Abstract:
A system and method for high speed and precision measurement of the distance between at least two near contact surfaces using heterodyne interferometry is disclosed. One of the surfaces is an optically transparent element and the other surface is a substantially non-transparent element. A laser source produces an output having two superimposed orthogonally polarized beams having S and P polarization, with a frequency difference between them. The polarized beams are split into measurement and reference beams without altering the characteristics of the polarized beams. The reference beams are caused to interfere, and a reference photo detector detects the reference beams and provides a reference signal. The measurement beam strikes the object of interest at an oblique angle after passing through a glass plate having a polarization coating on the bottom surface close to the object of interest. The oblique angle is such that the S polarization of the incident beam is reflected from the bottom surface of the polarization coated glass plate and the P polarization refracts through the glass plate. The P polarization reflects from the substantially non-transparent object of interest and refracts to the glass plate. The reflected S and P polarization beams from the bottom surface of the glass plate and the surface of the object are made to interfere. A measurement photo detector detects the measurement beams and provides a measurement signal. The distance between the bottom surface of the glass disk and the object surface based on the phase deference between the measurement and reference signals is determined from the measurement and reference photo detectors.

Description:
FIELD OF THE INVENTION 
     This present invention relates to high-speed, high precision measurement of the distance between two near contact surfaces one of which is an optical transparent element. In particular the invention relates to an apparatus and method for performing this measurement using a heterodyne interferometer. Moreover, this invention relates to an optical interferometer for measuring the flying height of a magnetic head. 
     BACKGROUND OF THE INVENTION 
     In magnetic data storage, there is a requirement for measuring the flying height of the slider assembly which is in near contact with a high-speed rotating disk in order to optimize the performance of the slider assembly. The flying height is the distance between the magnetic head pole and the surface of the rotating magnetic disk. Recently, the rapid increase in recording density has caused a continuous reduction in the flying height. The flying height is generally less than 250 nm depending on the design of the slider. The trend is toward an ultra-low flying height which is less than 25 nm. 
     Optical Flying Height Testing (OFHT) is the most popular testing technique in the field. OFHT are almost invariably based on interferometry. There are two known methods. The first method is measuring the flying height from a real magnetic disk through the backside of the slider. The second method is measuring the disk-slider spacing directly, using a transparent disk to replace the magnetic disk. The first method is not popular due to the fact that the accuracy of the measurement depends on the uniformity of the thickness of the slider. Moreover, the backside of the slider is not accessible on most production slider assemblies. Currently, the second method is more widely used in commercial applications. 
     Until now, there were three types of interferometry developed for flying height testing. The three types include white light interferometry, three-wavelength interferometry, and monochromatic interferometry. With a white light interferometer, a flying height of 700 to 300 nm can be detected accurately. Three-wavelength interferometer is an extension of white light interferometer. A flying height of less than 250 mm can be measured with a three-wavelength interferometer with acceptable errors. Both a white light interferometer and a three-wavelength interferometer are subject to multi-reflections in the disk-slider air bearing, which produces errors. When the flying height is reduced to 50 nm, the error caused by multi-reflections cannot be neglected. Therefore, the polarization interferometer was developed to measure ultra-low flying height. 
     In U.S. Pat. No. 4,606,638, Sommargren proposed a polarization phase modulated Fizeau interferometer in which the reference surface is the front surface of a plate polarizer. The modulated interference pattern is detected by a CCD array. The signal provides the absolute distance between the reference surface and the flying slider. Although this technique was an advance in providing the flying height, pitch angle and roll angle in a single measurement, it is not practicable because the measurement accuracy is dependent on the flatness of the glass disk surface which acts as a plate polarizer. 
