JP3106790B2 - Thin film characteristic value measuring method and apparatus - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thin film having a thickness, a refractive index,
The present invention relates to a method and an apparatus for measuring an absorption coefficient.

[0002]

2. Description of the Related Art Thin film characteristics (film thickness, refractive index, absorption coefficient)
In the prior art for optically measuring, there are an ellipsometry and an incident angle dependent reflection characteristic analysis. In each case, each characteristic value is obtained based on a physical phenomenon in which light incident on the resist film repeatedly reflects and interferes with the interface between the air and the thin film and the interface between the thin film and the substrate.

The polarization analysis method (for example, “Optical Measurement Handbook”, edited by Toshiharu Tada et al., Pp. 256-265, Asakura Shoten (1981)) irradiates light obliquely and reflects the reflection intensity of a P-polarized component and an S-polarized component. The film characteristic value is calculated from the phase difference. In order to determine the three unknowns of the film thickness, refractive index, and absorption coefficient of the film to be measured by ellipsometry, it is necessary to perform ellipsometry at two or more incident angles. Can not. Also, since the measurement sensitivity depends on the angle of incidence,
When the film to be measured is unknown, high-precision measurement cannot be performed unless measurement at different incident angles is repeated.

The incident angle dependent reflection characteristic analysis method is a method for obtaining each characteristic value from a change in reflection intensity when the incident angle is changed. JP-A-2-128106 discloses a method for mechanically scanning the entire optical system to measure the film thickness, refractive index, and absorption coefficient from the intensity reflectance at three incident angles.
However, it is not possible to measure at high speed because of the method of detecting reflected light by mechanical scanning. In order to measure with high accuracy, it is necessary to make the detection accuracy of the reflectance sufficiently high, and a complicated and expensive measuring device is required.

As a method for detecting the incident angle dependent reflection characteristic at high speed and in a stable manner, equitilt interference (Max Born et al.,
There is an optical principle called "optical principle II", pp 454-456, Tokai University Press. When the film to be measured is illuminated with convergent light over an appropriate incident angle range and the reflected light is passed through the detection lens, interference fringes occur on the rear focal plane of the detection lens. This is called equi-tilt interference fringes,
The intensity corresponds to the incident angle dependent reflection characteristics. Therefore, if the intensity distribution of equi-tilt interference fringes is imaged by an array sensor,
The incident angle dependent reflection characteristic can be detected without using a movable part, and the film thickness and the refractive index can be obtained. JP-B-62-49562, JP-A-64-75902, and JP-A-4-313006 detect equi-tilt interference fringes by an optical system using two opposing lenses for illumination and detection. It discloses a method of obtaining a film thickness and a refractive index from an incident angle at which the intensity has an extreme value.

In Japanese Patent Application Laid-Open No. 3-17505, illumination light and reflected light are passed through one lens whose optical axis is perpendicular to the film to be measured, and the film thickness and the refractive index are measured from concentric equi-tilt interference fringes. A method for doing so is disclosed. In this method, the convergent light is narrowed down by using a high NA lens, and the measurement of the film thickness and the refractive index in a very small area is enabled. However, none of the above-mentioned prior arts using equi-tilt interference fringes describes in detail the measurement of an absorbing thin film, that is, a thin film having an absorption coefficient other than 0.

[0007]

SUMMARY OF THE INVENTION The present invention provides an apparatus for detecting not only the thickness and refractive index of a thin film but also an absorption coefficient at high speed and stably by detecting equi-tilt interference fringes. .

It is an object of the present invention to provide a method for measuring the thickness of a thin film having absorptivity, the refractive index at a specific wavelength, and the absorption coefficient.

Another object of the present invention is to accurately detect the intensity distribution of equi-tilt interference fringes without including stray light in order to measure the thickness of an absorptive thin film and the refractive index and absorption coefficient at a specific wavelength. It is to provide an optical system which can be used.

It is another object of the present invention to provide a method for measuring the thickness of a photosensitive thin film such as a photoresist, and the refractive index and absorption coefficient at a photosensitive wavelength.

[0011]

To determine the absorption coefficient of a thin film, it is necessary to measure the incident angle dependent intensity reflectance whose absolute value is accurate. However, equidistant interference fringe intensity distribution of the film to be measured,
That is, the incident angle dependent reflected light intensity distribution I (θ) is the product of the illumination light intensity distribution S (θ) and the incident angle dependent intensity reflectance R (θ), and I (θ) does not match R (θ). . Therefore, in the present invention, in order to remove the influence of the illumination light intensity distribution S (θ), the incident angle dependent reflected light intensity distribution I _ref (θ) of the reference mirror sample whose refractive index and absorption coefficient at the measurement wavelength are known is detected. Then, the incident angle dependent intensity reflectance R _ref (θ) is theoretically calculated from the refractive index and the absorption coefficient of the reference mirror sample at the measurement wavelength. Then, the incident angle-dependent reflected light intensity distribution I (θ) of the film to be measured is divided by the incident angle-dependent reflected light intensity distribution I _ref (θ) of the reference mirror sample, and the incident angle dependent intensity of the reference mirror sample is calculated. By multiplying the reflectance R _ref (θ), the incident angle dependent intensity reflectance R (θ) of the film to be measured is obtained.

In the case where an optical system for detecting the equi-tilt interference fringes by passing illumination light and reflected light through one lens whose optical axis is perpendicular to the film to be measured is used, illumination passing near the optical axis of the lens is used. A light blocking plate for blocking light is inserted into the illumination optical system. Thus, the intensity distribution of the equi-tilt interference fringes is accurately detected without being affected by stray light.

In order to accurately measure the thickness of the photosensitive thin film and the refractive index and absorption coefficient at the photosensitive wavelength, not only the measurement at the photosensitive wavelength but also the measurement at the non-photosensitive wavelength are performed. First, equitilt interference fringes are detected at the non-photosensitive wavelength, and the film thickness, the refractive index and the absorption coefficient at the non-photosensitive wavelength are obtained. With the film thickness obtained here as a known value, next, equi-tilt interference fringes are detected at the photosensitive wavelength, and the refractive index and the absorption coefficient at the photosensitive wavelength are determined.

[0014]

[Function] Incident angle dependent reflected light intensity distribution I of a mirror sample for reference
_ref (θ) is the product of the illumination light intensity distribution S (θ) and the incident angle dependent intensity reflectance R _ref (θ). Therefore, if the incident angle dependent reflected light intensity distribution I (θ) of the film to be measured is divided by the incident angle dependent reflected light intensity distribution I _ref (θ) of the reference specular sample, the illumination light intensity distribution S (θ) is obtained. It can be erased and becomes R (θ) / R _ref (θ). In addition, the incident angle-dependent intensity reflectance R _ref (θ) of the mirror sample for reference
By multiplying by the theoretical calculation value of the above, the incident angle dependent intensity reflectance R (θ) of the film to be measured is obtained.

Near the optical axis of the lens, the surface of the lens is substantially perpendicular to the optical axis. For this reason, in an equi-tilt interference fringe detection optical system using a lens whose optical axis is perpendicular to the film to be measured, light reflected on the lens surface is detected as stray light. However, if a light shielding plate is inserted into the illumination optical system to block illumination light passing near the optical axis, no stray light is generated, and the intensity distribution of the equi-tilt interference fringes can be accurately detected.

The photosensitive thin film has a large absorption coefficient in the photosensitive wavelength region, and has almost zero absorption coefficient in the non-photosensitive wavelength region. When the measurement light is light having a photosensitive wavelength, the amplitude is attenuated while passing through the film because the film to be measured has absorptivity. For this reason, the amplitude of the light passing through the film to be measured is smaller than the light reflected on the surface of the film to be measured, and the contrast of the equi-tilt interference fringes is reduced. Since this interference fringe does not have much information on the film thickness, three unknowns of the film thickness, the refractive index, and the absorption coefficient cannot be simultaneously and accurately obtained.

On the other hand, if the measurement light is a non-photosensitive wavelength, there is no absorption in the film to be measured and the amplitude does not attenuate in the film. Therefore, it is possible to detect equi-tilt interference fringes having good contrast and sufficiently including information on the film thickness. Therefore, the thickness of the photosensitive thin film, the refractive index at the non-photosensitive wavelength, and the absorption coefficient are first determined from the interference fringes at the non-photosensitive wavelength. If the thickness determined here is a known value, the refractive index and the absorption coefficient at the photosensitive wavelength can be determined from the interference fringes at the photosensitive wavelength.

[0018]

Embodiments of the present invention will be described below. First, optical phenomena related to the present invention and the principle of a method of measuring thin film characteristic values (thickness d, refractive index n, absorption coefficient k) will be described. As shown in FIG. 4, the thin film is the thin film 21 on the substrate 23, and the interface 24 between the air and the thin film 21, the thin film 21 and the substrate 2
3 is assumed to be parallel.

