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Wafer bonding has been widely used for forming vacuum and hermetic packages for MEMS devices. Several techniques, including anodic silicon-glass, silicon-silicon fusion, metal compression, glass frit, eutectic, and solder bonding have been attempted and some have been demonstrated successfully. Solders have some advantages that make them an interesting area of research for wafer bonding. They offer lower permeability, lower temperature bonding, and the potential for die area savings through smaller bond rings. Some of the challenges of solder bonding are discussed. Then an overview of traditional solder wafer bonding as well as an advanced solder bonding technique called diffusion soldering is presented. Vacuum and Hermetic Packaging for MEMS Using Lead-Free Solder INTRODUCTION Many MEMS devices are experiencing substantial growth as they find their way into products that span consumer devices to national security. Packaging of these devices is still one of the most difficult and costly parts of the fabrication process, especially for packages that need a vacuum environment and/ or a hermetic seal. Vacuum packaging is critical for resonant devices to achieve high quality factor (Q) and also for thermal imaging devices to increase the thermal isolation of the sensor platform. Hermetic seals are needed for devices that do not require vacuum for proper operation, but still need to keep out unwanted contaminants, such as moisture. Water vapor accelerates the failure of many devices, so a clean, dry environment is necessary for long term reliable operation. Seals used to create vacuum packages can provide this environment because they must have excellent hermetic sealing capabilities in order to main-tain vacuum in the micro domain. Glass frit wafer bonding is widely used by industry for creating MEMS vacuum packages for many reasons. It is a proven process with demonstrated long term reliability and low vacuum levels. Its thermal coefficient of expansion can be tailored to match the device and package substrates to reduce residual stresses from thermal expansion mismatches. And it can planarize over topology, such as electrical feed-throughs, because the bonding temperature is above the glass transition temperature of the frit. There are several limitations with glass frit that spur research into other vacuum packaging approaches. The lower temperature glass frits contain lead, which is banned from importation into many countries. Also, glass frit is usually applied through a screen printing process, which limits the smallest definable bond ring to ~ 00 microns. Combining this with the difficulty of aligning the screen to the wafer, a large area of the die needs to be dedicated to the bond ring to achieve a good glass frit bond. Finally, glass as a material class does not offer the best hermetic sealing capability vs. width because of its high permeability [1]. Solders have several attractive qualities that make them interesting candidates for replacing glass frit in vacuum packaging applications. First, all solders are metals, which have the lowest permeability of common packaging materials. Second, there are numerous solder alloys that span a range of melting temperatures from near room temperature to many hundreds of deVacuum and Hermetic Packaging for MEMS Using Lead-Free Solder Warren C. Welch III, Jay S. Mitchell, and Khalil Najafi, Center for Wireless Integrated MicroSystems (WIMS) The University of Michigan Ann Arbor, Michigan, U.S.A. Page 6 Figure 1: Illustration of the cross section of the wafers just before tin solder bonding and after dicing. Figure 2: Picture of the bonded and diced leadfree flux-free solder bonded wafer. Table 1: Several Low Temperature Solders with their Melting Point and Composition. [2] Table 2: Several Diffusion Soldering material sets with the formation and service temperatures. [2] Composition, wt % Eutectic °C Ag Bi In Pb Sn Au Si ... 49.0 1.0 18.0 1 .0 . – 57 ... 57.0 ... ... 43.0 ... – 139 3.0 ... 97.0 ... ... . – 144 ... ... 99.4 ... ... 0.5 – 156 ... ... ... 38.0 6 .0 ... – 183 3.5 ... ... ... 96.5 ... – 1 ... ... ... ... 0 80 – 78 – – – – 97.1 .9 363 Material System Process Time and Temp. Remelt Temp. Copper Indium 4 min at 180 °C > 307 °C Copper Tin 4 min at 80 °C > 415 °C Silver Tin 60 min at 50 °C > 600 °C Silver Indium 1 0 min at 175 °C > 880 °C Gold Tin 15 min at 60 °C > 78 °C Gold Indium 0.5 min at 00 °C > 495 °C Nickel Tin 6 min at 300 °C > 400 °C grees Celsius (See Table 1). The range of melting temperatures allows the packaging engineer to pick an alloy with a melting temperature that suits the thermal requirements of the package. Lastly, solders melt and become liquid during the bonding process; this allows them to flow over and seal wafer topologies such as electrical feedthroughs. There are several challenges involved in using solder as a bonding mechanism for vacuum packaging. A typical soldering process uses fluxes to remove metal oxides and other contaminants to promote good wetting between the solder and the substrate. However, flux is not compatible with MEMS processing. The liquid flux can cause stiction issues with released MEMS devices and they leave behind residues that would ruin a vacuum package by outgassing. A flux-free solder process is necessary to vacuum package MEMS for these reasons, but it takes careful design of the bond joint and the solder process to achieve good wetting without flux. Also, the solder alloys must be lead-free to comply with emerging worldwide legislation banning lead in electronics components. The final challenge for solder wafer bonding lies in the fact that most wafer bonders have long thermal time constants, especially if the bonding chamber is pumped down to high vacuum. This increases the time the solder remains molten from tens of seconds in a typical solder process to several minutes in a standard wafer bonder process. This can be a problem because molten solder consumes most metals very rapidly through intermetallic formation [ ]. All these challenges necessitate careful design of the bond joint and careful selection of the materials involved. This article reviews some of the results of solder wafer bonding projects from around the world and highlights the research efforts from The University of