Abstract:
When rays of light converge inside a photosensitive material at angles larger than 70 degrees, one polarization of the light may fail to produce the desired image contrast in conventional exposure media. This invention describes means of suppressing the effects of the undesired polarization by using a class of photosensitive media that are insensitive to that polarization and more sensitive to the polarization conveying the desired image contrast as well as by means of optical configurations relevant in the context of semiconductor manufacturing using photolithography.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority pursuant to 35 U.S.C. 119(e) to U.S. Provisional Application No. 60/485,988 filed Jul. 10, 2003 which is incorporated herein by reference in its entirety including incorporated material. 
    
    
     FIELD OF THE INVENTION 
     The field of the invention is the field of photoresists and contrast agents for immersion photo lithographic applications. 
     RELATED PATENTS AND APPLICATIONS 
     This application is related to two applications by the same inventor filed on the same day as the present invention, both entitled “Photosensitive material for immersion photolithography”. 
     OBJECTS OF THE INVENTION 
     It is an object of the invention to produce a method, an apparatus, a system, a contrast enhancing material, and/or a photoresist material for exposing photoresist in a photoresist exposing tool where the photoresist is immersed in a high index transparent fluid for exposing finer features than possible when it is immersed a normal transparent material having index of refraction approximately equal to one. 
     SUMMARY OF THE INVENTION 
     A photoresist material and exposure method are disclosed wherein the photoresist material in a layer on the surface of a substrate is exposed only by a component of light polarized parallel with the substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  shows light entering a photoresist, the light having a polarization with a component perpendicular to the surface of the photoresist. 
         FIG. 1B  shows the electric fields of a first set of phases of the light of FIG.  1 A. 
         FIG. 1C  shows the electric fields of a second set of phases of light of FIG.  1 A. 
         FIG. 1D  shows light entering a photoresist, the light having polarization parallel to the surface of the photoresist. 
         FIG. 1E  shows the electric fields of a first set of phases of the light of FIG.  1 D. 
         FIG. 1F  shows the electric fields of a of a second set of phases of light of FIG.  1 D. 
         FIG. 2A  shows light propagating through a mask and an immersion photolithography system, the light having a polarization with a component perpendicular to the surface of the photoresist. 
         FIG. 2B  shows the electric fields of a first set of phases of the light of FIG.  2 A. 
         FIG. 2C  shows the electric fields of a second set of phases of light of FIG.  2 A. 
         FIG. 3  shows an embodiment of the invention. 
         FIG. 4  shows a polarization effected for an embodiment of the invention. 
         FIG. 5  shows a polarizer for an embodiment of the invention. 
         FIG. 6  shows placement positions for a polarizer for embodiments of the invention. 
         FIG. 7  shows a sketch of an embodiment of the invention. 
         FIG. 8  shows a sketch of an embodiment of the invention. 
         FIG. 9  shows a sketch of an embodiment of the invention. 
         FIG. 10  shows a sketch of a system embodying of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Images exposed in a photoresist are formed by light beams from the same point in a mask being imaged together and interfering in the photoresist. If the angle between the beams inside the photosensitive medium is less than 70 degrees, polarization is not a major issue.  FIG. 1A  shows the usual case of light beams  10  and  10 ′ representing light beams from a high numerical aperture (N.A.) imaging system impinging on a photoresist layer  12  supported by a substrate  14 . Normally, the light beams  10  and  10 ′ propagate through a medium  16  with index of refraction n nearly equal to 1. (The medium may be air, an inert gas like nitrogen or argon, or indeed a vacuum). As the light beams  10  and  10 ′ pass through the boundary at an angle with respect to the normal  11  to the surface of the substrate  14  and the photoresist  12 , between the medium  16  and the photoresist layer  12 , where the photoresist has an index of refraction greater than 1 (in the region of n=1.6) the light beams  10  and  10 ′ bend. If the light beams  10  and  10 ′ are polarized with a component of the electric field perpendicular to the surface of the photoresist (for example if the light beams are polarized in the plane determined by the light bean and the normal to the surface of the photoresist), as shown by the arrows  18  and  18 ′ representing the electric field of the light beams, bending of the light beams  10  and  10 ′ ensures that the electric fields  19  and  19 ′ of the light beams  10  and  10 ′ are much more parallel to the surface of substrate  14  and to each other inside the resist than outside the resist.  