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Investigations on the correlation between growth rate and gate oxide integrity of Czochralski-grown silicon
CRYSTAL GROWTH ELSEVIER
Journal of Crystal Growth 139 (1994) 37—46
Investigations on the correlation between growth rate and gate oxide integrity of Czochralski-grown silicon D. Zemke
a
P. Gerlach b, W. Zulehner
b
K. Jacobs
*,C
Kristallabor, Institut für Werkstoffivissenschaften (LS6), Universität Erlangen—Nürnberg, Martensstrasse ? D-91058 Erlangen, Germany b Central Research and Development, Wacker Chemitronic GmbH, D-84479 Burghausen, Germany Institut für Kristallographie und Materialforschung, Fachbereich Physik, Humboldt- Universitàt zu Berlin, invalidenstrasse 110, D-10115 Berlin, Germany a
(Received 19 August 1993; manuscript received in final form 17 December 1993)
Abstract Czochralski-grown silicon crystals show two concentric regions providing gate oxide layers of markedly different breakdown stability. The two regions are separated by a “wreath”, several millimeters in width, that is built-up from microdefects which act as nucleation centres for the formation of oxidation induced stacking faults. The core region of inferior gate oxide integrity (GOl) becomes smaller in diameter with decreasing pulling speed. The three regions have been extensively characterized by numerous analytical techniques. The most striking difference between the inner and outer regions, besides the GUI, is the enhanced oxygen precipitation in the core region occurring after annealing. An attempt is undertaken to analyze the defect structure in the different regions, its origin, correlation with growth and annealing processes, and its effect on the GOT.
1. Introduction Metal—oxide—semiconductor (MOS) silicon integrated circuit technology requires stable gate oxide layers, capable of withstanding high voltages. Gate oxide integrity (GOT) is one of the most important issues in current MUS technol-
ogy. With the increasing complexity of circuits, the total gate oxide area increases. Furthermore, there is a trend towards thinner gate oxide layers (down to about 50 A) and last, but not least, oxide layers grown on the walls of trenches are
*
Corresponding author,
0022-0248/94/$07.OO © 1994 Elsevier Science B.V. All rights SSDI 0022-0248(93)E0589-Y
becoming prevalent as standard components of
highly integrated circuits. The GUI depends on the quality of the oxide layer, the Si/Si02 interface, and also on the quality of the Si crystal itself. Details of the measurement and current understanding of the GUI are summarized in ref. [1]. Assuming high reproducibility of Si02 layer preparation, the defect structure of the Si crystal is then the deciding factor for the GUI yield. Studies on the effect of crystal pulling rate on the formation of “crystal-originated particles” and defects have recently been published by Ryuta et al. [2]. Considerable variation can be found between float-zone (FZ) and Czochralski (CZ) grown mareserved
38
D. Zemke ci vi. /Journal of Crystal (,rovii, / 19 (/994) 37—4~
terial, the FZ crystals showing better properties, but also between CZ materials from different vendors. A detailed understanding of the effect of intrinsic point defects on the GUI is as yet lacking. The GUI yield is greatly influenced by the growth rate of CZ crystals. A remarkable improvement of the GUI yield is achieved when the pulling speed is reduced from the common 0.8—1.8 mm/mm to values below about 0.6 mm / mm. In this paper experimental results on the relationship between Czochralski Si crystal growth parameters and GUI are presented. Results will be reported characterizing the nature, density and distribution of defects in CZ Si crystals, the influence of the growth rate and the correlation with the GUI.
Fig. I. Surface of a CZ Si wafer after wet oxidation and staining. Fhrve regions ovcurnng on crystals pulled with intermediate velocity can he clearly distinguished.
2. Influence of growth rate on axial and radial defect distribution in CZ Si crystals For the investigations here to be presented, a 6 inch CZ Si crystal has been grown in [0011 direction with intentionally varied pulling speed. The crystal is boron-doped and has initially an interstitial oxygen concentration between 6.2>< i0’~and 7.2 x 10t7 cm3. The growth rate was systematically lowered from 0.8 to 0.4 mm/mm and then again gradually increased to the initial value. To reveal the areas of different intrinsic point defect concentrations and agglomerations, both of which alter the precipitation of oxygen, oxygen was precipitated by annealing for 3 h at 780°C,followed by a second annealing for 16 h at 1000°C. The oxygen precipitates and their secondary defects were stained by the so-called Secco etchant [31,consisting of K 2Cr2U7 and HF. Fig. 1 is a photograph of the wafer surface after wet oxidation (2 h at 1100°Cin wet air) and staining. Three concentric regions can be clearly distinguished. These regions behave very differently in a variety of properties. Most important in the context of the present work is the variation in GUI. The outer circular ring provides material of high breakdown stability, while the core region is of inferior quality. These two regions are separated by a wreath of microdefects which cause
oxidation-induced stacking faults (USFs). Its width amounts to a few millimetres. The three regions have been extensively characterized by a variety of methods. The results will be presented in section 4. The diameter of the core region (and therefore also the quality of the wafer) depends on the pulling speed. Fig. 2 shows a longitudinal cross section through the crystal after oxygen precipitation and etching. The varying pulling speed is reflected in the changing diameter of the mi-
_______ ______
__________________________________________
_________
Fig. 2. Longitudinal cross section through a crystal grown with varying pulling speed after oxygcn precipitation and staining.
D. Zemke et al. /Journal of Crystal Growth 139 (1994) 37—46
48 vol% HF solution. The wafers were etched at room temperature for times between 30 s and 30 mm. A topography of etch pit density and dimen-
need
pulling speed in mm/mm
cone
2 1.6 1.2 0.8 0.4
/N
.
sions is obtained by means of a highly automated light-scattering technique differentiating between
0
E inner reg~on
0
hovde,
~ I
“s
\
8..-
onterregon
~I 00cr,
I
39
12
a
09000
Fig. 3. Correlation between pulling speed and annular diameters for the crystal shown in Fig. 2.
crodefect wreath. Fig. 3 is a sketch of the correlalion between pulling speed and extension of the different crystal regions. Decreasing the growth rate leads to a decrease of the core region area with inferior GUI. For 6 inch crystals, the wreath diameter eventually reaches the outer crystal diameter when the pulling speed exceeds 1.2 mm/mm, while it vanishes in the centre of the crystal when the pulling speed is lower than 0.5 mm/mm (not shown in the figure). Thus, an optimum between pulling speed and GUI yield has to be found, depending on the customer’s needs. An attempt to explain the observed phenomenon requires at first a more detailed investigation of the physical properties of the distinct crystal regions and characterization of the crystal properties on a microscopic or atomic scale.
