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H2 reactive ion etched β-SiC
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Solid-State Electronics Vol. 42, No. 2, pp. 253±256, 1998 Copyright # Elsevier Science Ltd. All rights reserved Printed in Great Britain 0038-1101/98 $19.00 + 0.00 S0038-1101(97)00224-4
SCHOTTKY CONTACTS ON CF4/H2 REACTIVE ION ETCHED b-SIC G. CONSTANTINIDIS1, J. KUZMIC2, K. MICHELAKIS1 and K. TSAGARAKI1 Foundation for Research and Technology Hellas, P.O. Box 1527, 711 10 Heraklion, Crete, Greece Institute of Electrical Engineering, Slovak Academy of Sciences, Dubravska cesta 9, 842 39 Bratislava, Slovakia 1
2
(Received 6 January 1997; in revised form 22 April 1997) AbstractÐCVD grown n-type b-SiC grown on a (100) Si substrate was reactive ion etched (RIE) in CF4/H2 gas mixtures. The etched surfaces were examined by SEM, FTIR-spectroscopy and AFM. Au Schottky diodes fabricated on the etched surface were compared to reference contacts on the nonetched surface. An oxidation step following the dry etching drastically improves the diode characteristics. # 1998 Elsevier Science Ltd. All rights reserved
1. INTRODUCTION
The multitude of extreme thermal and electronic properties of SiC produces numerous and novel prospects for both optoelectronic and for highpower, high-frequency, high-temperature, highspeed and radiation hard microelectronic devices[1± 5]. The ability of SiC to oxidize and form SiO2 has allowed compatibility with standard silicon-based fabrication processing. However, SiC oers higher resistance to thermal oxidation than Si and it takes up to several hours to obtain a suciently thick oxide on SiC[6,7]. SiC oxidation follows the Deal and Grove[8] parabolic model. It is considered that the growth of an oxide ®lm is limited by diusion through the ®lm of CO produced during the chemical reaction of carbon and oxygen[1]. SiC oxidation serves two purposes: (1) elimination of surface roughness and residual contaminants after polishing and (2) fabrication of gate insulators for MOSFETs. Etching is one of the critical steps in SiC device fabrication. Wet chemical etching of SiC requires molten salts and high temperature environment. Dry etching remains the main practical technique for SiC removal for device fabrication using conventional photoresist techniques. Reactive ion etching (RIE) of SiC is performed with either ¯uorine-based gases such as CF4/O2, SF6, CHF3/O2, CBrF3 and CCl2F2 or chlorine-based gases such as CCl4/O2. The basic mechanisms for the SiC etching process in ¯uorinated plasmas[9] consist of a combination of chemical (reaction with F and O) and physical (energetic ions) removal processes. The etching parameters that must be considered for most SiC device fabrication include the etch rate, etching selectivity to masking layer, etching anisotropy and the presence or absence of residues in the 253
etched area. Furthermore there are often speci®c device characteristics for the plasma etching process which depend on the device type and structure to be fabricated. 2. EXPERIMENT
The material used in this investigation was nonintentionally doped (2 1017 cmÿ3) single crystalline 6 mm thick n-type, b-SiC epitaxially grown on a (100) silicon substrate by the chemical vapor deposition process (CVD) at LETI[10]. After the surface had been mechanically polished with diamond pastes of various diminishing sizes (smaller paste size 0.1 m), the surface damage was removed by two consecutive dry oxidation steps at 13008C for 2.5 h each. The formed oxides were removed in (1:3) HF:H2O solution. The resulted surface was examined for its quality (smoothness and lack of surface defects) by scanning electron microscopy (SEM) (Fig. 1) and FTIR-spectroscopy (Fig. 2). Two thirds of this surface were etched in CF4/H2 under optimized conditions which produced extremely smooth etched surfaces[11]. The RIE experiment was performed in a commercial Vacutec VPS-1523 dual PECVD/RIE chamber. The RIE chamber is made of aluminum with a water cooled bottom aluminium electrode which is RF powered at a frequency of 13.56 MHz. The ¯ow of the reactive gases was separately controlled and mixed in a manifold before the mixture was introduced in the chamber. The optimized conditions of the experiment were: 50 mTorr pressure, 100 W rf power, 20 sccm of CF4 and 5 sccm of H2. The cathode selfbias was ÿ360 V and the duration of the etching was 60 min. The etched depth was 600 nm and the etched surface was examined by scanning electron
254
G. Constantinidis et al.
Fig. 1. SEM of ®nal surface.
