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Study of dielectric properties of laser processed BaTiO3 thin films on Si(100) with TiN buffer layer
Applied Surface Science 109r110 Ž1997. 366–370
Study of dielectric properties of laser processed BaTiO 3 thin films on Si ž100 / with TiN buffer layer N. Shu, Ashok Kumar ) , M.R. Alam, H.L. Chan, Q. You Department of Electrical Engineering, UniÕersity of South Alabama, Mobile, AL 36688, USA Received 4 June 1996; accepted 12 June 1996
Abstract High quality BaTiO 3 thin films on SiŽ100. substrates with TiN buffer layer were fabricated successfully by using pulsed laser deposition ŽPLD. technique. The high chemical stability and diffusion barrier properties of TiN were expected to prevent a reaction and interdiffusion between the BaTiO 3 and SiŽ100.. A KrF excimer laser Ž l s 248 nm. was used to ablate both the TiN and BaTiO 3 targets. The depositions were carried out both in high vacuum ŽF 10y6 Torr. and oxygen ambient at different temperatures. The film deposited at 6008C under 100 mTorr oxygen atmosphere showed lower dielectric losses of 0.3% and DC leakage current of 0.06 m A than those deposited under high vacuum. Dielectric properties studies indicated a dielectric constant of 223 at capacitance density of 56 nFrmm2.
1. Introduction Barium titanate ŽBaTiO 3 ., which is a perovskitetype ferroelectric material possessing a high dielectric constant, ferroelectricity, and large electro-optic coefficients, is promising for applications in electronic, electro-optic devices, and nonvolatile memories w1–3x. In order to prevent the formation of amorphous SiO 2 phase and interdiffusion of component atoms at the interface and to achieve the epitaxial growth of perovskite oxide films, a suitable buffer layer must be inserted between the Si substrate and the oxide. We have used a new conductive buffer layer of TiN because it has the following advantages: Ø low diffusion coefficients of metal atoms in the TiN layer,
)
Corresponding author. Tel.: q1-334-4607881; fax: q1-3344606028; e-mail: [email protected].
Ø low solubility, Ø chemical and thermal stability, Ø good adherence to both substrate and ferroelectric BaTiO 3 film, Ø good step coverage and absence of microcracks, Ø compatible thermal expansion coefficient with the substrate and BaTiO 3 layers. These can be expected to prevent a reaction and interdiffusion between the oxide and Si substrate. TiN can also act as a bottom electrode for the capacitor structure because of its high electrical conductivity.
2. Experimental The pulsed laser deposition ŽPLD. technique has been utilized to fabricate advanced materials w4–6x. A KrF excimer laser ŽLextra 200: wavelength 248
0169-4332r97r$17.00 Copyright q 1997 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 4 3 3 2 Ž 9 6 . 0 0 6 7 6 - 9
N. Shu et al.r Applied Surface Science 109 r 110 (1997) 366–370
nm, repetition rates up to 30 Hz. was used to ablate both buffer layers and targets. For the preparation of thin films, a laser beam with output energy up to 750 mJ was focused onto the target at an incidence angle of 458 to produce an energy density up to 10 Jrcm2 . The target was rotated at a period of 5 s to reduce the crater formation on the surface. The chamber was evacuated to a base pressure of 10y7 Torr by a turbomolecular pump with the aid of rotary vane pump. A mass flow controller was used to control the gas quantity leaking into the chamber when a gas atmosphere was needed. A SiŽ100. wafer of about 2.5 = 2.0 cm2 was ultrasonically cleaned in acetone and methanol, dipped into 5% HF solution, cleaned with methanol again, dried, and fixed onto a stainless-steel plate in a substrate holder. The substrate temperature, which was measured by a thermocouple, was varied from room temperature to 7008C. For the preparation of TiN film, a laser beam with output energy of 400 mJ and repetition rate of 10 Hz was focused onto the target at an incidence angle of 458 to produce an energy density up to 4 Jrcm2 . The substrate was heated to 6008C. Deposition was carried out in high vacuum Ž; 10y6 Torr.. For the growth of BaTiO 3 films, laser fluence of 3 Jrcm2
367
was used. Epitaxial BaTiO 3 films were grown at substrate temperature higher than 6008C in high vacuum Ž; 10y6 Torr. or in oxygen atmosphere of 100 mTorr on the TiNrSi layer. The crystallographic structure of the films was identified by X-ray diffraction ŽXRD. method. The surface morphology of the films was examined by using a high resolution scanning electron microscope ŽSEM.. The dielectric properties were measured as a function of frequency by using a low frequency ŽLF. impedance analyzer ŽHewlett–Packard 4192A.. The current–voltage Ž I– V . characteristics were measured by the semiconductor parameter analyzer ŽHewlett–Packard 4140B. for the metal–insulator–metal type capacitor structure.
