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The space-charge dosimeter
N U C L E A R I N S T R U M E N T S AND METHODS I 2 I
THE
(1974)
I 6 9 - 1 7 9 ; © N O R T H - H O L L A N D P U B L I S H I N G CO.
SPACE-CHARGE
DOSIMETER
General principles of a new method of radiation detection A N D R E W HOLMES-SIEDLE J. J. Thomson Physical Laboratory, University of Reading, Whiteknights, Reading RG6 2AF, England Received 22 February 1974 and in revised form 10 June 1974 W]hen dielectrics under a high field are exposed to high-energy radiation, carriers are separated and space charge results. The change in potential at the dielectric surface can be measured non-destructively and such a change is initially linear with radiation dose. This provides a principle for radiation dosimetry not previously exploited, here named space-charge (SC) dosimetry. One compact, direct-reading form, based on the metal-
oxide-semiconductor (MOS) transistor, is described and the principles for improving this form by orders of magnitude are set out. The present level of sensitivity [measuring a fraction of a Rad (SiO2)] is well adapted for use in radiology, beam diagnostics, space radiation monitoring and other applications requiring close-spaced integrating dosimeters. With development, the method could be used in personnel radiation monitoring.
1. Introduction
v e r y s m a l l single d e t e c t o r f o r m e a s u r i n g p h y s i o l o g i c a l X - r a y e x p o s u r e 3) a n d s e c o n d l y t o a c l o s e - s p a c e d d e t e c t o r a r r a y in a n X - r a y s p e c t r o m e t e r 4 ) . T h e techniq u e c o u l d , in the future, a s s u m e w i d e r i m p o r t a n c e if, as e x p e c t e d , the sensitivity o f s p a c e c h a r g e (SC) dosim e t e r s is i n c r e a s e d to m a k e t h e m useful in the m R a d range. T h i s w o u l d o p e n u p a w i d e r a n g e o f r a d i a t i o n m o n i t o r i n g a n d safety a p p l i c a t i o n s . F u r t h e r m o r e , w h i l e fig. 1 gives a n i m p r e s s i o n o f t h e c o m p a c t n e s s a n d c o n v e n i e n c e o f the system, m o d e r n m i n i a t u r i s a t i o n a n d p r o c e s s i n g t e c h n i q u e s c o u l d still m u c h f u r t h e r r e d u c e the sizes a n d i n c r e a s e the s o p h i s t i c a t i o n o f the s e n s o r
I n 1970 P o c h a n d H o l m e s - S i e d l e 1) p u b l i s h e d a b r i e f a c c o u n t o f a n e l e c t r o n i c a l l y - m e a s u r e d , solidstate, i n t e g r a t i n g d o s i m e t e r s y s t e m o f g r e a t c o m p a c t ness a n d h i g h d y n a m i c range. T h e d o s i m e t r i c p r i n c i p l e u s e d was novel, n a m e l y the b u i l d - u p a n d n o n - d e s t r u c t i v e e l e c t r o n i c sensing o f space c h a r g e in the silicondioxide portion of a metal-oxide-semiconductor (MOS), insulated-gate, field-effect t r a n s i s t o r device. T h e n o v e l t y was r e c o g n i z e d in a n i n d u s t r i a l a w a r d in 1971 2) a n d t w o a c c o u n t s h a v e since b e e n g i v e n by o t h e r w o r k e r s o f t h e a p p l i c a t i o n o f this p r i n c i p l e first to a
Fig. 1. A metal-oxide semiconductor (transistor) dosimeter system, one-dosimeter arrangement. The sensor head is at the end of the black cable. The black dot with thin wires attached is a detached sensor head, being a small transistor package (2 mm on a side) which holds the dosimeter pellet, made from layers of silicon, silicon-dioxide and metal. The coin, a US "quarter" ($0.25) is 25 mm high. The volume of the silicon pellet is less than 10-a cm ~. The volume of the radiation-sensitive region, a thin film of silicon dioxide grown on the single-crystal silicon-wafer is less than 10-10 cm 3. 169
170
ANDREW
HOLMES-SIEDLE
head and sensing circuitry shown in this figure. For several applications, including the radiobiological field, the important point to note in fig. 1 is that the sensor head is minute and can be operated at the end of many feet of wire. In fact, only two strands of very thin conductor, operating at less than 10 V, are required. This paper discusses the general principles and the optimisation of the SC dosimetric method. As the preliminary publication 1) was in a journal rarely found in archives, some sections of this reference are repeated in an abbreviated form in the appendix.
2. Basic principles When an insulator is irradiated by high-energy radiation while under an applied electric field, carriers are excited to and flow in the conduction band, producing a photocurrent. Except in the rare case of a near-perfect crystal, these carriers fall into traps in significant numbers. Frequently, the numbers of electrons and holes trapped are different by orders of magnitude. The net result, as shown in fig. 2, can thus be either the "persistent internal polarisation" of the sample s) or the build-up of a sheet of electronic space charge which is not balanced within the insulator by opposite charges. In either case, a variety of electrostatic methods could be used to measure these space charges non-destructively. In many dielectrics, the charges will remain unchanged for very long times. The build-up of space charge will initially be proportional to the number of electron-hole pairs created but the accelerating field will be gradually reduced by the built-in field due to the space charge, and proportionality will be lost. Under the right conditions, however, the growth curve will deviate only slightly from linearity over several decades of dose during which
the space charge is of measurable magnitude. This linearity may be maintained even further by the simple circuit techniques to be described. This fact had not been realised before the present investigation1). The authors found that a useful range of simple dosimetric operation with adequately linear response can be obtained if some external manifestation of the space charge, such as surface potential, can be measured, and changes in this parameter are calibrated against dose; the device is thus a relative dosimeter. It is an integrating dosimeter which, if the insulator slab is thin, will be very insensitive to particle type or irradiation energy, so long as the energy required to create an electron-hole pair or other mobile carrier is supplied. While preliminary experiments have shown the principle to operate in several dielectrics, prepared as thin slabs6), the most attractive form appears to be the MIS dosimeter, in which the field at the insulator surface is sensed by means of the image charge set up in a semiconductor electrode in contact with it. While not ideal in geometry or material composition, the commercial MOS transistor fulfils many of the requirements for an SC dosimeter, since it comprises a readymade small volume of low-leakage trap-filled insulator, well sealed from light and moisture and having a builtin field-sensitive electrode which allows a convenient read-out of surface potential. The intrinsic disadvantages of the commercial MOS transistor are the relatively poor control normally achieved with respect to trapping centres in the oxide, the restriction at present to silicon dioxide as the dosimetric material (even though lower atomic weights are desirable, for the simulation of tissue) and the interfering effects of the Si/SiO2 interface statesT).
3. The metal-insulator-semiconductor (MIS) dosimeter
HIGH-ENERGY RADIATION
3.1. DEVICE STRUCTURE
EXPOSURE
STORAGE
MEASUREMENT
Fig. 2. Principle of measurement of space-charge in insulators (holes more strongly trapped than electrons). In the case of the MOS device, the electrodes are metal and silicon, and serve both as contacts during exposure and as the electrostatic sensing system.
MIS devices used in solid-state electronics usually comprise a single-crystal silicon wafer, on which is deposited or grown, first, an amorphous oxide film of thickness 0.1~0.2/~m, and, subsequently, an evaporated metal island. This MIS capacitor or MIS diode has been used mainly as a vehicle for research on field effects in semiconductors. The insulated-gate field-effect transistor (IGFET) or MOS transistor is a three-terminal elaboration in which the conductivity of a channel beneath the silicon can be modulated. Fig. 3 is an example of an I G F E T structure in which, for the present purpose, the metal " d r a i n " and " g a t e " electrodes, normally separate, have been formed in one
THE SPACE-CHARGE DOSIMETER
171
t metal } O-Ijum to admit beta particles of E< IOkeV tox = IOeum ,
//
'
y ¢ ,l:ili.iliil ,Hilji[iilly: Insulator: activ~ sensor I region,showing radiation-induced trapped charge
"
I
l~-Typeconducting region (inversion
region) produced by radiati0n-induced charge in oxide
NOT TOSCALE
Fig. 3. Proposed MOS dosimeter sensing element. piece. The most convenient parameter which can be measured in this device is the gate voltage at which the field-induced inversion channel reaches some chosen current level. This voltage is termed the threshold voltage, VT ; for MIS capacitors, the unique voltage at which the semiconductor energy bands are not " b e n t " at the surface, called the flat-band voltage, VFB, can also be measmed automatically with a direct-reading capacitance bridge and auxiliary circuitryS). 3.2. RESPONSE TO IRRADIATION It has been known since 1962 9) that the irradiation of a metal-insulator-semiconductor (MIS) transistor by ionizing radiation leads to a shift (A VT) in the threshold voltage of the device, as shown in fig. 4a. Fig. 4b shows that the effect may also be detected by capacitance-voltage measurements on a transistor or capacitor. In the latter device, shift is commonly measured at the flat-band condition yielding A VFB. The commonest form of MIS device employs an amorphous, t h e r m a l l y - g r o w n silicon dioxide film. Tile mechanism commonly accepted for charge buildup in this film is shown in fig. 5 9). The degree of shift of the device characteristics along the voltage axis (A Vx or A VFB), is controlled mainly by total dose D, expressed hereafter in units of Rad, implying Rad (SiO2) ; local area density of holes trapped in the oxide (Nox, c m - 2) ; the thickness of the oxide (to~,/~m); and the irradiation bias (V i, V) applied across the oxide from gate to semiconductor during irradiation. Some
approximate relations for silicon dioxide are: -- A I / v B
or - A V v -
t-°~-N°---~
2.1 × 10 l°
+-f(Ni) + g ( O
(1)
where Nox(Cm-2) is the net area density of positive charges, each of value one electronic charge, which has been trapped during irradiation in the space-charge region of the oxide (assumed here to be very near to the silicon); that is: (2)
qNox = AQo x = -AQsl,
where A Qox and A Qsl are, respectively, the amount of
ICnNPDT ~
--', p'~k\ (a)
\l / ,"AFTER IRRADIATION / 5 ,
I ~" - " /
~ETAL
/ ~RRAD,AT,ON~OX,DE = SEMICONDUCTOR ----
V~ "X......J
_
+
!-
AVFB
.i
V~
//
=
//
(b)
+
1o
+
/
I'
"VT
"I
V¢
Fig. 4. Changes in operation of MIS devices, produced by ionizing radiation. ID = drain current, CINP~rT= capacitance. Other symbols explained in text.
