Abstract:
A method and apparatus for using a dual-beam interferometer to test surface flatness is provided. The interferometer directs two beams focused at distinct points on a testing surface, such as the surface of a magnetic recording disc. An offset distance “d” between the two beams is provided on the target surface. In the present invention, the separation distance “d” is adjustable. The feature of adjustable separation distance in the interferometer allows the interferometer to meet the different spatial frequency requirements of various applications. In operation, first and second reflected beams are returned to an intensity beam splitter, where they are split and then recombined into two new beams of substantially equal intensity. The second of the two new light beams is constructed by the interference of half intensity of the first and half intensity of second beams, and is sent to a photodiode. The photodiode generates signals in response to the changing interference fringes caused as a result of the modulation of the optical path length difference between the original first and second beams. A local height difference on the reflective surface is calculated relative to the separation distance “d”.

Description:
RELATED APPLICATIONS 
     This new application for letters patent claims priority from an earlier-filed provisional patent application entitled. That application was filed on Jun. 17, 2003 and was assigned Application No. 60/479,294. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to characterization of a flat surface. More specifically, the present invention pertains to the use of an optical interferometer to analyze the flatness of a flat surface. Further still, the present invention presents an apparatus and method for profiling a smooth surface, such as the surface of a magnetic recording disc. 
     2. Description of the Related Art 
     The computer industry employs magnetic discs for the purpose of storing information. In this respect, computer systems employ disc drive systems for transferring and storing large amounts of data between magnetic discs and the host computer. The magnetic discs are typically circular in shape, though other shapes are used. One or more discs may be used in a disc drive system, depending on the needs of the system and the capacity of the drive. 
     It is desirable that the surface of a magnetic disc be as flat as possible. Uniform flatness aids in maintaining a constant fly-height of the slider, where the magnetic read/write head operates over the disc surface. This, in turn, ensures accurate writing/reading of magnetic data by the read/write head to and from the disc. Flat surface topography also allows the slider and attached magnetic head to fly more closely to the disc surface, permitting a tighter concentration of magnetic data to be embedded in the disc. Thus, periodic surface characterization of magnetic discs is part of the quality control employed in the manufacturing process. 
     In order to accurately analyze surface topography in ultra-smooth surfaces, it is known to use an optical interferometer. An optical interferometer is a tool that provides the unique advantages of non-contact operation, high resolution, wide spatial frequency coverage and high throughput. However, conventional interferometers are extremely sensitive to environmental vibration. 
     In a conventional interferometer, the surface topography is inferred by measuring the optical path length difference between an object beam and a reference beam. The reference beam length is usually fixed to be a constant length. Environmental vibration can cause body movement between the interferometer and the test object, which in turn can introduce a spurious change of optical path length in the object beam. Stated another way, if the disc surface experiences vertical vibration, the optical path length difference between the object and reference beams can no longer be kept constant. This vibration-induced optical path length change will then be confounded with the signal of interest originating from the surface topography of the test object, e.g., a magnetic disc surface. 
     An effective solution to reduce the effect of environmental vibrations in interferometers is to translate the optical path length change caused by body movement into both the reference beam and the object beam. Such interferometers are known as common-path interferometers. There are three types of common-path interferometers; the heterodyne interferometer, the interferometer with a bifringent lens, and the scanning shearing interferometer. A common feature of these interferometer designs is the use of a single lens to deliver two beams to the object surface. The two beams are offset in striking the target surface. The two beams are typically generated by using a birefringent lens or a Wollaston prism. However, in these designs the separation distance “d” between the two beams as they strike the target surface is fixed. Moreover, the separation distance is limited by the numerical aperture of the lens and/or Wollaston prism. The maximum measurable spatial frequency is subsequently limited by these components. 
     Therefore, a need exists for an optical interferometer that insures a constant optical path length difference between the object and reference beams while the disc is experiencing the environmental vibration, which should cause the disc surface moving up and down. Still further, a need exists for an optical interferometer that permits adjustment of the separation distance between the two beams as they strike the target surface. 
     SUMMARY OF THE INVENTION 
     This disclosure describes a surface profiler using a dual-beam interferometer. The interferometer tool is designed to provide an optical, non-contact testing method for measuring and characterizing ultra-smooth surfaces. Examples of applications for the interferometer tool include the surfaces of magnetic recording discs and of semiconductor wafers. 
