated temperature but not necessarily so. Flip chip bonding uses solder bumps <NUM> deposited onto contact pads (e.g., the electrical contact 119B of the radiation absorption layer <NUM> of the chip <NUM> or the electrodes <NUM>). Any void space among the solder bumps <NUM> may be filled with an insulating material.

<FIG> schematically show a process of making the radiation detector <NUM>, according to the invention.

<FIG> schematically shows the wafer comprising the radiation absorption layer <NUM>, the plurality of electrical contacts 119A and 119B. The wafer may be tested before it is bonded to the electronics layer <NUM>. For example, each of the pixels in the wafer may be subject to electrical tests and optical inspections. A portion containing a defect may be identified on the wafer based on the test results.

<FIG> schematically shows that the electrical contacts 119B are aligned to the electrodes <NUM> of the electronics layer <NUM>.

<FIG> schematically shows that the radiation absorption layer <NUM> is bonded to the electronics layer <NUM> using a suitable bonding method. The electrical contacts 119B are now electrically connected to the electrodes <NUM>. After bonding, the radiation detector may be tested, for example, for electrical characteristics and functionalities. Portions containing at least one defect (e.g., a defective pixel) are identified based on testing results.

As shown in <FIG>, a portion of the wafer containing a defective pixel is removed from the wafer, and the removed portion of wafer is replaced by a chip configured to absorb the radiation, according to the invention. Different processes may be used to remove the portion. For example, elevated temperature may be applied to the portion to melt the solder bumps <NUM> under the portion, and the portion may be separated from the rest of the wafer by laser scribing. For example, the portion may be etched away from the wafer with the rest of the wafer is covered by a protective layer. A functional chip may be inserted and bonded to the electronics layer <NUM> to replace the removed portion.

<FIG> schematically shows a system comprising the radiation detector <NUM> described herein. The system may be used for medical imaging such as chest radiation radiography, abdominal radiation radiography, etc. The system comprises a radiation source <NUM>. Radiation emitted from the radiation source <NUM> penetrates an object <NUM> (e.g., a human body part such as chest, limb, abdomen), is attenuated by different degrees by the internal structures of the object <NUM> (e.g., bones, muscle, fat and organs, etc.), and is projected to the radiation detector <NUM>. The radiation detector <NUM> forms an image by detecting the intensity distribution of the radiation.

<FIG> schematically shows a system comprising the radiation detector <NUM> described herein. The system may be used for medical imaging such as dental radiation radiography. The system comprises a radiation source <NUM>. radiation emitted from the radiation source <NUM> penetrates an object <NUM> that is part of a mammal (e.g., human) mouth. The object <NUM> may include a maxilla bone, a palate bone, a tooth, the mandible, or the tongue. The radiation is attenuated by different degrees by the different structures of the object <NUM> and is projected to the radiation detector <NUM>. The radiation detector <NUM> forms an image by detecting the intensity distribution of the radiation. Teeth absorb radiation more than dental caries, infections, periodontal ligament. The dosage of radiation received by a dental patient is typically small (around <NUM> mSv for a full mouth series).

<FIG> schematically shows a cargo scanning or non-intrusive inspection (NII) system comprising the radiation detector <NUM> described herein. The system may be used for inspecting and identifying goods in transportation systems such as shipping containers, vehicles, ships, luggage, etc. The system comprises a radiation source <NUM>. Radiation emitted from the radiation source <NUM> may backscatter from an object <NUM> (e.g., shipping containers, vehicles, ships, etc.) and be projected to the radiation detector <NUM>. Different internal structures of the object <NUM> may backscatter radiation differently. The radiation detector <NUM> forms an image by detecting the intensity distribution of the backscattered radiation and/or energies of the backscattered radiation particles.

<FIG> schematically shows another cargo scanning or non-intrusive inspection (NII) system comprising the radiation detector <NUM> described herein. The system may be used for luggage screening at public transportation stations and airports. The system comprises a radiation source <NUM>. Radiation emitted from the radiation source <NUM> may penetrate a piece of luggage <NUM>, be differently attenuated by the contents of the luggage, and projected to the radiation detector <NUM>. The radiation detector <NUM> forms an image by detecting the intensity distribution of the transmitted radiation. The system may reveal contents of luggage and identify items forbidden on public transportation, such as firearms, narcotics, edged weapons, flammables.

<FIG> schematically shows a full-body scanner system comprising the radiation detector <NUM> described herein. The full-body scanner system may detect objects on a person's body for security screening purposes, without physically removing clothes or making physical contact. The full-body scanner system may be able to detect non-metal objects. The full-body scanner system comprises a radiation source <NUM>. Radiation emitted from the radiation source <NUM> may backscatter from a human <NUM> being screened and objects thereon, and be projected to the radiation detector <NUM>. The objects and the human body may backscatter radiation differently. The radiation detector <NUM> forms an image by detecting the intensity distribution of the backscattered radiation. The radiation detector <NUM> and the radiation source <NUM> may be configured to scan the human in a linear or rotational direction.

<FIG> schematically shows a radiation computed tomography (radiation CT) system. The radiation CT system uses computer-processed radiations to produce tomographic images (virtual "slices") of specific areas of a scanned object. The tomographic images may be used for diagnostic and therapeutic purposes in various medical disciplines, or for flaw detection, failure analysis, metrology, assembly analysis and reverse engineering. The radiation CT system comprises the radiation detector <NUM> described herein and a radiation source <NUM>. The radiation detector <NUM> and the radiation source <NUM> may be configured to rotate synchronously along one or more circular or spiral paths.

<FIG> schematically shows an electron microscope. The electron microscope comprises an electron source <NUM> (also called an electron gun) that is configured to emit electrons. The electron source <NUM> may have various emission mechanisms such as thermionic, photocathode, cold emission, or plasmas source. The emitted electrons pass through an electronic optical system <NUM>, which may be configured to shape, accelerate, or focus the electrons. The electrons then reach a sample <NUM> and an image detector may form an image therefrom. The electron microscope may comprise the radiation detector <NUM> described herein, for performing energy-dispersive radiation spectroscopy (EDS). EDS is an analytical technique used for the elemental analysis or chemical characterization of a sample. When the electrons incident on a sample, they cause emission of characteristic radiations from the sample. The incident electrons may excite an electron in an inner shell of an atom in the sample, ejecting it from the shell while creating an electron hole where the electron was. An electron from an outer, higher-energy shell then fills the hole, and the difference in energy between the higher-energy shell and the lower energy shell may be released in the form of a radiation. The number and energy of the radiations emitted from the sample can be measured by the radiation detector <NUM>.

Claim 1:
A method comprising the following consecutive steps:
attaching a wafer to a substrate (<NUM>), wherein the substrate comprises discrete electrodes (<NUM>), wherein the wafer comprises a radiation absorption layer (<NUM>) and a plurality of electrical contacts (119B), wherein each of the electrical contacts is connected to at least one of the discrete electrodes;
identifying a defective area of the wafer;
replacing a portion of the wafer with at least one chip configured to absorb radiation, wherein the portion comprises the defective area.