JPH09203615A - Method and apparatus for measuring depth - Google Patents

Abstract

PROBLEM TO BE SOLVED: To highly accurately measure a depth (difference in stages) of a recess of a pattern to be inspected in which protrusions and recesses appear periodically. SOLUTION: ±1-order diffraction light beams Lp, Lm generated by a laser light source 1, a reference grating 5 or the like are shed symmetrically on a periodically stepped pattern 9 on a wafer 8, and a combined flux DB including normally reflected light of the diffraction light Lp generated from the pattern 9 in parallel and -1-order diffraction light of the diffraction light Lm, and a combined flux DA including normally reflected light of diffraction light Lm generated in parallel and +1-order diffraction light of the diffraction light Lp are received by photoelectric detectors 11B, 11A, respectively. A depth of a recess of the periodically stepped pattern 9 is calculated based on detection signals SA, SB output from the photoelectric detectors 11A, 11B when the pattern 9 is scanned in an X-direction via a wafer stage 10, wavelengths of their diffraction light, a ratio in widths in the X-direction of the recess and a protrusion of the pattern 9 or the like.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a depth measuring method and a depth measuring apparatus for measuring a depth or a step of a concave portion of a pattern of periodic unevenness, and particularly to a trench structure or an alignment in a semiconductor device or the like. It is suitable for use when measuring the depth or step of a fine groove-shaped pattern such as a mark.

[0002]

2. Description of the Related Art For example, a semiconductor element is formed by stacking a multilayer circuit pattern on a wafer in a predetermined positional relationship. In a photolithography process for forming a circuit pattern of each layer, a reticle as a mask is exposed by an exposure apparatus. Pattern image is transferred to the photoresist layer on the wafer at a predetermined magnification. Then, when the pattern of the reticle is transferred to the second and subsequent layers of the wafer by the exposure apparatus, the position of the alignment mark formed on the wafer in the steps up to that time is detected, and the reticle of the reticle is detected based on this detection result. So-called overlay exposure is performed after the alignment between the pattern and the pattern already formed in each shot area of the wafer.

A conventional alignment mark on a wafer is
It is a pattern of irregularities formed at a predetermined pitch on the wafer along the normal measurement direction. In the following, such an uneven pattern periodically arrayed in a predetermined direction is also referred to as a groove pattern, and a fine pattern having a small uneven pitch among the groove patterns is also referred to as a fine groove pattern. When detecting an alignment mark composed of such a fine groove-shaped pattern by, for example, an alignment sensor of an imaging method using illumination light of a predetermined bandwidth, the boundary portion between the convex portion and the concave portion in the image of the alignment mark is detected. Since the portion corresponding to is a dark line, for example, the position of the dark line can be detected. In addition, in the alignment sensor, a light beam such as a laser beam is irradiated to an alignment mark formed of a fine groove-shaped pattern of a diffraction grating, and diffracted light from the pattern is detected. Laser step alignment method (LSA method) that detects the position based on the intensity, or a pair of diffracted light emitted parallel to the pattern by irradiating the pattern with coherent light beams from two directions. The two-beam interference method (L
(IA method), there is also a sensor that performs position detection based on diffracted light from a fine groove pattern in a predetermined direction.

For example, when position detection is performed based on the diffracted light from the fine groove pattern in a predetermined direction in this way, it is caused by the depth of the concave portion of the pattern, that is, the step between the convex portion and the concave portion. The intensity of the diffracted light to be changed greatly changes, and the SN ratio of the obtained detection signal may deteriorate in some cases. Therefore, it is necessary to control the manufacturing process so that the depth of the pattern falls within a predetermined range, but for that purpose, it is necessary to accurately measure the depth of the pattern.

In addition to the alignment mark on the wafer, when it is necessary to accurately control the depth of the recess of the groove pattern, it is necessary to measure the depth accurately. Conventionally, the depth (step) of a fine groove pattern has been measured by irradiating the fine groove pattern and its surroundings with a multi-wavelength light beam, receiving the specular reflection light, and measuring the depth from the spectral distribution. The method of calculating is used.

This is because the reflected light from the bottom (recess) of the fine groove-shaped pattern interferes with the reflected light from the upper surface (surrounding), and is caused by the optical path difference (twice the depth) of both reflected lights. It utilizes that the reflected light of a specific wavelength becomes strong or weak due to the phase difference.

[0007]

However, in the conventional technique as described above, since the detection is performed depending on the intensity of the reflected light from the concave portion of the groove-shaped pattern and the peripheral portion thereof, the obtained detection signal is not detected. There was the inconvenience that the SN ratio was not very good. Further, when the material of the groove-shaped pattern is a material whose reflectance changes within the wavelength range of the detection light, the reflectance change peculiar to the material with respect to the wavelength affects the reflectance change accompanying the depth change described above. However, there is an inconvenience that the accuracy of depth detection is significantly reduced.

[0008] In this regard, it is possible to input the change in reflectance peculiar to the material to the measuring device in advance and correct it. However, if there is a natural oxide film or a thin film for an etching mask for forming a step (derivation) on the surface of the object to be measured, the reflectance fluctuates with the change in wavelength due to the thin film interference effect, so such correction is not possible. Have difficulty. In view of such a point, the present invention aims to provide a depth measuring method capable of highly accurately measuring the depth (step) of a concave portion of a pattern to be tested in which irregularities are periodically repeated such as a fine groove pattern. And

A further object of the present invention is to provide a depth measuring device capable of carrying out such a depth measuring method.

