What is Megasonics cleaning?

Megasonics are emerging as an increasingly important, widely accepted cleaning method for contamination-sensitive products. A growing number of manufacturers in the integrated circuit, hard drive, raw silicon, mask, flat panel display, and other industries affected by contamination are turning to megasonic cleaning to help meet stringent cleanliness requirements.

Megasonic cleaning uses the piezoelectric effect to enable removal of submicron particles from substrates. A ceramic piezoelectric crystal is excited by high-frequency AC voltage, causing it to vibrate. This vibration generates an acoustic wave that is transmitted through a cleaning fluid, producing controlled cavitation. As the wave passes across the surface of an object, it causes particles to be removed from the material being cleaned. The technology was originally developed by the U.S. Navy as an element in anti-submarine warfare.

How does it work?

Megasonics work by generating controlled acoustic cavitation in the cleaning fluid. Acoustic cavitation is produced by the pressure variations in sound waves moving through a liquid.

Cavitation, the formation and activity of bubbles (or cavities), is believed to be an important mechanism in the actual particle removal process, because cavitation has sufficient energy to overcome particle adhesion forces and cause particles to be removed. Controlled megasonics cavitation becomes acoustic streaming, which pushes the particles away so they don't reattach to the material being cleaned.

The pressure amplitude, or megasonic power, required to achieve cavitation has been proven to depend on the pulse width, dissolved gas content in the cleaning fluid, and the power input. Megasonics cleaning is controlled by varying the power input. Pulsing the input power provides better control over cavitation than applying continuous input power.

Exposure time and megasonic power are the most significant variables affecting megasonics cleaning. As megasonic power or exposure time increases, particle redeposition decreases. Pulsed input power (pulsed-wave megasonics) achieves greater acoustic power levels in a cleaning bath than continuous input power (continuous-wave megasonics) at the same average input. Typical exposure times are 10 to 30 minutes.

Megasonics cleaning compared to ultrasonic cleaning

The difference between ultrasonic cleaning and megasonics cleaning lies in the frequency that is used to generate the acoustic waves. Ultrasonic cleaning uses lower frequencies; it produces random cavitation. Megasonics cleaning uses higher frequencies at 1000 kHz; it produces controlled cavitation.

An important distinction between the two methods is that the higher megasonic frequencies do not cause the violent cavitation effects found with ultrasonic frequencies. This significantly reduces or eliminates cavitation erosion and the likelihood of surface damage to the product being cleaned. Parts that would be damaged by ultrasonic frequencies or cavitation effects can often be cleaned without damage in a megasonic bath using the same solution.

With ultrasonics, cavitation occurs throughout the tank, and all sides of submerged parts are cleaned. With megasonics, only the side of the part that is facing the transducer(s) is cleaned.

Applications

The megasonics cleaning technique is effective for removing 0.15-micron particles from silicon wafers and other products, without damage. The method is currently being used by manufacturers of integrated circuits, flat panel displays, and hard disks, as well as by mask makers and raw silicon suppliers.

Megasonics cleaning may be used with a variety of chemistries. Although it is used primarily for particle removal, it may also be used to increase the efficiency of chemical cleaning with surfactants or detergents. Removal of other contaminants depends on the solutions in the tank.