Multilayer-film mirrors, lithography systems comprising same, and methods for manufacturing same

Multilayer-film (MLF) reflective mirrors are disclosed that have a highly precise surface profile. An exemplary MLF reflective mirror includes multilayer film in which layers of molybdenum (Mo) and layers of silicon (Si) are periodically deposited in an alternating manner on the surface of a mirror substrate. One or more selected regions of the multilayer film have been “shaved” away layer-wise as required to impart an in-plane distribution of removed material sufficient to correct a wavefront error in light reflected from the mirror. After such “layer-machining,” a single-layer film of Si (or Si-containing material) is applied to fill in the machined areas and restore the original contour, as designed, for the surface of the multilayer film. I.e., the Si film has a thickness distribution corresponding to the depth profile of material removed from the multilayer film. A capping layer can be deposited uniformly on the surface of the single-layer film.

FIELD

This disclosure relates to multilayer-film reflective mirrors in which a multilayer film is formed on a surface of a mirror substrate. The disclosure also relates to methods for manufacturing multilayer-film reflective mirrors, and to projection-exposure systems (notably, lithography systems) comprising at least one such multilayer-film reflective mirror.

BACKGROUND

In recent years, with advances in miniaturization of semiconductor integrated circuits, projection-exposure systems have been developed that use, instead of the ultraviolet (UV) light utilized by older projection-exposure systems, extreme ultraviolet (“EUV”) light. EUV light has substantially shorter wavelengths (for example, approximately 11 to 14 nm) than the UV light used previously. The shorter wavelength of EUV light improves the resolving power of optical systems that had reached their diffraction limits with respect to UV light. See Japan Laid-open Patent Document No. 2003-14893.

In configuring a projection-exposure system using EUV light (such a system is called an “EUV lithography” system, abbreviated “EUVL” system), there are no known materials that both transmit EUV light and exhibit sufficient refraction to such light to be useful as EUV lenses. Consequently, in an EUVL system, the constituent optical systems must be configured using EUV-reflective mirrors. But, in this wavelength range, the mirrors must be either oblique-incidence mirrors that utilize total reflection due to their refractive index being slightly smaller than 1, or multilayer-film mirrors in which the respective phases of multiple fronts of weakly reflected light at layer interfaces are superposed constructively in the reflected light to obtain high overall reflectance.

A EUV-reflective mirror employed in an EUVL system must be formed with a highly accurate and precise reflective-surface shape (surface “figure”) having extremely small figure errors with respect to wavefront aberration of reflected light. However, machining such a mirror is very difficult. Hence, techniques have been developed that are applied after the multilayer film has been formed on the reflective surface and that involve “shaving” away one layer at a time in selected regions of the multilayer-film reflective surface. This layer-shaving effectively corrects aberrations arising even from sub-nanometer figure errors. See International Patent Publication No. 01/41155.

In the case of a multilayer film comprising alternating layers of molybdenum (Mo) and silicon (Si), shaving the multilayer-film surface can result in an easily oxidized Mo layer being exposed to the atmosphere. Consequently, a single layer of Si or other oxidation-preventing substance, a ruthenium (Ru) layer, or other “capping” layer (to prevent oxidation of the exposed Mo layer) is normally applied at least in the shaved regions. The ruthenium (Ru) layer or other “capping” layer may also prevent carbon contamination in the multilayer film.

Because optical Ru layers are similar in many ways to Mo layers, depositing a Ru layer as a capping layer on the surface of a shaved multilayer film causes a considerable change in the phase of the reflected wavefront. The magnitude of the change depends upon the amount of film shaved away. The effect arises due to the fact that the Ru capping layer is conventionally formed over the entire surface, including in locations other than regions in which Mo-layer shaving has occurred. The capping layer also changes the reflectance of the shaved regions, causing irregularities in light propagation in the optical system.

SUMMARY

In view of the foregoing, objects of the invention are to provide multilayer-film reflective mirrors having a precise reference-surface shape, methods for manufacturing such multilayer-film reflective mirrors, and exposure systems comprising such multilayer-film reflective mirrors. These various aspects are described herein.

One aspect is directed to multilayer-film reflective mirrors, of which an embodiment comprises a mirror substrate defining a reflective surface comprising a multilayer film. The multilayer film comprises respective layers comprising Mo and respective layers comprising Si stacked in a periodic alternating manner on the reflective surface. In at least one selected region of the multilayer film, at least one layer is removed to impart, over the “top” surface of the multilayer film, a desired in-plane distribution of removed multilayer-film material. A single-layer film of Si, or of a material comprising Si, is situated on the top surface of the multilayer film to fill regions of the multilayer film from which material has been removed and to provide a “top” surface that substantially restores the as-designed contour of the reflective surface. The single-layer film has a thickness profile corresponding to the distribution of removed multilayer-film material. A substantially uniform-thickness capping layer is situated on the top surface of the single-layer film.

Another aspect is directed to methods for manufacturing multilayer-film reflective mirrors, of which an embodiment comprises forming a multilayer film on a figured surface of a mirror substrate, thereby forming a reflective surface having a contour. The multilayer film comprises multiple layer pairs of Mo and Si that are deposited periodically in an alternating manner. In at least one selected region of a “top” surface of the multilayer film, the multilayer film is “layer-machined” to remove at least one layer of the multilayer film to produce a desired in-plane depth distribution of removed multilayer-film material. Then, a single-layer film of Si, or of a material comprising Si, is applied to the layer-machined surface of the multilayer film to fill corresponding formed in the selected regions formed by localized removal of the multilayer-film material. The “top” surface of the single-layer film is planarized or otherwise provided with a contour that substantially restores the contour of the reflective surface. The single-layer film has a thickness distribution that substantially matches the depth distribution of the removed multilayer-film material. A capping layer is formed with a substantially uniform thickness on the top surface of the single-layer film.

