JP2510418B2 - Beam scanning interferometry film thickness measuring device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Object of the invention [Industrial field of use] The present invention relates to a film thickness measuring device for measuring the film thickness of a thin film by using an interferometry method.

In recent years, the technique of measuring the film thickness of a thin film or a polymer film attached to a substrate such as an optical component or LSI has become more important than ever.

[Prior Art] When a resist is applied to the surface of a thin film, for example, a mask blank of chromium (Cr), and this thin film is measured, conventionally, for example, a surface using a stylus as shown in FIG. Using a roughness measuring instrument, bring the tips of the diamond stylus 201 and skid 202 as shown in FIG. 8 (a) into contact with the chrome surface 203 as shown in FIG. 8 (b), and move it. Te membrane 2
This is to detect the change by crossing over 04 and electrically amplifying the change. For example, when the film diameter on the sample is l ₀ , the measurement result is obtained as shown in FIG. 8 (c).

However, such a mechanoelectric method is not always easy to measure with high precision, and the chrome surface 203 and the film 2
In the case where no step can be formed between the film and 04, that is, when the film 204 is formed on the entire chrome surface,
It is impossible to measure the film thickness with the above-mentioned measuring means. Therefore, the development of a thin film thickness measuring technique is desired.

Therefore, the inventor of the present invention has previously proposed a film thickness measuring device capable of measuring the film thickness of a thin film with high precision and in a non-contact manner (see Japanese Patent Application No. 040467 in 1985). In addition, a film refractive index measuring device that can easily measure the refractive index of a thin film with high accuracy and in a non-contact manner has also been proposed (see Patent Application No. 135756 in 1985).

This newly proposed interferometric film thickness measuring device and interferometric film refractive index measuring device, as shown in FIG. 9, change the optical path of laser light by mirrors 103 and 104, and then measure the light after passing through a pinhole 105. Illuminating the film 110, the reflected light I _A from the surface of the film 110 to be measured, the back surface of the film 110 to be measured or the reflective surface 112 of the substrate 113.
The second light of the reflected light I _B of interference because there is a difference in optical path length, and detects the interference light to photoelectric conversion by the light receiving unit 107 at the focal position of the lens 106, the rotary device to be measured film 110 114 It is configured to measure the intensity change of the interference light when it is rotated by.

[Problems to be Solved by the Invention] This newly proposed interferometric film thickness measuring device and interferometric film refractive index measuring device facilitate measurement of the film thickness and refractive index of a thin film with high accuracy and in a non-contact manner. Although it has a remarkable characteristic that can be measured, the measurement target film 110 needs to be rotated in the measurement, which causes the structure of the measurement device to be complicated.

The present invention has been made in view of the circumstances as described above, and provides a film thickness measuring device having a simple structure, which can easily measure the film thickness of a thin film with high accuracy and in a non-contact manner. That is the purpose.

(B) Configuration of the Invention [Means for Solving the Problems] To this end, the beam scanning interferometry film thickness measurement apparatus of the present invention illuminates a film body, which is an object to be measured, with laser light. A laser light source to be obtained, an interferometer that forms interference fringes by interfering the light reflected on the surface of the object to be measured and the light transmitted through the object to be measured and reflected on the back surface, and a light intensity measuring device that measures the intensity of the interference fringes. A rotating mirror, a motor for displacing the rotating mirror controlled by an input control pulse, a lens for converting scanning light from the rotating mirror into parallel light, and illumination of the parallel light to generate diffracted light. And a beam scanning device for changing the incident angle of the diffracted light on the object to be measured by the displacement of the rotating mirror.

 Hereinafter, details of the present invention will be described with reference to an embodiment.

In FIG. 1, reference numeral 1 is a film thickness measuring device. The film thickness measuring device 1 includes a laser light source device 2, a measuring optical system 10, and a light intensity measuring device 4. The measurement optical system 10 includes an interferometer 3 and a beam scanning device 5. Laser light source device 2
Has a laser light source 6, a mirror 7, and a mirror 8. As the laser light source 6, a helium neon laser, an argon laser, a krypton laser, a helium cadmium laser, a ruby laser or the like can be used.

The mirrors 7 and 8 are for changing the optical path and guide the laser light from the laser light source 6 to the rotating mirror of the beam scanning device 5.

