JP2013003094A - Radiation dosimeter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a radiation dosimeter which comprises two types of metal-oxide-semiconductor field-effect transistors (MOSFET) that can amplify a change in threshold voltage due to radiation and perform temperature compensation, and a method of operation.SOLUTION: The radiation dosimeter comprises a radiation integrated circuit (RADIC) including two radiation field-effect transistors (RADFET) and two MOSFETs that are integrated on the same substrate, or two RADFETs that are integrated on the same substrate and two resistors. The amplification of a threshold voltage change is achieved by using the amplification principle of an MOSFET inverter. Under ionizing irradiation, the gate of the first RADFET is forward biased and the gate of the second RADFET is biased off. An amplified differential threshold voltage change is measured in a reading mode. Measurement of milli-rad doses is enabled and temperature effects and drifts are substantially removed.

Description

  The present invention relates to a radiation dosimeter.

  A metal oxide semiconductor dosimeter is a MOS field effect transistor with a specially treated gate insulator to soften the radiation.

There are conventional radiation dosimeters for personnel such as thermoluminescence devices. This type of device uses a small crystal of CaF ₂ or LiF, which traps electrons and holes generated by ionizing radiation. When heated, the trap is emptied and light is emitted from the crystal, which is related to the accumulated dose.

  A MOSFET dosimeter is commonly known as a radiation field effect transistor (RADFET) and measures the shift in the threshold voltage of the RADFET caused by radiation.

  Radiation field effect transistors have many advantages associated with thermoluminescent devices: low cost, small size, light weight, ruggedness, accuracy, wide dosimetry range, direct reading in real time or time difference, information grasp, measurement Such as the possibility of monolithic integration with other sensors and / or circuits, signal conditioning, data processing, etc.

Thomson's Canadian Patent No. 1204885 dated May 20, 1986 measures a differential threshold between two IGFET sensors that have a pair of silicon insulated gate field effect transistors (IGFETs) and are exposed to the same radiation. A radiation dosimeter is disclosed. One of the two IGFET sensors is biased in its conductive region and the other is biased away from the conductive region or at a lower conductive level than the former. A dosimeter with these two IGFETs exhibits a sensitivity of about 2 mV / cGY when the gate bias is 3 volts and about 5 mV / cGY when the gate bias is higher than 10 volts. The sensitivity to temperature of this dual IGFET sensor has been found to be lower than 0.1 mV / ° C. At a temperature difference of −20 ° C. to + 50 ° C., that is, 70 ° C., which is the military temperature range, ΔV _T = 7 mV, ie, 1-3 cGY.

  A problem associated with this prior art device is that it is insufficiently sensitive as a dosimeter for use by medical, nuclear and other industrial personnel. The radiation dosimeter used by an individual must have a sensitivity of about 0.010 cGY (Rad).

  B. O'Connell, A.M. Keleher, W. Lane, L. Adams announced in June 1996 IEEE Tran. Nucl. Sci. Vol. 43, N3, published in the paper entitled “Stacked RADFET to increase radiation sensitivity”, stacking 15 individual PADFETs on the same chip provides a radiation sensitivity of 80 mV / cGY. It is shown that.

  V. Polyshuku and G. Sara Beilus is available from Radiation Physics and Chemistry Vol. 61, no. Stack connection using RADFET with a very thick gate oxide of 1.8 μm in the paper titled “MOS ionizing radiation dosimeter: low to high dose measurement” published in the review section of 3-6, 2001 An equation RADFET configuration is shown. As many as 14 transistors are stacked to increase sensitivity and minimum measured dose. With this configuration, the output voltage before irradiation was 18V. Sensitivity as high as 90 mV / cGY is obtained.

  Both groups insist on the possibility of measuring milli-Rad radiation dose. However, stacked RADFETs have many problems that limit their use in personal dosimeters. The problem is that one RADFET has a constant temperature coefficient. Metal oxide semiconductor field effect transistor devices have temperature threshold voltage dependence that must be taken into account so that only radiation induced threshold voltage shifts are measured with a dosimeter. . In stacked RADFETs, the temperature sensitivity is higher than N times the single one (N is the number of stacked RADFETs).

  The inventor of the present invention The temperature sensitivity of the stacked RADFET made by the O'Connell group was measured using their RADFET dosimeter. The temperature response of 15 MOSFETs laminated for a small constant current mode of 10 μA was 70 mV / ° C. When the measurement temperature is controlled to +/− 1 ° C., the minimum measured radiation dose is about 5 cGY or 5 Rad.

