Spin coating process

A process of spin coating a coating composition on the surface of a base plate, comprising supplying a material which is compatable with the coating composition, in either a liquid or vapor form, to the opposite surface of the base plate to the surface on which the coating composition has been supplied (hereinafter "back surface" or "back side") while rotating the base plate to remove the coating composition flowing to the back surface or to the edge portion of the plate. The material compatible with the coating composition may be applied to the surface of the base plate at the periphery thereof to reduce or remove the coating composition there.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a spin coating process and, more 
particularly, to a spin coating process for preventing a coating 
composition applied to the surface of a base plate from adhering to the 
back side thereof and to the edge or periphery of the plate. 
2. Description of the Prior Art 
A technique of forming a photoresist on a base plate by spin coating using 
an organic solvent as the solvent for the coating composition has widely 
been employed in the field of producing semiconductor devices, etc. For 
example, a photoresist coating layer of a uniform thickness is formed by 
placing a base plate, such as a glass sheet having thereon a thin metallic 
layer, a silicon wafer, etc., on a turntable of a spinner, applying 
dropwise a coating composition for a photoresist onto the turntable, and 
then rotating the turntable at high speed (usually at about 2,000 to about 
6,000 r.p.m.). 
However, attempts to coat a gelatino silver halide emulsion on a base plate 
such as a glass sheet and a glass sheet having thereon a thin metal layer 
by spin coating involve the following difficulties. That is, since the 
solids content of a gelatino silver halide emulsion is much lower than 
that of a photoresist coating composition, the dry thickness of the silver 
halide emulsion layer formed is much thinner than the thickness of the 
silver halide emulsion layer immediately after coating, namely, the dry 
thickness of the silver halide emulsion layer is usually about 1/50th the 
thickness of the silver halide emulsion layer immediately after coating. 
Therefore, if a gelatino silver halide emulsion is coated by spin coating 
at high speed as in the case of coating a photoresist coating composition, 
the dry thickness of the silver halide emulsion layer formed is extremely 
thin (for example, about 1/2 to about 1/10 micron). Also, when a gelatino 
silver halide emulsion is coated by spin coating, the emulsion layer 
formed contains a large quantity of bubble traces, similar in appearance 
to a comet, since fine bubbles enter the silver halide emulsion when it 
spreads over the base plate by the action of centrifugal force. 
Furthermore, when the temperature of a gelatino silver halide emulsion 
layer falls to about room temperature (e.g., about 20.degree.-30.degree. 
C.), the emulsion layer is set, but when the speed of rotation in spin 
coating is high, the emulsion is frequently set while the emulsion is 
being spread over a base plate by the rotation of the turntable. On the 
other hand, the periphery or edge of the set coating is subjected to a 
large centrifugal force and thus, the emulsion layer at the periphery is 
sometimes spun off. In this case, it would be desirable for the coating 
composition to be uniformly spun off at the periphery to expose the 
surface of the base plate at the periphery in a uniform width but the 
width or the amount of the emulsion layer thus spun off varies greatly at 
each position or area. However, when the solids content of a gelatino 
silver halide emulsion is increased to overcome these difficulties, the 
silver halide emulsion has an even greater tendency to set, which is 
undesirable. 
Therefore, when a silver halide emulsion is coated on a base plate by spin 
coating, the coating must be performed at much lower speed of rotation 
about 100 to about 1,000 r.p.m.) than that used in the above-described 
photoresist coating. Furthermore, when the viscosity of a coating 
composition is low or the solids content of a coating composition is low 
in the case of coating a photoresist coating composition on a base plate 
or when a coating layer having a thickness of about 5 to about 20 microns 
is required as in the case of coating a photoresist coating composition 
of, for example, a copper laminate for a printed circuit, the spin coating 
must be carried out at a low speed of rotation. 
However, when spin coating is performed by rotating the base plate at such 
a low speed of rotation (in particular, at a low speed of rotation of 
about 100 to about 500 r.p.m.), the coating composition accumulates at the 
periphery and further on the back surface of the plate. This phenomenon 
occurs more markedly at the periphery or the back surface at the corner 
portions when the base plate has a square or rectangular corner. If the 
coating composition adheres to the periphery or the back surface of the 
base plate as mentioned above, the coating composition thus adhered is 
scraped off after drying as fine pieces, which attach to the coated layer 
on the surface of the base plate to cause defects such as pinholes. 
Prevention of this problem is very important if fine images such as those 
required in production of semiconductor devices, are to be obtained. 
