Applied Surface Science 238 (2004) 336–340
Plasma-assisted SiC oxidation for power device fabrication P. Mandraccia,*, S. Ferreroa, S. Porroa, C. Ricciardia, G. Richierib, L. Scaltritob a
INFM and Dip. di Fisica, Politecnico di Torino, C.so Duca degli Abruzzi 24, 10129 Torino, Italy b International Rectifier Corporation Italia, Via Liguria 29, 10071 Borgaro T.se, Torino, Italy Available online 28 July 2004
Abstract In this work we show a plasma-assisted process for the deposition of good quality a-SiO2 layers on 4H–SiC as a possible alternative to thermal oxidation. We used the plasma enhanced chemical vapor deposition (PECVD) technique for the growth of a-SiO2 layers on 4H–SiC using SiH4 and CO2 as precursor gases in H2 dilution. We showed that good quality oxide ˚ /s, depending on the RF power. An layers could be obtained by this method, with a growth rate varying from 1.3 to 2.1 A estimation of the interface charge was obtained by high frequency capacitance voltage (HFCV) characteristic, obtaining values comparable to the ones typical of thermally grown oxides. This process was used for the growth of a-SiO2 insulating and protecting layers in the fabrication of Schottky diodes based on 4H–SiC, obtaining a breakdown voltage higher than 600 V. # 2004 Elsevier B.V. All rights reserved. PACS: 52.75.R; 77.84.B; 81.15.G; 84.30.J Keywords: Silicon oxide; Silicon carbide; PECVD
1. Introduction The thermal oxidation is a well-established method for the growth of SiO2 layers in the fabrication of electronic devices on Si and allows the creation of Si/ SiO2 interfaces with low interface charge. However, when dealing with the fabrication of devices based on silicon carbide (SiC), which is raising its importance in recent years especially in the field of power devices [1], the process of thermal oxidation shows several drawbacks. The temperature needed for the formation
*
Corresponding author. Tel.: þ39 011 5647354; fax: þ39 011 5647399. E-mail address:
[email protected] (P. Mandracci).
of SiO2 on SiC (1100–1200 8C) is considerably higher than the one required on Si, and the rate of oxide formation is considerably lower. The high temperature increases the process cost, can lead to strain formation and enhances the possibility of carbon enrichment at the film surface [2]. Moreover, the growth of a thick oxide film on SiC by thermal oxidation requires a thick SiC epilayer, due to the extraction of Si from the substrate for the SiO2 formation, and the excess carbon which is no more bonded to Si can bond to oxygen to form gaseous reaction products, such as CO, which have to diffuse through the growing oxide layer in order to be evacuated [3]. Most of the problems previously cited can be avoided by the use of plasma-assisted techniques for the growth of the oxide layer on SiC. In this kind of processes a precursor gas supplies the Si required
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.05.224
P. Mandracci et al. / Applied Surface Science 238 (2004) 336–340
for the oxide formation, avoiding the substrate consumption. Moreover, the chemical reactions that lead to the film formation occur at the growing film surface, rather than at the Si/SiO2 interface, so that diffusion through the film is not required for the evacuation of reaction products. Another important advantage is the deposition at lower temperatures (300–350 8C), thanks to the use of plasma for the activation of the growth reactions, reducing the stress formation. Although the use of a post-deposition thermal treatment at higher temperatures (900–950 8C) is often required to reduce the charge density at the SiC/SiO2 interface, in this case the heating is performed when the oxide layer is already formed and the problems related to C enrichment are strongly reduced. Several kinds of plasma-assisted techniques have already been used for the deposition of SiO2 on SiC, such as RPAO and RPECVD [2–5]. In most cases SiH4 or TEOS are used as Si precursor, while the most common choice for oxygen is O2 or N2O, although the latter has been found to lead to worse interface properties [2]. In this work we present some results on the growth of SiO2 on 4H–SiC by means of plasma enhanced chemical vapor deposition at a substrate temperature of 300 8C, using SiH4 and CO2 as precursor gases in H2 dilution. We also show the use of SiO2 layers grown by this technique for the production of insulating layers in the fabrication of Schottky diodes based on 4H–SiC.
2. Experimental The a-SiO2 films were grown on commercially available (CREE Research) 4H–SiC wafers of 2 in. diameter (n-type, Nd 1018 cm3), with an epitaxial layer (n-type, Nd 1015 cm3) of 7 mm thickness; a standard RCA-clean was performed on the substrates before the film growth. The deposition of a-SiO2 films, about 1 mm thick, was performed in a stainless steel PECVD reactor with capacitively coupled plasma excitation at 13.56 MHz at a substrate temperature of 300 8C. SiH4 and CO2 were used as Si and oxygen precursors, respectively, and hydrogen as diluent. The reactive gas flow ratio and hydrogen dilution were the following: [CO2]/([SiH4] þ [CO2]) ¼ 0.98 and [H2]/ ([H2] þ [CO2] þ [SiH4]) ¼ 0.71; the total gas flow was 141 sccm at a total pressure in the chamber of 80 Pa.
