- Email: [email protected]
metal interface
Thin Solid Films 447 – 448 (2004) 575–579
Investigation of the bonding states of the SiO2 aerogel filmymetal interface Sang-Bae Junga, Hyung-Ho Parka,*, Haecheon Kimb a Department of Ceramic Engineering, Yonsei University, 134 Shinchon-Dong, Seodaemun-Ku, Seoul 120-749, South Korea Semiconductor Technology Division, Electronics and Telecommunications Research Institute, 161 Kajong-Dong, Yusong-Ku, Taejon 305-350, South Korea
b
Abstract Due to a rapid decrease in the physical dimensions of today’s microelectronic devices, the RC-time-delay of the interconnection is now a serious problem. As a possible plan, lower resistive metal or lower dielectric constant material has to be introduced. For a low dielectric constant material, SiO2 aerogel may be a promising candidate for an interlayer dielectric (ILD) due to its relatively small dielectric constant. However, the formation of a SiO2 aerogel film on metal substrate may induce a modification of the metal surface because the aerogel film is made by the sol–gel process. Thus, this investigation focused on the interface formation of SiO2 aerogel film and substrate metal is important for the application of low-k material. It was revealed that aluminum silicate bond during aging of spun-on film was induced at the interface with Al and oxidized Al bond was increased after supercritical drying. Copper silicate bond was formed at the interface of the aged film and maintained after fabrication of SiO2 aerogel film. Cu(OH)2 bond, which did not exist at the Cu surface, was generated during film fabrication process. Measurement of leakage current of SiO2 aerogel film deposited on various substrates indicated the degradation of material property in AlySiO2 aerogelyCu structure. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Low dielectric; SiO2 aerogel film; Interface; Copper silicate bond; Leakage current; Al; Cu
1. Introduction Due to the rapid decrease in the dimensions of ultralarge-scaled-integration (ULSI) devices, interconnect RC delay time is a serious problem which cannot be neglected w1x. This delay time will be overcome both by changing the Al(Cu) metal of today’s technology by the lower resistivity Cu and by using a new low dielectric material. Among candidates for low dielectric constant materials, SiO2 aerogel is one of promising low-k materials due to its low dielectric constant and material adaptability. Highly porous SiO2 aerogel film is fabricated by the sol–gel polymerization and subsequent drying procedure, which excludes the collapse of pore structure w2x. Supercritical drying is the high temperature and high pressure process used with the supercritical fluid state. *Corresponding author. Tel.: q82-2-2123-2853; fax: q82-2-3655882. E-mail address: [email protected] (H.-H. Park).
This new material will confront problems related to the integration of the low-k materials with metal, which include chemical and electrical stability, adhesion, etc. The most probable areas to detect instabilities are the interfaces of low-k layers with adjacent materials; e.g. metals. It is expected that the formation of SiO2 aerogel film on metal substrate may induce a modification of metal surface because the aerogel film is made by sol– gel process. Furthermore, interfacial reaction during drying process could be expected due to high temperature. This interfacial reaction can affect the electrical properties of capacitor. In this study, the changes of the bonding states of the metal surface during film formation were investigated. Aluminum and copper were used as the metal substrate and the interfacial reaction after aging and supercritical drying was investigated. Furthermore, the effect of interface modification on the leakage current of dielectricymetal system was also discussed. As a comparison, SiO2 aerogel film was deposited on the Si substrate.
