- Email: [email protected]
The effect of swift heavy ion irradiation on threshold voltage, transconductance and mobility of DMOSFETs
Nuclear Instruments and Methods in Physics Research B 273 (2012) 40–42
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
The effect of swift heavy ion irradiation on threshold voltage, transconductance and mobility of DMOSFETs N. Pushpa a, K.C. Praveen a, A.P. Gnana Prakash a,⇑, P.S. Naik a, Ambuj Tripathi b, S.K. Gupta c, D. Revannasiddaiah a a b c
Department of Studies in Physics, University of Mysore, Manasagangotri, Mysore 570 006, India Inter University Accelerator Centre (IUAC), New Delhi 110 067, India Technical Physics Division, Bhabha Atomic Research Centre, Mumbai 400 085, India
a r t i c l e
i n f o
Article history: Available online 5 August 2011 Keywords: MOSFET Interface trapped charge Oxide trapped charge Threshold voltage Transconductance Mobility degradation
a b s t r a c t N-channel depletion MOSFETs were irradiated with 140 MeV Si10+ ions, 100 MeV F8+ ions and 48 MeV Li3+ ions in the dose range from 100 krad to 100 Mrad. The MOSFET parameters such as threshold voltage (VTH), transconductance (gm) and mobility of carriers (l) were determined by systematically studying the I–V characteristics before and after irradiation. The ion irradiated results were compared with Co60 gamma irradiated results in the same dose range. The degradation in VTH, gm and l was found to be more for the devices irradiated with Co-60 gamma radiation than that irradiated with swift heavy ions. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
N-channel depletion metal oxide semiconductor field effect transistors (MOSFETs) are extensively used in different radiation rich environments like space, military and high energy physics experiments such as Large Hadron Colliders (LHCs). The MOS devices are sensitive to ionizing radiation and prone to parametric and even functional damage up on irradiation [1–3]. The basic damage effects of ionizing radiation in MOS devices results from the generation of interface and oxide trapped charge in the gate oxide. This trapped charge degrades the important parameters of MOSFETs such as threshold voltage (VTH), transconductance (gm) and mobility of carriers (l) in the channel. Literature survey reveals the total dose effects of gamma, electron and proton irradiation on MOSFETs [1–7]. The high energy swift ion irradiation studies on MOS devices is sparse [1–4], therefore we made an attempt to understand the ion irradiation effects on MOS devices in this paper. Generally Co-60 gamma radiation is used to study total dose effects on semiconductor devices, but the time required to reach high doses like 100 Mrad for LHC application is very long, the another alternative is high energy ion irradiation. The main objective of this work is to investigate the effects of 140 MeV Si10+ ions, 100 MeV F8+ ions, 48 MeV Li3+ ions on VTH, gm and the mobility (l) of carriers in the N-channel of the MOSFETs. The ion irradiated results are also compared with that of Co-60 gamma irradiated MOSFETs.
The devices used for this work are two serially connected N-channels with independent dual gate depletion MOSFETs (BEL 3N187). The cross-sectional view of the N-channel depletion MOSFET and its details are given in our earlier paper [4]. The MOSFETs having identical electrical characteristics were irradiated with the ions examined here and Co-60 gamma radiation in the dose ranges from 100 krad to 100 Mrad. The ion irradiation was done at the 15 UD (16 MV) Pelletron Tandem Van de Graff Accelerator at the Inter University Accelerator Center (IUAC) in New Delhi, India. The experiments were performed at 300 K in an experimental chamber of diameter 1.5 m maintained at 107 mbar of vacuum. The MOSFETs were exposed in the fluence range of 0.7 109 ions/cm2 to 1.5 1013 ions/cm2 and its equivalent gamma dose ranges from 100 krad to 100 Mrad. The formula for conversion of fluence to dose (in rad) for different ions is:
