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MOS ionizing radiation dosimeters: from low to high dose measurement
Radiation Physics and Chemistry 61 (2001) 511–513
MOS ionizing radiation dosimeters: from low to high dose measurement G. Sarrabayrousea,*, V. Polischukb a b
LAAS-CNRS, 7 Avenue du Colonel Roche, 31077 Toulouse Cedex 04, France Kiev RaD Microdevices Institute, 3 Severo-Syretskaya, Kiev 254138, Ukraine
Abstract Metal-oxide-silicon dosimeters with a very thick gate oxide in a stack-connected configuration are studied with respect to their capability to measure a low dose. It is shown that if the temperature is controlled to 751C during reading a dose as low as 10@4 Gy can be measured with 10% accuracy. r 2001 Elsevier Science Ltd. All rights reserved. Keywords: Metal-oxide-silicon dosimeters; Radiation dosimetry; Low dose measurement; Electronic dosimeters
1. Introduction Metal-oxide-semiconductor dosimeters are MOS transistors with a specially processed gate insulator in order to make it radiation soft. They are of increasing interest because of their numerous advantages with respect to more conventional dosimeters such as TLDs: low cost, small size and weight, robustness, accuracy, large measurable dose range, sensitivity to low energy (o10 keV) radiation, real-time or delayed direct reading, information retention, possibility of monolithic integration with other sensors and/or circuitry capable of performing measurement, signal conditioning and data processing, possibility of use without power supply. These attractive features lend themselves to applications in fields such as space science, nuclear plant area monitoring, defence, emergency dosimetry, medical, equipment calibration and control. Furthermore recent results allow application in the 10 mGy range in personnel dosimetry. We have developed a special technological process leading to fabrication of a P-MOS transistor with a very thick gate insulator, a large gate area and a positive threshold voltage obtained without using adjustment by ion implantation. This allows us to obtain high intrinsic *Corresponding author. Fax: +33-5-61-33-62-08. E-mail address: [email protected] (G. Sarrabayrouse).
radiation sensitivity from a single transistor together with very small fading and low noise. The aim of this paper is to present the basic principle, the main characteristics of the MOS dosimeterFradiation sensitivity, stability with time and temperatureFand its limitations. In addition a stack-connected configuration, proposed by Conneely et al. (1997), has been studied. The main performance figures and the limitations of the dosimeter are given below.
2. Results and discussion The reading configuration is shown in Fig. 1. A constant current is injected in the stack and the associated output voltage Vs is measured before and after irradiation. During irradiation all pins may be tied together making the dosimeter a passive device. For a single transistor (i.e. n ¼ 1) the change in Vs following irradiation, represented in Fig. 2, is due to a positive charge generated in the oxide layer of the transistor and interface states at the oxide–semiconductor interface. For a 1.6 mm thick oxide layer the sensitivity was found to be 4.6 mV/cGy up to about 1 Gy, decreasing at higher doses. Due to the large gate area (3.5 105 mm2) electronic noise is very small. The minimum measurable signal is less than 50 mV so that a dose in the [1 cGy– 5 KGy] range can be measured with good accuracy.
0969-806X/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 6 X ( 0 1 ) 0 0 3 1 7 - 6
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G. Sarrabayrouse, V. Polischuk / Radiation Physics and Chemistry 61 (2001) 511–513
Fig. 3. Radiation response as a function of the stack length.
Fig. 1. Measurement configuration.
Fig. 2. Radiation response for a single-transistor dosimeter. (Measurements made in different laboratories; RX (10 KeV, 50 KeV ), 60Co.)
Fading of the dosimeter has been studied at various temperatures. At ambient temperature and following 10 Gy irradiation, fading has been found to be less than 8% after 1000 h. At 1001C, fading of about 20% has been observed and in this case if the dosimeter is read a long time after irradiation or if the dose rate is very small, measurement of the temperature and signal processing are both necessary for determination of the true absorbed dose.
