Method and apparatus for measuring thickness of thin films

Abstract: An apparatus (20) for measuring the thickness of a thin film layer (32) on substrate (28) includes a probe beam of radiation (24) focused substantially normal to the surface of the sample using a high numerical aperture lens (30). The high numerical aperture lens (30) provides a large spread of angles of incidence of the rays within the incident focused beam. A detector (50) measures the intensity across the reflected probe beam as a function of the angle of incidence with respect to the surface of the substrate (28) of various rays within the focused incident probe beam. A processor (52) functions to derive the thickness of the thin film layer based on these angular dependent intensity measurements. This result is achieved by using the angular dependent intensity measurements to solve the layer thickness using variations of the Fresnel equations. The invention is particularly suitable for measuring thin films, such as oxide layers, on silicon semiconductor samples.

Description: The present invention relates to an apparatus for evaluating a parameter of a sample which includes a thin film layer on a substrate, the apparatus comprising means for generating a probe beam of radiation, a lens arranged substantially symmetrically with respect to a normal to the surface of the sample at the thin film layer for focusing the probe beam at the said surface, so as to create in a predetermined plane a spread of angles of incidence with respect to the said surface, detector means responsive to radiation intensity and arranged to receive probe beam radiation reflected back through the lens, with the location of a ray on the detector means being a function of the angle of incidence of that ray with respect to the said surface, and the lens being so positioned that the reflected radiation varies in intensity as a result of interference effects created by the thin film layer and processor means coupled to the detector means for evaluating the sample on the basis of angle dependent intensity measurements, the detector means being selectively responsive to the reflected radiation in the said predetermined plane. Background In the prior art, there has been considerable interest in developing methods and apparatus for measuring the thickness of the thin films on substrates. This need is particularly acute in the semiconductor manufacturing industry where extremely thin films are deposited on silicon substrates. One technique which has been developed is classified as ellipsometry. In an ellipsometer, a probe beam, having a known polarization, is directed to reflect off the surface of the sample at an oblique angle. The thickness of the film layer on the substrate will affect the polarization state of the reflected beam. By measuring the polarization state of the reflected beam, information about the thickness of the layer can be derived. Ellipsometers are quite useful for accurately measuring the thickness of very thin films. However, the spatial resolution of ellipsometers is limited to areas as large as 20 - 30 microns in diameter. In semiconductor manufacturing, it is often necessary to obtain thickness measurements in a much smaller region, on the order of one micron in diameter. Ellipsometers cannot fulfil this need. Another drawback with ellipsometers is that the range of measurement is quite narrow. While ellipsometers work quite well with extremely thin layers, they become ineffective at thicknesses greater than about 1000 Angstroms. As noted above, ellipsometers rely on the measurement of the polarization state of a reflected probe beam. An entirely different class of layer thickness measurement devices found in the prior art rely on the interference effects (manifested in changes in surface reflectivity) created when a probe beam is reflected off the sample surface. Where a probe beam wavelength is selected which is at least partially transmitted by the thin film layer, reflections will occur both at the upper surface of the thin film layer and from the upper surface of the substrate. The interaction of the two reflected rays will create interference effects which vary based on layer thickness. These interference effects or changes in surface reflectivity can be detected by measuring the intensity of the reflected beam using a photodetector. Spectrophotometers are one of the devices within this class of detectors. In a spectrophotometer measurement, the sample is scanned with different wavelengths of light. Since the phase shift through the film is different for each wavelength, each wavelength will undergo different interference effects. By studying the different interference effects created by the different wavelengths, information about the thickness of the thin film layer can be derived. In many spectrophotometers, the wavelength of the probe beam is varied over a significant range to reduce ambiguities in the measurement. Examples of such systems are disclosed in U.S. Patent No. 3,824,017, issued July 16, 1974 to Gaylon, U.S. Patent No. 4,293,224, issued October 6, 1981 to Gaston, et al. and U.S. Patent No. 4,555,766, issued November 26, 1985 to Case, et al. As described in the Gaston patent, interferometric devices can be used in situ to monitor changing layer thickness during processing. As can be appreciated, as the thickness of the layer changes during a coating operation, interference patterns in the reflected probe beam will change. By monitoring this change in interference patterns, information about the changing layer thickness can be obtained. In the spectrophotometer techniques discussed above, known variables, such as changes in the wavelength of the probe beam or changes in the thickness of the layer are introduced to gain information about layer thickness. Another type of known variable which can be introduced is a change in the angle of incidence of the probe beam. Interference effects will vary as the angle of incidence the probe beam is changed. One example of a device where the angle of incidence of the input beam is varied can be found in U.S. Patent No. 4,453,828, issued June 12, 1984 to Hershel, et al. The latter techniques, which rely on the detection of interference effects, have better spatial resolution than the ellipsometry techniques discussed above. However, even the best interferometric techniques are limited to a resolution of about the 2-3 microns in diameter. More significantly, the interferometric techniques known heretofore are incapable of measuring thin films having a thickness less than 200Å. Finally, the interferometric techniques are not nearly as accurate as the ellipsometer techniques discussed above. A different interferometric