⇑ Corresponding author. Tel.: +91 9590583920. E-mail address: [email protected] (A.P. Gnana Prakash). 0168-583X/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2011.07.033
Dose ðradÞ ¼ Fluence ðcm2 Þ 1:6018 105 LET ðMeV cm2 =mgÞ where, LET is the Linear energy transfer of the radiation species [8]. The typical beam current was one particle-nano ampere (1 p-nA) for 48 MeV Li3+ ions, 0.125 p-nA for 100 MeV F8+ ions and 0.1 p-nA for 140 MeV Si10+ ions. The ion beam was scanned in an area of 10 mm 10 mm by magnetic scanner in order to get uniform dose. The N-channel MOSFETs were also exposed to Co-60 gamma radiation using the Gamma Chamber 5000 with a dose rate of 167 rad/s
41
N. Pushpa et al. / Nuclear Instruments and Methods in Physics Research B 273 (2012) 40–42 Table 1 The energy loss and range of 140 MeV Si10+ ion, 100 MeV F8+ ion, 48 MeV Li3+ ion and Co-60 gamma radiation in metal oxide semiconductor (MOS) structure. Source
Linear energy transfer in MeV cm2/g Al
Si
SiO2
Al
Si
SiO2
140 MeV Si10+ ions 100 MeV F8+ ions 48 MeV Li3+ ions Co-60 gamma (0.6 MeV electron)
8917 9307 412 1.5
9107 4427 408 1.6
9702 4723 440 1.7
46 64 254 –
53 73 310 –
50 68 285 –
3. Results and discussion Fig. 1 shows the transfer (ID–VGS) characteristics of 140 MeV Si10+ ion irradiated MOSFETs at VDS = 1 V. From this figure it can be seen that the ID swings towards negative gate voltage with the increase in Si10+ ion total dose. This means that the threshold voltage (VTH) decreases with increase in ion total dose. The VTH is defined as the negative gate voltage for which the drain current becomes 1 lA (VTH VGS @ ID = 1 lA). Fig. 2 shows the variation in VTH versus total dose for the different ions and Co-60 gamma irradiated MOSFETs. From the figure it is clear that the VTH degradation is more for gamma irradiated MOSFETs when compared to swift heavy ion irradiated MOSFETs. In other words, Co-60 gamma radiation creates more trapped charge in oxide layer of the MOSFETs than that of higher LET radiations like the ions examined here. We have studied the high energy ion irradiation effects on SiGe HBTs, Si BJTs and MOSFETs. We observed more degradation in DC electrical characteristics of gamma irradiated MOSFETs when compared to ion irradiated MOSFETs. Whereas in NPN transistors and SiGe HBTs, we observed more degradation in forward mode Gummel characteristics for ion irradiated transistors when compared to gamma irradiated transistors. Particularly in SiGe HBTs, ion irradiation creates more degradation in emitter–base (E–B) spacer oxide when compared to the gamma radiation. Therefore we observed more degradation in Gummel characteristics, current gain, transconductance and other electrical characteristics for ion irradiated HBTs when compared to gamma radiation. The degradation in shallow trench isolation oxide (STI) is studied by inverse mode Gummel characteristics, where emitter and collector terminals of HBT are interchanged while taking Gummel characteristics.
-4
1x10
-5
1x10
-6
1x10
0.020 0.017 0.015 0.013 0.010
ID (A)
-3
1x10
ID (A)
0.022
Pre-rad 100krad 300krad 600krad 1Mrad 3Mrad 6Mrad 10Mrad 30Mrad 60Mrad 100Mrad
-2
1x10
-7
1x10
0.007
-8
1x10
0.005
-9
1x10
0.003
140 MeV Silicon ion
-10
1x10
-6
-4
-2
0
VGS (V)
2
4
0.000
6
Fig. 1. Subthreshold characteristics of 140 MeV Si10+ ion irradiated MOSFET.
-1.0 -1.5
Threshold voltage, VTH (V)
at Pondicherry University in Puducherry, India. Identical total dose was given for different ion species and gamma radiation. The gate terminals of MOSFETs were biased at +2 V (VGS = +2 V) during irradiation. The electrical characterization of the un-irradiated and irradiated MOSFETs were performed using a computer interfaced Agilent 4155 Semiconductor Parameter Analyzer (see Table 1).
