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Modification of GaAs and copper surface by the formation of SiO2 aerogel film as an interlayer dielectric
Applied Surface Science 216 (2003) 98–105
Modification of GaAs and copper surface by the formation of SiO2 aerogel film as an interlayer dielectric Sung-Woo Parka, Sang-Bae Junga, Min-Gu Kanga, Hyung-Ho Parka,*, Hae-Cheon Kimb a
Department of Ceramic Engineering, Yonsei University, 134 Shinchon-Dong, Seodaemun-Ku, Seoul 120-749, South Korea b Semiconductor Technology Division, Electronics and Telecommunications Research Institute, 161 Kajong-Dong, Yusong-Ku, Taejon 305-350, South Korea
Abstract For the application of SiO2 aerogel film to GaAs based devices, the changes of interfacial chemical bonding state of HClcleaned (or S-passivated) GaAs surface were investigated using monochromatic X-ray photoelectron spectroscopy after immersing the GaAs in each constituent of SiO2 sol. A large amount of oxide was formed on the HCl-cleaned GaAs after the treatment using tetraethoxysilane and de-ionized water due to hydroxyl group in SiO2 sol, while the S-passivated GaAs was not oxidized. The interfacial surface of GaAs was also investigated after the removing SiO2 aerogel film formed by supercritical drying. The aerogel film showed 80% of porosity and low dielectric constant of 1.8. Normally the oxidation of GaAs was successfully suppressed with S-passivation, however during the supercritical drying due to the high temperature and pressure, S-passivation layer was completely decomposed, and GaAs-oxides and elemental As were generated somewhat, but less than the case of HCl-treated GaAs. Furthermore, the formation of SiO2 aerogel film on copper metal substrate was revealed to induce a modification of metal surface. The modified and oxidized state of copper surface formed during the formation of the aerogel film was found to be not greatly influenced on the leakage current behavior of SiO2 aerogel/Cu system. # 2003 Elsevier Science B.V. All rights reserved. Keywords: SiO2 aerogel; GaAs; Copper; Sulfur passivation; XPS
1. Introduction GaAs compound semiconductor is of rising importance for opto- and micro-electronics, especially, for light emitting diode (LED), laser diode (LD), and high frequency device like monolithic microwave integrated circuit (MMIC) [1]. Recent technology in GaAs devices has been focused to simultaneously integrate digital and RF circuits. This trend inevitably demands * Corresponding author. E-mail address: [email protected] (H.-H. Park).
multi-level metallization, Thus it is expected that parasitic resistance–capacitance (RC) coupling on propagation delay, cross-talk noise, and power dissipation become important. Whereas dielectric materials mainly determine capacitance, conductor materials affect resistance. SiO2 aerogel with low dielectric constant has an enormous potential to replace conventional dielectric material of GaAs devices, i.e., Si3N4/SiO2 or benzocyclobutene (BCB). Among various low-k candidate materials, SiO2 aerogel film has drawn an attention due to its low dielectric constant, high dielectric
0169-4332/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-4332(03)00488-4
S.-W. Park et al. / Applied Surface Science 216 (2003) 98–105
strength, and good gap filling capabilities for applying to Si based devices [2–5]. However, the control of interfacial state will be a key factor when applying to GaAs devices. As for conductor material, due to its high electrical conductivity and electro-migration resistance, copper is a promising material to replace aluminum in some microelectronic applications [6]. In this study, applicability of SiO2 aerogel film for GaAs devices was studied. Two kinds of GaAs surfaces, HCl-cleaned and sulfur-passivated were prepared for the purpose of investigating the stability of surface bonding state during aerogel fabrication process. Furthermore the change in the bonding state of copper metal surface with SiO2 aerogel film was analyzed.
