Abstract:
A scatterometer for detecting and analyzing wafer surface defects includes a light source generating a beam of light and a photodetector. Optics are used for splitting the beam of light into a reference beam and a detection beam. Optics also direct the reference beam and the detection beam to the photodetector through different optical paths. The optics direct the detection beam to the surface of the wafer and when incident upon a defect creates a scattered beam. The optics direct the scattered beam to the photodetector. A driver moves the surface of the wafer with respect to the detection beam. A computer coupled to the photodetector determines the presence of a defect on the surface by analyzing an interference pattern from the superposition of the reference beam and the scattered beam.

Description:
This invention was made with government support under contract F33615-96-C-5108 awarded by the United States Air Force. The government has certain rights in this invention. 
    
    
     RELATED APPLICATION 
     The application claims priority to provisional application 60/026,173 filed on Sep. 12, 1996. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to optical inspection systems and more particularly to optical instruments for measuring defects on a finished surface. 
     The semiconductor industry increasingly requires ever smaller semiconductor structures. This is due to the increased need for more capacity per chip. More capacity includes faster response times and more function per unit of area of each chip. Presently, the semiconductor industry projects an immediate move to manufacture 64 Mb, and in the near future 256 Mb, DRAMS. This increased structure density requires the detection and identification of smaller particles and defects that would render the wafer useless for this application. Detection of 2 nm substrate defects and 20 nm sized particles on unpatterned silicon wafers is necessary. 
     In addition, the industry is changing from 200 mm to 300 mm diameter wafers with fewer defects and rapid detection at all processing stages. To meet these needs, defect data must be processed in near-real-time to expedite correction of processing problems through statistical process control techniques. 
     Many surface roughness inspection systems are available; for example, high resolution microscopes such as the atomic force microscope and optical microscopes such as the phase contrast microscope. Other optical measurement systems such as ellipsometers, and mechanical contact methods that use a stylus are also used to measure surface roughness. For sub-micron resolution, most of these techniques are not suitable for in-process surface inspection. High resolution microscopes require cumbersome surface preparations and expensive operations. Standard optical microscopes do not have sufficient resolution and accuracy. Ellipsometry or spectroscopy also do not provide adequate surface roughness information. Mechanical stylus devices may damage the surface and thus are not even considered. 
     An additional possibility is the optical heterodyne (frequency-shifted) microscope. The heterodyne microscope is an interferometric microscope where a signal beam is frequency-shifted relative to a reference beam. The signal containing the optical phase information (i.e., surface roughness) can be electronically detected by comparing the phase of the beams from different portions of the water surface. Optical heterodyne microscopes can calculate the surface roughness to less than 0.1 nm. Optical heterodyne microscopes, however, have several drawbacks, including inadequate lateral resolution, slow scanning speed, critical focusing requirements of the beams, limited dynamic range, and inadequate information on the composition of surface defects. A serious drawback is in assessing the number of defects over a large wafer by scanning with a micron-sized area of view. This would require hours to examine a wafer even with a multiple array of detectors. Such slow processing time is unacceptable for in-process inspection for the semiconductor industry. 
     Conventional scatterometry is another method for measuring defects. The limitation with conventional scatterometry is that the intensity of scattering is measured at oblique angles, excluding the specular beam. Under this condition, the diameter of particles that can be detected must be greater than 100 nm (or at best 80 nm). Conventional scatterometry can rapidly scan large wafers to measure larger particle sizes, but has the disadvantage that the size of particles that can be measured is not small enough for the smaller structures to be built in the near future. 
     SUMMARY OF THE INVENTION 
