- Email: [email protected]
Vacuum–ultraviolet pulsed-laser deposition of silicon dioxide thin films
Applied Surface Science 127–129 Ž1998. 595–600
Vacuum–ultraviolet pulsed-laser deposition of silicon dioxide thin films Brian D. Jackson, Peter R. Herman
)
The UniÕersity of Toronto, Department of Electrical and Computer Engineering and the Ontario Laser and LightwaÕe Research Centre, Toronto, Ontario, Canada
Abstract Vacuum–ultraviolet pulsed-laser deposition ŽPLD., using the 157-nm molecular fluorine laser, has been demonstrated to produce particulate-free silica films from a fused silica target, in contrast with the 100-nm particulates found on films grown with the 193-nm ArF laser. XPS analysis has shown that films grown at low laser fluence Ž3–4 J cmy2 . in 2 = 10y4 Torr of dry air have significantly improved stoichiometry ŽSiO1.9 ., relative to films grown in vacuum ŽSiO1.75 .. Additionally, for films grown in vacuum, low ablation fluences Ž3–4 J cmy2 . produced films with higher oxygen content ŽSiO1.75 . than high ablation fluences Ž) 10 J cmy2 peak fluence – SiO1.65 .. FTIR has shown that the characteristic Si–O–Si asymmetric stretching mode ŽASM. absorption peak in the best films grown to date lies at ; 1050 cmy1, with a width of 90 cmy1 ŽFWHM., as compared with the 1060–1080 cmy1 position and 70–75 cmy1 width in a high quality thermal oxide. Process optimization and doping with optically-active and -passive ions will permit low-temperature growth of planar optical waveguides for use in optical integrated circuits. q 1998 Elsevier Science B.V. PACS: 81.15.Fg; 81.05.Kf; 85.40.Sz Keywords: Pulsed-laser deposition; Vacuum–ultraviolet; Silicon dioxide; Fluorine laser; Laser ablation; Thin film
1. Introduction Pulsed-laser deposition ŽPLD. is a versatile thin film deposition technique which has been successfully applied to an extremely wide range of materials w1x. The ability to deposit films with complex composition profiles at low substrate temperatures is a significant advantage of this technique over traditional techniques such as CVD and thermal evaporation. In the case of silica-based films, controlled ) Corresponding author. 10 King’s College Rd., Toronto, Ontario, Canada M5S 3G4. Tel.: q1-416-978-7722; fax: q1-416971-3020; e-mail: [email protected].
doping with erbium and germanium oxides will allow the formation of planar active optical waveguide structures for use in optical integrated circuits. Additionally, the ability to deposit these films at low temperature may allow deposition on temperaturesensitive structures such as polymers or processed semiconductor wafers. The works of Fogarassy et al. w2x and Slaoui et al. w3x have shown that relatively high quality silicon dioxide films can be grown by ablation from a silicon or silicon monoxide target in a background of ; 100 mTorr of oxygen. However, deposition temperatures of 4508C andror an 8008C rapid thermal anneal were required to produce the best quality
0169-4332r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 4 3 3 2 Ž 9 7 . 0 0 7 1 2 - 5
596
B.D. Jackson, P.R. Hermanr Applied Surface Science 127–129 (1998) 595–600
films, and this quality was below that provided by another low-temperature technique – VUV lamp-assisted CVD w4x. A reduced ambient pressure is desired to supply increased kinetic energy to the deposition species, and thereby improve growth at low temperatures. For example, the work of Chen et al. w5x has shown that deposition in the presence of an rf-induced oxygen plasma allows the growth pressure to be reduced to ; 1 mTorr for the case of 532-nm ablation of a silicon target. However, the best results obtained using this technique were of lower quality than the best ArF-PLD films. Alternatively, ablation of a fused silica target produces stoichiometric films in very low pressures of oxygen w2x. However, previous studies have shown that particulate generation is a major limitation when deposition takes place from a bulk fused silica target. In particular, the work of Fogarassy et al. w2x, using an ArF laser, found that the films were coated with 0.1–1.0 m m diameter spherical particulates, likely resulting from droplet emission from the target. Additionally, the work of Baeri et al. w6x, using a XeCl laser, found larger, irregularly shaped particles on the surfaces of the deposited films. In each of these cases, the generation of particulates could be linked to the weak optical absorption of the bulk silica target at the ablating wavelength. Previous works in our lab w7,8x have shown that the vacuum–ultraviolet F2 laser can be used to produce clean ablation patterns in UV-grade fused silica, in contrast with results of the longer wavelength ArF laser. In particular, the increased optical absorption in the bulk fused silica target at the 157-nm wavelength of the F2 laser is responsible for the reduced generation of particulates. These observations indicate that the F2 laser may be used to deposit particulate-free silica films from a fused silica target, enabling the deposition of high quality optical structures by PLD.
