Name: design and characterization methodology for efficient wide range tunable mems filters
Uploaded: 2018-02-04T13:00:00
Description: Watch this Scientific Journal Video about Design and Characterization Methodology for Efficient Wide Range Tunable MEMS Filters at JoVE.com

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 Fabr...

<ol>
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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 border="0" cellpadding="0" cellspacing="0" width="573">
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Laser Doppler Vibrometer</td>
			<td style="width:71px;">Polytec</td>
			<td style="width:119px;">Polytec MSA-500</td>
			<td style="width:167px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Scanning Electron Microscope</td>
			<td style="width:71px;">Zeiss</td>
			<td style="width:119px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Thermal Camera</td>
			<td style="width:71px;">X</td>
			<td style="width:119px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Power Supply&#160;</td>
			<td style="width:71px;">Egilent</td>
			<td style="width:119px;">(E3631A)</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Microscope</td>
			<td style="width:71px;">X</td>
			<td style="width:119px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Coventor</td>
			<td style="width:71px;">Coventor</td>
			<td style="width:119px;"></td>
			<td>Simulation Tool</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Cadence Virtuoso</td>
			<td style="width:71px;">Cadence</td>
			<td style="width:119px;"></td>
			<td>Simulation Tool</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Multisim</td>
			<td style="width:71px;">Multisim</td>
			<td style="width:119px;"></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 ...

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