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Oxygen precipitation in floating-zone silicon grown in hydrogen ambience and its application
Materials Science and Engineering B83 (2001) 106– 110 www.elsevier.com/locate/mseb
Oxygen precipitation in floating-zone silicon grown in hydrogen ambience and its application Huaixiang Li a,*, Chuanbo Li a, Yuejiao He a, Guirong Liu b, Yansheng Chen b, Shuzhen Duan b b
a Institute of Semiconductors, Shandong Teachers Uni6ersity, Jinan 250014, People’s Republic of China Department of Physical Chemistry, Uni6ersity of Science and Technology Beijing, Beijing 100083, People’s Republic of China
Received 12 September 2000; accepted 18 December 2000
Abstract Oxygen precipitates in the floating-zone silicon in hydrogen ambience [FZ(H) Si] and in the neutron transmutation doping (NTD) FZ(H) Si were investigated by infrared (IR) spectroscopy at room temperature. In the intermediate temperature range, 600–850°C, the apparent activation energies of 1.4 and 1.2 eV were derived from Arrhenius plots of the product of the absorbance at 1230 cm − 1 and the half-peak breadth for the formation of oxygen precipitates in the FZ(H) Si and in the NTD FZ(H) Si, respectively. The high temperature stability of the oxygen precipitates was only in the NTD FZ(H) Si. A denuded zone was obtained by denuding annealing and precipitating annealing the NTD FZ(H) Si wafers. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Oxygen precipitates; Silicon; NTD; Denuded zone
1. Introduction Czochralski (CZ) silicon wafers are widely used in the semiconductor industry for integrated circuit (IC) fabrication. CZ silicon wafers contain about 1018 oxygen atoms cm − 3, which are incorporated during crystal growth [1]. Since the oxygen in as-grown CZ-Si crystals is usually supersaturated, subsequent thermal treatments [2] cause precipitation of the oxygen and the oxygen related defects. Intrinsic gettering (IG) techniques that utilize oxygen related defects are likely to be important [3] for large diameter (300 mm) silicon wafers used in IC device fabrication, therefore, an understanding of oxygen precipitates behavior is important. In the floating-zone (FZ) technique a radio-frequency (RF) coil is used to melt a narrow zone in a vertically mounted polycrystalline silicon rod. Argon generally is used as ambience during the crystal growth. Since the container or crucible is not directly in contact with the * Corresponding author. Tel.: + 86-531-2962894. E-mail address: [email protected] (H. Li).
molten zone, the oxygen concentration in the FZ-Si is comparatively low and its typical value [1] is l015 –5× 1016 atoms cm − 3. The precipitation of interstitial oxygen might be hard to occur within this range of oxygen concentration. No technique has been developed to incorporate significant oxygen amounts, which is a prerequisite for successful ultra-large-scale-integration (ULSI) device manufacture. The properties of hydrogen impurities in silicon are currently of considerably interest. Hydrogen is the simplest element, but with the variety of its effects on the properties of the host and with its elusiveness it represents quite a challenge for the physics and technology of defects in silicon [4]. Hydrogen in bulk silicon can passivate dopants and deep-level defects [5]. Atomic hydrogen can bond defects and/or other impurity atoms in silicon to form many kinds of hydrogen-related defect-impurity complexes [6], which can give rise to many infrared (IR) vibrational bands. For example, a large number of SiH stretching, wagging and bending vibration IR bands in the 500 –2300 cm − 1 wave number range have been reported and hydrogen-defects complex donors [7] are formed within the annealing
0921-5107/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 1 ) 0 0 5 0 2 - 5
H. Li et al. / Materials Science and Engineering B83 (2001) 106–110
temperature range of 350– 600°C. We have previously shown that hydrogen can promote the formation of hydrogen–oxygen complexes donor [8] and catalyze oxygen precipitation in crystalline silicon [9]. In this work, we will present the formation and dissolution of oxygen precipitates in the crystalline silicon grown in hydrogen ambience by FZ technique.
2. Experimental A polycrystalline silicon rod was etched for 5–10 min in the solution of 49% HF:67% HNO3 =1:4 (v/v) and
Fig. 1. Typical Infrared absorbance spectra of FZ(H) Si with 10 mm thickness (A): sample annealed at 750°C for 4 h; (B): as-grown crystal.
