Ionizing radiation sensor

Abstract

The invention relates to semiconductor devices for conversion of the ionizing radiation into an electrical signal enabling determination of the radiation level and absorbed dose of gamma, proton, electronic and alpha radiations being measured. The ionizing radiation sensor is a p-i-n structure fabricated by the planar technology. The sensor contains a high-resistance silicon substrate of n-type conductivity, on whose front side there are p-regions; layer from SiO2; aluminum metallization; and a passivating layer. P-region, located in the central part of the substrate and occupying the most surface area, forms the active region of the sensor. At least two p-regions in the form of circular elements are located in the inactive region on the perimeter of the substrate around the central p-region and ensure a decrease in the surface current value and smooth voltage drop from the active region to the device perimeter.

Claims

1. An ionizing radiation sensor in a form of a p-i-n structure, containing: a high-resistance silicon substrate of n-type conductivity, on a front working side of which there are p-regions and a masking coating of SiO.sub.2; aluminum metallization; passivating layer; on a back side of the substrate, there are a high-doped layer of an n-region and an aluminum metallization; wherein at least one p-region is located in a central part of the substrate and occupies most of the surface area, forming an active region of the sensor, and at least two p-regions in a form of circular elements are located in an inactive region on a perimeter of the substrate to decrease a surface current value and ensure a smooth voltage drop from the active region to the substrate perimeter; in the SiO.sub.2 coating layer, there are windows to ensure a contact between the aluminum and the p-region; in the passivating layer, there are windows for connection of leads.

2. The sensor according to claim 1, wherein the sensor is fabricated by the planar technology with the use of contact photolithography.

3. The sensor according to claim 1, wherein a total area of the windows for ensuring of the contact between the aluminum and the p-region doesn't exceed 1% of a surface area of the active region of the sensor to prevent aluminum diffusion into silicon.

4. The sensor according to claim 1, wherein a number of windows for connection of the leads is equal to 4, the windows being located along edges of the substrate-one on each side.

5. The sensor according to claim 1, wherein the windows for connection of the leads are located in the inactive region of the substrate.

6. The sensor according to claim 1, wherein the p-region, located in the central part of the substrate, has shaped sections along edges in a form of grooves ensuring formation of inactive zones for location of the windows for connection of the leads.

7. The sensor according to claim 1, wherein a wafer of high-purity, floating-zone silicon with a specific resistance of 312 kOhm.Math.cm and thickness of 250-1,000 m is used as the silicon substrate.

8. The sensor according to claim 1, wherein a number of circular elements serving as guard rings, located at a distance from each other, the distance being increased from a substrate center to the perimeter, is equal to 4.

9. The sensor according to claim 8, wherein a width of the circular elements is equal to 25 m, a distance between a first and a second element being equal to 40 m, that between the second and a third being 50 m, between the third and a fourth being 70 m, wherein the first element is at a distance of 40 m from a boundary of the p region, the said values having a permissible tolerance of 20%.

10. The sensor according to claim 1, wherein the substrate has a working surface dimensions of 102102 mm.sup.2, dimensions of the active region being equal to 100100 mm.sup.2, a sensor thickness being equal to 2501,000 m, and an area occupied by the circular elements being equal to no more than 1 m on the substrate perimeter.

11. The sensor according to claim 1, said sensor ensuring an achievement of following electrical characteristics: a value of a reverse bias of 40200V until an achievement of a full depletion mode depending on a specific resistance and a thickness of the sensor; an operating mode characterized by the reverse bias at the full depletion; an operating voltage determined from a full depletion voltage value (V.sub.FD) of V.sub.op=V.sub.FD+20V; a breakdown voltage of not less than 2V.sub.FD;a dark current of no more than 200 nA/cm.sup.2 at the operating voltage; measurements of said parameters being taken at a temperature of 202 C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is further explained with the drawings, where

(2) FIG. 1 schematically shows the device applied for,

(3) FIGS. 2 and 3 show sections A-A and B-B of FIG. 1 respectively;

(4) FIG. 4 shows detail C of FIG. 1;

(5) FIG. 5 shows section D-D of FIG. 4;

(6) FIG. 6 is a diagram for taking of the electrical charge from the sensor for transmission to a recording device.

(7) The following is designated by items on the figures: 1high-resistance silicon substrate of n-type conductivity; 2p-region, located in the central part of the substrate, forming an active region of the sensor; 3p-regions being guard rings; 4SiO.sub.2 layer (coating); 5aluminum metallization forming one of the sensor electrodes; 6passivating (protective) layer 7windows for the contact between the metal (aluminum metallization) and the p-region, formed in the SiO.sub.2 layer, 8window for contacting with the p-n-region in the process of testing, located in the passivating layer over the p-region in the central part of the substrate; 9windows for connection of leads; 10n-region, located on the back side of the substrate; 11aluminum metallization on back side of the substrate forming the second electrode of the sensor, 12shaped sections along the substrate edges in the form of grooves ensuring the formation of the inactive regions for location of windows 9 for connection of leads.

