Laser micromachining of aluminum films on glass substrates is investigated using a time-resolved transmission imaging technique with nanosecond resolution. Micromachining is performed using a 7 ns pulse-width Nd:YAG laser operating at the 1064 nm wavelength for fluences ranging from 2.2 to 14.5 J/cm 2 . Transmission imaging uses a nitrogen laser-pumped dye laser with a 3 ns pulse-width and 500 nm wavelength. Images are taken from the back of the sample at various time delays with respect to the beginning of the ablation process, allowing the transient hole opening process to be observed and measured. Results show that for high fluences the holes begin opening during the laser pulse and that the major portion of the holes have opened within the first 50 ns of the process. The second stage of the process is slower and lasts between 100–200 ns. The rapid hole opening process can be attributed to melt expulsion due to recoil pressure on the surface of the melt pool rather than Marangoni flow. Recoil pressure may be due to vaporization at the free surface at low fluences and phase explosion (explosive liquid-vapor phase change) at higher fluences. Measurements of the transient shock wave position are used to estimate the pressure behind the shock wave and indicate pressures at high as 89 atm during ablation. The high pressure above the laser spot results in pressure on the molten surface, leading to expulsion of the molten pool in the radial direction.

Wicking materials with tunable wettability are of great importance for both fundamental research and practical applications such as heat pipes. In this work, we adopt recently developed titanium bulk micromachining[1] techniques to fabricate pillar arrays. Then we modify the micromachined pillars to form micro- & nano-textured (bitextured) titania structures (BTS). Further, we investigated how to plate gold on the modified surfaces to tune the wettability. A wicking material for heat pipe requires super wetting by common fluids such as water. We show theoretical studies and experimental work to investigate the wetting behavior of two different designs/samples. For heat pipe applications the BTS and plating gold not only increases the capillary pressure which enhances liquid pumping from condenser to evaporator, but also increases the heat transfer performance by extended surface and smaller pore sizes[2]. Testing results show that water can completely wet the micromachined Ti pillars (Design A: 5μm in diameter/5μm gap). The BTS helps increase the wetting speed by over 100% for this design. A second design with much larger diameter and gap (Design B: 100μm in diameter/50μm in gap) is also tested to compare with design A for wetting speed. Results show that Design B gives a wetting speed twice of Design A. Plating method is used to decrease pillar gap (from 50μm to 5μm) by growing gold on surfaces. This will help increase thermal conductivity of wicking material which is preferred for the evaporator and condenser regions of heat pipes. Wetting experiment is done on Sample B after plating with gold. Wetting results after Au plating show that wetting velocity decreases but is still significantly large.

Due to the high power density and local temperature increase on nanoscopic asperities of solid metal contacts, traditional MEMS contact switches suffer from contact welding, pitting, electromigration and oxidation. Particularly, when MEMS switches are used to handle high power, solid metal contacts pose serious limitation on the contact reliability. A self-healing RF MEMS switch, which utilizes liquid gallium contacts to take the place of the traditional solid metal-to-metal contacts, is proposed in this paper. Electrostatic actuation is used to drive a silicon nitride bridge with upper electrodes. When the bridge is pulled down, liquid gallium droplets work as an interface between the upper and lower contact electrodes. The loss of the gallium droplets can be avoided due to the unwettability of the material surrounding the contact electrodes. The switch is fabricated using a surface micromachining process. A coupled-field finite element analysis (FEA) is used to model the electric current, heating and thermal conduction of the contacts. The model includes deformable gallium droplets with 4 μm base diameter. The two sides of the droplets are connected to the upper and lower solid metal contact electrodes, respectively. By using the FEA models, the electric and thermal characteristics of the gallium droplets featuring a variety of geometric parameters have been studied. 1 A current handling capability of the liquid gallium contact is verified by the FEA models.

The thermal properties of microelectromechanical systems (MEMS) devices are governed by the structure and composition of the constituent materials as well as the geometrical design. With the continued reduction of the characteristic sizes of these devices, experimental determination of the thermal properties becomes more difficult. In this study, the thermal conductivity of polycrystalline silicon (polysilicon) microbridges are measured with the transient 3ω technique and compared to measurements on the same structures using a steady state joule heating technique. The microbridges with lengths from 200 microns to 500 microns were designed and fabricated using the Sandia National Laboratories SUMMiT™ V surface micromachining process. The differences between the two measurements, which arise from the geometry of the test structures, are explained by bond pad heating and thermal boundary resistance effects.