U.S. patent application number 09/756026 was filed with the patent office on 2002-09-12 for method for spin coating a high viscosity liquid on a wafer.
This patent application is currently assigned to Taiwan Semiconductor Manufacturing Co., Ltd.. Invention is credited to Huang, Der-Fang, Lee, Kun-I, Young, Bao-Ru.
Application Number | 20020127878 09/756026 |
Document ID | / |
Family ID | 25041714 |
Filed Date | 2002-09-12 |
United States Patent
Application |
20020127878 |
Kind Code |
A1 |
Young, Bao-Ru ; et
al. |
September 12, 2002 |
METHOD FOR SPIN COATING A HIGH VISCOSITY LIQUID ON A WAFER
Abstract
A method for spin-coating a high viscosity liquid on a wafer
surface capable of producing an improved uniformity in the coating
thickness and a reduced material usage is disclosed. In the method,
a liquid that has a high viscosity of at least 1000 cp is first
provided. A wafer is then rotated to a speed of less than 300 rpm
while simultaneously, a first volume of a high viscosity liquid is
dispensed onto the wafer surface forming a cup-shaped pattern. The
spinning of the wafer is then stopped and a second volume of the
high viscosity liquid is dispensed into a cavity formed in the
cup-shaped pattern to substantially fill the cavity. The wafer is
then rotated again to a high rotational speed of at least 3000 rpm
such that liquid in the cup-shaped pattern spreads out to
substantially cover an entire surface of the wafer resulting in
improved coating uniformity.
Inventors: |
Young, Bao-Ru; (I-Lan,
TW) ; Lee, Kun-I; (San-Chung, TW) ; Huang,
Der-Fang; (Hsin-Chu, TW) |
Correspondence
Address: |
TUNG & ASSOCIATES
Suite 120
838 W. Long Lake Road,
Bloomfield Hills
MI
48302
US
|
Assignee: |
Taiwan Semiconductor Manufacturing
Co., Ltd.
|
Family ID: |
25041714 |
Appl. No.: |
09/756026 |
Filed: |
January 5, 2001 |
Current U.S.
Class: |
438/782 ;
257/E21.259 |
Current CPC
Class: |
H01L 21/02282 20130101;
H01L 21/312 20130101; B05D 1/005 20130101 |
Class at
Publication: |
438/782 |
International
Class: |
H01L 021/31 |
Claims
1. A method for spin coating a high viscosity liquid on a wafer
surface comprising the steps of: providing a liquid having a
viscosity of at least 1000 cp; rotating a wafer having a top
surface to be coated in a first rotating step to a speed of less
than 300 rpm while simultaneously dispensing in a first step said
liquid onto said top surface of the wafer forming a cup-shaped
pattern at a center portion of said wafer; stopping the rotation of
said wafer; dispensing in a second step said liquid onto said top
surface of the wafer to sufficiently fill up a cavity in said
cup-shaped pattern; and rotating said wafer in a second rotating
step to a speed of at least 3000 rpm such that said liquid in said
cup-shaped pattern spreads out to substantially cover an entire top
surface of said wafer.
2. A method for spin coating a high viscosity liquid on a wafer
surface according to claim 1 further comprising the step of
rotating said wafer in said first rotating step preferably to a
speed between about 100 rpm and about 280 rpm.
3. A method for spin coating a high viscosity liquid on a wafer
surface according to claim 1 further comprising the step of
rotating said wafer in said first rotating step more preferably to
a speed between about 200 rpm and about 250 rpm.
4. A method for spin coating a high viscosity liquid on a wafer
surface according to claim 1 further comprising the step of forming
said cup-shaped pattern of said liquid to a total thickness of at
least 10 .mu.m.
5. A method for spin coating a high viscosity liquid on a wafer
surface according to claim 1 further comprising the step of
providing said liquid having a viscosity preferably of at least
3000 cp.
6. A method for spin coating a high viscosity liquid on a wafer
surface according to claim 1 further comprising the step of
providing said liquid in a material selected from the group
consisting of a photoresist liquid, a spin-on-glass liquid and a
low-k dielectric liquid.
