referring to fig1 a - c , an optical system 50 uses ists to measures the thickness of a thin film 10 disposed on a substrate 11 . similar optical systems are described in u . s . pat . no . 5 , 633 , 711 ( entitled measurement of material properties with optically indiced phonons ); u . s . pat . no . 5 , 546 , 811 ( entitled optical measurement of stress in thin film samples ); and u . s . ser . no . 08 / 783 , 046 ( entitled method and device for measuring film thickness , filed jul . 15 , 1996 ), the contents of which have been previously incorporated herein by reference . the system 50 features a microchip laser 52 that emits an excitation pulse 12 that passes through a beam - splitter 65 . the beam - splitter 65 reflects a portion of the pulse into a low - speed photo - detector 67 to generate an electrical pulse that triggers a data - acquisition system ( not shown in the figure ). the remainder of the excitation pulse 12 travels through the beam splitter 65 and impinges a spherical collimating lens 60 and a cylindrical focussing lens 64 that focus the pulse onto a phase mask 62 . referring to fig1 b - 1c , the phase mask 62 attaches to a motor - driven stage 16 that , during a measurement , “ dithers ” back and forth to translate the mask 62 horizontally relative to the propagation of the excitation pulse 12 . as indicated by the arrow 63 ′, dithering is typically performed in a time - dependent , periodic manner ( e . g ., a sinusoidal manner ). during dithering the excitation pulse 12 irradiates a pattern 66 on the phase mask 62 that consists of glass etched to include regions 88 a , 88 b having different thicknesses . as indicated by the arrow 67 in fig1 c , the regions 88 a , 88 b are spaced by a distance ( typically between 1 and 50 microns ) that is smaller than a spot size of the laser pulse . the spot size is typically elliptical with dimensions of approximately 20 × 200 microns ; the long axis of the ellipse extends orthogonal to the extension of the regions 88 a , 88 b . during typical applications , the phase mask translates back and forth over a range of about 0 . 01 mm - 2 . 0 mm with a velocity of about 50 microns / second . the range of the dither is typically similar to the spacing of the pattern on the phase mask , described above as being between 1 and 50 microns . in general , it is preferred that dithering the phase mask vary the spatial phase of the excitation pattern in an incremental fashion ranging from 0 ° to 180 °, or some multiple thereof . since the excitation pattern is preferably a sequence of parallel light and dark regions , this means that the light regions will move to where the dark regions were and the dark regions will move to where the light regions were , and back again , during dither . in this configuration , the pattern 66 diffracts the pulse 12 into two excitation pulses 12 a , 12 b that diverge from the mask 62 at an angle θ . during operation , dithering of the phase mask causes the position of the regions 88 a , 88 b to vary relative to the elliptical laser spot in a time - dependent manner . this process modulates the optical phase of each excitation pulse 12 a , 12 b and , consequently , the phase of the light and dark regions of the excitation pattern when the excitation pulses are overlapped on the sample surface . during a typical measurement , dithering occurs at a frequency ( typically 100 - 1000 hz ) that is higher than the data - acquisition frequency ( typically 10 - 500 hz ) of the ists measurement , so that several ists measurements are performed within each dither cycle . referring again to fig1 a , the excitation pulses 12 a , 12 b diverge from the phase mask 62 and irradiate an achromatic lens pair 70 having a focal length f . the lens pair 70 focuses the pulses 12 a , 12 b onto the film 10 at the same angle θ that the pulses diverge from the phase mask . the lens pair 70 overlaps the pulses on a surface 49 of the film 10 so that they optically interfere to form a spatially periodic excitation pattern 15 . the pattern 15 features between about 50 alternating light and dark regions . the lens pair 70 is separated from the phase mask by a distance of 2 f so that the excitation pattern 15 has a spatial frequency that matches the spatial frequency of the pattern 66 on the phase mask ( i . e ., the achromatic lens pair approximately performs 1 : 1 imaging ). dithering the phase mask as described above adjusts the spatial phase of the excitation pattern relative to the sample in a time - dependent manner , but does not change its spatial frequency . during an ists measurement , the film absorbs the light regions in the pattern 15 and , in response , thermally expands to launch counter - propagating acoustic waves . the acoustic waves form a “ transient grating ” that