Patent Publication Number: US-6713760-B2

Title: Analysis of semiconductor surfaces by secondary ion mass spectrometry and methods

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of application Ser. No. 10/358,939, filed Feb. 4, 2003, pending, which is a continuation of application Ser. No. 09/795,999, filed Feb. 28, 2001, now U.S. Pat. No. 6,528,786, issued Mar. 4, 2003, which is a continuation of application Ser. No. 09/309,208, filed May 10, 1999, now U.S. Pat. No. 6,232,600, issued May 15, 2001, which is a continuation of application Ser. No. 09/035,197, filed Mar. 5, 1998, now U.S. Pat. No. 5,920,068, issued Jul. 6, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to the bulk measurement of trace contaminants in the surface layers of semiconductor wafers and dies, as well as material composition as a function of depth. More particularly, the invention pertains to improvements in methods and apparatus for mass spectrographic analysis of wafer and semiconductor die surface layers. 
     2. State of the Art 
     Secondary ion mass spectrometry (SIMS) is known as a method for determining particular constituents of a semiconductor material and providing a quantitative measurement of each. 
     Generally, this method involves bombarding a sample with “primary” ions, e.g. oxygen ions, measuring the intensities of secondary ions emitted or sputtered from the sample, and calculating the quantity of each conductive impurity based on the secondary emission as compared to the emission of standard materials. The sputtering and analysis is typically conducted in an ultra-high-vacuum environment. 
     SIMS may be used to achieve parts-per-billion (ppb) detection limits for bulk analysis and for determining material composition as a function of depth, provided the sample size is sufficiently large. The extreme sensitivity of SIMS results from its ability to “consume” large amounts of sample material, and thus process a large number of atoms to detect. However, because of the high rate of material consumption from a very small sample, dynamic SIMS is generally not appropriate for analysis of a very thin oxide surface layer of a semiconductor die and plurality of semiconductor dice in wafer form. Typical semiconductor contaminants may include lithium, boron, sodium, potassium, iron, sulfur, and carbon, all of which are found in the oxide layer on the semiconductor die surface. For the case of surface contaminants on silicon, the oxide layer is generally not more than about 15 Å thick. However, several minutes are required to obtain a sufficient number of data points at the required analyte masses, so the method is not useful for this application as the oxide layer will be quickly consumed. 
     It is desirable to be able to detect the concentrations of boron, lithium and sodium to less than about 1×10 6  atoms per square centimeter of semiconductor die surface area. These detection limits are considerably lower than currently obtainable. 
     U.S. Pat. No. 4,874,946 of Kazmerski discloses a method and apparatus for mapping the chemical composition of a solid device, using a rasterable SIMS mass analyzer. 
     U.S. Pat. No. 4,611,120 of Bancroft et al. discloses a method for suppressing molecular ions in the secondary ion mass spectra of a commercial SIMS instrument. 
     U.S. Pat. No. 5,521,377 of Kataoka et al. discloses a method for analysis of a solid in a planar or depth-wise direction using sputtering with two ionizing beams and detecting a two-atom composite ion. 
     U.S. Pat. No. 5,502,305 of Kataoka and U.S. Pat. No. 5,442,174 of Kataoka et al. disclose methods for analysis of a solid in a planar or depth-wise direction using sputtering with an ionizing beam and detecting a three-atom composite ion. 
     U.S. Pat. No. 5,332,879 of Radhakrishnan et al. discloses the use of a pulsed laser beam to remove contaminant metals from the surface of a polyimide layer. The disclosure indicated high surface metal removal with “minimal” removal of the polyimide, i.e., 250-500 Å per pulse. Such ablation rates are far greater than useful in the analysis of surface contaminants in semiconductor devices, where the surface oxide layer is typically only about 15 Å in depth. 
