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P-epitaxial wafers
Journal Pre-proofs The effects of pre-annealing on oxygen precipitation in silicon P/P-epitaxial wafers Kyu hyung Lee, Don ha Hwang, Hee bog Kang, Bo young Lee PII: DOI: Reference:
S0022-0248(19)30576-7 https://doi.org/10.1016/j.jcrysgro.2019.125361 CRYS 125361
To appear in:
Journal of Crystal Growth
Received Date: Revised Date: Accepted Date:
13 August 2019 13 November 2019 15 November 2019
Please cite this article as: K. hyung Lee, D. ha Hwang, H. bog Kang, B. young Lee, The effects of pre-annealing on oxygen precipitation in silicon P/P-epitaxial wafers, Journal of Crystal Growth (2019), doi: https://doi.org/ 10.1016/j.jcrysgro.2019.125361
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier B.V.
The effects of pre-annealing on oxygen precipitation in silicon P/P- epitaxial wafers
Kyu hyung Leea*, Don ha Hwanga, Hee bog Kanga, and Bo young Leea
a
Research Center, SK Siltron Inc., 435 Suchul-daero, Gumi-si, Gyeongsangbuk-do, 39387, Korea
*E-mail : [email protected]
Abstract Herein, the effect of pre-annealing on oxygen precipitation in a silicon P/P- epitaxial wafer was investigated using an experimental approach. Czochralski-grown silicon wafers were annealed at 800 – 900 oC to enhance their bulk micro defects (BMDs), before the epitaxial growth process. The effect of the pre-annealing temperature on oxygen precipitation was greater than that of the pre-annealing time, and the effect of pre-annealing temperature varied between polished and epitaxial wafers. Regarding pre-annealed epitaxial wafers that were annealed at low temperatures, the pre-annealing time also had as large an effect on BMD formation as the pre-annealing temperature. These experimental results were used to determine the optimal pre-annealing temperature for enhancing oxygen precipitation in epitaxial wafers. The experimental results regarding the optimal conditions were consistent with prior hypotheses. Crystal originated particles (COPs) and COP-free crystals exhibited their highest BMD density values at similar pre-annealing temperatures.
Keywords : A1. Defects, A1. Light scattering tomography, A3. Chemical vapor deposition processes, B2. Semiconducting silicon
1. Introduction Silicon wafers have predominantly been used for the fabrication of ultra-large-scale integrated (ULSI) devices. USLI device technology requires low levels of surface defects and metal contamination in the silicon wafer. It has been well established that surface defects on the polished wafer, such as crystal originated particles (COPs) and scratches, can cause device failures, such as gate oxide breakdown. [1] As silicon wafers with epitaxially-grown layers can control these surface defects, they can significantly improve device performance [2]. As the device features continues to shrink, metallic contaminants as well as surface defects in silicon wafers that have not been affected in the past can have detrimental effects on both device performance and yield. [3-5] Since the early days of the semiconductor industry, gettering techniques have been studied to eliminate or minimize the impact of metallic contamination on an electrical device’s performance. Gettering techniques can be categorized as being either internal (or intrinsic) or external (or extrinsic). For a 300 mm wafer, external gettering techniques such as ion implantation methods, trapping metal impurities in a high-dose implanted region, or dislocation loops, have been studied for complementary metal-oxide-semiconductor image sensor (CIS) devices. [6-7] However, it is necessary to strengthen internal gettering, so as to minimize the risk of metal contamination, as external gettering techniques (including ion implantation) are not normally used for 300 mm epitaxial wafers. Furthermore, in logic devices, boron gettering by heavy-boron doping is not possible, because P/P- epitaxial wafers are commonly used. As a result, in P/P- epitaxial wafers used in logic devices, internal gettering by oxygen precipitates plays a dominant role regarding effective sinks for harmful metal impurities. In order to improve the gettering efficiency, it is essential to increase the density of bulk micro defects (BMDs), which act as gettering sites. Various methods to maintain BMD density have been studied, such as increasing oxygen concentration or doping nitrogen. [8-10] In addition to the incorporation of impurities such as oxygen and nitrogen, heat treatment methods that are used before the epitaxial process have been explored as a method for increasing BMD density. [11] Most of the grown-in BMD nuclei that are produced during crystal growth are dissolved, because the epitaxial process is conducted at a high temperature
(above 1100 °C). The pre-annealing process, conducted prior to epitaxial growth, plays a role in generating and growing BMD nuclei by making them larger than the critical size necessary for surviving the epitaxial process. As a result, pre-annealed epitaxial wafers exhibit high BMD density values after device processing, allowing them to provide sufficient gettering efficiency. Lightly boron-doped COP crystals (that include nitrogen) are the most commonly-used substrate for epitaxial wafers for logic devices. With increasing epitaxial layer thickness, defects induced by COPs on the surface of the substrate gradually become covered, before finally disappearing completely. [1213] However, in case of epitaxial layer thicknesses that are below 3 mm, it is possible that COPs on the surface of the substrate can be transformed into extremely shallow pits, with depths in the range of 1 nm. These epitaxial defects are merely morphological defects, and are therefore not detrimental to gate oxide integrity. They can, however, adversely affect the device’s yields, such as its overlay and defocus. Epitaxial wafers that use substrates without COP defects are free from this risk. However, in order to replace the epitaxial wafers using COP crystals, it is important that epitaxial wafers made using COP-free substrates have a high and radially-uniform distribution of oxygen precipitates. Ensuring this distribution is also advantageous regarding the epitaxial defects mentioned above. This paper details an examination of the effects of pre-annealing on oxygen precipitation of P/P- epitaxial wafers, using both nitrogen-doped COP-free crystals and nitrogen-doped COP crystals. Focus is given to the pre-annealing conditions in P/P- epitaxial wafers that can most effectively increase BMD density, in terms of annealing temperature and annealing time.
