- Email: [email protected]
Integration of platinum bottom electrode on poly-Si for ferroelectric thin films
Applied Surface Science 141 Ž1999. 77–82
Integration of platinum bottom electrode on poly-Si for ferroelectric thin films Eun-Suck Choi a , Soon-Gil Yoon b
a,c,)
, Won-Youl Choi b, Ho-Gi Kim
b
a Department of Materials Engineering, Chungnam National UniÕersity, Daeduk Science Town, Taejon 305-764, South Korea Department of Materials Engineering, Korea AdÕanced Institute of Science and Technology, Ku-Song Dong, Taejon, South Korea c CPRC (Ceramic Processing Research Center), Hanyang UniÕersity, Seoul 133-791, South Korea
Received 16 March 1998; accepted 8 October 1998
Abstract Platinum bottom electrodes are integrated directly on poly-Si by metalorganic chemical vapor deposition ŽMOCVD. and dc sputtering. Platinum films deposited directly on poly-Si by MOCVD at 4008C do not form platinum-silicide after annealing at 7008C in oxygen ambient. On the other hand, Pt films deposited on poly-Si at 3508C by dc sputtering were changed to form platinum-silicide at 7008C. Pt films deposited on poly-Si by MOCVD effectively prevent the diffusion of oxygen and silicon at high temperatures in oxygen ambient. MOCVD-Pt films for integration directly on poly-Si are potentially attractive for the ferroelectric random access memory ŽFRAM. application. q 1999 Elsevier Science B.V. All rights reserved. PACS: 66.30.Ny; 67.70.q n; 68.35.y p; 68.35.Fx Keywords: Platinum; Poly-Si; Ferroelectric thin films
1. Introduction There has been increasing interest in ferroelectric thin films, such as lead zirconium titanate ŽPZT. and Bi-layered SrBi 2Ta 2 O 9 ŽSBT., because of their fascinating properties and potential for applications in ferroelectric random access memories ŽFRAMs.. To achieve the desired perovskite PZT or SBT crystal phase, the film must be deposited at high temperatures and requires an oxygen environment to avoid oxygen deficiencies in the film structure w1–3x. The )
Corresponding author. Fax: q82-42-822-3206; E-mail: [email protected]
formation of thin film capacitors in integrated circuits requires that the ferroelectric material be deposited over a previously formed electrode. In order to fabricate optimum capacitors, electrode materials should have several requirements such as low electrical resistivity, high thermal stability, and good oxygen resistance. Platinum is one of the few metals that is relatively inert to O 2 and, therefore, satisfies some of the listed conditions. Thus, it has been used as a contact to sol–gel and sputter-deposited high dielectric thin films w4x. Platinum was frequently prepared by physical vapor deposition ŽPVD. processes such as sputtering w5x and electron-beam evaporation w6x. However, platinum prepared by the above methods
0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 6 2 3 - 0
E.-S. Choi et al.r Applied Surface Science 141 (1999) 77–82
78
reacts with Si to form a silicide at temperatures as low as 2008C, so platinum cannot be in direct contact with Si w7–9x. Furthermore, the adhesion of Pt to conventional dielectrics such as SiO 2 is poor. Thin adhesion layers, such as Ti and Ta, are commonly used between Pt and SiO 2 . Platinum deposition on SiO 2rSi substrates by MOCVD was reported by previous works w10,11x. In this study, we tried to deposit Pt film directly on poly-SirSiO2rSiŽ100. substrates by MOCVD without the adhesion layers. The characteristics of Pt films deposited on poly-Si by MOCVD and dc sputtering was evaluated with annealing at high temperatures in oxygen ambient.