     Time-of-flight secondary ion mass spectroscopy (TOF-SIMS) has also been found useful for bulk analysis of materials, provided the sample size is sufficiently large. The TOF-SIMS instrument directly measures the speeds of secondary ions by measuring the time taken to travel a given distance. Knowing the ion&#39;s energy, which is defined by the spectrometer&#39;s acceleration voltage, its mass can then be calculated. Typically, the time intervals are defined as the difference in time between pulsing the ion gun and the ion arrival at the detector. The mass range is then calibrated using at least three known mass peaks. 
     TOF-SIMS instruments have been found to provide some of the lowest detection limits in surface analysis, typically even lower than total reflection X-ray fluorescence (TXRF) with vapor phase decomposition (VPD). For the TOF-SIMS instrument, some representative detection limits are &lt;1×10 8  atoms/cm 2  for lithium, boron and sodium, and &lt;1×10 9  atoms/cm 2  for iron. 
     The TXRF instrument, on the other hand, is incapable of detecting elements lighter than sulfur, so critical elements such as sodium, carbon, lithium and boron cannot be detected. 
     Thus, the TOF-SIMS method would appear to be potentially useful for surface analysis, but instrumental constraints limit the sampling area to about 100×100 μm, and sampling of a relatively shallow oxide layer over the 100×100 μm area does not produce sufficient sample material for achieving the desired detection limits. 
     For TOF-SIMS, the detection limits are determined by the transmission and exceptance of the mass spectrometer, the sputter and ionization yield of the analyte, and the amount of material consumed during the analysis. These parameters may be categorized as the useful yield of the mass spectrometer and volume of analyte. Sampling of the maximum raster area of 100×100 μm to a depth of about 13 Å will produce about 3×10 11  particles. This is equivalent to between 0.3 to 30 (thirty) counts of a measured component at the 1 ppm level depending upon the ionization yields. It is critical to semiconductor device manufacture that bulk concentrations of some contaminants as low as 0.01 ppm and even 1 ppb be accurately detectable. Thus, current detection limits for certain contaminants must be reduced by a factor on the order of about 100 or more. 
     U.S. Pat. No. 5,087,815 of Schultz et al. discloses a method and apparatus for a TOF-SIMS isotopic ratio determination of elements on a surface. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a method for substantially increasing the sensitivity of a mass spectrographic analytical method for determining contaminant levels in the surface oxide layer of a semiconductor die or semiconductor dice in wafer form. 
     The present invention further provides a method for increasing the sensitivity of surface contaminant analysis of an oxide layer of a wafer by secondary ion mass spectroscopy (SIMS). 
     The present invention additionally provides a method for increasing the sensitivity of surface contaminant analysis by time-of-flight secondary ion mass spectroscopy (TOF-SIMS). 
     Related to the present invention is providing a method for determining the bulk concentration of contaminants in a surface oxide layer of a semiconductor material by SIMS or TOF-SIMS, wherein the surface area which may be sampled may be much greater than the electrostatic raster limits of the sputtering primary beam, and/or acceptance area limits of the spectrometer. 
     The present invention further includes apparatus for achieving the desired rastered sputtering and analysis of an enlarged area of a semiconductor wafer. 
     The present invention includes a method and means which enable sputtering to a uniform sampling depth and maintaining mass resolution irrespective of warpage or other non-planarity of a wafer. Thus, contaminant analysis of semiconductor wafers and semiconductor dice by SIMS or TOF-SIMS may be limited to the surface oxide layer. Additionally, advantages and novel features of this invention are set forth in part in the description infra. These advantages and features will become apparent to those skilled in the art upon examination of the following specification and drawings, or may be learned by practice of the invention. The various combinations of apparatus and/or methods which comprise the invention are pointed out in the appended claims. 
