Abstract:
Disclosed is a method of modifying of a surface of a substrate of a photolithographic mask for extreme ultraviolet radiation comprising the step of focusing femtosecond light pulses of a laser system onto the substrate so that a plurality of color centers is generated inside the substrate, wherein the color centers are distributed to cause a modification of the substrate surface.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Pursuant to 35 U.S.C. §119, this application claims the benefit of prior U.S. provisional application 61/324,467, filed Apr. 15, 2010. The contents of the prior application is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to the field of modifying a substrate surface of a photolithographic mask. 
     BACKGROUND 
     As a result of the constantly increasing integration density in the semiconductor industry, photolithographic masks have to project smaller and smaller structures. In order to fulfill this demand, the exposure wavelength of photolithographic masks has been shifted from the near ultraviolet across the mean ultraviolet into the far ultraviolet region of the electromagnetic spectrum. Presently, a wavelength of 193 nm is typically used for the exposure of the photoresist on wafers. As a consequence, the manufacturing of photolithographic masks with increasing resolution is becoming more and more complex, and thus more and more expensive as well. In order to use significantly smaller wavelength lithography system for the extreme ultraviolet (EUV) wavelength range (approximately 13.5 nm) are presently in development. 
     Photolithographic masks have to fulfill highest demands with respect to transmission, planarity, pureness and temperature stability. In particular, the surface of reflective masks for EUV radiation coated with the reflective structure has to be plane within the range of about 1 nm in order to avoid aberrations of the desired structure in the photoresist of the wafer. These challenges also apply for other EUV reflective optical elements e.g. mirrors used in the beam path of EUV lithography (EUVL) systems. 
     Well known methods exist for the final precision polishing of the optical surfaces (J. S. Taylor and R. Soufti: “Specification, fabrication, testing, and mounting of EUVL optical substrates”, in EUV Lithography, SPIE Press Monograph, Vol. PM178, Ed.: Vivek Bakshi, 2008, p. 702). Further, for correcting aberration errors based on material removal well known methods are small tool polishing (J. S. Taylor, M. Piscotty, and A. Lindquist, Eds., “Fabrication and testing of aspheres, trends in optics and photonics (TOPS)”, Vol. XXIV, Optical Society of America, Washington D.C. (1999) and R. A. Jones, Ed., “Selected papers on computer controlled optical surfacing”, Vol. MS40, SPIE Press, Bellingham, Wash. (1991), ion beam figuring (F. Frost, R. Fechner, B. Ziberi, D. Flamm, and A. Schindler, “Large area smoothing of optical surfaces by low-energy ion beams”, Thin Solid Films 459, p. 100-105 (2004) and L. N. Allen and R. E. Keim, “An ion figuring system for large optics fabrication”, Proc. SPIE 1168, p. 33-50 (1989)) and plasma-assisted surface etching (S. J. Hoskins, “Aspheric surface figuring of fused silica plasma assisted chemical etching”, SPIE Vol. 2542, Optomechanical and Precision Instrument Design, p. 220-230 (1995) and the U.S. Pat. No. 6,858,537 B2). 
     The U.S. Pat. No. 6,844,272 B2 describes drawbacks of the methods mentioned above. 
     Several patents disclose various methods for controllable deformation of surface of solid material trying to overcome drawbacks mentioned above, which are described hereinafter. 
     The above mentioned U.S. Pat. No. 6,844,272 B2 discloses a method and apparatus for figure error correction on optical or other precision surfaces by changing the local density of material in a zone at or near the surface. 
     The U.S. Pat. No. 6,844,272 B2 gives the graph of the Mo/Si bilayer thickness dependence on the annealing temperature as an example for the implementation of the disclosed method. Excimer laser radiation may be applied for localized energy deposition into the predetermined region. Unfortunately, the invention does not disclose the way of laser radiation implementation, such as pulse width, pulse energy, focusing conditions, etc. 
     The very similar idea for repairing defects in a multilayer coating layered onto a reticle blank used in an EUVL system is described in United States patent with U.S. Pat. No. 6,821,682 B2. 
     The feasibility of the idea is illustrated by heating the Mo/Si multilayer with electron beam. However, electron beam implementation needs high vacuum conditions and has low throughput. 
     The idea of using laser radiation for aberration corrections of optical elements installed in the system is disclosed in U.S. Pat. No. 7,352,452 B2. This document is based on the known effect of compaction of some optical materials (for example—fused silica) under UV radiation (cf. e.g. U.S. Pat. No. 6,205,818 B1). The drawback of the U.S. Pat. No. 7,352,452 B2 is that some optical materials used in EUVL (for example, ZERODUR®) are not transparent for UV radiation, so the method can not be used for aberration corrections of reflective optical elements made of those materials. Another drawback of the invention is low throughput. 
     The US 2007/0224522 A1 describes a method for flattening a concave or convex substrate of an EUV substrate of a photolithographic mask by generating expanded portions at the corresponding positions of the substrate using a ultrashort pulse laser system. The US 2008/0032206 A1 discloses both the formation of a plurality of expansion stress generation portions and contraction stress generation portions again using an ultrashort pulse laser system. The expansion stress generation portions are generated using pulse durations in the range of 1 μs, whereas the contraction stress generation portions are formulated using laser pulses in the picosecond range. Both, expansion stress generation portions and contraction stress generation portions induce binding modification of the quartz lattice. However, this document does not disclose which processes occur in the stress generation portions. Therefore, it is not clear whether the expansion and contraction stress generation portions are temporally stable or whether the stress generation portions may damage the lattice. 
     The method of controllable bending of the surface of solid material based on the focusing of femtosecond laser pulses on the surface is described by P. Bechtold and M. Schmidt, “Non-thermal micro adjustment using ultrashort laser pulses”, JLMN-Journal of Laser Micro/Nanoengineering, Vol. 2, No. 3 p. 183-188 (2007). This paper considers two options. One option includes the ablation of pre-stressed coatings from a substrate, thus releasing the stress and producing the bending. Another option comprises micro-shockwaves inducing into the material with high-energy ultrashort laser pulses. After rapid transformation of the shock wave to the wave of compression they induce near-surface plastic deformation which results in bending of the surface. 
