DESIGN CONSIDERATIONS
BY DANIEL D. EVANS JR.
The microelectromechanical systems (MEMS) component market size is forecasted to continue growing over the next few years. This article presents design considerations to provide a better understanding of the unique challenges facing MEMS packaging engineers, when compared to traditional IC packaging. Automatic MEMS assembly requires optimized equipment and processes.
 |
MEMS technology-based products continue at relatively low volumes compared to large markets such as memory or microprocessors. However, many market forecasts predict that MEMS will grow at a compound annual growth rate of 17 to 20 percent over the next four to five years. The long-term (1980-2000) growth rate (14.3 percent CAGR) of the semiconductor device industry has entered a period of secular decline (5 to 10 percent range).
Package Technology Segmentation
Package technology segmentation by interface to the real world is divided into two separate classes: capped and non-capped.
Capped product examples include accelerometers, gyros and RF switches. Fragile MEMS components typically are sealed from the environment using a lid or cap. Hermeticity is a common requirement for this type of component. The lids/caps can be implemented using discrete assembly, wafer-to-wafer-level bonding and wafer processing techniques.
Discrete Assembly. The discrete assembly of MEMS, electronics and lids/caps requires pick-and-place equipment that can handle a variety for bare die, substrate and lid components (Figure 1). Attach materials must be compatible with the overall assembly and not outgas onto the fragile MEMS structures.
 Figure 1. This MEMS accelerometer with multiple components to be assembled, includes a delicate sensing element. Courtesy of Silicon Designs. |
Wafer-to-wafer-level Bonding. Wafer-to-wafer-level bonding has been applied to MEMS for lids/caps. It requires precise alignment of wafers, and then bonding of the wafers in a gas or vacuum environment. Components are subsequently processed and singulated for final packaging. Electrical connections depend on the packaging configuration (Figure 2).
 Figure 2. Wafer-to-wafer-level bonding concept. |
Wafer Processing Techniques. Wafer processing techniques allow for vacuum-sealed environment in a wafer-processing step, but are not compatible with all MEMS sensor products. Figure 3 shows a MEMS structure before and after wafer-process-based cap sealing. This technique creates a hermetic seal with a lid that is strong enough to protect the microstructure during packaging process steps. Assembly attachment materials may outgas for this structure because the MEMS structure is completely sealed at the chip level.
 Figure 3. MEMS-based 3-axis accelerometer, shown here before wafer process cap seal (left) and after cap seal (right). |
Non-capped product examples include pressure sensors and nozzles that typically require direct contact between the MEMS and the environment. As such, they do not allow for a sealed lid/cap. However, an unsealed gel lid/cap may be used to protect other parts of the MEMS component, such as interconnect wires.
 Figure 4. MEMS-based pressure sensor cross section showing MEMS and seal gel. Courtesy of Motorola. |
A classic pressure sensor uses a diaphragm that strains as pressure is applied on both sides. An absolute pressure sensor has fixed (sealed) pressure on one side of the diaphragm, while a differential pressure sensor has direct contact with the environment on both sides. The package will vary dramatically between absolute and differential pressure sensors. An example package is shown in Figure 4.
MEMS Component Challenges
The particular MEMS application dictates the functional performance and reliability requirements of the packaged device. Packaging technical challenges include: cost, size, package stresses, electrical shielding, particles and hermeticity.
IC packaging is mature and approximately 30 to 95 percent of the whole manufacturing cost. MEMS package costs account for 70 to 90 percent of the device. Package stress, particle protection during manufacturing, hermeticity requirements and lower production volumes are the primary drivers for increased cost in MEMS packaging. Material selection is critical to minimize package stress and out-gas contamination of tiny MEMS structures.
Design modeling will help designers remove redundant features in MEMS component packages to help drive cost down. However, modeling total system performance is difficult when combining the package, components, adhesives, interconnections and possible effects on the package from board mounting, thermal conditions, etc.
