WO2010003527A1

WO2010003527A1 - Illumination system for a projection exposure apparatus in semiconductor lithography and projection exposure apparatus

Info

Publication number: WO2010003527A1
Application number: PCT/EP2009/004357
Authority: WO
Inventors: Damian Fiolka
Original assignee: Carl Zeiss SMT GmbH
Current assignee: Carl Zeiss SMT GmbH
Priority date: 2008-06-17
Filing date: 2009-06-16
Publication date: 2010-01-14
Anticipated expiration: 2010-12-17
Also published as: DE102008028416A1

Abstract

The invention relates to an illumination system for a projection exposure apparatus for semiconductor lithography, comprising a light source for generating a beam bundle and optical elements for conditioning the beam bundle, and comprising a beam shaping unit for setting different angular orientations of different partial beams of the beam bundle. In this case, the beam shaping unit has a micromirror array having a plurality of micromirrors that are arranged on a carrier element and can be tilted about a zero position, and the beam shaping unit is embodied in such a way that given a zero position of all the micromirrors at least two micromirrors are present for which partial beams that are reflected at the same angle relative to the surface of the two micromirrors leave the beam shaping unit in different directions. In addition, the invention relates to a projection exposure apparatus equipped with the corresponding illumination system.

Description

Illumination system for a projection exposure apparatus in semiconductor lithography and projection exposure apparatus

The invention relates to an illumination system for a projection exposure apparatus in semiconductor lithography and to a projection exposure apparatus in which the illumination system according to the invention is employed. Projection exposure apparatuses in semiconductor lithography are based on the principle that a structure on a mask, a so-called reticle, which structure is to be imaged onto a semiconductor wafer by means of a photolithographic method, is illuminated and subsequently imaged on the semiconductor wafer generally in demagnified fashion by means of a projection optical unit. In this case, the intensity distribution by means of which ^* the reticle is illuminated can depend on the structure to be imaged and change from reticle to reticle.

In order to correspondingly adapt the distribution of the illumination light on the reticle, various approaches have been presented in the past. Thus, it has been proposed, for example, to set the desired intensity distribution in the plane of the reticle by means of diaphragms that can be inserted into the apparatus. However, this procedure is associated with some disadvantages: since 20 to 30 recticles are usually used for producing a single semiconductor component, the requirement to change the corresponding diaphragms for setting the desired illumination light distribution arises very frequently. This change causes the manufacturing process to come to a standstill; furthermore, the available number of possible illumination light distributions is limited in the case of this procedure by the number of different diaphragms present. An alternative to this procedure consists in using, instead of diaphragms, areal active optical elements by means of which the desired distribution of the illumination light can be set dynamically. Thus, by way of example, by means of a so-called micromirror array having hundreds of individually drivable micromirrors arranged in rows and columns, it is possible to set practically any desired distribution of the intensity of the illumination light in a reticle or pupil plane of the projection exposure apparatus. In general, a micromirror array should be understood to mean a group of micromirrors, wherein this group can also be arranged one-dimensionally, that is to say linearly, or else three-dimensionally, that is to say spatially.

Figure 2 schematically illustrates an illumination system according to the prior art in which this technology is employed. In the systems of this type which are known from the prior art, in this case it is often necessary to set extreme tilting angles of the individual micromirrors.

It is an object of the present invention to specify an illumination system for a projection exposure apparatus and a projection exposure apparatus for semiconductor lithography in which a micromirror array is employed for setting the illumination light distribution, wherein the requirements made of the adjustment range of the individual micromirrors of the micromirror array are reduced by comparison with the prior art.

This object is achieved by means of the illumination system comprising the features of independent Patent Claim 1 and also by means of the projection exposure apparatus comprising the features of Patent Claim 10. The dependent claims concern advantageous developments and variants of the invention. The illumination system according to the invention for a projection exposure apparatus for semiconductor lithography exhibits a light source for generating a beam bundle. For conditioning the beam bundle, the illumination system contains optical elements, in particular a beam shaping unit for setting different angular orientations of different partial beams of the beam bundle. In this case, the beam shaping unit has a micromirror array having a plurality of micromirrors that are arranged on a carrier element and can be tilted about a zero position, which beam shaping unit is embodied in such a way that given a zero position of all the micromirrors at least two micromirrors are present for which partial beams that are reflected at the same angle relative to the surface of the two micromirrors leave the beam shaping unit (2) in different directions.

