Abstract:
In a system for specific deformation of optical elements in an imaging device, in particular in a projection exposure machine having a projection lens for micro-lithography, for the purpose of eliminating image errors or for active adjustment, piezoelectric elements are applied as actuators in the form of thin plates, films or layers to surfaces to be deformed, or integrated into them. In conjunction with an adaptronic servo loop having sensors, forces and/or moments are exerted on the optical elements for their specific deformation by means of a controlled activation of the piezoelectric elements as actuators.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The invention relates to an optical element deformation system in an imaging device for the purpose of eliminating image errors or for active adjustment.  
           [0002]    Very high imaging accuracies are required in many optical fields, in particular in micro-lithography. The aim is to minimize image errors even more. One of the main causes of image errors in this case is deformations of the optical elements. Such deformations can be introduced via mounts or be produced by mounts. Weight forces on optical elements can also lead to deformations and thus to image errors.  
           [0003]    Reference is made to WO 97/34171 in relation to the general prior art.  
           [0004]    Moreover, the precise adjustment of an imaging device, for example a projection lens, also requires individual optical elements to be actively adjusted in order, for example, to compensate errors in the lens in this way, or else to correct mounting inaccuracies.  
           [0005]    It is the object of the present invention to create a system of the type mentioned at the beginning with the aid of which specific deformations can be undertaken on an optical element in a very comprehensive way, and/or very specific and sensitive active adjustments are also possible.  
         SUMMARY OF THE INVENTION  
         [0006]    According to the invention, this object is achieved by means of the features mentioned in claim 1.  
           [0007]    According to the invention, piezoelectric elements are now used as actuators in the form of thin plates, films or layers which are fitted specifically to surfaces to be deformed, or integrated in them. A type of adaptronic servo loop is created in cooperation with sensors, as a result of which it is possible to examine detected deformations very precisely and to treat them appropriately. It is also possible in this way to introduce specific deformations very precisely into the optical element by means of forces and/or moments applied via the activation of the piezoelectric elements.  
           [0008]    Although the use of piezoelectric elements is already fundamentally known in optics, for which purpose reference is made, for example, to WO 97/09642, U.S. Pat. Nos. 4,202,605 and 5,986, 795, piezoelectric elements in the form of piezostacks respectively exerting deformation forces on optical elements, in the present case, by contrast, use is made specifically of thin plates, films or layers which are applied, for example in the form of coatings or via bonding methods, directly to the surfaces to be deformed and are arranged and aligned there for a specific purpose. At the same time, feedback via sensors and an adaptronic servo loop is provided in this case in such a way that it is possible in this way to act very precisely and within very small tolerances.  
           [0009]    It is possible to use as sensors, for example, capacitive ranging sensors, interferometers and the like, or, in a very advantageous development of the invention, also piezoelectric elements in the form of thin plates, films or layers such as, for example, ceramic films, which act as sensors and cooperate with the piezoelectric elements as actuators. The sensors and actuators can be arranged in an alternating fashion for this purpose.  
           [0010]    In this solution according to the invention, piezoelectric elements are used in two regards, specifically firstly deformations or vibrations which occur lead to changes in length of the piezoelectric elements which are thereby correspondingly bent in the manner of a strain gauge. These changes in the piezoelectric elements generate voltages which are correspondingly detected and evaluated. The piezoelectric elements act in this way as sensors.  
           [0011]    The reverse action is employed in the case of use as actuators: voltages are impressed via control lines on the piezoelectric elements and lead to corresponding changes in length and/or bending of the piezoelectric elements. These changes thereby initiate vibrations and/or deformations in the adjoining structure. In this case, the deformations are controlled or regulated via the adaptronic control circuit such that the desired effects are achieved in an extremely precise fashion such as, for example, the counteracting of detected deformations in order to comply with imaging accuracy, or compensating weight forces which lead to deformations or else to active adjustment, in order to counteract image errors detected or to correct them.  
