InGaN/GaN 다층양자우물 LED구조의 Cl2/Ar ICP 식각

Theories and Applications of Chem. Eng., 2002, Vol. 8, No. 1

화학공학의 이론과 응용 제8권 제1호 2002년

박형조, 최락준, 최창선,홍주형, 한윤봉*

전북대학교 화학공학부, 반도체물성연구센터

Inductively Coupled Cl2/Ar Plasma Etching of InGaN/GaN Multiple

Quantum Well Light-Emitting Diode Structures

H. J. Park, C. S. Choi, R. J. Choi, J. H. Hong, and Y. B. Hahn*

School of Chemical Engineering and Technology, Semiconductor Physics Research Center, Chonbuk National University, Chonju 561-756, Korea

I. INTRODUCTION

In recent years, there has been remarkable progress for blue LEDs and LDs by utilizing InGaN/GaN multiple quantum wells as active materials.1,2 All of the LEDs and a majority of the LDs have a ridge waveguide structure in which the mesas are formed by a dry etching technique. Hence, the fabrication of these GaN-based optoelectonic devices largely depends on dry etching. Plasma etching techniques have been predominantly used in the patterning of the III-nitrides. It is well known that a high-density plasma etcher can readily produce etch rates greater than 6000 □/min for single layer films of the III-nitrides under conditions of high ion flux and ion energy.3,4 However, little work has been reported for the dry etching of multiple layered heterostructures such as the InGaN/GaN MQW LED structures.

In this work, to elucidate the etch characteristics of the InGaN/GaN MQW LED structures grown by metal-organic chemical vapor deposition, a parametric study of ICP etching with Cl2/Ar discharges has been carried out. The effects of etch gas concentration, pressure, rf chuck and ICP source powers have been investigated in terms of etch rates, dc bias, plasma species, surface chemistry, and surface morphology

II. EXPERIMENTAL

InxGa1-xN/GaN multiple quantum well (MQW) LED structures were grown on c-plane sapphire substrate by metal-organic chemical vapor deposition system. Trimethylgallium (TMGa), trimethylindium (TMIn), ammonia (NH3), and silane (SIH4) were used as the precursors of Ga, In, N, and Si, respectively. A GaN nucleation layer of 25 nm thickness was grown on the cleaned substrate at 560 ℃, and a 4-mm-thick GaN:Si was then grown at 1100℃. The InGaN/GaN MQW LED structure

* Author for all correspondence; electronic mail: ybhahn@moak.chonbuk.ac.kr

was grown at 750 ℃, having a 6-period InGaN(2nm)/GaN(6nm) quantum well structure. A 0.25-mm-thick Mg-doped p-GaN was grown finally at 1100 ℃ on the top of MQWs.  Etching was performed in a planar-type inductively coupled plasma (ICP) system, in which the ICP source operated at 13.56 MHz. The ion energy was controlled by the applied radio frequency (rf) chuck power at 13.56 MHz. The Cl2/Ar mixture with total gas flow rate of 40 standard cubic centimeters per minute (sccm) was injected into the etcher through electronic mass flow controllers. For etch rate experiments, samples were patterned with AZ 4330 photoresist, etch depths were obtained from stylus profilometry measurements of etched samples after removal of the photoresist. Plasma species were analyzed using an optical emission spectroscopy (OES). Surface morphology and near-surface chemistry were examined using atomic force microscopy (AFM) operating in tapping mode with Si tip and Auger electron microscopy (AES), respectively.

III. RESULTS AND DISCUSSION

Figure 1 shows the effect of Cl2 concentration on the etch rate and dc bias (left), and the OES peak intensities of plasma species of Cl, Cl+, Cl2+ and Ar+ (right). The plasma conditions were: 700 W ICP source power, 150 W rf chuck power, and 10 mTorr. The etch rate increased with Cl2 concentration and remained relatively constant at higher concentrations (> 60 %). The dc bias increased with the Cl2 concentration, leading to a decrease in incident ion flux. The OES peak intensities of chlorine species increased substantially up to 25 % and almost saturated beyond 25 % Cl2, but the intensity of Ar+ decreased exponentially with Cl2 concentration.

Figure 2 shows the effect of the ICP source power on the etch rate and dc bias (left), and the OES peak intensities of plasma species (right). The etch rate and OES intensities of plasma species increased monotonically with the ICP source power, but the dc bias decreased with the power, typical in an ICP etching system. The increase in etch rate with the source power is mainly due to the increased ion fluxes onto the wafer surface. Although not illustrated, the etch rate and the dc bias increased with the rf chuck power applied to the wafer chuck.

Figure 3 shows the AFM images of the surface of InGaN/GaN quantum well structures, etched varying the rf chuck power at 700 W ICP, 10 mTorr and 50 % Cl2. The root mean square (rms) roughness increased with the chuck power because of an increase in ion-bombarding energy, however the etched samples showed overall quite smooth surface (rms = 0.62  1.26 nm) up to 150 W.

The experimental results lead to a conclusion that the dominant etch mechanism of ICP etching of the InGaN/GaN MQW LED structures is an energetic ion-enhanced chemical etching

화학공학의 이론과 응용 제8권 제1호 2002년

IV. CONCLUSIONS

Dry etching of InGaN/GaN multiple quantum well light emitting diode structures has been carried out with Cl2/Ar in an inductively coupled plasma discharge. The etch rate of the InGaN/GaN MQW LEDs increased up to 60 % Cl2 and remained relatively constant at higher Cl2 percentages, but increased monotonically with the ICP source and rf chuck powers. The attainable etch rate of the InGaN/GaN MQW structure was about 4,500 □/min at a moderate ICP condition: 700 W ICP source power, 100 W chuck power, 10 mTorr, and 50 % Cl2. Equirate removal of group III and group V elements was obtained for p-GaN, but some depletion of nitrogen from the etched surface for n-GaN. The surface of the etched InGaN/GaN MQW structure showed quite smooth surface (rms = 0.62  1.26 nm). The experimental results overall showed that the dominant etch mechanism of the ICP etching of the InGaN/GaN MQW LED structures is an energetic ion-enhanced chemical etching.

ACKNOWLEDGMENTS

This work was supported by Grant No. R01-2000-00330 from the Basic Research Program of the Korea Science and Engineering Foundation.

REFERENCES

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2. F. A. Ponce and D. P. Bour, Nature, 386, 351 (1997).

3. Y. B. Hahn, D. C. Hays, S. M. Donovan, C. R. Abernathy, J. Han, R. J. Shul, H. Cho, K. B. Jung, and S. J. Pearton, J. Vac. Sci. Technol. A, 17, 763 (1999).

4. Y. H. Im, J. S. Park, Y. B. Hahn, K. S. Nahm, and Y. S. Lee, J. Vac. Sci. Technol. A, 18, 2169 (2000).

Fig. 1 Effect of chamber pressure on etch rate (left) and plasma species (right).

Fig. 2 Effect of ICP source power on etch rate (left) and plasma species (right).

(c) 150 W, rms = 1.259 nm

(b) 100 W, rms = 1.154 nm

(d) 200 W, rms = 1.438 nm

(a) 50 W, rms = 0.615 nm

Fig. 3 Effect of rf chuck power on surface morphology of etched InGaN/GaN MQWs.