Packaging of MEMS: some thermodynamic and kinetic issues during eutectic bonding in the framework of the miniaturization trend

F. Hodaj
Grenoble Institute of Technology,

Keywords: MEMS packaging, brazing alloys, eutectic bonding, thermodynamics, interfacial reactivity, metastability


Micro Electro-Mechanicals Systems (MEMS) devices include movable parts for the sensor function. As chip dicing and subsequent operations can damage these sensitive micro- or nanometer-size devices, they need to be shielded from the outside environment to keep a clean surrounding atmosphere. For this purpose, packaging is essential to provide this physical protection. Wafer Level Packaging (WLP) consists in providing a cap to every chip on the wafer in one step, resulting in a cost reduction compared to the non-collective chip scale packaging process. Wafer bonding can be carried out by several techniques: direct bonding, anodic bonding, polymer bonding, glass frit bonding, metal bonding. By comparison, metal bonding provides increased hermeticity, reduced seal ring geometry, conductive surfaces, fine resolution without stringent surface preparation. Moreover, this bonding technique can operate for a wide temperature range process from about 250°C to about 450°C. In this work, some fundamental issues concerning the eutectic bonding during WLP are presented and discussed. The fundamental problems concerning this bonding technique in relation with some phenomena such as (i) the wetting process (non-reactive or reactive wetting) between liquid solders and different type of substrates, (ii) the interfacial reactivity as well as (iii) the degree of undercooling of liquid solder - metastable equilibria, and its consequences on the microstructure of the joint, are discussed in detail through different examples. The influence of some physicochemical and geometrical parameters and especially of the thickness of the joint on the microstructure of the joint and interfaces are also discussed from a thermodynamic and kinetic point of view. All these points will be illustrated by theoretical analysis of experimental results obtained in sessile drop experiments as well as in bonding experiments in the main MEMS WL packaging processes involving eutectic Sn-Cu, Au-Sn, Au-Si and Al-Ge brazing alloys with melting temperatures of 221, 278, 363 and 423°C respectively.