Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
  • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
  • G01N21/171—Systems in which incident light is modified in accordance with the properties of the material investigated with calorimetric detection, e.g. with thermal lens detection
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N25/00—Investigating or analyzing materials by the use of thermal means
  • G01N25/16—Investigating or analyzing materials by the use of thermal means by investigating thermal coefficient of expansion

Definitions

• the present invention is applicable to contactless characterization of materials used in engineering, to measure thermal properties at microscopic ranges, particularly dilation, thermal conductivity, thermal diffusion coefficients and film thicknesses, the characterization of optical properties such as adsorption and dispersion spectrums of very small devices in the range of nanometers to micrometers and measuring of adsorption spectrums of very small particles (in the range of nanometers to millimeters) .
• thermal parameters dielectric, thermal conductivity, thermal diffusion, etc.
• Contactless measuring techniques allow detection without damaging the sample or system as the measurement during operation without disturbing the device. Also they allow remote measurements of parts not accessible by contact.
• Photothermal techniques that allow heating the sample with a pulsed or modulated laser beam and measuring of the temperature increase by infrared radiation are known in the art, see US 5.667.300 and 7.060.980 and published US patent applications 2002/0031164 and 2002/0011852. These techniques have the drawback of requiring infrared detectors that must be cooled and that prevent the use of optic microscopes for detection. Similar variations measure the reflection change of the sample using a second laser and measuring the reflected intensity that due to changes of temperature varies in time, allowing assessing said temperature change, (D. Rochais et. al., J. Phys.
• the patent application publication (AR) N° 70.418 describes a technique to measure dilation based on the use of devices that determine the focus error.
• the measuring system of focus error similar to the one used to focus CD reading systems allows to determine material dilation.
• This system differs from the present invention in that it requires a 4 quadrant detector to determine the out of focusing based on algebraic operations between said detectors. These operations must be performed at the modulation frequency of the heating beam or higher, and for a high space resolution (for example, for an aluminum sample in the limit of a commercial optical microscope the beam would have been modulated at more than 800MHz) .
• the present invention used a single detector that allows using common electronics in the field of optical communications for detecting at these high frequencies.
• Said patent also proposes the use of multiple parallel detectors to sample different angles.
• This configuration has the benefit of requiring a very fast acquisition system, as if observed at microscopic ranges the heating beam must be modulated at very high frequencies (millions of Hertz) and therefore acquiring on each channel at even higher frequencies, which would be extremely expensive and voluminous if one also desires to increase the sensibility of the system using "lock-in" detection on each channel.
• the proposed configuration uses only one detection channel that allows working with only one detector and a single amplifier at very high frequencies.
• Confocal detection is a very popular microscopy technique for determining cross sections based on introducing a very small opening on the image plane of the sample to observe. Said opening has de property of mostly blocking the light from planes out of focus, providing in this manner only an image of the focal plane of the objective of the microscope used. This technique is very popular for detecting fluorescence in biology or to determine surface profiles on material. Confocal laser scanning microscopy, (See C.J.R. Sheppard and D. M. Shotton Bios Scientific Publishing Limited, 1997 ISBN 0387 91514 1) .
• the present invention relates to a method and apparatus for measuring the protrusion generated on the surface of a sample due to the thermal dilation produced by heating with an electromagnetic radiation beam and based on said result determining the thermal diffusion, optical absorption, thermal dilation or other parameters derived from the same.
• the method is based on measuring the curvature of the surface that occurs due to the thermal dilation based on the transmission of a measuring beam reflected on the protrusion and through a focusing optical system and one opening located out of focus.
• the protrusion changes the focus position and therefore transmission through the opening, that being out of focus allows quantifying the deformation magnitude.
• Figures 1A and IB are descriptive schemes of the method.
• Figure 1A shows the distribution of components before starting the heating beam.
• Figure IB shows the effect of heating on focusing the measuring beam.
• Figure 2 is a scheme of a possible embodiment of the apparatus for measuring the thermal properties of opaque surfaces according to the present invention.
• Figure 3 are graphics of the amplitude of the heating beam and the signal based on time indicating the characterizing parameters .
• Figure 4 are two graphics of the amplitude of the signal (A) and the phase of the signal (B) based on the modulation frequency divided by the critical frequency.