Assembly Automation Design Considerations
Automated assembly using pick-and-place equipment is considered by many to be the most flexible technique for creating a total MEMS package. Pick-and-place machines allow designers to choose components from multiple sources and attach them together in a single package. Although capping can be accomplished with wafer-to-wafer bonding and semiconductor processing techniques, these components are still relatively delicate and need to be handled with care.
Design areas to consider when assembling a MEMS package include: component presentation to the equipment; fiducial marks for the package and MEMS device; MEMS no-touch zones and special pick tool requirements; material compatibility with the component and application; cleanliness of the process; placement accuracy and alignment algorithms (global or relative placement); attachment methods and materials; single level or die stacking; flip chip or circuit up; and wire bond.
Component Presentation to the Equipment. MEMS components can be presented to the equipment in a variety of formats such as Waffle Pack, Gel Pak, Tape and Reel and wafer. The package may be lead-frame, strip, panel, or individual packages. Depending on the production volume and manufacturing strategy, assembly cells may stand alone with operators feeding material by hand or be fully automated material handling systems with inline integration of upstream and downstream equipment processes.
Fiducial marks for the package and MEMS device. Physical features or marks that are recognizable by the machines vision processing systems is critical for robust automated assembly. The location, size, shape, and materials used for fiducial marks should be reviewed early in the design process. Fiducial marks are ideally created during the same process step as the features, which are being aligned inside the package. Package fiducial marks are typical larger and less repeatable when compared to MEMS component fiducial marks. Alignment accuracy of the MEMS to package will depend on both the package and MEMS fiducial marks.
MEMS No-touch Zones and Special Pick Tool Requirements. Many MEMS have sensitive areas that are damaged if touched by a pick tool. Custom pick tools can be designed to avoid the sensitive areas of the MEMS. In some cases two- or four-sided collets are used to pick from the very edges of the MEMS device. The pick-and-place equipment must have the ability to align these custom pick tools over the die. Flat bottom tools may also be used if the tool can be positioned on a durable area of the MEMS.
Material Compatibility with the Component and Application. The materials are chosen for chemical, mechanical stress, electrical, and optical compatibility. The package, MEMS, and attach materials form a complex system that can affect the performance of the device adversely when exposed to the manufacturing processing and environmental conditions. Optical MEMS have created some of the most difficult challenges for the packaging community.
Cleanliness of the Process. Assembly, joining, and aging cannot generate contamination that will adversely affect the device. Attach materials and joining or curing processes must be chosen carefully to consider the affect on the device during manufacturing processes.
Placement Accuracy and Alignment Algorithms. Global placement relies on one set of package fiducial marks to which all components are placed. Relative placement algorithms may place some of the components relative to previously placed components. Relative placement algorithms will visually reference a component in the package and then place the next component relative to the found component location.
Single Level or Die Stacking. There is a growing trend to deliver more functional performance per square centimeter of package area. Manufacturers are now stacking die on top of each other where possible to save area. Die stacking requires relative placement and possibly adhesive dispensing on the same platform if stacking in a single pass through the assembly machine.
Flip Chip or Circuit Up/Wire Bond. The choice between flip chip or circuit up/wire bond affects many of the areas discussed above and must considered carefully for cost and reliability.
Conclusion
The forecast for the MEMS market is for faster growth than the semiconductor RAM and microprocessor markets. MEMS-based products cover a broad range of applications (Table 1). MEMS-based packages require a variety of different form factors and package styles. As such, equipment that serves MEMS assembly will also need flexibility to handle multiple assembly criteria.
 Table 1. MEMS uses in today's markets. |
Pick-and-place of singulated components will continue to provide the flexibility and low start-up cost to prove MEMS product designs at low production volumes.
Packaging design engineers, equipment and materials suppliers, and engineering design tool suppliers should work together to continue driving down the costs of MEMS-based products.
References
For a complete list of references, please contact the author.
DANIEL D. EVANS JR., senior scientist, may be contacted at Palomar Technologies Inc., 2230 Oak Ridge Way, Vista, CA 92081-8341; (760) 931-3406.