     In U.S. Pat. No. 5,218,424, Sommargren proposed a polarization interferometer. In a polarization interferometer, a coherent, single wavelength, linearly polarized beam passes through a phase shifter which varies the relative phase of the orthogonal polarization components of the beam by 0, π/2, π, 3π/2. Then the beam is directed to the glass disk at a Brewster&#39;s angle. The polarized beam P will pass through the glass disk and strike the slider without being reflected from glass disk surface. While the S polarized beam will be reflected by each glass-air interface. Therefore, the S polarization beam can be used as the reference beam and the phase difference between S and P polarization beams carries the flying height information. The phase change which is caused by the optical path difference between S and P polarization beams, therefore, can be determined by a four-step algorithm. By using the polarized laser beam, the error caused by multi-reflection is completely eliminated. Thus, the apparatus is capable of measuring extremely small gaps. Despite the advantage, this technique has some significant limitations, including the use of a very expensive complicated, high speed phase modulator. Moreover, the data processing rate is quite slow, around 15 HZ, which is not suitable for commercial applications. 
     In U.S. Pat. No. 5,660,441, Groot proposed a quad-beam polarization interferometer. Three of the four beams are S polarization, the other one is a P polarization. These four beams are parallel to each other and are directed to the glass disk at Brewster&#39;s angle. Except for the P polarized beam passing through glass disk without reflection, all the beams are reflected back from the surface of the glass disk. After being reflected from the slider ABS, the P polarized beam again passes through the glass disk. The two reflected S polarized beams are made to interfere thereby providing a reference. The P polarized beam is made to interfere with the remaining S polarized beam. Thus, the interfered beam contains the flying height information. The spacing between disk surface and slider ABS can be detected by comparing the two interfered beams. The main advantage of this technique is that it eliminates the phase modulator which was used by all the prior-art techniques. Therefore, the signal-processing rate can be very fast, and the error introduced by the phase shifting can be eliminated. This apparatus is capable of measuring ultra-low flying height with the required accuracy. In U.S. Pat. No. 5,751,427, Groot further improved his polarization interferometer by using a single linearly polarized beam. 
     All these prior art techniques are based on homodyne interferometry, using phase measurement to find out the flying height by detecting the optical path difference between reference beam and measurement beam. One phase measurement method, which is generally employed in all these prior art techniques, is the four-step algorithm. This method is subject to the intensity variation of the laser beam, which may be caused by an unstable laser source, birefringence of the high-speed rotating glass disk, disturbances from the surroundings, etc. Moreover, the reflection of the slider ABS will introduce a phase change in the measurement beam which strikes it. In the prior art technique, this phase change was assumed to be π. These will cause flying height measurement errors as large as 10 nm. It is acceptable when the flying height is above 250 nm. When flying height is reduced to less than 50 nm, it is necessary to precisely measure the phase change introduced by the ABS reflection. Usually, the correction of the phase change occurring at the ABS will be done by a measurement instrument independent of the flying height tester which is called an ellipsometer. The correction of the ABS reflection phase change is substantial disadvantage of all the prior art techniques. Although Groot integrated the ABS reflection phase change correction with the flying height testing into one instrument in his U.S. Pat. No. 5,757,427, the correction is still a separate measurement step that has to be done before the gap measurement. 
     SUMMARY OF THE INVENTION 
     Thus, the objective of the present invention is to provide an apparatus and method capable of measuring the flying height by removing various uncertainties in an OFHT using heterodyne interferometry without requiring any extra measurement other than flying height testing. The embodiment of the present invention has a laser source emitting two orthogonally polarized beams of different frequency by using a Zeeman laser source or by using an acousto optic modulator and recombining the beam by using optical components. The beam is sally filtered and expanded in diameter by using a spatial filter, by using a pinhole, or with optical fibers. The beam is made to strike the object at a single point or to scan on the object by using acousto optic deflector. The embodiment of the present invention has means to obtain parallel scanning beam and to focus the scanning beam on a single plane. The beam is divided in to two, one for the reference path and the other for the measurement path. The measurement beam strikes the glass disk at an oblique angle such that the P polarized beam refracts through the glass disk and is reflected from the surface of the object. The S polarization beam is reflected from the bottom surface of the glass disk. The reflected S and P polarized beams from the bottom surface of the disk and the object surface are made to interfere on the measurement photo detector. The reference beams are made to interfere at the reference photo detector. Upon comparison of the phase change between the reference and the measurement signal, the displacement and the distance between the disk surface and the object can be obtained using a phase meter and adopting heterodyne interfrometric principles. The reference signal can also be obtained by reflection from the surface of the object or the bottom surface of the glass disk by directing the beam at a particular oblique angle. 
