Jordan E. Brough and Dr. Larry Howell, Mechanical Engineering
Microelectromechanical Systems (MEMS) is a rapidly developing field which uses technology similar to that used to carve out the millions of tiny channels found in today’s computer processors. MEMS, however, uses this technology to make mechanical structures and mechanisms instead of electrical pathways. My effort over the past eight months has been to research the developing LIGA MEMS design (an electromolding process), fabricate a test structure via the LIGA technology, and in the process establish a working relationship between Sandia National Laboratories California (a leader in MEMS) and BYU’s microengineering team.
Our original plan to achieve these goals was to fabricate a LIGA MEMS device at Sandia National Laboratories California while I fulfilled an internship there (over the past summer) and then to carry out the testing and analysis of the product jointly at Sandia and Brigham Young University. Sandia National Laboratories California is one of two sites comprising Sandia National Laboratories – a national government laboratory dedicated to the stewardship and development of the nation’s nuclear stockpile. Sandia California specializes in the LIGA form of MEMS fabrication. Working with Sandia was advantageous because the costs associated with the technology required to make LIGA microstructures are extremely high.
Over the course of the project, two designs were developed and submitted for fabrication. The first design was a fully-compliant bistable switch (developed through the work of Dr. Howell and a graduate student at BYU) and the second was a mass flow regulator (developed by my “mentor” at Sandia Labs and myself over the past summer). Due to the complexity of these devices, as well as unforeseen difficulties with Sandia’s Fabrication Facilities, the fabrication of both devices has been delayed and will not be completed until October of 2001.
Delays and difficulties notwithstanding, I consider this research project to have been a success. Advances will be made on the designs mentioned and testing them will shed further light on many issues we have been investigating. Furthermore, a formal working relationship between Sandia Laboratories California and BYU has been established and several exciting new projects have started to materialize over the past few weeks which should ensure a rich and mutually beneficial relationship. Several improvements have been suggested as to the design and fabrication of MEMS mechanisms to help streamline a process which has been plagued by disjointed methods and entities. In addition, my grasp of several mechanical engineering subjects, including compressible fluid flow, has been enormously improved.
It was determined through our research efforts that a great deal of investigation into the material properties of the parts fabricated via the LIGA process is required. Because of the small nature of LIGA parts and the method of shaping the metal (electroplating), significant variations may be present within a single batch of parts that are not encountered in similar macro devices. Improvements are being sought in both the documentation and understanding of these parts’ properties. Research is also being done in developing more accurate fabrication methods to ensure better consistency of part properties. We also determined that standard procedures and increased interaction between designers and fabricators in the LIGA process would help this new technology gain speed.
Most of my time working on this research project was spent developing the design for the gas mass flow regulator. A rather complicated balance of compressible fluid flow properties was necessary to design a product that would work, and thus a large amount of time was spent researching compressible fluid flow and how its properties could work together with the current possibilities of LIGA fabrication. Due to the great amount of time spent on this aspect, the rest of this research paper will be dedicated to a brief explanation of how the device was designed.
We designed our mass flow regulator to regulate the flow of helium gas exiting a pressurized vessel (of 60 to 6000 psi) into standard atmospheric pressure (15 psi) and provide a constant flow rate of .0016 lbm/s while emptying a 6 in3, 6000 psi vessel in approximately 10 seconds. A device with somewhat similar specifications would be very useful in some instances of gas transfer within nuclear weapons. We used the principles of compressible fluid flow to balance the force created by a pressure-difference across a piston head against a spring force on the same piston so that for any given pressure in the vessel, the piston would move itself to open or close the exit outlet and regulate the flow of gas out of the pressurized vessel. The device is designed to be 1 inch in diameter and contains 3 inner layers, which are each .04 in thick. The thickness of the outer layers will depend on the testing apparatus, though they must also withstand internal pressure. (See Fig. 1)
In order to design the regulator we broke the motion of the piston into 4000 steps using Microsoft Excel and determined the geometry necessary to make the regulator function correctly at each point of its motion. We analyzed the motion of the piston in reverse (we began with the piston fully open and the tank empty and worked backwards) and determined what the neck area between the piston head and the sidewalls should be for each point. After creating the sidewall profile in this fashion, we then used a visual basic code to verify our design by simulating the results of emptying a 6000 psi tank through the regulator in forward motion (starting with a full vessel and proceeding to empty it). Intentional error was introduced in order to simulate fabrication errors. Several designs were discarded and modified as errors were discovered through our virtual testing, and the final design was a product of those efforts.
Two designs for fabrication in LIGA have been developed and submitted for fabrication, advancements in the LIGA design process have been made, additional inquiries into key properties of LIGA materials are in process, a formal working relationship has been established between Brigham Young University and Sandia National Laboratories California and several new and exciting projects have begun to materialize which could help further MEMS technology. This project has been especially beneficial to me, as it has helped me gain experience in the field that I am interested in and helped me establish important contacts with key researchers in the MEMS industry. This research of LIGA microstructures has been a great success and should provide continued opportunities.