Description of Related Art [0002] Phase shift masking has been applied to create small dimension features in integrated circuits. Typically the features have been limited to selected elements of the design, which have a small, critical dimension. See, for example, United States Patent No. 5,766,806.

[0003] Although manufacturing of small dimension features in integrated circuits has resulted in improved speed and performance, it is desirable to apply phase shift masking more extensively in the manufacturing of such devices. However, the extension of phase shift masking to more complex designs results in a large increase in the complexity of the mask layout problem. For example, when laying out phase shift windows on dense designs, phase conflicts will occur. One type of phase conflict is a location in the layout at which two phase shift windows having the same phase are laid out in proximity to a feature to be exposed by the masks, such as by overlapping of the phase shift windows intended for implementation of adjacent lines in the exposure pattern. If the phase shift windows have the same phase, then they do not result in the optical interference necessary to create the desired feature. Thus, it is necessary to prevent inadvertent layout of phase shift windows in phase conflict near features to be formed in the layer defined by the mask.

[0004] Another problem involves efficient layout of optical proximity correction OPC features, and other proximity correction features. In one system provided by the assignee of the present invention, known as iN-Phase 4.0 from Numerical Technologies,

Inc., San Jose, CA, a model OPC feature for OPC of gate shrink designs is featured, and OPC of the phase shift pattern along the gate region to correct for light imbalance is provided.

[0005] In the design of a single integrated circuit, millions of features may be laid out. The burden on data processing resources for iterative operations over such large numbers of features can be huge, and in some cases makes the iterative operation impractical. The layout of phase shift windows and the assignment phase shift values to such windows, along with the layout of complementary trim mask patterns, for circuits in which a significant amount of the layout is accomplished by phase shifting, is one such iterative operation which has been impractical using prior art techniques.

[0006] Because of these and other complexities, implementation of a phase shift masking technology for complex designs will require improvements in the approach to the design of phase shift masks.

SUMMARY OF THE INVENTION [0007] The present invention provides techniques suitable for use with complex phase shift mask patterns and complementary trim mask patterns, which allow for improved proximity correction.

[0008] Thus, a method for producing a computer readable definition of photolithographic mask used to define a target pattern is provided. For the purposes of this description, the phase shift mask patterns and trim mask patterns include shapes having boundaries that are defined by sets of line segments. The phase shift mask patterns include phase shift windows, and the trim mask patterns include trim shapes, which have boundaries defined by such sets of line segments. For a particular pair of phase shift windows used to define a target feature in a target pattern, each of the phase shift windows in the pair can be considered to have a boundary that includes at least one line segment that abuts the target feature. Likewise, a complementary trim shape used in definition of the target feature, for example by including a transmissive region used to clear an unwanted phase transition between the particular pair of phase shift windows, includes at least one line segment that can be considered to abut the target feature. According to the present invention, proximity correction is provided by adjusting the position of the at least one line segment on the boundary of a phase shift windows in said pair which abuts the target feature, and by adjusting the position of the

at least one line segment on the boundary of the complementary trim shape which abuts the target feature.

[0009] In one embodiment, the at least one line segment on the phase shift windows is defined by dissecting boundaries of the phase shift windows in the pair of phase shift windows at dissection points that are selected according to the shape of the phase shift windows. For example, the dissection points are selected so that they occur along the edge of the target feature at positions corresponding to corners of the phase shift windows. In addition, the at least one line segment on the trim shape is selected by dissecting boundaries of the trim shape at dissection points selected according to the shape of the trim shape. Again, for example the dissection points on the trim shape are selected so that they occur along the edge of the target feature at positions corresponding to corners of the trim shape.

[0010] In another embodiment, the pair of phase shift windows discussed in the preceding paragraph includes a complementary phase shift window by which a phase transition is produced that results in formation of at least a part of the target feature.

The line segments for the phase shift windows in the pair are defined by dissecting a boundary of the phase shift window at a dissection point at corner of the phase shift window which abuts an edge of the target feature caused by the phase transition.

Likewise, the line segments of the trim shape are defined by dissecting the boundary of the trim shape at dissection points at the corner of the trim shape which abuts an edge of the target feature caused by the phase transition.

[0011] Another embodiment of the invention provides a method for performing optical proximity correction for a target feature of integrated circuit layout. In this embodiment, the layout is accomplished using a full phase pattern that comprises a first phase shift window and the second phase shift window. The first and second phase shift windows have respective sides comprising at least one line segment abutting the target feature. A phase transition between the first and second phase shift windows cause an artifact to be trimmed. For example, the first and second phase shift windows may abut the sides of intersecting line segments in the target pattern, and create a phase transition at the inside corner of the intersection that would create an artifact to be trimmed. A trim shape comprising at least one line segment abutting the target feature is used to trim the artifact. According to the present invention, at least one line segment of the first phase shift window and at least one line segment of the trim shape that abut the

target feature are identified. Proximity correction is performed by adjusting the positions of both the identified at least one line segment of the first phase shift window and the identified at least one line segment of the trim shape. Said adjusting includes for example offsetting the at least one line segment of the phase shift window and the at least one line segment of the trim shape from adjacent line segments defining boundaries of the features, preferably in a direction which is orthogonal to the adjacent line segments that define boundaries of features.

