Normal incidence ellipsometer with complementary waveplate rotating compensators

An apparatus configured to direct an illumination source against a sample at a normal angle of incidence to evaluate the sample, the apparatus comprising:

• an illumination source configured to generate a probe beam of polychromatic light having a range of optical wavelengths for interacting with the sample;
• a polarizer optically coupled to the illumination source and configured to impart a known polarization state to the probe beam before the probe beam interacts with the sample;
• a first rotating compensator optically coupled to the polarizer and configured to induce a first phase retardation value in the polarization state of the probe beam over a first range of wavelengths;
• a second rotating compensator optically coupled to the first rotating compensator and configured to induce a second phase retardation value in the polarization state of the probe beam before the probe beam interacts with the sample over a second range of wavelengths, wherein the second range of wavelengths is complementary to the first range of wavelengths;
• the phase retardations in the polarization state of the probe beam induced by both first and second compensators before the probe beam reaches the sample and after it has been reflected by the sample;
• said first and second compensators being configured such that at least the first effective phase retardation value induced by the first compensator is within a primary range of effective retardations, and the second effective phase retardation value induced by the second compensator is outside of and complementary to said primary range;
• the polarizer configured to interact with the probe beam after the reflected probe beam has returned through the compensators;
• and
• a detector configured to measure the intensity of the probe beam after interaction with the sample as a function of the rotational angles of the compensators.

An ellipsometry method for evaluating a sample comprising:

• a) generating a beam of polychromatic light characterized by a range of wavelengths or a plurality of discrete wavelengths;
• b) polarizing the light to impart a known polarization state to the light beam;
• c) inducing phase retardations of the polarization state of the light beam over a first range of wavelengths with a first compensator, wherein the first range of wavelengths and the first compensator are selected such that at least a first effective phase retardation value induced over the first range of wavelengths lies within a range of effective phase retardations;
• d) inducing phase retardations of the polarization state of the light beam over a second range of wavelengths with a second compensator before at least a portion of the light beam interacts with the sample at normal incidence, wherein the second range of wavelengths is complementary to the first range of wavelengths, wherein the second range of wavelengths and the second compensator are selected such that at least a second effective phase retardation value induced over the second range of wavelengths range lies within the range of effective phase retardations;
• e) said first and second compensators being configured such that at least the first effective phase retardation value induced by the first compensator is within a primary range of effective retardations, and the second effective phase retardation value induced by the second compensator is outside of and complementary to said primary range;
• and
• f) measuring an intensity of the light beam after interaction of the light beam with the sample.