Name: real-time dc-dynamic biasing method for switching time improvement in severely underdamped fringing-field electrostatic mems actuators
Uploaded: 2014-08-15T17:00:00
Description: Watch this Scientific Journal Video about Real-Time DC-dynamic Biasing Method for Switching Time Improvement in Severely Underdamped Fringing-field Electrostatic MEMS Actuators at JoVE.com

The authors wish to thank Ryan Tung for his assistance and useful technical discussions.
The authors also wish to acknowledge the assistance and support of the Birck Nanotechnology Center technical staff. This work was supported by the Defense Advanced Research Projects Agency under the Purdue Microwave Reconfigurable Evanescent-Mode Cavity Filters Study. And also by NNSA Center of Prediction of Reliability, Integrity and Survivability of Microsystems and Department of Energy under Award Number DE-FC5208NA28617. The views, opinions, and/or findings contained in this paper/presentation are those of the authors/presenters and should not be interpreted as representing the official views or policies, either expressed or implied, of the Defense Advanced Research Projects Agency or the Department of Defense.

1. Fabrication of EFFA MEMS Fixed-fixed Beams (See Figure 3 for Summarized Process)
<ol>
	<li>UV lithography and chemical wet etch of silicon dioxide with buffered hydrofluoric acid (CAUTION27).
	<ol>
		<li>Use an oxidized, low resistivity silicon substrate.</li>
		<li>Fill a glass beaker with acetone28 (enough to submerge the sample), place the sample in the acetone filled beaker, and sonicate for 5 min in a water bath sonicator.</li>
		<li>Without drying, directly transfer the sample from the ac...

<ol>
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Low residual stress Au film deposition and a dry release with XeF2 are critically components in the successful fabrication of the device. Electrostatic fringing-field actuators provide relatively low forces when compared to parallel-plate field actuators. Typical MEMS thin film stresses of &#62;60 MPa will result in excessively high drive voltages which can potentially compromise the reliability of EFFA MEMS. For this reason the electroplating recipe is carefully characterized to yield a thin film with low bi-...

Microelectromechanical systems (MEMS) utilize several actuation mechanisms to achieve mechanical displacement. The most popular are thermal, piezoelectric, magnetostatic, and electrostatic. For short switching time, electrostatic actuation is the most popular technique1,2. In practice, critically-damped mechanical designs deliver the best compromise between initial rise time and settling time. Upon applying the DC bias and actuating the membrane down towards the pull-down electrode, the settling time is not a significant issue as the membrane will snap down and adhere to the dielectric coated actuation electrode. Several applications have benefited from the aforementioned electrostatic actuation design3-8. However, the presence of the dielectric coated pull-down electrode makes the actuator susceptible to dielectric charging and stiction.
MEMS membranes can utilize an underdamped mechanical design to achieve a fast initial rise time. An example of an underdamped mechanical design is the electrostatic fringing-field actuated (EFFA) MEMS. This topology has exhibited far less vulnerability to typical failure mechanisms that plague electrostatic based designs9-20. The absence of the parallel counter electrode and consequently the parallel electric field is why these MEMS are appropriately called &#8220;fringing-field&#8221; actuated (Figure 1). For the EFFA design, the pull-down electrode is split into two separate electrodes that are positioned laterally offset to the moving membrane, completely eliminating the overlap between the movable and stationary parts of the device. However, the removal of the substrate from beneath the movable membrane significantly reduces the squeeze film damping component thereby increasing the settling time. Figure 2B&#160;is an example of the settling time in response to standard step biasing. Transient, or DC-dynamic applied biasing in real-time can be used to improve the settling time20-26. Figures 2C and 2D qualitatively illustrate&#160;how a time varying waveform can effectively cancel the ringing. Previous research efforts utilize numerical methods to calculate the precise voltage and timings of the input bias to improve the switching time. The method in this work uses compact closed form expressions to calculate the input bias waveform parameters. Additionally, previous work focused on parallel plate actuation. While the structures are designed to be underdamped, squeeze-film damping is still available in this configuration. The actuation method presented in this work is fringing-field actuation. In this configuration squeeze-film damping is effectively eliminated. This represents an extreme case where the mechanical damping of the MEMS beam is very low. This paper describes how to fabricate the EFFA MEMS devices and perform the measurement to experimentally validate the waveform concept.

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			<td height="21" style="height:21px;width:251px;">Chemical</td>
			<td style="width:163px;">Company</td>
			<td style="width:137px;">Catalogue number</td>
			<td style="width:195px;">Comments (optional)</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Buffered oxide etchant</td>
			<td>Mallinckrodt Baker</td>
			<td>1178</td>
			<td>Silicon dioxide etch, Ti etch</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Acetone</td>
			<td>Mallinckrodt Baker</td>
			<td>5356</td>
			<td>wafer clean</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Isopropyl alcohol</td>
			<td>Honeywell</td>
			<td>BDH-140</td>
			<td>wafer clean</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Hexamethyldisilizane</td>
			<td>Mallinckrodt Baker</td>
			<td>5797</td>
			<td>adhesion promoter</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Microposit SC 1827 Positive Photoresist</td>
			<td>Shipley Europe Ltd</td>
			<td>44090</td>
			<td>Pattern, electroplating</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Microposit MF-26A developer</td>
			<td>Shipley Europe Ltd</td>
			<td>31200</td>
			<td>Develop SC 1827</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Tetramethylammonium hydroxide</td>
			<td>Sigma-Aldrich</td>
			<td>334901</td>
			<td>Bulk Si etch</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Hydrofluroic acid</td>
			<td>Sciencelab.com</td>
			<td>SLH2227</td>
			<td>Silicon dioxide etch</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Sulfuric acid</td>
			<td>Sciencelab.com</td>
			<td>SLS2539</td>
			<td>wafer clean</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Hydrogen peroxide</td>
			<td>Sciencelab.com</td>
			<td>SLH1552</td>
			<td>Wafer clean</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Transene Sulfite Gold TSG-250</td>
			<td>Transense</td>
			<td>110-TSG-250</td>
			<td>Au electroplating solution</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Baker PRS-3000 Positive Resist Stripper</td>
			<td>Mallinckrodt Baker</td>
			<td>6403</td>
			<td>Photoresist stripper</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Gold etchant type TFA</td>
			<td>Transense</td>
			<td>060-0015000</td>
			<td>Au etch</td>
		</tr>
	</tbody>
</table>

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 setup in Figure 4 is used to capture the deflection versus time characteristics of the MEMS bridges. By using the laser doppler vibrometer in its continuous measurement mode, the precise voltage and time parameters can be found to result in minimum beam oscillation for the desired gap height. Figure 5 illustrates an example beam deflection corresponding to the 60 V gap height. It is seen that virtually all of the oscillation is removed. Not only is the dynamic waveform useful for one...

Watch this Scientific Journal Video about Real-Time DC-dynamic Biasing Method for Switching Time Improvement in Severely Underdamped Fringing-field Electrostatic MEMS Actuators at JoVE.com

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

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 ...

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