1. Capabilities
2. About the REM dosimetry system,
inccluding a list of references and application notes
1. REM OXFORD;CAPABILITIES
IN RADIATION EFFECTS AND MONITORING
* THE RADFET, AN ADVANCED,
ULTRA-SMALL DOSIMETRY SYSTEM
* DEVICE PHYSICS MODELLING
OF RADIATION EFFECTS
* RADIATION-HARDENED HARDENED
SYSTEM DESIGN AND TEST
REM was formed in 1985 as “Radiation Experiments and Monitors”, a sole trader
supplying dosimetry systems and consulting in device physics applied to radiation effects. Over 30 years of experience in
this field were compiled in a definitive work “Radiation Effects Handbook” (Holmes-Siedle and Adams, 1st Edition
1993, 2nd Edition 2002). Contract work on radiation effects assessment included the radiation testing, modelling and “radiation-hardening”
of optical imaging devices, high-speed integrated circuits and large scintillator crystals. The laboratory aspects of this
are performed through the Centre for Radiation Damage Studies, formed by REM and Brunel
University, Uxbridge, Middlesex as a consulting organization for the aerospace and nuclear industry.
REM Oxford is the originator of the RADFET
dosimeter, in which the response to ionization of a p-channel field-effect transistor with a specially treated gate oxide
is used as a measure of integrated radiation dose. Where remote reading and very small detectors are required, the possibilities
of this micro-technology are large. In 1985, REM designed the "TOT500" a 1 x 1 mm chip, which formed the basis of the initial
US Army Tactical Dosimeter. Now REM and a sensor company, DST, have formed a team to build a DOSIMETER SYSTEM. The emphasis is on ever-smaller probes. The new sensor chip, the "TOT600", is available in quantity.
The size is such that the sensor can be contained in a catheter probe or deployed as a minute "pill". REM is also working
on software which makes RADFETs "smart" and on the use of the device in "edge-on" mode for resolving very
narrow beams of X-rays. A 2005 publication describes how to model the transport
of radiation into such a sensor using novel Monte Carlo computations. These efforts are internally-funded
but consulting contracts are also a part of REM's business model.
Although
the above scheme is formulated first for medicine, adaptations of the system are already working, in space
(cited as an "ESA Success Story"), in various ultra high-energy accelerators (SLAC, CERN, KEK) the nuclear
industry (Harwell, Mol) and the military field (US Army and Navy). Laboratories in Europe, USA, China, Korea
and Australia have been supplied with
commercial quantities of the RADFET sensor. Several RADFET reader designs are
available.
REM
was formed into a Limited Company in 1997. The managing director is Dr. Andrew Holmes-Siedle, winner of several awards
for the invention of the RADFET and for innovation in the management of radiation effects
Publications.
* A comprehensive bibliography of silicon dosimetry, still being kept up to date, was compiled
for the "Handbook of Radiation Effects" (OU Press 2002) by AHS and co-author.
* REM's "Electronic Introduction" to the RADFET, rfi05.doc, is available by email to any requestor.
* Application notes on RADFET handling and measurement,available to REM's clientele.
2. About the REM dosimetry system
SUMMARY OF REM'S "DOT" DOSIMETER SYSTEM BASED ON
THE RADFET: FUNCTIONALITY,
STABILITY AND APPLICATIONS
Andrew Holmes-Siedle
RADFET
n. An acronym for RADIATION-SENSING FIELD-EFFECT TRANSISTOR based
on the metal-oxide-silicon p-channel structure. An integrating dosimeter which measures dose (rad or Gy(Si)) by virtue of
the field effect caused by space charge trapped in an inorganic insulator (SiO2).The RADFET was invented in 1970
.by Andrew Holmes-Siedle (now sole owner of REM) and his co-worker, the late Waldemar Poch. Details were published in 1974.
The acronym was coined by Robert Hughes in 1986. Used for the last 20 years in aerospace industry, now has a future in medicine,
nuclear industry and science.
