Definition

Stiction/adhesion is a phenomenon in which two surfaces of a microsystem or of microstructures are adhered together due to various interfacial attractive forces and fail to separate. Thus anti-adhesion/stiction technology involves a variety of application approaches to overcome the interfacial adhesive forces by structure design and special surface film/coating fabrication.

Scientific Fundamentals

Mechanism

The stiction/adhesion phenomenon of two such surfaces is a consequence of the scaling law of a microsystem: the surface-to-volume ratio scales with the inverse of the microdevice dimensions and interfacial/surface forces of the micro/nanometer scale become dominant. The various interfacial forces include electrostatic force, intermolecular van der Waals forces , and meniscus capillary forces.

With the boom in development of micro/nano systems such as microelectromechanical system (MEMS) and nanoelectromechanical system (NEMS), stiction/adhesion between the substrate (usually silicon based) and the microstructure has become a dominant failure type. A variety of anti-adhesion/stiction approaches have emerged in the related fields of microelectronics, micro/nano tribology, and physical chemistry (Zhao 2003; Zhuang 2005; Delrio et al. 2005).

Stiction occurs when interfacial energy is higher than the mechanical restoring energy of the micro-structure after intimate contact. Actually, in addition to the interfacial adhesion forces, mechanical design of the microstructure, surface roughness of the substrate and the structure, and environmental conditions (relative humidity or temperature) all play an important role in stiction.

Anti-adhesion/stiction approaches are generally divided into three categories according to the physical causes of stiction: (a) structure design to reduce contact area or avoid intimate contact; (b) avoidance or reduction of meniscus capillary force; (c) surface modification to minimize the surface free energy of microstructure. The core concept is to avoid or decrease the dominant role of the interfacial adhesion interaction.

Structure Design

It has been proved that stiction may occur when flexible and smooth structures are brought in contact with the substrate (Zhao 2003; Zhuang 2005). Thus, mechanical design, including structure and surface topography, may be an easy way to avoid stiction failure. Firstly, contacting surfaces can be textured by a periodic array of small supporting posts, commonly known as dimples (Lee et al. 2003) to decrease the real contact area, thus reducing the total interfacial attractive forces. Secondly, the microstructure can be designed with a higher structural rigidity to obtain higher restoring energy (e.g., adopting a shorter and thicker beam instead of a flexible thinner one). Additionally, novel structure design can also be considered in real microsystems, such as a “ramps” structure in modern hard disk system, which will be discussed in more detail later.

Avoidance or Reduction of Meniscus Capillary Force

A thin liquid layer between two solid plates can result in significant adhesion forces. If the contact angle between liquid and the solid plates is less than 90° (hydrophilic surfaces), a net attractive force will act between the plates due to the liquid surface tension , namely capillary force (Zhuang 2005):

$$ {F_{{cap}}}(d) = \frac{{4\gamma r{{(\cos \theta )}^2}}}{{{d^2}}} $$
(1)

where \( \gamma \) is surface tension of liquid, \( \theta \) the contact angle on the solid plates, \( d \) the separation distance, and \( r \) the Kelvin radius given by Kelvin equation.

For microsystems, a thin liquid layer may result from the fabrication process or environment humidity during use. According to equation (1), effective anti-adhesion/stiction measures may include roughening or hydrophobic modification of the surfaces. The surface roughness can increase the minimum separation \( d \) and decrease the real contact area. The hydrophobic surface can increase the contact angle and prevent a water layer forming between the two surfaces. In engineering, many microdevices are designed to be encapsulated into an inert gas cell to avoid water condensation.

Surface Modification

It has been found that micro/nano-scale stiction is closely related to surface free energy of the used materials. The material surfaces with higher surface free energy have higher adhesion force and thereby higher tendency for stiction. Effective approaches may be to fabricate surface film/coatings with low surface-energy materials, such as a self-assembly monolayer (SAM) (Zhao 2003; Maboudian et al. 2000) or diamond-like carbon (DLC) coatings (Li et al. 2008).

The term SAM denotes a single layer of ordered special molecules adsorbed onto a substrate due to bondings between the surface and molecular head groups. The head groups of the SAM chemisorb on the substrate. The chains realize a hydrophobic property to reduce the surface energy significantly. Several classes of organic films have been explored. These include alkyi- and perfluoroalkytrichlorosilane SAMs, dichlorosilane- and alkene-based molecular films. Among these, the most widely used is the octadecyltrichlorosilane (OTS)-based film.

Key Applications

Anti-Adhesion/Stiction Approaches in Hard Disk Drives

In the context of hard disk drives, stiction refers to the tendency of a read/write head to stick to the platter. Stiction most likely occurs as a result of properties of the platters (smoothness and magnetic forces) as well as other forces including interfacial van der Waals force, the cohesion of lubricant thin film to form a meniscus, and so on.

Firstly, breakdown of lubricant thin film may cause the read/write head to stick to the platter. Today, with the increase of scanning speed and the decrease of flight height, the much tighter head-flatter space causes much higher internal operating temperatures, which often lead to an accelerated breakdown of the surface lubricants. On another hand, the several-nanometer head-flatter separation may also cause cohesion of the lubricant thin film, which may form a liquid meniscus of 10 nm (Izumisawa et al. 2002), much higher than the flight height. The resulting capillary force may cause severe stiction of the read/write head. Therefore, a suitable selection of lubricant (including surface free energy, molecular weight, and end groups) becomes more important for anti-stiction.

Another stiction case is that the read/write head “sticks” to the parking zone after shut-down of the computer. In 1995, a contact start/stop (CSS) technology was adopted in IBM to improve the anti-stiction performance. The CSS area was designed as a landing zone on the platter, usually near its inner diameter where no data is stored. The landing zone was made by producing an array of smooth nanometer-scale “bumps,” thus vastly decreasing the real contact area and, thus, the interfacial adhesion forces.

Today, modern hard drives have mostly solved this kind of stiction problem by a head unloading technology, which includes a “ramps” structure to “unload” the head from the disk surface during shut-down, similar to the concept of a “airport” for the head. These ramps ensure the head is not touching the platter when it stops or takes off to achieve to a high speed. This approach not only prevents stiction but also prevents abrasion from kicking up microscopic particulates that may later contaminate the drive mechanism.

Cross-References

Self-Assembled Monolayers

Surface Free Energy

Surface Tension

Van der Waals Forces