WO1996024034A1

WO1996024034A1 - Method for improving optical measurement of rough samples in ellipsometry and reflectometry

Info

Publication number: WO1996024034A1
Application number: PCT/US1996/001495
Authority: WO
Inventors: Mitchell E. Haller
Original assignee: Rudolph Research Corporation
Priority date: 1995-02-02
Filing date: 1996-02-01
Publication date: 1996-08-08

Abstract

A method and apparatus for optical measurements of rough samples (2) in ellipsometry and reflectometry wherein optical probe beams (A-D) are reflected from the sample (2). The method includes directing an optical probe beam (A-D) so that it is reflected from a sample (2), and directing the optical probe beam reflected (A'-D') from the sample, and wherein at least one of the sample (2) and the probe beam (A-D) is moved during the directing of the probe beam (A-D) so that a relative movement of the sample (2) and the probe beam (A-D) with respect to one another at the surface of the sample (2) from which the probe beam (A-D) is reflected exceeds an amount of roughness scale of the sample (2) so as to produce independent speckle patterns in the reflected beam (A'-D') during the directing. The reflected probe beam (A'-D') is detected with a photodetector (6) which responds to intensity of the reflected optical probe beam (A'-D') averaged over a predetermined time period.

Description

METHOD FOR IMPROVING OPTICAL MEASUREMENT OF ROUGH SAMPLES IN ELLIPSOMETRY AND REFLECTOMETRY

Field of the Invention

The present invention is directed to an improved method and apparatus for optical measurements of rough samples, especially thin films, in ellipsometry and reflectometry using optical probe beams which are reflected from the samples.

Background and Summary of the Invention

Ellipsometry, as defined by R.A. Azzam in N.M. Bashara in Ellipsometry and Polarized Light, published by North-Holland Physics Publishing, 1987 Edition, is an optical technique for the characterization and observation of events at an interface or film between two media and is based on exploiting the polarization transformation that occurs as a beam of polarized light is reflected from or transmitted through the interface or film. Two factors make ellipsometry particularly attractive: (1) its essential non-perturbing character (when the wavelength and intensity of the light beam are properly chosen) hence its suitability for in-situ measurements and (2) its remarkable sensitivity to minute interfacial effects, such as the formation of a sparsely distributed sub-monolayer of atoms or molecules. The great diversity of situations in nature and man-made systems where interfaces and films play an important role has led to the application of ellipsometry in a wide spectrum of fields such as physics, chemistry, materials and photographic science, biology, as well as optical, electronic, mechanical, metallurgical and biometrical engineering.

Ellipsometry is sometimes referred to as polarimetry, generalized polarimetry, or complete polarimetry. The latter names are more common especially when interaction with the sample involves transmission of light through the bulk of the sample and the polarization transformation depends on bulk sample properties as well as surface properties and films. Azzam and Bashara further state in their aforementioned book that ellipsometry can be generally defined as the measurement of the state of polarization of a polarized vector wave. Ellipsometry is generally conducted in order to obtain "information" about an "optical system" that modifies the state of polarization. In a general scheme of ellipsometry, a polarized light-wave is allowed to interact with an optical system under investigation. The interaction changes the state of polarization of the wave. Measurement of the initial and final states of polarization, repeated for an adequate number of different initial states, leads to the determination of the law of transformation of polarization by the system as described, for example, by its Jones or Mueller matrix. To extract more fundamental information about the optical system than is conveyed by its Jones or Mueller matrix, it is necessary to examine light-matter interaction within the system by the electromagnetic theory of light. In other words, it is necessary to study the details of the internal polarization- modifying processes that are responsible for the external behavior as described by the measured Jones or Mueller matrix of the system.

An operational diagram of a general ellipso eter arrangement as shown in Ellipsometry and Polarized Light is shown in Fig. 1 of the drawings. A beam from a suitable light source (L) is passed through a variable polarizer (P) to produce light of known polarization. This light interacts with the optical system (S) under study and its polarization is modified. The modified state of polarization at the output of the system is measured (analyzed) by a polarization analyzer (A) followed by a photodetector (D) . If the light interaction with the sample under study varies with wavelength, a monochromatic light source must be used or a means of isolating quasimonochromatic portions (with known wavelengths) of the light must be provided.