[0012] Another embodiment of the invention includes layout a first mask pattern including phase shift windows having boundaries defined by line segments in a first mask layout, and a second mask pattern including trim shapes having boundaries defined by line segments in the second mask pattern. In this embodiment, the combination of the first and second mask patterns is used for defining a target pattern in which at least a part of an exposure feature caused by a phase transition between a pair of phase shift windows in the first mask pattern is cleared by a transmissive region in a trim shape in the second mask pattern to define a portion of the target feature. Next, the invention includes adjusting the position of a line segment defining boundaries of the pair of phase shift windows in the first mask pattern that create said exposure feature to be cleared by the transmissive region, and the position of a line segment defining boundaries of the transmissive region in the second mask layout to provide for proximity correction for the target feature. Next, a result of the laying out and adjusting is stored in a computer readable medium.

[0013] In yet another embodiment, these techniques are applied to phase shift windows and trim shapes used for definition of inside corners of target features. In a further embodiment, these techniques are applied to phase shift windows and trim shapes used for definition of outside corners of target features.

[0014] According to another aspect of the present invention, a computer readable definition of the photolithographic mask is provided that define the pattern in a layer to be formed which includes a target feature having first and second outside corners form by intersections of first, second and third edges of the target feature. According to this embodiment of the invention, a method includes laying out a first mask pattern including phase shift windows and a second mask pattern including trim shapes, wherein the first and second mask patterns are used in combination for defining the first and second outside corners of the target feature. The first mask pattern including first

and second a shift windows having opposite phases and abutting the first and second edges of the target feature near the first outside corner, and the third phase shift window having the same phase as the first phase shift window and abutting the third edge of the feature near the second outside corner. A first phase transition occurs between the first and second patient windows in a location near the first outside corner and cause an exposure feature tending to extend in a line away from the first outside corner. A second phase transition occurs between the second and third phase shift windows in a location near the second outside corner, and causes an exposure feature tending to extend in a line away from the second outside comer. The second mask pattern includes a trim shape having a first transmissive region corresponding to a location of the first phase transition for clearing at least a portion of the exposure feature caused by the first phase transition such that the first outside corner is sharper in a resulting image, and has a second transmissive region corresponding to the location of the second phase transition for clearing at least a portion of the exposure feature caused by the second phase transition such that the second outside corner is sharper in the resulting image.

Proximity correction adjustments are applied to one or both of the first and second mask patterns. The result is stored in a computer readable medium.

[0015] Other aspects of the invention include an article of manufacture that comprises a computer readable storage medium with computer readable instructions stored thereon for executing the layout processes just described. Further, lithographic masks are provided that include one or more masks having phase shift mask patterns and trim mask patterns laid out as described. Also, a method for manufacturing integrated circuits is provided based upon use of mask patterns that are laid out as described.

[0016] In various embodiments of the invention, the trim mask pattern defines binary shapes only. In other embodiments, the trim mask pattern includes one or more of tricolor shapes, attenuated phase shift windows, and attenuated opacity shapes.

[0017] Further aspects and advantages of the present invention can be understood upon review of the figures, the detailed description and the claims which follow.

BRIEF DESCRIPTION OF THE FIGURES [0018] Fig. 1 is a legend showing symbols that will be used in subsequent figures.

[0019] Fig. 2 shows the placement of dissection and evaluation points on both phase and trim patterns for a straight line layout.

[0020] Fig. 3 shows the placement of dissection and evaluation points on both phase and trim patterns for an inner corner.

[0021]-Fig. 4 shows the placement of dissection and evaluation points on both phase and trim patterns for an outer corner.

[0022] Fig. 5 shows the placement of dissection and evaluation points on both phase and trim patterns for a complex pattern.

[0023] Fig. 6 is a process flow diagram for setting up and performing optical proximity correction (OPC) for a layout.

[0024] Fig. 7 shows a corner with the phase shifters separated by a cut prior to OPC correction.

[0025] Fig. 8 shows a layout with a corner with a single, uncut, phase shifter prior to OPC.

[0026] Fig. 9 shows a layout with a contact defined using phase shifters prior to OPC.

[0027] Fig. 10 is a simulation of the uncorrected layout of Fig. 7.

[0028] Fig. 11 is a simulation of the uncorrected layout of Fig. 8.

[0029] Fig. 12 is a simulation of the uncorrected layout of Fig. 9.

[0030] Fig. 13 shows a layout with two contacts, one with a phase conflict and the other with the conflict removed.

[0031] Fig. 14 is a simulation of the uncorrected layout of Fig. 13.

[0032] Fig. 15 shows a layout with two copies of feature each defined using a different phase shifter arrangement.

