Patent Publication Number: US-9417515-B2

Title: Ultra-smooth layer ultraviolet lithography mirrors and blanks, and manufacturing and lithography systems therefor

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/786,109 filed Mar. 14, 2013, and the subject matter thereof is incorporated herein by reference thereto. 
     The present application contains subject matter related to concurrently filed U.S. patent application Ser. No. 14/139,307. The related application is assigned to Applied Materials, Inc. and the subject matter thereof is incorporated herein by reference thereto. 
     The present application contains subject matter related to concurrently filed U.S. patent application Ser. No. 14/139,371. The related application is assigned to Applied Materials, Inc. and the subject matter thereof is incorporated herein by reference thereto. 
     The present application contains subject matter related to concurrently filed U.S. patent application Ser. No. 14/139,415. The related application is assigned to Applied Materials, Inc. and the subject matter thereof is incorporated herein by reference thereto. 
     The present application contains subject matter related to concurrently filed U.S. patent application Ser. No. 14/139,457. The related application is assigned to Applied Materials, Inc. and the subject matter thereof is incorporated herein by reference thereto. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to extreme ultraviolet lithography mirrors and blanks, and manufacturing and lithography systems for such extreme ultraviolet lithography mirrors and blanks. 
     BACKGROUND 
     Extreme ultraviolet lithography (EUV, also known as soft x-ray projection lithography) is a contender to replace deep ultraviolet lithography for the manufacture of 0.13 micron, and smaller, minimum feature size semiconductor devices. 
     However, extreme ultraviolet light, which is generally in the 5 to 40 nanometer wavelength range, is strongly absorbed in virtually all materials. For that reason, extreme ultraviolet systems work by reflection rather than by transmission of light. Through the use of a series of mirrors, or lens elements, and a reflective element, or mask blank, coated with a non-reflective absorber mask pattern, the patterned actinic light is reflected onto a resist-coated semiconductor wafer. 
     The lens elements and mask blanks of extreme ultraviolet lithography systems are coated with reflective multilayer coatings of materials such as molybdenum and silicon. Reflection values of approximately 65% per lens element, or mask blank, have been obtained by using substrates that are coated with multilayer coatings that strongly reflect light essentially at a single wavelength within a extremely narrow ultraviolet bandpass; e.g., 12 to 14 nanometer bandpass for 13 nanometer ultraviolet light. 
     There are various classes of defects in semiconductor processing technology which cause problems in lens elements and masks. Opaque defects are typically caused by particles on top of the multilayer coatings or mask pattern which absorb light when it should be reflected. Clear defects are typically caused by pinholes in the mask pattern on top of the multilayer coatings through which light is reflected when it should be absorbed. And phase defects are typically caused by scratches and surface variations beneath the multilayer coatings which cause transitions in the phase of the reflected light. These phase transitions result in light wave interference effects which distort or alter the pattern that is to be exposed in the resist on the surface of the semiconductor wafer. Because of the shorter wavelengths of radiation which must be used for sub-0.13 micron minimum feature size, scratches and surface variations which were insignificant before now become intolerable. 
     While progress has been made in reducing or eliminating particle defects and work has been done on repair of opaque and clear defects in lens elements and masks, to date nothing has been done to address the problem of phase defects. For deep ultraviolet lithography, surfaces are processed to maintain phase transitions below 60 degrees. Similar processing for extreme ultraviolet lithography is yet to be developed. 
     For an actinic wavelength of 13 nanometers, a 180 degree phase transition in the light reflected from the multilayer coating may occur for a scratch of as little as 3 nanometers in depth in the underlying surface. This depth gets shallower with shorter wavelengths. Similarly, at the same wavelength, surface variations more abrupt than one (1) nanometer rise over one hundred (100) nanometers run may cause similar phase transitions. These phase transitions can cause a phase defect at the surface of the semiconductor wafer and irreparably damage the semiconductor devices. 