     The second embodiment of the present invention is a means to measure an object surface smaller than the focused spot size of the beam by using a diaphragm before the measurement photo detector to allow a smaller portion of the interfered beam to pass through it. The smaller the diameters of the diaphragm or slot results in a smaller object being measured in the focused spot size. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a drawing showing a preferred embodiment of the invention setup to measure the distance of a flat object surface with respect to the surface of a rotating transparent disk using a stationary measurement beam and a reference beam taken directly from light source. 
     FIGS. 2A and 2B are drawings showing setups to produce two orthogonally polarized beams with a frequency difference between them and to superimpose the two beams. 
     FIGS. 3A and 3B are drawings showing the optical setup and working principles of a spatial filter. 
     FIG. 4 is a drawing illustrating the working principles of the preferred embodiment of the invention using a glass disk having polarization coating on its side closest to the object surface. 
     FIG. 5 is a drawing showing a preferred embodiment of the invention setup to measure the distance of a flat object surface with respect to the surface of a rotating transparent disk using a measurement beam and a reference beam which strike the bottom surface of the disk or the object surface at the same point but at a different angle. 
     FIG. 6 is a drawing showing the working principles of the preferred embodiment of the invention when the measurement and reference beams strike the bottom surface of the glass disk or the object surface at the same point but at a different angle. 
     FIG. 7 is a drawing showing a preferred embodiment of the invention setup to measure the distance of a flat object surface with respect to the surface of a rotating transparent disk using a scanning measurement beam and a reference beam taken directly from the light source. 
     FIG. 8 is a drawing showing a preferred embodiment of the invention setup to measure the distance of a flat object surface with respect to the surface of a rotating transparent disk using a scanning measurement beam and scanning reference beam which strike the bottom surface of the glass disk or the object surface at the same point but at a different angle. 
     FIGS. 9A and 9B are drawings illustrating the working principles for obtaining a parallel scanning beam by using the combination of an acousto optic deflector, and focusing optics or scanning lens. 
     FIG. 10 is a drawing illustrating the relative distance between optical components in the system. 
     FIG. 11 is a drawing showing a preferred embodiment of the invention setup to measure the distance of a flat object surface with respect to the surface of a rotating transparent disk using a collimated and stationary micro-sized measurement beam and a micro-sized reference beam taken directly from a light source. 
     FIG. 12 is a drawing showing a preferred embodiment of the invention setup to measure the distance of a flat object surface with respect to the surface of a rotating transparent disk using a collimated and stationary micro-sized measurement beam and a micro-sized reference beam which strike the bottom surface of the glass disk or the object surface at the same point but at a different angle. 
     FIG. 13 is a drawing showing a preferred embodiment of the invention setup to measure the distance of a flat object surface with respect to the surface of a rotating transparent disk using a collimated and stationary micro-sized scanning measurement beam and a micro-sized reference beam taken directly from the light source. 
     FIG. 14 is a drawing showing a preferred embodiment of the invention setup to measure the distance of a flat object surface with respect to the surface of a rotating transparent disk using a collimated and scanning micro-sized measurement beam and a micro-sized reference beam, which strike the bottom surface of the glass disk or the object surface at the same point but at a different angle. 
     FIG. 15 is a drawing showing the top view of the object with reference to the rotating glass disc. 