When the illumination light 32 enters the thin film 21 at an incident angle θ, reflection and transmission at the boundary surface 24 and reflection at the boundary surface 25 are repeated. Since the two boundary surfaces are parallel, the light reflected from the thin film becomes a parallel light beam 33 as shown in FIG. When this parallel light beam passes through the lens 6, it is condensed and interferes with one point P on the rear focal plane 34 of the lens. Since the position of the point P and the interference intensity change depending on the incident angle, the rear focal plane 34
Bright and dark fringes occur on the upper part, which are called equi-tilt interference fringes. Since the equi-tilt interference fringes correspond to the intensity reflectance of the thin film depending on the incident angle, by observing the interference fringes,
The thickness, refractive index, and absorption coefficient of the thin film are measured.

The intensity reflectance R of the thin film is theoretically calculated by the following equation. The subscript 'indicates a complex number, and i is a symbol of an imaginary unit.

[0021]

(Equation 1)

Where r 'is the amplitude (complex) reflectivity of the thin film, and r' * is the complex conjugate of r '. When the variables are defined as follows, r ′ is expressed by (Equation 2) to (Equation 8).

R ₀₁ ′: amplitude (complex) reflectance at the interface 24 between the air and the thin film 21 r ₁₂ ′: amplitude at the interface 25 between the thin film 21 and the substrate 23
(Complex) reflectance δ ': optical path difference generated by one round trip in the thin film λ: wavelength of illumination light d: film thickness of the thin film n ₀ : refractive index of air (= 1) n': complex refractive index of the thin film. n ′ = n−ik n ₂ ′: complex refractive index of the substrate. n ₂ ′ = n ₂ −ik ₂ n ₂ : refractive index of substrate at wavelength λ k ₂ : absorption coefficient of substrate at wavelength λ θ: incident angle θ ₁ ′: refraction angle in thin film θ ₂ ′: in substrate Angle of refraction

[0024]

(Equation 2)

[0025]

(Equation 3)

[0026]

(Equation 4)

Further, the amplitude reflectance at the boundary surface can be calculated by the Fresnel equation. When the illumination light 32 is P-polarized light,

[0028]

(Equation 5)

[0029]

(Equation 6)

When the illumination light is S-polarized light,

[0031]

(Equation 7)

[0032]

(Equation 8)

## EQU1 ## That is, the theoretical value Rth of the intensity reflectance of the thin film can be expressed by a function of Rth (n ₀ , d, n, k, n ₂ , k ₂ , θ, λ).

FIG. 5 shows an example in which a change in the intensity reflectance R depending on the incident angle θ when a thin film on a mirror-finished silicon substrate is illuminated with P-polarized light having a wavelength of 365 nm is theoretically calculated.
It can be seen that the change in the intensity reflectance R differs depending on the combination of the film thickness d, the refractive index n, and the absorption coefficient k. When the absorption coefficient k increases, light energy is absorbed in the thin film, so that the intensity reflectance R tends to decrease. If the absorption coefficient k is different, the phase change amount at the boundary surface is different, so that the incident angle at which the incident angle-dependent reflectance becomes an extreme value also moves. In other words, the method focusing on the incident angle at which the reflectance has an extreme value cannot measure the characteristic value of the absorbing thin film. In order to measure the characteristic value of a thin film having absorption, it is necessary to use the absolute value of the reflectance.

In order to measure the characteristic value of the thin film from the absolute value of the incident angle dependent intensity reflectance, the incident angle dependent intensity reflectance R (θ) of the film to be measured is actually measured, and the theoretical reflectance matching R (θ) is measured. Rth
(n ₀ , d, n, k, n ₂ , k ₂ , θ, λ) may be found. To do this, either one of the following (Equation 9) or (Equation 10) is performed.

[0036]

(Equation 9)

[0037]

(Equation 10)

Here, the illumination light is linearly polarized light of P-polarized light or S-polarized light, and its polarization direction is also known. The refractive index n _{0 of} air, the wavelength λ, the refractive index n ₂ of the substrate at the wavelength λ and the absorption coefficient k ₂ are also known, and these are excluded from the variables. The function M (d, n, k) is obtained by measuring the measured reflectance R (θ) and the theoretical reflectance Rth.
An evaluation function indicating the degree of coincidence of (θ, d, n, k).
Calculates the sum of the absolute values of the differences at a plurality of incident angles, and (Equation 10) calculates the sum of the squares of the differences. When the evaluation function M is minimum, the measured reflectance R (θ) and the theoretical reflectance Rth
(θ, d, n, k) is the best, and the set of d, n, and k at this time may be the measured value of the film to be measured at the wavelength λ.

An embodiment will be described in which both the improvement of the measurement resolution and the reduction of the calculation amount are achieved in the above-mentioned method. The maximum and minimum of the film thickness d, the refractive index n, and the absorption coefficient k that are possible in the film to be measured are indicated by subscripts of max and min, and the measurement resolution is _dr , _nr , kr _.
Then, the number of calculations C ₁ of (Equation 9) or (Equation 10) required to determine the three unknowns of the film thickness, the refractive index, and the absorption coefficient is

[0040]

[Equation 11]

## EQU1 ## Here, Nd, Nn, and Nk are decomposition points of each characteristic value. When d _r , n _r , and k _r are reduced and calculation is performed with high resolution, the number of decomposition points increases, and the number of calculations C ₁
Will be very huge. Therefore, the resolution is sequentially improved and the amount of calculation is reduced. That is, in the first stage, the measurement resolution _dr ,
Rough detection is performed by increasing n _r and k _r to some extent, and the film thickness d ₁ , refractive index n ₁ , and absorption coefficient k ₁ are obtained. In the second stage, the measurement range of the film to be measured is d ₁ ± Δd ₁ , n ₁ ± Δn ₁ , k ₁ ±
The film thickness d ₂ , the refractive index n ₂ , and the absorption coefficient k ₂ are searched while limiting the measurement resolution to Δk ₁ . Thereafter, this operation is repeated until the required measurement resolution is reached by sequentially reducing both the measurement range and the measurement resolution of the film to be measured.

As a result, the measurement resolution can be improved without performing enormous calculations. If the measurement resolution in the first stage is too coarse, it may not converge to a correct result. In order to prevent this, in the first stage of coarse detection, not only the case where the evaluation function M is minimized but also a condition such as satisfying twice or less of the minimum value, for example, a plurality of d, n, k Will be output. Then, the above-described second and subsequent steps are performed on each coarse detection result, and finally, a combination of d, n, and k that minimizes the evaluation function M may be obtained.

In order to measure the thickness d, the refractive index n, and the absorption coefficient k of the thin film by this method, the absolute value of the incident angle dependent intensity reflectance R (θ) of the measured film to be measured is accurately measured. There is a need to. One embodiment of the present invention in which R (θ) is measured stably and at high speed to obtain a thin film characteristic value will be described with reference to FIG.

FIG. 6 shows the incident angle dependent intensity reflectance R (θ) of the thin film.
Is a device for measuring a characteristic value of a thin film. The present measuring apparatus includes a light source 1, a collimating lens 2, a polarizing element 3, a monochromatic light transmission filter 4, a condensing illumination side lens 5, a detection side lens 6, and a detection side lens 6 on the rear focal plane 34 and in the incident plane. A light quantity accumulation type linear sensor 7 for detecting the intensity distribution in the direction;
Film memory 8 for storing the reflected light intensity distribution of the film to be measured, reference sample memory 9 for storing the reflected light intensity distribution of the reference sample, and dark level memory 1 for storing the dark level of the sensor
2. A reflectance calculation unit 10 for calculating the incident angle dependent intensity reflectance of the film to be measured, and a characteristic value determination unit 11 for obtaining a thin film characteristic value using the evaluation function M of (Equation 9) or (Equation 10). The angles at which the optical axes of the illumination side lens 5 and the detection side lens 6 enter the sample surface are both θd.

The light emitted from the light source 1 is transmitted to a collimating lens 2.
Is converted into parallel light, and the polarizing plate 3 and the monochromatic light transmitting filter 4
And becomes monochromatic light of linearly polarized light. Here, the P-polarized light is parallel to the paper surface. The P-polarized monochromatic light is converged by the illumination side lens 5 and illuminates the sample. First, the reflected light intensity distribution of the film to be measured 21 on the substrate 23 is measured. Where the incident angle θa
Is repeatedly reflected in the thin film, and the reflection angle θa
Are reflected as a parallel light beam 33a. FIG. 6 shows only two light beams for simplicity. When passing through the detection-side lens 6, the parallel light beam 33 a condenses on a point Pa on the rear focal plane 34 of the detection-side lens 6 due to the nature of the lens. Similarly, the illumination light beam 32b having the incident angle θb is focused on a point Pb on the back focal plane. As described above, the incident angle θ of the light beam and the condensing position on the back focal plane 34 have a one-to-one relationship, and the reflected light intensity distribution on the rear focal plane corresponds to the incident angle dependent reflected light intensity characteristic.

