2. 1971 Rearick, Maksimov, and Heydemann: Design of a hydrogen etching system for surface preparation 1971
FIG. 3. AFM image of a 4 ϫ 4 m2 area of the same ͑0001͒ 6H-SiC diced
wafer shown in Fig. 1, after hydrogen etching at 1650 ° C, 650 Torr, 10% FIG. 4. AFM image of a 1 ϫ 1 m2 area of the same ͑0001͒ 6H-SiC diced
H2 in 90% Ar at ϳ1100 SCCM flow for 60 min. The surface is free of wafer shown in Fig. 1, after hydrogen etching at 1650 ° C, 650 Torr, 10% H2
scratches and exhibits equally spaced unit cell height steps and terraces in 90%Ar at ϳ1100 SCCM flow for 1 h. The image shows a close-up view
indicating the slight misorientation of the wafer surface against the ͑0001͒ of the steps and terraces shown in Fig. 3. The texture on the ͑0001͒ terraces
basal plane. The small circular features visible on the step risers are likely is likely due to the formation of a native oxide layer in air after the hydrogen
single Burgers vector threading dislocations present in the wafers, intro- etch. The root mean square ͑rms͒ roughness of this surface is approximately
duced by the original SiC crystal growth process. The root mean square 2.7 Å.
͑rms͒ roughness of this surface is approximately 3.3 Å.
A ͑0001͒ 6H-SiC wafer was processed under the process
conditions mentioned above. Atomic force microscopy
͑AFM͒ images of 4 ϫ 4 and 1 ϫ 1 m2 areas of the processed
is mechanically clamped to water-cooled current wafer are shown in Figs. 3 and 4, respectively. The surface of
feedthroughs. The vacuum chamber is evacuated to a base the hydrogen etched sample is significantly smoother ͑Fig. 3,
pressure of approximately 5 Torr using a Welch Vacuum rms= 3.3 Å͒ than the as-received surface that exhibited
Technology 2561B dry pump. The wafer is heated to a regu- scratches ͑Fig. 1, rms= 25 Å͒. The postetch surface exhibits
lated temperature of 1650 ° C by passing a dc current of equally spaced unit cell steps and terraces reflecting the
300– 350 A through the Ta boat using an Elgar Electronics slight misorientation of the wafer surface from the ͑0001͒
Power Ten 62 series 3 kW power supply ͑8 V / 350 A͒. The basal plane. Similar images and roughness have been ob-
boat and wafer temperature is measured with a Mikron In- tained both at different areas of the same wafer and for wa-
frared M77S two color pyrometer through a quartz viewport fers subsequently etched under the same conditions, ensuring
located axially above the heater. The etching is performed for repeatability of the process with uniformity of 3.3± 0.3 A.
a determined duration ͑typically 60 min͒ under horizontal The process removed approximately 3.5 m of material, cor-
gas flow of ϳ1100 SCCM ͑SCCM denotes cubic centimeter responding to an etch rate of 3.5 m / h.
per minute at STP͒ composed of 10% H2 in 90% Ar to en- In conclusion, we described the construction of a low-
sure safe operation and stay below the hydrogen explosion cost, operator-friendly, repeatable hydrogen etching system
ratio. The total pressure in the chamber is maintained at and demonstrated its capability for the preparation of defect-
650 Torr by utilizing an MKS Instruments 253B throttling free SiC wafer surfaces ready for epitaxial growth. Studies
valve that is controlled by an MKS 651C pressure controller indicate that a similar hydrogen etch process can be applied
connected to an MKS 722A 1000 Torr capacitive pressure to other substrates, such as GaN.4
transducer. The controller was tuned to assure constant sys-
tem pressure over a range of 5 – 760 Torr, even with changes This material is based upon work supported by Dr. Colin
in gas inlet rate or wafer temperature. Two 1000 SCCM Wood, ONR, under Contract No. N00014-05-1-0238.
MKS M100B mass flow controllers are used to inject H2 and 1
͑0001͒ 6H-SiC research grade wafer, Intrinsic Semiconductor. Note:
Ar, respectively, and a regulated exhaust throttle valve con- Wafers from other commercial SiC vendors exhibit similar surface mor-
trols the pumping speed. The custom control system is de- phology.
2
signed to assure safe, repeatable operation utilizing National V. Ramachadran, A. R. Smith, R. M. Feenstra, and D. W. Greeve, J. Vac.
Sci. Technol. A 17, 1289 ͑1999͒.
Instruments LABVIEW process control and data acquisition 3
K. H. Ploog, O. Brandt, R. Muralidharan, A. Thamm, and P. Waltereit,
software that interfaces with the sensors, actuators, and con- J. Vac. Sci. Technol. A 18, 2290 ͑2000͒.
4
trollers. The total cost of the system is below $20 000. T. H. Myers et al., J. Cryst. Growth 246, 244 ͑2002͒.
JVST A - Vacuum, Surfaces, and Films