One way in which the light wave can interact with the optical system is by being reflected from a surface of the optical system (S) . This reflection causes the state of polarization to be changed abruptly. Such a change can be explained using the Fresnel reflection coefficients for the two linear polarizations parallel (p) and perpendicular (s) to the plane of incidence. Another way the light wave can interact with the optical system is transmission through the material of the optical system. When the polarization state change depends on the angle of interaction of the light beam and the sample under study, as for example with reflection from (or oblique transmission through) a sample, the incidence light should be as collimated as possible so only a single angle of incidence is measured at one time.

Azzam and Bashara explain that, since the time of Drude, reflection ellipsometry has been recognized as an important tool for the study of surfaces and thin films. Among the many useful applications of ellipsometry are: (1) measurement of the optical properties of materials and their frequency dependence (wavelength dispersion) , the materials may be in the liquid or solid phase, may be optically isotropic or anisotropic, and can be either in bulk or thin film form; (2) monitoring of phenomena on surfaces that involve either the growth of thin films starting from a submonolayer (e.g. by oxidation, deposition, adsorption or diffusion of impurities) , or the removal of such films (e.g. by desorption, sputtering or diffusion) ; and (3) measurement of physical factors that effect the optical properties such as electric and magnetic fields, stress or temperature.

A description of the principles of ellipsometry, and a discussion of the reflection process, the measurement process, and data reduction can be found in "Ellipsometry A Century Old New Technique" by Dr. Richard F. Spanier, Industrial Research. September 1975, which article is incorporated herein by reference. A diagram of a conventional ellipsometer from Dr. Spanier's article is shown in Fig. 2B. Many additional types of automated and manually operated ellipsometers are known in the art. Dr. Spanier states in the article that ellipsometry involves the measurement of tan φ, the change in the amplitude ratio upon reflection, and Δ, the change in the phase difference upon reflection. The quantities Δ and φ are functions of the optical constants of the surface, the wave length of the light used, the angle of incidence, the optical constants of the ambient medium, and for film-covered surfaces, the thicknesses and optical constants of the films.

Thus, in order to be able to compute the information about a sample's properties which cause a polarization state change in the reflected light, it is necessary to convert the polarization state change together with the angle of incidence and wavelength into physical properties of the sample according to some mathematical model. Properties such as refractive index, thickness, and absorption index of films on a surface or the optical constants of bare surfaces can be computed, for example. Similarly, in the case of transmitted light, properties such as the birefringence of the bulk material can be computed. Each ellipsometric measurement of polarization state change yields one value for Δ and one value for φ. Thus, at best, two of the properties of the surface (whether or not film covered) or two properties of the bulk (in the case of transmitted light) can be computed if values for the remaining properties are known from other surfaces. Frequently, in the art, one can compute more of these properties of film covered surfaces, for example, if one has values for Δ and values for φ, at more than one angle of incidence; preferably, at many angles of incidence. Theoretically, one property can be computed for each independent Δ and one property can be computed for each independent φ measured, but it is better to over determine the unknowns with extra values of Δ and φ. Accordingly, it is advantageous to measure as many angles of incidence on a particular sample as possible. However, until recently this has not been done frequently because it is so cumbersome to get the data by making separate successive measurements at each angle through the use of a scanning technique.

It has also been proposed to provide ellipsometers with a plurality of duplicate setups with multiple beams all of different, discrete angles in order to simultaneously obtain information for light at different angles of incidence. These ellipsometers essentially combine several ellipsometers of the known type and use them simultaneously. This technique is limited in the number of angles that can be simultaneously measured because of the need for a plurality of ellipsometers, which can add considerably to the initial cost and maintenance of such a system.

More recently, as described in the commonly owned U.S. Patent No. 5,166,752, a simultaneous multiple angle/multiple wavelength ellipsometer and method are disclosed for measuring a large plurality of angles of incidence at one time, quickly and without complicated operator intervention or scanning and wherein only one ellipsometer and a single beam are required. The disclosure of Patent No. 5,166,752 is hereby incorporated by reference.

While the improved ellipsometry method and ellipsometer of U.S. Patent No. 5,166,752 permit easy collection of a large multiplicity of data at different light wavelengths as well as angles, it has been found that surface roughness of a sample being studied degrades the accuracy of the measurements obtained with these and other known methods and apparatus. For example, in the case of the measurement of samples of thin films with optical probe beams in ellipsometry, the measured values for the thin films vary considerably with respect to modeled values for the films in the case the samples are rough. This is illustrated in Fig. 3 which shows ellipsometric measurements of a rough polysilicon film, the measured and modeled values for tangent φ being shown as a function of the angle of incidence of a focused laser beam reflected from the film. The variations in the measurements as compared to the modeled values, show the degradation due to surface roughness. There is a need in th art for an improved method and apparatus for making optica measurements on rough samples in ellipsometry, an reflectometry, which reduce or eliminate the aforementione limitations of the known methods and apparatus.