172
ANDREW
HOLMES-SIEDLE
charge residing in the oxide and the amount of image charge produced in the semiconductor7). Typical values of Nox are 102' c m - 2 , which, in eq. (1), yields a value for A VvB of about 1 V in an oxide of 0.2 pm thickness; f(Ni) is an "interface state creation" term (if correct oxide growth conditions are maintained, Ni, the interface-state density, can be kept
l o w - l e s s than 101° states cm - 2 - and this term can often be neglected); g(t) is a trapped-charge leakage or "annealing" term which, again with correct oxide preparation conditions, can be kept sufficiently low that it does not interfere over the normally desirable integration times (1-300 d). The change of threshold voltage with irradiation is not linear but follows a saturating characteristic, described by t o)
m . . . . . TION
HIGH-ENERG~ ' PARTICLEOR
AVT = ~ V ~ ( I - e - ~ ° ) , 4ETAL
where c( and fl are constants depending on the trapping parameters and thickness of the oxide and the precise profile and location of the space charge. Both constants are complicated functions of N o , the local area density of the species in the oxide (presumably a defect) on which the positive charge resides and /Y is also a function of the lifetime-mobility product for the electrons in the oxide. It is not yet clear whether the saturation of A VT occurs because of saturation of all traps (Nox = N D in a given element, dx, of oxide thickness) or because electrostatic conditions in the oxide cause electron loss to cease. However, there is
EXI~ING HOLE TRAPS
NEWINTERFACE STATES
Fig. 5. Mechanism of charge build-up in an insulator under an electric field.
°°t o
(3)
/
/
//
IC
A•
#
A
7.9 (I-e-21D)
.<1
~/ TA 5631 FIXED VI=-9V A TA 5388 FIXED VI = -9V ® TA 5388 USING COMPENSATIONCIRCUIT
¢"
0-1 103
~
~
I
10 4
,
L
I
IO s
I
IO s
i
,
I
10 7
i
IO s
DOSE O (Rods) Fig. 6. Shifts in threshold voltage (A VT) vs radiation dose (D). Curve A = theoretical relation for d V~ vs D with constants as shown; curve B = linear relationship between A VT and D, chosen to fit the data shown as circles. Triangles: data from irradiation of R C A developmental p-channel MOS transistors TA5388 and TA5631 at fixed irradiation bias, VI, o f - 9 V. Circles: irradiation o f TA5388 in compensation circuit as in fig. 7.
173
THE S P A C E - C H A R G E DOSIMETER
indirect evidence that an increase of ND leads to an inc,rease in A QoX under the same electrostatic and irradiation conditions6). Curve A in fig. 6 shows the growth of A VT for one of the more radiation-sensitive forms of commercial M][S device geometry, namely a p-channel transistor of which the gate oxide is formed from about 1000/k of steam-grown silicon dioxide overlaid by about the same thickness of phosphosilicate glass (doped organosihme method) 11). It can be seen that, for dose values up to about 10 4 Rad, the equation does not deviate greatly from a linear dependence of A VT against D, namely: A V T = K D . K, of course, will serve as a sensitivity constant for the dosimetric system. Experiment 11) confirms that MOS devices irradiated at a constant negative value of VI follow this curve closely and some data points are shown on the curve. Ciarlo and co-workers 4) also found adequately linear behaviour in MOS devices under constant positive bias. In the constant-bias mode, the dynamic range of the device as a dosimeter of linear response with dose (desirable in a direct-reading application) is thus bounded at the upper end by the " k n e e " of the curve for eq. (3), representing the incipient saturation of the charge build-up. At the lower end, the minimum detectable shift in VT or VFB provides the limitation. Both eq. (3) and experiment show that saturation is delayed at higher values of applied bias, VI. (Strictly, the equation implies that, for the same structure, higher changes in VVB are achieved at the same dose, without alteration to the degree of sublinearity exhibited.) This being so, we can extend the useful range of linearity by usJ[ng a characteristic of the MOS transistor demonstrated in fig. 7a. If the gate is shorted to the drain and the source-drain circuit is put in series with a battery (w)ltage E) and a large limiting resistor (RL), VI automatically increases as VT becomes more negative1). As fig. 6, curve B shows, with the correct device structure and the correct values of E and RL, linearity can, in fact, be extended for well over a decade further in dose. Moreover, in this "compensation circuit" configuration, the voltage drop across the channel, V¢h, is also a measure of the threshold shift, VT, hence: A Vch = AA VT
=
KD,
circuit only operates using negative gate bias in p-channel devices. For an increase in range, a sacrifice in sensitivity is thus made, since, according to Mitchell's theory for single dielectrics i 0), positive values of V~ give the greatest sensitivity. Thus the choice of biassing scheme must be based on the values of dose range which are to be measured, the requirements for simplicity in the circuit, the procedure for measurement and the nature of the dielectric chosen. 3.3. PRINCIPLES FOR OPTIMIZATION AS DOSIMETER
3.3.1. Electrical parameters To optimise with respect to sensitivity, the sensitivity constant, K of eqs. (4) and (5), has to be made as large as possible. A desirable, though arbitrarily chosen figure is K = 10 V/Rad. For readily available MOS transistors (tox~0.1 #m, ANox~ 1012 c m - 2 ) , K is about 10 -2 V/Rad. A dosimeter optimised for high sensitivity would possess an increased value of K,
i SE/O OIR l'NSS
E--
(a)
T
' SM I PLF IE IC D R I CUT I
__J~MOS SENSOR (b)
ZE OST A DR JU
(5)
where A is a constant which can be made near to unity by proper adjustment of RL. Vch can be read directly with a voltmeter, hence this linearity of A Vch vs dose allows the device to be used as a direct-reading dosimeter. A practical arrangement for this compensation circuit is shown in fig. 7b. Unfortunately, this compensatory function of the
DO I DE ZENER
_-
Fig. 7. Typical compensation and read-out circuit for p-channel MOS device. (a) simplified form, (b) as used in dosimetric experiments.
174
ANDREW
HOLMES-SIEDLE
obtained by increasing tox and Nox to their practical limits. These limits are discussed below. 3.3.2. Oxide thickness limits There are electrical-measurement reasons why to~ cannot be increased indefinitely: for a transistor of initial threshold voltage of zero, the ID--Va characteristic is determined 6) by the equation I D = k V 2 - /~seoxWV2 + I t ,
to×L which can also be expressed in a different form: 2/~seox WI/~ gm
--
toxL
where I v = source-drain current, I e = source-drain leakage current independent of
vo, p~ = "surface majority carrier mobility" in the channel, cox = dielectric constant of the oxide layer, L = source-drain spacing or channel length, W = width of channel, k
= Itseox W/(toxL),
gm = transconductance = dlo/d V c . Thus, for a given insulator type (i.e. fixed/~s, eox) and device geometry (namely fixed W / L ) , a limit on the value of tox is set by the requirement for ID to achieve values which are detectable above the constant, but low Ie, when the gate voltage is still (a) below the breakdown voltage of the oxide, and (b) at a level which meets the practical requirements of the supply voltage (E in fig. 7a). Similar requirements apply to detection of shifts in V w by means of capacitance measurements: the ultimate accuracy possible in measuring small values of A Vv in a transistor, while not, in simple theory, dependent on the value of gm for the device, may indeed be controlled by this due to the above leakage currents. Moreover, if the value of gm is too low, the linearity of the A VT VS dose relation at low values of dose will also be affected by radiationinduced changes in leakage current and i n / ~ , brought about by the creation of new interface states (see fig. 5 and ref. 9). Holmes-Siedle and Poch ~2) have shown by experiment that MOS dosimeters with thick films of thermal silicon dioxide are, as eq. (1) would predict, more sensitive in terms of shift (A Vx) per Rad. Moreover, although the thick (1.5/tm) and thin (0.15 llm) oxides were grown on the same silicon substrate, the effect was
greater than predicted by eq. (1), possibly because of impurity effects, discussed in the next section. Thus the limitation in thickness of the oxide would appear to be that discussed earlier in relation to the accuracy of the measurement of VT or VvB. The feasibility of using oxides of 1.5/~m thickness has been proved 12) and, judging by these experiments, the limit for detectability of Vv or Vv~ may well be in the region of 10/~m. Thus the sensitivity of the devices used in previous studies on linearity with dose 1'3'4) (tox about 0.1/~m), can be increased by two orders of magnitude by virtue of a dimensional change alone, which change can be achieved rather simply, for example, by prolonging the growth of the oxide in steam or adding a deposited dielectric layer to the thermally-grown oxide. 3.3.3. Optimisation of,space-charge density The exact nature of the species which acts as a trap for holes has not been demonstrated, although it is assumed widely that it is a structure such as a nonbridging oxygen atom 9) and that the space-charge density (No0 in some way controlled by the density of traps (ND), especially at saturation of A VT 9). In this discussion, we will thus assume that increase in No in a given region of the oxide will lead to an increase in A Qsi for a given dose, provided the correct conditions for net hole trapping exist in that part of the oxide (i.e. low electron density, see ref. 10). As explained in ref 9, positive irradiation bias leads to positive space-charge build-up near the silicon, a condition which gives strong imaging of that charge in the silicon. Per contra, the opposite bias gives positive space-charge near the metal, with consequently weak imaging in the silicon. Nevertheless, because of requirements for maximum extension of linearity (see section 3.3.1) the gate bias and silicon polarity used in the author's studies 1) was such that the charge sheet was built up near the metal electrode. For this mode, a high value of No near the metal electrode is clearly required and conditions should favour the growth of the space-charge region as far as possible towards the semiconductor. On the other hand, greater sensitivity can be obtained by using a positive gate bias and, in this case, the space charge layer should be confined as closely as possible to the region near the silicon. Hence, a high value of No within a few hundred Angstroms of the silicon is desirable, the value of No in the rest of the oxide being relatively unimportant. It can thus be seen that the preparation of an insulator film for dosimetric purposes must be carried out with a particular mode of operation in mind. Very recently, Hughes 13) has produced good correla-
THE S P A C E - C H A R G E