     In the past, lens elements and mask blanks for deep ultraviolet lithography have generally been of glass but silicon or ultra low thermal expansion materials have been proposed as alternatives for extreme ultraviolet lithography. Whether the lens element or mask blank is of glass, ultra low thermal expansion material, or silicon, the surface of the lens element or mask blank is made as smooth as possible by mechanical polishing with an abrasive. The scratches that are left behind in such a process are sometimes referred to as “scratch-dig” marks, and their depth and width depend upon the size of the particles in the abrasive used to polish the mask blank. For visible and deep ultraviolet lithography, these scratches are too small to cause phase defects in the pattern on the semiconductor wafer. However, for extreme ultraviolet lithography, scratch-dig marks are a significant problem because they will appear as phase defects. 
     Due to the short illumination wavelengths required for EUV lithography the lens elements and pattern masks used must be reflective mask instead of the transmissive masks used in current lithography. The reflective mask is made up of a precise stack of alternating thin layers of molybdenum and silicon, which creates a Bragg refractor or mirror. Because of the nature of the multilayer stack and the small feature size, any imperfections in the surface of the substrate on which the multilayer stack is deposited will be magnified and impact the final product. Imperfections on the scale of a few nanometers can show up as printable defects on the finished mask and need to be eliminated from the surface of the mask blank before deposition of the multilayer stack. 
     Common imperfections include pits, scratches, and particles. Common cleaning techniques remove many of the particles but either generate new pits or amplify existing pits. The pits can come from the polishing or cleaning process or can be from inclusions or flaws in the substrate material itself that are exposed during the cutting and polishing process. Further polishing can be used to remove the pits at the surface, but there is a risk that new pits will be exposed or caused in the process, which limits the usefulness of using polishing alone to smooth and planarize the substrate surface. Another method for substrate smoothing is laser or plasma annealing. These techniques melt and reflow a thin surface layer of the glass substrate, removing local defects. The problem is that they induce longer range roughness or ripples in the substrate surface and so do not provide the substrate flatness required for EUV mask blanks. 
     In view of the need for the increasingly smaller feature size of electronic components, it is increasingly critical that answers be found to these problems. In view of the ever-increasing commercial competitive pressures, along with growing consumer expectations, it is critical that answers be found for these problems. Additionally, the need to reduce costs, improve efficiencies and performance, and meet competitive pressures adds an even greater urgency to the critical necessity for finding answers to these problems. 
     Solutions to these problems have been long sought but prior developments have not taught or suggested any solutions and, thus, solutions to these problems have long eluded those skilled in the art. 
     SUMMARY 
     An embodiment of the present invention provides an extreme ultraviolet lens element or blank production system that includes: a first deposition system for depositing a planarization layer over a semiconductor substrate; a second deposition system for depositing an ultra-smooth layer over the planarization layer, the ultra-smooth layer having reorganized molecules; and a third deposition system for depositing a multi-layer stack over the ultra-smooth layer. 
     An embodiment of the present invention provides an extreme ultraviolet lithography system that includes: an extreme ultraviolet light source; a mirror for directing light from the extreme ultraviolet light source; a reticle stage for placing an extreme ultraviolet mask blank with a planarization layer and an ultra-smooth layer over the planarization layer; and a wafer stage for placing a wafer. 
     An embodiment of the present invention provides an extreme ultraviolet blank that includes: a substrate; a planarization layer over the substrate; an ultra-smooth layer over the planarization layer, the ultra-smooth layer having reorganized molecules; a multi-layer stack having an amorphous metallic layer; and capping layers over the multi-layer stack. 
     Certain embodiments of the invention have other steps or elements in addition to or in place of those mentioned above. The steps or element will become apparent to those skilled in the art from a reading of the following detailed description when taken with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an extreme ultraviolet (EUV) mirror or mask blank production system. 
         FIG. 2  is an EUV mask blank in accordance with an embodiment of the present invention. 
         FIG. 3  is an EUV mask. 
         FIG. 4  is a method for making the EUV mask blank with ultra-low defects. 
         FIG. 5  is an optical train for an EUV lithography system. 
         FIG. 6  is an EUV lithography system. 
     
    
    
     DETAILED DESCRIPTION 
     The following embodiments are described in sufficient detail to enable those skilled in the art to make and use the invention. It is to be understood that other embodiments would be evident based on the present disclosure, and that system, process, or mechanical changes may be made without departing from the scope of the present invention. 
