Semiconductor device manufacturing: process – Coating of substrate containing semiconductor region or of... – Insulative material deposited upon semiconductive substrate
Reexamination Certificate
2001-01-05
2002-08-20
Sherry, Michael J. (Department: 2829)
Semiconductor device manufacturing: process
Coating of substrate containing semiconductor region or of...
Insulative material deposited upon semiconductive substrate
Reexamination Certificate
active
06436851
ABSTRACT:
FIELD OF THE INVENTION
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
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.
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.
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.
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.
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.
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
.
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.
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.
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
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
.
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 &mgr;m, while the height of the center portion
44
is only about 3 &mgr;m. The large discrepancy between the thicknesses of the coating layer
40
results in a non-uniform coating layer
50
shown in FIG.
3
B 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. On that is conducted
Huang Der-Fang
Lee Kun-I
Young Bao-Ru
Kilday Lisa
Sherry Michael J.
Taiwan Semiconductor Manufacturing Co. Ltd.
Tung Randy W.
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