How Does Ultrasonic and Megasonic Cleaning Work?
The difference between ultrasonic and megasonic cleaning is the driving frequency of the system. The ultrasonic frequency range is generally recognized between 20 and 500 kHz. Megasonic frequencies are above 500 kHz.
There are 3 primary cleaning modes:
- Direct Field (DF) – Vibrations from oscillating transducers mounted to the cleaning tank are transferred to the cleaning liquid to create acoustic pressure waves. These waves, oscillating at the drive frequency of the tank, dislodge and remove particles on the substrate surface.
- Stable Cavitation (SC) – Micro-bubbles (or cavities) in the liquid are generated from the direct field sound pressure. The bubbles oscillate in size and shape causing the surrounding fluid to move. This results in strong shear forces in the vicinity of a solid surface, removing particles. This form of cavitation is also known as non-inertial cavitation.
- Transient Cavitation (TC) – When the acoustic field is strong enough, the bubbles may oscillate to the point where they can implode, resulting in shock waves that dislodge particles from a solid surface. This form of cavitation is also known as inertial cavitation.
Please view the video to understand the mechanisms which contribute to cleaning.
The level of each cleaning mode will depend on process conditions such as the drive frequency, electrical input power, gas concentration, chemistry, and temperature. For instance, it is generally recognized that transient cavitation is more prevalent at ultrasonic frequencies while stable cavitation dominates at “meg” frequencies.
The overall acoustic pressure contributing to cleaning is the sum of the pressure for each mode.
