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 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.
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 “fringing-field” 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 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 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.
1. Fabrication of EFFA MEMS Fixed-fixed Beams (See Figure 3 for Summarized Process)
2. Experimental Validation of Dynamic Waveform
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 gap height, but for all of the gap possible heights. This is demonstrated in Figure 6 and Figure 7 for both the pull-down and release operations, respectively. The calculated and measured dynamic wave form used to achieve the measurements in the previous figures is presented in Figures 8 and 9, respectively.
Figure 1. 2D sketch and SEM image of MEMS bridges used in this study. (A) 2D profile. (B) Top view of MEMS bridges. (C) SEM of actual fabricated device. Please click here to view a larger version of this figure.
Figure 2. Sketch of underdamped MEMS bridge in response to an input step and time varying response. (A) Unit step applied bias. (B) Response of underdamped MEMS bridge to unit step input. (C) Time varying/dynamic input bias. (D) Response of MEMS bridge to time varying input. Please click here to view a larger version of this figure.
Figure 3. Summarized process flow for the MEMS bridges. (A) Oxidized silicon substrate. (B) Bulk etch of silicon substrate. (C) Re-oxidation of silicon substrate. (D) Silicon dioxide etch to expose sacrificial silicon. (E) Gold deposition and patterning. (F) Etch of sacrificial silicon layer to release the MEMS bridge. Please click here to view a larger version of this figure.
Figure 4. Block diagram of the experimental setup used to apply the bias signal and capture the MEMS bridge deflection. Please click here to view a larger version of this figure.
Figure 5. Measured pull-down and release states of a MEMS bridge in response to a 60 V input bias. The black curve is the response from a step input. The red curve is the response to a dynamic input.
Figure 6. Measured intermediate pull-down gap heights of the MEMS bridge in response to a dynamic input. Please click here to view a larger version of this figure.
Figure 7. Measured intermediate release gap heights of the MEMS bridge in response to a dynamic input.
Figure 8. Calculated waveform for the input bias.
Figure 9. Actual waveform used to achieve minimum oscillation of the MEMS bridge.
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-axial mean stress. Additionally, this study uses silicon as the sacrificial layer type due to its relative lack of expansion and contraction (compared to photoresist) during process steps that require heat cycles. Finally, the dry release step with XeF2 facilitates high yield processing by virtually eliminating stiction.
The desired beam gap height corresponds to the overshoot gap height (Figure 2B) in response to the first step bias20. Once the beam attains the overshoot/desired gap height the second step bias (Figure 2C) is applied to hold the beam in this position. By knowing the mechanical quality factor of the MEMS bridge (which can be measured or calculated), the overshoot percentage and the time to reach the overshoot gap height can be calculated. These parameters are used to determine the amplitude and timing of the input voltage.
DC-dynamic drive signals used in this study improved the settling time from ~2 msec down to ~35 μsec for both up-to-down and down-to-up states. The calculated switching time using the heuristic model20 is 28 μsec for a beam with width w = 10 μm, length L = 400 μm, thickness t = 0.45 μm, lateral pull-down gap s = 8 μm, and residual tensile mean stress σ = 5 MPa. Switching time has a σ−1/2 relationship20. The consequence of this relationship is that relatively small variations in the residual stress can have a non-marginal impact on the switching time calculation. A relatively small difference of 2 MPa in residual stress can result in a switching time variation of 20%. Therefore a need exists for real-time optimization with the method presented in this paper due to the inevitability of process variation across a wafer.
The method presented in this work demonstrates significant improvements in switching time for electrostatic fringing field actuators where the substrate is removed. The details for fabrication of the EFFA MEMS tuners and the electrical testing are described in detail. The experimental method, in particular the dynamic biasing technique, will find utility in virtually any mechanically underdamped MEMS design in regards to improving the switching time performance.
The authors have nothing to disclose.
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.
Chemical | Company | Catalogue number | Comments (optional) |
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 |
Hydrofluroic acid | Sciencelab.com | SLH2227 | Silicon dioxide 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 |