2. Experimental procedures The substrates used in this study were (0 0 1) oriented and n-doped with Si to a net donor level of 5 1016 cm3. The wafers were degreased by immersing in boiling acetone for 10 min and in methanol for 5 min, and rinsed with de-ionized water (DIW) of the resistance of 18 MO. They were dried in blowing nitrogen. One set of degreased GaAs was cleaned with concentrated HCl for 3 min to remove native oxide on GaAs surface and immediately rinsed with DIW [7]. The other set was sulfur-passivated GaAs prepared by dipping the HCl-cleaned GaAs in (NH4)2S solution for 10 min and then rinsed with DIW [8]. HCl-cleaned GaAs and sulfur-passivated GaAs were separately immersed in each constituent of sol for 1 h to investigate the stability of the GaAs surface in sol. Due to a reverse reaction, ethyl alcohol (EtOH) could be generated in iso-prophyl alcohol (IPA)-based sol, thus the used solvents were chosen as IPA, EtOH, tetraethoxysilane (TEOS), and DIW [9]. The bonding modification of GaAs surface by sol was monitored using X-ray photoelectron spectroscopy (XPS). SiO2 aerogel film was fabricated on HCl-cleaned GaAs and sulfur-passivated GaAs as follows. SiO2 sols were synthesized from TEOS dissolved in IPA using a two-step acid/base catalyzed procedure. Final composition of TEOS:IPA:H2O:HCl:NH4OH was 1:3:4:1:8 103 :8:13 103 . In the optimized viscosity range, the sol was spin-deposited on each
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GaAs wafer at 3000 revolutions/min for 20 s. Then each spun-on film was aged in IPA and subsequently placed in an autoclave apparatus. Supercritical drying was conducted through additional solvent method using IPA. Final temperature and pressure were 250 8C and 1160 psi, respectively. Detailed experimental procedures are described elsewhere [10]. Copper metal foil was cleaned in acetone for 10 min and 0.5 M HNO3 solution for 5 min, and then rinsed with DIW and dried in a stream of argon gas. The monochromatic XPS (VG Scientific, ESCALAB 200i-XL) measurement was carried out to characterize the chemical bonding state of GaAs after each treatment under ultra high vacuum of 8 109 Torr. The excitation source for photoemission was Al Ka radiation of 1486.6 eV and take-off angle is 908. Planar and cross-sectional images of SiO2 aerogel film were taken by using scanning electron microscopy (SEM, Hitachi S 4200). Porosity of SiO2 aerogel film was calculated using Rutherford backscattering spectrometer (RBS) of a 2 MeV He2þ Pelletron accelerator and cross-sectional SEM [5,6]. The electrical properties of metal–insulator–semiconductor (MIS) structure were measured using HP 4298A impedance/gainphase analyzer at 1 MHz and HP 4145B semiconductor parameter analyzer.
3. Results and discussion SiO2 aerogel film could be successfully deposited on HCl-cleaned and sulfur-passivated GaAs. Fig. 1 shows the planar and cross-sectional SEM images of SiO2 aerogel film on HCl-cleaned GaAs. The particle size and pore size were 20–50 nm and 30–100 nm, respectively. Porosity was 80% and measured dielectric constant in MIS structure was 1.8. This value is much lower than that of BCB (2.5), which has been widely used material in GaAs-based devices [11]. Therefore, SiO2 aerogel film with low dielectric constant can give a superior insulating property to GaAs devices used at high frequency. Fig. 2 shows Ga 3d and As 3d core level spectra of HCl-cleaned GaAs surface treated with (a) IPA, (b) EtOH, (c) TEOS, and (d) DIW. Ga 3d and As 3d spectra in Fig. 2(a) and (b) revealed that GaAs surface was oxidized a little. GaAs surface is known to be inert in alcohols [12]. Therefore, the surface oxide might be
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Fig. 1. (a) Planar and (b) cross-sectional SEM images of SiO2 aerogel film on HCl-cleaned GaAs.
Fig. 2. Ga 3d and As 3d photoelectron spectra of HCl-cleaned GaAs after immersing in (a) IPA, (b) EtOH, (c) TEOS, and (d) DIW for 1 h.
