US 20050153235 A1
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 a material which may be applied to a semiconductor wafer surface which ensures that the photosensitive material is exposed principally by light polarized parallel to the semiconductor wafer surface.
1. A composition of matter, comprising:
a fluid for forming a photoresist layer on a surface of a substrate, the fluid comprising;
photosensitive molecules, each of the photosensitive molecules having an axis, wherein each photosensitive molecules aligns with its axis having a particular direction with respect to the normal to the surface of the substrate when the fluid is formed as a layer on the surface of the substrate, and wherein each photosensitive molecule has a different sensitivity to light of wavelength λ having a first linear polarization parallel to the axis than to light of a second linear polarization orthogonal to the first polarization such that a fluence of light of either one of the first or second polarizations causes a reaction in the photosensitive molecules which allows development of the layer as a photoresist layer, and that same fluence of light of the orthogonal polarization does not cause a reaction which allows development of the layer as a photoresist layer.
2. The composition of matter of
3. The composition of matter of
4. The composition of matter of
5. A method, comprising:
a) applying a layer of a first fluid for forming a photoresist layer on a surface of a semiconductor substrate, the first fluid comprising;
photosensitive molecules, each of the photosensitive molecules having an axis, wherein each photosensitive molecules aligns with its axis having a particular direction with respect to the normal to the surface of the substrate when the fluid is formed as a layer on the surface of the substrate, and wherein each photosensitive molecule has a different sensitivity to light of wavelength λ having a first linear polarization parallel to the axis than to light of a second linear polarization orthogonal to the first polarization such that a fluence of light of either one of the first or second polarizations causes a reaction in the photosensitive molecules which allows development of the layer as a photoresist layer, and that same fluence of light of the orthogonal polarization does not cause a reaction which allows development of the layer as a photoresist layer; and
b) solidifying the first fluid layer.
6. The method of 5, further comprising;
c) inserting the photoresist covered semiconductor wafer into a photolithographic exposure tool;
d) immersing the semiconductor wafer in a second fluid transparent to light of wavelength λ, the second fluid having an index of refraction for light of wavelength λ greater than 1.3; and
e) exposing the photoresist layer with patterned light of wavelength λ.
7. The method of 2, further comprising;
f) developing the photoresist layer.
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.
The field of the invention is the field of photoresists and contrast agents for immersion photolithographic applications.
This application is related to two applications by the same inventor filed on the same day as the present invention entitled “Photosensitive material for immersion photolithography” and “Immersion photolithography apparatus and system”.
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.
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.
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.
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
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
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
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® film. Polaroid® 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® 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.
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
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
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
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
A system for using the object of the invention is shown sketched in
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.