U.S. patent application number 12/168310 was filed with the patent office on 2010-01-07 for system and method for projection lithography with immersed image-aligned diffractive element.
This patent application is currently assigned to INTERNATIONAL BUSINESS MACHINES CORPORATION. Invention is credited to Jaione Tirapu Azpiroz, Dario Gil, David O. Melville, Alan E. Rosenbluth, Kehan Tian.
Application Number | 20100003605 12/168310 |
Document ID | / |
Family ID | 41464645 |
Filed Date | 2010-01-07 |
United States Patent
Application |
20100003605 |
Kind Code |
A1 |
Gil; Dario ; et al. |
January 7, 2010 |
SYSTEM AND METHOD FOR PROJECTION LITHOGRAPHY WITH IMMERSED
IMAGE-ALIGNED DIFFRACTIVE ELEMENT
Abstract
A novel system and method and computer program product for
exposing a photoresist film with patterns of finer resolution than
can physically be projected onto the film in an ordinary image
formed at the same wavelength. A hologram structure containing a
set of resolvable spatial frequencies is first formed above the
photoresist film. If necessary the photoresist is then sensitized.
An illuminating wavefront containing a second set of resolvable
spatial frequencies is projected through the hologram, forming a
new set of transmitted spatial frequencies that expose the
photoresist. The transmitted spatial frequencies include sum
frequencies of higher frequency than is present in the hologram or
illuminating wavefront, increasing the resolution of the exposing
pattern. These high spatial frequency transmitted waves can be
evanescent, or they can propagate at a steeper obliquity in a
higher index medium than is possible in a projected image. A
further method is described for designing lithographic masks to
fabricate the hologram and to project the illuminating wavefront.
In other embodiments, a simple personalization based on Talbot
fringes and plasmonic interference is performed.
Inventors: |
Gil; Dario; (Latonah,
NY) ; Melville; David O.; (New York, NY) ;
Rosenbluth; Alan E.; (Yorktown Heights, NY) ; Tian;
Kehan; (Poughkeepsie, NY) ; Azpiroz; Jaione
Tirapu; (Poughkeepsie, NY) |
Correspondence
Address: |
SCULLY, SCOTT, MURPHY & PRESSER, P.C.
400 GARDEN CITY PLAZA, SUITE 300
GARDEN CITY
NY
11530
US
|
Assignee: |
INTERNATIONAL BUSINESS MACHINES
CORPORATION
Armonk
NY
|
Family ID: |
41464645 |
Appl. No.: |
12/168310 |
Filed: |
July 7, 2008 |
Current U.S.
Class: |
430/1 ;
716/55 |
Current CPC
Class: |
G03H 1/0244 20130101;
G03H 1/02 20130101; G03H 1/0402 20130101; G03F 7/70466 20130101;
G03H 2222/47 20130101; G03H 2001/0094 20130101; G03H 1/08 20130101;
G03H 2001/2615 20130101; G03H 2240/56 20130101 |
Class at
Publication: |
430/1 ;
716/21 |
International
Class: |
G03H 1/04 20060101
G03H001/04; G06F 17/50 20060101 G06F017/50 |
Claims
1. A lithographic processing method of exposing a photosensitive
medium formed in a wafer of semiconductor material to form an image
of specified spatial frequency modulation, said method comprising:
forming, atop a semiconductor wafer stack and adjacent to a
photosensitive medium layer, a modulated diffractive hologram
structure having a first spatial frequency modulation in its
profile and refractive indices; and, illuminating the formed
diffractive hologram structure with a wave field having a second
spatial frequency modulation, which combines with the first spatial
frequency modulation to produce a specified spatial frequency
modulation specified for the image.
2. The method according to claim 1, wherein a diffractive hologram
structure includes a grating, wherein a formed image includes
alternating bright and dark regions of a specified spatial
frequency modulation formed in the photosensitive medium layer.
3. The lithographic method as claimed in claim 1, further
comprising: inhibiting said photosensitive medium layer from
exposure during forming of said diffractive hologram structure;
and, switching on sensitivity of said photosensitive medium layer
after forming said diffractive hologram structure.
4. The lithographic method as claimed in claim 1, wherein said
spatial frequency modulation produced by said hologram is a sum of
frequencies of said first and second spatial frequency
modulations.
5. The lithographic method as claimed in claim 1, wherein a second
spatial frequency modulation includes wave fields of evanescent
orders.
6. The lithographic method as claimed in claim 1, further
comprising: printing, using partially coherent projection
lithography, said hologram structure having spatial frequency
content of up to 2*P*NA/.lamda. diffraction orders, wherein P is
the period of the first spatial frequency modulation, NA is the
numerical architecture of the projection lens and .lamda. is the
wavelength of the first spatial frequency modulation used to print
the hologram.
7. The lithographic method as claimed in claim 6, wherein
amplitudes of said diffraction orders is an adjustable variable
when forming said hologram structure.
8. The lithographic method as claimed in claim 6, wherein said
forming of said hologram structure results in a patterned image
exploiting frequency doubling such that an amplitude bandlimit of
said printed image is 4*P*NA/.lamda..
9. The lithographic method as claimed in claim 1, further
including: implementing a patterned reduction mask for modulating
said wave field to form said second spatial frequency
modulation.
10. The lithographic method as claimed in claim 1, wherein said
modulated diffractive hologram structure is formed in a
photosensitive medium layer having an increased or decreased
transmission characteristic under irradiation.
11. The lithographic method as claimed in claim 1, wherein said
modulated diffractive hologram structure is formed in a
semiconductor film using imprint lithography.
12. The lithographic method as claimed in claim 1, wherein said
modulated diffractive hologram structure is formed in an opaque
semiconductor film by etching apertures into said opaque film.
13. The lithographic method as claimed in claim 1, wherein said
modulated diffractive hologram structure is formed in a
photosensitive medium layer by exposing and developing a
conventional photoresist layer.
14. The lithographic method as claimed in claim 1, wherein a
resulting lithography includes image patterns wherein the spatial
frequency content of an image pattern achieves a non-evanescent
spatial frequency of about 1.911/.lamda., where .lamda. is the
exposing wavefront wavelength.
15. The method according to claim 2, wherein a pitch spacing
between an alternating bright and dark region ranges between 50 nm
and the size of the image.
16. A method for designing a mask implemented for fabricating a
hologram structure in a photosensitive layer of a semiconductor
stack and for generating an optimized wavefield for illuminating
said hologram structure to achieve a specified image target
comprising alternating bright and dark pattern region in a
photosensitive material layer of said stack, said method
comprising: selecting a preliminary wave field and hologram
diffractive properties using a scalar model at an artificially
reduced spatial frequency wavelength; implementing wavefront
engineering method to generate a physical structure for said
hologram in a preliminary solution and a 1.sup.st stage wave field
for modulating said hologram; implementing local optimization to
provide an optimized design for said hologram and a wavefront at
the artificially decreased wavelength; incrementing the wavelength
in a small upward increment; refining, at said incremented
wavelength, said local optimization to tune in a solution for the
current incremented wavelength by adjusting variables of a hologram
profile, an index of the hologram features and a film stack beneath
the hologram; and, iterating between steps d)-e) to arrive at a
solution which is valid at an operating wavelength, said solution
including a 2.sup.nd stage wave field for modulating said hologram,
wherein a spatial frequency modulation in a resulting image
includes the same bright and dark regions as the image target; and,
generating a pair of lithographic masks suitable to generate
1.sup.st stage and 2.sup.nd stage wave fields at respective first
and second sets of spatial frequencies using said wavefront
engineering method.
17. The mask design method as claimed in claim 16, further
including: employing an electromagnetic optimizer when iterating
between steps d)-e), said optimizer maximizing lithographic process
window utilization and, maximizing a range of light dose and focus
fluctuations within which the bright and dark polarity patterns in
the developed image match the target patterns to within an
acceptable tolerance.
18. The mask design method as claimed in claim 16, wherein said
iterating between steps d)-e) results in a solution valid at a
specified operating wavelength.
19. The mask design method as claimed in claim 16, wherein said
hologram structure is a parent grating structure, said iterating
between steps d)-e) resulting in a solution having a minimized zero
order diffraction by the parent grating by an interlayer designed
to produce reflections capable of canceling a propagating 0.sup.th
order wavefront.
20. A method of exposing a photosensitive resist layer with an
image including a pattern of specified spatial frequency
modulation, said method comprising: forming a first parent grating
structure over a film stack, said film stack including a
photosensitive layer of material; exposing said first parent
grating structure with a wave field energy at a first modulation
frequency to form a diffractive hologram structure above said
photosensitive material layer; illuminating said formed diffractive
hologram structure with wave field energy at a second modulation
frequency to form an image pattern at said photosensitive material
layer, wherein said formed image pattern exhibits enhanced spatial
frequencies including a sum of first and second modulations
frequencies when the wave field energy at a second modulation
frequency is diffracted by said diffractive hologram structure.
21. The method of claim 20, wherein said hologram and illuminating
wave field are formed by reduction mask projection lithography, and
information in a combined image is bounded by an amplitude spatial
frequency limit at about 1.91/.lamda., where .lamda. is the
exposing wavefront wavelength.
22. The method of claim 21, further including, personalizing said
image pattern formed in said resist layer by said image pattern by
illuminating said diffractive hologram structure via a reduction
mask at said second modulation frequency.
23. The method of claim 21, further including: removing said
diffractive hologram structure; and personalizing said image
pattern formed in said resist layer by said image pattern by
illuminating said diffractive hologram structure via a reduction
mask at said second modulation frequency after said removing.
24. The method of claim 21, further including: using said
diffractive hologram structure to print many different patterns in
said photosensitive material layer by changing the exposing pattern
via a reduction mask at a second modulation frequency.
25. A program storage device readable by a machine, tangibly
embodying a program of instructions executable by the machine to
perform method steps for designing a mask implemented for
fabricating a hologram structure in a photosensitive layer of a
semiconductor stack and for generating an optimized wavefield for
illuminating said hologram structure to achieve a specified image
target comprising alternating bright and dark pattern region in a
photosensitive material layer of said stack, said method steps
comprising: a) selecting a preliminary wave field and hologram
diffractive properties using a scalar model at an artificially
reduced spatial frequency wavelength; implementing wavefront
engineering method to generate a physical structure for said
hologram in a preliminary solution and a 1.sup.st stage wave field
for modulating said hologram; implementing local optimization to
provide an optimized design for said hologram and a wavefront at
the artificially decreased wavelength; incrementing the wavelength
in a small upward increment; refining, at said incremented
wavelength, said local optimization to tune in a solution for the
current incremented wavelength by adjusting variables of a hologram
profile, an index of the hologram features and a film stack beneath
the hologram; and, iterating between steps d)-e) to arrive at a
solution which is valid at an operating wavelength, said solution
including a 2.sup.nd stage wave field for modulating said hologram,
wherein a spatial frequency modulation in a resulting image
includes the same bright and dark regions as the image target; and,
generating a pair of lithographic masks suitable to generate
1.sup.st stage and 2.sup.nd stage wave fields at respective first
and second sets of spatial frequencies using said wavefront
engineering method.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates generally to lithographic
formation of integrated circuit patterns, and more particularly to
a method for generating the spatial frequency modulation of a
lithographic pattern by projecting a light beam that has been
modulated with a set of spatial frequencies through a hologram
modulated by a signal having a second spatial frequency
modulation.
