Semiconductor device manufacturing: process – Chemical etching – Liquid phase etching
Reexamination Certificate
1999-09-21
2001-07-24
Utech, Benjamin L. (Department: 1765)
Semiconductor device manufacturing: process
Chemical etching
Liquid phase etching
C438S749000
Reexamination Certificate
active
06265322
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a selective growth technique for Group III-nitride materials and, more particularly, to the use of masking technique to form both polycrystalline and single crystal Group III-nitride films that can be preferentially processed to form selected growth areas.
BACKGROUND OF THE INVENTION
Group III nitrides, in particular, GaN, are of interest for many electronic and optoelectronic applications, including high temperature electronic devices, high-power microwave circuits, blue lasers and LEDs, as well as solar-blind photodetectors. For these applications, the semiconductor layers are grown epitaxially on a crystalline substrate. After the epitaxial growth, a masking layer is typically deposited on the surface and lithographically patterned. The pattern can then be transferred to the underlying semiconductor material by etching. There are several reasons why this patterning is needed, including device isolation, contact to underlying layers, and the formation of waveguides and laser facets.
GaN etching is commonly performed using a reactive ion etching (RIE) technique. An article entitled “Dry and Wet Etching for Group III-nitrides” by I. Adesida et al. appearing in
MRS Internet Journal of Nitride Semiconductor Research
, Vol. 4S1, No. G1.4, 1999, discussed in detail various prior art RIE and wet chemical etching techniques for GaN. One of the drawbacks of reactive ion etching includes the generation of ion-induced damage on the surface of the GaN film. Wet chemical etching produces significantly less damage and is often less costly and complex than RIE systems, thus making wet chemical etching an attractive alternative. However, epitaxial GaN (or Group III-nitrides in general) has been found to be resistant to wet chemical etching. In particular, the etch rates have been too slow for efficient processing. Further, the wet etch techniques have generally not been directional enough to produce sidewalls that mirror the masking material. To circumvent this problem, photo-enhanced electrochemical PPEC) has been developed. U.S. Pat. No. 5,773,369 issued to E. L. Hu et al on Jun. 30, 1998 discussed an exemplary PEC process for Group III nitrides where an ultra-violet light source and a metal mask layer is used. A bias is applied to the metal mask to effect the etch process. In general, a PEC process is ineffective in etching p-type and semi-insulating materials since there are not enough holes at the surface to allow the wet chemistry to progress and etch the material. Regardless of the process used, the etch depth is controlled by several factors, including—among others—the etch time, etch composition and sample temperature.
In some applications, it is desirable to perform an additional growth over already patterned semiconductor layers. With existing etch processes, it is very difficult to pattern any new layer selectively without damaging the previously grown layers. A need remains in the prior art, therefore, for a more controllable method of selectively regrowing and patterning Group III-nitride layers. Selective regrowth would, for example, enable the integration of various GaN-based devices on a single wafer (e.g., electronic devices with lasers or LEDs).
Summary of the Invention
The need remaining in the prior art is addressed by the present invention, which relates to a selective growth technique for Group III-nitride materials and, more particularly, to the use of masking technique to form both polycrystalline and single crystal Group III-nitride (for example, GaN) films that can be preferentially processed to form selected growth areas.
In accordance with the technique of the present invention, a selective growth and etch process is obtained by first depositing, then patterning, a suitable masking layer on a substrate. An epitaxial GaN (for example) film is then grown, using an appropriate process, on the patterned substrate. A single crystal material will form on the exposed substrate, with polycrystalline material forming on the masking layer. An etchant that is selective between the single crystal and polycrystalline material can then be used to remove the polycrystalline film. The remaining masking layer is then removed, leaving only the single crystal GaN material in the selected areas.
In a preferred embodiment of the present invention, the GaN is plasma deposited using a reactive nitrogen species. This process results in the required vertical growth of the GaN film over both the substrate and masking layer. The polycrystalline GaN film will include both N-polar and Ga-polar surfaces, while the single crystal material will exhibit only Ga-polar faces. In order to etch away the polycrystalline material, an etchant such as potassium hydroxide can be used, which is known to etch only the N-polar surface. The single crystal material will remain intact, and the masking layer material can then be removed. The thickness of the patterned, single crystal Ga layer is thus determined precisely by the thickness of the epitaxial growth, and is therefore more controllable than other prior art processes.
Other and further aspects of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings.
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Adesida, I., Youtsey C., Ping A.T., Khan, F., Romano, LT., Bulman, G. “Dry and Wet Etching for Group III-Nitrides” MRS Internet J. Nitride Semiconductor Res 451, G1.4 (1999).
Bishop S.G., Coleman J.J. “GAN: From Selective Area to Epitaxial Lateral Overgrowth” MRS Internet J. Nitride Semicond. Res. vol. 451, article 64:8 (1999).
Anselm Klaus Alexander
Cho Alfred Yi
Chu Sung-Nee George
Agere Systems Guardian Corp.
Deo Duy-Vu
Koba Wendy W.
Utech Benjamin L.
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