     FIG. 16 is a drawing showing the method to measure the vibration of a smaller portion of the focused beam on the object by using a diaphragm or slit. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a preferred embodiment of the invention employing an interferometry setup for measuring the distance of an object  28  surface that is nearly in contact with a surface  27  of a rotating transparent disk  25 . The apparatus in this embodiment is preferably suited to the prediction of the aerodynamic flight characteristics of a conventional slider  28  over the surface of a rotating magnetic storage medium. A spindle drives disk  25 . The size of the gap may be determined as a function of the speed of the rotating disk  25 . The position of the test object  28  with respect to disk  25  may be further clarified by reference to FIG. 15, which shows top a view of the disk  25 , indicating the position of the object  28 . 
     Referring to FIG. 1, the laser beam is preferably obtained from the source  1 , which provides a smooth laser beam  2  which contains two orthogonally polarized laser beams having a frequency difference between them. Source  1  can be of the type shown in FIG. 2A or  2 B, which will be explained in later sections. Non-polarization beam splitter  6  will split the beam  2  into two parts. The beam  15 , the reference beam, passes through focusing lens  21  and interferes on passing through a polarizer or analyzer  22 , falling on photodetector  23 . The interferometric signal, reference signal  41  is sent into phasemeter  24 . This optical path is used as a reference path for the flying height tester. The other optical path, which passes through beam splitter  6 , is used as a measurement path. Focusing lens  7  focuses the wavefront  8  on to the air bearing surface (ABS) of the slider  28 . 
     FIG. 4 illustrates the working principles of the optical element  25 . Optical element  25  is a transparent disk, which transmits wavefront  8 . Surfaces  26  and  27  are plano and parallel. Surface  26  has an antireflection coating so that essentially all of piano wavefront  8  is refracted onto disk  25  and essentially none of wavefront  8  is reflected by the surface  26 . Surface  27  of disk  25  is coated so that it is a surface polarizer. Therefore, when wavefront  8  is incident on the disk surface at an oblique angle θ, the surface  27  transmits the P polarization component and reflects the S polarization component of wavefront  8 . Thus, surface  27  acts as the reference surface for the flying height tester. The S polarization wavefront reflected back by surface  27  passes through transparent disk  25  and is refracted by surface  26 . The P polarization component of wavefront  8  is refracted by surface  27  and falls on the air bearing surface (ABS) of the slider  28 . It is then reflected back from the ABS, goes through transparent disk  25  and is refracted by surface  26 . 
     Referring to FIG. 5 since the disk-slider spacing is very small, the S and P polarization component of wavefront  8  will merge again when they are refracted by surface  27 , forming wavefront  9 . Focusing lens  10  collimates the beam  9  into beam  11 . Focusing lens  12  focuses the beam  11  into the photodetector  14 , after passing through polarizer or analyzer  13 , where the beam interferes. The interferometeric signal  40 , the measurement signal  41  is sent into phasemeter  24 . The output signal of phasemeter  24  represents the phase difference between reference beam  15  and measurement beam  9 . 
     In order to get the absolute flying height of the slider  28 , a zero flying height has to be defined before the measurement. First, slider  28  is loaded, and made to contact with surface  27 . Then, the transparent disk  25  starts rotating. At the same time, the phasemeter  24  starts counting the phase difference between signal  41  and  40 . The phase difference obtained when the disk rotates at a very low speed gives the zero flying height. The phase difference obtained when the disk rotates at a full speed gives the flying height. From the phase change, the displacement of the slider can be derived from formula:          Δ                 φ     =       (       2                 π     λ     )                   2      h                 cos                 θ                            
     where h is the spacing between disk surface  27  and ABS of slider  28 . θ is the incident angle of beam  8 , and λ is the wavelength of the laser beam. 