"DOT" (Dosimetry by Oxide Trapping). The DOT sensor is designed to detect the field produced when space-charge is trapped in the
gate oxide region of the FET. An electrical measurement [shift of threshold voltage] by the DOT reader then gives a relative
value of dose in a silicon environment (rad or Gy(Si)). The charge remains for many years.
REM OXFORD Ltd.
64A ACRE END ST., EYNSHAM, OXFORD
OX29 4PD, ENGLAND
phone +44 18 65 88
00 50 . fax +44 18 65 88 00 30
email: holmes.siedle@dial.pipex.com
Introduction
The RADIATION-SENSING FIELD-EFFECT TRANSISTOR (abbreviation: RADFET) is a microminiature type
of integrating radiation dosimeter. The sensor has a microscopically small sensor volume, which offers opportunities for radical
new designs of miniature radiation sensing system. The sensing principle is long-term charge storage. The p-MOSFET responds
to the field produced when space-charge is trapped in the oxide region. An electrical measurement [shift of threshold voltage]
then gives a relative value of dose in rad or Gy(Si). Compared to other detector systems, the RADFET system is compact and
easily coupled with computer power.
APPLICATIONS
Dosimetric experts recognize that, due to the microminiature dimensions of the ionization-sensitive
region [only 0.05 mm2] the dose resolution is not as good as for gas tubes, film badges and TLD dosimeters, which
achieve “millirad resolution”. Thus, the main applications are in high-dose applications such as radiotherapy
and space. The dosimetric information, trapped charge, is both generated and
stored in an oxide film only a micrometre thick and a fraction of a square millimetre in area. Because the sensor die is minute
and readout is convenient, radical new designs of dosimeter are possible. In radiotherapy, “Tactical dosimetry”
for military personnel and in spacecraft dosimetry, these special properties have already created a commercial market.
A table of uses for the RADFET system matched to the dose range and size, is given below. Associated
dosimeter types, already built, are quoted in brackets. A superscript “ R
” indicates a device designed and built by REM.
Controller for radiotherapy
(CA-1 catheter probe) R
Personal dosimeter for civil
accident and military battlefield (US
Army "Tie-Clip" device) R
Monitor on robots in a nuclear
power station environment (TELEMAN Tel-H dosimeter) R
Nuclear waste cleanup (TELEMAN
Tel-H dosimeter) R
Space vehicle health monitor
(ESA/PSI: Radiation Effects Monitor)
High-energy accelerator; dose
mapper for crystals (BaBar RADFET Monitoring Board) R
Response
to radiation: The DOT sensor is designed to detect the field produced when
space-charge is trapped in the gate oxide region of the FET. An electrical measurement [shift of threshold voltage] by the
DOT reader then gives a relative value of dose in a silicon environment (rad or Gy(Si)). The charge remains for many years.
The oxide dielectric responds to all forms of particle and photon radiation in proportion to the ionization
produced. Response to neutrons is relatively small. Radiation responses [photons]
are specified in individual REM data sheets for doses varying from 1 cGy to 1E8 cGy.
The net positive charge in the oxide trapped (ot) charge region is generated when electrons
escape to the external circuit. The effects of other trapping phenomena in the structure are reduced as much as possible in
the design of the dosimeter. Accurate calibrations in the radiation of choice are normally the responsibility of the user,
comparing the voltage shift to the response of a standard detector such as an air ionization dosemeter. Co-60 gamma rays are
commonly use by the suppliers to standardize a sensor lot. An eight-decade curve of these responses (“nominal growth curve”) is available
for general reference.
Reading out the charge - the reader box
Readers for RADFET dosimeter systems consist of circuits for tracking the threshold voltage
of the pMOS FET and applying electrical stress as required during exposure. The common electrical bias modes are shown in
Table 1. REM designs of reader (DOT system for "Dosimetry by Oxide Trapping")
range from simple, manually switched versions to systems controlled by PC and microprocessor. As Fig 1 suggests, this can
be a slim-built device using battery power and can connect to the Internet.