tions between charge build-up and alkali ion content of o~:ide films, which suggest that the charge sheet producing the flatband voltage shifts is not due to traps intrinsic to the amorphous S i C 2 network but due ekher to hole traps located on non-mobile alkali-metal impurities concentrated near to the semiconductor or even to the migration of sodium ions themselves, after they have been made mobile by the radiation.* There is thus a possibility that No can be strongly increased by judicious diffusion or implantation of alkali ions, followed by heat treatment to produce the correct chemical binding of these atoms to the SiC2 network. The author and co-workers had already found t 5) that values of Nox as large as l0 t 3 c m - 2 could be produced by trial-and-error methods in steam-grown S i C 2. Artificial doping could presumably increase Nox by an order of magnitude. For reasons not well understood, aluminium ion implantation has been found to enhance the VT shift for oxides biased negatively during irradiation 14'16) while depressing the shift under positive bias. A similar effect was also found to be produced by superimposing a second insulator layer on the grown oxide ~1,14-16). These methods can be used to increase the sensitivity of the negative-bias mode. 3.3.4. Temperature effects The trapped positive charge in MIS devices made from thermal SiC2 is completely annihilated by heating to 250-300 °C for a few hours9). This effect can be-an advantage or a disadvantage for a dosimetric system depending on the ambient in which it will be used. For example, the S i C 2 device in its present form could not be used in radiation beams which produced heating above about 150°C without some loss of sensitivity. On the other hand, as the appendix shows, a baking treatment will restore this form to the unirradiated state for re-use. The thermal annealing phenomenon appears not to be a thermal depopulation ot' holes (this probably requires a far higher temperature) but rather a thermally-activated injection of electrons from either interface ( E a c t ~ l eV) via interface states l v); in other materials, the temperature for annealing could thus be very different. The limitations on re-use after heavy irradiation are probably set by the creation of unannealable bulk defects in the * I f the latter case is true, then the concept of a sheet of trapped holes of concentration Nox, can be replaced by an effective concentration o f positive ions, Nl, activated by the irradiation, located at an effective distance, xi, from the semiconductor. The treatment to increase sensitivity remains similar in several but not all respects:
175
DOSIMETER
silicon, which could be significant for particles (for example a fluence of 1015 protons/cm 2 at 10 MeV could increase source-drain diode reverse leakages by an order of magnitude). Some types of oxide undergo a slight room-temperature a n n e a l i n g - an effect which would have to be avoided for long integration times (see section 3.2). 4. Some SC dosimeter designs 4.1. OPTIMISEDMIS DOSIMETER The considerations outlined in the previous section can be embodied in the following optimum form of MIS dosimeter, which is that shown in fig. 3. The gm value of this device is such that it would achieve an Io value of 3/~A at a V~ value of 10 V, thus leakage currents must be kept well below this level. 2 × 1 0 L 4 c m -2, tox = 1 0 p m , L = 7.5/~m. No =
k = 3x10-7/~V, W= 2.5ram,
To estimate sensitivity constant K, we will assume linearity to 104 Rad, followed by complete saturation. Combining eqs. (1) and (4) we obtain K -
tox Nox 2 × 101°D"
Using the value for N D in place of Nox, we obtain a sensitivity factor of 10 V/Rad. Using sensing circuitry which would measure changes of 5× 10 - 3 V, the smallest dose measurable would be l0 -4 Rad which makes the device potentially useful in the radiationsafety monitoring field. The m a x i m u m dose measurable would not, in this case, be controlled by saturation of traps by holes or by field conditions within the oxide; already, at 10 2 Rad, the value of A VT would be 100 V, which is probably near the practical limit at which leakage currents and breakdown in the device or noise in the sensing circuitry would obscure the level of the currents in the channel. While it may be difficult to produce a suitable dielectric film of thickness l0/~m by thermal growth, even in steam, it should be acceptable to use a thinner layer of thermal oxide (say 1/~m) and deposit another dielectric upon this to the required thickness. The results should follow the favourable trend for two-layer devices mentioned in section 3.3.3. 4.2. MIS DOSIMETERWITH SEPARATEINSULATOR By abandoning the mechanical convenience of depositing or growing an insulator film in intimate contact with the semiconductor, freedom is acquired
176
ANDREW
HOLMES-SIEDLE
to modify the atomic weight and trap structure of the insulator. A form of dosimeter in which the insulator is separated from the semiconductor is shown in fig. 8. Since the surface of the semiconductor must be controlled with respect to semiconductor surface states, a film of silicon dioxide must still be thermally grown to "passivate" the surface, but it can be made sufficiently thin so that imaging of charge stored in the lower face of the detached insulator slab is efficient and so that radiation-induced charge build-up in the grown film does not interfere. The latter effect can, of course, be disposed of if the slab is irradiated between two metal electrodes and only presented to the semiconductor for measurement purposes. The practical disadvantages of this method lie mainly in alignment and in the interfering effects of "contact electrification" and leakage incurred in any use of a demountable capacitor l s). The main advantages lie in the greater possibilities of varying the insulator, especially the use of materials of high atomic number with greater stopping power or materials which better simulate biological tissue, say boron or beryllium compounds. 4.3.
OTHER FORMS OF DOSIMETER NOT INVOLVING A SEMICONDUCTOR
As mentioned, the semiconductor portion of fig. 6 can be replaced by any metal electrode for application of field during irradiation. For sensing, an electrometer system other than the semiconductor surface can be used. Gross 18) has used a simple demountable capacitor for measuring charges trapped in dielectric slabs,
ACHABLE JLATORAND ALGATE
~/ASSIVATIOR iii!lrrnl,lll~l
;ON SURFACE
(O.Ol.,um,
m iii1[i MiglJlllllll fl]lrlqlM~l hllllll!~ ill~lllll]~]llllm $[11 illlllLmll n
',
FIELD[NOU ' CED CONDUCTIONREGION (~NANNEC)
~
METAL
[ITTFrTTnINSULATOR P-TYPEDIFFUSION [--"~'-] N-TYPE SILICON SUPPORT
Fig. 8. Design o f MIS dosimeter with separated insulator.
while, in preliminary experiments, the author has found a vibrating-capacitor method for measuring surface potential~ 9) to be suitable. A wide range of variation is possible for this configuration of dosimeter. One requirement for linearity will be a minimum amount of production of new traps by the radiation itself, since this may lead to superlinearity. This requirement would presumably eliminate materials undergoing strong radiolytic effects such as the alkali halides and many organic materials. However, Gross 5) and Kallman et al. 5) have observed early-time linearity of charge build-up in several organic materials, glasses and inorganic powders. Based on present knowledge 12, 18, 2o) alkali-containing oxide glasses should meet the requirements of variability of dopants, high trap content and ruggedness particularly well. 5. Discussion
While the advantages of the space-charge build-up system are fairly clear, it need not be supposed that further developments will be without some pitfalls. The MIS system has always presented problems of interface effects and of uncontrolled impurity effects, both of which must be rigorously controlled in the dosimetric device; in fact, techniques of controlling hole-trap density in insulators are as yet only in their infancy, but are developing14-16). The insulator is prone to electrical breakdown, even from stray electrostatic build-up on metal electrodes which are not grounded and this effect would have to be considered when designing for intense charged-particle beams. The MOS dosimeter device has, however, been used with success in 0.01-1.5 MeV electron beams, 0.5-20 MeV proton beams and fission reactors6). All the forms of SC dosimeter described require that a large difference exists between the effective mobility of holes and electrons in an insulator. In fact, in insulators, it is the holes which are commonly of very low mobility due to self-trapping effects, and the form investigated by the author indeed involved positive space-charge sheets. However, the sensing method could be applied equally well to charge sheets due to trapped electrons and to excess charge generated by processes other than differentials in mobility, for example Compton scattering 5) or absorption of non-penetrating ions and electrons. Thus special methods could be developed for the dosimetry of ion l=eams or gamma-rays. A particular advantage of the principle for laboratory radiation beams is the smallness of the sensitive element, making possible the mapping of a beam at an array of
THE SPACE-CHARGE
points simultaneously. The sensor array can be made very dense, using existing techniques of integrated circuitry and even possibly self-scanning. The possible applications thus cover a very wide range, for example from small single dosimeters which automatically trigger an alarm when a threshold dose is reached to very dense arrays which can image a radiation source of any energy from the band-gap energy to several millions of electron volts.
6. Conclusions A broad dosimetric principle, which has not been exploited before, is described and the name Space Charge Dosimetry is proposed for the principle. One embodiment of the principle is the metal-insulatorsemiconductor version and some designs are described. The main advantage of the method with respect to other relative methods of measuring integrated dose, for example thermoluminescence dosimetry, are the potentially great compactness (the sensor element can be as small as 10 - 4 cm-3), the possibility of remote, continuous and non-destructive electrical readout and the wide range of materials in which the charge build-up may be produced and possibly enhanced by doping or defect introduction. In the MOS dosimeter form, linearity, dynamic range and reproducibility are shown here to be as good as, or better than other solid-state integrating dosimetric systems such as thermoluminescent, fluorescent and colorimetric systems. The sensitivity of the first crude devices tested is comparable with the above systems while improvements in sensitivity by several orders of magnitude appear possible. The method is very well suited for beam diagnostics, radiographic research and space radiation monitoring. An optimised design is proposed, of a sensitivity such that less than 1 m R e m of ionising radiation could be sensed. Thus applications in radiation safety work are ultimately possible.
