Publications

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The primary goals of the present study are to: (1) determine how and why MEMS-scale friction differs from friction on the macro-scale, and (2) to begin to develop a capability to perform finite element simulations of MEMS materials and components that accurately predicts response in the presence of adhesion and friction. Regarding the first goal, a newly developed nanotractor actuator was used to measure friction between molecular monolayer-coated, polysilicon surfaces. Amontons law does indeed apply over a wide range of forces. However, at low loads, which are of relevance to MEMS, there is an important adhesive contribution to the normal load that cannot be neglected. More importantly, we found that at short sliding distances, the concept of a coefficient of friction is not relevant; rather, one must invoke the notion of 'pre-sliding tangential deflections' (PSTD). Results of a simple 2-D model suggests that PSTD is a cascade of small-scale slips with a roughly constant number of contacts equilibrating the applied normal load. Regarding the second goal, an Adhesion Model and a Junction Model have been implemented in PRESTO, Sandia's transient dynamics, finite element code to enable asperity-level simulations. The Junction Model includes a tangential shear traction that opposes the relative tangential motion of contacting surfaces. An atomic force microscope (AFM)-based method was used to measure nano-scale, single asperity friction forces as a function of normal force. This data is used to determine Junction Model parameters. An illustrative simulation demonstrates the use of the Junction Model in conjunction with a mesh generated directly from an atomic force microscope (AFM) image to directly predict frictional response of a sliding asperity. Also with regards to the second goal, grid-level, homogenized models were studied. One would like to perform a finite element analysis of a MEMS component assuming nominally flat surfaces and to include the effect of roughness in such an analysis by using a homogenized contact and friction models. AFM measurements were made to determine statistical information on polysilicon surfaces with different roughnesses, and this data was used as input to a homogenized, multi-asperity contact model (the classical Greenwood and Williamson model). Extensions of the Greenwood and Williamson model are also discussed: one incorporates the effect of adhesion while the other modifies the theory so that it applies to the case of relatively few contacting asperities.

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Polycrystalline silicon (polysilicon) surface micromachining is a new technology for building micrometer ({micro}m) scale mechanical devices on silicon wafers using techniques and process tools borrowed from the manufacture of integrated circuits. Sandia National Laboratories has invested a significant effort in demonstrating the viability of polysilicon surface micromachining and has developed the Sandia Ultraplanar Micromachining Technology (SUMMiT V{trademark} ) process, which consists of five structural levels of polysilicon. A major advantage of polysilicon surface micromachining over other micromachining methods is that thousands to millions of thin film mechanical devices can be built on multiple wafers in a single fabrication lot and will operate without post-processing assembly. However, if thin film mechanical or surface properties do not lie within certain tightly set bounds, micromachined devices will fail and yield will be low. This results in high fabrication costs to attain a certain number of working devices. An important factor in determining the yield of devices in this parallel-processing method is the uniformity of these properties across a wafer and from wafer to wafer. No metrology tool exists that can routinely and accurately quantify such properties. Such a tool would enable micromachining process engineers to understand trends and thereby improve yield of micromachined devices. In this LDRD project, we demonstrated the feasibility of and made significant progress towards automatically mapping mechanical and surface properties of thin films across a wafer. The MEMS parametrics measurement team has implemented a subset of this platform, and approximately 30 wafer lots have been characterized. While more remains to be done to achieve routine characterization of all these properties, we have demonstrated the essential technologies. These include: (1) well-understood test structures fabricated side-by-side with MEMS devices, (2) well-developed analysis methods, (3) new metrologies (i.e., long working distance interferometry) and (4) a hardware/software platform that integrates (1), (2) and (3). In this report, we summarize the major focus areas of our LDRD project. We describe the contents of several articles that provide the details of our approach. We also describe hardware and software innovations we made to realize a fully automatic wafer prober system for MEMS mechanical and surface property characterization across wafers and from wafer-lot to wafer-lot.

A new type of surface micromachined ratcheting actuation system has been developed at the Microelectronics Development Laboratory at Sandia National Laboratories. The actuator uses a torsional electrostatic comb drive that is coupled to an external ring gear through a ratcheting scheme. The actuator can be operated with a single square wave, has minimal rubbing surfaces, maximizes comb finger density, and can be used for open-loop position control. The prototypes function as intended with a minimum demonstrated operating voltage of 18V. The equations of motion are developed for the torsional electrostatic comb drive. The resonant frequency, voltage vs. displacement and force delivery characteristics are predicted and compared with the fabricated device's performance.