1. Field of the Invention
The present invention is concerned with new polymers which can be used to form anti-reflective compositions for use in the manufacture of microelectronic devices. The polymers comprise hydroxyalkyl cellulose reacted with an aryl isocyanate.
2. Description of the Prior Art
Integrated circuit manufacturers are consistently seeking to maximize substrate wafer sizes and minimize device feature dimensions in order to improve yield, reduce unit case, and increase on-chip computing power. Device feature sizes on silicon or other chips are now submicron in size with the advent of advanced deep ultraviolet (DUV) microlithographic processes.
However, a frequent problem encountered by photoresists during the manufacturing of semiconductor devices is that activating radiation is reflected back into the photoresist by the substrate on which it is supported. Such reflectivity tends to cause blurred patterns which degrade the resolution of the photoresist. Degradation of the image in the processed photoresist is particularly problematic when the substrate is non-planar and/or highly reflective. One approach to address this problem is the use of an anti-reflective coating (ARC) applied to the substrate beneath the photoresist layer.
Compositions which have high optical density at the typical exposure wavelengths have been used for some time to form these ARC layers. The ARC compositions typically consist of an organic polymer which provides coating properties and a dye for absorbing light. The dye is either blended into the composition or chemically bonded to the polymer. Thermosetting ARCs contain a cross-linking agent in addition to the polymer and dye. Cross-linking must be initiated, and this is typically accomplished by an acid catalyst present in the composition.
While these ARCs are effective at lessening the amount of light reflected back into the photoresist, most prior art ARC compositions are lacking in that they do not have a sufficiently high etch rate. As a result, prior art ARCs present significant limitations which make them difficult or impossible to use on submicron (e.g., 0.3 xcexcm) features. Accordingly, there is a need for improved ARCs which can be effectively utilized to form integrated circuits having submicron features while absorbing light at the wavelength of interest.
The present invention broadly comprises new polymers for use in forming anti-reflective compositions that are useful for the manufacture of microelectronic devices. These new polymers comprise a hydroxyalkyl cellulose reacted with an aryl isocyanate.
In more detail, the polymers comprise a moiety according to the formula: 
wherein each R is individually selected from the group consisting of xe2x80x94OH, xe2x80x94CH2OH, xe2x80x94Oxe2x80x94R1xe2x80x94Oxe2x80x94X, and xe2x80x94CH2xe2x80x94Oxe2x80x94R1xe2x80x94Oxe2x80x94X. Preferably, at least one of the R""s is xe2x80x94Oxe2x80x94R1xe2x80x94Oxe2x80x94X or xe2x80x94CH2xe2x80x94Oxe2x80x94R1xe2x80x94Oxe2x80x94X. Each R1 is individually selected from the group consisting of branched and unbranched, substituted and unsubstituted alkyl groups (preferably C1-C8, and more preferably C1-C4), and each X has the formula 
where Ar is selected from the group consisting of substituted and unsubstituted aryl (preferably C6-C12) groups. The most preferred aryl groups include phenyls, benzyls, 2-methoxy phenyls, and 2-nitrophenyls.
Preferably, the polymer comprises from about 41-66% by weight, and preferably from about 47-58% by weight of this isocyanate monomer, based upon the total weight of the polymer taken as 100% by weight.
The inventive polymers are formed by reacting a hydroxyalkyl cellulose polymer with an aryl isocyanate in a solvent system. Preferred such solvent systems include a solvent selected from the group consisting of propylene glycol methyl ether acetate (PGMEA), propylene glycol methyl ether (PGME), propylene glycol n-propyl ether, 2-heptanone, N-methylpyrollidinone, and mixtures thereof. Preferably the reaction is carried out for a time period of from about 2-24 hours (and more preferably about four hours), and at a temperature of from about 55-145xc2x0 C. (and more preferably from about 80-120xc2x0 C.).
Reacting the hydroxyalkyl cellulose polymer and aryl isocyanate under these conditions causes the carbon atom of the Cxe2x95x90O group of the isocyanate to bond to an xe2x80x94OH group of the cellulose polymer so as to yield a urethane linkage therebetween. Furthermore, carrying out the reaction in the presence of an excess amount of isocyanate will cause at least some of the isocyanate groups to react with at least some of the urethane linkages. It will be appreciated that this method of forming the polymer is particularly advantageous in that the entire reaction can be carried out within a single reaction vessel. That is, there is no need to isolate any intermediate compounds during this reaction.
The inventive polymers can then be used to prepare anti-reflective compositions by dissolving the polymer in a suitable solvent system. The solvent system should have a boiling point of from about 118-202xc2x0 C., and preferably from about 118-160xc2x0 C. The amount of polymer dissolved in the solvent system is from about 7.5-20% by weight polymer, and preferably from about 9-15% by weight polymer, based upon the total weight of the composition taken as 100% by weight. The solvent system should be utilized at a level of from about 80-92.5% by weight, and preferably from about 85-91% by weight, based upon the total weight of the composition taken as 100% by weight. Preferred solvent systems include a solvent selected from the group consisting of PGMEA, 2-heptanone, N-methylpyrollidinone, and mixtures thereof.
Preferably, the inventive compositions further comprise a crosslinking agent and a catalyst. The crosslinking agent can be separate from the polymer or, alternately, the polymer can include xe2x80x9cbuilt-inxe2x80x9d crosslinking moieties. Preferred crosslinking agents include aminoplasts (e.g., Powderlink(copyright) 1174, Cymel(copyright) 303LF). The crosslinking agent or moieties should be present in the composition at a level of from about 0.25-1.40% by weight, and preferably from about 0.3-1.2% by weight, based upon the total weight of the composition taken as 100% by weight. Thus, the compositions of the invention should crosslink at a temperature of from about 125-225xc2x0 C., and more preferably from about 150-205xc2x0 C.
Preferred catalysts include those selected from the group consisting of p-toluenesulfonic acid, pyridinium tosylate, 4,4xe2x80x2-sulfonyldiphenol, and mixtures thereof. The catalyst should be present in the composition at a level of from about 0.025-0.20% by weight, and preferably from about 0.03-0.15% by weight, based upon the total weight of the composition taken as 100% by weight.
The resulting ARC composition is subsequently applied to the surface of a substrate (e.g., silicon wafer) by conventional methods, such as by spin-coating, to form an anti-reflective coating layer on the substrate. The substrate and layer combination is baked at temperatures of at least about 150xc2x0 C. The baked layer will generally have a thickness of anywhere from about 250 xc3x85 to about 1500 xc3x85. Next, a photoresist can be applied to the ARC layer followed by exposing the photoresist to light at the desired wavelength, developing the exposed photoresist layer, and etching the developed photoresist layer all according to known procedures.
ARCs according to the invention have a high etch rate. Thus, the ARCs have an etch selectivity to resist (i.e., the ARC etch rate divided by the photoresist etch rate) of at least about 1.0, and preferably at least about 1.2, when HBr/02 is used as the etchant. Additionally, at 193 nm the inventive ARCs have a k value (i.e., the imaginary component of the complex index of refraction) of at least about 0.3, and preferably at least about 0.35, and have an n value (i.e., the ratio of the speed of light through a vacuum to the speed of light through the particular material) of at least about 1.5, and preferably at least about 1.6. That is, a cured layer formed from the inventive composition will absorb at least about 96%, and preferably at least about 99% of light at a wavelength of 193 nm. Furthermore, when subjected to a stripping test as defined herein, the inventive ARCs will have a stripping amount of less than about 10 xc3x85, and preferably less than about 5 xc3x85.