[0010]

According to the depth measuring method of the present invention, a concave portion of a pattern (9) to be inspected is formed on a substrate (8) so that irregularities are periodically repeated in a predetermined direction (X direction). In the depth measuring method for optically measuring the depth of the pattern, when the period of the pattern to be measured (9) is P, the first and second light beams (Lp , L
By irradiating m), an interference fringe having a period of amplitude distribution of 2P / N (N is a natural number) is formed on the test pattern (9) in the predetermined direction, and the pattern is formed by the first light beam (Lp). A first combined light flux (DB) obtained by combining the regular reflection light from the test pattern (9) and the Nth-order diffracted light from the test pattern (9) by the second light beam (Lm); Regular reflection light from the pattern to be inspected (9) by the light beam (Lm) and the pattern to be inspected (9) by the first light beam (Lp)
The second combined light flux (D
A) and the photoelectric conversion signals of the first and second combined light fluxes (DB, DA) obtained when the pattern to be measured (9) and its interference fringes are relatively scanned in the predetermined direction. , And the depths of the concave portions of the test pattern (9) are calculated based on the wavelengths of the first and second light beams (Lp, Lm).

According to the present invention, for example, when the natural number N is 1, interference fringes having an amplitude distribution pitch of 2P are formed by two light beams on the test pattern (9) having the period P. . As a result, of the reflected light and the diffracted light generated from the test pattern (9), the regular reflected light (0th-order diffracted light) of the first light beam (Lp) and the second light beam (Lm) are
The second-order diffracted light is diffracted in the same direction and becomes a first combined light flux (DB) that interferes with each other. Similarly, the specularly reflected light of the second light beam (Lm) and the first-order diffracted light of the first light beam (Lp) are also diffracted in the same direction and interfere with each other.
Is a combined luminous flux (DA).

These interferences occur due to the mutual phase difference between the generated 0th-order diffracted light (regularly reflected light) and the 1st-order diffracted light. The mutual phase difference between the first-order diffracted light and the first-order diffracted light can be obtained. Furthermore, both the 0th and 1st order diffracted light are
The pattern to be inspected as the above-mentioned periodic step pattern (9)
Of the first combined light flux (DB), because the intensity difference and the phase difference are generated in accordance with the reflectance, the ratio of the width between the convex portion and the concave portion, the step amount (phase difference of light), and the like. From the phase difference between the signal corresponding to the change and the signal corresponding to the change in the intensity of the second combined light flux (DA), the depth of the concave portion of the test pattern (9) can be obtained with high accuracy.

When the value of the natural number N is 2 or more,
Second-order or higher-order N-th order diffracted light is used instead of the above-mentioned first-order diffracted light, and in this case, the depth can be calculated as in the case of using the first-order diffracted light. Next, in the present invention, when calculating the depth of the concave portion of the test pattern (9), the phase and contrast of the photoelectric conversion signals of the first and second combined light fluxes (DB, DA), the first and the second Second light beam (Lp, L
It is desirable to use the wavelength m) and the ratio of the width of the concave portion and the convex portion of the test pattern (9) as the shape of the test pattern (9) in the predetermined direction. In this case, in particular, the ratio of the widths of the concave portions and the convex portions of the test pattern (9) in the predetermined direction can be sufficiently accurate even if the design value is used, and therefore the depth can be calculated with high accuracy.

Further, the depth measuring apparatus according to the present invention optically measures the depth of the concave portion of the pattern (9) to be inspected, which is formed on the substrate (8) so that irregularities are periodically repeated in a predetermined direction. In the depth measuring device, when the period of the pattern to be inspected (9) is P, an interference fringe having an amplitude distribution period of 2P / N (N is a natural number) in the predetermined direction on the inspected pattern (9). A light-transmitting optical system (1 to 7) for irradiating the pattern (9) to be inspected with the coherent first and second light beams to be formed;
Specular reflection light from the test pattern (9) with the first light beam and the test pattern (9) with the second light beam
The first combined light flux (D
B) from the first photoelectric conversion element (11B), the regular reflection light from the test pattern (9) by the second light beam and the test pattern (9) by the first light beam. A second photoelectric conversion element (11A) that receives a second combined light flux formed by combining the Nth-order diffracted light, a pattern to be inspected (9), and their interference fringes, which relatively scan in a predetermined direction. The scanning means (10) and the first and second photoelectric conversion elements (11B, 11A) when the pattern to be measured (9) and its interference fringes are relatively scanned by the relative scanning means.
And a depth calculation means (12) for calculating the depth of the concave portion of the test pattern (9) on the basis of the photoelectric conversion signals output from the photoelectric conversion signals and the wavelengths of the first and second light beams. Is. The above-described depth measuring method can be implemented by the depth measuring device of the present invention.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. In this example, the present invention is applied to an optical depth measuring device for measuring the depth (step) of a concave portion of an alignment mark on a semiconductor wafer used for alignment in an exposure apparatus.