Yet another aspect is directed to exposure systems (e.g., lithographic projection-exposure systems). An embodiment of such a system comprises a reflective optical system, of which at least one reflective mirror comprises a multilayer-film reflective mirror according to any of the instant embodiments, or manufactured using any of the method embodiments described herein.

In any of various embodiments of multilayer-film reflective mirrors as described herein, a single-layer film of Si or of a material comprising Si, is provided with a substantially planar “top” surface, or with a “top” surface having a particular surface profile such as one that is consistent with the figure profile of the reflective surface of the mirror. The single-layer film is situated on the “top” surface of the multilayer film of which one or more locations have experienced removal of at least one constituent layer to create an error-offsetting thickness profile. Since the single-layer film is on the surface of the multilayer film, the single-layer film also has a depth distribution that substantially conforms to the surface profile of the multilayer film. With such a configuration of the single-layer film, even if a uniform thickness of a capping layer is present on the “top” layer of the single-layer film, changes in phase and reflectance that otherwise would arise from variations in the amount of material removed from the multilayer film are prevented. Consequently, multilayer-film reflective mirrors having highly precise surface profiles are provided.

In the various method embodiments, a single-layer film of Si, or of a material comprising Si, is applied to the layer-machined multilayer film. The single-layer film has a substantially planar “top” surface or a top surface having a contour that is consistent with an as-designed contour of the reflective surface of the mirror. The single-layer film has a thickness profile that substantially corresponds with a depth profile of the multilayer film resulting from removal of at least one layer of the multilayer film in the selected areas of the multilayer film. The capping layer is applied at substantially uniform thickness on the “top” surface of the single-layer film. Hence, changes in phase and reflectance that otherwise would arise from removing material from the top surface of the multilayer film are prevented, and multilayer-film reflective mirrors having highly precise surface profiles are manufacturable.

According to yet another aspect, exposure systems are provided, of which an embodiment comprises at least one multilayer-film reflective mirror having a highly precise surface profile as disclosed herein. Such systems provide, among various benefits, satisfactory exposure of extremely fine pattern features.

DETAILED DESCRIPTION

The following description is set forth in the context of representative embodiments that are not intended to be limiting in any way.

The following description and other portions of this disclosure, including the claims, may use terms such as “up,” “down,”, “upper,” “lower,” “top,” “bottom,” “left,” “right,” “vertical,” “horizontal,” and the like. These terms are used herein to facilitate understanding of the relationships of various components with respect to an exemplary orientation. But, these terms are not intended to be construed in any manner that limits the disclosure to the literal meanings of these words. For example, as a result of simply turning a thing over, a “top” surface becomes a “bottom” surface, but the thing itself is unchanged.

A multilayer-film reflective mirror according to a first embodiment is explained referring to the drawings. The multilayer-film reflective mirror is used, for example, in an EUV exposure device or the like that uses extreme ultraviolet light (EUV light) as the exposure light.FIG. 1is an elevational section schematically depicting a portion of the multilayer-film reflective mirror2of this embodiment. The multilayer-film reflective mirror2comprises a multilayer film6, having a structure in which layers6acomprising molybdenum (Mo) and layers6bcomprising silicon (Si) are deposited periodically in an alternating manner on the surface of a mirror substrate made of a low-thermal-expansion glass polished to a precise shape (figure profile). A Si single-layer film7is deposited on the multilayer film6as an oxidation-prevention film. A capping layer8, comprising a ruthenium (Ru) layer, is deposited on the Si single-layer film7to prevent carbon contamination and oxidation of the Si single-layer film7. The multilayer film6comprises multiple layer-pairs of Mo layers6aand Si layers6b. InFIG. 1, however, only four layer-pairs of Mo and Si are shown.

Portions of the surface of the multilayer film6are removed (“shaved” or “machined” away) as required so as to impart an in-plane distribution of the amount removed, so as to correct the wavefront of light reflected from the surface. The Si single-layer film7has a thickness profile that substantially corresponds to the profile of removed material of the multilayer film6. The “top” surface of the Si single-layer film7is shown as being substantially flat (planar) in the figure but actually has a profile that substantially restores the as-designed surface profile of the reflective surface of the mirror (e.g., the surface profile of the multilayer film). The Ru capping layer8is deposited uniformly on the Si single-layer film7, desirably at uniform thickness.

In theFIG. 1embodiment the Si single-layer film7is effectively an intermediate layer situated substantially on the surface of the top-most Si layer6bof the multilayer film6. Hence, despite the presence of the Ru capping layer8in this embodiment, wherein the Ru capping layer is deposited uniformly on the surface of the Si single-layer film7, phase changes and reflectance changes that otherwise would arise from removal machining are substantially prevented.

Next, a method for manufacturing the first embodiment of the multilayer-film reflective mirror2is described, referring to the flowchart ofFIG. 2. First, a multilayer film6, having a structure in which Mo layers6aand Si layers6bare deposited periodically in an alternating manner, is formed on a mirror substrate4(desirably low-thermal-expansion glass), polished with high precision (step S10). Desirably, a magnetron-sputtering film-deposition device is used to deposit these layers on the reflective surface of the mirror substrate4. Thus formed, the multilayer film6has multiple layer-pairs, of which the period length is in the range of 6.9 nm to 7.5 nm.