     Referring to FIG. 2A, the linearly polarized beam  51  having a frequency of f o  from the laser source  50  will expand in diameter as well as be smoothened after passing through a spatial filter  52 . The beam  53  passes through acousto-optical modulator  54 . When beam  53  is incident on the acousto optic modulator  54  at Bragg&#39;s angle, two orthogonally polarized beams, zero order beam  56  and the first order beam  55  having a frequency difference between them, are obtained at the output aperture of  54 . In FIG. 2A, beam  55  has S polarization while beam  56  has P polarization. The frequency of beam  55  is modulated as f o  +f c , where f c  is the center frequency of acousto-optical modulator  54  and f o  is the frequency of the laser beam  53 . The frequency of beam  56  will remain the same as inital beam  53 . Since beams  55  and  56  have orthogonal polarization, when they pass through a polarized beam splitter  57 , beam  55  will be deflected, and beam  56  will pass through. The beam  56  is then directed into another polarized beam splitter  60  and passes through it. The deflected the beam  55  will be directed into polarized beam splitter  60  by mirror  58  and  59 . The beam splitter  60  and the deflecting mirror  58  or  59  are aligned such that the beams  55  and  56  merge together, forming beam  2 . After obtaining beam  2 , the beam can also be filtered. 
     Another preferred embodiment of source  1  is shown in FIG.  2 B. Here the laser source  50  is a Zeeman laser generator. A Zeemam laser generator provides a linearly polarized beam  51 , where there is a frequency difference between the two orthogonally polarized beams. Beam  51  is led into spatial filter  52 . At the output of  52 , a laser beam is obtained having the same characteristics of beam  2 . 
     FIG. 3A is the typical structure of a spatial filter. Objective lens  60  focuses the laser beam  51  into a tiny spot in the micro meter range. Pinhole  62  acts as a filter, filtering the spatial noise contained in the laser beam. After passing through the pinhole  62 , the scattered beam  61  is smoothened into beam  63 . Thus, the diverged beam  63  is collimated by a collimating lens  64 . The objective lens and the pinhole are carefully selected so that a smooth beam can be obtained. Once the objective lens has been decided, the diameter of the laser beam  2  is determined by the focal length of collimating lens  64 . FIG. 3B illustrates the working principles of the spatial filter. A single mode fibre can be used to replace the pinhole. The fibre has the same function as pinhole, and better filtering effectiveness can be obtained. 
     The embodiment shown in FIG. 1, takes the reference optical path directly from source  1 . Another preferred embodiment is as shown in FIG. 5, where the reference beam is made to pass through an optical path next to that of the measuring beam. As the reference beam and measurement beams travel nearly the same optical path, this embodiment can provide improved measurement accuracy. Non-polarization beam splitter  6  splits the beam  2  into two parts. Wavefront  8  is incident on the disk surface at oblique angle θ, and the surface  27  transmits the P polarization component and reflects the S polarization component of wavefront  8 . Thus, the surface  27  acts as the reference surface for the flying height test. The S polarization wavefront reflected back by the surface  27  passes through transparent disk  25  and is refracted by the surface  26 . The P polarization component of wavefront  8  is refracted by the surface  27  and falls on the air bearing surface (ABS) of the slider  28 . It is then reflected back from the ABS, passes through the transparent disk  25  and is refracted by the surface  26 . As the disk-slider spacing is very small, the S and P polarization components of wavefront  8  will merge again when they are refracted by surface  27 , thereby forming wavefront  9 . Focusing lens  10  collimates the beam  9  into the beam  11 . Focusing lens  12  is used to focus the beam  11  on to the photodetector  14 , after passing through the polarizer or analyzer  13 . The interferometeric signal  40 , the measurement signal, is sent into phasemeter  24 . Mirror  42  directs the beam  15  into transparent disk  25 , striking on the disk surface  26  at an angle φ. Angle φ is slightly larger or less than oblique angle θ. Therefore, beam  17  will be completely reflected back at disk surface  27  or at the object surface  28 , instead of being split into two polarization beams as what happens to the beam  8  which is incident to disk surface  26  at oblique angle θ. Focusing lens  16  having the same specification as the lens  7  focuses beam S into a tiny laser spot on the surface  27  or  28 . The alignment of the measurement and reference beams  8  and  17 , respectively, are such that they focus on the same point on the surface  27  or  28 . 
