- Email: [email protected]
Radiation immunity of pMOSFETs and nMOSFETs examined by means of MeV He single ion microprobe
ap~:~o surface science ELSEVIER
Applied Surface Science 104/105 (1996) 364-368
Radiation immunity of pMOSFETs and nMOSFETs examined by means of MeV He single ion microprobe M. K o h a , * K. Horita a B. Shigeta a T. M a t s u k a w a a A. Kishida a T. Tanii a S. Mori a I. O h d o m a r i a.b School of Science and Engineering, Waseda UniL'ersity, 3-4-10hkubo, Shinjuku-ku, Tokyo 169, Japan b Kagami Memorial Laboratory for Material Science and Technology, Waseda Unit,ersity, 2-8-6 Nishi-waseda, Shinjuku-ku, Tokyo 169, Japan Received 28 June 1995; accepted I I September 1995
Abstract
Radiation effects induced by MeV He single ions in pMOSFETs and nMOSFETs in commercially available CMOS4007 have been studied extensively. The key results from this study are: (1) pMOSFETs are more fragile than nMOSFETs in terms of threshold voltage shift, (2) nMOSFETs are more susceptible than pMOSFETs to the degradation of subthreshold swing. The different features in the radiation effects have been discussed comprehensively.
1. Introduction
In order to optimize device structures or processes for radiation hardness of CMOS devices it is inevitable to investigate radiation effects in both pMOSFETs and nMOSFETs fabricated in a same chip quantitatively. So far radiation hardening has been done based on the data obtained by irradiating a whole IC chip with energetic particles randomly [1-5]. However, the information obtained in this way has not been quantitative in the sense that identifying a site dependence of radiation immunity has never been possible. Random irradiation with energetic particles would induce also the so-called lateral
* Corresponding author. Tel.: + 81-3-52863380; fax: + 81-352725749; e-mail: [email protected].
non-uniformity effects, i.e., the influence of non-uniform distribution of trapped charges on device characteristics, which will obscure the proper interpretation of radiation effects [6,7]. So far we have succeeded in developing the single ion beam induced charge (SIBIC) imaging by using the single ion microprobe (SIMP) system which has made it possible to hit a local area of less than 3 /_tin square in a target with MeV He single ions one by one [8-11]. The SIBIC imaging enables us to 'observe' the feature of MOSFET without degradation of device characteristics. The sample feature observation before any tests is very important for the quantitative analysis of site-dependent radiation effects in MOSFETs. In this study, we have investigated site dependent radiation effects in CMOS devices induced by high energy He single ions and comparing the difference in pMOSFETs and nMOS-
0169-4332/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S01 6 9 - 4 3 3 2 ( 9 6 ) 0 0 172-9
M. Koh et al. / Applied Surface Science 104 / 105 (1996) 364-368
FETs. We first introduce an experimental setup and the method of sample positioning. Then we describe the difference in ion irradiation effects in the characteristics of pMOSFETs and nMOSFETs in a test CMOS chip.