pulses at a repetition rate of 1–2 Hz was used for these experiments. The laser beam passed through evacuated beamtubes into a general-purpose vacuum chamber. An aperture placed in the beamtube was imaged onto the target surface using an 8.6-cm MgF2 focusing lens, producing uniform illumination at fluences of 2–5 J cmy2 , with a total on-target energy of 5–10 mJ. The incident laser beam made an angle of 458 with the target normal, and the silicon substrates were positioned 2.5 cm from the target surface. An x–y translator was used to scan the target relative to the input laser beam to continually expose fresh target material to the laser beam. A turbomolecular pump, backed by a dual-stage rotary vane pump, produced a base pressure of ; 6 = 10y6 Torr. Argon flow through the chamber raised the ambient pressure to 10y5 –10y3 Torr during deposition. An ArF gas fill provided 193-nm radiation, with on-target energies of ; 20 mJ focused to ; 7–8 J cmy2 . Films were produced with a 1.5-Hz repetition rate in a background of 2 = 10y4 Torr of argon. Deposition experiments were limited to ; 5000–10,000 shots due to the limited lifetime of the fluorine gas fill in the absence of gas injection and the 1.5-Hz repetition
2. Demonstration of particulate-free F2-PLD of SiO 2 The initial investigation into F2-PLD of SiO 2 focused upon the reduction of particulate generation in 157-nm PLD relative to 193-nm PLD. A homebuilt F2 laser w9x producing 45-mJ, 15-ns FWHM
Fig. 1. AFM image of a ;15-nm thick F2-laser PLD SiO 2 film grown with an ablation fluence of ; 3.5 J cmy2 . The RMS surface roughness of this film was calculated to be 0.3 nm. Image area s1 m m2 .
B.D. Jackson, P.R. Hermanr Applied Surface Science 127–129 (1998) 595–600
597
based contaminant attributed to oil contamination of the deposition chamber used for these experiments. A second high-vacuum chamber was therefore constructed for accurate determination of the O : Si stoichiometry of the deposited films.
3. Optimization of 157-nm PLD of SiO 2
Fig. 2. AFM image of a ;15-nm thick ArF-laser PLD SiO 2 film grown with an ablation fluence of ; 7–8 J cmy2 . Note the ;1% surface coverage by ;100-nm diameter particulates. The RMS surface roughness of this film Žexcluding particulates. was calculated to be 0.23 nm. Image area s1 m m2 .
rate. This limited the deposited film thickness to ; 10–30 nm. The surface roughness of the deposited films was examined using a Nanoscope III atomic force microscope ŽAFM.. As seen in the AFM images in Figs. 1 and 2, the F2-deposited films were virtually particulate-free, while the ArF-deposited films were contaminated by 100-nm diameter particulates. Larger area scans showed the particle density on the ArF˚ y1 , deposited film to be 3 = 10 6 particles cmy2 A with a 100–200 = reduction in the particle density on the F2-deposited film. The background surface roughness of both films were very low Ž0.3 nm for F2 and 0.23 nm for ArF., indicating that these ; 15-nm thick films are nearly atomically-smooth. XPS chemical analysis showed all films to be highly contaminated Ž20–50 at%. with a carbon-
A second set of 157-nm PLD silica films were grown in a very-high-vacuum deposition chamber. A turbomolecular pump, backed by a liquid-nitrogentrapped dual-stage rotary vane pump, produced base pressures of ; 1.5 = 10y8 or 2.5 = 10y7 Torr Ždepending upon the pump being used.. Argon or dry air Žpure oxygen results not available at time of publication. were passed through the chamber during deposition to maintain a variety of ambient pressures, ranging from vacuum to 10y3 Torr. The target was rotated during deposition, and the substrate was located 2.5 cm from the target surface. The silicon substrates were heated to up to ; 4008C prior to deposition to reduce contamination, with deposition taking place at room temperature. XPS analysis of the chemical composition of the deposited films showed a ) 10-fold reduction in the carbon contamination Žto - 2 at% carbon., relative to the previous set of experiments, enabling reliable determination of the O : Si film stoichiometry. Table 1 summarizes the results obtained for high fluence Ž) 15 J cmy2 peak fluence. and low fluence Ž3–4 J cmy2 . ablation in vacuum and 2 = 10y4 or 1 = 10y3 Torr of dry air. In particular, films deposited in vacuum at high fluences were highly oxygen-deficient ŽSiO1.65 .. However, the low fluence case produced films with a significantly higher oxygen content ŽSiO1.75 .. Additionally, a relatively low ambient pressure of 2 = 10y4 Torr of dry air is observed to have significantly improved the stoichiometry of the deposited films ŽSiO1.90 – 1.95 ..