In other words, the carrier element is embodied in such a way that the reflective surfaces of the micromirrors do not lie in the same plane in a zero position of the micromirrors.

This can be achieved for example by the fact that the carrier element has at least two, in particular four, planar regions on which the micromirrors are arranged, wherein the planar regions are tilted with respect to one another by an angle that is different from zero.

The angle by which the abovementioned regions are tilted with respect to one another can be e.g. 8° -12° and dynamically adjustable by means of a suitable actuator system.

As an alternative or else in addition, the beam shaping unit (2) can have an additional optical element suitable for deflecting a portion of the partial beams of the beam bundle further in addition to the deflection already effected on account of the micromirror array. In this case, the additional optical element can be a wedge plate, for example. In this case, wedge plate is understood to mean an optical element composed of a material that is transmissive to the useful radiation, wherein the interfaces which the useful radiation passes through upon passing through the optical element do not run parallel to one another. Other optical elements, such as e.g. lenses or diffractive optical elements, can also be employed for the purpose mentioned.

In this case, the additional optical element can be embodied in such a way that it can be introduced into and moved out of the beam path e.g. by means of a suitable actuator system.

In a further variant of the invention, at least two light sources are present. In this case, the illumination system is designed in such a way that the beam bundles that emerge from the at least two light sources reach different regions of the micromirror array. These can be the already discussed planar partial regions that are tilted with respect to one another. This embodiment of the invention makes it possible to provide an overall higher optical power for the exposure process.

A further variant of the invention consists in the fact that in a zero position of the micromirrors, partial beams leave the beam shaping unit (2) in two different directions, wherein the partial beams in the first direction originate from a first group of the micromirrors and the partial beams in the second direction originate from a second group of the micromirrors, and wherein the number of the micromirrors in both groups is approximately identical in magnitude. In an alternative illumination system, the reflective elements can be mirror facets of a facet mirror, in particular for EUV lithography. In this case, the carrier element can be embodied as part of a curved shell or be composed of planar segments which form at least one part of a curved shell. The radius of the shell can in this case be 500 - 1800mm, preferably 900 - 1100 mm, in particular 1000 mm.

The facet mirror can be a field facet mirror or a pupil facet mirror.

Embodiments and variants of the invention are explained below with reference to the drawing.

In the figures:

Figure 1 shows a projection exposure apparatus according to the prior art,

Figure 2 shows a detail illustration of an illumination system according to the prior art,

Figure 3 shows a micromirror array according to the prior art in an enlarged illustration,

Figure 4 shows a micromirror array according to the invention in an enlarged illustration,

Figure 5 shows an alternative embodiment of the invention; and

Figure 6 shows an application of the invention in an illumination system for semiconductor lithography. Figure 1 illustrates a projection exposure apparatus 31 for semiconductor lithography according to the prior art. The apparatus serves for the exposure of structures onto a substrate coated with photosensitive materials, which substrate in general is composed predominantly of silicon and is referred to as a wafer 32, for the production of semiconductor components, such as e.g. computer chips.

In this case, the projection exposure apparatus 31 essentially comprises an illumination system 33, a device 34 for receiving and exactly positioning a mask provided with a structure, a so- called reticle 35, which is used to determine the later structures on the wafer 32, a device 36 for retaining, moving and exactly positioning precisely said wafer 32, and an imaging device, namely a projection objective 37, comprising a plurality of optical elements 38, which are borne by means of mounts 39 in an objective housing 40 of the projection objective 37.

In this case, the basic functional principle provides for the structures introduced into the reticle 35 to be imaged onto the wafer 32; the imaging is generally performed in demagnifying fashion.

After an exposure has taken place, the wafer 32 is moved further in the arrow direction, such that a multiplicity of individual fields, each with the structure prescribed by the reticle 35, are exposed on the same wafer 32. On account of the step-by-step advancing movement of the wafer 32 in the projection exposure apparatus 31, the latter is often also referred to as a stepper.

The illumination system 33 provides a projection beam 41, for example light or a similar electromagnetic radiation, required for the imaging of the reticle 35 on the wafer 32. A laser or the like can be used as a source for said radiation. The radiation is shaped in the illumination system 33 by means of optical elements in such a way that the projection beam 41, upon impinging on the reticle 35, has the desired properties with regard to diameter, polarization, shape of the wavefront and the like.

