U.S. patent application number 10/815312 was filed with the patent office on 2005-10-06 for photomask features with chromeless nonprinting phase shifting window.
This patent application is currently assigned to Matrix Semiconductor, Inc.. Invention is credited to Chen, Yung-Tin.
Application Number | 20050221200 10/815312 |
Document ID | / |
Family ID | 35054733 |
Filed Date | 2005-10-06 |
United States Patent
Application |
20050221200 |
Kind Code |
A1 |
Chen, Yung-Tin |
October 6, 2005 |
Photomask features with chromeless nonprinting phase shifting
window
Abstract
Aspects of the present invention provide for a novel photomask
for patterning features for an integrated circuit, the photomask
including a first area transmitting light in a first phase
surrounded by second area, the second area transmitting light in a
second phase, the second phase opposite the first phase. No
blocking material separates the first area from the second area.
After development of photoresist, the transition between the first
and second area causes formation of a residual photoresist feature
on the photoresist surface due to phase canceling of light. If the
first area is small enough, it is nonprinting, ie., the opposite
sides of the residual photoresist feature formed at its perimeter
merge, forming a contiguous photoresist feature, and thus a
corresponding patterned feature after etch.
Inventors: |
Chen, Yung-Tin; (Santa
Clara, CA) |
Correspondence
Address: |
MATRIX SEMICONDUCTOR, INC.
3230 SCOTT BOULEVARD
SANTA CLARA
CA
95054
US
|
Assignee: |
Matrix Semiconductor, Inc.
Santa Clara
CA
|
Family ID: |
35054733 |
Appl. No.: |
10/815312 |
Filed: |
April 1, 2004 |
Current U.S.
Class: |
430/5 ;
716/53 |
Current CPC
Class: |
G03F 1/36 20130101; G03F
1/34 20130101 |
Class at
Publication: |
430/005 ;
716/019; 716/021 |
International
Class: |
G06F 017/50; G03F
009/00 |
Claims
What is claimed is:
1. A phase shifting photomask comprising: a plurality of
transmitting nonprinting windows transmitting light in a first
phase; a transmitting area transmitting light in a second phase,
each transmitting window substantially entirely surrounded by and
in contact with the transmitting area with no blocking material
intervening, wherein the second phase is substantially opposite the
first phase, and wherein a first width of unbroken transmitting
area surrounds each transmitting window on all sides, the first
width sufficient for the unbroken transmitting area to print when
the photomask is used to expose photoresist.
2. The photomask of claim 1 wherein the first phase is about 180
degrees and the second phase is about 0 degrees.
3. The photomask of claim 2 wherein the shortest dimension of any
of the plurality of nonprinting transmitting windows parallel to
the plane of the photomask is no more than about 160
nm.times.S.
4. The photomask of claim 3 wherein the shortest dimension of any
of the plurality of nonprinting transmitting windows parallel to
the plane of the photomask is no more than about 120
nm.times.S.
5. The photomask of claim 3 wherein at least one of the plurality
of transmitting windows is rectangular.
6. The photomask of claim 5 wherein all of the transmitting windows
are rectangular.
7. The photomask of claim 3 wherein the transmitting windows are
uniformly spaced.
8. The photomask of claim 2 wherein the first width of unbroken
transmitting area is at least 100 nm.times.S.
9. The photomask of claim 3 wherein the first phase is about 0
degrees and the second phase is about 180 degrees.
10. The photomask of claim 9 wherein the shortest dimension of any
of the plurality of nonprinting transmitting windows in the plane
of the photomask is no more than about 160 nm.times.S.
11. The photomask of claim 10 wherein the shortest dimension of any
of the plurality of nonprinting transmitting windows in the plane
of the photomask is no more than about 120 nm.times.S.
12. The photomask of claim 9 wherein at least one of the plurality
of transmitting windows is rectangular.
13. The photomask of claim 12 wherein all of the transmitting
windows are rectangular.
14. The photomask of claim 9 wherein the transmitting windows are
uniformly spaced.
15. The photomask of claim 9 wherein the first width of unbroken
transmitting area is at least 100 nm.times.S.
16. A phase shifting photomask comprising: a transmitting
nonprinting window transmitting light in a first phase; and a
transmitting area substantially entirely surrounding and in contact
with the transmitting window on all sides with no blocking material
intervening, wherein the transmitting area transmits light in a
second phase, the second phase substantially opposite the first
phase, and wherein, when used to pattern photoresist, the
transmitting area is printing on all sides of the transmitting
window.
17. The photomask of claim 16 wherein the first phase is about zero
degrees and the second phase is about 180 degrees.
18. The photomask of claim 17 wherein a shortest dimension of the
first nonprinting transmitting window parallel to the plane of the
photomask is no more than about 160 nm.times.S.
19. The photomask of claim 17 wherein a shortest dimension of the
nonprinting transmitting window parallel to the plane of the
photomask is no more than about 120 nm.times.S.
20. The photomask of claim 17 wherein a width of the transmitting
area on all sides of the transmitting window is at least 100
nm.times.S.
21. The photomask of claim 16 wherein the first phase is about 180
degrees and the second phase is about zero degrees.
22. The photomask of claim 21 wherein the shortest dimension of the
nonprinting transmitting window parallel to the plane of the
photomask is no more than about 160 nm.times.S.
23. The photomask of claim 21 wherein the shortest dimension of the
nonprinting transmitting window parallel to the plane of the
photomask is no more than about 120 nm.times.S.
24. The photomask of claim 21 wherein a width of the transmitting
area on all sides of the transmitting window is at least 100
nm.times.S.
25. A phase shifting photomask comprising: a plurality of spatially
separate transmitting nonprinting windows transmitting light in a
first phase; and a transmitting area transmitting light in a second
phase, the second phase substantially opposite the first, the
transmitting area entirely surrounding and in contact with each of
the transmitting windows of the first plurality; wherein each
transmitting window is separated from its nearest neighbor in the
plurality by an unbroken length of transmitting area having at
least a first dimension, and wherein the smallest dimension of each
window is no more than about 160 percent of the first
dimension.
26. The photomask of claim 25 wherein the first phase is about zero
degrees and the second phase is about 180 degrees.
27. The photomask of claim 26 wherein the first dimension is at
least about 100 nm.times.S.
28. The photomask of claim 27 wherein the smallest dimension of
each window is less than about 160 nm.times.S.
29. The photomask of claim 26 wherein the plurality of transmitting
nonprinting windows are arranged in a grid pattern.
30. The photomask of claim 25 wherein the first phase is about 180
degrees and the second phase is about zero degrees.
31. The photomask of claim 30 wherein the first dimension is at
least about 100 nm.times.S.
32. The photomask of claim 31 wherein the smallest dimension of
each window is less than about 160 nm.times.S.
33. The photomask of claim 30 wherein the plurality of transmitting
nonprinting windows are arranged in a grid pattern.