107
then cleaned by de-ionized water. A single crystal silicon ingot was grown from the cleaned and dried rod in a hydrogen ambience (1 atm.) by the floating-zone technique. The growth direction, growth rate, and seed rotational speed were [111], 5.5 mm min − 1, and 15 rpm, respectively. The as-grown crystal silicon ingot in 36 mm diameter was n-type with resistivities of 350–450 Vcm. The interstitial oxygen in a concentration of 2.8×1016 atoms cm − 3 was determined at room temperature by IR absorption spectroscopy with a 1106 cm − 1 absorption line and a conversion factor of 3.14× 1017 atoms cm − 2. The carbon concentration in the as-grown ingot was 2.2× 1016cm − 3, found by 606 cm − 1 IR absorption line. The ingot was divided into two pieces, one was used as the as-grown crystal denoted as FZ(H) Si, and another was irradiated with neutron at room temperature in a light water reactor at China Institute of Atomic Energy. The total thermal neutron doses were 5.9× 1017 neutrons cm − 2. The thermal/fast neutron ratio was 10. The ingots of FZ(H) Si and NTD FZ(H) Si were cut into the studied samples with the thickness of about 10, 2, and 0.4 mm. These samples were ground, polished smooth and then cleaned by boiling in solution I (30% H2O2:27% NH4OH:H2O= 1:1:5, v/v), in solution II (30% H2O2:37% HCl:H2O= 1:1:5, v/v), and in deionized water. Annealing was carried out in a quartz tube in high-purity argon ambient and the temperature was controlled by a temperature controller (Model JWT-702) with a platinum–rhodium/platinum thermocouple. The annealed samples were reground and repolished to a mirror surface on both sides before the IR measurements. The IR spectra were obtained at room temperature using a Perkin Elmer 983G spectrometer. The incident direction was [111] and vacuum floating-zone silicon materials with the same thickness were used as control. The denuded zone (DZ) was detected using photo-metallurgical microscope after Secco solution (49% HF:0.l5 mol dm − 3K2Cr2O7,= 2:1, v/v) etching.
3. Results and discussion
3.1. Oxygen precipitates at 750 °C
Fig. 2. The relative absorbance intensity variation of the peaks at 1106 and 1230 cm − 1 in IR spectra for 2 mm thick samples annealed at 750°C for different hours: ---, relative absorbance at 1230 cm − 1 for FZ(H) Si; ---, relative absorbance at 1106 cm − 1 for FZ(H) Si;
— , relative absorbance at 1230 cm − 1 for NTD FZ(H) Si; —, relative absorbance at 1106 cm − 1 for NTD FZ(H) Si.
Fig. 1(A) shows IR absorbance spectra for FZ(H) Si annealed at 750°C for 4 h. The peak at 1106 cm − 1 corresponds to the interstitial oxygen and the broader band near 1230 cm − 1 is associated with the oxygen precipitates in the crystal silicon [10]. From the IR measurements, the intensities of 1230 cm − 1 bands increase while the intensities of the 1106 cm − 1 decrease with the increase of annealing time at 750°C. Similar variation tendency occurs in the IR absorbance spectra
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H. Li et al. / Materials Science and Engineering B83 (2001) 106–110
Table 1 The temperature dependence of the product (R) of the absorbance (ABS) at the1230 cm−1 and the half-peak breadth (HPB) from the IR-absorbance measurements Annealing temperature (°C)
600 650 700 750 800 850
FZ(H) Si
NTD FZ(H) Si
ABS
HPB
R
ABS
HPB
R
– 0.037 0.071 0.085 0.097 0.092
–
–
50 88 125 130 125
1.85 6.25 10.62 12.61 11.50
0.024 0.036 0.060 0.069 0.072 –
42 49 67 70 112 –
1.01 1.76 4.02 4.83 8.06 –
for NTD FZ(H) Si. Fig. 2 shows the variation of relative intensity in IR spectra for both samples, FZ(H) Si and NTD FZ(H) Si annealed at 750°C for various duration. Oxygen precipitates generally depends on the interstitial oxygen concentration in the silicon and the annealing conditions, such as temperature, duration. The interstitial oxygen, in the concentration of 2.8× 1016 atoms cm − 3 for the studied samples, is still supersaturated at 750°C from the point of solubility limit [11] of oxygen atoms in the silicon crystal. Fig. 2 shows that the interstitial oxygen precipitates in a couple of hours for both crystals, FZ(H) Si and NTD FZ(H) Si annealed at 750°C, which could be attributed to hydrogen catalysis as we have reported [9] before. The hydrogen was introduced in the process of FZ(R) Si crystal growth due to the hydrogen ambience. As shown in Fig. 1(B), the peaks of 1945, 2120 and 2210 cm − 1 correspond to the stretching vibration of SiH bonds [7] in the FZ(H) Si crystal. Under the effect of hydrogen catalysis, short time duration for the precipitation of interstitial oxygen in the FZ(H) Si forms a striking contrast to CZ silicon where the oxygen precipitation requires an annealing for a few dozen hours or for more. As for shorter time duration for oxygen precipitates in the NTD FZ(H) Si it seems that neutron irradiation also enhances the precipitation of interstitial oxygen.
