Microlithoqraphic projection exposure apparatus and microlithographic exposure method

BACKGROUND OF THE INVENTION

1. Field of the invention

The invention relates to a microlithographic projection exposure apparatus, and to a microlithographic exposure method.

2. Description of the related art

Microlithographic projection exposure apparatuses are used for the fabrication of microstructured components such as, for example, integrated circuits or LCDs. Such a projection exposure apparatus has an illumination device and a projection objective. In the microlithography process, the image of a mask (= reticle) illuminated with the aid of the illumination device is projected, by means of the projection objective, onto a substrate (e.g. a silicon wafer) that is coated with a light-sensitive layer (photoresist) and is arranged in the image plane of the projection objective, in order to transfer the mask structure to the light-sensitive coating of the substrate.

In the illumination device, the generation of light that is as unpolarized as possible is desirable for some applications, for which purpose it is necessary to depolarize the linearly polarized light emerging from the laser source.

For this purpose, DE 199 21 795 A1 discloses arranging a rotating, polarization-changing element in the form of a lambda/2 plate in an illumination beam path of a projection exposure apparatus for generating unpolarized radiation, the individual pulses of the laser used as the light source being synchronized with the rotary movement of the lambda/2 plate in such a way that the lambda/2 plate carries on rotating by 45° from pulse to pulse and the pulses, downstream of the lambda/2 plate, therefore cancel one another out in pairs in terms of their polarization effect.

US 2004/0262500 A1 discloses a method and an apparatus for the image- resolved polarimetry of a beam pencil generated by a pulsed radiation source (e.g. an excimer laser) e.g. of a microlithographic projection exposure apparatus, wherein two photoelastic modulators (PEM) that are excited at different oscillation frequencies and a polarization element e.g. in the form of a polariza- tion beam splitter are positioned in the beam path, the radiation source is driven for emission of radiation pulses in a manner dependent on the oscillation state of the first and/or the second PEM, and the radiation coming from the polarization element is detected in image-resolved fashion by means of a detector.

The above-mentioned photoelastic modulators (PEM) are optical components which are produced from a material exhibiting stress birefringence in such a way that an excitation of the PEM to effect acoustic vibrations leads to a periodically varying mechanical stress and thus to a temporally varying retardation. "Retardation" denotes the difference in the optical paths of two orthogonal (mutually perpendicular) polarization states. Photoelastic modulators (PEM) of this type are sufficiently known in the prior art, e.g. US 5,886,810 A1 or US 5,744,721 A1 and are produced and sold for use at wavelengths of visible light through to the VUV range (approximately 130 nm) e.g. by the company Hinds Instruments Inc., Hillsboro, Oregon (USA).

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a microlithographic projection exposure apparatus and a microlithographic exposure method by means of which a pulse-resolved modulation of the polarization state can be achieved without the need for movable, in particular rotating, optical components.

A microlithographic projection exposure apparatus according to the invention comprises: - a pulsed light source for generating pulsed light;

- an illumination device; and

- a projection objective, the illumination device illuminating an object plane of the projection objective and the object plane being imaged into an image plane of the projection objective by means of the projection objective; - at least one photoelastic modulator being arranged between the pulsed light source and the illumination device.

The photoelastic modulator can be subjected to a temporally varying retardation by means of suitable (e.g. acoustic) excitation in a manner known per se, which retardation may in turn be temporally correlated with the pulsed light, such that individual (e.g. successive) pulses of the pulsed light are subjected in each case to a defined retardation and hence to a defined alteration of the polarization state, in which case said alteration can also be different for individual pulses. In this case, movable (in particular rotating) optical components are avoided by virtue of the use according to the invention of a photoelastic modulator for generating this pulse-resolved variation of the polarization state, thereby avoiding in particular a stress birefringence that is induced in such components on account of e.g. centrifugal forces that occur, and an undesirable influencing of the polarization distribution that accompanies said stress bi- refringence.

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In accordance with an embodiment, by way of example, the above-mentioned temporal correlation can be effected in such a way that two successive pulses of the pulsed light cancel one another out in terms of their polarization effect after emerging from the photoelastic modulator, or are oriented orthogonally with respect to one another in terms of their polarization direction upon emerging from the photoelastic modulator, in order to generate unpolarized light in the illumination device as a result.

