U.S. patent application number 15/593732 was filed with the patent office on 2018-11-15 for apparatus for and method of controlling debris in an euv light source.
The applicant listed for this patent is ASML Netherlands B.V.. Invention is credited to Christianus W.J. Berendsen, Alexander I. Ershov, Igor V. Fomenkov, John Tom Stewart, IV.
Application Number | 20180330841 15/593732 |
Document ID | / |
Family ID | 64050834 |
Filed Date | 2018-11-15 |
United States Patent
Application |
20180330841 |
Kind Code |
A1 |
Ershov; Alexander I. ; et
al. |
November 15, 2018 |
APPARATUS FOR AND METHOD OF CONTROLLING DEBRIS IN AN EUV LIGHT
SOURCE
Abstract
Disclosed is an EUV system in which a source control loop is
established to maintain and optimize debris flux while not unduly
affecting optimum EUV generation conditions. One or more
temperature sensors, e.g., thermocouples may be installed in the
vessel to measure respective local gas temperatures. The respective
local temperature as measured by the one or more thermocouples can
be used as one or more inputs to the source control loop. The
source control loop may then adjust the laser targeting to permit
optimization of debris generation and deposition while not
affecting EUV production, thus extending the lifetime of the source
and its collector.
Inventors: |
Ershov; Alexander I.; (San
Diego, CA) ; Stewart, IV; John Tom; (San Diego,
CA) ; Fomenkov; Igor V.; (San Diego, CA) ;
Berendsen; Christianus W.J.; (Veldhoven, NL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ASML Netherlands B.V. |
Veldhoven |
|
NL |
|
|
Family ID: |
64050834 |
Appl. No.: |
15/593732 |
Filed: |
May 12, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G03F 7/7085 20130101;
G03F 7/70175 20130101; H05G 2/003 20130101; G03F 7/70033 20130101;
H05G 2/008 20130101; G21K 1/067 20130101; G03F 7/70916
20130101 |
International
Class: |
G21K 1/06 20060101
G21K001/06; H05G 2/00 20060101 H05G002/00 |
Claims
1. Apparatus for generating EUV radiation comprising: a vessel; a
laser adapted to generate laser radiation; a laser steering system
arranged to receive the laser radiation and adapted to steer the
laser radiation to an irradiation region within the vessel; a
target material delivery system adapted to deliver target material
to the irradiation region to be irradiated by the laser, the
irradiation of the target material by the laser generating the EUV
radiation; a target material steering system coupled to the target
material delivery system for adjusting a position of the target
material within the irradiation region; an EUV radiation metrology
system adapted to measure at least one operating parameter of the
EUV radiation and to generate a first signal indicative of a value
of the operating parameter; a temperature sensor arranged at a
position within the vessel and adapted to measure a temperature
within the vessel at the position and to generate a temperature
signal indicative of a value of the measured temperature; and a
controller adapted to receive the first signal and the temperature
signal and to generate a control signal based at least in part on
the measured temperature and to provide the control signal to at
least one of the laser steering system and the target material
steering system to adjust an angle at which the laser radiation
strikes the target material in the irradiation region.
2. Apparatus as claimed in claim 1 further comprising an EUV
optical element located within the vessel and wherein the position
at which the temperature sensor is arranged is gravitationally
above the EUV optical element.
3. Apparatus as claimed in claim 2 wherein the EUV optical element
comprises a collector mirror.
4. Apparatus as claimed in claim 1 wherein the position at which
the temperature sensor is arranged is on an internal surface of a
vessel wall of the vessel.
5. Apparatus as claimed in claim 1 wherein the temperature sensor
comprises a thermocouple.
6. Apparatus as claimed in claim 1 further comprising a second
temperature sensor arranged at a second position within the vessel
and adapted to measure a second temperature within the vessel at
the second position and to generate a second temperature signal
indicative of a value of the second measured temperature and
wherein the controller is adapted to receive the second temperature
signal and to generate the control signal based at least in part on
the second measured temperature.
7. Apparatus for generating EUV radiation comprising: a vessel; a
laser adapted to generate laser radiation; a laser steering system
arranged to receive the laser radiation and adapted to steer the
laser radiation to an irradiation region within the vessel; a
target material delivery system adapted to deliver target material
to the irradiation region to be irradiated by the laser, the
irradiation of the target material by the laser generating the EUV
radiation; an EUV optical element located within the vessel; a
first temperature sensor arranged at a first position within the
vessel gravitationally above the EUV optical element and adapted to
measure a first measured temperature within the vessel at the first
position and to generate a first temperature signal indicative of a
value of the first measured temperature; a second temperature
sensor arranged at a second position within the vessel and adapted
to measure a second temperature of a gas within the vessel at the
second position and to generate a second temperature signal
indicative of a value of the second measured temperature; and a
controller adapted to receive the first signal and the temperature
signal and to generate a control signal based at least in part on
the first measured temperature and the second measured temperature
to provide the control signal to the laser steering system to
adjust an angle at which the laser radiation strikes the target
material in the irradiation region.