0040-6090/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2003.07.019
576
S.-B. Jung et al. / Thin Solid Films 447 – 448 (2004) 575–579
2. Experimental procedure SiO2 aerogel films were fabricated on Si or metals. Al and Cu substrate were fabricated by e-beam deposition of the metals on thermal oxide grown on Si. Thickness of metal film was 150–200 nm. SiO2 sol was prepared as follows. Tetraethoxyorthosilicate (TEOS; Fluka Chemica, Switzerland) and 2-propyl alcohol (IPA; Yacuri Co., Japan) were mixed, and then a two-step acidybase catalyzed process was used. First, 1.8=10y4 mol of HCl in H2O was added to a stock solution. And next 8.13=10y4 mol of NH4OH in H2O was added successively. Final composition of TEOS: IPA: H2O: HCl: NH4OH was 1:3:4:1.8=10y3:8.13=10y3 w3x. Each sol was spin-deposited and aged in IPA for 12 h. The aged films were transferred to an autoclave apparatus. The solvent in the wet-gel pore could be removed without any damage to the SiO2 wet-gel structure by supercritical drying. The conditions of the supercritical drying process were 250 8C and 1160 psi with heating rate of 2 8Cymin in IPA solvent w4x. For the measurement of leakage current, circular Al dots were deposited on SiO2 aerogel film with the e-beam evaporator and the current–voltage (I–V) characteristic was measured using a HP 4145B semiconductor parameter analyzer. For the observation of interfacial bonding states, the aged spun-on film and supercritical-dried aerogel film was physically peeled from Al or Cu film and X-ray photoelectron spectroscopy (XPS, VG Scientific, ESCALAB 220i-XL) measurement was carried out. The excitation source was monochromatic Al Ka radiation and narrow scan spectra of all region of interest were recorded with 20 eV of pass energy. As a comparison, bare Al and Cu film by e-beam deposition was transferred to XPS chamber right after the deposition of metal and surface bonding states could be analyzed. The thickness of the film was investigated by field emission scanning electron microscope (SEM; Hitachi, S 4200) at electron energy of 10 kV. 3. Results and discussion Fig. 1 represents the interfacial bonding state changes of Al metal film during the fabrication process of SiO2 aerogel film. As shown in Fig. 1a, Al 2p spectrum is composed of the metallic Al bond at 72.8 eV and the Al–O bond at 75.5 eV w5x. By the binding energy difference of 2.7 eV between metallic Al and Al–O peaks, the peak at 75.5 eV is related to Al2O3, which is due to air exposure w6x. It appeared that the relative amount of the Al–O bond increased after aging the spun-on film in IPA for 12 h as shown in Fig. 1b, and this behavior was more pronounced after supercritical drying as given in Fig. 1c. Meanwhile, the center peak position of the Al–O bond was shifted to a higher binding energy by nearly 0.5 eV as given in Fig. 1b,c.
Fig. 1. Al 2p XPS core level spectra of (a) Al metal film and the interface of SiO2 aerogel and Al film after (b) aging in IPA for 12 h and (c) successive supercritical drying.
The presence of aluminum hydroxide can be considered, however, in that case, the binding energy difference of the Al 2p between metallic Al and aluminum hydroxide was reported to be nearly 3.8 eV, which is too large for our case w7x. Another possible assumption could be the formation of aluminum silicate. It was reported that binding energy difference between Al–O–Al and Al–O–Si bonds is 0.53 eV w8x. Therefore, it could be said that during the fabrication process of the SiO2 aerogel by supercritical drying, aluminum silicate can be formed on Al surface. O 1s and Si 2p XPS core level spectra are shown in Fig. 2. The peak at 531.3, and 531.7 eV in Fig. 2a is assigned to O–Al and O_ C bonds, respectively, w5,9x. From Figs. 1a and 2a, the Al film surface was made up of metallic Al bond together with the Al–O and O_ C bonds. The presence of the O_ C bond was also confirmed by C 1s spectrum (not given). After aging of spun-on film as shown in Fig. 2b, the peaks of O–Si bond at 532.8 eV and O–C bond at 534.15 eV from O 1s spectrum appeared due to remained silica on Al film; they corresponded to skeleton Si–O–Si network bond and residual Si–OR bond (R: alkyl group). This peak assignment was supported by the presence of the Si–O bond and the Si–O–C bond in Si 2p spectrum of Fig. 2b. Furthermore, the broadening of Si 2p spectrum toward lower binding energy was attributed to the existence of the Si–O–Al bond at 102.5 eV w10x. The amount of the Si–O–Al bond in Si 2p peak increased
S.-B. Jung et al. / Thin Solid Films 447 – 448 (2004) 575–579
577
Fig. 2. O 1s and Si 2p core level spectra of (a) Al metal film and the interface of SiO2 aerogel and Al film after (b) aging in IPA for 12 h and (c) successive supercritical drying.
