U.S. patent application number 10/198309 was filed with the patent office on 2004-01-22 for method for removing carbon contamination from optic surfaces.
Invention is credited to Graham, Samuel JR., Klebanoff, Leonard E..
Application Number | 20040011381 10/198309 |
Document ID | / |
Family ID | 30443096 |
Filed Date | 2004-01-22 |
United States Patent
Application |
20040011381 |
Kind Code |
A1 |
Klebanoff, Leonard E. ; et
al. |
January 22, 2004 |
Method for removing carbon contamination from optic surfaces
Abstract
A method using atomic hydrogen for removing carbon contamination
from optical surfaces. The method is particularly useful for
removing carbon and hydrocarbon contamination in-situ from the
surface of the multilayer optics used for extreme ultraviolet
lithography (EUVL) without degrading the quality of the optical
surface. Atomic hydrogen at pressures in the range of about
10.sup.-3 and 10.sup.-4 Torr without the potentially detrimental
heating of the optic is used to provide cleaning rates of about
6-60 .ANG./hr.
Inventors: |
Klebanoff, Leonard E.;
(Dublin, CA) ; Graham, Samuel JR.; (Dublin,
CA) |
Correspondence
Address: |
DONALD A. NISSEN
Sandia National Laboratories
MS 9031
7011 East Avenue
Livermore
CA
94550
US
|
Family ID: |
30443096 |
Appl. No.: |
10/198309 |
Filed: |
July 17, 2002 |
Current U.S.
Class: |
134/2 ; 134/201;
134/21; 257/E21.226; 359/838 |
Current CPC
Class: |
C03C 17/3482 20130101;
H01L 21/02046 20130101; B08B 7/0035 20130101; C03C 23/0075
20130101; G03F 7/70925 20130101; C03C 2218/32 20130101 |
Class at
Publication: |
134/2 ; 134/21;
134/201; 359/838 |
International
Class: |
C23G 001/00 |
Goverment Interests
[0001] This invention was made with Government support under
contract no. DE-AC04-94AL85000 awarded by the U.S. Department of
Energy to Sandia Corporation. The Government has certain rights in
the invention.
Claims
We claim:
1. A method for removing contaminants from a surface, comprising:
providing a vacuum chamber to house the contaminated surface; and
injecting atomic hydrogen at a pressure of between about 10.sup.-3
and 10.sup.-4 Torr into the vacuum chamber.
2. The method of claim 1, wherein the contaminated surface is the
surface of a multilayer optic.
3. The method of claim 2, w herein the multilayer optic is a
Si-capped Mo/Si multilayer optic or a Ru--B.sub.4C-capped Mo/Si
multilayer optic.
4. A system for removing contaminants from a surface, comprising: a
housing defining a vacuum chamber in which a surface to be cleaned
is located; and a source of atomic hydrogen capable of injecting
atomic hydrogen into the vacuum chamber, w herein pressure of
atomic hydrogen within the vacuum chamber is between 10.sup.-3 to
10.sup.-4 Torr, and wherein the surface is at a temperature of less
than about 50.degree. C. throughout the cleaning process.
5. The system of claim 4, w herein the surface is the surface of a
multilayer optic.
6. The system of claim 5, wherein the multilayer optic is a
Si-capped Mo/Si multilayer optic or a Ru--B.sub.4C-capped Mo/Si
multilayer optic.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0002] Not applicable.
FIELD OF THE INVENTION
[0003] This invention pertains to an in-situ method for removing
carbon contamination from optical surfaces and particularly for
removing carbon contamination from the surface of multilayer optics
used for extreme ultraviolet lithography (EUVL). The method is
directed to the use of atomic hydrogen at pressures significantly
greater than those used conventionally and without heating of the
optic.
BACKGROUND OF THE INVENTION
[0004] Degradation of reflectivity has long been a problem
associated with optical components (mirrors, gratings, etc.)
exposed to high energy radiation. In such systems, exposed optical
surfaces develop carbonaceous deposits as a result of
photoemission-induced cracking of hydrocarbons adsorbed on these
surfaces. This layer of contamination absorbs incident radiation
and reduces the reflectivity of optical surfaces. Carbon
contamination is of special concern in extreme ultraviolet
lithography (EUVL) since carbon effectively absorbs radiation at
13.4 nm, the wavelength most commonly employed in this technology.
For EUVL to be a viable next generation lithography technique,
optics must be capable of operating for extended periods of time
(years) with less than 2% loss in absolute reflectivity. To attain
this objective, carbon contamination must be limited to a layer of
less than about 20 .ANG. and preferably in the range of 5-10
.ANG..
[0005] Strategies for cleaning EUVL optical surfaces should have
the following attributes:
[0006] 1) The cleaning method should operate in-situ since ex-situ
cleaning requires optic removal and undesirable and possibly
excessive tool downtime.