modifies the film &# 39 ; s surface ( e . g ., by generating a time - dependent surface ripple or refractive index change ) at the acoustic frequency . once initiated , the acoustic waves are measured with a probe beam 20 emitted from a probe laser 54 . the probe beam 20 passes through a collimating lens 55 and a spherical lens 71 that focus the beam so that it irradiates the film &# 39 ; s surface at or near the pattern . a portion of the probe beam diffracts off the surface waves to form a diffracted signal beam 20 ′ that passes through a lens 76 and into a high - speed photodetector 80 . the photodetector 80 detects the diffracted beam 20 ′ to generate a light - induced electrical signal that is processed by the data - acquisition system to generate a data sweep that is modulated at the acoustic frequency . in other words , the term “ data sweep ” will be used herein to refer to a single measurement . during a typical measurement , a computer ( not shown in the figure ) averages multiple data sweeps ( typically 50 - 500 sweeps ) from a single spot to generate a signal waveform 100 , such as that shown in fig2 a . thus , in this application , the phase mask is dithered within the ists measurement . averaging data sweeps in this way results in data having high signal - to - noise ratios ( typically 1 part in 100 , 000 ). the signal waveform 100 shown in fig2 a was measured from a nominal 5185 - angstrom copper / 250 - angstrom / 1000 - angstrom oxide structure deposited on a silicon wafer , and shows an ists signal intensity ( in units of millivolts ) plotted as a function of time ( in units of nanoseconds ). the fourier transform 101 of the signal waveform 100 is indicated by the graph in fig2 b . these data indicate that the frequency of the signal waveform , and thus the frequency of the acoustic mode , in this case is about 270 mhz . to determine film thickness , this acoustic frequency is analyzed by a computer along with the inverse of the spacing between the light and dark regions of the excitation pattern ( i . e ., the wavevector ) and the film &# 39 ; s density and sound velocities as described in the above - mentioned references . the analysis process is described , e . g ., in u . s . pat . no . 5 , 633 , 711 , the contents of which are incorporated herein by reference . measurements can be made at single or multiple points along the sample surface . fig3 a and 3b show how dithering the phase mask improves the position - dependent thickness measurement using the above - described method . the y - axis for both figures has units of angstroms and the x - axis has units of mm . data for the figures were taken using the sample described in fig2 over a linear distance of 0 . 035 mm with dither ( fig3 a ) and without dither ( fig3 b ). to make the position - dependent thickness measurements , an ists measurement was taken every 0 . 5 microns . as is clear from the data , the measurement of film thickness using a dithered phase mask is more uniform ( having a total range of about 75 angstroms ) than a similar measurement made without dithering ( having a total range of greater than 200 angstroms ). those of ordinary skill will immediately appreciate that the thickness excursions in fig3 b are not an accurate representation of the thickness of the copper film , while those shown in fig3 a are more typical . other embodiments are within the scope of the invention described above . in other words , other components of the optical system can be moved or altered in order to achieve the desired variation of the spatial phase of the excitation pattern . in each case , the preference is to dither the component so as to cause the spatial phase of the excitation pattern to vary between 0 ° and 180 °. in particular , referring to fig4 a , the phase of the excitation pattern can be adjusted in a time - dependent manner by translating a sample mount 13 that dithers horizontally back and forth ( as indicated by the arrow 14 ). this method is analogous to dithering the phase mask , as it adjusts the spatial phase of the excitation pattern 15 relative to the sample &# 39 ; s surface 49 . in this case , the sample mount 13 translates over a distance and with a velocity similar to the values described for the dithered phase mask of fig1 a and 1b . referring to fig4 b , in another embodiment the excitation laser 52 , a mirror ( not shown ), a lens ( not shown ), or a combination of these or other components in the optical system are dithered ( as indicated by the arrow 17 ) to obtain the same result of dithering the phase mask as described above . again , the range and velocity of the dithering is similar to that described with reference to fig1 a and 1b . in general , dithering is accomplished by moving any element of the system ( e . g ., the phase mask ) with a motor - driven stage or equivalent that receives any type of electrical signal . the signal can be a sinusoidal , saw - tooth , square , random , or any other type of waveform between 0 . 