U.S. patent application number 09/745611 was filed with the patent office on 2001-05-24 for high efficiency photoresist coating.
Invention is credited to Chun, Jung-Hoon, Derksen, James, Han, Sangjun.
Application Number | 20010001746 09/745611 |
Document ID | / |
Family ID | 27367658 |
Filed Date | 2001-05-24 |
United States Patent
Application |
20010001746 |
Kind Code |
A1 |
Chun, Jung-Hoon ; et
al. |
May 24, 2001 |
High efficiency photoresist coating
Abstract
An improved method and apparatus for coating semiconductor
substrates with organic photoresist polymers by extruding a ribbon
of photoresist in a spiral pattern which covers the entire top
surface of the wafer. The invention provides a more uniform
photoresist layer and is much more efficient than are current
methods in the use of expensive photoresist solutions. A wafer is
mounted on a chuck, aligned horizontally and oriented upward. An
extrusion head is positioned adjacent to the outer edge of the
wafer and above the top surface of the wafer with an extrusion slot
aligned radially with respect to the wafer. The wafer is rotated
and the extrusion head moved radially toward the center of the
wafer while photoresist is extruded out the extrusion slot. The
rotation rate of the wafer and the radial speed of the extrusion
head are controlled so that the tangential velocity of the
extrusion head with respect to the rotating wafer is a
constant.
Inventors: |
Chun, Jung-Hoon; (Sudbury,
MA) ; Derksen, James; (West Chicago, IL) ;
Han, Sangjun; (Cambridge, MA) |
Correspondence
Address: |
WILSON SONSINI GOODRICH & ROSATI
650 PAGE MILL ROAD
PALO ALTO
CA
943041050
|
Family ID: |
27367658 |
Appl. No.: |
09/745611 |
Filed: |
December 20, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09745611 |
Dec 20, 2000 |
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09094443 |
Jun 10, 1998 |
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60050017 |
Jun 16, 1997 |
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60050159 |
Jun 19, 1997 |
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60055789 |
Aug 14, 1997 |
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Current U.S.
Class: |
438/780 ;
118/320; 427/240; 427/346; 430/270.1; 430/935; 438/782 |
Current CPC
Class: |
B05D 1/005 20130101;
B05D 1/002 20130101; G03F 7/162 20130101; B05C 11/08 20130101; G03F
7/16 20130101; H01L 21/6715 20130101; B05D 1/265 20130101; B05C
5/0245 20130101 |
Class at
Publication: |
438/780 ;
438/782; 427/240; 427/346; 430/270.1; 118/320 |
International
Class: |
H01L 021/312; B05D
003/12; G03F 007/004 |
Claims
The invention claimed is:
1. A method of applying a coating of photoresist to a circular
semiconductor wafer, the wafer having a top surface, a center, and
an outer edge, the method comprising extruding a ribbon of
photoresist, the ribbon having a width, the ribbon extruded in a
spiral pattern which covers the entire top surface of the
wafer.
2. A method according to claim 1, wherein the ribbon of photoresist
is extruded in a spiral pattern beginning at the outer edge of the
wafer and ending at the center of the wafer.
3. A method according to claim 1, wherein the ribbon of photoresist
is extruded in a spiral pattern beginning at the center of the
wafer and ending at the outer edge of the wafer.
4. A method according to claim 1, wherein the width of the
photoresist ribbon is between about one tenth and about one third
of the diameter of the semiconductor wafer.
5. A method of applying a coating of photoresist to a circular
semiconductor wafer, the wafer having a top surface, a center, a
diameter, and an outer edge, the method comprising the steps of (a)
mounting the wafer on a chuck, the top surface of the wafer aligned
horizontally and oriented upward, (b) positioning an extrusion head
adjacent to the outer edge of the wafer and above the top surface
of the wafer, the extrusion head configured to extrude photoresist
out an extrusion slot, the extrusion slot having a length bounded
by a first end and a second end, the extrusion head positioned with
the extrusion slot aligned radially with respect to the wafer, the
first end of the extrusion slot located adjacent to the outer edge
of the wafer, and the second end of the extrusion slot outside the
outer edge of the wafer, (c) rotating the wafer about its center,
(c) extruding a ribbon of photoresist from the extrusion slot, the
ribbon having a width which is substantially equal to the length of
the slot, and (e) while extruding photoresist from the extrusion
slot, and maintaining the extrusion slot aligned radially with
respect to the wafer, moving the extrusion head radially inward
from the outer edge of the wafer toward the center of the wafer
until the photoresist covers the entire top surface of the
wafer.
6. A method according to claim 5, wherein the length of the
extrusion slot is between about one tenth and about one third of
the diameter of the semiconductor wafer.
7. A method according to claim 5, wherein the photoresist is
extruded from the extrusion slot at a rate which is a constant
extrusion rate.
8. A method according to claim 5, wherein, with the wafer rotating
at a rotational speed, and the extrusion head moving at a radial
speed, the motion of the radially moving extrusion head with
respect to the rotating wafer is at a tangential velocity which is
a constant tangential velocity.
9. A method according to claim 5, wherein step (e) comprises
maintaining the extrusion slot a predetermined distance above the
top surface of the wafer.
10. A method according to claim 9, wherein step (e) comprises
determining the distance between the extrusion slot and the top
surface of the wafer, and adjusting the position of the extrusion
slot to maintain the predetermined distance.
11. A method according to claim 10, wherein step (e) comprises
determining the distance between the extrusion slot and the top
surface of the wafer using an optical sensor.
12. A method according to claim 5, wherein the photoresist ribbon
is extruded in a spiral pattern which covers the entire top surface
of the wafer.
13. A method according to claim 5, comprising the steps of (f)
removing the extrusion head, and (g) rotating the wafer at high
speed.
