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Anti-Stiction Coatings in MEMS Devices

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There have been many exciting predictions that the future of micromachines or microelectromechanical systems (MEMS) is just "around the corner", but this future has proven to be slow in coming. Despite the demonstration of numerous MEMS devices and product concepts each year, a very small number have actually succeeded in the market place. Two prominent examples are the digital mirror device (DMD) of Texas Instruments, Inc. and the MEMS accelerometer of Analog Devices, Inc. While these two products are vastly different, one can identify a common ingredient in their formula of success: both companies successfully developed special surface coating technologies [1, 2]. Indeed, the difficulty in controlling surface forces is a critical impediment to the fabrication and operation of many MEMS devices [3, 4]. This is a consequence of the scaling law: the surface-to-volume ratio scales with the inverse of device dimension and surface forces dominate at length scales < 1mm.

A well-known problem in the fabrication of MEMS devices from surface micromachining is stiction, which occurs when surface adhesion forces are higher than the mechanical restoring force of the micro-structure. When a device is removed from the aqueous solution after wet etching of an underlying sacrificial layer, the liquid meniscus formed on hydrophilic surfaces pulls the microstructure towards the substrate and stiction occurs. While this release-stiction problem may be alleviated by dry HF etching or supercritical CO2 drying, a more difficult problem is in-use stiction which occurs during operation when microstructures come into contact (intentionally or accidentally). In-use stiction may be caused by capillary forces, electrostatic attraction, and direct chemical bonding. To circumvent the stiction problem, many MEMS developers are forced to switch to bulk micro-machining, which is less capable and versatile than surface micromachining in terms of device function. Even for devices from bulk-micromachining, in-use stiction is still of concern, perhaps to a lesser extent.

One attractive approach to tackle the stiction problem is to provide low-energy surface coating in the form of an organic passivation layer on the inorganic surface [3, 4]. Such a coating can not only eliminate or reduce capillary forces and direct chemical bonding, but also reduce electro-static forces if the thin organic layer is directly applied to the semiconducting substrate, without the intervening oxide layer [3,4 ]. Texas Instruments uses a fluorinated fatty acid self-assembled monolayer (SAM) on the aluminum oxide surface in their DMD [1], while Analog Devices coats the surfaces of their inertia sensors using thermal evaporation of silicone polymeric materials at the packaging stage after the device is completely released. Another much advocated approach is the formation of siloxane self-assembled monolayers (SAMs) on the oxide terminated surface, but the difficulty of this chemistry and the poor reproducibility put significant limitations on its practical usage.

In view of the critical importance of anti-stiction coatings in MEMS products, we are developing a number of chemical processes that possess the following attributes: (1) the chemistry is simple and reproducible; (2) the coatings are of monolayer nature and are covalently bonded to the substrate; (3) the coating processes are compatible with dry or aqueous etching processes; (4) the monolayers are chemically and mechanically stable under conditions of processing and operation. 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.

Figure 1 shows a group of poly-Si cantilever beams completed released by the above coating process. The cantilever beam arrays (CBAs) are fabricated by the SUMMiT process of Sandia National Laboratory. A cross section of the structure is shown schematically in the upper part of the figure. The poly-Si beams, with thickness of 2.25 mm, are anchored at one end. The sacrificial PSG layer under each beam is ~2 mm thick. The bending of p-Si beams on each CBA sample is characterized by interference microscopy. Out-of-plane bending of the beams is shown as optical fringes; each fringe corresponds to the bending of l/2 or 311 nm. The lower part of figure 1 shows typical images of beam arrays (2 mm in length) with alkoxyl coating. All beams are completely released. The fringes reflect (downward) bending due to residual stress. The maximum bending is ~1.8 mm. Therefore, the above monolayer coating process eliminates release stiction. Electro-static actuation of these coated beams also reveals little in-use stiction with relative humidity as high as 90% [5].

  Fig. 1. Upper: schematic illustrate of a cantilever; lower: Interference microscope image of cantilever beam arrays with beam length of 2 mm. The image (composite of three frames) is obtained after the beams are released in an all-liquid etching-coating process.

REFERENCES:
[1] L. J. Hornbeck, US Patent 5602671 (1997).
[2] J. R. Martin and Y. Zhao, US Patent 694740 (1997).
[3] M. P. de Boer and T. M. Mayer, MRS Bulletin, 26, 302-304 (2001).
[4] R. Maboudian, Surf. Sci. Rep. 30, 207-269 (1998).
[5] Y. S. Jun, X.-Y. Zhu, J. Adh. Sci. Technol. (special MEMS issue) 17, 593-601 (2003)


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