Amongst other users, the European Space Agency has sponsored the development of several readers
suitable for monitoring aboard unmanned spacecraft (e.g. the SREM, developed by PSI, Villigen). Thomson Associates has licensed
an analogue reader, for use in medicine and sterilization, to several industrial firms. A student project at Harvard Medical School
gave rise to a PC-controlled reader, for which Dr David Gladstone won the Young Investigator prize of the American Association
of Physicists in Medicine of 1991. This was one of the first systems used in the clinical treatment of cancer. The BaBar project
at Stanford Linear Accelerator, California, has developed
a RADFET Monitoring Board. The Centre for Medical Radiation Physics (CMRP) at University
of Wollongong, Australia have developed the Clinical
Semiconductor Dosimetry System (CSDS) which is available for commercial development.
FUNCTIONALITY
In the production cycle RADFETs are tested for functionality and stability after assembly.
Mechanical damage to a single FET, generated during handling, does not affect other FETs on the same die. On the other hand,
“border states", which are sometimes produced during
processing, affect stability and tend to affect a segment of a wafer uniformly. This and other stability issues are discussed
below
STABILITY
1. Border (slow) states
The drift up (du) of threshold voltage signal with time due to border states is usually of significance only after irradiation. It is usually measured
as a function of the time interval after the reader has been switched on (i.e. the "expose” mode has been switched to
the “read” mode). The rate of border state drift decreases as the logarithm
of time (Holmes-Siedle et al 1983). The value of “the drift up in any twofold time increase (dutti)” or in
any decade of time will therefore be the same for both short and long periods. Before irradiation, the value of “dutti”
is often less than 0.001V but the value becomes significant as integrated dose accumulates. For example, at 1E4 rads, the
value is about 0.02 V.
2. Temperature: effects
All semiconductor devices are prone to strong effects of temperature on operation. In this respect, they are at a disadvantage versus other types such as air chambers and TLDs. Their other advantages such as size, simplicity and ruggedness have to
overcome this disadvantage.
The measured electrical parameters of a diode [leakage current, responsivity to radiation etc] tend
to rise with temperature and are also affected by radiation in complex ways. For
example the temperature sensitivity of the response of a dosimeter diode to radiation, before irradiation is about 1 percent
per deg C but after heavy irradiation it may be as high as 4 percent per de C. The electrical parameter which is changed by
temperature is a dynamic one [ the conductivity, which can be compared to the static parameter changed in the
MOSFET case]. Thus, radiotherapy under the control of a silicon diode requires good knowledge of the device temperature and
an accurate correction which changes in value per deg C with "wear and tear" on the diode. In the MOSFET, the correction needed
results from the high temperature coefficient of threshold voltage. At a current of 40 mA, the value
is about 5 mV per oC. This is a DC effect [change in a static potential] which gives access to several correction
strategies which are not possible with the diode. First of all, there is an operating point [one value of current] which gives
a “zero temperature coefficient"called a "ZTC point”. For example, at ID = 200 mA, the coefficient is less than 0.5mV per degree C. Corrections for temperature effects can be made by the "dual bias method" where a second MOSFET is irradiated. The threshold shift is corrected by analogue
electronics in the reader (see e.g. Soubra et al 1994). The third strategy is to monitor the temperature, say by thermistor,
and calculating the correction digitally in a microcontroller forming part of the reader. In all cases. there is a similar
problem of responses changing with "wear and tear" but all the correction strategies will result in only small sensitivity
to temperature near the end of life.
Grades of sensor.. A device
of "Acceptable stability, in the “excellent” range" with one type
R and one type K functional would receive the symbol Ae'' (RK). See
Appendices and other REM technical data for details of availability of given wafer configurations, such as RK and RKKR, at
a given time.