     Another approach for the modification of transparent dielectric materials includes focusing of ultrashort femtosecond laser pulses inside the material. Focusing the beam keeps its intensity below the damage threshold at the surface, but concentrates it enough at the focal point inside the material to cause multiphoton/avalanche ionization and structural changes of the material. In this case, near IR (infrared) (typically at about 800 nm or 1.06 μm) focused radiation is used to produce modified fields inside transparent material. 
     During last years this approach was used for three-dimensional data storage (E. N. Glezer, M. Milosavljevic, L. Huang, R. J. Finlay, T. H. Her, J. P. Gallan, and E. Mazur, “Three-dimensional optical storage inside transparent materials”, Opt. Lett., Vol. 21, No. 24, p. 2023-2025 (1996), direct writing waveguides in transparent media (M. Ams, G. D. Marshall, P. Dekker, M. Dubov, V. M. Mezentsev, I. Bennion, and M. J. Withford, “Investigation of ultrashort laser-phonic material interaction: Challenges for directly written glass photonics”, IEEE J. of selected topics in quantum electronics, Vol. 14, No. 5, September/October 2008, p. 1370-1379), waveguide couplers writing (A. M. Streltsov and N. F. Borrelli, “Fabrication and analysis of a directional coupler written in glass by nanojoule femtosecond laser pulses”, Opt. Lett., Vol. 26, No. 1, p. 42-43 (2001)), and nanogratings producing (Y. Shimotsuma, P. G. Kazansky, J. R. Qiu, and K. Hirao, “Self-organized nanogratings in glass irradiated by ultrashort light pulses”, Phys. Rev. Lett., Vol. 91, No. 24, p. 24705-1-247405-4, (2003), Y Shimotsuma et al., “Nano-modification inside transparent materials by femtosecond pulse laser”, Mod. Phys. Lett. B, Vol. 19, No. 5, p. 225-238, (2005), and R. S. Taylor, C. Hnatovsky, E. Simova, P. P. Rajeev, D. M. Rayner, and P. B. Corkum, “Femtosecond laser erasing and rewriting of self-organized planar nanocracks in fused silica glass”, Opt. Lett. B, Vol. 32, No. 19, p. 2888-2890, (2007)). 
     Non-linear photoionization of glasses leads also to the creation of laser-induced color centers. Color centers formation in fused silica and corresponding absorption spectra from DUV (deep ultraviolet) to near IR range are described by L. Skuja, H. Hosono, M. Hirano, “Laser-induced color centers in silica”, Proc. SPIE, Vol. 4347, p. 155-167 (2001). DUV attenuation was observed in the fields of modified fused silica, which were generated by ultrashort laser pulses at fluence below the threshold of laser-induced breakdown (S. Oshemkov, V. Dmitriev, E. Zait, and G. Ben-Zvi, “DUV attenuating structures in fused silica induced by ultrafast laser radiation”, Proc. CLEOE-IQEC, Munich 2007). 
     Summarizing the discussion of this section, there are some development efforts to understand the processes of non-linear photoionization in transparent dielectrics. However, presently there is no reliable and well understood method of using electromagnetic radiation for “polishing” optical surfaces in a controlled manner. 
     It is therefore one object of the present invention to provide a method and an apparatus for modifying a surface of a substrate of an EUV photolithographic mask that at least partially avoids the above-mentioned disadvantages. 
     SUMMARY 
     According to a first aspecta method of modifying a surface of a substrate of a photolithographic mask for extreme ultraviolet radiation is disclosed herein. The method comprises the step of focusing femtosecond light pulses of a laser system onto the substrate so that a plurality of color centers is generated inside the substrate, wherein the color centers are distributed to cause a modification of the substrate surface. 
     The inventors have found that in a non-linear photoionization process using excitation conditions which avoid optical breakdown of the substrate simultaneously with the formation of color centers a change of the height of the substrate surface on a nanometer scale occurs. Further, the inventors have found that there exists a strong correlation between the amount of absorption caused by generated color centers and the height of the substrate surface change. 
     This phenomenon can be physically explained as follows: The formulation of color centers in transparent materials as e.g. in the substrate of EUV photolithographic masks is connected with structural changes of the substrate, which leads to a localized densification or compaction of the substrate in the irradiated area. In this area, stress appears. This stress causes a modification of the height of the substrate surface. 
     The method disclosed herein has several advantages. The light pulses of a laser beam can be focused on a focal point having a spot diameter of essentially 1 μm. Consequently, the spatial resolution of the method of claim  1  is also in this range. As the femtosecond light pulses are extremely short, the method of claim  1  achieves a high processing throughput, which results in scanning an area of approximately 1 cm 2  per minute. In contrast to methods using an electron beam or an ion beam, the method disclosed herein does not need high vacuum and clean room facilities. 
     A benefit of the defined method is to enable measurements of the induced surface deformations in real time. This is achieved by investigating the distribution of the generated color centers by measuring their absorption in simple transmission loss experiments. Therefore, the method disclosed herein can be used for fast corrections of substrates for reflective optical elements for waviness errors or flatness imperfections of the substrate in a broad range of spatial scales. 
     The method can be used to modify both surfaces of the substrate of a photolithographic mask, the front surface substrate and/or the rear surface substrate. It is a further advantage of the defined method that both the modifications of the front and the rear substrate surface can be performed by focusing the femtosecond light pulses of a laser system through only one of the substrate surfaces. Presently, the femtosecond light pulses focused into the substrate preferably enter through the rear substrate surface in order to modify the front substrate surface of the photolithographic mask. 
     According to a further aspect, one object of the present application is solved by a method for compaction of a boundary layer of a substrate of a photolithographic mask for extreme ultraviolet radiation comprising the step of focusing femtosecond light pulses in the boundary layer from a rear substrate surface opposite to a front substrate surface so that a plurality of color centers is generated in the boundary layer, wherein the substrate comprises a front substrate surface with a multi-layer mirror structure. 
     It is already known that DUV (deep ultraviolet) radiation induces radiation damages in the photolithographic mask leading of a compaction of the mask (cf. U.S. Pat. No. 6,844,272 B2, col. 5, 1. 27-28). This problem will be even more serious for photolithographic masks for the EUV wavelength range. At the same time, the tolerances which do not induce aberrations of the photolithographic mask will decrease. The above method for compaction of a boundary layer can avoid this problem. The layer of the substrate of the photolithographic mask which is hit by EUV radiation is already compacted prior to the application of the mask, so that the EUV radiation can not introduce uncontrollable aberrations due to small deviations of the mask dimensions from predetermined dimensions. 
