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High-density microwave plasma for high-rate and low-temperature deposition of silicon thin film
Vacuum 59 (2000) 266}276
High-density microwave plasma for high-rate and low-temperature deposition of silicon thin "lm Y. Sakuma , L. Haiping , H. Ueyama, H. Shirai * Department of Functional Materials Science, Faculty of Engineering, Saitama University 255 Shimo-Okubo, Urawa-shi, Saitama, 338-8570, Japan Nihon Koshuha Co., Ltd. 1119 Nakayama, Yokohama, Kanagawa, 226-0011, Japan
Abstract A novel high-density microwave plasma utilizing a spokewise antenna was produced and applied for high-rate and low-temperature deposition of hydrogenated microcrystalline silicon (lc-Si : H) "lm. The plasma maintains a uniform state, i.e., high electron density, n '10 cm\ and low temperature, ¹ of 2}2.5 eV in pure Ar plasma over 20 cm in a diameter. High deposition rate was achieved of 47 As /s in the growth of highly crystallized and photoconductive lc-Si : H "lm at the axial distance, Z"6 cm from the quartz glass plate from SiH and Ar without the use of H dilution method at low substrate temperature, ¹ of 2503C. The e!ects of the total pressure, H dilution ratio, #ow rate of SiH , Fr(SiH ) and the axial distance, Z from the quartz glass plate on the lc-Si : H "lm deposition are discussed along with the plasma diagnostics. 2000 Elsevier Science Ltd. All rights reserved. Keywords: Microwave plasma; High-density plasma; Low temperature; lc-Si : H; SiH
1. Introduction In recent years, higher throughput of highly crystallized and photoconductive amorphous and microcrystalline silicon (a-Si : H, lc-Si : H) "lms have received an increasing interests not only for the large-area electronic device applications such as solar cell and thin "lm transistor (TFT) but also the innovation of a novel deposition technique. However, the deposition rate of lc-Si : H has been, in general, very low for obtaining both good crystallinity and good optoelectronic properties, as far as RF and very-high-frequency (VHF) glow discharge are used. To overcome this problem, so
* Corresponding author: Fax: #81-48-858-3676. E-mail address: [email protected] (H. Shirai). 0042-207X/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 0 ) 0 0 2 7 9 - 7
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far, the electron}cyclotron resonance (ECR) and helicon plasma CVD techniques were also used for high rate deposition of a-Si : H and a- SiGe : H and lc-SiC alloy "lms [1}3]. However, higher V electron temperature and ion bombardment damage the growing surface and interface including the inhomogeneities of the "lm thickness. In addition, for further large area processing applications, no magnetic "eld should be required, because of complex components, i.e., magnetic coil, waveguide, and a loop antenna. Recently, a high deposition rate of &50 As /s was achieved with high crystallinity and low defect density using a VHF glow discharge by combining the SiH depletion and high working pressure conditions [4]. Several problems, however, still remain to be unsolved, i.e., the powder formation and thicker incubation layer at the initial growth stage. To this end, the novel high-density and low-temperature plasma source is strongly required without the magnetic "eld. Recently, several high-density plasma sources are proposed and applied for ultra-large-scale integrated circuit (ULSI) technologies, such as surface wave plasma (SWP), inductive coupling plasma (ICP) and ultra-high-frequency plasma (UHF) [5}7]. We have developed a novel high-density microwave plasma utilizing a spokewise antenna for fast deposition of lc-Si : H "lm from SiH or dichlorosilane, SiH Cl . Highly crystallized and photoconductive lc-Si : H "lm could be obtained at &20 As /s [8}10]. In this paper, we demonstrate the fast deposition of lc-Si : H "lm of 47 As /s using the high-density SiH microwave plasma without the use of the H dilution along with the plasma diagnostics by Langmuir probe and optical emission spectroscopy (OES).
2. Experimental procedure Deposition of lc-Si : H "lm was carried out using high-density microwave discharge utilizing a spokewise antenna with an outer diameter 22 cm. The schematics of the deposition apparatus is shown in Fig. 1 used in this study. The microwave power is supplied to the spokewise antennas. The antenna's spoke length is "xed at 4 cm, that is of the microwave length. The resonance frequency of each spoke was individually turned by adjusting the space between the spoke and the stub plate. A microwave power is supplied to the antenna assembly and radiated into the discharge chamber through the 15 mm thick quartz plate. The optical emission spectroscopy (OES) was employed to investigate the spatial distribution of the Ar emission using a periscope with a 3 mm diameter, which collect the photon to a spectrometer. In addition, the OES intensities, SiH(288 nm), SiH (414 nm) and H (656 nm) were also monitored with the pressure, H dilution and Fr[SiH ] as ? variables. The deposition conditions were the #ow rate of SiH , Fr[SiH ]"10}30 sccm, Fr[H ]"0}10 sccm and Fr[Ar]"5 sccm and substrate temperature, ¹ "150}3003C, total pressure"50 mTorr and microwave power, P "600 W. The deposition parameters were pres sure, H dilution ratio, the axial distance, Z from the quartz glass plate. The "lm structure was characterized by X-ray di!raction (XRD) and Raman spectroscopy. The residual H content in the "lm was measured by Fourier-transform infrared spectrometer (FTIR). The "lm homogeneity was examined by the UV}visible spectroscopic ellipsometry (SE) and atomic force microscopy (AFM). Dark and photoconductivity measurements were also performed with a 100 lm wide coplanar Al electrode. The optical absorption spectra were measured by the combination of the standard transmission/re#ection and constant photocurrent method (CPM).
