U.S. patent number 3,876,883 [Application Number 05/429,438] was granted by the patent office on 1975-04-08 for method and system for focusing and registration in electron beam projection microfabrication.
This patent grant is currently assigned to International Business Machines Corporation. Invention is credited to Alec Nigel Broers, Marcus Barry Heritage.
United States Patent |
3,876,883 |
Broers , et al. |
April 8, 1975 |
Method and system for focusing and registration in electron beam
projection microfabrication
Abstract
A method and system for improved focusing and registration in an
electron beam device including an electron beam source, condenser
lenses, deflection coils, projection lenses, a mask and a target.
The deflection coils are located between second and final condenser
lenses and deflect the focused electron beam onto a projection mark
on the mask and onto a similar registration mark on the target to
provide superimposed images for registration purposes.
Inventors: |
Broers; Alec Nigel (Purdys
Station, NY), Heritage; Marcus Barry (Katonah, NY) |
Assignee: |
International Business Machines
Corporation (Armonk, NY)
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Family
ID: |
27544212 |
Appl.
No.: |
05/429,438 |
Filed: |
December 28, 1973 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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267844 |
Jun 30, 1972 |
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Foreign Application Priority Data
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Mar 28, 1973 [GB] |
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14834/73 |
Jun 4, 1973 [JA] |
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48-62080 |
Jul 6, 1973 [FR] |
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73.21785 |
Jul 23, 1973 [DT] |
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2332091 |
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Current U.S.
Class: |
250/492.1;
250/398; 250/491.1 |
Current CPC
Class: |
H01J
37/1471 (20130101); H01J 37/21 (20130101); H01J
37/3005 (20130101) |
Current International
Class: |
H01J
37/02 (20060101); H01J 37/147 (20060101); H01J
37/21 (20060101); H01J 37/30 (20060101); H01j
037/00 () |
Field of
Search: |
;250/492,491,398,311,396 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lawrence; James W.
Assistant Examiner: Anderson; B. C.
Attorney, Agent or Firm: Jones, II; Graham S. Goodwin; John
J.
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATION
This application is a continuation-in-part of copending patent
application Ser. No. 267,844, filed on June 30, 1972 now abandoned.
Claims
What is claimed is:
1. A method for operating an electron beam wafer exposure system
including a target whose position is adjustable, a source of an
electron beam, a projection mask for said target having a
registration hole therein, means for focusing said beam on said
mask, means for scanning said focused beam across said mask, means
for projecting radiation from said beam in the image form through
said mask onto said target, means for sensing beam radiation
projected onto said target, means for displaying a scanned input
connected to receive the output of said means for sensing for
display, means for providing scanning signals having outputs
connected to drive said means for scanning and said means for
displaying in synchronism, whereby radiation projected onto said
target through said mask is displayed upon said display, said
target having a registration mark thereon, the steps comprising
focusing said beam substantially only upon said registration hole
in said mask, viewing said display, adjusting the position of said
target until the display shows that said beam is focused upon said
hole, and then broadening the focus of said beam to cover
substantially all of said mask.
2. In a method for focusing and registration of an electron beam
projection system including a source of an electron beam, a
projection mask having a unique pattern of registration electron
beam windows therein, an adjustable focal length condenser lens
system located between said electron source and said projection
mask including electron lenses for collecting said electron beam
from said source and directing said beam onto said projection mask,
a target wafer having a unique registration marks on the surface
thereof, a projection lens system located between said projection
mask and said target wafer for collecting electron beams passing
through said projection mask and directing them onto said target
wafer, a deflection system located between said source and said
mask for scanning said electron beam across said mask, said
deflection system cooperating with said condenser lens system to
scan the focus of said electron beam across the plane of said
projection mask at one of said registration windows and said
projection lens system operating to focus the portion of said
electron beam passing through a said window in said mask onto the
surface of said target wafer, and electronic detection means for
sensing the image projected through said mask onto said wafer, the
improvement comprising, focusing said electron beam into a pencil
point beam upon a point upon said projection mask, scanning said
pencil point beam to hit a registration window in said mask,
viewing said detection means to determine whether said electron
beam is directed towards said registration mark on the surface of
said target wafer, and adjusting the position until said beam is
directed towards said mark, and then changing the focus of said
condenser lens system to adjust said beam to flood said projection
mask and to project through all of the windows therein upon said
target wafer.