     Further, it should be noted that the method of compaction of a boundary layer adjacent to a substrate surface layer and the method of modifying a substrate surface can be combined. For example, the imperfections of the substrate surface adjacent to the boundary layer are removed in a first step and then the boundary layer is compacted so that EUV radiation cannot introduce imperfections during the operation. 
     According to another aspect, one object of the present application is solved by a method for correcting a substrate surface of a photolithographic mask for extreme ultraviolet radiation, comprising the steps of analyzing the substrate surface, focusing light pulses of a laser system onto positions in the substrate where the substrate height is below a predetermined substrate height so that energy exceeding a threshold of optical breakdown is deposited at these positions, and focusing femtosecond light pulses of the laser system onto positions in the substrate where the substrate height exceeds the predetermined substrate height so that a plurality of color centers are generated at these positions. 
     In the optical breakdown the substrate material is locally melted and microcracks are always formed due to high local stress inducing local damage in the substrate of the photolithographic mask. In contrast at the color center generation no microcracks or local damage of the substrate occurs. The combination between both modes depends on the substrate material, the pulse duration, the repetition range and the details of the focusing conditions. 
     The above defined method combines the variation of the height of a substrate surface in both directions i.e. increasing and decreasing of the height. After having determined the imperfections of a photolithographic mask, it can be decided on an individual basis of each mask how to apply the defined method of the two method steps. 
     In a further aspect disclosed herein for modification of at least a portion of a substrate surface of a photolithographic mask for extreme ultraviolet radiation is provided, comprising at least one light source for generating light pulses of variable pulse duration, repetition rate and energy, at least one objective for focusing light pulses of a light beam, and at least one scanning unit for scanning the light beam across the substrate surface, wherein the pulse duration, the repetition rate, the energy and/or the focusing are selected such that color centers are generated in the portion of the substrate. 
     The apparatus defined above comprises essentially conventional components. Furthermore, in contrast to electron or ion beam systems, it can operate at atmospheric conditions. Consequently, it may be operated cost-efficiently. Moreover, it has a high throughput as it uses extremely short and high repetition rate light pulses. Furthermore, the loading times of electron and ion beam systems are avoided. 
     In still another aspect, one object of the present application is solved by a photolithographic mask for extreme ultraviolet radiation, comprising at least one substrate having at least one multi-layer structure and an absorbing structure on a first side, and at least one transparent conductive coating on a second side of the substrate opposite to the first side. 
     The substrates of EUV photolithographic masks as supplied by the manufacturers may have deviations of its flatness which are already much higher than can be the tolerated for EUV photolithographic masks in order to avoid aberrations. The manufacturing process of the photolithographic mask may impair this situation. In order to have a yield required for an economical process, the front substrate surface has to be flat within a range of essentially 1 nanometer in order to avoid aberrations generated by the mask. The rear substrate surface of photolithographic masks is typically coated with a metal coating, typically with a chromium layer. Consequently, the methods defined above cannot be applied for the corrections of flatness imperfections of the front substrate surface and/or the multi-layer mirror system at the end of the manufacturing process since the light pulses can neither penetrate the metal coating nor the multi-layer mirror structure. Replacing the metal coating at the rear substrate surface by a transparent conductive coating for the wavelength of the processing laser changes the situation. The methods defined above can now be applied to correct imperfections of the front substrate surface and/or the multi-layer mirror system of EUV photolithographic masks. 
     According to a further aspect a mirror for extreme ultraviolet radiation is provided, comprising at least one substrate having at least one multi-layer structure on a first side, and at least one transparent conductive coating on a second side of the substrate opposite to the first side. 
     Finally, in yet another aspect, the application concerns an apparatus for modification of at least a portion of a substrate surface of a photolithographic mask for extreme ultraviolet radiation, comprising means for generating light pulses of variable pulse duration, repetition rate and energy, means for directing and focusing light pulses of a light beam, and means for scanning the light beam across the substrate surface, wherein the pulse duration, the repetition rate, the energy and the focusing are selected such that color centers are generated in the portion of the substrate. 
     Further aspects, embodiments, and advantages follow. 
    
    
     
       DESCRIPTION OF DRAWINGS 
       In order to better understand the present disclosure and to appreciate its practical applications, the following Figures are provided and referenced hereafter. It should be noted that the Figures are given as examples only and in no way limit the scope of the invention, which are set forth in the claims. 
         FIG. 1  shows in cross-section a schematic view of an EUV photolithographic mask; 
         FIG. 2  depicts a schematic representation of a substrate of an EUV photolithographic mask having a transparent conductive coating at the rear substrate surface and an uneven front substrate surface with a single layer of the multi-layer system of  FIG. 1 ; 
         FIG. 3  schematically represents a block diagram of an apparatus for the modification of a substrate surface of an EUV photolithographic mask; 
         FIG. 4  schematically depicts a block diagram of an apparatus for determining the absorption of a substrate of an EUV photolithographic mask; 
         FIG. 5  schematically shows the processes leading to the generation of a color center in a quartz substrate; 
         FIG. 6  presents the dependence of the transmittance at a wavelength of 214 nm of a volume of the fused silica substrate scanned with the apparatus of  FIG. 3  on the scanning pitch at a pulse energy of 1.5 μJ; 
         FIG. 7  shows the dependence of the transmittance at a wavelength of 214 nm of a volume of the fused silica substrate on the pulse energy at a pitch of 0.5 μm; 
         FIG. 8  presents the dependence of a surface deformation on the attenuation of a volume of the fused silica substrate at a distance of 450 μm from the substrate surface; 
         FIG. 9  represents the dependence of a surface deformation on the attenuation of a volume of the fused silica substrate at a distance of 3.7 mm from the substrate surface; 
         FIG. 10  schematically shows the substrate of  FIG. 2  after removal of the flatness imperfections; 
         FIG. 11  schematically depicts a substrate of an EUV photolithographic mask with imperfections of the front substrate surface; 
         FIG. 12  schematically depicts a substrate of an EUV photolithographic mask with a boundary layer below the front substrate surface, a single MoSi layer and a transparent conductive coating on the rear substrate surface; 
         FIG. 13  schematically represents a substrate of an EUV photolithographic mask having a bump and a depression of the front substrate surface with respect to a predetermined height of the substrate; 
         FIG. 14  schematically shows the substrate of the EUV photolithographic mask of  FIG. 13  after correction of the front substrate surface using the optical breakdown to remove the depression and using the generation of color centers to remove the bump of the front substrate surface of  FIG. 13 ; 
         FIG. 15  schematically represent a substrate of an EUV photolithographic mask whose rear substrate surface has been corrected using the optical breakdown to remove the depression and using the generation of color centers to remove the bump of the rear substrate surface; 
         FIG. 16  schematically represents the EUV photolithographic mask without the absorbing structure of  FIG. 1 , but with a plan front substrate surface and imperfections of the upper MoSi layers of the multi-layer system of  FIG. 1 ; and 
         FIG. 17  schematically shows the EUV photolithographic mask of  FIG. 16  after correction of the upper MoSi layers by straining the front substrate surface using the methods of  FIG. 14 . 
