Model-based data conversion

Details Model-based data conversion Model-based data conversion

2002-06-11

2004-08-31

Siek, Vuthe (Department: 2825)

Computer-aided design and analysis of circuits and semiconductor

Nanotechnology related integrated circuit design

C716S030000, C716S030000, C716S030000, C430S005000

Reexamination Certificate

active

06785879

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to phase shifting masks and, in particular, to the use of model-based tools to facilitate phase assignment on the phase shifting masks.
2. Discussion of the Related Art
Lithography is a well-known process used in the semiconductor industry to form lines, contacts, and other known structures in integrated circuits (ICs). In conventional lithography, a mask (wherein the term “mask” as used herein can refer to a mask or a reticle) having a pattern of transparent and opaque regions representing such structures in one IC layer is illuminated. The emanating light from the mask is then focused onto a photoresist layer provided on a wafer. During a subsequent development process, portions of the photoresist layer are removed, wherein the portions are defined by the pattern. In this manner, the pattern of the mask is transferred to (i.e. printed on) the photoresist layer.
However, diffraction effects at the transition of the transparent regions to the opaque regions on the mask can render the corresponding printed edges on the wafer indistinct, thereby adversely affecting the resolution of the lithography process. Various techniques have been proposed to improve the resolution. One such technique, phase shifting, uses phase destructive interference of the waves of incident light. Specifically, phase shifting shifts the phase of a first region of incident light waves approximately 180 degrees relative to a second, adjacent region of incident light waves to create a feature between the first and second regions. Thus, a feature, as defined by exposed and unexposed portions of a photoresist illuminated through a mask, can be more closely defined by using phase shifting, thereby allowing greater structure density on the IC. Typically, features generated by phase shifting can then be protected from exposure by using a “trim” mask, which is used to expose the remaining field.
As the need for feature density increases, phase shifting is being applied to many features on the layout. In one embodiment, called a full phase approach, substantially all features of a layer can be defined using phase shifting. However, using phase shifting in dense layouts can result in phase conflicts. Phase conflicts can negate the optical interference necessary to create the desired feature(s). Therefore, assigning phase to the layout can constitute a time-intensive, but mandatory part of typical process flows in the production of many integrated circuits.
Other modifications can be made to the layout to optimize printing resolution. For example, a process called optical proximity correction (OPC) can be used to compensate for non-linear distortions caused by optical diffraction and resist process effects when transferring the pattern from the mask to the wafer. Advanced computer-implemented tools can simulate (i.e. predict) a real pattern transfer with a set of mathematical formulas (i.e. models). In simulating pattern transfer, a simulation tool can use one or multiple models with the layout to generate an OPC-modified layout.
FIG. 1
illustrates a standard process flow
100
including phase assignment as well as OPC. In step
101
of process flow
100
, a target layout can be provided to a phase shifting tool for analysis. In one embodiment, the iN-Phase™ tool, licensed by Numerical Technologies, Inc., can be used. The PSM type, e.g. double exposure alternating aperture phase shifting, can be designated in step
102
. Step
103
, in which phase shifting is applied to the target layout, can include the steps of placing the shifters in the layout (step
104
), assigning phase to the shifters (step
105
), and resolving any phase conflicts (step
106
). Step
105
may include algorithms for assigning phases to the shifters.
The algorithms used are based phase dependencies and, optionally, costs. By describing the dependencies in one or more data structures, the algorithms can attempt to solve the problem. See, e.g., U.S. patent application Ser. No. 09/823,380, filed Mar. 29, 2001, entitled, “Incrementally Resolved Phase-shift Conflicts In Layouts For Phase-Shifted Features”, and U.S. patent application Ser. No. 10/085,759, filed Feb. 28, 2002, entitled “Design And Layout Of Phase Shifting Photolithographic Masks”, both of which are incorporated by reference herein. Performing these algorithms to assign phase can be extremely time consuming and require a separate computer program and/or software to be maintained for performing phase assignment, as opposed to the simulation engine for model-based optical proximity correction.
After being output in step
107
, the phase shifted layout can then be input to a tool that applies OPC. In one embodiment, the tool for applying phase shifting is different than the tool for providing OPC. In another embodiment, the same tool can provide both phase shifting and OPC. For example, the iN-Phase™ tool, licensed by Numerical Technologies, Inc., can assign phase to shifters as well as perform OPC of a finished layout. In step
109
, the layout can be modified as appropriate. At this point, the final layout can be output in step
110
. Unfortunately, in either embodiment, significant time is spent reading (or streaming out) and then streaming in the phase shifted layout (steps
107
and
108
).
Therefore, a need arises for an accurate and time effective method and system for assigning phase to shifters in a model-based environment.
SUMMARY OF THE INVENTION
In accordance with one feature of the invention, shifters on a phase shifting mask (PSM) can be assigned their corresponding phase through the use of a simulation engine of the type normally used for model-based optical proximity correction (OPC). Specifically, instead of using an algorithm based on phase dependencies, the phase of a shifter can be assigned based on simulating the image contrast provided by each phase for that shifter. The higher the contrast, the better the lithographic performance of the shifter. Therefore, the phase providing the higher contrast can be selected for that shifter.
To provide this simulation-based phase assignment, a pre-shifter, or shifter shape, can be placed relative to a feature on the layout. In one embodiment, the pre-shifter can be placed close to all edges of the feature, thereby surrounding the feature. The pre-shifter can then be divided into a plurality of shifter tiles along the edges where the shifters abut the feature. This division can be done using an OPC engine's dissection capabilities. A first contrast and a second contrast can be calculated by assigning a first phase and a second phase, respectively, to a shifter tile. In multi-phase phase shifting masks, additional contrasts can be computed for each phase possibility. A phase for the shifter tile can be selected based on whichever contrast is higher (and will more accurately produce the feature at the target dimension). These steps can be repeated for multiple shifter tiles for the feature as well as other features on the layout. In another embodiment, all the evaluation points can be calculated at the same time using the 0 degree/180 degree options to determine which option give the best contrast.
In one embodiment, the pre-shifter can be cut at predetermined areas of the feature. For example, the predetermined areas can include corners of the feature. In this manner, phase conflicts can be avoided. In another embodiment, cuts are not made to the pre-shifter at line ends. Specifically, a trim mask (which will be used in conjunction with the PSM) can be used to define the line end, thereby rendering shifters unnecessary for this region.
The trim mask includes a plurality of trim regions for protecting the features defined by the PSM. In accordance with another feature of the invention, a trim region can be placed relative to a feature on the layout. This trim region, like the pre-shifter, can be divided into a plurality of trim tiles. In one embodiment, the dividing of the pre-shifter and the trim region can be based on dissection points p

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