- Email: [email protected]
Characterization of Czochralski Si crystals by HVEM
Ultl'amicroscopy 39 ( 1991 ) 160-170
ro
N o r , H toiiand
c'opy
Characterization of Czochralski Si crystals by HVEM R. O s h i m a a n d G . C , H u a 1 D~7~artment of Mawrial Physics, Faculty ~1"Engineering Seiem:e, Osaka Uniret:~io', Toyonaka, Osaka 500, JalmH
Received 26 March 1991
Based on in-situ obse~'ations of electron-irradiation-induced sccondaw defccts of wlrious Czochralski-grown silicon crystals (Cz-Si) above room temperature with an ultrahighwoltagc electron microscope, a new tool fl)r characterization of Cz-Si is proposed. It has been shown that the nucleation of the defects is associated with clusters of oxygen, which is an unavoidable impurity of Cz-Si. and also other impurities impinged from the electron beam incident surface. Characteristic features of sequences of the defect formation, growth and shrinkage wilh irradiation are also discussed on lhe basis of the reaction Fate theo~.
1. Introduction W i t h r e c e n t r e m a r k a b l e d e v e l o p m e n t s of large-scale i n t e g r a t e d circuits (LSI) and charge conversion devices ( C C D ) in s e m i c o n d u c t o r industries, studies of the b e h a v i o r of oxygen, which is an u n a v o i d a b l e impurity of g e n e r a l l y used C z o c h r a l s k i - g r o w n silicon crystals (Cz-Si), become increasingly i m p o r t a n t . In spite of the g r e a t n u m b e r of works r e g a r d i n g the states of s u p e r s a t u r a t e d oxygen a t o m s in the crystals so far, t h e i r u n d e r s t a n d i n g has b e e n yet insufficient. W h i l e the oxygen a t o m s i m p r o v e the m e c h a n i c a l s t r e n g t h of the wafers for inhibiting the slip by the m a c h i n i n g of the d r a w n single crystals, they are r e s p o n s i b l e for a variety of defect f o r m a t i o n s d u r i n g the t h e r m a l p r o c e s s e s of the device production. T h e c h a n g e in c o n c e n t r a t i o n of interstitial solute oxygen a t o m s of Si crystals has b e e n e x a m i n e d c o m m o n l y by infrared spectroscopy, but d e t a i l e d i n f o r m a t i o n on oxygen clusters is not easy to acquire by this m e t h o d . High-voltage e l e c t r o n microscopy ( H V E M ) has b e e n d e v e l o p e d as a powerful tool to observe thicker s p e c i m e n s exhibiting the bulk p r o p e r t i e s i Present address: Department of Physics, College of General Education, Osaka University, Toyonaka, Osaka 560, Japan.
of materials. On the o t h e r hand, studies of radiation d a m a g e of metals and alloys utilizing the higher densities of the h i g h - e n e r g y illuminating e l e c t r o n s than the c o n v e n t i o n a l e l e c t r o n a c c e l e r a tors have b e e n p e r f o r m e d extensively, and quantitative information on point defects has been derived It]. Several H V E M investigations of Si havc bccn also u n d e r t a k e n , and it is found that the e l e c t r o n - i r r a d i a t i o n - i n d u c e d s e c o n d a r y defects are interstitial-type dislocation loops with stacking faults on {113} p l a n e s [2-5]. Prcvious H V E M works on silicon by the p r e s e n t authors" g r o u p have also shown that the f o r m a t i o n of i r r a d i a t i o n i n d u c e d s e c o n d a r y defects is different b e t w e e n floating zone (Fz) and Cz silicon [6-1(I]. Differing from the i r r a d i a t i o n of Fz-Si in which the loops are f o r m e d after a few rain of incubation, it is c h a r a c t c r i s t i c that the loops of Cz-Si b e c o m c visible almost as soon as the start (11 irradiation. T h c spatial distributions of the defects were also differcnt b e t w e e n Fz-Si and Cz-Si. in contrast to Fz-Si, in which the loops are only o b s c r v c d to form in a thin layer n e a r thc electron b e a m incident surfacc, those of Cz-Si arc f o r m c d hom o g e n e o u s l y insidc the s p e c i m e n except for thin d e n t i d e d zone layers n e a r the surfaces. Only the defects r c l a t e d to i m p i n g e d impurity a t o m s from
0304-3991/91/S03.511 <> 19tJl Elsevier Science Publishers B.V. All rights reserved
R. Oshima, G.C. Hua / Characterization of Cz-Si by HVEM
the electron incident surface are observed at temperatures between room temperature and 400 ° C in the Fz-Si, while the attribution of oxygen atoms to the nucleation of loops is suggested in the bulk of Cz-Si. Since the previous experimental results have exhibited that the nucleation of the irradiation-induced secondary defects is dependent upon the species and states of impurity atoms in silicon, in situ high-voltage electron microscopy is expected to become one of the candidates for a tool for characterization of Czochralski-grown silicon crystals. The present work is aimed to look for a new technique for characterization of Cz-Si wafers based on the in situ observation results of the irradiation-induced lattice defects of various Si wafers in H V E M .
2. Experiment
The specimens used were prepared mostly from a Cz-Si crystal with a (001) or (111) orientation containing interstitial oxygen atoms of the order of 10LS/cm 3 and substitutional carbon atoms of about 10aS/cm 3. In order to dissociate oxygen clusters previously formed during the crystal growth processes and to disperse the oxygen atoms in the matrix, they were subjected to a solution heat treatment at 1300 ° C for 16 h, followed by rapid cooling to room temperature. The concentration of interstitial oxygen atoms after the heat treatment was determined by the 9 txm absorption band of infrared spectroscopy using an Analect RFX-65 Fourier transform infrared spectrometer. Wafers subjected to a simulation heat treatment of intrinsic gettering were also prepared. Other wafers whose surfaces were chemically contaminated with Fe or Ni were made to examine the effect of metal impurities on the nucleation of the defects. Three kinds of wafers of Cz-Si crystals drawn in magnetic fields (MCzSi) to control the growth conditions more stably were also used; their oxygen concentrations ranged from 1017 and to 1018/cm 3. It is noteworthy that such a wide range of control of the oxygen content is attained only by the MCz-Si. They were formed into disks of 3 mm diameter
161
and chemically thinned in a mixture of hydrofluoric acid and nitric acid for transmission electron microscopy. In situ observations using an ultrahigh-voltage electron microscope, HU-3000, at Osaka University have been carried out in a {220} dynamical condition at temperatures between 200 and 4 5 0 ° C at 2 MeV. The doses used were between 10 ~ and 1020 e / c m 2 s. The irradiated specimens were further investigated by a JEM200CX electron microscope operated at 160 kV.
