JPH0534610B2 - - Google Patents

Description

Claim 1: A set of electrical conductors disposed in an electrically insulating substrate, spaced apart from each other and extending between opposing first and second sides of the substrate; a detection layer made of a radiation detection material disposed above the first surface and doped with impurities to have conductivity; and a plurality of semiconductor regions disposed along the first surface and doped with impurities. a set of radiation detectors provided between the detection layer and the substrate and corresponding to the set of electrical conductors so as to electrically connect the set of electrical conductors to the detection layer; a first contact layer comprising an electrically insulating region located between the semiconductor regions; and a first contact layer provided on the side of the sensing layer opposite the substrate to control the flow of dark current. a low conductivity stop layer provided along the surface of the stop layer opposite to the detection layer, serving as a common terminal for all of the radiation detectors and serving as a terminal for each of these detectors; a second contact layer having a grid-like structure made of electrical conductors.

2. A semiconductor radiation detector according to claim 1, wherein the lattice-like structure comprises a layer of doped semiconductor on which a lattice-like metal compound is disposed.

3 said openings in said grid metal compound are in registration with said conductors and each have substantially the same dimensions as one of said radiation detectors, said array opposite said set of conductors; 3. A semiconductor radiation detector according to claim 2, adapted to receive incident radiation thereon from a surface of the semiconductor radiation detector.

4 having a radiation detection area constituting each of a plurality of detectors, a first surface receiving radiation and a second surface opposite to the first surface, wherein the first surface is incident on the radiation detection area; the first surface is electrically conductive and forms a common electrically conductive contact between the detection area and a reading device; the second surface has a plurality of conductors exiting therethrough, each of the conductors reading one of the areas; A semiconductor radiation detector arranged so as to be connected to the device, further comprising the detection area, having a predetermined thickness and impurity concentration, and absorbing radiation in a predetermined frequency range to generate a single electric charge. a first semiconductor layer that emits a signal indicating the presence of the semiconductor layer; a second semiconductor layer that is adjacent to the first semiconductor layer and is made of an electrical insulator formed of an intrinsic semiconductor material; first and second contacts adjacent to each other so as to be electrically connected to the semiconductor layer; a substrate adjacent to the first contact; and first and second contacts provided through the substrate, respectively. a plurality of conductors having ends, a first end conductively connected to the first contact, and a second end respectively connected to the reader; and the second contact. a metallized layer arranged on top and in conductive connection therewith, the metallization layer being thin enough that radiation in a predetermined frequency range incident thereon substantially passes through it, and having a connection to said reading device; and the second semiconductor layer has a sufficiently thick impurity concentration and a sufficiently low concentration of impurities, so that charge carriers are transferred from the second contact to the impurity conduction of the first semiconductor layer. Injection into the first semiconductor layer at the energy level of the band is substantially prevented. Semiconductor radiation detector.

5 The metallized layers are spaced apart and orthogonally arranged in the form of a grid, and radiation can enter the detector through the spaces between the members of the grid, and the space is 5. The semiconductor radiation detector of claim 4, wherein the semiconductor radiation detector forms a surface area over the second contact therebelow.

6. The first contact has a region of high conductivity in connection with the region of the second contact, and each of the regions of high conductivity is adjacent to the region of high conductivity. 6. A semiconductor according to claim 5, wherein the semiconductor layer is electrically conductively connected to the volume of the first semiconductor layer absorbing said radiation, whereby said detection areas are each formed to detect radiation in a predetermined frequency range. Radiation detector.

7 The above frequency range is about 14 in the long wavelength infrared
7. A semiconductor radiation detector according to claim 6, having a wavelength of .about.30 microns.

BACKGROUND OF THE INVENTION The present invention relates to radiation detectors, and in particular to suppressed impurity band detectors, used exclusively for the detection of longwave infrared radiation (LWIR).

