Method and apparatus for in-line quality control of wafers

A method of any one of the preceding claims wherein the reference resonant frequency curves are each characterized by the following reference parameters:

• mean value of resonant frequency of resonant peak,
• mean bandwidth of resonant peak,
• mean amplitude of resonant peak,
• and the defined set of values comprises functions of the standard deviations of each of the reference parameters.

An apparatus for in-line quality control of wafers comprising:

• A broad band acoustic actuator capable of exciting mechanical vibrations in a wafer simultaneously in multiple predetermined frequency ranges,
• At least one acoustic sensor capable for detecting mechanical vibrations in the wafer and converting the vibrations into electrical signals,
• At least two electrical generators jointly delivering a superposition of the electrical signals to the acoustic actuator,
• At least two electrical amplifiers each synchronized with the corresponding electrical generator and tuned to measure the vibrations of the wafer in one of the predetermined frequency ranges,
• Data acquisition and control system, and
• A means for fast coupling the wafer with the actuator.