The redissolution behavior of gettered iron was studied in p-type Czochralski-grown silicon with a doping level of 2.5×10exp14 cm−3 and an oxide precipitate density of 5×10exp9 cm−3. The concentrations of interstitial iron and iron–boron pairs were measured by deep level transient spectroscopy. It was found that the dependence of redissolved iron concentration on annealing time can be fitted by the function C(t)=C_0[1−exp(−t/tau_diss)], and the dissolution rate tau−1diss has an Arrhenius-type temperature dependence of tau−1diss=4.01×10exp4 × exp[−(1.47±0.10) eV/k_BT] s−1. Based on this empirical equation, we predict how stable the gettered iron is during different annealing sequences and discuss implications for optimization of internal gettering.Peer reviewe

Istratov, Andrei A.

Väinölä, Hele

Weber, Eicke R.

Zhang, Peng

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This is an electronic reprint of the original article.This reprint may differ from the original in pagination and typographic detail.Author(s): Zhang, Peng & Väinölä, Hele & Istratov, Andrei A. & Weber, Eicke R.Title: Thermal stability of internal gettering of iron in silicon and itsimpact on optimization of getteringYear: 2003Version: Final published versionPlease cite the original version:Zhang, Peng & Väinölä, Hele & Istratov, Andrei A. & Weber, Eicke R. 2003. Thermalstability of internal gettering of iron in silicon and its impact on optimization of gettering.Applied Physics Letters. P. 3. 0003-6951 (printed). DOI: 10.1063/1.1630158.Note: Copyright 2003 American Institute of Physics. This article may be downloaded for personal use only. Anyother use requires prior permission of the author and the American Institute of Physics.http://scitation.aip.org/content/aip/journal/aplAll material supplied via Aaltodoc is protected by copyright and other intellectual property rights, andduplication or sale of all or part of any of the repository collections is not permitted, except that material maybe duplicated by you for your research use or educational purposes in electronic or print form. You mustobtain permission for any other use. Electronic or print copies may not be offered, whether for sale orotherwise to anyone who is not an authorised user.Powered by TCPDF (www.tcpdf.org)Thermal stability of internal gettering of iron in silicon and its impact on optimization ofgetteringPeng Zhang, Hele Väinölä, Andrei A. Istratov, and Eicke R. Weber  Citation: Applied Physics Letters 83, 4324 (2003); doi: 10.1063/1.1630158 View online: http://dx.doi.org/10.1063/1.1630158 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/83/21?ver=pdfcov Published by the AIP Publishing  Articles you may be interested in Lattice location and thermal stability of implanted nickel in silicon studied by on-line emission channeling J. Appl. Phys. 115, 023504 (2014); 10.1063/1.4861142  Investigation of iron impurity gettering at dislocations in a SiGe/Si heterostructure J. Appl. Phys. 105, 073712 (2009); 10.1063/1.3093912  Internal gettering of iron in multicrystalline silicon at low temperature Appl. Phys. Lett. 93, 152108 (2008); 10.1063/1.2987521  Impact of the end of range damage from low energy Ge preamorphizing implants on the thermal stability ofshallow boron profiles J. Appl. Phys. 96, 4939 (2004); 10.1063/1.1776624  Lateral gettering of iron by cavities induced by helium implantation in silicon J. Appl. Phys. 88, 5000 (2000); 10.1063/1.1315328    This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:130.233.216.27 On: Wed, 08 Apr 2015 09:37:40Thermal stability of internal gettering of iron in silicon and its impacton optimization of getteringPeng Zhang,a) Hele Va¨ ino¨ la¨ ,b) Andrei A. Istratov, and Eicke R. WeberDepartment of Materials Science and Engineering, University of California at Berkeley and LawrenceBerkeley National Laboratory, MS 62R0203, 1 Cyclotron Road, Berkeley, California 94720~Received 14 July 2003; accepted 2 October 2003!The redissolution behavior of gettered iron was studied in p-type Czochralski-grown silicon with adoping level of 2.531014 cm23 and an oxide precipitate density of 53109 cm23. Theconcentrations of interstitial iron and iron–boron pairs were measured by deep level transientspectroscopy. It was found that the dependence of redissolved iron concentration on annealing timecan be fitted by the function C(t)5C0@12exp(2t/tdiss)# , and the dissolution rate tdiss21 has anArrhenius-type temperature dependence of tdiss2154.0131043exp@2(1.4760.10) eV/kBT# s21.Based on this empirical equation, we predict how stable the gettered iron is during differentannealing sequences and discuss implications for optimization