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Boron accumulation at epi-substrate silicon interface during epitaxial growth
32
,{httertals 5~ lem e a n d l'mgmeermg, HI>, I t)92,32-3O
Boron accumulation at epi-substrate silicon interface during epitaxial growth B. P i v a c R. Boskovtc Insntute, P.O. Box 1016, 41000 Zagreb (Croatia) A . B o r g h e s i , M . G e d d o a n d A . Sassella Dipartimento dt Fistca ",4 Volta", Umverstta degli Studt, 1-27100 Pavta (Italy)
M. Pedrotti M E M C Electromc Matenals S.p.A., 1-28100 Novara (Italy) (Received January 29, 1992)
Abstract Boron accumulalaon was observed close to the interface between an epitaxlaUy grown sihcon layer and a silicon substrate wafer and then analyzed. It was concluded that boron contamination interacting with the surface oxade on wafers led to boron accumulataon close to the interface. Such accumulalaon is shown to occur for eptlayers of standard thickness (approxamately 10 ~m), with boron being electrically unactave.
1. Introduction
2. Experimental details
Impunties present m the silicon substrate or in buried layers can be redistributed into the growing epilayer during a typical epitaxlal process, either by solid-state out-diffusion or by gas phase autodoping. An unintended impurity depth profile results, often extending for considerable distance into the epilayer. Consequent degradation in device characteristics impedes downscaling of bipolar complementary metal-oxtde-semiconductors (CMOS) and specific CMOS, where thin epistructures are often grown over heavily doped substrates and/or buried layer structures [1, 2]. As industry approaches ultra large-scale integration density levels, control of dopant profiles is critical. They depend on the dopant incorporation mechanism, on the epidopant species and on the substrate autodoping species. Use of a relatively low epitaxlal growth temperature is the general approach to minimizing solid-state outdiffusion for all dopant species. On the contrary, the autodoping phenomenon has been extensively studied [3-5] and is significantly different for various dopant species In this article we shall focus on the dopant accumulation phenomenon at the epi-substrate interface, observed in boron-doped silicon samples only (not in phosphorus-doped samples).
The silicon substrates used in this experiment were p-type (B-doped, resistivity about 40 f~ cm) wafers, or n-type (P-doped, resistivity about 4 Q cm), grown by the Czochralski (CZ) method in the (100) direcnon with a diameter of 100 mm and average wafer thickness of about 500 /~m. Oxygen concentration m the bulk was measured by the Fourier transform infrared technique and was found to be about 7 x 1017 atoms c m -3. Epitaxial layers about 100 /~m thick of the p-type silicon (B-doped, resistivity about 90 Q cm) and n-type silicon (P-doped, resistivity about 75 fl cm) were grown on p-type and n-type substrates respectively. The samples were cleaned in situ prior to epitaxial growth, in a pancake type, r.f. heated reactor, to remove native oxide from the surface. A prebake treatment was performed in ramp from room temperature to 1180 °C at a rate of 70 °C min-1 After surface etching and ramp down to deposttion temperature in 1 min, purge and deposition were performed. Deposition sources were trichlorosilane and diborane (for p-type) or phosphme (for n-type) diluted at 50 p.p.m, in H 2. Deposition was performed at atmospheric pressure with a growth rate of about 1.25/~m rain- 1. In order to grow layers 100 ~ m thick, wafers were held at a temperature of 1100 *C for about 80 min. Resistivity profiles across the interface were obtained by spreading resistance (SR) measurements
0921-5107/92/$5-00
© 1992 - Elsevaer Sequoia. All rights reserved
B. Pivac et al
/
Boron accumulation at epz-substrate sdicon interface
performed with computer assisted solid state measurement apparatus model SSM 130. Samples were beveled at an angle of 11" 32'. Measurements were made at 1 /~m steps in the z-direction (depth). The distance between probes was 100/zm. A spreading resistance profile across the substrate-epilayer interface for an n-type wafer is shown in Fig. 1, and for a p-type wafer in Fig. 2. (The figures show the variation in resistance in the vicinity of the interface; 0 does not therefore correspond to the wafer surface, but to an arbitrarily chosen point.) The rather steep change in the resistivity on passing from the substrate into the epilayer (Fig. 1) is in good agreement with the rather low diffusivity of phosphorus. On