tra measurements shows the rate (R) for the formation of oxygen precipitates, we can obtain the data listed in Table 1. Fig. 3 shows plots of natural logarithm of the product, ln(R), versus reciprocal absolute temperature, T − 1 Apparent activation energies, Ea, derived from the Arrhenius plots as shown in Fig. 3 are about 1.4 and 1.2 eV for the oxygen precipitation in the FZ(H) Si and in the NTD FZ(R) Si, respectively. In neutron irradiated silicon, NTD FZ(H) Si, there are a lot of vacancy type radiation defects and secondary defects such as V, V2, V4, V5, vacancy oxygen (V-O), and disordered regions[6]. Neutron radiation can promote oxygen atoms to diffuse towards disordered regions due to the stress field around them. The V-O defects are easy to form near the disordered regions [7]. With increase of annealing temperature, from 600 to 800°C, the oxygen atoms become more movable, the V-O defects can release vacancies and become interstitial oxygen atoms (Oi). The Oi and oxygen atoms agglomerated around
3.2. Temperature dependence of the absorbance at 1230 cm − 1 in IR spectra In the temperature range of 600– 850°C, the 1230 cm − 1 band in absorptive intensity increases with the increase at annealing temperature, which could give some kinetics for the oxygen precipitation in FZ(H) Si and NTD FZ(R) Si annealed at the intermediate temperature. The intensity of the 1230 cm − 1 bands in IR-absorbance spectra is suggested to indicate the relative content of the precipitates. Assume that the product of the absorbance (ABS) at l230 cm − 1 and the half-peak breadth (HPB) from the IR-absorbance spec-
Fig. 3. Arrhenius plot of the product (R) of the absorbance (ABS) at 1230 cm − 1 and the half-peak breadth (HPB) from the IR-absorbance measurements.
H. Li et al. / Materials Science and Engineering B83 (2001) 106–110
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3.3. Dissolution of the oxygen precipitates in the FZ(H) Si at high temperature
Fig. 4. IR-transmittance spectra for FZ(H) Si annealed at: (A) 750°C 4 h; (B) 750°C 4 h+ 1000 l h; (C)750°C 4 h +1000°C 4 h.
The FZ(H) Si samples were annealed at 750°C for 4 h to form enough oxygen precipitates which were indicated by the 1230 cm − 1 in IR spectra. If the annealed samples were once again annealed at 1000°C, the l230 cm − 1 band shrunk while the 1106 cm − 1 peak increased correspondingly with the time duration of second step heating treatment. There was no clear signal near 1230 and the 1106 cm − 1 peak was almost recovered to the position as in the as-grown crystal samples if the postheating time was over 4 h. As shown in Fig. 4, this variation is proved that the oxygen precipitates dissolves in the second step heating treatment. It can be explained that the solubility limit [1,11] of oxygen atoms in silicon crystal at 1000°C is greater than the real oxygen concentration in the FZ(R) Si so the precipitates dissolve.