of internal gettering. © 2003American Institute of Physics. @DOI: 10.1063/1.1630158#Iron is a ubiquitous and harmful metal impurity in sili-con integrated circuit manufacturing.1,2 Gettering has beenwidely used in semiconductor processing3,4 to remove metalimpurities from the device region. Among different getteringtechniques, internal gettering5–7 of metals by oxide precipi-tates formed in the bulk of silicon wafers was shown to bevery efficient for fast diffusers, such as iron. On the otherhand, it was established that the gettered iron can redissolveinto the silicon matrix if the wafer is reheated after gettering.Several groups have shown that the gettered iron can becompletely dissolved within several minutes if the wafer isheated to a sufficiently high temperature.8–10 However, verylittle is known about the kinetics of this dissolution process,and whether there is a binding energy between the oxideprecipitate and iron impurities which would slow down thedissolution of the gettered iron during short anneals at mod-erate temperatures, inherent in rapid thermal processing~RTP!. In this letter, we report studies of the kinetics of thedissolution of iron precipitated at oxide precipitates in p-typeCzochralski ~CZ!-grown silicon as a function of temperaturethat allowed us to determine the effective barrier for the dis-solution of the gettered iron. The experimental data are usedtogether with gettering simulations to predict how the redis-solution process affects the gettering stability during RTP.The samples were prepared from p-type 200 mm CZsilicon wafers with a boron doping level of 2.531014 cm23 and initial oxygen concentration of 6.5931017 cm23. The wafer was first annealed at 1230 °C todissolve all oxygen clusters and then subjected to oxygennucleation treatments at 650 °C for 14 h followed by 4 h800 °C and 16 h 1000 °C growth steps. Well-defined oxideprecipitates,11 with a density of approximately 53109 cm23 were obtained, which was confirmed by prefer-ential etching and then counting the pits. Based on the den-sity of the oxide precipitates and the decrease in interstitialoxygen concentration, assuming spherical SiO2 precipitateswith oxygen atomic concentration of Cp54.631022 cm23,an average radius of an oxide precipitate was estimated to ber’88 nm. The samples were intentionally contaminated byevaporating a thin film of iron on the back side of thesample, followed by a drive-in annealing of 50 min at 950 °Cin an ambient of 96% N214% H2 . After that, the set pointof the furnace was changed to cool the samples to 700 °C.The average cooling rate of the furnace in this temperaturerange was measured to be approximately 14 °C/min. Thenthe samples were kept at 700 °C for 30 min, after which thetemperature was decreased to 450 °C during the next 30 min.This gettering anneal was terminated by quenching thesamples to room temperature by dropping them on an alumi-num plate. The gettering efficiency was tested by deep leveltransient spectroscopy ~DLTS!, which confirmed that no de-tectable concentration (;1010 cm23) of ungettered intersti-tial iron was left in the sample. After etching off approxi-mately 50 mm from the surface by a HNO31HF1H2Osolution ~performed to remove surface iron silicide!, thesamples were annealed at 750 °C, 800 °C, 850 °C, or 900 °Cfor different times between several seconds and several min-utes ~dissolution annealing! in the same gas mixture as men-tioned above. The dissolution annealing was terminated byquenching the samples on an aluminum plate. The top layer~about 100 mm! was chemically removed, after which thealuminum Schottky diodes were deposited by thermal evapo-ration, and DLTS analyses were performed to measure thedissolved iron concentration in the samples.Figure 1 shows the typical result of dissolved iron con-centration versus dissolution annealing time at 800 °C. Theexperimental data points were fitted with an exponentialfunction: C(t)5C0@12exp(2t/tdiss)# , wheretdiss5~198683! s and C054.5831012 cm23; the solubilityof iron in silicon at this temperature.1 Note that the dissolu-tion time constant of 198 s at 800 °C, albeit short, is muchgreater than a prediction based on the assumption that thereis no dissolution barrier, i.e., that the redissolve process isonly limited by iron diffusivity and solubility in silicon anda!Electronic mail: pzhang@lbl.govb!Also at: Helsinki University of Technology, Electron Physics Laboratory,P.O. BOX 3500, FIN-02015 HUT, Finland.APPLIED PHYSICS LETTERS VOLUME 83, NUMBER 21 24 NOVEMBER 200343240003-6951/2003/83(21)/4324/3/$20.00 © 2003 American Institute of Physics This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:130.233.216.27 