14. A method of applying a coating of photoresist to a circular
semiconductor wafer, the wafer having a top surface, a center, a
diameter, and an outer edge, the method comprising the steps of a.
mounting the wafer on a chuck, b. positioning an extrusion head at
the center of the wafer and above the top surface of the wafer, the
extrusion head configured to extrude photoresist out an extrusion
slot, the extrusion slot having a length bounded by a first end and
a second end, the extrusion head positioned with the extrusion slot
aligned radially with respect to the wafer, the first end of the
extrusion slot located at the center of the wafer and the second
end of the extrusion slot located between the center of the wafer
and the outer edge of the wafer, c, rotating the wafer about its
center, d. extruding a ribbon of photoresist from the extrusion
slot, the ribbon having a width substantially equal to the length
of the slot, and e. while extruding photoresist from the extrusion
slot, and maintaining the extrusion slot aligned radially with
respect to the wafer, moving the extrusion head radially outward
toward the outer edge of the wafer until the photoresist covers the
entire top surface of the wafer.
15. An apparatus for applying a coating of photoresist to a
circular semiconductor wafer, the wafer having a top surface, a
center, a diameter, and an outer edge, the apparatus comprising
means for mounting a wafer with the top surface of the wafer
aligned horizontally and oriented upward, an extrusion head
positioned adjacent to the outer edge of the wafer and above the
top surface of the wafer, the extrusion head configured to extrude
photoresist out an extrusion slot, the extrusion slot having a
length bounded by a first end and a second end, the extrusion head
positioned with the extrusion slot aligned radially with respect to
the wafer, the first end of the extrusion slot located adjacent to
the outer edge of the wafer, and the second end of the extrusion
slot outside the outer edge of the wafer, means for rotating the
wafer about its center, means for extruding a ribbon of photoresist
from the extrusion slot, the ribbon having a width substantially
equal to the length of the slot, and means for, while extruding
photoresist from the extrusion slot, and maintaining the extrusion
slot aligned radially with respect to the wafer, moving the
extrusion head radially inward toward the center of the wafer until
the photoresist covers the entire top surface of the wafer.
16. An apparatus according to claim 15, comprising means for
rotating a wafer at a rotational speed, and moving the extrusion
head at a radial speed, such that the motion of the radially moving
extrusion head with respect to the rotating wafer is at a
tangential velocity which is a constant tangential velocity.
17. An apparatus according to claim 15, comprising means for
maintaining the extrusion slot a predetermined distance above the
top surface of the wafer.
18. An apparatus according to claim 17, wherein the means for
maintaining the extrusion slot a predetermined distance above the
top surface of the wafer comprises means for determining the
distance between the extrusion slot and the top surface of the
wafer, and means for adjusting the position of the extrusion slot
to maintain the predetermined distance.
19. An apparatus according to claim 18, wherein the means for
measuring the distance between the extrusion slot and the top
surface of the wafer comprises an optical sensor.
20. An apparatus according to claim 15, comprising means for
extruding a ribbon of photoresist in a spiral pattern beginning at
the center of the wafer and ending at the outer edge of the
wafer.
21. An apparatus according to claim 15, wherein the width of the
extrusion slot is between about one tenth and about one third of
the diameter of the semiconductor wafer.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority from our copending
provisional U.S. patent applications 60/050,017, filed Jun. 16,
1997; 60/050,159, filed Jun. 19, 1997; and 60/055,789, filed Aug.
14, 1997.
FIELD OF THE INVENTION
[0002] This invention relates to an improved method and apparatus
for coating semiconductor substrates with organic photoresist
polymers. In particular, this invention relates to an improved
method and apparatus for coating semiconductor substrates which
provides a more uniform photoresist layer and is much more
efficient than are current methods in the use of expensive
photoresist solutions.
BACKGROUND OF THE INVENTION
[0003] The manufacture of integrated circuits involves the transfer
of geometric shapes on a mask to the surface of a semiconductor
wafer. Thereafter, the semiconductor wafer corresponding to the
geometric shapes, or corresponding to the areas between the
geometric shapes, is etched away. The transfer of the shapes from
the mask to the semiconductor wafer typically involves a
lithographic process. This includes applying a solution of a
pre-polymer solution to the semiconductor wafer, the pre-polymer
being selected to form a radiation-sensitive polymer which reacts
when exposed to ultraviolet light, electron beams, x-rays, or ion
beams, for example. The solvent in the pre-polymer solution is
removed by evaporation, and the resulting polymer film is then
baked. The film is exposed to radiation, for example, ultraviolet
light, through a photomask supporting the desired geometric
patterns. The images in the photosensitive material are then
developed by soaking the wafer in a developing solution. The
exposed or unexposed areas are removed in the developing process,
depending on the nature of the radiation-sensitive material.
Thereafter, the wafer is placed in an etching environment which
etches away the areas not protected by the radiation-sensitive
material. Due to their resistance to the etching process, the
radiation sensitive-materials are also known as photoresists, and
the term photoresist is used hereinafter to denote the
radiation-sensitive polymers and their pre-polymers.
[0004] The photoresist film thickness required depends on the
desired resolution, defect protection, and step coverage. Thicker
films provide better adhesion, greater protection for reactive ion
erosion, and improved defect protection. However, thicker films
also result in lower resolution because they take longer to expose
and develop. Photoresist film thicknesses used in current
semiconductor manufacturing may be typically 0.5 to 4 .mu.m
thick.
[0005] Thickness uniformity of the photoresist layer is an
important criterion in the manufacture of integrated circuits. When
the radiation is focused through the mask onto the coating,
variations in thickness of the coating prevent the precise focus
over the entire surface of the wafer which is required to obtain
the sharpness necessary to ensure satisfactory reproduction of the
geometric patterns on the semiconductor wafer for advanced circuits
with line width dimensions approaching 0.25 .mu.m line widths and
smaller over a surface. Photoresist film thickness uniformity is
required to maintain good transfer of the mask pattern to the
photoresist. Uniformity is important to maintain a constant
exposure level across the surface of the wafer. Nonuniformities
cause position overlay errors when optical steppers attempt to
sense alignment marks beneath the photoresist film. Nonuniformities
also change the reflectivity of a photoresist deposited over an
oxide.