2. Experimental details Boron-doped (1.1 × 1015 and 1.2 × 1015 atoms/cm3) (100) Czochralski-grown silicon crystals were studied, with both high and low oxygen concentrations. The high oxygen materials had an interstitial oxygen concentration ([Oi]) of 6.0 ×1017 atoms/cm3 (12 ppma, New ASTM). The low oxygen materials had an [Oi] of 5.2 × 1017 atoms/cm3 (10.4 ppma, New ASTM). Both the high and low oxygen groups had only pure silicon in the vacancy-dominant crystal region for each whole wafer (pure or COP-free silicon crystal). Another group was also boron-doped (at 1.3 × 1015 atoms/cm3), this
group consisted of Czochralski-grown silicon with an [Oi] of 5.7 × 1017 atoms/cm3 (11.4 ppma, New ASTM). In contrast to the COP-free crystal group, this group had vacancy-rich regions, including COPs, across each whole wafer (COP crystal). All samples, both COP-free crystal and COP crystal, had diameters of 300 mm, and were doped with nitrogen at a concentration of 3.1 × 10 12 ~ 5.1 × 1012 atoms/cm3. Within each ingot, the sample positions of group 2 (P-2 & E-2) and 3 (P-2 & E-3) were within ~ 50 mm, while those of group 1 (P-1 & E-1) were separated by a distance of ~ 200 mm. Due to the difference in sample intervals for each ingot, the difference in the concentrations of interstitial oxygen, boron, and nitrogen within the samples of group 1 were relatively larger than those of groups 2 and 3 (Table 1).
Table 1. Parameters of the wafers used for these studies. Group
[Oi (New ASTM, ppma)
[N] (ea/cm3)
[B] (ea/cm3)
Crystal (substrate)
P-1
10.22~10.60
3.3~3.8E12
1.20~1.24E15
COP free
P-2
11.96~12.14
3.1E12
1.14E15
COP free
P-3
11.44~11.68
5.09E12
1.32E15
COP
E-1
9.95~10.46
3.3~3.8E12
1.20~1.24E15
COP free
E-2
11.78~12.10
3.1E12
1.14E15
COP free
E-3
11.38~11.49
5.09E12
1.32E15
COP
Remark
Polished wafer
Epitaxial wafer
Pre-annealing conditions were selected regarding the temperature and time at which both BMD nucleation and growth could be satisfied at simultaneously. These tests were conducted with nine variants of conditions: three different temperatures (800, 850, and 900 oC) and three different times (20, 40, and 60 min). The pre-annealing process was carried out in a vertical furnace of ambient nitrogen; the push-in and pull-out temperatures were both 700 oC. The ramp-up and ramp-down rates of these tests were 5 oC/min and 3 oC/min, respectively. After pre-annealing and polishing, epitaxial layers with a thickness of approximately 2.5 μm were grown at a high temperature (above 1100 oC). In order to confirm the pre-annealing effect, BMD evaluations were carried out both before and after the epitaxial growth process, namely on both polished and epitaxial wafers. BMD evaluation was performed in order of [Oi] (initial state), heat treatment for BMD formation, [Oi] (final state), and
BMD measurements. Fourier transform infrared spectroscopy (FTIR) was used to measure [Oi] before and after heat treatment for BMD formation. Both polished and epitaxial wafers were subjected to the same heat treatment, logic device simulation heat cycle (pad oxidation simulation at 850 oC for 1 hr and pad nitride simulation at 750 oC for 1 hr) followed by BMD growth heat treatment (1000 oC 16 hr). Finally, an etching method, an optical microscopic evaluation after preferential etching (etching BMD, or E-BMD), and laser scattering tomography (LST) were performed to evaluate each sample’s BMD density.
3. Results and Discussion
Fig. 1. (a) Initial [Oi], (b) LST BMD density, and (c) E-BMD density of polished wafers with different pre-annealing temperatures. Each sample was pre-annealed for 40 min.