Table 1 Oxygen : silicon stoichiometry of F2 -laser PLD SiO 2 films y2 .
High fluence Ž) 15 J cm Low fluence Ž3–4 J cmy2 .
Vacuum or 2 = 10y4 Torr argon
2 = 10y4 Torr dry air
1 = 10y3 Torr dry air
1.60 : 1 1.75 : 1
1.95 : 1 1.9 : 1
1.9 : 1 y
598
B.D. Jackson, P.R. Hermanr Applied Surface Science 127–129 (1998) 595–600
peaks at 5–8 cmy1 lower wavenumber relative to films deposited with low ablation fluences Ž3–4 J cmy2 ..
4. Discussion
Fig. 3. Infrared transmission spectra of F2-laser PLD SiO 2 films determined by referencing the sample transmission to the transmission of a blank substrate. L.F.s low fluence Ž3–4 J cmy2 ., H.F.s high fluence Ž )15 J cmy2 ..
FTIR transmission spectroscopy was used to further characterize the chemical structure of the deposited films. Characteristic Si–O–Si asymmetric stretching mode ŽASM. peak spectra are shown in Fig. 3, with the peak positions and widths summarized in Table 2. These spectra showed that the ASM peak, nominally at ; 1080 cmy1 , was shifted to lower wavenumber, and that the peak was broadened from the ; 70-cmy1 w4x width of a high quality thermal oxide. These shifts and broadening can be attributed to a combination of film stress, contamination, porosity, and O : Si substoichiometry. In particular, it is observed that the peaks of films deposited in air are 11–32 cmy1 narrower and 12–17 cmy1 higher in wavenumber relative to films deposited in vacuum. Additionally, high ablation fluences Ž) 15 J cmy2 . produced films with 4–24 cmy1 broader Table 2 Si–O–Si asymmetric stretch absorption peak data
Typical thermally-grown oxide High fluence Ž )15 J cmy2 . Vacuum 2=10y4 Torr dry air 1=10y3 Torr dry air Low fluence Ž ; 3.5 J cmy2 . Vacuum 2=10y4 Torr dry air
Peak position Žcmy1 .
Peak width Žcmy1 .
;1080
; 70 w4x
1030 1042 1047
122 95 90
1035 1047
102 91
The elimination of particulates in F2-laser PLD of silica films from a bulk fused silica target, as compared with ArF-laser PLD, can be directly linked to increased optical absorption at 157 nm. As summarized in Table 3, both the small-signal optical absorption coefficient and the effective absorption coefficient determined from ablation data are significantly larger at 157 nm than at 193 nm. Increased absorption in the solid phase is expected to reduce the initial generation of particulates, as has been shown previously by Singh et al. w11x for the case of PLD of YBCO. Further, increased plume absorption may cause the remaining particulates to be vapourized in the plume as noted by Koren et al. w12x, also for the case of YBCO ablation. In the present 157-nm study, plume absorption by molecular species will be substantially stronger than with 193-nm or longer wavelength sources. The better quality films observed for low-fluence deposition, noted by the narrowing and shift to higher wavenumber of the Si–O–Si ASM IR absorption peak, may be attributed to a reduced dissociation of silicon dioxide at low fluence. This reduced dissociation may explain the larger oxygen content of films grown in vacuum for a low ablation fluence. The addition of 2 = 10y4 Torr of dry air provided compensation for oxygen loss at both high and low fluence conditions as noted in Table 1. Background oxygen gas is clearly being incorporated into the film. This higher oxygen content also correlates with a narrowing and shift to higher wavenumber in the ASM peak ŽTable 2., providing a second indication
Table 3 Optical absorption coefficient of UV-grade fused silica
F2 ablation ArF ablation
Small signal a Žcmy1 .
a eff from ablation data Žcmy1.
14 w10x <1 w10x
2=10y5 w8x 1=10y5 w8x
B.D. Jackson, P.R. Hermanr Applied Surface Science 127–129 (1998) 595–600
of improved film quality provided by the oxygen gas. The observed substoichiometry of silica films deposited in vacuum is a result of the near-bandgap energy of the F2-laser photons. Although the photon energy lies ; 1.4 eV below the bandgap of pure silica, numerous defect states and blurring of the band-edge permit the 7.9-eV photons to photo-dissociate the bulk. Additionally, strong laser–plume interactions at 157 nm, including direct photo-dissociation of molecular oxygen, will create volatile oxygen species which are less likely to be incorporated into the deposited film. Therefore, the low oxygen content of films deposited under high laser fluence conditions may be the result of nonlinear absorption effects within the bulk target and increased plume absorption of the incident laser beam. The observed oxygen-deficiency of F2-deposited SiO 2 films is similar to results observed for YBCO deposition from a bulk or pressed ceramic target w13x. In that YBCO case, stoichiometric deposition by ArF-laser PLD required a higher background pressure of oxygen gas than was required for KrFlaser PLD, due to photo-dissociation of CuO by the 6-eV ArF-laser photons. Additionally, the work of Kurosawa et al. w14x has demonstrated that irradiation of silicon dioxide by sub-ablation-threshold fluences of 126-nm Ar2-laser radiation reduces the oxygen content of the target surface.
