JPS63263402A - Film thickness measuring method - Google Patents

Abstract

PURPOSE:To measure the thickness of a film with high accuracy by measuring the quantity of angle variation until the reflection intensity or transmission intensity of an electromagnetic wave reaches a certain extremal value while varying the angle between the irradiation direction of the electromagnetic wave and a light-transmissive film. CONSTITUTION:Homogeneous light from a laser oscillator 1 travels in the order of a rotary reflection mirror 2, the film (measuring object) 10, and a photodetector 5 and its reflection intensity is measured by the detector 5. The light quantity signal detected by the detector 5 is sent to a CPU 7. The CPU 7 measures continuously the quantity of variation in the incidence angle of the light when the quantity of light varies from a certain extremal value to another extremal value and the relations shown by equations I and II are applied to measure the film thickness (d) of the film 10. In the equations, (d) is the film thickness, lambda the wavelength of the electromagnetic wave (light), (n) the refractive index of the film 10, DELTAtheta1 and DELTAtheta2 variations in angle between the electromagnetic wave and film 10 from the time when the reflection intensity reaches a certain extremal value to the time when the intensity reach other two optional extremal values, and theta0 the incidence angle of the electromagnetic wave when the intensity reach the certain extremal values. Consequently,the film thickness is measured with high accuracy without finding the incidence angle directly.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method of measuring film thickness using electromagnetic wave interference.

[Prior Art] The following devices have been put into practical use as devices for measuring film thickness.

(1) Film thickness meter that uses absorption when electromagnetic waves are irradiated onto a film This is an application of the fact that the amount of electromagnetic waves absorbed by a film shows a correlation with the film thickness, and the absorption coefficient varies depending on the film constituent materials. Since the absorption coefficients are different, it is necessary to select a unique absorption coefficient for each substance to be measured, but it is possible to measure with an accuracy of about 1 to 10 μm. Specific examples include a fluorescent X-ray film thickness meter, a γ-ray film thickness meter, an infrared film thickness meter, a β-ray film thickness meter, and the like.

(2) Ultrasonic film thickness meter The film thickness is measured from the round trip time of ultrasonic waves, and the accuracy is only about 0.1 am.

(3) A capacitive film thickness meter utilizes the change in dielectric constant due to the difference in thickness when the object to be measured is placed between a pair of electrode plates;
Measurements can be made down to the micrometer level.

(4) Mechanical film thickness meter Mechanically measures changes in film thickness and converts the changes electrically using a differential transformer, etc. to measure film thickness, and can also measure to the nearest 10 μm. .

Various techniques have been put into practical use to measure film thickness in this way, such as those that utilize electromagnetic wave absorption, those that utilize changes in capacitance, and those that utilize ultrasound reflection, but none of these techniques However, the accuracy is on the order of 1 to 100 μm, and there is a problem in that it cannot be applied to measurements in fields where accuracy on the A (angstrom) scale is required.

Therefore, the present inventor proposed a method for measuring film thickness by irradiating electromagnetic waves to one point on the film while changing the angle of incidence on the film, and at an incident where the reflected or transmitted intensity caused by interference in the film takes an extreme value. We developed a technology to measure film thickness by measuring angles and filed a patent application (Japanese Patent Application No. 259598/1982).

Although the development of this technique has made it possible to measure film thickness with much higher precision than conventional techniques, there are still some problems that remain with this technique.

[Problems to be Solved by the Invention] When measuring film thickness using such a technique, it is a premise that the angle of incidence on the film is measured. However, as a practical matter, it is extremely difficult to accurately determine the angle of incidence as described below.For example, when measuring film thickness online, it is difficult to accurately determine the angle of incidence due to
There was a problem in that it was difficult to maintain the horizontality of the film due to vibrations, etc., and therefore the angle of incidence could not be determined accurately, resulting in inaccurate film thickness measurements.

The present invention was made in view of such problems, and
Its purpose is to measure the angle of incidence 1
It is an object of the present invention to provide a film thickness measuring method that allows film thickness to be measured with high precision even without the use of film thickness.

[Means for Solving the Problems] The present invention that achieves the above object includes two inventions.

A first invention is a method of measuring film thickness by irradiating electromagnetic waves to one point of a transparent film with a known refractive index, in which the angle between the irradiation direction of the electromagnetic waves and the transparent film is changed. At the same time, the changes in the reflected or transmitted intensity of the electromagnetic waves due to the interference effect in the transparent film are tracked, and the amount of angular change is measured until the intensity reaches a certain extreme value, and the following (4t) is carried out.
This film thickness measurement method has the gist of determining the film thickness by solving the two-dimensional simultaneous equations of equation (12).