On: Wed, 08 Apr 2015 09:37:40the time constant t51/(4pnrD)52.53 s. It has been previ-ously shown12 that the effective iron density of the precipi-tate sites, obtained from iron precipitation kinetics using thesame gettering temperatures and similar oxide precipitatedensity as in our experiments, is in a good agreement withthe oxide precipitate density. Therefore, it is easy to showthat if there were no dissolution barrier, the gettered ironwould redissolve in less than 10 s at the annealing tempera-ture of 800 °C in CZ silicon with the density of the oxideprecipitates as 53109 cm23, which corresponds to the in-terprecipitates distance 5.8 mm.Measurements of the dependence of dissolution kineticsof precipitated iron on the annealing temperature ~shown inFig. 2! revealed an Arrhenius dependence of iron dissolutionrate tdiss21 on annealing temperature T:tdiss2154.0131043exp~2EA /kBT ! s21, ~1!with the effective energy barrier for iron dissolution ofEA5~1.4760.10! eV, significantly larger than the activationenergy of ED50.67 eV ~Ref. 1! for Fe diffusion in silicon.Hence, our data indicate that there is a substantial bindingenergy between gettered iron and oxide precipitates.In the past, it was assumed that there is no barrier foriron dissolution except for its diffusion barrier, and thereforeany heat treatment dissolves the gettered iron almost in-stantly. Hence, only the final cooling treatment was assumedto be important for the overall internal gettering efficiencyand only this cooling had to be optimized. It is very likelythat the dissolution barrier, reported in this letter, has little orno impact on the behavior of iron during furnace annealswith inherently slow temperature ramping rates. However,one may be able to take advantage of incomplete iron disso-lution during RTP which often has rapid heating and coolingsteps combined with a short processing time at moderatetemperatures. In this case, gettering efficiency may be im-proved by optimizing not only the last cooling, but also allprevious processing steps.In order to address this problem, we used the experimen-tal data obtained above to perform modeling of gettering andredissolution of iron during a sequence of rapid heating, fol-lowed by a short heat treatment and fast cooling, typical forRTP. The gettering simulator used in this study was based onthe algorithm described in detail in Ref. 13, which usesHam’s law14 to describe diffusion-limited precipitation ofiron. The dissolution process was described by the followingequation:C~ t !5C~ t2Dt !1@S~ t !2C~ t2Dt !#3@12exp~2Dt/tdiss!# , ~2!where C is the dissolved iron concentration, S is the solubil-ity, and tdiss is given by Eq. ~1!.Figure 3 shows examples of how iron is predicted tobehave during a short RTA treatment at different tempera-tures with and without dissolution barrier. Two cases weremodeled, corresponding to high and low iron contaminationlevels. In Figs. 3~a!–3~c!, the amount of iron gettered at ox-ide precipitates is equal to or exceeds the equilibrium ironsolubility at the processing temperature and the initial con-dition is that all the supersaturated iron impurities are get-tered, i.e., the dissolved iron concentration equals the solu-bility at the starting temperature. The ramp rate is 25 °C/sfrom 600 °C to 750 °C–1100 °C and the length of the annealtime is 30 s. In the simulations, the oxide precipitate densityis set to 53109 cm23 and the precipitates are assumed to beFIG. 1. The dependence of the dissolved iron concentration on annealingtime at T5800 °C.FIG. 2. Temperature dependence of the iron dissolution time constant.FIG. 3. Dissolved iron concentration during 30 s rapid thermal anneal atthree different temperatures: ~a! and ~d! 750 °C, ~b! and ~e! 900 °C, and ~c!and ~f! 1050 °C simulated with ~solid line! and without ~dashed line! bind-ing energy. The dotted line presents the equilibrium iron solubility duringthe annealing treatment.4325Appl. Phys. Lett., Vol. 83, No. 21, 24 November 2003 Zhang et al. This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:130.233.216.27 On: Wed, 08 Apr 2015 09:37:40homogeneously distributed throughout the wafer. It is seen inFig. 3~a! that in the case of no barrier ~dashed line!, thedissolved iron closely follows the solubility ~dotted line! andalmost instantly reaches the level of 731011 cm23, whereasthe dissolved iron concentration calculated with the dissolu-tion barrier does not exceed 531010 cm23 by the end of the750 °C, 30 s anneal. At 900 °C @Fig. 3~b!#, we can still detectsome difference between the models but at 1050 °C @Fig.3~c!