Ex Parte 5900637 et al

Board of Patent Appeals and Interferences

Jun 30, 2008

90006470 (B.P.A.I. Jun. 30, 2008)

UNITED STATES PATENT AND TRADEMARK OFFICE
BEFORE THE BOARD OF PATENT APPEALS
AND INTERFERENCES
Ex parte MASSACHUSETTS INSTITUTE OF TECHNOLOGY
Appeal 2008-03 3 3
Reexamination Control 901006,470
Patent 5,900,637
Technology Center 2800
Decided: June 30,2008
Before JOHN C. MARTIN, LEE E. BARRETT, and MARK NAGUMO,
Administrative Patent Judges.
BARRETT, Administrative Patent Judge.
DECISION ON APPEAL
This is a decision on appeal under 35 U.S.C. $ 5 306 and 134(b) from
the Final Rejection of claims 1, 2, 4, 8-1 1, 13-16, 18, 22,23, and 25-28,
which are all of the pending claims.
We affirm.
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REEXAMINATION
A request for reexamination of U.S. Patent 5,900,637 ('637 patent),
entitled "Maskless Lithography Using a Multiplexed Array of Fresnel Zone
Plates," was filed on December 2, 2002, by Patent Owner Massachusetts
Institute of Technology. The '637 patent issued May 4, 1999, to Henry I.
Smith, based on Application 081866,550, filed May 30, 1997.
BACKGROUND
The claimed invention relates to a maskless ultraviolet (UV)
lithography system employing modulating means and phase zone plates.
Claim 1 is reproduced below (omissions from the original patent claim
are enclosed in brackets and additions are underlined, 37 C.F.R. fj 1.530(f)).
1. A maskless lithography system comprising:
a source of UV energy;
modulating means for modulating the UV energy from said
source of UV energy to create a plurality of individual beams of UV
energy; and
an array of [Fresnel] phase zone plates which focus said plurality
of individual beams of UV energy [an energy beam] into an array of
images in order to create a permanent pattern on an adjacent substrate;
said modulating means being positioned between said source of
UV energy and said array of phase zone plates.
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THE REFERENCES
Me yers
Johnson
5,589,983 Dec. 3 1, 1996
6,133,986 Oct. 17,2000
(filed Feb. 20, 1997, which claims the priority of Provisional
Application 6010 12,434, filed Feb. 28, 1996)
Janos Kirz, Phase. zone plates for x rays and the extreme uv, Journal of
the Optical Society of America, Volume 64, Number 3 (March 1974),
pages 30 1-09 (hereinafter "Kirz").
Henry I. Smith, A Maskless X-ray Projection Pattern Generator,
Massachusetts Instituted of Technology, Cambridge, MA, distributed
May 28, 1996, pages 179-80 (hereinafter "Smith").
THE REJECTIONS '
Claims 1, 2,4, 8-1 1, 13, 16, 18,22,23,25, and 26 stand rejected under
35 U.S.C. 5 103(a) as unpatentable over Johnson and Kirz.
Claims 14 and 27 stand rejected under 35 U.S.C. 5 103(a) as
unpatentable over Johnson and Kirz, further in view of Smith.
Claims 15 and 28 stand rejected under 35 U.S.C. 5 103(a) as
unpatentable over Johnson, Kirz, and Smith, further in view of Meyers.
1 A 5 103(a) rejection based on Smith in view of Kirz and Swanson
et al. U.S. Patent 4,895,790 (Swanson) (Final Rejection 3-4) was withdrawn
at page 2 of the Answer.
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FINDINGS OF FACT
The invention is fairly described in the summary of the invention:
The invention provides a system and method of performing
lithography without the need for a mask that contains the pattern to be
exposed. More specifically, it employs an array of Fresnel zone plates
to focus parallel beamlets of electromagnetic radiation so that they
converge to foci on a substrate. The beamlets can be individually
turned on and off by means of shutters that obstruct a beamlet, or by
deflecting small mirrors that would otherwise direct a beamlet to its
Fresnel zone plate. Pattern generation is accomplished by moving -the
substrate while multiplexing the individual bearnlets on or off by means
of electrical or optical signals.
Columri 2, lines 55-65.
A zone array is shown in Figure 1, reproduced below.
Z O N f PI A l f ARRAI,
S i ,
FIG. I
Figure 1 shows an array of "Fresnel zone plates" 102 supported on a
carbonaceous membrane 106 with vertical etched silicon joists 108 for rigid
mechanical support (col. 3,ll. 49-65).
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The '637 patent discloses:
The membrane 106 is made of thin carbonaceous material
because it is transparent to a beam source of 4.5 nm x-ray. If deep UV
radiation is used, the membrane can be made of glass, and the zone
plates could be made from phase zone plates, i.e. grooves cut into the
glass membrane
Column 3, line 66 to column 4, line 3.
The '637 patent also,explains that "'phase' zone plates" provide greater
focusing efficiency than do "'amplitude' zone plates." Column 5, lines 25-32.
