Patent Application
20100033696
The present invention relates to a method for producing an element having at least one arbitrarily freely formed surface (freeform surface) having a high accuracy of form and a low surface roughness, and to an apparatus for smoothing a freeform surface.
High performance illumination and projection objectives for illuminating and imaging the structures of a reticule onto a semiconductor component are used for microlithography for producing electrotechnical devices having extremely small structure sizes. In order to be able to obtain the corresponding imaging accuracies in the nanometers range, it is necessary to use optical elements such as lenses, mirrors and the like having a high accuracy of form and low surface roughnesses and to produce them accordingly. In the material processing methods available for this purpose, a distinction is drawn between shaping methods which maintain the surface roughness and those which result in a roughening of the surface during processing. Unfavorably, precisely the methods which enable high degrees of material removal, such as e.g. grinding or milling, are unfavorable with regard to the surface roughness since they lead to a severe roughening of the surface. Accordingly, after processing methods of this type, which are necessary, however, for processing the optical elements in tenable times, smoothing steps or smoothing processes are carried out, by which the roughenings caused by the shaping are eliminated again. However, since the smoothing steps usually in turn bring about an alteration of the form as a result of material removal, the form errors introduced thereby have to be corrected again in form correction steps. Accordingly, the result is a process chain comprising a sequence of form correction and smoothing steps until the desired accuracy of form and surface roughness are attained.
On account of the further increasing requirements made of the performance of the microlithography objectives, in addition to the spherical surface forms of the optical elements with spherical segment surfaces and aspherical yet rotationally symmetrical optical surfaces deviating from said spherical form, non-rotationally symmetrical optical surfaces arbitrarily freely formed, so-called freeform surfaces, are increasingly also required in order to ensure the corresponding imagings. In the case of freeform surfaces of this type, however, the problem arises that, in contrast to the spherical surfaces and aspherical, rotationally symmetrical surfaces, there are no suitable smoothing processes available which make it possible to set a surface roughness that remains constant over the entire surface in conjunction with a high accuracy of form at the corresponding surface.
DE 10 331 390 A1 discloses a method in which an optical element having two opposite spherical surfaces is pressed into an aspherical forming shell and is subsequently processed spherically. In the stress-relieved state, the processed surface relaxes from the spherical shape into an aspherical form.
An adaptation of the surface to be processed to the shaping tool is also known from JP 2000 084 838 A. Here semiconductor components having a non-planar surface are adapted to the planar shaping tool by corresponding deformation, such that the corresponding surface can be processed by the shaping tool.
It is an object of the present invention to provide a method and an apparatus which make it possible to produce elements, in particular optical elements, such as mirrors, lenses and the like, which have at least one arbitrarily freely formed surface, a so-called freeform surface, which satisfies extremely stringent requirements made of high accuracy of form and low surface roughness. In particular, elements of this type are intended to be suitable for being used as optical elements for microlithography objectives with the corresponding accuracy and roughness requirements. Furthermore, the intention is to enable the method to be applied in a simple manner and the apparatus to be produced and operated in a simple manner, and overall to enable effective production of the corresponding elements. Furthermore, it is also an object to provide corresponding optical elements and projection exposure apparatuses for microlithography comprising corresponding optical elements.
This object is achieved, according to various formulations, by a method, an apparatus, an optical element, and a projection exposure apparatus as recited e.g. in the enclosed independent claims. The dependent claims relate, inter alia, to advantageous configurations.