microscopy (SEM) (Fig. 3), FTIR-spectroscopy (Fig. 4) and AFM for its quality. Then the sample was cut into half by scribing, each half containing etched and non-etched regions. One half was subjected to an extra dry oxidation step at 13008C for 2.5 h. The formed oxide was
removed in (1:3) HF:H2O solution. The resulted surface was examined by SEM. The next step was the fabrication of (2000A Au) Schottky and (2000A Cr/1000A Au) ohmic contacts to both pieces with the same evaporation run for each type of contact. The diode geometry has circular dots of 0.13 mm2
Fig. 2. FT-IR spectra of initial/®nal surface.
Schottky contacts on ion etched-SiC
255
Fig. 3. SEM of non-etched/etched surface.
in the area separated from the ohmic region by an annulus. The fabrication of the metal contacts was performed by standard optical lithography (AZ 5214 photoresist), e-gun evaporation (commercial Temescal BJD 1800 evaporator at a background pressure of 2 10ÿ7 Torr) and lift-o techniques. The above processing sequence yielded ``four'' regions denoted as follows (with respect to their overall surface treatment):
Region 1 (S) = Polished/oxidized/oxide removal/ contacts Region 2 (SR) = Polished/oxidized/oxide removal/ CF4/H2 RIE/contacts Region 3 (SO) = Polished/oxidized/oxide removal/ 2nd oxidation/oxide removal/contacts Region 4 (SRO) = Polished/oxidized/oxide removal/CF4/H2 RIE/2nd oxidation/oxide removal/ contacts
Fig. 4. FT-IR spectra of non-etched/etched surfaces.
256
G. Constantinidis et al.
Table 1. Variation of ideality factor and Barrier height depending on the surface treatment Surface treatment S SR SO SRO
Ideality factor 2.30 2.08 1.70 1.58
B (eV) 0.47 0.58 0.63 0.69
Table 1 summarizes the results from the analysis of the I±V characteristics of the diodes.
3. DISCUSSION
This work was based on the optimization of the etching conditions of b-SiC in CH4/H2 gas mixtures that produced extremely smooth surfaces as can be seen from Figs 3 and 4. From the SEM analysis we could not really distinguish between the two surfaces, while the FT-IR spectra exhibited a ``¯atter'' restsrahlen region for the etched region than for the non-etched one. This is indicative of the more smooth nature of the etched surface[12]. The more smooth nature of the etched surface is conclusively con®rmed by the AFM analysis (average roughness of etched region 6 nm while that for the non-etched region 13 nm). Since the presence of surface defects critically aects the quality of the Schottky contact[13], the results from the diode characteristics (better ideality factor and barrier height for the diodes on the etched region) is an additional con®rmation of the smoothness of the etched region. In order to further improve the quality of the SiC surface prior to metal deposition, thermal oxidation was employed as an additional processing step following RIE. The improvement in the diode electrical characteristics is attributed mainly to the removal of surface roughness (in a similar process to that following mechanical polishing) and also to the removal of RIE residues (like F) that may be present on the surface after etching. The results obtained for the barrier height (Table 1) compare favorably with recently published ones on bSiC[14]. An ideality factor lower than 1.58 is di-
cult to obtain on b-SiC due to the inherent relative high defect density of the material. In conclusion, optimized RIE conditions in CH4/ H2 for the production of extremely smooth b-SiC coupled with an extra oxidation step have been established and may prove to be an important process factor for the fabrication of future devices. AcknowledgementsÐThis work was supported by EU Brite-Euram under the contract No. BRE2-CT92-0211. The authors would also like to thank Mrs. M. Androulidaki for the FT-IR measurements and Mrs. V. Papaioannou of the Aristotle University of Thessaloniki, Greece for the AFM measurements. REFERENCES
