WO1993024805A1

WO1993024805A1 - Interferometric method and apparatus to measure surface topography

Info

Publication number: WO1993024805A1
Application number: PCT/US1993/005242
Authority: WO
Inventors: James F. Biegen
Original assignee: Zygo Corporation
Priority date: 1992-06-03
Filing date: 1993-05-28
Publication date: 1993-12-09

Abstract

A non-contact method for measuring the material-dependent optical phase change on reflection that occurs using phase shifting interferometry. A phase shifting interference microscope utilizing an extended, quasi-monochromatic illumination source (12-18) is used to generate a two beam interference intensity pattern at a given field position on a detector array (30). A reference surface (24) of known surface characteristics and an unknown test surface (32) being profiled are axially translated relative to each other while the interference intensity pattern impinging on the detector array (30) is sampled, digitized, stored and then utilized to produce a digitized two-beam interference intensity pattern, the shape of which is characteristic of the particular interference configuration.

Description

INTERFEROMETRIC METHOD AND APPARATUS TO MEASURE SURFACE TOPOGRAPHY

FIELD OF THE INVENTION

The invention relates generally to precision optical metrology instrumentation, specifically to optical phase shifting interferometers and, more particularly, to increasing the accuracy of phase shifting interference microscopes used in measuring the topography of test surfaces whose index of refraction and hence optical phase change on reflection may vary significantly as a function of field position.

BACKGROUND OF THE INVENTION

When using phase shifting interference microscopy to measure the optical phase of a heterogeneous test surface comprised of two or more dissimilar materials, at least one of which has a complex index of refraction, measurement errors result because the amount of optical phase change on reflection varies with each individual material forming the test surface. Materials with complex indices of refraction produce an optical phase change on reflection that, in general, varies from material to material as well as from dielectrics, whose optical phase change on reflection is either 0 or π. For example, conventional phase shifting interferometric techniques cannot distinguish on one "Ξ_*- hand, an in optical phase difference between two points of different height on a test surface and, on the other, an optical phase difference between two points on a test surface of the same height but composed of disparate materials with at least one having a complex index of refraction. Use of the technique of phase shifting interferometry with an interference microscope to measure the profile of a test surface is known. Such use is described, for example, in Biegen, J.F. and Smythe, R.A. , "High-Resolution Phase Measuring Laser Interferometric Microscope for Engineering Surface Metrology", presented at the Fourth International Conference on Metrology and Properties of Engineering Surfaces at the National Bureau of Standards, Washington, D.C., Apr. 13-15, 1988. As applied to interference microscopes, the phase shifting interferometric technique provides effective and accurate test surface profile measurements so long as the test surface is homogeneous, i.e. comprised of a singular material having either a complex or a non-complex index of refraction. The optical phase change on reflection from a homogeneous test surface is constant as a function of field position and, therefore, does not affect the accuracy of the test surface profile measurement. When, however, the test surface to be profiled is heterogeneous and the optical phase change on reflection accordingly varies significantly as a function of field position, the optical phase map produced by phase shifting interferometry no longer represents an accurate geometrical profile of the test surface. In conventional phase shifting"interferometry there is no way to extract^~the optical phase change component induced by reflection of the illumination beam off the test surface from the total optical phase that is measured. This has been a fundamental limitation in the utility and practice of conventional phase shifting interferometry. Also known is the technique of using values of n and k — the real and imaginary parts of a material's complex index of refraction previously measured using instrumentation other than phase shifting interferometers, to correct subsequent phase shifting interferometry measurements. This prior art technique, however, has serious limitations. The n and k measurements are usually made on representative materials, rather than on the actual test surface, and even minor differences in material composition between the representative material and the test surface material can introduce large errors in the optical phase correction. Moreover, when correcting optical phase measurements carried out with a moderate to high numerical aperture microscope interferometer objective, both the quantities n and k and a measure of the average illumination beam angle of incidence to the test surface are needed. The average illumination beam angle, which is a function of the numerical aperture and of the illumination beam intensity distribution at the entrance pupil of the microscope interferometer objective, can only be found through empirical means with this technique and, as such, introduces a source of potentially significant measurement error in the test surface profile. Previous techniques for the direct measurement of optical phase change on reflection, as for example described in J. Bennett, "Precise Method for Measuring the Absolute Phase Change on Reflection", 54 J. Opt. Soc. Am. G12-24 (1964), are difficult, time consuming, limited in use to transparent films and, being experimental methods, produce results independent of the actual test surface so that even if the measurement results are correct there is no certainty that the result actually represents the material on the test surface. A method of directly measuring the phase change on reflection by overcoating the test surface with a homogeneous opaque material is described in Smythe, R. , Selberg, L. and Deck, L. , "Pole Tip Recession Measurement of Transducers on Thin Film Sliders for Rigid Disk Drives", presented at the International Disk Conference in Tokyo, Japan, April, 1992. Other prior art techniques that utilize the axial position of maximum fringe contrast for profiling the test surface^" emphasize the use of broad spectral bandwidth (i.e. white light) as the preferred illumination. See U.S. Patent No. 4,340,306 to Balasubramanian; M. Davidson et al., "An Application of Interference Microscopy to Integrated Circuit Inspection -and Metrology", 775 SPIE 233-247 (1987); G.S. Kino and S.T. Chim, "Mirau Correlation Microscope", 29 Applied Optics 3775-83 (1990); and B.S. Lee and T.C. Strand, "Profilometry With a Coherence Scanning Microscope", 29 Applied Optics 3784-88 (1990) . A broad illumination spectral bandwidth reduces crosstalk between vertically adjacent features, permitting depth slicing as in confocal scanning microscopy. A broad bandwidth also allows for a theoretically unlimited test surface feature measurement range with high vertical resolution. And with a large illumination spectral bandwidth, the axial interference region is small and independent of the objective magnification or numerical aperture so that vertical resolution is constant across the magnification range and is not a function of the objective depth of focus, as it is in confocal scanning microscopy. One disadvantage of broad illumination spectral bandwidth is that the axial position of maximum interference contrast will shift position as a function of material type. This renders the technique as inaccurate as conventional phase shifting interferometry when measuring heterogeneous test surfaces comprised of two or more dissimilar materials with at least one having a complex index of refraction. In contrast, the herein disclosed method and apparatus of the invention extend and improve the technique of conventional phase shifting interference microscopy by ^~additionally measuring the optical phase change on reflection from the surface of metals, semi-metals, and dielectrics being profiled, and thereby correct a systematic measurement error that occurs when utilizing conventional phase shifting interference microscopy.

