JPS6322471B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention comprises a semiconductor in the form of substantially parallel elongated strips of semiconductor on a substrate and an ambipolar drift of free minority charge carriers along said strip due to the generation of thermal radiation. The present invention relates to an associated thermal radiation imaging device.

British Patent Specification No. 1488258 discloses a device comprising a semiconductor in the form of an elongated strip of semiconductor material, capable of generating free charge carriers upon absorption of thermal radiation incident on said strip, and bias electrode means extending along said strip. The strips are spaced apart in the direction so that a bias current mainly of the majority charge carriers flows along the strip, and the bias current causes a simultaneous bipolar drift of the free minority charge carriers due to the generation of thermal radiation with respect to the bias current. A thermal radiation imaging device is described which is capable of being supported in opposite directions and having readout means in a simultaneous bipolar drift path between said spaced apart bias electrode means.

The semiconductor material forming the strip is typically mercury cadmium telluride. The readout means may be configured with first and second readout electrodes proximate to the strip forming an ohmic contact;
These electrodes may be made of metal, such as aluminum, and preferably extend across the strip, and one electrode may be common to the bias electrode.
When using such an imaging device, the voltage developed between the two readout electrodes is of the magnitude of the minority carrier concentration produced by thermal radiation. However, the read-out means may be metallic or may form a diode junction with the strip, and in use a semiconductor region (this region preferably traverses the strip) which junction is reverse biased by applying a suitable bias voltage. It is also possible to cut it and extend it. The current generated through the diode also has a magnitude of the minority carrier concentration generated by thermal radiation.

As described in the said British Patent Specification No. 1488258,
and in the particular form of device illustrated, the metal or semiconductor regions forming the readout means are mounted on or defined in the semiconductor strip, and the strip itself is adapted to bring the strip to the desired operating temperature. It is mounted in a conventional encapsulation system for cooling and suitable electrical imaging. It is customary to use wire bonding techniques to make electrical connections to thermal radiation imaging devices in such encapsulations. However, there are problems with bonding methods in which wires are connected directly to the readout means as described above in the imaging device strip. The location of the reading means is a sensitive area in the simultaneous bipolar path, and wire bonding in this area can lead to damage to the semiconductor material and cause significant recombination of charge carriers in this area. In extreme cases, even destruction of the semiconductor material can occur.

Further, in experiments leading to the invention, the inventors assembled multiple strips of such devices in parallel on a common substrate to form a two-dimensional sensitive area. It is desirable to closely space the strips from each other to reduce the insensitive area between parallel strips.
Also, to simplify imaging systems using such parallel strips, it is desirable to have the readout means and bias electrode means each aligned in a direction substantially perpendicular to the strip. in this way,
The close spacing of the strips from each other and the alignment of said means perpendicular to the strips can be satisfied by direct wire bonding to the readout means on each strip, in which case You will suffer from such drawbacks.

a thermal radiation coupling device substrate of the present invention, a generally parallel elongated strip of semiconductor material disposed on the substrate and of a semiconductor material capable of absorbing incident thermal radiation and generating free charge carriers; consisting of spaced apart bias electrodes which cause a bias current consisting primarily of majority charge carriers to flow in and along each strip;
bias electrode means capable of causing this bias current to maintain a simultaneous bipolar drift with free minority charge carriers generated by the incident thermal radiation and flowing in an opposite direction to the flow of said majority charge carriers; read means comprising an electrode and a first read electrode, each of said strips providing a continuous portion of a simultaneous bipolar drift path and a simultaneous bipolar drift path; The continuous portion extends from a region connected to the continuous portion, and is characterized in that the continuous portion is formed to have a narrower width and a larger electric field than the simultaneous bipolar drift path immediately preceding the continuous portion.

In such a device, the wiring to the reading means comprises the other part of the strip and extends from the area of the reading means, so that these wires can be used, for example, when wire bonding is used for encapsulating the device. This wiring can be bonded to the part opposite the sensitive area associated with the reading means. The parallel arrangement of the slots and the branching portions of the strips allows wiring of this type to extend between the semiconductor strips even when these strips are arranged at small intervals. This allows the strips to be arranged in such a way that the bias electrode means and reading means of these strips are each aligned in a direction substantially perpendicular to the strips, and the strips can be arranged in a manner that is significantly narrower than the width of the strips, e.g. They can be separated from each other and placed close to each other by a slot whose width is less than 1/2 or 1/4 of the width of .