1. Ivanov, P. A. and Chelnokov, V. E., Semicond. Sci. Technol., 1992, 7, 863±880. 2. Waldrop, J. R., Grant, R. W., Wang, Y. C. and Davis, R. F., J. Appl. Phys., 1992, 72, 4757±4760. 3. Bhatanagar, M., McLarty, P. K. and Baliga, B. J., IEEE Electron Dev. Lett., 1992, 13, 501±503. 4. Ioannou, D. E., Papanicolaou, N. A. and Norquist, P. E., IEEE Trans. Electron Dev., 1987, 34, 1694± 1699. 5. Papanicolaou, N. A., Christou, A. and Gipe, L. M., J. Appl. Phys, 1989, 65, 3526±3530. 6. Suzuki, A., Ashida, H. and Tanaka, T., J. Electrochem. Soc., 1986, 125, 131. 7. Fung, C. D. and Kopanski, J. J., J. Appl. Phys. Lett., 1984, 45, 757. 8. Deal, B. E. and Grove, A. S., J. Appl. Phys., 1965, 36, 3770. 9. Pan, W. S. and Steckl, A. J., in Amorphous and Crystalline Silicon Carbide, Spinger Proceedings in Physics, 1989, Vol. 34 (Edited by Harris, G. L. and Yang, Y. W.), p. 192. 10. Becourt, N., Cros, B., Ponthenier, J. L., Berjoan, R., Papon, A. M. and Jaussaud, C., Appl. Surf. Sci., 1993, 68, 461±466. 11. Zekentes, K., Constantinidis, G. and Kuzmic, J., Brite-Euram ``TECSICA'' under the contract No. BRE2-CT92-0211, midterm assessment report, pp. 11± 12, July, 1994. 12. Holm, R. T., Klein, P. H. and Nordquist, P. Jr., J. Appl. Phys, 1986, 60(4), 1479±1485. 13. Rhoderick, E. H., Metal-Semiconductor Contacts. 1978, Oxford, England, Claredon. 14. Porter, L. M. and Davis, R. F., Mater. Sci. Eng. B, 1995, 34, 83±105.
Solid-State Electronics Vol. 42, No. 2, pp. 253±256, 1998 Copyright # Elsevier Science Ltd. All rights reserved Printed in Great Britain 0038-1101/98 $19.00 + 0.00 S0038-1101(97)00224-4
SCHOTTKY CONTACTS ON CF4/H2 REACTIVE ION ETCHED b-SIC G. CONSTANTINIDIS1, J. KUZMIC2, K. MICHELAKIS1 and K. TSAGARAKI1 Foundation for Research and Technology Hellas, P.O. Box 1527, 711 10 Heraklion, Crete, Greece Institute of Electrical Engineering, Slovak Academy of Sciences, Dubravska cesta 9, 842 39 Bratislava, Slovakia 1
2
(Received 6 January 1997; in revised form 22 April 1997) AbstractÐCVD grown n-type b-SiC grown on a (100) Si substrate was reactive ion etched (RIE) in CF4/H2 gas mixtures. The etched surfaces were examined by SEM, FTIR-spectroscopy and AFM. Au Schottky diodes fabricated on the etched surface were compared to reference contacts on the nonetched surface. An oxidation step following the dry etching drastically improves the diode characteristics. # 1998 Elsevier Science Ltd. All rights reserved
1. INTRODUCTION
The multitude of extreme thermal and electronic properties of SiC produces numerous and novel prospects for both optoelectronic and for highpower, high-frequency, high-temperature, highspeed and radiation hard microelectronic devices[1± 5]. The ability of SiC to oxidize and form SiO2 has allowed compatibility with standard silicon-based fabrication processing. However, SiC oers higher resistance to thermal oxidation than Si and it takes up to several hours to obtain a suciently thick oxide on SiC[6,7]. SiC oxidation follows the Deal and Grove[8] parabolic model. It is considered that the growth of an oxide ®lm is limited by diusion through the ®lm of CO produced during the chemical reaction of carbon and oxygen[1]. SiC oxidation serves two purposes: (1) elimination of surface roughness and residual contaminants after polishing and (2) fabrication of gate insulators for MOSFETs. Etching is one of the critical steps in SiC device fabrication. Wet chemical etching of SiC requires molten salts and high temperature environment. Dry etching remains the main practical technique for SiC removal for device fabrication using