The beam scanning device 5 includes a rotating mirror 11, a lens 12 and a first
And the hologram 13 of.

The rotary mirror 11 can be rotationally displaced and reflects the laser light incident from the mirror 8. The rotational displacement converts the reflection angle, that is, the incident angle to the film 15 to be measured on the sample table 16. is there. The lens 12 collimates the laser light reflected by the rotating mirror 11 into parallel light and creates a first hologram 13
For illuminating.

The first hologram 13 is in the vicinity of the film 15 to be measured and generates diffracted light to illuminate the film 15 to be measured. When the second hologram 21, which will be described later, is used as the first hologram 13, the first hologram 13 and the second hologram 21 are arranged symmetrically with respect to an axis 17 perpendicular to the measured film 15.

The first hologram 13 and the second hologram 21 are created as follows.

As shown in FIG. 2, the beam from the laser light source 6 is reflected by the mirror 7 and then split into two by the half mirror 22. The beam reflected by the half mirror 22 is the objective lens 2
Mirror 2 after collimated by collimator lens 24
The optical path is changed at 5 and enters the objective lens 26 to become divergent light, which illuminates the photographic dry plate 13a. On the other hand, the laser light transmitted through the half mirror 22 is changed in its optical path by a mirror 27 and then converted into parallel light by an objective lens 28 and a collimator lens 31 to illuminate and expose a photographic dry plate 13a.

This photographic dry plate 13a is developed and subjected to reversal bleaching to form two holograms, which are set at the positions of the first hologram 13 and the second hologram 21. Thus, the beam scanning device 5 and the measuring optical system 10 described later are completed. The interferometer 3 includes a second hologram 21 and a lens 32.
The second hologram 21 reproduces parallel light by the laser light reflected from the film 15 to be measured or the sample table 16. The lens 32 forms an interference fringe at the focal position by the reproduction light. The light receiving surface of the photodetector 35 of the light intensity measuring device 4 is located at this focus position. The light intensity measuring device 4 includes a photodetector 35, an amplifier 36,
A data analysis device 37, an oscilloscope 38, a processing device 41, a pulse stage 42, and a pulse stage controller 43 are provided.

[Operation] The operation in the case of measuring the film thickness of the film-to-be-measured 15 in the film thickness measuring device configured as described above is as follows.

The beam from the laser light source 6 has its optical path changed by the mirrors 7 and 8 and then is irradiated onto the rotating mirror 11.

Rotating mirror rotated by rotating pulse stage 42
The beam oscillated by 11 is collimated by the lens 12 and then diffracted by the first hologram 13 to illuminate the film 15 to be measured. The light reflected on the front surface and the back surface of the measured film 15 interferes with each other, and after being diffracted by the second hologram 21, becomes parallel light and becomes a lens.
At 32, the light is focused on the photodetector 35 and detected. The signal photoelectrically converted by the photodetector 35 is amplified through the amplifier 36, and then input to the data analysis device 37 and the oscilloscope 38,
Data processing, waveform observation.

Each time the pulse stage controller 43 sends one pulse to the rotary pulse stage 42, a light intensity signal is detected and data processing is performed.

The rotating mirror 11 is used by being set on the rotating pulse stage 42, but various methods such as a galvano mirror, a polygon scanner, and a hologram scanner can be used as a method for oscillating the beam.

As a method of condensing the beam, a method of using a lens, a concave mirror or the like can be considered in addition to using a hologram element.

The relationship between the rotation angle of the rotating mirror 11 and the incident angle on the film to be measured 15 will be described.

FIG. 3 shows the relationship between the rotation angle δ of the rotating mirror 11, the lens 12, and the first hologram 13. When the focus O of the collimator lens 12 is ▲ ▼ = J, the deflection angle of the mirror is δ ₁ , the change on the lens 12 at that time is Y ₁ , and the change on the first hologram 13 is X ₁ , ▲ ▼ = Let K be the focus of the first hologram 13 and G be the focus. The deflection angle when changing X ₁ is θ ₁ , ▲
If ▼ ＝ K ₁ , then X ₁ ＝ Y ₁ / cos ψ… (1) Y ₁ ＝ Jtan δ ₁ … (2) Therefore The relationship between the deflection angle δ1 of the rotating mirror 11 and the deflection angle θ ₁ of the convergent light from the first hologram 13 is expressed by equation (3).