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Fig. 1. Schematic diagrams of the deposition apparatus of (a) the microwave plasma source and (b) spokewise antenna used in this study.
3. Results and discussion 3.1. kc-Si : H xlm growth Figs. 2(a) and (b) show the OES intensities, SiH, SiH and H and the intensity ratio, I a /I H ? & 1 plotted as a function of the pressure and Raman spectra of the "lms fabricated at di!erent H dilution ratio, respectively. The OES intensity, SiH and SiH have the maximum at &50 mTorr along with the minimum of the intensity ratio, I ? /I H . Highest deposition rate can be expected at & 1 P of 50 mTorr, because the OES intensity, SiH is originated from the one-electron impact excitation and is proportional to the deposition rate [11,12]. In addition, the deposition study of lc-Si : H "lm was made at the constant partial pressure of SiH with Fr[H ] as a variable. The contribution of the amorphous phase observed at 480 cm\ region increase with increasing the H dilution ratio, which indicate that the "lm crystallinity deteriorate with the H dilution (Fig. 2(b)). In addition, the OES intensity ratio, I ? /I H increase with deteriorating the "lm & 1 crystallinity. These results are completely contrary to the result of the conventional RF and VHF glow discharge. Therefore, to clarify which parameters, the pressure or H dilution ratio, is dominant for promoting the "lm crystallinity in this high-density plasma, similar deposition study
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Fig. 2. (a) Pressure dependences of OES signal intensities, SiH, SiH and H and the signal intensity ratio, I ? /I H . (b) ? & 1 Raman spectra of lc-Si : H samples fabricated at difefrent H dilution conditions. The OES intensity ratio, I ? /I H at & 1 each deposition condition is also included.
was performed at constant P "50 mTorr with Fr[H ] as a variable. The OES intensities, SiH, SiH, the intensity ratio, I ? /I H and the deposition rate are almost independent of Fr[H ]. & 1 In addition, no signi"cant change of the "lm crystallinity was observed. Thus, the total pressure is more dominant parameter for enhancing the "lm crystallinity in this high-density SiH plasma. Therefore, to study the e!ect of the pressure on the plasma, more precisely, the optical emission spectroscopy (OES) study was employed to investigate the spatial distribution of the Ar emission using a periscope with a 3 mm diameter, which collect the photon to a spectrometer. Figs. 3(a) and (b) show the axial distributions of the OES intensity, Ar I (750.4 nm, 4s P4p ) corresponding to the threshold energy above 13.5 eV and the saturation ion current monitored by Langmuir probe technique, respectively, at di!erent pressure. The OES Ar I (750.4 nm) intensity systematically decrease with the distance exponentially at each pressure condition. Interestingly, near the quartz glass plate, the OES intensity increase with increasing the pressure, whereas it decreases with the pressure in the axial distance, z'6 cm. In addition, the integrated OES intensity between 5 and 10 cm is plotted as a function of the pressure in the inset of Fig. 3(a), which has a maximum at around 50 mTorr. This is consistent with the result that the highest deposition rate and the maximum OES intensity was observed at 50 mTorr, as reported elsewhere [13]. Thus, the origin of the highest deposition rate observed at &50 mTorr is due to the axial distribution of plasma and the pressure. On the other hand, the saturation ion current is almost independent or slightly decrease with the axial distance, Z up to &50 mTorr, though the pressure dependence is not so systematic compared with that of the OES Ar I (750.4 nm) intensity. Fig. 4 shows the imaginary part of pseudodielectric function, 1e 2 spectra at di!erent Z position before and after 10 min exposure of H and Ar mixture plasma on c-Si wafer. Generally, the magnitude of 1e 2 is tightly correlated with the degree of the surface roughness or bulk void fraction, which is attributed to the
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Fig. 3. Axial distribution of (a) the OES Ar I (750.4 nm) intensity and (b) the saturation ion current at di!erent pressure conditions measured by OES and Langmuir probe.
Fig. 4. The imaginary part of pseudodilectric function, 1e 2 spectra of c-Si wafer before and after 10 min exposure of the H (10 sccm) and Ar (5 sccm) plasma at di!erent axial distance, Z.
plasma damage. Here, the di!erence of the magnitude of 1e 2 is mainly due to the degree of the surface roughness. The magnitude of 1e 2 decrease drastically at the axial distance, Z"4 and 15 cm positions, compared with that at Z"6}10 cm region. These results suggest that the distributions of neutral radical and ion strongly depend on the pressure and the axial distance, Z from the quartz glass plate and the "lm properties are also strongly in#uenced on the selection of the pressure and the position of the substrate holder.