[0033] Fig. 16 is a simulation of the corrected layout of Fig. 15.

[0034] Fig. 17 shows a portion of a complex layout.

[0035] Fig. 18 shows the phase shift pattern for the layout of Fig. 17 after OPC has been performed.

[0036] Fig. 19 shows the trim pattern for the layout of Fig. 17.

[0037] Fig. 20 shows the simulated exposure for the layout of Fig. 17 using the phase and trim patterns of Figs. 18-19.

[0038] Fig. 21 shows a portion of a complex layout.

[0039] Fig. 22 shows the phase shift pattern for the layout of Fig. 21 after OPC has been performed.

[0040] Fig. 23 shows the trim pattern for the layout of Fig. 21.

[0041] Fig. 24 shows the simulated exposure for the layout of Fig. 21 using the phase and trim patterns of Figs. 22-23.

[0042] Fig. 25 shows a portion of a layout where critical dimension within a region has been further reduced.

[0043] Fig. 26 shows a portion of the layout of Fig. 25 after the shifter widths are further adjusted to produce the reduced critical dimension region.

DETAILED DESCRIPTION Overview [0044] Optical proximity correction (OPC) involves modification of one or more layouts that will be used in the production of a layer of material to correct for optical effects of the exposure of a mask in lithography equipment for integrated circuit (IC) production. (Note, as used herein, the term mask includes the term reticle.) [0045] The OPC process involves adding and removing hammerheads, serifs, and the like to the pattern (composed of structures and/or features) in a layout so that a mask made from the pattern in the resulting corrected layout will more accurately produce the desired original, or reference, pattern from the target layout on the finished IC., Proximity correction for other artifacts of the lithographic process, including etch proximity effects for example, is also possible using such modifications.

[0046] More particularly, the application of OPC to layouts designed for producing substantially all portions of a layout pattern will be considered. The techniques described apply to other types of proximity correction as well. First, the types of layouts being considered will be discussed in greater detail. Next, the selection of dissection and evaluation points for OPC on such layouts will be considered in greater detail. Then, OPC of corners will be considered in greater detail with reference to the layout of the phase shift layout and phase assignment. Finally, example layouts together with trim masks and sample exposure results will be considered.

Layouts [0047] In one embodiment of the invention, substantially all portions of a pattern in a layout are being defined using a phase shifting mask. Masks for such layouts are sometimes referred to as"full phase"masks. In one embodiment, the layouts are defined according to the process described in United States Patent Application Serial No. 09/932,239 filed 17 Aug 2001, entitled"Phase Conflict Resolution for Photolithographic Masks"having inventors Christophe Pierrat and Michel Cote, and assigned to the assignee of the present invention, which is incorporated herein by reference.

[0048] In another embodiment, a phase shifting layout that produces substantial portions of a pattern of an IC using phase shifting comprises a phase shifting layout where substantially all features for a particular layer are defined using phase shifting. In another embodiment, a phase shifting layout that produces substantial portions of a pattern of an IC using phase shifting comprises a layout such that only features that are non-critical for the binary exposure are non-phase shifted.

[0049] In other embodiments, the relevant layout comprises a layout where at least one of : 'eighty percent (80%) of non-memory portions in one layer of material in the layout ; eighty percent (80%) of a part of the floorplan in one layer of material ; 'eighty percent (80%) of cells in a given area; 'ninety percent (90%) of a layer of material ; ninety five percent (95%) of a layer of material ; ninety nine percent (99%) of a layer of material ; one hundred percent (100%) of a layer of material; one hundred percent (100%) of a in a functional unit of the chip (e. g. ALU) in one layer of material; one hundred percent (100%) of features in a layer of material that are in the critical path of the design; one hundred percent (100%) of features in a layer of material above or below certain dimensions, e. g. all features with a critical dimension 50 Rm < CD < 100 am ;

'everything in a layer of material except those features that cannot be phase shifted due to phase conflicts that cannot be resolved; everything in a layer of material except test structures ; and one hundred percent (100%) of all non-dummy features, e. g. features providing structural support for processing purposes, and non-electrically functional features in a layer of material are defined using phase shifting.

[0050] In one embodiment, the percentages are computed based on the percentage of edges, or OPC edge segments, defined using phase shifting. Turning briefly to Fig. 2, the edge between the dissection point 210 and the dissection point 212 is defined by the trim mask while the remaining edges of the feature 200 are defined using the phase shifting mask. In other embodiments, the percentage is determined by area.

[0051] The remainder of this application will consider OPC on layouts meeting one or more of the above criterion.

Dissection and Evaluation Points and OPC Process [0052] The OPC process itself may be a rule based or a model based OPC process, or both. If a rule based process is used, the rules may be constructed through examination of the performance of model based OPC process according to the methods described herein. The trade off between the two is usually quality (model based generally produces better results) for time (rule based is generally faster).