     In the following description, numerous specific details are given to provide a thorough understanding of the invention. However, it will be apparent that the invention may be practiced without these specific details. In order to avoid obscuring the present invention, some well-known circuits, system configurations, and process steps are not disclosed in detail. 
     The drawings showing embodiments of the system are semi-diagrammatic and not to scale and, particularly, some of the dimensions are for the clarity of presentation and are shown exaggerated in the drawing FIGS. Similarly, although the views in the drawings for ease of description generally show similar orientations, this depiction in the FIGS. is arbitrary for the most part. Generally, the invention can be operated in any orientation. 
     Where multiple embodiments are disclosed and described having some features in common, for clarity and ease of illustration, description, and comprehension thereof, similar and like features will be described with similar reference numerals. 
     For expository purposes, the term “horizontal” as used herein is defined as a plane parallel to the plane or surface of a lens element or mask blank, regardless of its orientation. The term “vertical” refers to a direction perpendicular to the horizontal as just defined. Terms, such as “above”, “below”, “bottom”, “top”, “side” (as in “sidewall”), “higher”, “lower”, “upper”, “over”, and “under”, are defined with respect to the horizontal plane, as shown in the figures. The term “on” indicates that there is direct contact between elements. 
     The term “processing” as used herein includes deposition of material or photoresist, patterning, exposure, development, etching, cleaning, and/or removal of the material or photoresist as required in forming a described structure. 
     Embodiments of the present invention use various established techniques for depositing silicon, silicon oxide, and related films of compatible thermal expansion coefficient by CVD, PVD, ALD, and flowable CVD to fill the pits and bury the defects. Once deposited, the films surface maybe smooth and flat enough for further multilayer stack deposition, or may then be smoothed further using a variety of established smoothing or polishing techniques, including CMP, annealing, or ion beam polishing. 
     Referring now to  FIG. 1 , therein is shown an extreme ultraviolet (EUV) mirror or blank production system  100 . The EUV mirror or blank production system  100  includes a mask blank loading and carrier handling system  102  into which mask blanks  104  are loaded. An airlock  106  provides access to a wafer handling vacuum chamber  108 . In the embodiment shown, the wafer handling vacuum chamber  108  contains two vacuum chambers, a first vacuum chamber  110  and a second vacuum chamber  112 . Within the first vacuum chamber  110  is a first wafer handling system  114  and in the second vacuum chamber  112  is a second wafer handling system  116 . 
     The wafer handling vacuum chamber  108  has a plurality of ports around its periphery for attachment of various other systems. The first vacuum chamber  110  has a degas system  118 , a first physical vapor deposition system  120 , a second physical vapor deposition system  122 , and a preclean system  124 . 
     The second vacuum chamber  112  has a first multi-cathode source  126 , a flowable chemical vapor deposition (FCVD) system  128 , a cure chamber  130 , and an ultra-smooth deposition chamber  132  connected to it. 
     In an alternative embodiment, the FCVD system  128 , the cure chamber  130 , and the ultra-smooth deposition chamber  132  can be in a separate system from the EUV mirror or blank production system  100 . 
     The first wafer handling system  114  is capable of moving wafers, such as a wafer  134 , among the airlock  106  and the various systems around the periphery of the first vacuum chamber  110  in a continuous vacuum. The second wafer handling system  116  is capable of moving wafers, such as a wafer  136 , around the second vacuum chamber  112  while maintaining the wafers in a continuous vacuum. 
     Referring now to  FIG. 2 , therein is shown an EUV lens element or mask blank  200  in accordance with an embodiment of the present invention. The EUV lens element or mask blank  200  has an ultra-low expansion substrate  202  of glass or silicon. The top surface of the ultra-low expansion substrate  202  has imperfections  203 , such as bumps, pits, scratches, and particles, which result from chemical mechanical polishing (CMP) with an abrasive or other polishing method and handing of the substrate. The scratches that are left behind in such a process are sometimes referred to as “scratch-dig” marks, and their depth and width depend upon the size of the particles in the abrasive used to polish the EUV lens element or mask blank  200  to form a EUV mirror or mask. 
