Wineglass - on - a - Chip

This paper introduces free-standing, stem-supported silicon dioxide hemispherical shells (μ-wineglasses) that are thermallygrown and have high-quality-factor resonances due to unrestricted support stem diameter and low internal thermoelastic damping. The fabrication process offers a direct path to batch fabrication of hemispherical wineglass microstructures with ultra-thin conformal conductive coatings using atomic layer deposition (ALD). A novel assembly method forms capacitive electrodes for rapid electrical characterization of the silicon dioxide μ-wineglass resonators. A quality factor of 5,600 is measured for the m=4 resonance of a 740 μm diameter, 2 μm thick thermal oxide shell with 30 nm ALD TiN coating and 77 μm diameter silicon support stem. INTRODUCTION Three-dimensional microstructures hold great potential for a multitude of applications. An example of a desirable and truly 3D structure is a hemispherical wineglass (Figure 1). Such μwineglasses can yield very high mechanical quality factors at low frequencies (low kHz) due to their thin flexural shell and highlybalanced symmetry, all within a small die area. However, difficulty in fabricating free-standing, stem-supported wineglass hemispherical shells with capacitive transduction electrodes has prevented thorough mechanical characterization of these structures. This paper introduces thermally-grown silicon dioxide hemispherical shells released from microfabricated single-crystal silicon molds, which are subsequently blanketed with an ultra-thin atomic layer deposition (ALD) conductive coating for electrical actuation and sensing. The nearly ideal symmetry of the shells confines the vibration energy near the shell rim, minimizing acoustic radiation through the support into the substrate. Furthermore, the low coefficient of thermal expansion of thermally-grown oxide results in reduced internal thermoelastic damping [1]. The oxide wineglass-on-a-chip resonators share these advantageous features with ultra-high-performance macroscale hemispherical resonator gyroscopes (HRGs) [2]. Several reports have been made in literature on different approaches for fabricating microscale hemispherical shells. PMMA and boron-doped silicon shells were first fabricated in 1979 for thermonuclear fusion research [3]. Two years later, shells made from gold or oxide were fabricated by a similar technique [4]. However, these shells were fully detached from the substrate, and the resonance characteristics of these shells were not measured. A blow-molding method based on thermoplastic forming of bulk metallic glass has been used to fabricate 3D micro shells [5]. UCI has recently reported ‘inverted’ Pyrex wineglass structures fabricated by wafer-level glassblowing. The inverted Pyrex hemispherical structures are mechanically supported at the rim, limiting their use as high-Q resonators [6]. The same group has demonstrated stem-supported hemispherical shells through laser-cutting of individual glass spheres [7]. However, laser-cutting is a serial process that is difficult to implement and control at wafer-level. Furthermore, the stem support diameter is determined by the opening size in the silicon stencil wafer, which must be large to create large diameter shells. Our group recently reported polysilicon hemispherical shell resonators with integrated capacitive transducers for electrical operation [1]. The isolated electrodes are created by boron-doping of n-type silicon wafers, forming PN junctions for isolation. The capacitive gap is defined by the sacrificial oxide layer, whose thickness is limited to a few microns. In this work, a novel assembly method forms electrodes with large capacitive gaps surrounding the shell for rapid characterization of the oxide shell resonances, enabling full electrical characterization of microscale stem-supported hemispherical shells.