JPH04313006A - Film thickness measuring method - Google Patents

Abstract

PURPOSE:To obtain a film thickness measuring method which can measure the thickness of films with high accuracy even when the refractive index of the films are unknown, not to mention when the refractive index is known, and is the optimum for on-line measurement which must be completed at a high speed. CONSTITUTION:This film thickness measuring method which measures the thickness of films by irradiating a light transmissive film 3 having a known refractive index with electromagnetic waves measures the thickness of the film 3 in such a way that, after electromagnetic waves 1 having a single wavelength form an image on the surface of the film 3 through an image forming element 2, the formed image is again passed through an image forming element 4 and the image is taken by means of an image pickup element set to the focal point of the element 4. Then two positions where the signal intensity on the image pickup element becomes the extremal values due to the interference action of the electromagnetic waves in the film 3 are measured and the thickness of the film 3 is measured based on the information. When the refractive index of the film 3 is unknown, the thickness and refractive index of the film 3 are measured by measuring at least three positions where the signal intensity becomes the extremal values.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to a method of measuring film thickness with high precision using electromagnetic interference, and more specifically, it can be used not only when the refractive index of the film is known, but also when the refractive index is unknown. The present invention relates to a film thickness measurement method that can measure film thickness with high precision even when the film thickness is low, and is optimal for online measurement that requires high-speed measurement.

0002

[Prior art] On-line film thickness measurement technology that uses electromagnetic wave absorption (for example, β-ray film thickness meter, γ-ray film thickness meter,
Linear film thickness meters, infrared film thickness meters, etc.) have been put into practical use, but with any of these techniques it is difficult to measure film thicknesses of 10 μm or less with high precision.

On the other hand, a spectroscopic film thickness meter that measures film thickness using the interference effect of white light has also been put into practical use. However, with a spectroscopic film thickness meter, the film thickness cannot be determined unless the refractive index of the object to be measured is known, and since the refractive index changes depending on the wavelength, it is difficult to accurately measure the film thickness. Moreover, since an expensive spectroscopic device is required, the method also has the disadvantage of being expensive.

0004

Problems to be Solved by the Invention The present invention has been made to solve these technical problems, and its purpose is not only when the refractive index of the film is known, but also when the refractive index is unknown. It is an object of the present invention to provide a film thickness measuring method that can measure film thickness with high precision even when the film thickness is high.

[0005]

[Means for Solving the Problems] The present invention, which has achieved the above object, is a method for measuring film thickness by irradiating electromagnetic waves onto a transparent film with a known refractive index. After an image is formed on the film surface by the image element, the image is passed through the image element again and captured by an image sensor placed on the focal point of the image element, and the electromagnetic wave intensity on the image sensor is determined by the electromagnetic wave intensity. This film thickness measurement method is based on measuring at least two positions where the light-transmitting film has an extreme value due to interference, and measuring the film thickness based on this information. If the refractive index of the light-transmitting film is unknown, the film thickness and refractive index can be measured by measuring at least three positions where the film has an extreme value.

[0006]

[Operation] The present invention is constructed as described above, but in short, it utilizes the interference of electromagnetic waves of a single wavelength, and can eliminate the influence of changes in refractive index due to wavelength, as in spectroscopic analysis. Furthermore, even film thicknesses of 10 μm or less can be measured with high accuracy. Furthermore, since an expensive spectroscopic device is not required, there is also the advantage that an apparatus configuration for carrying out the present invention can be achieved at low cost. Furthermore, the device implementing the present invention can be configured without any mechanically movable parts, and compared to conventional film thickness meters that require mechanically movable parts,
A device capable of high-speed measurement can be realized, and can be made into an optimal form as an online film thickness meter.

FIG. 1 is a schematic diagram for explaining the configuration of the present invention, and FIG. 2 is a diagram for explaining the principle of electromagnetic wave interference in the configuration of the present invention. In FIGS. 1 and 2, PQRS is a transparent film 3 that exists between substances I and II (medium) and is composed of parallel planes on both outer surfaces.

First, as shown in FIG. 1, a single wavelength electron wave 1
passes through an imaging element 2 such as a lens, is collected at point B on the surface of the film 3, is reflected on the front and back surfaces of the film 3, and then is reflected by the imaging element 4 such as a lens. Plane 5 on focal point
Then, the electromagnetic waves reflected from the front and back surfaces of the film 3 form an interference image. Then, on the plane 5, an imaging device such as a CCD camera that can measure the intensity of electromagnetic waves at each position is arranged.

