Diamond temperature and radiation sensor

Abstract: A temperature sensor comprises a body (10; 20) of insulating diamond material and a pair of spaced apart electrical contacts (12, 14; 22, 24) applied to the body. The body has a predetermined thickness between the contacts, and the contacts have a predetermined area, so that the sensor has a corresponding capacitance between the contacts which is temperature dependent The diamond body may comprise a diamond crystal, or may be a layer (20) of diamond deposited on a substrate (22) by a CVD process. In the latter case, the substrate may be a semi-conductive silicon layer, which serves as one of the contacts. The other contact (24) may be a metallic contact, or may comprise a layer of doped amorphous semi-conductive material. The sensor is sensitive to ionising radiation as well as to changes in temperature, and is suitable for use in a temperature compensated radiation measuring apparatus.

Description: BACKGROUND OF THE INVENTION This invention relates to a temperature sensor formed from diamond material which is sensitive to temperature and also to ionising or nuclear radiation. A diamond temperature sensor is known from EP-A-0 392 467, while a radiation sensor is known from EP-A-0 314 458. SUMMARY OF THE INVENTION According to the invention a temperature sensor comprises a body of insulating diamond material and a pair of spaced apart electrical contacts applied to the body, the body of diamond material having a predetermined thickness between the contacts, and the contacts having a predetermined area, so that the sensor has a corresponding capacitance between the contacts which is temperature dependent. The diamond body may comprise a type Ia, type Ib or type IIa natural or synthetic diamond crystal. The diamond body may further comprise a layer of undoped diamond material deposited by a chemical vapour deposition process. The layer of diamond may be deposited on a conductive or semi-conductive substrate. The substrate may comprise doped silicon. The doped silicon substrate may function as one of the electrical contacts. At least one of the electrical contacts may be metallic and may comprise, for example, titanium, aluminium, nickel or chrome. At least one of the electrical contacts may comprise doped amorphous semiconductor material deposited on the diamond body. The invention extends to temperature measurement apparatus which comprises a temperature sensor as defined above and which further comprises an oscillator circuit, the sensor being connected as a feedback element in the circuit so that the output frequency of the oscillator varies with the temperature of the sensor. The invention further extends to radiation measurement apparatus which comprises a temperature sensor of the invention and further comprises charging means for applying an electrical charge to the sensor at a predetermined charging rate, and monitoring means for monitoring the resultant voltage on the sensor, thereby to measure the intensity of ionising radiation incident on the sensor. The invention still further extends to radiation measurement apparatus which comprises a temperature sensor of the invention and further comprises pulse or conduction mode measuring means for measuring pulse or conduction mode electrical signals due to ionising radiation incident on the sensor and for generating a first electrical signal corresponding thereto, capacitance measuring means for measuring the capacitance of the sensor as a function of temperature and for generating a second electrical output signal corresponding thereto, and compensating means responsive to the first and second electrical output signals to generate an output signal corresponding to the intensity of the radiation incident on the sensor, which is compensated for changes in temperature. The invention additionally extends to a method of measuring temperature including the steps of providing a temperature sensor which comprises a body of insulating diamond material and a pair of spaced apart electrical contacts applied to the body, the body of diamond material having a predetermined thickness between the contacts, and the contacts having a predetermined area, so that the sensor has a corresponding capacitance between the contacts which is temperature dependent; locating the sensor in an environment or against an object to be measured; measuring the capacitance between the contacts or an electrical parameter related thereto; and relating the measured capacitance or electrical parameter to a reference capacitance or electrical parameter to determine the temperature of the environment or the object. The method may include connecting the sensor as a feedback element in an oscillator circuit, and relating variations in the temperature measured by the sensor to corresponding variations in the oscillation frequency of the oscillator circuit. The invention includes a method of measuring radiation including the steps of providing a temperature sensor which comprises a body of insulating diamond material and