What is Megasonic Cleaning?
In a sonic cleaning system, a transducer generates high frequency vibrations in a fluid medium. These high frequency vibrations, as described below, create a scrubbing action on a substrate uniquely suited to cleaning devices with crevices or blind holes such as semiconductors or memory disks
How Does Megasonic Cleaning Work?
The transducer mounted on the outside of the tank produces high frequency waves that propagate through the fluid medium. Each point along the wave oscillates between a maximum and minimum pressure. When the pressure minimum is below the vapor pressure of the fluid medium, bubbles are formed in the fluid medium. When the pressure increases to maximum pressure, the bubbles implode, sending out an intense shockwave of energy as the fluid rushes in to fill the void left by the collapsed bubble. This energy is referred to as cavitation energy, and is well-suited for dislodging particles or contaminants from a substrate.
Megasonics vs. Ultrasonics
Many factors can affect the intensity of the cavitation energy in a sonic process, such as the surface tension of the fluid medium, or the distance of the substrate from the transducer. However, a critical factor is the frequency of the sonic waves. In a typical Ultrasonic cleaning process, the transducer emits a frequency of less than 100kHz. This lower frequency means that bubbles have a longer time to form and grow, creating more cavitation energy when they collapse. Megasonic processes utilize frequencies from 600-2000 kHz. These higher frequencies mean that bubbles have less time to grow and are smaller when they collapse, producing a proportionally smaller amount of cavitation energy.
As wafer structures become smaller and more complex, Ultrasonic energy is becoming increasingly undesirable in the Semiconductor market. The high cavitation energy produced in Ultrasonic cleaning can blow out sensitive structures and damage substrates. The gentler cleaning energy produced by Megasonic waves can dislodge particles without damaging sensitive devices.