Radiation detectors with scintillators

Disclosed herein is radiation detector, comprising a first photodiode comprising a first junction; and a first scintillator, wherein a first point in a first plane and inside the first scintillator is essentially completely surrounded in the first plane by an intersection of the first plane and the first junction. The first junction is a p-n junction, a p-i-n junction, a heterojunction, or a Schottky junction. The radiation detector further comprises a first reflector configured to guide essentially all photons emitted by the first scintillator into the first photodiode. The first scintillator is essentially completely enclosed by the first reflector and the first photodiode.

TECHNICAL FIELD

The disclosure herein relates to radiation detectors.

BACKGROUND

A radiation detector is a device that measures a property of a radiation. Examples of the property may include a spatial distribution of the intensity, phase, and polarization of the radiation. The radiation may be one that has interacted with an object. For example, the radiation measured by the radiation detector may be a radiation that has penetrated the object. The radiation may be an electromagnetic radiation such as infrared light, visible light, ultraviolet light, X-ray or γ-ray. The radiation may be of other types such as α-rays and β-rays. The radiation may comprise radiation particles such as photons (electromagnetic waves) and subatomic particles.

SUMMARY

Disclosed herein is a radiation detector comprising a first photodiode comprising a first junction and a first scintillator, wherein a first point in a first plane and inside the first scintillator is essentially completely surrounded in the first plane by an intersection of the first plane and the first junction.

According to an embodiment, the first junction is a p-n junction, a p-i-n junction, a heterojunction, or a Schottky junction.

According to an embodiment, the first photodiode is configured to measure a characteristic of photons emitted by the first scintillator and incident on the first photodiode.

According to an embodiment, the characteristic is energy, radiant flux, wavelength, or frequency.

According to an embodiment, the first scintillator is in direct physical contact with the first photodiode.

According to an embodiment, the first scintillator comprises sodium iodide.

According to an embodiment, the first scintillator comprises quantum dots.

According to an embodiment, the radiation detector further comprises a substrate, wherein the first scintillator is in a recess into a substrate surface of the substrate.

According to an embodiment, the recess has a shape of a truncated pyramid.

According to an embodiment, the first photodiode is in the substrate.

According to an embodiment, the first junction conforms to side and bottom walls of the recess.

According to an embodiment, the radiation detector further comprises a first reflector configured to guide photons emitted by the first scintillator into the first photodiode.

According to an embodiment, the first reflector is configured to reflect photons emitted by the first scintillator toward the first reflector.

According to an embodiment, the first reflector is not opaque to some radiation particles which are able to cause the first scintillator to emit photons when the radiation particles are incident on the first scintillator.

According to an embodiment, the first scintillator is essentially completely enclosed by the first reflector and the first photodiode.

According to an embodiment, the first reflector comprises a material selected from the group consisting of aluminum, silver, gold, copper, and any combinations thereof.

According to an embodiment, the first reflector is in direct physical contact with the first scintillator.

According to an embodiment, the first reflector is electrically connected to the first photodiode.

According to an embodiment, the radiation detector further comprises a second photodiode comprising a second junction and being adjacent to the first photodiode; and a second scintillator, wherein a second point in a second plane and inside the second scintillator is essentially completely surrounded in the second plane by an intersection of the second plane and the second junction.

According to an embodiment, the radiation detector further comprises a second reflector separate from the first reflector and configured to guide photons emitted by the second scintillator into the second photodiode.

According to an embodiment, the radiation detector further comprises a common electrode electrically connected to the first and second photodiodes.

Disclosed herein is a method comprising forming a first recess into a substrate surface of a substrate; forming a first junction in the substrate; and forming a first scintillator in the first recess, wherein a first point in a first plane and inside the first scintillator is essentially completely surrounded in the first plane by an intersection of the first plane and the first junction.

According to an embodiment, the first junction is a p-n junction, a p-i-n junction, a heterojunction, or a Schottky junction.

According to an embodiment, a first photodiode which comprises the first junction is configured to measure a characteristic of photons emitted by the first scintillator and incident on the first photodiode.

According to an embodiment, the characteristic is energy, radiant flux, wavelength, or frequency.