3. A method for operating an electron beam wafer exposure system
including a target whose position is adjustable, a source of an
electron beam, a projection mask for said target having plural
widely spaced registration holes therein, first means for focusing
said beam on said mask, means for scanning said focused beam across
said mask, means for projecting radiation from said beam in the
image form through said mask onto said target, means for sensing
beam radiation projected onto said target, means for displaying a
scanned input connected to receive the output of said means for
sensing for display, means for providing scanning signals having
outputs connected to drive said means for scanning and said means
for displaying in synchronism, whereby radiation projected onto
said target through said mask is displayed upon said display, said
target having a registration mark thereon, the steps comprising
focusing said beam substantially only upon said registration holes
in said mask, viewing said display, adjusting the scale relative
the position of said beam of said target until the display shows
that said beam is generally focused upon said holes, adjusting the
scale of the projection of said mask to match the spacing of said
registration marks upon said target, adjusting the alignment of the
projection through said holes upon said registration marks and then
broadening the focus of said beam to cover substantially all of
said mask.
4. A method in accordance with claim 3 wherein said first means is
adjusted to focus said beam upon said mask by adjusting the control
of said first means to yield a focused image of the scanned portion
of said mask upon said display, and adjusting said means for
projection to focus said beam passing through said mask upon said
target, prior to adjusting scale and alignment of said
projection.
5. A focusing and registration system for an electron beam
projection system comprising:
a source of an electron beam,
a projection mask having a unique pattern of registration electron
beam windows therein,
a condenser lens system located between said electron source and
said projection mask including electron lenses for collecting said
electron beam from said source and focusing said beam onto a point
on said projection mask,
a target wafer having a unique registration mark on the surface
thereof,
a projection lens system located between said projection mask and
said target wafer for collecting electron beams passing through
said projection mask and directing them onto said target wafer,
a deflection system located between said source and said mask for
scanning said electron beam across said mask, said deflection
system cooperating with said condenser lens system to scan the
focus of said electron beam across the plane of said projection
mask at one of said registration windows and said projection lens
system operating to focus the portion of said electron beam passing
through a registration window in said mask onto the surface of said
target wafer,
and electronic detection means for sensing the image projected
through said mask onto said wafer.
6. A focusing and registration system according to claim 5 wherein
said projection mask has holes therein in a predetermined pattern,
one of said holes being a unique registration hole.
7. A focusing and registration system according to claim 5 further
including a projection aperture located in said projection lens
system and where a first and second pair of deflection elements are
located in said condenser lens system in a plane conjugate to the
plane of said projection aperture through all projection lenses
between said aperture and said mask and at least one condenser
lens.
8. A focusing and registration system according to claim 5 wherein
said detection means is an electron detector responsive to
electrons scattered from the surface of or collected by said target
wafer.
9. A focusing and registration system according to claim 5 wherein
said condenser lens system includes a first magnetic condenser
lens, a second condenser lens and a final condenser lens and
wherein said deflection system is located between said second
condenser lens and said final condenser lens.
10. A focusing and registration system according to claim 5 wherein
said projection lens system includes a first magnetic projection
lens and a final magnetic projection lens, said aperture being
located between said first and final projection lenses and said
projection mask being located between said condenser lens system
and said first projection lens.
11. A focusing and registration system according to claim 5 wherein
said first and second pair of deflection elements are first and
second deflection coils arranged orthogonally with respect to each
other.
