Abstract:
The invention provides an apparatus for measuring a property of a sample (using, e.g., ISTS) that includes: 1) an excitation laser that generates an excitation laser beam; 2) an optical system aligned along an optical axis that separates the excitation laser beam into at least three sub-beams; 3) an imaging system aligned along the optical axis that collects the sub-beams and focuses them onto the sample to form an optical interference pattern that generates a time-dependent response in the sample; 4) a probe laser that generates a probe laser beam that diffracts off the time-dependent response to form a signal beam; 5) a detector that detects the signal beam and in response generates a radiation-induced electronic response; and 6) a processor that processes the radiation-induced electronic response to determine the property of the sample.

Description:
This is a divisional of application Ser. No. 08/885,555, filed Jun. 30, 1997 U.S. Pat. No. 6,016,202. 
    
    
     BACKGROUND 
     This invention relates to optical systems for measuring properties of a sample, e.g., the thickness of a thin film. 
     An optical measurement called four-wave mixing (FWM) has been used previously to measure a variety of different material properties, such as film thickness, and film delamination. In FWM measurements, two excitation laser beams are overlapped in time and space on a sample s surface to form a spatially varying optical interference pattern. The pattern consists of alternating “light” (i.e., constructive interference) and “dark” (i.e., destructive interference) regions; the spacing between these regions depends on the wavelength of the laser beams and the angle therebetween. In a class of FWM measurements called impulsive stimulated thermal scattering (ISTS), the excitation laser beam contains a series of short (e.g., a few hundred picoseconds) optical pulses. These pulses of radiation are absorbed by the sample in the light regions, but not in the dark regions, to excite a “transient grating”. This process heats and thermally expands the irradiated regions to launch coherent, counter-propagating acoustic waves whose wavelength and direction match those of the interference pattern. When ISTS is used to measure strongly absorbing films (e.g., metal films), the acoustic waves generate a time-dependent “ripple” on the film&#39;s surface that oscillates at the acoustic frequency. A probe beam then diffracts off the transient grating to form a series of signal beams, each of which represents a different diffracted order (e.g, the +/−1 and +/−2 orders). The signal beams oscillate in intensity at the acoustic frequency. One of the signal beams is detected and monitored to measure the properties of the sample. 
     Use of ISTS to measure film thickness and a variety of other properties is described, for example, in U.S. Pat. No. 5,633,711 (entitled MEASUREMENT OF MATERIAL PROPERTIES WITH OPTICALLY INDUCED PHONONS), U.S. Ser. No. 08/377,308 (entitled OPTICAL MEASUREMENT OF STRESS IN THIN FILM SAMPLES, filed Jan. 24, 1995), U.S. Ser. No. 08/783,046 (entitled METHOD AND DEVICE FOR MEASURING FILM THICKNESS, filed Jul. 15, 1996), and U.S. Ser. No. 08/885,786 (entitled METHOD AND APPARATUS FOR MEASURING THE CONCENTRATION OF IONS IMPLANTED IN SEMICONDUCTING MATERIALS, filed concurrently herewith), the contents of which are incorporated herein by reference. 
     There are several optical systems that generate the two excitation beams needed for ISTS. In a typical measurement, for example, a single excitation beam passes through a beam-splitter to form two excitation beams of roughly equal intensity. An imaging system (e.g., a lens) then collects these beams and then spatially overlaps them on the sample to form an optical interference pattern that includes the light and dark regions described above. 
     While useful for many types of FWM experiments, beam-delivery systems that generate two excitation beams, such as those using beam-splitters, suffer drawbacks. For example, these systems often have a relatively long depth of focus, i.e., the beams are overlapped for a relatively long distance. This makes it difficult to position the sample in the exact image plane of the imaging system. Another disadvantage is that the diffraction efficiency of the transient grating formed by two excitation beams is often quite small (e.g., on the order of 10 −4 −10 −5 ). This means that the diffracted signal beam is often weak and difficult to measure. A weak signal beam, in turn, makes it difficult to precisely measure the acoustic frequency and any corresponding property (e.g., film thickness). 
     SUMMARY 
     The above-mentioned disadvantages are overcome in part using a method and apparatus that include improvements to beam-delivery systems that generate excitation beams for FWM measurements. These improvements are described with respect to several different optical systems. 
