JPH0569361B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a microgroove depth measuring device for measuring the depth of a microgroove or hole formed in a Si substrate for use in a VLSI capacitor.

[Conventional technology]

Generally, the groove for VLSI capacitor is 3 deep.
~5μm, width 0.5~1μm, pitch several μm~several tens of μm
In order to measure the depth, the Si substrate was cut and the cross section was observed using a scanning electron microscope. but,
Since this method is a destructive measurement, a measurement method using an interferometer is being researched as a non-destructive method of measuring the groove depth.

FIG. 10 is a diagram showing this measurement method, in which light emitted from a white light source 1 is transmitted to a lens 3 via a multimode optical fiber 2 that serves as a pinhole to take out a part of the light. It is divided into two directions by a half mirror 4, and one side is guided to a Si substrate 5 in which a groove for a capacitor is formed, is reflected there, and is outputted again via the half mirror 4, and the other side is a total reflection mirror. 6, where it is reflected and output again via the half mirror 4.

A group delay of 2d/c occurs between the light reflected on the surface of the Si substrate 5 and the light reflected on the groove surface, where d is the depth of the groove and c is the speed of light. So 2d/
A light source with a coherence time shorter than c,
When the Si substrate 5 on which the capacitor groove is formed is irradiated, the light reflected from the Si substrate surface and the light reflected from the groove surface are systematically uncorrelated with each other. Therefore, when the total reflection mirror 6 is moved in the direction of the arrow indicated by the symbol "A", two interference peaks are detected by the detector 8 where the light is focused by the lens 7. When the alternating current component of this detection result is amplified by the amplifier 9 and recorded by the storage scope 10, the depth of the groove is converted into the distance between the positions of the total reflection mirror 6 giving two peaks.

[Problem that the invention seeks to solve]

However, in such conventional devices, the degree of spatial coherence of light is very small, and the spatial coherent distance within the cross section of the light beam is approximately
It will be about 10μm. For this purpose, the total reflection mirror 6
and Si substrate 5 must be fine-tuned with high precision on the order of 10 ^-3 to match the respective coherent regions of the combined light. Since the reflected light is a parallel beam as shown in FIG. 11, accurate fine adjustment is required each time the Si substrate 5 is fixed. Therefore, the workability is poor and the fine adjustment device is also large in size.

[Means for solving problems]

In order to solve such problems, this invention
This device is equipped with means for focusing the light that has passed through the half mirror onto the object to be measured, and means for guiding the light reflected from the object to be measured to the Michelson interferometer.

[Effect]

There is no need to impose strict positional accuracy when fixing the Si substrate.

〔Example〕

FIG. 1 shows an embodiment of the present invention, and the same symbols are used for the same parts and corresponding parts as in FIG. 10. 6a and 6b are total reflection mirrors, 11 is a lens,
12 is a half mirror, total reflection mirror 6a,
6b and the half mirror 12 constitute a Michelson interferometer.

In the device configured in this manner, the light emitted from the white light source 1 is incident on the optical fiber 2, and the light is made parallel by the lens 3 and then emitted. The emitted light passes through the half mirror 4 and is focused by a lens 11 having a relatively long focal point, and irradiates the Si substrate 5. Here, the lens 11 is installed so that its focus is on the surface of the Si substrate 5. The reflected light from the Si substrate 5 is turned into parallel light by the lens 11, guided to the half mirror 12 via the half mirror 4, where it is branched into two directions, arm A and arm B, and is split into two directions by the total reflection mirror 6.
It is reflected by a and 6b. The reflected light is multiplexed by a half mirror 12, enters a detector 8 via a lens 7, and is detected. Here, at least one of the total reflection mirrors, for example, the total reflection mirror 6b, is indicated by an arrow C
If the total reflection mirror 6 is moved in the direction, the AC component in the interference intensity caused by the movement of the total reflection mirror 6 is amplified by the amplifier 9 and sent to the storage oscilloscope 1.
Stored as 0.