a pair of spaced apart electrical contacts applied to the body, the body of diamond material having a predetermined thickness between the contacts, and the contacts having a predetermined area, so that the sensor has a corresponding capacitance between the contacts which is temperature dependent; applying an electrical charge to the sensor at a predetermined charging rate; and monitoring the resultant voltage on the sensor, thereby to measure the intensity of ionising radiation incident on the sensor. The invention further includes a method of measuring radiation including the steps of providing a temperature sensor which comprises a body of insulating diamond material and a pair of spaced apart electrical contacts applied to the body, the body of diamond material having a predetermined thickness between the contacts, and the contacts having a predetermined area, so that the sensor has a corresponding capacitance between the contacts which is temperature dependent; subjecting the sensor to ionising radiation; measuring pulse or conduction mode electrical signals due to the ionising radiation and generating a first electrical output signal corresponding thereto; measuring the capacitance of the sensor as a function of temperature and generating a second electrical output signal corresponding thereto; and processing the first and second electrical output signals to generate an output signal corresponding to the intensity of the radiation incident on the sensor, which s compensated for changes in temperature. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic sectional side view of a first embodiment of a diamond temperature sensor according to the invention; Figure 2 is a schematic sectional side view of a second embodiment of the temperature sensor; Figure 3 is a graph indicating the electrical characteristic of the sensor with temperature; Figures 4 and 5 illustrate circuits which can be used with the sensor of the invention to measure temperature; and Figure 6 illustrates a circuit which can be used with the sensor of the invention to measure radiation and/or temperature. DESCRIPTION OF EMBODIMENTS Figure 1 illustrates a temperature sensor according to the invention which comprises a body 10 of diamond material with electrical contacts 12 and 14 applied thereto. Conductors 16 and 18 are connected to the contacts 12 and 14, respectively. The body 10 is typically a 1 mm cube of insulating diamond material, such as a type Ia, Ib, or IIa natural or synthetic diamond crystal. The crystal is selected for low levels of impurities such as boron or phosphorous, which would impair its insulating properties. The contacts 12 and 14 are preferably metallic and may comprise, for example, titanium, aluminium, nickel, chrome or other metals. The contacts need not necessarily be ohmic. Figure 2 illustrates an alternative embodiment of the temperature sensor, in which a diamond body 20 is formed as a layer on a doped silicon substrate by a chemical vapour deposition (CVD) process. Again, the diamond layer has a low concentration of impurities which would increase its conductivity. The doped substrate 22 is conductive or semi-conductive and acts as a first electrical contact to the diamond layer. The substrate can be made from a silicon wafer with a thickness of about 100 microns or greater, and having a crystallographic orientation which is within 3 to 4° of one of the main crystallographic planes. A second, metallic contact 24 is applied to the diamond body 20. Instead of a metallic contact, a non-metallic contact could be provided, for example, by the deposition of doped amorphous semi-conductor material such as amorphous silicon or germanium on the body 20. The semi-conductor material is doped with boron or phosphorous to a sufficiently high concentration to give the semi-conductor material a resistivity of 10 3 Ω cm or less. A prototype sensor had a size of 1mm square, with the diamond layer 20 being about 100 microns thick. In both versions of the sensor, the device exhibits a capacitance between the electrical contacts which is temperature dependent. Figure 3 illustrates schematically the relationship between the capacitance of the sensor and temperature. A prototype of the first embodiment of the sensor according to Figure 1 had a capacitance of approximately 5 pF at 20°C, while a prototype of the second embodiment had a capacitance of about 50 pF. The capacitance C of the sensor is determined by the dielectric constant ε of the diamond material, according to the formula : C=εε 0 A d where A is the area of the contacts and d is the width or thickness of the diamond material between the contacts. ε o is a constant, while ε is a function of temperature: ε =5.701-5.351 x 10 -5 T +1.66 x 10 -7 T 2 where T is absolute temperature. Thus, it can be seen that the capacitance of the sensor changes as a function of temperature, in a non-linear way. The basic capacitance of the temperature sensors of the invention can be selected by adjusting the size (area) of the contacts and the width or