[0006] The small critical dimensions of microelectronic devices
require photoresist coating thickness typically to be uniform to
within 10 .ANG. (3.sigma.). As the critical dimension decreases
further, even better uniformities will be required.
[0007] The high cost of the photoresist pre-polymer solutions makes
it desirable to devise methods of improving the efficiency of the
coating process so as to minimize the amount of the polymer
solution required to coat a substrate.
[0008] Methods which have been used or proposed for coating wafers
include dip coating, meniscus coating, spray coating, patch
coating, bubble coating, chemical vapor deposition, and spin
coating. Only a few of these methods produce photoresist films with
the thicknesses and uniformities required for semiconductor
production. Of these methods, only spin coating has a production
rate fast enough to meet the demands of chip manufacturers. One
major shortcoming of spin coating, however, is that it can waste as
much as 90%, or more, of the photoresist applied to the wafer
surface.
[0009] About one million gallons of photoresist are consumed each
year at a cost of several hundred million dollars. As the critical
dimension of semiconductor devices becomes smaller, new deep UV
photoresists will be used. These new photoresists can cost five or
more times the cost of the i-line photoresists used currently.
Therefore, a new coating method is needed which wastes less
photoresist while producing uniform, defect-free coatings at a rate
comparable to that of spin coating.
OBJECTS AND SUMMARY OF THE INVENTION
[0010] One object of this invention is to provide an improved wafer
coating process and apparatus which provide greater coating
uniformity across the entire surface of the wafer.
[0011] Another object of the invention is to provide an improved
wafer coating process and apparatus which provide coating
uniformity with less waste and more efficient use of the
photoresist.
[0012] In a first aspect the invention provides a method of
applying a coating of photoresist to a circular semiconductor
wafer, the wafer having a top surface, a center, and an outer edge,
the method comprising extruding a ribbon of photoresist, the ribbon
having a width bounded by outer and inner sides, the ribbon
extruded in a spiral pattern which covers the entire top surface of
the wafer.
[0013] In a second aspect, the invention provides a method of
applying a coating of photoresist to a circular semiconductor
wafer, the wafer having a top surface, a center, a diameter, and an
outer edge, the method comprising the steps of mounting the wafer
on a chuck, the top surface of the wafer aligned horizontally and
oriented upward; positioning an extrusion head adjacent to the
outer edge of the wafer and above the top surface of the wafer, the
extrusion head configured to extrude photoresist out an extrusion
slot, the extrusion slot having a length bounded by a first end and
a second end, the extrusion head positioned with the extrusion slot
aligned radially with respect to the wafer, the first end of the
extrusion slot located adjacent to the outer edge of the wafer, and
the second end of the extrusion slot outside the outer edge of the
wafer; rotating the wafer about its center; extruding a ribbon of
photoresist from the extrusion slot, the ribbon having a width
bounded by outer and inner sides, the width of the ribbon
substantially equal to the length of the slot; and, while extruding
photoresist from the extrusion slot, and maintaining the extrusion
slot aligned radially with respect to the wafer, moving the
extrusion head radially inward from the outer edge of the wafer
toward the center of the wafer until the photoresist covers the
entire top surface of the wafer.
[0014] In a third aspect, the invention provides a method of
applying a coating of photoresist to a circular semiconductor
wafer, the wafer having a top surface, a center, a diameter, and an
outer edge, the method comprising the steps of mounting the wafer
on a chuck; positioning an extrusion head at the center of the
wafer and above the top surface of the wafer, the extrusion head
configured to extrude photoresist out an extrusion slot, the
extrusion slot having a length bounded by a first end and a second
end, the extrusion head positioned with the extrusion slot aligned
radially with respect to the wafer, the second end of the extrusion
slot located at the center of the wafer and the first end of the
extrusion slot located between the center of the wafer and the
outer edge of the wafer; rotating the wafer about its center;
extruding a ribbon of photoresist from the extrusion slot, the
ribbon having a width substantially equal to the length of the
slot; and, while extruding photoresist from the extrusion slot, and
maintaining the extrusion slot aligned radially with respect to the
wafer, moving the extrusion head radially outward toward the outer
edge of the wafer until the second end of the extrusion slot
reaches the outer edge of the wafer.
[0015] In a fourth aspect, the invention provides an apparatus for
applying a coating of photoresist to a circular semiconductor
wafer, the wafer having a top surface, a center, a diameter, and an
outer edge, the apparatus comprising means for mounting a wafer
with the top surface of the wafer aligned horizontally and oriented
upward; an extrusion head positioned adjacent to the outer edge of
the wafer and above the top surface of the wafer, the extrusion
head configured to extrude photoresist out an extrusion slot, the
extrusion slot having a length bounded by a first end and a second
end, the extrusion head positioned with the extrusion slot aligned
radially with respect to the wafer, the first end of the extrusion
slot located adjacent to the outer edge of the wafer, and the
second end of the extrusion slot outside the outer edge of the
wafer; means for rotating the wafer about its center; means for
extruding a ribbon of photoresist from the extrusion slot, the
ribbon having a width substantially equal to the length of the
slot; and means for, while extruding photoresist from the extrusion
slot, and maintaining the extrusion slot aligned radially with
respect to the wafer, moving the extrusion head radially inward
toward the center of the wafer until the photoresist covers the
entire top surface of the wafer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 illustrates a static dispense method employed to
dispense photoresist on a wafer surface in a spin coating
process.
[0017] FIG. 2 illustrates a forward radial dynamic dispense method
employed to dispense photoresist on a wafer surface in a spin
coating process.
