Abstract
Quartz crystal out-of-plane vibration units are critical components of QMEMS devices. However, the fabrication of their 3D sidewall electrode structures presents significant challenges, particularly within ultrafine etched grooves. These challenges seriously limit further miniaturization, which is critical for portable and wearable electronic applications. In this paper, we propose a novel 3D self-masking fabrication strategy that enables the precise formation of sidewall electrodes by using the etched beam structure as a self-aligned pattern transfer medium. Based solely on photolithography and wet etching processes, this approach overcomes the limitations of the conventional shadow mask technique by improving alignment accuracy, process efficiency, and fabrication yields. In addition, a predictive mathematical model was developed to guide process optimization, enabling adaptive and reliable fabrication. Sidewall electrodes were successfully achieved in etched grooves as narrow as 45 μm, closely matching the theoretical predictions. To validate the approach, an ultra-miniaturized out-of-plane vibration unit with a beam spacing of just 150 μm-the narrowest reported to date-was fabricated, representing an 80% reduction compared to previously documented structures. The unit exhibited a repeatability error below 1.13%, confirming the precision and reliability of the proposed fabrication strategy.