Coma aberration automatic measuring mark and measuring method

Two coma aberration automatic measuring marks M1 and M2 of a first-order diffraction grating are each composed of a plurality of elongated isosceles triangle patterns which are so arranged that the axis of symmetry passing on the center of each elongated isosceles triangle is parallel to one another, that a half P1, P2, and P3 of the elongated isosceles triangle patterns have the widths thereof which extend in a direction opposite to that of the remaining half P4, P5 and P6 of the elongated isosceles triangle patterns, and the elongated isosceles triangle patterns are located separately from one another, in a direction perpendicular to the axis of symmetry passing on the center of each elongated isosceles triangle, and at a pitch diffracting a measuring coherent light. The two first-order diffraction gratings M1 and M2 are located separately from each other in a direction of the axis of symmetry passing on the center of the elongated isosceles triangle, in such a manner that the elongated isosceles triangle patterns included in the two first-order diffraction gratings are in symmetry to each other, in connection with a line positioned between the two first-order diffraction gratings and which is perpendicular to the axis of symmetry passing on the center of the elongated isosceles triangle. The two first-order diffraction gratings are scanned by the measuring coherent light, and a relative distance R1 between diffraction lights generated by the two first-order diffraction gratings, is measured and compared with a distance R between the two first-order diffraction gratings.

BACKGROUND OF THE INVENTION
 The present invention relates to a coma aberration automatic measuring mark
 used for measuring only a coma aberration of various aberrations of a lens
 system used in a reduction projection exposure, and a method for measuring
 the coma aberration by using the coma aberration automatic measuring mark.
 One important factor is to quickly and simply measure a coma aberration of
 a reduction projection lens system. Now, a prior art method for measuring
 the coma aberration of the reduction projection lens system will be
 described with reference to FIGS. 1A and 1B.
 As shown in FIG. 1A, a prior art coma aberration automatic measuring mark
 is constituted of a plurality of strip-shaped patterns L1 L2, L3, L4 and
 L5 arranged in parallel to one another and separately from one another, at
 a pitch which is about a double of a wavelength of an exposure light. A
 line width of a projected pattern of each of the strip-shaped patterns L1,
 L2, L3. L4 and L5 is measured by for example a length measuring SEM
 (scanning electron microscope), and the amount of coma aberration is
 calculated from a difference in the line width between the strip-shaped
 patterns L1 and L5 positioned at opposite side ends of the projected coma
 aberration automatic measuring mark, as shown in FIGS. 1A and 1B.
 FIG. 1A illustrates a case having no coma aberration, and FIG. 1B
 illustrates a case having a coma aberration. FIG. 1C illustrates a case
 having a spherical aberration.
 In the prior art, however, when the coma aberration is very large, it is
 not possible to carry out an automatic measurement utilizing an image
 processing, and therefore, the coma aberration must be measured manually,
 with the result that the measurement needs a considerable time. Therefore,
 adjustment of the reduction projection lens system cannot be smoothly
 performed, and a satisfactory degree of reproduction cannot be obtained
 because of the manual measurement. In addition, it is difficult to
 separate the coma aberration from the other aberrations occurring in the
 reduction projection lens system.
 BRIEF SUMMARY OF THE INVENTION
 Accordingly, it is an object of the present invention to provide a coma
 aberration automatic measuring mark used for automatically measuring the
 coma aberration with a high degree of reproduction, and a method for
 measuring the coma aberration by using the coma aberration automatic
 measuring mark.
 The above and other objects of the present invention are achieved in
 accordance with the present invention by a coma aberration automatic
 measuring mark comprising a first-order diffraction grating composed of a
 plurality of elongated isosceles triangle patterns which are so arranged
 that the axis of symmetry passing on the center of each elongated
 isosceles triangle is parallel to one another, that a half of the
 elongated isosceles triangle patterns are located in a direction opposite
 to that of the remaining half of the elongated isosceles triangle
 patterns, and the elongated isosceles triangle patterns are located
 separately from one another, in a direction perpendicular to the axis of
 symmetry passing on the center of each elongated isosceles triangle, and
 at a pitch diffracting a measuring coherent light.
 Specifically, the pitch diffracting the measuring coherent light is about a
 double of a lens design wavelength.