The effects of varying both the pre-annealing temperature and the pre-annealing time on the polished wafer were investigated experimentally. Fig. 1 shows the oxygen precipitation results of polished wafers under pre-annealing temperatures of (a) 800, (b) 850, and (c) 900 oC (for a fixed 40
min period). As the pre-annealing temperature was increased, both the LST and etching BMD density values of groups P-2 and P-3 decreased, but the LST density values of group P-1 showed different results at a pre-annealing temperature of 850 oC (Figs. 1 (b) and (c)). Unlike other groups (P-2 and P3), the LST BMD density of group P-1 at 850 oC in Fig. 1 (b) was higher than that at 800 oC; this is likely caused by the differences in the initial [Oi] and [N] between these samples. In the polished wafers, as-grown BMD nuclei formed as the ingots cooled and BMD nuclei generated by the preannealing process can contribute to BMD formation after heat treatment, as there was no high temperature process, such as epitaxial growth. As-grown BMD nuclei density of the pre-annealed wafer at 850 oC having the initial [Oi] (10.43 ppma) and [N] (3.77 × 1012 cm-3) were higher than those of the wafer at 800 oC (10.36 ppma and 3.3 × 1012 cm-3, respectively), this difference affected the LST BMD density after logic device heat treatment. Although the initial [Oi] (10.47 ppma) of group P-1 when pre-annealed at 900 oC ([N] of 3.36 × 1012 cm-3) was higher than when it was pre-annealed wafer at 850 oC, the BMD density decreased at 900 oC. This is determined that the effect of BMD density due to the pre-annealing temperature of 900 oC was greater than that of as-grown BMD density.
Fig. 2. (a) BMD size distributions for group P-1 samples pre-annealed at 800 and 850 oC (for a 40 min period), and E-BMD images corresponding to group P-1 pre-annealed at (b) 800 and (c) 850 oC.
This increase in BMD density due to higher oxygen and nitrogen concentrations was not observed in E-BMD results of group P-1 at 850 oC, however, this was determined to be due to the BMD density and size. (Fig. 2 (a)). As the LST BMD density and size of the wafer pre-annealed at 850 oC were higher and larger respectively than those of the wafer that was pre-annealed at 800 oC, it was observed to have combined with the surrounding BMD after preferential etching, resulting in a larger size but a lower density (Figs. 2 (b) and (c)). At the same pre-annealing temperature conditions, no significant difference was visible in the BMD density of each group across pre-annealing times of 20, 40, or 60 min.
Fig. 3. (a) Initial [Oi], (b) LST BMD density, and (c) E-BMD density values of epitaxial wafers with different pre-annealing temperatures for 40 min.
Fig. 3 displays the results of oxygen precipitation on epitaxial wafers prepared with pre-annealing temperatures of (a) 800, (b) 850, and (c) 900 oC, respectively, for a 40 min period. As the epitaxial
wafers underwent high temperatures during epitaxial growth, BMD nuclei smaller than the critical size could be dissolved. Therefore, the BMD trends of the epitaxial wafers showed different results from those of the polished wafers, due to the dissolution of small-size nuclei. In the case of polished wafers, the BMD density of the wafer prepared with a pre-annealing temperature of 800 °C was the highest, and BMD density tended to decrease as the pre-annealing temperature increased. The epitaxial wafers that were pre-annealed at 800 oC showed the lowest BMD density, however, this increased at 850 oC before decreasing again at 900 oC. All three of the groups showed similar results.
Fig. 4. (a) Initial [Oi], (b) LST BMD density, and (c) E-BMD density of samples with E-2 with different pre-annealing conditions (temperatures of 800, 850, and 900 oC, and times of 20, 40, and 60 min).
The effect of changing the pre-annealing time on oxygen precipitation was investigated. Fig. 4 shows the oxygen precipitation results for different pre-annealing times for the group E-2. At a relatively low pre-annealing temperature (800 oC), the increasing trend in BMD density was clear as
the pre-annealing time increased. However, for pre-annealing temperatures of 850 and 900 oC, oxygen precipitation did not increase even when the pre-annealing time increased. This may be related the size of the BMD nuclei generated in the pre-annealing process prior to the epitaxial growth process. As the diffusivity of oxygen atoms in silicon at pre-annealing temperature of 800 oC is relatively slower than that of higher temperatures, the density of BMD nuclei that exceed the critical size to survive during epitaxial growth process is also lower than that of the higher pre-annealing temperatures. However, as the pre-annealing time increases, the diffusion distance of oxygen increases, which leads to an increase in BMD nuclei density that are above the critical size. The increase in BMD density with increasing time is not observed, as oxygen diffusion under preannealing conditions of 850 oC or higher are sufficient to grow nuclei above the critical size during only 20 minutes of pre-annealing [14-15].
Fig. 5. LST BMD density as a function of delta [Oi] in epitaxial wafers (group E-1, E-2, and E-3).
The relationship between the change in oxygen concentration (delta [Oi]; the difference between
the initial [Oi] and the residual [Oi] after heat treatment), and BMD density was also investigated. Fig. 5 shows the relationships between delta [Oi] and LST BMD density for all of the sample groups (groups E-1, E-2, and E-3). As delta [Oi] increased, the LST BMD density also increased, but this relationship appears to almost become saturated when delta [Oi] reaches about 1 ppma. This means that it is possible to produce wafers that require a specific BMD range by controlling the delta [Oi] levels. In the heat treatment used in this experiment, if the wafer were designed to have a delta [Oi] of 0.5 ppma or higher, a wafer with a BMD density of 3 × 109 ea/cm3 or higher would be produced.
Fig. 6. Contour plots of (a) delta [Oi] and (b) LST BMD density as a function of pre-annealing temperature and time for groups E-1, E-2, and E-3.