Range in lm
-2.0 -2.5 -3.0 -3.5 -4.0 -4.5
Co-60 Gamma 48 MeV Li ion 100 MeV F ion 140 MeV Si ion
-5.0 2
Pre-rad 1x10
3
1x10
4
1x10
5
1x10
Total dose (krad) Fig. 2. Threshold voltage degradation as a function of total dose for different swift heavy ions and Co-60 gamma irradiated MOSFETs.
STI oxide is made up of SiO2 and observed more degradation for gamma irradiated HBT than ion irradiated HBT. The different behaviour of E–B spacer oxide and STI oxide to different radiation is because; the EB spacer oxide is the oxide/nitride composite whereas, the STI is made up of silicon dioxide (SiO2). The effect of nitrogen near the insulator/silicon improves the radiation hardening by undergoing very small amount of ionization and creating less number of radiation induced G/R traps. Hence E–B spacer oxide is immune to gamma radiation and STI oxide undergoes more ionization for gamma radiation [8–11]. The insulating gate oxide layer of the MOSFETs studied in the present work is made up of SiO2. Therefore, we expect more charge yield in SiO2 after gamma irradiation when compared to ion irradiation. As a result, the number of electron–hole pairs produced by Co-60 gamma radiation at the interface and in the oxide layer is nearly an order of magnitude higher than that produced by the ions (Figs. 3 and 4). In Fig. 2 it can be seen that the VTH decreases up to 10 Mrad of Co-60 gamma dose and beyond 10 Mrad there is no much decrease in VTH up to a total dose of 100 Mrad and it is unexpected. A detailed investigation is required to further understand this phenomenon. The net threshold voltage shift (DVTH) is the sum of the shift due to the interface trapped charge (DVNit) and the oxide trapped charge (DVNot). DVNit and DVNot were calculated from the subthreshold measurements using the technique proposed by McWhorter and Winokur [12]. The oxide trapped charge density (DNot) and the interface trapped charge density (DNit) were calculated using the standard expression DNot = DVot Cox/q and DNit = DVit Cox/q, where q = 1.6 1019 C, DNot is the effective oxide-trapped charge density in cm2 as projected to the interface, Cox is the oxide capacitance per unit area and DNit is the effective interface-trapped charge density. The DNot variation of the MOSFETs as a function of total dose is shown in Fig. 3 for different irradiation types. Similarly, the variation of DNit as a function of total dose is shown in Fig. 4 for different radiation. From Figs. 3 and 4 it is clear that DNot is higher than DNit, and hence the VTH shifts towards negative voltage side. It
42
N. Pushpa et al. / Nuclear Instruments and Methods in Physics Research B 273 (2012) 40–42
1400
Co-60 Gamma 48 MeV Li ion 100 MeV F ion 140 MeV Si ion
1200
Mobility (cm2/V-s)
12
ΔNot(cm-2)
1x10
11
1x10
1000
Co-60 Gamma 48MeV Li ion 100 MeV F ion 140 MeV Si ion
10
1x10
2
1x10
3
4
1x10
600 400 200 0 0
5
1x10
800
1x10
Fig. 3. Oxide trapped density versus total dose for different swift heavy ions and Co-60 gamma irradiated MOSFETs.
4
6x10
4
8x10
5
1x10
Fig. 6. Mobility versus total dose for different swift heavy ions and Co-60 gamma irradiated MOSFETs.
gmPeak (called field effect mobility, lFE). Fig. 6 shows the variation of lFE for different high energy ions and Co-60 gamma irradiated MOSFETs for varying total doses. From this figure, it can be seen that the lFE of the irradiated devices decreases with increase in radiation total dose and it is observed about 80% degradation in lFE after a total dose of 100 Mrad. The degradation in l is mainly due to the interface-trapped charge (Nit) and the effect of oxidetrapped charges (Not) is negligible.