3. Analytical techniques applied In order to obtain more detailed information on the nature and distribution of defects in the distinct wafer regions, numerous analytical techniques have been applied. These techniques will be briefly described here. The wafers were preferentially etched using the so-called Secco etchant [3].Etching solutions have been prepared by dissolution of 44 g K 2Cr2U7 in 100 ml H2U and finally mixing with 200 ml of a
different classes of etch pits. Another method for the decoration of microdefects was to convert the wreath microdefects into oxidation-induced stacking faults by a wet oxidation at 1100°C for 2 h and to make visible the USFs Seccoleads etching. The growing oxide layer on thebysurface to the generation of excess interstitial Si atoms which precipitate at the wreath microdefects. The strain generated by the formation of interstitials is reduced by the formation of large stacking faults. The USFs are precipitates of silicon self-interstitials. Diverse analytical techniques have been applied without and with annealing. A precipitation test comprising a two-step annealing was applied to reveal the precipitation behaviour of the material. In the first step at 780°C, small oxygen precipitates are formed by homogeneous nudeation. In the second step at 1000°C,the oxygen nuclei grow to large Si02 particles. The annealing times at 780 and at 1000°Cmay be adjusted to the oxygen concentration if stronger or weaker precipitation is desired. In the present investigation, the annealing time at 780°Cwas always 3 h, whereas at 1000°Cannealing times of 16 h and 32 h were applied. The precipitation induced microdefects are made visible by Secco etching. The concentrations of oxygen, nitrogen and carhon were determined by means of Fourier transform infra-red spectroscopy. The spatial resolution is 0.5 mm. At room temperature the oxygen concentration can be determined in the (4—10) x 1017 cm~ concentration range with an levels accuracy 3. The detectable are of ± 0.25cm3 X i0’~cm 5>< 1015 for carbon and 8 x 1014 cm3 for nitrogen. The minority carrier diffusion length was determined from either photocurrent measurements, illuminating the backside of the wafer [4], or from measurements of the photovoltage excited under surface illumination [5]. The resistance was measured by means of a four-point DC probe [6].
40
D. Zemke ci al. /Journal of Crystal Growth 139 (1994) 37—46
The samples were also characterized by transmission electron microscopy [7], applying an acceleration voltage of I MeV. Areas of 1 mm2 could be inspected in different operating modes. The spatial resolution was about 1 nm. The positron annihilation technique was applied for the detection of vacancies [8]. 22Na was the positron source. The mean positron lifetime was typically around 220 ps. This value agrees exactly with data found in the literature for highly perfect Si [9]. The sensitivity limit of the method is about 2.5 X 1015 cm3.
4. Characterization of defects in different wafer regions The three distinct regions which can be observed on each wafer have been extensively characterized by the techniques described in section 3. All the test wafers have been etch-polished in order to remove surface damage and impurities, and have undergone a rapid thermal annealing procedure in order to kill shallow donors [10]. Fig. 4 shows the concentration of interstitial oxygen along various traces in the as-grown state and after different annealing procedures. In the cylindrical part of the as-grown crystal the interstitial oxygen is homogeneously distributed. Generally, it is lowered by annealing. The reduction is
8
_____________
_________
—
most pronounced in the core region (trace 1). The decrease of the interstitial oxygen concentration is due to the formation of precipitates. This is obviously most easily accomplished in the central region. The reason has yet to be discussed. Another characteristic difference between the distinct regions is their behaviour against the Secco etchant. In a solution at rest after 30 mm of etching time well-developed flow patterns are observed in the core region (Fig. 5). Following Yamagishi et al. [11], such flow patterns are correlated with so-called D defects. The D defects are probably vacancy agglomerates. The indirect conclusion is that in the core region more vacancies are present. Their original concentration could, however, be reduced by agglomeration or by recombination with interstitial Si atoms. Figs. 6a—c show the evolution of the etch pit density, developed by the Secco etchant. (The disturbed dark region in Figs. 6b and 6c is due to improper sample handling.) After 30 s (Fig. 6a), etch pits appear essentially only in the outer region. The highest density is generally observed within the wreath. With advancing time, however, the etch pit density in the core region increases and finally surpasses that in the outer region. The etch pits in the core region are also deeper. With increasing etching time also the size of the etch pits increases. As a mean dimension a value of 3.5 jzm after 30 s can be given, increasing up to
as-grown
-—
______________________
~:
>c~~
3 h 780°C+ 16 h 1000°C
.
~
3 h 780°C+ 32 h 1000°C
Fig. 4. Concentration of interstitial oxygen along various traces in a CZ Si crystal after different thermal treatments.
D. Zemke et al. /Journal of Crystal Growth 139 (1994) 37—46
I
Fig. 5. Flow patterns occurring in the core region of CZ Si crystals when etched for 30 mm in a quiet Secco etch solution.
about 17 ~im after 10 mm. Most etch pits show a characteristic dimension of about 3 ~m. Fig. 7a is a topograph of the minority carrier diffusion length distribution as obtained by backside photocurrent measurement on an as-grown wafer. The darkest regions are those with shortest carrier diffusion length or lifetime, respectively. As can be clearly seen, the wreath is a region of drastically reduced carrier lifetime. On the other hand, in the as-grown state there is no significant difference between core and outer region. Carrier diffusion lengths between 525 and 768 j~mhave been found; the most frequent value is 590 ~tm. In the wreath region it is 530 ~m. After a short-time low-temperature annealing step (3 h, 780°C), the so-called nucleation step, the appearance of the topograph becomes
41
almost homogeneous. Under these conditions homogeneous nucleation of oxygen, involving intrinsic point defects, takes place. At higher temperatures, initially homogeneously distributed oxygen precipitates at these nucleation centres. Fig. 7b shows the diffusion length topogram after this subsequent high-temperature annealing step (16 h, 1000°C). Now, the wreath is well developed again. Apparently, the nuclei generated in the first annealing step do not show electrical trap activity, neither in the wreath, nor in the other regions. Electrically active defects, present in the as-grown state in the wreath region, become inactive or are eliminated, respectively, in the low-annealing step. Nevertheless, the high-annealing step leads to the generation or accumulation of minority carrier traps in the wreath region. The diffusion length is also reduced in the core region after this annealing. There the higher density of oxygen precipitates obviously reduces the diffusion length. This is clearly seen in Fig. 8, where both the interstitial oxygen concentration and the diffusion length are shown across the wafer diameter. The two-step annealing leads to a reduction in both the diffusion length and the interstitial oxygen concentration. Resistance scans do not indicate any inhomogeneity in the distribution of electrically active dopants. Unly the concentration of thermal donors is slightly increased in the wreath region, provided the oxygen concentration is > 8 x 1017 cm3. Neither carbon nor nitrogen can be detected. Positron lifetime measurements show that in all regions the concentration of double vacancies is below 1016 cm3, and that of negatively charged double vacancies is below 1015 cm3. The results of all the measurements performed are summarized in Table 1.
