Samsung files patents ahead of LED spin-out
Korean manufacturing giant Samsung has had four US patent applications published on April 2, just as its LED manufacturing subsidiary springs into motion.
The four applications all come from research performed into GaN optoelectronics at Samsung Electro-Mechanics, which is spinning off its LED division in a joint venture with Samsung Electronics.
Two of the applications refer directly to LED manufacturing methods that might be used in the fabs of such an operation. The third covers patterning LEDs to improve light extraction, but perhaps most surprising innovation is a method for producing single-crystal GaN substrates in the fourth application.
In application number 20090087937 a Samsung team describes how it makes a single-crystal substrate on top of a ZnO buffer layer, itself deposited on a conventional substrate.
The ZnO is originally sputtered onto a substrate such as sapphire, after which the GaN layer is deposited before the ZnO layer is wet-etched away.
Application 20090087937 says that existing laser lift-off approaches for separating similar nitride substrates grown on sapphire are inappropriate for wafer sizes above 2-inch, as they cause warpage.
While 2-inch sapphire substrates are the most commonly deployed in the LED industry, Samsung is reputed to be looking far beyond this, to 6-inch wafers (see related story).
Samsung s LED surface patterning patent application, number 20090087994, also exploits wet-etching to minimize unwanted damage caused to the GaN surface by dry-etching. The company says it is using the approach to investigate photonic lattices and gratings for surface plasmon resonance.
Using solely reactive ion etching (RIE) and inductively coupled plasma reactive ion etching (ICP-RIE) to produce these structures damages GaN LEDs to the extent that they produce “almost no light”, the application says. Consequently Samsung has devised a method that uses a minimum of dry-etching to produce basic surface patterns, before wet-etching expands these to their desired extent.
In the LED manufacturing patents, US patent application number 20090085052 covers eutectic metal bonding of a GaN LED chip to a submount. Carefully selecting metals provides better thermal conductivity than epoxy adhesives and prevents the electron leakage seen when using silver to form such bonds.
Producing a low-contact resistance n-type LED electrode from a lanthanum-nickel alloy to improve light-emission efficiency is covered in application number 20090085043.