Low-Temperature Bonding of Laser Diode Chips on Silicon Substrates Using Plasma Activation of Au Films

A low-temperature bonding of vertical-cavity surface-emitting laser (VCSEL) chips on Si substrates was achieved by using plasma activation of Au films. After the surfaces of Au films were cleaned using an Ar radio frequency plasma, bonding was carried out by contact in ambient air with applied static pressure. The experimental results showed that surface morphological change (the reduction of asperity width) as well as removal of adsorbed organic contaminants by plasma treatment significantly improved the quality of joints. At a bonding temperature of 100degC, the die-shear strength exceeded the failure criteria of MIL-STD-883.

I. Introduction

Optical hybrid integration of optical components on Si substrate has opened many new possibilities for constructing small, reliable, and low-cost optical devices, such as optical modules and microsensors [1], [2]. In the bonding process, eutectic AuSn (80 wt%Au, 20 wt%Sn) solder (melting temperature: 280°C) [3] is commonly used for optoelectronic applications because the AuSn has very high yield strength and is typically free of thermal fatigue and creep movement. However, the AuSn bonding process has the following disadvantages. 1) Lack of plastic deformation of the AuSn results in its inability to release the stresses, which may cause the chip to crack. 2) The AuSn bonding process needs a temperature above 300°C to ensure complete melting and this high temperature processing degrades temperature-sensitive materials (e.g., plastics). Also it causes cracking or debonding of the chips made of materials having large coefficient of thermal expansion mismatches with the substrate.