SUMMARY OF THE INVENTION 
The object of this invention is, therefore, to provide a spin coating 
process for preventing a coating composition from adhering to the 
periphery and the back surface of a base plate to be coated. 
That is, this invention provides a spin coating process for coating a 
coating composition on the surface of a base plate, which comprises 
supplying a material compatable with the coating composition, in a liquid 
or a vapor form, onto the opposite surface of the base plate to the 
surface on which the coating composition has been supplied while rotating 
the base plate to remove the coating composition flowing to the periphery 
or the opposite surface and/or the edge of the base plate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The invention will now be explained in more detail by reference to the 
accompanying drawings. 
FIG. 1 is a cross sectional view showing an embodiment of spin coating in 
general. In the figure, a base plate 12 on the surface of which an 
emulsion is to be coated is placed on a turntable 10 rotatable by a rotary 
shaft 11. The base plate 12 is generally held on the surface of the 
turntable 10 by a vacuum applied to the base plate through perforations 
formed in the turntable but other appropriate means can be employed. Then, 
a coating composition is supplied onto the surface of the base plate 12 
thus held on the turntable 10 and thereafter the turntable 10 is rotated 
to form on the surface of the base plate 12 a coated layer having a 
desired thickness. Numeral 13 shows the coated layer formed on the surface 
of the base plate 12, numeral 14 shows the portion of the coating 
composition which has reached the edge of the base plate, and numeral 15 
shows the portion of the coating composition which has reached the back 
surface of the base plate 12 at the peripheral portion. 
FIG. 2 is a front view showing the back side of the base plate 12 to that 
coated by spin coating as shown in FIG. 1. When the base plate 12 is 
square and has corners, the proportion of the coating composition reaching 
the corner portion of the opposite side is very large as shown by the 
numeral 15. This is assumed to occur because when spin coating is 
performed by rotating the base plate 12 at a low speed (about 100 to about 
500 r.p.m.), the coating composition which was not spun off by the 
centrifugal force accumulates on, in particular, the corner portions of 
the base plate and the coating composition reaches the back side of the 
base plate around the edge thereof. 
FIG. 3 shows an embodiment of the spin coating process of the invention for 
removing the above described defects. That is, as shown in the figure, a 
nozzle 30 is disposed for supplying a material 31 (e.g., as a liquid) 
compatible with the coating composition coated on the base plate onto the 
back side of the rotating base plate 12. The head of the nozzle 30 is so 
bent that it is directed toward the peripheral area of the base plate 12 
and the above described liquid is ejected through the nozzle toward the 
back side of the base plate. However, since the liquid 31 ejected from the 
nozzle 30 and brought into contact with the back side of the rotating base 
plate 12 is spun off outwardly by the centrifugal force, the head of the 
nozzle 30 need not always be bent and directed outwardly. The liquid 31 
reaching the back side of the base plate 12 is spun away as liquid 
droplets 32 together with the coating composition adhering to the back 
side of the base plate 12 and the coating composition attached to the edge 
thereof by the centrifugal force. 
FIG. 4 is given to explain in more detail the process of this invention 
illustrated in FIG. 3. In the figure, numerals 41, 42, and 43 show the 
front side, the back side, and the edge respectively of the base plate 12 
and the accumulated portion 40 of the coating composition is formed on the 
edge 43 of the base plate. The liquid 31 supplied from the nozzle 30 
reaches the back side 42 of the base plate, flows ourwardly together with 
the coating composition attached to the back side portion due to the 
actions of centrifugal force and the velocity of the liquid as it is 
ejected from the nozzle, and further reaches the edge 43 due to the action 
of the surface tension of the liquid to dilute the coating composition 
attached thereto. Thus, the accumulation 40 of the liquid and the coating 
composition is formed on the edge portion 43 of the base plate and liquid 
droplets 32 are formed successively from the accumulation 40 due to the 
centrifugal force, and are then spun off outwardly. Thus, after a certain 
period of time has passed, this phenomenon occurs over the entire 
periphery of the base plate 12 and the solids component of the coating 
composition in the accumulation 40 is substantially absent. When the 
supply of the liquid from the nozzle is stopped in this state and the 
rotation of the base plate is further continued (preferably, at a higher 
rate of rotation), the coated layer 13 without any coating composition 
attached to the back side and the edge of the base plate is obtained. 
