1. Field of the Invention
The present invention relates to (1) a resist composition suitable for microprocessing techniques that exhibits excellent resolution and excellent resistance to surface roughness and side lobeunder use of a halftone mask; and (2) a patterning process using the resist composition.
2. Description of the Related Art
In recent years, a finer pattern rule is demanded as high integration and high-speed of LSIs have been achieved. Under these circumstances, there have been vigorously developed microprocessing techniques using deep-ultraviolet lithography or vacuum ultraviolet lithography. Photolithography using KrF excimer laser light at a wavelength of 248 nm as a light source has already used as a major technique for actually fabricating semiconductor devices. In order to achieve further pattern size reduction, use of ArF excimer laser light at a wavelength of 193 nm has been under review, and ArF excimer laser has already been used in some trial productions. The ArF excimer laser lithography is technically, however, still in its infancy, and has various outstanding problems for actually fabricating semiconductor devices.
The properties required for resist compositions applicable to ArF excimer laser lithography are transparency at a wavelength of 193 nm and dry etching resistance. As resist compositions exhibiting both of the properties, there were proposed resist compositions having as a base resin a poly(meth)acrylic acid derivative having a bulky acid cleavable protective group represented by 2-ethyl-2-adamantyl group or 2-methyl-2-adamantyl group (See Japanese Patent Application Laid-open (kokai) Nos. 09-73173 and 09-90637). After that, various compositions have been proposed, but most compositions share use of a resin having a highly transparent backbone and a carboxylic acid moiety protected with a bulky tertiary alkyl group.
Most of the tertiary alkyl groups generally used as protective groups for carboxylic acids, however, have low reactivity and absolutely lacks necessary properties for applications requiring high resolution such as forming micro grooves or micro holes.
The reactivity of the alkyl groups can be enhanced to some extent by increasing the temperature of a heat treatment conducted after exposure. But, this also promotes acid diffusion, and which deteriorates pitch dependency and mask fidelity. Thus the temperature increase does not result in the enhancement of resolution ultimately. If resolution is not enhanced, semiconductor devices with reduced size cannot be fabricated.
In order to enhance the resolution, improvement of exposure systems and resist compositions have been attempted.
A halftone phase shift mask is designed so that light-shading parts allow a small amount of light to pass through and the phase of the transmitted light is reversed relative to light-transmitting parts. The intensity of light at the edge of a pattern is decreased by the interference of lights having opposite phases. This results in enhancement of optical contrast and considerable enhancement of resolution. Thus the halftone phase shift mask is an indispensable tool in advanced lithography.
Resolution is also enhanced by using a composition comprising an acid-labile group that is apt to be deprotected by acid generated from an photoacid generator, specifically, a composition using an acetal protection group as a protection group for carboxylic acid.
When this composition is combined with the halftone phase shift mask, a small amount of light that passed through the light-shading parts effects the reaction of resist in non-pattern areas, and which causes surface roughness or a recess (side lobe) between adjacent patterns. The tendency of causing surface roughness or side lobe depends on mask transmittance, lighting conditions, and the design of a pattern. The tendency also depends strongly on a resist composition. In particular, when a highly reactive acetal protection group is used, irradiation of relatively weak light advances reaction. This results in insufficient resistance to surface roughness and side lobe.
Under these circumstances, there is a desire for a composition that comprises a highly reactive acid labile group and has excellent resistance to surface roughness and side lobe.
It is known that acid diffusion is controlled by adding a basic compound (see Japanese Patent Application Laid-open (kokai) No. 05-249662). For the same purpose, basic ammonium salts are also added (see Japanese Patent Application Laid-open (kokai) No. 06-242605). In resist solvents, the basic ammonium salts probably react with propylene glycol monomethyl ether acetate or ethyl lactate to form acetate or lactate. In contrast, tetramethyl ammonium hydroxide is an aqueous solution or a methanol solution, and addition of it as such solutions results in the presence of unwanted water or alcohol in resist solvents. This is not preferable and removal of the water or alcohol requires a complicated process of removing the low boiling point substances by concentration.
Japanese Patent Application Laid-open (kokai) No. 2003-5376 discloses a resist composition that has reduced pitch dependency, particularly reduced line-and-space pitch dependency, by combining a compound generating alkanesulfonic acid in which hydrogen atoms at the α position are substituted with fluorine atoms and an onium salt of nonfluorinated alkanesulfonic acid. The detailed mechanism of providing the advantageous effect is not described in the document, but it supposedly depends on that fluorine-containing sulfonic acid generated upon exposure reacts with the onium salt of nonfluorinated alkanesulfonic acid, generating nonfluorinated alkanesulfonic acid and an onium salt of fluorine-containing sulfonic acid, thereby replacing strong acid (fluorine-containing sulfonic acid) with weak acid (nonfluorinated alkanesulfonic acid).
But, depending on the type of the acid labile group of a polymer to be used, replaced weak acid can advance acidolysis or conversely the capability of restricting deprotection can function excessively, resulting in insufficient resolution.
Under the circumstances, there is a desire for developing a resist composition that exhibit both high reactivity and the capability of inhibiting reaction caused by faint light, and also exhibit high resolution and excellent resistance to surface roughness and side lobe under use of a halftone phase shift mask.