The invention relates generally to the manufacture of electronic devices. More specifically, this invention relates to methods of trimming photoresist patterns useful in shrink processes for the formation of fine lithographic patterns.
In the semiconductor manufacturing industry, photoresist materials are used for transferring an image to one or more underlying layers, such as metal, semiconductor and dielectric layers, disposed on a semiconductor substrate, as well as to the substrate itself. To increase the integration density of semiconductor devices and allow for the formation of structures having dimensions in the nanometer range, photoresists and photolithography processing tools having high-resolution capabilities have been and continue to be developed.
Positive-tone chemically amplified photoresists are conventionally used for high-resolution processing. Such resists typically employ a resin having acid-labile leaving groups and a photoacid generator. Exposure to actinic radiation causes the acid generator to form an acid which, during post-exposure baking, causes cleavage of the acid-labile groups in the resin. This creates a difference in solubility characteristics between exposed and unexposed regions of the resist in an aqueous alkaline developer solution. Exposed regions of the resist are soluble in the aqueous alkaline developer and are removed from the substrate surface, whereas unexposed regions, which are insoluble in the developer, remain after development to form a positive image.
One approach to achieving nm-scale feature sizes in semiconductor devices is the use of short wavelengths of light, for example, 193 nm or less, during exposure of chemically amplified photoresists. To further improve lithographic performance, immersion lithography tools have been developed to effectively increase the numerical aperture (NA) of the lens of the imaging device, for example, a scanner having a KrF or ArF light source. This is accomplished by use of a relatively high refractive index fluid (i.e., an immersion fluid) between the last surface of the imaging device and the upper surface of the semiconductor wafer. The immersion fluid allows a greater amount of light to be focused into the resist layer than would occur with an air or inert gas medium. When using water as the immersion fluid, the maximum numerical aperture can be increased, for example, from 1.2 to 1.35. With such an increase in numerical aperture, it is possible to achieve a 40 nm half-pitch resolution in a single exposure process, thus allowing for improved design shrink. This standard immersion lithography process, however, is generally not suitable for manufacture of devices requiring greater resolution, for example, for the 32 nm and 22 nm half-pitch nodes.
Considerable effort has been made to extend the practical resolution beyond that achieved with standard photolithographic techniques from both a materials and processing standpoint. For example, multiple patterning processes have been proposed for printing CDs and pitches beyond lower resolution limits of conventional lithographic tools. One such multiple patterning process is self-aligned double patterning (SADP), described for example in U.S. Patent Application Pub. No. 2009/0146322A1. In this process, a spacer layer is formed over pre-patterned lines. This is followed by etching to remove all spacer layer material on horizontal surfaces of the lines and spaces, leaving behind only material on the sidewalls of the lines. The original patterned lines are then etched away, leaving behind the sidewall spacers which are used as a mask for etching one or more underlying layers. Since there are two spacers for every line, the line density is effectively doubled.
For multiple patterning and other lithographic processes, the printing of isolated lines and posts having a duty ratio greater than two with a good process window is essential at the lithography stage. Achieving a good process window for isolated lines and posts through direct lithographic imaging is, however, extremely difficult due to poor aerial image contrast at defocus as compared with dense lines.
There is a continuing need in the art for improved photolithographic methods for the formation of fine patterns in electronic device fabrication and which avoid or conspicuously ameliorate one or more of the foregoing problems associated with the state of the art.