technique is described in U.S. Patent No. 4,660,980, issued April 28, 1987 to Takabayashi, et al. In this device, a laser beam, which consists primarily of light having S-polarization, is reflected off the surface of the sample at close to Brewster's angle. Assuming the layer is relatively thick, i.e., on the order of 50 microns, a plurality of repeating minima or interference fringes will be produced across the reflected probe beam. In the Takabayashi device, the number of minima or fringes are counted and used to derive information about layer thickness. Takabayashi is therefore limited to measuring relatively thick thin film layers which will generate the multiple fringe effects. In addition, because Takabayashi forces the input beam to have an angle of incidence near Brewster's angle, tight focusing can not be achieved and spatial resolution must be sacrificed. In IBM Technical Disclosure Bulletin, Vol. 31, No. 8, January 1989, at pages 363 to 368, there is described an apparatus and method for film thickness and optical constant determination. The apparatus includes a laser generating a beam of narrow band, polarized and collimated radiation which is focused at the surface of a sample consisting of a thin film layer on a substrate by a low f/number cylindrical lens arranged substantially symmetrically with respect to a normal to the surface of the sample at the thin film layer. The cylindrical lens creates in a predetermined plane a spread of angles of incidence at a normal to the thin film layer surface. Radiation reflected by the thin film layer back through the lens is collimated by the lens and directed onto a diode array in which each diode measures radiation from a small angular range and the reflectivity as a function of angle is obtained directly from the output of the array. The output of the array is supplied to signal processing means for determining the thin film layer thickness, the real part of the refractive index of the material of the thin film layer, and the imaginary part of the refractive index from reflectivity versus angle of incidence curves for Sand P- polarization reflectivities. EP-A-0 150 945 describes an apparatus and a method for measuring the thickness of a transparent film on a substrate, in which a circularly polarised beam is passed through a polarization plate mounted in a hollow shaft of a motor that rotates the shaft at a fixed rate so as to produce a beam having a continuously varying polarization angle. This beam is passed through a beam splitter that produces five such output beams. These five beams are parallel and are passed through a condensing lens to focus at a point on the surface of the thin film. It is arranged that the principal axis of the lens is at an angle to the normal at the film surface and that all five incident beams lie in the same plane of incidence. Because the plane of polarization rotates continuously, the component of the incident light polarized parallel to the plane of incidence varies cyclically between a maximum and zero. Similarly, the component of the incident light polarized perpendicular to the plane of incidence varies cyclically between a maximum and zero. These maxima and zeros are out of phase so that the maximum of one component occurs at the same time as the zero of the other component. Since the reflectivities for the two components are different, the reflected light varies cyclically between a maximum corresponding to the maximum of one component and a minimum corresponding to the maximum of the other component. A linear array of detectors is arranged to sense the five reflected beams and is coupled to a microprocessor based apparatus that senses and compares the maxima and minima in the reflected beams to determine the thickness of the film. According to one aspect of the present invention, an apparatus of the kind defined hereinbefore at the beginning is characterised in that the lens is a spherical lens and creates a spread of angles of incidence with respect to the said surface both in the said predetermined plane and in a further predetermined plane, the said predetermined planes being orthogonal to one another, and in that the detector means is selectively responsive to the reflected radiation in both of the predetermined planes, whereby the detector means is capable of responding simultaneously and independently to different polarization states of the reflected radiation. According to a further aspect of the invention there is provided a method of evaluating a parameter of a sample which includes a thin film layer on a substrate, the method comprising the steps of: generating a probe beam of radiation; so focusing the probe beam at the surface of the sample at the thin film layer as to create in a predetermined plane a spread of angles of incidence with respect to said surface and such that the reflected radiation varies in intensity as a result of interference effects created by the thin film layer; sensing and measuring the intensity of probe beam radiation reflected back from the sample as a function of the said angles of incidence; and evaluating the sample on the basis of the angle dependent intensity measurements, characterised in that the probe beam is so focused as to create a further spread of angles of incidence with respect to the said surface in a further predetermined plane which is orthogonal to the first said predetermined plane, the intensity of probe beam radiation reflected back from the sample as a function of the angles of incidence in the further predetermined plane is sensed simultaneously with the intensity of probe beam radiation reflected back in the first said predetermined plane and is also measured, with the sensed intensities of radiation reflected back in the said first and further planes being representative of different polarization states of the radiation, and the sample is evaluated on the basis of all the angle dependent intensity measurements. Preferred embodiments of the invention provide: apparatus for measuring the thickness of thin films which is accurate over a wide range of thin film thicknesses; an apparatus for measuring thin film thicknesses within a region on the order of 1 micron or less in diameter; apparatus for measuring the thickness of thin films from 50 to 50,000Å; apparatus for measuring the thickness of thin films which utilizes only a single probe beam wavelength; apparatus for accurately measuring the thickness of thin film layers which does not require any means for actively changing the incidence angle of the probe beam; apparatus for measuring the thickness of a thin film layer which does not have to be operated in situ , where the thickness of the thin film is changing; apparatus which is