JP2006058038A - Diffraction direction measuring method of diffraction grating of shearing interferometer for euv - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for measuring highly accurately the shear direction (diffraction direction of a diffraction grating) of a wave front of a shearing interferometer for EUV in consideration of the fact that, though the shearing interferometer is used for evaluating a projection exposure optical system using EUV light, the diffraction direction of the diffraction grating used for the interferometer is required to be measured highly accurately in order to measure the exposure optical system highly accurately, and that especially for measurement of an EUV wave front including astigmatism, the diffraction direction is required to be determined highly accurately. <P>SOLUTION: An angle between beams is measured by an autocollimator or the like by providing a process for measuring a relative position relation of a diffraction beam by improving separation of beams by visible light or for reducing the angle between the diffraction beams. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for measuring the diffraction direction of a diffraction grating used in an EUV shearing interferometer.

  2. Description of the Related Art Conventionally, a semiconductor device having a smaller semiconductor chip and a higher element density has been manufactured by using a lithography technique centered on a projection exposure technique. The basis of this flow is the pursuit of “narrower circuit line width”, and means for that is “higher NA” of the projection optical system and “shorter wavelength” of the exposure wavelength. At present, development using EUV (Extreme Ultra-Violet) as a technology for shortening the wavelength is actively performed.

  In general, a technique for measuring the accuracy of a highly accurate apparatus is indispensable for the development and practical application of the apparatus. As a method for measuring the imaging performance of a projection optical system using EUV light, it has been proposed to use a shearing interferometer or a point diffraction interferometer. See, for example, K.A. Goldberg et al .: “EUV interferometric testing and alignment of the 0.3 NA MET optics” Proceedings of SPIE Vol.5374, 2004; P.64-P.72.

  As with other interferometers, a shearing interferometer detects interference fringes formed by a measurement light beam that has passed through a test object using a two-dimensional detector such as a CCD camera. However, the two wavefronts forming the interference fringes are different from other interferometers in that both are the wavefronts of the same measurement light beam that has passed through the test object. In a shearing interferometer for EUV light, a diffraction grating is used as a splitting means for splitting (shearing) the wavefront of a measurement light beam that has passed through a test object into two. Then, based on the division direction (shear direction, diffraction direction of the diffraction grating) and the division amount (diffraction angle at the diffraction grating) of the divided wavefront, the wavefront (through the test object) from the interference fringe data output by the detector ( A transmitted wavefront or reflected wavefront) is reproduced. Therefore, in order to accurately reproduce the wavefront that has passed through the test object using the shearing interferometer, it is important to accurately obtain the diffraction direction of the diffraction grating (the shear direction of the interferometer) and the diffraction angle (the amount of shear). .

  By the way, when the wavefront to be measured is measured by a shearing interferometer and the wavefront is expressed by a Zernike polynomial, defocusing and astigmatism cannot be measured by a diffraction grating that diffracts in only one direction. Measured by changing For example, diffraction gratings in two orthogonal directions are used. In this regard, G. Harbers et al: “Analysis of lateral shearing interferograms by use of Zernike polynomials”, Applied Optics Vol. 35, No. 11, 1996; Thus, when measuring the wavefront to be detected using a shearing interferometer, it is necessary to use a plurality of diffraction gratings having different diffraction directions. Therefore, when actually measuring the test wavefront, a plurality of diffraction gratings having different diffraction directions are arranged on the diffraction grating plate, and the diffraction grating plate is moved to select a diffraction grating necessary for the measurement. In this case, it is important to grasp the diffraction direction of each of the plurality of diffraction gratings with high accuracy, but it is sufficient to grasp the relative value of each diffraction direction (that is, the angle difference between the diffraction directions) with high accuracy. Sometimes.

