This work was supported by the U.S. Army Research Laboratory, Adelphi, MD, USA, under Grant W91ZLK-12-P-0447. The resonance measurements were conducted with the help of Michael Stone and Anthony Brock. The thermal camera measurement was conducted with the help of Damon Conover from the George Washington University.

1. Selecting and Designing an Optimum Structure
<ol>
	<li>Select the fixed-fixed beam for wide range frequency tuning (compared to other candidates, it enables wide range tuning when it is heated due to its large temperature coefficient of frequency (TCF) and negligible thermal expansion constant).</li>
	<li>Design a longer beam if the purpose is tuning efficiency improvement. Design a shorter beam if the purpose is frequency hopping or signal tracking applications.</li>
</ol>
2. Modeling and Fabrication in Complementary Metal-oxide Semiconductor (CMOS)
<ol>
	<li>Design and create the 3D-model for the MEMS filter in a FEM-based program.</li>
	<li>Reconstruct the same layout in an integrated circuit (IC) design tool, layer by layer to create the gds file.</li>
	<li>Submit this gds file to the CMOS foundry for fabrication (we used CMOS 0.6 &#181;m technology).</li>
	<li>Continue with post-processing once the CMOS process is completed (note that the chips should have polysilicon, aluminum, and oxide layers).
	<ol>
		<li>Conduct the CHF3/O2 dry etch process via an inductively coupled plasma (ICP) etch system. Etch the SiO2 between aluminum layers and form the beams at the aspect ratio of 5.7. For this process, use the following parameters: CHF3 at 40 sccm, O2 at 5 sccm, pressure at 0.5 Pa, ICP power at 500 W, and sample power at 100 W with the 56 min total etch time.</li>
		<li>Apply the XeF2 etch process in the silicon substrate to create a 9 &#181;m depth cavity under the beams. For this process, use the XeF2 etching system for 3 cycles at 3T, for 60 s/cycle.</li>
	</ol>
	</li>
	<li>Characterize the devices with a scanning electron microscope (SEM) to ensure they are properly fabricated. For this step, change the beam accelerating voltage to 2.58 kV and the working distance to 9.5 mm.</li>
</ol>
3. Device Testing
Note: Device testing consist of many steps including the Joule heating test and frequency response test.
<ol>
	<li>Thermal camera test for embedded heaters
	<ol>
		<li>Position the thermal camera on top of the chip and test the embedded heaters to ensure they heat the beams.</li>
		<li>Connect the power supply to the chip package and apply a DC voltage on the embedded heaters from 0 V to 5.7 V in small increments to increase the temperature throughout the beams.</li>
		<li>Record the temperature profile throughout the chip package via a thermal camera during the heating process. Save the results in a numerical computing program and plot the heating profile.</li>
	</ol>
	</li>
	<li>Calibrating the LDV and test setup
	<ol>
		<li>Position the laser on top of the 120 &#181;m long beams.</li>
		<li>Connect the power supply between the two 120 &#181;m long beams to apply both 7 V DC and 3 V AC voltage for the resonance operation. Connect an additional DC bias voltage to the embedded heaters with a maximum of 5.7 V to apply Joule heating to the beams during the resonance operation.</li>
		<li>Move the laser to a different spot on the beam to get a low noise laser reflection. Make sure to increase the intensity of the blue bar to decrease the noise.</li>
		<li>Divide the screen into multiple views to calibrate and start the measurement setup.</li>
		<li>Go to Acquisition Settings, set the measurement mode to FFT, do not use any filter, and set the bandwidth to 2 MHz.</li>
		<li>Change the velocity so that it can support a maximum frequency of 2.5 MHz.</li>
		<li>Use the periodic chirp waveform. 
		NOTE: Here, Amplitude stands for AC voltage and Offset stands for DC voltage.</li>
		<li>Start the measurement with this new setup.</li>
		<li>Update the Acquisition Settings by changing the DC voltage to 1 V.</li>
		<li>Decrease the applied bias voltage in the Acquisition Settings window when the Ref1 shows the red alarm (this means that the signal is noisy).</li>
		<li>Move the laser to a different spot on the beam to further increase the signal-to-noise ratio. Occasionally, there may be bad spots on the beam that causes the red alarm on the vibration bar; in this case, continue to search for the best spot.</li>
	</ol>
	</li>
	<li>Testing 68 &#181;m long MEMS filters via LDV
	<ol>
		<li>Select the 68 &#181;m long MEMS filter for testing.</li>
		<li>Apply 25 V DC voltage and 5 V AC voltage together between the two 68 &#181;m long adjacent beams. Here, the DC voltage provides bending and the AC voltage enables the resonance operation.</li>
		<li>Apply an additional DC voltage to the embedded heaters placed in the 68 &#181;m long beam, and increase the voltage from 0 V to 5.7 V in small increments. This will provide frequency tuning based on the Joule heating.</li>
		<li>Observe and record the resonance frequency and phase response with respect to the applied bias voltage at each step and summarize the results in a table. Here, the total frequency tuning for this sample is around 874 kHz when the 5.7 V DC voltage is applied to the embedded heater. 
		NOTE: Simulations (on the right side) and real measurement (on the lefts side) are synchronized.</li>
	</ol>
	</li>
	<li>Higher modes measurement
	<ol>
		<li>Push the A/D button to go to the Acquisition Settings window demonstrated in Section 3.2 and change the velocity so that it can support very high frequencies.</li>
		<li>Measure the first and the second mode with their phase. 
		NOTE: The primary resonance displacement is in Y direction for mode-1 and it is in Z direction (that is towards the microscope) for mode-2.</li>
	</ol>
	</li>
</ol>
4. Avoiding Device Failure
<ol>
	<li>Low frequency square wave signal application to solve stiction
	<ol>
		<li>Apply a 1 Hz square wave signal to solve the stiction problem that results from electrostatic charging between the two adjacent beams.</li>
		<li>Go to the offset box and set the DC voltage to 1 V, while keeping the AC voltage at 1 V.</li>
		<li>Go to the frequency box and set the frequency to 1 Hz.</li>
		<li>Activate and apply this new setup on the beams.</li>
		<li>Observe the separation of the beams.</li>
	</ol>
	</li>
	<li>High thermal stress and burning
	<ol>
		<li>Use an extra sample for the thermal stress test.</li>
		<li>Increase the applied bias voltage on the embedded heater by small increments to find the maximum allowable voltage before the device fails due to high thermal stress.</li>
	</ol>
	</li>
</ol>
5. Boosting the Tuning Capability
<ol>
	<li>Apply a 25 V DC voltage and 5 V AC Voltage together between the two 68 &#181;m adjacent beams while increasing the applied bias voltage on the embedded heater from 0 V to 5.7 V, for a total 661 kHz frequency shift.</li>
	<li>Increase the applied bias voltage from 25 V to 35 V to add an additional spring softening effect between the two 68 &#181;m long adjacent beams, while applying a 1 V AC voltage and keeping the same bias voltage setup on the embedded heaters.</li>
	<li>Record the 32% improvement in total frequency shift as it should increase from 661 kHz to 875 kHz coming from this additional spring softening effect. 
	NOTE: To the best of our knowledge, changing the tuning capability of the MEMS resonators was achieved for the first time in this work.</li>
</ol>