the contrary, some quite unusual behavior is shown in Fig. 2. In addition to the large tail of boron that extends deep into epilayer, there is a significant drop in resistivity at the interface, which we attribute to boron accumulation in this region. This phenomenon is quite unexpected since substrate and epilayer were both p-type materials (B-doped) with similar resistivities. This boron accumulation is quantified to be about 3 x 1014 atoms cm-3. The accumulation was not found on the p-type epilayer grown on p + substrates. There is a striking difference between boron and phosphorus dopant redistribution at the substrateepilayer interface. While phosphorus follows the diffusion laws the behavior of boron is quite unusual. A SR profile reliability is obviously crucial in determining whether the differences in SR profiles obtained can be straightforwardly attributed to dopant variation. The SR technique for measuring resistivity profiles of
33
thin silicon layers was developed in 1963, and its use became popular in the late 1970s [6] owing to its major advantage of a fast tumround time. Since then a lot of research has checked reproducibility and many theoretical studies on converting SR to concentration for diffused profiles have been published [7, 8]. Godfrey et al. [9] compared SR and secondary ion mass spectrometry (SIMS) profiles for As and B after implanting and annealing. Recently Clapper et al. [10] made a comparison of boron profiles obtained by SR and SIMS and found very good agreement between the two techniques. Although SR could be considered semiquantitative, and SIMS data much more reliable, we should point out that SIMS sensitivity is about 10 t5 atoms cm-3. In our case variation in boron concentration across the interface is much less than SIMS sensitivity. It can further be argued that we registered only electrically active boron; the total boron concentration was much higher. Therefore we performed SIMS measurements with a C A M E C A IMS 4f spectrometer. However, our analysis showed that the boron concentration was below the sensitivity of the SIMS technique throughout the interface, and was therefore in agreement with SR data. In order to understand boron behavior, we performed the following experiment to determine whether the observed peak was a result of autodoping [5]. About 10 /~m of p-type epilayer was grown on the p-type substrate while several other p + substrates were present in the reactor. No sigruficant B segregation at the interface was observed in this case; the possibility of autodoping from adjacent p+ doped substrates was therefore excluded. The accumulation is more evident in the resistivity profile of p-p samples than in p-p+
102
o ==~
101
tj
2O 2
I
0
I
20
I 0
I
!
DEPTH (Urn)
Fig 1. SR profile across the cpl-substrate interface for n-type wafer,wath 100/zm of n-type epilayer
o
I
2O
I
40
I
I
60
D E P T H (tim)
Fig. 2. SR profile across the epi-substrate interface for p-type wafer, with 100/zm of p-type epllayer.
34
B_ Ptvac et al
/
Boron accumulatton at ept-substrate sthcon tnter~at e
samples, owing to the very low resistivity of p+ substrates, wtuch could mask this small peak. Subsequently we deposited a 10 ~m thick p-type epflayer on p-type substrates and ascertained that only low doped
102
101
t~
10 0
I
I
0
I
I
20
I
40
60
p-type substrates were present in the reactor. Ftgure 3 shows the SR profile for these samples, mdlcatlng no boron accumulation at the interface The samples were further annealed in air at 650 °C. Curves A and B in Fig 4 show SR data obtained for samples after annealing for 24 and 144 h respectively. Annealing at 650 °C caused a slgmficant B segregation at the interface which did not degrade w~th time. The results of SR measurements performed on the same samples after annealing at 1100 °C for 0.5, 1, 2 and 6 h are shown in Fig. 5 (curves A, B, C and D respectively). Again we observed that boron accumulation occurred after short periods of annealing (curves A, B and C). However, the accumulation disappeared completely upon longer annealing at the same temperature (curve D). In fact, only a boron dtffUSlOn tall remained in the epilayer after 6 h anneahng at 1100 °C. SR measurements after two-step annealing, the first for 144 h at 650 °C and the second for 0.5, 1, 2, and 6 h at 1100 °C, are reported in Fig. 6 (curves A, B, C and D respectively). The profiles clearly indicate that upon longer high-temperature anneahng boron slowly diffuses out of the segregation region into the ep~taxial layer (Fig. 6, curve D).
