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SiO2 Atomic Layer Deposition Using Tris(dimethylamino)silane and Hydrogen Peroxide Studied by in Situ Transmission FTIR Spectroscopy B. B. Burton,† S. W. Kang,⊥,‡ S. W. Rhee,‡ and S. M. George*,†,§ Department of Chemistry and Biochemistry, UniVersity of Colorado, Boulder, Colorado 80309-0215, Department of Chemical Engineering, Pohang UniVersity of Science and Technology, Pohang 790-784, South Korea, and Department of Chemical and Biological Engineering, UniVersity of Colorado, Boulder, Colorado 80309-0215 ReceiVed: July 26, 2008; ReVised Manuscript ReceiVed: February 25, 2009
The atomic layer deposition (ALD) of silicon dioxide (SiO2) was initially explored using a variety of silicon precursors with H2O as the oxidant. The silicon precursors were (N,N-dimethylamino)trimethylsilane) (CH3)3SiN(CH3)2, vinyltrimethoxysilane CH2dCHSi(OCH3)3, trivinylmethoxysilane (CH2dCH)3SiOCH3, tetrakis(dimethylamino)silane Si(N(CH3)2)4, and tris(dimethylamino)silane (TDMAS) SiH(N(CH3)2)3. TDMAS was determined to be the most effective of these precursors. However, additional studies determined that SiH* surface species from TDMAS were difficult to remove using only H2O. Subsequent studies utilized TDMAS and H2O2 as the oxidant and explored SiO2 ALD in the temperature range of 150-550 °C. The exposures required for the TDMAS and H2O2 surface reactions to reach completion were monitored using in situ FTIR spectroscopy. The FTIR vibrational spectra following the TDMAS exposures showed a loss of absorbance for O-H stretching vibrations and a gain of absorbance for C-Hx and Si-H stretching vibrations. The FTIR vibrational spectra following the H2O2 exposures displayed a loss of absorbance for C-Hx and Si-H stretching vibrations and an increase of absorbance for the O-H stretching vibrations. The SiH* surface species were completely removed only at temperatures >450 °C. The bulk vibrational modes of SiO2 were observed between 1000-1250 cm-1 and grew progressively with number of TDMAS and H2O2 reaction cycles. Transmission electron microscopy (TEM) was performed after 50 TDMAS and H2O2 reaction cycles on ZrO2 nanoparticles at temperatures between 150-550 °C. The film thickness determined by TEM at each temperature was used to obtain the SiO2 ALD growth rate. The growth per cycle varied from 0.8 Å/cycle at 150 °C to 1.8 Å/cycle at 550 °C and was correlated with the removal of the SiH* surface species. SiO2 ALD using TDMAS and H2O2 should be valuable for SiO2 ALD at temperatures >450 °C. I. Introduction Silicon dioxide (SiO2) is a common dielectric material in silicon microelectronic devices.1 High quality SiO2 has been formed by the thermal oxidation of silicon between 700-900 °C.2 SiO2 has also been deposited by chemical vapor deposition (CVD).3-10 Some SiO2 CVD approaches have utilized plasma11,12 or a NH3 catalyst.13 However, CVD is not conformal in high aspect ratio structures and displays void formation in trenches and vias. Atomic layer deposition (ALD) methods can be used to obtain conformality and atomic layer control of thin film growth. Atomic layer deposition (ALD) is a growth method based on sequential, self-limiting surface reactions.14 A variety of materials including oxides, nitrides, and various metals have been deposited using ALD.14,15 Despite its importance, SiO2 ALD has been difficult. SiO2 ALD using SiCl4 and H2O requires high temperatures >325 °C and large reactant exposures of >109 L (1 L ) 10-6 Torr s).16-18 In contrast, catalyzed SiO2 ALD can be accomplished using NH3 or NC5H5 at temperatures close to room temperature and * Corresponding author. † Department of Chemistry and Biochemistry University of Colorado. ‡ Pohang University of Science and Technology. § Department of Chemical and Biological Engineering, University of Colorado. ⊥ Current address: Vacuum Center, Division of Advanced Technology, Korea Research Institute of Standards and Science, Daejeon, 305-340, South Korea.