A multistrip device of such a geometry can be particularly compact if the strip section that continues the simultaneous bipolar drift path is narrower than the drift path section in front of the area of the reading means. Narrowing the simultaneous bipolar drift path in the area of the reading means causes a concentration of bias current in this area and therefore a high electric field, which increases the drift speed and response of the device and improves the characteristics of the device. Improve.

In order to insulate the semiconductor strip from the effects of electrical wiring provided within the encapsulation device, it is preferable to form these wiring directly on the substrate by metallization rather than on a portion of the semiconductor strip. The top edge of at least one end of each strip is rounded from the top edge along its side edges, and a metal layer extends from the top edge of the rounded end onto the substrate. It is advantageous to form the connection to the bias electrode and the first read electrode by. Since the side edges of the strips are not rounded, the strips can be placed close together and the rounded edges at the ends of the strips reduce problems encountered when depositing metal layers on these edges. Thus, it is possible to form crack-free and highly reliable wiring to the reading means and bias electrode means.

The reading means may be of known types including, for example, ohmic contacts or diode junctions as described above. In order to reduce the series resistance of the wiring to the reading means, the other part of the two parts is provided with a metal strip extending at least approximately to the inner edge of the slot between the two parts. A main conductive path for the wiring including the wiring is provided.

The invention further provides that each of the strips bifurcates into two parts separated from each other by a slot extending at least substantially parallel to the strip, one of the two parts forming the continuous part, and one of the two parts forming the continuous part; The other portion forms a continuation to said first read electrode, the first read electrode extending in a direction generally parallel to said slot and extending from one bias electrode of said bias electrode means at said slot. separated.

The device according to the invention described above can be used in a system comprising mechanical scanning means as described in GB 1488258. The device of the invention therefore obtains a thermal radiation imaging system by providing means for scanning the thermal radiation image along said strip in the same direction as the simultaneous bipolar drift and at a speed corresponding to the speed of this simultaneous bipolar drift. be able to.

However, the apparatus of the present invention is suitable for thermal radiation imaging systems using other forms of scanning, such as applying a scanning gradient voltage to the strip via a bias electrode to drive carriers generated by the thermal radiation towards the reading means. It can also be used in systems with

The present invention will be explained below with reference to the drawings. It should be noted that the figures are not drawn to scale, and the relative dimensions and proportions of certain parts may be exaggerated or reduced for ease of illustration and convenience. Furthermore, the same reference numerals are used in the figures to refer to the same parts of the same device or element, as well as to similar parts of different devices and elements.

The thermal radiation imaging device of FIGS. 1-3 comprises a plurality of photoconductive elements 1 on a substrate 2, the elements 1 being generally parallel elongated rectangular strips of semiconductor material of a predetermined conductivity type. is a semiconductor in the form of , which is capable of generating free charge carriers by absorption of thermal radiation incident on the strip. This semiconductor material can be, for example, n-type mercury cadmium telluride Hg _0.79 Cd _0.21 Te, which has a carrier concentration of less than 5×10 ¹⁴ cm ^-3 in the absence of incident radiation. For a material of this composition, the radiation absorption edge at an operating temperature of 77° is at a wavelength of approximately 11.5 microns. This material absorbs infrared radiation with a window of 8 to 14 microns and effectively generates electron-hole pairs, 77
The mobility of holes at the operating temperature of 〓 is 600cm ²
At V ^-1 sec ^-1 its lifetime is 2.5 milliseconds.
The electron mobility is approximately 2×10 ¹⁵ cm ² V ⁻¹ sec ⁻¹ .

Each strip 1 has a length of, for example, 1 mm and a width of
It can be 62.5 microns and 10 microns thick. The strips 1 may be separated by slots 3, the width of which may be, for example, 12.5 microns. FIG. 1 shows by way of example eight such separated strips 1.
It will be appreciated that different systems may require different numbers of strips as well as their length, width, thickness and spacing dimensions.

