JPH03226573A - Method for controlling thickness of thin film - Google Patents

Abstract

PURPOSE:To control not only limited film thicknesses but also film thicknesses over a wide range including the thicknesses of thinner films and the integer times thereof by utilizing the phenomenon that a change in the reflectivity or transmittivity of the light to be used has a singular point as forming of thin films. CONSTITUTION:The T'' (d) obtd. by differentiating, twice, the transmittance T of the light through the thin film having the thickness (d) expressed by equation (where lambda: the wavelength of the light to be used as a reference, (d): the thickness of the thin film to be formed, (n): the refractive index of the light of the thin-film forming material, ng: the refractive index of the light of a monitor substrate) with respect to the thickness (d) is made into zero and the film thickness dh to become the inflection point of the transmittance or reflectivity is determined. The wavelength lambdah=lambda<2>/mdh is determined from this film thickness dh, integer (m), etc. While the thin film is formed, light is projected to the substrate and the transmittance or reflectivity is measured. The value obtd. by differentiating this transmittance or reflectivity twice with respect to the thickness (d) is observed and the formation of the thin film is stopped where the value attains zero.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to a method for controlling the thickness of a thin film using an optical method, and in particular, a method for controlling the thickness of a thin film using an optical method. This invention relates to an improvement in a film thickness control method that takes advantage of the fact that a thin film has a singularity during its formation.

(Prior art) A method for controlling the thickness of a thin film using an optical method, taking vapor deposition film formation as an example, is to provide a monitor substrate for film thickness measurement in a chamber where vapor deposition is performed, and to connect the product to be vaporized and the monitor substrate. While forming a deposited film on both sides, light is projected onto the monitor substrate, and the transmitted light or monochromatic light in the reflected light is received to determine the transmittance or reflectance, and when these reach the target values, the deposition is stopped. The desired film thickness is obtained by finishing the coating.

By the way, to calculate the target value of transmittance or reflectance, the refractive index of the deposited film, the refractive index of the monitor substrate, and the wavelength λ of monochromatic light are used.
However, although the refractive index of the deposited film changes depending on the deposition conditions and the thickness of the film formed, this change was ignored and considered as a known constant, so the transmittance was Or, the target value of reflectance was not appropriate (or the accurate film thickness could not be obtained).

In order to compensate for these drawbacks and obtain an accurate film thickness, we developed a film using the property that the transmittance or reflectance becomes maximum or minimum when the product of film thickness and refractive index reaches λ/4. Japanese Patent Application Laid-Open No. 1988 (1982) describes a method for controlling the thickness of a deposited film in which the transmittance or reflectance is calculated when the product of the thickness and the refractive index reaches λ/4, and the target value of the transmittance or reflectance is corrected. -133
It has become publicly known in No. 05.

However, in these devices, the film thickness can be controlled so that the product of the deposited film thickness and the refractive index of light is 1/4 of the wavelength λ of the light used or an integral multiple thereof; Since it is not possible to control the product with the refractive index to be 1/8 or an integral multiple thereof, the application of film formation technology has become widespread, and it has become necessary to form films of various thicknesses. However, I was unable to meet that request.

(Technical Problems to be Solved) This invention was made with attention to these points. It is an object of the present invention to provide a film thickness control method that is capable of controlling a wider range of film thicknesses, rather than controlling only a limited number such as 1/4 of 1/4 or an integer multiple thereof.

(Means and actions to solve the problem) The reference wavelength λ of the light, the film thickness d of the thin film to be formed, the refractive index n of the light at the reference wavelength λ of the material forming the thin film, the monitor substrate For example, when the refractive index of light is ng, the transmittance T of light for a thin film having a thickness d is expressed as in equation (1).

(1) Since f(a), which is obtained by differentiating Ni twice with respect to the film thickness d, is very complicated, is it a formula? Although it is omitted when filling in,
Express it as shown in equation (2) and find the inflectional film thickness dh?

1゛electricity d) = 0 (2) Next, select m such that the product nd of the desired film thickness do and the light refractive index n of the thin film forming material is 17 m of the reference wavelength λ, and (3) The wavelength λh of the monochromatic light to be used is determined by the formula.

The method for determining λh is the same when measuring reflectance.

λh - λ2/mdh (3) Then, the light transmittance or reflectance of the thin film formed on the monitor substrate is measured using light of this wavelength λh, and this is differentiated twice with respect to the film thickness d? Observe it, and when it reaches zero, stop forming the thin film.

By the way, in order to obtain a film thickness in which the product of the desired film thickness do, the light of the thin film forming material, and the refractive index n is 1/m of the reference wavelength λ or an integral multiple thereof, even in the conventional method, the wavelength Just use the light of λ/m. However, for example, by using light with a wavelength of 4000 m, the product nd of the refractive index n and the film thickness becomes
Even if it is possible to obtain a film thickness of 1/4 of 400 nm, or 1100 nm, the film thickness is 1/8 of 400 nm, or 5Q nm.
To obtain a film thickness of , light with a wavelength of 2001 m is required,
Since a wavelength of 200 nm is not visible light, optical measurements are virtually impossible.

