U.S. patent application number 13/650778 was filed with the patent office on 2014-04-17 for method of and apparatus for in-situ repair of reflective optic.
This patent application is currently assigned to Cymer Inc.. The applicant listed for this patent is CYMER INC.. Invention is credited to Alexander I. ERSHOV.
Application Number | 20140102881 13/650778 |
Document ID | / |
Family ID | 50474414 |
Filed Date | 2014-04-17 |
United States Patent
Application |
20140102881 |
Kind Code |
A1 |
ERSHOV; Alexander I. |
April 17, 2014 |
METHOD OF AND APPARATUS FOR IN-SITU REPAIR OF REFLECTIVE OPTIC
Abstract
Method of and apparatus for repairing an optical element
disposed in a vacuum chamber while the optical element is in the
vacuum chamber. An exposed surface of the optical element is
exposed to an ion flux generated by an ion source to remove at
least some areas of the surface that have been damaged by exposure
to the environment within the vacuum chamber. The method and
apparatus are especially applicable to repair multilayer mirrors
serving as collectors in systems for generating EUV light for use
in semiconductor photolithography.
Inventors: |
ERSHOV; Alexander I.;
(Escondido, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
CYMER INC. |
San Diego |
CA |
US |
|
|
Assignee: |
Cymer Inc.
San Diego
CA
|
Family ID: |
50474414 |
Appl. No.: |
13/650778 |
Filed: |
October 12, 2012 |
Current U.S.
Class: |
204/192.34 ;
204/298.32; 204/298.36 |
Current CPC
Class: |
G03F 7/70925 20130101;
G03F 7/70975 20130101; G03F 7/70216 20130101 |
Class at
Publication: |
204/192.34 ;
204/298.36; 204/298.32 |
International
Class: |
B44C 1/22 20060101
B44C001/22; B32B 43/00 20060101 B32B043/00 |
Claims
1. A method of repairing an optical element disposed in a vacuum
chamber comprising the steps of: cleaning an exposed surface of the
optical element while the optical element is in the vacuum chamber
to remove at least some of a contaminant on the exposed surface to
produce a cleaned exposed surface; and exposing the cleaned exposed
surface while the optical element is in the vacuum chamber to an
ion flux generated by an ion gun to remove at least some areas of
the surface that have been damaged by exposure to the environment
within the vacuum chamber.
2. A method as claimed in claim 1 wherein said cleaning step is
performed by subjecting the exposed surface to hydrogen
radicals.
3. A method as claimed in claim 1 wherein said cleaning step
removes substantially all of the contaminant from the exposed
surface.
4. A method as claimed in claim 1 wherein said exposing step
removes substantially all areas of the surface that have been
damaged by exposure to the environment within the vacuum
chamber.
5. A method as claimed in claim 1 wherein said exposing step
comprises the additional steps of: generating the ion flux using an
ion gun; and directing the ion gun to cause ions to strike at least
some areas of the surface that have been damaged by exposure to the
environment within the vacuum chamber.
6. A method as claimed in claim 1 wherein said exposing step
comprises the additional steps of: generating the ion flux using an
ion gun; and directing the ion gun to cause ions to strike
substantially all areas of the surface that have been damaged by
exposure to the environment within the vacuum chamber.
7. A method of repairing a multilayer mirror in a system for
producing EUV light for semiconductor photolithography, the
multilayer mirror being disposed in a vacuum chamber in which a
source material is vaporized to produce the EUV light, comprising
the steps of: cleaning an exposed surface of the multilayer mirror
while the multilayer mirror is in the vacuum chamber to remove at
least some source material on the exposed surface to produce a
cleaned exposed surface; and exposing the cleaned exposed surface
while the multilayer mirror is in the vacuum chamber to an ion flux
generated by an ion gun to remove at least some areas of the
surface that have been damaged by exposure to the environment
within the vacuum chamber.
8. A method as claimed in claim 7 wherein said cleaning step is
performed by subjecting the exposed surface to hydrogen
radicals.
9. A method as claimed in claim 7 wherein said cleaning step
removes substantially all of the contaminant from the exposed
surface.
10. A method as claimed in claim 7 wherein said exposing step
removes substantially all areas of the surface that have been
damaged by exposure to the environment within the vacuum
chamber.