S.-W. Park et al. / Applied Surface Science 216 (2003) 98–105
induced through DIW rinsing after HCl cleaning or air exposure during the sample transfer. When treated with TEOS, GaAs surface comprises of Ga–As, Ga–O, As–As, and As–O bonds as shown in Fig. 2(c). When compared with Fig. 2(a) and (b), the amount of GaAsoxides are slightly large, and a large amount of Si–O bonds were found from the observation of silicon and oxygen spectra (not given). In Ga 3d spectra of Fig. 2(d), the spectrum was deconvoluted into Ga–As peak and Ga–O peak and in As 3d spectra, As–Ga peak, As–O peak, and As–As peak were shown. From the peak attribution, it could be said that very large amount of GaAs-oxides were generated on GaAs surface after DIW treatment. And As/Ga ratio was about 0.18 by compositional analysis. It can be inferred that GaAs surface is oxidized a lot and mainly covered with Ga oxide. GaAs-oxide is formed by
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hydroxyl ion and dissolved oxygen in DIW [13]. Due to more electropositive property of Ga, hydroxyl and oxygen ions preferentially bind with Ga and then elemental As is formed. This chemical bonding deserves much consideration because the constituents of sol are Si–OH, adsorbed water, and Si–OR (R ¼ CH3 , C2H5, etc.). Fig. 3 represents Ga 3d and As 3d core level spectra of S-passivated GaAs surface after treatment with (a) IPA, (b) EtOH, (c) TEOS, and (d) DIW. In Fig. 3, it can be found that Ga–S and As–S bonds formed a passivation layer at the uppermost surface. Note that there is no surface oxide irrespective of immersed solvent due to the passivation effect of GaAs–S bonds. This effective protection from surface oxidation enables to exclude the Fermi level pinning induced by elemental As.
Fig. 3. Ga 3d and As 3d photoelectron spectra of sulfur-passivated GaAs after immersing in (a) IPA, (b) EtOH, (c) TEOS, and (d) DIW for 1 h.
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Fig. 4. Ga 3d and As 3d photoelectron spectra of (a) HCl-cleaned and (b) sulfur-passivated GaAs surface after supercritical drying.
To fabricate SiO2 aerogel film on GaAs, supercritical drying has to be conducted. Whether passivation layer is still protective from the oxidation of GaAs surface during supercritical drying or not,
interfacial states between GaAs and aerogel after supercritical drying was investigated after the removal of aerogel film from GaAs substrate. Fig. 4 shows photoelectron spectra of (a) HCl-cleaned
Fig. 5. Leakage current behavior of SiO2 aerogel film on HCl-cleaned and sulfur-passivated GaAs.
S.-W. Park et al. / Applied Surface Science 216 (2003) 98–105
and (b) sulfur-passivated GaAs surface after supercritical drying. In Ga 3d and As 3d, it was observed that GaAs surface was made up of Ga–As bond, Ga–O bond, As–O bond, and As–As bond. Especially in Fig. 4(b), it could be found that Ga–S and As–S bonds were disappeared. Even though Ga–S and As–S bonds are known to be decomposed at 500 8C [14], however under the supercritical drying condition, anneal at 250 8C under 1160 psi, GaAs–S
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bonds were completely decomposed and this was confirmed from the absence of sulfur on the GaAs surface by XPS observation. However, oxidation of S-passivated GaAs was suppressed somewhat when compared with the case of HCl-cleaned GaAs. In both cases, the amount of Ga oxide is much larger than that of As oxide and elemental As is found. As same as the oxidation of GaAs, Ga oxide is preferentially generated during the supercritical drying and
Fig. 6. The change of interfacial copper surface during aerogel fabrication; (a) Cu 2p and (b) Cu LMM spectra.