12. A focusing and registration system according to claim 6 wherein
said registration hole in said projection mask and said
registration mark on said target wafer have the same geometrical
shapes.
13. A focusing and registration system according to claim 6 wherein
said registration hole in said projection mask and said
registration mark on said target wafer are dissimilar in geometric
shape.
14. A focusing and registration system according to claim 8 wherein
said electron detection means includes an electron detector
responsive to electrons from the surface of said target wafer and a
video display means connected to said electron detector for
visually displaying the projected image of said registration window
in said projection mask on said target wafer and the image of said
registration mark on the surface of said target wafer.
15. A focusing system for an electron beam projection system
comprising:
a source of an electron beam,
a projection mask having a unique pattern of windows therein,
a condenser lens system located between said electron source and
said projection mask including an electron lens for collecting said
electron beam from said source and focusing said beam onto a point
on said projection mask,
a target,
a projection lens system including at least one lens and an
aperture located between said projection mask and said target for
collecting the portion of said electron beam passing through said
projection mask and directing it onto said target,
a deflection system for scanning said electron beam across said
mask, including deflection elements located between said source and
said projection mask, said deflection system cooperating with said
condenser lens system to scan the focus of said electron beam
across the plane of said projection mask across at least one of
said windows and said projection lens system operating to focus
said electron beam passing through said window in said mask onto a
point on the surface of said target.
16. A focusing and registration system for an electron beam
electrical circuit manufacturing system comprising:
a source of an electron beam,
a projection mask having a unique registration pattern of windows
therein,
a condenser lens system located between said electron source and
said projection mask including electron lenses for collecting said
electron beam from said source and focusing said beam onto a point
on said projection mask,
a target electrical circuit material,
a projection lens system including at least one lens and an
aperture located between said projection mask and said target wafer
for collecting said electron beam passing through said projection
mask and directing said beam onto said target material,
a deflection system for scanning said electron beam across said
mask located in said condenser lens system in a plane conjugate to
the plane of said aperture through all projection lenses between
said aperture and said mask and at least one condenser lens, said
deflection system cooperating with said condenser lens system to
focus and scan said electron beam in the plane of said projection
mask across at least one of said registration windows and operating
with said projection lens system to focus the portion of said
electron beam passing through said window in said mask onto a point
on the surface of said target material.
17. An electron beam wafer exposure system including,
a target,
a source of an electron beam,
a projection mask for said target,
means for focusing said beam on said mask,
means for scanning said focused beam across said mask,
means for projecting radiation from said beam in the image formed
by said mask onto said target,
means for sensing beam radiation projected onto said target,
means for displaying a scanned input having a radiation input
connected to receive the output of said means for sensing for
display and having position inputs for receiving scanning
signals,
means for providing scanning signals having outputs connected to
drive said means for scanning and said position inputs of said
means for displaying in synchronism,
whereby radiation projected onto said target through said mask is
displayed upon said display.
18. In a method for focusing and registration of an electron beam
projection system including a source of an electron beam, a
projection mask having a unique pattern of registration electron
beam windows therein, an adjustable focal length condenser lens
system located between said electron source and said projection
mask including electron lenses for collecting said electron beam
from said source and directing said beam onto said projection mask,
a target wafer having a unique registration mark on the surface
thereof, a projection lens system located between said projection
mask and said target wafer for collecting electron beams passing
through said projection mask and directing them onto said target
wafer, a deflection system located between said source and said
mask for scanning said electron beam across said mask, said
deflection system cooperating with said condenser lens system to
scan the focus of said electron beam across the plane of said
projection mask at one of said registration windows and said
projection lens system operating to focus the portion of said
electron beam passing through a said window in said mask onto the
surface of said target wafer, and electronic detection means for
sensing the image projected through said mask onto said wafer, the
improvement comprising, focusing said electron beam into a pencil
point beam upon a point upon said projection mask, scanning said
pencil point beam to hit a registration window in said mask,
viewing said detection means to determine whether said electron
beam is directed towards said registration mark on the surface of
said target wafer, and adjusting the deflection of the beam until
said beam is directed through said window towards said mark,
adjusting power to said condenser lens system to focus said mask as
presented upon said display;
adjusting power to said projection lens system to focus the image
of said target wafer upon said display; adjusting the scale of the
projection of said mask shown upon said display to match the
spacing of the images of said registration windows with
corresponding registration marks upon said target as shown upon
said display; adjusting the alignment of the projection through of
said windows upon said marks; and then changing the focus of said
condenser lens system to adjust said beam to flood said projection
mask and to project through all of the windows therein upon said
target wafer.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to electron optical projection
systems for microfabrication and methods of focusing and
registration therefor, and use thereof.
