Coating processes – With post-treatment of coating or coating material – Movement of work treats coating
Reexamination Certificate
2000-12-29
2002-08-06
Barr, Michael (Department: 1762)
Coating processes
With post-treatment of coating or coating material
Movement of work treats coating
C427S240000, C427S377000, C427S294000, C118S050000, C118S052000, C118S600000, C438S782000
Reexamination Certificate
active
06428852
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates in part to a method and system for forming and coating a liquid composition on to a solid surface. More particularly, this invention relates to a process and system for coating a solid surface with a liquid polymer composition. The invention also relates to a method for conditioning liquid polymer compositions to optimize the uniformity of the thin films created during the coating process.
Liquid polymer compositions such as photoresists, antireflective coatings, and spin-on dielectrics are commonly used to coat a solid surface. For example, printed circuits presently are formed on the surface of a wafer such as silicon or gallium arsenide under cleanroom conditions. A commonly used process in microelectronic circuit manufacturing includes a step of applying a liquid photoresist onto the wafer surface and spinning the wafer so that the photoresist forms a thin, uniform coating on the wafer surface. The photoresist then is exposed to light through a patterned mask to transfer a circuit pattern from the mask onto the wafer. The exposed resist is developed to form an image of the desired circuit features on the wafer. Areas not coated with the developed photoresist are then processed further to form a semiconductor device.
Wafers having a diameter of 300 mm or larger are now in development. These diameters are larger than the diameters of commercially produced wafers. In order to attain a photoresist coating of the same thickness and uniformity as for the smaller diameter wafers, the 300 mm or larger wafers must be rotated at speeds that produce turbulent gas flow above the wafer. This turbulence may lead to non-uniform coating thickness, particularly adjacent to the edge of the wafer where the rotational speeds are the highest. A photoresist coating having non-uniform thickness is undesirable since it causes undesired variability in photoresist exposure and development.
To limit the turbulence above the wafer and thereby limit non-uniform photoresist coatings, it has been previously proposed to coat the wafer under partial vacuum. Unfortunately, it has been found that when the photoresist is dispensed into such a reduced pressure environment, the dissolved atmospheric gases in the photoresist liquid out gas by bubble formation. Bubble formation is undesirable because bubbles in the photoresist dispense lines and nozzle cause an inaccurate volume of photoresist to be dispensed on the wafer, thereby affecting final film uniformity on the wafer and the repeatability of the film coating from one wafer to the next. Those of ordinary skill in the art recognize that current typical dispense volumes are in the range of about 1 ml to about 10 ml of photoresist. Typically, each ml of resist is approximately 1 gram. Preferably, an approximately 3 ml dispense (3 grams) is used.
Bubbles are undesirable in the photoresist because they interfere with the exposure and development of the photoresist after it is dispensed onto the wafer. This distortion may result in breaks within the lines formed to define the features in the printed circuit. As the desired circuit elements and conductive lines of a printed circuit become smaller, the goal of the wafer manufacturers, even small bubbles may interfere with accurate transfer of lithographic patterns onto the wafer; generating defects in the lines used to define the printed circuit.
In the spin-coat process, an aliquot of the liquid composition, like a photoresist, is dispensed onto a stationary or slowly rotating substrate. The liquid composition contains solids at least partially dissolved in a volatile solvent. Such liquid compositions are typically true solutions; however, they can also include colloidal dispersions or suspensions. After the photoresist is dispensed onto the substrate, the substrate is rotated at speeds ranging from 1,000 to 7,000 rpm. During this high-speed spin up step, centrifugal forces spread the liquid composition across the substrate. Concurrent with this spin up step, the volatile solvent from the liquid composition evaporates. The result is a thin film of solid material deposited on the substrate. Uniform evaporation of solvent from the liquid composition aids in the formation of a uniform thin film.
In an optimized spin-coat process, the thickness of the deposited film is directly proportional to the viscosity and solvent fraction in the liquid composition deposited on the substrate. In addition, the final film thickness is proportional to the inverse square root of the final spin speed of the substrate. Lastly, optimum film uniformity from the spin-coat process occurs when the substrate is rotated at the highest possible spin speed at a constant temperature. The viscosity of the liquid composition changes with fluctuations in the temperature of the liquid. Accordingly, photoresist temperature is controlled to within ±1° C. to minimize temperature-related changes in viscosity and hence film thickness.
In a semiconductor spin-coat process, a silicon wafer is typically the substrate and a typical liquid composition is a photoresist. In this application a highly uniform coating of the photoresist film across the wafer is required to achieve accurate transfer of lithographic patterns of the desired circuit features to the wafer. A uniformity of from about 5 to about 100 angstroms is currently needed for deposited photoresist films. The typical thickness of the deposited film ranges from about 0.5 to about 2 &mgr;m. While typical substrates currently being produced are about 200 mm in diameter, 300 mm and 450 mm diameter substrates are considered to be the choice for future manufacturing of semiconductors to decrease manufacturing costs and to increase die yields.
Besides accurate control of the viscosity and solvent fraction of the liquid composition, it is also currently necessary to have laminar flow conditions above the rotating substrate in order to achieve optimum final film uniformity.
When interaction between the rotating substrate and the gases above the substrate occurs, turbulent flow conditions can be produced in the gas above the substrate. A dimensionless number called the Reynolds Number, Re, can be used to characterize the flow conditions above the substrate. When the Reynolds Number is below 3×10
5
the flow is laminar, and when it is higher turbulent conditions exist. The Reynolds Number can be calculated using the equation below:
Re
=
(
(
substrate
diameter
(
mm
)
)
/
(
2
*
1000
)
)
2
(
2
π
*
(
rotation
per
minute
/
60
)
)
(
kinematic
viscosity
(
m
2
/
sec
)
)
Using this equation, the Reynolds Number for a 300-mm substrate rotating at 4,000 rpm in a helium environment (kinematic viscosity 0.000123 m
2
/sec) would be 7.7×10
4
.
The presence of turbulent conditions in the gas environment overlying the substrate results in a non-uniform evaporation rate of solvent from the liquid composition during the spin up cycle. As stated above, non-uniform evaporation decreases the final film uniformity across the wafer. As can be determined from the equation, lower Reynolds Numbers can be currently achieved by using high kinematic viscosity gases, lower spin speeds, or smaller substrates.
Gases like helium, hydrogen, and neon have a high kinematic viscosity when compared to air or nitrogen gas. The kinematic viscosity is defined as gas viscosity divided by the gas density. The kinematic viscosity of gases increases with increasing temperature or independently increases with decreasing gas pressure. Thus, reduced pressures and higher temperatures can be used to further reduce the turbulence above a spinning substrate. In a spin-coat process, the combination of reduced pressure, higher gas temperature, and high kinematic viscosity gas will give the lowest Reynolds Number for the environment above the substrate.
In the spin-coat process, a majority of the liquid composition dispensed onto the substrate is not used to coat the subs
Boski Jill
Clarke Michael E.
Pillion John E.
Barr Michael
King Timothy J.
Mykrolis Corporation
Mykrolis Corporation
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