34. A phase shifting photomask comprising: a transmitting
nonprinting window having a first shifting degree; a second
transmitting area having a second shifting degree, the second
transmitting area entirely surrounding and in contact with the
first transmitting window, wherein the second transmitting area is
printing on all sides of the transmitting window; and wherein the
second shifting degree is substantially opposite the first shifting
degree.
35. The photomask of claim 34 wherein the first shifting degree is
about zero degrees and the second shifting degree is about 180
degrees.
36. The photomask of claim 35 wherein the shortest dimension of the
second transmitting area in the plane of the photomask is no more
than about 160 nm.times.S.
37. The photomask of claim 36 wherein the shortest dimension of the
second transmitting area in the plane of the photomask is no more
than about 120 nm.times.S.
38. The photomask of claim 22 wherein the second transmitting area
has a rectangular shape.
39. The photomask of claim 34 wherein the first shifting degree is
about 180 degrees and the second shifting degree is about zero
degrees.
40. The photomask of claim 39 wherein the shortest dimension of the
second transmitting area in the plane of the photomask is no more
than about 160 nm.times.S.
41. The photomask of claim 40 wherein the shortest dimension of the
second transmitting area in the plane of the photomask is no more
than about 120 nm.times.S.
42. The photomask of claim 39 wherein the second transmitting area
has a rectangular shape.
43. A method for forming a patterned feature on a wafer surface,
the method comprising: transmitting light through a phase shifting
photomask onto photoresist covering the wafer surface; forming an
isolated first residual photoresist feature between a first wafer
area exposed to light in a first phase and a second wafer area
exposed to light in a second phase, wherein the first phase is
substantially opposite the second phase, and wherein the second
wafer area entirely surrounds the first wafer area in the plane of
the wafer; and forming the patterned feature from the photoresist
feature.
44. The method of claim 43 wherein the first phase is about zero
degrees and the second phase is about 180 degrees.
45. The method of claim 44 wherein: the step of forming an isolated
photoresist feature comprises developing photoresist, and the step
of forming the patterned feature comprises etching, and the first
residual photoresist feature defines a closed shape having a
perimeter, and, after the developing step and before the etching
step, no portion of the wafer surface is exposed within the
perimeter.
46. The method of claim 45 wherein the shortest dimension of the
photoresist feature measured in the plane of the wafer surface is
no greater than about 150 nm.
47. The method of claim 44 further comprising forming a plurality
of residual photoresist features, wherein each photoresist feature
of the plurality is exposed to light in the first phase, and each
photoresist feature of the plurality is entirely surrounded by the
second wafer area.
48. The method of claim 47 wherein each of the plurality of
photoresist features defines a closed shape having a perimeter,
and, after the developing step and before the etching step, no
portion of the wafer surface is exposed within the perimeter.
49. The method of claim 48 wherein the plurality of photoresist
features is uniformly spaced.
50. The method of claim 48 wherein the plurality of photoresist
features is randomly spaced.
51. The method of claim 48 wherein a patterned feature is formed on
the wafer surface from each of the plurality of photoresist
features, and wherein the patterned features are portions of memory
cells forming a first memory level in a memory array, the first
memory level formed at a first height above a substrate.
52. The method of claim 51 wherein the memory array is a monolithic
three dimensional memory array, the array further comprising at
least a second memory level formed at a second height above the
substrate, the second height different from the first height.
53. The method of claim 43 wherein the first phase is about 180
degrees and the second phase is about zero degrees.
54. The method of claim 53 wherein: the step of forming an isolated
photoresist feature comprises developing photoresist, and the step
of forming the patterned feature comprises etching, and the first
residual photoresist feature defines a closed shape having a
perimeter, and, after the developing step and before the etching
step, no portion of the wafer surface is exposed within the
perimeter.
55. The method of claim 54 wherein the shortest dimension of the
photoresist feature measured in the plane of the wafer surface is
no greater than about 150 nm.
56. The method of claim 55 further comprising forming a plurality
of residual photoresist features, wherein each photoresist feature
of the plurality is exposed to light in the first phase, and each
photoresist feature of the plurality is entirely surrounded by the
second wafer area.
57. The method of claim 56 wherein each of the plurality of
photoresist features defines a closed shape having a perimeter,
and, after the developing step and before the etching step, no
portion of the wafer surface is exposed within the perimeter.
58. The method of claim 57 wherein the plurality of photoresist
features is uniformly spaced.
59. The method of claim 57 wherein the plurality of photoresist
features is randomly spaced.
60. The method of claim 57 wherein a patterned feature is formed on
the wafer surface from each of the plurality of photoresist
features, and wherein the patterned features are portions of memory
cells forming a first memory level in a memory array, the first
memory level formed at a first height above a substrate.
61. The method of claim 60 wherein the memory array is a monolithic
three dimensional memory array, the array further comprising at
least a second memory level formed at a second height above the
substrate, the second height different from the first height.
62. A method for forming photoresist features on a wafer surface
using a photomask, the method comprising: transmitting light
through a first mask area onto a first wafer area, the first mask
area having a first shifting degree; transmitting light through a
second mask area onto a second wafer area, the second mask area
having a second shifting degree, wherein the second mask area
entirely surrounds and is on all sides in contact with the first
mask area, and the first shifting degree is substantially opposite
the second shifting degree; and developing photoresist, wherein,
after the developing step, a closed residual photoresist feature
remains between the first wafer area and the second wafer area, and
wherein the closed residual photoresist feature is isolated and not
merged with any adjacent photoresist feature.
63. The method of claim 62 wherein the first shifting degree is
about zero degrees and the second shifting degree is about 180
degrees.
64. The method of claim 63 wherein, after the developing step, no
wafer surface is exposed within the first wafer area.
65. The method of claim 62 wherein the first shifting degree is
about 180 degrees and the second shifting degree is about zero
degrees.
66. The method of claim 65 wherein, after the developing step, no
wafer surface is exposed within the first wafer area.
67. A monolithic three dimensional memory array comprising: a
plurality of patterned features, the plurality of patterned
features patterned using a photomask comprising: a plurality of
spatially separate first transmitting windows, wherein the
transmitting windows transmit light in a first phase; and a
transmitting area of the photomask, each transmitting window
substantially surrounded by and in contact with the transmitting
area, wherein the transmitting area transmits light in a second
phase, the second phase substantially opposite the first phase.
68. The monolithic three dimensional memory array of claim 67,
wherein the patterned features comprise substantially coplanar
pillars.
69. The monolithic three dimensional memory array of claim 68
wherein the pillars have a diameter no more than about 150 nm.
Description
BACKGROUND OF THE INVENTION
[0001] The invention relates to a method for patterning fine
features for semiconductor devices using a phase shifting mask with
no blocking material separating the opposite phases.
