You might think from the title of this post that we’re talking about gems or jewelry, but we’re not. We’re talking about commercial sapphire – the type that is used to make LEDs. Not all commercial sapphire is high quality. In fact, the quality of the sapphire crystal impacts the quality of the sapphire wafer and the resulting LED. Sapphire producers must go through a qualification process in order for LED manufacturers to select the vendor. What are they looking for? Raja Parvez, President and CEO of Rubicon Technology shared information about what LED manufacturers look for when they come to Rubicon.
Rubicon Technology shared information about what LED manufacturers look for when they come to Rubicon.
Flatness: When a sapphire wafer is not flat it will become like a potato chip during processing. This prevents the wafer from being processed properly. The key tolerance for 6-inch and 8-inch is the flatness across the wafer. Thickness is not standardized yet and can range anywhere from 1-2 mm. The greater thickness also uses a larger amount of sapphire, but we’ll get to that in a later post.
Cleanliness: Surface morphology of the wafer needs to be clean and presents a uniform surface before depositing the epitaxial layer. The particulate count on the polished surface is very important too. A dirty wafer will cause issues when depositing the epitaxial layer. In addition, if impurities have been introduced in crystal development, colorization will be introduced rendering a colored crystal. This has a negative effect on commercial sapphire quality in contrast to gem quality sapphire that depend on impurities for their color such as red (with chromium impurities) for rubies and blue (with titanium and iron impurities) for sapphire. For example, commercial sapphire crystals with impurities result in pink wafers that interfere with LED performance.
Stress: LED manufacturers need stress-free wafers. Sapphire crystals go through temperature cycles of up to 1200ºC. That causes stress that can create cracks in the wafers and reduce yield. Rubicon’s ES2 technology produces almost stress-free crystals. During the crystal growth cycle, 50 percent of the time is taken to grow our crystal, and 50 percent is taken to cool down the crystal. During cooling, stresses are automatically released. Other wafer technologies introduce significant stress, so it’s common to put those wafers through an annealing furnace to reduce stress. This adds operational costs and time to production.