The Response of a High-Sensitivity Quartz Crystal Microbalance to Irradiation by MeV Ions

Abstract

Quartz crystal microbalances (QCMs) enable the detection of mass changes with high precision, down to 10 pg/cm2/s [1], and are widely used to monitor layer growth during thin film deposition or for sputtering studies [2, 3]. Depending on the application, the sensors are exposed to different thermal loads. During thin film deposition, thermal loads typically remain in the range of a few eV per impinging particle. In sputtering experiments, thermal loads increase to around 1 keV per particle, making the usage of stress- compensated (SC) cut crystals necessary. However, QCMs only detect net mass changes and cannot resolve contributions from individual elements. Therefore, preferential sputtering or implantation processes during erosion experiments cannot always be disentangled. To close this gap, complementary techniques are needed. Ion beam analysis (IBA) methods offer to quantify elemental composition [4]. Therefore, we combined a QCM with IBA to open up new possibilities for erosion studies [5]. However, IBA introduces ion energies in the MeV range, resulting in high localized thermal loads, presenting challenges for QCM operation. In this study, we investigated the behavior of a QCM under irradiation with MeV ions and observed a twofold response in resonance frequency: a fast spike ∆f1 followed by a slower exponential drift ∆f2, which appear during beam exposure. Both effects are reversed once the irradiation stops (see Figure 1). We systematically investigated this behavior for different beam energies and currents and found that the exponential component can be explained by a pure thermal effect as the crystal heats up. The fast spike is mainly due to thermal stress induced by the localized energy deposition. Finite-element method (FEM) simulations were used to calculate the change in resonance frequency due to thermal loads. These calculations provide a framework to interpret the observed effects and to assess the usability and limitations of combining QCM with IBA. References [1] R. Stadlmayr et al. Rev. Sci. Instrum. 91 125104 (2020) [2] G. Franceschi et al. Rev. Sci. Instrum. 91 065003 (2020) [3] C. Cupak et al. Appl. Surf. Sci. 570 151204 (2021) [4] P. Ström et al. J. Instrum. 17 P04011 (2022) [5] E. Pitthan et al. Materialia 27 101675 (2023)