FIG. 1B  shows electric fields  19  and  19 ′ for the case that light beams  10  and  10 ′ are out of phase (as shown in  FIG. 1A ) and  FIG. 1C  shows the case that light beams  10  and  10 ′ are in phase. The resultant electric field  19 ″ is much less for  FIG. 1B  than for  FIG. 1C , and the contrast between the in phase and out of phase case is relatively large. There is thus little difference in the intensity of the light absorbed between light polarized as shown and light polarized parallel to the surface of the photoresist. The situation with light polarized parallel to the surface is shown in  FIG. 1D  where the vectors  18 ′″,  18 ″″,  19 ′″, and  19 ″″ are all parallel to the surface and to each other. The electric fields when the light beams  10 ′″ and  10 ″″ are out of phase is shown in  FIG. 1E , and in  FIG. 1F  when the light beams are in phase. In the case of light polarized parallel to the surface there is no resultant electric field if the light beams are out of phase, and the resultant electric field is great when the beams are in phase. In both polarization cases, changes in the relative phases of the electric fields  19  and  19 ′ result in large changes in the magnitude of their sum and thus the intensity of the light, producing image contrast.  FIG. 1B  shows the electric field vectors  19  and  19 ′ of  FIG. 1A  where the light beams  10  and  10 ′are out of phase. The resultant electric field  19 ″ is perpendicular to the surface and small. In contrast, when the light beams  10  and  10  are in phase to produce a light spot on the resist,  FIG. 1C  shows that the electric fields  19  and  19 ′ add to produce electric field  19 ″ which is substantially larger, and nearly the same as would be the case if light beams  10  and  10 ′ were polarized parallel to the surface of photoresist  12 . 
     However, in  FIG. 2A  the light beams are shown propagating through a medium  26  with index that more or less matches the index of refraction of the photoresist  12 . Such immersion photolithography systems could be used to produce finer features in photoresist than available using the same wavelength of the light and types of optical systems as prior art systems. Light  23  illuminating a mask  24  produces as an example two light beams  20  and  20 ′ having polarization with the electric field shown by vectors  22  and  22 ′. The light beams  20  and  20 ′ resulting after passing through lens system  25  into medium bend little at the interface between the photoresist  12  and the medium  26 , and the electric fields  29  and  29 ′ of the light beams are at right angles in the medium in the case shown. In the case shown in  FIG. 2A , light beams  20  and  20 ′ are emitted from a part of the mask which is opaque, and they interfere at the surface of the mask to produce a minimum light intensity. When the light beams are brought together again in the photoresist, they are out of phase, and the resultant electric field  29 ″ in the photoresist is shown in  FIG. 2B , which shows that there is a significant resultant electric field perpendicular to the surface of the wafer. In addition, when the light beams  20  and  20 ′ are emitted from a transparent part of the mask  24 , one of the directions of the electric field vectors are reversed to give a maxima in intensity at the mask, but as shown in  FIG. 2C  the resultant field  29 ″ in the photoresist is not as great as the resultant field would be if the beams were polarized parallel to the resist surface, and in fact the resultant field has exactly the same magnitude as if the light were coming from an opaque portion of the mask. At 90 degrees the image contrast due to such variations in the relative phases  21  and  21 ′ resulting from displacement along the substrate surface disappears, and the contrast even reverses (ie dark will print light and vice-versa) for larger angles of incidence. 
       FIG. 2A  shows how a demagnifying immersion lens system can alter the electric field directions at the image, resulting in zero or reversed contrast when the polarization is not optimal. The illumination  23  (assumed off axis) diffracts from the mask  24  producing imaging rays  20  and  20 ′ with a relatively small angle between them. The polarizations of these rays are in directions  22  and  22 ′, also with a small angle between their axes. (In the example, the field vectors are shown anti-parallel for clarity. Note that they arise from a location of the mask where there must be a null light intensity). At a location where the mask is opaque, electric fields  22  and  22 ′ sum to a small resultant. However, when these rays are refracted by a high numerical aperture immersion lens  25  in contact with a high refractive index fluid  26  above a resist layer  12  on a substrate  14 , the angle between rays  20  and  20 ′ is increased as a direct result of the demagnification of the lens. The electric fields  29  and  29 ′ remain transverse to the directions of propagation of the rays  20  and  20 ′ in the fluid  26  and resist  12 . Those fields  29  and  29 ′ have a large angle between them and sum to a resultant  29 ″ that is larger than the resultant field formed by the fields  22  and  22 ′. When the fields  29  and  29 ′ are orthogonal, as shown in  FIG. 2A , the resultant field  29 ″ would have constant magnitude, independent of the position in the image, eliminating the image contrast that would have been produced by mask  24  without this effect.  FIGS. 2B  shows of  FIG. 2A , and  FIG. 2C  shows the fields  29  and  29 ′ and the resultant field  29 ″ in the configuration where the light beams  20  and  20 ′ are in phase and coming from a transparent part of the mask. Note that the magnitude of the electric fields  29 ″ and hence the absorbed power are equal in this case and there is no contrast. 
