JP2006287082A - Manufacturing method for optical system and euv exposure system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing an optical system in which the flare light quantity at a mirror surface does not fluctuate so much. <P>SOLUTION: Substrates are polished using an ion beam (Step S1). After the completion of polishing of the substrates, the mirror substrates are measured for undulation shape (Step S2). Subsequently, the in-field flare light quantity distribution of an optical system is computed based on the undulation shape distribution of each mirror (Step S3). Such a correction amount is computed for undulation shape distribution in mirror that unevenness in the light quantity of flare is reduced (Step S4). The mirror is polished again to be provided with the above undulation shape distribution (Step S5). These mirror substrates are coated with a multilayer film (Step S6), and a projection optical system is assembled (Step S7). This projection optical system is mounted on an aligner (Step S8), and exposure evaluation is carried out to measure flare amount distribution (Step S9). When the above steps were followed, unevenness of 2% was measured which is substantially the same value as of the flare distribution estimated from computation. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention is a projection mounted on an EUV exposure apparatus (also called an extreme ultraviolet exposure apparatus, and in the present specification and claims, refers to an exposure apparatus using ultraviolet rays having a wavelength of 50 nm or less). The present invention relates to a manufacturing method of an optical system such as an optical system, and an EUV exposure apparatus having a projection optical system manufactured by this method.

  In recent years, with the miniaturization of semiconductor integrated circuits, EUV light having a shorter wavelength (11 to 14 nm) is used in place of conventional ultraviolet rays in order to improve the resolving power of the optical system limited by the diffraction limit of light. Projection lithography techniques have been developed (see, for example, D. Tichenor, et al, SPIE 2437 (1995) 292: Non-Patent Document 1). This technique is recently called EUV (Extreme UltraViolet) lithography, and is expected as a technique that can obtain a resolution that cannot be achieved by conventional optical lithography using light having a wavelength of about 190 nm.

  In a wavelength region of EUV light, a transmission / refraction type optical element such as a conventional lens cannot be used, and an optical system utilizing reflection is used.

  An outline of the EUV exposure apparatus is shown in FIG. The EUV light 32 emitted from the EUV light source 31 enters the illumination optical system 33 and becomes a substantially parallel light beam via a concave mirror 34 that acts as a collimator mirror, and enters an optical integrator 35 including a pair of fly-eye mirrors 35a and 35b. Incident and a substantial surface light source is formed. The light from the substantial surface light source is deflected by the plane mirror 36 and then forms an elongated arc-shaped illumination area on the mask M. The light from the pattern of the illuminated mask M forms an image of the mask pattern on the wafer W through the projection optical system PL composed of a plurality of mirrors (six mirrors M1 to M6 in FIG. 3 exemplarily). .

  In order to form a desired pattern on the wafer, it is preferable to sufficiently reduce the aberration of the projection optical system. In particular, the wavefront aberration (rms value) of the projection optical system is reduced to 1/30 or less of the wavelength, and the pattern distortion is reduced. When the exposure line width is maintained at 1/10 or less, a fine pattern with high contrast can be formed.

  For example, when the exposure wavelength is 13.5 nm and the NA of the projection optical system is 0.25, the line width is about 45 nm when the wavefront aberration is 0.5 nm (rms) or less and the pattern distortion is 5 nm or less. A high-quality resist pattern can be formed.

  Furthermore, it is preferable to keep the flare of the projection optical system sufficiently small. In particular, uneven flare is not preferable because the line width of the resist pattern varies in the image plane. The flare is caused by a wavy shape error having a relatively high spatial frequency among the mirror shape errors that cause the wavefront aberration. In general, a wavy shape in the spatial frequency range of 1 mm to 1 μm affects the flare.