FIG. 1 is a block diagram conceptually showing the optical depth measuring apparatus of this embodiment. In FIG. 1, the coherent light beam LS emitted from the laser light source 1 is reflected by a mirror 2.
And then enters the reference grating 3. The reference grating 3 generates 0th-order diffracted light (straight-ahead light) L0, + 1st-order diffracted light Lp, −1st-order diffracted light Lm, and diffracted light of the 2nd-order or more. These light fluxes pass through the collimator lens 4 and reach the spatial filter 5 along mutually parallel optical paths, and the spatial filter 5 shields light fluxes other than the ± first-order diffracted lights Lp and Lm.

± first-order diffracted light L which has passed through the spatial filter 5
A light beam that has passed through the half mirror 6 among p and Lm (this light beam is also referred to as “± first-order diffracted light Lp, Lm”) is a semiconductor wafer (hereinafter, simply referred to as “wafer”) as an object to be inspected by the condenser lens 7. The alignment mark 9 on the (8) is symmetrically irradiated at a predetermined intersection angle. In the following, the condenser lens 7
In the following description, the Z axis is taken parallel to the optical axis AX, the X axis is taken parallel to the plane of FIG. 1 in the plane perpendicular to the Z axis, and the Y axis is taken perpendicular to the plane of FIG.

At this time, the wafer 8 of this example is held on the wafer stage 10, and the wafer stage 10 moves in the X direction.
The wafer 8 is positioned by stepping in the Y direction, and the position (focus position) of the wafer 8 in the Z direction is adjusted.
A movable mirror fixed to the upper end of the wafer stage 10 and an X-ray of the wafer stage 10 by an external laser interferometer 14.
The coordinates and the Y coordinate are constantly measured with a resolution of about 0.01 μm, the rotation angle (yaw) of the wafer stage 10 is also measured, and the measurement result is supplied to the main control system 13 that controls the operation of the entire apparatus. ing. The main control system 13 controls the positioning operation of the wafer stage 10 via the wafer stage drive system 15 based on the supplied measurement result and the target position of the wafer 8.

The alignment mark 9 on the wafer 8 of this example is a pattern to be inspected for the X axis, which is formed by repeating irregularities at a predetermined pitch in the X direction. It is called a step pattern 9 ".
At this time, the ± first-order diffracted lights Lp and Lm are irradiated so as to intersect symmetrically in the direction parallel to the X axis with respect to the optical axis AX at the incident angle θ on the periodic step pattern 9, and the periodic step pattern 9 is irradiated. Interference fringes having a predetermined pitch are formed in the X direction. That is, in the optical system including the collimator lens 4 and the condenser lens 7, the arrangement surface of the reference grating 3 and the formation surface of the periodic step pattern 9 are conjugate. The periodic step pattern 9
The period (pitch) in the X direction is P, and the wavelength of the light flux LS is λ
Then, the incident angles θ of the ± first-order diffracted lights Lp and Lm are set to satisfy the following equation.

Sin θ = λ / (2 · P) (1) In this case, the period in the X direction of the amplitude distribution of the interference fringes formed on the periodic step pattern 9 by the ± first-order diffracted lights Lp and Lm is 2P. Becomes Incident angle θ to satisfy equation (1)
Is adjusted by, for example, changing the cycle (pitch) of the reference grating 3 or changing the focal length of the collimator lens 4 or the condenser lens 7. In order to continuously and easily adjust the focal length in the latter case, the optical system including the collimator lens 4 and the condenser lens 7 may be a zoom lens system.

When the condition of the incident angle θ of the expression (1) is satisfied, the regular reflection light (0th order diffracted light) generated from the periodic step pattern 9 and the diffracted light of the 1st or higher order are extracted from the reference grating 3. The generated specularly reflected light of the −1st order diffracted light Lm and the reference grating 3
The + 1st-order diffracted light of the + 1st-order diffracted light Lp generated by the above becomes the first combined light beam DA which is emitted from the periodic step pattern 9 in the same direction and interferes with each other. At the same time, the specularly reflected light of the + 1st-order diffracted light Lp generated from the reference grating 3 and the −1st-order diffracted light of the −1st-order diffracted light Lm generated from the reference grating 3 are emitted from the periodic step pattern 9 in the same direction and are mutually crossed. The second combined light flux DB interferes with each other.

The combined light fluxes DA and DB travel in opposite directions on the optical paths of the + 1st-order diffracted light Lp and the -1st-order diffracted light Lm, pass through the condenser lens 7, and reach the half mirror 6 along the mutually parallel optical paths. The light flux reflected by the half mirror 6 (this light flux is also called “combined light flux DA and DB”)
Photoelectric detectors 11A each consisting of a photodiode or the like
And 11B. Then, the detection signals SA and SB obtained by photoelectrically converting the combined luminous fluxes DA and DB in the photoelectric detectors 11A and 11B are the step measuring unit 1.
2 is supplied. Further, the step measuring unit 12 is previously supplied with information necessary for calculating the depth from the main control system 13.

