Abstract: A method for monitoring consumption of a component, including the steps of emitting a radiation beam onto a first area of the component and detecting a portion of the radiation beam that is refracted by the component. A radiation level signal is generated based at least on a strength of the detected portion of the radiation beam, and a thickness of the component is determined based on the radiation level signal. The thickness of the component is compared to a predetermined thickness value, and a status signal is generated when the comparing step determines that the thickness of the component is substantially equal to or below the predetermined thickness value. When the comparing step determines that the thickness of the component is greater than the predetermined thickness value, the component is exposed to a process that can erode at least a portion of the component.

Abstract: A phase measuring apparatus for measuring phase characteristics of a film applied onto an object to be measured includes a shearing interference system for providing incident light onto the object or light reflected on the object with shearing interference, a detector for detecting shearing interference information, and a computing unit for calculating the phase characteristics of the film based on the shearing interference information.

Abstract: A method including: imaging test light emerging from a test object over a range of angles to interfere with reference light on a detector, wherein the test and reference light are derived from a common source; for each of the angles, simultaneously varying an optical path length difference from the source to the detector between interfering portions of the test and reference light at a rate that depends on the angle at which the test light emerges from the test object; and determining an angle-dependence of an optical property of the test object based on the interference between the test and reference light as the optical path length difference is varied for each of the angles.

Abstract: A method of monitoring a critical dimension of a structural element in an integrated circuit is provided comprising the following steps: collecting an optical interference endpoint signal produced during etching one or more layers to form the structural element; and determining based upon the optical interference endpoint signal the critical dimension of the structural element.

Abstract: A system and method for analyzing the characteristics of a thin film is provided whereby the in-plane birefringence of thin films is determined by measuring the interference fringes in the transmission or reflection spectra using unpolarized light and light linearly polarized along the MD and CD directions. The three spectra can be measured simultaneously or sequentially. The in-plane birefringence data can be used to characterize clear polymer films, which are principally made of biaxial oriented polymer, as the film is being continuously fabricated on a production line.

Abstract: An apparatus for inspecting a ball-bumped wafer is provided in which an wafer to be inspected, in which a plurality of chips having ball bumps are formed, is mounted on a wafer table; an inspection light is irradiated from a light projection optical system to the wafer mounted on the wafer table; an intensity of the reflected light from a surface, including a bump surface, of the wafer is detected by a detection optical system; and a shape, such as a height, a diameter and a position, of the ball bump is measured on the basis of the intensity of the inspection light.

Abstract: A semiconductor fabricating apparatus for etching a semiconductor wafer, which is placed in a chamber and which has a multiple-layer film composed of a first film formed on a surface thereof and a second film formed on the first film, using plasma generated in the chamber. The semiconductor fabricating apparatus includes a light detector that detects a change in an amount of light with a plurality of wavelengths obtained from the surface of the wafer for a predetermined time during which the second film is etched, and a detection unit that detects a thickness of the first film based on a specific waveform obtained from an output of the detector.

Abstract: Process control monitors are disclosed that are produced using at least some of the same process steps used to manufacture a MEMS device. Analysis of the process control monitors can provide information regarding properties of the MEMS device and components or sub-components in the device. This information can be used to identify errors in processing or to optimize the MEMS device. In some embodiments, analysis of the process control monitors may utilize optical measurements.

Abstract: An optical system is presented for use in a measurement system (100) for use in measurements of thin films of a workpiece (W), the system comprising an optical assembly (14), comprising illuminator assembly, a detector assembly, and a light directing assembly (FA-OF) for directing illuminating light to a plurality of measurement sites in the workpiece (W) arranged in an array of substantially concentric ring-like regions, such that an area defined by the measurement sites within one of the substantially concentric ring-like regions is substantially equal to that of the other substantially concentric ring-like region.

Abstract: Thermal expansion characteristics of test materials of ultra-low thermal expansion material are measured with a test beam that is split into a test material-measuring portion and an instrument-measuring portion. Both measuring portions propagate through common portions of a test arm. The test material-measuring portion encounters a test material, but the instrument-measuring portion does not. Thermal expansion characteristics of the test material are measured to high accuracy by manipulating the measures to distinguish displacements associated with the test material from displacements associated with the instrument structure.

