JPH11121343A - Aberration coefficient measuring method of reduced projecting aligner - Google Patents

Abstract

PROBLEM TO BE SOLVED: To easily measure the aberration coefficient of a stepper, by a method wherein a plurality of pitches are formed at shot center, and the spherical aberration coefficient for the whole area of shot is measured by the center value in the range of formation of each pattern. SOLUTION: In an optical system having a stepper, a line and space pattern is formed by the three-luminous flux coupling which is symmetrical to optical axis under the illumination approximate to the coherent having the abarration coefficient σ of a stepper of about 0.1 or lower. The line and space pattern is formed at the center of shot by changing focus in several kinds of pitches and shots in the direction of radiation and in the direction which orthogonally intersects with it. The center value of the best focus range is computed for each pattern formed, and if there is an aberation in the center value of the focus range, pattern pitches are different. Accordingly, an aberation can be computed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a method for measuring an aberration coefficient of an optical system of a reduction projection exposure apparatus (hereinafter, referred to as a stepper) used in a lithography process in semiconductor manufacturing.

[0002]

2. Description of the Related Art Conventionally, as a method of measuring aberration of a stepper, there has been a method using a Twyman type interferometer, for example. FIG. 3 is a schematic view of an optical system of such a conventional stepper. As shown in this figure, the parallel emission light from the collimator 13 is split by the translucent mirror 15 into two orthogonal directions,
One of the lights is reflected by the plane mirror 14 and then the translucent mirror 1
5 to the observation lens 18.

The other light passes through the lens 16 to be measured, is reflected by the convex spherical mirror 17 having its center at the focal point, passes through the lens 16 in the opposite direction again, becomes a parallel light beam, and becomes a translucent mirror 15. The light is reflected by the lens and reaches the observation lens 18. Here, since both light beams interfere with each other, the focal plane 1 of the observation lens 18 is changed.
By observing the pupil plane of the test lens 16 with an eye on 9, interference fringes can be seen.

In this case, the interference fringes pass through the lens 16 to be measured, and have an equal optical path difference between the actual wavefront deformed by the aberration and the reference spherical wavefront which is considered to converge without aberration to the center of the convex spherical mirror 17. , And the light is observed after passing through the test lens 16 twice, and the interval between the stripes means that the change in the wavefront aberration is １ ／ of the wavelength. This can be graphed to determine the wavefront aberration. In FIG. 3, reference numeral 11 denotes a light source, and reference numeral 12 denotes a pinhole.

[0005]

However, in the above-described conventional method for measuring aberration of a stepper, when actually measuring with a stepper, it is necessary to set a plane mirror or a translucent mirror. Has to be set in the optical path, and using this method on a stepper that has already been assembled involves a very difficult operation including setting and adjustment.

SUMMARY OF THE INVENTION It is an object of the present invention to eliminate the above-mentioned problems and to provide a simple and easy method for measuring the aberration coefficient of a stepper.

[0007]

According to the present invention, there is provided a method for measuring an aberration coefficient of an optical system of a reduction projection exposure apparatus, comprising the steps of: A plurality of patterns with a plurality of pitches are formed at the center of the shot by three-beam image formation symmetrical with respect to, and the spherical aberration coefficient over the entire shot is obtained from the center value of each pattern formation range.

[2] In an aberration coefficient measuring method for obtaining an aberration coefficient of an optical system of a reduction projection exposure apparatus, three light flux images symmetrical with respect to an optical axis under illumination close to coherent are used to form a plurality of pitches at a plurality of image heights. The pattern is formed, and the aberration coefficient is obtained from the center value of each pattern forming range. [3] In an aberration coefficient measuring method for obtaining an aberration coefficient of an optical system of a reduction projection exposure apparatus, formation of a pattern having a plurality of pitches at a plurality of image heights by forming three light beams symmetrical to an optical axis under illumination close to coherent. 2 with different pitches in the direction orthogonal to the direction of emission to the shot center.
The difference in the amount of positional deviation between the types of line and space patterns is obtained for a plurality of pitches.

[4] In an aberration coefficient measuring method for obtaining an aberration coefficient of an optical system of a reduction projection exposure apparatus, a pattern is formed at a plurality of image heights by imaging two light beams symmetrical to an optical axis under illumination close to coherent. In this embodiment, the difference in the amount of positional deviation between two types of line and space patterns having different pitches in the direction orthogonal to the direction radiated from the shot center is obtained for a plurality of pitches.

[0010]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a schematic view of an optical system of a stepper according to a first embodiment of the present invention.
In this figure, 21 is an aperture stop, 22 is a condenser lens, 23 is a mask, 24 is diffracted light, 25 is a projection lens, 26 is an aperture stop, and 27 is a wafer.

For example, if the aberration coefficient σ of the stepper is 0.1
A line and space pattern is formed by imaging three light beams symmetrical with respect to the optical axis as shown in FIG. The line and space pattern is formed by changing the focus with respect to several types of pitches and positions within the shot in the direction of emission and the direction perpendicular to the center of the shot.