# after 10 s anneal, the dissolved iron concentration al-ready reaches the solubility even in the presence of the dis-solution barrier. Hence, in laboratory experiments which fre-quently use relatively high iron contamination levels, thedissolution barrier has a significant impact on the total dis-solved iron concentration observed after the anneal.Figures 3~d!–3~f! illustrate the contamination level typi-cal for a production line, where iron concentration does notexceed 1011 cm23. In this set of simulations, the total ironconcentration in the sample is set to be 131011 cm23. Theinitial condition and ramping profiles were the same as inFigs. 3~a!–3~c!. As we can see from Fig. 3~d!, with the con-sideration of dissolution barrier, the iron concentration doesnot saturate after RTP treatment at 750 °C. At a higher tem-perature, for example, 900 °C @Fig. 3~e!# and 1050 °C @Fig.3~f!#, the interstitial iron concentration reaches 131011 cm23 ~i.e., the precipitated iron dissolves com-pletely! already during the temperature ramping. Thus, athigh-temperature annealing and low iron contamination lev-els, the existence of the dissolution barrier has little or noimpact on optimizing the gettering procedure.In conclusion, the analysis of the dissolution kinetics ofiron gettered by oxide precipitates in Cz silicon revealed thatthere is a strong binding between the gettered iron and oxideprecipitates. The effective dissolution energy barrier was de-termined to be EA5~1.4760.10! eV, as determined in thetemperature range from 750 °C to 900 °C. Considering thediffusion barrier of 0.67 eV for iron diffusion in silicon, ourresults give an effective binding energy of 0.80 eV betweenoxide precipitates and the gettered iron impurities. The na-ture of this barrier is not clear. It is not likely that the effec-tive binding energy of 0.80 eV is the result of retardation ofdiffusion by iron–boron paring similar to the effect of CuBpairing on copper diffusivity.15 This is because iron–boronpairs dissociate already at 200 °C, whereas our dissolutionexperiments were performed at much higher temperatures,from 750 °C to 900 °C. At these temperatures and with borondoping level as low as 1014 cm23, not only would FeB pairsdissociate instantly, but even their formation is unlikely sinceinterstitial iron is predominantly neutral in p2 silicon atthese temperatures. Additionally, precipitation kinetics ofiron can be accurately described by Ham’s law without anyadditional diffusion barrier. Hence, we conclude that the bar-rier stems from the microscopic interaction of iron with ox-ide precipitates. Additional studies are in progress to under-stand the nature of this interaction. The existence of thisbinding energy may be used to improve the efficiency ofinternal gettering during rapid thermal annealing, particularlyat temperatures below 800 °C or at high iron contaminationlevels. Our simulations demonstrated that short RTP annealsat moderate temperatures dissolve only a small fraction ofthe gettered iron.The authors acknowledge the support by SiWEDS. Oneof the authors ~H.V.! acknowledges support by Finnish Na-tional Technology Agency, Academy of Finland, OkmeticOyj, Micro Analog Systems Oy, and VTI Technologies Oy.The authors are grateful to Dr. R. Falster from MEMC forproviding the silicon wafers.1 A. A. Istratov, H. Hieslmair, and E. R. Weber, Appl. Phys. A: Mater. Sci.Process. 69, 13 ~1999!.2 A. A. Istratov, H. Hieslmair, and E. R. Weber, Appl. Phys. A: Mater. Sci.Process. 70, 489 ~2000!.3 S. M. Myers, M. Seibt, and W. Schro¨ter, J. Appl. Phys. 88, 3795 ~2000!.4 A. A. Istratov, H. Hieslmair, and E. R. Weber, MRS Bull. 25, 33 ~2000!.5 T. Y. Tan, E. E. Garnder, and W. K. Tice, Appl. Phys. Lett. 30, 175 ~1977!.6 D. Gilles, E. R. Weber, and S. K. Hahn, Phys. Rev. Lett. 64, 196 ~1990!.7 M. Aoki, A. Hara, and A. Ohsawa, J. Appl. Phys. 72, 895 ~1992!.8 E. G. Colas and E. R. Weber, Appl. Phys. Lett. 48, 1371 ~1986!.9 M. Aoki and A. Hara, J. Appl. Phys. 74, 1440 ~1993!.10 S. A. McHugo, E. R. Weber, M. Mizuno, and F. G. Kirscht, Appl. Phys.Lett. 66, 2840 ~1995!.11 K. F. Kelton, R. Falster, D. Gambaro, M. Olma, M. Cornara, and P. F. Wei,J. Appl. Phys. 85, 8097 ~1999!.12 H. Hieslmair, A. A. Istratov, S. A. McHugo, C. Flink, T. Heiser, and E. R.Weber, Appl. Phys. Lett. 72, 1460 ~1998!.13 H. Hieslmair, S. Balasubramanian, A. A. Istratov, and E. R. Weber, Semi-cond. Sci. Technol. 15, 567 ~2001!.14 F. S. Ham, J. Phys. Chem. Solids 6, 335 ~1958!.15 A. A. Istratov, C. Flink, H. Hieslmair, E. R. Weber, and T. Heiser, Phys.Rev. Lett. 81, 1243 ~1998!.4326 Appl. Phys. Lett., Vol. 83, No. 21, 24 November 2003 Zhang et al. This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:130.233.216.27 On: Wed, 08 Apr 2015 09:37:40