     The interferometer of the present invention is a common-path interferometer. The interferometer directs two beams focused at two distinct points on the testing surface. An offset distance “d” between the two beams is provided on the target surface. In the present invention, the separation distance “d” is adjustable. The interferometer requires neither a birefringent lens nor a Wollaston prism to generate the two separated beams; but uses instead known optical components. The feature of adjustable separation distance in the interferometer provides an efficient and accurate hardware low pass filter with which to meet the different spatial frequency requirements for various applications. Further, the reduced sensitivity to the environmental vibration qualifies this type of interferometer for applications requiring a portable device. 
     Generally, the optical interferometer first comprises a light source for generating a light beam. In one arrangement, the light beam is initially in the P-polarization state. The light beam is first directed to a first beam splitter. The beam splitter receives the light beam, and divides it into first and second beams. The first and second beams are of substantially equal intensity. 
     A half wave plate is provided for receiving the second beam from the first beam splitter. The half wave plate converts the second light beam from its P-polarization state to the S-polarization state. 
     The optical interferometer also comprises a polarizing cube beam splitter. The polarizing cube beam splitter receives and transmits the first light beam to the reflective surface, i.e., the test object. The polarizing cube beam splitter further receives and reflects the second light beam to the reflective surface. The first and second light beams are directed such that the first and second light beams are received at the reflective surface an offset distance “d” apart. 
     The first and second light beams are reflected back to the first beam splitter. Upon reflection, the light beams are split again and then the beams that travel in a same direction will be recombined. The process of splitting and recombining beams forms new first and second light beams. The new second light beam is constructed by the half intensity of the first beam and the half intensity of the second beam, and produces interference fringes as a result of the modulation of the optical path length difference between the new first and second beams. The new second light beam is directed to a photodiode. The photodiode receives the new second light beam, and converts the intensity of new second beam into electrical signals. These signals are representative of irregularities in the target surface and are later processed for analysis. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
         FIG. 1  presents a schematic diagram of parts comprising the dual beam optical interferometer of the present invention, in one embodiment. 
         FIG. 2  is a schematic representation of a target surface, such as the upper surface of a magnetic disc. Two beams as generated in the diagram of  FIG. 1  are seen striking the target surface in offset fashion. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  presents a diagram of a dual-beam, common path optical interferometer  100  of the present invention, in one embodiment. As the title implies, two beams  110  and  120  are generated through the interferometer  100 . The beams  110 ,  120  are directed towards a target surface  150  under analysis. In the exemplary arrangement of  FIG. 1 , the target surface  150  is a mirror-like, highly reflective, ultra-smooth disc surface, such as the surface of a magnetic data storage disc. However, it is understood that the present invention has utility in measuring smoothness of other smooth surfaces, such as silicon dioxide wafers. 
     In the present apparatus, a light source  200  is first provided. Preferably, the light source  200  defines a He—Ne laser. The laser  200  supplies a single, polarized laser beam  105 , in which the beam  105  is continuous. The beam  105  may be in either the P-polarization state or the S-polarization state, depending upon the configuration of other components as will be shown. 
     The polarized beam  105  is transmitted through an optical isolator  250 . The optical isolator  250  serves to direct the light beam  105 , and prevents the light beam  105  from returning to the laser  200  during the disc testing process. An example of a suitable optical isolator is product no. 501010 manufactured by Linos Photonics. 
     Once the polarized beam  105  is transmitted through the optical isolator  250 , it is directed to a first beam splitter  240 . The beam splitter  240  is an intensity beam splitter. The beam splitter  240  divides the single beam  105  into two parts of substantially equal intensity. The two beams are designated as beam one  110  and beam two  120 . Beam one  110  and beam two  120  each remain in their original state of polarization at this point. In the preferred embodiment for the method and apparatus of the present invention, the polarization state is the P-polarization state. 
     Each beam  110 ,  120  is transmitted to a mirror. Beam one  110  is transmitted through the beam splitter  240  to mirror one  112 , while beam two  120  is redirected at 90 degrees by the first beam splitter  240  to mirror two  212 . 
     As shown in the diagram of  FIG. 1 , mirror one  112  reflects beam one  110  at 45 degrees. Likewise, mirror two  212  reflects beam two  120  at 45 degrees. The result is that each beam  110 ,  120  is redirected at 90 degrees towards the same location, i.e., a polarizing cube beam splitter  160 . 