Abstract: A method for monitoring consumption of a component, including the steps of emitting a radiation beam onto a first area of the component and detecting a portion of the radiation beam that is refracted by the component. A radiation level signal is generated based at least on a strength of the detected portion of the radiation beam, and a thickness of the component is determined based on the radiation level signal. The thickness of the component is compared to a predetermined thickness value, and a status signal is generated when the comparing step determines that the thickness of the component is substantially equal to or below the predetermined thickness value. When the comparing step determines that the thickness of the component is greater than the predetermined thickness value, the component is exposed to a process that can erode at least a portion of the component.

Abstract: A method and apparatus for measuring wafers, thin films, or other planar layers are disclosed. This invention utilizes a tunable, monochromatic light source reflected from or transmitted through the layer to be measured. The wavelengths of light are selected such that the light is partially transmitted through the material to be measured so as optical interference is seen among the interfaces of the layer(s). The wavelengths are also controlled to sufficiently small increments to resolve these interference features. This apparatus relates to the need to monitor wafer thinning, film deposition, and other semiconductor device related processes.

Abstract: A standard pattern of a differential value of an interference light is set with respect to a predetermined film thickness of a first member to be processed. The standard pattern uses a wavelength as a parameter. Then, an intensity of an interference light of a second member to be processed, composed just like the first member, is measured with respect to each of a plurality of wavelengths so as to obtain a real pattern of an differential value of the measured interference light intensity. The real pattern also uses a wavelength as a parameter. Then, the film thickness of the second member is obtained according to the standard pattern and the real pattern of the differential value.

Abstract: A system and method for measuring the thickness of materials and coatings across a moving length of material such as sheet, film, or web by the use of non-contact optical interferometry is provided. Also, a system and method for evaluating the seal integrity in flexible packaging across a moving web by the use of non-contact optical interferometry is provided. Measurement of optical density and thickness, and the combination of various measurements in the production and process of manufacturing materials such as flexible packaging items that involve moving webs of material is disclosed. The present invention concerns the system and method involved in the collection and interpretation of data for these measurements and inspections.

Abstract: A film thickness measurement apparatus has an image pickup part (32) for acquiring a plurality of single-band images corresponding to a plurality of wavelengths, and the image pickup part (32) acquires a plurality of reference single-band images representing a pattern on a reference substrate. A correction factor setting part (51) performs setting of a plurality of correction factors in accordance with distances from a specified pixel by using a plurality of reference single-band images. Subsequently, the image pickup part (32) acquires a plurality of measurement single-band images representing a pattern on an objective substrate and corrects a value of the specified pixel for each of a plurality of measurement single-band images by using the value of the specified pixel and values of pixels surrounding the specified pixel and the correction factors.

Abstract: A thickness of a wafer during polishing operation is detected to accurately perform the polishing. A thickness measuring method, which measures the thickness of the wafer of wafer 7 in polishing a surface, comprises the steps of irradiating the thin film-like material during the surface polishing from a backside with probe light, measuring a reflectance spectrum with a dispersion type multi-channel spectroscope using a photodiode array which has particularly high sensitivity to light having a wavelength ranging from 1 to 2.4 ?m, and calculating the thickness on the basis of a wave form of the reflectance spectrum. The surface polishing is performed while the thickness of the wafer 7 is measured by the above-described thickness measuring method, and the polishing is finished when the thickness of the wafer 7 reaches a target thickness.

Abstract: A system for inspecting specimens such as semiconductor wafers is provided. The system provides scanning of dual-sided specimens using a diffraction grating that widens and passes nth order (n>0) wave fronts to the specimen surface and a reflective surface for each channel of the light beam. Two channels and two reflective surfaces are preferably employed, and the wavefronts are combined using a second diffraction grating and passed to a camera system having a desired aspect ratio. The system preferably comprises a damping arrangement which filters unwanted acoustic and seismic vibration, including an optics arrangement which scans a first portion of the specimen and a translation or rotation arrangement for translating or rotating the specimen to a position where the optics arrangement can scan the remaining portion(s) of the specimen. The system further includes means for stitching scans together, providing for smaller and less expensive optical elements.

Abstract: A coefficient of linear expansion measuring apparatus includes: two reflection plates between which a sample is put, a container to house them, which is filled with a gas having known rate of a refractive index variation, a temperature regulating member to set a temperature in the container variably, a light source to irradiate an irradiating light to reflecting surfaces of the reflection plates, a light receiving element to receive reflected lights in which the lights interferes each other and detecting a light intensity thereof, and a calculating member to calculate a coefficient of linear expansion of the sample, wherein: the calculating member calculates an optical path length variation between the reflecting surfaces from an output variation of the light receiving element, and calculates a length variation of the sample by correcting a part of the optical path length variation derived from the refractive index variation of the gas caused by the temperature variation.