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Therefore, systematic deposition study was performed at di!erent Z-position under the Fr[SiH ]: 10 sccm, Fr[H ]: 10 sccm and Fr[Ar]: 5 sccm and constant pressure of 50 mTorr condition. Figs. 5(a) and (b) show the deposition rate for lc-Si : H "lm growth plotted against the axial distance, Z, and Raman spectra of the "lms fabricated at corresponding Z-positions, respectively. The deposition rate systematically increase from 14 to 17 As /s with shortening the axial distance, Z from 10 to 4 cm. In addition, the "lm crystallinity systematically improved with shortening the axial distance, Z, though the crystalline orientation is random. Additionally, the 1e 2 spectra of the corresponding lc-Si : H "lms prepared at di!erent Z-position are also shown in Fig. 6. According to the spectroscopic analysis based on a two-layer model using Bruggeman e!ective medium approximation (Bruggeman EMA), the volume fractions of the surface roughness and crystallite phase were estimated. It was found that the change of the magnitude of 1e 2 as a function of the axial distance, Z, is mainly correlated to the degree of the surface roughness, which is also con"rmed by the AFM observation. The FTIR spectra of SiH (n"1, 2) stretching mode L region and the intensities at 630 and 2100 cm\ normalized by the "lm thickness are plotted as a function of the axial distance, Z (Figs. 7(a) and (b)). Two sharp FTIR peaks composed of 2080 and 2100 cm\ are observed, which correspond to the surface SiH bonds of a nanocrystallite grain boundary and bulk SiH bonds, respectively. The normalized FTIR intensities by the "lm thickness, corresponding to the residual hydrogen content in the "lm systematically increase with increasing the axial distance, Z, though a no signi"cant change of the SiH con"guration is L observed except the peak intensity. Therefore, the "lm crystallinity is systematically enhanced with shortening the axial distance, Z along with an increase of the deposition rate and less surface roughness in this high-density microwave plasma. From these results, the growth mechanism of the lc-Si network in this high-density SiH microwave plasma is considered as follows. Near the quartz glass plate, the large amounts of neutral radical and ion contribute to the "lm deposition and crystallization. They increase the e!ective temperature at the growing surface, which increase the chemical activity of the deposition precursor on the growing surface. These result in the permotion of the crystallization and decrease
Fig. 5. (a) The deposition rate and (b) Raman spectra of lc-Si : H "lms fabricated at di!erent axial distance, Z under the Fr[SiH ]: 10 sccm, Fr[H ]: 10 sccm and Fr[Ar]: 5 sccm and constant pressure of 50 mTorr condition.
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Fig. 6. The imaginary part of pseudodielectric function, 1e 2 spectra for the corresponding lc-Si : H "lms fabricated at di!erent axial distance, Z.
Fig. 7. (a) FTIR spectra of SiH stretching mode region and (b) the FTIR intensity measured at 630 and 2100 cm\ region normalized by the "lm thickness for lc-Si : H "lms fabricated at di!erent axial distance, Z.
of the hydrogen content. On the other hand, with increasing the axial distance, Z, the generation rate of higher energy electron decrease exponentially, as shown in Fig. 3(a). Therefore, a number density of neutral radical with higher energy decrease and the relaxation of Si-network is not enough compared with that at upper position. It is considered that the relaxation of Si-network is not promoted enough compared with the results of the upper position including an increase in ion energy with the axial distance, Z, though more detail study is needed. At this stage, several growth mechanisms in the high-density SiH plasma deposition are proposed in terms of atomic Si, SiH > V
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or neutral radicals such as SiH [5}8]. As far as the systematic deposition study is concerned, no signi"cant contribution of the ion species, SiH > were con"rmed to the deposition V rate, although the moderate ion bombardment improve the crystallinity. In this high-density plasma, the H dilution was not essential for lc-Si : H growth because a large amount of atomic H can be produced from SiH . In addition, there was no relationship between the OES intensity, SiH and the deposition rate. At this stage, it is considered that the high deposition rate is due to the enhanced generation e$ciency of the neutral radical such as SiH . Therefore, at this V stage, we have an idea that the neutral radical with higher energy mainly contribute to the "lm crystallinity. In addition, the high-energy ion deteriorate the "lm crystallinity. More detail study of the spatial distribution of ion energy is needed to investigate the role of ion for the "lm growth. 3.2. For further increase of the deposition rate For further improving the deposition rate without deteriorating the "lm crystallinity, we performed the "lm deposition by increasing Fr[SiH ] under the constant P of 50 mTorr. Figs. 8(a) and (b) demonstrate the deposition rate, OES intensity, SiH and the intensity ratio, I ? /IH H plotted as a function of Fr[SiH ] under the steady #ow of 5 sccm Ar plasma, respectively. & 1G The deposition rate, and OES intensity of SiH emission have linear relationship with Fr[SiH ]. whereas the SiH signal intensity was almost independent of Fr[SiH ]. This suggests that the input SiH is completely depleted and the number density of atomic Si does not necessarily determine the deposition rate. The OES intensity ratio, I ? /I H is also almost independent of Fr[SiH ] up to & 1 Fr[SiH ]"20 sccm and after that, it increases with Fr[SiH ]. No saturation can be observed in the deposition rate up to the 30 sccm supply of SiH , which suggest that we can expect further higher deposition rate of lc-Si : H "lm. However, the crystallinity gradually deteriorates with Fr[SiH ] as shown in Figs. 9(a) and (b). Therefore, to improve the "lm crystallinity under the
Fig. 8. (a) The deposition rate, OES signal intensity, SiH and (b) the OES intensity ratio, I ? /I H plotted as a function of & 1 Fr[SiH ] in SiH and Ar plasma at constant pressure of 50 mTorr. Here, (䢇) and (夹) indicate the results at Z"10 and 6 cm, respectively.
