Single trench alternating phase shift mask fabrication

Details Single trench alternating phase shift mask fabrication Single trench alternating phase shift mask fabrication

2002-06-07

2004-12-14

Norton, Nadine G. (Department: 1765)

Etching a substrate: processes

Masking of a substrate using material resistant to an etchant

C216S013000, C430S005000

Reexamination Certificate

active

06830702

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to semiconductor device fabrication, and more particularly to the use of phase shift masks (PSM's) in conjunction with such fabrication.
BACKGROUND OF THE INVENTION
Since the invention of the integrated circuit (IC), semiconductor chip features have become exponentially smaller and the number of transistors per device exponentially larger. Advanced IC's with hundreds of millions of transistors at feature sizes of 0.25 micron, 0.18 micron, and less are becoming routine. Improvement in overlay tolerances in photolithography, and the introduction of new light sources with progressively shorter wavelengths, have allowed optical steppers to significantly reduce the resolution limit for semiconductor fabrication far beyond one micron. To continue to make chip features smaller, and increase the transistor density of semiconductor devices, IC's have begun to be manufactured that have features smaller than the lithographic wavelength.
Sub-wavelength lithography, however, places large burdens on lithographic processes. Resolution of anything smaller than a wavelength is generally quite difficult. Pattern fidelity can deteriorate in sub-wavelength lithography. The resulting features may deviate significantly in size and shape from the ideal pattern drawn by the circuit designer. For example, as two mask patterns get closer together, diffraction problems occur. At some point, the normal diffraction of the exposure rays start touching, leaving the patterns unresolved in the resist. The blending of the two diffraction patterns into one results from all the rays being in the same phase. Phase is a term that relates to the relative positions of a wave's peaks and valleys. In
FIG. 1A
, the waves
102
and
104
are in phase, whereas in
FIG. 1B
, the waves
106
and
108
are out of phase.
One way to prevent the diffraction patterns from affecting two adjacent mask patterns is to cover one of the openings with a transparent layer that shifts one of the sets of exposing rays out of phase, which in turn nulls the blending. This is shown in
FIGS. 2A and 2B
. In
FIG. 2A
, the mask
202
causes an undesirable light intensity as indicated by the line
204
. In
FIG. 2B
, adding the phase shifter
206
to the mask
202
causes a desirable light intensity as indicated by the line
208
. This mask
202
in
FIG. 2B
with the phase shifter
206
added is a phase shift mask (PSM), which is a special type of photomask.
A typical photomask affects only one of the properties of light, the intensity. Where there is chromium, which is an opaque region, an intensity of zero percent results, whereas where the chromium has been removed, such that there is a clear or transparent region, an intensity of substantially 100 percent results. By comparison, a PSM not only changes the intensity of the light passing through, but its phase as well. By changing the phase of the light by 180 degrees in some areas, the PSM takes advantage of how the original light wave adds to the 180-degree wave to produce zero intensity as a result of destructive interference.
PSM's have gained increased popularity among manufacturers as the feature sizes they are tasked with printing become smaller, and the topography over which these features must be printed becomes more varied. PSM's offer their customers the opportunity to greatly improve the resolution capability of their steppers. This allows them to print smaller feature sizes using the same equipment and processes.
One particular type of PSM is referred to as an alternating PSM. The PSM of
FIG. 2B
was one example of an alternating PSM. In an alternating PSM, closely spaced apertures are processed so that light passing through any particular aperture is 180 degrees out of phase from the light passing through adjacent apertures. Any light that spills over into the dark region from the two edges that are out of phase will destructively interfere. This reduces the unwanted exposure in the center dark region.
FIG. 3
shows another example of an alternating PSM, and more specifically, a single-trench alternating PSM
300
. The PSM
300
includes two layers, a chromium layer
302
, and a quartz layer
304
. The chromium layer
302
is the same type of layer typically found in other, non-PSM photomasks, in which light is exposed therethrough to an underlying semiconductor wafer. Clear regions within the chromium layer
302
allow light to pass through, whereas opaque regions within the chromium layer
302
prevent light from passing through. The clear and opaque regions are arranged to correspond to a desired semiconductor design, or pattern. In the PSM
300
, there are clear regions
306
A,
306
B,
306
C,
306
D, and
306
E.
The quartz layer
304
is more generally a clear or transparent layer, in which single trenches are alternatively added beneath the clear regions of the chromium layer
302
to phase shift light passing through these clear regions. For instance, the alternating clear regions
306
A,
306
C, and
306
E of the chromium layer
302
do not have single trenches beneath them in the quartz layer
304
. Conversely, the alternating regions
306
B and
306
D of the chromium Elayer
302
have single trenches
308
A and
308
B beneath them in the quartz layer
304
. The PSM
300
is an alternating PSM in that only every other clear region of the chromium layer
302
has a phase shifter beneath them in the quartz layer
304
. The PSM
300
is a single-trench alternating PSM in that these phase shifters are the single trenches
308
A and
308
B, as compared to other types of phase shifters, such as double trenches, and so on.
The manner by which the PSM
300
of
FIG. 3
can be fabricated according to the prior art is summarized with reference to
FIGS. 4A
and
4
B. In
FIG. 4A
, the clear regions within the chromium layer
302
are already present, by a process of photoresist patterning, etching the chromium layer
302
, and then stripping the remaining photoresist. A new layer of photoresist
402
has been added, such as by a coating process, and patterned to correspond to where the single trenches
308
A and
308
B of
FIG. 3
will be made. In
FIG. 4B
, the quartz layer
304
is first dry etched, and then wet etched using sodium hydroxide (NaOH) to form the single trenches
308
A and
308
B. The photoresist layer
402
is then removed by a photoresist strip process to result in the PSM
300
of FIG.
3
.
This conventional approach to manufacturing the alternating single-trench PSM
300
has several disadvantages, however. To not damage the quartz layer
304
and/or the chromium layer
302
, as well as possibly for other reasons, the patterning of the photoresist resulting in the photoresist layer
402
of
FIG. 4A
must be accomplished by using laser-beam writing to properly expose the photoresist. This means that more conventional tools, such as e-beam writers, cannot be used to expose the photoresist to result in the photoresist layer
402
of FIG.
4
A. Furthermore, phase defects in the PSM
300
are difficult to repair when using the process summarized with reference to
FIGS. 4A and 4B
.
The depth of the quartz layer
304
after dry etching, and before wet etching, also cannot be accurately measured with the photoresist layer
402
remaining on top of the chromium layer
302
, which is problematic to ensure that the phase shift resulting from the PSM
300
is correct. Accurate measurement cannot be accomplished because the photoresist layer
402
is an inaccurate reference from which to measure the depth of the of the quartz layer
304
after dry etching. Furthermore, the wet etching of the quartz layer
304
, because it uses NaOH, may cause the photoresist layer
402
to peel, decreasing the likelihood that the single trenches
308
A and
308
B will be properly fabricated. A different approach to single-trench alternating PSM manufacture, described in U.S. Pat. No. 5,958,630, also suffers from at least some of these problems.
Therefore, there is a need for a process for fabricati

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