3. Results and discussion
3.1. Effect of pre-heat treatments on the nucleation of secondary defects 3.1.1. Effect of heat treatment at 1300 ° C A Cz specimen with (001) orientation subjected to a heat treatment at 1300 °C for 16 h to decompose oxygen clusters formed during the crystal growth was irradiated at 340 ° C with an electron dose of 1.2 × 1020 e / c m 2 s, and compared with the as-grown crystal. After the annealing the interstitial oxygen concentration increased from 1.2 × 10 is to 1.3 × 1018/cm 3, indicating that decomposition of oxygen clusters actually took place. The in situ observation of the annealed specimen showed that the secondary defects as well as the as-grown one were observed soon after the irradiation started. However, the loop density decreased remarkably by the heat treatment at 1300 ° C as seen in fig. 1, suggesting that not the oxygen clusters but a single oxygen atom would be involved in the defect nucleation. On continuing the irradiation, the secondary defects started to shrink and finally annihilated in the central area of the irradiation where the beam intensity was the strongest. 3.1.2. Effect of gettering heat treatment Commercial Cz-Si wafers are generally subjected to heat treatments for gettering of harmful impurities in the bulk and to keep the circuit construction surfaces free from the defect formation during the thermal processes, which is called an intrinsic gettering. As a simulation experiment wafers are annealed at 1150 ° C for 4 h and then
162
R. Oshima, G.C. ttua / Characterization 01"(':-Si by HVEM
at 800 ° C for 100 h. For comparison wafers which received a thermal donor killer treatment at 650 °C for 30 min were also investigated. Figs. 2 and 3 represent respectively results of in situ observations at 300 °C of the surface and the bulk of a wafer. Here, the "surface" and the "bulk" specimens were respectively prepared from the same wafers with the gettering heat treatments to leave only the surface region and the bulk by chemical polishing. It is very clear that the loops do not appear until 109 s of irradiation (fig. 2b) for the surface, while they are already visible by 25 s of irradiation (fig. 3a). The stereoscopy showed that the loops observed in the surface of the intrinsic gettering treatment were formed only near the beam incident surface as in the case of Fz-Si, suggesting that the defect formation is associated with an impurity impinging effect from the surface by the incident beam.
Taking into account the responsibility of oxygen clusters for the defect nucleation, the results indicate that the concentrations of oxygen clusters are much different between the surface area and the bulk. It is also seen in fig. 3 that some defects grow and some disappear with the continuation of irradiation in the central area; other defects formed near the beam incident surface are visible as small dot contrasts in (fig. 3d). Fig. 4 shows the change in the defect density of the bulk of the irradiated specimens of thickness 800 nm subjected to the gettering heat treatment. The defect density of 1014/cm 3 of the specimen of the gettering heat treatment is one order of magnitude larger than that of the as-grown one. Here, it must be noticed that one sixth of {113} defects are invisible in the (022) dark field micrographs because of the extinction rule of gb = 0. The decrease in the interstitial oxygen atoms with the
Fig. 1. D a r k field e l e c t r o n m i c r o g r a p h s s h o w i n g c h a n g e in the d e n s i t y o f e l e c t r o n - i r r a d i a t i o n - i n d u c e d d i s l o c a t i o n loops o f Cz-Si with o x y g e n - d i s p e r s e d t r e a t m e n t : (a) a s - g r o w n Cz-Si a f t e r 152 s i r r a d i a t i o n , (b) s o l u t i o n - t r e a t e d Cz-Si at 13(/0 ° C for 16 h a f t e r 223 s i r r a d i a t i o n . I r r a d i a t i o n s w e r e d o n e at 2 M V with a w e a k w i d e e l e c t r o n flux of 2 x 1(I 1'~ e / c m 2 s at 340 o C.
R. Oshima, G.C. Hua / Characterization of Cz-Si by HVEM
h e a t t r e a t m e n t was 0.15 × 1018/cm 3. F o r the m o ment, the effect of the d o n o r killer t r e a t m e n t on t h e d e f e c t f o r m a t i o n is not clear. 3.2. M C z - S i
T h r e e kinds of M C z wafers with d i f f e r e n t oxygen c o n c e n t r a t i o n s were e x a m i n e d . In the specim e n with the lowest oxygen c o n t e n t o f 2 x 1017/cm3, which is not a t t a i n e d by the conventional Cz m e t h o d , the s e c o n d a r y defects w e r e f o r m e d only a f t e r several m i n u t e s i n c u b a t i o n periods n e a r the e l e c t r o n b e a m i n c i d e n t surface by the i r r a d i a t i o n at 3 0 0 ° C , as shown in fig. 5, w h e r e no d e f e c t s a r e visible yet until an e l a p s e d time of 114 s of (a). T h e b e h a v i o r was very similar to that of Fz-Si. O n the o t h e r hand, the f o r m a tion of s e c o n d a r y defects in the s p e c i m e n with a
163
high oxygen c o n t e n t of 2.2 × 10~S/cm 3 was not d i f f e r e n t from that o f n o r m a l Cz-Si. No s o o n e r did the i r r a d i a t i o n start t h a n the s e c o n d a r y defects b e c a m e visible in the bulk of the s p e c i m e n . A s the i r r a d i a t i o n p r o c e e d e d , they s t a r t e d to shrink a n d s o m e of t h e m finally d i s a p p e a r e d before new d e f e c t s w e r e f o r m e d n e a r the b e a m i n c i d e n t surface, the p r o c e s s of which is shown in fig. 6. It was f o u n d that in the s p e c i m e n with a m e d i u m oxygen c o n t e n t o f 7 x 10~7/cm 3 no seco n d a r y d e f e c t s which a p p e a r e d w i t h o u t incubation p e r i o d s were f o r m e d in the c e n t r a l a r e a of the s t r o n g e s t b e a m intensity of the s p e c i m e n , while they were o b s e r v e d in the p e r i p h e r y o f the o b s e r v a t i o n area. Fig. 7 shows a (022) d a r k field m i c r o g r a p h of the s p e c i m e n t a k e n by 160 k e V T E M after the H V E M observation. In this case only the u p p e r - r i g h t a r e a was i r r a d i a t e d with a
Fig. 2. Sequence for irradiation of the surface of Cz-Si wafer subjected to intrinsic gettering heat treatment at 1150 ° C for 4 h and then at 800 o C for 100 h. Irradiation was done at 2 MV with a dose of 1 x 102o e/cm 2 s at 300 o C. The irradiation elapsed times are (a) 44, (b) 109, (c) 142, (d) 200, (e) 248 and (f) 301 s.
164
R. Oshima, G.C. Hua / Characterization Of"Cz-Si by HVEM
condensed beam in the H V E M , where the defects were formed near the beam incident surface after an incubation period. The present results also clearly indicate that the nucleation of the electron-irradiation-induced secondary defects is associated with the oxygen of the specimen.
3.3. Effect of impurity on the nucleation of the secondary defects Since previous results suggest that the nucleation of the secondary defects should be promoted also by impurities, the effects of contamination of the surface with heavy metals, such as Fe and Ni, were examined. In fig. 8 is shown an example of the case of contamination with Fe of 1.7 x 10~4/cm 3. The specimen was annealed at 1300 o C and quenched before the irradiation. The
loops already appear at 11 s after starting the irradiation, and the number is larger than in the uncontaminated specimen shown in fig. lb, though the present irradiation temperature was higher, Examples of the surface contamination with Ni are presented in fig. 9. It is clear that the number of loops is slightly larger in the specimen which is more contaminated. Accordingly, heavy metal impurities are also responsible for the nucleation of the irradiation-induced secondary defects. However, further systematic experiments seem to be necessary to obtain the nature of nuclei and quantitative results.
3.4. Consideration of the results" of in situ obsercation based on the reaction rate theory From the in situ H V E M observations of electron-irradiation-induced secondary defects of Cz-
Fig. 3. Sequence for irradiation of the bulk of Cz-Si wafer subjected to intrinsic gettering heat treatment at 1150 ° C for 4 h and then at 800 ° C for 100 h. The irradiation conditions were the same as in fig, 2. The irradiation elapsed times are (a) 25, (b) 76, (c) 128, (d) 193, (e) 222 and (D 300 s.
