The RADFET: TRANSDUCERS RESEARCH Transducers Group

Transcription

1 Page 1 of 5 TRANSDUCERS RESEARCH Transducers Group Introduction Research Teams Analog and Sensor Interface BioAnalytical Microsystems Chemical Microanalytics e-learning Instrumentation and software development, system integration and modelling Metal Insulator Silicon (MIS) devices Nanobiotechnology The RADFET: The basic principle of dosimetry is that ionising radiation imparts energy to matter and a knowledge of this energy provides information of the absorbed dose. A knowledge of the rate of absorption of this energy gives information on the dose rate. This is the basic principle of the RADFET. RADFETs are normally p-type MOSFETs, and a cross-section of a typical p-type MOSFET is shown in Figure 1. The region of interest for the dosimeter is the gate region. The principle of the MOSFET is such that when the gate voltage is below a certain value the device is off and above this value (threshold) the device is on. It is this threshold voltage which is used as the dosimetric parameter of the RADFET. MORE TO FOLLOW SOON Figure 1. Cross section of PMOS Transistor University College Cork When the RADFET is exposed to ionising radiation, electrons and holes are generated in the device. The important carriers generated are those in the gate oxide as shown in Figure 2. A certain number of the generated carriers recombine immediately after generation. However those which do not recombine drift under the electric field which is present in the oxide. If a positive bias is applied to the gate, then the electrons will travel to the gate electrode very quickly and leave the oxide. Figure 2. Radiation induced charge mechanisms in gate oxide region of PMOS Transistor

2 Page 2 of 5 The holes move more slowly towards the silicon substrate. Because of the nature of SiO 2, hole traps exist as an intrinsic part of the oxide, these hole traps are present in the oxide since its fabrication. The density of hole traps is also greater near the Si/SiO 2 interface. As the holes move towards the silicon a certian number of the holes get trapped and this causes the positive charge within the oxide to increase. Positive charge at the Si/SiO 2 interface also increases as a function of dose. As the positive charge in the bulk and interface of the oxide increases, the device becomes harder to switch on, i.e. its absolute threshold voltage becomes larger as shown in Figure 3. It is this change in threshold voltage due to positive charge which allows the absorbed dose to be measured. Figure 3. Change in Transfer Charcteristics of a RADFET as a result of irradiation Dosimeter Measurement Circuit: The typical circuit which is used to measure the change in threshold voltage as a function of dose is shown in Figure 4. In this circuit the device is biased in saturation with a current of typically 10µA flowing through the RADFET. The voltage, Vo, is the threshold voltage plus a small component which is dependent on the biasing current. The ease of use of this circuit is the main reason why it is used in many dosimeter applications. Figure 4. Reader Circuit Configuration Dosimeter Applications: Typical applications for PMOS RADFETs are:

3 Page 3 of 5 Application Dose Range (approximate) Spacecraft 10 rad to 0.5 Mrad Medicine 1 rad to 50 krad Personnel milli-rad to 1 krad For the various applications, different dosimeter radiation sensitivities are required. As the application goes from spacecraft to medicine to personnel the the sensitivity of the RADFET must increase significantly and the minimum detectable dose must also decrease dramatically. Currently Tyndall supplies RADFET parts commercially, the data sheets of which are available here. However to give an indication of the sensitivities, Table 1 shows the sensitivity and the minimum dose which may be detected for the RADFETs with gate oxides of a thickness of either 400nm or 1µm. Table 1. Sensitivities and Minimum Dose levels for Tyndall devices Important RADFET parameters: The radiation sensitivity of a RADFET may be increased by a number of methods, some of which include : 1. Increasing the bias voltage applied to the gate during irradiation. 2. Varying the processing conditions of the gate oxide during RADFET fabrication. 3. Increasing the gate oxide thickness, thereby increasing the volume of oxide which is available to trap oxide charge. 4. Change the gate dielectric, for a single thermal oxide to a dual layer which consists of a thermal oxide in conjunction with a deposited oxide. Figure 5. Threshold Voltage shift as a function of Dose, for a number of oxide thicknesses and irradiation biases

4 Page 4 of 5 Figure 5 shows a graph of the change in threshold voltage as a function of dose. This graph also shows the affect of irradiation bias and of oxide thickness. Three different oxide thicknesses are examined: 100nm, 400nm and 1µm. As may be seen, both increasing the oxide thickness and increasing the irradiation bias improves the response to radiation. There is a practical limit to the radiation bias which may be applied. Usually this is limited by the power supply available in a given application. There are also limits to how thick the oxide may be grown, due to the very long processing times required for oxides greater than 1µm. This is one of the reasons which prompted the evaluation of dual dielectrics. Some results for these are shown in Figure 6 which show that there is an advantage in using thick dual dielectrics. Figure 6. Threshold Voltage shift as a function of Dose, for a number of dual dielectric oxide thicknesses Other important parameters in obtaining a RADFET suitable for radiation dosimetry are the long term-fading and the read-time stability of the device. The long term fading is defined as the loss of information about dose when the RADFET is removed from the ionising radiation. This needs to be minimised for some applications, such as personnel dosimetry. However, in other applications it is not as significant. Figure 7 shows the fading of a 400nm gate oxide RADFET Fading is a function of both time and temperature, and is in fact linear with both log(time) and log(temperature). At room temperature the fading with time is very small.

5 Page 5 of 5 Figure 7. %Fading as a function of time, for a number of temperatures and biases The read-time stability is another important parameter. When the dosimeter is powered-up in the measurement circuit, trapped charges near the Si/SiO 2 interface may either charge or discharge and this causes the output voltage, Vo, to either increase or decrease. This in turn will give a variation in the estimation of the absorbed dose measurement, depending on the rate of change. Normally, this is examined between 10 seconds and 20 seconds after power up. Figure 8 shows a plot of this drift which is usually less than 1.5%. Figure 8. %Drift as a function of dose, for different biases For further information on developments in Tyndall RADFET process, contact Aleksandar Jaksic at the Tyndall Tyndall National Institute. All Rights Reserved E&OE Privacy Statement Updated: Wednesday, 03-Dec :09:44 GMT