Kianoush Naeli

Dimensional considerations in achieving large quality factors for resonant silicon cantilevers in air

This work aims to provide guidelines for designing rectangular silicon cantilever beams to achieve maximum quality factors for the fundamental flexural resonance at atmospheric pressure. The methodology of this work is based on experimental data acquisition of resonance characteristics of silicon cantilevers, combined with modification of analytical damping models to match the captured data. For this purpose, rectangular silicon cantilever beams with thicknesses of 5, 7, 8, 11, and 17 μm and lengths and widths ranging from 70 to 1050 μm and 80 to 230 μm, respectively, have been fabricated and tested. Combining the three dominant damping mechanisms, i.e., the air damping, support loss, and thermoelastic damping, the variation in the measured Q-factors with the cantilever geometrical dimensions is predicted. Also to better describe the experimental data, modified models for air damping have been developed. These modified models can predict the optimum length and thickness of a resonant cantilever to achieve...

Cantilever sensor with stress-concentrating piezoresistor design

A novel approach for enhancing the sensitivity of piezoresistive cantilever sensors is presented. Thin piezoresistive clamped-clamped silicon beams are released on the surface of the cantilever by a micromachining process sequence combining deep-reactive ion etching and anisotropic wet etching of silicon. A deflection of the cantilever sensor yields a stress concentration in these micromachined piezoresistive structures. Finite element simulations indicate an increase in both force and displacement sensitivity compared to a conventional cantilever beam with the same thickness. Devices have been fabricated and initial characterization has been performed. Under constant tip deflection of the cantilever, the new piezoresistor design shows an increase in the relative resistance change by a factor of 5.2 compared to a cantilever with the same thickness and conventional piezoresistor design

An iterative curve fitting method for accurate calculation of quality factors in resonators

A new method for eliminating the noise effect in interpreting the measured magnitude transfer characteristic of a resonator, in particular in extracting the Q-factor, is proposed and successfully tested. In this method the noise contribution to the measured power spectral density of resonator is iteratively excluded through a sequence of least-square curve fittings. The advantage of the presented method becomes more tangible when the signal to noise power ratio (SNR) is close to unity. A set of experiments for a resonant cantilever vibrating at different amplitudes has shown that when SNR is less than 10, the calculation results of conventional methods in extracting the Q-factor, i.e., the 3 dB bandwidth and single least-square curve fit, exhibit significant deviations from the actual Q-factor, while the result of the proposed iterative method remains in 5% margin of error even for a SNR of unity. This method is especially useful when no specific data is available about the measurement noise, except the assumption that the noise spectral density is constant over the measured bandwidth.

Cancellation of environmental effects in resonant mass sensors based on resonance mode and effective mass

A novel technique is developed to cancel the effect of environmental parameters, e.g., temperature and humidity, in resonant mass sensing. Utilizing a single resonator, the environmental cancellation is achieved by monitoring a pair of resonant overtones and the effective sensed mass in those overtones. As an eminent advantage, especially compared to dual-mode temperature compensation techniques, the presented technique eliminates any need for previously measured look-up tables or fitting the measurement data. We show that a resonant cantilever beam is an appropriate platform for applying this technique, and derive an analytical expression to relate the actual and effective sensed masses on a cantilever beam. Thereby, it is shown that in applying the presented technique successfully, the effective sensed masses must not be the same in the investigated pair of resonance overtones. To prove the feasibility of the proposed technique, flexural resonance frequencies of a silicon cantilever are measured before and after loading with a strip of photoresist. Applying the presented technique shows significant reductions in influence of environmental parameters, with the temperature and humidity coefficients of frequency being improved from -19.5 to 0.2 ppm degrees C(-1) and from 0.7 to -0.03 ppm %RH(-1), respectively.

Coupling High Force Sensitivity and High Stiffness in Piezoresistive Cantilevers with Embedded Si-Nanowires

Using a single-mask add-on process module, two usually non-coexistent properties of cantilevers, large stiffness and large force sensitivity, are summoned in a single device. To achieve this, a stress-concentrating (SC) principle is used: instead of decreasing the cantilever thickness, which is the conventional approach in increasing its force sensitivity, the stress is locally concentrated in piezoresistive beams or wires. Hence, with a negligible decrease in stiffness, a higher sensitivity is obtained. Results of finite element simulations and experimental data confirm the feasibility of the proposed design. Fabricated cantilevers with implemented stress-concentrating silicon nanowires show a more than 5-fold improvement in force sensitivity compared to a conventional cantilever with the same dimensions, while maintaining the same stiffness. With thinner wires, the sensitivity increases 8-fold at the expense of a 15% decrease in stiffness.

Fabrication and Characterization of Liquid and Gaseous Micro- and Nanojets

This paper reports on the fabrication and characterization of liquid and gaseous jets ejected from microfabricated nozzles with dimensions ranging from 500 nm to 12 μm. Unlike previous work reporting the fabrication of nano-orifices defined within the thickness of the substrates [1-4], the in-plane nanonozzles presented in this paper are designed to sustain the high pressures necessary to obtain substantial nanofluidic jet flows. This approach also allows important three-dimensional features of nozzle, channel and fluidic reservoir to be defined by design and not by fabrication constraints, thereby meeting important fluid-mechanical criteria such as a fully-developed flow. The shrinking jet dimensions demand new metrology tools to investigate their flow behavior. A laser shadowgraphy technique is used to visualize and image the jet flows. Micromachined heated and piezoresistive cantilevers are used to investigate the thrust and heat flux characteristics of the jets.Copyright

Geometrical Optimization of Resonant Cantilever Sensors

The influence of beam geometry on the quality factor Q of resonant cantilever sensors operated at atmospheric pressure has been investigated. The studied rectangular silicon cantilevers have lengths and widths ranging from 70-500 mum and 80-160 mum, respectively, and thicknesses of 5, 8, and 17 mum. Transverse cantilever vibrations are excited electromagnetically and the flexural resonance modes of the beams are detected on-chip by a piezoresistive Wheatstone bridge. Based on the experimental data, design guidelines for beam dimensions ensuring maximal Q-factors are established. In particular, an optimal length-to-thickness ratio of 15-20 is found; support loss limits the Q-factor for smaller length-to- thickness ratios, while air damping becomes dominant for larger ratios.

Micro-cantilever metrology tool for flow characterization of liquid and micro/nanojets

This paper reports the development of MEMS metrology tools to characterize liquid and gaseous jets ejected from micro/nanofabricated nozzles. To date few highly local measurements have been made on micro/nanojets, due in part to the lack of characterization tools and techniques to investigate their characteristics. Atomic force microscope cantilevers are well-suited for interrogating these flows due to their high spatial and temporal resolution. In this work, cantilever sensors with either integrated heating elements or piezoresistive elements have been fabricated to measure thrust, velocity, and heat flux characteristics of micro/nanojets.Copyright