[0007] 2) Since there are stringent requirements on the maximum
temperature and temperature gradients in the EUVL tool and
components contained therein, in particular multilayer optics,
cleaning processes that require significant heating can not be
used; the cleaning process should operate effectively at a
temperature as close to ambient as possible.
[0008] 3) The cleaning process should resuscitate contaminated
optical surfaces by restoring reflectivity uniformly over the optic
while, at the same time, maintaining surface and bulk optical and
physical properties of the multilayer optics.
[0009] Techniques for carbon gassification are well known in the
semiconductor processing industry. These methods include the use of
plasma discharges or sources of neutral atomic particles at
elevated temperatures and pressures (cf. U.S. Pat. No. 5,312,591
Method of Cleaning A Charged Beam Apparatus and Muiller et al.,
Cleaning of Carbon Contaminated Vacuum Ultra-violet Optics:
Influence on Surface Roughness and Reflectivity, Rev. of Sci.
Instrum., 63, 1428,1431, January 1992). While these methods have
been show n to remove carbon and hydrocarbon contamination they are
not amenable to cleaning of the multilayer thin film optics used
for EUVL. Carbon contamination on the surface of EUV optics is
expected to be highly nonuniform. Consequently, some regions of the
optic surface can be exposed to the cleaning procedure for a longer
period of time than others. Over exposure of the optic surface can
result in adverse effects to the optic (overshoot risks). Remote
radio frequency (RF) discharges used to produce oxygen species can
be effective in removing carbon deposits but induce surface
oxidation in the Si-capped Mo/Si reflective optics and erode the
surface of Ru--B.sub.4C-capped Mo/Si optics used in EUVL. Both of
these effects result in permanent loss in reflectivity of the
optics. By way of example, the addition of 4 .ANG. of an oxide film
beyond the thin film interference levels (.apprxeq.17 .ANG.) w ill
cause a loss in absolute reflectivity of about 1%. While
RF-hydrogen has been demonstrated to clean carbon contamination at
room temperature, a small but significant loss in reflectivity
from, as yet unknown causes, has been observed over exposure times
as short as 3 hours. This loss is believed to be related to the
diffusion and reaction of hydrogen ions produced by the RF-hydrogen
process with the outer layers of Si-capped Mo/Si optics.
[0010] One challenge in EUVL is that optics w ill be buried under
layers of surrounding hardware, such as mechanical frames and
cabling, as well as mechanical devices used to perform and monitor
the lithographic process. For reference, a state-of-the-art EUVL
tool is described in U.S. Pat. No. 6,031,598 to Tichenor et al.
Reactive gas phase species that encounter solid objects can be
quenched prior to reaching the optic surfaces that are to be
cleaned. The obscuring structures in the machine make it very
difficult to direct reactive species from the tool periphery, w
here they are generated, to the optics located in the interior of
the machine. Moreover, the integration of an RF source with
delicate electronics in an EUV lithographic toll presents
additional challenges.
[0011] Atomic hydrogen cleaning has been effectively demonstrated
in molecular epitaxy surface preparation of Si, GaAs and InP at
pressures in the range of about 10.sup.-5-10.sup.-6 Torr. However,
in all cases, both native oxide and hydrocarbon contamination are
typically removed at temperatures of several hundred degrees
Celsius (cf. Hirayama and Tatsumi, Si(111) Surface Cleaning Using
Atomic Hydrogen and SiH.sub.2 Studied Using Reflection High-Engergy
Eelectron Diffraction, J. Appl. Phys., 66 (2), July 1989, and
Sugaya and Kawabe, Low-Temperature Cleaning of GaAs Substrate by
Atomic Hydrogen Radiation, Jap. J. Appl. Phys., 30 (3A), March
1991, Akatsu et al., GaAs Wafer Bonding By Atomic Hydrogen Surface
Cleaning, J. Appl. Phys., 86 (12), December 1999). Consequently,
the art is in need of a method to generate reactive species inside
the optic mounting assembly in a manner that limits adverse effects
on the optics themselves.
SUMMARY OF THE INVENTION
[0012] Accordingly, the present invention is based, in part, on a
process that provides for removal of carbon and hydrocarbon
contamination from optical surfaces with substantially no
degradation of the properties of the optical surface. In
particular, the invention provides a method for removing carbon and
hydrocarbon contamination from the Si-capped and
Ru--B.sub.4C-capped multilayer Mo/Si optics used for EUVL. The
invention is particularly suited for photolithography systems with
optic surfaces that are otherwise inaccessible unless the system is
dismantled. Further, the invention provides for cleaning at near
room temperature with the cleaning species being generated near the
contaminated optical surfaces. Moreover, the process is compatible
with the sensitive electronics generally found in an EUVL tool.