DEPTH (tml) Fig. 3 SR profile across the epl-substrate interface for p-type wafer, with 10/,tm of p-type as-grown epdayer.
B
L A
A
> 0t~ ;0
•
0
i
i
4
8 DEPTH
!
!
12
m
1
16
(#m)
Fig. 4 SR profiles across the epl-substrate interface for p-type wafer, with 10 # m of p-type epflayer, annealed for 24 h (curve A) and 144 h (curve B) at 650 oC m mr
0
4
8 DEPTH
12
16
(#m)
Fig. 5 SR profile for sample of curve B in Fig. 3, annealed for 0.5, 1, 2 and 6 h (curves A, B, C and D respectavely) at 1100 oC In air.
B. Pivac et al
/
Boron accumulaaon at epl -substrate sdtcon interface
oxidized silicon surface is a prerequisite for boron accumulation, which originates from the gas phase. This could explain the phenomenon that we observed, since we used oxidized wafers which were cleaned m situ prior to epi-deposition. However, our SR data show a much wider region of boron accumulation (typically of the order of several microns) than the findings reported in earher works, and we did not have U H V in our system A second possible explanation, which agrees better with our findings, could be as follows. The atmospheric species containing boron are the sources of contamination. These species appear to be boron suboxide fragments, such as BO and ( B O ) 2 . The presence of sihcon oxide on the surface of wafers during the m - s i t u cleanlng process caused further oxidation of the suboxlde, according to
D
r12
(BO)2 + SiO2 = B203 + SiO
I
0
i
4
i
35
I
8
I
I
12
I
I
16
D E P T H (~m) 6 SR profile for sample of curve B m Fig. 3 annealed for 0.5, 1, 2 and 6 h (curves A, B, C and D respectively) at 1100 °C
(1)
This reaction is expected to proceed [18], and at temperatures greater than 750 °C SiO will leave the surface. At higher temperatures, as in our case (ll00°C), further reducUon of boric oxide may occur, following the reaction B203 + 3Si = 2B + 3SiO
(2)
Fig.
In air.
3. Discussion
Similar effects have been observed m samples grown in slightly different conditions. Several researchers [11-17] have observed that a strongly p-doped surface layer forms on silicon wafers annealed at high temperatures (higher than 1000 *C) in ultrahigh vacuum (UHV) systems. The acceptor impurity was identified as boron by mass spectrometry and photohiminescence. The authors [17] concluded that silicon surface contamination with boron during cleaning and initial heat treatment leads to boron accumulation in regions close to the surface. The source of such contamination is not yet completely clear. Henderson [15] has suggested that boron may accumulate during wet chemical pretreatment of wafers, while Allen et al. [11] proposed that vapor phase transport of boron species from borosilicate components of the vacuum system occurs during pump down. Kubiak et al. [16] and Iyer et al. [17] observed a boron spike after repeated prebakes of epilayer grown on surfaces briefly exposed to air in the wafer load lock, but no spike occurred if the epilayers remained within UHV. The authors suggested that an
This reaction has been proposed by several authors [19, 20] for boron doping and has been treated in detail by De Fresart et al. [21]. Their work shows that unless the substrate temperature is significantly above 800 °C, the reaction does not proceed to completion. The activation energy of the process has been calculated to be E a = 4.5 + 1.0 eV. This is in agreement with our experiment since our deposition temperature was 1100 °C. Thus boron accumulation at the surface will be buried at the interface beneath the grown epilayer. This explains the difference between samples with layers 10 # m or 100 p m thick. Since the time of deposition of 10 p m thick layer is short, the reaction described by eqn. (2) cannot be completed, and accumulated