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Extraction of the carrier generation and recombination lifetime from the forward characteristics of advanced diodes
Materials Science and Engineering B102 (2003) 189 /192 www.elsevier.com/locate/mseb
Extraction of the carrier generation and recombination lifetime from the forward characteristics of advanced diodes A. Poyai a,b,*, E. Simoen a, C. Claeys a,b, E. Gaubas c, A. Huber d, D. Gra¨f d a
IMEC, Kapeldreef 75, B-3001 Leuven, Belgium E.E. Department, KU Leuven, Kasteelpark Arenberg 10, B-3001 Leuven, Belgium c Institute of Material Research and Applied Sciences, Vilnius University, Sauletkio av. 10, LT-2040 Vilnius, Lithuania d Wacker Siltronic A.G., P.O. Box 1140, D-84479 Burghausen, Germany b
Received 13 April 2002; received in revised form 19 August 2002; accepted 21 October 2002
Abstract This paper proposes to extract the generation and recombination lifetime simultaneously from the recombination current in forward operation. The method is applied to n /p-well junctions fabricated in advanced processing schemes on silicon wafers with different grown-in defect concentrations. The value of the recombination lifetime is confirmed by the data obtained from the crosssectional microwave absorption (MWA) technique. It is also shown that the obtained generation lifetime agrees well with the one found from the reverse current after correction for the generation width and the electric field. In conclusion, the proposed method gives reasonable values for the generation and recombination lifetime in advanced silicon p /n junctions. Mostly, these junctions will be dominated by the processing-induced defects. # 2003 Elsevier B.V. All rights reserved. Keywords: Current /voltage; Capacitance /voltage; Generation lifetime; Recombination lifetime PACS numbers: 72.80.cw
1. Introduction
2. Experimental
Several techniques have been developed to measure both the carrier generation (tg) and recombination (tr) lifetime in the bulk of a silicon wafer [1]. tg and tr can, for example, be derived from the reverse and forward diode characteristics, respectively [2]. Straightforward calculation can yield lifetimes differing several orders of magnitude from the values measured with other techniques [3]. For modern, scaled junctions, this discrepancy is related to the high electric field and the shallowjunction effect in the reverse and forward characteristics, respectively. To overcome this problem, a method to extract tg and tr from the recombination current in forward operation is proposed.
Shallow junctions in a retrograde p-well have been fabricated according to the standard IMEC’s 0.18 mm CMOS processing scheme in a variety of 200 mm Czochralski (Cz) p-type wafers, the main features of which are given in Table 1. Polysilicon encapsulated local oxidation of silicon (PELOX) [5] was used for isolation. A retrograde p-well was obtained by a deep (1.2 /1013 cm 2 at 180 keV) and a shallow (1 /1013 cm 2 at 35 keV) boron ion implantation, followed by dopant activation annealing at 850 8C. The n-region was made by an arsenic ion implantation (4 /1015 cm 2 at 70 keV) and activated by an RTP step at 1070 8C. This resulted in a junction depth of around 0.1 mm. A cobalt silicide with titanium capping layer (12 nm Co/8 nm Ti) was applied with a maximum silicidation temperature of 850 8C before aluminum metallization. Also, indicated in Table 1 are tg and tr of the starting material, extracted from capacitance /time (C /t ) measurements on MOS capacitors [1] and derived from the
* Corresponding author. Present address: TMEC, NECTEC, 51/4, Moo 1, Wangtakien Distric, Amphur Muang, Chachoengsao 24000, Thailand. Tel.: /66-38-857-100; fax: /66-38-857-175. E-mail address: [email protected] (A. Poyai). 0921-5107/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0921-5107(02)00654-2
A. Poyai et al. / Materials Science and Engineering B102 (2003) 189 /192
190 Table 1 Description of the wafer types Label
Substrate, r (V cm)
Center Oi (1017 at. cm 3) average
MOS tg (ms)
MWA tr (ms)
Material
M1 M2 M3
10 /18 10 /18 7 /13
6.46 5.91 6.94
2.17 2.11 1.85
30 35 40
High COP /high Oi High COP /low Oi low COP-high Oi
The interstitial oxygen (Oi) is calculated according to the new ASTM standard [4].
cross-section microwave absorption (MWA) technique [6]. The static current /voltage (I /V ) characteristics of different geometry rectangular (SQ) and meander (ME) diodes were measured. This was performed at wafer level in the dark and in the voltage range from /2 to / 1 V. The bias was applied to the back p-type substrate and the current was measured at the grounded top n contact. The capacitance/voltage (C /V ) measurements were performed on the same diodes at a frequency of 100 kHz.
