METHOD AND APPARATUS OF USING A TWO BEAM INTERFERENCE MICROSCOPE FOR INSPECTION OF INTEGRATED CIRCUITS AND THE LIKE

Abstract: A specially adapted Linnik microscope is used in combi­ nation with a video camera, a wafer transport stage and data processing electronics to form a novel inspection apparatus based on the use of the two beam interference microscope. The apparatus can utilize either broad band or narrow band light to develop a plurality of interference images taken at dif­ ferent axial positions relative to the surface under investiga­ tion. The point-by-point brightness along scan lines across such images is then used to develop data which is proportional to the degree of coherence (or to the fringe amplitude, the var­ iance of the fringes, or the amplitude of oscillation of the fringes) as the optical path difference is varied in a two beam optical or acoustic microscope.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates generally to precision optical inspection methods and apparatus, and more particularly to a method and apparatus for performing microscopic inspection and measurement of integrated circuit wafer geometry using interference microscopy in combination with electronic image processing. Discussion of the Prior Art It has long been desired that means be provided to inspect and measure the characteristics of microminiature surfaces such as those formed in integrated circuit wafers. One such characteristic of interest is the line widths of the various traces produced on a wafer surface during IC device manufacture. One prior art technique for integrated circuit metrology includes the use of an ordinary microscope with some form of electronic detector positioned at the image plane. For example, video cameras, scanning slits (see U.S. Patent No. 4,373,817), shearing systems and linear arrays, have all been used as detectors with ordinary microscopes. However, the capability of the ordinary microscope is limited in that it can only measure the intensity of the optical wave amplitude and cannot measure the complex phase of the amplitude. As a consequence, the three-dimensional nature of integrated circuit surfaces makes use of the classical microscope impractical for precision surface inspections and measurements of this type. Other prior art techniques have used confocal laser scanning microscopes to obtain three-dimensional data relating to integrated circuit surfaces. FR-A-2 143 204 discloses an inspection system employing television techniques for evaluation of speckles produced by a laser beam. This inspection system uses a comparison between two video signals each resulting from irradiation of a photosensitive screen with coherent light consisting of two interfering beams, the light of one beam having been scattered from a surface to be inspected and imaged on the screen, the light of the other beam having been reflected by a reference surface. The point-by-point variations in the difference between the two resultant speckle patterns are used to provide a picture representative of the deformation of the surface. By US-A-4 340 306, a method for developing a topographical map of an object surface using a white light interferometer and detecting the occurence of zero path length differences at various points on the object by looking for maxima in the fringe intensity has been disclosed. This method, however, is not suited for constructing synthetic images. In addition, problems may arise, because this method is too sensitive to the relative reflectivities with respect to an object extending vertically. A further thorough treatment of the subject may be found in T. Wilson and C. Shepard (1984), Theory and Practice of Scanning Optical Microscopy, Academic Press. Aside from the complexity and relatively high cost associated with the use of confocal laser devices and techniques, the fact that such techniques use monochromatic light makes them subject to inaccuracies caused by destructive interference for certain thicknesses of transparent films often found in semiconductor devices. Summary of the Present Invention It is therefore a principal object of the present invention to provide an improved method and apparatus for accomplishing three-dimensional inspection of integrated circuits and the like. Another object of the present invention is to provide an improved synthetic imaging technique utilizing a two beam interference microscope. Still another object of the present invention is to provide a method and apparatus by which the top width, bottom width and height of an integrated circuit line may be accurately measured. Briefly, a preferred embodiment of the present invention includes a specially adapted Linnik microscope in combination with a video camera, a wafer transport stage and data processing electronics to form a novel inspection apparatus based on the use of the two beam interference microscope. The apparatus can utilize either broad band or narrow band light to develop a plurality of interference images taken at different axial positions relative to the surface under investigation. The point-by-point brightness along scan lines across such images is then used to develop data which is proportional to the degree of coherence (or to the fringe amplitude, the variance of the fringes, or the amplitude of oscillation