5. Conclusions and future work For the first time, F2-laser PLD has been demonstrated to produce virtually particulate-free silica films from a bulk fused silica target, in contrast with ArF-laser PLD. The near-bandgap energy of the F2-laser photons provides strong optical absorption of bulk silica in comparison with 193-nm radiation, resulting in the observed reduction of particulates. Although films produced at high fluences in vacuum have been shown to be ; 20% deficient in oxygen ŽSiO1.65 ., a relatively low ambient pressure of 2 = 10y4 Torr of dry air has been used to improve the stoichiometry by ; 80% ŽSiO1.9 – 1.95 .. Additionally, FTIR spectroscopy has shown that films grown at low fluences, in 2 = 10y4 Torr of dry air, have characteristic absorption peaks at 1047 cmy1 which
599
are slightly broader Ž; 90 cmy1 vs. 80 cmy1 . than those of the best previous PLD-grown SiO 2 films. The significant improvements in film quality obtained here by relatively coarse tuning of laser fluence and gas pressure indicate that, with further optimization of the growth parameters, F2-laser PLD will produce high quality silica films. Elevated substrate temperatures are expected to significantly improve the structure of the deposited films. Additionally, increased partial pressures of oxygen gas is also available to improve film quality, albeit, limited by absorption of the 157-nm laser beam by molecular oxygen to ; 100 mTorr Žfor a 5-cm lens-to-target distance.. Alternatively, generation of an activated oxygen background ŽO or O 3 . may improve the oxidation process and provide the means for low pressure deposition. Care must be taken to avoid damage to the growing film resulting, for example, from ion bombardment in plasma-assisted growth. Once optimized, the deposition process may be applied to the low-temperature growth of silica films for optical waveguide structures and passivation layers in electronics.
Acknowledgements The authors would like to thank Prof. S. Zukotynski, Dr. R.N.S. Sohdi and Prof. M.C. Goh of the University of Toronto for the use of diagnostic equipment. Funding for this project has been supplied by the Natural Sciences and Engineering Research Council of Canada and the Ontario Laser and Lightwave Research Centre.
References w1x Chrisey, Hubler ŽEds.., Pulsed Laser Deposition of Thin Films, Wiley, New York, 1994. w2x E. Fogarassy, A. Slaoui, C. Fuchs, J.P. Stoquert, Appl. Surf. Sci. 54 Ž1992. 180–186. w3x A. Slaoui, E. Fogarassy, C. Fuchs, P. Siffert, J. Appl. Phys. 71 Ž1992. 590–596. w4x P. Bergonzo, U. Kogelshatz, I.W. Boyd, Appl. Surf. Sci. 69 Ž1993. 393–397. w5x T.P. Chen, T.-I. Bao, L. I, Appl. Phys. Lett. 63 Ž18. Ž1993. 2475–2477.
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w6x P. Baeri, R. Reitano, N. Marino, Appl. Surf. Sci. 86 Ž1995. 128–133. w7x P.R. Herman, B. Chen, J.D. Moore, R. Sia, Mater. Res. Soc. Symp. Proc. 236 Ž1994. 53–58. w8x P.R. Herman, K. Beckley, B. Jackson, K. Kurosawa, D. Moore, T. Yamanishi, J. Yang, Proc. SPIE 2992 Ž1997. 86–95. w9x P.C. Hill, P.R. Herman, R. Sia, J. Appl. Phys. 73 Ž1993. 5274–5276. w10x H.R. Philipps, in: E.D. Palik ŽEd.., Handbook of Optical
w11x w12x w13x w14x
Constants of Solids, Academic Press, New York, 1985, pp. 749–763. R.K. Singh, D. Bhattacharya, J. Narayan, Appl. Phys. Lett. 61 Ž4. Ž1995. 483–485. G. Koren, R.J. Baseman, A. Gupta, M.I. Lutwyche, R.B. Laibowitz, Appl. Phys. Lett. 56 Ž21. Ž1990. 2144–2146. R.W. Dreyfus, Appl. Surf. Sci. 86 Ž1995. 29–34. K. Kurosawa, K. Nakamai, Y. Tagikawa, W. Sasaki, Y. Kato, M. Okuda, Nucl. Instr. Meth. Phys. Res. B 116 Ž1996. 410–415.