Fig. 6 displays contour plots of delta [Oi] and LST BMD density as a function of pre-annealing temperature and time for the epitaxial wafers. The influence of the pre-annealing temperature on delta [Oi] and BMD density differed slightly depending on the pre-annealing time, but the overall trends in oxygen precipitation are similar. As the pre-anneal temperature increased, both the delta [Oi] and BMD density rose as well, peaking at around 875 oC and then decreasing at higher temperatures (Figs. 6 (a) and (b)). However, there is a slight difference in oxygen precipitation between the epitaxial wafer made using COP-free crystal and the epitaxial wafer using COP crystal, due to the differences in vacancy concentrations between the two. It was confirmed that the effect of the pre-annealing time on BMD density was weaker than that of pre-annealing temperature. In other words, as the pre-annealing time increased, the BMD density gradually increased, but the absolute change in BMD density was small. As shown in Fig. 6, the preannealing temperature with the highest oxygen precipitation increased as the pre-annealing time decreased. This occurred because a higher pre-annealing temperature was required for BMDs to exceed the critical size of the epitaxial growth process when the pre-annealing time was short. It was also observed that the pre-annealing temperatures with the highest delta [Oi] values were slightly shifted to the higher temperatures, as the initial [Oi] level was lower (20 min pre-annealing conditions for E-1 and E-2 wafers). A pre-annealing temperature of 880 oC was selected as the optimal preannealing condition, in consideration of the initial [Oi] level and oxygen precipitation results. Additional tests were then conducted using the group E-1 samples.
Fig. 7. (a) Delta [Oi] and (b) LST BMD density values for group E-1 at different pre-annealing temperatures (fixed period of 40 min). The dotted line in (a) indicates a fitting curve for the preannealing temperature at 850 oC.
Fig. 7 shows the delta [Oi] and LST BMD density results of Group E-1 samples at different preannealing temperatures (for a fixed 40 min period), including 880 oC. As shown in Fig. 7 (a), the delta [Oi] for the pre-annealed wafer at 880 oC was equal to or higher than those of the other conditions. Considering the initial [Oi], the delta [Oi] levels of the wafer that was pre-annealed at 880 oC were higher than those of the pre-annealed wafers at 800 and 900 oC, and were similar to the group that was pre-annealed 850 oC. The trends in the LST BMD density were also similar to those observed in the delta [Oi] results (Fig. 7 (b)). These results are in good agreement with the expectation that epitaxial wafers pre-annealed at 880 oC will have the highest oxygen precipitation. It is thought that the effect of the pre-annealing temperature on the oxygen precipitation may be well revealed under shorter preannealing conditions.
4. Conclusions Pre-annealing effects on oxygen precipitation were investigated regarding BMD density and delta [Oi] evaluations. The BMD density results of polished wafers differed from those of epitaxial wafers. The polished wafers showed higher BMD densities in the pre-annealed group at low temperatures (800 oC) because there was no high temperature process in which the BMD nuclei could be dissolved. Unlike the polished wafers, the epitaxial wafers exhibited higher BMD densities at pre-annealing temperatures above 800 oC. The pre-annealing temperature had a greater effect on oxygen precipitation in P/P- epitaxial wafers than the pre-annealing time. Based on the delta [Oi] and BMD density results, it was deduced that the optimal pre-annealing temperature for enhancing oxygen precipitation in the epitaxial wafer was approximately 880 oC. Both epitaxial wafers using COP-free crystal and also epitaxial wafers using COP crystal showed their highest BMD density values at a similar pre-annealing temperature. In
additional experiments, the results of oxygen precipitation were consistent with expectations that epitaxial wafers pre-annealed at 880 oC would show the highest oxygen precipitation.
Acknowledgments The authors would like to thank SK Siltron Inc. for the materials, processing, measurement, and analytical cooperation.
References [1] M. Porrini, P. Collareta, G. Borionetti, D. Gambaro, I. Fini, Mater. Sci. Eng. B 73 (2000) 139-144. [2] E. Dornberger, D. Temmler, W. von Ammon, J. Electrochem. Soc. 149 (2002) G226-G231. [3] T. M. Pan, F. H. Ko, T. S. Chao, C. C. Chen, K. S. Chang-Liao, Electrochem. Solid-State Lett. 8 (2005) G201-G203. [4] C. Bigot, A. Danel, S. Thevenin, Solid State Phenom.108-109 (2005) 297-302. [5] F. Russo, G. Nardone, M. L. Polignano, A. D’Ercole, F. Pennella, M. D. Felice, A. D. Monte, A. Matarazzo, G. Moccia, G. Polsinelli, A. D’Angelo, M. Liverani, F. Irrera, ECS J. Solid State Sci. Technol. 6 (2017) P217-P226. [6] K. Saga, R. Ohno, D. Shibata, S. Kobayashi, K. Sueoka, ECS J. Solid State Sci. Technol. 4 (2015) P131-P136. [7] K. Kurita, T. Kadono, R. Okuyama, S. Shigemastu, R. Hirose, A. Onaka-Masada, Y. Koga, H. Okuda, Phys. Status Solidi A 214 (2017) 1700216. [8] K. Nakai, Y. Inoue, H. Yokota, A. Ikari, J. Takahashi, A. Tachikawa, K. Kitahara, Y. Ohta, W. Ohashi, J. Appl. Phys. 89 (2001) 4301-4309. [9] A. Borghesi, B. Pivac, A. Sassella, A. Stella, J. Appl. Phys. 77 (1995) 4169-4244. [10] G. Kissinger, G. Raming, R. Wahlich, T. Muller, Phys. B 407 (2012) 2993-2997. [11] K. Sueoka, M. Akatsuka, M. Yonemura, S. Sadamitsu, E. Asayama, T. Ono, Y. Koike, H. Katahama, Solid State Phenom. 69-70 (1999) 63-72. [12] H. Shimizu, Y. Sugino, N. Suzuki, S. Kiyota, K. Nagasawa, M. Fujita, K. Takeda, S. Isomae, Jpn. J. Appl. Phys. 36 (1997) 2565-2570. [13] R. Schmolke, D. Feijoo, R. Ostermeir, R. Schauer, S. Furukawa, D. Fräf, Electrochem. Soc. Proc. 98-1 (1998) 855-866. [14] R. C. Newman, J. Phys. : Condens. Matter 12 (2000) R335-R365. [15] G. Kissinger, Oxygen Precipitation in Silicon, in : Y. Yoshida, G. Langouche (Eds.), Defects and Impurities in Silicon materials, Springer, Heidelberg, 2015, pp. 273-341.