R. Oshima, G.C. Hua / Characterization ()[ Cz-Si by HVEM
Si with interstitial-type nature it has been shown that they are immediately formed uniformly in the bulk except near the surface layers as soon as the irradiation (that is, the observation) starts and begin to shrink after a certain growth period. In the special case of strong irradiation of more than 2 × 10 20 e / c m 2 s, the defects sometimes appear no longer. The behavior also depends on the concentrations and the states of oxygen in the specimen. In order to consider the behavior of the secondary defects with irradiation, computer simulation was carried out based on the reaction rate theory. Since the interstitial-type loops start to shrink after some growth period, it is indicated that the concentrations of vacancies and interstitials in the specimen do not reach a steady state even several minutes after the beginning of the irradia-
165
tion. This can be explained if some vacancy-type clusters are formed gradually and act as sinks of interstitials with the irradiation. Divacancies will be the major defects in high-energy particle irradiations of Si at room temperature [11-12]. Other larger vacancy clusters, such as trivacancies, tetravacancies and pentavacancies, have also been identified after electron and neutron irradiation [13-18]. The pentavacancies, for example, were found to be stable up to 450 ° C. Accordingly, it is reasonable to conclude that such larger vacancy clusters are also formed during strong electron irradiation as in the present case. As a result of the formation of the vacancy clusters, the number of sinks for interstitials is increased to reduce the concentration of the interstitials. Consequently, the loops formed in the beginning absorb more vacancies than interstitials, and they will shrink. The decrease in the interstitial concentration,
®
.
Fig. 4. (022) dark field electron micrographs showing change in the density of electron-irradiation-induced dislocation loops of Cz-Si wafers with intrinsic gettering heat treatment. (a) As-grown Cz-Si and (b) at 1150°C for 4 h and then at 8 0 0 ° C for 100 h. The irradiation conditions were: at 2 MeV with a wide weak dose of 2 × 10 i9 e / c m 2 s at 300 ° C.
166
R. Oshima, G.C. Hua / Characterization of" Cz-Si by H V E M
and the retardation of reaching the steady state due to the formation of vacancy clusters, may be demonstrated by computer simulation using reaction rate equations. A simplification is at first introduced by assuming that only divacancies are formed as multi-vacancy clusters during the irradiation. Also assumed is a simple case in which only interstitial dislocation loops are formed as secondary defects, and oxygen clusters which act as the nucleation sites of the loops already exist in the specimen before irradiation with a volume density of C L, and the loops grow at such latent nucleation sites. The equations describing the reactions between point defects and the loops are
written as follows: dCi/dt
4rrriv (Di +
= p-
Dv)CiC
v
4(,rrACL)'/Z(CiL)'/2DikiCi
- - D i C s f C i - 4,rrriv2( D i + D v 2 ) C i C v 2 ,
(l) dCv/dt
= p -
4,rr riv ( D i +
- 4(
rrAC L)
_ DvC~fC v _
Dv)CiC
v
t/2(CiL ) I / 2 D , . k v C v 8rr rvv( 2 D v ) ( Cv ) 2
+ 4"rrriv2( D i + D v 2 ) C i C v 2 +
2kd~Cv2,
(2)
Fig. 5. Sequence for irradiation of MCz-Si with interstitial oxygen content of 2.0 × 1017/cm 3. Irradiation conditions were: at 2 MV with a dose of 1 × 1020 e / c m 2 s at 300 ° C. The thickness of the wedge-shaped specimen was about 800 nm at the center of the irradiation area. The irradiation elapsed times were (a) 114, (b) 217, (c) 251 and (d) 317 s.
R. Oshima, G.C. Hua / Characterization of Cz-Si by H V E M
167
Fig. 6. Sequence for irradiation of MCz-Si with interstitial oxygen content of 2.2 × 10~S/cm 3. Irradiation conditions were the same as in fig. 5. The irradiation elapsed times were (a) 5, (b) 18, (c) 107, (d) 203, (e) 258 and (f) 304 s.
dCvz/dt = 4wG.,( 2Dv)( Cv) 2 - 4 vr riv 2 ( D i + Ov2 ) C iCv2
- O v z C s f C v 2 - kdsCv2 ,
dCiL/dt =
4(
ACL)1/2(G
)t/2
X ( DikiC i - DvkvCv), R L = (ACiL/'ITCL)
1/2.
(3)
(4) (5)
In these equations, Ci, C v, Cv2 and Cic are respectively the numbers per unit volume of interstitials, vacancies, divacancies and atoms absorbed in the loops. R c is the loop radius. P is the production rate of interstitials and vacancies in defect pairs per unit volume per unit time, which can be calculated using a displacement cross section of 58 b for 2 MeV electrons and irradiation electron doses, and in the present case
the order of 10 . 2 and 10 . 3 depending on the beam intensity. The second terms in eqs. (1) and (2) describe the recombination of interstitials and vacancies. The diffusivities of interstitials D~, vacancies Dv and divacancies Dr2 are calculated from their migration energies, respectively, riv means the effective radius of the vacancy-interstitial recombination volume, and other terms r in the equations indicate the radii of corresponding recombination reactions indicated by the subscripts. The third terms of eqs. (1) and (2) denote the absorption of interstitials and vacancies by the loops. A is the reciprocal of the area density on the loop plane of the atoms precipitated in the loops, calculated using a {113} defect model of Tan [19]. k i and k v denote the absorption efficiency of interstitials and vacancies by the loops, which depend upon both the geometry and the
168
R. Osh#na. G.C. Hua / Characterization of Cz-Si by ttVEM
a r r a n g e m e n t of the loops. A l t h o u g h their exact solutions are not easy to obtain, in a few simple cases they have b e e n d e r i v e d [20-21]. T h e third t e r m s of eqs. (1) a n d (2) d e s c r i b e respectively the a n n i h i l a t i o n of interstitials and vacancies at the s p e c i m e n surface, w h e r e C~f is the a t o m i c fraction of the sink sites of the surface, a s s u m e d to be identical for the a b s o r p t i o n of interstitials a n d vacancies, a n d a p p r o x i m a t e l y t a k e n as C s f = 5 ( 1 / H ) 2 for s p e c i m e n thickness H [22]. T h e o t h e r t e r m s d e s c r i b e the i n t e r a c t i o n s of defect species with divacancies, kas is a dissociation coefficient of a divacancy. In the calculations, for simplicity, it has b e e n a s s u m e d h e r e that if one c o n s t i t u e n t vacancy o f a divacancy j u m p s one lattice d i s t a n c e to a s e c o n d n e a r e s t n e i g h b o r of the o t h e r constituent vacancy, the i n t e r a c t i o n b e t w e e n the two vacancies b e c o m e s zero; the activation energy for this j u m p has b e e n a s s u m e d to be the b i n d i n g e n e r g y of the divacancy r e p o r t e d , 1.6 eV. T h e density o f the oxygen clusters which act as the n u c l e a t i o n sites of the loops has b e e n a s s u m e d to be 1013/cm 3, which is e s t i m a t e d from the density of the o b s e r v e d loops. T h e r e a c t i o n radii have
b e e n a s s u m e d to be: riv dv,, = 2r~ and riv2 3r~, w h e r e r 0 is the closest d i s t a n c e b e t w e e n two silicon a t o m s of 0.235 nm. A l t h o u g h the p r e s e n t s i m u l a t i o n was u n a b l e to r e p r o d u c e c o m p l e t e l y the e x p e r i m e n t a l results, a s u g g e s t e d result is given in fig. 10, w h e r e an e x t r e m e and r a t h e r u n r e a l i s t i c case that vacancies are m o r e a b s o r b e d at the loops than interstitials is tentatively a s s u m e d in o r d e r to save the comp u t a t i o n time a n d also to e x a g g e r a t e the effect of increasing vacancy clusters resulting in the relative r e d u c t i o n of sink efficiency of the f o r m e d loops for interstitials. In this calculation, not only divacancies but also A c e n t e r s ( v a c a n c y - o x y g e n pair) are t a k e n into account. It is shown that the loops grow in the beginning, t h e n start to shrink and a n n i h i l a t e finally, t h o u g h the time scale much d e v i a t e s from the actual one. F o r the i n t e r p r e t a tion of the e x p e r i m e n t a l results quantitatively, c o m p u t e r simulation taking into a c c o u n t all the possible r e a c t i o n s which t a k e p l a c e actually during the i r r a d i a t i o n is necessary. By c o m p a r i n g such s i m u l a t i o n with e x p e r i m e n t a l findings, a t h o r o u g h u n d e r s t a n d i n g of the b e h a v i o r of p o i n t =
=
Fig. 7. (022) dark field electron micrograph showing defect formation of MCz-Si with interstitial oxygen content of 7× l()~7/cm~. The HVEM irradiation was done at the upper-right area at 2 MV with a dose of 1 × 10z° e/cm ~ s at 300°C for 306 s, where the defects were formed near the beam incident surface. The defects around the irradiated area are present in the bulk.