 In an embodiment of the coma aberration automatic measuring mark, the
 elongated isosceles triangle patterns includes a first group of elongated
 isosceles triangle patterns and a second group of elongated isosceles
 triangle patterns, which are located in symmetry to each other, in
 connection with a line which is positioned between the first group of
 elongated isosceles triangle patterns and the second group of elongated
 isosceles triangle patterns and which is perpendicular to the axis of
 symmetry passing on the center of the elongated isosceles triangle.
 This coma aberration automatic measuring mark is adapted for not only a
 measurement of a coma aberration but also a measurement of a telecentric
 property of an optical axis.
 According to a second aspect of the present invention, there is provided a
 method for measuring a coma aberration by using a coma aberration
 automatic measuring mark, wherein the coma aberration automatic measuring
 mark comprising at least two first-order diffraction gratings each of
 composed of a plurality of elongated isosceles triangle patterns which are
 so arranged that the axis of symmetry passing on the center of each
 elongated isosceles triangle is parallel to one another, that a half of
 the elongated isosceles triangle patterns are located in a direction
 opposite to that of the remaining half of the elongated isosceles triangle
 patterns, and the elongated isosceles triangle patterns are located
 separately from one another, in a direction perpendicular to the axis of
 symmetry passing on the center of each elongated isosceles triangle, and
 at a pitch diffracting a measuring coherent light, the at least two
 first-order diffraction gratings being located separately from each other
 in a direction of the axis of symmetry passing on the center of the
 elongated isosceles triangle, in such a manner that the elongated
 isosceles triangle patterns included in the at least two first-order
 diffraction gratings are in symmetry to each other, in connection with a
 line which is positioned between the at least two first-order diffraction
 gratings and which is perpendicular to the axis of symmetry passing on the
 center of the elongated isosceles triangle, and
 wherein the method includes the steps of scanning the at least two
 first-order diffraction gratings by the measuring coherent light,
 measuring a relative distance between diffraction lights generated by the
 at least two first-order diffraction gratings, and comparing the measured
 relative distance with a distance between the at least two first-order
 diffraction gratings.
 Specifically, a distance between the at least two first-order diffraction
 gratings located in symmetry to each other is set to a distance which is
 sufficiently longer than a wavelength of a measuring coherent light and
 which is on the order which can make negligible an Abbe error occurring in
 a coordinate measuring system and a distortion occurring in a reduction
 projection lens.
 This method can be used for measuring a telecentric property of an optical
 axis, in place of measuring the coma aberration.
 Here, the triangular patterns constituting the coma aberration automatic
 measuring mark are not limited to the elongated isosceles triangle
 patterns, but are sufficient if the triangular patterns are elongated
 triangular patterns which can be so located that a half of the elongated
 triangular patterns are located in a direction opposite to that of the
 remaining half of the elongated triangular patterns, and the elongated
 triangular patterns are located in parallel to each other and separately
 from one another, at a pitch diffracting a measuring coherent light, in a
 direction substantially perpendicular to the long axis that extends from
 the vertex between two long sides of three sides of the elongated triangle
 to the shortest side of the three sides of the elongated triangle,
 perpendicularly to the shortest side of the elongated triangle.
 The above and other objects, features and advantages of the present
 invention will be apparent from the following description of preferred
 embodiments of the invention with reference to the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION
 Now, embodiments of the present invention will be described with reference
 to the drawings.
 FIG. 2 is a diagrammatic view of an embodiment of the coma aberration
 automatic measuring mark in accordance with the present invention. FIG. 3
 is a diagrammatic view illustrating a method in accordance with the
 present invention for measuring the coma aberration by using the
 embodiment of the coma aberration automatic measuring mark in accordance
 with the present invention.
 The present invention is characterized by making it possible to
 quantitatively measure only a coma aberration of various aberrations of a
 lens system used in a reduction projection exposure.
 As shown in FIG. 2, the embodiment of the coma aberration automatic
 measuring mark in accordance with the present invention is generally
 designated by the reference sign M, and includes a first-order diffraction
 grating composed of a plurality of elongated isosceles triangle patterns
 P1, P2, P3, P4, P5 and P6 which are arranged at a pitch which is about a
 double of a lens design wavelength. Specifically, the plurality of
 elongated isosceles triangle patterns P1, P2, P3, P4, P5 and P6 are so
 arranged that the axis of symmetry passing on the center of each elongated
 isosceles triangle is parallel to one another, that a half P1, P2 and P3
 of the elongated isosceles triangle patterns are located in a direction
 opposite to that of the remaining half P4, P5 and P6 of the elongated
 isosceles triangle patterns, and the elongated isosceles triangle patterns
 are located separately from one another, in a direction perpendicular to
 the axis of symmetry passing on the center of each elongated isosceles
 triangle, and at a pitch diffracting a measuring coherent light. Here, the
 elongated isosceles triangle patterns P1, P2, P3, P4, P5 and P6 has the
 length which is about five times to ten times the width. However, the
 length is not limited to this range.