  G. Sara Beluth, D.C. Bukdar, V. Polyshuku, and S.H. Ciscos is a member of Radiation Physics and Chemistry Vol. 61, no. In a paper entitled “Laminated MOS Field Effect Ionizing Radiation Dosimeter: Possibilities and Limitations” published in 3-6, 2003, pp 737-739 It has been proposed to lower the temperature sensitivity of the stacked RADFET by measuring. This paper actually shows only computer simulation. Temperature sensitivity and threshold voltage drift at the MTC point are not measured.

  Another problem with stacked RADFETs is the high output voltage, which can be as high as about 18 volts. Therefore, it is difficult to amplify a small change in threshold voltage caused by radiation using an operational amplifier.

  In the present invention, in order to increase the radiation sensitivity, R.I. H. The amplification principle of the MOSFET inverter described in the book Crawford “MOSFET in Circuit Design”, New York: McGraw-Hill, 1967 was used.

Accordingly, an object of the present invention is to provide a radiation integrated circuit as a personal radiation dosimeter having milli-Rad sensitivity and temperature compensation performance by applying the amplification principle of an inverter.
The present invention relates to a solid dosimeter that measures very low doses of 0.01 cGY to 2 cGY ionizing radiation, more particularly a radiation integrated circuit (RADIC) based on RADFET or MOSFET or a circuit based on RADFET and resistor. About. During any RADIC irradiation, the first RADFET (left) of the reference circuit is unbiased and the second RADFET (right) of the inverter circuit is biased. Thus, the threshold voltage of the second RADFET varies with dose to a much higher degree than that of the first RADFET. During the measurement, the change in the threshold voltage of the second RADFET (right) is amplified by the inverter circuit. The change in output voltage will be equal to the change in amplified differential threshold voltage as expressed in the equation below.
ΔUout = AuΔUt

  Accordingly, the present invention solves the problem of low radiation sensitivity of conventional double IGFETs and stacked connection RADFET dosimeters.

  A second object of the present invention is that this radiation integrated circuit has minimal temperature effects and is relatively insensitive to temperature changes. This is accomplished by measuring the differential threshold voltage from the two RADFETs. In order to ensure that the effects of temperature on the two RADFETs are equal, the RADFET and MOSFET or the circuit with the two and the RADFET are made on the same silicon substrate or chip. The thickness of the gate oxide of each RADFET is preferably 1 μm.

2 shows a dual IGFET dosimeter that is ready to measure a differential threshold voltage. The reading structure of a stacked connection type RADFET radiation dosimeter is shown. An inverter having a MOSFET as a load is shown. 1 shows an inverter having a resistor as a load. 1 shows a block diagram of a radiation integrated circuit for a basic embodiment of the invention in a configuration that is ready to receive irradiation. FIG. FIG. 2 shows a block diagram of a radiation integrated circuit for a basic embodiment of the present invention in reading mode. The block diagram of the radiation circuit for 2nd embodiment of this invention of the structure in the state which accepts irradiation is shown. FIG. 4 shows a block diagram of a radiation circuit for a second embodiment of the present invention in reading mode. The response of the radiation circuit according to the radiation dose of the gamma ray for 2nd embodiment of this invention is shown.

ただし、βは、下の式であたえられる。

The invention will be better understood by reference to the following detailed description of the invention and the accompanying drawings.
FIG. 3 shows an inverter made of two MOSFETs. In the figure, T1 is a management MOSFET transistor, and T2 is a loading MOSFET transistor. The R.C. referenced in the background section of this invention. H. The amplification factor of this inverter, based on prior art from Crawford's book, is expressed as:

However, β is given by the following equation.

ただし、Ｒ_Ｄ‖ｒ_ｄは、並列接続の装荷抵抗器Ｒ_Ｄと動的ドレイン抵抗ｒ_ｄの等価抵抗である。

FIG. 4 shows an inverter made up of a MOSFET and a resistor. In the figure, T1 is a management transistor, and _RD is a loading resistor. R. H. The amplification factor of this second inverter, based on prior art from the Crawford book, is proportional to the magnitude of the slope or the transition conductivity and the loading resistance and is given by

However, _{R D} ‖R _d is the equivalent resistance of the loading resistor _{R D} and a dynamic drain resistance _{r d} connected in parallel.

  FIG. 5 shows a basic embodiment of the present invention. The RADFET Q2 has a gate G2, a drain D2, and a source S2 connected to each other and further connected to a common source S. RADFET Q1 has its gate G1 biased by battery 4 and its drain D1 and source S1 connected to a common source S. The two MOSFETs Q3 and Q4 have their drains D3 and D4, gates G3 and G4, and sources S3 and S4 connected to a common source S.