R. Oshima, G.C. Hua / Characterization of Cz-Si by HVEM
169
......
.
....
2 50mn
.......
Fig. 8. S e q u e n c e for i r r a d i a t i o n of Cz-Si w a f e r i n t e n t i o n a l l y c o n t a m i n a t e d with 1.7× 1014 F e / c m 3. The i r r a d i a t i o n c o n d i t i o n s were: at 2 M V with a dose of 2.0 x 102o e / c m 2 s at 400 ° C. The i r r a d i a t i o n e l a p s e d times were (a) 11, (b) 75, (c) 163 and (d) 352 s.
Fig. 9. Effect of Ni c o n t a m i n a t i o n of Cz-Si w a f e r on the f o r m a t i o n of e l e c t r o n - i r r a d i a t e d - i n d u c e d defects. The i r r a d i a t i o n c o n d i t i o n s were: at 2 M V with a dose of 2 × 102o e / c m 2 s at 200 ° C. (a) W i t h 1.3 × 1013 N i / c m 3 after 100 s i r r a d i a t i o n and (b) with 1.1 x 1015 N i / c m 3 after 89 s irradiation.
170
R. Oshirna, G.C. Hua / Characterization of Cz-Si by HVEM 104
i0 le
A center
E
10 '4
/ ~
io 2
ioIO
i0 ~ o
IOS
p
~o2
LOo -J
ing specimen materials. We are also grateful to Emeritus Prof. F.E. Fujita for stimulating and valuable discussions. We are very obliged to Emeritus Prof. H. Fujita, Prof. H. Mori, Drs. K. Yoshida, M. Komatsu and T. Sakata of the Research Center for Ultra-High-Voltage Electron Microscopy of Osaka University for their kind cooperation in the course of this study. Also, the helpful discussions with Dr. T. Ezawa and the assistance of Mr. T. Kawano are appreciated.
}oi0-16
/
/
i0-I
/ ,oe
J
Irradiation
,;time/s
Fig. 10. C o m p u t e r simulation of the v a r i a t i o n of the concentration of defects and the size of loops with 2 MeV electron
irradiation. Parameters used are: an electron dose 2 × 102° e / c m z s, temperature 300 ° C, specimen thickness 500 nm, migration energies of a single vacancy 0.3 eV, a divacancy 1.3 eV, and an interstitial 0.02 eV, binding energy of a divacancy 1.6 e g , k i = 0.5 and k v = 2.5.
defects and their interactions with impurities in silicon will be obtained.
4. Conclusion The in situ ultrahigh-voltage electron mi= croscopy (UHVEM) of various silicon wafers has exhibited characteristic features of the irradiation-induced secondary defects which are very dependent upon the species and states of impurity atoms of the crystals. Accordingly, the U H V E M technique is promising as a tool for characterization of Cz-Si subjected to various treatments, which is important in preparing defect-free silicon wafers for semiconductor industries.
Acknowledgements The authors would like to thank Kyusyu Electric Metals Co. and Sony Corporation for provid-
References [1] M. Kiritani, in: Point Defects, Eds. M. Doyama and S. Yoshida (Univ. Tokyo Press, Tokyo, 1977) p. 247. [2] M.D. Matthews and S.T. Ashby, Phil. Mag. 27 (1973) 1313. [3] H. F611, Inst. Phys. Conf. Set. 23 (1975) 233. [4] I.G. Salisbury and M.H. Loretto, Phil. Mag. 39 (1979) 317. [5] S. Furuno, K. lzui and H. Otsu, Jpn. J. Appl. Phys. 18 (1979) 203. [6] H. Asahi, R. Oshima and F.E. Fujita, Inst. Phys. Conf. Set. 59 (1981) 455. [7] R. Oshima, S. Sadamitsu and F.E. Fujita, Physica B 116 (1983) 606. [8] R. Oshima, M. Hasebe, G.C. H u a and F.E. Fujita. in: Proc. Int. Symp. on ln-situ Experiments with HVEM, Ed. H. Fujta (Osaka Univ., Osaka 1985) p. 207. [9] M. Hasebe, R. Oshima and F.E. Fujita, Jpn. J. Appl. Phys. 25 (1986) 159. [10] G.C. Hua, R. Oshima and F.E. Fujita, J. Mater. Sci. 25 (1990) 328. [11] B.C. Svensson, J. Svensson, J. Lindstrom, G. Davies and J.W. Corbett, Appl. Phys. Lett. 51 (1987) 2257. [12] P. Mascher, S. Dannefaer and D. Kerr, Mater. Sci. Forum 38-41 (1989) 1157. [13] G.D. Watkins and J.W. Corbett, Phys. Rev. A 138 (1965) 543. [14] Y.H. Lee and J.W. Corbett, Phys. Rev. B 9 (1974) 4351. [15] W. Jung and G.S. Newell, Phys. Rev. 132 (1963) 648. [16] K.L. Brower, Rad. Eft. 8 (1971) 213. [17] Y.H. Lee, Y.M. Kim and J.W. Corbett, Rad. Eft. 15 (1972) 77. [18] Y.H. Lee and J.W. Corbett, Phys. Rev. 8 (1973) 2810. [19] T.Y. Tan, H. F611 and S.M. Hu, Phil. Mag. A 44 (1981) 127. [20] A. Seeger and U. G6sele, Rad. Eft. 82 (1984) 19. [21] U. G6sele and A. Seeger, Rad. Eft. 82 (1984) 35. [22] T. Ezawa, PhD Thesis, Osaka Univ. (1984).
ro
N o r , H toiiand
c'opy
Characterization of Czochralski Si crystals by HVEM R. O s h i m a a n d G . C , H u a 1 D~7~artment of Mawrial Physics, Faculty ~1"Engineering Seiem:e, Osaka Uniret:~io', Toyonaka, Osaka 500, JalmH
Received 26 March 1991
Based on in-situ obse~'ations of electron-irradiation-induced sccondaw defccts of wlrious Czochralski-grown silicon crystals (Cz-Si) above room temperature with an ultrahighwoltagc electron microscope, a new tool fl)r characterization of Cz-Si is proposed. It has been shown that the nucleation of the defects is associated with clusters of oxygen, which is an unavoidable impurity of Cz-Si. and also other impurities impinged from the electron beam incident surface. Characteristic features of sequences of the defect formation, growth and shrinkage wilh irradiation are also discussed on lhe basis of the reaction Fate theo~.