Abstract: Apparatus for determining the thickness of a configuration having flat, parallel surfaces that are transparent, or nearly so, to pre-specified types of energy. Embodiments comprise a mechanism for illuminating a front surface with an energy source and mechanisms for measuring reflections of the illumination from a parallel back surface. The energy is contained in a spectrum of wavelengths, the energy being refracted in components at unique wavelengths, e.g., different colored light bands, and similarly reflected from the back surface. The measuring mechanisms, e.g., spectrometers, determine the relative lateral displacement between two spectral lines in the refracted and reflected beams to enable determination of thickness. Other characteristics of the material of the configuration may be ascertained, e.g., chemical composition is ascertained by measuring the intensity of responses at multiple wavelengths and comparing this to responses of known materials.

Abstract: A method including: imaging test light emerging from a test object over a range of angles to interfere with reference light on a detector, wherein the test and reference light are derived from a common source; for each of the angles, simultaneously varying an optical path length difference from the source to the detector between interfering portions of the test and reference light at a rate that depends on the angle at which the test light emerges from the test object; and determining an angle-dependence of an optical property of the test object based on the interference between the test and reference light as the optical path length difference is varied for each of the angles.

Abstract: Systems and methods of the present invention measure at least one reflecting surface of an object disposed along an optical path. In some embodiments a measured optical interference signal for each of at least three wavelengths of reflected light may be used to determine a modulation of frequency components of a Fourier series. Frequency components of a Fourier series may be transformed to spatial components that describe intensities and positions of light reflected along an optical path. Systems and methods of the present invention permit rapid measuring and may monitor corneal thickness during surgery. The invention may do so by integrating an ablation device and a measurement apparatus into a single system. An integrated scanning and monitoring system may include an ablative light source producing an ablative beam and a measurement light source producing a measurement beam.

Abstract: A method and apparatus for determining the thickness of slabs of materials using an interferometer.

Abstract: A method and device for measuring the wall thickness of a pipe in a pipe-rolling mill wherein a Fabry-Pérot interfero-meter has its mirror spacing set by providing an input to a linear activity for one of the mirrors from a controller receiving an input from a photodiode at the output side of the interferometer. A second control circuit regulates the amplification of that photodiode with at least one parameter derived from the rolling system, for example, the input optical signal to the interferometer or a disturbance value representing for example the temperature of the rolled product and obtained through an optical pyrometer. The incoming optical signal may be tapped to another photodiode also with a variable amplification amplifier and both amplifiers may be controlled by a second controller.

Abstract: A method and apparatus for measuring the optical thickness and absolute physical thickness of an optically transparent object utilizes a reflective interferometric process. A broadband optical signal is directed toward the object to be measured, and a pair of signals reflected off of the object are processed to determine the optical thickness of the object. When used with an optical fiber preform, the technique can be used to measure the outer diameter of the preform and control the drawing process. If the index of refraction of optically transparent object is known, the absolute physical thickness can also be determined.

Abstract: A characteristic of a surface is measured by illuminating the surface with optical radiation over a wide angle and receiving radiation reflected from the surface over an angle that depends on the extend of the illumination angle. An emissivity measurement is made for the surface, and, alternatively, if a reflectivity measurement is made, it becomes more accurate. One application is to measure the thickness of a layer or layers, either a layer made of transparent material or a metal layer. A one or multiple wavelength technique allow very precise measurements of layer thickness. Noise from ambient radiation is minimized by modulating the radiation source at a frequency where such noise is a minimum or non-existent. The measurements may be made during processing of the surface in order to allow precise control of processing semiconductor wafers, flat panel displays, or other articles. A principal application is in situ monitoring of film thickness reduction by chemical-mechanical-polishing (CMP).

Abstract: A phase measuring apparatus for measuring phase characteristics of a film applied onto an object to be measured includes a shearing interference system for providing incident light onto the object or light reflected on the object with shearing interference, a detector for detecting shearing interference information, and a computing unit for calculating the phase characteristics of the film based on the shearing interference information.