     It is, therefore, one object of the present invention to provide an improved surface defect detection system that can provide accurate and rapid surface defect information in an in-process manufacturing environment for relatively small defects. 
     The present invention uses optical heterodyne detection to measure light scattered from a particle on a wafer surface. The system includes an illuminator, such as a laser or an alternative &#34;white&#34; light source, and a photodetector. Optics are used to split the beam of light into a reference beam and a separate detection beam, focused on a wafer surface. The optics direct the reference beam and the detection beam at a photodetector through different optical paths. A motor drive is coupled to the wafer and moves the surface of the wafer with respect to the detection beam. A computer interfaced to the photodetector can determine the presence of a defect on the surface of the wafer by analyzing the interference pattern created by the superposition of the reference beam and the detection beam. 
     A specular reflection of one of the beams may serve as the &#34;reference&#34; beam. This reference beam is used to interfere with a beam of light scattered from a particle on the wafer surface. This beam of scattered light is produced by an illuminating beam from the direction opposite that of the reference beam. (The scattering light used is, therefore, back-scattered.) This system detects the scattered beam as an interference pattern caused by the interference between the reference beam and the scattered beam. This interference pattern can be detected with a large linear detector array. Each element of the array detector corresponds to the line of the illumination. The light measured by the detector element consists of a large DC component and a small AC component. The AC component is the changing interference pattern. This AC component is due to a Doppler shift of the scattered beam, caused by the motion of the particle as the wafer is scanned past the illumination beam. A Doppler frequency of the order of 40 kHz, for example, will be produced by a wafer scanning speed of 1 cm/second. The frequency of the reference beam is not affected by the motion of the wafer since the reference beam is specularly reflected by the moving wafer. With this heterodyne detection, weak scattering from particles in the range of 10 nm to 100 nm in size can be detected. 
     The amplitude of the signal is proportional to the volume density of the particle. The detector output is thus a measurement of the volume of the contaminant per the corresponding illumination area. In one implementation, the optics has a beamsplitter and, at least, a pair of mirrors. The beamsplitter splits the beam of light and directs the reference beam to one mirror and the detection beam to the other mirror. Both mirrors direct the beams to a particular point on the surface of the wafer at near grazing incidence from the opposite directions. Light scattered by the detection beam interfering with a surface detect is superposed with the reference beam and this beam is directed to the photodetector. The interference pattern of the two beams is then analyzed. The volume of any defect may be determined by analyzing the amplitude of the interference pattern and by repeating the process at different wavelengths, the composition of the defect may be determined. 
     In a further embodiment if a light source having several wavelengths is used with a spectrometer as the light detector, the composition of the defect can be determined. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other features and advantages of the present invention will become apparent from the following detailed description which should be read in conjunction with the drawings in which: 
     FIG. 1 is a diagrammatic representation of a scatterometer according to the present invention; 
     FIG. 2 is a diagrammatic representation of an alternative embodiment of a scatterometer having different optics according to the present invention; 
     FIG. 3 is a variation of the scatterometer of FIG. 2 having a spectrometer for determining the composition of particles; 
     FIG. 4 is diagrammatic representation of a scatterometer having a different optical arrangement; 
     FIG. 5 is a diagrammatic representation of a scatterometer having yet another optical arrangement; 
     FIG. 6 is a cross-sectional view of a scanning stage to be used with the optical arrangement of FIG. 5. 
     FIG. 7 is a diagrammatic representation of an apparatus to determine a compensation factor for the present invention; and 