3.4. High temperature stability of the oxygen precipitates and denuded zone
Fig. 5. Optical micrographs of cleaved cross-section of denuded wafers, 0.43 mm thickness, etched in a Secco solution of HF (49%): K2Cr2O7 (0.l5 mol dm − 3)= 2:1, (v/v) for 10 s; annealing conditions for samples are: (A) 940°C 2 h+750°C 4 h; (B) 940°C 2 h+750°C 4 h + 1100°C 2 h.
disordered regions can diffuse into them. In this way, neutron radiation can promote the formation of oxygen precipitation by considering the phenomenon as diffusion limited [1]. The inference is in accordance with the difference of the apparent activation energies of the rate (R) for the FZ(H) Si and the NTD FZ(H) Si as shown in Fig. 3.
The intensity of 1230 cm − 1 bands in IR spectra remained unchanged for the NTD FZ(H) Si when the samples with oxygen precipitates were re-annealed for 2 to 6 h. It was evident that there was no dissolution of the oxygen precipitates at 1000°C and that neutron irradiation played a role to retard the dissolution of the precipitates in silicon. In contrast to the FZ(H) Si, the reason why the precipitates in the NTD Si are stable can be attributed to the hydrogen exhausted partially during the NTD as proved before [9]. Furthermore, 0.43 mm thick wafers of the NTD FZ(H) Si were heat-treated for 940°C 2 h denuding annealing and 750°C 4 h precipitating annealing, a denuded zone and defective zone could be obtained as shown in Fig. 5 A. Moreover, the denuded zone, defective zone, and the 1230 cm − 1 in the IR spectrum remain distinctly after a re-annealing at 1100°C for 2 h as shown in Fig. 5 B and in Fig. 6, respectively. In addition, the wafers of NTD FZ(H) Si with denuded zone were used to fabricate the silicon target vidicon in a factory test. The main working characteristic parameters of the NTD FZ(H) Si target vidicon were as follows: working current, 450 mA; working voltage of the modulation electrode, 45 V, maximum signal current, 1.7 mA; center resolution of the vidicon, 550 TVL.
4. Conclusion Single crystal silicon ingots were grown in hydrogen ambience by floating-zone technique. The concentration of interstitial oxygen in the as-grown crystal silicon was
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post-heating treatment. By denuding annealing and precipitating annealing the NTD FZ(H) Si wafers, a denuded zone layer and bulk micro-defect region have been obtained and they can be expected to act as an intrinsic getter.
Acknowledgements The financial supports of the project by the Natural Science Foundation of China (No: 69971014, 69890221) and Natural Science Foundation of Shandong Province (No: Y98G03099), are gratefully acknowledged. Fig. 6. Infrared transmittance spectra of NTD FZ(H) Si wafers annealed at 940°C 2 h+750°C 4 h+ 1100°C 2 h.
References
about 2.8× 1016 atoms cm − 3 by IR 1106 cm − 1 absorption line and a conversion factor of 3.14×1017 atoms cm − 2. Neutron transmutation doping was used to the FZ(H) Si with the total thermal neutron doses of 5.9×l017 neutrons cm − 2. Oxygen precipitation has been studied by IR-absorption band near l230 cm − 1 after annealing FZ(H) Si and NTD FZ(H) Si. In the temperature range of 600– 850°C, apparent activation energies of the formation of oxygen precipitates were 1.4 and 1.2 eV for FZ(H) Si and NTD FZ(H) Si, respectively, which were derived from Arrhenius plots of the product of the absorbance at 1230 cm − 1 and half-peak breadth in the IR-absorbance spectra. These results indicate that neutron radiation can promote the formation of oxygen precipitation. A dissolution of the oxygen precipitates occurred in the FZ(H) Si but not in the NTD FZ(H) Si after a high temperature (1000°C)
[1] A. Borghesi, B. Pivac, A. Sassella, A. Stella, J. Appl. Phys. 77 (1995) 4169 – 4244. [2] H. Abe, I. Suzuki, H. Koya, J. Electrochem. Soc. 144 (1997) 306 – 311. [3] T. Ono, G.A. Rozgonyi, C. Au, T. Messina, R.K. Goodall, H.R. Huff, J. Electrochem. Soc. 146 (1999) 3807 – 3811. [4] P. Deak, M. Heiurich, L.C. Snyder, J.W. Corbett, Mat. Sci. Eng. B4 (1989) 57 – 62. [5] S.J. Pearton, J.W. Corbett, T.S. Shi, Appl. Phys. A 43 (1987) 153 – 195. [6] X.T. Meng, J.W. Xiong, Y.C. Du, Phys. Stat. Sol. (a) 143 (1994) k67 – k70. [7] X.T. Meng, G.G. Qin, Y.C. Du, Y.F. Zhang, J. Appl. Phys. 63 (1988) 5606 – 5608. [8] H.X. Li, Y.S. Chen, G.R. Liu, S.Z. Duan, Rare Metals 15 (1996) 288 – 291. [9] H. Li, C. Xue, L. Chen, W. Zhou, G. Liu, Y. Chen, S. Duan, Mat. Sci. Eng. B72 (2000) 105 – 108. [10] S.M. Hu, Appl. Phys. Lett. 48 (1986) 115 – 117. [11] K.F. Kelton, R. Falster, D. Gambaro, M. Olmo, M. Cornara, P.F. Wei, J. Appl. Phys. 85 (1999) 8097 – 8111.