FIG. 3shows certain features of a magnetron-sputtering film-deposition device that comprises a substrate-holder12accommodated within a vacuum chamber10evacuated to a suitable vacuum level. The substrate holder12holds the mirror substrate4. A rotational driving mechanism (not shown) holds the mirror substrate4while rotating it about a rotational axis AX.

The magnetron-sputtering film-deposition device further comprises a film-thickness-distribution correction plate14that is accommodated within the vacuum chamber10. The film-thickness-distribution correction plate14is positioned in the vicinity of the mirror substrate4and is configured to move in the directions of the arrow in the drawing by means of a correction-plate-driving mechanism16. By moving the film-thickness-distribution correction plate14in the depicted directions and adjusting the amount of film-deposition particles reaching the mirror substrate4, the thickness of the film deposited on the mirror substrate4is controlled.

The magnetron-sputtering film-deposition device also comprises a first cathode18, using a molybdenum (Mo) plate20as a first target material, and a first target shutter22. Process gas is introduced into the vacuum chamber10, and by applying a voltage to the first cathode18, a plasma is generated in the vicinity of the Mo plate20. By means of this plasma, the Mo plate20is sputtered, and the sputtered particles of Mo accumulate on the mirror substrate4. The first target shutter22is opened during Mo-film deposition, and is closed during Si-film deposition, described below.

The magnetron-sputtering film-deposition device further comprises a second cathode24, using a silicon (Si) plate26as a second target material, and a second target shutter28. Process gas is introduced into the vacuum chamber10, and by applying a voltage to the second cathode24, a plasma is generated in the vicinity of the Si plate26. By means of this plasma, the Si plate26is sputtered, and the sputtered particles of Si accumulate on the mirror substrate4. The second target shutter28is opened during Si-film deposition, and is closed during Mo-film deposition.

Next, the surface of the multilayer film6formed in step S10is removed, imparting an in-plane distribution of the amount of the multilayer film6removed (step11).

Normally, whenever multiple reflective mirrors are used in a reflecting-optical system of an EUV exposure system, the figure error (FE) allowed in each reflective mirror, with respect to the wavefront error (WFE) of the reflecting-optical system, is given by Equation (1):
FE=WFE/2/√{square root over (n)}(RMS)  (1)

Here, n is the number of reflective mirrors of the optical system. In a reflecting-optical system, both incident light and reflected light are affected by figure errors, resulting in wavefront errors (aberrations) being proportional to twice the figure error. Hence, Equation (1) includes division by 2. The figure error (FE) allowed for each reflective mirror is, for wavelength λ and number n of reflective mirrors, given by Equation (2):
FE=λ/28/√{square root over (n)}(RMS)  (2)

For example, if the wavelength (λ) is 13 nm, the figure error allowed for each reflective mirror in a reflecting-optical system comprising four reflective mirrors is 0.23 nm RMS, and the figure error allowed for each reflective mirror in a reflecting-optical system comprising six reflective mirrors is 0.19 nm RMS. Whenever a multilayer-film reflective mirror2of the instant embodiment is used in an EUV exposure device, the allowed figure error is similarly calculated. However, it is extremely difficult to manufacture a mirror substrate having a reflective surface with such a highly precise surface figure. Even if the mirror substrate is polished to high precision, subsequent deposition of a multilayer film on it may cause the reflected wavefront to exhibit errors in wavefront shape.

Technology has been developed in which, by shaving off the surface of the multilayer film one layer at a time in selected locations, effective correction of sub-nanometer figure errors is possible (see International Patent Publication no. 01/41155). For example, consider a case in which one layer-pair is locally removed, as shown inFIG. 5, from the surface of a multilayer film in which two types of material A and B are layered in an alternating manner with a fixed period length d, as shown inFIG. 4. The optical-path length OP in a layer-pair having thickness d, for a beam propagating in a direction perpendicular to the surface of the multilayer film (FIG. 4), is OP=nAdA+nBdB. Here, dAand dBdenote the respective thicknesses of the layers, wherein dA+dB=d. Further, nAand nBare the respective refractive indices of the materials A and B.

InFIG. 5the optical-path length OP′ of the portion of the thickness d removed from the multilayer-film layer-pair in the uppermost surface is given by OP′=nd. Here n is the refractive index of a vacuum, where n=1. That is, by removing the one or more of the uppermost layers of the multilayer film, the optical-path length traversed by a beam passing therethrough is changed. This is, in effect, optically equivalent to modifying the surface figure by the amount of change of the optical-path length. The change in optical-path length (i.e., the change in surface shape)is given by=OP′−OP.

Because, in the EUV wavelength range, the refractive indices of materials are close to unity,is a small quantity. Thus, by locally shaving off one or more layers as required, layer-by-layer from selected location(s) on the surface of the multilayer-film reflective mirror, the surface shape can be precisely corrected. For example, consider a case in which a Mo/Si multilayer film is used at a wavelength of 13.5 nm. Because the light is substantially directly incident on the mirror surface, it is assumed that the thickness d of a layer-pair is 6.9 nm, that the thickness dMoof each Mo layer is 2.415 nm, and that the thickness dSiof each Si layer is 4.485 nm. The refractive index nMoof Mo at a wavelength of 13.5 nm is 0.92, and the refractive index nSiof Si at this wavelength is 0.998. Using these numbers, the change in optical-path length is calculated. The optical-path length OP, prior to shaving off a layer-pair from a region of the surface of the multilayer-film reflective mirror, is 6.698 nm; the optical-path length OP′ after shaving off a layer-pair is 6.9 nm, and the change in optical-path length=OP′−OP=0.202 nm.

