STICTION IN SURFACE MICROMACHINING

Due to the smoothness of the surfaces in surface micromachining, large adhesion forces between fabricated structures and the substrate are e ncountered. Four major adhesion mechanisms have been analysed: capilla ry forces, hydrogen bridging, electrostatic forces and van der Waals f orces. Once contact is made adhesion forces can be stronger than the r estoring elastic forces and even short, thick beams will continue to s tick to the substrate. Contact, resulting from drying liquid after rel ease etching, has been successfully reduced. In order to make a fail-s afe device stiction during its operational life-time should be anticip ated. Electrostatic forces and acceleration forces caused by shocks en countered by the device can be large enough to bring structures into c ontact with the substrate. In order to avoid in-use stiction adhesion forces should therefore be minimized. This is possible by coating the device with weakly adhesive materials, by using bumps and side-wall sp acers and by increasing the surface roughness at the interface. Capill ary condensation should also be taken into account as this can lead to large increases in the contact area of roughened surfaces.