As described above, the imaging optical system is generally evaluated by the aberration of the wavefront that has passed through the optical system. However, when measuring the wavefront aberration using a shearing interferometer, the element that determines the measurement accuracy of the wavefront. The shear direction (the diffraction direction when a diffraction grating is used) and the shear amount are important. A method for measuring the diffraction direction and the shear amount is disclosed in Japanese Patent Application Laid-Open No. 2004-037429, and a diagram of a measuring apparatus is shown in FIG. The main point of the measurement method is that an index is placed on the pupil plane of the interferometer (or the aperture stop that is its conjugate plane), and this index is observed with an observation device (for example, a CCD) placed at a subsequent conjugate position. The diffraction direction (shear direction) and the amount of shear are determined from the deviation.
JP2004-037429A

However, it has been found that the above method cannot measure the diffraction direction with the accuracy required to measure astigmatism with high accuracy.
Therefore, an object of the present invention is to provide a method for measuring the shear direction of the wave front of a shearing interferometer for EUV (the diffraction direction of a diffraction grating) with high accuracy.

The first means for solving the above problems is as follows.
A method for measuring the diffraction direction of a diffraction grating used in a shearing interferometer in the EUV wavelength region,
Irradiating the diffraction grating with a visible light laser beam to produce zero-order and higher-order diffraction beams;
A diffraction direction measuring method for photoelectrically detecting a relative position of the zero-order diffracted beam and the high-order diffracted beam in a predetermined plane.

  In this method, a visible light laser beam is applied to the diffraction grating, and the relative position of the higher-order diffracted beam to the zero-order diffracted beam is detected photoelectrically. Therefore, the diffraction angle is increased, the degree of beam position separation is increased, the measurement accuracy is improved, and there is no need to insert an index into the measurement apparatus, so that the measurement itself is simplified.

The second means for solving the above problems is as follows.
For the first means,
The beam relative position is detected photoelectrically using either a two-dimensional analog sensor, an analog use of a two-dimensional discrete sensor, or an aperture scanning method.

  For detecting the position of the beam, for example, a light receiving unit such as a CCD having a discrete sensor inevitably lacks the accuracy of detecting the position of the beam. Therefore, the relative position of the beam is detected photoelectrically by a method in which the light receiving unit (pixel) is selected from a non-discrete sensor (referred to as an analog sensor in the present invention) or a method of continuously scanning the aperture. As a result, the relative position of the beam can be measured with high accuracy.

The third means for solving the above problem is as follows:
A method for measuring the diffraction direction of a diffraction grating used in a shearing interferometer in the EUV wavelength region,
Irradiating the diffraction grating with a visible light laser beam to produce zero-order and higher-order diffraction beams;
Transform so that the angle between the beam formed by the zero-order diffraction beam and the high-order diffraction beam is small,
A diffraction direction measuring method for detecting the converted zero-order diffraction beam and high-order diffraction beam.

Thus, by reducing the angle between the beams to be detected, a simple detection device can be used, and highly accurate measurement can be easily performed.
The fourth means for solving the above problem is as follows.
As a method of reducing the inter-beam angle, the zero-order diffracted beam and the first-order diffracted beam generated by diffraction are folded back by a folding optical element, and then passed through the diffraction grating again.

  According to this method, it is easy to reduce the inter-beam angle. In particular, when the folded optical element and the diffraction direction have a specific relationship, two beams that are substantially parallel are obtained by re-diffraction, and the measurement of the inter-beam angle is performed. Becomes easier.

The fifth method for solving the above problem is as follows.
As a method of reducing the inter-beam angle, a first-order diffracted beam generated by diffraction is passed through a lens or a mirror.

This method is a method that can easily reduce the beam-to-beam angle.
The sixth method for solving the above problem is as follows.
When carrying out the third to fifth methods,
A zero-order diffracted beam and a high-order diffracted beam are detected by a collimator or an interferometry, and an inter-beam angle is obtained.

  When the inter-beam angle to be detected becomes small, the inter-beam angle can be easily measured by a collimator. Similarly, the angle between beams can be easily measured by forming interference fringes and analyzing the interference fringes. These measuring apparatuses and methods are existing established techniques, and if applied to the present invention, highly accurate angle measurement can be easily performed.