     The problems introduced by the immersion lithography set up can be addressed in several ways. 
     The novel problem of the relative phases of the light in an immersion photolithography system is particularly noxious when using phase shift mask technology. However, the problem has been solved by the inventor by using light polarized parallel to the surface of the photoresist (i.e. out of the page in the  FIGS. 1A ,  1 D and  2 A as shown). 
       FIG. 3  shows a schematic side view of a set up for a tangential polarization system. Representative light sources  30 ,  30 ′, and  30 ″ are shown, with light  32  from light source  30  having electric field polarization  33  (pointing out of the plane of the page) shown schematically impinging on to an outer part of a condensing lens system  34 . The exit pupil of light source  30  is imaged through the mask  36  and through the outer portion of the exposing imaging immersion lens  38 . Patterned light  35  and  35 ′ diverging from the mask  36  is imaged by the exposure lens  38  on to the substrate  14 . If a polarizer is placed in the path of light  32 , or if light source  30  produces polarized light as shown by the horizontal polarization vector  33 , such that the light impinging on to imaging lens  38  is tangentially polarized as shown by vector  33 , light falling on to substrate  14  will be polarized parallel to the surface of substrate  14 . 
       FIG. 4  shows a plan view of an example of the desired polarization  33  of the light impinging on to lens  38  in the absence of mask  36 . In this case, the outer region of lens  38  consists of an annulus  31  illuminated with the polarization  33  which is tangential to the outer edge of lens  38 . A preferred embodiment of the invention which produces approximately the desired polarization uses a 4 quadrant polarizer  60  with polarization direction sketched by the hatching  64  which approximates a tangential polarizer. Such a polarizer is sketched in  FIG. 5  in plan view when place near the condensing lens  34  as shown in FIG.  6 . Other preferred embodiments use from two to sixteen polarizers. Light falls transmitted through the center of the lens may or may not be blocked, but is not important for the contrast as explained before. 
     A tangentially (or otherwise) transmitting polarizing element may also be placed elsewhere in the projection lens system. Other preferred embodiments of the invention places a polarizer just after the output element of the lens system  38 , which is just above the wafer, or immersed in the index matching fluid, or near the condenser lens  34 . Preferred positions of tangential polarizers  60  or  62  are shown schematically in FIG.  6 . 
     In another solution to the problem of contrast is immersion lithographic systems, photoresists can be devised that preferentially are sensitized by light polarized in the plane of the photoresist film and are relatively unaffected by the component of light polarized perpendicular to the plane of the photoresist film. Such materials are known, but are not well known in the UV photoresist communities. There is every reason to believe that a photoresist with the required properties could be devised, given reasonable experimental effort and the teaching of this specification. When such a material is used as a photoresist, the image projection system can employ essentially unpolarized light or light polarized sub-optimally in terms of the discussion above. Such a polarization sensitive resist would show contrast even when the interfering electric fields were oriented orthogonally as in FIG.  2 . 
     It is well known in the art that many molecules have one or more axis, and that light polarized in particular direction with respect to these axis is either absorbed or not absorbed. For example, there are many molecules that have an axis of elongation, where one dimension of the molecule is much larger than the other two dimensions. Such molecules are often sensitive to light having polarization parallel to the axis of elongation. A classical picture may be drawn of such molecules, where the electron acts like an electron in an elongated box. It takes a longer time for the electron to run from end to end along the long axis of the box than to run back and forth along a short axis of the box. If the time taken to run back and forth equates to a frequency of the incoming light, the molecule will absorb the light polarized along the axis. The electron will resonate at too high a frequency if it oscillates back and forth along a short axis, and hence the molecule may not substantially absorb light. At shorter wavelengths, other polarization selection rules may apply. 