By means of the beams 41, an image of the reticle 35 is generated and transferred to the wafer 32 in correspondingly demagnified fashion by the projection objective 37, as has already been explained above. The projection objective 37 has a multiplicity of individual refractive, diffractive and/or reflective optical elements 38 such as e.g. lenses, mirrors, prisms, terminating plates and the like.

Figure 2 schematically shows a more detailed illustration of the illumination system 33 of the projection exposure apparatus shown in Figure 1. In this case, the primary light source 1 used in the illumination system 33 is a laser, which can be embodied for example as an F₂ laser having an operating wavelength of 157 nm, or as an ArF excimer laser having an operating wavelength of 193 nm. Other primary light sources having a longer or shorter operating wavelength can also be employed.

After emerging from the primary light source 1, the light beam enters into a beam expanding optical unit 7, which generates a substantially parallel beam bundle having a larger cross section. In this case, the beam expanding optical unit 7 can contain elements that reduce the coherence of the illumination light. The expanded, parallel and homogeneous beam is split into a plurality of parallel partial beams in the further course in the microlens array 3. The partial beams impinge on the micromirrors of the micromirror array 21 in the beam shaping unit 2.

In the beam shaping unit 2, in this case a predefined angle distribution of the individual partial beams is set by means of a corresponding angle setting of the individual micromirrors, which are not designated more specifically in Figure 2, such that a predefined intensity distribution of the illumination light arises after passing through the diffuser 4 and the lens 5 in the pupil plane 6. In this case, micromirrors are typically adjustable in an angular range of -10° to +10°.

Figure 3 shows the micromirror array 21 in an enlarged illustration in order to clarify the problem area. It comprises the carrier element 210 and the tiltable micromirrors 211, 212, 213, 214 arranged on the carrier element 210. The partial beams 221, 222, 223 and 224 that are reflected as partial beams 221', 222', 223' and 224' at the micromirrors 211, 212, 213, 214 will be considered by way of example in Figure 2. The light spots generated thereby in the downstream pupil plane are illustrated in the upper part of Figure 2 by the corresponding assignments to the partial beams 221', 222', 223', 224'. It can be discerned from Figure 2 that, in order to achieve the desired light distribution in the pupil plane as illustrated, the two micromirrors 211 and 214 have to be tilted by a comparatively large angle relative to their zero position (the latter is essentially parallel to the surface of the carrier element 210) . This has the effect that some of the micromirrors 211, 212, 213, 214 arranged on the carrier element 210 have to be set in a wide adjustment range in a direction relative to the zero position of the micromirror, whereas angle positions in the other direction relative to the zero position are never used for these micromirrors. On account of the extreme tiltings of the micromirrors necessary because of this, controlling or complying with predefined angle positions becomes difficult.

A first variant according to the invention for solving the problem area outlined with reference to Figure 3 consists in using an arrangement such as is illustrated in Figure 4.

Figure 4 shows a micromirror array 21, the carrier element 210' of which is divided into two regions that are tilted relative to one another. What is achieved by this tilting is that the micromirrors 211, 212, 213, 214 arranged on the carrier element 210' already have a certain angle offset, such that extreme angle positions of the micromirrors do not have to be set. It becomes clear from Figure 4 that, according to the invention, the intensity distribution - identical to the illustration in Figure 3 - of the illumination light in the pupil plane can be achieved with a significantly smaller tilting angle of the micromirrors 211, 212, 213 and 214. A variable setting of the tilting of the two partial regions of the carrier element 210 relative to one another can be achieved by means of the actuator unit 230, which can be realized for example as a linear motor, adjusting screw or alternatively piezo-drive. The actuator arrangement 230 engages via suitable lever elements 235 on the partial regions of the carrier element 210', which can be connected to one another for example by means of a hinge (not illustrated) . In this case, the tilting angle of the two partial regions relative to one another can be set in particular in a range of 8° to 12°; settings outside this range are also conceivable. It is also conceivable to divide the carrier element 210 or 210' into more than two, in particular into four, partial regions with corresponding offset angles, wherein each partial region is assigned a pupil partial region. In other words, beams from a specific partial region of the micromirror array 21 also in each case reach only the correspondingly assigned pupil region. Overall, the required angle adjustment range of the individual micromirrors 211, 212, 213, 214 can thereby be approximately halved.

