JPS63229309A - Method and apparatus for measuring depth of fine pattern - Google Patents

Abstract

PURPOSE:To individually measure the depth or difference in level of a minute hole or groove with good accuracy, by projecting the light from a wavelength variable light source on the surface to be measured as a minute spot and detecting the interference fringe of the zero order light of the reflected light therefrom and the reflected light from a reference surface. CONSTITUTION:The luminous flux from a wavelength variable light source 1 is divided into two by a light splitting interference means 3 and one of them is condensed in a start spot form to be projected on two surfaces 10a, 10b to be measured forming difference in level of the minute pattern provided to the surface of a specimen 10 as sample light by a projection optical system 9 while the other luminous flux is projected on a reference surface as reference light. The respective reflected sample lights from two surfaces 10a, 10b to be measured of the projected spot light go toward an interference means 3 through the optical system 9. The reflected reference light from the reference surface 8 interferes through the interference means 3. The intensity of the interference fringe generated by interference is detected by a photoelectric detection means 14 while the wavelength of the light source 1 is changed and the light path difference between two surfaces to be measured, that is, the depth of the fine pattern can be calculated from the detected spectrum distribution.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention relates to a method for measuring the depth of a minute groove or a minute hole, etc. formed on a semiconductor IC circuit board, etc. The present invention relates to a measuring method and apparatus suitable for measuring the depth of minute differences in unevenness, minute holes, grooves, etc.

(Prior art) With the increasing integration of semiconductor IC circuits, semiconductor devices are
The conventional two-dimensional planar structure is being converted to a three-dimensional three-dimensional structure. For example, in order to form the capacitance part of an IC on a silicon substrate, technology has advanced to form grooves or holes called trenches with an opening of 1 to 2 μm and a depth of 4 to 5 μm using processing methods such as etching. As a result, it has become necessary to individually measure the depth of these etched holes and grooves. As a method for measuring the depth, Japanese Patent Application Laid-Open Nos. 60-136324 and 6
1-107104, a method of measuring from the spectral intensity distribution of diffracted light, and a method of measuring from the spectral intensity distribution of diffracted light,
As disclosed in Japanese Patent No. 5708, various proposals have been made, including a method of measuring a single hole by adding an imaging optical system and a spatial filter to a step measuring device using optical interferometry.

(Problems to be Solved by the Invention) However, in the former method of measuring from the spectral intensity distribution of diffracted light, the light is incident on a wide area and the average value of the depth of the three-dimensional pattern is measured. Therefore, it is difficult to measure the depth or level difference of a single hole or groove in a specific area. On the other hand, with the latter step measurement method, it is possible to measure the depth of a single hole or groove. Therefore, even if a dye laser is used as a wavelength-tunable coherent light source, the dye laser has a narrow variable range and may not be able to perform measurements with sufficient accuracy.

That is, the method disclosed in Japanese Patent Application Laid-Open No. 61-235708 utilizes interference between reflected light from the inside of a groove or hole and weak diffracted light from the surface, so it is difficult to sufficiently detect interference light. Can not. Moreover, in a dye laser, the range in which the usable wavelength can be changed with one dye is approximately 50nI11 to 1100n at most. However, with the method described above that uses interference between the diffracted light from the surface and the reflected light from inside the groove or hole, the wavelength variable range required for measurement must be sufficiently wide, especially when the depth is shallow. Information on a relatively wide spectral range is required.

Therefore, the dye of the dye laser should be combined with the solvent for at least 2
It is necessary to change more than one species, but the dye exchange requires a lot of time and has the disadvantage that handling is extremely complicated.

The purpose of the present invention is to solve the problems of the above-mentioned known measuring methods, and to provide a measuring method that can efficiently measure the depth and level difference of minute holes and grooves individually and with high precision and reproducibility. The purpose is to provide equipment.

[Structure of the invention]

(Means for Solving the Problems) In order to solve the above problems, in the present invention, the light beam from the wavelength tunable light source is divided into two, and one light beam is focused in a spot shape to form a spot on the sample surface. The sample beam is projected as a sample beam onto two measurement surfaces having a step formed in the same direction, and the other beam is projected onto the reference surface as a reference beam, and the spot-shaped sample light is reflected by the two measurement surfaces onto which it is respectively projected. The reflected sample light reflected from the reference surface is caused to interfere with the reflected reference light reflected from the reference surface, the intensity of each interference fringe produced by the interference is photoelectrically detected while changing the wavelength of the light source, and each detected spectrum is The means of solving the problem is to change the optical path difference between the reference beam and the sample beam so that at least two peaks occur in the distribution, and to determine the optical path difference between the two measurement surfaces from each of the obtained spectral distributions. It is.

