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Three-level charge pumping study of radiation-induced defects at SiSiO2 interface in submicrometer MOS transistors
,tOURNA
ELSEVIER
L OF
Journal of Non-Crystalline Solids 187 (1995) 211-215
Three-level charge pumping study of radiation-induced defects at Si-SiO2 interface in submicrometer MOS transistors J e a n - L u c A u t r a n a'*, B e r n a r d B a l l a n d a, D a n i e l B a b o t b aLaboratoire de Physique de la Mati~re, URA CNRS 358, bdtiment 502, lnstitut National des Sciences Appliqubes de Lyon, 20 avenue Albert Einstein, 69621 Villeurbanne, France bEquipe Contrfle Non Destructif par Rayonnements lonisants, bdtiment 303, lnstitut National des Sciences Appliqubes de Lyon, 20 avenue Albert Einstein, 69621 Villeurbanne, France
Abstract The charge pumping response of interface traps and near-interfacial oxide traps (border traps) induced by Co 6° gamma rays in submicrometer (0.5 lam channel length) metal-oxide-semiconductor transistors has been studied. Using an improved three-level charge pumping technique, the energy distribution of interface trap parameters (emission times, capture cross-sections and interface state density) has been determined after irradiation in both the upper and lower parts of the silicon band gap on n-channel devices. The influence of border traps on three-level charge pumping measurements is demonstrated for the first time. Good agreement has been found between standard charge pumping and three-level charge pumping characteristics in terms of'breakpoint frequency' at which the charge recombined per cycle deviates from the fast interface state response. The distance of border traps from the interface has been estimated to be ~ 15-20/~ from a trap-to-trap tunneling model. In addition, a new technique is presented based on three-level charge pumping measurements to determine a border trap distribution in the silicon band gap.
1. Introduction The increase of trapped charge in the oxide and the increase of interface state density, Dit , are the most important phenomena produced by radiation exposure in the Si/SiO2 system. But ionizing radiation can also induce a complete change in the nature of the electrically active defects at the interface in the oxide layer in terms of energy distri-
This work is partially supported by IBM Microelectronics, IC Manufacturing Plant of Corbeil-Essonnes, France. * Correspondingauthor. Tel: +33 72 43 87 33. Telefax: +33 72 43 85 31. E-mail: [email protected].
bution of Dit or in terms of trapping-detrapping properties I-1,2]. Recently, it has been reported by Paulsen et al. [3] that the standard charge pumping technique can provide an interesting way to separate the contribution of the near-interface oxide traps (called border traps too I-4,5]) that exchange charge with the semiconductor and the Si/SiO2 interface traps (fast states). In this work, we have used both standard charge pumping (CP) [6] and 3-level charge pumping (3CP) [7-10] techniques on irradiated submicrometer metal-oxide-semiconductor field effect transistors (MOSFETs) in order to determine the energy distributions of interface trap parameters (emission times, capture cross-
0022-3093/95/$09.50 © 1995- Elsevier Science B.V. All rights reserved SSDI 0 0 2 2 - 3 0 9 3 ( 9 5 ) 0 0 1 4 0 - 9
212
J.-L. Autran et al. 1 Journal
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187 (1995) 211b215
sections, Dil) and to examine the charge pumping response of interface and border traps. We also discuss a new method, based on the three-level charge pumping technique, for the determination of a Di, distribution related to the fastest border traps measureable above the interface-trap background in the silicon band gap.