7. A method for spin coating a high viscosity liquid on a wafer
surface according to claim 1 further comprising the step of
dispensing in said second step said liquid in an amount that is
less than 50% of the total liquid dispensed in said first and
second step.
8. A method for spin coating a high viscosity liquid on a wafer
surface according to claim 1 further comprising the step of
rotating said wafer in said second rotation step until said
cup-shaped pattern of liquid spreads out to a film having a
thickness between about 3 .mu.m and about 10 .mu.m.
9. A method for spin coating a high viscosity liquid on a wafer
surface according to claim 1 further comprising the step of
rotating said wafer in said second rotation step until said
cup-shaped pattern of liquid spreads out preferably to a film
having a thickness between about 4 .mu.m and about 6 .mu.m.
10. A method for spin coating a high viscosity liquid on a wafer
surface according to claim 1 further comprising the step of
ramping-up in said second rotation step said wafer to a speed of at
least 3000 rpm within a time period of less than 5 sec.
11. A method for spin coating a high viscosity liquid on a wafer
surface according to claim 1 further comprising the step of
ramping-up in said second rotation step said wafer to a speed of at
least 3000 rpm within a time period preferably of less than 2
sec.
12. A dual-step method for coating a high viscosity liquid on a
wafer surface comprising the steps of: filling a liquid dispenser
in a spin coating apparatus with a liquid having a viscosity of at
lest 1000 cp; providing a wafer having an active surface to be
coated; rotating said wafer to a first rotational speed of less
than 300 rpm while simultaneously dispensing in a first dispensing
step said liquid onto said active surface of the wafer forming a
coating layer having a concave cross-section; reducing said first
rotational speed to 0 rpm; dispensing a second dispensing step said
liquid into a cavity formed by said concave cross-section and
substantially filling said cavity; and rotating said wafer from
said first rotational speed of 0 rpm to a second rotational speed
of at least 3000 rpm within a time period of less than 5 sec. so
that said liquid substantially covers the entire active surface of
the wafer.
13. A dual-step method for coating a high viscosity liquid on a
wafer surface according to claim 12 further comprising the step of
rotating said wafer preferably to a first rotational speed between
about 100 rpm and about 280 rpm.
14. A dual-step method for coating a high viscosity liquid on a
wafer surface according to claim 12 further comprising the step of
rotating said wafer more preferably to a first rotational speed
between about 200 rpm and about 250 rpm.
15. A dual-step method for coating a high viscosity liquid on a
wafer surface according to claim 12 further comprising the step of
forming said coating layer having a concave cross-section to a
total thickness of at least 10 .mu.m.
16. A dual-step method for coating a high viscosity liquid on a
wafer surface according to claim 12 further comprising the step of
rotating said wafer at said second rotational speed until said
liquid spreads out to a film thickness of between about 3 .mu.m and
about 10 .mu.m.
17. A method for spin coating a high viscosity liquid on a wafer
surface according to claim 12 further comprising the step of
providing said liquid having a viscosity preferably of at least
3000 cp.
18. A method for spin coating a high viscosity liquid on wafer
surface according to claim 12 further comprising the step of
providing said liquid in a material selected from the group
consisting of a photoresist liquid, a spin-on-glass liquid and a
low-k dielectric liquid.
19. A method for spin coating a high viscosity liquid on a wafer
surface according to claim 12 further comprising the step of
dispensing in said second step said liquid in an amount that is
less than 50% of the total liquid dispensed in said first and
second steps.
20. A method for spin coating a high viscosity liquid on a wafer
surface according to claim 12 further comprising the step of
rotating said wafer at said second rotational speed until said
liquid spreads out to a film thickness of between about 4 .mu.m and
about 6 .mu.m.
Description
FIELD OF THE INVENTION
[0001] The present invention generally relates to a method for
coating a high viscosity liquid and more particularly, relates to a
method for spin coating a high viscosity liquid on a semiconductor
wafer by a dual-step process for achieving reduced material usage
and improved coating uniformity.