3. Results and discussions Fig. 1 shows the XRD patterns of BaTiO 3 films grown on SiŽ100. substrates at 6008C and 1008C in 100 mTorr oxygen atmosphere. When deposition was carried out at low temperature of 1008C, film had the amorphous structure, but at high temperature of 6008C, highly oriented BaTiO 3 films grew. The film structures are found to be strongly dependent on
Fig. 1. X-ray diffraction patterns of BaTiO 3 thin films on SiŽ100. substrates at different deposition temperatures.
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N. Shu et al.r Applied Surface Science 109 r 110 (1997) 366–370
Fig. 2. X-ray diffraction patterns of BaTiO 3 thin films on SiŽ100. substrates at 6008C in high vacuum and 100 mTorr oxygen.
the substrate temperature. Fig. 2 shows the XRD patterns of BaTiO 3 films grown at 6008C in both high vacuum and 100 mTorr oxygen atmosphere on the Si substrates. It indicates that films are more
crystalline, have mainly ²001: and ²002: planes, and no diffraction from randomly oriented grains or impurity phases are observed. It also shows that films are mainly polycrystalline in nature. Fig. 3
Fig. 3. X-ray diffraction patterns of BaTiO 3 thin films on SiŽ100. substrates at 6008C with TiN buffer layer.
N. Shu et al.r Applied Surface Science 109 r 110 (1997) 366–370
369
Table 1 Results of dielectric properties of BaTiO 3 thin films Samples
Capacitance ŽnFrmm2 .
tan d loss Ž%.
Leakage current Ž m A.
Dielectric constant
BaTiO 3rSi Ž6008C, 100 mTorr O 2 . BaTiO 3rTiNrSi Ž6008C, 100 mTorr O 2 . BaTiO 3rTiNrSi Ž6008C, high vacuum. BaTiO 3rSi Ž6008C, high vacuum.
54 56 166 117
8 0.3 173 118
30 0.06 200 50
215 223 661 466
shows the XRD patterns of a BaTiO 3 film grown on SiŽ100. substrate with TiN buffer layer. Films have the same crystal structure as the films deposited without TiN buffer layer. The films deposited with buffer layer have more crystalline quality structure compared to the previous XRD results. Generally, for MOS Žmetal–oxide–semiconductor. capacitor applications in DRAM Ždynamic random access memory. and other applications as well, the tan d loss should be less than 1% with the dielectric constant higher than 200. The BaTiO 3 films deposited at 6008C in 100 mTorr oxygen environment show the highly oriented peaks of BaTiO 3 phase with TiN buffer layers. The dielectric properties of the BaTiO 3 films deposited at 6008C in high vacuum and 100 mTorr oxygen atmosphere are
shown in Table 1. The BaTiO 3 film deposited at 6008C in 100 mTorr oxygen with TiN buffer layer has the best results with dielectric constant of about 223, loss factor of about 0.3%, and DC leakage current of about 0.06 m A Žat 40 V. at a frequency of 100 kHz. This film shows lower losses and lower DC leakage current than those deposited under vacuum. The reason is the oxygen deficiency in BaTiO 3 deposited in vacuum, which makes BaTiO 3 semiconducting and therefore a leaky capacitor is obtained. In oxygen atmosphere, BaTiO 3 is stoichiometric with no semiconducting behaviors. On the other hand, TiN acted as a good diffusion barrier and electrode between BaTiO 3 oxide and Si substrate. Fig. 4 shows the relationship between applied voltage and DC leakage current for the BaTiO 3 films deposited at
Fig. 4. Current versus voltage characteristics of BaTiO 3 thin films on SiŽ100. substrate.