All of the original claims of the '637 patent (i.e., claims 1, 16, 29, and
30) recite "Fresnel zone plates," which term as used in the '637 patent appears
to be generic to amplitude zone plates and phase zone plates. During this
reexamination proceeding, the phrase "Fresnel zone plates" in claims 1 and
16, the only currently pending independent claims, has been changed to
"phase zone plates." Amendment Under 37 C.F.R. 1.11 1, filed May 3,2004,
at 2-3.
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Figure 2, reproduced below, shows one arrangement of the system.
/ 2o
210
If 1A SOURCE
FIG. 2
Figure 2 shows the beam source 2 10 passing collimated beams 2 12 of
x-rays through zone plates 202 of a zone plate array 200 to create focused
individual beamlets 21 3, which may be turned on and off by shutters 21 8
located between the zone plate array and the substrate 204, under control of
computer 230 (col. 4,ll. 16-27).
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An alternative arrangement is shown in Figure 3, reproduced below.
ARRAY O f MIRRORS
I I I I I I I l l
UPSTREAM
2 4 1 f l 1 1 1 1 1 MIRROR OF, TT j02
FIG. 3
Figure 3 shows a source of energy 3 10, an array of mirrors 305
between the source of energy 3 10 and the array 300 of Fresnel zone
plates 302 which acts as a modulating means to form the energy into
individual beamlets for each Fresnel zone plate and to turn the beamlets on
and off (col. 4,ll. 29-45).
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Johnson patent
Figure 2 of Johnson is shown below.
FIG. 2
Figure 2 illustrates a lithography printer having a projection system I
which focuses an image source 1 1 (e.g., an array of mirrors) onto a microlens
array 2, where each microlens images a microspot onto a substrate surface 12.
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Johnson describes Figure 2 as follows:
FIG. 2 illustrates an embodiment which is very similar to the
microscopy system of FIG. 1, but which could function as a
lithography printer. (In this figure as well as later figures, elements
.corresponding to those in an earlier figure will generally be denoted
with the same reference numeral.) This system also contains a low-
resolution, double-telecentric projection system 1, but in this
embodiment the projection system functions to focus an image source
11 onto the microlens array 2. The image source comprises an array of
light-modulating source elements (e.g., spots or pads of variable
reflectivity), with each source element being imaged onto a
corresponding microlens element. The image source could be a Digital
Micromirror Device (or DMD, Ref. 3), with each source element
comprising an individual micromirror pixel element. Each microlens
images the projection aperture 7 onto a corresponding microspot on the
printing surface 12, and each source element controls the exposure
level over the corresponding microspot. mi he image source 11 is
illuminated in reflection mode from the illumination system 9, using a
beam splitter 13 to merge the illumination into the light path.
Column 4, lines 28-48.
The light source is described as follows: "A practical embodiment of
the microlithography. system might use a continuous deep-UV laser light
source such as a frequency-quadrupled 266 nm Nd:YAG laser . . . and a DMD
image source . . . ." (Col. 6,ll. 1-4.)
I
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Johnson describes fabricating the microlenses by reactive ion etching
(cols. 12- 13) and states:
Numerous alternatives to reactive ion-etched microlenses exist
for either the mastering microlens elements or the replica array.
Possibilities include molded microlenses, distributed-index planar
microlenses, micro-Fresnel lenses (or binary optics), and melted-resin
arrays . . . .
Column 13, lines 34-38.
The difference between Johnson and the subject matter of
claims 1 and 16 is that Johnson discloses that the focusing elements are
"rnicrolenses" or "micro-Fresnel lenses (or binary optics)" without indicating
that they can take the form of "phase zone plates," as claimed.
Kirz
Kirz describes "Phase zone plates for x rays and the extreme uv"
(page 30 1, title), where "phase zone plates" are also referred to in the article
as "phase-reversal zone plates."
It is described that "[iln the extreme ultraviolet and the x-ray domain
the choice of optical elements is limited, and Fresnel zone plates have found
increasing use here." (Page 301, left col.)
Kirz states that "[phase-reversal zone plates (phase zone plates)] are
superior to Fresnel zone plates in both their light collection, and in their
i signal-to-noise characteristics." (Page 30 1, abstract.)
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Thus, in Kirz, which was published in 1974, the term "Fresnel zone
plate" refers to an amplitude zone plate and not to a phase zone plate.
However, as noted above, Appellant's '637 patent, filed in 1997, applies the
term "Fresnel" to phase zone plates and amplitude zone plates. Furthermore,
Swanson, filed in 1987 and discussed infra, describes "Fresnel phase zone
plate profiles" (Swanson, col. 2,ll. 59).2
The Examiner's rejections
The Examiner finds that Johnson teaches a UV light source for a
lithography system (referring to col. 6,ll. 1-3), a modulating means for
modulating the UV light source to create a plurality of individual beams of
UV energy (referring to the array of mirrors described at col. 4,ll. 27-45), an
array of microlenses to focus .the individual beams of UV energy onto a
substrate, and the modulating means being located between the source of UV
energy and ,the array of microlenses (Final Rejection 4).
