JPH01134203A - Film thickness measuring instrument - Google Patents

Abstract

PURPOSE:To obtain a simple, film thickness measuring method which has a wide measurement range by branching incoherent light after reflection into two and inputting them to the same light intensity detector while giving them an optical path difference, and measuring film thickness from the optical path difference when the light beams interfere with each other. CONSTITUTION:The incoherent light 20 is projected on the film 1 to be measured and reflected incoherent light beams 21-1 and 21-2 are branched into two light beams 23 and 24. Those two light beams 23 and 24 are reflected by mirrors 7 and 8 and inputted to the same light intensity detector 9, but the mirror 8 for one light beam 24 is vibrated to generate the variable optical path difference between the two light beams 23 and 24. Then the optical path difference when the same pieces of flux of the two light beams 23 and 24 interfere with each other on the light intensity detector 9 is found and the film thickness (t) of the object film 1 is measured according to the value of the optical path difference.

Description

[Detailed Description of the Invention] [Summary] This invention relates to a method of measuring film thickness using interference of incoherent light, and aims to enable simple measurement of film thickness and expansion of the range of film thickness measurement. Interference between the same wave packets of incoherent light emitted from the light source and reflected by the front surface of the film to be measured and the bottom surface of the film, with the same wave packets shifted by an amount corresponding to the thickness of the film. A method of measuring the film thickness of the film using the method, wherein the incoherent light is split into two parts, the first
．． The second incoherent light is guided to the same light intensity detector, and an optical path difference is set between the optical path length of the first incoherent light and the optical path length of the second incoherent light. variably given, and the above 1. The film thickness is measured based on the optical path difference between the same wave packets of the second incoherent light that interfere with each other.

[Industrial application field]

The present invention relates to a method of measuring film thickness using interference of incoherent light.

[Conventional technology]

A measurement method using optical interference is generally used to measure the thickness of a thin film formed in a semiconductor manufacturing process.

The conventional film thickness measurement method uses coherent light, changes the angle of incidence on the film to be measured, counts the number of brightnesses observed during this process, and calculates the film thickness by substituting the counted value into a predetermined formula. It is calculated.

(Problems to be solved by the invention) Conventional film thickness measurement methods require work such as changing the incident angle and counting the number of brightnesses that appear during this changing process, making measurement cumbersome. .

Furthermore, when the film thickness exceeds 1 μm, it becomes difficult to measure the film thickness, and the measurement range has been limited to 1 μm or less.

Additionally, a laser light source that emits coherent light was required.

SUMMARY OF THE INVENTION An object of the present invention is to provide a film thickness measuring method that can easily measure film thickness and expand the range of film thickness measurement.

(Means for Solving the Problems) The present invention provides light that is emitted from the same source, is reflected at the front surface of the film to be measured, and is also reflected at the bottom surface of the film, and remains the same in a state shifted by an amount corresponding to the thickness of the film. of incoherent light with a wave packet,
A method of measuring the film thickness of the film using interference between the same wave packets, wherein the incoherent light is split into two, first and second incoherent lights, and the same light intensity is detected. while variably giving an optical path difference between the optical path length of the first incoherent light and the optical path length of the second incoherent light; This configuration measures the film thickness based on the optical path difference when the same wave packets of the second incoherent light interfere with each other.

[Effect]

By splitting the incoherent light reflected by the film into two and guiding them to the same light intensity detector, and by providing an optical path difference between each optical path length of the split incoherent light, the film At the time of reflection, the same wave packets, which were different by an amount corresponding to the film thickness, are brought into alignment, causing interference.

Interference occurs at two points, and the film thickness is measured from the dimension between these two points.

Since interference between the same wave bundles of incoherent light is used, film thickness measurement becomes easy, and film thicknesses exceeding 1 μm can be measured.

〔Example〕

FIG. 1 is a diagram illustrating the film thickness measuring method of the present invention.

1 is the film to be measured whose thickness is to be measured, d2 is the top surface, 3 is the bottom surface, and t is the thickness. The refractive index n of the film 1 is known from another measurement result.

4 is a light source, which is a general type that emits incoherent light. The light source 4 is inexpensive.

5 is a Michelson interferometer device, which includes a half mirror 6, a fixed mirror 7, a vibrating mirror 8, and a light intensity detector 9 for detecting the intensity of light.

Reference numeral 10 denotes a vibration device, which includes a piezoelectric element 11 and a piezoelectric element drive circuit 12. A photographing mirror 8 is attached to the piezoelectric element 11.

