Two research institutions in South Korea conducted a feasibility study, and laboratory-scale production showed that LEDs with sapphire substrates combined with cavity design are better than LEDs with patterned sapphire substrates. Their process of producing cavity-type sapphire substrates is robust and scalable (see Figure 1).

The process starts with a photoresist pattern, and a cylindrical photoresist is first made into a dome shape through a reflow process. Subsequently, an 80-nm-thick amorphous aluminum oxide layer was subjected to atomic layer deposition at a temperature of 120°C on all exposed surfaces.

The aluminum oxide partly covers the sapphire and partly covers the photoresist. Figure 1. The manufacturing process used to form the cavity substrate involves the complete crystallization of amorphous aluminum oxide to turn it into sapphire. This greatly simplifies the subsequent GaN growth.

The next step is to perform heat treatment in an oxidizing atmosphere. This allows oxygen to diffuse internally through the porous alumina layer and diffuse the oxidation by-products outwards. The result is a dome-shaped cavity. At the same time, the amorphous alumina becomes completely crystalline, starting from the sapphire contact area and finishing on top of the dome of the photoresist. One advantage of this method is that the crystalline phase of amorphous alumina is sapphire.

Therefore, since rechargeable work light oxide crystallizes into sapphire during the heat treatment, no additional processing steps are required to expose the sapphire seed layer and start GaN growth (see Figure 2). Figure 2. Scanning electron microscopy shows the hexagonal dome embedded in the cavity formed by GaN growth.

https://www.nbsunlux.com/product/flood-light/rechargeable-work-light-flood-light/