2. Experimental A schematic diagram of the SIMP system is illustrated in Fig. 1. The beam line is connected to a tandem accelerator (NEC 5SDH) with a terminal voltage of 1.0 MV. As shown in Fig. 1, each single ion is extracted one by one from a continuous ion beam by chopping. If the ion current, the slit opening and the switching velocity are set that after chopping several times only one ion may pass through the slit, single ion can be successfully extracted. Extracted single ion is focused with a high precision quadrupole magnet and irradiated within an area corresponding to the micro beam spot size. Number of the incident ions is controlled by an electron multiplier tube (EMT) by detecting secondary electrons (SEs) emitted upon ion incidence. Signals from the EMT are fed to the chopper controller which keeps on sending the beam chopping signals to the deflector until the desired number of single ions are detected. A device to be irradiated is mounted on a high precision goniometer, which is movable in three directions under the control of a host computer. I - V character-
365
istics of MOSFETs can be measured in situ by combining HP4140B system with the SIMP. In this study, I - V characteristics were recorded before exposure and after irradiation of a certain number of ions. Throughout this work, 2.0 MeV He single ions have been used. The number of irradiated ions has been varied from 0 to 102//zm 2. The irradiated area was 240 X 240/zm 2. During irradiation all pins were grounded. A commercial inverter CMOS TC4007UBP produced by Toshiba, which contains three pMOSFETs and three nMOSFETs, was used as a test target in this study. Fig. 2 shows an optical micrograph of the CMOS4007 after removal of plastic package with heated fuming acid. Since this sample is made for commercial purpose, detailed information on device structure, e.g. oxide thickness and doping concentration are not clear. The radiation tests have been performed on each FET. Sample positioning was performed by using the SIBIC imaging method. Fig. 3 shows a coarse SIBIC image of the entire chip which consists of 16 X 16 pixel array. The scanned area was 800 X 800 /xm 2 and the number of incident ions per pixel was only two. The image clearly reveals each MOSFET. After the coarse imaging, much clearer SIBIC image of the desired transistor was taken for higher precision sample positioning. The radiation damage during these SIBIC imaging is negligible for the subsequent
Tr.n.,°r ,,n* .,gh prec,.io. /I P'CI'T
o,,eot,,,. I
,.,,.ctor
"'"
j..--
r
I
l
if ....
"'"°Ul]II
il"""'l
chopper controller circuit
Fig. 1. Schematic drawing of the experimental system.
/
I I N'°'"
L_J
366
M. Koh et al. /Aplflied Surlace Science 104 / 105 (1996) 364-368
!
~
. . . . . . . . . . °~g, ,+, <, o
Nm ,,
.[
200 ~ m Fig. 2. Optical micrograph of the tcst device.
analyses. The analysis of the ion irradiation effects in the MOSFETs has been done by repeatedly taking the SIBIC images. Details of sample positioning using the SIBIC imaging are introduced in the other literatures [10,1 I].
3. Results and discussion in this section, we describe the different responses of pMOSFETs and nMOSFETs in the test CMOS4007 (Toshiba) against ion irradiation. Fig. 4 shows the threshold voltage shift AVth for both pMOSFETs and nMOSFETs as a ['unction of ion dose at room temperature. The threshold voltage was defined as the zero current intercept of square root of the l~ versus V, curves in the saturation mode. The threshold voltage Vth shfl'ted to the negative direction for both pMOSFETs and nMOSFETs proportional to the number of He ions. There are two major causes in generation of radiation induced interface charges which are responsible for Vth shift. They are oxide trapped holes and trapped charges by interface states [12]. The slope represents fragility of the target devices against ion irradiation. Since the slope for pMOSFETs is steeper than that for nMOSFETs, pMOSFETs are more fragile than nMOSFETs in the threshold voltage shift at room temperature. The reason for the steeper slope, i.e., larger AV,1 for pMOSFETs, can be explained as follows. It is known that the net ,AVth is the algebraic sum of voltage shifts due to the trapped holes AVN,,t and the trapped charges by the interface states AVN~~ [13]. It also should be noted that the Pb-centers responsible for the interface states have amphoteric nature. They are donors in the lower part of the bandgap and acceptors in the upper part of the bandgap. Their occupancy depends on the value of the surface potential. The donor-like interface states are responsi-
Dose
0
25 F
-10(
L ~ "
[ions/IJrn
50 i
2]
75 i - O - n-ch ~ p-ch
00
:~ -200 - -300 400 \., 500 ~ m
Fig. 3. SIBIC image of the entire chip which is composed of 16 × 16 pixels. The scanned area is 800 × 800 gin-'.
~00
~
~
Fig. 4, Threshc,ld voltage shift A~h for both pMOSFBTs and
nMOSFETs induced by ion irradiation.