.
Oxygen precipitation in floating-zone silicon grown in hydrogen ambience and its application Huaixiang Li a,*, Chuanbo Li a, Yuejiao He a, Guirong Liu b, Yansheng Chen b, Shuzhen Duan b b
a Institute of Semiconductors, Shandong Teachers Uni6ersity, Jinan 250014, People’s Republic of China Department of Physical Chemistry, Uni6ersity of Science and Technology Beijing, Beijing 100083, People’s Republic of China
Received 12 September 2000; accepted 18 December 2000
Abstract Oxygen precipitates in the floating-zone silicon in hydrogen ambience [FZ(H) Si] and in the neutron transmutation doping (NTD) FZ(H) Si were investigated by infrared (IR) spectroscopy at room temperature. In the intermediate temperature range, 600–850°C, the apparent activation energies of 1.4 and 1.2 eV were derived from Arrhenius plots of the product of the absorbance at 1230 cm − 1 and the half-peak breadth for the formation of oxygen precipitates in the FZ(H) Si and in the NTD FZ(H) Si, respectively. The high temperature stability of the oxygen precipitates was only in the NTD FZ(H) Si. A denuded zone was obtained by denuding annealing and precipitating annealing the NTD FZ(H) Si wafers. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Oxygen precipitates; Silicon; NTD; Denuded zone
1. Introduction Czochralski (CZ) silicon wafers are widely used in the semiconductor industry for integrated circuit (IC) fabrication. CZ silicon wafers contain about 1018 oxygen atoms cm − 3, which are incorporated during crystal growth [1]. Since the oxygen in as-grown CZ-Si crystals is usually supersaturated, subsequent thermal treatments [2] cause precipitation of the oxygen and the oxygen related defects. Intrinsic gettering (IG) techniques that utilize oxygen related defects are likely to be important [3] for large diameter (300 mm) silicon wafers used in IC device fabrication, therefore, an understanding of oxygen precipitates behavior is important. In the floating-zone (FZ) technique a radio-frequency (RF) coil is used to melt a narrow zone in a vertically mounted polycrystalline silicon rod. Argon generally is used as ambience during the crystal growth. Since the container or crucible is not directly in contact with the * Corresponding author. Tel.: + 86-531-2962894. E-mail address: [email protected] (H. Li).
molten zone, the oxygen concentration in the FZ-Si is comparatively low and its typical value [1] is l015 –5× 1016 atoms cm − 3. The precipitation of interstitial oxygen might be hard to occur within this range of oxygen concentration. No technique has been developed to incorporate significant oxygen amounts, which is a prerequisite for successful ultra-large-scale-integration (ULSI) device manufacture. The properties of hydrogen impurities in silicon are currently of considerably interest. Hydrogen is the simplest element, but with the variety of its effects on the properties of the host and with its elusiveness it represents quite a challenge for the physics and technology of defects in silicon [4]. Hydrogen in bulk silicon can passivate dopants and deep-level defects [5]. Atomic hydrogen can bond defects and/or other impurity atoms in silicon to form many kinds of hydrogen-related defect-impurity complexes [6], which can give rise to many infrared (IR) vibrational bands. For example, a large number of SiH stretching, wagging and bending vibration IR bands in the 500 –2300 cm − 1 wave number range have been reported and hydrogen-defects complex donors [7] are formed within the annealing
0921-5107/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 1 ) 0 0 5 0 2 - 5
H. Li et al. / Materials Science and Engineering B83 (2001) 106–110
temperature range of 350– 600°C. We have previously shown that hydrogen can promote the formation of hydrogen–oxygen complexes donor [8] and catalyze oxygen precipitation in crystalline silicon [9]. In this work, we will present the formation and dissolution of oxygen precipitates in the crystalline silicon grown in hydrogen ambience by FZ technique.
