U.S. patent application number 11/108673 was filed with the patent office on 2006-10-19 for liquid immersion lithography system with tilted liquid flow.
This patent application is currently assigned to ASML Holding N.V.. Invention is credited to Aleksandr Khmelichek, Louis Markoya, Harry Sewell.
Application Number | 20060232753 11/108673 |
Document ID | / |
Family ID | 36602526 |
Filed Date | 2006-10-19 |
United States Patent
Application |
20060232753 |
Kind Code |
A1 |
Khmelichek; Aleksandr ; et
al. |
October 19, 2006 |
Liquid immersion lithography system with tilted liquid flow
Abstract
A liquid immersion lithography system including a projection
optical system for directing electromagnetic radiation onto a
substrate, and a showerhead for delivering liquid flow between the
projection optical system and the substrate. The showerhead
includes an injection nozzle and a retrieval nozzle located at
different heights. The liquid flow is tilted relative to the
substrate. A direction from the injection nozzle to the retrieval
nozzle is tilted at approximately 1 to 2 degrees relative to the
substrate.
Inventors: |
Khmelichek; Aleksandr;
(Brooklyn, NY) ; Markoya; Louis; (Sandy Hook,
CT) ; Sewell; Harry; (Ridgefield, CT) |
Correspondence
Address: |
STERNE, KESSLER, GOLDSTEIN & FOX PLLC
1100 NEW YORK AVENUE, N.W.
WASHINGTON
DC
20005
US
|
Assignee: |
ASML Holding N.V.
Veldhoven
NL
|
Family ID: |
36602526 |
Appl. No.: |
11/108673 |
Filed: |
April 19, 2005 |
Current U.S.
Class: |
355/30 ;
355/53 |
Current CPC
Class: |
G03F 7/70341
20130101 |
Class at
Publication: |
355/030 ;
355/053 |
International
Class: |
G03B 27/52 20060101
G03B027/52 |
Claims
1. A liquid immersion lithography system, comprising: a projection
optical system to direct electromagnetic radiation onto a
substrate; and a showerhead to deliver liquid flow between the
projection optical system and the substrate, wherein the showerhead
includes an injection nozzle and a retrieval nozzle located at
different heights.
2. The liquid immersion lithography system of claim 1, wherein the
liquid flow is tilted relative to the substrate.
3. The liquid immersion lithography system of claim 1, wherein a
direction from the injection nozzle to the retrieval nozzle is
tilted at approximately 1 to 2 degrees relative to the
substrate.
4. A liquid immersion lithography system comprising: a projection
optical system to expose a substrate; and an injection nozzle and a
retrieval nozzle to deliver tilted liquid flow between the
projection optical system and the substrate.
5. The liquid immersion lithography system of claim 4, wherein the
tilted liquid flow is tilted at approximately 1 to 2 degrees
relative to the substrate.
6. An exposure system comprising: in order of light propagation, an
illumination source, a condenser lens, a contrast device and
projection optics; a liquid delivery system to provide liquid to an
exposure area below the projection optics; and means for providing
tilted liquid flow of the liquid.
7. An exposure system comprising: in order of light propagation, an
illumination source, a condenser lens, a contrast device and
projection optics; a liquid delivery system to provide liquid to an
exposure area of a substrate; and means for tilting the substrate
relative to a horizontal.
8. The system of claim 6, wherein the means for tilting also tilts
the projection optics.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention is related to liquid immersion
lithography, and more particularly, to efficient recirculation of
liquid in immersion lithography systems.
[0003] 2. Description of the Related Art
[0004] An integrated circuit ("IC") integrates a large number of
electronic circuit elements, including transistors. The circuit
elements are manufactured and interconnected on a semiconductor
substrate, e.g., on a single crystalline silicon wafer. During
manufacturing, the wafers undergo cycles of film deposition and
lithography, in addition to other processing. Film deposition is
the process of depositing a layer of material, e.g., insulating or
metallic, over the entire substrate; lithography is the process of
patterning the deposited layer. The first step in lithography
involves coating the wafer with photoresist that is sensitive to
particular radiation, typically ultra-violet light. During the next
step--exposure--the substrate is exposed to a radiation pattern
stored on a mask, also called a reticle. Radiation locally changes
the physical or chemical properties of the photoresist, and the
exposed (or unexposed) areas are selectively dissolved during a
developing step that leaves behind a pattern of photoresist. The
patterned photoresist provides a pattern for a subsequent etching
step. The etching step removes undesired areas of the deposited
layer, leaving behind structures associated with circuit elements,
such as wires, resistors and transistors, and the like.