1. Introduction W i t h r e c e n t r e m a r k a b l e d e v e l o p m e n t s of large-scale i n t e g r a t e d circuits (LSI) and charge conversion devices ( C C D ) in s e m i c o n d u c t o r industries, studies of the b e h a v i o r of oxygen, which is an u n a v o i d a b l e impurity of g e n e r a l l y used C z o c h r a l s k i - g r o w n silicon crystals (Cz-Si), become increasingly i m p o r t a n t . In spite of the g r e a t n u m b e r of works r e g a r d i n g the states of s u p e r s a t u r a t e d oxygen a t o m s in the crystals so far, t h e i r u n d e r s t a n d i n g has b e e n yet insufficient. W h i l e the oxygen a t o m s i m p r o v e the m e c h a n i c a l s t r e n g t h of the wafers for inhibiting the slip by the m a c h i n i n g of the d r a w n single crystals, they are r e s p o n s i b l e for a variety of defect f o r m a t i o n s d u r i n g the t h e r m a l p r o c e s s e s of the device production. T h e c h a n g e in c o n c e n t r a t i o n of interstitial solute oxygen a t o m s of Si crystals has b e e n e x a m i n e d c o m m o n l y by infrared spectroscopy, but d e t a i l e d i n f o r m a t i o n on oxygen clusters is not easy to acquire by this m e t h o d . High-voltage e l e c t r o n microscopy ( H V E M ) has b e e n d e v e l o p e d as a powerful tool to observe thicker s p e c i m e n s exhibiting the bulk p r o p e r t i e s i Present address: Department of Physics, College of General Education, Osaka University, Toyonaka, Osaka 560, Japan.
of materials. On the o t h e r hand, studies of radiation d a m a g e of metals and alloys utilizing the higher densities of the h i g h - e n e r g y illuminating e l e c t r o n s than the c o n v e n t i o n a l e l e c t r o n a c c e l e r a tors have b e e n p e r f o r m e d extensively, and quantitative information on point defects has been derived It]. Several H V E M investigations of Si havc bccn also u n d e r t a k e n , and it is found that the e l e c t r o n - i r r a d i a t i o n - i n d u c e d s e c o n d a r y defects are interstitial-type dislocation loops with stacking faults on {113} p l a n e s [2-5]. Prcvious H V E M works on silicon by the p r e s e n t authors" g r o u p have also shown that the f o r m a t i o n of i r r a d i a t i o n i n d u c e d s e c o n d a r y defects is different b e t w e e n floating zone (Fz) and Cz silicon [6-1(I]. Differing from the i r r a d i a t i o n of Fz-Si in which the loops are f o r m e d after a few rain of incubation, it is c h a r a c t c r i s t i c that the loops of Cz-Si b e c o m c visible almost as soon as the start (11 irradiation. T h c spatial distributions of the defects were also differcnt b e t w e e n Fz-Si and Cz-Si. in contrast to Fz-Si, in which the loops are only o b s c r v c d to form in a thin layer n e a r thc electron b e a m incident surfacc, those of Cz-Si arc f o r m c d hom o g e n e o u s l y insidc the s p e c i m e n except for thin d e n t i d e d zone layers n e a r the surfaces. Only the defects r c l a t e d to i m p i n g e d impurity a t o m s from
0304-3991/91/S03.511 <> 19tJl Elsevier Science Publishers B.V. All rights reserved
R. Oshima, G.C. Hua / Characterization of Cz-Si by HVEM
the electron incident surface are observed at temperatures between room temperature and 400 ° C in the Fz-Si, while the attribution of oxygen atoms to the nucleation of loops is suggested in the bulk of Cz-Si. Since the previous experimental results have exhibited that the nucleation of the irradiation-induced secondary defects is dependent upon the species and states of impurity atoms in silicon, in situ high-voltage electron microscopy is expected to become one of the candidates for a tool for characterization of Czochralski-grown silicon crystals. The present work is aimed to look for a new technique for characterization of Cz-Si wafers based on the in situ observation results of the irradiation-induced lattice defects of various Si wafers in H V E M .
2. Experiment
The specimens used were prepared mostly from a Cz-Si crystal with a (001) or (111) orientation containing interstitial oxygen atoms of the order of 10LS/cm 3 and substitutional carbon atoms of about 10aS/cm 3. In order to dissociate oxygen clusters previously formed during the crystal growth processes and to disperse the oxygen atoms in the matrix, they were subjected to a solution heat treatment at 1300 ° C for 16 h, followed by rapid cooling to room temperature. The concentration of interstitial oxygen atoms after the heat treatment was determined by the 9 txm absorption band of infrared spectroscopy using an Analect RFX-65 Fourier transform infrared spectrometer. Wafers subjected to a simulation heat treatment of intrinsic gettering were also prepared. Other wafers whose surfaces were chemically contaminated with Fe or Ni were made to examine the effect of metal impurities on the nucleation of the defects. Three kinds of wafers of Cz-Si crystals drawn in magnetic fields (MCzSi) to control the growth conditions more stably were also used; their oxygen concentrations ranged from 1017 and to 1018/cm 3. It is noteworthy that such a wide range of control of the oxygen content is attained only by the MCz-Si. They were formed into disks of 3 mm diameter
161
and chemically thinned in a mixture of hydrofluoric acid and nitric acid for transmission electron microscopy. In situ observations using an ultrahigh-voltage electron microscope, HU-3000, at Osaka University have been carried out in a {220} dynamical condition at temperatures between 200 and 4 5 0 ° C at 2 MeV. The doses used were between 10 ~ and 1020 e / c m 2 s. The irradiated specimens were further investigated by a JEM200CX electron microscope operated at 160 kV.