2. Experimental A polycrystalline silicon rod was etched for 5–10 min in the solution of 49% HF:67% HNO3 =1:4 (v/v) and
Fig. 1. Typical Infrared absorbance spectra of FZ(H) Si with 10 mm thickness (A): sample annealed at 750°C for 4 h; (B): as-grown crystal.
107
then cleaned by de-ionized water. A single crystal silicon ingot was grown from the cleaned and dried rod in a hydrogen ambience (1 atm.) by the floating-zone technique. The growth direction, growth rate, and seed rotational speed were [111], 5.5 mm min − 1, and 15 rpm, respectively. The as-grown crystal silicon ingot in 36 mm diameter was n-type with resistivities of 350–450 Vcm. The interstitial oxygen in a concentration of 2.8×1016 atoms cm − 3 was determined at room temperature by IR absorption spectroscopy with a 1106 cm − 1 absorption line and a conversion factor of 3.14× 1017 atoms cm − 2. The carbon concentration in the as-grown ingot was 2.2× 1016cm − 3, found by 606 cm − 1 IR absorption line. The ingot was divided into two pieces, one was used as the as-grown crystal denoted as FZ(H) Si, and another was irradiated with neutron at room temperature in a light water reactor at China Institute of Atomic Energy. The total thermal neutron doses were 5.9× 1017 neutrons cm − 2. The thermal/fast neutron ratio was 10. The ingots of FZ(H) Si and NTD FZ(H) Si were cut into the studied samples with the thickness of about 10, 2, and 0.4 mm. These samples were ground, polished smooth and then cleaned by boiling in solution I (30% H2O2:27% NH4OH:H2O= 1:1:5, v/v), in solution II (30% H2O2:37% HCl:H2O= 1:1:5, v/v), and in deionized water. Annealing was carried out in a quartz tube in high-purity argon ambient and the temperature was controlled by a temperature controller (Model JWT-702) with a platinum–rhodium/platinum thermocouple. The annealed samples were reground and repolished to a mirror surface on both sides before the IR measurements. The IR spectra were obtained at room temperature using a Perkin Elmer 983G spectrometer. The incident direction was [111] and vacuum floating-zone silicon materials with the same thickness were used as control. The denuded zone (DZ) was detected using photo-metallurgical microscope after Secco solution (49% HF:0.l5 mol dm − 3K2Cr2O7,= 2:1, v/v) etching.
3. Results and discussion
3.1. Oxygen precipitates at 750 °C
Fig. 2. The relative absorbance intensity variation of the peaks at 1106 and 1230 cm − 1 in IR spectra for 2 mm thick samples annealed at 750°C for different hours: ---, relative absorbance at 1230 cm − 1 for FZ(H) Si; ---, relative absorbance at 1106 cm − 1 for FZ(H) Si;
— , relative absorbance at 1230 cm − 1 for NTD FZ(H) Si; —, relative absorbance at 1106 cm − 1 for NTD FZ(H) Si.