[0018] FIG. 3 illustrates a reverse radial dynamic dispense method
employed to dispense photoresist on a wafer surface in a spin
coating process.
[0019] FIG. 4 is an assembly drawing of a side view of an extrusion
head of the invention.
[0020] FIG. 5 is a front view of a front plate of an extrusion head
of the invention.
[0021] FIG. 6 is a front view of a rear plate of an extrusion head
of the invention.
[0022] FIG. 7 is a front view of a shim of an extrusion head of the
invention.
[0023] FIG. 8 is a front view of a shim against a back plate.
[0024] FIG. 9 is a cross sectional view of an assembled extrusion
head of the invention.
[0025] FIG. 10 is a perspective view of an assembled extrusion head
of the invention.
[0026] FIG. 11 is a cross sectional view of the lips of an
extrusion head with a substrate moving beneath the lips of the
extrusion head.
[0027] FIGS. 12, 13 and 14 are a front view, top view and rear
view, respectively of an extrusion spin coating assembly of the
invention.
[0028] FIG. 15 is a block diagram of an embodiment of a control
system in the extrusion spin coating assembly of the invention.
[0029] FIGS. 16, 17, 18 and 19 illustrate the configuration of an
extrusion spin coating assembly during several steps of the
extrusion spin coating process of the invention.
[0030] FIG. 20 is a diagram which illustrates certain parameters of
extrusion spin coating motion according to the invention.
[0031] FIG. 21 illustrates an extrusion spin coating spiral pattern
according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0032] FIGS. 1, 2 and 3 shows three primary methods currently
employed to dispense photoresist on a wafer surface in a spin
coating process. The method depicted in FIG. 1 is referred to as
"static dispense." In static dispense, the photoresist is dispensed
directly into the center of a stationary wafer 10, producing a
circular pool of photoresist 12. Alternatively, the entire surface
of the wafer 10 may be flooded with photoresist. Often, the wafer
10 is rotated slowly after a static dispense to begin spreading the
photoresist 12 over the wafer 10 surface.
[0033] The methods illustrated in FIGS. 2 and 3 are referred to as
"dynamic dispenses," because the wafer 10 is rotating slowly while
the photoresist 14, 16 is dispensed. During forward radial
dispense, illustrated in FIG. 2, the dispense nozzle 20 is
initially located at the center of the wafer 10 and moves radially
outward as the photoresist 14 is deposited. For reverse radial
dispense, illustrated in FIG. 3, the dispense nozzle begins at the
outer edge of the wafer and moves radially inward. In both FIGS. 2
and 3 the dispense nozzle 20 is illustrated at the end of travel
after having deposited photoresist on the slowly spinning wafer 10.
Both forward and reverse radial dispense produce a spiral pattern
14, 16 of photoresist. The geometry of the spiral 14, 16, i.e.
number of turns of the spiral and volume of photoresist per unit
length along the spiral, is determined by the angular rotation of
the wafer 10, the radial velocity of the nozzle 20 with respect to
the wafer 10, and the volumetric flow of the photoresist during the
dispense. Dynamic dispenses use less photoresist, but static
dispenses produce a more uniform film.
[0034] After the photoresist is deposited on the wafer, the wafer
is accelerated to create a centrifugal force which spreads the
photoresist toward the edge of the wafer. The wafer may be spun at
an intermediate speed for a few seconds before being accelerated to
the final high-speed spin. When the bulk of the photoresist reaches
the edge of the wafer, most of the photoresist is flung off in many
tiny droplets. It has been shown that while the acceleration rate
does not affect the final film thickness, higher acceleration rates
do tend to produce more uniform films.
[0035] Once the wafer is spun up to the final high speed, the wafer
continues to spin to cause the photoresist to reach the desired
thickness. Photoresist continues to flow outward and off the wafer
in concentric waves. Simultaneously, the solvent in the photoresist
evaporates quickly because of high convection over the wafer
surface. As the solvent fraction in the photoresist decreases, the
viscosity of the photoresist gradually increases, causing the
outward flow of photoresist to diminish until it almost ceases.
Subsequent thinning of the photoresist comes almost entirely from
solvent evaporation. When the solvent is mostly evaporated,
typically after about 30 seconds, spinning is stopped, and the
wafer is soft baked at a high temperature to evaporate the
remaining solvent from the photoresist.
[0036] In each of the dispense methods depicted in FIGS. 1, 2 and
3, the photoresist is dispensed onto the wafer in a thick puddle or
ribbon, and must be spread by some means, e.g. slow spin, to spread
the photoresist to cover the wafer and to reduce the photoresist to
a thin layer. In the method of the invention, the photoresist is
applied in a thin uniform layer over the entire surface of the
wafer. This eliminates the need for the slow spin step, and
requires less photoresist to be deposited on the wafer to achieve
the desired final thickness and uniformity.
[0037] The method of the invention employs extrusion slot coating
to dispense a thin ribbon of photoresist over the entire surface of
the wafer. Extrusion slot coating is a member of the class of
pre-metered coating methods. With extrusion slot coating, the
coating thickness can be controlled by the photoresist dispense
rate, the efficiency can be near 100%, and the thickness uniformity
is very good.
[0038] In extrusion slot coating, the photoresist is extruded onto
the wafer through a narrow slot. FIGS. 4-11 illustrate an
embodiment of an extrusion head 30 which may be used in the
invention. The extrusion head 30 may also be referred to as an
extrusion die. FIG. 4 shows a side assembly view of the extrusion
head 30 which is constructed of a stainless steel U-shaped shim 31
sandwiched between a stainless steel front plate 32 and a stainless
steel back plate 33. FIGS. 5, 6 and 7 show a front view of the
front plate 32, back plate 33, and shim 31, respectively. FIG. 8
shows a front view of the shim 31 against the back plate 33.
