Film thickness measuring apparatus and a method for measuring a thickness of a film

A system to measure a thickness of a film, which is disposed on or above a semiconductor wafer, at a plurality of locations of the film, the system comprising:

• an electron gun to irradiate an electron beam, having a plurality of energies, on the film, wherein the electron gun is adapted to irradiate an electron beam having a first energy and an electron beam having second energy;
• an electrode, coupled to the semiconductor wafer, to detect substrate currents in the wafer, wherein the electrode detects:
• a first plurality of substrate currents generated in response to the electron beam, having the first energy, irradiating the plurality of locations of the film, wherein each substrate current of the first plurality of substrate currents corresponds to a substrate current detected in the wafer and generated in response to the electron beam, having the first energy, irradiating one of the locations of the plurality of locations;
• and
• a second plurality of substrate currents generated in response to the electron beam, having the second energy, irradiating the plurality of locations of the film, wherein each substrate current of the second plurality of substrate currents corresponds to a substrate current detected in the wafer and generated in response to the electron beam, having the second energy, irradiating one of the locations of the plurality of locations;
• and
• a processor, coupled to the electrode, to determine the thickness of the film at the plurality of locations using (i) the amount of each substrate current of the first plurality of the substrate currents, (ii) the amount of each substrate current of the second plurality of substrate currents, and (iii) reference data, wherein the reference data is data which provides a correlation between a substrate current and a thickness of a film.