2. Experimental Platinum films were deposited onto polySirSiO 2rSi substrates by MOCVD and dc sputtering. The precursor for Pt deposition by MOCVD is Žmethylcyclopentadienyl.trimethylplatinum ŽŽCH 3 . 3ŽCH 3 C 5 H 4 .Pt.. Silicon wafers, used to eliminate the native oxide, were etched with the following schedule: wafers were etched for 10 s using the HF 2.5% solution and then rinsed with deionized water for 5 min in ultrasonic cleaner. After rinsing, wafers were again etched for 5 s with a solution with a mole ratio of HF 2.5% and C 2 H 5 OH of 1 to 6 and then blown with nitrogen Ž99.9999% purity.. The growth system of MOCVD used for the Pt deposition consists of a vertical cold-wall reactor w10x and the detailed deposition conditions by MOCVD and dc sputtering are summarized in Table 1. The Pt films deposited by MOCVD and dc sputtering were annealed at 7008C
Fig. 1. X-ray diffraction patterns of Ža. MOCVD and Žb. dc sputtered Pt films annealed at 7008C in oxygen ambient.
for 1 h in oxygen ambient. The film thickness and the surface morphologies were determined from the cross-sectional and surface images by scanning electron microscopy ŽSEM, Akashi DS-130C., respectively. X-ray diffraction ŽXRD, Rigaku DrMAX-RC. using Cu K a radiation with an Ni filter was used to determine the crystal phase and the preferred orientation of the films. The film composition was determined by Auger electron spectroscopy ŽAES,
Table 1 Deposition conditions of Pt films by MOCVD and dc sputtering Deposition parameter
MOCVD
dc sputtering
Deposition temperature Substrate Deposition time Deposition rate Ar gas flow rate Bubbling temperature dc power Base pressure Deposition pressure Oxygen flow rate
4008C Poly-Si 25 min – 20 sccm 108C – – 5=10y1 Torr 30 sccm
3508C Poly-Si 20 min 10 nmrmin 10 sccm – 10 W 5=10y6 Torr 3=10y3 Torr –
Fig. 2. SEM surface and cross-sectional images of Ža,c. MOCVD and Žb,d. dc sputtered platinum films annealed at 7008C in oxygen ambient, respectively.
E.-S. Choi et al.r Applied Surface Science 141 (1999) 77–82
79
MgK a radiation and a take-off angle of 908 was used to check the chemical nature of platinum–silicide. 3. Results and discussion
Fig. 3. AES depth profiles of Ža. MOCVD and Žb. dc sputtered Pt films annealed at 7008C in oxygen ambient.
Perkin-Elmer SAM 4300., Secondary ion mass spectroscopy ŽSIMS, Cameca-ims 4 f., and Rutherford backscattering spectroscopyŽRBS, NEC 3SDH. with 2.24 MeV 4 He 2q ions using a detecting angle of 1658. An X-ray photoelectron spectroscopy ŽXPS, VG Escalab 200-R. using a non monochromatized
Fig. 1Ža,b. show the X-ray diffraction ŽXRD. patterns of the MOCVD and dc sputtered Pt films annealed in oxygen ambient for 1 h at 7008C, respectively. Platinum films by MOCVD do not form platinum silicide after annealing at 7008C in oxygen ambient as shown in Fig. 1Ža.. However, dc sputtered Pt films were changed to form platinum silicide at 7008C. When the ferroelectric films were deposited on Ptrpoly-Si in oxygen ambient at high temperatures, the oxidation of poly-Si was a great obstacle in the integration of memory devices. Fig. 2Ža,c,b,d. shows the SEM surface and crosssectional images of MOCVD and dc sputtered Pt films annealed at 7008C for 1 h, respectively. The annealed films show a clear interface from the cross-sectional images of MOCVD-Pt films and a smooth and dense structure from the surface images. However, dc sputtered Pt films show rough surface morphologies indicating the formation of platinum silicide and a severe interdiffusion between Pt and Si elements. Fig. 3Ža,b. shows the AES depth profiles of MOCVD and dc sputtered Pt films annealed at 7008C in oxygen ambient. As shown in Fig. 3Ža., silicon and platinum show little interdiffusion and oxygen was not detected at interface between Pt layer and poly-Si. However, in case of dc sputtered
Fig. 4. Secondary ion mass spectroscopy depth profiles of Ža. room temperature dc sputtered, Žb. 4008C MOCVD, and Žc. 7008C annealed MOCVD platinum films.