The inventors have recognized that the object mentioned above can be achieved by at least dividing the processing steps in two, to be precise on the one hand into a shaping step and on the other hand into a smoothing step. Furthermore, according to the invention the procedure is such that, in the smoothing step in particular, a stress or resultant elastic deformation of the element to be processed is brought about, which enables processing by means of known smoothing processes for spherical, plane or aspherical, in particular rotationally symmetrical, surfaces. It is ensured in this way that shaping methods with a high degree of material removal can be used in the shaping step, with the result that short processing times are ensured. The roughnesses possibly introduced as a result of this are then eliminated in a subsequent smoothing process, wherein the stress or deformation of the element to be processed establishes an intermediate form that makes it possible to use smoothing processes for spherical, almost spherical, plane or aspherical, rotationally symmetrical surfaces. In this case, the inventors have recognized that the deformation or stress of the element to be processed that was previously used for forming aspherical optical surfaces can be modified in a novel manner for achieving the objective mentioned above.
Unlike in the above-described method in DE 10 331 390 A1, according to the method of the present invention, firstly an arbitrarily formed workpiece is brought to a form approximated to the desired freeform surface by any shaping methods whatever, in order that the surface to be processed is subsequently brought to a spherical, almost spherical, plane or rotationally symmetrical shape by force action. This intermediate form is then smoothed in the second processing step. Accordingly, in the method according to the present invention, unlike in the known method according to DE 10 331 390 A1, the shaping is not carried out in a stressed, deformed state, but rather in particular the smoothing of a freeform surface already present.
What is essential in the present invention, therefore, is that the processing is divided into two independent, separate steps, namely on the one hand the shaping step and on the other hand the smoothing step, and that an arbitrarily freely formed, non-rotationally symmetrical surface can be fashioned in the shaping step, which surface can then be correspondingly set in terms of the surface roughness in the subsequent smoothing step. At the same time, the method according to the invention ensures that the method can be carried out in a simple manner since the element to be processed has to be clamped into a processing apparatus only during the second processing step, that is to say the smoothing step, in which apparatus actuators act on the opposite side with respect to the surface to be processed, in such a way that, from the arbitrarily freely formed surface to be processed, which is usually not rotationally symmetrical, a rotationally symmetrical or ideally spherical or almost spherical surface is produced by elastic deformation or stress, which surface can then be processed in a simple manner.
In the case of the force introduction for stressing or deforming the element to be processed, mechanically, piezoelectrically, pneumatically and/or hydraulically acting actuators can be provided with the result that both tensile and compressive forces can act on the optical element. This has the effect that the surface to be processed can both be bulged outward and pressed inward.
The force introduction or the distribution of the actuators at the opposite side with respect to the surface to be processed can be such that the entire element to be processed can be influenced by the actuators, that is to say that the actuators are distributed over the entire side opposite to the surface to be processed.
In this case, the actuators can be arranged in or at the receptacle of the processing apparatus in such a way that the actuators in a manner adjacent to one another cover the entire receptacle. In particular, the actuators can be arranged in an array in rows and columns.
The required tensile and/or compressive forces can be determined by a preceding simulation calculation from the sought target form of the surface and the surface contour set in the first processing step. For this purpose, it is merely necessary to determine in advance what intermediate form the surface to be processed is intended to assume during the second processing step, that is to say the smoothing step.
The first and second processing steps, that is to say the shaping step and the smoothing step, can be run through repeatedly one after another in the method according to the invention, wherein different material processing methods can be used in particular during the individual processing steps in each cycle in order thus to progressively approximate to the target form with the accuracy of form and surface roughness sought.
The second processing step, that is to say the smoothing of the freeform surface, can be followed by a third processing step, in which a material processing is performed which enables a slight correction of the form whilst simultaneously maintaining the surface roughness. Corresponding methods can be for example ion beam methods, ion beam figuring (IBF) or magnetorheological processing methods (MRF). Although these methods merely have only a small degree of material removal, they have the advantage that they do not lead to a roughening of the surface. Accordingly, they are suitable for slight form adaptations after smoothing or setting of the surface roughness has already been effected.
Said third processing step can be effected after stress relief of the element to be processed, that is to say cancellation of the corresponding force introduction. In particular, the third processing step can be preceded by a measuring step for measuring or assessing the surface obtained with regard to accuracy of form and surface roughness, as moreover can also be carried out during, before or after every other processing step of the method according to the invention, for example by means of interferometric measurements.