capable of determining the refractive index of a thin film as well as the thickness of the thin film layer; and a method for refining the accuracy of the thin film thickness measurement by providing an additional detector for recording the total power of the reflected probe beam. The preferred apparatus is particularly suited for measuring layer thicknesses deposited on the silicon substrates found in the semiconductor industry. The preferred apparatus includes a means for generating a probe beam of radiation having a single wavelength. A laser is utilized to generate the beam because it provides accurate wavelength information, can be tightly focused, and has enough power to accurately allow measurement of the reflectivity. The laser beam is focused substantially normal to the surface of the sample using a high numerical aperture lens. The high numerical aperture lens allows focusing to a spot diameter of about 1 micron. The use of the high numerical aperture lens also creates a ray pattern within the incident focused beam having a wide range of angles of incidence with respect to the surface of the sample. Information derived from various rays having different angles of incidence within the focused probe beam can be used to derive information about layer thickness. The latter result is achieved by measuring the intensity variations across the reflected probe beam as a function of the angle of incidence of the various rays within the focused incident probe beam. The angle of incidence of the rays within the focused probe beam with respect to the surface of the sample is related to the position of the rays in the reflected probe beam. More specifically, the ray in the center of the reflected probe beam will correspond to the center ray in the incident beam. Assuming the probe beam is focused normal to the surface of sample, the center ray will have an incidence angle of 0°. In contrast, the portions of the reflected probe beam at the outer edges thereof correspond to the highest angles of incidence. The highest angle of incidence is dependent on the focusing power or numerical aperture of the lens. The maximum angle ϑ m is given by the following equation: (1) Sine ϑ m = (numerical aperture) The location of any ray in the reflected probe beam corresponds to the sine of the angle of incidence of the associated ray in the focused incident probe beam. As is known from the prior art patents discussed above, if measurements can be made over varying angles of incidence, information about layer thickness can be derived. This result is a consequence of the fact that the interference effects vary for different angles of incidence. The interference effects (or changes in surface reflectivity) can be seen as changes of the intensity across the reflected probe beam. The expected reflectance of linear polarized light from a thin film substrate of a given thickness can be calculated using Fresnel's equations. Thus, the angular dependent intensity measurements can be compared to expected values derived from the Fresnel equations to give information about the thickness of the thin film layer. In the preferred embodiment, the light in the probe beam is initially linearly polarized such that when it is tightly focused normal to the surface of the sample, the incident rays will have substantial components of S and P polarized light relative to the sample surface. The detector means comprises 2 orthogonal arrays, oriented in a manner to isolate and detect the interference effects from the S and P polarized components of the incident beam. By measuring angular dependent intensity profiles for each of the polarizations states, measurement ambiguity is reduced and accuracy is improved. In addition to improving accuracy, information about the reflectance of the isolated P-polarized light near Brewster's angle can be used to derive information as to the refractive index of either the thin film layer or the substrate. As noted above, the use of a high numerical aperture lens will generate a relatively large spread of incidence angles in the rays of the focused incident probe beam. In the preferred embodiment, the incidence angles will include not only the normal but rays at Brewster's angle. At Brewster's angle, the intensity of the reflected P polarized light does not vary significantly with changes in layer thickness. Therefore, a measurement of the reflectivity of rays incident at or about Brewster's angle can give information about the index of refraction of either the substrate or the thin film layer. The subject apparatus is capable of making thickness measurements over a wide range of thin film thicknesses. It is believed this system will provide information for layers ranging from 50Å to 50,000Å. Equally significant is the fact that a spatial resolution on the order of 1 micron in diameter can be achieved. In this manner, highly localized thickness measurements can be made over a broad range of layer thicknesses. The accuracy of measurement of layer thickness can be enhanced if the total reflected power of the probe beam is also measured. More specifically, it is known that the reflectivity of the surface of the sample will vary approximately sinusoidally with the thickness of the thin film layer. However, information about layer thickness cannot be derived from this measurement alone since any particular reflectivity measurement would be consistent with a number of different layer thickness. These ambiguities can be removed if a very good approximation of layer thickness is first made using the device described above. Once this measurement is made, data taken from a reflectivity measurement based on the total power of the reflected beam can be used to reduce the uncertainty in layer thickness even further. This added sensitivity is a result of the fact that the signal to noise ratio of the measurement of the full power of the beam will be much better than the initial detector measurement where only portions of the beam, at discrete points corresponding to specific angles of incidence, are measured. Further objects and advantages of the subject invention will be apparent from the following detailed description taken in conjunction with the drawings in which: Brief Description of the Drawings Figure 1 is a simplified block diagram of the apparatus of the subject invention. Figure 2 is a diagram of the focusing of the probe beam, enlarged with respect to Figure 1. Figure 3 is a plan view of the surface of a photodetector which can be used to carry out the method of the subject invention. Figures 4A-C include three graphs plotting the reflectivity of S and P polarized light with respect to the angle of incidence of the incoming probe beam at three different