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E.-S. Choi et al.r Applied Surface Science 141 (1999) 77–82
Fig. 5. Rutherford backscattering spectra of Ža. MOCVD-PtrpolySi and Žb. dc sputtered Ptrpoly-Si annealed at 7008C in oxygen ambient.
formed is SiO 2 is about 4.7 = 10 22 atomrcc. The relative ratio of oxygen existing at interface between platinum and silicon as compared with SiO 2 structure could be estimated. The oxygen contents existing at interface between platinum and poly-Si were about 2.47 = 10 20 Ž0.5%., 6.69 = 10 20 Ž1.4%., and 1.33 = 10 21 atomrcc Ž2.8%. as shown in Fig. 4Ža,b,c., respectively. The oxygen content at poly-Si surface in case of as-deposited MOCVD-Pt and annealed MOCVD-Pt films shows only a little increase as compared with that of as-sputtered Pt films. From these results MOCVD-Pt annealed at 7008C in oxygen ambient plays an important role in blocking the diffusion of oxygen through the platinum layer. Platinum in as-deposited films by MOCVD and annealed at 7008C was diffused into the polysilicon layer as compared with that in room temperature sputtered films. Fig. 5Ža,b. shows the Rutherford backscattering spectra of MOCVD-Pt and dc sputtered Pt films annealed at 7008C in oxygen ambient, respectively. As shown at Fig. 5Žb., the thin peak at the high energy edge of the Pt signal indicates the presence of unreacted Pt. The backscattering spectrum obtained from dc sputtered PtrSi was similar to that for a
Pt films ŽFig. 3Žb.., plenty of oxygens were detected at the poly-Si layer and a great diffusion of silicon into Pt layers occurred. Silicon existing in Pt layers formed a SiO x phase after reaction with oxygen diffused through Pt layers during oxygen ambient annealing. These results suggested that Pt layers by dc sputtering did not block the diffusion of silicon into the Pt layer and of oxygen through the Pt layer. Fig. 4Ža,b,c. shows the secondary ion mass spectroscopy depth profiles of dc sputtered Pt films in argon ambient at room temperature, as-deposited MOCVD-Pt at 4008C, and MOCVD-Pt annealed at 7008C in oxygen ambient. SIMS depth profiles of as-sputtered Pt films in argon ambient at room temperature were performed to compare with the degree of oxidation of poly-Si when platinum was deposited at 4008C by MOCVD and annealed at 7008C in oxygen ambient, respectively. Silicon oxide ŽSiO x . can be formed by the reaction with oxygen at interface between platinum and poly-Si. The theoretical atomic density of oxygen in SiO 2 if silicon oxide
Fig. 6. XPS Pt 4f spectra after argon ion-etching of Ža. MOCVD and Žb. dc sputtered platinum films annealed at 7008C in oxygen ambient.
E.-S. Choi et al.r Applied Surface Science 141 (1999) 77–82
Fig. 7. SEM Ža. surface and Žb. cross-sectional images of BST films deposited on MOCVD-Ptrpoly-Si at 6008C.
81
The reason why platinum deposited on poly-Si by MOCVD do not form platinum silicide as compared with dc sputtering can be explained as follows: as shown in Fig. 3Žb., platinum-silicide was formed by diffusion of silicon into platinum layers. It has been suggested that the formation of metal-silicide is initiated by grain boundary diffusion of Si inhomogeneously into the metal overlayer, followed by silicide formation at the grain surfaces w13x. Platinum films deposited at 4008C by MOCVD show denser microstructure and larger grain size as compared with those by dc sputtering w14x. The activation energy for the diffusion of Si through platinum layers can be increased due to the dense microstructure at grain boundaries of platinum films. The increase of kinetic barrier for diffusion of Si through platinum by dense microstructure resulted in the suppression of silicide formation. Fig. 7 shows SEM surface and cross-sectional images of ŽBa,Sr.TiO 3 ŽBST. films deposited by MOCVD on MOCVD-Ptrpoly-Si substrates at 6008C. The interface between platinum and poly-Si was clear after deposition of BST in oxygen ambient. From these experimental results platinum films deposited on poly-Si by MOCVD were concluded not to form platinum-silicide even at high temperatures in oxygen ambient.