11. A method as claimed in claim 7 wherein said exposing step
comprises the additional steps of: generating the ion flux using an
ion gun; and directing the ion gun to cause ions to strike at least
some areas of the surface that have been damaged by exposure to the
environment within the vacuum chamber.
12. A method as claimed in claim 7 wherein said exposing step
comprises the additional steps of: generating the ion flux using an
ion gun; and directing the ion gun to cause ions to strike
substantially all areas of the surface that have been damaged by
exposure to the environment within the vacuum chamber.
13. Apparatus comprising: a vacuum chamber; an optical element
disposed within the vacuum chamber; an ion source disposed within
the vacuum chamber; and an actuator mechanically coupled to the ion
source and arranged to aim the ion source toward at least a portion
of an exposed surface of the optical element in response to a
control signal.
14. Apparatus as claimed in claim 13 wherein the ion source
generates a beam and wherein a cross-section size of the beam at
the exposed surface is in the range of about 2 cm to about 50
cm.
15. Apparatus as claimed in claim 13 further comprising a scanning
control system connected to supply the control signal to the
actuator.
16. Apparatus for producing EUV light for semiconductor
photolithography, the apparatus comprising: a vacuum chamber in
which a source material is vaporized to produce the EUV light; a
multilayer mirror disposed in the vacuum chamber; an ion source
disposed within the vacuum chamber; and an actuator mechanically
coupled to the ion source and arranged to aim the ion source toward
at least a portion of an exposed surface of the multilayer mirror
in response to a control signal.
17. Apparatus as claimed in claim 16 wherein the ion source
generates a beam and wherein a cross-section size of the beam at
the exposed surface is in the range of about 2 cm to about 50
cm.
18. Apparatus as claimed in claim 16 further comprising a scanning
control system connected to supply the control signal to the
actuator.
Description
FIELD
[0001] The present disclosure relates to optical elements designed
to operate in environments in which they are subject to
contamination and wear. An example of such an environment is the
vacuum chamber of an apparatus for generating extreme ultraviolet
("EUV") radiation from a plasma created through discharge or laser
ablation of a source material. In this application, the optical
elements are used, for example, to collect and direct the radiation
for utilization outside of the vacuum chamber, e.g., for
semiconductor photolithography.
BACKGROUND
[0002] Extreme ultraviolet light, e.g., electromagnetic radiation
having wavelengths of around 50 nm or less (also sometimes referred
to as soft x-rays), and including light at a wavelength of about
13.5 nm, can be used in photolithography processes to produce
extremely small features in substrates such as silicon wafers.
[0003] Methods for generating EUV light include converting a target
material from a liquid state into a plasma state. The target
material preferably includes at least one element, e.g., xenon,
lithium or tin, with one or more emission lines in the EUV range.
In one such method, often termed laser produced plasma ("LPP"), the
required plasma can be produced by using a laser beam to irradiate
a target material having the required line-emitting element.
[0004] One LPP technique involves generating a stream of target
material droplets and irradiating at least some of the droplets
with laser light pulses. In more theoretical terms, LPP light
sources generate EUV radiation by depositing laser energy into a
target material having at least one EUV emitting element, such as
xenon (Xe), tin (Sn), or lithium (Li), creating a highly ionized
plasma with electron temperatures of several 10's of eV.
[0005] The energetic radiation generated during de-excitation and
recombination of these ions is emitted from the plasma in all
directions. In one common arrangement, a near-normal-incidence
mirror (often termed a "collector mirror" or simply a "collector")
is positioned to collect, direct (and in some arrangements, focus)
the light to an intermediate location. The collected light may then
be relayed from the intermediate location to a set of scanner
optics and ultimately to a wafer.
[0006] In the EUV portion of the spectrum it is generally regarded
as necessary to use reflective optics for the collector. At the
wavelengths involved, the collector is advantageously implemented
as a multi-layer mirror ("MLM"). As its name implies, this MLM is
generally made up of alternating layers of material over a
foundation or substrate.