[0002] Patterned features making up integrated circuits are
conventionally formed using photolithography and etch techniques. A
photomask, which transmits light in some areas and blocks it in
others, is formed, the blocking areas corresponding to the pattern
to be formed on the wafer surface (or its inverse.) The surface to
be patterned, for example a semiconductor or dielectric layer, is
covered with a layer of photoresist, a photoreactive material.
Light is projected onto the photoresist surface using the
photomask, selectively exposing areas of photoresist. The wafer is
then subjected to a developing process, in which exposed
photoresist (or unexposed photoresist, in the case of negative
photoresist) is removed, leaving patterned photoresist behind.
[0003] The remaining patterned photoresist then typically serves to
protect underlying material during a subsequent etch process,
creating features in the same pattern as the remaining
photoresist.
[0004] Over the years integrated circuits have become denser and
patterned features smaller. As projected features become smaller,
the limits of resolution are reached and it becomes more difficult
to project patterns with sharp edges. Poor resolution can lead to
incomplete patterning and to incomplete etching or overetching,
causing device flaws.
[0005] Alternating phase shifters, which invert the phase of light
in some areas of the photomask, increasing contrast in light
intensity at the photoresist surface, are a powerful tool to
improve resolution and sharpen edges.
[0006] The use of alternating phase shifters in photomasks,
however, has disadvantages. When alternating phase shifters are
used, projected light is either incident, in what will be called
zero degree phase, or inverted, in what will be called 180 degree
phase (this is sometimes also called .pi. phase.) As will be more
fully described, as conventionally used, light in opposite phases
is transmitted on opposite sides of an obscured area. The
configuration of some patterns leads to phase conflicts, in which
rules dictate that the same area must see light of opposite phases.
To date, this has meant that use of alternating phase shifters has
been limited to only certain types of patterns.
[0007] There is a need, therefore, for increased flexibility of
phase shifting photomasks.
SUMMARY OF THE INVENTION
[0008] The present invention is defined by the following claims,
and nothing in this section should be taken as a limitation on
those claims. In general, the invention is directed to an improved
method for using phase shifters in a photomask for
photolithography.
[0009] A first aspect of the invention provides for a phase
shifting photomask comprising a plurality of transmitting
nonprinting windows transmitting light in a first phase; a
transmitting area transmitting light in a second phase, each
transmitting window substantially entirely surrounded by and in
contact with the transmitting area with no blocking material
intervening, wherein the second phase is substantially opposite the
first phase, and wherein a first width of unbroken transmitting
area surrounds each transmitting window on all sides, the first
width sufficient for the unbroken transmitting area to print when
the photomask is used to expose photoresist.
[0010] Another aspect of the invention provides for a phase
shifting photomask comprising a transmitting nonprinting window
transmitting light in a first phase; and a transmitting area
substantially entirely surrounding and in contact with the
transmitting window on all sides with no blocking material
intervening, wherein the transmitting area transmits light in a
second phase, the second phase substantially opposite the first
phase, and wherein, when used to pattern photoresist, the
transmitting area is printing on all sides of the transmitting
window.
[0011] An embodiment of the invention provides for a phase shifting
photomask comprising a plurality of spatially separate transmitting
nonprinting windows transmitting light in a first phase; a
transmitting area transmitting light in a second phase, the second
phase substantially opposite the first, the transmitting area
entirely surrounding and in contact with each of the transmitting
windows of the first plurality; wherein each transmitting window is
separated from its nearest neighbor in the plurality by an unbroken
length of transmitting area having at least a first dimension, and
wherein the smallest dimension of each window is no more than about
160 percent of the first dimension.
[0012] Yet another embodiment of the invention provides for a phase
shifting photomask comprising a transmitting nonprinting window
having a first shifting degree; a second transmitting area having a
second shifting degree, the second transmitting area entirely
surrounding and in contact with the first transmitting window,
wherein the second transmitting area is printing on all sides of
the transmitting window; and wherein the second shifting degree is
substantially opposite the first shifting degree.
[0013] Another aspect of the invention provides for a method for
forming a patterned feature on a wafer surface, the method
comprising transmitting light through a phase shifting photomask
onto photoresist covering the wafer surface; forming an isolated
first residual photoresist feature between a first wafer area
exposed to light in a first phase and a second wafer area exposed
to light in a second phase, wherein the first phase is
substantially opposite the second phase, and wherein the second
wafer area entirely surrounds the first wafer area in the plane of
the wafer; and forming the patterned feature from the photoresist
feature.
[0014] Still another aspect of the invention provides for a method
for forming photoresist features on a wafer surface using a
photomask, the method comprising transmitting light through a first
mask area onto a first wafer area, the first mask area having a
first shifting degree; transmitting light through a second mask
area onto a second wafer area, the second mask area having a second
shifting degree, wherein the second mask area entirely surrounds
and is on all sides in contact with the first mask area, and the
first shifting degree is substantially opposite the second shifting
degree; and developing photoresist, wherein, after the developing
step, a closed residual photoresist feature remains between the
first wafer area and the second wafer area, and wherein the closed
residual photoresist feature is isolated and not merged with any
adjacent photoresist feature.
[0015] Another aspect of the invention provides for a monolithic
three dimensional memory array comprising a plurality of patterned
features, the plurality of patterned features patterned using a
photomask comprising: a plurality of spatially separate first
transmitting windows, wherein the transmitting windows transmit
light in a first phase; and a transmitting area of the photomask,
each transmitting window substantially surrounded by and in contact
with the transmitting area, wherein the transmitting area transmits
light in a second phase, the second phase substantially opposite
the first phase.
[0016] Each of the aspects and embodiments of the invention can be
used alone or in combination with one another.
[0017] The preferred aspects and embodiments will now be described
with reference to the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1a is a cross section of a portion of a conventional
binary photomask.
[0019] FIG. 1b shows the electrical field in the plane of the
photomask for the photomask of FIG. 1a.
[0020] FIG. 1c shows the light intensity at the surface of the
photoresist for light projected through the photomask of FIG.
1a.
[0021] FIG. 2a is a cross section of a portion of a conventional
alternating phase shifting photomask.
[0022] FIG. 2b shows the electrical field in the plane of the
photomask for the photomask of FIG. 2a.
[0023] FIG. 2c shows the light intensity at the surface of the
photoresist for light projected through the photomask of FIG.
2a.
[0024] FIG. 3 illustrates phase assignment for a line-and-space
pattern using an alternating phase shifting mask.
[0025] FIG. 4 illustrates phase conflict for rectangular shapes
arranged in a grid using an alternating phase shifting mask.
[0026] FIG. 5 shows the electrical field in the plane of the
photomask when an unshifted region is immediately adjacent a
shifted region.
[0027] FIG. 6 illustrates a masked feature having an interior
nonprinting window surrounded by blocking material.
[0028] FIG. 7 shows successful phase assignment for rectangular
shapes arranged in a grid pattern using nonprinting interior
alternating phase shifters.
[0029] FIG. 8 shows a prior art phase shifting mask and the
residual photoresist features created by it.