<ol><li>Göktaş, H., Turner, K., Zaghloul, M. <a target="_blank" href="http://ieeexplore.ieee.org/document/7762782/">Enhancement in CMOS-MEMS Resonator for High Sensitive Temperature Sensing.</a> IEEE Sensors J. 17 (3), 598-603 (2017).</li>
<li>Davila, A. P., Jang, J., Gupta, A. K., Walter, T., Aronson, A., Bashir, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Microresonator+mass+sensors+for+detection+of+Bacillus+anthracis+Sterne+spores+in+air+and+water%5BTitle%5D)+AND+%22Biosens.+Bioelectron%22%5BJournal%5D)">Microresonator mass sensors for detection of Bacillus anthracis Sterne spores in air and water.</a> Biosens. Bioelectron. 22 (12), 3028-3035 (2007).</li>
<li>Lee, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Suspended+microchannel+resonators+with+piezoresistive+sensors%5BTitle%5D)+AND+%22Lab+Chip%22%5BJournal%5D)">Suspended microchannel resonators with piezoresistive sensors.</a> Lab Chip. 11 (4), 645-651 (2011).</li>
<li>Arash, H., Pourkamali, S. <a target="_blank" href="http://ieeexplore.ieee.org/document/6111426/">Fabrication and Characterization of MEMS-Based Resonant Organic Gas Sensors.</a> IEEE Sensors J. 12 (6), 1958-1964 (2012).</li>
<li>Göktaş, H., Zaghloul, M. <a target="_blank" href="http://ieeexplore.ieee.org/document/6985576/">Tuning In-Plane Fixed-Fixed Beam Resonators with Embedded Heater in CMOS Technology.</a> IEEE Electron Device Lett. 36 (2), 189-191 (2015).</li>
<li>Li, C. S., Hou, L. J., Li, S. S. <a target="_blank" href="http://ieeexplore.ieee.org/document/6112657/">Advanced CMOS-MEMS Resonator Platform.</a> IEEE Electron Device Lett. 33 (2), 272-274 (2012).</li>
<li>Li, M. H., Chen, W. C., Li, S. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Mechanically+Coupled+CMOS-MEMS+Free-Free+Beam+Resonator+Arrays+With+Ehanced+Power+Handling+Capability%5BTitle%5D)+AND+%22IEEE+Trans.+Ultrason.+Ferroelect.+Freq.+Control%22%5BJournal%5D)">Mechanically Coupled CMOS-MEMS Free-Free Beam Resonator Arrays With Ehanced Power Handling Capability.</a> IEEE Trans. Ultrason. Ferroelect. Freq. Control. 59 (3), 346-357 (2012).</li>
<li>Lopez, J. L., et al. <a target="_blank" href="http://ieeexplore.ieee.org/abstract/document/5109806/">A CMOS-MEMS RF-Tunable Bandpass Filter Based on Two High-Q 22-MHz Polysilicon Clamped-Clamped Beam Resonators.</a> IEEE Electron Device Lett. 30 (7), 718-720 (2009).</li>
<li>Khine, L., Palaniapan, M. <a target="_blank" href="http://iopscience.iop.org/article/10.1088/0960-1317/19/1/015017/meta">High-Q bulk-mode SOI square resonator with straight-beam anchors.</a> J. Micromech. Microeng. 19 (1), (2009).</li>
<li>Manca, N., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Programmable+mechanical+resonances+in+MEMS+by+localized+joule+heating+of+phase+change+materials%5BTitle%5D)+AND+%22Adv.+Mater%22%5BJournal%5D)">Programmable mechanical resonances in MEMS by localized joule heating of phase change materials.</a> Adv. Mater. 25 (44), 6430-6435 (2013).</li>
<li>Rúa, A., et al. <a target="_blank" href="http://aip.scitation.org/doi/abs/10.1063/1.4716191?journalCode=jap">Phase transition behavior in microcantilevers coated with M1-phase VO2 and M2-phase VO2:Cr thin films.</a> J. Appl. Phys. 111 (10), 104502(2012).</li>
<li>Remtema, T., Lin, L. <a target="_blank" href="http://www.sciencedirect.com/science/article/pii/S0924424701006033">Active frequency tuning for micro resonators by localized thermal stressing effects.</a> Sens. Actuators A, Phys. 91 (3), 326-332 (2001).</li>
<li>Göktaş, H., Zaghloul, M. <a target="_blank" href="http://onlinelibrary.wiley.com/doi/10.1002/2015RS005893/abstract">The Implementation of Low Power and Wide Tuning Range MEMS filters for Communication Applications.</a> Radio Sci. 51 (10), 1636-1644 (2016).</li>
<li>Göktaş, H., Zaghloul, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((The+Novel+Microhotplate%3A+A+Design+Featuring+Ultra+High+Temperature%2C+Ultra+Low+Thermal+Stress%2C+Low+Power+Consumption+and+Small+Response+Time%5BTitle%5D)+AND+%22Sensor+Comm%22%5BJournal%5D)">The Novel Microhotplate: A Design Featuring Ultra High Temperature, Ultra Low Thermal Stress, Low Power Consumption and Small Response Time.</a> Sensor Comm. , (2013).</li>
<li>Kushmerick, J. G., et al. <a target="_blank" href="https://link.springer.com/article/10.1023/A%3A1009082530479">The influence of coating structure on micromachine stiction.</a> Tribol Lett. 10 (1), (2001).</li>
<li>Kim, J. M., et al. <a target="_blank" href="http://iopscience.iop.org/article/10.1088/0960-1317/12/5/327/meta">Continuous anti-stiction coatings using self-assembled monolayers for gold microstructures.</a> J. Micromech. Microeng. 12 (5), 688-695 (2002).</li>
<li>Bhattacharya, E., et al. <a target="_blank" href="http://onlinelibrary.wiley.com/doi/10.1002/pssa.200461149/abstract">Effect of porous silicon formation on stiction in surface micromachined MEMS structures.</a> Phys. Stat. Sol. (A). 202 (8), 1482-1486 (2005).</li>
<li>Koppaka, S. B., Phinney, L. M. <a target="_blank" href="http://ieeexplore.ieee.org/document/1416917/">Release Processing Effects on Laser Repair of Stiction-Failed Microcantilevers.</a> J. Microelectromech. Syst. 14 (2), 410-418 (2005).</li>
<li>Ghatkesar, M. K., et al. <a target="_blank" href="https://www.semanticscholar.org/paper/Higher-modes-of-vibration-increase-mass-sensitivit-Ghatkesar-Barwich/b50ed2d5233ea10271788baa70517f97d465d3aa">Higher modes of vibration increase mass sensitivity in nanomechanical microcantilevers.</a> Nanotechnology. 18 (44), 445502(2007).</li>
<li>Göktaş, H., Mona, Z. <a target="_blank" href="http://ieeexplore.ieee.org/document/6688202/">High Sensitivity CMOS Portable Biosensor with Flexible Microfluidic Integration.</a> IEEE SENSORS. , (2013).</li>
</ol>