[0007] The optical element must be placed within the vacuum chamber
with the plasma to collect and redirect the EUV light. The
environment within the chamber is inimical to the optical element
and so limits its useful lifetime, for example, by degrading its
reflectivity. An optical element within the environment may be
exposed to high energy ions or particles of source material. The
particles of source material can contaminate the optical element's
exposed surface. Particles of source material can also cause
physical damage and localized heating of the MLM surface. The
source materials may be particularly reactive with a material
making up at least one layer of the MLM, e.g., molybdenum and
silicon. Temperature stability, ion-implantation and diffusion
problems may need to be addressed even with less reactive source
materials, e.g., tin, indium, or xenon.
[0008] There are techniques which may be employed to increase
optical element lifetime despite these harsh conditions. For
example, protective layers or intermediate diffusion barrier layers
may be used to isolate the MLM layers from the environment. The
collector may be heated to an elevated temperature of, e.g., up to
500.degree. C., to evaporate debris from its surface. The collector
surface may be cleaned using hydrogen radicals. An etchant may be
employed e.g., a halogen etchant, to etch debris from the collector
surfaces and create a shielding plasma in the vicinity of the
reflector surfaces. These latter two techniques can remove
contaminating source material from the collector surface, but they
are generally ineffective to remove damaged original collector
material from the surface of the collector. There remains a need to
extend collector lifetime by cleaning the collector surface and
removing damaged collector material from the collector surface, all
preferably without having to remove the collector from its
operating environment. With this in mind, applicant discloses
arrangements for in-situ cleaning and repair of the surfaces of
optical elements.
SUMMARY
[0009] The following presents a simplified summary of one or more
embodiments in order to provide a basic understanding of the
embodiments. This summary is not an extensive overview of all
contemplated embodiments, and is not intended to identify key or
critical elements of all embodiments nor delineate the scope of any
or all embodiments. Its sole purpose is to present some concepts of
one or more embodiments in a simplified form as a prelude to the
more detailed description that is presented later.
[0010] According to one aspect, the invention is a method of
repairing an optical element disposed in a vacuum chamber
comprising the steps of cleaning an exposed surface of the optical
element while the optical element is in the vacuum chamber to
remove at least some of a contaminant on the exposed surface to
produce a cleaned exposed surface, and exposing the cleaned exposed
surface while the optical element is in the vacuum chamber to an
ion flux generated by an ion gun to remove at least some areas of
the surface that have been damaged by exposure to the environment
within the vacuum chamber. The cleaning step may be performed by
subjecting the exposed surface to hydrogen radicals and may remove
substantially all of the contaminant from the exposed surface.
[0011] The exposing step may remove substantially all areas of the
surface that have been damaged by exposure to the environment
within the vacuum chamber. The exposing step may comprise the
additional steps of generating the ion flux using an ion gun and
directing the ion gun to cause ions to strike at least some areas
of the surface that have been damaged by exposure to the
environment within the vacuum chamber, or all of the surface.
[0012] Another aspect of the invention is a method of repairing a
multilayer mirror in a system for producing EUV light for
semiconductor photolithography, the multilayer mirror being
disposed in a vacuum chamber in which a source material is
vaporized to produce the EUV light, comprising the steps of
cleaning an exposed surface of the multilayer mirror while the
multilayer mirror is in the vacuum chamber to remove at least some
source material on the exposed surface to produce a cleaned exposed
surface and exposing the cleaned exposed surface while the
multilayer mirror is in the vacuum chamber to an ion flux to remove
at least some areas of the surface that have been damaged by
exposure to the environment within the vacuum chamber.
[0013] Yet another aspect of the invention is an apparatus
comprising a vacuum chamber, an optical element disposed within the
vacuum chamber, an ion source disposed within the vacuum chamber,
and an actuator mechanically coupled to the ion source and arranged
to aim the ion source toward at least a portion of the exposed
surface of the optical element in response to a control signal.
[0014] Still another aspect of the invention is apparatus for
producing EUV light for semiconductor photolithography, the
apparatus comprising a vacuum chamber in which a source material is
vaporized to produce the EUV light, a multilayer mirror disposed in
the vacuum chamber, an ion source disposed within the vacuum
chamber, and an actuator mechanically coupled to the ion source and
arranged to aim the ion source toward at least a portion of the
exposed surface of the optical element in response to a control
signal.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 shows a schematic, not to scale, view of an overall
broad conception for a laser-produced plasma EUV light source
system according to an aspect of the present invention.
[0016] FIG. 2 is a scanning electron microscope photograph of the
collector surface damage.
[0017] FIG. 3 is transmission electron microscope photograph of
collector surface damage.
