Chapter 20
A Top-Surface Imaging Approach Based on the Light-Induced Formation of Dry-Etch Barriers 1
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U. Schaedeli , M . Hofmann , E. Tinguely , and N . Münzel
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Materials Research, Ciba-Geigy Inc., 1723 Marly/Fribourg, Switzerland OCG Microelectronic Materials AG, Klybeckstrasse 141, Basel, CH-4002, Switzerland 2
It is believed that optical lithography will continue to play an important role for the manufacturing of future generation IC devices. Surface imaging processes might be required to overcome the small depth of focus associated with new high numerical aperture exposure tools. Our new concept is based on the light induced formation of sites for the fixing of reactive monomers in the top zones of a highly absorbing resist film. These sites, which typically consist of photolytically generated Bronsted acid or radicals, are able to oligomerize silicon-containing, reactive monomers during a baking step. The net result is the formation of a silicon gradient in the top zone of the exposed resist film, which subsequently acts as a dry etch barrier. Negative tone images were obtained by exposing resist films, mainly composed from a linear matrix polymer, a sulfonium salt type photoacid generator, and silicon-containing epoxy monomers, with light of 193 nm or 254 nm wavelength.
Historically, the production of integrated circuits has relied exclusively on photolithography as the method for transferring mask patterns onto silicon chips. It is believed that optical lithography will continue to play an important role for the manufacturing of future generation microdevices, where resolution well below 0.2 micrometer will be needed. Surface imaging processes, where the actual interaction between light and matter is limited to the near surface of a resist film, might be required to overcome the small depth of focus, associated with the new high numerical aperture exposure tools. There has been a continuous trend towards shorter wavelengths to meet the need for smaller feature size. At the forefront of the technology roadmap stands deep-UV lithography (DUV), and 193 nm lithography as the most likely candidate for becoming the technology of choice for future advanced lithography.
0097-6156/95/0614-0299$12.00/0 © 1995 American Chemical Society
Reichmanis et al.; Microelectronics Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
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MICROELECTRONICS TECHNOLOGY
V/w)/A Si-Wafer
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I DUV-expose through mask
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Scheme 1. Process sequence involved in the image forming process.
Figure 1. SEM micrograph of the latent image as formed after post exposure bake on a resist film composedfrom9 (25 parts), PS (75 parts), TPST (5 parts), and MA (1 part).
Reichmanis et al.; Microelectronics Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
Downloaded by CORNELL UNIV on September 1, 2016 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0614.ch020
20.
SCHAEDELI ET AL.
Top-Surface Imaging Approach
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Top surface imaging concepts can mainly be separated in two distinct categories: (I) two layer approaches, where a relatively thin image forming layer, which acts as an etch barrier, is located above of a relatively thick planarizing layer (1-6) and (Π) single layer approaches, where the etch barrier is formed in the top zones of a resist film (7-14). In both cases, the image of the mask, which has been defined in the top zones of the film, is subsequently transferred down to the semiconductor substrate by means of dry etching. In approach (I), the etch barrier forming substance is typically an organosilicon compound, and can be present in the original resist formulation. This is contrasted to approach (Π), in which a silylation process is needed after the image defining step to allow for the formation of the dry etch barrier. The major drawback of materials based on approach (I) is their need for additional process steps. Qn the other hand, the silylation process in approach (Π) needs to be very strictly controlled in order to obtain reproducible lithographic results. The inherent advantages of approaches (I) and (Π) are combined to design a new resist approach which allows for a process consisting of a minimum number of steps (single layer) and avoids the need for a silylation process. In addition, the concept should have enough flexibility to allow imaging at both 248 nm and 193 nm wavelengths, without the needforintroducing changes in the chemistry involved in the image forming process.
Results and Discussion Imaging Concept. The process steps involved in the image forming sequence of our top surface imaging approach are outlined in Scheme 1. In afirststep (A), a resist film which consists of a matrix polymer, a photoacid or photo radical generator, a siliconcontaining reactive monomer and an optional light absorber is imaged through a mask. The net result is the generation of sites for fixing the polymerizable monomers in the uppermost zones of the highly absorbing resist film. These sites are composed of photolytically generated Bronsted acid or radicals. In a second step (B), the wafer containing the illuminated resist film is heated. Thereby the reactive, silicon-containing monomers, which can be considered as a highly boiling solvent, start migrating in the resist matrix. Eventually, these monomers reach the top layers of the film. In areas which have been irradiated before, and which do therefore contain the fixing sites, immobilization of the reactive monomer takes place due to rapid oligomerization. If sufficiently high doses are applied, the reactive monomers cannot leave the film in these irradiated areas. The result is the formation of a silicon gradient in the illuminated zones of the film. However, in the unirradiated zones, where no initiators had been introduced, the reactive monomers can leave the resist film. Whereas the original film thickness is retained in the irradiated zones of the film, a volume loss is observed in the unirradiated zones, leading to the formation of a clearly visible topographic image of the mask pattern (Figure 1). In the third step (C), the mask pattern, which was generated in the near surface region of the resist film, is transferred down to the semiconductor substrate by means
Reichmanis et al.; Microelectronics Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
MICROELECTRONICS T E C H N O L O G Y
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Downloaded by CORNELL UNIV on September 1, 2016 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0614.ch020
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