Reference numeral 13 denotes a display device, which is supplied with the output from the detector 9 and the output from the drive circuit 12, and displays the thickness of the film 1 so that it can be read almost directly, as will be described later.

Next, we will explain the optical path.

Light 11j120 from the light source 4 is incident on the film 1 at an incident angle φ1.

A portion of the incident light is reflected by the upper surface 2 and becomes a light beam 21-1.

Another part is refracted and enters IIl, is reflected by the lower surface 3, and is refracted by the upper surface 2, becoming light Ia21-2.

The refraction angle is φ. It is. The light rays 21-1 and 21-2 actually overlap and coincide to form the light ray 21 as shown in FIG. The light beams 21-1, 21-2 are directed to the half mirror 6 of the Michelson interferometer device 5.

For convenience of explanation, it is assumed that the light ray 21-1 is transmitted through the half mirror 6, and the light ray 21-2 is reflected by the half mirror 6.

The light beam 21-1 transmitted through the half mirror 6 heads toward the fixed mirror 7, is reflected there, travels backward, and returns to the half mirror 6.
The light beam is reflected by the light beam 22-1 and enters the light intensity detector 9.

The light ray 21-2 reflected by the half mirror 6 heads toward the vibrating mirror 8, is reflected there, travels backward, and returns to the half mirror 6.
The light beam passes through the beam, becomes a light beam 22-2, and enters the light intensity detector 9.

In reality, the light ray 21 is transmitted through the half mirror 6 into a first light ray 23 and a second light ray 24 reflected there.
It is branched into.

The light beam 24 is reflected by the fixed mirror 7 and travels backward, and a part of the light beam 24 is reflected by the half mirror 6 and enters the detector 9.

The light beam 25 is reflected by the movable mirror and travels backward, and the negative portion thereof passes through the half mirror 6 and enters the detector 9.

The light 122 incident on the detector 9 consists of rays 22-1 and 22-2.
These interfere on the detector 9.

Next, interference will be explained.

The present invention utilizes interference of incoherent light, and first, interference of incoherent light will be briefly explained.

Generally, the electric field components of two lights are: U+ (XI, t)= A eXl)j (Q) t−kX+ −11/ +
) exa-t/ r +U2 (XI, t)= B eXDJ (Q) t-kxz 11/2)
These two interference strengths I, expressed as exp-t/'l: 2, are 1 = (Lll + U2) 2 = (A2+
82) eXl)-2t/r+2ABcos(k(x+
-X2 )+ (v+-'4f2))exp -2
It is expressed as t/r.

If the two lights are incoherent, W −ψ =
This is arbitrary, and if k(x-x2)=nλ, it is expressed as 1-(A2+82)eXp-2t/Z:.

If A=8, 1-2 A 2exp -2t/r It is expressed as

Here, vl-1412=O1, that is, the corresponding same wave packet when the incoherent light is split into two and the - branched incoherent light and another branched incoherent light are matched without phase difference. When looking at comradely interference, that is, self-interference, the interference intensity ■ is expressed as 1 = 4 A 2exp -2t/r.

Therefore, there is no correlation in phase between the two branched incoherent lights, but if we look at the same wave packets, that is, self-wave packets, which occur about 109 times per second in each of the two incoherent lights. This results in so-called self-interference.

The film thickness measuring method of the present invention utilizes this self-interference as described below.

FIG. 2 shows a negative wave packet 30 in a ray 20 at a certain point in time.
It schematically shows how the image moves to the detector 9 over time.

On the light beam 1!21-1, the wave packet is located at the position indicated by 30-1, and on the light beam 21-1, the wave packet is located at the position indicated at 30-2.

Wave packet 30-2 lags behind wave packet 30-1 by a distance of 111E. This delay is due to the fact that the light ray 21-1 is reflected by the upper surface 2 of the film 1, whereas the light ray 21-2 is II! This is because the light rays are reflected by the lower surface 3 of 11, and there is an optical path difference between the two light rays. The distance is related to the thickness t of the membrane 1 and is 2
n t cosφ. It is expressed as

Here, n, φ. is known, so Therefore, if 2 above is known, t can be found.

Hereafter, the wave packet 30-. 30-2 is this distance l! It is propagated while maintaining tI1.

