A FULLY PACKAGED PIEZOELECTRIC SWITCH WITH LOW-VOLTAGE ACTUATION AND ELECTROSTATIC HOLD

Matthieu Cueff1, Emmanuël Defaÿ1, Patrice Rey1, Gwenaël Le Rhun1, François Perruchot1, Christine Ferrandon1, Denis Mercier1, Frédéric Domingue1, Aurélien Suhm1, Marc Aïd1,

Lianjiu Liu2, Sergio Pacheco2, Mel Miller2 1CEA, Léti, MINATEC, F38054 Grenoble, France 2Freescale Semiconductor, Tempe, Arizona, USA

ABSTRACT This paper reports RF characterization of a fully packaged RF MEMS piezoelectric switch. The switch demonstrates better than 0.8 dB insertion loss at 2 GHz and 30 dB isolation up to 10 GHz. The presented device combines a piezoelectric actuation and a low electrostatic hold voltage to improve contact force. Actuation voltages of the switch are 5V for both piezoelectric actuation and electrostatic hold. This actuation was sufficient to obtain contact resistance lower than 2 ohms. The switch is packaged by wafer-level packaging technology using gap control, AuSn eutectic bonding and post-process Thru-Silicon Vias. INTRODUCTION Development of wireless communication requests RF switches with high performance characteristics and wide bandwidth. RF MEMS switches present an alternative to PIN diodes and GaAs field effect transistors because they offer very low insertion loss and good isolation at high frequency ranges [1]. They also have a cut-off frequency far better than electrical solutions. Most of the works are focused on electrostatic actuation because the fabrication is easier. However, electrostatic actuation requires high operation voltages of more than tens volts [1], which is not compatible with low voltage applications. Piezoelectric actuation is a way to reduce actuation voltages. Piezoelectric actuators can be actuated at a few volts instead of tens volts for electrostatic actuation with a comparable air gap [2]. This is particularly interesting for low-voltage applications like mobile phone devices: micro-mirrors [3], micro-switches [4] or acoustic transducers for example. In the case of micro-switches, a higher isolation can be obtained since the gap between the CPW line and the contact can be increased because of the large displacement of piezoelectric actuators. In a previous study, properties of our sol gel Lead Zirconate Titanate (PZT) material on the 100 nm range were developed [5]. The use of very thin layers of PZT is a way to facilitate piezoelectric material integration. Furthermore, during process, high temperature is used, which induces the

depolarisation of the PZT layer. To repolarise the piezoelectric material, it is necessary to apply an important electric field. Because of the thickness of the PZT layer, only 5V are needed to apply 500 kV/cm on a 100 nm-thick PZT layer. This level of voltage is compatible with actuation voltages for mobile phone devices. However, the piezoelectric material integration is complicated and few fully packaged switches have been realized up to now [6]. DESIGN The CPW lines and piezoelectric actuators have been fabricated on two separate wafers and assembled by wafer-level packaging techniques. Input pads were transferred to the top of the packaging wafer using Thru-Silicon Vias (TSV). The RF switch is actuated by elastic-piezoelectric bimorph cantilevers. These actuators support a suspended metal line as shown in Figure 1, which can be switched on a transmission line under the top wafer to make a RF contact.

Figure 1. Microscope picture of the cantilevers and the suspended line.

The suspended line is supported by a SiN bridge. Cantilevers present a hole to reduce the rigidity of the beam and to increase the quality of the contact. Cantilevers are 180 µm-long. Figure 2 presents cross-section views of the switch. Figure 2.a is a cross section view of the mechanical structure tip with the contact and electrostatic hold. Figure 2.b is a cross-section view of the suspended line and the contact on the top wafer. Figure 2.c is the cross-section view of one actuator.

978-1-4244-5764-9/10/$26.00 ©2010 IEEE

212

Figure 2. Schematic view of the switch with the suspended line : a) Tip of the mechanical structure, b) suspended line,

c) cross-section view of one cantilever.

The piezoelectric stack is represented with PZT in purple and platinum in pink. The SiN elastic layer is in light green. SiO2 spacers are represented in blue. RF Au transmission line (a. and b.) is represented in yellow. Under the beam (c.), the dark green layer is TiN. The orange layer is SiO2 to protect TiN during release process. As shown in Figure 3, actuators are stable over a temperature range from 25 °C to 80 °C. The temperature stress variation is compensated by the TiN TCE (Temperature Coefficient of Expansion). Temperature induced deformation of the beam is lower than 0.2 µm between 25 °C and 80 °C.

-1

-0,8

-0,6

-0,4

-0,2

0

0,2

0,4

0,6

0,8

1

20 30 40 50 60 70 80

Working temperature (°C)

Delta

def

lect

ion

at th

e fre

e en

d (µ

m) L = 180µm

Figure 3. Deflection versus working temperature.

