AVS2010 Session MN-FrM: Characterization for MEMS and NEMS

RF-MEMS DC contact and capacitive switches are being developed, involving a broad range of designs in series and shunt configurations. The work done until now has facilitated significant maturing of the technology to overcome technical challenges such as reliability, packaging, and high power operation. In particular, the reliability of RF-MEMS capacitive switches has been limited mainly by the electrical charging of the oxide or nitride dielectric layers used until now, which exhibit discharging times in the hundred of seconds range, resulting in failure of the switches. Therefore, it is critical to develop dielectric layers with fast charging/discharging performance. In this respect, the novel ultrananocrystalline diamond (UNCD) films developed and patented at Argonne National Laboratory exhibit a unique fast charging (50-100 µsec)/discharging (≤ 100 µsec) behavior, which provides the reliability required by RF-MEMS switches. The charges are transported through a large network of grain boundaries, which occupy a large percentage of the total area of the films, characterized by a nanostructure formed by 2-5 nm grains with ~ 0.5 nm wide grain boundaries.

This paper focuses on a description of materials, materials integration strategies, device architecture and performance of prototype monolithically integrated RF-MEMS mm-wave shunt capacitive switches/CMOS devices in coplanar waveguides, using UNCD as the dielectric layer. The RF-UNCD MEMS switches are based on a MEMtronics, Inc. switch design, fabricated on sapphire wafers with high-voltage CMOS devices, provided by Peregrine Semiconductor, using standard lithography and surface micromachining techniques. Small signal measurements were performed in the frequency range of 1-20 GHz. Measurements of the UNCD dielectric layer charging and discharging were performed using both MEMS and metal-insulator-metal (MIM) capacitor device configurations, using standard I-V and C-V techniques. The charging and discharging time constants for RF-MEMS switches with UNCD dielectric were 5-6 orders of magnitude faster (≤ 100 µsec) than the discharging times (100s of seconds) exhibited by conventional oxide or nitride dielectric materials. Although static power consumption is an issue with UNCD-based capacitors, they can be used in applications, which demand little to no degradation in performance and allow microwatts of power consumption.

This work was mainly supported by DARPA, under contract MIPR 06-W238. Use of the Center for Nanoscale Materials was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

Nanocrystalline materials exhibit unique mechanical properties depending on the nature of the bond, co-ordination number of atoms at grain boundaries. Tetragonal sp³-bonded diamond has the highest known atomic density. The nature of the bond and its high density enable diamond to have superior physical properties such as the highest Young’s modulus, melting temperature and acoustic velocity of all materials. Recently, conformal thin diamond films have been grown at CMOS-compatible temperatures in the form known as ultrananocrystalline diamond (UNCD). We have measured the Young’s modulus (E), Poisson’s ratio and the quality factors (Q) for microfabricated overhanging ledges and fixed-free beams composed of UNCD films grown at lower temperatures¹. The overhanging ledges exhibited periodic undulations due to residual stress. This was used to determine a biaxial modulus of 838 ± 2 GPa. Resonant excitation and ring down measurements of the cantilevers were conducted under ultra high vacuum (UHV) conditions on a customized atomic force microscope to determine E and Q. At room temperature we found E = 790 ± 30 GPa, which is ~20 % lower than the theoretically predicted value of polycrystalline diamond, an effect attributable to the high density of grain boundaries in UNCD. From these measurements, Poisson’s ratio for UNCD is estimated for the first time to be 0.057±0.038. We also measured the temperature dependence of E and Q in these cantilever beams from 60 K to 450 K. Above ~ 150 K, temperature dependence of modulus is slightly higher than that of single crystal diamond averaged over all directions despite the presence of large fraction of disordered carbon at grain boundaries and below 150 K changes in modulus are extremely small. This is the first such measurement for UNCD and strongly suggests that the nanostructure plays a significant role in modifying the thermo-mechanical response of the material. We have measured a very low temperature coefficient of frequency at room temperature which has important technological applications including resonant mass sensors and filters. The room temperature Q varied from 5000 to 16000 and showed a moderate increase as the cantilevers were cooled below room temperature and it saturates resembling the plateaus observed in many disordered systems. The results suggest that defects in the grain boundaries significantly contribute to the observed dissipation.

1. “Mechanical stiffness and dissipation in ultrananocrystalline diamond resonators” Adiga et al, V 79, pp 245403.1-8, Physical Review B, 2009

The highly nonlinear nature of the crack propagation is studied using a completely nonlinear beam theory model. A model is described that is highly accurate for various deflections of microcantilevers and agrees with the experimental data with nanometer accuracy. The micro cantilever beams used are fabricated by SUMMiT V technology and the dimensions of the beams are 500 x 20 x 2 microns. A nonlinear method is also proposed to calculate the strain energy release rate of stiction failed micro cantilever beams. It is shown that at higher deflections the previous methods fail to accurately determine the stored strain energy and the strain energy release rate. The proposed method is used to accurately determine the beam’s profile and strain energy release rate at higher deflections, and it is in good agreement with the other methods at small deflections.

The method developed here is completely applicable to both Mode I and mixed mode I & II crack development problems. It is shown that when it comes to micro cantilevers the so called Mode I crack propagation turns out to be a mixture of Mode I and Mode II. Longitudinal stresses developed inside the cantilevers increases rapidly as the beam gets nonlinear and starts to play a more important role in the crack length and strain energy release rate. The strain energy release rate can be formulated both using the elastic energy stored in the beam and also using the definition of stress intensity. In this paper, the two common methods for calculation strain energy release rate are discussed and stress intensity factors determined. The obtained stress intensity factors once again verify the fact that there is some Mode II component present during the crack propagation. These results also show that the strain energy release rate is not a constant value and increases as the crack develops. A highly linear relationship is observed between stress intensity components which are in good agreement with the analytical models of the phase angle. It is observed that the phase angle is almost a constant value for all crack heights.