3. Results and discussion The following method to obtain tr and tg from the recombination current in n /p-well junctions is proposed. Combining a number of SQ and ME diodes allows to separate the volume or bulk current density (JA (A cm 2)) which scales with the diode area (A ), from the perimeter density (JP (A cm 1)), assumed proportional with perimeter (P ). In principle, also a corner component (JC (A/corner)) can be separated. More detail on the analysis procedure can be found in [7]. Fig. 1a shows JA versus bias for diodes on silicon wafers with different grown-in defects and interstitial oxygen (Oi) concentrations as indicated in Table 1. No marked impact is observed of the crystal-originated particle (COP) or Oi density on the current. This points out that the current characteristics of deep sub-micron diodes, fabricated in state-of-the-art Cz substrates according to our 0.18 mm CMOS scheme are dominated by the processing-induced defects.
In general, JA can be written as [8]
qni WA JA JdA qV tg 2tr exp 2kT
exp
qV 1 kT
(1)
with q the elementary charge, ni the intrinsic carrier concentration, k Boltzmann’s constant and T the absolute temperature. WA ( /oSi/CjA, oSi is the silicon permittivity, CjA the area junction capacitance) is the area depletion width. CjA is derived from the junction capacitance (Cj) of different geometry diodes, as outlined elsewhere [9]. Cj can be calculated from the measured capacitance (C ) by taking the series resistance (Rs) into account [10]. The second term of Eq. (1) in the first parenthesis is the area bulk generation current density (JgbA) for reverse bias, while it is the area bulk recombination current (JrbA) for forward bias (VF). The area saturation current density (JA0) can be obtained from JA by dividing with (exp(qV /kT ) /1). Fig. 1b shows the forward area saturation current density versus bias for the low COP /high Oi case (M3). It shows that at low forward bias (VF B/0.15 V) JA0 is dominated by the recombination current while JA0 at VF /0.15 /0.4 V is expected to be controlled by the area diffusion current density (JdA). By extrapolating JA0 in the linear part to VF /0 and Vbi ( /1 V), the values of 56 and 10 pA cm 2 were found, respectively. The built-in potential of the junction (Vbi) has been extracted from a 1=Cj2 versus V plot, by extrapolating the linear fit to the x -axis. It should be remarked here that JA0 at VF /0 V is close to the value that is derived from the forward bias
Fig. 1. (a) Area current density versus bias for different Cz materials (high COP /high Oi (M1), high COP /low Oi (M2) and low COP /high Oi (M3)) and (b) forward area saturation current density (JA0) versus bias for M3.
A. Poyai et al. / Materials Science and Engineering B102 (2003) 189 /192
Fig. 2. qniWA/JrbA0 versus exp(qV /2kT ) of low COP /high Oi (M3) material.
method, which is obtained by combining the diffusion current Id of different geometry diodes. Id is calculated from the forward I /V characteristics by taking the ideality factor (m ) into account [11]. By considering JA0 in Fig. 1b, it indicates that JA0 at VF /0.15 /0.4 V is composed of the diffusion and recombination current. This gives rise to a reduction of JA0 with increasing forward bias (smaller depletion width). From this, it is derived that JA0 at V /Vbi is JdA. The saturation area bulk recombination current density (JrbA0) equal to JA0 /JdA can be derived from Eq. (1), yielding after rearranging, qni WA qV tg : 2tr exp (2) 2kT JrbA0 By plotting (qniWA/JrbA0) versus exp(qV /2kT ) as shown in Fig. 2, 2tr and tg can be obtained from the slope and the crossing point at exp(qV /2kT ) /0, respectively. The values of 5.18 and 79.28 ms were found for tr and tg, respectively. The lifetimes obtained from this method are spatially averaged ones and correspond with a low electric field. Both tr and tg after processing are lower than the ones before processing, indicating the dominance of the processing-induced defects. Are these values for tr and tg correct? One way of tackling this issue is by an independent evaluation of tr and tg using a different technique. Therefore, MWA measurements were performed on the same devices.