The pitch is changed to the resolution limit, and the position in the shot is changed from the shot center to the maximum exposure position. When the pitch is large, three-beam imaging can be realized by reducing the NA (numerical aperture of the projection lens) (decreasing the aperture stop 26). Then, for each of the formed patterns, the best focus, that is, the center value of the focus range in which the pattern is formed well is obtained.

If the center value of the focus range has an aberration, it differs depending on the pattern pitch or the position in the shot, so that the aberration can be obtained from this. By the way, when a line-and-space pattern is formed by imaging three light beams symmetrically with respect to the optical axis under coherent illumination, the light intensity distribution can be expressed by the following equation.

I = a + b1 · cos [(2πX / P) + kW ₁ ] + b2 · cos [(2πX / P) −kW ₋₁ ] + c1 · cos [(4πX / P) + kW ₁ −kW ₋₁ ] (1) Here, P is the pitch, k is the wave number, W1 is the wavefront aberration of the + _1st- order diffracted light, and W- ₁ is the wavefront aberration of the _-1st- order diffracted light. The wavefront aberration is obtained by setting the in-shot position and the pupil plane position to (r,
ω) and (ρ, φ) are represented by the following equations. Here, r and ρ are standardized values.

W = Σa _{1, m, n} r ^{21 + m} ρ ⁿ cos ^m (φ−ω) (2) Among these aberrations, n such as spherical aberration, field curvature and astigmatism is an even number Causes the best focus difference due to the pitch, the best focus difference due to the position in the shot, and the best focus difference in the direction of emission and the direction orthogonal to the center of the shot.

The spherical aberration due to the entire surface within the shot can be obtained as follows. Of the aberrations expressed by the above equation (2), the spherical aberration at the shot center can be expressed by the following equation. W _a = b ₃ ρ ⁴ + c ₆ ρ ⁶ + d ₁₀ ρ ⁸ + (3-1) W ₁ = W _-1 (3-2) Here, considering the aberration due to defocus, W _def = (1/2) NA ² ρ ² z (4) NA is the numerical aperture of the projection lens, and z is the focus position.

From the above equations (1) and (3-2), the best focus position of the pattern formed by the three-beam image formation by coherent illumination is: W _a + W _def = 0 (5) The focus position z _b can be expressed as follows. z _b = (− 2 / NA ² ) (b ₃ ρ ² + c ₆ ρ ⁴ + d ₁₀ ρ ⁶ +...) (6) The best focus, that is, ρ is in the range of 0 to 1 by changing the pattern pitch, that is, The center value of the focus range in which the pattern is formed favorably is obtained, and a graph with the horizontal axis being ρ and the vertical axis being the best focus position is fitted by a polynomial of the following form. More aberration coefficients can be obtained.

Z _b = A 0 + αρ ² + βρ ⁴ γρ ⁶ + (7) Next, when the aberrations that vary depending on the position within the shot are also included, they can be obtained as follows. Such aberrations can be developed by applying the above equation (2) to the direction radiating to the shot center and the directions W _m and W _S orthogonal thereto, as follows.

W _m = (b ₁ + b ₅ ) r ² ρ ² + b ₃ ρ ⁴ + (c ₁ + c ₅ ) r ⁴ ρ ² + (c ₃ + c ₈ ) r ² ρ ⁴ + c ₆ ρ ⁶ +... ··· (8) W _s = b ₁ r ² ρ ² + b ₃ ρ ⁴ + c ₁ r ⁴ ρ ² + c ₃ r ² ρ ⁴ + c ₆ ρ ⁶ + ······ (9) Radiation to the shot center The best focus position in the direction of movement and the direction perpendicular thereto can be obtained by replacing W _a in the above equation (5) with W _m and W _s . Therefore, the best focus positions z _bm and z _bs can be expressed as follows. it can.

Z _bm = −2 W _m / (NA ² ρ ² ) (10) z _bs = −2 W _s / (NA ² ρ ² ) (11) The above equations (10) z _bm and (11) z _bs Are both polynomials of the form z _b = A0 + αr ² + βρ ² + γr ⁴
+ Δr ² ρ ² + ερ ⁴ +... (12) From this, the pattern pitch is changed to the resolution limit at a plurality of positions within the exposure range, and radiated to the shot center when ρ is in the range of 0 to 1. Find the best focus of the line and space pattern in the direction of
Each is fitted with a two-variable polynomial of ρ and r in the above equation (12), and coefficients α, β, γ, δ, ε.
The aberration coefficient can be obtained by comparing this with Expressions (8) and (9).

Next, a second embodiment of the present invention will be described. For example, under coherent illumination in which the aberration coefficient σ of the stepper is about 0.1 or less, a line-and-space pattern having two pattern pitches is formed by imaging three light beams symmetrically with respect to the optical axis. The difference between the two types of patterns is calculated. If there is an aberration, the difference in the amount of positional deviation differs depending on the pattern pitch or the position in the shot. When the pitch is large, three-beam imaging can be realized by reducing the NA (reducing the aperture stop 26).