FIG. 2 shows the state of light incident on the Si substrate 5 and reflected. FIG. 3 shows the propagation of reflected light when the lens 11 is placed in front of the Si substrate 5. S
1 incident light and S2 are reflected lights. When the surface of the Si substrate 5 is perpendicular to the axis of the lens 1 and placed on the focal point, the reflected light S2 propagates in the opposite direction along the same optical path as the incident light S1. . Next, when the Si substrate 5 is arranged at an angle and is fixed so as to be near the focal point of the lens 11, the reflected light becomes a parallel ray after passing through the lens 11, and is parallel to the incident light S1. Become. Furthermore, since it has been proven from wave theory that light becomes parallel and converges near the focal point, it can be assumed that the light reflected from the Si substrate 5 placed near the focal point comes out from the focal point. Therefore, even if the Si substrate 5 is tilted to such an extent that the reflected light passes through the lens 11, the reflected light passes through the lens 11, becomes a parallel light beam, and propagates in parallel with the incident light S1.

Therefore, even if the position of the Si substrate 5 is slightly shifted from the focal position when fixing the Si substrate 5, the reflected light S2 enters the Michelson interferometer in parallel with the incident light S1. For this reason, the reflected lights propagated through arms A and B within the interferometer are combined without any deviation, so the interference output does not decrease. When the substrate 5 shifts, the interference portion shifts, the interference area decreases, and the output decreases. Therefore, in the past, it was necessary to make fine adjustments to the optical system each time the Si substrate was fixed, but with the device of this invention, even if the position of the Si substrate shifts on the order of microns, the reflected light is reliably reflected by Michelson interference. enters the meter. Once you have fine-tuned the total reflection mirror in the Michelson interferometer, there is no need to fine-tune it every time you replace the Si substrate 5, even if the installation error is on the micron order. .

The measurement principle in a configuration in which the irradiation unit for the Si substrate 5 to be measured and the Michelson interferometer are separated will be explained. In Fig. 4, Si with a groove 5a for a capacitor is shown.
When the substrate 5 is irradiated with light beams 15a and 16a, reflected lights 15b and 16a are reflected from the Si substrate surface 5b and the surface of the groove 5a.
16a, a group delay time difference corresponding to 2d/c occurs. Therefore, when irradiating with a light source that has a coherent time shorter than 2d/c, the luminous flux 1
5b and the light beam 16b are separated and have no correlation. When this reflected light is incident on a Michelson interferometer, the interference is as shown in FIG.

In FIG. 5, a is a diagram showing the time relationship of the light flux of arm A. Since the total reflection mirror 6a of arm A is fixed, the relative time of the light flux propagating through arm A and incident on the detector 8. As the luminous flux 15b
Let T=0 be the luminous flux 15c at arm A for
The luminous flux 16c at arm A with respect to the luminous flux 16b is T
Set the relationship so that =2d/c.

FIGS. 5b to 5d are diagrams showing the time relationship of the luminous flux of arm B. Since the total reflection mirror 6b moves in the direction of arrow C in FIG. 1, the luminous flux 15
The luminous flux at arm B corresponding to b, 16b is 15
d and 16d, the light beams 15c and 16c and the light beams 15d and 16d interfere as follows as the total reflection mirror 6b moves. First, the luminous flux 15c and the luminous flux 16d interfere (symbols in FIG. 5), and then the luminous flux 16c and the luminous flux 16d (symbol) and the luminous flux 16
c and the beam 16d interfere (symbol), and finally the beam 1
6c and the light beam 15d interfere (symbol). Therefore, three interference peaks are generated, and the time difference between each peak is 2d/c (2d in distance). Therefore, the time difference between each peak, that is, the total reflection mirror 6b
By measuring the moving distance of the groove, it is possible to measure the depth d of the groove. Note that FIG. 6 is a diagram showing this interference state.

