Asymmetric surface roughness measurements and meniscus modeling of polysilicon surface micromachined flaps

Abstract

Polycrystalline silicon (polysilicon) films are primary structural materials for microelectromechanical systems (MEMS). Due to relatively high compliance, large surface-to-volume ratio, and small separation distances, micromachined polysilicon structures are susceptible to surface forces which can result in adhesive failures. Since these forces depend on surface properties especially surface roughness, three types of microhinged flaps were fabricated to characterize their roughness and adhesive meniscus properties. The flaps enabled access to both the top and bottom surfaces of the structural polysilicon layers. Roughness measurements using an atomic force microscope revealed that MEMS surfaces primarily exhibit non-Gaussian surface height distributions, and for the release procedures studied, the bottom surface of the structural layers was significantly smoother and prone to higher adhesion compared to the top surface. A non-symmetric surface roughness model using the Pearson system of frequency curves was coupled with a capillary meniscus adhesion model to analyze the effects of surface roughness parameters (root-meansquare, skewness, and kurtosis), relative humidity, and surface contact angle on the interfacial adhesion energy. Using the measured roughness properties of the flaps, four different surface pairs were simulated and compared to investigate their effects on capillary adhesion. It was found that since the base polysilicon layer (poly0) was rougher than the base silicon nitride and the structural layer on poly0 was also rougher than that on silicon nitride, depositing MEMS devices on poly0 layer rather than directly on silicon nitride will reduce the adhesion energy.

An improved meniscus surface model for contacting rough surfaces

“…and the total adhesive force between two contacting rough surfaces is obtained by Table 2 Polysilicon surface on silicon nitride substrate [22] At equilibrium, an external force F separating the surfaces must be applied…”

“…The fabrication process of these samples and the details of the AFM measurements were described in Ref. [22]. Fig.…”

A Micro–Macroapproach to Predict Stiction due to Surface Contact in Microelectromechanical Systems

“…Surface sample C was considered in [35] to predict the adhesion energy resulting from a meniscus and results cannot be compared with our predictions as dry environment was assumed.…”

“…This justifies its study in the present paper. Non-exhaustive typical applications are the support-spring of comb-drive actuators [25] (in which stiction between comb teeth could be induced by mechanical shocks) or the micro-machined flaps [35] (in which stiction could occur between the movable plates and substrate).…”

“…The original purpose of this cantilever beams array [49,50] is to study the stiction phenomenon between polysilicon surfaces. In [35], micromachined flaps were fabricated using a three-layer polysilicon surface micromachining process; silicon wafers, deposited silicon nitride and alternating layers of low pressure chemical vapor deposition of sacrificial (silicon dioxide) and structural (polysilicon) layers. This process was followed by a released procedure; etching of the silicon oxide layer using 49% HF, DI water and isopropyl alcohol (IPA) rinses and finally drying.…”