4. Conclusions sample of platinum-silicide grown for 30 min in O 2 at 6008C w12x. However, spectrum of MOCVD-Pt shown in Fig. 5Ža. shows the different shape from that of dc sputtered Pt and a little diffusion of platinum into Si. Fig. 6Ža,b. show the XPS Pt 4 f spectra to ascertain the formation of platinum silicide at films annealed at 7008C in oxygen ambient after deposition by MOCVD and dc sputtering. The XPS spectra in the Pt 4 f peak region were obtained after argon ion etching for 1200 s Žthe approximate interface between platinum and poly-Si.. The spectra are deconvoluted into peaks assuming a Gaussian-type shape. Fig. 6Ža. shows that platinum films deposited by MOCVD only indicate the presence of platinum after annealing at 7008C. However, as shown in Fig. 6Žb., dc sputtered Pt films show the apparent presence of platinum silicide as well as platinum.
Platinum films deposited directly on poly-Si by MOCVD at 4008C do not form platinum silicide after annealing at 7008C in oxygen ambient. On the other hand, Pt films deposited by dc sputtering at 3508C were changed to form platinum silicide at 7008C. Pt films deposited on poly-Si by MOCVD effectively block the diffusion of oxygen and silicon at high temperatures in oxygen ambient. Direct integration of platinum on poly-Si by MOCVD is potentially attractive for the application of ferroelectric random access memory ŽFRAM. devices.
Acknowledgements This work was supported Žin part. by the Korea Science and Engineering Foundation ŽKOSEF.
82
E.-S. Choi et al.r Applied Surface Science 141 (1999) 77–82
through the Ceramic Processing Research Center ŽCPRC. at Hanyang University.
References w1x C.K. Kwok, S.B. Desu, L. Kammerdiner, MRS Symp. Proc. 200 Ž1990. 83. w2x S. Krishnakumar, S.C. Esener, C. Fan, V.H. Ozguz, M.A. Title, C. Cozzolino, S.H. Lee, MRS Symp. Proc. 200 Ž1990. 91. w3x L.N. Chapin, S.A. Myers, MRS Symp. Proc. 200 Ž1990. 153. w4x B.A. Tuttle, R.W. Schartz, D.H. Doughty, J.A. Voight, MRS Symp. Proc. 200 Ž1990. 159.
w5x R.C. Sun, T.C. Tisone, P.D. Cruzan, J. Appl. Phys. 40 Ž1973. 1009. w6x S.Y. Cha, H.C. Lee, W.J. Lee, H.G. Kim, Jpn. J. Appl. Phys. 34 Ž1995. 5220. w7x A. Hiraki, M.A. Nicolet, J.W. Mayer, Appl. Phys. Lett. 18 Ž1971. 178. w8x H. Muta, D. Shinoda, J. Appl. Phys. 43 Ž1972. 2913. w9x J.H. Kwon, S.G. Yoon, J. Electrochem. Soc. 144 Ž1997. 2848. w10x J.H. Kwon, S.G. Yoon, Thin Solid Films 303 Ž1997. 136. w11x J.M. Poate, T.C. Tisone, Appl. Phys. Lett. 24 Ž1974. 391. w12x R.J. Blattner, C.A. Evans, S.S. Lau, J.W. Mayer, B.M. Ullrich, J. Electrochem. Soc. 122 Ž1975. 1732. w13x R.M. Tromp, G.W. Rubloff, E.J. van Loenen, J. Vac. Sci. Technol. A 4 Ž1986. 865. w14x E.S. Choi, S.G. Yoon, unpublished.