In this state, the main control system 13 scans the wafer stage 10 in the X direction at a constant speed via the wafer stage drive system 15, and at this time, the laser interferometer 14 continuously measures at a predetermined sampling frequency. The information of the X-coordinate and the Y-coordinate of the wafer stage 10 to be sequentially transferred is sequentially measured by the step measuring unit 1.
Feed to 2. In response to this, the level difference measuring unit 12 uses the detection signals SA and SB during the period in which the wafer stage 10 is scanned, the X coordinate of the wafer stage 10, and the like, as will be described later, and the depth of the concave portion of the periodic level difference pattern 9 is increased. The height (step) is calculated, and the calculation result is supplied to an external host computer or the like via the main control system 13. The calculation result is fed back to, for example, the step of forming the periodic step pattern 9, and the depth of the periodic step pattern in that step is adjusted.

Next, in the present example, the detection signals SA, SB
Also, a calculation principle for obtaining the depth (step) of the concave portion of the periodic step pattern 9 from the above and other information, and an example of the calculation method will be described with reference to FIGS. First, the regular reflection light (0th-order diffracted light) from the periodic step pattern 9 and 1
Although there is a phase difference due to a step or the like of the pattern with respect to the second-order diffracted light, the positional deviation of the reference grating 3 and the periodic step pattern 9 in the X direction (± first-order diffracted light Lp, Lm
A phase difference is also generated due to the positional deviation between the interference fringes formed by the above and the periodic step pattern 9. Therefore, the light intensities of the combined luminous fluxes DA and DB, and thus the intensities of the detection signals SA and SB, are changed by changing the relative positional relationship between the reference grating 3 and the periodic step pattern 9, for example, by setting the wafer stage 10 to the periodic step pattern. It is also changed by scanning in the period direction of 9 (X direction).

FIG. 2 shows such a wafer stage 10.
An example of intensity change of the detection signals SA and SB due to scanning is shown.
2 (a) and 2 (b), the horizontal axis represents the wafer stage.
Position in the X direction, the vertical axis indicates the detection signal.
It is the strength of No. SA and SB. The periodic step pattern of this example
Since the cycle of the channel 9 in the X direction is P, as shown in FIG.
In addition, the detection signals SA and SB are both of the wafer stage 10
A sine wave with a period P is generated for the position in the X direction.
No. SA and SB are maximum intensity A respectively_maxAnd B _maxWhen
Position of the wafer stage 10 when
The positions of the difference pattern 9 usually do not match. Therefore, the detection signal
No. SA and SB are maximum intensity A respectively_maxAnd B_maxWhen
Wafer stage 10 (periodic step pattern
The position difference (position shift amount) of 9) in the X direction is 2 · ΔX
I do. ΔX has a dimension of length, but correct this
The angle is calculated using the conversion formula that sets the period P of the chord wave to 2π [rad].
The degree (phase) δ [rad] is as follows.

Δ = 2π · ΔX / P (2) Using the phase δ of the equation (2), the detection signals SA and SB
The phase difference of is represented by 2δ. This phase difference 2δ depends on the step amount of the periodic step pattern 9 and each width of the convex portion and the concave portion, and the phase difference 2δ between the detection signals SA and SB.
According to the analysis by the inventor of the present application, it is possible to measure the depth (step) of the concave portion of the periodic step pattern 9 conversely from the above, and the widths of the convex portion and the concave portion of the periodic step pattern 9. found. Hereinafter, a specific description will be given.

FIG. 3 is an enlarged view of the periodic step pattern 9 of FIG.
FIG. 3 is a large diagram, and as shown in FIG.
The concave portion 9a and the convex portion 9b are arranged at a cycle P in the X direction.
It is a pattern. Further, the width of the recess 9a in the X direction is
a, the width of the convex portion 9b in the X direction is b (P = a + b),
The depth of the concave portion 9a, that is, the step between the concave portion 9a and the convex portion 9b is d.
And Also, the amplitude reflection of the concave portion 9a and the convex portion 9b
The rate is φ_aAnd φ _bAnd In addition,
Amplitude reflectance φ_a, Φ_bOf the periodic step pattern 9
In the same plane in the depth direction (Z direction in FIG. 3) (in the reference plane)
It represents the amplitude of the reflected light at. For example, this criterion
If the surface is the surface of the convex portion 9b of the periodic step pattern 9,
Amplitude reflectance φ at recess 9a_aOn the surface of the recess 9a
Equivalent to the round-trip optical path difference (phase difference) of the step d
Multiplied by exp (4πid / λ) which is the factor
You. In addition, with the exception of the below-mentioned, this phase difference 4πd
/ Λ (= ω) is the amplitude reflectance φ_aAnd φ_bPlace with
It is all of the phase difference, φ_aAnd φ_bThe phase difference ω with
Thus, the step amount d can be obtained.

Generally, the amplitude distribution of the diffracted light generated from each of the concave portion 9a and the convex portion 9b in FIG. 3 in the diffraction direction is expressed as a sinc function. For example, a recess 9a having a width a
When the amplitude distribution of the diffracted light generated at the diffraction angle Θ with respect to the periodic direction (X direction) of the periodic step pattern 9 is represented by the function ψA (u) of the variable u corresponding to the order of the diffraction angle Θ,
It looks like this:

ΨA (u) = φ _a · sin (πau) / (πu) (3) Here, the variable u corresponding to the diffraction order is calculated as follows using the wavelength λ of the diffracted light with respect to the diffraction angle Θ. Have a relationship. u = sin Θ / λ (4) Similarly, the amplitude distribution ψB (u) of the diffracted light generated from the convex portion 9b having the width b is as follows.