According to an embodiment, the first junction conforms to side and bottom walls of the first recess.

According to an embodiment, said forming the first junction comprises ion implantation.

According to an embodiment, said forming the first scintillator in the first recess comprises forming a scintillator layer on the substrate surface of the substrate; and polishing a layer surface of the scintillator layer until the substrate surface is exposed to a surrounding ambient.

According to an embodiment, the method further comprises forming a first reflector on the first scintillator, wherein the first reflector is configured to guide photons emitted by the first scintillator into a first photodiode which comprises the first junction.

According to an embodiment, the first reflector is configured to reflect photons emitted by the first scintillator toward the first reflector.

According to an embodiment, the first reflector is not opaque to some radiation particles which are able to cause the first scintillator to emit photons when the radiation particles are incident on the first scintillator.

According to an embodiment, the first scintillator is essentially completely enclosed by the first reflector and the first photodiode.

According to an embodiment, the first reflector comprises a material selected from the group consisting of aluminum, silver, gold, copper, and any combinations thereof.

According to an embodiment, the first reflector is in direct physical contact with the first scintillator.

According to an embodiment, the first reflector is electrically connected to the first photodiode.

According to an embodiment, the method further comprises forming a second recess into the substrate surface of the substrate; forming a second junction in the substrate; and forming a second scintillator in the second recess, wherein a second point in a second plane and inside the second scintillator is essentially completely surrounded in the second plane by an intersection of the second plane and the second junction.

According to an embodiment, the method further comprises forming a second reflector on the second scintillator, wherein the second reflector is separate from the first reflector, and wherein the second reflector is configured to guide photons emitted by the second scintillator into a second photodiode which comprises the second junction.

DETAILED DESCRIPTION

FIG. 1-FIG. 9schematically show the structure, fabrication process and operation of a radiation detector700, according to an embodiment. Specifically, with reference toFIG. 1, the fabrication process may start with a substrate100. The substrate100may be a semiconductor substrate. For example, the substrate100may comprise silicon (Si) which may be lightly doped with P-type dopants such as boron atoms. The substrate100may have a surface100aand a surface100b. The surface100amay be opposite from the surface100b.

In an embodiment, a common electrode110may be formed on the surface100aof the substrate100. The common electrode110may comprise gold (Au). If gold is used, the common electrode110may be formed using a physical vapor deposition (PVD) process such as sputtering deposition.

With reference toFIG. 3, in an embodiment, recesses310may be formed into the surface100bof the substrate100. Specifically, in an embodiment, the recesses310may be formed as follows. An etch mask (e.g., a stencil or a pattern formed by lithography) with apertures may be placed on the surface100bof the substrate100so that the apertures are at the locations where the recesses310are to be formed. Portions of the substrate100exposed through the apertures are etched away, resulting in the recesses310(FIG. 3). The etching may be an anisotropic wet etching using an etchant such as potassium hydroxide (KOH), or dry etching.

In an embodiment, the surface100bof the substrate100may be a (100) silicon crystallographic plane. As a result, the recesses310resulting from the wet etching have truncated pyramid shapes with flat bottom walls and angled side walls as shown inFIG. 3. Other shapes of the recesses310may be possible depending on the method of forming the recesses310.

With reference toFIG. 4, in an embodiment, junctions may be formed in the substrate100. For example, N-type Si regions410may be formed in the substrate100and on the side walls and bottom walls of the recesses310. Specifically, the N-type Si regions410may be formed by an ion implantation process. More specifically, a stencil (not shown) with apertures may be placed on the surface100bof the substrate100ofFIG. 3such that the recesses310are exposed through the apertures. Then, the stencil may be used as a shadow mask for doping the areas of the substrate100exposed through the apertures. The dopants used in the doping may be N-type dopants such as phosphorus atoms. The stencil blocks the dopant ions from reaching the area of the substrate100between the recesses310but exposes other areas of the substrate100(including the recesses310) to dopant ions through the apertures. After the ion implantation, the stencil may be removed and an annealing process may be performed resulting in the N-type Si regions410(FIG. 4). In an alternative embodiment, instead of using the stencil, a pattern mask (not shown) may be used for the ion implantation process. The pattern mask may be formed on the substrate100ofFIG. 3by photolithography.