12
1x10
-2
4
4x10
Total dose (krad)
Total Dose (krad)
ΔNit (cm )
4
2x10
11
1x10
10
1x10
4. Summary
Co-60 Gamma 48MeV Li ion 100 MeV F ion 140 MeV Si ion
9
1x10
2
1x10
3
4
1x10
5
1x10
1x10
Total Dose (krad) Fig. 4. Interface trapped density versus total dose for different swift heavy ions and Co-60 gamma irradiated MOSFETs.
can also be observed the values of DNot and DNit are more for gamma irradiated MOSFETs when compared to swift heavy ion irradiated MOSFETs. Fig. 5 shows the variation in peak transconductance (gmPeak) with total dose for ions examined here and Co-60 gamma irradiated MOSFETs. The peak transconductance (gmPeak) was extracted from the ID versus VGS data taken at VDS = 0.1 V and the mobility of carriers in the N-channel were estimated from the gmPeak. It can be seen from Fig. 5 that the gmPeak decreases with increase in total dose and more degradation was observed in gamma irradiated MOSFETs compared to ion irradiated MOSFETs. The mobility (l) of carriers (electrons) in the channel was estimated from the
The electrical characteristics of N-channel depletion MOSFETs were studied before and after 140 MeV Si10+ ions, 100 MeV F8+ ions, 48 MeV Li3+ ions and Co-60 gamma irradiation for the total doses ranging from 100 krad to 100 Mrad. We observed that the VTH of the irradiated MOSFETs decreases significantly after 100 Mrad of total dose for different radiation examined here. However, more degradation was observed for gamma irradiated MOSFETs when compared to ion irradiated MOSFETs. More deterioration in MOSFETs after Co-60 gamma irradiation is attributed to more charge yield in MOS structure when compared to swift heavy ion irradiation. Around 80% degradation was observed in transconductance (gm) and mobility of carriers after receiving a total dose of 100 Mrad. Acknowledgment This work is carried out under the Research Project sanctioned by DAE-BRNS, Government of India (Project No. 2009/37/35/BRNS/ 2275). References
Peak Transconductance (gmPeak)
0.0007
Co-60 Gamma 48 MeV Li ion 100 MeV F ion 140 MeV Si ion
0.0006 0.0005 0.0004 0.0003 0.0002 0.0001 0.0000 Pre-rad 1x102
3
1x10
4
1x10
5
1x10
Total dose (krad) Fig. 5. Peak transconductance versus total dose for different swift heavy ions and Co-60 gamma irradiated MOSFETs.
[1] N. Pushpa, K.C. Praveen, A.P. Gnana Prakash, Y.P. Prabhakara Rao, A. Tripati, G. Govindaraj, D. Revannasiddaiah, Nucl. Instr. Meth. A, 613 (2010) 280. [2] A.P. Gnana Prakash, S.C. Ke, K. Siddappa, Semicond. Sci. Tech. 18 (2003) 1037. [3] A.P. Gnana Prakash, S.C. Ke, K. Siddappa, Radiat. Eff. Defects Solids 158 (9) (2003) 635. [4] A.P. Gnana Prakash, John D. Cressler, S.C. Ke, K. Siddappa, Indian J. Phys. 78 (11) (2004) 1187. [5] S. Kaschieva, S.N. Dmitriev, Ch. Angelov, Nucl. Instr. Meth. B 206 (2003) 452. [6] K. Hayama, K. Takakura, T. Ohtani, T. Kudou, H. Ohyama, A. Mercha, E. Simoen, C. Claeys, Mater. Sci. Semicond. Process. 11 (5–6) (2008) 314. [7] W.J. Stapor, L.S. August, D.H. Wilson, T.R. Oldham, K.M. Murray, IEEE Trans. Nucl. Sci. 32 (6) (1985) 4399. [8] J.D. Cressler, G. Niu, Silicon–germanium heterojunction bipolar transistors, Artech House, Norwood, MA, 2003. [9] P. Paillet, J.R. Schwank, M.R. Shaneyfelt, V. Ferlet-Cavrois, R.L. Jones, O. Flament, E.W. Blackmore, IEEE Trans. Nucl. Sci. 49 (6) (2002) 2656. [10] A.K. Sutton, B.M. Haugerud, A.P. Gnana Prakash, B. Jun, J.D. Cressler, C.J. Marshall, P.W. Marshall, R. Ladbury, F. Guarin, A.J. Joseph, IEEE Trans. Nucl. Sci. 52 (6) (2005) 2358. [11] K.C. Praveen, N. Pushpa, John D Cressler, A.P. Gnana Prakash, J. Nano Electron. Phys. 3 (1) (2011) 348. [12] P.J. McWhorter, P.S. Winkur, Appl. Phys. Lett. 48 (2) (1986) 133.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
The effect of swift heavy ion irradiation on threshold voltage, transconductance and mobility of DMOSFETs N. Pushpa a, K.C. Praveen a, A.P. Gnana Prakash a,⇑, P.S. Naik a, Ambuj Tripathi b, S.K. Gupta c, D. Revannasiddaiah a a b c
