Sapphire is more than just a gemstone. This typically blue variety of corundum can be used to make nanostructures with antireflection properties which allow for high optical transmission and low energy loss. These characteristics make sapphire nanostructures well-suited for applications in composite windows with broadband transmission and high hardness. However, developing sapphire nanostructures through various etching techniques proves challenging due to the chemical and mechanical stability of sapphire.

Chen et al. improved upon an etching method known as reactive ion etching, which previously had limited success. They overcame problems with poor etch selectivity by using a multi-level etch mask.

“We demonstrate a fabrication process that can enhance the etch selectivity and create nanostructures for materials that are traditionally difficult to micromachine. The key insight is that the limitations in material etching reaction can be overcome through etch mask design. This general approach can be applied to other alumina-based ceramics and materials that are difficult to etch,” author Yi-An Chen said.

This improvement may have applications in manufacturing photonics, optoelectronics and multifunctional materials. The authors examined the etching rates and chemistry of different masking materials and studied the etching of sapphire nanostructures using single and multilayer masks to attempt to increase the maximum etching depth. They succeeded and increased depth from 25 to 230 nanometers.

The authors intend on furthering the research by using more mask layers with optimized etching parameters and reducing the defect density of nanostructures.

Source: “Increasing etching depth of sapphire nanostructures using multilayer etching mask,” by Yi-An Chen, I-Te Chen, and Chih-Hao Chang, Journal of Vacuum Science and Technology (2019). The article can be accessed at