Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Radiation sensitive composition or product or process of making
Reexamination Certificate
2000-10-12
2004-01-20
Ashton, Rosemary (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Radiation sensitive composition or product or process of making
C296S067000, C296S900000
Reexamination Certificate
active
06680157
ABSTRACT:
BACKGROUND OF THE INVENTION
The technical field of the invention is lithography and, in particular, methods and materials for advanced lithography using ultraviolet light or particle beams to pattern resist materials.
Generally speaking, “photoresists” are radiation sensitive films used in lithography for transfer of images to substrates. They can form negative or positive images. Conventional manufacturing of integrated circuits has been enabled by high-performance organic polymeric resists that are spin-coated onto a wafer surface. After coating with a photoresist, the coated substrate is exposed to a source of activating radiation. This radiation exposure causes a chemical transformation in the regions of the coated surface that are exposed. After the radiation exposure step, the photoresist is treated with a developer solution to dissolve or otherwise remove either the radiation-exposed or unexposed areas of the resist coating, depending upon the type of photoresist used.
Resist “outgassing” is a concern in advanced lithography and occurs when volatile organic molecules or molecular fragments volatilize from the resist film. This can occur during or after exposure. The outgassing rates for organic resist materials can range from 10
10
to 10
13
molecules/cm
2
-sec or higher. This outgassing can cause a reduction in the resist film thickness in the exposed region and/or lead to the deposition of organic molecules on the exposure system. Deposition on the lens of the ultraviolet exposure apparatus can change the optical properties of the lens, ultimately affecting the amount of light transmitted through the lens and the imaging quality. Similarly, degradation of electron-beam lithography performance can occur when outgassing of photoresists during exposure degrades the high vacuum needed for this exposure mode. Because of the highly energetic nature of the radiation source, control of resist outgassing during exposure is of particular importance in UV lithography, especially when sub-200 micrometer (e.g., 193 nm, 157 nm and extreme ultraviolet) wavelengths or particle beams (e.g., e-beams) are used.
Examples of resist materials that outgas are poly-methyl methacrylate, poly-t-butyl methacrylate, and poly-t-butyl acrylate, each of which are commonly used in resist technology. It is believed that the organic molecules released by these resists arise from polymer fragmentation due to absorbed exposure energy (either photon or electron). The polymer fragmentation can be caused either by polymer main chain scission, polymer side chain fragmentation, or polymer blocking group deprotection. The first two are a direct result of photon absorption or electron impact while the latter arises from a chemical reaction of photogenerated acids.
At the longer wavelengths currently used in commercial semiconductor lithography, outgassing is largely caused by the break-up of the photoacid generators (PAGs) present in the photoresist material. Various formulations have been proposed to make PAGs less volatile. However, as the wavelength of the radiation becomes shorter in advanced lithography systems, the polymer backbone of the resist itself becomes a primary source of volatile organic molecules. None of the resist systems proposed to date for advanced lithography systems have addressed the basic problem of outgassing at sub-200 nanometer wavelengths.
A need therefore exists for methods and materials that overcome or reduce the outgassing problem as it effects advanced resist technology.
SUMMARY OF THE INVENTION
The present invention pertains to polymeric compositions useful for the suppression or elimination of outgassing of volatile components generated from photoresist polymers during lithographic construction. In resists of the invention, an aromatic compound is mixed with a photoresist composition, such that the aromatic compound suppresses or eliminates outgassing of volatile components upon exposure of the resist to radiation. The aromatic additive is preferably an aromatic polymer and in at least some instances can be substituted with at least one electron-donating group or electron-withdrawing group to enhance its stabilizing effects. In one embodiment, the aromatic compound can be an additive to a resist composition. In another embodiment, the aromatic compound can be incorporated into the polymeric backbone of the resist composition.
The present invention also encompasses methods of lithography based on the surprising discovery that the addition of aromatic compounds can suppress or eliminate the release of volatile by-products generated during advanced lithographic processes. For example, the introduction of poly-p-hydroxystyrene into sub-200 nanometer resist compositions prevents or greatly reduces the release of the volatile by-products. The methods of the invention are applicable to advanced resists formulated for use at sub-200 nm wavelengths, such as 157 nm, as well as extreme ultraviolet (EUV), X-ray and/or particle beam systems.
DETAILED DESCRIPTION OF THE INVENTION
The features and other details of the invention will now be more particularly described and pointed out by the following specifications and examples. All percentages by weight identified herein are based on the total weight of the photosensitive resist composition unless otherwise indicated. It will be understood that the particular embodiments of the invention are shown by way of illustration and not as limitations of the invention. The principle features of this invention can be employed in various embodiments without departing from the scope of the invention.
From a resist viewpoint, advanced energy sources can be categorized as one of two types. Highly penetrating energy beams, such as X-rays, high-kV electrons, and high-energy particle beams, are only partially absorbed by the resist layer with much of the exposure energy passing into the underlying substrate. On the other hand, highly absorbing energy beams, such as sub-200 nanometer UV radiation, EUV radiation and low-kV electrons, are almost fully absorbed by the resist. Both categories of energy sources, however, exacerbate the outgassing problem.
More specifically, there has been considerable interest recently in the use of shorter wavelengths of light in lithography to achieve finer resolution. Unfortunately, most conventional photoresist materials absorb far (or deep) ultraviolet radiation strongly. This is most pronounced at sub-200 micron UV wavelengths (e.g., at 157 nanometers) and extreme ultraviolet (EUV) wavelengths, where the radiation takes the form of soft X-rays (e.g., at wavelengths of 10 to 20 nanometers). While this is advantageous from the standpoint of resist speed (i.e. the exposure dose required to form a pattern) and the associated printing rate, it poses a problem for projection lithography because of the highly non-uniform absorption of this radiation through the photoresist thickness. In present photoresist materials, EUV radiation will not penetrate much beyond a film thickness of about 0.10 to 0.15 micrometers. Yet, to fabricate holes and other structures in semiconductor materials such as silicon, as well as metals, or dielectrics, the photoresist layer must be thick enough, preferably about 0.5 to about 1.0 micrometers, to withstand etching and/or other processing steps. Accordingly, in order to make use of the increased resolution afforded by advanced lithography systems in the processing and fabrication of small structures, photoresist materials need to address the problem of photonic penetration/etch resistance and yet remain compatible with conventional lithographic processing techniques.
An example of extended optical wavelengths is lithography employing 157 nm ultraviolet radiation. Patterning of resists with 157 nm radiation from an F
2
excimer laser represents the next evolutionary step in photolithography. Medium-field, 157 nm systems have already been realized and the first full-field, 157 nm system can be expected in the very near future. Resists for this technology must be capable initially of 100 nanometer reso
Ashton Rosemary
Engellenner Thomas J.
Massachusetts Institute of Technology
Mollaaghababa Reza
Nutter & McClennen & Fish LLP
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