M. Koh et al. / Applied Surface Science 104/105 (1996) 364-368 6.0
i
i
I
--C-- n-oh
5.0
<[:;:! ~"
-D-P-c/
4.0
3.0
1.0
0.0 0
25 Dose
50
75
100
[ I o n s / ~ m 2]
Fig. 5. Subthreshold swing shift AS for both pMOSFETs and nMOSFETs induced by ion irradiation.
ble for the VN~t shift of pMOSFETs, while the acceptor-like interface states are responsible for the WNit shift of nMOSFETs. Therefore AVNit is negative for pMOSFETs and positive for nMOSFETs, while AVNot induced by trapped holes is always negative [14]. For pMOSFETs, sign of AVNit is the same as AVNo t, which causes larger AVth. For nMOSFETs, however, it is expected that the opposite signs of mWNi t and AVNot result in a relatively small shift in Vth. Thus pMOSFETs are more fragile than nMOSFETs in the threshold voltage shift. Fig. 5 shows the subthreshold swing shift AS for both pMOSFETs and nMOSFETs as a function of ion dose. The subthreshold swing is defined as the change in gate voltage necessary to reduce transistor current by one decade. It is well known that the subthreshold swing S is proportional to the average interface state density, Dit, in the weak inversion region. The change in interface state density induced by an ion irradiation AD~, is given by
AOit = [Co×/(kTIn(IO))]AS,
(l)
where Cox is the oxide capacitance, and k the Boltzmann constant [15]. Although the exact amount of interface state density cannot be estimated because of the lack of information on the precise sample structure, a tendency of the change in mean interface state density can be understood by evaluating the change in the subthreshold swing before and after the ion irradiation. As shown in Fig. 5, the subthreshold swing S became larger for both pMOSFETs and nMOSFETs, as the number of He ions was increased, and AS for nMOSFETs was higher than
367
that for pMOSFETs. This suggests that a larger number of interface states were induced by high energy ions in nMOSFETs than pMOSFETs. The difference in AS between nMOSFETs and pMOSFETs can be attributed to the asymmetrical distribution of radiation induced interface states in the midgap, which was found by Ma [16,17]. According to his work, the density of electron beam induced interface states in the upper half of the band gap is higher than that in the lower half of the band gap and the peak is located at 0.2-0.3 eV above midgap. Similar distribution has been recognized for y-ray induced interface states [18-20]. Since the interface states in the upper half of the bandgap are responsible for the degradation of S in nMOSFETs and the one in the lower half for the degradation in pMOSFETs, it is clearly concluded that nMOSFETs are more susceptible than pMOSFETs to the degradation of the subthreshold swing S.
4. Summary We have investigated the radiation effects induced by MeV He single ions in pMOSFETs and nMOSFETs in commercially available CMOS4007 using the SIMP. It has been concluded that, (1) pMOSFETs are more fragile than nMOSFETs in threshold voltage shift and (2) nMOSFETs are more susceptible than pMOSFETs to the degradation of subthreshold swing S. These effects have been fully explained on the basis of different contribution of oxide trapped charges and the charges at interface states depending on the channel conduction type.
Acknowledgements This work is partly supported by a Grant-in-Aid for Specially Promoted Research, the Ministry of Education, Science and Culture.
References [1] R.W. Tallon, M.R. Ackermann, W.T. Kemp, M.H. Owen and D.P. Saunders, IEEE Trans. Nucl. Sci. NS-32 (1985) 4393.
368
M. Koh et al. /Applied Surface Science 104/105 (1996) 364-368
[2] G.J. Brucker, E.G. Stassinopoulos, O. Van Gunten, L.S. August and T.M. Jordan. IEEE Trans. Nucl. Sci. NS-29 (1982) 1966. [3] E.G. Stassinopoulos, G.J. Brucker and O. Van Gunten, IEEE Trans. Nucl. Sci. NS-31 (1984) 1444. [4] W.J. Stapor, L.S. August, D.H. Wilson, T.R. Oldham and K.M. Murray, IEEE Trans. Nucl. Sci. NS-32 (1985) 4399. [5] T.R. Oldham and J.M. McGarrity, IEEE Trans. Nucl. Sci. NS-28 (1981) 3975. [6] N.S. Saks and M.G. Ancona, IEEE Trans. Nucl. Sci. NS-34 (1987) 1348. [7] M.A. Xapsos, R.K. Freitag, C.M. Dozier, D.B. Brown, G.P. Summers, E.A. Bruke and P. Shapiro, IEEE Trans. Nucl. Sci. NS-37 (1990) 1677. [8] I. Ohdomari, M. Sugimori, M. Koh, K. Noritake, Y. Takiguchi and H. Shimizu, Nucl. Instr. Meth. B 72 (1992) 436. [9] I. Ohdomari, M. Sugimori. M. Koh. K. Noritake, M. Ishikawa and H. Shimizu, Nucl. Instr. Meth. B 54 (1991) 71. [10] M. Koh, K. Hara, K. Horita, B. Shigeta, T. Matsukawa, A. Kishida, T. Tanii, M. Goto and I. Ohdomari, Nucl. Instr. Meth. B 93 (1994) 82.
