Computer-aided design and analysis of circuits and semiconductor – Nanotechnology related integrated circuit design
Reexamination Certificate
2001-03-29
2003-06-24
Siek, Vuthe (Department: 2825)
Computer-aided design and analysis of circuits and semiconductor
Nanotechnology related integrated circuit design
C716S030000
Reexamination Certificate
active
06584610
ABSTRACT:
BACKGROUND OF THE INVENTION
1. The Field of the Invention
This invention relates to the field of printed circuit manufacturing. In particular, this invention relates to inserting and assigning phases to phase shifters on masks used to fabricate integrated circuits.
2. Description of Related Art
Conventional integrated circuit (IC) fabrication involves many steps in common with other processes that impose physical structures in a layer on a substrate, such as laying ink in patterns on a page, or laying chrome in patterns on a quartz substrate. Some of the important steps viewed at a high level are depicted in FIG.
1
.
In step
110
, engineers use a functional computer aided design (CAD) process, to create a schematic design, such as a schematic circuit design consisting of individual devices coupled together to perform a certain function or set of functions. The schematic design
115
is translated into a representation of the actual physical arrangement of materials upon completion, called a design layout
125
, with a physical CAD process
120
. If multiple layers are involved, as is typical for an IC, a design layout is produced for each layer, e.g., design layouts
125
a
,
125
b
, etc.
FIG. 2
shows a sample design layout. A fabrication CAD process
130
produces one or more fabrication layouts
135
, such as masks for each design layout
125
a
. The one or more fabrication layouts
135
are then used by a substantiation process
140
to actually produce physical features in a layer, called here the printed features layer
149
.
One recent advance in optical lithography called phase shifting generates features in the printed features layer
149
that are smaller than the features on the mask
135
a
projected onto the printed features layer
149
. Such fine features are generated by the destructive interference of light in adjacent separated windows in the mask called shifters.
FIG. 3
shows two adjacent shifters,
310
and
312
, in a mask
300
. The shifters
310
and
312
are light transmissive areas on the mask separated by an opaque area
311
with a width of Wm
313
when projected onto the printed features layer
149
. The projection of Wm onto the printed features layer
149
is limited by the resolution of the optical process. However, if the light of a single wavelength passing through one of the shifters, e.g.
310
, is out of phase (by 180 degrees or n radians) with the light of the same wavelength passing through the other shifter, e.g.
312
, then an interference pattern is set up on the printed features layer
149
during the substantiation process
140
. This interference generates a printed feature
350
having a width Wp
353
that is less than the width Wm
313
of the opaque area projected onto the printed features layer
149
. In other embodiments, the width
313
and width
353
are much closer and can be equal. In each case, the width
353
of the printed feature is less than that produced by the same optical system without phase shifting.
The use of phase shifting puts extra constraints on the fabrication layouts
135
, and hence on the design layout, e.g.
125
a
. These constraints are due to several factors. One factor already illustrated is the need for finding space on the mask, e.g.,
135
a
, for the two shifters,
310
and
312
, as well as for the opaque area
311
between them. This precludes the one mask from placing additional features on the printed features layer
149
in the region covered by the projection of the two shifters
310
and
312
and the opaque area
311
. Another factor is that overlapping or adjacent shifters on a single mask, used, for example, to generate neighboring phase-shifted features, generally do not have different phases. Adjacent shifters with different phases will produce a spurious feature.
Currently, design layouts
125
may provide the space needed for placement of phase shifters through design rules, but shifters are actually placed and simultaneously assigned a phase in the conventional fabrication design steps, not shown, in attempts to produce the fabrication layouts. As complex circuits are designed, such as by combining many standard cells of previously designed sub-circuits, shifters of different phases may overlap or become adjacent in the layouts, leading to phase-shift conflicts. It is generally recognized that resolving phase-shift conflicts should be done globally, after the whole circuit is laid out, because swapping the phases of a pair of shifters to resolve one conflict can generate a new conflict with another neighboring feature already located in the design or one added later. The conventional IC design systems try to reassign phases of individual pairs to resolve the conflicts at the end of the design process when all the phase conflicts are apparent. For example, iN-Phase™ software from NUMERICAL TECHNOLOGIES, INC.™ of San Jose, Calif., uses this conventional technique.
For example,
FIG. 4
shows a T-junction element
440
that is desirably formed with narrow phase-shifted features
443
,
442
and
444
as well as with wide noncritical features
441
and
445
.
FIG. 4A
shows a pair of shifters
410
and
420
needed to form the vertical phase-shifted feature
443
of element
440
.
FIG. 4A
also shows another shifter
415
disposed opposite shifter
410
to form the left half
442
of the horizontal phase-shifted feature of element
440
. Similarly,
FIG. 4A
also shows a fourth shifter
425
disposed opposite shifter
420
to form the right half
444
of the horizontal phase-shifted feature of element
440
. Shifters
415
and
425
are so close that they violate a design rule requiring at least a minimum spacing X between adjacent shifters. That is, separation
427
is less than X.
In the conventional fabrication CAD process, not shown, the shifters
410
,
420
,
415
and
425
are placed as shown and assigned phases, but the phase-shift conflict is not addressed until all the elements of the design layout have been accounted for. Then the design rule is applied in which shifters
415
and
425
are replaced by a single shifter
430
.
However, there is no assignment of phase for shifter
430
that can simultaneously be opposite to the phases assigned to shifters
410
and
420
, because shifters
410
and
420
are already opposite to each other. Thus such a design has a conflict that cannot be solved by changing the phases assigned to the shifters. Some re-arrangement of shifters or features or both is needed. In this example, however, the feature
440
from the physical design layout does not allow shifter
430
to be moved and does not allow another shifter to be inserted. Thus the fabrication CAD process
130
cannot move or change the shifters enough to resolve the conflict.
When a phase-shift conflict is irresolvable by the fabrication CAD process
130
, then the physical CAD process
120
is run again to move or reshape the features, such as those of element
440
. Process flow with an irreconcilable phase-shift conflict is represented in
FIG. 1
, which shows that fabrication layouts
135
are produced along the arrow marked “Succeed” if the fabrication CAD process
130
succeeds, but that control returns to the physical CAD process
120
along the arrow marked “Fail” if the fabrication CAD process
130
fails, such as if it fails to resolve all phase conflicts.
While suitable for many purposes, the conventional techniques have some deficiencies. As designs, such as designs for IC circuits, become more complex, the time and effort involved in performing the physical CAD process
120
and the fabrication CAD process
130
increase dramatically, consuming hours and days. By resolving phase-shift conflicts at the end of this process, circumstances that lead to irresolvable phase-shift conflicts are not discovered until the end of these time consuming processes. The discovery of such irresolvable phase-shift conflicts induces the design engineers to start over at the physical CAD process
120
. The processes
120
and
130
are repeated until final design layouts and fab
Wang Yao-Ting
Wu Shao-Po
Bever Hoffman & Harms LLP
Harms Jeanette S.
Lin Sun James
Numerical Technologies Inc.
Siek Vuthe
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