5. Discussion Summarizing, the three characteristic features of the core region that make it distinct from the outer region are: (i) the inferior stability of the gate oxide against electrical breakdown; (ii) the decrease of the interstitial oxygen concentration due to precipitation by annealing;
42
D. Zemkc eta!. /Journai of Crystal Growth 139 (1994) 37—46
(a)
(iii) the occurrence of flow patterns in a quiet etch solution (which may be correlated with defects that are associated with vacancy agglomerations). The pronounced oxygen precipitation in the core region is, besides the different GUI, the most striking difference between the outer and the inner circular regions. These results may lead to the conclusion that the oxygen precipitation behaviour plays the dominant role in the integrity of thin gate oxides formed using an otherwise high quality oxidation process. Oxygen precipitation is extensively discussed by Wilkes [12]. During crystal growth most of the oxygen is homogeneously incorporated into the crystal at bonded interstitial sites bridging two Si atoms. According to Livingstone et al. [13] the equilibrium concentration of interstitial oxygen as a function of temperature is given by c~=7.1 x 10~~ exp(—1.2 eV/kT) (cm3). This gives c~’ 1.87 X 1018 cm3 at the temperature of fusion. Due to the strong decrease of solid solubility with decreasing temperature, all asgrown crystals are supersaturated, and subsequent intermediate temperature annealing results in precipitation of a second solid phase [14]. Growing oxygen precipitates or Si0 7 particles exert considerable strain on the surrounding lattice. The strain is released by generation of secondary defects, both by emission and precipitation of silicon self-interstitials and by plastic deformation of the lattice, i.e. by generation and moving of dislocations. The latter relaxation mechanism is effective only above about 900°C. Emission and precipitation of silicon self-interstitials lead to extrinsic stacking faults, the edges of which are partial dislocation loops, whereas plasdeformation results in small perfect double lattice planes, the edges of which are perfect dislocation loops. The small double lattice planes are punched out of the surrounding lattice planes by the growing SiU2 particles and are pushed into the 12 different K 110> directions (“punching”, “punched dislocation loops”). Typically, 50 dislocation loops are punched into the twelve different (110> directions by an emitting particle (~4 in each [110] direction) in a CZ Si of common oxygen content during a 3 h/780°C+ 16 =
(b)
tic
(C)
Fig. ti. Evolution of etch pit density, generated by the Secco etchant, with increasing etch time. The dark region in (b) and (c) is due to improper handling (tweezer). Etch times: (a)30 5: (h) 1 mm; (c) 10 mm.
.
.
.
.
D. Zemke eta!. /Journal of Crystal Growth 139 (1994) 37—46
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43
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Fig. 7. Minority carrier lifetime distribution over the wafer area in the as-grown state (a), and after two step-annealing (3 h, 780°C/16h, 1000°C)(b). The minority carrier diffusion length range is between 525 and 768 ~zmin the as-grown state, and between 488 and 577 ~zm after annealing.
A
as-grown
6
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~
~
after two step anneal
0
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o
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outer region O~concentration
two step anneal: 3 h 780 C + 16 h 1000 C
diffusion length
Fig. 8. Concentration of interstitial oxygen and minority carrier diffusion length across a wafer diameter before and after annealing.
44
D. Zemke et al. /Journal of Crystal Growth 139 (1994) 3 7—46
h/1000°Cprecipitation test. However, not every SiU2 particle emits perfect dislocation loops, Additionally, as-grown microdefects, the wreath microdefets, e.g., act as heterogeneous nucleation sites for the precipitation of oxygen. The three types of precipitation induced microdefects the S1U2 particles, the stacking faults and the punched dislocation loops become visible by preferential etching, e.g. by Secco etching. In the as-grown concentrations around crystals, (6—7) x dissolved 1017 cm3oxygen have been typically found. They can be significantly reduced by annealing. In the literature two mechanisms are discussed for the oxygen precipitation [15,10]: (1 +x) Six. + 2 U. Si0 2 +~ Si., (1) —
—
there is a higher concentration of Si vacancies in the core region, as suggested by the etch behaviour, the second mechanism could be effective. This is, however, conflicting with the positron annihilation experiments. For the second mechanism to be effective, the concentrations of interstitial oxygen and of Si vacancies must be comparable. Annealing leads to oxygen reduction in a concentration of several 1017 oxygen atoms 3. The range vacancy concentration detected, per cm however, is at least one order of magnitude lower. Thus, the first mechanism appears more probable. This means that a reason has to be found for easier oxide precipitate formation in the core region.