[11] M. Koh, K. Hara, K. Horita, B. Shigeta, T. Matsukawa, A. Kishida, T. Tanii, M. Goto and I. Ohdomari, Jpn. J. Appl. Phys. 33 (1994) L962. [12] E.H. Nicollian and J.R. Brews, MOS Physics and Technology (Wiley, New York, 1982) p. 549. [13] P.J. McWhorter and P.S. Winokur, Appl. Phys. Lett. 48 (1986) 133. [14] P.M. Lenahan and P.V. Dressendorfer, J. Appl. Phys. 55 (1984) 3495. [15] H. Gesch, I.-P. Leburton and G.E. Dorda, IEEE Trans. Electron Devices ED-29 (1982) 913. [16] T.P. Ma, G.A. Scoggan and R. Leone, Appl. Phys. Lett. 27 (1975) 61. [17] G.A. Scoggan and T.P. Ma, J. Appl. Phys. 48 (1977) 294. [18] J.M. Benedetto and H.E. Boesch, Jr., IEEE Trans. Nucl. Sci. NS-31 (1984) 1461. [19] M. Gaitan and T.J. Russell, IEEE Trans. Nucl. Sci. NS-31 (1984) 1256. [20] K. Naruke, M. Yoshida, K. Maeguchi and H. Tango, IEEE Trans. Nucl. Sci. NS-30 (1983) 4054.
Applied Surface Science 104/105 (1996) 364-368
Radiation immunity of pMOSFETs and nMOSFETs examined by means of MeV He single ion microprobe M. K o h a , * K. Horita a B. Shigeta a T. M a t s u k a w a a A. Kishida a T. Tanii a S. Mori a I. O h d o m a r i a.b School of Science and Engineering, Waseda UniL'ersity, 3-4-10hkubo, Shinjuku-ku, Tokyo 169, Japan b Kagami Memorial Laboratory for Material Science and Technology, Waseda Unit,ersity, 2-8-6 Nishi-waseda, Shinjuku-ku, Tokyo 169, Japan Received 28 June 1995; accepted I I September 1995
Abstract
Radiation effects induced by MeV He single ions in pMOSFETs and nMOSFETs in commercially available CMOS4007 have been studied extensively. The key results from this study are: (1) pMOSFETs are more fragile than nMOSFETs in terms of threshold voltage shift, (2) nMOSFETs are more susceptible than pMOSFETs to the degradation of subthreshold swing. The different features in the radiation effects have been discussed comprehensively.
1. Introduction
In order to optimize device structures or processes for radiation hardness of CMOS devices it is inevitable to investigate radiation effects in both pMOSFETs and nMOSFETs fabricated in a same chip quantitatively. So far radiation hardening has been done based on the data obtained by irradiating a whole IC chip with energetic particles randomly [1-5]. However, the information obtained in this way has not been quantitative in the sense that identifying a site dependence of radiation immunity has never been possible. Random irradiation with energetic particles would induce also the so-called lateral
* Corresponding author. Tel.: + 81-3-52863380; fax: + 81-352725749; e-mail: [email protected].