We have nothing to disclose.

One of the critical steps in building MEMS filters is to design the device based on the application area. The beam should be longer or thinner for better tuning efficiency (ppm/mW), but shorter or thinner for frequency hopping or signal tracking applications. In the same way, clear signal detection via LDV is critical in device testing which is why it is better to design the beam with at least 3-4 &#181;m thickness. Otherwise the signal will be noisy, even with a 100X lens, and it takes multiple points testing with noise...

There is a growing demand for MEMS filters due to their high reliability, low power consumption, compact design, high quality factor, and low cost. They are widely used as sensors and as core parts in wireless communication. Temperature sensors1, bio-sensors2,3, gas-sensors4, filters5,6,7, and oscillators are the most popular application areas. The most popular electrostatic MEMS filters are fixed-fixed beam5,8, cantilever2, tuning fork6, free-free beam6,7, flexural-disk design7, and square shape design9. 
There are many critical steps in realizing a MEMS filter, such as design methodology (application-based structure optimization, wide range frequency tuning range, and avoiding failures) and characterization (fast prototyping, avoiding parasitic capacitances, and detecting higher modes). Frequency tuning capability is required to compensate for any frequency changes due to fabrication tolerances, or ambient temperature variations. Different techniques10,11,12 have been reported in the literature to address this requirement; however, they have some drawbacks such as limited frequency tuning capability, low center frequency, additional post processing requirements, and external heater10,11.
In this study we present wide range frequency tuning by the Joule heating method5,13 over a limited frequency tuning range via an elastic modulus change12 (increasing the DC bias voltage between two adjacent beams) and a material phase transition method10,11. Moreover, the optimum structure selection and the application-based design were summarized in G&#246;kta&#351; and Zaghloul13. Here, we show how to tune the resonance frequency of a fixed-fixed beam by increasing the DC voltage applied to the embedded heater with the help of the LDV. The finite element analysis (FEM) simulation is synchronized with the LDV measurement in the same frame for the sake of visualizing the tuning mechanism. This includes the Joule heating and bending profile throughout the beam.
We also present the possible failures (burnt devices and stiction) and their proposed solutions. The Joule heating method in combination with the high thermal stress of the fixed-fixed beam provides wide range frequency tuning but at the same time can result in burnt devices at a certain temperature level. This is attributed to the high thermal stress between different materials14. The solution is to increase the DC voltage between the two adjacent beams, which in turn increases the tuning range (by 32%), and eliminates the need for high temperature. This "tuning the tuning-range" method was first demonstrated in G&#246;kta&#351; and Zaghloul5, explained in more detail in G&#246;kta&#351; and Zaghloul13, and re-presented here. Stiction, on the other hand, can take place during the fabrication process or resonance operation. There have been many techniques proposed to address this problem such as applying surface coating to reduce adhesion energy15,16, increasing surface roughness17, and the laser repair process18. In contrast, we present a simple technique where a low frequency square wave signal was applied between two attached beams and the separation was successfully recorded by LDV. This method can eliminate extra cost and reduce design complexity. 
Another critical step in building a state of the art MEMS filter is characterization and verification. Characterization with a network analyzer is one of the most popular and widely used methods; however, it has some drawbacks. Even small parasitic capacitance can kill the signal and so this usually requires an amplifier circuit3,6,8 for noise elimination, and it can only detect first mode resonance. On the other hand, characterization with LDV is free from this parasitic capacitance issue, and can detect much smaller displacement. This enables fast prototyping, while eliminating the need for amplifier design. Furthermore, LDV can detect higher mode resonance of MEMS filters. This feature is very promising, especially in the field of highly sensitive biosensors. A higher cantilever mode can provide much more sensitivity19. The higher mode measurement of a fixed-fixed beam with LDV is shown and applied to FEM simulation measurement. The premature results from the FEM simulation offer up to 46 times improvement in sensitivity compared to the first mode of the fixed-fixed beam.