In the design of high quality radiation detectors, a natural goal is to create a detector that is most sensitive to incident radiation within a certain desired frequency. One of the most common problems encountered by designers of high quality radiation detectors is the phenomenon known collectively as dark current. This phenomenon is a result of various mechanical phenomena in the operation within the detector, resulting in the flow of current within the detector in the absence of incident radiation.

The dark current flowing through the detector is generally considered as electrical noise, and the magnitude of the noise is directly proportional to the strength of the dark current. This dark current noise is inversely proportional to the detector's signal-to-noise ratio, making the detector less sensitive to changes in the incident radiation.

One of the well-known mechanisms of this dark current is the generation of thermal charge carriers. This mechanism works to absorb thermal energy by the semiconductor by freeing donor impurity electrons from its atoms. These electrons enter the conductor band and are swept by the electric field to the positive detector electrical contact. In normal operation, an electric field is created in the detector by a potential difference, which is primarily provided by an external integrated circuit reader, such as a hybrid thin film or charge coupled device. Additional electrons are injected into the detector by the negative potential contact of the reader. As a result, the two mechanisms cooperate to generate a current in the detector in the absence of incident radiation, ie, a dark current.

One of the well-known methods of extinguishing components thermally induced by dark current is
There are methods to cool radiation detectors to a few degrees above absolute temperature. However, it is difficult to house such a cryogenically cooled detector in an assembly compactly and at low cost.

Another common method used to reduce dark current is to interpose a relatively high resistance layer between a highly doped and therefore low resistance sensing layer and one electrical contact of the detector. This is the way to do it. This high resistance layer blocks the conductive path of the impurity band conduction mechanism,
As a result, dark current is reduced. Therefore, generally a relatively high resistance layer is used as a stop layer, and therefore
A detector using such a layer is known as a suppressed impurity band detector.

One of the special problems with this restrained impurity band detector is the physical mounting of the electrical contacts and associated reading device. Typical arrays of this detector are of very small dimensions, with spacing of less than 100 microns between each detector, and common wiring connections are often difficult. This problem is further compounded by the large number, thousands or more, of independent detector elements contained within an array.

As one method for solving this wiring problem, the following integrated circuit reader is used. That is,
The device is sized to be comparable to these portions of the radiation detector and is typically arranged such that each contact of the reader is in registration with each detector element. . The detector and reader are then packed together such that they are physically coupled to each other, thereby making the reader electrically connected to each detector element. That is,
Radiation detectors that are compatible with integrated circuit reading technology can have a number of advantages over detectors that are not compatible.

However, one of the problems with this type of reading technology is that
This reduces the fill factor of the detector. The film factor is a measure of the surface of the array that can receive the incident radiation.
It is one. The installation of electrical contacts and associated reading devices typically reduces the fill factor of a given array due to partial occlusion of the radiation receiving surface.

Many conventional detectors, known as stop impurity transducers (BITs), have all of their electrical contacts exposed to the radiation receiving surface. This arrangement has severe limitations on the physical characteristics of the readout circuit, and as a result, such detectors are often not compatible with integrated circuit readout techniques.

In response to the obvious drawbacks caused by this type of detector, an alternative form has been devised.
That is, in this case all electrical connection points have been moved to the opposite side to the side receiving the radiation. This device, commonly known as a Reverse Illumination Inhibition Impurity Transducer (RIBIT), is compatible with integrated circuit reading technology. This inverted detector is described in US Pat. No. 4,507,674, assigned to the assignee of this patent. The low foil factor of BIT detectors has been overcome by this RIBIT study. The RIBIT array's foil actors can be approached as a single unit,
It is possible to install a very large number of hybrid compatible detectors per focal plane. But this RIBIT
The structure has certain material and processing issues that make its manufacture experimental. In the RIBIT structure, to create the back contact area, a first epitaxial layer must be fabricated on a bulk silicon substrate with a heavily implanted surface area. The crystalline and electrical properties of the epitaxial film formed over this area are difficult to control and result in poor detector performance. The RIBIT process also requires a V-groove etch in both epitaxial layers to provide a means of contacting the heavily implanted areas to the substrate.