Highlights l The BMD effects of pre-annealing in polished and epitaxial wafers were different l The effect of pre-annealing temperature on the BMD was larger than time l Both COP free and COP crystals show the highest BMD at same pre-annealing temperature
Author contribution statements Kyu hyung Lee : Conceptualization, Validation, Investigation, Writing - Original Draft, Writing Review & Editing Don ha Hwang : Supervison, Conceptualization, Methodology Hee bog Kang : Project administration, Formal analysis Bo young Lee : Resources, Data Curation
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S0022-0248(19)30576-7 https://doi.org/10.1016/j.jcrysgro.2019.125361 CRYS 125361
To appear in:
Journal of Crystal Growth
Received Date: Revised Date: Accepted Date:
13 August 2019 13 November 2019 15 November 2019
Please cite this article as: K. hyung Lee, D. ha Hwang, H. bog Kang, B. young Lee, The effects of pre-annealing on oxygen precipitation in silicon P/P-epitaxial wafers, Journal of Crystal Growth (2019), doi: https://doi.org/ 10.1016/j.jcrysgro.2019.125361
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier B.V.
The effects of pre-annealing on oxygen precipitation in silicon P/P- epitaxial wafers
Kyu hyung Leea*, Don ha Hwanga, Hee bog Kanga, and Bo young Leea
a
Research Center, SK Siltron Inc., 435 Suchul-daero, Gumi-si, Gyeongsangbuk-do, 39387, Korea
*E-mail : [email protected]
Abstract Herein, the effect of pre-annealing on oxygen precipitation in a silicon P/P- epitaxial wafer was investigated using an experimental approach. Czochralski-grown silicon wafers were annealed at 800 – 900 oC to enhance their bulk micro defects (BMDs), before the epitaxial growth process. The effect of the pre-annealing temperature on oxygen precipitation was greater than that of the pre-annealing time, and the effect of pre-annealing temperature varied between polished and epitaxial wafers. Regarding pre-annealed epitaxial wafers that were annealed at low temperatures, the pre-annealing time also had as large an effect on BMD formation as the pre-annealing temperature. These experimental results were used to determine the optimal pre-annealing temperature for enhancing oxygen precipitation in epitaxial wafers. The experimental results regarding the optimal conditions were consistent with prior hypotheses. Crystal originated particles (COPs) and COP-free crystals exhibited their highest BMD density values at similar pre-annealing temperatures.
Keywords : A1. Defects, A1. Light scattering tomography, A3. Chemical vapor deposition processes, B2. Semiconducting silicon
1. Introduction Silicon wafers have predominantly been used for the fabrication of ultra-large-scale integrated (ULSI) devices. USLI device technology requires low levels of surface defects and metal contamination in the silicon wafer. It has been well established that surface defects on the polished wafer, such as crystal originated particles (COPs) and scratches, can cause device failures, such as gate oxide breakdown. [1] As silicon wafers with epitaxially-grown layers can control these surface defects, they can significantly improve device performance [2]. As the device features continues to shrink, metallic contaminants as well as surface defects in silicon wafers that have not been affected in the past can have detrimental effects on both device performance and yield. [3-5] Since the early days of the semiconductor industry, gettering techniques have been studied to eliminate or minimize the impact of metallic contamination on an electrical device’s performance. Gettering techniques can be categorized as being either internal (or intrinsic) or external (or extrinsic). For a 300 mm wafer, external gettering techniques such as ion implantation methods, trapping metal impurities in a high-dose implanted region, or dislocation loops, have been studied for complementary metal-oxide-semiconductor image sensor (CIS) devices. [6-7] However, it is necessary to strengthen internal gettering, so as to minimize the risk of metal contamination, as external gettering techniques (including ion implantation) are not normally used for 300 mm epitaxial wafers. Furthermore, in logic devices, boron gettering by heavy-boron doping is not possible, because P/P- epitaxial wafers are commonly used. As a result, in P/P- epitaxial wafers used in logic devices, internal gettering by oxygen precipitates plays a dominant role regarding effective sinks for harmful metal impurities. In order to improve the gettering efficiency, it is essential to increase the density of bulk micro defects (BMDs), which act as gettering sites. Various methods to maintain BMD density have been studied, such as increasing oxygen concentration or doping nitrogen. [8-10] In addition to the incorporation of impurities such as oxygen and nitrogen, heat treatment methods that are used before the epitaxial process have been explored as a method for increasing BMD density. [11] Most of the grown-in BMD nuclei that are produced during crystal growth are dissolved, because the epitaxial process is conducted at a high temperature