  Since flare increases in proportion to the number of mirrors, the shape undulation of each mirror must be reduced as the number of mirrors increases. For example, in order to suppress flare to several percent or less in a projection optical system composed of six mirrors, it is necessary to reduce the waviness of the mirror to 0.1 nm (rms) or less. Further, in order to reduce the flare unevenness, it is necessary to control the mirror shape waviness with an accuracy smaller than 0.1 nm (rms).

  A manufacturing process of a conventional projection optical system is shown in FIG. First, an aspherical mirror substrate is produced, and a multilayer film is coated on the surface. A projection optical system is assembled by combining a plurality of aspherical mirrors, and the projection optical system is installed in an EUV exposure machine to complete. The flare amount can be detected by exposing a pattern having a predetermined line width formed on the mask onto the resist and measuring the spread of the line width obtained by developing the resist.

D. Tichenor, et al, SPIE 2437 (1995) 292 Proc. Of SPIE Vol 5374 p.897-905

  However, in order to reduce the shape waviness error of the mirror to the design value or less by a single machining, a high level machining measurement technique is required, and it is extremely difficult to control the spatial distribution of the waviness shape with high accuracy. For this reason, the projection optical system produced by the conventional process may not obtain desired optical performance in optical performance evaluation.

  The present invention has been made in view of such circumstances, and it is an object of the present invention to provide a method for manufacturing an optical system in which the fluctuation of the flare light amount on the image plane is small, and an EUV exposure apparatus using the optical system. .

A first means for solving the above-described problems is an optical system manufacturing method (claim 1) having the following steps.
(1) Process to shape a plurality of mirrors constituting an optical system based on design values
(2) Measuring the shape error of the mirror that affects flare
(3) A step of calculating flare light amount unevenness on the image plane based on the shape error data
(4) A step of calculating a mirror shape error correction amount for at least one of the mirrors so as to improve the uniformity of flare unevenness on the image plane.
(5) Based on the mirror shape correction amount, the step of reworking the mirror for which the mirror shape error correction amount has been calculated.In this means, before forming the reflective film on the actually shaped mirror, The shape error affecting the flare is measured, and the flare light amount unevenness on the image plane is calculated based on the measurement data. Then, a mirror shape error correction amount that improves the uniformity of flare unevenness on the image plane is calculated for at least one mirror, and the calculated mirror is reprocessed based on the mirror shape correction amount. . Therefore, the flare light amount unevenness on the image plane can be reduced.

A method for examining the effect of the mirror shape (mainly surface roughness) on the flare light intensity unevenness is described in, for example, Proc. Of SPIE Vol 5374 p.897-905 (Non-patent Document 2), The outline of one example will be described below. The object plane is divided into predetermined areas, the areas are defined as O _i (i = 1 to n), and the image plane area corresponding thereto is defined as I _i .

Assuming that there are M mirrors in the optical system, each mirror is divided into predetermined regions, and the surface roughness in these regions is defined as _rmj . However, m corresponds to each mirror, m = 1 to M, j corresponds to each mirror region, and takes a value from 1 to a predetermined number for each mirror. Given the image area O _i of the object plane is imaged in the region I _i of the image plane, the amount of flare F _Ii in the region I _i of the image plane,
F _Ii = F _Ii (r ₁₁ , r ₁₂ ,..., R ₂₁ , r ₂₂ ,..., R _M1 , r _M2 ,...)
It can be expressed by the function Such a function form can be obtained from the flare amount F _Ii obtained as a result of changing each r _mj .

  The smaller the amount of flare, the better. However, it is unavoidable to have a certain value, and in that case, the unevenness of the flare becomes a problem rather than the size. That is, if the flare amount varies at the image plane position, the imaging accuracy varies at the image plane position, which is not preferable. Therefore, the mirror is reworked so as to eliminate the fluctuation of the flare amount.

For example, suppose that the flare amount is significantly different from other locations in I _{k on} the image plane (in practice, I _k is not one, but has a predetermined range, but it is simplified for the sake of explanation. Think of it as an area).