(2)
In a pure and defect-free silicon crystal, the lack of precipitation centres is the greatest obstacle for oxygen precipitation. They must be gener-
with x 1. The lower indices indicate the lattice sites the atoms occupy; V51 is a vacant Si site. If
ated by homogeneous nucleation, which is difficult to perform and needs therefore a large su-
I
I
I
(1 +x) Si~~ + 2 U~+x ~
SiU2,
Table 1
Characterization of concentrical wafer regions Analytical technique/ wafer treatment
Outer region
30 mm Secco etch, without motion
Low flow pattern density, microdefect at tip
Time dependent Secco
30 s: many small etch pits, slowly increasing in size
Highest etch pit density, first appearance after I mm
30 s; low EPD; EPD increasing, finally exceeding that in outer region
Minority carrier lifetime
Same as in core region
Slightly reduced in comparison with both the other regions
Same as in outer region; reduced after annealing
Resistance measurement
Radially homogeneous after rapid thermal annealing As-grown; axially decreasing with rod length As in core Slightly increased
etch
[N] determination
Below detection limit
[C] determination
Below detection limit
Positron annihilation TEM Annealing/O concentration
GOl
Wreath region
Core region Higher flow pattern density, at tip
As in outer region
3 negatively charged vacancy pairs < 10° cm~ Neutral vacancy pairs below lOIS cm As-grown: no defects detectable No defects No defects Irregular dislocations Homogeneous 0 distribution before annealing Weaker 0 precipitation
Steep 0, gradient
Increased 0 precipitation; 0 significantly reduced
High
Not usable
Medium
D. Zemke et a!. /Journal of Crystal Growth 139 (1994) 3 7—46
percooling of some hundred degrees below the equilibrium solubility temperature. In crystal regions where vacancies are present, the formation of the first Si02 molecules or Si~U~ complexes is facilitated because the vacancies provide the empty volume necessary for their formation. In contrast, in regions of a surplus of silicon interstitials, the formation of SiU2 or SiXUY is impeded. Comparing the concentration oxygen precip3) in typicalofCZ silicon after itates (101110t4 cm standard technical precipitation processes with the vacancy concentrations (5 x 1013_2 x 1014 cm3), measured by Zimmermann in float-zone silicon [161, it seems reasonable to assume that such vacancy concentrations are able to promote the nucleation of oxide. In an area of higher density of nuclei for precipitation, i.e. in the core region, the precipitation of oxygen at higher temperatures is increased owing to the shorter diffusion distances between the oxygen atoms and the nearest nuclei. The “wreath” around the core region seems to be supersaturated with self-interstitials, which, in many cases, form extrinsic microdefects. In wafer oxidation processes, the extrinsic microdefects act as nuclei for the formation of large oxidation induced extrinsic stacking faults. Uutside the “wreath” (in the outer annular region), there is probably also a surplus of silicon self-interstitials, but at a lower concentration level, which does not enable the formation of extrinsic microdefects. The shrinkage of the core region and of the wreath by slower pulling looks like the result of an increasing out-diffusion of vacancies and selfinterstitials, where the vacancies show the higher starting concentration and the higher diffusivity compared to that of the self-interstitials. If this picture is correct, then the recombination between vacancies and self-interstitials consumes only a small portion of the two intrinsic point defects. However, direct and final evidence of this model is as yet lacking. The inner region contains the flow pattern generating microdefects, the origin of which is possibly correlated with the agglomerations of vacancies, i.e. with D defects (D defects
45
agglomerates of vacancies [171). Such a vacancy agglomerate should act particularly as a heterogeneous nucleation site for the precipitation of fast diffusing interstitial impurities (e.g. U, Cu, Ni, Fe, Li, Na). If the temperature of the cooling crystal falls below the solubility limit of such an element then this element should start to precipitate at the D defect. By uptake of interstitial atoms, the vacancy agglomerate shrinks, eventually vanishes a precipitate, i.e. an SiU2 particle or, lessleaving probably, a metal silicide particle, or both. The particles may grow further and exert strain on the surrounding lattice. The increasing strain would be released by emission of silicon self-interstitials and/or by punching of small dislocation loops. The supersaturated self-interstitials would precipitate forming extrinsic stackingfaults. However, there is a strong counter-argument against the existence of extrinsic secondary defects (perfect dislocation loops, extrinsic stacking faults): In oxidation processes, they should act as nuclei for the formation of oxidation induced stacking faults, as in the case of the wreath microdefects. However, no USFs occur in the inner region. So, the flow pattern defects must be different from the wreath microdefects. This is also confirmed by the fact that the wreath microdefects do not generate flow patterns in a Secco etch at rest. Any metal content of the flow pattern defects would cause defects in a growing gate oxide. Une possibility is that the metal silicides grow further during gate oxidation and penetrate the gate oxide. A second possibility is the formation of mixed silica and metal oxides in the gate oxide. Especially in the case of sodium oxide, such a mixed oxide would have a lower viscosity and a weaker insulating resistance than pure silica. Additional mechanisms, surface roughening by microdefect formation e.g., for defect formation are possible.
6. Conclusions Czochralski-grown silicon crystals show two concentric regions that provide gate oxide layers
46
L). Zemke ci a!. /Journal of Crystal Growth /39 (1994) 37—46
of markedly different breakdown stability. The two regions are separated by an annular “wreath” of several millimetres’ width that is formed by stacking faults in high density after wet oxidation. The wreath diameter, and thus the relative areas of different gate oxide integrity, depend on the crystal pulling speed. With decreasing pulling speed the core region of inferior gate oxide stability becomes smaller in diameter. A significant difference in the behaviour of the inner and the outer region is the enhanced precipitation of oxygen in the core region occurring during annealing. The oxygen precipitation appears to influence the growth and structure of thin gate oxides, and with this the gate oxide integrity. Further, the inner region contains the flow pattern defects that, with high probability, contam precipitates of impurities, most probably oxygen precipitates, but also precipitates of fast diffusing metals. The metal silicides may grow further and penetrate the gate oxide during the growth of the gate oxide. It is also possible that mixed oxides of S1U7 and metal oxides are formed, having lower viscosity and insulating power and an oxidation behaviour different from that of pure Si02. Such a site in a gate oxide would surely cause a breakdown at lower voltage. However, none of the models has been proven to be absolutely conclusive owing to the small
dimensions of the involved microdefects. Further investigations are necessary. 7. References [I] W. Bergholz. W. Mohr, W. Drewes and I-I. Wendt. Mater. Sci. Eng. B 4 (1989) 359. [2] J. Ryuta. E. Morita, T. Tanaka and Y. Shimanuki. Jap. J. AppI. Phys 31(1992) L293. [3] F. Secco dAragona, J. Electrochcm. Soc. 119 (1972) 948. [4] H. FOIl, Life Time Mapping with the Elymat Technique. Symp. on Advances in Science and Technology of Silicon Material. 1992. [51G. Puppe, Wacker Chemitronic GmhH. private communication. [6] German standard DIN 50 431. [7] M. Reiche, Max-Planck-Institut für Mikrostrukturanalyse, Halle. private communication. [8] R. Krause, Martin-Luther-Universität Hallc, private communication. [9] 5. Dannefear. Phys. Status Solidi (a) 102 (1987) 481. [10] CF. Cerofilini and L. Meda, Physical Chemistry of. in and on Silicon, Springer Series in Material Science 8 (Springer, Berlin, 1989). [11] H. Yamagishi, I. Fusegawa. N. Fujimaki and M. Katayama. Semicond. Sci. Technol. 7(1992) Al35. [12] JO. Wilkes, J. Crystal Growth 65 (1983) 214. [13] F.M. Livingstone. S. Messoloras, R.C. Newman, B.C. Pike, R.J. Stewart, M.J. Binns. W.P. Brown and JO. [14] [15] [16] [17]
Wilkes, J. Phys. C (Solid State Phys.) 17 (1984) 6253. W. Kaiser, Phys. Rev. 105 (1957) 1751. S. Hahn. Mater. Sci. Eng. B 4 (1989) 11. H. Zimmermann, AppI. Phys. Lett. 59(1991) 3133. P,J. Roksnoer and MB. van den Boom, J. Crystal Growth 53(1981) 563.