Now, the detection side lens 6 is a lens whose aberration is well corrected and the sine condition is satisfied, and its focal length is f. In this case, as shown in FIG. 7, the distance x from the optical axis 36 of the detection-side lens 6 to the focal point P and the angle θp formed by the optical axis 36 and the parallel light beam 33 are:

[0047]

(Equation 12)

There is a relationship as follows. The sine condition of (Equation 12) is a condition necessary for the lens to have good imaging performance, and the microscope objective lens is designed to satisfy this condition. That is, a microscope objective lens may be used as the detection lens 6. Since θp = θ−θd,

[0049]

(Equation 13)

Is as follows. Therefore, the reflected light intensity distribution I on the back focal plane 34 of the detection side lens 6 detected by the linear sensor 7
By transforming (x) into a variable according to (Equation 13), the incident angle dependent reflected light intensity characteristic I (θ) is obtained. However, when a mercury lamp or the like is used as the light source 1, the illuminance distribution 31 of the illumination light cannot be made uniform as compared with the case where a laser is used.
The illumination light intensity distribution S (θ) at 32b and the incident angle therebetween is not uniform. Therefore, the incident angle dependent reflected light intensity characteristic I
(θ) and the incident angle dependent intensity reflectance R (θ) do not have a simple proportional relationship, but are expressed by the following equation.

[0051]

[Equation 14]

Therefore, to obtain R (θ), the reference sample 22
Is placed on the sample surface, the reflected light intensity distribution I _ref (x) on the back focal plane 34 is measured, and converted into a variable according to (Equation 13) to obtain I
_{Find ref} (θ). The reference sample 22 may be any mirror surface sample having a known refractive index n _ref and absorption coefficient k _{ref at} the measurement wavelength λ. For example, a mirror-surface silicon wafer may be used. Assuming that the incident angle dependent intensity reflectance of the reference sample is R _ref (θ),

[0053]

(Equation 15)

Is represented by When Equation 14 is divided by Equation 15 from each side, the following equation is obtained.

[0055]

(Equation 16)

That is, the ratio between the incident angle dependent reflected light intensity characteristic I (θ) of the film 21 to be measured and the incident angle dependent reflected light intensity characteristic I _ref (θ) of the reference sample 22 is obtained, thereby obtaining the illumination light intensity distribution. S (θ) can be eliminated.

Here, the incident angle dependent reflectance characteristic R _ref (θ) of the reference sample 22 is defined by the following equation (1)
Theoretical calculation can be performed by 7) to (Equation 20).

R _ref ': interface 2 between air and reference sample 22
The amplitude (complex) reflectance at 6 λ: the wavelength of the illumination light n ₀ : the refractive index of air (= 1) n _ref ′: the complex refractive index of the substrate at the measurement wavelength λ. n
_ref ′ = n _ref −ik _ref n _ref : refractive index of the substrate at the measurement wavelength λ k _ref : absorption coefficient of the substrate at the measurement wavelength λ θ: incident angle θ _ref ′: refraction angle at the reference sample 22

[0059]

[Equation 17]

[0060]

(Equation 18)

[0061]

[Equation 19]

[0062]

(Equation 20)

R _ref ': interface 2 between air and reference sample 22
The amplitude (complex) reflectance at 6 λ: the wavelength of the illumination light n ₀ : the refractive index of air (= 1) n _ref ′: the complex refractive index of the substrate at the measurement wavelength λ. n
_ref ′ = n _ref −ik _ref n _ref : refractive index of the substrate at the measurement wavelength λ k _ref : absorption coefficient of the substrate at the measurement wavelength λ θ: incident angle θ _ref ′: refraction angle at the reference sample 22 Incident angle dependent intensity reflectance R of reference sample 22
_By multiplying the theoretical calculated value of _ref (θ) by both sides of Equation 16,

[0064]

(Equation 21)

And R (θ) is obtained. That is, the measured value of the incident angle dependent reflected light intensity characteristic I (θ) of the film to be measured 21 is used as the measured value of the incident angle dependent reflected light intensity characteristic I of the reference sample 22.
_By dividing by _ref (θ) and multiplying by the theoretical calculation value R _ref (θ) of the incident angle dependent intensity reflectance of the reference sample 22, the incident angle dependent intensity reflectance R (θ) of the film to be measured can be obtained. .
FIG. 8 shows an outline of the above processing.

Considering the non-uniformity of the optical element in the optical system, such as the transmittance distribution depending on the light beam passing position of the illumination lens 5 and the detection lens 6, etc., as a function of the incident angle θ, T (θ)
(Equation 14) and (Equation 15) are represented by the following equations.

[0067]

(Equation 22)

[0068]

(Equation 23)

Also in this case, if the operation of (Equation 21) is performed, S
T (θ) can be erased similarly to (θ).

The reflectance calculating section 10 in FIG. 6 obtains the incident angle dependent intensity reflectance R (θ) of the film to be measured by the above processing. Then, the film thickness d of the film to be measured, the refractive index n, and the absorption coefficient k are obtained by the characteristic value determining unit 11.

According to this embodiment, the illumination light intensity distribution S (θ)
Is not uniform, the incident angle dependent characteristic intensity reflectance R (θ) of the film to be measured can be obtained stably and at high speed, and the film characteristic value can be obtained. For this reason, various light sources can be used for thin film characteristic value measurement, and adjustment of the illumination optical system is also facilitated. In addition, since the nonuniformity T (θ) of the transmittance due to the difference in the transmission position of each optical element is also corrected, there is an advantage that the manufacturing accuracy of the optical element can be reduced.

In order to obtain the incident angle dependent intensity reflectance R (θ) of the film to be measured with higher accuracy, it is sufficient to perform dark level correction. For this purpose, the sample is retracted from the position where the illumination light 32 is condensed, and the output of the linear sensor 7 at this time is stored as a dark level intensity distribution Ib (x) in the dark level memory 12 shown in FIG. This is converted into a variable according to (Equation 13) and Ib (θ)
And R (θ) is calculated by the following equation.

[0073]

(Equation 24)

By the dark level correction, it is possible to correct the variation of the dark level for each detection element of the linear sensor 7. As a result, the incident angle dependent intensity reflectance R (θ) of the film to be measured is determined with high accuracy, and the measurement accuracy of the film characteristic value can be improved.

FIG. 9 shows the measurement procedure of this embodiment. Same figure
As shown in (a), the substrate 23 on which the film to be measured is formed and the reference sample 22 are mounted on the mounting table 64, and the measurement is performed in the following procedure.

Procedure 1: (1) The mounting table 64 is retracted, and the dark level intensity distribution Ib (x) of the linear sensor 7 is detected. (Same figure
(b)) Step 2: The reference sample is moved to the measurement position, and the back focal plane reflected light intensity distribution I _ref (x) is detected. (FIG. 9C) Step 3: The film substrate 23 to be measured is moved to the measurement position, and the back focal plane reflected light intensity distribution I (x) is detected. (FIG. 4D) Step 4: Each detection signal is transformed into a variable by (Equation 13), and I (θ),
I _ref (θ) and Ib (θ) are obtained.

Step 5: Calculate the incident angle dependent intensity reflectance R (θ) of the film to be measured in (Equation 24).

Step 6: Calculation of (Equation 9) or (Equation 10) is performed to determine the film characteristic value.

By measuring according to this procedure, even if the light amount of the light source changes gradually with time, R (θ) can be accurately obtained, and the film characteristic value can always be measured with high accuracy. Further, the measurement can be performed efficiently without exchanging the film substrate 23 to be measured and the reference sample 22. Of course, the film substrate 23 to be measured and the reference sample 22 may be replaced manually. In short-time measurement such as multi-point measurement on the same substrate, I _ref (x) and I b (θ) are measured first and stored in a memory, and the reflected light intensity distribution I (x) of the film to be measured is obtained. If reading is performed every time measurement is performed, measurement can be performed more efficiently.

FIG. 10 shows another embodiment for improving the measurement accuracy. In this embodiment, the film 2 to be measured is
The reflected light intensity distribution I (x) of the back focal plane 1 is detected a plurality of times by the linear sensor 7, and the average thereof is stored in the film memory 8 to be measured. Since the white noise component superimposed by the electric circuit for driving the linear sensor 7 can be reduced by the averaging effect, the S / N ratio is improved, and the measurement accuracy of each characteristic value can be improved. Of course, if the reflected light intensity distribution and the dark level of the reference sample are similarly detected, the measurement accuracy is improved.

An embodiment in which the light quantity fluctuation of the light source itself is corrected and measured will be described with reference to FIG. In this embodiment, a light amount fluctuation correction unit 80, a photoelectric conversion type light amount detector 58, a light amount detector 58
A half mirror 55 for guiding illumination light to the embodiment is added to the embodiment of FIG. FIG. 12 shows the principle of light quantity correction. (a)
Is a scanning cycle signal φ of the linear sensor 7, (b) is an output signal of the linear sensor 7, (c) is an output signal of the light amount detector 58,
(d) is a signal obtained by integrating the output signal of the light quantity detector 58 for the scanning cycle time of the linear sensor 7.

