U.S. patent number RE36,113 [Application Number 08/635,565] was granted by the patent office on 1999-02-23 for method for fine-line interferometric lithography.
This patent grant is currently assigned to The University of New Mexico. Invention is credited to Steven R. J. Brueck, An-Shyang Chu, Saleem Zaidi.
United States Patent |
RE36,113 |
Brueck , et al. |
February 23, 1999 |
Method for fine-line interferometric lithography
Abstract
In microelectronic processing, the method of producing complex,
two-dimensional patterns on a photosensitive layer with dimensions
in the extreme submicron range. A photosensitive layer is first
exposed to two beams of coherent radiation to form an image of a
first interference pattern on the surface of the layer. The layer
is subsequently exposed to one or more interference pattern(s) that
differ from the first interference pattern in some way, such as by
varying the incident angle of the beams, the optical intensity, the
periodicity, rotational orientation, translational position, by
using complex amplitude or phase masks in one or both of the
coherent beams, or a combination of the above. Desired regions of
the complex pattern thus produced are isolated with a further
exposure of the photosensitive layer using any conventional
lithography.
Inventors: |
Brueck; Steven R. J.
(Albuquerque, NM), Zaidi; Saleem (Albuquerque, NM), Chu;
An-Shyang (Albuquerque, NM) |
Assignee: |
The University of New Mexico
(Albuquerque, NM)
|
Family
ID: |
25483540 |
Appl.
No.: |
08/635,565 |
Filed: |
April 22, 1996 |
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
945776 |
Sep 16, 1992 |
05415835 |
May 16, 1995 |
|
|
Current U.S.
Class: |
430/311; 430/1;
430/394; 430/397; 430/945; 430/952; 355/53; 355/77; 250/492.1 |
Current CPC
Class: |
G03F
7/704 (20130101); G03F 7/001 (20130101); G03F
7/2022 (20130101); G03F 7/70408 (20130101); Y10S
430/146 (20130101); Y10S 430/153 (20130101) |
Current International
Class: |
G03F
7/20 (20060101); G03F 007/20 () |
Field of
Search: |
;430/1,2,311,394,396,397,945,952 ;250/492.11,492.21
;355/18,53,43,67,77 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Zhou: "Precise Periodicity Control in the Fabrication of
Holographic Gratings" in Appl. Optics, 20(8), Apr. 1, 1981, pp.
1270-1272..
|
Primary Examiner: Duda; Kathleen
Attorney, Agent or Firm: Snell & Wilmer L.L.P.
Government Interests
.Iadd.The U.S. Government has a paid-up license in this invention
and the right in limited circumstances to require the patent owner
to license others on reasonable terms as provided for by the terms
of Contract No. 502-MC-91 with Semantech and Contract No. AFOSR
F490-89-C-0028 with the Air Force Office of Scientific Research.
Claims
We claim:
1. In microelectronic processing, the method of producing a
two-dimensional complex pattern on a photosensitive layer said
pattern containing structures with dimensions in the extreme
submicron range, comprising the steps of:
a) exposing the photosensitive layer for a first time to two beams
of coherent radiation which form an image of a first interference
pattern on the surface of said layer;
b) exposing the photosensitive layer for at least one subsequent
time to two beams of coherent radiation which form an image of at
least one subsequent interference pattern, such that said
subsequent interference pattern or patterns referenced to the
photosensitive layer are each different from the first pattern;
c) isolating desired regions of said complex pattern with a further
exposure of the photosensitive layer using any conventional
lithography.
2. The method of claim 1 wherein the photosensitive layer is
rotated between exposures such that each subsequent interference
pattern differs in rotational orientation relative to said first
interference pattern.
3. The method of claim 1 wherein the photosensitive layer is
translated between exposures such that each subsequent interfere
pattern is offset from said first interference pattern.
4. The method of claim 1 wherein the photosensitive layer is both
rotated and translated between exposures such that each subsequent
interference pattern different from said first interference pattern
in both rotational orientation and in translational position.