OBJECTS OF THE INVENTION Accordingly, it is a principal object of the present invention to increase the accuracy of phase shifting interference microscopy for use with a test surface comprised of two or more materials, at least one of which has a complex index of refraction. It is a particular object of the invention to accurately profile a test surface comprised of materials with complex indices of refraction. It is another object of the invention to measure the optical phase change on reflection from a test surface comprised of materials with complex indices of refraction. A further object of the invention is to use previously measured optical phase change on reflection values to correct any subsequent optical phase measurements of test surfaces in an application of conventional phase shifting interferometry.

SUMMARY OF THE INVENTION

Briefly described, the present invention provides an apparatus and technique for measuring the profile and optical phase change on reflection from surfaces of materials whose index of refraction is complex. The apparatus comprises a phase shifting interference microscope having an extended, quasi-monochromatic illumination source. The microscope objective is equal or near-equal path, with a moderate to high numerical aperture. A solid-state camera array having multiple detector pixel sites is located in an image plane of the interference microscope. On the receptive surface of the camera array, in the image plane, the test and reference surfaces are imaged together with the two-beam interference intensity pattern which represents the optical path difference, or optical phase difference, between the test and reference surface wavefronts. Using an equal or near-equal path two-beam interferometer having an extended, quasi-monochromatic * illumination source, the ^"■'" interference intensity varies as the geometrical path diϊference between the test surface and the reference surface is varied axially along the so- called z-axiε, generally an imaging axis of the microscope. The resultant intensity function has the form of a cosine wave whose amplitude is modulated by a slowly varying function known as the modulus of the complex degree of coherence, or coherence modulus. The axial extent of the coherence modulus is a function of the diameter of the illumination source and its spectral bandwidth, of the numerical aperture of the microscope interferometer objective, and of the wavelength of the illumination b">.am. In operation, during an initial macro focusing procedure, the test surface is moved or translated axially along the z-axis, inside focus, until the relative separation between the test and reference surfaces is greater than the axial extent of the coherence modulus, i.e. to a position at which the interference contrast is essentially zero. Then, by means of a piezoelectric transducer (PZT) crystal or the like, the test surface is linearly translated axially at a constant velocity along the z-axis in the direction of increasing interference contrast so as to vary the geometrical path separation between the unknown test surface and the known reference surface until the interference intensity values along the entire axial extent of the coherence modulus have been scanned. For a high ^" numerical aperture microscope interferometer objective, the coherence modulus axial extent is on the order of a few micrometers. The scanned intensity values received at the detector array are converted to electrical signals, sampled at a predetermined sampling frequency, digitized, and stored sequentially in a computer or processor memory. The stored intensity values obtained at any given pixel site of the detector array represent the two-beam interference intensity pattern of the particular interferometer configuration as a function of the geometrical path separation between the test and reference surfaces at a specific portion of the test surface, typically represented by the coordinates x and y. The digitized two-beam interference intensity pattern data is then analyzed to locate and identify the point along the axial scan of maximum interference contrast in the detected intensity pattern. For an extended, quasi-monochromatie- illumination source, the axial position of the coherence modulus center — i.e. the position of maximum interference contrast — is constant, is independent of material type, and occurs at that point on the interference intensity pattern at which the geometrical path difference between the test and reference surfaces is zero. The material-dependent optical phase change on reflection from the test surface produces a phase shift of the axially-varying cosine function relative to the center of the coherence modulus. Having determined the center position of the coherence modulus from the digitized intensity data stored in computer memory, the material-dependent optical phase change on reflection from the test surface can then be obtained by calculating the phase of the two-beam interference intensity pattern at the position of the center of the coherence modulus using conventional phase shifting interferometry algorithms. One phase shifting interferometry algorithm uses the value of the interference intensity measured at the coherence modulus center with at least two other intensity values spaced 1/8 wavelength — i.e., 1/4 fringe — apart from the previous intensity value to calculate the test surface optical phase change on reflection component. Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for the purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.