In addition, the refractive index n of the material forming the thin film changes as the wavelength of the light changes, so if the wavelength of the light used is too different, the target value of transmittance or reflectance should not be changed. For these two reasons, it is necessary that the appropriate wavelength to be used is not much different from the reference wavelength for determining the desired film thickness.For this reason, the wavelength λh to be used is determined as described above. Using this light, we can observe the transmittance of transmitted light or the reflectance of reflected light from a thin film,
Formation of the thin film is stopped when the second differential becomes zero, and the desired film thickness can be obtained.

(Example) An example of an apparatus for performing vapor deposition in a vacuum chamber will be described with reference to the drawings.

In Fig. 1, a vacuum chamber 1 is used to deposit a thin film on the target product.
A crucible 3 containing vapor deposition material 2 is installed at the bottom of the crucible, and a shutter 4 is installed above the crucible 3 to block the evaporated material.

Lenses 5, which are objects to be film-formed, are arranged and held in the upper part of the vacuum chamber 1 by a holder (not shown), and a monitor substrate 6 for observing the deposition status is held at the center thereof.

A light source 7 is arranged at the lower side of the vacuum chamber 1, facing a mirror 8 provided below the center of the vacuum chamber 1, and a mirror 8? : A lower window 9 facing the room is provided. Also,
An upper window 10 is provided at the center of the upper surface of the vacuum chamber 1, and above the upper window 10, an interference filter 11 for a reference wavelength or an interference filter 12 used for measurement can be detachably mounted using a holder (not shown). It is.

A light receiver 13 is provided in the upper center of the filter mounting section so as to face the filter mounting section, and its output is connected to be input to a signal processing device 14 via an optical fiber.

With this equipment arrangement, the light from the light source 7 is directed to the mirror 8,
An optical path is formed through the lower window 9, the monitor board 6, the upper window 10, and the filter 11 or 121 to the light receiver 13, and only the monochromatic light that has passed through the filter 11 or 12 is received.
Measured by the signal processing device 14. The signal processing device 14 has a built-in computer (not shown), and performs measurements of the transmittance of monochromatic light, double differentiation of its temporal change curve, inspection of inflection points, operation commands to the shutter 4, and the like.

Refractive index of light n=3 for light with reference wavelength λ=400 nm
The product of the film thickness do and the refractive index n=3 is λ=
In order to deposit on the surface of the lens 5 so as to have a thickness of 1/8 of 400 nm, it is necessary to select m in equation (3).

For this reason, substitute λ-4001mY into equation (1) and convert it into 2
h-46 is obtained from equation (2) in which the equation obtained by differential differentiation is set to 0. In addition, when m = 9 in equation (3), t(
4) Let the formula be

λh - λ2/8dh (4) (4)
Substitute λ=4QQnm and dh=4Qt into the equation, and set 5 to λh.
Get 00. The transmitted light wavelength of the interference filter to be used has now been determined.

A lens 5 and an unused monitor board 6 are placed in a holder (not shown) in the vacuum chamber 1, and an interference filter 12 with a transmitted light wavelength of 5QQnm is attached.

The output of the light source 7 is adjusted, and the initial input to the light receiver 13 is set in the signal processing device.

Evaporation is started, shutter 4 is opened, and vapor deposition is performed.

In the signal processing device 14, the output of the light receiver 13 is measured and compared with the initial input to observe the change in transmittance.

Find the second derivative from the change in transmittance and find the point where it becomes zero. When the second differential value becomes zero, 5 Q n m
Figure 2 shows the flow of the work after obtaining a film thickness of 100 mL and then stopping at the zero point.

As another example, the reference wavelength λ and the constant m may be appropriately selected (the thin film to be formed may also be made of a material other than vapor deposition). The same thing can be done by measuring the rate.

(Effects) As described above, according to the present invention, the wavelength of light used for measuring the monitor board is determined, and light of this wavelength is used to measure the thin film at the point where the second differential of transmittance or reflectance becomes zero. By stopping the formation, it is possible to control the film thickness in a wide range, not only with a limited film thickness of 1/4 of the wavelength used or an integral multiple thereof, but also with a thinner film or an integral multiple thereof.

[Brief explanation of drawings]

The drawings show an embodiment of the invention, with FIG. 1 being a schematic diagram and FIG. 2 being a flow diagram. In the drawing, 6 is a monitor board, 7 is a light source, 12 is an interference filter, and 14 is a signal processing device.

Claims (1)

[Claims] A method of controlling the thickness of a thin film using an optical method, which takes advantage of the singularity of the transmittance of light transmitted through the thin film or the reflectance of light reflected from the thin film that accompanies thin film formation. In the method of controlling the wavelength λ of the reference light, the thin film d to be formed, the light refractive index n of the thin film forming material, and the light refractive index ng of the monitor substrate, the transmittance or reflection of transmitted light is determined. By setting the value obtained by differentiating the light reflectance twice with respect to the film thickness d as zero, find the film thickness dh that is the inflection point of the transmittance or reflectance, and calculate this dh, the reference wavelength λ of the light, and this wavelength. The wavelength λh of the light to be used is determined from the integer m obtained by dividing λ by the product of the desired film thickness do and the light refractive index n of the thin film-forming substance, and the light with the wavelength λh is used while forming the thin film. light is projected onto a monitor substrate, the transmittance of the transmitted light or the reflectance of the reflected light is measured, and thin film formation is stopped when a point where the second differential of these changes becomes zero is detected. film thickness control method.