non-uniformity effects, i.e., the influence of non-uniform distribution of trapped charges on device characteristics, which will obscure the proper interpretation of radiation effects [6,7]. So far we have succeeded in developing the single ion beam induced charge (SIBIC) imaging by using the single ion microprobe (SIMP) system which has made it possible to hit a local area of less than 3 /_tin square in a target with MeV He single ions one by one [8-11]. The SIBIC imaging enables us to 'observe' the feature of MOSFET without degradation of device characteristics. The sample feature observation before any tests is very important for the quantitative analysis of site-dependent radiation effects in MOSFETs. In this study, we have investigated site dependent radiation effects in CMOS devices induced by high energy He single ions and comparing the difference in pMOSFETs and nMOS-
0169-4332/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S01 6 9 - 4 3 3 2 ( 9 6 ) 0 0 172-9
M. Koh et al. / Applied Surface Science 104 / 105 (1996) 364-368
FETs. We first introduce an experimental setup and the method of sample positioning. Then we describe the difference in ion irradiation effects in the characteristics of pMOSFETs and nMOSFETs in a test CMOS chip.
2. Experimental A schematic diagram of the SIMP system is illustrated in Fig. 1. The beam line is connected to a tandem accelerator (NEC 5SDH) with a terminal voltage of 1.0 MV. As shown in Fig. 1, each single ion is extracted one by one from a continuous ion beam by chopping. If the ion current, the slit opening and the switching velocity are set that after chopping several times only one ion may pass through the slit, single ion can be successfully extracted. Extracted single ion is focused with a high precision quadrupole magnet and irradiated within an area corresponding to the micro beam spot size. Number of the incident ions is controlled by an electron multiplier tube (EMT) by detecting secondary electrons (SEs) emitted upon ion incidence. Signals from the EMT are fed to the chopper controller which keeps on sending the beam chopping signals to the deflector until the desired number of single ions are detected. A device to be irradiated is mounted on a high precision goniometer, which is movable in three directions under the control of a host computer. I - V character-
365
istics of MOSFETs can be measured in situ by combining HP4140B system with the SIMP. In this study, I - V characteristics were recorded before exposure and after irradiation of a certain number of ions. Throughout this work, 2.0 MeV He single ions have been used. The number of irradiated ions has been varied from 0 to 102//zm 2. The irradiated area was 240 X 240/zm 2. During irradiation all pins were grounded. A commercial inverter CMOS TC4007UBP produced by Toshiba, which contains three pMOSFETs and three nMOSFETs, was used as a test target in this study. Fig. 2 shows an optical micrograph of the CMOS4007 after removal of plastic package with heated fuming acid. Since this sample is made for commercial purpose, detailed information on device structure, e.g. oxide thickness and doping concentration are not clear. The radiation tests have been performed on each FET. Sample positioning was performed by using the SIBIC imaging method. Fig. 3 shows a coarse SIBIC image of the entire chip which consists of 16 X 16 pixel array. The scanned area was 800 X 800 /xm 2 and the number of incident ions per pixel was only two. The image clearly reveals each MOSFET. After the coarse imaging, much clearer SIBIC image of the desired transistor was taken for higher precision sample positioning. The radiation damage during these SIBIC imaging is negligible for the subsequent
Tr.n.,°r ,,n* .,gh prec,.io. /I P'CI'T
o,,eot,,,. I
,.,,.ctor
"'"
j..--
r
I
l
if ....
"'"°Ul]II
il"""'l
chopper controller circuit
Fig. 1. Schematic drawing of the experimental system.
/
I I N'°'"
L_J
366
M. Koh et al. /Aplflied Surlace Science 104 / 105 (1996) 364-368
!
~
. . . . . . . . . . °~g, ,+, <, o
Nm ,,
.[
200 ~ m Fig. 2. Optical micrograph of the tcst device.
analyses. The analysis of the ion irradiation effects in the MOSFETs has been done by repeatedly taking the SIBIC images. Details of sample positioning using the SIBIC imaging are introduced in the other literatures [10,1 I].