ΨB (u) = φ _b · sin (πbu) / (πu) (5) These amplitude distributions ψA (u) and ψB (u) are respectively a · in the 0th-order diffracted light (u = 0). φ _a and b ・ φ _b
Becomes Then, as shown in FIG. 3, the concave portion 9a and the convex portion 9
The amplitude distribution ψ (u) of the diffracted light with the diffraction angle Θ from the periodic step pattern 9 in which b is periodically arranged in the X direction at the pitch (pitch) P is as follows.

Ψ (u) = {ψA (u) + ψB (u) · exp (πiPu)} · Pir (u) (6) However, the function Pir (u) in the expression (6) is a periodic step pattern. The number of repetitions of 9 in the X direction (an integer of 2 or more) is m
Is represented by the following equation. Pir (u) = sin (mπPu) / sin (πPu) (7) When deriving equation (6), the center of the concave portion 9a was used as the reference of the phase of the diffracted light, but of course the center of the convex portion 9b was used as the reference. It doesn't matter.

The function Pir (u) in the equation (6) is called the "periodic term" of the diffraction grating, and if the repetition number m of the periodic step pattern 9 is large, the variable u corresponds to the j-th order diffracted light. Has a non-zero value m only at the position where u = j / P (j is an integer), and becomes almost 0 at other positions. In this example, since only the 0th-order diffracted light and the 1st-order diffracted light from the periodic step pattern 9 are used, the function Pir
(u) may be a constant value. Also, the function exp in equation (6)
(πiPu) becomes 1 in the 0th order diffracted light (u = 0) and becomes −1 in the ± 1st order diffracted light (u = ± 1 / P).

From this, the amplitude ψ _{0 of the 0th-order} diffracted light and the amplitude ψ ₁ of the _1st- order diffracted light generated from the periodic step pattern 9 as shown in FIG. 3 are as follows. ψ ₀ = a · φ _a + b · φ _b (8) ψ ₁ = a ′ · φ _a −b ′ · φ _b (9) However, the coefficients a ′ and b ′ are respectively expressed as follows.

A ′ = P · sin (πa / P) / π (10) b ′ = P · sin (πb / P) / π (11) Thus, the amplitudes ψ ₀ and ψ ₁ of the diffracted light are amplitudes. FIG. 4 shows the process derived from the reflectances φ _a and φ _b in polar coordinates on the complex plane. That is, the horizontal axis of FIGS. 4A to 4C is the Re axis (real axis) representing the real part, and the vertical axis is the Im axis (imaginary axis) representing the imaginary part. The amplitude reflectance φ _a is shown as _a real number on the Re axis. However, if the phase difference between the amplitude reflectances φ _a and φ _b (defined as ω as described above) is unchanged, the amplitude reflectance φ _a
The result that is derived by using as a general complex number does not change.

FIG. 4A shows the amplitude reflectance φ _a ,
It is shown that the amplitude ψ _{0 of the 0th-order} diffracted light is determined by the formula (8) from φ _b . FIG. 4B shows the amplitude reflectances φ _a and φ.
_{From b,} it is shown that the amplitude ψ _{1 of the first-order} diffracted light is determined by the expression (9). In this case, the phase difference between the amplitude reflectance φ _a and the amplitude reflectance φ _b is ω. FIG.
(C) shows the amplitude ψ obtained from FIGS. 4 (a) and (b).
₀ and ψ ₁ are displayed on the same complex plane (polar coordinates), and the phase δ in FIG. 4C is the detection signal SA of FIG.
, And SB, which are 1/2 of the phase difference 2δ of the change in the light amount of the combined light beams DA and DB, respectively. Its phase δ
Is the amount of positional deviation 2 between the detection signals SA and SB shown in FIG.
After measuring ΔX, it can be calculated from the equation (2).

Further, the ratio of the magnitudes of the amplitudes ψ ₁ and ψ ₀ (|
ψ ₁ |: | ψ ₀ |) can also be measured from a signal obtained by photoelectrically converting the diffracted light. That is, the maximum values A _max and B _max of the detection signals SA and SB in FIG.
Is the intensity when the amplitudes ψ ₀ and ψ ₁ are added in the same phase, and the minimum values A _min and B _min of the detection signals SA and SB are obtained by adding the amplitudes ψ ₀ and ψ ₁ in the opposite phase. Since it is the strength in the state, it becomes as follows.

A _max = B _max = (│ψ ₀ │ + │ψ ₁ │) ² (12) A _min = B _min = (│ψ ₀ │-│ψ ₁ │) ² (13) Also, (12) Since the expressions (13) are established, the contrast γ of the detection signals SA and SB is commonly defined as follows. γ = (A _max −A _min ) / (A _max + A _min ) (14)

By substituting the equations (12) and (13) into the equation (14), the contrast γ becomes as follows. γ = 2 · | ψ ₀ | · | ψ ₁ | / (| ψ ₀ | ² + | ψ ₁ | ² ) (15) Therefore, the contrast γ is measured, and the absolute value of the amplitude ψ ₁ and the amplitude ψ ₀ By substituting the following expression (16) including the value β of the ratio with the absolute value into the expression (15), the value β of the ratio is expressed as the expression (17).

| Ψ ₁ | = β · | ψ ₀ | (16) β = {1 ± (1-γ ² ) ^1/2 } / γ (17) The sign of ± in the equation (17) is unique. Can't decide,
Generally, the intensity of the 1st-order diffracted light is weaker than that of the 0th-order diffracted light,
The minus sign is adopted. However, as will be described later, a + sign may be used in some cases. Then, using the phase δ and the value β of the ratio determined by the equation (17), the amplitude ψ ₁ can be expressed by the following equation.