thickness of the diamond material between the contacts. A typical range of thicknesses of the diamond material for both the above described embodiments is in the range of 10 microns to 1 mm. In the case of the first embodiment of the sensor, employing a diamond crystal, the sensor will generally not be more than 1 or 2 mm in size, but sensors of the second embodiment can be much larger, with an area of several square centimeters, and a correspondingly greater capacitance, if desired. Although it is convenient to provide a sensor with a relatively large capacitance, which reduces the sensitivity of temperature measurement apparatus incorporating the sensor to stray capacitance, it will be appreciated that increasing the size of the sensor will also reduce its speed of response, since there will be a greater mass of material to be heated by the object or environment which is being measured. Thus, a compromise must be reached according to the intended application of the sensor. In the described embodiments of the sensor, charge carriers are mobilised or localised as a function of changes in temperature, which is different from the mechanism of operation of other sensors such as radiation sensors. In a radiation sensor, carriers are generated by the interaction of ionising particles with the structure of the diamond material. However, the described sensor responds in both modes, which can be used to advantage, as described below. The temperature sensor of the invention may be used in conjunction with a nulling capacitance bridge, typically with a reference diamond capacitor, to measure temperature. Figures 4 and 5 show circuits which can be used with the sensor of the invention to measure temperature and to provide an output voltage corresponding to the measured temperature. Figure 4 illustrates a voltage controlled oscillator (VCO) circuit in which the temperature sensor C is used as a feedback element The VCO is preset to run at a nominal operating frequency, which varies according to variations in the capacitance of the sensor with temperature. In the illustrated circuit, the capacitance of the sensor is given by the following formula: C = T ( R 1 + R 2 ) 4 RR 2 ν V out where T is the period of oscillation of the VCO. From the above relationship between the capacitance of the sensor and the absolute temperature T (not to be confused with the period of oscillation above) a temperature value can be calculated. The circuit of Figure 5 is a relatively simple square wave oscillator in which the temperature sensor C is used as a non-linear feedback element. This circuit generates a square wave output waveform witb a frequency which is determined by the capacitance of the sensor C and the resistors in the circuit, according to the relationship: C = T 2 R ln(1+ 2 R 1 R 2 ) where T is the period of oscillation of the square wave generator. Again, from the capacitance value thus obtained, a temperature value can be calculated. In Figure 6, a simple circuit comprising field effect transistors (FETs) T 1 and T 2 is fed with a square pulse train from an oscillator. This causes the transistor T 1 to switch at the repetition rate of the pulse train, charging the temperature sensor C at a constant rate. When the sensor C is subjected to nuclear or ionising radiation, it discharges at a rate proportional to the intensity of the incident radiation. The transistor T 2 monitors the charge on the sensor C and provides an output voltage V out , corresponding thereto. It will be appreciated that the fact that the sensor of the invention is sensitive both to ionising radiation and to temperature can be utilised to provide a measuring device which measures both temperature and incident radiation, or a radiation measuring device which is temperature compensated. In the latter case, a reference temperature compensation capacitor could be provided in the circuits of Figures 4 and 5, with an appropriate compensation circuit. The described temperature sensor is physically similar to known radiation sensors which operate in a pulse mode or conduction mode, and can therefore be connected to a composite measuring circuit which measures both the capacitance of the sensor as a function of temperature, and pulse or conduction mode signals resulting from radiation incident on the sensor. The temperature dependent capacitance measurement is used to compensate for variations in the pulse or conduction mode output of the sensor due to variation in temperature. Because the diamond body of the temperature sensor is non conductive, it can be used in strong magnetic fields without being susceptible to errors due to induced currents. The temperature sensor of the invention is thus suited to use in conjunction with superconductors, where conventional thermistors may be unsuitable. Due to the excellent thermal conductivity of diamond, the temperature sensors of the invention can provide a relatively rapid response compared with prior art sensors, which may use electrically insulating materials which are also thermal insulators.