  In the present invention, when measuring the diffraction direction of the diffraction grating used in the shearing interferometer in the EUV wavelength region, the diffraction angle is increased using a visible light laser, and detection is performed within the plane where the required angular accuracy can be obtained. A method in which the element is analog is used. This makes it possible to measure the diffraction angle with the required accuracy. As another method, a visible light laser is used to easily generate a diffracted beam having a sufficient amount of light, and then the angle between the beams formed by these beams is reduced to easily diffract the beam with high accuracy. It is possible to measure the direction. When the diffraction direction of the diffraction grating can be measured with high accuracy, the reproduction accuracy of the wavefront using the shearing interferometer, particularly the reproduction accuracy of the wavefront including astigmatism is improved.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[First Embodiment]
A first embodiment according to claims 1 and 2 of the present invention will be described with reference to FIG.

  In a diffraction grating used for an EUV shearing interferometer, a transmission portion and a shielding portion are alternately arranged at a pitch of several μm. The diameter to which the EUV light is actually irradiated varies depending on the positional relationship with the condensing point of the measurement beam in the interferometer, but is several mm or less.

Although FIG. 1 shows the case of diffraction in the X direction, the same can be done for other directions.
A diffraction grating 1 whose diffraction direction is to be measured is irradiated with a He-Ne laser 2 having a wavelength of 632.8 nm. The grating pitch of the diffraction grating is 2 μm, and the diameter of the laser beam is 1 mm. When passing through the diffraction grating, the beam is diffracted to a high order, but only the zero-order diffraction (not diffracted) beam 11 and the first-order diffraction beam 12 (broken line) are shown in FIG. The primary diffraction angle θ is θ = 0.32 rad from 2 μm · sin θ = 632.8 nm. A two-dimensional sensor is placed on the measurement surface separated from the diffraction grating by a distance Lmm. When the position detection accuracy of the sensor is R μm and the required diffraction angle detection accuracy is D seconds, (3600 * 180 / 3.14) * 2 * (R * 10 ⁻³ ) / (L * 0.32) <= D Accordingly, if 1.29 * 10 ³ (R / D) <= L (mm), the diffraction direction (the angle formed by the straight line connecting the two beams with the X axis in FIG. 1) can be measured with a predetermined angular accuracy. . For example, if R = 0.5 and D = 0.3, L≈2 m. In this case, the distance on the XY plane between the zero-order diffraction beam and the first-order diffraction beam is approximately L * 0.32 = 6.4 * 10 ² mm. The “predetermined surface” in the present invention is a measurement surface for obtaining the necessary resolution in this way.

The relative positions of the two diffracted beams are measured by two two-dimensional sensors 13 whose distances are accurately determined. The sensor is a two-dimensional analog sensor, such as a PSD (Position Sensitive Detector) or a quadrant sensor (also referred to as a four-quadrant sensor). Since these sensors basically do not have a light receiving unit having discrete pixels as a unit, they are not subject to resolution limitations limited by the size of the pixels. Regarding PSD and 4-quadrant sensors, Fujimura: Optical Application Technology III-5 “Optical Measurement” Japan Opto-Mechatronics Boundary Edition 19
1991 See 153-154. In addition, among the products on the market, there are S4444 manufactured by Hamamatsu Photonics, Ltd. as PSD, and S4349 in the quadrant sensor.

  Regarding the relative position measurement of the two diffracted beams, it is possible to measure with high accuracy by using a device such as a CCD having a discrete light receiving section and scanning a minute amount thereof. In the present invention, such a method is called analog use of discrete elements. 2A, 2B, 2C, and 2D are pixel arrangement examples when the element is scanned by a minute amount. The pixel and the non-light receiving portion of the pixel can be measured by scanning, and high-precision measurement can be performed.

  Next, the positional relationship between the two sensors will be described. Basically, the position coordinates of the zero-order diffraction beam 11 and the position coordinates of the first-order diffraction beam 12 are obtained by placing a two-dimensional sensor in the XY plane (in the observation plane) at the origin and the position of X = L · tan θ. However, in this method, the diffracted beam may deviate from the measurement area of the two-dimensional sensor. In such a case, the position of the two-dimensional sensor is changed so as to enter the sensor measurement region, and the change in the position of the sensor at that time is accurately obtained. The position change amount is measured using, for example, a linear encoder or an interferometer, and the position of the diffracted beam is measured based on the position change amount of the sensor and the light receiving position in the sensor where the position has changed.

