WO2002082008A1  A method and apparatus of two wavelength interferometry for measuring accurate height of small step composed of two different materials  Google Patents
A method and apparatus of two wavelength interferometry for measuring accurate height of small step composed of two different materialsInfo
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 WO2002082008A1 WO2002082008A1 PCT/KR2002/000609 KR0200609W WO2002082008A1 WO 2002082008 A1 WO2002082008 A1 WO 2002082008A1 KR 0200609 W KR0200609 W KR 0200609W WO 2002082008 A1 WO2002082008 A1 WO 2002082008A1
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 G—PHYSICS
 G01—MEASURING; TESTING
 G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
 G01B11/00—Measuring arrangements characterised by the use of optical means
 G01B11/02—Measuring arrangements characterised by the use of optical means for measuring length, width or thickness
 G01B11/06—Measuring arrangements characterised by the use of optical means for measuring length, width or thickness for measuring thickness, e.g. of sheet material
 G01B11/0608—Height gauges

 G—PHYSICS
 G01—MEASURING; TESTING
 G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
 G01B9/00—Instruments as specified in the subgroups and characterised by the use of optical measuring means
 G01B9/02—Interferometers for determining dimensional properties of, or relations between, measurement objects
 G01B9/02001—Interferometers for determining dimensional properties of, or relations between, measurement objects characterised by manipulating or generating specific radiation properties
 G01B9/02007—Two or more frequencies or sources used for interferometric measurement

 G—PHYSICS
 G01—MEASURING; TESTING
 G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
 G01B9/00—Instruments as specified in the subgroups and characterised by the use of optical measuring means
 G01B9/02—Interferometers for determining dimensional properties of, or relations between, measurement objects
 G01B9/02055—Interferometers for determining dimensional properties of, or relations between, measurement objects characterised by error reduction techniques
 G01B9/02056—Passive error reduction, i.e. not varying during measurement, e.g. by constructional details of optics
 G01B9/02057—Passive error reduction, i.e. not varying during measurement, e.g. by constructional details of optics by using common path configuration, i.e. reference and object path almost entirely overlapping

 G—PHYSICS
 G01—MEASURING; TESTING
 G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
 G01B9/00—Instruments as specified in the subgroups and characterised by the use of optical measuring means
 G01B9/02—Interferometers for determining dimensional properties of, or relations between, measurement objects
 G01B9/02083—Interferometers for determining dimensional properties of, or relations between, measurement objects characterised by particular signal processing and presentation
 G01B9/02084—Processing in the Fourier or frequency domain when not imaged in the frequency domain

 G—PHYSICS
 G01—MEASURING; TESTING
 G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
 G01B9/00—Instruments as specified in the subgroups and characterised by the use of optical measuring means
 G01B9/02—Interferometers for determining dimensional properties of, or relations between, measurement objects
 G01B9/0209—Nontomographic low coherence interferometers, e.g. low coherence interferometry, scanning white light interferometry, optical frequency domain interferometry or reflectometry
Abstract
Description
A method and apparatus of two wavelength interferometry for measuring accurate height of small step composed of two different materials.
Technical Field
The present invention relates to a measurement method capable of compensating an error generated when the height of a step composed of different materials is measured using twowavelength whitelight interferometry. Specifically, the invention relates to a measurement method capable of mathematically modeling the effect of the difference in phase change caused by different metals on the measuring error and interpreting the mathematical expression, thereby compensating the measuring error with only onetime measurement. Background Art
The step height of a material is measured using a monochrome light (single wavelength) scanning interferometer or a white light (multi wavelength) scanning interferometer. In general, phase change occurs when illuminating light is inputted into a material and then reflected. In case that a step is composed of the same kind of materials, the measuring error caused by phase change is not generated because phase changes by the two materials are identical to each other. When the step is composed of two different materials, however, phase changes by the materials are different from each other so that the height of the step composed of the two different materials cannot be accurately measured. In case where the height of a step composed of different materials is measured using the monochrome light scanning interferometer, the difference in phase change of the materials for the frequency of illuminating light use was confirmed in advance and the confirmed phase change difference was reflected on the measured result. In this case, however, compensation can be carried out only when the frequency of the illuminating light used and phase change of each material are known and, if the material is changed or the frequency of the illuminating light varies, the phase change occurs differently to make the compensation difficult. In addition, accurate phase change rate cannot be calculated when white light is used as the illuminating light. In this case, the phase change rate is predicted on an average to compensate so that accurate compensation is difficult to perform. Thus, the white light is not used as the illuminating light in most cases.