In this case, when the speed of rotation in spin coating is low, the 
centrifugal force applied to a coating composition on a support at the 
periphery is low and hence the coating composition accumulates at the 
peripheral portion and the edge portion of the support, which results in 
an increase in the thickness of the layer at these portions. That is, in 
FIG. 1, the thickness of the thick layer portion 16 of the coated layer 13 
becomes about 2 to 10 times the thickness of the center portion of the 
coated layer 13 and the width of the thick portion becomes sometimes about 
2 to 6 mm. This tendency appears at the peripheral portion of the base 
plate 12 and is particularly marked at the corner portions. The tendency 
occurs in the case of coating a photoresist by spin coating but the 
tendency is particularly remarkable in the case of coating a gelatino 
silver halide emulsion as compared with the case of coating a photoresist 
since the wetability of the base plate (such as a glass sheet and a glass 
sheet having a metal thin film on the surface thereof) by the gelatino 
silver halide emulsion is poor as compared with an organic solvent used 
for coating the photoresist. For removing the disadvantage, the thought 
might be to improve the wetability by the gelatino silver halide emulsion 
by adding a surface active agent thereto but even though such is an 
improvement, it is yet insufficient. Furthermore, the addition of a large 
amount of a surface active agent to a gelatino silver halide emulsion 
results in a deterioration of adhesivity and the photographic properties 
of the emulsion and hence it is undesirable to employ a large amount of 
surface active agent. The reason that the coated layer at the periphery of 
a base plate becomes thick is believed to be due to the fact that the 
emulsion possesses a poor wetability for the base plate and the 
temperature of the emulsion decreases during rotation with the viscosity 
of the emulsion greatly increasing. 
When the thickness of the thick layer is low, there is no practical problem 
but if the thickness of the thick layer at the periphery of a base plate 
is extremely high, an uneven gap forms between a photomask and the coated 
layer in the case of exposing the coated layer in a close contact 
relationship, which results undesirable effects occuring. Also, when such 
a coated layer is used for projection printing, an accurate focus is not 
obtained. 
The thick portion of the coated layer at the periphery can be removed or 
the thickness of the thick portion can be reduced in the following manner. 
This approach is explained by reference to the embodiment illustrated in 
FIG. 5. In the figure, numeral 33 shows a coated layer formed on the 
surface of a base plate 12 and numeral 50 shows the peripheral portion of 
the base plate 12. That is, according to the embodiment shown in the 
figure, the liquid 31 supplied is transferred to the front side of the 
base plate 12 at the periphery by increasing the supply rate of the liquid 
from the nozzle 30 and/or reducing the speed of rotation of the base plate 
to dissolve the coated layer or dilute the coating composition at the 
peripheral portion 50. By the means described above, the coated layer at 
the peripheral portion 50 is removed or the thickness thereof is reduced. 
Such a condition is obtained only when a certain relationship exists 
between the supply rate of the liquid 31 from the nozzle 30 and the speed 
of rotation of the base plate 12 but it is difficult to define numerically 
this relationship. That is, this condition depends upon the kind of liquid 
used, the kind of coating composition (difference in viscosity, 
wetability, etc.,), the wetability of the base plate, the form of the edge 
of the base plate, the thickness of the base plate, etc., and hence the 
optimum condition must be selected each time. However, this is very simple 
and can be easily determined by controlling the speed of rotation of the 
base plate and/or the supply rate of the liquid. 
The liquid thus transferred to the front side of the base plate 12 
dissolves the thick coated layer formed at the periphery of the base plate 
12 or dilutes the coating composition at the periphery portion. 
Thereafter, by increasing the speed of rotation of the base plate 12 
and/or stopping the supply of the liquid from the nozzle 30, the thick 
coated layer formed at the periphery of the base plate 12 is removed or 
the thickness of the thick layer portion is sufficiently reduced. 
In other method of removing the thick coated layer formed on the periphery 
of a base plate, a nozzle for supplying a liquid to the front side of the 
base plate at the periphery is disposed in addition to the nozzle for 
supplying a liquid to the back side of the base plate and the liquid is 
supplied from the nozzles to the peripheral portions to remove the coating 
composition at the peripheral portions. 
FIG. 6 shows a preferred embodiment of the abovedescribed method. That is, 
the embodiment shown is for removing the coating composition accumulated 
on the back side and the edges of the base plate 12 by supplying a liquid 
to the back side of the base plate by means of a nozzle 30 as well as for 
removing the coated layer on the right side of the base plate at the 
periphery with a constant width, that is, for forming a regular square 
coated layer. 
FIG. 6a shows an embodiment wherein a hollow pyramid-like cover 60 is 
disposed on or above a square base plate 12. In this case, the pyramid 
cover is so disposed that the center thereof substantially coincides with 
the center of the base plate. Also, the pyramid-like cover can be rotated 
together with the base plate. 