<table><tbody><tr><td>Laser Doppler Vibrometer</td><td>Polytec</td><td>Polytec MSA-500</td><td></td></tr><tr><td>Scanning Electron Microscope</td><td>Zeiss</td><td></td><td></td></tr><tr><td>Thermal Camera</td><td>X</td><td></td><td></td></tr><tr><td>Power Supply&amp;nbsp;</td><td>Egilent</td><td>(E3631A)</td><td></td></tr><tr><td>Microscope</td><td>X</td><td></td><td></td></tr><tr><td>Coventor</td><td>Coventor</td><td></td><td>Simulation Tool</td></tr><tr><td>Cadence Virtuoso</td><td>Cadence</td><td></td><td>Simulation Tool</td></tr><tr><td>Multisim</td><td>Multisim</td><td></td><td>Simulation Tool</td></tr></tbody></table>

design and characterization methodology for efficient wide range tunable mems filters

Here, we demonstrate the advantages of the laser Doppler vibrometer (LDV) over conventional techniques (the network analyzer), as well as the techniques to create an application-based microelectromechanical systems (MEMS) filter and how to use it efficiently (i.e., tuning the tuning-capability and avoiding both failure and stiction). LDV enables crucial measurements that are impossible with the network analyzer, such as higher mode detection (highly sensitive biosensor application) and resonance measurement for very small devices (fast prototyping). Accordingly, LDV was used to characterize the frequency tuning range and resonance frequency at different modes of the MEMS filters built for this study. This wide range frequency tuning mechanism is based simply on Joule heating from the embedded heaters and relatively high thermal stress with respect to the temperature of a fixed-fixed beam. However, we demonstrate that another limitation of this method is the resulting high thermal stress, which can burn the devices. Further improvement was achieved and shown for the first time in this study, such that the tuning capability was increased by 32% through an increase in the applied DC bias voltage (25 V to 35 V) between the two adjacent beams. This important finding eliminates the need for the extra Joule heating at the wider frequency tuning range. Another possible failure is through stiction and requirement of structure optimization: We propose a simple and easy technique of low frequency square wave signal application that can successfully separate the beams and eliminates the need for the more sophisticated and complicated methods given in the literature. The above findings necessitate a design methodology, and so we also provide an application-based design.

Stiction was avoided by applying low frequency square wave signal and this was verified by using LDV (Figure 1). Possible failure due to high thermal stress14 when applying relatively higher bias DC voltage to the embedded heaters was verified under microscope (Figure 2). The FEM program was used to derive the higher modes for the beam (Figure 3). Changing the tuning capabilit...

Watch this Scientific Journal Video about Design and Characterization Methodology for Efficient Wide Range Tunable MEMS Filters at JoVE.com

Design and Characterization Methodology for Efficient Wide Range Tunable MEMS Filters

A protocol for a fixed-fixed beam design using a laser Doppler vibrometer (LDV), including the measurement of frequency tuning, modification of tuning capability, and avoidance of device failure and stiction, is presented. The superiority of the LDV method over the network analyzer is demonstrated due to its higher mode capability.

Here, we demonstrate the advantages of the laser Doppler vibrometer (LDV) over conventional techniques (the network analyzer), as well as the techniques ...

design-characterization-methodology-for-efficient-wide-range-tunable

Research

JoVE Journal

Engineering

6.1K Views.  Harran University. A protocol for a fixed-fixed beam design using a laser Doppler vibrometer (LDV), including the measurement of frequency tuning, modification of tuning capability, and avoidance of device failure and stiction, is presented. The superiority of the LDV method over the network analyzer is demonstrated due to its higher mode capability.

Video: Design and Characterization Methodology for Efficient Wide Range Tunable MEMS Filters

Fabrication of Silica Ultra High Quality Factor Microresonators

Whispering gallery resonant cavities confine light in circular orbits at their periphery.1-2 The photon storage lifetime in the cavity, quantified by the quality factor (Q) of the cavity, can be in excess of 500ns for cavities with Q factors above 100 million. As a result of their low material losses, silica microcavities have demonstrated some of the longest photon lifetimes to date1-2. Since a portion of the circulating light extends outside the resonator, these devices can also be used to probe the surroundings. This interaction has enabled numerous experiments in biology, such as single molecule biodetection and antibody-antigen kinetics, as well as discoveries in other fields, such as development of ultra-low-threshold microlasers, characterization of thin films, and cavity quantum electrodynamics studies.3-7
The two primary silica resonant cavity geometries are the microsphere and the microtoroid. Both devices rely on a carbon dioxide laser reflow step to achieve their ultra-high-Q factors (Q&gt;100 million).1-2,8-9 However, there are several notable differences between the two structures. Silica microspheres are free-standing, supported by a single optical fiber, whereas silica microtoroids can be fabricated on a silicon wafer in large arrays using a combination of lithography and etching steps. These differences influence which device is optimal for a given experiment.
Here, we present detailed fabrication protocols for both types of resonant cavities. While the fabrication of microsphere resonant cavities is fairly straightforward, the fabrication of microtoroid resonant cavities requires additional specialized equipment and facilities (cleanroom). Therefore, this additional requirement may also influence which device is selected for a given experiment.
Introduction
An optical resonator efficiently confines light at specific wavelengths, known as the resonant wavelengths of the device. 1-2 The common figure of merit for these optical resonators is the quality factor or Q. This term describes the photon lifetime (&tau;o) within the resonator, which is directly related to the resonator's optical losses. Therefore, an optical resonator with a high Q factor has low optical losses, long photon lifetimes, and very low photon decay rates (1/&tau;o). As a result of the long photon lifetimes, it is possible to build-up extremely large circulating optical field intensities in these devices. This very unique property has allowed these devices to be used as laser sources and integrated biosensors.10
A unique sub-class of resonators is the whispering gallery mode optical microcavity. In these devices, the light is confined in circular orbits at the periphery. Therefore, the field is not completely confined within the device, but evanesces into the environment. Whispering gallery mode optical cavities have demonstrated some of the highest quality factors of any optical resonant cavity to date.9,11 Therefore, these devices are used throughout science and engineering, including in fundamental physics studies and in telecommunications as well as in biodetection experiments. 3-7,12
Optical microcavities can be fabricated from a wide range of materials and in a wide variety of geometries. A few examples include silica and silicon microtoroids, silicon, silicon nitride, and silica microdisks, micropillars, and silica and polymer microrings.13-17 The range in quality factor (Q) varies as dramatically as the geometry. Although both geometry and high Q are important considerations in any field, in many applications, there is far greater leverage in boosting device performance through Q enhancement. Among the numerous options detailed previously, the silica microsphere and the silica microtoroid resonator have achieved some of the highest Q factors to date.1,9 Additionally, as a result of the extremely low optical loss of silica from the visible through the near-IR, both microspheres and microtoroids are able to maintain their Q factors over a wide range of testing wavelengths.18 Finally, because silica is inherently biocompatible, it is routinely used in biodetection experiments.
In addition to high material absorption, there are several other potential loss mechanisms, including surface roughness, radiation loss, and contamination loss.2 Through an optimization of the device size, it is possible to eliminate radiation losses, which arise from poor optical field confinement within the device. Similarly, by storing a device in an appropriately clean environment, contamination of the surface can be minimized. Therefore, in addition to material loss, surface scattering is the primary loss mechanism of concern.2,8
In silica devices, surface scattering is minimized by using a laser reflow technique, which melts the silica through surface tension induced reflow. While spherical optical resonators have been studied for many years, it is only with recent advances in fabrication technologies that researchers been able to fabricate high quality silica optical toroidal microresonators (Q&gt;100 million) on a silicon substrate, thus paving the way for integration with microfluidics.1
The present series of protocols details how to fabricate both silica microsphere and microtoroid resonant cavities. While silica microsphere resonant cavities are well-established, microtoroid resonant cavities were only recently invented.1 As many of the fundamental methods used to fabricate the microsphere are also used in the more complex microtoroid fabrication procedure, by including both in a single protocol it will enable researchers to more easily trouble-shoot their experiments.

