METHOD AND APPARATUS FOR DETERMINING THE THERMAL EXPANSION OF A MATERIAL

• US 20100208242A1
• Filed: 02/12/2010
• Published: 08/19/2010
• Est. Priority Date: 02/12/2009
• Status: Active Grant

• Current Assignee: Consejo Nacional de Investigaciones Científicas y Técnicas
• Original Assignee: Consejo Nacional de Investigaciones Científicas y Técnicas
• Inventors: BALZAROTTI, Francisco, MARTÍNEZ, Oscar Eduardo, DOMENE, Esteban Alejo, BRAGAS, Andrea Veronica, MINGOLO, Nélida
• Granted Patent: US 8,622,612 B2
• Time in Patent Office: Days
• US Class Current: 356/72
• CPC Class Codes: G01B 21/085 using thermal means
• CPC Class Codes: G01N 21/171 with calorimetric detection, e.g. with thermal lens detection
• CPC Class Codes: G01N 21/8422 Investigating thin films, e.g. matrix isolation method
• CPC Class Codes: G01N 25/16 by investigating thermal coefficient of expansion

Abstract

A focus error signal resulting from the photothermically-induced expansion is measured in a sample of material under analysis. A laser is disposed as a periodically modulated heating source which is directed to the sample and a device for focus error measuring which is directed to de surface being heated. A device measuring focus error generates a signal representative of the displacement of the surface of material in perpendicular direction due to the expansion produced by the periodic heating, which is filtered, either analogically or digitally, to discriminate the displacement component at the frequency in which it was modulated or at any other related frequency, such any harmonic or a sum with any other modulation. The focus error signal, appropriately calibrated, gives a precise and sensitive measure of the magnitude the expansion. Said magnitude and its dependence with the modulation frequency allows the determination of physical properties such as the thermal expansion or thermal diffusivity coefficient, the thickness of a coating film or the absorption coefficient of the light from the heating beam. By varying the wave length of the directed radiation it is possible to determine the absorption spectrum of the sample even for very small sized particles in which the fraction of absorbed energy is very little.

Claims

1. A method for determining the thermal expansion of a material through the steps of:

inducing the heating of material and measuring the displacement of the surface of material due to the thermal expansion;

characterized in that the step of measuring the displacement comprises sending a measurement beam on the material and determining the focus error of said measuring beam resulting from said displacement.

2. A method in accordance with claim 1, characterized in that the step of inducing the heating comprises irradiating a modulated electrical or magnetic field in time at different frequencies of the wave lengths of visible light.

3. A method in accordance with claim 2, characterized in that said electric or magnetic field belongs to the group consisting of X rays, infrared radiation and other part of the electromagnetic spectrum, different from the wave lengths of visible light.

4. A method in accordance with claim 2, characterized in that the step of inducing the heating comprises irradiating el material with a periodically modulated heating beam.

5. A method in accordance with claim 1, characterized in that it further comprises, the step of determining at least one additional physical property of material from said determination of thermal expansion.

6. A method in accordance with claim 5, characterized in that said additional physical property belongs to the group consisting of thermal properties at microscopic scales, including thermal expansion coefficients, thermal conductivity and thermal diffusivity and film thicknesses;

optical properties including absorption coefficient of incident light in the heating beam, absorption spectra and dispersion; and

coating film thicknesses.

7. A method in accordance with claim 4, characterized in that it further comprises, the step of determining thermal diffusivity of material from the measurement of thermal expansion as a function of the modulation frequency.

8. A method in accordance with claim 7, characterized in that it has the additional steps of:

displacing the beams with relation to a sample and obtaining a spatial map for thermal diffusivity, optical absorption or the thickness of the film on the surface of the sample.

9. A method in accordance with claim 7, characterized in that it has the additional steps of:

varying the wave length of the heating beam and measuring the localized absorption spectrum of the sample.

10. A method in accordance with claim 9 for the determination of an optical property of a material comprising a microscopic or nanoscopic structure, characterized in that it places said microscopic or nanoscopic structures on a transparent substrate of known thermal properties, varying the wave length of the heating beam and measuring the absorption spectrum and dispersion of said microscopic or nanoscopic structure, optical properties such as absorption spectra and dispersion of devices.

11. A method in accordance with claim 6, characterized for the additional step of determining the thermal expansion coefficient of material from the determination of the thermal expansion, the dimensions of the heating beam and the fraction of absorbed energy.

12. A method in accordance with claim 6, characterized for the additional step of determining the thickness of the film covering the substrate of material by measuring the phase of the expansion modulation as a function of the frequency and determining the frequency at which said phase takes maximum or minimum values.

13. A method in accordance with claim 1, characterized in that the step of determining the focus error comprises introducing astigmatism in said measurement beam and measuring the elipticity of the beam reflected by the material.

14. An apparatus for determining the thermal expansion of a material by the use of the method of claim 1, characterized in that it comprises a mechanism of heating with a periodic modulated source and a focus error measuring device capable of determining the displacement of the surface due to the thermal expansion.

15. An apparatus in accordance with claim 14, characterized in that the mechanism of heating comprises a periodically modulated laser beam source while said focus error measuring device comprises a laser beam source with a different frequency as regards the heating laser beam.

16. An apparatus in accordance with claim 15, characterized in that the mechanism of heating comprises a periodically modulated laser beam source directed to the material while said focus error measuring device comprises a laser beam source also directed to the material and a detector of the beam reflected by said material.

17. An apparatus in accordance with claim 16, characterized in that between the laser beam source of the focus error sensing device and the material there is a means introducing astigmatism in said laser beam and said detector of the reflected beam is capable of measuring the elipticity of the reflected beam.

18. An apparatus in accordance with claim 16, characterized in that the laser heating beam source and the laser beam source of the device measuring focus error are of different frequencies and are jointly directed towards the same surface of material.

19. An apparatus in accordance with claim 16, characterized in that the source of laser heating beam and the laser beam source of the device measuring focus error are directed on opposite surfaces of the material.

20. An apparatus in accordance with claim 16, characterized in that said detector of the reflected beam comprises a digital camera.

21. An apparatus in accordance with claim 16, characterized in that it further includes a detector of error of the beam transmitted by said material.