3. Results and discussion in this section, we describe the different responses of pMOSFETs and nMOSFETs in the test CMOS4007 (Toshiba) against ion irradiation. Fig. 4 shows the threshold voltage shift AVth for both pMOSFETs and nMOSFETs as a ['unction of ion dose at room temperature. The threshold voltage was defined as the zero current intercept of square root of the l~ versus V, curves in the saturation mode. The threshold voltage Vth shfl'ted to the negative direction for both pMOSFETs and nMOSFETs proportional to the number of He ions. There are two major causes in generation of radiation induced interface charges which are responsible for Vth shift. They are oxide trapped holes and trapped charges by interface states [12]. The slope represents fragility of the target devices against ion irradiation. Since the slope for pMOSFETs is steeper than that for nMOSFETs, pMOSFETs are more fragile than nMOSFETs in the threshold voltage shift at room temperature. The reason for the steeper slope, i.e., larger AV,1 for pMOSFETs, can be explained as follows. It is known that the net ,AVth is the algebraic sum of voltage shifts due to the trapped holes AVN,,t and the trapped charges by the interface states AVN~~ [13]. It also should be noted that the Pb-centers responsible for the interface states have amphoteric nature. They are donors in the lower part of the bandgap and acceptors in the upper part of the bandgap. Their occupancy depends on the value of the surface potential. The donor-like interface states are responsi-
Dose
0
25 F
-10(
L ~ "
[ions/IJrn
50 i
2]
75 i - O - n-ch ~ p-ch
00
:~ -200 - -300 400 \., 500 ~ m
Fig. 3. SIBIC image of the entire chip which is composed of 16 × 16 pixels. The scanned area is 800 × 800 gin-'.
~00
~
~
Fig. 4, Threshc,ld voltage shift A~h for both pMOSFBTs and
nMOSFETs induced by ion irradiation.
M. Koh et al. / Applied Surface Science 104/105 (1996) 364-368 6.0
i
i
I
--C-- n-oh
5.0
<[:;:! ~"
-D-P-c/
4.0
3.0
1.0
0.0 0
25 Dose
50
75
100
[ I o n s / ~ m 2]
Fig. 5. Subthreshold swing shift AS for both pMOSFETs and nMOSFETs induced by ion irradiation.
ble for the VN~t shift of pMOSFETs, while the acceptor-like interface states are responsible for the WNit shift of nMOSFETs. Therefore AVNit is negative for pMOSFETs and positive for nMOSFETs, while AVNot induced by trapped holes is always negative [14]. For pMOSFETs, sign of AVNit is the same as AVNo t, which causes larger AVth. For nMOSFETs, however, it is expected that the opposite signs of mWNi t and AVNot result in a relatively small shift in Vth. Thus pMOSFETs are more fragile than nMOSFETs in the threshold voltage shift. Fig. 5 shows the subthreshold swing shift AS for both pMOSFETs and nMOSFETs as a function of ion dose. The subthreshold swing is defined as the change in gate voltage necessary to reduce transistor current by one decade. It is well known that the subthreshold swing S is proportional to the average interface state density, Dit, in the weak inversion region. The change in interface state density induced by an ion irradiation AD~, is given by
AOit = [Co×/(kTIn(IO))]AS,
(l)
where Cox is the oxide capacitance, and k the Boltzmann constant [15]. Although the exact amount of interface state density cannot be estimated because of the lack of information on the precise sample structure, a tendency of the change in mean interface state density can be understood by evaluating the change in the subthreshold swing before and after the ion irradiation. As shown in Fig. 5, the subthreshold swing S became larger for both pMOSFETs and nMOSFETs, as the number of He ions was increased, and AS for nMOSFETs was higher than
367
that for pMOSFETs. This suggests that a larger number of interface states were induced by high energy ions in nMOSFETs than pMOSFETs. The difference in AS between nMOSFETs and pMOSFETs can be attributed to the asymmetrical distribution of radiation induced interface states in the midgap, which was found by Ma [16,17]. According to his work, the density of electron beam induced interface states in the upper half of the band gap is higher than that in the lower half of the band gap and the peak is located at 0.2-0.3 eV above midgap. Similar distribution has been recognized for y-ray induced interface states [18-20]. Since the interface states in the upper half of the bandgap are responsible for the degradation of S in nMOSFETs and the one in the lower half for the degradation in pMOSFETs, it is clearly concluded that nMOSFETs are more susceptible than pMOSFETs to the degradation of the subthreshold swing S.
