Lawrence E. Felton

Chip scale packaging of a MEMS accelerometer

Accelerometers fabricated with MEMS technology are widely used in automotive crash detection and vehicle dynamic control systems. Recently, there has been increased interest in the use of MEMS accelerometers in smart handheld devices. The most demanding new requirement for this application is package size. This paper details the fabrication a 2 axis accelerometer in a 4 mm/spl times/4 mm/spl times/1.5 mm thick lead frame chip scale package. The fabrication of the device depends on three technologies: (1) an integrated MEMS technology that yields the MEMS structure and all associated signal processing circuitry on a single die; (2) the development of a wafer scale capping process that provides protection for the MEMS sensor and renders it suitable for transfer molding; and (3) modifications to the standard lead frame chip scale package (LFCSP) assembly process to accommodate the capped MEMS sensor. The performance of the accelerometer is assessed and compared to the same device in a leadless chip carrier.

Next generation low stress plastic cavity package for sensor applications

MEMS inertial sensors are widely used in automotive safety systems. For example, accelerometers are used to detect crashes and gyroscopes are used to detect roll over events. One of the principal performance issues for these inertial sensors is the stability of the device output over temperature. The origin of the offset drift with temperature is the deformation of the MEMS structure that results from thermal strains. The sensors signal conditioning electronics process the thermal deformations just as they would deformations due to acceleration. Because the thermal strains are temperature dependent, the device output has temperature dependence. Several approaches have been developed to improve the device performance over temperature. These include modifying the design of the MEMS structure to reduce its susceptibility to thermal strains, the use of silicone gels to decouple the MEMS structure from the package stress, modifying the package design to reduce the package stress, and the use of electronic temperature dependent correction of the output. Many commercially available sensors use a combination of several of these approaches. One of the most straightforward approaches to improving sensor performance is to use a cavity package rather than an overmolded package. In an overmolded package, the entire sensor is encapsulated in the epoxy mold compound. In this case, there is significant coupling between the package and the sensor, so the thermal strains are readily transmitted to the senor. In a cavity package, they only mechanical connection between the sensor and the package is through the die attach. Thus, the package thermal strains cannot readily act on the device. Several cavity package technologies are available commercially. In the past, both ceramic and metal have been used for inertial sensor. They offered a very low stress package and excellent reliability. But these packages are no longer viable for high volume sensor manufacturing because their costs are too high. Another barrier to their use is that the vast majority of the semiconductor assembly infrastructure is used for transfer molded plastic packaging. What is needed is a low-cost cavity package for sensor applications. Pre-molded plastic cavity packages are ideal solution for application for stress sensitive inertial sensors. These packages have several features that make them ideal for the application: first, they are much lower cost than a ceramic or metal package of the same size; second, the packages can be molded onto a lead-frame in an array format that is compatible with standard transfer molded assembly lines. Thus, the cost of assembly for the packages can be very low. Finally, the packages are lead-free, which is an important environmental concern

Very thin packaging of capped MEMS accelerometer device

Analog Devices Inc. (ADI) has been a major supplier of accelerometer devices in hermetic packages used for crash detection and vehicle dynamic control in the automotive industry. A major market pull from the handheld customers makes it imperative to develop thin plastic packaging solution for MEMS accelerometer. This paper details the backgrinding process developed to successfully thin the wafer-level-capped MEMS accelerometer. Functionality is maintained after packaging the thinned device, a significant effect in performance is observed. Three packaging solutions are evaluated to produce a 4times4times1 mm3 leadframe chip scale package (LFCSP). The performance of the thinned accelerometer device using the thin packaging options is compared. Assembly issues of the thin packages are also discussed