First, measure ∠FGN first. Incident angle to sample ∠EGN ∠EGN = ∠FGN + ∠DGF−θ ₁ (4)

This is up to ∠DGN, and those exceeding this are represented by ∠EGN = ∠DGN + θ ₁ (5).

Therefore, with K as the boundary, before and after that, equations (4) and (5)
Use each formula properly.

As shown in FIG. 4, a part of the laser light illuminating the film to be measured 15 is reflected on the surface of the film to be measured 15 (reflected light I _A ),
Remaining part is transmitted through the measured film 15 reaches the back surface 20 of the measured film 15, reflected by the back surface or surface of the sample stage 16 of the measured film 15 (reflected light I _B). Thus, there is a difference between the optical path length of the reflected light I _{A and} the optical path length of the reflected light I _B of the laser light that illuminates the film to be measured 15, and the difference d between the optical path lengths is d = ▲ ▼- It is ▲ ▼.

The reflected light I _A and the reflected light I _B having different optical path lengths interfere with each other to generate an interference fringe at the focal position of the lens 32. A photodetector 35 is located at the focal position of the lens 32, and the photodetector 35 detects the interference light and photoelectrically converts the interference light to generate a voltage according to the intensity of the interference light. This voltage is amplified by the amplifier 36 and input to the processing device 41 to detect the intensity of the interference light. Since the intensity of the interference light changes in accordance with the change in the incident angle of the laser light on the film to be measured 15, the rotating mirror 11 is rotated to change the incident angle of the laser light on the film to be measured 15 to generate the interference light. The intensity is measured, the rotation angle at which the extreme value of the change in the intensity of the interference light is generated is obtained, and the rotation angle at which this extreme value is generated is input to an arbitrary computer (not shown) to obtain the thickness of the film 15 to be measured. .

The incident light at the incident angle θ _{1 and} the incident light at the incident angle θ ₂ may have the same wavelength or different wavelengths.

Next, the principle of obtaining the film thickness of the film 15 to be measured will be described.

[A] First, the case where the wavelengths of both the incident light at the incident angle θ _{1 and} the incident light at the incident angle θ ₂ are λ is as follows.

Consider a case where the laser light is incident on the measured film 15 having the refractive index n and the thickness h as shown in FIG. 4 at an angle θ. Let I _{A be} the reflected light from the front surface of the film to be measured 15 and I _{B be} the reflected light from the back surface.
If the optical path difference d between I _A and I _B is ▲ ▼-▲ ▼ = d (6) Is represented by

The intensity of the interference light in this case is Is represented by

If the optical path difference at the incident angle θ ₁ is d ₁ and the optical path difference at the angle θ ₂ is d ₂ , the position where the intensity of the interference fringes is max and min is d ₁ −d ₂ = (1/2) mλ (M is an integer) (9) Therefore Thickness h is Can be represented by

Therefore, the refractive index n of the film to be measured and the wavelength λ of the incident light are given, and if the incident angles θ ₁ and θ ₂ are obtained from the extreme values of the change of the interference light given by the equation (8), the film to be measured 15 Thickness h
Can be requested.

[B] then the wavelength of the incident light at an incident angle theta ₁ is lambda _1, in the case of _two wavelengths of the incident light lambda at the incident angle theta ₂ is as follows.

The phase at wavelength λ ₁ , angle θ ₁ , wavelength λ ₂ , angle θ ₂ is Is represented. As a result, the film thickness h is Therefore, the refractive index of the film to be measured and the wavelengths λ ₁ and λ _{2 of the} incident light are given, and the incident angles θ ₁ and θ ₂ are divided from the extreme value of the change in the intensity of the interference light given by the equation (8). If the film to be measured 15
Can be obtained.

[C] Further, even when the refractive index is unknown in [A] and [B], the film thickness is obtained as follows. In this case, since it is necessary to first obtain the refractive index of the film to be measured, the principle of obtaining the refractive index will be described.

(A) First, it is as follows when the wavelengths of both the incident light at the incident angle θ _{1 and} the incident light at the incident angle θ ₂ are on.