boron (in B203 molecules) therefore remains electrically inactive. In the case of the layer 100 kLm thick and/or in the case of the 10 p m layer annealed at 1100°C for less than 2 h, the reaction described was completed and the boron present became electrically active. However, for longer annealing times, i.e. 6 h at 1100 °C, boron diffuses into the bulk of material and the peak disappears. In conclusion, we demonstrated that for homoepitaxial, low doped silicon layers, obtained with atmospheric pressure chemical vapor deposition, a boron accumulation occurred at the interface. A model that explains the observed phenomenon predicts boron oxide formation at the interface prior to epitaxial
36
B ]gtva~ el al
/
Borott tic ~ttmtdaHon al ept-~ttl)sttate sth~oft tnterhu c
growth. A similar phenomenon, however, was not observed for phosphorus-doped epltaxml layers obtamed m the same way, probably owing to lack ot phosphorus oxide formation. References 1 M R Gouldlng and J O Borland, Semiconductor Internanonal, (May 1988)90 2 N Endo, N Kasai, H. 1Otapma and A Ishltanl, in S. Broydo and C M. Osburn (eds.), ULS1 Science and Technology-1987, The Electrochermcal Society, Princeton, NJ, 1987, p 64, 3 G R Snnlvasan, In H R. Huff and E Slrtl (eds.), Semtconductor Sthcon 1977. The Electrochemical Society, Pnnceton, NJ, 1977, p. 218 4 G R Srmlvasan, m T O. Sedgwlck and H Lydtm (eds), Chemical Vapor Deposition 1979, The Electrochemical Society, Princeton, NJ, 1979, p 77 5 G. K Ackerman and E Ebert, J Electrochem So~, 130 (1983) 1910 6 R. G Mazur and D H. Dickey, J Electrochem Soc, 113 (1966) 255. 7 J R. Ehrstein led ), Proc Sp~'eadmg Resistance Syrup, Semiconductor Measurement Technology. NBS Special Pub. 40010, Washington DC, 1974_
8 H L Bcrkovltz and R A Lux. I Llectro~hem Ao, , 12A' (1981}1137 9 D J_ Godtrcy, R D Gro~c.,,, M G Dow~,ctt and A F W Willoughby, Phvswa, 129B ( 1985 ) 181 10 R A Clapper, D (3_ Schimmel, J C C T~al, F S_ Jabara, k A Stevle and P M Kahara..I l"5le~lro¢hem So~ , 1 ~7~ 19901 1877 11 E G Allen, T M Buck and J T Low. J Appl Phys, 31 (1960) 979_ 12 J D Mottram, A Thanadakls and D C Northrop, J Phvs D, 8(1975)1316 13 L N Aleksandrov, R N Lovyagln, P A Slmonov and I_ S Bzmkovskaya, Phys Status Sohdt A, 45 (1978) 521 14 M. Llehr, M Renler, R. A Wachnlk and G S_ Scllla, 3 Appl Phys, 61/1987)4619 15 R C Henderson, J_ Electrochem Soc , 119 (1972) 772. 16 R A A Kubiak, W Y. Leong, M G. Dowsett, D S. McPhafl, R. Houghton and E H Parker, J Vac Sct 7e~hnol A, 4 (1986) 191)5 17 S S. Iyer, S L Delage and G J. Scllla, Appl Phys Lett, 52 (1988)486 18 NBS Tables ot Chemical Thermodynamic Properties, J Phys Chem Re] Data Suppl 2, 11 (1982)267 19 R. Ostrom and F G. Allen, Appl. Phys Lett, 48 (1987) 221. 20 T Tatsuml, H Hirayama and N Alzakl, Appl Phys Lett, 50 (1987) 1234 21 E de Fresart, S S Rhee and K L. Wang, Appl Phys Lett, 49 (1986)847
,{httertals 5~ lem e a n d l'mgmeermg, HI>, I t)92,32-3O
Boron accumulation at epi-substrate silicon interface during epitaxial growth B. P i v a c R. Boskovtc Insntute, P.O. Box 1016, 41000 Zagreb (Croatia) A . B o r g h e s i , M . G e d d o a n d A . Sassella Dipartimento dt Fistca ",4 Volta", Umverstta degli Studt, 1-27100 Pavta (Italy)
M. Pedrotti M E M C Electromc Matenals S.p.A., 1-28100 Novara (Italy) (Received January 29, 1992)
Abstract Boron accumulalaon was observed close to the interface between an epitaxlaUy grown sihcon layer and a silicon substrate wafer and then analyzed. It was concluded that boron contamination interacting with the surface oxade on wafers led to boron accumulataon close to the interface. Such accumulalaon is shown to occur for eptlayers of standard thickness (approxamately 10 ~m), with boron being electrically unactave.