     FIG. 8 is a diagrammatic representation of another embodiment to determine a compensation factor for the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the following figures, like reference numerals will be used to represent like components. The figures illustrate a scatterometer used to detect defects on the surface of a semiconductor wafer. However, other uses for the scatterometer of the present invention would be evident to those skilled in the art. 
     Referring now to FIG. 1, a scatterometer 10 has a light source 12 and optics 14 which directs light from light source 12 to a wafer 16 having a surface 18. A photodetector 20 receives light reflected from the surface 18 of wafer 16 through optics 14. As is further described below, a computer 22 coupled to photodetector 20 is used to calculate the presence of a defect on surface 18. A surface defect may be a substrate defect or a contaminant particle on the surface of the wafer. Computer 22 may also be used to calculate the size and composition of a surface contaminant particle. 
     A driver 24 is shown coupled to a scanning stage 17. Scanning stage 17 moves wafer 16 with respect to the light directed by optics 14. 
     Light source 12 may, for example, be a multiple line argon laser or a line scanning dye laser. Light source 12 may also be a &#34;white light&#34; source containing a broad spectrum of light such as a xenon arc lamp. 
     Optics 14 are used to generate two beams of light; a reference beam R and a detection beam D. Optics 14 directs the two beams through different optical paths to photodetector 20. 
     Optics 14 have a mask 26, a number of mirrors M 1  through M 4 , two beamsplitters B 1  and B 2  and two lenses L 1  and L 2 . 
     Mask 26 has a slit (not shown). The slit is preferably a line of about 10 microns by 20 mm. Light passing through the slit is directed to beamsplitter B 1 . The beam of light from light source 12 in its slit shape is divided by beamsplitter B 1 . Reference beam R is directed to surface 18 of wafer 16 by mirror M 1  and mirror M 2  Between mirrors M 1  and M 2 , a converging lens L 1  is used to focus an image of the slit on surface 18. 
     Detection beam D is split from the slit-shaped beam of light by beamsplitter B 1 . Detection beam D is directed to the surface 18 by mirror M 3 , beamsplitter B 2  and mirror M 4 . A lens L 2  between beamsplitter B 2  and mirror M 4  is used to direct an image of slit of mask 26 to the surface 18 at point P. Reference beam R and detection beam D are directed to the same point P from opposite directions. 
     Photodetector 20 may be a charge coupled device (CCD) camera. Alternate detectors, such as silicon photodetectors, may also be used. CCD&#39;s have a noise level greater than that of silicon photodetectors. However, using the appropriate procedure, a cooled CCD camera may be used for the detection of particles smaller than 80 nm. The photodetector may also be a spectroscopic detector. As will be described below, a spectroscopic detector in conjunction with a &#34;white light&#34; source is capable of determining the composition of a particle on the wafer surface. Photodetector 20 changes the analog photonic signals into digital signals. In the case of a spectrometer, this will provide information such as the relative intensity of the various wavelengths for the scattered light. 
     A computer 22 receives digitized information from photodetector 20 to calculate the size of the particle or defect on surface 18 as further described below. 
     Referring now to FIG. 2, optics 14 have been changed from FIG. 1. Optics 14 have a mask 26, beamsplitters B 1  and B 2 , lenses L 1  and L 2  and mirrors M 2  and M 4 . Lenses L 1  and L 2  and mirrors M 2  and M 4  act in a similar manner to that of FIG. 1 except their orientation is slightly changed since the light beam incident upon them is different than FIG. 1. Mirrors M 2  and M 4  direct light to point P on surface 18 at near grazing angles as in FIG. 1. Light source 12 generally directs light to beamsplitter B 1  at an angle normal to the surface 18 of wafer 16. 
     Referring now to FIG. 3, the optics configuration is nearly identical to that of FIG. 2. Light source 12, however, does not direct light at an angle normal to the surface 18 of wafer 16 as in FIG. 2. Light source 12 preferably emits a wide spectrum of light wavelengths. The photodetector 20 of FIG. 2 has been replaced by a spectrometer 28. Spectrometer 28 allows analysis of the wavelength of light reflected from the surface. If adequate background information on possible contaminant materials on the surface 18 of wafer 16 is known, the constituents of the spectrum of reflected light can be analyzed to determine the particle composition. This, of course, assumes that the signal is scattered from a single particle. 
