Abstract:
Apparatus for analyzing thin surface layers. An acoustic wave generating laser beam is amplitude modulated with continuous wave modulation of a frequency in the megahertz to gigahertz range and an optical system directs the modulated radiation to a surface of a thin surface layer. This in turn causes an acoustic wave that is sensed and analyzed to provide an indication of properties of thin surface layer.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Application No. 60/555,427 filed on Mar. 23, 2004 entitled Device and Method for High Sensitivity Laser Ultrasonic Characterization of Micro- and Nanoscale Materials, the disclosure of which is incorporated herein by reference. 

   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not applicable 
   BACKGROUND OF THE INVENTION 
   This invention relates to a laser based system to measure the physical and mechanical properties of thin films, plates, and coating materials. Currently existing laser ultrasonic techniques for the characterization of such materials use pulse laser sources for the generation of acoustic waves. Short laser pulses from femto second to nanosecond second lasers generate broad bandwidth acoustic waves, and detection of these waves is accompanied by the presence of broadband noise in the measurement system. This degrades the signal-to-noise ratio of the measurement limits the accuracy with which the relevant material properties can be determined. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention relates to a narrow bandwidth laser based system which uses a high frequency modulated continuous wave laser source to generate narrow bandwidth acoustic waves combined with a narrow-bandwidth detection scheme. According to the present invention, an acoustic microscopy technique is presented which uses a continuous wave (CW) amplitude modulated laser source for the generation of narrow band acoustic waves for use in analysis of thin materials and micro- and nanoscale plates, membranes, and coatings. An acoustic wave generating laser beam is amplitude modulated with continuous wave modulation of megahertz-gigahertz frequency range and an optical system directs the modulated radiation to a surface of a test specimen. This in turn causes an acoustic wave that is optically sensed using an interferometer and analyzed in the time domain and/or frequency domain to provide an indication of properties of the test specimen. 
   This invention allows for the displacement sensitivity to be improved over other laser based ultrasonic inspection techniques through a narrowing of the bandwidth of the detection system. The energy in the generated acoustic signal is centered at the frequency of modulation of the laser generation source. The effective bandwidth of the acoustic signal is inversely proportional to the length of time that the surface is illuminated. The bandwidth of the optical detection system may then be reduced to match that of the acoustic signal, thus allowing for a substantial improvement in the signal to noise ratio of the detection system. This narrow band measurement is made, for example, using a lock-in amplifier or vector network analyzer, and the bandwidth can be easily selected based on the signal to noise ratio requirements for a given application. This system is capable of modulating the amplitude of the laser source, and hence generating acoustic waves, over a broad range of frequencies from the low megahertz to tens of gigahertz. 
   This invention allows for the generation of high frequency acoustic waves with short wavelengths that are suitable for inspecting small scale systems such as thin films and coatings. It is well suited to measure the mechanical properties of thin films such as the elastic moduli and density, as well as the dimension properties such as thickness. It is also suitable for the inspection of other small-scale structures such as micro- or nanoscale beams, membranes, or plates. The invention also can be used to generate and detect acoustic waves in macroscopic systems for nondestructive evaluation and determination of physical and mechanical properties. These applications include the detection of subsurface or surface breaking cracks and the detection of subsurface voids or disbonds. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     These and other features of the present invention are illustrated in the detailed description below in conjunction with the drawing of which: 
       FIG. 1  illustrates the creation and sensing of an acoustic wave from a CW modulated laser in a material using laser beams; 
       FIG. 2  illustrates a system for the purpose of  FIG. 1 ; 
       FIGS. 3   a  and  3   b  illustrate the steps of time domain processing of the acoustic response according to the invention; 
       FIGS. 4   a  and  4   b  illustrate frequency domain processing of the acoustic response according to the invention; 
       FIG. 5  illustrates the response of a material to pulse versus CW modulated laser radiation; 
       FIGS. 6   a - 6   g  illustrate the measured frequency response of a standard material. 
