There are five major methods of sealing packages: seam or projection welding, high temperature (AuSn) or low temperature soldering, low temperature glass sealing, and epoxy sealing - all but epoxy form hermetic seals . Of these methods, welding is the most expensive because of the fixturing/tooling needed. Sealing glass requires higher temperatures in the range of 330-450°C, whereas both epoxy and seam welding are done at lower temperatures. KovarTM or Alloy 42, electroplated with nickel and gold, is used for most lids for hermetic MEMS applications. For AuSn solder sealing, the lids often have a solder preform tacked on, making solder quantity control and alignment easy. The nickel underlayer serves as a very effective barrier to corrosion, while the gold surface layer preserves solderability of the nickel surface. For glass sealing, glass is usually pre-applied onto for the lid for MEMS, opto-electronic and other devices, and glass is often used in larger array packaging. Material properties of some solder preform materials are included in Section 12.4.1. The application of glass lids for optical systems is discussed in the case study in Section 12.11.2.
12.3.1 Optical Applications
Optical communications and sensors require hermetically sealed packages that allow transmission of optical, infrared, and ultraviolet signals. Sapphire, germanium and special glasses such as BK-7 allow the direct transmission of optical data. For maximum transmission efficiency, anti-reflective coatings are applied to the transparent component of the sealing lid. These coatings are nanometer thick layers of highly refractive fluoride and oxide compounds that are alternately evaporated or sputtered. These antireflective coatings can be tailored to allow specific wavelengths to pass through the lid with less than a 1% reflection loss of signal strength . Properties of window materials that allow optical and infrared transmission are shown in Table 12.7.
Table 12.7 Optical and IR transmission of materials as MEMS "windows"
Modulus of elasticity, Gpa
Wavelength transmittance, (im
12.4 Die Attach Materials and Processes
The methods used to attach a MEMS device to a package are the same as those used with Integrated Circuit devices. Such "die attach" media serve several functions: mechanical support, heat dissipation, and possibly electrical contact between the MEMS device and the package.
Electrically conductive attachment materials include silver-filled epoxies, silver-filled glasses and low melting solders. The stability and reliability of the attachment material is largely dictated by the ability of the material to withstand thermome-chanical stresses created by the differences in the CTE between the MEMS silicon and the package base material. Silicon has a CTE between 2 and 3 ppm/°C while most package bases have higher CTE between 6 and 20 ppm/°C.
MEMS packages use solders, adhesives or epoxies for die attach. Each method has advantages and disadvantages that affect the overall MEMS reliability. Generally, when a solder is used, the silicon die would have a gold backing. Au-Sn (80-20) solder generally is used and forms an Au-Sn eutectic when the assembly is heated to approximately 250°C in the presence of a forming gas. When this method is applied, a single rigid assembled part with low thermal and electrical resistances between the MEMS device and the package is obtained. One problem with this attachment method is that the solder attach is rigid (and brittle) which means it is critical for the MEMS device and the package CTEs to match since the solder cannot absorb the stresses.
Adhesives and epoxies are comprised of a polymer bonding material filled with metal flakes such as silver since it has good electrical conductivity and has been shown not to migrate through the die attachment material [19, 20]. These die attachment materials have the advantage of lower process temperatures. Generally, temperatures between 100 and 200°C are required to cure the material. They also have a lower built-in stress from the assembly process as compared to solder attachment. Furthermore, since the die attachment does not create a rigid assembly, shear stresses caused by thermal cycling and mechanical forces are relieved to some extent [21, 22]. One particular disadvantage of the soft die attachment materials is that they have a significantly higher electrical resistivity which is 10 to 50 times greater than solder and a thermal resistivity which is 5 to 10 times greater than solder. Lastly, humidity has been shown to increase the aging process of the die-attachment material [20, 23].