JPH06307811A - Dislocation and gap sensing method - Google Patents

Abstract

PURPOSE:To enhance the S/N ratio of a signal by making close the intensity of each diffracted light which is to be interfered with and which has a different order, heightening the interferential striation contrast of the diffracted light, and widening the amplitude of the beat signal. CONSTITUTION:Two sets of monochromatic light fluxes having slightly differing two wavelengths in frequencies f1 and f2 are used as an incident illumination light to be cast onto each diffraction lattice, and +1'th order and 0'th order diffracted lights and 0'th and -1'th order diffracted lights in each set are interfered so that two beat signals per set are sensed, and also the beat signal phase difference in each set is determined. The X-direction dislocation and Z-direction gap of two objects are sensed from the sum and difference of the beat signal phase difference between the sets, wherein the duty ratio (x) of the diffraction lattice should be no less than 0.5.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention measures the overlay accuracy of patterns exposed on an exposure device such as a semiconductor ultra-fine processing device [proximity (proximity) of SOR aligner stepper, liquid crystal stepper, etc.] and a photosensitive substrate. The present invention relates to a method for detecting a position shift and a gap using an optical heterodyne interferometry method using a diffraction grating in registration precision measurement and the like.

[0002]

2. Description of the Related Art In ultra-precision alignment of aligners for synchrotron radiation lithography, photosteppers and the like, it is necessary to detect the positional deviation between two objects to be aligned with high accuracy. 62-261
No. 003 and Japanese Patent Laid-Open No. 64-89323 propose a position shift detection method using optical heterodyne interference light.

In both of these, the -1st and -1st order diffracted lights extracted from the diffraction gratings of the two objects are made to interfere with each other, and the phase difference of the beat signals generated in each of these two objects is detected. Since it measures the amount of positional deviation between both objects and is not affected by the intensity change due to the gap change, it is expected that the positional deviation detection accuracy and resolution will be dramatically improved.

In fact, with regard to the resolution, a high resolution of about 5 nanometers, which is higher than the stage control resolution, was obtained. But,
In practical use, the phase fluctuates under the influence of atmospheric fluctuations, and detection accuracy is usually degraded by a little less than an order of magnitude. For this reason, two light fluxes and four light fluxes are used to make a symmetrical optical system arrangement so that the laser optical path is close to the optical axis, and measures to reduce the influence of atmospheric fluctuations, mechanical vibrations, and thermal vibrations are taken as much as possible.

On the other hand, regarding X-ray lithography exposure, the practical gap amount is estimated to be about 30 μm.
In order to transfer a finer pattern, resolution evaluation is performed by a gap of about 10 μm that brings a mask and a wafer closer to each other, and an ultrafine pattern is being obtained. For this reason, precise gap control in the Z direction is required together with alignment, which is an important key point for achieving fine processing.

Therefore, the inventors of the present invention have proposed a method capable of detecting a positional deviation in the X direction and a gap in the Z direction. Ie slightly different frequencies (eg f ₁ and f ₂ )
Two sets of two monochromatic light beams of two wavelengths are used as incident illumination light that is applied to each diffraction grating of two objects (mask and wafer),
Interference of these diffraction orders of different diffracted light in each set (e.g. 0-order diffracted light of the frequency f ₁ and -1-order diffracted light of the frequency f _2, and the frequency f ₁ of the + 1st-order diffracted light and 0-order diffracted light of the frequency f ₂₎ At least two beat signals in each set are detected, and beat signal phase differences φ _xz and φ _xz ′ within these sets are obtained, and two objects are obtained from the sum and / or difference of the beat signal phase differences between these sets. The X-direction positional deviation and the Z-direction gap are detected.

[0007]

However, when the absolute value of the diffracted order becomes large, the intensity of the diffracted light decreases, and when the diffracted lights of different orders are made to interfere with each other as described above, the interference fringes of the diffracted light obtained. Since the contrast becomes smaller and the amplitude of the beat signal also becomes smaller, the signal S / N ratio becomes lower.

The present invention has been made in view of the above problems of the prior art, and makes the intensities of diffracted lights of different orders to be interfered close to each other to increase the interference fringe contrast of the diffracted lights, Expand the amplitude of the signal to increase the signal S
It is intended to improve the / N ratio.

[0009]

Before describing the structure of the present invention, the diffraction order of the diffracted light obtained from the diffraction grating of the present application will be defined in advance.

FIGS. 1 and 2 show the state of coincidence of the incident angle and the diffraction angle of the reflection diffraction grating. First, when the monochromatic light of the frequency f is incident on the reflection diffraction grating 36 having the grating pitch P with an inclination of the incident angle θi from the optical axis, the diffracted light of the diffraction order m, n = 0, which is specular reflection, is centered Those diffracting toward the optical axis side are m, n = -1, -2, -3, and so on, and those diffracting to the minus order or the opposite side are m and n =
+1, +2, +3, etc. are positive orders (the diffraction angles corresponding to these diffraction orders are θm and θn). The relationship between the incident angle θi and the diffraction angles θm and θn at this time is
According to the basic formula of the diffraction grating, the following equations 1 and 2 are obtained.