Wave packet 30-. A part of the light beam 30-2 passes through the half mirror 6. This wave packet is 30-1. ．． 30-2. Indicated by

Wave packet 30-. The remainder of the beam 30-2 is reflected by the half mirror 6. This wave packet 3O-1b, 30-2. Indicated by b.

The remainder of the wave packet 30-1, 30-2 is reflected by the half mirror 6. These wave packets are shown as 3O-1b and 3o-2b.

Wave packet 30-1. ．． 30-2. is reflected back by the fixed mirror 7, reflected by the half mirror 6, and becomes a wave packet 30-
', 30-2. ' and enters the detector 9.

Wave packet 30-1. ．． 30-. 2. is reflected back by the vibrating mirror 8, passes through the half mirror 6, and becomes the wave packet 3O-1
b', 30 2b' and enters the detector 9.

Here, the wave packet 30-1a' and the wave packet 30-2b' are self-wave packets having the same origin, and when they match, they interfere.

Similarly, the wave packet 30-2a' and the wave packet 3O-1b' are self-wave packets, and when they match, they interfere.

Next, film thickness measurement will be specifically explained.

The above-mentioned Michelson interferometer device 5 is adjusted so that the optical path length between the vibrating mirror 8 and the half mirror 6 is equal to the optical path length 1 between the fixed mirror 7 and the half mirror 6. The vibrating mirror 8 is at the mai level 1fP at this time, and the optical path length 2 is at the reference optical path length 2 at this time.

ref Also, by the above adjustment, the optical path length L1 of the first optical path A-B-A-C from the half mirror 6 to the detector 9 is equal to the optical path length L2 of the second optical path A-D-A, -C. .

Further, the piezoelectric element 11 is driven by the drive circuit 12, and the vibrating mirror 8 is moved as shown by the arrow x1. It is vibrated in the direction of the light beam 24 as shown by ×2. This allows the optical path length e (
A-D) is 2 2 re around 2
f is repeatedly varied in the lengthening and shortening directions.

The optical path length 21 remains unchanged.

FIG. 3 shows the display state on the display device 13. The horizontal axis is the optical path length series 2, and the vertical axis is the light intensity (interference intensity).

Here, there is a predetermined relationship between the output voltage of the drive circuit 12 and the amount of deformation of the piezoelectric element 11, and this relationship is known. Also, the deformation opening of the piezoelectric element 11 is the amplitude of the vibrating mirror 8. Therefore, in FIG. 3, the horizontal axis is the optical path length 22 obtained by applying the output voltage of the drive circuit 12 to the above relationship. Thereby, the thickness of the membrane 1 can be substantially directly read, as will be described later.

For example, a line 42 having two peaks 40 and 41 is displayed on the display device 113. The reason for this will be explained later.

W is the amplitude of the vibrating mirror 8, in which the beaks 41.
The width is set to include 42 mm.

Next, the relationship between the optical path length, L2, and interference will be explained.

Optical path length L1. Since A-C parts of L2 are common,
Let us consider the optical path lengths of 2 and 2.

■ When the optical path length is equal to the optical path length
re4=之,).

The wave packet on the light beam 22 is in the state shown in FIG.

Wave packets 30-18' and 3o-2b' differ by a distance of 2, and no interference occurs between these self-wave packets.

The wave packet 30-2a' and the wave packet 3O-1b' are also different from each other in the same manner as described above, and no interference occurs between these self-wave packets.

The intensity of the light detected by the detector 9 is 11.

- The vibrating mirror 8 is displaced from the reference position IP in the direction of arrow x1.

■-1 If the displacement amount of the vibrating mirror 8 is Δd, the optical path length is 2
2 becomes 2 ref +2Δd. As a result, the wave packet 3
0-1. The distance 2 between ' and wave packet 30-2a' is l
l-2Δd, and the wave packet is 30-1. ' is the wave packet 30-2.
approach '.

However, as long as the two wave packets do not match, no interference occurs and the intensity of the light from the detector 9 remains at 11.

-2 When the displacement Δd becomes exactly 2/2, as shown in FIG. 4, the wave packet 30-1. The distance between ' and wave packet 30-2a' becomes zero, and wave packet 30-1b' and wave packet 30-
2a' perfectly matches and interference occurs.

Interference also occurs between self-wave packets of other wave packets of incoherent light.

As a result, the intensity of the light from the detector 9 increases to I2.

■-3 When the displacement Δd exceeds Il/2, the wave packet 3O-1
b' is the wave packet 30-2. ', the interference disappears,
The intensity of the light from the detector 9 decreases to I.