 As mentioned above, the patterns P1, P2, P3, P4, P5 and P6 which constitute
 the coma aberration automatic measuring mark M in accordance with the
 present invention are formed in the form of an elongated triangle, in
 place of a line and space pattern shown in FIG. 1. This is because the
 triangular pattern is more susceptible to influence of the coma
 aberration, than the strip or elongated rectangular pattern in the line
 and space pattern, and therefore, it is possible to sensitively measure
 the coma aberration.
 As shown in FIG. 2, in the embodiment of the coma aberration automatic
 measuring mark M in accordance with the present invention, the elongated
 isosceles triangle patterns P1, P2, P3, P4, P5 and P6 are divided into a
 first group which includes a first half of the elongated isosceles
 triangle patterns (P1, P2, P3) and a second group which includes the
 remaining half of the elongated isosceles triangle pattern (P4, P5 and
 P6), and the direction of the first group of the elongated isosceles
 triangle patterns P1, P2, P3 is opposite to the direction of the second
 group of the elongated isosceles triangle pattern P4, P5 and P6. In
 addition, the elongated isosceles triangle patterns P1, P2, P3, P4, P5 and
 P6 are located separately from one another in a direction perpendicular to
 the axis of symmetry passing on the center of each elongated isosceles
 triangle, and at a "pitch which is about a double of a lens design
 wavelength", which is sufficient to diffract a measuring coherent light.
 Incidentally, the number of the elongated isosceles triangle patterns
 included in the coma aberration automatic measuring mark M in accordance
 with the present invention, is in no way limited to 6 as shown in FIG. 2,
 but also can be freely selected if the first-order diffraction grating is
 constituted.
 Now, a method for measuring the coma aberration by using the coma
 aberration automatic measuring mark M in accordance with the present
 invention, will be described with reference to FIG. 3. Two coma aberration
 automatic measuring marks M as shown in FIG. 2, namely, two coma
 aberration automatic measuring marks M1 and M2 as shown in FIG. 3 are
 located separately from each other by a distance R in a direction of the
 axis of symmetry passing on the center of each elongated isosceles
 triangle pattern, and in symmetry to each other in connection of an axis
 of symmetry "S" which is positioned between the two coma aberration
 automatic measuring marks M1 and M2 (and which is perpendicular to the
 axis of symmetry passing on the center of the elongated isosceles
 triangle).
 In this case, furthermore, the two coma aberration automatic measuring
 marks M1 and M2 are so located that the direction of the elongated
 isosceles triangle patterns P1, P2, P3, P4, P5 and P6 included in the coma
 aberration automatic measuring mark M1 are opposite to the direction of
 the corresponding elongated isosceles triangle patterns included in the
 coma aberration automatic measuring mark M2. Therefore, in the two coma
 aberration automatic measuring marks M1 and M2 in symmetry to each other,
 the elongated isosceles triangle patterns P1, P2, P3, P4, P5 and P6
 included in the coma aberration automatic measuring mark M1 and the
 elongated isosceles triangle patterns included in the coma aberration
 automatic measuring mark M2 are in symmetry in connection of the axis of
 symmetry "S".
 In addition, the distance "R" between a center of the coma aberration
 automatic measuring mark M1 and a center of the coma aberration automatic
 measuring mark M2 is set to a distance which is sufficiently longer than a
 wavelength of a measuring coherent light and which is on the order which
 can make negligible an Abbe error occurring in a coordinate measuring
 system and a distortion occurring in a reduction projection lens.
 In order to measure the coma aberration by using the coma aberration
 automatic measuring mark M in accordance with the present invention, the
 two coma aberration automatic measuring marks (two first-order diffraction
 gratings) M1 and M2 separated from each other by the distance "R" are
 scanned by the coherent light (laser beam) as shown in FIG. 3, and as
 shown in FIGS. 4 and 5, a relative distance "R1" between diffraction
 lights "K" generated by the two coma aberration automatic measuring marks
 (two first-order diffraction gratings) M1 and M2, is measured, and then
 compared with the normal distance "R" between the two first-order
 diffraction gratings.
