Abstract:
Objectives, such as projection objectives for semiconductor lithography, are disclosed. An objective generally has an optical axis and optical elements mounted in an objective housing. Projection exposure apparatuses having an objective are also disclosed. In addition, guides and adjusting systems for an optical element in an objective are disclosed. Further, related components and methods are disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of PCT/EP2006/011371, filed Nov. 28, 2006, which claims benefit of German Application No. 10 2005 057 860.8, filed Dec. 3, 2005. The contents of PCT/EP2006/011371 are hereby incorporated by reference. 
    
    
     FIELD 
     The disclosure relates to an objective, such as a projection objective for semiconductor lithography, having an optical axis and having optical elements mounted in an objective housing. The disclosure also relates to a projection exposure apparatus having an objective. The disclosure furthermore relates to a guide and adjusting system for an optical element. In addition, the disclosure relates to related components and methods. 
     BACKGROUND 
     An objective, such as a projection objective designed to be used in semiconductor lithography to produce semiconductor elements, generally contains optical elements, e.g. lenses and mirrors. It is often desirable to be able to very precisely align the optical elements because there are often extremely stringent performance characteristics on the imaging accuracy of the objective. It is known to use manipulators in an objective to position the optical elements. 
     SUMMARY OF THE DISCLOSURE 
     The disclosure can provide an objective, such as a projection objective for semiconductor lithography, designed so that the optical elements can be moved or adjusted in a plurality of degrees of freedom with extremely high accuracy. The movement or adjustment can be achieved by one or more adjusting devices. The design of the adjusting devices can be relatively simple. 
     In some embodiments, the objective has an optical axis and optical elements mounted in an objective housing. At least one optical element is mounted in an inner part connected to an outer part via an intermediate part. Adjusting devices are provided, which make it possible to realize, depending on their actuation, relative movements between the outer part and the intermediate part and between the intermediate part and the inner part in a plane perpendicular to the optical axis (z), parallel to the optical axis (z), and in directions tilted relative to the optical axis (z). 
     In contrast to at least some known mountings of an optical element in an inner part and in an outer part, an extremely high adjustment accuracy can be achieved via the intermediate part according to the disclosure and the connection of the intermediate part to the inner part and the outer part. In some instances, adjustments or displacements in one direction or the alteration of one degree of freedom can have no effect on the other degrees of freedom. 
     By virtue of the configuration or mounting of the optical element according to the disclosure, the optical element can be guided and manipulated not only in all translational degrees of freedom but also in rotational degrees of freedom. In this case, the individual displacements or adjustments of the optical element can be performed such without alteration in a direction or a rotation or torsion in which no alteration is desired. 
     In some embodiments, the adjusting devices are supported on the outer part and in each case act on a lever mechanism, the center of rotation of which is fixedly connected to the intermediate part. With a lever mechanism, step-down and step-up ratios of the adjustments can also be achieved in a simple manner depending on the length of the individual levers. 
     In certain embodiments, the lever mechanism has three levers and is supported on the intermediate part in such a way that in the event of actuation of the adjusting devices, a relative movement occurs between the outer part and the intermediate part only in a plane perpendicular to the optical axis (z) and/or in a direction parallel to the optical axis (z) and/or tiltings with respect to the optical axis (z) only occur between the inner part and the intermediate part. 
     If the optical element is a lens, the inner part, outer part and intermediate part can be in each case concentrically arranged rings, with the adjusting devices generally arranged in a manner distributed at a distance of 120° with respect to one another on the circumference. In this case, the center of rotation of all three lever mechanisms for the adjusting devices may each be fixedly connected to the intermediate ring. 
     Optionally, if the intermediate part is formed from a plurality of ring segments, such as three ring segments, the possibilities with regard to adjusting the optical element can become even greater. This holds true particularly when each ring segment has a dedicated adjusting device. The individual ring segments can form a closed ring, in which case they may be connected to one another by a rigid or an elastic connecting part. 