As a result, as shown in Figure 3, e2 = f12 ref + −’− A beak 40 appears at the position.

■ When the vibrating mirror 8 is displaced from the reference position P in the direction of arrow X2.

■-1 If the displacement amount of the vibrating mirror 8 is Δd, the optical path length z
2 becomes 2ref-2Δd. As a result, the wave packet 30−
The distance ff12 between wave packet 2b' and wave packet 30-1a' is f
l-2Δd, and the wave packet 30-2b' becomes the wave packet 30-1.
' approach.

However, as long as the two wave packets do not match, no interference occurs and the intensity of the light from the detector 9 remains at 11.

-2 When the displacement Δd becomes T degrees Il/2, the distance between 3O-2b' and the wave packet 30-1a' becomes zero, and the distance between the wave packet 3O-2b' and the wave packet 30-1a' becomes zero, as shown in FIG. -1
a' perfectly matches, and interference occurs.

Interference also occurs between self-wave packets of other wave packets of incoherent light.

As a result, the intensity of the light from the detector 9 increases to I2.

■-3When the displacement Δd exceeds 272, the wave packet 30-'
is the wave packet 30-1. ', the interference is weakened, and the intensity of the light from the detector 9 decreases to 11.

As a result, as shown in Figure 3, I12 = [2raf--'- A beak 41 appears at the position.

The distance between the beaks 40 and 41 is becomes.

Therefore, by reading the distance between the beaks 40 and 41 from the display device 13, and performing a simple calculation by substituting this value into the equation 0, the thickness of the film 1 can be immediately determined.

Note that the amplitude W of the vibrating mirror 8 is determined to be the width at which the beak 40°41 appears. This width is not required to be exact.

Further, since the present invention utilizes self-interference of incoherent light, the measurement range is not limited to 1 μm or less as in the conventional method, and film thicknesses exceeding 1 μm can be measured with high accuracy. The measurement range is 0 μl to 10 μl.

Further, the light source 4 may be of any type as long as it emits incoherent light.

〔Effect of the invention〕

As explained above, according to the present invention, film thickness can be easily measured using a general light source. Moreover, since it uses self-interference of incoherent light, the measurement range is not limited to 1 μm or less, and the film thickness is 1 μm or less.
The film thickness can be measured even when the thickness is µm or more.

[Brief explanation of the drawing]

Fig. 1 is a diagram explaining the film thickness measurement method of the present invention, Fig. 2 is a diagram showing the wave packet of incoherent light in Fig. 1, Fig. 3 is a diagram showing an example of display on a display device, Fig. 4 FIG. 5 is a diagram illustrating another aspect of interference between identical wave packets. FIG. 5 is a diagram illustrating another aspect of interference between identical wave packets. In the figure, 1 is the film to be measured, 2 is the top surface, 3 is the bottom surface, 4 is incoherent light, 5 is the Michelson interferometer device, 6 is the half mirror 1, 7 is the fixed mirror, 8 is the vibrating mirror, 9 is the light intensity Detector, 10 is a vibration device, 11 is a piezoelectric element, 12 is a piezoelectric element drive circuit, 13 is a display device 20.21.21-1.21-2.22°22-1.2
2-2 is an incoherent ray, 23 is a first ray, 24 is a second ray, 30.30-7.30-2, 30-1a, 3O-2a. 3O-1b, 3o-2b, 30-', 30-2.
'. a 30-, b', 3o-2b' are wave packets, 40.41
indicates a peak. Figure 1 for explaining the film thickness measurement method of the present invention.
Figure 3 Figure 5 shows an example of the display of the display device

Claims (1)

[Claims] The light is emitted from the same light source (4) and is reflected by the front surface (2) of the film to be measured (1), and is also reflected by the bottom surface (3) of the film, with a deviation corresponding to the thickness of the film. The same wave packet (30-_1, 3
A method for measuring the film thickness of the film by using interference between the same wave packets of the incoherent light having a wavelength of incoherent light (23, 24) and guide them to the same light intensity detector (9) (5, 6, 7, 8)
At the same time, an optical path difference is variably provided between the optical path length of the first incoherent light and the optical path length of the second incoherent light (10, 11, 12), and the optical path length of the first incoherent light and the second incoherent light are The same wave packet of coherent light (30-_2_a_', 30-_
1_b_', 30-_1_a_', 30-_2_b_'
) A film thickness measuring method characterized by measuring the film thickness based on the optical path difference when comrades interfere.