Journal of Crystal Growth 139 (1994) 37—46
Investigations on the correlation between growth rate and gate oxide integrity of Czochralski-grown silicon D. Zemke
a
P. Gerlach b, W. Zulehner
b
K. Jacobs
*,C
Kristallabor, Institut für Werkstoffivissenschaften (LS6), Universität Erlangen—Nürnberg, Martensstrasse ? D-91058 Erlangen, Germany b Central Research and Development, Wacker Chemitronic GmbH, D-84479 Burghausen, Germany Institut für Kristallographie und Materialforschung, Fachbereich Physik, Humboldt- Universitàt zu Berlin, invalidenstrasse 110, D-10115 Berlin, Germany a
(Received 19 August 1993; manuscript received in final form 17 December 1993)
Abstract Czochralski-grown silicon crystals show two concentric regions providing gate oxide layers of markedly different breakdown stability. The two regions are separated by a “wreath”, several millimeters in width, that is built-up from microdefects which act as nucleation centres for the formation of oxidation induced stacking faults. The core region of inferior gate oxide integrity (GOl) becomes smaller in diameter with decreasing pulling speed. The three regions have been extensively characterized by numerous analytical techniques. The most striking difference between the inner and outer regions, besides the GUI, is the enhanced oxygen precipitation in the core region occurring after annealing. An attempt is undertaken to analyze the defect structure in the different regions, its origin, correlation with growth and annealing processes, and its effect on the GOT.
1. Introduction Metal—oxide—semiconductor (MOS) silicon integrated circuit technology requires stable gate oxide layers, capable of withstanding high voltages. Gate oxide integrity (GOT) is one of the most important issues in current MUS technol-
ogy. With the increasing complexity of circuits, the total gate oxide area increases. Furthermore, there is a trend towards thinner gate oxide layers (down to about 50 A) and last, but not least, oxide layers grown on the walls of trenches are
*
Corresponding author,
0022-0248/94/$07.OO © 1994 Elsevier Science B.V. All rights SSDI 0022-0248(93)E0589-Y
becoming prevalent as standard components of
highly integrated circuits. The GUI depends on the quality of the oxide layer, the Si/Si02 interface, and also on the quality of the Si crystal itself. Details of the measurement and current understanding of the GUI are summarized in ref. [1]. Assuming high reproducibility of Si02 layer preparation, the defect structure of the Si crystal is then the deciding factor for the GUI yield. Studies on the effect of crystal pulling rate on the formation of “crystal-originated particles” and defects have recently been published by Ryuta et al. [2]. Considerable variation can be found between float-zone (FZ) and Czochralski (CZ) grown mareserved
38
D. Zemke ci vi. /Journal of Crystal (,rovii, / 19 (/994) 37—4~
terial, the FZ crystals showing better properties, but also between CZ materials from different vendors. A detailed understanding of the effect of intrinsic point defects on the GUI is as yet lacking. The GUI yield is greatly influenced by the growth rate of CZ crystals. A remarkable improvement of the GUI yield is achieved when the pulling speed is reduced from the common 0.8—1.8 mm/mm to values below about 0.6 mm / mm. In this paper experimental results on the relationship between Czochralski Si crystal growth parameters and GUI are presented. Results will be reported characterizing the nature, density and distribution of defects in CZ Si crystals, the influence of the growth rate and the correlation with the GUI.
Fig. I. Surface of a CZ Si wafer after wet oxidation and staining. Fhrve regions ovcurnng on crystals pulled with intermediate velocity can he clearly distinguished.
2. Influence of growth rate on axial and radial defect distribution in CZ Si crystals For the investigations here to be presented, a 6 inch CZ Si crystal has been grown in [0011 direction with intentionally varied pulling speed. The crystal is boron-doped and has initially an interstitial oxygen concentration between 6.2>< i0’~and 7.2 x 10t7 cm3. The growth rate was systematically lowered from 0.8 to 0.4 mm/mm and then again gradually increased to the initial value. To reveal the areas of different intrinsic point defect concentrations and agglomerations, both of which alter the precipitation of oxygen, oxygen was precipitated by annealing for 3 h at 780°C,followed by a second annealing for 16 h at 1000°C. The oxygen precipitates and their secondary defects were stained by the so-called Secco etchant [31,consisting of K 2Cr2U7 and HF. Fig. 1 is a photograph of the wafer surface after wet oxidation (2 h at 1100°Cin wet air) and staining. Three concentric regions can be clearly distinguished. These regions behave very differently in a variety of properties. Most important in the context of the present work is the variation in GUI. The outer circular ring provides material of high breakdown stability, while the core region is of inferior quality. These two regions are separated by a wreath of microdefects which cause
oxidation-induced stacking faults (USFs). Its width amounts to a few millimetres. The three regions have been extensively characterized by a variety of methods. The results will be presented in section 4. The diameter of the core region (and therefore also the quality of the wafer) depends on the pulling speed. Fig. 2 shows a longitudinal cross section through the crystal after oxygen precipitation and etching. The varying pulling speed is reflected in the changing diameter of the mi-
_______ ______
__________________________________________
_________
Fig. 2. Longitudinal cross section through a crystal grown with varying pulling speed after oxygcn precipitation and staining.
D. Zemke et al. /Journal of Crystal Growth 139 (1994) 37—46
48 vol% HF solution. The wafers were etched at room temperature for times between 30 s and 30 mm. A topography of etch pit density and dimen-
need
pulling speed in mm/mm
cone
2 1.6 1.2 0.8 0.4
/N
.
sions is obtained by means of a highly automated light-scattering technique differentiating between
0
E inner reg~on
0
hovde,
~ I
“s
\
8..-
onterregon
~I 00cr,
I
39
12
a
09000
Fig. 3. Correlation between pulling speed and annular diameters for the crystal shown in Fig. 2.
crodefect wreath. Fig. 3 is a sketch of the correlalion between pulling speed and extension of the different crystal regions. Decreasing the growth rate leads to a decrease of the core region area with inferior GUI. For 6 inch crystals, the wreath diameter eventually reaches the outer crystal diameter when the pulling speed exceeds 1.2 mm/mm, while it vanishes in the centre of the crystal when the pulling speed is lower than 0.5 mm/mm (not shown in the figure). Thus, an optimum between pulling speed and GUI yield has to be found, depending on the customer’s needs. An attempt to explain the observed phenomenon requires at first a more detailed investigation of the physical properties of the distinct crystal regions and characterization of the crystal properties on a microscopic or atomic scale.