[0005] Highly integrated circuits require small circuit elements.
Since the radiation pattern shapes the circuit elements, the
smallest feature size depends on the resolution achieved in the
lithography exposure step, or the resolution of the projection
device used to project the radiation pattern onto the substrate.
According to the Raleigh criterion, this resolution is proportional
to the wavelength .lamda. of the projected light and to an
adjustment factor k.sub.1, and inversely proportional to the sine
function of the marginal, or capture, angle .theta. of the
projection optics, where:
Resolution=k.sub.1*.lamda./sin(.theta.)
[0006] The resolution can be decreased, i.e., improved, in one of
three ways. First, the wavelength .lamda. of the projected light
can be decreased. A shorter wavelength, however, may require new
photoresist and a number of changes in the projection device, such
as using a different light source and light filters, and special
lenses for the projection optics. Second, the resolution can be
decreased by decreasing the adjustment factor k.sub.1. Decreasing
k.sub.1 may also require the use of different photoresist and high
precision tools. Third, the marginal angle .theta. can be increased
by increasing the size of the projection optics. The effect of this
increase, however, is limited by the sine function above.
[0007] One way to reduce the wavelength .lamda. of the projected
light is through the use of immersion lithography, where a liquid
is injected between the projection optics and the wafer, taking
advantage of the higher refractive index of the liquid compared to
air (and, therefore, resulting in a smaller effective wavelength
.lamda.).
[0008] One of the persistent problems in immersion lithography
relates to ensuring purity and lack of contamination of the
immersion liquid. The immersion liquid is generally recirculated,
using some form of an injection system to inject the liquid into
the volume between the projection optics and the substrate, and
extracted using some form of a extraction, or suction system to
extract the liquid from the exposure area back into recirculation.
However, the liquid can get contaminated, for example, due to
pickup of particles from the air, or due to pickup of material from
the photo resist that is being exposed. Normally, filtering systems
are in place to remove the contaminants. However, not all of the
liquid that is injected into the exposure area can actually be
recirculated. This is due to the surface tension that exists
between the liquid and the substrate surface. Although most of the
liquid can be extracted, using the suction pressure of the
extraction/recirculation system, some droplets of liquid remain on
the surface of the way of the substrate, together with their
contaminants. Increasing the suction pressure generally does not
help past a certain point, since this will increase the
recirculation speed, but will not address the problems caused by
the surface tension of the liquid.
[0009] Even with the current designs, many liquid injection and
extraction systems utilize a fairly complex showerhead design to
deliver and extract the immersion liquid. However, even in such
complex designs, the problem of the surface tension of the liquid
is not entirely solved.
[0010] Accordingly, what is needed is an approach that ensures that
all of the injected liquid is collected by the extraction system in
an immersion lithography tool.
SUMMARY OF THE INVENTION
[0011] The present invention is directed to liquid immersion
lithography system with tilted liquid flow that substantially
obviates one or more of the problems and disadvantages of the
related art.
[0012] There is provided a liquid immersion lithography system
including a projection optical system for directing electromagnetic
radiation onto a substrate, and a showerhead for delivering liquid
flow between the projection optical system and the substrate. The
showerhead includes an injection nozzle and a retrieval nozzle
located at different heights. The liquid flow is tilted relative to
the substrate. A direction from the injection nozzle to the
retrieval nozzle is tilted at approximately 1 to 2 degrees relative
to the substrate.
[0013] In another aspect, a liquid immersion lithography system
includes a projection optical system for exposing a substrate, an
injection nozzle and a retrieval nozzle for delivering tilted
liquid flow between the projection optical system and the
substrate. The liquid flow is tilted at approximately 1 to 2
degrees relative to the substrate.
[0014] In another aspect, an exposure system includes, in order of
light propagation, an illumination source, a condenser lens, a mask
(or contrast device) and projection optics. A liquid delivery
system provides liquid to an exposure area below the projection
optics. The exposure system also includes means for providing
tilted liquid flow of the liquid.