191
However, the tr derived from the diode I /V is considerably lower than the one obtained by MWA. This indicates that tr in the normally doped p-type bulk of the wafer, derived from MWA is higher than in the highly doped p-well region (forward I /V method). In fact, from MWA analysis (Fig. 3a), a recombination lifetime profile is derived, with lower tr values closer to the sample surface and approaching the data of the diode analysis, which corresponds to the near-surface ( B/1 mm) p-well region. This confirms that the tr derived by this approach yields a reasonable estimate. Alternatively, it is possible to derive tg from JgbA, which may serve as a further validation of the proposed technique. tg has been calculated by taking both the generation width [12] and the electric field enhancement factor [13] into account. The generation lifetime obtained from the forward current in Fig. 2 is close to the one found from the reverse current after correction for generation width and electric field enhancement factor (Fig. 3b). This indicates that the proposed method to obtain tg and tr from the recombination current gives reasonable values. At the same time, our electric field correction procedure is also consistent with the low-field forward current analysis of tg.
4. Conclusions It has been demonstrated that the proposed method to obtain tg and tr from the recombination current gives reasonable values and overcomes lifetime discrepancy due to the high electric field and the shallow-junction effect in scaled junctions. tr is in agreement with the one measured from MWA, while tg coincides with the one derived from the generation current after correcting for the generation width and electric field enhancement factor. It is furthermore concluded that the lower tg and tr after processing, compared with the starting values of the substrates, point to the dominance of processinginduced defects in the near-surface p-well region. This is also derived from the fact that the I /V characteristics are not influenced by the COP and Oi densities.
Fig. 3. (a) Recombination lifetime versus the depth below the surface derived from the cross-sectional MWA technique for low COP /high Oi (M3) material and (b) generation lifetime versus depletion width for diodes processed on low COP /high Oi (M3) material.
192
A. Poyai et al. / Materials Science and Engineering B102 (2003) 189 /192
References [1] D.K. Schroder, Semiconductor Material and Device Characterization, 2nd ed., Wiley, New York, 1998. [2] Y. Murakami, T. Shingyouji, J. Appl. Phys. 75 (1994) 3548. [3] C. Claeys, E. Simoen, A. Poyai, A. Czerwinski, J. Electrochem. Soc. 146 (1999) 3429. [4] A. Baghdadi, W. Bullis, M. Croarkin, Y.-Z. Li, R. Scace, R. Series, P. Stallhofer, M. Watanabe, J. Electrochem. Soc. 136 (1989) 2015. [5] G. Badenes, R. Rooyackers, I. DeWolf, L. Deferm, Jpn. J. Appl. Phys. 36 (1997) 1325. [6] E. Gaubas, E. Simoen, C. Claeys, A. Poyai, Solid State Phenomena 69 /70 (1999) 155.
[7] C. Claeys, A. Poyai, E. Simoen, A. Czerwinski, J. Katcki, J. Electrochem. Soc. 146 (1999) 1151. [8] S.M. Sze, Physics of Semiconductor Devices, Wiley, New York, 1981. [9] A. Czerwinski, E. Simoen, C. Claeys, K. Klima, D. Tomaszewski, J. Gibki, J. Katcki, J. Electrochem. Soc. 145 (1998) 2107. [10] A. Poyai, E. Simoen, C. Claeys, Appl. Phys. Lett. 80 (2002) 1192. [11] J. Vanhellemont, E. Simoen, C. Claeys, Appl. Phys. Lett. 66 (1995) 2894. [12] A. Czerwinski, E. Simoen, A. Poyai, C. Claeys, in: T. Abe, W.M. Bullis, S. Kobayashi, W. Lin, P. Wagner (Eds.), Third International Symposium on Defects in Silicon, vols. PV 99-1, The Electrochemical Society, Pennington, NJ, 1999, pp. 88 /99. [13] A. Poyai, E. Simoen, C. Claeys, IEEE Trans. Electron Devices 48 (2001) 2445.