Ψ ₁ = β · ψ ₀ · exp (iδ) (18) When the phase δ and the value β of the ratio are obtained as described above, the process of deriving the amplitude of the diffracted light from the amplitude reflectance is reversed. The amplitude reflectances φ _a and φ _b of the concave portions 9a and the convex portions 9b of the periodic step pattern 9 can be obtained from the amplitude of the diffracted light by tracing Specifically, the simultaneous equations of the equations (8) and (9) may be solved based on the known parameters.

Generally, in processing a semiconductor integrated circuit,
The controllability of the pattern line width is superior to that of the processing controllability of the step difference to be measured. Therefore, the widths a and b of the concave portions 9a and the convex portions 9b of the periodic step pattern 9 shown in FIG. 3 are almost the designed values, and the designed values can be used as known values. Similarly, the coefficients a'and b'become known values from the expressions (10) and (11). Therefore, (8)
The unknown variables (variables that have not been measured) in the simultaneous equations of Equations (9) are amplitude reflectances φ _a and φ _b , and the simultaneous equations can be solved for φ _a and φ _b . As a result, the amplitude reflectances φ _a and φ _b are expressed as follows. The equation (18) is substituted for the amplitude ψ ₁ .

Φ _a = (b ′ · ψ ₀ + b · ψ ₁ ) / (a · b ′ + a ′ · b) (19) φ _b = (a ′ · ψ ₀ −a · ψ ₁ ) / (a · b ′ + a ′ · b) (20) In the equations (19) and (20), the phase of the amplitude ψ ₀ is not known, but as a final result, the phase difference between the amplitude reflectances φ _a and φ _b (= Since it is sufficient to know ω), the phase of the amplitude ψ ₀ (the angle formed by the amplitude ψ ₀ with the Re axis in FIG. 4) may be any value.

Therefore, the values (complex numbers) of the amplitude reflectances φ _a and φ _b can be obtained from the equations (19) and (20). Then, the respective phases ω _a and ω _b are obtained from the real number part and the imaginary number part of the amplitude reflectances φ _a and φ _b . That is,
_Let the real and imaginary parts of the amplitude reflectance φ _a be Re (φ
As _a) and Im (phi _a), when the real part and the imaginary part of the amplitude reflectance phi _b respectively and Re (φ _b) and Im (φ _b),
The phases ω _a and ω _b are represented as follows.

Ω _a = tan ⁻¹ {Im (φ _a ) / Re (φ _a )} (21) ω _b = tan ⁻¹ {Im (φ _b ) / Re (φ _b )} (22) and 2 The difference between the two phases ω _a and ω _b (ω _b −ω _a ) is ω
And this phase difference ω is the amplitude reflectance φ _a , φ _b
It becomes the phase difference of. Further, as described above, since the step d of the periodic step pattern 9 of FIG. 3 and the phase difference ω usually have a relation of (4πd / λ = ω), the step d Is calculated. The above is the calculation principle of the step d in this example.

Next, an example of a procedure for calculating the step d will be described. First, the wafer stage 10 of FIG. 1 is scanned in the periodic direction (X direction) of the periodic step pattern 9 as described above, and along with this, the photoelectric detectors 11A and 11B photoelectrically convert the combined luminous fluxes DA and DB, respectively. The obtained detection signals SA and SB are taken into the step measuring unit 12. At the same time, the X-coordinate and Y-coordinate of the wafer stage 10 measured by the laser interferometer 14 are also taken into the step measuring unit 12 via the main control system 13. In the step measuring unit 12, as shown in FIG. 2, based on the relationship between the position X of the wafer stage 10 and the detection signals SA and SB, the internal phase difference detection unit detects the detection signals SA and SB.
The positional deviation amount 2 · ΔX of SB is obtained, and the phase difference 2δ between the detection signals SA and SB is calculated using this positional deviation amount and the equation (2).

Further, in the step measuring section 12, the internal peak hold section and the bottom hold section, as shown in FIG. 2, determine the maximum values A _max , B _max and the minimum values A _min , B _min of the detection signals SA, SB. Detect and maximum of these A
The contrast γ of the detection signals SA and SB is calculated by substituting the _max and the minimum value A _min into the equation (14). It should be noted that the contrasts of both detection signals are equal unless the periodic step pattern 9 has a great degree of asymmetry.
If the contrasts are different, the maximum value B _max and the minimum value B _min are substituted for the maximum value A _max and the minimum value A _{min in} the equation (14) to obtain the contrast of the detection signal SB. The average value of the two contrasts may be newly set as the contrast γ.