[0053] The model based OPC process will now be considered in greater detail as follows. First, some terminology and information about model based OPC will be discussed. Next, setup and dissections for: a straight line, an inner corner, an outer corner, and a more complex pattern, will be considered. Finally, a process flow for model based OPC on layouts where substantially all portions of a pattern in a layout are being defined using a phase shifting mask will be considered.

Model Based OPC Terminology [0054] Model based OPC involves the simulation of the optical effects of a user selected lithography process to a given layout at evaluation points placed within a pattern of a layout.

[0055] In one embodiment, the lithography process is modeled using an optical model generated by the ModelGen (TM) software produced by Numerical Technologies, Inc., of San Jose, California. The ModelGen software allows for the description of the characteristics of the optical lithography system being used including, among other things, the wavelength of light (X), the numerical aperture (N. A.), the coherency of the light (e. g.,) for each exposure, the type of illumination (off-axis, quadrapole, etc.), etc.

[0056] That optical model can be calibrated for the particular process and lithographic stepper or scanner by making a test exposure and measuring the critical dimension (CD) of test features. The calibration can be accomplished by providing the test measures to the ModelCal (TM) software produced by Numerical Technologies, Inc. of San Jose, CA. Other embodiments of the invention can support optical models generated or be used in conjunction with software from tools from Avant! Corporation, Fremont, California, and Mentor Graphics Corporation, Wilsonville, Oregon.

[0057] Together with an optical model, calibrated or otherwise, OPC parameters are provided for the positioning of dissection and evaluation points by the OPC process.

The OPC parameters include measurements that can be given as a segment length, e. g.

60 nm, 120 nm, etc. The segment length describes how far apart, and thus how frequently, or infrequently, line segments that make up features or structures in the pattern in the layout are dissected and evaluated. The OPC software may allow different segment lengths for different types of features, e. g. outer corner, inner corner, straight line, etc.

[0058] In one embodiment, the TROPiC (TM) software, the iN-Phase (TM), the iN- Tandem (TM) software and/or the Photolynx (TM) software from Numerical Technologies, Inc., are used to provide model based OPC as described more fully below. In other embodiments, the OPC process is performed using software provided by Avant! and/or Mentor Graphics.

[0059] Fig. 1 is a legend showing symbols that will be used in subsequent figures.

More specifically, Fig. 1 includes a legend 110. Dissection points in subsequent figures for both the trim pattern and the phase shifting layer will be shown as an eight cornered "Petronas towers"shape of a square intersected with a rotated square. Dissection points for the trim patterns will be shown as a diamond. Dissection points for the phase shift pattern will be shown as a square. Evaluation points for trim layouts will be shown as an X and evaluation points for phase layouts as plus marks. These symbols will facilitate a

better understanding of the placement of dissection and evaluation points used by embodiments of the invention.

Straight Line Dissection [0060] Fig. 2 shows the placement of dissection and evaluation points on both phase and trim patterns for a straight line layout. Fig. 2 includes a target feature 200, a straight line. The feature 200 is an element of a target layout, which the phase shift mask and trim mask are designed to implement. This feature 200 presents a target that includes elements formed by the phase transition between the shifters 202 and 204, and an element, that is the line end, defined by the trim 206. The shifter 202 and the shifter 204 represent opposite phase, light transmissive regions on a phase shifting layout for producing the feature 200. The phase shifting layout will make use of a dark field mask and the area between, and around, the shifter 202 and the shifter 204 will be non- transmissive (opaque), e. g. chrome, etc.

[0061] The binary trim layout includes the trim 206 shown as a dashed line (the dashed outline follows, and is obscured by, the solid line of the feature 200 along the top edge of the feature 200). The binary trim layout can be a bright field mask for use in conjunction with the phase shifting mask to define the feature 200. Opaque areas in the trim mask protect features of the target layout formed by the phase shift mask that are part of the target feature. Transmissive areas in the trim mask are used to clear unwanted exposure features caused by the phase shift mask. Also, transmissive areas are used to refine or delimit features caused by the phase shift mask so that they more closely match the target layout. For example, transmissive areas in the trim mask are used to define line ends for lines that are formed using phase transitions in the phase shift mask.

[00621 Turning to OPC of the phase layout. Dissection and evaluation will occur where the phase layout abuts the target layout, e. g. the feature 200. Fig. 2 shows a portion of the dissection and evaluation points for the shifter 202 and the shifter 204.

The dissection and evaluation points are placed along the full length of the edges of the shifters where the shifters abut the feature 200 in some embodiments of the invention; see e. g. Fig. 4 and Fig. 5.

[0063] Turning to OPC of the trim layout, the trim 206 (opaque rectangle overlying feature 200 and portions of shifters 202 and 204) includes dissection and evaluation

points where the trim 206 abuts the feature 200, e. g. along the top edge of the feature 206. In this example, the dissection points 210,212 and evaluation point 211 for the trim pattern and the phase shift pattern along the top edge of feature 200 are at positions in the respective patterns that correspond to the same location on the target feature.