exposures ∼103-104 L.18-22 Unfortunately, the use of halides results in the release of corrosive HCl. In addition, the HCl can react with the amine catalyst to form chloride salts that can lead to film contamination.21 To avoid using halides, SiO2 ALD was also accomplished using Si(OCH2CH3)4, and H2O along with NH3 as a catalyst.23 This ALD system reacted at room temperature but required large exposures of ∼1010 L for the surface reactions to reach completion.23 A variety of other precursor combinations have also been used for SiO2 ALD. Si(NCO)4 and either H2O or N(C2H5)3 have been used to deposit SiO2 at 25 °C.24,25 The reported required exposure times were 120 s when using H2O24 and 180 s when using N(C2H5)3.25 Subsequent attempts utilized CH3OSi(NCO)3 and H2O2 at 25 °C but also required large exposure times between 300-450 s.26 The use of ozone (O3)27,28 and O2/N2 plasma29 have also been investigated to reduce growth temperatures and exposures. SiO2 ALD using SiH2Cl2 and O3 was accomplished at 300 °C but required large exposures on the order of 109 L.27 Subsequently, both SiH2(N(CH3)2)2 and SiH(N(CH3)2)3 were studied with O3 at 275 °C.28 This study revealed that high amounts of hydrogen were incorporated into the film most likely because of the unreactive SiH species in the parent molecule.28 In addition, the reactant mixture of ClSi(N(CH3)2)3 and Si(N(CH3)2)4 was used with O2/N2 plasma and was determined to deposit SiO2 between 100-250 °C with dose times of 1 s for each reactant.29
10.1021/jp806638e CCC: $40.75 2009 American Chemical Society Published on Web 04/17/2009
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Recently, a new SiO2 deposition method using silanols with Al catalysts has been developed that is known as rapid SiO2 ALD.30-32 Rapid SiO2 ALD has the ability to produce deposition rates >100 times higher than conventional SiO2 ALD approaches while maintaining self-limiting behavior.32 Rapid SiO2 ALD using tris(tert-butoxy)silanol deposited SiO2 layers with thickness up to ∼120 Å in one silanol exposure in the temperature range of 225-250 °C.32 Rapid SiO2 has also been accomplished using tris(tert-pentoxy)silanol (TPS) in the temperature range of 125-300 °C.30 TPS displayed a maximum growth per silanol exposure of 165 Å in the temperature range of 125-150 °C. The mechanism of rapid SiO2 ALD using TPS involves the insertion of silanol monomers to grow siloxane chains that crosslink by forming Si-O-Si bonds.30 In this study, a variety of silicon precursors were investigated with the goal to develop an improved thermal SiO2 ALD process that provides atomic layer thickness control, operates with efficient exposures, and avoids the use of halogen precursors. The silicon precursors contain new ligands and different combinations of ligands including vinyl (Si-CHdCH2), methoxy (Si-OCH3), dimethylamino (Si-N(CH3)2), methyl (SiCH3), and hydrogen (Si-H). These ligands span a range of sizes and Si-R bond energies that explore the effects of steric hindrance and activation barriers on the silicon reactivity. The silicon precursors investigated were (N,N-dimethylamino)trimethylsilane (DMATMS) (CH3)3SiN(CH3)2, vinyltrimethoxysilane (VTMOS) CH2dCHSi(OCH3)3, trivinylmethoxysilane (TVMOS) (CH2dCH)3SiOCH3, tetrakis(dimethylamino)silane (TKDMAS) Si(N(CH3)2)4, and tris(dimethylamino)silane (TDMAS) SiH(N(CH3)2)3. These precursors all have reasonable vapor pressures. For the initial survey, H2O was utilized as the oxidant. This survey of various silicon precursors revealed that TDMAS displayed the highest reactivity. Unfortunately, the SiH* surface species deposited by TDMAS were found to be fairly unreactive using H2O as the oxidant. To remove the SiH* surface species, this study explored 50 wt.% H2O2 and a wide range of substrate temperatures from 150-550 °C. The overall proposed SiO2 ALD reaction using TDMAS and H2O2 can be written as:
HSi(N(CH3)2)3 + 5/2H2O2 f SiO2 + 3HN(CH3)2 + 3H2O (1) In situ Fourier transform infrared (FTIR) spectroscopy was used to determine the surface species following the TDMAS and H2O2 exposures and also to verify that each surface reaction reaches completion. Monitoring the surface species with in situ FTIR spectroscopy versus temperature identified the presence of SiH* surface species at 450 °C. Figure 5 shows the absorbance for the absolute FTIR spectra following saturation exposures of TDMAS and H2O2 for the 1st, 5th, and 25th ALD cycles at 550 °C. The spectra are again shifted for clarity in presentation. Each TDMAS saturation exposure results in absorbance from Si-H stretching vibrations and a much weaker absorbance from C-Hx stretching vibrations. The absorbance from Si-H stretching vibrations is absent after the 1st, 5th, and 25th H2O2 exposure at 550 °C. In contrast to the results at 250 °C in Figure 2, there are no SiH* species that build up in the SiO2 film. Figure 5 also reveals that no hydrogen-bonded SiOH* species are observed after the 5th or 25th H2O2 exposures in Figures 5d and 5f. Only the isolated SiOH* species is observed in Figures 5d and 5f. The absence of hydrogen-bonded SiOH* species is attributed to dehydroxylation that produces H2O at 550 °C. The absorbance from Si-O combination bands was also apparent by the 25th ALD cycle in Figures 5e and 5f. After 25 ALD cycles, the absorbance of the Si-O combination bands at 550 °C in Figure 5 is stronger than at 250 °C in Figure 2. The adsorption of TDMAS and H2O2 versus reactant exposure was examined during the 10th ALD cycle at 550 °C. Figure 6a shows the normalized integrated absorbance of the C-Hx, Si-H, and O-H stretching vibrational features versus reactant exposure at 550 °C. A reactant exposure of 1.0 × 106 L was sufficient for the completion of the TDMAS reaction as confirmed by the removal of absorbance from the O-H stretching vibrations. TDMAS exposures larger than 1.0 × 106 L yielded no further increase in absorbance from C-Hx or Si-H stretching vibrations. This behavior indicates that the surface reactions are selflimiting. Figure 6b shows the normalized integrated absorbance of the C-Hx, Si-H, and the O-H stretching vibrations versus H2O2 exposure during the 10th ALD cycle at 550 °C. A reactant
Figure 6. Normalized integrated absorbance of the O-H, Si-H, and C-H stretching vibrations during the 10th reaction cycle of SiO2 ALD on ZrO2 nanoparticles at 550 °C for (a) TDMAS exposure and (b) H2O2 exposure.
exposure of 5 × 106 L was sufficient for the removal of absorbance from the C-Hx stretching vibrations. However, the complete removal of absorbance from Si-H stretching vibrations required H2O2 exposures of 5.5 × 107 L. The absorbance from O-H stretching vibrations was also saturated after H2O2 exposures >1 × 107 L. This behavior again is consistent with a self-limiting surface reaction. SiO2 ALD was then performed at 550 °C with reactant exposures of 1.2 × 106 L and 7.5 × 107 L for TDMAS and H2O2, respectively. Figure 7 displays the absolute FTIR spectra for the Si-O vibrational modes between 900-1300 cm-1 observed on ZrO2 nanoparticles after the 1st, 3rd, 5th, and 15th ALD cycles at 550 °C. All FTIR spectra were referenced to the spectrum following the 10 ALD cycles of Al2O3 ALD prior to the SiO2 ALD. The peak bulk Si-O vibrational features are observed at ∼1050 and ∼1200 cm-1. These features are consistent with bulk Si-O vibrational features that have been observed previously.42,47 The spectral region below 850 cm-1 was inaccessible because of absorption from Zr-O vibrational modes in the ZrO2 nanoparticles. Figure 7 shows that the absorbance from Si-O bulk vibrational features grows progressively with number of ALD cycles. The SiO2 ALD thickness cannot easily be determined
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Figure 7. Absolute FTIR spectra of absorbance from bulk Si-O vibrational features during SiO2 ALD after the 1st, 3rd, 5th, 10th, and 15th reaction cycle on ZrO2 nanoparticles at 550 °C.