     FIG. 6 shows the relationship of the two laser beams and the relative positon of the two laser spots focused from beam  17  and beam  8 . After being reflected back completely from surface  27  or  28 , beam  17  is refracted by disk surface  26  into beam  18 . Focusing lens  19  collimates the beam  18 . The lens  21  focuses it into the photodetector  23  after passing through the polarizer or analyser  22 , where the two orthogonal polarized components of beam  18  interfere. Interferometric signal  41  is sent into phasemeter  24 , serving as the reference signal. As reference the beam passes along an optical path very close to that of the measurement beam, the errors introduced by the high-speed rotating disk, disturbance from external environment, etc., can be compensated effectively. 
     Embodiments shown in FIGS. 1 and 5, can measure the spacing at a in single position of the slider ABS each time. If the spacing of more than one point is necessary, the slider controller  30  drives the slider  28  accordingly. The measurement step described above has to be repeated at each measurement point. 
     Other preferred embodiments of this invention are illustrated in FIGS. 7 and 8, which provides a beam scanning function that can access any point on the ABS of slider  28  and the rotating glass disk. The optical system and working process of embodiment shown in FIG. 7 is similar to that of the embodiment shown in FIG. 1, except that the polarized beam splitter is replaced by an acousto-optical deflector  3 , which can scan the beam in both X and Y axes, and the focusing lens  7  and  10  are replaced by scanning lens or achromatic lens  57  and  60 . The crystal of the acousto optic deflector  3  is made such that the input beam to the acousto optic deflector includes both S and P polarization and the output beam includes zero order beam  4  and first order beam scanning beam  5  which both have S and P polarization states. The first order beam  5  from the acousto optic deflector  3  is used as the scanning measurement beam. The zero order beam  4  has the same characteristics as the beam  2 . The zero order beam  4  of deflector  3  is directed into focusing lens  21 . Beam  4  then passes through polarizer  22  so that the beam interferes and the interference signal  41  is captured by the photo detector  23 . The interferometric signal  41 , the reference signal detected by photodetector  23 , is sent into phasemeter  24 . First order beam  5  is focused by scanning lens or achromatic lens  57  on to the object surface  28 . Beam  5  strikes the disk surface  26  at oblique angle θ. The incident angle of beam  58  to disk surface  26  is maintained as oblique angle θ during the scanning. 
     FIG. 9A shows the working principles of the scanning lens. The deflector, scanning lens and the measured target have to be placed according to the forward working distance (FWD) and backward working distance (BWD) of the scanning lens. The scanning lens will make all the scanning beams parallel to each other and focus the scanning beam into a tiny spot. Moreover, the scanning lens makes all the scanning beams focus onto a flat plane, which maintains the measuring spot in focus during the scanning measurement. In this embodiment, the deflector  3 , scanning lens  57  and the transparent disk have to be placed according to the forward working distance(FWD) and backward working distance (BWD) of scanning lens  57 , as shown in FIG. 10. L 1  =FWD of scanning lens  57 . L 2  +L 3  =BWD of scanning lens  57 . Beam  58  is refracted by disk surface  26 , changed into beam  59 . Scanning lens  60  is used to change the diverging beam  59  into collimated beam  11 . All the scanning beams converge at a single point, by placing disk surface  27 , scanning lens  60  and focusing lens  20  according to the FWD and BWD of the scanning lens  60 . As shown in FIG. 10, L 4  +L 5  =BWD of scanning lens  60 , and L 6  =FWD of scanning lens  60 . If an achromatic or focusing lens is used in place of scanning lens, the FWD and the BWD will be equal to the focal length of the achromatic or focusing lens. If focusing or achromatic lens are used, the position accuracy may not be as accurate as scanning lens. Focusing lens  12  focuses the collimated beam  11  into one small spot falling on to detector  14 . Before detector  14 , a polarizer causes the two orthogonal polarized components of beam  11  to interfere. Interferometerice signal  40 , the measurement signal, from detector  14  is sent to phasemeter  24 . 