BACKGROUND OF THE INVENTION
[0002] In the manufacturing processes for integrated circuits, a
lithography process is frequently used for reproducing circuits and
structures on a semiconductor substrate. As a first step in a
lithography process, a photoresist layer is first coated onto a
semiconductor substrate such that an image can be projected and
developed on the substrate. The photoresist material is a liquid
that is coated in a very thin layer on top of the semiconductor
substrate. In a conventional process for applying a photoresist
coating material to a semiconductor substrate, a spin coating
apparatus is normally used. The spin coating apparatus is a sealed
chamber constructed by an upper compartment, a lower compartment
and a circular-shaped, rotating platform that has a diameter
slightly smaller than the diameter of a semiconductor substrate.
The rotating platform is a vacuum chuck since vacuum is applied to
the platform for holding the semiconductor substrate securely
during a spin coating process. The rotating platform is positioned
in the coating machine such that a semiconductor substrate may be
placed on top horizontally. During the coating process, the bottom
or the uncoated surface of a semiconductor substrate contacts the
rotating platform. A suitable vacuum is then applied to the bottom
surface of the substrate such that it stays securely on the vacuum
chuck even at high rotational speed. The rotating motion of the
vacuum chuck is achieved by a shaft which is connected to the
vacuum chuck and powered by a motor.
[0003] In a typical photoresist coating process, a desirable amount
of a liquid photoresist material is first applied to a top surface
of the semiconductor substrate from a liquid dispenser that is
mounted on a track while the substrate is rotated at a low speed on
the vacuum chuck. The photoresist liquid spread radially outward
from the center of the semiconductor substrate where it is applied
towards the edge of the semiconductor substrate until the entire
top surface of the substrate is covered with a thin layer. Excess
photoresist liquid spun off the rotating wafer during the
photoresist coating process. The rotational speed of the vacuum
chuck and the amount of the photoresist liquid applied at the
center of the semiconductor substrate can be determined and
adjusted prior to and during an application process such that a
predetermined, desirable thickness of the photoresist is obtained.
The rotational speed of the vacuum chuck is normally increased at
the end of the application process to ensure that the entire
surface of the substrate is evenly coated with the photoresist
material.
[0004] A typical process flow chart illustrating a spin coating
process 10 is shown in FIG. 1. In a conventional deposition process
10, a liquid material is first dispensed in step 12 by depositing a
predetermined amount of liquid at or near the center of the wafer.
The amount of the liquid can be suitably controlled by adjusting
the flow rate through a dispensing nozzle from which the liquid is
dispensed. The flow rate can, in turn, be controlled by a pressure
existing in a liquid reservoir tank.
[0005] The wafer turns on a wafer pedestal at a rotational speed
between 2000 and 3000 RPM when the liquid material is dispensed at
the center of the wafer. The liquid material is then spun-out in
step 14 by centrifugal forces from the center toward the edge of
the wafer uniformly over the entire wafer surface. After all the
liquid material is spun-out and the edge of the wafer is fully
covered, the solvent contained in liquid has at least partially
vaporized and form a solid coating on the wafer surface. After the
spin-out step 14 is completed, an edge bead rinse process of step
16 is carried out at the edge of the wafer surface, i.e. an area of
approximately 2-3 mm from the edge of the wafer, to wash away
material deposited at such area. At this stage of the process, the
material has mostly solidified and thus the edge bead rinse process
is not always effective. After the edge bead rinse step 16, the
backside of the wafer is rinsed by a different jet of cleaning
solvent to wash away any material deposited at undesirable
locations. This is shown as step 18 in FIG. 1. The wafer is then
spun-dry in step 20 to complete the coating process.
[0006] A typical apparatus 22 for coating photoresist on a
semiconductor substrate is shown in FIG. 2. The apparatus 22
consists of a drain cup 28 and a rotating platform 30, i.e. a
vacuum chuck, positioned at the center of the drain cup for
supporting a semiconductor wafer 26 on a top surface 24 of the
vacuum chuck 20. The vacuum chuck can be rotated by a shaft 32
which is connected to an electric motor (not shown). The drain cup
28 is provided with a spent photoresist drain pipe 34. The spent
photoresist drain pipe 34 is used to drain away photoresist liquid
that spun off the substrate during a coating operation.