 In a case having a coma aberration, the relative distance "R1" between the
 diffraction lights "K" generated by the two first-order diffraction
 gratings M1 and M2, is converted into the amount of coma aberration. A
 plurality of pairs of symmetrical coma aberration automatic measuring
 marks M are previously provided under the same condition as shown in FIG.
 3, or alternatively, the same pair of symmetrical coma aberration
 automatic measuring marks M are measured a plurality of times, so that the
 results of measurement is averaged to minimize an error to a possible
 extent.
 In other words, according to the present invention, it is possible to
 quantify the coma aberration by a high speed automatic measurement with
 the result that the coma aberration in the reduction projection exposure
 lens can be measured precisely for a short time. In addition, if the coma
 aberration automatic measuring marks M are located not only in the
 vertical direction as shown in FIG. 1 but also in a horizontal direction
 and also in a sagitall direction and in a meridional direction, at a place
 where the coma aberration of a lens is to be measured, it is possible to
 clarify the distribution of the coma aberration in the plane of the lens.
 If the distribution of the coma aberration thus obtained in the plane of
 the lens, is fed back to adjustment of the reduction projection lens
 system, it is possible to correct the coma aberration in an actual
 pattern, with the result that it is possible to minimize the influence
 such as the size thinning of the device pattern and the unevenness of the
 line width, which are caused by the coma aberration.
 Next, the specific method for measuring the coma aberration by using the
 coma aberration automatic measuring mark M in accordance with the present
 invention, as shown in FIG. 3, will be described with reference to FIG. 5,
 which is a diagrammatic view illustrating the diffraction lights "K"
 generated by the coma aberration automatic measuring marks M1 and M2 in
 accordance with the present invention, in the case having a coma
 aberration.
 The coma aberration is the aberration in which the contrast lowers like a
 comet, and therefore, the patterns (P1 and P2 in the example shown in FIG.
 5) positioned at one end of the coma aberration automatic measuring mark M
 are thinned. As a result, the projected patterns P1 to P3 and the
 projected patterns P4 to P6 of the coma aberration automatic measuring a
 mark M become asymmetrical, as shown in FIG. 5.
 Therefore, a peak position S1 of the diffraction lights generated by the
 coma aberration automatic measuring mark M when it is scanned by the
 coherent light, is deviated from a normal peak position S. Since the
 elongated isosceles triangle patterns P1, P2, P3, P4, P5 and P6 included
 in the coma aberration automatic measuring mark M1 and the elongated
 isosceles triangle patterns included in the coma aberration automatic
 measuring mark M2 are in symmetry in connection of the axis of symmetry
 "S", the distance R1 between the peak position S1 of the diffraction
 lights generated by the coma aberration automatic measuring mark M1 and
 the peak position S1 of the diffraction lights generated by the coma
 aberration automatic measuring mark M2 become longer than the distance R
 between the normal peak position S of the diffraction lights generated by
 the coma aberration automatic measuring mark M1 and the normal peak
 position S of the diffraction lights generated by the coma aberration
 automatic measuring mark M2 when no coma aberration exists. Therefore, the
 amount of coma aberration is measured by measuring the distance between
 the respective peak positions S1 of the diffraction lights "K" generated
 by the two coma aberration automatic measuring marks M1 and M2.
 FIG. 6 is a diagrammatic view illustrating the diffraction lights generated
 by the coma aberration automatic measuring mark in accordance with the
 present invention, in the case having a spherical aberration.
 The spherical aberration is the aberration in which the contrast lowers
 simply. Therefore, the patterns (P1 and P2, and P5 and P6 in the example
 shown in FIG. 6) positioned at both ends of the coma aberration automatic
 measuring mark M are thinned. As a result, the projected patterns P1 to P3
 and the projected patterns P4 to P6 of the coma aberration automatic
 measuring mark M1 become asymmetrical, as shown in FIG. 6.
 As mentioned above, in the case having the spherical aberration, the ends
 of the patterns are thinned by the spherical aberration. However, if the
 coma aberration automatic measuring mark in accordance with the present
 invention is used, the diffraction light generated by the coma aberration
 automatic measuring mark does not become asymmetrical, and therefore, the
 peak position of the diffraction lights is not deviated from the normal
 peak position S, as shown in FIG. 6. The reason for this is that, in the
 two coma aberration automatic measuring marks M1 and M2 in symmetry to
 each other, the elongated isosceles triangle patterns P1. P2, P3, P4, P5
 and P6 included in one coma aberration automatic measuring mark M1 and the
 elongated isosceles triangle patterns included in the other coma
 aberration automatic measuring mark M2 are in symmetry in connection of
 the axis of symmetry "S" which is perpendicular to the axis of symmetry
 passing on the center of each elongated isosceles triangle pattern.