The linear sensor 7 accumulates the light energy applied to each pixel during the period Tn, and outputs the signal W in the next period.
Output n. For this reason, if the light amount fluctuates as shown in (c), the accumulated light energy also fluctuates, and Wn does not become constant. Therefore, as shown in (d), the output obtained by integrating the output of the light amount detector 58 by the scanning period is converted to the scanning period signal φ.
, And a correction value Cn is obtained. Since the correction value Cn corresponds to the total amount of light in the cycle Tn, a change in the output signal Wn due to a change in the amount of light can be corrected by dividing the signal Wn by the correction value Cn.

FIG. 13 shows a light amount fluctuation correcting section 8 for performing the above correction.
0 is an example. FIG. 9A includes an A / D converter 83, an integration circuit 81, a sample and hold circuit 84, an A / D converter 85, and a digital divider 86. FIG. 1B includes an integrating circuit 81, a sample and hold circuit 84, an analog divider 87, and an A / D converter 88. The integration circuit 81 performs an analog integration of the output signal of the linear sensor 7 for one scanning cycle time of the linear sensor 7, and the sample and hold circuit 84 samples the output signal of the integration circuit 81 in synchronization with the scanning cycle signal φ. , And outputs the correction value Cn.
(a) shows the output signal Wn of the linear sensor 7 and the correction value Cn as A
After the / D conversion, division is performed with the digital signal. (b)
Is for dividing the output signal Wn of the linear sensor 7 and the correction value Cn by an analog signal and then performing A / D conversion.

While correcting the output signal of the linear sensor 7 as described above, the dark level intensity distribution Ib (x), the reflected light intensity distribution I _ref (x) of the reference sample, and the reflected light intensity distribution of the film substrate 23 to be measured. If I (x) is detected, the influence of the light quantity fluctuation can be reduced, so that the incident angle dependent intensity reflectance R (θ) of the film to be measured can be determined with high accuracy. Thereby, the measurement accuracy of the characteristic value of the film to be measured is improved.

Hereinafter, another embodiment of the present invention will be described. In this embodiment, the thickness d, the refractive index n, and the absorption coefficient k of the photosensitive thin film are obtained with high accuracy. Although the film thickness d is a three-dimensional dimension and does not depend on the wavelength, the refractive index n and the absorption coefficient k
Is the physical property value of the film material, and the value differs depending on the wavelength. In particular, the values of the refractive index n and the absorption coefficient k at the exposure wavelength of the photosensitive thin film have industrial significance.

For example, in the semiconductor manufacturing industry, a photoresist, which is a kind of photosensitive material, is used, and an original pattern is transferred to a photoresist film applied on a wafer surface to form a fine circuit pattern. In the manufacturing process, in order to perform stable transfer,
Process control by film thickness measurement is usually performed. However, the transfer conditions depend not only on the film thickness but also on the refractive index n and the absorption coefficient k at the exposure wavelength.

FIG. 14A shows the spectral characteristics of the absorption coefficient of the photoresist. The solid line shows the characteristics before exposure and the broken line shows the characteristics after exposure. The wavelength region where the absorption coefficient is almost 0 is a non-photosensitive wavelength region, and the photoresist film does not absorb light energy and does not change its chemical composition even when irradiated with light of wavelength λa. That is, the photoresist is not exposed to light having the wavelength λa. Therefore, in an exposure line at a semiconductor factory, indoor illumination is performed with yellow light in a non-photosensitive wavelength range to prevent unnecessary exposure. On the other hand, the wavelength region where the absorption coefficient is large before the exposure is the photosensitive wavelength region of the photoresist, and when the photoresist film is exposed by irradiating light of wavelength λb of the photosensitive wavelength region, the light energy is absorbed and the composition changes. As shown, the absorption coefficient decreases. In other words, pattern transfer is a chemical composition difference between a region where the photoresist film is exposed to light in a photosensitive wavelength range to reduce the absorption coefficient by exposing the region to the original absorption coefficient without being exposed,
It can be said that this is the work of transferring the original pattern to the resist film.

As part of stabilizing the pattern transfer, the exposure apparatus is controlled to irradiate a constant light energy. However, in this control, if the value of the absorption coefficient of the photoresist before exposure is different, the absorption coefficient after exposure (that is, the chemical composition of the photoresist film) fluctuates, which causes a pattern dimension error. Similarly, a change in the refractive index at the exposure wavelength also causes an error. Therefore, in the pattern transfer step, it is important to measure and manage the refractive index and absorption coefficient at the exposure wavelength of the photoresist film before exposure.

However, there are the following difficulties in measuring the thickness of the photosensitive thin film, the refractive index at the exposure wavelength, and the absorption coefficient. As shown in FIG. 14B, when the measuring light beam 32 is irradiated on the photosensitive film 21 to be measured, the film light to be measured reflected 33c reflected on the film surface is incident on the film to be measured.
The light passing through the film to be measured 33 which is emitted after being reflected at the interface with
Divided into d. As described above, the thin film characteristic value measuring method of the present invention measures a thin film characteristic value from equi-tilt interference fringes generated by interference between the measured film reflected light 33c and the measured film passing light 33d. In this case, the contrast of the interference fringes increases as the amplitude of the film-to-be-measured reflected light 33c and the amplitude of the light-to-be-measured film passing light 33d are equal, and the measurement accuracy is improved.

On the other hand, when the measuring light 32 is light having the photosensitive wavelength λb, the amplitude is attenuated while passing through the film because the film 21 to be measured has absorptivity. Therefore, the amplitude of the light 33d passing through the film to be measured is smaller than the reflected light 33c of the film to be measured, and the contrast of the interference fringes is reduced.

Also in the theoretical calculation example of FIG. 5, the greater the absorption coefficient, the smaller the change in the equi-tilt angle interference fringe, that is, the change in the incident angle dependent intensity reflectance. Interference fringes whose contrast has been reduced due to absorption in the film do not have much information about the film thickness. Therefore, when measuring at the photosensitive wavelength, it is difficult to accurately measure the three unknowns of the thickness, the refractive index, and the absorption coefficient of the photosensitive thin film having absorptivity.

Therefore, in order to accurately measure the characteristic value of the photosensitive thin film, not only the measurement at the photosensitive wavelength but also the measurement at the non-photosensitive wavelength are performed. As shown in FIG. 14 (c), the non-photosensitive wavelength λa
When the measurement light 32 is used, the light 33d passing through the film to be measured is not attenuated in the film, so that it is possible to detect interference fringes of good contrast sufficiently including information on the film thickness. Then measurement light 3
First, the thickness of the film 21 to be measured, the refractive index at the non-photosensitive wavelength λa, and the absorption coefficient are obtained from the interference fringe when the wavelength 2 is the non-photosensitive wavelength λa. Next, the wavelength of the measurement light 32 is changed to the photosensitive wavelength λb.
Measured as Here, the film thickness obtained by the measurement at the non-photosensitive wavelength λa may be treated as a known value, and the refractive index and the absorption coefficient at the photosensitive wavelength λb may be obtained.

The present invention focuses on the fact that the absorption coefficient of the photosensitive thin film has spectral characteristics, and obtains the refractive index and the absorption coefficient at the film thickness and the exposure wavelength by measuring at two wavelengths, the non-photosensitive wavelength and the photosensitive wavelength. Method. Hereinafter, the present invention will be referred to as a two-wavelength measurement method.

FIG. 15 shows an embodiment of a thin film characteristic value measuring apparatus based on the above two-wavelength measuring method. In the present embodiment, an optical system capable of exchanging the non-photosensitive wavelength transmitting monochromatic optical filter 4a and the photosensitive wavelength transmitting monochromatic optical filter 4b, and capable of processing a detection signal at each wavelength. The detection signal processing system in the non-photosensitive wavelength measurement includes a measured film memory 8a that stores the reflected light intensity distribution of the measured film, a reference sample memory 9a that stores the reflected light intensity distribution of the reference sample, and a dark level of the sensor. The memory comprises a dark level memory 12a, a reflectance calculating unit 10a for calculating an incident angle dependent intensity reflectance of the film to be measured, and a characteristic value determining unit 11a for calculating a thin film characteristic value.

On the other hand, the detection signal processing system in the photosensitive wavelength measurement includes a film memory 8b for storing the reflected light intensity distribution of the film to be measured, a reference sample memory 9b for storing the reflected light intensity distribution of the reference sample, and a sensor. A dark level memory 12b for storing a dark level, a reflectance calculating unit 10b for obtaining an incident angle dependent intensity reflectance of a film to be measured, and a characteristic value determining unit 1 for obtaining a thin film characteristic value.
1b. The characteristic value determining section 11b for the photosensitive wavelength uses the film thickness d obtained by the characteristic value determining section 11a for the non-photosensitive wavelength as a known value to determine the refractive index n and the absorption coefficient k at the photosensitive wavelength. Of course, the characteristic value determining unit 11b for the photosensitive wavelength treats the film thickness d obtained by the characteristic value determining unit 11a for the non-photosensitive wavelength as a rough detection result, determines the film thickness again by limiting the film thickness range to d ± Δd. May be.