[0007] In the operation of the conventional spin coater 22 of FIG.
2, the rotating platform 30 is first loaded with a semiconductor
wafer 26 on top. A liquid dispenser 18 then approaches the center
of the wafer 26 and applies a predetermined amount of a liquid
photoresist material to the center of the substrate. The rotating
platform 30 then spins to spread out the photoresist material to
evenly cover the top surface of the wafer 26. Extra photoresist
material 36 is thrown off the substrate surface and drained away by
the drain pipe 34.
[0008] In the conventional spin-coating process of FIG. 1 utilizing
the apparatus of FIG. 2, the process results in a significant waste
of the coating material and furthermore, a non-uniform coating when
a highly viscous liquid is being coated. By highly viscous, it is
meant that any liquid material that has a viscosity of higher than
1000 cp.
[0009] A typical non-uniformity measured on a coated film is shown
in FIGS. 3A and 3B, while a typical spin-coating process recipe for
a high viscosity photoresist material is shown in Table 1.
1 TABLE 1 Step # Time (sec.) Speed (rpm) Dispense 1 6 0 YES 2 12
200 YES 3 7 600 YES 4 4 800 YES 5 10 1000 NO 6 23 3000+ NO
[0010] In the conventional spin-coating process shown in Table 1
and FIGS. 3A and 3B, droplets of a photoresist material that has a
high viscosity such as 4000 cp are first deposited onto the top
surface of wafer 26. The droplets of the photoresist material form
a cup-shaped coating layer 40 when the droplets are dispensed by
the liquid dispenser 18. As seen in Table 1, steps 1, 2, 3 and 4,
the photoresist liquid droplets are dispensed onto the top surface
of the wafer 26. Except for step 1 where the rotational speed is 0
rpm, the rotational speed of the wafer platform is increased, or
ramped up during steps 2-4 from 0 rpm to 800 rpm, while the
photoresist droplets are dispensed onto the wafer surface. The
total dispense time for the liquid droplets is about 29 sec. during
steps 1-4.
[0011] Due to the high rotational speed of the wafer when the
liquid is dispensed, a pronounced cup-shaped formation of the
coating layer 40 is produced. An edge hump 42 formed in the coating
layer 40 is significantly higher than the center portion 44. For
instance, the height of the edge hump 42 may be at least 10 .mu.m,
while the height of the center portion 44 is only about 3 .mu.m.
The large discrepancy between the thicknesses of the coating layer
40 results in a non-uniform coating layer 50 shown in FIG. 3B after
layer 40 is spun out, as shown in steps 5 and 6 of Table 1. The
edge hump 42 remains as edge hump 52, even though at a smaller
height. The center portion 54 became significantly thinner than the
original center portion 44 prior to the spinning process, i.e. one
that is conducted initially at 1000 rpm and then ramped up to 3000
rpm or higher. For instance, the final spin-out speed may be
between about 3000 rpm and about 5000 rpm. It should be noted that
the initial spin-out is carried out for about 10 sec. at 1000
rpm.
[0012] FIG. 3B illustrates the non-uniformity problem resulted from
the conventional spin-coating of high viscosity liquid coating
material, such as a high viscosity photoresist liquid. The
non-uniformity can not be improved by merely increasing the
dispense volume of the liquid. Moreover, the high cost of the
liquid coating material further prohibits any waste of the material
during a spin coating process.
[0013] The conventional single-step spin-coating process is
therefore inadequate and frequently results in a non-uniform
coating of the high viscosity liquid material. The non-uniformity
is partially caused by a large difference in the surface tension of
the coating material and of the wafer. It was also observed that
the degree of non-uniformity becomes worse when the acceleration
time (for instance, 10 sec. shown in step 5) required to reach the
maximum spin speed.
[0014] It is therefore an object of the present invention to
provide a method for spin-coating a high viscosity liquid on a
wafer surface that does not have the drawbacks or shortcomings of
the conventional spin-coating methods.
