Sapphire glass, an artificially synthesized single-crystal alumina material, is highly favoured in high-end optics, electronic displays, aerospace exploration and other fields due to its outstanding physical and chemical properties, including hardness second only to diamond, high-temperature resistance and chemical inertness. However, behind these outstanding properties lies an extremely precise and complex preparation process.

1.Crystal growth: From powder to single crystal

Sapphire glass is made from high-quality single-crystal alumina. At present, the commonly used methods in industry include the Kyropoulos method and the heat exchange method. The Kyropoulos method involves melting high-purity alumina raw materials at a temperature in excess of 2050°C. The molten material is then subjected to a gradual pulling process, whereby the seed crystals are extracted. The temperature gradient is utilized to direct the directional growth of the crystals. The heat exchange method is an effective technique for achieving crystallization from the bottom to the top through bottom cooling, making it particularly suitable for the preparation of large-sized crystals.

2. Cutting and orientation: Establish the precision of the optical axis

Following the completion of crystal growth, directional cutting is required. Due to the anisotropy of sapphire (differing hardness and refractive index in different crystal directions), it is essential that the cutting direction is strictly aligned with the optical axis of the target application. For instance, sapphires used as LED substrates are usually cut along the C-axis, while when used as watch crystals, both strength and optical uniformity need to be taken into account. The cutting process utilizes an inner circular diamond blade. The surface of the cut sapphire sheet is not smooth enough to meet the optical grade requirements, meaning that further processes are required.

3. Grinding and polishing: From micron to atomic level

Rough grinding typically employs a diamond slurry with a particle size of 20μm to remove the cutting damage layer, and then gradually transitions to finer abrasives. The crucial polishing stage employs chemical mechanical polishing (CMP), which takes advantage of the synergistic effect of the chemical reaction of the silica slurry in an alkaline environment and mechanical grinding to reduce the surface roughness from the nanometer level to the atomic level. The surface roughness of sapphire glass treated with CMP is equivalent to one-tenth of that of ordinary optical glass. This exceptional level of flatness is the key factor that allows sapphire to offer a high level of hardness and excellent optical transmittance.

4. Coating treatment: Endow the surface with new functions

The polished sapphire glass has a robust optical foundation, but it still requires functional coating. In order to meet the demand for anti-reflection, multi-layer films of silica and titanium dioxide are usually alternately deposited on the surface. By precisely controlling the film thickness and refractive index, the reflectivity is reduced to below 0.5%. The quality of the coating process directly affects the adhesion and durability of the film.

5. Strict testing: Ensure each sapphire plate is of high quality

Quality inspection is an integral part of the process. Following the process of crystal growth, it is essential to meticulously observe stress birefringence using a polarizing microscope. The polished sapphire surface must be inspected for flatness using a laser interferometer, with an error within λ/4. Tests for key performance indicators such as light transmittance, scratch resistance and acid and alkali resistance should be conducted in batches.