     It has been discovered that the imperfections in the EUV lens element or mask blank  200  can be eliminated by deposition of a planarization layer  204 . The planarization layer  204  can be formed by depositing a flowable CVD film or depositing silicon, silicon oxide, or related films by CVD, PVD, or similar processes. This step buries particles, fills in scratches or indentations, and repairs other defects that are on the ultra-low expansion substrate  202 . 
     In the case of flowable CVD films, no further processing may be required to achieve an acceptably smooth, flat surface on the ultra-low expansion substrate  202 . For silicon, silicon oxide, or related films, smoothing after deposition may be required. This smoothing can be done by a variety of polishing methods including, but not limited to CMP, chemical polishing, ion beam polishing or annealing. These smoothing techniques can also be applied to the flowable CVD film if further smoothing is required. 
     However, the planarization layer  204  has still been found to have a roughness of up to 1.0 nm RMS. 
     It has been discovered that the roughness of the planarization layer  204  can be further reduced by the application of an ultra-smooth layer  205  over the planarization layer  204 . The term ultra-smooth is defined as a local roughness under 0.2 nm RMS. 
     During the deposition of the ultra-smooth layer  205 , there is a reorganization of the film to result in ultra-smoothness. The reorganization is due to a reflow process, a sputtering and redeposition process, or other process where the molecules of the film are reorganized to level out the surface to the ultra-smoothness. 
     The ultra-smooth layer  205  can impart improved mechanical and chemical properties to the surface of the planarization layer  204  to aid in the integration of the subsequent processing steps. The ultra-smooth layer  205  include films such as a high density plasma (HDP) oxide, boron doped phosphorous glass, amorphous silicon, or a metal film. 
     It has been found that the planarization layer  204  having a local roughness of about 0.5 nm RMS can be further smoothed by the application of a HDP oxide layer to about 0.15 nm RMS. 
     A multi-layer stack  206  of thin films is formed above the planarization layer  204  to form a Bragg reflector. Due to the transmissive nature of the optics and illuminating wavelengths used in EUV, reflective optics are used and the multi-layer stack  206  may be made of alternating layers of reflective materials, such as molybdenum and silicon, which are much thinner than the planarization layer  204  and the ultra-smooth layer  205 . 
     It has been found that the planarization layer  204  and the ultra-smooth layer  205  can be formed in a different system from the multi-layer stack  206  because the need to maintain a vacuum between the processes is not an essential condition to forming the layers and the stack. 
     A capping layer  208  is formed above the multi-layer stack  206 . The capping layer  208  can be a material such as ruthenium (Ru) or a non-oxidized compound thereof to help protect the multi-layer stack  206  from any chemical etchants to which the EUV mask blank  200  is exposed during mask processing. Other material such as titanium nitride, boron carbide, silicon nitride, ruthenium oxide, and silicon carbide may also be used in the capping layer  208 . 
     An absorber layer  210  is placed over the capping layer  208 . The absorber layer  210  is of a material having a high absorption coefficient for a particular frequency of EUV light (about 13.5 nm) and may be a material such chromium, tantalum or nitrides thereof. 
     An anti-reflective coating (ARC)  212  is deposited on the absorber layer  210 . The ARC  212  can be of a material such as tantalum oxynitride or tantalum boron oxide. 
     A backside chucking layer  214  is formed on the rear surface of the ultra-low expansion substrate  202  for chucking the substrate in an electrostatic chuck (not shown). 
     Referring now to  FIG. 3 , therein is shown a EUV mask  300 . The EUV mask  300  is square and has a pattern  302  on the top surface thereof. 
     Referring now to  FIG. 4 , therein is shown a method  400  of manufacturing an EUV mirror or mask blank. The method  400  includes: forming a planarization layer on a substrate in a step  404 ; forming an ultra-smooth layer over the planarization layer in a step  406 ; and forming a multi-layer stack on the ultra-smooth layer in a step  408 . 