[0015] It is another object of the present invention to provide a
method for spin-coating a high viscosity liquid on a wafer surface
that is capable of producing improved coating uniformity across the
wafer surface.
[0016] It is a further object of the present invention to provide a
method for spin-coating a high viscosity liquid on a wafer surface
that does not waste a large volume of the coating material.
[0017] It is another further object of the present invention to
provide a method for spin-coating a high viscosity liquid on a
wafer surface that can be used to coat liquids with a viscosity of
at least 1000 cp.
[0018] It is still another object of the present invention to
provide a method for spin-coating a high viscosity liquid on a
wafer surface wherein the liquid may be a photoresist material, a
spin-on-glass material or a low-k dielectric material.
[0019] It is yet another object of the present invention to provide
a method for spin-coating a high viscosity liquid on a wafer
surface by a dual-step coating process.
[0020] It is still another further object of the present invention
to provide a method for spin-coating a high viscosity liquid on a
wafer surface by first coating the wafer while the wafer is
spinning, and then coating the wafer while it is in a stationary
state.
[0021] It is yet another further object of the present invention to
provide a method for spin-coating a high viscosity liquid on a
wafer surface by a dual-step coating process wherein a wafer is
first coated while spinning at a speed of between 200 rpm and 300
rpm, and then coating the wafer while it is stationary.
SUMMARY OF THE INVENTION
[0022] The present invention discloses a method for spin-coating a
high viscosity liquid on a wafer surface that is capable of
producing improved coating uniformity and reduced material
usage.
[0023] In a preferred embodiment, a method for spin-coating a high
viscosity liquid on a wafer surface can be carried out by the
operating steps of first providing a liquid that has a viscosity of
at least 1000 cp; rotating a wafer that has a top surface to be
coated in a first rotating step to a speed of less than 300 rpm
while simultaneously dispensing in a first step the liquid onto the
top surface of the wafer forming a cup-shaped pattern at a center
portion of the wafer; stopping the rotation of the wafer;
dispensing in a second step the liquid onto the top surface of the
wafer to sufficiently fill a cavity in the cup-shaped pattern; and
then rotating the wafer in a second rotating step to a speed of at
least 3000 rpm such that the liquid in the cup-shaped pattern
spreads out to substantially cover an entire top surface of the
wafer.
[0024] The method for spin coating a high viscosity liquid on a
wafer surface may further include the step of rotating the wafer in
the first rotating step preferably to a speed between about 100 rpm
and about 280 rpm, and more preferably to a speed between about 200
rpm and about 250 rpm. The method may further include the step of
forming the cup-shaped pattern of the liquid to a total thickness
of at least 10 .mu.m, or the step of providing the liquid that has
a viscosity preferably of at least 3000 cp. The method may further
include the step of providing the liquid in a material selected
from the group consisting of a photoresist liquid, a spin-on-glass
liquid and a low-k dielectric liquid. The method may further
include the step of dispensing in the second step the liquid in an
amount that is less than 50% of the total liquid dispensed in the
first and second step.
[0025] The method for spin-coating a high viscosity liquid on a
wafer surface may further include the step of rotating the wafer in
the second rotating step until the cup-shaped pattern of liquid
spreads out to a film that has a thickness of between about 3 .mu.m
and about 10 .mu.m, and more preferably between about 4 .mu.m and
about 6 .mu.m. The method may further include the step of ramping
up in the second rotation step the wafer to a speed of at least
3000 rpm within a time period of less than 5 sec., or preferably
within a time period of less than 2 sec.
[0026] The present invention is further directed to a dual-step
method for coating a high viscosity liquid on a wafer surface which
can be carried out by the operating steps of first filling a liquid
dispenser in a spin-coating apparatus with a liquid that has a
viscosity of at least 1000 cp; providing a wafer that has an active
surface to be coated; rotating the wafer to a first rotation speed
of less than 300 rpm while simultaneously dispensing in a first
dispensing step the liquid onto the active surface of the wafer
forming a coating layer that has a concave cross-section; reducing
the first rotational speed to 0 rpm; dispensing in a second
dispensing step the liquid into a cavity formed by the concave
cross-section and substantially filling the cavity; and rotating
the wafer from the first rotational speed of 0 rpm to a second
rotational speed of at least 3000 rpm within a time period of less
than 5 sec. so that the liquid substantially covers the entire
active surface of the wafer.
