Drying and gas or vapor contact with solids – Process – Gas or vapor contact with treated material
Reexamination Certificate
2000-08-04
2002-05-07
Wilson, Pamela (Department: 3749)
Drying and gas or vapor contact with solids
Process
Gas or vapor contact with treated material
C034S428000, C034S429000, C427S374100
Reexamination Certificate
active
06381873
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of semiconductor lithography, in particular to a method and apparatus for coating a substrate with a polymer solution and for drying the applied coating. More specifically, the invention relates to the formation of photo-, electron-, and X-ray resists on the surfaces of semiconductor wafers.
BACKGROUND OF THE INVENTION
Resists are organic or inorganic materials which are applied either directly onto the surface of semiconductor substrate or onto the surface of a topological layer preformed on the substrate for the formation of a selected latent image of a pattern which later is turned into a functional layer of a chip to be produced. In a majority of cases, the materials applied on the semiconductor substrate are organic materials. There exist a great variety of such materials. Among them, tens of polymer compositions are commercially used as polymer-type resists. Types and properties of some of them are described, by Wayne M. Moreau in: “Semiconductor Lithography. Principles, Practices, and Materials”, Plenum Press, New York, London, 1988.
In the process of development of the latent image formed in the resist as a result of exposure to light or other radiant energy, the exposed or non-exposed areas are removed by subsequent etching processes to form the so called mask having a configuration of the aforementioned pattern. The aforementioned process is known as semiconductor lithography. The mask produced by the semiconductor lithography plays a crucially important role for protecting the masked areas during processing of opened areas in subsequent processes such as doping by ion implantation, coating in lift-off lithography, etching, etc. Films from which masks are formed are normally have a thickness within the range of from fractions of micron to several microns.
It is understood that protective property of the mask is one of the most important factors in the quality of the entire chip manufacturing process.
The properties of the mask, in turn, to a great extent depend on technological operations used in the manufacture of the masking film, such as application of a coating material on a substrate from a solution, uniform spreading of the applied material over the substrate, and drying of the applied coating for the formation of a coating film.
Quality of the masking films is determined by such factors as uniformity of thickness and properties, presence of defects such as pinholes, surface cracks, structural nonuniformity caused by foreign particles, etc., and adhesion.
It is understood that processes used for the manufacture of the mask should exclude formation of the aforementioned defects and combination with high adhesion of the film to the substrate.
It is obvious that a main process that determines the final quality of the mask is drying. In the context of the present invention, the term “drying” means removal of the solvent from the liquid polymer coating applied onto the surface of the substrate. It is understood that physical and chemical processes, which accompany removal of the solvent from resist also, may lead to conversions in the polymer itself. Such conversion, however, occur at temperatures higher than the temperature of drying. It should be noted in this connection, that the step of drying can be divided into a process of drying itself at a temperature that does not cause the aforementioned conversions and a process of baking at a temperature that is maintained to cause such conversion as hardening.
More specifically, drying out of a material includes the processes of penetration of the solvent from the polymer itself into free volumes in the polymer, such as voids, microcracks, or air bubbles with subsequent transition of the solvent from the liquid state into a vapor phase a gaseous phase (vapor). Kinetic characteristics (time behavior) of the above-indicated processes determine the mechanism of removal of the solvent from the coating.
The initial stage of drying, which is normally carried out at room temperatures, is characterized by a high content of the solvent in the polymer coating. As the coating becomes dry, the yield of the solvent into a gaseous phase is retarded, whereby the rate of drying is reduced. It is known, however, that by holding the polymer coating only at room temperatures, it is impossible to obtain high values of protective properties such as adhesion, defect-free condition, etc.
Therefore, for obtaining the above properties, drying should include a high-temperature stage (i.e., the stage at a temperature above room temperature but below the glass transition temperature for the polymer −T
g
. After completion of the phase separation at the first stage of drying with the formation of a polymer matrix that contains the solvent, the increase in the process temperature accelerates diffusion of the solvent into the polymer (as a rule, a constant of diffusion depends on the temperature exponentially). As a result, the solvent is rapidly removed from the polymer matrix. Increase in the polymer temperature also decreases its viscosity, which in addition to the aforementioned accelerated diffusion, leads to a decrease in internal stress. This, in turn, affects uniformity of adhesion over the coating area. It is understood, however, that the high-temperature stage of drying should not exceed the level at which such undesirable thermodestruction or thermopolymerization may occur.
It is known that the initial stage of drying of a polymer at an increased temperature is accompanied by a sharp increase in the rate of solvent removal. After having reached its maximum, this rate then drops to zero.
As has been mentioned above, evaporation of the solvent from the external surface of the coating occurs in parallel with a phase transition (evaporation) into free volumes of the polymer matrix, such as microcracks, voids, etc. This process is known as internal vapor formation.
It has been proven experimentally that connection exists between internal vapor formation and protective properties, for example, of a photoresist coating. Many factors influence kinetic characteristics of the process of the internal formation. The following are examples of these factors: concentration of the solvent in the polymer, solvent vapor pressure, geometry of microcavities, density of distribution of microcracks and microcavities in the coating volume, coefficient of diffusion of the solvent, viscosity and surface tension of the polymer coating, and temperature of the coating.
The increase in the rate of internal vapor formation leads to an increase in concentration of defects and to a decrease in adhesion of the coating to the substrate. These phenomena are caused by an increase in the gas pressure of solvent vapor in microcracks and microcavities and by subsequent opening of the aforementioned microcracks and microcavities to the interface between the substrate and the coating film and to the external surface of the film.
It is obvious that surface microcracks as well as microcavities and microcracks located near the surface of the coating film affect adhesive and protective properties of the coating film to a lesser degree than those located inside the film and on the interface between the coating and the substrate. It is understood that aggregation of such defects distributed across the film cross section may lead to the formation of pinholes in the coating film.
At the first stage of drying the internal vapor generation in the protective coating with a high concentration of the solvent does not essentially affects protective properties of the polymer coating. This is explained by favorable conditions for removal of the solvent at this stage of the drying, such as a relatively high rate of diffusion of the solvent molecules in the polymer film and high mobility of the polymer molecules enhancing closing of microcracks and microcavities.
Decrease in the concentration of he solvent is accompanied by an increase in the viscosity of the polymer in the coating film and decrease in planarization ability
Kristul Joseph
Peremychtchev Vladimir
Rakhvalsky Igor
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