DETAILED DESCRIPTION OF THE DRAWINGS

In the drawings, wherein like reference characters denote similar elements throughout the several views: Fig. 1 is a diagrammatic depiction of the basic functional components of a phase shifting interference microscope apparatus constructed in accordance with the present invention and including an extended, quasi-monochromatic illumination source; Fig. 2 is an enlarged representation of portions of the interferometer objective of the apparatus of Fig. 1, showing^"the relation between the reference and test surfaces; Fig. 3 is a graph showing a two-beam interference intensity pattern as a function of axial separation between a dielectric test surface and a dielectric reference surface using an extended, quasi-monochromatic illumination source in accordance with the present invention; and Fig. 4 is a graph showing a two-beam interference intensity pattern as a function of axial separation between a metallic test surface and a dielectric reference surface using an extended, quasi-monochromatic illumination source in accordance with the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to Fig. 1, reference numeral 10 designates an embodiment of a phase shifting interference microscope apparatus constructed in accordance with the present invention. A laser illumination source 12" emits a spatially and temporally coherent, collimated illumination beam 13 that is refracted by a negative lens 14 to emerge therefrom as a divergent illumination beam 15, which subsequently impinges on a ground glass surface of a rotating diffuser disc 17 or other means of creating a time-averaged, extended quasi-monochromatic source, such as a liquid crystal. The diffuser disc 17 is rotated by an electric motor 16 to create a time-averaged, extended, quasi-monochromatic source 18 — i.e. a source in which Δλ/λ₀ << 1, wherein λ₀ is the mean wavelength and Δλ is the spectral bandwidth of the source —on the ground glass surface of the rotating disc 17. The time-averaged, extended quasi- monochromatic source 18 produces an illumination beam 40 having the properties of the source 18. Illumination beam 40 is reflected toward the microscope interferometer objective 22 by ^~a beamsplitter 20 to define a reflected illumination beam 43. The objective 22 is of the equal or near- equal path type with a moderate to high numerical aperture. The reflected illumination beam 43 from the beamsplitter 20 is refracted by the objective lens 21 to form the refracted illumination beam 45. The refracted illumination beam 45 from the objective lens 21 impinges a beamsplitter surface 28 that is carried by the objective 22 in fixed positional relation to the lens 21. The beamsplitter surface 28 reflects a portion of the refracted beam and transmits a portion of the refracted beam to thereby split the beam 45 into a partially reflected illumination beam 47 and a partially transmitted illumination beam 49, respectively. The reflected beam 47 impinges on a reference surface 24 of known topography and material characteristics that is carried by the microscope objective 22 intermediately between, in the apparatus 10 herein disclosed, and in fixed positional relation to the lens 21 and beamsplitter surface 28; the resulting reflection of the reflected beam 47 from the reference surface 24 defines the reference surface imaging beam 53 which is directed back to the beamsplitter surface 28. The transmitted beam 49 "from the beamsplitter surface 28 is similarly directed into reflective incidence with a test surface 32 of unknown topography and/or material characteristics and the resulting test surface imaging beam 51 reflected from a portion of the test surface impinges on the beamsplitter surface 28 coincident with the reference surface imaging beam 53 to thereby combine the test and reference surface imaging beams and form a reflected imaging beam 55. Thus, the test surface imaging beam 51 is formed by reflection of the image of the extended source 18 from that portion of the test surface 32 to be profiled, and the reference surface imaging beam 53 is formed by reflection of the extended image source from the reference surface 24. The reflected imaging beam 55, representing the interference between the two beams reflected from the test and reference surfaces, is directed from the beamsplitter surface 28 towards the objective lens 21 by which the beanu 55 is refracted to form the refracted imaging beam 57. The refracted beam 57 passes through the beamsplitter 20, emerging as the transmitted imaging beam 59 which impinges on a detector array 30 to form the simultaneous image of the time-averaged, extended, quasi-monochromatic source 18, of the reference surface 24 and test surface 32, and of the two-beam interference waveform resulting from the combination of the wavefronts reflected from the test and reference surfaces. The detector array 30 may, by way of example, be implemented by a CCD or like solid state camera or detector located in an image plane of the interference microscope. The array 30 thus receives, and converts into an electrical signal, a two-beam interference intensity value which represents the relationship between the interference intensity pattern and the geometrical path difference between the reference surface 24 and test surface 32. A piezoelectric transducer 34, . in response to an electrical signal from an electronic or otherwise implemented controller 41, drives the microscope interferometer objective 22 — which carries in relatively fixed positional relation the objective lens 21, the reference surface 24 and the beamsplitter surface 28 — through an axial translation linearly toward and/or away from the test surface 32, thereby varying the geometrical path difference between the test surface 32 and the reference surface 24 and, correspondingly, producing a two-beam interference intensity pattern formed at the imaging or interference plane 36 of the detector array 30. The axial translation of the microscope objective 22 is preferably at a constant linear rate. Of course, embodiments in which the test surface 32 is axially translated, in lieu of the microscope objective 22, are also within the intended scope of the invention. The analog electrical signal generated by the detector array 30 is sampled and digitized by the controller 40 and sent to a processor or computer 38 for data storage and subsequent analysis, the results of which are displayed on a monitor of the computer 38. The sampling rate should be sufficiently high as to enable suitable reconstruction of the interference intensity pattern; typically, at least two samples per fringe should be collected, although it will be readily recognized that the more samples collected per fringe, the more accurate the achievable reconstruction of the intensity pattern. The intensity pattern at the detector array image/interference plane may also be displayed directly on an image monitor 42. The test surface 32 is - preferably positioned relative to the objective 22, during ^" initial focusing, so as to be located just outside of the axial region of interference before the piezoelectric transducer 34 receives an electrical ^" signal from the controller 40 for linearly translating the objective 22. In response to that electrical signal, the piezoelectric transducer 34 expands, and thereby effects movement of the microscope interferometer objective 22 toward the test surface 32. This movement results in a corresponding charge or variation in the geometrical path difference as between the test surface 32 and the reference surface 24, which in turn produces a varying interference intensity pattern such for example, as those depicted in Figs. 3 and 4. Before describing the use and analysis of the interference intensity pattern data received, sampled and stored in accordance with the invention, it should be pointed out that it is intended, and generally contemplated, that the foregoing procedure for generating the interference intensity pattern be repeated for a plurality of surface locations or areas or portions of the test surface. This will normally be done by predeterminately moving the test surface along the "x,y" coordinate axes — i.e. generally perpendicular to the direction of z-axis relative axial movement or translation between the test and reference surfaces — so as to change the location at which the transmitted beam 49 from the beamsplitter surface 28 reflectingly impinges on the test surface. By repeatedly moving the test surface in this manner, the entire test surface portion of interest can be topographically mapped and the material characteristics of that portion may be determined in accordance with the invention. Referring now to Fig. 2, the total optical phase Teasured, 0_totaι(^χ' Y) ' by a phase shifting interference microscope such as that herein described and shown in Fig. 1 is the sum of the test surface optical phase change on reflection φ_t(x, y) , the reference surface optical phase change on reflection ø_r(x, y) , and the term 47r[z_t(x,y) - z_r(x,y)]/λ₀ which represents the geometrical phase difference produced as a result of the relative separation between the test surface 32 and the reference surface 24. λ₀ is the mean wavelength of the illumination beam, and is selected so as to accommodate the intended sensitivity of the measurements to be attained in use. The term z_t(x,y) is the test cavity geometrical path length from the beamsplitter 28 to the test surface 32-. and is proportional to the actual test surface profile, H(x,y) . The term z_r(x,y) is the reference cavity geometrical path length from the beamsplitter 28 to the reference surface 24. In actual practice z_r(x,y) , the reference cavity geometrical path length, and ø_r(x,y), the reference surface optical phase change on reflection, are both assumed to be constant and, independent of the x, y coordinate location of the portion of the test surface being profiled, commonly referred to as the field position. Thus, ø_r(x,y) = <ρ_r and z_r(x,y) = z_r and, accordingly, the total optical phase measured at the detector array 30 may be represented as:

0_totat(^χ'y) = 4ττ[z_t(x,y) - z_r]/λ₀ + ø_t(x,y) - ø_rEq. 1

Neglecting the constant and linear offset terms introduced by the optical phase difference term 4π[z_t(x,y) - z_r]/λ₀, and the reference surface phase change^"on reflection term φ_r, the relationship of the total measured optical phase, 0_totai(^χ'y)' ^{to the} actual test surface profile may be written as:

H(x,y) = λ₀[φ_totat(x,y) - φ_t(x,y) ]/4τrEq. 2

In order for the test surface optical phase change on reflection term ø_t(x,y) to have no affect on the accuracy of the actual test surface profile measurement H(x,y), the term ø_t(x,y) must either (1) be a function known a priori to the optical phase 1 measurement, so that it can be subtracted out, or

2 (2) be constant, independent of field position.

3 Only when at least one of these conditions are met

4 will Eq. 2 give accurate results. Prior art phase

5 shifting interference microscopes determine only the

6 total optical phase measured, ø_total(X_/Y) , and neglect

7 the test surface optical phase change on reflection

8 contribution, ø_t(x,y) , because of the difficulty in

9 obtaining accurate, readily achieved measurements of

10 ø_t(X_/Y)« The consequence is that a significant 1 source of error in determining H(x,y) is ignored.

12 The method and apparatus of the present invention on 3 the other hand, can advantageously determine both

14 the total measured optical phase, ø_totaι(x,y), and the

15 test surface optical phase change on reflection,

16 ø_t(x,y) and, accordingly, represents a considerable

17 Ξ_^ improvement over prior art procedures and apparatus

18 for methods of phase shifting interference

19 microscopy.

20 The following mathematical expressions have

21 been simplified from those hereinabove to exclude

22 any explicit field (x, y) dependence. Although

23 lacking such explicit field dependence, they are not

24 intended to suggest that none exists. The inventive

25 method herein described, however, is applicable at

26 all points in the field (x, y) of the test surface.

27 The equation for two-beam interference with 2 P. partially coherent illumination for a singular point 2.' in the test surface field is

3j

31 I (Δz ) = I₁ + I₂ + 2 (I, " ^) Re [γ₁₂ (Δz) ] Eq. 3

32

33 where I₁ is the test beam intensity, I₂ is the

34 reference beam intensity, and Δz = z_t - z_ . The symbol Re[ ] refers to the real part of the expression contained within the brackets and γ₁₂(Δz) is the complex degree of coherence, whose modulus satisfies the relation 0 < |γ₁₂(Δz)| < 1. Where the illumination source is quasi-monochromatic and extended, the function representing the complex degree of coherence γ₁₂(Δz) is given by

Eq.4

V (Δz) 12

I (ξ,n) <_^> dn

where I_s(ξ,J7) is the intensity distribution at the entrance pupil of the microscope interferometer objective 22 with spatial coordinates ξ and η . R_t and R_r are the optical path lengths from a point at the entrance pupil along the corresponding test and reference beam paths to the image/interference plane 36. When the intensity distribution at the circular entrance pupil of the microscope interferometer objective 22 is uniform and there is no lateral or radial shear between the test and reference beams, the integrals in Eq. 4 can be evaluated to define the real part of the complex degree of coherence function as