Figure 7 shows the waveform observed when measuring the depth of the groove using the system shown in Figure 1, and Figure 7a
Figure 7b shows the case where a Si substrate without a capacitor groove is used, and Fig. 7b shows the case where a Si substrate with a groove is used. The distance between each peak was 2.7 μm, which agreed with the value measured by a scanning electron microscope.

FIG. 8 shows the change in output voltage when a grooved Si substrate is placed in the measurement system and the substrate is tilted with respect to the optical axis. In the figure, white circles indicate measurements made using the apparatus shown in FIG. 1, and black circles indicate measurements made using a conventional measurement system. An incident angle of 0° indicates that the light was incident perpendicularly to the Si substrate. As can be seen from the figure, when using the conventional measurement system, the permissible angle for Si substrate placement is extremely small at ±10 ^-3 , but with this invention it is ±0.8 degrees, so the mounting error that occurs each time the board is fixed is not a problem.

FIG. 9 is a diagram showing another embodiment, 20,
21 is a lens, 22 is a multi-mode optical fiber, and in this way, the irradiating part for the Si substrate 5 to be measured and the Michelson interferometer, which is the measuring part, are connected to the multi-mode optical fiber which is a light guide. 22 and the lenses 20 and 21, the irradiation part can be made movable, and the irradiation part can be installed in a capacitor groove manufacturing system to observe the depth of the groove in real time when forming the groove. I can do it.

In this invention, it is necessary to use a point light source as the light source in order to focus the light beam to a small diameter on the Si substrate to be measured and to obtain a uniform light intensity distribution. The light from the light source may be passed once through an optical fiber and the light from the output end thereof may be used, as in the example shown in FIG. 2, or the light may be passed through a pinhole.

〔Effect of the invention〕

As explained above, this invention converts light from a point light source into parallel light, passes it through a half mirror, focuses it on the measurement target, and guides the reflected light to a Michelson interferometer. Because of this, there is no need to make fine adjustments to the mounting position every time the object to be measured is fixed, which has the effect of improving work efficiency, simplifying the device, and making it more compact.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of a measurement system showing an embodiment of the present invention, Fig. 2 is a perspective view showing the state of light converging on the measurement object, and Fig. 3 is a diagram of the light incident on the measurement object and reflected. Figure 4 is a side view showing the process, Figure 4 is an enlarged sectional view of the groove to be measured, Figure 5 is a diagram showing the interference relationship of light, Figure 6 is a diagram showing the time relationship of the luminous flux, Figure 7 is a diagram showing the time relationship of the luminous flux. FIG. 8 is a diagram showing the relationship between the incident angle and relative output, FIG. 9 is a block diagram showing another embodiment, FIG. 10 is a block diagram showing a conventional example, and FIG. 11 is a diagram showing the interference state. FIG. 11 is a perspective view showing the state of light converging on the measurement target in FIG. 10; 1... White light source, 2, 22... Optical fiber,
3, 7, 11, 20, 21...lens, 4, 12
...Half mirror, 5...Si substrate, 5a...groove,
5b...Surface, 6, 6a, 6b... Total reflection mirror, 8... Detector, 15a, 15b... Luminous flux.

Claims (1)

[Claims] 1. A point light source of white light, a lens that converts the light emitted from the point light source into a parallel light beam, a half mirror placed in the optical path of the parallel light beam, and a point light source that outputs the white light through the half mirror. A Michelson interferometer includes a lens that focuses parallel light rays on the surface of the measurement target, at least one of the two total reflection mirrors is variable in position, and the Michelson interferometer is supplied with light output from the half mirror. A microgroove depth measuring device consisting of a detector that detects the output of a son interferometer. 2. The microgroove depth measuring device according to claim 1, wherein the Michelson interferometer is equipped with an optical fiber and a guide via lenses provided at both ends of the optical fiber. .