     In accordance with this disclosure, a first aspect of the invention comprises the steps of: 
     (a) scan-sputtering a large area of a semiconductor wafer surface while continuously moving, i.e. scanning the wafer with a supporting mechanical stage in a first direction, and mechanically rastering the wafer in a second direction, whereby the scanning speed is controlled to limit the sputtering to a sampling depth not generally exceeding the depth Q of the surface oxide layer, and the rate of sputtered secondary ionic emission directed to the SIMS detector simultaneously satisfies the SIMS consumption rate; the total sputtered area is generally at least about 10 4  μm 2  for a surface oxide layer sputtered to a depth of about 15  Å; 
     (b) directing a stream of secondary ions produced by the primary ionizing beam into a SIMS for mass spectrographic analysis at a rate satisfying the SIMS sample consumption rate for a time period sufficient for high analytical sensitivity; and 
     (c) computing the total mass of each of the selected detected ions. 
     In a second aspect of the invention, a combination of mechanical scanning/rastering and primary beam electrostatic rastering is used to move the sputtering and sampling operations over the wafer surface at a speed responsive to a controlled sampling depth and sample consumption over a large area. 
     In a third aspect of the invention, mechanical scanning and rastering are used to sample a large area of a semiconductor material with a continuous sputtering and analysis by a time-of-flight secondary ion mass spectrometer (TOF-SIMS). 
     In a fourth aspect of the invention, a combination of mechanical rastering and primary beam rastering, e.g. electrostatic rastering is applied to a time-of-flight secondary ion mass spectrometer (TOF-SIMS) whereby a large area of controlled limited depth may be sputtered at a speed which simultaneously (a) limits total sputtering depth generally to the surface oxide layer and (b) provides a large quantity of secondary ions for high resolution of measured atoms. 
     In one preferred method, an area, typically limited by the electrostatic rastering capability of the TOF-SIMS instrument to about 100 μm 2 , is repetitively sputtered and analyzed. The primary beam is then moved to a new area by mechanical stage rastering, and the new area is repetitively sputtered and analyzed using electrostatic rastering. The process is repeated until sufficient sample material is consumed for each “slice” of the surface oxide layer to provide the desired detection limits. The total area sampled is limited only by the total area of the wafer or die being analyzed, and the time available for analysis. 
     Thus, the detection limits are reduced to low levels. For example, the detection limits of lithium, boron, and sodium may be extended to less than about 1×10 6  atoms/cm 2 . 
     The detection limits of trace iron may be at somewhat higher concentrations because of the peak shape of the neighboring SiO 2 . 
     In a variant of the above method, mechanical rastering is performed such that a thin slice of an enlarged area, e.g. 
     400-1600 μm is sputtered and analyzed before the next slice is sputtered and analyzed. 
     In each of the embodiments described and illustrated of the present invention, it should be kept in mind that removal of the surface layer by sputtering is conducted in a continuous scanning motion rather than by stepping along a series of stationary raster points. The sputter depth is controlled by the scanning speed, and the sputter rate (mass per unit time) is controlled by the ion gun characteristics. 
     Because of the much greater area which is rastered, sputtered and analyzed, the analysis is more sensitive to non-planarity, e.g. warpage, of the wafer. Non-uniform extraction fields and some loss of mass resolution inaccuracy in analysis may result. This would have adverse effects on detection limits of Fe since the neighboring peak of Si 2  is at the same nominal mass as Fe and will overlap Fe without sufficient mass resolution. 
     Thus, in another aspect of the present invention, a method and apparatus are provided for counteracting the effects of wafer warpage (non-planarity) upon analytical results. In accordance with the invention, a non-invasive laser interferometer is incorporated into the SIMS or TOF-SIMS spectrometer to measure non-planarity as a function of wafer location, and permit correction of the stage elevation or the sample electrical potential relative to the extraction potential. The sample elevation measurements may be made prior to the mass spectrometric analysis and then used to control e.g. the stage elevation during rastering/sputtering, or the sample elevation measurements may be made during the rastering/sputtering operation for continuous in-situ correction. Because of the required physical separation, e.g. about 1-3 cm., of the sputter beam and the interferometer laser beam in the latter case, the sample surface elevation at the sputter location may be calculated and correction made therefor by a computer program of the process controller, based on distant measurements and assuming a particular warpage shape. Alternatively, the wafer may be first scanned by the interferometer; the data may be stored and used to provide correction during the sputtering/analyzing operation. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     The invention is illustrated in the following figures, wherein the elements are not necessarily shown to scale. 