8. Apparatus as claimed in claim 7 wherein the EUV optical element
comprises a collector mirror.
9. Apparatus as claimed in claim 7 wherein the first temperature
sensor is arranged is on an internal vessel wall of the vessel and
the second temperature sensor is arranged is on an internal vessel
wall of the vessel.
10. Apparatus as claimed in claim 7 wherein the first temperature
sensor comprises a first thermocouple and the second temperature
sensor comprises a second thermocouple.
11. Apparatus for generating EUV radiation comprising: a vessel; a
laser adapted to generate a laser beam; a laser steering system
arranged to receive the laser beam and adapted to direct the laser
beam to and adjust a tilt of the laser beam in an irradiation
region within the vessel; a target material delivery system adapted
to deliver target material to the irradiation region to be
irradiated by the laser beam, the irradiation of the target
material by the laser beam generating the EUV radiation; a
temperature sensor arranged at a position within the vessel and
adapted to measure a temperature within the vessel at the position
and to generate a temperature signal indicative of a value of the
measured temperature; and a controller adapted to receive the
temperature signal and to generate a control signal based at least
in part on the value of the temperature signal and to provide the
control signal to the laser steering system to adjust the tilt of
the laser beam.
12. Apparatus as claimed in claim 11 wherein the tilt is adjusted
to maintain the temperature below a predetermined maximum
value.
13. Apparatus as claimed in claim 11 further comprising a second
temperature sensor arranged at a second position within the vessel
and adapted to measure a second temperature within the vessel at
the second position and to generate a second temperature signal
indicative of a second value of the measured temperature and
wherein the controller is further adapted to receive the second
temperature signal and to generate the control signal based at
least in part on the second value of the second temperature signal
and to provide the control signal to the laser steering system to
adjust the tilt of the laser beam.
14. Apparatus as claimed in claim 13 further comprising an EUV
optical element located within the vessel and wherein the position
at which the temperature sensor is arranged is gravitationally
above the EUV optical element.
15. Apparatus as claimed in claim 14 wherein the EUV optical
element comprises a collector mirror.
16. Apparatus as claimed in claim 14 wherein the vessel has a
vessel wall having an internal surface and the position at which
the temperature sensor is arranged is on the internal surface of
the vessel wall.
17. Apparatus as claimed in claim 13 wherein the temperature sensor
comprises a thermocouple and the second temperature sensor
comprises a thermocouple.
18. Apparatus for generating EUV radiation comprising: a vessel; a
laser adapted to generate a laser beam; a laser steering system
arranged to receive the laser beam and adapted to direct the laser
beam to and adjust a tilt of the laser beam in irradiation region
within the vessel; a target material delivery system adapted to
deliver target material to the irradiation region to be irradiated
by the laser beam, the irradiation of the target material by the
laser beam generating the EUV radiation; a plurality of temperature
sensors arranged at respective positions within the vessel and
adapted to measure a temperature within the vessel at the
respective position and to generate a plurality of temperature
signals indicative of values of the measured temperatures; and a
controller adapted to receive the plurality of temperature signals
and to generate a control signal based at least in part on the
value of the temperature signals and to provide the control signal
to the laser steering system to adjust the tilt of the laser
beam.
19. Apparatus as claimed in claim 18 further comprising an EUV
optical element located within the vessel and wherein the position
at which at least one of the plurality of temperature sensors is
arranged is gravitationally above the EUV optical element.
20. Apparatus as claimed in claim 18 wherein each of the plurality
of temperature sensors comprises a thermocouple.
Description
FIELD
[0001] The present disclosure relates to apparatus for and methods
of generating extreme ultraviolet ("EUV") radiation from a plasma
created through discharge or laser ablation of a target material in
a vessel. In such applications optical elements are used, for
example, to collect and direct the radiation for use in
semiconductor photolithography and inspection.
BACKGROUND
[0002] Extreme ultraviolet radiation, e.g., electromagnetic
radiation having wavelengths of around 50 nm or less (also
sometimes referred to as soft x-rays), and including radiation 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 radiation include converting a
target material to 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 portion of the
electromagnetic spectrum. The target material can be solid, liquid,
or gas. 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 one or more laser radiation pulses. Such LPP sources generate
EUV radiation by coupling laser energy into a target material
having at least one EUV emitting element, creating a highly ionized
plasma with electron temperatures of several 10's of eV.
[0005] For this process, the plasma is typically produced in a
sealed vessel, e.g., a vacuum chamber, and the resultant EUV
radiation is monitored using various types of metrology equipment.
