Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making electrical device
Reexamination Certificate
2000-02-02
2001-03-13
Le, Hoa Van (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Making electrical device
C430S311000
Reexamination Certificate
active
06200736
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to photoresist developer compositions and methods of use, and more particularly to metal ion-based, aqueous developer compositions for developing electron beam exposed positive photoresists and method therefore.
2. Description of Related Art
Shrinking semiconductor device feature sizes and increasing circuit densities necessitates improvements in semiconductor fabrication processes and materials, in particular submicron lithography tooling and high-resolution resist compositions. Device fabrication requirements such as high-resolution, and tight overlay creates a need for resists and resist/developer combinations exhibiting high dry etch resistance, high-resolution, high speed, and adequate process and line width control.
The smallest feature (i.e., opening or space) that can be produced in a photoresist layer is referred to as its resolution capability. The smaller the feature produced, the better the resolution capability. Presently, features on an integrated circuit require wafer resist resolution capabilities of about 0.25 microns (&mgr;m). The effort to pack ever-increasing functional density on a semiconductor die, however, results in smaller, more densely packed, device elements. Speed and power consumption requirements of these high density integrated circuits further drive the device designer to use increasingly smaller dimensions. It is anticipated that the smallest feature size in an integrated circuit device will approach 0.13 &mgr;m within the next five years.
Fabrication of these semiconductor devices using photolithographic processes includes forming an image on a semiconductor wafer using a mask. The mask includes a transparent substrate, generally quartz, and a thin layer of patterned material, typically 800 Å of chrome. A photoresist is applied over the mask surface to a thickness of between about 2000 Å-6000 Å. The patterned image on the mask is typically four to five times larger than the circuit to be imaged onto the semiconductor wafer. The reduced image is formed on the semiconductor wafer photoresist by passing actinic radiation through the mask, and focusing the reduced image on the wafer photoresist.
The fabrication of a 0.13 micron (&mgr;m), or 130 nanometer (nm), device on the wafer necessitates improvements in mask making processes and materials. Ideally, a 4× mask must have a resolution of about 0.52 &mgr;m in order to provide 130 nm feature resolution on the wafer. However, proximity effects caused by diffusion of radiation in the wafer photoresist reduces the minimum feature size required on the mask to approximately 0.26 &mgr;m. In some cases, even smaller feature sizes may be required in order to correct optical proximity effects. The Semiconductor Industry Association, a trade association of semiconductor device manufacturers, indicates a minimum feature size of 0.20 &mgr;m on the mask in order to accommodate a 130 nm wafer minimum feature size.
The minimum feature size formed in a photoresist is determined by, among other things, the wavelength of the exposing energy. Resolutions on the order of 130 nm on a wafer require short wavelength radiation such as extreme ultraviolet (where the wavelength, &lgr;, is approximately 13 nm), x-rays (&lgr; of approximately 0.1-5 nm), and high energy electron beams. Accordingly, masks having less than a 0.20 &mgr;m minimum feature size include fabrication processes which must be restricted to these energetic, short wavelength radiation sources.
As electron beam energies provide sufficiently short wavelengths, high resolution masks are typically patterned using a computer-controlled, electron beam writing tool. The electron beam exposes preselected portions of an electron-beam sensitive photoresist deposited on a glass or quartz plate overlaid with a thin layer of metal or metal oxide. The electron beam resist is developed and the exposed metal or metal oxide is etched in the pattern of the desired circuit to produce a mask (for full-wafer exposure), or a reticle containing the pattern for a few semiconductor dies, or one die.
Presently available electron beam resists include poly(methylmethacrylate) (“PMMA”), poly(methyl-isopropenylketone) (“PMIPK”), poly(butene-1-sulfone) (“PBS”), poly(trifluoroethyl chloroacrylate) (“TFECA”), poly(&agr;-cyanoethylacrylate-&agr;-amidoethylacrylate) copolymer (“PCA”, and poly-(2-methylpentene-1-sulfone) (“PMPS”). These resists are inconvenient in that the PMMA is very insensitive to electron beam and actinic radiation, and the PBS and PMPS degrade at higher temperatures. Such thermal degradation results in low dry etch resistance, thereby requiring that the mask's underlying chrome layer be wet etched. One problem with wet etching, however, is that the etchant undercuts the exposed metal film, widening the feature size by approximately 0.12 &mgr;m on each side. In the case of a space, or channel, in the mask's metal film, the channel width is increased by 0.12 &mgr;m on both sides thereby increasing the channel width by 0.24 &mgr;m over the original, as-exposed feature size. The resulting feature is larger than the desired minimum feature size of 0.20 &mgr;m required to obtain 130 nm resolutions on the wafer. Accordingly, these sulfone-based electron beam resists are unsuitable for masks where a resolution capability of less than 0.20 &mgr;m is required.
Another problem with the above listed electron beam resist compositions is that they require an organic solvent, typically methylisobutyl ketone (MIBK), methylisoamyl ketone (MIAK), methylpropyl ketone (MPK), ethoxyethyl acetate, or 2-methoxyethylene. Moreover, these resists require that the developer solutions also be based on an organic solvent. These solvents represent significant health hazards, and/or flammability hazards, therefore requiring special handling, and disposal considerations. The federal and local regulations governing the use, and disposal of these solvents include abatement equipment, special storage facilities, and monitoring requirements significantly increase the manufacturing costs associated with using an electron beam resist.
Yet another problem with electron beam resists is their relatively low contrast. The pattern resolution attainable with a given resist for a given set of processing conditions is determined, in part, by the resist contrast (&ggr;). Referring to Prior Art
FIG. 1
, for a positive resist, the film thickness of the irradiated region decreases gradually with increasing radiation exposure, until eventually the clearing dose D
c
is reached, resulting in complete removal of the film upon development. Accordingly, D
c
defines the “sensitivity” of a positive resist.
Contrast (&ggr;
p
) is related to the rate of degradation of molecular weight of the exposed resist and is defined as:
&ggr;
p
=1/[log
10
D
c
−log
10
D
0
] Equation (1)
where D
o
is the dose at which the developer begins to attack the irradiated film and is defined as the intersection of the extrapolated linear portions of the normalized remaining film thickness versus dose plot. A higher contrast value renders non-exposed portions of the resist less susceptible to photodissolution resulting from scattered reflected radiation by a developer. As a result, higher resolutions are characterized by features having crisp, clean edges thereby starkly delimiting the exposed resist regions from unexposed resist regions.
Still referring to
FIG. 1
, the parameters defining the resist characteristics include the “dark loss.” Dark loss represents the thickness of unexposed resist that is removed by the developer. When dark loss is large, thicker resist films must be initially applied so that the resulting thinner, developed film is able to adequately protect the underlying metal film area of the mask during dry etch. The capability of a particular resist relative to resolution and thickness is measured by its “aspect ratio.” The aspect ratio is calculated as the ratio of the as-applied
Etec Systems, Inc.
Le Hoa Van
Skjerven, Morrill, Macpherson, Franklin & Friel
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