     FIG. 1 is a front diagrammatic view of an exemplary secondary ion mass spectrometer (SIMS) apparatus of the invention for high resolution detection of component concentrations in a thin surface oxide layer of a semiconductor die or wafer; 
     FIG. 2 is a perspective view of a portion of a semiconductor wafer illustrating a method of the invention for determining the component concentrations in the surface oxide layer thereof; 
     FIG. 3 is a lateral diagrammatic view of a laser interferometer useful in the invention; 
     FIG. 4 is a plan view of a portion of a semiconductor wafer, illustrating an exemplary scanning/rastering pattern of the invention; 
     FIG. 5 is a side diagrammatic view of an alternative grazing angle interferometer used in the present invention; 
     FIG. 6 is a graphical representation of the grazing property of the grazing angle interferometer used in the present invention; 
     FIG. 7 is a graphical representation of the increase in reflection of the beam as the grazing incidence angle is increased; and 
     FIG. 8 is a table of wafer surfaces capable of measurement by a grazing incidence interferometer. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to the drawings of FIGS. 1-4, and particularly to drawing FIG. 1, a secondary ion mass spectrometer (SIMS) analysis system  10  of the present invention is illustrated. The SIMS analysis system  10  includes a primary sputtering ion generator or ion gun  12 , a sample stage  14  having a stage controller  16 , and a secondary ion analyzer  18 . A primary ion beam  20  is directed from the ion generator  12  onto the surface  22  of a semiconductor die or wafer  24  mounted on the stage  14 , and a stream of secondary ions, represented by line  26 , is sputtered from the die  24  by the primary ion beam  20 , extracted by the extraction field between the wafer  24  and the extraction plate  151  and directed through an electric field  28  and a magnetic field  32  for separation of ions, i.e. resolution of the ion stream. The stream  26 A of partially resolved secondary ions is then directed through a collector slit  36  and deflection electrode  38  to a spectrometric detector  30 . The detector  30  is conformed to detect the resolved secondary ionic emission stream  26 A and relay measured values via signal conductor  34  to a controller/analyzer/recorder  40 . Other time-dependent inputs to the controller/analyzer/recorder  40  include an indication of the sampling location from the stage controller  16  via conductor  42 . The controller/analyzer/recorder  40  controls the ion gun  12  via signal conduit  43 . 
     The controller/analyzer/recorder  40  receives the measured values of current measurements from signal conductor  34 , determines the particular secondary ions thereof, and sums the counts to provide a “bulk” concentration of each ion, and/or a concentration of each as a function of X-Y location and/or an interval of depth Z in the wafer or die  24 . In accordance with the method of the invention as generally practiced, the wafer or die  24  has a surface oxide layer  44  in which are the contaminant atoms/molecules of interest, and the depth of sputtering does not generally exceed the depth  46  (FIG. 2) of the surface oxide layer. 
     As used herein, the term “rastering” refers to the movement of a primary ion beam, laser light beam, or other apparatus or process in a line over the surface of a wafer or other planar material, and lateral movement, in turn, to each of a plurality of subsequent lines, whereby a significant area of the surface is subjected to the apparatus or process of the present invention. 
     The functions of the controller/analyzer/recorder  40  need not be confined to a single instrument, but may be distributed, for example, in the ion generator  12 , stage controller  16 , spectrometric detector  30 , and/or controller/analyzer/recorder  40 . 
     The primary ion beam  20  may be a stream of e.g. O 2   +  ions, Cs +  ions, or other ions, as known in the art. As also known in the art, the primary ion beam  20  may be laterally focusable by an electrostatic lens  48  to incrementally raster the ionization of the wafer surface  22 . The maximum area of the wafer surface  22  which is electrostatically rasterable under the constraints of currently available SIMS equipment has dimensions of about 100×100 μm. In some configurations of the present invention, the primary ion beam is not rastered. 