In addition to generating EUV radiation, the processes used to
generate plasma also typically generate undesirable by-products in
the plasma chamber which can include out-of-band radiation, high
energy ions and debris, e.g., atoms and/or clumps/microdroplets of
residual target material.
[0006] The energetic radiation 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
at least a portion of the radiation to an intermediate location.
The collected radiation may then be relayed from the intermediate
location to a set of optics, a reticle, detectors and ultimately to
a silicon wafer.
[0007] In the EUV portion of the spectrum it is generally regarded
as necessary to use reflective optics for the optical elements in
the system including the collector, illuminator, and projection
optics box. These reflective optics may be implemented as normal
incidence optics as mentioned or as grazing incidence optics. 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
(the MLM stack) over a foundation or substrate. System optics may
also be configured as a coated optical element even if it is not
implemented as an MLM.
[0008] The optical element must be placed within the vessel with
the plasma to collect and redirect the EUV radiation. 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 target material. The
particles of target material, which are essentially debris from the
laser vaporization process, can contaminate the optical element's
exposed surface. Particles of target material can also cause
physical damage to and localized heating of the MLM surface.
[0009] In some systems H.sub.2 gas at pressures in the range of 0.5
to 3 mbar is used in the vacuum chamber as a buffer gas for debris
mitigation. In the absence of a gas, at vacuum pressure, it would
be difficult to protect the collector adequately from target
material debris ejected from the irradiation region. Hydrogen is
relatively transparent to EUV radiation having a wavelength of
about 13.5 nm and so is preferred to other candidate gases such as
He, Ar, or other gases which exhibit a higher absorption at about
13.5 nm.
[0010] H.sub.2 gas is introduced into the vacuum chamber to slow
down the energetic debris (ions, atoms, and clusters) of target
material created by the plasma. The debris is slowed down by
collisions with the gas molecules. For this purpose a flow of
H.sub.2 gas is used which may also be counter to the debris
trajectory and away from the collector. This serves to reduce the
damage of deposition, implantation, and sputtering target material
on the optical coating of the collector.
[0011] The process of generating EUV light may also cause target
material to be deposited on the walls of the vessel. Minimizing
target material deposition on the vessel walls is important for
achieving an acceptably long lifetime of EUV sources placed in
production. Also, maintaining the direction of target material flux
from the irradiation site and directionality of power dissipation
into the buffer gas is important for ensuring that the waste target
material mitigation system works as intended and can acceptably
manage by-products associated with vaporization of the target
material.
SUMMARY
[0012] 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 set limits on 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.
[0013] According to one aspect, a source control loop is
established to maintain and optimize debris flux while not unduly
affecting optimum EUV generation conditions. One or more
temperature sensors, e.g., thermocouples, may be installed in the
vessel to measure respective local gas temperatures. The respective
local temperature as measured by the one or more thermocouples can
be used as one or more inputs to the source control loop. The
source control loop may then adjust the drive laser targeting,
i.e., targeting of the laser used to vaporize target material, to
permit optimization of debris generation and deposition while not
affecting EUV production, thus extending the lifetime of the source
and its collector.
[0014] According to one aspect there is disclosed an apparatus for
generating EUV radiation in which the apparatus includes a vessel,
a laser adapted to generate laser radiation, and a laser steering
system arranged to receive the laser radiation and adapted to steer
the laser radiation to an irradiation region within the vessel. The
apparatus also includes a target material delivery system adapted
to deliver target material to the irradiation region to be
irradiated by the laser, the irradiation of the target material by
the laser generating the EUV radiation. A target material steering
system coupled to the target material delivery system for adjusting
a position of the target material within the irradiation region.
The apparatus also includes an EUV radiation metrology system
adapted to measure at least one operating parameter of the EUV
radiation and to generate a first signal indicative of a value of
the operating parameter, a temperature sensor arranged at a
position within the vessel and adapted to measure a temperature
within the vessel at the position and to generate a temperature
signal indicative of a value of the measured temperature, and a
controller adapted to receive the first signal and the temperature
signal and to generate a control signal based at least in part on
the measured temperature and to provide the control signal to at
least one of the laser steering system and the target material
steering system to adjust interaction of the laser radiation and
the target material in the irradiation region.
[0015] The apparatus may further include an EUV optical element
located within the vessel and wherein the position at which the
temperature sensor is arranged may be gravitationally above the EUV
optical element. The EUV optical element may be a collector mirror.
The temperature sensor may be arranged on an internal wall of the
vessel. The temperature sensor may be or include a thermocouple.
The apparatus may also include a second temperature sensor arranged
at a second position within the vessel and adapted to measure a
second temperature within the vessel at the second position and to
generate a second temperature signal indicative of a value of the
second measured temperature and the controller may be adapted to
receive the second temperature signal and to generate the control
signal based at least in part on the second measured
temperature.