     In one improvement, a beam-delivery system includes a phase mask that generates three beams, rather than two, to form the transient grating in the sample. In a typical embodiment, the three beams are equally spaced in a linear fashion (i.e., the center, right, and left beams) prior to being imaged onto the sample. When focused with an imaging system, the center beam propagates down a central optical axis of the beam-delivery system, while the right and left beams converge toward the same spot at the same angle but on opposite sides of the center beam. The beams are overlapped on the sample so that two different transient gratings are simultaneously formed: (1) a grating is formed by the center beam and each of the right and left beams; and (2) a grating is formed by the right and left beams. The laser beams forming these gratings are separated by different angles (i.e., θ and θ/2), and thus they have different spatial frequencies and will simultaneously excite acoustic waves having different acoustic frequencies. 
     In another improvement, the excitation laser beam is collimated prior to irradiating the phase mask that generates the excitation beams. Collimating the beam in this way reduces variations in the distance separating the light and dark regions of the interference pattern. This consequently improves the precision to which the acoustic wave is measured in the sample. In another improvement, multiple diffracted signal beams, rather than just a single beam, are collected and imaged onto the photodetector. This increases the intensity of the measured signal and thus further improves the precision of the measurement. 
     In one aspect, the invention provides an apparatus for measuring a property (e.g., thickness) of a sample that includes: 1) an excitation laser that generates an excitation laser beam; 2) a beam-delivery system, aligned along an optical axis, that separates the excitation laser beam into at least three sub-beams; 3) an imaging system aligned along the optical axis that collects the sub-beams and focuses them onto the sample to form an optical interference pattern that generates a time-dependent response in the sample; 4) a probe laser that generates a probe laser beam oriented to diffract off the time-dependent response to form at least one signal beam; 5) a detector that detects at least one signal beam (and in some cases, two signal beams) and in response generates a radiation-induced electronic response; and 6) a processor that processes the radiation-induced electronic response to determine the property of the sample. 
     In a typical embodiment, the three sub-beams are generated with a transmissive phase mask, and one of the sub-beams propagates along the optical axis of the optical system. For example, the phase mask may contain a pattern that is oriented along the optical axis and partially diffracts the excitation laser beam to form at least two of the sub-beams, and partially transmits the excitation laser beam to form the remaining sub-beam. In this case, the sub-beam transmitted by the pattern is oriented along the optical axis of the optical system, and the diffracted sub-beams are oriented on each side of the transmitted sub-beam. Each of the sub-beams diffracted by the pattern has a roughly equal intensity that is typically between 20% and 40% of the excitation laser beam, and the sub-beam transmitted by the pattern has an intensity that is between 20% and 60% of the excitation laser beam. 
     In other embodiments, the interference pattern contains interference fringes separated by a distance of between 3 and 20 microns. Such an interference pattern is generated with a phase mask that contains multiple patterns, each of which generates sub-beams that diverge from the phase mask at a different angle. 
     In another aspect, an apparatus includes: 1) an excitation laser that generates an excitation laser beam; 2) a beam-delivery system, aligned along an optical axis, that separates the excitation laser beam into at least two sub-beams; 3) an imaging system aligned along the optical axis that collects the two sub-beams and focuses them onto the sample to form an optical interference pattern that generates a time-dependent response in the sample; 4) a probe laser that generates a probe laser beam oriented to diffract off the time-dependent response to form at least two signal beams; 5) an optic oriented in the optical axis that transmits the excitation sub-beams, reflects the probe beam towards the sample, and reflects at least one signal beam; 6) a detector that detects at least one signal beam and in response generates a radiation-induced electronic response; and 7) a processor that processes the radiation-induced electronic response to determine the property of the sample. In a typical embodiment, the optic is a beam-splitter oriented to reflect at least two signal beams toward the photodetector. 
     The apparatus described above measures a property of a sample, such as film thickness, film delamination, thermal diffusivity, elastic moduli, loss moduli, and concentration of implanted ions. These properties can be measured from a wide range of samples, e.g., metal thin films, semiconductor films, semiconductor substrates, and polymeric materials. In another aspect, the apparatus accurately positions a sample along an optical axis. In this case, for example, the apparatus generates a data set by measuring data at a number of positions along the optical axis, and then processes the data set to position the sample along the optical axis. In one case, the processing step includes fitting the data set with a function, e.g., a Gaussian or Lorentzian function or a derivative or analog thereof. 