(above 1100 °C). The pre-annealing process, conducted prior to epitaxial growth, plays a role in generating and growing BMD nuclei by making them larger than the critical size necessary for surviving the epitaxial process. As a result, pre-annealed epitaxial wafers exhibit high BMD density values after device processing, allowing them to provide sufficient gettering efficiency. Lightly boron-doped COP crystals (that include nitrogen) are the most commonly-used substrate for epitaxial wafers for logic devices. With increasing epitaxial layer thickness, defects induced by COPs on the surface of the substrate gradually become covered, before finally disappearing completely. [1213] However, in case of epitaxial layer thicknesses that are below 3 mm, it is possible that COPs on the surface of the substrate can be transformed into extremely shallow pits, with depths in the range of 1 nm. These epitaxial defects are merely morphological defects, and are therefore not detrimental to gate oxide integrity. They can, however, adversely affect the device’s yields, such as its overlay and defocus. Epitaxial wafers that use substrates without COP defects are free from this risk. However, in order to replace the epitaxial wafers using COP crystals, it is important that epitaxial wafers made using COP-free substrates have a high and radially-uniform distribution of oxygen precipitates. Ensuring this distribution is also advantageous regarding the epitaxial defects mentioned above. This paper details an examination of the effects of pre-annealing on oxygen precipitation of P/P- epitaxial wafers, using both nitrogen-doped COP-free crystals and nitrogen-doped COP crystals. Focus is given to the pre-annealing conditions in P/P- epitaxial wafers that can most effectively increase BMD density, in terms of annealing temperature and annealing time.
2. Experimental details Boron-doped (1.1 × 1015 and 1.2 × 1015 atoms/cm3) (100) Czochralski-grown silicon crystals were studied, with both high and low oxygen concentrations. The high oxygen materials had an interstitial oxygen concentration ([Oi]) of 6.0 ×1017 atoms/cm3 (12 ppma, New ASTM). The low oxygen materials had an [Oi] of 5.2 × 1017 atoms/cm3 (10.4 ppma, New ASTM). Both the high and low oxygen groups had only pure silicon in the vacancy-dominant crystal region for each whole wafer (pure or COP-free silicon crystal). Another group was also boron-doped (at 1.3 × 1015 atoms/cm3), this
group consisted of Czochralski-grown silicon with an [Oi] of 5.7 × 1017 atoms/cm3 (11.4 ppma, New ASTM). In contrast to the COP-free crystal group, this group had vacancy-rich regions, including COPs, across each whole wafer (COP crystal). All samples, both COP-free crystal and COP crystal, had diameters of 300 mm, and were doped with nitrogen at a concentration of 3.1 × 10 12 ~ 5.1 × 1012 atoms/cm3. Within each ingot, the sample positions of group 2 (P-2 & E-2) and 3 (P-2 & E-3) were within ~ 50 mm, while those of group 1 (P-1 & E-1) were separated by a distance of ~ 200 mm. Due to the difference in sample intervals for each ingot, the difference in the concentrations of interstitial oxygen, boron, and nitrogen within the samples of group 1 were relatively larger than those of groups 2 and 3 (Table 1).
Table 1. Parameters of the wafers used for these studies. Group
[Oi (New ASTM, ppma)
[N] (ea/cm3)
[B] (ea/cm3)
Crystal (substrate)
P-1
10.22~10.60
3.3~3.8E12
1.20~1.24E15
COP free
P-2
11.96~12.14
3.1E12
1.14E15
COP free
P-3
11.44~11.68
5.09E12
1.32E15
COP
E-1
9.95~10.46
3.3~3.8E12
1.20~1.24E15
COP free
E-2
11.78~12.10
3.1E12
1.14E15
COP free
E-3
11.38~11.49
5.09E12
1.32E15
COP
Remark
Polished wafer
Epitaxial wafer
Pre-annealing conditions were selected regarding the temperature and time at which both BMD nucleation and growth could be satisfied at simultaneously. These tests were conducted with nine variants of conditions: three different temperatures (800, 850, and 900 oC) and three different times (20, 40, and 60 min). The pre-annealing process was carried out in a vertical furnace of ambient nitrogen; the push-in and pull-out temperatures were both 700 oC. The ramp-up and ramp-down rates of these tests were 5 oC/min and 3 oC/min, respectively. After pre-annealing and polishing, epitaxial layers with a thickness of approximately 2.5 μm were grown at a high temperature (above 1100 oC). In order to confirm the pre-annealing effect, BMD evaluations were carried out both before and after the epitaxial growth process, namely on both polished and epitaxial wafers. BMD evaluation was performed in order of [Oi] (initial state), heat treatment for BMD formation, [Oi] (final state), and
BMD measurements. Fourier transform infrared spectroscopy (FTIR) was used to measure [Oi] before and after heat treatment for BMD formation. Both polished and epitaxial wafers were subjected to the same heat treatment, logic device simulation heat cycle (pad oxidation simulation at 850 oC for 1 hr and pad nitride simulation at 750 oC for 1 hr) followed by BMD growth heat treatment (1000 oC 16 hr). Finally, an etching method, an optical microscopic evaluation after preferential etching (etching BMD, or E-BMD), and laser scattering tomography (LST) were performed to evaluate each sample’s BMD density.
3. Results and Discussion
Fig. 1. (a) Initial [Oi], (b) LST BMD density, and (c) E-BMD density of polished wafers with different pre-annealing temperatures. Each sample was pre-annealed for 40 min.
The effects of varying both the pre-annealing temperature and the pre-annealing time on the polished wafer were investigated experimentally. Fig. 1 shows the oxygen precipitation results of polished wafers under pre-annealing temperatures of (a) 800, (b) 850, and (c) 900 oC (for a fixed 40
min period). As the pre-annealing temperature was increased, both the LST and etching BMD density values of groups P-2 and P-3 decreased, but the LST density values of group P-1 showed different results at a pre-annealing temperature of 850 oC (Figs. 1 (b) and (c)). Unlike other groups (P-2 and P3), the LST BMD density of group P-1 at 850 oC in Fig. 1 (b) was higher than that at 800 oC; this is likely caused by the differences in the initial [Oi] and [N] between these samples. In the polished wafers, as-grown BMD nuclei formed as the ingots cooled and BMD nuclei generated by the preannealing process can contribute to BMD formation after heat treatment, as there was no high temperature process, such as epitaxial growth. As-grown BMD nuclei density of the pre-annealed wafer at 850 oC having the initial [Oi] (10.43 ppma) and [N] (3.77 × 1012 cm-3) were higher than those of the wafer at 800 oC (10.36 ppma and 3.3 × 1012 cm-3, respectively), this difference affected the LST BMD density after logic device heat treatment. Although the initial [Oi] (10.47 ppma) of group P-1 when pre-annealed at 900 oC ([N] of 3.36 × 1012 cm-3) was higher than when it was pre-annealed wafer at 850 oC, the BMD density decreased at 900 oC. This is determined that the effect of BMD density due to the pre-annealing temperature of 900 oC was greater than that of as-grown BMD density.
Fig. 2. (a) BMD size distributions for group P-1 samples pre-annealed at 800 and 850 oC (for a 40 min period), and E-BMD images corresponding to group P-1 pre-annealed at (b) 800 and (c) 850 oC.
This increase in BMD density due to higher oxygen and nitrogen concentrations was not observed in E-BMD results of group P-1 at 850 oC, however, this was determined to be due to the BMD density and size. (Fig. 2 (a)). As the LST BMD density and size of the wafer pre-annealed at 850 oC were higher and larger respectively than those of the wafer that was pre-annealed at 800 oC, it was observed to have combined with the surrounding BMD after preferential etching, resulting in a larger size but a lower density (Figs. 2 (b) and (c)). At the same pre-annealing temperature conditions, no significant difference was visible in the BMD density of each group across pre-annealing times of 20, 40, or 60 min.
Fig. 3. (a) Initial [Oi], (b) LST BMD density, and (c) E-BMD density values of epitaxial wafers with different pre-annealing temperatures for 40 min.
Fig. 3 displays the results of oxygen precipitation on epitaxial wafers prepared with pre-annealing temperatures of (a) 800, (b) 850, and (c) 900 oC, respectively, for a 40 min period. As the epitaxial
wafers underwent high temperatures during epitaxial growth, BMD nuclei smaller than the critical size could be dissolved. Therefore, the BMD trends of the epitaxial wafers showed different results from those of the polished wafers, due to the dissolution of small-size nuclei. In the case of polished wafers, the BMD density of the wafer prepared with a pre-annealing temperature of 800 °C was the highest, and BMD density tended to decrease as the pre-annealing temperature increased. The epitaxial wafers that were pre-annealed at 800 oC showed the lowest BMD density, however, this increased at 850 oC before decreasing again at 900 oC. All three of the groups showed similar results.
Fig. 4. (a) Initial [Oi], (b) LST BMD density, and (c) E-BMD density of samples with E-2 with different pre-annealing conditions (temperatures of 800, 850, and 900 oC, and times of 20, 40, and 60 min).
The effect of changing the pre-annealing time on oxygen precipitation was investigated. Fig. 4 shows the oxygen precipitation results for different pre-annealing times for the group E-2. At a relatively low pre-annealing temperature (800 oC), the increasing trend in BMD density was clear as
the pre-annealing time increased. However, for pre-annealing temperatures of 850 and 900 oC, oxygen precipitation did not increase even when the pre-annealing time increased. This may be related the size of the BMD nuclei generated in the pre-annealing process prior to the epitaxial growth process. As the diffusivity of oxygen atoms in silicon at pre-annealing temperature of 800 oC is relatively slower than that of higher temperatures, the density of BMD nuclei that exceed the critical size to survive during epitaxial growth process is also lower than that of the higher pre-annealing temperatures. However, as the pre-annealing time increases, the diffusion distance of oxygen increases, which leads to an increase in BMD nuclei density that are above the critical size. The increase in BMD density with increasing time is not observed, as oxygen diffusion under preannealing conditions of 850 oC or higher are sufficient to grow nuclei above the critical size during only 20 minutes of pre-annealing [14-15].