Of the light that exits the object plane O _k and forms an image on the image plane I _k , the light that contributes to the image formation through the aperture stop is reflected by each mirror at different locations. For the mirror to be reworked to make it uniform at the position, if the area where this light is reflected is different from the area where light emitted from other areas of the object is reflected, the machining range is selected. Is narrow. In general, a mirror near the aperture stop reflects light from most areas of the object surface, so even if it is reprocessed, only the flare amount in most areas of the image plane changes. It is not suitable for adjusting only the flare amount of a predetermined portion.

  In this way, the mirror to be adjusted is determined (may be determined in advance). For example, assume that the determined mirror is the sixth mirror (M6 mirror).

, The change amount of F _Ii when each of r ₆₁ , r ₆₂ ,... Is changed by the unit amount is known. Each _{r 61, r 62,} ... by adjusting the _near to the variation amount of change in _{F Ik} is to _adjust, each such as the amount of change in _{F Ii} other than _{F Ik} decreases _{r 61, r 62,} ... Find a combination. As described above, this work is facilitated by selecting a mirror that has a narrow area for reflecting the light imaged on the area _Ik of the imaging plane.

In this way, the mirror shape correction amount error may be obtained so that the obtained combination of r ₆₁ , r ₆₂ ,... Can be obtained. This operation is common to the following means. By the method as described above, the variation in the flare amount at the image plane position can be reduced.

A second means for solving the above problem is a method for manufacturing an optical system (Claim 2) including the following steps.
(1) A step of forming a plurality of mirrors constituting an optical system based on a design value, and subsequently forming a reflective film on the surface of the mirror
(2) Assembling the optical system and measuring the flare light amount unevenness on the image plane
(3) A step of calculating a mirror shape error correction amount for at least one of the mirrors so as to improve the uniformity of flare unevenness on the image plane.
(4) The step of peeling off the reflective film from the mirror for which the mirror shape error correction amount has been calculated, and reworking the mirror based on the mirror shape correction amount. Before forming the mirror, the mirror shape was measured, and the mirror was reworked to reduce the flare light unevenness by calculation. On the other hand, in this means, after the mirror is completed, the actual flare light amount unevenness on the image surface is measured in a state of being incorporated in an actual exposure apparatus, and based on this, the flare light amount unevenness on the image surface is measured. The mirror shape error correction amount that improves the uniformity is obtained, and based on this, the reflection film is peeled off from the mirror for which the mirror shape error correction amount is calculated, and the mirror is reworked. Thus, since the mirror shape error correction amount that improves the uniformity of the flare light amount unevenness on the image plane is obtained based on the actual flare light amount unevenness, an accurate mirror shape correction amount can be obtained.

A third means for solving the above problem is a method for manufacturing an optical system (Claim 3) including the following steps.
(1) A step of forming a plurality of mirrors constituting an optical system based on a design value, and subsequently forming a reflective film on the surface of the mirror
(2) Assembling the optical system and measuring the flare light amount unevenness on the image plane
(3) A step of calculating a mirror shape error correction amount for at least one of the mirrors so as to improve the uniformity of flare unevenness on the image plane.
(4) The step of processing the reflection film of the mirror for which the mirror shape error correction amount has been calculated based on the mirror shape correction amount In the second means, when the mirror is reprocessed, After peeling, the mirror was reworked. On the other hand, this means corrects the mirror shape by processing the reflective film without peeling off the reflective film. This is effective when the amount of correction of the mirror shape is small, and it is not necessary to peel off the reflective film, so that the process is simpler than that of the second means.

  A fourth means for solving the problem is any one of the first to third means, and a mirror for calculating the mirror shape error correction amount includes a plurality of mirrors constituting the optical system. Among the mirrors, when viewed along the optical path, at least one of the mirror closest to the object plane, the intermediate image plane, and the image plane is defined (claim 4).

