The influence of the AlN film texture on the wet chemical etching

Abstract

The influence of the aluminum nitride (AlN) film texture on the chemical etching in KOH solution was invested. The AlN films with the different texture and crystal quality were prepared by sputtering. It is found that the chemical etching behaviors, including the etch rate, the activation energy, the surface morphology and the anisotropy, are strongly dependent on the film texture. There is a faster etching in the case of mixed (1 0 0) and (0 0 2) texture and a lower rate in the case of only (0 0 2) texture. The etch rate also decreases with the crystal quality. The sample with the only (0 0 2) texture forms discontinuous column structure after etching and exhibits lower porosity compared to that of the mixed (1 0 0) and (0 0 2) texture. Due to the strong anisotropy of the AlN wurtzite structure, the morphology of the film deposited at 700 °C shows the homogeneous pyramid shape after etching. The cross-section micrographs of etching patterns indicate that the anisotropy of the chemical etching is improved with the improving of the crystal quality.

Introduction

With the tremendous progress of III-nitrides research in terms of both fundamental understanding as well as devices applications, aluminum nitride (AlN) attracts increasing interest due to the band gap of approximately 6.2 eV and some excellent properties. The direct wide band gap is very attractive for the ultraviolet light-emitting diodes [1] and the photo detectors [2]. The high thermal conductivity (340 W m⁻¹ K⁻¹) makes AlN suitable for the high power applications [3]. Furthermore, the polycrystalline AlN films have been utilized to fabricate the piezoelectric sensors/actuators [4] and the high-frequency electro-acoustic devices [5] due to the piezoelectric properties and high acoustic velocity. In the devices fabrication, the patterning of the materials is a key step because a number of device performances may be affected by the etching process. Generally, the reactive ion etching using chlorine can be highly anisotropic, an ideal characteristic for producing vertical profiles [6]. However, due to the strong physical component and the high chemical activity of the chlorine plasma, the dry etching has low etch-selectivity between materials and can cause subsurface damage by ion bombardment. In contrast, the wet etching, produces negligible damage, can be highly selective, is relatively inexpensive, and can be done with simple equipment. In addition, wet chemical etching is a reasonable, reliable, and simple method to analyze the defects and crystal polarity in III-nitrides [3]. Unfortunately, there was relatively little success in developing wet etching solutions for AlN because of their excellent chemical stability. The chemical etching was strongly dependent on the crystal quality and the etch temperature. Only KOH or NaOH containing solution can be etch epitaxial and single crystal AlN at the temperature below 80 °C [3], [7]. The hot H₃PO₄ [8], and AZ400 K photoresist developer [9] were also reported can etch the polycrystalline AlN. However, more detail information about etching behaviors of polycrystalline AlN films is required for the micro-device fabrication.

In this paper, the influence of the film texture on the chemical etching in KOH solution was invested. The AlN films with the different texture and crystal quality were prepared by sputtering. It is found that the etch rate, the activation energy and the morphology of etching surface depend on the films texture and the crystal quality. The micrographs of the patterns were observed from the top view and the cross-section view to detect the etching evolution and anisotropy.