3-D packaging of MEMS sensors for development of miniaturised inertial measurement unit

In this paper a 3D packaging solution for bare die MEMS sensors for the implementation of an inertial measurement unit (IMU) is presented. An IMU is an array of inertial sensors, accelerometers, magnetometers and gyroscopes, which provide inertial data in 3-axes for a system. Miniaturisation of these systems is essential in the realisation of embedded ubiquitous computing many applications. State of the art commercially available IMU devices are bulky and do not meet the size requirements necessary to realise a fully unobtrusive ambient system. This paper will discuss the work carried out in the miniaturisation of an existing 25 mm form factor IMU to a 10mm form factor. The 25mm module has been developed at the Tyndall Institute using innovative PCB technology in conjunction with surface mount devices while the current module has been miniaturised using a combination of bare die sensors and a flexible substrate. The aim of the current work is the establishment and characterisation of the assembly parameters and sequence for the fabrication of a prototype IMU with limited functionality. The prototype IMU development involved flexible circuit design with two bare die, capped-accelerometer and conditioning circuitry. The paper will describe in detail the process and assembly steps that were investigated in the development of initial prototype. The processes involved were fabrication of flexible substrate, hole formation to accommodate the cap of the sensors, interconnection technology and finally the folding of the substrate into the desired shape to allow the orthogonal array of sensors needed for the prototype IMU to have two degrees of freedom in operation. The work established the assembly route, which will be used for the development of a fully functional IMU. This version of IMU is the miniaturised version of already developed 25mm module based on standard PCB technology. The final aim of the work is to develop these highly miniaturised IMU's as a distributed sensor node in a wireless sensor network configuration providing inertial data over a 3D space.