The wavelength of light for lithography has been reduced into the deep ultraviolet (DUV) range to produce the feature size necessary for current and future electronics devices. The electronics industry is developing new resists that are tailored to the DUV range. One such resist class is chemically amplified resists.
The main components of chemically amplified resist formulations are a photoacid generator compound, a polymer resin and a solvent capable of dissolving the photogenerator and the resin. For many positive chemically amplified resists, the polymer resin contains acid labile groups which makes the polymer resin insoluble in an aqueous developer. Upon irradiation, the photoacid generator compound produces an acid which cleaves the acid labile groups resulting in a polymer resin that is aqueous soluble. Chemically amplified resists have generated a great deal of interest and there are numerous patents available discussing these compositions such as, for example, U.S. Pat. Nos. 5,069,997; 5,035,979; 5,670,299; 5,558,978; 5,468,589; and 5,389,494.
One group of polymers which can be used as resins in chemically amplified resists are acetal derivatized polymers. The alkali solubility of phenolic resins are greatly inhibited by converting the hydroxyl groups to acetal groups. Typically, acetal phenolic resins are produced by reacting a phenolic resin with a vinyl ether in the presence of an acid catalyst.
The acetal resins are then formulated with a photoacid generator compound and solvent to form a chemically amplified resist product. Upon irradiation, the generated acid cleaves the acetal groups, and a phenolic resin is created which is soluble in an aqueous developer.
One problem with producing acetal derivatized polymers is that currently there are only a very limited number of bulk vinyl ethers that can be used to produce the acetals. It is possible to generate other acetal functionalities by synthesizing intermediates, but this would require a series of reactions which makes the overall process for producing acetal polymers relatively complicated, expensive, and unlikely to be reproducible. Thus, the limited number of bulk vinyl ethers severely constrains the use of acetal polymers because only a small number of acetal groups can be readily and inexpensively generated.
It would be advantageous to be able to generate reproducibly and economically a large variety of acetal groups on polymers. Various resist properties such as alkaline solubility, etch resistance, film shrinkage, temperature stability, adhesion, sensitivity can be altered by choosing appropriate acetal functionalities. Furthermore, if more than one acetal group can be readily generated on the polymer, this would provide further ability to tailor the polymer resin to a specific application. For example, one acetal group may be used to increase the sensitivity, while another acetal group may be used to increase the etch resistance. The host of applications for acetal polymers can be greatly expanded if a large variety of acetal groups on the polymers can be produced readily.
The present inventors have developed a reproducible process for readily producing a wide variety of acetal derivatized polymers.
It is another object of this invention to provide a new, inexpensive, reproducible process for generating a large variety of acetal derivatized polymers that have applications as resins for photoresists.
It is a further object of this invention to provide a process for generating a large variety of acetal derivatized polymers based on mixed acetals. A still further object of this invention is to provide a variety of mixed acetals derivatized polymers that have applications as resins for photoresist. Yet another object of this invention is to provide new photoresist compositions containing mixed acetal derivatized polymers and the use of such new photoresist compositions in photolithography imaging processes to produce microelectronic devices. The present invention has many important advantages over the prior art. First, the present invention provides a new class of polymers based on mixed acetals. Desired properties of the polymer resin can be tailored by choosing among appropriate acetal functionalities. Also, the relative proportions of the different acetals in the polymer can be readily varied by changing the relative proportions of reagents in the feedstock. Changing the proportion of the mixed acetals can further tailor the properties of the polymer resin. In addition, the reproducibility of the mixed acetal polymers compositions are excellent. Reproducible polymer compositions are very important in order to produce commercially viable resist formulations because the properties of the resist must not change from batch to batch. Furthermore, the mixed acetal process is relatively inexpensive because intermediates needed for the reaction are commercially available and only one synthesis reaction is required to produce the final acetal polymer. Previously, a number of synthesis reactions would generally be required to produce the final single acetal polymer if the acetal group was not from one of the readily available vinyl ethers.
The present invention also provides many additional advantages which shall become apparent as described below.