JPH0296640A - Method for measuring dispersion of refractive index and thickness of film - Google Patents

Abstract

PURPOSE:To calculate the dispersion of the refractive index of a membrane and the thickness of said membrane by a spectral method by selecting the estimated dispersion of a refractive index smooth in wavelength dependence among the estimated dispersions of the refractive indice of a plurality of respective membrane thickness estimated values. CONSTITUTION:A sample 2 to be measured wherein a membrane 2a is formed on a substrate 2b is used and the reflection spectrum of the sample 1 is measured by a microscopic spectral device 1 to set a plurality of membrane thickness estimated values with respect to the membrane 2a and a plurality of the estimated dispersions of the refractive indice corresponding to the respective estimated values are subsequently calculated. Then, on the basis of a predetermined standard showing the smoothness of the wavelength dependence of the estimated dispersions of refractive indice, the estimated dispersion of the refractive index of the membrane 2a is selected from a plurality of the estimated dispersions of the refractive indice and the membrane thickness estimated value corresponding to said selected dispersion is set to the estimated value of the thickness of the membrane 2a. When the membrane thickness estimated value corresponding to the estimated dispersion of the refractive index is different from a real membrane thickness value, the wavelength dependence thereof does not become smooth and, therefore, a measured value can be selected on the basis of smoothness.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a method for measuring so-called refractive index dispersion and film thickness of a thin film, and particularly to a method capable of simultaneously measuring film thickness and refractive index dispersion.

(Prior art) Film thickness measurement methods using spectroscopy are widely used to optically measure thin films such as silicon oxide films formed on silicon substrates in inspection steps in semiconductor manufacturing processes. It's been done.

In this gm thickness measurement method, visible white light is first irradiated onto the sample surface, and the spectral reflection spectrum of the interference light of the reflected light from the surface of the thin film and the reflected light from the interface between the substrate surface and the thin film is detected. Since the value of (refractive index)×(film thickness) is calculated from this spectral reflection spectrum, the film thickness can be determined using the known refractive index of the thin film.

(Problem to be solved by the invention) L/)' L., aj of several μnth 52'1;
However, there is a problem in that the refractive index depends on the method of forming the thin film, etc.9, and the refractive index is adjusted to a value different from the inherent refractive index of the thin film material. Therefore, there was a problem in that the thickness of the thin film could not be measured using spectroscopy alone.

Further, there are cases where it is desired to measure the wavelength dependence of the refractive index, that is, the refractive index dispersion itself as an optical constant. but,
Since spectroscopy basically only provides the value of the product of refractive index and film thickness, it has been impossible to determine the refractive index dispersion of a thin film whose film thickness and refractive index dispersion are unknown.

(Purpose of the Invention) The present invention is intended to solve the above-mentioned problems in the prior art, and is to obtain the refractive index dispersion and film thickness of a thin film by spectroscopy for a thin film whose refractive index dispersion and film thickness are unknown. An object of the present invention is to provide a method for measuring refractive index dispersion and a method for measuring film thickness.

Means for Solving Problem C1) In order to solve the above-mentioned problem, in the first configuration of the present invention, the refractive index dispersion of the back film is measured using a sample to be measured in which a front film is formed on a substrate. In the method for measuring refractive index dispersion, (a) measuring the spectral reflection spectrum of the sample to be measured, (b) setting a plurality of film thickness estimates for the surface film, (c) the spectral reflection spectrum A plurality of estimated refractive index dispersions of the surface film corresponding to each of the plurality of film thickness estimation values are calculated based on (
d) Based on a predetermined criterion indicating the smoothness of the wavelength dependence of the estimated refractive index dispersion, one of the plurality of estimated refractive index dispersions is selected, and this is used as the refractive index dispersion of the surface film. Measured value.