Department of Studies in Physics, University of Mysore, Manasagangotri, Mysore 570 006, India Inter University Accelerator Centre (IUAC), New Delhi 110 067, India Technical Physics Division, Bhabha Atomic Research Centre, Mumbai 400 085, India
a r t i c l e
i n f o
Article history: Available online 5 August 2011 Keywords: MOSFET Interface trapped charge Oxide trapped charge Threshold voltage Transconductance Mobility degradation
a b s t r a c t N-channel depletion MOSFETs were irradiated with 140 MeV Si10+ ions, 100 MeV F8+ ions and 48 MeV Li3+ ions in the dose range from 100 krad to 100 Mrad. The MOSFET parameters such as threshold voltage (VTH), transconductance (gm) and mobility of carriers (l) were determined by systematically studying the I–V characteristics before and after irradiation. The ion irradiated results were compared with Co60 gamma irradiated results in the same dose range. The degradation in VTH, gm and l was found to be more for the devices irradiated with Co-60 gamma radiation than that irradiated with swift heavy ions. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
N-channel depletion metal oxide semiconductor field effect transistors (MOSFETs) are extensively used in different radiation rich environments like space, military and high energy physics experiments such as Large Hadron Colliders (LHCs). The MOS devices are sensitive to ionizing radiation and prone to parametric and even functional damage up on irradiation [1–3]. The basic damage effects of ionizing radiation in MOS devices results from the generation of interface and oxide trapped charge in the gate oxide. This trapped charge degrades the important parameters of MOSFETs such as threshold voltage (VTH), transconductance (gm) and mobility of carriers (l) in the channel. Literature survey reveals the total dose effects of gamma, electron and proton irradiation on MOSFETs [1–7]. The high energy swift ion irradiation studies on MOS devices is sparse [1–4], therefore we made an attempt to understand the ion irradiation effects on MOS devices in this paper. Generally Co-60 gamma radiation is used to study total dose effects on semiconductor devices, but the time required to reach high doses like 100 Mrad for LHC application is very long, the another alternative is high energy ion irradiation. The main objective of this work is to investigate the effects of 140 MeV Si10+ ions, 100 MeV F8+ ions, 48 MeV Li3+ ions on VTH, gm and the mobility (l) of carriers in the N-channel of the MOSFETs. The ion irradiated results are also compared with that of Co-60 gamma irradiated MOSFETs.
The devices used for this work are two serially connected N-channels with independent dual gate depletion MOSFETs (BEL 3N187). The cross-sectional view of the N-channel depletion MOSFET and its details are given in our earlier paper [4]. The MOSFETs having identical electrical characteristics were irradiated with the ions examined here and Co-60 gamma radiation in the dose ranges from 100 krad to 100 Mrad. The ion irradiation was done at the 15 UD (16 MV) Pelletron Tandem Van de Graff Accelerator at the Inter University Accelerator Center (IUAC) in New Delhi, India. The experiments were performed at 300 K in an experimental chamber of diameter 1.5 m maintained at 107 mbar of vacuum. The MOSFETs were exposed in the fluence range of 0.7 109 ions/cm2 to 1.5 1013 ions/cm2 and its equivalent gamma dose ranges from 100 krad to 100 Mrad. The formula for conversion of fluence to dose (in rad) for different ions is:
⇑ Corresponding author. Tel.: +91 9590583920. E-mail address: [email protected] (A.P. Gnana Prakash). 0168-583X/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2011.07.033