of the fringes) as the optical path difference is varied in a two beam optical or acoustic microscope. Among the advantages of the present invention are that it provides a much simpler and more economical technique than those using the confocal microscope. Another advantage is that it can use white light rather than monochromatic light and as a result, can have a signal-to-noise ratio which is not degraded by coherent speckle effects which affect any coherent optical system. Moreover, by using white light the possibility of destructive interference for certain thicknesses of transparent films is eliminated. Furthermore, the theoretical resolution along the optical axis appears to be better than that for a confocal microscope because the short coherence length of white light effectively reduces the depth of focus of the instrument. Empirically, it also appears that the present invention substantially improves the lateral resolution of the microscope, at least for the purpose of measuring linewidths of integrated circuits. These and other objects and advantages of the present invention will no doubt become apparent to those skilled in the art after having read the following detailed description of the preferred embodiments which are illustrated in the several figures of the drawing. In the Drawing Fig. 1 is a schematic diagram depicting the basic functional components of the present invention; Fig 2 is an isometric diagram illustrating an integrated circuit line and five inspection levels; Figs. 3 through 7 are actual photographic depictions of interference images taken at the levels 5 through 1 respectively, of Fig. 2; Fig. 8 is a flow diagram functionally depicting operation of the electronic processing electronics of the present invention; Fig. 9 is an RMS profile of a central column developed in accordance with the present invention; and Fig. 1⊘ is a depiction of a CRT display in accordance with the present invention. Detailed Description of the Preferred Embodiment Interference microscopes can measure the topography of reflective surfaces using standard techniques so long as the undulations in relief are within the depth of field of the imaging system, and so long as the topography is not so jagged as to confuse the fringe counting algorithm. The basic formula is where Δ h is the difference in height between two points in the image, Δ is the phase difference, and λ is the wave length of light. The standard applications of the Linnik microscope in this context are given in "Incident-Light Microscope Inteferometer for the Orthoplan and Metalloplan", Instruction Manual for Use of the Linnik Microscope Attachment by Ernst Leitz Gmbh, Wetzlar (1980); and in LEITZ, "Incident-Light Interference Illuminator for the Orthoplan/Metalloplan, a module which uses the wave length of light for measurement (1980)". However, these standard techniques break down when any of the following three conditions are present: 1. The topographic fluctuations on the object surface within the field of view exceed the depth of focus of the microscope; 2. The object consists of transparent structures formed on an opaque substrate; or 3. The object surface has steep cliffs or walls the vertical extent of which exceeds a half wavelength of light. When any of these cases occur, as they often do in integrated circuit devices, the standard use of the Linnik or other two beam interference microscopes simply does not give useful data because the fringe counting algorithm provides hopelessly confused and incorrect results when combined with equation 1. The analysis process of the present invention described hereinafter overcomes the difficulties encountered by the standard techniques of Linnik interference microscopy, and when implemented in electronic hardware offers new capabilities for automated inspection of semiconductor devices. The basic concept of the present invention is that broad band illumination (white light) has a very short coherence length, and by measuring the degree of coherence between an object beam and a reference beam at each point in an image, a powerful light sectioning technique may be developed. The principle can be illustrated with scalar defraction theory. However, the basic ideas apply in general even when scalar defraction theory does not provide a good approximation. Consider the wave equation of light in a homogeneous medium: where c is the speed of light in the medium and U (x, t) is the usual scalar wave approximation to the electric field. U may be written as a Fourier integral in the form where The spectral density is where α is a normalization constant and δ is a delta function. The degree of first order coherence is where the brackets 〈 〉 denote ensemble average. If the wave U is a sum of two constituent waves: U = U₁ + U₂, (7) then the degree of coherence between U₁ and U₂ may be defined analogously as Referring now to Fig. 1 of the drawing, apparatus in accordance with the present invention is schematically shown to include a spotting microscope 10, a LINNIK microscope 12, an X,Y stage 14 for carrying a wafer 16 and a piezo-electric vertical motion system 18 between a set up position beneath microscope 10 and an inspection position beneath microscope 12, a pair of videocameras 20 and 22, data processing electronics 24, a CRT display 26 and an