Re[γ₁₂(Δz )] = cos(4πΔz/λo + φt)sinc[π(r/F)²Δz/λo] Eq. 5 where F is the focal length of the microscope _ interferometer objective 22 and r is the radius of the entrance pupil. The intensity at any point in the field (x, y) is then given by substituting Eq. 5 into Eq.3, which yields

l(Δz) = Ii + +2 [i7Ϊ₂)cos(4πΔz/λo + φ_t)sinc[π(r/F)²Δz/λo] E^Q- ⁶

Fig. 3 is a plot, prepared using Eq. 6, of the two-beam axial interference intensity 66 as a function of Δz when the test surface 32 is formed of a dielectric material. Both the interference intensity pattern 66 and the coherence modulus function 68 are centered at the axial position of zero geometrical path difference (i.e. Δz = 0) between the test surface 32 and the reference surface 24. Fig. 4 shows the same plot where the test surface 32 is formed of a non-dielectric material and, therefore, exhibits a phase change on reflection of φ_t . It will be seen that the interference intensity pattern 70 is shifted in phase relative to the axial position— of zero geometrical path difference by ø_tλ₀/4π, while the coherence modulus function 72 remains centered and, therefore, is independent of the material properties of the test surface 32. This results from the use of an extended, quasi-monochromatic illumination source and enables, in accordance with the present invention, ready measurement of the phase change due solely to reflection from the test surface itself. As can be seen from Eq. 6, the axial extent of the coherence modulus 72 depicted in Fig. 4 is a function of the radius of the entrance pupil of the microscope interferometer objective 22 divided by the focal length of the objective 22. Thus, the smaller the focal length of the microscope objective 22, the smaller the axial extent of the sine coherence modulus 72. At Az = 0, Eq. 6 reduces to

l(Δz = 0) = I, + |₂ +2V(ϊ7ii)cos(φ_|) ^ ⁷

yielding an equation whereby the test surface optical phase change on reflection, ø_t, can be obtained uniquely at any point in the test surface field (x, y) . The two-beam interference intensity pattern detected by the detector array 30 and digitized and stored in the computer 38 can be analyzed by any number of methods to provide ø_t, the test surface optical phase change on reflection at any given point in the test surface field (x, y) . In a preferred embodiment of the invention, Eq. 4 may be used to analytically derive a coherence modulus function forming a "best fit" of the stored two-beam interference intensity data. Taking the intensity value I_A found at the zero slope position of the analytically-derived coherence modulus function "best fit" to the stored two-beam intensity interference ^"data and four other intensity values I_B, I_c ,I_D I_& each spaced λ/8 apart, and using the standard five bucket phase algorithm, the equation

tanffe)=2(l_β-|_D)/(,_{A +},_E._2|c) Eq.8

returns the value of the test surface optical phase change on reflection ø_t. The value φ_χ thereby obtained is then subtracted from the total measured optical phase ø_total/ yielding a test surface profile measurement of significantly enhanced accuracy than heretofore attainable. In an alternate embodiment of the inventive method, Eqs. 3 and 4 are used to analytically derive a two-beam interference equation which is then "best fit" to the stored two-beam interference intensity data to thereby directly determine the value of the test surface optical phase change on reflection φ_t . A third embodiment contemplates analysis of the stored two-beam interference intensity data to determine the axial positions of maximum fringe contrast across the test surface field (x, y) at each pixel position of the detector array 30. Using this last method, the only data used are the points of maximum interference contrast, and no measurement of the test surface optical phase change on reflection φ_t is required. The actual test surface profile is then the relative axial position of maximum fringe contrast of the two-beam interference intensity data at any given pixel site and is independent of any test surface optical phase change on reflection. In its broadest form, therefore, the present ^" invention provides a non-contact method of profiling a test surface of unknown topography and composition, including the steps of producing a first illumination beam and a second illumination beam from an extended, quasi-monochromatic source; establishing on a detector an imaging beam having an intensity which results from the interference between a first wavefront and a second wavefront, the first wavefront being formed by reflection of the first illumination beam from at least one point on a reference surface of known topography and composition and the second wavefront being formed by reflection of the second illumination beam from a corresponding point on a test surface of unknown topography and composition; moving one of the reference surface and the test surface relative to the other said surface over a- predetermined linear range of motion so as to translate one of the first and second wavefronts relative to the other said wavefront and thereby vary the imaging beam intensity and create a time-varying interference intensity pattern on the detector; and identifying a point of maximum interference contrast of the interference intensity pattern on the detector at a position along the predetermined linear range of motion to provide the topography of the test surface, at the said corresponding point, independent of any composition-dependent phase shift introduced by the reflection of the second illumination beam f-rom the test surface. The invention may also be broadly recited as a non-contact method o determining an optical phase shift on reflection from a test surface of unknown topography and composition, wherein the method includes the steps of producing a first illumination beam and a second illumination beam from an extended, quasi-monochromatic source; establishing on a detector an interference beam having an intensity which results from the interference between a first wavefront and a second wavefront, the first wavefront being formed by reflection of the first illumination beam from at least one point on a reference surface of known topography and composition and the second wavefront being formed by- reflection of the second illumination beam from a corresponding point on a test surface of unknown topography and composition; moving one of the reference surface and the test surface relative to the other said surface over a predetermined linear range of motion so as to translate one of the first and second wavefronts relative to the other said wavefront and thereby vary the interference beam intensity and create an interference intensity pattern on the detector; identifying a point of maximum interference contrast of the interference intensity pattern on the detector at a position along the predetermined linear range of motion; and determining a phase shift introduced by said reflection of the second illumination beam from the test surface by analyzing the interference intensity pattern at the point of maximum interference contrast to thereby determine the composition- dependent optical phase shift on reflection from the test surface. — While there have been shown and described and pointed out fundamental novel features of the invention as applied to preferred embodiments thereof, it will be understood that various omissions and substitutions and changes in the form and details of the disclosed methods and apparatus may be made by those skilled in the art without departing from the spirit of the invention. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.