Vacuum–ultraviolet pulsed-laser deposition of silicon dioxide thin films Brian D. Jackson, Peter R. Herman
)
The UniÕersity of Toronto, Department of Electrical and Computer Engineering and the Ontario Laser and LightwaÕe Research Centre, Toronto, Ontario, Canada
Abstract Vacuum–ultraviolet pulsed-laser deposition ŽPLD., using the 157-nm molecular fluorine laser, has been demonstrated to produce particulate-free silica films from a fused silica target, in contrast with the 100-nm particulates found on films grown with the 193-nm ArF laser. XPS analysis has shown that films grown at low laser fluence Ž3–4 J cmy2 . in 2 = 10y4 Torr of dry air have significantly improved stoichiometry ŽSiO1.9 ., relative to films grown in vacuum ŽSiO1.75 .. Additionally, for films grown in vacuum, low ablation fluences Ž3–4 J cmy2 . produced films with higher oxygen content ŽSiO1.75 . than high ablation fluences Ž) 10 J cmy2 peak fluence – SiO1.65 .. FTIR has shown that the characteristic Si–O–Si asymmetric stretching mode ŽASM. absorption peak in the best films grown to date lies at ; 1050 cmy1, with a width of 90 cmy1 ŽFWHM., as compared with the 1060–1080 cmy1 position and 70–75 cmy1 width in a high quality thermal oxide. Process optimization and doping with optically-active and -passive ions will permit low-temperature growth of planar optical waveguides for use in optical integrated circuits. q 1998 Elsevier Science B.V. PACS: 81.15.Fg; 81.05.Kf; 85.40.Sz Keywords: Pulsed-laser deposition; Vacuum–ultraviolet; Silicon dioxide; Fluorine laser; Laser ablation; Thin film
1. Introduction Pulsed-laser deposition ŽPLD. is a versatile thin film deposition technique which has been successfully applied to an extremely wide range of materials w1x. The ability to deposit films with complex composition profiles at low substrate temperatures is a significant advantage of this technique over traditional techniques such as CVD and thermal evaporation. In the case of silica-based films, controlled ) Corresponding author. 10 King’s College Rd., Toronto, Ontario, Canada M5S 3G4. Tel.: q1-416-978-7722; fax: q1-416971-3020; e-mail: [email protected].
doping with erbium and germanium oxides will allow the formation of planar active optical waveguide structures for use in optical integrated circuits. Additionally, the ability to deposit these films at low temperature may allow deposition on temperaturesensitive structures such as polymers or processed semiconductor wafers. The works of Fogarassy et al. w2x and Slaoui et al. w3x have shown that relatively high quality silicon dioxide films can be grown by ablation from a silicon or silicon monoxide target in a background of ; 100 mTorr of oxygen. However, deposition temperatures of 4508C andror an 8008C rapid thermal anneal were required to produce the best quality
0169-4332r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 4 3 3 2 Ž 9 7 . 0 0 7 1 2 - 5
596
B.D. Jackson, P.R. Hermanr Applied Surface Science 127–129 (1998) 595–600
films, and this quality was below that provided by another low-temperature technique – VUV lamp-assisted CVD w4x. A reduced ambient pressure is desired to supply increased kinetic energy to the deposition species, and thereby improve growth at low temperatures. For example, the work of Chen et al. w5x has shown that deposition in the presence of an rf-induced oxygen plasma allows the growth pressure to be reduced to ; 1 mTorr for the case of 532-nm ablation of a silicon target. However, the best results obtained using this technique were of lower quality than the best ArF-PLD films. Alternatively, ablation of a fused silica target produces stoichiometric films in very low pressures of oxygen w2x. However, previous studies have shown that particulate generation is a major limitation when deposition takes place from a bulk fused silica target. In particular, the work of Fogarassy et al. w2x, using an ArF laser, found that the films were coated with 0.1–1.0 m m diameter spherical particulates, likely resulting from droplet emission from the target. Additionally, the work of Baeri et al. w6x, using a XeCl laser, found larger, irregularly shaped particles on the surfaces of the deposited films. In each of these cases, the generation of particulates could be linked to the weak optical absorption of the bulk silica target at the ablating wavelength. Previous works in our lab w7,8x have shown that the vacuum–ultraviolet F2 laser can be used to produce clean ablation patterns in UV-grade fused silica, in contrast with results of the longer wavelength ArF laser. In particular, the increased optical absorption in the bulk fused silica target at the 157-nm wavelength of the F2 laser is responsible for the reduced generation of particulates. These observations indicate that the F2 laser may be used to deposit particulate-free silica films from a fused silica target, enabling the deposition of high quality optical structures by PLD.
