Resist coating apparatus

A liquid state resist is dropped to form a coating on a semiconductor substrate mounted on a rotary chuck disposed in a resist coating vessel in which a sealed solvent vapor atmosphere is created. An adjusting plate is provided in the coating vessel above the rotary chuck in a stationary or movable manner to weaken or substantially reduce a swirly flow of the solvent vapor which tends to occur in the coating vessel when the chuck is rotated. Thus a substrate coated with a resist film of uniform thickness is obtained.

BACKGROUND OF THE INVENTION 
This invention relates generally to resist processing apparatuses and more 
particularly to a resist coating apparatus for applying a resist on a 
semiconductor substrate in a solvent atmosphere. 
In semiconductor device manufacturing processes, various kinds of 
patterning are carried out by using photoresists, electron beam resists, 
etc. (hereinafter called merely resists or resist) by an exposing, a 
developing or an etching method. It is required for such a manufacturing 
process to coat evenly the resist on the substrate, and therefore, various 
kinds of resist coating apparatus for achieving even coating of the resist 
have been proposed. 
One typical example of conventional apparatus provides a resist coating 
apparatus in which a liquid state resist is dropped on the central portion 
of the substrate in the solvent atmosphere. Then the substrate is rotated 
so that the resist is evenly spread on the entire surface of the substrate 
by the centrifugal force of the rotating substrate. 
With the conventional resist coating apparatus of the described type, 
however, a vortex or swirly flow of solvent vapor occur in the coating 
vessel which swirly flow adversely causes vapor pressure difference in the 
coating vessel, resulting in uneven drying speed of the resist on the 
substrate. This causes unevenness of the viscosity of the resist and 
finally unevenness of the thickness of the resist film coated on the 
surface of the substrate. 
The unevenness of the thickness of the coated resist film adversely affects 
the improvement of the quality and the yield of the substrate because the 
dimension or size of the pattern formed on the substrate changes in 
accordance with the thickness of the coated resist film. 
SUMMARY OF THE INVENTION 
An object of this invention is to substantially eliminate difficulties and 
drawbacks as described which are encountered with a prior art technique 
and to provide a resist coating apparatus capable of applying a coat of a 
resist film of even thickness on the entire surface of a substrate in a 
solvent vapor atmosphere. 
Another object of this invention is to provide a resist coating apparatus 
provided with a member for reducing or eliminating swirly flow of the 
solvent vapor in a sealed coating vessel to apply a coat of a resist film 
of even thickness on the entire surface of a semiconductor substrate, thus 
improving the quality and the yield of the same. 
These and other objects can be achieved according to this invention by 
providing an apparatus for coating resist on a substrate, which apparatus 
comprises a vessel with an open top covered by a cover plate, a sealing 
member for creating a sealed solvent vapor atmosphere in the vessel, a 
circular rotary chuck on which the substrate is mounted and which is 
disposed in the vessel at substantially the central portion of the vessel 
to be rotatable through a rotating shaft extending upward through the 
bottom of the vessel and operatively connected to an external driving 
source, a nozzle member disposed directly above the central portion of the 
substrate on the rotary chuck and adapted to drop liquid state resist 
thereon, and a baffle or adjusting plate positioned above the rotary chuck 
and extending in a radial direction of the chuck for substantially 
reducing the swirly flow of the solvent vapor created above the rotary 
chuck in the vessel when the chuck is rotated. 
According to the construction of the resist coating apparatus described 
above, and particularly, the location of the adjusting plate, the swirly 
flow of the solvent vapor created in the sealed coating vessel is baffled 
to weaken or substantially reduce the swirly flow, whereby a substrate 
with a coated resist having even film thickness can be fabricated, thus 
improving the quality and the yield of the substrate. 
Preferred embodiments of this invention will be described hereunder in 
detail with reference to the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
For a better understanding of this invention, a conventional resist coating 
apparatus and problems or drawbacks encountered therewith will first be 
described hereunder with reference to FIGS. 1, 2, and 3 before a 
description of preferred embodiments of this invention. 
FIG. 1 shows one example of a conventional resist coating apparatus 
generally comprising a coating vessel 1, in which an annular trough or 
receptacle 2 of substantially channel-shaped cross section is disposed at 
the lower end of the side wall of the coating vessel 1 and contains an 
organic solvent 3 such as ethyl alcohol. The solvent 3 reaches at least 
the lower end of the coating vessel side wall to maintain a sealed 
condition in the coating vessel 1 and to create the solvent atmosphere 
therein. A rotary chuck 5 for supporting a substrate 4 thereon is disposed 
at the center of the interior of the coating vessel 1 and is supported by 
a vertical shaft 6 extending downwardly through the bottom of the coating 
vessel and coupled to driving means, not shown, for rotating the chuck 5, 
i.e., the substrate 4 disposed thereon at a specific rotational speed. 