Further, in a second configuration of the present invention, in the film thickness measuring method of measuring the III thickness of the surface film using a test sample having a surface film formed on a substrate, (a) spectroscopy of the test sample measuring a reflection spectrum; (b) setting a plurality of film thickness estimates for the surface film; (c) based on the spectral reflection spectrum, the surface corresponding to each of the film thickness estimation values of the number of layers; Multiple estimated bending W1 of the membrane! (d) selecting one of the plurality of estimated refractive index dispersions according to a predetermined criterion indicating smoothness of the wavelength dependence of the estimated refractive index dispersion, and (e) selecting one of the plurality of estimated refractive index dispersions; The estimated film thickness value corresponding to the estimated refractive index dispersion is taken as a measured value of the film thickness of the surface film.

Note that in step (d) of the first or second configuration, ? ! After obtaining a fitting function that creates a predetermined functional form h for the estimated refractive index dispersion, a statistical amount indicating the degree of mismatch between the fitting function and the corresponding estimated refractive index dispersion is obtained, and Control! One estimated refractive index dispersion may be selected from among the plurality of estimated refractive index dispersions using the smallest i'tffi as a predetermined criterion.

In this specification, "refractive index dispersion" is a term meaning the wavelength dependence of the refractive index, and has the same meaning as the characteristic simply called "dispersion."

(effect)? ! A plurality of estimated refractive index dispersions obtained corresponding to each of the film thickness estimation values are calculated so that the corresponding film thickness estimation value is the true yA.
When the film thickness values are different, the wavelength dependence is not smooth.

On the other hand, since true refractive index dispersion is a physical property of the surface film, it should show a smooth change with respect to wavelength.

Therefore, if one estimated refractive index dispersion is selected according to a predetermined criterion indicating the smoothness of the wavelength dependence of the estimated refractive index dispersion, it can be used as the measured value of the refractive index dispersion.

Further, the estimated film thickness value corresponding to the selected estimated refractive index dispersion should be extremely close to the true film thickness value, and can be used as the measured value of the film thickness.

When selecting the estimated refractive index dispersion, various criteria can be considered to judge its smoothness.
The estimated refractive index dispersion is fitted with a fitting function having a predetermined functional form, the statistical amount (standard lI difference, error, etc.) indicating the discrepancy between this fitting function and the estimated refractive index dispersion is determined, and the smallness of this concavity is calculated. can be used as a standard.

(Embodiment) FIG. 1 is a schematic configuration diagram of an apparatus to which an embodiment of the present invention is applied. In the figure, a microspectroscopy device 1 is connected to a sample to be measured 2.
This is a device that determines film thickness and refractive index dispersion by irradiating white light onto the surface of the film and dispersing the reflected light. The sample 2 to be measured has a 'ag12a such as a silicon oxide film formed on a substrate 2b such as a silicon substrate. The surface of this substrate 2b is required to be a mirror surface within the measurement range of the film thickness of the thin IA 2a.

During measurement, incident light emitted from a white light source 10 such as a halogen lamp is focused by a condensing element 12, reflected by a half mirror 16, and transmitted through an objective lens 18 to the upper surface of the object to be measured FI2. is irradiated. The reflected light LR reflected by the sample to be measured 2 is transmitted through the objective lens 18. The light enters a spectrometer 32 via a half mirror 16 and a pinhole mirror 20. Further, a part of the reflected light R is sequentially reflected by the pinhole mirror 20 and the mirror 22, and enters the camera unit 26 via the imaging lens 24.

The operator operates the sample table (not shown) on which the sample 2 to be measured is placed while viewing the image of the surface of the sample 2 to be measured which is imaged by the camera unit 26 and projected on a CRT (not shown). Position at the appropriate measurement location.

FIG. 2 is a schematic sectional view of main parts showing a longitudinal section of the sample to be measured 2. FIG. Medium A. A part of the incident light L1 that has passed through (that is, air) is reflected by the cap F12a, and the reflected light LR1
becomes. In addition, the remaining incident light L1 passes through the thin film 2a,
It is reflected by the surface of the substrate 2b and becomes reflected light 'R2. Here, the incident light L1 is applied to the surfaces of the thin film 2a and the substrate 2b.
is assumed to be incident perpendicularly, and the substrate 2b and the film 2a
Assuming that R does not absorb light, the energy reflectance R is expressed by the following equation.