Dose ðradÞ ¼ Fluence ðcm2 Þ 1:6018 105 LET ðMeV cm2 =mgÞ where, LET is the Linear energy transfer of the radiation species [8]. The typical beam current was one particle-nano ampere (1 p-nA) for 48 MeV Li3+ ions, 0.125 p-nA for 100 MeV F8+ ions and 0.1 p-nA for 140 MeV Si10+ ions. The ion beam was scanned in an area of 10 mm 10 mm by magnetic scanner in order to get uniform dose. The N-channel MOSFETs were also exposed to Co-60 gamma radiation using the Gamma Chamber 5000 with a dose rate of 167 rad/s
41
N. Pushpa et al. / Nuclear Instruments and Methods in Physics Research B 273 (2012) 40–42 Table 1 The energy loss and range of 140 MeV Si10+ ion, 100 MeV F8+ ion, 48 MeV Li3+ ion and Co-60 gamma radiation in metal oxide semiconductor (MOS) structure. Source
Linear energy transfer in MeV cm2/g Al
Si
SiO2
Al
Si
SiO2
140 MeV Si10+ ions 100 MeV F8+ ions 48 MeV Li3+ ions Co-60 gamma (0.6 MeV electron)
8917 9307 412 1.5
9107 4427 408 1.6
9702 4723 440 1.7
46 64 254 –
53 73 310 –
50 68 285 –
3. Results and discussion Fig. 1 shows the transfer (ID–VGS) characteristics of 140 MeV Si10+ ion irradiated MOSFETs at VDS = 1 V. From this figure it can be seen that the ID swings towards negative gate voltage with the increase in Si10+ ion total dose. This means that the threshold voltage (VTH) decreases with increase in ion total dose. The VTH is defined as the negative gate voltage for which the drain current becomes 1 lA (VTH VGS @ ID = 1 lA). Fig. 2 shows the variation in VTH versus total dose for the different ions and Co-60 gamma irradiated MOSFETs. From the figure it is clear that the VTH degradation is more for gamma irradiated MOSFETs when compared to swift heavy ion irradiated MOSFETs. In other words, Co-60 gamma radiation creates more trapped charge in oxide layer of the MOSFETs than that of higher LET radiations like the ions examined here. We have studied the high energy ion irradiation effects on SiGe HBTs, Si BJTs and MOSFETs. We observed more degradation in DC electrical characteristics of gamma irradiated MOSFETs when compared to ion irradiated MOSFETs. Whereas in NPN transistors and SiGe HBTs, we observed more degradation in forward mode Gummel characteristics for ion irradiated transistors when compared to gamma irradiated transistors. Particularly in SiGe HBTs, ion irradiation creates more degradation in emitter–base (E–B) spacer oxide when compared to the gamma radiation. Therefore we observed more degradation in Gummel characteristics, current gain, transconductance and other electrical characteristics for ion irradiated HBTs when compared to gamma radiation. The degradation in shallow trench isolation oxide (STI) is studied by inverse mode Gummel characteristics, where emitter and collector terminals of HBT are interchanged while taking Gummel characteristics.
-4
1x10
-5
1x10
-6
1x10
0.020 0.017 0.015 0.013 0.010
ID (A)
-3
1x10
ID (A)
0.022
Pre-rad 100krad 300krad 600krad 1Mrad 3Mrad 6Mrad 10Mrad 30Mrad 60Mrad 100Mrad
-2
1x10
-7
1x10
0.007
-8
1x10
0.005
-9
1x10
0.003
140 MeV Silicon ion
-10
1x10
-6
-4
-2
0
VGS (V)
2
4
0.000
6
Fig. 1. Subthreshold characteristics of 140 MeV Si10+ ion irradiated MOSFET.
-1.0 -1.5
Threshold voltage, VTH (V)
at Pondicherry University in Puducherry, India. Identical total dose was given for different ion species and gamma radiation. The gate terminals of MOSFETs were biased at +2 V (VGS = +2 V) during irradiation. The electrical characterization of the un-irradiated and irradiated MOSFETs were performed using a computer interfaced Agilent 4155 Semiconductor Parameter Analyzer (see Table 1).
Range in lm
-2.0 -2.5 -3.0 -3.5 -4.0 -4.5
Co-60 Gamma 48 MeV Li ion 100 MeV F ion 140 MeV Si ion
-5.0 2
Pre-rad 1x10
3
1x10
4
1x10
5
1x10
Total dose (krad) Fig. 2. Threshold voltage degradation as a function of total dose for different swift heavy ions and Co-60 gamma irradiated MOSFETs.