[0011]

[Equation 1] sin θ _m −sin θ _i = m · λ / P

[0012]

[Equation 2] sin θ _n −sin θ _i = n · λ / P

Explaining on the basis of the above definitions, the present invention is
Two sets of two monochromatic two light fluxes with slightly different frequencies
The two sets of two objects are used as incident illumination light that strikes each diffraction grating, and the diffracted lights of different diffraction orders in each set interfere with each other to detect the beat signals of the two sets and beats in these sets. In the position shift / gap detection method for detecting the signal phase difference and detecting the X direction position shift and the Z direction gap of the two objects from the sum and / or difference of the beat signal phase differences between the sets, the duty of the diffraction grating is used. For the ratio x [value obtained by (P−s) / P when the pitch of the diffraction grating is P and the aperture width between the gratings is s], the intensity of diffracted light of different diffraction orders to be interfered is calculated. The basic feature is to bring them closer to each other and adjust to a ratio x that can enhance the interference fringe contrast of the obtained diffracted light.

By the above method, when the diffracted lights causing interference in each set are the + 1st and 0th orders and the 0th and -1st orders,
The duty ratio x of the diffraction grating is preferably 0.5 or more.

With respect to the duty ratio of the diffraction grating,
It can be said that it is also desirable from the results of the experiments described later to select a ratio that is 0.5 or more and the absolute value in the diffracted light order is the third order or the fourth order that is the missing order.

[0016]

A plane wave having a wavelength λ is incident on a plurality of diffraction gratings arranged adjacent to each other on two objects (for example, a wafer and a mask) at equal intervals P (pitch) and having an opening s therebetween. Here, if the direction cosines of the incident light and the diffracted light are l ₀ and l, respectively, and N is the number of diffraction gratings, the intensity I (l−l ₀ ) of the diffracted light is expressed by the following equation 3. , P and s.

[0017]

[Equation 3]

Here, the meaning of each term in the above equation will be specifically described. Each of (c) in FIGS. 3 to 5 shows three types of diffracted light intensity distributions in which the duty ratio x obtained from the above equation is 0.5, 0.66, and 0.75. Here, it is understood that when the pitch length P is fixed and the opening s is narrowed, the intensity differences of m = 0 and m = 1 are close to each other. The contents of each term in the above equation are as follows.

The second item on the right side is (a) in each of FIGS. 3 to 5.
And is an interference term generated as a result of interference of a plurality of diffracted lights from N (N = 10 in this example) apertures. If the number of diffraction gratings N is increased, the intensity of the main maximum becomes N ²
, The half width showing the spread of strength becomes narrower, and the submaximum on the side becomes negligible when N is increased.
The direction cosine 1 of the main maximum of the diffracted light is determined only by P (pitch). Here, it is divided by N for standardization. This function is ±
The main maximum comes at mπ.

On the other hand, the third item on the right side is shown in FIGS.
(B), and shows an extremely gentle interference curve having a main maximum and a sub-maximum. This term represents the intensity distribution of a single diffraction grating of the opening s determined by the duty ratio x, and is determined by the unit opening. In other words, the distribution of the intensity of diffracted light of each order is determined by this term.

Furthermore, the diffraction intensity distribution of each order shown in each of these figures (c) is expressed by the product of (a) and (b) from the above equation, and the intensity distribution in the diffraction order direction is determined by (b). Here, m represents the diffraction order, and the direction of the main maximum of the direction cosine 1 is as shown in the following expression 4.

[0022]

[Equation 4]

When the incident angle is θ _i and the diffraction angle is θ _m , l = s
Since in θ _m and l ₀ = sin θ _i can be set, the above formula becomes a familiar diffraction basic formula.

As the opening width s is made narrower (that is, the duty ratio x is made larger) in this way, as is apparent from FIGS. 3 to 5, the intensity difference of m = 0 and m = ± 1 becomes large. As they approach each other, the contrast of the obtained diffracted light increases.

[0025]

EXAMPLE An example of the present invention will be described below. The present inventors prepared a mask M and a wafer W provided with diffraction gratings having a constant pitch P but different aperture widths s, and different duty ratios x, and the frequencies are slightly different. Diffraction light when a beat signal is detected by illuminating these diffraction gratings by arranging monochromatic two light fluxes (frequency f ₁ and f ₂ ) as shown in FIG. A strength comparison experiment was conducted. In the figure, the surface on which the diffraction grating image of the diffraction grating 36 is obtained by the objective lens 35 located at the rear focal length F from the diffraction grating 36 is called a pupil plane EP70 (this surface is the diffraction grating 3).
It is also called the Fourier plane because it is the plane on which the Fourier transform image of 6 was obtained). The pupil plane EP70 is located at the front focal length F of the objective lens 35.