[0003] 2. Description of the Prior Art
[0004] The resolution of a lithographic image is limited by the
wavelength of the light that forms it. Currently, source
wavelengths shorter than .lamda.=193 nm (e.g., as provided by an
ArF excimer light source) are not contemplated for IC manufacture
until the future era of soft x-ray lithography. Fortunately,
wavelength is reduced inside a medium, and a favorable reduction of
as much as 1.8.times. can potentially be obtained for propagating
waves within photoresist films. This corresponds to an upper limit
of 1.8 for the resist refractive index for example. Current resists
typically have refractive indices of around 1.5 to 1.7 when
sensitized.
[0005] Considerably larger reductions in the effective wavelength
are possible in images formed from evanescent waves. However,
evanescent waves can only exist in extremely thin films, and even
the somewhat larger thicknesses that are typically given to
photoresist films (of order 0.1-1.0 microns) are microscopic on the
scale of the lens elements which propagate the light from mask to
wafer. With the possible exception of imprint templates for
specialized applications, practical semiconductor manufacturing
requires that circuit patterns first be fabricated on 4.times.
expanded masks, and then de-magnified to final size using a
projection lens. The macroscopic size of conventional projection
lenses makes them unsuitable for exploiting waves that become
evanescent in any medium between the resist film stack and the
mask.
[0006] In fact, until recently, lithographic projection lenses were
incapable of introducing spatial frequencies into the image that
corresponded to wavelengths shorter than the vacuum wavelength.
This is because the waves had to propagate through an air space
between the last lens element and the wafer film stack (with air
having essentially the same n=1 index as vacuum). A solution has
recently been developed to overcome this limitation, namely filling
the intervening space with water, whose index is about 1.44 at 193
nm. Note that a spatial frequency component at the 1.44 limit would
propagate parallel to the wafer surface in such water immersion
systems, with no projected component of propagation along the
lens-to-wafer axis. In practice image spatial frequencies are
therefore gated by a smaller effective index of about 1.35, since
diffraction-limited transfer from a lens of reasonable diameter to
an image of reasonable field size entails an upper limit on the
propagation angle of order 70.degree.. Because of this optical
design margin, the maximum NA in lithography lenses is limited to
roughly 0.93 times the refractive index of the coupling medium.
[0007] Increasing the refractive index of the immersion fluid is
challenging with a 193 nm source wavelength, because high index
fluids tend to have high absorption in the deep UV. Viscosity,
defects, and evaporatively or absorptively driven thermal
fluctuations also become concerns, as does chemical stability and
compatibility under UV loading. It is not clear whether these
problems can be solved with any 193 nm immersion fluid of index
higher than water. Moreover, the lens elements in 193 nm systems
also face stringent materials requirements, and their refractive
index is currently limited to n=1.56 or below. The optical design
margins noted above show that lens element refractive indices would
gate the NA to a maximum of about 1.45 even if an acceptable high
index immersion liquid were possible. This is because the final
lens element generally cannot have a concave exit surface since the
immersion cavity would then become impractically thick along the
optical axis, greatly stiffening the already difficult-to-surmount
materials requirements that a hypothetical high-index immersion
fluid must meet.
[0008] In fact, it is currently believed that the materials
challenges involved in significantly raising the refractive index
of the final lens element are at least as difficult as those faced
in raising the index of the coupling medium. Achieving the
necessary freedom from minute levels of index in-homogeneity and
uncorrectable birefringence is extremely difficult at 193 nm, given
the element sizes and tolerance levels needed for advanced
lithography lenses. Thus, with existing methods of projection
lithography the spatial frequency content of the projected image
ends up being gated by an effective wavelength of about
.lamda./1.35, potentially limiting the miniaturization of future
semiconductor designs. There are a few known approaches for
addressing this problem. First, the spatial frequency limit can be
improved by numerical factors through techniques like multiple
patterning, or frequency doubling (e.g. by using phase shift masks
or oblique illumination).
[0009] The first class of methods, so-called multiple patterning
methods, are theoretically capable of solving the index-limited
resolution problem of conventional single exposure/patterning
methods. (Multiple patterning methods involve using more than one
mask exposure and pattern transfer to print a single level of the
integrated circuit, instead of the single exposure and patterning
that are traditionally used.) Multiple patterning can avoid the
spatial frequency limit because the frequency content of a printed
chip level can theoretically be stepped down arbitrarily by
carrying out the pattern transfer of successive closely-neighboring
features in separate steps, with each separate image including only
one of the features at a time. However, such processes require that
the patterns transferred in any additional exposure be very narrow
in order to allow subsequently transferred features to fit in the
gaps between them. Unfortunately, even with one dimensional (1D)
layouts, the tight transfer process control needed to print a
narrow feature from a wide image will usually prevent a doubling of
minimum manufacturable resolution from being feasible by this
means--resolution improvements are typically quite a bit less than
2.times.. Even under ideal conditions, achievement of more than a
doubling of resolution would require three or more transfer steps,
which would cause cost to rise very substantially. Note that with
two-dimensional (2D) patterns, a fill doubling of resolution in
every cross-section (e.g. along both the x and y axes) is not even
theoretically possible if only two exposures are used.
[0010] Current lithographic practice exploits frequency doubling to
improve resolution. Frequency doubling is based on the fact that
photoresist responds in an identical manner to the positive and
negative half-cycles of each amplitude harmonic. Since exposing
intensity is the square of the electric field, the spatial
frequency bandlimit in the exposing intensity image is twice that
of the incoming amplitude spatial frequencies. This means, for
example, that one can print a periodic array of patterns with twice
as fine a periodicity by alternately tiling each repeat with
positive or negative phase. It is possible to take advantage of
this enhancement in resolution without insisting that the imaged
pattern take the form of positive and negative replicas in exact
balance; however when such a balance is present the patterns have
no amplitude in the DC order, also known as the zero order, and
classic frequency doubling obtains.
[0011] Frequency doubling can be implemented (when patterns are
compatible) by appropriately applying opposite phase shifts to
neighboring features on the reduction mask; alternatively, a
broadly equivalent effect can be achieved by illuminating the mask
from an oblique set of directions. These techniques have become
widely used, and have significantly improved the resolution
capabilities of semiconductor technology. However, this classic
approach is inherently unable to provide more than a single
doubling of resolution. New methods will be needed further extend
the spatial frequency content of lithographic images. Such methods
should preferably be compatible with the current practice of
exploiting frequency doubling.
[0012] One antecedent projection lithographic technology in the
prior art include: Thin Film Interference Talbot Lithography (also
known as In-Situ Interference Lithography). In Thin-film
interference, e.g., Talbot lithography, employs a so-called
"parent" grating (of lines and spaces) that is fabricated on an
initial layer of, for example, photoresist as shown in FIG. 1.
Interference lithography is a well-known method for fabricating
such a grating. As shown in FIG. 1, when illuminated from above by
a normally incident plane wave, it is known that (in the
conventional Fresnel diffraction regime) such a grating 10 will
produce duplicate images of itself in periodically spaced planes
below the grating lines, and further that denser images having a
two-times finer pitch will be produced in intermediate planes that
lie midway between the planes of the duplicate images. Such
three-dimensional (3D) interference structures are known as Talbot
fringes.
[0013] In a Talbot fringe thin-film interferometry method shown in
FIG. 1A, a parent diffraction grating pattern 10 is formed on a
layer of photoresist 11 using well known techniques, for example by
projecting an image of a master grating, or by imprint lithography,
or by two-beam interferometric lithography. The photoresist 11 is
prevented from being exposed when this grating 10 is formed.
Prevention may be accomplished by using imprint lithography, with
use of a 2.sup.nd wavelength, or, sensitivity may be switched on
via diffusion from a substrate underlayer. The photoresist 11 is
formed over a dielectric layer 12 (e.g., an oxide, nitride or
oxynitride) formed on a Silicon or Silicon-containing material
substrate 15. In the structure shown in FIG. 1, an anti-reflective
coating 13 such as an absorbing film or a graded index film may be
formed as a layer between the dielectric and substrate. Then, using
well known techniques, through flood illumination (e.g., with a
plane wave) of the parent grating 10, a daughter grating 20 is
ultimately patterned as shown in FIG. 1B. If the grating is
properly spaced away from the photosensitive film, line/space
fringes of doubled frequency are produced. Frequency doubling
results from suppression of the zero order for example. In current
implementations, the incident illumination is formed by light at
wavelengths ranging between about 157 nm and 300 nm. The dimensions
of the grating are often of only millimeter scale in experiments
but can be 1 cm or larger. The grating pitch is limited by what
conventional methods can fabricate, which in the case of projection
methods might be in the range of 150 nm.
[0014] Extension of the Talbot concept to dimensions of current
lithographic interest requires operation outside the scalar Fresnel
regime. It has been shown that very small features can be printed
in a sensitive layer beneath a parent grating that has been
designed to diffract the waves into the .+-.1st orders exclusively,
thus eliminating all propagation in the normal incidence direction
(0th order) and so producing an ideal two-beam interference pattern
with half the pitch, as illustrated in FIG. 2. As shown in FIG. 2,
elimination of the zero order creates a frequency-doubled fringe
pattern 30 that is formed from alternated regions of positive and
negative amplitude, as in phase mask lithography. Under ideal
circumstances, the minimum achievable spatial frequency, "P" as
indicated in FIG. 2, with this technique (in configurations where
the standoff is large enough to suppress evanescent waves) is
dictated by the index of refraction of the resist medium, according
to the equation 1) as follows:
1 P min = 2 n resist .lamda. , 1 ) ##EQU00001##
where P.sub.min is the smallest pitch that can be achieved in the
printed line/space pattern. The factor of "2" in equation 1)
represents the gain from frequency doubling, while n.sub.resist
(which may be as large as 1.7 or 1.8, but can potentially be
larger), represents an improvement in resolution beyond the
effective value of 1.35 which gates current projection lithography.
Talbot lithography provides this gain because it does not require
propagation of the Talbot fringes through an immersion liquid;
instead these frequency-doubled fringes are created at the upper
surface of the wafer film stack shown in FIG. 1.
