OPTICAL DEVICE FOR GENERATING A LINE FOCUS ON A SURJACE

BACKGROUND OF THE INVENTION

The present invention relates to an optical system for generating a line focus on a surface from a light beam having a cross-section with orthogonally arranged long and short axes.

The invention further relates to a scanning system for producing a scanning line focus on a surface from a light beam having a cross-section with orthogonally arranged long and short axes.

Still further, the present invention relates to a method for laser processing of a substrate, using a scanning system for creating a scanning line focus on said substrate from a light beam having a cross-section with orthogonally arranged long and short axes.

The present invention is useful, for example, in annealing of large substrates, in the field of light (e.g. laser) induced crystallization of substrates, in the field of flat panel display, such as liquid crystal display (LCD) (for example: thin film transistor displays (TFT) etc.) or luminescence display (anorganic or organic light emitting diode (LED, OLED), electroluminescence (EL)) manufacturing processes. Furthermore, the present invention may be used for the fabrication of thin film photovoltaic devices.

In particular, the present invention is useful in order to subsequently crystallize amorphous Silicon (a-Si) films forming polycrystalline Silicon (p-Si). Polycrystalline Silicon thin films are widely used in microelectronics and display techniques as mentioned above. P-Si has a higher charge carrier mobility as compared to a-Si which is useful for the fabrication of higher speed switching or integration of higher quality driver electronics on the display substrate. Furthermore, p-Si has a lower absorption coefficient for light in the visual

spectral range enabling p-Si to be used as a rear electrode for LCD-applications allowing backlight to be transmitted. Lastly, the defect density of p-Si is lower as compared with a- Si which is a prerequisite for the fabrication of high efficient solar cells.

The conversion of a-Si into p-Si may be employed by heat treatment at around 1000 ⁰ C. Such a procedure may only be used for a-Si on heat resistant substrates such as quartz. Such materials are expensive compared to normal float glass for display purposes. Light induced crystallization of a-Si allows the formation of p-Si from a-Si without destroying the substrate by the thermal load during crystallization. Amorphous Silicon may be deposited by a low cost process such as sputtering or chemical vapour deposition (CVD) on substrates such as glass, quartz or synthetics. The crystallization procedures are well known as excimer laser crystallization (ELC), sequential lateral solidification (SLS) or thin beam crystallization procedure (TDX). The modern TDX-procedure delivers improved crystal quality at highest charge carrier mobility. This procedure uses a very thin line beam with a width of 5 to 10 μm and a length of more than 30 cm.

Optical systems according to the prior art, which create a line focus or a line beam, as it is for example used in silicon annealing of large substrates, use refractive optical systems comprising cylindrical lenses as imaging and focusing optical elements. In particular, US 5,721,416 discloses an optical device for generating a sharp illuminating line on an illuminating plane from a high-power laser beam. The sharp illuminating line includes long and short axes. The optical device comprises an anamorphic setup of imaging and homogenizing optical systems for the separate imaging and homogenizing of the laser beam in the directions of the long and short axes. For imaging and homogenizing the laser beam in the direction of the short axes, a slit is illuminated homogeneously and the slit is imaged on the illumination plane by reducing optics.

According to the US 5,721,416 the reducing optics consist of two lenses.

Despite an arrangement according to the US 5,721,416 seems to fulfil all requirements in order to fabricate high quality poly crystalline Silicon (p-Si) thin films by laser- crystalhzation of amorphous Silicon (a-Si) films on a substrate, inhomogeneous grain growth has been detected This undesired grain growth has been found to result from inherent deficits of the line focus using an arrangement as disclosed in US 5,721,416

Therefore, it is a need for an optical system for creating a line focus which ovei comes the afore-mentioned drawback

SUMMARY OF THE INVENTION

It is the object of the present invention to provide an optical device of the kind specified initially which permits creating illumination lines having a high aspect ratio (length divided by width of the line) and a greater usable depth of field with improved performance

It is another object of the present invention to provide a scanning system for producing a scanning line focus on a surface from a light beam having a cross-section with orthogonally arranged long and short axes which delivers a scanning line focus with improved performance

It is still another object of the present invention to provide a method for laser processing of a substrate, using a scanning system for creating a scanning line focus on said substrate from a light beam having a cross-section with orthogonally arranged long and short axes which may deliver high quality polycrystalline Silicon films when laser annealing an amorphous Silicon film deposited on a substrate

Measurements show that the main reasons for the insufficient crystal quality of laser crystallized p-Si are the variance of the focus along the line beam in depth and of the projection quality In other words ¹ The line focus lies in some parts of the line beam in

front of the surface of the substrate and in some parts below the surface of the substrate. Using a system similar to that shown in US 5,721,416 the focus has been measured to vary along the long axis with the depth of the substrate by more than ± 10 μm per meter length of the line beam. It has been found that this variance results from residual surface figure errors from the optical elements of the imaging optical device.

