Flip-chip integration of selectively oxidized 850 nm VCSEL arrays

Abstract

Combinations of dense rrumbers of high performance photonic and microelectronic devices originating from disparate technologies are needed to achieve the next generation of information processing, interconnect, and intelligcnt sensing microsystems. Previous flip-chip VCSELs bonded to microelectronic circuitry were accomplished as isolated individual VCSEL islands, and often emitted at longer wavelengths for emission through the GaAs substratc [ 1,2]. However, 850 nm VCSELs are the most mature commercial tcchnology, and high count/density 2-dimensional arrays are desired for emerging applications. We report the fabrication techniques for flip-chip integration of selectively oxidized 850 nm 8x8 individually addressable VCSEI, arrays.For robust integration of 2-dimensional 850 nm VCSEL arrays to microelectronics, there are three challenges to overcome: 1) reliable bonding; 2) mechanically stable packaging; and 3 ) through substrate light emission. To achieve robust bonding ofVCSELs to Si or 111-V elcctronic circuitry, we have developed a silver post flip-chip process after Goossen [ 3 ] . The selectively oxidized 2-dimensional VCSEL arrays are completely fabricated [4] anti tested prior lo flip-chipping. The top facet of the VCSEL mesas are coated with a Au fjlm, then a Ag post, and finally another thin Au layer to provide the bonding contact. l h e flip-chip test coupon has matching electrical interconnections which extend Au contact pads out beyond the periphery of the VCSEL array die as shown in Fig. 1. (Eventually this coupon will be replaced with an integrated circuit driver chip.) To ensure mechanical stability between the VCSEL die aild the underlying coupon we cmploy an unfilled Dexter epoxy for under fill that readily wicks from the edges into the -6pm gap under the -2x2 nim' VCSEL dic. As evident from the cross section electron microscope image in Fig. 2, the epoxy completely fills in without voids, and secures the die to the coupon before substrate removal.Finally, to enable 850nm emission we remove the optically absorbing GaAs substrate by wet chemical jet etching. A 100 nm thick AlAs layer is utilized as an etch stop to preserve the VCSEL epitaxial material. Flip-chipped VCSEL assemblies have been tested on a probe station, as depicted in the inset of Fig. 3. l h e representative performance of a flip-chipped bottom emitting 850nm VCSEL with an oxide aperture of 14x14 pm2 is shown in Fig. 3 . We will also report on cfforts to improve uniformity and yield, as well as the performance of fully packaged flip-chipped VCSEL arrays. Rcfereticcs[I] U.