5. The method of claim 1 wherein at least one of said beams of the
second or subsequent exposures of the photosensitive layer is
varied in amplitude such that each subsequent interference pattern
differs from said first interference pattern.
6. The method of claim 1 wherein at least one of said beams of the
second or subsequent exposures of the photosensitive layer is
varied in phase such that each subsequent interference pattern
differs from said first interference pattern.
7. The method of claim 1 wherein at least one of said beams of the
second or subsequent exposures of the photosensitive layer is
varied in phase and amplitude such that each subsequent
interference pattern differs from said first interference
pattern.
8. The method of claim 1 wherein the periodicity of the
interference pattern of at least one said second or subsequent
exposures of the photosensitive layer is varied such that each
subsequent interference pattern differs from said first
interference pattern.
9. In microelectronic processing, the method of producing a single
isolated line of extreme submicron dimensions on a photosensitive
layer comprising the steps of:
a) exposing the photosensitive layer for a first time to two beams
of coherent radiation such that an image of an interference pattern
is formed on said layer;
b) isolating a portion of a single line within said interference
pattern by a second exposure of the photosensitive layer using
conventional optical lithography.
10. In microelectronic processing, a method of producing
interdigitated structures on a photosensitive layer, comprising the
steps of:
a) exposing a defined area of the photosensitive layer with a first
interference pattern, having a period p1, said defined area being
bounded by two side edges approximately parallel to the lines of
constant exposure dose and by top and bottom edges approximately
perpendicular to the lines of constant exposure dose;
b) exposing a second defined area containing the top edge of the
first defined area with a second interference pattern of period p2
equal to twice p1 and with lines of constant exposure parallel to
those of the first interference pattern, said second interference
pattern being positioned relative to the first interference pattern
such that every other unexposed region of the first exposure
pattern within the second defined area is exposed;
c) exposing a third defined area containing the bottom edge of the
fist defined area with a third interference pattern of period p2
equal to twice p1 and with lines of constant exposure parallel to
those of the first interference pattern, said third interference
pattern being positioned relative to the first and second
interference patterns such that every other unexposed region of the
first exposure pattern within the third defined area is exposed,
said unexposed regions being connected to unexposed regions
alternate to those exposed in step b.
11. The method of claim 10 wherein the second and third exposures
of steps b and c are replaced by a single second exposure of period
p2 equal to twice p1 and with lines of constant exposure parallel
to those of the first interference pattern, and further, in which
both interfering beams of the second exposure pass through a mask
with two transparent holes that map the second exposure into two
areas at the photosensitive layer containing said top edge and said
bottom edge, respectively, there further being a net phase shift of
1/2 period between the two resulting interference patterns at the
photosensitive layer caused by optical path length differences in
the transparent mask areas, said interference patterns being
disposed to simultaneously expose every other unexposed region of
said first exposure within the illuminated areas. .Iadd.
12. An apparatus for producing a two-dimensional complex pattern on
a photosensitive layer, said pattern containing structures with
dimensions in the extreme submicron range, comprising:
a movable table;
a wafer positioned on said movable table, said wafer having a
surface; and,
a source of coherent radiation which forms subsequent images of
interference patterns on said surface of said wafer, said source
providing at least two beam paths, said radiation having an
amplitude, phase, angle, intensity and
periodicity..Iaddend..Iadd.13. The apparatus of claim 12, wherein
said table communicates with a means for rotation and a means for
translation..Iaddend..Iadd.14. The apparatus of claim 12, wherein
said wafer has a photosensitive layer and a
substrate..Iaddend..Iadd.15. The apparatus of claim 12, wherein
said source has a means for varying the amplitude of said
radiation, a means for varying the phase of said radiation, a means
for varying the angle of said radiation, a means for varying the
optical intensity of said radiation and a means for varying the
periodicity of said interference pattern..Iaddend..Iadd.16. The
apparatus of claim 12, further comprising a means for dividing said
coherent radiation into said beam paths, each of said beam paths
having coherent radiation of essentially equal intensity at said
wafer, thereby assuring a high contrast exposure..Iaddend..Iadd.17.