  Since these mirrors have a high degree of freedom when correcting the flare amount distribution, the flare amount can be controlled efficiently.

  An EUV exposure apparatus characterized in that a fifth means for solving the above-mentioned problems has a projection optical system manufactured by any one of the first to fourth means. (Claim 5).

  In this means, the fluctuation of the flare light quantity on the image plane can be reduced.

  According to the present invention, it is possible to provide an optical system manufacturing method in which the flare light amount variation on the image plane is small, and an EUV exposure apparatus using this optical system.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings, taking as an example a projection optical system composed of six aspherical mirrors as shown in FIG. This projection optical system is an optical system that forms an intermediate image in the middle of the optical path, and a third mirror (M3 mirror) is disposed at a position closest to the intermediate image along the optical path from the image side.
Example 1
FIG. 1 is a process diagram showing an embodiment corresponding to the first means, and includes a process of correcting a mirror shape error based on the result of calculation of the flare light quantity distribution.

  First, an aspherical mirror substrate that constitutes the projection optical system was fabricated. As the substrate material, ceramic glass having a thermal expansion coefficient of 1 ppb / K or less was used. The substrate was polished using an ion beam (step S1), and the waviness shape affecting the flare was measured with a microscopic interferometer. A microscopic interferometer is a device that measures the undulation of a minute region of several mm or less by an interferometric method, and can measure the shape of a frequency band (1 mm to 1 μm) that affects flare.

  Polishing was performed until the waviness shape error of each mirror was 0.1 nm (rms) or less. Furthermore, the waviness shape error of each mirror was controlled to be uniform within the surface, and polishing was performed so that the in-plane distribution was ± 10% or less.

  After the substrate processing was completed, the waviness shape of the mirror substrate was measured (step S2). Measurement was performed on a plurality of points on the mirror surface, and wave shape distribution information was obtained. This measurement was performed on all mirrors. Next, the in-field flare light amount distribution of the optical system was calculated based on the wavy shape distribution of each mirror (step S3).

  Next, the in-field flare light amount distribution of the optical system was calculated based on the waviness shape distribution of each mirror. As a result, the average value of the flare light amount was 9%, and the distribution range was 7% to 11%.

  Next, the waviness distribution of the mirror was calculated so that the unevenness in the amount of flare light was reduced. This swell distribution was calculated for M3 mirrors out of 6 mirrors. Since the M3 mirror is disposed in the vicinity of the intermediate image plane, the flare distribution on the image plane can be controlled with a high degree of freedom by providing a wavy shape distribution on the mirror plane. The measured swell shape of the M3 mirror had an average value of 0.10 nm (rms) and an in-plane distribution of 0.06 to 0.13 nm (rms). The waviness shape on the M3 mirror obtained by calculation had an average value of 0.11 nm (rms) and an in-plane distribution in the range of 0.09 to 0.15 nm (rms). That is, it was found that the flare light amount distribution can be made uniform by giving the above-mentioned swell shape to the entire surface of the M3 mirror. The difference between the calculated waviness shape (frequency band shape affecting flare, for example, the frequency band is 1 mm to 1 μm) and the measured waviness shape is used as a mirror (waviness) shape error correction amount (step S4). .

  The M3 mirror was re-polished to provide the waviness shape distribution (step S5). When the waviness shape was measured with a microscopic interferometer after polishing, a waviness shape was formed with an error range of ± 10% with respect to the target waviness shape distribution. When the flare light amount distribution of the optical system was calculated based on this data, the average value was 10%, and the distribution was within the range of 9% to 11%. That is, the predicted flare unevenness has improved from 4% to 2%.

Next, a multilayer film was coated on these mirror substrates (step S6), and the projection optical system was assembled (step S7). When this projection optical system is mounted on an exposure apparatus (step S8), exposure evaluation is performed, and flare amount distribution is measured (step S9). As a result, 2% unevenness, which is almost the same as the flare distribution predicted from the calculation, is measured. It was done. With this exposure apparatus, a desired resist pattern could be obtained.
(Example 2)
FIG. 2 is a process diagram showing an embodiment corresponding to the second means, and includes a process of correcting a mirror waviness shape error based on the result of flare light quantity distribution measurement.