Another method of measuring the relative position of the diffracted beam is to perform photoelectrically by scanning the beam with a narrow slit (e.g., 1 [mu] width) or aperture edge. A method of scanning the slit is schematically shown in FIG. FIG. 3 is a plan view of the measurement surface, as viewed from the beam. A slit plate 20 having orthogonal slits 25 (a) and 25 (b) is arranged on the measurement surface, and the position of the slit plate 20 is interferometers 23 (a), (b), (c), ( It is controlled in the X and Y planes by d). Reference numerals 24 (a) and (b) denote measuring mirrors. The slit plate is moved in the X direction from the state shown in the figure, the zero-order diffracted beam 11 and the first-order diffracted beam 12 are sequentially scanned by the slit 25 (a), and the beam passing through the slit is electrically connected by a photomultiplier tube (not shown). Output as a signal. When scanning in the X direction is completed, the slit 25 (b) is moved while controlling the position by the interferometer based on the scanning output in the X direction, the beam 11 is scanned in the Y direction, and then the slit plate is moved in the X direction. The beam 12 is scanned. The electrical output by scanning is as shown in FIGS. 4 (a) and 4 (b). As a result, the X and Y coordinate values of the beams 11 and 12 are determined. When the X and Y coordinates of the two beams are determined, the inclination of the line segment connecting the two beams, that is, the diffraction direction of the beam is determined.
[Second Embodiment]
This example is an explanation relating to an embodiment according to claims 3 and 4 of the present invention.

First, the principle of measurement will be described with reference to FIG.
The diffraction grating 1 is irradiated with a visible light laser beam 2 to excite the zero-order diffraction beam 11 and the first-order diffraction beam 12. The two beams are folded back by the folding prism 45 (folding optical element) and passed through the diffraction grating 1 again. The folding prism 45 has a right-angled isosceles triangle section, and is arranged so as to be orthogonal to the ridge line 47 of the folding prism 45. Therefore, the folded zero-order diffracted beam 46 exits from the prism 45 in parallel with the incident beam 11, but the folded first-order diffracted beam 49 generally has a direction determined by the positional relationship between the diffraction direction and the ridge line 47 of the folding prism 45. Get out from 45. When these beams 46 and 49 are again passed through the diffraction grating 1 and observed through the half mirror 48, a plurality of beams of the folded zero-order diffracted beam 46 and the folded first-order diffracted beam 49 are diffracted. Observed. Here, consider a case where the diffraction direction of the primary diffraction beam 12 is orthogonal to the ridge line 47 of the folding prism 45. In this case, the folded zero-order diffracted beam 46, the incident first-order diffracted beam 12, and the folded first-order diffracted beam 49 are in the same plane. When passing through the diffraction grating 41 again, the first-order diffracted beam 46 of the folded zero-order diffracted beam 46 and the zero-order diffracted beam 49 of the folded first-order diffracted beam 49 become parallel. Therefore, for example, when observed with a collimator, only one point is observed on the observation surface of the collimator. Therefore, the folding prism 45 is rotated by a small angle, and the position change of the image point on the observation surface at that time is observed. When the rotation angle of the folding prism 45 when the image point becomes one point is measured, the diffraction direction of the diffraction grating 1 becomes a direction orthogonal to the ridge line 47 of the folding prism 45. Of course, there are means other than using a collimator when using this phenomenon. This point will be supplemented in later explanation.

  The folding prism does not necessarily have a right-angled isosceles triangle, but may have a trapezoidal shape having sides orthogonal to the zero-order diffraction beam. Moreover, it is not necessary to be a prism, and a reflecting mirror may be used.

