Thin-shelled composite mirrors have been recently proposed as both deformable mirrors
for aberration correction and as variable radius of curvature mirrors for phase diversity,
auto focus, and adaptive optical zoom. The requirements of actuation of a composite
mirror far surpass those for MEMS deformable mirrors. This paper will discuss the
development of a finite element model for a 0.2 meter carbon fiber reinforced polymer
mirror for use as a variable radius of curvature mirror in conjunction with a MEMS
deformable mirror for aberration correction.
Thin-shelled composite mirrors have been recently proposed as both deformable
mirrors for aberration correction and as variable radius-of-curvature mirrors for
adaptive optical zoom. The requirements on actuation far surpass those for other
MEMS or micro-machined deformable mirrors. We will discuss recent progress
on developing the actuation for these mirrors, as well as potential applications.
Long-term reliability testing of Micro-Electro-Mechanical Systems (MEMS) is important to the acceptance of these devices for critical and high-impact applications. In order to make predictions on aging mechanisms, these validation experiments must be performed in controlled environments. Additionally, because the aging acceleration factors are not understood, the experiments can last for months. This paper describes the design and implementation of a long-term MEMS reliability test bed for accelerated life testing. The system is comprised of a small environmental chamber mounted on an electrodynamic shaker with a laser Doppler vibrometer (LDV) and digital camera for data collection. The humidity and temperature controlled chamber has capacity for 16 MEMS components in a 4x4 array. The shaker is used to dynamically excite the devices using broadband noise, chirp or any other programmed signal via the control software. Driving amplitudes can be varied to maintain the actuation of the test units at the desired level. The actuation is monitored optically via the LDV which can report the displacement or velocity information of the surface. A springmass accelerated aging experiment was started using a controlled environment of 5000 ppmv humidity (roughly 13% at room temperature), temperature of 29 °C, and ±80μm maximum displacement of the mass. During the first phase of the experiment, the resonant frequency was measured every 2 hours. From 114.5 to 450 hours under stress, measurements were taken every 12 hours and after that every 24 hours. Resonant frequency tracking indicates no changes in the structures for 4200 hours of testing.
Several applications of metallic MEMS devices require that the component can endure cyclic stresses. Mechanisms for fatigue failure may be altered at length scales where the size of the component becomes comparable with the microstructure. For this reason, it is necessary to characterize the fatigue performance of MEMS-scale structures and understand the role of microstructure on potential failure modes. A new specimen configuration has been designed which allows for simple gripping and actuation using a fixed-free beam in bending. The cross-section of the beam is tapered to create a finite width gage section of constant maximum stress, as can be derived from elastic beam theory. This method has been applied to characterize the fatigue behavior of LIGA Nickel with a nominal cross-section of 26×260 microns, replicating the dimensions of a potential accelerometer device. The common stress-life approach was used to characterize the number of cycles to failure for a range of applied cyclic stresses. We found that the stress-life curve was similar to what has been observed for conventional Nickel. The endurance limit (defined in this study as the stress required to cause failure in ~10M cycles, below which the device has practically infinite life) was found to be 35-40% of the ultimate tensile strength. The surface condition of specimens at various stages in the fatigue life, characterized by scanning electron microscopy, revealed that failure initiated as microcracks within localized persistent slip bands (PSBs).
A new replication technology that produces, high aspect ratio ceramic or metal microparts by micromolding and sintering nanoparticle preforms is presented. In this LIGA replication technique, an epoxy based nanoparticle slurry is cast into sacrificial plastic micromolds produced by injection molding. The epoxy is allowed to cure and, if desired, excess epoxy is polished off to produce individual micropart preforms. The micromold is then dissolved in methylene chloride and the micropart preforms are sintered in either air (oxide ceramics) or 4% hydrogen in argon (nickel). This presentation will discuss the effects of the epoxy formulation, the microcasting procedure, and the sintering schedule on the materials properties of the final sintered microparts. It will be shown that this replication technique produces ceramic or metal microparts with micron size features and mechanical properties comparable to those of macroscopic materials.