FIG. 6b shows an embodiment of dissolving away the thick portion of the 
coated layer by flowing a liquid 71 down along the side walls of the 
pyramid cover from a liquid reservoir 70 disposed above the pyramid cover. 
In this manner, the periphery portions 73 of the base plate having the 
exposed surfaces with a uniform width and the square coated layer 72 
formed on the plate are obtained as shown in FIG. 6c. 
According to the above-described method, when a base plate 12 has, for 
example, a square shape, a square coated layer corresponding to the shape 
is obtained but it is not always necessary to form the coated layer 
corresponding to the shape of the base plate. For example, when the base 
plate 12 is square, the shape of the coated layer formed may be circular. 
Such a mode can be practiced by disposing a nozzle as the nozzle for 
supplying a liquid to the back side of the base plate 12 and supplying a 
liquid to the front side of the base plate at the periphery. 
This invention can be appropriately employed with any film forming coating 
composition and as set forth herein, two examples thereof are a 
photoresist coating composition and a silver halide coating composition. 
When this invention is employed in coating a photoresist coating 
composition, a suitable coating composition viscosity can range from about 
10 to about 200 cps and when this invention is employed in coating a 
silver halide emulsion coating composition, a suitable viscosity can range 
from about 1 to about 100 cps. Appropriate temperatures which can be used 
in applying the coating composition are those at which the above 
viscosities are achieved and such in general will range from above 
15.degree. to about 40.degree. C. for a photoresist coating composition 
and from about 20.degree. to about 50.degree. C. for the silver halide 
emulsion coating composition. Appropriate rates of rotation can also be 
used in coating these compositions and suitable rates of rotation during 
coating for a photoresist composition can range from about 100 to about 
3000 rpm for a photoresist coating composition and from about 50 to about 
2000 rpm for a silver halide emulsion coating composition. 
In the case of coating a photoresist on a base plate, the materials 
generally used as the solvents for photoresists can be used as the liquid 
used in this invention. Examples of suitable liquids are, for example, 
ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, 
cyclohexanone, diisobutyl ketone, etc.; esters such as ethyl acetate, 
butyl acetate, n-amyl acetate, methyl formate, ethyl propionate, dimethyl 
phthalate, ethyl benzoate, etc.; aromatic hydrocarbons such as toluene, 
xylene, benzene, ethylbenzene, etc.; halogenated hydrocarbons such as 
tetrachloromethane, trichloroethylene, chloroform, 1,1,1-trichloroethane, 
monochlorobenzene, chloronaphthalene, etc.; ethers such as 
tetrahydrofuran, diethyl ether, ethylene glycol monomethyl ether, ethylene 
glycol monoethyl ether acetate, etc.; dimethylformamide; 
dimethylsulfoxide; and the like. Furthermore, solvents other than those 
described above, which can dissolve the coating composition used or are 
compatible with the coating composition can be used in this invention as 
well. 
Moreover, a typical example of the liquid employed in the case of coating a 
silver halide emulsion on a base plate is warm water at about 30.degree. 
to about 60.degree. C. Furthermore, a lower alcohol such as methanol, 
ethanol, propanol, etc.; or acetone may be also used for this purpose. 
Also, a surface active agent may be incorporated in these liquids for 
improving the wetability of the liquids for the base plate. 
In this invention, liquids described herein can be also used in a vapor 
form. On comparing the effects between the liquid form and the vapor form, 
the use of a liquid is more preferred since in this case the liquid more 
easily and more completely removes the coating composition accumulated on 
the back side and the edge of a base plate and also more effectively 
reduces the thickness of the thick coated layer formed on the base plate 
at the periphery. 
The liquid or the vapor may be supplied to a base plate at any time during 
rotation of the base plate after supplying thereto a coating composition. 
That is, the liquid or the vapor may be supplied to the rotating base 
plate before or after drying the coating compositions formed on the front 
side thereof and on the back side and the edge of the base plate. However, 
it is preferred to supply the liquid or the vapor to the rotating base 
plate before drying the coating layer formed thereon since the coating 
composition accumulated on the back side and the edge of the base plate 
can be easily removed and also the coated layer formed on the periphery of 
the base plate can be effectively improved. In particular, when the 
coating composition is a gelatino silver halide emulsion, it is desirable 
to supply the liquid to the base plate when the silver halide emulsion 
supplied thereto is set. 
While the invention has been described in detail and with reference to 
specific embodiments thereof, it will be apparent to one skilled in the 
art that various changes and modifications can be made therein without 
departing from the spirit and scope thereof.