Thus, by locally shaving off a layer-pair, the surface shape is corrected by the equivalent of 0.2 nm. In the case of a Mo/Si multilayer film, because the refractive index of each Si layer to EUV light is close to unity, the change in optical-path lengthdepends mainly on the presence or absence of the Mo layers, and depends hardly at all on the presence or absence of the Si layers. Hence, when removing one or more layers of a multilayer film, there is no need to control the thickness of the Si layers accurately. In the above-described example, the thickness of Si layers is 4.485 nm, and it is sufficient to stop the removal machining midway through the thickness of a Si layer. That is, by performing layer-removal machining with a precision of several nanometers, surface-shape correction can be performed in 0.2-nm increments.

Hence, in step S11(FIG. 2), first the reflected wavefront of EUV light from the multilayer-film reflective mirror2, after depositing the multilayer film6, is measured. If the measured reflected wavefront has an error with respect to the desired wavefront, the amount of the multilayer film6that should be locally machined away to correct the wavefront is determined. Based on the determined amount of removal, removal-machining of the multilayer film6is performed in the selected area(s).FIG. 6shows an example configuration of a multilayer-film reflective mirror2after local removal-machining of the surface of the multilayer film6, but before depositing the Si single-layer film7and the Ru capping layer8. The multilayer film6comprises multiple layer-pairs each comprising a respective Mo layer6aand a respective Si layer6b. InFIG. 6, however, only four layer-pairs of Mo and Si are shown.

FIG. 7is a graph of reflectance and phase changes exhibited by the multilayer-film reflective mirror2, as functions of the depth of layer-removal machining (i.e., the amount of film machined) in step S11. The curve L1indicates the rate of change of reflectance for EUV light at a wavelength of 13.5 nm, and the curve L2indicates changes in phase. The change in phase when the machining reaches the depth of the first layer-pair (period length 6.9 nm) of the multilayer film6is approximately 8°. The corresponding change in the wavefront for λ=13.5 nm is 8°/360°×13.5 nm=0.30 nm. For comparison, if 6.9 nm of the surface of the mirror substrate had been removed by machining, the change in wavefront would be equal to twice the period length, or 13.8 nm. In the instant embodiment the effect on the wavefront of the amount of film machined on the surface of the multilayer film6is 0.30 nm/13.8 nm=1/46 as great.

Next, referring toFIG. 2, a Si single-layer film7is deposited on the surface of the multilayer film6that has been subjected to local layer-removal machining in step S11. The “top” surface of the Si single-layer film7is shown as being substantially planar, but it actually has a profile that substantially restores the as-designed surface profile of the reflective surface of the mirror. I.e., the Si film7is formed with a depth (thickness) profile that substantially conforms to the depth profile of material previously removed from the multilayer film6(step S12). The Si single-layer film7functions as an oxidation-prevention film for the Mo layers, thereby preventing oxidation of the Mo layer6athat has been exposed on the surface as a result of layer-removal machining in step S11. As an alternative to the Si single-layer film7, a single-layer film of a material comprising Si, such as for example SiO2or another compound, may be deposited.

FIG. 8is a flowchart depicting an embodiment of a method for depositing the Si single-layer film7. First, a Si single-layer film7of a prescribed thickness is deposited on the surface of a multilayer film6which has been subjected to localized layer-removal machining in step S11of the process inFIG. 2(step S20). That is, a Si single-layer film7is deposited uniformly, to a thickness that is equal to or greater than the amount of machined film locally removed, onto the surface of the multilayer film6.

Next, an amount of the Si single-layer film7, deposited in step S20, is removed according to the thickness of multilayer film6previously removed, i.e., according to the thickness of the multilayer film that had been locally machined away, so that the “top” surface of the Si single-layer film7substantially restores the as-designed profile of the surface of the multilayer film (step S21). That is, the Si single-layer film is formed having a depth (thickness) profile that substantially conforms to the depth profile of the multilayer film after layer-machining, thereby offsetting the effect of the layer-machining. The “top” surface of the Si single-layer film7is at substantially the same level as the multilayer film6prior to layer-removal machining.

Next, a Ru capping layer8is deposited uniformly at a thickness of approximately 2 nm on the surface of the Si single-layer film7deposited in step S12(step S13). The Ru capping layer8prevents carbon contamination of the multilayer film6, and prevents oxidation of the multilayer film6and of the Si single-layer film7.

As an alternative capping layer (protective layer) to the Ru capping layer, a single layer of rhodium (Rh), niobium (Nb), platinum (Pt), or molybdenum (Mo) may be deposited. Further alternatively, a single layer of an alloy comprising Ru, Rh, Nb, Pt, or Mo may be deposited, or a single layer of TiO2, SiO2, ZrO2, MoSi2, or SiC may be deposited. Further alternatively, two or more of these layers may be formed as a capping layer. In addition, an adjustment layer may be formed below a single capping layer, or below a multi-layer capping layer, to facilitate formation of these layers and promote exhibition of their functions.

According to the multilayer-film reflective mirror and method of manufacture thereof of the first embodiment, a Si single-layer film having a flat surface is deposited on the surface of a multilayer film previously subjected to localized layer-removal machining. The Si single-layer film has a film thickness corresponding to the amount of the multilayer film previously removed. Hence, even if a Ru capping layer is deposited uniformly on the surface of the Si single-layer film, phase changes and reflectance changes due to the removal machining can be prevented.