The Examiner finds ,that Johnson's statement that the microlenses may
be "micro-Fresnel lenses (or binary optics)" (col. 13,ll. 37-38) teaches
2 A "Fresnel phase zone plate" is described in Rostaei et al. U.S. Patent
5,786,582 (col. 6,ll. 28-29) (1998). Also, Fukui et al. U.S. Patent 5,909,423
(1 999) describes a holographic element that "may comprise a Fresnel zone
plate on which transparent and opaque rings are alternately formed, as shown
in FIG. 10A, or may comprise a Rayleigh-Wood type Fresnel zone plate on
which transparent rings with squared wave shaped cross section are
alternately formed, as shown in FIG. 10B" (col. 15,ll. 62-67).
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"Fresnel zone plates" (Final Rejection 4), and might even teach the claimed
"phase zone plates" (Ans. 7).
The Examiner finds that Kirz teaches that "phase zone plates" are more
efficient than "Fresnel zone plates"' (Final Rejection 4).
The Examiner concludes that it would have been obvious to one of
ordinary skill in the art to replace the Fresnel zone plate in Johnson with a
phase zone plate in view of Kirz (Final Rejection 4).
DISCUSSION
Appellant has the burden on appeal to the Board to demonstrate error in
the Examiner's position. See In re Kahn, 441 F.3d 977,985-86, 78 USPQ2d
1329, 1335 (Fed. Cir. 2006) ("On appeal to the Board, an applicant can
overcome a rejection [under 5 1031 by showing insufficient evidence ofprima
facie obviousness or by rebutting the prima facie case with evidence of
secondary indicia of nonobviousness.") (quoting In re Rouffet, 149 F.3d 1350,
1355 (Fed. Cir. 1998)).
Claims stand or fall together with the independent claims
Patent Owner argues only the merits of the rejection of the two
independent claims 1 and 16. Thus, the rejections of the dependent claims
stand or fall with claims 1 and 16.
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Obviousness
The rejections are not based on the Johnson provisional
Patent Owner argues for the first time in the Reply Brief that the
disclosure in the Johnson patent at column 13, lines 34-38, is not entitled to
the priority date of Provisional Application 6010 12,434 (Johnson provisional)
because the Johnson provisional only teaches, at page 20, the use of a Fresnel
lens in the collimating element, a separate element from the microlens, and
fails to teach that micro-Fresnel lenses are an alternative to the microlenses
(Reply Br. 5-6). The Examiner does not respond to this argument.
It is not necessary for the Examiner to rely on the filing date of the
Johnson provisional. The Johnson patent was filed on February 20, 1997,
which is still before the May 30, 1997, filing date of the '637 patent and is
valid prior art unless antedated. Patent Owner does not address this fact, nor
attempt to antedate the Johnson patent. Although not referred to by Patent
Owner, we recognize that the article by the inventor Henry I. Smith,
A proposal for maskless, zone-plate-array nanolithography, J. Vac. Sci.
Technol. B 14(6), NovIDec 1996 (copy enclosed), listed as a reference in the
Johnson patent, contains much of the same disclosure as the '637 patent.
However, even if we were to assume that this paper proves conception, a
proper declaration under 37 C.F.R. tj 1.13 1 would still be necessary to prove
due diligence from prior to the filing date of the Johnson patent to the filing
date of the '637 patent application. Only at this point would the Johnson
provisional become relevant.
13
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Also, we disagree with Patent Owner's argument. Page 17 of the
Johnson provisional describes that the microlens may formed as shown in
Figure 12, page 18, which one of ordinary skill in the optics art would
recognize as a Fresnel lens, or, since it is a microlens, a micro-Fresnel lens.
page 17 discloses that the microlens may be formed by "binary optics." Thus,
the Johnson provisional discloses micro-Fresnel lenses and binary optics.
Does Johnson teach "Fresnel zone plates"?
The Examiner finds that the Johnson patent teaches that the
microlenses may be "micro-Fresnel lenses (or binary optics)" (col. 13,
11.37-38) and that this teaches "Fresnel zone plates," but does not teach
"phase zone plates," The Examiner finds that Kirz teaches that "phase zone
plates" are more efficient than "Fresnel zone plates" and concludes that it
would have been obvious to one of ordinary skill in the art to replace the
Fresnel zone plate in Johnson with a phase zone plate in view of Kirz.
The dispositive issue is whether the Examiner is correct in equating
"micro-Fresnel lenses (or binary optics)" to "Fresnel zone plates." Patent
Owner does not dispute that it would have been obvious to substitute a "phase
zone plate" for a "Fresnel zone plate" in view of Kirz.
Patent Owner argues that Examiner erred in finding that Jolmson
describes "Fresnel zone plates" at column 13, lines 34-38, because Johnson
actually describes "micro-Fresnel lenses (or binary optics)" (col. 13,
11. 37-38), not "zone plates" (Br. 8). It is argued that Fresnel lenses are sloped
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lenses, while "a Fresnel zone plate is a lens having a substantially uniform
thickness with. grooves cut therein" (Br. 8) as shown in these figures:
Figure 1
Fresnel Lens
Figure 2
Fresnel Zone Plate
The Examiner responds (Ans. 6) that Patent Owner's definition of a
Fresnel lens as represented by the above figures is incorrect because a Fresnel
lens is composed of divided annular zones according to this description in the
Handbook of Optics, Volume II, Devices, Measurements, and Properties
(Optical Society of America 2d ed. 1995):
A Fresnel lens is constructed from many divided annular zones,
as shown in Fig. 22. Fresnel lenses are closely related to Fresnel zone
plates. Both zone patterns are the same. However, unlike a Fresnel
zone plate, the Fresnel lens has smooth contours in each zone, which
delay the phase of the optical beam by 27r radians at the thickest point.
