Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Post imaging radiant energy exposure
Reexamination Certificate
1999-12-02
2003-01-07
Huff, Mark F. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Post imaging radiant energy exposure
C430S311000, C430S315000, C430S324000, C430S325000, C430S330000, C216S049000, C438S692000, C427S337000
Reexamination Certificate
active
06503693
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to methods for altering physical and chemical characteristics of photoresist. In particulars the present invention relates to altering physical and chemical characteristics of exposed and developed photoresist features.
BACKGROUND OF THE INVENTION
In the course of manufacturing integrated circuits, semiconductor wafers typically are coated with photoresist. Ultraviolet light is passed through a mask of pre-defined features, both clear and opaque, that are subsequently transferred into the photoresist. The photoresist changes chemically as a result of the UV exposure. These changes typically alter the solubility characteristics of the photoresist. As a result of the alteration of the solubility characteristics, during a developing process, portions of the photoresist may be removed, resulting in a pattern of photoresist features determined by the mask on the wafer.
After defining and developing the pattern of photoresist features, the wafer may then be processed. The processing may be carried out according to a number of processing steps, including etching away areas determined by the pattern, doping impurities, and/or other processes. All or portions of the photoresist features may then be removed. The process of defining a mask of photoresist features and-carrying out processing on the wafer may be repeated until the desired integrated circuit device is fabricated.
The relentless drive towards ever smaller sub-micron feature sizes continues. Smaller feature sizes have resulted in more and faster components being included in a chip of a given size. However, smaller feature sizes have created numerous technical challenges for microlithographic processing.
The technical challenges include developing and implementing materials and processes capable of accurately and reproducibly creating the desired small lithographic structures. One means for doing so includes utilizing ever smaller wavelengths of light to expose the photoresist. Smaller wavelengths have made it possible to resolve smaller features. However, the photoresist needs to be capable of interacting with the light to produce a desired result, such as changing the solubility characteristics of the resist.
Along these lines, traditional I-line resists, based on novolac resins, have functioned well for feature sizes down to about 0.35 &mgr;m. The density of aromatic groups in these resins typically provides adequate plasma etch resistance. However, new photoresist polymers and photoimaging mechanisms have had to be designed in order to overcome problems associated with optical resolution below this level.
Because I-line wavelength light (about 365 nm) results in adverse diffraction effects in resolving features sizes smaller than about 0.35 &mgr;m, light of about 248 nm and about 193 nm, deep UV (DUV) light, is increasingly being used for lithography. The intensity of light at these wavelengths from known sources, however, is much lower than the I-line. Photoresists for use with such wavelengths typically are know as deep UV (DUV) photoresists.
DUV resists based on a “chemical amplification” mechanism have become increasingly popular because of their high sensitivity and demonstrated ability to resolve patterns at about 0.25 &mgr;m and smaller. These systems typically are based on poly(vinyl phenol) polymers or copolymers in which phenol or carboxylic acid groups are partially “blocked” or protected by moieties that can be chemically cleaved to regenerate the phenol or carboxylic acid. Thereby, DUV photoresists provide a solubility difference for development.
Photo-acid generators (PAG's) provide a UV-activated source of strong acid for the catalytic cleavage reaction. It is this catalytic mechanism that is primarily responsible for the high sensitivity of these systems. The polymers, however, are inherently less plasma etch resistant than those of I-line resists due to the lower concentration of aromatic groups in the former. Highly aromatic polymers like those in I-line photoresists are not transparent enough at the DUV exposure wavelengths.
Newer 193 nm resists exhibit the above described problems to an even greater degree. This at least in part results from the lack or near lack of aromatic groups in these polymers, necessitated because of the extremely high absorbance of 193 nm light by aromatic groups.
A new class of 193 nm resists based on cyclo-olefin polymers has been developed to try to address the problem of etch selectivity. However, the etch performance of these newer resists is still inferior to that of I-line resists.
Researchers have expended a great deal of effort in studying the relationships between chemical structure and plasma etch resistance. Except for a handful of post-imaging processes that are described below, most attention has been directed at building etch resistance into the original photoresist polymer. Often, the result of post-imaging processes includes compromising lithographic quality.
Often, photoresists are subject to high temperature processing. Such processing can introduce problems including flow or distortion of features formed of the resist as the resist reaches its glass transition temperature, or basically melts, during exposure to the high temperatures. One method of mitigating the flow or distortion that may occur during high temperature plasma etching of photoresists involves a process known as photostabilization. Photostabilization typically is carried out after the photoresist pattern is formed and prior to etching and/or ion implant.
A preferred method of photoresist stabilization that results in fast stabilization times is disclosed in U.S. Pat. No. 4,548,688, the entire contents of which are hereby incorporated herein by reference. In the preferred method, the photoresist is exposed to UV radiation while its temperature is elevated upon increase in the degree of polymerization due to exposure to the radiation. Additionally, the elevated temperature at any instant during exposure is kept below the flow temperature of the resist at that instant.
Photostabilization has resulted in advantages to the processing of I-line photoresists, and some benefit to deep UV photoresists. The benefits appear to result from crosslinking reactions that take place, which causes, among other things, resist to flow. See Jordhamo and Moreau,
DUV Hardening of DUV Resists
, SPIE, Vol. 2724, pp. 588-600 (1996), the entire contents of which are hereby incorporated by reference.
Although photostabilization of DUV photoresists produces beneficial effects, serious adverse effects exist with the process in the form of shrinkage of features during photostabilization. As described below, shrinkage results in many problems in the subsequent processing and has implications to the final device structure.
FIG. 1
illustrates how features formed of photoresist may shrink during processing.
Shrinkage apparently results from multiple causes particularly related to the DUV class of photoresists. Deep UV photoresist resins, typically polymers or copolymers of hydroxy styrene, usually have higher molecular weights and glass transition temperatures than the novolac resins used in I-line photoresists. As a result, the former do not pack as efficiently during the coating process as do the latter. Inefficient packing appears to result in substantial amounts of free volume in the photoresist.
During thermal processing subsequent to feature formation, such as photostabilization, this free volume in the photoresist may be eliminated as the polymer chains are packed more efficiently, resulting in shrinkage. Concurrently, trapped solvents may also be released, further compounding the mass loss. And possibly most importantly, photostabilization, or any thermal processing at sufficiently high temperatures, can cause decomposition of the photo acid generator. Decomposition of the PAG results in formation of strong acid and deprotection of the solubility inhibiting groups on the photoresist polymer. This can result in further mass loss and further shrinkage. It is not
Hallock John Scott
Mohondro Robert Douglas
Axcelis Technologies Inc.
Chacko-Davis Daborah
Huff Mark F.
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