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Fig. 9. (a) Raman and (b) XRD spectra of the lc-Si : H samples fabricated at ¹ of 200 and 2503C.
30 sccm SiH supply, the "lm deposition was carried out at higher ¹ of 2503C. The crystallinity is greatly improved in comparison with that of ¹ : 2003C maintaining high deposition rate of &40 As /s. In addition, XRD study revealed that the XRD peak at (2 2 0) was preferentially enhanced with an increase in ¹ (Fig. 9(b)). As a consequence, the high deposition rates of 40 and 47 As /s are achieved at the axial distance, Z"10 and 6 cm, from the quartz glass plate, respectively, under the Fr[SiH ]: 30 sccm, Fr[Ar]: 5 sccm, P : 50 mTorr and P : 600 W
condition. 3.3. Optoelectrical properties of the high-density microwave kc-Si : H xlm The optical absorption, a spectra are demonstrated for lc-Si : H samples in Fig. 10 including those of typical a-Si : H and c-Si. The magnitude of a at &1 eV region is decreased of approximately one order of magnitude in the Fr[SiH ]/Fr[H ]/Fr[Ar]"10/10/5 (sccm) sample in com parison with the results of the 10/0/5 sample, which imply that the defect density is lowered by adding H . In addition, the 10/10/5 spectra shifts slightly to higher energy region compared with the result of the 10/0/5, suggesting that H addition contributes to the reduction of the defect density as well as the improvement of surface roughness and/or bulk homogeneities. In addition, the optical absorption spectrum of the 30/10/5 sample shows similar behavior to that of the 10/10/5, in spite of high deposition rate of 40 As /s. The a spectra in all samples, however, shifts to infrared region compared with those of a-Si : H and c-Si. In addition, spectroscopic ellipsometry study revealed that the magnitude of the 1e 2 was around 10}15, which was almost independent of the "lm thickness. This suggests that the contributions of surface roughness and bulk inhomogeneities are very large from the beginning of the growth. Dark and photoconductivity under 100 mW/cm (AM1) exposure are 10\ and 10\ S/cm at room temperature, respectively, with no light-induced degradation. The dark conductivity activation energy, E , is 0.55}0.6 eV, which N shows almost intrinsic behavior.
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Fig. 10. Optical absorption spectra of several lc-Si : H samples determined by the combination of standard transmission/re#ection (T&R) and CPM measurements.
4. Conclusion A high-density and low-temperature microwave plasma discharge utilizing a spokewise antenna assembly is applied for high rate deposition of lc-Si : H from SiH and Ar. Among various deposition parameters, the total pressure is most dominant for determining the deposition rate and "lm crystallinity in this high-density microwave SiH plasma. Highly crystallized and photocon ductive, lc-Si : H "lms were fabricated at high deposition rate of 40 and 47 As /s at the axial distance, Z"10 and 6 cm, respectively, under the 30 sccm supply of SiH by combining SiH depletion and low pressure. The deposition condition we found in this work are completely opposite to those used for conventional RF and VHF glow discharge methods. These features of the microwave plasma utilizing a spokewise antenna source has a high potential for the future giant-microelectronics processing such as solar cell device.
Acknowledgements The authors would like to express their thanks to Mr. K. Kainuma, Miss. Y. Wasai and Mrs. N. Nabatova-Gabin in Atago Co., Ltd for permitting the use of the Raman spectrometer and spectroscopic ellipsometer. This work is supported in part by the Mazda Foundation's Reseach, the Suzuki Foundation and the Tokuyama Science Foundation.
