Study and development of advanced packaging technique for semiconductor lasers

Abstract

The development of Erbium-Doped Fiber Amplifiers (EDFA) in long-haul telecommunications applications resulted in the rapid evolution of high power semiconductor lasers. In these applications, the demand for increased channel density in fiber-optic communication has pushed forward the use of single-mode ridge-waveguide laser diodes (LDs). One of the major concerns in these single-mode LD packages is the thermal management of the module. These semiconductor lasers usually generate large heat fluxes, up to the range of MWcm" 2 , that adversely affect their performance and reliability. Thus, the aim of packaging is to dissipate heat as efficiently as possible, in order to operate the LD at its maximum optical performance, without exceeding the permitted device temperature. As a first stage packaging, die-attachment of LDs onto a submount/heatsink is adopted and face-down bonding approach is recommended to meet the large heat generation rate. Some of the packaging-related issues faced in this bonding configuration are precision bonding alignment, low bonding yield, high bonding stress, good bonding integrity, as well as long-term reliability issues. This thesis introduces the development of a modified bonding technique for single-mode ridge-waveguide LDs. The effects of bonding parameters on the microstructure, bonding integrity of the solder joint, and LD performances were analyzed. Reliability tests such as thermal cycling and accelerated aging tests were also performed to prove the package's long-term stability. The solder joint possessed good long-term reliability, with little intermetallic compounds (IMC) growth at the interfaces. The formation of Au-rich P and i ATTENTION: The Singapore Copyright Act applies to the use of this document. Nanyang Technological University Library CJ phase Au/Sn compounds in the solder joint limit the interfacial IMC growth. Mechanical testing exhibited excellent bonding integrity, with brittle fracture occurring within the LD. No degradation of threshold current I t h could be observed and a 3X optical improvement over unbonded LDs was attained due to improved thermal management. Band structure theory of semiconductor lasers was conducted to understand the temperature-dependence characteristics of LDs. Transient and steady-state thermal analysis of semiconductor lasers showed that the heat generated in the LD and its associated thermal resistance R t h was induced by the radiative energy transfer of free carriers. The dependence of R t h on different bonding configurations and its correlation with the output power was discussed. Finite Element (FE) modeling was also carried out to provide theoretical understanding on the temperature distribution and thermal dissipation for different bonding configurations. Face-down bonded LDs demonstrated improved thermal management with-80% of heat flow towards the heatsink while faceup bonded LDs could only manage-50%. The analytical method using electronic band structure calculations and FE modeling gave close fitting to the experimental result...