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Minority carrier lifetime scan maps applied to iron concentration mapping in silicon wafers
Materials Science and Engineering B91– 92 (2002) 216– 219
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Minority carrier lifetime scan maps applied to iron concentration mapping in silicon wafers O. Palais a,*, E. Yakimov b, S. Martinuzzi a a
UMR TECSEN, Faculte´ des Sciences et Techniques de Marseille St Je´roˆme, Uni6ersity of Marseilles, Case 231, 13397 Marseille, Cedex 20, France b Institute of Microelectronics Technology, Russian Academy of Science, Chernogolo6ka 142432, Russia
Abstract Mapping of iron concentration is obtained in p-type monocrystalline and multicrystalline silicon by means of contactless lifetime measurements. The recombination rate induced by interstitial iron (Fei ) and iron boron pairs (FeB), which is very sensitive to injection level, is appreciated thanks to comparison between deep level transient spectroscopy (DLTS) and lifetime measurement by the micro-wave phase-shift technique (mW-PS). Calibration is given for samples doped in the range 5 ×1013 – 3×1016 cm − 3. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Crystalline silicon; Iron; Lifetime; Mapping; Microwave; Phase-shift; DLTS; Recombination centers
1. Introduction In processed silicon wafers, impurity concentrations in the 1010 cm − 3 range (and below) could be deduced from minority carrier lifetime ~ measurements when the impurity atoms are the main source of recombination centers. Moreover, in some cases, like for iron in p-type silicon, it is possible to ascribe the recombination centers to a specific impurity and to evaluate its concentration. Indeed, the interstitial iron concentration [Fei ] can be evaluated by the measurement of ~ before and after sample annealing at 210 °C for 10 min, which dissociates the iron boron pairs in p-type silicon for some hours depending on the doping level. Large variations of ~ result from this dissociation as the capture cross section for minority carriers of Fei atoms and FeB pairs differ strongly. At low injection levels the pair dissociation induces a marked decrease of ~. However, that is not true at high injection level and [Fei ] is given by the equation [1]: [Fei ]=K
n
1 1 − ~Fei ~FeB
(cm − 3)
* Corresponding author. E-mail address: [email protected] (O. Palais).
(1)
where K depends on the injection level (relation between excess carrier density and dopant concentrations). Such a dependence must be known in order to evaluate correctly [Fei ]. In this paper the dependence of K on dopant concentration was measured and simulated for a given injection level. It is shown that the contactless microwave phase-shift technique (mW-PS) allows to evaluate recombination center densities in the range 109 to 1010 cm − 3 [2]. Iron concentration scan maps (actually interstitial iron associated with boron) are deduced by the technique with a lateral resolution of 50 mm and a sensitivity of a few 109 cm − 3.
2. Experimental We used p-type float zone (FZ) and Czochralski (CZ) grown monocrystalline silicon wafers, boron doped in the range 5× 1013 –3× 1016 cm − 3 intentionally iron contaminated and also multicrystalline wafers (mc-Si). The iron contamination results from vacuum iron deposition and annealing at 900 °C for 1 h under argon –hydrogen atmosphere followed by quenching in ice water.
0921-5107/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 1 ) 0 0 9 9 8 - 9
O. Palais et al. / Materials Science and Engineering B91–92 (2002) 216–219
Lifetime measurements were made by the contactless microwave phase-shift technique (mW-PS) which was already described elsewhere [3]. A great advantage of this technique is that it works at practically constant injection level giving more precise lifetime measurements in case of iron contamination or contamination whose the electrical activity depends on the excitation level. Such a technique was suitable to obtain scan maps of ~ with a lateral resolution of 50 mm, when the samples were moved by a X – Y stage powered by step motors. Scan maps of ~ before and after annealing at 210 °C for 10 min can be transformed in [Fei] mappings provided the value of K in Eq. (1) was determined. In iron contaminated samples, [Fei ] was evaluated using deep level transient spectroscopy (DLTS). The DLTS measurements were done with a Sula spectrometer and by means of Al– Si metal– insulator – semiconductor diodes. Such diodes were used with a back ohmic contact obtained by In– Ga paste in order to avoid iron precipitation (no annealing).