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References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
Watanabe T, Tanaka M, Azuma K, Nakatani M, Sonobe T, Shimada T. Jpn J Appl Phys 1987;26:L288. Imajo N. J Non-Cryst Solids 1996;198}199:681. Scheib M, Schroder B, Oechsner H. J Non-Cryst Solids 1996;198}200:895. Guo L, Kondo M, Fukawa M, Saitoh K, Matsuda A. Jpn J Appl Phys 1998;37:1116. Goto M, Toyoda H, Kitagawa M, Hirao T, Sugai H. Jpn J Appl Phys 1997;34:3714. Goshima K, Toyoda H, Kojima T, Nishitani M, Kiatagawa M, Yamazoe H, Sugai H. Jpn J Appl Phys 1999;38:3655. Yoshida R, Sumiya S, Merarki B, Ito M, Hori M, Goto T, Samukawa S, Tsukada T. (IEEE of Japan) Dry Process Symp Proc 1998, p. 73. Shirai H, Arai T, Ueyama H. Jpn J Appl Phys 1998;37:1078. Shirai H, Sakuma Y, Ueyama H. Thin Solid Films 1998;337:12. Shirai H, Sakuma Y, Ueyama H. Thin Solid Films 1999;338:1. Matsuda A, Tanaka K. Thin Solid Films 1982;92:171. Fukuda Y, Sakuma Y, Azuma K, Shirai H, Thin Solid Films (2000) (in press) Shirai H, Sakuma Y, Ueyama H, Jpn J Appl Phys 1999;38:6629.
High-density microwave plasma for high-rate and low-temperature deposition of silicon thin "lm Y. Sakuma , L. Haiping , H. Ueyama, H. Shirai * Department of Functional Materials Science, Faculty of Engineering, Saitama University 255 Shimo-Okubo, Urawa-shi, Saitama, 338-8570, Japan Nihon Koshuha Co., Ltd. 1119 Nakayama, Yokohama, Kanagawa, 226-0011, Japan
Abstract A novel high-density microwave plasma utilizing a spokewise antenna was produced and applied for high-rate and low-temperature deposition of hydrogenated microcrystalline silicon (lc-Si : H) "lm. The plasma maintains a uniform state, i.e., high electron density, n '10 cm\ and low temperature, ¹ of 2}2.5 eV in pure Ar plasma over 20 cm in a diameter. High deposition rate was achieved of 47 As /s in the growth of highly crystallized and photoconductive lc-Si : H "lm at the axial distance, Z"6 cm from the quartz glass plate from SiH and Ar without the use of H dilution method at low substrate temperature, ¹ of 2503C. The e!ects of the total pressure, H dilution ratio, #ow rate of SiH , Fr(SiH ) and the axial distance, Z from the quartz glass plate on the lc-Si : H "lm deposition are discussed along with the plasma diagnostics. 2000 Elsevier Science Ltd. All rights reserved. Keywords: Microwave plasma; High-density plasma; Low temperature; lc-Si : H; SiH
1. Introduction In recent years, higher throughput of highly crystallized and photoconductive amorphous and microcrystalline silicon (a-Si : H, lc-Si : H) "lms have received an increasing interests not only for the large-area electronic device applications such as solar cell and thin "lm transistor (TFT) but also the innovation of a novel deposition technique. However, the deposition rate of lc-Si : H has been, in general, very low for obtaining both good crystallinity and good optoelectronic properties, as far as RF and very-high-frequency (VHF) glow discharge are used. To overcome this problem, so
* Corresponding author: Fax: #81-48-858-3676. E-mail address: [email protected] (H. Shirai). 0042-207X/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 0 ) 0 0 2 7 9 - 7
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far, the electron}cyclotron resonance (ECR) and helicon plasma CVD techniques were also used for high rate deposition of a-Si : H and a- SiGe : H and lc-SiC alloy "lms [1}3]. However, higher V electron temperature and ion bombardment damage the growing surface and interface including the inhomogeneities of the "lm thickness. In addition, for further large area processing applications, no magnetic "eld should be required, because of complex components, i.e., magnetic coil, waveguide, and a loop antenna. Recently, a high deposition rate of &50 As /s was achieved with high crystallinity and low defect density using a VHF glow discharge by combining the SiH depletion and high working pressure conditions [4]. Several problems, however, still remain to be unsolved, i.e., the powder formation and thicker incubation layer at the initial growth stage. To this end, the novel high-density and low-temperature plasma source is strongly required without the magnetic "eld. Recently, several high-density plasma sources are proposed and applied for ultra-large-scale integrated circuit (ULSI) technologies, such as surface wave plasma (SWP), inductive coupling plasma (ICP) and ultra-high-frequency plasma (UHF) [5}7]. We have developed a novel high-density microwave plasma utilizing a spokewise antenna for fast deposition of lc-Si : H "lm from SiH or dichlorosilane, SiH Cl . Highly crystallized and photoconductive lc-Si : H "lm could be obtained at &20 As /s [8}10]. In this paper, we demonstrate the fast deposition of lc-Si : H "lm of 47 As /s using the high-density SiH microwave plasma without the use of the H dilution along with the plasma diagnostics by Langmuir probe and optical emission spectroscopy (OES).