     By virtue of this configuration, it is possible e.g. to combine an adjustment of the optical element in an axis perpendicular to the optical axis with an adjustment parallel to the optical axis. Furthermore, an individual arrangement of degrees of freedom may be utilized for constructing a redundancy within the objective in respect of the failure of individual degrees of freedom in the field. In other words, if one possibility of adjustment fails at other optical elements in the system, its task may, if appropriate, be concomitantly undertaken by the apparatus according to the disclosure. 
     A further considerable advantage of the configuration according to the disclosure is that the number of degrees of freedom to be driven can be minimized via an individual arrangement of the manipulable degrees of freedom within the objective. This leads, therefore, to a minimization of the actuator system and hence adjusting devices, to a reduced sensor system and likewise to a minimization of control electronics. Production costs and structural space can also be reduced in this way. 
     Exemplary embodiments of the disclosure from which further features according to the disclosure emerge are described in principle below with reference to the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a basic illustration of a projection exposure apparatus with a projection objective; 
         FIG. 2  shows a plan view of a lens as optical element to be adjusted; 
         FIG. 3  shows a section according to the line III-II in  FIG. 2 ; 
         FIG. 4  shows a section through the optical element according to  FIG. 2  for illustrating the sensor system; 
         FIG. 5  shows an enlarged representation of the left-hand half of  FIG. 3  with illustration of the adjusting forces and adjusting directions; 
         FIG. 6  shows an enlarged representation of the left-hand half of  FIG. 3  with a monolithically produced removed part for the adjusting device instead of a lever mechanism; 
         FIG. 7  shows a plan view corresponding to the representation of  FIG. 2  in a different embodiment; 
         FIG. 8  shows a section through  FIG. 7  according to the line VIII-VIII; 
         FIG. 9  shows a plan view in a similar configuration to that illustrated in  FIG. 7 ; and 
         FIG. 10  shows a section in a similar configuration to that illustrated in  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a projection exposure apparatus  1  for microlithography. This serves for the exposure of structures on a substrate coated with photosensitive materials, which substrate generally predominantly includes silicon and is referred to as a wafer  2 , for the production of semiconductor components, such as e.g. computer chips. 
     In this case, the projection exposure apparatus  1  essentially includes an illumination device  3 , a device  4  for receiving and exactly positioning a mask provided with a grating-like structure, a so-called reticle  5 , via which the later structures are imaged on a reduced scale on the wafer  2 , a device  6  for the mounting, movement and exact positioning of the wafer  2 , and an imaging device, namely a projection objective  7 , having a plurality of optical elements, such as e.g. lenses and mirrors, which are mounted in an objective housing of the projection objective  7  via holders  9 . For simplification, only one optical element  20  is illustrated in  FIG. 1 . 
     After exposure has been effected, the wafer  2  is moved further step by step in the arrow direction, so that a multiplicity of individual fields, each having the structure predefined by the reticle  5 , can be exposed on the same wafer  2 . On account of the step-by-step advancing movement of the wafer  2  in the projection exposure apparatus  1 , the latter is often also referred to as a stepper. 
     The illumination device  3  provides a projection beam  11  for imaging the reticle  5  on the wafer  2 , for example light or an electromagnetic radiation. A laser, by way of example, may be used as a source of the radiation. The radiation is shaped via optical elements in the illumination device  3  such that the projection beam  11  has the desired properties with regard to diameter, polarization, shape of the wavefront and the like when it impinges on the reticle  5 . 
     An image of the reticle  5  is generated via the projection beam  11  and is transferred to the wafer  2  in correspondingly demagnified fashion by the projection objective  7 , as has already been explained above. The projection objective  7  has a multiplicity of individual refractive, diffractive and/or reflective optical elements, such as e.g. lenses, mirrors, prisms, terminating plates and the like. 