2. Experimental
procedure
u.l
z The n-channel interdigital MOSFETs used in this study have been fabricated at IBM Microelectronics (Corbeil-Essonnes) using a 0.5 urn CMOS technology. The source and the drain form a single comb-shaped junction with the substrate that interpenetrates the polysilicon gate comb. As a consequence, there is a large gate area (14000 urn’) in spite of a short effective channel length (0.5 urn). Charge-voltage (Q-V) and charge pumping measurements can be made directly on the same devices. The samples were irradiated using the INSA Co60 gamma-ray source with a dose rate of 59 krad (SiOJ per hour. For convenience, the MOSFETs were kept unbiased (floating) during irradiation. There was a delay of about 50 h at room temperature between the end of the exposure and the post-irradiation characterization. This delay was chosen to reduce the possibility that the postirradiation interface trap buildup might influence our results. In the 3CP method introduced by Saks and Ancona [7,8], a three-level waveform, as shown schematically in Fig. 1 (inset) is applied to the gate of the MOSFET under test to select a time window and an energy window in the silicon band gap for studying the electrical response of the traps in emission or capture regimes. Recently, a new 3CP procedure, used in this study and based on the use of a high performance arbitrary function generator, has been proposed [9,10]. 3CP and Q-V measurements involve a LeCroy 9101 generator, a Keithley 617 electrometer and an IBM computer. The experimental relation between the gate voltage, Vo, and the surface potential, @s, is needed to calculate the energy distribution of interface-trap parameters. The method used in this study to calculate
IO-8
107
106
IO-5
101
IO-3
1”.z
IW1
INTERMEDIATE LEVEL DURATION t3 ( s 1
Fig. 1. Recombined charge per cycle Qi, as a function of thirdlevel parameters t, and V, in electron emission for a 10 Mrad irradiated MOSFET. The emission times of fast interface states are extracted from these data by a graphic construction for each value of V,. Inset: Three-level waveform used in 3CP experiment. Duration t, and voltage V, refer to the intermediate level.
the &(Vo) curve consists in resolving Poisson’s equation with the appropriate (post-irradiation) doping profile and then calculating the theoretical capacitance-voltage (C-V) curve by the method introduced by Panagraphi [11,12]. An excellent agreement is found between the theoretical curve and experimental C-V data obtained from static Q-I’ measurements made on the gate-substrate capacitor (source-drain and substrate are grounded [13]). This agreement ensures a good accuracy for the energy trap positions. Thus, problems relating to potential errors in inversion regimes can be eliminated [lo]. The complete procedure and various experimental results have been detailed elsewhere [lo] and a recent discussion of this problem has been made by Nicollian [l].
3. Results Fig. 1 shows the variations of the recombined charge per cycle Qit pumped into the substrate with the third level parameters (duration t3 and voltage level V,) for a 10 Mrad irradiated device in electron emission regime. Generally, a saturation of Qit has been observed for long tJ: this feature corresponds to an equilibrium state (constant interface trapped charge) since all traps above the Fermi level (fixed
J.-L. Autran et al. /Journal of Non-Crystalline Solids 187 (1995) 211-215
by V3) have emitted their electron [7]. In Fig. 1, one can see that Q~t remains non-constant for most values of v3 after the beginning of the saturation regime and abnormally increases for t3 above ,-~ 1 ms. We attribute this increase to radiation-induced traps located in the interfacial region that exchange charges with the semiconductor, i.e. border traps [4,5]. This assumption has also been recently reported by Paulsen et al. within the framework of a CP experiment [-3]. For irradiated devices or ultrathin tunnel oxide non-volatile memories, the authors have observed an increase of Q~t when the frequency of the gate square pulses tends towards small values. As shown in Fig. 2, a similar experiment has been performed on our 10 Mrad irradiated devices in order to compare CP and 3CP results. A 'breakpoint frequency' is observed between 300 and 600 Hz; this value can be interpreted as the inverse of the time constant of the charge exchange mechanism between the border traps and the semiconductor. According to 3CP data of Fig. 1, a good agreement is found between the two techniques. Emission times of border traps can be estimated in the decade between 1 and 10 ms for V3 values between 0.8 and 0.24V; they are above 10 ms for V3 values less than 0.16 V. Different models of border traps (and/or 'slow states') have been developed in the literature [13]. They
L)
generally consider border traps as defects which can trap charges by a tunneling mechanism directly from the silicon or indirectly via fast interface traps. We have used the model proposed by Roy [3,14] for the estimation of the tunneling distance d of the traps from the Si/SiO2 interface. With an average value ofDit ~ 7 x 101~ eV- ~cm 2, we estimate d to be between 15 and 20/% for a 10 Mrad irradiated device. A similar result is obtained in the case of a 1 Mrad dose exposure. Before examining how to estimate a border trap distribution with 3CP, we now investigate by 3CP the change in fast interface state properties after irradiation. From Qit(t3, V3) curves, the energy distribution of emission times t~ can be determined in the upper part of the band gap for electron traps and in the lower part of the band gap for hole traps [7,8]. Using the Shockely-Read-Hall formalism [ 13], it is then possible to calculate the corresponding capture cross-section distribution. In Fig. 3, t~(E) and a(E) spectra are plotted for virgin and 10 Mrad irradiated devices. Fig. 3(a) shows that irradiation induces a non-uniform increase of the emission times in the silicon band gap. For electron traps, this increase is approximately one decade; a slope of ,,~q/kT ( ~ 38 eV ~ at 300 K) is observed before and after irradiation. Thus, corresponding capture cross-sections displayed in Fig. 3(b) are
l. 80
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FREQUENCY f (Hz) Fig. 2, Recombinedcharge per cycle Qit versus the frequencyfofthe pulses for a non-irradiated devicesand for a 10 Mrad irradiated device.