2. Experimental procedure Deposition of lc-Si : H "lm was carried out using high-density microwave discharge utilizing a spokewise antenna with an outer diameter 22 cm. The schematics of the deposition apparatus is shown in Fig. 1 used in this study. The microwave power is supplied to the spokewise antennas. The antenna's spoke length is "xed at 4 cm, that is of the microwave length. The resonance frequency of each spoke was individually turned by adjusting the space between the spoke and the stub plate. A microwave power is supplied to the antenna assembly and radiated into the discharge chamber through the 15 mm thick quartz plate. The optical emission spectroscopy (OES) was employed to investigate the spatial distribution of the Ar emission using a periscope with a 3 mm diameter, which collect the photon to a spectrometer. In addition, the OES intensities, SiH(288 nm), SiH (414 nm) and H (656 nm) were also monitored with the pressure, H dilution and Fr[SiH ] as ? variables. The deposition conditions were the #ow rate of SiH , Fr[SiH ]"10}30 sccm, Fr[H ]"0}10 sccm and Fr[Ar]"5 sccm and substrate temperature, ¹ "150}3003C, total pressure"50 mTorr and microwave power, P "600 W. The deposition parameters were pres sure, H dilution ratio, the axial distance, Z from the quartz glass plate. The "lm structure was characterized by X-ray di!raction (XRD) and Raman spectroscopy. The residual H content in the "lm was measured by Fourier-transform infrared spectrometer (FTIR). The "lm homogeneity was examined by the UV}visible spectroscopic ellipsometry (SE) and atomic force microscopy (AFM). Dark and photoconductivity measurements were also performed with a 100 lm wide coplanar Al electrode. The optical absorption spectra were measured by the combination of the standard transmission/re#ection and constant photocurrent method (CPM).
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Fig. 1. Schematic diagrams of the deposition apparatus of (a) the microwave plasma source and (b) spokewise antenna used in this study.
3. Results and discussion 3.1. kc-Si : H xlm growth Figs. 2(a) and (b) show the OES intensities, SiH, SiH and H and the intensity ratio, I a /I H ? & 1 plotted as a function of the pressure and Raman spectra of the "lms fabricated at di!erent H dilution ratio, respectively. The OES intensity, SiH and SiH have the maximum at &50 mTorr along with the minimum of the intensity ratio, I ? /I H . Highest deposition rate can be expected at & 1 P of 50 mTorr, because the OES intensity, SiH is originated from the one-electron impact excitation and is proportional to the deposition rate [11,12]. In addition, the deposition study of lc-Si : H "lm was made at the constant partial pressure of SiH with Fr[H ] as a variable. The contribution of the amorphous phase observed at 480 cm\ region increase with increasing the H dilution ratio, which indicate that the "lm crystallinity deteriorate with the H dilution (Fig. 2(b)). In addition, the OES intensity ratio, I ? /I H increase with deteriorating the "lm & 1 crystallinity. These results are completely contrary to the result of the conventional RF and VHF glow discharge. Therefore, to clarify which parameters, the pressure or H dilution ratio, is dominant for promoting the "lm crystallinity in this high-density plasma, similar deposition study
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Fig. 2. (a) Pressure dependences of OES signal intensities, SiH, SiH and H and the signal intensity ratio, I ? /I H . (b) ? & 1 Raman spectra of lc-Si : H samples fabricated at difefrent H dilution conditions. The OES intensity ratio, I ? /I H at & 1 each deposition condition is also included.
was performed at constant P "50 mTorr with Fr[H ] as a variable. The OES intensities, SiH, SiH, the intensity ratio, I ? /I H and the deposition rate are almost independent of Fr[H ]. & 1 In addition, no signi"cant change of the "lm crystallinity was observed. Thus, the total pressure is more dominant parameter for enhancing the "lm crystallinity in this high-density SiH plasma. Therefore, to study the e!ect of the pressure on the plasma, more precisely, the optical emission spectroscopy (OES) study was employed to investigate the spatial distribution of the Ar emission using a periscope with a 3 mm diameter, which collect the photon to a spectrometer. Figs. 3(a) and (b) show the axial distributions of the OES intensity, Ar I (750.4 nm, 4s P4p ) corresponding to the threshold energy above 13.5 eV and the saturation ion current monitored by Langmuir probe technique, respectively, at di!erent pressure. The OES Ar I (750.4 nm) intensity systematically decrease with the distance exponentially at each pressure condition. Interestingly, near the quartz glass plate, the OES intensity increase with increasing the pressure, whereas it decreases with the pressure in the axial distance, z'6 cm. In addition, the integrated OES intensity between 5 and 10 cm is plotted as a function of the pressure in the inset of Fig. 3(a), which has a maximum at around 50 mTorr. This is consistent with the result that the highest deposition rate and the maximum OES intensity was observed at 50 mTorr, as reported elsewhere [13]. Thus, the origin of the highest deposition rate observed at &50 mTorr is due to the axial distribution of plasma and the pressure. On the other hand, the saturation ion current is almost independent or slightly decrease with the axial distance, Z up to &50 mTorr, though the pressure dependence is not so systematic compared with that of the OES Ar I (750.4 nm) intensity. Fig. 4 shows the imaginary part of pseudodielectric function, 1e 2 spectra at di!erent Z position before and after 10 min exposure of H and Ar mixture plasma on c-Si wafer. Generally, the magnitude of 1e 2 is tightly correlated with the degree of the surface roughness or bulk void fraction, which is attributed to the
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Fig. 3. Axial distribution of (a) the OES Ar I (750.4 nm) intensity and (b) the saturation ion current at di!erent pressure conditions measured by OES and Langmuir probe.