We describe the use of a carbon dioxide laser reflow technique to fabricate silica resonant cavities, including free-standing microspheres and on-chip microtoroids. The reflow method removes surface imperfections, allowing long photon lifetimes within both devices. The resulting devices have ultra high quality factors, enabling applications ranging from telecommunications to biodetection.

Whispering gallery resonant cavities confine light in circular orbits at their periphery.1-2 The photon storage lifetime in the cavity, quantified by the ...

Terahertz Microfluidic Sensing Using a Parallel-plate Waveguide Sensor

Refractive index (RI) sensing is a powerful noninvasive and label-free sensing technique for the identification, detection and monitoring of microfluidic samples with a wide range of possible sensor designs such as interferometers and resonators 1,2. Most of the existing RI sensing applications focus on biological materials in aqueous solutions in visible and IR frequencies, such as DNA hybridization and genome sequencing. At terahertz frequencies, applications include quality control, monitoring of industrial processes and sensing and detection applications involving nonpolar materials.
Several potential designs for refractive index sensors in the terahertz regime exist, including photonic crystal waveguides 3, asymmetric split-ring resonators 4, and photonic band gap structures integrated into parallel-plate waveguides 5. Many of these designs are based on optical resonators such as rings or cavities. The resonant frequencies of these structures are dependent on the refractive index of the material in or around the resonator. By monitoring the shifts in resonant frequency the refractive index of a sample can be accurately measured and this in turn can be used to identify a material, monitor contamination or dilution, etc.
The sensor design we use here is based on a simple parallel-plate waveguide 6,7. A rectangular groove machined into one face acts as a resonant cavity (Figures 1 and 2). When terahertz radiation is coupled into the waveguide and propagates in the lowest-order transverse-electric (TE1) mode, the result is a single strong resonant feature with a tunable resonant frequency that is dependent on the geometry of the groove 6,8. This groove can be filled with nonpolar liquid microfluidic samples which cause a shift in the observed resonant frequency that depends on the amount of liquid in the groove and its refractive index 9.
Our technique has an advantage over other terahertz techniques in its simplicity, both in fabrication and implementation, since the procedure can be accomplished with standard laboratory equipment without the need for a clean room or any special fabrication or experimental techniques. It can also be easily expanded to multichannel operation by the incorporation of multiple grooves 10. In this video we will describe our complete experimental procedure, from the design of the sensor to the data analysis and determination of the sample refractive index.

The procedure for implementing a refractive index sensor for terahertz frequencies based on a grooved parallel-plate waveguide geometry is described here. The method yields a measurement of the refractive index of a small volume of liquid through monitoring of the shift in the resonant frequency of the waveguide structure

Refractive index (RI) sensing is a powerful noninvasive and label-free sensing technique for the identification, detection and monitoring of microfluidic ...

Simulation, Fabrication and Characterization of THz Metamaterial Absorbers

Metamaterials (MM), artificial materials engineered to have properties that may not be found in nature, have been widely explored since the first theoretical1 and experimental demonstration2 of their unique properties. MMs can provide a highly controllable electromagnetic response, and to date have been demonstrated in every technologically relevant spectral range including the optical3, near IR4, mid IR5 , THz6 , mm-wave7 , microwave8 and radio9 bands. Applications include perfect lenses10, sensors11, telecommunications12, invisibility cloaks13 and filters14,15. We have recently developed single band16, dual band17 and broadband18 THz metamaterial absorber devices capable of greater than 80% absorption at the resonance peak. The concept of a MM absorber is especially important at THz frequencies where it is difficult to find strong frequency selective THz absorbers19. In our MM absorber the THz radiation is absorbed in a thickness of ~ &lambda;/20, overcoming the thickness limitation of traditional quarter wavelength absorbers. MM absorbers naturally lend themselves to THz detection applications, such as thermal sensors, and if integrated with suitable THz sources (e.g. QCLs), could lead to compact, highly sensitive, low cost, real time THz imaging systems.

This protocol outlines the simulation, fabrication and characterization of THz metamaterial absorbers. Such absorbers, when coupled with an appropriate sensor, have applications in THz imaging and spectroscopy.

Metamaterials (MM),  artificial materials engineered to have properties that may not be found in  nature, have been widely explored since the first ...

Real-Time DC-dynamic Biasing Method for Switching Time Improvement in Severely Underdamped Fringing-field Electrostatic MEMS Actuators

Mechanically underdamped electrostatic fringing-field MEMS actuators are well known for their fast switching operation in response to a unit step input bias voltage. However, the tradeoff for the improved switching performance is a relatively long settling time to reach each gap height in response to various applied voltages. Transient applied bias waveforms are employed to facilitate reduced switching times for electrostatic fringing-field MEMS actuators with high mechanical quality factors. Removing the underlying substrate of the fringing-field actuator creates the low mechanical damping environment necessary to effectively test the concept. The removal of the underlying substrate also a has substantial improvement on the reliability performance of the device in regards to failure due to stiction. Although DC-dynamic biasing is useful in improving settling time, the required slew rates for typical MEMS devices may place aggressive requirements on the charge pumps for fully-integrated on-chip designs. Additionally, there may be challenges integrating the substrate removal step into the back-end-of-line commercial CMOS processing steps. Experimental validation of fabricated actuators demonstrates an improvement of 50x in switching time when compared to conventional step biasing results. Compared to theoretical calculations, the experimental results are in good agreement.