       FIG. 2  and subsequent figures reveal the mounting of an adjustable optical element, in this case a lens  20 , in the projection objective  7  together with associated adjusting devices. The mounting and adjusting device has three concentrically arranged rings, namely an outer ring  21  as outer part, which is fixedly connected to the objective housing  10  of the projection objective  7 , an inner ring  22  as inner part and an intermediate ring  23  as intermediate part. The inner ring  22  is fixedly connected to the optical element  20 . The intermediate ring  23  is mounted relative to the inner ring  22  and the outer ring  21  via guide systems. 
     As can be seen from  FIG. 2 , six guide systems are provided e.g. in a manner distributed over the circumference at a distance of 60°. Other numbers may also be provided as desired for the relative connection and guidance. Each guide system has two leaf springs  24  lying in a plane perpendicular to the optical axis, namely the x/y plane, one leaf spring  24  in each case being arranged on a circumferential side. In the present case, this therefore means on the top side and on the underside of the intermediate ring  23  given a perpendicular position of the optical element  20 . The two leaf springs  24  of each guide system are likewise connected to the inner ring  22  on the top side and the underside. The two leaf springs  24  may in each case be embodied as a concentric ring or else as individual ring segments arranged in a manner distributed over the circumference. Perpendicular to the two leaf springs  24 , each guide system has a guide element  25  formed from a plurality of individual ring segments arranged in a manner distributed over the circumference or from a plurality of individual elements, e.g. elastic pins, distributed over the circumference. As can be seen from  FIG. 3 , the longitudinal axis of the guide element  25  extends parallel to the optical axis, namely to the z-axis. 
     On account of their configurations and position, the two leaf springs  24  have a high stiffness in the x/y plane, but in contrast a low stiffness in the optical axis, and thus permit movements in the optical axis or parallel to the optical axis. 
     By contrast, the guide element  25  has a high stiffness in the z-direction and a low stiffness in the x/y plane and thus permits movements in the x/y plane. 
     Three manipulators or actuators  26  are arranged at a distance of 120° and are mounted in each case on and in the outer ring  21 . The number of actuators  26  is also to be regarded only by way of example. The actuating forces of the actuators  26  are transmitted via a lever mechanism  27  to the inner ring  22  and thus to the optical element  20 . 
     Each lever mechanism  27  has three articulation points. One articulation point  28  is connected to the actuator  26  in an articulated manner, while a second articulation point  29  is connected to the inner ring  22  in an articulated manner. A third articulation point  30 , which is likewise articulated, is situated on the intermediate ring  23  and constitutes a fixed center of rotation for the articulation points  28  and  29 . 
     For determining the adjustment of the optical element  20 , sensors  31   a ,  31   b  and  32   a ,  32   b  are provided, as can be seen from  FIG. 4 . The sensors are arranged between the inner ring  22  and the outer ring  21 , the two sensors  31   a  and  31   b  measuring relative movements of the inner ring  22  with respect to the outer ring  21  in the optical axis (z-direction) and the two sensors  32   a  and  32   b  measuring relative movements between the inner ring  22  and the outer ring  21  in the x/y plane. A wide variety of possibilities may be used as sensors. In general, capacitive or incremental displacement sensors or else sensors based on the piezoelectric effect will be used. Their method of operation is generally known, for which reason it will not be discussed in greater detail here. 
     The static weight force Fg of the optical element  20  together with the inner ring  22  is supported via spring elements  33  (see  FIG. 3  and enlarged illustration in  FIG. 5 ). Helical springs, by way of example, may be used as the spring elements  33 , a plurality of helical springs being arranged in cutouts  34  of the inner ring  22  in a manner distributed on the circumference. 
     The functioning of the adjustment of the optical element  20  is explained below with reference to  FIG. 5 . Each of the three actuators  26  distributed on the circumference generates a radial force Fa on the associated lever mechanism  27 , which force is supported via the linking of the lever mechanism  27  at the intermediate ring  23  with the force Fz at the articulation point  30 , which simultaneously serves as a pivot. The height offset Ha between the point of application of the radial force Fa at the articulation point  29  and the articulation point  30  at the intermediate ring  23  causes a moment at the lever mechanism, which is supported via a vertical force Fb at the articulation point  29 . 