[0016] According to another aspect there is disclosed an apparatus
for generating EUV radiation, the apparatus including a vessel, a
laser adapted to generate laser radiation, and a laser steering
system arranged to receive the laser radiation and adapted to steer
the laser radiation to an irradiation region within the vessel. The
apparatus also includes a target material delivery system adapted
to deliver target material to the irradiation region to be
irradiated by the laser, the irradiation of the target material by
the laser generating the EUV radiation. And an EUV optical element
located within the vessel. A first temperature sensor is arranged
at a first position within the vessel gravitationally above the EUV
optical element and adapted to measure a first measured temperature
within the vessel at the first position and to generate a first
temperature signal indicative of a value of the first measured
temperature. A second temperature sensor is arranged at a second
position within the vessel and adapted to measure a second
temperature of a gas within the vessel at the second position and
to generate a second temperature signal indicative of a value of
the second measured temperature. A controller is adapted to receive
the first signal and the temperature signal and to generate a
control signal based at least in part on the first measured
temperature and the second measured temperature to provide the
control signal to the laser steering system to adjust an angle at
which the laser radiation strikes the target material in the
irradiation region. The EUV optical element may be or include a
collector mirror. The first temperature sensor may be arranged is
on an internal wall of the vessel and the second temperature sensor
may arranged is on an internal wall of the vessel. The first
temperature sensor may be a thermocouple and the second temperature
sensor may be a second thermocouple.
[0017] According to another aspect there is disclosed an apparatus
for generating EUV radiation, the apparatus including a vessel, a
laser adapted to generate a laser beam, and a laser steering system
arranged to receive the laser beam and adapted to direct the laser
beam to and adjust a tilt of the laser beam in irradiation region
within the vessel. The apparatus also includes a target material
delivery system adapted to deliver target material to the
irradiation region to be irradiated by the laser beam, the
irradiation of the target material by the laser beam generating the
EUV radiation, a temperature sensor arranged at a position within
the vessel and adapted to measure a temperature within the vessel
at the position and to generate a temperature signal indicative of
a value of the measured temperature, and a controller adapted to
receive the temperature signal and to generate a control signal
based at least in part on the value of the temperature signal and
to provide the control signal to the laser steering system to
adjust the tilt of the laser beam. The tilt may be adjusted to
maintain the temperature below a predetermined maximum value. The
apparatus may also include a second temperature sensor arranged at
a second position within the vessel and adapted to measure a second
temperature within the vessel at the second position and to
generate a second temperature signal indicative of a second value
of the measured temperature and the controller may be further
adapted to receive the second temperature signal and to generate
the control signal based at least in part on the second value of
the second temperature signal and to provide the control signal to
the laser steering system to adjust the tilt of the laser beam.
[0018] The apparatus as claimed may also include an EUV optical
element located within the vessel and wherein the position at which
the temperature sensor is arranged is gravitationally above the EUV
optical element. The EUV optical element may be or include a
collector mirror. The position at which the temperature sensor is
arranged is on an internal wall of the vessel. The temperature
sensors may be or include thermocouples.
[0019] According to another aspect there is disclosed an apparatus
for generating EUV radiation, the apparatus including a vessel, a
laser adapted to generate a laser beam, and a laser steering system
arranged to receive the laser beam and adapted to direct the laser
beam to and adjust a tilt of the laser beam in irradiation region
within the vessel. The apparatus also includes a target material
delivery system adapted to deliver target material to the
irradiation region to be irradiated by the laser beam, the
irradiation of the target material by the laser beam generating the
EUV radiation 18. The apparatus also includes a plurality of
temperature sensors arranged at respective positions within the
vessel and adapted to measure a temperature within the vessel at
the respective position and to generate a plurality of temperature
signals indicative of values of the measured temperatures and a
controller adapted to receive the plurality of temperature signals
and to generate a control signal based at least in part on the
value of the temperature signals and to provide the control signal
to the laser steering system to adjust the tilt of the laser beam.
The apparatus may also include an EUV optical element located
within the vessel and the position at which at least one of the
plurality of temperature sensors is arranged may gravitationally
above the EUV optical element. Each of the plurality of temperature
sensors may be or include a thermocouple.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a schematic, not-to-scale view of an overall broad
conception for a laser-produced plasma EUV radiation source system
according to an aspect of the present invention.
[0021] FIG. 2 is a schematic, not-to-scale view of a portion of the
system of FIG. 1.
[0022] FIG. 3 is a diagram of the geometry of a possible
interaction of a laser beam and a droplet of target material in a
system such as the system of FIG. 1.
[0023] FIG. 4 is a not-to-scale perspective diagram showing a
possible arrangement of temperature sensors in a vessel used in a
laser-produced plasma EUV radiation source system according to an
aspect of the present invention.
DETAILED DESCRIPTION
[0024] 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.