Referring to FIG. 4, the front plate 32 and back plate 33 are
grounded and polished on their inner edges, facing the shim 31, to
provide good seal with the shim 31 and a smooth surface for
extrusion. Photoresist enters the extrusion head 30 through a port
34 in the top of the back plate 33. The port 34 directs the
photoresist through a tube 35 to a flow channel 36 (FIGS. 4, 6).
The flow channel 36 is as wide as the opening of the "U" 37 of the
shim 31 (FIGS. 7, 8).
[0039] FIG. 9 is a sectional view of the extrusion head 30
illustrated in FIG. 4. The void created by the u-shape of the shim
31 leaves a narrow gap 38 between the front plate 32 and back plate
33 through which photoresist can flow. At the base of the extrusion
head 30, the gap 38 continues downward between two narrow "lips"
41, 42 which extend the inner surface of the front plate 32 and
back plate 33.
[0040] FIG. 10 is a perspective view of the extrusion head
illustrated in FIG. 4. The gap 38 extends across the opening of the
"U" 37 (FIGS. 7, 8) of the shim 31 to form an extrusion slot 39 in
the extrusion head 30.
[0041] FIG. 11 is a cross sectional view of the lips 41, 42 of an
extrusion head 30 with a substrate 50 moving beneath the extrusion
lips 41, 42. Photoresist is extruded out the slot 39 at the base of
the lips 41, 42 onto the top surface 51 of the substrate 50. The
width of the gap 38 between the front plate 32 and rear plate 33,
indicated as d, is equal to the thickness of the shim 31 (FIGS. 4,
9). The coating gap between the lips 41, 42 and the substrate 50 is
filled with a bead 46 of coating fluid coming from the slot 39.
When the substrate 50 is moved perpendicular to the slot 39,
keeping the coating gap constant, fluid is drawn out of the bead 46
and remains as a thin film on the substrate 50. The width of the
extruded film, w (FIGS. 19, 20) is approximately equal to the
length of the extrusion slot 39, i.e. the opening of the "U" 37 of
the shim 31 (FIGS. 7, 8). The average thickness of the extruded
film, h, is 1 h = Q wv
[0042] where v is the coating speed, and Q is the fluid dispense
rate. The menisci 44, 45 at the leading and trailing edges of the
coating bead 46 are pinned to the corners of the extrusion head
lips 41, 42. The corners of the extrusion head lips 41, 42 should
have a radius of curvature less than approximately 50 .mu.m to keep
the menisci 44, 45 pinned. The capillary, viscous, and inlet
pressures in the coating bead 46 must balance the external pressure
to maintain stability in the coating bead 46. A slight vacuum at
the leading edge of the coating bead 46 can be used to stabilize
the coating bead 46 when coating thinner films or at higher coating
speeds. The extrusion head lips 41, 42 are normally of equal length
(G.sub.1=G.sub.2) and the extrusion head 30 is perpendicular to the
substrate 50. For very thin coatings, however, it is sometimes
beneficial to have one of the lips extend beyond the other
(G.sub.1.noteq.G.sub.2) or to have the extrusion head 30 slightly
tilted from perpendicular to the substrate 50, thereby tilting the
coating slot 39 with respect to the substrate 50.
[0043] The description of the extrusion spin coating assembly 100
will be with reference to FIGS. 12, 13 and 14, which illustrate
front, top and rear views, respectively, of an extrusion spin
coating assembly 100 according to the invention. Components of the
extrusion spin coating assembly 100 illustrated in FIGS. 12, 13 and
14 include a coating module 110 and a positioning system 130. Not
illustrated in FIGS. 12, 13 and 14, but described with reference to
FIG. 15, is a control system 210 which includes a positioning
controller 220 and a spinner controller 280.
[0044] The coating module 110 includes a spinner assembly 111 which
includes a spinner servomotor (not illustrated, reference numeral
113 in FIG. 15) connected to a vertical shaft 112. The vertical
shaft 112 supports a Teflon vacuum chuck 114. The spinner assembly
111 can be moved vertically using a chuck elevator servomotor (not
illustrated, reference numeral 115 in FIG. 15). The chuck elevator
servomotor is equipped with an elevator motor brake (not
illustrated, reference numeral 135 in FIG. 15). With the spinner
assembly 111 at its lowest position, the chuck 114 is surrounded by
a catch cup 116 (sectional view illustrated). The catch cup 116 is
a circular cup having an open top 117. The upper portion 120 of the
cup wall 118 tilts inward to facilitate retaining waste photoresist
within the catch cup 116. The catch cup 116 serves three purposes.
The catch cup 116 catches and drains waste photoresist out a liquid
waste drain 122. The catch cup has an exhaust vent 118 through
which evaporated solvent is removed. The catch cup 116 directs the
flow of air over a spinning wafer to avoid turbulence. Both the
exhaust vent 118 and waste drain 122 exit the bottom 124 of the
catch cup 116. Means for removing waste photoresist and exhausted
vapors are well known to those skilled in the art and are therefore
not illustrated.
[0045] The spinner assembly 111 has a centering device including
eight Teflon pins 138 for centering wafers on the chuck 114, and
three vertical pins (not illustrated) for supporting loose wafers
before and after processing. The centering pins 138 are controlled
by a centering solenoid (not illustrated, reference numeral 119 in
FIG. 15). Sensors on the coater module 110 indicate chuck 114
vertical home position (not illustrated, reference numeral 121 in
FIG. 15), vacuum state (on/off) (not illustrated, reference numeral
123 in FIG. 15), and centering pin position (not illustrated,
reference numeral 125 in FIG. 15). These features of the coating
module 110 are well know to those skilled in the art and are
therefore not illustrated.
[0046] A coater module 110 suitable for use with the invention is a
90SE coater module which is commercially available from Silicon
Valley Group, Inc. The 90SE coater module is one component of a
90SE Wafer Processing track also commercially available from
Silicon Valley Group, Inc.
