JPS59180308A - Film thickness measuring device - Google Patents

Abstract

PURPOSE:To measure film thickness accurately by observing the beat frequency of frequencies of two laser oscillators which are obtained by moving a substrate having a film whose thickness is to be measured. CONSTITUTION:The film 6 which transmits laser light and whose thickness is to be measured is formed on the substrate 7, and an annular laser resonator having the surface of the substrate 7 as part of a reflecting surface is provided. The substrate 7 is moved by a driving source 11 in a specific direction at a constant speed to divide the laser oscillation frequencies of the laser resonator into two frequencies by an optical path difference. Then, the two laser oscillation frequencies are put together by a half-mirror 5 and inputted to a detector 12 to obtain a signal of beat frequency. This signal is amplified by an amplifier 13 and inputted to a counter 15 through a filter 14 to observe and display it. The thickness of the coating film is known accurately and easily from the observation result.

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field of the Invention The present invention relates to a film thickness measuring device that can accurately measure the thickness of a magnetic film, etc. of a magnetic disk.

Prior Art and Problems Conventionally, the thickness of this type of film to be measured has been measured using an X-ray film thickness meter or a laser interference film thickness meter.

However, when using the former method, an X-ray film thickness meter, all measurements are performed by comparison with a reference plate, so there are concerns about absolute accuracy) and handling is troublesome. C,
If the latter type of laser interference film thickness meter is used, absolute measurements cannot be made in principle and only relative changes in film thickness will be captured. It has the disadvantage that it cannot escape the risk of measurement errors that are multiples of integers.

OBJECTS OF THE INVENTION The present invention is intended to solve the above-mentioned drawbacks, and it is an object of the present invention to provide a film thickness measuring device that can directly, accurately and easily measure the thickness of a symmetrical film formed on the surface of a substrate. It is an object.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will now be described in detail with reference to the drawings. Figure 1 shows the measurement principle of the present invention, in which 1 is a laser tube, 2 and 3 are mirrors, and 4 and 5 are nozzles. - Humira, 6
is a coating film (film to be measured for film thickness) formed on the surface of a substrate 7 made of aluminum or the like having good reflectance. Laser tube 1
．． Mirror 2. The /...-fmirror 4 constitutes a ring-shaped laser resonator in association with the surface of the substrate 7.

Now, assuming that the coating film 6 is moving at a high speed of τ in the direction of the arrow together with the rotating disc-shaped substrate 7, the light is refracted in the left-hand direction and the right-hand direction of the ring-shaped laser resonator. By passing through the coating film 6 outside the frequency range (%ki 1), a difference is created in the optical path length, resulting in oscillation at two frequencies / + + f2. In this case, the right-handed light with a frequency of 12 and the left-handed light with a frequency f reflected by the mirror 6 are combined into a half mirror.
5, and the thickness of the coating film B can be calculated from the beat frequency Δf obtained from these two frequencies.

Next, the formula used for this calculation will be explained.

In order to perform the above calculation, the thickness and speed of the coating film must be determined.

It is necessary to find the relational expression of the refractive index, but as a similar problem, the following is the relational expression of the laser current meter for the case of Fig. 2 in the reference literature (Laser Application Technology P149~, Haruhiro Kobayashi, Nikkan Kogyo Shimbun). The formula is shown.

Here, V: flow velocity? L = refractive index 6 2 speed of light Δf: beat frequency In this equation, (fL2-1) is multiplied as a whole, and this has an important practical meaning. That is, when the substrate 7 is rotated with the configuration shown in FIG. 1, the air on the surface of the coating film 6 also moves with a velocity distribution in which it is dragged. If a difference occurs in the optical path length due to the transport of this air layer, the effect of the air layer will be observed at the same time as the effect of the film thickness, and correction will be necessary. The details of this will be described later.

By the way, although the shape of Figure 2 is different from Figure 1, it can be considered that Figure 1 is completely simplified. Moving medium 8 here
In order to deepen our understanding of the effect of optical delay in Figure 2, we will prove Equation (1), which holds true in the case of Figure 2.