177
DOSIMETER
part of the complementary-symmetry (CMOS) circuits on the R C A TA5388 series of integrated circuits. Several compact, self-contained, working models of the dosimeter were built, some suitable for installation in a spacecraft electronics bay. Two 9 V transistor radio batteries could be used as an internal source of power. The length of cable connecting the transistor to the control unit was not critical and lengths up to 100 ft were used without adverse effects. A switch on the side of the enclosure changed the sensitivity of the instrument; the full-scale reading on the meter could be adjusted to 20 Rad, 100 R, or 500 Rad. Fig. 9 shows the result of irradiating a single pchannel transistor connected as in fig. 7. The same transistor was irradiated four times, followed by an annealing procedure after each irradiation. This procedure, which included an overnight bake at 250 °C restored the transistor to nearly its original condition. The value of A VT decreased somewhat following the first irradiation but there was almost no change between the third and fourth test runs. Also, it can be seen that the curves corresponding to the four runs are essentially parallel and close to being linear. The dosimeter system provides a convenient method for calibrating one radiation source against another under a particular set of unusual conditions. For example, it was desired to know the ionizing dose absorbed in silicon integrated circuit pellets when encapsulated in the small metal "can s" used in semiconductor packaging. A comparison was required between two radiation sources, an 85 keV X-ray generator and a 6°Co gamma-ray source. The amount of attenuation due to the can and the nature of the secondary-electron bombardment generated by the
o
RCATA 5388
l: I-0
7fRuNs
t,.
Appendix The original description of a working MIS dosimeter system was published in R C A Engineer 1), a journal not widely found in archives. Thus, a greatly abbreviated account of the experimental data given in that article is given here to clarify the more general discussion given above. The M O S transistor. A large part of the development work on dosimeter circuits was done with two commercially available devices- the M E M 520 transistor, a discrete p-channel device made by General Instruments; and the p-channel transistors which form
o~ 0.1
~// ¢t:
O-OI 0'1
H°.3 ~, 4
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8S KeY X RAYS
../_ ............ 1.0 IO I00 EXPOSURETIME IN'MINUTES
Fig. 9. R e s p o n s e o f p-channel M O S transistor (part o f the R C A T A 5388 device) irradiated in the c o m p e n s a t i n g circuit configuration o f fig. 7. Between each r u n as m a r k e d , the device was heated for several h o u r s in an air oven at 250 °C.
178
ANDREW
HOLMES-SIEDLE
can differs widely in the two cases. However, unlike larger dosimeters, the MOS dosimeter could be mounted inside an identical can and its response monitored directly. When the transistor used to obtain the data in fig. 9 was annealed by heating and exposed to the calibrated 6°Co source at Forth Monmouth, N.J., the resulting dose vs threshold-shift curve was essentially parallel to the curves obtained with the X-ray generator. The 6°Co curve and the curves of runs 3 and 4 in fig. 9 could be made to coincide if the dose corresponding to a 10 min exposure in the X-ray machine under the specified conditions was taken to be 104 Rad. An important feature of the circuit, as shown in the main text, is that the dose-indicating voltmeter can be electrically reset to zero after each exposure. This assumes that the dose being measured is only a fraction of the total dose that the MOS transistor can absorb without reaching the limit imposed by saturation effects. For example, if the dosimeter were used to measure a series of dose increments of 250 Rad each, then 160 successive exposures would be required to reach the 4 x 104 Rad level (the upper limit of the curves in fig. 9). The incremental change in threshold voltage of about 0.010 V for each dose increment of 250 Rad would remain nearly constant. The gain of the operational amplifier under these circumstances could be adjusted to provide exactly 2.5 V at the output so that a digital voltmeter could then provide a direct reading in Rad. Recent experiments have indicated that the essentially linear portion of the dose versus threshold-shift curves, as shown in fig. 9 can be extended to at least 1.5 x l0 s Rad, and perhaps to even higher levels. Thus, the number of successive 250 Rad exposures mentioned previously could be extended from 160 to over 600. When the MOS tlansistor begins to show serious non-linear response due to saturation effects, it can be subjected to the annealing procedure mentioned. Sensitivity considerations. Ideally the sensitivity of this type of dosimeter could be extended indefinitely by simply increasing the gain of the operational amplifier. However, a number of factors impose an upper limit on the useful gain: 1) the basic stability of the MOS transistor, 2) the sensitivity of the transistor to temperature changes, 3) the constancy of the voltage supply, and 4) the stability of the other circuit components. When using the system with maximum amplifier gain (20 Rad full scale), the resulting instability in the output reading was approximately 5% of full scale.
An application particularly well suited to the capabilities of the dosimeter in its present state of development is the in-flight measurement of the radiation dose accumulated by spacecraft components while actually orbiting in the Van Allen regions. The small size, low power drain, adaptability to conventional telemetry systems, and basic simplicity are major advantages of this system over any other dose measuring system. Also, the range of dose levels involved in space applications are well within the capabilities of presently available MOS transistors. A more sensitive version would, for the same reason, be very well adapted for actively monitoring integrated doses in the new components of aircraft or other vehicles exposed to radiation. Mapping the beam from an X-ray, gammaray, or particle accelerator is another application that fits the capabilities of the MOS dosimeter extremely well. The distribution of the beam intensity close to the window of an X-ray generator and within the beam of a particle accelerator was very rapidly mapped with an array of detectors. An attempt to map the beam of the same X-ray generator with small commercial LiF thermoluminescent dosimeter (TLD) rods demonstrated the superiority of the MOS dosimeter for this application. With the latter device, measurement of the dose at a particular location could generally be repeated within 1 or 2% variation. With T E D rods, the variation was usually 10% or more. in addition, the time required for the complete test was considerably less when using the MOS device. Under certain circumstances it may not be feasible to provide an electrical connection between the sensor element and the control unit. Dose measurements can still be made, however, by using certain types of MOS transistors which accumulate a reduced but usable MEM 520
I0-0
Vz:O
RUN.N o . I . ~ ~"
r.0
>o 0q
/
RUNN~/J_.~RUN No.3
COBALT-60 GAMHARAYS
o=
o.oj
iOs
I0 4
IO ~
IO 6
RADS
Fig. ]0. Response of p-channel MOS transistor (General I n s t r u m e n t s M E M 520) irradiated at V1 = 0 (gate shorted to source a nd drain). H e a t t re a t me nt between runs as for fig. 9.
THE S P A C E - C H A R G E DOSIMETER
anaount of trapped charge even in the absence of an applied electric field. Fig. 10 shows how the threshold ve,ltage shift of a sample of the M E M 520 device varied when irradiated without bias. Under the " n o - b i a s " condition, the threshold shift versus dose relationship became non-linear at about 104 Rad. However, higher doses or dose increments could be determined from calibration curves of the type shown in fig. 10. After one or two exposure and annealing cycles, as for fig. 9, threshold shift versus dose relationship became suitably reproducible from run to run. Sensors of this kind can, therefore, be used in the same manner as T L D devices to measure the accumulated dose at locations where it would be impracticable to provide an electrical connection. The experiments described indicate that the MOS dosimeter has numerous advantages over competitive devices by virtue of small size, low power drain, basic simplicity, wide dynamic range and low cost. The sensitivity at present (about 10- 3 V/Rad) is inadequate for radiological safety monitoring applications, but can probably t~e improved by several orders of magnitude.
References 1) W. J. Poch and A. G. Holmes-Siedle, RCA Engineer 16 (3) (1970) 56. ~), The IR-100 Awards, Industrial Research, New Product Annual, (Sept. 15, 1971) p. 38.
179
3) M. Rosenstein and R. H. Schneider, to be published. 4) D. H. Ciarlo, R. Kalibjian, K. Mayeda and R. A. Boster, IEEE Trans. Nucl. Sci. NS-19 no. 1 (1972) 350. 5) See, for example, B. Gross, J. Appl. Phys. 36 (1965) 1635; H. Kallmann and B. Rosenberg, Phys. Rev. 97 (1955) 1596; A. yon Hippel and E. S. Rittner, J. Chem. Phys. 14 (1946) 370; A. von Hippel, E. P. Gross, J. G. Gelatus and M. Geller, Phys. Rev. 91 (1953) 568. 6) A. G. Holmes-Siedle, unpublished work. 7) See, for example, A. S. Grove, Physics and technology of semiconductor devices (J. Wiley, New York, 1967). 8) K. H. Zaininger, RCA Rev. 27 (1966) 341. 9) See, for example, A. G. Holmes-Siedle and K. H. Zaininger, 1EEE Trans. Reliability R-17, no. 1 (1968) 34; K. H. Zaininger and A. G. Holmes Siedle, RCA Rev. 28 (1967) 208; C. W. Gwyn, J. Appl. Phys. 40 (1969) 4886; E. H. Snow, A. S. Grove and D. J. Fitzgerald, Proc. IEEE 55 (1967) 1168. 10) j. p. Mitchell, IEEE Trans. Electron Devices ED-11 (1967) 764. 11) W. Poch and A. G. Holmes-Siedle, IEEE Trans. Nucl. Sci. NS-16, no. 6 (1969) 227. 12) A. G. Holmes-Siedle and W. Poch, unpublished work. la) H. L. Hughes, R. D. Baxter and B. Phillips, IEEE Trans. Nucl. Sci. NS-19, no. 6 (1972) 256. 14) H. L. Hughes, Proc. IEEE Symp. on Reliability physics, Las Vegas (March 1971). 15) W. Dennehy, A. G. Holmes-Siedle and K. ZainingOr, IEEE Trans. Nucl. Sci. NS-14, no. 6 (1966) 276. 16) A. G. Holmes Siedle, I. Groombridge and C. Emms, Verhandl. DPG, (VI) 8 (1973) 813. 17) V. Danchenko, U. D. Desai and S. S. Brashears, J. Appl. Phys. 39 (1968) 2417. is) B. Gross, Phys. Rev. 107 (1957) 368. 19) R. Williams, J. Appl. Phys. 39 (1968) 3731. 9_o) G. Sigel, J. Phys. Chem. Solids 32 (1971) 2373.