Fig. 4. The imaginary part of pseudodilectric function, 1e 2 spectra of c-Si wafer before and after 10 min exposure of the H (10 sccm) and Ar (5 sccm) plasma at di!erent axial distance, Z.
plasma damage. Here, the di!erence of the magnitude of 1e 2 is mainly due to the degree of the surface roughness. The magnitude of 1e 2 decrease drastically at the axial distance, Z"4 and 15 cm positions, compared with that at Z"6}10 cm region. These results suggest that the distributions of neutral radical and ion strongly depend on the pressure and the axial distance, Z from the quartz glass plate and the "lm properties are also strongly in#uenced on the selection of the pressure and the position of the substrate holder.
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Therefore, systematic deposition study was performed at di!erent Z-position under the Fr[SiH ]: 10 sccm, Fr[H ]: 10 sccm and Fr[Ar]: 5 sccm and constant pressure of 50 mTorr condition. Figs. 5(a) and (b) show the deposition rate for lc-Si : H "lm growth plotted against the axial distance, Z, and Raman spectra of the "lms fabricated at corresponding Z-positions, respectively. The deposition rate systematically increase from 14 to 17 As /s with shortening the axial distance, Z from 10 to 4 cm. In addition, the "lm crystallinity systematically improved with shortening the axial distance, Z, though the crystalline orientation is random. Additionally, the 1e 2 spectra of the corresponding lc-Si : H "lms prepared at di!erent Z-position are also shown in Fig. 6. According to the spectroscopic analysis based on a two-layer model using Bruggeman e!ective medium approximation (Bruggeman EMA), the volume fractions of the surface roughness and crystallite phase were estimated. It was found that the change of the magnitude of 1e 2 as a function of the axial distance, Z, is mainly correlated to the degree of the surface roughness, which is also con"rmed by the AFM observation. The FTIR spectra of SiH (n"1, 2) stretching mode L region and the intensities at 630 and 2100 cm\ normalized by the "lm thickness are plotted as a function of the axial distance, Z (Figs. 7(a) and (b)). Two sharp FTIR peaks composed of 2080 and 2100 cm\ are observed, which correspond to the surface SiH bonds of a nanocrystallite grain boundary and bulk SiH bonds, respectively. The normalized FTIR intensities by the "lm thickness, corresponding to the residual hydrogen content in the "lm systematically increase with increasing the axial distance, Z, though a no signi"cant change of the SiH con"guration is L observed except the peak intensity. Therefore, the "lm crystallinity is systematically enhanced with shortening the axial distance, Z along with an increase of the deposition rate and less surface roughness in this high-density microwave plasma. From these results, the growth mechanism of the lc-Si network in this high-density SiH microwave plasma is considered as follows. Near the quartz glass plate, the large amounts of neutral radical and ion contribute to the "lm deposition and crystallization. They increase the e!ective temperature at the growing surface, which increase the chemical activity of the deposition precursor on the growing surface. These result in the permotion of the crystallization and decrease
Fig. 5. (a) The deposition rate and (b) Raman spectra of lc-Si : H "lms fabricated at di!erent axial distance, Z under the Fr[SiH ]: 10 sccm, Fr[H ]: 10 sccm and Fr[Ar]: 5 sccm and constant pressure of 50 mTorr condition.
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Fig. 6. The imaginary part of pseudodielectric function, 1e 2 spectra for the corresponding lc-Si : H "lms fabricated at di!erent axial distance, Z.
Fig. 7. (a) FTIR spectra of SiH stretching mode region and (b) the FTIR intensity measured at 630 and 2100 cm\ region normalized by the "lm thickness for lc-Si : H "lms fabricated at di!erent axial distance, Z.