Thermal stability of internal gettering of iron in silicon and its impact on optimization of gettering

Peng Zhang

Hele Väinölä

Andrei A. Istratov

Eicke R. Weber

Crossref

Aaltodoc

This is an electronic reprint of the original article.
This reprint may differ from the original in pagination and typographic detail.
Author(s): Zhang, Peng & Väinölä, Hele & Istratov, Andrei A. & Weber, Eicke R.
Title: Thermal stability of internal gettering of iron in silicon and its
impact on optimization of gettering
Year: 2003
Version: Final published version
Please cite the original version:
Zhang, Peng & Väinölä, Hele & Istratov, Andrei A. & Weber, Eicke R. 2003. Thermal
stability of internal gettering of iron in silicon and its impact on optimization of gettering.
Applied Physics Letters. P. 3. 0003-6951 (printed). DOI: 10.1063/1.1630158.
Note: Copyright 2003 American Institute of Physics. This article may be downloaded for personal use only. Any
other use requires prior permission of the author and the American Institute of Physics.
http://scitation.aip.org/content/aip/journal/apl
All material supplied via Aaltodoc is protected by copyright and other intellectual property rights, and
duplication or sale of all or part of any of the repository collections is not permitted, except that material may
be duplicated by you for your research use or educational purposes in electronic or print form. You must
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otherwise to anyone who is not an authorised user.
Powered by TCPDF (www.tcpdf.org)
Thermal stability of internal gettering of iron in silicon and its impact on optimization of
gettering
Peng Zhang, Hele Väinölä, Andrei A. Istratov, and Eicke R. Weber 
 
Citation: Applied Physics Letters 83, 4324 (2003); doi: 10.1063/1.1630158 
View online: http://dx.doi.org/10.1063/1.1630158 
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/83/21?ver=pdfcov 
Published by the AIP Publishing 
 
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J. Appl. Phys. 105, 073712 (2009); 10.1063/1.3093912 
 