THE
(1974)
I 6 9 - 1 7 9 ; © N O R T H - H O L L A N D P U B L I S H I N G CO.
SPACE-CHARGE
DOSIMETER
General principles of a new method of radiation detection A N D R E W HOLMES-SIEDLE J. J. Thomson Physical Laboratory, University of Reading, Whiteknights, Reading RG6 2AF, England Received 22 February 1974 and in revised form 10 June 1974 W]hen dielectrics under a high field are exposed to high-energy radiation, carriers are separated and space charge results. The change in potential at the dielectric surface can be measured non-destructively and such a change is initially linear with radiation dose. This provides a principle for radiation dosimetry not previously exploited, here named space-charge (SC) dosimetry. One compact, direct-reading form, based on the metal-
oxide-semiconductor (MOS) transistor, is described and the principles for improving this form by orders of magnitude are set out. The present level of sensitivity [measuring a fraction of a Rad (SiO2)] is well adapted for use in radiology, beam diagnostics, space radiation monitoring and other applications requiring close-spaced integrating dosimeters. With development, the method could be used in personnel radiation monitoring.
1. Introduction
v e r y s m a l l single d e t e c t o r f o r m e a s u r i n g p h y s i o l o g i c a l X - r a y e x p o s u r e 3) a n d s e c o n d l y t o a c l o s e - s p a c e d d e t e c t o r a r r a y in a n X - r a y s p e c t r o m e t e r 4 ) . T h e techniq u e c o u l d , in the future, a s s u m e w i d e r i m p o r t a n c e if, as e x p e c t e d , the sensitivity o f s p a c e c h a r g e (SC) dosim e t e r s is i n c r e a s e d to m a k e t h e m useful in the m R a d range. T h i s w o u l d o p e n u p a w i d e r a n g e o f r a d i a t i o n m o n i t o r i n g a n d safety a p p l i c a t i o n s . F u r t h e r m o r e , w h i l e fig. 1 gives a n i m p r e s s i o n o f t h e c o m p a c t n e s s a n d c o n v e n i e n c e o f the system, m o d e r n m i n i a t u r i s a t i o n a n d p r o c e s s i n g t e c h n i q u e s c o u l d still m u c h f u r t h e r r e d u c e the sizes a n d i n c r e a s e the s o p h i s t i c a t i o n o f the s e n s o r
I n 1970 P o c h a n d H o l m e s - S i e d l e 1) p u b l i s h e d a b r i e f a c c o u n t o f a n e l e c t r o n i c a l l y - m e a s u r e d , solidstate, i n t e g r a t i n g d o s i m e t e r s y s t e m o f g r e a t c o m p a c t ness a n d h i g h d y n a m i c range. T h e d o s i m e t r i c p r i n c i p l e u s e d was novel, n a m e l y the b u i l d - u p a n d n o n - d e s t r u c t i v e e l e c t r o n i c sensing o f space c h a r g e in the silicondioxide portion of a metal-oxide-semiconductor (MOS), insulated-gate, field-effect t r a n s i s t o r device. T h e n o v e l t y was r e c o g n i z e d in a n i n d u s t r i a l a w a r d in 1971 2) a n d t w o a c c o u n t s h a v e since b e e n g i v e n by o t h e r w o r k e r s o f t h e a p p l i c a t i o n o f this p r i n c i p l e first to a
Fig. 1. A metal-oxide semiconductor (transistor) dosimeter system, one-dosimeter arrangement. The sensor head is at the end of the black cable. The black dot with thin wires attached is a detached sensor head, being a small transistor package (2 mm on a side) which holds the dosimeter pellet, made from layers of silicon, silicon-dioxide and metal. The coin, a US "quarter" ($0.25) is 25 mm high. The volume of the silicon pellet is less than 10-a cm ~. The volume of the radiation-sensitive region, a thin film of silicon dioxide grown on the single-crystal silicon-wafer is less than 10-10 cm 3. 169
170
ANDREW
HOLMES-SIEDLE
head and sensing circuitry shown in this figure. For several applications, including the radiobiological field, the important point to note in fig. 1 is that the sensor head is minute and can be operated at the end of many feet of wire. In fact, only two strands of very thin conductor, operating at less than 10 V, are required. This paper discusses the general principles and the optimisation of the SC dosimetric method. As the preliminary publication 1) was in a journal rarely found in archives, some sections of this reference are repeated in an abbreviated form in the appendix.
2. Basic principles When an insulator is irradiated by high-energy radiation while under an applied electric field, carriers are excited to and flow in the conduction band, producing a photocurrent. Except in the rare case of a near-perfect crystal, these carriers fall into traps in significant numbers. Frequently, the numbers of electrons and holes trapped are different by orders of magnitude. The net result, as shown in fig. 2, can thus be either the "persistent internal polarisation" of the sample s) or the build-up of a sheet of electronic space charge which is not balanced within the insulator by opposite charges. In either case, a variety of electrostatic methods could be used to measure these space charges non-destructively. In many dielectrics, the charges will remain unchanged for very long times. The build-up of space charge will initially be proportional to the number of electron-hole pairs created but the accelerating field will be gradually reduced by the built-in field due to the space charge, and proportionality will be lost. Under the right conditions, however, the growth curve will deviate only slightly from linearity over several decades of dose during which
the space charge is of measurable magnitude. This linearity may be maintained even further by the simple circuit techniques to be described. This fact had not been realised before the present investigation1). The authors found that a useful range of simple dosimetric operation with adequately linear response can be obtained if some external manifestation of the space charge, such as surface potential, can be measured, and changes in this parameter are calibrated against dose; the device is thus a relative dosimeter. It is an integrating dosimeter which, if the insulator slab is thin, will be very insensitive to particle type or irradiation energy, so long as the energy required to create an electron-hole pair or other mobile carrier is supplied. While preliminary experiments have shown the principle to operate in several dielectrics, prepared as thin slabs6), the most attractive form appears to be the MIS dosimeter, in which the field at the insulator surface is sensed by means of the image charge set up in a semiconductor electrode in contact with it. While not ideal in geometry or material composition, the commercial MOS transistor fulfils many of the requirements for an SC dosimeter, since it comprises a readymade small volume of low-leakage trap-filled insulator, well sealed from light and moisture and having a builtin field-sensitive electrode which allows a convenient read-out of surface potential. The intrinsic disadvantages of the commercial MOS transistor are the relatively poor control normally achieved with respect to trapping centres in the oxide, the restriction at present to silicon dioxide as the dosimetric material (even though lower atomic weights are desirable, for the simulation of tissue) and the interfering effects of the Si/SiO2 interface statesT).
3. The metal-insulator-semiconductor (MIS) dosimeter
HIGH-ENERGY RADIATION
3.1. DEVICE STRUCTURE
EXPOSURE
STORAGE
MEASUREMENT
Fig. 2. Principle of measurement of space-charge in insulators (holes more strongly trapped than electrons). In the case of the MOS device, the electrodes are metal and silicon, and serve both as contacts during exposure and as the electrostatic sensing system.
MIS devices used in solid-state electronics usually comprise a single-crystal silicon wafer, on which is deposited or grown, first, an amorphous oxide film of thickness 0.1~0.2/~m, and, subsequently, an evaporated metal island. This MIS capacitor or MIS diode has been used mainly as a vehicle for research on field effects in semiconductors. The insulated-gate field-effect transistor (IGFET) or MOS transistor is a three-terminal elaboration in which the conductivity of a channel beneath the silicon can be modulated. Fig. 3 is an example of an I G F E T structure in which, for the present purpose, the metal " d r a i n " and " g a t e " electrodes, normally separate, have been formed in one
THE SPACE-CHARGE DOSIMETER
171
t metal } O-Ijum to admit beta particles of E< IOkeV tox = IOeum ,
//
'
y ¢ ,l:ili.iliil ,Hilji[iilly: Insulator: activ~ sensor I region,showing radiation-induced trapped charge
"
I
l~-Typeconducting region (inversion
region) produced by radiati0n-induced charge in oxide
NOT TOSCALE
Fig. 3. Proposed MOS dosimeter sensing element. piece. The most convenient parameter which can be measured in this device is the gate voltage at which the field-induced inversion channel reaches some chosen current level. This voltage is termed the threshold voltage, VT ; for MIS capacitors, the unique voltage at which the semiconductor energy bands are not " b e n t " at the surface, called the flat-band voltage, VFB, can also be measmed automatically with a direct-reading capacitance bridge and auxiliary circuitryS). 3.2. RESPONSE TO IRRADIATION It has been known since 1962 9) that the irradiation of a metal-insulator-semiconductor (MIS) transistor by ionizing radiation leads to a shift (A VT) in the threshold voltage of the device, as shown in fig. 4a. Fig. 4b shows that the effect may also be detected by capacitance-voltage measurements on a transistor or capacitor. In the latter device, shift is commonly measured at the flat-band condition yielding A VFB. The commonest form of MIS device employs an amorphous, t h e r m a l l y - g r o w n silicon dioxide film. Tile mechanism commonly accepted for charge buildup in this film is shown in fig. 5 9). The degree of shift of the device characteristics along the voltage axis (A Vx or A VFB), is controlled mainly by total dose D, expressed hereafter in units of Rad, implying Rad (SiO2) ; local area density of holes trapped in the oxide (Nox, c m - 2) ; the thickness of the oxide (to~,/~m); and the irradiation bias (V i, V) applied across the oxide from gate to semiconductor during irradiation. Some
approximate relations for silicon dioxide are: -- A I / v B
or - A V v -
t-°~-N°---~
2.1 × 10 l°
+-f(Ni) + g ( O
(1)
where Nox(Cm-2) is the net area density of positive charges, each of value one electronic charge, which has been trapped during irradiation in the space-charge region of the oxide (assumed here to be very near to the silicon); that is: (2)
qNox = AQo x = -AQsl,
where A Qox and A Qsl are, respectively, the amount of
ICnNPDT ~
--', p'~k\ (a)
\l / ,"AFTER IRRADIATION / 5 ,
I ~" - " /
~ETAL
/ ~RRAD,AT,ON~OX,DE = SEMICONDUCTOR ----
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_
+
!-
AVFB
.i
V~
//
=
//
(b)
+
1o
+
/
I'
"VT
"I
V¢
Fig. 4. Changes in operation of MIS devices, produced by ionizing radiation. ID = drain current, CINP~rT= capacitance. Other symbols explained in text.