More specifically, in this example, the dissection point 210 and the dissection point 212 are used in defining OPC segments in both layers. However, more generally, the placement of the dissection points and evaluation points can be determined for particular layouts based on the edge segments where the transmissive feature of the trim pattern abuts the target layout and the dissection/evaluation segment length criterion being used. In this example, the trim 206 is used to define an end of the feature 200 between the points 210 and 212, and to protect the rest of the feature 200 formed by the phase transition between shifters 202 and 204.

[0064] Summarizing, according to one embodiment of the invention, the trim and phase shift patterns are dissected only where they abut the original (target) layout. Once the dissection and evaluation points have been placed, OPC (and other proximity correction adjustments) can be performed. The dissection of an inner corner will now be considered.

Inner Corner Dissection [0065] Fig. 3 shows the placement of dissection and evaluation points on both phase shift patterns and trim patterns for an inner corner. More specifically, Fig. 3 includes a feature 300 and shows respective portions of the phase shift and trim patterns that will be used to define the feature 300. The phase shift pattern includes two shifters ; a shifter 302 and a shifter 304. Shifters on the opposite side of the feature 300 that would allow the feature 300 to be completely defined by phase shifting are not shown. One side of the outline of the trim 306 on the trim pattern in the corner is shown as a dashed line.

(Note: the trim 306 follows, and is obscured by, the boundary of the feature 300 on the inside corner through points 322,318,320. See, e. g. Fig. 19 and Fig. 23 for full examples of the shape of trim patterns.) As in Fig. 2, several dissection and evaluation points are omitted from the figure to focus on the inner corner area ; see Fig. 4 and Fig. 5 for more complete examples.

[0066] The gap between the shifter 302 and the shifter 304 and can also be referred to as a"cut."In a dark field phase mask produced from the layout, this gap will be

nontransmissive on the phase shift mask. The corresponding trim mask typically includes a transmissive area in the region of the cut to clear any unwanted exposure caused by the phase transition between the shifter 302 and the shifter 304. In one embodiment of the invention, the size and shaping of the cuts on the phase shift mask are designed to support easy mask manufacturability and/or satisfaction of design rule checking. In one embodiment, the gap areas make use of 45 degree and ninety degree angles. In another embodiment, an inner corner cut is formed by a substantially square shaped notch with a forty-five (45) degree angle straight line out of one corner to separate the two shifters. The separation between the shifters along the cut is designed to obey any minimum spacing rules for one or more of mask manufacturability and/or design rule checking. Unless stated otherwise, the cuts and angles shown in the figures will be at 45 and 90 degrees. Other embodiments of the invention can support handling of patterns with features aligned and positioned with non-Manhattan geometries (e. g. non 45/90 degree angles); however, such embodiments may make use of shifters and cuts while using Manhattan geometries in the cut shapes to the extent possible.

[0067]. Fig. 3 highlights several dissection points for the phase shift pattern, more specifically the dissection point 310, the dissection point 312, the dissection point 314, and the dissection point 316 are shown. The dissection point 314 and the dissection point 316 have been positioned proximate to the junction of the respective shifter 304 and shifter 302 and the feature 300. Additionally, the dissection point 310 and the dissection point 312 have been positioned in line with the point (31Oa and 312a, respectively) where the full width of the shifter begins, as shown by the dotted-and- dashed line extending from the dissection points to the full width of the respective shifter 304 and shifter 302. However, in some embodiments those dissection points are only substantially inline with the beginning of the full width of the shifter. In other embodiments, those dissection points are further into the full width portion of the shifter by a small amount, e. g. a few nanometers.

[0068] Fig. 3 shows distinct trim pattern dissection points where the trim 306 meets the feature 300, e. g. the dissection point 320 and the dissection point 322. Accordingly, in this embodiment the trim pattern dissection segments will run from the dissection point 318 to the dissection point 322 and the dissection point 320. This supports differing OPC corrections for the phase and trim patterns. However, if desired the

dissection point 314 and the dissection point 316 can be used in OPC of the trim pattern.

The dissections for an outer corner will now be considered.

Outer Corner Dissection [0069] Fig. 4 shows the placement of dissection and evaluation points on both phase and trim patterns for an outer corner. Here a feature 400 will be defined using the shifter 402 and the shifter 404. The outline of the trim pattern is not explicitly shown; however, several of the dissection points are positioned at the intersection of the trim pattern and the feature 400. Specifically, contrast dissection point 406 and dissection point 408.

Dissection point 406 is positioned where the shifter 404 meets the feature 400 while the dissection point 408 is positioned where the trim 410 meets the feature 400. As noted, this facilitates different OPC corrections for the phase and trim patterns. Note also the corresponding dissection points on the other edge of the corner, not labeled.