[0030] FIG. 9 illustrates adjacent shifting areas separated by very
thin, nonprinting transmitting areas according to a prior art
photomask.
[0031] FIG. 10a shows a photomask according to the present
invention in plan view.
[0032] FIG. 10b shows a photomask according to the present
invention in cross section.
[0033] FIG. 10c shows the electrical field in the plane of the
photomask for the photomask of FIG. 9a.
[0034] FIG. 10d shows the light intensity at the surface of the
photoresist for light projected through the photomask of FIG.
9a.
[0035] FIG. 10e shows, in cross section, residual photoresist
features formed on the wafer surface using the photomask of FIG.
9a.
[0036] FIGS. 11a through 11d illustrate formation of a photomask
having masked features with interior nonprinting windows.
[0037] FIGS. 12a through 12d illustrate formation of a photomask
formed according to the present invention.
[0038] FIGS. 13a and 13b show, in cross section, two projected
photoresist features formed using the same photomask with different
exposure dose.
[0039] FIGS. 14a-14d show different illumination apertures.
[0040] FIG. 15 shows a photomask according to the present invention
in plan view.
[0041] FIG. 16a shows, in plan view, a transmitting window
surrounded by a first width of unbroken transmitting area according
to the present invention.
[0042] FIG. 16b shows, in plan view, a transmitting window not
surrounded by a first width of unbroken transmitting area.
DETAILED DESCRIPTION OF THE INVENTION
[0043] One approach to allow alternating phase shifting photomasks
to be used to pattern a wider variety of shapes is taught in Chen,
U.S. patent application Ser. No. 10/728,436, filed Dec. 5, 2003,
hereinafter the '436 application. The '436 application is the work
of the same inventor as the present invention, is owned by the
assignee of the present invention, and is hereby incorporated by
reference. The present invention is an improved method to solve the
same problem.
[0044] The problem addressed by both the present invention and the
'436 application will be described.
[0045] The '436 application used the term maskedfeature. A masked
feature referred to a feature in a photomask, for example a line, a
rectangle, or any other shape. A masked feature in a photomask
substantially entirely or partially obscures light, so that when
light is projected through the photomask, a corresponding feature
in the photoresist is shielded from light, while the area outside
of the obscured area is exposed. This corresponding feature in
photoresist will be called a projected photoresist feature. The
projected photoresist feature will be roughly the same shape as the
masked feature, though corners on projected photoresist features
tend to be rounded. Typically a linear dimension in a masked
feature is four or five times the size of the corresponding
dimension in the projected photoresist feature, depending on the
stepper used.
[0046] In this description, a photomask feature will refer to a
feature in a photomask, which may be a line, a rectangle, or any
other shape. This is a broader term than masked feature, in that a
photomask feature may not include any blocking material. A
photomask feature in a photomask does not transmit sufficient light
for the corresponding area on the photoresist surface to be fully
exposed, and the wafer surface beneath it will not be exposed after
development of the photoresist. The area outside of the projection
of the photomask feature is exposed, creating a photoresist
feature.
[0047] A photoresist feature is a discrete feature formed by
exposing and developing photoresist. Such a photoresist feature is
surrounded by exposed wafer surface after development of
photoresist. This term is broader than the term "projected
photoresist feature" because it may be formed by projection, as is
the case for a projected photoresist feature, or, as will be seen,
by residual photoresist features formed at phase transition
boundaries.
[0048] The simplest photomask is a binary photomask 10, shown in
FIG. 1a. A plate of a transmitting material 12, for example quartz,
makes up the bulk of the photomask. A blocking material 14,
typically chromium, is formed in areas where light is to be
obscured. FIG. 1b shows the electrical field in the plane of the
photomask. The electrical field is either positive (1.0),
non-existent (0), or negative (-1.0). Where light is transmitted it
is in a first phase, here referred to as zero degree phase. Where
light is blocked, there is no electrical field. (In FIGS. 1b and
1c, the X-axis is horizontal position, corresponding with
horizontal position across the section of photomask shown in FIG.
1a.)
[0049] FIG. 1c shows the actual intensity distribution of light at
the photoresist surface. It will be seen that, due to interference
effects, the edges of lighter and darker areas are not perfectly
defined, and even in the center of the obscured area, the light
intensity at the photoresist surface is not zero. (A value of zero
on the Y-axis of FIG. 1c indicates zero intensity. The value of 1.0
is unitless and arbitrarily assigned, and the other values assigned
relative to it. This is a standard representation of image
intensity, as will be known to those skilled in the art.)
[0050] FIG. 2a illustrates a conventional alternating phase
shifting photomask 16. This photomask is also made up of a plate of
transmitting material 12, with regions of blocking material 14. In
region 18, light is transmitted as in the binary mask. In region
20, however, the transmitting material 12 is etched such that light
passing through it is shifted 180 degrees. An area of a photomask
which inverts the phase of incident light, such as transmitting
area 12, will be called a phase shifter.
[0051] FIG. 2b shows the electrical field in the plane of the
photomask: Where light is transmitted with no phase shifting, it is
in the first phase, zero degree phase. Where light is blocked,
there is no electrical field. Where light is transmitted with phase
shifting, it is in 180 degree phase, opposite the first phase. It
will be understood that while light in 180 degree phase is
perfectly opposite light in zero degree phase, some small deviation
can be tolerated; for example light can be in 179 or 183 degree
phase rather than 180 degree phase and have substantially the same
effect. For purposes of this description, within ten degrees of 180
degrees will be considered to be substantially opposite zero
degrees. Similarly, within ten degrees of zero degrees will be
considered to be substantially opposite 180 degrees. (In FIGS. 2b
and 2c, the X-axis is horizontal position, corresponding with
horizontal position across the section of photomask shown in FIG.
2a.)
[0052] FIG. 2c shows the intensity distribution of light at the
photoresist surface. While the electrical field shown in FIGS. 1b
and 2b can be either positive or negative, light intensity at the
photoresist surface is only zero or positive, since the exposure
intensity is proportional to the square of the electric field. The
transition from a positive to a negative electrical field in the
photomask creates a forced zero of light intensity reaching the
photoresist surface, effectively causing dark areas to appear
"darker", and making edges sharper.
[0053] It will be seen that for a conventional alternating phase
shifting mask, opposite phases are used on opposite sides of an
obscured region. Phase assignment--the process of determining which
phase is to be used in which transmitting area of the photomask--is
straightforward for some patterns, such as the alternating
line-and-space pattern pictured in FIG. 3. Zero degree phase is
assigned to the left of line A, 180 degrees between lines A and B,
zero degrees between lines B and C, etc.
[0054] Other patterns present difficulties, however. FIG. 4 shows a
photomask including rectangular masked features arranged in a grid
pattern. Suppose all areas between rectangles in row A' and in row
B' are assigned to 180 degree phase, and all areas between
rectangles in row B' and in row C' are assigned to 0 degree phase.