The robust device design of fringing-field electrostatic MEMS actuators results in inherently low squeeze-film damping conditions and long settling times when performing switching operations using conventional step biasing. Real-time switching time improvement with DC-dynamic waveforms reduces the settling time of fringing-field MEMS actuators when transitioning between up-to-down and down-to-up states.

Mechanically underdamped electrostatic fringing-field MEMS actuators are well known for their fast switching operation in response to a unit step input ...

Using Micro-Electro-Mechanical Systems (MEMS) to Develop Diagnostic Tools

Using Micro-Electro-Mechanical Systems MEMS to Develop Diagnostic Tools

I'm U De Mercy, I'm the principal investigator of Bio Stick MAMs in Medicine Labs. I did my PhD at Stanford University in electrical engineering. I did a postdoc at Harvard Medical School at MGH working on bio acoustic MAMs or mostly bio MAs.And now I'm continuing as a faculty at Harvard Medical School at Harvard MIT Health Sciences and Technology. And continuing with my research that I enjoy very much. The work that I did during my PhD was very much related to droplets and using acoustics to generate precise size droplets and control their location and position to deposit very sensitive polymers.Now we apply similar technologies to encapsulate cells in droplets and then position them on the surfaces for various applications in tissue engineering to print cells on the surfaces to pattern cells. Also, there are applications where you want to see that you can encapsulate few cells or a single cell or from the same population and to see the differences between the same population from one cell to another there being able to encapsulate cells in droplets at high throughput rates like 10, 000 cells per second kind of rates becomes very important and very useful to understand biological problems. So my research currently has two legs, I would say.One is this cell encapsulation work that I just talked about. How do we pack a cell in a droplet that's, and how can we do this repeatedly and reliably and without harming the cells? So after the cell is ejected or encapsulated in the droplet, we should be able to precisely located on a surface the cell should be functional and alive and viable.It shouldn't be harmed by the effects of ejection and so forth. So currently we have a system where we use acoustic drop focused waves to generate these droplets from open pools where we can encapsulate down to single cells in these really small droplets that are comparable to the cell size. That's very exciting in terms of applying this to cell printing and tissue engineering.The other side of my research is again using these MAMs micro electromechanical system technologies to develop low cost diagnostic tools. Mostly this research uses microfluidic approaches where we can introduce whole blood, very small volumes such as blood from a prick finger prick, less than 10 microliters that you can introduce to a chip. And from that blood we can encapsulate or you can capture certain sub-cellular populations from whole blood.Why is this important? Why should this be cheap for global health applications On top of the mountain in Africa, you want to be able to tell for a HIV patient, for instance, how many CD four T lymphocytes this patient has. Because World Health Organization says that below 200 CD four cells per microliter, you have to start treating the patients In the developed world, you use hundreds of thousands dollars of vert flow cytometers to be able to get this data.And it takes of course time to use and skill of course to be able to use these like huge table size machines. Our small chip can be introduced this small finger brick 10 microliters of whole blood and it'll capture the CD four cells using the surface protein affinities. And then you can rapidly count these cells that are captured because you know that there are CD four cells which are attached to CD four proteins antibodies on the surface of the chip.By controlling the flow rates and shear, you can make sure that the specificity and efficiency for these cell types are optimized. And there's of course always non specific binding, but by our sheer approaches we minimize these effects and make sure that between plus minus 10%errors, which is sufficient to make a diagnostic or prognostic decision on top mountain in Africa. So this has interesting applications for global health as well as these low cost techniques that are disposable can impact the developed world because now these fast blood tests which are really low cost can impact the tests that we use in the developed world.If they're optimized to higher levels of efficiency and specificity, which our initial data shows that they can be, then it'll definitely impact our lives. When I first finished my PhD, I knew more about microfluidics and that in MAMs more than anything else I could see, especially with the droplet application, I could see that if I could print cells and encapsulate cells or manipulate the few cells, single cells, that it'll have great applications in biotechnology area. And I was very interested in working on things that will actually impact people's lives.I used to apply these technologies to semiconductor industry, but then I wanted to to be useful directly to people. So I, that's what directed me towards problems in healthcare. And then that's why I made a big shift and came to a hospital, mass general hospital to the postdoc.And there, you know, I got exposed more and more to problems and it seems, and it's all clear that one of the biggest problems of the world today is global health and there comes illnesses like tuberculosis, HIV that kills thousands of people per day. And these people die not because the drugs are not there, but because there aren't enough diagnostic tools which is much more expensive than the existing drugs. So I know the technology side of the things very well.I get, I got exposed to the biological problems and the more I learned about them, the more I could see that I could make an impact. And that's how it all grew from there. And it's still continuing now.Like I seen biotechnology and in medicine there are like so many problems that actually directly cause our result into loss of people's lives and the technologies, the technology aspect of it and being able to apply it to real world medical problems is a great, I think path to follow. This is like how this whole biotechnology area I think is kind of growing and it's becoming very impactful. If you think about it, HIV cancer, those are big killers in the world.And current micro technologies, current approaches could maybe benefit from early detection of cancer by capturing these cells from blood or making it really cheap, which will make it available to masses. So all these things coming together, I think this whole microtechnology applications in healthcare in medicine could impact the future of human beings. So that's how all these things kind of come together I guess.And being in the Harvard MIT health sciences and technology where on one end you have the technology and on the other end you direct aspect, direct access to patients, direct access to doctors in Brigham and Women's Hospital where I work is a great environment. cause every person that you talk to have their problems and you could maybe it's coming from a different background, provide interesting solutions to those existing real world problems. So this is, I think how I see the whole perspective of how these medical problems meet the technology side.And we try to impact people's lives in a positive way. In single cell cap encapsulation or in cell encapsulation, the major challenge is to be able to encapsulate reliably and repeatably single cells. So you're ejecting 10, 000 hundred thousand cells per second.And how do you make sure that each droplet that you eject has a single cell? This statistics where you are changing the droplet size and minimizing the, optimizing the cell size over the droplet size, which goes into the old problem of packing spheres in a volume and what is the most efficient way to do it. So that's the biggest challenge in the cell printing area for from the technological side.The other aspect of it is now you can say print and precisely locate these cells. How can you generate these three dimensional tissues and how can you keep them alive and how can you transplant them? There comes in the biological end of the problems where you want to now exactly mimic the tissue.We have current approaches to be able to print an eyelet, a pancreas eyelet, and to be able to print using smooth muscle cells, a bladder tissue, directly mimicking what is in human or red bladder so that then we can make this tissue and test how well it performs compared to the real world existing native tissues. So that's the major challenge I guess by taking these novel technologies, how can you make this the ideal tissue that can be replaced, that can be transplanted, that's the, you know, biggest problem from the beginning because the control on single cell gives you the capability to precisely locate them. And then how do you kind of grow it from there?So it's something transplantable there where the impact of human life comes. So that's on that end, what the challenges are in the tissue engineering side of the things in southern encapsulation and in diagnostics using microfluidics, the challenge is to be able to capture a cell out of billions of cells. It's like you have a sugar particle in a salt can and you're trying to pull out that sugar particle and that's like one in a billion.So you, you're processing microliters to milliliters of whole blood and you want to be able to specifically and efficiently without any technological problems like clogging or any biological problems like non-specific binding, you want to be able to isolate at one cell out of billion cells in one microliter of whole blood, you have a few millions of cells. And for the CD four T lymphocyte case in that one microliter, we are after about thousand cells per microliter. So it's like one cell in a thousand kind of a problem where we have shown that it's doable.But when you want to go after circulating tumor cells for cancer, then the challenge becomes one in a billion. So to some summarize it, the challenge is how do we capture this one cell out of billions of other cells around it? And what is the technological aspects?What are the flow rates? What is the device design? What, what are the flow rates?What, what are the volumes of blood that has to be processed and how do you make sure that the cells you capture are the ones that you actually wanted to capture? All these aspects kind formed the whole challenge that can be summarized in one sentence, which is how do you capture this one rare cell out of billion others? So it's really a needle in a haystack kind of problem, which is exciting.And I think this is moving the technologies forward to address these problems is the current technological challenge. Being able to bring them to a level where they do the job that they're supposed to do is one thing. And then taking it to the clinic and making it a product has all different expertise such as being able to start a company, being able to patent these things and all those other aspects come into the picture.And I think we as scientists primarily don't have all of those skills that takes the products from desktop to products. So there comes in again, the importance of collaborations with people who have different backgrounds. And there are many technologies out there that I think could be very impactful and useful.They sometimes don't make it to the clinic or to the use of humankind either because it just didn't happen, wasn't the right time or other effects came into the picture that I just talked about. Or sometimes there isn't a direct link that you couldn't see that the technology could actually solve that problem. And the person who deals with the problem, or let's say the biologists or people in medicine are used to doing it one way for many years.And the people on the technology side mostly if you're not having a biotechnology focus, are not aware of the problems in medicine. So bringing these two sides together is a serious interdisciplinary research. And I think in the past years, the whole emphasis both at NIH level and that we see in research labs and universities is to generate interdisciplinary research.So people end up getting PhDs where they have to know multiple fields. For instance, for my PhD, I had to know acoustics, I had to know mems, I had to know microfluidics and I had to apply this for polymers. So now you see kinda very depth of knowledge in one field where you had to know actually three other fields quite well to be able to solve that problem.So I think the answer is in interdisciplinary research combined with people with business skills to make it available to use of the people, it's a whole big process and some inefficiencies that come in at certain locations. Cause that low percent of tech transfer.

