**7. Technology, experimental results and discussion**

An actual configuration having the same dimensions described in the previous sections has been realized and preliminary tested. A photo of the structure is given in Fig. 16. SU-8 polymeric sides have been realized by photo-lithography to be used as a support for both the ground planes of the CPW, and the suspended metal bridge. Silicon oxide has been deposited as a dielectric, and the actuation has been performed by means of the central conductor of the CPW.

The realization of double-clamped RF MEMS capacitive shunt switches has been performed by means of negative photo-resist SU-8 for the realization of the ground planes of the coplanar configuration, elevated with respect to the wafer, while positive S1818 photo-resist has been used as a sacrificial layer.

RF MEMS Switches have been manufactured on a 4 inch high-resistivity (ρ > 5000 ohm cm) silicon wafer <100> oriented, having a thickness of 400 μm. For the realization of the devices, a 4 mask sequence has been considered, and the entire fabrication process is subdivided in five steps:

Dynamics of RF Micro-Mechanical

Capacitive Shunt Switches in Coplanar Waveguide Configuration 227

Fig. 17. Test-fixture structure of the RF MEMS switch manufactured by means of SU-8 photo-lithography. The input and output ports are connected to a vector network analyzer

A further confirmation of the influence of the developed technological processing, and specifically the contribution from the gold stiffness, is evidenced from the mechanical response simulation plotted in Fig. 18 for a laterally actuated beam. In that case, the imposed residual stress is σ = 60 MPa, leading to two major effects: (i) the increase of the actuation voltage up to values greater than 90 volt, and (ii) a deformation of the bridge,

It is worth noting that, looking at the shape of the bridge predicted in Fig. 18, an easier and more uniform actuation in the central part by using the lateral pads could be obtained because of the higher residual stress. On the other hand, the price to be payed in terms of the increase in the actuation voltage is not acceptable for many applications, and a trade-off

The only difference generated by changing the width of the bridge for the studied structure concerns with its RF response. Actually, the central capacitance defined by the bridge, the central conductor of the CPW and the dielectric between them changes the frequency of resonance for the device under test, while no change is recorded for the actuation voltages. Actually, wider beams experience lower frequencies of resonance when the switch is

Further theoretical and experimental results have been obtained by using another shunt capacitive switch manufactured by using the silicon technology and developed at FBK-irst [57]. Top view and lateral dimensions of the device are shown in the following Fig. 19. The device is a clamped-clamped beam obtained on silicon wafer by means of an eight-mask sequence of technological steps, and the final release of the suspended bridge was obtained removing the sacrificial layer via an ashing process. The configuration is also characterized

has to be obtained, usually accounting for actuation voltages not exceeding 50 volt.

by means of coplanar probes for on-wafer characterization.

which results in a more rigid shape.

actuated and it works in isolation [56].


Actuation voltages in the order of 20-25 volt have been obtained in different samples, almost in agreement with the expected value. A possible small under-evaluation of the gold membrane stiffness should be considered in the actual device. In fact, by imposing =25 MPa we obtain *Vthreshold*=21 volt ca. for a uniform beam with the same dimensions and holes. Moreover, because of the stiffness the beam could be also a bit upward with respect to the expected flat geometry and an increased gap could favour and increase in the threshold voltage. Other possible sources of spread with respect to the predicted threshold voltage maybe due to non-uniformity in the Au deposition over the entire 4 inch wafer, and the successive electro-plating process.

An optical characterization of the manufactured device has been also performed, revealing both a good shape of the beam and the resolution of the holes, which have no residuals after the ashing process. Results are given in Fig. 16.

Fig. 16. Optical microscopy characterization of the RF MEMS switch realized by means of SU-8 photo-lithography with evidence for the optimized profile of the beam after the removal of the sacrificial layer.

ii. definition of the SiO2 to be used for obtaining the capacitive configuration, with the aim

iii. creation of lateral supports made by SU-8 for the double-clamped structure. SU-8 2002 (Microchem Corp., USA) has been used for our purposes. In this configuration, polymer lateral pedestals are obtained, to be metalized for obtaining both the ground planes and

iv. reduction of the sacrificial layer has been obtained by means of a purposely designed mask, in order to avoid peaks in the shape coming from the lithography, which act as

v. as a final step, the switches are obtained by means of the release of the sacrificial layer

Actuation voltages in the order of 20-25 volt have been obtained in different samples, almost in agreement with the expected value. A possible small under-evaluation of the gold membrane stiffness should be considered in the actual device. In fact, by imposing =25 MPa we obtain *Vthreshold*=21 volt ca. for a uniform beam with the same dimensions and holes. Moreover, because of the stiffness the beam could be also a bit upward with respect to the expected flat geometry and an increased gap could favour and increase in the threshold voltage. Other possible sources of spread with respect to the predicted threshold voltage maybe due to non-uniformity in the Au deposition over the entire 4 inch wafer, and the

An optical characterization of the manufactured device has been also performed, revealing both a good shape of the beam and the resolution of the holes, which have no residuals after

Fig. 16. Optical microscopy characterization of the RF MEMS switch realized by means of SU-8 photo-lithography with evidence for the optimized profile of the beam after the

i. realization of the central conductor of the CPW.

of a high ratio in the ON/OFF states.

the support for the suspended bridge.

successive electro-plating process.

removal of the sacrificial layer.

the ashing process. Results are given in Fig. 16.

discontinuities in the next metallization process.

by using a modified reactive ion etching (RIE) process.

