Etch-a-sketch Resonator

This paper demonstrates AFM-based post-release piezoelectric domain engineering [1] on a Lithium Niobate (LN) MEMS resonator. We “Etch-A-Sketch” a poling pattern on the membrane resonator that redefines the electro-mechanical coupling of the mechanical vibration modes. This technique introduces a new resonance peak at 681 MHz to the original un-poled admittance spectrum of the resonator. In addition, this method only requires 80V (magnitude) to achieve domain inversion, which is over 100× lower than the poling voltage required for bulk LN. FABRICATION PROCESS AND DEVICE CONCEPT Figure 1. Fabrication of Etch-a-Sketch resonator: (a) Lithium Niobate thin-film bonded to Si substrate. (b) Top gold electrodes are defined by lift-off, (c) ion milling etches the LN to define device geometry, and (d) XeF 2 is used to release the resonator. Fabrication of the Etch-a-Sketch resonator begins with a 1μm thin[2] white LN thin film directly bonded on a Si substrate, with crystal z-axis normal to plane of the wafer. Z-cut LN does not provide modes with high coupling coefficient, keff, that can be easily excited using interdigitated transducers (IDT). However, we chose this cut for a proof-of-concept demonstration as domain inversion is well-studied on z-axis LN [1]. Top electrodes are defined by lift-off, followed by ion milling through the LN film with a photoresist mask to define the resonator geometry [3]. Finally, the membrane resonator is released from the substrate using XeF2 (Fig. 1). Fig. 2 shows an SEM of the released device, where the IDT fingers oriented perpendicular to the crystal y-axis Figure 2. SEM of the z-cut LN Etch-A-Sketch resonator. Figure 3. “Writing” the resonator frequency by AFM tip poling checkerboard patterns on the LN resonator. Green and blue arrows mark +y-axis and +z-axis domain directions respectively. are spaced by 4.5μm. The width (x-direction) of the resonator is 72μm. Before performing domain patterning, the admittance of the resonator is measured as baseline for comparison with the admittance after poling. An Asylum AFM system was used to perform the domain patterning of the resonator. The membrane was scanned continuously between the IDT electrodes in contact mode while applying periodic voltage to the AFM tip to invert the domain direction of the membrane in a checkerboard pattern (Fig. 3). A low spring constant (8N/m) solid platinum AFM tip (Model No.: 25Pt400B, Rocky Mountain Nanotech.) was used to minimize the stress on the membrane during domain writing, while providing good electrical contact. In addition to excellent spatial control (25nm resolution), a key benefit of Etch-a-Sketch poling is the focused E-field from the sharp AFM tip which penetrates the thin film LN. This reduces the voltage required to achieve domain inversion relative to bulk LN poling by over 100×. In this work, 80V (magnitude) was sufficient to surpass the coercive field of LN [4], and ensured high contrast domain inversion. The poled domain pattern was verified by piezoresponse force microscopy (PFM). Fig. 4 shows the phase of the piezo-response which exhibits a clear checkerboard pattern. Figure 4. Phase response of the PFM after domain poling. 20um y x S G Out of Phase IDT Finger