Therefore, an optical device for generating a line focus on an illumination surface from a light beam having a cross-section with orthogonally arranged long and short axes according to the invention devised to meet the above object is provided. The optical device comprises an imaging optical device for imaging said light beam having extensions in transverse first and second directions (the long and short axes) transverse to the propagation direction of the light beam on said illumination surface, wherein said imaging optical device having a locally varying line focus, i.e. a variance of the locations of the foci with respect to the propagation direction of the light beam. This means, the foci are in some areas in front of the illumination surface, in some areas behind the illumination surface and only selectively the focus is on the illumination surface as will in general be desired. Therefore, the present invention provides a line focus correction device for locally correcting said line focus such that said focus of said line beam follows the contour as desired. This means that the line focus correction device according to the present invention corrects the variance of the focus with depth as far as possible. Variances of below 1 μm per illuminated area are possible.

The line focus correction device in principle may be part of said imaging optical device itself. Nevertheless, the line focus correction device is formed by a separate component having no additional imaging function, e.g. an optical component being inserted additionally or an optical component having no optical function such as a window. The reason is that the shape of a separate line focus correction device may be amended quite easily while the shape of an existing imaging component having a quite low aberration may hardly be corrected.

In a first modification of the invention said line focus correction device is located close to the surface, i.e. the image plane. The optical sensitivity of the line focus correction device is lower as compared to an arrangement of said line focus correction device within or close to the imaging device itself.

In a preferred modification when said imaging optical device has a locally varying line focus in particular along said long axis (and preferably no or negligible variance of the focus along the short axis), said line focus correction device corrects said line focus along said long axis. In the case where the variance of the line focus does not vary significantly along the short axis, said line focus correction device only corrects said line focus along said long axis.

According to a further preferred embodiment said illumination surface is a plane, said line focus correction device corrects said line focus such that said line focus is located on said plane. Such an embodiment may be used for laser crystallization of thin amorphous Silicon films deposited on a substrate, wherein the a-Si film has a thickness of around between 50 to 100 nm. The line focus may be corrected to lie on the surface of said a-Si film. Alternatively, the line focus may also be adjusted to be located in a predetermined distance below said surface. It has to be noted that the thicker the amorphous film used the deeper the locations of the foci within the bulk of the a-Si film may be without significantly heating the substrate during the annealing procedure.

In a preferred modification of this embodiment said light beam is emitted from a high power laser providing optical power of more than 150 Watts, preferably providing optical power of more than 250 Watts, delivering an extension of the beam in the short axis direction of around 5-10 μm and an extension of the beam in the long axis direction of 730 mm or more. As high power lasers preferably excimer lasers are used which preferably emit radiation in the spectral range between 130 nm and 390 nm. Instead of excimer lasers also pulsed CO ₂ -lasers, diode lasers, solid state lasers or frequency doubled solid state lasers may be used.

In a still preferred modification of the optical device according to the invention, said line focus correction device is a coplanar glass plate being locally abrasively treated or deformed forming a surface contour such that said line focus correction device correcting said line focus as being predetermined. Such a focus correction device may be fabricated after having determined the actual location of the focus in the illuminated area of an optical device without introduction of said line focus correction device. Sometimes the designer of said line focus correction device will know from dimensioning of the optical system which aberrations (e.g. bow tie errors, coma, spherical aberrations) are existent. He therefore will be able to calculate the shape of said line focus correction device without having measured said actual local variation of said focus line.

Instead of an optical coplanar glass plate also a lens (e.g. having no or one planar surface) or as the case may be a prism may be used. For the fabrication of a lens or a prism forming said line focus correction device preferably a transparent material is used. Such transparent materials may be from the group of glasses, semiconductors or crystals, such as float glass, quartz, Silicon or CaF ₂ .

A preferred modification of the optical device described at last consists in that said (e.g. coplanar) glass plate or said lens or said prism is treated by ion beam figuring. Such a procedure allows the abrasion of highly precise contours into such a glass plate, a lens or a prism. Alternatively, laser-treatment, lapping or polishing, such as robotics polishing or magneto-rheological polishing, may also be used for the fabrication of highly precise surfaces.