Figure 9. TEM image after 50 reaction cycles of SiO2 ALD on ZrO2 nanoparticles at 550 °C showing a film thickness of ∼100 Å.
Figure 8. TEM image after 50 reaction cycles of SiO2 ALD on ZrO2 nanoparticles at 250 °C showing a film thickness of ∼50 Å.
from this absorbance. The exact surface area of the ZrO2 nanoparticles is not known. The maximum absorbance is also limited by pinholes in the tungsten grid holding the ZrO2 nanoparticles.49,50 Consequently, the absorbance begins to saturate after 10 and 15 ALD cycles as observed in Figure 7. 3. Transmission Electron Microscopy of SiO2 ALD. TEM was used to investigate the conformality of the SiO2 ALD on ZrO2 nanoparticles. The TEM images were recorded after 50 ALD cycles of SiO2 ALD at temperatures ranging from 150-550 °C. Before SiO2 ALD, sequential exposures of TMA and H2O were also used to deposit an Al2O3 ALD adhesion layer with a thickness of ∼10 Å on the ZrO2 nanoparticles at 177 °C. The 50 ALD cycles of SiO2 ALD were all performed under conditions where the surface reactions reached completion as given by Figures 3 and 6. Figure 8 displays the TEM image of ZrO2 nanoparticles coated by 50 reaction cycles of SiO2 ALD on the Al2O3 ALD adhesion layer at 250 °C. The total thickness of this coating is ∼50 Å. The TEM image reveals that the coating is very conformal on the underlying ZrO2 nanoparticles. The TEM
Figure 10. Temperature dependence of the SiO2 ALD growth per cycle and the Si-H integrated absorbance during the 10th reaction cycle after the H2O2 exposure.
image in Figure 9 displays the SiO2 ALD film deposited by 50 reaction cycles on ZrO2 nanoparticles at 550 °C. The total thickness of this film is ∼100 Å. After 50 ALD cycles, the thickness observed at 550 °C is larger than the thickness observed at 250 °C. The TEM image also reveals that the coating deposited at 550 °C is very conformal on the ZrO2 nanoparticles. Similar TEM results were observed using this SiO2 ALD surface chemistry on TiO2 nanoparticles at 500 °C.51 Figure 10 displays the growth per cycle versus deposition temperature determined by the ex situ TEM analysis. This growth per cycle averages over all the reaction cycles including the initial reaction cycles on the ZrO2 nanoparticles. The SiO2 ALD growth per cycle increases at higher temperatures. Figure 10 also shows the integrated absorbance for the Si-H stretching vibrations measured after saturation H2O2 exposures during the 10th ALD cycle. The decrease in the integrated absorbance for the Si-H stretching vibrations after saturation H2O2 exposures correlates with the increase in the SiO2 ALD growth per cycle.
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Burton et al.
(A1)
SiOH* + HSi(N(CH3)2)3 f SiO-SiH(N(CH3)2)2*+HN(CH3)2 (2) (A2)
SiN(CH3)2* f SiNdCH2*+CH4
(3)
All surface sites are proposed to undergo reaction A1 where TDMAS reacts with a surface hydroxyl to release dimethylamine. Following reaction A1, β-hydride elimination between neighboring methyl groups on some dimethylamino ligands can release CH4 as given by reaction A2 in eq 3. The subsequent H2O exposure was determined to react with N(CH3)2* or NdCH2* surface species and leave a surface hydroxyl as given by reactions B1 and B2 in eqs 4 and 5, respectively. However, H2O was unable to react with the SiH* surface species. H2O2 was required to react with the SiH* surface species, produce a surface hydroxyl group, and release H2O as given in reaction B3 in eq 6.
