JP2592211B2 - Interference film thickness measuring device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for detecting the thickness of a thin film by interference of light, and more particularly, to non-destructively detecting the thickness of a thin film in a transparent body.

[0002]

2. Description of the Related Art Generally, in order to measure a film thickness step of an extremely thin film such as a semiconductor oxide film or an unevenness, a method using a Michelson interferometer based on the principle of interference of light reflected on the surface is used. Since this measurement utilizes reflected light from an object to be measured, it can be measured with light having relatively high intensity, and thus has been established as a highly accurate measurement method.

FIG. 11 shows a basic configuration of a Michelson interferometer. The light beam L emitted from the white light source 104 is split into two directions by a beam splitter (BS) 106, and one of the light beams is reflected by a mirror 107 to be used as reference light. The other is reflected by the surface S1 and the surface S2 of the DUT 103, and these become phase conjugate light. The reflected reference light and phase conjugate light are received by the photodetector (photodiode in this case) 109 by the BS 106, and their interference state is detected by the photodetector 109 as an electric signal.

In order to measure the step (or film thickness) d ₁ between the surface S 1 and the surface S 2 of the object 103, first, the reflected light s ₁ and the reflected light R _{1 on} the surface S 1 of the object 103 are measured. to interfere to adjust the position of the mirror 107 in the x-axis fine movement stage (the position of the mirror and x _1). Next,
The position of the mirror 107 is adjusted so that the reflected light s ₂ and the reflected light R _{2 on the} surface S2 interfere with each other, and the position of this mirror is changed to x
When _2, the difference between the position x ₁ and the position x ₂ is equal to the step d _1. Here, the detection of the interference state is represented as follows.

The distance L from the BS 106 to the mirror 107
0, distance L1 from BS106 to surface S1, mirror 1
07, the reflectances s ₁ ′ of the surfaces S1 and S2,
When s ₂ ′ is used, the light intensity P ₁ at the BS 106 due to the interference between the reflected light s ₁ and the reflected light R ₁ is expressed as “s ₁ ′ ² + r ² ” when L 1 ≠ L 0. When L1 = L0, it is represented by (s ₁ ′ + r) ² , and a difference occurs by “2s ₁ ′ · r”. This was detected by the photodetector to the position on the mirror and x _1. By also performing the interference between the reflected light R ₁ and the reflected light s ₂ in the same manner, obtained a position x _2. Thus, the step d ₁ is required.

As a thin film measuring apparatus using the above-mentioned Michelson interferometer, there are “Japanese Patent Laid-Open No. 1-134203” and “Japanese Patent Laid-Open No. 3-110405”. FIG. 12 shows an example of a case where a multilayer thin film is measured. This device also uses a white light source 104, and the light from this light source is reflected by the front surface 102 and the bottom surface of the thin film. The reflected light 121-1 and the reflected light 121-2 are converted by the half mirror 106 into light 12
4 and light 123 directed to the fixed mirror 107. These lights 124 and 123 are reflected by mirrors 108 and 107.
And reflected light 121-1 and reflected light 121-of the thin film
2 as the interference light 122-1 and 122-2 with the photodetector 1
09 is detected.

Here, the distance l ₁ between the half mirror 106 and the fixed mirror 107 is set equal to the distance l ₂ between the half mirror 106 and the vibration mirror 108. When the oscillating mirror 108 is oscillated by the oscillating device 110 by the piezoelectric drive circuit 112 around the center, the interference light 122-1,
The intensity of 122-2 changes. When the distance l ₁ is equal to the distance l ₂ , the interference light 122-1 is strengthened. When the optical path length of the reflected light 121-2 of the thin film is equal on both the fixed mirror 107 side and the vibration mirror 108 side, the interference light 122-1 is generated. Light 122-2 is enhanced. This is detected by the detector 109 and output to the display device 113. The detection signal is displayed on the display device 113 in synchronization with the signal from the piezoelectric drive circuit 112 as a trigger. Thus, the interference state according to the vibration position is monitored by the display device 113, and the film thickness is measured from the interval between the peak positions of the signal of the detector.