The apparatus of claim 12, further comprising a phase-amplitude
mask, said mask intercepting at least one of said beam
paths..Iaddend..Iadd.18. A method for producing a two-dimensional
complex pattern on a photosensitive layer, with dimensions in the
extreme submicron range, in a stepwise manner by reducing the field
aperture to increase the source coherence..Iaddend.
Description
This application is a reissue of Ser. No. 07/945,776, now U.S. Pat.
No. 5,415,835..Iaddend.
FIELD OF INVENTION
This invention relates to microelectronic circuits and more
particularly to the use of interferometric patterning in optical
lithography to produce complex, high density integrated circuit
structures.
BACKGROUND OF THE INVENTION
The miniaturization of integrated circuits has been underway ever
since the first demonstration of an integrated circuit Using
dynamic random access memory (DRAM) as a benchmark, current
expectations of device generations, dates of peak production, and
lithography critical dimension are: (4 Mb, 1994, 0.8 .mu.m); (16
Mb, 1997, 0.5 .mu.m); (64 Mb, 1999, 0.35 .mu..mu.); (256 Mb, 2003,
0.25 .mu.m) and (1 Gb, 2006, 0.15 .mu.m) [projections from R. J.
Kopp, Semiconductor International 15, 34-41 (1992)]. In the
industry news section in the same issue of this trade magazine
(page 11), there is a report of a MICROTECH 2000 workshop
cosponsored by the National Advisory Committee on Semiconductors
(NACS) and the Office of Science and Technology Policy (OSTP). The
reported recommendation relative to lithography is: "An
experimental lithography capability that can print features of 0.10
to 0.15 .mu.m will be required by 1994 in sufficient volumes to
allow essential process and manufacturing equipment development
This need may require new electron-beam mask or direct wafer
writing tools, or a capability in advanced X-Ray or phase-shift
optical lithography. Research and development for several
lithography alternatives will have to be supported for the next
several few years to determine what system is best suited for
production."
Imaging optical lithography, in which a mask image is projected
onto a photoresist layer on the wafer, dominates today's
manufacturing. Two equations describing the optical diffraction of
the optical system determine the characteristics of the image. The
minimum resolution, r, is proportional to the lens numerical
aperture, or
and the depth of focus (DOF)
where 1 is the wavelength and NA the lens numerical aperture. These
simple equations point out some of the difficulties in extending
optical lithography to the extreme submicrometer regime, ie., about
0.1 .mu.m or 100 nm. Refractive optics are available only up to
approximately 200 nm at shorter wavelengths almost all materials
become strongly absorptive and unusable. There are several efforts
underway to use reflective optics at short wavelengths. However,
there remain significant materials problems, particularly at X-Ray
wavelengths and the NAs of these systems are significantly lower
than for refractive systems, giving away some of the wavelength
advantage for imaging small areas.
Considerable interest and attention have been given to new X-ray
lenses based on grazing incidence filamentary propagation through
hollow "waveguides." This remains a difficult problem without a
demonstration of a high-efficiency, high numerical aperture,
manufacturable lens with a field-of-view that can accommodate
today's growing field sizes. From the experience of longer
wavelength optical lithography using refractive lenses, the optical
train can easily be the most complex and expensive part of a
lithography tool.
The progression to short wavelengths to improve the minimum
resolution carries a concomitant penalty in the reduction of the
depth-of-focus (DOF). This has motivated efforts at multilayer
resists with strong absorption layers, as well as efforts at
improved planarization of circuits to eliminate topographic
variations that would cause different parts of the circuit to image
at different heights. This small DOF is a major concern for
submicrometer lithography.
