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1139-GG03-33

Metal Wafer Bonding for MEMS Applications V. Dragoi1, G. Mittendorfer1, F. Murauer1, E. Cakmak2 and E. Pabo2 1 EV Group, DI E. Thallner Str. 1, 4782 – St. Florian/Inn, Austria 2 EV Group Inc., 7700 S. River Parkway, Tempe, AZ 85284, U.S.A.

ABSTRACT Metal layers can be used as bonding layers at wafer-level in MEMS manufacturing processes for device assembly as well as just for electrical integration of different levels. One has to distinguish between two main types of processes: metal diffusion bonding and bonding with formation of an interface eutectic alloy layer or an intermetallic compound. The different process principles determine also the applications area for each. From electrical interconnections to wafer-level packaging (with emphasis on vacuum packaging) metal wafer bonding is a very important technology in MEMS manufacturing processes.

INTRODUCTION Wafer bonding plays an important role in Micro-Electro-Mechanical Systems (MEMS) applications as a technique used for joining two substrates. The large device variety required the development of various wafer bonding processes. Among the wafer bonding processes currently used for industrial applications can be mentioned direct bonding (also named fusion or molecular bonding - adhesion appears between the molecules on the two surfaces) [1], anodic bonding (used to bond a Si wafer to a glass wafer - adhesion appears through an oxide layer grown at the interface) [2], adhesive bonding (uses intermediate layers, typically polymers) [3, 4], eutectic bonding (adhesion appears through an eutectic alloy layer grown at the interface) [5] or intermetallic bonding [6] and thermocompression bonding (metal bond - adhesion occurs between two metal surfaces pressed together under heating) [7]. Wafer bonding process selection is based on various criteria related to the materials (substrates types, bonding temperature) as well as to the desired application (type of bond – mechanical connection, electrical or thermal conductivity of interface, optical properties and device working temperature). This paper presents a review of wafer boding processes based on the use of metal layers and introduces the process selection criteria for metal wafer bonding. BASICS Apart from direct (fusion) bonding for which adhesion between two substrates is generated through molecular bonds, processes using “bonding layers” are extensively used in MEMS manufacturing . The choice of intermediate layers for wafer bonding is made based on required processing temperatures as well as on other materials characteristics (e.g. specific outgasing, transparency, thermal/electrical/optical conductivity).

Wafer bonding using metal bonding layers is an important technique used for applications requiring good thermal conductance and applications in which electrical conductivity is required (e.g. for 3D TSV – Through-Si Vias - applications). Two main types of principles are governing metal bonding: alloying (eutectic alloy or intermetallic compound formation), and metal diffusion by thermo-compres