AVS1996 Session MM-WeA: Micro-Analysis with MEMS

Wednesday, October 16, 1996 2:00 PM in Room 204B

Wednesday Afternoon

Start	Invited?	Item
2:00 PM	Invited	MM-WeA-1 Microfabricated Instruments for In Situ Analysis of Planetary Surfaces M. Hecht (Jet Propulsion Laboratory) Analysis of the surface characteristics of planetary bodies other than the earth involve in situ measurement of three states of matter, particles, and fields, in a wide range of hostile environments. Mass, volume, and power are at a premium in planetary missions, and there is a tremendous opportunity for the substitution of MEMS and microinstruments for conventional instruments as long as performance and reliability are not compromised. As a general rule, microfabricated devices are more resistant to mechanical and thermal shock than their conventional counterparts, but they typically suffer from reduced signal to noise ratios due to limited aperature and sampling abilities. Scaling laws vary from technology to technology. As an example, consider miniature electrostatic energy analyzers. Trajectories scale transparently from large to small systems if all voltages are kept constant. Field strengths increase leading to increased concern about breakdown, but perturbations of trajectories due to magnetic fields are reduced. Pumping requirements are less since scattering probabilities are reduced due to shorter path lengths. Lower signals can be compensated for by the use of arrays of parallel analyzers. Specific instrument development programs using microfabrication will be discussed, including quadrupole mass analyzers, various pressure gauges, diode laser spectrometers, LIGA-fabricated x-ray optics, and fiber-based chemical probes. The optimal use of microfabricated instruments occurs when the strategy for deployment of instruments is optimized around the sensor technology. A prime example is the Mars MicroProbe to be flown under NASA's New Millennium Program in 1998. A complete surface science station weighing under 2 kg is deployed directly from space, containing microinstruments for meteorology, accelerometry, heat conductivity, and soil chemistry.
2:40 PM	Invited	MM-WeA-3 Micro-analysis with MEMS A. Feinerman (University of Illinois, Chicago) Microfabrication techniques have advanced to the point where conductors, semiconductors, and insulators can be positioned in complex three-dimensional arrangements with very high precision. This is equivalent to a conventional machinist operating miniature milling machines and lathes with micron-sized bits. This flexible machining capability allows electric and magnetic fields to be created that can accelerate, focus, steer, and/or align charged particles because the fields occupy a volume of space rather than just existing next to a surface. Specific fabrication techniques developed at UIC include: stacking silicon chips with pyrex fibers, selective anodic bonding, a LIGA lathe, and a helix generator. These techniques are being used to integrate charged particle sources, electrodes, and detectors into various miniature instruments including: a sub-cm scanning electron microscope, a 10 cm time of flight mass spectrometer, a 10 cm nuclear magnetic resonance instrument, and a 5 m linear accelerator/undulator capable of producing hard x-rays. Analytical instruments of this size will allow the analytical laboratory to be brought to the sample, which will be essential when the sample must be observed in situ, e.g., at a toxic waste site or in outer space. The initial design and data on each of these instruments as well as their fabrication methods will be discussed.
3:20 PM	Invited	MM-WeA-5 Microfabricated Devices for Chemical and Biochemical Analysis J. Ramsey (Oak Ridge National Laboratory) Important problems in chemistry and biology will benefit from the ability to perform automated, rapid, and precise procedures on minute quantities of material in a highly parallel fashion. Microfabrication techniques have been employed to try to address some of these daunting problems. There is also promise that microfabricated components can be integrated into a single device to accomplish a chemical or biochemical procedure. The advantages of integrated devices that perform chemistry and chemical analysis may be quite similar to those realized by the microelectronics industry through the integrated circuit. Potential advantages include low cost, compact devices with high speed processing while improving operational simplicity and reliability and having the added benefit of parallel architectures for solving large problems. Moreover, integration of chemical processing and analysis functions allows automated manipulation of samples and reagents at volumes orders of magnitude smaller than is feasible manually or robotically. Miniaturized devices that have been fabricated by our group and others primarily involve electrically driven separation techniques. Recently, monolithic devices that integrate chemical and biochemical reactions with analysis have been demonstrated in out laboratory. Devices that perform automated chemical kinetics experiments and DNA restriction fragment analysis will be discussed. Both devices allow chemistry or biochemistry, and analysis to be accomplished at a volumetric scale four to six orders of magnitude smaller than conventional laboratory procedures with a speed advantage of one to two orders of magnitude.
4:00 PM		MM-WeA-7 Investigation of Surface Adhesion Phenomena in MEMS Devices M. Houston, R. Maboudian (University of California, Berkeley) Adhesion, friction, and wear (sometimes grouped into the term stiction) are prevalent problems in a majority of MEMS devices. Since gravity is negligible at the dimensions of most micromachines, understanding of surface interactions in MEMS is important for controlling stiction phenomena. This presentation will discuss the use of electrostatically-actuated cantilever beam arrays, in conjunction with other surface characterization techniques such as X-ray photoelectron spectroscopy and atomic force microscopy, to measure the surface forces present between polycrystalline silicon surfaces and to manipulate those forces by utilizing various surface treatments. To date, hydrofluoric acid, ammonium fluoride, and self-assembled monolayers have been used to attain dramatic reductions in the work of adhesion (over three orders of magnitude) on treated structures compared with untreated surfaces. For example, conventional polysilicon structures with hydrophilic oxide surfaces exhibit a work of adhesion in a humid environment of 1.4x10\super 5\ \mu\J/m\super 2\, about twice the surface energy of water. In contrast, however, structures rendered hydrophobic by one of the treatments described above exhibit works of adhesion in the range 3-30 \mu\J/m\super 2\. The factors leading to such a remarkable reduction in the adhesion properties of these microstructures have been found to be a complex combination of the surface chemical composition and the topography of the contacting surfaces, and can also depend on the surrounding ambient.
4:20 PM		MM-WeA-8 Towards Miniaturization of a Vacuum Pump J. Sniegowski, D. Aeschliman, M. Rodgers, C. Wong (Sandia National Laboratories) At Sandia, we are developing a miniaturized, planar vacuum pump ("micro-vacuum pump") operating on the jet ejector principle. An ejector pump has been selected because it has no moving parts, valves or seals. This design is adaptable to multiple staging to achieve lower pressures. A planar configuration is needed when fabricated using micromachining techniques. Because scaling is a major element in our design consideration, our initial development is to focus on evaluating planar pump designs of millimeter scale. Pump performance data are obtained for several ejector- driven vacuum pump configurations. A maximum single-stage compression ratio K, where K is the ratio of exhaust pressure to pump suction pressure, of 3.6 was observed for a supersonic ejector-driven pump of 3 mm throat height and 3 mm channel depth at an exhaust pressure of 3.5 psia. Peak K was lower at both lower and higher exhaust pressures. Based on these data, a K of 50 or greater should be possible for a five-stage pump of the same geometric design and physical scale exhausting to atmosphere. From these preliminary designs, we have also developed a micro-fabricated planar version with a largest dimension of 0.8 cm. The micro-devices are comprised of two micromechanical wafers. The ejector body is a deep (50 micron) trench etched into the first silicon wafer. The first wafer also contains the access ports for the ejector input, vacuum, and exhaust. The second wafer serves as a cap to fully enclose the flow channels of the ejector pump. These wafers are then to be bonded together. The fabrication of these devices is complete through the definition of the flow features of the ejector body. This work was supported by the US DOE under contract DE-AC04-94AL8500.