2. Prior Art
Electron beams such as used in electron beam tubes have been
focused by lenses and deflected by magnetic fields in the prior
art. A typical example of such a system is described in U.S. Pat.
No. 2,991,361 issued July 4, 1961 to Karl-Heinz Herrmann.
Another example of focusing an electron beam is set forth in U.S.
Pat. 3,319,110, Electron Focus Projection & Scanning System,
issued May 9, 1967 to Kurt Schlesinger.
A distinction of the present invention over the prior art is the
provision of a registration hole in a mask and a registration mark
on a target which is used in combination with a deflection coil for
providing radiation focused to form an image of the hole upon the
registration mark on the target and employing such registration in
an electron beam projection system to prepare for exposure of the
entire target.
U.S. Pat. No. 3,118,050 issued to J. S. Hetherington Jan. 14, 1964
shows an electron beam system with a microfabrication target and
without a beam deflection system. The source of electrons passes
through a single variable focus condenser or collimating type of
magnetic lens. The lens floods the beam at once over all of the
area of a projection mask having fiducial notches in the edge
thereof through which rays of the electron beam may pass. The
condenser type lens has an adjustable D.C. supply connected to
opposite ends of the coil, which apparently can be adjusted to
collimate the electrons passing through the lens. A focusing system
including a pair of magnetic lenses is located between the mask and
the work piece holder. The focusing lenses are also connected to
adjustable D.C. supplies. The holder includes spaces for
workpieces, and around the periphery of such spaces are fiducial
notches each containing a terminal insulated from the holder. A
balanced pair comparator circuit is employed to help to register
the holder, while the holder is moved by adjusting of micrometer
screws, until the balanced pair indicates proper positioning. The
only means for viewing the position of the work holder and work is
through a binocular microscope. The balanced pair and the binocular
microscope are totally independent means for measuring the position
of the holder via the balanced pair and the work as well, via the
optical microscope.
No prior art has been found however, which deals with the problem
of locating the specific orientation of the work in the holder
relative to a mask to be used. In addition an optical microscope
type of sensor does not provide sufficient magnification for the
small kinds of microcircuits being developed in electronics today.
Also, any radiation projected by Hetherington floods the entire
surface of the workpiece causing radiation exposure of the entire
surface of the workpiece when the workpiece has not yet been
registered. This is unacceptable in cases where the radiation must
be shielded from the workpiece until after it has been registered
in the proper position. Furthermore, Hetherington does not indicate
how the radiation is to be applied during registration other than
to say the beam is directed at the work and that the beam varies up
to a maximum voltage, a maximum pulse rate and a maximum current,
and that intensity can be varied. No suggestion is made as to how
the beam can be prevented from performing work upon the workpiece
during holder registration. Thus, Hetherington registers only the
holder and thus fails to register the work itself, and apparently
exposes the workpiece to harmful radiation, prior to alignment.
In addition none of the prior art suggests scanning a pencil beam
focused upon a mask with windows through the mask onto a
workpiece.
SUMMARY OF THE INVENTION
An object of the present invention is an improvement to an electron
optical projection system wherein a projection pattern is focused
and accurately registered on an unexposed wafer.