Further, the step difference measuring unit 12 substitutes the contrast γ into the equation (17) to obtain the amplitudes ψ ₁ and 0 of the _first-order diffracted light.
The value β of the magnitude ratio to the amplitude ψ ₀ of the next-order diffracted light is obtained, and this value and the above-mentioned phase difference δ are substituted into the equation (18), and the amplitude ψ ₁
Find the exact (complex number) relationship between and the amplitude ψ ₀ . In addition, for example, an operator previously sets a width (a, b) of each of the concave portions 9a and the convex portions 9b of the periodic step pattern 9 on the step measuring portion 12 via a console (not shown) and the main control system 13.
The period P of the periodic step pattern 9 and the wavelength λ of the light flux LS
Enter the value of. Then, in the level difference measuring unit 12, the input widths a and b and the period P are substituted into the equations (10) and (11) to calculate the values of the coefficients a ′ and b ′, and the calculated coefficient a ′. , B ′ are substituted into the equations (19) and (20) to calculate the values (complex numbers) of the amplitude reflectances φ _a and φ _b . Furthermore,
(21), (22) an amplitude reflectance from the equation phi _a, the respective phases of phi _b omega _a, obtains the omega _b, the phase difference omega (= omega _b
−ω _a ) is calculated.

This phase difference ω corresponds to the reciprocal optical path difference (phase difference) of the step d of the periodic step pattern 9 as described above, and ω = 4πd / λ, that is, the step difference measuring unit according to the following relationship. In step 12, the step d is finally obtained. d = ω · λ / (4π) (23) By the way, in this example, the detected phase difference ω is a value within the range of 0 to 2π [rad], and the step d is from 0 to λ / 2. It will be a value within the range. Therefore, the step d is λ
For a periodic step pattern of ½ or more, for example, a step pattern where the step d is actually 0.7λ, the step is measured as 0.2λ (= 0.7λ−λ / 2). Will be. However, the measurement error λ / 2 (0.
The difference between 2λ and 0.7λ is an amount that can be sufficiently discriminated by the conventional depth measuring device. For example, a rough depth measurement is performed by the conventional depth measuring device, and the step d is, for example, 0.
After it is found to be about 7λ ± 10%, the problem of the measurement error of λ / 2 can be solved by a method of performing precise measurement using the measuring apparatus of this example.

That is, when the equation (23) is developed, an integer k
Using (k = 0,1,2, ...), the step d is generally expressed as follows. d = (2π · k + ω) · λ / (4π) (24) Then, a rough depth measurement is first performed to determine the value k _{0 of the} integer k and the value ω ₀ of the phase difference ω. Then, the value ω ₁ of the phase difference ω is determined using the depth measuring device of this example. At this time, when the phase difference ω ₁ is near 0 or 2π, the phase difference ω ₀ initially obtained and the phase difference ω _{1 are} almost 2π.
It may be different. In such a case, for example, close to the phase difference omega ₁ is 2 [pi, when the phase difference omega ₀ is close to 0, instead of the integer k _0, integer (k ₀ -1) (24) is substituted into integer k of Formula , On the contrary, the phase difference ω ₁ is close to 0, and the phase difference ω ₀
Is close to 2π, instead of the integer k ₀ , the integer (k
_By substituting ₀ + 1) into the integer k in equation (24), accurate depth measurement is performed.

The "size" of the amplitude reflectances φ _a and φ _b of the concave portions 9a and the convex portions 9b of the periodic step pattern 9 shown in FIG. 3 differs depending on the material (reflectance) of the periodic step pattern 9. However, in this example, the amplitude reflectances φ _a and φ _b
Irrespective of the "magnitude" (reflectance) of the above, the step amount d can be obtained based only on the phase difference ω. Therefore, even if the material of the concave portions 9a and the convex portions 9b of the periodic step pattern 9 is different, the step can be accurately measured. Also, suppose that the periodic step pattern 9
If the material of the concave portion 9a and the material of the convex portion 9b are materials that give different phase changes to the reflected light (materials having a complex index of refraction), this phase change deteriorates the step measurement accuracy of the pattern. Is generally very small and not a problematic amount. Of course, more strictly, it is also possible to calculate this phase change from the complex refractive index of the material (usually known) of the concave portion and the convex portion and correct the measured phase difference (pattern step).

Further, in the present example, since the detected light flux is monochromatic, naturally there is no problem of the reflection spectrum of the pattern to be inspected, which has been a problem in the conventional depth measuring apparatus. It is possible to measure the depth of the inspection pattern with high accuracy. In the above-described embodiment,
As an example of the pattern to be inspected, a pattern in which the boundary (sidewall) between the concave portion 9a and the convex portion 9b is vertical as in the periodic step pattern 9 in FIG. 3 is shown. However, of course, highly accurate measurement is possible. In this case, the concave portion input to the step measuring unit 12 in FIG.
The respective widths a and b of the convex portion are not values satisfying (a + b = P), but the step difference of the pattern may be calculated based on the values of these widths a and b as in the above-described embodiment.