electronic controller and operator interface console 28. The spotting microscope 10 and camera 20 are used to allow an operator to visually locate, using CRT 26, a particular wafer surface feature (such as a resist line, for example) to be inspected. Once located and aligned with the optical axis of microscope 10, a predetermined movement of the wafer-carrying stage 14 will place the feature directly within the field of view of microscope 12. In a two-beam interference microscope (such as the LINNIK microscope), a light wave from a source 31 reaching the image plane 36 is a sum of two constituent waves; one reflecting off the surface of the object 16, and the other reflecting off the surface of a reference mirror 34. Thus wave energy emitted from source 31 is split by reference mirror 34 into object wave energy passing from the object 16 through an object channel 30 to the image plane 36, and into reference wave energy passing from the reference surface formed by reference mirror 34 through a reference channel 32 to the image plane 36. Fringes are seen in the image at 36, even when white light is used to illuminate the object. If broad band illumination (white light) is used, strongest fringing occurs when the path difference between the reference channel 32 and the object channel 3⊘ is very small, on the order of a fraction of the average wavelength, because the coherence length of white light is very short. When the degree of coherence is high between the reference channel and the object channel, the fringes are strong. Conversely, when the degree of coherence is low, the fringes are weak. In the preferred embodiment, white light Kohler illumination is provided by a Xenon arc lamp 31, and a shutter 33 is included to flip the reference beam in and out. The fringe rate and direction can be controlled on commercially available Linnik microscopes by moving the microscope objective in the reference channel off-axis. Accordingly, in the preferred embodiment the lens 35 is positioned to make the fringes which appear at image plane 36 be parallel to camera 22's raster direction and the fringe spacing equal to 32 horizontal raster rows of camera 22. The connection between degree of coherence and fringe intensity may be described as follows where U₁ is the object wave and U₂ is the reference wave. At the image plane, the superposition of the object wave and the reference wave results in the light intensity 〈|U₁ + U₂|²〉 = 〈|U₁|²〉 + 〈|U₂|²〉 + 2Re〈U₁ *U₂〉 (9) The overall path length difference between the reference channel and the object channel can be varied so as to introduce a phase difference between the object channel and the reference channel. In the narrow bandwidth approximation, the phase shift will be the same for all frequencies of the light. In this case, intensities at the image plane are of the form 〈|U₁ + e i U₂|²〉 = 〈|U₁|²〉 + 〈|U₂|²〉 + 2Re[e i 〈U₁ *U₂〉] (1⊘) The variance in equation 1⊘, calculated by letting  vary from -π to π is easily found to be Variance of 〈|U₁ +e i U₂|²〉 = 2|〈U₁ *U₂〉|² (11) and therefore the degree of coherence may be expressed as In the present case, the illumination is actually broad band and the phase shift is different for the different frequencies of the light. However, in this case it is found that the following functional form for 〈U₁*U₂〉 is a good approximation for images taken in a two beam interference microscope: 〈U₁*U₂〉=me R(ℓ) e iℓK(ℓ) , ℓ = path difference, (13) where R(ℓ) and K(ℓ) are slowly varying over the distance 2π/K(ℓ), and therefore equation (12) is still deriveable provided that  is made to vary through 2π by letting ℓ vary from -π/K(ℓ) to π/K(ℓ) in the calculation of the variance. The parameter "m" in equation (13) is a complex constant. Therefore, one can define an easily measurable quantity C(x,t) which may be taken as a practical measure of the degree of coherence as C(x,t) = √ ½Variance over path ℓ of 〈|U₁ + U₂|²〉 (14) If U₁ and U₂ are not coherent, then C=⊘. In general, assuming that R(ℓ) and K(ℓ) are slowly varying in equation (13), it can be shown that C= (〈|U₁|²〉〈|U₂|²〉) ½ G (15) The technique of the present invention is to synthetically construct images the brightness of which at each image point is proportional to C. This amounts to imaging by means of a coherence probe. The interference microscope 12 is set up in the following way prior to calculation of C. With a first surface mirror (not shown) as the object (at 16), the focus in the object channel 3⊘ and the reference channel 32 are adjusted so that both the reference mirror 34 and the object mirror are simultaneously in focus. Then the path difference is adjusted until the maximum degree of coherence is obtained between the object wave and the reference wave. This reference position is then the center point in the variation of path difference used to measure the degree of coherence. If fringe data is being used, as described below the setup is a little different. In such case, a window in the center of the image plane 36 is selected as the area of interest, and after focusing the reference and object mirrors, the path is adjusted so that the fringe amplitude is the greatest at center of the window. The object mirror is then replaced by an object such as a silicon wafer 16 having an integrated circuit formed in its upper surface. All