High-Aspect-Ratio Micro-and Nanostructures:Simulation,Fabrication,and Application

High-aspect-ratio (HAR) micro- and nanostructures are of central interest for microelectromechanical systems (MEMS) fabrication and possess many potential traits for many applications, such as biosensers, photonic crystals, three-dimensional (3-D) microbatteries, and interconnection of 3-D integrated circuits (ICs). For example, an array of metals was formed through a silicon wafer in a high aspect ratio so that such through-silicon-metal-vias would interconnect multiple circuit layers for 3-D IC, which offers significant improvements over two-dimensional (2-D) IC on response time, integration density, and reliability. For another example, a dense array of HAR metal posts was fabricated to serve as the 3-D electrodes for 3-D microbatteries, which produce more energy and power than traditional 2-D planar electrodes do on a given footprint area while sustaining high discharge rates. However, the fabrication of higher aspect-ratio structures raises a lot of new challenges. For the interconnection of 3-D IC, an aspect ratio up to 100:1 is expected, which exceeds most current available etching techniques'ability. Moreover, the void-free electroplating to fill such deep and narrow through-holes is also quite challenging to most known electroplating techinuqes.Aiming at the 3-D microbatteries project, this paper studies several key techniques involved in HAR fabrication, including deep reactive ion etching (DRIE), photo-electrochemical etching of silicon, and electroplating. Five topics are discussed, including DRIE simulation, DRIE-based fabrication of HAR structures, photo-electochemical etching-based fabrication of HAR structures, fabrication of HAR metal structures, and application of HAR structures-3-D microbatteries.First, we develop a 3-D DRIE process model and integrate it into our voxel-based MEMS process simulation system to realize the 3-D simulation of DRIE. For the simulation of DRIE in nanoscale, a series of modifications and improvements have been made with respect to the gas transportation and microloading effect. The simulation results agree well with fabrication results. Secondly, we perform a series of experiments to study the influence of DRIE process parameters related to the etching of deep trenches, HAR through-silicon-vias, and nanoposts. To overcome the drawbacks of nanopatterning techniques and restrictions on their combinability with microfabrication, we develop an approach for the fabrication of micro/nano dual-scale structures by coupling the DRIE black silicon effect and profile-controllable DRIE process. By regulating etching/passivation parameters, the black silicon effect typical in DRIE is not only controlled but also utilized to realize a sophisticated surface treatment with respect to the characteristics of liquids and optics, which results in a superhydrophobic and anti-reflected dual-functional surface.Thirdly, to improve the etching uniformity and yield of photo-electrochemical etching, we have successfully obtained 5μm diameter and 500μm deep (aspect ratio =100:1) through-silicon-vias with almost 100% yield over the entire process area (>2cm2), by optimizing the process parameters and regulating the equipment. To overcome the restriction in the crystal direction of etching patterns, we use a new surfactant-added TMAH etching technique to fabricate the initial sharp pits for the subsequent photo-electrochemical etching, successfully obtaining several arbitrary shapes of HAR structures (non-<110> patterns).Fourthly, to overcome the trapped gas problem in HAR electroplating, we develop an intermittent vaccum degassing-assisted electroplating technique, which enables us to obtain high quality HAR (up to 100:1) metal filling without any defect.Finally, we study the design, fabrication, and packaging of a new 3-D microbattery-zinc-air microbattery. By using the improved photo-electrochemical etching and vacuum degassed electroplating, we have fabricated dense arrays of zinc electrodes with an aspect ratio up to 95:1. More importantly, the yield of electrodes is almost 100%.