1. Introduction
2. Experimental details
Impunties present m the silicon substrate or in buried layers can be redistributed into the growing epilayer during a typical epitaxlal process, either by solid-state out-diffusion or by gas phase autodoping. An unintended impurity depth profile results, often extending for considerable distance into the epilayer. Consequent degradation in device characteristics impedes downscaling of bipolar complementary metal-oxtde-semiconductors (CMOS) and specific CMOS, where thin epistructures are often grown over heavily doped substrates and/or buried layer structures [1, 2]. As industry approaches ultra large-scale integration density levels, control of dopant profiles is critical. They depend on the dopant incorporation mechanism, on the epidopant species and on the substrate autodoping species. Use of a relatively low epitaxlal growth temperature is the general approach to minimizing solid-state outdiffusion for all dopant species. On the contrary, the autodoping phenomenon has been extensively studied [3-5] and is significantly different for various dopant species In this article we shall focus on the dopant accumulation phenomenon at the epi-substrate interface, observed in boron-doped silicon samples only (not in phosphorus-doped samples).
The silicon substrates used in this experiment were p-type (B-doped, resistivity about 40 f~ cm) wafers, or n-type (P-doped, resistivity about 4 Q cm), grown by the Czochralski (CZ) method in the (100) direcnon with a diameter of 100 mm and average wafer thickness of about 500 /~m. Oxygen concentration m the bulk was measured by the Fourier transform infrared technique and was found to be about 7 x 1017 atoms c m -3. Epitaxial layers about 100 /~m thick of the p-type silicon (B-doped, resistivity about 90 Q cm) and n-type silicon (P-doped, resistivity about 75 fl cm) were grown on p-type and n-type substrates respectively. The samples were cleaned in situ prior to epitaxial growth, in a pancake type, r.f. heated reactor, to remove native oxide from the surface. A prebake treatment was performed in ramp from room temperature to 1180 °C at a rate of 70 °C min-1 After surface etching and ramp down to deposttion temperature in 1 min, purge and deposition were performed. Deposition sources were trichlorosilane and diborane (for p-type) or phosphme (for n-type) diluted at 50 p.p.m, in H 2. Deposition was performed at atmospheric pressure with a growth rate of about 1.25/~m rain- 1. In order to grow layers 100 ~ m thick, wafers were held at a temperature of 1100 *C for about 80 min. Resistivity profiles across the interface were obtained by spreading resistance (SR) measurements
0921-5107/92/$5-00
© 1992 - Elsevaer Sequoia. All rights reserved
B. Pivac et al
/
Boron accumulation at epz-substrate sdicon interface
performed with computer assisted solid state measurement apparatus model SSM 130. Samples were beveled at an angle of 11" 32'. Measurements were made at 1 /~m steps in the z-direction (depth). The distance between probes was 100/zm. A spreading resistance profile across the substrate-epilayer interface for an n-type wafer is shown in Fig. 1, and for a p-type wafer in Fig. 2. (The figures show the variation in resistance in the vicinity of the interface; 0 does not therefore correspond to the wafer surface, but to an arbitrarily chosen point.) The rather steep change in the resistivity on passing from the substrate into the epilayer (Fig. 1) is in good agreement with the rather low diffusivity of phosphorus. On the contrary, some quite unusual behavior is shown in Fig. 2. In addition to the large tail of boron that extends deep into epilayer, there is a significant drop in resistivity at the interface, which we attribute to boron accumulation in this region. This phenomenon is quite unexpected since substrate and epilayer were both p-type materials (B-doped) with similar resistivities. This boron accumulation is quantified to be about 3 x 1014 atoms cm-3. The accumulation was not found on the p-type epilayer grown on p + substrates. There is a striking difference between boron and phosphorus dopant redistribution at the substrateepilayer interface. While phosphorus follows the diffusion laws the behavior of boron is quite unusual. A SR profile reliability is obviously crucial in determining whether the differences in SR profiles obtained can be straightforwardly attributed to dopant variation. The SR technique for measuring resistivity profiles of