thickness levels of an oxide on a silicon wafer. Figure 5 illustrates the variation of reflectivity of S-polarized light in a focused incident probe beam with respect to different oxide thicknesses on silicon at various angles of incidence. Figure 6 illustrates variations of reflectivity of P-polarized light in a focused incident probe beam as a function of different thickness on silicon at various angles of incidence. Figure 7 is a flow chart illustrating the steps the processor takes to derive layer thickness in accordance with the subject invention. Figure 8 is a flow chart illustrating the steps the processor takes to derive both layer thickness and index of refraction in accordance with the subject invention. Figure 9 is a schematic diagram of the preferred embodiment of the subject invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Figure 1, there is illustrated, in schematic form, the basic apparatus 20 for carrying out the method of the subject invention. Apparatus 20 includes a laser 22 for generating a probe beam of radiation 24. Probe beam 24 is reflected downward by a beam splitter 26 towards the sample 28 having a thin film layer 32 deposited thereon. In accordance with the subject invention, beam 24 is focused on the surface of the sample 28 with a high numerical aperture lens 30. Preferably, beam 24 is focused substantially normal to the surface of the sample in order to minimize the spot size and maximize resolution. Figure 2 is an enlarged view of the probe beam 24 shown focused onto the sample 28 by lens 30 and illustrating some of the individual rays within the incident focused probe beam. As can be seen, where beam 24 is directed normal to the surface, the incident focused beam will include a center ray 36 substantially normal to the surface of the sample. The focused beam will also include outer rays, 38A and B, having a maximum angle (a) of incidence with respect to the surface of the sample. As set forth in equation (1) above, the angle of incidence of the outer rays 38 is dependent upon the numerical aperture of lens 30. Intermediate rays have angles of incidence proportional thereto. For example, symmetric rays 40A and 40B illustrate an angle of incidence of 30° while symmetric rays 42A and 42B represent an angle of incidence of 15°. Lens 30 is selected to have a high numerical aperture thereby maximizing the spread of angles of incidence. In practice, a lens creating a spread of angles (from the center ray to the outermost ray) of at least 30° is desired. A lens having a numerical aperture of 0.5 will provide such a spread of rays. In the preferred embodiment, a lens having a .95 numerical aperture is used which gives a spread of greater than 70°. After reaching the sample, beam 24 is reflected back upwardly through lens 30, past beam splitter 26 and falls on photodetector 50. The surface of photodetector 50 is shown in Figure 3. Photodetector 50 includes a plurality of individual discrete detecting elements, the outputs of which are supplied to processor 52. In accordance with the subject invention, detector 50 and processor 52 function to measure the intensity of the reflected probe beam as a function of the angle of incidence of various rays within the focused incident probe beam. This result can be appreciated through an optical pathway analysis illustrated in Figures 2 and 3. As can be seen, center element 54 of detector 50 will receive and measure the light intensity of the ray 36 passing through the center of lens 30. Accordingly the output of detector element 54 would correspond to a ray having an angle of incidence of zero degrees. Similarly, the radially outer elements 56A-D on detector 50 receive and measure the light intensity of rays such as 38A and 38B, which have the greatest angle of incidence. As noted above, this angle of incidence is known based upon the numerical aperture of lens 30. Discrete elements 58A-D and 60A-D correspond to angles of incidence between zero and the highest angle. In the illustrated embodiment, elements 58A-D are located in a manner to receive and measure rays 40A-B having an angle of incidence of 30° while discrete elements 60A-D are located in a manner to receive and measure rays such as 42A-B having an angle of incidence of 15°. The detector surface shown in Figure 3 illustrates only a few discrete detecting elements. As can be appreciated, additional information can be obtained by increasing the number of detecting elements in a manner to gain information with respect to a larger number of discrete angles of incidence of the focused probe beam. For example, additional elements could be used to measure other angles of incidence. In the embodiment illustrated in Figures 2 and 3, two arrays of detector elements, oriented orthogonally with respect to each, are provided. Two orthogonally disposed arrays of detector elements are desirable in order to isolate information about S and P polarized light as discussed below. In the preferred embodiment, probe beam 24 consists of linearly polarized light normally incident onto the sample surface. When the normally incident, linearly polarized light is tightly focused by lens 30, the light rays incident on the sample surface will have substantial components of S and P polarized light with respect to the sample surface. Light which is substantially all S polarized will lie along one axis. Conversely, light which is substantially all P polarized will lie along another axis, orthogonal to the first. Axes between these two orthogonal axes will contain a mixture of both components. It has been found that if two orthogonal arrays of detectors, as shown in Figure 3, are properly oriented, the S and P polarization components can be isolated from the probe beam. The isolated S and P-polarization intensity information can be used to calculate the thickness of the thin film layer on a sample as well as to derive information about the index of refraction of the sample as discussed in greater detail below. Figures 4A-C illustrate the type of measurements which might be generated with an apparatus 20 of the subject invention. Each of the graphs illustrates the reflectivity of the surface of the sample for the S and P polarized light as a function of the angle of incidence in degrees. The reflectivity of the sample is proportional to the intensity of the reflected probe beam which is directly measured by the discrete elements on the photodetector 50. A measurement of surface reflectivity of the surface is derived by calibrating the output of the photodetector. Figures 4A-4C illustrate reflectivity curves for an oxide layer on a silicon semiconductor substrate taken with thin film oxide layers of 5,000 angstroms, 1,000 angstroms and 100 