One-step quartz-MEMS micro-fabrication strategy for a grooved tuning fork

Grooved tuning forks with hierarchical structures have become some of the most widely used piezoelectric quartz microelectromechanical system devices; however, fabricating these devices requires multi-step processes due to the complexity of etching of quartz, particularly in specific orientations of the crystal lattice. This paper proposes a one-step fabrication strategy that can form a complete hierarchical structure with only a single etching process using novel lithography patterns. The core principle of this strategy is based on the effect of the size of the groove patterns on quartz etching, whereby trenches of varying depths can be created in a fixed etching time by adjusting the width of the hard mask. Specifically, the device outline and grooved structure can be completed using a seamlessly designed etching pattern and optimized time. Furthermore, the etching structure itself influences the etching results. It was found that dividing a wide trench by including a wall to separate it into two narrow trenches significantly reduces the etching rate, allowing for predictable tuning of the etching rate for wider grooves. This effectively increases the usability and flexibility of the one-step strategy. This was applied to the manufacture of an ultra-small quartz grooved tuning fork resonator with a frequency of 32.768 kHz in a single step, increasing production efficiency by almost 45% and reducing costs by almost 30% compared to current methods. This has great potential for improving the productivity of grooved tuning fork devices. It can also be extended to the fabrication of other quartz crystal devices requiring hierarchical structures.