 Incidentally, the embodiment shown in FIGS. 2 to 6 has been applied to the
 measurement for the coma aberration, but can be applied for measuring a
 telecentric property of an optical axis.
 Namely, in the case of the coma aberration, the "center of gravity" of the
 detected signal (corresponding to the peak position S of the diffraction
 light) does not appreciably change, regardless of whether it is in the
 best focused condition or in a defocused condition.
 However, when the telecentric property of the optical axis is deviated, the
 peak position of the measured signal changes between the defocused
 condition of a "+" direction and the defocused condition of a "-"
 direction.
 Therefore, the projected patterns of the coma aberration automatic
 measuring mark become asymmetrical because of the deviation of the
 telecentric property of the optical axis. This asymmetry is "0" (zero) in
 the best focused condition, but becomes large in the defocused condition.
 Fundamentally, the asymmetry generated in the defocused condition of the
 "+" direction is reverse to the asymmetry generated in the defocused
 condition of the "-" direction.
 Accordingly, the two first-order diffraction gratings (each of which is the
 coma aberration automatic measuring mark M shown in FIG. 2) are scanned,
 and the distance R between the diffraction lights generated by the two
 first-order diffraction gratings is measured, and a gradient component of
 the distance between the peak positions caused by the defocusing, is
 obtained. If it is assumed that the gradient component thus obtained
 corresponds to the deviation of the telecentric property, the telecentric
 property can be automatically measured.
 Therefore, if the lens is adjusted on the basis of the measurement result,
 it is possible to minimize the unevenness of the line width which is
 caused by the coma aberration and the deviation of the telecentric
 property.
 As mentioned above, according to the present invention, since the coma
 aberration automatic measuring mark is formed of triangular patterns, the
 coma aberration automatic measuring mark is susceptible to influence of
 the coma aberration, and therefore, it is possible to sensitively measure
 the coma aberration.
 Therefore, the triangular patterns constituting the coma aberration
 automatic measuring mark are not limited to the elongated isosceles
 triangle patterns, but are sufficient if the triangular patterns are
 elongated triangular patterns which can be so located that a half of the
 elongated triangular patterns are located in a direction opposite to that
 of the remaining half of the elongated triangular patterns, and the
 elongated triangular patterns are located in parallel to each other and
 separately from one another, at a pitch diffracting a measuring coherent
 light, in a direction substantially perpendicular to the long axis that
 extends from the vertex between two long sides of three sides of the
 elongated triangle to the shortest side of the three sides of the
 elongated triangle, perpendicularly to the shortest side of the elongated
 triangle. In this case, preferably the vertex between the two long sides
 of the elongated triangular patterns and a center of the shortest side of
 the elongated triangular patterns are located at the same pitch,
 respectively. For example, the elongated triangular patterns are
 preferably so arranged that the long axis that extends from the vertex
 between two long sides of three sides of each triangle to the shortest
 side of the three sides of the same triangle, perpendicularly to the
 shortest side of the same triangle, is in parallel to each other, a half
 of the elongated triangular patterns are located in a direction opposite
 to that of the remaining half of the elongated triangular patterns, and
 the elongated triangular patterns are located separately from one another,
 at a pitch diffracting a measuring coherent light.
 Furthermore, according to the present invention, at least two coma
 aberration automatic measuring marks, each of which is constituted of a
 first-order diffraction grating composed of a plurality of elongated
 triangular patterns located at a pitch diffracting a measuring coherent
 light, are located in symmetry to each other, and are scanned by the
 coherent light so that the distance between diffraction lights generated
 by the two first-order diffraction gratings, is measured and compared with
 a normal distance between the two first-order diffraction gratings. Thus,
 only the coma aberration can be automatically measured with a high degree
 of reproduction.
 Furthermore, since the present invention can be applied for measuring the
 coma aberration and for measuring a telecentric property of an optical
 axis, the degree of versatility can be elevated.
 The invention has thus been shown and described with reference to the
 specific embodiments. However, it should be noted that the present
 invention is in no way limited to the details of the illustrated
 structures but changes and modifications may be made within the scope of
 the appended claims.