Laser ultrasonics-based material analysis system and method using matched filter processing

• US 5,604,592 A
• Filed: 06/07/1995
• Issued: 02/18/1997
• Est. Priority Date: 09/19/1994
• Status: Expired due to Fees

• Current Assignee: Brown University
• Original Assignee: Textron Defense Systems Division of AVCO Corporation
• Inventors: Cunningham, James F., Gozewski, Paul F., Klimek, Daniel E., Woodroffe, Jaime A., Kotidis, Petros A., Borsody, Charles
• Primary Examiner(s): Gonzalez, Frank
• Assistant Examiner(s): MERLINO, AMANDA H
• Application Number:
  US08/482,782
• Time in Patent Office: 622 Days
• Field of Search: 356/357, 356/432, 73/655, 73/657
• US Class Current: 356/493
• CPC Class Codes: G01N 21/1717 with a modulation of one or more physical properties of the sample during the optical investigation, e.g. electro-reflectance
• CPC Class Codes: G01N 2291/011 Velocity or travel time
• CPC Class Codes: G01N 2291/015 Attenuation, scattering
• CPC Class Codes: G01N 2291/0237 Thin materials, e.g. paper, membranes, thin films
• CPC Class Codes: G01N 2291/0251 Solidification, icing, curing composites, polymerisation
• CPC Class Codes: G01N 2291/02827 Elastic parameters, strength or force
• CPC Class Codes: G01N 2291/02854 Length, thickness
• CPC Class Codes: G01N 2291/0421 Longitudinal waves
• CPC Class Codes: G01N 2291/0422 Shear waves, transverse waves, horizontally polarised waves
• CPC Class Codes: G01N 2291/0423 Surface waves, e.g. Rayleigh waves, Love waves
• CPC Class Codes: G01N 2291/0427 Flexural waves, plate waves, e.g. Lamb waves, tuning fork, cantilever
• CPC Class Codes: G01N 2291/044 Internal reflections (echoes), e.g. on walls or defects
• CPC Class Codes: G01N 2291/045 External reflections, e.g. on reflectors
• CPC Class Codes: G01N 2291/102 one emitter, one receiver
• CPC Class Codes: G01N 2291/2697 Wafer or (micro)electronic parts
• CPC Class Codes: G01N 29/041 on the surface of the material, e.g. using Lamb, Rayleigh or shear waves
• CPC Class Codes: G01N 29/07 by measuring propagation velocity or propagation time of acoustic waves
• CPC Class Codes: G01N 29/228 related to high temperature conditions
• CPC Class Codes: G01N 29/2418 using optoacoustic interaction with the material, e.g. laser radiation, photoacoustics photoacoustic cells G01N21/1702; measuring characteristics of vibrations by using radiation-sensitive means G01H9/00; acousto-optical conversion techniques for short-range imaging G01S15/8965; sound-producing devices using laser bundle G10K15/046
• CPC Class Codes: G01N 29/2456 Focusing probes focusing arrangements G01N29/221
• CPC Class Codes: G01N 29/326 : compensating for temperature variations
• CPC Class Codes: G01N 29/348 : with frequency characteristics, e.g. single frequency signals, chirp signals measuring frequency of mechanical vibrations or acoustic waves in general G01H1/06, G01H3/04; measuring frequency or analysing frequency spectra G01R23/00
• CPC Class Codes: G01N 29/4427 : with stored values, e.g. threshold values
• CPC Class Codes: H01S 3/08004 : incorporating a dispersive element, e.g. a prism for wavelength selection H01S3/0811, H01S3/08022 take precedence
• CPC Class Codes: H01S 3/106 : by controlling devices placed within the cavity H01S3/10076, H01S3/13 take precedence
• CPC Class Codes: H01S 5/02208 : characterised by the shape of the housings
• CPC Class Codes: H01S 5/02255 : using beam deflecting elements
• CPC Class Codes: H01S 5/141 : using a wavelength selective device, e.g. a grating or etalon H01S5/146 takes precedence

Abstract

Disclosed herein is an interferometric-based materials analysis system (10) that employs a novel combination of laser beam shaping and pointing techniques, the use of a low cost, rugged, and compact diode laser (22) as a detection laser, and the use of signal processing techniques that compensate for inherent instabilities and short-term drift in the diode laser. A matched filter processing technique is disclosed for processing interferometrically-obtained data points from a target being analyzed. The matched filter technique is shown to be especially useful for detecting and analyzing Lamb modes within thin targets, such as a silicon wafer undergoing a rapid thermal processing cycle. Also disclosed is a method and apparatus for interferometrically monitoring a target to determine, in accordance with predetermined criteria, an occurrence of a period of time that is optimum for obtaining a data point. In response to detecting such a period an impulse source, such as an impulse laser (14), is triggered to launch an elastic wave within the target so that a data point can be obtained. A plurality of data points so obtained are subsequently processed, such as by the matched filter technique, to determine a property of interest of the target. The property of interest may be, by example, the temperature of the target or the metallurgical status of the target.

Claims

1. A system for determining a characteristic of a target, comprising:

means for launching an elastic wave within the target;

an interferometer for detecting a displacement of a surface of the target in response to said launched elastic wave;

means, responsive to said detected displacement, for determining a time of flight of said elastic wave within the target; and

means for correlating the determined time of flight with a characteristic of the target, wherein

said interferometer includes at least one photodetector, and wherein said correlating means is comprised of;

means for detecting a waveshape of a signal output from said photodetector; and

matched filter processing means comprising means for storing a plurality of predetermined waveshapes, each of said plurality of predetermined waveshapes corresponding to a different target thickness;

means for comparing said detected waveshape to said stored plurality of predetermined waveshapes; and

means for correlating a thickness of the target with a predetermined thickness that is associated with a selected one of said stored plurality of waveshapes that best matches said detected waveshape.

2. A system for determining a characteristic of a target, comprising:

means for launching an elastic wave within the target;

an interferometer including at least one photoresponsive detector for indicating a displacement of a surface of the target in response to the launched elastic wave;

signal processing means, responsive to said detected displacement, for determining a time varying characteristic of the elastic wave within the target and for relating the determined time varying characteristic with a property of interest of the target, wherein said signal processing means is comprised of;

means for determining a waveshape of a signal output from said detector;

matched filter processing means comprising means for storing a plurality of predetermined waveshapes;

calibration curve processing means comprising means for storing a plurality of calibration curves each relating an elastic wave time varying characteristic to a property of interest of the target;

means for selecting one of said stored plurality of predetermined waveshapes based on a thickness of the target and on a type of material that comprises the target;

means for determining a time varying characteristic of the determined waveshape in cooperation with the selected one of the stored plurality of predetermined waveshapes; and

means for relating the determined time varying characteristic of the detected waveshape with the property of interest of the target in cooperation with one of the stored plurality of calibration curves.