Integration of platinum bottom electrode on poly-Si for ferroelectric thin films Eun-Suck Choi a , Soon-Gil Yoon b
a,c,)
, Won-Youl Choi b, Ho-Gi Kim
b
a Department of Materials Engineering, Chungnam National UniÕersity, Daeduk Science Town, Taejon 305-764, South Korea Department of Materials Engineering, Korea AdÕanced Institute of Science and Technology, Ku-Song Dong, Taejon, South Korea c CPRC (Ceramic Processing Research Center), Hanyang UniÕersity, Seoul 133-791, South Korea
Received 16 March 1998; accepted 8 October 1998
Abstract Platinum bottom electrodes are integrated directly on poly-Si by metalorganic chemical vapor deposition ŽMOCVD. and dc sputtering. Platinum films deposited directly on poly-Si by MOCVD at 4008C do not form platinum-silicide after annealing at 7008C in oxygen ambient. On the other hand, Pt films deposited on poly-Si at 3508C by dc sputtering were changed to form platinum-silicide at 7008C. Pt films deposited on poly-Si by MOCVD effectively prevent the diffusion of oxygen and silicon at high temperatures in oxygen ambient. MOCVD-Pt films for integration directly on poly-Si are potentially attractive for the ferroelectric random access memory ŽFRAM. application. q 1999 Elsevier Science B.V. All rights reserved. PACS: 66.30.Ny; 67.70.q n; 68.35.y p; 68.35.Fx Keywords: Platinum; Poly-Si; Ferroelectric thin films
1. Introduction There has been increasing interest in ferroelectric thin films, such as lead zirconium titanate ŽPZT. and Bi-layered SrBi 2Ta 2 O 9 ŽSBT., because of their fascinating properties and potential for applications in ferroelectric random access memories ŽFRAMs.. To achieve the desired perovskite PZT or SBT crystal phase, the film must be deposited at high temperatures and requires an oxygen environment to avoid oxygen deficiencies in the film structure w1–3x. The )
Corresponding author. Fax: q82-42-822-3206; E-mail: [email protected]
formation of thin film capacitors in integrated circuits requires that the ferroelectric material be deposited over a previously formed electrode. In order to fabricate optimum capacitors, electrode materials should have several requirements such as low electrical resistivity, high thermal stability, and good oxygen resistance. Platinum is one of the few metals that is relatively inert to O 2 and, therefore, satisfies some of the listed conditions. Thus, it has been used as a contact to sol–gel and sputter-deposited high dielectric thin films w4x. Platinum was frequently prepared by physical vapor deposition ŽPVD. processes such as sputtering w5x and electron-beam evaporation w6x. However, platinum prepared by the above methods
0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 6 2 3 - 0
E.-S. Choi et al.r Applied Surface Science 141 (1999) 77–82
78
reacts with Si to form a silicide at temperatures as low as 2008C, so platinum cannot be in direct contact with Si w7–9x. Furthermore, the adhesion of Pt to conventional dielectrics such as SiO 2 is poor. Thin adhesion layers, such as Ti and Ta, are commonly used between Pt and SiO 2 . Platinum deposition on SiO 2rSi substrates by MOCVD was reported by previous works w10,11x. In this study, we tried to deposit Pt film directly on poly-SirSiO2rSiŽ100. substrates by MOCVD without the adhesion layers. The characteristics of Pt films deposited on poly-Si by MOCVD and dc sputtering was evaluated with annealing at high temperatures in oxygen ambient.