[0008]

In the above-described measurement, for example, when the dielectric film is a single layer or a multi-layer film, the thickness of each layer is approximately known and is substantially uniform. Any film thickness is effective. However, if the condition is not satisfied, it is impossible to determine which layer and which layer is the interference signal due to the reflected light beam in the above-described measurement, so that the film thickness cannot be measured.

In the latter measurement, since one fixed mirror and one vibrating mirror are provided, the moving amount of the vibrating mirror needs to cover the entire thickness of the object to be measured. Assuming a film thickness t and a refractive index n in the case of a single layer, nt / cos φ ₀ , and in the case of a multilayer film, “(total film thickness + the thickest film thickness) × nt / cos
Only the _value multiplied by φ ₀ is necessary. And the measurement accuracy deteriorates due to the linearity of the mirror moving device. Further, since the trigger is performed in the signal processing based on the drive signal of the moving device of the oscillating mirror, the reproducibility of the output waveform is lost due to the fluctuation (jitter).

As described above, in the above-described measurement, it is not very convenient when the thin film to be measured is not on the surface of the object to be measured but in the lower layer. In view of the above problems, an object of the present invention is to provide a film thickness measuring device capable of performing good measurement even when a thin film to be measured is below the surface of a measurement object.

[0011]

In order to solve the above-mentioned problems, an interference film thickness measuring apparatus according to the present invention comprises a light source which emits light including white light, and a light source which emits white light. First and second mirrors arranged to correspond to the difference in optical path length, a fine movement stage for oscillating the first and second mirrors to change the difference in optical path length, To the first and second mirrors and to the first and second mirrors.
Optical means for causing each of the reflected light of the mirrors to interfere with the reflected light from the measurement target; a first photodetector for detecting an interference state between the reflected light of the first mirror and the reflected light from the measurement target; A second photodetector that detects an interference state between the reflected light of the second mirror and the reflected light from the measurement target, and based on a detection output of one of the first and second photodetectors. The thickness of the object to be measured is measured from the interval between the detection outputs of the other photodetector.

The light source may include a white light source and a light source having a long coherent length, that is, a light source having a narrow spectrum width.

[0013]

In the interference film thickness measuring apparatus according to the present invention, white light is given to the object to be measured and the first and second mirrors, and the reflected light of the first and second mirrors and the object to be measured by the optical means. Interference with reflected light from The state of interference between the reflected light of the first mirror and the reflected light from the measurement object is a first photodetector,
The interference state between the reflected light from the second mirror and the reflected light from the measurement target is detected by the second photodetector.

Here, the first and second mirrors are vibrated by the fine movement stage, and the optical path length of the white light changes, so that the interference between the reflected light and the reflected light from the object to be measured changes. If the reflected light from the first (or second) mirror and the reflected light from the measurement target are equal to the optical path length from the light source, the detection output from the photodetector shows a peak.

When the thin film to be measured is covered with a transparent body, the reflected light from the measured object is a component of light reflected on the surface of the transparent body (hereinafter, a first component) and a component of light reflected on the upper surface of the thin film. (Hereinafter, a second component) and a component of light reflected on the lower surface of the thin film (hereinafter, a third component). And
If the difference between the optical path lengths from the light source to the first and second mirrors is adjusted to about the thickness of the transparent body, the second and third mirrors will be located near the position of the mirror where the interference state with the first component shows a peak. The state of interference with the third component shows a peak due to the reflected light of the other mirror. These peaks are detected by the first and second photodetectors, and the peak position of the interference state of the second and third components can be detected using the peak of the interference state with the first component as a mark. The thickness of the thin film is detected from the difference between the vibration positions of the fine movement stage indicating the peak of the interference state between the second and third components.