     The improved optical systems described above reduces measurement errors and increases signal strengths when compared to existing systems used in FWM measurements. For example, the three-beam system has a relatively sharp effective depth of focus (i.e., depth of field), making it possible to position a sample precisely in the image plane of the imaging system. This minimizes errors in the measurement that are associated with incorrectly positioning the sample along the focus. Measurement errors caused by incorrect positioning are even further reduced by collimating the excitation beam prior to irradiating the phase mask. In addition, since the three-beam optical system simultaneously excites and measures two acoustic frequencies in a sample, two data points can be collected in the time normally required to collect one data point. This reduces the time required to make ISTS measurements requiring multiple data points. 
     The system also increases the magnitude of the measured signal waveform. For example, the diffraction efficiency of a transient grating formed with three beams is large relative to that formed with only two beams. This increases the magnitude of each the diffracted signal beams. The signal measured with the photodetector is even further increased by collecting multiple signal beams, rather than just a single beam. These improvements result in better signal-to-noise ratios in the measured data, and thus increase the precision of an ISTS-based measurement. 
     Still other advantages will be apparent from the following detailed description, and from the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic drawing of the excitation, probe, and signal beams used in a three-beam optical system; 
     FIG. 2 is a front view of a beam-delivery system that includes a phase mask that diffracts an incident excitation laser beam to form three sub-beams; 
     FIG. 3 is a plot of the signal waveform amplitude measured from an aluminum film as a function of the distance the sample is placed from the focus of an imaging system (z) for the two-beam and three-beam optical systems; 
     FIG. 4 is a plot of the signal waveform amplitude measured from a ti:tungsten film as a function of z for the two-beam and three-beam optical systems; 
     FIG. 5 is a plot showing the signal waveform amplitude measured from an aluminum film as a function of z using the three-beam optical system; 
     FIGS. 6A and 6B are time and frequency-domain plots of a signal waveform measured with a three-beam optical system from, respectively, aluminum and ti:nitride films; 
     FIG. 7 is a schematic drawing of a three-beam optical system for measuring a property of a sample with ISTS; and 
     FIG. 8 is a schematic drawing of a modified three-beam optical system for measuring a property of a sample with ISTS. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a three-beam configuration  11  that measures a property (e.g., film thickness) of a sample  10  using ISTS. In the three-beam configuration  11  three excitation sub-beam beams  12   a-c  are overlapped in time and space on the sample s surface to generate a transient grating. The three-beam configuration  11  features: (1) a center sub-beam  12   c  that propagates along an optical axis  23 ; and (2) left  12   a  and right  12   b  sub-beams that converge on the sample s surface at an angle of θ/2 relative to the optical axis. The left  12   a  and right  12   b  sub-beams have roughly equal intensity (typically between 20% and 40% of the incident excitation laser beam) that is typically greater than that of the center sub-beam  12   c.  All sub-beams are separated from an incident laser beam generated from an excitation laser (not shown in the figure). Each beam contains a short laser pulse typically having a duration of a few hundred picoseconds and a wavelength (typically in the infrared or visible spectral regions) that is strongly absorbed by the sample. 
     Once overlapped, the three optical excitation beams interfere to form a spatially varying interference pattern  15  containing alternating light (constructive interference)  17  and dark (destructive interference)  19  regions. In ISTS, radiation is absorbed strongly by the sample  10  in the light regions  17 , but not in the dark regions  19 . The absorbed radiation impulsively heats the sample in those regions during the short duration of the excitation pulse and initiates a time-dependent acoustic response that has been described in detail in the above-mentioned references. This response is measured in its entirety by irradiating sample near or on the interference pattern  15  with a probe pulse  20 . The probe pulse  20  (typically several hundred microseconds) is longer than the duration of the time-dependent response, and is partially reflected to form a reflected beam  20 , and partially diffracted to form a pair of signal beams  25   a,    25   b  that are the +1 and −1 diffracted orders of the transient grating. Higher diffracted orders (e.g., the +/−2, +/−3 orders) are also formed, but are weaker than the +/−1 orders. At least one of the signal beams is detected with a photodetector (not shown in the figure) to generate a signal waveform. The signal waveform is then analyzed to measure a property of the sample. 
