U.S. patent application number 16/215822 was filed with the patent office on 2019-04-25 for compact storage ring extreme ultraviolet free electron laser.
The applicant listed for this patent is Lyncean Technologies, Inc.. Invention is credited to Michael Feser, Roderick J. Loewen, Ronald D. Ruth.
Application Number | 20190123507 16/215822 |
Document ID | / |
Family ID | 63167439 |
Filed Date | 2019-04-25 |
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United States Patent
Application |
20190123507 |
Kind Code |
A1 |
Ruth; Ronald D. ; et
al. |
April 25, 2019 |
COMPACT STORAGE RING EXTREME ULTRAVIOLET FREE ELECTRON LASER
Abstract
A high power extreme ultraviolet (EUV) beam is produced. An
electron beam is injected in a compact electron storage ring
configured for emission of free-electron laser (FEL) radiation. The
electron beam is passed through a magnetic undulator on each of a
plurality of successive revolutions of the electron beam around the
compact electron storage ring. The electron beam is induced to
microbunch and radiate coherently while passing through the
magnetic undulator. A portion of the free-electron laser radiation
at an extreme ultraviolet wavelength produced by an interaction of
the electron beam through the magnetic undulator is outputted.
Inventors: |
Ruth; Ronald D.; (Stanford,
CA) ; Loewen; Roderick J.; (Redwood City, CA)
; Feser; Michael; (Orinda, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Lyncean Technologies, Inc. |
Fremont |
CA |
US |
|
|
Family ID: |
63167439 |
Appl. No.: |
16/215822 |
Filed: |
December 11, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
15438611 |
Feb 21, 2017 |
10193300 |
|
|
16215822 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01S 3/0903 20130101;
G03F 7/70025 20130101; H05H 9/048 20130101; H01S 4/00 20130101;
H05H 2007/041 20130101 |
International
Class: |
H01S 4/00 20060101
H01S004/00; H01S 3/09 20060101 H01S003/09 |
Claims
1. A system for producing a high power extreme ultraviolet (EUV)
beam, including: a compact electron storage ring configured for
emission of free-electron laser (FEL) radiation; an electron
injector configured to insert an electron beam into the compact
electron storage ring; a plurality of bending magnets and a
plurality of quadrupole magnets interspersed along the compact
electron storage ring, wherein at least a corresponding one of the
quadrupole magnets is between any two of the bending magnets along
the compact electron storage ring; a magnetic undulator configured
to allow the electron beam to pass through the magnetic undulator
where the electron beam is induced to microbunch and radiate
coherently; and an exit aperture configured to output a portion of
the free-electron laser radiation at an extreme ultraviolet
wavelength produced by an interaction of the electron beam through
the magnetic undulator, wherein the output portion of the
free-electron laser radiation is provided to a lithography system
as a light source for the lithography system and an average power
of the output portion of the free-electron laser radiation is
greater than 250 W.
2. The system of claim 1, wherein the magnetic undulator is a
transverse gradient undulator that includes a plurality of
transverse gradient undulator components and each of the transverse
gradient undulator components includes a periodic structure of
mechanically coupled set of magnetic components with alternating
poles that create alternating transverse magnetic fields along at
least a portion of a length of the transverse gradient undulator to
generate at least a portion of the free-electron laser radiation
when the electron beam travels along at least the portion of the
length of the transverse gradient undulator, and for each of the
plurality of transverse gradient undulator components, the included
corresponding mechanically coupled set of magnetic components with
alternating poles is uniformly tilted in a transverse direction to
a path of the electron beam.
3. The system of claim 1, wherein an undulator parameter K of the
magnetic undulator is less than 1.
4. The system of claim 1, wherein an electron beam emittance
(.epsilon.) of the system is greater than
.lamda..sub.FEL/4.pi..
5. The system of claim 1, wherein an equilibrium relative energy
spread is greater than a FEL .rho. parameter of the system.
6. The system of claim 1, wherein for each of the plurality of
transverse gradient undulator components, the corresponding
transverse gradient undulator component is uniformly tilted for the
entire length of the corresponding transverse gradient undulator
component.
7. The system of claim 1, wherein a spacing between sections of the
magnetic undulator is configured to diminish an impact of an FEL
action on electron beam parameters.
8. The system of claim 1 further comprising, one or more magnets
configured to laterally disperse electrons of the electron beam
according its energy before entering the magnetic undulator.
9. The system of claim 1, wherein EUV FEL of the system is operated
in an exponential gain region of a FEL gain curve.
10. The system of claim 1, wherein the electron beam is in a steady
state.
11. The system of claim 10, wherein the steady state is reached
between a cooling of the electron beam in the compact storage ring
and a heating of the electron beam due to incoherent and FEL
processes.
12. The system of claim 1, wherein FEL radiation is initiated via
Self Amplified Stimulated Emission.
13. The system of claim 1, wherein FEL radiation is initiated using
an external separate coherent EUV source.
14. The system of claim 1, wherein a portion of the output FEL
radiation is seeded back to the compact electron storage ring.
15. The system of claim 14, wherein the portion of the output FEL
radiation seeded back to the compact electron storage ring is
delayed by a multiple of a revolution time of the electron beam
around the compact electron storage ring.
16. The system of claim 14, wherein based on an inverse of a total
gain of the FEL radiation, the seeded portion of the output FEL
radiation is selected to reach a steady state.
17. The system of claim 16, wherein a power, an intensity or a
parameter of the seeded portion of the output FEL radiation is
selected or adjusted to maintain the inverse relationship of the
total gain of the FEL radiation required to reach the steady
state.
18. The system of claim 1, wherein a number of electron bunches
stored in the compact storage ring is adjusted and set to achieve a
desired total output power.
19. The system of claim 1, wherein for a portion of a plurality of
times the electron beam passes through the magnetic undulator, the
electron beam is steered within the magnetic undulator to reduce
the emission of the FEL radiation during the portion of the
plurality of times the electron beam passes through the magnetic
undulator.