On the other hand, in case of a step composed of the same kind of materials, the white light is often used because unnecessary stray diffraction is not generated due to short coherent range (range where interference occurs) thereof. The basic principle applied to this is that the position of a measurement surface or a reference mirror is accurately moved in the direction of an optical axis to obtain a transfer distance to a position at which the intensity of interferogram is the highest and the obtained transfer distance is converted into the height of the measurement surface. Compared to monochrome laser light, the coherent range of the white light is limited to several micrometers so that the position of the envelope peak is clear. Thus, problems with respect to 2π ambiguity are not generated when the absolute phase of the measurement surface is calculated. In addition, superior interferogram from which stray diffraction was removed can be acquired because unnecessary diffraction is not generated from an optical system. The whitelight scanning interferometer is being widely studied in response to the extension of industrial demand requiring precise examination of a surface and the recent rapid improvement of computation capability of a microcomputer.
As described above, while the whitelight interferometer is widely used for measuring the height of a step composed of the same kind of metal materials owing to its advantage of short coherent range, it is not suitable for different kinds of metal materials because of the difference in phase change in the different metals. That is, when the height of a step composed of different metals is measured using the whitelight interferometer, the measuring error of 1040nm is brought about due to the difference in phase change generated when the white light is reflected from the different metals. The error is compensated using monochrome light in most cases, which requires a difficult correction process. Furthermore, in case where the height of the step composed of different metals is measured using the whitelight scanning interferometer, the phase change difference is generated in all wavelengths due to the different metals so that analysis becomes very complicated. Accordingly, measurement methods using illuminating light having a wide wavelength band such as the white light are barely applied to the measurement of the height of a step composed of different materials. Disclosure of Invention
An object of the present invention is to provide an algorithm for overcoming the difference in phase change generated in different materials while making effective use of the advantage of white light in the measurement of the height of a step composed of the different materials using the whitelight scanning interferometer. Another object of the present invention is to provide a measurement method capable of realizing the algorithm and a measurement system to which the measurement method is applied.
To accomplish the objects of the present invention, the invention analyzes phase change that occurs when the height of a step composed of different materials is measured with the whitelight scanning interferometer, develops mathematical modeling for overcoming the phase change, and proposes a measurement method and system for realizing the modeled algorithm, thereby realizing a twowavelength whitelight interferometer capable of compensating a step composed of the different materials. Brief Description of the Drawings FIG. 1 illustrates the fringe peak and envelope peak of a whitelight interferogram;
FIG. 2 illustrates the height h of a step composed of a metal A and a metal B in monochrome light interferometry;
FIG. 3 illustrates phase changes of lights reflected from metal surfaces according to wavelengths;
1 ^{d} FIG. 4 A illustrates analysis of the error caused by the envelope peak, z , (= )
2 dk (phase change error);
1 d_{φ} FIG. 4B illustrates analysis of the error caused by the envelope peak, z_{φ}(= )
2 dk
(result of calculation of phase change error);
FIG. 5A illustrates the spectrum of the white light interferogram using Fourier transform (whitelight interferogram); FIG. 5B illustrates the spectrum of the whitelight interferogram using Fourier transform (result of Fourier transform);
FIG. 6 illustrates a configuration of a twowavelength whitelight interferometer; FIG. 7A illustrates analysis of a twowavelength whitelight interferogram (two wavelength whitelight interferogram : I(z)); FIG. 7B illustrates analysis of the twowavelength whitelight interferogram
(frequency conversion of the two wavelength whitelight interferogram : J(k)=FFT[I(z)]); and
FIG. 8 illustrates results of compensation of the 94nm VLSI standard step sample using the two wavelength whitelight interferometer (h=95.3nm). Best mode for Carrying Out the Invention
