Multivariable measuring ellipsometer method and system

• US 6,002,485 A
• Filed: 11/28/1997
• Issued: 12/14/1999
• Est. Priority Date: 11/27/1996
• Status: Expired due to Fees

• Current Assignee: Heureka Company Limited
• Original Assignee: Heureka Company Limited
• Inventors: Masao, Katsuya
• Primary Examiner(s): Rosenberger, Richard A.
• Application Number:
  US08/969,346
• Time in Patent Office: 746 Days
• Field of Search: 356/369, 356/381, 356/382
• US Class Current: 356/369
• CPC Class Codes: G01B 11/065 using one or more discrete wavelengths

Abstract

The present invention discloses an ellipsometer that is able to provide multivariable measurement and analysis of optical constants of sample, comprising: a semiconductor laser diode that serves as the light source; a Pertie device attached to the semiconductor laser diode; a collimator for making the light parallel; a polarizer for transforming the light into a suitable linearly polarized state and radiating onto the sample; an analyzer for converting the light reflected from the sample into suitable linearly polarized light; and, a photosensor; wherein, the wavelength of light radiated from the semiconductor laser can be changed by changing the case temperature of the semiconductor laser by a Pertie device. This ellipsometer is used to perform measurement using multiple wavelengths to obtain an initial model of the optical constants of each layer after which, by starting from this model, the optical constants of each layer are systematically calculated from measured values of tan Ψ and cos Δ at a fixed wavelength in the form of optimization problems in which a multilayer film model is optimized so that the error terms involved in calculating film thickness are minimized. As a result, ellipsometric measurement can be performed that allows calculation of multiple variables.

Claims

1. An ellipsometer for measuring samples having multilayer films that require multivariable measurements, comprising:

a semiconductor laser diode in the form of a light source that radiates light;

a Pertie device for changing the case temperature of said laser diode;

a collimator for transforming incident light into a parallel beam;

a polarizer for transforming said light into suitable linearly polarized light and radiating said light onto said sample;

an analyzer for converting reflected light into linearly polarized light;

a photosensor for measuring said reflected light; and

calculating means for calculating optical constants of each layer of the multilayer films from measured values of tan ψ and

cos Δ

at a fixed wavelength in the form of optimization problems for minimizing error terms involved in calculating film thickness.

2. The ellipsometer of claim 1, wherein the case temperature of semiconductor laser can be changed by changing the current direction or current flowing to Pertie device.

3. The ellipsometer of claim 2, wherein the case temperature of semiconductor laser diode is changed to measure spectra of tan Ψ

and cos Δ

as function of wavelength of the light source.

4. A method to find the model of the set of refractive index and film thickness of each layer of a multilayer film in an ellipsometer, said ellipsometer having a semiconductor laser diode in the form of a light source that radiates light, a Pertie device for changing the case temperature of said laser diode, a collimator for transforming incident light into a parallel beam, a polarizer for transforming said light into suitable linearly polarized light and radiating said light onto said sample, an analyzer for converting reflected light into linearly polarized light, and a photosensor for measuring said reflected light, said method comprising the steps of:

(a) from the knowledge about the sample, assuming the initial values of optical constants of sample, then set an initial model of sample;

(b) from said model of sample, calculating spectra of tan ψ and

cos Δ

as a function of wavelength of light; and

(c) calculating the difference between the said actually measured spectra of tan ψ and

cos Δ and

the said calculated spectra of tan ψ and

cos Δ

,

wherein the set of film thickness and refractive index of each layer of a multilayer film are calculated according to the least squares method so as to minimize the said difference, wherein said calculated set of film thickness and refractive index of each layer of a multilayer film is the model of sample.

5. A method to calculate the set of film thickness and refractive index of each layer of a multilayer film, comprising the steps of:

(a) measuring tan ψ and

cos Δ

of a sample at a constant wavelength of the light;

(b) from the knowledge about the sample, assuming the set of initial values and the range of values of optical constants of sample;

(c) calculating the film thickness of arbitrary layer from the set of optical constants of sample and measured tan ψ and

cos Δ

; and

(d) calculating the gradient vector and Hessian matrix of imaginary part of said film thickness at a said set of optical constants,

wherein if said gradient vector is almost zero and said Hessian matrix is positive constant and said imaginary part of film thickness is almost zero, then the set of film thickness and refractive index of each layer of a multilayer film are determined value;

otherwise a new set of optical constants are searched, and return to the step of calculating the film thickness of the arbitrary layer.

6. The method of claim 5, wherein said set of initial values of optical constants of sample are a model of sample.

7. The method of claim 5, wherein said searching is made by the method of steepest searching.

8. The method of claim 5, wherein said searching is made by the quasi-Newton method.