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N. Shu et al.r Applied Surface Science 109 r 110 (1997) 366–370
6008C in high vacuum and 100 mTorr oxygen environment. It is obvious from the Fig. 4 that BaTiO 3 deposited at 6008C in 100 mTorr oxygen with the TiN buffer layer has the smallest DC leakage current of 0.06 m A among all the films w7x.
dielectric properties analysis. Funding of this research was supported by Alabama NASA EPSCoR, Alabama UniversitiesrTVA, DOE EPSCoR and NSF SIP programs. References
4. Conclusions In summary, highly oriented barium titanate ŽBaTiO 3 . film, deposited on the silicon Ž100. substrate at 6008C, in 100 mTorr oxygen partial pressure with the titanium nitride ŽTiN. buffer layer, exhibited good crystallinity, a dielectric constant of 223, a dissipation factor of 0.3%, and a leakage current density of 0.06 m Arcm2 at 40 V. The buffer layer TiN acted as a good diffusion barrier and electrode between BaTiO 3 oxide and SiŽ100. substrate.
Acknowledgements The authors would like to thank Dr. Vivek Mehrotra, at Philips Laboratory, for his help with
w1x M.B. Lee, M. Kawasaki, M. Yoshimoto and H. Koinuma, Appl. Phys. Lett. 66 Ž1995. 1331. w2x H.F. Cheng, M.H. Yeh, K.S. Liu and I.N. Lin, Jpn. J. Appl. Phys. 32 Ž1993. 5656. w3x T. Nose, H.T. Kim and H. Uwe, Jpn. J. Appl. Phys. 33 Ž1994. 5259. w4x A. Kumar, U. Ekanayake, D. Kjendal, N. Shu and R.B. Inturi, in: Proc. Symp. on High Temperature Coating II, TMS Annual Meeting, Anaheim, CA, Feb. 4–8, 1996 Ž1996. pp. 349–360. w5x R.B. Inturi, A. Kumar, U. Ekanayake, N. Shu and J.A. Barnard, in: Proc. Symp. on Polycrystalline Thin Film Structure, Properties and Applications II, MRS Fall Meeting, Boston, MA, Nov. 27 – Dec. 1, 1995, Vol. 403 Ž1996. pp. 283–290. w6x D.B. Chrisey and G.K. Hubler, Pulsed Laser Deposition of Thin Films ŽWiley, 1994.. w7x A. Kumar, N. Shu, U. Ekanayake, D. Kjendal and G. Wattuhewa, Int. Symp. on Manufacturing Practices and Technologies, ACS Spring Meeting, New Orleans, LA, Nov. 5–8, 1995.
Study of dielectric properties of laser processed BaTiO 3 thin films on Si ž100 / with TiN buffer layer N. Shu, Ashok Kumar ) , M.R. Alam, H.L. Chan, Q. You Department of Electrical Engineering, UniÕersity of South Alabama, Mobile, AL 36688, USA Received 4 June 1996; accepted 12 June 1996
Abstract High quality BaTiO 3 thin films on SiŽ100. substrates with TiN buffer layer were fabricated successfully by using pulsed laser deposition ŽPLD. technique. The high chemical stability and diffusion barrier properties of TiN were expected to prevent a reaction and interdiffusion between the BaTiO 3 and SiŽ100.. A KrF excimer laser Ž l s 248 nm. was used to ablate both the TiN and BaTiO 3 targets. The depositions were carried out both in high vacuum ŽF 10y6 Torr. and oxygen ambient at different temperatures. The film deposited at 6008C under 100 mTorr oxygen atmosphere showed lower dielectric losses of 0.3% and DC leakage current of 0.06 m A than those deposited under high vacuum. Dielectric properties studies indicated a dielectric constant of 223 at capacitance density of 56 nFrmm2.
1. Introduction Barium titanate ŽBaTiO 3 ., which is a perovskitetype ferroelectric material possessing a high dielectric constant, ferroelectricity, and large electro-optic coefficients, is promising for applications in electronic, electro-optic devices, and nonvolatile memories w1–3x. In order to prevent the formation of amorphous SiO 2 phase and interdiffusion of component atoms at the interface and to achieve the epitaxial growth of perovskite oxide films, a suitable buffer layer must be inserted between the Si substrate and the oxide. We have used a new conductive buffer layer of TiN because it has the following advantages: Ø low diffusion coefficients of metal atoms in the TiN layer,
)
Corresponding author. Tel.: q1-334-4607881; fax: q1-3344606028; e-mail: [email protected].
Ø low solubility, Ø chemical and thermal stability, Ø good adherence to both substrate and ferroelectric BaTiO 3 film, Ø good step coverage and absence of microcracks, Ø compatible thermal expansion coefficient with the substrate and BaTiO 3 layers. These can be expected to prevent a reaction and interdiffusion between the oxide and Si substrate. TiN can also act as a bottom electrode for the capacitor structure because of its high electrical conductivity.