  Next, a suitable optical system for carrying out the measuring method of this embodiment will be described. FIG. 6 shows an example. The laser beam 2 is irradiated to the diffraction grating 1 that requires measurement of the diffraction direction to excite the zero-time diffraction beam 11 and the first-order diffraction beam 12. In the diffraction grating, other high-order diffracted beams are also excited, so that only the necessary order beams are applied to the folding prism 45 using the lens 58 (a), the mask 59 (a), and the lens 58 (b). Make it incident. The folding prism 45 is attached to the rotation mechanism 60. The zero-time diffracted beam and the first-order diffracted beam 49 folded by the folding prism 45 reach the mask 59 (a) through the lens 58 (b). The folded zero-order diffracted beam 46 passes through the opening of the mask 59 (a) and reaches the diffraction grating 1 again, but the folded first-order diffracted beam 49 is blocked by the mask 59 (a) depending on the rotation angle of the prism 45. Sometimes. However, when the ridgeline of the prism 45 and the diffraction direction of the diffraction grating 1 are close to a right angle, the folded primary beam 49 passes through the other opening of the mask 59 (a) and reaches the diffraction grating again. The high-order diffracted beam is excited by the re-diffraction, but among the beams diffracted again by the diffraction grating 1, only the re-diffracted first-order beam of the folded zero-order diffracted beam can pass through the lens 58 (c), A mask 59 (b) is disposed. Then, the re-diffracted primary beam of the zero-order diffracted beam excited by the first diffraction passes through the opening of the mask 59 (b) without depending on the incident condition with respect to the folding prism 45. On the other hand, the re-diffracted zero-order beam of the first-order diffracted beam excited by the first diffraction passes through the opening of the mask 59 (b) when the diffraction direction and the ridge line direction of the prism 45 are close to each other. Therefore, a photoelectric conversion element (not shown) is placed after the opening of the mask 59 (b) to detect the amount of light. Then, when the rotation angle of the prism 45 (which may be considered as the direction angle of the ridge line 47 in the XYZ coordinate system) resulting in the maximum output is obtained, the diffraction direction of the diffraction grating is determined. Here, the photoelectric conversion element is placed after the mask 59 (b) to measure the amount of light. However, as described above, the lens 58 (d) is placed after the mask 59 (b) and the parallel beam is again emitted. The re-diffracted zero-order beam and the re-diffracted primary beam may be observed by the autocollimator 95. Alternatively, the diffraction direction may be calculated by detecting the angle between the two beams by causing interference between the two beams and analyzing the interference fringes.

  Further, instead of forming a parallel beam by the lens 58 (d), a method of condensing the beam may be used (indicated by a dotted line in the figure). In this case, a two-dimensional sensor is used in place of the autocollimator, the condensing positions of the two folded diffraction beams are detected, and the two beams are obtained from the condensing position information based on the optical characteristics of the prism 45, the lens 58, and the half mirror 48. Find the angle between them and calculate the diffraction direction of the diffraction grating.

Furthermore, a method for obtaining the difference in diffraction angles of a plurality of diffraction gratings with high accuracy using such an optical system will be described.
First, the diffraction grating (a) 71 and the diffraction grating (b) 72 are attached to the diffraction grating plate 73 as shown in FIG. The diffraction grating plate 73 is movably attached to the position in FIG.

  The first diffraction grating 71 is moved to the measurement position (the position of the diffraction grating 1 in FIG. 6), and the diffraction direction is measured by the above method. Next, the second diffraction grating 72 is moved to the measurement position, and the diffraction direction of the second diffraction grating is similarly measured. At this time, a reflecting mirror 81 (FIG. 9) is attached to the end face of the diffraction grating plate 73. When the first and second diffraction gratings are switched, the interferometer 82 performs direction control during movement. Yes. As a result, the direction of the diffraction grating when switched is controlled. Therefore, after the measurement of the first diffraction direction, the diffraction grating plate 73 is moved, and at the same time, when the rotation mechanism 60 is rotated by a predetermined angle, the grating directions of the first and second diffraction gratings (accordingly, diffraction). (Direction) and the angle of the rotation mechanism are accurately adjusted, and the relative angle can be obtained more accurately.