Instead of local abrasion of material in order to amend the contour of said line focus correction device in said predetermined manner treatments adding material such as evaporation, sputtering or chemical vapour deposition of (optical) material are possible. The deposing material may pass masks or photo resist masks or the material may be grown locally.

Instead of local abrasion of material or the deposition of material also deformation of the optical line focus correction device resulting in the predetermined surface contour of the device is possible Such a (in general mechanical) deformation may be irreversible Nevertheless, it is also possible to deform the surface contour of said line focus correction device in a reversible manner Actuators may be used in order to give said line focus correction device said contour as desired Such actuators may be piezoelectric crystals, mechanical actuators, electrostπctive actuators, electromechanical actuators, electro magnetic actuators or pneumatic actuators

Such actuators may in particular serve for adjusting the surface contour of said line focus correction device continuously m order to eliminate time dependent faults such as thermal drift, long term drift, ageing of the optical elements as well as ageing and altering of the coating characteristics of the optical elements The actuators may also serve for the correction of said line focus correction device in response of amendments of the substrate itself, e g in response of substrate bending as well as different heights or slopes of said surface of said substrate.

A further possibility of conditioning said line focus correction device consists in locally amending the refractive force of the optical element forming said line focus correction device In particular, the desired refractive index characteristics may be performed by ion beam treatment, by ionizing radiation beam treatment or by chemical treatment

In a preferred modification of the embodiment as being described above, said (coplanar) glass plate or said lens or said prism is abrasively treated forming a plurality of lenses Some of the lenses may be of concave, some of the lenses may be of convex type Some of the lenses may be spherical, some of the lenses may be aspheπcal There may be one or more cylindrical lenses Some of the lenses or all lenses may be arranged separately or adjacent to each other

Assuming a very narrow beam line, i.e. an illumination line with a high aspect ratio (small extension along the short axis direction and wide extension along the long axis direction) it is preferred that at least two of said plurality of lenses are arranged adjacent to each other along said long axis. Most preferably, a one dimensional sequence of adjacent lenses are arranged in the direction of said long axis.

A further preferred embodiment of the optical device according to the invention may comprise a plurality of cylindrical lenses. The lenses may preferably be curved in the direction of the short axis. The cylindrical lenses may preferably be arranged adjacent to each other along said long axis in the form of a line of cylindrical lenses. One or more lines of cylindrical lenses may be arranged in the short axis direction forming an array of lenses.

According to the description above, the plurality of lenses arranged adjacent to each other are formed by surface treatment of one single glass plate, one lens (with e.g. one planar surface and one, e.g. cylindrical Iy, bent surface) or one prism. Still another preferred modification bases on more than one single glass plates, lenses or prisms. As the case may be, said more than one single glass plates, lenses or prisms itself may form one or more of said plurality of lenses cited above.

Instead of one or more refracting elements forming said line focus correcting device also one or more reflecting elements may form said line focus correcting device. Said reflecting element or said reflecting elements may consist of or comprise a metal. Materials having low thermal expansion coefficients may be preferred such as or low thermal expansion coefficients glasses, glassceramics, silicon or quartz. One may be a material which is sold under the trademark "Zerodur".

A preferred embodiment of the invention therefore may comprise a plurality of mirrors being arranged adjacent to each other along said long axis. Similar to an optical device comprising a focus correcting device with refracting elements at least one of said mirrors may be a spherical mirror or an aspherical mirror. In particular, at least one of said mirrors

may be a cylindrical mirror. Similar to an embodiment described above said plurality of mirrors may form an array.

In an exceptionally preferred embodiment of the invention said line focus correction device comprises at least two sub line focus correction devices. Each of said sub line focus correction devices is able to independently correct said line focus in a predetermined manner. Each of them may be constructed similar to a line focus correction device as described above. All or some of said sub line focus correction devices may be operatively connected in order to correct said line focus together. Different aberrations of the imaging device may be corrected by different sub line focus correction devices or groups of sub line focus correction devices.

Some of said sub line focus correction devices may also preferably be replaceable among each other. The replacement may be performed by displacement or rotation of the focus correction device comprising said at least two sub line focus correction devices or by displacement or rotation of one or more of said sub focus correction devices. Such a replacement allows the correction of said line focus for different optical arrangements of said imaging device, for example optical arrangements having different geometries, positions, or shapes. Furthermore, redundant sub line focus correction devices may help to increase operating life time of the system, in particular of said focus correction device according to the invention.

