Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Electron beam imaging
Reexamination Certificate
1999-12-28
2002-03-19
Young, Christopher G. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Electron beam imaging
C430S313000, C430S328000, C430S942000
Reexamination Certificate
active
06358670
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for increasing the etch resistance of photoresists which are suitable for use in the production of microelectronic devices such as integrated circuits. More particularly, the invention provides a process for increasing the etch resistance of the upper surface of photoresists by a surface-intensive dose of electron beam radiation. Such imparts increased surface etch resistance to the photoresist without causing as much shrinkage in the bulk of the film as a uniform electron beam irradiation dose.
2. Description of the Related Art
The art of forming images for the production of microelectronic devices is well known. In this regard, photoresist compositions are widely used image-forming compositions for microelectronic device manufacturing processes. Generally, in these processes a thin coating of a radiation sensitive photoresist composition is first applied to a substrate material. The coated substrate is then treated to evaporate any solvent in the photoresist composition and to fix the coating onto the substrate. The coated surface of the substrate is next subjected to an imagewise exposure to actinic radiation. This radiation exposure causes a chemical transformation in the exposed areas of the coated surface. After imagewise exposure, the coated substrate is contacted with a developer solution to dissolve and remove either the radiation-exposed or the unexposed areas of the coated surface of the substrate.
The production of positive photoresists is well known in the art as shown in U.S. Pat. Nos. 3,666,473; 4,115,128 and 4,173,470. Photoresists are either positive working or negative working. In a negative working resist composition, the imagewise light struck areas harden and form the image areas of the resist after removal of the unexposed areas with a developer. In a positive working resist the exposed areas are the non-image areas. The light struck parts are rendered soluble in aqueous alkali developers. The ability to reproduce very small dimensions, is extremely important in the production of large scale integrated circuits on silicon chips and similar components. As the integration degree of semiconductor devices becomes higher, finer photoresist film patterns are required. Positive photoresists have been found to be capable of much higher resolution and have almost universally replaced negative resists for this purpose. Positive working resists often contain aqueous alkali soluble polyvinyl phenol or phenol formaldehyde novolak resins together with light sensitive materials, usually a substituted naphthoquinone diazide compound. The resins and sensitizers are dissolved in an organic solvent and are applied as a thin film coating to a substrate suitable for the particular application desired. The resin component of photoresist formulations is soluble in an aqueous alkaline solution, but the photosensitizer is not. Upon imagewise exposure of the coated substrate to actinic radiation, the exposed areas of the coating are rendered more soluble than the unexposed areas. This difference in solubility rates causes the exposed areas of the photoresist coating to be dissolved when the substrate is immersed in an alkaline developing solution, while the unexposed areas are substantially unaffected, thus producing a positive image on the substrate.
The uncovered substrate is thereafter subjected to an etching process.
Frequently, this involves a plasma etching against which the resist coating must be sufficiently stable. The photoresist coating protects the covered areas of the substrate from the etchant and thus the etchant is only able to etch the uncovered areas of the substrate. Thus, a pattern can be created on the substrate which corresponds to the pattern of the mask or template that was used to create selective exposure patterns on the coated substrate prior to development.
Chemical amplification resist films have been developed, which have been found to have superior resolution. 248 nm and 193 nm photoresists are based on chemically amplified deprotection. With this mechanism, a molecule of photogenerated acid catalyzes the breaking of bonds in a protecting group of a polymer. During the deprotecting process, another molecule of the same acid is created as a byproduct, and continues the acid-catalytic deprotection cycle. The chemistry of a 193 mn photoresist is based on polymers such as, but not limited to, acrylates, cyclic olefins with alicyclic groups, and hybrids of the aforementioned polymers which lack aromatic rings, which contribute to opacity at 193 nm. It has thus been known to utilize photoresists based on methacrylate resins for the production of microstructures by means of 193 nm radiation. However, chemically amplified resist films have not played a significant role in the fine pattern process using deep UV because they lack sufficient etch resistance, thermal stability, post exposure delay stability and processing latitude. A typical chemical amplification photoresist film comprises a polymer, a photoacid generator, and other optional additives. The polymer is required to be soluble in the chosen developer solution, and have high thermal stability and low absorbance to the exposure wavelength in addition to having excellent etch resistance.
It would be desirable to overcome the etch resistance problems and to provide a photoresist film superior in etch resistance. There have been several attempts to solve this problem. One attempt to improve the etching resistance proposes to treat the substrate having a finished, developed, image-structured photoresist coating with specific alkyl compounds of magnesium or aluminum, in order to introduce the given metals in the resist material as etching barriers (See U.S. Pat. No. 4,690,838). The use of metal-containing reagents, however, is generally not desired in microlithography process, due to the danger associated with contamination of the substrate with metal ions.
It has been determined that the etch resistance of imagewise exposed and developed photoresists may be increased by an overall flood exposure with an electron beam. One proposal has been to irradiate an exposed and developed photoresist with a uniform electron dose prior to plasma etch processing. However, this has been determined to cause excess shrinkage in the bulk of the film. It has been determined, that the etch resistance of imagewise exposed and developed photoresists can be enhanced using a non-uniform, surface-intensive dose of electrons. This non-uniform dose, or surface cure treatment imparts etch resistance to the surface of the photoresist without causing as much film shrinkage as a uniform dose. This reduced shrinkage is particularly desirable in the advanced lithography resists which can shrink up to 35% of their original thickness during e-beam processing.
The surface cure via flood electron beam treatment uses a non-uniform dose distribution as a function of photoresist depth. The electrons initiate crosslinking in the resists, making the resists more mechanically robust and thus more etch resistant. By delivering fewer electrons to the bulk of the resist layer and more electrons to the surface (e.g., about the top third of the thickness), the surface is preferentially more crosslinked than the bulk of the resist, resulting in a hardened “shell” of resist. This hardened shell is able to maintain the photoresist dimensions during a plasm etch process.
In general, a photosensitive resist is applied to a substrate, imagewise exposed to actinic radiation, and developed in a developer solution to create the desired pattern of resist. The patterned resist is then overall flood exposed to a large area electron source, at dose levels which can range from 5 &mgr;C/cm
2
to 50,000 &mgr;C/cm
2
. In this manner, a resist image is now sufficiently stable to permit plasma etching. In this way, a photoresist image is provided with an enhanced etch resistance rate without needing to treat the resist coating with metal compounds.
SUMMARY OF THE INVENTION
Ross Matthew
Wong Selmer
Electron Vision Corporation
Roberts & Mercanti LLP
Young Christopher G.
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