J.-L. Autran et al. / Journal o f Non-Crystalline Solids 187 (1995) 211-215
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.--a .
.
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TRAP ENERGY Et - Ei (eV)
Fig. 4. Energy distributions of interface state density in silicon
~ 10_17 ~ 10-18 -0.5
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band gap obtained for transistors irradiated to different doses and a n n e a l e d for 50 h at r o o m temperature; t 3 = 6 ms.
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Fig. 3. E n e r g y distribution of electron ( a b o v e midgap) and hole (below midgap) e m i s s i o n times (a) and capture cross-sections (b) in the silicon band gap for a non-irradiated M O S F E T (triangles) and for a M O S F E T i r r a d i a t e d to 10 M r a d and a n n e a l e d for 50 h at r o o m temperature (open circles).
weakly dependent on energy and significantly decrease (about one decade) for irradiated devices with respect to virgin devices. This evolution is consistent with experimental observations in conductance technique [1536]. In the case of hole traps, the changes in 6 and ~ are not constant with respect to the trap energy level. The fact that capture cross-sections seem to be energy dependent before (and after) irradiation is perhaps due to the fabrication process that induces specific defects in the interfacial region. These defects may have an electrical behavior different from those of intrinsic interface defects [17], but this difference is presently unclear. Fig. 4 shows Dit(E) spectra obtained from Qit(V3) characteristics in emission (equilibrium) regime for various radiation exposure doses (the duration of t3 -- 6 ms determines the emission rate of the technique [17]). The curves exhibit two 'humps' of Dit at approximately ~ E v + 0.35 eV and ~ Ev + 0.75 eV. These local increases of density in the band gap are very well correlated with
the dose of irradiation and have been observed by many authors [2]. But this classical approach of 3CP measurement does not allow us to distinguish fast interface trap density from border trap density in the band gap. To evaluate the contribution of these two categories of traps to the charge pumping response in emission regime, as shown in Fig. 1, we have performed two consecutive acquisitions of Qit(lZ3) curves with two different values of /:3. The first value (1.2 ms) was chosen to involve the greater part of fast states during the charge pumping cycle (see Fig. 1), whereas the second value 40 ms allows a part of border traps (the fastest traps that can communicate during this delay with the silicon) to participate in the recombination process. Subtracting the two curves and expressing the derivative of this result with respect to the surface potential and solving for Dit , we obtain a border trap distribution in the silicon band gap, similar to the fast state distribution, which refers to a given emission window. Fig. 5 shows such a border trap distribution estimated with this new method for an irradiated device. Two peaks of density that correspond preferentially to border traps clearly appear in the band gap: one above midgap ('-,Ev + 0.74 eV), the other below midgap ('-,Ev + 0.36eV). Similar energy positions of Dit peaks induced by irradiation have been often reported by other authors using various techniques I-2,18]; but, in this
215
J.-L. Autran et al. / Journal o f Non-Crystalline Solids 187 (1995) 211-215
~, 1012
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T R A P E N E R G Y E t - Ei (eV) Fig. 5. Energy distribution of border trap density for emission times between 1.2 and 40 ms. This curve is calculated from the difference of two 3CP measurements, for two distinct values (1.2 and 40 ms) of the intermediate level in emission regime.
case, we c a n u n a m b i g u o u s l y say t h a t t h e s e p e a k s are d u e to b o r d e r t r a p s i n s t e a d o f fast i n t e r f a c e traps, in v i e w o f t h e e m i s s i o n w i n d o w u s i n g in this method.