2. Experimental The pulsed laser deposition ŽPLD. technique has been utilized to fabricate advanced materials w4–6x. A KrF excimer laser ŽLextra 200: wavelength 248
0169-4332r97r$17.00 Copyright q 1997 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 4 3 3 2 Ž 9 6 . 0 0 6 7 6 - 9
N. Shu et al.r Applied Surface Science 109 r 110 (1997) 366–370
nm, repetition rates up to 30 Hz. was used to ablate both buffer layers and targets. For the preparation of thin films, a laser beam with output energy up to 750 mJ was focused onto the target at an incidence angle of 458 to produce an energy density up to 10 Jrcm2 . The target was rotated at a period of 5 s to reduce the crater formation on the surface. The chamber was evacuated to a base pressure of 10y7 Torr by a turbomolecular pump with the aid of rotary vane pump. A mass flow controller was used to control the gas quantity leaking into the chamber when a gas atmosphere was needed. A SiŽ100. wafer of about 2.5 = 2.0 cm2 was ultrasonically cleaned in acetone and methanol, dipped into 5% HF solution, cleaned with methanol again, dried, and fixed onto a stainless-steel plate in a substrate holder. The substrate temperature, which was measured by a thermocouple, was varied from room temperature to 7008C. For the preparation of TiN film, a laser beam with output energy of 400 mJ and repetition rate of 10 Hz was focused onto the target at an incidence angle of 458 to produce an energy density up to 4 Jrcm2 . The substrate was heated to 6008C. Deposition was carried out in high vacuum Ž; 10y6 Torr.. For the growth of BaTiO 3 films, laser fluence of 3 Jrcm2
367
was used. Epitaxial BaTiO 3 films were grown at substrate temperature higher than 6008C in high vacuum Ž; 10y6 Torr. or in oxygen atmosphere of 100 mTorr on the TiNrSi layer. The crystallographic structure of the films was identified by X-ray diffraction ŽXRD. method. The surface morphology of the films was examined by using a high resolution scanning electron microscope ŽSEM.. The dielectric properties were measured as a function of frequency by using a low frequency ŽLF. impedance analyzer ŽHewlett–Packard 4192A.. The current–voltage Ž I– V . characteristics were measured by the semiconductor parameter analyzer ŽHewlett–Packard 4140B. for the metal–insulator–metal type capacitor structure.
3. Results and discussions Fig. 1 shows the XRD patterns of BaTiO 3 films grown on SiŽ100. substrates at 6008C and 1008C in 100 mTorr oxygen atmosphere. When deposition was carried out at low temperature of 1008C, film had the amorphous structure, but at high temperature of 6008C, highly oriented BaTiO 3 films grew. The film structures are found to be strongly dependent on
Fig. 1. X-ray diffraction patterns of BaTiO 3 thin films on SiŽ100. substrates at different deposition temperatures.
368
N. Shu et al.r Applied Surface Science 109 r 110 (1997) 366–370
Fig. 2. X-ray diffraction patterns of BaTiO 3 thin films on SiŽ100. substrates at 6008C in high vacuum and 100 mTorr oxygen.
the substrate temperature. Fig. 2 shows the XRD patterns of BaTiO 3 films grown at 6008C in both high vacuum and 100 mTorr oxygen atmosphere on the Si substrates. It indicates that films are more
crystalline, have mainly ²001: and ²002: planes, and no diffraction from randomly oriented grains or impurity phases are observed. It also shows that films are mainly polycrystalline in nature. Fig. 3
Fig. 3. X-ray diffraction patterns of BaTiO 3 thin films on SiŽ100. substrates at 6008C with TiN buffer layer.
N. Shu et al.r Applied Surface Science 109 r 110 (1997) 366–370
369
Table 1 Results of dielectric properties of BaTiO 3 thin films Samples
Capacitance ŽnFrmm2 .
tan d loss Ž%.
Leakage current Ž m A.
Dielectric constant
BaTiO 3rSi Ž6008C, 100 mTorr O 2 . BaTiO 3rTiNrSi Ž6008C, 100 mTorr O 2 . BaTiO 3rTiNrSi Ž6008C, high vacuum. BaTiO 3rSi Ž6008C, high vacuum.
