Thick Oxide Layers on N and P SiC Wafers by a Depo-Conversion Technique
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enhancement at the contact periphery of devices. In order for this field oxide to be effective it should have good dielectric properties and sufficient thickness to sustain the high breakdown voltage. However, the SiC oxidation rate is too slow to obtain thick oxide layers via the conventional thermal oxidation techniques currently in practice [1,2,3]. Even though alternate techniques such as oxide deposition by LPCVD and poly-silicon conversion have been reported [4,5], the high voltage characteristics of deposited oxide vs a conventional thermal oxide has not been studied in detail. In this paper, we report the possibilities of using simple depoconverison technique for obtaining thick oxide layers with high breakdown strength on n and ptype 6H-SiC wafers. EXPERIMENT The 6H-SiC wafers used in our experiments were from Cree Research (substrate doping 1.6x10' 8 cm"3 ) with a 10 ýtm thick epilayer of- 6x1015cMn3 doping concentration forp-type and - 3.9x 1015 cm 3 for n-type SiC respectively. The 30 mm diameter wafer was cut into 10 mm x 10 mm square pieces to obtain several samples for experiments. The as-received SiC samples were first cleaned using TCE at 85'C for 15 min. followed by ultrasonic cleaning in acetone and methanol respectively. Then the native surface oxide was chemically etched using a 20% HIF solution prior to a modified RCA cleaning process. After the RCA cleaning process the SIC wafers were etched in a 5% -F solution for 10 seconds to remove the surface oxide -
57 Mat. Res. Soc. Symp. Proc. Vol. 572 ©1999 Materials Research Society
caused by the RCA process. De-ionized water rinses were used after every step. In order to obtain the depo-converted oxide, a Si film is deposited on the SiC wafer and converted to silicon dioxide by oxidation. The Si films were RF sputtered from a single crystal Si target in a vacuum chamber under Ar ambient and the sputtered Si films were converted to oxide by wet oxidation at 1050'C for 3 hours to ensure total conversion of the deposited Si. All samples with converted oxide had a very uniform surface morphology, indicating that depo-conversion method can be used to obtain thick oxide films. MOS capacitor (MOS-C) structures were fabricated, to characterize the electrical properties of the depo-converted oxide, by evaporating Al on both sides of the sample. Moreover, the usefulness of the depo-converted oxide as a field oxide was demonstrated by fabricating a 1kV 6H-SiC Schottky diode with oxide layer edge termination. RESULTS AND DISCUSSION C-V characteristics Capacitance-voltage measurements were made on MOS capacitors with different oxide thicknesses on n and p type SiC at room temperature using a Kiethley 590 capacitance-voltage (C-V) analyzer at 100 kHz. Voltage sweeps applied to the MOS capacitor were made from accumulation to depletion/inversion. Typical C-V characteristics of MOS capacitors with thermal and converted oxides fabricated on n and p-type SiC are shown in Fig. 1. 1
-I- -
'
'N
Depo-converted oxide
Thermal oxide
-4000A
-370A
... 0.5
epo-con
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