172
ANDREW
HOLMES-SIEDLE
charge residing in the oxide and the amount of image charge produced in the semiconductor7). Typical values of Nox are 102' c m - 2 , which, in eq. (1), yields a value for A VvB of about 1 V in an oxide of 0.2 pm thickness; f(Ni) is an "interface state creation" term (if correct oxide growth conditions are maintained, Ni, the interface-state density, can be kept
l o w - l e s s than 101° states cm - 2 - and this term can often be neglected); g(t) is a trapped-charge leakage or "annealing" term which, again with correct oxide preparation conditions, can be kept sufficiently low that it does not interfere over the normally desirable integration times (1-300 d). The change of threshold voltage with irradiation is not linear but follows a saturating characteristic, described by t o)
m . . . . . TION
HIGH-ENERG~ ' PARTICLEOR
AVT = ~ V ~ ( I - e - ~ ° ) , 4ETAL
where c( and fl are constants depending on the trapping parameters and thickness of the oxide and the precise profile and location of the space charge. Both constants are complicated functions of N o , the local area density of the species in the oxide (presumably a defect) on which the positive charge resides and /Y is also a function of the lifetime-mobility product for the electrons in the oxide. It is not yet clear whether the saturation of A VT occurs because of saturation of all traps (Nox = N D in a given element, dx, of oxide thickness) or because electrostatic conditions in the oxide cause electron loss to cease. However, there is
EXI~ING HOLE TRAPS
NEWINTERFACE STATES
Fig. 5. Mechanism of charge build-up in an insulator under an electric field.
°°t o
(3)
/
/
//
IC
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#
A
7.9 (I-e-21D)
.<1
~/ TA 5631 FIXED VI=-9V A TA 5388 FIXED VI = -9V ® TA 5388 USING COMPENSATIONCIRCUIT
¢"
0-1 103
~
~
I
10 4
,
L
I
IO s
I
IO s
i
,
I
10 7
i
IO s
DOSE O (Rods) Fig. 6. Shifts in threshold voltage (A VT) vs radiation dose (D). Curve A = theoretical relation for d V~ vs D with constants as shown; curve B = linear relationship between A VT and D, chosen to fit the data shown as circles. Triangles: data from irradiation of R C A developmental p-channel MOS transistors TA5388 and TA5631 at fixed irradiation bias, VI, o f - 9 V. Circles: irradiation o f TA5388 in compensation circuit as in fig. 7.
173
THE S P A C E - C H A R G E DOSIMETER
indirect evidence that an increase of ND leads to an inc,rease in A QoX under the same electrostatic and irradiation conditions6). Curve A in fig. 6 shows the growth of A VT for one of the more radiation-sensitive forms of commercial M][S device geometry, namely a p-channel transistor of which the gate oxide is formed from about 1000/k of steam-grown silicon dioxide overlaid by about the same thickness of phosphosilicate glass (doped organosihme method) 11). It can be seen that, for dose values up to about 10 4 Rad, the equation does not deviate greatly from a linear dependence of A VT against D, namely: A V T = K D . K, of course, will serve as a sensitivity constant for the dosimetric system. Experiment 11) confirms that MOS devices irradiated at a constant negative value of VI follow this curve closely and some data points are shown on the curve. Ciarlo and co-workers 4) also found adequately linear behaviour in MOS devices under constant positive bias. In the constant-bias mode, the dynamic range of the device as a dosimeter of linear response with dose (desirable in a direct-reading application) is thus bounded at the upper end by the " k n e e " of the curve for eq. (3), representing the incipient saturation of the charge build-up. At the lower end, the minimum detectable shift in VT or VFB provides the limitation. Both eq. (3) and experiment show that saturation is delayed at higher values of applied bias, VI. (Strictly, the equation implies that, for the same structure, higher changes in VVB are achieved at the same dose, without alteration to the degree of sublinearity exhibited.) This being so, we can extend the useful range of linearity by usJ[ng a characteristic of the MOS transistor demonstrated in fig. 7a. If the gate is shorted to the drain and the source-drain circuit is put in series with a battery (w)ltage E) and a large limiting resistor (RL), VI automatically increases as VT becomes more negative1). As fig. 6, curve B shows, with the correct device structure and the correct values of E and RL, linearity can, in fact, be extended for well over a decade further in dose. Moreover, in this "compensation circuit" configuration, the voltage drop across the channel, V¢h, is also a measure of the threshold shift, VT, hence: A Vch = AA VT
=
KD,
circuit only operates using negative gate bias in p-channel devices. For an increase in range, a sacrifice in sensitivity is thus made, since, according to Mitchell's theory for single dielectrics i 0), positive values of V~ give the greatest sensitivity. Thus the choice of biassing scheme must be based on the values of dose range which are to be measured, the requirements for simplicity in the circuit, the procedure for measurement and the nature of the dielectric chosen. 3.3. PRINCIPLES FOR OPTIMIZATION AS DOSIMETER
3.3.1. Electrical parameters To optimise with respect to sensitivity, the sensitivity constant, K of eqs. (4) and (5), has to be made as large as possible. A desirable, though arbitrarily chosen figure is K = 10 V/Rad. For readily available MOS transistors (tox~0.1 #m, ANox~ 1012 c m - 2 ) , K is about 10 -2 V/Rad. A dosimeter optimised for high sensitivity would possess an increased value of K,
i SE/O OIR l'NSS
E--
(a)
T
' SM I PLF IE IC D R I CUT I
__J~MOS SENSOR (b)
ZE OST A DR JU
(5)
where A is a constant which can be made near to unity by proper adjustment of RL. Vch can be read directly with a voltmeter, hence this linearity of A Vch vs dose allows the device to be used as a direct-reading dosimeter. A practical arrangement for this compensation circuit is shown in fig. 7b. Unfortunately, this compensatory function of the
DO I DE ZENER
_-
Fig. 7. Typical compensation and read-out circuit for p-channel MOS device. (a) simplified form, (b) as used in dosimetric experiments.
174
ANDREW
HOLMES-SIEDLE
obtained by increasing tox and Nox to their practical limits. These limits are discussed below. 3.3.2. Oxide thickness limits There are electrical-measurement reasons why to~ cannot be increased indefinitely: for a transistor of initial threshold voltage of zero, the ID--Va characteristic is determined 6) by the equation I D = k V 2 - /~seoxWV2 + I t ,
to×L which can also be expressed in a different form: 2/~seox WI/~ gm
--
toxL
where I v = source-drain current, I e = source-drain leakage current independent of
vo, p~ = "surface majority carrier mobility" in the channel, cox = dielectric constant of the oxide layer, L = source-drain spacing or channel length, W = width of channel, k
= Itseox W/(toxL),
gm = transconductance = dlo/d V c . Thus, for a given insulator type (i.e. fixed/~s, eox) and device geometry (namely fixed W / L ) , a limit on the value of tox is set by the requirement for ID to achieve values which are detectable above the constant, but low Ie, when the gate voltage is still (a) below the breakdown voltage of the oxide, and (b) at a level which meets the practical requirements of the supply voltage (E in fig. 7a). Similar requirements apply to detection of shifts in V w by means of capacitance measurements: the ultimate accuracy possible in measuring small values of A Vv in a transistor, while not, in simple theory, dependent on the value of gm for the device, may indeed be controlled by this due to the above leakage currents. Moreover, if the value of gm is too low, the linearity of the A VT VS dose relation at low values of dose will also be affected by radiationinduced changes in leakage current and i n / ~ , brought about by the creation of new interface states (see fig. 5 and ref. 9). Holmes-Siedle and Poch ~2) have shown by experiment that MOS dosimeters with thick films of thermal silicon dioxide are, as eq. (1) would predict, more sensitive in terms of shift (A Vx) per Rad. Moreover, although the thick (1.5/tm) and thin (0.15 llm) oxides were grown on the same silicon substrate, the effect was
greater than predicted by eq. (1), possibly because of impurity effects, discussed in the next section. Thus the limitation in thickness of the oxide would appear to be that discussed earlier in relation to the accuracy of the measurement of VT or VvB. The feasibility of using oxides of 1.5/~m thickness has been proved 12) and, judging by these experiments, the limit for detectability of Vv or Vv~ may well be in the region of 10/~m. Thus the sensitivity of the devices used in previous studies on linearity with dose 1'3'4) (tox about 0.1/~m), can be increased by two orders of magnitude by virtue of a dimensional change alone, which change can be achieved rather simply, for example, by prolonging the growth of the oxide in steam or adding a deposited dielectric layer to the thermally-grown oxide. 3.3.3. Optimisation of,space-charge density The exact nature of the species which acts as a trap for holes has not been demonstrated, although it is assumed widely that it is a structure such as a nonbridging oxygen atom 9) and that the space-charge density (No0 in some way controlled by the density of traps (ND), especially at saturation of A VT 9). In this discussion, we will thus assume that increase in No in a given region of the oxide will lead to an increase in A Qsi for a given dose, provided the correct conditions for net hole trapping exist in that part of the oxide (i.e. low electron density, see ref. 10). As explained in ref 9, positive irradiation bias leads to positive space-charge build-up near the silicon, a condition which gives strong imaging of that charge in the silicon. Per contra, the opposite bias gives positive space-charge near the metal, with consequently weak imaging in the silicon. Nevertheless, because of requirements for maximum extension of linearity (see section 3.3.1) the gate bias and silicon polarity used in the author's studies 1) was such that the charge sheet was built up near the metal electrode. For this mode, a high value of No near the metal electrode is clearly required and conditions should favour the growth of the space-charge region as far as possible towards the semiconductor. On the other hand, greater sensitivity can be obtained by using a positive gate bias and, in this case, the space charge layer should be confined as closely as possible to the region near the silicon. Hence, a high value of No within a few hundred Angstroms of the silicon is desirable, the value of No in the rest of the oxide being relatively unimportant. It can thus be seen that the preparation of an insulator film for dosimetric purposes must be carried out with a particular mode of operation in mind. Very recently, Hughes 13) has produced good correla-
THE S P A C E - C H A R G E
