Impact of Film Stress and Film Thickness Process Control on GaAs-TiAu Metal Adhesion
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https://doi.org/10.1007/s11664-020-08521-z Ó 2020 The Minerals, Metals & Materials Society
Impact of Film Stress and Film Thickness Process Control on GaAs-TiAu Metal Adhesion MICHAEL K. CONNORS ,1,2 JENNIFER P. COLETTA,1 and MICHAEL J. SHEEHAN1 1.—Lincoln Laboratory, Massachusetts Institute of Lexington, MA 02420, USA. 2.—e-mail: [email protected]
Technology,
244
Wood
St.,
The fabrication of GaAs-based optoelectronic ridge-waveguide devices requires deposition of a topside-contact metallization for proper device operation. Fabrication delays occurring during the processing of TiAu-contact pads have been linked to poor adhesion and metal blister formation, factors that negatively affect the final device yield. In this study, we examined sputterdeposited Ti and Au films to determine the impact of film-thickness process control and film stress as measured by wafer bow. We theorized that competing stress relaxation forces between the Ti and Au films would produce a post-deposition change in wafer bow, which affects the Au film, setting the stage for blister creation. We now report the development of a reduced-stress sputter-deposited TiAu-contact metallization and demonstrate the utility of the modified process with fabrication of blister-free ridge-waveguide devices with high device yield. Key words: GaAs, film stress, metal blister, blister, adhesion, device yield
INTRODUCTION The fabrication of GaAs-based ridge-waveguide optoelectronic devices requires the deposition of a topside-contact metal. Optimized metal film composition is a function of semiconductor contact layer conductivity, as well as device design.1 Two suitable non-alloyed-contact metals for highly doped ptype GaAs are TiAu and TiPtAu. In this study, a sputter-deposited TiAu (50-nm Ti/200-nm Au) metallization was used as the contact metal for GaAsbased slab-coupled optical waveguide (SCOW) laser devices which have a (> 5 9 1019) p-type-contact layer. The metal deposition process included an in situ sputter etch to promote a clean semiconductor–metal interface with enhanced adhesion.2,3 The initial Ti layer provided adhesion enhancement while also functioning as a barrier between the Au layer and GaAs surface, eliminating undesirable
(Received December 17, 2019; accepted September 24, 2020)
Ga-Au interactions. The top Au layer provided an oxide resistant electrical contact suitable for probe testing, wire bonding, and solder-based packaging. The contact-metal film was sputter-deposited on a pre-cleaned photoresist-free sample surface, creating a continuous metal sheet, to ensure optimal adhesion. A photolithographic process was subsequently used to delineate contact-metal pads, and the removal of excess deposited Au and Ti was achieved with wet chemical etching, thus isolating the devices as well as successfully completing the fabrication process. Fabricated devices must have low metal–semiconductor contact resistance (< 1 9 103 X cm2) and a defect-free appearance with good adhesion to ensure proper device operation and to meet packaging requ
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