Suppose further that all areas between rectangles in column A and
in column B are assigned to 180 degree phase, and all areas between
rectangles in column B and in column C are assigned to 0 degree
phase.
[0055] It will be seen that in framed areas marked with a question
mark ("?"), either phase could be appropriate, depending on whether
the row rule or the column rule is followed. If either phase is
assigned, a region of zero degree phase will be immediately
adjacent a region of 180 degree phase. In the transition from zero
degree phase to 180 degree phase, the electrical field must pass
through zero, as shown in FIG. 5. (The X-axis on this figure again
corresponds to horizontal position across the photomask.) Where the
electrical field is zero, the light intensity at the photoresist
surface will be zero, unintentionally creating a region of
unexposed photoresist, leading to creation of a residual
photoresist feature after development of the photoresist. In this
case, the residual photoresist feature is unwanted.
[0056] The '436 application solved this problem by teaching masked
features like the one shown in FIG. 6, each including an interior
nonprinting window. For such a feature, light transmitted through
the nonprinting window 22 is in a first phase, while light
transmitted through a transmitting area 24 outside the masked
feature is in a second phase opposite the first phase. For example,
the window may be a phase shifter, such that light transmitted
through the masked feature is in 180 degree phase, while the
transmitting area 24 outside the window 22 is in zero degree phase.
Alternatively, the window 22 may transmit light in zero degree
phase, while the transmitting area outside the masked feature 24 is
in 180 degree phase. The window is described as "nonprinting"
because its dimensions are selected so that it will not print, i.e.
such that light transmitted through it will not expose photoresist
within the perimeter of the corresponding photoresist feature
enough for the wafer surface within the feature to be exposed after
development of photoresist. In the '436 application, the interior
nonprinting window 22 was substantially surrounded by blocking
material 26. A blocking material is one that transmits 15 percent
or less of incident light, for example chromium or molybdenum
silicide.
[0057] Turning to FIG. 7, it will be seen that masked features with
interior nonprinting windows, as taught in the '436 application,
transmitting light in one phase inside the masked feature and light
in an opposite phase outside the masked feature, can be printed
with no phase conflict. Each masked feature F of FIG. 7 includes a
window W, the window W comprising a phase shifter. Thus the windows
W are assigned 180 degree phase. The transmitting area 34 commonly
and substantially entirely surrounding the masked features F is
assigned zero degree phase. Clearly the phases could be inverted if
desired.
[0058] The techniques of the '436 application provide a powerful
tool to pattern very fine features. The photomask used in the '436
application, however, is relatively complex and expensive to make.
The present invention uses similar concepts, but omits the
surrounding blocking material (blocking material 26, shown in FIG.
6) used in the '436 application, resulting in a photomask which is
simpler and cheaper to make.
[0059] It has been described that when a nonshifting area and a
shifting area in a photomask are immediately adjacent, at the
transition from zero degree phase to 180 degree phase, the
electrical field must pass through zero, as shown in FIG. 5. The
light intensity at the corresponding point on the photoresist
surface is also zero, so a residual photoresist feature remains on
the wafer surface after the photoresist is developed.
[0060] Prior art chromeless phase edge techniques have made use of
this residual photoresist feature. Turning to FIG. 8, a photomask
is shown consisting of parallel lines, where the lines are
alternately shifting regions 30 and nonshifting regions 32, each
having a width 34. Such an arrangement is described in prior art
which is mentioned in Lee et al., U.S. Pat. No. 5,240,796. Each of
the shifting and nonshifting regions is printing, exposing a
corresponding area on the photoresist surface. FIG. 8 further
shows, below the photomask in cross-section, the wafer and
photoresist surface after developing. It will be seen that residual
photoresist lines 36 form on the wafer surface 38 at the phase
transition boundaries, creating patterned lines after etch. (Dotted
lines relate the phase transition in the photomask with the
resulting residual photoresist feature.)
[0061] In other prior art, for example Chen et al., U.S. Pat. No.
6,482,555 (hereinafter the '555 patent), several phase shifting
windows are placed in close proximity to each other, with no
blocking material used, as in FIG. 5b or FIG. 7 of that patent, and
as shown in FIG. 9. Turning to FIG. 9, the transmitting area
between the phase shifting areas 60 of the '555 patent has width
62. This width 62 is so small that the residual photoresist
features created by adjacent shifting areas 60 merge, forming a
single, large photoresist feature. Thus the transmitting area
between phase shifters is nonprinting.
[0062] The present invention is a photomask comprising nonprinting
transmitting windows transmitting light in a first phase entirely
surrounded by a printing transmitting area transmitting light in a
second phase opposite the first phase, with no blocking material
intervening. The transition between the phases causes a closed
residual photoresist feature to be formed at the perimeter of such
a nonprinting transmitting window. The dimensions of the
nonprinting transmitting window are chosen such that opposite sides
of the residual photoresist feature merge, so that, during normal
use of the photomask, photoresist in the interior of the feature is
not exposed sufficiently to expose the wafer surface after the
photoresist is developed. A patterned feature can be created from
each residual photoresist feature.
[0063] To contrast with other photomasks mentioned herein: In the
present invention, a photomask comprises shifting areas immediately
adjacent to nonshifting areas, with no blocking material
intervening (unlike the '436 application). In aspects of the
present invention, either the shifting area or the nonshifting area
is nonprinting, so that opposite sides of a single closed residual
photoresist feature formed by the nonprinting area merge (unlike
the alternating shifting and non-shifting stripe photomask
described in Lee et al.). The nonprinting area is entirely
surrounded by an area that is printing (unlike the '555 patent.)
and creates a photoresist feature that does not merge with an
adjacent photoresist feature. FIG. 10a, for example, shows a plan
view of a section of a photomask formed according to the present
invention. The areas labeled 180.degree. are shifting, while the
area labeled 0.degree. is nonshifting. Clearly, the phases could be
inverted if desired.
[0064] To summarize, an examplary photomask according to the
present invention comprises a transmitting nonprinting window
transmitting light in a first phase; and a transmitting area
substantially entirely surrounding and in contact with the
transmitting window on all sides with no blocking material
intervening, wherein the transmitting area transmits light in a
second phase, the second phase substantially opposite the first
phase, and wherein, when used to pattern photoresist, the
transmitting area is printing on all sides of the transmitting
window. A transmitting window is a continuous area of a photomask
which transmits light and includes no blocking material which is
surrounded on all sides by some other material.
[0065] FIG. 10b shows the same section of photomask in cross
section, the cross section taken along line L-L'. Shifting areas
have been thinned to shift incident light by about 180 degrees,
while nonshifting areas do not shift incident light. No blocking
material separates shifting areas from nonshifting areas.