     
    
    
     DETAILED DESCRIPTION 
     In the following, the present invention will now be described more fully hereinafter with reference to the accompanying figures, in which exemplary embodiments of the invention are illustrated. However, the present invention may be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and will convey the scope of the invention to persons skilled in the art. 
       FIG. 1  shows a schematic cross-sectional view of a photolithographic mask  100  for an exposure wavelength of 13.5 nm. Different from presently applied photolithographic masks, the mask  100  is a reflective optical element based on a multi-layer mirror structure. The multi-layer mirror system of photolithographic mask  100  is deposited on a front substrate surface  115  of a suitable substrate  110 , such as fused silica substrate. Other transparent dielectrics, glass materials or semiconducting materials may also be applied as substrates for photolithographic masks as for example ZERODUR®, ULE® or CLEARCERAM®. 
     The multi-layer mirror system comprises  40  pairs of alternating molybdenum (Mo)  130  and silicon (Si) layers  140  (referred to in the following as MoSi layers). The thickness of each Mo layer  130  is 4.15 nm and that of the Si layer  140  amounts to 2.80 nm. In order to protect the multi-layer structure, a capping layer  150  of silicon with a native oxide of 7 nm depth is arranged on top of the structure. In the multi-layer mirror system, the Mo layers  130  act as scattering layers, whereas the silicon layers function as separation layers. For the scattering layers instead of Mo other elements with a high Z number may utilized, such as cobalt (Co), nickel (Ni), tungsten (W), rhenium (Re) and iridium (Ir). 
     The multi-layer structure on the substrate  110  acts a mirror for XUV electromagnetic radiation. In order to become a photolithographic mask  100 , a buffer structure  160  and an absorbing structure  170  are additionally deposited on the capping layer  150 . The buffer layer  160  may be deposited to protect the multi-layer mirror structure during processing, for example etching or repairing of the absorbing structure  170 . Possible buffer structure materials are for example of fused silica (SiO 2 ), silicon-oxygen-nitride (SiON), ruthenium (Ru), chromium (Cr), and/or chromium nitride (CrN). The absorbing structure  170  comprises a material having a large absorption constant for photons in the XUV wavelength range. Examples of these materials are chromium (Cr) and/or tantalum nitride (CrN). A thickness of about 50 nm is sufficient to absorb essentially all XUV photons  180  incident on the absorbing structure  170 . In contrast, the majority of the photons  180  incident on the capping layer  150  is reflected as photons  190 . In this context as well as on further positions of this description the term “essentially” means a numeric value of a quantity within its measurement limit. 
     The substrate has typical lateral dimensions of 152 mm×152 mm and a thickness or height of essentially 6.35 mm. The rear surface  125  of the substrate  110  or the rear substrate surface  125  has a thin metallic coating  120 . Typically this coating  120  comprises chromium. The metallic coating  120  is used to fix the photolithographic mask  100  at the EUV scanner by the application of electrostatic forces. 
     In the inventive methods described in the following energy from light pulses of a light beam is locally deposited in the substrate  110  of the photolithographic mask  100 . However, the light pulses of the light beam can neither penetrate the front substrate surface  115 , as they are absorbed by the multi-layer mirror structure, nor the rear substrate surface  125 , as they are also absorbed by the metallic coating  120  on the rear substrate surface  125 . 
     It is therefore one aspect of the inventive concept to replace the metallic coating  120  on the rear substrate surface  125  of the photolithographic mask  100  by a transparent conductive coating  222 , as it is shown in  FIG. 2 . Such a transparent conductive coating  222  may for example comprise indium tin oxide (ITO). Alternative materials for transparent conductive coatings are for example fluorine tin oxide (FTO) and/or aluminum zinc oxide (AZO) and/or antimony tin oxide (ATO). These materials can easily be applied to the rear substrate surface  125  of a fused silica substrate  110  and have a conductivity which is high enough to fix the photolithographic mask  100  to the EUV scanner. The transparent conductive coating  222  enables to irradiate the completely manufactured photolithographic mask  100  with light pulses of a laser beam through the rear substrate surface  125 . 
     An EUV mirror may have the structure of the photolithographic mask  100  unless it does not have the capping structure  160  and the absorbing structure  170 . Therefore, also for an EUV mirror the metallic coating may  120  be replaced by a transparent conductive coating  222 , so that the light pulses are able to enter into the substrate from the rear substrate surface. 
       FIG. 2  shows a substrate  210  of a photolithographic mask  200  where the front substrate surface  215  of the substrate  210  is not flat but has two bumps. A single MoSi layer is arranged on the front substrate surface  215  which reproduces the imperfections of the front substrate surface  215 . The further 39 Mo Si layers and the absorbing structure of  FIG. 1  have been removed. The further coating of the remaining MoSi layers will typically also reproduce imperfections of the front substrate surface  215 . In rare lucky cases the coating of the remaining 39 MoSi layers will smooth the imperfections of the front substrate surface  215 , so that the upper layer shows less imperfections than the MoSi layers close to the front substrate surface  215  of the substrate  210  of the photolithographic mask  200 . In contrast, there is a much higher probability that coating of the remaining MoSi layers will aggravate the imperfections of the front substrate surface  215 . 