When light at the speed of light C (light from a ring laser) passes in the length direction through a medium 8 moving in the length direction at a flow velocity υ, as shown in FIG. The oscillation frequency f of the laser is expressed by the following equation.

／−□ ・・・・・・・・・・・・・・・・・・・・・
...(2) Here, D; optical path length for one round: Because of the order, the oscillation frequency f in the case of v-0. is as follows.

1; length of the part other than the medium (see Figure 2) Here, the speed ω of light passing through a medium other than the refractive index moving at a speed τ is:
The Lorentz transformation is given by the following equation. (Reference section, Relativity and Electrodynamics P49, Tokyo Tosho) ω=C±1(1--) ・・・・・・・・・・・・
・・・・・・・・・・・・(4)% tL2 Therefore, the optical path length in the moving medium is expressed as 6. The laser oscillation frequency in this case is as shown in the following equation.

According to the above equation, when the moving directions of the light and the medium are the same (in the case of the left turn in FIG. 1), the frequency f is given by the following equation.

(5) Since ヱ(1 in the formula, the next component is taken out by Taylor expansion, the following formula is obtained.

! (In step 1, perform Taylor expansion on the right-hand side again to extract the first-order component.

Furthermore, the following equation can be obtained from equation (3).

f+=fo(1+Qiha 辷U) ・・・・・・・・・・・・
・・・・・・・・・・・・(6) C(β+rbL) Similarly, the following formula is obtained.

f2=fo(1”-1 history) ・・・・・・・・・・・・
(7) C(λ+nL) Therefore, the beat frequency Δf is as follows, and the same result as in equation (1) can be obtained.

d Δf"f, -f2=c(xL-hl,) "2-1)
``Next, the actual calculation formula will be explained.

In order to substitute the form shown in FIG. 1 into equation (1), a transformation is performed based on FIG. 4 when light enters obliquely in the direction of movement.

When the angle of incidence is θ, ψ can be obtained by Snell's law as follows.

sinθ n=7-,', ψ=si%-1 (blade material)...
・・・・・・(8) as'le ψ
The vector velocity τ in the optical axis direction within the n medium (coating film 6) is as follows.

Ya = Vos rest ψ body ・−・・−・・・・・・・・・・
(9) The length L of the medium is as follows, where h is the film thickness.

h L-□・・・・・・・・・・・・・・・・・・・・・
...α0)608ψ However, ψ=86%-1 (is called) Next, feasibility will be examined using this α0 formula.

That is, try to calculate how much beat frequency can be obtained by approaching the actual conditions using Equation I. However, the conditions are as follows.

7o = 4.7 X 10 Hz (Hs-Na laser) B = 2 θ = ta%-' n = 6145゜ψ = 5i
n-' (μ company) = 26.56゜h -1μm Ji = 0.5 m When the disk (substrate 7) is rotated at 600 Or, p, m and measured at the position of radius de = 0.1 flL, Vo is It will look like this:

Vo-πr ・w-5,1416X O-I X 'b
'a' = 31.416''/s = s9o, sHz That is, Δf is a measurable frequency.

Next, we will consider errors caused by the air layer around the medium.

The wind speed distribution on the disk needs to be determined by aerodynamic analysis, but in order to roughly understand the degree of influence, we can use the above-mentioned feasibility assuming that an air layer with a thickness of 1 yam is moving uniformly at a speed of τ. Calculations were performed under the same conditions as in the study, and the following results were obtained.

=456Hz From this, it was found that even though the refractive index of the air layer is very close to 1, the effect of the air layer that is nearly 1000 times the thickness of the coating film is the same as that of the coating film.

The following methods can be considered to eliminate this influence.

1. Depressurize the air layer completely or measure in vacuum.

2. Measure the degree of influence of the air layer using a blank aluminum plate, etc., and use it as a correction coefficient.