[0066] FIG. 10c shows the electric field in the plane of the
photomask, and FIG. 10d shows the light intensity at the
photoresist surface. FIG. 10e illustrates (in cross-section)
residual photoresist features formed on the wafer surface after
developing the photoresist. It will be seen that, if the dimensions
of the shifting area and the nonshifting areas are chosen
appropriately, the opposite sides of a single residual photoresist
features formed at the phase transition perimeter are so close
together that they are in contact, and merge.
[0067] Thus, using such a photomask, a patterned feature can be
formed by transmitting light through a phase shifting photomask
onto photoresist covering the wafer surface; forming an isolated
first residual photoresist feature between a first wafer area
exposed to light in a first phase and a second wafer area exposed
to light in a second phase, wherein the first phase is
substantially opposite the second phase, and wherein the second
wafer area entirely surrounds the first wafer area in the plane of
the wafer; and forming the patterned feature from the photoresist
feature. In this case the isolated first photoresist feature
defines a closed shape having a perimeter, and, after the
developing step and before the etching step, no portion of the
wafer surface is exposed within the perimeter. The photoresist
feature is described as isolated because it is not merged with an
adjacent photoresist feature.
[0068] Such a photomask is relatively simple to make, compared to
the photomask used in the '436 application.
[0069] The photomask of the '436 application requires two
patterning steps, one to pattern the blocking material, the second
to etch the quartz to form the shifting area. Turning to FIG. 11a,
patterning begins with quartz layer 40 coated with a layer of
blocking material (typically chromium) 42. Photoresist 44 is
deposited on chromium layer 42, then patterned in the shape of the
blocking material defining masked features with interior
nonprinting windows, like the photomask shown in FIG. 7. FIG. 11b
shows the photomask after etch of chromium layer 42 and removal of
photoresist. (It will be noted that in cross-sectional figures
representing patterning of a photomask, the photomask appears
upside down relative to its presentation in figures representing
the photomask in use; in FIG. 2a, for example, where the blocking
material appears below the photomask, as opposed to FIG. 11a, where
it is shown above. This represents the orientation of the photomask
during these respective activities.)
[0070] For the second patterning step, photoresist is again applied
and patterned, this time exposing only the interior windows of the
masked features, as shown in FIG. 11c. An etch step thins quartz
layer 40, creating shifting areas 46, as shown in FIG. 11d, which
shows the completed photomask.
[0071] Both photolithography steps performed to create the
photomask of the '436 application present challenges; the first
includes patterning of complex shapes, while the second requires
precise alignment.
[0072] Forming the photomask according in the present invention,
for example the photomask shown in FIG. 10a, is significantly
simpler. Turning to FIG. 12a, patterning again begins with quartz
layer 40 coated with chromium layer 42. Photoresist 44 is deposited
on chromium layer 42, then patterned to excavate rectangular holes
in the photoresist. FIG. 12b shows patterned chromium remaining
after the etch step. In this example, a continuous area of chromium
remains with rectangular holes etched in the chromium, the surface
of quartz layer 40 exposed in the holes. A second etch step
follows, in which the remaining chromium 42 serves as a hard mask
while the quartz layer 40 is etched to produce rectangular shifting
areas 46 shown in FIG. 12c. FIG. 12d shows the completed photomask
after the chromium layer 42 has been stripped.
[0073] Only a single, relatively simple patterning step is required
to make this photomask. It will be noted that none of chromium
layer 42 remained in the completed photomask shown in FIG. 12d. It
was included in this description because standard photomask blanks
as purchased normally include this layer. If quartz or some other
suitable photomask substrate with no chromium layer was used as the
starting point, then photoresist could be deposited directly on the
quartz, the photoresist patterned and the quartz etched, without
the intermediate chromium etch.
[0074] For clarity, an example has been provided of how to make a
photomask according to the present invention. Many variations will
be readily apparent to those skilled in the art. This example
described a photomask having shifting transmitting areas surrounded
by a nonshifting transmitting area; clearly the phases could be
inverted. Negative photoresist is also well known in the art. When
developed, the exposed areas of negative photoresist remain, while
the obscured areas are removed. For brevity, the present
application describes the use of positive photoresist. Those
skilled in the art will appreciate that negative photoresist could
be used either in formation of the photomask or when patterning the
wafer surface. A regular rectangular pattern was described; clearly
other patterns can be envisioned.
[0075] When describing dimensions in a photomask, it is usual to
speak of those dimensions in terms of projected dimensions; i.e.
rather than describing the actual size of a photomask feature in
the photomask, one describes the size of the photomask feature
multiplied by the stepper magnification. Stepper magnification is
typically 4 or 5; i.e. a linear dimension of a chromium area in a
photomask is typically about four or five times larger than the
size of the same feature as projected onto the photoresist
surface.
[0076] This description will follow this convention. A photomask
feature dimension will be described as ".times.S", or multiplied by
a projection scaling factor S. The projection scaling factor S is
the stepper magnification. Suppose, for example, the stepper
magnification and the projection scaling factor is four. If the
actual physical dimension of a photomask feature in a photomask is,
for example, 1000 nm, the dimension of the photomask feature will
be described as 250 nm.times.S. For a stepper magnification and
projection scaling factor S of five, and an actual physical
dimension of 1000 nm of a photomask feature will be described as
200 nm.times.S.
[0077] Stepper magnification is determined by optics, and is the
single most important factor controlling the relationship between
the size of a photomask feature and the size of the resulting
projected photoresist feature. It is not, however, the only factor.
As is well known in the art, varying exposure dose also changes the
size of projected photoresist features. Longer exposure results in
more light energy reaching the photoresist surface, and more
thorough exposure of photoresist. When the photoresist is
developed, more photoresist is removed; thus for larger exposure
dose, dimensions of the eventual etched features are smaller. FIG.
13a and FIG. 13b, for example, show projected photoresist features
47 after development. The photoresist feature 47 of FIGS. 13a and
13b could be formed from the same photomask feature in the same
photomask during separate photolithography operations. The narrower
photoresist feature 47 of FIG. 13b is created when a higher
exposure dose is applied.
[0078] It will be recalled that in aspects of the present
invention, residual photoresist features are formed using a
photomask fabricated according to the present invention, having
first transmitting windows transmitting light in a first phase
surrounded by a second transmitting area transmitting light in a
second phase, the second phase substantially opposite the first
phase. The size of the first transmitting windows are selected such
that the opposite sides of a single residual photoresist feature
formed at the perimeter of a window merge. The residual photoresist
feature forms at the transition between opposite phases; ie. at the
border between the projection of a first transmitting window and
the second transmitting area surrounding it, and not entirely
within the boundaries of the first transmitting window. Thus a
linear dimension of a photoresist feature defined by merged
residual photoresist lines may be larger than the linear dimension
.times.S of the photomask feature used to create it.
[0079] Many interrelated factors determine the actual dimension a
transmitting window must have in its shortest dimension to be
nonprinting, according to the present invention. In general, the
size of the window is proportional to the wavelength of incident
light, and is inversely proportional to numerical aperture.