Now suppose that substances I and II are air, and FIG.
As shown in FIG.
Consider the case where electromagnetic wave 1 is incident on point B on Q at an incident angle θ. Note that the film thickness of the film 3 is d, the wavelength of the electromagnetic wave 1 in air is λ, the refractive index of the film 3 with respect to air is n, and the refraction angle is Ψ.

A part of the electromagnetic wave 1 irradiated on the film 3 is reflected at point B and travels in the direction of arrow L, and the other electromagnetic wave 1 enters the film 3 at a refraction angle Ψ and heads toward point C, and then It is reflected at point C, refracted at point D on plane PQ, and then heads in the direction of arrow M. The path BL of the electromagnetic wave 1 reflected at point D is parallel to the path DM of the electromagnetic wave 1 that goes in the direction of arrow M via points C and D. The electromagnetic waves 1 traveling along these two paths BL and DM interfere at a point E on the plane 5 after passing through the imaging element 4.

Here, two paths BL in the electromagnetic wave 1,
The optical distance difference Δ of BCDM can be expressed as in Equation 1, where K is the foot of a perpendicular line drawn from point D to route BL.

0012

[Math 1]

[0013] Since there is a relationship between θ and Ψ as shown in formula 2,
When formula 2 is substituted into formula 1 and rearranged, formula 3 is derived.

[0014]

[Math 2]

[0015]

[Math 3]

Let us now assume that the vertical distance from the optical axis 6 of the imaging element 4 to the point E is x, and that an fθ lens is used as the imaging element 4. If the inclination angle is α, these relationships can be expressed as in Equation 4. Note that although the case where an fθ lens is used as the imaging element 4 is shown here, other lenses such as f tan
A θ lens (ordinary convex lens) may be used, and in this case, the relationship in Equation 4 is x=f. It becomes like tan(θ-α).

[0017]

[Math 4]

Now, when observing the interference image on the plane 5, the interference intensity changes depending on the distance x. and the distance x is x
At m, the intensity takes a maximum value (or minimum value) due to the interference of the reflected wave, and as the distance x increases, the maximum value (or minimum value) is reached sequentially when the next distance x is xm+1, xm+2. Suppose I took it. Distance x measured sequentially at this time
Optical distance difference Δ for each of m, xm+1, xm+2
The relationships of Equations 5 and 6 hold between m, Δm+1, and Δm+2.

[0019]

[Math 5]

[0020]

[Math 6]

Therefore, from Equations 2 to 6, Equations 7 and 8 can be obtained.

[0022]

[Math 7]

[0023]

[Math. 8]

On the other hand, from the relationship between Equations 7 and 8, Equation 9 can be obtained.

[0025]

[Math. 9]

Next, consider the general formulas 7 to 9. As shown in Figure 3, when x is changed, the interference intensity reaches an extreme value at distance xp, and then when it is changed further, it passes through the maximum and minimum points r times and becomes Assume that it takes the extreme value again at xp+2r, passes through the maximum and minimum points r times, and takes the extreme value again when changing it further. At this time, the relationship between the above formulas 7 to 9 is generally expressed by the following formula 1.
It can be expressed as a relationship between 0 and 12.

[0027]

[Math. 10]

[0028]

[Math. 11]

[0029]

[Math. 12]

Therefore, if the refractive index n is known, find two positions where the interference intensity takes the extreme value (that is, the value of the distance x),
The film thickness d can be determined using Equation 10 or 11. Further, when the refractive index n is unknown, three positions where the interference intensity takes an extreme value are found, first the refractive index n is found using Equation 12, and then the film thickness can be found using Equation 10 or 11.

In calculating Equations 10 to 12, reflected waves are used in the cases shown in FIGS. 1 and 2, but the present invention is not limited to the case where reflected waves are used. As shown, Equations 10 to 12 can be obtained in the same way even when using transmitted waves.

[0032]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 5 is a schematic explanatory diagram showing an example of a film thickness measuring apparatus constructed to carry out the present invention. The laser oscillated from the laser oscillator 11 is transmitted through two lenses 16a, 1
The beam diameter is expanded by the beam expander 6b. The light beam whose beam diameter has been expanded forms a spot on the sample 13 (film to be measured) by the lens 12 . At this time, the positional relationship between the lens 12 and the sample 13 is such that the lens 1
The focal point of the sample 13 is arranged so that its focal point almost coincides with the surface of the sample 13.