[0025] With initial reference to FIG. 1 there is shown a schematic
view of an exemplary EUV radiation source, e.g., a laser produced
plasma EUV radiation source 10 according to one aspect of an
embodiment of the present invention. As shown, the EUV radiation
source 10 may include a pulsed or continuous laser source 22, which
may for example be a pulsed gas discharge CO.sub.2 laser source
producing a beam 12 of radiation at 10.6 .mu.m or 1 .mu.m. The
pulsed gas discharge CO.sub.2 laser source may have DC or RF
excitation operating at high power and at a high pulse repetition
rate.
[0026] The EUV radiation source 10 also includes a target delivery
system 24 for delivering target material in the form of liquid
droplets or a continuous liquid stream. In this example, the target
material is a liquid, but it could also be a solid or gas. The
target material may be made up of tin or a tin compound, although
other materials could be used. In the system depicted the target
material delivery system 24 introduces droplets 14 of the target
material into the interior of a vacuum 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. The EUV light source may
also include a beam steering system 32 as will be explained in more
detail below in conjunction with FIG. 2.
[0027] In the system shown, the components are arranged so that the
droplets 14 travel substantially horizontally. The direction from
the laser source 22 towards the irradiation region 28, that is, the
nominal direction of propagation of the beam 12, may be taken as
the Z axis. The path the droplets 14 take from the target material
delivery system 24 to the irradiation region 28 may be taken as the
X axis. The view of FIG. 1 is thus normal to the XZ plane. The
orientation of the EUV radiation source 10 is preferably rotated
with respect to gravity as shown, with the arrow G showing the
preferred orientation with respect gravitationally down. This
orientation applies to the EUV source but not necessarily to
optically downstream components such as a scanner and the like.
Also, while a system in which the droplets 14 travel substantially
horizontally is depicted, it will be understood by one having
ordinary skill in the art the other arrangements can be used in
which the droplets travel vertically or at some angle with respect
to gravity between and including 90 degrees (horizontal) and 0
degrees (vertical).
[0028] The EUV radiation source 10 may also include an EUV light
source controller system 60, which may also include a laser firing
control system 65, along with the beam steering system 32. The EUV
radiation source 10 may also include a detector such as a target
position detection system which may include one or more droplet
imagers 70 that generate an output indicative of the absolute or
relative position of a target droplet, e.g., relative to the
irradiation region 28, and provide this output to a target position
detection feedback system 62.
[0029] The target position detection feedback system 62 may use the
output of the droplet imager 70 to compute a target position and
trajectory, from which a target error can be computed. The target
error can be computed on a droplet-by-droplet basis, or on average,
or on some other basis. The target error may then be provided as an
input to the light source controller 60. In response, the light
source controller 60 can generate a control signal such as a laser
position, direction, or timing correction signal and provide this
control signal to the laser beam steering system 32. The laser beam
laser beam steering system 32 can use the control signal to change
the location and/or focal power of the laser beam focal spot within
the chamber 26. The laser beam steering system 32 can also use the
control signal to change the geometry of the interaction of the
beam 12 and the droplet 14. For example, the beam 12 can be made to
strike the droplet 14 off-center or at an angle of incidence other
than directly head-on.
[0030] As shown in FIG. 1, the target material delivery system 24
may include a target delivery control system 90. The target
delivery control system 90 is operable in response to a signal, for
example, the target error described above, or some quantity derived
from the target error provided by the system controller 60, to
adjust paths of the target droplets 14 through the irradiation
region 28. This may be accomplished, for example, by repositioning
the point at which a target delivery mechanism 92 releases the
target droplets 14. The droplet release point may be repositioned,
for example, by tilting the target delivery mechanism 92 or by
shifting the target delivery mechanism 92. The target delivery
mechanism 92 extends into the chamber 26 and is preferably
externally supplied with target material and a gas source to place
the target material in the target delivery mechanism 92 under
pressure.
[0031] Continuing with FIG. 1, the radiation source 10 may also
include one or more optical elements. In the following discussion,
a collector 30 is used as an example of such an optical element,
but the discussion applies to other optical elements as well. The
collector 30 may be a normal incidence reflector, for example,
implemented as an MLM with additional thin barrier layers, for
example B.sub.4C, ZrC, Si.sub.3N.sub.4 or C, deposited at each
interface to effectively block thermally-induced interlayer
diffusion. Other substrate materials, such as aluminum (Al) or
silicon (Si), can also be used. The collector 30 may be in the form
of a prolate ellipsoid, with a central aperture to allow the laser
radiation 12 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 radiation may be output from the EUV radiation
source 10 and input to, e.g., an integrated circuit lithography
scanner 50 which uses the radiation, for example, to process a
silicon wafer workpiece 52 in a known manner using a reticle or
mask 54. The silicon wafer workpiece 52 is then additionally
processed in a known manner to obtain an integrated circuit
device.