[0080] The shortest dimension for a transmitting window to be
nonprinting is also affected by the aperture of the condensing
lens. This value can either be a single aperture radius .sigma., or
can be an inner aperture radius .sigma..sub.i and an outer aperture
radius .sigma..sub.o. FIG. 14a shows a conventional aperture which
has a single aperture radius .sigma.. An annular aperture is shown
in FIG. 14b, in which the center area is obscured and light is
transmitted through an annulus having an outer aperture radius
.sigma..sub.o and an inner aperture radius .sigma..sub.i.
[0081] A quadrupole aperture, shown in FIG. 14c, is obscured except
for four holes arranged as shown. The outer aperture radius
.sigma..sub.o is the distance to the outside edge of each hole,
while the inner aperture radius .sigma..sub.i is the distance to
the inside edge of each hole. A dipole aperture, shown in FIG. 14d,
is obscured except for two holes arranged as shown. The outer
aperture radius .sigma..sub.o is the distance to the outside edge
of each hole, while the inner aperture radius .sigma..sub.i is the
distance to the inside edge of each hole. Aperture radius is
generally unitless, and is expressed as a proportion of the entire
lens. For example, for a conventional aperture, .sigma. is
typically about 0.7.
[0082] The relationship of all of these factors can be summarized
as follows: 1 D ' ( 1 + i + o 2 ) NA
[0083] In this equation, D represents the size the shortest
dimension of a transmitting window must have in order to be
nonprinting; .lambda. is the wavelength of incident light,
.sigma..sub.i is the inner aperture radius and .sigma..sub.o is the
outer aperture radius; and NA is the numerical aperture. Thus D is
proportional to wavelength .lambda., and is inversely proportional
to aperture radius .sigma. (or inner aperture radius .sigma..sub.i
and outer aperture radius .sigma..sub.o) and numerical aperture
NA.
[0084] Conventionally, light of two wavelengths are in general use:
248 nm (KrF) and 193 nm (ArF).
[0085] In general, for 248 nm photolithography, at any point at
which a transmitting window has a dimension no more than about 160
nm.times.S, it will be nonprinting. Similarly, for 193 nm
photolithography, at any point at which a transmitting window has a
dimension no more than about 120 nm.times.S, it will be
nonprinting.
[0086] In practical terms a window having a shortest dimension
smaller than about 50 nm will be ineffective; preferably the window
of the present invention should have no dimension less than about
60 nm.
[0087] FIG. 15 is a plan view of an exemplary photomask formed
according to the present invention. The photomask comprises a
plurality of transmitting nonprinting windows 50 transmitting light
in a first phase; a transmitting area 52 transmitting light in a
second phase, each transmitting window 50 substantially entirely
surrounded by and in contact with the transmitting area 52 with no
blocking material intervening, wherein the second phase is
substantially opposite the first phase, and wherein a first width
of unbroken transmitting area 52 surrounds each transmitting window
on all sides, the first width sufficient for the unbroken
transmitting area 52 to print when the photomask is used to expose
photoresist.
[0088] In this example, windows 50 have linear dimension D.sub.1,
and are separated by distance D.sub.2. D.sub.1 is chosen so that
opposite sides of a single residual photoresist feature formed on
the photoresist surface at the projected perimeter of a window
merge, and the window 50 is nonprinting. D.sub.2 is chosen so that
the space between the windows prints, that is, so that the
photoresist in this area is exposed sufficiently that it is removed
during developing. Because the window 50 is a closed feature, its
interior receives less overall light than does the transmitting
area 52 between windows. For this reason dimension D.sub.1 can be
larger than first dimension D.sub.2 while window 50 is nonprinting
and transmitting area 52 is printing. The exposure dose must be
selected so that the windows 50 are nonprinting. (This example
shows windows 50 square and uniformly spaced. Clearly many other
arrangements are possible. Transmitting windows 50 can be
rectangular but not square, or any other shape. Transmitting
windows 50 can be randomly spaced.)
[0089] Describing the transmitting area surrounding a transmitting
window on all sides as "unbroken" for a first width means that
within that width only the transmitting area exists, and that no
other material, such as blocking material or another transmitting
window transmitting light in a different phase, is present. For
example, turning to FIG. 16a, transmitting window 50, which
transmits light in a first phase, is surrounded on all sides by a
first width D.sub.3 of unbroken transmitting area 52, which
transmits light in a second phase substantially opposite the first
phase. The boundary of first width D.sub.3 is denoted by the
dotted-line frame 51. No other material exists within first width
D.sub.3 of transmitting window 50. The first width D.sub.3 is
sufficient for the unbroken transmitting area 52 to print when the
photomask is used to expose photoresist; in preferred embodiments,
the first width D.sub.3 is at least 100 nm.times.S.
[0090] In contrast, in FIG. 16b, transmitting window 54, which in
this example transmits light in the first phase, is less than a
first width D.sub.3 from transmitting window 50, and thus
transmitting window 50 in FIG. 16b is not surrounded on all sides
by first width D.sub.3 of unbroken transmitting area 52. An example
of such a configuration appears in the '555 patent, in which the
width of the transmitting area separating adjacent transmitting
shifting windows is not sufficient for the transmitting area
between those windows to print when the photomask is used to expose
photoresist.
[0091] In other words, FIG. 15 shows a section of a photomask. The
photomask comprises a plurality of spatially separate transmitting
nonprinting windows 50 transmitting light in a first phase; a
transmitting area 52 transmitting light in a second phase, the
second phase substantially opposite the first, the transmitting
area 52 entirely surrounding and in contact with each of the
transmitting windows 50 of the first plurality; wherein each
transmitting window is separated from its nearest neighbor in the
plurality by an unbroken length of transmitting area having at
least a first dimension D.sub.2, and wherein the smallest dimension
D.sub.1 of each window is no more than about 160 percent of the
first dimension D.sub.2. As described earlier, because the window
50 is a closed feature, its interior receives less overall light
than does the transmitting area 52 between windows. For this reason
dimension D.sub.1 can be larger than first dimension D.sub.2 while
window 50 is nonprinting and transmitting area 52 is printing. It
will be evident that the smallest dimension D.sub.1 of each window
is measured in a plane parallel to the plane of the photomask.
[0092] Monolithic three dimensional memory arrays such as the one
taught in Herner et al., U.S. patent application Ser. No.
10/326470, "An Improved Method for Making High Density Nonvolatile
Memory," filed Dec. 19, 2002, hereby incorporated by reference,
include a plurality of substantially evenly spaced pillars. These
pillars can comprise polycrystalline silicon, called polysilicon.
The pillars are portions of memory cells, and the memory cells
formed in the same patterning steps generally form a portion of a
memory level at a first height above a substrate. Such a monolithic
three dimensional memory array further comprises at least a second
memory level formed at a second height above the substrate, the
second height different from the first height.
