Sapphire is an extremely versatile material with a growing list of applications in a wide range of industries. Sapphire suits optical applications because of its scratch resistance and its transmission characteristics. You’ll find sapphire components such as lenses and windows in medical equipment, lasers, satellites, aircraft, flame detectors, smart phones, cameras and watches. Recent advances in sapphire crystal growth technology and fabrication have improved the performance, purity, and availability of sapphire for all types of applications.
Recently, Novus Light Today published an article by Dr. Jonathan Levine, Director of Technical Business Development at Rubicon Technology, about sapphire quality. His article gives a thorough review of the measures of sapphire quality for optical applications. Levine writes that the quality of a sapphire is determined by how closely the grown crystal matches the ideal structure with respect to the arrangement of atoms within the lattice, dislocations, defects, and stress. Root causes for these problems often originate from insufficient purity of the starting material and the growth process itself.
The effects of these variables in the final product are commonly quantified by three metrics: chemical analysis, X-ray rocking curves, and optical transmission. Additionally, the observance of bubbles in the crystal provides a baseline from which crystal quality is determined because bubbles serve as scattering centers for any light transmitted through a sapphire optic, thus reducing its performance.
This week, we look at the first two metrics, chemical analysis and X-ray rocking curves.
Powdered aluminum oxide
Purity of the crystal is highly important. According to Levine, the presence of certain elements can vary drastically between suppliers, and sapphire manufacturers must exercise proper quality control. For example, titanium (Ti) and chromium (Cr) impurities can result in pink crystals. In nature, these impurities lead to rubies and other variations of sapphire depending on the impurity. Levine says trace amounts of these elements must be kept below 1 ppm. Levine includes a graphic about other elements that can cause issues including silicon (Si), potassium (K), chlorine (Cl), iron (Fe), lithium (Li), and sodium (Na). The data was collected using glow discharge mass spectroscopy (GDMS).
Typically, a company can buy two types of raw material for crystal growth that can have impurities. Levine says it can be purified alumina powder and/or Verneuil sapphire. Rubicon has developed a new in-house purification process that converts the raw powder into densified pellets for crystal growth without an increase in cycle time or decrease in crystal yield. This process enables Rubicon to eliminate impurities in the alumina power that they use to make crystal.
Levine includes another useful metric for analyzing sapphire, rocking curve data obtained via X-ray diffraction. A rocking curve helps measure various stresses in a crystal. Levine says the width of the resulting peak is highly sensitive to strain and defects within the crystal. A narrow peak, indicated by its full width at half maximum (FWHM) measured in arcseconds, signifies a high quality crystal free of low-angle grain boundaries and lattice strain. A standard narrow rocking curve for Rubicon’s ES2 sapphire windows is shown below.
Sample rocking curve data from Rubicon ES2 sapphire.
What can introduce a poor rocking curve? Levine says that high thermal gradients, fast growth rates, and impurities contributed by the surrounding insulation can introduce defects and stress into the crystal that subsequently yield poor results in rocking curve data. He adds that accurately controlling the temperature gradient and maintaining a stable growth interface throughout the entire process can help make higher quality sapphire.
For Further Reading
Novus Light Today, Optical-Grade Sapphire, Where Quality Matters, http://www.novuslight.com/optical-grade-sapphire-where-quality-matters_N1596.html#sthash.giGipxT1.dpuf