About My Research
To date, my research has primarily focused on the surface engineering of semiconductor materials with the ultimate goal of improving the reliability of MicroElectroMechannical Systems (MEMS). These micromechanical devices are made by a variety of techniques, including surface micromachining, bulk micromachining, LIGA, HEXSIL, and many others. Irrespective of their fabrication process, these micro scale machines all have the common property that their surface area to volume ratios are very large. From a surface engineering point of view, this means that forces and interactions that are related to surface area will dominate forces and interactions that are related to volume. This is sometimes evidenced when microstructure surfaces spontaneously stick together, and remain adhered. Experimental evidence (and force calculations) show that the capillary force is usually the main surface force responsible for adhesion. When adhesive forces exceed restoring forces internal to the micromechanical structure, a phenomonon known as stiction is likely to occur. Stiction refers to unintentional adhesion of compliant micromechanical surfaces either to each other or their substrate. Additionally, in the microscale, friction is not independent of adhesion (or sticking), so the terms were combined to describe the phenomena. Stiction is a major factor which limits the reliability of MEMS.
Engineering approaches to avoid release stiction (stiction that occurrs during the release step as a result of liquid capillary forces) include supercritical drying techniques, vapor release techniques, and freeze sublimation. However, these techniques are not capable of addressing the problem of in-use stiction (stiction that occurrs after the release, i.e., while the device is in operation) and other approaches must be investigated. In fact, the most effective treatments to date involve chemical modification of the MEMS surfaces. I will leave out the details, but this is where my research is focused.
My Research Projects
Vapor Phase Processes for Anti-Stiction Monolayers for MEMS: This project seeks to develop vapor phase methods for monolayer deposition on micromachine surfaces. Current methodologies for the application of anti-stiction monolayers are liquid phase processes. In other words, the released MEMS must be immersed in some solution containing the reactive precursors. This type of processing, while very effective at the research level, is not very desireable for commercial processes. Issues regarding the scale-up from chip level processing (currently) to whole wafer processing present a huge activation barrier. Other issues such as reproducibility, chemical handling, cost, safety, and yield also present obstacles for liquid phase processing. It is therefore our goal to develop methods for vapor phase processing, which should (in principle) maintain the high yields, be controllable for reproducibility, and be easy to scale up. Moreover, there would be no solvent issues, and chemical handling would be kept to an absolute minimum.
Thermal Stability of Anti-Stiction Monolayer Systems: This project seeks to understand the degradation mechanism of anti-stiction monolayers in oxidizing ambients. In order to be truly effective, any anti-stiction film must survive all the subsequent processing steps that the MEMS undergo. Often, there is a high temperature step, post-release, where the MEMS are put into a protective package. This exposure to high temperature (typically around 400C, some much higher or lower) poses a threat to the organic monolayer that is to prevent stiction. Currently, what happens to these monolayer systems under these high temperature conditions is not well understood. It is therefore the goal to understand the mechanismd at work here, in the hopes that imptovements can be made to the design of the monolayer systems.
Alkene Based Process for an Anti-Stiction Monolayer for Polysilicon MEMS: This project seeks to develop an anti-stiction monolayer process which uses the reaction of a primary alkene with hydrogen terminated silicon. The motivation for this revolves around some limitations of chlorosilane chemistry (the current standard anti-stiction monolayer system)
Hard Coatings for MEMS: This project seeks to address the second main limitation of MEMS. After stiction, the next problem likely to limit the relaibility of MEMS is wear. Currently, the mechanism of wear on the microcale is not well inderstood, but is known to depend on a number of factors including surface roughness,surface compostiion and relative humidity. Although the mechanism is not well understood, the prevailing solution to the problem of wear is to develop hard coatings for MEMS. Since silicon (the main material from which micromachines are made) is relatibely hard, few materials can be used. It must also be kept in mind that the materials must be compatible with the micromachining processes. With this in mind, the approach here is to use silicon carbide as the protective layer. Since it is chemically more inert than silicon, and typically has lower frictional properties, it seems to be a good choice.
I attended the 2003 AIChE annual meeting in San Francisco, CA, and I gave two presentations. The slides are given below in pdf format.
I did not attend the 2002 annual meeting because I was doing an internship at Sandia National Labs.
I attended the 2001 AIChE annual meeting in Reno, NV.
I had the pleasure of attending the AIChE annual meeting 2000 in Los Angeles, CA. I gave two talks about stiction (DDMS monolayer) and tribology.