     Each of the embodiments shown in FIGS. 1, 2 and 3 work in a similar manner. A beam of light is generated by light source 12. A slit in mask 26 shapes the beam into a narrow line. A beamsplitter B 1  splits the beam of light into a reference beam R and a detection beam D. In FIG. 1, mirrors M 1  and M 3  assist in directing the light to mirrors M 2  and M 4 . Lens L 1  and L 2  are used to focus the beam to a narrow line on the surface 18 of wafer 16 at point P. Mirrors M 2  and M 4  direct the beam at near grazing angles to point P. A grazing angle would be nearly parallel to the surface of wafer 16. A near grazing angle of about 20° was used in FIGS. 1-3. 
     Reference beam R reflects from mirror M 2  to point P. From point P, the reference beam R reflects from surface 18 to mirror M 4  through lens L 2  and is reflected by beamsplitter B 2  to photodetector 20 or spectrometer 28 as in FIG. 3. No Doppler effect is present in the reference beam R since the distance between the light source and the spectrometer or photodetector remains constant, even while driver 24 moves wafer 16. 
     At the same time reference beam R is generated, beamsplitter B 1  also divides the beam from light source 12 into a detection beam D. Detection beam D is directed to lens L 2  and mirror M 4  by beamsplitter B 2 . Lens L 2  focuses the beam onto point P on surface 18. Mirror M 4  directs detection beam D at a near grazing angle. If a particle or defect is present on the surface 18 of wafer 16, light is backscattered by the particle illuminated by detection beam D. The backscattered light propagates along the same path followed by reference beam R to spectrometer 28 or photodetector 20. That is, light backscatters from a defect on surface 18 at point P back to mirror M 4  and through lens L 2  and is reflected by beamsplitter B 2  to photodetector 20 or spectrometer 28. The backscattered beam and the reference beam R are coherently superimposed. The light power arriving at either photodetector 20 or spectrometer 28 consists of a large DC component and a small AC component. The AC component is an interference pattern formed by the interference between the reference beam and the backscattered beam. The modulation of the interference pattern varies according to the size of the defect. 
     Referring now to FIG. 4, optics 14 are used to generate a reference beam R and a detection beam D. The reference beam is not directed at the surface 18 of wafer 16 as in FIGS. 1 through 3. Optics 14 use three lenses L 3 , L 4  and L 5 , beamsplitters B 3  and B 4 , and a mirror M 5 . Light source 12 generates a beam of light which is collimated by lens L 3 . The light may be in the form of a slit as described above in FIGS. 1 through 3. Light from lens L 3  is directed at beamsplitter B 3 . Beamsplitter B 3  divides the light beam into a reference beam R and a detection beam D. Reference beam R is directed to photodetector 20 through a lens L 4  which focuses reference beam R at a point Q before mirror M 5 . Mirror M 5  directs reference beam R to a beamsplitter B 4  where the reference beam is reflected from beamsplitter B 4  and through lens L 5 . Lens L 5  focuses the beam from beamsplitter B 4  onto the surface of photodetector 20. 
     Returning to beamsplitter B 3  in the optical path, detection beam D is directed to a point P on the surface 18 of wafer 16. Light from the detection beam is scattered at point P if a particle or defect is present on surface 18 of wafer 16. The scattered light is directed through beamsplitter B 4  where it is superimposed with the reference beam R. The superimposed beam is focused by lens L 5  to photodetector 20. 
     Photodetector 20 is located at an extreme grazing angle relative to the direction in which wafer 16 is driven by driver 24. The longitudinal axis 30 of wafer is held at an angle 32 with respect to the direction that the wafer is driven represented by arrow 34. The angle 32 between the reflected detection beam D and the light scattered towards lens L 5  is preferably about 15°. 
     With the arrangement of optics 14 as shown in FIG. 4, the signal-to-noise ratio for the particle detection compared to the system shown in FIG. 1, may be improved up to two times. This is due to the wafer surface being illuminated only by the detection beam D. The detection beam intensity can nearly be doubled in such configurations. However, the overall signal-to-noise ratio of the system is limited by the optical power that can be used. The maximum optical power is determined so that the wafer is not damaged and the photodetector does not saturate. 
     Referring now to FIG. 5, motion normal to the wafer surface is also possible using the different optics 14. Optics 14 has a beamsplitter B 1  similar to that shown in FIGS. 1 through 4. Optics 14 has a right angle prism 38, a porro prism 40 and an aluminized right angle prism 42. Lenses L 6  through L 9  are also used to shape the reference beam R and detection beam D. 
