Thermal Considerations

For independent small signal circuits, the temperature of the device junction does not increase substantially during operation, and thermal dissipation from the MEMS is not a problem. However, with highly integrated System-in-Package applications, MEMS devices share thermal paths with other components, such that the temper­ature rise in the device junctions can be substantial causing the circuits to operate in an unsafe region. Clearly, such distributed thermal dissipation requirements for such highly integrated packages can place severe design constraints on the package design.

Three thermal resistances that must be minimized: the resistance through the package substrate, the resistance through the die-attach material, and the resistance through the carrier or package base. Furthermore, the thermal resistance of each is dependent on the thermal conductivity and the thickness of the material. A package base made of metal or metal composites has very low thermal resistance and does not add substantially to the total resistance. When electrically insulating materials are used for bases, metal-filled via holes are routinely used, under the MEMS, to provide a thermal path to the heat sink. Although thermal resistance is a consider­ation in the choice of the die attachment material, adhesion and bond strength are even more important. To minimize the thermal resistance through the die-attachment material, the material must be thin, void-free, and the two surfaces to be bonded should be smooth [37].

12.9 Hermeticity and Getter Materials 12.9.1 Hermeticity and Pressurized Packaging

The definition of hermeticity comes from the United States Department of Defense specification for Microelectronic Packaging (MIL-STD-883) stating that a seal that prevents the entry of contaminants or reactive gasses into the internal cavity is deemed hermetic. In practice, small gas molecules may enter the cavity over long time scales through diffusion or permeation. Table 12.12 comes from the Military Standard and defines the leak rate limit by package volume. To test to this specifi­cation, the package is placed in a pressurized Helium atmosphere for the specified time followed by a dwell time between pressure release and measurement. The unit is then placed in a calibrated Helium mass spectrometer that measures its helium leak rate.

Table 12.12 Leak rate testing to cavity size from MIL-STD-883

12.9.2 Hermeticity and Vacuum Packaging

Some special applications require wafer-level packaging with hermetic cavities under vacuum, creating the need for bonding equipment that can be used in high- or ultra-high-vacuum environments. These include:

High-accuracy accelerometers and gyroscopes;

MEMS switches and oscillators;

High-frequency resonators; and

Optical switches and infrared (IR) imaging sensors

A wafer bonding technique developed to accommodate these requirements uses getter materials to ensure suitable vacuum (total pressure < 1 x 103 mbar) and long-term stability in MEMS devices. Gettering is essential for removing unwanted gases out of the MEMS package to create the desired final atmosphere.

Date: 2015-02-28; view: 1258