First, the detection principle when the duty ratio x of the diffraction grating in FIG. 6 is 0.66 will be described. That is, there is an angle difference between the θ ₁ inner incident angle and the θ ₂ outer incident angle with respect to the optical axis direction, and the two light fluxes are arranged in two sets symmetrically with respect to the optical axis. The angle difference of θ = (θ ₁ −θ ₂ ) is an angle equal to the ± first-order diffraction angle. First, a pair of two light fluxes f ₁ and f ₂ left-shifted from the optical axis direction is at a position apart from the optical axis by F · sin θ ₁ and F · sin θ _{2 on the} left side in this figure,
The intervals of the two light fluxes are (-1, -2) order, (0, -1) order,
The intervals are adjusted to obtain the (+1, 0) order and (+2, +1) order optical heterodyne interference.

At the pupil plane EP70, the light is once converged (beam waist) and is incident, and the objective lens 35 irradiates the diffraction grating 36. Although the incident illumination light has a broadened intensity distribution, the center of the intensity distribution is the beam waist position. As a result, specularly reflected (0th-order) diffracted light is extracted at a position symmetrical with respect to the optical axis of FIG. 6 in the incident direction, that is, at the other (shaded) two-beam incident position. The image of the diffracted light is the pupil plane EP7 of the objective lens 35.
It is converted into a Fourier transform image on 0. The intensity distribution of the diffracted light on the pupil plane of the frequency components of f ₁ and f ₂ is shown in the lower part of FIG. 6 (b). As shown in the figure, the third order is the missing order, and
The next, ± 1st, and ± 2nd-order intensity distributions are arranged at positions shifted to the right from the optical axis.

Similarly, two laser beams of f ₁ and f ₂ frequency components from the right direction shown by the diagonal lines are F · s on the right side of the optical axis in FIG.
pupil surface EP70 at a position separated by inθ ₁ and F · sin θ ₂
Then, the light is condensed once and is irradiated onto the diffraction grating 36 by the objective lens 35. As a result, the regular reflection (0th order) diffracted light of the f ₁ and f ₂ frequency components is extracted at a position symmetrical with respect to the optical axis in this incident direction, and is converted into a Fourier transform image on the pupil plane EP70. . This intensity distribution is shown in the upper part of FIG. As shown in the figure, the 0th order, ± 1st order, and ± 2nd order intensity distributions are arranged at positions shifted left from the optical axis.

As in the case described above, the interference beat light of each diffracted light is visualized in FIG. 6 (c) and is indicated by a circle and a shaded circle. Thus, a set of diffraction orders of f ₁ , f ₂ frequency components m = + 1, n =
−1 diffracted light and diffraction orders of frequency components of f ₂ and f ₁ n, m =
Optical heterodyne interference fringe with diffracted light of 0 and another set of f
_1, f ₂ diffraction order m of the frequency components, n = 0 of the diffracted light and f _2, f ₁ diffraction order n = -1 frequency components, optical heterodyne interference fringes of the diffracted light of m = + 1 at the same time, yet light It can be obtained without being mixed in the position symmetrical with respect to the axis. Note that here, the interference light of the (-1, -2) th order is not received because it has low intensity.

Further, | f ₁ obtained by the interference of the above-mentioned diffracted lights
The position where the interference fringes of the optical heterodyne beat signal of −f ₂ | are obtained is also shown in FIG. 6 (c).

The above experiment was repeated to examine the influence of the opening width s on the diffraction intensity. FIG. 7 is a diagram showing changes in the diffracted light intensity depending on the ratio of the openings s of the plurality of diffraction gratings. In the figure, the vertical axis indicating the intensity ratio is logarithmic, and the horizontal axis is the duty ratio x [x = (P−s) / P]. The intensity of the diffracted light is expressed by the following equations 5 and 6 for m ≠ 0 and m = 0.

[0032]

[Equation 5]

[0033]

[Equation 6]

In the figure, the absolute value of the order m = 0, 1,
The change in each duty ratio x regarding the intensity of the diffracted light in the cases of 2, 3, and 4 is shown. m = 0 decreases with (1-x) ² . When m = 1, the maximum duty ratio x is 0.5. Further, when the duty ratio is x = 0.66, m = 3 is the missing order. Similarly, when the duty ratio is x = 0.75, the fourth order is an absent order, and the opening s is narrowed, so that the diffracted light intensity difference between the 0th order and the 1st order is narrowed. Further, when the duty ratio x changes in the actual process, the contrast of the interference light changes.