In the case where the light source includes a white light source and a light source having a long coherent length or a narrow spectrum width, the latter light source is used to obtain the interference state with the first component, the second and third light sources. The angle of the first and second mirrors and the difference in optical path length are adjusted while monitoring the interference state with the components with the first and second photodetectors, and the difference in optical path length is about the thickness of the transparent body. Can be adjusted so that

[0017]

An embodiment of the present invention will be described with reference to the drawings.
The description of the same or equivalent components as those of the above-described conventional example will be simplified or omitted.

FIG. 1 shows the configuration of an optical system of an interference film thickness measuring apparatus according to the present invention. The light source of this device is composed of a white light source 104a and a light source 104b,
As the white light source 104a, a light source that emits a light beam having a very short interference length and being nearly parallel is used. The light source 104b has L
An ED, an SLD, or the like is used to obtain light having a long coherent length (several tens times the light of the white light source 104a), that is, a light having a narrow spectrum width. White light source 10
White light from the light source 4a and light from the light source 104b are directed to the half mirror 106 by the half mirror 104c. A bandpass filter may be placed after the white light source 104a instead of the light source 104b.

Half mirror (beam splitter) 106
Converts white light (or light from the light source 104b) into the object 10 to be measured.
3 and mirrors R1 and R2, and mirror R
The reflected light (reference light) of each of R1 and R2 is measured 103
To interfere with the reflected light from the
3 and the distances to the mirrors R1 and R2 are almost the same. The mirrors R1 and R2 are arranged on the fine movement x stage 110 at a distance of d, and the optical path lengths of the light reflected by these are different. The distance d can be adjusted by the adjustment stage 110b, and the fine movement x
Mirrors R1 and R2 while maintaining distance d on stage 110
The whole vibrates.

With the arrangement shown in the figure, white light (or light from the light source 104b) is partially
And reflected toward the measurement target 103, and a part of the mirror R
Head to 1, R2. Then, the light reflected by the measurement target 103 and the light reflected by the mirrors R1 and R2 are directed to the mirror 130. The light from the half mirror 106 includes the reflected light from the mirror R1 and the reflected light from the mirror R2,
3 and the reflected light interfered. Mirror 13
0 is composed of two mirrors, among the light from the half mirror 106, the interference light with the reflected light of the mirror R1, and the mirror R
The interference light with the reflected light of the two
1, for branching to D2.

FIG. 2 shows an example of a sample in which the object 103 to be measured is a liquid crystal panel.
a (refractive index n ₂ ) has a structure sandwiched between glasses 103b and 103c (refractive index n ₁ , refractive index n ₃ ). The reflectances s ₁ , s ₂ , and s of the surface S1 of the glass 103b, the surface S2 of the liquid crystal layer 103a, the surfaces S3 and S4 of the glass 103c.
_3, assuming that with s _4, the surface S2, as compared to the reflected light of the surface S1, light reflection surface S3 are become very weak.
There are also so many reflections. Further, the thickness d ₂ of the liquid crystal layer 103a is usually about 4 μm,
Since the thickness is very small compared to the thickness of about 1.1 mm of the layers 03b and 103c, it is very difficult to measure with the conventional method. Using such an object to be measured, the liquid crystal layer 1
Explaining the measurement of the film thickness d ₂ of 03a.

First, the distance d is set to the thickness d of the glass 103b.
_The value is adjusted so as to be approximately “d ₃ × n ₁ ” according to ₃ . At this time, the light from the light source 104b is used instead of the white light source 104a. Of the light of the light source 104b,
The interference state between the reflected light of the surface S1 of the glass 103b and the reflected light of the mirror R1 is monitored by the photodiode D1 so that the detection output of the photodiode D1 shows a peak. Next, the state of interference between the reflected light of the surface S2 of the liquid crystal layer 103a and the reflected light of the mirror R2 will be described with reference to a photodiode D
The distance d is increased while monitoring in step 2, and the distance d becomes "d ₃
× n ₁ ”, that is, the mirror R2 is positioned at the distance d when the detection output of the photodiode D2 is at the peak.