FABRICATION 5 k.cm resistivity wafers were used to improve RF performances. Piezoelectric cantilevers with their suspended lines were realized on the actuators wafer. RF transmission lines and caps were realized on the cap wafers. Then both wafers were sealed with AuSn process. Actuators wafer First a 80 nm-thick PECVD SiO2 layer is deposited to protect the beam during the release process. Then a 100 nm-thick TiN layer is deposited for the thermal stress compensation.

The elastic material used is silicon nitride (SiN). 1 µm-thick of stress controlled SiN was deposited. The main reason is that it gives the opportunity to balance the whole residual stresses in the released beam. Stress compensation was previously presented at MEMS 2009 [5]. The piezoelectric stack is deposited on a 100 nm-thick PECVD SiO2 film which is used as a barrier for lead diffusion in SiN. The 120 nm-thick PZT was deposited by a sol gel method on the standard stack composing the bottom electrode (100 nm of sputtered Pt) and the adhesion layer underneath (10 nm of sputtered TiO2). Sol gel films were prepared from a commercial source, containing 10% excess of the lead precursor. Each spin coating gives at the end of the process a PZT thickness between 50 and 60 nm. The final thickness is then obtained by adjusting the number of spin coating. The standard process is spin coating, calcination at 400 °C under air during 5 min and a final thermal annealing at 700 °C under oxygen during 1 min, which induces the crystallisation of the PZT films into the desired perovskite phase. The top electrode is also made of 30 nm-thick sputtered Pt. The PZT exhibits a piezoelectric coefficient reaching -5 C/m2. Piezoelectric actuation results were previously presented at MEMS 2009 [5]. It has been demonstrated that the elastic-piezoelectric bimorphs exhibited deflection higher than 5 µm at 5V. The coplanar RF transmission line is made with a 1.4 µm-thick gold layer. The beam is released by XeF2 etching of the silicon substrate. Cap wafer On the top wafer, a 500 nm-thick thermal oxide layer was grown to increase RF characteristic of the transmission line. 1 µm of SiN was deposited to form a stop layer for future SiO2 etching: SiO2 of the bumps implemented to avoid contact sticking and 5 µm of PECVD SiO2 to make spacers. These SiO2 walls fix the gap between top and bottom wafers. 1 µm of gold was deposited for the RF transmission line and electrostatic hold. 100 nm of SiN was deposited on electrostatic hold to isolate electrodes from the top electrode of the cantilever. Under-layers insuring adhesion and barriers layers were deposited and etched. The solder material made of eutectic 80 w.% Au – 20 w.% Sn was electroplated. It has two functions: electrical contact between both wafers and hermetical solder joints for dies. Top and bottom wafers were bonded at 310 °C. Our process prevents the solder to spread outside dedicated areas. The assembly was then thinned and TSV were processed. A 3 µm-thick SiO2 layer was deposited for the isolation of vias. Then copper was deposited by electroplating to form the vias and contacts for probes. The gap between the contact of the suspended line on the bottom wafer and the transmission line on the top wafer is 5 µm.

a)

b)

c)

213

RF RESULTS AND DISCUSSION For RF measurements, a SOLT (Short, Open Load, Thru) calibration was used. Substrates losses Because the switch is fully packaged, insertion losses are dependent on substrates and contact insertion losses. Two wafers type were evaluated: standard resistivity wafers (resistivity between 5 and 20 .cm) and high resistivity wafers (5 k.cm). Three configurations were tested:

- Both wafers exhibit standard resistivity - Bottom wafer exhibits standard

resistivity, top wafer is a high resistivity wafer

- Both wafers with high resistivity Figure 4 shows a test device. A continuous line under the cap wafer is represented in blue. Contacts on the cap wafer are represented in green. TSV are represented in black. It is the same geometry as the switch without the suspended line. There is no actuator on the bottom wafer.

Figure 4. Test device for RF transmission.

At 2 GHz, insertion losses were 2.1 dB for standard resistivity wafers, 1.4 dB for mixed resistivity wafers, and 0.7 dB for high resistivity wafers. Substrates insertion losses are highly dependent on both wafers resistivity. Even if the RF line is on the top wafer, because the gap between top and bottom wafers is smaller than the distance between ground lines and signal line, the electromagnetic signal propagates in both top and bottom wafers [7]. To evaluate with a good accuracy insertion losses of the contact, we have to fix losses of the packaging, including TSV and lines. Some switches were glued to the top wafer during the sealing process by capillary force or during the thinning process. Because these forces are very strong, contact resistance is far weaker than what can be obtained with electrostatic hold. Insertion losses of these process contacted switches were measured to be 0.6 dB at 2 GHz, which is consistent with substrate insertion losses evaluation.

Fully packaged switch measurement RF measurement results of the packaged switch at both 2 and 10 GHz are shown in Table 1.

Table 1. Measured RF results at 2GHz and 10 GHz. The first

line corresponds to the off-state isolation. The other correspond to on-state insertion losses.

