Method of characterizing a beam of electromagnetic radiation in ellipsometer and the like systems

• US 9,952,141 B1
• Filed: 06/22/2016
• Issued: 04/24/2018
• Est. Priority Date: 06/25/2015
• Status: Active Grant

• Current Assignee: J.A. Woollam Co., Inc.
• Original Assignee: J.A. Woollam Co., Inc.
• Inventors: Hale, Jeffrey S., Herzinger, Craig M., Liphardt, Martin M.
• Primary Examiner(s): Stock, Jr., Gordon J
• Application Number:
  US14/999,748
• Time in Patent Office: 671 Days
• Field of Search: 356364-370
• CPC Class Codes: G01N 21/211 Ellipsometry optical thickness measurement G01B11/06
• CPC Class Codes: G01N 2201/127 Calibration; base line adjustment; drift compensation

Abstract

An approach to characterizing beams of electromagnetic radiation such as are applied in ellipsometer and the like systems, involving considering the beam to be comprised of a number of spatially distributed beam rays, each of which is represented mathematically as an effectively independent source.

Claims

1. A method of directly characterizing samples with a beam of electromagnetic radiation, which does not require use of a separately investigated diffraction producing structured sample, comprising the steps of:

a) providing a source of a beam of electromagnetic radiation and considering a beam produced thereby as a composite of a plurality of spatially distributed beam rays;

b) causing said produced beam of electromagnetic radiation to pass through a polarization state generator and become a polarized beam of electromagnetic radiation;

c) causing the resulting polarized electromagnetic beam to interact with a non-structured calibration sample at an effective set central ray angle of incidence, and an effective set central ray plane of incidence, and then enter a polarization state detector prior to a data detector, in which data detector data is produced;

d) for each of the plurality of spatially distributed beam rays performing a calibration procedure that effectively treats each beam ray as a separate source of electromagnetic radiation, said calibration procedure involving a simultaneous regression of said data produced in step c) onto a mathematical model of said calibration sample and of mathematical matrix representations that account for all other beam affecting elements with which said beam interacts for each beam ray, such that said values for parameters in mathematical matrix representations that account for all other beam affecting elements for each beam ray with which said beam interacts, are established and fixed;

e) replacing said calibration sample with a sample and causing a polarized electromagnetic beam produced as in step b) to interact with said sample at an effective set central ray angle of incidence, and an effective set central ray plane of incidence, and then enter a polarization state detector prior to a data detector, in which data detector data is produced;

f) performing a simultaneous regression of said data acquired in step e) onto a mathematical model for said sample and said calibrated mathematical matrix representations that account for all other beam affecting elements with which said beam interacts, for each of said plurality of beam rays, to arrive at values for parameters in said sample characterizing mathematical model.

2. A method as in claim 1, wherein at least one weighting factor is applied to at least one of said calibrated and fixed mathematical matrix representations that account for said all other beam affecting elements, in addition to said sample, with which said beam interacts.

3. A method as in claim 2, in which said at least one weighting factor accounts for the effects of the shape of an aperture placed into the path of said plurality of beam rays.

4. A method as in claim 2, in which said at least one weighting factor accounts for the effects of different attenuations encountered by different beam rays in said beam of electromagnetic radiation that is considered as a composite of a plurality of spatially distributed beam rays.

5. A method as in claim 1, wherein the beam of electromagnetic radiation is comprised of a plurality of wavelengths and in step e) data is produced in said data detector for at least some of said plurality of wavelengths, and wherein said step f) simultaneous regression of said data acquired in step e) onto a mathematical model for said sample and said calibrated mathematical matrix representations that account for all other beam affecting elements with which said beam interacts, for each of said plurality of beam rays and wavelengths, provides at values for parameters in said sample characterizing mathematical model for said at least some of said plurality of wavelengths .

6. A method as in claim 1, wherein said all other beam affecting elements for each beam ray with which said beam interacts, are present between at least one selection from the group consisting of:

said source and said polarization state generator;

said polarization state generator and said polarization state detector;

said polarization state detector and said data detector.

7. A method as in claim 1, wherein said calibrated mathematical matrix representations that account for all other beam affecting elements with which said beam interacts account for at least one selection from the group consisting of:

Fresnel refraction;

reflections;

aberrations;

phase changes;

coordinate system rotations;

attenuations;

polarization changes;

angle of incidence changes; and

plane of incidence changes.

8. A method as in claim 1 in which a preferred number and configuration of spatially distributed beam rays in said composite of a plurality thereof in said electromagnetic beam is determined by repeating steps a)-f) using different selected numbers and/or configurations of spatially distributed beam rays, and determining where convergence occurs and further changes therein do not result in significant changes in values of determined sample values for parameters in said sample characterizing mathematical model.