(Function) The light beam from the light source (1) whose wavelength can be changed is divided into two by the beam splitting and interference means (3), and one of the light beams is condensed into a starting spot by the projection optical system (9) as sample light. The light beam is projected onto two measurement surfaces (10a, 10b) forming a micropattern step provided on the sample surface, and the other beam is projected onto the reference surface (8) as a reference beam. The respective reflected sample lights reflected by the two measurement surfaces (LOA, 10b) of the projected spot light are directed to the beam splitting interference means (3) through the projection optical system (9), and are reflected by the reflected reference light from the reference surface. and interfere with each other via the beam splitting interference means (3). The intensity of interference fringes caused by the interference is detected by a photoelectric detection means (14) while changing the wavelength of the light source (1), and the optical path difference between the two measurement surfaces is determined from the detected spectral distribution.

Now, the optical path difference between the reference surface (8) and the sample surface (10) is d,
The mutually adjacent peak wavelengths obtained from the spectral distribution of the detection signal from the photoelectric detection means (14) are expressed as λ1. λ
2, the optical path difference d is d - λ3, S2/2 (λ, -
λ2).

Therefore, the optical path difference between the reference surface (8) and one measurement surface (10a) and the other measurement surface (
10b), an optical path length variable means (4) for changing the optical path length of either the reference beam or the sample beam is provided. As a result, even if the wavelength variable range of the light source is narrow, two peak wavelengths can be detected.
Measurement surface (10a), reference surface (8) and measurement surface (10b)
) and the reference surface (8), and from the difference in each optical path difference, it is possible to measure the step difference between the two measurement surfaces, that is, the depth of the fine pattern.

(Example) Next, an example of the present invention will be described based on the attached drawings.

FIG. 1 is a schematic configuration diagram of an optical system showing an embodiment of the present invention to which the principles of an interference microscope are applied. In FIG. 1, light from, for example, a dye laser 1 used as a coherent variable wavelength light source is expanded by a beam expander 2, and then divided into two beams by a beam splitting prism 3 having a splitting surface 3a of a half mirror. The divided surface 3a
One of the light beams (reference light) that has passed through passes through an optical path difference prism 4 and a variable light amount filter 5, which will be described in detail later, and then is focused by an objective lens 6 onto a reflective surface (reference surface) 8 of a mirror 7. . Further, the other light beam (sample light) reflected by the dividing surface 3a is focused by the objective lens 10 onto a sample surface 10 placed on a movable sample stage 11.

Note that the objective lens 6 on the reference light side is a lens having the same aberration as the objective lens 9 on the sample side.
When the N and A (numerical aperture) of the sample light are small and the sample light is substantially aberration-free, there is no problem even if there is no objective lens on the reference light side. The reference light reflected by the reference surface 8 and the sample light reflected by the sample surface 10 travel backwards through the objective lenses 6 and 9, respectively, and match at the splitting surface 3A of the beam splitting prism 3 and interfere. Further, the interference light is imaged by the imaging lens 12 onto an aperture 13 having a circular opening provided at a position conjugate with the sample surface, and the interference light that has passed through the aperture 13 is detected by a photoelectric conversion element. It is photoelectrically detected by the device 14.

On the other hand, the illumination light from the white light St5 for observing the sample surface 10 is condensed by a condenser lens 16, and a beam splitter 17 provided on the optical path between the beam expander 2 and the beam splitting prism 3 , and illuminates the sample surface 10 via the beam splitting prism 3 and the objective lens 9. The image of the sample surface 10 illuminated by the illumination light is formed by the objective lens 9, the beam splitting prism 3, and the imaging lens 12.
An image is formed on the field diaphragm 19 via the overhead viewing prism 18, and the image is observed through the eyepiece lens 20.

The optical path difference prism 4 provided on the reference optical path on the side of the reference surface 8 is composed of two wedge-shaped prisms that are moved relative to each other in the vertical direction in FIG. This is to change the optical path length of the reference light between the dividing surface 3a and the reference surface 8 to arbitrarily change the optical path difference between the reference light on the reference surface 8 side and the sample light on the sample surface 10 examples.

Further, the variable light amount filter 5 is used to appropriately reduce the amount of light at the reference destination reflected by the reference surface 8, thereby improving the visibility of interference fringes detected by the detector 14.