  Both RADFETs Q1 and Q2 and both MOSFETs Q3 and Q4 receive the same ionizing radiation. These MOSFETs have a thin oxide thickness of 100 nm or less, and these RADFETs have a thin oxide thickness of 1 μm or less. Since the radiation sensitivity is proportional to the thickness of the oxide, the MOSFET is much less sensitive than the RADFET. It has been found that the biased RADFET Q1 is much more sensitive to radiation than the unbiased RADFET Q2.

FIG. 6 shows a basic embodiment of the invention in reading mode. The figure shows the same RADFETs Q1 and Q2 and the same MOSFETs Q3 and Q4 of the radiation integrated circuit (further RADIC1) ready to measure radiation dose. The sources S1 and S2 of the two RADFETs are connected to each other and grounded. The gate G2 of RADFET Q2 is connected to its own drain D2, which is also connected to the gate G1 of RADFET Q1. The gate G3 and the drain D3 of the MOSFET Q3 are connected to each other. The gate G4 and the drain D4 of the MOSFET Q4 are also connected to each other. The source S3 of the MOSFET Q3 is connected to the drain D1 of the RADFET Q1, and the source S4 of the MOSFET Q4 is connected to the drain D2 of the RADFET Q2. The power supply U _dd is connected to the drains D3 and D4 of the two MOSFETs. The two MOSFETs must be the same. The two RADFETs must also be the same. Two MOSFETs and two RADFETs are made on the same die. Thus, the two RADFETs must have the same temperature change characteristics, the same initial threshold voltage, and the same oxide charge prior to irradiation. The two MOSFETs must also have the same temperature characteristics and the same threshold voltage. The RADFET Q1 and the MOSFET Q3 are inverters that can amplify changes in the threshold voltage of the RADFET Q1. Amplification of the change in threshold voltage is given by the following equation:

  When the parameters of RADFET are W1 = 1200 μm, L1 = 50 μm, dox = 1 μm, and the parameters of MOSFET are W2 = 20 μm, L2 = 2400 μm, dox = 0.1 μm, the degree of amplification of the change in threshold voltage Is 17.

The output voltage U _out is measured between the drains D1 and D2 of the RADFET. Prior to irradiation, the voltage U _out is measured as a first amplified difference threshold. After irradiation, the output voltage U _out is measured again. The output voltage U _out is equal to the amplified difference threshold voltage .DELTA.U _T due to the difference or received radiation amount of the output voltage before and after irradiation.

  RADFET Q2 and MOSFET Q4 are reference circuits having the same temperature and drift characteristics as the inverter. Therefore, the temperature effect of this radiation integrated circuit is eliminated.

  FIG. 7 shows a second embodiment of the present invention. The gate G6, drain D6, and source S6 of RADFET Q6 are connected to each other and further connected to a common source S6. RADFET Q5 has its gate G5 biased by battery 4 and its drain D5 and source S5 connected to a common source S.

  Both RADFETs Q5 and Q6 receive the same ionizing radiation. These RADFETs have a thin oxide thickness of 1 μm or less. During irradiation, RADFET Q5 is biased by battery 4 and RADFET Q6 is unbiased. Therefore, RADFET Q5 will have a much higher radiation sensitivity than RADFET Q6.

FIG. 8 shows a second embodiment of the present invention in the reading mode. FIG. 8 shows the same RADFETs Q5 and Q6 and the same resistors 2 and 3 of the radiation integrated circuit 2 (further RADIC 2) prepared for measuring the radiation dose. The sources S1 and S2 of the two RADFETs are connected to each other and grounded. The gate G6 of RADFET Q6 is connected to its own drain D6, which is also connected to the gate G5 of RADFET Q5. Resistor 2 is connected to drain D5 of RADFET Q5, and resistor 3 is connected to drain D6 of RADFET Q6. The power supply U _dd is connected to the two resistors 2 and 3.

The two RADFETs Q5 and Q6 must be the same and are made of the same die. Thus, the two RADFETs Q5 and Q6 must have the same temperature change characteristics, the same initial threshold voltage, and the same oxide charge prior to irradiation. The RADFET Q5 and the resistor 2 are inverters that can amplify changes in the threshold voltage of the RADFET Q5. Amplification of the change in threshold voltage is given by the following equation:

  When the parameters of the RADFET are W1 = 4000 μm, L = 40 μm, and R1 = 1000 kΩ, the amplification factor of the change in the threshold voltage is 15.

  RADFET Q6 and resistor 3 are reference circuits having the same temperature and drift characteristics as the inverter. Therefore, the temperature effect of this radiation integrated circuit is minimized. The measurement temperature sensitivity of RADIC2 is 0.5 mV / C.