[0032] The arrangement of FIG. 1 also includes a temperature sensor
34, e.g., a thermocouple positioned within the chamber 26 to
measure the local temperature, i.e., temperature at the sensor, of
the gas within the chamber 26. FIG. 1 shows one temperature sensor
but it will be apparent that additional temperature sensors may be
used. The temperature sensor 34 generates a signal indicative of
the measured temperature and supplies it as an additional input to
the controller 60. The controller 60 bases the control signal it
supplies to the beam steering system 32 at least in part on this
temperature signal.
[0033] As discussed below, it has been found that controlling the
offset of beam impingement on the droplet with respect to the
center of mass of the droplet ("tilt") can optimize debris control
without sacrificing EUV generation performance. Specifically, it
has been found that negative tilt (deliberately causing the beam to
strike the droplet slightly to one side of the center of mass of
the droplet) can minimize target material deposition in areas of
the chamber 26 where it is desired to avoid deposition of target
material without materially affecting EUV radiation production. It
has also been determined that the temperature distribution in the
chambers bears a correlation to the distribution of target material
debris. The controller 60 thus uses the input from the temperature
sensor 34 as at least a partial basis to generate a control signal.
This is in conjunction with a control loop responsible for
optimizing EUV generation. It has been determined that debris
production and EUV generation are essentially decoupled so that
successful EUV production can be achieved with limited debris
production.
[0034] Continuing to FIG. 2, it can be seen that the beam steering
system 32 may include one or more steering mirrors 32a, 32b, and
32c. Although three mirrors are shown, it is to be appreciated that
more than three or as few as one steering mirror may be employed to
steer the beam. Moreover, although mirrors are shown, it is to be
appreciated that other optics such as prisms may be used and that
one or more of the steering optics may be positioned inside the
chamber 26 and exposed to plasma-generated debris. See for example
U.S. Pat. No. 7,598,509 filed on Feb. 21, 2006, and titled LASER
PRODUCED PLASMA EUV LIGHT SOURCE, the entire contents of which are
hereby incorporated by reference herein. For the embodiment shown,
each of the steering mirrors 32a, 32b, and 32c may be mounted on a
respective tip-tilt actuator 36a, 36b, and 36c which may move each
of the steering mirrors 32a, 32b, and 32c independently in either
or both of two dimensions.
[0035] It has been noted that very small changes in Y-axis tilt of
the CO.sub.2 laser beam can lead to very significant changes in
target material deposition without affecting EUV generation. FIG. 3
is a diagram to illustrate the concept of Y tilt as applied to the
geometry of the interaction of the CO.sub.2 laser beam 12 and the
droplet 14. The Z axis is the direction along the nominal (no
Y-tilt) propagation of the laser beam. Droplets travel along the
X-axis, which is perpendicular to Z-axis and is horizontal in the
global frame of reference. The Z-coordinate of the droplet travel
path is Z=0. Y-tilt leads to the beam hitting slightly to one side
of the center of the droplet as it travels through the beam focal
spot. Thus, in the situation shown in FIG. 3, the beam 12 strikes
the droplet (has a focal point at) to one side of the droplet 14
(below the nominal beam path or Z axis). This is described as
negative Y-tilt. Y-tilt is measured as a displacement of the
location the beam strikes the droplet from the location the beam
strikes the droplet in the zero Y tilt condition. For example, a
value of negative Y-tilt might typically be on the order of -10
microns. In FIG. 3, the droplet is shown as spherical but it will
be understood that the droplet shape will not necessarily be
spherical and may assume other shapes, for example, if flattened by
a prepulse. The displacement is thus measured from the center of
mass of the droplet.
[0036] The relative orientation of CO.sub.2 beam and the droplet
controls the flow of target material debris. If the CO.sub.2 beam
is dead center on the droplet then the target material debris tends
to propagate in the direction of beam propagation parallel to Z
axis. Shifting the center of the beam relative to the center of the
droplet causes the flux of debris to be tilted, that is, to
propagate with a component normal to the Z axis. The actual Y-tilt
of the beam is negligibly small compared to the tilt of target
material debris flux caused by the laser-droplet misalignment. An
actual Y-tilt on the order of 20 microrads has been found to cause
a shift in debris direction on the order of 0.1 rad or 5000 times
larger.
[0037] It is not that important how the beam-droplet misalignment
is achieved. It can be achieved by shifting the position of the
center of the beam by steering the beam or it can be achieved by
shifting the position of the droplet by manipulating its release
point. It is also possible where the droplet trajectory has a
vertical component to achieve the desired displacement/misalignment
by controlling the timing of droplet release with respect to pulse
timing by itself or in conjunction with droplet displacement and/or
laser shift.