The substrate 2 may be made of sapphire, for example 0.5
The semiconductor strip 1 can be fixed by a micron-thick epoxy adhesive layer (see FIG. 3). On the upper surface of each of these semiconductor strips 1 is a passivating layer 5 which may be approximately 0.1 micron thick and which consists primarily of mercury,
It consists of oxides of cadmium and tellurium.
At the opposite ends of the upper surface of each strip 1, where bias electrodes 6 and 7 are present, this passivating layer 5 is removed. These electrodes may consist of approximately 1 micron thick deposited gold layers, each of which forms ohmic contact with the semiconductor surface. These layers can then extend over the semiconductor surface over a distance of, for example, 100 microns from the end of the strip 1. As shown in FIG. 3, the bias electrodes 6 and 7 can be sunk over a short distance, e.g. 1 or 2 microns, into the semiconductor surface, as described in British Patent Application No.
Metal lift disclosed in No. 2027986A
It can be formed using an OFF method and an ion-etching method.

Metal layers forming the bias electrodes 6 and 7 also extend over the substrate 2, and these metal layers also act as connections to these electrodes. These electrode connections 6 and 7 on the substrate widen and diverge slightly to form an area in which, for example, gold wire connections can be formed when the device is installed in the housing. As shown in Figure 3, each strip 1
The upper edges at each end of the strip are rounder than the edges along the sides of the strip. The metal layer forming the electrode connections 6 and 7 extends over this rounded upper edge on the substrate 2. Ion-etching methods can be used to form the parallel semiconductor strips 1 from a single semiconductor, and the connections and individual electrodes for each strip 1 from the semiconductor and a metal layer deposited on the substrate 2. In this case, the method disclosed in UK Patent Application No. 2027556A may be used.

A DC bias voltage is applied between these electrodes 6 and 7 spaced apart in the direction along each strip 1,
A bias current of mainly majority charge carriers (electrons in this example) is caused to flow along the strip. This bias current makes it possible to maintain simultaneous bipolar drifts of radiation-generated minority carriers (holes in this example) in opposite directions. The operation of this device will be described later with reference to FIG.

The simultaneous bipolar drift path between the spaced apart bias electrodes 6 and 7 is provided with reading means comprising an electrode 8, which reading means may be of any known type. These reading means may be surface-adjacent regions of opposite conductivity type (p-type in this example) forming a pn diode junction with the bulk of the semiconductor strip 1. This area and semiconductor strip 1
exhibit their conductivity type characteristics at the intended operating temperature of the device, but do not necessarily exhibit these characteristics at room temperature. In the particular case where n-type mercury cadmium telluride is used in strip 1 and the intended operating temperature is 77°, the presence of a new pn diode junction is not evident at room temperature. Instead of using this pn junction, the reading means may comprise a shotgun (metal-semiconductor) diode junction.

However, in the embodiment shown in FIGS. 1 to 3, the readout means do not include diode junctions, but include spaced electrodes (8 and 6) and (8 and 7) forming ohmic contact with the semiconductor strip 1. It simply has. The apparatus shown in Figures 1 to 3 is for each strip 1.
It has reading means at both ends of the strip so that it can be read with a strip biased in either direction. That is, it is possible to perform either reading using electrodes (8 and 6) by passing a bias current from electrode 6 to 7, or reading using electrodes (8 and 7) by passing a bias current from electrode 7 to 6. . Therefore, if the characteristics of the device are better when biased in one direction than when biased in the other direction, then this one bias direction is selected for operation of the device.

However, it is not necessary to provide reading means at both ends of the strip 1. Therefore, FIGS. 4 and 5
In the embodiment shown, the electrode 7 at the end of the strip 1
does not have a reading means adjacent to it.

In the embodiment shown in FIGS. 4 and 5, the metal layer providing the individual electrodes 7 of each strip 1 extends over the ends of each strip 1 and also on the substrate 2.