[0018] FIG. 4 is flow chart showing the steps of a process for
cleaning and repairing the collector 30 of FIG. 1.
[0019] FIG. 5.1 is a schematic of one embodiment of an arrangement
for repair of the collector 30 of FIG. 1.
DETAILED DESCRIPTION
[0020] Various embodiments are now described with reference to the
drawings, wherein like reference numerals are used to refer to like
elements throughout. In the following description, for purposes of
explanation, numerous specific details are set forth in order to
promote a thorough understanding of one or more embodiments. It may
be evident in some or all instances, however, that any embodiment
described below can be practiced without adopting the specific
design details described below. In other instances, well-known
structures and devices are shown in block diagram form in order to
facilitate description of one or more embodiments.
[0021] With initial reference to FIG. 1 there is shown a schematic
view of an exemplary EUV light source, e.g., a laser produced
plasma EUV light source 20 according to one aspect of an embodiment
of the present invention. As shown, the EUV light source 20 may
include a pulsed or continuous laser source 22, which may for
example be a pulsed gas discharge CO.sub.2 laser source producing
radiation at 10.6 .mu.m. The pulsed gas discharge CO.sub.2 laser
source may have DC or RF excitation operating at high power and
high pulse repetition rate.
[0022] The EUV light source 20 also includes a target delivery
system 24 for delivering target material in the form of liquid
droplets or a continuous liquid stream. The target material may be
made up of tin or a tin compound, although other materials could be
used. The target delivery system 24 introduces the target material
into the interior of a chamber 26 to an irradiation region 28 where
the target material may be irradiated to produce plasma. In some
cases, an electrical charge is placed on the target material to
permit the target material to be steered toward or away from the
irradiation region 28. It should be noted that as used herein an
irradiation region is a region where target material irradiation
may occur, and is an irradiation region even at times when no
irradiation is actually occurring.
[0023] Continuing with FIG. 1, the light source 20 may also include
one or more optical elements such as a collector 30. The collector
30 may be a normal incidence reflector, for example, implemented as
an MLM, that is, a SiC substrate coated with a Mo/Si multilayer
with additional thin barrier layers deposited at each interface to
effectively block thermally-induced interlayer diffusion. Other
substrate materials, such as Al or Si, can also be used. The
collector 30 may be in the form of a prolate ellipsoid, with an
aperture to allow the laser light to pass through and reach the
irradiation region 28. The collector 30 may be, e.g., in the shape
of a ellipsoid that has a first focus at the irradiation region 28
and a second focus at a so-called intermediate point 40 (also
called the intermediate focus 40) where the EUV light may be output
from the EUV light source 20 and input to, e.g., an integrated
circuit lithography tool 50 which uses the light, for example, to
process a silicon wafer workpiece 52 in a known manner. The silicon
wafer workpiece 52 is then additionally processed in a known manner
to obtain an integrated circuit device.
[0024] As described above, one of the technical challenges in the
design of an optical element such as the collector 30 is extending
its lifetime. The surface of the collector, which is usually a
coating, becomes contaminated with source material, e.g., tin.
In-situ cleaning using hydrogen radicals in a known manner can be
used to remove this contamination from the collector surface.
In-situ collector cleaning is very desirable because it
dramatically reduces both tool downtime and the expense of
collector replacement. Hydrogen radical-based cleaning does not,
however, repair damage to the collector surface. This type of
damage can be quite severe, as can be seen in FIGS. 2 and 3, which
are a scanning electron microscope photograph and a transmission
electron microscope photograph, respectively, of collector surface
damage.
[0025] Thus, the surface damage of the collector coating is still
present after cleaning has been performed. This surface damage can
lead to accelerated degradation of collector reflectivity after the
cleaning. In order to improve collector lifetime after cleaning so
that reflectivity approaches that of a new collector, the collector
surface damage needs to be repaired in-situ. As used herein,
"repair" and its cognates refer to removing some or all of the
collector material that has been damaged by exposure to the
conditions in the chamber environment.
[0026] To achieve this end, in one embodiment, the present
invention is a method of in-situ cleaning and repairing of a
collector surface. The method will now be described in conjunction
with FIG. 4, which is a flowchart of one embodiment of the method.