After the above preparation, measurement of the film thickness d ₂ is started.

The white light source 104 is used instead of the light source 104.
a, mirrors R1 and R2
When the whole is vibrated (about ± (d4 / 2) + α), the photodiodes D1 and D2 are driven in accordance with the vibration of the mirrors R1 and R2.
The detection output of No. 2 changes. 3A and 3B show the state at that time, wherein FIG. 3A shows the vibration position of the fine movement x-stage 110 (mirrors R1 and R2), and FIG.
1 shows a detection output, and (c) shows a detection output of the photodiode D2. By the above adjustment, when the vibration position is substantially zero, the detection outputs of the photodiodes D1 and D2 both show peaks. When the vibration position becomes the thickness d ₂ (negative direction), a peak due to interference between the reflected light from the surface S3 of the liquid crystal layer 103a and the reflected light from the mirror R2 is detected (however, the refractive index is ignored). ).

FIG. 4 (a) ~ (d), the detection output of the photodiode D1, D2 (FIG. 3 (b), (c) ) to the waveform shaping, the signal processing to obtain a film thickness d ₂ It is a conceptual diagram. (B) the signals w ₁ obtained by waveform shaping the detection output of the photodiode D1, indicating the (e) is a signal w ₃ that waveform shaping detection output of the photodiode D1. The signal w ₂ (FIG. 4C) obtained by delaying the signal w ₁ by δ and the fine movement x
An AND with the drive signal t ₂ (FIG. 4A) of the stage 110 is generated to generate a synchronization signal T. This can be obtained thickness d ₂ from the pulse interval ΔW of signal w ₃ as a trigger.

FIG. 5 shows an example of a circuit for this signal processing. The input CK1 of the synchronizing signal T, which is to obtain an output signal OUT of the distance ΔW of pulses corresponding giving signal w ₃ to the input CK2 to the thickness d _2. FIG. 6 shows a timing chart of the operation.
FIG. 6C shows the state of each flip-flop.

Now, the photodiodes D1, D2
It is difficult to adjust the distance d between the mirrors R1 and R2 so that the detection outputs (FIGS. 3 (b) and 3 (c)) have peaks at the same timing. . FIG. 7 shows this state, where (a) shows the vibration position of the fine movement x stage 110 (mirrors R1 and R2),
(G) is the detection output of the photodiode D1, (b) to
(F) shows the detection output of the photodiode D2. In the case of (c), the detection output timings of the photodiodes D1 and D2 are synchronized, but (b)
Indicates a case where the vehicle is late, and (c) indicates a case where the vehicle advances.
In the case of (c), the circuit of FIG. 5 can also process,
In the cases of (a) and (d) to (e), it is necessary to change the number of flip-flop stages. However, the circuit shown in FIG. 8 can handle such a case.

FIGS. 9A and 9B show a timing chart of the operation, wherein FIG. 9A shows a drive signal t _{2 for} the fine movement x stage 110, and FIG. 9B shows a signal w ₁ obtained by shaping the detection output of the photodiode D1. . Then, using the signal w ₂ obtained by delaying the signal w ₁ by [pi / 2 (FIG. 9 (c)), taking the AND between this and the driving signal t ₂ of the fine x stage 110 make a synchronization signal T (FIG. 9 (D)). Photodiode D1
The detection output to the input CK2 signal w ₃ that waveform shaping of the output signal OUT of the distance ΔW of pulses corresponding to the film thickness d ₂ is given to the input CK1 of the synchronizing signal T is obtained. FIG. 9 (e)
~ (I) is shows the case where the deviation of the above the signal w ₃ occurs respectively, it is also possible to detect the distance ΔW of signal w ₃ from the rise of the synchronizing signal T pulse in the case of these. Thus, the film thickness d ₂ can be obtained from the pulse interval ΔW.