The twowavelength whitelight interferometer can be realized through various interference optical systems including Micheolson, Mirau, Linnik and so on. A method of measuring the height of a step composed of different metals through the twowavelength whitelight interferometer to which Micheolson interference optical system is applied is explained below. Here, it is assumed that the numerical aperture (NA) value of the optical system is small, and parameters are defined as follows. Z_{0} : Actual position of an object to be measured Z_{m} : Peak of a whitelight interferogram Z_{env} : Envelope peak of the whitelight interferogram (Z_{em} = Z_{m})
Z_{fπn}ge : Fringe peak of the whitelight interferogram (z_{fl}  z_{m}  φ_{m} 12k_{0})
1 d_{φ} Zφ : EΠΌΓ of the envelope peak caused by phase change of a metal ( z_{ό} (= ) )
2 dk φ_{m} : Phase value appearing in the whitelight interferogram h : True value of the height of a step composed of metals
H : Step height value measured using the fringe peak of the whitelight interferogram hj: Step height value measured using the monochrome light interferometry at frequency ki (wavelength λi) h_{2}: Step height value measured using the monochrome light interferometry at frequency k_{2} (wavelength λ_) Δh : h_{2} hι k_{0}: Central frequency of white light (ko = 2π/ λ_) i: Frequency of light for measuring the step height hi (ki = 2π/ λj) k : Frequency of light for measuring the step height h_{2} (k_{2} = 2π/ λ_)
When it is assumed that the numerical aperture (NA) value is as small as it can be ignored and the height of the object to be measured is z_{0}, a variation in the optical intensity of interferogram with respect to a scan distance z is represented by the following equation. (Reference : "The Mirau correlation microscope" by G. Kino and S. Chim,
App. Opt, 29(26), 37753783 (1990))
I(∑) = + _{r}(k) cos(2k(z  z_{0}) + φ(k))]F(k)dk (Equation 1) In Equation 1, r(k) is reflectivity, φ(k) is phase change generated when the metal reflects light, F(k) is the spectrum of light, k_{0} is the central frequency of the light (k_{0} = 2π I λ_{0} , λ_{0} is the central wavelength of the light) and Ak is the frequency band of whitelight used. The phase change φ(k) is induced by Fresnel equation. When it is assumed that the light is inputted into the object to be measured perpendicularly, the reflectivity is as follows.
r = — (Equation 2)
In Equation 2, n_{l} , n_{l} are refractive indexes of an incident material and a reflecting material, respectively. In general, the incident material is air whose reflective index n, is 1. In case that the reflecting material is a metal, the metal has energy loss according to the photoelectric effect caused by reflection of light so that the refractive index of the metal is represented by n_{t} = n  ik that is a complex number. Due to this refractive index of the complex number of the metal, the phase change φ is determined as follows. 7 tan φ  —  (Equation 3) n^{~} + k^{~}  1
If Equation 1 is integrated in consideration of the phase change of the Equation 3, general whitelight interferogram equation as described below is acquired.
I(z) = g(z  z,„ ) cos(2£_{0} (z  z_{m} ) + φ_{m} ) (Equation 4)
In Equation 4, a background light component I_{0} is omitted for the simplification of the equation, and g(z  z_{m} ) is the envelope function and φ_{m} is the average value of phase changes for the whitelight wavelength band. The whitelight interferogram reproduced on the basis of Equation 4 is represented as shown in FIG. 1. Accordingly, if all of peaks of the interferogram generated in the overall measurement range are detected, the threedimensional shape of the object to be measured can be restored. As shown in
FIG. 1, the whitelight interferogram includes the envelope peak that is the highest point of the envelope function and the fringe peak that is the maximum value of the
interferogram itself. The envelope peak z_{eιn}, and the fringe peak z_{frmge} are represented as z_{em} = z_{m} and z_{frin} = z_{m}  φ_{m} /2k_{0} , respectively. In case where the fringe peaks are
applied to a conventional measurement method according to an optical phase interferometry to measure the height h of a step composed of different metals A and B, as
shown in FIG, the measuring error of (φ^{B}  φ^{A})/2k_{0} is generated. The error of the fringe
peaks is corrected by grasping characteristics of the object to be measured and then correcting phase change using data of an optical handbook or by vising a conventional experimental method ("Effects of phase changes on reflection and their wavelength dependence in optical profilometry" by T. Doi and K. Toyoda, App. Opt, 36, 7157 (1997)). These correction methods require excessively large quantity of calculations. In addition, it is difficult to apply the methods to the actual object to be measured. Meanwhile, studies on the error of the envelope peak caused by a step height of different metals have been made only when monochrome light is used as the illuminating light and the optical system has a high numerical aperture value. Thus, there have been hardly carried out studies on the case of employing the white light as the illuminating light because of complicate characteristic that phase change depends on wavelength.