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 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
130.233.216.27 On: Wed, 08 Apr 2015 09:37:40
Thermal stability of internal gettering of iron in silicon and its impact
on optimization of gettering
Peng Zhang,a) Hele Va¨ ino¨ la¨ ,b) Andrei A. Istratov, and Eicke R. Weber
Department of Materials Science and Engineering, University of California at Berkeley and Lawrence
Berkeley National Laboratory, MS 62R0203, 1 Cyclotron Road, Berkeley, California 94720
~Received 14 July 2003; accepted 2 October 2003!
The redissolution behavior of gettered iron was studied in p-type Czochralski-grown silicon with a
doping level of 2.531014 cm23 and an oxide precipitate density of 53109 cm23. The
concentrations of interstitial iron and iron–boron pairs were measured by deep level transient
spectroscopy. It was found that the dependence of redissolved iron concentration on annealing time
can be fitted by the function C(t)5C0@12exp(2t/tdiss)# , and the dissolution rate tdiss21 has an
Arrhenius-type temperature dependence of tdiss
2154.0131043exp@2(1.4760.10) eV/kBT# s21.
Based on this empirical equation, we predict how stable the gettered iron is during different
annealing sequences and discuss implications for optimization of internal gettering. © 2003
American Institute of Physics. @DOI: 10.1063/1.1630158#
Iron is a ubiquitous and harmful metal impurity in sili-
con integrated circuit manufacturing.1,2 Gettering has been
widely used in semiconductor processing3,4 to remove metal
impurities from the device region. Among different gettering
techniques, internal gettering5–7 of metals by oxide precipi-
tates formed in the bulk of silicon wafers was shown to be
very efficient for fast diffusers, such as iron. On the other
hand, it was established that the gettered iron can redissolve
into the silicon matrix if the wafer is reheated after gettering.
Several groups have shown that the gettered iron can be
completely dissolved within several minutes if the wafer is
heated to a sufficiently high temperature.8–10 However, very
little is known about the kinetics of this dissolution process,
and whether there is a binding energy between the oxide
precipitate and iron impurities which would slow down the
dissolution of the gettered iron during short anneals at mod-
erate temperatures, inherent in rapid thermal processing
~RTP!. In this letter, we report studies of the kinetics of the
dissolution of iron precipitated at oxide precipitates in p-type
Czochralski ~CZ!-grown silicon as a function of temperature
that allowed us to determine the effective barrier for the dis-
solution of the gettered iron. The experimental data are used
together with gettering simulations to predict how the redis-
solution process affects the gettering stability during RTP.
The samples were prepared from p-type 200 mm CZ
silicon wafers with a boron doping level of 2.5
31014 cm23 and initial oxygen concentration of 6.59
31017 cm23. The wafer was first annealed at 1230 °C to
dissolve all oxygen clusters and then subjected to oxygen
nucleation treatments at 650 °C for 14 h followed by 4 h
800 °C and 16 h 1000 °C growth steps. Well-defined oxide
precipitates,11 with a density of approximately 5
3109 cm23 were obtained, which was confirmed by prefer-
ential etching and then counting the pits. Based on the den-
sity of the oxide precipitates and the decrease in interstitial
oxygen concentration, assuming spherical SiO2 precipitates
with oxygen atomic concentration of Cp54.631022 cm23,
an average radius of an oxide precipitate was estimated to be
r’88 nm. The samples were intentionally contaminated by
evaporating a thin film of iron on the back side of the
sample, followed by a drive-in annealing of 50 min at 950 °C
in an ambient of 96% N214% H2 . After that, the set point
of the furnace was changed to cool the samples to 700 °C.
The average cooling rate of the furnace in this temperature
range was measured to be approximately 14 °C/min. Then
the samples were kept at 700 °C for 30 min, after which the
temperature was decreased to 450 °C during the next 30 min.
This gettering anneal was terminated by quenching the
samples to room temperature by dropping them on an alumi-
num plate. The gettering efficiency was tested by deep level
transient spectroscopy ~DLTS!, which confirmed that no de-
tectable concentration (;1010 cm23) of ungettered intersti-
tial iron was left in the sample. After etching off approxi-
mately 50 mm from the surface by a HNO31HF1H2O
solution ~performed to remove surface iron silicide!, the
samples were annealed at 750 °C, 800 °C, 850 °C, or 900 °C
for different times between several seconds and several min-
utes ~dissolution annealing! in the same gas mixture as men-
