Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2002-11-14
2004-09-28
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
Reexamination Certificate
active
06797441
ABSTRACT:
BACKGROUND
1. Field of the Invention
The invention relates to the process of fabricating semiconductor chips. More specifically, the invention relates to a method and an apparatus for assigning different phases to selected cuts on a complementary mask to clear phase conflicts generated by a phase shifting mask during an optical lithography process used in fabricating a semiconductor chip.
2. Related Art
Recent advances in integrated circuit technology have largely been accomplished by decreasing the feature size of circuit elements on a semiconductor chip. As the feature size of these circuit elements continues to decrease, circuit designers are forced to deal with problems that arise as a consequence of the optical lithography process that is typically used to manufacture integrated circuits. This optical lithography process begins with the formation of a photoresist layer on the surface of a semiconductor wafer. A mask composed of opaque regions, which are generally formed of chrome, and light-transmissive clear regions, which are generally formed of quartz, is then positioned over this photoresist layer. (Note that the term “mask” as used in this specification is meant to include the term “reticle.”) Light is then shone on the mask from a visible light source, an ultraviolet light source, or more generally some type of electromagnetic radiation source together with suitably adapted masks and lithography equipment.
This image is reduced and focused through an optical system containing a number of lenses, filters, and mirrors. The light passes through the clear regions of the mask and exposes the underlying photoresist layer. At the same time, opaque regions of the mask block the light leaving underlying portions of the photoresist layer unexposed.
The exposed photoresist layer is then developed, through chemical removal of either the exposed or non-exposed regions of the photoresist layer. The end result is a semiconductor wafer with a photoresist layer having a desired pattern. This pattern can then be used for etching underlying regions of the wafer.
Printing Problems with Cuts in Close Proximity
As integration densities continue to increase, it is becoming desirable to use phase shifters to define more and more features within a layout. This can lead to problems in some situations. For example,
FIG. 1A
illustrates the phase shifters for a dark field alternating aperture phase shifting mask
100
. The phase shifters (shown interposed against the original layout shown with a lighter stipple than the rest of the field and with slanted lines to indicate phase shifters) are set on a dark field, e.g. chromium. The lightly stippled areas between the phase shifters correspond to the intended, or original layout.
Referring to
FIG. 1A
, a desired feature
102
can be formed using zero-degree phase shifter
104
and 180-degree phase shifter
106
. At the right end of feature
102
, there is a small region
108
, or cut, between phase shifters
104
and
106
that is not part of the layout and is not intended to print. Region
108
will leave a dark area that must be cleared by a complementary, or trim, mask during the optical lithography process, additionally there may be a loss of definition in the areas surrounding this region.
A complementary mask
101
(also known as a trim mask) provides a corresponding cut
110
at the appropriate location to fully expose the photoresist in region
108
. Note that other cuts in complementary mask
101
expose other regions that are not intended to print between phase shifters on phase shifting mask
100
.
In particular, note that cuts
112
and
114
are in close proximity to each other on the complementary mask
101
. This can cause unwanted exposure in a region between cuts
112
and
114
. This can be seen by examining
FIG. 2A
, which illustrates an aerial image
200
created by exposure through complementary mask
101
. An areas' intensity of exposure is shown using colors with the most intensity shown as red, orange, yellow, green, blue, and violet in descending intensity. Consider areas
202
and
204
, which correspond to cuts
112
and
114
, respectively, in complementary mask
101
. Note that the close proximity of cuts
112
and
114
to each other causes unwanted exposure of the region between cuts
112
and
114
.
This unwanted exposure results in a lack of definition in the printed image between cuts
112
and
114
. This lack of definition is apparent in corresponding region
208
in
FIG. 2B
, which presents an aerial image
206
of how the layout will print. Notably, the lighter blue color in region
208
of the feature shows that the line may be broken. In
FIGS. 2A and 2B
, the exposure conditions used were &lgr;=193 nm, NA=0.85, &sgr;=0.4, dosage ratio 3:1 (trim:phase ratio in mJ/cm
2
).
Hence, what is needed is a method and an apparatus for clearing phase conflicts on a phase shifting mask without the problems described above.
SUMMARY
One embodiment of the invention provides a system that assigns different phases to selected cuts on a complementary mask used to clear phase conflicts generated by a phase shifting mask during an optical lithography process used in fabricating a semiconductor chip. The system first receives the complementary mask, which uses cuts to clear phase conflicts generated by the phase shifting mask. Next, the system identifies the cuts on the complementary mask that may not clear the phase conflicts, and then assigns a different phase to selected cuts on the complementary mask, so that the selected cuts are out-of-phase with other cuts on the complementary mask. Assigning a different phase to the selected cuts ensures that the cuts on the complementary mask clear the phase conflicts generated by the phase shifting mask. And assists in the definition of the adjacent features.
In a variation of this embodiment, identifying the cuts that may not clear the phase conflicts involves locating cuts that are close enough to each other to cause unwanted exposure in the region between the cuts during the exposure through the complementary mask. In some instances the unwanted exposure may be an over exposure (e.g. feature definition suffers), in other instances the unwanted exposure may be an under exposure (e.g. conflict not fully cleared.)
In a further variation, the system varies the phase between the cuts to eliminate the unwanted exposure in the region between the cuts during the exposure through the complementary mask.
In a further variation, identifying the cuts that may not clear the phase conflicts involves locating isolated cuts that are too small to clear the phase conflicts.
In a further variation, assigning the different phase to selected cuts on the complementary mask involves adding out-of-phase assist features to the complementary mask in proximity to the isolated cuts.
In a further variation, identifying the cuts on the complementary mask that may not clear the phase conflicts involves simulating exposures through the phase shifting mask and the trim mask to produce a simulation result, and then examining the simulation results.
REFERENCES:
patent: 6730463 (2004-05-01), Heissmeier et al.
Numerical Technologies Inc.
Park Vaughan & Fleming LLP
Rosasco S.
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