The present invention relates to photolithographic processing, and in particular to mask data preparation techniques for lithographic processes using multiple mask photolithography.
Most integrated circuits are created by exposing a pattern of features contained on a mask (sometimes referred to as a reticle) onto a silicon wafer that is coated with photosensitive materials. The exposed wafer is then chemically and mechanically processed to create corresponding objects or circuit elements on the wafer. Other circuit patterns are then exposed onto the wafer to build up the integrated circuit layer by layer.
As the circuit elements become smaller and smaller, the ability of a photolithographic printing system to print the features on the wafer becomes increasingly diminished. Optical and other process distortions occur such that the pattern contained on the mask will often not match the pattern of circuit elements that is printed on the wafer. To address this problem, numerous resolution enhancement techniques, such as optical and process correction (OPC) and other tools, such as phase shifting masks, subresolution assist features, etc., have been developed to enhance the fidelity with which a desired pattern can be printed on a wafer. One technique that is becoming increasingly used to print tightly packed features on a wafer is known as double patterning. In double patterning, a layout is parsed into two sets of polygons, with each set following design rules that allow it to be individually printable. Each set of polygons is then patterned onto the wafer. The phrase “double patterning” usually refers to a bright field, positive toned process, in which the polygons to be patterned are separated into two sets of dark (light blocking) polygons in an otherwise clear transparent field, and exposure to a mask with one set of polygons is completely processed (i.e. wafer coated with resist, and resist then exposed, developed and etched) before repeating the entire process again with a mask with the second set of polygons. Because the entire patterning sequence is repeated, this is called “double patterning”, as opposed to “double exposure”, where two exposures are made with different masks before the wafer is processed.
The present invention relates to improvements in multiple mask photolithographic printing techniques such as double patterning and double exposure processing techniques.
The present invention is a method of preparing data to create photolithographic masks or reticles for use in a multiple mask photolithographic process. A desired layout pattern of features is divided into two or more data layers or groups, wherein each group or data layer defines the data for a different mask or reticle. In one embodiment of the invention, a coloring algorithm is used to divide a target layout pattern into the two or more groups of features. The features in each group are inspected for features that are within a predetermined distance of each other and assigned to the same group. Features so located are broken up into smaller features that can be separated into groups to maintain the distance between features assigned to the same group.
In one embodiment of the invention, the division of a feature into two or more smaller features is made by adding a cutting box to the feature. The polygons of a feature that includes a cutting box are extended to overlap in the region of the cutting box to ensure proper printing during the photolithographic process.
This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.
The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:
With double patterning, the desired pattern of features to be printed on a wafer is divided among two or more masks wherein each mask prints one set of features on the wafer. The masks print sets of features that are interleaved such that the features printed by both masks have a pitch that is closer than that obtainable with a single mask alone. For example, as shown in
In practice, many desired feature patterns can be divided into two separate groups and printed with different masks.
The present invention is a technique to overcome the problems associated with designating features in a desired layout in a manner that allows a double patterning or double exposure printing technique to be used with more desired feature patterns.
For example, a new feature 60e is adjacent a feature 50b on one side and a feature 50c on the other side in the original target pattern. To separate the features in the original target pattern 50 into different groups for use in creating separate masks, a coloring algorithm can be run with the new target pattern 60 as an input.
The following is one example of a script of computer executable instructions that may be stored on a computer-readable media such as a hard drive, CD-ROM, memory card or transmitted over a wired or wireless communication link to a computer system that executes the instructions to perform the present invention. The script creates a new target layer (s32_newTarget) that is used by a coloring algorithm, as described below, to divide the features into two different data layers (mask—0, mask—1). The resulting data layers are then checked by a design rule checking algorithm (DRC) that confirms that all features are spaced appropriately for the mask.