[Third embodiment]
This example is an explanation relating to an embodiment according to claim 5 of the present invention.
This will be described with reference to FIG.

  The diffraction grating 1 is irradiated with a visible light laser beam 2 to excite the zero-order diffraction beam 11 and the first-order diffraction beam 12. In FIG. 7A, a lens 91 having a zeroth-order diffracted beam as an optical axis is placed, and the first-order diffracted beam 12 is refracted to reduce the angle with the beam 11.

  In FIG. 7B, the first-order diffracted beam is bent by a mirror 92 to reduce the angle with the beam 11. After reducing the angle formed by the two beams in this manner, the angle formed by the two beams (including the direction) is measured by an autocollimator or an interference fringe observation method. The angle formed by the two beams before the change, that is, the diffraction direction, can be obtained from the position of the lens 91 and mirror 92 in the optical system and the optical characteristics thereof.

  The measurement of the beam angle that has become smaller may be performed as in the second embodiment.

  Increasing the density of semiconductor devices is an indispensable technology for the development of portable electronic devices, and the present invention is one of the circuit thinning techniques indispensable for increasing the density. Therefore, the present invention is a technique that is surely used in the electronics industry.

It is a conceptual diagram of the relative position measuring method of two diffracted beams. It is a figure which shows the positional relationship at the time of the scanning of a discrete light receiving element. It is a conceptual diagram which measures the relative positional relationship of two beams with an aperture scanning system. It is an example of an output obtained by beam scanning. It is a beam angle conversion principle view using a return optical element. It is a figure which shows the example of the angle measurement system between beams using a return | turnback optical element. It is a figure which shows the example of the angle conversion optical element between beams. It is an example of a diffraction grating plate. It is a perspective view of a diffraction grating plate.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Diffraction grating 2 ... Laser beam 11 ... Zero order diffraction beam 12 ... First order diffraction beam 13 ... Two-dimensional sensor 20 ... Slit Plate 23 ... Interferometer 24 ... Measuring mirror 25 ... Slit 45 ... Folding prism 47 ... Edge line 48 ... Half mirror
58 ... Lens 59 ... Mask 60 ... Rotating mechanism 73 ... Diffraction grating plate 81 ... Reflector 91 ... Lens 92 ... Miller
95 ・ ・ ・ ・ Autocollimator

Claims (6)

A method for measuring the diffraction direction of a diffraction grating used in a shearing interferometer in the EUV wavelength region,
Irradiating the diffraction grating with a visible light laser beam to produce zero-order and higher-order diffraction beams;
A diffraction direction measuring method, wherein a beam relative position of the zero-order diffraction beam and the high-order diffraction beam in a predetermined plane is detected photoelectrically. The method of measuring a diffraction direction according to claim 1,
A diffraction direction measuring method, wherein the beam relative position is detected photoelectrically using any one of a two-dimensional analog sensor, an analog use of a two-dimensional discrete sensor, and an aperture scanning method. A method for measuring the diffraction direction of a diffraction grating used in a shearing interferometer in the EUV wavelength region,
Irradiating the diffraction grating with a visible light laser beam to produce zero-order and higher-order diffraction beams;
Transform so that the angle between the beam formed by the zero-order diffraction beam and the high-order diffraction beam is small,
A diffraction direction measuring method, wherein the converted zero-order diffraction beam and high-order diffraction beam are detected. A diffraction direction measuring method according to claim 3,
A diffraction direction measuring method, wherein the zero-order diffracted beam and the first-order diffracted beam generated by diffraction are folded by a folding optical element and then passed through the diffraction grating again to reduce the angle between the beams. A diffraction direction measuring method according to claim 3,
A diffraction direction measuring method characterized in that the angle between the beams is reduced by passing the first-order diffracted beam generated by diffraction through a lens or a mirror. The method of measuring a diffraction direction according to claim 3, wherein
A diffraction direction measuring method, wherein the zero-order diffracted beam and the high-order diffracted beam are detected by a collimator or an interferometry, and the inter-beam angle is obtained.