JPS6199805A - Step measuring device - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a step measuring device that detects the presence of a reflective surface at a focal position and measures, in particular, minute steps or film thickness.

[Background of the invention]

In recent years, as the integration of semiconductor devices has progressed, the surface structures of semiconductor devices have become finer. Therefore, in measuring the surface structure of semiconductor devices, lateral resolution on the order of microns has come to be required. Furthermore, non-contact, non-destructive testing is desirable in order to return the measured samples to the original production line. Measurement of surface structure,
In other words, methods for measuring differences in level include the stylus method, the method of observing interference fringes of light, the method of using the focus position of a microscope,
There are optical cutting methods, observation using a scanning electron microscope, tipped needle methods, etc., but each of the above methods has the following drawbacks. In the stylus method, which has good step sensitivity, the sample and the tip of the stylus are in contact with each other, and because the tip of the stylus is thin, strong pressure is applied to the sample, which may destroy the surface of the sample. Furthermore, even in a non-contact manner, observation using a scanning electron microscope is a type of destructive inspection, as damage and charge-up to the sample may occur due to the electron beam. Other methods that use light can be non-contact, non-destructive inspections, but the method that observes optical interference fringes measures the level difference from the deviation of the interference fringes, so it is possible to measure deviations when the surface structure changes on the micron order. This makes it difficult to measure the difference in level. In the method of using the focus position of the microscope, the focus position of the microscope is visually adjusted for the upper and lower sides of the step, and the height of the step is determined from the difference in distance between the focus positions. Since this is done visually, individual differences occur and accurate measurement is difficult. In addition, in the light cutting method, a slit light is irradiated obliquely onto a step, and the height of the step is estimated from the length of the shadow of the slit light. This method also requires irradiation of light from an oblique direction, so there is no lateral resolution. On the other hand, since the lateral resolution of the tip probe method can be on the order of microns, it has the potential to meet the measurement requirements of semiconductor devices. Since it is used as a probe needle, it has sufficient resolution in the lateral direction, and its purpose is to detect displacement, but it can also be used as a step measurement device if the sample is scanned in the lateral direction. However, this device cannot perform accurate measurements unless the intensity distribution of the light reflected from the sample surface is symmetrical with respect to the optical axis.

[Purpose of the invention]

An object of the present invention is to provide a non-contact, non-destructive level difference measuring device that has a lateral resolution on the order of microns and can perform stable level difference measurements even when the intensity distribution of reflected light is disturbed.

[Summary of the invention]

The present invention focuses collimated light into a minute spot and irradiates it onto a measurement surface, and uses light interference to detect with high accuracy whether or not the measurement surface is on the narrowed-down correct focal plane. FIG. 4 shows how the wavefront of the light returned from the objective lens 2 changes when the distance between the reflecting surface and the objective lens 2 changes. The reflective surface 1-1 is located at the focal point of the objective lens 2, and the bride surface of the returned light is perpendicular to the optical axis. The reflective surfaces 1-2 and 1-3 are deviated from the focal position, and their respective wavefronts are curved as shown in 4-2 and 4-3. Therefore, when the reference beams with wavefronts perpendicular to the optical axis are caused to interfere with each other, no interference fringes appear when the reflecting surface is 1-1, but when the reflecting surface is away from the focal position, the interference fringes appear perpendicular to the optical axis. Since there is an optical path difference with respect to the surface, concentric interference fringes as shown in FIG. 5 are produced. Let us now consider the change in interference fringes when the phase of the reference light is changed. It is assumed that the phase of the reference light changes in the same way all around. When the reflecting surface is located at 1-1, the brightness changes uniformly as the phase of the reference light changes. On the other hand, when the reflecting surface is deviated from the focal position like 1-2 or 1-3, when the phase of the reference light changes, the dark parts of the interference fringes become brighter and the brighter parts become dark, which is repeated.

