Dry etching of sapphire substrate for device separation in chlorine-based inductively coupled plasmas

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Abstract

In this study, sapphire wafers were etched using magnetized inductively coupled plasmas (MICP) and their etch characteristics were investigated as a function of gas combination of Cl2/BCl3, operation pressure, and additive gases such as Ar, Xe and SiCl4. The characteristics of plasmas were estimated using a Langmuir probe and optical emission spectroscopy, and the profiles of the etched sapphire wafers were evaluated with a scanning electron microscopy (SEM). The increase of BCl3 in Cl2/BCl3 increased the etch rate and improved the etch selectivity over photoresist, SiO2 and Cr until 80% BCl3 was reached. The decrease of operating pressure also increased the sapphire etch rate. The maximum etch rate over 3300 °C min−1 could be obtained using 20%Cl2/80%BCl3 and, by the addition of 10%Ar or 10%Xe in this mixture, the etch rates increased further to over 3500 °C min−1 at 2.0 Pa of operating pressure, 1.6 kW of inductive power, −250 V of bias voltage, and 70 °C of substrate temperature. When the sapphire etching was performed with 10% Ar in 20%Cl2/80%BCl3, sharp sidewall trenches required for stress concentration during the device separation could be observed on the sapphire etch profiles.

Introduction

Sapphire wafers are currently used in the optoelectronics industries as the substrates due to its high chemical and thermal stability. One of the problems in using sapphire wafers to optoelectronic devices such as GaN-based devices is the difficulty in cutting and backside mechanical polishing after completing the device due to the differences in the crystal orientation and the hardness of sapphire itself [1]. Especially, to obtain reliable device separation, more than 50 °C wide scribe line width is required for mechanical cutting using a diamond wheel or for scribing using a diamond scriber. However, if the device separation can be replaced by the dry etching, the scribe line width could be reduced to 5–10 °C, therefore the yield per wafer could be increased by 30%.

To replace for the mechanical cutting or scribing, high sapphire etch rates with high etch selectivities over mask materials are required. Recently, the sapphire etching techniques, ion beam etching (IBE) [2], chemical wet etching after ion implantation [3], reactive ion etching [4], [5], laser-assisted etching [6] and inductively coupled plasma (ICP) [1]. In this study, a high density plasma etching equipment, a magnetized inductively coupled plasma (MICP) etcher, has been used and the effects of Cl2/BCl3 gas combination and additive gases on the etch rates and etch selectivities has been studied to obtain high etch rates and high etch selectivities over mask materials. Also, the effect of gas combination on the sidewall etch profile has been studied to obtain a sharp sidewall trench for easier device separation using a simple roller.

Section snippets

Experiment

To etch sapphire, a specially designed MICP etcher was fabricated that can hold a permanent magnetic bucket inside the chamber and Helmholtz-type axial electromagnets around the chamber wall. The details of the characteristics of the plasmas and the magnet configurations used in this experiment can be found elsewhere [7].

The specimens were both-side polished sapphire wafers with (0001) orientation. These wafers were patterned using a conventional photoresist (AZ9262), SiO2, and Cr to measure

Results and discussion

Fig. 1 shows the effect of BCl3 in Cl2/BCl3 gas mixtures on the sapphire etch rates and the etch selectivities over photoresist, SiO2, and Cr while inductive power, bias voltage, operating pressure, total flow rates, and substrate temperatures were fixed at 1.6 kW, −250 V, 2.73 Pa, 100 sccm, and 70 °C, respectively. As shown in the figure, the addition and increase of the BCl3 to about 80% increased the sapphire etch rates from 600 to 2800 °C min−1 and the further increase of BCl3 decreased the

Conclusion

In this study, sapphire wafer was etched using magnetized inductively coupled Cl2/BCl3 based plasmas, and the effects of various gas combination, additive gas and operating pressure on the sapphire etch rates, etch profiles, and the etch selectivities over photoresist, SiO2 and Cr were investigated as the application for the device separation by dry etching.

Eighty percent BCl3 in Cl2/BCl3 showed the highest sapphire etch rates and etch selectivities over mask materials due to the enhanced

Acknowledgements

This work was supported by Korea Science and Engineering Foundation (1999-2-30100-002-3), Video Industrial R&D Association of Korea, and NRL program by Ministry of Science & Technology.

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