[0027] The dual-step method for coating a high viscosity liquid on
a wafer surface may further include the step of rotating the wafer
preferably to a first rotational speed of between about 100 rpm and
about 280 rpm, and more preferably to a first rotational speed of
between about 200 rpm and about 250 rpm. The method may further
include the step of forming the coating layer that has a concave
cross-section to a total thickness of at least 10 .mu.m. The method
may further include the step of rotating the wafer at the second
rotational speed until the liquid spreads out to a film thickness
of between about 3 .mu.m and about 10 .mu.m. The method may further
include the step of providing the liquid that has a viscosity
preferably of at least 3000 cp, while the liquid may be a
photoresist liquid, a spin-on-glass liquid or a low-k dielectric
liquid. The method may further include the step of dispensing in
the second step the liquid in an amount that is less than 50% of
the total liquid dispensed in the first and second steps. The
method may further include the step of rotating the wafer at the
second rotational speed until the liquid spreads out to a film
thickness of between about 4 .mu.m and about 6 .mu.m.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] These and other objects, features and advantages of the
present invention will become apparent from the following detailed
description and the appended drawings in which:
[0029] FIG. 1 is a process flow chart for a conventional
spin-coating process on a wafer.
[0030] FIG. 2 is a cross-sectional view of a conventional
spin-coater utilized in spin-coating a liquid.
[0031] FIG. 3A is an enlarged, cross-sectional view of a cup-shaped
coating layer formed on a wafer surface by the conventional
spin-coating process.
[0032] FIG. 3B is the coating layer of FIG. 3A after spun-out at a
rotational speed of 3000 rpm.
[0033] FIG. 4A is an enlarged, cross-sectional view of a coating
layer formed by the present invention method while the wafer is
rotating at a slower speed.
[0034] FIG. 4B is an enlarged, cross-sectional view of the present
invention coating layer with the cavity filled by the liquid
material while the wafer is stationary.
[0035] FIG. 4C is an enlarged, cross-sectional view of the present
invention coating layer after spun-out to cover the entire wafer
surface.
[0036] FIG. 5 is a graph illustrating the defect count prior to and
after the implementation of the present invention novel method.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0037] The present invention discloses a method for spin-coating a
high viscosity liquid on a wafer surface by a dual-step coating
process resulting in a significantly improved coating uniformity
and reduced material usage. While the present invention novel
method is particularly suited for coating a high viscosity
photoresist material such as polyimide, it can be used for coating
any other high viscosity material such as spin-on-glass or low-k
dielectric.
[0038] The method can be carried out by first providing a liquid to
be coated that has a high viscosity, then rotating a wafer to a
speed of less than 300 rpm and simultaneously dispensing the liquid
onto the top surface of the wafer forming a cup-shaped pattern. The
spinning of the wafer is then stopped and a second amount of the
liquid is dispensed into the cup-shaped pattern of the coating
layer previously formed to fill a cavity in the pattern. The wafer
is then spun again to a speed of at least 3000 rpm such that the
liquid in the cup-shaped pattern spreads out to substantially cover
the entire surface of the wafer.
[0039] The present invention utilizes a novel second coating step
for the high viscosity liquid, i.e. while the wafer is held
stationary such that a cavity in a cup-shaped coating layer can be
filled with the liquid prior to the spun-out process for spreading
the liquid over the entire wafer surface.
[0040] The present invention novel dual-step coating process not
only reduces the required dispense volume of the liquid, thus
achieving material savings, but also improves the coating
uniformity over the entire wafer surface. The spun-out speed for
the coating material is increased to the main speed in a time
period that is shorter than that required in the conventional
method, resulting in an improved coating uniformity of the high
viscosity liquid material. It has been found that the dispense e
high viscosity photoresist material, such as a polyimide, can be
reduced from 2.5 cc to 1.5 cc per wafer which leads to a
significant cost saving.