Another object of the present invention is to provide an electron
beam projection system including a set of deflection coils located
proximate to condenser lenses to focus and deflect the electron
beam onto registration marks.
The foregoing and other objects, features, and advantages of the
present invention will be apparent from the following more
particular description of a preferred embodiment of the invention
as illustrated in the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows in a schematic manner the imaging portion of an
electron beam column for optical projection and control circuits
therefore in accordance with the principles of the present
invention.
FIG. 2 is an illustration of superimposed registration images for
the cases where the marks and thereby their images are similar and
dissimilar.
FIG. 3 is a plan view of a mask for an electron beam projection
microfabrication system.
FIG. 4 is a video display of a semiconductor wafer target
registration mark with a shadow of a registration grid projected by
the electron beam from a mask, with both images out of focus.
FIG. 5 is a view similar to FIG. 4 with the shadow grid in
focus.
FIG. 6 is a view similar to FIG. 5 with the wafer also in
focus.
FIG. 7 (A-D) is a set of video displays of projections of four
registration windows in a mask upon a single wafer where excessive
scale causes a mismatch of registration.
FIG. 8 (A-D) is a set of video displays of the marks in FIG. 7
(A-D) after the scale has been readjusted, by reducing
magnification in the projection system.
FIG. 9 (A-D) is a set of video displays of the marks in FIG. 8
(A-D) after registration has been realigned.
DESCRIPTION OF A PREFERRED EMBODIMENT
In electron projection optics, as in light optics, the function of
the condenser lens system is to illuminate the projection mask and
then collect all the electron beams passing through the mask and
focus them into the entrance pupil of the projection lens
system.
Because of the extreme tolerances required in the fabrication of
microcircuits by electron beam projection optics, it is very
important that the electron image of the mask be focused and that
the projection mask and the target wafer be registered with respect
to each other. In the past, electron beam tube projection systems
have been used in the fabrication of microcircuits on semiconductor
wafers in a manner similar to the way light optics in microcircuit
cameras are used to photographically reproduce circuit patterns
contained on light masks onto semiconductor wafers.
Some examples of the use of electron beam projection technology in
the fabrication of microcircuits are described in the publication
"Electron Optical Microminiaturization of Stencils" by H, Koops, G.
Mollenstedt and R. Speidel, Optik 28 (5) pgs. 518-531 (1968/69) and
"an Electron Imaging System For the Fabrication of Integrated
Circuits" by T. W. O'Keeffe, J. Vine and R. M. Handy, Solid State
Electronics, Pergamon Press 1969, Vol. 12, pp. 841-848. These
publications discuss the use of electron beam optics in
microminiaturization and are hereby incorporated by reference.
Accordingly, the fundamentals of electron beam tube operation
including electron beam sources, magnetic lenses, focusing,
deflection systems and the like are presumed to be known to those
of described skill in the art and will not be explained or describe
in unnecessary detail in the present disclosure.
An embodiment of the present invention is shown schematically in
FIG. 1. The structure of FIG. 1 is capable of illuminating a
suitable projection mask with electrons and imaging the mask onto a
semiconductor target wafer to fabricate a microcircuit in a manner
described in the prior art. In FIG. 1, however, an additional mode
of operation is shown and will be described wherein the electron
beam optics can be operated in a scanning and in a preliminary
focusing and registration mode, as well.
Referring to FIG. 1, an electron beam tube microcircuit fabrication
structure 24 is shown in the focusing and registration mode, also
referred to as the probe mode. The structure 24 evacuated to about
10.sup.-.sup.6 Torr includes a conventional electron beam gun 28 of
about 20KV which produces beams of electrons, for example, from a
tungsten cathode. The electron beam is directed through blanking
electrodes 25, 26 aperture 27 a first magnetic condenser lens 30
powered by adjustable constant current power supply 70 and a second
magnetic condenser lens 32 powered by adjustable constant current
power supply 71 which lenses focus the electron beam. In the
present invention, prior to the final condenser lens 38 powered by
adjustable constant current power supply 72, a set of orthogonal X
deflection coils 34 and Y deflection coils 36 are positioned in a
plane which is an image of aperture 44 through lenses 38 and 42.
