The thermal NTM methods have the shortcoming of the formation of the heat-affected zone around the laser scanning path, which could reduce the strength of the silicon wafers. Although the chemical wet etching method and electrochemical machining method have the advantages of almost no damage, they still have significant disadvantages of low processing speed, hard to be controlled, and high chemical disposal costs. The commonly used NTM processes are as follows: chemical NTM methods (such as chemical wet etching method and electrochemical machining method) and thermal NTM methods (such as laser-based machining methods). In recent years, the non-traditional machining (NTM) processes for silicon wafers have been widely investigated and rapidly developed,. In addition, the traditional micro-machining process also induces residual stress on the material surface, which has negative impacts on the mechanical and optical properties. Microcracking and chipping being commonly generated on the machined surface lead to lower product quality and a higher rejection rate of machined parts.

However, brittle materials (e.g., silicon wafers, sapphire wafers, and SiC wafers) are difficulted to machine due to their low fracture toughness. In addition, bone micro-machining applications are commonly used in dentistry and orthopedic surgeries. For example, micro-features are needed to be machined into optical components (lenses and fibers) for sensor applications. Brittle materials are widely used in these industries, such as single crystal materials, amorphous materials, and ceramics. Micro-machining has been widely used in the energy industry, aerospace industry, and biomedical industry, ,. The tools for the micro-machining process are usually smaller than 500 μm in diameter. Micro-machining is the machining technology that fabricates high-precision devices or features in the micron size ranges. Results show that the ultrasonic vibration could reduce the cutting force, improve the cutting surface quality, and suppress the residual compressive stress, especially under conditions with high tool rotation speed. The effects of ultrasonic vibration, tool rotation speed, and depth of cut on tool trajectory, material removal rate (MRR), cutting force, cutting surface quality, and residual stress were investigated. To fill this knowledge gap, rotary ultrasonic surface micro-machining of the silicon wafer (one of the most versatile brittle materials) was conducted in this study. Up till now, there is still no investigation on the effects of ultrasonic vibration (UV) and input variables (such as tool rotation speed and depth of cut) on cutting force and machined surface quality in RUSμM of brittle materials. Due to the small diameter cutting tool (<500 μm) and high accuracy requirements, the impact of input parameters in the rotary ultrasonic surface micro-machining (RUSμM) process on tool deformation and cutting quality is extremely different from that in rotary ultrasonic surface machining (RUSM) with relatively large diameter cutting tool (∼10 mm).

Therefore, rotary ultrasonic micro-machining (RUμM) is investigated to conduct the surface micro-machining of brittle materials. As one of the nontraditional machining processes, rotary ultrasonic machining is an effective method to reduce the issues generated by traditional machining processes of brittle materials. To reduce and further solve these problems, a new micro-machining process is needed. Traditional ultra-precision micro-machining can lead to workpiece cracking, low machined surface quality, and reduced tool life. In recent years, there is an increasing demand for high-precision micro-machining of these brittle materials to produce precision functional parts. Many brittle materials, such as single-crystal materials, amorphous materials, and ceramics, are widely used in many industries such as the energy industry, aerospace industry, and biomedical industry.

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