If the angles θ ₁ , θ ₂ , and θ _{3 at} the adjacent extrema of the interference fringes are the phases, It is expressed as above. Taking the ratio of each difference from these three equations and developing and solving, the refractive index n becomes It is expressed as above.

Therefore, from the extreme value of the change in the intensity of the interference light, the incident angles θ ₁ , θ ₂ ,
If θ ₃ is known, the refractive index n of the film to be measured can be obtained,
The film thickness is obtained by substituting this n into the equation (11) or the equation (14).

(B) Next, when the wavelength of the incident light at the incident angle θ ₁ is λ ₁ , the wavelength of the incident light at the incident angle θ ₂ is λ ₂ , and the wavelength of the incident light at the incident angle θ ₃ is λ ₃ , is there.

The phase at wavelength λ ₁ , angle θ ₁ , wavelength λ ₂ , angle θ ₂ , wavelength λ ₃ , angle θ _{3 at} the extreme value of the interference fringes is Is represented. Taking the ratio of each difference from these three equations and developing and solving, the refractive index n becomes _{^{a = (4λ 2 3 · λ}} 2 1 -λ 2 1 · λ 2 2 -λ 2 2 · λ 2 3) 2 -4λ 2 1 · λ 2 2 · λ 2 3 b = 2 (4λ 2 3 · λ 2 ₁₋ λ ² ₁・ λ ² ₂₋ λ ² ₂・ λ ² ₃ ) × (λ ² ₂・ λ ² ₃・ sin ² θ ₁ -4λ ² ₃・ λ ² ₁・ sin ² θ ₂ + λ ² ₁・ λ ² ₂ · sin ² θ ₃ ) + 4λ ² ₁ · λ ² ₂ · λ ² ₃ × (sin ² θ ₁ + sin ² θ ₃ ) C = (λ ² ₂ · λ ² ₃ · sin ² θ ₁ -4λ ² ₃ · expressed as ^{_{λ 2 1 · sin 2 θ 2}} + λ 2 1 · λ 2 2 · sin 2 θ 3) 2 -4λ 2 1 · λ 2 above ^{_{2 · λ 2 3 × sin 2}} θ 1 · sin 2 θ 3 formula Be done.

Therefore, if the wavelengths λ ₁ , λ ₂ , λ ₃ of the incident light on the film to be measured are given and the incident angles θ ₁ , θ ₂ , θ ₃ are known from the extreme value of the change in the intensity of the interference light, The refractive index n of the film to be measured can be obtained. The film thickness h is obtained by substituting this n into the formula (11) or the formula (14).

[Example] As a sample, a glass substrate was coated with chromium (Cr) to form a substrate having a reflective surface, and magnesium fluoride (MgF ₂ ) was coated on the reflective surface to form a film to be measured. did.
When the thickness of the film to be measured was measured using a He-Ne laser, a value of d = 0.64 μm was obtained. When the same sample was measured by a stylus type surface roughness measuring device, a thickness of about 0.63 μm was obtained. It can be seen that the values obtained by both methods are in good agreement.

[Other Method] A method in which this method is further developed is also conceivable. For example, a method of improving the contrast as shown in FIG. 5 can be considered. It is the same as the embodiment shown in FIG. 1 up to the point where the interference light is diffracted and emitted from the hologram 21, but here, this beam is returned to the original direction by the mirror 44, passes through the hologram 21, and is measured. The hologram reflected by the film 15
After diffracting at 13, the light is returned to the rotating mirror 11 at the lens 12, the reflected beam is reflected by the beam splitter 45, and is condensed on the photodetector 35 by the condensing lens 46 to detect the light intensity. In this method, since the laser beam passes back and forth through the film to be measured 15, the sensitivity is improved, and a signal with good contrast is obtained.
Since the position detection sensitivity of is improved, highly accurate measurement becomes possible.

There are various conceivable methods for oscillating the beam in addition to those described above. For example, as shown in FIG. 6, a method using a concave mirror 48, a method using a lens 47 (FIG. 7), etc. can be considered.

1 to 7, the same components are designated by the same reference numerals.

(C) Effect of the Invention With the above configuration, highly accurate film thickness measurement can be performed even for an extremely thin film having an extremely small thickness.