STI oxide is made up of SiO2 and observed more degradation for gamma irradiated HBT than ion irradiated HBT. The different behaviour of E–B spacer oxide and STI oxide to different radiation is because; the EB spacer oxide is the oxide/nitride composite whereas, the STI is made up of silicon dioxide (SiO2). The effect of nitrogen near the insulator/silicon improves the radiation hardening by undergoing very small amount of ionization and creating less number of radiation induced G/R traps. Hence E–B spacer oxide is immune to gamma radiation and STI oxide undergoes more ionization for gamma radiation [8–11]. The insulating gate oxide layer of the MOSFETs studied in the present work is made up of SiO2. Therefore, we expect more charge yield in SiO2 after gamma irradiation when compared to ion irradiation. As a result, the number of electron–hole pairs produced by Co-60 gamma radiation at the interface and in the oxide layer is nearly an order of magnitude higher than that produced by the ions (Figs. 3 and 4). In Fig. 2 it can be seen that the VTH decreases up to 10 Mrad of Co-60 gamma dose and beyond 10 Mrad there is no much decrease in VTH up to a total dose of 100 Mrad and it is unexpected. A detailed investigation is required to further understand this phenomenon. The net threshold voltage shift (DVTH) is the sum of the shift due to the interface trapped charge (DVNit) and the oxide trapped charge (DVNot). DVNit and DVNot were calculated from the subthreshold measurements using the technique proposed by McWhorter and Winokur [12]. The oxide trapped charge density (DNot) and the interface trapped charge density (DNit) were calculated using the standard expression DNot = DVot Cox/q and DNit = DVit Cox/q, where q = 1.6 1019 C, DNot is the effective oxide-trapped charge density in cm2 as projected to the interface, Cox is the oxide capacitance per unit area and DNit is the effective interface-trapped charge density. The DNot variation of the MOSFETs as a function of total dose is shown in Fig. 3 for different irradiation types. Similarly, the variation of DNit as a function of total dose is shown in Fig. 4 for different radiation. From Figs. 3 and 4 it is clear that DNot is higher than DNit, and hence the VTH shifts towards negative voltage side. It
42
N. Pushpa et al. / Nuclear Instruments and Methods in Physics Research B 273 (2012) 40–42
1400
Co-60 Gamma 48 MeV Li ion 100 MeV F ion 140 MeV Si ion
1200
Mobility (cm2/V-s)
12
ΔNot(cm-2)
1x10
11
1x10
1000
Co-60 Gamma 48MeV Li ion 100 MeV F ion 140 MeV Si ion
10
1x10
2
1x10
3
4
1x10
600 400 200 0 0
5
1x10
800
1x10
Fig. 3. Oxide trapped density versus total dose for different swift heavy ions and Co-60 gamma irradiated MOSFETs.
4
6x10
4
8x10
5
1x10
Fig. 6. Mobility versus total dose for different swift heavy ions and Co-60 gamma irradiated MOSFETs.
gmPeak (called field effect mobility, lFE). Fig. 6 shows the variation of lFE for different high energy ions and Co-60 gamma irradiated MOSFETs for varying total doses. From this figure, it can be seen that the lFE of the irradiated devices decreases with increase in radiation total dose and it is observed about 80% degradation in lFE after a total dose of 100 Mrad. The degradation in l is mainly due to the interface-trapped charge (Nit) and the effect of oxidetrapped charges (Not) is negligible.
12
1x10
-2
4
4x10
Total dose (krad)
Total Dose (krad)
ΔNit (cm )
4
2x10
11
1x10
10
1x10
4. Summary
Co-60 Gamma 48MeV Li ion 100 MeV F ion 140 MeV Si ion
9
1x10
2
1x10
3
4
1x10
5
1x10
1x10
Total Dose (krad) Fig. 4. Interface trapped density versus total dose for different swift heavy ions and Co-60 gamma irradiated MOSFETs.
