text: U.S. Pat. No. 4,401,131 A1 discloses a wafer-cleaning device, wherein the wafer is held by a vacuum spin chuck and rotated. Simultaneously cleaning liquid is supplied between a transducer-plate and the wafer. The transducer plate is agitated by piezo elements. Liquid is supplied between the wafer and the transducer through a central opening in the transducer plate. Ultrasonic is supplied across the whole wafer. This device has the disadvantage that only one side of the wafer can be treated at one time and the other side is mechanically touched by the spin chuck.
  U.S. Pat. No. 5,979,475 A1 discloses a wafer-cleaning device comprising two parallel plates optionally having an ultrasonic vibrator for treating both sides of a wafer sandwiched therebetween. The wafer floats on liquid cushions generated by liquid introduced through central openings of each plate. The wafer is not held by any further element and is therefore allowed to rotate freely. This device has the disadvantage that the wafer floating sandwiched between two plates is very hard to control and therefore most of the time a very instable system.
  An object of the invention is to simultaneously treat a wafer with liquid on both sides with a device, which is easy to control in order to achieve stable treating conditions.
  Another object of the invention is to apply ultrasonic wave energy to a wafer for sufficient cleaning but however avoiding to high local ultrasonic energy.
  Yet another object of the invention is to treat a wafer with a very low amount of liquid because of environmental and cost reduction purposes.