The two-wavelength measurement method is not only useful for measuring a photosensitive thin film, but also effective for reducing the amount of calculation for determining film characteristic values. Possible thickness d, refractive index n,
Maximum absorption coefficient k, showed minimal max, with subscripts min, the measurement resolution d _r, n _r, and k _r. 3 for one wavelength measurement
(Equation 9) or necessary to determine the three unknowns
The number of calculations C _{1 in} (Equation 10) is, as shown in (Equation 11),

[0097]

[Equation 11]

Is represented by Here, Nd, Nn, and Nk are decomposition points of each characteristic value. On the other hand, in the two-wavelength measurement, if the absorption coefficient at the non-photosensitive wavelength is treated as 0, and the film thickness in the photosensitive wavelength measurement is treated as known, the number of calculations C ₂ becomes

[0099]

(Equation 25)

Is obtained. here

[0101]

(Equation 26)

Therefore, when both Nd and Nk are larger than 2, C ₁ > C ₂ is satisfied. In thin film property measurement, Nd and Nk are generally much larger than 2, so that 2
By measuring the wavelength, the amount of calculation can be greatly reduced and the characteristic value can be determined at high speed.

Next, another embodiment for accurately measuring the characteristic value of an absorptive photosensitive thin film will be described.

In this embodiment, the measurement at the photosensitive wavelength is also performed after the exposure to the measurement light for a sufficiently long time. FIG. 14 (a)
As shown in (2), when the photoresist film is exposed by irradiating the photoresist film with light having the photosensitive wavelength λb, the light energy is absorbed, the composition is changed, and the absorption coefficient is reduced. In this state, FIG.
As shown in FIG. 4D, even when the measuring light 32 having the photosensitive wavelength λb is used, the attenuation of the light 33d passing through the film to be measured in the film is reduced, so that the interference fringe including the information on the film thickness can be detected. . Therefore, the interference fringes s when the measurement light 32 having the photosensitive wavelength λb starts to be irradiated and the interference fringes e when the film 21 to be measured is sufficiently exposed to the measurement light 32 and the absorption coefficient is reduced are detected. And from first interference fringes e, determine the thickness d of the post-exposure, the refractive index n _e, the absorption coefficient k _e. Next, assuming the film thickness d obtained here as a known value, the refractive index n _s at the start of measurement from the interference fringe _s.
And determine the absorption coefficient k _s.

When the film thickness fluctuates before and after the exposure,
Restricting the film thickness range to d ± Δd, the refractive index n _s and the absorption coefficient k _s
At the same time, the film thickness at the start of the measurement may be determined again. The present invention focuses on the fact that the absorption coefficient of a photosensitive thin film decreases after exposure, and uses a detection result before and after exposure to determine a refractive index and an absorption coefficient at a film thickness and an exposure wavelength. Less than,
The present invention will be referred to as a pre- and post-exposure measurement method.

FIG. 16 shows an embodiment of a thin film characteristic value measuring apparatus based on the above-mentioned pre- and post-exposure measuring methods.

In this embodiment, an optical system similar to that of FIG. 6, a reference sample memory 9 for storing the reflected light intensity distribution of the reference sample, a dark level memory 12 for storing the dark level of the sensor, and a film characteristic value after exposure are stored. It consists of a signal processing system to be obtained and a signal processing system to obtain a film characteristic value at the start of measurement. The signal processing system for calculating the film characteristic value after exposure includes a film memory 8e for storing the reflected light intensity distribution of the film after exposure, a reflectance calculator 10e, and a characteristic value determiner 11e. A signal processing system for obtaining a film characteristic value at the start of measurement includes a measured film memory 8s for storing a reflected light intensity distribution at the start of measurement, a reflectance calculation unit 10s, and a characteristic value determination unit 11s.

The characteristic value determining section 11s at the start of measurement uses the film thickness d obtained by the characteristic value determining section 11e after exposure as a known value to determine the refractive index n and the absorption coefficient k at the start of measurement.
Of course, the characteristic value determining unit 11s treats the film thickness d obtained by the characteristic value determining unit 11e as a rough detection result, and sets the film thickness range to d ±
The film thickness may be determined again along with the refractive index n and the absorption coefficient k at the start of the measurement, limited to Δd.

An embodiment in which the present invention is realized by another optical system will be described with reference to FIG.

In this embodiment, the illumination side lens 5 shown in FIG.
And the incident angle θd of the optical axis of the detection-side lens 6 is set to 0 degree, and the illumination-side lens 5 and the detection-side lens 6 are replaced with one objective lens 41 whose optical axis is perpendicular to the film to be measured. is there.

The optical system of this measuring apparatus includes a light source 1, a collimating lens 2, a polarizing element 3, a non-photosensitive wavelength transmitting monochromatic optical filter 4a, a photosensitive wavelength transmitting monochromatic optical filter 4b, a half mirror 55, and a photoelectric conversion type. It comprises a light quantity detector 58, a half mirror 43, an objective lens 41, a relay lens 42, and a light quantity accumulation type linear sensor 7. The relay lens 42
The intensity distribution on the rear focal plane 34 of the objective lens 41 is projected on the sensor surface of the linear sensor 7. The signal processing system includes a light amount variation correction unit 80, an averaging unit 13, a film memory 8 for storing the reflected light intensity distribution of the film 21 to be measured, a reference sample memory 9 for storing the reflected light intensity distribution of the reference sample 22, A dark level memory 12 for storing a dark level of the sensor; a reflectance calculating unit 10 for obtaining an incident angle dependent intensity reflectance of the film 21 to be measured;
Alternatively, it comprises a characteristic value determining unit 11 for obtaining a thin film characteristic value by using the evaluation function M of Expression 10.

As can be seen from Equation 3, the present optical system can detect interference fringes near the incident angle of 0 ° where an optical path difference easily occurs, and thus has the advantage of improving the measurement sensitivity. Another advantage of this embodiment is that the adjustment of the optical system is facilitated.

Next, another embodiment for improving the measurement accuracy by using the above optical system will be described. First, the cause of accuracy deterioration will be described. One objective lens 41 whose optical axis is perpendicular to the film to be measured
In the optical system using, stray light due to the illumination light 32e passing near the optical axis as shown in FIG. 18 causes a measurement error. Because the vicinity of the optical axis of the objective lens 41 is almost perpendicular to the optical axis, the light reflected by the surface of the objective lens 41 of the illumination light 32e passing near the optical axis is detected by the linear sensor 7 as stray light. Therefore, the reflected light intensity distribution 45 actually detected by the linear sensor 7 has an increased intensity around the optical axis with respect to the reflected light intensity distribution 44 to be detected originally. This causes a measurement error of the film characteristic value. Since this stray light is also generated by reflection between the objective lens 41 and the film 21 to be measured, it cannot be completely removed even if the dark level is detected with the sample retracted.

On the other hand, since the surface away from the optical axis of the objective lens 41 is inclined with respect to the optical axis, the reflected light of the illumination light 32c passing therethrough cannot pass through the relay lens, and therefore does not become stray light.

In order to remove stray light due to the illumination light 32e passing near the optical axis of the objective lens 41, it is effective to apply a coating for preventing reflection of the wavelength of the measurement light to the surface of the objective lens 41. This method may be used when the measurement light has a predetermined wavelength. However, as described above, in order to measure a photosensitive thin film, when measurement light having two wavelengths, photosensitive and non-photosensitive, is used, it is necessary to prevent reflection at both wavelengths. Realizing such an anti-reflection coating is not technically easy, and even if it can be done, the production cost increases. In addition, since the types of photosensitive thin films and the exposure wavelengths vary widely, measuring a large number of types of photosensitive thin films with a single measuring device requires an antireflection coating over a wide wavelength range, which is difficult to achieve. is there. Therefore, a stray light prevention method other than the antireflection coating is required.

FIG. 19 shows an embodiment of the stray light prevention method without using the antireflection coating. In this embodiment, the principle diagram of a method of blocking stray light near the optical axis of the objective lens 41 by blocking the illumination light rays near the optical axis and detecting the reflected light intensity distribution on the back focal plane by the linear sensor 7 is shown. Is shown. In the embodiment shown in FIG. 9A, the illumination light passing near the optical axis is shielded by the light shielding plate 46, reflected by the half mirror 43, and made incident on the objective lens 41. The illumination light 32f is reflected by the film 21 to be measured, detected by the area 7f of the linear sensor 7, and
2g is detected in region 7g. Since the vicinity of the optical axis is not illuminated, the reflected light intensity distribution on the back focal plane can be detected without including stray light.

In the embodiment shown in FIG. 11B, half of the back focal plane including the illumination light passing near the optical axis is shielded by the light shielding plate 47, reflected by the mirror 48, and made incident on the objective lens 41. Things. The illumination light 32f is reflected by the film to be measured 21 and detected by the linear sensor 7. Also in this embodiment, since the vicinity of the optical axis is not illuminated, the reflected light intensity distribution on the back focal plane can be detected without including stray light.