In an embodiment, scintillators may be formed in the recesses310. Specifically, for example, a scintillator material may be deposited on the structure ofFIG. 4resulting in, as shown inFIG. 5, a scintillator layer510t. The scintillator material emits photons (such as visible light photons) in response to radiation (e.g., X-ray) incident on the scintillator material. In an embodiment, the scintillator layer510tmay comprise sodium iodide (NaI) or quantum dots.

With reference toFIG. 5, in an embodiment, forming the scintillators may include polishing a surface510t′ of the scintillator layer510tuntil the surface100bof the substrate100is exposed to the surrounding ambient, resulting in, as shown inFIG. 6, scintillators510in the recesses310(FIG. 4).FIG. 7schematically shows a cross-sectional view of the structure ofFIG. 6along a plane6′-6′. In other words,FIG. 7schematically shows a cross-sectional view of the structure ofFIG. 6across a plane6′-6′ which comprises the line6′-6′ and is perpendicular to the page ofFIG. 6.

InFIG. 6, it should be noted that each N-type region410forms with a substrate portion100dof the P-type substrate100a photodiode410+100dwhich includes a p-n junction410jat the interface of the N-type region410and the P-type substrate portion100d.FIG. 6shows3such photodiodes410+100d. The p-n junction410jmay conform to the side and bottom walls of the recess310(FIG. 4) which the associated scintillator510now occupies.

In an embodiment, all photodiodes410+100dmay share (i.e., be electrically connected to) the common electrode110. InFIG. 7(i.e., in the plane6′-6′), it should be noted that a point M in the plane6′-6′ and inside a scintillator510is essentially completely surrounded in the plane6′-6′ by an intersection410j′ of the plane6′-6′ and the p-n junction410jof the associated photodiode410+100d. “Essentially completely” means completely or almost completely.

With reference toFIG. 8, in an embodiment, reflectors710may be formed on the scintillators510, according to an embodiment. The reflectors710may be formed using a photolithographic process. In an embodiment, the material and thickness710aof each reflector710may be such that the reflector710is not opaque to at least some radiation particles of a radiation720from an object730. In an embodiment, the material and thickness710aof each reflector710may be such that the reflector710reflects photons emitted by the associated scintillator510towards the reflector710. Specifically, the reflectors710may comprise aluminum, silver, gold, copper, or any combinations thereof. The thickness710amay be around 10 micrometers (μm).

In an embodiment, the reflectors710may be formed in direct physical contact one-to-one with the scintillators510. In an embodiment, the reflectors710may be formed in direct physical contact one-to-one with the N-type regions410of the photodiodes410+100d. As a result, each reflector710is electrically connected to the associated photodiode410+100d. In an embodiment, the reflectors710may be formed such that each scintillator510is essentially completely enclosed by an N-type Si region410and a reflector710. In other words, each scintillator510is essentially completely enclosed by a photodiode410+100dand a reflector710.

Specifically, in an embodiment, the reflectors710may be formed as follows. A photoresist layer (not shown) may be formed on the structure ofFIG. 6. The photoresist layer may be patterned exposing the scintillators510and the N-type Si regions410but covering areas between the N-type Si regions410. Then, a physical vapor deposition process (e.g., sputter deposition) may be performed so as to deposit a suitable material such as aluminum (Al) on surfaces of the structure ofFIG. 6not covered by the patterned photoresist layer, resulting in the reflectors710(FIG. 8). After that, the patterned photoresist layer may be removed resulting in the radiation detector700ofFIG. 8.FIG. 9schematically shows a top view of the radiation detector700ofFIG. 8.

In an embodiment, each photodiode410+100dmay be configured to detect radiation particles incident thereon (e.g., incident photons emitted by the associated scintillator510) and may be configured to measure a characteristic (e.g., energy, radiant flux, wavelength, and frequency) of the incident radiation particles. In an embodiment, a characteristic (e.g., total energy) of the radiation particles incident on the associated scintillator510may be determined based on the measured characteristic (e.g., total energy) of the photons emitted by the associated scintillator510and incident on the photodiode410+100d.

