Xu Bowen, Saygin Verda, Brown Keith A, Andersson Sean B
Department of Mechanical Engineering, Boston University, Boston, Massachusetts 02215, USA.
Rev Sci Instrum. 2020 Dec 1;91(12):123705. doi: 10.1063/5.0026069.
The atomic force microscope (AFM) is widely used in a wide range of applications due to its high scanning resolution and diverse scanning modes. In many applications, there is a need for accurate and precise measurement of the vibrational resonance frequency of a cantilever. These frequency shifts can be related to changes in mass of the cantilever arising from, e.g., loss of fluid due to a nanolithography operation. A common method of measuring resonance frequency examines the power spectral density of the free random motion of the cantilever, commonly known as a thermal. While the thermal is capable of reasonable measurement resolution and speed, some applications are sensitive to changes in the resonance frequency of the cantilever, which are small, rapid, or both, and the performance of the thermal does not offer sufficient resolution in frequency or in time. In this work, we describe a method based on a narrow-range frequency sweep to measure the resonance frequency of a vibrational mode of an AFM cantilever and demonstrate it by monitoring the evaporation of glycerol from a cantilever. It can be seamlessly integrated into many commercial AFMs without additional hardware modifications and adapts to cantilevers with a wide range of resonance frequencies. Furthermore, this method can rapidly detect small changes in resonance frequency (with our experiments showing a resolution of ∼0.1 Hz for cantilever resonances ranging from 70 kHz to 300 kHz) at a rate far faster than with a thermal. These attributes are particularly beneficial for techniques such as dip-pen nanolithography.
原子力显微镜（AFM）因其高扫描分辨率和多样的扫描模式而广泛应用于众多领域。在许多应用中，需要精确测量悬臂梁的振动共振频率。这些频率变化可能与悬臂梁质量的变化有关，例如由于纳米光刻操作导致的流体损失。一种常用的测量共振频率的方法是检测悬臂梁自由随机运动的功率谱密度，通常称为热噪声。虽然热噪声能够实现合理的测量分辨率和速度，但一些应用对悬臂梁共振频率的变化很敏感，这些变化很小、很快或者两者皆有，而热噪声在频率或时间上的性能无法提供足够的分辨率。在这项工作中，我们描述了一种基于窄范围频率扫描来测量AFM悬臂梁振动模式共振频率的方法，并通过监测甘油从悬臂梁上的蒸发来进行演示。它可以无缝集成到许多商业AFM中，无需额外的硬件修改，并且适用于具有广泛共振频率的悬臂梁。此外，这种方法能够快速检测共振频率的微小变化（我们的实验表明，对于共振频率在70 kHz至300 kHz范围内的悬臂梁，分辨率约为0.1 Hz），其速度比热噪声测量快得多。这些特性对于诸如蘸笔纳米光刻等技术特别有益。
