Method of N2O growth of an oxide layer on a silicon carbide...

Details Method of N2O growth of an oxide layer on a silicon carbide... Method of N2O growth of an oxide layer on a silicon carbide...

2001-10-01

2004-07-27

Niebling, John F. (Department: 2812)

Semiconductor device manufacturing: process

Coating of substrate containing semiconductor region or of...

C438S770000, C257S077000

Reexamination Certificate

active

06767843

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to the fabrication of semiconductor devices and more particularly, to the fabrication of oxide layers on silicon carbide (SiC).
BACKGROUND OF THE INVENTION
Devices fabricated from silicon carbide are typically passivated with an oxide layer, such as SiO
2
, to protect the exposed SiC surfaces of the device and/or for other reasons. However, the interface between SiC and SiO
2
may be insufficient to obtain a high surface mobility of electrons. More specifically, the interface between SiC and SiO
2
conventionally exhibits a high density of interface states, which may reduce surface electron mobility.
Recently, annealing of a thermal oxide in a nitric oxide (NO) ambient has shown promise in a planar 4H—SiC MOSFET structure not requiring a p-well implant. See M. K. Das, L. A. Lipkin, J. W. Palmour, G. Y. Chung, J. R. Williams, K. McDonald, and L. C. Feldman, “High Mobility 4H—SiC Inversion Mode MOSFETs Using Thermally Grown, NO Annealed SiO
2
, ” IEEE Device Research Conference, Denver, Colo., Jun. 19-21, 2000 and G. Y. Chung, C. C. Tin, J. R. Williams, K.
McDonald, R. A. Weller, S. T. Pantelides, L. C. Feldman, M. K. Das, and J. W. Palmour, “Improved Inversion Channel Mobility for 4H—SiC MOSFETs Following High Temperature Anneals in Nitric Oxide,” IEEE Electron Device Letters accepted for publication, the disclosures of which are incorporated by reference as if set forth fully herein. This anneal is shown to significantly reduce the interface state density near the conduction band edge. G. Y. Chung, C. C. Tin, J. R. Williams, K. McDonald, M. Di Ventra, S. T. Pantelides, L. C. Feldman, and R. A. Weller, “Effect of nitric oxide annealing on the interface trap densities near the band edges in the 4H polytype of silicon carbide,” Applied Physics Letters, Vol. 76, No. 13, pp. 1713-1715, March 2000, the disclosure of which is incorporated herein as if set forth fully. High electron mobility (35-95 cm
2
/Vs) is obtained in the surface inversion layer due to the improved MOS interface.
Unfortunately, NO is a health hazard having a National Fire Protection Association (NFPA) health danger rating of 3, and the equipment in which post-oxidation anneals are typically performed is open to the atmosphere of the cleanroom. They are often exhausted, but the danger of exceeding a safe level of NO contamination in the room is not negligible.
Growing the oxide in N
2
O is possible as described in J. P. Xu, P. T. Lai, C. L. Chan, B. Li, and Y. C. Cheng, “Improved Performance and Reliability of N
2
O -Grown Oxynitride on 6H—SiC,” IEEE Electron Device Letters, Vol. 21, No. 6, pp. 298-300, June 2000, the disclosure of which is incorporated by reference as if set forth fully herein. Xu et al. describe oxidizing SiC at 1100° C. for 360 minutes in a pure N
2
O ambient and annealing in N
2
for 1 hour at 1100° C.
Post-growth nitridation of the oxide on 6H—SiC in N
2
O at a temperature of 1100° C. has also been investigated by Lai et al. P. T. Lai, Supratic Chakraborty, C. L. Chan, and Y. C. Cheng, “Effects of nitridation and annealing on interface properties of thermally oxidized SiO
2
/SiC metal-oxide-semiconductor system,” Applied Physics Letters, Vol. 76, No. 25, pp. 3744-3746, June 2000, the disclosure of which is incorporated by reference as if set forth fully herein. However, Lai et al. concluded that such treatment deteriorates the interface quality which may be improved with a subsequent wet or dry anneal in O
2
which may repair the damage induced by nitridation in N
2
O. Moreover, even with a subsequent O
2
anneal, Lai et al. did not see any significant reduction in interface state density as compared to the case without nitridation in N
2
O.
SUMMARY OF THE INVENTION
Embodiments of the present invention provide methods for fabricating a layer of oxide on a silicon carbide layer by forming the oxide layer on the silicon carbide layer by oxidizing the silicon carbide layer in an N
2
O environment. Preferably, a predetermined temperature profile and/or a predetermined flow rate profile of N
2
O are provided during the oxidation. The predetermined temperature profile and/or predetermined flow rate profile may be constant or variable and may include ramps to steady state conditions. The predetermined temperature profile and/or the predetermined flow rate profile are selected so as to reduce interface states of the oxide/silicon carbide interface with energies near the conduction band of SiC.
In particular embodiments of the present invention, the predetermined temperature profile may result in an oxidation temperature of at least about 1200° C. In particular embodiments, the oxidation temperature is about 1300° C. In further embodiments of the present invention, the duration of the oxidation may vary depending on the thickness of the oxide layer desired. Thus, oxidation may be carried out for from about 15 minutes to about 3 hours or longer.
In additional oxidation embodiments of the present invention, the predetermined flow rate profile includes one or more flow rates of from about 2 Standard Liters per Minute (SLM) to about 6 SLM. In particular embodiments, the flow rates are from about 3.5 to about 4 Standard Liters per Minute.
In further embodiments, formation of the oxide layer may be followed by annealing the oxide layer in inert gas such as Ar or N
2
or combinations thereof. The post formation anneal may also be carried out in a hydrogen containing environment, such as H
2
or combinations of H
2
and one or more inert gases such as Ar or N
2
. Such an annealing operation may be carried out, for example, for about one hour.
In still further oxidation embodiments of the present invention, the predetermined flow rate profile provides a velocity or velocities of the N
2
O of from about 0.37 cm/s to about 1.11 cm/s. In particular embodiments, the predetermined flow rate profile provides a velocity or velocities of the N
2
O of from about 0.65 cm/s to about 0.74 cm/s.
In additional oxidation embodiments, methods for fabricating a layer of oxide on a silicon carbide layer include forming the oxide layer on the silicon carbide layer in an N
2
O environment at a predetermined temperature profile which includes an oxidation temperature of at least about 1200° C. and at a predetermined flow rate profile for the N
2
O. The predetermined flow rate profile may be selected to provide an initial residence time of the N
2
O of at least about 11 seconds.
In particular oxidation embodiments of the present invention, the initial residence time may be from about 11 seconds to about 33 seconds. In still further embodiments of the present invention, the initial residence time is from about 19 seconds to about 22 seconds.
Additionally, a total residence time of the N
2
O may be from about 28 seconds to about 84 seconds. In such oxidation embodiments of the present invention, the total residence time may also be from about 48 seconds to about 56 seconds.


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