conventional photoresist techniques. Reactive ion etching (RIE) of SiC is performed with either ¯uorine-based gases such as CF4/O2, SF6, CHF3/O2, CBrF3 and CCl2F2 or chlorine-based gases such as CCl4/O2. The basic mechanisms for the SiC etching process in ¯uorinated plasmas[9] consist of a combination of chemical (reaction with F and O) and physical (energetic ions) removal processes. The etching parameters that must be considered for most SiC device fabrication include the etch rate, etching selectivity to masking layer, etching anisotropy and the presence or absence of residues in the 253
etched area. Furthermore there are often speci®c device characteristics for the plasma etching process which depend on the device type and structure to be fabricated. 2. EXPERIMENT
The material used in this investigation was nonintentionally doped (2 1017 cmÿ3) single crystalline 6 mm thick n-type, b-SiC epitaxially grown on a (100) silicon substrate by the chemical vapor deposition process (CVD) at LETI[10]. After the surface had been mechanically polished with diamond pastes of various diminishing sizes (smaller paste size 0.1 m), the surface damage was removed by two consecutive dry oxidation steps at 13008C for 2.5 h each. The formed oxides were removed in (1:3) HF:H2O solution. The resulted surface was examined for its quality (smoothness and lack of surface defects) by scanning electron microscopy (SEM) (Fig. 1) and FTIR-spectroscopy (Fig. 2). Two thirds of this surface were etched in CF4/H2 under optimized conditions which produced extremely smooth etched surfaces[11]. The RIE experiment was performed in a commercial Vacutec VPS-1523 dual PECVD/RIE chamber. The RIE chamber is made of aluminum with a water cooled bottom aluminium electrode which is RF powered at a frequency of 13.56 MHz. The ¯ow of the reactive gases was separately controlled and mixed in a manifold before the mixture was introduced in the chamber. The optimized conditions of the experiment were: 50 mTorr pressure, 100 W rf power, 20 sccm of CF4 and 5 sccm of H2. The cathode selfbias was ÿ360 V and the duration of the etching was 60 min. The etched depth was 600 nm and the etched surface was examined by scanning electron
254
G. Constantinidis et al.
Fig. 1. SEM of ®nal surface.
microscopy (SEM) (Fig. 3), FTIR-spectroscopy (Fig. 4) and AFM for its quality. Then the sample was cut into half by scribing, each half containing etched and non-etched regions. One half was subjected to an extra dry oxidation step at 13008C for 2.5 h. The formed oxide was
removed in (1:3) HF:H2O solution. The resulted surface was examined by SEM. The next step was the fabrication of (2000A Au) Schottky and (2000A Cr/1000A Au) ohmic contacts to both pieces with the same evaporation run for each type of contact. The diode geometry has circular dots of 0.13 mm2
Fig. 2. FT-IR spectra of initial/®nal surface.
Schottky contacts on ion etched-SiC
255
Fig. 3. SEM of non-etched/etched surface.
in the area separated from the ohmic region by an annulus. The fabrication of the metal contacts was performed by standard optical lithography (AZ 5214 photoresist), e-gun evaporation (commercial Temescal BJD 1800 evaporator at a background pressure of 2 10ÿ7 Torr) and lift-o techniques. The above processing sequence yielded ``four'' regions denoted as follows (with respect to their overall surface treatment):
Region 1 (S) = Polished/oxidized/oxide removal/ contacts Region 2 (SR) = Polished/oxidized/oxide removal/ CF4/H2 RIE/contacts Region 3 (SO) = Polished/oxidized/oxide removal/ 2nd oxidation/oxide removal/contacts Region 4 (SRO) = Polished/oxidized/oxide removal/CF4/H2 RIE/2nd oxidation/oxide removal/ contacts
Fig. 4. FT-IR spectra of non-etched/etched surfaces.