3. Results
217
In p-type silicon, iron gives two recombination centers corresponding to Fei and FeB. The first one induces a deep level in the band gap at Ev + 0.43 eV, and the second one two levels: a shallow level at Ev +0.09 eV and a level at Ec − 0.29 eV. This is summarized in Fig. 1. As said in the Section 1, at room temperature recombination due to Fei is ten times more efficient than that due to FeB, but this is true only at low injection level. Indeed, Shockley Read Hall (SRH) theory shows that lifetime associated with deep level increases with injection level. Consequently, the recombination strength of Fei atoms decreases when the injection level increases and can become less efficient than that of FeB pairs [6]. In order to determine this variation we measured the iron concentration in each investigated sample by DLTS measurements. We also computed the associated lifetimes versus the injection level given by the SRH theory considering Fei level and both levels of FeB pairs [7], with, respectively, ~1 for the acceptor level associated with FeB0/ − and ~2 for the donor level associated with FeB0/ + . Thus, ~FeB is given by Eq. (2):
3.1. Computed 6alues of K
1 [FeB0]+ [FeB + ] [FeB0]+ [FeB − ] = + ~FeB ~1 ~2
The variation of the coefficient K with the doping level is computed according to the following considerations.
For calculations the following capture cross-section values were used: for Fei |n(Fei )= 4× 10 − 14 cm2 [4] and |p(Fei )= 1× 10 − 15 cm2. For FeB (0.09) |n(FeB1)=3.5× 10 − 14 cm2 and |p(FeB1)=3×10 − 14 cm2. For FeB (0.29) |n(FeB2)=3× 10 − 15 cm2 [5] and |p(FeB2)=1.6× 10 − 13 cm2. This is summarized in Fig. 1. Fig. 2 gives the computed variations of lifetime associated with FeB and Fei recombination centers versus the injection level Dp/p0, where p0 and Dp are, respectively, the doping level and the excess carrier concentration, for various values of p0. The curves indicate that even at low injection level, ~FeB depends strongly on dopant concentration. From the results of Fig. 2 it is possible to evaluate the term between brackets of Eq. (1), and using the [Fei ] value obtained by DLTS, to determine the values of K as a function of the doping level, for a given injection level. Fig. 3 shows the variation of K on more than three orders of magnitude when the doping level varies from 1012 to 1017 cm − 3. This proves that it is indispensable to take into account the injection level for iron concentration evaluation by means of ‘electrical’ techniques. Moreover, we can distinguish between two different doping range for K, for low doping level where K is negative (i.e. ~Fei B ~FeB) and for high doping level where K is positive (i.e. ~Fei \ ~FeB). Between these two areas there is an indetermination area due to the recom-
Fig. 1. Iron levels in p-type boron doped silicon.
Fig. 2. Calculated lifetime dependence versus the excitation level for FeB pairs and Fei, in samples with various dopant concentrations and [Fei ] =1 ×1012 cm − 3.
(2)
218
O. Palais et al. / Materials Science and Engineering B91–92 (2002) 216–219
This remark suggests that it is possible to adjust the injection level versus the doping level to obtain a maximum of sensitivity for iron detection and suppress the indetermination.
3.2. Experimental 6alues of K
Fig. 3. Variation of K versus the doping level for a given injection level of 4 × 1014 cm − 3. Dots correspond to experimental measurements and straight lines to computed values.
By measuring the lifetime before and after the dissociation of FeB pairs for each sample we obtain the value of the term between brackets in Eq. (1). This leads to the experimental values of K represented in Fig. 2 by full dots for a given injection level. An acceptable agreement is obtained between the experimental value (dots) and the computed variations of K.
3.3. Iron scan maps As an example, we give in Fig. 4 an iron concentration map of an intentionally iron contaminated Cz sample doped to 7× 1015 cm − 3. As expected in such a sample the iron concentration is quite homogeneous (the value of K for the scan map is 140× 106 cm − 3 s). Fig. 5 shows the iron concentration map of an as grown mc-Si doped to 3×1016 cm − 3 (K=52×106 cm − 3 s). This map indicates that [Fei ] is higher in the grain, while at grain boundaries less iron is found, probably because the precipitate iron is not detected by the lifetime measurements.
