ANTI-STICTION COATINGS FOR HIGH RELIABILITY MEMS

Transcription

1 ANTI-STICTION COATINGS FOR HIGH RELIABILITY MEMS Introduction Nilesh Gunda, Santosh K. Jha and Suri A. Sastri Surmet Corporation, 33 B Street Burlington, MA (USA). Micro-electromechanical system (MEMS) is a rapidly growing technology with a forecasted annual growth rate exceeding that of the semiconductor electronics industry. Several studies have projected the MEMS market size in tens of billions of dollars in a few years from now. Micro-electromechanical systems (MEMS) are very small, integrated devices that combine electrical and mechanical components and replicate the structure and function of meter-scale devices used in day-to-day life. They range in size from the submicron to the millimeter level. Inkjet-printer cartridges, accelerometers, miniature robots, microengines, inertial sensors, micromirrors, microactuators, optical scanners, fluid pumps, and transducers are some of the examples of current day MEMS device applications. Newer applications are emerging as the existing technology is applied to the miniaturization and integration of conventional devices. Figure 1 shows the secondary electron micrographs (SEM) of some MEMS devices. (a) (c) c (d) Figure 1: Examples of MEMS devices; (a) Micro-machined gear, Micro-mirrors, (c) Electrostatic lateral output motor, and (d) Single-crystal Si micro-tweezer. Despite the exciting growth predictions for the future of MEMS technologies, the difficulty in controlling surface forces is a critical impediment to the fabrication and operation of many MEMS devices. Surface phenomena such as wear and stiction

2 (permanent adhesion) of the micro moving parts, often restrict the operational environment and limit the lifetime of these devices. Protecting MEMS against friction, wear and stiction is thus major challenge. These problems occur in both air and vacuum, although the extent of degradation and specific mechanism may differ from one application to the next. Relative humidity significantly affects both wear and stiction properties of MEMS devices. Either very low or high relative humidity has adverse effect on the performance of these components. Coating requirements for MEMS As mentioned above, stiction is a major problem in MEMS devices. Stiction (i.e., unintentional adhesion) occurs when surface adhesion forces (viz., capillary, van der Waals and electrostatic) are higher than the mechanical restoring force of the miniaturized surfaces of the structural components. As a result of stiction, surfaces can permanently adhere to each other causing device failure- a phenomenon known as in-use stiction. Micro-machining can address the stiction problem to some extent, but is not an optimal solution because of the possible adverse effect functionality of the device. In order to alleviate the stiction-related problems without compromising with functionality of MEMS device, both the topography and the chemical composition of the contacting surfaces must be controlled. The most effective approach to achieve this would be to develop suitable surface coatings technology for MEMS. The application of low-energy surface coatings would be required to eliminate or reduce capillary, chemical bonding and electrostatic forces between the contacting microstructure surfaces of the MEMS device. For a coating system to be suitable for the MEMS application, it needs to have the following characteristics. The coating should have excellent adhesion on the surfaces of the micro-components; it should be hydrophobic in nature and thin enough not to bridge structural features of the MEMS device. The coating also has to permeate through microscopically small openings and diffuse onto under surfaces. In addition, the coating film should have stability in MEMS operating environments (viz. air and vacuum); and it should retain its lubricating properties for times longer than required for a particular application to avoid premature failure of the devices. This is specially critical in MEMS devices for biomedical applications. Efforts had been underway in big way to develop anti-stiction and wear resistant coatings for MEMS applications. Anti-Stiction MEMS Coatings Surmet Corporation has developed an innovative non line of sight coating application technique to alleviate stiction problems in MEMS devices. Surmet has been collaborating with Scientists at the Wright-Patterson Air Force Research Labs (WPAFB) who have the capability to evaluate performance of MEMS devices with and without surface treatments. Modifications had to be made to Surmet s high performance UltraC TM coating technology, which is a low stress DLC type ultra hard amorphous carbon coating with unique characteristics. The process modification allowed the coating to permeate through microscopically small openings and diffuse onto under surfaces, all the while maintaining excellent adhesion to the MEMS surfaces and providing ultra low friction and excellent anti-wear properties. With this approach Surmet has successfully coated devices to provide both the wear and anti-stiction properties to moving microstructures in MEMS. The so-called anti-stiction MEMS coating was applied by Surmet s conformal-plasma induced chemical vapor deposition (CPI-CVD) process to deposit a controlled thickness coating. Surface morphology of the DLC coated Si substrates are presented in Figure 2. Surmet s modified coating process has been exploited to coat several functional MEMS

3 device structures as shown in Figures 3 and 4. The coated components were evaluated for electro mechanical performance at the Air Force Labs. The devices performed much better in both air and vacuum environments as shown in Figure 5. The coating had not only imparted the wear resistance to the component, but was also able to significantly alleviate the stiction related problems. This coating exhibits a very low friction coefficient (0.05) and reduces the wear of polysilicon substrate to negligible levels compared to uncoated polysilicon. Surmet has also used this novel process termed C-MEMS to deposit thin and thick permeating fluorocarbon based coatings with contact angles exceeding 110 degrees, which would be considered strongly hydrophobic. Efforts are underway to collaborate with US Air Force and a major commercial MEMS producer to refine the coating technology and integrate it with MEMS fabrication technology. (a) Figure 2: Anti-stiction carbon coatings for MEMS application; (a) 59 nm DLC on (100) Si substrate, and 59 nm DLC on polycrystalline Si substrate. Figure 3: DLC coating on a MEMS Electrostatic Lateral Output Motor

4 (a) (c) Figure 4: Coating on MEMS Devices; (a) Die coated with ~60 nm of Ultra C (devices are stuck), die coated with ~15 nm Ultra C (devices are released), and (c) Device with broken slider showing Ultra C coating underneath. Lifetime (min.) A: Uncoated in Air B: Uncoated in Vacuum C: UltraC-100nm D: UltraC-15 nm A B C D Condition Figure 5: Performance of electrostatic lateral output motors with Surmet s coatings. Coated motors performed much longer, compared to uncoated ones in both air and vacuum. Surmet s efforts in this area has demonstrated that solid lubricant hard coatings with the ability to penetrate intricate side wall and under surface spaces in 3D, are capable of extending the operating life of MEMS devices. The ultra thin and lubricious carbon

5 coatings are also effective in high humidity environments where normal DLC coatings fail. This could have important ramifications in the future for MEMS industry. In addition, these hard coatings provide a functionally superior surface for bound and mobile monolayers, which have potential to further increase life of MEMS devices here on earth and in space environments. Acknowledgement The authors are grateful to Dr. Jeff Zabinsky and his Tribology group at the Wright- Patterson Air Force Laboratory (WPAFB) for their SBIR funding support and technical collaboration during this research program. For more information: Dr. Suri Sastri, Surmet Corporation, 33 B Street, Burlington, MA (USA); Tel: (781) ; Fax: (781) ; Website: