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Surface Coatings Solve Drift Problems in MEMS

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One of the barriers to full commercialization of complex microsystems is reliability. Among a number of process related issues, the difficulty in controlling surface forces is a critical impediment to the fabrication and operation of MEMS devices. This is a consequence of the scaling law. The surface-area to volume ratio scales with the inverse of device dimension. As a result, there are a number of reliability problems related to surfaces; one of these is Drift.

Drift
is a common but hard to deal with problem. As the MEMS industry matures and more MEMS products come to the market place, how to avoid or control drift in device performance becomes a pressing issue. One source of drift is due to charging. which usually leads to instantaneous changes in driving voltage. Another long term problem is changes in surface characteristics. We focus on the latter problem here.

One can identity at least two surface processes that may contribute to drift in device performance. The first is the effect of adsorption on electrostatic actuation. Key MEMS materials, such as silicon and silicon nitride, are terminated by surfaces with high surface energy. This may lead to the accumulation of background molecules, such as water. The presence of such a thin film changes the dielectric properties and thus electrostatic actuation. Consider a parallel-plate capacitive micro-actuator. The normal stress s for a given driving voltage on the capacitive plate is given by:

electrostatic actuation

where n is the dielectric constant; V is the applied voltage; d is the distance between the two plates without bias voltage; and x is displacement from equilibrium position. The uncontrolled adsorption of thin molecular films on the surface changes the distance-dependent profile of dielectric constant and thus, the stress-voltage relationship. One can estimate the effect as follows: for a gap distance of d = 1 mm and with the adsorption of a 1 nm thick adsorbate film, the maximum drift in driving voltage to give the same normal stress is 0.1%. In reality, the drift is significantly smaller because n is much smaller than the maximum value, i.e., infinity.
The second and more important effect is adsorption induced change in surface stress. The surface free energy or interfacial tension, g, is a function of molecular concentration as given by the Gibbs adsorption equation. Surface stress (s) is related to g as given by the Shuttleworth equation:

surface stress

where e is the elastic surface strain. Change in s with adorbate concentration is so significant that it is actually one of the most sensitive mechanisms for chemical sensing. Both experiment and theory have shown that the change in surface stress upon the chemisorption of one monolayer of atoms/molecule can be as high as a few Newtons per meter. For a typical poly-Si cantilever with thickness of 1-2 micrometer, this magnitude of change in surface stress can lead to bending on the order of micrometer. In addition to surface stress, adsorption can also induce substantial albeit slow chemical changes, such as oxidation in the near surface region of a microstructure. This leads to longterm changes in stress and spring constants. Therefore, uncontrolled adsorption onto the high-energy surface of microstructures can lead to drift in surface stress and spring constants. This is likely the main reason for drift in device performance.

To solve the drift problem induced by surface adsorption, one needs to control the surface chemical properties. This can be achieved by the formation of low-energy, passivation coatings on the surface. With this goal in mind, we have developed a number of surface coatings with the following characteristics: covalently bonded monolayer, low surface energy, thermally and mechanically stable. The design principle of the coating process is illustrated in scheme 1. A key component in the coating chemical is a molecule containing two major parts, R & X. The R group is selected to provide low surface energy, i.e., "wax" or "Teflon" like, while the X group is chosen to selectively react with the solid surface of interest for covalent linkage. The attachment of these molecules to the solid surface is a specially designed process which provides kinetic control; the reaction self-terminates after a saturated monolayer coverage is reached. This is very different from self-assembled monolayers (SAMs). Such a selective and kinetically controlled reaction ensures that the coating is uniform and conformal with solid surfaces in a MEMS device. The thickness of the coating is chosen to be 1-2 nm.



Scheme 1. Design of low energy surface coating for MEMS devices. R represents a molecular group to give low surface energy and X is a functional group for selective attachment to the solid surface.


Based on these principles, we have developed a number of surface coatings for MEMS applications. Coating 1: This coating is applied via gas exposure in a CVD (chemical vapor deposition) type of reaction for pre-released MEMS devices. It is ideally suited for bulk-micromachined devices or surface micromachined devices obtained from dry etch-release. This coating process is compatible with existing equipment, such as cluster tools, on a fab-line. The coating is typically applied before packaging. For some applications, the coating process may be combined with the packaging step. Typical water contact angle of the coated surface is in the range of 94 ­ 100o, indicating the hydrophobic nature of the surface. The coatings are thermal stability at temperatures as high as 400oC for brief heating in air or long term heating in inert environment. Coating 2: This coating is optimized for thermal stability and is applied in the liquid phase to a variety of MEMS materials. It is designed for MEMS devices obtained from aqueous HF etching. After liquid release-etch, the sample is transfer to a coating liquid for the application of the monolayer coating. The reaction is again kinetically controlled to achieve conformal coating and monolayer coverage. The MEMS device is dried after the completion of the coating reaction. Surface energy and thermal stability are both similar to those of coating 1. Coating 3: This coating is specifically designed for silicon based MEMS devices obtained from aqueous HF release etch. It is optimized for exceptionally low surface energy (water contact angle in the range of 110 ­ 125o) and low friction coefficient (~0.01). It is ideally suited for p-Si based devices from surface micromachining. After liquid release-etch, the sample is transferred to a liquid for the application of the monolayer coating. The reaction is kinetically controlled to achieve conformal coating and monolayer coverage. The MEMS device is dried after the completion of the coating reaction. This coating is thermally stable in air at temperature < 200oC. In inert atmosphere or vacuum, the coating is expected to withstand temperatures as high as 400oC.
These passivation coatings have been successfully applied to two MEMS products for optical applications. Drift in driving voltage is significantly reduced or eliminated after the application of the low energy passivation coatings.


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