Surface texturing and chemical treatment methods for reducing high adhesion forces at micromachine interfaces

Microelectromechanical systems (MEMS) is a relatively new field dealing with the design and manufacturing of miniaturized devices (micromachines) using techniques adapted mainly from the integrated-circuit industry. Micromachine fabrication comprises the growth, deposition, and selective removal of thin films. Early indications suggest that although significant opportunities exist for MEMS, there are several obstacles preventing the evolution of micromachines from the research environment to the application world. Among challenging problems is the identification and analysis of microscopic processes encountered at MEMS interfaces during fabrication and operation that often render the devices dysfunctional. The development of high adhesion forces at micromachine interfaces during release-etch drying and/or operation often leads to permanent adhesion of contacting surfaces, a phenomenon referred to as stiction, hence affecting the micromachine yield and operation life. In this publication, an appraisal of the important issues involved in micromachine stiction is presented, accompanied by an assessment of the contribution of various surface forces (e.g., van der Waals, capillary, electrostatic, and asperity deformation forces) to the total stiction force arising at MEMS interfaces. The critical micromachine stiffness required to overcome stiction is interpreted in terms of the composite surface roughness and material properties. In addition, various surface modification techniques compatible with standard surface micromachining, such as surface roughening (texturing) and deposition of low surface energy films (e.g., diamond-like carbon coatings and self-assembled monolayer films) are presented, and their efficacy to reduce etch-release and in-use micromachine stiction is discussed in light of experimental and analytical results.