[0015] In another aspect, an exposure system includes, in order of
light propagation, an illumination source, a condenser lens, a mask
and projection optics. A liquid delivery system provides liquid to
an exposure area of a substrate. The substrate is tilted relative
to a horizontal.
[0016] Additional features and advantages of the invention will be
set forth in the description that follows. Yet further features and
advantages will be apparent to a person skilled in the art based on
the description set forth herein or may be learned by practice of
the invention. The advantages of the invention will be realized and
attained by the structure particularly pointed out in the written
description and claims hereof as well as the appended drawings.
[0017] It is to be understood that both the foregoing general
description and the following detailed description are exemplary
and explanatory and are intended to provide further explanation of
the invention as claimed.
BRIEF DESCRIPTION OF THE DRAWINGS/FIGS.
[0018] The accompanying drawings, which are included to provide a
further understanding of the exemplary embodiments of the invention
and are incorporated in and constitute a part of this
specification, illustrate embodiments of the invention and together
with the description serve to explain the principles of the
invention. In the drawings:
[0019] FIG. 1 illustrates one embodiment of the invention that uses
a tilted showerhead for injection and extraction of the immersion
liquid.
[0020] FIG. 2 illustrates a close-up view of the tilted showerhead
arrangement of FIG. 1.
[0021] FIG. 3 is another illustration of the exposure area of a
liquid immersion lithography system with a liquid in the exposure
area.
[0022] FIG. 4 illustrates a meniscus region A of FIG. 3.
[0023] FIG. 5 illustrates a meniscus region B of FIG. 3.
[0024] FIG. 6 illustrates a three-dimensional isometric view of the
embodiment illustrated in FIGS. 1-5.
[0025] FIG. 7 shows an exemplary photolithographic system that uses
the tilted showerhead.
DETAILED DESCRIPTION OF THE INVENTION
[0026] Reference will now be made in detail to the embodiments of
the present invention, examples of which are illustrated in the
accompanying drawings.
[0027] The inventors have discovered a rather unexpected
phenomenon:
[0028] when the liquid flow in an immersion lithography system is
tilted, even by a relatively small angle, as little as one or two
degrees, the tilt, and the corresponding effect of gravity on the
liquid flow, is sufficient to overcome the residual surface tension
forces acting on the liquid. Accordingly, with such a tilted
arrangement, the pooling of the immersion liquid in certain
portions of the exposure area can be avoided, reducing the
possibility of contamination.
[0029] FIG. 7 shows an exemplary photolithographic system that uses
the tilted showerhead. As shown in FIG. 7, the lithographic system
700 (shown in side view) includes a light source (illumination
source) 710, such as a laser or a lamp, illumination optics 712
(such as a condenser lens), and a reticle (i.e., a, mask, or a
contrast device) 714, which is usually mounted on a reticle stage
(not shown). Note that the reticle 714 can be a plate with an
exposure pattern on it, or a spatial light modulator array, such as
used in maskless lithography. Light from the reticle 714 is imaged
onto a wafer 718 using projection optics 716. The wafer 718 is
mounted on a wafer stage 720.
[0030] FIG. 1 illustrates one embodiment of the invention that uses
a tilted showerhead for injection and extraction of the immersion
liquid. As shown in FIG. 1, the projection optics 716 is mounted
above a substrate, such as the semiconductor wafer 718. One portion
of a showerhead is used for liquid injection, and a second portion
of the showerhead is used for liquid extraction.
[0031] FIG. 1 thus illustrates a cross-sectional view (at the top)
and a plan view (at the bottom) of one embodiment of the immersion
lithography system with tilted liquid flow arrangement. A lower
portion of the projection optical system 716 is located above the
wafer 718 (note that the last element of the projection optical
system 716 can be a prism, or a lens, or a glass window). Also
shown in FIG. 1 is a immersion head, or shower head 110, which is
shown in cross section. Liquid flow enters the exposure area
through an injection nozzle 106, and exits through a retrieval, or
suction, nozzle 108. Note the different heights of the injection
nozzle 106 and the retrieval nozzle 108 in the exposure area
illustrated in more detail in FIG. 2, which provides for a height
differential, and therefore for a tilted liquid flow.