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elemental As is liberated. To sum up, although sulfur passivation plays an important role in the obstruction of oxidation when GaAs comes in contact with sol, Ga–S and As–S were thermally decomposed and oxidation was preceded during the supercritical drying. Fig. 5 represents the leakage current behavior of SiO2 aerogel film formed on HCl-cleaned and S-passivated GaAs. The electrical property of S-passivated GaAs is a little more stable than the HCl-cleaned, because the leakage behavior was not only dependent on film itself, but also interfacial state. This was found to be higher than that observed with Si system because of interfacial GaAs-oxides which were formed during the supercritical drying. Thus for the amelioration of electrical property of aerogel/GaAs system, an introduction of chemically inert ultra thin barrier layer seems to be needed. The change of interfacial copper surface during aerogel fabrication was investigated by XPS analysis. Fig. 6 represents (a) Cu 2p and (b) Cu LMM spectra. Various surface state of copper was prepared; as received, after aging for 12 h in IPA, and after removal of aerogel thin film, respectively. In copper surface, four chemical bonding states are observed; Cu, CuO,
Cu2O, and Cu(OH)2 [15,16]. And the satellite peak is observed due to the presence of Cu2þ state. The metallic Cu state has a binding energy of 932.8 eV. In Fig. 6(a), a large amount of Cu(OH)2 was observed due to exposure to air [17]. However after spin coating and consequently aging for 12 h in IPA solvent, Cu(OH)2 was dissolved to mainly metallic Cu and Cu–O. And after supercritical drying, metallic Cu was oxidized. In photoelectron spectra, the binding energy of Cu2O is almost same as that of metallic Cu (about 0.1 eV of difference) and it is very hard to distinguish them, but there is a significant difference in Cu LMM Auger kinetic energy. In Fig. 6(b), the oxidation of Cu substrate after supercritical drying procedure could be confirmed. Fig. 7 shows the leakage current behavior of Cu/ aerogel/Si and Cu/aerogel/Cu systems. At low applied voltage, the leakage current density of Cu/aerogel/Cu system is almost one order lower than that of Cu/ aerogel/Si system. However after 30 V of applied voltage, both systems show almost same leakage current value. As previously shown in Fig. 6, Cu under the aerogel used for bottom electrode contains surface oxides such as Cu2O and CuO. However this complex oxidation state of Cu interface seems to be better than
Fig. 7. Leakage current behavior of Cu/aerogel/Si and Cu/aerogel/Cu systems.
S.-W. Park et al. / Applied Surface Science 216 (2003) 98–105
Si interface with aerogel. The difference between them should be dependent on the quality of interface. However it can be said that the modification of copper surface during the application of SiO2 aerogel as IMD is not important on the leakage current behavior of aerogel/Cu system.
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Acknowledgements This work was supported by Electronics and Telecommunications Research Institute (ETRI) and Brain Korea 21 project.
References 4. Conclusions SiO2 aerogel film with 80% porosity and dielectric constant of 1.8 was successfully deposited on GaAs. From the investigation of surface reaction between GaAs and constituents of sol, it was found that HClcleaned GaAs was oxidized with TEOS and DIW, but S-passivated GaAs was not. However after supercritical drying, S-passivated GaAs was also oxidized from thermal decomposition of GaAs–S bonds during the drying. Ga-oxide was dominant and elemental As was also observed. Both aerogel/GaAs systems showed almost same leakage current behavior and it can be concluded that S-passivation treatment is effective for the prevention of oxidation of GaAs. And for the amelioration of electrical property of aerogel/GaAs system, an introduction of chemically inert ultra thin barrier layer stronger than S–GaAs passivation bond seems to be needed. Interfacial bonding state of copper was characterized after the formation of SiO2 aerogel and complex oxidation state of copper, CuO and Cu2O was found. However it was revealed that this complex oxidation of copper interface does not affect greatly on the leakage current behavior of Cu/aerogel/Cu system from the comparison with Cu/aerogel/Si system.