3. Results and discussion
3.1. Effect of pre-heat treatments on the nucleation of secondary defects 3.1.1. Effect of heat treatment at 1300 ° C A Cz specimen with (001) orientation subjected to a heat treatment at 1300 °C for 16 h to decompose oxygen clusters formed during the crystal growth was irradiated at 340 ° C with an electron dose of 1.2 × 1020 e / c m 2 s, and compared with the as-grown crystal. After the annealing the interstitial oxygen concentration increased from 1.2 × 10 is to 1.3 × 1018/cm 3, indicating that decomposition of oxygen clusters actually took place. The in situ observation of the annealed specimen showed that the secondary defects as well as the as-grown one were observed soon after the irradiation started. However, the loop density decreased remarkably by the heat treatment at 1300 ° C as seen in fig. 1, suggesting that not the oxygen clusters but a single oxygen atom would be involved in the defect nucleation. On continuing the irradiation, the secondary defects started to shrink and finally annihilated in the central area of the irradiation where the beam intensity was the strongest. 3.1.2. Effect of gettering heat treatment Commercial Cz-Si wafers are generally subjected to heat treatments for gettering of harmful impurities in the bulk and to keep the circuit construction surfaces free from the defect formation during the thermal processes, which is called an intrinsic gettering. As a simulation experiment wafers are annealed at 1150 ° C for 4 h and then
162
R. Oshima, G.C. ttua / Characterization 01"(':-Si by HVEM
at 800 ° C for 100 h. For comparison wafers which received a thermal donor killer treatment at 650 °C for 30 min were also investigated. Figs. 2 and 3 represent respectively results of in situ observations at 300 °C of the surface and the bulk of a wafer. Here, the "surface" and the "bulk" specimens were respectively prepared from the same wafers with the gettering heat treatments to leave only the surface region and the bulk by chemical polishing. It is very clear that the loops do not appear until 109 s of irradiation (fig. 2b) for the surface, while they are already visible by 25 s of irradiation (fig. 3a). The stereoscopy showed that the loops observed in the surface of the intrinsic gettering treatment were formed only near the beam incident surface as in the case of Fz-Si, suggesting that the defect formation is associated with an impurity impinging effect from the surface by the incident beam.
Taking into account the responsibility of oxygen clusters for the defect nucleation, the results indicate that the concentrations of oxygen clusters are much different between the surface area and the bulk. It is also seen in fig. 3 that some defects grow and some disappear with the continuation of irradiation in the central area; other defects formed near the beam incident surface are visible as small dot contrasts in (fig. 3d). Fig. 4 shows the change in the defect density of the bulk of the irradiated specimens of thickness 800 nm subjected to the gettering heat treatment. The defect density of 1014/cm 3 of the specimen of the gettering heat treatment is one order of magnitude larger than that of the as-grown one. Here, it must be noticed that one sixth of {113} defects are invisible in the (022) dark field micrographs because of the extinction rule of gb = 0. The decrease in the interstitial oxygen atoms with the
Fig. 1. D a r k field e l e c t r o n m i c r o g r a p h s s h o w i n g c h a n g e in the d e n s i t y o f e l e c t r o n - i r r a d i a t i o n - i n d u c e d d i s l o c a t i o n loops o f Cz-Si with o x y g e n - d i s p e r s e d t r e a t m e n t : (a) a s - g r o w n Cz-Si a f t e r 152 s i r r a d i a t i o n , (b) s o l u t i o n - t r e a t e d Cz-Si at 13(/0 ° C for 16 h a f t e r 223 s i r r a d i a t i o n . I r r a d i a t i o n s w e r e d o n e at 2 M V with a w e a k w i d e e l e c t r o n flux of 2 x 1(I 1'~ e / c m 2 s at 340 o C.
R. Oshima, G.C. Hua / Characterization of Cz-Si by HVEM
h e a t t r e a t m e n t was 0.15 × 1018/cm 3. F o r the m o ment, the effect of the d o n o r killer t r e a t m e n t on t h e d e f e c t f o r m a t i o n is not clear. 3.2. M C z - S i
T h r e e kinds of M C z wafers with d i f f e r e n t oxygen c o n c e n t r a t i o n s were e x a m i n e d . In the specim e n with the lowest oxygen c o n t e n t o f 2 x 1017/cm3, which is not a t t a i n e d by the conventional Cz m e t h o d , the s e c o n d a r y defects w e r e f o r m e d only a f t e r several m i n u t e s i n c u b a t i o n periods n e a r the e l e c t r o n b e a m i n c i d e n t surface by the i r r a d i a t i o n at 3 0 0 ° C , as shown in fig. 5, w h e r e no d e f e c t s a r e visible yet until an e l a p s e d time of 114 s of (a). T h e b e h a v i o r was very similar to that of Fz-Si. O n the o t h e r hand, the f o r m a tion of s e c o n d a r y defects in the s p e c i m e n with a
163
high oxygen c o n t e n t of 2.2 × 10~S/cm 3 was not d i f f e r e n t from that o f n o r m a l Cz-Si. No s o o n e r did the i r r a d i a t i o n start t h a n the s e c o n d a r y defects b e c a m e visible in the bulk of the s p e c i m e n . A s the i r r a d i a t i o n p r o c e e d e d , they s t a r t e d to shrink a n d s o m e of t h e m finally d i s a p p e a r e d before new d e f e c t s w e r e f o r m e d n e a r the b e a m i n c i d e n t surface, the p r o c e s s of which is shown in fig. 6. It was f o u n d that in the s p e c i m e n with a m e d i u m oxygen c o n t e n t o f 7 x 10~7/cm 3 no seco n d a r y d e f e c t s which a p p e a r e d w i t h o u t incubation p e r i o d s were f o r m e d in the c e n t r a l a r e a of the s t r o n g e s t b e a m intensity of the s p e c i m e n , while they were o b s e r v e d in the p e r i p h e r y o f the o b s e r v a t i o n area. Fig. 7 shows a (022) d a r k field m i c r o g r a p h of the s p e c i m e n t a k e n by 160 k e V T E M after the H V E M observation. In this case only the u p p e r - r i g h t a r e a was i r r a d i a t e d with a
Fig. 2. Sequence for irradiation of the surface of Cz-Si wafer subjected to intrinsic gettering heat treatment at 1150 ° C for 4 h and then at 800 o C for 100 h. Irradiation was done at 2 MV with a dose of 1 x 102o e/cm 2 s at 300 o C. The irradiation elapsed times are (a) 44, (b) 109, (c) 142, (d) 200, (e) 248 and (f) 301 s.
164
R. Oshima, G.C. Hua / Characterization Of"Cz-Si by HVEM
condensed beam in the H V E M , where the defects were formed near the beam incident surface after an incubation period. The present results also clearly indicate that the nucleation of the electron-irradiation-induced secondary defects is associated with the oxygen of the specimen.
3.3. Effect of impurity on the nucleation of the secondary defects Since previous results suggest that the nucleation of the secondary defects should be promoted also by impurities, the effects of contamination of the surface with heavy metals, such as Fe and Ni, were examined. In fig. 8 is shown an example of the case of contamination with Fe of 1.7 x 10~4/cm 3. The specimen was annealed at 1300 o C and quenched before the irradiation. The
loops already appear at 11 s after starting the irradiation, and the number is larger than in the uncontaminated specimen shown in fig. lb, though the present irradiation temperature was higher, Examples of the surface contamination with Ni are presented in fig. 9. It is clear that the number of loops is slightly larger in the specimen which is more contaminated. Accordingly, heavy metal impurities are also responsible for the nucleation of the irradiation-induced secondary defects. However, further systematic experiments seem to be necessary to obtain the nature of nuclei and quantitative results.
3.4. Consideration of the results" of in situ obsercation based on the reaction rate theory From the in situ H V E M observations of electron-irradiation-induced secondary defects of Cz-
Fig. 3. Sequence for irradiation of the bulk of Cz-Si wafer subjected to intrinsic gettering heat treatment at 1150 ° C for 4 h and then at 800 ° C for 100 h. The irradiation conditions were the same as in fig, 2. The irradiation elapsed times are (a) 25, (b) 76, (c) 128, (d) 193, (e) 222 and (D 300 s.