       FIGS. 7   a - 7   g  illustrate a time domain response of a standard material, reconstructed from the frequency domain data of  FIGS. 6   a - 6   g , for reference purposes; 
       FIGS. 8   a - 8   c  illustrate use of the invention in detection of a fault in a material using time domain processing; 
       FIGS. 9   a - 9   g  illustrate the use of the invention in analysis of a thin gold layer; and 
       FIGS. 10   a  and  10   b  illustrate the processing of data in a thin plate to obtain the dispersion (velocity at each frequency) curve. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention presents a technology for acoustic microscopy using lasers amplitude modulated with continuous wave modulation at frequencies into the GHz frequency range. The signal to noise ratio of optical detection systems is inversely proportional to the bandwidth of the system. By sinusoidally modulating a CW laser beam applied to the surface of the target material, it is possible to produce very narrow-bandwidth acoustic waves and hence narrow the bandwidth of the optical detection system to match that of the acoustic waves. The signal-to-noise ratio of such a CW modulated laser system represents an improvement of several orders of magnitude over that of previous laser ultrasonic systems which use pulsed laser sources to generate high frequency, broad bandwidth acoustic waves. 
   The CW amplitude modulated laser beam such as laser beam  12  in  FIG. 1  is applied to a thin material or a surface layer of a thin or other material  14  where it creates both thermal waves  16 , surface acoustic waves (SAW)  18  and bulk waves  20 . The applied beam  12  is for example, in the wavelength range of 1500 nanometers (nm) and may be modulated at up to 40 gigahertz using, for example, an electroabsorption or Mach Zehnder modulator. Sources at 1,300 and 1,064 nm, for example, may also been used. The material  14  responds to heating to create acoustic waves on  18  which can be detected by a detection beam  22 , typically from a second laser source, described below. The surface perturbations caused by the SAW wave  18  are interferometrically detected and the detection signal used to analyze properties of the material  14  as described herein below. 
     FIG. 2  illustrates the apparatus for performing the acoustic testing of a sample material  14 . A laser  30  which may be a DBF diode laser is CW amplitude modulated using an electro-absorption, Mach-Zehnder, or electro-optic modulator. The signal to the modulator is provided by a RF signal generator  32  at a modulation frequency in the megahertz-gigahertz range, this range can be from less than 1 MHz −40 GHz, or more. The laser  30  provides an output radiation typically in the near infrared wavelengths, for example 1550 nm. The output of laser  30  is amplified by an amplifier  34 , typically an erbium fiber amplifier that also controls the power in the beam applied to the material. The output of the amplifier is applied to an optical path  36  consisting of a scanning mirror  38 , relay lenses  40 , dichroic mirror  42  and focusing objective lens  44  which focuses the radiation, typically to spot size of 100&#39;s of nanometers to a few microns on the sample  14 . The gimbal scanning mirror  38  allows for the excitation laser source to be scanned within the field of view of the microscope objective under control of a controller  45 . 
   A viewing system consisting of a prism  46  can slide into the optical path  36  and direct a portion of the radiation reflected from sample  14  through an imaging lens  48  to a video camera  50 . A monitor  52  detects the signal from the video camera  50  and allows the operator to view the sample surface for sample alignment via stage  54 . A second or detection laser  60  is placed in an interferometer such as a Michelson interferometer  62 . Its output is typically in the visible radiation range. Laser  60  applies its radiation through a beam splitter  64  to the mirror  42  to provide the detection beam  22  in  FIG. 1 . The detection beam is phase modulated by the acoustic wave produced by the modulated excitation source on the sample surface. 
   The interferometer is formed by an orthogonally placed reference mirror  66  on an actuator  67 . The actuator on the reference mirror used to control the path length of the reference beam. The reference and detection beam return to the beamsplitter  64  and interfere allowing for the phase modulation on the reference beam to be converted to an intensity modulation, which is subsequently detected by the photodetector  68 . The signal from the photo detector  68  is applied, along with the signal from the RF signal generator  32  to a lock-in amplifier or vector network analyzer  70 . The magnitude and phase of the acoustic signal are detected using the lock-in amplifier or vector network analyzer. This signal is fed to a processor  71  which performs signal manipulations and processing algorithms described below. 