     In one aspect of the invention, a movable stage  14  with a stage controller  16  is provided to support the wafer or die  24  and to provide both a controllable scanning motion in a first direction and a rastering motion of the wafer or die in a second direction. The stage  14  is preferably movable in the lateral X and Y axes at a controllable variable speed, such as between about 200 and 1000 μm/sec., and is movable along the Z axis. The surface area which may be sampled is thus not limited by the electrostatic beam-raster. 
     In another aspect of the present invention, a movable stage  14  with a stage controller  16  is provided to support the die or wafer  24  and to provide a controllable scanning and raster pattern in combination with the electrostatic rastering of the electrostatic lens  48 , greatly increasing the total rasterable area thereby. 
     In drawing FIGS. 1 and 2, stage  14  is an automated sample stage which permits e.g., scanning and rastering of wafers of any size typically used in the industry, such as 8 inches (20.3 cm.) in diameter. The wafer surface  22  is sputtered by primary ion beam  20  while scanning at a rate whereby the total sputtering depth does not generally exceed the depth  46  of the surface oxide layer  44  on the wafer surface. Thus, a bulk analysis at lower concentration limits may be obtained for the surface oxide layer  44 . 
     An example of a raster pattern is shown in drawing FIG. 2 wherein straight-line scanning along line or path  94  is followed by rastering to the adjacent line  96  and reversing the scanning direction. This is repeated across the surface  22  until the entire area to be sampled is sputtered to the control depth  45  and analyzed. 
     If desired, the scanning may be at a speed and/or primary beam strength whereby only a fraction of the surface oxide layer  44  is removed and analyzed per pass. Thus, a bulk analysis of the trace components may be obtained which will have limits equal to or lower than conventional bulk analyses. 
     For determination of composition as a depth function, the wafer surface  22  is repeatedly and progressively scanned in multiple passes, and the secondary ion stream from each pass is analyzed and correlated as a function of X-Y position and depth (Z position). 
     Electrostatic beam rastering may be added to a mechanical scanning/rastering. In this configuration, a beam rasterable area is scan-sputtered (as opposed to point-sputtered). Control of the rastering and scanning motions is combined whereby a much larger surface area may be scanned and rastered. 
     As shown in drawing FIG. 4, a surface  22  of a wafer  24  is sputter-sampled and analyzed by an apparatus of the present invention. Sector A comprises the maximum area electrostatically rasterable by the ion gun  12  and is shown as a square pattern having dimensions  100  and  102  of e.g. 100 μm×100 μm. Sector A is bounded by lines  104 ,  106 ,  108  and  110 . Adjacent sectors B, C, and D are shown in part, each having dimensions equal to sector A. 
     The exemplary raster pattern of the ion beam starts at point  120  adjacent corner  112 , and progresses along line or path  114  to point  122  adjacent corner  116 . At that location, the beam is rastered along line  118  to point  124 , and sputtering continues along line  126  to point  128 . The ion beam  20  is continuously moved at a speed and primary beam strength sufficient to remove and ionize the surface  22  to the desired depth. The secondary ion stream  26  is directed into the secondary ion analyzer  18 . The scanning and rastering of the primary ion beam  20  is continued over the entire sector A to point  130  adjacent corner  132 . 
     The wafer  24  is then mechanically rastered by the movable sample stage  14  in direction  134  so that sector B occupies the area previously occupied by sector A. At the same time, the primary ion beam  20  is moved in directions  134  and  136  to focus on point  138  adjacent corner  140 , and begin a scanning, sampling and analysis of sector B. These operations may be continued for any number of sectors, limited only by the size of the wafer  24  and the time available for the analysis. 