after supercritical drying. The center peak position of O 1s spectrum after supercritical drying was shifted to a lower binding energy compared with O 1s center of Fig. 2b. This is because in Si–O–Al configuration, charge transfer to oxygen is larger than that in Si–O–Si configuration due to lower electronegativity of Al w11x. As a result, Al–O and Al–O–Si bond, which existed during aging process, increased after supercritical drying. There are many hydroxyl ions in the sol due to base catalyst addition and even after spin coating, the hydroxyl ions exist in the wet-gel pore. Because supercritical drying was conducted at 250 8C and 1160 psi, the hydroxyl ion in the wet-gel pore proceeded the oxidation of Al film surface by the thermal activation. Fig. 3 shows the evolution of oxygen bonds on Cu metal film during the formation of SiO2 aerogel. Cu can be oxidized with chemical formula of CuO, Cu2O and Cu(OH)2 and each binding energy in O 1s spectrum is 529.6 eV, 530.4 eV and 532.3 eV, respectively w12,13x. The O 1s spectrum of Fig. 3a, was composed of O_ C bond at 531.7 eV and O–Cu bond at 530.4 eV and this O–Cu bond with Cu2O form was generated during air exposure. After aging spun-on film on Cu in Fig. 3b, the O–Si bond and O–C bond appeared similar to Fig. 2b. OH–Cu bond at 532.3 eV with Cu(OH)2 form appeared w13x. Bond fraction of oxidized Cu with chemical formula of Cu(OH)2 is larger than that of
Fig. 3. O 1s core level spectra of (a) Cu metal film and the interface of SiO2 aerogel and Cu film after (b) aging in IPA for 12 h and (c) successive supercritical drying.
578
S.-B. Jung et al. / Thin Solid Films 447 – 448 (2004) 575–579
Fig. 4. Cu 2p3y2 XPS core level spectra of (a) Cu metal film and the interface of SiO2 aerogel and Cu film after (b) aging in IPA for 12 h and (c) successive supercritical drying.
Cu2O in Fig. 3b. From Auger electron LMM spectra, photoelectron from metallic Cu was detected (not given). This implies that relative amount of OH–Cu and O–Cu bond is not affected by bond distribution at the interface. Also, bond fraction of Cu2O in Fig. 3a is 6.2% from composition of oxygen and curve fitting of O 1s peak, which is small in Cu surface. So, it is concluded that OH–Cu bond was generated by oxidizing Cu surface with the aid of hydroxyl ion. Even after supercritical drying, interfacial bonding nature was maintained as shown in Fig. 3c. Fig. 4 represents the Cu 2p3y2 core level spectra changes during SiO2 aerogel fabrication process. In Fig. 4a, metallic Cu bond at 932.7 eV and Cu–O bond at 932.8 eV in Cu2O form was found. The satellite line, which is the characteristic of Cu2q copper oxide formation, and Cu–OH bond at 935.1 eV was not observed w12,14x. This result agrees with Fig. 3a, where the only oxidized Cu bond is Cu2O. So, as-deposited Cu surface was made up of metallic Cu, Cu2O, C_ O and C–O bonds. The presence of C_ O and C–O bond was confirmed from C 1s spectra (not given). From O 1s spectra in Fig. 3b,c it can be found that the amount of O–Cu bond is relatively small. Therefore, the peak near 932.8 eV can be assigned to mostly metallic Cu bond in Fig. 4b,c. Also, existence of Cu–OH bond after aging
and supercritical drying agrees well with Fig. 3b,c. After aging and supercritical drying, another peak at 933.7 eV binding energy was found. Normally Cu–O bond in CuO form can be found at 933.6 eV, but it is not present in our samples due to the absence of satellite peak for Cu2q w12x. Cu–O–Si bond has been reported to have a binding energy of 933.7 eV in the absence of CuO w15x. So, it is concluded that copper silicate bond was formed after aging and supercritical drying. Also due to the similar electronegativity of Si and Cu, Cu–O–Si bond could not be discriminated in O 1s spectra and Cu–O– Si peak was possibly included in O–Si bond. It can be found that the amount of Cu–O–Si bond is small when compared with the case of the SiO2 aerogel on Al substrate. It has been known that Cu showed a poor adhesion on oxide due to high interfacial energy w16,17x. Therefore, the reduced silicate bond can be explained by poor adhesion property of Cu. As a result, the Cu surface with Cu2O was oxidized into Cu(OH)2 formula and formed copper silicate during aging process and supercritical drying. Leakage current behavior of SiO2 aerogel film in metal–insulator–semiconductor (MIS) or metal–insulator–metal (MIM) structure was investigated, and the results are given in Fig. 5. Al was used as a top electrode material in MIS or MIM structure. From Fig. 5a,b it was revealed that SiO2 aerogel film deposited on Si or Al shows almost the same leakage current behavior. However, SiO2 aerogel film deposited on Cu showed an increase of leakage current by approximately 1 order of magnitude, as shown in Fig. 5c. Formation of silicate bond can increase the adhesion of insulating layer. The
Fig. 5. Leakage current behavior of SiO2 aerogel film in (a) MIS structure using Al dot, (b) AlySiO2 aerogelyAl and (c) AlySiO2 aerogelyCu MIM structure.