In this case, the offset angle per segment is set in such a way that in the zero position of the micromirrors 211, 212, 213, 214, the partial beams obtain an angle of NA(MAX) /2 with respect to the pupil coordinate system (beams parallel to the optical axis and upstream of the pupil lens lie in the center of the pupil) . In this case, NA(MAX) /2 corresponds to the maximum radius of the pupil plane. If the offset position is set to NA(MAX) /2, that is to say {R/2}, then the angle range of approximately 2*R/2, that is to say R, proceeding from the zero position, can be achieved from it in all directions. An optimum utilization of the adjustment range thus results.

Figure 5 shows an alternative configuration of the beam shaping unit 2, in which a tilted arrangement of individual regions of the carrier element 210 relative to one another can be dispensed with. In the variant illustrated in Figure 5, the desired necessary angle setting of the individual partial beams is achieved by virtue of the fact that an additional optical element 200 is present, which performs the required deflection of the corresponding partial beams. In the example illustrated in Figure 5, the additional optical element 200 is a wedge plate in which the desired effect is achieved refractively . The refractive effect of the wedge plate 200 results in the beam profiles that are depicted in dashed fashion and are designated by the reference symbols 223' ' and 224 ' '. The beam profiles 223' and 224' correspond to the profiles that would arise without the use of a wedge plate. It is furthermore conceivable to provide the wedge plate 200 with a suitable mechanism by means of which said wedge plate can for example be brought into the beam path or removed from the latter or else be tilted in the beam path itself. The use of a plurality of wedge plates or optical elements 200 which are assigned to different regions of the micromirror array 21 can also be advantageous for specific applications. Furthermore, the two concepts outlined with reference to Figures 4 and 5 need not necessarily be used alternatively, but rather can, of course, also be combined.

Figure 6 shows, in a roughly schematic partial illustration, a variant of the invention in which the latter is applied in an illumination system of a projection exposure apparatus for EUV lithography. In this case, the short-wave optical radiation required for the exposure of the wafer (not illustrated) is generated by the plasma source 600 and pre-shaped or concentrated by the collector mirror 601. After passing through a diaphragm 602, the optical radiation firstly reaches the field facet mirror 603, which has a multiplicity of mirror facets 610 mounted in movable fashion. In this case, the mirror facets 610 are formed as substantially rectangular bodies whose side on which light impinges is generally embodied in spherical fashion. The mirror facets 610 can be composed, in particular, of silicon or else some other material; the reflective side on which light impinges can also be embodied as a multilayer layer. The mobility of the mirror facets 610 is ensured by virtue of the fact that they are mounted in a carrier element 612 in movable fashion in each case by means of a holding element 611. In this case, the actuation of the movable mirror facets 610 can be effected from that side of the carrier element 612 which is remote from the mirror facets 610. In this case, the holding elements 611 should be configured as far as possible such that the distance between the pivot, that is to say essentially the mounting in the carrier element 612, and the mirror facet 610 itself is kept small in order to ensure a tilting of the mirror facets 610 in such a way that the tilting is associated only with a small lateral offset of the mirror facet 610. In other words, what is intended to be achieved by means of the geometrical configuration of the holding element 611 is that large tilting angles of the mirror facets 610 can be achieved in conjunction with small deflections of the entire mirror facet 610 on a trajectory curve. According to the invention, the carrier element 612 of the field facet mirror 603 is embodied in such a way that a certain angular offset of the individual mirror facets 610 with respect to one another is already achieved by virtue of the geometrical shape of said carrier element. This reduces the tilting angles of the mirror facets 610 that are additionally required for setting a desired illumination setting, for example, upon the actuation of the individual mirror facets 610. As illustrated in figure 6, the carrier element 612 can in this case be composed in segments from substantially planar segments which are arranged at an angle different than zero degrees with respect to one another, and in particular form a half shell by virtue of their arrangement with respect to one another. Moreover, it also conceivable to realize the carrier element 612 overall not from individual segments but rather as a half shell embodied more or less spherically. As can be seen from figure 6, the pupil mirror 604 arranged downstream in the light path, said pupil mirror usually being equipped with mirror facets 620 embodied rather in round fashion, can also be embodied in a manner analogous to the field facet mirror 603. After passing through the pupil mirror 604, the optical radiation reaches the reticle 650, which is imaged onto a wafer in the downstream projection optical unit (not illustrated) . In the example shown, in contrast to the carrier element 612 of the field facet mirror 603 that is constructed in segmented fashion, the carrier element 613 of the pupil mirror 604 is constructed spherically or in free form fashion; other combinations of the basic forms of the carrier elements 612 and 613 are also conceivable. The actuability of the mirror facets 610 and 620, respectively, has the advantage in both cases that, in contrast to the systems realized protypically hitherto, desired illumination settings are no longer achieved by using diaphragms but rather by means of a corresponding deflection of the light. The loss of intensity of the radiation used for the exposure of the wafers, said loss being inevitably associated with the use of diaphragms, is thereby avoided, which constitutes a considerable advantage particularly in EUV lithography.

