Package design of pressure sensors for high volume consumer applications

Abstract

In this abstract we outline the critical design aspects for the design of high volume pressure sensor for MEMS applications. Pressure sensor designs by their nature require the active device to be in contact with the pressure to be measured, this requirement has until now restricted the possibility of applying large volume semiconductor package manufacturing techniques to pressure sensors and hence their large scale deployment in consumer applications. The current designs of pressure sensors rely on either a separate protective membrane, a Gel-Liquid to transmit the pressure to the active device or a Gel applied on top of the sensor. The active part of the sensor is normally made up of two separate chips , the active membrane which flexes in response to external pressure the movement of which is translated into electrical signals by resistive or capacitive elements and a support chip with trapped between them a reference pressure volume. In this paper the design of a novel LGA based MEMS Pressure sensor and barometer/altimeter design are demonstrated. A single chip sensor solution is outlined with no need for the trapped reference volume between the support chip and the sensor membrane. In this single chip sensor solution the reference pressure volume is trapped within the chip itself improving both the chip dimensions and overall reliability by eliminating the joint between the two chips. The Packaging for a this single chip sensor is based on the now well established LGA sensor platform, the problem of interfacing the pressure sensor to external pressure is solved by using an exposed silicon in the over moulded package which allows the pressure sensor access to the outside pressure, reducing the overall package size for the absolute pressure sensor to 3 x 3 x 1 mm. MEMS sensor devices in general depend on their mechanical performance to translate mechanical movement into an electrical signal, the interface to the outside world, the package, is a mechanical structure that has a large influence on the device performance and must be thought of a part of the device not just as protective housing. In the case of the pressure sensor the flexible silicon membrane is influenced by the thermal stresses in the package. The design aspects that have influence on the performance of the MEMS devices are outlined, the Package and sensor devices were extensively modeled using FEA thermo-mechanical simulation with the results showing a good correlation to the final device performance. Problems encountered during the development related to device manufacturability are outlined and the solutions detailed. The final performance and reliability data is also presented demonstrating the robustness of the design solution.