The sensor: The construction of the sensor is that of a conventional p-channel Metal-Oxide-Silicon
(MOS) transistor with an Al gate. The silicon die, sawn from a single-crysta wafer after processing, measures 1 mm x 1 mm
x 0.5mm and carries four MOSFETs. Of these, the two type K FETs are sensitive
to "kilorad” levels of dose; and the two type R give access to “rad” levels or even lower dose values (1
rad = 0.01 Gy). The response, r, of the sensor is the “change in threshold
voltage, DVT ", a function of integrated radiation dose.
We could also obtain this response by measuring the gate capacitance in a manner which gives us the "change in flatband
voltage, DVFB”. The value of responsivity, r
is proportional to the square of the thickness, tox, of the gate oxide layer and varies in a complex way with dose.
The amount of charge trapped also varies with the oxide field applied during exposure, often called “irradiation bias,
VI”, which can be positive " Vi+" or zero "Vi0" by choice. In the construction of the sensor, the values of tox are manipulated by the sensor designer. The oxide thickness
is usually much larger than for commercial MOSFETs.
Electrical
properties: RADFETs are enhancement pMOS transistors. I‑V
characteristics resemble those of pMOS
switching FETs. Dosimetry involves tracking threshold voltage, V(T), at
10 to 250 uA. For values of V(T) see Table. The shift of V(T) given by a given amount of charge trapping is dependent on the
thickness of the gate oxide.
Typical RADFET sensor, a summary of the construction
Wafer
- REM TOT601B Mask Set : Si Wafer, 4 inch diameter. Each die carries
dual pMOSFETs in perfect symmetry, a p+n photodiode and a MOScapacitor.
Chip size: less than 1x1
mm
Gate
Oxide thickness: 0.1 to 1.25 micrometres.
Typical
radiation responses: Vi+; 2 mV/cGy Vi0; 0.5 mV/cGy
Chip
carrier (header): 6-way polymeric substrate, width 1 to 9 mm, length - variable.
Encapsulation: Black epoxy
Cable
and connections; 6-way commercial Flat Flexible Cables and sockets. Commercial FFC in various lengths, and sockets are
available from catalogue stockists. The polymeric carriers fit the FFC sockets
Chip carriers and encapsulation: Sensor chips are mounted and wired to a chip carrier [a "header"]
and covered with the minimum possible amount of opaque epoxy resin or a plane lid. Low atomic weights are used wherever possible.
The geometries and connection schemes (“pinouts”) are shown in individual REM data sheets.
REM's "CC" series of dosimeters is designed in "chip-on-board"
technology, optimized for producing low-cost commercial devices. It is possible to trade dosimeter size against the number
of FETS available per package. REM's polymeric chip carriers are engineered to fit into small orifices, the current target
being a catheter with internal diameter less than 2mm. With small sensors, it is often possible to make the necessary buildup
cap by drilling the appropriate hole in a block of tissue-equivalent plastic or aluminium. Using a REM chip, Gladstone and co-workers (1991) designed a special ultra-miniature probe
to fit within a “flexineedle” and make dose measurements within a living tumour.
Conclusion
The concept of the RADIATION-SENSING FIELD-EFFECT
TRANSISTOR, an integrating dosimeter based on the metal-oxide-silicon p-channel structure, was made in a practical form in
1970 and put in the public domain in 1974 by REM and the present author. The acronym RADFET was coined by Robert Hughes in
1986 and REM has specific permission to use this name. Used for the last 20 years in aerospace industry, now has a future
in medicine, nuclear industry and science. This summary review of the device and its literature by the inventor indicates
that REM is pursuing that future. There is a stimulating level of commercial competition
from later comers who read the literature and saw the possibilities outlined by REM.