parts of the object surface which are at the same "level" as the surface of the reference mirror will now produce a scattered wave which is relatively coherent with respect to the reference wave, and those image points end up being bright in the final image at 36 (Fig. 1). The very brightest points are those where the object locally is a horizontally reflective surface because at those points the object wave and reference wave match is best. Parts of the object which are at a different level than the reference mirror appear dark. Sectioning can then be accomplished by moving the wafer 16 up or down to obtain successive images corresponding to respective object planes passing through the wafer 16, as illustrated in Fig. 2. The degree of coherence C can be measured in a Linnik microscope in several ways. One way is to vary the path length of the reference channel 32; for example, through one or more wavelengths centered on the reference position, and while doing this, calculating electronically the oscillation variance in intensity at each point in the image plane of the microscope. The amplitude of oscillation is proportional to C. Alternatively, for object surface features which do not vary too quickly in one direction (such as in the case of a semiconductor integrated circuit line) the interference fringes may be adjusted so that they lie perpendicular to a line to be inspected. The fringe spacing may also be adjusted to any convenient value. In this case C is simply the amplitude of the fringes within the window, i.e., C = Fringe Amplitude in window (16) The advantage of this technique is that, as will be further explained below, only one image is required to make a measurement of C at all points across the line. Figs. 3-7 show photographs of actual fringe data taken at the different object elevations generally depicted in Fig. 2. The object in this case was a silicon wafer having a one micron high resist line 4⊘ formed on its upper surface. The vertical and horizontal white lines depicted in the photographs are an electronic overlay produced on the display CRT 26 and can be ignored for this discussion. The resist portion 4⊘ is in the center of each photo, and as depicted in Fig. 2, the five photos of Figs. 3-7 are taken at different positions of the object along the vertical (or Z) axis. More specifically, the plane of focus of the image shown in Fig. 3 is slightly above the top surface of the resist line 4⊘, i.e., at level 5 in Fig. 2. Within the central window 42, framed in black in the photos, the fringe intensity is shown to be weak. Fig. 4 shows the wafer raised a few thousand Angstroms to bring the top of the resist into focus (at level 4 of Fig. 2). Fringes 43 in the window 42 are now strong in the central portion of the image (corresponding to the top surface of the resist) but are weak on either side (where no resist is present). Fig. 5 shows the wafer again raised an additional few thousand Angstroms so that the focal plane is between the top level of the resist and the silicon substrate (level 3 in Fig. 2). Here again, the fringes are weak in both the resist region and the silicon region since neither is in focus. In Fig. 6 the wafer has again been raised another few thousand Angstroms to level 2 to bring the silicon surface 44 (Fig. 2) into focus. Here the fringes 45 are strong on the silicon and are fairly weak in the resist portion of the image. In Fig. 7, only the resist shows strong fringing due to the reflection of the light off the bottom of the resist layer. The process by which fringe amplitudes are used to measure line widths on integrated circuits is shown in the flow chart of Fig. 8. The boxes describe the algorithms used. The "column in window" referenced in Fig. 8 refers to the columns of pixels, one of which is shown at 46 in Fig. 2, scanned by the processor 24 to determine the variance values over the length of the window 42, to thereby calculate the fringe coherence. More particularly, the image 36 is scanned by video camera 22, which develops an analog raster scan thereof for input to the processing electronics 24. The first processing step is to convert the analog data into 8 bit digital form and to store the data in a computer memory. A "window" such as is illustrated at 42 in Fig. 2, is then scanned a pixel column at a time, as illustrated at 46 and the data corresponding to each column is moved by DMA transfer to a high speed arithmetic processor which calculates the variance of each columnar array and stores its RMS value in memory. After data is collected across the window 42 the stage is incremented in the Z direction to another level, another scanning operation is completed and the data stored. This operation is repeated at levels separated by approximately 5⊘⊘ Angstroms until sufficient data is obtained to evaluate all desired surfaces. The several sets of scan data stored in memory is then itself scanned at the centermost point X R in "X" of the resist line 4⊘ (along the Z axis in Fig. 2) to determine the level having the highest RMS value, and such level is determined to coincide with and thus identify the top of the scanned line. One such electronic scan is depicted in Fig. 9 wherein the ordinate represents the RMS value of the central column data and the abscissa represents the inspection level (or