The term stiction comes from “static friction” and it has long been a factor in a wide variety of technologies, including suspension linkages for cars, polished glass, hard disk drives, and precision gage blocks. It occurs when two smooth surfaces are initially brought into contact and forces (such as those described below) prevent subsequent separation.
In a MEMS (microelectromechanical system) device, objects that could come in contact include elements such as actuators, proof masses, and sensing fingers (Figure 1 depicts lateral stiction). Such contact may occur as part of the device's normal operation or may unintentionally occur as the result of an external force such as a mechanical shock. Either way, however, once contact has occurred the device needs a reliable way to ensure that it can separate the surfaces again in order to keep functioning properly.
To ensure that a device can overcome stiction, the design needs to address the primary stiction forces properly. Forces that bring the surfaces into contact are electrostatic attraction (if powered) and surface work of adhesion. The force of electrostatic attraction is proportional to 1/d2 and surface work of adhesion is proportional to 1/d3 (where d is the distance between the surfaces at rest). Surface work of adhesion in MEMS is primarily due to van der Waals and hydrogen bond forces.
Release from stiction will occur if the restoring forces exceed the forces that allow the surfaces to stay in contact: Frelease > Fcontact. Release forces in a MEMS device include the mechanical properties of the MEMS design (separating spring constant) and, when packaged in a gas or fluid, squeeze film damping.
Veryst recommends reducing the surface work of adhesion, a popular approach for ensuring that stiction can be overcome. Such reduction can be achieved through both design and manufacturing methods to reduce Fcontact. One design method for reducing Fcontact involves reducing the surface area of contact through the inclusion of stoppers or bumpers.
Manufacturing methods also include reducing surface area through surface roughening techniques, sometimes called ‘nano-texturing.’ All of these techniques act to reduce the total surface area of contact.
Anti-stiction coatings are applied to reduce the surface work of adhesion. SAM (self-aligned monolayer) coatings, often with fluorine termination, are the most popular anti-stiction coatings. Veryst also recommends coating structures that are properly designed with bumps and stoppers.
Veryst has deep expertise in MEMS reliability and MEMS failure mechanisms, offering failure analysis, reliability testing plans, and fabrication line audits. Expertise in both the physics of failure and in MEMS design for reliability simulations is a powerful combination that can result in optimized field performance.