4. Conclusion
To our knowledge, this is the first 3-level charge pumping study carried out on irradiated devices, in spite of the fact that this technique can provide various information about interface traps concerning most of the band gap on a single device. We have observed the electrical behavior of interface traps and border traps by this technique and by standard charge pumping analysis in submicrometer n-channel MOSFETs, irradiated or not. For fast interface traps, a strong evolution of capture cross-sections has been observed after irradiation; the change of a appears to be a function of energy trap level in the silicon band gap. For border traps, we have developed a modified 3CP m e t h o d t h a t p r o v i d e s t h e w a y to e s t i m a t e q u a n t i t a t i v e l y a t r a p d i s t r i b u t i o n in the silicon b a n d gap. A n a d d i t i o n a l s t u d y is p l a n n e d to i n v e s t i g a t e b o r d e r t r a p s in m o r e d e t a i l w i t h this n e w a p p r o a c h o f charge pumping.
References [1] E.H. Nicollian, J. Electron. Mater. 21 (1992) 721. [2] T.P. Ma, Microelectron. Eng. 22 (1993) 197, and references therein. [3] R.E. Paulsen, R.R. Siergiej, M.L. French and M.H. White, IEEE Electron Device Len. 13 (1992) 627. [4] D.M. Fleetwood, IEEE Trans. Nucl. Sci. NS-39 (1992) 269. [5] D.M. Fleetwood, P.S. Winokur, R.A. Reber, T.L. Meisenheimer, J.R. Schwank, M.R. Shaneyfelt and L.C. Riewe, J. Appl. Phys. 73 (1993) 5058, and references therein [6] G. Groeseneken, H.E. Macs, N. Beltran and R.F. De Keersmaecker, IEEE Trans. Electron Devices 31 (1984) 42. [7] N.S. Saks and M.G. Ancona, IEEE Trans. Electron Devices 37 (1990) 1057. [8] M.G. Ancona and N.S. Saks, J. Appl. Phys. 71 (1992) 4415. [9] J.L. Autran and B. Balland, Rev. Sci. Instrum. 65 (1994) 2141. [10] J.L. Autran, B. Balland and L.M. Gaborieau, IBM J. Res. Develop. (1995) to be published. [11] R.F. Pierret and G. Panigraphi, J. Appl. Phys. 41 (1970) 2260. [12] G. Panigraphi, Electron. Lett. 9 (1973) 43. [13] B. Balland, Instabilities in Silcon Devices, ed. G. Barbottin and A. Vapaille (Elsevier, Amsterdam, 1989) ch. 10. 1-14] A. Roy, PhD dissertation, Lehigh University (1989). [15] W. Chen and T.P. Ma, J. Appl. Phys. 70 (1991) 860. [16] W. Chen, A. Balasinski and T.P. Ma, IEEE Trans. Nucl. Sci. 39 (1992) 2152. [17] J.L. Autran, F. Seigneur, C. Plossu and B. Balland, J. Appl. Phys. 74 (1993) 3932. [18] P.J. McWhorter, D.M. Fleetwood, R.A. Pastorek and G.T. Zimmerman, IEEE Trans. Nucl. Sci. 36 (1989) 1792.
ELSEVIER
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Journal of Non-Crystalline Solids 187 (1995) 211-215
Three-level charge pumping study of radiation-induced defects at Si-SiO2 interface in submicrometer MOS transistors J e a n - L u c A u t r a n a'*, B e r n a r d B a l l a n d a, D a n i e l B a b o t b aLaboratoire de Physique de la Mati~re, URA CNRS 358, bdtiment 502, lnstitut National des Sciences Appliqubes de Lyon, 20 avenue Albert Einstein, 69621 Villeurbanne, France bEquipe Contrfle Non Destructif par Rayonnements lonisants, bdtiment 303, lnstitut National des Sciences Appliqubes de Lyon, 20 avenue Albert Einstein, 69621 Villeurbanne, France
Abstract The charge pumping response of interface traps and near-interfacial oxide traps (border traps) induced by Co 6° gamma rays in submicrometer (0.5 lam channel length) metal-oxide-semiconductor transistors has been studied. Using an improved three-level charge pumping technique, the energy distribution of interface trap parameters (emission times, capture cross-sections and interface state density) has been determined after irradiation in both the upper and lower parts of the silicon band gap on n-channel devices. The influence of border traps on three-level charge pumping measurements is demonstrated for the first time. Good agreement has been found between standard charge pumping and three-level charge pumping characteristics in terms of'breakpoint frequency' at which the charge recombined per cycle deviates from the fast interface state response. The distance of border traps from the interface has been estimated to be ~ 15-20/~ from a trap-to-trap tunneling model. In addition, a new technique is presented based on three-level charge pumping measurements to determine a border trap distribution in the silicon band gap.