[0015] In the scalar Fresnel regime, suppression of the zero order
simply means matching the areas of the positive and negative
transmission regions of the grating. Restriction of the propagating
orders to .+-.1 will often improve doubled-frequency contrast in
the Fresnel regime. The classical scalar structure with such a
diffraction pattern is a grating whose transmission profile is
given a sinusoidal form, producing a pure intensity sinusoid (plus
DC) at the doubled frequency. However, if the doubled frequency is
printed at a depth below the grating that is sufficient to damp out
evanescent waves; one may employ variant profiles that include
additional non-propagating high frequencies, so long as the
propagating spectrum has the desired two-beam form. If n.sub.resist
is 1.8, the resulting doubled spatial frequency will be
considerably finer than can be achieved with current prior-art
projection lithography. Note that the grating must be fabricated on
the film stack without exposing the imaging resist layer. In the
prior art, this has been accomplished by forming the Talbot fringes
using a different wavelength from that used to print the grating,
so that resists with different spectral sensitivities can be used
in the two steps.
[0016] One drawback to Talbot lithography is that the printed
features are restricted to periodic line/space patterns. Most
useful semiconductor structures involve more complex features.
While there is a significant subset of semiconductor devices whose
design layouts are relatively simple (and moreover such devices may
be of particular importance at the ultra-high resolutions where
difficulties in device scaling make large and complex circuit
layouts more problematic), pure line/space patterns are only of
limited utility.
[0017] A second difficulty arises in fabricating the parent
grating. This must contain structure as fine as half the desired
output Talbot spatial frequency, which for a wavelength of 193 nm
and a photoresist index of refraction n.sub.resist=1.8, must be of
the order of I 100 nm if one wishes to approach the limit allowed
by equation 1. Some of the state-of-the-art techniques described
above can provide such fine resolutions, which can then be reduced
further through Talbot lithography; however such spatial
frequencies for the parent grating fall near the limit of current
lithographic technology, and can prove difficult to manufacture. A
more fundamental difficulty is that future semiconductor
technologies will require printing pitches that are considerably
smaller than the wavelength, and under such circumstances
electromagnetic effects due to interaction of the incident light
with the resist topography produces an undesired increase of the
energy diffracted into the 0th order, hence critically degrading
the double-frequency Talbot image in this regime. It has been shown
that the 0th order tends to increase rapidly as the grating pitch
decreases below 180 nm. For this reason a standard grating
structure will generally be unable to provide the sub-wavelength
frequency-doubled fringe pattern that is needed to achieve a
resolution superior to current projection technology.
[0018] A further antecedent projection lithographic technology in
the prior art include: Thin Film Thin Film Interference Lithography
using Surface Plasmons. In this technique, complex electromagnetic
effects also arise when light is transmitted through grating arrays
(1D or 2D) of pinholes or slits in metallic films, particularly
metals whose dielectric constant has a large negative real part.
The waves that are excited can be understood as evanescent spatial
frequencies, allowing the gratings to be handled with a wave-based
numerical methodology.
[0019] Recently, a contact lithography process was demonstrated
where surface plasmons were launched on a grating array
manufactured out of silver, a plasmonic metal, in such a way that
they could interfere to produce a standing wave that could be used
as an aerial image to expose a thin resist. FIG. 3 shows an example
in the form of a 2D FDTD (Finite Difference Time Domain)
simulation, where a 60 nm layer of silver 40, for example, has been
patterned with 60 nm openings 45, for example, on a 300 nm period.
The silver is then exposed with 436 nm wavelength light. This
causes surface plasmons to be created on the top and bottom
surfaces. On the bottom surface, counter propagating surface
plasmons interfere to present a standing wave 48 that can be used
to expose the underlying resist.
[0020] Thus far only gratings have been proposed for plasmonic
lithography, limiting this technique to very specialized
applications. And, as with Talbot parent gratings for
sub-wavelength spatial frequencies, analysis of the plasmonic
gratings is numerically intensive.
Contrast Enhancement Layers
[0021] A known method for improving contrast in a projected image
of given shape (i.e. composed of a given set of bright and dark
features) is to project the image through a bleachable film that is
placed just above the resist layer which is used to capture the
image. The exposing set of bright and dark features is imprinted
into the bleachable film as a transmission profile, so that the
film transmits more light where bright features pass through it,
and less light where the features are dark. The sidewalls of the
transmitted image are also sharpened. Such a bleachable film is
known as a Contrast Enhancement Layer (CEL). Variant CEL materials
are known that exhibit a thresholded behavior in their bleaching.
CELs thus comprise photosensitive films that can sharpen the
sidewalls of an exposing pattern after being optically imprinted
with an initial pattern. A CEL layer does not change the pitch of
the transmitted image (by design the basic pattern of bright and
dark features is unchanged with CELs).
Solid Immersion Lithography
[0022] It is known in the art that one can avoid the need to
propagate an NA>1 image through an immersion medium by using
so-called solid immersion lithography. Here the flat outside
surface of the final lens element is placed in direct contact with
the resist stack on the wafer. (A lens element with such a flat
surface is referred to as "plano".) Though this contact might well
be accomplished using a high index liquid, the thickness of the
contacting layer is made sufficiently small as to cement the lens
to the film stack, and such microscopic thicknesses in the
cementing layer ease many of the requirements that a high index
coupling medium needs to fulfill in the macroscopic thicknesses
required for conventional immersion operation.
[0023] A significant problem with the solid immersion approach
however, is that large-NA projection systems can only be optically
corrected to the diffraction limit over fields that are quite
small; typically somewhat smaller than a single chip, and far
smaller than a silicon wafer. This means that a relatively small
instantaneous image must be scanned across the wafer (in synchrony
with a scanned mask) in order to expose a fall chip area, and
further that this chip-exposure must be stepped out across the
wafer for later batch processing of the printed chip array.
Unfortunately, a microscopic liquid layer prevents relative motion
between the lens and wafer, and it cannot be rapidly applied or
released.
[0024] Thus, it is the case that lithographic technology is
currently constrained by limits on the spatial frequency content of
projected images. However, reduction mask projection technology
provides a number of advantages, including a well-developed
logistical infrastructure for efficiently manufacturing circuit
structures of prescribed shape, and the ability to exploit phase
tiling to double the spatial frequency limit. Solid immersion
lithography is relatively impractical, and can only be expected to
extend the spatial frequency limit of projection systems by a small
margin since the refractive index of the final lens element that is
contacted to the resist stack is currently limited to n=1.56 or
below. Non-projection systems have various drawbacks, e.g., Talbot
lithography (a/k/a in-situ interference lithography). Talbot
lithography is relatively inflexible in the patterns it can
produce, and it cannot easily provide high contrast frequency
doubling as feature sizes become strongly sub-wavelength, due to
EMF enhancement of the zero order at these dimensions.
[0025] As known, Plano final surfaces are highly desirable in
immersion systems that use a macroscopic coupling fluid, and in
solid immersion systems such surfaces are inherently required. From
a fundamental point of view, the reason that the refractive index
of the final lens element imposes a limit on the resolution of
solid immersion lenses is that the power in the final surface needs
to be zero (i.e., the element must be piano), in order that the
final surface can be optically contacted to the wafer stack. In
lenses with a conventional macroscopic coupling space, it is
possible to include power in a macroscopically flat final surface
by applying a diffractive structure, i.e. by forming a Fresnel lens
of concentric rings in the flat surface. However, such a lens
element (alternately referred to as a diffractive or holographic
element) is not suitable as a final contacted lens surface in a
lithographic system that seeks to overcome the limitations imposed
by the refractive index of the exit space by placing the final
element in close proximity to the wafer. One reason is that a
microscopic offset between the macroscopic Fresnel lens and the
macroscopic image field implies that the Fresnel lens and image
field must have almost exactly the same size. However, the Fresnel
lens increases the NA from, for example, 1.35 to 1.8, and this must
necessarily demagnify the field size in the same ratio. In a
conventional configuration, the holographic element can be spaced
away from the image field and given a larger diameter, but this is
not possible in a solid immersion system where materials
limitations force the high index space between the hologram and
image to be quite thin. A related problem is that the aberrations
in a Fresnel lens of such high power would be impossible to correct
in a telecentric system. A Fresnel lens is given its power by a
radial variation in spacing between diffracting fringes, and when
the Fresnel lens is placed in contact with the image, its
diffracting fringes take the form of a locally varying grating,
whose aberrations vary over the field in a complicated way that
differs from those of the conventional refractive surfaces in the
remainder of the lens that might otherwise be used to correct
it.
[0026] One could contemplate resolving this problem by using purely
holographic imaging, i.e. by creating a customized
(information-rich) hologram of the desired wafer pattern, which due
to the microscopic standoff would essentially take the form of a
moderately defocused waveform of the image. Such a configuration
would involve no projection lens, so difficulties in stepping or
scanning a contacted wafer would not arise. The hologram would
inject its information into the film stack at only a microscopic
distance from the imaging layer.
[0027] The drawback to such a holographic approach is that the
problem of fabricating an ultra-high-resolution image is simply
re-posed as that of fabricating the hologram. For this reason, the
conventional holograms do not offer the desired path to improved
resolution, since the resolution needed to fabricate them is in
general no coarser than the resolution attainable in the diffracted
patterns that they can form.
[0028] It would be highly desirable to provide an improved system
and method for forming lithographic images of integrated circuit
patterns, and more particularly, what is needed is an improved
holographic projection lithographic system for high resolution
patterning at sub-resolution dimensions.
[0029] It would be highly desirable to provide an improved system
and method for forming lithographic images that implements
holographic elements for frequency re-doubling.
SUMMARY OF THE INVENTION
[0030] It is an object of the present invention to provide a system
and method that produces super resolution lithographic images in
substrates that employ immersed diffractive holographic
elements.
[0031] The present invention relates generally to a semiconductor
method that includes forming a diffractive hologram above a
photosensitive medium, e.g., a resist film, and then projecting a
second pattern through the hologram to expose the resist.
[0032] In one embodiment, the resist is inhibited from exposure
when forming the diffractive hologram prior to exposing the
resist.
[0033] In one aspect of the invention, resolution is increased
because sum frequencies are generated when the modulated input beam
is diffracted by hologram.
[0034] In a further aspect of the invention, the same hologram is
used to print many different patterns, simply by changing the
exposing pattern. Thus, many different patterns may result from the
optimized use of spatial frequencies.