Fig. 17. Test-fixture structure of the RF MEMS switch manufactured by means of SU-8 photo-lithography. The input and output ports are connected to a vector network analyzer by means of coplanar probes for on-wafer characterization.

A further confirmation of the influence of the developed technological processing, and specifically the contribution from the gold stiffness, is evidenced from the mechanical response simulation plotted in Fig. 18 for a laterally actuated beam. In that case, the imposed residual stress is σ = 60 MPa, leading to two major effects: (i) the increase of the actuation voltage up to values greater than 90 volt, and (ii) a deformation of the bridge, which results in a more rigid shape.

It is worth noting that, looking at the shape of the bridge predicted in Fig. 18, an easier and more uniform actuation in the central part by using the lateral pads could be obtained because of the higher residual stress. On the other hand, the price to be payed in terms of the increase in the actuation voltage is not acceptable for many applications, and a trade-off has to be obtained, usually accounting for actuation voltages not exceeding 50 volt.

The only difference generated by changing the width of the bridge for the studied structure concerns with its RF response. Actually, the central capacitance defined by the bridge, the central conductor of the CPW and the dielectric between them changes the frequency of resonance for the device under test, while no change is recorded for the actuation voltages. Actually, wider beams experience lower frequencies of resonance when the switch is actuated and it works in isolation [56].

Further theoretical and experimental results have been obtained by using another shunt capacitive switch manufactured by using the silicon technology and developed at FBK-irst [57]. Top view and lateral dimensions of the device are shown in the following Fig. 19. The device is a clamped-clamped beam obtained on silicon wafer by means of an eight-mask sequence of technological steps, and the final release of the suspended bridge was obtained removing the sacrificial layer via an ashing process. The configuration is also characterized

Dynamics of RF Micro-Mechanical

**8. Conclusion** 

proposed model.

applied DC voltage.

**9. Acknowledgment** 

Hoboken, 2003.

**10. References** 

Capacitive Shunt Switches in Coplanar Waveguide Configuration 229

Analytical and numerical modelling for the mechanical response of a shunt capacitive RF MEMS switch have been compared by using an uni-dimensional theory, and 2D and 3D simulations performed by means of a commercial software package (COMSOL Multiphysics). Two actual configurations have been experimentally studied to validate the

As a result, it has been demonstrated that RF MEMS mechanics can be predicted in a convenient way by uni-dimensional phenomenological models if evaluations about switching times, threshold voltage and preliminary dynamics have to be studied, without involving cumbersome simulations with a computer. In fact, the most part of the previous quantities depend on the equations to be used for the correct definition of the spring constant, and we demonstrated that the analytical approach based on the knowledge of the materials and of the geometry fulfils the most part of the quantities to be obtained. The actuation velocity and the switching times have been also defined and predicted by means of the analytical approach based on the Mechanical Energy considerations, including the

2D and 3D simulations are really useful for configurations having a very peculiar shape, especially for combining mechanical and RF predictions, being based on the same geometry, and this will be very useful to get a figure of merit for the RF MEMS technology (not yet available) based on different input conditions. Concerning the evaluation of the actuation voltage, the main parameter to be defined is the residual stress of the structure, because it dramatically influences the mechanical response of the bridge. On the other hand, with the proper knowledge of the technology used, the evaluation can be easily based on the unidimensional approach by defining effective quantities for simple configurations. The relative influence of surface forces and charging contributions has been discussed, to demonstrate that, under proper geometrical and material constraints, only charging effects can be really responsible for un-reliable structures, and a very simple solution to prevent

this contribution is to tailor the switch in order to have contact-less actuation pads.

MEMS Redundancy Switch" ESA ITT AO/1-5288/06/NL/GLC contract No.20847

*Microwave Magazine*, Vol.2, No.4, pp.59-71 (2001).

The activity has been partially funded by the ESA/ESTEC Contract on "High Reliability

[1] G. M. Rebeiz and J. P. Muldavin: "RF MEMS Switches and Switch Circuits", *IEEE* 

[2] G. M. Rebeiz, Guan-Leng Tan and J. S. Hayden: "RF MEMS Phase Shifters, Design and Applications", *IEEE Microwave Magazine*, Vol.3, No.2, pp.72-81 (2002) [3] G. M. Rebeiz, "*RF MEMS, Theory, Design and Technology*", John Wiley and Sons,

[4] Harrie A.C. Tilmans: "MEMS components for wireless communication", *invited paper at XVI Conference on Solid State Transducers*, Prague, Czech Republic, Sept. 15-18, 2002 [5] E.K. Chan, E.C. Kan and R.W. Dutton: "Nonlinear Dynamic Modeling of Micromachined

Switches", *Proceed. Of IEEE MTT-Symposium*, pp.1511-1514 (1997).

by the following parameters: gap between bridge and floating metal *g*=2.85 µm, SiO2 dielectric thickness on the central conductor of the CPW *d*=0.1 µm, beam thickness *t*=4 µm (grown by electroplating). By using the above values and those given in Fig. 19, we get an actuation voltage V=40 V ca. for the lateral actuation. The experimental value was Vexp=(41±2) V by using ten devices measured onto the same wafer in different positions

Fig. 18. Simulated deformation for the same bridge experimentally tested with a higher value of the residual stress (σ=60 MPa). An actuation voltage around 95 volt has been predicted.

Fig. 19. Shunt capacitive switch manufactured by means of the eigth mask process developed at FBK-irst. Dimensions are in µm.