Extraction of the carrier generation and recombination lifetime from the forward characteristics of advanced diodes A. Poyai a,b,*, E. Simoen a, C. Claeys a,b, E. Gaubas c, A. Huber d, D. Gra¨f d a
IMEC, Kapeldreef 75, B-3001 Leuven, Belgium E.E. Department, KU Leuven, Kasteelpark Arenberg 10, B-3001 Leuven, Belgium c Institute of Material Research and Applied Sciences, Vilnius University, Sauletkio av. 10, LT-2040 Vilnius, Lithuania d Wacker Siltronic A.G., P.O. Box 1140, D-84479 Burghausen, Germany b
Received 13 April 2002; received in revised form 19 August 2002; accepted 21 October 2002
Abstract This paper proposes to extract the generation and recombination lifetime simultaneously from the recombination current in forward operation. The method is applied to n /p-well junctions fabricated in advanced processing schemes on silicon wafers with different grown-in defect concentrations. The value of the recombination lifetime is confirmed by the data obtained from the crosssectional microwave absorption (MWA) technique. It is also shown that the obtained generation lifetime agrees well with the one found from the reverse current after correction for the generation width and the electric field. In conclusion, the proposed method gives reasonable values for the generation and recombination lifetime in advanced silicon p /n junctions. Mostly, these junctions will be dominated by the processing-induced defects. # 2003 Elsevier B.V. All rights reserved. Keywords: Current /voltage; Capacitance /voltage; Generation lifetime; Recombination lifetime PACS numbers: 72.80.cw
1. Introduction
2. Experimental
Several techniques have been developed to measure both the carrier generation (tg) and recombination (tr) lifetime in the bulk of a silicon wafer [1]. tg and tr can, for example, be derived from the reverse and forward diode characteristics, respectively [2]. Straightforward calculation can yield lifetimes differing several orders of magnitude from the values measured with other techniques [3]. For modern, scaled junctions, this discrepancy is related to the high electric field and the shallowjunction effect in the reverse and forward characteristics, respectively. To overcome this problem, a method to extract tg and tr from the recombination current in forward operation is proposed.
Shallow junctions in a retrograde p-well have been fabricated according to the standard IMEC’s 0.18 mm CMOS processing scheme in a variety of 200 mm Czochralski (Cz) p-type wafers, the main features of which are given in Table 1. Polysilicon encapsulated local oxidation of silicon (PELOX) [5] was used for isolation. A retrograde p-well was obtained by a deep (1.2 /1013 cm 2 at 180 keV) and a shallow (1 /1013 cm 2 at 35 keV) boron ion implantation, followed by dopant activation annealing at 850 8C. The n-region was made by an arsenic ion implantation (4 /1015 cm 2 at 70 keV) and activated by an RTP step at 1070 8C. This resulted in a junction depth of around 0.1 mm. A cobalt silicide with titanium capping layer (12 nm Co/8 nm Ti) was applied with a maximum silicidation temperature of 850 8C before aluminum metallization. Also, indicated in Table 1 are tg and tr of the starting material, extracted from capacitance /time (C /t ) measurements on MOS capacitors [1] and derived from the
* Corresponding author. Present address: TMEC, NECTEC, 51/4, Moo 1, Wangtakien Distric, Amphur Muang, Chachoengsao 24000, Thailand. Tel.: /66-38-857-100; fax: /66-38-857-175. E-mail address: [email protected] (A. Poyai). 0921-5107/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0921-5107(02)00654-2
A. Poyai et al. / Materials Science and Engineering B102 (2003) 189 /192
190 Table 1 Description of the wafer types Label
Substrate, r (V cm)
Center Oi (1017 at. cm 3) average
MOS tg (ms)
MWA tr (ms)
Material
M1 M2 M3
10 /18 10 /18 7 /13
6.46 5.91 6.94
2.17 2.11 1.85
30 35 40
High COP /high Oi High COP /low Oi low COP-high Oi
The interstitial oxygen (Oi) is calculated according to the new ASTM standard [4].
cross-section microwave absorption (MWA) technique [6]. The static current /voltage (I /V ) characteristics of different geometry rectangular (SQ) and meander (ME) diodes were measured. This was performed at wafer level in the dark and in the voltage range from /2 to / 1 V. The bias was applied to the back p-type substrate and the current was measured at the grounded top n contact. The capacitance/voltage (C /V ) measurements were performed on the same diodes at a frequency of 100 kHz.
