Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2001-06-06
2003-05-27
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
Reexamination Certificate
active
06569583
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 using phase shifter cutbacks to resolve conflicts between phase shifters during creation of a mask to be used in an optical lithography process for manufacturing an integrated circuit.
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 generally 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 photo resist layer coated wafer. (Note that the term “mask” as used in this specification is meant to include the term “retical.”) Light is then shone on the mask from a visible light source or an ultraviolet light source.
This light is generally reduced and focussed through an optical system that contains 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, the light is blocked by opaque regions of mask, leaving underlying portions of the photoresist layer unexposed.
The exposed photoresist layer is then developed, typically through chemical removal of the exposed
on-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.
One problem in performing the optical lithography process arises from conflicts between phase shifters. Phase shifters are often incorporated into a mask in order to achieve line widths that are smaller than the wavelength of the light that is used to expose the photoresist layer through the mask. During phase shifting, destructive interference caused by two adjacent clear areas on a mask is used to create an unexposed area on the photoresist layer. This is accomplished by exploiting the fact that light passing through a mask's clear regions exhibits a wave characteristic having a phase that is a function of the distance the light travels through the mask material. By placing two clear areas adjacent to each other on the mask, one of thickness t
1
and the other of thickness t
2
, one can obtain a desired unexposed area on the underlying photoresist layer caused by interference. By varying the thickness t
1
and t
2
appropriately, the light exiting the material of thickness t
2
is 180 degrees out of phase with the light exiting the material of thickness t
1
. Phase shifting is described in more detail in U.S. Pat. No. 5,858,580, entitled “Phase Shifting Circuit Manufacture Method and Apparatus,” by inventors Yao-Ting Wang and Yagyensh C. Pati, filed Sep. 17, 1997 and issued Jan. 12, 1999, which is hereby incorporated by reference.
As can be seen in
FIG. 1A
, when two phase shifters are located in close proximity to each other, conflicts can arise. In
FIG. 1A
, a first phase shifter comprising a zero-degree phase region
102
and a 180-degree phase region
104
is used to produce a small line width in a gate region
103
of polysilicon line
101
. Similarly, a second phase shifter comprising a zero-degree phase region
114
and a 180-degree phase region
112
is used to produce a small line width in a gate region
113
of polysilicon line
111
.
Unfortunately, when the first phase shifter and the second phase shifter are located in close proximity to each other, conflicts can arise between them as is illustrated in FIG.
1
A. In existing systems, this can cause the system to halt with a phase conflict error.
What is needed is a method and an apparatus for resolving conflicts between phase shifters.
Another problem arises during the process of generating phase shifters and associated trim. A phase shifter located on a phase shifting mask will often be generated along with associated trim located on a second mask. During exposure of the second mask, this trim protects a region that is to be exposed by the phase shifter during exposure of the phase shifting mask. Unfortunately, design rules typically cause patches to be added to the shifter and to the associated trim and these patches can cause conflicts with other features on the masks. Note that in existing systems, phase shift regions cannot overlap with field polysilicon.
What is needed is a method and an apparatus for generating phase shifters and trim that satisfy design rules while minimizing conflicts with other mask features.
SUMMARY
One embodiment of the invention provides a system that uses phase shifter cutbacks to resolve conflicts between phase shifters during creation of a mask to be used in an optical lithography process for manufacturing an integrated circuit. The system works by locating a plurality of phase shifters, including a first phase shifter and a second phase shifter, on a phase shifting mask, and then identifying a conflict area wherein a conflict is likely to occur between the first phase shifter and the second phase shifter on the phase shifting mask. The system resolves this conflict by cutting back one or both of the first phase shifter and the second phase shifter, so that the first phase shifter and the second phase shifter do not interfere with each other in the conflict area.
In one embodiment of the invention, identifying the conflict area involves expanding the size of each shifter in the plurality of phase shifters, so that each shifter covers an area defined by a halo surrounding each shifter. The system then retrieves environment information for each shifter by examining the area covered by each expanded shifter. This enables the system to obtain inter-cell environment information, such as the location of inter-cell polysilicon features. The system uses this environment information not only to identify conflict areas between phase shifters but also to produce better shifter shapes. Finally, the system restores the size of each shifter. As a result of the above-described process, the system can produce better shifter shapes, which can improve yield and manufacturability.
In one embodiment of the present invention, the system uses a rule-based shape generation process known as “priority placement” to generate shapes for special cases.
In one embodiment of the invention, cutting back one or both of the first phase shifter and the second phase shifter involves reducing the size of the first phase shifter and/or the second phase shifter so that the first phase shifter and the second phase shifter do not interfere with each other in the conflict area.
In one embodiment of the invention, identifying the conflict area involves looking for conflicts between phase shifters located in different predefined cells, wherein no conflicts are assumed to exist between phase shifters within the same predefined cell.
In one embodiment of the invention, identifying the conflict area involves looking for conflicts between phase shifters located in the same predefined cell.
In one embodiment of the invention, resolving the conflict involves cutting back a shifter endcap extension for the first phase shifter and/or the second phase shifter.
In one embodiment of the invention, resolving the conflict involves unifying portions of the first phase shifter and the second phase shifter.
In one embodiment of the invention, the system additionally generates a third phase shifter within the phase shifting mask, wherein generating the third phase shifter involves ensuring that design rules are satisfied in defining dimensions for the third phase shifter. After this thi
Cho Seonghun
Wu Shao-Po
Numerical Technologies Inc.
Park Vaughan & Fleming LLP
Rosasco S.
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