[0035] According to a first aspect of the invention, there is
provided a lithographic processing method of exposing a
photosensitive medium formed in a wafer of semiconductor material
to form an image of specified spatial frequency modulation, said
method comprising:
[0036] forming, atop a semiconductor wafer stack and adjacent to a
photosensitive medium layer, a modulated diffractive hologram
structure having a first spatial frequency modulation in its
profile and refractive indices; and,
[0037] illuminating the formed diffractive hologram structure with
a wave field having a second spatial frequency modulation, which
combines with the first spatial frequency modulation to produce a
specified spatial frequency modulation specified for the image.
[0038] Further to this aspect, a diffractive hologram structure
includes a grating, wherein a formed image pattern includes
alternating bright and dark regions of a specified spatial
frequency modulation formed in the photosensitive medium layer.
[0039] Further to this aspect, the lithographic method further
includes: inhibiting the photosensitive medium layer from exposure
during forming of the diffractive hologram structure; and,
switching on sensitivity of the photosensitive medium layer after
forming the diffractive hologram structure. In one embodiment,
avoiding premature exposure of the primary photosensitive layer
includes giving the hologram formation layer a much higher
sensitivity.
[0040] Moreover, further to this aspect, the spatial frequency
modulation produced by the hologram is a sum of frequencies of the
first and second spatial frequency modulations. Moreover, in an
alternate embodiment, the second spatial frequency modulation
includes spatial frequencies of evanescent wave orders.
[0041] In a further embodiment, the hologram structure is printed
using partially coherent projection lithography, having spatial
frequency content of up to 2*P*NA/.lamda. diffraction orders along
each radial direction, wherein P is the period of the first spatial
frequency modulation, NA is the numerical architecture of the
projection lens and .lamda. is the wavelength of the first spatial
frequency modulation used to print the hologram.
[0042] In a further embodiment, the amplitudes of the diffraction
orders is an adjustable variable when forming the hologram
structure.
[0043] In a further embodiment, the forming of the hologram
structure results in a patterned image exploiting frequency
doubling such that an amplitude bandlimit of the printed image is
4*P*NA/.lamda..
[0044] In a further embodiment, a patterned reduction mask for
modulating the wave field is implemented to form the second spatial
frequency modulation.
[0045] In a further embodiment, modulated diffractive hologram
structure is formed in a photosensitive medium layer having an
increased or decreased transmission characteristic under
irradiation.
[0046] In a further embodiment, the modulated diffractive hologram
structure is formed in a semiconductor film using one or more of:
imprint lithography, or by etching apertures into the opaque film,
a photosensitive medium layer by exposing and developing a
conventional photoresist layer.
[0047] In a further embodiment, the amplitude spatial frequency
content of an image pattern achieves a non-evanescent spatial
frequency of about 1.91/.lamda., where .lamda. is the vacuum
wavelength of the exposing wavefront.
[0048] In a further embodiment, a pitch spacing between an
alternating bright and dark region ranges between the size of an
exposed region of a semiconductor material, and one half the
wavelength of the exposing light within a photosensitive material,
or, for example, in the range between 1 cm and 50 nm.
[0049] According to a second aspect of the invention, there is
provided a method for designing a mask implemented for fabricating
a hologram structure in a photosensitive layer of a semiconductor
stack and for generating an optimized wavefield for illuminating
the hologram structure to achieve a specified image target
comprising alternating bright and dark pattern region in a
photosensitive material layer of the stack, the method comprising:
[0050] a) selecting a preliminary wave field and hologram
diffractive properties using a scalar model at an artificially
reduced spatial frequency wavelength; [0051] b) implementing
wavefront engineering method to generate a physical structure for
the hologram in a preliminary solution and a 1.sup.st stage wave
field for modulating the hologram; [0052] c) implementing local
optimization to provide an optimized design for the hologram and a
wavefront at the artificially decreased wavelength; [0053] d)
incrementing the wavelength in a small upward increment; [0054] e)
refining, at the incremented wavelength, the local optimization to
tune in a solution for the current incremented wavelength by
adjusting variables of a hologram profile, an index of the hologram
features and a film stack beneath the hologram; and, [0055] f)
iterating between steps d)-e) to arrive at a solution which is
valid at an operating wavelength, the solution including a 2.sup.nd
stage wave field for modulating the hologram, wherein a spatial
frequency modulation in a resulting image includes the same bright
and dark regions as the image target; and, [0056] g) generating a
pair of lithographic masks suitable to generate 1.sup.st stage and
2.sup.nd stage wave fields at respective first and second sets of
spatial frequencies using the wavefront engineering method.
[0057] In each of these embodiments of the invention, the mask
design method employs an electromagnetic optimizer when iterating
between steps d)-e), the optimizer maximizing lithographic process
window utilization and, maximizing a range of light dose and focus
fluctuations within which the bright and dark polarity patterns in
the developed image match the target patterns to within an
acceptable tolerance.
[0058] In the mask design method, the iterating between steps d)-e)
results in a solution valid at a specified operating
wavelength.
[0059] In a further alternate embodiment, the hologram structure is
a parent grating structure, the iterating between steps d)-e)
resulting in a solution having a minimized zero order diffraction
by the parent grating by an interlayer designed to produce
reflections capable of canceling a propagating 0th order
wavefront.
[0060] According to a further aspect of the invention, there is
provided a method of exposing a photosensitive resist layer with an
image including a pattern of specified spatial frequency
modulation, the method comprising:
[0061] forming a first parent grating structure over a film stack,
the film stack including a photosensitive layer of material;
[0062] exposing the first parent grating structure with a wave
field energy at a first modulation frequency to form a diffractive
hologram structure above the photosensitive material layer;
[0063] illuminating the formed diffractive hologram structure with
wave field energy at a second modulation frequency to form an image
pattern at the photosensitive material layer,
[0064] wherein the formed image pattern exhibits enhanced spatial
frequencies including a sum of first and second modulations
frequencies when the wave field energy at a second modulation
frequency is diffracted by the diffractive hologram structure.
[0065] Further to this aspect of the invention, there is further
comprising a step of personalizing the image pattern formed in the
resist layer by the image pattern by illuminating the diffractive
hologram structure via a reduction mask at the second modulation
frequency.
[0066] Further to this aspect of the invention, there is further
comprising steps of:
[0067] removing the diffractive hologram structure; and
[0068] personalizing the image pattern formed in the resist layer
by the image pattern by illuminating the diffractive hologram
structure via a reduction mask at the second modulation frequency
after the removing.
[0069] Further to this aspect of the invention, there is further
comprising steps of:
[0070] using the diffractive hologram structure to print many
different patterns in the photosensitive material layer by changing
the exposing pattern via a reduction mask at a second modulation
frequency.
[0071] In a further aspect of the invention, all embodiments of the
invention require electromagnetic (E&M) optimization.
BRIEF DESCRIPTION OF THE DRAWINGS
[0072] The objects, features and advantages of the present
invention will become apparent to one skilled in the art, in view
of the following detailed description taken in combination with the
attached drawings, in which:
[0073] FIG. 1 depicts generally, a prior art Thin Film
Interferometry concept wherein Talbot fringes of two-times finer
pitch are used to print a daughter grating from a parent
grating;
[0074] FIG. 2 depicts a prior art Talbot lithographic technique
capable of producing an ideal two-beam interference pattern having
elimination of the zero order to create a frequency-doubled fringe
pattern in a resist;
[0075] FIG. 3 depicts a thin film interference lithographic
technique using Surface Plasmons;
[0076] FIG. 4 depicts an example wafer stack structure employing a
holographic image-aligned element for frequency re-doubling
according to one embodiment of the invention;
[0077] FIG. 5 depicts a procedure for manufacturing the structure
of the present invention;
[0078] FIG. 6 depicts an exemplary stack structure showing the
resulting amplitude transmission of the hologram including a
spatial frequency filter to help engineer a solution;
[0079] FIG. 7 depicts a flowchart showing a procedure that can be
used to design masks for producing the stage 2 exposing wavefront,
and fabricating the hologram;
[0080] FIG. 8 illustrates method steps for forming a Line/space
Talbot grating that represents a simple example of a
quasi-universal hologram.
[0081] FIG. 9 illustrates method steps for fabricating a Line/space
Talbot grating, without personalizing the image (as in FIG. 8);
[0082] FIG. 10 depicts employment of an optimizable filter
resulting in zero order suppression;
[0083] FIG. 11 depicts a hologram structure that has been optimized
in this way to produce Talbot fringes of 60 nm pitch, along with a
calculated final image;
[0084] FIG. 12 depicts an example starting design for
quasi-universal hologram to double the resolution of a projection
lithography system (e.g., a 1-D doubler); and,
[0085] FIG. 13 depicts an example Fresnel regime solution that
creates diffraction orders of uniform amplitude to fill a doubled
NA.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0086] The present invention is directed to a system and method for
producing super resolution lithographic images in substrates that
employs holographic elements for image patterning.
[0087] A hologram structure containing a set of resolvable spatial
frequencies is first formed above the photoresist film. If
necessary the photoresist is then sensitized. An illuminating
wavefront containing a second set of resolvable spatial frequencies
is projected through the hologram, forming a new set of transmitted
spatial frequencies that expose the photoresist
[0088] A key idea of the present invention is to deploy a
conveniently large portion of the information content in the final
image on a reduction mask, rather than encoding it entirely in a
diffractive hologram. This enables exploitation of the extensive
logistical infrastructure that has been developed in the
semiconductor industry for flexibly encoding design information in
reduction masks.
[0089] In an example embodiment of the invention, that advantage is
maximized by, in a 1st stage, patterning the diffractive structure
onto the wafer in a single quasi-universal design which allows many
different high-resolution final patterns to be produced via
projection of different 2nd-stage reduction masks onto the
diffractive structure. (In certain cases, a high reflectance
hologram may instead be positioned below the resist layer, for
single-level device applications that allow such a buried grating
structure in the substrate.) Because the structure formed in the
first stage is diffractive, it serves a different role from
so-called Contrast Enhancement Layers as they cannot print patterns
at the sum spatial frequency that the present invention identifies
between the exposing and imprinted versions of the pattern (for
example, at double the frequency).
[0090] In example embodiments of the present invention, the spatial
frequency information encoded in the 2.sup.nd stage reduction mask
is sufficiently extensive that the remaining spatial frequency
content (which must be deployed in the wafer-mounted diffractive
element) will be strongly bandlimited; sufficiently bandlimited
that the element can be formed using a standard lithographic
projection system (particularly when nonlinear enhancement of the
structure's frequency content occurs during post-expose steps in
the hologram fabrication process). In these embodiments an
ultra-high resolution image is fabricated using two stages of
exposure with images of conventional resolution. In most of these
embodiments there is no need to form intermediate features in
either stage that are dramatically finer than 1/2 the period of the
maximum projected spatial frequency. In this respect, the present
invention offers an advantage over conventional double-exposure
methods for pitch reduction.