256
G. Constantinidis et al.
Table 1. Variation of ideality factor and Barrier height depending on the surface treatment Surface treatment S SR SO SRO
Ideality factor 2.30 2.08 1.70 1.58
B (eV) 0.47 0.58 0.63 0.69
Table 1 summarizes the results from the analysis of the I±V characteristics of the diodes.
3. DISCUSSION
This work was based on the optimization of the etching conditions of b-SiC in CH4/H2 gas mixtures that produced extremely smooth surfaces as can be seen from Figs 3 and 4. From the SEM analysis we could not really distinguish between the two surfaces, while the FT-IR spectra exhibited a ``¯atter'' restsrahlen region for the etched region than for the non-etched one. This is indicative of the more smooth nature of the etched surface[12]. The more smooth nature of the etched surface is conclusively con®rmed by the AFM analysis (average roughness of etched region 6 nm while that for the non-etched region 13 nm). Since the presence of surface defects critically aects the quality of the Schottky contact[13], the results from the diode characteristics (better ideality factor and barrier height for the diodes on the etched region) is an additional con®rmation of the smoothness of the etched region. In order to further improve the quality of the SiC surface prior to metal deposition, thermal oxidation was employed as an additional processing step following RIE. The improvement in the diode electrical characteristics is attributed mainly to the removal of surface roughness (in a similar process to that following mechanical polishing) and also to the removal of RIE residues (like F) that may be present on the surface after etching. The results obtained for the barrier height (Table 1) compare favorably with recently published ones on bSiC[14]. An ideality factor lower than 1.58 is di-
cult to obtain on b-SiC due to the inherent relative high defect density of the material. In conclusion, optimized RIE conditions in CH4/ H2 for the production of extremely smooth b-SiC coupled with an extra oxidation step have been established and may prove to be an important process factor for the fabrication of future devices. AcknowledgementsÐThis work was supported by EU Brite-Euram under the contract No. BRE2-CT92-0211. The authors would also like to thank Mrs. M. Androulidaki for the FT-IR measurements and Mrs. V. Papaioannou of the Aristotle University of Thessaloniki, Greece for the AFM measurements. REFERENCES
1. Ivanov, P. A. and Chelnokov, V. E., Semicond. Sci. Technol., 1992, 7, 863±880. 2. Waldrop, J. R., Grant, R. W., Wang, Y. C. and Davis, R. F., J. Appl. Phys., 1992, 72, 4757±4760. 3. Bhatanagar, M., McLarty, P. K. and Baliga, B. J., IEEE Electron Dev. Lett., 1992, 13, 501±503. 4. Ioannou, D. E., Papanicolaou, N. A. and Norquist, P. E., IEEE Trans. Electron Dev., 1987, 34, 1694± 1699. 5. Papanicolaou, N. A., Christou, A. and Gipe, L. M., J. Appl. Phys, 1989, 65, 3526±3530. 6. Suzuki, A., Ashida, H. and Tanaka, T., J. Electrochem. Soc., 1986, 125, 131. 7. Fung, C. D. and Kopanski, J. J., J. Appl. Phys. Lett., 1984, 45, 757. 8. Deal, B. E. and Grove, A. S., J. Appl. Phys., 1965, 36, 3770. 9. Pan, W. S. and Steckl, A. J., in Amorphous and Crystalline Silicon Carbide, Spinger Proceedings in Physics, 1989, Vol. 34 (Edited by Harris, G. L. and Yang, Y. W.), p. 192. 10. Becourt, N., Cros, B., Ponthenier, J. L., Berjoan, R., Papon, A. M. and Jaussaud, C., Appl. Surf. Sci., 1993, 68, 461±466. 11. Zekentes, K., Constantinidis, G. and Kuzmic, J., Brite-Euram ``TECSICA'' under the contract No. BRE2-CT92-0211, midterm assessment report, pp. 11± 12, July, 1994. 12. Holm, R. T., Klein, P. H. and Nordquist, P. Jr., J. Appl. Phys, 1986, 60(4), 1479±1485. 13. Rhoderick, E. H., Metal-Semiconductor Contacts. 1978, Oxford, England, Claredon. 14. Porter, L. M. and Davis, R. F., Mater. Sci. Eng. B, 1995, 34, 83±105.