The coloring algorithm operates to assign different properties to the features that are adjacent each feature in the input data layer. Using polygon 60e as an example, a coloring algorithm assigns a first property to the polygon 50b on the left vertical edge of polygon 60e and assigns another property to the polygon 50c on the right vertical edge of polygon 60e. The property assigned to each feature can be of any type that allows the features to be grouped. For example, the features can be assigned a color (e.g. blue or green), a number (1 or 0), a logical value (true or false) or any other identifier whereby features to be grouped have the same property. In the embodiment of the invention described, the property is assigned by grouping features into one of two different data layers in a layout database.
In one embodiment of the present invention, the coloring algorithm used is Mentor Graphics' PSMGate program. PSMGate is typically used to assign phase shift values to features in order to create gates at the boundaries of features having opposite phase values. In this case, the features on each side of the “virtual gate” input layer such as the polygons 60 shown in
First, each polygon among the polygons to be parsed is assigned a unique identification number (e.g., 71, 72, 73, . . . n for n polygons—see
These relationships are used to construct a graph, with the polygons 71 through n as nodes and “gates” as connections between the nodes. See
A node in the graph is selected as the starting point. A “Depth-First Search” is then carried out, assigning the nodes to either group A or group B as the search progresses through the graph. A sequence of steps follows (as shown below), and a colored graph (indicated by plain or hashed nodes) is shown in
Once a polygon has been assigned to either group A or B, it is not reassigned. Conflicts occur when a newly assigned polygon in another branch of the tree has a portion that connects to a polygon that is already assigned. When an assigned polygon is encountered, the algorithm currently does nothing, and instead moves on with the next node in the search. Such coloring conflicts are generally referred to as “phase conflicts” when the coloring algorithm is used to assign phase shift values to polygons. However, as used herein the term coloring conflict can encompass any two polygons that are assigned to the same group and are within a predetermined distance of each other.
Although the polygon assignment algorithm does not identify these conflicts as they are created, they are easily detected after the assignment is finished by using a DRC check for minimum spacing among the polygons assigned to collection group A or collection group B.
Note also that there need not be a single layer of “virtual gates.” The gates can actually be on multiple data layers as well, some assigned a higher priority than others (indicated by the data layer used to store them). Coloring can first be done using a graph constructed using only the high priority “gates,” then re-colored using all the gates.
From the initial assignment of features into two different groups or data layers, a determination can be made if features in each group or data layer do not have the minimum separation required for printing with a single mask. These features can be readily identified by software programs which determine the distance between adjacent features. If the distance is less than or equal to some predetermined amount, and the features are assigned to the same group or data layer, a user can be alerted to the fact that a coloring conflict exists. In most cases, coloring conflicts occur in features having a “U” or other bent shape that enclose other polygons within their interior. As can be seen in the example shown in
To rectify the coloring conflicts, one embodiment of the present invention introduces one or more separation points into a feature to divide the feature into two or more parts. With a feature divided, the coloring algorithm is reapplied to the desired target pattern in an attempt to separate the features into two or more groups without coloring conflicts. In one embodiment, the separation points are called cutting boxes and are defined as polygons having a length and width.
Once the cutting boxes have been placed into the layout, a new virtual gate layer can be defined in the same manner as shown in
Although the present invention has been described with respect to its preferred embodiments, it will be appreciated that changes could be made without departing from the scope of the invention. For example, although the locations of cutting boxes are determined manually based on an identification of coloring conflicts that occur with an initial analysis of a layout, it will be appreciated that other techniques, such as software algorithms, could be used to determine where the cutting boxes should be placed. For example, in one embodiment, it is possible to determine if a feature includes an even number of polygons within its boundaries. If so, it is likely that a coloring conflict will occur with this feature and the feature can be divided into two or more smaller features with one or more cutting boxes. In addition, although the disclosed embodiment of the invention uses two masks to print a target layout pattern, it will be appreciated that the invention could be applied to designating a target pattern into three or more groups or data layers, where each group or data layer is used in making a mask for use with a multiple mask printing technique. Finally, although the disclosed embodiments primarily illustrate the grouping of features into bright field masks for double patterning processing, it will be appreciated that the invention is equally useable with processing techniques of the type shown in
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