   The measurement system described thus far allows for the measurement of the real and imaginary components of the acoustic wave field, or, equivalently, the magnitude and phase of the acoustic waves generated by the modulated source. This data must then be processed to obtain acoustic wave velocity information or to detect material defects. In the inspection of thin films, for example, SAW velocity is important. SAWs that propagate on a film/substrate system are dispersive. The penetration depth of SAWs depends on their wavelength. High frequency (short wavelength) SAWs interact primarily with the near surface region while low frequency (long wavelength) SAWs penetrate further into the material. The SAW velocity depends on the elastic moduli, Poisson&#39;s ratios, densities, and thicknesses of each of the coating layers and the substrate. Using theoretical models for SAW propagation in layered media, the properties of thin films are found if the dispersion curves can be determined experimentally. 
   With reference to  FIG. 3  there is shown a schematic of the experimental configuration showing one detection point  72  and several excitation points  74  evenly spaced on the surface of the specimen that result from activation of mirror  38 . The detector may be held fixed and the source scanned with the real and imaginary parts of the acoustic wave field measured at each excitation point. In another embodiment, the source may be held fixed and the detection point scanned in equally spaced increments. At a given excitation frequency the real and imaginary components of the acoustic wave field are obtained as a function of space using processor  71  and the flow chart of step  75   a - 75   d  in  FIG. 3 . This date is Fourier transformed and the magnitude taken giving the magnitude at each spatial frequency. Peaks in spatial frequency correspond to acoustic modes in the system. The temporal excitation frequency w is divided (step  75   a ) by the peak spatial frequency k, giving the velocity v at w through v=w/k. This process can be repeated at several temporal frequencies to obtain the dispersion curve for the system as shown below. 
   With reference to  FIG. 4  there is shown an alternate processing flow diagram. In this case the source and receiver point are held fixed at points  73  and  75  and the temporal frequency is scanned over the frequency range of interest in step  80 . The result is that the real and imaginary components of the acoustic wave field are measured over the entire frequency range. An inverse Fourier transform of the data is then taken at step  82  resulting in a synthesized time domain response of the system. This is the response of the system to a “pulse like” excitation source given by the inverse Fourier transform of the (real and imaginary components) excitation laser source in step  84 . This synthesized time domain trace may be processed through standard techniques to obtain velocity dispersion curves, or it may be used, for example, to detect transient acoustic responses associated with material defects or inhomogeneities. 
   Referring now to  FIG. 5 , theoretical results are shown for surface acoustic waves generated using pulsed and CW modulated laser sources. These were calculated by numerically solving the equations of thermoelasticity, and show the displacement of the sample surface after illuminating it with the given source. The amount of laser power that is used in the calculation is fixed such that the two laser sources produce the same surface temperature rise. For thermoelastic generation of acoustic waves, there exists some temperature T max  (typically taken as the melting point) that the sample surface is kept below in order to avoid damage or ablation. For a given laser pulse shape, this limits the maximum allowable absorbed power density at the surface. As an example, laser heating with a 5 ns Gaussian laser pulse is compared with that produced by a 60 MHz CW laser source. The laser spot size is taken as 3 microns. It is found that, for the same absorbed power density in each case, the CW laser heats the material to a temperature of approximately 2.5 times higher than the pulsed laser. This is due to the fact that heat builds up in the sample between cycles until the sample reaches steady state. With the CW laser power scaled down by a factor of 2.5 both of the laser sources produce equivalent surface heating. The scaled pulse shapes are then convolved in processor  71  with the impulse response of an aluminum semi-infinite half space (with the source and receiver slightly offset on the sample surface) to find the acoustic response of the sample. The resulting signals are shown in  FIG. 5 . As is evident in the pulsed laser case, the laser source produces a strong surface acoustic wave (SAW)  77 . The CW response is shown at  79 . For laser powers that produce equivalent surface heating, the SAW displacement amplitude produced by pulsed generation is a factor of about 2.5 higher than that of CW generation, but the bandwidth of the CW signal can be substantially reduced through detection with an RF lock-in amplifier or vector network analyzer. Using a sufficiently long integration time, the bandwidth can be reduced by six orders of magnitude for the narrowband case over the broadband case resulting in a SNR increase of three orders of magnitude for this particular example. SNR is an important issue in laser based systems, which have substantially lower sensitivity than conventional contact transducers, and this type of SNR increase could open up the possibility of using these non-contact systems for a much wider range of inspection applications. 