tioned above. The dissolution annealing was terminated by
quenching the samples on an aluminum plate. The top layer
~about 100 mm! was chemically removed, after which the
aluminum Schottky diodes were deposited by thermal evapo-
ration, and DLTS analyses were performed to measure the
dissolved iron concentration in the samples.
Figure 1 shows the typical result of dissolved iron con-
centration versus dissolution annealing time at 800 °C. The
experimental data points were fitted with an exponential
function: C(t)5C0@12exp(2t/tdiss)# , where
tdiss5~198683! s and C054.5831012 cm23; the solubility
of iron in silicon at this temperature.1 Note that the dissolu-
tion time constant of 198 s at 800 °C, albeit short, is much
greater than a prediction based on the assumption that there
is no dissolution barrier, i.e., that the redissolve process is
only limited by iron diffusivity and solubility in silicon and
a!Electronic mail: pzhang@lbl.gov
b!Also at: Helsinki University of Technology, Electron Physics Laboratory,
P.O. BOX 3500, FIN-02015 HUT, Finland.
APPLIED PHYSICS LETTERS VOLUME 83, NUMBER 21 24 NOVEMBER 2003
43240003-6951/2003/83(21)/4324/3/$20.00 © 2003 American Institute of Physics This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
130.233.216.27 On: Wed, 08 Apr 2015 09:37:40
the time constant t51/(4pnrD)52.53 s. It has been previ-
ously shown12 that the effective iron density of the precipi-
tate sites, obtained from iron precipitation kinetics using the
same gettering temperatures and similar oxide precipitate
density as in our experiments, is in a good agreement with
the oxide precipitate density. Therefore, it is easy to show
that if there were no dissolution barrier, the gettered iron
would redissolve in less than 10 s at the annealing tempera-
ture of 800 °C in CZ silicon with the density of the oxide
precipitates as 53109 cm23, which corresponds to the in-
terprecipitates distance 5.8 mm.
Measurements of the dependence of dissolution kinetics
of precipitated iron on the annealing temperature ~shown in
Fig. 2! revealed an Arrhenius dependence of iron dissolution
rate tdiss
21 on annealing temperature T:
tdiss
2154.0131043exp~2EA /kBT ! s21, ~1!
with the effective energy barrier for iron dissolution of
EA5~1.4760.10! eV, significantly larger than the activation
energy of ED50.67 eV ~Ref. 1! for Fe diffusion in silicon.
Hence, our data indicate that there is a substantial binding
energy between gettered iron and oxide precipitates.
In the past, it was assumed that there is no barrier for
iron dissolution except for its diffusion barrier, and therefore
any heat treatment dissolves the gettered iron almost in-
stantly. Hence, only the final cooling treatment was assumed
to be important for the overall internal gettering efficiency
and only this cooling had to be optimized. It is very likely
that the dissolution barrier, reported in this letter, has little or
no impact on the behavior of iron during furnace anneals
with inherently slow temperature ramping rates. However,
one may be able to take advantage of incomplete iron disso-
lution during RTP which often has rapid heating and cooling
steps combined with a short processing time at moderate
temperatures. In this case, gettering efficiency may be im-
proved by optimizing not only the last cooling, but also all
previous processing steps.
In order to address this problem, we used the experimen-
tal data obtained above to perform modeling of gettering and
redissolution of iron during a sequence of rapid heating, fol-
lowed by a short heat treatment and fast cooling, typical for
RTP. The gettering simulator used in this study was based on
the algorithm described in detail in Ref. 13, which uses
Ham’s law14 to describe diffusion-limited precipitation of
iron. The dissolution process was described by the following
equation:
C~ t !5C~ t2Dt !1@S~ t !2C~ t2Dt !#
3@12exp~2Dt/tdiss!# , ~2!
where C is the dissolved iron concentration, S is the solubil-
ity, and tdiss is given by Eq. ~1!.
Figure 3 shows examples of how iron is predicted to
behave during a short RTA treatment at different tempera-
tures with and without dissolution barrier. Two cases were
modeled, corresponding to high and low iron contamination
levels. In Figs. 3~a!–3~c!, the amount of iron gettered at ox-
ide precipitates is equal to or exceeds the equilibrium iron
solubility at the processing temperature and the initial con-
dition is that all the supersaturated iron impurities are get-
tered, i.e., the dissolved iron concentration equals the solu-
bility at the starting temperature. The ramp rate is 25 °C/s
from 600 °C to 750 °C–1100 °C and the length of the anneal
time is 30 s. In the simulations, the oxide precipitate density