20. The system of claim 1, wherein the output portion of the FEL
radiation is provided to the lithography system via a mirror.
21. The system of claim 1, wherein the system is installed in a
semiconductor fabrication is facility.
22. A method for producing a high power extreme ultraviolet (EUV)
beam, including: injecting an electron beam in a compact electron
storage ring configured for emission of free-electron laser (FEL)
radiation, and the compact electron storage ring includes a
plurality of bending magnets and a plurality of quadrupole magnets
interspersed along the compact electron storage ring, and at least
a corresponding one of the quadrupole magnets is between any two of
the bending magnets along the compact electron storage ring;
passing the electron beam through a magnetic undulator on each of a
plurality of successive revolutions of the electron beam around the
compact electron storage ring, wherein the electron beam is induced
to microbunch and radiate coherently while passing through the
magnetic undulator, and the magnetic undulator is a transverse
gradient undulator that includes a plurality of transverse gradient
undulator components and each of the transverse gradient undulator
components includes a periodic structure of mechanically coupled
set of magnetic components with alternating poles that create
alternating transverse magnetic fields along at least a portion of
a length of the transverse gradient undulator to generate at least
a portion of the free-electron laser radiation when the electron
beam travels along at least the portion of the length of the
transverse gradient undulator, and for each of the plurality of
transverse gradient undulator components, the included
corresponding mechanically coupled set of magnetic components with
alternating poles is uniformly tilted in a transverse direction to
a path of the electron beam; and outputting a portion of the
free-electron laser radiation at an extreme ultraviolet wavelength
produced by an interaction of the electron beam through the
magnetic undulator, wherein the output portion of the free-electron
laser radiation is provided to a lithography system as a light
source for the lithography system and an average power of the
output portion of the free-electron laser radiation is greater than
250 W.
23. The method of claim 22, wherein the magnetic undulator includes
a transverse gradient undulator.
Description
CROSS REFERENCE TO OTHER APPLICATIONS
[0001] This application is a continuation of co-pending U.S. patent
application Ser. No. 15/438,611 entitled COMPACT STORAGE RING
EXTREME ULTRAVIOLET FREE ELECTRON LASER filed Feb. 21, 2017 which
is incorporated herein by reference for all purposes.
BACKGROUND OF THE INVENTION
[0002] The production of upcoming generations of semiconductor
circuits for a broad range of applications will require the next
generation of lithographic tools which utilize extreme ultraviolet
(EUV) lithography. There has been much development using plasma
based sources to reach the 100 W scale in terms of usable EUV power
and efforts are under way to reach 250 W of usable power, but it is
unclear if this level can be achieved reliably using the plasma
based technology. The low usable EUV power poses significant
challenges in terms of demands for ultra-sensitive photoresists
leading to shot noise induced roughness and limitations of high
wafer throughput. EUV sources that produce higher power (e.g., an
average power in the range of 1 kW to 3 kW) would address the
current challenges for EUV lithography and provide a viable path
for high volume manufacturing at smaller node sizes, which is
currently unattainable with plasma-based sources. Thus, there
exists a need for practical high power EUV source.
BRIEF DESCRIPTION OF THE DRAWINGS
[0003] Various embodiments of the invention are disclosed in the
following detailed description and the accompanying drawings.
[0004] FIG. 1 is a diagram illustrating an embodiment of a compact
electron storage ring configured to accept an insertion device
suitable for a free-electron laser (FEL).
[0005] FIG. 2 is a diagram illustrating an embodiment of a Compact
Storage Ring high power EUV beam generator system utilizing an
undulator magnet insertion device to create FEL radiation initiated
by Self Amplified Stimulated Emission (SASE).
[0006] FIG. 3 is a diagram illustrating an embodiment of a Compact
Storage Ring high power EUV beam generator system utilizing an
undulator magnet insertion device to create FEL radiation seeded
with an external coherent source at EUV wavelength or a multiple
thereof.
[0007] FIG. 4 is a graph illustrating example gain curves for
different seeding power levels in which the FEL output power
increases as a function of position in an undulator.
[0008] FIG. 5 is a diagram illustrating an embodiment of a Compact
Storage Ring high power EUV beam generator system utilizing a
regenerative self-seeded FEL.
[0009] FIGS. 6A-6B are diagrams illustrating embodiments of
magnetic undulator insertion devices used to produce FEL
radiation.
[0010] FIG. 7 is a block diagram illustrating an embodiment of a
system for performing EUV lithography.
DETAILED DESCRIPTION
[0011] The invention can be implemented in numerous ways, including
as a process; an apparatus; a system; a composition of matter; a
computer program product embodied on a computer readable storage
medium; and/or a processor, such as a processor configured to
execute instructions stored on and/or provided by a memory coupled
to the processor. In this specification, these implementations, or
any other form that the invention may take, may be referred to as
techniques. In general, the order of the steps of disclosed
processes may be altered within the scope of the invention. Unless
stated otherwise, a component such as a processor or a memory
described as being configured to perform a task may be implemented
as a general component that is temporarily configured to perform
the task at a given time or a specific component that is
manufactured to perform the task. As used herein, the term
`processor` refers to one or more devices, circuits, and/or
processing cores configured to process data, such as computer
program instructions.
[0012] A detailed description of one or more embodiments of the
invention is provided below along with accompanying figures that
illustrate the principles of the invention. The invention is
described in connection with such embodiments, but the invention is
not limited to any embodiment. The scope of the invention is
limited only by the claims and the invention encompasses numerous
alternatives, modifications and equivalents. Numerous specific
details are set forth in the following description in order to
provide a thorough understanding of the invention. These details
are provided for the purpose of example and the invention may be
practiced according to the claims without some or all of these
specific details. For the purpose of clarity, technical material
that is known in the technical fields related to the invention has
not been described in detail so that the invention is not
unnecessarily obscured.