[0047] The positioning system 130 is supported by an aluminum
baseplate 132 which is mounted above the coater module 110. The
baseplate 132 has a center cut-out 134 positioned over the coater
module 110. First and second vertical support plates 134, 136
mounted above the baseplate support a cross-support 137 on which a
two-axis positioning system 150 is mounted. The positioning system
150 includes an x-axis positioning table 152 and a z-axis
positioning table 162. The x-axis positioning table 152 includes an
x-axis table motor 154 and x-axis table base 156. Likewise, the
z-axis positioning table 162 includes a z-axis table motor 164 and
z-axis table base 166. The z-axis positioning table 162 also
includes a z-axis brake (not illustrated, reference numeral 133 in
FIG. 15). The z-axis positioning table 162 is mounted on the
carriage 158 of the x-axis positioning table 152. The x-axis
positioning table 152 moves in a horizontal plane, parallel to the
surface 51 of a wafer 50 mounted on the chuck 114, and the z-axis
positioning table 162 moves in a vertical direction perpendicular
to the plane of the surface 51 of a wafer 50 mounted on the chuck
114. A positioning system suitable for use in the x-axis and z-axis
positioning tables 152, 162 of the invention is the Parker Daedal
Motion Table driven by 5-pitch ball screws.
[0048] An extrusion head 30 is mounted at the bottom of an aluminum
extrusion head support 172 which, in turn, is mounted on the z-axis
positioning table 162. The z-axis positioning table 162 has
sufficient range of motion to move the extrusion head 30 from a
position above the base plate 132, down, through the center cut-out
134 in the baseplate 132, to the proximity of a wafer 50 on the
chuck 114.
[0049] An optical sensor 174 is mounted on the extrusion head
support 172. The optical sensor 174 is used to measure the gap
between the extrusion head 30 and a wafer 50 mounted on the chuck
114. A sensor suitable for use in an embodiment of the invention is
a Philtec RC140L reflectance compensated optical displacement
sensor. The optical sensor 174 shines a light on the surface of the
wafer 50, measures the reflected light, and generates a voltage
proportional to the intensity of the measured light. The spot size
of the Philtec sensor is 6 mm and has a bandwidth from DC to 100
Hz. The voltage-distance curve of the Philtec sensor is generally
non-linear, but has a linear region when the sensor-wafer distance
is between, for example, 5.51 and 6.17 mm (0.217 and 0.243 inch).
The optical sensor 174 is positioned on the extrusion head support
172 so that all measurements fall within the linear range of the
optical sensor 174.
[0050] Means for controlling flow of the photoresist includes a
photoresist pump (not illustrated) and a photoresist shutoff valve
129. Such arrangements are well know to those skilled in the art,
and therefore is not fully illustrated in FIGS. 12, 13 or 14.
However, the following description of the control system 210 of the
extrusion spin coating assembly 100 includes reference to the
photoresist pump (not illustrated, reference numeral 127 in FIG.
15) and the photoresist shutoff valve 129.
[0051] FIG. 15 is a block diagram which illustrates an embodiment
of a control system 210 suitable for controlling the extrusion spin
coating assembly 100 of the invention. The control system 210
includes a computer 212, a positioning controller 220 and a spinner
controller 280. The computer 212 downloads programs to the
positioning controller 220, the spinner controller 280 and the
photoresist dispense pump 127 via serial interfaces 213, 214, 215.
The positioning controller 220 sends commands to the photoresist
dispense pump 127 to start and stop photoresist flow and to control
the photoresist shutoff valve 129. The positioning controller 220
also controls the position of the x-axis positioning table 152 via
the x-axis motor 154 and z-axis positioning table 162 via the
z-axis motor 164, and the chuck elevator servomotor 115. The
positioning controller 220 receives the output of the optical
sensor 174, calculates the distance between the extrusion head 30
and the wafer 50, and uses the results to control the z-axis
positioning table 162 via the z-axis motor 164.
[0052] A computer suitable for use in the control system 210 is an
IBM-compatible PC. Suitable for use as the positioning controller
220 is the Parker Compumotor AT6450 Servo Controller, including the
optional ANI analog input PC card and the AUX board. Suitable for
use as the spinner controller 280 is The Pacific Scientific SC 755.
Although the computer 212, positioning controller 220 and spinner
controller 280 are shown separately in the block diagram of FIG.
15, in an embodiment which includes the Parker Compumotor AT6450
and Pacific Scientific SC755 controllers, the Compumotor AT6450
plugs into the motherboard of the PC. The invention also
contemplates an embodiment in which both the positioning controller
220 and spinner controller 280 functions are provided by a single,
combined controller.
[0053] The positioning controller 220 includes a positioning
controller processor and several inputs and outputs. The inputs and
outputs include a 14-bit analog to digital (A/D) converter, several
discrete digital inputs and outputs, and servomotor outputs (the
processor and inputs and outputs are well known to those skilled in
the art and are not individually illustrated). The output of the
optical sensor 174 is coupled to the A/D converter input 224. The
positioning controller 220 discrete digital inputs are optically
isolated interfaces, and include a chuck position home indicator
input 242 coupled to the chuck position home sensor 121; a vacuum
on/off status indicator input 244 coupled to the vacuum on/off
sensor 123 on the vacuum chuck 114; a centering pin in/out position
indicator input 246 coupled to the centering pin position sensor
125; and one or more manual positioning command inputs 248 coupled
to operator manual positioning switches 126.
[0054] The positioning controller 220 outputs include an x-axis
servomotor output 226 which is coupled to the x-axis servomotor
154; a z-axis servomotor output 228 which is coupled to the z-axis
servomotor 164; and an elevator motor output 230 which is coupled
to the elevator servomotor 115.