Fig. 1(A) shows IR absorbance spectra for FZ(H) Si annealed at 750°C for 4 h. The peak at 1106 cm − 1 corresponds to the interstitial oxygen and the broader band near 1230 cm − 1 is associated with the oxygen precipitates in the crystal silicon [10]. From the IR measurements, the intensities of 1230 cm − 1 bands increase while the intensities of the 1106 cm − 1 decrease with the increase of annealing time at 750°C. Similar variation tendency occurs in the IR absorbance spectra
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H. Li et al. / Materials Science and Engineering B83 (2001) 106–110
Table 1 The temperature dependence of the product (R) of the absorbance (ABS) at the1230 cm−1 and the half-peak breadth (HPB) from the IR-absorbance measurements Annealing temperature (°C)
600 650 700 750 800 850
FZ(H) Si
NTD FZ(H) Si
ABS
HPB
R
ABS
HPB
R
– 0.037 0.071 0.085 0.097 0.092
–
–
50 88 125 130 125
1.85 6.25 10.62 12.61 11.50
0.024 0.036 0.060 0.069 0.072 –
42 49 67 70 112 –
1.01 1.76 4.02 4.83 8.06 –
for NTD FZ(H) Si. Fig. 2 shows the variation of relative intensity in IR spectra for both samples, FZ(H) Si and NTD FZ(H) Si annealed at 750°C for various duration. Oxygen precipitates generally depends on the interstitial oxygen concentration in the silicon and the annealing conditions, such as temperature, duration. The interstitial oxygen, in the concentration of 2.8× 1016 atoms cm − 3 for the studied samples, is still supersaturated at 750°C from the point of solubility limit [11] of oxygen atoms in the silicon crystal. Fig. 2 shows that the interstitial oxygen precipitates in a couple of hours for both crystals, FZ(H) Si and NTD FZ(H) Si annealed at 750°C, which could be attributed to hydrogen catalysis as we have reported [9] before. The hydrogen was introduced in the process of FZ(R) Si crystal growth due to the hydrogen ambience. As shown in Fig. 1(B), the peaks of 1945, 2120 and 2210 cm − 1 correspond to the stretching vibration of SiH bonds [7] in the FZ(H) Si crystal. Under the effect of hydrogen catalysis, short time duration for the precipitation of interstitial oxygen in the FZ(H) Si forms a striking contrast to CZ silicon where the oxygen precipitation requires an annealing for a few dozen hours or for more. As for shorter time duration for oxygen precipitates in the NTD FZ(H) Si it seems that neutron irradiation also enhances the precipitation of interstitial oxygen.
tra measurements shows the rate (R) for the formation of oxygen precipitates, we can obtain the data listed in Table 1. Fig. 3 shows plots of natural logarithm of the product, ln(R), versus reciprocal absolute temperature, T − 1 Apparent activation energies, Ea, derived from the Arrhenius plots as shown in Fig. 3 are about 1.4 and 1.2 eV for the oxygen precipitation in the FZ(H) Si and in the NTD FZ(R) Si, respectively. In neutron irradiated silicon, NTD FZ(H) Si, there are a lot of vacancy type radiation defects and secondary defects such as V, V2, V4, V5, vacancy oxygen (V-O), and disordered regions[6]. Neutron radiation can promote oxygen atoms to diffuse towards disordered regions due to the stress field around them. The V-O defects are easy to form near the disordered regions [7]. With increase of annealing temperature, from 600 to 800°C, the oxygen atoms become more movable, the V-O defects can release vacancies and become interstitial oxygen atoms (Oi). The Oi and oxygen atoms agglomerated around
3.2. Temperature dependence of the absorbance at 1230 cm − 1 in IR spectra In the temperature range of 600– 850°C, the 1230 cm − 1 band in absorptive intensity increases with the increase at annealing temperature, which could give some kinetics for the oxygen precipitation in FZ(H) Si and NTD FZ(R) Si annealed at the intermediate temperature. The intensity of the 1230 cm − 1 bands in IR-absorbance spectra is suggested to indicate the relative content of the precipitates. Assume that the product of the absorbance (ABS) at l230 cm − 1 and the half-peak breadth (HPB) from the IR-absorbance spec-
Fig. 3. Arrhenius plot of the product (R) of the absorbance (ABS) at 1230 cm − 1 and the half-peak breadth (HPB) from the IR-absorbance measurements.
H. Li et al. / Materials Science and Engineering B83 (2001) 106–110
109
3.3. Dissolution of the oxygen precipitates in the FZ(H) Si at high temperature
Fig. 4. IR-transmittance spectra for FZ(H) Si annealed at: (A) 750°C 4 h; (B) 750°C 4 h+ 1000 l h; (C)750°C 4 h +1000°C 4 h.
