Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Radiation sensitive composition or product or process of making
Reexamination Certificate
1999-02-20
2001-03-13
Baxter, Janet C. (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Radiation sensitive composition or product or process of making
C430S913000, C430S914000, C430S325000, C522S015000
Reexamination Certificate
active
06200728
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to new photoresist compositions that contain a blend of photoacid generator compounds. Compositions of the invention are highly useful as deep U.V. photoresists.
2. Background
Photoresists are photosensitive films for transfer of images to a substrate. They form negative or positive images. After coating a photoresist on a substrate, the coating is exposed through a patterned photomask to a source of activating energy such as ultraviolet light to form a latent image in the photoresist coating. The photomask has areas opaque and transparent to activating radiation that define an image desired to be transferred to the underlying substrate. A relief image is provided by development of the latent image pattern in the resist coating. The use of photoresists is generally described, for example, by Deforest, Photoresist Materials and Processes, McGraw Hill Book Company, New York (1975), and by Moreau, Semiconductor Lithography, Principals, Practices and Materials, Plenum Press, New York (1988).
Known photoresists can provide features having resolution and size sufficient for many existing commercial applications. However for many other applications, the need exists for new photoresists that can provide highly resolved images of submicron dimension.
Various attempts have been made to alter the make-up of photoresist compositions to improve performance of functional properties. Among other things, a variety of photoactive compounds have been reported for use in photoresist compositions. See, e.g., U.S. Pat. No. 4,450,360 and European Application 615163.
Relatively recently interest has increased in photoresists that can be photoimaged with deep U.V. radiation. Such photoresists offer the potential of forming images of smaller features than may be possible at longer wavelength exposure. As is recognized by those in the art, “deep U.V. radiation” refers to exposure radiation having a wavelength in the range of about 350 nm or less, more typically in the range of about 300 nm or less. While a number of deep U.V. resists have been reported, the need clearly exists for new deep U.V. resists that can provide highly resolved fine line images as well as acceptable photospeed and other lithographic properties. Particular interest exists in resists that can be imaged with sub-250 nm wavelengths such as KrF radiation (ca. 248 nm) or sub-200 nm wavelengths such as ArF radiation (193 nm).
SUMMARY OF THE INVENTION
We have now discovered novel blends or mixtures of photoacid generators compounds (“PAGs”) that can formulated in photoresist compositions to provide excellent lithographic properties, particularly chemically-amplified positive-acting resists. Preferred PAG blends can be photoactivated upon exposure to deep U.V. radiation, particularly 248 nm.
In a first aspect of the invention, PAG blends are provided that photogenerate acids of differing strengths. More particularly, preferred PAG blends comprise at least one PAG that generates a strong acid upon photoactivation, and at least one PAG that generates a comparatively weak acid upon photoactivation. Typically, the “strong” and “weak” photogenerated acids of the blend differ in pKa values (determined by Taft parameter calculation as discussed in detail below) by at least about 1 or 1.5, more typically a pKa difference of at least about 2 or more. A typical “strong” photogenerated acid of a blend of the invention has a pKa (Taft parameter calculation) of about −1 or less, more typically a pKa of about −2, −3 or lower. A typical “weak” photogenerated acid of a blend of the invention has a pKa (Taft parameter calculation) of about −2, −1, 0 or higher.
For instance, illustrative preferred “strong” photogenerated acids include perfluorinated alkylsulfonic acids e.g. perfluorooctanesulfonic acid, perfluorohexanesulfonic acid, perfluorobutanesulfonic acid, perfluoro(4-ethyl)cyclohexanesulfonic acid, trifluoromethanesulfonic acid and the like. Additional suitable “strong” photogenerated acids include aromatic sulfonic acids that are substituted with electron withdrawing groups such as fluoro, nitro, cyano and trifluoromethyl. Suitable “strong” photogenerated acids for use in the blends of the invention may include pentafluorobenzenesulfonic acid, 2-trifluoromethylbenzenesulfonic acid, 3-trifluoromethylbenzenesulfonic acid, 4-trifluoromethylbenzenesulfonic acid, and bis(trifluoromethyl)benzenesulfonic acid, particularly 3,5-bis(trifluoromethyl) benzenesulfonic acid.
Preferred “weak” photogenerated acids include e.g. alkylsulfonic acids that are not substituted with electron withdrawing groups such as fluoro, or have minimal electron withdrawing group substitution, e.g. only one or two electron withdrawing substituents. Cycloalkylsulfonic acids are particularly suitable “weak” acids, such as cyclohexanesulfonic acid, adamantanesulfonic acid, camphor sulfonic acid and the like. PAGs that may be employed to provide such acids include e.g. onium salts such as iodonium salts, sulfonium salts and the like; imidosulfonates;
sulfonate esters; and non-ionic halogenated compounds that generate a halo-acid (e.g.
HBr) upon photoactivation.
In a further aspect, the invention provides PAG blends that generate acids of differing size upon photoactivation. More specifically, in this aspect of the invention, preferred PAG blends comprise at least one PAG that generates a “large” acid upon photoactivation, and at least one PAG that a comparatively “small” acid upon photoactivation. Typically, the “large” and “small” photogenerated acids of the blend differ in size by at least about 25 or 30 cubic angstroms (i.e. Å
3
), more typically by at least about 40 or 50 cubic angstroms (i.e. Å
3
). A typical “large” photogenerated acid of a blend of the invention has a volume of at least about 155 or 160 Å
3
), more preferably a volume of at least about 170, 180 or 190 Å
3
. A typical “small” photogenerated acid of a blend of the invention has a volume of about less than 155 or 150 Å
3
, more preferably a volume of about 130 or 140 Å
3
or less. Sizes of photogenerated acids may be readily determined by standard computer-based analyses, as are well-known and further discussed below.
PAG blends also are provided that combine both aspects of the invention, where the blend comprises PAGs that generate acids that differ both in acid strength and size. For example, PAG blends are provided that comprise at least one PAG that generates upon photoactivation a strong acid that is large (or small), and at least one PAG that generates upon photoactivation a weak acid that is small (or large if the strong acid is small).
However, in at least some aspects of the invention, it is preferred that two PAGs of a blend differ only in either size or strength of the photogenerated acid. Thus, e.g., in this aspect of the invention, if the blend members generate photoacids that differ in size as discussed above, then those photogenerated acid have similar acid strengths, e.g. a pKa (Taft parmater calculation) difference of 0.5 or less. Similarly, in this aspect of the invention, if the blend members generate photoacids that differ in strength as discussed above, then those photogenerated acid have similar size, e.g. less than about 25 or 20 Å
3
difference in size.
Without being bound by theory, it is believed that larger photogenerated acids will diffuse more slowly (relative to a small acid) through a photoresist layer after exposure and prior to development. Such diffusion, particularly into unexposed resist layer areas, can limit resolution of the developed image. It is also believed that a strong photogenerated acid can provide enhanced photospeed (relative to a weak acid).
It has been found that selective blending of members of a PAG mixture of the invention can provide the optimal balance of properties selected for a particular resist containing the PAG blend.
Photoresist compositions are also provided that comprise a PAG blend of the invention. P
Cameron James F.
Mori James Michael
Orsula George W.
Thackeray James W.
Ashton Rosemary
Baxter Janet C.
Corless Peter F.
Edwards & Angell LLP
Frickey Darryl P.
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