[0038] It has been determined that the rate of target material
deposition on a given portion of the vessel for negative Y-tilt can
be made markedly less than deposition rate for positive Y-tilt. The
temperatures as measured by temperature sensors and are indicative
of the rate of target material deposition were lower for the lower
deposition rate, negative Y-tilt condition than for the higher
deposition rate, positive Y-tilt condition. At the same time, the
amount of Y-tilt involved (about 10 microns) and did not affect EUV
production. Thus at negative tilt, which is tilting the CO.sub.2
beam away from location of the temperature sensor, the target
material deposition rate becomes very small, while at the positive
Y-tilt (towards location of the temperature sensor), the target
material accumulation rate reaches has a very high value.
[0039] It is presently preferred that the distance from the plasma
to the temperature sensor location be in the range of about 200 mm
to about 250 mm. The temperature sensor may be a "bare"
thermocouple that has a metal junction exposed to the environment.
Materials such as those making up such thermocouples have high
recombination probability for H-radicals, and as a result this type
of thermocouple measures a higher temperature value which is the
sum of the gas temperature and extra heating due to H-radical
recombination. The other type is a "glass" thermocouple in which
the metal junction is inserted into a glass capillary to protect it
from direct contact with the environment. The recombination
probability for H-radicals on glass is much lower (about 1000 times
lower) than on bare metal, so the glass thermocouple reads a lower
value, determined only by gas temperature. As a practical matter,
however, in the application in which the thermocouple is exposed to
debris accumulation, a glass thermocouple will become coated with
target material debris relatively quickly so that the difference in
measured temperature is not significant. In the presently preferred
embodiment bare thermocouples are used.
[0040] In order to create the desired control loop at least one
thermocouple should be installed in the vessel in the areas where
it is desired to minimize debris accumulation, i.e., minimize the
flow of debris towards that area. As an example, one such area is
may be the vessel walls directly above the collector. Target
material debris accumulation in this area creates the risk that
target material will drip onto the collector. FIG. 4 shows an
example where the thermocouples 34 are positioned around the
circumference of internal walls 44 of a rotationally symmetric
vessel 26 at a position between the collector 30 and the
intermediate location 40. FIG. 4 shows an arrangement in which six
thermocouples 34 are used but it will be understood that fewer or
more thermocouples may be used and that different arrangements and
positioning of the thermocouples may be used. Each thermocouple is
preferably configured as a small diameter wire (less than 1 mm in
diameter) that protrudes into the gas for about 2 cm from the wall
44. For such a thermocouple, even if the wire protrudes somewhat
into the path of EUV propagation, the total EUV loss will be
negligible. The solid double arrow in FIG. 4 shows the direction of
debris propagation. The outline arrows show a preferred arrangement
for causing H.sub.2 to flow away from the collector 30. Elements 42
are scrubbers for removing contaminants from the H.sub.2. Arrow G
indicates the direction of gravity. Also shown is a line demarking
a division between the source 10 and the scanner 50.
[0041] The thermocouple temperature readings are supplied to the
controller 60 as inputs. FIG. 4 shows one such connection 46 but it
will be understood that each of the thermocouples 34 is connected
to supply a signal to the controller 60. The controller 60 then
controls the beam steering system 32 to adjust the beam tilt such
that to minimize the readings from the thermocouples installed in
the areas where it is desired to minimize debris accumulation. In
one embodiment, the control loop made up of the thermocouple,
controller, and beam steering system can be conceptualized as
operating to minimize the error signal Y.sub.err according to the
relationship
Y.sub.err=.SIGMA.(T.sub.i,protected)/.SIGMA.(T.sub.i,all),
[0042] where .SIGMA./(T.sub.i, protected) is the sum of temperature
readings over the area directly above collector and
.SIGMA./(T.sub.i, all) is the sum of all readings around the wall
circumference.
[0043] The control loop adjusts Y-tilt to minimize the error
signal. The total tilt Y.sub.tilt CO2 is the sum of Y-tilt value
set by the main control system operating to optimize EUV
production, Y.sub.tilt EUV, and the Y-tilt value correction set by
the "debris loop" as described above, Y.sub.tilt Debris, thus:
Y.sub.tilt CO2=Y.sub.tilt EUV+Y.sub.tilt Debris.
[0044] Preferably a maximum absolute value of Debris Y should be
limited to a predetermined value, preferably in the range of about
10 microns to about 20 microns. Of course, the preferred value will
depend on the implementation of the "tilt." For example, if the
target is flat, it may be possible to implement "tilt" by creating
a displacement between the midpoint of the flat target and where it
is struck by the beam to effect a change in the debris
distribution.
[0045] It is believed that the measurement of local temperature
provides useful information on the local concentration of Sn. The
physical equations of convection and diffusion apply both to heat
and Sn concentration. Also, the heat source and the Sn source
largely coincide at the irradiation site. Therefore it is
reasonable to infer that the measured temperature provides an
indication of Sn concentration. This correlation is not necessarily
uniform but is sufficient to provide an input to the control loop
if care is taken to ensure measuring temperature at a location
where the correlation can be expected to be strong. In principle,
it is necessary only to locate the temperature sensor in a position
in which there will be a significant signal to noise for the
metrology and controller.