[0055] The positioning controller 220 discrete digital outputs
include a photoresist valve on/off output 254 which is coupled to
the photoresist shutoff valve 129; a centering solenoid output 256
which is coupled to the centering solenoid 119 which controls the
centering pins 138; a vacuum solenoid output 258 which is coupled
to the vacuum solenoid 131; a z-axis motor brake output 260 which
is coupled to the z-axis brake 133 in the z-axis positioning table
162; an elevator motor brake output 262 which is coupled to the
elevator motor brake 135; a trigger output 264 to the photoresist
dispense pump 127; and logical outputs 266 to the spinner
controller 280.
[0056] The spinner controller 280 runs the coating and spin cycles
in response to signals received from the positioning controller
220. The spinner controller 280 includes a spinner controller
processor, a servomotor output, and an encoder (the processor and
encoder are well known to those skilled in the art and are not
individually illustrated). The spinner controller 280 outputs
include a spinner motor output 286 which is coupled to the spinner
motor 113 The output of the spinner controller 280 also includes a
simulated encoder signal 288 which is coupled to the positioning
controller. The simulated encoder signal 288 allows electronic
gearing of the spinner motor 113 speed to control the x-axis
positioning of the extrusion head 30 performed by the positioning
controller 220.
[0057] The extrusion head 30 and the positioning tables 152, 162
must be aligned with respect to a wafer 50 mounted on the chuck 114
to obtain reliable coating. Three alignments are required. These
alignments will be described with reference to FIGS. 12, 13 and 14.
A first alignment adjusts the path of the extrusion slot 39 so that
the extrusion slot 39 passes directly over the center of a wafer 50
mounted on the chuck 114. This alignment is needed to completely
cover the center area of the wafer 50. The extrusion head 30 is
positioned over the center of the wafer 50 by sliding the vertical
support plates 134, 136 forward or backward over the base plate
132. The motion of the vertical support plates 134, 136 is
constrained by a guide on the base plate 132. Adjustment bolts at
the rear of each of the vertical support plates 134, 136 allow fine
tuning of the position of the vertical support plates 134, 136
before the vertical support plates 134,136 are fastened into
place.
[0058] The second alignment adjusts the angle of the x-axis with
respect to the wafer surface 51. This alignment maintains a
constant gap between the wafer 50 and the extrusion head 30 as the
x-axis positioning table 152 changes position. The angle of the
x-axis with respect to the wafer surface 51 can be changed by
rotating the cross-support 138 about a first pivot 179 at one end
of the cross-support 137. Fine and coarse adjustment bolts 184, 186
allow adjustments of the angle between the x-axis and the wafer
surface 51 of 1.64.times.10.sup.-5 radians per turn of the fine
adjustment bolt 184. The angle of the x-axis with respect to the
wafer surface 51 can be determined by scanning across the wafer
surface 51 with the optical sensor 174. During the scan, with the
z-axis fixed, measurements of the optical sensor 174 output and the
x-position are recorded. A linear regression of these data pairs
provides the angle between the wafer surface 51 and the x-axis.
[0059] The third alignment adjusts the bottom edge of the extrusion
head 30, i.e. the extrusion slot 39, until it is parallel with the
x-axis and the wafer surface 51. This alignment is crucial for
maintaining a constant gap across the width of the extrusion head
30. The angle between the bottom edge of the extrusion head 30 and
the x-axis can be adjusted using a wafer-extruder parallelism
adjustment bolt 176. The wafer-extruder parallelism adjustment bolt
176 pivots the extrusion head support 172 about a wafer-extruder
parallelism adjustment pivot 178 at the base of the z-axis
positioning table 162. The angle between the x-axis and the bottom
of the extrusion head 30 can be measured using a linear variable
differential transformer (LVDT) sensor. The LVDT sensor is secured
to the wafer surface 51 with the measurement tip pointing
vertically up. Next, the extrusion head 30 is lowered until the
lips 41, 42 of the extrusion head 30 move the LVTD sensor to a
reference position. After the x-axis and z-axis positioning table
152, 162 positions are recorded, the procedure is repeated for
several other positions along the extrusion head lips 41, 42. The
slope of the extrusion head 30 with respect to the x-axis is
determined using a linear regression of these data pairs.
[0060] The optical sensor 174 may be calibrated in a two-step
process. First, a voltage offset (i.e., zero-gap bias) voltage is
determined by measuring the output voltage of the optical sensor
174 at several small gap distances using precision shims placed
between the extrusion head 30 and the wafer surface 51. A linear
regression analysis of the gap distance and sensor voltage data is
used to calculate the voltage offset (i.e., sensor voltage at a
zero gap). Second, the relationship of the sensor voltage and the
height of the extrusion slot 39, in the linear range of the optical
sensor 174, is determined by raising the extrusion slot 39 in
selected increments (e.g., 10 encoder counts equals 12.7 .mu.m) and
recording the sensor voltage at each position. A linear regression
of the data pair provides the slope of the curve representing
sensor voltage versus z-axis position of the extrusion slot 39. The
extrusion head 30 must be aligned with respect to the x-axis and
wafer surface, as described above, prior to calibrating the optical
sensor 174 so that errors will not arise from the angle between the
extrusion head 30 and the wafer surface 51.
[0061] The extrusion spin coating process will be described with
reference to FIGS. 16-19. The alignment and calibration procedures
described above may be performed periodically or prior to a series
of runs as determined to be necessary based on experience with the
equipment used.
[0062] Referring to FIG. 16, the vacuum chuck 114 is raised through
the cut out 134 in the base plate 132, and the wafer 50 is placed
on the chuck 114. The wafer 50 is centered on the chuck 114 using
the centering pins 138 (FIG. 13). The chuck vacuum (not
illustrated) is turned on to secure the wafer 50. The chuck 114 is
lowered, lowering the wafer 50 into the coating position, and the
extrusion head 30 is lowered into position at the edge of the wafer
50 with the desired gap between the wafer 50 and the extrusion head
lips 41, 42 as illustrated in FIG. 17. The chuck 114 is then
rotated at an initial rotational speed which is the desired coating
speed. The photoresist shutoff valve 129 is opened and the
photoresist pump 127 is triggered to begin dispensing photoresist.