3. Analytical techniques applied In order to obtain more detailed information on the nature and distribution of defects in the distinct wafer regions, numerous analytical techniques have been applied. These techniques will be briefly described here. The wafers were preferentially etched using the so-called Secco etchant [3].Etching solutions have been prepared by dissolution of 44 g K 2Cr2U7 in 100 ml H2U and finally mixing with 200 ml of a
different classes of etch pits. Another method for the decoration of microdefects was to convert the wreath microdefects into oxidation-induced stacking faults by a wet oxidation at 1100°C for 2 h and to make visible the USFs Seccoleads etching. The growing oxide layer on thebysurface to the generation of excess interstitial Si atoms which precipitate at the wreath microdefects. The strain generated by the formation of interstitials is reduced by the formation of large stacking faults. The USFs are precipitates of silicon self-interstitials. Diverse analytical techniques have been applied without and with annealing. A precipitation test comprising a two-step annealing was applied to reveal the precipitation behaviour of the material. In the first step at 780°C, small oxygen precipitates are formed by homogeneous nudeation. In the second step at 1000°C,the oxygen nuclei grow to large Si02 particles. The annealing times at 780 and at 1000°Cmay be adjusted to the oxygen concentration if stronger or weaker precipitation is desired. In the present investigation, the annealing time at 780°Cwas always 3 h, whereas at 1000°Cannealing times of 16 h and 32 h were applied. The precipitation induced microdefects are made visible by Secco etching. The concentrations of oxygen, nitrogen and carhon were determined by means of Fourier transform infra-red spectroscopy. The spatial resolution is 0.5 mm. At room temperature the oxygen concentration can be determined in the (4—10) x 1017 cm~ concentration range with an levels accuracy 3. The detectable are of ± 0.25cm3 X i0’~cm 5>< 1015 for carbon and 8 x 1014 cm3 for nitrogen. The minority carrier diffusion length was determined from either photocurrent measurements, illuminating the backside of the wafer [4], or from measurements of the photovoltage excited under surface illumination [5]. The resistance was measured by means of a four-point DC probe [6].
40
D. Zemke ci al. /Journal of Crystal Growth 139 (1994) 37—46
The samples were also characterized by transmission electron microscopy [7], applying an acceleration voltage of I MeV. Areas of 1 mm2 could be inspected in different operating modes. The spatial resolution was about 1 nm. The positron annihilation technique was applied for the detection of vacancies [8]. 22Na was the positron source. The mean positron lifetime was typically around 220 ps. This value agrees exactly with data found in the literature for highly perfect Si [9]. The sensitivity limit of the method is about 2.5 X 1015 cm3.
4. Characterization of defects in different wafer regions The three distinct regions which can be observed on each wafer have been extensively characterized by the techniques described in section 3. All the test wafers have been etch-polished in order to remove surface damage and impurities, and have undergone a rapid thermal annealing procedure in order to kill shallow donors [10]. Fig. 4 shows the concentration of interstitial oxygen along various traces in the as-grown state and after different annealing procedures. In the cylindrical part of the as-grown crystal the interstitial oxygen is homogeneously distributed. Generally, it is lowered by annealing. The reduction is
8
_____________
_________
—
most pronounced in the core region (trace 1). The decrease of the interstitial oxygen concentration is due to the formation of precipitates. This is obviously most easily accomplished in the central region. The reason has yet to be discussed. Another characteristic difference between the distinct regions is their behaviour against the Secco etchant. In a solution at rest after 30 mm of etching time well-developed flow patterns are observed in the core region (Fig. 5). Following Yamagishi et al. [11], such flow patterns are correlated with so-called D defects. The D defects are probably vacancy agglomerates. The indirect conclusion is that in the core region more vacancies are present. Their original concentration could, however, be reduced by agglomeration or by recombination with interstitial Si atoms. Figs. 6a—c show the evolution of the etch pit density, developed by the Secco etchant. (The disturbed dark region in Figs. 6b and 6c is due to improper sample handling.) After 30 s (Fig. 6a), etch pits appear essentially only in the outer region. The highest density is generally observed within the wreath. With advancing time, however, the etch pit density in the core region increases and finally surpasses that in the outer region. The etch pits in the core region are also deeper. With increasing etching time also the size of the etch pits increases. As a mean dimension a value of 3.5 jzm after 30 s can be given, increasing up to
as-grown
-—
______________________
~:
>c~~
3 h 780°C+ 16 h 1000°C
.
~
3 h 780°C+ 32 h 1000°C
Fig. 4. Concentration of interstitial oxygen along various traces in a CZ Si crystal after different thermal treatments.
D. Zemke et al. /Journal of Crystal Growth 139 (1994) 37—46
I
Fig. 5. Flow patterns occurring in the core region of CZ Si crystals when etched for 30 mm in a quiet Secco etch solution.
about 17 ~im after 10 mm. Most etch pits show a characteristic dimension of about 3 ~m. Fig. 7a is a topograph of the minority carrier diffusion length distribution as obtained by backside photocurrent measurement on an as-grown wafer. The darkest regions are those with shortest carrier diffusion length or lifetime, respectively. As can be clearly seen, the wreath is a region of drastically reduced carrier lifetime. On the other hand, in the as-grown state there is no significant difference between core and outer region. Carrier diffusion lengths between 525 and 768 j~mhave been found; the most frequent value is 590 ~tm. In the wreath region it is 530 ~m. After a short-time low-temperature annealing step (3 h, 780°C), the so-called nucleation step, the appearance of the topograph becomes
41
almost homogeneous. Under these conditions homogeneous nucleation of oxygen, involving intrinsic point defects, takes place. At higher temperatures, initially homogeneously distributed oxygen precipitates at these nucleation centres. Fig. 7b shows the diffusion length topogram after this subsequent high-temperature annealing step (16 h, 1000°C). Now, the wreath is well developed again. Apparently, the nuclei generated in the first annealing step do not show electrical trap activity, neither in the wreath, nor in the other regions. Electrically active defects, present in the as-grown state in the wreath region, become inactive or are eliminated, respectively, in the low-annealing step. Nevertheless, the high-annealing step leads to the generation or accumulation of minority carrier traps in the wreath region. The diffusion length is also reduced in the core region after this annealing. There the higher density of oxygen precipitates obviously reduces the diffusion length. This is clearly seen in Fig. 8, where both the interstitial oxygen concentration and the diffusion length are shown across the wafer diameter. The two-step annealing leads to a reduction in both the diffusion length and the interstitial oxygen concentration. Resistance scans do not indicate any inhomogeneity in the distribution of electrically active dopants. Unly the concentration of thermal donors is slightly increased in the wreath region, provided the oxygen concentration is > 8 x 1017 cm3. Neither carbon nor nitrogen can be detected. Positron lifetime measurements show that in all regions the concentration of double vacancies is below 1016 cm3, and that of negatively charged double vacancies is below 1015 cm3. The results of all the measurements performed are summarized in Table 1.