33
thin silicon layers was developed in 1963, and its use became popular in the late 1970s [6] owing to its major advantage of a fast tumround time. Since then a lot of research has checked reproducibility and many theoretical studies on converting SR to concentration for diffused profiles have been published [7, 8]. Godfrey et al. [9] compared SR and secondary ion mass spectrometry (SIMS) profiles for As and B after implanting and annealing. Recently Clapper et al. [10] made a comparison of boron profiles obtained by SR and SIMS and found very good agreement between the two techniques. Although SR could be considered semiquantitative, and SIMS data much more reliable, we should point out that SIMS sensitivity is about 10 t5 atoms cm-3. In our case variation in boron concentration across the interface is much less than SIMS sensitivity. It can further be argued that we registered only electrically active boron; the total boron concentration was much higher. Therefore we performed SIMS measurements with a C A M E C A IMS 4f spectrometer. However, our analysis showed that the boron concentration was below the sensitivity of the SIMS technique throughout the interface, and was therefore in agreement with SR data. In order to understand boron behavior, we performed the following experiment to determine whether the observed peak was a result of autodoping [5]. About 10 /~m of p-type epilayer was grown on the p-type substrate while several other p + substrates were present in the reactor. No sigruficant B segregation at the interface was observed in this case; the possibility of autodoping from adjacent p+ doped substrates was therefore excluded. The accumulation is more evident in the resistivity profile of p-p samples than in p-p+
102
o ==~
101
tj
2O 2
I
0
I
20
I 0
I
!
DEPTH (Urn)
Fig 1. SR profile across the cpl-substrate interface for n-type wafer,wath 100/zm of n-type epilayer
o
I
2O
I
40
I
I
60
D E P T H (tim)
Fig. 2. SR profile across the epi-substrate interface for p-type wafer, with 100/zm of p-type epllayer.
34
B_ Ptvac et al
/
Boron accumulatton at ept-substrate sthcon tnter~at e
samples, owing to the very low resistivity of p+ substrates, wtuch could mask this small peak. Subsequently we deposited a 10 ~m thick p-type epflayer on p-type substrates and ascertained that only low doped
102
101
t~
10 0
I
I
0
I
I
20
I
40
60
p-type substrates were present in the reactor. Ftgure 3 shows the SR profile for these samples, mdlcatlng no boron accumulation at the interface The samples were further annealed in air at 650 °C. Curves A and B in Fig 4 show SR data obtained for samples after annealing for 24 and 144 h respectively. Annealing at 650 °C caused a slgmficant B segregation at the interface which did not degrade w~th time. The results of SR measurements performed on the same samples after annealing at 1100 °C for 0.5, 1, 2 and 6 h are shown in Fig. 5 (curves A, B, C and D respectively). Again we observed that boron accumulation occurred after short periods of annealing (curves A, B and C). However, the accumulation disappeared completely upon longer annealing at the same temperature (curve D). In fact, only a boron dtffUSlOn tall remained in the epilayer after 6 h anneahng at 1100 °C. SR measurements after two-step annealing, the first for 144 h at 650 °C and the second for 0.5, 1, 2, and 6 h at 1100 °C, are reported in Fig. 6 (curves A, B, C and D respectively). The profiles clearly indicate that upon longer high-temperature anneahng boron slowly diffuses out of the segregation region into the ep~taxial layer (Fig. 6, curve D).