Briefly, major issues facing extension of conventional lithography
to the extreme submicron regime (0.1 .mu.m) include: source
technology (issues are uniformity, spectral bandwidth,
repeatability, reliability, etc.); the imaging system (again
refractive optics become impossible below .about.200 nm and
reflective optics have inherently smaller numerical apertures); the
mask technology (there are significant issues related to vibration,
heating and distortion in X-Ray masks which must be fabricated on
pellicle substrates because of the strong X-Ray absorption of most
materials); and the resist technology.
For many years periodic line and space gratings in the extreme
submicron range have been fabricated by use of two interfering
coherent beams. For two beams incident at angles .theta. and
-.theta. to the surface normal, the period of the interference
pattern is .lambda.(2 sin .theta.). For readily available
wavelengths (361-nm Ar-ion laser) and angles
(.theta..about.75.degree. ) this gives a period as small as 187 nm.
The resulting grating pattern is a periodic line and space array;
the critical dimensions of the lines are adjustable using
nonlinearities in the expose and develop processes to roughly 1/3
of this dimension or 60 nm.
SUMMARY OF THE INVENTION
The present invention provides complex, two-dimensional patterns in
integrated circuits through the use of multiple grating exposures
on the same or different photoresist layers and the use of complex
amplitude and phase masks in one or both of the beams of
illuminating coherent radiation. ("Complex, two-dimensional
patterns" as used herein means a pattern of multiple,
interconnected and/or unconnected straight or curved lines or
bodies spaced apart from each other. "Extreme submicron range"
means distances of the order of 0.1 .mu.m or 100 nm or less between
lines.) Interferometric lithography may be combined with
conventional lithography for the production of extreme
submicrometer structures and the flexible interconnect technology
necessary to produce useful structures. Generally, a critical
dimension (CD) of the order of 60 nm with a pitch of 187 nm is
obtainable through the process of the invention. Although with the
use of a KrF excimer laser at 248 nm, a pitch of 124 nm and a CD of
41 nm can be attained. Further extension to a ArF excimer laser at
193 nm will proportionately reduce these dimensions. In general, as
laser technology continues to evolve and results in shorter
wavelength coherent sources, this technique can be adapted to
produce still smaller structures.
In view of the close tolerances involved in producing patterns and
microelectronic integrated circuits in accordance with the present
invention, accurate alignment and position sensing is important. In
that connection, the arrangements shown and described in Brueck et
al, U.S. Pat. No. 4,987,461, and in Brueck et al. U.S. patent
application Ser. No. 07/599,949, filed on Oct. 10, 1990, now U.S.
Pat No. 5,343,292, may be used to particular advantage.
The photoresist exposure process of the invention takes advantage
of the fact that, in terms of dimensional thicknesses of
photoresists (typically 1-2 .mu.m), there are no DOF limitations
for the two interfering coherent optical beams. That is, for two
interfering plane waves, there is no z or depth dependence of the
pattern in the direction bisecting their propagation directions.
For coherent optical beams, the depth of field dependence is set
usually by the shorter of the beam cofocal parameter or, less
usually, the laser coherence length. For larger laser spots the
confocal parameter is many centimeters. For typical cw lasers, i.e.
Ar-ion lasers at 361 nm, the laser coherence length is on the order
of meters. The DOF of interferometric lithography is essentially
unlimited on the micrometer scale of the thin-films employed in
semiconductor manufacturing.
Another feature of the process in accordance with the invention
involves the provision of large dimensions over which a sub-micron
structure may be fabricated. Interferometrically defined gratings
have long been available with dimensions up to 5.times.25 cm.sup.2
or larger, approximately a factor of 10 larger in linear dimension
than the typical field sizes of today's integrated circuits.
Further, this can be achieved at ultraviolet wavelengths for which
photoresist is already well developed.