[0070] Turning more closely to the bottom portion of the feature 400, three dissection points: the dissection point 420, the dissection point 422, and the dissection point 424, are shown. The dissection point 420 and the dissection point 424 form an OPC segment along the feature 400 for the trim pattern. However, because in this example the segment is too small relative to the selected OPC parameters, no evaluation point is placed. The dissection point 422 forms an OPC segment for the phase shift pattern with the dissection point 422a further up the feature 400 where the shifter 402 and the feature 400 abut in the phase shift pattern.

[0071] Also note that the cut between the shifter 402 and the shifter 404 is of a similar shape and structure to the cut used between the shifters in Fig. 3. Here, the 45 degree opening stretches out of the upper corner and towards a square end. The shape of the cut is designed to be design rule checker (DRC) clean and/or to support better mask manufacturability.

[0072] Before turning to a process flow for the OPC process a more complex dissection will now be considered.

Complex Dissection [0073] Fig. 5 shows the placement of dissection and evaluation points on both phase and trim patterns for a complex pattern including the feature 500. Four shifters, the

shifter 502, the shifter 504, the shifter 506, and the shifter 508 will be used to define the feature 500. As in Fig. 4, the outline of the trim pattern 530 is shown as a dashed line.

[0074] Fig. 5 illustrates the use of different dissection points for the phase and trim pattern at the inside corner cut of the shifter 506 and the shifter 508. Fig. 5 includes separate dissection points for where the trim pattern meets the original feature.

Specifically, Fig. 5 includes the dissection point 512 and the dissection point 516 for the trim pattern. Note in this example that the trim evaluation point 520 is approximately midway between the two dissection points. In contrast, the dissection point 514 is at the intersection of the shifter 508 with the feature 500 and is a phase shift pattern dissection point. The corresponding dissection and evaluation points for the shifter 506 are shown without reference numerals, for clarity of illustration. Compare area around the trim dissection point 516 with the area around the corner 535 where there is only phase layer.

Note the absence of overlapping OPC segments since only the phase layer will be adjusted.

[0075] Now, a process for performing OPC on layouts where substantially all portions of a pattern in the layout are being defined using a phase shifting mask will be considered in greater detail.

Process Flow [0076] Fig. 6 is a process flow diagram for setting up and performing optical proximity correction (OPC) for a layout. This process flow could be used to setup the OPC dissections seen in Figs. 2-5 and then to perform the appropriate optical proximity correction. This process is designed for use on layouts where substantially all portions of a pattern in the layout are being defined using a phase shifting mask.

[0077] The process starts at step 600, as OPC parameters and the layout (phase shift and trim patterns) to be corrected are loaded. In one embodiment, the OPC parameters include information about the process, e. g. an optical model, optionally calibrated, is provided at this stage. Additionally, settings such as the frequency of evaluation and dissection for different types of line segments within the layout can be provided. As noted previously, a model based OPC process is considered here, if a rule based OPC is used, the process can be suitably adapted. Loading the layout may involve loading the layout data, e. g. from a GDS-II stream format file, a mask electron beam exposure system (MEBES) format, and/or some other suitable format. In some embodiments, the

layout may be in the memory of the computer system performing the process of Fig. 6.

For example, if a set of one or more UNIX (R) workstations is being used to perform the process of Fig. 6, the layout might be loaded from a network attached storage device into memory.

[0078] In some embodiments, the layout loaded at step 600 is a non-phase shifted layout. In such embodiments, a conversion process may be performed to convert the input layout to a layout where substantially all portions of a pattern in the layout are being defined using a phase shifting mask prior to continuing the process at step 610.

This conversion can be performed according to some embodiments as described in United States Patent Application No. 09/932,239. In one embodiment, the conversion to a phase shifting layout and the process of Fig. 6 can both be performed by a suitable version of the iN-Phase (TM) software from Numerical Technologies, Inc.

[0079] The process continues at step 610 with definition of dissection and evaluation points. In some embodiments step 610 and step 620 operate in parallel. In other embodiments, the order is swapped with step 620 performed prior to step 610. In still other embodiments, the steps are combined into a single step that defines evaluation and trim points for both layers.

[0080] More specifically, the dissection points are placed along edge segments of the trim pattern and the phase shift pattern according to the OPC parameters. For example, the OPC parameter might specify that all edge segments are to be dissected every 120 nm. Other embodiments, might allow greater selectivity, e. g. for corners, etc.

Once the dissection points are placed, the evaluation points can be placed in relation to or on the edge between a pair of dissection points. Returning to Fig. 2, at step 610, the only portion of trim abutting the feature 200 is the top edge and the length of that edge is such that only the two dissection points can be placed. Then an evaluation point is positioned between those dissection points. As noted, the evaluation point need not be placed directly on the edge segment and multiple evaluation points could be used for a given segment.

[0081] In either case, at step 630, optical proximity correction can be performed. In one embodiment a model based OPC process is used. In another embodiment a rule based OPC process is used. In another embodiment a hybrid, or mixed mode, OPC process is used. In one embodiment, the mixed mode OPC process of United States Patent Application No. 09/514,551 entitled"Method And Apparatus For Mixed-Mode

Optical Proximity Correction"filed 28 Feb 2000, and assigned to the assignee of this application, is applied to the layout at step 630.