S.-B. Jung et al. / Thin Solid Films 447 – 448 (2004) 575–579
amount of aluminum silicate bond is more than that of copper silicate bond through the observation of interfacial bonding state. Loosely bounded insulator–metal contact can induce the rough interface and increase the leakage current. The deposition of SiO2 aerogel film on Si might generate silicon suboxide, which contributes the adhesion to insulator. So, it can be assumed that existing interfacial bonding states gave the same contribution in each case of Si and Al; i.e. MIS and MIM with lower electrode Al. However, AlySiO2 aerogelyCu structure showed a degraded leakage current property due to the adhesion. Furthermore, a drift of the Cu ion in porous material during thermal anneal also has to be considered for the degradation of leakage behavior of SiO2 aerogelyCu system w18x. 4. Conclusions Interfacial bonding states during SiO2 aerogel film fabrication process and the leakage current property of SiO2 aerogel film on metal substrates were investigated. It proved that Al–O and Al–O–Si bond, which existed after aging, increased after supercritical drying due to thermal activation. Cu–O–Si bond was generated at the interface of SiO2 aerogel film and Cu. The Cu(OH)2 bond, which did not exist in bare Cu surface, was generated by oxidizing Cu surface and maintained during the process. These oxidized bonds were expected to induce from the hydroxyl ion in the sol or wet-gel pore. It could be concluded that SiO2 aerogel film formed the interfacial silicate bond on metal substrate with small quantity of metal oxide. Leakage current of SiO2 aerogel film deposited on Si or Al showed an almost same property. However, leakage current in AlySiO2 aerogely Cu structure increased and it might be due to a poor adhesion of Cu.
579
Acknowledgments The authors wish to acknowledge the financial support of Electronics and Telecommunications Research Institute (ETRI). References w1x K. Mosig, T. Jacobs, K. Brennan, M. Rasco, J. Wolf, R. Augur, Microelectron. Eng. 64 (2002) 11. w2x L.W. Hrubesh, L.E. Keene, V.R. Latorre, J. Mater. Res. 8 (1993) 1736. w3x S.B. Jung, H.H. Park, Thin Solid Films 420–421 (2002) 503. w4x M.H. Jo, H.H. Park, Appl. Phys. Lett. 72 (1998) 1391. w5x A.C. Miller, F.P. McCluskey, J. Vac. Sci. Technol. A 9 (1991) 1461. w6x M.J. Vasile, B.J. Bachman, J. Vac. Sci. Technol. A 7 (1989) 2992. w7x S.J. Ding, Q.Q. Zhang, D.W. Zhang, J.T. Wang, Y.D. Zhou, W.W. Lee, Appl. Surf. Sci. 178 (2001) 140. w8x O. Renault, L.G. Gosset, D. Rouchon, A. Ermolieff, J. Vac. Sci. Technol. A 20 (2002) 1867. w9x S. Massey, D. Roy, A. Adnot, Nucl. Instrum. Methods Phys. Res. Section B, in press. w10x M. Kundu, N. Miyata, M. Ichikawa, J. Appl. Phys. 93 (2003) 1498. w11x R.C. Evans, An Introduction to Crystal Chemistry, 2nd ed, Cambridge University Press, New York, 1966, p. 68. w12x A.S.W. Wong, R.G. Krishnan, G. Sarkar, J. Vac. Sci. Technol. A 18 (2000) 1619. w13x Y.S. Kim, Y. Shimogaki, J. Vac. Sci. Technol. A 19 (2001) 2642. w14x A. Rajagopal, C. Gregoire, J.J. Lemaire, J.J. Pireaux, M.R. Baklanov, S. Vanhaelemeersch, K. Maex, J.J. Waeterloos, J. Vac. Sci. Technol. B 17 (1999) 2336. w15x A.A. Galuska, J. Vac. Sci. Technol. A 10 (1992) 381. w16x Y. Wu, E. Garfunkel, T.E. Madey, J. Vac. Sci. Technol. A 14 (1996) 1662. w17x C.H.F. Peden, K.B. Kidd, N.D. Shinn, J. Vac. Sci. Technol. A 9 (1991) 1518. w18x P.T. Liu, T.C. Chang, Y.L. Yang, Y.F. Cheng, F.Y. Shih, J.K. Lee, E. Tsai, S.M. Sze, Jpn. J. Appl. Phys. 38 (1999) 6247.