     Light source 12 is used to generate a beam of light in a predetermined pattern similar to that as described above. The light beam from light source is directed to beamsplitter B 1  and divided into a detection beam D and a reference beam R. Reference beam R is reflected by right angle prism 38 and to porro prism 40. Right angle prism 38 changes the direction of reference beam R by 90 degrees. Porro prism 40 changes the direction of reference beam R by 180 degrees. The reference beam is directed to lens L 6  by porro prism 40. Lens L 6  focuses reference beam R on the aluminized right angle prism 42. The aluminized right angle prism 42 directs light through lens L 7  and to photodetector 20. 
     Detection beam D is formed at beamsplitter B 1 . Detection beam D is directed through lens L 8  where detection beam D is focused to the aluminized right angle prism 42. Right angle prism 42 directs the detection beam to lens L 9  where it is collimated to illuminate surface 18 of wafer 16. Scattered beam S from a particle or defect on surface 18 is directed through lens L 9 . Scattered beam S is directed to photodetector 20 through lens L 7 . Lens L 7  focuses scattered beam S to the surface of photodetector 20. In a similar manner to FIGS. 1 through 4, the superimposed scattered beam S and reference beam R are used to calculate the size of the defect or particle on surface 18 of wafer 16. A computer 22 is used to perform the calculation. 
     The system as shown in FIG. 5 demonstrates that the entire optical system may easily be contained within a housing 44 and moved relative to wafer 16 rather than moving the wafer. Wafer 16 may also be moved relative to a stationary housing 44. 
     The relative motion for producing the Doppler shift in the scattered light, which is necessary for the heterodyne effect, can be accomplished in a variety of ways. A simple linear motion of either the wafer 16 or the housing 44 may be used. An oscillatory motion using either a sinusoidal or a ramp motion is also possible. The oscillatory motion can be simplified by using an image sampling arrangement, wherein several data samples need be taken at each scan position of wafer 16. For example, four images shifted at a fraction of a wavelength such as 1/4 of a wavelength may be obtained. If each of these images are called D1, D2, D3 and D4 respectively, the equivalent to the above filtering may be obtained. The steps to determine the defect signal include calculating the difference for each pair of data. That is, Dc=D1-D3 and Ds=D2-D4. The defect signal is the square root of the sum of the square of Dc and Ds. The intensity of the defect signal is proportional to the volume of the defect. 
     Referring now to FIG. 6, one type of scanning stage 17 such as that shown in FIG. 5 is shown that is capable of moving wafer 16 fractions of a wavelength. Scanning stage 17 becomes a wafer mounting/transport mechanism having a driver 24 securely attached to a baseplate 51, a flexible diaphragm 53, and a mounting plate 56. Baseplate 51 is rigidly attached at contact point 54 to the flexible diaphragm 53. Mounting plate 56 is attached to flexible diaphragm 53. Wafer 16 is held flat against the surface of mounting plate 56 by a vacuum that is maintained in the open volume 55. Tension rods 52 are used to clamp flexible diaphragm 53 between the mounting plate 56 and baseplate 51. 
     Driver 24 may consist of either a piezoelectric or magnetostrictive drive system. When activated, driver 24 moves the center of the flexible diaphragm 53 in small incremental steps. This motion is reduced by the lever arm distance to a point of contact 57. This results in an extremely small, well-controlled motion of the wafer surface 18. Thus, as driver 24 moves upward, diaphragm 53 pivots at 54 which in turn moves contact point 57 upward. Thus as the the driver arm moves upward, point 57 only moves upward a predetermined fraction of the driver arm movement. Using this mechanism, the wafer can be easily translated in small, incremental steps of much less than the wavelength of light. 
     The method for determining the detection signal may be performed as described above with respect to FIG. 5. In the simplest form, however, a signal defect may be obtained from the configurations of FIGS. 1, 2 and 3 by a simple narrow bandpass filter. This, however, is only effective if no phase jitter is in the detected interference beam, i.e., that is between the scattered beam and the reference beam. 
     Each of the systems of FIGS. 1 through 5 use similar calculations to determine the presence of a defect. Equation (1) describes the optical power detected I by a single element of an array detector for a given wavelength, λ. 
     