FIGS. 9 and 10show the configuration of a multilayer-film reflective mirror100according to the comparative example. The multilayer-film reflective mirror100shown inFIG. 9includes a Ru capping layer102of thickness 2 nm deposited uniformly directly on a multilayer film6previously subjected to localized layer-removal machining. In the multilayer-film reflective mirror104shown inFIG. 10, a Si single-layer film106having a thickness of 2 nm and a Ru capping layer108having a thickness of 2 nm are deposited uniformly on the multilayer film6previously subjected to localized layer-removal machining. The multilayer film6comprises multiple layer-pairs each comprising a respective Mo layer6aand a respective Si layer6b. InFIGS. 9 and 10, only four layer-pairs of Mo and Si are shown.

The graph inFIG. 11shows reflectance and phase change, as functions of machined-film thickness (nm), of the multilayer-film reflective mirrors100and104shown inFIGS. 9 and 10. The curve L3is the reflectance of EUV light of wavelength 13.5 nm, and the curve L4is the phase change. The reflectance and phase change shown in the graph ofFIG. 11fluctuate greatly compared to the reflectance and phase change of a multilayer-film reflective mirror prior to depositing a Ru capping layer102or a Si single-layer film106and Ru capping layer108(seeFIG. 7). This fluctuation is due to the fact that the Ru capping layers102and108, which optically are substantially equivalent to the Mo layers6aof the multilayer film6, are deposited at positions at which a Mo layer6ais not meant to be deposited. Hence, despite the fact that the reflected wavefront of the multilayer-film reflective mirror has been corrected by layer-removal machining of the surface of the multilayer film6, the desired correction of the reflected wavefront is not obtained. Substantial fluctuations in reflectance also may give rise to transmission irregularities.

In contrast, the graph ofFIG. 12shows reflectance and phase change, as functions of thickness of film machined away, of the multilayer-film reflective mirror2of this embodiment. The curve L5represents reflectance for EUV light of wavelength 13.5 nm, and the curve L6represents the phase change. As shown inFIG. 12, there are no large fluctuations in reflectance or phase change such as are seen in the graph ofFIG. 11, and the reflectance profile and phase-change profile are substantially the same as the reflectance profile and phase-change profile, respectively, immediately after performing localized layer-removal machining of the multilayer film6, shown inFIG. 7. InFIG. 12, the change in phase when the film-machining amount reaches one layer-pair (period length 6.9 nm) of the multilayer film6is approximately 6.66°. The corresponding change in wavefront is 6.66°/360°×13.5 nm (wavelength)=0.25 nm. Normally, if 6.9 nm of the mirror substrate is removed by machining, the change in the wavefront is twice the period length, or 13.8 nm. Thus, the effect on the wavefront of machining the film at the surface of the multilayer film6is 0.25 nm/13.8 nm=1/55 as great.

That is, in a multilayer-film reflective mirror2of this embodiment, a Si single-layer film7having a contour-restoring “top” surface is deposited on the surface of the multilayer film6that previously had been subjected to localized layer-removal machining. The Si layer has a thickness profile that corresponds to (conforms to) to the depth profile of the layer-machining, so that the Ru capping layer8is deposited where a top-most Mo layer6aotherwise would have been deposited. Further, since no significant reflectance changes or phase changes occur due to the thickness of the newly deposited Si single-layer film7, high-precision correction of the surface shape can be performed.

In the method for manufacturing a multilayer-film reflective mirror according to the first embodiment, the Si single-layer film7of prescribed thickness is deposited on the surface of the multilayer film6that had previously been subjected to layer-removal machining. The “top” surface of the Si single-layer film7is then “planarized,” by which is meant that the top surface is formed to have a surface profile that is substantially the same as the as-designed surface profile of the reflective surface of the mirror. This profile restoration can be performed after forming the Si single-layer film57, or the Si single-layer film7can be deposited in such a manner that the surface of the Si single-layer film7is at substantially the same level as the surface of the multilayer film6had been prior to the layer-removal machining. That is, the Si single-layer film7(the intermediate-layer) is deposited on the surface of the at least one region from which material of the multilayer film has been removed.

In the first embodiment, the reflected wavefront error is minute even if the Si single-layer film7has thickness variations from the ideal surface contour. For example, suppose that, as shown inFIG. 13, the “top” surface of the Si single-layer film7does not have the desired contour (e.g., has an error of ±0.3 nm from the as-designed contour). By depositing the Ru capping layer8on the “top” surface of the Si single-layer film7, the ±0.3 nm error in the thickness of the Si single-layer film7is retained. The multilayer film6comprises multiple layer-pairs of Mo layers6aand Si layers6b,but inFIG. 13only four layer-pairs of Mo layers6aand Si layers6bare shown.

In this case,FIG. 14is a graph showing changes in reflectance versus the amount of layer-machining performed on the multilayer-film reflective mirror2ofFIG. 13.FIG. 15is a graph showing changes in phase versus the amount of layer-machining performed on the multilayer-film reflective mirror2ofFIG. 13.FIGS. 14 and 15show changes in reflectance and phase accompanying various thicknesses of the multilayer film machined away. Thickness errors (THK ERROR) of the Si single-layer film7of ±0.3 nm, ±0.2 nm, and ±0.1 nm are very small compared to a case in which no thickness error is present (i.e., thickness error=0 nm).

Phase changes when the thickness error (THK ERROR) of the Si single-layer film7is ±0.3 nm are ±2.5°, compared to the phase changes when no thickness error is present. The change in the wavefront at this time is ±2.5°/360°×13.5 nm=±0.09 nm. Normally, with a thickness error of 0.3 nm, the change in the wavefront is twice the thickness error, or 0.6 nm. Therefore, the effect on wavefront of the thickness error in the Si single-layer film7is 0.09 nm/0.6 nm=approximately 1/7.

