MEMS devices after fabrication typically posses significant residual stresses in each material layer. This is largely as a result of the different thermal properties of each material/layer when subjected to the elevated fabrication process temperatures. Many MEMS designers actually use this residual stress as a design feature to allow 2-D micro machined geometries to "flip up" when strategic anchor points are released from the wafer substrate.
The ANSYS initial stress function (ISTRESS) allows the direct specification of a constant state of residual stress in each material present in the model. This feature is applicable to beam, shell, and solid structural elements. Can be used in static or time transient structural analysis. Can only be used in the first load step of a time transient analysis. The stress state can be applied directly to selected materials either by using ISTRESS command, GUI dialog box or ASCII file input. This allow Sx, Sy, Sz, Sxy, Syx, Sxz to be specified in the current system of units. Here is the ISTRESS dialog box:
An alternative, more flexible method of applying the initial stress is to use the ISFILE command. This allows the initial stress state for each element to be read in from an ASCII file. Initial stresses can also be written out to a file using the ISWRITE command.
The following example illustrates the effect of initial stress in the structural polysilicon layer of a MEMS optical grating device (MOEMS grating). The first image shows the solid model,the second image shows the deformed grating. Deformation here is of the order of 50 nm, and is enough to render the device useless!