BRIEF DESCRIPTION OF THE DRAWINGS
Reference is now made to the drawings in which like reference
numerals refer to like parts and in which
FIGS. 1 and 2 are diagrammatic views of alternative versions of
apparatus employed to carry out the process of the invention;
FIG. 3 is a scanning electron microscopic (SEM) view of an exposed
and developed latent image in a photoresist from which patterns may
be formed in a semiconductor wafer;
FIGS. 4-7 are SEM views of different complex two-dimensional
patterns produced from the developed photoresist image in FIG. 3 in
a semiconductor wafer depending on the kind of transfer process
used;
FIGS. 8-14 are SEM views of other complex two-dimensional patterns
fabricated in semiconductive material in accordance with the
invention;
FIG. 15 is a view of a cross section of a phase-amplitude mask in
accordance with an embodiment of the invention;
FIGS. 16-19 are schematic views of exposure stages illustrating the
process of making an interdigitated or interleaved structure in
accordance with an embodiment of the invention;
FIG. 20 is a SEM view of an interdigitated structure produced by
the method outlined, and
FIG. 21 is an illustration of an embodiment of the invention when
used in combination with conventional imaging lithography to
produce a single, isolated line constituting the pattern.
DETAILED DESCRIPTION OF THE INVENTION
Referring to FIGS. 1 and 2, a wafer 11 having a photosensitive
layer 13 and substrate 14 is positioned on a movable table 15. The
table 15 is supported on a shaft 17 and is arranged to be rotated
and translated in two-dimensions respectively via controls 19 and
21 which control mechanical rotational and translational motion
producing motors and linkages generally indicated by the numeral
23. The motors and linkages 23 and controls 19 and 21 need not be
shown in detail since they are well known in the art and may be of
any suitable well known construction.
Coherent optical beams 25 and 27 provided by any suitable well
known source or sources are directed at a variable angle A from the
vertical or system axis 29 toward each other and toward the
photoresist layer 13 to form an interference pattern on the
photosensitive layer 13. The arrangement shown in FIG. 2 is
identical to that of FIG. 1 with the addition of a phase-amplitude
mask 31 in the path of beam 25 or a phase-amplitude mask 33 in the
path of both beams 25 and 27 in their interference region at the
surface of the photosensitive layer 13, or both The beams 25 and 27
of coherent radiation may be lasers and may be provided in any
suitable well known manner so that they are from the same source
and are essentially equal in intensity at the wafer which assures a
high contrast exposure.
In accordance with the invention the complex interference pattern
produced on the photoresist layer or layers is varied by (a)
rotating the wafer, (b) translating the wafer, (c) both rotating
and translating the wafer, (d) changing the angle A, (e) varying
the number of exposures, (f) varying the optical intensity. (g)
using a phase/amplitude mask in one or both illuminating beams of
coherent radiation, or (h) employing any combination of (a)-(g).
Further flexibility is offered by a combination of any of (a)-(g)
with conventional or imaging lithography techniques as are well
known.
As an alternative method, first a single or multiple set of
interferometric exposures are carried out in photosensitive layer.
The subsequent pattern is then developed and transferred to a
semiconductor substrate by any of the well known commercially
available techniques. This substrate is then again recoated with a
photoresistive layer, and single or multiple exposure processes can
be repeated with the aid of the alignment position sensing
arrangement described in Brueck, et al. in U.S. Pat No.
4,987,461.
Referring to FIG. 3, the image depicted is a rectilinear array of
circular dots on the photosensitive layer about 300 nm apart from
each other in the x and y axes. The photoresist layer is developed
and transferred into the Si sample by a plasma-etch process. The
interiors of the circles are etched into the Si. A potentially very
large scale application of structures such as this is in the
fabrication of field-emission flat panel displays which require
large fields (up to large-screen television size or greater) of
submicrometer field emitter tips. This lithography is preferably be
carried out on glass plates which are much less polished than
today's Si wafers.
In accordance with the invention, the image of FIG. 3 is produced
on the photosensitive layer 13 by two exposures of the layer to an
interference pattern produced by the two coherent optical beams 25
and 27 as follows: With a wavelength of 488 nm for each laser beam
25 and 27 and angle A=50 degrees, the photosensitive layer is
subjected to a first exposure with the period of the interference
pattern being 0.3 microns and a second exposure with the same
period and other parameters but with the wafer rotated 90 degrees
about the axis 29. The length of each exposure depends upon the
nature of the photoresist and the optical wavelength and intensity
and, as an illustration, for a photoresist comprising KTI 1350, is
about 60 seconds.