Next, the multilayer-film reflective mirror of a second embodiment of the invention is explained, referring to the drawings. The multilayer-film reflective mirror of the second embodiment is used in, for example, an EUV exposure system or the like that uses EUV light as the exposure light.FIG. 16is an elevational section schematically depicting a portion of the multilayer-film reflective mirror52of this embodiment. The multilayer-film reflective mirror52comprises a multilayer film56comprising multiple Mo layers56aand Si layers56bdeposited periodically in an alternating manner on a mirror substrate54made of low-thermal-expansion glass polished to a precise surface profile (shape). The multilayer-film reflective mirror also includes a Si single-layer film57, deposited as an oxidation-prevention film on the multilayer film56. The localized depth of machining of the surface of the Si single-layer film57is according to an in-plane distribution. A capping layer58, comprising a layer of ruthenium (Ru), is deposited on the Si single-layer film57to prevent carbon contamination and oxidation of the Si single-layer film57. The multilayer film56comprises multiple layer-pairs of Mo layers56aand Si layers56b, but inFIG. 15only four layer-pairs of Mo and Si are shown.

The multilayer film56is formed by depositing multiple layer-pairs of Mo layers56aand Si layers56busing the magnetron-sputtering film-deposition device shown inFIG. 3. The multilayer film56is formed on the reflective face (surface) of the mirror substrate54, with a period length in the range from 6.9 nm to 7.5 nm.

The surface of the multilayer film56is machined, as described above, so as to produce a desired an in-plane distribution of the removed amount that serves to correct the reflected wavefront. The Si single-layer film57has a thickness that, in any location of the film, is 0.4 nm to 1.2 nm thinner than the corresponding depth of material actually removed from the multilayer film56. The Ru capping layer58is deposited at substantially uniform thickness on the “top” surface of the Si single-layer film57. Thus, the Ru capping layer and the Si single-layer film57collectively restore the as-designed contour of the reflective surface of the mirror.

By making the film thickness of the Si single-layer film57in the range of 0.4 nm to 1.2 nm thinner than the removed thickness of the multilayer film56, fluctuations in reflectance and phase change with thickness errors of the Si single-layer film57are reduced.FIG. 17is a graph showing reflectance, as a function of thickness of film machined away, for the multilayer-film reflective mirror52of this embodiment. The reflectance changes are shown for cases in which there is no thickness error (THK ERROR of 0 nm) and in which thickness errors of ±0.3 nm, ±0.2 nm, and ±0.1 nm arise during deposition of the Si single-layer film57.FIG. 18is a graph of phase changes for the multilayer film52of this embodiment. Phase changes are shown for cases in which there is no thickness error (THK ERROR of 0 nm) and in which thickness errors of ±0.3 nm, ±0.2 nm, and ±0.1 nm arise during deposition of the Si single-layer film57.

The fluctuations in reflectance and phase change shown in the graphs ofFIGS. 17 and 18reveal smaller spread compared to the graphs inFIGS. 14 and 15, pertaining to the first embodiment. That is, inFIGS. 17 and 18, the phase change caused by a thickness error in the Si single-layer film57of ±0.3 nm is from −0.8° to +1.4° relative to the phase change when there is no thickness error in the Si single-layer film, and the wavefront change at this time is ±0.045 nm. Hence, the effect of a thickness error in the Si single-layer film57on the wavefront is approximately 1/14, and when the thickness of the Si single-layer film57is from 0.4 nm to 1.2 nm thinner than the removed thickness of the multilayer film56at the respective locations, fluctuations in reflectance and phase change with thickness errors are small compared with cases in which the thickness of the Si single-layer film57is equal to the removed thickness of the multilayer film56.

Whenever the thickness of the Si single-layer film57is made thinner by 0.4 nm than the corresponding removed thickness of the multilayer film56, the effect on the wavefront of any thickness errors of the Si single-layer film57is minimized. Also, if the thickness of the Si single-layer film57is 1.2 nm less than the removed thickness of the multilayer film56, the effect on reflectance fluctuations of thickness errors of the Si single-layer film57is minimized. Hence, if the thickness of the Si single-layer film57is denoted d1(nm), and the removed thickness of the multilayer film56is denoted d2(nm), the Si single-layer film57should be deposited so as to satisfy the condition d2−0.4≦d1≦d2−1.2.

According to the second embodiment, the thickness of the Si single-layer film is less by 0.4 nm to 1.2 nm than the removed thickness of the multilayer film. As a result, even if an error occurs in the thickness of the Si single-layer film, the changes caused thereby to reflectance and phase change of the multilayer-film reflective mirror can be minimized, and a multilayer-film reflective mirror having a highly precise surface shape can be provided.

In the multilayer-film reflective mirror of the second embodiment, the Si single-layer film is deposited 0.4 nm to 1.2 nm thinner than the thickness of multilayer film actually removed. Alternatively, the uppermost Si layer alone of the multilayer film may be deposited 0.4 nm to 1.2 nm thinner than the other Si layers in the multilayer film. Thus, the Si single-layer film may be deposited at a thickness substantially equal to the removed thickness of the multilayer film.

