고밀도 플라즈마를 이용한 반도체 재료 식각기술

한 윤 봉

전북대학교 화학공학부

Etching Technology of Semiconductor Materials in

High Density Plasmas

Yoon Bong Hahn

School of Chemical Engineering and Technology, Chonbuk National University, Chonju 561-756, Korea

INTRODUCTION

In the fabrication of high frequency transistors as well as optoelectronic devices, it is critically important to accurately control the pattern size, with minimal damage created during the patterning process. The III-V compounds such as GaAs, GaSb and AlGaAs are used for high electron mobility transistors (HEMTs), heterojunction bipolar transistors (HBTs), lasers and light-emitting diodes (LEDs).1 The III-nitrides are used for photonic device applications such as laser diodes and light-emitting diodes.2 Currently, all of the LEDs and a majority of the lasers are ridge wave guide structures in which the mesas are formed by dry etching. The trend toward decreasing feature size has become an important issue and to this end various types of dry etching techniques have been under development.

In this paper, a parametric study of dry etching of III-V compounds and III-nitrides with Cl2-, ICl- and IBr-based inductively coupled plasma (ICP) chemistries is reported. The effects of gas concentration, reactor pressure, rf chuck power and ICP source power on etch rates and selectivity have been investigated. The ICP discharges are well suited for achieving controllable etch rates (500-1500 □/min) and high selectivities (up to 80) for InN over AlN and GaN.

EXPERIMENTAL

The samples of III-V and III-nitride semiconductors, grown by either metal organic molecular beam epitaxy or metal organic chemical vapor deposition, were patterned with Apiezon wax and etched in an ICP etcher. The rf chuck power was varied between 50 and 350 W, and ICP source power between 300 and 1000 W. Etch rates were calculated from stylus profilometry measurements of the etched samples. The selectivity was calculated for InN over AlN and GaN. Surface morphology and near-surface chemistry were examined using Atomic Force Microscopy and Auger Electron Spectroscopy, respectively.

RESULTS AND DISCUSSION

Figures 1 and 2 show the effect of Cl2 concentration on etch rates of InN, AlN and GaN in Cl2/He, Cl2/Ar and Cl2/Xe discharges. It is seen that the effects of noble gas additives are strongly dependent on the particular III-nitride material: the highest etch rates for InN were obtained with Cl2/Xe (Fig. 1, top) and for AlN (bottom) and GaN (Fig. 2, top) with Cl2/He. It is also seen that etch rates of AlN and GaN were much lower in Cl2-based plasmas compared to InN. These results indicate that etch mechanism is dependent on the material bond strength and the particular plasma chemistry employed, and optimization of the ICP etching process is crucial for obtaining the best results.

The effect of rf chuck power on the etch rates, dc bias, and ion flux at the sheath edge are shown in Figs. 3 and 4. The etch rates of InN with Cl2/Xe and Cl2/He discharges increased up to 250 W and decreased at higher power (Fig 3, top), but increased monotonically with Cl2/Ar as the rf power increased. The increase in etch rate can be attributed to enhanced sputter  desorption  of  etch products as well as dominant physical sputtering of InN surface. The etch rates of AlN increased monotonically as the rf power increased with all Cl2-based discharges. However, GaN etching in Cl2/He and Cl2/Ar showed relatively constant etch rates with some fluctuations, and increased rapidly in the Cl2/Xe chemistry as the rf power increased (Fig. 4, top).

In order to reduce the currently high contact resistance in GaN- based heterostructure field transistors, and eventually heterojunction bipolar transistors, it is expected that InN-based contact layers will be necessary. In such a case, the ability to selectively etch InN relative to the other nitrides will be crucial. The effect of plasma composition showed an overall trend of decrease in selectivities for InN over AlN and GaN as the concentrations of ICl and IBr increase (Fig. 5). However, the selectivity of InN over AlN showed maximum values depending on rf chuck power, while that of InN over GaN increased overall as the chuck power increased (Fig. 6). The maximum values of selectivity obtained in this work were ~ 14 for InN/GaN and ~ 30 for InN/AlN with ICl and IBr plasmas.

Although not illustrated, ICP etching of GaAs, GaSb and AlGaAs was also performed in ICl/Ar and IBr/Ar chemistries. GaSb and AlGaAs showed maximum etch rates for both plasma chemistries as a function of interhalogen percentage, while GaAs showed increased etch rates with plasma composition in both chemistries. Etch rates of all materials increased substantially with increasing rf chuck power, but rapidly decreased with chamber pressure.

CONCLUSIONS

In Cl2-based discharges, the etch rates were greatly affected by plasma composition, rf chuck power and ICP source power. The highest etch rates for InN were obtained with Cl2/Xe, while the highest rates for AlN and GaN were obtained with Cl2/He. In ICl- and IBr-based chemistries, the etch rates of InN and AlN were relatively independent of plasma composition, while GaN showed increased etch rates with interhalogen concentration. It was confirmed that the efficient breaking of the III-nitrogen bond is crucial for higher etch rates in all cases. The Cl2 plasmas produced greater selectivity than the ICl- and IBr-based plasmas.

ACKNOWLEDGEMENTS

This work is supported by the KOSEF through the Semiconductor Physics Research Center at Chonbuk National University.

References

1. GaAs: Materials Devices and Circuits, ed. M. Howes and D. V. Morgan (Wiley & Sons, N. Y. 1985).

2. S. Nakamura, in GaN and Related Materials, ed. S. J. Pearton (Gordon and Breach, N. Y. 1997).

3. Y. B. Hahn et al., J. Vac. Sci. Technol. A, 17(3), 768 (1999).

Fig. 1 Effect of Cl2 % on etch        Fig. 2 Effect of Cl2 % on etch rates

rates of InNand AlNwith Cl2/He,      of GaNand on dc bias and ion flux

Cl2/Ar and Cl2/Xe.	with Cl2/He, Cl2/Ar and Cl2/Xe.

Fig. 3 Effect of rf chuck power on	Fig. 4 Effect of rf chuck power on

etch rates of InN (top) and AlN(bottom)     etch rates of GaN(top) and on dc

with Cl2/He, Cl2/Ar and Cl2/Xe.	bias and ion flux (bottom).

Fig. 5 Effect of IBr (top) and ICl (bottom)   Fig. 6 Effect of rf chuck power on

concentrations on the selectivity for InN      the selectivity for InN over AlN

over AlN and GaN.	and GaN with IBr and ICl.