Dielectric Coating Thermal Stabilization During GaAs-Based Laser Fabrication for Improved Device Yield

Abstract

The quality and yield of GaAs-based ridge waveguide devices fabricated at MIT Lincoln Laboratory were negatively impacted by the random lot-to-lot appearance of blisters in the front-side contact metal. The blisters signaled compromised adhesion between the front-side contact metal, underlying SiO 2 dielectric coating, and semiconductor surface. A thermal-anneal procedure developed for the fabrication of GaAs slab coupled optical waveguide (SCOW) ridge waveguide devices stabilizes the SiO 2 dielectric coating by means of outgassing and stress reduction. This process eliminates a primary source of adhesion loss, as well as blister generation, and thereby significantly improves device yield. Stoney's equation was used to analyze stress-induced bow in device wafers fabricated using this stabilization procedure. This analysis suggests that changes in wafer bow contribute to the incidence of metal blisters in SCOW devices.

Impact of Film Stress and Film Thickness Process Control on GaAs-TiAu Metal Adhesion

“…These adhesion failures generated concerns about the quality of the metal-semiconductor electrical contact resistance and ultimately, device performance and reliability. Previous published studies had linked blister formation to inadequate surface cleanliness and SiO 2 dielectric thermal instability, factors that must also be managed to optimize device yield [6,7]. Moreover, a direct correlation between the appearance of blisters and contact pad processing time was also previously observed.…”

“…The full GaAs SCOW fabrication sequence has been described in detail previously [6,7]. A ridge waveguide was dry etched into the semiconductor utilizing an ICP-RIE process.…”

“…A ridge waveguide was dry etched into the semiconductor utilizing an ICP-RIE process. Once the ridge was defined, the surface was coated with a 200 °C low stress Samco PECVD SiO 2 dielectric layer and annealed at 450 °C for 45 s for stress reduction and improved film stability [7]. Photolithographically patterned contact vias were etched utilizing CF 4 in a parallel plate RIE system.…”

“…The sputter-deposition system used in In order to simulate the SCOW device fabrication procedure and replicate the generation of contact-padmetal blisters on GaAs substrates, a streamlined-fabrication process was developed. The surface of two 20-mm x 30mm GaAs wafer test samples (A, B) were coated with a 300-nm, 200 °C plasma enhanced chemical vapor deposition (PECVD) SiO 2 dielectric layer, and annealed at 450 °C for 45 s to reduce stress and improve film stability [7]. Subsequently, the samples were coated with the SCOW sputter-deposited TiAu (50-nm Ti /200-nm Au) contact metallization and annealed immediately (Sample A), and seven days post-deposition (Sample B).…”