[1] J. Birkeland, IEEE MTT-S Int., Atlanta, June 14–18, 1993, Microwave Symposium Digest 2 (1993), p. 1105. [2] H. Treichel, G. Ruhl, P. Ansmann, R. Wurl, C. Muller, M. Dietlmeier, G. Majer, in: Proceedings of the Dielectrics for ULSI Multilevel Interconnection Conference (DUMIC), vol. 333D, The Institute for Microelectronics Interconnection (IMIC), Santa Clara, Feb 16–17, 1998, p. 201. [3] S.P. Murarka, Solid State Tech. 39 (3) (1996) 83. [4] M.H. Jo, H.H. Park, D.J. Kim, S.H. Hyun, S.Y. Choi, J.T. Paik, J. Appl. Phys. 82 (1997) 1299. [5] M.H. Jo, J.K. Hong, H.H. Park, J.J. Kim, S.H. Hyun, S.Y. Choi, Thin Solid Films 308–309 (1997) 490. [6] H. Miyazaki, K. Hinode, Y. Homma, K. Mukai, Jpn. J. Appl. Phys. 48 (1987) 329. [7] M.G. Kang, S.H. Sa, H.H. Park, K.S. Suh, K.H. Oh, Thin Solid Films 308–309 (1997) 634. [8] M.G. Kang, H.H. Park, K.S. Suh, J.L. Lee, Thin Solid Films 290–291 (1996) 328. [9] C.J. Brinker, G.W. Scherer, Sol–Gel Science, 1st ed., Academic Press, San Diego, 1990, pp. 108–136. [10] D.J. Kim, S.H. Hyun, J. Korean Ceram. Soc. 33 (1996) 485. [11] E. Mills, Microelectron. Eng. 33 (1997) 327. [12] W. Kern, RCA Rev. 32 (1971) 64. [13] Y. Hirota, J. Appl. Phys. 75 (3) (1994) 1798. [14] K.H. Oh, S.H. Sa, M.G. Kang, H.H. Park, Surf. Coatings Technol. 100 (1998) 222. [15] J. Chastain, Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer, Eden Prairie, MN, 1992. [16] Y.S. Kim, Y. Shimogaki, J. Vac. Sci. Technol. A 19 (5) (2001) 2642. [17] T.L. Barr, J. Vac. Sci. Technol. 14 (1977) 660.
Modification of GaAs and copper surface by the formation of SiO2 aerogel film as an interlayer dielectric Sung-Woo Parka, Sang-Bae Junga, Min-Gu Kanga, Hyung-Ho Parka,*, Hae-Cheon Kimb a
Department of Ceramic Engineering, Yonsei University, 134 Shinchon-Dong, Seodaemun-Ku, Seoul 120-749, South Korea b Semiconductor Technology Division, Electronics and Telecommunications Research Institute, 161 Kajong-Dong, Yusong-Ku, Taejon 305-350, South Korea
Abstract For the application of SiO2 aerogel film to GaAs based devices, the changes of interfacial chemical bonding state of HClcleaned (or S-passivated) GaAs surface were investigated using monochromatic X-ray photoelectron spectroscopy after immersing the GaAs in each constituent of SiO2 sol. A large amount of oxide was formed on the HCl-cleaned GaAs after the treatment using tetraethoxysilane and de-ionized water due to hydroxyl group in SiO2 sol, while the S-passivated GaAs was not oxidized. The interfacial surface of GaAs was also investigated after the removing SiO2 aerogel film formed by supercritical drying. The aerogel film showed 80% of porosity and low dielectric constant of 1.8. Normally the oxidation of GaAs was successfully suppressed with S-passivation, however during the supercritical drying due to the high temperature and pressure, S-passivation layer was completely decomposed, and GaAs-oxides and elemental As were generated somewhat, but less than the case of HCl-treated GaAs. Furthermore, the formation of SiO2 aerogel film on copper metal substrate was revealed to induce a modification of metal surface. The modified and oxidized state of copper surface formed during the formation of the aerogel film was found to be not greatly influenced on the leakage current behavior of SiO2 aerogel/Cu system. # 2003 Elsevier Science B.V. All rights reserved. Keywords: SiO2 aerogel; GaAs; Copper; Sulfur passivation; XPS
1. Introduction GaAs compound semiconductor is of rising importance for opto- and micro-electronics, especially, for light emitting diode (LED), laser diode (LD), and high frequency device like monolithic microwave integrated circuit (MMIC) [1]. Recent technology in GaAs devices has been focused to simultaneously integrate digital and RF circuits. This trend inevitably demands * Corresponding author. E-mail address: [email protected] (H.-H. Park).