of the hydrogen content. On the other hand, with increasing the axial distance, Z, the generation rate of higher energy electron decrease exponentially, as shown in Fig. 3(a). Therefore, a number density of neutral radical with higher energy decrease and the relaxation of Si-network is not enough compared with that at upper position. It is considered that the relaxation of Si-network is not promoted enough compared with the results of the upper position including an increase in ion energy with the axial distance, Z, though more detail study is needed. At this stage, several growth mechanisms in the high-density SiH plasma deposition are proposed in terms of atomic Si, SiH > V
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or neutral radicals such as SiH [5}8]. As far as the systematic deposition study is concerned, no signi"cant contribution of the ion species, SiH > were con"rmed to the deposition V rate, although the moderate ion bombardment improve the crystallinity. In this high-density plasma, the H dilution was not essential for lc-Si : H growth because a large amount of atomic H can be produced from SiH . In addition, there was no relationship between the OES intensity, SiH and the deposition rate. At this stage, it is considered that the high deposition rate is due to the enhanced generation e$ciency of the neutral radical such as SiH . Therefore, at this V stage, we have an idea that the neutral radical with higher energy mainly contribute to the "lm crystallinity. In addition, the high-energy ion deteriorate the "lm crystallinity. More detail study of the spatial distribution of ion energy is needed to investigate the role of ion for the "lm growth. 3.2. For further increase of the deposition rate For further improving the deposition rate without deteriorating the "lm crystallinity, we performed the "lm deposition by increasing Fr[SiH ] under the constant P of 50 mTorr. Figs. 8(a) and (b) demonstrate the deposition rate, OES intensity, SiH and the intensity ratio, I ? /IH H plotted as a function of Fr[SiH ] under the steady #ow of 5 sccm Ar plasma, respectively. & 1G The deposition rate, and OES intensity of SiH emission have linear relationship with Fr[SiH ]. whereas the SiH signal intensity was almost independent of Fr[SiH ]. This suggests that the input SiH is completely depleted and the number density of atomic Si does not necessarily determine the deposition rate. The OES intensity ratio, I ? /I H is also almost independent of Fr[SiH ] up to & 1 Fr[SiH ]"20 sccm and after that, it increases with Fr[SiH ]. No saturation can be observed in the deposition rate up to the 30 sccm supply of SiH , which suggest that we can expect further higher deposition rate of lc-Si : H "lm. However, the crystallinity gradually deteriorates with Fr[SiH ] as shown in Figs. 9(a) and (b). Therefore, to improve the "lm crystallinity under the
Fig. 8. (a) The deposition rate, OES signal intensity, SiH and (b) the OES intensity ratio, I ? /I H plotted as a function of & 1 Fr[SiH ] in SiH and Ar plasma at constant pressure of 50 mTorr. Here, (䢇) and (夹) indicate the results at Z"10 and 6 cm, respectively.
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Fig. 9. (a) Raman and (b) XRD spectra of the lc-Si : H samples fabricated at ¹ of 200 and 2503C.
30 sccm SiH supply, the "lm deposition was carried out at higher ¹ of 2503C. The crystallinity is greatly improved in comparison with that of ¹ : 2003C maintaining high deposition rate of &40 As /s. In addition, XRD study revealed that the XRD peak at (2 2 0) was preferentially enhanced with an increase in ¹ (Fig. 9(b)). As a consequence, the high deposition rates of 40 and 47 As /s are achieved at the axial distance, Z"10 and 6 cm, from the quartz glass plate, respectively, under the Fr[SiH ]: 30 sccm, Fr[Ar]: 5 sccm, P : 50 mTorr and P : 600 W
condition. 3.3. Optoelectrical properties of the high-density microwave kc-Si : H xlm The optical absorption, a spectra are demonstrated for lc-Si : H samples in Fig. 10 including those of typical a-Si : H and c-Si. The magnitude of a at &1 eV region is decreased of approximately one order of magnitude in the Fr[SiH ]/Fr[H ]/Fr[Ar]"10/10/5 (sccm) sample in com parison with the results of the 10/0/5 sample, which imply that the defect density is lowered by adding H . In addition, the 10/10/5 spectra shifts slightly to higher energy region compared with the result of the 10/0/5, suggesting that H addition contributes to the reduction of the defect density as well as the improvement of surface roughness and/or bulk homogeneities. In addition, the optical absorption spectrum of the 30/10/5 sample shows similar behavior to that of the 10/10/5, in spite of high deposition rate of 40 As /s. The a spectra in all samples, however, shifts to infrared region compared with those of a-Si : H and c-Si. In addition, spectroscopic ellipsometry study revealed that the magnitude of the 1e 2 was around 10}15, which was almost independent of the "lm thickness. This suggests that the contributions of surface roughness and bulk inhomogeneities are very large from the beginning of the growth. Dark and photoconductivity under 100 mW/cm (AM1) exposure are 10\ and 10\ S/cm at room temperature, respectively, with no light-induced degradation. The dark conductivity activation energy, E , is 0.55}0.6 eV, which N shows almost intrinsic behavior.
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Fig. 10. Optical absorption spectra of several lc-Si : H samples determined by the combination of standard transmission/re#ection (T&R) and CPM measurements.
4. Conclusion A high-density and low-temperature microwave plasma discharge utilizing a spokewise antenna assembly is applied for high rate deposition of lc-Si : H from SiH and Ar. Among various deposition parameters, the total pressure is most dominant for determining the deposition rate and "lm crystallinity in this high-density microwave SiH plasma. Highly crystallized and photocon ductive, lc-Si : H "lms were fabricated at high deposition rate of 40 and 47 As /s at the axial distance, Z"10 and 6 cm, respectively, under the 30 sccm supply of SiH by combining SiH depletion and low pressure. The deposition condition we found in this work are completely opposite to those used for conventional RF and VHF glow discharge methods. These features of the microwave plasma utilizing a spokewise antenna source has a high potential for the future giant-microelectronics processing such as solar cell device.
Acknowledgements The authors would like to express their thanks to Mr. K. Kainuma, Miss. Y. Wasai and Mrs. N. Nabatova-Gabin in Atago Co., Ltd for permitting the use of the Raman spectrometer and spectroscopic ellipsometer. This work is supported in part by the Mazda Foundation's Reseach, the Suzuki Foundation and the Tokuyama Science Foundation.
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