Long-Term Aging Study of a Silicon Nitride Nanomechanical Resonator

TL;DR

This study investigates the long-term stability of silicon nitride nanomechanical resonators over 135 days, revealing aging behaviors similar to quartz oscillators and highlighting surface adsorption effects on frequency stability.

Contribution

It provides the first detailed long-term aging characterization of SiN nanomechanical resonators, linking surface chemistry to frequency drift and aging mechanisms.

Findings

Aging follows double-logarithmic and drift-reversal trends.

Aging magnitude is comparable to temperature compensated quartz oscillators.

Surface adsorption, especially carbon and oxygen, significantly affects frequency stability.

Abstract

Short-term changes in the resonance frequency of silicon nitride (SiN) nanomechanical resonators can be measured very precisely due to low thermomechanical fluctuations resulting from large mechanical quality factors. These properties enable high-performance detection of quasi-instantaneous stimuli, such as sudden exposure to radiation or adsorption of mass. However, practical use of such sensors will eventually raise questions regarding their less-studied longer-term stability, notably for calibration purposes. We characterize aging of an as-fabricated SiN membrane by continuously tracking changes of its resonance frequency over 135 days in a temperature-controlled high vacuum environment. The aging behavior is consistent with previously reported double-logarithmic and drift-reversal aging trends observed in quartz oscillators. The aging magnitude (300 ppm) is also comparable to typical temperature compensated quartz oscillators (TCXO), after normalization to account for the greater importance of interfaces in our thin (90 nm) resonator, compared to several microns thick TCXOs. Possible causes of aging due to surface adsorption are investigated. We review models on how water adsorption and desorption can cause significant frequency changes, predominantly due to chemisorption stress. Chemical species adsorbed on the resonator surface are also identified by X-ray photoelectron spectroscopy (XPS). These measurements show a significant increase in carbon every time the sample is placed under vacuum, while subsequent exposure to air causes an increase in oxidated carbon. Developing models for the contribution of carbon and oxygen to the membrane stress should therefore be an important future direction. Other contaminants, notably alkaline and halide ions, are detected in smaller quantities and briefly discussed.