multi-level metallization, Thus it is expected that parasitic resistance–capacitance (RC) coupling on propagation delay, cross-talk noise, and power dissipation become important. Whereas dielectric materials mainly determine capacitance, conductor materials affect resistance. SiO2 aerogel with low dielectric constant has an enormous potential to replace conventional dielectric material of GaAs devices, i.e., Si3N4/SiO2 or benzocyclobutene (BCB). Among various low-k candidate materials, SiO2 aerogel film has drawn an attention due to its low dielectric constant, high dielectric
0169-4332/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-4332(03)00488-4
S.-W. Park et al. / Applied Surface Science 216 (2003) 98–105
strength, and good gap filling capabilities for applying to Si based devices [2–5]. However, the control of interfacial state will be a key factor when applying to GaAs devices. As for conductor material, due to its high electrical conductivity and electro-migration resistance, copper is a promising material to replace aluminum in some microelectronic applications [6]. In this study, applicability of SiO2 aerogel film for GaAs devices was studied. Two kinds of GaAs surfaces, HCl-cleaned and sulfur-passivated were prepared for the purpose of investigating the stability of surface bonding state during aerogel fabrication process. Furthermore the change in the bonding state of copper metal surface with SiO2 aerogel film was analyzed.
2. Experimental procedures The substrates used in this study were (0 0 1) oriented and n-doped with Si to a net donor level of 5 1016 cm3. The wafers were degreased by immersing in boiling acetone for 10 min and in methanol for 5 min, and rinsed with de-ionized water (DIW) of the resistance of 18 MO. They were dried in blowing nitrogen. One set of degreased GaAs was cleaned with concentrated HCl for 3 min to remove native oxide on GaAs surface and immediately rinsed with DIW [7]. The other set was sulfur-passivated GaAs prepared by dipping the HCl-cleaned GaAs in (NH4)2S solution for 10 min and then rinsed with DIW [8]. HCl-cleaned GaAs and sulfur-passivated GaAs were separately immersed in each constituent of sol for 1 h to investigate the stability of the GaAs surface in sol. Due to a reverse reaction, ethyl alcohol (EtOH) could be generated in iso-prophyl alcohol (IPA)-based sol, thus the used solvents were chosen as IPA, EtOH, tetraethoxysilane (TEOS), and DIW [9]. The bonding modification of GaAs surface by sol was monitored using X-ray photoelectron spectroscopy (XPS). SiO2 aerogel film was fabricated on HCl-cleaned GaAs and sulfur-passivated GaAs as follows. SiO2 sols were synthesized from TEOS dissolved in IPA using a two-step acid/base catalyzed procedure. Final composition of TEOS:IPA:H2O:HCl:NH4OH was 1:3:4:1:8 103 :8:13 103 . In the optimized viscosity range, the sol was spin-deposited on each
99
GaAs wafer at 3000 revolutions/min for 20 s. Then each spun-on film was aged in IPA and subsequently placed in an autoclave apparatus. Supercritical drying was conducted through additional solvent method using IPA. Final temperature and pressure were 250 8C and 1160 psi, respectively. Detailed experimental procedures are described elsewhere [10]. Copper metal foil was cleaned in acetone for 10 min and 0.5 M HNO3 solution for 5 min, and then rinsed with DIW and dried in a stream of argon gas. The monochromatic XPS (VG Scientific, ESCALAB 200i-XL) measurement was carried out to characterize the chemical bonding state of GaAs after each treatment under ultra high vacuum of 8 109 Torr. The excitation source for photoemission was Al Ka radiation of 1486.6 eV and take-off angle is 908. Planar and cross-sectional images of SiO2 aerogel film were taken by using scanning electron microscopy (SEM, Hitachi S 4200). Porosity of SiO2 aerogel film was calculated using Rutherford backscattering spectrometer (RBS) of a 2 MeV He2þ Pelletron accelerator and cross-sectional SEM [5,6]. The electrical properties of metal–insulator–semiconductor (MIS) structure were measured using HP 4298A impedance/gainphase analyzer at 1 MHz and HP 4145B semiconductor parameter analyzer.
3. Results and discussion SiO2 aerogel film could be successfully deposited on HCl-cleaned and sulfur-passivated GaAs. Fig. 1 shows the planar and cross-sectional SEM images of SiO2 aerogel film on HCl-cleaned GaAs. The particle size and pore size were 20–50 nm and 30–100 nm, respectively. Porosity was 80% and measured dielectric constant in MIS structure was 1.8. This value is much lower than that of BCB (2.5), which has been widely used material in GaAs-based devices [11]. Therefore, SiO2 aerogel film with low dielectric constant can give a superior insulating property to GaAs devices used at high frequency. Fig. 2 shows Ga 3d and As 3d core level spectra of HCl-cleaned GaAs surface treated with (a) IPA, (b) EtOH, (c) TEOS, and (d) DIW. Ga 3d and As 3d spectra in Fig. 2(a) and (b) revealed that GaAs surface was oxidized a little. GaAs surface is known to be inert in alcohols [12]. Therefore, the surface oxide might be
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Fig. 1. (a) Planar and (b) cross-sectional SEM images of SiO2 aerogel film on HCl-cleaned GaAs.