[0013] In one embodiment, the invention is directed to a method for
cleaning contaminated optical surfaces that includes:
[0014] providing a vacuum chamber to house the contaminated
surface; and
[0015] injecting atomic hydrogen at a pressure of between about
10.sup.-3 to 10.sup.-4 Torr into the vacuum chamber.
[0016] In another embodiment, a system for cleaning carbon
contaminated optic surfaces that includes:
[0017] a housing defining a vacuum chamber in which a surface to be
cleaned is located; and
[0018] a source of atomic hydrogen capable of injecting atomic
hydrogen into the vacuum chamber, wherein the pressure of atomic
hydrogen within the vacuum chamber is between about 10.sup.-3 to
10.sup.-4 Torr, and wherein the surface is at a temperature of less
than about 50.degree. C. throughout the cleaning process.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a schematic illustration of an atomic hydrogen
source.
[0020] FIGS. 2a and 2b show the etch rate dependence of a
photoresist coated surface (2a) and a sputtered carbon coating (2b)
on hydrogen pressure.
[0021] FIGS. 3a and 3b are Auger depth profiles of a sputtered
carbon-coated silicon surface before exposure to atomic hydrogen
(3a) and after a 5-hr exposure (3b).
[0022] FIGS. 4a and 4b show Auger analyses of a silicon surface of
a Mo/Si multilayer optic prior to exposure to atomic hydrogen (4a)
and after a 3-hr exposure (4b).
[0023] FIGS. 5a and 5b show Auger analysis of a Si-capped Mo/Si
optic prior to (5a) and following (5b) exposure to atomic hydrogen
for 20 hours.
[0024] FIGS. 6a and 6b show Auger analysis of a Ru-capped Mo/Si
optic prior to (6a) and following (6b) exposure to atomic hydrogen
for 20 hours.
DETAILED DESCRIPTION OF THE INVENTION
[0025] It is well known in the art that atomic hydrogen can be used
to clean contaminated surfaces and particularly carbon contaminated
surfaces. However, as discussed above prior art atomic cleaning
methods employ elevated temperatures (typically >200.degree. C.)
and very low pressures (.apprxeq.10.sup.-5 to 10.sup.-6 Torr) in
order to facilitate a greater percentage of hydrogen atoms reaching
the contaminated surface. The dependence on elevated temperatures
is an unacceptably rigorous condition for cleaning contaminated
optical surfaces in an EUVL tool because of degradation of the
optical properties of the surface. The present invention employs
atomic hydrogen to clean optical surfaces but under much less
rigorous conditions. The inventors have found that thick (>100
.ANG.) layers of carbon and photoresist material (>1000 .ANG.)
can be removed from an optical surface by subjecting an optical
surface coated with these materials to atomic hydrogen at pressures
between about 10.sup.-3 to 10.sup.-4 Torr. Moreover, in contrast to
prior atomic hydrogen cleaning methods, it is not necessary to heat
the optics, which for the multilayer reflective optics used for
EUVL prevents degradation of the optical properties by
interdiffusion of the components of the multilayer structure. It
has been found that optical surfaces coated with carbon
contamination can be cleaned in about 3-5 hrs without any damage to
the underlying multilayer surface. Finally, the method is
particularly suited for photolithography systems with optic
surfaces that are otherwise inaccessible unless the system is
dismantled.
[0026] The experiments below are provided to illustrate and
exemplify the invention and are not intended to be limiting.
Modifications and variations may become apparent to those skilled
in the art, however these modifications and variations come within
the scope of the appended claims. Only the scope and content of the
claims limit the invention.
[0027] Atomic hydrogen was produced by a source arranged in a
configuration such as illustrated in FIG. 1. Hydrogen gas (H.sub.2)
was admitted through an inlet in the source and passed over a
filament heated to a temperature of about 2400.degree. C. to create
atomic hydrogen (H) that flowed from the source and into a vacuum
chamber that contained grounded samples consisting of; 1) a Si
wafer coated with about 1000 .ANG. of photoresist, 2) a Si wafer
coated with a 100 .ANG. layer of sputtered carbon, 3) a bare
Si-capped Mo/Si multilayer optic, and 4) a bare Ru-B.sub.4C-capped
Mo/Si multilayer optic. These samples were placed about 8 inches
downstream from an atomic hydrogen source. Following an exposure
lasting from 3-5 hours the samples were removed from the vacuum
chamber and the surfaces analyzed by Auger spectroscopy and
reflectometry to determine their surface composition and
reflectivity at an EUV wavelength of 13.4 nm. Because of the
possibility that IR radiation emitted from the filament could be
absorbed by the walls of the source causing the walls to heat up
and, in turn, emit IR radiation which could irradiate the samples,
the walls of the source were water cooled. Surfaces to be cleaned
were placed far enough away from the atomic hydrogen source so that
heating of the sample surface by IR radiation emitted by the atomic
hydrogen source was negligible. In a typical EUVL tool the optics
are water cooled further reducing the possibility of unwanted
surface heating.