• Figure 5 is a graphic of the amplitude of the signal based on the out of focusing in relation with the confocal configuration .
• Figure 6 is a graphic of the phase of the signal based on the frequency when the substrate is covered by a layer of another material. The different graphics correspond to different material thicknesses.
• Figure 7 shows the preferred embodiment of the apparatus using fiber optics to combine the heating and measuring lasers and to configure the opening of the disfocal detection.
• Figure 8 is a graphic with the results obtained on the amplitude of the signal with the method of the invention when testing a sample of fused quartz with a 30nm chromium coating.
• Figure 9 is a graphic with the results obtained on the phase delay of the signal in the assay of Figure 8.
• Figure 10 is a graphic that shows how diffusion data is recovered based on a numeric adjustment according to the data of Figures 8 and 9.
• the method of the present invention measures the protrusion generated on the surface of a sample due to the thermal dilation produced by heating with an electromagnetic radiation beam and based on said measuring determining the thermal diffusion, optical absorption, thermal dilation or others parameters derived thereof.
• Figure 1 shows the method.
• Figure 1A shows a possible distribution of the components, wherein the sample to be analyzed 40 is illuminated by a laser or other measuring or detection electromagnetic radiation beam 10 that is focused on the surface of the sample by an adequate optics, in this case a beam divider 33 and one lens 32 located at a distance L2 of the sample, though it can be any suitable combination of optical elements that perform this function.
• the beam is reflected on the surface of the sample and directed through a collection optical system that in this view is comprised by lenses 32 and 31 and is focused on a plane 60 at a distance L0 of the focusing system.
• Lines 11 indicate the path of two rays representative of said measuring or detection beam.
• An opening of dimensions similar to the ones of the focused measuring beam is located at a distance LI of the lens. Said distance is different (longer or shorter) from the distance L0 corresponding to is confocal location, so that part of the energy carried by the reflected beam is obstructed on said opening.
• a detector 52 located behind the opening determines the transmitted power that will be maximum power in case the opening is located on the plane 60 confocally. Location out of focus is an essential part of this method of disfocal detection.
• the method is completed when starting a heating beam 9 as shown in Figure IB and producing a protrusion of a radius R by thermal dilation on the sample.
• the heating beam should be centered in relation with the measuring beam.
• the protrusion is spontaneously produced as heating is inhomogeneous .
• On the center of the beam the intensity is higher and therefore heating is higher and consequently dilation is higher.
• the side projection of the protrusion depends on the size of the beam and modulation frequency, as when modulating at a low frequency, heat has more time to laterally propagate before the beam is turned off. Therefore the dimensions and delay to produce the protrusion depend on the modulation frequency, the size of the beam and the thermal diffusion of the sample under study.
• the measuring beam reflected is out focused by this protrusion and remains focused on a plane different from the original plane 60 as indicated by the representative rays 12. If the opening was behind the plane 60 (L2 higher than L0) the signal transmitted by the opening 50 and captured by the detector 52 increases. Conversely, if the opening was located in front (L2 lower than L0) a signal reduces. This signal variation allows measuring the curvature acquired by the surface based on solving the propagation of beams by optical systems conventionally.
• the located heating means preferably a laser
• the located heating means is modulated in time at a controlled frequency, it produces the periodic dilation of the material at the modulation frequency.
• Said dilation conventionally depends on the power absorbed, spatial dimensions of the heating means and the frequency of heating as described in the state of the ar. Said dilation produces a protrusion on the heated zone, as it is higher at the center heating beam than at the periphery of the same.
• the maximum departure of the focus plate rearward will take place (away from the original image plane) . If the opening was retracted rearward, material dilation increases the transmission through the opening. When the material shrinks, transmission decreases.
• FIG. 2 shows a possible embodiment of the present invention.
• a fiber coupling system 19 two lasers are injected in a fiber optic 20. Both lasers are in this embodiment collinearly at the outlet of the fiber.
• An optical system 300 produces an image of the fiber outlet on a plane very close to the surface of the sample.
• This scheme shows a possible embodiment of said optical system 300 consisting of two lenses, a lens 31 that collimates the outlet beam of the fiber and another beam 32 that focuses it close to the surface of the sample.
• Distances LI of the fiber to the optical system and L2 of the optical system 300 to the sample 40 allow adjusting the level of out of focusing of the system.
• One of the beams acting on the sample known as heating beam
• the second beam known as testing, preferably of a wavelength different from that of the heating beam, is reflected on the surface of the protrusion and returns to the fiber but out focused in relation with the beam reflected on the undeformed surface.
