Over the past decade, silicon microelectromechanical systems (MEMS) technology has gradually increased its foothold in mechanical engineering. Favored for their low cost, reliability, and small size (qualities inherited from the integrated circuit manufacturing process), simple MEMS devices began finding their way into consumer applications almost two decades ago.
At the time, that market was benefiting from microscale automotive sensors and inkjet modules, considerably more complex microsystems were being considered for use in space applications, where miniaturization is a prime goal in the design of military and nonmilitary payloads alike.
Such payloads are limited in terms of weight and volume, so when a new function needs to be added to the system, it must be accomplished through miniaturization.
Research at Sandia National Laboratories
It was not surprising then, that beginning in the late 1980s, Sandia National Laboratories began to look at MEMS for solutions in its continuing mission to improve and modernize ordnance systems required for the U.S. nuclear stockpile.
Sandia then had a state-of-the-art microelectronics fabrication facility that would provide the physical environment, and much of the engineering talent, to take on this new initiative.
With the growing interest in MEMS technology, part of these new facilities was turned over to the new enterprise, and a core group of scientists and engineers began work on advanced microsystems. While focused on the needs of the U.S. nuclear weapon complex, these innovations would also spur new developments in the commercial arena, such as automotive, consumer products, and telecommunications applications.
Sandia's new MEMS team began work on a number of relatively complex designs in the early 1990s, and in 1994 demonstrated a micro steam engine that used resistive heating to provide steam from a single drop of water.
Engineers are finding that the adhesion properties and interactions of polysilicon and other materials are leading to reliability problems and high failure rates, which have to be addressed.
Remarkably, it worked without seals because the attractive interfacial force between surfaces was sufficient to prevent the loss of steam. The increased surface-to-volume ratio with decreasing size led to the self-sealing nature of the design.
This was an important early indicator that, as structures are scaled to smaller and smaller size, elements of mechanical, optical, and chemical understanding needed to be revisited. In fact, the micro steam engine gave early notice that, if you really want to excel in MEMS, you need to understand the dominant transport processes and material interactions at the micro and, the nanoscale level.
New Fixes
Certain processes, such as chemical-mechanical polishing to planarize polysilicon layers, were inherited from standard microelectronics fabrication procedure. But new "fixes" had to be found as the fledgling MEMS industry moved farther from its parent technology, and the design assumptions derived from observation of large-scale phenomena became less dependable.
At about this time, Sandia also was tackling a couple of major problems in the design of micromechanical actuators (mechanical control devices). While MEMS sensors were already a marked success, the micro-actuators of the time suffered from low torque and an inherent difficulty in coupling tools to engines.
Its solution was revolutionary: a new, four-layer polysilicon micromachining process that made it possible to make the more complex devices that were needed to resolve the actuator limitations. The process incorporated three movable levels of polysilicon in addition to a stationary layer for a total of four layers of polysilicon. These were separated by sacrificial oxide layers, and an additional friction-reducing layer of silicon nitride was placed between the bearing surfaces.
The smaller gear of the actuator was successfully operated at speeds in excess of 300,000 rpm, and the larger gear as fast as 5,000 rpm. Unfortunately, the performance of this device appeared to degrade over time. This was confirmed by scanning electron microscopy images taken after only 477,000 cycles.
Learning from Failure
The search for an in-depth understanding of wear mechanisms in dynamic silicon MEMS—so elusive and yet so important-would drive an ambitious wave of leading-edge research into microscale science and engineering, distinct from that which prevailed at the mesoscale.
For example, for structures with thicknesses of a few tenths to several micrometers and lateral dimensions of tens to hundreds of micrometers, significant forces are required to pull apart two surfaces in contact and to initiate motion.
Additionally, controlling surface adhesion for materials with high surface energies like polysilicon requires special consideration. As a result of this, adhesion and electrostatic models have been added to Sandia's simulation codes to model such structural deformations.
Modeling vibrational energy associated with grain boundary interactions and "non-continuum" heat transfer in gases have all proven to be important in predicting the overall, systemic behavior of MEMS devices. By incorporating more reliable models in design, the number of design-testing cycles has been reduced, and pretest predictions are becoming more reliable.
The relative infancy of MEMS manufacturing disciplines, and to some extent the restrictions posed by corporate proprietary protection of its intellectual property and the painfully slow emergence of industry standards, has resulted in slower MEMS technology development and infusion into the commercial sector than was expected in the 1990s.
What has emerged in the past decade is the recognition that more data on materials and on the underlying physics are needed to move MEMS technology forward.
[Adapted from "MEMS from the Nanoscale Up," by Arthur C. Ratzell III, for Mechanical Engineering, March 2007.]
What has emerged in the past decade is the recognition that more data on materials and on the underlying physics are needed to move MEMS technology forward.
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