Surface Adhesion (Stiction) in Encapsulated Silicon MEMS

Summary - We are undertaking a systematic experimental and theoretical evaluation of stiction between intermittently contacting silicon surfaces in an ultra-clean encapsulation process. This includes evaluating the magnitude of stiction forces, nature of sidewall contact, and the conditions and mechanisms that result in this behavior. The uniquely stable environment and lack of native oxide intrinsic to the wafer-scale epitaxial encapsulation (epi-seal) process are leveraged to enable reliable collision and contact models, which help build our understanding of stiction in this process. In addition, we are developing a series of dynamic mechanical anti-stiction solutions, and studying the mechanisms by which they mitigate stiction. Further investigation is being undertaken to examine stiction under various harsh environment conditions, such as high shock impact and elevated temperature. 

Introduction to Stiction in MEMS

Stiction, or sticking friction, is a fundamental issue in MEMS and NEMS technology that can pose a serious problem to any device with intermittently contacting surfaces. These types of contact may occur either by design, as in a micro/nano mechanical relay, or by accident during fabrication or device operation. Stiction and surface adhesion are especially problematic in microscale and smaller devices because of the high surface area to volume ratio. As a result of this scaling, surface forces that are typically too weak to consider at macroscale dimensions, (e.g. van der Waals forces) come to play a significant role, or even dominate at the microscale.  The problem is further exacerbated by typical design goals or restrictions requiring narrow transduction gaps, high displacements for increased sensitivity, or high shock survivability.

A vast array of forces may contribute to stiction in MEMS devices. Practically, however, a small number of forces are responsible for most stiction forces: capillary attraction, electrostatic force, hydrogen bonding, and van der Waals forces [4]. Van der Waals forces, in particular, are unavoidable, as they exist between any pair of surfaces in close proximity [5], but all of these forces are common in MEMS devices during fabrication or use.

Current Efforts and Results

In order to study the nature of the stiction forces in encapsulated MEMS devices, a series of test structures were designed. The test devices were designed with several key features: actuation to force a surface contact, defined contact geometry, measurement capability, and design within standard process rules. In addition, the test structures were designed to resemble a generic inertial sensor to maintain relevance to real devices. 

Figure 1. Electrostatically actuated stiction test structure

Results show that the stiction forces are not overwhelming compared to the typical forces in a MEMS device, and therefore that pure silicon-silicon contact does not result in irreversible attraction [1]. In addition, use of dynamic and Hertizian contact models suggests that the contact is dominated by a consistent distribution of asperities, rather than by the micron-scale contact areas. Repeated tests over several variations of the process shows that the results are consistent, and not highly sensitive to small changes. 

Figure 2. Measured stiction forces in encapsulated MEMS devices and
based on hot-switching measurements.

To overcome stiction forces, a series of dynamic bump stops have been developed and tested. These bump stops allow for reduced impact forces, and provide additional stored energy to promote release between two adhered surfaces. The results indicate that the dynamic bump stops significantly improve resistance to stiction related failures [2]. 

Figure 3. Spring anti-stiction bump stop test device

Upcoming Research Efforts

Efforts are continuing in developing more advanced understanding of stiction in encapsulated MEMS devices, and in developing additional strategies to combat these forces. We would like to better understand the stiction failures experienced by many MEMS devices at high impact loadings in excess of 1000g, and at high temperatures where typical chemical anti-stiction coatings degenerate. 

Figure 4. High temperature testing set-up for encapsulated MEMS devices.

[1] D. B. Heinz, V. A. Hong, E. J. Ng, C.-H. Ahn, Y. Yang, and T. W. Kenny, "Characterization of stiction forces in ultra-clean encapsulated MEMS devices," 2014 IEEE 27th International Conference on Micro Electro Mechanical Systems (MEMS), pp. 588-591, (2014)
[2] D.B. Heinz, V.A. Hong, T.S. Kimbrell, J. Stehle, C.H. Ahn, E.J. Ng, Y. Yang, G. Yama, G.J. O'Brien, and T.W. Kenny, "Stiction forces and reduction by dynamic contact in ultra-clean encapsulated MEMS devices," 2015 28th IEEE International Conference on Micro Electro Mechanical Systems (MEMS), pp. 393-396, (2015)