The FZ(H) Si samples were annealed at 750°C for 4 h to form enough oxygen precipitates which were indicated by the 1230 cm − 1 in IR spectra. If the annealed samples were once again annealed at 1000°C, the l230 cm − 1 band shrunk while the 1106 cm − 1 peak increased correspondingly with the time duration of second step heating treatment. There was no clear signal near 1230 and the 1106 cm − 1 peak was almost recovered to the position as in the as-grown crystal samples if the postheating time was over 4 h. As shown in Fig. 4, this variation is proved that the oxygen precipitates dissolves in the second step heating treatment. It can be explained that the solubility limit [1,11] of oxygen atoms in silicon crystal at 1000°C is greater than the real oxygen concentration in the FZ(R) Si so the precipitates dissolve.
3.4. High temperature stability of the oxygen precipitates and denuded zone
Fig. 5. Optical micrographs of cleaved cross-section of denuded wafers, 0.43 mm thickness, etched in a Secco solution of HF (49%): K2Cr2O7 (0.l5 mol dm − 3)= 2:1, (v/v) for 10 s; annealing conditions for samples are: (A) 940°C 2 h+750°C 4 h; (B) 940°C 2 h+750°C 4 h + 1100°C 2 h.
disordered regions can diffuse into them. In this way, neutron radiation can promote the formation of oxygen precipitation by considering the phenomenon as diffusion limited [1]. The inference is in accordance with the difference of the apparent activation energies of the rate (R) for the FZ(H) Si and the NTD FZ(H) Si as shown in Fig. 3.
The intensity of 1230 cm − 1 bands in IR spectra remained unchanged for the NTD FZ(H) Si when the samples with oxygen precipitates were re-annealed for 2 to 6 h. It was evident that there was no dissolution of the oxygen precipitates at 1000°C and that neutron irradiation played a role to retard the dissolution of the precipitates in silicon. In contrast to the FZ(H) Si, the reason why the precipitates in the NTD Si are stable can be attributed to the hydrogen exhausted partially during the NTD as proved before [9]. Furthermore, 0.43 mm thick wafers of the NTD FZ(H) Si were heat-treated for 940°C 2 h denuding annealing and 750°C 4 h precipitating annealing, a denuded zone and defective zone could be obtained as shown in Fig. 5 A. Moreover, the denuded zone, defective zone, and the 1230 cm − 1 in the IR spectrum remain distinctly after a re-annealing at 1100°C for 2 h as shown in Fig. 5 B and in Fig. 6, respectively. In addition, the wafers of NTD FZ(H) Si with denuded zone were used to fabricate the silicon target vidicon in a factory test. The main working characteristic parameters of the NTD FZ(H) Si target vidicon were as follows: working current, 450 mA; working voltage of the modulation electrode, 45 V, maximum signal current, 1.7 mA; center resolution of the vidicon, 550 TVL.
4. Conclusion Single crystal silicon ingots were grown in hydrogen ambience by floating-zone technique. The concentration of interstitial oxygen in the as-grown crystal silicon was
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post-heating treatment. By denuding annealing and precipitating annealing the NTD FZ(H) Si wafers, a denuded zone layer and bulk micro-defect region have been obtained and they can be expected to act as an intrinsic getter.
Acknowledgements The financial supports of the project by the Natural Science Foundation of China (No: 69971014, 69890221) and Natural Science Foundation of Shandong Province (No: Y98G03099), are gratefully acknowledged. Fig. 6. Infrared transmittance spectra of NTD FZ(H) Si wafers annealed at 940°C 2 h+750°C 4 h+ 1100°C 2 h.
References
about 2.8× 1016 atoms cm − 3 by IR 1106 cm − 1 absorption line and a conversion factor of 3.14×1017 atoms cm − 2. Neutron transmutation doping was used to the FZ(H) Si with the total thermal neutron doses of 5.9×l017 neutrons cm − 2. Oxygen precipitation has been studied by IR-absorption band near l230 cm − 1 after annealing FZ(H) Si and NTD FZ(H) Si. In the temperature range of 600– 850°C, apparent activation energies of the formation of oxygen precipitates were 1.4 and 1.2 eV for FZ(H) Si and NTD FZ(H) Si, respectively, which were derived from Arrhenius plots of the product of the absorbance at 1230 cm − 1 and half-peak breadth in the IR-absorbance spectra. These results indicate that neutron radiation can promote the formation of oxygen precipitation. A dissolution of the oxygen precipitates occurred in the FZ(H) Si but not in the NTD FZ(H) Si after a high temperature (1000°C)
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