pulses at a repetition rate of 1–2 Hz was used for these experiments. The laser beam passed through evacuated beamtubes into a general-purpose vacuum chamber. An aperture placed in the beamtube was imaged onto the target surface using an 8.6-cm MgF2 focusing lens, producing uniform illumination at fluences of 2–5 J cmy2 , with a total on-target energy of 5–10 mJ. The incident laser beam made an angle of 458 with the target normal, and the silicon substrates were positioned 2.5 cm from the target surface. An x–y translator was used to scan the target relative to the input laser beam to continually expose fresh target material to the laser beam. A turbomolecular pump, backed by a dual-stage rotary vane pump, produced a base pressure of ; 6 = 10y6 Torr. Argon flow through the chamber raised the ambient pressure to 10y5 –10y3 Torr during deposition. An ArF gas fill provided 193-nm radiation, with on-target energies of ; 20 mJ focused to ; 7–8 J cmy2 . Films were produced with a 1.5-Hz repetition rate in a background of 2 = 10y4 Torr of argon. Deposition experiments were limited to ; 5000–10,000 shots due to the limited lifetime of the fluorine gas fill in the absence of gas injection and the 1.5-Hz repetition
2. Demonstration of particulate-free F2-PLD of SiO 2 The initial investigation into F2-PLD of SiO 2 focused upon the reduction of particulate generation in 157-nm PLD relative to 193-nm PLD. A homebuilt F2 laser w9x producing 45-mJ, 15-ns FWHM
Fig. 1. AFM image of a ;15-nm thick F2-laser PLD SiO 2 film grown with an ablation fluence of ; 3.5 J cmy2 . The RMS surface roughness of this film was calculated to be 0.3 nm. Image area s1 m m2 .
B.D. Jackson, P.R. Hermanr Applied Surface Science 127–129 (1998) 595–600
597
based contaminant attributed to oil contamination of the deposition chamber used for these experiments. A second high-vacuum chamber was therefore constructed for accurate determination of the O : Si stoichiometry of the deposited films.
3. Optimization of 157-nm PLD of SiO 2
Fig. 2. AFM image of a ;15-nm thick ArF-laser PLD SiO 2 film grown with an ablation fluence of ; 7–8 J cmy2 . Note the ;1% surface coverage by ;100-nm diameter particulates. The RMS surface roughness of this film Žexcluding particulates. was calculated to be 0.23 nm. Image area s1 m m2 .
rate. This limited the deposited film thickness to ; 10–30 nm. The surface roughness of the deposited films was examined using a Nanoscope III atomic force microscope ŽAFM.. As seen in the AFM images in Figs. 1 and 2, the F2-deposited films were virtually particulate-free, while the ArF-deposited films were contaminated by 100-nm diameter particulates. Larger area scans showed the particle density on the ArF˚ y1 , deposited film to be 3 = 10 6 particles cmy2 A with a 100–200 = reduction in the particle density on the F2-deposited film. The background surface roughness of both films were very low Ž0.3 nm for F2 and 0.23 nm for ArF., indicating that these ; 15-nm thick films are nearly atomically-smooth. XPS chemical analysis showed all films to be highly contaminated Ž20–50 at%. with a carbon-
A second set of 157-nm PLD silica films were grown in a very-high-vacuum deposition chamber. A turbomolecular pump, backed by a liquid-nitrogentrapped dual-stage rotary vane pump, produced base pressures of ; 1.5 = 10y8 or 2.5 = 10y7 Torr Ždepending upon the pump being used.. Argon or dry air Žpure oxygen results not available at time of publication. were passed through the chamber during deposition to maintain a variety of ambient pressures, ranging from vacuum to 10y3 Torr. The target was rotated during deposition, and the substrate was located 2.5 cm from the target surface. The silicon substrates were heated to up to ; 4008C prior to deposition to reduce contamination, with deposition taking place at room temperature. XPS analysis of the chemical composition of the deposited films showed a ) 10-fold reduction in the carbon contamination Žto - 2 at% carbon., relative to the previous set of experiments, enabling reliable determination of the O : Si film stoichiometry. Table 1 summarizes the results obtained for high fluence Ž) 15 J cmy2 peak fluence. and low fluence Ž3–4 J cmy2 . ablation in vacuum and 2 = 10y4 or 1 = 10y3 Torr of dry air. In particular, films deposited in vacuum at high fluences were highly oxygen-deficient ŽSiO1.65 .. However, the low fluence case produced films with a significantly higher oxygen content ŽSiO1.75 .. Additionally, a relatively low ambient pressure of 2 = 10y4 Torr of dry air is observed to have significantly improved the stoichiometry of the deposited films ŽSiO1.90 – 1.95 ..