[0028] The cleaning rate was determined as a function of pressure.
The results are shown in FIGS. 2a and 2b for cleaning photoresist
(FIG. 2a) and sputtered carbon (FIG. 2b) from a coated surface.
These data show that the rate of cleaning (etching) these surfaces
reaches a maximum at between 10.sup.-3-10.sup.-4 Torr. In both
cases rates of surface cleaning of from 6-60 .ANG./hr were
observed. At the conclusion of the cleaning experiments, the
temperatures of the samples was determined to be about 50.degree.
C., only slightly above ambient (.apprxeq.22.degree. C.), and well
below the 70.degree. C. limit for long-term stability of Mo/Si
optics.
[0029] Referring now to FIG. 3, an Auger analysis of the surface of
a silicon sample coated with sputtered carbon is show n in FIG. 3a.
After exposure to atomic hydrogen at a pressure of about
9.times.10.sup.-4 Torr for about 4.3 hours, the sample was again
analyzed by Auger depth profiling. As show n in FIG. 3b, the carbon
coating is nearly gone resulting a carbon etch rate of about 20
.ANG./hr.
[0030] EUV-based contamination of optical surfaces is not expected
to be uniformly distributed over the optical surface. Consequently,
bare portions of the optical surface can be exposed to atomic
hydrogen for varying amounts of time. A series of experiments were
undertaken to determine if any degradation in optical performance
would be induced by direct exposure of the optical surface to
atomic hydrogen.
[0031] A bare Si-capped Mo/Si multilayer optic w as exposed to
atomic hydrogen at a pressure of about 2.times.10.sup.-4 for 3
hours. Prior to beginning the experiment an Auger sputter profile
of the Si surface was taken (FIG. 4a). After the 3 hour exposure an
Auger sputter profile of the Si surface was taken again (FIG. 4b).
A comparison of these two Auger patterns shows a slight increase in
surface silicon oxide (less than 3 .ANG.). Reflectometry at 13.4 nm
showed a peak reflectance of about 66.6.+-.0.1% prior to exposure
and 66.5.+-.0.1% afterward. Within experimental error, the surface
reflectivity was unchanged in spite of exposure to atomic hydrogen
for 3 hours.
[0032] In order to explore in more detail the effects of exposure
of an optic surface to atomic hydrogen, uncoated (i.e., lacking a
carbon surface layer) multilayer optics having either a Si or
Ru--B.sub.4C capping layer were exposed to atomic hydrogen at a
pressure of about 9.times.10.sup.-4 Torr for about 20 hrs. It
should be noted that these exposure conditions is equivalent to
about 40 cleaning cycles or about the number of cleaning cycles
that an optic would undergo over about 5-7 years, the designated
life of the optic.
[0033] Results of Auger depth profiling of the exposed surfaces are
shown in FIGS. 5 and 6. FIG. 5 shows the growth of a very thin
silicon oxide film (.apprxeq.10 .ANG.) on the Si surface.
At-wavelength reflectometry at 13.4 nm shows a loss in absolute
surface reflectivity on the order of about 1%. Comparison of FIGS.
6a and 6b shows that the composition of the Ru surface was
substantially unchanged during the 20 hr. exposure to atomic
hydrogen. At-wavelength reflectometry (13.4 nm) of these surfaces
showed a loss in reflectivity of about 0.6%. These data show
reflectivity losses well within the specification of 2% for an EUVL
tool. The small losses in reflectivity experienced by exposure of
an uncoated multilayer optic to atomic hydrogen for extended
periods of time are substantially less than seen with RF-discharge
cleaning methods where losses in reflectivity of about 1% or
greater are experienced for exposures less than 3 hrs.
[0034] In summary, atomic hydrogen has been shown to efficiently
remove surface contamination (sputtered carbon and hydrocarbon
material) from both the Si and Ru surfaces of multilayer optics
with little adverse effect on the EUV reflectivity of the surfaces.
In contrast to prior art atomic hydrogen cleaning methods, the
cleaning rate disclosed here is most efficient at atomic hydrogen
pressures of between 10.sup.-3 and 10.sup.-4 Torr. Moreover, prior
art atomic hydrogen cleaning methods have required heating of the
component being cleaned to several hundred degrees Celsius, such is
not the case here. While the temperature of the optic being cleaned
rose slightly (to .apprxeq.50.degree. C.), presumably due to
radiative heating by the atomic hydrogen source, no heating of the
optics was found to be necessary for efficient cleaning.
Furthermore, the cleaning method disclosed here has been show n to
produce negligible surface damage, even to uncoated surfaces, in
contrast to prior cleaning methods.
* * * * *