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As we all know, ultrasonic cleaning is a common efficient and fast physical cleaning method. It has the advantages of remarkable cleaning effect and easy to operate. It reduces the possibility of secondary pollution in the complex chemical cleaning process, and can also achieve the purpose of cleaning complex devices. Therefore, it is widely used in various industrial, medical, civil and other fields. The core principle of ultrasonic cleaning is to remove and peel the stains on the object surface by using the cavitation effect of ultrasonic in liquid.
What is ultrasonic cavitation effect? The high-frequency mechanical vibration of ultrasonic wave will produce tiny bubbles in the liquid. These tiny bubbles will expand rapidly and then close suddenly. When they close, they will generate strong shock wave. This dynamic process of expansion, closure and shock is called the "cavitation effect" of ultrasound. "Cavitation effect" not only provides enough energy for the chemical and physical reactions in the cleaning process, but also the instantaneous high pressure constantly impacts the surface of the cleaned workpiece, and also makes the foreign matter dirt on the surface of the cleaned object and the gap peel off rapidly. However, too strong ultrasonic cavitation will also cause some damage to the surface of the object. For example, in the cleaning of precision components such as semiconductor devices and optical precision parts, due to some microstructure on the surface of the device, the traditional ultrasonic cleaning will cause some damage to the surface. At the same time, with higher ultrasonic frequency, the cavitation bubbles and the removable impurity particles are smaller. The frequency of traditional ultrasonic cleaning transducer is between 20khz-100khz. The cavitation effect in this frequency range is large, which will damage precision components, and it is difficult to remove impurity particles smaller than 1 micron.
When the ultrasound with frequency higher than 400kHz propagates in the liquid, it can form a very thin acoustic boundary layer with large velocity gradient near the surface of the cleaned part, and its impurity particles fall off from the surface of the device under the vibration of liquid in mega-hertz scale, which can clean the micron and submicron impurity particles on the surface of the component, so as to realize the ultra-precision cleaning process. In addition, due to the extremely low cavitation effect, the ultra-high frequency ultrasonic cleaning process will not damage the surface of the cleaned parts, which can effectively solve the corrosion or damage caused by the cleaning of precision components. Therefore, ultrasonic devices that can emit ultrasound in megahertz are widely used in the field of semiconductor manufacturing, and in addition to cleaning, they can also play an important role in key processes such as chemical mechanical polishing, development, photoresist stripping, lift-off, etching and so on. The ultrasonic with the frequency in megahertz is also called "megasonic" in the industry.
Figure 1. The Demostration of Shower Type Megasonic Nozzle
The existing megasonic cleaning technology is mainly divided into tank type megasonic cleaning system (batch type), shower type megasonic cleaning system and cover type megasonic cleaning system. The tank type megasonic cleaning is similar to the traditional ultrasonic cleaning. Generally, the megasonic emission device is placed at the bottom of the water tank, and the wafer is placed in the water tank during cleaning. This method has the advantages of convenient use and batch cleaning of products, so it is also called "batch type" megasonic cleaning. At present, megasonic wet processes for batch wafers such as cleaning, CMP, etching, development mainly use tank type.
Shower type megasonic cleaning is to make the megasonic transducer into a nozzle. The megasonic energy falls onto the cleaned object (such as wafer) with the fluid. Compared with the batch type megasonic cleaning placed at the bottom of the tank, it has higher acoustic transmission efficiency and can avoid secondary pollution. It is especially suitable for the high-precision wet process on a single wafer. However, the disadvantage of shower type megasonic cleaning is that the effective megasonic area is small (around 10mm) and it needs a large flow of cleaning fluid to work, resulting in a lot of waste of cleaning fluid.
Cover type megasonic cleaning is a more optimized way based on the improvement of shower type megasonic cleaning system and tank type megasonic cleaning system. Usually, the megasonic emission device is very close to the surface of the cleaning object (wafer), leaving only a gap of no more than 3mm, and the fluid flows through the gap to complete the cleaning. Because the megasonic emission surface is very close to the wafer, its acoustic energy transmission efficiency is very high. The power density of the megasonic emission surface only needs 2W/cm2 to achieve a great performance, while the shower type megasonic cleaning system usually needs 3-4W/cm2, and the tank type megasonic cleaning system needs 5W/cm2 or even higher power density. In addition, cover type megasonic cleaning system only needs a small amount of cleaning fluid to complete the cleaning process. Compared with the other two types megasonic cleaning system, it can greatly save the use of cleaning fluid. In order to make the sound field obtained by the whole wafer surface uniform when the wafer rotates, the megasonic emission device can be realized by making the megasonic device with fan-shaped structure, so as to ensure that the megasonic energy received from the edge to the center of the wafer decreases linearly when the wafer rotates, so as to obtain the front uniform sound energy. The typical cove type megasonic cleaning system is shown in the figure. The megasonic emission device is fixed, the cleaned object (wafer) rotates, and the cleaning fluid is continuously delivered to the surface of the cleaned object (wafer) by a separate liquid feeding device.
With nearly 40 years of technical accumulation in power ultrasonic industry, especially high-frequency transducers for liquid process, and years of special product research and development, Siansonic has launched a series of megasonic cleaning products with various specifications and types, such as tank type megasonic cleaning system(batch type), shower type megasonic cleaning system and cover type megasonic cleaning system, to provide megasonic solutions to companies in semiconductor manufacturing industry all over the world.