[0013] Producing a high power EUV beam is disclosed. In some
embodiments, the system includes a compact and/or low-energy
electron storage ring configured for emission of free electron
laser (FEL) radiation. In some embodiments, the compact electron
storage ring has a circumference that is on the order of 30 meters.
For example, compact electron storage ring fits inside a 60 square
meter area. In some embodiments, the compact electron storage ring
has a circumference that is at least 10 meters but less than 60
meters. In some embodiments, the energy of the electron storage
ring is less than 500 MeV. The use of the compact/low-energy
electron storage ring allows for a significant reduction in the
size of the system as compared to other high power EUV beam
sources. The reduction in size allows associated financial costs to
be reduced and allows the system to be installed in a wider type of
environments, including under a floor of a semiconductor
fabrication facility. The system further includes an electron
injector configured to output an electron beam into the compact
electron storage ring and a magnetic undulator (e.g., magnetic
undulator insertion device consisting of one or more discrete
undulator magnets) configured to allow the electron beam to pass
through the magnetic undulator. The magnetic undulator may include
a plurality of undulators (e.g., the magnetic undulator includes a
plurality of different component magnetic undulators). An exit
aperture is configured to output at least a portion of the FEL
radiation at an EUV wavelength produced by an interaction of the
electron beam through the magnetic undulator device.
Miniaturization of the high power EUV beam source through the use
of the electron storage ring is enabled by various system aspects
and inventions. Examples of aspects that differ substantially from
prior and standard conventions in the field of FEL design include:
an undulator K parameter that is less than 1, an electron beam
emittance that is greater than .lamda..sub.FEL/4.pi., operation of
the EUV FEL in the exponential gain region of an FEL gain curve
(i.e., not in the saturation region), and/or keeping EUV output
power below 10% of the FEL saturation power, as described further
in the specification.
[0014] Free electron laser (FEL) based sources, using various
extensions of FEL techniques can be utilized to produce high usable
EUV power. FEL schemes use the property that a magnetic undulator
(e.g., periodic structure of alternating poles of magnets) acts
upon a relativistic electron beam to produce electron micro-bunches
that coherently radiate, substantially increasing the radiated
power compared to conventional incoherent undulator radiation.
[0015] In some embodiments, a self-consistent FEL is utilized
rather than utilizing a "phase-merging enhanced harmonic
generation" FEL, a proposal where an electron beam is bunched with
a "modulator" undulator then afterwards converted through a
dispersive section (e.g., where it can interact more efficiently
with a "radiator" undulator to emit harmonic radiation). For
example, much like a klystron rather than true FEL, phase-merging
enhanced harmonic generation induces an energy modulation on an
electron beam before inducing it to radiate (e.g., using two
separate undulators). A self-consistent FEL has an insertion device
that provides a mechanism to both microbunch and coherently radiate
within the same device (e.g., concurrently in a single undulator
magnet device).
[0016] Typically to generate FEL in prior approaches, a new
relativistic electron pulse is created for each FEL interaction in
a linear accelerator (LINAC) where the electron pulse must be very
well conditioned to produce FEL radiation (e.g., low electron-beam
emittance and energy spread). These FEL sources produce high peak
power radiation flashes with a repetition rate of the LINAC and
with the sequence of bunches within each pulse of the LINAC.
However, to achieve high average power, high repetition rates are
required, necessitating a superconducting LINAC and associated very
high capital cost and high power consumption. Typically these
LINACs are physically too large for installation in semiconductor
fabrication plants for use in photolithography without significant
changes to the construction or layout of the entire facility.
[0017] In some embodiments, instead of implementing the FEL in a
linear geometry following a LINAC, the FEL is incorporated within
an electron storage ring to make efficient use of the electron
bunches without the need to generate and accelerate new electron
bunches for every pass through the FEL undulator. For example, the
electron bunches circulate in a storage ring and traverse the FEL
undulator periodically and continuously without the need to
regenerate the electron bunches for every FEL undulator
interaction.
[0018] However, the emission of incoherent synchrotron radiation
from a conventional weak undulator in a storage ring does not
produce sufficient power for EUV applications. For example a 330
MeV electron storage ring with a 7 m long undulator produces only a
few watts of incoherent power in the full spectrum with a single
bunch. When this is filtered down to 2% BW at 13.5 nm, which is
required for EUV application, the power is less than one Watt
average power. Adding more bunches to the storage ring can linearly
increase the radiated power which may achieve an average power
significantly exceeding one Watt. However, this is far below the
necessary power for EUV lithography applications. Increasing the
magnetic field of the undulator increases the incoherent radiated
power but also increases the bandwidth of the output power so this
is not an efficient path to reach high output power in a narrow
spectral bandwidth. On the other hand, the energy stored within the
electron bunch is very substantial. For example, a 330 MeV electron
bunch with 2.5 nC of charge in a storage ring has on the order one
joule of stored energy. With a repetition rate (revolution
frequency of the storage ring) of order 10 MHz, the average
circulating power is on the order of 10 MW. With only 10 bunches,
the circulating power is on the order of 100 MW. Therefore, if the
FEL mechanism extracts just 0.001% of the power at EUV wavelength,
it would yield, as an example, 1 kW of EUV power.
[0019] Although typical large electron storage rings are hundreds
of meters in circumference, in some embodiments, a compact
low-energy electron storage ring is utilized. For example, a
compact electron storage ring with a circumference that is on the
order of 30 meters is utilized. In some embodiments, the compact
electron storage ring has a circumference that is at least 10
meters but less than 60 meters. In some embodiments, the energy of
the electron storage ring is less than 500 MeV. The compact
electron storage ring implementation achieves a 1 kW to 3 kW (or
higher) average power EUV source, which is more compact, affordable
and has a lower operating cost, than prior art LINAC-based
proposals, suitable for application in semiconductor fabrication or
other applications requiring high average power. In some
embodiments, the use of a storage ring substantially increases the
interaction rate (to >10 MHz) for a circulating pulse to radiate
power through multiple passes in a single magnetic undulator by
many orders of magnitude.
