THz MEMS Switch Design
<p>(<b>a</b>) 3-D model of an electro-static actuated MEMS switch in a series configuration. The switch’s OFF state isolation is provided an air gap between the actuator’s tip and the CPW section’s tip. When applying external DC bias, the electrostatic force between the bias pad and the actuator will pull the actuator down and provide an RF signal path. (<b>b</b>) A photomicrograph of a fabricated switch.</p> "> Figure 2
<p>The OFF and ON states of the RF MEMS switch under different DC bias conditions. The cantilever to bias pad gap <span class="html-italic">g</span> and dimple to CPW distance <math display="inline"><semantics> <msub> <mi>g</mi> <mn>1</mn> </msub> </semantics></math> are both marked. (<b>a</b>) The Switch is under OFF state. (<b>b</b>) The Switch is under ON state.</p> "> Figure 3
<p>The strongest electric field is between the CPW and the adjacent DC bias pad.</p> "> Figure 4
<p>The cross-section view of a CPW. In this work, both high resistivity silicon and fused quartz substrate have thickness h of 500 <math display="inline"><semantics> <mi mathvariant="sans-serif">μ</mi> </semantics></math>m. The CPW is evaporated gold with a thickness t of 400 nm. The signal line width <span class="html-italic">w</span> and signal-ground gap <span class="html-italic">g</span> are impacted by the substrate’s relative permittivity.</p> "> Figure 5
<p>The parasitic capacitance of MEMS switch under OFF state and the switch’s equivalent circuit model.</p> "> Figure 6
<p>Three dimensional (3-D) model of high resistivity silicon-based MEMS switch. The initial design (<b>a</b>) has a larger parallel area at the actuator tip, which causes larger capacitance. To reduce such capacitance for higher isolation performance, its actuator tip is trimmed (<b>b</b>).</p> "> Figure 7
<p>Three dimensional (3-D) model of fused quartz-based MEMS switch. In the initial design (<b>a</b>) the actuator has the same width as the CPW’s signal line, which causes roughly 2 fF parasitic capacitance at the tip. To reduce such capacitance, the actuator’s width was reduced and the tip is trimmed to further reduce the capacitance (<b>b</b>).</p> "> Figure 8
<p>Estimated switch isolation performance dominated by actuator tip capacitance.</p> "> Figure 9
<p>(<b>a</b>) Impedance variation caused by CPW structure elevation. (<b>b</b>) impedance tuning is realized by adjusting the DC bias pad size.</p> "> Figure 10
<p>Simplified transmission line model of MEMS switch at the ON state.</p> "> Figure 11
<p>A comprehensive transmission line circuit model for ON state MEMS switch.</p> "> Figure 12
<p>A comprehensive transmission line circuit model for OFF state MEMS switch.</p> "> Figure 13
<p>The THz MEMS switches were modeled as transmission line circuits and simulated using AWR Microwave Office (marked as T line). The circuit simulation results are compared with ANSYS HFSS finite element analysis. (<b>a</b>) Silicon based design at the ON state. (<b>b</b>) Silicon based design at the OFF state. (<b>c</b>) Quartz based design at the ON state. (<b>d</b>) Quartz based design at the OFF state.</p> "> Figure 14
<p>A simplified silicon-based MEMS switch fabrication flow. The fabrication process starts with circuit layer deposition using a lift-off technique in (<b>a</b>,<b>b</b>). After that, two aluminum sacrificial layers are deposited and dimple position is prepared with another lift-off in (<b>c</b>). Etching and plating were used to form the switch’s anchor in (<b>d</b>). After the gold seed layer and a photoresist patterning, the beam was plated in (<b>e</b>). After a series of wet etch steps, the switch is finally released by CPD in (<b>f</b>).</p> "> Figure 15
<p>(<b>a</b>) The Al-Au compounds observed (<b>b</b>) Al-Au compounds are reduced with 50 nm chromium barrier layer applied between the gold and aluminum layer (<b>c</b>) 80 nm chromium barrier layer prevented Al-Au to produce.</p> "> Figure 16
<p>On wafter two-ports probing set up used for MEMS switch RF measurement is shown in this diagram.</p> "> Figure 17
<p>SEM image of (<b>a</b>) on-wafer TRL calibration kit and (<b>b</b>) an example silicon switch.</p> "> Figure 18
<p>The THz MEMS switch HFSS simulation measurement comparison. (<b>a</b>) Silicon switch ON state measurement and simulation. (<b>b</b>) Silicon switch OFF state measurement and simulation. (<b>c</b>) Quartz switch ON state measurement and simulation. (<b>d</b>) Quartz switch OFF state measurement and simulation.</p> ">
Abstract
:1. Introduction
2. Mechanical Design
3. Electrical Design
4. Switch Fabrication Challenge & Solution
5. Switch Calibration & Measurement
6. Discussion & Conclusions
Author Contributions
Funding
Conflicts of Interest
Abbreviations
MEMS | Micro-electromechanical systems |
CPW | Coplanar waveguide |
CMOS | Complementary metal-oxide-semiconductor |
BiCMOS | Bipolar Complementary metal-oxide-semiconductor |
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Substrate Material | Relative Permittivity | g (m) | w (m) |
---|---|---|---|
Silicon | 11.9 | 4 | 7 |
SiC | 9.7 | 4 | 9 |
AlN | 9.2 | 4 | 10 |
Quartz | 4.0 | 4 | 35 |
Elevation Height (m) | Impedance () |
---|---|
0 | 50 |
0.4 | 63.6 |
0.8 | 64.3 |
1.2 | 70 |
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Feng, Y.; Tsao, H.-y.; Barker, N.S. THz MEMS Switch Design. Micromachines 2022, 13, 745. https://doi.org/10.3390/mi13050745
Feng Y, Tsao H-y, Barker NS. THz MEMS Switch Design. Micromachines. 2022; 13(5):745. https://doi.org/10.3390/mi13050745
Chicago/Turabian StyleFeng, Yukang, Han-yu Tsao, and N. Scott Barker. 2022. "THz MEMS Switch Design" Micromachines 13, no. 5: 745. https://doi.org/10.3390/mi13050745