Here, N: refractive index of air N: refractive index of thin film 2a N: refractive index of substrate 2b δ: =4πN1D/λ D: thickness of thin film 2a λ: wavelength Note that an equation equivalent to equation (1) is: For example, "Applied Physics Selection 3. Thin Films" (Awa Kanehara et al., Shokabo, 1981) No. 19
It is described as the formula (5,168) on page 8.

Among the parameters included in equation (1), the refractive index N,
N 1 and N 2 each have wavelength dependence (ie, “dispersion”) on the wavelength of the incident light. Further, the dispersion of the refractive index N0 of the air AQ and the refractive index N2 of the substrate 2b is known. Therefore, by the spectrometer 32, different wavelengths λ
, λ2. When the reflectances R, R2, . . . are obtained, each reflectance R, R, . It is expressed by the formula where the thickness is the unknown: R1=R1(N1 (λ1), D) ・(
2a) R-R(N (λ), D) ・
...(2b) Here, N (λ ) means the value of the refractive index N1 at the wavelength λ1.

With only relational expressions such as equations (2a) and (2b), there are more unknowns than the number of equations, N1 (λ1) and N1 (λ2).

Since the number of ... is always one more, these values cannot be determined by numerical solution. However, if the film thickness values are given, the refractive index N1 (λ1), N1 (λ2),...
・The value of can be found.

Therefore, in this invention, firstly, the estimated 1l ID of discipline thickness. ,
Dl,... are set, and each of these estimated film thicknesses DO2D
For each I, "", refractive index N1 (λ,), N1 (λ2)
Find the value of ,... That is, each S1 estimated value oo,
D1. ..., the estimated refractive index dispersion of the thin layer 1112b is calculated for each time. Then, from among the plurality of estimated refractive index dispersions obtained in this way, the optimum one is selected according to a predetermined criterion, and this is used as the measured value of the refractive index dispersion of the film 2b.

Further, the estimated film thickness value corresponding to this optimal estimated refractive index dispersion is used as the measured value of the film thickness.

Here, the "predetermined criterion" is "the refractive index N1 changes continuously with respect to the wavelength λ, and the change is monotonous.

It is determined based on the assumption that For example, each estimated film thickness Do, D1 . The estimated refractive index dispersion for ... can be approximated by a predetermined polynomial with the wavelength λ as a variable, and the smallest approximation error can be used as a "predetermined criterion".

Hereinafter, such a method will be explained along with a specific example.

FIG. 3 is a flowchart showing the procedure of one embodiment of the present invention. First, in step S1 of FIG. 3A,
A standard sample P (not shown) whose absolute reflectance is known is loaded into the microspectroscope 1, and the spectroscopic spectrum of the reflected light LR is determined. The standard sample P is 1! in the step to be described again! f! 2b
The standard sample P is for obtaining spectral data used in determining the reflectance of
A silicon substrate or the like on which a is not formed is used.

When such reflected light LR from the standard sample P is input to the spectrometer 32, a detector such as a COD (not shown) in the spectrometer 32 detects the energy E of the reflected light LR depending on the wavelength λ. , As a result, the spectroscopic spectrum E, (λ)
is required. The spectroscopic spectrum E, (λ) of the standard sample P thus measured is stored in the microcomputer 34. Further, the known absolute reflectance R, (λ) of the standard sample P is stored in the microcomputer 34 via an input means such as a keyboard (not shown).

Next, in step S2, the sample to be measured 2 is loaded in place of the standard sample PIF into the microspectroscopy device 1, and the reflected light L
The optical spectrum E(λ) of R is measured. Sample to be measured 2
The optical spectrum E(λ) of the microcomputer 34
is memorized.

