Understanding light propagation in fibrous tissue plays a fundamental role in the development of novel and minimally invasive diagnosis techniques. For this purpose, we have developed a polarimetric microscope that operates in the backscattering geometry. Our apparatus has been thoroughly calibrated and verified with
experiments and Monte Carlo simulations on well characterized colloidal suspensions. In this study, we have investigated the feasibility of retrieving structural information on multiply scattering, fibrous electrospun scaffolds fabricated of Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDFhfp) nanofibers having diameters ranging from 500 to 1000 nm. These nanofibers display various degrees of structural alignment and the structural anisotropy manifests itself in optical birefringence. We probed these scaffolds with a focused near-infrared light beam at three pairs of cross-polarized states and recorded images of the Stokes vector elements of the light backscattered at the surfaces of the scaffolds. Our results demonstrate that it is possible to structurally differentiate the scaffolds by analyzing the spatial variations of the Stokes vectors/polarization ellipses as a function of the polarization state of the probing beam. Visualizing the rate of retardance induced by the birefringent fibers together with the distribution of the degree of polarization unveils the orientation of these fibers and their respective degree of organization, which was compared to results obtained by small angle x-ray scattering (SAXS). This study contributes to a better understanding of the interaction of the light with multiply scattering fibrous matter such as tissue, which is particularly challenging in the backscattering geometry but fundamental to make the diagnosis of cancer possible.
This work reports on mechanical tests and irradiations made on silicon bulk-acoustic wave resonators. The resonators were based on a tuning fork geometry and actuated by a piezoelectric aluminum nitride layer. They had a resonance frequency of 150 kHz and a quality factor of about 20,000 under vacuum. The susceptibility of the devices to radiation-induced degradation was investigated using Co60γ-rays and 50 MeV protons with space-relevant doses of up to 170 krad. The performance of the devices after irradiation indicated a high tolerance to both ionizing damage and displacement damage effects. In addition, the device characteristics were evaluated after mechanical shock and vibration tests and only small effects on the devices were observed. In all experiments, no significant changes of the resonance characteristics were observed within the experimental uncertainty, which was below 100 ppm for the resonance frequency. The results support the efforts toward design and fabrication of highly reliable MEMS devices for space applications.
The mechanical stability of silicon MEMS dies is strongly influenced by the microfabrication processes, especially grinding, dicing and etching, which leave characteristic damage (defects, cracks, dislocations…) in the substrate material. Specially designed mechanical tests are used to assess the resistance of micro-structures to monotonic and cyclic loading. We report on the development progress of a micromechanical test bench for reliability assessment of microstructures in 2-, 3- and 4-point bending configurations. Strain distributions and defects in micron-sized silicon devices can be investigated by in-situ testing in combination with high-resolution x-ray diffraction measurements for experimentally assessing the strain distribution.
This work reports on irradiations made on silicon bulk-acoustic wave resonators. The resonators were based on a tuning
fork geometry and actuated by a piezoelectric aluminum nitride layer. They had a resonance frequency of 150 kHz and a
quality factor of about 20000 under vacuum. The susceptibility of the devices to radiation induced degradation was
investigated using 60Co γ-rays and 50 MeV protons with space-relevant doses of up to 170 krad. The performance of the
devices after irradiation indicated a high tolerance to both ionizing damage and displacement damage effects. The results
support the efforts towards design and fabrication of highly reliable MEMS devices for space applications.
For laser micro processing with short and ultra-short pulses the threshold fluence is affected by the incubation and changes with the number of pulses applied. In general the incubation effect is described by a power function including the incubation coefficient S. Beside the threshold fluence also the energy penetration depth is subject to the incubation effect; moreover it is a main cause for the change of the threshold fluence with increasing pulse number.
The behavior of the threshold fluence can be explained by varying absorption (due to changes in the surface reflectivity), chemical changes of the surface (e.g. due to oxidation) or changes in the microstructure of the material whereas the behavior of the energy penetration depth could be explained by the latter two effects but should not be affected by a change in the absorption. To try to distinguish between these three effects a systematic ablation study with 10 ps pulses at 1064nm wavelength on copper and iron under different gases atmospheres and pressures was done.
The results show on the one hand the change of the energy penetration depth is the main cause of the incubation and that on the other hand an adapted model better fits the trend of the threshold fluence and the penetration depth as a function of the number of pulses applied. The influence of the gas (air, oxygen, nitrogen and argon) is only marginal whereas a reduction of the pressure from normal atmosphere down to 50 mbar results in a 25% increase of the maximum removal rate. Induced changes in the microstructure were detected by a high resolution X-ray diffraction analysis on single crystal (111-orientation) copper and iron samples.
We report on the susceptibility of structural MEMS materials to proton radiation damage. Radiation tests at space relevant doses were conducted on MEMS resonators. The two materials examined were single crystal silicon and SU-8, which are both in widespread use in microsystems. The resonance frequency was monitored for measuring minute changes of the Young’s modulus. No radiation-induced changes of the elasticity were observed in the silicon devices up to fluences of 1013 cm-2, corresponding to a total ionizing dose (TID) of over 5.5 MRad for 10 MeV protons. The SU-8 resonators showed a variation of less than ±5.5% at doses of up to 1.4 Mrad (TID). Chemical and structural analyses of the polymer were performed using infrared absorption spectroscopy and x-ray diffraction methods. We discuss possible mechanisms for the observed changes of the elasticity of SU-8.
MEMS Reliability, especially the study of the reliability of their physical characteristics, is an area that is still in its
infancy [1]. However, reliable MEMS exists already and are produced in hundreds of millions MEMS devices and some
of them are even intended to use in safety critical applications. The wide variety of materials and physical principles used
make it difficult to give general statements about MEMS reliability. However, in several cases reliability is not even
studied, confirmed or modeled. Consequently, the lack of long-term reliable devices reduces their level of acceptance
considerably.
The aging of MEMS is always connected with the occurrence of defects and their mobility. The creation rate and the
mobility of the defects are precursors for the aging behavior. The mobility of defects will be enhanced by greater stress
gradients. Both, the stress gradient and the defects can be easily determined by means of High resolution X-Ray
techniques (HRXRD).
The idea behind is now to connect mechanical stress, thermals load and even radiation damage which lead to the
corresponding signal drift of MEMS devices with the structural properties like defect density and mobility. High
resolution X-ray diffraction techniques (HRXRD) such as the rocking curve (RC) and the reciprocal space maps (RSM)
are well suited to detect this features, leading to the drift of the MEMS devices. High Resolution X-ray diffraction
(HRXRD) techniques are therefore very powerful tools to study aging through the determination of the stresses and
defects in the devices.
We are convinces that these advanced state-of-the art X-ray methods will serve as a useful tool for setting up a
fundamental understanding of the reliability and also aging problems of MEMS.
The microtribometer fabricated is designed to observe the wear of removable flat silicon test inserts, coated with thin film layers such as DLC and moving in an oscillating manner relatively to each other. For observing the low wear of DLC layers in a reasonable amount of time, high oscillating speed is essential and can be achieved by reducing the mass in motion. The silicon microtribometer reaches oscillating frequencies of 10 Hz while applying a normal force on the test inserts up to 9.6 N, the maximal displacement amplitude being 1.5 mm. The two silicon main parts of the microtribometer guide test inserts along one direction in a back and forth motion while avoiding any side friction, the actuation being done by an external linear motor. For such application crystalline silicon presents, compare to other materials, the advantage of the invariance of its behavior over time.
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