Ultrasonic cleaning by acceleration causes water molecules to be accelerated and strike the workpiece, and the impact causes particles to be stripped from the workpiece. The higher the frequency, the more effective the ultrasonic cleaning is, and it is effective in removing very fine particles with weak adhesion. The acceleration increases in proportion to the square of the multiple of the frequency, and the wavelength also decreases, making it effective against finer particles.
This is mainly a cleaning action at medium and high frequencies.
When ultrasonic waves are irradiated, standing waves are generated in the liquid in accordance with the frequency. (λ is one wavelength)
While dirt is removed, there is a possibility of damaging the workpiece. In order to reduce the influence of standing waves, oscillation of the workpiece and multi-frequency oscillation are also taken into consideration. It is necessary to determine the balance between the ease of stain removal and damage to the workpiece.
Batch cleaning of semiconductor wafers
This method cleans multiple numbers of semiconductor wafers at the same time by placing them in an ultrasonic cleaning tank in the MHz band.
(1) Many wafers can be cleaned at one time, which saves time.
(2) Relatively less cleaning liquid is required compared to the single-wafer method.
(3) Easier use of specific cleaning solution with double tanks.
When chemicals are used in a batch-type process, the dip-type cleaning method is often used, in which a double-layered structure of a cleaning tank and a quartz tank is used for indirect irradiation. The use of a quartz tank also avoids elution of metal ions and impurities and maintains cleanliness.
However, batch-type cleaning has been presented as a problem due to the increase in the size of equipment as substrates become larger and the re-adhesion of particles as substrate patterns become finer.
Next-generation cleaning for low damage and high cleanliness
In 2006, Honda Electronics developed the world's first quartz transducer unit type ultrasonic cleaner. In this system, semiconductor wafers are cleaned by superimposing ultrasonic waves on a quartz transducer unit. Compared to the batch type and nozzle type, the amount of chemicals used can be reduced, and the unit is clean because only quartz glass and chemical-resistant resin come in contact with the liquid while no rubber materials such as packing come in contact with the liquid. In addition, by changing the shape of the quartz transducer, it is possible to perform low-damage cleaning, large-area cleaning, and cleaning of hard-to-remove bevels (edge surfaces) and notches (notches on wafer edge surfaces), etc., to effectively clean wafers according to customer needs.
The world's first flowing water cleaning system developed
We were the first company in the world to develop this system in 1986. High-frequency ultrasonic waves in the MHz range are superimposed on the flowing water radiated from the tip of the nozzle to precisely clean the workpiece. The radiated water acts as a transport medium for the ultrasonic waves and at the same time, for the dirt that has been stripped off by the ultrasonic waves.