     When fixing the photolithographic mask  200  at an EUV scanner the irregularities of the front substrate surface  215  lead to aberrations compared with the essentially ideal photolithography mask  100 . The aberrations caused by the uneven front substrate surface  215  can neither be removed by varying the position of the wafer nor by changing the reproduction scale. As already discussed, deviations from the flatness of the front substrate surface  215  in the range of 1 nanometer are already sufficient to induce aberrations. In contrast to  FIG. 1 , the photolithographic mask  200  of  FIG. 2  has on its rear substrate surface  225  a transparent conductive coating  222  instead of a metal coating  120 . 
       FIG. 3  depicts a schematic block diagram of an apparatus  300  which can be used to correct flatness imperfections of the front substrate surface  215  of the photolithographic mask  200 . The apparatus  300  comprises a sample holder  320  which may be movable in three dimensions. The movement of the sample holder  320  in two dimensions in the plane of the sample holder  320  is indicated in  FIG. 3  by crossed arrows. The photolithographic mask  310  may be fixed to the sample holder  320  by using various techniques as for example clamping. The photolithographic mask  310  may be the photolithographic mask  200  mounted upside down, so that its rear substrate surface  225  is directed towards the objective  350 . 
     The apparatus  300  includes a pulse laser source  330  which produces a beam or light beam  335  of pulses or light pulses. The laser source  330  generates light pulses of variable duration. The pulse duration may be as low as 10 fs but may also be continuously increased up to 100 ps. The pulse energy of the light pulses generated by the pulsed laser source  330  can also be adjusted across a huge range reaching from 0.01 μJ per pulse up to 10 mJ per pulse. Further, the repetition rate of the light pulses comprises the range from 1 Hz to 100 MHz. In a preferred embodiment the light pulses may be generated by a Ti:Sapphire laser operating at a wavelength of 800 nm. However, the methods described in the following are not limited to this laser type, principally all laser types may be used having a photon energy which is smaller than the band gap to the substrate of the photolithographic mask  310  and which are able to generate pulses with durations in the femtosecond range. Therefore, for example Nd-YAG laser or dye laser systems may also be applied. 
     The steering mirror  340  directs the pulsed laser beam  335  into the focusing objective  350 . The objective  350  focuses the pulsed laser beam  335  through the rear substrate surface into the substrate of the photolithographic mask  310 . The NA (numerical aperture) of the applied objectives depends on the predetermined spot size of the focal point and the position of the focal point within the substrate of the photolithographic mask  310  relative to the rear substrate surface. The NA of the objective  350  may be up to 0.9 which results in a focal point spot diameter of essentially 1 μm and a maximum intensity of essentially 10 20  W/cm 2 . 
     The apparatus  300  also includes a controller  370  and a computer  375  which manage the translations of the two-axis positioning stage of the sample holder  320  in the plane of (x and y directions). The controller  370  and the computer  375  also control the translation of the objective  350  perpendicular to the plane of the sample holder  320  (z direction) via the one-axis positioning stage  355  to which the objective  350  is fixed. It should be noted that in other embodiments of the apparatus  300  the sample holder  320  may be equipped with a three-axis positioning system in order to move the photolithographic mask  310  to the target location  360  and the objective  350  may be fixed, or the sample holder  320  may be fixed and the objective may be moveable in three dimensions. Although not economical, it is also conceivable to equip both the objective  350  and the sample holder  320  with three-axis positioning systems. It should be noted that a manual positioning stages can also be used for the movement of the photolithographic mask  310  to the target location  360  of the pulsed laser beam  335  in x, y and z directions and/or the objective may have manual positioning stages for a movement in three dimensions. 
     The computer  375  may be a microprocessor, a general purpose processor, a special purpose processor, a CPU (central processing unit), a GPU (graphic processing unit) or the like. It may be arranged in the controller  370 , or may be a separate unit such as a PC (personal computer), a workstation, etc. The computer  375  may further comprise I/O (input/output) units like a keyboard, a touchpad, a mouse, a video/graphic display, a printer, etc. In addition, the computer  375  may also comprise a volatile and/or a non-volatile memory. The computer  375  may be realized in hardware, software, firmware or any combination thereof. Moreover, the computer  375  may control the laser source  330  (not indicated in  FIG. 3 ). 
     Further, the apparatus  300  may also provide a viewing system including a CCD (charge-coupled device) camera  365  which receives light from an illumination source arranged to the sample holder  320  via the dichroic mirror  345 . The viewing system facilitates navigation of the photolithographic mask  310  to the target position  360 . Further, the viewing system may also be used to observe the formation of a modified area on the rear substrate surface of the photolithographic mask  310  by the pulse laser beam  335  of the light source  330 . 
       FIG. 4  shows an apparatus  400  used for measuring the transmittance of the substrate of the photolithographic mask  410 . The photolithographic mask  410  may again be the photolithographic mask  200  of  FIG. 2  mounted upside down on the sample holder  420 . The probe source  430  may be any light source from which a wavelength range suitable for the detection of transmittance changes in the substrate of the photolithographic mask  410  can be filtered out. In the embodiment of apparatus  400  a Deuterium lamp is used. It should be noted that the wavelength of the probe light source  430  depends on the material used as substrate for the photolithographic mask  410 . The objective  440  directs the light beam of the probe light source  430  onto the substrate of the photolithographic mask  410 . The beam of the probe light source  430  may be CW (continuous wave) or may be pulsed. The photodetector  450  can be of any type suitable to detect the radiation of the probe light source  430 . The apparatus  400  uses a silicon photodiode as photodetector  450 . 
       FIG. 4  shows schematically in the upper part a configuration of the apparatus  400  in which the probe light source  430  radiates its light through the rear substrate surface of the photolithographic mask  410  similar than pulsed laser source  330 . The multi-layer mirror structure on the front substrate surface of the photolithographic mask  410  reflects the beam of the probe light source  430  onto the photodetector  450 . This arrangement of the probe system is the presently preferred configuration to measure the absorption of the color centers generated by the pulsed laser source  330 . The lower part of  FIG. 4  presents an alternative configuration to determine the transmittance change or the attenuation of the light beam of the probe light source  430  induced by the generated color centers. In this configuration the probe light source  430  radiates its light in the plane of the sample holder  420  into the substrate of the photolithographic mask  410 . 