In still a preferred modification of the optical device according to the invention, said imaging optical device comprises a reducing optical device which images said light beam with a reduced ratio on said illumination surface. Said reducing optical device may comprise one or more lenses, for example cylinder lenses, as e.g. described in US 5,721,416. Instead of cylindrical lenses also one or more mirrors may be used. The usage of mirrors has the advantage that the so-called bow tie error, which limits the width of the line focus, may be reduced. Furthermore, using reflecting imaging and/or reducing optical

devices the aberrations are in general less severe as compared to an optical device using refracting imaging and/or reducing optical devices.

Still another preferred modification of the optical device according to the invention comprises a field limiting element such as for example the slit disclosed in US 5,721,416. Instead of a slit also a single sided field stop having a predetermined shape and being arranged in the propagation direction of the laser beam may be used. Said reducing optical device images said homogenized beam of said field limiting optical element in reduced form in the direction of said short axis while said homogenized beam may be directed through said field limiting element and/or the imaging/reducing system without any modification in the long axis direction.

It is a further modification when said reducing optical device is telecentric at the side facing the image. An optical system is telecentric if, for each image point, it provides a principal ray parallel to the optical axis. In the case of an optical system which is telecentric at the image side the exit pupil is located at an infinite distance.

According to another aspect of the invention, a scanning system for producing a scanning line focus on an illumination surface from a light beam is provided, wherein said light beam propagates in a propagation direction and said light beam has an extension in a first dimension transverse to the propagation direction and an extension in a second direction transverse to said first dimension and said propagation direction (said first dimension and said second direction are in general named as long and short axis), said scanning system comprises an imaging optical device for imaging said light beam on said illumination surface, wherein said imaging optical device having residual surface figure errors resulting in a line focus locally differing from said illumination surface, and a line focus correction device for locally correcting said line focus in a predetermined manner.

According to another aspect of the invention, a method for laser processing of a substrate, using a scanning system for creating a scanning line focus on said substrate from a light

beam having a cross-section with orthogonally arranged long and short axes, is provided, wherein said scanning system comprises an imaging optical device for imaging said light beam on said illumination surface, wherein said imaging optical device having residual surface figure errors resulting in a line focus locally differing from said surface, and a line focus correction device for locally correcting said line focus in a predetermined manner

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described further, by manner of example, with refeience to the accompanying drawings, in which ¹

FIG. 1 are diagrammatic presentations of an optical device according to the invention for producing a sharp illuminating line, FIG. Ia illustrating the beam path for generating the so-called short beam axis and FIG Ib the beam path for generating the long beam axis of the illuminating line;

FIG 2 is a perspective view of a line focus correction device in the form of a coplanar glass plate according to the invention;

FIG. 3 is a top view of the coplanar glass plate according to FIG. 2;

FIG. 4 is a cut along A-A of the coplanar glass plate according to FIGs 2 and 3,

FIG. 5 is a cut along B-B of the coplanar glass plate according to FIGs 2 and 3;

FIG 6 the location of the focus on a panel of an illuminating line according to the prior art. FIG. 6a shows a perspective view of the line focus on the panel, FIG. 6b is a cut through of the location of the focus on the panel along the long beam axis,

FIG. 7 is a perspective view of a line focus correction device in the form of a lens according to the invention;

FIG. 8 is a perspective view of a line focus correction device in the form of a mirror glass plate according to the invention;

DETAILED DESCRIPTION OF THE INVENTION

The optical arrangement 2 illustrated in FIG. 1 comprises an excimer laser 10 which emits a pulsed beam 12 in per se known manner. The dimensions of the excimer laser beam 12 emitted, for instance, may be 15 times 40 mm. The excimer laser 10 may preferably emit radiation in the spectral range between 130 nm and 390 nm. Instead of an excimer laser also a pulsed CO ₂ -laser, a diode laser, a solid state laser or a frequency doubled solid state laser may be used.

This laser beam is to be processed by means of the optical device 2 which will be specified below to yield an illuminating line 14.

The beam 12 emitted by the excimer laser 10 first passes an attenuator (not shown), if desired, and then enters a set of anamorphic cylindrical lenses 16. The optical system according to FIGs. Ia and Ib, on the whole, is an anamorphic system in the sense that the processing of the laser beam 12 in the directions of the long and short axes, respectively, takes place largely independently. An example for such an anamorphic system and in particular a typical set of anamorphic cylindrical lenses 16 may be seen in US 5,721,416.