In the central zone, the contour is usually smooth enough that it acts as
a refractive element. Toward the edges, zone spacing can because
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close to the wavelength of light, so the Fresnel lens exhibits diffiactive
properties. [Endnotes omitted.]
Handbook of Optics at page 7.18. Figure 22 is reproduced below.
zone
zone
FIGURE 22 Fresnel lens construction. M divided
annular zones m u r at radii r, in the same manner as
e Fresnel zone plalr. The profiles of cach zone are
g iven by d(r) , and they are optimized to yield the
innximum efficiency in lhc focused beam.
The Examiner states:
According to this definition, Johnson's micro-Fresnel lenses
might even anticipate the phase zone plates claimed in the appealed
claims. This is not clear. But the "binary optics" indicated by the
parenthesis at line 38 in column 13 of Johnson as being equivalent to
micro-Fresnel lenses are defined in lines 2-4 of page 7.21 of the
HANDBOOK as being "stepped approximations to the MFL [micro-
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Fresnel lens] smooth zone contour." Thus, these binary optic
,equivalents of the micro-Fresnel lenses clearly constitute Fresnel zone
plates . . . .
Ans. 7. The sentence in the Handbook of Optics to which the Examiner refers
reads: "Note that binary optics, which are described in Farn and Veldkamp's
Chap. 8, (Vol. 11) on 'Binary Optics[,]' are stepped approximations to the NIFL
smooth-zone contour." Handbook of Optics at 7.21. We understand the
Examiner's position to be that because the surfaces of the binary optics
I approximation of a micro-Fresnel lens are stepped rather than smooth, the
binary optics approximation constitutes a micro-Fresnel zone plate rather than
a micro-Fresnel lens. The Examiner's position is consistent with Swanson's
description of a similar optical structure as a Fresnel phase zone plate.3
Figures 1 A- 1C of Swanson are reproduced on the next page:
Swanson is assigned to Massachusetts Institute of Technology, the
owner of the patent under reexamination.
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FRESNEL ZONE PLATE PHASE PROFILES
RADIAL DISTRIBUTION
Figures 1 A- 1 C are schematic illustrations of three different Fresnel
zone plate profiles. Swanson explains that "FIGS. 1 b and 1 c show Fresnel
phase zone plate profiles quantized to two and four phase levels, respectively"
(Swanson, col. 2,ll. 58-60). Each level is a different phase zone.
Patent Owner replies that a "binary optic" is a stepped approximation of
a micro-Fresnel lens, while a "Fresnel zone plate is a lens having a
substantially uniform thickness with groove cut therein" (Reply Br. 7) as
shown in the following figures:
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Slope Lens Stepped Slope
Figure 1
Fresnel Lens Figure 2
Fresnel Lens
(Binary Optics)
Figure 3
Fresnel Zone Plate
. Appellant's response is unpersuasive for a number of reasons. In the
first place, neither of the above Figures 1 and 2 includes the annular zones
that are characteristic of a Fresnel lens. Handbook of Optics at p. 7.18. A
Fresnel lens is depicted in Figure 22 of -the Handbook of Optics, reproduced
supra. Second, and more important, Appellant has not cited any evidence that
contradicts the Examiner's finding that a binary optic approximation of a
Fresnel lens is a Fresnel zone plate. In particular, Appellant has not directed
our attention to any evidence of record that supports its argument that "a
Fresnel zone plate is a lens having a substantially uniform thickness with
grooves cut therein" (Br. 8). ' To the contrary, the evidence indicates that those
of ordinary skill in the art would recognize and accept Swanson's description
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of the four-level structure depicted in Swanson's Figure IC as a "Fresnel
phase zone plate" (col. 2,ll. 48-60).~
For the foregoing reasons, Appellant has failed to show that the
Examiner erred in finding that Johnson's "micro-Fresnel lens (or binary
optics)" language describes a Fresnel zone plate. In fact, it is evident from
Figure 1 C of Swanson that Johnson more particularly discloses a Fresnel
phase zone plate.
Accordingly, the rejection of claims 1,2,4, 8- 1 1, 13, 16, 18,22,23, 25
and 26 for obviousness over Johnson in view of Kirz is sustained, as is the
rejection of claims 14 and 27 for obviousness over Johnson in view of Kirz
and Smith and the rejection of claims 15 and 28 for obviousness over Johnson
in view of Kirz, Smith, and Meyers.
CONCLUSION
The rejections of claims l , 2 , 4 , 8-1 1, 13- 16, 18,22, 23, and 25-28 are
sustained and the decision of the Examiner is affirmed.
AFFIRMED
Attachment:
Henry I. Smith, A proposal for maskless, zone-plate-array nanolithography,
J. Vac. Sci. Technol. B 14(6), NovIDec 1996.
4 Figure 3 of the Brief appears to represent a phase zone plate rather
than an amplitude zone plate.