Disclosure of Invention

An object of the present invention is to provide such a improved method and apparatus for making optical measurements on rough samples in ellipsometry and reflectometry which greatly reduce the oscillations or variations of the optical measurements of rough samples, bringing the measured results more in line with the theoretical model.

The present invention is based upon an understanding of the interaction of the optical probe beam with the rough surface of a sample. Namely, the interaction of the surface roughness of a sample with a focused laser beam, for example, produces a speckle pattern in the reflected laser beam which is characteristic of the reflecting surface. The speckle pattern can give a high contrast maximum or minimum in the measured reflectance thus distorting the optical measurement to produce the aforementioned oscillations in the measured values from the modeled values as depicted in Fig. 3.

The present invention solves this problem in obtaining optical measurements of rough samples in ellipsometry and reflectometry. According to the method of the invention for improving optical measurements of rough samples in ellipsometry and reflectometry using optical probe beams which are reflected from the samples, the method comprises directing an optical probe beam so that it is reflected from a sample, and detecting the optical probe beam reflected from the sample, and wherein the method includes moving at least one of the sample and the probe beam during said directing so that a relative movement of the sample and the probe beam with respect to one another at the surface of the sample from which the beam is reflected exceeds an amount of a roughness scale of the sample so as to produce independent speckle patterns in the reflected beam during said directing. The reflectance measurements having independent speckle patterns are added together so that the aximums and minimums in the measured reflectance average out, giving a true reading of the mean reflectance as the measured value which more closely follows the modeled value.

In a disclosed embodiment of the invention, the method includes placing the sample on a supporting device during the optical measurement and moving the supporting device and the sample thereon relative to the optical probe beam while reflecting the optical probe beam from the surface of the sample. The movement of the surface of the sample is in a direction in the plane of the surface of the sample in the disclosed embodiment but could be in another direction. Further, according to the disclosed embodiment the step of detecting the optical probe beam includes detecting the reflected probe beam with a photodetector which responds to the intensity of reflected optical probe beam averaged over a predetermined time period. The aforesaid amount of the relative movement of the optical probe beam and the sample during said directing occurs within said predetermined time period according to the invention so that the reflected light from the independent speckle patterns is averaged. Thus, a true reading of the mean reflectance is obtained.

An apparatus of the invention for optical measurement of a rough sample comprises a device for directing an optical probe beam for reflection from a surface of a sample, a detector for detecting the optical probe beam reflected from the sample, and a device for moving at least one of the sample and the probe beam so that a relative movement of the sample and the probe beam with respect to one another at the surface of the sample from which the probe beam is reflected exceeds an amount of a roughness scale of the sample so as to produce independent speckle patterns in the reflected beam during the directing. According to the disclosed embodiment, the device for oscillating includes a support for the sample and a device for oscillating the support and a sample thereon relative to the probe beam which is reflected from the supported sample. In the disclosed form of the invention, the support for the sample is an X-Y sample stage having stepper motors for moving the sample stage in the X and Y directions of a Cartesian coordinate system, respectively. The device for moving further includes means for dithering at least one of the stepper motors for effecting the oscillating of the support and sample supported thereon relative to the probe beam directed at and reflected from a surface of the sample. The disclosed apparatus in the illustrated embodiment of the invention is an ellipsometer with means for measuring a change in polarization state of the probe beam reflected from the sample. The ellipsometer in the disclosed embodiment includes a device for directing an optical probe beam for reflection from a surface of the sample. The device includes means for simultaneously directing the optical probe beam to interact with the sample at different angles of incidence. The detector of the ellipsometer includes a plurality of detectors for detecting the reflected probe beam reflected at each of a plurality of different angles of incidence from the sample surface. As a result of this combination of features, the method and apparatus of the invention permit the rapid, easy collection of a large multiplicity of data for different angles or ranges of angles within the whole range of angles for rough samples such as rough films of polysilicon, as well as smooth films on rough substrates, such as sputtered aluminum while avoiding degradation of the ellipsometric measurements due to surface roughness.

