Advances in ultrasonic technology over the past several years have resulted in new and promising tools which are beginning to find use in new applications. Among these are several in CMP (chemical mechanical polishing, or planarization) and post-CMP operations—specifically, ultrasonic transducers which operate at multiple frequencies ranging up to nearly 300 kHz and ultrasonic generators capable of producing a variety of waveforms to excite these transducers in modes not previously possible.

In CMP, a slurry of abrasive particles in a liquid containing a number of chemical additives is used to polish wafers. The quality of polishing, which is critical to yield, depends upon the quality of the slurry and the integrity and cleanliness of the polishing equipment. Contamination of the process by larger foreign particles or small particles that have agglomerated (formed into a rounded mass) during storage and transfer of the polishing slurry may cause micro-scratches on the polished wafer surfaces.

Ultrasonics can play a role in both assuring the quality of the slurry and maintaining the polishing equipment in proper operating condition and free of foreign particles. Historically, ultrasonic energy has been demonstrated to be effective in breaking up and suspending small particles which have agglomerated as a result of settling during storage. One proven application is the suspension of photographic and photo-resist emulsions for a number of uses. This process parallels the shaking action used to remix pigments in paint that have settled out during storage. The difference is that particles found in CMP slurry are typically around 1/100^thëthe size of those found in paint. Conventional agitation may not provide sufficient mechanical energy to provide the needed mixing on a micro scale to break up agglomerated particles in slurry, which are typically less than 100 nm (10^-7 meters) in diameter.

In the ultrasonic suspension of particles, it is the mechanical effects of cavitation and implosion delivered by the surrounding liquid that impart motion to the particles to break the bonds that hold them together. The effectiveness of ultrasonic energy in breaking the bonds is dependent on there being a sufficiently large pressure differential applied to two adhering particles to cause one to move while the other stays in a relatively fixed position. As particles become smaller, achieving the required pressure differential becomes more difficult at a relatively low frequency.

The only way to increase the pressure differential without changing frequency is to increase the power or displacement of the ultrasonic wave Figure 1). Although this may be effectivX in breaking particles away from one another, the resulting large displacements may jlso cause particles to achieve velocities that can result in collisions between particles, which may cause sintering or permanent fusing of particles together.