When a photosensitive thin film is measured at a photosensitive wavelength, the chemical composition of the film to be measured gradually changes due to illumination light. For this reason, it is necessary to secure the amount of reflected light that can be detected by the linear sensor 7 at a sufficient S / N ratio while suppressing the exposure of the photosensitive thin film. In the embodiment shown in FIG. 19B, the light reflected from the film to be measured 21 reaches the linear sensor 7 directly. Therefore, even if the illumination light amount applied to the film to be measured 21 is the same, a larger reflected light amount can be obtained as compared with the embodiment of FIG. Since the reflected light intensity distribution on the back focal plane can be measured accurately without exposing the film to be measured much, the configuration of the optical system shown in FIG. .

FIG. 1 shows an embodiment of a thin film characteristic value measuring apparatus according to the present invention which prevents stray light near the optical axis based on the principle of the embodiment of FIG. 19 (b). The present embodiment shows a configuration of an optical system that can apply a two-wavelength measurement method for measuring a photosensitive thin film and can simultaneously perform measurement at two wavelengths of photosensitive / non-photosensitive.

First, an embodiment of an optical system for preventing stray light near the optical axis will be mainly described. The optical system shown in FIG.
It consists of a light quantity detection system, a back focal plane detection system, and a sample plane detection system.
In the figure, the illumination light is shown by a solid line, and the light reflected from the sample surface is shown by a broken line. The illumination system includes a mercury lamp light source 1, a condenser lens 48, a heat ray absorbing filter 49 for absorbing infrared rays, a shutter 50, a lens 51, a filter 4c, a monochromatic optical filter 4d for transmitting non-photosensitive wavelengths, a light shielding plate 47, a lens 52, and a visual field. It comprises an aperture 53, a lens 54, a polarizing plate 3, a right angle mirror 60, a field lens 61, an imaging lens 62, and an infinity correction system objective lens 41. Light quantity detection system is half mirror 5
5, a dichroic mirror 56, a non-photosensitive wavelength transmitting monochromatic optical filter 57, a non-photosensitive wavelength light quantity detector 58a, and a photosensitive wavelength light quantity detector 58b.

The back focal plane detecting system includes an objective lens 41, an image forming lens 62, a right angle mirror 60, a field lens 61,
A relay lens 67, a dichroic mirror 68, a monochromatic light filter 69 for transmitting a non-photosensitive wavelength, a linear sensor 7a for detecting a back focal plane intensity distribution at a non-photosensitive wavelength, and a linear sensor 7b for detecting a rear focal plane intensity distribution at a photosensitive wavelength. . The sample surface detection system includes an objective lens 41, an imaging lens 62, a right angle mirror 60, a field lens 61, a half mirror 66, a non-photosensitive wavelength transmitting monochromatic light filter 71, and a TV camera 72.
Consists of The substrate 23 on which the film 21 to be measured is formed, the reference sample 22, and the sample surface light amount detector 63 are mounted on the mounting table 64 on the moving mechanism 65, and can be arbitrarily moved to the measurement positions. Further, the mounting table 64 can be retracted from the measurement position. The sample surface light amount detector 63 is installed to measure the irradiation light amount of the measurement light having the photosensitive wavelength.

The light source 1 forms an image by the condenser lens 48 and makes it incident on the lens 51. The focal point of the lens 51 is apart from the light source image forming position to the lens 51,
7 is located at the rear focal position of the lens 51. Light shield plate 47
Is projected on the orthogonal plane 34b of the right-angle mirror 60 by the lens 52 and the lens 54. The orthogonal plane 34b is formed by the field lens 61 and the image forming lens 62 by the objective lens 4
1 has a conjugate positional relationship with the back focal plane 34. Therefore, the light-shielding plate 47 and the rear focal plane 34 of the objective lens 41 have a conjugate positional relationship, and by adjusting the position of the light-shielding plate 47, illumination light can be prevented from reaching around the optical axis of the objective lens 41. And stray light can be prevented.

The shutter 50 is provided so as not to irradiate unnecessary light to the non-measurement film. Also, the film to be measured 2
If you want to observe the sample surface without exposing 1, filter 4
c may be replaced with the non-photosensitive wavelength light transmitting filter 4d.

The light quantity detectors 58a and 58b are orthogonal to the orthogonal plane 34b.
In each case, a detector having a photosensitive surface capable of detecting all illumination light beams passing through the light shielding plate 47 is used as the photoelectric conversion surface. The detection signals of the light amount detectors 58a and 58b are for observing the fluctuation of the light amount of the light source 1, and if this detection signal is used, the fluctuation of the reflected light intensity distribution detection signal due to the fluctuation of the light amount can be corrected, and the measurement accuracy can be improved.

The light source image formed before the lens 51 is re-formed by the lenses 51 and 52 at the position where the field stop 53 is installed. The field stop 53 is projected by a lens 54 and a field lens 61 onto a first image plane 70 of the sample surface. The first imaging position 70 on the sample surface is a position where an image is formed on the sample surface by the objective lens 41 and the imaging lens 62 of the infinity correction system. Accordingly, the field stop 53 is in a conjugate positional relationship with the sample surface, and by adjusting the diameter of the field stop 53, the area of the sample surface to which the illumination light is irradiated can be limited.

The light reflected by the film 21 to be measured forms an image of the sample surface on the first imaging surface 70 by the objective lens 41 and the imaging lens 62 of the infinity correction system. The sample surface image on the first image forming surface 70 is reflected by the right-angle mirror 60 and the half mirror 66, and then formed on the TV camera 72 by the image forming action of the field lens 61. In this embodiment, since the illumination light is obliquely incident on the sample, if the height position of the sample surface changes, the TV
The sample surface image picked up by the camera 72 moves in the left-right direction on the paper. Therefore, by detecting the position in the horizontal direction of the sample surface image detected by the TV camera 72, it is possible to realize automatic focusing in which the height position of the sample surface is kept constant.

The light reflected by the film to be measured 21 produces a reflected light intensity distribution (equiangular interference fringes) corresponding to the incident angle dependent intensity reflectance on the rear focal plane 34 of the objective lens 41 of the infinity correction system. . The reflected light intensity distribution on the back focal plane 34 first forms an image on the orthogonal plane 34 b of the right-angle mirror 60 by the imaging lens 62 and the field lens 61. However, the imaging position is on the opposite side of the illumination light passing position. The reflected light intensity distribution on the focal plane 34 after being imaged on the orthogonal plane 34b is reflected by the right-angle mirror 60 and then imaged on the linear sensors 7a and 7b by the lens 67. And the linear sensors 7a, 7b
The characteristic value of the film to be measured is obtained from the detection signal of (1).

Next, in this embodiment, a method of simultaneously performing two-wavelength measurement for measuring a photosensitive thin film at two wavelengths of photosensitive and non-photosensitive will be described.

In order to perform two-wavelength measurement at the same time, the filter and the like used in the above optical system may have the following characteristics. FIG. 2A shows the spectral intensity characteristics of the mercury lamp light source 1. As shown in the figure, the light emitted from the mercury lamp is
There are wavelengths called emission lines having a very strong light emission intensity, and these are generally abbreviated to i-line (365 nm), e-line (546.1 nm), and the like in English. FIG. 4D shows an example of the spectral characteristic of the absorption coefficient of a photoresist used for exposing a semiconductor.

Here, the photosensitive wavelength is i-line, and the non-photosensitive wavelength is e.
A description will be given of an embodiment in which the two-wavelength measurement described above in the present invention is simultaneously performed as a line.

As the filter 4c of the illumination system shown in FIG. 1, a filter having the characteristic shown in FIG. The filter 4c is an i-line monochromatic light filter, but in this type of filter, a transmission band called a sub-transmission band is generated on the long wavelength side in principle. In this embodiment, this sub-transmission band is positively used. FIG. 3C shows the spectral intensity characteristics when the light from the mercury lamp light source 1 passes through the filter 4c. In the photosensitive wavelength region, the light becomes monochromatic light of only the i-line, but a plurality of bright lines remain in the non-sensitive wavelength. The film 21 to be measured is illuminated with illumination light having such spectral characteristics, and reflected light is detected.

The dichroic mirror 68 of the back focal plane detecting system shown in FIG. 1 has a characteristic shown in FIG. In this case, the transmission side of the dichroic mirror 68 becomes i-line monochromatic light having a photosensitive wavelength. Therefore, the linear sensor 7b
Then, the reflected light intensity distribution on the back focal plane 34 of the objective lens 41 can be detected with i-line monochromatic light. Dichroic mirror 6
Since a plurality of bright lines of a non-photosensitive wavelength are included on the reflection side of 8, a non-photosensitive wavelength transmitting monochromatic optical filter 69 shown in FIG. In this way, the linear sensor 7a emits monochromatic light of the e-line and outputs the rear focal plane 3 of the objective lens 41.
4 can be detected. Optical elements having the same characteristics may be used for the dichroic mirror 56 and the non-photosensitive wavelength transmitting monochromatic optical filter 57 of the light amount detection system.