54 56 166 117
8 0.3 173 118
30 0.06 200 50
215 223 661 466
shows the XRD patterns of a BaTiO 3 film grown on SiŽ100. substrate with TiN buffer layer. Films have the same crystal structure as the films deposited without TiN buffer layer. The films deposited with buffer layer have more crystalline quality structure compared to the previous XRD results. Generally, for MOS Žmetal–oxide–semiconductor. capacitor applications in DRAM Ždynamic random access memory. and other applications as well, the tan d loss should be less than 1% with the dielectric constant higher than 200. The BaTiO 3 films deposited at 6008C in 100 mTorr oxygen environment show the highly oriented peaks of BaTiO 3 phase with TiN buffer layers. The dielectric properties of the BaTiO 3 films deposited at 6008C in high vacuum and 100 mTorr oxygen atmosphere are
shown in Table 1. The BaTiO 3 film deposited at 6008C in 100 mTorr oxygen with TiN buffer layer has the best results with dielectric constant of about 223, loss factor of about 0.3%, and DC leakage current of about 0.06 m A Žat 40 V. at a frequency of 100 kHz. This film shows lower losses and lower DC leakage current than those deposited under vacuum. The reason is the oxygen deficiency in BaTiO 3 deposited in vacuum, which makes BaTiO 3 semiconducting and therefore a leaky capacitor is obtained. In oxygen atmosphere, BaTiO 3 is stoichiometric with no semiconducting behaviors. On the other hand, TiN acted as a good diffusion barrier and electrode between BaTiO 3 oxide and Si substrate. Fig. 4 shows the relationship between applied voltage and DC leakage current for the BaTiO 3 films deposited at
Fig. 4. Current versus voltage characteristics of BaTiO 3 thin films on SiŽ100. substrate.
370
N. Shu et al.r Applied Surface Science 109 r 110 (1997) 366–370
6008C in high vacuum and 100 mTorr oxygen environment. It is obvious from the Fig. 4 that BaTiO 3 deposited at 6008C in 100 mTorr oxygen with the TiN buffer layer has the smallest DC leakage current of 0.06 m A among all the films w7x.
dielectric properties analysis. Funding of this research was supported by Alabama NASA EPSCoR, Alabama UniversitiesrTVA, DOE EPSCoR and NSF SIP programs. References
4. Conclusions In summary, highly oriented barium titanate ŽBaTiO 3 . film, deposited on the silicon Ž100. substrate at 6008C, in 100 mTorr oxygen partial pressure with the titanium nitride ŽTiN. buffer layer, exhibited good crystallinity, a dielectric constant of 223, a dissipation factor of 0.3%, and a leakage current density of 0.06 m Arcm2 at 40 V. The buffer layer TiN acted as a good diffusion barrier and electrode between BaTiO 3 oxide and SiŽ100. substrate.
Acknowledgements The authors would like to thank Dr. Vivek Mehrotra, at Philips Laboratory, for his help with
w1x M.B. Lee, M. Kawasaki, M. Yoshimoto and H. Koinuma, Appl. Phys. Lett. 66 Ž1995. 1331. w2x H.F. Cheng, M.H. Yeh, K.S. Liu and I.N. Lin, Jpn. J. Appl. Phys. 32 Ž1993. 5656. w3x T. Nose, H.T. Kim and H. Uwe, Jpn. J. Appl. Phys. 33 Ž1994. 5259. w4x A. Kumar, U. Ekanayake, D. Kjendal, N. Shu and R.B. Inturi, in: Proc. Symp. on High Temperature Coating II, TMS Annual Meeting, Anaheim, CA, Feb. 4–8, 1996 Ž1996. pp. 349–360. w5x R.B. Inturi, A. Kumar, U. Ekanayake, N. Shu and J.A. Barnard, in: Proc. Symp. on Polycrystalline Thin Film Structure, Properties and Applications II, MRS Fall Meeting, Boston, MA, Nov. 27 – Dec. 1, 1995, Vol. 403 Ž1996. pp. 283–290. w6x D.B. Chrisey and G.K. Hubler, Pulsed Laser Deposition of Thin Films ŽWiley, 1994.. w7x A. Kumar, N. Shu, U. Ekanayake, D. Kjendal and G. Wattuhewa, Int. Symp. on Manufacturing Practices and Technologies, ACS Spring Meeting, New Orleans, LA, Nov. 5–8, 1995.