3. Results and discussion The following method to obtain tr and tg from the recombination current in n /p-well junctions is proposed. Combining a number of SQ and ME diodes allows to separate the volume or bulk current density (JA (A cm 2)) which scales with the diode area (A ), from the perimeter density (JP (A cm 1)), assumed proportional with perimeter (P ). In principle, also a corner component (JC (A/corner)) can be separated. More detail on the analysis procedure can be found in [7]. Fig. 1a shows JA versus bias for diodes on silicon wafers with different grown-in defects and interstitial oxygen (Oi) concentrations as indicated in Table 1. No marked impact is observed of the crystal-originated particle (COP) or Oi density on the current. This points out that the current characteristics of deep sub-micron diodes, fabricated in state-of-the-art Cz substrates according to our 0.18 mm CMOS scheme are dominated by the processing-induced defects.
In general, JA can be written as [8]
qni WA JA JdA qV tg 2tr exp 2kT
exp
qV 1 kT
(1)
with q the elementary charge, ni the intrinsic carrier concentration, k Boltzmann’s constant and T the absolute temperature. WA ( /oSi/CjA, oSi is the silicon permittivity, CjA the area junction capacitance) is the area depletion width. CjA is derived from the junction capacitance (Cj) of different geometry diodes, as outlined elsewhere [9]. Cj can be calculated from the measured capacitance (C ) by taking the series resistance (Rs) into account [10]. The second term of Eq. (1) in the first parenthesis is the area bulk generation current density (JgbA) for reverse bias, while it is the area bulk recombination current (JrbA) for forward bias (VF). The area saturation current density (JA0) can be obtained from JA by dividing with (exp(qV /kT ) /1). Fig. 1b shows the forward area saturation current density versus bias for the low COP /high Oi case (M3). It shows that at low forward bias (VF B/0.15 V) JA0 is dominated by the recombination current while JA0 at VF /0.15 /0.4 V is expected to be controlled by the area diffusion current density (JdA). By extrapolating JA0 in the linear part to VF /0 and Vbi ( /1 V), the values of 56 and 10 pA cm 2 were found, respectively. The built-in potential of the junction (Vbi) has been extracted from a 1=Cj2 versus V plot, by extrapolating the linear fit to the x -axis. It should be remarked here that JA0 at VF /0 V is close to the value that is derived from the forward bias
Fig. 1. (a) Area current density versus bias for different Cz materials (high COP /high Oi (M1), high COP /low Oi (M2) and low COP /high Oi (M3)) and (b) forward area saturation current density (JA0) versus bias for M3.
A. Poyai et al. / Materials Science and Engineering B102 (2003) 189 /192
Fig. 2. qniWA/JrbA0 versus exp(qV /2kT ) of low COP /high Oi (M3) material.
method, which is obtained by combining the diffusion current Id of different geometry diodes. Id is calculated from the forward I /V characteristics by taking the ideality factor (m ) into account [11]. By considering JA0 in Fig. 1b, it indicates that JA0 at VF /0.15 /0.4 V is composed of the diffusion and recombination current. This gives rise to a reduction of JA0 with increasing forward bias (smaller depletion width). From this, it is derived that JA0 at V /Vbi is JdA. The saturation area bulk recombination current density (JrbA0) equal to JA0 /JdA can be derived from Eq. (1), yielding after rearranging, qni WA qV tg : 2tr exp (2) 2kT JrbA0 By plotting (qniWA/JrbA0) versus exp(qV /2kT ) as shown in Fig. 2, 2tr and tg can be obtained from the slope and the crossing point at exp(qV /2kT ) /0, respectively. The values of 5.18 and 79.28 ms were found for tr and tg, respectively. The lifetimes obtained from this method are spatially averaged ones and correspond with a low electric field. Both tr and tg after processing are lower than the ones before processing, indicating the dominance of the processing-induced defects. Are these values for tr and tg correct? One way of tackling this issue is by an independent evaluation of tr and tg using a different technique. Therefore, MWA measurements were performed on the same devices.