     Another preferred embodiment of the scanning flying height tester is shown in FIG.  8 . The optical system and working process of FIG. 8 is similar to that of embodiment described in FIG. 7, except that the reference beam is made to scan and pass through an optical path very close to that of the scanning measuring beam. The first order beam  5  from acoustical deflector  3  follows the same optical system as that of the embodiment illustrated in FIG.  7 . Zero order beam  4  of the deflector  3  is blocked by a beam blocking mechanism  100 . The reference beam is obtained from the scanning first order beam  5  by using a non-polarizing beam splitter  101  and directing the beam into lens  16  with mirror  42 . This beam passes through an optical system the same as that of beam  17  described in the embodiment shown in FIG.  5 . 
     FIG. 11 to FIG. 14 show preferred embodiments of the present invention using micro-collimated laser beams. In these embodiments, the beam  102  coming out of source  1  contains two orthogonally polarized laser beams with a frequency difference between them. Moreover, the diameter of beam  102  is in the range of micrometers. Source  1  of the present embodiments are the same as that described in FIG.  2 . FIG. 3A shows the layout of a spatial filter. In these embodiment a micro-lens is used as the collimating lens  64 . Therefore, a collimated beam with diameter in the range of micrometers can be obtained at the output of spatial filter or fiber optics by using a micro collimating lens. 
     Referring to FIG. 11, beam  102  is split into two parts, beam  15  and beam  8  by a non-polarization beam splitter  6 . The two orthogonal polarized components of beam  15  are made to interfere by passing through polarizer  22 . The interferometric signal  41  detected by photodetector  23  is sent to phasemeter  24 . Signal  41  is used as a reference signal. Beam  8  is incident upon the disk surface  26  at oblique angle. The S and P polarization components of the beam  8  are split at disk surface  27 . The P polarization component is reflected back by the ABS of slider and S polarization component of beam  8  is reflected back by disk surface  27 . The two reflected polarization beams merge when they are refracted by disk surface  26  into beam  9 , since the measured spacing is very small. The two orthogonally polarized components of beam  9  are made to interfere by passing through a polarizer  13 . The interferometric signal  40  detected by photodetector  14  is sent into phasemeter  24 . 
     Referring to FIG. 12, beam  102  is split into two parts beam  15  and beam  8  by non-polarization beam splitter  6 . Mirror  42  directs the beam  15  into the transparent disk  25 , striking the disk surface  26  at an angle φ. Angle φ is slightly larger or less than the oblique angle θ. Therefore, beam  17  will be completely reflected back at disk surface  27  or at the slider surface  28 , instead of being split into two polarization beams as happens to the beam  8  which is incident on disk surface  26  at oblique angle θ. Beam  18  is directed to the photodetector  23  and is made to interfere by passing through polarizer or analyzer  22 . The θ interferometric signal  41 , the reference signal detected by photodetector  23 , is sent to phasemeter  24 . Beam  8  is incident on the disk surface  26  at oblique angle θ. The S and P polarization components of the beam  8  are split at disk surface  27 . The P polarization component is reflected back by the ABS of slider and the S polarization component of beam  8  is reflected back by disk surface  27 . The two reflected polarization beams merge when they are refracted by disk surface  26  into beam  9 , since the measured spacing is very small. The two orthogonally polarized components of beam  9  are made to interfere by passing through a polarizer  13 . The interferometric signal  40 , the measurement signal detected by photodetector  14 , is sent into phasemeter  24 . 
     Referring to FIG. 13, beam  102  passes through collimating lens  32  and is incident on the acousto-optical deflectors. The zero order beam  4  of deflector  3  is deflected by mirror  42  and it is used as reference beam. The two orthogonal polarized components of beam  4  are made to interfere by passing through polarizer  22 . The interfrometric signal  41  detected by photodetector  23  is sent to phasemeter  24 . Signal  41  is used as reference signal. Referring to FIG. 9B the acousto optic deflector  3  is placed in-between two focusing or collimating lenses such that the distance between the two focusing lenses  32  and  33  is equal to the sum of the focal length of the lens  32  and  33 . The scan field can be increased by either placing the acousto optic deflector  3  close to the lens  32 , or by using large focal lengths for lenses  32  and  33 . This optical system will produce collimated and parallel scanning beams  8  in X and Y directions. Beam  8  is incident on the disk surface  26  at oblique angle θ. The S and P polarization components of the beam  8  are split at disk surface  27 . The P polarization component is reflected back by the ABS of the slider, and the S polarization component of beam  8  is reflected back by disk surface  27 . The two reflected polarization beam remix, when they are refracted by disk surface  26  into beam  9 , since the measured spacing is very small. The two orthogonally polarized components of beam  9  are made to interfere by passing through a polarizer  13 . Lens  12  will focus the scanning beam  9  into the photo-detector  14 . The interferometric signal  40 , measurement signal detected by photodetector  14 , is sent into phasemeter  24 . 