     Color centers generated by the light source  330  of the apparatus  300  absorb photons from the light beam of the probe light source  430 . This absorption can be detected by a reduction in the transmittance (attenuation) of the light beam of the probe light source  430  at the photodetector  450  compared to an area or volume having no color centers. The focal point of the light beam of the probe light source  430  should be as small as possible since this quantity limits the spatial resolution of the transmittance change within the substrate of the photolithographic mask  410 . 
     The sample holder  420  is again movable in the plane of the sample holder  420  as indicated by the crossed arrows using a two-axis position state. The optical elements  430 ,  440  and  450  or at least the probe light source  430  and the objective  440  may be moveable in a direction perpendicular to the plane of the sample holder  420  (not shown in  FIG. 4 ). Alternatively, the optical elements may be fixed and the sample holder  420  may comprise a three-axis positioning stage. In a further embodiment, the sample holder  420  may be fixed and the optical elements  430 ,  440  and  450  may be moveable in three directions. 
     Instead of using the transmittance change to detect the distribution of the color centers generated by the pulsed laser source  330  of the apparatus  300 , the emission of the excited color centers could also or in addition to the detection of the transmittance change be used to identify the distribution of the generated color centers. For these alternatives a second photodetector may be used which may be arranged in the same plane as the photodetector  450  but may have an angle of for example 90° to the photodetector  450  (not shown in  FIG. 4 ) Alternatively, the photodetector  450  may be rotated by for example 90° without using a second photodetector (also not shown in  FIG. 4 ). In a further embodiment, the second photodetector and/or the photodetector  450  may be arranged outside of the plane of the sample holder  420 . 
     The optical elements  430 ,  440  and  450  may also be controlled by the controller  370  and/or the computer  375 . Further, the apparatus  300  and the apparatus  400  may be combined in a single apparatus. The combined configuration has the advantage that it is not necessary to adjust the photolithographic mask several times to the sample holder. 
     The apparatus  300  of  FIG. 3  is used to locally generate color centers in the substrate  210  of the photolithographic mask  200  of  FIG. 2 .  FIG. 5  schematically illustrates the essential processes at the generation of color centers using the pulsed laser source  330  of the apparatus  300  at the example of fused silica. The bandgap energy  520  of fused silica between the valence band  510  and the conduction band  530  is about 9 eV. The wavelength of the laser source  330  is essentially 800 nm which corresponds to about 1.5 eV. This means that a single photon of the pulsed laser source  330  can not bridge the energy gap between the valence band  510  and the conduction band  530 . Still much less a single photon of the pulsed laser source  330  can break up a bond between a silicon atom and an oxygen atom forming the structure of fused silica of the substrate  210  of the photolithographic mask  200 . 
     The intensity in the focal point of the laser beam is however high enough so that non-linear photoionization processes may occur. One of several non-linear processes is multiphoton ionization, which occurs when a single electron in the valence band  510  of fused silica simultaneously absorbs several photons and thus bridging the gap  520  between the valence band  510  and the conduction band  530  of fused silica. 
     Another possible option includes self-focusing of the beam and spectral broadening of the high intensity laser pulse (white light generation) which leads to the increase of the single photon energy and to the increase the efficiency of the multiphoton ionization process (O. M. Efimov, K. Gabel, S. V. Gernov, L. B. Glebev, S. Granthman,. Richardson, and M. J. Soioileau, “Color center generation in silicate glasses exposed in infrared femtosecond pulses”, J. Opt. Soc. Am., Vol. 15, p. 193-199 (1998)). 
     In the focal point of an intense light pulse a multitude of electrons are excited from the valence band  510  to the conductor band  530  of fused silica. The excited electrons in the conduction band  530  are also subjected the strong electric field of the laser pulses. The combined interaction of the electrons in the conduction band  530  and the intense electric field of the light pulses can break the bonding of an oxygen atom to one of the silicon atoms. If this occurs, two color centers  540 ,  550  are generated. In the E′ center  540  a hole is trapped in an oxygen vacancy. In the non-bridging oxygen hole center (NBOHC)  550  an oxygen atom has an excessive electron. The excessive electron at the oxygen atom gives raise to strong absorption bands in the UV (ultraviolet) and in the visible wavelength range. The inventive principles of the present application use these absorption bands in order to identify or to detect the generated color centers. Preferably, the absorption band in the visible wavelength range is used as there is a variety of probe light sources and photodetectors in this wavelength range. 
     In the presence of hydrogen and/or chlorine both color centers  540  and  550  decay by forming a bonding with these elements. The time scale of this decay depends on the concentration of these and/or other impurities, on the temperature, and the illumination conditions in particular the DUV (deep ultraviolet) radiation. At room temperature, the time constant is in the range of ten days. It should be noted that in contrast to conditions of optical breakdown the generation of color centers does not lead to microcracks in the substrate of photolithographic masks. 
     It should further be noted that the generation of the color centers occurs below the threshold of breakdown which results in a local mechanical damage of the substrate material. 
     As explained above, the pulsed laser source  330  generates color centers  540 ,  550  in the substrate  210  of the photolithographic mask  200  mounted on the sample holder  320  of the apparatus  300 . By scanning the pulsed laser source  330  across the rear substrate surface  225  of the substrate  200  and by changing the focal point relative to the rear substrate surface  225  color centers may be generated with a predetermined three-dimensional distribution at any position within the substrate  200 . At these scans the pitch or the distance between two different strikes of laser pulses on the rear substrate surface  225  of the photolithographic mask  200  mounted upside down on the sample holder  320  of the apparatus  300  can be varied.  FIG. 6  presents a curve showing the dependence of the transmittance of fused silica at the wavelength of 214 nm as a function of the pitch determined with the apparatus  400  of  FIG. 4 . The pulse energy is 1.5 μJ and is identical for all scans. The pulse width or pulse duration is 200 fs at a repetition rate of 100 kHz. This curve shows a liner relationship between the transmittance and the pitch. The curve of  FIG. 6  indicates that the larger the distance between the strikes of subsequent laser pulses the less color centers are within the beam path of the probe light source  430 , the less the transmitted light is attenuated, since less color centers absorb photons from the beam of the probe light source  430 . At very small pitches the transmittance tends to saturate. 