[0046] For example, it is preferred that the thermocouple be
shielded from incoming EUV radiation to prevent sensing the wrong
temperature. Also, the boundary conditions at the wall may cause
the correlation to break down. Sn concentration at a wall may
approach zero if Sn is permitted or caused to condense on the wall,
and the wall may be temperature controlled. It is thus preferred
that the thermocouple be placed far enough away from the nearest
wall to prevent the boundary conditions at the wall from unduly
distorting the temperature measurement. It is presently preferred
that the temperature be measured at about 2 cm from the wall, but
the temperature could be measured as far as 25 cm from the wall or
farther. Each thermocouple is preferably configured as a small
diameter wire (less than 1 mm in diameter) that protrudes into the
gas for the preferred length. It also may be preferred for some
applications to position another thermocouple at the opposite side
of the chamber to develop a measurement of temperature near the
wall in the presence of a higher or lower SN concentration.
[0047] As described above, the interaction between the incoming
beam and the target material is controlled to affect the dispersion
of target material in the chamber. The temperature at a given
location is measured as an indication of the target material
concentration at that location. The measured temperature is then
used as an input to a control loop to control the beam/target
material interaction to obtain a desired target material
concentration at the location.
[0048] As an example in some systems using droplets of Sn the laser
sequentially supplies two pulses to each droplet, a first pulse
called a prepulse and a second pulse called a main pulse. The
purpose of the prepulse is to precondition the droplet and the
purpose of the main pulse is to vaporize the droplet after it has
been conditioned by the prepulse. For example, if the prepulse
strikes the droplet head-on then the target expands flat into what
is referred to as a "flat target" which will present a flat face to
the main pulse which is not tilted. The position where the droplet
is vaporized is preferably the primary focus of the collector. In
other words, to obtain a good, focused image of the vaporization
event to be relayed to the scanner it is preferred to have the main
pulse impact the target at the primary focus.
[0049] As described above, "tilt" may be achieved on by displacing
the position where the prepulse strikes the droplet. This causes
the target to expand at an angle, thus resulting in a tilted target
for the main pulse. To create the displacement the droplet may be
at the primary focus when struck and the laser is displaced, or the
laser may be directed at the primary focus and there is a
displacement between the droplet and the primary focus. The tilt is
what determines the "tilted" debris emission. There are, however,
different ways to affect the ion distribution (e.g., target shape;
target tilt, main pulse displacement). Whichever method is used,
the debris pattern as measured by the thermocouples is used as
feedback in the control loop.
[0050] In other words, the beam-droplet interaction may be altered
by adjusting the pointing (tilt) of the CO2 beam. It is also
possible to adjust the beam-droplet interaction by adjusting the
horizontal or vertical position at which the beam strikes the
droplet. This can be accomplished by controlling the target
delivery control system 90 to cause the target delivery mechanism
92 to change the release point of the droplet. It can also be
accomplished by changing the relative timing of the droplet release
and generation of the pulse in systems in which the droplet travels
vertically. Controlling the droplet/beam interaction in this manner
may have the advantage of reducing the amount of shift in the
position of the plasma caused by the operation of the control
loop.
[0051] After adjustment of either Y-tilt of CO.sub.2 beam or the
position of the droplet/beam interaction, both CO.sub.2 beam and
the droplet/beam interaction position can be moved (tilted or
shifted) simultaneously to a new location (without affecting mutual
alignment between the two) if adjustment in the plasma position is
desired. This simultaneous shift of Y-tilt and the droplet/beam
interaction position will not affect debris flux or EUV production.
If further adjustment in either of the loop is required, however,
the process can be repeated.
[0052] In yet another embodiment, the temperature reading can be
used to direct the flow of debris in a predictable way, such as
collinear with the Z-axis, for example, to minimize the target
material deposition on the walls everywhere in the vessel. In this
case the control loops could operate to minimize the value:
Y.sub.err=.SIGMA.(T.sub.i,all).
[0053] It is also possible to use this control loop to minimize
debris flux onto the collector. In this case, the positions for the
one or more temperature sensors should be shifted towards collector
(along the Z axis). Also it is possible to install the temperature
sensors in the areas which are determined to be the most sensitive
indicators for debris based on computational fluid dynamics (CFD)
simulations.
[0054] Thus, by using very small and simple temperature sensors
such as thermocouples in the EUV source it is possible to design a
debris control loop that allows for stabilizing, minimizing, and
directing the entrained target material debris in the H.sub.2
flows. The ability to direct flows away from surfaces which are
positioned gravitationally above the collector could in particular
be used to extend collector lifetime.
[0055] 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.
* * * * *