The extrusion head 30 is moved radially with respect to the wafer
50. As the extrusion head 30 moves toward the center of the wafer
50, the rotational speed of the chuck 114 is increased and the
extrusion head speed is increased at a rate proportional to the
increase in the rotational speed in order to maintain the coating
speed of the extrusion head 30 over the wafer 50 constant. When the
leading edge of the extrusion head 30 reaches the center of the
wafer 50, illustrated in FIG. 18, the speed of rotation of the
wafer 30 is held constant until the trailing edge of the extrusion
head 30 reaches the center of the wafer 50. When the entire wafer
50 is covered with photoresist, the photoresist pump 127 is
triggered to stop dispensing photoresist, and the photoresist
shutoff valve 129 is closed. Typically, it is necessary to continue
extruding photoresist and continue moving the extrusion head 30
until the trailing edge of the extrusion head 30 reaches the center
of the wafer 50 in order to cover the entire wafer 50 with
photoresist. When the photoresist pump 127 and shutoff valve 129
are triggered to stop dispensing photoresist, a residual amount of
photoresist which is already in the extrusion head 30 (and possibly
also in tubing leading to the extrusion head 30) may continue to
flow and be deposited on the wafer 50. In such cases, the
photoresist pump 127 and shutoff valve 129 may be triggered to stop
dispensing photoresist a short time prior to covering the entire
wafer 50, thereby allowing such residual photoresist to finish
covering the wafer 50.
[0063] The chuck 114 then lowers the wafer 50 into the catch cup
116, and the extrusion head 30 is raised from the coating area as
illustrated in FIG. 19. The wafer 50 is then spun at high speed to
remove excess photoresist and achieve the desired coating
uniformity. The chuck 114 stops spinning and is raised through the
center cut out 134 in the base plate 132. The vacuum is turned off
and the wafer 50 removed from the chuck 114.
[0064] FIG. 20 is a diagram which illustrates certain parameters of
extrusion spin coating motion according to the invention. In FIG.
20, a wafer 50, has a radius R, and is rotating about its center at
an angular velocity of .OMEGA.. An extrusion head 30 is above the
wafer 50, with the extrusion slot 39 radially aligned with respect
to the wafer 50. The extrusion slot 39 has a width w, and is moving
radially with respect to the wafer 50 at a velocity u. The distance
between the center of the wafer 50 and the trailing edge of the
extrusion head 30 is r.
[0065] The tangential velocity of any point on the surface of the
wafer 50, at a distance r from the axis of rotation shown in FIG.
20 is:
v=.OMEGA.r
[0066] With the trailing edge of the extrusion head 30 at a
distance r from the axis of rotation, a spiral extrusion pattern
can be made by moving the extrusion head 30 inward one length of
the extrusion slot 39 for each revolution of the wafer 50, The
extrusion head 30 speed along the diameter of the wafer 50 is then:
2 u = w 2
[0067] Solving for .OMEGA. and substituting yields: 3 u = wv 2
r
[0068] For radially inward motion, u=-dr/dt, and a differential
equation for the position of the extrusion head can be obtained as
follows: 4 r t = - wv 2 r
[0069] Integrating this equation using the initial condition
r=r.sub.0 at time t=0 yields: 5 r = ( r 0 2 - wvt ) 1 / 2
[0070] The wafer rotation speed can be expressed as a function of
time as: 6 = v ( r 0 2 - wvt ) 1 / 2
[0071] and the head speed can be expressed as a function of time
as: 7 u = wv 2 .PI. ( r 0 2 - wvt ) 1 / 2
[0072] FIG. 21 illustrates an extrusion spin coating spiral pattern
202 according to one aspect of the invention. The spiral pattern
202 results from the extrusion head 30 starting at the outer edge
52 of the wafer 50 and moving radially inward toward the center of
the wafer 50. A first shaded region 204 represents wasted
photoresist at the outer edge of the wafer 50, and a second shaded
region 206 represents a double thickness of photoresist extruded in
the center region of the wafer 50. It is necessary to start the
process with the extrusion head 50 just off the outer edge 52 of
the wafer 50 to cover the entire outer edge 52 with the extruded
spiral pattern 202 without unnecessary overlap or double thickness
around the outer edge 52 of the wafer 50. This results in the first
shaded region 204 of wasted photoresist. Likewise, it is necessary
to continue to extrude photoresist after the leading edge of the
extrusion head 30 reaches the center of the wafer 50 until the
entire wafer 50 is covered. Typically, it will be necessary to
continue the process until the trailing edge of the extrusion head
30 reaches the center to cover the entire center region of the
wafer 50. The overlap in the second shaded region 206 at the center
of the wafer 50 is inevitable because of the finite width of the
extrusion head 30. However, the amount of wasted and excess
photoresist is relatively small, and the efficiency of the
extrusion spin coating process far exceeds the efficiency of prior
spin coating processes.
[0073] FIG. 21 illustrates an extrusion spin coating spiral pattern
which results from starting the extrusion head at the outer edge of
the wafer and, while spinning the wafer, moving the extrusion head
radially inward toward the center of the wafer. The method and
apparatus of the invention may instead start the extrusion head at
the center of the wafer and move the extrusion head radially
outward toward the outer edge of the wafer.
[0074] It will be readily apparent to those skilled in the art that
this invention is not limited to the embodiments described above.
Different configurations and embodiments can be developed without
departing from the scope of the invention and are intended to be
included within the scope of the claims.
* * * * *