     In ISTS the three sub-beams  12   a-c  are focused onto the sample with an imaging system that overlaps them in a cylinder-shaped volume having a long axis (called the Raleigh length) that increases with the focal length of the imaging system and the wavelength of the sub-beams. The magnitude of the Raleigh length determines the sharpness of the imaging system s focus: a relatively short Raleigh length indicates a sharp focus. Only when the sample is positioned along the Raleigh length will the sub-beams be overlapped to form a transient grating on its surface. Thus, a signal waveform is only measured when the sample is positioned along the Raleigh length. 
     Within the Raleigh length the spatial periodicity (i.e., the distance between the light and dark regions) of the interference pattern can vary slightly, causing changes in the frequency of the excited acoustic wave. Since many properties of the sample can be calculated from this frequency, such a variation can result in an erroneous measurement, even when the sample appears to be properly positioned. It is therefore desirable to have a beam-delivery system that: 1) generates beams that are sharply focused over a short Raleigh length; and 2) minimizes the dependence of frequency on the sample s position within the Rayliegh length. 
     FIG. 2 shows beam-delivery system  9  that realizes both of these improvements. The system  9  includes a phase mask  62  containing a diffracting pattern  66  that generates the excitation sub-beams  12   a-c.  Here, an incident optical excitation beam  12  is focused along the x axis onto the pattern  66  using a cylindrical lens  64 . The beam  12  is well collimated along the y axis using a collimating lens  60 . The pattern partially diffracts the incident beam into the +1 and −1 orders (to form the left  12   a  and right  12   b  sub-beams) and partially transmits the incident beam  12  to generate the center sub-beam  12   c.  Higher diffracted orders are generated by the phase mask, but are typically spatially filtered before impinging the sample. These orders are therefore not shown in the figure. The diffracted sub-beams emerge along the x axis at an angle θ/2 relative to the transmitted sub-beam. This angle depends on the periodicity of the irradiated pattern  66  (described in more detail below), and will determine the periodicity of the interference pattern used to excite the sample. These sub-beams are then collected and focused onto the sample to form an optical interference pattern that is an image of the irradiated pattern  66 . A related process is described in U.S. Ser. No. 08/377,310 (entitled SIMPLIFIED DEVICE AND METHOD FOR TIME-RESOLVED OPTICAL MEASUREMENTS, filed Jan. 24, 1997) the contents of which are incorporated herein by reference. A phase mask typically contains multiple patterns  67 , each of which diffracts sub-beams at a different angle and thus forms a different interference pattern on the sample. 
     Phase masks that produce three beams from a single impinging beam are made using techniques well known in the optical arts. For example, the diffraction pattern  66  is typically an alternating series of grooves etched into a glass substrate. The depth of the grooves relative to the wavelength of the incident radiation determines how much radiation is diffracted into to sub-beams, while the periodicity of the grooves determines the angle at which the sub-beams emerge from the pattern. To achieve the three-beam configuration, the depth of the grooves is chosen to be slightly less than optimal so that some of the incident radiation passes directly through the mask, rather than being diffracted into the non-zeroth orders. 
     FIGS. 3 and 4 illustrate how the effective focus of the three-beam configuration is significantly sharper than that of the two-beam configuration. These figures plot the amplitude of a diffracted signal waveform as a function of the position of a sample from the image plane of the imaging system (z) for both the two-beam and three-beam configurations. The image plane of the imaging system occurs at z=0. 
     The figures show, respectively, data measured from aluminum and ti:tungsten, two films commonly used in the microelectronics industry. The data show a relatively strong dependence of the signal magnitude on z for the three-beam configuration, indicating the relatively sharp focus of this configuration. For example, for both aluminum and ti:tungsten the full-width, half-maximum (FWHM) of the peak was about 50 microns. Conversely, in both FIGS. 3 and 4, the magnitude of the signal generated in the two-beam configuration has a relatively weak dependence on z: the FWHM is greater than 300 microns. In fact, in FIG. 3, the dependence of the signal waveform amplitude on z is bi-modal, making it even more difficult to determine the exact position of the image plane of the two-beam configuration. 