Fig. 4. Iron concentration mapping of 8 × 8 mm2 area in 7 ×1015 cm − 3 boron doped Cz sample obtained from Eq. (1) and the corresponding K coefficient.
4. Conclusion This paper demonstrates the ability of mW-PS technique to build high resolution iron scan maps in p-type silicon. The sensitivity is high enough to reach very small concentration as 109 cm − 3 for any doping level provided the actual coefficient K which links the iron concentrations to the lifetime values is known. The variation of such a coefficient versus the doping level of the sample for a defined injection level is given.
Acknowledgements Contrat JOR3-CT970165: ADEME-CNRS ECODEV France and European Union— DGXII and JOR 3-C980228. Fig. 5. Iron density mapping of 8 ×8 mm2 area in 3 × 1016 cm − 3 boron doped mc-Si sample obtained from Eq. (1) and the corresponding K coefficient.
bination activities of Fei and FeB, which are approximately the same, consequently the difference in Eq. (1) between ~Fei and ~FeB is very close to zero.
References [1] J. Lagowski, P. Edelman, M. Dexter, W. Henley, Semicond. Sci. Technol. 7 (1992) A185 – A192. [2] O. Palais, Solid State Phenomena 78 – 79 (2001) 267 –274.
O. Palais et al. / Materials Science and Engineering B91–92 (2002) 216–219 [3] O. Palais, J. Gervais, E. Yakimov, S. Martinuzzi, Eur. Phys. J. AP 10 (2000) 157 –162. [4] G. Zoth, W. Bergholz, J. Appl. Phys. 67 (11) (1990) 1.
219
[5] D. Waltz, J.P. Joly, G. Kamarinos, Appl. Phys. A 62 (1996) 345. [6] D.K. Schroder, Electrochem. Soc. Proc. 17 (2000) 365. [7] S.C. Choo, Phys. Rev. B 1 (2) (1970) 687.
www.elsevier.com/locate/mseb
Minority carrier lifetime scan maps applied to iron concentration mapping in silicon wafers O. Palais a,*, E. Yakimov b, S. Martinuzzi a a
UMR TECSEN, Faculte´ des Sciences et Techniques de Marseille St Je´roˆme, Uni6ersity of Marseilles, Case 231, 13397 Marseille, Cedex 20, France b Institute of Microelectronics Technology, Russian Academy of Science, Chernogolo6ka 142432, Russia
Abstract Mapping of iron concentration is obtained in p-type monocrystalline and multicrystalline silicon by means of contactless lifetime measurements. The recombination rate induced by interstitial iron (Fei ) and iron boron pairs (FeB), which is very sensitive to injection level, is appreciated thanks to comparison between deep level transient spectroscopy (DLTS) and lifetime measurement by the micro-wave phase-shift technique (mW-PS). Calibration is given for samples doped in the range 5 ×1013 – 3×1016 cm − 3. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Crystalline silicon; Iron; Lifetime; Mapping; Microwave; Phase-shift; DLTS; Recombination centers
1. Introduction In processed silicon wafers, impurity concentrations in the 1010 cm − 3 range (and below) could be deduced from minority carrier lifetime ~ measurements when the impurity atoms are the main source of recombination centers. Moreover, in some cases, like for iron in p-type silicon, it is possible to ascribe the recombination centers to a specific impurity and to evaluate its concentration. Indeed, the interstitial iron concentration [Fei ] can be evaluated by the measurement of ~ before and after sample annealing at 210 °C for 10 min, which dissociates the iron boron pairs in p-type silicon for some hours depending on the doping level. Large variations of ~ result from this dissociation as the capture cross section for minority carriers of Fei atoms and FeB pairs differ strongly. At low injection levels the pair dissociation induces a marked decrease of ~. However, that is not true at high injection level and [Fei ] is given by the equation [1]: [Fei ]=K
n
1 1 − ~Fei ~FeB
(cm − 3)
* Corresponding author. E-mail address: [email protected] (O. Palais).