  First, an aspherical mirror substrate constituting the projection optical system was manufactured (step S11). As the substrate material, ceramic glass having a thermal expansion coefficient of 1 ppb / K or less was used. The substrate was polished using an ion beam, and the waviness shape affecting flare was measured with a microscopic interferometer. Polishing was performed until the waviness shape error of each mirror was 0.1 nm (rms) or less. Further, the waviness shape error of each mirror was controlled so as to be uniform within the surface, and polishing was performed so that the in-plane distribution was ± 10% or less.

  After the substrate processing was completed, the mirror substrate was coated with a multilayer film (step S12), and the projection optical system was assembled (step S13). When this projection optical system is mounted on an exposure apparatus (step S14) and exposure evaluation is performed to obtain a flare amount distribution (step S15), the average flare light amount is 9%, and the distribution is 7% to 11%. Range.

  Next, the waviness distribution of the mirror was calculated so as to reduce the unevenness in the amount of flare light (step S16). This swell distribution was calculated for M3 mirrors out of 6 mirrors. The measured swell shape of the M3 mirror had an average value of 0.10 nm (rms) and an in-plane distribution of 0.06 to 0.13 nm (rms). The wavy shape on the M3 mirror obtained by calculation was 0.11 nm (rms), and the in-plane distribution was in the range of 0.09 to 0.15 nm (rms) on average. The difference between the calculated waviness shape (the shape of the frequency band affecting flare, for example, the frequency band is 1 mm to 1 μm) and the measured waviness shape is used as the mirror shape error correction amount (step S16).

  The M3 mirror was removed from the optical system (step S17), and the multilayer film was removed by wet etching (step S18). An acid solution containing nitric acid as a main component was used for wet etching. The mirror was re-polished based on the mirror shape correction amount to provide the waviness shape distribution (step S19). When the waviness shape was measured with a microscopic interferometer after polishing, a waviness shape was formed with an error range of ± 10% with respect to the target waviness shape distribution.

Next, the M3 mirror substrate was again coated with a multilayer film (step S20), and the projection optical system was assembled (step S21). When this projection optical system was mounted on an exposure apparatus (step S22), exposure evaluation was performed and a flare amount distribution was acquired (step S23), the observed flare amount unevenness was 2% or less. With this exposure apparatus, a desired resist pattern could be obtained.
(Example 3)
FIG. 3 is a process diagram showing an embodiment corresponding to the third means, and includes a process of correcting the optical performance based on the result of the flare light quantity distribution measurement. Most of the steps are the same as in the example of FIG. 2, but only the differences will be described.

  In this example, the waviness shape distribution calculation of the mirror was performed based on the measurement result of the flare light amount unevenness (step S35). The steps up to here are the same as those of the embodiment shown in FIG. 2, but in the embodiment shown in FIG. 2, the mirror substrate is re-polished, whereas in this embodiment, the surface of the multilayer film is polished. (S38) is different. The scattered light of a normal multilayer mirror is generated by the undulation shape of a plurality of interfaces existing in the multilayer film. On the other hand, by polishing the surface of the multilayer film of the multilayer film mirror and providing a predetermined undulation shape on the outermost film surface of the multilayer film, a small part of incident light is scattered, and from below The reflected light that comes back can also be scattered. That is, the flare caused by the light passing through the portion provided with the predetermined undulation shape can be increased. Therefore, when there is a portion where the amount of flare on the imaging surface is small compared to other portions, the amount of flare is reduced by forming undulations on the surface of the multilayer film of the reflecting mirror through which the light beam that forms an image passes. The flare distribution can be made uniform.