     In addition to showing a sharpening of effective focus, FIGS. 3 and 4 indicate how the overall magnitude of the diffracted signal is larger in the three-beam configuration than in the two-beam configuration. For example, in FIG. 3 the maximum signal waveform amplitude for the three-beam configuration is roughly 3 times that measured using the two-beam configuration. In FIG. 4, the signal waveform amplitude is increased by nearly a factor of 5 for the three-beam configuration. The measurements for the two configurations were carefully monitored to ensure that factors that might otherwise affect the signal amplitude, such as laser pulse energy and acoustic wavelength, were identical. 
     FIG. 5 shows the signal waveform amplitude for the three-beam configuration plotted as a function of z along the entire Raleigh length of the imaging system. Several peaks are evident in the data, with the center peak  30  having the largest amplitude, and the side peaks  31 ,  31  decreasing in amplitude in a symmetrical fashion. Mathematically, this distribution of intensity (D(x,z), where x and z are the axes described in FIG. 2) is described by equation (1), below: 
     
       
           D ( x,z )= I   0 +2 I   +/−1 [(1+cos (2 kx  sin (θ/2)))−4/π sin (φ 0 ) cos ( kz  sin (θ/2)) sin ( kz (cos (θ/2)−1))]  (1) 
       
     
     where I 0  is the intensity of the zeroth order beam, I +/−1  is the intensity of the right and left sub-beams, k=2π/λ, where λ is the wavelength of the excitation beam, θ/2 is the angle between the right and left sub-beams and the optical axis, and φ 0  is the phase shift associated with the phase mask (typically π/1.3 for a phase mask that generates three beams, and π for a phase mask that generates two beams). Equation 1 neglects the Gaussian envelope that describes the intensity distribution of most laser beams; this effect can be included simply be multiplying the equation by a Gaussian function. Eqn. 1 indicates that the spatial frequency f D(x,z) is proportional to: 
     
       
         f˜−2k(sin (θ/4)) 2   (2) 
       
     
     Thus, the effective focus of the imaging system is sharpened by decreasing this frequency, i.e., increasing the angle of the diffracted sub-beams. This is done by increasing the periodicity of the grooves in the diffracting mask. 
     Eqn. 1 also predicts the increase in diffraction efficiency (shown in FIGS. 3 and 4) observed in the three-beam configuration (where φ 0 =π/1.3) relative to the two-beam configuration (φ 0 π). In the three-beam configuration D(x,z) is maximized at a slight distance (typically a few microns) away from the image plane (z=0). The value of D(x,z) at this position is nearly 50% larger than the maximum value of D(x,z) for the two-beam configuration that occurs at the image plane. 
     Once generated, data like that shown in FIG. 5 can be fit to determine where to properly position the sample. In one embodiment, the central peak in the data ( 30  in FIG. 5) is determined and then fit to a Gaussian function. The z value that gives the best fit indicates the optimal location for the sample, i.e., the position that maximizes the diffraction efficiency of the grating pattern. 
     The three-beam mask can also be used to increase the rate of data collection. A transient grating formed by overlapping two laser beams will have a periodicity T of 
     
       
         T=λ sin −1  (θ/4)  (3) 
       
     
     where λ is the wavelength of the excitation radiation and θ is the angle between the beams forming the grating. As described above, in the three-beam configuration a transient grating is formed from interference between the center sub-beam and each of the right and left sub-beams, and from interference between the right and left sub-beams. Thus, two different gratings (formed by beams separated by θ and θ/2) are simultaneously formed. This results in simultaneous excitation of two different acoustic frequencies. FIGS. 6A and 6B illustrate this point, showing signal waveforms plotted in the time and frequency domains measured using ISTS in the three-beam configuration. The data were collected from aluminum (FIG. 6A) and ti:nitride (FIG. 6B) films, and clearly show the presence of two acoustic frequencies that are measured in the same time period normally required to measure a single acoustic frequency. Measuring two frequencies simultaneously in this way has several advantages. In general, each frequency can be analyzed to determine a property (e.g., film thickness) of the sample being measured; analyzing two frequencies therefore increases the accuracy to which the property is determined. In measurements where multiple frequencies need to be measured from a sample to determine a property (e.g., an acoustic dispersion), simultaneous measurement of two frequencies significantly expedites the measurement process. 