[0020] The utilized storage ring meets the FEL emission conditions
for the selected undulator with its equilibrium parameters and the
coherent radiation is extracted in such a manner that the electron
bunch in the ring can be stored in steady-state operation without
detrimental change in its equilibrium properties. For example, the
storage ring satisfies the requirement of storing the electron
bunch with a beam quality (emittance) and relative energy spread
which are low enough and beam charge high enough to enable FEL
radiation generation in the undulator both initially and in the
steady state operation during FEL emission. In some embodiments,
the requirements on the emittance of the electron beam may be
significantly relaxed compared to LINAC based EUV FELs.
[0021] In some embodiments, a compact low-energy electron storage
ring is combined together with a short period undulator (e.g., 1
cm), which is operated so that it emits FEL radiation at EUV
wavelengths (e.g. 13.5 nm). The generation of FEL radiation
typically has required a sufficiently low emittance electron beam
with a sufficiently low relative-energy spread. In some
embodiments, the value required for the emittance may be
substantially larger than what was traditionally required for prior
approaches. Prior EUV FEL designs operate in the power saturation
regime, which as a consequence increases the relative energy spread
and degrades substantially the required low emittance of the
electron beam, thus requiring that the electron bunch be used only
once and then discarded or decelerated in a controlled way to
recover a portion of its energy and cost. However, when utilizing
at least some of the embodiments described herein (e.g., an FEL
within a compact storage ring), this emission may be limited far
below saturation to substantially preserve the low energy spread
and electron beam emittance, but at the same time deep in the
amplification regime of the FEL process to harness significantly
more power than the incoherent emission from the same undulator.
The electron bunch is stored at an equilibrium energy of, for
example, 330 MeV. At each circulation of the beam, it passes
through an undulator and emits EUV radiation after an FEL gain of
many gain lengths, for example 7 gain lengths, which yield about a
factor of 1000 gain. The gain length is the physical length of the
undulator device for which the EUV output power increases by a
factor of e (Euler's number) in the exponential gain regime (linear
in lin/log plot). The output power is purposely kept below the
natural saturation power of the FEL. For example, the output may be
10% or less of the saturation power.
[0022] The lost energy is restored by the RF accelerating system of
the storage ring (e.g., analogous to the restoration of the energy
lost due to incoherent radiation). Because the electron bunch is
continually stored in the storage ring, there is no other energy
lost except that due to the typical incoherent synchrotron
radiation and the additional incoherent EUV radiation. The beam
emittance and energy spread reach an equilibrium state controlled
by the natural and induced radiation emission of the storage ring
which is restored by the RF accelerating system thereby maintaining
a constant energy of the electron beam.
[0023] In a free electron laser (FEL) an electron beam is passed
through an undulator magnet which causes emission of
electromagnetic radiation at the characteristic wavelength of the
undulator, .lamda..sub.FEL, given by following equation.
.lamda. FEL = .lamda. u 2 .gamma. 2 ( 1 + K 2 2 ) ##EQU00001##
where ##EQU00001.2## K = eB 0 .lamda. u 2 .pi. m e c .apprxeq.
0.934 B 0 [ T ] .lamda. u [ cm ] ##EQU00001.3##
and .lamda..sub.u is the undulator wavelength, .gamma. is the ratio
of the electron total energy to the electron rest energy, e is the
electron charge, B.sub.0 is the magnetic field, m.sub.e is the mass
of the electron and c is the speed of light. K is the undulator
parameter K. The square brackets enclose the units used for the
numerical formula.
[0024] A magnetic undulator includes of a series of bending magnets
that alternate in sign with a wavelength of .lamda..sub.u, which
cause an electron beam to oscillate in an approximately sinusoidal
fashion as it passes between the poles. The oscillating electron
beam emits synchrotron radiation or undulator radiation
incoherently when passing through the undulator. Typical prior
undulators for FELs have had been designed with an undulator
parameter K that is greater than 1. However, in at least some of
the described embodiments, an undulator parameter K of less than
one (e.g., substantially less than 1) is utilized.
[0025] The gain of a FEL depends upon the parameters of the
electron beam and the undulator. The gain length may be calculated
by computer simulations or by the use of calculations which take
into account three dimensional effects. In cases where 3D effects
start to be important, the gain length is increased. However, as an
example, the one dimensional power gain length L.sub.G is given by
following equation,
L G = .lamda. u 4 .pi. 3 .rho. ##EQU00002##
where .lamda..sub.u is the period of the undulator, and .rho. is
the Pierce parameter.
[0026] Prior FEL designs have traditionally imposed various
restrictions on parameters, including that: (1) the relative energy
spread within the bunch being less than .rho., and (2) the
emittance of the bunch being less than .lamda..sub.FEL/4.pi.. The
restriction of item (1) is important in that the FEL saturation
occurs when this condition is satisfied at the end of the gain
process. However, this condition may be alleviated by the
utilization of the Transverse Gradient Undulator as discussed
herein in order to fulfill the condition locally within the
undulator. In this way the steady state relative energy spread may
be significantly larger than .rho.. The restriction of item (2) is
relaxed in at least some of the described embodiments with proper
design of the 3D FEL. A 3D FEL design process is utilized in order
to arrange the conditions for FEL emission.
[0027] Example parameters for a FEL electron storage ring optimized
for 13.5 nm output is shown in the following Table 1. The
parameters have been specifically selected so that the storage ring
optimization permits the inclusion of an undulator or undulators
for the purpose of use with one or more of the described
embodiments. The prior designs typically only take into account
incoherent emission. In the case of a storage ring with an
integrated FEL, the system provides for additional energy lost by
the FEL action by supplying more RF power to the ring. In addition,
the system is optimized with the FEL emission as part of the design
process. For example, the undulator is sufficiently long to provide
sufficient FEL gain on each pass of the bunch through the undulator
(e.g., such gain could be more than 10 but less than 1000). The
length of the undulator may differ depending upon the embodiment.