To obtain the so-called short axis of the illuminating line (i.e. to reduce the beam diameter to the width of the illuminating line of, typically 5 to 10 μm), first of all, as shown in FIG. Ia, a field limiting element 18 is illuminated with the laser beam 20 generated by the set of cylindrical lenses 16. This field limiting element 18 may be illuminated homogeneously in

efficient manner (i.e. making the best possible use of the radiation energy supplied by the laser 10). Nevertheless, the field limiting element 18 may also be illuminated deviating from a homogeneous manner dependent on the processing needs.

In the further course of forming the short axis A ₃ of the illuminating line 14 the beam 22 which exits said field limiting element 18 is imaged on a substrate 28 in the illumination plane 30 by means of a transmissive system comprising two cylindrical projection lenses 24, 26. This optical system may be a reducing optics so that a reduced image of the opening of said field limiting element 18 is obtained, e.g. at a ratio of 10:1. Optical systems of this kind permit edge definitions down to a few μm to be achieved, depending on the optical resolution (NA) of the reducing optics. A sharp edge definition is accompanied by an excellent depth of field. Thus an edge definition of 3μm and a depth of field of typically +/- 20 μm are obtainable for an NA of illumination of 0.1 on the image side.

As already explained in the foregoing an optical arrangement 2 as shown in FIGs. Ia and Ib may be used for laser annealing of thin semiconductor films being deposited on said substrate 28. In particular, said arrangement 2 may be used in order to convert an amorphous Silicon layer into a polycrystalline thin film.

In some cases, during laser annealing the substrate 28 has to be kept in a vacuum chamber. The laser beam 32 emitted from the reducing optics 24, 26 in this case is conducted through a window 34 of said vacuum chamber (not shown).

It is inevitable that the optical elements 16, 24, 26, 34 of the arrangement 2 according to FIGs. Ia and Ib do not behave like ideal optical elements as desired. This leads to optical surface figure errors of the whole arrangement 2. These surface figure errors result in a line focus 14 locally deviating from a predetermined course or shape. In particular, the line focus 14 will partly be in front of the surface 30 of the substrate 28 and in some parts the line focus 14 will be inside the substrate 28 as e.g. illustrated in FIGs. 6a and 6b, which show a perspective view of the location of the focus 14 on a panel 28 of an illuminating

line according to the prior art and a cut through of this location 14 along the long beam axis A|, respectively.

The invention now introduces a line focus correction device for locally correcting said line focus 14 in a predetermined manner. In the arrangement 2 shown in FIG. 1 the window 34 is used as a line focus correction device which is used for the correction of the surface figure errors of the optical elements 16, 18, 24, 26 of said optical arrangement 2 described above.

According to the invention one surface 36 of said window 34, which in principle consists of a coplanar glass plate 34, is locally abrasively treated forming a plurality of lenslets 38a, 38b, ...38i, ...38z (cylindrical segments) being arranged adjacent to each other along the long axis A|. These lenslets 38a, 38b, ...38i, ...38z, which may be of concave type as may e.g. been demonstrated by the lenslet (cylindrical segment) of FIG. 4 or of convex type as e.g. the segment shown in FIG. 5. Said surface 36 or specifically said lenslets 38a, 38b,... 38i,... 38z locally correct the line focus 14 along the long axis Ai to follow a predetermined course or shape in the long axis Ai direction. This predetermined course or shape may be the surface 30 of said substrate 28 (as is indicated with the position "0" or the straight line indicated with reference number 40 in FIGs. 6a and 6b, respectively). In some cases the desired focus line can also be positioned within the substrate 28 (depth axis value below "0" in FIG. 6b) or in front of said substrate 28 (depth axis value above "0" in FIG. 6b). It is obvious for a person skilled in the art that the focus line, as desired, does not necessarily need to follow a straight line as for facility reasons has been shown in FIGs. 6a and 6b. Also a curved focus line may be predetermined dependent on the specific application the line beam is used for.

Furthermore, instead of using the top surface 36 of the window 34 as a surface figure error correction surface also the bottom surface 42 may be used. Instead of using the window 34 as a focus line correction device also a separate optical device may be used. In particular, another lens or another mirror being additionally introduced as an optical element or being

already part of said optical arrangement 2, such as e.g. lens 26 or lens 24 or one of the lenses 16, may be used as said line focus correction device.

FIG. 7 shows another embodiment of a line focus correction device in the form of a lens 44 consisting of a plurality of lenslets 46a, 46b, .. 46j used for the above described anamorphic imaging of said incoming laser beam 12 and comprising a plurality of cylindrical segments 38a, 38b, 38c, ... 38z arranged adjacent to each other in the long axis A| direction.