(1)
where K depends on the injection level (relation between excess carrier density and dopant concentrations). Such a dependence must be known in order to evaluate correctly [Fei ]. In this paper the dependence of K on dopant concentration was measured and simulated for a given injection level. It is shown that the contactless microwave phase-shift technique (mW-PS) allows to evaluate recombination center densities in the range 109 to 1010 cm − 3 [2]. Iron concentration scan maps (actually interstitial iron associated with boron) are deduced by the technique with a lateral resolution of 50 mm and a sensitivity of a few 109 cm − 3.
2. Experimental We used p-type float zone (FZ) and Czochralski (CZ) grown monocrystalline silicon wafers, boron doped in the range 5× 1013 –3× 1016 cm − 3 intentionally iron contaminated and also multicrystalline wafers (mc-Si). The iron contamination results from vacuum iron deposition and annealing at 900 °C for 1 h under argon –hydrogen atmosphere followed by quenching in ice water.
0921-5107/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 1 ) 0 0 9 9 8 - 9
O. Palais et al. / Materials Science and Engineering B91–92 (2002) 216–219
Lifetime measurements were made by the contactless microwave phase-shift technique (mW-PS) which was already described elsewhere [3]. A great advantage of this technique is that it works at practically constant injection level giving more precise lifetime measurements in case of iron contamination or contamination whose the electrical activity depends on the excitation level. Such a technique was suitable to obtain scan maps of ~ with a lateral resolution of 50 mm, when the samples were moved by a X – Y stage powered by step motors. Scan maps of ~ before and after annealing at 210 °C for 10 min can be transformed in [Fei] mappings provided the value of K in Eq. (1) was determined. In iron contaminated samples, [Fei ] was evaluated using deep level transient spectroscopy (DLTS). The DLTS measurements were done with a Sula spectrometer and by means of Al– Si metal– insulator – semiconductor diodes. Such diodes were used with a back ohmic contact obtained by In– Ga paste in order to avoid iron precipitation (no annealing).
3. Results
217
In p-type silicon, iron gives two recombination centers corresponding to Fei and FeB. The first one induces a deep level in the band gap at Ev + 0.43 eV, and the second one two levels: a shallow level at Ev +0.09 eV and a level at Ec − 0.29 eV. This is summarized in Fig. 1. As said in the Section 1, at room temperature recombination due to Fei is ten times more efficient than that due to FeB, but this is true only at low injection level. Indeed, Shockley Read Hall (SRH) theory shows that lifetime associated with deep level increases with injection level. Consequently, the recombination strength of Fei atoms decreases when the injection level increases and can become less efficient than that of FeB pairs [6]. In order to determine this variation we measured the iron concentration in each investigated sample by DLTS measurements. We also computed the associated lifetimes versus the injection level given by the SRH theory considering Fei level and both levels of FeB pairs [7], with, respectively, ~1 for the acceptor level associated with FeB0/ − and ~2 for the donor level associated with FeB0/ + . Thus, ~FeB is given by Eq. (2):
3.1. Computed 6alues of K
1 [FeB0]+ [FeB + ] [FeB0]+ [FeB − ] = + ~FeB ~1 ~2
The variation of the coefficient K with the doping level is computed according to the following considerations.
For calculations the following capture cross-section values were used: for Fei |n(Fei )= 4× 10 − 14 cm2 [4] and |p(Fei )= 1× 10 − 15 cm2. For FeB (0.09) |n(FeB1)=3.5× 10 − 14 cm2 and |p(FeB1)=3×10 − 14 cm2. For FeB (0.29) |n(FeB2)=3× 10 − 15 cm2 [5] and |p(FeB2)=1.6× 10 − 13 cm2. This is summarized in Fig. 1. Fig. 2 gives the computed variations of lifetime associated with FeB and Fei recombination centers versus the injection level Dp/p0, where p0 and Dp are, respectively, the doping level and the excess carrier concentration, for various values of p0. The curves indicate that even at low injection level, ~FeB depends strongly on dopant concentration. From the results of Fig. 2 it is possible to evaluate the term between brackets of Eq. (1), and using the [Fei ] value obtained by DLTS, to determine the values of K as a function of the doping level, for a given injection level. Fig. 3 shows the variation of K on more than three orders of magnitude when the doping level varies from 1012 to 1017 cm − 3. This proves that it is indispensable to take into account the injection level for iron concentration evaluation by means of ‘electrical’ techniques. Moreover, we can distinguish between two different doping range for K, for low doping level where K is negative (i.e. ~Fei B ~FeB) and for high doping level where K is positive (i.e. ~Fei \ ~FeB). Between these two areas there is an indetermination area due to the recom-
Fig. 1. Iron levels in p-type boron doped silicon.