Investigation of the bonding states of the SiO2 aerogel filmymetal interface Sang-Bae Junga, Hyung-Ho Parka,*, Haecheon Kimb a Department of Ceramic Engineering, Yonsei University, 134 Shinchon-Dong, Seodaemun-Ku, Seoul 120-749, South Korea Semiconductor Technology Division, Electronics and Telecommunications Research Institute, 161 Kajong-Dong, Yusong-Ku, Taejon 305-350, South Korea
b
Abstract Due to a rapid decrease in the physical dimensions of today’s microelectronic devices, the RC-time-delay of the interconnection is now a serious problem. As a possible plan, lower resistive metal or lower dielectric constant material has to be introduced. For a low dielectric constant material, SiO2 aerogel may be a promising candidate for an interlayer dielectric (ILD) due to its relatively small dielectric constant. However, the formation of a SiO2 aerogel film on metal substrate may induce a modification of the metal surface because the aerogel film is made by the sol–gel process. Thus, this investigation focused on the interface formation of SiO2 aerogel film and substrate metal is important for the application of low-k material. It was revealed that aluminum silicate bond during aging of spun-on film was induced at the interface with Al and oxidized Al bond was increased after supercritical drying. Copper silicate bond was formed at the interface of the aged film and maintained after fabrication of SiO2 aerogel film. Cu(OH)2 bond, which did not exist at the Cu surface, was generated during film fabrication process. Measurement of leakage current of SiO2 aerogel film deposited on various substrates indicated the degradation of material property in AlySiO2 aerogelyCu structure. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Low dielectric; SiO2 aerogel film; Interface; Copper silicate bond; Leakage current; Al; Cu
1. Introduction Due to the rapid decrease in the dimensions of ultralarge-scaled-integration (ULSI) devices, interconnect RC delay time is a serious problem which cannot be neglected w1x. This delay time will be overcome both by changing the Al(Cu) metal of today’s technology by the lower resistivity Cu and by using a new low dielectric material. Among candidates for low dielectric constant materials, SiO2 aerogel is one of promising low-k materials due to its low dielectric constant and material adaptability. Highly porous SiO2 aerogel film is fabricated by the sol–gel polymerization and subsequent drying procedure, which excludes the collapse of pore structure w2x. Supercritical drying is the high temperature and high pressure process used with the supercritical fluid state. *Corresponding author. Tel.: q82-2-2123-2853; fax: q82-2-3655882. E-mail address: [email protected] (H.-H. Park).
This new material will confront problems related to the integration of the low-k materials with metal, which include chemical and electrical stability, adhesion, etc. The most probable areas to detect instabilities are the interfaces of low-k layers with adjacent materials; e.g. metals. It is expected that the formation of SiO2 aerogel film on metal substrate may induce a modification of metal surface because the aerogel film is made by sol– gel process. Furthermore, interfacial reaction during drying process could be expected due to high temperature. This interfacial reaction can affect the electrical properties of capacitor. In this study, the changes of the bonding states of the metal surface during film formation were investigated. Aluminum and copper were used as the metal substrate and the interfacial reaction after aging and supercritical drying was investigated. Furthermore, the effect of interface modification on the leakage current of dielectricymetal system was also discussed. As a comparison, SiO2 aerogel film was deposited on the Si substrate.