By using the filter and the dichroic mirror having the above characteristics and using the optical system having the configuration shown in FIG. 1, the detection of the reflected light intensity distribution at the back focal plane and the detection of the amount of light at the photosensitive wavelength and the non-photosensitive wavelength can be performed simultaneously. As a result, switching between the photosensitive wavelength and the non-photosensitive wavelength becomes unnecessary, and the thickness, refractive index, and absorption coefficient of the photosensitive thin film can be measured at high speed. It should be noted that the monochromatic optical filter 71 for transmitting light-insensitive wavelength inserted into the sample surface detection system of FIG.
Also, if a material having the characteristics shown in FIG. 3F is used, the influence of chromatic aberration can be reduced and a good sample surface image can be obtained.

FIG. 3 shows an embodiment of the signal processing system of the thin film measuring apparatus of FIG. The non-photosensitive wavelength signal processing system includes a light amount fluctuation correction unit 80 for inputting signals from the linear sensor 7a and the light amount detector 58a, a film memory 8a for storing the reflected light intensity distribution of the film to be measured, and a reflected light of the reference sample. A reference sample memory 9a for storing the intensity distribution, a dark level memory 12a for storing the dark level of the linear sensor 7a, a reflectance calculator 10a for obtaining an incident angle dependent intensity reflectance of a film to be measured, and a characteristic value determination for obtaining a thin film characteristic value. It comprises a part 11a.

The reflectivity calculating section 10a includes a difference circuit 90a,
The calculation of Equation 24 is performed by a reference circuit 91a, a division circuit 92a, a theoretical reflectance calculation unit 93a for a reference sample at a non-photosensitive wavelength, and a multiplication circuit 94a. The signal processing system for the photosensitive wavelength has a similar configuration. However, the characteristic value determining unit 11b for the photosensitive wavelength
Calculates the refractive index n and the absorption coefficient k at the photosensitive wavelength using the film thickness d obtained by the non-photosensitive wavelength characteristic value determining unit 11a as a known value. Of course, the characteristic value determining unit 11 for the photosensitive wavelength
b is the film thickness d obtained by the non-photosensitive wavelength characteristic value determination unit 11a.
May be treated as a rough detection result, the film thickness range may be limited to d ± Δd, and the film thickness may be determined again together with the refractive index and the absorption coefficient at the photosensitive wavelength.

In the above embodiment, the measurement of the photosensitive wavelength and the non-photosensitive wavelength are performed at the same time, but the wavelength may be limited by replacing the filter as shown in FIG. In this case, only one set of the light amount detector of the optical system and the linear sensor for detecting the back focal plane reflected light intensity distribution may be used.
In this case, the signal processing system could be omitted as appropriate. Further, the light source 1 is a mercury lamp, but any other light source such as a mercury xenon lamp may be used as long as it emits light having a photosensitive / non-sensitive wavelength of the film to be measured.

In the above embodiment, the linear sensor having the photoelectric conversion elements arranged one-dimensionally is used as the reflected light intensity distribution detector on the back focal plane. However, the TV camera having the photoelectric conversion elements arranged two-dimensionally is used. May be used. If a two-dimensional detector is used, a two-dimensional image of the back focal plane can be observed, and thus there is an advantage that adjustment of the optical system is facilitated.

In the embodiments, the critical illumination method in which a light source image is formed on a sample surface has been described as an illumination method. However, in the present invention, the influence of uneven illuminance can be eliminated by using a reference sample. Koehler illumination that forms an image of a light source on a focal plane may be used.

As described above, according to the present invention, the absolute value of the incident angle dependent intensity reflectance of the film to be measured can be accurately obtained by detecting the equi-tilt interference fringes.
It becomes possible to measure the refractive index and the absorption coefficient with high accuracy and at high speed. Also in the measurement of photosensitive thin films that are industrially important, the refractive index and absorption coefficient at the photosensitive wavelength can be measured with high precision and high speed by detecting at two wavelengths, the photosensitive wavelength and the non-photosensitive wavelength, focusing on the spectral characteristics of the photosensitive material. Can be measured.

[0140]

According to the present invention, the thickness of a thin film, the refractive index,
The absorption coefficient can be measured with high accuracy, and is particularly effective for measuring a photosensitive thin film such as a photoresist. For example, in the semiconductor manufacturing industry, variations in the thickness, refractive index, and absorption coefficient of a photoresist film that degrade pattern dimensional accuracy can be easily detected, so that photoresist quality control and operation monitoring of a coating device can be performed with higher precision than before. . This can greatly contribute to improvement in semiconductor manufacturing yield. In addition, a similar effect can be obtained in various industrial fields using a lithography technique using a photosensitive thin film.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a thin film characteristic value measuring device of the present invention.

FIG. 2 is an explanatory diagram of optical element characteristics used in the thin film characteristic value measuring device of the present invention.

FIG. 3 is a functional block diagram of a signal processing system of the thin film characteristic value measuring device of the present invention.

FIG. 4 is a diagram illustrating the principle of equi-tilt interference.

FIG. 5 is a theoretical calculation example of an incident angle dependent intensity reflectance.

FIG. 6 is a configuration diagram of an apparatus that obtains an incident angle dependent intensity reflectance of a film to be measured by using a mirror surface reference sample and measures a thin film characteristic value.

FIG. 7 is an explanatory diagram of a relationship between an incident angle of a parallel light beam and a condensing position on a back focal plane.

FIG. 8 is a principle diagram of a method for obtaining an incident angle dependent intensity reflectance of a film to be measured by using a mirror reference sample.

FIG. 9 is an explanatory diagram of a procedure for obtaining an incident angle dependent intensity reflectance of a film to be measured.

FIG. 10 is a configuration diagram of a thin film characteristic value measuring device that improves measurement accuracy by averaging processing.

FIG. 11 is a configuration diagram of a thin film characteristic value measuring device that improves measurement accuracy by correcting illumination light amount fluctuation.

FIG. 12 is a principle diagram of illumination light amount fluctuation correction.

FIG. 13 is a functional block diagram of an illumination light amount fluctuation correction circuit.

FIG. 14 is a diagram illustrating the principle of a two-wavelength measurement method and a measurement method before and after exposure for measuring a photosensitive thin film.

FIG. 15 is a configuration diagram of a thin film characteristic value measuring apparatus to which a two-wavelength measuring method is applied.

FIG. 16 is a configuration diagram of a thin film characteristic value measuring apparatus to which a method before and after exposure is applied.

FIG. 17 is a configuration diagram of a thin film characteristic value measuring device using a lens whose optical axis is perpendicular to a film to be measured in a detection optical system.

FIG. 18 is an explanatory diagram of stray light generated in a detection optical system using a lens whose optical axis is perpendicular to a film to be measured.

FIG. 19 is an explanatory diagram of a method for preventing stray light generated in a detection optical system using a lens whose optical axis is perpendicular to a film to be measured.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Light source 2 ... Collimating lens 3 ... Polarizing plate
DESCRIPTION OF SYMBOLS 4 ... Monochromatic light transmission filter 4a ... Monochromatic light filter for non-photosensitive wavelength transmission 4b ... Monochromatic light filter for photosensitive wavelength transmission 4c ... Filter 4d ... Non-photosensitive wavelength transmission filter 5 ... Illumination side lens 6 ... Detection side lens 7
... Linear sensor 7a ... Linear sensor for non-photosensitive wavelength light detection 7b ... Linear sensor for photosensitive wavelength light detection 8 ...
Membrane memory to be measured 9 ... Reference sample memory 10 ... Reflectance calculator 11 ... Characteristic value determiner 12 ... Dark level memory 13 ... Averaging unit 21 ...
Film under measurement 22 Reference sample 23 Substrate on which film to be measured is formed 24 Interface between air and film to be measured 25 Interface between substrate on which film to be measured is formed and film to be measured
31: Irradiation unevenness of illumination light 32: Illumination light 33: Reflected light 33c: Reflected light of the film to be measured 33d: Light passing through the film to be measured 34: Rear focal plane of the detection side lens 36 ... Optical axis of the detection side lens 41 ... Objective lens 42 ... Relay lens 43: half mirror 44: reflected light intensity distribution not including stray light
45: reflected light intensity distribution including stray light 46: light shielding plate 47: light shielding plate 49: heat ray absorption filter 50
… Shutter 51… Lens 52… Lens 53… Field stop
54 ... lens 55 ... half mirror 56 ... dichroic mirror 57 ... monochromatic light filter for non-photosensitive wavelength transmission 58 ... light quantity detector 58a ... non-photosensitive wavelength light quantity detector 58b ... photosensitive wavelength light quantity detector 60 ... right angle mirror 61 ... field lens 62 ... Imaging lens 63 ... Sample surface light quantity detector 64
… Mounting table 65… Moving mechanism 66… Half mirror
67: relay lens 68: dichroic mirror 69: monochromatic optical filter for non-photosensitive wavelength transmission 70:
Half mirror 71: Monochromatic light filter for non-photosensitive wavelength transmission 72: TV
Camera 73: dark level storage device 80: light quantity fluctuation correction unit 81: integration circuit 83: A / D converter
84 sample-hold circuit 85 A / D converter 86 digital divider 87 analog divider 88 A / D converter 80 light intensity fluctuation correction unit
Reference numeral 90: difference circuit 91: difference circuit 92: division circuit 93: theoretical reflectance calculation unit of a reference sample 94: multiplication circuit