These and other objects, features and advantages of the present invention will become more apparent from the following description when taken in connection with the accompanying drawings which show, for purposes of illustration only, one embodiment in accordance with the present invention.

Brief Description of Drawings

Fig. 1 is an operational diagram of a general ellipsometer or arrangement wherein L, P, S, A, and D represent a light source, control polarizer, optical system under measurement, variable polarization analyzer, and photodetector, respectively.

Fig. 2A is a schematic diagram illustrating a known reflection ellipsometry arrangement wherein the incoming collimated polarized light is reflected at an angle equal to its angle of incidence.

Fig. 2B is a schematic diagram of a conventional nulling or photometric ellipsometer which measures changes in the state of polarized light reflected at a single angle of incidence from the surface of samples resting on a sample mount. Fig. 3 shows the results of ellipsometer measurements, φ as a function of angle of incidence, using the conventional ellipsometer of Fig. 2B and a conventional ellipsometry method for a rough film of polysilicon, the oscillations of the measured values being compared with the modeled values. Fig. 4 is a schematic illustration of a portion of an ellipsometer according to an embodiment of the invention.

Fig. 5 is a simplified diagram of the ellipsometer of Fig.

4 illustrating a lens system for focusing, a lens system for refocusing reflected light and a linear detector array for detecting the reflected light from the surface of each of a plurality of different angles of incidence.

Fig. 6 is a schematic diagram of a support of the apparatus for supporting the sample in the ellipsometer of Fig. 4, the support including an X-Y sample stage with stepper motors for moving the sample stage in X and Y directions of a Cartesian coordinate system, respectively, a microprocessor controller being provided for dithering the stepper motors for effecting movement of the sample stage and sample supported thereon relative to a probe beam being directed at and reflected from the surface of the sample.

Fig. 7 shows the results of ellipsometric measurements, φ as a function of angle of incidence with the apparatus of Figs. 4-6 on a rough polysilicon film, the same film employed in the measurements shown in Fig. 3, except with the sample being moved during the measurement in accordance with the present invention, the variations in the measured values from the modeled values being greatly reduced as compared with the results using the conventional ellipsometer and ellipsometry method. Description of the Disclosed Embodiment

The ellipsometer 1 of the invention shown in Figs. 4-6 comprises means for directing polarized light onto the surface 2 and means for analyzing the polarization state of the light reflected from the surface. The means for directing includes a light source, beam shaping optics with an optional optical narrow band filter, a polarizer, a compensator and a variable aperture as in the conventional ellipsometer of Fig. 2B. In addition, the means for directing further includes means for simultaneously directing polarized light from a single beam of light from the light source onto the surface 2 at different angles of incidence. This means for simultaneously directing the light at different angles of incidence onto the surface 2 comprises a focusing lens system 7. The lens system 7 has an effective aperture to focal length ratio for focusing the light on the surface 2 with angles of incidence which vary over a range of angles of at least one or two degrees. More particularly, in the illustrated preferred embodiment the range of angles of incidence α is 30°. Larger angles could be employed for directing rays at the sample 2.

The focusing lens system 7 focuses the polarized light, which may be from a HeNe laser for example, down to a single small spot or point on the surface 2. In the illustrated embodiment, the spot has a diameter of lOμ. The schematic illustration of Fig. 5 depicts several rays A, B, C and D having widely varying angles of incidence which are focused on a single, small spot on the surface 2. Thus, the light directed on the small spot on surface 2 contains rays at many angles of incidence above and below the angle of incidence of the central ray through the focusing lens. Each one of the incoming rays is reflected at an angle equal to its angle of incidence with a polarization state of each of the rays being altered by that reflection, see rays A¹, B¹, C¹ and D¹ in Fig. 5. A detector array 6 is employed to detect a plurality of rays reflected from the surface 2 individually over the different, narrow ranges of angles of incidence to simply and quickly obtain data at a plurality of angles of incidence. The means for analyzing includes the detector array as well as the analyzer and other elements as shown in Fig. 4 and in some embodiments additional lenses represented by lens 8 in the reflected light.

As shown in Fig. 5, the diameter d of the lenses 7 and 8 corresponds to their effective diameter. In the illustrated embodiment the lens 7 and 8 each have a diameter d of 18mm and a focal length 1 of 34mm. Other effective lens diameters and focal lengths could be employed so long as a range of angles of incidence, preferably at least 30°, is provided. The lens diameter and focal length are chosen with a view toward maximizing the number of angles of incidence of the light beams which strike the surface 2. The refocusing lens or lenses 8 directs the reflected light toward the detector array 6. However, a refocusing lens nee not be employed as the reflected light could be made to directl impinge upon an array of detectors. It is important that th lenses 7 and 8 do not themselves alter the polarization state o the light.