[0032] The dimension of the gap between the lowest element of the
projection optics 716 (which can be either a lens or a prism) and
the surface of the substrate 718 is usually on the order of
approximately one millimeter (typically ranging between about 0.5
millimeters and 2 millimeters). Thus, it is possible to leave one
of the nozzles (either injection or extraction) in its original
place and correspondingly raise or lower the other nozzle, so as to
create a tilt.
[0033] The relative arrangement of the two nozzles is such that
there is a natural gravity-induced liquid flow from the injection
showerhead to the extraction showerhead. As noted above, the tilt
can be relatively small, even as little as one or two degrees. Note
that the important aspect is the relative arrangement of the
injection and extraction nozzles, such that there is a relative
tilt of the liquid flow compared to the plane of the wafer 102
(which is normally horizontal).
[0034] FIG. 2 illustrates a close up view of the tilted nozzle
arrangement. Note in particular the bottom surface of the
showerhead, labeled X in FIG. 2, which is tilted at approximately
one or two degrees as shown in the figure. Note also that the
suction nozzle 108 is located below a bottom surface 202 of the
projection optical system 716 by a distance t (see left hand side
of the figure).
[0035] FIG. 3 is another illustration of the exposure area of a
liquid immersion lithography system, similar to FIG. 2, but showing
a liquid 302 in the exposure area, as would be the case during
actual operation of the device. Note also a withdrawal pressure
p.sub.w, and two regions A and B, which include two meniscus
regions, discussed further below.
[0036] FIG. 4 illustrates a meniscus region A from FIG. 3. Note a
height "h," which refers to a gap height, and the shape of the
meniscus, which is outward.
[0037] The pressure in the liquid adjacent to the meniscus
(p.sub.m) will be reduced due to the effect of surface tension. The
exact pressure at this location will depend on the detailed shape
of the meniscus and include effects related to the contact angles.
However, an order of magnitude estimate of this pressure depression
is given by: p m = p amb - p w - 4 .times. .sigma. h ##EQU1##
[0038] where p.sub.amb is the ambient pressure, p.sub.w is the
withdrawal pressure, .sigma. is the surface tension and h is the
gap height (see FIG. 4). (See generally J. Fay, Introduction to
Fluid Mechanics, MIT Press, Cambridge, Mass. (1994), which is
incorporated herein by reference.
[0039] FIG. 5 illustrates a region B from FIG. 3 with a gap height
"H" between the showerhead 110 and the wafer 718. Note that the
meniscus is inward shaped, with the pressure in the liquid given
by: p M = p amb - p w - 4 .times. .sigma. H ##EQU2##
[0040] Because H>h, p.sub.M>p.sub.m and liquid will start
flowing from the side that has the bigger gap.
[0041] FIG. 6 is another illustration of the embodiment illustrated
in FIGS. 1-5, in this case, a three-dimensional isometric view.
Shown in FIG. 6 is the wafer 718 positioned below the projection
system 716 (only a portion of which is shown). The showerhead 110
is visible in the figure, with the liquid 302 flowing under the
projection optical system 716.
[0042] In another embodiment, it is possible to accomplish the
tilting effect by tilting the wafer surface. Normally, in
conventional systems, the wafer 718 is kept perfectly horizontal
(or, as horizontal as practical), to ensure good image quality.
However, it is possible to tilt the wafer 718 (and,
correspondingly, the rest of the exposure optics, such that the
wafer surface is tilted by approximately one or two degrees, so as
to encourage liquid flow in the direction from the injection port
to the extraction port. Such an approach is more complicated to
implement than the embodiment described above, since tilting the
entire lithographic tool may be undesirable or mechanically
problematic. However, such a tilting of the entire lithographic
tool will accomplish the same purpose--creation of a preferred
direction of liquid flow even in the absence of suction pressure
for extraction.
[0043] As yet a third embodiment, the tilting effect can be
simulated using forced air flow. In other words, even if the
injection and extraction ports are level with each other, and the
substrate is also oriented perfectly horizontally, an air pressure
gradient in the direction from the injection port to the extraction
port will also achieve a similar effect--that is, overcoming the
surface tension forces that otherwise impede liquid flow.
CONCLUSION
[0044] While various embodiments of the present invention have been
described above, it should be understood that they have been
presented by way of example, and not limitation. It will be
apparent to persons skilled in the relevant art that various
changes in form and detail can be made therein without departing
from the spirit and scope of the invention.
* * * * *