Abstract: The present invention provides a method of measuring the thickness of a thin film or thin layer by a spectroscopic measurement, which is applicable to the measurement of a multiple layered film whose layers have different refractive indices. According to the method, an interference light from the film is measured to create a measured spectrum. The waveform of the measured spectrum can be approximately represented by a linear sum of base spectrums. Accordingly, various constructed spectrums are created using base spectrums each having a cycle interval as a parameter. Then, the constructed spectrum that minimizes the square error against the measured spectrum is identified. The least square error is calculated for each of predetermined cycle intervals. A graph is drawn to represent the relation between the least square error and the cycle interval. The correspondence between the layers and the plural minimum points of the least square error appearing on the graph is determined.

Abstract: An apparatus capable of measuring topography and transparent film thickness of a patterned metal-dielectric layer on a substrate without contact with the layer. A broadband interferometer measures an absolute phase of reflection at a plurality of wavelengths from a plurality of locations within a field of view on the metal-dielectric patterned layer on the substrate, and produces reflection phase data. An analyzer receives the reflection phase data and regresses the transparent film thickness and the topography at each of the plurality of locations from the reflection phase data. In this manner, the apparatus is not confused by the phase changes produced in the reflected light by the transparent layers, because the thickness of the transparent layers are determined by using the reflection phase data from multiple wavelengths. Further, the surface topography of the layer, whether it be opaque or transparent is also determinable.

Abstract: A wavefront splitting element includes a diffraction grating for splitting light into multiple beams including transmitting light and reflected light.

Abstract: When a semiconductor wafer placed in a chamber and having films thereon is etched using plasma generated in the chamber, a change in the amount of lights with at least two wavelengths, obtained from the wafer surface during the processing, is detected. The time between the time at which the amount of a light with one of two wavelengths is maximized and the time at which the amount of a light with the other wavelength is minimized is compared with a predetermined value to determine the state of etching processing.

Abstract: A detection apparatus for detecting the process end point in the removal process of a layer on a wafer in an IC or other semiconductor device manufacturing process. This point can be detected in-situ and at high precision even when there is a pattern on the surface, or when there is no distinct change in the polishing layer, or when there is disturbance caused by a difference in the detection position or the slurry. Two or more characteristic quantities are extracted from a signal waveform obtained by irradiating a substrate surface with white light and detecting the reflected signal light or the transmitted signal light or both, fuzzy rules, etc., are used in performing detection by using these two or more characteristic quantities to perform a logical operation, and tuning is performed.

Abstract: A standard pattern of a differential value of an interference light is set with respect to a predetermined film thickness of a first member to be processed. The standard pattern uses a wavelength as a parameter. Then, an intensity of an interference light of a second member to be processed, composed just like the first member, is measured with respect to each of a plurality of wavelengths so as to obtain a real pattern of an differential value of the measured interference light intensity. The real pattern also uses a wavelength as a parameter. Then, the film thickness of the second member is obtained according to the standard pattern and the real pattern of the differential value.

Abstract: The invention relates to an apparatus based on the telecentric imaging system for forming an image of a linear zone (17) of an object (1). The apparatus comprises a non-telecentric camera (9) consisting of an objective (8) and an image plane (19) formed by a row of photosensitive cells (15) as well as telecentric imaging means (18) between the objective and the object. The telecentric means comprise a concave strip mirror (6), which is aligned with said row of cells and with the aperture of said objective located in its focal plane, as well as a strip-like plane mirror (5) between the parabolic mirror and the objective, whereby the radiation being reflected from the objective continues via the parabolic mirror and the planar mirror further to the objective and from there to the image plane. In the apparatus there is further a scattered light source (7) which lightens the object. The apparatus is adapted for measuring the dimensions of the width parts of the object.

Abstract: A system for determining at least one condition of a substrate includes an optical waveguide for transmitting light from a light source. The optical waveguide can be embedded in the substrate and operatively coupled to an interferometric system. The interferometric system is operatively coupled to a processor. The interferometric system provides the processor with information relating to transmissions through the wave guide, which are indicative of substrate conditions and/or operations being performed relative to the substrate.