R. Oshima, G.C. Hua / Characterization ()[ Cz-Si by HVEM
Si with interstitial-type nature it has been shown that they are immediately formed uniformly in the bulk except near the surface layers as soon as the irradiation (that is, the observation) starts and begin to shrink after a certain growth period. In the special case of strong irradiation of more than 2 × 10 20 e / c m 2 s, the defects sometimes appear no longer. The behavior also depends on the concentrations and the states of oxygen in the specimen. In order to consider the behavior of the secondary defects with irradiation, computer simulation was carried out based on the reaction rate theory. Since the interstitial-type loops start to shrink after some growth period, it is indicated that the concentrations of vacancies and interstitials in the specimen do not reach a steady state even several minutes after the beginning of the irradia-
165
tion. This can be explained if some vacancy-type clusters are formed gradually and act as sinks of interstitials with the irradiation. Divacancies will be the major defects in high-energy particle irradiations of Si at room temperature [11-12]. Other larger vacancy clusters, such as trivacancies, tetravacancies and pentavacancies, have also been identified after electron and neutron irradiation [13-18]. The pentavacancies, for example, were found to be stable up to 450 ° C. Accordingly, it is reasonable to conclude that such larger vacancy clusters are also formed during strong electron irradiation as in the present case. As a result of the formation of the vacancy clusters, the number of sinks for interstitials is increased to reduce the concentration of the interstitials. Consequently, the loops formed in the beginning absorb more vacancies than interstitials, and they will shrink. The decrease in the interstitial concentration,
®
.
Fig. 4. (022) dark field electron micrographs showing change in the density of electron-irradiation-induced dislocation loops of Cz-Si wafers with intrinsic gettering heat treatment. (a) As-grown Cz-Si and (b) at 1150°C for 4 h and then at 8 0 0 ° C for 100 h. The irradiation conditions were: at 2 MeV with a wide weak dose of 2 × 10 i9 e / c m 2 s at 300 ° C.
166
R. Oshima, G.C. Hua / Characterization of" Cz-Si by H V E M
and the retardation of reaching the steady state due to the formation of vacancy clusters, may be demonstrated by computer simulation using reaction rate equations. A simplification is at first introduced by assuming that only divacancies are formed as multi-vacancy clusters during the irradiation. Also assumed is a simple case in which only interstitial dislocation loops are formed as secondary defects, and oxygen clusters which act as the nucleation sites of the loops already exist in the specimen before irradiation with a volume density of C L, and the loops grow at such latent nucleation sites. The equations describing the reactions between point defects and the loops are
written as follows: dCi/dt
4rrriv (Di +
= p-
Dv)CiC
v
4(,rrACL)'/Z(CiL)'/2DikiCi
- - D i C s f C i - 4,rrriv2( D i + D v 2 ) C i C v 2 ,
(l) dCv/dt
= p -
4,rr riv ( D i +
- 4(
rrAC L)
_ DvC~fC v _
Dv)CiC
v
t/2(CiL ) I / 2 D , . k v C v 8rr rvv( 2 D v ) ( Cv ) 2
+ 4"rrriv2( D i + D v 2 ) C i C v 2 +
2kd~Cv2,
(2)
Fig. 5. Sequence for irradiation of MCz-Si with interstitial oxygen content of 2.0 × 1017/cm 3. Irradiation conditions were: at 2 MV with a dose of 1 × 1020 e / c m 2 s at 300 ° C. The thickness of the wedge-shaped specimen was about 800 nm at the center of the irradiation area. The irradiation elapsed times were (a) 114, (b) 217, (c) 251 and (d) 317 s.
R. Oshima, G.C. Hua / Characterization of Cz-Si by H V E M
167
Fig. 6. Sequence for irradiation of MCz-Si with interstitial oxygen content of 2.2 × 10~S/cm 3. Irradiation conditions were the same as in fig. 5. The irradiation elapsed times were (a) 5, (b) 18, (c) 107, (d) 203, (e) 258 and (f) 304 s.
dCvz/dt = 4wG.,( 2Dv)( Cv) 2 - 4 vr riv 2 ( D i + Ov2 ) C iCv2
- O v z C s f C v 2 - kdsCv2 ,
dCiL/dt =
4(
ACL)1/2(G
)t/2
X ( DikiC i - DvkvCv), R L = (ACiL/'ITCL)
1/2.
(3)
(4) (5)
In these equations, Ci, C v, Cv2 and Cic are respectively the numbers per unit volume of interstitials, vacancies, divacancies and atoms absorbed in the loops. R c is the loop radius. P is the production rate of interstitials and vacancies in defect pairs per unit volume per unit time, which can be calculated using a displacement cross section of 58 b for 2 MeV electrons and irradiation electron doses, and in the present case
the order of 10 . 2 and 10 . 3 depending on the beam intensity. The second terms in eqs. (1) and (2) describe the recombination of interstitials and vacancies. The diffusivities of interstitials D~, vacancies Dv and divacancies Dr2 are calculated from their migration energies, respectively, riv means the effective radius of the vacancy-interstitial recombination volume, and other terms r in the equations indicate the radii of corresponding recombination reactions indicated by the subscripts. The third terms of eqs. (1) and (2) denote the absorption of interstitials and vacancies by the loops. A is the reciprocal of the area density on the loop plane of the atoms precipitated in the loops, calculated using a {113} defect model of Tan [19]. k i and k v denote the absorption efficiency of interstitials and vacancies by the loops, which depend upon both the geometry and the
168
R. Osh#na. G.C. Hua / Characterization of Cz-Si by ttVEM
a r r a n g e m e n t of the loops. A l t h o u g h their exact solutions are not easy to obtain, in a few simple cases they have b e e n d e r i v e d [20-21]. T h e third t e r m s of eqs. (1) a n d (2) d e s c r i b e respectively the a n n i h i l a t i o n of interstitials and vacancies at the s p e c i m e n surface, w h e r e C~f is the a t o m i c fraction of the sink sites of the surface, a s s u m e d to be identical for the a b s o r p t i o n of interstitials a n d vacancies, a n d a p p r o x i m a t e l y t a k e n as C s f = 5 ( 1 / H ) 2 for s p e c i m e n thickness H [22]. T h e o t h e r t e r m s d e s c r i b e the i n t e r a c t i o n s of defect species with divacancies, kas is a dissociation coefficient of a divacancy. In the calculations, for simplicity, it has b e e n a s s u m e d h e r e that if one c o n s t i t u e n t vacancy o f a divacancy j u m p s one lattice d i s t a n c e to a s e c o n d n e a r e s t n e i g h b o r of the o t h e r constituent vacancy, the i n t e r a c t i o n b e t w e e n the two vacancies b e c o m e s zero; the activation energy for this j u m p has b e e n a s s u m e d to be the b i n d i n g e n e r g y of the divacancy r e p o r t e d , 1.6 eV. T h e density o f the oxygen clusters which act as the n u c l e a t i o n sites of the loops has b e e n a s s u m e d to be 1013/cm 3, which is e s t i m a t e d from the density of the o b s e r v e d loops. T h e r e a c t i o n radii have
b e e n a s s u m e d to be: riv dv,, = 2r~ and riv2 3r~, w h e r e r 0 is the closest d i s t a n c e b e t w e e n two silicon a t o m s of 0.235 nm. A l t h o u g h the p r e s e n t s i m u l a t i o n was u n a b l e to r e p r o d u c e c o m p l e t e l y the e x p e r i m e n t a l results, a s u g g e s t e d result is given in fig. 10, w h e r e an e x t r e m e and r a t h e r u n r e a l i s t i c case that vacancies are m o r e a b s o r b e d at the loops than interstitials is tentatively a s s u m e d in o r d e r to save the comp u t a t i o n time a n d also to e x a g g e r a t e the effect of increasing vacancy clusters resulting in the relative r e d u c t i o n of sink efficiency of the f o r m e d loops for interstitials. In this calculation, not only divacancies but also A c e n t e r s ( v a c a n c y - o x y g e n pair) are t a k e n into account. It is shown that the loops grow in the beginning, t h e n start to shrink and a n n i h i l a t e finally, t h o u g h the time scale much d e v i a t e s from the actual one. F o r the i n t e r p r e t a tion of the e x p e r i m e n t a l results quantitatively, c o m p u t e r simulation taking into a c c o u n t all the possible r e a c t i o n s which t a k e p l a c e actually during the i r r a d i a t i o n is necessary. By c o m p a r i n g such s i m u l a t i o n with e x p e r i m e n t a l findings, a t h o r o u g h u n d e r s t a n d i n g of the b e h a v i o r of p o i n t =
=
Fig. 7. (022) dark field electron micrograph showing defect formation of MCz-Si with interstitial oxygen content of 7× l()~7/cm~. The HVEM irradiation was done at the upper-right area at 2 MV with a dose of 1 × 10z° e/cm ~ s at 300°C for 306 s, where the defects were formed near the beam incident surface. The defects around the irradiated area are present in the bulk.
