Stiction (a word coined from the combination of static and friction) is the force required to cause one body which is in contact with another to begin to move. The magnitude of stiction forces becomes especially important for the moving parts of microelectromechanical machines (MEMs). These devices find ever-growing usage in a wide variety of applications from advanced medical devices to aerospace and defense systems.

Robert K. Lowry

The heart of such devices is various combinations of exceedingly small sets of gears, cantilever beams, and other moving parts. Proper MEMs function depends on these tiny components initiating motion instantly upon command. The slightest hesitation which might occur to overcome a stiction force compromises function of the device and the system depending on it. There are four main causes of stiction:

Capillary forces. These occur when a liquid fills the tiny space between two movable parts. A liquid contact angle Ө_c <90º between two gear teeth or beams (Fig. 1) sets up an attractive force between the surfaces “bridged” by the liquid. The most common cause of a stiction-induced capillary force is condensed liquid water.

Figure 2. Hydrogen bonding in water

Hydrostatic forces. Two adjacent surfaces need not be bridged by liquid to suffer from stiction. A few molecular layers of adsorbate are sufficient to impart hydrostatic forces between two surfaces when hydrogen bonds (Fig. 2) form between oxygen atoms of adsorbate. Water s the most likely (but not the only) cause of hydrostatic hydrogen bonding.

Water is a polar molecule, with two hydrogen atoms bonded to an oxygen atom at an angle of 104º. Because of the orbital geometry of the electrons around the molecule, the oxygen atom has two unshared pairs of electrons. This imparts a partial negative charge to the oxygen atom while the hydrogen atoms have a partial positive charge.

This property of molecular water not only encourages its molecules to adsorb on surfaces, but once adsorbed, the partial negative charge of oxygen atoms in water molecules on one surface tends to attract the partial positive charge of hydrogen atoms in water molecules on an adjacent surface. These hydrogen bonds effectively “glue” adjacent surfaces together.

The effect is not limited to water; any polar adsorbate containing oxygen and hydrogen atoms, including many common organic solvents used in electronics processing, can also adsorb and cause the problem. Also see Van der Waals forces later in this article.

Electrostatic forces. The magnitude of these forces between adjacent surfaces is expressed by Coulomb’s Law where q₁ and q₂ are the magnitude of static charges on two adjacent surfaces, r is the distance between them, and k is a proportionality constant. When the value of F from Coulomb’s Law is negative, the forces between the surfaces are attractive and there is stiction to be overcome to get them to move.

To avoid stiction problems induced by electrostatic forces, completed materials processing must not leave static charge on movable MEMs surface structures. Alternatively, the fabricated structures must receive surface treatments designed to dissipate accumulation of static charge.

Van der Waals forces. These forces are intermolecular in nature. Atoms within covalent molecules are firmly bound to each other by covalent electronic bonds. However, positive atomic nuclei within a molecule can experience weak attractions between themselves and electrons of other atoms that are outside their own radius and external to their own molecule. These are also called dispersive forces. These forces are very weak, but can create a stiction effect under the right conditions.

Keep everything dry. Capillary forces and hydrostatic forces are the two principal causes of stiction in MEMs devices. If a MEMs enclosure has 0.5v% moisture content, water can condense onto surfaces at temperatures <-2ºC, imparting liquid to surfaces and creating a condition favorable for capillary forces.

Even very low relative humidities, down to ≈30%, create the threshold condition for adsorbing three monolayers of water molecules onto surfaces. Three monolayers imparts enough adsorbate onto surfaces to create a condition favorable for hydrostatic forces.

Stiction in MEMs and nanomachines devices can only be avoided by keeping critical moving parts free of both condensed and adsorbed water that may be sourced either from materials outgassing or poor seal integrity of the enclosure.

In future articles of this series we will discuss the theoretical foundations of water transmission both into and within a system as well as materials and processes (M&P) used to control water transmission, condensation, and absorbtion.

Robert K. Lowry (321-777-9949, www.electronic-materials.com) is a consultant/materials scientist with 39 years microelectronic industry experience and is also a principal with Arthur Jonath Associates (http://www.jonathassociates.com/).

Richard C. Kullberg (719-966-4296, rckullberg@vacuumenergyinc.com, brings 30 years of materials science and microelectronic industry experience to his product and business development work with Vacuum Energy, Inc. (http://www.h2getters.com/).