[0082] The result, at step 640, is a corrected layout that can be output, e. g. to screen, disk, network attached storage, Internet, etc. The output layout might even be directly transmitted to a mask making machine, e. g. input in GDS-II stream format and output in MEBES for use in mask making. Other formats may be used in some embodiments of the invention, including proprietary formats used by such companies as Hitachi, Toshiba, Jeol, Leica, etc.

[0083] In one embodiment, an instance based (IB) representation of the layout is used for performing the process of Fig. 6. An IB representation is described more fully in United States Patent application No. 09/835, 313 having a filing date of 13 Apr 2001 and inventors Chin-Hsen (Michael) Lin, et. al., and assigned to the assignee of the present invention.

[0084] Now corner OPC will be considered in greater detail with reference to several simulations.

Corner OPC [0085] Turning to Figs. 7-8, a corner from a layout is shown. Specifically, Fig. 7 shows a layout 700 and Fig. 8 a layout 800. Except for the configuration of the phase shifters, the layouts are identical. In the layout 700, the corner is defined using two opposite phase shifters separated by a cut. In the layout 800, a single, uncut phase shifter is used. Neither layout has yet been corrected with OPC.

[0086] Simulations of the two layouts are shown in Figs. 10-11, respectively. The two simulations were performed with identical parameters and a coherency (a) = 0.5.

Fig. 10 shows a simulation 1000 of the layout 700 and Fig. 11 shows a simulation 1100 of the layout 800. Note that without application of OPC, the image contour for the printed image is closer to the target layout for the layout 700 than the layout 800. (Note, the phase shifters are shown as yellow lines and the trim pattern outline as a dark blue/black line in Figs. 10-11.) [0087] Fig. 9 further considers the tradeoffs between the number of cuts and the amount of OPC that will be needed. Fig. 9 illustrates a layout 900 for a landing area; the phase shifters defining the contact have been cut at every corner. The result: sharper

corner definitions as seen in Fig. 12 showing a simulation 1200 for that uncorrected layout 900.

[0088] Fig. 13-14 further illustrate this with Fig. 13 showing a layout 1300 including two landing areas, a left landing area 1310 and a right landing area 1320.

Again, maximizing the number of cuts, right landing area 1320, improves corner definition without OPC as shown in Fig. 14 that includes the simulation 1400.

[0089] However, there can be circumstances where leaving the phase conflict can produce better results as shown in Figs. 15-16. As shown, Fig. 15 includes the layout 1500 and the left hand copy of the feature includes a phase conflict 1510 where the shifter is not of the appropriate phase to facilitate the definition of the feature. Contrast that with the right hand copy of the feature where all phase conflicts have been removed. The dissection and evaluation points used for both arrangements are shown as well.

[0090] The layout 1500 was dissected and corrected according to the process of Fig. 6 using parameters of 120 nm/120 nm/120 nm/120 nm indicating the minimum segment length, the maximum segment length, the outer corner segment length, and the inner corner segment length, respectively. An outline of the OPC correction is visible in dark blue/black inside the simulation 1600 results of Fig. 16. (Both the corrected trim and corrected phase shift patterns are shown in dark blue/black.) [0091] Here, a layout with a phase conflict produces better results. Accordingly, some embodiments of the invention when defining the phase shift pattern attempt to maximize the number of cuts for contacts and corners to reduce the need for OPC correction. In contrast, other embodiments, may perform simulations for a given feature, structure, or pattern, to select a phase assignment strategy for the given feature. For example, if the OPC parameters are known when the phase shifting layout is defined a simulation of the type used to generate Figs. 10-12,14 and 16 can be used and the critical dimension and corner variance computed. However, this approach is likely remaining computationally infeasible for layouts for real world designs. As such, other embodiments may use rule or shape tables to select strategies.

[0092] For example, a rule table might specify that contacts should receive the maximum number of cuts where possible, while another more specific rule might identify shapes like the ones of Fig. 15 where a phase conflict should be maintained.

[0093] In some embodiments, one or more cost functions are defined to select when and how to cut the phase shift regions that will define the features of the layout. In some embodiments the cost functions are designed to minimize the number of divisions taken, e. g. favor layout like the left landing area 1310 over the right landing area 1320.

However, some embodiments of the invention use cost functions that take into account features and patterns on other layers. For example, if a contact was positioned inside the right landing area 1320, then the cost function might favor maximizing the number of cuts to improve containment of the contact.

[0094] Finally, two additional layouts will be considered along with detailed examples of the phase mask after optical proximity correction and the trim masks as well.

Further Examples Example 1 [0095] Fig. 17 shows a portion of a layout 1700, the layout 1700 extends past the area shown. The layout includes a plurality of features being defined using phase shifters. Several aspects of the layout bear mention.