The difference in level is measured by detecting the change in the interference fringes and detecting the focal position. In addition, heterodyne interferometry is used to determine the state of interference fringes, using light with a frequency shift in the reference light.
The brightness or darkness of the interference fringes is determined based on the frequency change of the frequency displacement, and the focal position is detected and the step is measured. That is, the level difference measuring device according to the present invention includes coherent light, a light splitting element that splits the coherent light, and a first light splitting element that splits the coherent light.
a converging optical system that converges the coherent light of 1 to a minute spot on the surface of the measurement sample; a movable stage that changes the relative position of the convergence optical system and the surface of the measurement sample; and an optical path length of the divided second coherent light or a mechanism for changing the frequency; and a second mechanism for condensing the first coherent light reflected from the surface of the measurement sample and changing the optical path length or frequency.
The device is equipped with a photodetector that interferes with coherent light and detects interference fringes generated when the interference occurs, and a signal processing circuit that processes the signal of the photodetector.

[Embodiments of the invention]

Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a diagram showing the configuration of an embodiment of the level difference measuring device according to the present invention, FIG. 2 is a diagram showing the dependence of the optical power of the photodetector on the position of the reflecting surface in the above embodiment, and FIG. FIG. 7 is a diagram showing the configuration of another embodiment of the invention.

In FIG. 1, coherent light 6 emitted from a laser light source 5 is collimated using a lens 7-1, 7-2 and a pinhole 8. collimated coheren]・
The light 9 is amplitude-divided by a light splitting element (for example, a semi-transparent mirror) 10, and the divided first coherent light is narrowed down to a minute spot by the objective lens 2, and the reflected surface 1, which is the sample surface.
irradiated on top.

The sample is placed on a moving stage 11 that can be moved in three dimensions. The light reflected by the reflecting surface 1 passes through the light splitting element 1° and becomes one of the lights to be interfered with. The second coherent light transmitted through the light splitting element 10 is reflected by the plane mirror 12, returns to the light splitting element 1o, and is reflected again to become the other light for interference, a reference light.

In order to change the optical path length of the reference light, the plane mirror 12 is driven by an electrostrictive element 13 that vibrates in a direction perpendicular to the reflecting surface.

The interference light 14 is split by a light splitting element 15, the reflected light is detected by a photodetector 17-1, 17-2, and the transmitted light is detected by a photodetector 16, respectively. The photodetector 16 has a wide light-receiving area and can receive the entire interference fringe, and a voltage proportional to the amount of received light is generated at the output of the photodetector 16. The vibration caused by the electrostrictive element 13 is sinusoidal, and the amplitude is less than a fraction of the wavelength of the coherent light used. The frequency is W. It is.

Figure 2 shows only the WQ frequency component of the photodetector 16 extracted by the signal processing circuit 18, and (a) shows that the distance between the objective lens 2 and the reflective surface 1 is as shown in Figure 4 1-3. (b) shows the case where it is the same as the focal length as shown in Fig. 4 1-1, and (C) shows the case where it is longer than the focal length as shown in Fig. 4 1-2. ing. In both figures, the same scale is used for the horizontal and vertical axes. When the reflecting surface 1 is at the focal point, the amplitude of the signal is the largest, as shown in FIG. 2(b). The reason is that the photodetector 1
This is because the phases of the intensities of the interference light are aligned on the light receiving surface 6.

When the reflective surface 1 deviates from the focal position, interference fringes are generated and the brightness and darkness change alternately, so the amplitude does not become large. Therefore, while the reflecting surface l is moved up and down by the movable table 11, the output W of the photodetector 16 is measured. The operation of detecting the position of the movable base 1 where the amplitude of the component is maximum and the operation of moving the movable base 11 in the lateral direction are performed alternately, and the detection is performed with respect to the lateral movement distance of the movable base 11. By plotting the height, it becomes possible to measure the difference in level of the reflective surface 1. The signal processing device 18 processes W of the output of the photodetector 16. The frequency component of is extracted, the maximum value of its amplitude is detected, and at the same time, the position of the movable table 11 in the x, y, and z directions where the maximum value is realized is stored.