     Referring now to  FIG. 5 , therein is shown an optical train  500  for an EUV lithography system. The optical train  500  has a plasma source  502  for creating the EUV light and collecting it in a collector  504 . The collector  504  provides the light to a field facet mirror  508  which is part of an illuminator system  506  which further includes a pupil facet mirror  510 . The illuminator system  506  provides the EUV light to a reticle  512  (which is the fully processed version of the mask blank  104  of  FIG. 1 ), which reflects the EUV light through projection optics  514  and onto a wafer  516 . 
     Referring now to  FIG. 6 , therein is shown an EUV lithography system  600 . The EUV lithography system  600  includes an EUV light source area  602 , a reticle stage  604  and a wafer stage  606  as adjuncts to the optical train  500 . 
     Embodiments of the present invention planarize and smooth EUV lens element and mask blank substrates so as to remove all pits, defects, and particles on the substrate surface so that the surface is atomically flat and smooth. The idea is to deposit defect free material on the surface of the EUV lens element or mask blank substrate that can then be processed without inducing any defects to achieve an atomically flat and smooth surface. 
     The first step is to fill any pits that are present; this can be done by depositing a flowable CVD film or by depositing silicon, silicon oxide, or related films via CVD, PVD, or a similar process. This step will also bury particles and other defects that are on the EUV mask blank substrate surface. In the case of flowable CVD films, no further CMP or other smoothing processing would be required to achieve an acceptably smooth, flat surface on the EUV mask blank substrate. 
     For the silicon, silicon oxide, or related films smoothing after deposition will likely be required. This smoothing can be done by a variety of polishing methods including, but not limited to CMP, chemical polishing, ion beam polishing, or annealing. These techniques can also be applied to the flowable CVD films if further smoothing is required. 
     One advantage of this method is that it is substrate independent and so it can be used on a variety of substrates and qualities of substrates. It has the potential to make it possible to use glass substrates that have the required properties for EUV lens element and mask blanks but do not have atomically flat, smooth surfaces after polishing. This independence makes it possible to use different substrate suppliers and minimizes the impact of unexpected changes to the substrate preparation and polishing by the suppliers. 
     Embodiments of the invention provide an atomically flat, low defect, smooth surface for an EUV lens element or mask blank. However, embodiments of the invention could also be used to manufacture other types of blanks, such as for mirrors. Over a glass substrate, embodiments of the invention can be used to form an EUV mirror. 
     Further, embodiments of the invention can be applied to other atomically flat, low defect, smooth surface structures used in UV, DUV, e-beam, visible, infrared, ion-beam, x-ray, and other types of semiconductor lithography. Embodiments of the invention can also be used in various size structures that can range from wafer-scale to device level and even to larger area displays and solar applications. 
     Another approach would be to use flat highly thermally conducting surfaces to grow the multilayer stack on. Historically, glass is used as the substrate for masks, due to the transmissive nature of the optics and illuminating wavelengths used. EUV is absorbed by all materials, thus reflective optics is used. However, reflectivity is not 100% (&lt;70% for current Mo/Si stack), and the absorbed part of the radiation will heat up the substrate. Current mask glass substrate composition is optimized to give zero thermal expansion coefficient, at the operating temperature, to avoid pattern distortion during resist exposure. If substrates more thermally conducting than glass are used, for example metallic or silicon, heat from EUV exposure can be transferred into a cooled chuck thus eliminating the need for a specialized glass. Furthermore, the mask substrate surface can be smoothed using semiconductor compatible processes such as deposition of a layer such as described above (silicon, silicon dioxide) or by CMP or a combination of both. 
     The resulting method, process, apparatus, device, product, and/or system is straightforward, cost-effective, uncomplicated, highly versatile, accurate, sensitive, and effective, and can be implemented by adapting known components for ready, efficient, and economical manufacturing, application, and utilization. 
     Another important aspect of the present invention is that it valuably supports and services the historical trend of reducing costs, simplifying systems, and increasing performance. 
     These and other valuable aspects of the present invention consequently further the state of the technology to at least the next level. 
     While the invention has been described in conjunction with a specific best mode, it is to be understood that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the aforegoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations that fall within the scope of the included claims. All matters hithertofore set forth herein or shown in the accompanying drawings are to be interpreted in an illustrative and non-limiting sense.