4. Summary We have investigated the radiation effects induced by MeV He single ions in pMOSFETs and nMOSFETs in commercially available CMOS4007 using the SIMP. It has been concluded that, (1) pMOSFETs are more fragile than nMOSFETs in threshold voltage shift and (2) nMOSFETs are more susceptible than pMOSFETs to the degradation of subthreshold swing S. These effects have been fully explained on the basis of different contribution of oxide trapped charges and the charges at interface states depending on the channel conduction type.
Acknowledgements This work is partly supported by a Grant-in-Aid for Specially Promoted Research, the Ministry of Education, Science and Culture.
References [1] R.W. Tallon, M.R. Ackermann, W.T. Kemp, M.H. Owen and D.P. Saunders, IEEE Trans. Nucl. Sci. NS-32 (1985) 4393.
368
M. Koh et al. /Applied Surface Science 104/105 (1996) 364-368
[2] G.J. Brucker, E.G. Stassinopoulos, O. Van Gunten, L.S. August and T.M. Jordan. IEEE Trans. Nucl. Sci. NS-29 (1982) 1966. [3] E.G. Stassinopoulos, G.J. Brucker and O. Van Gunten, IEEE Trans. Nucl. Sci. NS-31 (1984) 1444. [4] W.J. Stapor, L.S. August, D.H. Wilson, T.R. Oldham and K.M. Murray, IEEE Trans. Nucl. Sci. NS-32 (1985) 4399. [5] T.R. Oldham and J.M. McGarrity, IEEE Trans. Nucl. Sci. NS-28 (1981) 3975. [6] N.S. Saks and M.G. Ancona, IEEE Trans. Nucl. Sci. NS-34 (1987) 1348. [7] M.A. Xapsos, R.K. Freitag, C.M. Dozier, D.B. Brown, G.P. Summers, E.A. Bruke and P. Shapiro, IEEE Trans. Nucl. Sci. NS-37 (1990) 1677. [8] I. Ohdomari, M. Sugimori, M. Koh, K. Noritake, Y. Takiguchi and H. Shimizu, Nucl. Instr. Meth. B 72 (1992) 436. [9] I. Ohdomari, M. Sugimori. M. Koh. K. Noritake, M. Ishikawa and H. Shimizu, Nucl. Instr. Meth. B 54 (1991) 71. [10] M. Koh, K. Hara, K. Horita, B. Shigeta, T. Matsukawa, A. Kishida, T. Tanii, M. Goto and I. Ohdomari, Nucl. Instr. Meth. B 93 (1994) 82.
[11] M. Koh, K. Hara, K. Horita, B. Shigeta, T. Matsukawa, A. Kishida, T. Tanii, M. Goto and I. Ohdomari, Jpn. J. Appl. Phys. 33 (1994) L962. [12] E.H. Nicollian and J.R. Brews, MOS Physics and Technology (Wiley, New York, 1982) p. 549. [13] P.J. McWhorter and P.S. Winokur, Appl. Phys. Lett. 48 (1986) 133. [14] P.M. Lenahan and P.V. Dressendorfer, J. Appl. Phys. 55 (1984) 3495. [15] H. Gesch, I.-P. Leburton and G.E. Dorda, IEEE Trans. Electron Devices ED-29 (1982) 913. [16] T.P. Ma, G.A. Scoggan and R. Leone, Appl. Phys. Lett. 27 (1975) 61. [17] G.A. Scoggan and T.P. Ma, J. Appl. Phys. 48 (1977) 294. [18] J.M. Benedetto and H.E. Boesch, Jr., IEEE Trans. Nucl. Sci. NS-31 (1984) 1461. [19] M. Gaitan and T.J. Russell, IEEE Trans. Nucl. Sci. NS-31 (1984) 1256. [20] K. Naruke, M. Yoshida, K. Maeguchi and H. Tango, IEEE Trans. Nucl. Sci. NS-30 (1983) 4054.