Fig. 2. Ga 3d and As 3d photoelectron spectra of HCl-cleaned GaAs after immersing in (a) IPA, (b) EtOH, (c) TEOS, and (d) DIW for 1 h.
S.-W. Park et al. / Applied Surface Science 216 (2003) 98–105
induced through DIW rinsing after HCl cleaning or air exposure during the sample transfer. When treated with TEOS, GaAs surface comprises of Ga–As, Ga–O, As–As, and As–O bonds as shown in Fig. 2(c). When compared with Fig. 2(a) and (b), the amount of GaAsoxides are slightly large, and a large amount of Si–O bonds were found from the observation of silicon and oxygen spectra (not given). In Ga 3d spectra of Fig. 2(d), the spectrum was deconvoluted into Ga–As peak and Ga–O peak and in As 3d spectra, As–Ga peak, As–O peak, and As–As peak were shown. From the peak attribution, it could be said that very large amount of GaAs-oxides were generated on GaAs surface after DIW treatment. And As/Ga ratio was about 0.18 by compositional analysis. It can be inferred that GaAs surface is oxidized a lot and mainly covered with Ga oxide. GaAs-oxide is formed by
101
hydroxyl ion and dissolved oxygen in DIW [13]. Due to more electropositive property of Ga, hydroxyl and oxygen ions preferentially bind with Ga and then elemental As is formed. This chemical bonding deserves much consideration because the constituents of sol are Si–OH, adsorbed water, and Si–OR (R ¼ CH3 , C2H5, etc.). Fig. 3 represents Ga 3d and As 3d core level spectra of S-passivated GaAs surface after treatment with (a) IPA, (b) EtOH, (c) TEOS, and (d) DIW. In Fig. 3, it can be found that Ga–S and As–S bonds formed a passivation layer at the uppermost surface. Note that there is no surface oxide irrespective of immersed solvent due to the passivation effect of GaAs–S bonds. This effective protection from surface oxidation enables to exclude the Fermi level pinning induced by elemental As.
Fig. 3. Ga 3d and As 3d photoelectron spectra of sulfur-passivated GaAs after immersing in (a) IPA, (b) EtOH, (c) TEOS, and (d) DIW for 1 h.
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Fig. 4. Ga 3d and As 3d photoelectron spectra of (a) HCl-cleaned and (b) sulfur-passivated GaAs surface after supercritical drying.
To fabricate SiO2 aerogel film on GaAs, supercritical drying has to be conducted. Whether passivation layer is still protective from the oxidation of GaAs surface during supercritical drying or not,
interfacial states between GaAs and aerogel after supercritical drying was investigated after the removal of aerogel film from GaAs substrate. Fig. 4 shows photoelectron spectra of (a) HCl-cleaned
Fig. 5. Leakage current behavior of SiO2 aerogel film on HCl-cleaned and sulfur-passivated GaAs.
S.-W. Park et al. / Applied Surface Science 216 (2003) 98–105
and (b) sulfur-passivated GaAs surface after supercritical drying. In Ga 3d and As 3d, it was observed that GaAs surface was made up of Ga–As bond, Ga–O bond, As–O bond, and As–As bond. Especially in Fig. 4(b), it could be found that Ga–S and As–S bonds were disappeared. Even though Ga–S and As–S bonds are known to be decomposed at 500 8C [14], however under the supercritical drying condition, anneal at 250 8C under 1160 psi, GaAs–S
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bonds were completely decomposed and this was confirmed from the absence of sulfur on the GaAs surface by XPS observation. However, oxidation of S-passivated GaAs was suppressed somewhat when compared with the case of HCl-cleaned GaAs. In both cases, the amount of Ga oxide is much larger than that of As oxide and elemental As is found. As same as the oxidation of GaAs, Ga oxide is preferentially generated during the supercritical drying and
Fig. 6. The change of interfacial copper surface during aerogel fabrication; (a) Cu 2p and (b) Cu LMM spectra.