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Hitoshi Kubota 292 Yoshida-cho, Totsuka-ku, Yokohama-shi, Kanagawa Pref. JP-A-64-12208 (JP, A) JP-A-64-75902 (JP, A) JP-A-1-262404 (JP, A) JP-A-2-128106 (JP, A) JP-A-3-17505 (JP) JP-A-4-109147 (JP, A) JP-A-4-313006 (JP, A) JP-A-5-141924 (JP, A) JP-B-62-49562 (JP, B2) (58) Field surveyed (Int.Cl. ⁷ , DB name) G01B 11/06 G01N 21/45 G01N 21/59

Claims (22)

(57) [Claims] 1. A method for measuring a characteristic value of a thin film from an equi-tilt interference fringe of a thin film sample, comprising the steps of: detecting an intensity distribution of the equi-tilt interference fringe of the thin film sample; From the intensity distribution detected value of the equi-tilt interference fringes and the theoretical calculation value of the incident angle dependent intensity reflectance characteristic at the measurement wavelength of the reference sample, the incident angle dependent intensity reflectance characteristic of the thin film sample is calculated, and the film thickness of the thin film is calculated. A method for measuring characteristic values of a thin film, comprising measuring a refractive index and an absorption coefficient. 2. The method according to claim 1, wherein the incident angle-dependent intensity reflectance characteristic of the thin film sample and a set of an arbitrary film thickness, refractive index, and absorption coefficient are substituted to calculate the incident light. A thin film characteristic value measuring method comprising comparing angle-dependent intensity reflectivity characteristics and outputting a set of a film thickness, a refractive index, and an absorption coefficient as a measurement result when the two coincide with each other. 3. The method according to claim 1, wherein an incident angle-dependent intensity reflectance of the thin film sample is determined by using a detected intensity distribution on an oblique interference fringe observation surface when the sample is retracted. A method for measuring a characteristic value of a thin film, comprising calculating a characteristic. 4. The thin film characteristic value measuring method according to claim 1, wherein the intensity distribution of the equi-tilt interference fringes is detected a plurality of times, and the average value is used as a detected value of the intensity distribution of the equi-tilt interference fringes. Thin film characteristic value measuring method. 5. The method for measuring a characteristic value of a thin film according to claim 1, wherein the detected values of the intensity distributions of the equi-tilt interference fringes of the thin film sample and the reference sample are corrected using the detected value of the illumination light amount. And calculating an incident angle dependent intensity reflectance characteristic of the thin film. 6. A thin film characteristic value measuring method according to claim 1, wherein a film thickness is obtained from an intensity distribution detected value of the equi-tilt angle interference fringe at a wavelength at which the absorption coefficient of the thin film sample is small, and the film thickness value and the thin film sample are determined. A method for measuring a characteristic value of a thin film, comprising measuring a refractive index and an absorption coefficient from an intensity distribution detection value of an equitilt interference fringe at a wavelength having a large absorption coefficient. 7. A method for measuring characteristic values of a thin film according to claim 6, wherein the detected values of the intensity distributions of the equitilt interference fringes at the wavelength where the absorption coefficient of the thin film sample is small and the wavelength where the absorption coefficient is large are simultaneously detected. Thin film characteristic value measurement method. 8. The method of measuring thin film characteristics according to claim 1, wherein a film thickness is obtained from a detected value of the intensity distribution of equi-tilt interference fringes at a non-photosensitive wavelength of the photosensitive thin film sample. A method for measuring a characteristic value of a thin film, comprising measuring a refractive index and an absorption coefficient from an intensity distribution detected value of an equi-tilt interference fringe at a photosensitive wavelength of a sample. 9. A thin film characteristic value measuring method according to claim 8, wherein the intensity distribution detection values of the equi-tilt interference fringes at the non-photosensitive wavelength and the photosensitive wavelength of the photosensitive thin film sample are simultaneously detected. Value measurement method. 10. A method for measuring characteristic values of a thin film according to claim 1, wherein the photosensitive thin film sample is sufficiently exposed to light having a photosensitive wavelength to reduce the absorption coefficient, and the intensity distribution of the equi-tilt interference fringes is used to determine the value. The film thickness of the photosensitive thin film sample at the start of exposure is determined using the film thickness value and the intensity distribution detection value of the equi-tilt interference fringes of the photosensitive thin film sample at the start of exposure. A method for measuring characteristic values of a thin film, comprising measuring a refractive index and an absorption coefficient at a thickness and a photosensitive wavelength. 11. The thin film characteristic value measuring method according to claim 1, wherein the thin film sample is illuminated with convergent light that does not include illumination light from an almost vertical direction using an objective lens whose optical axis is perpendicular to the thin film sample. A thin film characteristic value measuring method characterized by detecting an intensity distribution of equi-tilt interference fringes of a thin film sample. 12. A method for measuring a characteristic value of a thin film according to claim 11, wherein the illumination light and the reflected light from the thin film sample do not pass through the same position on the back focal plane of the objective lens. A method for measuring characteristic values of a thin film, comprising detecting an intensity distribution of a thin film. 13. A method for measuring a characteristic value of a thin film from equidistant interference fringes of a thin film sample, wherein the film thickness is obtained from a detected intensity distribution value of the equitilt interference fringes at a wavelength having a small absorption coefficient of the thin film sample. And measuring a refractive index and an absorption coefficient from an intensity distribution detection value of an equi-tilt interference fringe at a wavelength having a large absorption coefficient of the thin film. 14. A method for measuring a characteristic value of a thin film according to claim 13, wherein the intensity distribution detected values of the equi-tilt interference fringes at the wavelength where the absorption coefficient of the thin film sample is small and the wavelength where the absorption coefficient is large are simultaneously detected. Thin film characteristic value measurement method. 15. A method for measuring a characteristic value of a thin film from equidistant interference fringes of a thin film sample, wherein a film thickness is obtained from an intensity distribution detection value of the equitilt interference fringes at a non-photosensitive wavelength of the photosensitive thin film sample. And measuring a refractive index and an absorption coefficient from a detected value of an intensity distribution of equi-tilt interference fringes at a photosensitive wavelength of a photosensitive thin film sample. 16. A thin film characteristic value measuring method according to claim 15, wherein the intensity distribution detection values of the equi-tilt interference fringes at the photosensitive wavelength and the non-photosensitive wavelength of the photosensitive thin film sample are simultaneously detected. Value measurement method. 17. A light source, an optical element for limiting a wavelength, a polarizing element, an illumination lens for irradiating convergent light to a sample, a detection lens for capturing divergent light reflected from the sample, and the detection lens. A thin film characteristic value measuring device having a device for detecting an intensity distribution on a back focal plane of the thin film sample, wherein the detected value of the back focal plane intensity distribution of the thin film sample and the back focal plane intensity of the reference sample whose refractive index and absorption coefficient are known A device that calculates the incident angle dependent intensity reflectance characteristic of the thin film sample from the distribution detection value and the theoretical calculation value of the incident angle dependent intensity reflectance characteristic of the reference sample, and the film thickness of the thin film from the incident angle dependent intensity reflectance characteristic, A thin film characteristic value measuring device comprising a device for determining a refractive index and an absorption coefficient. 18. A thin film characteristic value measuring apparatus according to claim 17, wherein the illumination lens and the detection lens are shared by a single objective lens whose optical axis is perpendicular to the thin film sample, and the illumination light from a substantially vertical direction. An optical system for illuminating a thin film sample with convergent light that does not include a thin film. 19. An apparatus according to claim 18, wherein the illumination light and the reflected light from the thin film sample do not pass through the same position on the back focal plane of the objective lens. Thin film characteristic value measuring device. 20. The thin film characteristic value measuring device according to claim 17, further comprising an optical element for limiting an illumination wavelength to a wavelength having a small absorption coefficient and a wavelength having a large absorption coefficient of the thin film sample. . 21. The thin film characteristic value measuring device according to claim 17, wherein an optical system for simultaneously detecting the intensity distribution of equi-tilt interference fringes at a wavelength where the absorption coefficient of the thin film sample is small and a wavelength where the absorption coefficient is large is provided. Thin film characteristic value measuring device. 22. The thin film characteristic value measuring device according to claim 17, wherein a thin film reference sample is simultaneously mounted on a mounting table.