2. Experimental Platinum films were deposited onto polySirSiO 2rSi substrates by MOCVD and dc sputtering. The precursor for Pt deposition by MOCVD is Žmethylcyclopentadienyl.trimethylplatinum ŽŽCH 3 . 3ŽCH 3 C 5 H 4 .Pt.. Silicon wafers, used to eliminate the native oxide, were etched with the following schedule: wafers were etched for 10 s using the HF 2.5% solution and then rinsed with deionized water for 5 min in ultrasonic cleaner. After rinsing, wafers were again etched for 5 s with a solution with a mole ratio of HF 2.5% and C 2 H 5 OH of 1 to 6 and then blown with nitrogen Ž99.9999% purity.. The growth system of MOCVD used for the Pt deposition consists of a vertical cold-wall reactor w10x and the detailed deposition conditions by MOCVD and dc sputtering are summarized in Table 1. The Pt films deposited by MOCVD and dc sputtering were annealed at 7008C
Fig. 1. X-ray diffraction patterns of Ža. MOCVD and Žb. dc sputtered Pt films annealed at 7008C in oxygen ambient.
for 1 h in oxygen ambient. The film thickness and the surface morphologies were determined from the cross-sectional and surface images by scanning electron microscopy ŽSEM, Akashi DS-130C., respectively. X-ray diffraction ŽXRD, Rigaku DrMAX-RC. using Cu K a radiation with an Ni filter was used to determine the crystal phase and the preferred orientation of the films. The film composition was determined by Auger electron spectroscopy ŽAES,
Table 1 Deposition conditions of Pt films by MOCVD and dc sputtering Deposition parameter
MOCVD
dc sputtering
Deposition temperature Substrate Deposition time Deposition rate Ar gas flow rate Bubbling temperature dc power Base pressure Deposition pressure Oxygen flow rate
4008C Poly-Si 25 min – 20 sccm 108C – – 5=10y1 Torr 30 sccm
3508C Poly-Si 20 min 10 nmrmin 10 sccm – 10 W 5=10y6 Torr 3=10y3 Torr –
Fig. 2. SEM surface and cross-sectional images of Ža,c. MOCVD and Žb,d. dc sputtered platinum films annealed at 7008C in oxygen ambient, respectively.
E.-S. Choi et al.r Applied Surface Science 141 (1999) 77–82
79
MgK a radiation and a take-off angle of 908 was used to check the chemical nature of platinum–silicide. 3. Results and discussion
Fig. 3. AES depth profiles of Ža. MOCVD and Žb. dc sputtered Pt films annealed at 7008C in oxygen ambient.
Perkin-Elmer SAM 4300., Secondary ion mass spectroscopy ŽSIMS, Cameca-ims 4 f., and Rutherford backscattering spectroscopyŽRBS, NEC 3SDH. with 2.24 MeV 4 He 2q ions using a detecting angle of 1658. An X-ray photoelectron spectroscopy ŽXPS, VG Escalab 200-R. using a non monochromatized
Fig. 1Ža,b. show the X-ray diffraction ŽXRD. patterns of the MOCVD and dc sputtered Pt films annealed in oxygen ambient for 1 h at 7008C, respectively. Platinum films by MOCVD do not form platinum silicide after annealing at 7008C in oxygen ambient as shown in Fig. 1Ža.. However, dc sputtered Pt films were changed to form platinum silicide at 7008C. When the ferroelectric films were deposited on Ptrpoly-Si in oxygen ambient at high temperatures, the oxidation of poly-Si was a great obstacle in the integration of memory devices. Fig. 2Ža,c,b,d. shows the SEM surface and crosssectional images of MOCVD and dc sputtered Pt films annealed at 7008C for 1 h, respectively. The annealed films show a clear interface from the cross-sectional images of MOCVD-Pt films and a smooth and dense structure from the surface images. However, dc sputtered Pt films show rough surface morphologies indicating the formation of platinum silicide and a severe interdiffusion between Pt and Si elements. Fig. 3Ža,b. shows the AES depth profiles of MOCVD and dc sputtered Pt films annealed at 7008C in oxygen ambient. As shown in Fig. 3Ža., silicon and platinum show little interdiffusion and oxygen was not detected at interface between Pt layer and poly-Si. However, in case of dc sputtered
Fig. 4. Secondary ion mass spectroscopy depth profiles of Ža. room temperature dc sputtered, Žb. 4008C MOCVD, and Žc. 7008C annealed MOCVD platinum films.