angstroms thick. As can be seen, the reflection curves are substantially different for each of the different layer thicknesses. This information can therefore be used to derive information about layer thickness. Information about layer thickness is specifically derived using the measured reflectance information in a mathematical model or set of equations which relate reflectivity, angle of incidence and layer thickness. The equations utilized are based on the well known Fresnel equations. The principal equation for the reflectivity of a sample with respect to S polarized light is as follows: (2) R = r L + r S + 2 r L r S cos (2 k₁ t) 1 + r L r S + 2 r L r S cos (2 k₁ t) where R is the reflectivity of the light derived from the isolated intensity measurement, the index of refraction in air is taken as one and t is the thickness of the layer. The constant k₁ in equation (2) is given by the formula: (3) k₁ = 2π λ n L cos ϑ L where λ is the wavelength of the probe beam. All r values are for the reflectivity of polarized light where r L is the reflectance from the surface of thin film layer and r S is the reflectance from the surface of the substrate. For S polarized light, the reflectivities from the surface thin film layer (r L ) and the substrate (r S ) are given by the following equations: (4) r L = n L cos ϑ L - cos ϑ O n L cos ϑ L + cos ϑ O and (5) r S = n S cos ϑ S - n L cos ϑ L n S cos ϑ S + n L cos ϑ L where n L is the index of refraction of the thin film layer, n S is the index of refraction of the substrate, ϑ O is the angle of incidence at the interface between the air and surface of the thin film layer, ϑ L is the angle of incidence in the thin film layer and ϑ S is the angle of incidence in the substrate. For P polarized light, the reflectivities from the surface of the thin film layer (r L ) and the substrate (r S ) are given by the following equations: (6) r L = n L cos ϑ L - 1 cos ϑ O n L cos ϑ L + 1 cos ϑ O and (7) r S = n S cos ϑ S - n L cos ϑ L n S cos ϑ S + n L cos ϑ L Figures 5 and 6 are computer generated graphs of reflectance curves for the S and P polarized light generated using the equations set forth above. The multiple curves in each of the graph represent angles of incidence (ϑ) from 0° to 60° in 5° increments. The graphs demonstrate how reflectivity varies with oxide thickness. Figure 7 is a flow chart of the algorithm the processor uses to derive the thickness of a thin film layer based upon the measured reflectance values that are a function of the angle of incidence. The measured reflectance values are obtained in step 100 by apparatus 20 as discussed. In step 102, the processor solves equation (2) using the measured reflectance values and the substitutions from equations (4) through (7). As should be clear from Figures 5 and 6, any particular reflectively measurement will correspond to a number of different layer thicknesses. Thus, the result in step 102 will include a number of possible solutions for the thickness of the thin film. In step 104, a statistical analysis is performed to select a single, most likely solution for layer thickness from the group of solutions derived in step 104. This solution will typically be accurate to within 20 angstroms. The accuracy of the result can be further improved if an analysis is made which takes into account such factors as light absorption by the sample and the effect of a finite detector size. The amount of absorption varies from material to material. If the materials are known, various correction factors can be incorporated into equation (2) to further refine the determination. However, such correction factors make an analytical solution of the equations difficult. Thus, a numerical solution must be sought using a least squares fitting routing as set forth in step 106. Such a numerical analysis is greatly facilitated due to the fact that an approximate solution for layer thickness has already been determined. Thus, in step 106, a least squares fitting routine is used to solve a set of Fresnel equations, corrected for absorption characteristics of the particular sample being tested and limited by the likely solution for layer thickness obtained in step 104. If the solution is refined in this manner, the accuracy of the measurement can be significantly improved. This measurement can be made on films ranging in thickness from 50 angstroms to 50,000 angstroms with a spatial resolution on the order of 1 micron. In many situations, in particular thin films having a thicknesses less than 1000 angstroms, there is a need to know the thickness with even greater accuracy. In accordance with the subject invention, greater accuracy can be achieved by making an additional measurement of the total power of the reflected probe beam as shown in step 108. As will be discussed below with reference to Figure 9, this measurement is preferably made by using a separate photodetector. However, an approximation of this measurement could be made by adding the outputs of all of the discrete elements 54 through 60 of the detector shown in Figure 3. Assuming that the detector is underfilled by the beam (i.e., the entire reflected beam falls within the confines of the detector elements) a measurement proportional to the total power of the beam can be made. The analysis of layer thickness based on this additional information is similar to that stated above. More specifically, equation (2) is modified by the substitutions set forth below in order to relate layer thickness to the reflectance of the full beam from the thin film layer (L) and substrate (S). (8) r L = n L - 1 n L + 1 (9) r S = n S - n L n S + n L (10) K₁ = 2 π n L <λ> where <λ> is an effective wavelength, slightly greater than λ, which accounts for the 3-dimensional beam spreading of the focused beam within the layer. When comparing layer thickness to reflectivity based on the latter equations, one finds that the curve is highly sinusoidal giving a large number of possible thickness solutions for any given measurement of total reflected power. This problem is overcome if the thickness of the thin film layer is first established to within a narrow range in step 106. This information reduces the possible solutions of the latter modified Fresnel equations so that the measurement of the total reflected power of the beam derived in step 108 can now be used to refine the calculation of layer thickness to a much greater degree of accuracy in step 110. The reason the full power measurement provides greater accuracy is that the signal to noise ratio of the measurement is on the order of 10 to 100 times better than the measurement of any of the discrete elements on detector 50. As can be appreciated, the reflected beam is spread out over the