  The waviness distribution given to the surface of the multilayer film was calculated so as to reduce the unevenness of the flare light amount. This swell distribution was calculated for M3 mirrors out of 6 mirrors. The measured swell shape of the M3 mirror had an average value of 0.10 nm (rms) and an in-plane distribution of 0.06 to 0.13 nm (rms). The wave shape on the M3 mirror obtained by calculation was 0.5 nm (rms) and the in-plane distribution was in the range of 0.1 to 0.75 nm (rms). That is, it was found that the flare light amount distribution can be made uniform by giving the above-mentioned swell shape to the entire surface of the M3 mirror. The difference between the calculated waviness shape (the shape of the frequency band affecting flare, for example, the frequency band is 1 mm to 1 μm) and the measured waviness shape is used as the mirror shape error correction amount (step S36).

  Based on the mirror shape error correction amount, a projection optical system was assembled using an M3 mirror that had been polished so that the swell as described above was formed on the multilayer film surface. When this projection optical system was mounted on an exposure apparatus and exposure evaluation was performed to obtain a flare amount distribution, the flare amount unevenness was 2% or less. With this exposure apparatus, a desired resist pattern could be obtained.

  In the above-described embodiment, the shape is corrected by polishing. However, the shape may be corrected by another processing method. In the embodiment described above, the waviness shape of the M3 mirror is corrected, but the mirror to be corrected is not limited to M3. However, it is preferable to correct at least one of the mirror closest to the image plane or the object plane when viewed along the optical path, or the mirror closest to the intermediate image plane. These mirrors have a high degree of freedom for correcting the flare light amount distribution.

It is process drawing which shows the Example corresponding to a 1st means. It is process drawing which shows the Example corresponding to a 2nd means. It is process drawing which shows the Example corresponding to a 3rd means. It is a figure which shows the outline | summary of an EUV exposure apparatus. It is a figure which shows the manufacturing process of the conventional projection optical system.

Claims (5)

The manufacturing method of the optical system characterized by having the following processes.
(1) Process to shape a plurality of mirrors constituting an optical system based on design values
(2) Measuring the shape error of the mirror that affects flare
(3) A step of calculating flare light amount unevenness on the image plane based on the shape error data
(4) A step of calculating a mirror shape error correction amount for at least one of the mirrors so as to improve the uniformity of flare unevenness on the image plane.
(5) A step of reworking the mirror for which the mirror shape error correction amount is calculated based on the mirror shape correction amount The manufacturing method of the optical system characterized by having the following processes.
(1) A step of forming a plurality of mirrors constituting an optical system based on a design value, and subsequently forming a reflective film on the surface of the mirror
(2) Assembling the optical system and measuring the flare light amount unevenness on the image plane
(3) A step of calculating a mirror shape error correction amount for at least one of the mirrors so as to improve the uniformity of flare unevenness on the image plane.
(4) A step of peeling the reflective film from the mirror for which the mirror shape error correction amount has been calculated and reworking the mirror based on the mirror shape correction amount The manufacturing method of the optical system characterized by having the following processes.
(1) A step of forming a plurality of mirrors constituting an optical system based on a design value, and subsequently forming a reflective film on the surface of the mirror
(2) Assembling the optical system and measuring the flare light amount unevenness on the image plane
(3) A step of calculating a mirror shape error correction amount for at least one of the mirrors so as to improve the uniformity of flare unevenness on the image plane.
(4) Process of processing the reflection film of the mirror for which the mirror shape error correction amount is calculated based on the mirror shape correction amount The mirror for calculating the mirror shape error correction amount is at least one of a plurality of mirrors constituting the optical system, the mirror closest to the object plane, the intermediate image plane, and the image plane when viewed along the optical path. The method of manufacturing an optical system according to any one of claims 1 to 3, wherein: An EUV exposure apparatus comprising a projection optical system manufactured by the optical system manufacturing method according to claim 1.

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