(B1)
SiN(CH3)2* + H2O f Si-OH* + HN(CH3)2
(4) (B2)
SiNdCH2* + H2O f Si-OH* + HNdCH2 (5) (B3)
Figure 11. Proposed surface reactions based on FTIR results for SiO2 ALD using sequential exposures of TDMAS and H2O2 at 550 °C.
This correlation indicates that the SiH* species limit the SiO2 ALD probably by blocking surface sites needed for film growth. A steeper increase in the SiO2 ALD growth per cycle is observed in Figure 10 at >350 °C. This increase is partially attributed to some SiO2 CVD resulting from the partial decomposition of the TDMAS precursor. Previous investigations of Ta(N(CH3)2)5, determined that N(CH3)2 ligands could decompose at temperatures as low as 277 °C.48 The difference FTIR spectra in Figure 4 also displayed a broadened absorption feature from 1300-1700 cm-1 compared with Figure 1 that may be consistent with new surface species resulting from some TDMAS decomposition. Although the FTIR results are consistent with the deposition of SiO2 films, some nitrogen or carbon could be present in the films. The Si-N vibration observed in Si3N4 appears at ∼850 cm-1.12 The Si-C vibration is observed at ∼800 cm-1.52 Both of these vibrations are concealed by the Zr-O bulk vibrations of the ZrO2 nanoparticles. Additional confirmation of the composition of the SiO2 films deposited using TDMAS and H2O2 requires analysis by X-ray photoelectron spectroscopy or Auger electron spectroscopy. C. SiO2 ALD Growth Mechanism at 550 °C. The FTIR results can be used to propose a growth mechanism for SiO2 ALD using TDMAS and H2O2/H2O at 550 °C. Figure 11 shows the proposed reactions during SiO2 ALD with TDMAS and H2O2/H2O. The possible surface reactions during the TDMAS exposure are given by reactions A1 and A2 in eqs 2 and 3. The asterisks designate the surface species.
SiH* + H2O2 f Si-OH* + H2O
(6)
Equations 2-6 can explain most of the observed vibrational frequencies during the TDMAS and H2O2 reactions. The primary surface species are N(CH3)2* and SiH* following the TDMAS exposures and SiOH* following the H2O2/H2O exposures. The SiH* surface species play a critical role in determining the SiO2 ALD growth per cycle. These SiH* surface species are completely removed only with H2O2 at higher temperatures of >450 °C. 4. Conclusions SiO2 ALD was explored using a variety of silicon precursors with H2O as the oxidant. The silicon precursors were DMATMS, VTMOS, TVMOS, TKDMAS, and TDMAS. The low reactivity of VTMOS and TVMOS indicated that H2O and SiOH* surface species do not easily react with Si-OR ligands. The low reactivity DMATMS, VTMOS, and TVMOS also suggested that breaking Si-C bonds to form Si-O bonds may have an activation barrier that is kinetically limiting. Although the N(CH3)2 ligand was shown to react very favorably with H2O and SiOH* surface species, TKDMAS was not a favorable precursor for SiO2 ALD. The low reactivity of TKDMAS was attributed to steric repulsion and high stability resulting from four N(CH3)2 ligands around the Si center in TKDMAS. TDMAS demonstrated reasonable reactivity with H2O because the N(CH3)2 ligands are reactive and the presence of the Si-H bond lowers the steric repulsion around the Si center compared with TKDMAS. However, the SiH* surface species were