Wet and dry etching of Sc2O3

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Abstract

Wet chemical and plasma etch processes were developed for pattering of Sc2O3 films on GaN. Chlorine-based plasma chemistries produced a significant chemical enhancement of removal rate over pure Ar sputtering. The etching was anisotropic and did not significantly alter the surface composition of the Sc2O3 films. Reaction-limited wet etching in the HNO3/HCl/HF system was investigated as a function of solution formulation and temperature. The activation energy for the wet etching ranged from 8 to 14 kcal/mol and the etch rates were independent of solution agitation.

Introduction

GaN metal-oxide semiconductor field effect transistors (MOSFETs) have shown improved high temperature performance relative to the more conventional GaN metal semiconductor FETs (MESFETs) due to their lower gate leakage current and higher breakdown voltage [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. To this point, Ga2O3 (Gd2O3) [1], [4], [6], SiO2 [7], [8], AlN [4], [5] and SiNx [9] have been reported as gate dielectrics for GaN. In addition, AlGaN/GaN heterostructure FETs has shown greatly improved gate leakage characteristics when SiO2 is used under the gate metal [12]. The search for higher dielectric constant gate oxides has prompted interest in Gd2O3 and Sc2O3 [13], [14]. We have previously reported GaN transistors with SiO2/Gd2O3 dielectric stacks that are thermally stable to 1000 °C and quasi-amorphous Gd2O3 films grown by molecular beam epitaxy with breakdown field strength ∼3 MV cm−1 and interface state density, Dit∼3×1011cm−2eV−1 [15]. In addition, Sc2O3/GaN MOS diodes have shown reverse breakdown voltage of ∼20 V, breakdown fields of ∼0.6 MV cm−1 and interface state densities of ∼1012 cm−2 eV−1 measured by the a.c. conductance method [16].

A necessary process for fabrication of high performance GaN MOSFETs is an etch to remove the oxide in the source/drain regions so that ohmic metallization can be deposited. There is no available information on wet or dry etch processes for Sc2O3, especially those that are selective to GaN (see e.g. [17]). In this paper, we report a study of etch rates, surface morphologies and compositions of Sc2O3 films patterned by dry etching in different plasma chemistries and also show that the HCl/HNO3/HF system is useful for wet etching of these films.

Section snippets

Experimental

The single crystal Sc2O3 films were deposited on GaN epi-layer (∼1.5 μm thick) on sapphire substrates. The GaN was grown by MBE (SVT Associates). The Sc2O3 was deposited at 600 °C using a 4 N solid source of scandium and an ECR oxygen plasma. The typical thickness of the Sc2O3 was ∼2500 Å.

A wide variety of wet chemical solutions were used for etching of the Sc2O3, including HNO3, HF, HCl and H2SO4 with solution temperature up to 80 °C. The dry etching was performed in a Plasma Therm 790 reactor in

Plasma etching

Fig. 1 (left) shows the Sc2O3 etch rates as a function of source power in all three plasma chemistries for fixed rf chuck power. The d.c. self-bias developed on the sample chuck under these conditions is shown in Fig. 1 (right). As the source power is increased the plasma becomes more conductive as the ion density is also increased, which reduces the self-bias [18]. The etch rates with SF6/Ar and CH4/H2/Ar are slower than the sputter rate with pure Ar and only Cl2/Ar shows a practical removal

Summary and conclusions

Wet and dry etch processes have been developed for Sc2O3, a promising gate dielectric for GaN. Plasma etching with some degree of chemical enhancement is possible in Cl2/Ar, whereas both SF6/Ar and CH4/H2/Ar show impractically low etch rates. The etched Sc2O3 surface shows minor amounts of chlorine residues after the plasma process. Maximum selectivites of 4 for Sc2O3 over GaN were achieved. Wet chemical etching in the HNO3/HCl/HF was found to produce effective removal of the Sc2O3 with

Acknowledgements

The work at UF is partially supported by grants from NSF (CTS-991173, DMR-0101438 and ONR-N00014-98-1-0204).

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    Present address: Samsung Electronics, Kyungki-Do, North Korea.

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