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We are developing innovative heat pipes based on Nano-Structured Titania (NST) with a potential for high heat carrying capacity and high thermal conductivity. These heat pipes have a flat geometry as opposed to a cylindrical geometry found in conventional heat pipes. The flatness will enable a good contact with microprocessor chips and thus reduce the thermal contact resistance. We refer to it as a Thermal Ground Plane (TGP) because of its flat and thin geometry. It will provide the ability to cool the future generations of power intensive microprocessor chips and circuit boards in an efficient way. It also brings the potential to function in high temperature (>150°C) fields because of its high yield strength and compatibility [1]. The TGP is fabricated with Titanium. It adopts the recently developed high aspect ratio Ti processing techniques [2] and laser packaging techniques. The three main components of the TGP are 1) a fine wick structure based on arrays of high aspect ratio Ti pillars and hair like structures of Nano-Structured Titania (NST), 2) A shallow Ti cavity welded onto the wick structure and 3) the working fluid, water, sealed between the cavity and the wick. The heat carrying capacity and the thermal conductivity of a heat pipe are generally determined by the speed of capillary flow of the working fluid through its wick. The TGP wick has the potential to generate high flow rates and to meet the growing challenges faced by electronics cooling community. The TGP wick structure, developed by etching high aspect ratio pillars in a titanium substrate and growing nano scale hairs on the surface of the pillars, is super hydrophilic and capable of wicking water at velocities ∼ 10−2 m/s over distances of several centimeters. The thermal conductivity of the current TGP device was measured to be k = 350 W/m·K. The completed TGP device has the potential of attaining a higher conductivity by improving the wicking material and of carrying higher power density. Washburn equation [3] for dynamics of capillary flow has been employed to explain the results of our experiments. The experiment shows a good agreement with Washburn equation.

Micromachined titanium pillars with nanoscale titania structures on surfaces are adopted to form wicking material for a proposed flat heat pipe system. This unique bitextured titania structure (BTS) provides a suprerhydrophilic surface and improves the wicking ability of the titanium based pillar arrays. Experimental studies on optimizing the wicking behaviors of BTS pillar arrays are reported in this work. Oxidization and plating techniques are applied to modify the surface properties and pillar/gap dimensions of the wicks. A titanium based flat heat pipe is proposed to integrate the BTS wicks and study the concept of this titanium based flat heat pipe architecture. A thermal conductivity ~500W/m-K (with contact resistance included) is achieved at around 118°C. The chamber of this Ti heat pipe is formed by hermetically welding two substrates together using laser welding technique. The hermiticity performance of the chamber is studied with helium leaking approaches. A leaking rate of 3.4x10 -10 atm.cm 3 /s is achieved for a chamber of ~170μL in volume.

This paper reports a new packaging method for a wide range of MEMS applications on both the wafer and device scale. Titanium is used as the packaging material in this work and both Si-MEMS and Ti-MEMS devices are integrated into a 350 µm titanium substrate. A Nd:YAG pulsed laser is used as a localized heating source to micro-weld a 350 µm titanium cap to the substrate. Simulation of the heat conduction of a two-dimensional time-dependent pulse laser between the substrate and cap was carried out using COMSOL to investigate pulsed laser melting properties. To avoid thermal distortion of the welding, several geometries at the cap and substrate interface were investigated to minimize laser intensity in order to achieve the required melting depth.