Megasonic Cleaning Technology: Development, Mechanism, and Application in Semiconductor Wet Processes

Megasonic cleaning technology is developed based on ultrasonic cleaning technology. Compared to the 20kHz to 200kHz operating frequency range of ultrasonic cleaning, megasonic cleaning has a higher frequency range of 800kHz to 3MHz. The two technologies differ in their cleaning mechanisms and applicable environments. Ultrasonic cleaning primarily relies on cavitation to remove contaminants from the surface of objects and is suitable for cleaning larger particles; while megasonic cleaning mainly relies on acoustic currents to remove contaminants adhering to the surface of objects and is suitable for cleaning micro- and nano-sized particles.

Research on the mechanism of megasonic cleaning technology began in the 1990s. In 1993, researcher Schwartzman introduced a high-frequency acoustic field into the SC1 and SC2 processes. The transient shock waves generated by the cavitation effect significantly improved the efficiency of surface contaminant removal. This breakthrough promoted the engineering application of acoustic-assisted cleaning technology. In 1995, Busnaina's team experimentally demonstrated that the removal efficiency of suspended particles was significantly correlated with the solution ion concentration, particle size distribution, acoustic energy density, and treatment duration. In 1997, Olim et al. established a mathematical model showing a cubic proportionality between particle removal rate and particle size, deducing that there is a cleaning blind zone for submicron (< 0.1 μm) particles. Notably, megasonic waves have shown unique advantages in cleaning polished wafer surfaces. Their ability to remove 0.2 μm particles fills the technological gap in traditional ultrasonic cleaning, achieving a multiplied surface treatment effect through the synergistic effect of physical cavitation and chemical reaction. In current engineering practice, the frequency parameters of megasonic waves need to be optimized within the 0.8-3MHz range. Due to the medium attenuation effect in high-frequency acoustic energy transmission, excessively increasing the frequency will lead to a decrease in energy conversion efficiency. The three existing megasonic cleaning technologies are shown in the figure below: (a) immersion type megasonic diaphragm, (b) spray type megasonic nozzle, and (c) adhesive type megasonic cleaning head.

In semiconductor wet processes, the mainstream megasonic cleaning methods are mainly divided into three modes: immersion, spray, and bonding. Immersion-type equipment typically integrates the acoustic wave generation module at the bottom of the cleaning tank, achieving silicon wafer process requirements through batch processing. This mode, due to its high efficiency and batch processing advantages, is widely used in wet etching, development, and post-CMP cleaning processes on wafer surfaces. Its upgraded bonding system, by controlling the distance between the acoustic source and the substrate to within 3mm, not only reduces acoustic energy consumption to 2W/cm², but also significantly reduces cleaning consumables usage by more than 70%.

In comparison, while spray-type equipment uses flow-guided acoustic nozzles, which theoretically have the advantage of preventing contaminant re-adhesion, it still requires a power output of 3.5W/cm² due to energy loss caused by the distance from the sound source. More significantly, this mode requires several times the flow rate of cleaning fluid compared to other systems, which presents a clear limitation in scenarios with strict cost control.

This innovative bonding solution integrates the technological features of two generations of systems. Through a three-dimensional curved acoustic array design, it achieves a uniform gradient distribution of the acoustic energy field as the wafer rotates. A matching liquid supply assembly continuously replenishes the chemical medium on the contact surface, simultaneously achieving the dual goals of precision cleaning and resource conservation. Its typical structure consists of a fixed acoustic wave assembly and a rotating workpiece. A separate liquid supply device continuously delivers cleaning fluid to the surface of the object being cleaned (semiconductor wafer). This design demonstrates unique advantages in advanced manufacturing processes.