80
E.-S. Choi et al.r Applied Surface Science 141 (1999) 77–82
Fig. 5. Rutherford backscattering spectra of Ža. MOCVD-PtrpolySi and Žb. dc sputtered Ptrpoly-Si annealed at 7008C in oxygen ambient.
formed is SiO 2 is about 4.7 = 10 22 atomrcc. The relative ratio of oxygen existing at interface between platinum and silicon as compared with SiO 2 structure could be estimated. The oxygen contents existing at interface between platinum and poly-Si were about 2.47 = 10 20 Ž0.5%., 6.69 = 10 20 Ž1.4%., and 1.33 = 10 21 atomrcc Ž2.8%. as shown in Fig. 4Ža,b,c., respectively. The oxygen content at poly-Si surface in case of as-deposited MOCVD-Pt and annealed MOCVD-Pt films shows only a little increase as compared with that of as-sputtered Pt films. From these results MOCVD-Pt annealed at 7008C in oxygen ambient plays an important role in blocking the diffusion of oxygen through the platinum layer. Platinum in as-deposited films by MOCVD and annealed at 7008C was diffused into the polysilicon layer as compared with that in room temperature sputtered films. Fig. 5Ža,b. shows the Rutherford backscattering spectra of MOCVD-Pt and dc sputtered Pt films annealed at 7008C in oxygen ambient, respectively. As shown at Fig. 5Žb., the thin peak at the high energy edge of the Pt signal indicates the presence of unreacted Pt. The backscattering spectrum obtained from dc sputtered PtrSi was similar to that for a
Pt films ŽFig. 3Žb.., plenty of oxygens were detected at the poly-Si layer and a great diffusion of silicon into Pt layers occurred. Silicon existing in Pt layers formed a SiO x phase after reaction with oxygen diffused through Pt layers during oxygen ambient annealing. These results suggested that Pt layers by dc sputtering did not block the diffusion of silicon into the Pt layer and of oxygen through the Pt layer. Fig. 4Ža,b,c. shows the secondary ion mass spectroscopy depth profiles of dc sputtered Pt films in argon ambient at room temperature, as-deposited MOCVD-Pt at 4008C, and MOCVD-Pt annealed at 7008C in oxygen ambient. SIMS depth profiles of as-sputtered Pt films in argon ambient at room temperature were performed to compare with the degree of oxidation of poly-Si when platinum was deposited at 4008C by MOCVD and annealed at 7008C in oxygen ambient, respectively. Silicon oxide ŽSiO x . can be formed by the reaction with oxygen at interface between platinum and poly-Si. The theoretical atomic density of oxygen in SiO 2 if silicon oxide
Fig. 6. XPS Pt 4f spectra after argon ion-etching of Ža. MOCVD and Žb. dc sputtered platinum films annealed at 7008C in oxygen ambient.
E.-S. Choi et al.r Applied Surface Science 141 (1999) 77–82
Fig. 7. SEM Ža. surface and Žb. cross-sectional images of BST films deposited on MOCVD-Ptrpoly-Si at 6008C.
81
The reason why platinum deposited on poly-Si by MOCVD do not form platinum silicide as compared with dc sputtering can be explained as follows: as shown in Fig. 3Žb., platinum-silicide was formed by diffusion of silicon into platinum layers. It has been suggested that the formation of metal-silicide is initiated by grain boundary diffusion of Si inhomogeneously into the metal overlayer, followed by silicide formation at the grain surfaces w13x. Platinum films deposited at 4008C by MOCVD show denser microstructure and larger grain size as compared with those by dc sputtering w14x. The activation energy for the diffusion of Si through platinum layers can be increased due to the dense microstructure at grain boundaries of platinum films. The increase of kinetic barrier for diffusion of Si through platinum by dense microstructure resulted in the suppression of silicide formation. Fig. 7 shows SEM surface and cross-sectional images of ŽBa,Sr.TiO 3 ŽBST. films deposited by MOCVD on MOCVD-Ptrpoly-Si substrates at 6008C. The interface between platinum and poly-Si was clear after deposition of BST in oxygen ambient. From these experimental results platinum films deposited on poly-Si by MOCVD were concluded not to form platinum-silicide even at high temperatures in oxygen ambient.