The detector array 6 is a linear, multiple element detecto wherein each of the detector elements 9 can detect a narro range of angles of incidence of the rays that illuminate th sample. In the disclosed embodiment the array 6 is a solid- state photosensitive detector array wherein the separat detector elements 9 are all integrated on one circuit chip. Particularly, the detector elements comprise a linear array of photodiodes. While integrated on a single circuit chip, the individual photodiodes can function as separate detectors. The linear array of the disclosed embodiment comprises 128 detector elements arranged in a row to provide data for 128 different angles of incidence where the full array is illuminated by the reflected light. The number of individual detector elements 9 could be more or less than that in the disclosed embodiment and the detector elements need not be integrated on a single chip but could be discrete detectors. By using a plurality of detector elements, it is possible to simultaneously detect the light reflected from the surface for each of a plurality of different angles of incidence. The physical size of each of the detector elements is less than the expanse of the reflected rays so that each element detects only a certain narrow range of angles of incidence on the illuminating side. The photodiodes of the detector array 6 each responds to the intensity of light it receives averaged over a predetermined time period which is a function of the response time of the photodiodes and the time of their exposure to the light be detected.

Surface roughness of a sample, such as a thin film, degrades the measurements of the sample with the conventional ellipsometer and method as noted above with reference to Fig. 3. To avoid this problem, according to the method and apparatus of the present invention, at least one of the sample and the optical probe beam being reflected from the sample is moved relative to the other during the directing of the optical probe beam for reflection from the sample. The relative movement of the sample and the optical probe beam with respect to one another at the surface of the sample from which the optical probe beams is reflected is selected to exceed an amount of a roughness scale of the sample during the aforesaid predetermined time period of detection so as to produce independent speckle patterns in the reflected beam during the directing of the beam to the sample. As a result, the photodiodes will average the intensity of light from the independent speckle patterns to provide a measurement of the true average sample reflectance, e.g. the high contrast maximums and minimums of the individual speckle patterns will be averaged.

This relative movement of the sample and the optical probe beam with respect to one another at the surface of the sample from which the beam is reflected is achieved in the illustrated embodiment by placing the sample 2 on a supporting device 11 and moving the supporting device and the sample thereon relative to the optical probe beam by a driving mechanism 21 under the control of a microprocessor 14 as shown in Fig. 4 while reflecting the optical probe beam from the surface of the sample during the directing of the probe beam to the sample. The supporting device 11 is an X-Y sample stage which, as shown in Fig. 6, has stepper motors 12 and 13 forming the driving mechanism 21 for moving the sample stage in the X and Y directions of a Cartesian coordinate system, respectively. The motor 12 moves both of the upper and lower stages, A and B, of the sample stage 11 in the X-direction while the motor 13 moves upper stage A relative to the lower stage B on which it is supported and carried. The microprocessor 14 includes means for dithering the stepper motors 12 and 13 for effecting movement of the supporting device 11 and sample 2 supported thereon in the X-Y plane relative to the optical probe beam being directed at and reflected from the surface of the sample in the z plane as shown in Fig. 6.

By way of example, for ellipsometric measurements of a rough polysilicon film 2 supported on the supporting device 11 of the ellipsometer 1, the roughness scale of the film is a function of the grain size of the polysilicon, which is typically only a few microns in width in a direction along the surface of the film, e.g. in the X-Y plane. The roughness scale of the sample determines the amount of the relative motion required to produce independent speckle patterns during the measurement. According to this embodiment of the invention, the optical probe beam spot on the surface of the film 2 should traverse at least a few grains of the film within the response time and time of detection of the photodiodes to randomize the speckle pattern, e.g. average out the maximums and minimums from the independent speckle patterns in the reflected beam during the measurement time period.

The photodiodes of the detector ray each respond to the intensity of light of the reflected optical probe beam averaged over a predetermined time period,of .001 second in this example. According to the invention the aforementioned relative movement during this time period was an amount which exceeded an amount of the roughness scale of the sample 2 so as to produce independent speckle patterns in the reflected beam during this time period. With a grain size of only a few microns for the polysilicon film, e.g. 2 to 5 μ grain size, a relative motion which exceeded this amount, preferably at least 10-50 μ relative motion during the millisecond interval that the optical probe beam was directed to, reflected from and detected by the respective photodiodes, was needed to give a true reading of the mean reflectance of the sample.

