Micropackaging Using Thin Films as Mechanical Components

The development and testing of simulation software based on variational principles to examine the behavior of structures undergoing large scale elastic deflections is described. The programmes are used in the design of micro mechanical beams for packaging applications, specifically in the domain of passive alignment of optical components. The fabrication process is described for 5 μm thick silicon carbide beams which are then tested by bending them. Introduction Optoelectronic subsystems are becoming increasingly important to minimise the costs of assembly and packaging [1-18]. The mechanical properties of beams made from thin films can be used to advantage for precise assembly. For example 3 μm thick silicon nitride microclips hold single mode optic fibres in place in silicon V-shaped grooves [10]. This paper describes the proposed use of pairs of thin film microcantilevers to precisely locate an optic component such as a filter or a mirror in an optical bench, or electronic components. In the configuration shown in Figure 1 the precision of the lithographic process for the cantilevers would determine the precise location of the component in the package, and to first order the etched shape in the substrate is unimportant. To analyse this proposed configuration, simulation software based on variational principles has been developed to examine the behavior of structures undergoing large scale elastic deflections. The design software consists of spreadsheet front end to enter parameters, along with Visual Basic (VBA) code and Frontline 'solver' software to run simulations [16]. Experiments with test structures on silicon-carbide coated silicon wafers have involved (i) etching of the SiC, (ii) wet etching of the silicon using the SiC as an etch mask, and (iii) bending SiC beams using the force produced by a surface profiler ( Dektak ) [13,18]. Simulation of large-scale deflections of microbeams A variety of Microsystems involve small elastic deflections of cantilevers and membranes e.g. accelerometers and pressure sensors [1]. Small angle deflections are considerably more amenable to analysis than large-scale deflections. Many proposed micropackages involve elastic deflections large enough to make the point of application of a force change hence making the system geometrically nonlinear [9]. Conventional analytical analysis of such a system is an arduous task resulting in a set of problem specific equations whose solution requires numerical methods. We report a simple method, which is applicable to a variety of problems. It can deal with large deflections and some coupled physics problems where an electric field may be applied. Lumped mass analysis involves splitting a continuous section into a series of discrete elements whose properties approximate those of the original bar. It is used widely in vibration analysis to investigate system resonances. The continuous beam is split into a series of hinged torsional springs joined by rigid members of zero mass as in Figure 2. When the beam undergoes deformation the rigid members can rotate relative to each other and elastic energy is stored in the torsional springs. The total elastic energy stored in the bar is simply the sum of spring energies. Fig.1. Cross section of the package structure. The springs are such as to give the same bending rigidity per unit length as the continuous bar. We can then write the torsional spring constant as EI k s = ∆ M kθ =