Table 1 Oxygen : silicon stoichiometry of F2 -laser PLD SiO 2 films y2 .
High fluence Ž) 15 J cm Low fluence Ž3–4 J cmy2 .
Vacuum or 2 = 10y4 Torr argon
2 = 10y4 Torr dry air
1 = 10y3 Torr dry air
1.60 : 1 1.75 : 1
1.95 : 1 1.9 : 1
1.9 : 1 y
598
B.D. Jackson, P.R. Hermanr Applied Surface Science 127–129 (1998) 595–600
peaks at 5–8 cmy1 lower wavenumber relative to films deposited with low ablation fluences Ž3–4 J cmy2 ..
4. Discussion
Fig. 3. Infrared transmission spectra of F2-laser PLD SiO 2 films determined by referencing the sample transmission to the transmission of a blank substrate. L.F.s low fluence Ž3–4 J cmy2 ., H.F.s high fluence Ž )15 J cmy2 ..
FTIR transmission spectroscopy was used to further characterize the chemical structure of the deposited films. Characteristic Si–O–Si asymmetric stretching mode ŽASM. peak spectra are shown in Fig. 3, with the peak positions and widths summarized in Table 2. These spectra showed that the ASM peak, nominally at ; 1080 cmy1 , was shifted to lower wavenumber, and that the peak was broadened from the ; 70-cmy1 w4x width of a high quality thermal oxide. These shifts and broadening can be attributed to a combination of film stress, contamination, porosity, and O : Si substoichiometry. In particular, it is observed that the peaks of films deposited in air are 11–32 cmy1 narrower and 12–17 cmy1 higher in wavenumber relative to films deposited in vacuum. Additionally, high ablation fluences Ž) 15 J cmy2 . produced films with 4–24 cmy1 broader Table 2 Si–O–Si asymmetric stretch absorption peak data
Typical thermally-grown oxide High fluence Ž )15 J cmy2 . Vacuum 2=10y4 Torr dry air 1=10y3 Torr dry air Low fluence Ž ; 3.5 J cmy2 . Vacuum 2=10y4 Torr dry air
Peak position Žcmy1 .
Peak width Žcmy1 .
;1080
; 70 w4x
1030 1042 1047
122 95 90
1035 1047
102 91
The elimination of particulates in F2-laser PLD of silica films from a bulk fused silica target, as compared with ArF-laser PLD, can be directly linked to increased optical absorption at 157 nm. As summarized in Table 3, both the small-signal optical absorption coefficient and the effective absorption coefficient determined from ablation data are significantly larger at 157 nm than at 193 nm. Increased absorption in the solid phase is expected to reduce the initial generation of particulates, as has been shown previously by Singh et al. w11x for the case of PLD of YBCO. Further, increased plume absorption may cause the remaining particulates to be vapourized in the plume as noted by Koren et al. w12x, also for the case of YBCO ablation. In the present 157-nm study, plume absorption by molecular species will be substantially stronger than with 193-nm or longer wavelength sources. The better quality films observed for low-fluence deposition, noted by the narrowing and shift to higher wavenumber of the Si–O–Si ASM IR absorption peak, may be attributed to a reduced dissociation of silicon dioxide at low fluence. This reduced dissociation may explain the larger oxygen content of films grown in vacuum for a low ablation fluence. The addition of 2 = 10y4 Torr of dry air provided compensation for oxygen loss at both high and low fluence conditions as noted in Table 1. Background oxygen gas is clearly being incorporated into the film. This higher oxygen content also correlates with a narrowing and shift to higher wavenumber in the ASM peak ŽTable 2., providing a second indication
Table 3 Optical absorption coefficient of UV-grade fused silica
F2 ablation ArF ablation
Small signal a Žcmy1 .
a eff from ablation data Žcmy1.
14 w10x <1 w10x
2=10y5 w8x 1=10y5 w8x
B.D. Jackson, P.R. Hermanr Applied Surface Science 127–129 (1998) 595–600
of improved film quality provided by the oxygen gas. The observed substoichiometry of silica films deposited in vacuum is a result of the near-bandgap energy of the F2-laser photons. Although the photon energy lies ; 1.4 eV below the bandgap of pure silica, numerous defect states and blurring of the band-edge permit the 7.9-eV photons to photo-dissociate the bulk. Additionally, strong laser–plume interactions at 157 nm, including direct photo-dissociation of molecular oxygen, will create volatile oxygen species which are less likely to be incorporated into the deposited film. Therefore, the low oxygen content of films deposited under high laser fluence conditions may be the result of nonlinear absorption effects within the bulk target and increased plume absorption of the incident laser beam. The observed oxygen-deficiency of F2-deposited SiO 2 films is similar to results observed for YBCO deposition from a bulk or pressed ceramic target w13x. In that YBCO case, stoichiometric deposition by ArF-laser PLD required a higher background pressure of oxygen gas than was required for KrFlaser PLD, due to photo-dissociation of CuO by the 6-eV ArF-laser photons. Additionally, the work of Kurosawa et al. w14x has demonstrated that irradiation of silicon dioxide by sub-ablation-threshold fluences of 126-nm Ar2-laser radiation reduces the oxygen content of the target surface.