surface of the detector such that any of the discrete elements will receive only a fraction of the power of the beam and thus be sensitive to beam shape and beam position fluctuations. In contrast, the full power measurement takes advantage of the full laser power and thus is much less sensitive to small beam shape and beam position fluctuations. In another aspect of the subject invention, the index of refraction of either the thin film layer or the substrate can be calculated along with the thickness of the thin film layer. This capability is a result of the fact that the high numerical aperture lens can be used to create rays having an angle of incidence at or near Brewster's angle. Referring to Figure 6, it will be seen that the reflectance of the sample with respect to P-polarized light varies quite significantly with changes in oxide thickness for virtually all of the angles of incidence. However, as the angle of incidence approaches Brewster's angle, (ϑ equals about 55°96 for silicon dioxide) the reflectivity curve becomes almost flat. Thus, the reflectivity measurement of P-polarized light at Brewster's angle is substantially independent of layer thickness. However, reflectivity will vary with changes in the index of refraction of either the layer or substrate. An algorithm for deriving layer thickness and the index of refraction of either the substrate or the thin film layer is shown in Figure 8. This approach assumes that the layer thickness is known within some range and that the index of refraction of either the substrate or thin film layer is known. As set forth in step 200, reflectance measurements are made, including a measurement of P-polarized light at Brewster's angle. In step 202, an approximate solution for the unknown index of refraction (either of the substrate or of the thin film layer) can be calculated using equations (2), (6) and (7) and an estimated layer thickness. Using the approximate solution for the index of refraction, a group of solutions for the actual thickness of the thin film layer is calculated in step 204 using equation (2) and an approach identical to Step 102 in Figure 7. From these possible solutions, one solution is selected based on the fact that there is some prior knowledge of a thickness range. Using the calculated approximate thickness and calculated approximate index of refraction, a least squares fitting routine is used with the Fresnel equations, corrected for absorption and detector size, to generate the best solution for layer thickness and index of refraction in step 206 (as in Step 106 in Figure 7). As noted above, the accuracy of the thickness measurement of the thin film can be further enhanced by taking a measurement of the full power of the reflected probe beam in step 208. This measurement is used to calculate layer thickness in step 210 using equation (2) (as modified by equations 8-10) and limited by the solution for layer thickness calculated in step 206. Having described the basic theory and operation of the subject's invention, one preferred apparatus 300 for carrying out the measurements will be described with reference to Figure 9. In Figure 9, parts equivalent to the structure shown in Figure 1 are numbered the same. As noted above, laser 22 generates an output beam 24. Laser 22 can be a helium neon laser having a 5 milliwatt power output and a polarization oriented at 45°. Beam 24 is then passed through a polarizing filter 302 oriented at 45° to insure only 45° polarized light is transmitted. This light is then passed through a faraday rotator 304 for rotating the polarization an additional 45° to prevent spurious reflected helium neon radiation from being reflected back into the helium neon laser. This arrangement reduces instabilities in the power output of the laser caused by spurious reflected HeNe radiation. Beam 24 is then passed through a polarizing filter 306 oriented at 90° to further reduce spurious reflected radiation from going back into the laser. The beam 24 is then reflected downward by a metallic coated glass turning mirror 308 and down into a beam expander spatial filter arrangement. This arrangement includes an input lens 310 and an output lens 312. A pin-hole filter 314 is provided to improve the beam profile and its uniformity. The beam that exits lens 312 is expanded by the ratio of the focal lengths of lens 310 and lens 312. The beam is then turned with a metallic coated glass beam splitter 316 having an 20% transmission and 80% reflection. This beam splitter is used to combine the HeNe beam and white light from source 318. The white light is used to image the sample using a vision system described in greater detail below. The probe beam is then directed downward towards the sample by a metallic coated glass beam splitter 320 having a 60% transmission and 40% reflection. This beam splitter also functions to turn the white light down towards the sample. Upon return from the sample, part of the probe beam passes up through the splitter 320 and will be collected by the detector 50. Some of the white light from source 318 will be reflected back to the vision system. The probe beam is focused on the sample through the objective lens 30. Lens 30 has a high numerical aperture so the various rays in the focused beam will have a wide spread of angles of incidence. The position of lens 30 is controlled by an autofocus mechanism 322. As noted above, beam 24 is focused to a spot size about 1 micron in diameter on the surface of sample 28. The sample 28, which may be a silicon semiconductor with a thin film oxide layer, is mounted on a stage which provides X, Y and rotational positional movement. The top stage 324 provides rotational movement as well as a vacuum chuck for holding down the wafer. Stages 326 and 328 provide X and Y linear movement. Light from the HeNe probe beam and white light that is reflected from the sample passes through beam splitter 320. The transmission of white light is restricted by a band pass filter 332 which transmits only the 633 nm HeNe wavelength radiation. A portion of the HeNe probe beam is then redirected by a metallic coated glass beam splitter 334 to detector 50. Beam splitter 334 has a 20% transmission and an 80% reflection. The reflected light from beam splitter 334 is expanded by lens 336 and imaged on detector 50 which was described above with reference to Figure 1. Detector 50 can be defined by a diode array, CCD camera, fiber optic sensor array. Preferably, detector 50 has detector elements which are arranged to isolate the information from about the reflectivity of the S and P polarization light incident on the surface of the sample. The output of detector 50 is supplied to the processor 52. As mentioned above, the