stage position along the Z axis). As illustrated, the peak at level 4 corresponds to the top of the resist line 4⊘ of Fig. 2 while the peak at level 2 corresponds to reflection off the wafer substrate 44. The horizontal distance between the two peaks is thus an indication of the vertical thickness of the resist line 4⊘. The memory is thereafter scanned at an X position X S over only the substrate to find the scan line having the brightest level and this is taken to be the bottom of the line. By subtracting the top level from this bottom level information the height of the line can be determined. The next step is to apply an edge finding algorithm to the memory location corresponding to the top of the line in order to determine the raw top width. An edge finding algorithm is then applied to the memory location corresponding to the bottom of the line to determine the raw bottom width. The final results for top and bottom width are calculated from the formulas top width = A₁ * raw top width + B₁ * raw bottom width + C₁ bottom width = A₂ * raw top width + B₂ * raw bottom width + C₂. The constants A, B, and C are determined by calibration procedure. Once these widths are determined they can be reported to the CRT and displayed to the user. Fig. 1⊘ shows an artist's rendition of the synthetic images produced on a CRT screen by imaging C. The upper and lower righthand quadrants 5⊘ and 52 show cross sections of the line, each row corresponding to a different level (scan line). The elevated resist 54 appears as a cloud above the silicon substrate. An electronic line 56 has been drawn in the right upper quadrant through the algorithm's choice as the best row to call the top of the line. The upper left hand quadrant 58 shows this row expanded vertically to fill the entire quadrant and to thereby look like a top down view. The lower right hand quadrant 52 showns a line 6⊘ drawn through the algorithm's choice as the best candidate for the bottom substrate level. The lower left quadrant 62 depicts an expanded top down view of the line 6⊘. The edge finding algorithm then uses a threshold technique to find the edges in both the upper and lower left hand quadrants. In this way the height is known (by the difference in stage position between the top and bottom rows), the top width is known by the distance between the edges in the upper left quadrant, the bottom width is known by the distance between the edges in the bottom left quadrant, and the wall angles may be calculated. For best results, calibration to scanning electron microscrope results are required. In operation the technique consists of first aligning a semiconductor IC line (such as 4⊘ in Fig. 2) in the field of view of the spotting microscope 1⊘. The wafer is then moved a fixed offset to the right, as illustrated in Fig.1, so that the same line is now viewed by the Linnik microscope 12. The interference fringes are then pre-adjusted to lie perpendicular to the line direction, and the fringe spacing is adjusted to have two complete fringes within the window (of variance calculation) 42 (Figs. 3-7) when a reflective plane in the object is in focus (see Fig. 4 for example). The wafer is then dropped in the z direction so that the highest point in the line is several thousand Angstroms below the focal plane, and the resulting image at 36 is digitized by the electronics 24. The fringe amplitude is then calculated for each scan column 46 (Fig. 2) in the window 42, and the result is stored in memory. One method of calculating the fringe coherence is to calculate the variance of each column array 46 (Fig. 2) across the window 42. To accomplish this for each array 46, the arithmetic processor in the electronics 24 calculates: RMS = RM√ Variance The stage is then moved up a small distance (about 5⊘⊘ Angstroms) and another image is digitized, fringe amplitudes calculated, and results stored. The process of moving the stage up, digitizing the image, and storing the fringe amplitudes is repeated until the stage has scanned a sufficient distance to place the lowest point of the line above the optics focal plane so that the entire depth of the line has been sectioned. The top and bottom levels of the line are then determined by finding the brightest scan rows at the appropriate positions in the image. Then the edges of the line at the top and bottom levels are determined by an edge finding algorithm. Finally, the output data is calculated by means of a calibration formula which calculates the top and bottom width by the formulas: Top width = A₁ * (Raw top width) + B₁ * (Raw bottom width) + C₁ Bottom width = A₂ * (Raw top width)+B₂ * (Raw bottom width)+C₂ where A₁, A₂, B₁, B₂, C₁ and C₂ are determined by a calibration procedure, and the results maybe reported to the CRT 26. The results consist of the top width and bottom width, as well as the positions of the four edges used in the calculation of the vertical wall angles. Although the present invention has been disclosed in terms of a preferred embodiment, it is anticipated that, following a reading of this disclosure, numerous alterations and modifications thereof will become apparent to those skilled in the art. It is therefore intended that the appended claims be interpretted as covering all such embodiments as fall within the true spirit and scope of the invention.