Step S3 and subsequent steps are processes performed inside the microcomputer 34. First, in step S3, step 81. From the result of N2, the measured reflectance value R(λ) of the sample to be measured 2 is calculated according to the following formula.

Equation (3) is the reflectance R,,R and the energy E of the reflected light,
It is derived from the fact that E is proportional to each other, and its proportionality constant does not depend on the type of sample, and its value remains constant when measured using the same device.

The reflectance measurement value R(λ) obtained in this way is shown in FIG. 4A. Reflectance measurement [aR(λ)] exhibits remarkable wavelength dependence due to interference between reflected light LR1 from the film 2a and reflected light R2 from the substrate 2b (see FIG. 2).

Next, in step S4, a plurality of film thickness estimates [Do, ol, . . . ] are set for i1!12a, and the estimated refractive index dispersion corresponding to each is determined. Figure 38 shows step S
4 is a flowchart showing the procedure in step 4 in more detail. First, in step 541, the estimated film thickness is set to o□. Then, in steps 542 to 846, the film thickness estimation 1laDo and the wavelength λ are determined. ~1 for λ1! ! The estimated refractive index NDo (λo) to NDo (λ·) of J2a is calculated. Estimated refractive index 1'JD.

■ (λo) ~ NDo (λi) is as described above (7) (1) 4
It is expressed as the value of the refractive index N1 in the GC, but (1)
Since the equation includes a triangular numerical value 5 in 2 (δ/2), the procedure requires the following steps 343 to 345.

In step 843, taU quasi-determined go-oo, wavelength λ-
Substituting λ0 into equation (1), the total reflectance $'jli! for a predetermined range of values of the refractive index N1 is calculated. Find ICR.

The range of this refractive index N1 is i1g! 2a, and the value of the refractive index N1 is set discretely for each fixed width, for example.

Note that, as described above, the refractive index N. , N2 are known.

Since equation (1) includes trigonometric functions, the calculated value of the inverse 0-J ratio C
R changes as shown in FIG. 4B with respect to the refractive index N1.

In the figure, white circles are points that exemplarily show the calculated reflectance 11icR. In FIG. 4A, wavelength λ. If the value of the measured reflectance R for the
As can be seen from Figure B, there are multiple refractive indices n C□ ”-n Ck (hereinafter referred to as
It is called "calculated refractive index." ) exists. Step S44
Then, the values of these calculated refractions 1 n CO-n Ck are determined.

In step 845, these calculated refractive indices ncm ”−
From among n Ck, unsuitable ones are excluded, and the optimum calculated refractive index is selected as the value of the estimated refractive index NO(λ0). FIG. 4C is a diagram showing this selection method, and is a partially enlarged view of FIG. 4A. That is, as shown in FIG. 4C, first, the wavelength λ that is currently used against the Song Dynasty. minute difference Δ
λ plus wavelength λ. , (=λ0+Δλ), the reflectance measurement value ROa (−R+ΔRo) is determined from the calculation result in step S3. On the other hand, the estimated film thickness. , wavelength λO
Under the conditions of a, the reflectance RC-RC for each calculated refractive index nC6-nCk is calculated from equation (1).

In FIG. 4C, examples of the values of these reflectances RCo-RC are shown by white circles. At this time, the refractive index N1 in equation (1) corresponds to the p-relative refractive index nC□-nCk, and the reflectance R corresponds to the reflectance RCo-RCk. Since the refractive index N can be considered to be constant in the range of a small wavelength difference Δλ, the reflectance value calculated using the correct refractive index at the wavelength λOa should match the measured value ROa. Therefore, in the reflectance RCo-RC, the measurement I
The value closest to ROa is selected, and the corresponding calculated refractive index value is adopted as the estimated refractive index NDo (λ0).

In the example of FIG. 4C, since RC2#Roa, the calculated refractive index n
c is adopted as the value of H1 constant refractive index NDo (λ0).

In addition, in equation (1), the wavelength λ and the film thickness are (D/λ
), so in step S45, in order to select the optimum value from among the calculated refractive indexes nC''nCk, the wavelength λ is included as described above.

