Publications

Abstract not provided.

Thermal actuators have proven to be a robust actuation method in surface-micromachined MEMS processes. Their higher output force and lower input voltage make them an attractive alternative to more traditional electrostatic actuation methods. A predictive model of thermal actuator behavior has been developed and validated that can be used as a design tool to customize the performance of an actuator to a specific application. This tool has also been used to better understand thermal actuator reliability by comparing the maximum actuator temperature to the measured lifetime. Modeling thermal actuator behavior requires the use of two sequentially coupled models, the first to predict the temperature increase of the actuator due to the applied current and the second to model the mechanical response of the structure due to the increase in temperature. These two models have been developed using Matlab for the thermal response and ANSYS for the structural response. Both models have been shown to agree well with experimental data. In a parallel effort, the reliability and failure mechanisms of thermal actuators have been studied. Their response to electrical overstress and electrostatic discharge has been measured and a study has been performed to determine actuator lifetime at various temperatures and operating conditions. The results from this study have been used to determine a maximum reliable operating temperature that, when used in conjunction with the predictive model, enables us to design in reliability and customize the performance of an actuator at the design stage.

Low energy electron microscopy (LEEM) is used to investigate the dynamics of Pb overlayer growth on Cu(100). By following changes in surface morphology during Pb deposition, we measure the amount of Cu transported to the surface as the Pb first alloys into the surface during formation of the c(4 × 4) phase and subsequently dealloys during conversion to the c(2 × 2) phase. We find that the added coverage of Cu during alloying is consistent with the proposed model for the c(4 × 4) alloy phase, but the added coverage during dealloying is not consistent with the accepted model for the c(2 × 2) phase. To account for the discrepancy, we propose that Cu atoms are incorporated in the c(2 × 2) structure. Island growth and step advancement during the transition from the c(2 × 2) to c(5√2 × √2) R 45° structure agrees with this model. We also use LEEM to identify the order and temperature of the two-dimensional melting phase transitions for the three Pb/Cu(100) surface structures. Phase transitions for the c(5√2 × √2) R 45° and c(4 × 4) structures are first-order, but the c(2 × 2) transition is second-order. We determine that rotational domains of the c(5√2 × √2) R 45° structure coarsen from nanometer- to micron-sized dimensions with relatively mild heating (∼ 120°C), whereas coarsening of c(4 × 4) domain requires considerably higher temperatures (∼ 400°C). In studies of three-dimensional island formation, we find that the islands grow asymmetrically with an orientational dependence that is directly correlated with the domain structure of the underlying c(5√2 × √2) R 45° phase.

Abstract not provided.

The authors use low energy electron microscopy to identify a correlation between the growth shape of three-dimensional Pb islands on Cu(100)and the domain structure of the underlying Pb overlayer. Deposition of 0.6 monolayer Pb on Cu(100) produces a compressed c(2x2) overlayer, designated c(5{radical}2x{radical}2)R45{degree}, with periodic rows of anti-phase boundaries. They found that heating the surface to temperatures above 100 C coarsens the orientational domains of this structure to sizes that are easily resolved in the low energy electron microscope. Three-dimensional Pb islands, grown on the coarsened domains, are found to be asymmetric with orientations that correlate with the domain structure. Once nucleated with a preferred growth orientation, islands continue to grow with the same preferred orientation, even across domain boundaries.