SUMMARY OF THE INVENTION The above-mentioned problems are overcome and other advantages are created by an emissive sealed impurity band detector. According to one preferred embodiment of the invention, the detector is formed as an array of detector elements;
It includes a layered semiconductor structure with electrical contacts on its front and back sides. This contact shape allows radiation to be received on its front side and an integrated circuit reader can be mounted on its back side, which is connected to the front and rear contacts.

The detector includes two insulating semiconductor layers, each of which serves as one substrate and the other as a stop layer. A radiation detection layer is provided between them. In this embodiment of the invention,
Although these two layers are made of silicon, other semiconductor materials may be used in accordance with the principles of the present invention. This detection layer is doped and modified to a valence band structure to reduce the band gap between this conduction band and the subsequent valence band, elevating photons of the incident radiation to electrons from the valence band to the conduction band. be done. This stop layer is so thin that advancing electrons can pass through it without going back into the valence band.

This electrical connection between the detection layer and the stop layer is made with the help of two contact layers, one of which is placed between the detection layer and the substrate and the other on the opposite side of the detection layer. disposed on the surface of the stop layer. The two contacts are made of silicon and doped to make them conductive. In the first contact layer, areas of doped material are interspersed between areas of undoped electrically insulating areas to form each detector element of the array of detector elements. In the second layer, doping takes place uniformly and electrically connects all detector elements in common. This second
The layers are very thin and do not interfere significantly with the incoming radiation. The layers mentioned above are deposited one on top of the other by epitaxial growth methods.

Prior to stacking each layer, backside contacts are formed in the substrate. This is done by running multiple metal conductors where the board joins each detector element, with one end of each conductor leading to the back of the board, which is also the back of the detector. Re,
Connected to a reader. The other end of each conductor is led to the opposite surface of the substrate and connected to the next first contact layer. the electrical contacts on the front side of the detector are formed as a single metallization in the form of a grid;
Serves as a common contact to the detector elements.
The thickness of this grating line is very thin and does not significantly suppress the incident radiation.

When using this detector, the insulating properties of the stop layer prevent dark current from flowing. This electrical contact shape and arrangement achieves high foil factor and does not suffer from the disadvantages inherent in the complex physical construction of the inverted arrest detectors described above.

[Brief explanation of the drawing]

The present invention will be described below with reference to the drawings, and the same reference numerals are used for common parts in each drawing. 1 is a perspective view of a restrained impurity band infrared detector with electrodes disposed on opposite sides in accordance with one embodiment of the present invention; FIG. 2 is a cross-section of the detector along line 2--2 of FIG. 1; FIG.

[Example] The suppressed impurity band detector 10 is shown in FIGS. 1 and 2.
One example is shown. The detector is made to be particularly sensitive to longwave infrared (LWIR) radiation. Generally, LWIR radiation is approximately
It is thought to be of a frequency corresponding to the wavelength range of 14 to 30 microns. Therefore, the impurity used in the detector is an element that reduces the width of the forbidden band between the valence band and the conduction band, which corresponds to the wavelength energy of the LWIR radiation.

The operation of the detector 10 is based on the use of a doped detection layer with a relatively thin undoped insulating layer that inhibits the flow of dark current. In the presence of incident radiation of a wavelength suitable for use with detector 10, electrons are elevated into the conduction band of the detection layer and, as described below, are elevated by the electric field provided by an external reading device connected to detector 10. Run through the restraining layer. It should be noted that in the detection layer, the doping has already reduced the band gap sufficiently, allowing electrons to move up and into the conduction band, while in the stop layer, the doping The problem is that the layers that are not connected maintain the original relatively large band gap. This stop layer is sufficiently thin that electrons can easily pass through it without the possibility of dropping back into the valence band of the stop layer. The thickness of the stop layer is very thick and is sufficient to block the advance of dark current. In the presence of an electric field applied by the reader, the detector 10 has the characteristics of a variable resistor, in which case the current produced by the applied voltage varies depending on the intensity of the incident radiation. do. The structure of the detector 10 with the novel features of the present invention will be described in more detail below.