... (11) ... - (12) where d: film thickness (unknown) λ: wavelength of electromagnetic wave (known) n: refractive index of the transparent film (known) Δθ3. Δθ, the change in the angle between the electromagnetic wave and the transparent film from when the reflection intensity or transmission intensity shows a certain extreme value to any other two extreme values (known), Δθ1. Each of Δθ2 includes (rt-1) and (ra-2) angles at which the reflected intensity or transmitted intensity takes an extreme value.

θ. : Incident angle of electromagnetic waves when it shows a certain extreme value (unknown) The second invention is a case where a translucent film with an unknown refractive index is targeted, and the irradiation direction of electromagnetic waves and the translucent film are While changing the angle formed with Measure the amount of change in the angle until it shows the value, and calculate the following (ll
) to (13), the gist of this film thickness measurement method lies in determining the film thickness as well as the refractive index.

... (11) ... (12) ・-(13) where d: film thickness (unknown) λ: wavelength of electromagnetic wave (known) n: refractive index of translucent film (known) ΔθhΔθ2. Δθ3: Change in the angle between the electromagnetic wave and the transparent film from when the reflected intensity or transmitted intensity shows a certain extreme value until it shows any other three extreme values (known). Δθ1. Δθ2. Each of Δθ includes (rl-1), (rz-2), and (rs-1) angles at which the reflected intensity or transmitted intensity takes an extreme value.

θ. : Incident angle of electromagnetic waves (unknown) when a certain extreme value is indicated by □ [Operation] The basic principle of electromagnetic wave interference, which plays an important role in the above configuration of the present invention, will be explained with reference to the drawings.

First, in Figure 2, PQR3 is substance 11! ! (media)
A transparent [
10 (transparent film).

As shown in FIG. 2, consider the case where an electromagnetic wave is incident from point A to point B on surface PQ of film 1o at an incident angle θ. Sho membrane 1
The film thickness of 0 is d1, the wavelength of electromagnetic waves in vacuum is λ, and the refractive index of the film is n1 material! The refractive index of is set to no, and the refraction angle is set to ψ.

A part of the electromagnetic waves irradiated to the film 10 is reflected at point B and travels in the direction indicated by the arrow, and the other electromagnetic waves enter the film 10 at a refraction angle ψ and head toward point C, and at the ridge point C. The path BL of the electromagnetic wave reflected at point 0 B, which is reflected and goes in the direction of arrow M via points on plane PQ, is parallel to the path DM of the electromagnetic wave, which goes in the direction of M via points C and D. .

The electromagnetic waves traveling through these two paths BL and DM interfere on the focal point Fo of the lens 11 after passing through the lens 11.

Here, there are two paths BL in electromagnetic waves.

The optical distance difference Δ of BCDM can be expressed as in the following equation (1), where K is the foot of the perpendicular line drawn from the dot to the route BL.

...(1) Between θ and ψ, sin θ= −−s l nψ −
(2) n.

Since there is a relationship, the following equation (3) can be obtained by substituting equation (2) into equation (1) and rearranging.

Δx 2ndφcosψ −(3)
Now, as shown in FIG. 4, when the angle θ (incident angle) between the electromagnetic wave irradiation direction and the transparent film is continuously changed, a certain incident angle θ is reached. Assume that the extreme value is taken at the angle of incidence θ1, and as the value is further changed, the point of the extreme value (including the maximum value and the minimum value) is passed through several times, and the r3th extreme value is taken at the incident angle θ1.

At this time, the incident angle θ. , θ, the optical distance difference Δ.

, Δ1, the following relationship (4) holds true.

Δ. −Δ, = λ・rl/2 (however, θ, 〉θ.) Therefore, from the above equations (2), (3), and (4), the following (5) is obtained.
We can obtain the formula.

(5) In the same way, pass through the extreme points several times from a certain incident angle θ0, and then set the incident angle θ2. Assuming that θ takes the third extreme value of r2+r, the following equations (6) and (7) are obtained.

...(6) ... (7) Here, 90@〉θ, 〉θ2〉θ1〉θ. Binding, incident angle θ
0 and other incident angles θ1. θ2. θ, (i.e., the amount of angular change) − as shown in the following equations (a), (9), and (lo), respectively. Δθ3. If Δθ, then the above (5
) to (7) can be expressed as the following equations (11) to (13).