The main limitation comes from temperature sensitivity during reading. When Vsub is properly adjusted a region exists in the current–voltage curve where this dependency is a minimum (Minimum Temperature Coefficient (MTC) region). For the devices under study an MTC of 0.05 mV/1C has been found so that to achieve a 10% accuracy the temperature should be controlled to better than 751C during reading. To increase the sensitivity and the minimum measurable dose up to 14 transistors have been stacked. With this, the maximum output voltage before irradiation was 18 V. The response with dose is shown in Fig. 3 as a function of the stack length. A sensitivity as high as 90 mV/cGy has been obtained. In the stack-connected configuration there still exists a MTC current. Biasing the stack at this current leads to the same limitation as for the single transistor dosimeter. Electronic noise has been found to increase as n1:78 . Within the range 1–100 cGy shown on Fig. 3, the smallest measurable signal variation was 0.5 mV, the implication being that a dose of 10@4 Gy could be measurable.
3. Conclusion Stack-connected MOS radiation dosimeters have been studied using P-MOS transistors with a very thick gate oxide, a large gate area and a positive threshold voltage. A stack of up to 14 transistors has been made which can measure doses in the range 10@4–5 103 Gy, with good accuracy and acceptable output voltage value if the temperature is monitored to 751C during reading. The main limitation comes from electronic noise at high sensitivity (long stack), temperature sensitivity and to parameters variability which can induce a large temperature sensitivity due to a variable MTC current.
G. Sarrabayrouse, V. Polischuk / Radiation Physics and Chemistry 61 (2001) 511–513
513
Acknowledgements
References
The authors acknowledge the skilful assistance of Robert Plana and Laurent Bary in measurement of noise and of Francoise Rossel, Bernard Rousset, Hughes Granier and Jean Christophe Marrot during device processing.
Conneely, C., O’Connell, B., Hurley, P., Lane, W., Adams, L., 1997. Strategies for millirad sensitivity in PMOS dosimeters. In: Barbottin, G., Dressendorfer, P. (Eds.), Proceedings of the RADECS-97, Cannes, 15–19 Sept. 1997, IEEE, Pitscataway, NJ, pp. 288–293.
MOS ionizing radiation dosimeters: from low to high dose measurement G. Sarrabayrousea,*, V. Polischukb a b
LAAS-CNRS, 7 Avenue du Colonel Roche, 31077 Toulouse Cedex 04, France Kiev RaD Microdevices Institute, 3 Severo-Syretskaya, Kiev 254138, Ukraine
Abstract Metal-oxide-silicon dosimeters with a very thick gate oxide in a stack-connected configuration are studied with respect to their capability to measure a low dose. It is shown that if the temperature is controlled to 751C during reading a dose as low as 10@4 Gy can be measured with 10% accuracy. r 2001 Elsevier Science Ltd. All rights reserved. Keywords: Metal-oxide-silicon dosimeters; Radiation dosimetry; Low dose measurement; Electronic dosimeters
1. Introduction Metal-oxide-semiconductor dosimeters are MOS transistors with a specially processed gate insulator in order to make it radiation soft. They are of increasing interest because of their numerous advantages with respect to more conventional dosimeters such as TLDs: low cost, small size and weight, robustness, accuracy, large measurable dose range, sensitivity to low energy (o10 keV) radiation, real-time or delayed direct reading, information retention, possibility of monolithic integration with other sensors and/or circuitry capable of performing measurement, signal conditioning and data processing, possibility of use without power supply. These attractive features lend themselves to applications in fields such as space science, nuclear plant area monitoring, defence, emergency dosimetry, medical, equipment calibration and control. Furthermore recent results allow application in the 10 mGy range in personnel dosimetry. We have developed a special technological process leading to fabrication of a P-MOS transistor with a very thick gate insulator, a large gate area and a positive threshold voltage obtained without using adjustment by ion implantation. This allows us to obtain high intrinsic *Corresponding author. Fax: +33-5-61-33-62-08. E-mail address: [email protected] (G. Sarrabayrouse).
radiation sensitivity from a single transistor together with very small fading and low noise. The aim of this paper is to present the basic principle, the main characteristics of the MOS dosimeterFradiation sensitivity, stability with time and temperatureFand its limitations. In addition a stack-connected configuration, proposed by Conneely et al. (1997), has been studied. The main performance figures and the limitations of the dosimeter are given below.