Claims: A method of inspecting an object and generating synthetic image data including the steps of: (a) using an interference optical system including an object channel (30) and a reference channel (32) for simultaneously inspecting an object (16) and a reflective reference surface (34) and developing a plurality of images formed by the interference between object wave energy passing from said object (16) and through said object channel (30) to an image plane (36) and reference wave energy passing from said reference surface (34) and through said reference channel (32) to said image plane (36); (b) measuring for each image the degree of coherence between said object wave energy and said reference wave energy at each pixel point of a predetermined array of pixel points in said image plane (36) and developing coherence data corresponding to each said pixel point, and proportional to the brightness of each said pixel point; (c) using said coherence data to generate synthetic image data representative of a particular characteristic of said object (16); characterized in that: (d) each said image is formed in response to a change in position of either said object (16) or said reference surface (34); (e) for each said image the absolute value of the degree of coherence between said object wave energy and said reference wave energy is determined by calculating the variance along each column of an array of mxn pixels in said image plane (36), where m and n are integers, and generating absolute value coherence data corresponding to each said column; (f) the brightness of each pixel element of a synthetic image developed using said synthetic image data is proportional to said absolute value coherence data. A process for generating synthetic image data representative of a cross-section of an at least partially reflective irregular surface of an object (16), including the steps of: (a) illuminating the irregular object surface with light from a source (31) of illumination; (b) illuminating a reflective reference surface (34) with light from said source (31) of illumination; (c) collecting object light reflected from said object surface and directing said object light along a first optical axis having a first optical path length; (d) collecting reference light reflected from said reference surface (34) and directing said reference light along a second optical axis at least a portion of which is parallel to said first optical axis, said second optical axis having a second optical path length different from said first optical path length; (e) focussing the light directed along said first and second optical axes to form a fringed image pattern resulting from interference of said object light and on said reference light; (f) scanning said image pattern to develop a series of data corresponding to the fringe amplitude along a selected scan line; characterized in that: (g) the position of said object (16) along said first optical axis is incrementally changed, each time repeating steps (c) through (f); and (h) the plurality of series of data is processed to develop said synthetic image data corresponding to a cross-sectional profile of said object surface taken in a plane including said scan lines. A process as recited in claim 2, further comprising the step of: (i) displaying said synthetic image data to visually depict a cross-sectional profile of said object surface taken in the plane including said scan lines. A process as recited in claim 3, wherein said reference surface (34) is formed by an optically flat mirror and said object surface is formed by a portion of a semiconductor wafer having an elongated strip (40) of raised surface extending therethrough, and further comprising the steps of: (j) orientating said wafer so that said selected scan line is directed substantially orthogonal relative to the length of said elongated strip (40); and (k) determining the position of said object along said first optical axis when the value of said data corresponding to the crossing of a first particular scan line over said raised surface is at a maximum relative to the corresponding data of the other scan lines and identifying this portion as corresponding to the top surface of said strip (40). A process as recited in claim 4, further comprising the step of: (l) detecting the width of the top of said raised surface by measuring the length of the portion of said first particular scan line over which said data is at a maximum. A process as recited in claim 5, further comprising the steps of: (m) determining the position of said object (16) along said first optical axis when the value of said data corresponding to the crossing of another particular scan line over portions of said surface other than said raised surface are at a maximum relative to the corresponding data of the other scanned lines; and (n) determining the width of the base of said raised surface by measuring the separation between the portions of said other particular scan lines over which said data is at maximum. A process as recited in claim 6, further comprising the step of: (o) determining the height of said raised surface above the adjacent wafer surface by measuring the distance between the position at which the object (16) is positioned along said first optical axis, when the width of the top of said raised surface is detected and the position along said first optical axis, when the width of the base of said raised surface is detected. A process as recited in claim 7, further comprising the step of: (p) calculating the slopes of the side walls of said raised surface, in the plane including said scan lines, as a function of the height of the top of the raised surface above the base thereof and the difference in width of the top and the base along the corresponding scan lines. A method of inspecting an object and generating synthetic image data including the steps of: (a) using an interference optical system including an object channel (30) and a reference channel (32) for simultaneously