It is known that the envelope peak largely depends on the spectrum of light, reflectivity and phase change of the object to be measured and the numerical aperture value of the optical system. To investigate effects of phase change on the envelope peak of the whitelight interferogram, the interference term φ(k) in Equation 1 where the numerical aperture of the optical system is assumed to be very small is defined as follows.
Φ(k) = φ(k) + 2k(z  z_{0}) (Equation 5) φ(k) represents the phase change of the object to be measured according to wavelengths. If φ(k) is a constant, the envelope peak z_{em}, becomes identical to the position of the object, z_{0}. However, the phase change φ(k) is intensive according to
wavelengths, and variations of representative metals with respect to k are shown in FIG. 3. (Reference : Edward D. Palik, Handbook of Optical Constants of Solids Nol I, Academic Press, (1985)). Referring to FIG. 3, it can be confirmed that the phase change φ(k) is lineai without having a sharp change in the visual ray range. From this characteristic, the phase change φ(k) can be assumed as follows.
φ(k) ~ φ(k_{0}) + (k  k_{0}) — (Equation 6) dk
When Equation 6 is introduced into Equation 5, the following result is obtained.
φ(k) s φ(k_{0} )  k_{0} A + 2k(z  (z_{Q}  1^)) (Equation 7) dk 2 dk
If it is assumed that the interference term Φ(k) , the spectrum distribution of the light, F(k), and reflectivity γ(k) induced to Equation 7 are not sharply changed, the whitelight interferogram represented by Equation 1 is generalized into Equation 4. However, as confirmed in the interference term Φ(k) of the following Equation 8, the position of the object, z_{0} , is moved by the slope component of phase change with respect to k, 0.5dφ/dk . From this, the envelope peak z_{m} has the value corresponding to the
movement from the position of the object, z_{Q} , to the phase change rate z_{φ} as
represented in Equation 8.
^{z}m = ^{z}o  JT ^{= z}° ^{~ Z}Φ (Equation 8)
2 dk
Consequently, the movement value z , acts as an error of the envelope peak. Due
to this, the error of (d_{φ} ^{A} I dk  d_{φ} ^{B} I dk)/2 is generated when the height h of the step
composed of the different metals A and B is measured using the envelope peak. The present invention proposes a self compensation method for compensating the aforementioned error of the envelope peak caused by the phase change rate and shows results obtained by applying this compensation method to actual measurements below.
The measuring error of the envelope peak, (d_{φ} ldk ~ d_{φ} /dk)/2 , generated when the step height h is measured, has the physical meaning as shown in FIG. 4A. If the phase changes of the metals A and B forming the step with respect to k is assumed as shown in FIG. 4A, it can be confirmed that the measuring error generated in this case is caused by a difference between the slopes of phase change of the two metals. Accordingly, if the phase change slope difference is mathematically represented and analyzed to correct the error caused by the phase change, the height of the step composed of the different metals can be accurately measured.
The measurement and correction of the phase change difference start with setting the minimum frequency /c, and the maximum frequency k_{2} from the frequency band of the white light. Here, k_{x} < k_{2} . The frequency k defined in the present invention is a wavenumber and k = 2π I λ . The first step of error correction is explained below.
When the height of a step composed of metals is measured using the two frequencies k and k_{2} through the monochrome light interferometry, the step height A, measured by the frequency k and the step height h_{2} measured by the frequency k_{2} are represented by the actual step height h and the following Equation 9.