           [0012]    In an advantageous refinement of the invention, it can also be provided that, to damp vibrations, all the piezoelectric elements are configured as sensors in the quiescent state or in a phase of slight excitation of vibrations. Disturbances which occur are then picked up by all the piezoelectric elements, and all, or else only specific piezoelectric elements can then subsequently be activated as actuators by appropriate control action. This refinement has the advantage that it is not necessary to identify specific piezoelectric elements as sensors or as actuators. All that need be provided is an appropriate regulating or control device,  
           [0013]    In a further advantageous refinement of the invention, it is possible, for example, for the deforming surface to be provided over its entire area, or virtually over its entire area with a piezoelectric layer or film. Structures are then shaped in this layer, for example using lasers. The most varied patterns can be created in this way. If the structures of the most varied type are respectively provided with terminals in this case, the most varied deformation zones or regions and intensities of deformation can be achieved at any desired sites by appropriate activation. It is then also possible in this case to control the individual structures completely separately, as a result of which completely diverse deformations and force effects can be achieved as a function of the desired corrections and/or results 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    Further advantageous refinements and developments of the invention emerge from the subclaims and from the exemplary embodiments described below in principle with the aid of the drawing, in which.  
         [0015]    [0015]FIG. 1 shows a perspective illustration of the principle of a mirror with a rear which is to be deformed,  
         [0016]    [0016]FIG. 2 shows a top view of a surface to be deformed,  
         [0017]    [0017]FIG. 3 shows a top view of a surface to be deformed, in another embodiment,  
         [0018]    [0018]FIG. 4 shows an enlarged illustration of two piezoelectric elements in accordance with FIG. 3, once as actuator and once as sensor,  
         [0019]    [0019]FIG. 5 shows a top view of a surface to be deformed, with annular piezoelectric elements,  
         [0020]    [0020]FIG. 6 shows a side view of FIG. 5,  
         [0021]    [0021]FIG. 7 shows a top view of a surface to be deformed, with radially extending piezoelectric elements,  
         [0022]    [0022]FIG. 8 shows a side view of FIG. 7,  
         [0023]    [0023]FIG. 9 shows a mirror in the case of which a deformation occurring owing to weight force is compensated, and  
         [0024]    [0024]FIG. 10 shows a lens in the case of which a deformation occurring owing to weight force is compensated. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]    In accordance with FIG. 1, a mirror  1  is provided with piezoelectric elements  3  on its rear  2  as surface to be deformed. Two piezoelectric elements  3 , which form the shape of a cross, are provided in accordance with the illustration.  
         [0026]    Capacitive ranging sensors  4  at a short distance from the rear  2  are provided in order to detect the state of deformation. The piezoelectric elements  3  are piezoelectric ceramic films (PZT films) which are integrated in the mirror material or bonded onto the rear  2 , and which can, for example, have a thickness of a few μm. When the PZT films are being driven, they experience in accordance with the piezoelectric effect a strain which impresses forces on the mirror via the connection with the mirror substrate, and this leads to deformation of the mirror. Multifarious “patterns” are conceivable for generating any desired states of deformation in any desired angular orientation. As a rule, the piezoelectric elements will be provided for low corrugations, although these are not, of course, restrictive.  
         [0027]    Provided in FIG. 2 by way of example from the multiplicity of possible patterns are piezoelectric elements in strip form which, distributed appropriately over the periphery in a radial direction, run on the surface to be deformed and in this way produce a type or spoked form.  
         [0028]    [0028]FIG. 3 shows a very abstract pattern with a multiplicity of sections  5  in each of which a piezoelectric element  3   a  is arranged as sensor, and the piezoelectric element  3   b  is arranged as actuator.  