     Thus, for example, a first analysis area of 200×200 μm may be scanned at a speed to complete sputtering in 0.5 second. The analysis depth will be about 13 Å which is close to the depth of the surface layer. A 300 nÅ primary probe beam will consume the sputtered mass (about 3×10 11  particles) from this sputtered area (4×10 4  μm 2 ) in about 0.5 seconds, and yield accurate bulf analyses of contaminants at low concentration levels. Following sputtering of this 200×200 μm area, the wafer is moved, i.e. rastered, by the mechanical stage to a second area, typically of equal size and adjacent the first area, which is then scanned. A plurality of e.g. 100 or more area may be thus sampled and analyzed, depending upon the wafer size, greatly increasing the sensitivity to contaminant levels. As described above, the primary ion beam electrostatic raster is not used; all movement is controlled by the mechanical stage. 
     Alternatively, the electrostatic rastering and mechanical rastering devices may be combined to provide a linear scanning motion for continuous sputtering and a raster step motion for moving the primary ion beam  20  to a new line of sputtering. A single pass, for bulk analysis, or multiple passes for depth analysis, may be performed. 
     Other raster patterns may be used, and the number of sectors chosen to raster by this method may be any convenient number which provides the desired analytical accuracy. 
     Consumption of the substrate underlying the surface oxide layer  44  is avoided, increasing the concentrations of contaminant ions to be measured and reducing the analytical limits by a factor of about 10 to 100 or even more. Thus, surface analysis may be achieved with the same sensitivity as a typical SIMS bulk analysis. 
     For surface analysis of large areas such as on a wafer of 8 inches or greater diameter, any surface non-planarity resulting from e.g. warping, will affect the extraction voltage, and hinder mass resolution and thus abundance sensitivity. Such warpage on an 8 inch wafer is typically specified to be less than about 40 μm across the wafer. Thus, the allowable non-yield planarity would yield up to a 2% change in extraction field for extraction gate. Unless corrected, the resulting error may cause an unacceptable loss in mass resolution and abundance sensitivity. 
     Thus, in a further feature of the invention, an interferometry apparatus and method are combined with compensation apparatus and methods for counteracting sample non-planarity. 
     Drawing FIGS. 1-3 illustrate this feature as configured in accordance with the known Michelson type of interferometer  50 . A light beam  54  generated by a collimated light source  52  such as a laser is directed at a beamsplitting surface  56  of a beamsplitter  60 . A first split beam  62  is reflected from the beamsplitting surface  56  to the wafer surface  22  and is reflected back to the beamsplitter  60 , passing through it to a detector  70  as beam  64 . A second split beam  66  passes through the beamsplitter  60  and anti-reflection surface  58  to a reflecting surface  68  of an axially movable mirror  72 , from which the beam  66  is reflected back to the beamsplitter  60 . The reflected beam  66  is further reflected from the beamsplitting surface  56  and passes to the detector  70  as part of beam  64 . Depending upon the difference in distance traveled by the first split beam  62  and by the second split beam  66 , the two beam portions received as beam  64  by the detector  70  will interfere constructively or destructively. Compensating plate  74  compensates for the thickness of the beamsplitter  60 , and may be unnecessary when the generated light beam  54  is monochromatic or quasi-monochromatic. In a preferred embodiment, the generated light beam  54  is a coherent laser beam. 
     The function of the beam splitter is to superimpose the mirror images onto each other and determine the phase difference and hence the difference in distance traveled by the first and second split beams  62 ,  66 . The difference in traveled distance is twice the control distance  76 . Sample distance  78  equals the total of control distance  76  and mirror distance  80 . The detector  70  measures the change in amplitude of the beam  64  from which the difference in phase between beams  62  and  64  may be calculated and counts interference fringes. A warped wafer  24  will change the sample distance  78  and this change in distance relative to the mirror distance  80  (or the control distance  76 ) will be detected and quantified by the detector  70 . Mirror controller  90  provides precise axial movement  88  of mirror  72 , controlling distance  76  for determining the phase difference and may be used for phase modulation. Signal lines  82 ,  84 , and  86  interconnect the light source  52 , detector  70 , mirror controller  90  and the controller/recorder/analyzer  40 . Conductor  42  provides for vertical movement of stage  14 , by which the stage controller  16  raises or lowers the wafer/die  24  to compensate for measured warpage or other source of non-planarity. Alternatively, the DC bias of the wafer/die  24  relative to the extraction plate  151  may be varied, i.e. raised or lowered to maintain a constant extraction field. 