In accordance with an embodiment, a polarization-influencing optical element is furthermore arranged in the illumination device.

In the context of the present application, a polarization-influencing optical element should be understood to mean, in principle, any element which has the property of converting an input polarization state of light impinging on said element into a different polarization state, whether it be by rotating the preferred direction of polarization of the light, filtering out the light component of a specific polarization state or converting a first polarization state into a second polarization state. Furthermore, the polarization state can be changed, in principle, both in transmission and in reflection or absorption of the light compo- nent of a polarization state.

In accordance with an embodiment, said polarization-influencing optical element has four polarizer elements, which are arranged offset by 90° relative to one another in the circumferential direction about an optical axis of the illumi- nation device, two polarizer elements that lie opposite one another being transmissive to light of a first polarization direction and the remaining two polarizer elements being transmissive to light of a second polarization direction perpendicular thereto.

By means of such a polarization-influencing optical element, in conjunction with a pulse-resolved change in the polarization direction that is carried out according to the manner described above, it is possible to change between a horizontal illumination setting polarized in the vertical direction and a vertical illumina-

tion setting polarized in the horizontal direction, such that it is possible to switch between an illumination setting optimized for vertical structures and an illumination setting optimized for horizontal structures without movable parts and in rapid succession.

The polarization-influencing optical element is preferably arranged in a pupil plane of the illumination device.

In accordance with an embodiment, said polarization-influencing optical ele- ment is a polarization filter.

In accordance with a further aspect, the invention also relates to a microlitho- graphic exposure method, in which pulsed light generated by means of a pulsed light source is fed to an illumination device of a projection exposure ap- paratus, pulses of the pulsed light being subjected to defined changes in their polarization state before entering into the illumination device, wherein said changes in the polarization state are effected using at least one photoelastic modulator arranged in the beam path of the pulsed light.

In accordance with a further aspect, the invention also relates to the use of a photoelastic modulator in a microlithographic projection exposure apparatus for the defined, pulse-resolved changing of the polarization state of light passing through the projection exposure apparatus.

Further configurations of the invention can be gathered from the description and also the subclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in more detail below on the basis of exemplary embodiments illustrated in the accompanying figures.

In the drawings:

Figure 1 shows a schematic illustration of part of a microlithographic projection exposure apparatus in accordance with a first em- bodiment of the invention;

Figure 2 shows a schematic illustration of part of a microlithographic projection exposure apparatus in accordance with a second embodiment of the invention;

Figure 3 shows a schematic illustration of a polarization-influencing optical element used in the projection exposure apparatus of Figure 2, in plan view; and

Figures 4a-b show schematic illustrations for elucidating the effect of the element from Figure 3 upon irradiation with light of different polarization directions.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

For elucidating the apparatus according to the invention for influencing the polarization distribution in a microlithographic projection exposure apparatus in accordance with a first embodiment, Fig. 1 firstly shows a pulsed light source 110, which generates polarized light. The pulsed light source 110 is typically an excimer laser, e.g. an ArF laser having an operating wavelength of 193 nm. Situated downstream of the pulsed light source 110 in the direction of light propagation running in the direction of the arrow in Fig. 1 is a photoelastic modulator (PEM) 120 and, disposed downstream thereof in the direction of light propagation an illumination device 130 of a microlithographic projection exposure apparatus.

The PEM 120 is excited to effect acoustic vibrations by means of an excitation unit 140 in a manner known per se, such that a retardation that varies temporally with a modulation frequency forms in the PEM 120, said modulation frequency being dependent on the mechanical dimensioning of the PEM and typi- cally being in the region of a few 10 kHz. It is assumed in Fig. 1, then, that the pressure direction or the vibration direction is arranged at an angle of 45° relative to the polarization direction of the laser light that is emitted by the pulsed light source 110 and impinges on the PEM 120. The excitation of the PEM 120 by the excitation unit 140 is correlated with the emission from the pulsed light source 110 by means of suitable trigger electronics.

The pulsed light source 110 generates a first pulse, then, at a point in time at which the retardation in the PEM 120 is precisely zero. A second pulse is generated by the pulsed light source 110 at a point in time at which the retardation in the PEM 120 amounts to half the operating wavelength, that is to say λ/2.