A tube 8 for supplying the liquid resist under pressure into the coating 
vessel 1 is connected at one end to a resist supply device, not shown, and 
at the other end thereof, the tube 8 extends into the coating vessel 1 
through a cover 12 thereof. To the end of the tube 8 inserted into the 
coating vessel 1 is mounted a resist dropping nozzle 7 at a position 
substantially coaxial with the shaft 6. 
With the resist coating apparatus of the character described above, the 
liquid state resist is dropped from the nozzle member 7 directly onto the 
central portion of the substrate 4 disposed on and fixed to the upper 
surface of the rotary chuck 5. When, under this condition, the substrate 4 
is rotated by rotating the shaft 6 at a rotational speed of 400 rpm, for 
example, the liquid state resist is spread outwardly and evenly over the 
entire surface of the substrate 4 by the centrifugal force acting thereon. 
However, according to this operation of the device shown in FIG. 1, as 
shown in FIG. 2, a swirly flow of the solvent vapor is caused in the 
coating vessel 1, and this swirly flow causes the pressure difference of 
the vapor described hereinbefore which will causes the resist to form a 
film having uneven thickness. 
For example, in a case where it is required to coat a mask substrate having 
a diameter of 5 inches with resist having viscosity of 50 cp, it is 
necessary to rotate the chuck with a rotational speed of 400 rpm for 200 
seconds in order to form a resist film having a thickness of 5,000 .ANG. 
on the substrate. However, according to this method, for example, when the 
film thickness of the resist on the substrate is measured at portions 
along the arrow direction shown in FIG. 3B, it was found as shown in FIG. 
3A that the thicknesses of the coated resist film are different by about 
250 .ANG. at positions between the peripheral portion of the substrate and 
the central portion thereof because of the swirly flow of the solvent 
vapor. 
This invention has succeeded in overcoming the above described problems 
encountered in the prior art by providing an improved resist coating 
apparatus, one example of which, as shown in FIG. 4, has substantially the 
same construction as that of the prior art apparatus shown in FIG. 1 
except for a thin plate type adjusting member 10 disposed vertically in 
the coating vessel 1 at a position above the circular rotary chuck 5 so as 
to extend in the radial direction of the chuck 5. The adjusting plate 10 
serves to effectively reduce the swirly flow of the solvent vapor caused 
by the high speed rotation of the chuck 5 on which the substrate 4 is 
mounted. It is necessary that the adjusting plate 10 have a lower edge 
substantially parallel to the upper surface of the chuck 5. 
In order to obtain the most favorable shape or dimension of the adjusting 
plate, an experiment was carried out by using a coating apparatus having 
the following dimensions or sizes as shown in FIG. 5. 
L.sub.1 : diameter of the rotary chuck 5, 
H.sub.1 : distance between the upper surface of the substrate 4 mounted on 
the chuck 5 and the lower surface of the cover plate 12 of the coating 
vessel 1, 
h.sub.1 : vertical height of the adjusting plate 10, and 
l.sub.1 : horizontal length of the adjusting plate 10. 
The experiment was performed by changing the ratios R.sub.1 =h.sub.1 
/H.sub.1 and R.sub.2 =l.sub.1 /L.sub.1 under the condition that a positive 
type resist to be exposed to electron beams with a viscosity of 50 cp was 
dropped and applied as coating on a blank substrate having a diameter of 5 
inches and the substrate was rotated at a rotational speed of 400 rpm for 
200 seconds. 
The results of the experiment are indicated by the graph shown in FIG. 7, 
which shows the relationship, with the ratio R.sub.1 as a parameter, 
between the ratio R.sub.2 and the area A of a portion having a difference 
in the resist film thickness of more than 15 .ANG. from the average resist 
film thickness as shown in FIGS. 6A and 6B. 
From FIG. 7, it will be seen that the most favorable result is obtained in 
the case where the ratio R1 approaches nearly 1 (R.sub.1 .div.1), that is; 
the lower edge of the adjusting plate approaches as closely as possible 
the upper surface of the substrate, and, otherwise, in the case where the 
ratio R.sub.2 approaches nearly 0.5 (R.sub.2 .div.0.5), that is, the 
horizontal length l.sub.1 of the adjusting plate is equal to the radius 
L.sub.1 /2 of the rotary chuck. These results were supported by 
observation of the swirly flow of CO.sub.2 gas evaporated from dry ice 
used instead of the solvent. 
