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The combination of high quality complex oxide films with silicon technology opens the pathway to realize innovative devices. In our group, we focus on the growth of epitaxial piezoelectric films on silicon wafers for the fabrication of microelectronics and microelectromechanical systems (MEMS) employed for several applications such as filters, microsensors and microactuators.
Epitaxial ferroelectric thin films exhibit properties, such as piezoelectric coefficients, polarization and dielectric constant generally superior to polycrystalline films [1]. For this reason, several advantages are expected from piezoelectric devices based on epitaxial thin films, such as high forces at low operational voltage in actuators, high frequency operation in filters and high energy-conversion efficiency in transducers. Moreover, the epitaxial growth can reduce the fatigue effect and cyclic depolarization [2], an improvment that is necessary for a reliable use in devices.

The main challenges for the integration of oxides on silicon are the control of the silicon/oxide interface at the atomic level, the growth of high quality epitaxial oxide films and the choice of microfabrication processes, allowing the excellent properties of the oxide layers to be retained, to maximize the performance of fabricated devices.

Epitaxial piezoelectric MEMS

Our epitaxial piezoelectric MEMS are based on a piezoelectric Pb(Zr_0.2Ti_0.8)O₃ layer grown on a silicon substrate through two oxide layers: SrTiO₃ used as buffer and metallic SrRuO₃ used as bottom electrode.

The growth of epitaxial (001) SrTiO₃ films on 2" (001) silicon wafers is performed using molecular beam epitaxy (MBE), through a complex multi-step procedure monitored in situ using a RHEED technique [3]. These SrTiO₃-buffered silicon samples are used as substrates to grow subsequently epitaxial thin films of metallic SrRuO₃ and ferroelectric Pb(Zr_0.2Ti_0.8)O₃ by off-axis magnetron sputtering.

X-ray θ-2θ diffractograms display only (00l) peaks, confirming the c-axis orientation of the oxide stack and revealing a PZT c-axis lattice parameter of 4.13 � (Figure 1).

Figure 1: θ-2θ diffractograms of a PZT(150 nm)/SRO(30 nm) heterostructure (top), θ-2θ diffractograms of the STO buffer layer on (001) silicon (bottom).

Finite size oscillations around the SRO (001) and (002) reflections attest of the high crystalline coherence of the bottom electrode. Rocking curve measurements reveal the good crystalline quality of the piezoelectric layer with a full width half maximum of 0.5� measured around the PZT (001) peak. Detailed diffraction analyses confirm the epitaxial relationship between the oxide layers and the substrate: PZT[001]//SRO[001]//STO[001]//Si[001] and PZT[100]//SRO[100]//STO[100]//Si[110].
Local measurements of the d₃₃ piezoelectric coefficient, performed with an atomic force microscope, reveal the ferroelectric behavior of the PZT layer: the estimated piezoelectric coefficient d₃₃ is of the order of 50 pm/V and the coercive field is ~100 kV/cm. I-V and P-V measurements reveal a remnant polarization around 70 �C/cm² (Figure 2).

Figure 2: The current-voltage loop and the corresponding polarization at 100 Hz as a function of the voltage were determined using a ferroelectric tester TF analyzer 2000 system.

By using a silicon micromachining process, developed in collaboration with the Insitute of Microengineering (IMT) in Neuchatel, we realize piezoelectric membranes and cantilevers of different sizes and shape as shown in Figure 3.

Figure 3: (a) SEM image of a circular membrane (top view) with a diameter of 1000 µm. (b) Optical image of a cantilever (1000x2500x40 µm³)

The static and dynamic behaviors of membranes and cantilevers are investigated by optical and electric impedance measurements. The preliminary experimental results are very encouraging and reveal the potential of epitaxial piezolectric MEMS devices both in static and dynamic working operations.

Wang Y, Ganpule C, Liu B T, Li K, Mori K, Hill B, Wuttig M, Ramesh R, Finder J, Yu Z, Droopad R and Eisenbeiser K Epitaxial ferroelectric Pb(Zr,Ti)O₃ thin films on Si using SrTiO₃ template layers Appl. Phys. Lett. 80, 97-99 (2002)
Guerrero C, Ferrater C, Roldan J, Trtik V, Sanchez F and Varela M Epitaxial ferroelectric PbZr_xTi_1-xO₃ thin films for non-volatile memory applications Microelectron. Reliab. 40, 671-674 (2000)
Reiner J W, Garrity K F, Walker F J, Ismail-Beigi S and Ahn C H Role of strontium in oxide epitaxy on silicon (001) Phys. Rev. Lett. 101, 105503 (2008)

Epitaxial oxide growth on silicon :

Epitaxial piezoelectric MEMS