θ amount - θ0 amount Δθ - (8) θ2 - θ
0=Δθ2...(9) θ, -θ. =Δθ
3 ... (10) ... (11) ... - (12) ... - (13,) In the above equations (11) to (!3), the angle change amount Δθ1
．． Δθ2. Δθ3 can be determined by measurement, and the wavelength λ and the number of extreme values rl+r2+r can also be determined. Therefore, if the refractive index n is known, (11),
By combining equations (12) as simultaneous equations,
Even if the incident angle θ0 is unknown, the film thickness d can be determined. Also, if the refractive index n is unknown, (11), (1
2) By combining Equations (j3) as simultaneous equations, the film thickness d can be determined together with the refractive index n even if the incident angle θ0 is unknown.

In this way, the incident angle θ. Since the film thickness d can be determined without directly determining the thickness d, it is possible to avoid problems such as errors in measurement results caused by the inclination of the object to be measured (film 10).

In calculating the above equations (11) to (13), 's
Although reflected waves were used as shown in Figure 2, the present invention is not limited to the use of reflected waves, but can also be applied to cases where transmitted waves are used, for example, as shown in Figure 3. Above (11
) to (13) can be obtained.

Next, a specific calculation method for actually determining the film thickness d using equations (11) to (13) will be described.

(1) When the refractive index n is known, follow the procedure below.

From the relationship between equations (11) and (12) above, the following equation (14) is obtained.
A formula is required.

(14) The unknown angle of incidence θG in the above equation (14) can be easily determined by numerical calculation such as the interval reduction method or the Newton-Roughsin method.

Here, the interval reduction method will be explained.

The left side of the above equation (14) is set as f(θ0 (nl) [the following equation (15)], and further set as shown in the following equation (16).

...(15) ・f(θ0)) ...(16) Above (16)
In the formula, the initial value θ. (.), θ. , 1. .

is set appropriately and repeated until θ(1(n)) converges.

θ. , rl, converges, θ. , n)=θ. The film thickness d is obtained by substituting the convergence value into the above equation (11) or (12).

(1) When the refractive index n is unknown, follow the procedure below.

From the relationship between the above t12) and equation (13), the following (17) is obtained.
A formula is required.

...-(17) However, 10-・sin' (θ.◆Δθ3) "3 "2°r3 In the above equations (17) and (18), put θ.tsuθ.i), and in the case of (I) θ can be obtained by the interval reduction method in the same manner as θ. Once θ is obtained, the refractive index n is obtained using the above equations (17) and (18), and when the refractive index is obtained, further (11) The film thickness d can be determined using any of the equations (13).

The inventor has confirmed through experiments that when determining the film thickness according to the present invention, it is better to take the value of r, ~r, as large as possible to reduce the angle change amount Δθ1. Δθ3. Δ
The influence of measurement error of θ3 can be reduced, and the film thickness d
The measurement accuracy has increased.

On the other hand, since the method of the present invention makes use of the interference effect of electromagnetic waves in principle, in order to maximize the effects of the present invention, it is advantageous if interference with electromagnetic waves can be clearly obtained. According to various experiments carried out by the authors, if the electromagnetic wave used in the present invention is coherent with a single wavelength and has a vibration component perpendicular to the plane of incidence (S-polarized light), The result was that interference was obtained clearly and the amount of change in angle could be determined more accurately. This is thought to be because the intensity of reflection from the film 10 of S-polarized electromagnetic waves increases and decreases monotonically in response to changes in the angle of incidence, and interference occurs successfully for all angles of incidence. Compared to the case where S-polarized electromagnetic waves are used, when an electromagnetic wave with a vibration component (P-polarized light) parallel to the plane of incidence is used, the reflection intensity of the electromagnetic waves from the film 10 decreases up to the Brewster angle θ6. ,
It increases rapidly when the Brewster angle θ is exceeded, and the desired interference could not be obtained in this vicinity.

In addition, when the P-polarized electromagnetic wave is used, the incident angle θ is the angle θ of the Brewster. Find the following (19
) It is well known that the refractive index n can be determined by the formula (
Brewster's law), the purpose of the present invention is to determine the angle of incidence θ. Since the film thickness d and the refractive index n are determined without directly determining the relationship d and the refractive index n, the relationship expressed by equation (19) does not meet the purpose of the present invention.

nmtanθm (19) [Example] FIG. 1 is a schematic explanatory diagram of a film thickness measuring device configured to carry out the method of the present invention.