Gain is also influenced by the storage ring in that the system
produces an equilibrium emittance that is sufficiently small in
both transverse degrees of freedom. The system produces a relative
energy spread that must be of order or less than the FEL .rho.
parameter. The system produces an equilibrium bunch length which is
sufficiently small to provide high peak current. The system also
produces equilibrium transverse emittances which permit FEL
radiation. The focusing of the electron bunch throughout the
undulator system is sufficient to provide the necessary electron
beam density. As shown in Table 1, the emittance is significantly
larger (factor of seven larger than .lamda..sub.FEL/4.pi.) than
typically required in prior systems. Due to the larger emittance,
gain length in the example will be increased somewhat from prior
conventional designs, but the increased gain length may be
compensated by suitable adjusting the interaction length as part of
the design constraint of the compact storage ring. Additionally,
note that undulator parameter K is less than 1 as compared to prior
FEL design that require K to be greater than 1. The parameters
shown in Table 1 are merely illustrative examples and the
parameters in the table may not have been optimized for high
average power performance of various embodiments.
TABLE-US-00001 TABLE 1 Overall Gross Parameters Value Electron
Energy, E [MeV] 330.0 Circumference, C [m] 23.9 Undulator length
[m] 6.5 Undulator period [cm] 1.0 Undulator Parameter (K) 0.5
Electron Bunch Charge [nC] 2.0 Electron Bunch length (rms) 2 mm
.gamma.* emittance 5 micron Relative energy spread 0.0003 1-D FEL
.rho. Parameter 0.0006 Energy loss per turn, U.sub.0 [keV] 1.9 RF
voltage, V.sub.RF [MV] 0.324 RF frequency, f.sub.RF [MHz] 1428
Harmonic number 114
[0028] FIG. 1 is a diagram illustrating an embodiment of a compact
electron storage ring configured to accept an insertion device
suitable for a FEL. Electron storage ring 100 is a compact
low-energy storage ring. In some embodiments, electron storage ring
100 has a circumference that is on the order of 30 meters. For
example, electron storage ring 100 fits inside a 60 square meter
area. In some embodiments, electron storage ring 100 has a
circumference that is at least 10 meters but less than 60 meters.
In some embodiments, the energy of electron storage ring 100 is
less than 500 MeV. The shown storage ring 100 includes magnetic
undulator 202, bending magnets 102, 104, 106, and 108, quadrupole
magnets 110, 112, 114, 116, and 118 for beam focusing, and an RF
Cavity 120 for replacing energy lost by synchrotron radiation and
also to keep the electrons in tight bunches longitudinally. Not all
components have been labeled and only a select number of the
components have been labeled to illustrate the embodiment clearly.
Injector 126 generates electron beams that are bent by bending
system 128 (e.g., using one or more magnets) for insertion in the
storage ring. Septum 124 receives the electron beam and pulsed
kicker magnet (kicker) 122 is used to inject the electron beam into
the storage ring. In various embodiments, the use of pulsed kicker
magnet 122 is optional.
[0029] Injector 126 provides sufficient energy to the electron beam
to be injected and stored in the storage ring. The final energy of
the injector may be either the design energy or a lower energy
which is subsequently increased after storage in the storage ring.
The injector may be one of several different types that are
familiar to those skilled in the art. In some embodiments, injector
126 includes a linear accelerator (LINAC). In some embodiments,
injector 126 includes a LINAC utilized on multiple passes by
bending the beam to pass through the LINAC more than once. In some
embodiments, injector 126 includes a Microtron. The emittance of
the injected beam is not required to be a very low emittance. The
injected emittance is sufficiently low so that the beam may
circulate enough times in the electron storage ring to come to
equilibrium after cooling to the smaller equilibrium emittance
sufficient for FEL emission. The injected energy spread does not
have to be as low as that in the storage ring which is required for
FEL emission. However, the injected energy spread is sufficient to
permit the electron beam to circulate enough times in the electron
storage ring to come to equilibrium after cooling to the smaller
equilibrium energy spread sufficient for FEL emission.
[0030] In some embodiments, operation of the storage ring FEL
begins by the injection of electrons into the storage ring. In some
embodiments, injector 126 creates electron bunches below the energy
of the storage ring. In this embodiment, the storage ring is ramped
in energy after injection to the desired final energy, for example
330 MeV in the sample parameters shown in Table 1. The operation of
the EUV may be off from time to time for injecting the storage ring
and reacceleration to the design storage ring energy.
[0031] In some embodiments, the storage ring may be injected with
an electron beam with an energy that is equal to that desired for
EUV operation. In this embodiment, the desired number of bunches
may be injected and operation may be commenced thereafter. From
time to time' additional electrons may be injected, avoiding the
main beam and without disturbing the operation at EUV, for example
after the intensity of the electron beam is reduced by several
percent. This additional beam cools down to be absorbed into the
primary circulating beam. This type of injection is sometimes
referred as "top up" or trickle charge injection. The reduction in
EUV power may be avoided by utilizing a corresponding increase in
the seed power. This may be controlled by a feedback system in
order to fulfill stability requirements for EUV output power.
[0032] The electrons circulate counter clockwise in storage ring
100. Bending magnets have a dipole magnet field and may have
additional quadrupole and sextupole fields. Additional magnets (not
shown) include quadrupole (e.g., for focusing which keep the
electron beam near a stable orbit that closes after one turn) and
sextupole (e.g., used for correcting the chromaticity) magnets. The
number of bending magnets 102, 104, 106, and 108, and the angular
bend of each adds to a total bending of 360 degrees. The total
number, position and strength of bending magnets, quadrupoles and
RF cavities may be optimized to achieve the desired parameters of
the ring including effects due to the emission of FEL radiation.