     En route to the polarizing cube beam splitter  160 , beam one  110  passes through a long working distance objective (“LWO one”)  114 . LWO one  114  serves to focus beam one  110  onto a target surface  150 . Beam one  110  passes through the polarizing cube beam splitter  160  before hitting the target surface  150 . Because the beam  110  is in its P-polarization state, it is transmitted essentially straight through the cube beam splitter  160  and onto the target surface  150 . 
     Referring back now to beam two  120 , beam two  120  moves from mirror two  212  and also moves towards a long working distance objective. In this case, the long working distance objective is “LWO two”  124 . However, beam two  120  passes through a half-wave plate (“HWP”)  126  before it is focused onto the target surface  150  by LWO two  124 . The HWP  126  is aligned so that the transmitted beam  120 ′ consists primarily of S-polarized light. Thus, the light  120 ′ received and focused by the long working distance objective two  124  is in the S-polarization state. 
     The S-polarized beam  120 ′ is received by the cube beam splitter  160 . The S-polarized beam  120 ′ is not transmitted through the cube beam splitter  160 , but is reflected onto the target surface  150  at a designated angle. In the arrangement shown in  FIG. 1 , the reflection angle is 45 degrees. 
     In the diagram of  FIG. 1 , it can be seen that beam one  110  and beam two  120 ′ do not strike the target surface  150  at the same location. In this respect, a distance “d” is defined by the separation between the two target strikes. This distance is created by virtue of placement of mirror one  112 . 
     In one arrangement, the objective LWO one  114  and mirror one  112  are built together as a block assembly. The block assembly is shown schematically in phantom at  118 . The assembly  118  is connected to a piezoelectric translator (not shown). The piezoelectric translator provides movement of the block  118  parallel to beam one  110  with an accuracy and resolution of less than 1 nanometer resolution. Bilateral movement of the block  118  is shown by arrow “a.” This allows the apparatus  100  to accurately control and adjust the separation distance “d” between beam one  110  and beam two  120  as the beams  110 ,  120  strike the target surface  150 . 
       FIG. 2  is a schematic representation of a target surface  150 , such as the upper surface of a magnetic disc. Two beams  110 ,  120 ′ as generated in the diagram of  FIG. 1  are seen striking the target surface  150  in offset fashion. Beam one  110  and beam two  120 ′ reflect off the target surface  150 . The reflected beams are shown as  210  and  220 , respectively. Thus,  FIG. 2  is an enlarged view of a portion of  FIG. 1 . In  FIG. 1 , the target surface  150  appears planar. However, in the enlarged view of  FIG. 2 , a surface irregularity is visible. 
     It will be understood by those of ordinary skill in the art that a magnetic disc surface is not always perfectly planar, but may have topographical variations. In the view of  FIG. 2 , a topographical variation is demonstrated by local amplitude “dH.” A magnetic disc having a significant surface amplitude dH within a short wavelength is considered defective. 
     After striking the mirror-like surface  150 , each beam  110 ,  120 ′ is reflected back towards the polarizing cube beam splitter  160 . The beams  210 ,  220  reflect back from the focal points along their respective original paths. Thus, reflected beam one  210  returns through the LWO one  114 , against mirror one  112 , and back to the original beam splitter  240 . Reflected beam two  220  reflects against the polarizing cube beam splitter  160 , passes through the LWO two  214 , reflects again against mirror two  212 , and returns to the intensity beam splitter  240 . Beam two  220  returns to its original polarization state after transmitting through half wave plate  126 . Therefore, the beams  210  and  220  can interfere with each other once they recombine again at beam splitter  240 . The two reflected beams  210 ,  220  are each split at the original beam splitter  240 . The reflected first beam  210  splits into beams  410  and  411  Beam  410  travels back towards optical isolator  250 , while beam  411  reflects to a photodiode  300 . In similar fashion, the second reflected beam  220  also splits into two beams, to wit, beams  420  and  421 . Beam  420  is reflected towards the optical isolator  250 , while beam  421  travels on to the photodiode  300 . Each beam  410 ,  411  and  420 ,  421  is comprised in approximately 50/50 ratios of the reflected first  210  and second  220  beams. A new recombined first beam  310  is thus formed by beams  410  and  420 , and a new recombined second beam  320  is thus formed by beams  411  and  421  at the intensity beam splitter  240 . 
     The newly constituted first beam  310  travels towards to the laser  200 . However, the new first beam  310  is blocked by the optical isolator  250  before it returns into the laser  200 . The newly constituted second beam  320  travels towards the photodiode  300 . This new second beam  320  received at the photodiode  300  produces interference fringes as a result of the modulation of the optical path length difference between the two beams  210 ,  220 . 
     The photodiode  300  captures these moving or changing fringes, which are observed as temporal variations in light intensity. The photodiode  300  then delivers a voltage signal proportional to the temporal light intensity change. This voltage signal “s,” in turn, can be analyzed by subsequent digital signal processing as is known in the art. 
     The signals, I, detected by the photodiode  300  are described by:
 