The difference between the two types of pattern pitches can be measured as a conventional method, for example, as follows. First, as a first layer, a line and space resist pattern is formed by exposure and development, and then the substrate is etched to form a line and space under the substrate. Next, a resist pattern is formed by exposing and developing the 1/2 layer line & space of the first layer so as to match the line & space of the first layer.

FIG. 2 shows a sectional view of the pattern formed in this manner. As shown in this figure, by measuring the widths (shift amount 1) 33 and (shift amount 2) 34 of the underlying line patterns on both sides of the resist pattern 32 on the substrate 31, the first layer and the second layer are measured. The shift is determined as 1/2 of the difference. The positional deviation obtained in this manner appears when there is an odd aberration other than 1 in the above equation (2). This shift amount xs can be expressed by the following equation.

[0024] xs = (1 / λ) × { ^{_{[b 4 (ρ 1 3 P 1}} -ρ 2 3 P 2) + c 7 (ρ 1 5 P 1 -ρ 2 5 P 2) + ·· ] r + [c ^{_{4 (ρ 1 3 P 1 -ρ}} 2 3 P 2) + ·· ] ^{r 3 + ···} ... (13} ) the formula (11) is a polynomial as a variable r, the coefficient of each term α, β, γ... can be expressed as follows.

[0025] ^{xs = A0 + αr + βr 3} + γr 5 + ··· ... (14) than this, radiation to the shot center of the pattern of the two types of lines and spaces of different pitch up shot center to a maximum exposure position of the exposure in the area Is obtained for a plurality of image heights, and from the graph in which the horizontal axis is r and the vertical axis is the positional shift between the two types of patterns, the above equation (1) is obtained.
Fit by the function of 4), and coefficient α, β, γ ...
Ask for.

A0 is an offset component of the superposition of the first layer and the second layer. Here, if up to the third order aberration coefficients in the above equation (13) it is only b _4, can be obtained quickly. Also, if it is a fifth-order or higher aberration,
Since the aberration coefficient increases, the difference in the amount of pattern displacement between two types of lines and spaces having different pitches is obtained, and the coefficients α, β, γ... Thus, the aberration coefficient can be obtained.

In the above description, pattern formation is performed by three light beam imaging. However, two light beam imaging symmetrical with respect to the optical axis using oblique incidence illumination or a phase shift method is used to form two patterns. The aberration coefficient can be measured from the positional deviation amount difference between them. It should be noted that the present invention is not limited to the above embodiment, and various modifications can be made based on the gist of the present invention, and these are not excluded from the scope of the present invention.

[0028]

As described in detail above, according to the present invention, the setting of the aberration coefficient of the stepper is easy and simple.

[Brief description of the drawings]

FIG. 1 is a schematic view of an optical system of a stepper according to a first embodiment of the present invention.

FIG. 2 is a sectional view of a formed pattern showing a method for measuring an aberration coefficient of a stepper according to a second embodiment of the present invention.

FIG. 3 is a schematic view of an optical system of a conventional stepper.

[Explanation of symbols]

 21, 26 aperture stop 22 condenser lens 23 mask 24 diffracted light 25 projection lens 27 wafer 31 substrate 32 resist pattern 33 shift amount 1 34 shift amount 2

Claims (4)

[Claims] 1. A method of measuring an aberration coefficient of an optical system of a reduction projection exposure apparatus, comprising: forming a pattern having a plurality of pitches at the center of a shot by forming three light beams symmetrical with respect to an optical axis under illumination close to coherent. And measuring the spherical aberration coefficient over the entire shot from the center value of each pattern formation range. 2. A method for measuring an aberration coefficient of an optical system of a reduction projection exposure apparatus, comprising: a three-beam image symmetric with respect to an optical axis under illumination close to coherent; And measuring an aberration coefficient from a center value of each pattern formation range. 3. A method for measuring an aberration coefficient of an optical system of a reduction projection exposure apparatus, comprising: a method of forming a plurality of pitches at a plurality of image heights by forming three light beams symmetrical with respect to an optical axis under nearly coherent illumination. Reduction projection exposure wherein a difference between positional deviation amounts of two types of line & space patterns having different pitches in a direction orthogonal to a direction radiating from a shot center is obtained for a plurality of pitches. Method for measuring aberration coefficient of device. 4. An aberration coefficient measuring method for obtaining an aberration coefficient of an optical system of a reduction projection exposure apparatus, wherein a pattern is formed at a plurality of image heights by two light flux images symmetrical with respect to an optical axis under illumination close to coherent. An aberration coefficient of the reduced projection exposure apparatus, wherein a difference between positional deviation amounts of two types of line and space patterns having different pitches in a direction orthogonal to a direction radiating from a shot center is obtained for a plurality of pitches. Measuring method.