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MAT
. Matthew E. Connors
Gauthier & Connors LLP
Suite 2300
225 Franklin Street
Boston MA 021 10
A proposal for maskless, zone-plate-array nanolithography
Henry I . smitha)
Deparlment of Elec~rical Engineering and Cornputer Science, Mussachuse~~s lnslilule of techno log)^,
Cambridge, Massachuse~~s 021 39
(Received 6 June 1996; accepted 16 August 1 996)
We propose a novel form of x-ray projection lithography that: (1) requires no mask, and hence can
be considered an x-ray pattern generator; (2) is, in principle, capable of reaching the limits of the
lithographic process. The new scheme utilizes an array of Fresnel zone plates, and matrix-addressed
micromechanical shutters to turn individual x-ray beamlets on or off In response to commands from
a control computer. Zone plate resolution is approximately equal to the minimum zone width, which
can approach 10 nm. Zone plates are narrow-band lensing elements: For a diffraction limited focus,
the source bandwidth AAIA should be less than or equal to the reciprocal of the number of zones N.
An undulator having Nu magnetic sections emits collimated radiation in a bandwidth AAIA = l/Nu .
Nu is usually in the range 35-100. We present a system design based on 25 nm lithographic
resolution using h=4.5 nm. For N = 100 the zone-plate diameter is 10 pm. Each zone plate of the
array would be responsible only for exposure within its "unit cell." To fil l in a f i l l pattern, the stage
holding the sample would be scanned in X and Y while the beamlets are multiplexed on and off. A
microundulator designed for installation on a commercial compact synchrotron can provide 87 mW
within a 2% bandwidth around 4.5 nrn in a divergence cone of 0.28 rnrad. The calculated efficiency
of first-order focus for a zone plate operating at 4.5 nrn is 31%, using 130 nrn of spent U as the
absorberlphase shifter. An exposure rate of - l cm2/s at 25 nm resolution appears feasible. O 1996
American Vacuum Society.
I. INTRODUCTION
The relationship between mask-substrate gap G and
'minimum feature size Win conventional x-ray lithography is
given by ,
where A is the x-ray wavelength (-1 nm) and cu is in the
range 1 - 1 .5.'-3 For feature sizes below 50 nm, the gap must
be below 4 pm. Although such small gaps, and even mask-
substrate contact, are feasible in research, it is questionable
whether this would be acceptable in future manufacturing.
Thus, one is persuaded to consider x-ray projection, espe-
cially for feature sizes below 50 nm. At x-ray and extreme
UV (EUV) wavelengths there appear to be only three pos-
sible approaches to projection lithography: imaging with
multilayer mirrors, in-line holography, and imaging with
zone plates. The latter is the simplest and probably the most
practical. In this article a novel maskless version of zone-
plate-based lithography is proposed which appears to be
highly attractive from the points-of-view of efficiency,
throughput, and flexibility. It relies upon recent advances in
micromechanics and spatial-phase-locked e-beam lithogra-
phy, both of which are undergoing rapid development.
II. WHY ZONE PLATES FOR NANOLI'THOGRAPHY
The imaging properties of Freznel zone plates have been
understood since the late 19th century.4 Zone plates have
been used for many years in x-ray microscopy at the "water
window" around 2.4 nm.'-' In this application they work
a)~lec t ronic mail: hismith@nano.mit.edu
extremely well, revealing details, for example, in biological
specimens, that are not observable with either electron mi-
croscopy or conventional optical microscopy. Burge,
Browne, and Charalarnbous were the first to propose the use
of a zone plate in x-ray lithography.10 To circumvent the
problem of the very limited field-of-view of zone plates,
Hector and smith" and ~ e l d m a n l ~ proposed two different
lithography schemes using arrays of zone plates. Both
schemes require a mask and two zone-plate arrays in tandem.
Because the focusing efficiency of zone plates in the x-ray
regime is in the range 10%-33%, the need for two tandem
arrays implies a focusing efficiency of only 1%-9%, at best.
The appropriate wavelength to use for sub-1 00 nm lithog-
raphy is either 4.5 nm, at the carbon K absorption edge, or
around 1 nm.' At the CK edge, resists such as PMMA, which
are composed primarily of C and H, attenuate only about 2
dBlpm, and hence can be quite thick. Early, Schattenburg,
and smithi3 and Ocola et aL2 showed that at a wavelength
around 1 nm the ranges of photoelectrons and Auger elec-
trons do not prevent one from achieving resolutions below
30 nm. The intrinsic resolution at the 4.5 nm wavelength is
-5 nm, which is probably at or just beyond the practical
limit of the lithographic process itself. For the zone-plate-
array scheme described here, 4.5 nm is the optimal wave-
length from the points-of-view of resolution, source charac-
teristics, and zone plate fabrication, as described below.