As described hereinbefore, according to the first embodiment of this 
invention, the swirly flow of the solvent atmosphere caused by the 
rotation of the chuck 5 in the resist coating vessel 1 can be effectively 
reduced by providing the adjusting plate 10, whereby the vapour pressure 
in the coating vessel 1 is maintained substantially uniform, and, 
accordingly, the resist 11 can be uniformly coated on the entire surface 
of the substrate 4. A pattern can thus be formed on the substrate 4 with 
uniform dimension and the quality and yield of the substrate can be 
remarkably improved. 
FIG. 8A shows a result of an experiment performed for forming a resist 
having a film thickness of 5,000 .ANG. and a viscosity of 50 cp on a blank 
substrate having a diameter of 5 inches by using the coating apparatus of 
this example with the rotary chuck rotating at a rotational speed of 400 
rpm for 200 seconds, the film thickness being measured along the arrow 
direction in FIG. 8B. According to the result of this experiment shown in 
FIG. 8A, it is found that the difference in the resist film thickness at 
positions between the peripheral portion and the central portion of the 
substrate is merely approximately 15 .ANG.. 
In the above described example, the adjusting plate 10 may be arranged so 
as to be stationarily suspended from the lower surface of the cover plate 
12 of the coating vessel 1, but in a preferred modification, the adjusting 
plate 10 of rectangular or other shape is arranged to be movable in 
association with a moving mechanism, not shown. The plate 10 may also be 
supported in an inclined manner with respect to the rotary chuck 5 at a 
position where it will not disturb the dropping of the liquid state resist 
from the nozzle member 7. A plurality of adjusting plates 10 of the 
character described above may be used in a preferred modification. 
FIG. 9 shows another embodiment of the resist coating apparatus according 
to this invention wherein like reference numerals are used for members or 
elements corresponding to those shown in FIG. 4. Detailed descriptions 
relating to such members and elements will not be repeated. 
In this second embodiment of the invention, an adjusting plate 10' is 
substituted for the adjusting plate 10 of the first embodiment. This 
adjusting plate 10' comprises a disc disposed above and parallelly to the 
rotary chuck 5 in the coating vessel 1. 
The disc shaped adjusting plate 10' is retracted to the peripheral portion 
by a disc moving mechanism when the liquid resist is dropped on the 
substrate 4 from the nozzle member 7 positioned directly above the same 
and after the resist has been dropped, the disc 10' is moved to its 
position above the rotary chuck 5. 
In order to determine the most favorable dimension of the disc plate 10', 
an experiment was carried out by using a coating apparatus having the 
following dimensions or sizes as shown in FIG. 10. 
L.sub.2 : diameter of the rotary chuck 5, 
H.sub.2 : distance between the upper surface of the substrate 4 mounted on 
the chuck 5 and the lower surface of the cover plate 12 of the coating 
vessel 1, 
l.sub.2 : diameter of the adjusting disc plate 10', and 
h.sub.2 : distance between the lower surface of the disc plate 10' and the 
lower surface of the cover plate 12. 
Under the provision of the relationships R.sub.3 =h.sub.2 /H.sub.2, R.sub.4 
=l.sub.2 /L.sub.2 and R.sub.4 .gtoreq.1, the dimension of the adjusting 
disc plate 10' is determined by considering the relationship between the 
ratio R.sub.3 and the area A at which the difference in the resist film 
thickness occurs. 
FIG. 11 is a graph representing the result of this experiment, which 
indicates the fact that the difference in the film thickness of the resist 
coating on the substrate is gradually eliminated or reduced as the ratio 
R.sub.3 approaches the value "1", that is, the lower surface of the 
adjusting disc plate approaches the rotary chuck. 
As described hereinabove, according to the preferred embodiments of resist 
coating apparatus of this invention, an adjusting plate is located above 
the rotary chuck supporting the substrate thereon in a resist coating 
vessel when the chuck is rotated to substantially reduce swirly flow of 
solvent vapour in the coating vessel, so that the vapour pressure of the 
solvent atmosphere in the coating vessel is made uniform, thus forming a 
resist having substantially an even film thickness on the substrate. 
Accordingly, a pattern having a uniform dimension can be formed on the 
substrate and the quality and the yield of the substrate can be remarkably 
improved.