[0091] A key enabling technology for all embodiments of the
invention is an accurate electromagnetic solver that can be used in
an efficient way by an optimization algorithm. Such solvers, also
referred to as E&M codes, have become well-known in the field
of semiconductor lithography as means for calculating the effect of
the finite thickness of mask topography on the light transmitted
through the apertures of masks. Most often the E&M codes used
in semiconductor lithography solve Maxwell's equations using either
a finite-difference time domain method, or a coupled wave
algorithm. Optimization algorithms are also well-known in the art,
being referred to by such terms as algorithms for nonlinear
constrained optimization, or algorithms for NLP (Non-Linear
Programming). Software codes incorporating these algorithms are
available commercially. When an optimizer is used with an E&M
solver to adjust structural or electromagnetic inputs to
substantially achieve a specified output, the combined set of
software is referred to herein as an E&M optimizer. Unlike the
electromagnetic effects that arise in reduction masks (which are
physically fabricated at 4.times. the scale of the final image),
the interaction of the incoming field with the wafer-mounted
diffractive structure cannot usually be understood as a simple
perturbation on a classic thin-screen diffraction problem.
[0092] In some cases the nonlinearity involved in developing an
exposed image to form the hologram can be exploited to produce
higher spatial frequencies. Talbot imaging can be carried out in a
way that constitutes a special case of this approach, and in this
respect provides a solution to the problem of achieving adequate
resolution during hologram fabrication, in the special case of a
grating pattern. By exposing an upper imaging layer with a pair of
plane waves that propagate at +NA and -NA (achievable, for example,
by projecting a phase mask), one can form a parent grating of
doubled frequency (since no zero order is present). If the parent
grating in turn yields no significant transmission into its own
zero order upon illumination by a plane wave (after activation of
the second imaging layer), then another frequency doubling will be
achieved in the transmitted Talbot fringes. True superresolution is
thus obtained. Unfortunately, such line and space patterns are only
useful for very specialized devices.
[0093] FIG. 4 depicts an example wafer stack structure 50 employing
the holographic image-aligned element for frequency re-doubling
according to one embodiment of the invention. As will be described
in greater detail herein below, the method for forming the wafer
stack structure employing the holographic image-aligned element
employs a frequency doubling in forming the hologram.
[0094] In one example embodiment of the invention, as shown in FIG.
4, a diffractive element (e.g., hologram) 55 is fabricated atop a
wafer (e.g., silicon) stack to form the integral structure 50. The
present invention accommodates the limited field sizes of
lithography lenses, i.e., the invention can function when the
element is physically affixed to a scanned or stepped wafer. This
means that unlike a conventional diffractive optical element, the
diffractive structure of the present invention does not exhibit
conventional optical power along the lens axis; instead its design
must locally encapsulate information about only the region of the
final designed image that image forming waves can reach. This
region of influence is very small since the image is formed in a
plane that is microscopically offset from the diffractive
structure.
[0095] That is, in a first embodiment, a projection system is used
to illuminate the formed diffractive structure (hologram) 55 with a
wavefront that has been modulated by a patterned reduction mask 60.
The total image then shows enhanced spatial frequency content 70.
Spatial frequency enhancement includes not only the frequency
doubling seen in Talbot fringes 65, and the high frequencies that
are produced in evanescent orders, but also the summed frequencies
70 that are generated by the interaction of the incoming wave with
the diffractive hologram. Such summed frequencies 70 have not
previously been contemplated for use in lithography. Past efforts
to exploit evanescent waves have been limited to short-range
propagation from 1:1 masks, while Talbot-fringe images have been
limited to simple line-and-space patterns. In embodiments that do
not involve evanescent waves, the maximum amplitude spatial
frequency obtained in the image is given by equation 1) provided
above, where n.sub.resist might be, for example, 1.8 but may range
from about 1.5 to 2.0. In some applications, the circuit being
fabricated may only attain minimum pitch along a single direction;
for example, in the "x" or horizontal direction. Such circuits may
be designed using coarser ground rules in the orthogonal "y"
direction. For such fabrication applications, the equation 1) limit
immediately summarizes the resolution capability of the present
invention in this embodiment.
[0096] The present invention further provides the capability of
fabricating other kinds of circuits that achieve the equation 1)
limit in the full 2D spatial frequency domain. If both the hologram
and illuminating wave field are formed by reduction mask projection
that reaches the spatial frequency limit of 1.35/.lamda., the
information in the combined image will then be bounded by an
amplitude spatial frequency limit no larger than about
1.91/.lamda.. Given realistic resist indices, this implies that the
limited resolution of projection technology will enable achieving
the equation 1) limit in the combined image. As another example,
with the hologram and illuminating wave field formed using a
non-immersion "dry" system (coupling index=1) whose frequency
content will be limited to about 0.93/.lamda., the spatial
frequency of the combined image may reach 1.32/.lamda.. That is,
the invention enables one to achieve the projection limit for
immersion systems using only non-immersion tooling. Such a system
can achieve resolutions that approach the equation 1) limit when
full resolution is not needed in both axes.
[0097] Further details regarding the provision of alternate
embodiments for frequency doubling will be discussed in further
detail herein below.
[0098] FIG. 5 depicts a procedure 75 for manufacturing the
structure of the present invention. Referring to FIG. 5, as
depicted at step 80, the hologram element may be formed from any
medium that can be modulated, and which can in turn modulate an
incoming beam (wavefront). For example, the hologram element 55 (of
FIG. 4) may be fabricated in a film whose transmission is altered
under irradiation (either increased or decreased), or it may be
fabricated using imprint lithography, or by etching apertures into
an opaque film, or by exposing and developing a conventional resist
layer, whose developed profile may modulate the transmitted field
in amplitude as well as phase, with the amplitude and phase range
being determined by the imaginary and real parts respectively of
the resist material (which may be modified using additives such as
dyes), as well as by the resist thickness. When the hologram is
formed in a resist layer one may use standard equipment for
semiconductor lithography to expose the hologram pattern, in which
case a suitable material is standard photoresist, in a thickness
comparable to the depth of focus of the lithographic projection
lens, which for current state of the art equipment, is a thickness
of about 50 nm.
[0099] Referring back to FIG. 5, in one embodiment, the hologram
modulation is formed by first exposing a photosensitive medium with
an image (a stage 1 of the full process) as depicted at step 85;
however, it would be desirable that the hologram's modulation be
fixed during a second stage (e.g., stage 2) exposure of the imaging
layer as depicted at step 90. If the hologram is formed in a
conventional photoresist film, the modulation can be fixed by
simply developing the resist. This freezing of the modulation is
not essential; if the hologram modulation does in fact change
during the stage 2 exposure, the effect of this alteration can be
taken into account (and to some extent compensated) during the
design process to be described in further detail below.
[0100] In FIG. 5, as depicted at step 80, the hologram element must
ordinarily be fabricated on the film or film stack including an
image resist layer however, in a manner without fully exposing the
imaging resist layer. If the hologram is fabricated by exposing and
developing a conventional resist film, this may be accomplished,
for example, by using thermal or chemical diffusion (e.g., from an
underlayer) to activate sensitivity in the stage 2 resist film
after the hologram has been printed, or by using imprint
lithography, or by using different wavelengths during the stages 1
and 2 exposures. Alternatively, the stage 2 resist layer can be
given a weaker dose sensitivity than the resist layer in which the
hologram is formed, so that the stage 2 layer remains essentially
unexposed during hologram formation. If hologram fabrication does
give rise to a non-negligible exposure in the stage 2 resist layer,
the effect can be taken into account (and corrected, at least to
some extent) during a design procedure to be described in greater
detail herein below. The films in the film stack may include any
materials used in semiconductor fabrication, so long as the
materials do not undesirably alter the transistors in the
underlying semiconductor wafer. Suitably inert materials include
spin-on polymers and dielectric films. The required film
thicknesses and refractive indices (and thus the specific materials
choices) are determined using an optimizer in conjunction with an
E&M solver. Generally the indices would be in the range of 1.35
to 2.0, and more usually in the range of about 1.5 to 1.8. The
films which are between the hologram and the stage 2 resist layer
must transmit a substantial portion of the stage 2 exposure light.
If the stage 2 exposure is carried out using conventional
semiconductor lithography equipment, it will not be possible to
achieve perfect spatial coherence during this exposure, and as a
result it might be desirable to limit the total thickness of the
stage 2 resist layer and the films which are between this layer and
the hologram to the range of about 100 nm to 200 nm.
[0101] The various alternative methods for controlling sensitivity
during the first and second stage exposures, i.e., preparing the
stage 2 exposure, is depicted as step 90 in FIG. 5. In a further
step 95 shown in FIG. 5, the stage 2 spatial frequency projection
(e.g., of the 2nd-stage illuminating wave field) through the
hologram in a resist layer is performed. The projected image, in
one embodiment, may exhibit frequency doubling. The hologram is
then removed and the resist developed to provide the final pattern
at step 98.
[0102] In each of the embodiments described herein, one can exploit
frequency doubling in either of the two projected images, or in the
final image. Other kinds of fabrication nonlinearities may be
exploited to produce higher spatial frequencies in the hologram
than are present in the image which prints it. For example, sharp
edges in the fabricated hologram may be a source of sharp
(predominantly evanescent) edge fields that are highly
localized.
Correcting Fabrication Compromises in the Hologram by Adjusting the
Stage 2 Wave Field
[0103] Although processing effects can add higher spatial
frequencies to the fabricated hologram, it may be at the expense of
being able to fully control lower spatial frequencies. A degree of
control can be recovered by including a model of the
hologram-formation process in the optimization algorithm that
designs it. In one embodiment, the number of controlling degrees of
freedom in the image that prints the hologram is limited. However,
distortions in the hologram spectrum can partially be compensated
using adjustments in the 2nd-stage illuminating wave field. Such
compensation is necessarily imperfect in most cases because a
larger number of spatial frequencies are usually being controlled
than are present in the illuminating wave field alone. Fortunately,
however, in other embodiments, there may be no need to produce a
specific set of spatial frequency values in the combined image.
Instead, the combined image is printed in a layer of binary resist,
and merely require of the image that it be above threshold in a
sufficiently complete portion of those regions designated to have
bright polarity, and that the image be below threshold in a
sufficiently complete portion of dark-polarity regions; in other
words, it is required that the spatial frequency modulation
produced in the resist have the proper bright and dark regions. It
is noted that, consistent with CD tolerances, a limited shrinkage
in the extent of each bright and dark regions may be allowed to
provide a transition of finite width. In addition, it has become
common even with current lithographic processes to adjust the
designed shape of circuit features to accommodate lithographic
limitations. Finally, it is noted that the non-evanescent spatial
frequency limit of 1.91/.lamda. exceeds that of most resist and
spacer films, meaning that the two optical exposures often have
"extra" spatial frequencies that can be traded off in the design.
