Method and apparatus for characterizing thin films

A method comprising:

• generating, with a heterodyne light source, a first source beam of heterodyne light toward a sample comprising a first test layer;
• polarizing, with a first analyzer, a first portion of the first source beam, thereby forming a first reference beam;
• measuring, with a first light sensor, an intensity signal of the first reference beam;
• expanding, with a beam expander, a second portion of the first source beam, thereby forming an expanded beam, wherein the expanded beam is incident on the first test layer at a first incidence angle and a portion of the expanded beam is reflected by the first test layer, thereby forming a first reflected beam;
• polarizing, with a second analyzer, the first reflected beam, thereby forming a first test beam;
• measuring, with a first pixel of a second light sensor, a first intensity signal of the first test beam, and measuring, with a second pixel of the second light sensor, a second intensity signal of the first test beam;
• determining a difference between a phase of the first intensity signal of the first test beam and a phase of the intensity signal of the first reference beam, and determining a difference between a phase of the second intensity signal of the first test beam and the phase of the intensity signal of the first reference beam;
• and
• determining a refractive index, an extinction coefficient, and a thickness of the first test layer at a first region along the first test layer based on the difference between the phase of the first intensity signal of the first test beam and the phase of the intensity signal of the first reference beam, and determining the refractive index, the extinction coefficient, and the thickness of the first test layer at a second region along the first test layer, spaced from the first region, based on the difference between the phase of the second intensity signal of the first test beam and the phase of the intensity signal of the first reference beam.

An apparatus comprising:

• a heterodyne light source configured to generate a first source beam of heterodyne light toward a first test layer;
• a beam splitter between the heterodyne light source and the first test layer and configured to reflect a first portion of the first source beam and transmit a second portion of the first source beam;
• a first analyzer configured to polarize the first portion of the first source beam, thereby forming a first reference beam;
• a first light sensor configured to measure an intensity of the first reference beam, wherein the first analyzer is between the beam splitter and the first light sensor;
• a beam expander between the beam splitter and the first test layer and configured to expand and collimate the second portion of the first source beam, thereby forming an expanded beam;
• a second analyzer configured to polarize a portion of the expanded beam that is reflected by the first test layer, thereby forming a first test beam;
• a second light sensor comprising a first pixel and a second pixel configured to measure an intensity of the first test beam, wherein the second analyzer is between the first test layer and the second light sensor;
• and
• a characterization circuit coupled to the first light sensor and the second light sensor and configured to determine a difference between a phase of the intensity of the first test beam and a phase of the intensity of the first reference beam, and determine a refractive index, an extinction coefficient, and a thickness of the first test layer based on the difference between the phase of the intensity of the first test beam and the phase of the intensity of the first reference beam.