In this embodiment, the interference light 14 is further divided into a light splitting element 15.
and detected by photodetectors 17-1 and 17-2. The light-receiving surfaces of these two detectors are made small,
Receives part of the interference light. Photodetector 17-1.17-2
are arranged so that they are not equidistant from the optical axis in a plane perpendicular to the optical axis. The signal processing device 19 includes a photodetector 17-
The phase difference between the output of No. 1 and the output of No. 17-2 is detected, and the position of the moving table 11 in the X, Y, and Z directions when the two phases match is stored. When the phases match, the reflecting surface 1 and the focal position match. However, the signal from the photodetector 16 is processed in advance to determine the height of the movable table 11 at which the reflecting surface 1 and the focal position match, and the movable table 11 is set at that height, and then the Apply the method using phase difference. The method of using photodetectors 17-1, 17-2 is therefore used to further refine the processing results of photodetector 16.

Since a phase difference of about 2 to 3 degrees can be detected, when a 100x microscope objective lens is used as the objective lens 2, a step resolution of about 10X can be obtained. Although both of the signal processing circuits 18 and 19 are used in this embodiment, when the level difference is low, it is possible to measure the level difference by using only one of them. In addition, the signal processing circuit 19 detects a photodetector 17-1.
and 17-2, and based on that, move the moving platform 1.
It is also possible to apply feedback to the moving mechanism at a height of 1 to always make the phase difference zero. By plotting the height of the movable base 11 while moving the movable base 11 in the lateral direction, it is possible to know the state of the level difference.

Now consider a case where the reflective surface 1 is slightly tilted. It is assumed that the coherent light 9 has a Gaussian distribution symmetrical to the optical axis, and that the center of the focal plane and the reflective surface 1 coincide. At this time, the intensity distribution of the reflected light returning from the objective lens 2 is not symmetrical with respect to the optical axis. However, the wavefront is still perpendicular to the optical axis. Therefore, regardless of the inclination of the reflecting surface 1, the phase difference between the two interference lights incident on the photodetector 16 is the same on the light receiving surface, and at this time, the amplitude of the output of the photodetector 16 is maximized. Since the maximum value of this amplitude is detected, there is no influence from changes in the intensity distribution. Similarly, in the processing of the signals from the photodetectors 17-1 and 17-2, since the phase difference is taken into account, there is no effect even if the intensity distribution of reflected light changes.

In another embodiment shown in FIG. 3, coherent light emitted from a laser light source 5 is collimated using a lens 7-1, 7-2 and a pinhole 8, and the collimated coherent light 9 is sent to a light splitting element (for example, a half Transparent mirror) Divide the amplitude by 10. The divided first coherent light is passed through the objective lens 2.
The beam is narrowed down to a minute spot and irradiated onto a sample having a reflective surface l.

The reflecting surface 1 is installed on a movable table 11 that is movable in three dimensions. The light reflected by the reflective surface 1 is transmitted to the light splitting element 10
and becomes the first coherent light.

The coherent light 9 transmitted through the light splitting element 10 is frequency-shifted by, for example, an AO (acoust, ooptic) modulator. When the modulation frequency is W, the first-order frequency-shifted light is directed toward the plane mirror 21, is reflected, returns along the same optical path, and enters the AO modulator again. The frequency of the light emitted from the AO modulator ends up being shifted by two degrees. The frequency-shifted light is reflected by the light splitting element 10 and superimposed on the first coherent light as second coherent light. These interference lights 14 are split by a light splitting element 15, and some are detected by a photodetector 16, and the others are detected by photodetectors 17-1 and 17-2. The photodetector 16 has a wide light-receiving area and receives the entire interference fringe, and its output generates a voltage proportional to the amount of received light. Depending on the distance between the objective lens 2 and the reflective surface 1, the signal voltage takes the form shown in FIGS. 2(a), (b), and (C), similar to the first embodiment.

However, the frequency of all the signals is the same, 2W.

The amplitude is maximum when the reflective surface 1 of the sample is at the focal point. The reason for this is that the phases of the intensities of the interference light on the light receiving surface of the photodetector 16 are aligned, and when the reflecting surface 1 deviates from the focal position, interference fringes are generated and the brightness and darkness alternately change, so the amplitude does not become large. Therefore, while the reflective surface 1 is moved up and down by the moving table 11, the photodetector 16
The operation of detecting the position of the moving table 11 where the amplitude of the output of By plotting the height of 11, it becomes possible to measure the step difference in the sample. The signal processing device 18 detects the maximum amplitude value of the signal from the photodetector 16, and determines the amplitude of the moving platform 11 by X%'Y% at the maximum value.
It has a function to memorize the position in the Z direction.