In particular, corresponding devices for monitoring and measuring the surface, which make it possible intermittently or continuously to compare the surface achieved with the target form, can be provided directly at the processing tools.
During the first processing step or steps, that is to say the shaping steps, processing methods with a relatively large degree of material removal are used in order to ensure effective production of the desired element having a freeform surface. Suitable methods here include methods which have a degree of material removal of the order of magnitude of at least 50 nm, preferably at least 100 nm, in particular at least 20 μm, per processing step. Possible methods here include grinding or milling methods, in particular five-axis milling, honing or lapping. It should be taken into consideration here that the removal values and methods used should not be regarded as absolute, but rather are to be understood in relative terms depending on the surface quality sought and the tenable outlay and the area of use and merely indicate starting points or preferred ranges. It is also evident from this that material removal per processing step can be taken to mean either the material removal when passing over the surface once, e.g. during milling, or the total removal when passing over the surface repeatedly in one process step.
During the first processing step, the element to be processed can be either in a stressed state or in a stress-relieved state.
In the second processing steps, that is to say the smoothing methods, material processing methods with a small degree of material removal of the order of magnitude of at most 1 μm, in particular at most 100 nm, per processing step are used, which enable in particular a low surface roughness of the order of magnitude of ≦1 nm, preferably ≦0.1 nm RMS roughness. The statements made regarding the removal values in the first processing step are applicable here in the same way.
In particular, the method involves using material processing methods which make it possible to produce a freeform surface having an accuracy of form in which the root mean square deviation from the target form is ≦10 nm, preferably ≦1 nm. At the same time, the surface roughness is intended to be values of the RMS roughness, in which the root mean square deviations from a central line are employed, of ≦1 nm or ≦0.1 nm. The measuring method used can include all measuring methods known to the person skilled in the art for determining surface forms and roughnesses, in particular standard methods defined in corresponding DIN or ISO standards, preferably interferometric measurements.
The method according to the invention can be used in such a way that, as early as in the design of the corresponding element, that is to say for example of an optical element for a microlithography objective, the corresponding processing steps, that is to say in particular the stress or deformation during the second processing step, are concomitantly taken into account as boundary conditions.
The present invention correspondingly comprises an apparatus for carrying out the method, wherein the apparatus has at least one receptacle for mounting the element to be processed and a smoothing tool for the smoothing processing of the freeform surface. According to the invention, at least one actuator for exerting a tensile and/or compressive force on the element to be processed is provided in the receptacle, such that the element to be processed can be elastically stressed into an intermediate form. The freeform surface to be smoothed can correspondingly be processed by means of a smoothing tool for spherical, plane or aspherical surfaces, which is readily possible with smoothing tools that are correspondingly present, wherein the surface to be processed is provided by the intermediate form.
The present invention likewise relates to an optical element processed or produced by the method according to the invention or the apparatus according to the invention.
Such an optical element is distinguished by an accuracy of form of the freeform surface within the range of ≦10 nm root mean square (RMS) and/or a surface roughness within the range of ≦1 nm RMS roughness.
Preferably, the accuracy of form can be ≦1 nm root mean square, and the surface roughness can have ≦0.1 nm RMS roughness.
Since the freeform surface which is processed according to the invention and has corresponding accuracy of form and roughness values is preferably an optically active surface, the method according to the invention or the corresponding apparatus or the optical elements produced thereby are particularly suitable for use in projection exposure apparatuses for microlithography. Consequently, this invention likewise also relates to a corresponding projection exposure apparatus and in particular a projection objective or an illumination system of a projection exposure apparatus comprising a corresponding optical element in particular for EUV microlithography with light or generally electromagnetic radiation in the range of the extreme ultraviolet wavelength range.