Claims: A temperature sensor characterised in that it comprises a body (10; 20) of insulating diamond material and a pair of spaced apart electrical contacts (12, 14; 22, 24) applied to the body, the body of diamond material having a predetermined thickness between the contacts, and the contacts having a predetermined area, so that the sensor has a corresponding capacitance between the contacts which is temperature dependent. A temperature sensor according to claim 1 characterised in that the diamond body comprises a type Ia, type Ib or type IIa natural or synthetic diamond crystal (10). A temperature sensor according to claim 1 characterised in that the diamond body comprises a layer (20) of undoped diamond material deposited by a chemical vapour deposition process. A temperature sensor according to claim 3 characterised in that the layer (20) of diamond is deposited on a conductive or semi-conductive substrate (22). A temperature sensor according to claim 4 characterised in that the substrate (22) comprises doped silicon. A temperature sensor according to claim 4 or 5 characterised in that the substrate (22) functions as one of the electrical contacts. A temperature sensor according to any one of claims 1 to 6 characterised in that at least one of the electrical contacts (12, 14; 22, 24) is metallic. A temperature sensor according to claim 7 characterised in that the at least one electrical contact (12, 14; 22, 24) comprises titanium, aluminium, nickel or chrome. A temperature sensor according to any one of claims 1 to 8 characterised in that at least one of the electrical contacts (24) comprises doped amorphous semiconductor material deposited on the diamond body (20). Temperature measurement apparatus characterised in that it comprises a temperature sensor (C) according to any one of claims 1 to 9 and further comprising an oscillator circuit, the sensor being connected as a feedback element in the circuit so that the output frequency of the oscillator varies with the temperature of the sensor. Radiation measurement apparatus characterised in that it comprises a temperature sensor (C) according to any one of claims 1 to 9 and further comprising charging means (T 1 ) for applying an electrical charge to the sensor at a predetermined charging rate, and monitoring means (T 2 ) for monitoring the resultant voltage on the sensor, thereby to measure the intensity of ionising radiation incident on the sensor. Radiation measurement apparatus characterised in that it comprises a temperature sensor (C) according to any one of claims 1 to 9, pulse or conduction mode measuring means for measuring pulse or conductive mode electrical signals due to ionising radiation incident on the sensor and for generating a first electrical signal corresponding thereto, capacitance measuring means for measuring the capacitance of the sensor as a function of temperature and for generating a second electrical output signal corresponding thereto, and compensating means responsive to the first and second electrical output signals to generate an output signal corresponding to the intensity of the radiation incident on the sensor, which is compensated for changes in temperature. A method of measuring temperature characterised in that it includes the steps of providing a temperature sensor which comprises a body (10; 20) of insulating diamond material and a pair of spaced apart electrical contacts (12, 14; 22, 24) applied to the body, the body of diamond material having a predetermined thickness between the contacts, and the contacts having a predetermined area, so that the sensor has a corresponding capacitance between the contacts which is temperature dependent; locating the sensor in an environment or against an object to be measured; measuring the capacitance between the contacts or an electrical parameter related thereto; and relating the measured capacitance or electrical parameter to a reference capacitance or electrical parameter to determine the temperature of the environment or the object. A method according to claim 13 characterised in that it includes connecting the sensor (C) as a feedback element in an oscillator circuit, and relating variations in the temperature measured by the sensor to corresponding variations in the oscillation frequency of the oscillator circuit. A method of measuring radiation characterised in that it includes the steps of providing a temperature sensor which comprises a body (10; 20) of insulating diamond material and a pair of spaced apart electrical contacts (12, 14; 22, 24) applied to the body, the body of diamond material having a predetermined thickness between the contacts, and the contacts having a predetermined area, so that the sensor has a corresponding capacitance between the contacts which is temperature dependent; applying an electrical charge to the sensor at a predetermined charging rate; and monitoring the resultant voltage on the sensor, thereby to measure the intensity of ionsing radiation incident on the sensor. A method of measuring radiation characterised in that it includes the steps of providing a temperature sensor which comprises a body (10; 20) of insulating diamond material and a pair of spaced apart electrical contacts (12, 14; 22, 24) applied to the body, the body of diamond material having a predetermined thickness between the contacts, and the contacts having a predetermined area, so that the sensor has a corresponding capacitance between the contacts which is temperature dependent; subjecting the sensor to ionising radiation; measuring pulse or conduction mode electrical signals due to the ionising radiation and generating a first electrical output signal corresponding thereto; measuring the capacitance of the sensor as a function of temperature and generating a second electrical output signal corresponding thereto, and processing the first and second electrical output signals to generate an output signal corresponding to the intensity of the radiation incident on the sensor, which is compensated for changes in temperature.

Inventors: JONES BARBARA LYNN, JONES, BARBARA LYNN
Applicants: DE BEERS IND DIAMOND, DE BEERS INDUSTRIAL DIAMOND DIVISION (PROPRIETARY) LIMITED
Application year: 1993
Main classification: G01T1
IPC codes: G01J5, G01T1, H01L21, G01K7