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Why is megasonic cleaning critical to chip manufacturing?

2024-08-16 08:47

We know that any little particle contamination on the wafer surface will affect the subsequent chip performance. As semiconductor components become more and more tightly packed, conductive paths become finer, and structures become smaller and smaller, tiny contaminating particles on the wafer surface can impede etching and lead to component defects. Mega acoustic cleaning is particularly useful for silicon wafer cleaning. The process uses high-frequency acoustic waves to clean the wafer surface and can be integrated into a wet bench station. As product complexity increases, megasonic cleaning is becoming increasingly important for silicon wafer processing. In wafer cleaning, ultrasonic and megasonic cleaning are often seen when removing particles from the wafer surface. Ultrasonic cleaning, which we have already introduced in ‘Why Ultrasonic Cleaning is Commonly Used in Chip Manufacturing and Chip Packaging’, can only clean large particles. 
For smaller particles, megasonic cleaning is required. As shown in Figure 1, the larger the frequency, the smaller the particles to be cleaned.
Fig. 1 Relationship between frequency and size of cleaned particles
However, particles can often be effectively removed by combining a chemical solution with an acoustic field (in the MHz range). Megasonic cleaning is a gentler cleaning mechanism that is widely used for wafer surface cleaning during device fabrication. For example, megasonic cleaning is often used after RCA cleaning to produce optimal cleaning results and reduce the use of additional chemicals. When used in conjunction with traditional cleaning methods, megasonic cleaning results in cleaner wafers, which reduces the number of particles.
1. How to clean wafer particles
The generation of high frequency sound waves through a transducer is the key way in which MegaSonic Cleaning handles product cleaning. The piezoelectric transducer generates sound waves that alternate between low and high pressure. Micro-cavitation bubbles appear in response to the sound waves and remove particles and contaminants from exposed surfaces. Mega sonic cleaning technology relies heavily on megahertz frequencies for effective cleaning, as the smaller, high-energy cavitation bubbles remove submicron particles while leaving the product intact. In contrast, the larger cavitation bubbles generated by lower ultrasonic frequencies can damage sensitive devices (which is why they are listed as a restricted process for aerospace products). The higher the frequency, the smaller the cavitation bubbles, so the smoother the cleaning and the less or no damage to the structure to be cleaned. Tiny bubbles move through small surface areas, crevices and unusual shapes, providing an effective cleaning action.
Fig. 2 Microscopic damage caused by ultrasonic low frequency cleaning 
Figure 3 Megasonic cleaning without damage
During megasonic cleaning, a high-frequency generator produces an electrical signal in the megahertz range. A transducer immersed in the cleaning solution converts the electrical signal into sound waves. As the sound waves travel through the cleaning bath, they create tiny cavitation bubbles. The bubbles form in the troughs of the low-pressure sound waves and burst at the high-pressure peaks of the sound waves. When the bubbles burst, they release a high-energy jet of cleaning fluid that impacts the wafer surface and removes contaminants. The jet is powerful enough to remove foreign material from the wafer without damaging the silicon.
Chemical cleaning methods are effective at removing contaminants, but particles often remain on the wafer surface due to static electricity and surface tension. Chemical action and rinsing are not sufficient to remove many ultra-small particles. Mega-acoustic cleaning and microscopic bubble and jet action can remove these particles so that they can be rinsed away and the cleaned wafer is almost completely free of contaminating particles.

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