accuracy of the thickness measurement can be further enhanced by measuring the full power of the reflected probe beam. In the embodiment illustrated in Figure 9, a separate detector 340 is provided for that purpose. A portion of the probe beam is directed to the detector 340 by metallic coated glass beam splitter 342. The only requirement of detector 340 is that the probe beam substantially underfill the detecting surface so that the full power of the beam can be measured. In the illustrated embodiment, an autofocus mechanism is provided. Autofocus mechanism comprises a bi-cell photo detector 344, lens 346 and a chopper wheel 348. Autofocusing using these types of components is known in the art. In order to insure proper calibration, the output power of the laser 22 must be monitored. This result is achieved using a detector 360 upon which a part of the probe laser beam 24 is directed. A polarizing filter 362 and a band pass filter 364 removes white light from the HeNe probe beam. Lens 366 is used to image the beam onto detector 360. In the illustrated embodiment, apparatus 300 includes a vision system to allow the operator to locate points on the sample. The vision system includes the white light source 318, a collimating lens 370 and a polarizing filter 372. Polarizing filter 372 is designed to polarize the white light perpendicular to the polarization state of the probe beam thus allowing separation of the white light and the helium neon beams by the downstream optics. A portion of the white light returning from the sample is directed downward to a camera 374 by a 50/50 beam splitter 376. A tunable polarization filter 378 is provided to control the amount of probe beam radiation falling on the video camera 374. Lens 380 focuses the image provided for the camera. The basic operation of the apparatus 300 is the same as set forth above with respect to Figures 1-3. The detector 50 and processor 52 function to measure the intensity of the reflected probe beam as a function of the angle of incidence of the rays in the incident focused probe beam. Calculations of layer thickness are performed using the equations and algorithms discussed above. In summary, there has been provided a new and improved method and apparatus for measuring the thickness of a thin film layer on a substrate. In a preferred embodiment, a probe beam of radiation is focused substantially normal to the surface of the sample using a high numerical aperture lens. A high numerical aperture lens provides a large spread of angles of incidence of the rays within the incident focused beam. A detector is provided for measuring the intensity across the reflected probe beam as a function of the angle of incidence with respect to the surface of the substrate of various rays within the focused incident probe beam. A processor functions to derive the thickness of the thin film layer based on these angular dependent intensity measurements. This result is achieved by using the angular dependent intensity measurements to solve the layer thickness using variations of the Fresnel equations. In one embodiment, the focused probe beam includes substantial components of S and P polarized light. In addition, the detector is designed to isolate the S and P polarized components to facilitate evaluation of layer thickness. Preferably, an additional detector is provided for measuring the full power of the reflected probe beam. The additional signal information provided from the latter detector is used to refine the accuracy of the derived thickness measurement.

Claims: An apparatus for evaluating a parameter of a sample which includes a thin film layer (32) on a substrate (28), the apparatus comprising means (22) for generating a probe beam (24) of radiation, a lens (30) arranged substantially symmetrically with respect to a normal to the surface of the sample at the thin film layer (32) for focusing the probe beam (24) at the said surface, so as to create in a predetermined plane a spread of angles of incidence with respect to the said surface, detector means (50) responsive to radiation intensity and arranged to receive probe beam radiation reflected back through the lens (30), with the location of a ray on the detector means (50) being a function of the angle of incidence of that ray with respect to the said surface, and the lens (30) being so positioned that the reflected radiation varies in intensity as a result of interference effects created by the thin film layer (32) and processor means (52) coupled to the detector means (50) for evaluating the sample (28,32) on the basis of angle dependent intensity measurements, the detector means (50) being selectively responsive to the reflected radiation in the said predetermined plane, characterised in that the lens is a spherical lens (30) and creates a spread of angles of incidence with respect to the said surface both in the said predetermined plane and in a further predetermined plane, the said predetermined planes being orthogonal to one another, and in that the detector means (50) is selectively responsive to the reflected radiation in both of the predetermined planes, whereby the detector means (50) is capable of responding simultaneously and independently to different polarization states of the reflected radiation. Apparatus according to claim 1, characterised in that the said reflected radiation is linearly polarized radiation with the plane of polarization corresponding to one of the said predetermined planes. An apparatus according to claim 2, characterised in that the radiation in said probe beam (24) is linearly polarized prior to being focused by said lens (30), and in that the detector means (50) functions to isolate and measure the angle dependent intensity of the reflected S and P polarized components of said focused incident probe beam (24) and said processor means (52) utilizes the angle dependent intensity measurement of the S and P polarized light to determine the parameter of the sample (28,32). An apparatus according to claim 3, characterised in that said focused incident probe beam has at least one ray having a substantial component of P polarized light included near Brewster's angle with respect to the thin film layer (32) and in that said processor means (52) further functions to determine information as to the index of refraction of said thin film layer based on the measurement of the intensity of reflected P polarized light incident on the said surface near Brewster's angle. An apparatus according to claim 4, characterised in that said incident probe beam is focused to a diameter of about one micron or less on the said surface. An apparatus according to claim 1, characterised in that said focused incident probe beam