During registration rather than have the electron beam impinge the
entire projection mask 40, the condenser lens system is readjusted
to provide a pinpoint focus upon mask 40. The deflection coils 34
and 36 are energized by waveforms adapted to cause the electron
beam to be directed through the final condenser lens 38 in such a
direction that the electron beam is focused at a specific location
on projection mask 40 at which there is located a unique
registration pattern in the form of a hole 41 of a selected
configuration.
In light beam projection systems for microcircuit fabrication, the
projection mask is usually a transparent substrate on which the
desired circuit is graphically layed out using light opaque
material. In electron beam projection systems, the projection mask
such as 40 in FIG. 1 and FIG. 3 with circuit apertures 39 and
registration mark apertures 41 can be analogous to light optics.
Here the mask is preferably a photolithographically manufactured
very thin (0.2 mil thick) self-supporting, electro-formed grid, or
pattern of copper gold or nickel which forms the electron opaque
sections where desired. The grid is formed on a substrate and then
lifted off it to produce the self-supporting foil. The electron
beam is to pass through the openings in the mask and impinge on the
target wafer thereby exposing the upper surface of the target
wafer, which may be silicon or silicon oxide coated with an
electron sensitive resist, with the desired circuit pattern. Yet
again, it may be a cathode mask such as described in the
aforementioned reference of O'Keeffe et al. where now the mask
itself is the source of electrons for the projection mode of
operation. The mask 40 is rotatably mounted to be turned by worm
gear 78.
In circuit fabrication, it is important that the projection mask be
properly registered and aligned with the target wafer. In the
embodiment of the present invention illustrated in FIg. 1, the
deflection coils (or plates) 34 and 36 are provided to deflect the
electron beam over the mask 40 and wafer 48 when a deflection
current (or voltage) is applied to the coils. The value of the
deflection current is a function of the type of deflection coils
employed, the geometry of the projection system and the design of
the magnetic lenses. This will vary from system to system and the
deflection current in a given system can be determined by one
having ordinary skill in electron beam technology. In the general
case, including projection systems without a physical aperture, one
or two sets of orthogonal coils may be positioned above, within or
after the lens 38 previous to the mask such that the direction of
the principal ray of the spot focused at the mask plane is such
that the ray passes through the center of the effective entrance
pupil of the projection optics.
In FIG. 1, the lens 38 previous to the mask focuses the electron
beam at the plane of the mask. The focused spot, which is smaller
than any dimension of the registration mark is scanned by the coils
34 and 36 over a registration hole configuration 41 in the mask 40.
This may be any of the three types of mask described except that
when the cathode mask is used, the accelerating electrostatic
potential of the electron gun 28 is the same as the electrostatic
potential of the cathode mask. The focused beam that passes through
the registration hole 41 then passes on through the projection
optics, which consists of the projection lens 42 powered by
constant current power supply 73, aperture 44 shifting deflection
coils 79 and 80 associated with aperture 44 powered by variable
supplies 74, 75 for shifting the focus point transversely a small
amount and final projection lens 46 powered by variable supply 76,
and onto the target wafer 48 where another registration mark is
located. The projection mask 40 and target wafer 48 are in
alignment when the images of the registration marks on the wafer
surface are superimposed with the shadow images 41 in FIG. 2 of the
registration mark mask openings 41.
The aforesaid is accomplished in the following manner. A signal,
which may take the form of backscattered electrons detected by the
electron detector 50, is amplified and displayed by a video display
52 scanned in synchronism with the deflection coils 34 and 36. The
scanning system includes X generator 55 and Y generator 54, which
respectively drive X and Y amplitude and variable offset control
units 56 and 58 having X and Y control knobs 62 and 64 respectively
for amplitude which are shown mechanically ganged by line 93 for
display magnification control and X and Y offset control knobs 63
and 65 respectively. Outputs are provided from X and Y units 56 and
58 to X and Y current amplifiers 57 and 59 connected to coils 34
and 36 respectively. Separate outputs from control units 56 and 58
are connected to the scanning input circuits of video display 52.