_{h = h} f^{j} _?!_ h_ = h ^  (Equation 9)
In Equation 9, the upper added letters A and B represent phase changes generated in the metals A and B, respectively, and the lower added numerals 1 and 2 mean the measured results in the spectrum frequencies k_{λ} and k_{2} of the light, respectively. Since
the phase changes φ_{x} , φ_{λ} , φ_{2} and φ_{2} are unknown values, the values z, and h_{2}
measured according to the monochrome light interferometry have results different from the actual step height h .
In the second step of the error correction, the monochrome light filter is removed and the step height is measured using the envelope peak of the whitelight scanning interferometry. Here, the measured result is represented as H. For analysis of the measured
result H, the error value z_{φ} of the envelope peak, defined by Equation 8, is simplified as
follows.
z . ≡ 1 dφ ≤ — I — φ_{2} — φ —_{x} ( _{r}E_quat .i.on , 10)
* 2 dk 2 k_{2}  k_{x} When the phase change error z_{φ} assumed by Equation 10 is employed, the
measured step height H of the two materials A and B using the envelope peak z_{m} is represented as follows.
H _{(Equation π)}
The point of the phase change correction method is to compensate the error term of the envelope peak represented in Equation 11 using the step height values h_{x} and h_{2} obtained by the monochrome light interferometry. For this, the difference between the step heights in Equation 9 is represented by the following Equation 12 by introducing Aφ .
Ah = h_{2}  h_{x} ≡ [(φ_{2}  φ )  (φ_{2}  φ_{x} )] (Equation 12) k_{0} where k_{Q} is the central frequency of the light. In Equation 12, k and k are replaced by k_ for simplification of the equation.
It is known that the error caused by the replacement does not affect the measurement and explanation about this is omitted. When Equation 12 is introduced into Equation 11, the following step height calculation equation is obtained. _{h =} nΛhΑzl l (Equation 13)
2 k_{2}  k_{x} From Equation 13, it can be confirmed that the step height h of the different metals is accurately measured using the results h_{x} and h_ measured by the monochrome light interferometry and the measured result H obtained from the envelope peak of the white light scanning interferometer. As shown in FIG. 4B, — (d_{φ} ldk  d_{φ} I dk) represented
as the phase change difference can be easily obtained using the errors of the step height,
(φ_{x}  _{x} )lk and (φ_{2}  _{2} )lk_{2} , measured at the two frequencies k_{x} and k_{2} .
However, the correction method using the monochrome light interferometry, described above, requires threetime measurements so that the measurement operations become complicated. Thus, an external environmental variation occurring during the measurements may become a measuring error.
As shown in FIG. 1, the fringe peak among the peaks of the whitelight interferogram is decided by the phase of the interferogram, φ_{m} , which indicates the maximum intensity position of the interferogram. If the frequency band Ak of the white light used becomes narrow, the correlation distance of the interferogram is increased in the space so that the interference range becomes wide. In this case, the fringe peak of the whitelight interferogram becomes identical to the fringe peak of the monochrome light interferometer. Accordingly, the fringe peak of the whitelight interferogram can be analyzed as the average position of fringe peaks of all wavelengths within the frequency band Ak , and the phase change φ(k) with respect to the frequency of the light in Equation 1 means integration for the section of the frequency band Ak .
As shown in FIG. 5, if the measured whitelight interferogram is dispersed using Fourier transform, the phases φ(k_{x} ) and φ(k_{2}) can be calculated at the specific frequencies k_{x} and k_{2} . Also, it can be analogized that and the heights h_{x} and h_{2} required for Equation 13 are substituted by the calculated phase values. Consequently, when a single whitelight interferogram is dispersed, two fringe peaks and one envelope peak can be calculated simultaneously and the accurate metal stepheight h can be measured using Equation 13. When a general whitelight interferogram is dispersed, however, the interferogram is distributed all over the visual ray range, as shown in FIG. 5B, so that calculation of the phase of a specific wavelength component is easily affected by external disturbance. To overcome this shortcoming, twowavelength whitelight interferometry using two lights are applied, as shown in FIG. 6.