[0035]

According to the position shift and gap detection method of the present invention described in detail above, the contrast of the interference diffracted light detected at the time of alignment is increased, and the alignment accuracy can be improved. Further, the optical heterodyne is strong in intensity modulation because it detects the phase, but amplitude modulation makes it difficult to detect the phase in the gap direction with high accuracy. However, in the present invention using the diffraction grating with the narrowed opening s, the light amount of the light flux passing through the first object is reduced and the effect of multiple interference generated between the two objects can be reduced, which is also advantageous for gap detection. is there.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of diffraction orders of diffracted light when incident on the left side of the optical axis.

FIG. 2 is an explanatory diagram of a diffraction order of diffracted light when the light is incident on the right side of the optical axis.

FIG. 3 is a graph showing a diffracted light intensity distribution when the duty ratio x is 0.5.

FIG. 4 is a graph showing a diffracted light intensity distribution when the duty ratio x is 0.66.

FIG. 5 is a graph showing a diffracted light intensity distribution when the duty ratio x is 0.75.

FIG. 6 shows F on both sides of the pupil plane optical axis with a duty ratio x of 0.66.
FIG. 5 is an explanatory diagram of a detection principle showing an optical heterodyne interference model when alignment two-flux light beams spaced apart by sin θ are incident and the diffraction grating is irradiated with the light by an objective lens.

FIG. 7 is a graph showing changes in diffracted light intensity depending on the ratio of openings s of a plurality of diffraction gratings.

[Explanation of symbols]

 35 Objective Lens 36 Diffraction Grating 70 Pupillary EPM Mask W Wafer

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[Procedure amendment]

[Submission date] April 20, 1994

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0006

[Correction method] Change

[Correction content]

Therefore, the inventors of the present invention have proposed a method capable of detecting a positional deviation in the X direction and a gap in the Z direction. Ie slightly different frequencies (eg f ₁ and f ₂ )
Two sets of two monochromatic light beams of two wavelengths are used as incident illumination light that is applied to each diffraction grating of two objects (mask and wafer),
Interference of these diffraction orders of different diffracted light in each set (e.g. 0-order diffracted light of the frequency f ₁ and -1-order diffracted light of the frequency f _2, and the frequency f ₁ of the + 1st-order diffracted light and 0-order diffracted light of the frequency f ₂₎ At least two beat signals in each set are detected, and the beat signal phase variations Δφ _xz and Δφ in these sets are detected.
_xz ' is obtained, and the X-direction positional shift and the Z-direction gap of the two objects are detected from the sum and / or difference of the beat signal phase fluctuation amounts between these sets.

[Procedure Amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0013

[Correction method] Change

[Correction content]

Explaining on the basis of the above definitions, the present invention is
Two sets of two monochromatic two light fluxes with slightly different frequencies
The two sets of two objects are used as incident illumination light that strikes each diffraction grating, and the diffracted lights of different diffraction orders in each set interfere with each other to detect the beat signals of the two sets and beats in these sets. The signal phase fluctuation amount is obtained, and 2 is calculated from the sum and / or difference of the beat signal phase fluctuation amounts between these pairs.
In a positional shift and gap detection method for detecting a positional shift in the X direction and a gap in the Z direction of two objects, when the duty ratio x of the diffraction grating is [Pitch of diffraction grating is P and aperture width between gratings is s, (P -S) / P]], the intensities of diffracted lights of different diffraction orders to be caused to interfere are brought close to each other and adjusted to a ratio x capable of enhancing the interference fringe contrast of the obtained diffracted lights. It is a basic feature.

Claims (3)

[Claims] 1. A monochromatic 2 of two wavelengths having slightly different frequencies.
Two sets of light fluxes are used as incident illumination light that strikes the diffraction gratings of the two objects, and diffracted lights of different diffraction orders in these sets are made to interfere with each other to detect the beat signals of the two sets. In the position deviation and gap detection method, the beat signal phase difference in the set of the two objects is obtained, and the X direction position deviation and the Z direction gap of the two objects are detected from the sum and / or the difference of the beat signal phase differences between the groups. Positional deviation characterized by adjusting the duty ratio of the diffraction grating so that the intensities of diffracted lights of different diffraction orders to be interfered are made close to each other and the interference fringe contrast of the obtained diffracted light is enhanced. And a gap detection method. 2. The position shift and gap detection method according to claim 1, wherein the diffracted light causing interference in each set is:
The position shift and gap detection method according to claim 1, wherein the duty ratio of the diffraction grating is set to 0.5 or more when the order is + 1st order and 0th order, and 0th order and -1st order. 3. The position shift and gap detection method according to claim 1, wherein the duty ratio of the diffraction grating is 0.5 or more, and the absolute value of the diffracted light order lacks the third or fourth order. The position shift and gap detection method according to claim 1, wherein a ratio that is an order is selected.