The multilayer film of each of the multilayer-film reflective mirrors in each of the above-described embodiments is configured from alternating layers of Mo and Si. Alternatively, configurations using materials other than Mo and Si are possible. For example, the multilayer-film reflective mirrors configured from multiple layers, each comprising a different material selected from the group consisting of Ru, Mo, Rh, Si, Be, B4C, and combinations thereof are possible. Also, a Si single-layer film is deposited as an oxidation-prevention film. Alternatively, silicon compounds such as, for example, SiO2or SiC or other materials may be deposited. In any event, it is desirable that the material actually used be one that exhibits minimal absorption of EUV light and has a refractive index near unity in the EUV wavelength range.

In the multilayer film of a multilayer-film reflective mirror according to any of the above-described embodiments, a magnetron-sputtering film-deposition device was used for film deposition. Alternatively, film deposition may be performed using any of various film-deposition devices other than a magnetron-sputtering film-deposition device. For example, an ion-beam-sputtering film-deposition device is possible.

An EUV exposure system, according to a third embodiment of the invention is explained, referring toFIG. 19. The depicted system is a demagnifying (reducing) projection-exposure device. In the depicted system, the entire optical path is maintained in a state of high vacuum. The EUV exposure system comprises an illumination-optical system IL including an EUV light source. EUV light (in general, wavelengths from 5 to 20 nm are targeted; specifically, the 13 nm and 11 nm wavelengths are used) emitted from the illumination-optical system IL is reflected by the return mirror301, and irradiates a reticle302on which is formed a pattern.

The reticle302is a reflection-type reticle, and is held by a chuck303afixed to a reticle stage303. The reticle stage303is configured to perform movements of 100 mm or more in the scanning direction, and is configured to perform minute movements in a direction perpendicular to the scanning direction and in the optical-axis direction. The position of the reticle stage303in the scanning direction and in the direction perpendicular to the scanning direction is precisely controlled using a laser interferometer (not shown). The position in the optical-axis direction is controlled using a reticle-focus sensor, which comprises a reticle-focus optical transmitting system304and a reticle-focus optical receiving system305.

In the reticle302, a multilayer film (for example, Mo/Si or Mo/Be), which reflects EUV light, is deposited. An absorption layer (e.g., nickel (Ni) and aluminum (Al)) on this multilayer film is patterned. EUV light reflected by the reticle302is incident in the optical lens-barrel314.

In the optical lens-barrel314are positioned multiple (four in this embodiment) mirrors306,307,308,309. At least one of these mirrors306-309comprises a multilayer-film reflective mirror of the first or second embodiment, or a multilayer-film reflective mirror manufactured using the method of the first embodiment. In theFIG. 19embodiment, the projection-optical system comprises four mirrors; alternatively, six or eight mirrors may be used in the projection-optical system, in which event the numerical aperture (NA) of the projection-optical system can be made larger.

EUV light entering optical lens-barrel314is reflected by the mirror306, and then reflected in succession by the mirrors307,308,309. The EUV light exits the optical lens-barrel314and is incident on the wafer310. The demagnification (reduction) ratio of the projection-optical system314is, for example, 1/4 or 1/5. In the vicinity of the optical lens-barrel314is placed an off-axis microscope315used for performing alignment of the wafer310.

The wafer310is held by a chuck311afixed to a wafer stage311. The wafer stage311is positioned in a plane perpendicular to the optical axis, and is configured to enable motion over, for example, 300 to 400 mm in the plane perpendicular to the optical axis. The wafer stage311is also configured to perform minute movements in the optical-axis direction. The position of the wafer stage311in the optical-axis direction is controlled by a wafer auto-focus sensor comprising a wafer auto-focus optical-transmitting system312and a wafer auto-focus optical-receiving system313. The position of the wafer stage311in a plane perpendicular to the optical axis is controlled precisely using a laser interferometer (not shown).

At the time of exposure, the reticle stage303and wafer stage311are scanningly moved synchronously at respective velocities of which a ratio is equal to the demagnification ratio of the projection-optical system. An example is (movement velocity of reticle stage303):(movement velocity of wafer stage311)=4:1 or 5:1.

According to the EUV exposure system of this third embodiment, at least one of the mirrors of the projection-optical system314comprises a multilayer-film reflective mirror of the first or second embodiment or a multilayer-film reflective mirror manufactured by the manufacturing method of the first embodiment. Consequently, satisfactory exposures can be performed using an optical system comprising mirrors having precise surface shapes.

In the third embodiment, at least one of the mirrors306-309comprises a multilayer-film reflective mirror of the first or second embodiment, or comprises a multilayer-film reflective mirror manufactured using the method of the first embodiment. However, any of the mirrors of the illumination-optical system IL, the return mirror301, the reticle302, and the like may comprise a multilayer-film reflective mirror of the first or second embodiment, or may comprise a multilayer-film reflective mirror manufactured using the method of the first embodiment.

A multilayer-film reflective mirror according to this invention can be used in X-ray optical systems other than an optical system of an EUV exposure system. For example, the multilayer-film reflective mirror can be used with similar advantageous effect in a high-precision reflective optical system used in wavelength regions other than the X-ray region.

FIG. 20is a flowchart of an exemplary microelectronic-fabrication method in which systems and methods according to the invention can be applied readily. The fabrication method generally comprises the main steps of wafer production (wafer manufacturing or preparation), reticle (mask) production or preparation; wafer processing, device (chip) assembly (including dicing of chips and rendering the chips operational), and device (chip) inspection. Each step usually comprises several sub-steps.

Among the main steps, wafer processing is key to achieving the smallest feature sizes (critical dimensions) and best inter-layer registration. In the wafer-processing step, multiple circuit patterns are layered successively atop one another on the wafer, forming multiple chips destined to be memory chips or main processing units (MPUs), for example. The formation of each layer typically involves multiple sub-steps. Usually, many operative microelectronic devices are produced on each wafer.