0040-6090/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2003.07.019
576
S.-B. Jung et al. / Thin Solid Films 447 – 448 (2004) 575–579
2. Experimental procedure SiO2 aerogel films were fabricated on Si or metals. Al and Cu substrate were fabricated by e-beam deposition of the metals on thermal oxide grown on Si. Thickness of metal film was 150–200 nm. SiO2 sol was prepared as follows. Tetraethoxyorthosilicate (TEOS; Fluka Chemica, Switzerland) and 2-propyl alcohol (IPA; Yacuri Co., Japan) were mixed, and then a two-step acidybase catalyzed process was used. First, 1.8=10y4 mol of HCl in H2O was added to a stock solution. And next 8.13=10y4 mol of NH4OH in H2O was added successively. Final composition of TEOS: IPA: H2O: HCl: NH4OH was 1:3:4:1.8=10y3:8.13=10y3 w3x. Each sol was spin-deposited and aged in IPA for 12 h. The aged films were transferred to an autoclave apparatus. The solvent in the wet-gel pore could be removed without any damage to the SiO2 wet-gel structure by supercritical drying. The conditions of the supercritical drying process were 250 8C and 1160 psi with heating rate of 2 8Cymin in IPA solvent w4x. For the measurement of leakage current, circular Al dots were deposited on SiO2 aerogel film with the e-beam evaporator and the current–voltage (I–V) characteristic was measured using a HP 4145B semiconductor parameter analyzer. For the observation of interfacial bonding states, the aged spun-on film and supercritical-dried aerogel film was physically peeled from Al or Cu film and X-ray photoelectron spectroscopy (XPS, VG Scientific, ESCALAB 220i-XL) measurement was carried out. The excitation source was monochromatic Al Ka radiation and narrow scan spectra of all region of interest were recorded with 20 eV of pass energy. As a comparison, bare Al and Cu film by e-beam deposition was transferred to XPS chamber right after the deposition of metal and surface bonding states could be analyzed. The thickness of the film was investigated by field emission scanning electron microscope (SEM; Hitachi, S 4200) at electron energy of 10 kV. 3. Results and discussion Fig. 1 represents the interfacial bonding state changes of Al metal film during the fabrication process of SiO2 aerogel film. As shown in Fig. 1a, Al 2p spectrum is composed of the metallic Al bond at 72.8 eV and the Al–O bond at 75.5 eV w5x. By the binding energy difference of 2.7 eV between metallic Al and Al–O peaks, the peak at 75.5 eV is related to Al2O3, which is due to air exposure w6x. It appeared that the relative amount of the Al–O bond increased after aging the spun-on film in IPA for 12 h as shown in Fig. 1b, and this behavior was more pronounced after supercritical drying as given in Fig. 1c. Meanwhile, the center peak position of the Al–O bond was shifted to a higher binding energy by nearly 0.5 eV as given in Fig. 1b,c.
Fig. 1. Al 2p XPS core level spectra of (a) Al metal film and the interface of SiO2 aerogel and Al film after (b) aging in IPA for 12 h and (c) successive supercritical drying.
The presence of aluminum hydroxide can be considered, however, in that case, the binding energy difference of the Al 2p between metallic Al and aluminum hydroxide was reported to be nearly 3.8 eV, which is too large for our case w7x. Another possible assumption could be the formation of aluminum silicate. It was reported that binding energy difference between Al–O–Al and Al–O–Si bonds is 0.53 eV w8x. Therefore, it could be said that during the fabrication process of the SiO2 aerogel by supercritical drying, aluminum silicate can be formed on Al surface. O 1s and Si 2p XPS core level spectra are shown in Fig. 2. The peak at 531.3, and 531.7 eV in Fig. 2a is assigned to O–Al and O_ C bonds, respectively, w5,9x. From Figs. 1a and 2a, the Al film surface was made up of metallic Al bond together with the Al–O and O_ C bonds. The presence of the O_ C bond was also confirmed by C 1s spectrum (not given). After aging of spun-on film as shown in Fig. 2b, the peaks of O–Si bond at 532.8 eV and O–C bond at 534.15 eV from O 1s spectrum appeared due to remained silica on Al film; they corresponded to skeleton Si–O–Si network bond and residual Si–OR bond (R: alkyl group). This peak assignment was supported by the presence of the Si–O bond and the Si–O–C bond in Si 2p spectrum of Fig. 2b. Furthermore, the broadening of Si 2p spectrum toward lower binding energy was attributed to the existence of the Si–O–Al bond at 102.5 eV w10x. The amount of the Si–O–Al bond in Si 2p peak increased
S.-B. Jung et al. / Thin Solid Films 447 – 448 (2004) 575–579
577
Fig. 2. O 1s and Si 2p core level spectra of (a) Al metal film and the interface of SiO2 aerogel and Al film after (b) aging in IPA for 12 h and (c) successive supercritical drying.