Systematic processing distortions may make it infeasible to achieve
a precise target value in every spatial frequency in the fabricated
hologram, but the value of one spatial frequency can be traded off
against another when optimizing against the fabrication model.
[0104] FIG. 6 depicts a spatial frequency filter that can be
engineered to provide a solution. Particularly, the system 100
depicted in FIG. 6 employs a hologram 105 and a spacer layer 108
acting as the filter. The refractive index of the spacer may range
between 1.35 and 2.0. Essentially, any spin-on polymer or
dielectric material can be used, as long as it transmits the stage
2 exposure wavelength. The spacer 108 functions as a filter
enabling removal of spatial frequencies whose amplitudes would be
deleterious to the resulting image 65. These filtered frequencies
can then be given whatever values arise in setting the amplitudes
of the retained lower spatial frequencies to their proper values in
the face of processing distortions. The bandlimit of the retained
frequencies still comfortably exceeds that available from standard
reduction technology.
[0105] Fundamentally, the image-aligned hologram acts as a lens
element, and as such it is likely to exhibit large aberrations
(referring to the degradation imposed on an incident image that
merely needs to be re-imaged to a new conjugate by the hologram).
The hologram would generally be positioned close to the focal plane
of the projection lens that illuminates it; typically within the
lens depth of focus, or a small multiple thereof, which for
state-of-the-art lithographic equipment would mean a position
within about 100 nm of the focal plane. As the hologram is
approximately positioned at the focal plane, the illuminating wave
field is designed to precisely correct the hologram aberrations on
a point-by-point basis. Thus, the image-aligned character of the
hologram is a key factor in the viability of this approach. If the
reduction mask includes features with three (3) different phases,
it is possible to project an arbitrarily phased wave field (with
arbitrary amplitude variation) onto the hologram. This can be used
to correct for the focal shift between the hologram and exposed
image, as well as for the hologram aberrations. Since waves
propagate from the hologram at virtually all angles, the standoff
between the hologram and imaging layer should typically be smaller
than the transverse coherence length of the illuminating wavefront,
in order to avoid blurring the image.
Design of the Hologram and 2nd Stage Illuminating Field
[0106] Detailed simulation is an important element of any viable
procedure for designing the hologram and illuminating wavefield.
The required simulations generally include simulation of the
lithographic process involved in the substep of fabricating the
hologram, and simulation of the diffraction step by which the
extended frequency final image is created. In one embodiment, the
amplitudes of key spatial frequencies in the hologram and stage 2
wavefront as primary problem variables are used. In embodiments
where the hologram is fabricated using optical lithography, these
spatial frequency variables are the amplitudes of the diffraction
orders that are captured by the projection lens during each
exposure (stage 1 and stage 2). Additional structural variables can
be used; for example, the refractive indices and thicknesses of
films between the hologram and resist layer (as well as the indices
of the hologram and resist film themselves). It is also possible to
use other process adjustments as variables (e.g. development
parameters that might be used to tune the hologram profile). The
illumination used in the stage 2 exposure would typically be highly
coherent, but the exposure used to fabricate the hologram (in
embodiments where it is fabricated optically) may be partially
coherent, meaning that a number of (typically adjustable) source
variables would be involved.
[0107] Considering a one dimensional periodic pattern that is
bilaterally symmetric for purposes of illustration, the total
number of independent diffraction orders that can be used to print
the hologram is 2*P*NA/.lamda., where .lamda. is the wavelength, P
is the period, and NA is the numerical aperture of the projection
lens. The amplitudes of these orders constitute one set of
variables that can be adjusted to obtain the desired image. This
expression takes into account the restriction imposed by bilateral
symmetry, but also considers the additional degrees of freedom
represented by orders that can be collected by including off-axis
directions in the source shape. (Lithographic exposure tools
typically limit these directions to the bright-field range
subtended by the NA.)
[0108] The intensity distribution that prints the hologram contains
intensity spatial frequency harmonics out to 2*NA/.lamda. (for a
total harmonic count of 2*P*NA/.lamda.). This intensity spatial
frequency bandlimit is twice the bandlimit of the amplitude
distribution produced by each source point--a consequence of the
fact that intensity is the square of amplitude. However, because
these intensity harmonics (2*P*NA/.lamda. in number) are produced
by spatial frequency doubling, it is not necessarily possible to
adjust all of their magnitudes independently, even though an equal
number of independent variables (the 2*P*NA/.lamda. order
amplitudes) are available in the collected wavefront. However, even
when this possibility is ignored, at a minimum, the intensity
pattern which exposes the hologram has at least P*NA/.lamda.
degrees of freedom that are fully controllable. The superposition
of these amplitudes is referred to as the first (or stage 1)
spatial frequency modulation.
[0109] The connection between this stage 1 exposing pattern and the
resulting amplitude transmission of the hologram depends on the
specific type of hologram that is employed. The exposing pattern is
referred to as the "stage 1a modulation", and the resulting spatial
frequency modulation of the hologram's transmission as "stage 1b
modulation". The exposure-to-transmission relationship may be
highly nonlinear, but this is usually acceptable, as long as the
hologram fabrication is modeled accurately. This is in contrast to
the very specific nonlinearities that are required in double
patterning techniques like so-called double-expose/double-etch
processes, or processes that use CEL in two exposures. In these
techniques it is necessary that the processes involved in
transferring the image into the medium have a strong nonlinearity
of a very specific type, namely one that shifts the effective print
threshold to a level either barely above the intensity minimum, or
just below the intensity maximum. Such a nonlinearity is required
in order that the printed features be biased very small, in one
polarity or the other. These prior-art double-patterning methods
need to produce such a bias in features of one polarity in order
that the separations of opposite polarity be large enough that the
features of the other exposure have room to "squeeze through".
[0110] The present invention avoids the need to create large gaps
for the second stage patterns to pass between; instead, the second
stage patterns are deliberately diffracted through the first stage
patterns that are imprinted in the hologram.
[0111] The stage 2 wavefront (also referred to as the stage 2 [or
second] spatial frequency modulation) is given a fairly high
spatial coherence in order to allow reasonable thickness in the
film stack. For example, to filter evanescent frequencies, a film
stack of about 200 nm thickness may be used; in that case, the
hologram will not be in close focus, so it would be desirable to
restrict the illuminating pupil fill during the stage 2 exposure to
a radius of about 10% of the NA. (The illuminating pupil fill
refers to the range of directions that illuminates a patterned mask
that, in turn, is projected through the hologram during the second
stage exposure. Though the pupil is only sparsely filled when the
stage 2 mask is illuminated, the mask generally spreads this light
to such an extent as to substantially fill the collection pupil of
the projection tens.) Under these conditions the number of degrees
of freedom (order amplitudes) available in the stage 2 exposure is
P*NA/.lamda..
[0112] The illuminating coherence can be reduced if evanescent
waves are used, introducing additional degrees of freedom.
[0113] Thus, the two stages together offer a minimum of
2*P*NA/.lamda. independently adjustable degrees of freedom, which
in a 1-Dimensional example is sufficient to fully control the image
produced by a hypothetical lens having double the NA of the
projection lens actually used. In most cases, additional degrees of
freedom would be available in many forms; from an increased set of
collected diffraction orders during the partially coherent stage 1
exposure, from the detailed source design of the stage 1 exposure,
from non-linearities such as frequency doubling that arise in
forming the hologram, and from adjustments of thicknesses and
refractive indices of the film stack. Moreover, the present
invention makes available the usual lithographic option of
exploiting frequency doubling in the final image when the patterns
are compatible; in such cases the 2*P*NA/.lamda. amplitude
bandlimit of the final image becomes a filly exploited
4*P*NA/.lamda. bandlimit in intensity. As noted above, the two
stage lithographic procedure usually makes many extra degrees of
freedom available for controlling these spatial frequencies. If the
image does not make use of evanescent waves, the film indices can
be used to filter extraneous high frequencies from the image--in
this case, the amplitudes arising in these frequencies are
unimportant. It should be noted that, in addition to frequency
doubling in the final image, frequency doubling may be exploited
during formation of the hologram, enabling resolution extension
into the evanescent regime.
[0114] Design generation generally requires intensive optimization
of the hologram and stage 2 orders together. Typically this
optimization would be local rather than global, due to the
complexity of the problem, though the local optimization can be
supplemented with non-local search steps. Local optimization in
turn requires a starting (preliminary) solution, which is then
refined to a fully satisfactory solution obeying all required
constraints. Such a starting solution can be found for the present
invention when the core optimization algorithm is embedded in a
continuation procedure. This continuation procedure begins the
design in an artificially scalar regime, then transforms the
problem to the ultra-high NA scale of the physical image.
[0115] FIG. 7 illustrates an example methodology 200 which may be
implemented to design masks for producing the stage 2 exposing
wavefront and for fabricating the hologram. The procedure begins by
temporarily assuming an artificially reduced wavelength
(several-fold reduced) at step 203, such as by artificially
initializing k to a lower value, e.g., about 1/5th its true value.
At this fictitious short wavelength the geometries become
sufficiently coarse as to be approximately analyzable in the
Fresnel regime. Furthermore, low spatial frequencies in the
holograms are defined as one set of variables (stage 1b), and the
spatial frequencies in the 2nd exposure as another (stage 2).
Typically each set would contain N=P*NA/.lamda. variables.
[0116] It is understood that materials constants for the various
structures are constrained to limits appropriate to the true
physical wavelength. Then, as depicted at 206, one chooses a
preliminary wave field and hologram diffractive properties (stage
1b modulation) using a simple scalar model. Thus, at this step, up
to 2N diffraction orders of the desired final image are decided. It
is understood that intensity orders may be only approximately
determined, and multiple amplitude choices may be available to
provide it, e.g. frequency doubling. Many options to choose from as
many different images yield valid printed pattern.
[0117] In the Fresnel regime, a suitable physical structure for the
hologram may thus be devised using so-called "wavefront
engineering" algorithms, as have been developed to design reduction
masks having specified diffractive properties such as described in
the reference to A. E. Rosenbluth, et al. entitled "Optimum Mask
and Source Patterns to Print a Given Shape," Journal of
Micro/Nanolithography, MEMS, and MOEMS 1, No.1 (2002), p. 13, the
entire contents and disclosure of which is wholly incorporated by
reference herein. When calculating the image using an artificially
shortened wavelength, the algorithm correspondingly applies an
artificial filter that trims all spatial frequencies from the image
which would not be present if the wavelength assumed its physical
value. In reduction masks, wavefront engineering algorithms allow
binary patterns to be employed in producing gray-level spectra. In
the context of the present invention, restriction to a binary
hologram constitutes a processing distortion. In alternate
embodiments of the invention, the imaging layer is offset from the
imaging layer by a spacer film of thickness sufficient to damp out
evanescent waves. In the Fresnel regime such waves will not be
evanescent, but an optimizer ignores their contribution when
assessing image quality. The fact that many spatial frequencies in
the hologram will not propagate to the imaging layer when
illuminated by a bandlimited field means that the binary character
of the hologram can be maintained.