     Apart from the pitch, the energy of the light pulses of the pulsed laser source  330  is also an important parameter.  FIG. 7  presents the variation of the transmittance as a function of the pulse energy. The pitch is always 0.5 μm. As in  FIG. 6 , the pulse duration is 200 fs and the repetition rate amounts to 100 kHz. The curve of  FIG. 7  shows that the variation of the transmittance increases as a function of the pulse energy. This indicates that the higher the pulse energy of the pulsed laser source  330  the more color centers are generated which absorb photons from the probe light source  430  during the transmittance measurement.  FIG. 7  also indicates that there is a threshold for the generation of color centers; in the example of  FIG. 7  no color centers are generated below a pulse energy of 0.3 μJ. 
     As already discussed above in the context of  FIG. 5 , the generation of the color centers  540  and  550  results in the formation of an oxygen vacancy. This oxygen vacancy induces stress in the substrate  210  of the photolithographic mask  200  around the vacancy.  FIG. 8  shows the deformations of the front substrate surface  215  of the photolithographic mask  200  as a function of the color center density generated at a depth of 450 μm below the front substrate surface  215  of the substrate  210 . The distribution of the color centers are generated with the pulsed laser source  330 . The generated color center distribution is probed with the probe light source  430  by measuring the attenuation of the probe light beam in the substrate  210 . The attenuation corresponds to the change in the transmittance (1−transmittance). 
       FIG. 8  depicts a linear relationship between the generated color center density and the surface deformation. The stress induced by the femtosecond light pulses of the pulsed laser source  330  gives rise to a reduction of the height of the rear substrate surface  225  of about 20 nm at an attenuation of about 8% of the light beam of the probe light source  430 . A transmittance change or attenuation of the probe light source of about 3% results in a surface height reduction of the front substrate surface  215  of the substrate  210  of approximately 5 nm. This shows that the stress introduced by the generation of color centers results in an increase of the density (compaction) of fused silica around the color centers. 
       FIG. 9  reproduces the experimental result of  FIG. 8  with the difference that the femtosecond light pulses mm generate the color center distribution in a depth of 3.7 mm below the front substrate surface  215  of the substrate  210  of the photolithographic mask  200 . The measured data in  FIG. 9  indicate again a linear relationship between the color density distribution and the surface height reduction of the front substrate surface  215  of the substrate  210 . 
     The comparison of the results of  FIGS. 8 and 9  reveals that a color center distribution having a larger distance to the rear substrate surface  225  induces less surface deformation than a color center distribution generated closer to the rear substrate surface  225 . Or in other words, the highest surface modification results from a color center distribution as close as possible to the respective substrate surface. 
     The front substrate surface  215  of the substrate  210  carries the multi-layer mirror structure of manufactured the photolithographic mask  200 . At the correction of the front substrate surface  215  by focusing the femtosecond light pulses through the rear substrate surface  225 , the focal point of the femtosecond light pulses has to be adjusted relative the front substrate surface  215  in such a way so that no damage of the multi-layer mirror structure on the front substrate surface  215  will occur. 
       FIG. 10  schematically shows the correction of the flatness imperfections of the front substrate surface  215  of photolithographic mask  200 . The photolithographic mask  1000  corresponds to the photolithographic mask  200  prior to the application of one of the inventive principles. The rear substrate surface  1025  has a conductive transparent coating  1022 . In the substrate  1010  of the photolithographic mask  1000  color centers are generated by scanning focused femtosecond light pulses of the pulsed laser source  330  in planes of various distance from the front substrate surface  1015  across the regions having the bumps, so that the distribution of the generated color centers correlates to the local deviation of the front substrate surface  1015  from the predetermined the front substrate surface  1015 . The color center distribution of  FIG. 10  is formed using pulses which have a duration of 200 fs and a repetition rate of 100 kHz. The pulse energy is 1.5 μJ at a focal point diameter of 2.5 μm and a pitch of 0.2 μm. The minimum distance of the focal point of the femtosecond light pulses from the front substrate surface  1015  is 400 μm and the distance between the different planes of generated color centers is 100 μm. As indicated in  FIG. 10 , the generated color center distribution essentially removes the flatness imperfections of the front substrate surface  215  of the photolithographic mask  200 . 
       FIG. 11  schematically depicts that the generation of a distribution of color centers by the pulsed laser source  330  may also be used to correct imperfections of the front substrate surface  1125  by focusing the light pulses into the substrate  1110  close to the front substrate surface  1125 . After the generation of a corresponding distribution of color centers the bumps of the front surface substrate  1125  disappear. As already discussed in the context of  FIGS. 3 and 4 , the generation of the color center distribution may be carried out in a number of steps controlled by the detection of changes in the transmittance of the probe light source  430  within the substrate  1110  by the photodetector  450 , i.e. in a closed feedback loop. 
       FIG. 12  illustrates a further aspect of the present invention. A small fraction of the EUV radiation reflected from the multi-layer mirror structure (MoSi layers) may reach the front substrate surface  1215  of the photolithographic mask  1200 . As already discussed in section  3 , the EUV radiation may damage the structure of the fused silica a boundary layer  1230  of the substrate  1210  of the photolithographic mask  1200 . This process will cause aberrations of the photolithographic mask  1200  during the operation since the dimensions of the multi-layer mirror system of the photolithographic mask  1200  change temporally and locally (not indicated in  FIG. 12 ). This problem can be avoided if the boundary layer  1230  of the substrate of the photolithographic mask  1200  is scanned at the end of the manufacturing process of the photolithographic mask  1200  with the pulsed laser source  330  of the apparatus  300  of  FIG. 3 . In order to be able to focus the pulsed laser source  330  into the boundary layer  1230  through the rear substrate surface  1225 , the rear substrate surface  1225  has to have a transparent conductive coating  1222 . 