inspecting an object (16) and a reflective reference surface (34) and developing an image formed by the interference between object wave energy passing from said object (16) and through said object channel (30) to an image plane (36) and reference wave energy passing from said reference surface (34) and through said reference channel (32) to said image plane (36); (b) evaluating the fringe pattern formed by the interference between said object wave energy and said reference wave energy for measuring the degree of coherence between said object wave energy and said reference wave energy at each pixel point of a predetermined array of pixel points in said image plane and developing coherence data corresponding to each said pixel point; (c) using said coherence data to generate synthetic image data representative of a particular characteristic of said object (16); characterized in that: (d) said synthetic image data is developed by calculating the intensity of the interference fringes (43) in said image pattern and by calculating the local variance of the image intensity caused by the fringes (43). Object inspection apparatus, including: (a) interference means including an object channel (30) and a reference channel (32) for simultaneously inspecting an object (16) and a reference surface (34), and for developing corresponding image formed by the interference between object wave energy passing from said object (16) and through said object channel (30) to an image plane (36), and reference wave energy passing from said reference surface (34) and through said reference channel (32) to said image plane (36); (b) means for measuring the degree of coherence between said object wave energy and said reference wave energy at a predetermined array of pixel points in said image plane (36) and for developing coherence data corresponding to each said pixel point; (c) means responsive to said coherence data and operative to generate synthetic image data representative of particular characteristics of said object (16); characterized in that: (d) said interference means includes a microscope means (12); (e) stage means (14) for holding and selectively moving said object (16) relative to the object plane of said object channel is provided. Object inspection apparatus as recited in claim 10, wherein said means for measuring the degree of coherence includes means for converting the light intensity of each said image at each pixel point in said array into corresponding signal values, and said means responsive to said coherence data includes means for measuring the variance of the signal values along each column of each said array to develop a plurality of series of variance values related to each scan of each said image, and means for using said plurality of series of variance values to develop said synthetic image data. Object inspection apparatus as recited in claim 11, wherein said means responsive to said coherence data further includes display means (26) responsive to said synthetic image data and operative to generate a visual image corresponding to said particular characteristics of said object (16). Object inspection apparatus as recited in claim 10, wherein said interference microscope means is a Linnik microscope (12) having an off-axis, movable objective lens (35) forming a part of the reference channel (32). Apparatus for detecting particular characteristics of the surface of a micro-miniature device, comprising: (a) stage means (14) for holding and selectively moving said micro-miniature device; (b) an optically flat mirror means (34) disposed in a predetermined relationship relative to the upper surface of said device; (c) a source (31) of illumination for simultaneously illuminating said device and said mirror means (34); (d) means for collecting object light reflected from the upper surface of an illuminated portion of said device and for directing it along a first optical axis having a first predetermined length; (e) means for collecting reference light reflected from the surface of said mirror means and for directing it along a second optical axis having a second predetermined length different from that of said first optical axis; (f) means for focussing light reflected along said first and second optical axes to form a fringed image resulting from interference between said object light and said reference light; (g) means associated with said stage means (14) for incrementally moving said device in step-wise fashion along said first optical axis and between extremes on opposite sides of the object plane of said means for collecting object light; (h) means for scanning said fringe image along a scan line during the interval between each incremental motion of said device and for developing a series of data corresponding to the fringe amplitude along each scan line; (i) means for storing the data developed during each said scan; and (j) means for processing said stored date to develop information representative of a cross-section of the surface of said device lying in the plane including each said scan line. Apparatus as recited in claim 14, wherein said mirror means (34), said source (31) of illumination, said means for collecting object light, said means for collecting reference light, and said means for focussing light are formed by a two-beam interference microscope (12). Apparatus as recited in claim 15, wherein said means for incrementally moving said device includes a piezo-electric crystal vertical motion system. Apparatus as recited in claim 16, wherein said means for scanning includes a videocamera means (22). Apparatus as recited in claim 14, wherein said means for collecting reference light includes an off-axis movable objective lens means (35) for allowing pre-selection of difference between said first predetermined length and said second predetermined length.

Inventors: DAVIDSON, MARK
Applicants: KLA INSTRUMENTS CORPORATION
Application year: 1987
Main classification: G01B11
IPC codes: G01B11, G02B21, G01B9, G01N21