Biology

Fabrication and Characterization of Superconducting Resonators

Superconducting microwave resonators are of interest for a wide range of applications, including for their use as microwave kinetic inductance detectors (MKIDs) for the detection of faint astrophysical signatures, as well as for quantum computing applications and materials characterization. In this paper, procedures are presented for the fabrication and characterization of thin-film superconducting microwave resonators. The fabrication methodology allows for the realization of superconducting transmission-line resonators with features on both sides of an atomically smooth single-crystal silicon dielectric. This work describes the procedure for the installation of resonator devices into a cryogenic microwave testbed and for cool-down below the superconducting transition temperature. The set-up of the cryogenic microwave testbed allows one to do careful measurements of the complex microwave transmission of these resonator devices, enabling the extraction of the properties of the superconducting lines and dielectric substrate (e.g., internal quality factors, loss and kinetic inductance fractions), which are important for device design and performance.

Superconducting microwave resonators are of interest for detection of light, quantum computing applications and materials characterization. This work presents a detailed procedure for fabrication and characterization of superconducting microwave resonator scattering parameters.

Superconducting microwave resonators are of interest for a wide range of applications, including for their use as microwave kinetic inductance detectors ...

Fabrication of 3D Carbon Microelectromechanical Systems (C-MEMS)

A wide range of carbon sources are available in nature, with a variety of micro-/nanostructure configurations. Here, a novel technique to fabricate long and hollow glassy carbon microfibers derived from human hairs is introduced. The long and hollow carbon structures were made by the pyrolysis of human hair at 900 &#176;C in a N2 atmosphere. The morphology and chemical composition of natural and pyrolyzed human hairs were investigated using scanning electron microscopy (SEM) and electron-dispersive X-ray spectroscopy (EDX), respectively, to estimate the physical and chemical changes due to pyrolysis. Raman spectroscopy was used to confirm the glassy nature of the carbon microstructures. Pyrolyzed hair carbon was introduced to modify screen-printed carbon electrodes ; the modified electrodes were then applied to the electrochemical sensing of dopamine and ascorbic acid. Sensing performance of the modified sensors was improved as compared to the unmodified sensors. To obtain the desired carbon structure design, carbon micro-/nanoelectromechanical system (C-MEMS/C-NEMS) technology was developed. The most common C-MEMS/C-NEMS fabrication process consists of two steps: (i) the patterning of a carbon-rich base material, such as a photosensitive polymer, using photolithography; and (ii) carbonization through the pyrolysis of the patterned polymer in an oxygen-free environment. The C-MEMS/NEMS process has been widely used to develop microelectronic devices for various applications, including in micro-batteries, supercapacitors, glucose sensors, gas sensors, fuel cells, and triboelectric nanogenerators. Here, recent developments of a high-aspect ratio solid and hollow carbon microstructures with SU8 photoresists are discussed. The structural shrinkage during pyrolysis was investigated using confocal microscopy and SEM. Raman spectroscopy was used to confirm the crystallinity of the structure, and the atomic percentage of the elements present in the material before and after pyrolysis was measured using EDX.