Abstract: Disclosed is a thin-film inspection device with two or more light sources. This device can control the wavelength and intensity of light and illuminate the lights with different incident angles, and are used to control the interference intensity, thereby determining optimal inspection conditions to obtain a reliable inspection result even when different kinds of thin films coexist. The thin films are formed on a flat plate and have different indices of refraction and thicknesses. An incident light control unit is disposed between the illumination unit and the flat plate for controlling the light to be incident on the patterns. A sensor unit detects a reflection light from the patterns. A reflection light control unit is disposed between the flat plate and the sensor unit and controls the light to be detected by the sensor unit. A control unit controls the movement of the illumination unit and the sensor unit.

Abstract: A high-resolution and high-speed film thickness and thickness uniformity measurement method is disclosed in this invention. The disclosed method includes a step a) of measuring a film thickness at a single point on the top surface of the substrate using an interferometry with a measuring light beam having a range of wavelengths. The method further includes a step b) of selecting an optimal wavelength from the range of wavelengths applied for measuring the film thickness at the single point. The method further includes a step c) of measuring reflection intensities by scanning over a plurality of points with a measuring light beam of the optimal wavelength over the top surface of the substrate. The method further includes a step d) of calculating a film thickness at the plurality of points applying the optimal-wavelength reflection intensities at the plurality of points over the top surface of the substrate.

Abstract: At each measurement time, measurement light is supplied from a measurement light source 11, and interference light obtained when reflected light from a semiconductor wafer W and reference light from a reference light generating section 14 are coupled is detected by a photodetector 15. A thickness calculating section 16 obtains a light intensity distribution representing the correlation between the light intensity of the interference light and the reference optical path length, selects a wafer upper surface peak and wafer lower surface peak from a plurality of light intensity peaks in the light intensity distribution using a predetermined selection criterion, and calculates the thickness of the semiconductor wafer W from the optical path length difference between the light intensity peaks.

Abstract: A method and system using spectral interference of light from plasma emissions collected at near grazing incidence to in-situ monitor and control the film thickness of a non-opaque film. Embodiments of this invention are particularly useful to all substrate processing chambers equipped to form an in-situ plasma within the chamber and which are used to deposit or etch non-opaque films. One embodiment of the method of the present invention forms a plasma within a substrate processing chamber to deposit a non-opaque film on a wafer substrate within the chamber. During the plasma deposition process, a plurality of wavelengths of radiation including those reflected from the top and bottom layer of the film being deposited upon a wafer surface are collected through an existing viewport, and conveyed to a spectrometer for measurements via an optical fiber attached near this viewport. These measurements are analyzed to determine the film's thickness.

Abstract: A system and method are provided for obtaining mapping profiles of transparent objects having a plurality of reflective surfaces. The object, the surfaces of which are to be mapped, is placed in an unequal path interferometer including a reference surface located a predetermined distance from the object. Coherent light is supplied in the interferometer from a tunable source; and multiple optical interferograms for each of the plurality of reflective surfaces are simultaneously recorded. These interferograms are simultaneously extracted through the use of a dynamically generated weighted least-square fitting technique; which separates interferograms from a set of superimposed interferograms to obtain a given interferogram for any one of the surfaces, free from errors resulting from the existence of the other interferograms.

Abstract: An apparatus and a method are provided which allow two opposite plane surfaces of a body to be interferometrically measured simultaneously using light from a single light source. From a parallel light beam (P) produced by a light source (1) partial light beams (A, B) having positive and negative diffraction angles are produced using a beam splitter (8) in the form of a diffraction grating. The partial light beams strike the respective surfaces (90, 91) of the body (9) to be measured and are reflected thereat. The reflected partial light beams (A, B) are interfered with the throughgoing partial light beam (P) having an order of diffraction of zero and the thus produced interference patterns are digitized and subtracted from each other, whereby the parallelism of both surfaces (90, 91) of the body can be determined.

Abstract: An apparatus and method of chemical mechanical polishing (CMP) of a wafer employing a device for determining, in-situ, during the CMP process, an endpoint where the process is to be terminated. This device includes a laser interferometer capable of generating a laser beam directed towards the wafer and detecting light reflected from the wafer, and a window disposed adjacent to a hole formed through a platen. The window provides a pathway for the laser beam during at least part of the time the wafer overlies the window.

Abstract: A method for determining a film thickness in a semiconductor substrate is provided. The method initiates with providing multiple layers on the semiconductor substrate. Then, two reflectance spectra are generated where each of the two reflectance spectra are associated with different time periods while an upper layer is being removed. Next, a difference between the two reflectance spectra is calculated. Then, a curve is defined from the difference between the two reflectance spectra. Next, the defined curve is fitted by a known parametric function to determine the film thickness. An endpoint detector and a CMP system are also provided.