Of these, the latter two methods are practical.

An embodiment of a film thickness measuring device implementing the above-mentioned principle is shown in FIG.

In the figure, 11 is a motor (drive source) for driving the disc-shaped substrate 7;
12 is a detector, 16 is an amplifier, 14 is a filter, and 15 is a counter.

The motor 11 rotates the substrate 7 at a constant speed.

As mentioned above, the left-handed and right-handed rings of the ring-shaped laser resonator have different optical path lengths and oscillate at two frequencies f + 112, but these are combined by the half mirror 5 and then sent to the detector 12. A signal with a beat frequency Δf is obtained.

This signal is amplified by amplifier 16 and filter 14
It enters Wotoshikakunta 15 and is observed and displayed. It is possible to determine the thickness of the coating film 6 by calculation from the value of Δf thus obtained.

Note that the rotational speed accuracy of the substrate 7 and laser oscillation conditions in such an apparatus are as follows.

Regarding the rotational speed accuracy of the substrate 7: As is clear from equation (1), the beat frequency is only coupled to the rotational speed in a linear relationship, so there should be no need to measure absolute peak summer for the rotational speed. However, since the measurement frequency is relatively low, around 500ffz,
Vibrations caused by rotation must be removed to avoid measurement errors.

Regarding laser oscillation conditions: As is well known, in order to oscillate, the gain of the laser tube must exceed the sum of losses at each reflecting surface. For this purpose, a long laser tube is more advantageous, but it is inconvenient to handle, and the denominator λ in equation (1) becomes smaller, resulting in a decrease in detection sensitivity.

Another important issue is reflection on the paint surface.

If there is surface reflection, two optical paths will be created and two beams will be distributed, making laser oscillation difficult.

In order to completely solve this problem, we used a linearly polarized laser tube, set the angle of incidence on the coating surface to Brewster's angle, and made the polarization plane 1,000 degrees parallel to the reflective surface to reduce the reflection on the coating surface. It is necessary to minimize it. This is advantageous since most external mirror type gas lasers are linearly polarized with a Brewster window.

Effects of the Invention As described above, according to the present invention, light having a frequency of sf++f2 is synthesized (by a half mirror), and these two frequencies are put into a detector, and the resulting beat frequency is observed by an amplifier, a counter, etc. Since direct observation has become possible through this method, it is now possible to accurately and easily determine the thickness of the coating film as a result of this observation. Furthermore, in the case of the present invention, there is no risk of radiation or the like, and the reproducibility is good.

[Brief explanation of drawings]

The drawings show an embodiment of the film thickness measuring device according to the present invention. FIG. 1 is a diagram explaining the principle, FIG. 2 is a schematic diagram of the device that derived equation (1), and FIG. A partially enlarged explanatory diagram, FIG. 4 is an explanatory diagram of incident and one reflection of light on the coating film, and FIG. 5 is an explanatory diagram of the configuration of the film thickness measuring device. In the figure, 1 is a laser tube, 2 and 3 are mirrors, 4°5 is a half mirror, 16 is a coating film (film to be measured for film thickness), 7 is a substrate made of aluminum or the like with good reflectivity, and 11 is a motor (drive source) , 12 is a detector (frequency synthesis means), 16 is an amplifier,
14'# filter, 15 a counter. Patent applicant Fujitsu Ltd. Representative Patent Attorney Gobe Tamamushi (3 others)

Claims (1)

[Claims] A ring-shaped laser resonator having a substrate surface on which a film to be measured for film thickness through which laser light is transmitted is formed as a part of the reflecting surface is provided, and the laser oscillation frequency of the ring-shaped laser resonator is changed to two due to the occurrence of an optical path difference. a driving source for moving the substrate in a fixed direction at a constant speed so as to separate the substrates, a means for synthesizing the two laser oscillation frequencies obtained by moving the substrate, and a bead obtained by the synthesis; 1. A film thickness measuring device characterized by being provided with observation means for observing frequency.