1. Introduction The increase of trapped charge in the oxide and the increase of interface state density, Dit , are the most important phenomena produced by radiation exposure in the Si/SiO2 system. But ionizing radiation can also induce a complete change in the nature of the electrically active defects at the interface in the oxide layer in terms of energy distri-
This work is partially supported by IBM Microelectronics, IC Manufacturing Plant of Corbeil-Essonnes, France. * Correspondingauthor. Tel: +33 72 43 87 33. Telefax: +33 72 43 85 31. E-mail: [email protected].
bution of Dit or in terms of trapping-detrapping properties I-1,2]. Recently, it has been reported by Paulsen et al. [3] that the standard charge pumping technique can provide an interesting way to separate the contribution of the near-interface oxide traps (called border traps too I-4,5]) that exchange charge with the semiconductor and the Si/SiO2 interface traps (fast states). In this work, we have used both standard charge pumping (CP) [6] and 3-level charge pumping (3CP) [7-10] techniques on irradiated submicrometer metal-oxide-semiconductor field effect transistors (MOSFETs) in order to determine the energy distributions of interface trap parameters (emission times, capture cross-
0022-3093/95/$09.50 © 1995- Elsevier Science B.V. All rights reserved SSDI 0 0 2 2 - 3 0 9 3 ( 9 5 ) 0 0 1 4 0 - 9
212
J.-L. Autran et al. 1 Journal
of Non-CrystallineSolids
187 (1995) 211b215
sections, Dil) and to examine the charge pumping response of interface and border traps. We also discuss a new method, based on the three-level charge pumping technique, for the determination of a Di, distribution related to the fastest border traps measureable above the interface-trap background in the silicon band gap.
2. Experimental
procedure
u.l
z The n-channel interdigital MOSFETs used in this study have been fabricated at IBM Microelectronics (Corbeil-Essonnes) using a 0.5 urn CMOS technology. The source and the drain form a single comb-shaped junction with the substrate that interpenetrates the polysilicon gate comb. As a consequence, there is a large gate area (14000 urn’) in spite of a short effective channel length (0.5 urn). Charge-voltage (Q-V) and charge pumping measurements can be made directly on the same devices. The samples were irradiated using the INSA Co60 gamma-ray source with a dose rate of 59 krad (SiOJ per hour. For convenience, the MOSFETs were kept unbiased (floating) during irradiation. There was a delay of about 50 h at room temperature between the end of the exposure and the post-irradiation characterization. This delay was chosen to reduce the possibility that the postirradiation interface trap buildup might influence our results. In the 3CP method introduced by Saks and Ancona [7,8], a three-level waveform, as shown schematically in Fig. 1 (inset) is applied to the gate of the MOSFET under test to select a time window and an energy window in the silicon band gap for studying the electrical response of the traps in emission or capture regimes. Recently, a new 3CP procedure, used in this study and based on the use of a high performance arbitrary function generator, has been proposed [9,10]. 3CP and Q-V measurements involve a LeCroy 9101 generator, a Keithley 617 electrometer and an IBM computer. The experimental relation between the gate voltage, Vo, and the surface potential, @s, is needed to calculate the energy distribution of interface-trap parameters. The method used in this study to calculate
IO-8
107
106
IO-5
101
IO-3
1”.z
IW1
INTERMEDIATE LEVEL DURATION t3 ( s 1
Fig. 1. Recombined charge per cycle Qi, as a function of thirdlevel parameters t, and V, in electron emission for a 10 Mrad irradiated MOSFET. The emission times of fast interface states are extracted from these data by a graphic construction for each value of V,. Inset: Three-level waveform used in 3CP experiment. Duration t, and voltage V, refer to the intermediate level.
