Better MEMS, NIL with surface engineering
11/01/2007
Advanced thin-film deposition methods and tools promise to facilitate device production and operation
BY BORIS KOBRIN, APPLIED MICROSTRUCTURES
“Stickiness” may be a sought-after attribute in Website design, but stiction and adhesion are major problems for performance of microelectromechanical systems (MEMS) and production in nanoimprint lithography (NIL). Although organic monolayer films have proven to solve the problem, traditional methods of deposition introduce their own complications. Now an advancement in coating creation promises to get MEMS performance and NIL production “unstuck”-and facilitate their uptake, too.
The vapor solution for MEMS
Thin organic monolayer films have commonly been used to reduce surface energy in micro-structures and thereby improve the devices’ performance and reliability. Most notably, a film layer just a few angstroms thick can reduce failures attributed to “release” and “in-use” stiction-the adhesion of compliant micromechanical structure surfaces in close proximity-by orders of magnitude. Such anti-stiction coatings are self-assembled monolayers (SAMs) that can uniformly coat such complex structures as high-aspect-ratio comb drives in MEMS inertial sensors, areas under the mirrors in MEMS displays, and membranes of MEMS microphones.
The conventional method of SAMs deposition, from a liquid phase, requires a sequence of wet chemical reactions performed in a wet station. It is expensive and environmentally unfriendly involving use (and waste) of expensive solvents, and it’s unsafe for operators because of the possibility of direct contact with chemicals. On the other hand, vacuum processing and the vapor-phase deposition, implemented by methods such as Applied Microstructures’ Molecular Vapor Deposition (MVD), have proven to produce higher-quality films thanks to tight control of moisture in the process environment.
Ultra-thin hydrophobic coatings repel moisture, which is the main cause of the capillary stiction in MEMS devices. Some nanofilms can also lubricate surfaces to avoid device failures caused by mechanical impact of movable micromechanical parts.
Low-surface-energy coatings have enabled defect-free operation of MEMS inkjet nozzles in industrial printing applications. Passivation of inkjet nozzle faceplates with such coatings prevents contamination with ink, which reduces printing defects and enhances inkjet nozzle operational lifetime. For other microfuidic devices (e.g., lab-on-a-chip and biosensors) MVD systems have enabled surface modification of microchannels. Further, MVD technology has recently demonstrated passivation of buried microchannels up to 6 meters with controllable wetting coatings. The capability to coat microchannels buried in a fully assembled device implies simplified manufacturing and cost reduction.
Breaking NIL’s mold-resist bond
Use of MVD for creation of release layers for nanoimprint lithography (NIL)-an emerging application-has already demonstrated yield-improvement and cost-reduction advantages in development labs worldwide. NIL, a printing process, requires mechanical contact between mold and resist (polymer material), thus resist adhesion to the mold is one of its main challenges. Resist remaining on the mold creates a defect that affects not only the substrate from which it is pulled, but also all subsequently printed substrates because of an air gap formed between a mold and a substrate.
The main approach to overcoming the problem is to apply a low-surface-energy coating to the mold surface, which can dramatically reduce adhesive forces between mold and resist materials. Imprint molds coated with a thin and conformal nano-layer perform hundreds of replication cycles without recoating and provide high fidelity of sub-50nm features.
“Ruggedizing” MEMS
Yield and operating life of MEMS devices depend on survivability of surface modification coatings, especially where mechanical impacts, thermal treatments, or exposure to chemicals are involved. This is the case, for example, for MEMS displays where movable mirrors touch the substrate; micro-engines (e.g., used as actuators for micro-needles); and MEMS microvalves and micropumps where gears experience strong friction impact. In MEMS inkjet nozzles, a faceplate modified with MVD passivation coating can prevent defects caused by dried ink contamination.
The durability of MVD coatings has been improved using layers deposited in-situ that enhance the adhesion of the coating to the substrate material and thus increase the density of the bonding sites. Such coatings have proven stable for periods of many weeks while immersed in DI water and different types of inks, and in thermal treatment up to 400ºC in air. Their durability has been tested using inkjet dry wipe test (HP-990 Maintenance Blades tester) and shown stable for tens of thousands of wipe cycles.
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Although silicon is an excellent structural material for MEMS, small area contacts (e.g., between rough polycrystalline surfaces) can generate sufficiently high contact pressures to create plastic deformations that lead, in turn, to significant wear. Recently we have found that carbon-doped Al2O3 deposited via MVD provides, in sidewall wear test microstructures, the highest degree of wear prevention documented to date. The test structures, based on a double-comb drive design, were fabricated in the Sandia SUMMiT V process. One set of combs is used to apply a static load on a beam, which is pulled against a fixed post; the other set is used for tangential actuation to rub the beam against the fixed post. The coated microstructures have survived without failure for over 7x106 cycles-seven times longer than the SiC-coated structures tested in a similar way (see table, below).
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Tools for molecular vapor deposition
Applied Microstructures’ MVD-100 tool, designed for R&D and small-batch production, launched in 2004 (40 units are installed worldwide). Two years later, the company introduced its MVD-150, a production tool with batch-processing capabilities for high-volume MEMS manufacturing.
The MVD-150 provides factory automation (supporting the SECS/GEM communications protocol), a robotic interface, and a wide range of processing capabilities. These features combine to reduce labor, improve throughput and yield, and enhance device-to-device uniformity; and vacuum processing, especially, helps eliminate safety concerns associated with handling solvents and corrosive chemistries. And, it accommodates standard cassettes of 25 8-inch wafers and, alternatively, a custom jig for shaped products such as fully assembled inkjet nozzles, packaged MEMS or IC chips, or a cassette of nanoimprint stamps.
 Applied Microstructures’ MVD-150 production system. |
Finally, the machine is poised to facilitate the transition of MEMS from mainly industrial markets to higher-volume consumer markets. For one thing, it allows designers their choice of a wide range of nanofilms and substrate materials for advanced-device creation. Use of plastic and metal materials instead of silicon in MEMS is expected to drive down manufacturing costs. And, coating-enabled improvement of MEMS operation and NIL-stamp operating life will result in greater reliability and wide availability of yet smaller devices.
Dr. Boris Kobrin is senior director of marketing and business development, Applied Microstructures Inc. You can reach him at tel: (408) 907-2885 or e-mail at [email protected].