2 TABLE 2 Step # Time (sec.) Speed (rpm) Dispense 1 4 0 YES 2 4 220
YES 3 8 250 YES 4 6 0 YES 5 1 300 NO 6 1 300 NO 7 1 1000 NO 8 25
3000+ NO
[0041] Referring now to Table 2 and FIGS. 4A-4C, wherein a present
invention novel method of dual-step spin-coating of a high
viscosity photoresist material is illustrated. At the beginning of
the dual-step coating process, as shown in FIG. 4A and steps 1, 2
and 3 in Table 2, the photoresist liquid is dispensed onto the
wafer 26 from a liquid dispenser 18 forming a cup-shaped coating
layer 60. The cup-shaped coating layer 60 is formed with an edge
hump 62 which has a thickness significantly larger than the center
portion 64 of the coating layer. The first coating step is
conducted while the wafer is rotated at a low rotational speed,
i.e. at less than 250 rpm, and preferably, at less than 300 rpm, as
shown in steps 2 and 3 in Table 2. Step 1 illustrates a ramp-up
step wherein the wafer is turned from 0 rpm to 220 rpm.
[0042] According to the present invention novel method, the
rotation of the wafer is then stopped, as shown in step 4 of Table
2, wherein the rpm is 0. Additional high viscosity liquid is then
applied into the cavity 66 formed in the cup-shaped coating layer
60 to substantially fill the cavity such that a leveled surface 68
on the coating layer is obtained. This is shown in step 4 which is
conducted for a time period of 6 sec. in Table 2 while the high
viscosity liquid is dispensed into the cavity 66.
[0043] After the cavity 66 in the coating layer 60 is filled with
the additional high viscosity liquid material, the wafer 26 is spun
to spread out the coating layer 60 resulting in a substantially
uniform thickness and a substantially leveled top surface 70, as
shown in FIG. 4C. The initial thickness "L" shown in FIG. 4A is at
least 10 .mu.m, or preferably at least 15 .mu.m. After the spun-out
step of the present invention method, the final uniform thickness
"1" is about 5 .mu.m, or a thickness in the range between about 3
.mu.m and about 8 .mu.m.
[0044] The present invention novel method therefore allows a more
uniform coating layer to be formed of a high viscosity material on
a wafer surface in a spin-coating apparatus. The total volume of
the liquid material dispensed in forming the uniform thickness
layer is reduced, i.e. from 2.5 cc of photoresist material to 1.5
cc for coating a single wafer.
[0045] FIG. 5 shows a graph illustrating poor coating counts on
wafers coated with a high viscosity photoresist material, i.e. a
polyimide, during a time period of approximately one year. It is
noted that the present invention novel method was implemented at
time 00/3 resulting in zero/poor coating counts in the last 3
measurements, i.e. a 700% reduction from the highest poor coating
counts during the year.
[0046] Table 2 further shows that the present invention novel
method spun-out process is executed differently than that in the
conventional spin-coating technique. The high spun-out speed of
3000 rpm+(i.e. 3000 rpm.about.5000 rpm) is rapidly ramped up in a
short time period of 3 sec. For instance, between steps 5 and 8,
the rotational speed of the wafer for spreading out the coating
layer is increased from 300 rpm to 3000 rpm in a short 3 sec. time
duration. This was also determined as a factor in achieving the
present invention desirable result of uniform coating
thickness.
[0047] The present invention novel method for spin-coating a high
viscosity liquid on a wafer surface has therefore been amply
described in the above description and in the appended drawings of
FIGS. 4A-4C and 5.
[0048] While the present invention has been described in an
illustrative manner, it should be understood that the terminology
used is intended to be in a nature of words of description rather
than of limitation.
[0049] Furthermore, while the present invention has been described
in terms of a preferred embodiment, it is to be appreciated that
those skilled in the art will readily apply these teachings to
other possible variations of the inventions.
[0050] The embodiment of the invention in which an exclusive
property or privilege is claimed are defined as follows.
* * * * *