the &(Vo) curve consists in resolving Poisson’s equation with the appropriate (post-irradiation) doping profile and then calculating the theoretical capacitance-voltage (C-V) curve by the method introduced by Panagraphi [11,12]. An excellent agreement is found between the theoretical curve and experimental C-V data obtained from static Q-I’ measurements made on the gate-substrate capacitor (source-drain and substrate are grounded [13]). This agreement ensures a good accuracy for the energy trap positions. Thus, problems relating to potential errors in inversion regimes can be eliminated [lo]. The complete procedure and various experimental results have been detailed elsewhere [lo] and a recent discussion of this problem has been made by Nicollian [l].
3. Results Fig. 1 shows the variations of the recombined charge per cycle Qit pumped into the substrate with the third level parameters (duration t3 and voltage level V,) for a 10 Mrad irradiated device in electron emission regime. Generally, a saturation of Qit has been observed for long tJ: this feature corresponds to an equilibrium state (constant interface trapped charge) since all traps above the Fermi level (fixed
J.-L. Autran et al. /Journal of Non-Crystalline Solids 187 (1995) 211-215
by V3) have emitted their electron [7]. In Fig. 1, one can see that Q~t remains non-constant for most values of v3 after the beginning of the saturation regime and abnormally increases for t3 above ,-~ 1 ms. We attribute this increase to radiation-induced traps located in the interfacial region that exchange charges with the semiconductor, i.e. border traps [4,5]. This assumption has also been recently reported by Paulsen et al. within the framework of a CP experiment [-3]. For irradiated devices or ultrathin tunnel oxide non-volatile memories, the authors have observed an increase of Q~t when the frequency of the gate square pulses tends towards small values. As shown in Fig. 2, a similar experiment has been performed on our 10 Mrad irradiated devices in order to compare CP and 3CP results. A 'breakpoint frequency' is observed between 300 and 600 Hz; this value can be interpreted as the inverse of the time constant of the charge exchange mechanism between the border traps and the semiconductor. According to 3CP data of Fig. 1, a good agreement is found between the two techniques. Emission times of border traps can be estimated in the decade between 1 and 10 ms for V3 values between 0.8 and 0.24V; they are above 10 ms for V3 values less than 0.16 V. Different models of border traps (and/or 'slow states') have been developed in the literature [13]. They
L)
generally consider border traps as defects which can trap charges by a tunneling mechanism directly from the silicon or indirectly via fast interface traps. We have used the model proposed by Roy [3,14] for the estimation of the tunneling distance d of the traps from the Si/SiO2 interface. With an average value ofDit ~ 7 x 101~ eV- ~cm 2, we estimate d to be between 15 and 20/% for a 10 Mrad irradiated device. A similar result is obtained in the case of a 1 Mrad dose exposure. Before examining how to estimate a border trap distribution with 3CP, we now investigate by 3CP the change in fast interface state properties after irradiation. From Qit(t3, V3) curves, the energy distribution of emission times t~ can be determined in the upper part of the band gap for electron traps and in the lower part of the band gap for hole traps [7,8]. Using the Shockely-Read-Hall formalism [ 13], it is then possible to calculate the corresponding capture cross-section distribution. In Fig. 3, t~(E) and a(E) spectra are plotted for virgin and 10 Mrad irradiated devices. Fig. 3(a) shows that irradiation induces a non-uniform increase of the emission times in the silicon band gap. For electron traps, this increase is approximately one decade; a slope of ,,~q/kT ( ~ 38 eV ~ at 300 K) is observed before and after irradiation. Thus, corresponding capture cross-sections displayed in Fig. 3(b) are
l. 80
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Slow states + (border traps) + ++++++++,t +
12.0 10.5 9,0
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t .60
.
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FREQUENCY f (Hz) Fig. 2, Recombinedcharge per cycle Qit versus the frequencyfofthe pulses for a non-irradiated devicesand for a 10 Mrad irradiated device.
J.-L. Autran et al. / Journal o f Non-Crystalline Solids 187 (1995) 211-215
214 10.3
.--a .
.