     Hb represents the distance between the articulation points  29  and  30  in the plane perpendicular to the optical axis. 
     The position of the centers of rotation at the articulation points  30  of all the lever mechanisms  27  arranged in a manner distributed over the circumference is fixedly defined with respect to one another for the intermediate ring  23 . If a rotationally symmetrical arrangement is taken into account, then no parasitic forces occur and the three supporting forces Fz are equal in magnitude. With a symmetrical construction, the vertical forces Fb are likewise equal in magnitude, as a result of which all the actuator forces Fa are also equal in magnitude. 
     The weight force Fg is supported by the spring elements  33  arranged in a manner distributed on the circumference, the respective supporting force being Fg/n. In this case, n denotes the number of spring elements  33  arranged in a manner distributed on the circumference. 
     The following possibilities of adjustment are possible with the actuators  26  and the lever mechanisms  27 : 
     Case 1—Adjustment in the Z-direction (Optical Axis): 
     As a result of a uniform actuation of all three actuators  26 , the intermediate ring  23  remains in its position. All the lever mechanisms  27  pivot about their articulation point  30  as a pivot, as a result of which the inner ring  22  moves in the z-direction. 
     Case 2—Adjustment in the x/y Plane: 
     If the actuators  26  are moved in such a way that the articulation points  28  maintain their relative distance from one another, then, analogously to the displacement of all the articulation points  28 , the intermediate ring  23  with the lever mechanisms  27  and the inner ring  22  is also displaced arbitrarily in the x/y plane. For this purpose, the three sensors  32   a  and  32   b  arranged in a manner distributed on the circumference are arranged such that they can detect the movements in the x/y plane. As a result of the actuators being subjected to displacements with different magnitudes at the articulation points  28 , it is possible to effect adjustment in each case in the desired direction in the x/y plane, it merely being desirable to take care to ensure that all the articulation points  28  maintain their relative distance from one another during the displacement. For this purpose, the different angular positions with respect to the respective displacement direction have to be taken into consideration, of course, given an arrangement of the actuators at a distance of 120° from one another. The sensors  31   a  and  31   b , of which there are likewise in each case three arranged in a manner distributed on the circumference, measure the displacement in the z-direction and/or displacements in the x/y plane. 
     Via controlled operation and a combination of case  1  and case  2 , the optical element  20  can thus be displaced arbitrarily and without high outlay in the x-/y-/z-direction. 
     Step-down and step-up ratios can be initiated by way of the distances and vertical positions of the articulation points  28 ,  29  and  30  of the lever mechanism  27  with respect to one another and thus their geometrical design. This depends, in particular, on the quantities Ha and Hb of the two articulation points  29  and  30  and the distance between the articulation points  28  and  30 . 
     In principle, any drive unit which operates appropriately sensitively is possible for the actuators  26 . These are desirably self-locking, however. One possibility in this respect is e.g. a piezo-stepper drive. 
     Instead of the leaf springs  24 , articulated-joint beams arranged uniformly on the circumference are also possible in the context of the guide system. The same applies to the guide element  25 , which may be provided as a rotationally symmetrical bending or articulated-joint beam. A guide with removed sections, e.g. L-sections, is also possible. 
     A guide system together with the lever mechanism  27  as a removed part in a monolithic configuration can be seen from  FIG. 6 . For this purpose, in a manner corresponding to the number of actuators  26 , monolithic blocks  35  are provided between the inner ring  22  and the outer ring  21 , each block  35  being supported on the intermediate ring  23 . Furthermore, the block  35  is connected to the inner ring  22  and the outer ring  21 . By removing sections with corresponding extended holes at their ends, corresponding diameter reductions and thus elastic connections are achieved which replace the guide system with the leaf springs  24  and the guide element  25  and the lever mechanism  27  with the articulation points  28 ,  29  and  30 . For illustration purposes, however, the corresponding parts with the same reference symbols have been inserted into the block  35  since, after all, the block  35  in principle produces the same construction and primarily the same effect. The removed sections and the cutouts or holes may be introduced e.g. via a laser. 