5. Discussion Summarizing, the three characteristic features of the core region that make it distinct from the outer region are: (i) the inferior stability of the gate oxide against electrical breakdown; (ii) the decrease of the interstitial oxygen concentration due to precipitation by annealing;
42
D. Zemkc eta!. /Journai of Crystal Growth 139 (1994) 37—46
(a)
(iii) the occurrence of flow patterns in a quiet etch solution (which may be correlated with defects that are associated with vacancy agglomerations). The pronounced oxygen precipitation in the core region is, besides the different GUI, the most striking difference between the outer and the inner circular regions. These results may lead to the conclusion that the oxygen precipitation behaviour plays the dominant role in the integrity of thin gate oxides formed using an otherwise high quality oxidation process. Oxygen precipitation is extensively discussed by Wilkes [12]. During crystal growth most of the oxygen is homogeneously incorporated into the crystal at bonded interstitial sites bridging two Si atoms. According to Livingstone et al. [13] the equilibrium concentration of interstitial oxygen as a function of temperature is given by c~=7.1 x 10~~ exp(—1.2 eV/kT) (cm3). This gives c~’ 1.87 X 1018 cm3 at the temperature of fusion. Due to the strong decrease of solid solubility with decreasing temperature, all asgrown crystals are supersaturated, and subsequent intermediate temperature annealing results in precipitation of a second solid phase [14]. Growing oxygen precipitates or Si0 7 particles exert considerable strain on the surrounding lattice. The strain is released by generation of secondary defects, both by emission and precipitation of silicon self-interstitials and by plastic deformation of the lattice, i.e. by generation and moving of dislocations. The latter relaxation mechanism is effective only above about 900°C. Emission and precipitation of silicon self-interstitials lead to extrinsic stacking faults, the edges of which are partial dislocation loops, whereas plasdeformation results in small perfect double lattice planes, the edges of which are perfect dislocation loops. The small double lattice planes are punched out of the surrounding lattice planes by the growing SiU2 particles and are pushed into the 12 different K 110> directions (“punching”, “punched dislocation loops”). Typically, 50 dislocation loops are punched into the twelve different (110> directions by an emitting particle (~4 in each [110] direction) in a CZ Si of common oxygen content during a 3 h/780°C+ 16 =
(b)
tic
(C)
Fig. ti. Evolution of etch pit density, generated by the Secco etchant, with increasing etch time. The dark region in (b) and (c) is due to improper handling (tweezer). Etch times: (a)30 5: (h) 1 mm; (c) 10 mm.
.
.
.
.
D. Zemke eta!. /Journal of Crystal Growth 139 (1994) 37—46
______________________________
43
MsX
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__
_______________
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~ 000J{...,.,,.,
—
.
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.,.~
~,
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X ~CPN [mm]
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Fig. 7. Minority carrier lifetime distribution over the wafer area in the as-grown state (a), and after two step-annealing (3 h, 780°C/16h, 1000°C)(b). The minority carrier diffusion length range is between 525 and 768 ~zmin the as-grown state, and between 488 and 577 ~zm after annealing.
A
as-grown
6
800
as-grown
1:: -700
~
~
after two step anneal
0
—
o
V
—
outer region
ring
core region
ring
300
outer region O~concentration
two step anneal: 3 h 780 C + 16 h 1000 C
diffusion length
Fig. 8. Concentration of interstitial oxygen and minority carrier diffusion length across a wafer diameter before and after annealing.
44
D. Zemke et al. /Journal of Crystal Growth 139 (1994) 3 7—46
h/1000°Cprecipitation test. However, not every SiU2 particle emits perfect dislocation loops, Additionally, as-grown microdefects, the wreath microdefets, e.g., act as heterogeneous nucleation sites for the precipitation of oxygen. The three types of precipitation induced microdefects the S1U2 particles, the stacking faults and the punched dislocation loops become visible by preferential etching, e.g. by Secco etching. In the as-grown concentrations around crystals, (6—7) x dissolved 1017 cm3oxygen have been typically found. They can be significantly reduced by annealing. In the literature two mechanisms are discussed for the oxygen precipitation [15,10]: (1 +x) Six. + 2 U. Si0 2 +~ Si., (1) —
—
there is a higher concentration of Si vacancies in the core region, as suggested by the etch behaviour, the second mechanism could be effective. This is, however, conflicting with the positron annihilation experiments. For the second mechanism to be effective, the concentrations of interstitial oxygen and of Si vacancies must be comparable. Annealing leads to oxygen reduction in a concentration of several 1017 oxygen atoms 3. The range vacancy concentration detected, per cm however, is at least one order of magnitude lower. Thus, the first mechanism appears more probable. This means that a reason has to be found for easier oxide precipitate formation in the core region.
