Study On Development Of Self-Sensing AFM Head And Hybrid Measurement System

With the development of micro-nanometer manufacturing technology, the demand for the measurement technology has been increased. As an important measurement technology, the atomic force microscope (AFM) is able to conduct dimensional measurement for micro-nano structural surfaces and devices with a nano or sub-nanometer resolution. Nowadays, the performances of AFMs has been improved in some aspects such as accuracy, range, stability and usability, etc. Focusing on this theme, a self-sensing AFM measuring head is developed and integrated to the large range nanometer positioning stage to realize measurements in a 25 mm×25 mm×5 mm range with nanometer magnitude resolution. Meanwhile, hybrid measurement method combining AFM and white light scanning interferometry is studied to enhance the measurement efficiency of the AFM. The main achievements of this research work (listed chronologically) are the following:1. The theoretical and finite element model of a novel tuning fork probe is analyzed. A two degrees-of-freedom mechanical equivalent model of the probe is established according to its unique structure. The model explains that the probe has two first-order intrinsic resonant frequencies. In order to analyze the probe's electrical properties, an electrical equivalent model is created, based on the parasitic capacitance in the probe and the parallel resonance interference resulting from the parasitic capacitance. By the application of the finite element analytic method, the mechanical and electronic properties of the probe are further studied. According to the modal and harmonic response analysis, the work principle of the probe is explained, and one suitable working frequency, among several intrinsic resonant frequencies of the probe, is determined.2. An AFM head based on the self-sensing tuning fork probe is developed. By self-sensing a nA-current amplification and a parasitic capacitance compensation, the probe's signal doesn't require any optical sensors to detect. The dynamic operation methods for the head are established. Based on an auxiliary tracking oscillation type frequency modulation module, some time-consuming adjustment procedures can be omitted, and the working stability of the system is enhanced. One method for calibrating the probe's electromechanical coupling factor is proposed to quickly calculate the amplitude of the cantilever.3. Through the combination of the measuring head, and the large range nanometer positioning stage, a large range AFM system is developed. Because of the dual-feedback mode, the system's measurement range can perform millimeter magnitude measurement range in three directions. An easily operated method is proposed to calibrate the displacement positioning platform driven by the piezoelectric actuators. The measurement data can be traced back to the definition of meter through three embedded laser interferometers. Based on the advantage of the self-sensing measuring head, and the large range nanometer positioning stage, a hybrid measurement method integrated with white light scanning interferometry is proposed. With the help of the white light scanning interferometry, the measurement efficiency of AFM can be enhanced.4. The performance of the system is tested, and two application experiments are carried out. A series of standard samples are measured. The results demonstrate the system's large range measurement capability, a nanometer magnitude resolution, and good measurement repeatability. An optical sinusoidal surface sample is characterized. The contour profile and the roughness information of the sample are directly separated depending on two platforms. A sample is measured in a mm magnitude range, firstly by using white light scanning interferometry for selecting the regions of interest (ROI) in aμm magnitude range; then, the ROI is further measured by AFM to enhance the measurement efficiency.