Electron detector 50 comprises an electron scintillator receiving
electrons 66 and a light pipe connecting the scintillator to a
photo multiplier 51 which drives video amplifier 53 to control the
intensity of video display 52. Knobs 62, 63, 64 and 65 make it
possible for the beam to be adjusted to scan certain portions of
the large mask 40 shown in cross section by reducing the amplitude,
or the entire mask 40 and all apertures therein with larger
amplitude control settings. The zero offset can be used to adjust
the beam location for work in any given small area when a small
amplitude signal is applied. In general, the signal may be detected
by any of the methods known to those skilled in the art of scanning
electron microscopy. Assuming that the surface of target wafer 48
is coincident with the projection image plane, video display 52
will produce two superimposed images -- one of the surface of
target wafer 48 with the registration mark and any other surface
feature perfectly focused. The second image will be a shadow image
of mask 40 due to the chopping of the electron beams in the mask
plane by the mask itself. It is to be noted that because the
projection optics shown in FIG. 1 remain the same in both the
projection mode and the focusing mode as shown, the correspondence
between the two superimposed video images is the same as between
the mask and its projected image except that dimensions appearing
the same in both video images are in fact related by the projection
magnification.
Alignment of mask 40 to target wafer 48 is achieved with reference
to FIG. 2 by using registration patterns 41 or 41A in mask 40 and
shaped marks 43 on target wafer 48 which may, but need not be
similar in shape to the holes in the mask. FIG. 2 shows examples of
a similar hole pattern 41' and mark 43 and a dissimilar hole
pattern 141' and mark 143, both examples being shown superimposed
to indicate mask and wafer alignment. Registration of mask and
wafer is complete when the video output shows the images of both
marks superimposed as shown in FIG. 2 for all registration points
on the object. The ultimate limit on the accuracy of registration
is determined by the size of the electron beam probe in the image
plane, which itself is only limited by the edge resolution in the
projection optics. Any displacement in the mask from its
appropriate conjugate plane results in a defocusing of the mask
shadow image. Similarly, displacement of the wafer results in a
defocusing of the wafer image. Thus, the wafer 48 and the mask 40
can be focused one to the other as appropriate and also the two can
be accurately registered.
Focusing and Registration Procedure
To achieve correct focusing of the beam upon the mask and the wafer
(workpiece) as well as correct registration, three separate
conditions must be satisfied. (1) The mask and wafer planes must be
conjugate with respect to projection lenses 42 and 46 (i.e. the
mask and wafer are in focus on the video display because the
scanning beam is in focus as it reaches each of those planes.) (2)
The demagnification of the projection system must be maintained to
a very high degree of accuracy. (3) The mask and wafer must be
aligned in both X and Y translations and rotation.
Satisfaction of the above conditions is obtained by the procedures
outlined below. FIGS. 4-9 (D) show video displays with a mesh grid
alignment mask and a cross marked on the substrate in various
stages of adjustment. The image of the mesh grid is superposed upon
the image of the cross on the wafer plane.
First Condition Step: Focusing
To satisfy the first condition above, the objective is to correct
the defocussing.
Step (1a) is to adjust the current to condenser coil 38 until the
mask grid image 95 in FIG. 4 is in focus as in FIG. 5.
Step (1b) is to adjust current in projection coil 46 to focus the
larger image of the wafer marking 96 as in FIG. 6.
Demagnification Adjustment
Now the mask and wafer planes are both in focus. However, the
projection demagnification if inaccurate must be adjusted. If it is
inaccurate, then the result will be similar to that shown in FIG.
