Cl2/Ar 유도결합 플라즈마를 이용한 GaN 박막의 식각

조범철,1) 임연호, 박진수, 한윤봉*

전북대학교 화학공학부, 반도체과학기술학과1)

Inductively Coupled Plasma Etching of GaN Films

with Cl2/Ar Discharges

Bum-Chul Cho,1) Yeon-Ho Im, Jin-Su Park, and Yoon Bong Hahn*

School of Chemical Engineering and Technology, and

1)Department of Semiconductor Technology and Science, Chonbuk National University, Chonju 561-756, Korea

INTRODUCTION

The most significant advancement in dry etching of III-nitrides has been the utilization of high density plasmas. The main applications of the III-nitrides are photonic devices such as laser diodes and light-emitting diodes (LEDs).1-2 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. Hence, most of the previous etching studies have been focused on obtaining the relatively large etch depths typical of ridge or facet heights, where the final surface morphology on the field is less important. Moreover, an n-type ohmic contact is deposited on the etched surface, and loss of nitrogen from this surface actually improves contact resistance. In recent years the majority of the high density plasma etching has been performed using inductively coupled plasma (ICP) etch systems because of its superior uniformity, control and lower cost of ownership.3-5 However, little work has been done on the ICP etching of doped GaN films in Cl2/Ar discharges, especially in terms of varying the rf frequency of the capacitively coupled plasma source (or the rf chcuk power source), which is applied to control ion energy.

In this paper, a parametric study of ICP etching of undoped, n- and p-type GaN films in Cl2-based plasmas is reported. The effects of etch gas concentration, ICP source power, rf chuck power and reactor pressure on etch rates have been investigated under two different rf frequencies of 100 kHz and 13.56 MHz, in which the rf chuck power source operates. Results were discussed in terms of etch rates, dc bias voltage, surface morphology and surface chemistry of the etched samples.

EXPERIMENTAL

Dry Etching of GaN films was performed in a planar type Vacuum Science ICP system(VSICP-1250A), in which the ICP source operates at 13.56 MHz. The temperature of the back-side cooled chuck was held at 20 oC. The

ion energy was controlled by the applied rf chuck power at different excitation frequencies of 100 kHz  and  13.56 MHz. The Cl2/Ar mixture  with

total gas loads of 20 standard cubic centimeters per minute (sccm) was injected into the reactor through electronic mass flow controllers (MFCs). For etch rate experiments, samples were masked with Apiezon wax; etch depths were obtained from stylus profilometry measurements of etched samples after removal of the wax. Surface morphology and near-surface chemistry were examined using Atomic Force Microscopy (AFM) operating in tapping mode with Si tip and Auger Electron Spectroscopy (AES), respectively.

RESULTS AND DISCUSSION

The effect of ICP source power on the etch rates of undoped, n- and p-type gallium nitrides and dc-bias voltage was examined with two different rf frequencies of 100 kHz and 13.56 MHz for the capacitively coupled plasma source, and the results are shown in Fig. 1. During these experiments the reactor pressure, rf chuck power and gas feed rate were held constant at 40 mTorr, 150 W and 5 sccm Cl2/15 sccm Ar, respectively. The etch rates increased with the ICP source power in 100 kHz of the rf chuck power source (top), while they increased up to 500 W and remained relatively constant beyond 500 W in 13.56 MHz (bottom). It is also seen that the attainable maximum etch rates with lower frequency are somewhat greater than those with higher frequency. This may be attributed to higher ion bombarding energy with lower frequency: the dc bias voltage with 100 kHz was ranged from - 360 to - 475 eV when the ICP source power changed from 400 W to 700 W, but - 215 to - 380 eV with 13.56 MHz. The highest etch rates with the 100 kHz frequency were obtained at moderately high ICP power of 700 W: ~ 9300 □/min of n-GaN, ~ 5300 □/min of p-GaN and ~ 7100 □/min of undoped GaN (top). However, the 13.56 MHz frequency of rf chuck power source produced relatively constant etch rates ranged 3300 ~ 3800 □/min of all the GaN films at ICP powers greater than 500 W (bottom). The decrease in dc biases with the ICP source power in both frequencies were mainly due to increased ion density at higher ICP source powers.

Figure 2 shows the effect of rf chuck power on the etch rates and dc bias. The etch rates and dc bias voltages increased with both frequencies as the rf chuck power or ion-bombarding energy increased, typical results in the ICP etching systems. The increase in etch rate with the chuck power might be explained by enhanced sputter desorption of etch products as the ion energy increased.

The effect of reactor pressure on the etch rates and dc bias is shown in Fig. 3 for both frequencies of 100 kHz (top) and 13.56 MHz (bottom) of the rf chuck power source. It is seen that the etch rates of n-GaN are somewhat greater than those of undoped and p-type GaN with the rf frequency of 100 kHz (top), while all materials show overall similar etch rates with 13.56 MHz (bottom). The etch rates of the GaN films with 100 kHz excitation frequency increased substantially with increasing the pressure, indicating chemical etch mechanism is dominant. By contrast the etch rates of all materials with 13.56 MHz increased up to 20 mTorr and then decreased at higher pressures.

The surface morpholgy of ethced n-type and undoped GaN films were examined using AFM. Figure 4 shows the effect of reactor pressure on the surface morpholgy of the n-GaN etched at 25 % Cl2 (5 sccm Cl2/15 sccm Ar), 700 W ICP and 150 W rf chuck powers with rf frequencies of 100 kHz (left) and 13.56 MHz (right). The 100 kHz frequency of the rf chuck power source produced overall smoother morpholgy than that with 13.56 MHz especially at lower pressures (< 20 mTorr).

CONCLUSIONS

Dry etching of undoped, n- and p-type GaN films was carried out in Cl2-based inductively coupled plasmas using different rf excitation frequencies of 100 kHz and 13.56 MHz, in which the rf chuck power source operates. The etch rates with lower frequency of 100 kHz are somewhat greater than those with higher frequency of 13.56 MHz due to higher ion bombarding energy with lower frequency. The highest etch rates with the 100 kHz frequency were obtained at moderately high ICP power of 700 W: ~ 9300 □/min of n-GaN, ~ 5300 □/min of p-GaN and ~ 7100 □/min of undoped GaN. The 13.56 MHz frequency of rf chuck power source produced maximum etch rates of ~ 7900 □/min of n-GaN, ~ 5800 □/min of p-GaN and 6100 □/min of undoped GaN at 20 mTorr, 700 W ICP and 150 W rf power. The surface roughness showed quite smooth morphology (rms 1.1  1.3 nm).

ACKNOWLEDGEMENTS

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

REFERENCES

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Figure.2 Effect of rf chuck power on the etch rates of undoped, n- and p-type GaN films with different frequencies of 100kHz(top)  and 13.56MHz(bottom) of rf chuck power source

Figure.1 Effect of ICP source power on the etch rates of undoped, n- and p-type GaN films with different frequencies of 100kHz(top) and 13.56MHz(bottom) of rf chuck power source

Figure.3 Effect of reactor pressure on the etch rates of undoped, n- and p-type GaN films with different frequencies of 100kHz(top) and 13.56MHz(bottom) of rf chuck power source

Figure.4 AFM images of the surface of n-GaN films etched varying Pressure at different frequencies of 100kHz(left) and 13.56MHz

(right) the ICP Conditions are 700W ICP, 150 W rf and 5 sccm Cl2/15 sccm Ar