The pattern shown in FIG. 3 may then be transformed into the
grating structure shown in FIGS. 4-7 by any of several well known
processes as follows: FIG. 4--by plasma etching into silicon; FIG.
5--by reactive ion etching into GaAs; FIG. 6--by wet chemical
etching into silicon, and FIG. 7--by ion beam milling into
silicon.
Turning to FIGS. 8-14, the patterns shown therein are produced by
plasma etching from photoresists having complex images thereon
produced by the imaging scheme of the present invention as
indicated in the following table 1:
TABLE 1 ______________________________________ Period Rotation Beam
Angle FIG Exposure (mm) (deg) (deg)
______________________________________ 8 First 1.0 micron 0-deg
14-deg Second 2.0 0-deg 7-deg Third 1.0 90-deg 14-deg Fourth 2.0
90-deg 7-deg 9 First 1.0 micron 0-deg 14-deg Second 1.5 0-deg
9.4-deg Third 1.0 90-deg 14-deg Fourth 1.5 90-deg 9.4-deg 10 First
0.6 micron 0-deg 24-deg Second 0.7 0-deg 20.4-deg Third 0.8 0-deg
17.8-deg Fourth 0.6 90-deg 24-deg Fifth 0.7 90-deg 20.4-deg Sixth
0.8 90-deg 17.8-deg 11 First 0.95 micron 0-deg 15-deg Second 1.0
0-deg 14-deg Third 0.95 90-deg 15-deg Fourth 1.0 85-deg 14-deg 12
First 0.95 0-deg 15-deg Second 1.0 5-deg 14-deg Third 0.95 90-deg
15-deg Fourth 1.0 85-deg 14-deg 13 First 0.95 micron 0-deg 15-deg
Second 1.0 5-deg 14-deg Third 0.95 90-deg 15-deg Fourth 1.0 90-deg
14-deg 14 First 0.95 micron 0-deg 15-deg Second 1.0 0-deg0 14-deg
Third 0.95 90-deg 15-deg Fourth 1.0 90-deg 14-deg
______________________________________
It is clear from the images produced in connection with FIGS. 3
thru 14 that, in accordance with the invention, many other complex
patterns may be produced in the manner described. The examples
discussed in connection with FIGS. 3-14 by no means exhaust the
rich array of possibilities of patterns including those required
for highly repetitive integrated circuit elements such as DRAMs Of
course, for these applications an aperiodic wiring pattern must
ultimately be superimposed on this structure in any suitable
well-known manner. For DRAMS even the interconnection patter is
highly regular since these circuits are usually addressed in a
matrix fashion. Only at the periphery of the DRAM region do highly
aperiodic patterns occur.
It is understood that in accordance with the invention a very wide
range of structures that can be fabricated. The aerial image for
each exposure is simply a sine function:
where the amplitude A, period 2p/q and phase f are set by the
incident optical beams. Nolinearities in the exposure, develop and
etch processes result in a higher-order terms in a Fourier series
expansion at the same period and phase as the original image. That
is:
where S(x) is the resulting pattern on the wafer and the
coefficients A.sub.n are the result of these nonlinear processes.
Most often, the A.sub.n will be a monotonically decreasing function
of n. Finally, with multiple exposures the result for the pattern
is:
This is a two-dimensional Fourier transform, and thus, in
accordance with the invention, any pattern definable by the
transform can be synthesized. As a practical matter, this is
restricted in the range of .vertline.9m.vertline. to 4.pi./2 and,
of course, there is no independent control of each A.sub.nm.
Nevertheless, the transform provides the basic rule giving rise to
the very large variety of patterns that may be realized through
patterning in accordance with the principles of the invention.