[0093] The substantially evenly spaced pillars of Herner et al.
have a pitch of between about 220 nm and 280 nm, preferably about
260 nm, and are patterned, for example, using light having a
wavelength of 248 nm. A photomask according to the present
invention paired with a quadrupole aperture is highly effective for
patterning regularly spaced pillars.
[0094] It will be recalled that light is projected through a
photomask having a photomask feature to create a corresponding
photoresist feature. The photoresist feature is then processed,
typically by etching, to create a patterned feature. Pillars, for
example the pillars of Herner et al., are the patterned features
created from the photomask features 50 of FIG. 15.
[0095] The photomask of FIG. 15 can advantageously be used to
pattern the pillars of Herner et al. When used for this purpose,
referring to FIG. 15, photomask feature dimension D.sub.1 is
between about 50 nm.times.S and about 160 nm.times.S, preferably
between about 90 nm.times.S and about 140 nm.times.S, most
preferably between about 130 and 140 nm.times.S. The shortest
dimension of the transmitting window 50 is no more than about 160
nm.times.S. Photomask feature dimension D.sub.2, then, is between
about 210 nm.times.S and about 100 nm.times.S, preferably between
about 170 nm.times.S and about 120 nm.times.S, most preferably
between about 130 nm.times.S and 120 nm.times.S. With proper
exposure dose, these photomask dimensions should produce
photoresist features having a width of about 130 nm and separated
by a gap of about 130 nm. Dose varies from design to design,
photomask to photomask, and machine to machine, and it is routine
to for some experimentation to be required to identy optimum dose.
Preferably the width of the photoresist features is no more than
about 150 nm, and the gap is no less than about 110 nm. The
photoresist features are then etched to form the patterned
features, which will be pillars, as described in Herner et al. As
noted in Herner et al., while the masked feature is rectangular,
the cross-section of the patterned feature will tend to be
substantially cylindrical. The dimensions given here assume that
the light has a wavelength of 248 nm.
[0096] It will also be understood by those skilled in the art that,
depending on the materials etched and the etch processes used,
photoresist feature dimensions may not be the same as actual
patterned feature dimensions. It is routine to adjust exposure dose
and etch processes to achieve optimum results.
[0097] Monolithic three dimensional memory arrays are described in
Johnson et al., U.S. Pat. No. 6,034,882, "Vertically stacked field
programmable nonvolatile memory and method of fabrication";
Johnson, U.S. Pat. No. 6,525,953, "Vertically stacked field
programmable nonvolatile memory and method of fabrication"; Knall
et al., U.S. Pat. No. 6,420,215, "Three Dimensional Memory Array
and Method of Fabrication"; Lee et al., U.S. patent application
Ser. No. 09/927648, "Dense Arrays and Charge Storage Devices, and
Methods for Making Same," filed Aug. 13, 2001; Herner, U.S.
application Ser. No. 10/095962, "Silicide-Silicon
Oxide-Semiconductor Antifuse Device and Method of Making," filed
Mar. 13, 2002; Vyvoda et al., U.S. patent application Ser. No.
10/185507, "Electrically Isolated Pillars in Active Devices," filed
Jun. 27, 2002; Walker et al., U.S. application Ser. No. 10/335089,
"Method for Fabricating Programmable Memory Array Structures
Incorporating Series-Connected Transistor Strings," filed Dec. 31,
2002; Scheuerlein et al., U.S. application Ser. No. 10/335078,
"Programmable Memory Array Structure Incorporating Series-Connected
Transistor Strings and Methods for Fabrication and Operation of
Same," filed Dec. 31, 2002; Vyvoda, U.S. patent application Ser.
No. 10/440882, "Rail Schottky Device and Method of Making", filed
May 19, 2003; and Cleeves et al., "Optimization of Critical
Dimensions and Pitch of Patterned Features in and Above a
Substrate," U.S. patent application Ser. No. 10/728,451, filed Dec.
5, 2003, all assigned to the assignee of the present invention and
hereby incorporated by reference.
[0098] A monolithic three dimensional memory array is one in which
multiple memory levels are formed above a single substrate, such as
a wafer, with no intervening substrates. The layers forming one
memory level are deposited or grown directly over the layers of an
existing level or levels. In contrast, stacked memories have been
constructed by forming memory levels on separate substrates and
adhering the memory levels atop each other, as in Leedy, U.S. Pat.
No. 5,915,167, "Three dimensional structure memory." The substrates
may be thinned or removed from the memory levels before bonding,
but as the memory levels are initially formed over separate
substrates, such memories are not true monolithic three dimensional
memory arrays.
[0099] These monolithic three dimensional memory arrays are highly
dense structures. Thus photomasks made according to the present
invention can advantageously be used to pattern any of the lines,
pillars, or other tightly-packed structures formed at any level of
these arrays.
[0100] It will be evident, however, that the techniques described
herein are in no way limited to three dimensional memory arrays,
and can be used to pattern any fine features, including
conventional two-dimensional and non-memory devices.
[0101] The examples provided in this description of photomasks
formed according to the present invention have largely depicted
regular patterns, for example evenly spaced rectangles of uniform
size. Regular patterns offer the advantage that it is simpler to
determine exactly what size photomask feature and exposure dose are
required to create a patterned feature of a desired size; once the
determination is made, it applies to all of the elements in the
pattern. Nonetheless, the methods and photomasks of the present
invention can be applied to features of any shape, and the
patterned features need not be uniformly sized or spaced.
[0102] An area of a photomask can be described as having a shifting
degree. A shifting degree describes the shifting characteristics of
a transmitting area of a photomask. If light transmitted through an
area of a photomask is not phase shifted, for example, that area
has a shifting degree of about zero degrees. If light transmitted
through an area of a photomask is phase shifted to the opposite
phase, that area has a shifting degree of about 180 degrees.
[0103] To summarize, FIG. 15 shows a phase shifting photomask
comprising a transmitting nonprinting window 50 having a first
shifting degree; a second transmitting area 52 having a second
shifting degree, the second transmitting area 52 entirely
surrounding and in contact with the first transmitting window 50,
wherein the second transmitting area 52 is printing on all sides of
the transmitting window 50; and wherein the second shifting degree
is substantially opposite the first shifting degree.
[0104] Photomasks formed according to the present invention have
been described which include first transmitting windows
transmitting light in a first phase surrounded by a second
transmitting area transmitting light in a second phase, the second
phase substantially opposite the first phase. In the examples
provided, no blocking material separated the first transmitting
windows from the second transmitting area. It should be noted,
however, that photomask features with no blocking material formed
according to the present invention can be combined in a single
photomask with conventional photomask features that include
blocking material. More specifically, the presence of blocking
material in any part of a photomask does not preclude it from
falling within the scope of the invention.
[0105] The foregoing detailed description has described only a few
of the many forms that this invention can take. For this reason,
this detailed description is intended by way of illustration, and
not by way of limitation. It is only the following claims,
including all equivalents, which are intended to define the scope
of this invention.
* * * * *