# Measurement of Sub-Surface Microstructures Based on a Developed Ultrasonic Atomic Force Microscopy
## 1 Introduction  
With the continuous progress of ultraprecision machining technology, various ultra-precision components are widely used in aerospace, quantum communications, new energy, biomedical and other important fields related to the national economy and people's livelihood [1, 2]. In addition to the stringent requirements for machining accuracy, surface/subsurface integrity and service reliability are also required by ultra-precision components. The surface and subsurface defects of ultra-precision components are key factors affecting their service reliability [3]. Thus, high-precision measurement of surface and sub-surface defects of ultra-precision components is not only a key requirement for ensuring the quality of ultra-precision processing, but also a frontier direction in the field of micro- and nanometrology. Studies have shown that besides the inherent defects of the material itself, the complex mechanical, physical, and chemical interaction process between the tool and the material during the ultra-precision machining process are the main reasons for inducing defects of the machined parts [4]. The defect areas caused by the machining process mainly exist on the surface and subsurface of the workpiece [5]. Among them, surface defects are easy to detect by many existing measurement technologies and can also be removed by subsequent processes such as ultra-precision polishing. However, sub-surface defects are hidden underthe surface layer and thus difficult to be detected and removed, which is a key factor affecting the performance and reliability of ultra-precision components [6, 7]. Considering the scale of subsurface defect could be down to micrometric or even nanometric orders, traditional inspection technologies are facing severe technical challenges. It is important to develop new measurement methods suitable for high-resolution measurement and characterization of subsurface micro/nano-scale defects, so as to qualify the ultra-precision processing and reliability of ultra-precision processing components [4, 8, 9]. Many studies have shown that ultrasonic detection method is able to use the interactions between a focused ultrasound wave and materials to measure subsurface defects by analyzing the ultrasonic echo signals [9, 10, 11]. The ultrasonic detection has the advantages of wide materials applicability. However, the resolution of ultrasonic detection can only reach a few tens of microns, and it is quite challenging to measure micro/nano-scale subsurface defects due to the inevitable effect of diffraction limit in ultrasonic wave. On the other hand, atomic force microscopy (AFM) [12] can be completely free of the influence of diffraction limit, and has the advantage of achieving high resolution in the nanometric and even sub-nanometer scale [13]. However, considering that the interface between the AFM probe and the sample occurs only on the surface layer, conventional AFM can only be used for surface topography measurement and does not have the capability to obtain the information of subsurface structures [14]. Therefore, the advantage of subsurface detection capability of ultrasonic inspection and the advantage of high resolution measurement of AFM technique were considered to form the ultrasonic AFM technique. In recent days, some studies on combining AFM with ultrasonic excitation have been demonstrated and practiced to detect microstructures with tens or even hundreds of nanometers below the material surface. [PERSON] et al. [15] reconstructed the PDMS thin film materials embedded with silicon nanowires based on the dynamic model of the probe-sample coupling system by applying ultrasonic multi-frequency excitation to the AFM probe. [PERSON] et al. [16] carried out related research work on acoustic scanning probe measurement technology of the sample mechanism. They obtained information on the nanostructure of HOPG 200 nm below the surface by means of high harmonic acoustic field excitation of the AFM.