Fabrication of 3D Carbon Microelectromechanical Systems C-MEMS

Long and hollow glassy carbon microfibers were fabricated based on the pyrolysis of a natural product, human hair. The two fabrication steps of carbon microelectromechanical and carbon nanoelectromechanical systems, or C-MEMS and C-NEMS, are: (i) photolithography of a carbon-rich polymer precursor and (ii) pyrolysis of the patterned polymer precursor.

A wide range of carbon sources are available in nature, with a variety of micro-/nanostructure configurations. Here, a novel technique to fabricate long ...

Microparticle Manipulation by Standing Surface Acoustic Waves with Dual-frequency Excitations

We demonstrate a method for increasing the tuning ability of a standing surface acoustic wave (SSAW) for microparticles manipulation in a lab-on-a-chip (LOC) system. The simultaneous excitation of the fundamental frequency and its third harmonic, which is termed as dual-frequency excitation, to a pair of interdigital transducers (IDTs) could generate a new type of standing acoustic waves in a microfluidic channel. Varying the power and the phase in the dual-frequency excitation signals results in a reconfigurable field of the acoustic radiation force applied to the microparticles across the microchannel (e.g., the number and location of the pressure nodes and the microparticle concentrations at the corresponding pressure nodes). This article demonstrates that the motion time of the microparticle to only one pressure node can be reduced ~2-fold at the power ratio of the fundamental frequency greater than ~90%. In contrast, there are three pressure nodes in the microchannel if less than this threshold. Furthermore, adjusting the initial phase between the fundamental frequency and the third harmonic results in different motion rates of the three SSAW pressure nodes, as well as in the percentage of microparticles at each pressure node in the microchannel. There is a good agreement between the experimental observation and the numerical predictions. This novel excitation method can easily and non-invasively integrate into the LOC system, with a wide tenability and only a few changes to the experimental set-up.

A protocol for manipulating the microparticles in a microfluidic channel with a dual-frequency excitation is presented.

We demonstrate a method for increasing the tuning ability of a standing surface acoustic wave (SSAW) for microparticles manipulation in a lab-on-a-chip ...

Rapid Repetition Rate Fluctuation Measurement of Soliton Crystals in a Microresonator

Temporal solitons have attracted great interest in the past decades for their behavior in a steady state, where the dispersion is balanced by the nonlinearity in a propagation Kerr medium. The development of dissipative Kerr solitons (DKSs) in high-Q microcavities drives a novel, compact, chip-scale soliton source. When DKSs serve as femtosecond pulses, the repetition rate fluctuation can be applied to ultrahigh precision metrology, high-speed optical sampling, and optical clocks, etc. In this paper, the rapid repetition rate fluctuation of soliton crystals (SCs), a special state of DKSs where particle-like solitons are tightly packed and fully occupy a resonator, is measured based on the well-known delayed self-heterodyne method. The SCs are generated using a thermal-controlled method. The pump is a frequency fixed laser with a linewidth of 100 Hz. The integral time in frequency fluctuation measurements is controlled by the length of the delay fiber. For a SC with a single vacancy, the repetition rate fluctuations are ~53.24 Hz within 10 &#181;s and ~509.32 Hz within 125 &#181;s, respectively.

Here, we present a protocol to generate soliton crystals in a butterfly-packaged micro-ring resonator using a thermal tuned method. Further, the repetition rate fluctuations of a soliton crystal with a single vacancy are measured using a delayed self-heterodyne method.

Temporal solitons have attracted great interest in the past decades for their behavior in a steady state, where the dispersion is balanced by the ...

Characterizing Viscoelastic Properties of 3D-Printed Polymer Metamaterials

Characterizing Dissipative Elastic Metamaterials Produced by Additive Manufacturing

Viscoelastic behavior can be beneficial in enhancing the unprecedented dynamics of polymer metamaterials or, in contrast, negatively impacting their wave control mechanisms. It is, therefore, crucial to properly characterize the viscoelastic properties of a polymer metamaterial at its working frequencies to understand viscoelastic effects. However, the viscoelasticity of polymers is a complex phenomenon, and the data on storage and loss moduli at ultrasonic frequencies are extremely limited, especially for additively manufactured polymers. This work presents a protocol to experimentally characterize the viscoelastic properties of additively manufactured polymers and to use them in the numerical analysis of polymer metamaterials. Specifically, the protocol includes the description of the manufacturing process, experimental procedures to measure the thermal, viscoelastic, and mechanical properties of additively manufactured polymers, and an approach to use these properties in finite-element simulations of the metamaterial dynamics. The numerical results are validated in ultrasonic transmission tests. To exemplify the protocol, the analysis is focused on acrylonitrile butadiene styrene (ABS) and aims at characterizing the dynamic behavior of a simple metamaterial made from it by using fused deposition modeling (FDM) three-dimensional (3D) printing. The proposed protocol will be helpful for many researchers to estimate viscous losses in 3D-printed polymer elastic metamaterials that will improve the understanding of material-property relations for viscoelastic metamaterials and eventually stimulate the use of 3D-printed polymer metamaterial parts in various applications.

Additively manufactured polymers have been widely used for producing elastic metamaterials. The viscoelastic behavior of these polymers at ultrasonic frequencies remains, however, poorly studied. This study reports a protocol to estimate the viscoelastic properties of 3D-printed polymers and show how to use them to analyze the metamaterial dynamics.

Viscoelastic behavior can be beneficial in enhancing the unprecedented dynamics of polymer metamaterials or, in contrast, negatively impacting their wave ...