Ill. ARRAY WRITING STRATEGIES
The proposed lithography scheme is shown schematically
in Fig. 1 . It does not require a mask, and employs a single
zone-plate array. At a wavelength of 4.5 nm, a focusing ef-
4318 J. Vac. Sci. Technol. El 14(6), NovlDec 1996 0734-211X196114(6)14318151$10.00 01996 American Vacuum Society 4318
4319 Henry I. Smith: Zone -p la t ea r r ay nanol i thography 431 9
and-scan" strategy is a linear-scan strategy described by
~e1dman.I' It employs a close-packed array of zone plates,
rotated in such a way that all pixels can be addressed when
the substrate is scanned along one direction only.
In order that lithographic features which cross boundaries
between unit cell,s are free of stitching errors it is necessary
that the zone plates be arranged in the array with a placement
precision much finer than a pixel diameter. Spatial-phase-
locked e-beam lithography14 could be used to accomplish
(b) this.
micromechanical IV. ZONE-PLATE ARRAY DESIGN
shutters
The principle of operation of Fresnel zone plates has been
described in detail e ~ s e w h e r e . ~ - ' ~ " ~ A zone plate can be
thought of as a structure of circular symmetry in which the
local spatial period depends on radius in such a way that
first-order diffracted radiation from any radius value crosses
the axis at the same point, the focal length. For a plane wave
(d incident the equation that describes the relationship among
the first-order focal length f, the zone number n, and the
zone radius R, follows from the Pythagorean theorem and
.the condition for constructive interference,
ehutter open shutter open ehutter closed
(R,)' + f 2 = u+ n ~ / 2 ] ~ , (2)
serpentine wrlting
FIG. I . Schematic view of the proposed maskless x-ray projection system.
(a) Perspective view of an array of Fresnel zone plates on a (1 10) Si sub-
strate. Each zone plate, wh~ch defines a "unit cell," is supported on a thin
carbonaceous membrane, with vertical, anisotropically etched Si (I I I)
"joists" for rigid mechanical support. Each zone plate is responsible for
exposure only within its unit cell (<16X lo4 pixels). (b) Cross section illus-
trating the focusing onto a resist-coated substrate, and "downstream" mi-
cromechanical shutters which turn beams on and off in response to com-
mands from a control computer. (c) Illustration of one possible writing
scheme, with the micromechanical shutters located "upstrenm" and the
substrate scanned in X and Y by a fast piezoelectric system, thereby filling
in the full pattern.
ficiency of 31% is feasible. The system is, in effect, a mask-
less zone-plate-array 'pattern generator. Each zone plate of
the array is able to focus a collimated beam of x rays to a fine
spot on a resist coated substrate. To write a pattern, the sub-
strate is scanned under the array, while the individual beam-
lets are turned on and off as needed by means of microme-
chanical shutters, one associated with each zone plate. These
shutters can either be "downstream," as depicted in Fig.
I (b), or "upstream," as in Fig. I (c).
There are various ways in which the scanning and writing
can be done. One is to employ a square array of zone plates,
as depicted, with each zone plate "responsible" for pattern-
ing only within its "unit cell." The scanning in this case
would be serpentine, with the stage moving only a distance
equal to the zone-plate diameter ( ~ 1 0 ,um) in X and Y until
all pixels within all the unit cell are addressed, and either
written (i.e., shutter open) or not (i.e., shutter closed). The
entire array would then be stepped a distance equal to its
diameter and scanning repeated. An alternative to this "step-
where A is the wavelength. Letting p represent the "pitch"
or period of the outermost zones', the angle of convergence to
focus 0 for a plane wave incident is given by
sin 0==Alp. (3)
Just as transmission diffraction gratings can be based on
periodic obstruction or periodic phase shifting, so also zone
plates can be based on obstruction ("amplitude" zone
plates) or phase shifting ("phase" zone plates), and all in-
termediate types as well. Pure phase zone plates have a fo-
cusing efficiency of 40% whereas amplitude zone plates fo-
cus only 10% of the incident radiation into the positive first-
order focus. Because zone plates are based on diffraction
they are subject to chromatic aberration. That is, different
wavelengths are focused at different axial distances. A zone
plate will produce a diffraction-limited focal spot only for
radiation in a bandwidth (BW) given by
where N is the total number of zones.
The starting point of the design of a zone-plate-array pat-
tern generator is the x-ray source since its bandwidth dictates
other system parameters. The bandwidth of line radiation
from inner-shell atomic transitions is sufficiently narrow in
many cases for our purposes, however, such sources gener-
ally do not have sufficient brightness (i.e., photons emitted
per unit area per. unit solid angle). Expressed another way,
such sources have sufficient temporal coherence but inad-
equate spatial coherence for high throughput. Synchrotron
radiation has good collimation (i.e., spatial coherence) but
inadequate spectral brightness (i.e., power in a narrow band-
width). The optimal source for the zone-plate-array pattern
generator is an undulator attached to a synchrotron. Such
JVST B - Microelect ronics a n d Nanomete r S t r u c t u r e s
4320 Henry I. Smith: Zone-plate-array nanolithography 4320
sources, which consist of a linear array of alternating mag- The power incident on a resist-coated substrate P' is
netic fields inserted into a straight section of a synchrotron given by
orbit, have bandwidths given by an equation identical to Eq.