is set to 53109 cm23 and the precipitates are assumed to be
FIG. 1. The dependence of the dissolved iron concentration on annealing
time at T5800 °C.
FIG. 2. Temperature dependence of the iron dissolution time constant.
FIG. 3. Dissolved iron concentration during 30 s rapid thermal anneal at
three different temperatures: ~a! and ~d! 750 °C, ~b! and ~e! 900 °C, and ~c!
and ~f! 1050 °C simulated with ~solid line! and without ~dashed line! bind-
ing energy. The dotted line presents the equilibrium iron solubility during
the annealing treatment.
4325Appl. Phys. Lett., Vol. 83, No. 21, 24 November 2003 Zhang et al.
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homogeneously distributed throughout the wafer. It is seen in
Fig. 3~a! that in the case of no barrier ~dashed line!, the
dissolved iron closely follows the solubility ~dotted line! and
almost instantly reaches the level of 731011 cm23, whereas
the dissolved iron concentration calculated with the dissolu-
tion barrier does not exceed 531010 cm23 by the end of the
750 °C, 30 s anneal. At 900 °C @Fig. 3~b!#, we can still detect
some difference between the models but at 1050 °C @Fig.
3~c!# after 10 s anneal, the dissolved iron concentration al-
ready reaches the solubility even in the presence of the dis-
solution barrier. Hence, in laboratory experiments which fre-
quently use relatively high iron contamination levels, the
dissolution barrier has a significant impact on the total dis-
solved iron concentration observed after the anneal.
Figures 3~d!–3~f! illustrate the contamination level typi-
cal for a production line, where iron concentration does not
exceed 1011 cm23. In this set of simulations, the total iron
concentration in the sample is set to be 131011 cm23. The
initial condition and ramping profiles were the same as in
Figs. 3~a!–3~c!. As we can see from Fig. 3~d!, with the con-
sideration of dissolution barrier, the iron concentration does
not saturate after RTP treatment at 750 °C. At a higher tem-
perature, for example, 900 °C @Fig. 3~e!# and 1050 °C @Fig.
3~f!#, the interstitial iron concentration reaches 1
31011 cm23 ~i.e., the precipitated iron dissolves com-
pletely! already during the temperature ramping. Thus, at
high-temperature annealing and low iron contamination lev-
els, the existence of the dissolution barrier has little or no
impact on optimizing the gettering procedure.
In conclusion, the analysis of the dissolution kinetics of
iron gettered by oxide precipitates in Cz silicon revealed that
there is a strong binding between the gettered iron and oxide
precipitates. The effective dissolution energy barrier was de-
termined to be EA5~1.4760.10! eV, as determined in the
temperature range from 750 °C to 900 °C. Considering the
diffusion barrier of 0.67 eV for iron diffusion in silicon, our
results give an effective binding energy of 0.80 eV between
oxide precipitates and the gettered iron impurities. The na-
ture of this barrier is not clear. It is not likely that the effec-
tive binding energy of 0.80 eV is the result of retardation of
diffusion by iron–boron paring similar to the effect of CuB
pairing on copper diffusivity.15 This is because iron–boron
pairs dissociate already at 200 °C, whereas our dissolution
experiments were performed at much higher temperatures,
from 750 °C to 900 °C. At these temperatures and with boron
doping level as low as 1014 cm23, not only would FeB pairs
dissociate instantly, but even their formation is unlikely since
interstitial iron is predominantly neutral in p2 silicon at
these temperatures. Additionally, precipitation kinetics of
iron can be accurately described by Ham’s law without any
additional diffusion barrier. Hence, we conclude that the bar-
rier stems from the microscopic interaction of iron with ox-
ide precipitates. Additional studies are in progress to under-
stand the nature of this interaction. The existence of this
binding energy may be used to improve the efficiency of
internal gettering during rapid thermal annealing, particularly
at temperatures below 800 °C or at high iron contamination
levels. Our simulations demonstrated that short RTP anneals
at moderate temperatures dissolve only a small fraction of
the gettered iron.
The authors acknowledge the support by SiWEDS. One
of the authors ~H.V.! acknowledges support by Finnish Na-
tional Technology Agency, Academy of Finland, Okmetic
Oyj, Micro Analog Systems Oy, and VTI Technologies Oy.
The authors are grateful to Dr. R. Falster from MEMC for
providing the silicon wafers.
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Thermal stability of internal gettering of iron in silicon and its impact on optimization of gettering

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