2. Results and discussion The reading configuration is shown in Fig. 1. A constant current is injected in the stack and the associated output voltage Vs is measured before and after irradiation. During irradiation all pins may be tied together making the dosimeter a passive device. For a single transistor (i.e. n ¼ 1) the change in Vs following irradiation, represented in Fig. 2, is due to a positive charge generated in the oxide layer of the transistor and interface states at the oxide–semiconductor interface. For a 1.6 mm thick oxide layer the sensitivity was found to be 4.6 mV/cGy up to about 1 Gy, decreasing at higher doses. Due to the large gate area (3.5 105 mm2) electronic noise is very small. The minimum measurable signal is less than 50 mV so that a dose in the [1 cGy– 5 KGy] range can be measured with good accuracy.
0969-806X/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 6 X ( 0 1 ) 0 0 3 1 7 - 6
512
G. Sarrabayrouse, V. Polischuk / Radiation Physics and Chemistry 61 (2001) 511–513
Fig. 3. Radiation response as a function of the stack length.
Fig. 1. Measurement configuration.
Fig. 2. Radiation response for a single-transistor dosimeter. (Measurements made in different laboratories; RX (10 KeV, 50 KeV ), 60Co.)
Fading of the dosimeter has been studied at various temperatures. At ambient temperature and following 10 Gy irradiation, fading has been found to be less than 8% after 1000 h. At 1001C, fading of about 20% has been observed and in this case if the dosimeter is read a long time after irradiation or if the dose rate is very small, measurement of the temperature and signal processing are both necessary for determination of the true absorbed dose.
The main limitation comes from temperature sensitivity during reading. When Vsub is properly adjusted a region exists in the current–voltage curve where this dependency is a minimum (Minimum Temperature Coefficient (MTC) region). For the devices under study an MTC of 0.05 mV/1C has been found so that to achieve a 10% accuracy the temperature should be controlled to better than 751C during reading. To increase the sensitivity and the minimum measurable dose up to 14 transistors have been stacked. With this, the maximum output voltage before irradiation was 18 V. The response with dose is shown in Fig. 3 as a function of the stack length. A sensitivity as high as 90 mV/cGy has been obtained. In the stack-connected configuration there still exists a MTC current. Biasing the stack at this current leads to the same limitation as for the single transistor dosimeter. Electronic noise has been found to increase as n1:78 . Within the range 1–100 cGy shown on Fig. 3, the smallest measurable signal variation was 0.5 mV, the implication being that a dose of 10@4 Gy could be measurable.
3. Conclusion Stack-connected MOS radiation dosimeters have been studied using P-MOS transistors with a very thick gate oxide, a large gate area and a positive threshold voltage. A stack of up to 14 transistors has been made which can measure doses in the range 10@4–5 103 Gy, with good accuracy and acceptable output voltage value if the temperature is monitored to 751C during reading. The main limitation comes from electronic noise at high sensitivity (long stack), temperature sensitivity and to parameters variability which can induce a large temperature sensitivity due to a variable MTC current.
G. Sarrabayrouse, V. Polischuk / Radiation Physics and Chemistry 61 (2001) 511–513
513
Acknowledgements
References
The authors acknowledge the skilful assistance of Robert Plana and Laurent Bary in measurement of noise and of Francoise Rossel, Bernard Rousset, Hughes Granier and Jean Christophe Marrot during device processing.
Conneely, C., O’Connell, B., Hurley, P., Lane, W., Adams, L., 1997. Strategies for millirad sensitivity in PMOS dosimeters. In: Barbottin, G., Dressendorfer, P. (Eds.), Proceedings of the RADECS-97, Cannes, 15–19 Sept. 1997, IEEE, Pitscataway, NJ, pp. 288–293.