(4), except that N in this case is the number of periods of the P' = P E ( ~ T / ~ ) ( F ) ,
alternating magnetic field N u . In modem undulators, perma-
(9)
nent niagnets are used with magnetic-field strengths -0.35
T, and Nu between 35 and 100. The number of zones in the
zone plates is therefore tied directly to the number of periods
in the undulator. Synchrotrons, both superconducting16 and
n ~ n s u ~ e r c o n d u c t i n ~ , ~ ~ designed specifically for lithography,
are available commercially and can be provided with undu-
lators. The following design is based on the specifications of
one such ~ n d u 1 a t o r . l ~ ~ ' ~
For p = 5 0 nm (i.e., 25 nm zone widths) and A=4.5 nm,
sin O=0.09. Making the approximation sin O=tan.O, we have
Alp=RNlf. Substituting for f in Eq. (2) and solving for RN
we obtain
Thus, for N = 35- I00 a n d p = 50 nm, the zone plate diameter
D is in the range 3.5-10 pm. Substituting Eq. (5) in Eq. (2),
the focal length is given by
f = N ~ ~ / A , (6)
to within 0.2% accuracy. Thus, f is in the range 19-56 pm.
The minimum focal spot size is approximately equal to the
width of the outermost zone, i.e., p/2. We take this to be the
pixel diameter. If, for simplicity, we ignore the space taken
up by the joists in Fig. 1 (in a final design they niay not be
needed) the number of pixels per unit cell is given by
pixels per unit cell= ~ ~ / ( ~ / 2 ) ~ = 1 6 ~ . (7)
Thus, there are < 1.6X 1 o5 pixels per unit cell.
The focal spot of a zone plate will be smeared out beyond
its diffraction-limited value of p / 2 because of the angular
divergence of the source (i.e., nonperfect collimation). By
straightforward geometry one can show that the focal spot is
enlarged by the factor G, given by
G = 1 +(2NpAc$/A), (8)
where Ac$ is the source divergence. Taking the value pro-
vided by a commercial undulator, A c $ = 2 . 8 ~ 1 0 - ~ rad,17
N = 50, p = 50 nm, and h=4.5 nm we find that the focal spot
is smeared out by the factor 1.3, i.e., from about 25 to 33 nm.
V. THROUGHPUT
We first calculate the limit on throughput imposed by the
incident x-ray flux, and then consider the problem of multi-
plexed parallel addressing. Although undulators installed at
existing synchrotron facilities can provide adequate flux19 we
consider instead a type of undulator that could be installed
on a compact synchrotron, suitable for .manufacturing. For
example, a "microundulator" with a period of 14 mm, in-
stalled on the Aurora 2 synchrotron,17 would provide a first-
order peak at 4.5 nm, and a flux of 1 . 9 7 ~ 1 0 ' ~ photonsls, or
87 mW, in a 2% bandwidth, suitable for diffraction-limited
focusing by zone plates of -50 zones.I8
where E is the efficiency of first-order focusing of the zone
plate, and F accounts for loss due to various factors, includ-
ing the fraction of area taken up by the joists and the attenu-
ation of the membrane supporting the zone plate array. This
membrane can also serve as the vacuum window. It would be
made of diamond or other strong carbonaceous material. In
our laboratory we use SiN, membranes, 1.5 p m thick and
spanning a diameter of 20 mm, as vacuum windows.20 We
show below that the overall size of the zone-plate array is
<1 .3x 1.3 mm. Hence, it should be possible to make the
membrane significantly thinner than 1 p m . In order to allow
for future ingenuity in the design of either the array of zone
plates or the source, we assume a generous factor, F=0.9.
As discussed below, we can take ~ = 0 . 3 1 , in which case
P '= 19 mW, which is spread out to fill the zone-plate array.
For a resist with a sensitivity of 19 m!/cm2, a maximum
throughput of 1 cm2/s is predicted.
Actually, at sub-50 nm feature sizes, one should estimate
throughput taking into account the stochastic nature of the
resist exposure process.21 A resist sensitivity of 19 m ~ / c m ~
corresponds to an incident flux of 4.3 photons/nm2, or
-2700 photons per pixel (25x25 nm2). This is a very large
number by the usual lithographic ~ r i t e r i o n . ~ ' For example, if
50% of the incident radiation is absorbed in a 100-nm-thick
resist film, this corresponds to 950 ~ / c m ~ , which is approxi-
mately the sensitivity of PMMA. It is generally understood
that low-sensitivity resists such as PMMA will be required at
sub-50 nm resolution.
One can expect further improvements in undulators and
perhaps alternative sources (some of which are already in the
conceptual stage). Hence, the calculated throughput, which is
already attractive considering the fineness of the features
projected, could be further enhanced. At 1 cm2/s the equiva-
lent "data rate" is 1.6X 10" Hz.
Up to the point where the undulator flux is the limiter, the
throughput T is given by
where d is the pixel diameter, R is the rate at which micro-
mechanical shutters can be switched, and M is the number of
shutters that can be addressed in parallel. Because the shut-
ters would have very small masses, it should be possible to
switch them at rates of several megahertz. If we assume 10
MHz switching and d = 2 5 nm, T=6.3X M cm2/s.