The embodiment shown in FIG. 1 is constructed as described above.
The fine pattern of grooves and holes formed on the sample surface IO is observed through an observation optical system consisting of a white light source 15, an objective lens 9, an imaging lens 12, an overhead prism 18, and an eyepiece 20. In addition, while observing through the eyepiece 20, the part where the laser beam from the dye laser 1 is focused through the objective lens 9 and projected as a laser spot, the sample surface 10 along with the sample stage 13 is focused on the objective lens 9. It is set by one-dimensional movement within a plane perpendicular to the optical axis. The laser beam from the dye laser 1 is expanded by a beam expander and then split in amplitude into two beams by the splitting surface 3a of the beam splitting prism 3, and one of the reference beams passes through the objective lens 6 onto the reference surface 8. The light is focused on the reference surface, reflected by the reference surface, and sent back to the reference optical path through the objective lens 6. The other divided sample light is focused onto the sample surface 10 through the objective lens 9 which has the same aberration as the objective lens 6, and the light reflected from the sample surface IO passes through the objective lens 9 and passes through the sample optical path. go backwards. The reference light reflected from the reference surface 8 and the sample light reflected from the sample surface 1O interfere with each other at the splitting surface 3a of the beam splitting prism 3. The interference light that has interfered at the dividing surface 3a is imaged on the aperture 13 having a circular opening via the imaging lens 12, and further, the aperture 1
The interference light that has passed through the detector 3 is photoelectrically detected by the detector 14.

By the way, the phase of interference fringes obtained by light interference changes by changing the wavelength of the coherent light emitted from the dye laser 1, and the brightness and darkness of the fringes due to this change is detected by the detector 13.
photoelectrically detected by

From the changes in the photoelectric signal from the detector 14, we can see in Figure 2 (a
) is obtained.

At this time, the peak wavelength (or bottom wavelength) is detected from the spectrum, and the two adjacent peak wavelengths (
For example, from λ1, λ2), the optical path difference d between the reference surface 8 and the sample surface 10 is determined by the following equation.

d = λ1, λ2/2 (λ1 - λt) = λ1, λz/2Δλ (1) (However, Δ
λ=λ, -λ2) Note that the peak of the frequency spectrum shown in FIG. 2(a) can be captured using the MEM method (maximum entropy method) or Fourier transform.

As is clear from the above equation (1), when the optical path difference d is increased, two adjacent peak wavelengths λ1. The difference in λ2, Δλ=λ, -λ2, that is, the interval between the peak wavelengths λ1 and λ2, can be made small as shown in FIG. 2(b). Therefore, by adjusting the optical path difference prism 4 to change the optical path length of one of the split beams to the reference surface 8, and increasing the optical path difference d between the reference surface 8 and the sample surface 10, the spectrum can be The interval between the two peak wavelengths (for example, λ1, λ2) can be made small, and even with a coherent light source such as the dye laser 1, which has only a narrow variable wavelength range, the two peak wavelengths can be sufficiently detected and the value of the optical path difference d can be reduced. It becomes possible to obtain accurately. In addition, the other divided luminous flux is shown in Fig. 3 (
As shown in a), the optical path difference d1 between the reference surface 8 and the sample surface 10a when projected onto the surface 10a of the sample surface 10,
As shown in FIG. 3(b), if the optical path difference d2 between the reference surface 8 and the bottom surface 10b of the groove when projected onto the bottom surface 10b of the groove (or hole) on the sample surface 10 is determined, The depth h of the hole) can be calculated from the difference in optical path difference between the two, that is, h=ld+ail (2).

Now, the laser beam split by the beam splitting prism 3 and directed toward the sample surface 10 is focused by the objective lens 9 and projected onto the sample surface 10 as a laser spot. In order for this laser spot to enter the hole (or groove) as shown in FIG. 3(b), the laser beam must be focused so that the spot diameter is smaller than the hole diameter. In order to converge the spot diameter to a small size, it is necessary to increase N and A (numerical aperture) of the objective lens 9, and the depth of focus becomes shallow accordingly. Therefore, it is preferable to change N and A of the objective lens 9 according to the diameter of the hole through which the laser spot is projected, and to appropriately change the spot diameter and the depth of focus at the focal position of the objective lens 9.

Further, since the bottom surface 10b of the hole (or groove) has extremely fine irregularities, the reflected light from the bottom surface lob is generally diffracted and spread. However, only the interference light of the zero-order light component is detected by the detector 14 through a circular aperture 13 provided at a position conjugate with the sample surface 1o with respect to the objective lens 10 and the imaging lens 12. At this time, the intensity of the reflected light is considerably lower than that of the reflected light from the reference surface 8, so if left as is, the visibility of the interference fringes will deteriorate.

Therefore, as shown in FIG.
The filter 5 is provided in the parallel light beam between the beam splitting prism 3 and the objective lens 6, and can reduce the amount of light on the reference surface 8 side and increase the contrast of interference fringes.