9. A method as in claim 1, in which mathematical simulation is first applied to determine a relatively optimum beam of electromagnetic radiation configuration of a plurality of spatially distributed beam rays, comprising the steps of:

a′

) mathematically simulating a source of a beam of electromagnetic radiation and considering a mathematically simulated beam produced thereby as a composite of a plurality of mathematically simulated spatially distributed beam rays;

b′

) mathematically simulating passage of said mathematically simulated beam of electromagnetic radiation through a mathematically simulated polarization state generator so that it is becomes a mathematically simulated polarized beam of electromagnetic radiation;

c′

) mathematically simulating interaction of said resulting mathematically simulated polarized electromagnetic beam with a mathematically simulated calibration sample at an effective set central ray angle of incidence, and an effective set central ray plane of incidence, and then enter a mathematically simulated polarization state detector prior to a mathematically simulated data detector, in which mathematically simulated data detector mathematically simulated data is produced;

d′

) for each of the plurality of spatially distributed mathematically simulated beam rays performing a calibration procedure that effectively treats each mathematically simulated beam ray as a separate mathematically simulated source of electromagnetic radiation, said calibration procedure involving a simultaneous regression of said mathematically simulated data produced in step c) onto a mathematical model of said mathematically simulated calibration sample and of mathematical matrix representations that account for all other beam affecting elements with which said mathematically simulated beam interacts, for each mathematically simulated beam ray, such that said values for parameters in mathematical matrix representations that account for all other beam affecting elements for each beam ray with which said beam interacts, are established and fixed;

e′

) mathematically simulating replacement of said mathematically simulated calibration sample with a mathematically simulated sample and causing said mathematically simulated polarized beam of electromagnetic radiation interaction with a mathematically simulated sample at an effective set central ray angle of incidence, and an effective set central ray plane of incidence, and then entry into a mathematically simulated polarization state detector prior to entering a mathematically simulated data detector wherein mathematically simulated data is produced;

f′

) performing a simultaneous regression of said mathematically simulated data acquired in step e) onto a mathematical model for said sample and said calibrated mathematical matrix representations that account for all other beam affecting elements with which said mathematically simulated beam interacts, for each of said plurality of mathematically simulated beam rays, to arrive at values for parameters in said sample characterizing mathematical model;

g′

) repeating steps a′

-f′

using different selected numbers and/or configurations of mathematically simulated spatially distributed beam rays, and determining where a convergence in values of determined mathematically simulated sample values for parameters therein occurs;

h′

) selecting a convergence indicated number and/or configuration of mathematically simulated spatially distributed beam rays, and applying an actual beam of electromagnetic radiation that comprises a plurality of spatially distributed beam rays comprising said number and/or configuration, in an investigation of an actual sample in steps a)-f).

10. A method as in claim 1, in which mathematical simulation is first applied to determine a relatively optimum beam of electromagnetic radiation configuration of a plurality of spatially distributed beam rays, including the steps of:

a) assuming a number and/or configuration of mathematically simulated spatially distributed beam rays and determining values for parameters that characterize a simulated sample;

b) assuming a number and/or configuration of mathematically simulated spatially distributed beam rays which is different than that assumed in step a), and again determining values for parameters that characterize said simulated sample;

c) selecting a criteria for an acceptable convergence of resulting values for parameters that characterize said simulated sample produced in steps a) and b), and repeating steps a) and b) until the results therefrom meet said convergence criteria.

11. A method as in claim 1, wherein said calibration sample comprises a selection from the group consisting of:

one non-structured calibration sample, optionally comprising at least one thin layer; and

two non-structured calibration samples, each optionally comprising at least one thin layer.

12. A method as in claim 11, wherein the Matrix calculations in steps d) and d′

) are done on a per beam Basis, while the regression is done on the combination of those calculations, wherein the incoherent addition of matrix calculation produces a single parameterized model which is regressed onto said single data set as opposed to on a per beam-ray basis, said at least one non-structured calibration sample and said simultaneous regression determines all System Parameters for all the beam-rays, to account for at least one selection from the group consisting of;

Fresnel refraction;

reflections;

aberrations;

phase changes;

coordinate system rotations;

attenuations;

polarization changes;

angle of incidence changes; and

plane of incidence changes.

13. A method as in claim 1, wherein said calibration sample comprises a selection from the group consisting of:

one non-structured calibration sample, optionally comprising at least one thin layer; and

two non-structured calibration samples, each optionally comprising at least one thin layer;

and wherein data is obtained from said at least one non-structured calibration sample in step c, using at least one selection from the group consisting of;

at least two different central beam angles-of-incidence; and

at least two different central beam planes-of-incidence.