EMBODIMENT OF THE INVENTION

(8) The ionizing radiation sensor (sensing element) is a p-i-n structure fabricated by the planar technology. The sensor contains a high-resistance silicon substrate of n-type conductivity 1 (see FIG. 1-5), on the front (working) side of which there are p-regions 2, 3; SiO.sub.2 layer (coating) 4; aluminum metallization 5; and passivating (protective) layer 6 from phosphate-silicate glass (SiO.sub.2+P.sub.2O.sub.5). Thickness of the layers is determined by their manufacturing process and is as a rule no more than 0.5+1.1 m.

(9) P-region 2, located in the central part of the substrate and occupying the most surface area, forms an active region of the sensor. At least two p-regions 3 are in the form of circular elements (guard rings) are located in the inactive region on the substrate perimeter around central p-region 2 ensuring a decrease in the surface current value and smooth voltage drop from the active region to the device perimeter. In SiO.sub.2 layer 4, there are windows 7 for the contact between the metal (aluminum metallization) and the p-region; in the passivating layer over the p-region, located in the central part of the substrate, there is window 8 for contacting with the p-n-region in the process of testing and window 9 for connection of leads. On the substrate side opposite to the front surface, there is high-doped n*-layer 10 layer with a thickness of 2+4 m, doped with up to 10.sup.19 atoms of the donor impurity per cm.sup.3, and aluminum metallization layer 11 with thickness of 0.9+1.1 m.

(10) Total area of windows 7 for the contact between the metal (aluminum metallization) and the p-region doesn't exceed 1% of the surface area of the detector active region for prevention of aluminum diffusion into silicon.

(11) Number of windows 9 for connection of leads is equal to 4, the windows being located along the edges of the substrateone on each side. Windows for connection of leads are located in the inactive region of the substrate. P-region 2, located in the central part of the substrate, has shaped sections along the edges in the form of grooves 12 (see FIG. 1) ensuring formation of inactive regions for location of windows 9 for connection of the leads. A high-purity, floating-zone silicon wafer with a specific resistance of 3+12 kOhm.Math.cm and thickness of 250-1,000 m is used as a silicon substrate. Number of circular elements (guard rings), located at a distance from each other, and the distance being increased from the substrate center to the perimeter, is equal to 4. Number and configuration of the guard rings is determined considering the manufacturing process specifics. The system of guard rings must ensure smooth voltage drop from the active region to the sensor edge.

(12) In one embodiment, the width of the circular elements 3 is equal to 25 m, the distance between the first and the second element being equal to 40 m, that between the second and the third being 50 m, between the third and the fourth being 70 m, wherein the first element is at a distance of 40 m from the boundary of the sensitive p-region, the said values varying in the range of 20%. Accuracy of the said dimensions during fabrication of the sensor is determined by the accuracy of mask plate fabrication and is 0.1 m. The substrate can be selected with the working surface dimensions of up to 102102 mm.sup.2, the dimensions of the active region being equal to 100100 mm.sup.2, the sensor thickness being equal to 250+1,000 m (determined by the wafer thickness), and the area occupied by the circular elements being equal to no more than 1 mm on the substrate perimeter. This embodiment of the sensor ensures achievement of the following electrical characteristics: value of the reverse bias of 40+200V until the achievement of a full depletion mode depending on the specific resistance and thickness of the sensor; operating mode characterized by the reverse bias at the full depletion; operating voltage determined from the full depletion voltage value (V.sub.FD) of V.sub.op=V.sub.FD+20V; breakdown voltage of not less than 2V.sub.FD; dark current of no more than 200 nA/cm.sup.2 at the operating voltage; the measurements of the said parameters being taken at a temperature of 202 C.

(13) The sensors are fabricated by the planar technology being a set of manufacturing operations helping to form the structures of planar semiconductor sensors on one side of a wafer cut from a silicon monocrystal of up to 150 mm in diameter. Specifically, the invention can be embodied by a technology close to that presented in publications of Kemmer (Kemmer J. Fabrication of low noise silicon radiation detectors by the planar process//Nuclear Instruments and Methods. 1980.-V.169.-P.499-502.).