after supercritical drying. The center peak position of O 1s spectrum after supercritical drying was shifted to a lower binding energy compared with O 1s center of Fig. 2b. This is because in Si–O–Al configuration, charge transfer to oxygen is larger than that in Si–O–Si configuration due to lower electronegativity of Al w11x. As a result, Al–O and Al–O–Si bond, which existed during aging process, increased after supercritical drying. There are many hydroxyl ions in the sol due to base catalyst addition and even after spin coating, the hydroxyl ions exist in the wet-gel pore. Because supercritical drying was conducted at 250 8C and 1160 psi, the hydroxyl ion in the wet-gel pore proceeded the oxidation of Al film surface by the thermal activation. Fig. 3 shows the evolution of oxygen bonds on Cu metal film during the formation of SiO2 aerogel. Cu can be oxidized with chemical formula of CuO, Cu2O and Cu(OH)2 and each binding energy in O 1s spectrum is 529.6 eV, 530.4 eV and 532.3 eV, respectively w12,13x. The O 1s spectrum of Fig. 3a, was composed of O_ C bond at 531.7 eV and O–Cu bond at 530.4 eV and this O–Cu bond with Cu2O form was generated during air exposure. After aging spun-on film on Cu in Fig. 3b, the O–Si bond and O–C bond appeared similar to Fig. 2b. OH–Cu bond at 532.3 eV with Cu(OH)2 form appeared w13x. Bond fraction of oxidized Cu with chemical formula of Cu(OH)2 is larger than that of
Fig. 3. O 1s core level spectra of (a) Cu metal film and the interface of SiO2 aerogel and Cu film after (b) aging in IPA for 12 h and (c) successive supercritical drying.
578
S.-B. Jung et al. / Thin Solid Films 447 – 448 (2004) 575–579
Fig. 4. Cu 2p3y2 XPS core level spectra of (a) Cu metal film and the interface of SiO2 aerogel and Cu film after (b) aging in IPA for 12 h and (c) successive supercritical drying.
Cu2O in Fig. 3b. From Auger electron LMM spectra, photoelectron from metallic Cu was detected (not given). This implies that relative amount of OH–Cu and O–Cu bond is not affected by bond distribution at the interface. Also, bond fraction of Cu2O in Fig. 3a is 6.2% from composition of oxygen and curve fitting of O 1s peak, which is small in Cu surface. So, it is concluded that OH–Cu bond was generated by oxidizing Cu surface with the aid of hydroxyl ion. Even after supercritical drying, interfacial bonding nature was maintained as shown in Fig. 3c. Fig. 4 represents the Cu 2p3y2 core level spectra changes during SiO2 aerogel fabrication process. In Fig. 4a, metallic Cu bond at 932.7 eV and Cu–O bond at 932.8 eV in Cu2O form was found. The satellite line, which is the characteristic of Cu2q copper oxide formation, and Cu–OH bond at 935.1 eV was not observed w12,14x. This result agrees with Fig. 3a, where the only oxidized Cu bond is Cu2O. So, as-deposited Cu surface was made up of metallic Cu, Cu2O, C_ O and C–O bonds. The presence of C_ O and C–O bond was confirmed from C 1s spectra (not given). From O 1s spectra in Fig. 3b,c it can be found that the amount of O–Cu bond is relatively small. Therefore, the peak near 932.8 eV can be assigned to mostly metallic Cu bond in Fig. 4b,c. Also, existence of Cu–OH bond after aging
and supercritical drying agrees well with Fig. 3b,c. After aging and supercritical drying, another peak at 933.7 eV binding energy was found. Normally Cu–O bond in CuO form can be found at 933.6 eV, but it is not present in our samples due to the absence of satellite peak for Cu2q w12x. Cu–O–Si bond has been reported to have a binding energy of 933.7 eV in the absence of CuO w15x. So, it is concluded that copper silicate bond was formed after aging and supercritical drying. Also due to the similar electronegativity of Si and Cu, Cu–O–Si bond could not be discriminated in O 1s spectra and Cu–O– Si peak was possibly included in O–Si bond. It can be found that the amount of Cu–O–Si bond is small when compared with the case of the SiO2 aerogel on Al substrate. It has been known that Cu showed a poor adhesion on oxide due to high interfacial energy w16,17x. Therefore, the reduced silicate bond can be explained by poor adhesion property of Cu. As a result, the Cu surface with Cu2O was oxidized into Cu(OH)2 formula and formed copper silicate during aging process and supercritical drying. Leakage current behavior of SiO2 aerogel film in metal–insulator–semiconductor (MIS) or metal–insulator–metal (MIM) structure was investigated, and the results are given in Fig. 5. Al was used as a top electrode material in MIS or MIM structure. From Fig. 5a,b it was revealed that SiO2 aerogel film deposited on Si or Al shows almost the same leakage current behavior. However, SiO2 aerogel film deposited on Cu showed an increase of leakage current by approximately 1 order of magnitude, as shown in Fig. 5c. Formation of silicate bond can increase the adhesion of insulating layer. The
Fig. 5. Leakage current behavior of SiO2 aerogel film in (a) MIS structure using Al dot, (b) AlySiO2 aerogelyAl and (c) AlySiO2 aerogelyCu MIM structure.