     The scan of the light pulses of the pulsed laser source  330  can be repeated in planes having various distances to the front substrate surface  1215 , so that the pulsed laser source  330  generates color centers within the boundary layer  1230 . Similar to  FIG. 10 , pulses which have a duration of 200 fs and a repetition rate of 100 kHz are used to generate the compaction layer  1230 . The pulse energy is again 1.5 μJ at a focal point diameter of 2.5 μm and a pitch of 0.3 μm. The minimum distance of the focal point of the femtosecond light pulses from the front substrate surface  1215  is again in the range 400 μm and the distance between the different planes of generated color centers is 70 μm. As already discussed in the context of  FIG. 6 , the distribution of the color centers generated by the femtosecond light pulses of the pulsed laser source  330  should keep a minimum distance to the front substrate surface  1215  in order to avoid damaging of the multi-layer mirror structure on the front substrate surface  1215  (not indicated in  FIG. 12 ). As already described in the context of the discussion of  FIGS. 5 to 9 , the color centers compact the boundary layer  1230 . Therefore the EUV radiation reaching the boundary layer  1230  through the multi-layer mirror structure during operation can no longer damage the lattice of the fused silica of the boundary layer  1230  of the substrate  1210 . Consequently, the dimensions of the photolithographic mask  1200  are not changed during its operation and aberrations are avoided. 
     The three-dimensional distribution of the generated color centers can again be measured with the apparatus  400  of  FIG. 4 . The color centers may be homogenously distributed within the boundary layer  1230 , or their density can diminish with increasing distance from the front substrate surface  1215 . Further, the density of the generated color centers may be smaller or may be even zero below the absorbing structure as the probability is very low that EUV photons may penetrate both, the absorbing structure and the subjacent multi-layer mirror structure (cf.  FIG. 1 ). 
     The methods of correcting the front substrate surface  1215  and of compacting the boundary layer  1230  may be combined. In an embodiment, the front substrate surface  1215  may be corrected in a first step and then the boundary layer  1230  may be compacted by for by example homogeneously generating color centers in the boundary layer  1230 . In an alternative approach the sequence of steps may be reversed. In another embodiment, both the modification of the front substrate surface  1215  and compacting the boundary layer  1230  may be performed in one individual operation. 
       FIG. 13  illustrates still a further aspect of the present invention. In this figure the front substrate surface  1315  of the substrate  1310  of the photolithographic mask  1300  has both a bump (right part) and a depression (left part) with respect to a predetermined front substrate surface (indicated by the dashed line). The front substrate surface  1315  may now be planed by taking the point having the smallest substrate thickness as reference level, and removing all deviations from this height using the above described method of color center generation. However, this process may take a long time, and more important will reduce the overall thickness of the substrate  1310  below the predetermined value. The numerical value of this deviation has to be individually determined and compensated for each photolithographic mask in order to avoid aberrations. 
     Therefore, it may be desirable to remove the flatness imperfections of the front substrate surface  1315  and at the same time to keep the overall thickness of the substrate  1310  of the photolithographic mask  1300  at the predetermined value. This can be achieved if the bump can be removed and the depression can be filled. In the context of the discussion of  FIG. 10 , it has already been described how to remove the bump of the front substrate surface  1315 . As in  FIGS. 10 and 12  the photolithographic mask  1300  has on its rear substrate surface  1225  a transparent conductive coating  1222 . 
       FIG. 14  illustrates how to remove the depression of the front substrate surface  1315  of the substrate  1310  of the photolithographic mask  1300 . In the second part of this specification, it has already been discussed that the application of localized femtosecond light pulses can also lead to a breakdown of the fused silica of the substrate  1410  of the photolithographic mask  1400 . This breakdown leads to the expansion of a damaged zone and the formation of expansion stress in the material. Thus, the generation of breakdown zones which correlates to the deviation of the depression from the predetermined substrate height can increase the front substrate surface  1315  of  FIG. 13 . In the breakdown mode the pulse duration of the femtosecond light pulses is 500 fs and repetition rate is 100 kHz. The focal point diameter is 2.5 μm at a pitch of 3 μm. The distance of the plane of femtosecond light pulses from the front substrate surface  1415  is 400 μm and the distance between the layers of the breakdown zones generated by the femtosecond light pulses is 100 μm. As indicated in  FIG. 14 , the front substrate surface  1415  of the substrate  1410  corresponds essentially to the predetermined front substrate surface  1415  in  FIG. 14 . 
     The principles discussed in the context of  FIGS. 13 and 14  can also be applied to correct imperfections of the rear surface substrate  1525  of the substrate  1510  of the photolithographic mask  1500 , as it is schematically illustrated in  FIG. 15 . Similar to photolithographic masks  200 ,  1000 ,  1100 ,  1200 ,  1300 ,  1400 , the photolithographic mask  1500  also has a transparent conductive coating  1522  on the imperfect rear substrate surface  1525 . By means of the application of femtosecond light pulses of the pulsed light source  330  according to the above discussed methods the imperfect rear substrate surface  1525  can be smoothed to an essentially flat surface, as is indicated by the reference number  1540  in  FIG. 15 . 
     As already indicated during the discussion of  FIG. 2 , the coating of a multi-layer mirror structure comprising approximately 40 MoSi layers may lead to imperfections in the top MoSi layers below the capping layer  1650  even when the rear substrate surface  1515  is essentially flat. Small fluctuations in the layer thickness of individual MoSi layers may add up and may result in the imperfections schematically illustrated in  FIG. 16 . The absorbing structure of the photolithographic mask  1600  has been omitted (cf.  FIG. 1 ). The situation presented in  FIG. 16  will result in serious aberrations of the photolithographic mask  1600  since the major part of the EUV photons is reflected from top MoSi layers below the capping layer  1650  which have the largest imperfections. 
     The inventive principle may also be applied to significantly improve imperfections of the multi-layer mirror system of the photolithographic mask  1600 .  FIG. 17  illustrates a distortion of an originally flat front substrate surface  1615  to a distorted front substrate surface  1715 , so that the top MoSi layers below the capping layer  1750  are essentially smoothed by using some of the methods discussed above. The photolithographic mask  1700  shows significantly less aberrations than the photolithographic mask  1600  of  FIG. 16 . 
     As a consequence, the combination of both, the application of local breakdown and the local generation of color centers can correct front substrate surfaces and/or imperfections of the multi-layer system of photolithographic masks.