     Referring to FIG. 14, beam  102  pass through collimating lens  32  and is incident on acousto-optical deflector  3 . Blocking means  100  blocks the zero order beam  4  from the deflector  3 . The first order beam  5  passes through collimating lens  33 . Collimating lens  33  and collimating lens  32  have the same focal length. Lenses  32  and  33  and deflector  3  are placed according to the focal length of the lenses  32 ,  33 . The combination of collimating lens  32 , acousto-optical deflector  3  and collimating lens  33  provides a parallel scanning beam  5 , as shown in FIG.  9 B. Beam  5  passes through a non-polarizing beam splitter  101 , which divides the scanning beam  5  into two beams, beam  8  and beam  15 , with either equal or different intensity. Beam  8  is incident an the disk surface  26  at oblique angle θ. The S and P polarization components of the beam  8  are split at disk surface  27 . The P polarization component is reflected back by the ABS of slider and S polarization component of beam  8  is reflected back by disk surface  27 . The two reflected polarization beams merge when they are refracted by disk surface  26  into beam  9 , since the measured spacing is very small. Lens  12  focuses the scanning beam  9  into the photodetector  14 . The two orthogonally polarized components of beam  9  are made to interfere by passing through a polarize or analyzer  13 . The interferometric signal  40 , measurement signal detected by the photodetector  14 , is sent into phasemeter  24 . The zero order beam  4  is blocked by beam blocking mechanism  100 . Mirror  42  directs the reference beam  15  into transparent disk  25 , striking the disk surface  26  at an angle φ (beam  17 ). Angle φ is slightly larger or less than the oblique angle θ. Therefore, beam  17  will be completely reflected back at disk surface  27  or the surface  28  of the object, instead of being split into two polarization beams as happens to the beam  8  which is incident on disk surface  26  at oblique angle θ. Lens  20  focuses the scanning beam  9  into the photo-detector  23 . The two orthogonally polarized components of beam  18  are made to interfere by passing through polarizer or analyzer  22 . The interferometric signal  41 , the reference signal detected by photodetector  23 , is sent to phasemeter  24 . 
     Second Embodiment of the Present Invention 
     The second embodiment of the present invention increases the resolution of the system without reducing the spot size of the focused beam on the object. Referring to FIG. 16, the measurement beam  11  passes through a diaphragm or slot  200 , which allows only a certain portion of the beam  11  to pass through and the rest of the beam is blocked. A small portion of the measurement beam  201  is focused on the measurement photo detector  14  by the focusing lens  12  after passing through an analyzer or polarizer  13 . A smaller size for slot  200  will result in higher resolution for the system. By this method, a portion of the object smaller than the focal spot size of the beam can be measured. For example, if the focal spot size on the object is 10 micrometer and the collimated beam size of the beam  11  is 10 mm, the photo detector measures the average of the entire focused spot on the object. In order to measure the 1-micrometer area of the object by using 10 micrometer focused spot size, only 1 mm of the collimated beam  11  will pass through the diaphragm, and the rest of the beam will be blocked. In this case, the photodetector provides the average of 1-micro meter on the object, which corresponds to the 1 mm of the beam  11  passing through the diaphragm  200 . With this method, the resolution of the object area to be measured can be increased without reducing the focal spot size of the beam on the object.