   Referring now to  FIGS. 6   a - 6   g  and  7   a - 7   f  there is illustrated processing to establish a base line for the acoustic response of the system using a half space aluminum plate  90  for a reference standard. This provides a calibration standard for use in analysis of other acoustic responses to other thin surface layers or thin materials.  FIGS. 6   a - 6   f  illustrate the magnitude of the acoustic wave field measured in the experiment as a function of frequency. This is found by taking the square root of the sum of the squared real and imaginary components of the signal. It is observed that displacements in the femtometer range can be measured. The processing technique of  FIG. 4  has been used to obtain the time domain response of the system. The reconstructed signals at six different points in the application of a CW modulated laser beam to the aluminum half space  90  are given in  FIGS. 7   a - 7   f . The signals are in agreement with those that would be expected from a pulsed laser source. However, they are instead obtained from a CW modulated source which is scanned in frequency over the bandwidth of interest with each measurement being made at an extremely narrow bandwidth. The bandwidth for these measurements was 0.7 Hz and may be easily controlled through the integration time of the lock-in amplifier  34 . The displacement sensitivity surpasses that which is possible with comparable surface heating using a pulsed source. 
     FIGS. 8   a  and  8   b  represent similar processing conducted on a semi-infinite substrate  94  of  FIG. 8   c  at different test beam application points D. This figure illustrates one of the advantages of converting the signal from the frequency domain back to the time domain. The small arrival labeled “Edge reflection of SAW 83” is a due to the presence of a defect in the material surface. In the frequency domain data, this defect is not clearly evident as the frequency components of this signal overlap the frequency components of the larger, direct SAW arrival. Time domain reconstruction can be very useful in detecting signals scattered from defects or interfaces. Upon conversion to the time domain, these signals can be time gated and subsequently analyzed. These signals were obtained by inverse Fourier transforming the frequency domain data measured at a bandwidth of 0.7 Hz at each point in processor  71 . 
     FIGS. 9   a - 9   f  illustrate the application of a surface acoustic wave producing laser beam on a 240 nanometer thick gold film  98  on a fused silica substrate  100 . The wave forms are reconstructed from frequency domain data taken over the range of 100 KHz to 200 MHz. The bandwidth of the optical detection system at each frequency was 0.7 Hz at each measurement point. The dispersion in the waveforms is clearly seen; as the source to receiver distances D are increased, the waveforms are seem to spread out in time. Conventional processing of the time domain data allows for the dispersion curve to be obtained. In addition, comparison with theoretical dispersion curves, through application of an inversion algorithm, e.g., an optimizations routine is then used to determine the mechanical or physical properties (thickness, for example) of the film. 
     FIG. 10  illustrates an example of obtaining a dispersion curve through the method outlines in  FIG. 3 . The sample was a 50 micron tungsten plate. Measurements were taken at 60 spatial points in source to receiver distance increments of 10 microns. At each measurement temporal frequency, a Fourier transform was performed on the experimental data obtained from all of the spatial positions. An example of the result of this is shown in  FIG. 10   a , for a temporal frequency of 44.7 MHz. Peaks in the spatial frequency curves then correspond to acoustic modes in the system. In a free-standing thin plate, acoustic plate waves or Lamb modes are generated, and at each temporal frequency, more than one mode is excited. These are labeled in  FIG. 10   b  as  100 ,  102 , and  104  and these correspond to the first antisymmetric plate mode, the first symmetric plate mode, and the second antisymmetric plate mode. These modes have different acoustic wave velocities and thus can be effectively separated in the spatial frequency domain as shown in  FIG. 10   a . The corresponding dispersion curve shown in  FIG. 10   b  can be used to find the thickness or mechanical properties of the plate through comparison with theoretical dispersion curves using a standard optimization routine.