4. Conclusions sample of platinum-silicide grown for 30 min in O 2 at 6008C w12x. However, spectrum of MOCVD-Pt shown in Fig. 5Ža. shows the different shape from that of dc sputtered Pt and a little diffusion of platinum into Si. Fig. 6Ža,b. show the XPS Pt 4 f spectra to ascertain the formation of platinum silicide at films annealed at 7008C in oxygen ambient after deposition by MOCVD and dc sputtering. The XPS spectra in the Pt 4 f peak region were obtained after argon ion etching for 1200 s Žthe approximate interface between platinum and poly-Si.. The spectra are deconvoluted into peaks assuming a Gaussian-type shape. Fig. 6Ža. shows that platinum films deposited by MOCVD only indicate the presence of platinum after annealing at 7008C. However, as shown in Fig. 6Žb., dc sputtered Pt films show the apparent presence of platinum silicide as well as platinum.
Platinum films deposited directly on poly-Si by MOCVD at 4008C do not form platinum silicide after annealing at 7008C in oxygen ambient. On the other hand, Pt films deposited by dc sputtering at 3508C were changed to form platinum silicide at 7008C. Pt films deposited on poly-Si by MOCVD effectively block the diffusion of oxygen and silicon at high temperatures in oxygen ambient. Direct integration of platinum on poly-Si by MOCVD is potentially attractive for the application of ferroelectric random access memory ŽFRAM. devices.
Acknowledgements This work was supported Žin part. by the Korea Science and Engineering Foundation ŽKOSEF.
82
E.-S. Choi et al.r Applied Surface Science 141 (1999) 77–82
through the Ceramic Processing Research Center ŽCPRC. at Hanyang University.
References w1x C.K. Kwok, S.B. Desu, L. Kammerdiner, MRS Symp. Proc. 200 Ž1990. 83. w2x S. Krishnakumar, S.C. Esener, C. Fan, V.H. Ozguz, M.A. Title, C. Cozzolino, S.H. Lee, MRS Symp. Proc. 200 Ž1990. 91. w3x L.N. Chapin, S.A. Myers, MRS Symp. Proc. 200 Ž1990. 153. w4x B.A. Tuttle, R.W. Schartz, D.H. Doughty, J.A. Voight, MRS Symp. Proc. 200 Ž1990. 159.
w5x R.C. Sun, T.C. Tisone, P.D. Cruzan, J. Appl. Phys. 40 Ž1973. 1009. w6x S.Y. Cha, H.C. Lee, W.J. Lee, H.G. Kim, Jpn. J. Appl. Phys. 34 Ž1995. 5220. w7x A. Hiraki, M.A. Nicolet, J.W. Mayer, Appl. Phys. Lett. 18 Ž1971. 178. w8x H. Muta, D. Shinoda, J. Appl. Phys. 43 Ž1972. 2913. w9x J.H. Kwon, S.G. Yoon, J. Electrochem. Soc. 144 Ž1997. 2848. w10x J.H. Kwon, S.G. Yoon, Thin Solid Films 303 Ž1997. 136. w11x J.M. Poate, T.C. Tisone, Appl. Phys. Lett. 24 Ž1974. 391. w12x R.J. Blattner, C.A. Evans, S.S. Lau, J.W. Mayer, B.M. Ullrich, J. Electrochem. Soc. 122 Ž1975. 1732. w13x R.M. Tromp, G.W. Rubloff, E.J. van Loenen, J. Vac. Sci. Technol. A 4 Ž1986. 865. w14x E.S. Choi, S.G. Yoon, unpublished.