Literature and Application Notes
A.G. Holmes-Siedle, "The Space Charge Dosimeter ‑ General Principles of a New Method
of Radiation Dosimetry", Nucl. Instrum. Methods 121, 169 (1974) (original literature
reference, putting MOSFET dosimetr in the public domain and giving priority of
invention of many variants to REM).
R.C. Hughes. “Theory of response of radiation sensing field-effect transistors in zero-bias
operation”. J. Appl. Phys., 60 (3) 1216-7 (1986) (origin of name “RADFET”).
A. Holmes Siedle and L. Adams, "RADFETs: A Review of the Use of Metal‑Oxide‑Silicon
Devices as Integrating Dosimeters" Radiation Physics and Chemistry, 28, (2) 235 ‑ 244 (1986) (first general review)
A. Holmes‑Siedle, L. Adams and G. Ensell. "MOS dosimeters ‑ improvement of responsivity
(dosimetres MOS ‑ amelioration de reponsivite)". RADECS '91, Montpellier,
France, 9-12 Sept 1991, IEEE Catalogue No. 91 THO400-2 (development of thick oxides).
D.J. Gladstone, L.M. Chin and A.G. Holmes ‑ Siedle, "
MOSFET Radiation Detectors used as Patient Radiation Dose Monitors during Radiotherapy", Paper S3, 33rd Ann. Mtg. Am,
Assoc. of Physicists in Medicine, San Francisco, July 21-25 1991, submitted to
Medical Physics (use in testicular cancer).
A.B. Rosenfeld,
M.L. Lerch, T. Kron, E. Brauer-Krisch, A. Bravin, A. Holmes-Siedle and B.J. Allen (2001). "Feasibility study of online high-spatial-resolution
MOSFET dosimetry in static and pulsed X-ray radiation fields". IEEE Transactions on Nuclear Science, NS-48 (6) 2061-8 (December
2001) (the micron resolution of dose profiles and the use of new reader).
R.C. Hughes, D. Huffman, J.V. Snelling, T.E. Zipperian, A.J. Ricco and C.A. Kelsey, "Miniature
Radiation Dosimeter for in vivo Radiation Measurements", Int. J. Radiation Oncology
Biol. Phys, 14, 963-7 (1988) (First medical application).
D.J. Gladstone, L.M. Chin and A.G. Holmes ‑ Siedle, " MOSFET
Radiation Detectors used as Patient Radiation Dose Monitors during Radiotherapy", Paper S3, 33rd Ann. Mtg. Am, Assoc. of Physicists
in Medicine, San Francisco, July 21-25 1991, submitted to Medical Physics (use in testicular cancer).
S.J. Kronenberg and G.J. Brucker . "The use of hydrogenous material for sensitizing pMOS dosimeters
to neutrons" IEEE Trans. Nucl. Sci. NS-42, 20-6 [also see pp33-40] (February 1995) (tactical dosimetry).
A bibliography of research on RADFETs: Appendix D of A. Holmes-Siedle and L. Adams, "Handbook of Radiation Effects" (Oxford University
Press, 2nd Edition 2002). Book available by mail order from
REM in the UK and Europe or (in USA) from
mike@icsrad.com.
Application notes on RADFETs available from REM
1. rfi05genl.doc. Technical Note REM-05-01TR "REM'S INTEGRATING DOSIMETER SYSTEM BASED ON THE RADFET: AN INTRODUCTION
(Jan 2005)
2. rfi05sum01.doc Technical Note No.REM-05-02SUMM,"SUMMARY
OF REM'S "DOT" DOSIMETER SYSTEM BASED ON THE RADFET:FUNCTIONALITY, STABILITY AND APPLICATIONS" (January 2005)
3. tosp02-9-504xcc8resp.doc, Technical Note No. REM-02-9-504xCC8resp, "NOTES ON TOT500/CC8 RADFETs - RESPONSE
TO GAMMA RAYS (December 12, 2002)
File rfi05sumWEB03.doc August 11, 2005 ahs
end of web page at www.radfet.com
