Ken Gilleo, ET-Trends LLC
Microelectronic Package Elements and Attributes
Microelectronic packages generally provide a set of related elements: one or more devices to be packaged; interconnection from the devices to the package, also called first-level interconnect; a surrounding or containing structure to provide both mechanical support and electrical, chemical, and environmental protection; a joining structure (2nd level interconnect) to attach the package to the board or system.
These elements are chosen to provide acceptable levels of the desired attributes. For example, the interconnection and joining structure must provide a geometric and spatial translation from the chip-scale elements of the device to the circuit-board scale of the system. Since semiconductor devices and circuit boards typically have significantly different thermomechanical expansion properties, the package should mechanically decouple the two systems to prevent structural failure during assembly and operation. The package must provide the level of protection required by the device and application. In addition, the package as a whole must provide qualities such as ease of handling and assembly, standardization, testability, thermal management, electrical performance, and cost.
Integrated MEMS Gyroscope (Analog Devices Photo)
MEMS Added Packaging Requirements
Adapting microelectronic packaging to MEMS devices adds several challenging packaging requirements. The typical 3-dimensional structures and moving elements of many MEMS devices generally require some sort of cavity package to provide free space (“headroom”) above the active surface of the MEMS device.
The MEMS device might also require thermal isolation within the package, and a mounting method that minimizes mechanical stress on the device, especially for sensors. The interior of the cavity must generally be free of contaminants, including excessive outgassing of materials. The cavity may be evacuated, or be filled with atmosphere-controlling agents such as getters.
At the same time, non-electronic access to the device might be required, for example, from optical fibers or though a window. Some MEMS devices require controlled or selective access for gasses, liquids, biological agents, or other materials. For example, some Bio-MEMS systems will need packages that can handle fluids.
MEMS accelerometer (Analog Devices Photo)
MEMS Added Packaging Considerations
In addition to these more demanding requirements, MEMS devices are vulnerable to damage during what would otherwise be normal micropackaging procedures. The presence of 3-dimensional mechanical structures on the wafer surface that can move, adds fragility to unpackaged devices. Movable MEMS structures may make contact and permanently stick together (stiction effect) if roughly handled. Normal microelectronic pre-packaging steps, such as wafer dicing, may introduce contamination or damage. Cavity surface contamination can later impair MEMS performance.
Determining the reliability of MEMS devices is also complicated by the packaging. Reliability may depend on the package design, materials, handling, and the residual atmosphere within the cavity. The device and package must be a highly integrated system, much more so than standard electronics.
The cost of MEMS packaging has become a critical issue for many applications, as production volumes grow and MEMS becomes more pervasive. Until recently, choices have been too limited. The surface features and cavity requirements eliminate the low-cost transfer-molded non-hermetic plastic packaging used for most IC’s. Traditional full-hermetic cavity packages for electronic devices have generally been metal or ceramic enclosures built to meet the demanding requirements of military specifications, and priced accordingly, making the package the largest cost contributor. The conventional hermetic package has become cost-prohibitive for most MEMS in high-volume consumer and automotive applications shipping millions of units per month. MEMS requires new packaging strategies.
MEMS microchain and gear close-up. (Sandia Photos)
MEMS Packaging Directions
New packaging initiatives for MEMS to reduce packaging costs and provide the right attributes include new materials, designs, and methods of sealing. Thermoplastics, such as Liquid Crystal Polymers (LCP), noted for high temperature and excellent gas barrier properties, are a leading candidate material. Near-Hermetic Packages (NHP) of molded LCP promise to be a low cost alternative package for MEMS, MOEMS, and some RF applications.
While the NHP concept may not suit every MEMS device and application, many believe that it will become the preferred cost-effective solution for sensors, the largest market. LCP is easily injection-molded to mass-produce cavity packages with moisture barrier properties that are about 10 times better than epoxies. Plastic, metal, ceramic or glass lids can be sealed to LCP cavity packages using adhesives, direct heat, or ultrasonics.
Several companies are now sampling NHP plastic packages to MEMS fabricators. We predict that the thermoplastic NHP will have design wins in early 2004, with implementation by mid-year. The time is right for “Green,” lead-free and halogen-free, thermoplastic packaging.