This research reports the developments and characterizations of two types of NEMS gas detectors, based on nano-scaled titanium oxide (TiO₂) and monolayer protected gold nanoclusters (MPCs) as a sensing film of chemiresistors. For sensing films based on chemically synthesized MPCs, the diversified selectivity (>10 compound detection based on various MPC shell structures), the fast response (<10 seconds) and the low power consumption (25 μW) are its main advantages. However, the irreversible adsorption due to the strong affinity between MPCs and vapor molecules could be problematic. On the other hand, nano-structured TiO₂ film, which is operated at elevated temperature, is inherently immune from irreversible condensation. The TiO₂ sensing film is fabricated through E-beam lithography in the present study. Various shapes of nano-structures are precisely aligned and placed in between microelectrodes to mimic nano-crystalline structures for the enhancement of the detector sensitivity and robustness. In addition, these NEMS gas detectors are conditioned with integrated heaters at a controlled temperature to minimize any interfering gas adsorption in between analysis. Various performance evaluations including the characterizations of two types of detectors at different temperatures, test chemicals, detection limits, and the compatibility with a gas chromatography system, are investigated and to be presented.

Dielectric charging is among the major reliability issues that have prevented the commercialization of RF-MEMS Capacitive switches in spite of the extensive study performed on the topic. Moreover, a little work has been performed to study the effect of the relative humidity (RH) and environment gases on the dielectric charging process.

In this work we present the effect of RH and the environment gases on the charging/discharging processes in PECVD silicon nitride films based on Kelvin Probe Force Microscopy (KPFM) methodology. The measurements have been performed in ambient air and under N2 flow, both under different RH levels (from 6% to 40% RH). In addition, the influence of the dielectric film thickness, SiN deposition conditions and the substrate nature on the charging process have been investigated under different environment conditions. This has been done through depositing SiN films with different thicknesses ranges from 100nm to 400nm over bare silicon substrates and over evaporated Au layers and using both Low Frequency(LF) and High Frequency(HF) PECVD deposition modes.

For both measurements performed in ambient air and under N2 flow, the surface potential decay with time follows the stretched exponential law. The decay time constant decreases strongly as the RH increases (1.230E+03 sec and 650 sec for 6%RH and 40%RH respectively, for the HF 200nm thick SiN film deposited over evaporated Au). The measured decay time constant is found to be shorter in case of N2 than in ambient air measurements. The surface potential distribution is represented by the Full Width at Half Maximum (FWHM) for charges which have been injected in a single point with the AFM tip. The FWHM becomes smaller as RH decreases. Charge injection duration is controlled to range from 10 ms to 100 sec. The FWHM is found to be always larger in air comparing to FWHM measured in N2 flow for different charge injection durations. Moreover, FWHM increases almost linearly with increasing the charge injection time for different RH measurements. Charge lateral diffusion has been observed at larger RH only and is attributed to the more hydrophobic SiN material at smaller RH levels which prevents water condensation at the surface and thereby inhibits lateral charge migration due to the electrical conductivity of a possible water film. The FWHM is found to be smaller in thinner SiN films than in thicker ones and the relaxation time is found to be larger in the thinner SiN films, independently of the substrate nature. Finally, the decay time constant is found to be smaller in case of dielectric films deposited over Au layers comparing to films deposited over bare silicon substrates.

Aspect ratio dependent etching (ARDE) is a common issue in reactive ion etching. This phenomenon results in narrow, deeply etched features exhibiting slower etch rate and a more reentrant profile than larger, more open features. The common mechanism indicated for this effect is ion flux. The narrow features restrict the ability of ions that are not perfectly perpendicular to make it to the bottom of the feature and drive the etch. Recent data obtained at the University of Michigan Lurie Nanofabrication Facility suggests that, in some circumstances, there is a feature size effect that is independent of aspect ratio. The mechanism proposed and studied herein is local thermal loading due to exothermic etch reactions.

The evolution of faster etch rates in silicon is a key enabling factor in MEMS production. However, this fast etching results in thermal management concerns. In an effort to understand these temperature effects better, as well as to determine the evolution of aspect ratio dependence, we performed a variety of rate tests at various stages of etching on different size features. A surprising result was that, after an initial substrate warm up time where etch rates increased slightly, etching rates were flat until aspect ratios approached 10:1 after which they began to drop off slightly. Even more notable is a very significant rate variance with feature size even at very early stages of the etch. This appeared in a regime where ion flux and ARDE should not be significant. This strongly suggests a mechanism of feature size dependence separate from aspect ratio.

The mechanism proposed for this etch rate variance is local thermal loading. This agrees with data collected. Temperature is a significant factor in determining etch rate. The fluorine silicon reaction is exothermic. This has been shown to elevate the temperature of the substrate over the first few minutes of the process. The thermal load will also result in heating and a higher temperature at the etch interface within the features. Larger features will have a higher local thermal load and thus get hotter. This heating accelerates the etch and likely inhibits the ensuing etch resistant fluorocarbon deposition step that is characteristic of the Bosch Process.

We will demonstrate the evolution of this thermal loading and its effects on etch rate within an etch step and through the first few etch cycles of a Bosch Process. We will then evaluate the effectiveness of possible methods of mitigating this effect.