Instead of changing the film thickness by a small difference Δλ, the film thickness is estimated.

may be changed by a minute difference ΔD. That is, step 84
5, the estimated film thickness. The reflectance RCo-RC may be calculated by adding a small difference ΔD to the film thickness estimation ID (=Do+ΔD) and the wavelength a λ0. At this time, the wavelength λ at which the reflectance measurement value R8 is determined as shown in FIG. 4C. , (=λ+Δλ) is determined by the following formula.

λ −λ D/(D +ΔD) ... (4) Oa
OOO Also, the comparison of the reflectance measurement [ROa and r+ i value RCo...RC, at this time is the deviation ΔR (-R8, -R8) and ΔRC6 (=RCo) from the original value, respectively.
-CRo), -...ΔRC, (=RCk-CRk) may be compared. Here, CRo-CR is the film thickness estimation 1i
1'jD, wavelength λ. This is the reflectance meter value calculated using equation (1) for the out-of-frame refraction kO rate nc - nc. At this time, the deviation ΔRCo...ΔR
Among C, the one closest to the deviation ΔRo is selected, and the calculated refractive index corresponding to this is the estimated refractive index ND0 (
λ0).

As described above, one film thickness estimate is obtained. and one wavelength λ. One estimated refractive index ND(λ) is determined for . Steps S42 to S46 are then repeated to determine the wavelength range λ0 to λj for one film thickness estimate D0.
The estimated refractive index dispersion NDo (λ) at is determined.

This wavelength range λ0 to λj is set, for example, in the range of 200 to 800 steps, every several ni. Note that the estimated refractive index ND for one wavelength (for example, λ0) will be described below.

The value of (λ0) will be referred to as the "estimated refractive index", and its wavelength dependence NDQ (λ) will be referred to as the "estimated refractive index dispersion".

In the loop from steps 841 to S47, the estimated film thickness is calculated.

The estimated refractive index dispersion NDo (λ
>~ND(λ) is determined. Since the thickness of the thin film 2a can be estimated to some extent before measurement, the film thickness estimation 1'fDo-
The value of Di is also determined around this predicted value, and is set, for example, every several nl.

FIG. 5 is a diagram showing an example of estimated refractive index dispersion.

In step S5 of FIG. 3A, these estimated refractive index dispersions ND. Based on (λ) to NDi (λ), the measured value of the GL thickness and the measured value of the refractive index dispersion are determined simultaneously.

FIG. 3C is a flowchart showing details of the procedure of step S5. In step 851, each estimated refractive index molecule 11ND. (λ) to NDi (λ), least squares fitting is performed using a predetermined function G(λ) regarding the wavelength λ.

As the function G(λ), for example, the Cauchy equation shown below can be applied.

G(λ) −A+B/λ +C/λ (5) where A, B, and C are constants.

Through this fitting, each estimated refractive index dispersion ND0
(λ) ~ NDi (λ), the fitting functions G, Q(λ) ~ G, , (λ) are determined, and step 8
52, its standard deviation σ. ~σi are determined respectively.

In step S53, each estimated refractive index dispersion ND.

(λ) ~ NDi (λ), each ±1.5
Estimated refractive index values that deviate from σ0 to ±1.5σi−1 are removed as unnecessary. This is because such an estimated refractive index value deviates significantly from other values due to errors during measurement, and cannot be considered a reliable value. It goes without saying that this range of ±1.5σ0 to ±1.5σi can be changed as appropriate. Figures 5A to 5C show the estimated refractive index dispersion NDQ (λ) ~ excluding such unnecessary values.
An example of ND2 (λ) is shown.

Next, in step 854, these estimated refractive index dispersions ND
For 0 (λ) to ND, (λ), fitting functions GfiQ(λ) to Gbi(λ) having the same functional form as the function G(λ) in equation (5) are found again.

In step 355, each estimated refractive index dispersion ND.

