Coating apparatus – Program – cyclic – or time control – Having prerecorded program medium
Reexamination Certificate
1999-11-29
2002-08-27
Beck, Shrive P. (Department: 1762)
Coating apparatus
Program, cyclic, or time control
Having prerecorded program medium
C118S699000, C118S704000, C118S052000, C118S320000, C118S321000
Reexamination Certificate
active
06440218
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to methods and apparatus for applying a coating solution such as SOG (Spin On Glass, also called a silica coating material), photoresist or polyimide resin to substrates such as semiconductor wafers, glass substrates for photomasks, glass substrates for liquid crystal displays or glass substrates for optical disks (hereinafter referred to simply as substrates or as wafers). More particularly, the invention relates to a technique of supplying a coating solution to the surface of each substrate to form a coating film in a desired thickness thereon.
(2) Description of the Related Art
In a conventional coating solution applying method of the type noted above, a coating solution supplied to a substrate forms a substantially circular drop (hereinafter referred to as a core) on the substrate. The drop increases in diameter for a time. Subsequently, the coating solution begins to flow in a plurality of rivulets (hereinafter referred to as fingers) extending radially from the core toward the edge of the substrate. After the fingers reach the edge of the substrate, a large quantity of the coating solution supplied to the core flows through the fingers to scatter to the ambient. Thus, a long time and a large quantity of the coating solution are consumed before the entire substrate surface is covered by the coating solution. In order to cover the entire substrate surface, it is necessary to take into account the quantity of coating solution scattering to the ambient, and to supply the coating solution in a correspondingly increased quantity.
To overcome the above disadvantage, a method has been proposed in which, by way of pretreatment, a solvent is dripped onto a substrate to spin-coat the entire surface thereof first. This step is taken to facilitate spreading of the coating solution over the substrate surface. Then, the coating solution is dripped onto the substrate to spin-coat the surface thereof.
The conventional method noted above has the following drawbacks.
The solvent used in the pretreatment tends to stagnate in recessed parts of a circuit pattern formed on the substrate. The solvent trapped under a coating film formed could turn into bubbles when the film is baked. This results in irregularities of the coating film obtained ultimately.
In addition, while the pretreatment with the solvent facilitates spreading of the coating solution, thereby reducing consumption of the latter, the solvent per se has to be consumed in a large quantity.
SUMMARY OF THE INVENTION
The present invention has been made having regard to the state of the art noted above, and its object is to provide a coating solution applying method in which a solvent is sprayed in advance of supplying a coating solution, to avoid irregularities due to the solvent, and to drastically reduce the quantity of coating solution needed to form a coating film while suppressing solvent consumption.
The above object is fulfilled, according to this invention, by a method of applying a coating solution to a surface of a substrate to form a coating film of desired thickness thereon, comprising the steps of:
(a) spraying a solvent over the surface of the substrate maintained still or spun at a first rotational frequency;
(b) supplying the coating solution to a region centrally of the surface of the substrate maintained still or spun at a second rotational frequency;
(c) accelerating the substrate to a third rotational frequency before the coating solution supplied at step (b) above spreads and entirely covers the surface of the substrate; and
(d) spinning the substrate at a fourth rotational frequency for a predetermined period of time to adjust thickness of a coating film covering the surface of the substrate.
First, before supplying the coating solution to the surface of the substrate, the solvent is sprayed over the surface of the substrate maintained still or spun at the first rotational frequency (step (a)). The solvent sprayed before application of the coating solution produces the effect of reducing the angle of contact between the coating solution and the surface of the substrate. When the coating solution is supplied to the substrate subsequently, the coating solution may spread extremely smoothly over the substrate surface. Since the solvent is not supplied in droplets but is sprayed in mist, the solvent may cover a large area on the substrate within a short time. Even if recesses such as of a circuit pattern are formed on the surface of the substrate, the solvent hardly stagnates in such portions.
After the solvent is sprayed, the coating solution is supplied to a region centrally of the surface of the substrate maintained still or spun at the second rotational frequency (step (b)). In the initial spreading stage of the coating solution, numerous fingers develop from a circular core of the coating solution, and begin to extend radially toward the edge of the substrate. When the numerous fingers reach the edge of the substrate, the coating solution would flow through the fingers to scatter to the ambient. Thus, a large quantity of coating solution would be wasted. To avoid such a situation, the substrate is accelerated to the third rotational frequency before the coating solution supplied spreads and entirely covers the surface of the substrate (step (c)).
Under this rotational frequency control, the coating solution exhibits a behavior as shown in FIG.
4
.
Where, as in the prior art, the second rotational frequency is maintained, the core Ra and fingers Rb shown as a hatched region in
FIG. 4
enlarge and extend toward the edge of substrate W, as shown in a two-dot chain line, under the centrifugal force. However, as the spin of the substrate is accelerated to the third rotational frequency, the fingers Rb are subjected to a force of inertia, i.e. a force acting in the direction opposite to the direction of spin. The resultant of centrifugal force and inertia bends the fingers Rb circumferentially,. thereby enlarging widths thereof. The leading ends of fingers Rb extend under the centrifugal force toward the edge of substrate W (as shown in a dotted line in FIG.
4
). The diameter of core Ra also increases. Moreover, since the solvent has been sprayed before the coating solution is applied, the fingers Rb are readily bent to great degrees circumferentially. Though not shown in the schematic view, with the solvent applied beforehand, the fingers Rb are formed more thinly and in a greater number than in the prior art. The diameter of core Ra also increases more quickly than in the prior art.
Thus, the fingers Rb not only extend toward the edge of substrate W, but greatly increase in width in a circumferential direction. The regions between the fingers Rb diminish rapidly, to shorten the time taken for the coating solution to cover the entire surface of the substrate. The shortened coating time means that only a short time is taken from start of the supply of the coating solution to finish of the supply after the coating solution covers the entire surface of substrate W. In other words, a reduced time is taken from arrival of fingers Rb at the edge of substrate W to finish of the coating solution supply. Thus, a correspondingly reduced quantity of the coating solution scatters to the ambient through the fingers Rb. Thereafter the fourth rotational frequency is maintained for a predetermined period (step (d)) to dispel a superfluous part of the coating solution. In this way, a reduced quantity of coating solution is required to form a coating film of desired thickness.
By accelerating the substrate before the coating solution covers the entire surface thereof, a force of inertia may be applied to the rivulets of coating solution extending radially from the circular drop of coating solution, thereby rapidly diminishing the regions between the radially extending rivulets of coating solution. In addition, since the solvent has been sprayed before the coating solution is applied, thin rivulets of coating solution may be formed in an increased
Goto Shigehiro
Matsunaga Minobu
Nakano Kayoko
Sanada Masakazu
Arent Fox Kintner Plotkin & Kahn
Beck Shrive P.
Crockford Kirsten A.
Dainippon Screen Mfg. Co,. Ltd.
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