Further advantages, characteristics and features of the present invention will become clear from the following detailed description of an exemplary embodiment with reference to the accompanying drawings, in which, in a purely schematic manner here
The semifinished product for an optical lens 1 as shown in
The spherical surface 2, which corresponds to a spherical segment, is intended to have an arbitrarily freely formed surface instead of the spherical form, in which case the deviations from the spherical surface 2 can be realized by arbitrary elevations or depressions or recesses. Although the deviations from the spherical form can be very small, they are reproduced in an excessively exaggerated manner in order to illustrate the method principle in the schematic illustrations that follow.
Since the arbitrarily formed surface 2′ of the optical element 1, on account of its non-rotationally symmetrical form, cannot be smoothed with the sufficient quality by customary smoothing processes, in a second processing step the optical element 1 is accommodated in a clamping frame 5 of a corresponding processing apparatus. The clamping frame 5 has a multiplicity of actuators 6, which are illustrated as spring elements for applying tensile and/or compressive stresses in
In the case of the spherical surface 2″, provision is made for example of a full shell tool 7 for a full shell smoothing process, in which the smoothing tool 7 bears on the spherical surface 2″ over the whole area and can perform a uniform smoothing of the surface.
After the second processing step, that is to say the smoothing process, has been carried out, the surface 2″ can be examined with regard to its surface quality by a measuring device 8, which can for example be arranged on the processing tool 7 or be integrated in the latter. In this way it is possible to ascertain continuously or by short interruptions during the smoothing process whether the required surface roughnesses have already been set.
After the conclusion of the second processing step, that is to say of the smoothing process, a renewed pass through the two processing steps can be effected if for example the measuring device 8 has ascertained that although the surface roughness has been set in a corresponding manner, a small form error is still present. On the one hand, therefore, a further shaping first processing step for producing an intermediate product in accordance with
If it is ascertained after one or a plurality of second processing steps that both the accuracy of form and the surface roughness lie within the range sought, then the optical element 1 is removed from the clamping frame 5 after the last second processing step, with the result that the optical element 1 can relax. In this case, as is shown in
If the surface 2′″ still has deviations with regard to the desired target form, then it is possible, finally, to carry out a third processing step with a material processing method that achieves a degree of material removal while at the same time maintaining the surface roughness set. In this respect, ion beam methods, ion beam figuring (IBF), or magnetorheological methods, such as MRF (magnetorheological finishing), are examples. This is illustrated by the schematic tool 9 in
The surface 2′″ can in turn be examined or measured by a corresponding measuring apparatus 10, similar to the measuring apparatus 8 after or during the second processing step, during or after the third processing step, in order to ascertain whether the desired form has been set. This can, of course, also be carried out after the first or during the first processing step for shaping. Interferometric methods, in particular, can be used here as measuring methods.
If, however, it is ascertained after the second processing step for example that the surface 2′″ in the stress-relieved state corresponds to the target stipulations, then a third processing step can be dispensed with.
Although the present invention has been described in detail on the basis of the exemplary embodiment illustrated, it is clearly evident to the person skilled in the art that the invention is not just restricted to an embodiment of this type. Rather, modifications and changes, in particular with regard to omitting individual features presented or combining the features presented in a different manner, are conceivable without departing from the overall invention sought to be covered by the appended claims.
The invention is therefore distinguished in particular by the following non-restrictive features:
The above description of the preferred embodiments has been given by way of example. From the disclosure given, those skilled in the art will not only understand the present invention and its attendant advantages, but will also find apparent various changes and modifications to the structures and methods disclosed. It is sought, therefore, to cover all such changes and modifications as fall within the spirit and scope of the invention, as defined by the enclosed claims, and equivalents thereof.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102007013563.9 | Mar 2007 | DE | national |
This is a Continuation of International Application PCT/EP2008/053398, with an international filing date of Mar. 20, 2008, which was published under PCT Article 21(2) in German, and the complete disclosure of which, including amendments, is incorporated into this application by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/EP2008/053398 | Mar 2008 | US |
| Child | 12563766 | US |