The parameters may be optimized including 3D FEL effects so that,
in particular, the emittance may not be less than
.lamda..sub.FEL/4.pi.. The total number, position and strength of
septums 124 and kickers 122 may be optimized to achieve the desired
injection into the storage ring. The magnetic field may be selected
for each magnet so as to create a stable configuration that permits
an electron beam to circulate periodically in the steady state. In
some embodiments, storage ring 100 includes a sequence of magnets
to disperse the electrons laterally according to their energy
before entering an undulator.
[0033] Electrons which are stored in the storage ring radiate a
significant amount of so called `synchrotron radiation` which also
serves to damp the electrons towards this closed stable orbit,
which in turn cools the distribution of electrons by decreasing the
electron beam emittance. In addition, because of this constant
energy loss, RF cavity 120 is provided with sufficient power to
replace the energy lost due to synchrotron radiation. This
continuous loss and acceleration also serves to damp the electron
energy towards the stable periodic orbit, on which the amount of
radiation is exactly canceled by the acceleration system. This
results in the cooling, or decrease, of the relative energy spread
of the beam. In addition to this cooling action, the electron beam
is also heated by the emission of discrete photons. The competition
between these effects yield Gaussian distributions in the
transverse and longitudinal directions. Thus, electron storage ring
100 may be used to optimize the `emittance` of the beam in both the
transverse degrees of freedom, as well as the longitudinal degree
of freedom (relative energy spread and bunch length). Typical use
of electron storage rings have been traditionally for the
production of incoherent x-rays for research applications. However,
such prior uses has not been optimized or considered for coherent
FEL emission at EUV wavelength. Generally the prior designs based
on traditional storage rings will not generate very high average
power FEL EUV radiation.
[0034] The FEL in the storage ring may be operated in various
alternative ways. In some embodiments, radiation is initiated by
SASE (Self Amplified Stimulated Emission). In some embodiments, FEL
is seeded with an external coherent source at 13.5 nm. In some
embodiments, FEL is seeded with an external coherent source that
has a multiple of the desired EUV wavelength. In some embodiments,
FEL is self-seeded by selecting a small fraction of the output
energy of one pulse and then using that energy to seed the next
pulse (e.g., regenerative amplifier). The selected fraction of the
output energy may be tuned to reach the desired output power.
[0035] FIG. 2 is a diagram illustrating an embodiment of a Compact
Storage Ring high power EUV beam generator system utilizing an
undulator magnet insertion device to create FEL radiation initiated
by Self Amplified Stimulated Emission (SASE). System 200 includes
the compact electron storage 100 of FIG. 1 and magnetic undulator
202. Undulator 202 is adjusted in length so that it is long enough
(e.g., 12 gain lengths) for SASE to develop. The length of
undulator 202 is also adjusted with selection of electron bunch
parameters such that the output power is below saturation. The EUV
output exiting output aperture 130 includes incoherent undulator
radiation and also SASE coherent radiation. The evolution of the
bunching due to the FEL is shown in along the undulator. The beam
is shown with one transverse position and the longitudinal
position. Initially the beam is not bunched and is comprised of a
statistical random distribution of particles. After progression
along the undulator, the density is starting to be modulated at the
wavelength of FEL emission. Finally at the end of the undulator,
the bunching is more extreme, but still not yet fully bunched. The
FEL is significantly below saturation. After the bunch passes
through the undulator it circulates and arrives once again at the
beginning. The bunching is fully washed out by the entrance to the
undulator due to the variation of the path lengths of the particles
as they circulate once in the bending and focusing system.
[0036] FIG. 3 is a diagram illustrating an embodiment of a Compact
Storage Ring high power EUV beam generator system utilizing an
undulator magnet insertion device to create FEL radiation seeded
with an external coherent source at EUV wavelength or a multiple
thereof. System 300 includes the compact electron storage 100 of
FIG. 1 and magnetic undulator 302. FEL amplifies coherent EUV light
supplied from EUV seed system 304 that generates coherent EUV
light. The coherent EUV seed pulse is injected from the right after
it is created by the EUV seed system 304. After it traverses
undulator 302 and interacts with the electron bunch, it is
amplified to emerge on the left and a portion of the amplified EUV
exits out output aperture 130. The EUV pulse of seed system 304 is
timed appropriately to be superimposed on the electron bunch as
they transverse undulator 302. The length of undulator 302 (e.g.,
about 2 gain lengths plus gain sufficient to reach a sufficient
power for operation, for example 10% of the FEL saturation power)
does not have to be as long as undulator magnet 202 of FIG. 2 due
to the externally supplied coherent EUV light. A larger seed power
results in a shorter undulator length required for power
production. For example, if a power of 1 kW is desired, then a gain
of 1000 (e.g., seven gain lengths) is desired, provided that 1 W of
coherent EUV average power is used to seed the FEL. In this case,
the total gain length required is 9 gain lengths which may include
2 gain lengths to form the microbunching (lethargy) and an
additional 7 gain lengths to provide a factor of 1000 power
increase.
[0037] FIG. 4 is a graph illustrating example gain curves for
different seeding power levels in which the FEL output power
increases as a function of position in an undulator. The
co-propagating emitted radiation may cause bunching of the electron
beam at the emission wavelength. This in turn can induce further
emission due to the coherent emission of the bunched beam. This
process can grow exponentially with a characteristic gain length
until the energy lost is approximately equal to the FEL .rho.
parameter, after which the emission saturates. Graph 400 shows
example typical gain curves for FEL power increase as a function of
position in the undulator. For SASE radiation (e.g., generated
using system 200 of FIG. 2), as shown by curve 402, there are three
regions, early gain, exponential (high) gain and finally
saturation. For seeded operation (e.g., generated using system 300
of FIG. 3), three curves (i.e., 404, 406 and 408) are shown
corresponding to different seed power to illustrate the process of
seeding. There is an initial delay, called lethargy (e.g., during
which the beam is forming microbunches) before the onset of the
exponential (high) gain due to the process of bunching prior to
saturation. This delay is about 2 gain lengths, after which the
power grows exponentially according to ex where x is the distance
measured in gain lengths. The dashed line represents a desired
power in the exponential gain regions of curves 402-408, below the
saturation.