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elemental As is liberated. To sum up, although sulfur passivation plays an important role in the obstruction of oxidation when GaAs comes in contact with sol, Ga–S and As–S were thermally decomposed and oxidation was preceded during the supercritical drying. Fig. 5 represents the leakage current behavior of SiO2 aerogel film formed on HCl-cleaned and S-passivated GaAs. The electrical property of S-passivated GaAs is a little more stable than the HCl-cleaned, because the leakage behavior was not only dependent on film itself, but also interfacial state. This was found to be higher than that observed with Si system because of interfacial GaAs-oxides which were formed during the supercritical drying. Thus for the amelioration of electrical property of aerogel/GaAs system, an introduction of chemically inert ultra thin barrier layer seems to be needed. The change of interfacial copper surface during aerogel fabrication was investigated by XPS analysis. Fig. 6 represents (a) Cu 2p and (b) Cu LMM spectra. Various surface state of copper was prepared; as received, after aging for 12 h in IPA, and after removal of aerogel thin film, respectively. In copper surface, four chemical bonding states are observed; Cu, CuO,
Cu2O, and Cu(OH)2 [15,16]. And the satellite peak is observed due to the presence of Cu2þ state. The metallic Cu state has a binding energy of 932.8 eV. In Fig. 6(a), a large amount of Cu(OH)2 was observed due to exposure to air [17]. However after spin coating and consequently aging for 12 h in IPA solvent, Cu(OH)2 was dissolved to mainly metallic Cu and Cu–O. And after supercritical drying, metallic Cu was oxidized. In photoelectron spectra, the binding energy of Cu2O is almost same as that of metallic Cu (about 0.1 eV of difference) and it is very hard to distinguish them, but there is a significant difference in Cu LMM Auger kinetic energy. In Fig. 6(b), the oxidation of Cu substrate after supercritical drying procedure could be confirmed. Fig. 7 shows the leakage current behavior of Cu/ aerogel/Si and Cu/aerogel/Cu systems. At low applied voltage, the leakage current density of Cu/aerogel/Cu system is almost one order lower than that of Cu/ aerogel/Si system. However after 30 V of applied voltage, both systems show almost same leakage current value. As previously shown in Fig. 6, Cu under the aerogel used for bottom electrode contains surface oxides such as Cu2O and CuO. However this complex oxidation state of Cu interface seems to be better than
Fig. 7. Leakage current behavior of Cu/aerogel/Si and Cu/aerogel/Cu systems.
S.-W. Park et al. / Applied Surface Science 216 (2003) 98–105
Si interface with aerogel. The difference between them should be dependent on the quality of interface. However it can be said that the modification of copper surface during the application of SiO2 aerogel as IMD is not important on the leakage current behavior of aerogel/Cu system.
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Acknowledgements This work was supported by Electronics and Telecommunications Research Institute (ETRI) and Brain Korea 21 project.
References 4. Conclusions SiO2 aerogel film with 80% porosity and dielectric constant of 1.8 was successfully deposited on GaAs. From the investigation of surface reaction between GaAs and constituents of sol, it was found that HClcleaned GaAs was oxidized with TEOS and DIW, but S-passivated GaAs was not. However after supercritical drying, S-passivated GaAs was also oxidized from thermal decomposition of GaAs–S bonds during the drying. Ga-oxide was dominant and elemental As was also observed. Both aerogel/GaAs systems showed almost same leakage current behavior and it can be concluded that S-passivation treatment is effective for the prevention of oxidation of GaAs. And for the amelioration of electrical property of aerogel/GaAs system, an introduction of chemically inert ultra thin barrier layer stronger than S–GaAs passivation bond seems to be needed. Interfacial bonding state of copper was characterized after the formation of SiO2 aerogel and complex oxidation state of copper, CuO and Cu2O was found. However it was revealed that this complex oxidation of copper interface does not affect greatly on the leakage current behavior of Cu/aerogel/Cu system from the comparison with Cu/aerogel/Si system.
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