Substrate Transfer for GaN based LEDs grown on Silicon

This paper focuses on the substrate transfer process which is applied after the fabrication of LEDs on 4 inch Si (111) wafers comprising p and n contact formation to the GaN layer. After applying a passivation layer, a bonding metal is deposited. The wafer is then bonded to a Si carrier substrate using metallic bonding. Next, the original Si (111) substrate is completely removed by grinding and wet etching. GaN-LEDs are thus transferred to a new carrier substrate. The last step is etching of the transferred GaN layer from the back to open the contacts. A surface roughening technique on the backside of the transferred GaN layer to improve the light extraction efficiency of GaN-LEDs is also investigated. All the issues of the substrate transfer process steps such as permanent Cu/Sn bonding, thinning by grinding and wet etching will be discussed in detailed. A typical issue occurring during processing of GaN-LEDs on Si substrates is high stress and related large wafer bow originating from the GaN layer and the thick Cu/Sn metal bonding layer. Such a large wafer bow causes problems for some automatic handling tools and processes like lithography. Solutions to manage the stress and wafer bow have been investigated.