 I=I   1   +I   2 +2√{square root over ( I   1   ·I   2 )}·cos(φ)  (1)
 
where, I 1  and I 2  are the intensities of beam  411  and beam two  421 , respectively, and Ø is the phase difference between the two beams  411  (or  210 ),  421  (or  220 ). The phase difference Ø is a function of the optical path length difference, ΔL, between the two beams  210 ,  220 , which is presented in the equation: 
             ϕ   =       2   ⁢           ⁢   π   ⁢           ⁢   Δ   ⁢           ⁢   L     λ             (   2   )             
 
     where, λ is the wavelength of the laser light. 
     Based on the geometry of  FIG. 2 , which shows a magnified view of the beams&#39; focusing area, ΔL can be described by:
 
Δ L= 2( d+dH )  (3)
 
     where d is the separation of beam one  110  and beam two  120 ′, and dH is the height difference between the two focal points of beam one  110  and beam two  120 ′ on the object surface  150 . Equation (2) can then be rewritten as: 
             ϕ   =           4   ⁢           ⁢   π   ⁢           ⁢   d     λ     +       4   ⁢           ⁢   π   ⁢           ⁢   dH     λ       =     Φ   +       4   ⁢           ⁢   π   ⁢           ⁢   dH     λ                 (   4   )             
 
     The first term in the equation (4) is a constant because the beam separation d is pre-determined based on the minimum spatial wavelength required to be detected. Therefore, the phase angle Ø is a function of dH, which is itself a function of the local surface slope. By solving equations (1) and (4) based on the intensity value I detected from photodiode  300 , the local height difference dH can be obtained. Subsequently, the local slope dS can be calculated by:
 
dS=dHId  (5)
 
     If we assume that the surface profile can be described by f(x), as shown in  FIG. 2 , then df/dx=slope, or df/dx≈dS. Here, dx is d, the separation of beam one  110  and beam two  120 ′. The profile or topography of the surface  150  can then be calculated by integration of the slope information. 
     In equation (1), there are two other unknowns, to wit, I 1  and I 2 , that must be resolved before equation (1) can be solved. These two unknowns can be obtained by using I max  and I min  techniques. The I max  and I min  techniques are described in J. Wang and I Grant, “ ESPI, Phase Mapping, NDT The Techniques Applied to Real - Time, Thermal Loading, ” Applied Optics 34, 3620–3627 (1995). 
     With the current optical setup, the approach for obtaining I max  and I min  can be determined by moving the block assembly  118  backward and forward with the piezoelectric translator in order to vary the optical path length difference between the two beams, ΔL, such that a full cycle or more of moving interference fringes are generated. As long as the moving distance is greater than laser light wavelength, a full cycle moving fringe will be generated. The intensities of the moving fringes can be detected by the photodiode  300 . From there, the I max  and I min  can then be obtained. We can then re-write Equation (1) as: 
         I=I   a   +I   b ·cos(φ)  (6)         where   ⁢           ⁢     I   a       =         I   1     +     I   2       =             I   max     +     I   min       2     ⁢           ⁢   and   ⁢           ⁢     I   b       =       2   ⁢         I   1     ·     I   2           =           I   max     -     I   min       2     .                 
     The profiling dynamic range is determined by the local height difference, dH, which is caused by the slope of the surface topography. The maximum dH which can be observed without causing a phase unwrapping problem is given by the second term of Equation (2) when it is set equal to π. 
             π   =         4   ⁢           ⁢   π   ⁢           ⁢   dH     λ     ⁢           ⁢   or             (   7   )               dH   =     λ   4             (   8   )             
 
     A He—Ne laser has a known wavelength of 0.6328 μm. When using a He—Ne laser, the maximum dH is 0.133 μm. This range is much greater than the maximum local slope on an ultra-smooth surface, such as a hard disc surface. For instance, a typical hard disc, whose surface topography in the circumferential direction can be depicted by a sinusoidal function with 5 μm amplitude, or 10 μm peak-to-peak in amplitude, has a maximum dH of 0.004 μm for a radius of 25.4 mm and a sampling interval d of 10 μm. Therefore, this interferometer does not require phase unwrapping for most applications involving smooth surfaces. This increases the accuracy of the measurement and reduces the data processing time. 
     Various applications may be made with the dual beam interferometer of the present invention. Because the body movement between the profiling interferometer  100  and the testing object  150  will have little or no effect on the surface topography measurement, this type profiler  100  is well-suited to portable applications. For instance, the profiler  100  could be used for measuring HMS_Wq of the disc  150  on all kind of spindles, include measuring the discs in assembled hard disk drives. The profiler  100  may also be used for measuring disc edge roll-off without the need for an ultra-flat motion stage. 
     While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. For instance, the light source  200  may generate a continuous light beam  105  that is in the S-polarization state rather than the P-polarization state. In this instance, the half wave plate  126  would be in the path of beam one  110  rather than in the path of beam two  120 .