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TRAP ENERGY Et - Ei (eV)
Fig. 4. Energy distributions of interface state density in silicon
~ 10_17 ~ 10-18 -0.5
, -0,4
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band gap obtained for transistors irradiated to different doses and a n n e a l e d for 50 h at r o o m temperature; t 3 = 6 ms.
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Fig. 3. E n e r g y distribution of electron ( a b o v e midgap) and hole (below midgap) e m i s s i o n times (a) and capture cross-sections (b) in the silicon band gap for a non-irradiated M O S F E T (triangles) and for a M O S F E T i r r a d i a t e d to 10 M r a d and a n n e a l e d for 50 h at r o o m temperature (open circles).
weakly dependent on energy and significantly decrease (about one decade) for irradiated devices with respect to virgin devices. This evolution is consistent with experimental observations in conductance technique [1536]. In the case of hole traps, the changes in 6 and ~ are not constant with respect to the trap energy level. The fact that capture cross-sections seem to be energy dependent before (and after) irradiation is perhaps due to the fabrication process that induces specific defects in the interfacial region. These defects may have an electrical behavior different from those of intrinsic interface defects [17], but this difference is presently unclear. Fig. 4 shows Dit(E) spectra obtained from Qit(V3) characteristics in emission (equilibrium) regime for various radiation exposure doses (the duration of t3 -- 6 ms determines the emission rate of the technique [17]). The curves exhibit two 'humps' of Dit at approximately ~ E v + 0.35 eV and ~ Ev + 0.75 eV. These local increases of density in the band gap are very well correlated with
the dose of irradiation and have been observed by many authors [2]. But this classical approach of 3CP measurement does not allow us to distinguish fast interface trap density from border trap density in the band gap. To evaluate the contribution of these two categories of traps to the charge pumping response in emission regime, as shown in Fig. 1, we have performed two consecutive acquisitions of Qit(lZ3) curves with two different values of /:3. The first value (1.2 ms) was chosen to involve the greater part of fast states during the charge pumping cycle (see Fig. 1), whereas the second value 40 ms allows a part of border traps (the fastest traps that can communicate during this delay with the silicon) to participate in the recombination process. Subtracting the two curves and expressing the derivative of this result with respect to the surface potential and solving for Dit , we obtain a border trap distribution in the silicon band gap, similar to the fast state distribution, which refers to a given emission window. Fig. 5 shows such a border trap distribution estimated with this new method for an irradiated device. Two peaks of density that correspond preferentially to border traps clearly appear in the band gap: one above midgap ('-,Ev + 0.74 eV), the other below midgap ('-,Ev + 0.36eV). Similar energy positions of Dit peaks induced by irradiation have been often reported by other authors using various techniques I-2,18]; but, in this
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~, 1012
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EMISSION WINDOW 1.2 m s < te < 40 ms
G}
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peak o~
Z
peak 13
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-0.4
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
T R A P E N E R G Y E t - Ei (eV) Fig. 5. Energy distribution of border trap density for emission times between 1.2 and 40 ms. This curve is calculated from the difference of two 3CP measurements, for two distinct values (1.2 and 40 ms) of the intermediate level in emission regime.
case, we c a n u n a m b i g u o u s l y say t h a t t h e s e p e a k s are d u e to b o r d e r t r a p s i n s t e a d o f fast i n t e r f a c e traps, in v i e w o f t h e e m i s s i o n w i n d o w u s i n g in this method.
4. Conclusion
To our knowledge, this is the first 3-level charge pumping study carried out on irradiated devices, in spite of the fact that this technique can provide various information about interface traps concerning most of the band gap on a single device. We have observed the electrical behavior of interface traps and border traps by this technique and by standard charge pumping analysis in submicrometer n-channel MOSFETs, irradiated or not. For fast interface traps, a strong evolution of capture cross-sections has been observed after irradiation; the change of a appears to be a function of energy trap level in the silicon band gap. For border traps, we have developed a modified 3CP m e t h o d t h a t p r o v i d e s t h e w a y to e s t i m a t e q u a n t i t a t i v e l y a t r a p d i s t r i b u t i o n in the silicon b a n d gap. A n a d d i t i o n a l s t u d y is p l a n n e d to i n v e s t i g a t e b o r d e r t r a p s in m o r e d e t a i l w i t h this n e w a p p r o a c h o f charge pumping.
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