FIG. 8 shows another embodiment of a line focus correction device in the form of a curved mirror 48 comprising a plurality of cylindrical segments 38a, 38b, 38c, ... 38z arranged adjacent to each other in the long axis Ai direction.

Preferably planar or nearly planar surfaces (e.g. a surface of a mirror or a lens having a comparably large radius) of optical devices are used in order to form said correcting surfaces. Abrasion of such surfaces is quite easy from a fabrication point of view. Furthermore, such surfaces are arranged closer to the focus line to be generated as compared with other surfaces.

A line focus correction device may be fabricated using the following steps:

Assuming that only a focus shift and herewith a focus position correction along the optical axis shall be applied, in a first step the focus position error is to be predetermined on the surface 36 of the window 34. Subsequently cylindrical segments as explained above are applied onto said surface 36 in order to correct the error. In a first case where the distance of the focus of one point along the focus line is to be enlarged a concave cylindrical lenslet is to be applied. In a second case, where the focal distance of a point along the focus line is to be reduced a convex cylindrical lenslet is to be applied. The radius of the cylindrical segment determines the absolute value of the correction. By applying of segments of aspherical cylindrical lenses not only the position of the focus line and therefore its linearity maybe influenced but also the sharpness of the imaging.

Not only partially or locally false surface contours (e.g. shapes of optical elements deviating locally or in segments or parts from a predetermined ideal shape) of the optical system may be corrected but also systematic deviations from a predetermined shape. Such a systematic deviation may be for example the so called bow tie error, namely a variation of the focal distance towards the edge of the field in the long axis Ai direction. Another systematical error may be aspheric aberration or coma of the projection system. Such a correction may be done without having measured the error in advance. This is possible, when the influence of the error is determined by the geometry of the imaging system itself. Instead of cylindrical segments a continuously varying function of the cylindrical radii and aspherical constants may be used.

The application of the correction function on the correction surface may be applied with all kinds of abrasive or adding methods such as ion beam figuring (IBF), robotics polishing or magneto-rheological polishing (MRF), evaporation using specific masks or by evaporation of areas masked with a photo resist and subsequent lift-off. Furthermore, one may think of locally amending the refractive index of a transparent material for example by illuminating said material with ultraviolet light or by bombardment of said material with charged particles or by local chemical treatment of said material.

Not the complete correction may be applied to one surface area only. For example, it may be possible that one side, e.g. the front side surface, of a planar window carries the correction of a systematic error, such as for example the bow tie error, and the other side, e.g. the back side surface, of the window carries the correction of local deficiencies of the optical system. It is also possible to have correction surfaces applied to different optical elements.

The correction may also be caused by mechanical or electrical treatment of the material of the optical elements such as by mechanical leverage, piezoelectric actuators or electromechanical, electrostrictive or pneumatic actuators. Furthermore, the correction may vary in dependence of time and in dependence of other parameters in order for example to

compensate ageing of optical elements or coatings or in order to compensate the thermal drift. Furthermore the mechanism may be used in order to adjust the optical system with respect to deficiencies and singularities of the substrate to be processed. For example curvature or corrugation and different heights or substrate positions may be corrected.

Furthermore, on the surface of a larger optical element, such as a window, additional corrections for different applications may be implemented. The respective areas of the correction elements to be hit by the laser beam may be amended. Different substrate geometries may be addressed. Additionally, identical correction surfaces may be implemented separately in order to produce redundance. Such redundant correction devices may be introduced into the optical path in a case where contamination or thermal load puts a risk on the correction device just used.

Transparent optical elements may be used in order to fabricate a correction device according to the invention. Instead of a transparent optical element such as a window, a lens or a prism, also reflective elements, such as a mirror, may be introduced. A transparent line focus correction device may carry an anti reflection coating. A reflective line focus correction device may carry a reflective coating.

Quartz, all kinds of glasses, crystals, such as calcium fluoride or silicon or germanium may be used for the fabrication of transparent elements, in particular in the infrared spectral range. Reflective elements may consist of Quartz, all kinds of glasses but also materials with very low expansion coefficients such as ULE or Zerodur, crystals (CaF ₂ ), semiconductors (SiGe) or metals may be used. The useable spectral range may include the far infrared range as well as x-rays. Excimer lasers, solid-state lasers, diode-laser, gas lasers etc. may be used.