Further, when the concave portion of the test pattern has asymmetry (taper or the like), unlike the example shown in FIG. 2, the detection signal S obtained by photoelectrically converting the combined light fluxes DA and DB is obtained.
Although the contrasts of A and SB do not match, the use of the average value of both makes it possible to measure the depth as accurately as a pattern without asymmetry. In addition, in the above-described embodiment, by irradiating the test pattern having the period P with two light fluxes, the interference fringes having the period of 2P in the measurement direction are formed. However, by irradiating the test pattern with the period P with two light fluxes, interference fringes having an amplitude distribution period of 2P / N (N is an integer of 2 or more) in the measurement direction may be formed. In this case, assuming that the two light fluxes are the first and second light beams, the regular reflection light (0th order diffracted light) from the test pattern of the first light beam and the test pattern of the second light beam. And the N-th order diffracted light from the light beam form a first combined light beam that interferes with each other, and the regular reflection light from the test pattern of the second light beam and the test pattern of the first light beam. The second combined light flux is generated in parallel with the Nth-order diffracted light from and is interfered with each other. Therefore, 0
The depth can be detected in the same manner as in the case of using the combined light flux of the second-order diffracted light and the first-order diffracted light.

As described above, the present invention is not limited to the above-described embodiments, and various configurations can be taken without departing from the gist of the present invention.

[0054]

According to the depth measuring method of the present invention, the photoelectric conversion signals of the first and second combined light fluxes, which are the diffracted light from the pattern to be inspected, respectively, and the wavelength of the light beam for detection,
Since the depth of the recess of the test pattern is calculated based on the shape of the test pattern, the depth of the recess (step) of the test pattern in which unevenness is periodically repeated like a fine groove pattern. Has the advantage that it can be measured with high accuracy. In particular, according to the present invention, the depth (step amount) of the concave portion can be accurately measured even in the case of a test pattern whose reflectance changes with wavelength due to properties peculiar to the material or thin film interference.

When calculating the depth of the concave portion of the test pattern, the phases and contrasts of the photoelectric conversion signals of the first and second combined light beams, the wavelengths of the first and second light beams, and When using the ratio of the widths of the recesses and protrusions of the test pattern in the predetermined direction (measurement direction), usually the processing of the widths of the recesses and protrusions in the measurement direction can be performed accurately. The depth can be accurately calculated by using the design value as the width.

Further, the depth measuring apparatus of the present invention has an advantage that the depth measuring method of the present invention can be carried out.

[Brief description of drawings]

FIG. 1 is a partial cross-sectional view showing a depth measuring device used in an example of an embodiment of the present invention.

FIG. 2 is a detection signal SA, SB obtained by the measuring device of FIG.
It is a waveform diagram showing an example.

FIG. 3 is an enlarged cross-sectional view showing a step structure of the periodic step pattern 9 of FIG.

FIG. 4 shows the amplitude reflectances φ _a and φ in the embodiment.
It is explanatory drawing which shows the process which the amplitude (psi) ₀ , (psi) _{1 of} diffracted light is derived from _b by the polar coordinate of a complex plane.

[Explanation of symbols]

 1 Laser Light Source 3 Reference Lattice 4 Collimator Lens 5 Spatial Filter 6 Half Mirror 7 Condensing Lens 8 Wafer (Semiconductor Wafer) 9 Periodic Step Pattern 10 Wafer Stage 11A, 11B Photoelectric Detector 12 Step Difference Measuring Section 13 Main Control System

Claims (3)

[Claims] 1. A depth measuring method for optically measuring a depth of a concave portion of a pattern to be inspected formed so that irregularities are periodically repeated in a predetermined direction on a substrate, wherein a period of the pattern to be inspected is P By irradiating the test pattern with the coherent first and second light beams, the cycle of the amplitude distribution in the predetermined direction is 2P / N (N is a natural number). Interference fringes are formed, and the specular reflection light from the pattern to be inspected by the first light beam and the Nth order diffracted light from the pattern to be inspected by the second light beam are combined. The light flux, the specularly reflected light from the test pattern by the second light beam, and the N from the test pattern by the first light beam.
The first and second combined light beams obtained when a second combined light beam formed by combining the second-order diffracted light is received and the pattern to be measured and the interference fringes are relatively scanned in the predetermined direction. The depth measuring method, wherein the depth of the concave portion of the pattern to be inspected is calculated based on the photoelectric conversion signal and the wavelengths of the first and second light beams. 2. The depth measuring method according to claim 1, wherein the phase and the contrast of the photoelectric conversion signals of the first and second combined light fluxes are used when calculating the depth of the concave portion of the test pattern. And a wavelength ratio of the first and second light beams and a ratio of widths of the concave portion and the convex portion of the test pattern in the predetermined direction are used. 3. A depth measuring device for optically measuring a depth of a concave portion of a pattern to be inspected, which is formed on a substrate such that irregularities are periodically repeated in a predetermined direction. , The period of the amplitude distribution in the predetermined direction is 2P / N (N
Is a natural number), and a light-transmitting optical system for irradiating the pattern to be inspected with coherent first and second light beams to form an interference fringe, and a pattern from the pattern to be inspected by the first light beam. A first photoelectric conversion element for receiving a first combined light flux obtained by combining regular reflection light and N-th order diffracted light from the pattern to be inspected by the second light beam; A second photoelectric conversion element for receiving a second combined light flux obtained by combining regular reflection light from the pattern to be inspected and N-th order diffracted light from the pattern to be inspected by the first light beam; Relative scanning means for relatively scanning the pattern and the interference fringes in the predetermined direction, and the first and second relative scanning means for relatively scanning the test pattern and the interference fringes. Photoelectric conversion signal output from photoelectric conversion element And a depth calculation means for calculating the depth of the recess of the pattern to be measured based on the wavelengths of the first and second light beams.