For example, each photodiode410+100dmay be configured to count numbers of radiation particles incident thereon whose energy falls in a plurality of bins of energy, within a period of time. All the photodiodes410+100dmay be configured to count the numbers of radiation particles incident thereon within a plurality of bins of energy within the same period of time. When the incident radiation particles have similar energy, the photodiodes410+100dmay be simply configured to count numbers of radiation particles incident thereon within a period of time, without measuring the energy of the individual radiation particles.

Each photodiode410+100dmay 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, or to digitize an analog signal representing the total energy of a plurality of incident radiation particles into a digital signal. The photodiodes410+100dmay be configured to operate in parallel. For example, when one photodiode410+100dmeasures an incident radiation particle, another photodiode410+100dmay be waiting for a radiation particle to arrive. The photodiodes410+100dmay not have to be individually addressable.

The radiation detector700described here may have applications such as in an X-ray telescope, X-ray mammography, industrial X-ray defect detection, X-ray microscopy or microradiography, X-ray casting inspection, X-ray non-destructive testing, X-ray weld inspection, X-ray digital subtraction angiography, etc. It may be suitable to use this radiation detector700in place of a photographic plate, a photographic film, a PSP plate, an X-ray image intensifier, or another semiconductor X-ray detector.

In an embodiment, an operation of the radiation detector700may be as follows. Assume that the radiation detector700is exposed to the radiation720(e.g., X-ray) that have earlier interacted with the object730(e.g., an animal). As a result, the radiation720carries information of the object730.

For radiation particles of the radiation720that propagate in the direction of a photodiode410+100dof the radiation detector700, because the reflector710associated with the photodiode is not opaque to at least a portion of the radiation720as described above, at least some of these radiation particles pass through the associated reflector710and enter the scintillator510associated with the photodiode. In response, the associated scintillator510emits photons in directions.

Because each scintillator510is essentially completely enclosed by the associated reflector710and the associated photodiode410+100das described above, each photon emitted by the scintillator510may either enter the photodiode410+100dwith no interaction with the reflector710or bounce off the reflector710before entering the photodiode410+100d. In other words, all or almost all the photons emitted by the scintillator510are prevented by the reflector710from not entering the photodiode410+100d. In yet other words, the reflector710all or almost all the photons emitted by the scintillator510into the photodiode410+100d.

When the photons emitted by the scintillators510are guided by the reflectors710respectively into the photodiodes410+100d, these photons create in the photodiodes electrical signals that represent the information (e.g., an image) of the object730. In an embodiment, these electrical signals may be read out of the photodiodes and processed by the electronics structures of the radiation detector700before being sent out to a computer (not shown) for further processing and displaying the information (e.g., an image) of the object730.

In summary, when the radiation detector700is exposed to the radiation720which has earlier interacted with the object730, at least some radiation particles of the radiation720propagating in the direction of each photodiode410+100dpass through the associated reflector710and cause the associated scintillator510to emit photons in all directions. These emitted photons are guided by the associated reflector710into the photodiode resulting in the corresponding electrical signal in the photodiode. The resulting electrical signals in the photodiodes provide some information (e.g., an image) of the object730.

In the embodiments described above, with reference toFIG. 8andFIG. 9, the substrate100is doped P-type while the regions410are doped N-type. In an alternative embodiment, the substrate100may be doped N-type while the regions410may be doped P-type.

In the embodiments described above, the junction of each photodiode is a p-n junction. In general, the junction of each photodiode may be a p-n junction, a p-i-n junction, a heterojunction, a Schottky junction, or any suitable junction.

In the embodiments described above, the reflectors710are present in the radiation detector700. In an alternative embodiment, the reflectors710may be omitted (i.e., not present) in the radiation detector700.

In the embodiments described above, with reference toFIG. 6,FIG. 7, andFIG. 8, the radiation detector700includes 6 photodiodes410+100darranged in an array of 2 rows and 3 columns. In general, the radiation detector700may include any number of photodiodes410+100darranged in any way.