When the film thickness of the DUT is in the sub-micron order instead of in the micron order, the interval of the light incident angle on the DUT where the extreme value of the light intensity of the interference fringes occurs becomes large. In order to measure the film thickness of the measurement object with high accuracy, it is necessary to be able to increase the range of the light incident angle on the measurement object.

In order to meet this requirement as much as possible, it is desirable that the light incident lens has a large aperture and a small focal length, that is, a small F number. Conventionally, a microscope objective lens has been used as a lens satisfying such requirements. There is. However, in reality, in order to set the microscope objective lens near the object to be measured, there is a margin around the periphery of the microscope objective lens with the mounting frame, or there is a spatial restriction, and the aperture of the microscope objective lens is used. Since the possible range is limited, the range of light incident angle that can be realized is about 60 ° at maximum, which limits the measurable film thickness of the measured object. However, when the hologram lens (hologram 13) of the present invention is used in place of the microscope objective lens, theoretically, the light incident angle should be selected in the range of about 90 °, and actually about 5 ° to 85 °. Thus, the ultrathin film can be measured. As described above, according to the film thickness measuring apparatus utilizing the interference of the present invention, the film thickness of the thin film can be easily measured with high accuracy and in a non-contact manner. In addition, the film thickness can be measured even when there is no step between the substrate and the film to be measured.

Moreover, since it is not necessary to rotate the film to be measured, the structure of the device can be simplified.

[Brief description of drawings]

FIG. 1 is a structural explanatory view showing a film thickness measuring apparatus according to an embodiment of the present invention, FIG. 2 is a structural explanatory view showing a hologram forming optical system, and FIG. 3 is a rotational angle of a rotating mirror and a film to be measured. 4 is an enlarged explanatory view showing an optical path in a film to be measured, FIG. 5 is a structural explanatory view showing a film thickness measuring device according to another embodiment, and FIG. 6 is further shown. FIG. 7 is a structural explanatory view showing a film thickness measuring device according to another embodiment, FIG. 7 is a structural explanatory view showing a film thickness measuring device according to still another embodiment, and FIG. 8 is a stylus type film thickness measuring device. FIG. 9 and FIG. 9 are configuration explanatory views showing another conventional film thickness measuring device. 1 ... Film thickness measuring device, 2 ... Laser light source device, 3 ... Interferometer, 4 ... Light intensity measuring device, 5 ... Incident angle conversion device, 6 ... Laser light source, 7 ... Mirror, 8 ... …mirror,
10 …… Measuring optical system, 11 …… Rotating mirror, 12 …… Lens,
13 …… first hologram, 13a …… photo plate, 15 …… measured film, 16 …… sample stage, 17 …… axis, 21 …… second hologram, 22 …… half mirror, 23 …… objective Lens, 24 ……
Collimator lens, 25 …… Mirror, 26 …… Objective lens,
27 ... Mirror, 28 ... Objective lens, 31 ... Collimator lens, 32 ... Condensing lens, 35 ... Photodetector, 36 ... Amplifier, 37 ... Data analysis device, 38 ... Oscilloscope, 41
...... Processor, 42 …… Rotating pulse stage, 43 …… Balus stage controller, 44 …… Mirror, 47 …… Lens, 48 …… Concave mirror

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Yasutaka Kikuchi 2-20-9 Shimura, Itabashi-ku, Tokyo Union Optical Co., Ltd. (72) Inventor Michio Namiki 2-20-9 Shimura, Itabashi-ku, Tokyo Union Optical Co., Ltd. (56) Reference JP-A-53-3363 (JP, A) JP-A-58-38808 (JP, A) JP-A-57-113308 (JP, A)

Claims (1)

(57) [Claims] 1. A laser light source capable of illuminating a film body, which is an object to be measured, with a laser beam, and the light reflected on the surface of the object to be measured and the light transmitted through the object to be measured and reflected on the back surface, An interferometer that forms interference fringes, a light intensity measuring device that measures the intensity of the interference fringes, a rotating mirror, a motor that displaces the rotating mirror controlled by an input control pulse, and a rotating mirror. A lens that converts scanning light into parallel light and a hologram that receives illumination of the parallel light to generate diffracted light and illuminates the object to be measured is provided to the object to be measured by displacement of the rotating mirror. And a beam scanning device for changing the incident angle of the diffracted light.