Abstract: The present invention consists of a technique and device for measuring the thickness variation and shape of wafers or other polished opaque plates. A combination of two improved phase-shifting Fizeau interferometers is used to simultaneously measure the single-sided distance maps between each side of the wafer and the corresponding reference flat, with the thickness variation and shape being calculated from these data. Provisions are made to determine and eliminate the shape and tilt of the reference surfaces, and also to facilitate the correct overlay of the two single-sided measurements for the calculation of thickness variation and shape.

Abstract: Longitudinal and shear ultrasonic waves are generated inside a material by irradiating a laser beam onto a first surface, e.g., incident surface, of the material. An ultrasonic longitudinal wave and a mode converted wave reflected by a second surface, e.g., a bottom surface, of the material are detected, and times of flight of the ultrasonic longitudinal wave and the mode converted wave are measured. A thickness of the material is measured based on the times of flight and a correlation, obtained in advance, between longitudinal and shear wave velocities of the material and temperature of the material.

Abstract: The invention discloses a substrate holder (8) that is configured to receive a substrate (20) and can be utilized to determine the thickness deviation of a substrate from the standard thickness of a specific substrate type. The substrate holder (8) comprises a one-piece frame having a flat upper surface (42). An opening (30) that defines a peripheral rim (32) is provided in the substrate holder (8). Receiving elements (34) on which spheres are provided are shaped onto the peripheral rim (32) of the opening (30). A substrate (20) placed into the substrate holder (8) thus comes to rest on the upper surfaces of the spheres. The support elements (34) are arranged on the peripheral rim of the opening (30) in such a way that they lie at the vertices of an equilateral triangle.

Abstract: An invention for detecting an endpoint during a chemical mechanical polishing (CMP) process is provided. A reflected spectrum data sample is received that corresponds to a plurality of spectrums of light reflected from an illuminated portion of the surface of a wafer. The reflected spectrum data sample is normalized using a normalization reference comprising a first reflected spectrum data sample obtained earlier during the CMP process. In addition, the normalization reference is updated during the process using a second reflected spectrum data sample obtained earlier during the CMP process. The second reflected spectrum data sample is obtained after the first reflected spectrum data sample. In this manner, an endpoint is determined based on optical interference occurring in the reflected spectrum data.

Abstract: This invention aims to measure film thickness and film thickness distribution to high precision in a wide range of transparent films. As one example, in a CMP process, the film thickness of an outermost surface layer formed on a step pattern of an actual product can be measured so that high precision film thickness control can be performed. To achieve an increase of processing throughput, the film thickness of an optically transparent film formed on an actual device pattern is controlled to high precision by incorporating a film thickness measuring unit, which performs frequency analysis of a spectral distribution, in a polishing apparatus. As a result, an increase of processing throughput is realized.

Abstract: A device is provided for spatially resolved measurement of the thickness of a layer located on a sample carrier (7), said device comprising a light source (1-3) emitting polychromatic radiation with a predetermined spectral composition, illumination optics (4-6) illuminating the sample carrier (7) with radiation from the light source (1-3), detector optics (6, 5, 8) picking up radiation reflected by a line-shaped portion of the sample carrier (7) and guiding said radiation to a polychromator (9, 11) as a line-shaped beam, said polychromator (9, 11) separating the line-shaped beam into a field-shaped spectrum, and a camera (12), which receives the field-shaped spectrum, the polychromator (9, 11) being tuned to the spectral composition of the radiation from the light source.

Abstract: The present invention is directed to a fiber optic media thickness sensor used in a print media or document processing device. The invention is further directed to a method for measuring media thickness in a media processing device using a fiber optic sensor.

Abstract: At each measurement time, reflected light from a semiconductor wafer W or the like by measurement light from a measurement light source 11 is coupled to reference light from a reference light generating section 14, and interference light is detected by a photodetector 15. A raw thickness value calculating section 16b selects two light intensity peaks corresponding to the upper and lower surfaces of the wafer W from a light intensity distribution between an interference light intensity and a reference optical path length and calculates a raw thickness value. A statistical thickness value calculating section 16c executes statistical processing including data sorting, determination whether the raw thickness value falls within an allowable numerical value range, and determination of a thickness change line, thereby obtaining a statistical thickness value.