In principle, it is also possible - in addition to the variant shown - to embody further mirrors in an EUV projection exposure apparatus, for example, the collector mirror 601 of the plasma source, in a manner similar to that of the mirrors 603 or 604 shown.

The radius of the spheres in which the individual segments approximately run or on which the mirror facets 610 or 620 are directly arranged with their holding elements 611 or 615 can in this case lie in a range of approximately 500-1800mm, preferably in a range of 900-llOOmm, in particular at about 1000 mm.

Claims

Patent claims:

1. Illumination system (33) for a projection exposure apparatus (31) for semiconductor lithography, comprising a light source (1) for generating a beam bundle and optical elements for conditioning the beam bundle, and comprising a beam shaping unit (2) for setting different angular orientations of different partial beams of the beam bundle, wherein the beam shaping unit (2) has a plurality of reflective elements that are arranged on a carrier element (210) and can be tilted about a zero position, characterized in that the beam shaping unit (2) is embodied in such a way that given a zero position of all the reflective elements (211, 212, 213, 214) at least two reflective elements (211, 212, 213, 214) are present for which partial beams that impinge on the beam shaping unit (2) at the same angle relative to the optical axis leave the beam shaping unit (2) in different directions.

2. Illumination system (33) according to Claim 1, characterized in that the beam shaping unit has a micromirror array (21) having micromirrors (211, 212, 213, 214) as reflective elements .

3. Illumination system (33) according to Claim 2, characterized in that the carrier element (210) has at least two, in particular four, planar regions on which the micromirrors (211 , 212, 213, 214) are arranged, wherein the planar regions are tilted with respect to one another by an angle that is different from zero.

4. Illumination system (33) according to Claim 2, characterized in that the beam shaping unit (2) has an additional optical element (200) suitable for deflecting a portion of the partial beams of the beam bundle further in addition to the deflection already effected on account of the micromirror array (21) .

5. Illumination system (33) according to Claim 2, characterized in that the additional optical element (200) is a wedge plate .

6. Illumination system (33) according to either of the preceding Claims 4 and 5, characterized in that the additional optical element (200) is embodied in such a way that it can be introduced into and moved out of the beam path.

7. Illumination system (33) according to Claim 3, characterized in that the angle by which the regions are tilted with respect to one another is 8°-12°.

8. Illumination system (33) according to Claim 3, characterized in that the angle by which the regions are tilted with respect to one another is dynamically adjustable.

9. Illumination system (33) according to any of the preceding claims, characterized in that at least two light sources are present .

10. Illumination system (33) according to any of the preceding Claims 2 - 9, characterized in that in a zero position of the micromirrors (211, 212, 213, 214), partial beams leave the beam shaping unit (2) in two different directions, wherein the partial beams in the first direction originate from a first group of the micromirrors (211, 212, 213, 214) and the partial beams in the second direction originate from a second group of the micromirrors (211, 212, 213, 214), and wherein the number of the micromirrors (211, 212, 213, 214) in both groups is approximately identical in magnitude.

11. Illumination system (33¹) according to Claim 1, characterized in that reflective elements are mirror facets (610, 620) of a facet mirror (603, 604) .

12. Illumination system (33') according to Claim 11, characterized in that the carrier element (612, 613) is embodied as part of a curved shell.

13. Illumination system (33') according to Claim 11, characterized in that the carrier element (612, 613) is composed of planar segments which form at least one part of a curved shell.

14. Illumination system (33¹) according to Claim 12 or 13, characterized in that the radius of the shell is 500 - 1800mm, preferably 900 - 1100 mm, in particular 1000 mm.

15. Illumination system (33') according to any of the preceding Claims 11 - 13, characterized in that the facet mirror is a field facet mirror (603) or a pupil facet mirror (604) .

16. Projection exposure apparatus (31) for semiconductor lithography comprising an illumination system (33) according to any of Claims 1-15.

17. Projection exposure apparatus according to Claim 16, characterized in that it is a projection exposure apparatus for EUV semiconductor lithography.