     One example of such molecules which is very well known is the example of Polaroid R  film. Polaroid R  film contains linear molecules which absorb light polarized with electric fields in the long direction of the molecule. The energy of the absorbed photon is degraded into heat in Polaroid R  film, but in theory could be transferred to another photoresist molecule, which would expose the photoresist molecule and allow development. The molecules in Polaroid film are aligned so that only one polarization is absorbed, and the alignment is effected by stretching the film. Various methods of aligning photoresist and other films will be discussed below. 
     In the case that a localized bond absorbs light of a short wavelength to produce a photochemical change necessary for a photoresist, the bond angle with respect to the molecular axis will determine the polarization absorbed and effective for exposure of the photoresist. 
       FIG. 7  shows an embodiment of the invention where photoresist  12  comprises oriented molecules  72  oriented with their absorption axis parallel to the surface of the photoresist. 
     Other molecules can be thought of as planar molecules, where one dimension is much smaller than the other two. The sheets of carbon ions in graphite are one such example. In such molecules, light polarized perpendicular to the plane of the molecules is substantially less absorbed than light polarized in the plane of the molecules. 
     It is well known that materials such as liquid crystal materials placed on a substrate align in particular directions with respect to the surface of the substrate. It is also known that very thin layers of matrices of elongated molecules are known to have the elongated molecules align parallel to the surface of the substrate. Planar molecules are also known to align with their planes aligned parallel to the surface of the substrate. Photosensitive molecules which act as sensitizers or photacid generators or photobase generators in photoresists may also be engineered to align in such a fashion, which would lead to the resist being sensitive to light polarized parallel to the surface of the substrate and being insensitive to light polarized perpendicular to the surface of the substrate. 
     A preferred embodiment of the invention is to provide a material fluid comprising a matrix and photosensitive molecules admixed in the matrix, each of the photosensitive molecules having an axis, wherein each photosensitive molecule is insensitive to light of wavelength λ having a particular polarization with respect to the axis, and wherein each photosensitive molecules aligns with its axis having a particular direction with respect to the surface of the substrate when the matrix is formed as a layer on the surface of the substrate, and wherein the photosensitive molecules are insensitive to a light component of wavelength λ with polarization perpendicular to the surface, and wherein light of wavelength λ having polarization parallel to the surface causes a reaction in the photosensitive molecules which allows development of the layer as a photoresist layer. In the method of the invention, the fluid is applied as a layer on to the surface of a semiconductor substrate, and the molecules either align spontaneously with respect to the substrate, or are aligned by methods as known in the art. The layer is solidified by evaporation or by polymerization, as examples, and the oriented layer then acts as herein described. 
     In another preferred embodiment, a photosensitive molecule with an axis oriented perpendicularly to that above absorbs light and causes a reaction that inhibits the development of the layer as a photoresist material. The total photoresist formulation would then contain another photosensitive material (which may or may not have oriented axes within the matrix) that accelerates the chemical reactions needed for development. The balance between acceleration and inhibition gives rise to a net polarization sensitivity which selects the images formed by light polarized parallel to the plane of the resist film. 
     In another preferred embodiment sketched in  FIG. 8 , the photoresist film stack contains a polarization-selective contrast enhancing layer  84  between the photosensitive material  12  and the immersing medium  26 . The polarization selective layer is dichroic, allowing transmission of light polarized parallel to the surface of the photoresist  12  into the photosensitive medium  12  but attenuating or reflecting light with a polarization component normal to the surface of the photoresist layer  12 . Molecules or other entities  82  are shown in  FIG. 8  which would absorb electric field energies with components perpendicular to the surface of the photoresist  12 . 
     In another preferred embodiment, such a layer  84  is also effective as a top anti-reflection coating for the desired polarization. 
     In another preferred embodiment, layer  84  is formed as a sheet  92  in  FIG. 9 , and laminated to substrate  14  and photoresist  12  from a roller  94  or other lamination apparatus as is known in the art. 