       FIGS. 7 and 8  describe additional aspects. The formulated problem, as described in the exemplary embodiment above, where individual degrees of freedom can be driven actively, is in principle the same. In the exemplary embodiment above, however, it was emphasized that the tilting degrees of freedom of a z-tilting manipulator or actuator are replaced by x/y degrees of freedom. A more general form of the problem solution is possible with the exemplary embodiment described below. 
     As can be seen, an inner ring  22  and an outer ring  21  are likewise present here. However, the intermediate ring is divided into three ring segments  23   a ,  23   b  and  23   c . The outer ring  21  is once again fixedly connected to the objective housing, while the inner ring  22  carries the optical element, namely the lens  20 . 
     Guide systems and actuators may be fitted in various ways between the two rings  21 ,  22  and the ring segments  23   a ,  23   b  and  23   c . In this case, leaf springs  24   a ,  24   b  and  24   c  correspond to the leaf springs  24  of the first exemplary embodiment and constitute in the same way the adjustable connection between the inner ring  22  and the individual ring segments  23   a ,  23   b ,  23   c  of the correspondingly divided intermediate ring. The leaf springs  24   a ,  24   b ,  24   c  guide the ring segments  23   a ,  23   b ,  23   c  in the radial direction. Connecting parts  36  connect the three ring segments  23   a ,  23   b ,  23   c  to form a closed ring, which thus corresponds to the intermediate ring  23  of the first exemplary embodiment. The connecting parts  36  may optionally be embodied as rigid connections, elastic connections and also as actuating drives. 
       FIG. 9  shows a configuration wherein the connecting parts  36  are in each case provided with an actuating drive  37 . The actuating drive  37  may be embodied e.g. as a linear motor with a guide element which enables displacements in the circumferential direction. A wide variety of devices may be used as linear drives, such as e.g. mechanical, electromechanical, hydraulic or piezoelectric devices. In order to compensate for angular movements, articulated-joint connections  38  will be provided between the actuating drive  37  and the connecting parts  36 . The construction and method of operation of an actuating drive are generally known, for which reason they are not discussed in any greater detail here. 
     In the same way as in the first exemplary embodiment, here as well a lever mechanism  27  is in each case present between the inner ring  22 , the outer ring  21  and in each case one of the three ring segments  23   a ,  23   b ,  23   c . The lever mechanisms  27  can likewise be actuated by an actuator (not illustrated). Guide elements  25   a  having the same function as in the first exemplary embodiment serve in the same way for the guidance in the z-direction of the three ring segments  23   a ,  23   b ,  23   c.    
     The structural principle according to  FIGS. 7 and 8  makes it possible to construct different actuators which are in each case adapted to the desired characteristics of adjusting the objective. Individual degrees of freedom can be driven actively here or else be embodied as passive guides or even be completely inhibited. By virtue of the intermediate ring being divided into three ring segments, the possibilities of variation are significantly greater here. This is possible in particular because the three ring segments can be provided with actuators individually or else independently of one another and the number of possibilities of adjustment thus rises accordingly. 
     The arrangement of the guide systems, as described above, can also be reversed. This means that a relative movement between the outer ring  21  and the intermediate ring  23  only takes place in the z-direction, while the relative movement between the inner ring  22  and the intermediate ring  23  takes place in the x/y plane. As is evident in this respect from  FIG. 10 , in this case the leaf springs  24  are arranged between the intermediate ring  23  and the outer ring  21  and a guide element  25 ′, corresponding to the guide element  25  according to  FIG. 3 , is fixedly arranged on the intermediate ring  23  and projects into a cutout in the inner ring  22 . 
     The guide systems and drives or actuators are also interchangeable or can be combined with one another.