Fig. 5. LST BMD density as a function of delta [Oi] in epitaxial wafers (group E-1, E-2, and E-3).
The relationship between the change in oxygen concentration (delta [Oi]; the difference between
the initial [Oi] and the residual [Oi] after heat treatment), and BMD density was also investigated. Fig. 5 shows the relationships between delta [Oi] and LST BMD density for all of the sample groups (groups E-1, E-2, and E-3). As delta [Oi] increased, the LST BMD density also increased, but this relationship appears to almost become saturated when delta [Oi] reaches about 1 ppma. This means that it is possible to produce wafers that require a specific BMD range by controlling the delta [Oi] levels. In the heat treatment used in this experiment, if the wafer were designed to have a delta [Oi] of 0.5 ppma or higher, a wafer with a BMD density of 3 × 109 ea/cm3 or higher would be produced.
Fig. 6. Contour plots of (a) delta [Oi] and (b) LST BMD density as a function of pre-annealing temperature and time for groups E-1, E-2, and E-3.
Fig. 6 displays contour plots of delta [Oi] and LST BMD density as a function of pre-annealing temperature and time for the epitaxial wafers. The influence of the pre-annealing temperature on delta [Oi] and BMD density differed slightly depending on the pre-annealing time, but the overall trends in oxygen precipitation are similar. As the pre-anneal temperature increased, both the delta [Oi] and BMD density rose as well, peaking at around 875 oC and then decreasing at higher temperatures (Figs. 6 (a) and (b)). However, there is a slight difference in oxygen precipitation between the epitaxial wafer made using COP-free crystal and the epitaxial wafer using COP crystal, due to the differences in vacancy concentrations between the two. It was confirmed that the effect of the pre-annealing time on BMD density was weaker than that of pre-annealing temperature. In other words, as the pre-annealing time increased, the BMD density gradually increased, but the absolute change in BMD density was small. As shown in Fig. 6, the preannealing temperature with the highest oxygen precipitation increased as the pre-annealing time decreased. This occurred because a higher pre-annealing temperature was required for BMDs to exceed the critical size of the epitaxial growth process when the pre-annealing time was short. It was also observed that the pre-annealing temperatures with the highest delta [Oi] values were slightly shifted to the higher temperatures, as the initial [Oi] level was lower (20 min pre-annealing conditions for E-1 and E-2 wafers). A pre-annealing temperature of 880 oC was selected as the optimal preannealing condition, in consideration of the initial [Oi] level and oxygen precipitation results. Additional tests were then conducted using the group E-1 samples.
Fig. 7. (a) Delta [Oi] and (b) LST BMD density values for group E-1 at different pre-annealing temperatures (fixed period of 40 min). The dotted line in (a) indicates a fitting curve for the preannealing temperature at 850 oC.
Fig. 7 shows the delta [Oi] and LST BMD density results of Group E-1 samples at different preannealing temperatures (for a fixed 40 min period), including 880 oC. As shown in Fig. 7 (a), the delta [Oi] for the pre-annealed wafer at 880 oC was equal to or higher than those of the other conditions. Considering the initial [Oi], the delta [Oi] levels of the wafer that was pre-annealed at 880 oC were higher than those of the pre-annealed wafers at 800 and 900 oC, and were similar to the group that was pre-annealed 850 oC. The trends in the LST BMD density were also similar to those observed in the delta [Oi] results (Fig. 7 (b)). These results are in good agreement with the expectation that epitaxial wafers pre-annealed at 880 oC will have the highest oxygen precipitation. It is thought that the effect of the pre-annealing temperature on the oxygen precipitation may be well revealed under shorter preannealing conditions.
4. Conclusions Pre-annealing effects on oxygen precipitation were investigated regarding BMD density and delta [Oi] evaluations. The BMD density results of polished wafers differed from those of epitaxial wafers. The polished wafers showed higher BMD densities in the pre-annealed group at low temperatures (800 oC) because there was no high temperature process in which the BMD nuclei could be dissolved. Unlike the polished wafers, the epitaxial wafers exhibited higher BMD densities at pre-annealing temperatures above 800 oC. The pre-annealing temperature had a greater effect on oxygen precipitation in P/P- epitaxial wafers than the pre-annealing time. Based on the delta [Oi] and BMD density results, it was deduced that the optimal pre-annealing temperature for enhancing oxygen precipitation in the epitaxial wafer was approximately 880 oC. Both epitaxial wafers using COP-free crystal and also epitaxial wafers using COP crystal showed their highest BMD density values at a similar pre-annealing temperature. In
additional experiments, the results of oxygen precipitation were consistent with expectations that epitaxial wafers pre-annealed at 880 oC would show the highest oxygen precipitation.
Acknowledgments The authors would like to thank SK Siltron Inc. for the materials, processing, measurement, and analytical cooperation.
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Highlights l The BMD effects of pre-annealing in polished and epitaxial wafers were different l The effect of pre-annealing temperature on the BMD was larger than time l Both COP free and COP crystals show the highest BMD at same pre-annealing temperature
Author contribution statements Kyu hyung Lee : Conceptualization, Validation, Investigation, Writing - Original Draft, Writing Review & Editing Don ha Hwang : Supervison, Conceptualization, Methodology Hee bog Kang : Project administration, Formal analysis Bo young Lee : Resources, Data Curation
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