S.-B. Jung et al. / Thin Solid Films 447 – 448 (2004) 575–579
amount of aluminum silicate bond is more than that of copper silicate bond through the observation of interfacial bonding state. Loosely bounded insulator–metal contact can induce the rough interface and increase the leakage current. The deposition of SiO2 aerogel film on Si might generate silicon suboxide, which contributes the adhesion to insulator. So, it can be assumed that existing interfacial bonding states gave the same contribution in each case of Si and Al; i.e. MIS and MIM with lower electrode Al. However, AlySiO2 aerogelyCu structure showed a degraded leakage current property due to the adhesion. Furthermore, a drift of the Cu ion in porous material during thermal anneal also has to be considered for the degradation of leakage behavior of SiO2 aerogelyCu system w18x. 4. Conclusions Interfacial bonding states during SiO2 aerogel film fabrication process and the leakage current property of SiO2 aerogel film on metal substrates were investigated. It proved that Al–O and Al–O–Si bond, which existed after aging, increased after supercritical drying due to thermal activation. Cu–O–Si bond was generated at the interface of SiO2 aerogel film and Cu. The Cu(OH)2 bond, which did not exist in bare Cu surface, was generated by oxidizing Cu surface and maintained during the process. These oxidized bonds were expected to induce from the hydroxyl ion in the sol or wet-gel pore. It could be concluded that SiO2 aerogel film formed the interfacial silicate bond on metal substrate with small quantity of metal oxide. Leakage current of SiO2 aerogel film deposited on Si or Al showed an almost same property. However, leakage current in AlySiO2 aerogely Cu structure increased and it might be due to a poor adhesion of Cu.
579
Acknowledgments The authors wish to acknowledge the financial support of Electronics and Telecommunications Research Institute (ETRI). References w1x K. Mosig, T. Jacobs, K. Brennan, M. Rasco, J. Wolf, R. Augur, Microelectron. Eng. 64 (2002) 11. w2x L.W. Hrubesh, L.E. Keene, V.R. Latorre, J. Mater. Res. 8 (1993) 1736. w3x S.B. Jung, H.H. Park, Thin Solid Films 420–421 (2002) 503. w4x M.H. Jo, H.H. Park, Appl. Phys. Lett. 72 (1998) 1391. w5x A.C. Miller, F.P. McCluskey, J. Vac. Sci. Technol. A 9 (1991) 1461. w6x M.J. Vasile, B.J. Bachman, J. Vac. Sci. Technol. A 7 (1989) 2992. w7x S.J. Ding, Q.Q. Zhang, D.W. Zhang, J.T. Wang, Y.D. Zhou, W.W. Lee, Appl. Surf. Sci. 178 (2001) 140. w8x O. Renault, L.G. Gosset, D. Rouchon, A. Ermolieff, J. Vac. Sci. Technol. A 20 (2002) 1867. w9x S. Massey, D. Roy, A. Adnot, Nucl. Instrum. Methods Phys. Res. Section B, in press. w10x M. Kundu, N. Miyata, M. Ichikawa, J. Appl. Phys. 93 (2003) 1498. w11x R.C. Evans, An Introduction to Crystal Chemistry, 2nd ed, Cambridge University Press, New York, 1966, p. 68. w12x A.S.W. Wong, R.G. Krishnan, G. Sarkar, J. Vac. Sci. Technol. A 18 (2000) 1619. w13x Y.S. Kim, Y. Shimogaki, J. Vac. Sci. Technol. A 19 (2001) 2642. w14x A. Rajagopal, C. Gregoire, J.J. Lemaire, J.J. Pireaux, M.R. Baklanov, S. Vanhaelemeersch, K. Maex, J.J. Waeterloos, J. Vac. Sci. Technol. B 17 (1999) 2336. w15x A.A. Galuska, J. Vac. Sci. Technol. A 10 (1992) 381. w16x Y. Wu, E. Garfunkel, T.E. Madey, J. Vac. Sci. Technol. A 14 (1996) 1662. w17x C.H.F. Peden, K.B. Kidd, N.D. Shinn, J. Vac. Sci. Technol. A 9 (1991) 1518. w18x P.T. Liu, T.C. Chang, Y.L. Yang, Y.F. Cheng, F.Y. Shih, J.K. Lee, E. Tsai, S.M. Sze, Jpn. J. Appl. Phys. 38 (1999) 6247.