Novel environmentally friendly chemical mechanical polishing for zinc oxide in an alkaline environment using silica and ceria mixed abrasives

Abstract

Zinc oxide (ZnO) is widely used in high-performance semiconductor and optoelectronic devices. Nevertheless, ZnO is a hard-brittle solid, becoming a difficult-to-process material, and strong acids and alkalis are usually employed in the slurry. Moreover, the surface roughness (Sa) is normally more than 0.6 nm, and the material removal rate (MRR) in chemical mechanical polishing (CMP) is generally lower than 78.8 nm min⁻¹. To solve this challenge, a novel, environmentally friendly CMP technique was developed, and the new slurry contains silica and ceria mixed abrasives, sodium carbonate, hydrogen peroxide, and disodium ethylenediaminetetraacetate (EDTA). After CMP, a surface roughness (Sa) of 0.514 nm is acquired, and the MRR is 137.182 nm min⁻¹. To the best of our knowledge, both the Sa and MRR are the best for CMP on ZnO, compared with those reported hitherto. X-ray photoelectron spectroscopy (XPS) reveals that the peaks at 284.8 and 286.3 eV after CMP on ZnO are assigned to C–C or C–H bonds of the carbon backbone and C–O bonds of carboxylate of EDTA, respectively, according to the C 1s fine spectra. These peaks indicate the occurrence of complexation between Zn²⁺ and EDTA. Fourier transform infrared (FTIR) spectroscopy shows that a peak appears at 3415 cm⁻¹, corresponding to the O–H stretching vibration originating from Zn–OH of Zn(OH)₂. The peak at 1414 cm⁻¹ is derived from the symmetric stretching vibration of COO– in EDTA. The emergence and shift of this peak from the free state in general at 1400 cm⁻¹ verify the formation of a Zn–EDTA complex. The CMP mechanism is elucidated by XPS and FTIR. Firstly, ZnO was hydrated, forming Zn(OH)₂. Then, ZnO and Zn(OH)₂ were dissolved in an alkaline environment, and Zn²⁺ ions were released. The released Zn²⁺ ions were complexed by EDTA, generating a water-soluble complex and inhibiting the redeposition on the surface of ZnO. Mixed silica and ceria removed the softened ZnO and Zn(OH)₂, and a fresh surface of ZnO was exposed. This cycle continues, and microscopic protrusions are preferentially removed. Finally, an ultrasmooth surface is obtained. Our outcomes pave a new way for achieving an ultrasmooth surface on hard-brittle ZnO with relatively high MRR using environmentally friendly CMP in an alkaline environment.