In situ thin film thickness measurement using ultrasonics waves

The fact that the velocity of an ultrasonic Lamb wave traveling in a silicon wafer is changed by the thin film coating on the wafer surface can be used as a monitoring method for basically any type of film-opaque, transparent, metal or insulator. We excite and detect Lamb waves using PZT transducers on one end of quartz buffer pins that are in contact with the back side of the wafer. The time of flight of the ultrasonic wave is calculated and measured to be linearly related to the film thickness. The acoustic sensors are easily implemented into plasma or CVD environments. We have demonstrated the technique in an aluminum sputtering system. We measure Al film thickness with a resolution of ±100 Å. Even better resolution can be achieved for SiO₂, copper and tungsten films. This system has a variety of potential applications not only in film thickness measurement, but also in characterization of film properties and multi-layer deposition process control