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

Summary

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.

Abstract

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.

Introduction

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 technique^1,2. 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 h....

Protocol

1. Fabrication of EFFA MEMS Fixed-fixed Beams (See Figure 3 for Summarized Process)

1. UV lithography and chemical wet etch of silicon dioxide with buffered hydrofluoric acid (CAUTION²⁷).
   1. Use an oxidized, low resistivity silicon substrate.
   2. 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.
   3. Without drying, directly transfer the sample from the acetone beaker to a beaker filled with isopropyl alcohol²⁹ and sonicate for 5 min in a water-bath sonicator.
   4. Dry the sample with n....

Results

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.......

Discussion

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 >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-.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Chemicals
Buffered oxide etchant	Mallinckrodt Baker	1178	Silicon dioxide etch, Ti etch
Acetone	Mallinckrodt Baker	5356	Wafer clean
Isopropyl alcohol	Honeywell	BDH-140	Wafer clean
Hexamethyldisilizane	Mallinckrodt Baker	5797	Adhesion promoter
Microposit SC 1827 Positive Photoresist	Shipley Europe Ltd	44090	Pattern, electroplating
Microposit MF-26A developer	Shipley Europe Ltd	31200	Develop SC 1827
Tetramethylammonium hydroxide	Sigma-Aldrich	334901	Bulk Si etch
Sulfuric acid	Sciencelab.com	SLS2539	Wafer clean
Hydrogen peroxide	Sciencelab.com	SLH1552	Wafer clean
Transene Sulfite Gold TSG-250	Transense	110-TSG-250	Au electroplating solution
Baker PRS-3000 Positive Resist Stripper	Mallinckrodt Baker	6403	Photoresist stripper
Gold etchant type TFA	Transense	060-0015000	Au etch
Equipment
Mask aligner		Karl Suss MJB-3	Pattern photoresist
Sputter coater		Perkin Elmer 2400 Sputterer	Deposit metal
Thermal oxidation furnace		Pyrogenic Oxidation Furnace	Grow silicon dioxide
Reactive Ion Etch		Plasmatech RIE	Plasma ash
Xenon difluoride dry etcher		Xactix Xenon Difluoride Etcher	Selective dry isotropic silicon etch
Surface profilometer		Alpha-Step IQ	Step height measurement
Probe ring		Signatone	Holds DC probe manipulators
DC manipulators		Signatone S-900 Series Micropositioner	Applies potential difference to device
Laser doppler vibrometer		Polytec OFV-551/MSA-500 Micro System Analyzer	Switching time measurement
Digital function generator		Agilent E4408B Function Generator	Creates the DC-dynamic waveform
High voltage linear amplifier		Single channel high voltage linear amplifier A400	Facilitates high voltage
Digital oscilloscope		Agilent DS05034A Digital Oscilloscope	Verify the dynamic waveform parameters