Abstract:
A microlithographic projection exposure apparatus and method are provided. In some embodiments, a microlithographic projection exposure apparatus includes a light source to generate pulsed light, an illumination device, a projection objective, and at least one photoelastic modulator between the pulsed light source and the illumination device. The illumination device is configured to illuminate an object plane of the projection objective. The projection object projects an image of an object in the object plane of the projection objective to the image plane of the projection objective.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of international patent application PCT/EP2007/057776, filed Jul. 27, 2007, which claims the benefit of German patent application serial number DE 10 2006 038 643.4, filed Aug. 17, 2006. International patent application PCT/EP2007/057776 is incorporated herein by reference in it entirety. 
       FIELD 
       [0002]    The disclosure relates to a microlithographic projection exposure apparatus, and to a microlithographic exposure method. 
       BACKGROUND 
       [0003]    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 typically has an illumination device and a projection objective. In the microlithography process, in general, an illumination device is used to illuminate a mask (reticle) whose image is then projected onto a substrate, such as a silicon wafer, via a projection objective. Usually, the substrate is coated with a light-sensitive layer (photoresist) which is arranged in the image plane of the projection objective so that the mask structure is transferred to the light-sensitive coating of the substrate. 
       SUMMARY 
       [0004]    In some embodiments, the disclosure provides a microlithographic projection exposure apparatus and a microlithographic exposure method by which a pulse-resolved modulation of the polarization state can be achieved without involving movable, such as rotating, optical components. 
         [0005]    In certain embodiments, the disclosure provides a microlithographic projection exposure apparatus that includes a pulsed light source configured to generate pulsed light, an illumination device, a projection objective, and at least one photoelastic modulator between the pulsed light source and the illumination device. The illumination device is configured to illuminate the object plane of the projection objective, and the projection objective is configured to project an image from the object plane of the projection objective to the image plane of the projection objective. 
         [0006]    The photoelastic modulator can be subjected to a temporally varying retardation by suitable (e.g. acoustic) excitation. The 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. The alteration can also be different for individual pulses. Movable, such as rotating, optical components can be avoided by using a photoelastic modulator to generate the pulse-resolved variation of the polarization state. This can avoid, for example, stress birefringence induced in such components on account of, for example, centrifugal forces that occur, as well as undesirable influencing of the polarization distribution associated with such stress birefringence. 
         [0007]    In some embodiments, 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. 
         [0008]    In certain embodiments, a polarization-influencing optical element is included in the illumination device. 
         [0009]    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 the 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 component of a polarization state. 
         [0010]    In some embodiments, the 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 illumination 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. 
         [0011]    With 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, for example, to change between a horizontal illumination setting polarized in the vertical direction and a vertical illumination 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. 
         [0012]    In certain embodiments, the polarization-influencing optical element is arranged in a pupil plane of the illumination device. 
         [0013]    In some embodiments, the polarization-influencing optical element is a polarization filter. 
         [0014]    In certain embodiments, the disclosure provides a microlithographic exposure method, in which pulsed light is provided to an illumination device of a projection exposure apparatus. Pulses of the pulsed light are subjected to changes in their polarization state before entering into the illumination device. The changes in the polarization state are effected using at least one photoelastic modulator arranged in the beam path of the pulsed light. 
         [0015]    In some embodiments, the disclosure provides a photoelastic modulator in a microlithographic projection exposure apparatus. The photoelastic modulator provides pulse-resolved changes of the polarization state of light passing through the projection exposure apparatus. 
         [0016]    Further configurations of the disclosure can be gathered from the description, drawings and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0017]    The disclosure is explained in more detail below on the basis of exemplary embodiments illustrated in the accompanying figures, in which: 
           [0018]      FIG. 1  shows a schematic illustration of part of a microlithographic projection exposure apparatus; 
           [0019]      FIG. 2  shows a schematic illustration of part of a microlithographic projection exposure apparatus; 
           [0020]      FIG. 3  shows a schematic illustration of a polarization-influencing optical element used in the projection exposure apparatus of  FIG. 2 , in plan view; and 
           [0021]      FIGS. 4   a - b  show schematic illustrations for elucidating the effect of the element from  FIG. 3  upon irradiation with light of different polarization directions. 
       
    
    
     DETAILED DESCRIPTION  
       [0022]      FIG. 1  shows a pulsed light source  110 , which generates polarized light. The pulsed light source  110  is typically an excimer laser, such as an ArF laser having an operating wavelength of 193 nm. A photoelastic modulator (PEM) is downstream of the pulsed light source  110  in the direction of light propagation running in the direction of the arrow in  FIG. 1 . An illumination device  130  of a microlithographic projection exposure apparatus is downstream of PEM  120  in the direction of light propagation. 
         [0023]    In general, a PEM is an optical component that is 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 a temporally varying retardation. “Retardation” denotes the difference in the optical paths of two orthogonal (mutually perpendicular) polarization states. PEMs are disclosed, for example, in U.S. Pat. No. 5,886,810 or U.S. Pat. No. 5,744,721. PEMs configured to be used at wavelengths of visible light through to the VUV range (approximately 130 nm) are commercially available from, for example, by the company Hinds Instruments Inc., Hillsboro, Oreg. (USA). 
         [0024]    The PEM  120  is excited to effect acoustic vibrations via an excitation unit  140  such that a retardation that varies temporally with a modulation frequency forms in the PEM  120 . In general, the modulation frequency is dependent on the mechanical dimensioning of the PEM  120  and typically is in the region of a few tens of 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  via suitable trigger electronics. 
         [0025]    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 the 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 tens of 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. 
         [0026]    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 superimposition in the illumination device. 
         [0027]    In some embodiments, the excitation of the PEM  120  by the excitation unit  140  is correlated with the emission from the pulsed light source  110  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 . 
         [0028]    In  FIG. 2 , elements with like reference numbers are as noted above with respect to  FIG. 1 . In  FIG. 2 , a polarization-influencing optical element  232  is situated in a pupil plane of the illumination device  230 , the construction of the optical element being explained in more detail with reference to  FIG. 3 . 
         [0029]    The polarization-influencing optical element  232  has an arrangement including 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, for example, in 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 P 1  to P 4  in  FIG. 3 . The polarizer elements  233  to  236  themselves can be constructed from polarization-selective beam splitter layers joined to one another in pairs in a known manner (described in US2005/0140958 A1 mentioned above). 
         [0030]    As illustrated schematically in  FIG. 4   a  and  4   b,  via the method already described with reference to  FIG. 1 , the polarization direction of the light emerging from the pulsed light source  210  is modified before entering into the illumination device  230  in such a way that the polarization direction varies between the x-direction ( FIG. 4   a ) and the y-direction ( FIG. 4   b ) 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. 4   a,  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. 4   b,  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 . 
         [0031]    As a result, in  FIGS. 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. 
         [0032]    A diffractive optical element (DOE)  231 , which is used in the illumination device  230  and is depicted schematically in  FIG. 2 , can be designed such that only the four partial elements  233  to  236  in the pupil plane are illuminated (so-called quadrupole DOE). 
         [0033]    Even though the disclosure 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 features 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 disclosure, and the scope of the disclosure is only restricted within the meaning of the accompanying patent claims and the equivalents thereof.