[0038] In the case of an FEL within a storage ring, the electron
beam passes through the undulator each time that it circulates
around the ring. Therefore, if there is only one bunch of electrons
stored in the ring the pulses of EUV light are produced at the
revolution frequency of the storage ring which is typically of
order 10 MHz. In addition, if there are many bunches in the storage
ring, then pulse pattern of emitted radiation follows the pattern
of the bunches around the storage ring and repeats with a frequency
of the revolution frequency. If the EUV average power desired is 1
kW, then the energy emitted in the form of EUV radiation caused by
the FEL action on each turn is 100 micro joules based on a
revolution frequency of 10 MHz. If there are 10 bunches within the
ring, then the amount of energy extracted from each bunch would be
correspondingly lower, 10 micro joules for the case considered
above. For an example ring of the appropriate energy, the typical
stored energy of each bunch is of order 1 joule. Therefore, in the
given example, the necessary extracted energy is 10 to 100 parts
per million of the energy stored in the ring on each turn.
[0039] FIG. 5 is a diagram illustrating an embodiment of a Compact
Storage Ring high power EUV beam generator system utilizing
self-seeded regenerative amplifier FEL. System 500 includes the
compact electron storage 100 of FIG. 1 and magnetic undulator 502.
The length of undulator 502 is long enough to permit amplification
of the self-seeded pulse. As shown in the Figure, a portion of the
output of the EUV FEL itself is isolated and fed back into the
undulator 502 to act as a seed (e.g., instead of using a separate
seed system). The EUV output of system 500 exiting output aperture
130 includes incoherent undulator radiation and also coherent
radiation amplifying the seed pulse. Note that the seed pulse
overlaps the electron beam. To seed the same bunch which created
the seed, the total delay of the seed is equal to a multiple of the
revolution time of the electron beam. For example, if the delay is
two times the revolution period, then the even revolutions are
seeded by even revolutions and the odd revolutions are seeded by
odd revolutions (e.g., the first revolution seeds the third
revolution which seeds the fifth revolution etc. and similarly with
the second revolution, the fourth and the sixth, etc.). Mirrors 504
reflects a portion of the EUV FEL output and this reflected portion
is reflected on mirrors 506, 508 and 510 to be fed back into the
storage ring. For example, if the gain is 1000, and if one percent
is circulated back to seed the FEL, then on the next cycle the
radiation seed gets amplified by 1000 and the total round trip
output is increased by a factor of 10. Multiple mirrors are
utilized to increase the length of distance traveled by the
reflected beam within a compact physical space. By adjusting the
distance traveled by the reflected beam, the total time delay of
the reflected beam is controlled. In order for the seed power to
selectively seed the same bunch which created it, the total time
delay of the mirrors to provide the EUV seed equals an integer
multiple of the revolution time of the electron bunch. The mirrors
shown in FIG. 5 are merely illustrative examples. In various
embodiments, different number of mirrors, mirror geometries and/or
other mirror configurations may be utilized.
[0040] In the embodiment shown in FIG. 5, to avoid power saturation
due to increasing power with each pass through the undulator
magnet, the fraction of radiation power returned for the seeding
may be adjusted to a value equal to the inverse of the gain of the
FEL to operate the FEL in the steady state condition. In some
embodiments, a feedback system is utilized to control the output
power so that it is well below the saturation power of the FEL.
This may be accomplished by the attenuation or enhancement (e.g.,
adjust intensity) of the fraction of seed power, which is inserted
at the beginning of the FEL (e.g., the intensity of the seed is
adjusted to maintain the inverse relationship in which the fraction
of seed power is equal to the inverse of the total gain of the
FEL). Alternatively, properties of the electron beam may be altered
during operation to modify the amount of gain in the FEL in order
to control the output power, so that it is below the saturation
power of the FEL (e.g., parameters of the electron beam are
adjusted to maintain the inverse relationship in which the fraction
of seed power is equal to the inverse of the total gain of the FEL
so as to reach steady state). Alternatively, the properties of the
electron beam may automatically reach an equilibrium state
producing constant output power (e.g., the inverse relationship, in
which the fraction of seed power is equal to the inverse of the
total gain of the FEL so as to reach steady state, evolves
automatically by evolution of the parameters of the electron beam).
The specific total output power desired may be adjusted by adding
or reducing the number of bunches in the storage ring. In the
example shown in Table 1, the number of bunches may range from 1 to
114. Therefore, the output power could be adjusted over a range of
a factor of 114. For example, if the average FEL output power for
one bunch were 30 W, then the average power could range from 30 W
with one bunch to 1200 W with 40 bunches to 3420 W with 114
bunches. In some embodiments, the average power may be selected to
the appropriate level desired by the selection of the number of
electron bunches stored in the storage ring. In some embodiments
the bunch to bunch stability may be maintained by the use of bunch
by bunch feedback system.
[0041] In some embodiments, with the use of SASE, the output power
of the FEL may fluctuate and the coherence of each pulse is
independent so that on the average the illumination will not show
coherent effects provided that it is accomplished on multiple
cycles of the storage ring. In some embodiments, with the use of
the external seed generation system, the coherence of the output
follows that of the seed. In some embodiments, with the use of
self-seeding, the output may develop a single coherent phase, which
could lead to undesirable interference effects in the EUV
lithography application. Coherent effects in the utilization of the
EUV beam may be altered and eliminated by the utilization of a
variation of the total length of the seed return path by of order
one wavelength at times scales which are short compared to an
illumination time. For example, if the illumination time is one
second, the path length may be oscillated by more than one
wavelength with a frequency of 1 kHz so that any fringe effects are
averaged over 1000 cycles. In some embodiments, length of seed of
the external seed generation system may be likewise varied to alter
the phase of the seed pulse. Alternatively, an optical system after
the EUV output of the source can be designed to modify beam
parameters.