As shown in Figure 5, the off-state isolation is 43.6 dB at 2 GHz and higher than 30 dB at 10 GHz, thanks to the 5 µm gap.

-80-70-60-50-40-30-20-10

0

0 2 4 6 8 10

Frequency (GHz)

S21

(dB

)

Figure 5. Measured off-state isolation.

Figure 6 presents on-state insertion losses of the switch. The on-state insertion losses were 0.83 dB with only the 5V piezoelectric actuation. Adding a nominal 5V electrostatic hold voltage, the on-state insertion losses are reduced at 0.74 dB. In comparison, using only the electrostatic actuation, the required voltage increases to 20V to actuate the switch. Even if the gap is large, electrostatic actuation is low due to the compliance of the cantilevers.

-1,4

-1,2

-1,0

-0,8

-0,6

-0,4

-0,2

0,0

0 2 4 6 8 10Frequency (GHz)

S21

(dB

)

Piezo at 5V

Piezo at 5V and electrostatic at 5V

Electrostatic at 20V

Figure 6. On-state insertion losses with 5V piezoelectric actuation (pink), 5V piezoelectric and 5V electrostatic actuation (blue), 20V electrostatic actuation (brown).

Actuation 2GHz 10 GHz V piézo (V) V hold (V) S21 (dB) S21 (dB)

Isolation 0 0 -43.58 -29.79

Insertion losses 0 20 -0.76 -1.05 -5 0 -0.83 -1.14 -5 5 -0.74 -1.02

214

On-state return losses are higher than 25 dB, as shown in Figure 7.

-40

-35

-30

-25

-20

-15

-10

0 2 4 6 8 10

Frequency (GHz)

S11

(dB

)

Piezo at 5V

Piezo at 5V and electrostatic at 5V

Electrostatic at 20V

Figure 7. On-state return losses with 5V piezoelectric actuation (pink), 5V piezoelectric and 5V electrostatic

actuation (blue), 20V electrostatic actuation (brown). Blue and brown curves are overlayed.

Packaging insertion losses were estimated at 0.6 dB. Contact insertion losses can be estimated at 0.14 dB with nominal voltages and 0.23 dB with only the piezoelectric actuation. In a DC-contact serie switch, contact resistance can be evaluated with the following equation (1):

0

221

2111

ZRSS S=−− (1)

where Rs is the contact resistance, S11 the return losses, S21 insertion losses and Z0 characteristic impedance, designed to be 50 . With formula (1), contact resistances are evaluated at 2.5 with only the piezoelectric actuation and 1.5 with piezoelectric actuation and electrostatic hold. CONCLUSION Our fully packaged piezoelectric switch exhibits a good isolation and low actuation voltages. It can be commutated with only 5V piezoelectric actuation. Contact insertion losses are reduced by applying an electrostatic hold voltage. The contact quality is quite good even if the contact force is small. To improve insertion losses of the fully packaged switch, RF lines and packaging processes remain to be enhanced. REFERENCES [1] G. M. Rebeiz, “RF MEMS : Theory, Design,

and Technology”, Wiley-Interscience, 2003. [2] H.C. Lee and Y.Y. Park, “Piezoelectrically

Actuated RF MEMS DC Contact Switches With Low Voltage Operation”, IEEE Microwave and Wireless Components Letters, vol. 15 no. 4, pp. 202-204, april 2005.

[3] T. Bakke, I.B. Johansen, A. Vogl, F. Tyholdt, D. Wang, “A novel ultra-planar, long-stroke,

and low-voltage Piezoelectric micromirror”, Proc. of MicroMechanics Europe 2009 Conference, Toulouse, France, September 20-22, 2009, A25.

[4] R.G. Polcawich, D. Judy, J.S. Pulskamp, S. Trolier-McKinstry, M. Dubey, “Advances in Piezoelectrically Actuated RF MEMS Switches and Phase Shifters”, Proc. of IEEE Microwave Symposium, Honolulu, Hawaï, USA, June 3-8, 2007, pp.2083-2086.

[5] E. Defaÿ, G. Le Rhun, F. Perruchot, P. Rey, A. Suhm, M. Aïd, L.J. Liu, S. Pacheco, M. Miller, “Piezoelectric PZT thin films in the 100nm range: A solution for actuators embedded in low voltage devices”, Proc. of IEEE MEMS 2009, Sorrento, Italy, January 25-29, 2009, pp. 619-622.

[6] J.H. Park, H.C Lee, Y.H. Park, Y.D. Kim, C.H. Ji, J. Bu, H.J. Nam, “A fully wafer-level packaged RF MEMS switch with low actuation voltage using a piezoelectric actuator”, Journal of Micromechanics and Microengineering, vol. 16, pp.2281-2286, 2006.

[7] B. Guigues, “Ferroelectric tunable capacitors with (Ba,Sr)TiO3 for radio frequencies applications”, thesis, Centrale Paris, December 16th, 2008, http://tel.archives-ouvertes.fr/tel-00373537/en/

215