The detector 10 includes a radiation detection layer 20 and a stop layer 22.
and front and bottom detector contact layers 24 and 16, respectively, formed on the substrate 12. Electrical contact from the detector 10 to the integrated circuit reader 36 is provided by a plurality of metal conductors 14 and backside metal compounds 32 and 34.
and on the other hand by the front lattice metallization.
It will be done. LWIR conceptually indicated by arrow 28
When radiation is incident on the front surface of the detector 10, a substantially transparent contact layer 24 and an underlying stop layer 2 are formed.
2 and reach the radiation detection layer 20. Absorption of radiation by detector impurities is now sensed by electrical circuitry (not shown) in reader 36 as a change in the electrical resistance of detector 10.

Considering the overall configuration, radiation detector 10 is assembled on substrate 12 . Substrate 12 is typically made of intrinsic silicon and is approximately 29 mils thick. Metal conductor 14 known as bias
passes through the substrate 12, and a metal, typically aluminum, is deposited in defined areas on the top surface of the substrate 12. A sufficiently strong thermal gradient is then applied to the substrate 12 to melt the metals described above. This molten metal moves into the substrate material and descends completely. As this molten metal descends through the substrate 12, a portion of the metal is deposited onto the substrate, thereby selectively doping the substrate and forming a continuous aluminum conductor 14 from the front surface to the back surface of the substrate 12. Ru. The exposed edges of conductor 14 on the front surface of substrate 12 are polished by conventional methods to eliminate surface defects that would prevent uniform epitaxial layer growth on the top surface of substrate 12. A plurality of conductors 14 are connected to the substrate 1
Exiting from the rear surface of 2, a back metallized pad 32, typically made of aluminum, is formed to contact conductor 14. On the surface of each pad 32 are metallization points 34, typically made of indium, which conductively connect the underlying pad 32 and conductor 14 to an integrated circuit reader 36.

To make the preferred electrical contact of the conductor 14 to the internal radiation detection layer 20, a contact layer 16 is made of substantially pure crystalline silicon, which is typically 3 microns thick on the top surface of the substrate 12. grown epitaxially to any thickness. Layer 16 is doped with an acceptor impurity such as boron as follows. First, boron is added to layer 16, typically
It is planted in checkerboards at a depth of 0.1 to 0.2 mils. This planted part is the substrate 12
It is in registration relationship with the end of the metal conductor 14 disposed on the front surface of the metal conductor 14 . After planting, the as-built device is annealed to repair any flaws that may have occurred in the crystalline structure of layer 16. During this annealing, the implanted boron atoms migrate down through the layer 16 and into the conductor 14 below.
contact with the exposed end of the As shown in the drawings, layer 16 is heavily doped adjacent each conductor 14 such that layer 16 has a first heavily doped area 16a and a second area of substantially pure silicon. It is divided into a zone 16b. Area 16a
contains acceptor impurities, the concentration of which is typically 1×10 ¹⁹ acceptor atoms per cubic centimeter. Each area 16a is thus conductive and in electrical contact with the end of the corresponding conductor 14. The extent of the surface area of layer 16 thus planted is determined by design specifications. This percentage of planted area can vary widely from 1% to typically 75%.

A sensing layer 20 is grown epitaxially over the contact layer 16 and has a thickness typically between 4 and 4.
It is 25 microns. Layer 20 is doped with an acceptor type impurity, such as gallium, suitable to impart p-type semiconductor properties to layer 20. The concentration of acceptor type impurity atoms in layer 20 is typically 1× per cubic centimeter.
There are 10 ^{and 18} acceptor atoms.

Gallium has an ionization energy of LWIR
It is one of the elements that corresponds to the energy of radiation.
Thus, LWIR radiation 28 entering layer 20 ionizes the electrons bound to the gallium atoms, which are then free to enter the conduction band.