[0042] FIGS. 6A-6B are diagrams illustrating embodiments of
magnetic undulator insertion devices used to produce FEL radiation.
FIG. 6A shows a profile view and side view of top and bottom
portions of magnetic undulator 602. FIG. 6B shows a profile view
and a side view of top and bottom portions of transverse gradient
magnetic undulator 604. A plurality of Magnetic undulator 602 or
magnetic undulator 604 may be included in any of magnetic undulator
202, 302 and/or 502 of FIGS. 2, 3 and 5, respectively. Magnetic
undulator 602 creates alternating vertical magnetic fields that
produce a sinusoidal horizontal deflection in the example shown.
The undulator may be oriented to bend vertically or horizontally.
For example, rather than having top and bottom portions in a
vertical configuration that bend a beam horizontally, the undulator
may be configured in a horizontal configuration with left and right
portions that bend a beam vertically. The poles may be shaped to
influence the quadrupole field within the undulator.
[0043] The FEL emission has the most significant effect on the
energy distribution within the electron bunch. At each wavelength
of 13.5 nm, the emission induces a shifted sine wave much smaller
that the natural energy spread. If this process induces an
additional energy spread, even though the emission is far below the
saturation level, then the equilibrium energy spread of the storage
ring may be increased. For example, if the natural energy spread is
2.times.10.sup.-4 and the FEL .rho. parameter is 6.times.10.sup.-4,
then FEL emission will take place. However, if this energy spread
grows beyond 6.times.10.sup.-4 as the system reaches equilibrium,
then the FEL gain will be reduced on subsequent revolutions of the
electron bunch or the FEL may have an equilibrium output from each
bunch that is reduced. This effect may be compensated by the use of
transverse gradient magnetic undulator 604 of FIG. 6B which has a
transverse gradient in the deflecting field. Undulator 604 has a
variation of the undulator parameter transversely caused by tilting
the poles of the undulator. This variation can be automatically
matched to the increased energy spread by having a fixed dispersion
of transverse beam position proportional to the energy within the
undulator. In other words, electrons with different energies will
automatically enter the undulator at different transverse positions
such that the conditions for coherent emission is maintained along
the path through the undulator. Thus, the FEL resonance condition
can be maintained in spite of increases in the energy spread within
the electron beam, even at steady-state.
[0044] The tipping angle of the poles produces a linear dependence
of the vertical bending field with the horizontal position x. The
change of the field with x depends upon the choice of angle. The
poles may be also shaped to influence the quadrupole field in the
undulator. In a storage ring the dispersion of the electron beam
with energy spread is controlled by the detailed design of the
magnet lattice, the sequence and strength of bending and quadrupole
focusing magnets. This lattice may be adjusted to provide a
dispersion in position with energy spread within the undulator.
Alternatively, additional magnets may be used both upstream and
downstream of the undulator to provide a dispersion in position
with energy spread within the undulator. In either case, this may
be used to achieve a matching of the undulator condition of the
previously discussed .lamda..sub.FEL, equation, locally in the
dispersed direction. Thus, the equilibrium energy spread may be
allowed to exceed the .rho. parameter while still achieving EUV FEL
emission.
[0045] In some embodiments, the undulator magnet insertion device
includes a sequence of undulator sections with either spaces or
magnets in between them. The spacing of these separate undulator
sections may be adjusted so that the FEL action is undisturbed. The
spacing of these separate undulator sections may be adjusted to
affect the other parameters of the beam so as to improve the
performance of the FEL as operated within the storage ring. Such
spacing may diminish the output of the FEL in a single pass while
improving the overall performance of the system in the steady
state. For example, a shift of FEL phase may be included between
undulator sections such that the effect of the FEL on the steady
state electron beam parameters (e.g., relative energy spread) is
decreased or diminished. While this may or may not lead to
decreased power emitted by that bunch of the FEL, the overall
performance may be enhanced or maintained by increasing the number
of bunches or the overall current in the storage ring.
[0046] In some embodiments, the electron beam, including a sequence
of electron bunches, is steered within the undulator in order to
inhibit the emission of EUV FEL radiation temporarily on a sequence
of revolutions. For example, such a beam steering may be included
such that the effect of the FEL on the steady state electron beam
parameters (e.g. relative energy spread) is decreased. For example,
the beam is steered on 90% of the revolutions in order to inhibit
the emission of FEL radiation, while on the remaining 10% the beam
is not steered so that the FEL emission will occur. In this
example, to achieve the same output power, the gain of the FEL may
be increased by a factor of 10 on those passes that are not
steered. During those revolutions where the beam is steered and the
FEL emission is suppressed, the electron beam continues to be
cooled towards equilibrium values. Such cooling may be advantageous
in controlling the steady state values of beam parameters (e.g. the
relative energy spread).
[0047] FIG. 7 is a block diagram illustrating an embodiment of a
system for performing EUV lithography. EUV source 702 has been
installed on a subfloor under a floor of a semiconductor
manufacturing facility clean room. The compact size EUV source 702
enabled by the use of a compact storage ring (e.g., compact storage
ring 100 of FIG. 1) has allowed EUV source 702 to be small enough
to fit within a typically sized semiconductor manufacturing
facility. Examples of EUV source included in EUV source 702 include
system 200, system 300 or system 500 of FIGS. 2, 3 and 5. The EUV
beam output generated by EUV source 702 is reflected by mirror 704
up to EUV optics 706 of EUV lithography system/stepper 708 (e.g.,
lithography scanner) for use as the light source of EUV
lithography. In some embodiments, the same beam output generated by
EUV source 702 is provided a plurality of lithography
steppers/scanners.
[0048] Although the foregoing embodiments have been described in
some detail for purposes of clarity of understanding, the invention
is not limited to the details provided. There are many alternative
ways of implementing the invention. The disclosed embodiments are
illustrative and not restrictive.
* * * * *