Manufacturing of RF MEMS Switches on LTCC for Space Applications
Di Nardo, S.¹; Di Paola, E.¹; Farinelli, P.²; Mannocchi, G.³; Comparini, M.³; Di Marcantonio, U.³; Feudale, M.³; Fina, A.^3
1Thales Alenia Space Italia; ²DIEI-University of Perugia; ³Thales Alenia Space

In the last years a lot of work has been done on RF-MEMS switches by several research teams. Thales Alenia Space Italia (TAS-I) started, in the year 2000, its first MEMS project just on RF-switches, as prime contractor of the ESA/ESTEC program "Microwave Electrostatic Micro-Machined Devices for On-Board Applications". In this project the RF-MEMS switches were realized on silicon wafers (the process has been set-up in the Microfabrication Lab of ITC-IRST located in Trento-Italy) and reached the required electrical performance.
MEMS switches are very sensitive to humidity and contaminants like gases, organic compounds, etc. and therefore they must be packaged using hermetic seals and in nitrogen or vacuum environments.

During the time TAS-I acquired a strong heritage on packaging working with different materials and different layouts. One of this techniques, that is the in house developed LTCC technology (available at L`Aquila plant), has been used by TAS-I to package RF-MEMS switches built on silicon. For this intent a micro-package made of a Low Loss LTCC multilayer body, with a cavity for the MEMS lodging, containing the electrical connections (control pads and MS-CPW transition RF lines) was developed, manufactured and tested with a partial support of a ESA study contract. The micropackage was found to be fully hermetic and showed very good RF performance.

However, apart from the good results obtained in the past both on switches and on LTCC packaging, it is unavoidable that the connections between the silicon MEMS chip and the LTCC package introduce electrical RF losses and require additional assembly process steps. So, in order to reduce the cost and to improve the RF performances, it is very appealing to integrate the RF MEMS switches directly inside the LTCC package. In fact, with this approach, the number of connections/transitions between several items is reduced and this leads to better RF performances, higher integration and lower productions costs. The first step to reach this goal is to set-up a MEMS manufacturing process directly on LTCC.

In the final paper the building of RF MEMS switches directly onto LTCC substrates, by using standard multilayer thin film equipment, will be reported. The devices have been manufactured and evaluated at the L’Aquila (Italy) plant of Thales Alenia Space. The electromagnetic modelling and simulations have been performed by the DIEI-University of Perugia.
The MEMS switches have been designed on coplanar transmission lines to work with RF signals up to 20 GHz: clamped-clamped beam switches in series and shunt configuration have been taken into account.

The use of coplanar waveguides leads to a more difficult design with respect to microstrip line due to the less accurate available simulation tools, but, on the other hand, presents also some advantages: first of all the contacts RF-signal to ground do not need via-holes due to the CPW ground planes on the substrate surface. Secondly coplanar transmission lines gives one degree of freedom in the choice of the line dimensions more than microstrip and it allows easier RF probe measurements. However all the RF-MEMS reported in this work are based on CPW transmission line and can easily be adjusted for a microstrip design.

Figure 1. Top view of a shunt bridge geometry (a) and of a series bridge geometry (b). Different cell types have been designed changing the lengths LC, L1 and L2. In the shunt configuration the RF line is uninterrupted so, when the bridge is actuated, a short circuit of the RF signal to ground is obtained and the signal propagation is closed. In the series configuration the RF line is normally interrupted so only when the bridge is actuated the propagation of the signal is obtained. From the mechanical point of view the two configurations have identical features.

The MEMS are grown on LTCC multilayer based on standard DuPont GreenTape 951 system.

Figure 2. MEMS switches directly built onto LTCC manufactured at AAS-I (L’Aquila plant) under electrical test.

Preliminary electromechanical test have been performed and the results confirmed the expected performance: voltage driver between 15V and 30V. More details will be reported in the complete paper.

REFERENCES [1] G.M. Rebeiz, "RF-MEMS theory, design and technology, Wiley-Interscience", (2003).

[2] S. Pamidighantan, R. Puers, K. Baert, H. A. C. Tilmans, "Pull-in voltage analysis of electrostatically actuated beam structures with fixed-fixed and fixed-free end conditions", Journal of Micromechanics and microengineering, 12, 458-464, (2002).

[3] S. Di Nardo, L. Colicchia, U. Di Marcantonio, A. Fina, R. Marcelli, P. Mezzanotte, R. Sorrentino: "A compact, low cost LTCC micropackage for RF MEMS switches", Microwave Technology and Techniques workshop proceedings; 8-9 October 2002, ESA-ESTEC, Noordwijk, The Netherlands

[4] R. Marcelli, B. Margesin; P. Mezzanotte, S. Di Nardo, R. Sorrentino, P. Russer, F. Vitulli, L. Vietzorreck: "LTCC-Packaged Single Pole Double Through (SPDT) Switch in MEMS Technology for High Reliability Application", 4th Round Table on Micro/Nano Technologies for Space proceedings; 20-22 May 2003, ESA-ESTEC, Noordwijk, The Netherlands

[5] P. Farinelli, F. Giacomozzi, G. Mannocchi, R. Marcelli, B. Margesin, P. Mezzanotte, S. Di Nardo, P. Russer, R.o Sorrentino, F. Vitulli, L. Vietzorreck: "RF-MEMS switch on silicon substrate for use in the space environment: technology implications for high reliability design", MTT workshop, 11 may 2004, ESA-ESTEC, Noordwijk, The Netherlands

[6] P. Farinelli, F. Giacomozzi, G. Mannocchi, R. Marcelli, B. Margesin, P. Mezzanotte, S. Di Nardo, P. Russer, R. Sorrentino, F. Vitulli, L. Vietzorreck: "RF-MEMS SPDT Switch on Silicon Substrate for Space Applications", SiRf meeting, Sep 8-10, 2004, Atlanta, GA

[7] S. Di Nardo, E. Di Paola, P. Farinelli, G. Mannocchi, U. Di Marcantonio, R. Sorrentino:"Manufacturing And Characterization of Rf-Mems Switches Built on Alumina by Using Standard Multilayer Thin Film Equipment",5th ESA Round Table on Micro/Nano Technologies for Space, 3 - 5 October 2005, ESTEC Conference Centre Noordwijk, The Netherlands

[8] S. Di Nardo, E. Di Paola, G. Mannocchi, U. Di Marcantonio, P. Farinelli, M. Feudale, R. Marcelli, B. Margesin, P. Mezzanotte, F. Vitulli, L. Vietzorreck: "RF MEMS Reliability for Space Applications", Proceedings of Workshop MEMSWAVE 2006, Orvieto, June 27-29, pp.131-134, (2006).

[9] S. Catoni, S. Di Nardo, P. Farinelli, F. Giacomozzi, G. Mannocchi, R. Marcelli, B. Margesin, P. Mezzanotte, V. Mulloni, R. Sorrentino, F. Vitulli, L. Vietzorreck: "Reliability and Power Handling Issues in Ohmic Series and Shunt Capacitive RF MEMS Switches", Proceedings of Workshop, MEMSWAVE 2006, Orvieto, June 27-29, pp.123-126, 2006.