(2)
In a pure and defect-free silicon crystal, the lack of precipitation centres is the greatest obstacle for oxygen precipitation. They must be gener-
with x 1. The lower indices indicate the lattice sites the atoms occupy; V51 is a vacant Si site. If
ated by homogeneous nucleation, which is difficult to perform and needs therefore a large su-
I
I
I
(1 +x) Si~~ + 2 U~+x ~
SiU2,
Table 1
Characterization of concentrical wafer regions Analytical technique/ wafer treatment
Outer region
30 mm Secco etch, without motion
Low flow pattern density, microdefect at tip
Time dependent Secco
30 s: many small etch pits, slowly increasing in size
Highest etch pit density, first appearance after I mm
30 s; low EPD; EPD increasing, finally exceeding that in outer region
Minority carrier lifetime
Same as in core region
Slightly reduced in comparison with both the other regions
Same as in outer region; reduced after annealing
Resistance measurement
Radially homogeneous after rapid thermal annealing As-grown; axially decreasing with rod length As in core Slightly increased
etch
[N] determination
Below detection limit
[C] determination
Below detection limit
Positron annihilation TEM Annealing/O concentration
GOl
Wreath region
Core region Higher flow pattern density, at tip
As in outer region
3 negatively charged vacancy pairs < 10° cm~ Neutral vacancy pairs below lOIS cm As-grown: no defects detectable No defects No defects Irregular dislocations Homogeneous 0 distribution before annealing Weaker 0 precipitation
Steep 0, gradient
Increased 0 precipitation; 0 significantly reduced
High
Not usable
Medium
D. Zemke et a!. /Journal of Crystal Growth 139 (1994) 3 7—46
percooling of some hundred degrees below the equilibrium solubility temperature. In crystal regions where vacancies are present, the formation of the first Si02 molecules or Si~U~ complexes is facilitated because the vacancies provide the empty volume necessary for their formation. In contrast, in regions of a surplus of silicon interstitials, the formation of SiU2 or SiXUY is impeded. Comparing the concentration oxygen precip3) in typicalofCZ silicon after itates (101110t4 cm standard technical precipitation processes with the vacancy concentrations (5 x 1013_2 x 1014 cm3), measured by Zimmermann in float-zone silicon [161, it seems reasonable to assume that such vacancy concentrations are able to promote the nucleation of oxide. In an area of higher density of nuclei for precipitation, i.e. in the core region, the precipitation of oxygen at higher temperatures is increased owing to the shorter diffusion distances between the oxygen atoms and the nearest nuclei. The “wreath” around the core region seems to be supersaturated with self-interstitials, which, in many cases, form extrinsic microdefects. In wafer oxidation processes, the extrinsic microdefects act as nuclei for the formation of large oxidation induced extrinsic stacking faults. Uutside the “wreath” (in the outer annular region), there is probably also a surplus of silicon self-interstitials, but at a lower concentration level, which does not enable the formation of extrinsic microdefects. The shrinkage of the core region and of the wreath by slower pulling looks like the result of an increasing out-diffusion of vacancies and selfinterstitials, where the vacancies show the higher starting concentration and the higher diffusivity compared to that of the self-interstitials. If this picture is correct, then the recombination between vacancies and self-interstitials consumes only a small portion of the two intrinsic point defects. However, direct and final evidence of this model is as yet lacking. The inner region contains the flow pattern generating microdefects, the origin of which is possibly correlated with the agglomerations of vacancies, i.e. with D defects (D defects
45
agglomerates of vacancies [171). Such a vacancy agglomerate should act particularly as a heterogeneous nucleation site for the precipitation of fast diffusing interstitial impurities (e.g. U, Cu, Ni, Fe, Li, Na). If the temperature of the cooling crystal falls below the solubility limit of such an element then this element should start to precipitate at the D defect. By uptake of interstitial atoms, the vacancy agglomerate shrinks, eventually vanishes a precipitate, i.e. an SiU2 particle or, lessleaving probably, a metal silicide particle, or both. The particles may grow further and exert strain on the surrounding lattice. The increasing strain would be released by emission of silicon self-interstitials and/or by punching of small dislocation loops. The supersaturated self-interstitials would precipitate forming extrinsic stackingfaults. However, there is a strong counter-argument against the existence of extrinsic secondary defects (perfect dislocation loops, extrinsic stacking faults): In oxidation processes, they should act as nuclei for the formation of oxidation induced stacking faults, as in the case of the wreath microdefects. However, no USFs occur in the inner region. So, the flow pattern defects must be different from the wreath microdefects. This is also confirmed by the fact that the wreath microdefects do not generate flow patterns in a Secco etch at rest. Any metal content of the flow pattern defects would cause defects in a growing gate oxide. Une possibility is that the metal silicides grow further during gate oxidation and penetrate the gate oxide. A second possibility is the formation of mixed silica and metal oxides in the gate oxide. Especially in the case of sodium oxide, such a mixed oxide would have a lower viscosity and a weaker insulating resistance than pure silica. Additional mechanisms, surface roughening by microdefect formation e.g., for defect formation are possible.
6. Conclusions Czochralski-grown silicon crystals show two concentric regions that provide gate oxide layers
46
L). Zemke ci a!. /Journal of Crystal Growth /39 (1994) 37—46
of markedly different breakdown stability. The two regions are separated by an annular “wreath” of several millimetres’ width that is formed by stacking faults in high density after wet oxidation. The wreath diameter, and thus the relative areas of different gate oxide integrity, depend on the crystal pulling speed. With decreasing pulling speed the core region of inferior gate oxide stability becomes smaller in diameter. A significant difference in the behaviour of the inner and the outer region is the enhanced precipitation of oxygen in the core region occurring during annealing. The oxygen precipitation appears to influence the growth and structure of thin gate oxides, and with this the gate oxide integrity. Further, the inner region contains the flow pattern defects that, with high probability, contam precipitates of impurities, most probably oxygen precipitates, but also precipitates of fast diffusing metals. The metal silicides may grow further and penetrate the gate oxide during the growth of the gate oxide. It is also possible that mixed oxides of S1U7 and metal oxides are formed, having lower viscosity and insulating power and an oxidation behaviour different from that of pure Si02. Such a site in a gate oxide would surely cause a breakdown at lower voltage. However, none of the models has been proven to be absolutely conclusive owing to the small
dimensions of the involved microdefects. Further investigations are necessary. 7. References [I] W. Bergholz. W. Mohr, W. Drewes and I-I. Wendt. Mater. Sci. Eng. B 4 (1989) 359. [2] J. Ryuta. E. Morita, T. Tanaka and Y. Shimanuki. Jap. J. AppI. Phys 31(1992) L293. [3] F. Secco dAragona, J. Electrochcm. Soc. 119 (1972) 948. [4] H. FOIl, Life Time Mapping with the Elymat Technique. Symp. on Advances in Science and Technology of Silicon Material. 1992. [51G. Puppe, Wacker Chemitronic GmhH. private communication. [6] German standard DIN 50 431. [7] M. Reiche, Max-Planck-Institut für Mikrostrukturanalyse, Halle. private communication. [8] R. Krause, Martin-Luther-Universität Hallc, private communication. [9] 5. Dannefear. Phys. Status Solidi (a) 102 (1987) 481. [10] CF. Cerofilini and L. Meda, Physical Chemistry of. in and on Silicon, Springer Series in Material Science 8 (Springer, Berlin, 1989). [11] H. Yamagishi, I. Fusegawa. N. Fujimaki and M. Katayama. Semicond. Sci. Technol. 7(1992) Al35. [12] JO. Wilkes, J. Crystal Growth 65 (1983) 214. [13] F.M. Livingstone. S. Messoloras, R.C. Newman, B.C. Pike, R.J. Stewart, M.J. Binns. W.P. Brown and JO. [14] [15] [16] [17]
Wilkes, J. Phys. C (Solid State Phys.) 17 (1984) 6253. W. Kaiser, Phys. Rev. 105 (1957) 1751. S. Hahn. Mater. Sci. Eng. B 4 (1989) 11. H. Zimmermann, AppI. Phys. Lett. 59(1991) 3133. P,J. Roksnoer and MB. van den Boom, J. Crystal Growth 53(1981) 563.