• Lines 11 indicate that return path for two representative rays of said measuring beam reflected on the sample before dilating (without heating) that after covering the optical system 300 is focused close but not on the same surface of the fiber outlet 20.
• Dotted lines 12 indicate the return path of the beam in the presence of a protrusion produces by heating, that due to the out of focusing produced by the protrusion and the out of focusing of the original alignment provides a change on the coupling magnitude on the fiber.
• coupling increases, and the amplitude of said coupling depends on the level of out of focusing and the curvature radius R of the protrusion.
• the portion reinjected on the fiber of the measuring beam emerge the other end 50 and is detected by a detector 52 after filtered by a filter 51 that prevents the heating beam reinjected remaining portion passing. Changes on the curvature radius of the protrusion produce changes on the amplitude detected by 52.
• FIG. 3 shows a temporal sequence on the graphic A representing the oscillation of the heating beam (or a component at a frequency f if heating is periodic but not harmonic) .
• the cycle period of heating beam is
• Graphic B of Figure 3 shows the cycle followed by the amplitude collected by detector 52. Peak to peak amplitude will depend on the modulation frequency, the size of the heating beam, the intensity of the heating beam and the measuring beam and the properties of the material including thermal diffusion, thermal expansion coefficient, density, calorific capacity and optical reflectivity at the heating and testing wavelength. The signal depends on the frequency only through a function which amplitude is shown in Figure 4A.
• the critical frequency f 0 is expressed as follows:
• the signal amplitude depends on the level of initial out of focusing as shown in Figure 5 that shows the amplitude of the signal (in arbitrary units) based on the out of focusing related to the confocal position measured in units of focus depth (LI if the fiber moves or L2 if the sample moves) .
• LI out of focus displacement
• L2 an out of focus displacement
• the signal amplitude is null in case of confocal configuration.
• Figure 7 shows the best complete embodiment of the system including the combination system of two lasers within the fiber.
• a heating laser 9 and a measuring laser 10 coupled to respective fibers are combined by means of a coupling multiplexor or 2x2 coupler 14 similar to those used on optical communication systems but adapted to the wavelengths of the lasers used.
• Beam samples can be taken with couplers 15 to monitor operation.
• a circulator 16 or a 1x2 coupler takes the return on the fiber and sends it to a detector 52 connected to a lock-in amplifier 53 or other electronic filter. Saud lock-in amplifier or filter detects the component of the signal at the desired frequency to determine the phase and consequently the parameters of interest as previously described.
• Lasers 9 and 10 are controlled by the supplies 61 that are modulated by function generators 62 at different frequencies fl and f2. Alternatively they can be externally modulated with modulators.
• a fiber optic 20 connects the lasers to the optical system 300 comprising in this case a microscope adaptor system 310 that generates an image of the fiber outlet on an inlet plane of the microscope such as, for example, the camera port, and a microscope 320 that generates a new image of the fiber close to the plane of the sample as shown in Figure 1.
• Sample 40 is scanned transversally with a microscope motorized slide 330.
• the whole assembly is controlled by a computer 500 that registers the signal obtained with the lock-in amplifier 53, the modulation frequency f and the position of the sample to compose the desired phase information based on the frequency and position to obtain a complete map of the properties measured on the sample.
• the beam can be scanned on the sample instead of moving the sample by an optical scanner using conventional techniques in the field of scattering microscopy.
• Figure 8 show the results of a test conducted with the configuration described by Figure 7. It shows the amplitude of the signal based on the frequency (at a range from 50Hz to 20.000Hz) and the out of focusing on the sample (with +/- 6mm displacements) .
• the sample used is a fused quartz substrate covered with a thin layer of chromium of a thickness of 30nm. The layer acts as a heat absorbent, transmitted to the substrate without disturbing the measurement thanks to its extremely thin thickness (more than 100 times smaller than the size of the beam) .
• the heating beam used had a wavelength of 980nm and a power of lOmW. El measuring beam is of 1.550nm of wavelength and lOmW of power.
• the size of the beam on the sample is of 20 micrometers.
• the signal is measured with an infrared photodiode typical of optical communications and a lock-in amplifier Stanford Research model 830DSP.
• the upper curve of Figure 10 corresponds to the error obtained in the adjustment based on the assumed critical frequency, indicating the value of the critical frequency that minimizes the error.
• the other two curves are the amplitudes and phases measures based on the frequency and their respective adjustments for the optimum value of critical frequency.
• a small opening is used as indicated in the description of the method ( Figure 1) .

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Citations (13)

• 2011
  • 2011-06-17 AR ARP110102121 patent/AR088019A1/es not_active Application Discontinuation
• 2012
  • 2012-06-15 WO PCT/IB2012/053043 patent/WO2012172524A1/en active Application Filing

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