R. Oshima, G.C. Hua / Characterization of Cz-Si by HVEM
169
......
.
....
2 50mn
.......
Fig. 8. S e q u e n c e for i r r a d i a t i o n of Cz-Si w a f e r i n t e n t i o n a l l y c o n t a m i n a t e d with 1.7× 1014 F e / c m 3. The i r r a d i a t i o n c o n d i t i o n s were: at 2 M V with a dose of 2.0 x 102o e / c m 2 s at 400 ° C. The i r r a d i a t i o n e l a p s e d times were (a) 11, (b) 75, (c) 163 and (d) 352 s.
Fig. 9. Effect of Ni c o n t a m i n a t i o n of Cz-Si w a f e r on the f o r m a t i o n of e l e c t r o n - i r r a d i a t e d - i n d u c e d defects. The i r r a d i a t i o n c o n d i t i o n s were: at 2 M V with a dose of 2 × 102o e / c m 2 s at 200 ° C. (a) W i t h 1.3 × 1013 N i / c m 3 after 100 s i r r a d i a t i o n and (b) with 1.1 x 1015 N i / c m 3 after 89 s irradiation.
170
R. Oshirna, G.C. Hua / Characterization of Cz-Si by HVEM 104
i0 le
A center
E
10 '4
/ ~
io 2
ioIO
i0 ~ o
IOS
p
~o2
LOo -J
ing specimen materials. We are also grateful to Emeritus Prof. F.E. Fujita for stimulating and valuable discussions. We are very obliged to Emeritus Prof. H. Fujita, Prof. H. Mori, Drs. K. Yoshida, M. Komatsu and T. Sakata of the Research Center for Ultra-High-Voltage Electron Microscopy of Osaka University for their kind cooperation in the course of this study. Also, the helpful discussions with Dr. T. Ezawa and the assistance of Mr. T. Kawano are appreciated.
}oi0-16
/
/
i0-I
/ ,oe
J
Irradiation
,;time/s
Fig. 10. C o m p u t e r simulation of the v a r i a t i o n of the concentration of defects and the size of loops with 2 MeV electron
irradiation. Parameters used are: an electron dose 2 × 102° e / c m z s, temperature 300 ° C, specimen thickness 500 nm, migration energies of a single vacancy 0.3 eV, a divacancy 1.3 eV, and an interstitial 0.02 eV, binding energy of a divacancy 1.6 e g , k i = 0.5 and k v = 2.5.
defects and their interactions with impurities in silicon will be obtained.
4. Conclusion The in situ ultrahigh-voltage electron mi= croscopy (UHVEM) of various silicon wafers has exhibited characteristic features of the irradiation-induced secondary defects which are very dependent upon the species and states of impurity atoms of the crystals. Accordingly, the U H V E M technique is promising as a tool for characterization of Cz-Si subjected to various treatments, which is important in preparing defect-free silicon wafers for semiconductor industries.
Acknowledgements The authors would like to thank Kyusyu Electric Metals Co. and Sony Corporation for provid-
References [1] M. Kiritani, in: Point Defects, Eds. M. Doyama and S. Yoshida (Univ. Tokyo Press, Tokyo, 1977) p. 247. [2] M.D. Matthews and S.T. Ashby, Phil. Mag. 27 (1973) 1313. [3] H. F611, Inst. Phys. Conf. Set. 23 (1975) 233. [4] I.G. Salisbury and M.H. Loretto, Phil. Mag. 39 (1979) 317. [5] S. Furuno, K. lzui and H. Otsu, Jpn. J. Appl. Phys. 18 (1979) 203. [6] H. Asahi, R. Oshima and F.E. Fujita, Inst. Phys. Conf. Set. 59 (1981) 455. [7] R. Oshima, S. Sadamitsu and F.E. Fujita, Physica B 116 (1983) 606. [8] R. Oshima, M. Hasebe, G.C. H u a and F.E. Fujita. in: Proc. Int. Symp. on ln-situ Experiments with HVEM, Ed. H. Fujta (Osaka Univ., Osaka 1985) p. 207. [9] M. Hasebe, R. Oshima and F.E. Fujita, Jpn. J. Appl. Phys. 25 (1986) 159. [10] G.C. Hua, R. Oshima and F.E. Fujita, J. Mater. Sci. 25 (1990) 328. [11] B.C. Svensson, J. Svensson, J. Lindstrom, G. Davies and J.W. Corbett, Appl. Phys. Lett. 51 (1987) 2257. [12] P. Mascher, S. Dannefaer and D. Kerr, Mater. Sci. Forum 38-41 (1989) 1157. [13] G.D. Watkins and J.W. Corbett, Phys. Rev. A 138 (1965) 543. [14] Y.H. Lee and J.W. Corbett, Phys. Rev. B 9 (1974) 4351. [15] W. Jung and G.S. Newell, Phys. Rev. 132 (1963) 648. [16] K.L. Brower, Rad. Eft. 8 (1971) 213. [17] Y.H. Lee, Y.M. Kim and J.W. Corbett, Rad. Eft. 15 (1972) 77. [18] Y.H. Lee and J.W. Corbett, Phys. Rev. 8 (1973) 2810. [19] T.Y. Tan, H. F611 and S.M. Hu, Phil. Mag. A 44 (1981) 127. [20] A. Seeger and U. G6sele, Rad. Eft. 82 (1984) 19. [21] U. G6sele and A. Seeger, Rad. Eft. 82 (1984) 35. [22] T. Ezawa, PhD Thesis, Osaka Univ. (1984).