         [0029]    It may be seen from the enlarged illustration in FIG. 4 that the piezoelectric elements are provided as sensors  3   a  with control lines  6 , and the piezoelectric elements  3   b  are provided as actuators with control lines  7 . Control lines  6  and  7  make the connection to a computer/evaluation unit  8  and form an adaptronic servo loop. In this case, of course, all the sensors  3   a  and actuators  3   b  are appropriately linked to one another via the evaluation and control unit  8 . The pattern illustrated in FIG. 3 with the sensors  3   a  and the actuators  3   b  can be formed by bonding an appropriate multiplicity of individual piezoelectric elements. Alternatively, the entire surface to be deformed, in this case the rear  2  of the mirror  1 , can be coated with a film, after which corresponding structures of arbitrary shape, contour and configuration can be introduced, for example by etching operations or by laser processing, thus producing a corresponding multiplicity of individual structures which are independent of one another and which are each provided with control lines  6  and  7 . Virtually any desired deformation can be achieved by means of such refinements. Such deformations can be in the nanometer range, for example,  
         [0030]    If a lens is provided for a deformation as optical element instead of a mirror, the piezoelectric elements  3  must, of course, be arranged in regions which are not optically active. Furthermore, the peripheral surface of the lens is also available in this case for the purposes of deformation.  
         [0031]    Two exemplary embodiments relating to specific deformation of a mirror  1  and by means of which the focus and/or the mirror focal length can be varied are provided in FIGS.  5  to  8 . The aim is to obtain a mirror for optical systems in particular for a wavelength of lambda ≧157 nm, which permits a mirror focal length which is to be varied in operation, in order an this way firstly to obtain a type of zoom system and, secondly, also to be able to use tracking of the focal length to correct system-internal changes in length such as,, for example, temperature drift, aging phenomena, material fatigue, changes in pressure and the like. Use is made for this purpose of, for example, piezoelectric elements  3  which are applied in annular form to the underside  2  of the mirror (see FIGS. 5 and 6) or which run radially and produce a spoked shape (FIGS. 7 and 8). By driving these elements, the surface of the mirror can be deformed specifically with the desired effect. Here, as well, the deformation is performed via an adaptronic servo loop having sensors, for example, likewise in the form of piezoelectric elements.  
         [0032]    Both in the case of the exemplary embodiment according to FIGS.  1  to  3 , and according to FIGS.  5  to  8 , the piezoelectric elements  3  can also be used for active adjustment of deformation in order, for example, to raise the imaging accuracy of a projection exposure machine having a projection lens  10  (indicated only by dashes in FIG. 1) in which the optical element to be deformed is installed, this being done to improve the overall imaging accuracy, or to compensate errors occurring at another site  
         [0033]    Two examples are shown in FIGS. 9 and 10, specifically for a mirror  1  (FIG. 9) and for a lens  9  (FIG. 10), the aim in each case being to compensate weight effects at an angle to the vertical. In general, the optical axis is vertical in the case of semiconductor lenses, or parallel to gravity. In this position, the influence of gravity produces predominantly rotationally symmetrical deformations of the optical elements, and thus also rotationally symmetrical image errors. However, it is also provided to position the optical axes of the elements horizontally or at a specific angle to the direction of gravity in lithographic systems. This produces deformations of the optical elements which are not rotationally symmetrical,  
         [0034]    A force and/or a moment can be applied in each case to the appropriate optical element by activating the piezoelectric elements  3 , which are applied in the form of PZT films. In this case, the magnitudes of the forces and moments to be applied are selected such that their action is directed opposite to the action of gravity, and the weight force effect is thereby eliminated or at least substantially reduced.  
         [0035]    As may be seen from FIG. 9, for this purpose the piezoelectric elements  3  are bonded for this purpose onto the rear  2  of the mortar  1  and, when activated under control, neutralize the deformation by gravity, which is illustrated by dashes in a greatly exaggerated fashion in FIG. 9. In addition or instead of an arrangement on the rear  2  of the mirror  1 , it is also possible for piezoelectric elements to be arranged in the edge region or on the peripheral wall of the mirror  1 .  
         [0036]    The deformation of the lens  9 , reducing on the weight force, is reduced in a similar way in accordance with FIG. 10, by piezoelectric elements  3  which are arranged distributed over the periphery of the lens. Here, as well, the shape of a lens  9  deformed by the weight force is illustrated by dashes and in a greatly exaggerated fashion.