Fitting function Qb for (λ) to NO, (λ), respectively. (λ) ~ Gbi(λ) where,
n=1. . . . , 1 . Determine the rate variance measurement. Also, the corresponding estimated film thickness. is determined as the measured value of film thickness. In the example of FIG. 5, the estimated refractive index dispersion ND (λ) and the estimated film thickness D1 of FIG. 5B are
It is adopted as each measurement value.

As shown in FIGS. 5A and 5C, a part of the wavelength dependence of the refractive index that deviates largely from the fitting function is not valid as refractive index dispersion, which is a physical property of the film 2a. The above procedure is based on the physical and qualitative consideration of refractive index dispersion, and the error E expressed by equation (6) is calculated. has been established as a specific criterion. As a result, it is possible to simultaneously obtain the estimated refractive index dispersion ND,(λ) and the estimated film thickness D1, which change continuously with respect to the wavelength λ and whose change is monotonous.

Note that the present invention is not limited to the above-mentioned embodiments, and the following modifications are also possible.

■ The substrate 2b is not limited to a silicon substrate, but may also be a GaA substrate.
Other semiconductor substrates such as S, chromium-plated quartz substrates, etc. may also be used. Further, the thin film 2a may be a silicon oxide film, a silicon nitride film, a resist, or the like. In addition, this invention has a film thickness of 1100n to 1.5μm.
It is particularly suitable for measurements on thin films of approximately

(2) In step 856, the criterion for finally determining the measured values of film thickness and refractive index dispersion is not limited to the error expressed by equation (6), but other criteria may also be applied.

For example, the fitting function Gbo(λ) ~ Gbi(λ
) for each estimated refractive index dispersion NDo (λ) ~ ND, (
λ) (7) [! The deviation may also be used as a criterion. That is, the fitting function Gbo(λ) to Gb,(λ) and the estimated refractive index dispersion ND. (λ) to NO, other statistics indicating the degree of mismatch with (λ) may be used as the standard. In this way, the smoothness of the estimated refractive index dispersion can be evaluated in a window-like manner by using the statistical value indicating the degree of mismatch with the fitting function as a standard.

Further, the error EEo expressed by the following equation may be used as a reference.

EEo= J@1 (Σ IND (λ )−ND (λ
), 20nk
n k+1(NO(λo)
-NDo(λj))) /(j+1)...
・(1) Here, n=1. ... This error EE is within the wavelength range λ. ~λj1. : m mark showing the degree of unevenness with respect to the average slope of the refractive index ND, (λ).

That is, the above-mentioned criterion may be anything that indicates the smoothness of the wavelength dependence of the refractive index. Note that when calculating the error using equation (7), the fitting function Gbo(λ
) to Gbi(λ), step S54 in FIG. 3C is unnecessary.

(2) In the above embodiment, the case of vertical incidence was explained, but it goes without saying that the present invention can be applied to cases other than vertical incidence. In this case, equation (1) representing the anti-tA rate includes the angle of incidence.

In addition, although the substrate 2b was assumed not to absorb light, the substrate 2b
Even when there is absorption in b, the present invention is applicable by substituting the complex refractive index into equation (1). For example, the reflectance when the substrate 2b has absorption is expressed by the following equation.

R12=(c+Cb×CC a +2(cCcos6R C ” CbCc 5in6 R) ) /(1+C×C( +2(ccos6R +C,5in6R)) =−(8) Here, Ro
Amplitude refractive index Ca, Cb of double reflected light: Complex reflectance r of the interface between the thin film 2a and the substrate 2b. Real part and imaginary part Cc: Air layer A. The real part C, C, of the fluorine anti-reflection rA>* r of the interface between and the thin film 2a: the complex reflectance r.

, the real and imaginary parts of the product of rl, i.e. Co-C, C6 c, -c, c.