The length of the simultaneous bipolar drift path in front of the reading means, in which the total integration of the minority carriers generated by the radiation can be carried out, is appropriate for the lifetime of the minority carriers in the semiconductor material, the electric field and the semiconductor material, and is usually It is limited to a distance determined by the ambipolar mobility, which is close to the minority carrier mobility. This distance must therefore be taken into account when positioning the reading means.

According to the invention, in each region of the reading means (8 and 6) and (8 and 7), each of the strips 1 is divided into two parts separated from each other by a slot 13 (a second (see Figures and Figure 3). A slot 13 extends from said area in a direction substantially parallel to the strip 1.

One part 11 provides a continuation of the simultaneous bipolar drift path from said region to the adjacent bias electrode 6 or 7.

The electrode 8 extends from the read area in a direction substantially parallel to the slot 13 and forms a metal strip connection of the read electrode 8 supported by the other part 12. This connection has at least a section 12 as a mechanical support for the metal strip. This electrode connection is separated from the adjacent bias electrode 6 or 7 by a slot 13. This strip connection 8 is sunk over a short distance into the semiconductor surface and extends over the substrate 2 through the more rounded edge of the strip 1, forming an area for wire connections. This metal strip pattern can be formed simultaneously from the same metal layer as electrodes 6 and 7, and slot 13 can be formed at the same time as slot 3. slot 1
3 may have a width of 12.5 microns, for example.

In this way, a compact device shape is obtained in which the bias electrodes 6, 7 and the read electrodes 8 are each substantially arranged in a direction substantially perpendicular to the strip 1.

Portions 11 and 12 are extensions of their respective strips beyond the area of the reading means. The continuation of the simultaneous bipolar drift path within the portion 11 is therefore narrower than the main part of said drift path before the region of the reading means. This causes current convergence within portion 11 which increases the drift rate and responsivity of the device as discussed above. but,
Portion 11 should not be too narrow. The reason is that the sides of section 11 have a high carrier recombination effect which reduces the lifetime of minority carriers within section 11. The section 11 is therefore preferably wider than the section 12, at least half the width of the main part of the simultaneous bipolar drift path before this region. In the apparatus of Figures 1-3, portions 11 and 12 may have widths of, for example, 35 microns and 15 microns, respectively.

If the adjacent bias electrode 6 or 7 is very close to the read electrode 8, the responsivity and detectability of the element can be reduced. Therefore,
Preferably, the continuation of the drift path within section 11 is longer than its width. In the arrangement shown in FIGS. 1 to 3, the bias electrode 6 and the reading electrode 8
The distance between the region adjacent to the drift path can be, for example, 50 microns. In the particular form shown in FIGS. 1 and 2, the read electrode 8
does not extend beyond the inner end of slot 13 and contacts the drift passage on one side. The advantage of this is that the lateral extent of the electrode strip is precisely defined by the parallel slots 13 and 3. However, other configurations are also possible in which the read electrode 8 extends beyond the inner edge of the slot 13 and has more extensive contact with the drift path. In the embodiment shown in FIG. 6, metal strip 8 extends perpendicularly across strip 1 and beyond slot 13 to form a read electrode.

During operation, the device is kept at a low temperature and installed according to the specific application. Although such mounting is not shown, it is important to note that infrared radiation (e.g. 8-14
It usually consists of attachment to the substrate 2 in an evacuated enclosure with windows for guiding the micron band). This enclosure comprises means for maintaining the substrate 2 with the semiconductor strip 1 at the required operating temperature (for example 77°). One such form of attachment consists of a Dewar-type encapsulation commonly employed in infrared detector technology.

The operation of the apparatus of the present invention will be explained below with reference to FIG. Via bias electrodes 6 and 7 and connecting conductors, each strip 1 is connected in series to a DC bias power supply 21 and a variable resistor 22, so that a constant bias current consisting mainly of majority carriers (electrons in this case) flows through the strip 1. It is made to flow from bias electrode 6 to bias electrode 7 in the longitudinal direction. The bias power supply 21 is connected to all the electrodes 6 and 7, but in order to simplify the drawing, only one strip 1 is shown in connection in FIG.

