Meaning
Basal surfaces of aluminum oxide crystals represent a specific class of semiconductor growth platforms. The sapphire c-plane is the most common substrate for blue and white light emitting diodes due to its hexagonal symmetry. It offers high chemical stability and mechanical hardness.
Precision polishing creates the flat surface required for epitaxial layer growth.
Lattice Symmetry
Hexagonal atomic arrangements on this specific facet match the structure of nitride semiconductors. Using the sapphire c-plane minimizes the internal strain that occurs during the cooling of the wafer. If the lattice mismatch is too high, the resulting device will have a poor production yield.
Substrate Growth
Furnaces pull large boules that are then sliced into thin discs. Because the sapphire c-plane is perpendicular to the growth axis in many methods, the resulting wafers are highly uniform. The demonstrated rate of production for these substrates has increased as larger boules become easier to grow without cracking.
Surface Quality
Polishing steps remove the subsurface damage caused by the initial sawing of the crystal. A sapphire c-plane must be atomically flat before it enters a metal-organic chemical vapor deposition reactor. Even a small scratch on the surface can lead to a catastrophic failure of the entire wafer during the pilot phase.
Atomic force microscopy is used to audit the finish of every batch. High purity sapphire ensures that no trace metals contaminate the growth process.