can also be observed the values of DNot and DNit are more for gamma irradiated MOSFETs when compared to swift heavy ion irradiated MOSFETs. Fig. 5 shows the variation in peak transconductance (gmPeak) with total dose for ions examined here and Co-60 gamma irradiated MOSFETs. The peak transconductance (gmPeak) was extracted from the ID versus VGS data taken at VDS = 0.1 V and the mobility of carriers in the N-channel were estimated from the gmPeak. It can be seen from Fig. 5 that the gmPeak decreases with increase in total dose and more degradation was observed in gamma irradiated MOSFETs compared to ion irradiated MOSFETs. The mobility (l) of carriers (electrons) in the channel was estimated from the
The electrical characteristics of N-channel depletion MOSFETs were studied before and after 140 MeV Si10+ ions, 100 MeV F8+ ions, 48 MeV Li3+ ions and Co-60 gamma irradiation for the total doses ranging from 100 krad to 100 Mrad. We observed that the VTH of the irradiated MOSFETs decreases significantly after 100 Mrad of total dose for different radiation examined here. However, more degradation was observed for gamma irradiated MOSFETs when compared to ion irradiated MOSFETs. More deterioration in MOSFETs after Co-60 gamma irradiation is attributed to more charge yield in MOS structure when compared to swift heavy ion irradiation. Around 80% degradation was observed in transconductance (gm) and mobility of carriers after receiving a total dose of 100 Mrad. Acknowledgment This work is carried out under the Research Project sanctioned by DAE-BRNS, Government of India (Project No. 2009/37/35/BRNS/ 2275). References
Peak Transconductance (gmPeak)
0.0007
Co-60 Gamma 48 MeV Li ion 100 MeV F ion 140 MeV Si ion
0.0006 0.0005 0.0004 0.0003 0.0002 0.0001 0.0000 Pre-rad 1x102
3
1x10
4
1x10
5
1x10
Total dose (krad) Fig. 5. Peak transconductance versus total dose for different swift heavy ions and Co-60 gamma irradiated MOSFETs.
[1] N. Pushpa, K.C. Praveen, A.P. Gnana Prakash, Y.P. Prabhakara Rao, A. Tripati, G. Govindaraj, D. Revannasiddaiah, Nucl. Instr. Meth. A, 613 (2010) 280. [2] A.P. Gnana Prakash, S.C. Ke, K. Siddappa, Semicond. Sci. Tech. 18 (2003) 1037. [3] A.P. Gnana Prakash, S.C. Ke, K. Siddappa, Radiat. Eff. Defects Solids 158 (9) (2003) 635. [4] A.P. Gnana Prakash, John D. Cressler, S.C. Ke, K. Siddappa, Indian J. Phys. 78 (11) (2004) 1187. [5] S. Kaschieva, S.N. Dmitriev, Ch. Angelov, Nucl. Instr. Meth. B 206 (2003) 452. [6] K. Hayama, K. Takakura, T. Ohtani, T. Kudou, H. Ohyama, A. Mercha, E. Simoen, C. Claeys, Mater. Sci. Semicond. Process. 11 (5–6) (2008) 314. [7] W.J. Stapor, L.S. August, D.H. Wilson, T.R. Oldham, K.M. Murray, IEEE Trans. Nucl. Sci. 32 (6) (1985) 4399. [8] J.D. Cressler, G. Niu, Silicon–germanium heterojunction bipolar transistors, Artech House, Norwood, MA, 2003. [9] P. Paillet, J.R. Schwank, M.R. Shaneyfelt, V. Ferlet-Cavrois, R.L. Jones, O. Flament, E.W. Blackmore, IEEE Trans. Nucl. Sci. 49 (6) (2002) 2656. [10] A.K. Sutton, B.M. Haugerud, A.P. Gnana Prakash, B. Jun, J.D. Cressler, C.J. Marshall, P.W. Marshall, R. Ladbury, F. Guarin, A.J. Joseph, IEEE Trans. Nucl. Sci. 52 (6) (2005) 2358. [11] K.C. Praveen, N. Pushpa, John D Cressler, A.P. Gnana Prakash, J. Nano Electron. Phys. 3 (1) (2011) 348. [12] P.J. McWhorter, P.S. Winkur, Appl. Phys. Lett. 48 (2) (1986) 133.