In the method shown in FIG. 4 a specific source material, tin, is
referenced, but it will be understood by one of ordinary skill in
the art that the method is equally applicable to systems in which
another type of source material is used.
[0027] In a first step S1 the system is cleaned using a prior art
hydrogen radical-based technique or any other suitable technique.
The cleaning step Si is carried out until it is determined in step
S2 that a predetermined amount, e.g., preferably substantially all,
of the tin has been removed. The determination on whether enough
tin has been removed can be made using any desirable method. For
example, the completion of step Si can be determined by
measurement, for example, by measuring the amount of residual tin,
or simply continuing to perform step S1 for an amount of time that
has been determined to be sufficient to remove enough of the tin
residue.
[0028] Once it is determined in step S2 that enough of the tin has
been removed from the collector surface, an ion source or gun such
as ion gun 100 as shown in FIG. 5 is introduced into the vacuum
chamber 26 in step S3. As used herein, "ion source" or "ion gun"
means a device specifically adapted to produce ions predominantly
in a given direction without the production of contaminant
particles, and specifically excludes devices or arrangements that
produce ions ancillary to some other process such as the production
of EUV light. In step S4 the ion gun repairs the collector surface
by removing some or all of the damaged areas of the surface of
collector 30. Ion gun 100 accomplishes this by generating a flux of
ions towards the surface of collector 30. The energy of the ions is
selected to provide effective sputtering of the surface (typically
200-1000 V). Because the damage to the surface of collector 30 is
localized to just beneath the surface, that is, in the first 50 nm
or so, the ion bombardment will remove all the damaged area and
leave the undamaged inner layers of the collector 30.
[0029] As shown in FIG. 5, the ion gun 100 is arranged in fluid
communication with a source of process gas 110, which process gas
can be Ar or any other suitable gas, such as Kr or He. The flow of
process gas into the ion gun 100 could be in the range of 0.1-10
slm.
[0030] The vacuum chamber 26 is maintained at vacuum using a pump
120, which is preferably a high-throughput turbo-molecular pump
(>500 L/sec pumping speed).
[0031] The ion gun 100 can be positioned near the primary focus of
the collector 30 and pointed towards the collector surface. It
creates a flux of ions towards the collector, with the
cross-section size of the beam at the collector surface preferably
in the range of 2-50 cm depending on the particular design of the
ion gun 100. The ion beam can be scanned over the collector surface
by aiming or tilting the ion gun 100 under the control of scanning
control system 130. The scanning control system 130 can control the
ion gun to repair the entire surface of the collector 30 or just
the selected areas of the surface of collector 30. The scanning
control system 130 accomplishes this by aiming the ion gun 100, for
example, by tilting the head of ion gun 100 by controlling an
actuator 140 that is mechanically coupled to the ion gun 100. As
used herein, an actuator is any device for causing motion in
response to a control signal. The scanning control system 130 may
be implemented by any device that can generate control signals such
as in response to user input or a control program, for example a
processor suitably programmed in a manner that will be readily
apparent to one having ordinary skill in the art.
[0032] The ion gun 100 bombards the selected areas of the surface
of the collector 30 until a predetermined amount of the surface has
been removed. This can be accomplished by measuring a parameter
which depends on the amount of removed material, e.g.,
reflectivity. It can also be accomplished by bombarding the
selected area of the surface with ions for an amount a time that
has been determined to be sufficient to ensure removal of enough
material.
[0033] The above description includes examples of one or more
embodiments. It is, of course, not possible to describe every
conceivable combination of components or methodologies for purposes
of describing the aforementioned embodiments, but one of ordinary
skill in the art may recognize that many further combinations and
permutations of various embodiments are possible. Accordingly, the
described embodiments are intended to embrace all such alterations,
modifications and variations that fall within the spirit and scope
of the appended claims. Furthermore, to the extent that the term
"includes" is used in either the detailed description or the
claims, such term is intended to be inclusive in a manner similar
to the term "comprising" as "comprising" is construed when employed
as a transitional word in a claim. Furthermore, although elements
of the described aspects and/or embodiments may be described or
claimed in the singular, the plural is contemplated unless
limitation to the singular is explicitly stated. Additionally, all
or a portion of any aspect and/or embodiment may be utilized with
all or a portion of any other aspect and/or embodiment, unless
stated otherwise.
* * * * *