includes at least one ray included near Brewster's angle with respect to either the thin film layer (32) or the substrate (28). An apparatus according to claim 1, characterised in that said lens (30) has a numerical aperture of at least 0.5. An apparatus according to claim 1, characterised in that the variation in the angle of incidence of the rays of the focused incident probe beam, from the center to the outer edge, is at least 30 degrees. An apparatus according to claim 1, characterised in that the incident probe beam (24) is focused to a diameter of about one micron or less on the said surface. An apparatus according to claim 1, characterised in that said detector means (50) measures the intensity across the reflected probe beam at a plurality of discrete points, from the center of the reflected probe beam to the outer edge thereof. An apparatus according to claim 3, characterised in that said processor means (52) utilizes the angle dependent intensity measurement of the S and P polarized light to determine the thickness of the thin film layer (32). An apparatus according to claim 1, characterised in that said processor means (52) determines the thickness of the thin film layer (32) using a mathematical model which relates the angle dependent intensity measurement made by the detector means (50), angle of incidence, polarization of the rays of the focused incident probe beam and layer thickness. An apparatus according to claim 1, characterised in that said detector means (50,340) further functions to measure the total power of the reflected probe beam, and in that said processor means (52) determines the thickness of the thin film layer (32) based upon both the angle dependent intensity measurements and the total reflected power measurements made by the detector means (50). An apparatus according to claim 13, characterised in that said processor means (52) utilizes the angle dependent intensity measurements to approximate the thickness of the thin film layer (32) and utilizes the total reflected power measurement to reduce the uncertainty of the result. An apparatus according to claim 13 or 14, characterised in that the lens (30) has a numerical aperture of at least 0.5. An apparatus according to claim 13 or 14, characterised in that said incident probe beam (24) is focused to a diameter of about one micron or less on the said surface. A method of evaluating a parameter of a sample which includes a thin film layer (32) on a substrate (28), the method comprising the steps of: generating (22) a probe beam (24) of radiation; so focusing (30) the probe beam (24) at the surface of the sample at the thin film layer (32) as to create in a predetermined plane a spread of angles of incidence with respect to said surface and such that the reflected radiation varies in intensity as a result of interference effects created by the thin film layer (32); sensing and measuring the intensity of probe beam radiation reflected back from the sample as a function of the said angles of incidence; and evaluating the sample on the basis of the angle dependent intensity measurements, characterised in that the probe beam (24) is so focused as to create a further spread of angles of incidence with respect to the said surface in a further predetermined plane which is orthogonal to the first said predetermined plane, the intensity of probe beam radiation reflected back from the sample as a function of the angles of incidence in the further predetermined plane is sensed simultaneously with the intensity of probe beam radiation reflected back in the first said predetermined plane and is also measured, with the sensed intensities of radiation reflected back in the said first and further planes being representative of different polarisation states of the radiation, and the sample is evaluated on the basis of all the angle dependent intensity measurements. A method according to claim 17, characterised in that the said reflected radiation is linearly polarized radiation with the plane of polarization corresponding to one of the said predetermined planes. A method according to claim 17, characterised in that said incident probe beam (24) is focused in a manner to include at least one ray that is inclined near Brewster's angle with respect to either the thin film layer (32) or the substrate (28). A method according to claim 17, characterised in that the incident probe beam (24) is focused in a manner such that the variation in the angle of incidence of the rays with respect to the said surface (28) is at least 30 degrees. A method according to claim 17, characterised in that the incident probe beam (24) is focused to a diameter of about one micron or less on the said surface. A method according to claim 18, characterised in that the probe beam (24) is linearly polarized prior to being focused, and in that the two orthogonal planes are oriented such that the angle dependent intensity variations of the S and P polarized components of said focused incident probe beam can be isolated whereby the thickness of the thin film layer (32) can be determined utilizing the angle dependent intensity measurement of the S and P polarized light. A method according to claim 22, characterised in that the incident probe beam has at least one ray having a substantial component of P polarized light inclined near Brewster's angle with respect to the thin film layer (32) and in that the index of refraction of said thin film layer (32) is determined on the basis of measurement of the intensity of reflected P polarized light incident on the sample near Brewster's angle. A method according to claim 18, characterised in that the thickness of the thin film is calculated using a mathematical model which relates the angle dependent intensity measurements, angle of incidence and polarization of the rays of the focused, incident probe beam with layer thickness. A method according to claim 17, characterised by the steps of: measuring the total power of the reflected probe beam radiation; and determining the thickness of the thin film layer (32) on the basis of both the angle dependent intensity measurements and the total reflected power measurements. A method according to claim 25, characterised in that the angle dependent intensity measurements are utilized to approximate the thickness of the thin film layer (32) and the total reflected power measurement is utilized to reduce the uncertainty of the result.

Inventors: GOLD NATHAN, OPSAL JON, ROSENCWAIG ALLAN, WILLENBORG DAVID L, GOLD, NATHAN, OPSAL, JON, ROSENCWAIG, ALLAN, WILLENBORG, DAVID L.
Applicants: THERMA WAVE INC A DELAWARE COR, THERMA-WAVE, INC. (A DELAWARE CORPORATION)
Application year: 1990
Main classification: G01B11
IPC codes: G01N21, G01B11