When N and A of the objective lens 9 are small and the depth of the hole (or groove) is within the depth of focus, the surface 10a of the sample surface 10
Similarly to the reflected light from the reference surface 8, the reflected light from the bottom surface 1ffb of the groove interferes as a plane wave, and the interference fringes become a single one.
It is photoelectrically detected at 4. However, if the depth 11 of the hole (or groove) is deeper than the focal depth of the laser spot as shown in FIG.
The reflected light that has passed through becomes a spherical wave, which interferes with the plane wave of the reflected light from the reference surface 8. Therefore, the interference fringes are not single, but concentric fringes as shown in FIG. Even in this case, the detector 13 detects only the single color part at the center of the interference fringes through the circular opening of the aperture 12, so normal detection is possible as in the case where the depth h is within the focal depth. It is.

In the embodiment shown in FIG. 1, the optical path difference prism 4 is provided on the reference beam side, but it may also be provided in the sample optical path between the beam splitting prism 3 and the objective lens 9.

In addition, in the embodiment shown in FIG. 1 above, if N and A of the objective lens 9 of the sample light example are small and there is substantially no aberration, the objective lens 6 on the reference light side may be deleted. Good too. In this case, the optical path difference prism 4 is used to
Instead of changing the optical path length between a and the reference surface 8, the optical path length may be changed by directly displacing the reference surface 8 in the optical axis direction.

At this time, if a piezo actuator using a piezoelectric effect is used to displace the reference surface 8, extremely precise positioning is possible and the optical path length can be changed arbitrarily. Of course, in this case, the optical path difference prism is also unnecessary.

〔Effect of the invention〕

As described above, according to the present invention, the light from the wavelength-tunable light source is projected onto the measurement surface as a minute spot, and the interference fringes between the 0th-order light of the reflected light and the reflected light from the reference surface are detected. Therefore, clear interference fringes can be obtained, and the phase difference corresponding to the depth of a single hole or groove can be accurately determined. Moreover, since the optical path difference between the reference surface and the measurement surface can be freely changed, even if the wavelength tunable range of the illumination light is narrow, multiple peaks can be obtained in the spectral distribution of the interference fringe. It has the advantage of being able to measure depth with high precision using a narrow wavelength tunable light source.

[Brief explanation of the drawing]

Fig. 1 is a schematic configuration diagram of an optical system showing an embodiment of the present invention, Fig. 2 is a spectral diagram of an interference light detection signal, and (a) shows a case where the optical path difference between the reference light and the sample light is relatively small. , (b) shows the case where the optical path difference is large, and FIG. 3 is a cross-sectional explanatory diagram showing the state where the laser spot is projected onto the sample surface.
(a) shows the state in which the laser beam is projected onto the sample surface, (b) shows the state in which it is projected toward Omukai, and Fig. 4 shows the state in which the laser beam is projected when the depth of the hole is deeper than the focal depth of the laser spot. FIG. 5 is a plan view showing the state of interference fringes caused by the spherical wave beam of the sample light and the reference light shown in FIG. 4. (Explanation of symbols of main parts) 1... Dye laser (light source) 3... Beam splitting prism (beam splitting interference means) 4...
- Optical path difference prism (optical path length variable means) 5... Variable light amount filter 8... Reference surface 9... Objective lens (projection optical system) 10... Sample surface 11... Sample stage 12... Imaging lens 13...aperture

Claims (2)

[Claims] (1) The light beam from the wavelength-tunable light source is divided into two, and one light beam is focused into a spot and projected as sample light onto two measuring surfaces with a step formed on the sample surface. Projected onto a reference surface as a reference light, each reflected sample light reflected from the two measurement surfaces and the reflected reference light reflected from the reference surface are caused to interfere with each other, and the intensity of each interference fringe caused by the interference is calculated from the light source. Photoelectric detection is performed while changing the wavelength of the reference light and the sample light, and the optical path difference between the reference light and the sample light is changed so as to produce at least two peaks in each detected spectral distribution, and from each of the obtained spectral distributions, the two measurement planes are 1. A method for measuring the depth of a fine pattern, characterized in that the step is measured by determining an optical path difference. (2) a light source whose wavelength can be changed; a movable sample stage for changing the measurement position on the sample surface; and a projection optical system that focuses the light beam from the light source into a fine spot and projects it onto the sample surface. dividing the light beam from the light source into two, reflecting one of the divided light beams on a reference surface, and forming the other light beam in a spot shape on the sample surface via the projection optical system; a beam splitting and interference means for interfering the reflected light with the reflected light from the sample surface; an optical path length variable means for changing the optical path length of either one of the two split and reflected beams; A device for measuring the depth of a fine pattern, comprising: photoelectric detection means for photoelectrically converting and detecting interference fringes.