The monochromatic light emitted from the laser oscillator 1 is transmitted to the rotating reflector 2.
The process progresses in the order of -> lenses 3a, 3b -> film 10 (measurement object) - condensing lens 4, and the reflection intensity (light amount) is measured by the light amount detector 5. The rotating reflector ut2 is configured to be swung by a galvanometer 8, and the traveling direction of the monochromatic light reflected by the rotating reflector 2 can be arbitrarily changed.

On the other hand, rotation reflex! The reflection point G of the monochromatic light reflected by i2 is
It is configured to coincide with the focal point of the lens 3a, and all the monochromatic light reflected at the reflection point G becomes parallel to the optical axis of the lens 3a by passing through the lens 3a. Also lens 3a
and lenses 3b are arranged so that their respective optical axes coincide,
However, after passing through the lens 3a, the parallel ray passes through the lens 3b and then passes through the focal point of the lens 3b. The surface of the film 10 is positioned on the focal point of the lens 3b, so that all the monochromatic light reflected by the rotating mirror 2 reaches the same point H on the surface of the film 10, which causes the measurement position to shift. There isn't.

A rotary encoder 13 is connected to the rotary reflector 2, and the rotation angle of the rotary reflector 2 can be measured by the rotary encoder 13.

Note that the point Fo at which the reflection intensity of the monochromatic light on the light amount detector 5 is measured is made to coincide with the focal point of the focusing lens 4, as described in connection with FIG. 2.

The light amount signal detected by the light amount detector 5 is then sent to the amplifier 6.
The signal is sent to the CPU 7, amplified, and further sent to the CPU 7. CPU
In 7, the amount of change in the rotation angle of the rotary reflecting mirror 2 (i.e., the amount of angular change in the angle of incidence) when the amount of light changes from one extreme value to another is continuously measured, ) formula [more specifically, formulas (11) to (18)],
The thickness d of the film 10 (including the refractive index n if the refractive index n is unknown) is measured.

Furthermore, the optical system shown in FIG. 1 can be configured as shown in FIG. 5 (schematic explanatory diagram). In addition, 12 in Figure 5
1 is a reflecting mirror, and other parts corresponding to those in FIG. 1 are given the same reference numerals.

In the configuration shown in FIG. 5, monochromatic light emitted from a laser oscillator 1 has its traveling direction changed by a rotating reflector 2, and then passes through lenses 3a and 3b and enters a film 10. membrane 10
The reflected light passes through lens 3b again and is reflected! The traveling direction is changed at i12, the light passes through the condenser lens 4, and then the light amount is detected by the light amount detector 5.

The method of the present invention can also be implemented in such a configuration. Note that in the configuration shown in FIG. 5 as well, lenses 3a, 3b and lens 4 are used similarly to the optical system shown in FIG.
The focal points are arranged to coincide with the center of rotation of the rotating reflecting mirror 2 (coinciding with the irradiation point of the laser beam), the measurement point H5 of the film 110, and the light amount detector 5, respectively.

FIG. 6 is a schematic explanatory diagram showing another embodiment of the present invention.

The basic configuration of this embodiment is almost the same as the configuration shown in FIG. 5, and corresponding parts are given the same reference numerals to avoid redundant explanation.

In this embodiment, a laser oscillator 1 and a rotating reflector 2 are used.
A polarizing plate 15 is interposed between the two. The polarizing plate 15 is rotatably supported by a bearing 16 and rotated around the optical axis by a stepping motor 19 via gears 17 and 18.

The polarizing plate 15 has a function of converting circularly polarized monochromatic light from the laser oscillator 1 into linearly polarized monochromatic light, and by rotating the polarizing plate 15, vibration components perpendicular to the incident plane of the film 10 are converted. The film can be irradiated with monochromatic light while switching to (P-polarized light) or a parallel vibration component (S-polarized light). P
Switching between polarized light and S-polarized light using the polarizing plate 15 is performed using C20.
This is done by driving the stepping motor 19 in accordance with a command signal from the driver.

By adopting the configuration shown in FIG. 6, it becomes possible to arbitrarily irradiate @10 with S-polarized monochromatic light, thereby achieving clear interference and making the effects of the present invention even more remarkable. .

In the configuration shown in FIG. 6, it is also possible to irradiate the film 10 with P-polarized monochromatic light, which is limited to S-polarized light; (e.g., when the film 10 is irradiated with P-polarized monochromatic light)
This is because the refractive index n of 0 can be determined in advance according to the above equation (19).