The output voltage U _out is equal to the amplification by received radiation amount difference threshold voltage .DELTA.U _T.

The radiation sensitivity of this radiation circuit (S = ΔU _out / D) is 240 mV / cGY when the biased voltage during irradiation is Ubias = 3.3V. Considering the measured radiation sensitivity, it is about 0.01 cGY or 10 mRad when the temperature is controlled to ± 1 ° C.

  FIG. 9 shows changes in the experimental output voltage according to the irradiation dose of gamma rays for the radiation integrated circuit (RADIC2).

  Thus, it is understood that the radiation integrated circuits (RADIC1 and RADIC2) of the present invention are more sensitive and accurate dosimeter circuits than prior art dual IGFET dosimeters and stacked connection RADFET dosimeters.

  Embodiments of the invention that claim privilege monopoly are defined in the claims.

Claims (9)

A pair of radiation field effect transistors each having a gate, source and drain, comprising a thick oxide and integrated on the same substrate;
A pair of metal oxide field effect transistors each having a gate, source, and drain, comprising a thin oxide and integrated on the same substrate;
Means for applying a voltage to the drain of each metal oxide field effect transistor;
Means for forward biasing the gate of the second radiation field effect transistor and zero biasing the gate of the first radiation field effect transistor under the influence of ionizing radiation;
Means for measuring the amplified differential threshold voltage of the radiation integrated circuit. A pair of radiation field effect transistors each having a gate, source, and drain and integrated on the same substrate;
A pair of the same resistors of 200 kΩ or more,
Means for applying a voltage to the first and second resistors;
Means for forward biasing the gate of the second radiation field effect transistor and zero biasing the gate of the first radiation field effect transistor under the influence of ionizing radiation;
Means for measuring the amplified differential threshold voltage of the radiation integrated circuit. further,
Means for connecting the drain of the first metal oxide field effect transistor to the gate and connecting the drain of the second metal oxide field effect transistor to the gate;
Means for connecting the drain of the first metal oxide field effect transistor and the drain of the second metal oxide field effect transistor to a power source;
Means for connecting the gate of the first radiation field effect transistor to the drain;
Means for connecting the drain of the first radiation field effect transistor to the gate of the second radiation field effect transistor;
The source of the first metal oxide field effect transistor is connected to the drain of the first radiation field effect transistor, and the source of the second metal oxide field effect transistor is connected to the drain of the second radiation field effect transistor. Means for connecting to,
Means for reading an amplified threshold difference between the drains of both radiation field effect transistors to obtain an initial amplified differential threshold corresponding to the amount of radiation accumulated prior to irradiation. Radiation dosimeter described in 1. further,
Means for connecting a power supply to the first and second resistors, means for connecting to the gate and drain of the first radiation field effect transistor;
Means for connecting the drain of the first radiation field effect transistor to the gate of the second radiation field effect transistor;
Means for connecting the first resistor to the drain of the first radiation field effect transistor and connecting the second resistor to the drain of the second radiation field effect transistor;
3. A radiation dosimeter according to claim 2, comprising means for reading the amplified threshold difference between the drains of the first and second radiation field effect transistors to obtain the radiation dose accumulated after irradiation.   4. The radiation dosimeter according to claim 1, wherein the radiation field effect transistor and the metal oxide field effect transistor have an aluminum gate or a polysilicon gate.   The radiation dose meter according to claim 2 or 4, wherein the radiation field effect transistor has an aluminum gate or a polysilicon gate.   4. The radiation dosimeter according to claim 1, wherein the radiation field effect transistor has an oxide thickness of about 0.5 [mu] m or more. In a method of measuring ionizing radiation dose,
(1) Radiation integrated circuit 1 (RADIC1) having two radiation field effect transistors and two metal oxide field effect transistors or Radiation integrated circuit 2 (RADIC2) having two radiation field effect transistors and two resistors ) To measure the initial amplified differential threshold voltage;
(2) Zero zero is applied to the first radiation field effect transistor that is biased forward by the gate of the second radiation field effect transistor for radiation integrated circuits 1 and 2 and is exposed to ionizing radiation. Bias;
(3) The amplified differential threshold voltage after irradiation between the two radiation field effect transistors is measured, and the amplification factor for the radiation integrated circuit is given by:

(4) subtracting the amplified differential threshold voltage measured after irradiation from the amplified differential threshold voltage measured before irradiation;

A method consisting of:   Radiation integrated circuits 1 and 2 have a differential threshold voltage change amplification equal to 17 and 15, and these values are a parameter for metal oxide field effect transistors, radiation field effect transistors, and resistors. Radiation dosimeter according to claim 1 or 2, which can be between 2 and 100 accordingly.

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