5. Conclusions and future work For the first time, F2-laser PLD has been demonstrated to produce virtually particulate-free silica films from a bulk fused silica target, in contrast with ArF-laser PLD. The near-bandgap energy of the F2-laser photons provides strong optical absorption of bulk silica in comparison with 193-nm radiation, resulting in the observed reduction of particulates. Although films produced at high fluences in vacuum have been shown to be ; 20% deficient in oxygen ŽSiO1.65 ., a relatively low ambient pressure of 2 = 10y4 Torr of dry air has been used to improve the stoichiometry by ; 80% ŽSiO1.9 – 1.95 .. Additionally, FTIR spectroscopy has shown that films grown at low fluences, in 2 = 10y4 Torr of dry air, have characteristic absorption peaks at 1047 cmy1 which
599
are slightly broader Ž; 90 cmy1 vs. 80 cmy1 . than those of the best previous PLD-grown SiO 2 films. The significant improvements in film quality obtained here by relatively coarse tuning of laser fluence and gas pressure indicate that, with further optimization of the growth parameters, F2-laser PLD will produce high quality silica films. Elevated substrate temperatures are expected to significantly improve the structure of the deposited films. Additionally, increased partial pressures of oxygen gas is also available to improve film quality, albeit, limited by absorption of the 157-nm laser beam by molecular oxygen to ; 100 mTorr Žfor a 5-cm lens-to-target distance.. Alternatively, generation of an activated oxygen background ŽO or O 3 . may improve the oxidation process and provide the means for low pressure deposition. Care must be taken to avoid damage to the growing film resulting, for example, from ion bombardment in plasma-assisted growth. Once optimized, the deposition process may be applied to the low-temperature growth of silica films for optical waveguide structures and passivation layers in electronics.
Acknowledgements The authors would like to thank Prof. S. Zukotynski, Dr. R.N.S. Sohdi and Prof. M.C. Goh of the University of Toronto for the use of diagnostic equipment. Funding for this project has been supplied by the Natural Sciences and Engineering Research Council of Canada and the Ontario Laser and Lightwave Research Centre.
References w1x Chrisey, Hubler ŽEds.., Pulsed Laser Deposition of Thin Films, Wiley, New York, 1994. w2x E. Fogarassy, A. Slaoui, C. Fuchs, J.P. Stoquert, Appl. Surf. Sci. 54 Ž1992. 180–186. w3x A. Slaoui, E. Fogarassy, C. Fuchs, P. Siffert, J. Appl. Phys. 71 Ž1992. 590–596. w4x P. Bergonzo, U. Kogelshatz, I.W. Boyd, Appl. Surf. Sci. 69 Ž1993. 393–397. w5x T.P. Chen, T.-I. Bao, L. I, Appl. Phys. Lett. 63 Ž18. Ž1993. 2475–2477.
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w6x P. Baeri, R. Reitano, N. Marino, Appl. Surf. Sci. 86 Ž1995. 128–133. w7x P.R. Herman, B. Chen, J.D. Moore, R. Sia, Mater. Res. Soc. Symp. Proc. 236 Ž1994. 53–58. w8x P.R. Herman, K. Beckley, B. Jackson, K. Kurosawa, D. Moore, T. Yamanishi, J. Yang, Proc. SPIE 2992 Ž1997. 86–95. w9x P.C. Hill, P.R. Herman, R. Sia, J. Appl. Phys. 73 Ž1993. 5274–5276. w10x H.R. Philipps, in: E.D. Palik ŽEd.., Handbook of Optical
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Constants of Solids, Academic Press, New York, 1985, pp. 749–763. R.K. Singh, D. Bhattacharya, J. Narayan, Appl. Phys. Lett. 61 Ž4. Ž1995. 483–485. G. Koren, R.J. Baseman, A. Gupta, M.I. Lutwyche, R.B. Laibowitz, Appl. Phys. Lett. 56 Ž21. Ž1990. 2144–2146. R.W. Dreyfus, Appl. Surf. Sci. 86 Ž1995. 29–34. K. Kurosawa, K. Nakamai, Y. Tagikawa, W. Sasaki, Y. Kato, M. Okuda, Nucl. Instr. Meth. Phys. Res. B 116 Ž1996. 410–415.