tions between charge build-up and alkali ion content of o~:ide films, which suggest that the charge sheet producing the flatband voltage shifts is not due to traps intrinsic to the amorphous S i C 2 network but due ekher to hole traps located on non-mobile alkali-metal impurities concentrated near to the semiconductor or even to the migration of sodium ions themselves, after they have been made mobile by the radiation.* There is thus a possibility that No can be strongly increased by judicious diffusion or implantation of alkali ions, followed by heat treatment to produce the correct chemical binding of these atoms to the SiC2 network. The author and co-workers had already found t 5) that values of Nox as large as l0 t 3 c m - 2 could be produced by trial-and-error methods in steam-grown S i C 2. Artificial doping could presumably increase Nox by an order of magnitude. For reasons not well understood, aluminium ion implantation has been found to enhance the VT shift for oxides biased negatively during irradiation 14'16) while depressing the shift under positive bias. A similar effect was also found to be produced by superimposing a second insulator layer on the grown oxide ~1,14-16). These methods can be used to increase the sensitivity of the negative-bias mode. 3.3.4. Temperature effects The trapped positive charge in MIS devices made from thermal SiC2 is completely annihilated by heating to 250-300 °C for a few hours9). This effect can be-an advantage or a disadvantage for a dosimetric system depending on the ambient in which it will be used. For example, the S i C 2 device in its present form could not be used in radiation beams which produced heating above about 150°C without some loss of sensitivity. On the other hand, as the appendix shows, a baking treatment will restore this form to the unirradiated state for re-use. The thermal annealing phenomenon appears not to be a thermal depopulation ot' holes (this probably requires a far higher temperature) but rather a thermally-activated injection of electrons from either interface ( E a c t ~ l eV) via interface states l v); in other materials, the temperature for annealing could thus be very different. The limitations on re-use after heavy irradiation are probably set by the creation of unannealable bulk defects in the * I f the latter case is true, then the concept of a sheet of trapped holes of concentration Nox, can be replaced by an effective concentration o f positive ions, Nl, activated by the irradiation, located at an effective distance, xi, from the semiconductor. The treatment to increase sensitivity remains similar in several but not all respects:
175
DOSIMETER
silicon, which could be significant for particles (for example a fluence of 1015 protons/cm 2 at 10 MeV could increase source-drain diode reverse leakages by an order of magnitude). Some types of oxide undergo a slight room-temperature a n n e a l i n g - an effect which would have to be avoided for long integration times (see section 3.2). 4. Some SC dosimeter designs 4.1. OPTIMISEDMIS DOSIMETER The considerations outlined in the previous section can be embodied in the following optimum form of MIS dosimeter, which is that shown in fig. 3. The gm value of this device is such that it would achieve an Io value of 3/~A at a V~ value of 10 V, thus leakage currents must be kept well below this level. 2 × 1 0 L 4 c m -2, tox = 1 0 p m , L = 7.5/~m. No =
k = 3x10-7/~V, W= 2.5ram,
To estimate sensitivity constant K, we will assume linearity to 104 Rad, followed by complete saturation. Combining eqs. (1) and (4) we obtain K -
tox Nox 2 × 101°D"
Using the value for N D in place of Nox, we obtain a sensitivity factor of 10 V/Rad. Using sensing circuitry which would measure changes of 5× 10 - 3 V, the smallest dose measurable would be l0 -4 Rad which makes the device potentially useful in the radiationsafety monitoring field. The m a x i m u m dose measurable would not, in this case, be controlled by saturation of traps by holes or by field conditions within the oxide; already, at 10 2 Rad, the value of A VT would be 100 V, which is probably near the practical limit at which leakage currents and breakdown in the device or noise in the sensing circuitry would obscure the level of the currents in the channel. While it may be difficult to produce a suitable dielectric film of thickness l0/~m by thermal growth, even in steam, it should be acceptable to use a thinner layer of thermal oxide (say 1/~m) and deposit another dielectric upon this to the required thickness. The results should follow the favourable trend for two-layer devices mentioned in section 3.3.3. 4.2. MIS DOSIMETERWITH SEPARATEINSULATOR By abandoning the mechanical convenience of depositing or growing an insulator film in intimate contact with the semiconductor, freedom is acquired
176
ANDREW
HOLMES-SIEDLE
to modify the atomic weight and trap structure of the insulator. A form of dosimeter in which the insulator is separated from the semiconductor is shown in fig. 8. Since the surface of the semiconductor must be controlled with respect to semiconductor surface states, a film of silicon dioxide must still be thermally grown to "passivate" the surface, but it can be made sufficiently thin so that imaging of charge stored in the lower face of the detached insulator slab is efficient and so that radiation-induced charge build-up in the grown film does not interfere. The latter effect can, of course, be disposed of if the slab is irradiated between two metal electrodes and only presented to the semiconductor for measurement purposes. The practical disadvantages of this method lie mainly in alignment and in the interfering effects of "contact electrification" and leakage incurred in any use of a demountable capacitor l s). The main advantages lie in the greater possibilities of varying the insulator, especially the use of materials of high atomic number with greater stopping power or materials which better simulate biological tissue, say boron or beryllium compounds. 4.3.
OTHER FORMS OF DOSIMETER NOT INVOLVING A SEMICONDUCTOR
As mentioned, the semiconductor portion of fig. 6 can be replaced by any metal electrode for application of field during irradiation. For sensing, an electrometer system other than the semiconductor surface can be used. Gross 18) has used a simple demountable capacitor for measuring charges trapped in dielectric slabs,
ACHABLE JLATORAND ALGATE
~/ASSIVATIOR iii!lrrnl,lll~l
;ON SURFACE
(O.Ol.,um,
m iii1[i MiglJlllllll fl]lrlqlM~l hllllll!~ ill~lllll]~]llllm $[11 illlllLmll n
',
FIELD[NOU ' CED CONDUCTIONREGION (~NANNEC)
~
METAL
[ITTFrTTnINSULATOR P-TYPEDIFFUSION [--"~'-] N-TYPE SILICON SUPPORT
Fig. 8. Design o f MIS dosimeter with separated insulator.
while, in preliminary experiments, the author has found a vibrating-capacitor method for measuring surface potential~ 9) to be suitable. A wide range of variation is possible for this configuration of dosimeter. One requirement for linearity will be a minimum amount of production of new traps by the radiation itself, since this may lead to superlinearity. This requirement would presumably eliminate materials undergoing strong radiolytic effects such as the alkali halides and many organic materials. However, Gross 5) and Kallman et al. 5) have observed early-time linearity of charge build-up in several organic materials, glasses and inorganic powders. Based on present knowledge 12, 18, 2o) alkali-containing oxide glasses should meet the requirements of variability of dopants, high trap content and ruggedness particularly well. 5. Discussion
While the advantages of the space-charge build-up system are fairly clear, it need not be supposed that further developments will be without some pitfalls. The MIS system has always presented problems of interface effects and of uncontrolled impurity effects, both of which must be rigorously controlled in the dosimetric device; in fact, techniques of controlling hole-trap density in insulators are as yet only in their infancy, but are developing14-16). The insulator is prone to electrical breakdown, even from stray electrostatic build-up on metal electrodes which are not grounded and this effect would have to be considered when designing for intense charged-particle beams. The MOS dosimeter device has, however, been used with success in 0.01-1.5 MeV electron beams, 0.5-20 MeV proton beams and fission reactors6). All the forms of SC dosimeter described require that a large difference exists between the effective mobility of holes and electrons in an insulator. In fact, in insulators, it is the holes which are commonly of very low mobility due to self-trapping effects, and the form investigated by the author indeed involved positive space-charge sheets. However, the sensing method could be applied equally well to charge sheets due to trapped electrons and to excess charge generated by processes other than differentials in mobility, for example Compton scattering 5) or absorption of non-penetrating ions and electrons. Thus special methods could be developed for the dosimetry of ion l=eams or gamma-rays. A particular advantage of the principle for laboratory radiation beams is the smallness of the sensitive element, making possible the mapping of a beam at an array of
THE SPACE-CHARGE
points simultaneously. The sensor array can be made very dense, using existing techniques of integrated circuitry and even possibly self-scanning. The possible applications thus cover a very wide range, for example from small single dosimeters which automatically trigger an alarm when a threshold dose is reached to very dense arrays which can image a radiation source of any energy from the band-gap energy to several millions of electron volts.
6. Conclusions A broad dosimetric principle, which has not been exploited before, is described and the name Space Charge Dosimetry is proposed for the principle. One embodiment of the principle is the metal-insulatorsemiconductor version and some designs are described. The main advantage of the method with respect to other relative methods of measuring integrated dose, for example thermoluminescence dosimetry, are the potentially great compactness (the sensor element can be as small as 10 - 4 cm-3), the possibility of remote, continuous and non-destructive electrical readout and the wide range of materials in which the charge build-up may be produced and possibly enhanced by doping or defect introduction. In the MOS dosimeter form, linearity, dynamic range and reproducibility are shown here to be as good as, or better than other solid-state integrating dosimetric systems such as thermoluminescent, fluorescent and colorimetric systems. The sensitivity of the first crude devices tested is comparable with the above systems while improvements in sensitivity by several orders of magnitude appear possible. The method is very well suited for beam diagnostics, radiographic research and space radiation monitoring. An optimised design is proposed, of a sensitivity such that less than 1 m R e m of ionising radiation could be sensed. Thus applications in radiation safety work are ultimately possible.