     FIG. 7 shows an optical system  50  that uses the three-beam configuration to measure the properties of a sample  10  using ISTS. A similar system for making these measurements is described in SIMPLIFIED DEVICE AND METHOD FOR TIME-RESOLVED OPTICAL MEASUREMENTS (U.S. Ser. No. 08/377,310, filed Jan. 24, 1995). 
     The system  50  includes an excitation laser  52  that generates sub-nanosecond optical pulses that initiate the time-dependent responses in the sample  10 , and a probe laser  54  that measures these responses. The excitation laser, for example, can be a Q-switched Nd:YAG laser such as that described in U.S. Pat. No. 5,393,413. The probe laser, for example, can be a modulated diode laser. 
     The excitation laser  52  generates a beam  12  that is separated into three sub-beams  12   a-c  with a beam-delivery system  9 . The beam-delivery system  9  includes a first collimating lens  60  that collimates the beam  12 , and a cylindrical lens  64  that focuses the beam along one axis onto a phase mask  62 . A portion of the beam  12  is reflected by a glass cover slip  65  and detected with a low-speed photodetector  67  that generates an electrical pulse for triggering a data-acquisition system (not shown in the figure). The phase mask includes a series of patterns  66 , each of which generates a different interference pattern  15  on the sample  10 . After impinging a pattern  66  on the phase mask  62 , the excitation beam  12  is partially diffracted into sub-beams  12   a,    12   b,  and partially transmitted to form sub-beam  12   c.  Higher diffracted orders are typically generated by the phase mask and are spatially filtered using a beam block (not shown in the figure). Sub-beams  12   a,    12   b  diverge at an angle θ/2 relative to sub-beam  12   c,  which is transmitted along an optical axis  23  of the system. The angle of divergence of the diffracted sub-beams  12   a,b  determines the periodicity of the interference pattern as described in equation 1, above. 
     A first imaging lens pair  70  collects the three sub-beams  12   a-c  and focuses and overlaps them onto the surface  49  of the sample  10  to form the interference pattern  15 . Absorption of the pattern  68  initiates an acoustic wave through ISTS. Different grating patterns are formed on the sample simply by translating the phase mask  62  so that a new pattern is irradiated with the incident excitation beam  12 . 
     The probe laser  54  generates a probe pulse  20  that is collimated with a second collimating lens  55  and focused onto the interference pattern  15  with a second spherical lens  71  to measure the time-dependent response of the sample. As described above, this response diffracts a portion of the probe pulse  20  to form a signal beam  25   b.  A beam-stop  81  blocks a reflected portion  20  of the probe beam. A second imaging lens  76  collects the signal beam  25   b  and focuses it onto a high-speed (e.g., 1 GHz) photodetector  80  to generate a signal waveform. The waveform is then analyzed with a computer (not shown in the figure) to determine a property of the sample. 
     FIG. 8 shows another optical system  100  for performing ISTS measurements using either the two-beam or three-beam configuration. This system includes fewer optics and occupies less space than the system shown in FIG. 7, and can therefore be used to make measurements in environments that require compact instrumentation. The system  100 , for example, could be attached directly to a tool used during fabrication of a microelectronic device, such as a metal-film deposition chamber, to make in situ measurements. These types of measurements are described in more detail in U.S. Ser. No. 08/783,046 (entitled METHOD AND DEVICE FOR MEASURING FILM THICKNESS, filed Jul. 15, 1996). 
     In a typical embodiment, the system  100  includes a phase mask  162  and pattern  166  that diffracts an incident beam  112  into three sub-beams  112   a-c  as described above. The sub-beams  112   a-c  pass through a beam-splitter  113  coated to only reflect a probe laser beam  120 . After passing through the beam-splitter  133 , a lens pair  170  collects the sub-beams  112   a-c  and overlaps them onto a sample  110  to form an interference pattern  115  that generates acoustic waves through ISTS as described above. 