As described above, the measurement is performed with the distance d between the mirrors R1 and R2, and the mirrors R1 and R2 are oscillated about the thickness of the liquid crystal layer inside the object to be measured. An interference state with the reference light is detected. The detection output is used as a trigger to detect the interference state between the detection output of the photodiode D1 and the light reflected on the upper and lower surfaces of the thin film inside the measurement object. You can ask for it. In particular, the measurement can be performed with high accuracy because the vibration is small.

The present invention is not limited to the above-described embodiment, but can be variously modified.

For example, FIG. 10 specifically shows the configuration of the light source, and uses a lens 131 instead of the mirror 130 to transmit the interference light with the reflection light of the mirror R1 and the interference light with the reflection light of the mirror R2. This is applied to the diodes D1 and D2. In this way, if the arrangement is such that a beam is applied to the photodiodes D1 and D2 without branching by a mirror, detection can be performed similarly.

Although FIGS. 1 and 10 show the measurement using one white light beam, two beams are reflected by each mirror so that the differences in the optical path lengths are different. May be.

[0033]

As described above, according to the present invention, the reflected light from the object to be measured is a component of the reflected light on the transparent body surface (first component) and a component of the reflected light on the thin film upper surface (second component). ), The peak of the interference state of each component (second component) of the reflected light on the lower surface of the thin film is detected by the first and second photodetectors, and the peak of the interference state with the first component is used as a mark to detect the interference. By detecting the peak position of the interference state between the second and third components, it is possible to detect the thickness of the thin film under the transparent body in a nondestructive manner even when the measurement target is covered with the transparent body.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an optical system of the present invention.

FIG. 2 is a diagram showing a sample to be measured.

FIG. 3 is a diagram showing detection outputs of photodiodes D1 and D2 when mirrors R1 and R2 are vibrated.

FIG. 4 is a conceptual diagram of signal processing.

FIG. 5 is a diagram illustrating an example of a circuit for signal processing.

FIG. 6 is a diagram showing the operation of the circuit of FIG. 5;

FIG. 7 is a diagram showing detection outputs of photodiodes D1 and D2 when mirrors R1 and R2 are slightly displaced.

FIG. 8 illustrates an example of a circuit for signal processing.

9 is a diagram showing a timing chart of the operation of the circuit in FIG.

FIG. 10 is a diagram showing a configuration of a modification.

FIG. 11 is a diagram showing a basic configuration of a Michelson interferometer.

FIG. 12 is a configuration diagram of a conventional example.

[Explanation of symbols]

103 ... Measurement object, 103c ... Liquid crystal film, 103a, c ...
Glass, 104a, 104b ... light source, 104c, 106
... half mirror, 110 ... vibration x stage, D1, D2
... photodiodes, R1, R2 ... mirrors

 ──────────────────────────────────────────────────続 き Continuation of front page (72) Inventor Saraji Ichie 1126 No. 1 Ichinomachi, Hamamatsu-shi, Shizuoka Prefecture Inside Hamamatsu Photonics Co., Ltd. (56) References 2-52205 (JP, A) JP-A-59-131106 (JP, A) JP-A-55-29708 (JP, A)

Claims (2)

(57) [Claims] A light source that emits light including white light; a first mirror and a second mirror that are arranged so that a difference in an optical path length of the white light corresponds to a thickness of a transparent body on a surface to be measured; A fine movement stage for vibrating the first and second mirrors so as to change the difference in the optical path length; and providing the white light to the object to be measured and the first and second mirrors; Optical means for causing each of the reflected lights of the second mirror to interfere with the reflected light from the object to be measured; and first light for detecting an interference state between the reflected light of the first mirror and the reflected light from the object to be measured. A detector, and a second photodetector that detects an interference state between the reflected light of the second mirror and the reflected light from the measurement target, and the first or second photodetector is provided. Is the interval between the detection outputs of the other photodetector based on the detection output of An interference film thickness measuring device for measuring the film thickness of the object to be measured. 2. The apparatus according to claim 1, wherein the light source includes a white light source and a light source having a long coherent length, that is, a light source having a narrow spectrum width.