         I=|a+b(λ,t)exp (i4πvt/λ)+θ(t)!|.sup.2 
    
     
         I≈|a|.sup.2 +2|a||b(λ,t)|cos (4πvt/.lambda.)+θ(t)!                                           Equation (1) 
    
     where a is the amplitude of the reference beam (R), and b(λ,t) is the amplitude of the scattered beam which is modified by (1) the intensity distribution of the illumination beam (I) on the wafer surface and (2) the coherence length of the light. In the case of a laser source, the coherence length is such that it does not affect the function b(λ,t). The scanning speed of the particle with respect to the beam is v. That is, v=v 0  cosα where v 0  is the actual speed of the moving wafer surface and &#34;α&#34; is the complement of the incident angle of the grazing beam. θ(t) is &#34;phase jitter&#34; which may be introduced by any &#34;wobbling&#34; motion of the wafer, as explained below. 
     In a normal scatterometer, one must detect the light power signal of the form |b(λ,t)| 2 , which becomes smaller than photodetector noise if the particle size is less than 80 nm. The present invention uses a heterodyne detection method. The signal is &#34;amplified&#34; to 2|a||b(λ,t)| which is much greater than the conventional |b(λ,t)| 2 . The DC component and noise are filtered out by a narrow band-pass filter which will pass only the AC component. The center frequency of the band-pass filter is determined from the wavelength of light, the wafer scanning speed, and the angle of incidence. 
     Mechanical scanning of the wafer may introduce a &#34;jittering&#34; motion of the wafer surface with respect to the reference beam. This &#34;jittering&#34; introduces a &#34;phase jittering&#34;, represented by θ(t), into the AC component of Equation 1 above. A narrow band-pass filter cannot be used in extracting the desired AC signal when the phase of the signal &#34;jitters&#34;. For example, if a very narrow band-pass filter were used for such a signal, part of the signal would spread beyond the bandwidth and consequently the signal would have less signal-to-noise. If the bandwidth of the filter were widened to include the &#34;jittering signal&#34; then the total noise increases due to the wider bandwidth. This also reduces the signal-to-noise ratio. One way to overcome this problem is to obtain a sinusoidal wave (or rectangular wave) with &#34;phase jitter&#34; that has a frequency identical to that of the AC signal to be detected. The sinusoidal wave may then be used to extract the desired AC signal in a common filtering operation. That is, the sinusoidal wave may be multiplied by the AC signal to be detected and then the product is integrated. 
     Referring now to FIG. 7, a Michelson interferometer may be used to determine the sinusoidal wave representation of any &#34;phase jitter&#34; due to the motion of scanning stage 17. A driver 24 is used to move scanning stage 17 having a wafer mounted thereto. Light is directed to a beamsplitter B 5  by a source 46. The light source in this instance is a laser. Beamsplitter B 5  directs light to a retro-reflector 48 on the surface of scanning stage 17. Beamsplitter B 5  also directs light to another retro-reflector 50 a predetermined distance from beamsplitter B 5 . Retro-reflectors 48 and 50 reflect light to beamsplitter B 5  which directs the superimposed beams to photodetector 20. The sinusoidal wave measured by the combination of the reflected light from the scanning stage 17 and from reflector 50 may be used as a filter to determine the AC component with the best signal-to-noise ratio to measure the AC interference signal generated in the scatterometers of FIGS. 1 through 5. 
     With the configuration of FIG. 7, the detected optical power fluctuates sinusoidally as the scanning stage, to which reflector 48 is mounted, moves. The frequency of the sinusoidal wave (ω r ) is given as 
     
         ω.sub.r =(2π/λ.sub.r)2V                    (2) 
    
     The frequency of the AC component (ω s ) which is caused by the Doppler frequency shift (due to the particle motion) of the signal is 
     
         ω.sub.s =(2π/λ.sub.s)Vcos(α)         (3) 
    
     Since the wavelength of the Michelson interferometer (λ r ), the wavelength of the particle illumination beam (λ s ) and the illumination angle (α) are design parameters, these parameters can be selected to obtain a desired relationship between the two frequencies. For example, by choosing λ s  =λ r  and α=120°, one can establish a relationship ω r  =4 ω s . With this frequency relation, the AC component with the highest S/N can be determined by using the sinusoidal wave as a &#34;matched&#34; filter. Note that the mechanical &#34;jittering&#34; caused by the lack of smoothness of the velocity V is common for both waves. Therefore, by using the ω r  wave as a reference signal for the filtering, one can eliminate the effect of any phase jitter in the signal. From the ω r  wave, the desired cosine and sine waves (the quadratic wave) for the filtering can be generated. Then by conventional quadratic filtering the desired defect signal may be obtained. That is, the defect signal may be obtained by multiplying the cosine component and the sine component of the reference signal with the detected signal, integrating each component and taking the square-root of a squared sum of each component. 
     By choosing the proper incident angle β and wavelength of beam D in FIG. 7 with respect to the incident angle α and the wavelength of beam D in FIG. 4, the reference signal can be obtained. The frequency of the reference signal may be different from that of the detected signal but is related. For example, the parameters may be chosen so that the frequency of the reference signal is two times higher than that of the detected signal. The reference signal contains both cosine and sine components so that a single detector is sufficient to obtain both components. 
     Referring now to FIG. 8, optical power fluctuation may be determined by a configuration similar to that of FIG. 4. Driver 24 drives wafer 16 in a direction parallel to the longitudinal axis 30 of wafer. A grating 36 may be employed on the edge of scanning stage 17. Grating 36, when illuminated at a certain angle, diffracts a beam toward photodetector 20. The amount needed for the angle, β, between detection beam D and photodetector 20 is determined by the relative position of beamsplitter B 3  and photodetector 20. As in FIG. 6 the phase &#34;jitter&#34;can be determined in a similar manner. The frequency (ω s ) of the wave derived from this interferometer is 
     
         ω.sub.s =(2π/λ.sub.s)Vcos(β)          (4) 
    
     While the best mode for carrying out the present invention has been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention as defined by the following claims.