This bias current can maintain the simultaneous bipolar drift of free minority charge carriers (holes in this example) in the opposite direction, ie from electrode 7 to electrode 6, generated by the incident thermal radiation. A suitable range of bias voltage between electrodes 6 and 7 is 1 volt to 10 volts. Within the n-type material of the above composition
When a voltage drop of 15V/cm occurs, the ambipolar mobility of this minority carrier
is approximately 400cm ² V ^-1 sec ^-1 .

Scanning the radiation pattern and focusing the image of a unit area of this pattern onto the strip 1 can be carried out in a manner similar to that described in GB 1,488,258. Means for scanning the thermal radiation image along such a strip 1 in the same direction as the simultaneous bipolar drift direction of the minority carriers and at a speed approximately equal to this simultaneous bipolar drift velocity is illustrated in schematic form in FIG. be. This scanning means can be composed of a pair of rotating mirrors 25 and 26 and a lens system 27. This scanning means then scans the image area of the radiation pattern sent from the scene 28 on the surface of one or more semiconductor strips 1 at an angle of 5000 cm.sec ^-1 to 20000 cm.
Move at a speed in the range cm・sec ^-1 .

The image is thus scanned across the surface of the semiconductor strip 1 at a speed corresponding to the simultaneous dipolar drift velocity of the minority carriers, so that the minority carriers generated by the radiation are
It accumulates in the part of the shape strip that is hit by the radiation. Since the minority carriers generated by these accumulated radiations pass through the strip portion 11 between the electrodes 8 and 6, conductivity modulation occurs in this portion 11. The image signal is tapped off via an output circuit 29 which is connected in known manner between electrodes 8 and 6 and which amplifies and processes the voltage changes occurring between electrodes 8 and 6 as a result of the conductivity modulation. Although the output circuit is shown for only one strip 1 in order to simplify the drawing, in reality, an individual output circuit 29 is provided for each strip 1, and this is connected to the electrodes 6 and 8 of each strip. Connect between.

Obviously many modifications can be made within the scope of the invention. For example, the composition of an n-type material, mercury cadmium telluride, can be modified to create a device capable of imaging radiation in the 3-5 micron band, for example. It is also possible to form the photoconductive strip 1 using semiconductor materials other than mercury cadmium telluride.

In the embodiment described above, the metal strip 8 is part 1.
The slot 13 extends over the entire upper surface of the slot 13, and extends at least almost to the inner end of the slot 13. The metal strip 8 therefore provides the main electrical path for the read connection comprising the section 12. In such a case, the portion 12 does not necessarily need to be a conductive part for electrical connection, but may simply serve as a mechanical support for electrical connection. Also, if the read connection of the system in which the device is used can have a series resistor of high resistance, then the metal strip 8 need not extend to the inner edge of the slot 13 and can be placed on the part 12, e.g. or extends only as far as 7 extends on portion 11;
Thereby, the part of the read connection between this short strip 8 and the simultaneous bipolar drift path may be formed solely by a conductive path in the semiconductor part 11.

In the embodiment described above, the strip 1 is formed as a separate semiconductor body on an insulating substrate, for example of sapphire. FIG. 6 shows another arrangement, also according to the invention.
Here, the strip 1 is composed of a common semiconductor portion 10, and the strips are integrally connected via a common portion that supports an electrode 6. And neither the electrodes 6 nor 8 extend over the substrate 2 (which, as in the previous example, can be sapphire). However, although this also relates to the present invention,
In one variant, the strip 1 of FIG. 6 can be an epitaxial layer of a conductivity type material deposited on a neutral substrate 2 or on a substrate 2 of cadmium telluride, for example. When doing this in FIG. 6, the epitaxial material is removed from slots 3 and 13 to provide a unit structure, and the metal parts constituting the bias electrodes and reading electrodes 6, 7, and 8 are placed in the remaining epitaxial layer parts. limit. Slots 3 and 13 together separate adjacent electrodes 6 and 8, and the conductors are applied to the parts of electrode patterns 6, 7 and 8 on the epitaxial layer remote from active strip 1. In order to remove any undesired injected minority carriers (holes) from the simultaneous bipolar drift path adjacent to the main bias electrode, which becomes the anode, a rectifying junction is provided at the electrode connection adjacent to this bias electrode. It provides a drain for the minority carriers, effectively separating the first stage of the simultaneous bipolar drift path from the bias electrode. When attaching an electrode to such a rectifying junction, the slot can be used in the same manner as in the case of the reading means.