Claims

CLAIMS _,

What is claimed is: 1. A non-contact method of profiling a test surface of unknown topography and composition, comprising the steps of: (a) producing a first illumination beam and a second illumination beam from an extended, quasi-monochromatic source; (b) establishing on a detector an imaging beam having an intensity which results from the interference between a first wavefront and a second wavefront, the first wavefront being formed by reflection of the first illumination beam from at least one point on a reference surface of known topography and composition and the second wavefront being formed by reflection of the second illumination beam from a corresponding point on a test surface of unknown topography and composition; (c) moving one of the reference surface and the test surface relative to_ the other said surface over a predetermined linear range of motion so as to translate one of the first and second wavefronts^" relative to the other said wavefront and thereby vary the imaging beam intensity and create a time-varying interference intensity pattern on the detector; and (d) identifying a point of maximum interference contrast of said interference intensity pattern on the detector at a position along said predetermined linear range of motion to provide the topography of the test surface, at said corresponding point, independent of any composition- dependent phase shift introduced by said reflection of the second illumination beam from the test surface. 2. A method in accordance with claim 1, further comprising the step of: (e) determining a composition- dependent phase shift introduced by said reflection of the second illumination beam from the test surface by analyzing the interference intensity pattern at said point of maximum interference contrast. 3. A method in accordance with claim 2, wherein said step (e) comprises determining the phase shift introduced by said reflection of the second illumination beam from the test surface by calculating the phase of the interference intensity pattern at said point of maximum interference contrast, said phase shift being indicative of a compositional characteristic of the test surface at said corresponding point. 4. A method in accordance with claim 2, wherein said step (e) comprises determining the phase shift introduced by said reflection of the second illumination beam from the test surface using the interference intensity on the detector at three positions along said predetermined linear range of motion and each comprising an interference intensity measurement spaced one-eighth wavelength of the interference intensity pattern from another of said measurements, one of said three positions being said point of maximum interference contrast. 5. A non-contact method of determining an optical phase shift on reflection from a test surface of unknown topography and composition, comprising the steps of: (a) producing a first illumination beam and a second illumination beam from an extended, quasi-monochromati source; (b) establishing on a detector an interference beam having an intensity which results from the interference between a first wavefront and a second wavefront, the first wavefront being formed by reflection of the first illumination beam from at least one point on a reference surface of known topography and composition and the second wavefront being formed by reflection of the second illumination beam from a corresponding point on a test surface of unknown topography and composition; (c) moving one of the reference surface and Che test surface relative to the other said surface over a predetermined linear range of motion so as to translate one of the first and second wavefronts relative to the d€her said wavefront and thereby vary the interference beam intensity and create an interference intensity pattern on the detector; (d) identifying a point of maximum interference contrast of said interference intensity pattern on the detector at a position along said predetermined linear range of motion; and (e) determining a phase shift introduced by said reflection of the second illumination beam from the test surface by analyzing the interference intensity pattern at said point of maximum interference contrast to thereby determine the optical phase shift on reflection from the test surface. 6. A method in accordance with claim 5, further comprising the step of determining a compositional characteristic of the test surface at said corresponding point from said determined optical phase shift on reflection. 7. A method in accordance with claim 5, wherein said step (e) comprises determining the phase shift introduced by said reflection of the second illumination beam from the test surface by calculating the phase of the interference intensity pattern at said point of maximum interference contrast, said phase shift being indicative of a compositional characteristic of the test surface at said corresponding point. 8. A method in accordance with claim 5, wherein said step (e) comprises determining the phase shift introduced by said reflection of the second illumination beam from the test surface using the interference intensity on the detector at three positions along said predetermined linear range of motion and each comprising an interference intensity measurement spaced one-eighth wavelength of the interference intensity pattern from another of said measurements, one of said three positions being said point of maximum interference contrast. 9. A method in accordance with claim 5, wherein said step (d) comprises sampling, at a predetermined sampling frequency, the interference intensity pattern on the detector to define a plurality of samples of the interference intensity pattern, and determining the point of maximum interference contrast from said plural samples. 10. A method of profiling surface characteristics of a test surface of unknown topography using a phase shifting interferometric microscope, comprising the steps of: (a) directing a beam of extended, quasi-monochromatic illumination to a beamsplitter in which the quasi-monochromatic illumination beam is split to form a reflected illumination beam and a transmitted illumination beam; (b) reflecting the reflected illumination beam off a reference surface having known surface characteristics and mounted in fixed relation to the beamsplitter to form a reflected reference surface image beam; (c) reflecting the transmitted illumination beam off a portion of a test surface located in predetermined relation to the beamsplitter to form a reflected test surface image beam; (d) combining the reflected ^"Ξ_____ reference surface image beam and the reflected test surface image beam to form a reflected imaging beam having an interference intensity, said reflected imaging beam interference intensity being a function of material characteristics of the test surface portion and of a first distance between the test surface portion and the beamsplitter; (e) moving the beamsplitter and the reference surface concomitantly over a predetermined linear range of motion so as to vary said first distance between the test surface portion and the beamsplitter and thereby vary said reflected imaging beam interference intensity and define an interference intensity pattern as the distance between the test surface and the beam modifying means varies, said interference intensity pattern having a point of maximum interference contrast occurring at a position along said range of motion at which a second distance defined between said reference surface portion and said beamsplitter is equal to said first distance defined between the test surface portion and the beamsplitter; (f) sampling, at a predetermined sampling frequency, the interference intensity pattern to define a plurality of samples of the interference intensity pattern; (g) determining the point of maximum interference contrast from said plural samples; and