A stop layer 22 is epitaxially grown on the detection layer 20 . Stop layer 22 consists of substantially pure intrinsic silicon. The crystal structure of the stop layer 20 thus created is similar to that of the doped detection layer 2, provided that it is substantially free of impurity atoms.
It has an electrical resistance much higher than 0.

As previously explained, this relatively high resistance stop layer 22 serves to block the impurity band conducting component of the dark current. In order to perform this function completely, the blocking layer 22 must be connected to the radiation detecting layer 2.
0 and one of the electrical contacts of each sensing element of the radiation detector 10. As described above, a plurality of heavily doped regions 16a
However, together with the conductor 14, it is necessary to form a set of contacts so that a plurality of detection areas can be scanned by the reader 36. reading device 36
In order to create circuit continuity between the detector 10 and the detector 10, it is necessary to provide a ground electrical connection to the detector 10, and this connection is provided by the contact layer 24.

In this embodiment of the invention, the ground connection is formed by ion implantation, typically 0.2 microns deep, into the surface of the stop layer 22 opposite the sensing layer 20. , a p-type acceptor impurity, typically boron. After implantation, the surface of layer 22 is annealed to repair any physical defects in the crystal structure that may have been created during implantation. During this annealing, the implanted boron atoms migrate downward into layer 22. This annealing time is determined as follows: Due to the downward movement of the boron atoms, the stop layer 22 is not yet completely encapsulated, so that the layer 22 is substantially adjacent to the detection layer 20. It is divided into a region of pure crystalline silicon and an upper region containing p-type acceptor impurity boron atoms. This upper area is the contact layer 2
form 4. The concentration of acceptor impurity atoms in layer 24 is typically 1×10 ¹⁹ acceptor atoms per cubic centimeter, making layer 24 electrically conductive.

A thin layer of metallization, typically aluminum, is deposited over the contact layer 24 and the grid 30
It has two sets of spaced apart parallel members, with the members of one set arranged at right angles to the members of the other set.

As shown in the figures, LWIR radiation enters the detector 10 through the transparent front contact layer 24. Also, since the grating 30 is formed as a thin layer, the LWIR
Substantially transparent to radiation. Thus, the present invention achieves a high foil factor comparable to that of RIBIT devices, yet does not have the above-mentioned problems associated with the complex manufacturing process of RIBIT devices.

After passing through the substantially transparent front contact layer 24, the LWIR radiation 28 then passes through the transparent stop layer 24.
2 and enters the detection layer 20. As mentioned above,
Since the doping level of layer 20 is very high,
All of the LWIR radiation is absorbed by layer 20. The results of this are favorable. As a result of the absorption of radiation into the layer 20, the electrons of the impurity atoms are lifted from the valence band to the conduction band, thus generating charge carriers.

The sensing layer 20 has some resistance and is capable of conducting a current that flows in response to a bias voltage provided by a reading device such as an integrated circuit multiplexer. When the incident radiation 28 is absorbed, the resistance of the detection layer 20 changes. This results in a change in the current flowing through the detector 10, which change is then sensed by the reader 36.

As mentioned above, substantially all of the incident radiation 28 is absorbed by layer 20. Front and rear contact layers 24
The small amount of radiation absorbed by layers 20 and 16 has a negligible effect, and the conductivity in this area
It is much higher and harmless than that of . Additional radiation-introduced charge carriers occurring in the contact layers 24 and 16 are therefore not detected.

In terms of the structure of the essentially pure crystalline silicon stop layer 22, the layer 22 has a relatively high electrical resistance and therefore the mobility of charge carriers in this area is limited within the layer 20. much smaller than in
The small amount of radiation absorbed by layer 22 has a negligible effect on the operation of detector 10.

The embodiments of the invention described above are merely illustrative, and variations thereof will be readily apparent to those skilled in the art. For example, one variation is to replace p-type impurities in those layers of the device that need to be doped with n-type impurities. Accordingly, the invention is not to be limited by the embodiments disclosed herein, but only by the scope of the appended claims.