7, (A-D) where the mask size projected onto the wafer is too small.
While mask projection 41A' is aligned with its wafer mark, wafer
marks 96B and 96D are both too high relative to the mask
projections and wafer marks 96C and 96D are too far to the right of
their corresponding mask projections. Thus, it is obvious that the
mask demagnification must be changed to achieve similar relative
positioning of all marks. Thus, the next step is to adjust current
to the demagnification coils 42 and 46 from power supply units 73
and 76, so that the grid image remains focussed but the display
shows similar relative positioning of all marks. Since the mark
projections 41A',41B',41C' and 41D' are widely spaced on the
projection onto the wafer the scale can be adjusted to an extremely
high order of accuracy. Of course, the scan of the coils 34 and 36
must look at each one of those registration patterns alone without
scanning the intermediate areas. A problem has arisen as shown in
FIG. 8 (A-D) in the course of demagnification, since marks 96A and
41A' are no longer aligned correctly because as the whole pattern
shrank, for example, 96A moved up away from 96D while 96D moved
towards 96A and a correct position.
Alignment
The alignment of the mask image and wafer can be adjusted in X, Y
and rotation. The rotation is handled mechanically either by
turning worm gear 78 attached to the support for mask 40 or by worm
91 attached to turn the table 92 supporting wafer 48.
While X, Y alignment is originally adjusted by knobs on micrometer
drives 82, 83 to move intermediate support tables 84, 85, now
deflection coils 79, 80 can be supplied a slightly different
current by power supplies 74, 75 respectively to align the grids
with the crosses properly as shown in FIG. 9 (A-D). Note that the
above steps may be required to be performed in a cyclical sequence
of iterations of correction to achieve proper alignment.
Exposure
After the alignment step, the work is in position ready for
exposure of the work through the mask and now the electron beam can
expose the work piece in the desired kind of a way by flooding or
scanning as desired through the mask. Note, that the registration
marks 43 on the wafer are made initially, with an extra thickness
layer if required to prevent subsequent operations such as etching
from removing them, as for example where photo resist is being
developed to provide windows for etching, which windows would
expose the marks for etching also. See Hatzakis U.S. Pat. No.
3,519,788.
An added feature of the invention is that the probe mode of
operation as shown in FIG. 1 can be used to determine if any
aberrations exist in the projection optics. When optimum focusing
of both the shadow image of the mask opening and the wafer surface
has been achieved, the relative difference in definition of the
wafer surface features and the shadow image across the field of
view gives an indication of the defocusing effect of coma,
astigmatism and field curvature in the projection optics. Similarly
comparing the distortion of the shadow image, if any exists, with
that of the surface features of the wafer, can yield the projection
distortion coefficient.
When the X or Y generator 54, 55 is retracing, a signal on line 97
or 98 respectively operates blanking amplifier 90 via line 99 to
operate blanking electrodes to deflect the electron beam away from
the aperture in aperture 27. Amplifier 90 can be operated manually
also for timing of exposures in projection mode.
Variation in the configuration of the projection optics using
magnetic lenses is possible and the structure is not necessarily
limited to that shown in the drawings. For example, the final
condenser lens and the first projection lens can be merged into one
single field lens with the mask situated in the center of the
focusing field. Under these circumstances, it is necessary to
introduce an additional lens between the second condenser lens and
the field lens in order to focus the probe at the mask plane.
Except for this difference, all other operations in the probe mode
are similar to those described. In general, the required size of
the intermediate source image (after second condenser lens 32) in
the projection and the probe modes will be different. Thus, it is
expected that the changes in strength of the first and second
condenser lenses will be required when switching form one mode to
the other. Again, the changes in lens strengths are within the
skill of workers in electron beam technology.
While the invention has been particularly shown and described with
reference to a preferred embodiment, it will be understood by those
skilled in the art that the foregoing and other changes in form and
details may be made therein without departing form the spirit and
scope of the invention.
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