In this embodiment, the interference light is further detected by a photodetector 17-1.17-
The signal processing device 19 detects the phase difference between the outputs of these photodetectors, and determines the position of the moving table 11 in the z direction when the phases of the two match. Therefore, as in the previous embodiment, the photodetector 1
The signal processing of 7-1.17-2 is used to further refine the processing results of the photodetector 16.

In the above embodiment, since it is possible to detect a phase difference of about 2 to 3 degrees, the microscope objective lens 100 is used as the objective lens 2.
When using a multiplier, a step resolution of about 10^ can be obtained. Furthermore, although both of the signal processing circuits 18 and 19 are used in this embodiment, when the level difference is low, it is possible to measure the level difference even if only one of them is used.

Coherent light has a symmetrical Gaussian distribution with respect to the optical axis,
Assuming that the center of the focal plane and the reflective surface 1 coincide, if the reflective surface 1 is slightly tilted, the intensity distribution of the reflected light returning from the objective lens 2 will not be symmetrical with respect to the optical axis.

However, the wavefront is perpendicular to the optical axis. Therefore, the phases of the intensities of the interference lights incident on the photodetector 16 are all aligned.

The amplitude becomes maximum when the phases of the intensities of the interference lights are aligned, and since this maximum value is detected, there is no influence of changes in the intensity distribution due to the inclination of the reflecting surface 1. Similarly, photodetector 17-
1 and 17-2 are also concerned with the phase difference, so
There is no effect even if the intensity distribution of reflected light changes.

〔Effect of the invention〕

As described above, the level difference measuring device according to the present invention includes a coherent light, a light splitting element that splits the coherent light, and a converging optical system that focuses the split first coherent light onto a minute spot on the measurement sample surface. a movable stage that changes the relative position between the converging optical system and the measurement sample surface; a mechanism that changes the optical path length or frequency of the divided second coherent light; and a first coherent beam reflected from the measurement sample surface. A photodetector that collects light, causes it to interfere with a second coherent light whose optical path length or frequency has been changed, and detects interference fringes that occur when the interference occurs; and a photodetector that processes the signal of the photodetector. By being equipped with a signal processing circuit, if the coherent light is narrowed down sufficiently with an objective lens, the lateral resolution can be reduced to about 1 μm, making it possible to measure the surface of semiconductor devices whose planar structure changes on the order of microns. be. Furthermore, in the present invention, since the phase of the interference light is detected to be aligned, it is possible to eliminate errors that occur when the intensity distribution of the reflected light on the sample surface changes and is not symmetrical with respect to the optical axis. Furthermore, even if the sample surface is composed of various side slopes and the reflectance differs locally, the present invention detects the phase of the interference light rather than the intensity of the reflected light, so measurement can be performed.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the dependence of the detector output on the position of the reflecting surface of one embodiment of the step measuring device according to the present invention, FIG. 3 is a diagram showing the configuration of another embodiment of the present invention, and FIG. FIG. 5 is a diagram showing the relationship between the position of the reflecting surface and the wavefront of reflected light, and FIG. 5 is a diagram showing interference fringes. 1... Measurement sample surface, 2... Lens, 10.15-=light splitting element, 11... Moving table, 12... Plane mirror, 13
°°° electrostrictive element, 16.17-1.17-2... photodetector, 18.19°°° signal processing circuit, addition... AO modulator.

Claims (1)

[Claims] Coherent light, a light splitting element that splits the coherent light, a converging optical system that converges the divided first coherent light onto a minute spot on the surface of the measurement sample, and a relative position between the convergence optical system and the surface of the measurement sample. a moving table that changes the
a mechanism for changing the optical path length or frequency of the divided second coherent light; and a second coherent beam that collects the first coherent light reflected from the surface of the measurement sample and changes the optical path length or frequency. A level difference measuring device comprising: a photodetector that detects interference fringes generated when the interference occurs; and a signal processing circuit that processes a signal from the photodetector.