The results of measurements with the ellipsometer and ellipsometry method of the invention are illustrated in Fig. 7 wherein it is seen that the measured values for φ correspond much more closely to the modeled values, for the same rough polysilicon film as compared with the measurements with a conventional ellipsometer and method the results of which shown in Fig. 3. In the example referred to above, to achieve a relative motion of at least 10-50 μ during the millisecond interval of the detection with the photodiodes, the surface of the sample reflecting the optical probe beam was continuously, linearly moved in the plane of the surface, the X-Y plane during measurement at a rate of at least .010 to .050 meters/second. Instead, or in addition to movement in the X-Y plane, the surface of the sample could be moved in the Z-direction for achieving the relative movement with the optical probe beam.

While the aforementioned example has been with respect to reflectance measurements of a rough film, such as polysilicon, in ellipsometry, particularly using a simultaneous multiple angle ellipsometer, other rough samples including smooth films on rough substrates, such as sputtered aluminum, can also be measured more accurately by the ellipsometer and method of the invention and the invention is applicable to other types of ellipsometers and also in reflectometry, per se, for improving the optical measurement of a rough sample. Therefore, I do not wish to be limited to the details shown and described herein, but intend to cover all such changes and modifications as are encompassed by the scope of the appended claims.

Claims

CLAIMSI Claim:

1. A method for improving optical measurements of rough samples in a reflection ellipsometer or in reflectometry using optical probe beams which are reflected from rough surfaces of the samples, said method comprising directing an optical probe beam so that it is reflected from a rough surface of a sample, and detecting light of the optical probe beam reflected from said sample with a photodetector which responds to intensity of reflected light it receives over a predetermined time period to provide a measure of an average reflectance of said sample, wherein said step of directing includes focusing said probe beam on said rough surface of said sample to a relatively small spot in relation to a roughness scale of said rough surface such that the roughness of said sample can distort the measured reflectance of said sample which would be measured by said photodetector from a theoretical modeled value therefor if said probe beam were focused on a single spot on said rough surface of said sample during said predetermined time period of said detecting, and wherein said method further includes effecting relative movement between said sample and said optical probe beam at said rough surface of the sample from which the beam is reflected during said predetermined time period of said detecting, an amount of said relative movement during said predetermined time period exceeding said roughness scale of said rough surface of said sample so as to produce a measured reflectance of said sample by said detecting which closely follows said modeled value therefor.

2. A method according to claim 1, wherein said step of effecting relative movement includes placing said sample on a supporting device and moving said supporting device and said sample thereon relative to said optical probe beam while reflecting said optical probe beam from said surface of the sample during said directing.

3. The method according to claim 1, wherein said relative movement is in a direction along said surface of said sample.

4. The method according to claim 1, wherein said sample is a thin film of a material having a grain size of only a few microns as said roughness scale, and wherein the amount of said relative movement between the sample and the optical probe beam with respect to one another at the surface of the sample from which the optical probe beam is reflected during said predetermined time period of said detecting is at least several tens of microns.

5. The method according to claim 4, wherein said sample is a polysilicon film.

6. The method according to claim 1, wherein said roug surface of said sample is a smooth film on a rough substrate.

7. The method according to claim 1, wherein said focused probe beam interacts with the sample at different angles of incidence and said step of detecting is performed for each of a plurality of different, narrow ranges of angles of incidence of said different angles of incidence.

8. The method according to claim 1, wherein said optical probe beam is polarized light, said method including measuring a change in polarization state of the probe beam reflected from the surface of the sample.

9. The method according to claim 1, wherein the focused probe beam is directed onto the surface of the sample at a single spot with a cone of polarized light derived from a single beam, said single spot and the surface of the sample moving relative to one another during said predetermined time period of said detecting.

10. The method according to claim 9, wherein said single spot has a diameter on the order of ten microns.

11. The method according to claim 1, including supporting said sample to be optically measured on an X-Y sample stage having stepper motors for moving said sample stage in X and Y directions of a Cartesian coordinate system, respectively, and wherein said step of effecting said relative movement includes dithering at least one of said stepper motors for moving said support and said sample thereon relative to said probe beam directed at and reflected from said rough surface of said sample.