The PEM 120 therefore acts on said second pulse as a lambda/2 plate, such that the polarization direction of the second light pulse upon emerging from the PEM 120 is rotated by 90° with respect to its polarization direction upon entering into the PEM 120. Since the PEM 120 is operated at a frequency of a few 10 kHz and the period duration of the excited vibration of the PEM 120 is therefore long in comparison with the pulse duration (approximately 10 nanoseconds) of the pulsed light source 110, a quasi-static retardation acts on the light from the pulsed light source 110 in the PEM 120 during the pulse duration.

Since the two pulses described above are oriented orthogonally with respect to one another in terms of their polarization direction when emerging from the PEM 120, they cancel one another out in pairs in terms of their polarization effect after emerging from the PEM 120 or upon entering into the illumination device 130. Consequently, unpolarized light is produced as a result of the super- imposition in the illumination device.

In accordance with a further embodiment, the excitation of the PEM 120 by the excitation unit 140 is correlated with the emission from the pulsed light source

1 10 in such a way that a first pulse passes through the PEM 120 at a point in time at which the retardation in the PEM amounts to one quarter of the operating wavelength, that is to say λ/4, (which leads e.g. to left circularly polarized light), and a second pulse is generated by the pulsed light source 110 at a point in time at which the retardation in the PEM 120 is of identical magnitude and opposite sign, that is to say amounts to -λ/4 (which then leads to right circularly polarized light) or vice versa. Consequently, the superimposition of a multiplicity of such pairs of light pulses likewise produces unpolarized light when emerging from the PEM 120 or entering into the illumination device 130.

Fig. 2 shows a further embodiment of the invention, parts that are essentially functionally identical with respect to Fig. 1 being designated by reference numerals increased by 100. In accordance with Fig. 2, a polarization-influencing optical element 232 is situated in a pupil plane of the illumination device 230, the construction of said optical element being explained in more detail with reference to Fig. 3.

The polarization-influencing optical element 232 has an arrangement comprising four polarizer elements 233 to 236 which are arranged offset by 90° in each case in a light-opaque carrier 237. Such a polarization-influencing optical element is described e.g. in the laid-open patent application US2005/0140958 A1. The polarization directions or transmission directions of the individual polarizer elements 233 to 236 are designated on the basis of the double-headed arrows P1 to P4 in Fig. 3. The polarizer elements 233 to 236 themselves can be con- structed from polarization-selective beam splitter layers joined to one another in pairs in a known manner (described in US2005/0140958 A1 mentioned above).

As illustrated schematically in Fig. 4a and 4b, by means of the method already described with reference to Fig. 1 , the polarization direction of the light emerging from the pulsed light source 210 is then modified before entering into the illumination device 230 in such a way that the polarization direction varies between the x-direction (Fig. 4a) and the y-direction (Fig. 4b) in pulse-resolved

fashion. In the first-mentioned case, that is to say upon setting the polarization direction in the x-direction in accordance with Fig. 4a, by virtue of the effect of the polarization-influencing optical element 232, the light polarized in this way is transmitted only by the polarizer elements 234 and 236, whereas it is blocked by the polarizer elements 233 and 235. By contrast, upon setting the polarization direction in the y-direction in accordance with Fig. 4b, the light polarized in this way is transmitted only by the polarizer elements 233 and 235, whereas it is blocked by the polarizer elements 234 and 236.

As a result, in the embodiment described with reference to Fig. 2 to 4, a horizontal illumination setting polarized in the vertical direction (y-direction) varies in pulse-resolved fashion with a vertical illumination setting polarized in the horizontal direction. In this way, it is possible to switch between an illumination setting optimized for vertical structures and an illumination setting optimized for horizontal structures without movable parts and in rapid succession.

Furthermore, a diffractive optical element (DOE) 231 , which is used in the illumination device 230 and is depicted schematically in Fig. 2, is preferably designed such that only the four partial elements 233 to 236 in the pupil plane are illuminated (so-called quadrupole DOE).

Even though the invention has been described on the basis of specific embodiments, numerous variations and alternative embodiments can be deduced by the person skilled in the art, e.g. by combination and/or exchange of fea- tures of individual embodiments. Accordingly, it goes without saying for the person skilled in the art that such variations and alternative embodiments are also encompassed by the present invention, and the scope of the invention is only restricted within the meaning of the accompanying patent claims and the equivalents thereof.