Method and apparatus for evaluating the thickness of thin films

An apparatus for evaluating the thickness of a thin film on the surface of a sample comprising:

• means for generating a linearly polarized probe beam of radiation;
• means for focusing the probe beam substantially normal to the surface of the sample such that various rays within the focused probe beam create a spread of angles of incidence;
• a quarter-wave plate for retarding the phase of one polarization state in the probe beam with respect to the phase of the other polarization state in the probe beam;
• a linear polarizer for creating interference between the two polarization states in the probe beam after the probe beam has been reflected from the surface of the sample;
• a photodetector having four radially disposed quadrants for measuring the power of the reflected probe beam after it has passed through the quarter-wave plate and polarizer, said photodetector generating an output from each quadrant that is proportional to the power of the probe beam incident on that quadrant;
• and
• processor means functioning to derive a measurement value by calculating the difference between the sums of the outputs of diametrically opposed quadrants, said measurement value being used to evaluate the thickness of the thin film on the sample.

A method for evaluating a parameter of a thin film on the surface of a sample comprising:

• focusing a probe beam of radiation substantially normal to the surface of the sample such that various rays within the focused probe beam create a spread of angles of incidence;
• retarding the phase of one polarization state in the probe beam with respect to the phase of the other polarization state in the probe beam;
• interfering the two polarization states in the probe beam after the probe beam has been reflected from the surface of the sample;
• thereafter measuring the power of the reflected probe beam along two orthogonal axes and generating an output that integrates the intensity of various rays having different angles of incidence, said output having two components corresponding to said two orthogonal axes;
• and
• evaluating a parameter of the thin film on the sample based on the measured output.

A method for evaluating the thickness of a thin film on the surface of a sample comprising:

• focusing a probe beam of linearly polarized radiation substantially normal to the surface of the sample such that various rays within the focused probe beam create a spread of angles of incidence;
• retarding the phase of one polarization state in the probe beam with respect to the phase of the other polarization state in the probe beam by 90 degrees;
• polarizing the probe beam after the probe beam has been reflected from the surface of the sample using a linear polarizer;
• thereafter measuring the power of the reflected probe beam in four radially disposed quadrants;
• generating an output signal from each quadrant that is proportional to the power of the probe beam incident on that quadrant;
• and
• deriving a measurement value by calculating the difference between the sums of the outputs of diametrically opposed quadrants, said measurement value being used to evaluate the thickness of the thin film on the sample.