[0118] However, feature size constraints in the hologram will put
additional burden on the content of the illuminating field, so some
control of the hologram profile may be desirable (i.e. non-binary
hologram). This may be achieved without increasing the spatial
frequency bandlimit of the image that forms the hologram, by, for
example, exposing the hologram in a film or film stack having
structured sensitivity, as is done in dual damascene processes.
[0119] Thus, continuing, simultaneous quadratic equations may be
solved to obtain values for the stage 1 and 2 variables. Example:
With bilaterally symmetric patterns, 2.sup.(N-1) solution classes,
in general. Then, there is generated binary mask and hologram
shapes that contain these frequencies (implementing "Wavefront
engineering".)
[0120] As indicated at step 209, FIG. 7, there is initialized
hologram formation mask transmission values to the hologram
modulation values, which may be binary. Once the preliminary
solutions for the hologram formation mask ("1a values") and
wavefront (modulation stage 1b) have been optimized at the
artificially decreased wavelength, the wavelength is incremented
upwards in a small step. Particularly, as indicated at 211, FIG. 7,
a processing loop is entered in which an E&M optimizer is
employed, as described herein above, to fine tune the solution as
the wavelength is incremented upwards as shown at step 214. That
is, the implemented E&M optimizer is used to adjust collected
mask orders (stage 1a and stage 2 masks), plus the film stack, in
order to maximize suitability of diffracted "image", e.g. to match
target shape. The increase of wavelength .lamda. is slight, each
iteration, and the loop is repeated, until wavelength .lamda.
reaches its true physical value.
[0121] The preliminary solution retains an almost workable
character even at the new wavelength because the wavelength
increment is kept small. This proximity to a local minimum in turn
allows the local optimizer, at step 211, to tune in a solution for
the new wavelength by refinement of the preliminary solution. Note
that the adjusted variables include not only the hologram profile
(which may be multi-stepped), but also the index of the hologram
features and the film stack beneath the hologram. By iterating this
procedure by the loop of steps 211 and 214, a solution is arrived
which is valid at the operating wavelength. Note that during each
iteration, the optimizer need not create a specific set of spatial
frequency amplitudes in the image; instead it is only necessary
that the spatial frequency modulation in the image contain the same
bright and dark regions as the target pattern. The optimizer can,
for example, use maximization of lithographic process window as its
objective, i.e. the optimizer maximizes the range of dose and focus
fluctuations within which the bright and dark polarity patterns in
the developed image match the target patterns to within an
acceptable tolerance.
[0122] At step 215, FIG. 7, after convergence is complete, a pair
of lithographic masks to create the two sets of spatial frequencies
involved in the final solution is then generated using e.g., the
wavefront engineering method described in above-mentioned reference
to A. E. Rosenbluth, et al. entitled "Optimum Mask and Source
Patterns to Print a Given Shape". That is, wavefront engineering
methods are then used to render stage 1a and stage 2 reduction
masks.
Quasi-Universal Holograms to Produce a Range of Images Within a
Restricted Class
[0123] As it is not uncommon to adopt circuit design practices
which impose restrictions on the shape and layout of circuit
features in order to accommodate limitations in lithographic
technology, in the context of the current invention, this kind of
compromise allow use of more universal designs in the hologram. In
some embodiments, the hologram may be manufactured by Talbot or
interference lithography, thereby lowering cost. In certain
embodiments of this kind, the information content of the
illuminating wave field can also be restricted, e.g. to project
only relatively coarse illuminating images onto the hologram. For
example, as shown in FIG. 8, a simple parent grating, i.e., a
line/space hologram 250, can be illuminated with a coarser set of
binary shapes, thereby deploying the printed Talbot-like fine lines
within (lower resolution) bounding boxes of arbitrary shape.
[0124] FIG. 8 illustrates method steps for forming a Line/space
Talbot grating 275 including forming, according to well-known
fabrication techniques, a parent grating structure 250 in a top
layer 250' of a film stack by exposing it to a stage 1 wave field,
as depicted in FIG. 8. The thickness of layer 250' might be in the
range of 25 nm to 100 nm, with 55 nm being a typical value. Any
standard photosensitive material and development process might be
used, with typical refractive indices of around 1.7, for example,
in a high sensitivity formulation in which 20 millijoules are
required to expose one square centimeter of target area. This
hologram structure 250 is formed by exposing a resist layer 250' at
a processing step (a) with a stage 1 wave field (wavefront)
represented as .lamda..sub.1 in FIG. 8. When the hologram is used
to print selective sections of pure line/space Talbot fringes, the
stage 1 wave field may take the form of two interfering plane waves
that are incident at equal and opposite angles. Then, at processing
step (b), FIG. 8, the hologram 250 is selectively exposed in order
(processing step 0) by a stage 2 wave field represented by
.lamda..sub.2 (the 2nd stage exposure to illuminate the hologram).
The stage 2 wave field, which may be formed by standard
lithographic means, causes selected regions of hologram 250 to be
illuminated, while others are left dark. An interlayer 270 may be
used to remove evanescent high frequencies from the pattern that
exposes layer 260. The interlayer might have a thickness in the
range of 0 (i.e. no interlayer) to about 200 nm, a typical
thickness being for example 50 nm, and may have a similar
refractive index to resist layer 250'. The resulting image pattern
is formed in a photosensitive material layer 260, e.g., a resist
pattern, having increased spatial frequency shown formed in a
resist layer 260. A low sensitivity photoresist may be used for
layer 260, for example a photoresist requiring 120 millijoules to
expose one square centimeter of resist area. The refractive index
of layer 260 might typically be about 1.7. The parent grating
structure and inter layer are then removed and the resist is
developed at step (c) to form daughter grating comprising fine-line
Talbot bars 275, in the completed structure shown at (d). That is,
at the next step (c), the parent grating and developed in step (c)
form fine-line Talbot bars 275, e.g., at selected positions at step
(d). The selected positions at which grating bars are formed are
those beneath the portions of hologram 250 that are illuminated. It
is understood that the bars of pattern 275 may be formed only at
selected positions as shown in connection with FIG. 9.
[0125] FIG. 9 illustrates same method steps as implemented in FIG.
8 for fabricating a Line/space Talbot grating 275', however, rather
than personalizing the image formed from a Talbot grating before
removing the hologram (as in step (c) of FIG. 8), the grating 275
is exposed to make a uniform Talbot array 275. The resulting
structure 275 is then trimmed with a patterned exposure after
removing the hologram (fabricated as shown at a processing step (d)
in FIG. 9 implementing a second exposure (a third stage
wavefront)). Thus, in an alternative embodiment, a uniform array of
fine lines 275 (printed with Talbot fringes) may be trimmed using a
second exposure that is made after the parent hologram has been
removed, to produce an array 275' where the fine lines are only
present in selected regions. In this example embodiment, the stage
1 wave field might take the form of two interfering plane waves
that are incident at equal and opposite angles, while the stage 2
wave field may be a normally incident plane wave, and the stage 3
wavefront (wave field) would project exposing light onto those bars
of uniform grating 275 whose positions are not selected for pattern
275'.
[0126] It is understood that these two specialized forms of
2nd-stage illumination (coarse envelope, and post-trim) may be used
together, and further that the hologram need not be restricted to a
line/space parent grating in order to employ these personalization
techniques.
[0127] However, it should also be pointed out that if the
personalization technique of FIG. 9 is used in isolation, the
resulting method constitutes something of an exception to the other
systems described here. The difference is that in the approach
depicted in FIG. 9, the wavefront used to expose the hologram is
the rather trivial one of a null pattern, and the personalizing
information is only applied after the hologram is removed. However,
as mentioned above, the FIG. 9 personalization can also be used in
embodiments where the wavefront exposing the hologram contains a
non-null pattern.
[0128] Another embodiment commensurate in scope with the
embodiments depicted in FIG. 8 and FIG. 9 requires a resist layer
that can retain a latent image (without losing sensitivity) during
processing to remove the hologram. One then forms such a latent
image using a sub-threshold dose, and next superposes on the resist
a second latent image after the hologram has been removed, giving
the combined exposure sufficient magnitude to bring the total image
above threshold for development. The sidewalls of the 2nd-stage
image are arranged to fall along some of the sidewalls of the
initial image, allowing the sidewalls of the total image to have a
steeper slope than in a conventional projected image. (The
steepness would be about midway between that of the 2nd-stage
shape-defining image, and the steeper but rigidly laid out
sidewalls of the 1st-stage Talbot image.)
[0129] It should be understood that even the simplest holograms,
such as a parent grating, will require optimized EMF behavior to
extend resolution beyond current projection limits. As noted above,
even embodiments of the present invention that resemble Talbot
lithography will now require that the zero order be suppressed by a
hologram that has been designed using sophisticated electromagnetic
simulation as a component of the optimization procedure.
Additionally, as mentioned above, optimization may involve changes
in the refractive index of the hologram layers (within allowed
limits), as well as in their profiles. For example, minimization of
the zero order diffracted by the parent grating may involve the
optimized design of interlayers that will produce reflections
capable of canceling the propagating 0th order as illustrated in
FIG. 10. FIG. 10 particularly depicts employment of an optimizable
filter 300 formed to result in zero order suppression by the
cancellation shown by the encircled opposing arrows.
[0130] FIG. 11 shows a hologram structure 350' that has been
optimized in this way to produce Talbot fringes 400 of 60 nm pitch,
along with the calculated final image. As shown in FIG. 11, an
example resulting printed structure 400 is formed out of an example
parent structure 350' having a pitch of about 120 nm, for example,
and the resultant printed pitch 400 is 60 nm, when used with a
resist (n,k) value of (1.73,0), for example.
[0131] A parallel set of embodiments are also possible with
plasmonic lithography; these differ from the previous embodiments
in that the hologram is formed from a plasmonic metal, capable of
generating plasmon polaritons along its exit surface.
[0132] The present invention rather provides a process that uses
plasmonic interference to enhance the resolution of projection
lithography. A plasmonic metal is deposited on a resist layer and
then patterned with a transmission hologram design. The features of
that design are such that when the metal/resist stack is
illuminated with a designed set of spatial frequencies of
appropriate wavelength, counter-propagating surface plasmons are
generated that interfere to produce standing waves which in turn
expose the underlying resist. A thin lift-off layer could be
located below the plasmonic metal and above the resist layer to
facilitate simpler lift-off of the remaining metal, using enhanced
plasmonic coupling to transfer the high frequency plasmonic
modulation.