     There are a number of techniques art of aligning molecules in thin films or layers known to the inventor. (In the sense of this specification, a thin layer or film is a layer or film with one spatial dimension very much smaller than two other spatial dimensions.) Among them are: 
     1. Applying a liquid crystal material as a thin layer on a surface of a substrate or as a thin layer between two surfaces. The liquid crystal material molecules may be linear molecules which spontaneously align with their long axis parallel to the interface between the liquid crystal material and the substrate. If the surface of the substrate is prepared by methods known in the art, the liquid crystal molecules will in fact align in one direction so that they are all parallel to one another and parallel to the interface. This situations holds in the well known liquid crystal displays, wherein light propagating normal to the surface and polarized parallel to or perpendicular to the axis of the molecules interacts differently and has different propagation constants for each polarization (ie the index of refraction is different for the different polarizations). The molecules in such displays may be rotated by the effect of an electric field to align with their axis perpendicular to the surface, so that light propagating perpendicular to the surface has polarization perpendicular to the axis of all the molecules, and the difference in propagation constants disappears. Such molecules may be solidified by polymerization or by being incorporated in a monomer which is polymerized, and the orientation is therefore fixed. Molecules in such a film are of use in a film such as layer  84  in  FIG. 8  or  92  in FIG.  9 . The liquid crystal material used in liquid crystal displays is generally transparent to visible light polarized both perpendicular and parallel to the axis of the molecules. However, from general principles of optics, the difference in refractive index arises from a difference in the wavelength dependence of the absorption coefficient, and it is expected that such liquid crystal materials will have a spectral region where one polarization is absorbed, and the orthogonal polarization is not absorbed. Such linear molecules where the differing absorption of the light occurs at the wavelengths needed for the lithography are preferred for the present invention. Such wavelengths are preferably less than 260 nm, more preferably less than 200 nm and most preferably less than 160 nm. 
     2. Applying a Langmuir-Blodgett film to the surface of a substrate. It is well known that long chain molecules having a hydrophilic and a hydrophobic end my be closely stacked in a single layer with their long axis perpendicular to a surface, and that multiple layers may be built up by repeating the application process. Such molecules may also be in principle designed to absorb light having a component of the electric field vector perpendicular to the surface. Normally, such monolayer or multilayer films are adhered to the surface of a substrate, but techniques known in the art may produce solidified films which are separated from the surface and are free standing. 
     Note that the examples drawn from the art of liquid crystal devices and Langmuir-Blodget films are for illustrative use only, and that the present invention differs radically from their prior art uses in that light is not propagating in the layer substantially normal to the surface of the layer. Rather, in the immersion photolithographic art of the invention, the light is propagating in the layer in a direction substantially different from the normal to the surface of the layer. The light used for exposing the photoresist film is preferably propagating in the photoresist with an angle greater than 35 degrees to the normal, more preferably greater than 40 degrees, and most preferably greater than 45 degrees. 
     Light propagating substantially parallel to the surface of the layer, such as in the art of film optical waveguides, is also radically different than the art of the invention. Light does not propagate parallel to the surface in a film until it is many wavelengths thick (eg a single mode fiber has a core diameter about 6 microns for light of communication frequencies). In contrast, photoresist thicknesses are typically around a wavelength of light. (EG. a single mode fiber has a core diameter about 6 microns for light of communication frequencies). 
     At the thinnest film thickness where the light can propagate the angle between propagation direction and normal to the surface of the film is 90 degrees. For thicker films, sketches of the light propagation show light bouncing from side to side of the film, where the light is totally reflected by total internal reflection. If the angle between the light propagation direction and the surface normal is too little, the light escapes the film. For the very large index of refraction difference between glass (n=1.5) and air (n=1.0), the critical angle is about 45 degrees. However, the angle is usually much less since the index differences between core and cladding of the optical waveguide are usually much less than 0.5. 
     While the thickness of the contrast enhancing film  84  of the invention is limited only by the distance between surface of the photoresist  12  and the lens element  25  in  FIG. 8 , the most preferred embodiment of the invention is a film thickness just sufficient to attenuate light having a polarization normal to the surface so that adequate contrast is achieved. 
     A system for using the object of the invention is shown sketched in  FIG. 10. A  substrate  14  is shown immersed in an immersion fluid  26 . Apparatus  103  and  104  is used to add and remove the immersion fluid from the system as needed to provide a layer of fluid between the final lens element of lens  38  and the wafer  14 . The immersion fluid  26  contacts both an area of the wafer  14  which is to be exposed and the final lens element of lens  38  so that the light passing between lens  38  and wafer  14  to expose photoresist  12  passes only through only high index of refraction material. A wafer insertion and withdrawal tool  105  is shown, as is a mask insertion and withdrawal tool  106  for insertion and withdrawal of mask holder  107 . A computer system  108  is shown connected to the system controls the apparatus for handling the immersion fluid, the wafer handling tool  105 , the mask handling tool  106 , and the illumination system  30 . A computer program contained in a computer readable medium  109  controls the computer  108 . The computer readable medium may be included in the computer  108  as sketched, or may be stored separate from the computer in any location as known to one skilled in the computer art. 
     Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.