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Fig. 1. Schematic diagram of a Schottky diode with a-SiO2 insulating layer grown by PECVD.
This growth conditions, that are result of a previous study reported elsewhere [6], were kept fixed for all samples, while the RF power was varied from 10 to 30 W. A post-deposition thermal treatment at 950 8C in oxygen atmosphere for 5 h was performed on the grown film, in order to reduce the interface charge. The thickness of the films was measured by a Tencor profiler, in order to obtain the deposition rate, while the net oxide charge at the SiC/SiO2 interface was estimated by high frequency capacitance voltage (HFCV) measurements at room temperature. The a-SiO2-covered 4H–SiC wafers were processed to obtain Schottky diodes with the structure showed schematically in Fig. 1. The Schottky barriers were formed by evaporation of titanium or nickel, leaving a metal-oxide overlap for electric field termination, while ohmic contact were realized on the back of the wafer by a triple evaporation of titanium, nickel and silver. The I–V characteristic was measured by an SMU237 Keithley Source Measure Unit (high voltage source up to 1100 V) and an SMU238 Keithley Source Measure Unit (high current source up to 1 A).
3. Results and discussion The RF power used in the deposition process was varied from 10 to 30 W, keeping constant gas fluxes and total pressure, in order to find the maximum deposition rate. The results of this correlation are showed in Fig. 2, where the deposition rate as a
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Fig. 2. Deposition rate of PECVD-grown a-SiO2 as a function of RF power.
Fig. 3. High frequency capacitance voltage characteristic of a-SiO2 film grown on 4H–SiC.
P. Mandracci et al. / Applied Surface Science 238 (2004) 336–340
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Fig. 4. I–V characteristic of 4H–SiC based Schottky diodes with a-SiO2 insulating layer grown by PECVD.
function of RF power is reported. As can be seen, there is an approximately linear increase of the deposition ˚ /s as the power is raised from 10 rate from 1.4 to 2.2 A to 20 W, while after this value the growth speed decreases. This behavior can be due to the full dissociation of silane for high power levels, while the etching effect of hydrogen can still be enhanced by the increase of ion energy in the discharge. The optical characterization of the films, already reported elsewhere [6], showed an optical gap of 5.7 eV. The charge trapped at the 4H–SiC/SiO2 interface can be estimated from the HFCV measurement showed in Fig. 3. The shift of the CV curve toward positive voltage indicates an oxide charge of approximately 6 1010 cm2, which is comparable to typical values of SiO2 layers on SiC obtained by thermal oxidation [7]. The electrical characterization of the oxide was already reported in a previous work [6] and the current–density versus electric field (J–E) characteristic curve showed a breakdown field of about 9.9 MV cm1. The electrical characterization of the Schottky diodes is shown in Fig. 4, where the current voltage characteristic curve is reported. The ideality factor of the diodes was calculated from the slope and the intercept of the linear region in the semi-logarithmic
plots of the measured forward I–V characteristics and was found to be about 1.27. The breakdown voltage (defined as the value of reverse voltage at 1 mA of reverse current) was determined from the measured reverse I–V characteristics and was found to be higher than 600 V.
4. Conclusions The PECVD technique has been found suitable for the growth of good quality a-SiO2 films on 4H–SiC, at relatively high deposition rates. The obtained material shows good electrical quality, with a level of interface charges that is comparable to the one of thermally grown oxides. The PECVD-grown oxide allowed the fabrication of 4H–SiC based Schottky diodes, which showed good electrical characteristics with breakdown voltages higher than 600 V. References [1] M. Bhatanagar, B.J. Baliga, IEEE Trans. Electron. Develop. 40 (1993) 645. [2] A. Golz, S. Gross, R. Janssen, E. Stein von Kamienski, H. Kurz, Mater. Sci. Eng. B 46 (1997) 363.
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[3] G. Lucovsky, H. Niimi, A. Golz, H. Kurz, Appl. Surf. Sci. 123– 124 (1998) 435. [4] A. Golz, S. Gross, R. Janssen, E. Stein von Kamienski, H. Kurz, Diamond Relat. Mater. 6 (1997) 1420. [5] C.E. Viana, N.I. Morimoto, O. Bonnaud, Micr. Reliab. 40 (2000) 613.
[6] P. Mandracci, S. Ferrero, C. Ricciardi, L. Scaltrito, G. Richieri, C. Sgorlon, Thin Solid Films 427 (2003) 142. [7] D. Alok, P.K. McLarty, B.J. Baliga, Appl. Phys. Lett. 64 (1994) 2845.
