Design and simulation of low voltage driven RF MEMS switch - used to improve MEMS switch defects

In recent years, radio frequency microelectronic system (RF MEMS) devices have attracted widespread attention for their small size and low power consumption. In particular, the phase shifters and antennas constructed by MEMS switches are key technologies for realizing phased array radars with tens of thousands of units, which are of great significance in the military. In the field of communications, they are also used in mobile phones due to their advantages such as ultra-low loss, high isolation, and low cost. However, RF MEMS switches generally have the problems of high driving voltage and long switching time, which are inferior to FET field effect tube switches and PIN diode switches. Compared with the achievements made abroad, domestic research is still in its infancy. The following will make some improvements to the defects of MEMS switches.

1 General considerations for RF MEMS switches
When the beam or membrane of the MEMS switch is attracted by the electrostatic force and deflects downward to a certain extent, the threshold voltage is reached, and the beam or membrane quickly deflects to the lower plate. The voltage depends on the material parameters, switch size and structure. The material of the beam or membrane needs to have a relatively good Young's modulus and yield strength. The larger the Young's modulus, the higher the resonant frequency, ensuring high-speed stability and switch life; the size effect of the electrostatic driving force should be considered in the size design; the natural vibration frequency of the structure affects the maximum operating speed of the switch. From a structural point of view, the ways to reduce the driving voltage are: reducing the distance between the plates; increasing the driving area; reducing the elastic coefficient of the beam or membrane. Common structures include series and parallel cantilever beam switches, torsion arm switches and capacitive switches. The first three are resistive contact types. When the metal contacts the outside of the signal line, there are many problems such as large insertion loss, and the insulating medium of the capacitive contact switch also has the problem of breakdown. Studies have shown that the higher the applied voltage, the shorter the life of the switch. The reduction in driving voltage will inevitably lead to a slower switching speed. How to meet the requirements of driving voltage and switching speed at the same time is the current difficulty.

2 Simulation and optimization of RF MEMS switches
For capacitive switches, the driving voltage decreases as the length of the bridge membrane increases. The greater the residual stress of the bridge membrane, the greater the driving voltage. Au with a Young's tensor of 78 GPa and a Poisson's ratio of 0.44 is usually used as the bridge membrane material. In order to obtain good isolation, the switch requires a large permittivity. Here, S3N4 with a dielectric constant of 7.5 is selected as the dielectric layer, the bridge membrane unit is Solid98, a 5 V voltage is applied, the dielectric is air, and a 0 V voltage is applied to the lower plate. Then ANSYS is used to model, divide the grid, load and solve the electrostatic coupling and modal analysis. The switch deformation under a voltage of 5 V is about 0.2 μm, which does not meet the low-voltage drive requirements. The first five modes of the switch are extracted as shown in Figure 1.

It can be seen that the resonant frequency of the switch from low order to high order is getting larger and larger, which are 79.9 kHz, 130.3 kHz, 258.8 kHz, 360.7 kHz, and 505.6 kHz respectively. The first-order mode is far away from other modes, that is, it is not easily disturbed by the outside world. Only by controlling the switching frequency below the resonant frequency of the first-order mode can its stable operation be guaranteed. Since the actual switching time is still not ideal, holes are dug in the membrane to reduce the damping of the compression mold, thereby increasing the switching speed. Although the capacitance ratio of the off state is reduced, the holes can reduce the weight of the beam and obtain a higher mechanical resonance frequency. The final model has a total of 100 holes dug, and the two ends are bent to reduce the driving voltage. The simulation obtains a capacitive switch with a deformation of more than 1μm and a stable switching time of less than 5μs under a voltage of 5 V, as shown in Figure 2.

Considering the dielectric breakdown problem that still exists in capacitive switches, the structure is improved here by combining the torsion arm lever with the perforated capacitor film. While reducing the driving voltage and increasing the switching speed, the capacitance ratio is not affected, and the electrical breakdown is suppressed to a certain extent. Its working principle is: when the push electrode is voltageed, the lever is lifted, the distance between the dielectric film and the contact film increases, resulting in a very small coupling capacitance, and the signal passes through the transmission line; when the pull electrode is voltageed, the lever is pulled down, the coupling capacitance becomes larger, and the microwave signal is reflected. The material selection is still mainly Au and S3N4, and A1 can be used to replace Au in some parts. The design of the structure and size is estimated by the transcendental equation and the capacitance equation under the switch on and off. The lower plate is 25×25 (the unit system adopts μMKSV, the length unit is μm, the same below), with an insulating dielectric layer attached to it, the hole is 3.4×3.4, the lever is 100x30, the structural layer is 20×20, and the plate thickness is 1. The results shown in Figure 3 were obtained by ANSYS simulation.

After performing electrostatic coupling and modal analysis in ANSYS, 3D electromagnetic field simulation of the switch was performed using ANSOFT HFSS to further obtain its insertion loss and isolation, determine the structure of the coplanar waveguide and contact film, and thus improve the RF performance of the switch. When modeling, the bending of the switch was ignored, the material properties and air radiation boundary were defined, and the wave port was used for simulation to solve the insertion loss of the on state and the isolation of the off state respectively. When the dielectric layer is thin, the switch has good isolation near 10 GHz, and the insertion loss is less than 1 dB.

3 Preparation process of RF MEMS switch
Reasonable selection of the process for growing dielectric film has a great influence on the performance of the switch. The RF MEMS switch in this paper needs to grow a layer of silicon nitride film on the surface of the substrate. Generally, the LP-CVD process is selected, while the PECVD process is preferably selected for the dielectric film. The performance requirements of the metal film are relatively low, and the sputtering method can be used. Considering that the substrate requires leakage current and loss to be as small as possible, high-resistance silicon and silicon dioxide are selected as the substrate, and the latter guarantees the insulation requirements. The gold signal line and the lower electrode are formed by positive photoresist stripping, and the aluminum upper electrode is obtained by electron beam evaporation. However, from the perspective of feasibility, the process realization of some solutions is still difficult for domestic processing technology, and the processing conditions can only be achieved at the expense of the performance of the microsystem.

4 Conclusion
This paper mainly innovates in structure, and obtains theoretical solutions through computer-aided design simulation analysis, which meets the original design intention to a certain extent, but is still immature in terms of technology. Lower driving voltage and higher switching frequency are still urgent problems to be solved. In addition, how to ensure the reliability and practicality of actual products is also a future research focus.