The laser light reflected from the front and back surfaces of the sample 13 enters the fθ lens composed of lenses 14a and 14b, and then interferes with the image sensor surface of the imaging device 15, which is placed on the focal plane of the fθ lens. make a statue Furthermore, imaging device 1
For example, a commercially available one-dimensional CCD camera is usually used as the camera 5, but the camera is not limited to this.

Information on the interference image on the image sensor surface is input to the arithmetic processing unit 17. An example of the configuration of the arithmetic processing unit 17 is shown in FIG. The interference image information input to the arithmetic processing unit 17 is first sent to the image memory circuit and stored therein. In the image memory circuit, the image is stored in a form such as shown in FIG. 3 that the relationship between the interference intensity and the distance x becomes clear. The information in the image memory circuit is sent to the extreme value detection circuit, and the position where the interference intensity takes the extreme value (xp, xp+1 shown in Fig. 3) is sent to the extreme value detection circuit.
, xp+2..., etc.) are calculated.

[0035] These positional information are sent to the arithmetic circuit, and the arithmetic operations according to Equations 10 to 12 are performed to determine the film thickness d or the film thickness d and the refractive index n, and these values are output from the output circuit. Ru.

The device shown in FIG. 5 utilizes the interference of reflected waves as shown in FIG. 2, but the device shown in FIG. 4 utilizes the interference of transmitted waves. It can also be configured. In the device configuration shown in FIG.
Lens 12, lens 1 forming a beam expander
6a, 16b, lenses 14a, 1 forming the fθ lens
The configuration such as 4b may be included in a mechanism for correcting aberrations and the like.

In FIG. 5, the interference image is expressed as fθ on the image sensor surface.
Although it was formed using a lens, other lenses such as ftan
As mentioned above, a θ lens or the like may be used. In addition, when focusing the light from the laser oscillator 11 on the sample 13, the lens 12 was used, but the point is that the sample 13
It is sufficient to use an optical system in which spots are connected on 3, and other optical systems can also be used.

[0038]

[Effects of the Invention] As described above, according to the present invention, it is possible to measure the film thickness with high accuracy not only when the refractive index of the film is known, but also when the refractive index is unknown. became. In particular, the apparatus for carrying out the present invention can be configured to enable high-speed measurement, and can be optimally configured as an online film thickness meter.

[Brief explanation of drawings]

FIG. 1 is a schematic explanatory diagram for explaining the configuration of the present invention.

FIG. 2 is a diagram for explaining the principle of electromagnetic wave interference in the configuration of the present invention using reflected waves.

FIG. 3 is a graph showing waveforms for determining Equations 10 to 12.

FIG. 4 is a diagram for explaining the principle of electromagnetic wave interference in the configuration of the present invention using transmitted waves.

FIG. 5 is a schematic explanatory diagram showing an example of a film thickness measuring device configured to carry out the present invention.

FIG. 6 is a flowchart showing the circuit configuration of the arithmetic processing unit 17.

[Explanation of symbols]

1 Electromagnetic waves 2,4 Imaging element 3. Membrane 5 Plane 6 Optical axis

Claims (2)

[Claims] Claim 1: A method of measuring film thickness by irradiating electromagnetic waves onto a transparent film with a known refractive index, in which electromagnetic waves of a single wavelength are imaged on the film surface by an imaging element, and then the image is formed. passes through the imaging element again and is imaged by an imaging element placed on the focal point of the imaging element, and the signal intensity on the imaging element takes at least two extreme values due to the interference effect of electromagnetic waves on the transparent film. A method for measuring film thickness, characterized in that the film thickness is measured based on this information. 2. A method of measuring film thickness by irradiating electromagnetic waves onto a transparent film of unknown refractive index, in which electromagnetic waves of a single wavelength are imaged on the film surface by an imaging element, and then the image is formed. passes through the imaging element again and is imaged by an imaging element placed on the focal point of the imaging element, and the signal intensity on the imaging element takes an extreme value of at least 3 A method for measuring film thickness, characterized in that the film thickness and refractive index are measured based on this information.