(h) determining a phase shift introduced by said reflection of the transmitted illumination beam off the test surface portion by analyzing the interference intensity pattern at said point of maximum interference contrast to thereby determine a topographical surface characteristic of the test surface portion. 11. A method of profiling surface characteristics of a test surface of unknown topography using a phase shifting interferometric microscope, comprising the steps of: (a) directing a beam of extended, quasi-monochromatic illumination through arr objective of the microscope; (b) refracting the illumination beam through the microscope objective and directing the refracted beam to a beamsplitter mounted in fixed relation to the objective; (c) splitting the refracted illumination beam in the beamsplitter to form a reflected illumination beam and a transmitted illumination beam; (d) reflecting the reflected illumination beam off a reference surface having known surface characteristics and mounted in fixed relation to the objective and to the beamsplitter to form a reflected reference surface image beam; (e) reflecting the transmitted illumination beam off a portion of a test surface located in predetermined relation to the beamsplitter to form a reflected test surface image beam; (f) combining the reflected reference surface image beam and the reflected test surface image beam to form a reflected imaging beam; (g) refracting the reflected imaging beam to form a refracted imaging beam having an interference intensity, said refracted imaging beam interference intensity being~a function of material characteristics of the test surface portion and of a first distance between the test surface portion and the beamsplitter; (h) moving the microscope objective, the beamsplitter and the reference surface concomitantly over a predetermined linear range of motion so as to vary said first distance between the test surface portion and the beamsplitter arid thereby vary said refracted imaging beam interference intensity and define an interference intensity pattern as the distance between the test surface and the beam modifying means varies, said interference intensity pattern having a point of maximum interference contrast occurring along said range of motion at which a second distance defined between said reference surface portion and said beamsplitter is equal to said first distance defined between the test surface portion and the beamsplitter; (i) determining the point of maximum interference contrast; and (j) determining a phase shift in said interference intensity pattern introduced by said reflection of the transmitted illumination beam off the test surface portion by analyzing the interference intensity pattern^" at said point of maximum interference contrast to thereby determine a topographical surface characteristic of the test surface portion. 12. The method of claim 11, wherein said step (f) is performed in the beamsplitter. 13. The method of claim 11, wherein said step (b) is performed in the objective of the microscope. 14. The method of claim 11, further comprising the step of repeating each of steps (a) through (j) over multiple test surface portions so as to determine the topographical surface characteristic at a -selected surface region of the test surface. 15. The method of claim 11, further comprising the steps of sampling, at a predetermined sampling frequency, the interference intensity pattern to define a plurality of samples of the interference intensity pattern, and wherein the determinations of said s^f ϊps (i) and (j) are carried out using the plural samples of said interference intensity pattern. 16. The method of claim 11, wherein said step (i) comprises the steps of: (a) detecting, at a solid-state camera having a plurality of pixel sites, the interference intensity pattern; (b) converging the interference intensity pattern into an electrical signal; and (b) digitizing, at a predetermined sampling rate, the electrical signal to form a plurality of digitized samples of the interference intensity pattern. 17. The method of^" claim 16, further comprising the step of storing the plural digitized samples, and wherein said steps (i) and (j) are carried out on said digitized samples. 18. The method of claim 17, further comprising the steps of: visually displaying, on a first video display, the interference intensity pattern; and visually displaying, on at least one of the first video display and a second video display, the determinations made in steps (i) and (j⁾- 19. The method of claim 18, wherein said step (a) comprises -the steps of: transmitting a temporally coherent, collimated illumination beam from a laser illumination source; directing the temporally coherent ^" beam at a negative lens and refracting the temporally coherent beam to form a divergent illumination beam; directing the divergent illumination beam at a rotatable diffuser disc; rotating the diffuser disc so as to form, on a surface of said diffuser disc, a ti e- averaged, extended quasi-monochromatic illumination source; directing the time-averaged, extended, quasi-monochromatic illumination source at an illumination beamsplitter to form an illumination beam of extended, quasi-monochromatic light; and directing the illumination beam of extended, quasi-monochromatic light to the microscope objective. 20. A method of profiling the surface characteristics of a test surface using a phase shifting interferometric microscope, comprising the steps of: (a) directing a beam of extended, quasi-monochromatic illumination toward the microscope objective; (b) refracting the illumination beam through the microscope objective so as to direct the illumination beam toward a beamsplitter mounted in fixed relation to the microscope objective; (c) splitting the illumination beam in the beamsplitter to form a reflected illumination beam and a transmitted illumination beam; (d) directing the reflected ^"- illumination beam so as to cause it to impinge on and reflect from a reference surface having known surface characteristics and mounted in fixed relation to the microscope objective and the beamsplitter, said reflection from the reference surface producing a reflected reference surface image beam directed toward the beamsplitter; (e) directing said transmitted illumination beam so as to cause it to impinge on and reflect from a portion of a test surface to be profiled, said reflection from the test surface producing a reflected test surface image beam directed toward the beamsplitter into coincidence with the reflected reference surface image beam so as to combine the reflected test surface image beam and the reflected reference surface image"-beam to form a reflected imaging beam; (f) refracting the reflected imaging beam in the microscope objective to form a refracted imaging beam comprising a two-beam interference intensity formed of the combination of the reflected test surface image beam and the reflected reference surface image beam, said interference intensity being a function of material characteristics of the test surface portion and of a path length distance between the test surface portion and the beamsplitter; (g) moving the microscope objective, the beamsplitter and the reference surface concomitantly over a predetermined linear range of motion so as to vary the path length distance between the test surface portion and the beamsplitter and thereby define an interference intensity pattern having a point of maximum fringe contrast; ~' (h) directing the refracted imaging beam to an optical detector for detecting the interference intensity pattern over the range of motion in step (g) ; (i) sampling, at a predetermined sampling frequency, the detected interference intensity pattern to define a plurality of samples of the interference intensity pattern; (j) storing the plural samples; (k) analyzing the stored samples for determining the point of maximum fringe contrast; and (1) repeating-each of said steps (a) to (k) over multiple portions of the test surface so as to profile a selected surface region of the test surface. 