191
However, the tr derived from the diode I /V is considerably lower than the one obtained by MWA. This indicates that tr in the normally doped p-type bulk of the wafer, derived from MWA is higher than in the highly doped p-well region (forward I /V method). In fact, from MWA analysis (Fig. 3a), a recombination lifetime profile is derived, with lower tr values closer to the sample surface and approaching the data of the diode analysis, which corresponds to the near-surface ( B/1 mm) p-well region. This confirms that the tr derived by this approach yields a reasonable estimate. Alternatively, it is possible to derive tg from JgbA, which may serve as a further validation of the proposed technique. tg has been calculated by taking both the generation width [12] and the electric field enhancement factor [13] into account. The generation lifetime obtained from the forward current in Fig. 2 is close to the one found from the reverse current after correction for generation width and electric field enhancement factor (Fig. 3b). This indicates that the proposed method to obtain tg and tr from the recombination current gives reasonable values. At the same time, our electric field correction procedure is also consistent with the low-field forward current analysis of tg.
4. Conclusions It has been demonstrated that the proposed method to obtain tg and tr from the recombination current gives reasonable values and overcomes lifetime discrepancy due to the high electric field and the shallow-junction effect in scaled junctions. tr is in agreement with the one measured from MWA, while tg coincides with the one derived from the generation current after correcting for the generation width and electric field enhancement factor. It is furthermore concluded that the lower tg and tr after processing, compared with the starting values of the substrates, point to the dominance of processinginduced defects in the near-surface p-well region. This is also derived from the fact that the I /V characteristics are not influenced by the COP and Oi densities.
Fig. 3. (a) Recombination lifetime versus the depth below the surface derived from the cross-sectional MWA technique for low COP /high Oi (M3) material and (b) generation lifetime versus depletion width for diodes processed on low COP /high Oi (M3) material.
192
A. Poyai et al. / Materials Science and Engineering B102 (2003) 189 /192
References [1] D.K. Schroder, Semiconductor Material and Device Characterization, 2nd ed., Wiley, New York, 1998. [2] Y. Murakami, T. Shingyouji, J. Appl. Phys. 75 (1994) 3548. [3] C. Claeys, E. Simoen, A. Poyai, A. Czerwinski, J. Electrochem. Soc. 146 (1999) 3429. [4] A. Baghdadi, W. Bullis, M. Croarkin, Y.-Z. Li, R. Scace, R. Series, P. Stallhofer, M. Watanabe, J. Electrochem. Soc. 136 (1989) 2015. [5] G. Badenes, R. Rooyackers, I. DeWolf, L. Deferm, Jpn. J. Appl. Phys. 36 (1997) 1325. [6] E. Gaubas, E. Simoen, C. Claeys, A. Poyai, Solid State Phenomena 69 /70 (1999) 155.
[7] C. Claeys, A. Poyai, E. Simoen, A. Czerwinski, J. Katcki, J. Electrochem. Soc. 146 (1999) 1151. [8] S.M. Sze, Physics of Semiconductor Devices, Wiley, New York, 1981. [9] A. Czerwinski, E. Simoen, C. Claeys, K. Klima, D. Tomaszewski, J. Gibki, J. Katcki, J. Electrochem. Soc. 145 (1998) 2107. [10] A. Poyai, E. Simoen, C. Claeys, Appl. Phys. Lett. 80 (2002) 1192. [11] J. Vanhellemont, E. Simoen, C. Claeys, Appl. Phys. Lett. 66 (1995) 2894. [12] A. Czerwinski, E. Simoen, A. Poyai, C. Claeys, in: T. Abe, W.M. Bullis, S. Kobayashi, W. Lin, P. Wagner (Eds.), Third International Symposium on Defects in Silicon, vols. PV 99-1, The Electrochemical Society, Pennington, NJ, 1999, pp. 88 /99. [13] A. Poyai, E. Simoen, C. Claeys, IEEE Trans. Electron Devices 48 (2001) 2445.