Thus, the throughput would take on the maximum value of 1
cm2/s, set by the undulator flux, if 16 000 zone-plate shutters
are addressed and mult~plexed in parallel (i.e., an array of
126X 126). This appears to be feasible. The area occupied by
16 000 unit cells (~gnoring joist area) is <1.3X1.3 mm2.
Thus, it may be possible to avoid use of joists except on the
perimeter of the zone plate array.
J. Vac. Scl. Technol. B, Vol. 14, No. 6, NovlDec 1996
4321 Henry I. Smith: Zone-plate-array nanolithography
First-order focus efficiency Q k = 4.5 nm.
31% 8 130 nm
0 1M) 200 300 400 500
(a) Thickness (nm)
Zero-order efficiency Q l = 4.5 nm.
. I I , b I I I , . . I I , I I . . , I .
(bl
0 100 200 300 400 500
Thickness (nm)
Frc. 2. (a) Plots o f the fraction o f incident 4.5 nm x radiation that is focused
in the first-order focal 'spot, for zone plates made o f gold and uranium, as a
function o f the thickness o f the absorber. Note the 31% efficiency for a 130
nm thickness o f U. (b) Plob o f the zeroth-order efficiency for gold and U at
4.5 nm wavelength. At 130 nm thickness,.uranium is nearly an ideal phase
shiker.
VI. FABRICATION OF ZONE PLATES
~lectron-beam lithography provides the optimal path to
fabricating zone plates.22 As first demonstrated by Shaver
el ~ 1 . : ~ it is necessary to eliminate various sources of distor-
tion. This can be done most effectively by comparing the
electron-beam scan raster to a distortion free reference grid
made using interferometric lithography.24
Figure 2(a) is a plot of the first-order efficiency of zone
plates made of uranium and gold as a function of the thick-
ness of the absorber. Note that the efficiency of a gold zone
plate never exceeds 10% whereas the efficiency of zone
plates made of uranium reaches a maximum of 3 1% at a
uranium thickness of 130 nm. This is because uranium is a
nearly ideal phase shifter at A=4.5 nm. This is further illus-
trated in Fig. 2(b) in which the zeroth-order (i.e., the radia-
tion that is propagated straight ahead) is attenuated about
98% at 130 nm thickness. Spent uranium is available in large
quantities. Its fluoride UF, is a gas, hence the material can be
reactively etched in fluorocarbon plasmas, presumably at
high resolution and aspect ratios compatible with the 130 nm
thickness. Uranium does have the problem that it is pyro-
phoric, i.e., it will ignite in air; however, techniques for
working with uranium have been developed at various labo-
ratories around the world and this proposed peaceful use
should be entirely welcome.
VII. SUMMARY
We have proposed a system for performing nanolithogra-
phy (i.e., lithography below 100 nm feature sizes) that em-
ploys an array of Fresnel zone plates in conjunction with an
undulator source and micromechanical shutters, all relatively
new technologies, but separately proven. The size of the ar-
ray of zone plates depends on how many shutters can be
addressed in parallel. For the design example presented, i.e.,
25 nm resolution, this number need not exceed 1.6x lo4
since the throughput at that point is limited by the undulator
flux. Such' an array would occupy an area of about 1.3 x 1.3
mm2. Of course, it can be larger than this if necessary to
match the area of an expanded undulator beam, in which
case the shutters would be operated at a rate slower than 10
MHz.
From Eq. (7), and assuming a square array of zone plates,
measuring 1.3X 1.3 mm2, a pixel diameter of 25 nm, and
N = 100, an exposure rate of 1 cm2/s corresponds to a sub-
strate scanning rate of 24 crnts. Laser interferometer control
of the stage should be sufficient at least in the case of a
linear-scan strategy.
The maskless nanolithography system proposed here can
be operated in a He gas environment for temperature homo- .
geneity and control, which is a significant advantage over
E W and charged-particle projection systems which must be
operated in vacuum.
The 4.5 nm photon is probably the ideal particle for nano-
lithography. The difference in energy between the C, emis-
sion line and the binding energy of the K shell electron in
carbon, the predominant species in most resists, is only 7 eV.
Thus, there are no proximity effects with 4.5 nm photons. In
the late 1970s, Flanders demonstrated the replication of 18
nm lines and spaces in PMMA using x rays of 4.5 nm, with
the mask in contact with the resist.25 The absorption in resist
of 4.5 nm photons can be easily increased above that of
PMMA by adding elements other than H or C, i.e., absorp-
tion can be tailored as lithographic considerations require
(e.g., to absorb 50% in 100 nm).
ACKNOWLEDGMENTS
Grateful appreciation is extended to D. Atwood, E. H.
Anderson, F. Cerrina, M. L. Schattenburg, W. Toby, E.
Toyota, and S. D. Hector for help at various stages in the
development of the ideas represented in this proposed mask-
less x-ray projection system. This work was supported by the
Joint Services Electronics Program and the Defense Ad-
vanced Research Projects Agency.
JVST B - Microelectronics and Nanometer Structures
4322 Henry I. Smith: Zone-plate-array nano l i t hography
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J. Vac. Scl . Technol . B, Vol. 14, No. 6, NovlDec 1996