DEPTH (tml) Fig. 3 SR profile across the epl-substrate interface for p-type wafer, with 10/,tm of p-type as-grown epdayer.
B
L A
A
> 0t~ ;0
•
0
i
i
4
8 DEPTH
!
!
12
m
1
16
(#m)
Fig. 4 SR profiles across the epl-substrate interface for p-type wafer, with 10 # m of p-type epflayer, annealed for 24 h (curve A) and 144 h (curve B) at 650 oC m mr
0
4
8 DEPTH
12
16
(#m)
Fig. 5 SR profile for sample of curve B in Fig. 3, annealed for 0.5, 1, 2 and 6 h (curves A, B, C and D respectavely) at 1100 oC In air.
B. Pivac et al
/
Boron accumulaaon at epl -substrate sdtcon interface
oxidized silicon surface is a prerequisite for boron accumulation, which originates from the gas phase. This could explain the phenomenon that we observed, since we used oxidized wafers which were cleaned m situ prior to epi-deposition. However, our SR data show a much wider region of boron accumulation (typically of the order of several microns) than the findings reported in earher works, and we did not have U H V in our system A second possible explanation, which agrees better with our findings, could be as follows. The atmospheric species containing boron are the sources of contamination. These species appear to be boron suboxide fragments, such as BO and ( B O ) 2 . The presence of sihcon oxide on the surface of wafers during the m - s i t u cleanlng process caused further oxidation of the suboxlde, according to
D
r12
(BO)2 + SiO2 = B203 + SiO
I
0
i
4
i
35
I
8
I
I
12
I
I
16
D E P T H (~m) 6 SR profile for sample of curve B m Fig. 3 annealed for 0.5, 1, 2 and 6 h (curves A, B, C and D respectively) at 1100 °C
(1)
This reaction is expected to proceed [18], and at temperatures greater than 750 °C SiO will leave the surface. At higher temperatures, as in our case (ll00°C), further reducUon of boric oxide may occur, following the reaction B203 + 3Si = 2B + 3SiO
(2)
Fig.
In air.
3. Discussion
Similar effects have been observed m samples grown in slightly different conditions. Several researchers [11-17] have observed that a strongly p-doped surface layer forms on silicon wafers annealed at high temperatures (higher than 1000 *C) in ultrahigh vacuum (UHV) systems. The acceptor impurity was identified as boron by mass spectrometry and photohiminescence. The authors [17] concluded that silicon surface contamination with boron during cleaning and initial heat treatment leads to boron accumulation in regions close to the surface. The source of such contamination is not yet completely clear. Henderson [15] has suggested that boron may accumulate during wet chemical pretreatment of wafers, while Allen et al. [11] proposed that vapor phase transport of boron species from borosilicate components of the vacuum system occurs during pump down. Kubiak et al. [16] and Iyer et al. [17] observed a boron spike after repeated prebakes of epilayer grown on surfaces briefly exposed to air in the wafer load lock, but no spike occurred if the epilayers remained within UHV. The authors suggested that an