21. The method of claim 20, wherein said step (a) comprises the steps of; (a) transmitting a temporally coherent, collimated illumination beam from a laser illumination source; (b) directing the collimated illumination beam at a negative lens and refracting the collimated illumination beam to form a divergent illumination Εeam; (c) directing the divergent illumination beam at a rotatable diffuser disc; (d) rotating the diff ser-disc so as to form, on a surface of said diffuser disc, a time- averaged illumination source defining an illumination beam of extended, quasi-monochromatic light; and (e) directing the time averaged, extended, quasi-monochromatic illumination source at an illumination beamsplitter to form an illumination beam of extended, quasi-monochromatic light directed toward the microscope objective. 22. A phase shifting interference microscope operable for profiling surface characteristics of a test surface of unknown topography, said interference microscope comprising: a source of an illumination beam of extended, quasi-monochromatic light; means for refracting said illumination beam; means for directing said illumination beam toward said refracting- means so that said illumination beam passes ^"through said refracting means to form a refracted illumination beam; beam modifying means for receiving and for dividing said refracted illumination beam into a partially reflected illumination beam and a partially transmitted illumination beam, said beam modifying means being located in predetermined relation to the test surface so that said transmitted illumination beam impinges on and is reflected from a portion of the test surface to form a reflected test surface image beam; ~^~ a reference surface having known surface characteristics and located in fixed relation to said refraction means and to said beam modifying means^"and so that said partially reflected illumination beam impinges on a portion of said reference surface and is reflected therefrom to form a reflected reference surface image beam; said beam modifying means receiving said reflected reference surface image beam from the reference surface and said reflected test surface image beam from the test surface and combining said reflected test surface image beam and said reflected reference surface image beam to define a reflected imaging beam that passes through said refracting means to form a refracted imaging beam having an intensity, said refracted imaging beam intensity being a function of material characteristics of the test surface portion and of a first distance defined between the test surface portion and said beam modifying means and having a phase; means for moving said beam modifying means, said refracting means and said reference surface concomitantly over a predetermined linear range of motion relative to the test surface so as to vary said first distance defined between the test surface portion and the beam modifying means and to thereby vary said refracted imaging beam intensity and define an interference intensity pattern as the distance between the test surface and the beam modifying means is varied, said interference intensity pattern having a point of maximum interference contrast occurring at a position along said range of motion at which a second distance defined between said reference surface portion and said beam modifying means is equal to said first distance defined between the test surface portion and said beam modifying means; and means for determining said point of maximum interference contrast and for determining a phase shift introduced by said reflection of the partially transmitted illumination beam from the test surface portion by analyzing said interference intensity pattern at said point of maximum interference contrast so as to thereby determine a topographical surface characteristic of the test surface portion. 23.^" The microscope of claim 22, wherein said refracting means comprises a microscope objective. 24. The microscope of claim 23, wherein said microscope objective comprises a high numerical aperture interferometer objective. 25. The microscope of claim 22, wherein said beam modifying means comprises a first beamsplitter. 26. The microscope of claim 25, wherein said illumination beam directing means comprises a second beamsplitter. 27. The microscope of claim 22, wherein said moving means comprises a piezoelectric transducer. 28. The microscope of claim 22, further comprising means for sampling, at a predetermined sampling frequency, said interference intensity pattern to define a plurality of samples of said interference intensity pattern. 29. The microscope of claim 28, wherein said sampling means further comprises^': detecting means fixedly mounted in an image plane for receiving said interference intensity pattern, said detecting means comprising a solid-state camera having plural detector pixel sites; means for converting the received interference intensity pattern into an electrical signal to be sampled; and means for converting said signal samples into digital data representative of said interference intensity pattern. 30. The microscope of claim 29, wherein said illumination beam directing means comprises a beamsplitter and said microscope objective causes said refracted imaging beam to be transmitted through said beamsplitter to form a transmitted imaging beam which impinges on said detecting means at a detector pixel site. 31. The microscope of claim 30, wherein said determining means comprises a digital computer for processing said digital data and having a digital computer memory for storing said digital data. 32. The microscope of claim 31, further comprising a first video screen for visually displaying said interference intensity pattern received by said detecting means in said image plane, and a second video screen for visually displaying at least one of the processed digital data from said digital computer and the stored digital data in said digital computer memory. 33. The microscope of claim 22, wherein said extended, quasi-monochromatic light source comprises: a laser illumination source for transmitting a temporally~ coherent, collimated illumination beam; a negative lens for receiving said collimated illumination beam and for refracting said collimated illumination beam to form a divergent illumination beam; a rotatable diffuser disc for receiving said divergent illumination beam; and means for rotating said diffuser dis^'c so as to form on a surface of said diffuser disc a time-averaged illumination source defining said illumination beam of extended, quasi-monochromatic light.