[0133] In one embodiment, the 2nd stage modulation provides an
adjustable set of relatively coarse envelope boundary shapes that
enclose fine plasmonic fringe areas in the printed image. Here much
of the pattern tuning is carried out by varying the 2nd stage
patterns.
[0134] Another embodiment (non-plasmonic, in general) that aims for
a degree of universality in the hologram is based on period-halving
of periodic patterns that are more complex than those produced with
Talbot imaging (i.e., more complex than simple line/space
gratings). For exemplary purposes, there is considered the simple
case of resolution doubling along only one dimension. If an input
periodic pattern were successfully reproduced at 1/2 scale (as is
the function of this embodiment), the amount of information
contained in the output pattern would not be not fundamentally
changed, since the same set of features would be present in both
the input and output
[0135] FIG. 12 depicts a starting design for quasi-universal
hologram 500 to double the resolution of a projection lithography
system. For purposes of illustration, a 1-D (doubler) structure is
shown; that is, a structure which only doubles the resolution along
the left-to-right axis of the pattern, leaving the resolution along
the front-to-back axis unchanged. In this embodiment, the optical
information content of the image is no larger than that collected
from the object which, in turn, implies that the hologram which
carries out the pattern shrinkage need not contribute
pattern-specific information to the image. With periodic patterns
it is possible to use the stage 2 exposure to provide all of the
doubled-resolution pattern information without shrinking the
exposure field, since this information is contained in the
description of the unit cell, and a periodic hologram can demagnify
the cell by 2.times. without changing the size of the exposure
field if each hologram period spans two image periods, with the
stage 1 exposure having a periodicity matched to that of the
hologram. The spatial bandlimit of the light exiting the hologram
will be twice that of the stage 2 exposure, but this will not
increase the information content of the exit light since all even
diffraction orders in the exit light will have zero amplitude, due
to the two-fold tiling of the half-scale structure within each
hologram period. To design such a quasi-universal hologram, the
continuation method discussed above is modified to consider a suite
of lithographic patterns, which might include all levels needed to
fabricate a particular multilevel array of circuit devices. This
approach requires a starting design in the Fresnel regime. An
example of a starting design for such a universal hologram is shown
schematically in FIG. 12.
[0136] The example structure depicted in FIG. 12 is derived from a
2:1 de-magnifying lenslet that is modulated by a +1/-1 grating 550.
The hologram may be defined using binary or trinary Fresnel-like
lenslets and splitting modulation, rather than lenslets with the
focusing profiles shown. If a sufficient degree of universality is
obtained in the hologram, it may be economical to fabricate it
using imprint lithography. Many circuit applications in this regime
are likely to permit a degree of feature variation at the
half-harmonic, which eases the problem of correcting aberrations in
the lenslet. (The full content of the mask area within the doubled
field of the 2nd stage exposure remains available for control of
aberrations at that scale.)
[0137] In many embodiments of the invention, it may be desirable to
initially define the Fresnel-regime starting design in the
frequency domain. "Wavefront engineering" may be used to design the
reduction mask shapes that create the final optimized set of
frequencies for each stage. Using a 1D example, for purposes of
illustration, if the design period generates N diffraction orders
during each exposure (i.e. in both the 1st stage exposure to form
the hologram, and the 2nd stage exposure to illuminate the
hologram), it is seen that specification of the 2N-1 diffraction
orders of the combined image involves 2N-1 quadratic equations in
2N unknowns. As noted above, it is preferable to define the
requirements of the combined image by means of the relatively
flexible requirements that govern lithographic printing in binary
resists. Attempting to maintain specific values in all 2N-1 image
spatial frequencies would unnecessarily constrain the problem.
However, such a specification is nonetheless useful in defining the
starting design. In general, the resulting 2N-1 quadratic equations
will have 2.sup.(N-1) classes of solutions, each consisting of a
nominally infinite set of feasible amplitudes to employ in the two
exposures. (The reason that each set is infinitely degenerate is
that one can always multiply the amplitudes in one set of spatial
frequencies by an arbitrary constant, so long as the amplitudes in
the other set are multiplied by the reciprocal of the constant.)
This multiplicity of solutions eases the overall design problem,
since a large number of distinct starting solutions are available
for refinement. The system of quadratic equations can be solved by
a well-known homotopy method. (This homotopy is of course different
from the continuation method used to optimize the system for EMF
effects.) In the homotopy to solve the simultaneous equations, all
complicating terms of the polynomials are multiplied by a homotopy
parameter which is initially set to zero. With the complicating
terms zeroed out the polynomial roots are obtained analytically.
Next the parameter is increased in slow steps until it reaches a
value of 1, and in the process all roots are tracked using a local
method (e.g. Newton). Techniques are known for taking root
splitting and degeneracy into account.
[0138] Note that in embodiments where the image is offset from the
hologram, the hologram profile need not actually resemble the
superposed spatial frequencies that are being controlled to
construct tie desired image. For example, the hologram may have a
binary profile whose evanescent high frequency components do not
propagate to the resist layer.
[0139] FIG. 13 shows an example of spatial frequency sets, for both
the hologram (stage 1b, shown as pattern 601), and the stage 2
exposure (shown as pattern 602), which create an Airy disk image
610 equivalent to that from a hypothetical lens having twice the NA
of the projection lens. Particularly, FIG. 13 depicts an example
Fresnel regime solution 600 that creates diffraction orders of
uniform amplitude to fill a doubled NA. In this example, each
exposure uses seven (7) collected diffraction orders. In the
Fresnel regime, the hologram can be defined by a transmission
profile that is assumed shift invariant, and more specifically by
the Fourier transform of this transmission profile. If the designed
frequency content of the hologram includes a zero order, three
diffraction orders in the positive direction, and three in the
negative direction, then the controlled spectrum of the hologram
will include seven (7) diffraction orders. However, if a
symmetrical hologram structure is assumed for simplicity, it will
then include only four (4) independently controlled spatial
frequency orders, since the positive-direction and
negative-direction orders will have equal amplitudes. If hologram
formation and stage 2 exposure are both carried out using the same
projection lithography tool, the stage 2 exposure will likewise
consist of seven (7) diffraction orders, of which four (4) will
have independent values if the stage 2 exposure is likewise assumed
symmetric for purposes of discussion. The image pattern that is
formed when the stage 2 exposure transmits through the hologram
will then be symmetrical, and will contain thirteen (13) controlled
spatial frequencies, namely a zero order, six (6) diffraction
orders in the positive direction, and six (6) matching orders in
the negative direction. As discussed above, the product of the
Fourier transform of the hologram spatial frequencies and the
Fourier transform of the stage 2 spatial frequencies must give the
Fourier transform of the desired image spatial frequencies, and in
this way the Fresnel-domain design problem is reduced to solving a
set of polynomial equations, which may be solved by standard
methods, such as homotopy.
[0140] In the example of FIG. 13, the desired image consists of a
sharp impulse response function equivalent to that which would be
produced by a hypothetical lens having double the numerical
aperture (and therefore double the resolution) of the lens actually
used in the stages 1 and 2 exposures. More specifically, the
hologram and stage 2 exposure produce a transmitted image pattern
consisting of thirteen (13) diffraction orders of equal amplitude,
as would be formed if a uniform wavefront were focused through a
hypothetical lens having twice the numerical aperture of the lens
actually used in the stages 1 and 2 exposures. As noted above, the
Fresnel solutions for the hologram and stage 2 exposures are not
discrete, but instead come in groups each including a continuously
variable degree of freedom that can be tuned through an infinite
set of solutions which all provide the same transmitted field
within a constant of proportionality. In the case of the FIG. 13
example, one finds after algebraically solving the simultaneous
equations that there are twenty (20) such solution families. A
representative member of each family can be obtained by requiring
that equal contributions be made by the extreme spatial frequency
in the hologram and the extreme spatial frequency in the stage 2
exposure when the extreme sum frequency of the image is formed.
[0141] More particularly, FIG. 13 depicts a simple example plot 600
showing the superpositions of the spatial frequencies in the
hologram 601 and the 2nd exposure 602, e.g., in a starting design.
A "stage 1 " pattern is plotted as a sum of these spatial
frequencies, but the hologram may actually include many other
spatial frequencies as well. Initially, in the design process, the
stage 1 wavelength is only set at 1/5th its final physical value.
It is understood that in the starting design, the Fourier transform
of all spatial frequencies represents a transmission function. The
resulting hologram will not actually resemble the plotted sum of
the relevant low frequency components when the wavelength is
initialized to a low value, and in addition complex electromagnetic
interactions may strongly impact the hologram profile as the
wavelength is adjusted to take on its physical value. Thus, while
the example 600 shown in FIG. 13 utilizes an Airy disk @ 2.times.
NA, it is found that each exposure provides 7 collected diffraction
orders; the Image contains 13 diffraction orders, filling twice as
large an NA. In this example, an image is formed directly under
hologram (100 nm-200 nm offset being more typical, particularly
when the hologram uses phase modulation).
[0142] The present invention has been described with reference to
flow diagrams and/or block diagrams of methods, apparatus (systems)
and computer program products according to embodiments of the
invention. It will be understood that each flow and/or block of the
flow diagrams and/or block diagrams, and combinations of flows
and/or blocks in the flow diagrams and/or block diagrams, can be
implemented by computer program instructions. These computer
program instructions may be provided to a processor of a general
purpose computer, special purpose computer, embedded processor or
other programmable data processing apparatus to produce a machine,
such that the instructions, which execute via the processor of the
computer or other programmable data processing apparatus, create
means for implementing the functions specified in the flow diagram
flow or flows and/or block diagram block or blocks.
[0143] These computer program instructions may also be stored in a
computer-readable memory that can direct a computer or other
programmable data processing apparatus to function in a particular
manner, such that the instructions stored in the computer-readable
memory produce an article of manufacture including instruction
means which implement the function specified in the flow diagram
flow or flows and/or block diagram block or blocks.
[0144] The computer program instructions may also be loaded onto a
computer-readable or other programmable data processing apparatus
to cause a series of operational steps to be performed on the
computer or other programmable apparatus to produce a computer
implemented process such that the instructions which execute on the
computer or other programmable apparatus provide steps for
implementing the functions specified in the flow diagram flow or
flows and/or block diagram block or blocks.
[0145] Wile there has been shown and described what is considered
to be preferred embodiments of the invention, it will, of course,
be understood that various modifications and changes in form or
detail could readily be made without departing from the spirit of
the invention. It is therefore intended that the invention be not
limited to the exact forms described and illustrated, but should be
constructed to cover all modifications that may fall within the
scope of the appended claims.
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