Fig. 2. Calculated lifetime dependence versus the excitation level for FeB pairs and Fei, in samples with various dopant concentrations and [Fei ] =1 ×1012 cm − 3.
(2)
218
O. Palais et al. / Materials Science and Engineering B91–92 (2002) 216–219
This remark suggests that it is possible to adjust the injection level versus the doping level to obtain a maximum of sensitivity for iron detection and suppress the indetermination.
3.2. Experimental 6alues of K
Fig. 3. Variation of K versus the doping level for a given injection level of 4 × 1014 cm − 3. Dots correspond to experimental measurements and straight lines to computed values.
By measuring the lifetime before and after the dissociation of FeB pairs for each sample we obtain the value of the term between brackets in Eq. (1). This leads to the experimental values of K represented in Fig. 2 by full dots for a given injection level. An acceptable agreement is obtained between the experimental value (dots) and the computed variations of K.
3.3. Iron scan maps As an example, we give in Fig. 4 an iron concentration map of an intentionally iron contaminated Cz sample doped to 7× 1015 cm − 3. As expected in such a sample the iron concentration is quite homogeneous (the value of K for the scan map is 140× 106 cm − 3 s). Fig. 5 shows the iron concentration map of an as grown mc-Si doped to 3×1016 cm − 3 (K=52×106 cm − 3 s). This map indicates that [Fei ] is higher in the grain, while at grain boundaries less iron is found, probably because the precipitate iron is not detected by the lifetime measurements.
Fig. 4. Iron concentration mapping of 8 × 8 mm2 area in 7 ×1015 cm − 3 boron doped Cz sample obtained from Eq. (1) and the corresponding K coefficient.
4. Conclusion This paper demonstrates the ability of mW-PS technique to build high resolution iron scan maps in p-type silicon. The sensitivity is high enough to reach very small concentration as 109 cm − 3 for any doping level provided the actual coefficient K which links the iron concentrations to the lifetime values is known. The variation of such a coefficient versus the doping level of the sample for a defined injection level is given.
Acknowledgements Contrat JOR3-CT970165: ADEME-CNRS ECODEV France and European Union— DGXII and JOR 3-C980228. Fig. 5. Iron density mapping of 8 ×8 mm2 area in 3 × 1016 cm − 3 boron doped mc-Si sample obtained from Eq. (1) and the corresponding K coefficient.
bination activities of Fei and FeB, which are approximately the same, consequently the difference in Eq. (1) between ~Fei and ~FeB is very close to zero.
References [1] J. Lagowski, P. Edelman, M. Dexter, W. Henley, Semicond. Sci. Technol. 7 (1992) A185 – A192. [2] O. Palais, Solid State Phenomena 78 – 79 (2001) 267 –274.
O. Palais et al. / Materials Science and Engineering B91–92 (2002) 216–219 [3] O. Palais, J. Gervais, E. Yakimov, S. Martinuzzi, Eur. Phys. J. AP 10 (2000) 157 –162. [4] G. Zoth, W. Bergholz, J. Appl. Phys. 67 (11) (1990) 1.
219
[5] D. Waltz, J.P. Joly, G. Kamarinos, Appl. Phys. A 62 (1996) 345. [6] D.K. Schroder, Electrochem. Soc. Proc. 17 (2000) 365. [7] S.C. Choo, Phys. Rev. B 1 (2) (1970) 687.