Additional flexibility in pattern generation may be introduced
through the use of amplitude and/or phase masks for one or both of
the exposure beams. Phase/amplitude masks may take any desired form
depending on the desired pattern. The mask 41 shown in FIG. 15 has
two thickness-varied (i.e., path length-varied on the scale of the
frequency of the coherent beam radiation), phase modification
sections 43 and 45 and two amplitude or shadow or stenciled
sections 47 and 49. Of course, a mask need only have phase or
amplitude portions or both.
An example of patterning employing a mask is provided in the highly
useful, interleaved or interdigitated structure shown in the
embodiment of the invention of FIGS. 16-19. The end result pattern
shown in FIG. 19 is produced by first exposing a 1-.mu.m pitch
grating over the entire area of the photoresist to produce the
exposed photoresist image pattern shown in FIG. 167. Next, two
sequential exposures are made through a simple shadow mask (e.g., a
mask such as is shown in FIG. 15 with either shadow portion 47 or
49) at twice the pitch (2 .mu.m) over the top and bottom halves of
the wafer as shown in FIGS. 17 and 18.
The wafer is then translated by 1 .mu.m between these two later
exposures so that alternate lines of the original grating are
eliminated above and below the pattern to produce the pattern shown
in FIG. 19.
The structure shown in the SEM of FIG. 20 was fabricated by the
foregoing process. The following Table II shows the steps taken to
produce the image shown in FIG. 20. In this case, the image in the
photosensitive layer 13 is essentially the same as the image
produced by plasma etching, and in producing the image, the wafer
was not rotated about axis 29 and instead was translated and
apertures were located in the position 33 as shown in FIG. 2.
TABLE II ______________________________________ Beam Period
Translation Angle Aperture FIG No. Exp (.mu.m) (.mu.m) (deg.)
location ______________________________________ 20 first 1.0 0.0 14
none second 2.0 0.0 7 top third 2.0 1.0 7 bottom
______________________________________
Such an interdigitated structure with submicrometer spaces of about
100 nm between the fingers has application. for example, as a large
area submicrometer particle detector by fabricating an
interdigitated metal grid structure and monitoring the conductivity
induced by small numbers of particles shorting out the fingers.
Except for the arrangement of the present invention, no other
technique exists that can be used to economically fabricate these
interleaved structures over very large areas with extreme
sub-micrometer dimensions. These structures are also useful for
high-speed optical detectors where the transit times across the
sub-micrometer gap determines the detector speed. Indeed, this
interdigitated structure is commonly used for a wide array of
sensors. The capability provided by interferometric lithography of
the present invention will enhance the functionality of many of
these devices.
Reference is now made to FIG. 21 which shows an embodiment of the
invention used in combination with conventional lithography. In
general, the combining of the interferometric lithography of the
present invention with conventional imaging lithography adds other
possibilities to the structures that may be fabricated. As one
example, FIG. 21 illustrates the fabrication of an isolated line
with a submicrometer critical dimension (CD) using a relatively
coarse pitch (say 1-2 .mu.m) grating structure and isolating a
single line with a box defined by conventional lithography.
Specifically, as shown in the figure, a grating 51 is exposed on
the photosensitive layer using a 1 .mu.m pitch. The next exposure
is made via a mask to provide a 1.5 .mu.m wide box 53 which masks
out the other lines of the grating. The end result is the desired
single line 55 which will result after appropriate fabrication such
as plasma etching.
Single lines have immediate use, for example, as the gate structure
in high-speed field-effect transistors (FET). Commercial devices
currently have gate dimensions of .about.0.25 .mu.m, fabricated by
c-beam lithography. Laboratory research devices have been made with
gates as small as 5 nm using focused ion-beam lithography. Both of
these are serial processes in which each gate must be written
sequentially resulting in low throughput and yield. The present
invention offers the possibility of parallel writing of
submicrometer gates throughout a large field of view circuit or set
of circuits, very much as integrated circuits are conventionally
fabricated. This will result in dramatically reduced manufacturing
cost and improved yield.
* * * * *