By widening the gap 3 over almost its entire length, the strip 1 can be made narrower between the reading means than adjacent to the reading means, and on one side of each slot 1 the reading connection 8 can be made to stand out. However, such a construction results in a large insensitive so-called "dead" space between the strips 1, which is generally undesirable.

It is not necessary that the strip 1 extends substantially in a straight line. For example, each strip 1 may be twisted around an imaginary straight line. In this case, the imaginary straight lines between the different strips are made approximately parallel to each other. Such a serpentine strip is shown in published UK patent application no. CB2019649.

[Brief explanation of the drawing]

1 is a plan view showing an example of a thermal radiation imaging device according to the present invention; FIG. 2 is an enlarged plan view showing an example of the end portions of two elements of the device shown in FIG. 1 according to the present invention; FIG. is a sectional view taken along the line - in FIG. 2, FIG. 4 is a plan view showing a modification of some elements of the thermal radiation imaging device according to the present invention, and FIG. FIG. 6 is a perspective view of a portion of the imaging system in simplified form; FIG. 6 is a plan view of a portion of another thermal radiation imaging device according to the invention; 1... Strip, 2... Board, 3... Slot, 4
... layer, 5... passivation layer, 6, 7... bias electrode, 8... reading electrode, 10... common semiconductor body,
11... Strip portion between electrodes 8 and 6, 12
...part, 13...slot, 21...DC bias power supply, 22...variable resistor, 25, 26...rotating mirror, 27
...lens system, 28...scene, 29...output circuit.

Claims (1)

Claims: 1. A substrate 2, a substantially parallel elongated strip 1 of semiconductor of a semiconductor material provided on the substrate 2 and capable of absorbing incident thermal radiation and generating free charge carriers; It consists of bias electrodes 6, 7 spaced apart in a direction along bias electrode means capable of maintaining simultaneous bipolar drift with the free minority charge carriers flowing in the opposite direction to the flow of the majority charge carriers; one bias electrode 6, 7 of the bias electrode means and a first reading electrode 8; each of said strips 1 providing a simultaneous bipolar drift path and a continuous portion of the simultaneous bipolar drift path, said first read electrode 8 providing a simultaneous bipolar drift path and said continuous portion. A thermal radiation imaging device, characterized in that the continuous portion extends from a region where the two are connected, and the continuous portion is formed to have a narrower width and a larger electric field than a simultaneous bipolar drift path immediately preceding the region. 2. Apparatus according to claim 1, wherein said continuous portion extends to one bias electrode 7 of said bias electrode means. 3. Device according to claim 2, in which the first reading electrode 8 is in contact with one side edge of the simultaneous bipolar drift path. 4 Each said strip 1 has two strips separated from each other by a slot 13 extending at least substantially parallel thereto.
It branches into parts 11 and 12, one part 11 of the two parts forms the continuous part, the other part 12 of the two parts forms a continuous part to the first reading electrode 8, and the first reading The electrode extends in a direction substantially parallel to the slot 13 and extends from one bias electrode 7 of the bias electrode means to the slot 13.
An apparatus according to any one of claims 1 to 3, separated in . 5. Apparatus according to claim 4, wherein each of said strips 1, first read electrode 8 and bias electrodes 6, 7 are formed by one common semiconductor section. 6 the upper edge of at least one end of each strip 1 is rounded off from the upper edge along its side edge;
Bias electrodes 6, 7 are provided by metal layers 6, 7, 8 extending from above the upper edge of this rounded end onto the substrate.
and forming a connection to the first read electrode 8. 7. Connect the bias electrodes 6, 7 and reading means 8, 7 of different strips 1 to these strips 1 respectively.
6. A device according to any one of claims 1 to 5, wherein the device is substantially aligned in a direction substantially perpendicular to .