1 and 1000 hz . the motor - driven stage can be replaced with any movable component , such as a piezoelectric device . as shown in fig4 c , dithering can alternatively be accomplished by placing an electro - optic ( e . g ., pockels cell ) or acousto - optic ( e . g ., bragg cell ) modulator 47 in the path of one of the excitation pulses shown in fig4 b . in this case , as is known in the art , the modulator varies the phase of the incident pulse 12 and simultaneously generates two excitation pulses 12 a , 12 b . when overlapped on the sample , these pulses form an excitation pattern that varies as a function of time . in other embodiments , optical systems other than that shown in fig1 that use a phase mask to generate an excitation field to excite acoustic waves and a reflection - mode geometry to measure the waves can be used . such systems , for example , are described in u . s . ser . no . 08 / 885 , 555 ( entitled improved transient - grating method and apparatus for measuring material properties ), the contents of which are incorporated herein by reference . likewise , the phase mask can be replaced with a similar diffractive optical element , such as an amplitude mask , a diffraction grating , an electro - optic modulator , or some combination or equivalent thereof to generate the beams that form the excitation pattern . the diffractive element can include patterns that simultaneously generate more than one spatial frequency , an excitation pattern that is non - periodic , or an excitation pattern that contains light regions that are shapes other than a series of parallel lines . for example , the diffractive element may generate an excitation pattern consisting of a series of concentric circles , ellipses , or other shapes . other acceptable patterns for diffractive masks are described in u . s . pat . no . 5 , 734 , 470 , entitled device and method for time - resolved optical measurements , the contents of which have been previously incorporated herein by reference . in such cases , the dither of components may result in a change in the excitation pattern which is somewhat different from that described for the parallel light and dark regions . nevertheless , a movement of such excitation patterns at a frequency somewhat greater than the data acquisition frequency can be achieved with beneficial effects . likewise , any suitable laser can be used to generate excitation and probe radiation . typically the excitation and probe lasers are , respectively , a diode - pumped nd : yag / cr + 4 : yag microchip laser and a diode laser , although other lasers can be used . for example , the excitation laser can be a titanium : sapphire , chromium : lisaf , ring , or fiber laser . in still other embodiments the signal waveform can be analyzed to determine properties of the sample other than thickness . for example , the acoustic frequency can be used to determine adhesion , density , stiffness , elasticity , surface roughness , and other mechanical or physical properties of a thin film . in addition , delamination , portions of the signal waveform other than frequency can be analyzed to determine other properties about the sample . for example , the shape of the waveform can be analyzed to determine the degree of adhesion , surface roughness , or composition of one or more of the films in the structure . other properties that can be measured from the waveform include properties of ion - implanted silicon wafers , such as the concentration and energy of the implanted ions . measurement of these properties are described , for example , in u . s . ser . no . 08 / 783 , 046 ( entitled method and apparatus for measuring ions implanted in semiconductor materials ) and u . s . ser . no . 08 / 885 , 555 ( entitled improved method and apparatus for measuring ions implanted in semiconductor materials ), the contents of which are incorporated herein by reference . the method and apparatus of the invention can be used to measure a variety of structures . for example , the method is particularly effective in determining the thickness of metal films used in the microelectronic industry . such metal films include but are not limited to aluminum , copper , tungsten , titanium , tantalum , titanium : nitride , tantalum : nitride , gold , platinum , niobium , and alloys thereof . these metals may be included in single - layer and multi - layer structures . other materials that can be measured include semiconductors ( e . g ., silicon , gaas , and derivatives thereof ) polymers , diamond - like coatings , and buried transparent films .