D: n thickness of 14 m2a δ:=4πNID/λ N,: bending tfl of R1112a (Effects of the Invention) As explained above, according to the present invention, a plurality of iwW1 fixed stations are determined based on the measurement of the spectral reflection spectrum. A plurality of estimated refractive index dispersions are obtained for each, and the one with a smooth wavelength dependence is selected as the measured value of the refractive index dispersion, so that the refractive index dispersion and Fi! This method has the advantage that the measured value of refractive index dispersion can be determined by spectroscopy for a thin film of unknown thickness.

Further, by using the estimated film thickness value corresponding to the selected estimated refractive index dispersion as the measured value of the film thickness, there is an effect that the measured value of the film thickness can also be obtained.

Furthermore, by determining a fitting function for each of multiple refractive index dispersions and selecting the above estimated refractive index dispersion based on the statistics indicating the discrepancy with the fitting function, the smoothness of the refractive index dispersion is quantitatively evaluated. There is an effect that it can be done.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the configuration of a microspectroscopy apparatus for determining film thickness and refractive index dispersion by applying an embodiment of the present invention. FIG. 2 is a schematic cross-sectional view of the main part of a sample to be measured. FIG. 4 is an explanatory diagram showing measured values and calculated values of reflectance in the example. FIG. 5 is an explanatory diagram showing refractive index dispersion in the example. DESCRIPTION OF SYMBOLS 1...Microspectroscopy device, 2...Measurement sample, 2a.
...Thin film, 2b...Substrate, R...Reflectance measurement value, CR...Reflectance tl value, D...Film thickness,
D0~D2...Estimated film thickness, λ...Wavelength, ND, (λ)~ND2 <λ)...Estimated refractive index dispersion,
Gbo(λ) ~ Gb2(λ)...Fitting function substitute

Claims (4)

[Claims] (1) A method for measuring refractive index dispersion in which the refractive index dispersion of the surface film is measured using a test sample having a surface film formed on a substrate, the method comprising: (a) measuring the spectral reflection spectrum of the test sample; (b) after setting a plurality of film thickness estimates for the surface film; (c) based on the spectral reflection spectrum, determining a plurality of film thickness estimates of the surface film corresponding to each of the plurality of film thickness estimation values; While calculating the estimated refractive index dispersion, (d) selecting one from the plurality of estimated refractive index dispersions based on a predetermined criterion indicating smoothness of wavelength dependence of the estimated refractive index dispersion, A method for measuring refractive index dispersion, characterized in that this is used as a measured value of refractive index dispersion of the surface film. (2) The selection of the estimated refractive index dispersion in step (d) involves determining a fitting function having a predetermined functional form for each of a plurality of estimated refractive index dispersions, and then selecting the fitting function and the corresponding estimated refractive index. 2. The method for measuring refractive index dispersion according to claim 1, wherein a statistic indicating the degree of mismatch with the dispersion is determined, and the measurement is performed using the smallest statistic as a predetermined standard. (3) A film thickness measurement method for measuring the film thickness of the surface film using a test sample having a surface film formed on a substrate, the method comprising: (a) measuring the spectral reflection spectrum of the test sample; (b) after setting a plurality of film thickness estimates for the surface film; (c) based on the spectral reflection spectrum, a plurality of estimated refractive indices of the surface film corresponding to each of the plurality of film thickness estimates; (d) selecting one of the plurality of estimated refractive index dispersions according to a predetermined criterion indicating smoothness of the wavelength dependence of the estimated refractive index dispersion; and (e) selecting one of the plurality of estimated refractive index dispersions; A method for measuring film thickness, characterized in that the estimated film thickness value corresponding to the estimated refractive index dispersion is used as a measured value of the film thickness of the surface film. (4) Selection of the estimated refractive index dispersion in step (d) involves determining fitting functions each having a predetermined functional form for a plurality of estimated refractive index dispersions, and then selecting the fitting function and the corresponding estimated refractive index. 4. The film thickness measuring method according to claim 3, wherein a statistic indicating the degree of discrepancy with the variance is obtained, and the measurement is performed using the smallest statistic as a predetermined standard.