not removed using H2O as an oxidant. The removal of SiH* surface species required H2O2 as an oxidant and reaction temperatures >450 °C. Using TDMAS and H2O2, SiO2 ALD films were grown efficiently using reactant exposures of ∼1 × 106 L at temperatures >450 °C. The primary surface species on the surface determined by in situ FTIR spectroscopy after the TDMAS and H2O2 exposures were SiH* and SiOH*, respectively. The SiO2 ALD growth was observed using in situ FTIR spectroscopy. The growth of SiO2 ALD films on ZrO2 nano-
SiO2 Atomic Layer Deposition particles was also measured using TEM. The SiO2 ALD growth per cycle was found to increase progressively with reaction temperature from 0.8 Å/cycle at 150 °C to 1.8 Å/cycle at 550 °C. This increase was correlated with the progressive removal of SiH* surface species at higher temperatures. The SiO2 ALD films were deposited with high conformality on ZrO nanoparticles. SiO2 ALD using TDMAS and H2O2 is expected to be useful for efficient SiO2 ALD at temperatures >450 °C. Acknowledgment. This research was supported by the National Science Foundation under Grants CHE-0408554 and CHE-0715552. Additional support was provided by Ferro Electronic Material Systems. This research was also supported by the Korea Science and Engineering Foundation (KOSEF) through the National Research Laboratory Project and the POSTECH Core Research Program. References and Notes (1) Wolf, S.; Tauber, R. N. Silicon Processing for the VLSI Era; Lattice Press: Sunset Beach, CA, 1986. (2) Sze, S. M. VLSI Technology; McGraw-Hill: New York, 1983. (3) Adams, A. C.; Capio, C. D. J. Electrochem. Soc. 1979, 126, 1042. (4) Becker, F. S.; Pawlik, D.; Anzinger, H.; Spitzer, A. J. Vac. Sci. Technol. B 1987, 5, 1555. (5) Ong, T. P.; Tobin, P.; Mele, T. J. Appl. Phys. 1995, 77, 6055. (6) Ehrlich, D. J.; Melngailis, J. Appl. Phys. Lett. 1991, 58, 2675. (7) Park, B.; Conti, R.; Economikos, L.; Chakravarti, A.; Ellenberger, J. J. Vac. Sci. Technol. B 2001, 19, 1788. (8) Sakaue, H.; Nakano, M.; Ichihara, T.; Horiike, Y. Jpn. J. Appl. Phys. 2 1991, 30, L124. (9) Watanabe, K.; Tanigaki, T.; Wakayama, S. J. Electrochem. Soc. 1981, 128, 2630. (10) Miller, K. A.; John, C.; Zhang, K. Z.; Nicholson, K. T.; McFeely, F. R.; Holl, M. M. B. Thin Solid Films 2001, 397, 78. (11) Adams, A. C.; Alexander, F. B.; Capio, C. D.; Smith, T. E. J. Electrochem. Soc. 1981, 128, 1545. (12) Parsons, G. N.; Souk, J. H.; Batey, J. J. Appl. Phys. 1991, 70, 1553. (13) Klaus, J. W.; George, S. M. J. Electrochem. Soc. 2000, 147, 2658. (14) George, S. M.; Ott, A. W.; Klaus, J. W. J. Phys. Chem. 1996, 100, 13121. (15) Ritala, M.; Leskela, M. Handbook of Thin Film Materials; Academic Press: San Diego, CA, 2001. (16) Sneh, O.; Wise, M. L.; Ott, A. W.; Okada, L. A.; George, S. M. Surf. Sci. 1995, 334, 135. (17) Klaus, J. W.; Ott, A. W.; Johnson, J. M.; George, S. M. Appl. Phys. Lett. 1997, 70, 1092. (18) Klaus, J. W.; Sneh, O.; Ott, A. W.; George, S. M. Surf. ReV. Lett. 1999, 6, 435. (19) Klaus, J. W.; George, S. M. Surf. Sci. 2000, 447, 81. (20) Klaus, J. W.; Sneh, O.; George, S. M. Science 1997, 278, 1934. (21) Du, Y.; Du, X.; George, S. M. Thin Solid Films 2005, 491, 43.
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