Typical wafer-processing steps include: (1) thin-film formation (by, e.g., sputtering or CVD) involving formation of a dielectric layer for electrical insulation or a metal layer for connecting wires or electrodes; (2) oxidation step to oxidize the substrate or the thin-film layer previously formed; (3) microlithography to form a resist pattern for selective processing of the thin film or the substrate itself; (4) etching or analogous step (e.g., dry-etching) to etch the thin film or substrate according to the resist pattern; (5) doping as required to implant ions or impurities into the thin film or substrate according to the resist pattern; (6) resist stripping to remove the remaining resist from the wafer; and (7) wafer inspection. Wafer processing is repeated as required (typically many times) to fabricate the desired microelectronic devices on the wafer.

FIG. 21provides a flowchart of typical steps performed in microlithography, which is a principal step in the wafer-processing step shown inFIG. 20. The microlithography step typically includes: (1) resist-application step, wherein a suitable resist is coated on the wafer substrate (which an include a circuit element formed in a previous wafer-processing step); (2) exposure step, to expose the resist with the desired pattern by microlithography; (3) development step, to develop the exposed resist to produce the imprinted image; and (4) optional resist-annealing step, to enhance the durability of and stabilize the resist pattern.

The process steps summarized above are all well known and are not described further herein.

EXAMPLES

Referring toFIG. 1, a Mo/Si multilayer film6was formed of 50 layer-pairs on a planar mirror substrate. Each Mo layer had a thickness of 2.415 nm, each Si layer had a thickness of 4.485 nm, and the period length was 6.9 nm. After layer-machining the surface of the multilayer film6, a Si single-layer film7was applied to re-fill the machined areas. The “top” surface of the Si single-layer film was planarized to restore the intended planar contour of the mirror surface. Atop the Si single-layer film7was formed a Ru capping layer8having a uniform film thickness of 2 nm. The target reference “height” to which the Si single-layer film7was formed was the highest point of the surface of the Mo/Si multilayer film6prior to layer-machining.

In this example, made according to the first embodiment, the changes in phase and reflectance, as functions of layer-machining amount, are shown inFIG. 12, showing that precise wavefront control is achievable. The Ru capping layer ensures a durable multilayer-film reflective mirror that is resistant to contamination and oxidation.

Referring toFIG. 13, a Mo/Si multilayer film6was formed of 50 layer-pairs on a planar mirror substrate. Each Mo layer had a thickness of 2.415 nm, each Si layer had a thickness of 4.485 nm, and the period length was 6.9 nm. After layer-machining the surface of the multilayer film6, a Si single-layer film7was used for re-filling the machined areas so as to re-planarize the “top” surface. Atop the Si single-layer film7was formed a Ru capping layer8having a uniform film thickness of 2 nm. However, thickness errors were introduced in the course of forming the Si single-layer film7. Also, errors were introduced to the “top” surface of the Si single-layer film while machining it to a profile opposite the profile of the layer-machined multilayer film. Consequently, the profile of the “top” surface of the Si single-layer film was not completely planar, and had a thickness error of ±0.3 nm. The target reference “height” to which the Si single-layer film7was formed was the highest point on the surface of the Mo/Si multilayer film6prior to layer-machining.

In this example, made according to the second embodiment, the changes in phase and reflectance, as functions of layer-machining amount, are as shown in FIGS.14and15, which show that precise wavefront control is achievable. The re-filling thickness error was ±0.3 nm, but the resulting wavefront error was within ±0.09 nm. Thus, the effect of the re-filling error was held to a small amount. The Ru capping layer ensured a durable multilayer-film reflective mirror that is resistant to contamination and oxidation.

Referring toFIG. 16, a Mo/Si multilayer film56was formed on a planar mirror substrate. The multilayer film had 50 layer-pairs, in which each Mo layer had a thickness of 2.415 nm, each Si layer had a thickness of 4.485 nm, and the period length was 6.9 nm. After layer-machining the surface of the multilayer-film, a Si single-layer film57was applied to re-fill the machined areas. The Si single-layer film57was planarized, and a Ru capping layer of uniform thickness was formed atop the Si single-layer film57. The Si single-layer film57was deposited so as to have a “top” surface located 0.8 nm below the maximum contour “height” of the multilayer film56. Thickness errors were introduced during re-filling with the Si single-layer film57, and machining errors were introduced when machining the Si single-layer film57to have a profile opposite the layer-machined profile of the multilayer-film. Consequently, the “top” surface of the Si single-layer film57was not completely planar, and had a thickness error of ±0.3 nm. The target reference “height” to which the Si single-layer film57was formed was 0.8 nm lower than the height of the uppermost layer of the Mo/Si multilayer film56prior to layer-machining.

According to the third embodiment, the changes in phase and reflectance, as functions of layer-machining amount, are as shown inFIGS. 17 and 18, which show that precise wavefront control is achievable. The re-filling thickness error was ±0.3 nm, but the resulting wavefront error was within ±0.09 nm. Consequently, the effect of the re-filling error was held to a small amount. The Ru capping layer ensured a durable multilayer-film reflective mirror that is resistant to contamination and oxidation.

Whereas the invention has been described in connection with representative embodiments, it will be understood that the invention is not limited to those embodiments. On the contrary, the invention is intended to encompass all modifications, alternatives, and equivalents as may be included within the spirit and scope of the invention, as defined by the appended claims.