(14) The planar technology is based on creation of regions with different types of conductivity or with different concentrations of the same impurity, together forming the sensor's structure, in the near-surface layer of the substrate. Regions of the structures are formed by local introduction of impurities in the substrate (by means of gas phase diffusion or ion implantation) through a mask (typically from a SiO.sub.2 film), formed by photolithography. By successive conduction of oxidizing (creation of a SiO.sub.2 film), photolithography and doping processes, a doped region of any required configuration is obtained, as well as regions with other type of conductivity (or other impurity concentration). The planar technology enables simultaneous manufacturing of a great number (up to several hundreds and even thousands) of identical discrete semiconductor devices (e.g. sensors) or integrated circuits on one wafer in a single process. Batch processing ensures a good repeatability of the devices parameters and high efficiency at relatively low unit cost.

(15) The ionizing radiation sensor works as follows. Quanta of the X-ray and low-energy gamma radiation, entering the sensor's material, react with it that results in the productiondepending on the incident quantum energyof a photoelectron, Compton electron or an electron-positron pair. Probability of this process is 0.01+0.03, but taking into account that the probability of detection of a charged particle (electron, positron, proton, alpha-particle etc.) is equal to 1, this is quite enough for consistent detection of the ionizing gamma radiation, even at the background level, with an accuracy of not less than 20% for 1+2 minutes of measuring. Charged particles penetrate into the active region of the sensor and generate electron-hole pairs in it. Charge carriers (electrons and holes) disperse under the action of the electric field applied to the semiconductor sensor and move to the electrodes. As a result, there is an electrical impulse in the external circuit of the semiconductor detector detected by a charge-sensitive preamplifier, converted into a voltage drop at its output and then transmitted to a signal processor (see FIG. 6 for example).

(16) A test specimen, in which the semiconductor sensor (detector) is a high-voltage p-i-n diode in the form of a single-sided structure fabricated by the planar technology on a high-purity, floating-zone melting silicon substrate with a specific resistance of 3+4 kOhm.Math.cm, with the overall dimensions of 1212 mm and thickness of 450 m, was created to check the performance efficiency of the sensor. The construction of the manufactured sensor corresponds to the embodiment, presented in FIG. 1-5. Flat signal p+-n junction represents an ion-implanted p+-region with an increased concentration of boron atoms. Circular guard p+-n junctions by the same method as flat signal p+-n junction, located in the central part of the substrate, are arranged around the flat signal p+-n junction, occupying the most part of the substrate (size of the active region was 1010 mm.sup.2). Perimeter of the region, occupied with the guard rings, was not more than 1 mm. Metal electrodes were made of aluminum. On the substrate side opposite to the front surface, there is high-doped n*-layer 10 layer with a thickness of 2+4 m, doped with up to 10.sup.19 atoms of the donor impurity per cm.sup.3, and aluminum metallization layer 11 with a thickness of 0.9+1.1 m.

(17) Set of 4 working mask plates (m/p) for the contact photolithography was used in manufacturing of the sensor by the planar technology, the first of which is a mask plate for formation of a p+-region, the second is for formation of contacts to the p+-region of the diode and to guard rings on the perimeter of the wafer front side; the third is for Al-metallization; and the fourth is for formation of contacts to metallization. The masks are listed in the order of their use in the process. Thus, the minimum width of the perimeter rings was 25 m in the first m/p; the minimum contact size in the second m/p for the formation of contacts to the p+-diode and guard rings on the perimeter of the front side of the wafer was 2525 m.sup.2; on the perimeter to the guard rings1040 and 4010 m.sup.2; the minimum width of the rings on the diode perimeter in the third m/p for Al-metallization was 20 m; dimensions of the fourth m/p for the formation of contacts to the central region of metallization are not critical.

(18) The manufactured device had the following electrical characteristics:

(19) Operating modereverse bias at the full depletion.

(20) Operating voltage is determined from the full depletion voltage value (V.sub.FD)V.sub.op=V.sub.FD+20V;

(21) Breakdown voltage, not less than 2.Math.V.sub.FD;

(22) Dark current at operating voltage, no more than 200 nA/cm;

(23) All measurements were taken at a temperature of 202 C. Test structures for determination of the specific resistance of the p-region by the four-point method are located on the wafer. Connection of the guard rings was not provided for.

(24) The invention thus provides a sensor, which can be used in various devices for detection and/or measurement of the ionizing radiation. The sensor has small dimensionspossibility of being used in portable, self-contained devices; reliable detection of any ionizing radiation in combination with a wide operating temperature range; high sensitivity (possibility of operation in a gamma-quants counting mode); high radiation resistance of the detector material; wide measuring range; elimination of the necessity for periodic servicing; low power consumption; and low-voltage power supply.