Researchers have demonstrated that rotary ultrasonic machining can drill high-quality holes in single-crystal silicon by modifying the material removal mechanism to a hybrid ductile-brittle mode. The study reveals ultrasonic vibration assistance reduces cutting forces by up to 29% while improving the hole quality, minimizing tool wear and enabling effective control of edge chipping damage with appropriate machining parameters. The findings could benefit semiconductor, photovoltaic, and microelectronics manufacturing where silicon components require precision machining.
Silicon is the foundation of modern electronics. It is used in semiconductors, solar cells, sensors, microelectromechanical systems (MEMS), and many other high-tech products. Yet despite its widespread use, silicon remains one of the most challenging materials to machine.
"Silicon is extremely hard, but its crystal structure also makes it susceptible to brittle fracture during machining," explains Kh M Asif Raihan of Kansas State University. "When conventional drilling or cutting methods are used, cracks, chipping, and tool wear can occur easily, making it difficult to achieve the surface quality and dimensional accuracy required for high-end applications."
To address this challenge, researchers from Kansas State University investigated a manufacturing technique known as rotary ultrasonic machining (RUM Unlike conventional drilling, RUM superimposes high-frequency ultrasonic vibrations onto a rotating diamond tool. The vibration causes the cutting tool to repeatedly engage and disengage from the material within a short period of time and with a high impact force. The resulting intermittent contact can alter the localized stress conditions, changing how silicon deforms and fractures during machining.
The team studied the drilling of holes in single-crystal silicon, a material widely used in semiconductor manufacturing. Through machining experiments, microscopic imaging, motion simulations, and nanoindentation testing, the researchers sought to understand not only whether RUM works, but also why it works.
The study revealed that silicon removal occurs through a hybrid ductile-brittle mechanism. Although silicon is normally considered a brittle material, the intermittent loading-unloading cycles can suppress crack propagation by limiting the energy available for crack growth. In addition, the high localized contact pressures may induce phase transformations and localized ductile zones that exhibit greater resistance to fracture than the original single-crystal silicon. The combined effects promote a hybrid ductile-brittle material removal mode that improves machining performance while maintaining surface integrity.
"Our experiments showed evidence of localized plastic deformation occurring alongside brittle fracture," says Kh M Asif Raihan. "This mixed-mode of material removal helps explain why RUM can produce high-quality features in single-crystal silicon, a material that has traditionally been difficult to machine."
The altered material removal behavior was also reflected in the machining performance. Their findings showed that ultrasonic vibration assistance reduced cutting forces by as much as 29% compared with conventional machining conditions. Lower cutting forces are important because they reduce mechanical stress on both the silicon workpiece and the cutting tool.
The researchers found that the best machining performance was achieved using a combination of high spindle speed, high ultrasonic power, and low feedrate. Under these conditions, the machined holes exhibited improved geometrical accuracy, including lower roundness error, reduced taper and improved dimensional consistency, reduced edge chipping, and better overall surface quality.
Additional experiments demonstrated that the benefits of ultrasonic vibration assistance persisted even when the feedrate was increased substantially beyond the original experimental range. While higher feedrates generally increased cutting force, ultrasonic vibration continued to reduce machining loads without significantly compromising surface quality, highlighting the process's potential for higher-productivity silicon manufacturing.
An additional advantage was the low level of tool wear during RUM. Most diamond abrasive particles on the cutting tool remained intact after machining. Compared with conventional machining conditions, RUM reduced severe wear signatures by lowering the mechanical stress acting on both the abrasive particles and the bonding matrix, indicating the potential for longer tool life and improved process reliability.
The findings could have implications for industries that depend on precision silicon components. Semiconductor equipment commonly contains silicon-based gas distribution plates, electrostatic wafer chucks, microfluidic channels, and other components that require accurate hole-making operations with strict quality requirements. Improved machining methods could help manufacturers reduce defects, improve productivity, and lower manufacturing costs.
Beyond silicon, the researchers believe the insights gained from this work may also support the machining of other hard and brittle materials used in advanced manufacturing applications.
"This study improves our understanding of how ultrasonic vibration changes the way silicon breaks during machining," says Meng Zhang, corresponding author of the study. "The knowledge can help guide the development of more efficient manufacturing processes for semiconductor and high-precision engineering applications.
The paper, "Rotary ultrasonic machining of single-crystal silicon", was published in Advanced Manufacturing.
Raihan K, Obembe I, Wieland D, Jones C, Zhang M. Rotary ultrasonic machining of single-crystal silicon. Adv. Manuf. 2026(3):0011, https://doi.org/10.55092/am20260011