[Effects of the Invention] As described above, according to the present invention, by employing the above-described configuration, it has become possible to measure the film thickness with high precision without directly determining the incident angle.

[Brief explanation of drawings]

Fig. 1 is a schematic explanatory diagram of a film thickness/refractive index measuring device configured to carry out the method of the present invention, Figs. 2 and 3 are schematic explanatory diagrams showing the principle of the present invention, and Fig. 4 is a ( Graphs showing waveforms for determining equation (13), and FIGS. 5 and 6 are schematic explanatory diagrams showing other examples of film thickness measuring devices configured to carry out the method of the present invention. 1... Laser oscillator 2... Rotating reflector 5...
Light amount detector 6...Amplifier 7...Central processing circuit 8...Galvanometer 10--Membrane (measurement target) 13...Rotary encoder 15--Polarizing plate Fig. 1 Fig. 2 Fig. 3 Hand thread seller Hosei () (spontaneous) 1. Display of the case 1986 Patent Application No. 99083 2 Name of the invention Film thickness measurement method 3 Person making the amendment Relationship to the case Patent applicant Toyobo Co., Ltd., 2-2-8 Dojimahama, Kita-ku, Osaka (316) Company Representative: 3 Takizawa, Department 4, Agent: Shinko Building, 2-3-7 Dojima, Kita-ku, Osaka 530 Column 6 of “Detailed Description of the Invention” of the Specification, Contents of Amendment

Claims (4)

[Claims] (1) In a method of measuring film thickness by irradiating an electromagnetic wave to one point of a transparent film with a known refractive index, while changing the angle between the irradiation direction of the electromagnetic wave and the transparent film, Track changes in reflected or transmitted intensity of electromagnetic waves due to interference in the transparent film, measure the amount of angular change until the intensity reaches a certain extreme value, and calculate the following equations (11) and (12). A film thickness measurement method characterized by determining film thickness by solving two-dimensional simultaneous equations. ▲There are mathematical formulas, chemical formulas, tables, etc.▼ ・・・(11) ▲There are mathematical formulas, chemical formulas, tables, etc.▼ ・・・(12) However, d: Film thickness (unknown) λ: Wavelength of electromagnetic wave (known) n: Refractive index of translucent film (known) Δθ_1, Δθ_2: The angle between the electromagnetic wave and the translucent film from when the reflected intensity or transmitted intensity shows a certain extreme value to when it shows any other two extreme values. change (known). Δθ_1 and Δθ_2 include (r_1-1) and (r_2-2) angles at which the reflected intensity or transmitted intensity takes an extreme value, respectively. θ_0: Incident angle of electromagnetic wave when it shows a certain extreme value (unknown) (2) The film thickness measuring method according to claim 1, wherein the electromagnetic wave has a single wavelength and consists of a vibration component perpendicular to the plane of incidence. (3) In a method of measuring film thickness by irradiating electromagnetic waves to one point of a transparent film with an unknown refractive index, the angle between the irradiation direction of the electromagnetic waves and the transparent film is changed, and the Tracking changes in reflected or transmitted intensity of electromagnetic waves due to interference in a transparent film, and determining the amount of angular change from when the intensity shows a certain extreme value to when it shows any other three extreme values. A film thickness measuring method characterized in that the film thickness is determined together with the refractive index by measuring and solving three-dimensional simultaneous equations of the following equations (11) to (13). ▲There are mathematical formulas, chemical formulas, tables, etc.▼ ...(11) ▲There are mathematical formulas, chemical formulas, tables, etc.▼ ...(12) ▲There are mathematical formulas, chemical formulas, tables, etc.▼ ...(13) However, d : Film thickness (unknown) λ: Wavelength of electromagnetic wave (known) n: Refractive index of translucent film (known) Δθ_1, Δθ_2, Δθ_3: From when the reflection intensity or transmission intensity shows a certain extreme value, any other arbitrary value The change in the angle between the electromagnetic wave and the transparent film until it reaches three extreme values (known). Δθ_1, Δθ_2, and Δθ_3 each include (r_1-1), (r_2-2), and (r_3-1) angles at which the reflected intensity or transmitted intensity takes an extreme value. θ_0: Incident angle of electromagnetic wave when it shows a certain extreme value (unknown) (4) The film thickness measuring method according to claim 3, wherein the electromagnetic wave has a single wavelength and consists of a vibration component perpendicular to the plane of incidence.