     The acoustic waves are measured with the probe beam  120  that first passes through a clear hole  139  drilled into a mirror  140 . The beam-splitter  113  then reflects the probe beam  120  so that it is collinear with the center excitation sub-beam  112   c  and passes through the lens pair  170 , where it is focused onto the interference pattern  115 . There, the probe beam  120  is diffracted to form a pair of signal beams  125   a,b  that propagate back through the lens pair  170  and are reflected by the beam-splitter  113  onto opposite sides of the clear hole  139  drilled into the mirror  140 . The mirror reflects both signal beams  125   a,b  into a photodetector that in response generates a signal waveform that is analyzed as described above. 
     The optical sources and detection electronics which may be used with the optical systems described above have been described previously in the references described above. Briefly, in order to impulsively generate material motions in a sample (using, e.g., ISTS), the excitation laser beam is pulsed, and may be generated using a light source which is Q-switched, mode-locked, or both. The pulse duration must be short enough to impulsively stimulate material motions in the film. For example, in order to excite acoustic processes, the output pulse typically has a duration of between 100 picoseconds and 1 nanosecond. The energy of the output pulse is typically between 3 and 20 microjoules. In addition, the repetition rate of the pulses must be high enough to allow suitable data averaging (when necessary), but low enough to allow the thermal properties of the sample to recover between laser shots. Typically, the repetition rate is between 1 and 2000 Hz, with the rate being adjusted for measurement of different types of samples. For samples which easily damage, such as thin metal films, it may be desirable to reduce the repetition rate of the excitation laser. 
     Once excited, the time-dependent properties of the sample are recorded by monitoring the time-dependent diffraction of the probe pulse, which is typically derived from a single-mode laser producing between 0.1 and 1 Watt peak power in the visible or infrared frequency range. The pulse typically has a temporal duration on the order of between 1 and 100 microseconds. Alternatively, a continuous-wave (cw) probe beam can be used. 
     Light sources other than Nd:YAG lasers may be used to optically excite the film. Suitable lasers include, among others, Nd:YLF, ion (e.g., argon and krypton), Ti:Sapphire, diode, CO 2 , holmium, excimer, dye, and metal-vapor lasers. Similarly, light sources other than diode lasers may be used as the probe laser. Alternative lasers include ion lasers. Pulsed light sources which may be used to generate the probe beam include Q-switched Nd:YAG, Nd:YLF, Ti:Sapphire, diode, CO 2 , holmium, excimer, dye, and metal-vapor lasers. 
     In addition, optical elements other than transmissive phase masks can be used to generate the excitation sub-beams. For example, amplitude masks (i.e., a mask that uses opaque bars rather than transparent grooves to diffract radiation) can be used, as can phase and amplitude and phase masks operating in a reflective mode. In still other embodiments a series of beam-splitters and optical delay lines known in the art can be used to generate the excitation sub-beams. 
     The samples which may be monitored with the method and apparatus described herein may be bulk (e.g., solids such as metal or semiconductors), thin films (e.g., polymer, semiconductor, or metal films), fluids, surfaces or other samples exhibiting time-dependent material motions. Typical samples include metal films used in the semiconductor industry, such as aluminum, tungsten, titanium, ti:tungsten, ti:nitride, chromium, and cobalt films. The material properties that can be determined in these samples include mechanical, physical (e.g., thickness), elastic, (depth-dependent and/or anisotropic) diffusive, adhesion-based, thermal (e.g., thermal diffusivities) and viscous properties associated with the damping of acoustic waves. In addition, electron relaxation lifetimes, electron-hole recombination times, exciton lifetimes, and bi-exciton lifetimes may additionally be determined in, for example, metal and semiconductor samples. 
     In bulk systems the dispersion of the acoustic properties allows the frequency dependence of the mechanical (e.g., bulk or shear) modulus to be determined. Additionally, measurement of the thermal diffusion dynamics allows the thermal diffusivity for the acoustic information that is obtained allows determination of the degree of adhesion, delamination properties, and the elastic, shear, and longitudinal moduli, as well as anisotropies in these properties. Depth-dependent properties and residual stresses in thin films can also be determined. 
     Still other embodiments are within the scope of the following claims.