An apparatus using the two white lights having different central wavelengths, shown in FIG. 6, is roughly described. Lights emitted from the two white light sources 100 and 110 having central wavelengths λ_{x} and λ_{2} , respectively are inputted into an optical combiner 120. The combined light is inputted into an optical divider 160 through a parallel beam lens unit 130. The combined light inputted into the optical divider 160 is incident on an object 190 including a step composed of two different metals through an object lens unit 140. The incident combined light is reflected from the object to be transmitted to an interferogram acquisition unit 180 through the object lens unit 140, the optical divider 160 and an image lens unit 170, thereby obtaining the whitelight interferogram.
The two illuminating lights used for the experiment using the apparatus have their central wavelengths, λ_{x} = 650nm and λ_{2} = 550nm and the bandwidth Aλ  70nm . The illuminating lights are used in order to solve the problem of the conventional white light interferometry that the whitelight interferometry is difficult to divide a specific wavelength component and vulnerable to external disturbance because it includes all of the visual ray range. The whitelight interferogram generated from the illuminating lights is as shown in FIG. 7A. FIG. 7B shows the spectrum obtained by Fouriertransforming the whitelight interferogram. In the spectrum of the interferogram shown in FIG. 7B, peaks appear at the central wavelengths of the two illuminating lights, λ = 650nm and λ = 550nm , as expected. It can be confirmed that the illuminating lights are not affected by external disturbance because the lights are concentrated on the specific wavelengths λ_{x} and λ_ .
As shown in FIG. 7A_{5} the algorithm for detecting peaks on the space is not suitable for detecting the envelope peak of the twowavelength whitelight interferogram. (Reference with respect to the algorithm is invited to the article entitled "Wavelet transform as a processing tool in whitelight interferometry" by P. Sandoz appearing in Otp. Lett, 22, 1065 (1997), which is not explained in detail in the present invention.) However, frequency domain analysis that is an algorithm proposed by Groot is suitable for detecting the envelope peak from the twowavelength whitelight interferogram because it Fouriertransforms the obtained interferogram to use the phase at each frequency. In the present invention, detailed explanation for the frequency domain analysis is omitted and reference is invited to the article entitled "Threedimensional imaging by subNyquist sampling of whitelight interfere grams" in Opt. Lett, 18, 1462 (1993).
When the obtained whitelight interferogram I(z) is Fouriertransformed into J(k)=FFT[I(z)], the phase at the frequency domain has the following relationship. ZJ(k) = φ(k)  2(k  k_{0} )z_{m} (Equation 14)
From Equation 14, it can be known that the envelope peak z_{m} is identical to the slope value with respect to the frequency k and phase changes φ_{x} and φ_{2} can be calculated at frequencies k and k_{2} arbitrarily set.
In application of the abovedescribed details on the measuring system, the phases φ (k_{x} ) and φ (k_{2}) of the two frequencies k_{x} ( 2π I λ_{x} ) and k_{2} (= 2π/λ_{2}) are first calculated in the spectrum of FIG. 7B to extract two fringe peaks, and then slope values are calculated from all phases in the domains the two frequencies include to extract the envelope peak. Accordingly, the frequency domain analysis is applied to the interferogram obtained from the twowavelength whitelight interferometer, as described above, to calculate all of h_{x} , h_{2} , H and h with only onetime measurement.
Table 1 represents measurement results of a step height of metals using the two wavelength whitelight interferometry. Samples used for measurements include a 94.0nm standard step sample and a step sample composed of chrome and gold coated on a glass, which are fabricated in VLSI Co., an expert at making metal step samples, and guaranteed by NIST. The step heights of chrome and gold were measured by a contact measurement instrument and they were confirmed to have 76.0nm and 67.0nm, respectively. The step heights H , h_{x} and h_{2} measured using the envelope peak and two fringe peaks obtained from the twowavelength interferogram are represented in Table 1 , and the measured step height h selfcorrected is calculated from H , h and h_{2} as 95.3nm, 71.9nm and 60.4nm. The measuring errors are 1.3nm, 4.1nm and 6.4nm, respectively. Especially, the step height of the 94.0nm standard sample composed of two different metals has the small error value of 1.3nm. The measurement result of the standard sample is shown in FIG. 8. From the aforementioned experimental results, the measuring error of tens of nanometers, generated when the height of the step composed of different metals is measured by the conventional optical phase interferometry, can be reduced to several nanometers by using the twowavelength whitelight interferometry proposed by the present invention.
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