177
DOSIMETER
part of the complementary-symmetry (CMOS) circuits on the R C A TA5388 series of integrated circuits. Several compact, self-contained, working models of the dosimeter were built, some suitable for installation in a spacecraft electronics bay. Two 9 V transistor radio batteries could be used as an internal source of power. The length of cable connecting the transistor to the control unit was not critical and lengths up to 100 ft were used without adverse effects. A switch on the side of the enclosure changed the sensitivity of the instrument; the full-scale reading on the meter could be adjusted to 20 Rad, 100 R, or 500 Rad. Fig. 9 shows the result of irradiating a single pchannel transistor connected as in fig. 7. The same transistor was irradiated four times, followed by an annealing procedure after each irradiation. This procedure, which included an overnight bake at 250 °C restored the transistor to nearly its original condition. The value of A VT decreased somewhat following the first irradiation but there was almost no change between the third and fourth test runs. Also, it can be seen that the curves corresponding to the four runs are essentially parallel and close to being linear. The dosimeter system provides a convenient method for calibrating one radiation source against another under a particular set of unusual conditions. For example, it was desired to know the ionizing dose absorbed in silicon integrated circuit pellets when encapsulated in the small metal "can s" used in semiconductor packaging. A comparison was required between two radiation sources, an 85 keV X-ray generator and a 6°Co gamma-ray source. The amount of attenuation due to the can and the nature of the secondary-electron bombardment generated by the
o
RCATA 5388
l: I-0
7fRuNs
t,.
Appendix The original description of a working MIS dosimeter system was published in R C A Engineer 1), a journal not widely found in archives. Thus, a greatly abbreviated account of the experimental data given in that article is given here to clarify the more general discussion given above. The M O S transistor. A large part of the development work on dosimeter circuits was done with two commercially available devices- the M E M 520 transistor, a discrete p-channel device made by General Instruments; and the p-channel transistors which form
o~ 0.1
~// ¢t:
O-OI 0'1
H°.3 ~, 4
RUNNo2/~-~/INITIAL THRESHOLDVOLTAGES ,,/'t~ RUN No.I. 3-773 /"~ RUN No.2.4 . 4 9 4 /// RUN No.3.4-437 / / RUN No.4. 4-440
8S KeY X RAYS
../_ ............ 1.0 IO I00 EXPOSURETIME IN'MINUTES
Fig. 9. R e s p o n s e o f p-channel M O S transistor (part o f the R C A T A 5388 device) irradiated in the c o m p e n s a t i n g circuit configuration o f fig. 7. Between each r u n as m a r k e d , the device was heated for several h o u r s in an air oven at 250 °C.
178
ANDREW
HOLMES-SIEDLE
can differs widely in the two cases. However, unlike larger dosimeters, the MOS dosimeter could be mounted inside an identical can and its response monitored directly. When the transistor used to obtain the data in fig. 9 was annealed by heating and exposed to the calibrated 6°Co source at Forth Monmouth, N.J., the resulting dose vs threshold-shift curve was essentially parallel to the curves obtained with the X-ray generator. The 6°Co curve and the curves of runs 3 and 4 in fig. 9 could be made to coincide if the dose corresponding to a 10 min exposure in the X-ray machine under the specified conditions was taken to be 104 Rad. An important feature of the circuit, as shown in the main text, is that the dose-indicating voltmeter can be electrically reset to zero after each exposure. This assumes that the dose being measured is only a fraction of the total dose that the MOS transistor can absorb without reaching the limit imposed by saturation effects. For example, if the dosimeter were used to measure a series of dose increments of 250 Rad each, then 160 successive exposures would be required to reach the 4 x 104 Rad level (the upper limit of the curves in fig. 9). The incremental change in threshold voltage of about 0.010 V for each dose increment of 250 Rad would remain nearly constant. The gain of the operational amplifier under these circumstances could be adjusted to provide exactly 2.5 V at the output so that a digital voltmeter could then provide a direct reading in Rad. Recent experiments have indicated that the essentially linear portion of the dose versus threshold-shift curves, as shown in fig. 9 can be extended to at least 1.5 x l0 s Rad, and perhaps to even higher levels. Thus, the number of successive 250 Rad exposures mentioned previously could be extended from 160 to over 600. When the MOS tlansistor begins to show serious non-linear response due to saturation effects, it can be subjected to the annealing procedure mentioned. Sensitivity considerations. Ideally the sensitivity of this type of dosimeter could be extended indefinitely by simply increasing the gain of the operational amplifier. However, a number of factors impose an upper limit on the useful gain: 1) the basic stability of the MOS transistor, 2) the sensitivity of the transistor to temperature changes, 3) the constancy of the voltage supply, and 4) the stability of the other circuit components. When using the system with maximum amplifier gain (20 Rad full scale), the resulting instability in the output reading was approximately 5% of full scale.
An application particularly well suited to the capabilities of the dosimeter in its present state of development is the in-flight measurement of the radiation dose accumulated by spacecraft components while actually orbiting in the Van Allen regions. The small size, low power drain, adaptability to conventional telemetry systems, and basic simplicity are major advantages of this system over any other dose measuring system. Also, the range of dose levels involved in space applications are well within the capabilities of presently available MOS transistors. A more sensitive version would, for the same reason, be very well adapted for actively monitoring integrated doses in the new components of aircraft or other vehicles exposed to radiation. Mapping the beam from an X-ray, gammaray, or particle accelerator is another application that fits the capabilities of the MOS dosimeter extremely well. The distribution of the beam intensity close to the window of an X-ray generator and within the beam of a particle accelerator was very rapidly mapped with an array of detectors. An attempt to map the beam of the same X-ray generator with small commercial LiF thermoluminescent dosimeter (TLD) rods demonstrated the superiority of the MOS dosimeter for this application. With the latter device, measurement of the dose at a particular location could generally be repeated within 1 or 2% variation. With T E D rods, the variation was usually 10% or more. in addition, the time required for the complete test was considerably less when using the MOS device. Under certain circumstances it may not be feasible to provide an electrical connection between the sensor element and the control unit. Dose measurements can still be made, however, by using certain types of MOS transistors which accumulate a reduced but usable MEM 520
I0-0
Vz:O
RUN.N o . I . ~ ~"
r.0
>o 0q
/
RUNN~/J_.~RUN No.3
COBALT-60 GAMHARAYS
o=
o.oj
iOs
I0 4
IO ~
IO 6
RADS
Fig. ]0. Response of p-channel MOS transistor (General I n s t r u m e n t s M E M 520) irradiated at V1 = 0 (gate shorted to source a nd drain). H e a t t re a t me nt between runs as for fig. 9.
THE S P A C E - C H A R G E DOSIMETER
anaount of trapped charge even in the absence of an applied electric field. Fig. 10 shows how the threshold ve,ltage shift of a sample of the M E M 520 device varied when irradiated without bias. Under the " n o - b i a s " condition, the threshold shift versus dose relationship became non-linear at about 104 Rad. However, higher doses or dose increments could be determined from calibration curves of the type shown in fig. 10. After one or two exposure and annealing cycles, as for fig. 9, threshold shift versus dose relationship became suitably reproducible from run to run. Sensors of this kind can, therefore, be used in the same manner as T L D devices to measure the accumulated dose at locations where it would be impracticable to provide an electrical connection. The experiments described indicate that the MOS dosimeter has numerous advantages over competitive devices by virtue of small size, low power drain, basic simplicity, wide dynamic range and low cost. The sensitivity at present (about 10- 3 V/Rad) is inadequate for radiological safety monitoring applications, but can probably t~e improved by several orders of magnitude.
References 1) W. J. Poch and A. G. Holmes-Siedle, RCA Engineer 16 (3) (1970) 56. ~), The IR-100 Awards, Industrial Research, New Product Annual, (Sept. 15, 1971) p. 38.
179
3) M. Rosenstein and R. H. Schneider, to be published. 4) D. H. Ciarlo, R. Kalibjian, K. Mayeda and R. A. Boster, IEEE Trans. Nucl. Sci. NS-19 no. 1 (1972) 350. 5) See, for example, B. Gross, J. Appl. Phys. 36 (1965) 1635; H. Kallmann and B. Rosenberg, Phys. Rev. 97 (1955) 1596; A. yon Hippel and E. S. Rittner, J. Chem. Phys. 14 (1946) 370; A. von Hippel, E. P. Gross, J. G. Gelatus and M. Geller, Phys. Rev. 91 (1953) 568. 6) A. G. Holmes-Siedle, unpublished work. 7) See, for example, A. S. Grove, Physics and technology of semiconductor devices (J. Wiley, New York, 1967). 8) K. H. Zaininger, RCA Rev. 27 (1966) 341. 9) See, for example, A. G. Holmes-Siedle and K. H. Zaininger, 1EEE Trans. Reliability R-17, no. 1 (1968) 34; K. H. Zaininger and A. G. Holmes Siedle, RCA Rev. 28 (1967) 208; C. W. Gwyn, J. Appl. Phys. 40 (1969) 4886; E. H. Snow, A. S. Grove and D. J. Fitzgerald, Proc. IEEE 55 (1967) 1168. 10) j. p. Mitchell, IEEE Trans. Electron Devices ED-11 (1967) 764. 11) W. Poch and A. G. Holmes-Siedle, IEEE Trans. Nucl. Sci. NS-16, no. 6 (1969) 227. 12) A. G. Holmes-Siedle and W. Poch, unpublished work. la) H. L. Hughes, R. D. Baxter and B. Phillips, IEEE Trans. Nucl. Sci. NS-19, no. 6 (1972) 256. 14) H. L. Hughes, Proc. IEEE Symp. on Reliability physics, Las Vegas (March 1971). 15) W. Dennehy, A. G. Holmes-Siedle and K. ZainingOr, IEEE Trans. Nucl. Sci. NS-14, no. 6 (1966) 276. 16) A. G. Holmes Siedle, I. Groombridge and C. Emms, Verhandl. DPG, (VI) 8 (1973) 813. 17) V. Danchenko, U. D. Desai and S. S. Brashears, J. Appl. Phys. 39 (1968) 2417. is) B. Gross, Phys. Rev. 107 (1957) 368. 19) R. Williams, J. Appl. Phys. 39 (1968) 3731. 9_o) G. Sigel, J. Phys. Chem. Solids 32 (1971) 2373.