     Movement of the stage  14  by the stage controller  16  may be continuously controlled in each of the X, Y and Z axes, so that the primary ion beam  20  may be scanned and rastered over the desired X-Y sample area  98  and the ion gun may also be maintained at a relatively constant distance from the wafer surface  22 . Thus, the surface  22  will be sputtered to a relatively constant depth  45 , constant extraction field, and sputtering will be substantially confined to the oxide layer  44 , which is a desired result of this method. 
     While a Michelson type of laser interferometer is illustrated in drawing FIGS. 1 and 3, the interferometer may alternatively be a “grazing incidence” interferometer of the Twyman-Green type or may be of another configuration, e.g. Mach-Zehnder, Shearing, or Fabry-Perot interferometer. Also, by further using phase modulation on the above-referenced interferometers, the absolute position of the wafer can be evaluated using well known Fourier transform calculations. The various types of interferometer generally differ in the beam geometries and in analytical treatment of the response. Incorporation of a particular type of interferometer into a SIMS or TOF-SIMS apparatus should be made with measurement accuracy and ease of operation in mind. 
     Placement of an interferometer in a mass spectrometer will require that the first split beam  62  be separated from the primary ion beam  20 . This separation distance  92  may be e.g about 0.5-2 cm. This problem may be overcome by initially scanning the wafer surface  22  with the interferometer  50 , storing the planarity results, and providing continuous or semi-continuous correction during the sputtering operation. Alternatively, in situ remote interferometric measurements may be taken during the sputtering operations and corrections made based on calculated deviations from planarity. 
     Referring to drawing FIGS. 5 through 8, another type of interferometer, a “grazing incidence” interferometer  100  is illustrated for use in the present invention. By using a grazing incidence interferometer  100  and by using “phase modulation” techniques the basic data generated from scanning the surface of a wafer is readily transferred to a computer for analysis through the use of well known Fourier series phase calculation techniques to determine the degree and direction the wafer is warped. 
     As illustrated in drawing FIG. 5, the grazing incidence interferometer  200  typically includes a laser light source  202 , objective  204 , coherence limiter  206 , GALVO  208 , fold mirror  210 , fold mirror  212 , collimator lens  214 , fold mirror  216 , prism  218 , fresnel lens  220 , fold mirror  222 , fold mirror  224 , video (TV) camera  226 , video output  228 , and computer interface  230 . Laser beam  150  is illustrated emanating from laser  202 , dividing into beams  150 ′, which are received by video camera  226  after being reflected from the wafer  24  supported on a suitable member (not shown). 
     Referring to drawing FIG. 6, graphically represented, a laser beam  150  is illustrated grazing off the surface of a wafer  24  at a relatively large angle to the surface normal through prism  218 . Using this method of “grazing incidence” for the laser beam off the surface of the wafer and by using “phase modulation” techniques the basic data is readily transferred from the video camera (not shown) to the computer interface for analysis by the computer using Fourier series phase calculation techniques to determine the surface variations and direction of the surface variation. By striking a surface at a shallow angle with the laser beam, objects which are considered quite diffuse become near-perfect reflectors of the laser beam. 
     Referring to drawing FIG. 7, illustrated is the increase in reflection as the angle of grazing incidence is increased. 
     Referring to drawing FIG. 8, illustrated in TABLE 1, are wafer surfaces of wafers of varying materials and associated wafer surface conditions for which grazing interferometers have been used to measure. 
     It will be evident to those skilled in the art that various changes and modifications may be made in the methods and apparatus as disclosed herein without departing from the spirit and scope of the invention as defined in the following claims.