This reaction has been proposed by several authors [19, 20] for boron doping and has been treated in detail by De Fresart et al. [21]. Their work shows that unless the substrate temperature is significantly above 800 °C, the reaction does not proceed to completion. The activation energy of the process has been calculated to be E a = 4.5 + 1.0 eV. This is in agreement with our experiment since our deposition temperature was 1100 °C. Thus boron accumulation at the surface will be buried at the interface beneath the grown epilayer. This explains the difference between samples with layers 10 # m or 100 p m thick. Since the time of deposition of 10 p m thick layer is short, the reaction described by eqn. (2) cannot be completed, and accumulated boron (in B203 molecules) therefore remains electrically inactive. In the case of the layer 100 kLm thick and/or in the case of the 10 p m layer annealed at 1100°C for less than 2 h, the reaction described was completed and the boron present became electrically active. However, for longer annealing times, i.e. 6 h at 1100 °C, boron diffuses into the bulk of material and the peak disappears. In conclusion, we demonstrated that for homoepitaxial, low doped silicon layers, obtained with atmospheric pressure chemical vapor deposition, a boron accumulation occurred at the interface. A model that explains the observed phenomenon predicts boron oxide formation at the interface prior to epitaxial
36
B ]gtva~ el al
/
Borott tic ~ttmtdaHon al ept-~ttl)sttate sth~oft tnterhu c
growth. A similar phenomenon, however, was not observed for phosphorus-doped epltaxml layers obtamed m the same way, probably owing to lack ot phosphorus oxide formation. References 1 M R Gouldlng and J O Borland, Semiconductor Internanonal, (May 1988)90 2 N Endo, N Kasai, H. 1Otapma and A Ishltanl, in S. Broydo and C M. Osburn (eds.), ULS1 Science and Technology-1987, The Electrochermcal Society, Princeton, NJ, 1987, p 64, 3 G R Snnlvasan, In H R. Huff and E Slrtl (eds.), Semtconductor Sthcon 1977. The Electrochemical Society, Pnnceton, NJ, 1977, p. 218 4 G R Srmlvasan, m T O. Sedgwlck and H Lydtm (eds), Chemical Vapor Deposition 1979, The Electrochemical Society, Princeton, NJ, 1979, p 77 5 G. K Ackerman and E Ebert, J Electrochem So~, 130 (1983) 1910 6 R. G Mazur and D H. Dickey, J Electrochem Soc, 113 (1966) 255. 7 J R. Ehrstein led ), Proc Sp~'eadmg Resistance Syrup, Semiconductor Measurement Technology. NBS Special Pub. 40010, Washington DC, 1974_
8 H L Bcrkovltz and R A Lux. I Llectro~hem Ao, , 12A' (1981}1137 9 D J_ Godtrcy, R D Gro~c.,,, M G Dow~,ctt and A F W Willoughby, Phvswa, 129B ( 1985 ) 181 10 R A Clapper, D (3_ Schimmel, J C C T~al, F S_ Jabara, k A Stevle and P M Kahara..I l"5le~lro¢hem So~ , 1 ~7~ 19901 1877 11 E G Allen, T M Buck and J T Low. J Appl Phys, 31 (1960) 979_ 12 J D Mottram, A Thanadakls and D C Northrop, J Phvs D, 8(1975)1316 13 L N Aleksandrov, R N Lovyagln, P A Slmonov and I_ S Bzmkovskaya, Phys Status Sohdt A, 45 (1978) 521 14 M. Llehr, M Renler, R. A Wachnlk and G S_ Scllla, 3 Appl Phys, 61/1987)4619 15 R C Henderson, J_ Electrochem Soc , 119 (1972) 772. 16 R A A Kubiak, W Y. Leong, M G. Dowsett, D S. McPhafl, R. Houghton and E H Parker, J Vac Sct 7e~hnol A, 4 (1986) 191)5 17 S S. Iyer, S L Delage and G J. Scllla, Appl Phys Lett, 52 (1988)486 18 NBS Tables ot Chemical Thermodynamic Properties, J Phys Chem Re] Data Suppl 2, 11 (1982)267 19 R. Ostrom and F G. Allen, Appl. Phys Lett, 48 (1987) 221. 20 T Tatsuml, H Hirayama and N Alzakl, Appl Phys Lett, 50 (1987) 1234 21 E de Fresart, S S Rhee and K L. Wang, Appl Phys Lett, 49 (1986)847