[0096] Particularly, the cut 1710 should be contrasted with the cut 1740. In the case of the cut 1710, a diagonal cut of the type used for the cut 1740 was not possible so a straight line cut was used instead. Also, the adjustment for proximity correction in cut 1710 includes offsetting of a line segment 1711 in a direction orthogonal to adjacent line segment 1712 on the phase shift pattern.

[0097] In a similar vein, cuts are extended where appropriate, e. g. the cut 1720 is an extended corner cut that was made contiguous with a no longer distinctly visible cut for the endcap.

[0098] The finished layout also includes several phase conflicts, two of which, the phase conflict 1750 and the phase conflict 1760, are called out. Another phase conflict also exists near the cut 1730. If the cut 1730 had been moved from the bottom right corner of the contact to the upper right corner, the phase conflict in that region would be removed.

[0099] In this example, the phase conflicts remaining in the layout will be compensated for-to the extent possible-by OPC according to the process of Fig. 6. In Fig. 18 the layout 1800 for the OPC corrected phase shift pattern is shown. The OPC

was performed only on portions of the phase shift pattern that abut the location of the target layout.

[0100] Similarly, Fig. 19 shows the layout 1900 for the OPC corrected trim layout.

[0101l Finally, Fig. 20 includes the simulation 2000 of the printed image of the phase layout 1800 used in combination with the trim layout 1900. The target layout is shown as a dark blue/black line.

10102] Reviewing the simulation 2000, it is apparent that the phase conflicts were not an issue as the features in the layout were defined. Additionally, measurements can be taken to verify whether or not the simulated printed image meets specifications. In this example it does.

Example 2 [0103] Fig. 21 shows a portion of a layout 2100, the layout 2100 extends off the shown edges. The layout includes a plurality of features being defined using phase shifters, shown. The portion of the layout shown has been completely phase shifted without conflicts.

[0104] Figs. 22 and 23 show the phase shift pattern and the trim pattern respectively after OPC has been performed. More specifically, the layout 2200 in Fig. 22 illustrates the phase shift pattern after OPC and the layout 2300 in Fig. 23 illustrates the trim pattern after OPC.

[0105] Finally, Fig. 24 includes the simulation 2400 of the printed image of the phase layout 2200 used in combination with the trim layout 2300. The target layout is shown as a dark blue/black line.

[0106] Reviewing the simulation 2400, it is apparent that the phase conflicts were not an issue because the layout was defined within specification. This can be determined by taking measurements from simulation 2400 and comparing them against requirements, e. g. acceptable CD variance, electrical connectivity, etc. Additionally, it is useful to note that the location of the cut at the corner of the"T"junction is not detectable, due in part to OPC.

Representative Alternative Embodiments [0107] Embodiments of the invention can operate on layouts where portions of the phase shifting layout have been targeted for different critical dimension sizes. For

example, Fig. 25 illustrates a portion of a layout where a feature 2500 is defined using phase shifting by a shifter 2510, a shifter 2520, a shifter 2530, and a shifter 2540.

Additionally, a region 2550 is indicated where the shifter 2510 and the shifter 2540 have been modified to produce a smaller critical dimension (CD) than is used for the feature 2500. However, the placement of evaluation and dissection points for the phase shift pattern within the region 2550 is the same as described above.

[0108] Furthermore, the shifter widths abutting the region 2550 can be further adjusted to reduce the need for OPC while printing the narrow critical dimension feature in the region 2550. For example, if necessary the outer edges of the shifters could be made wider than the shifters in the surrounding region, not shown. Or conversely, the shifter width in the region 2550 could be made narrower, see Fig. 26. More specifically, Fig. 26 includes the feature 2500 defined by the shifter 2520, the shifter 2530, a shifter 2610 and a shifter 2640. The shifter 2610 and the shifter 2640 are narrower in the region 2650 where reduced critical dimension is desired.

[0109] The appropriate sizing, e. g. shifter widths, can depend on the available field as well as other features surrounding the shifters in the region 2550. Also, in some embodiments, a preferred shifter width for reduced critical dimension features may be selected, e. g. nX, where n > 0.0. Then, to the extent practical based on the layout, the shifter widths in regions like the region 2550 are modified so that the intervening control chrome is a narrower size and the widths correspond to the preferred shifter width.

[0110] In variants of the above embodiments, the target layout may be reduced at critical areas where reduced feature size is desired prior to application of the phase shifting process. In such an event, the resultant layout will look similar to Fig. 25 and/or Fig. 26.

[0111] Additionally, although the description has primarily focused on examples of defining a polysilicon, or"poly", layer within an IC, phase shifting can be used to define other layers of material, e. g. interconnects, metal, etc.

[0112] Embodiments of the invention can also operate on cuts shaped other than as described, for example the invention can be applied to cuts where the notch is a simple diagonal opening without the addition of a square head, or base. However, experimental test runs have shown that such layouts frequently, but not always, result in OPC corrections inward towards the target layout for the first segment. For example,

returning to Fig. 3, if the shifter 302 met the dissection point 316 from a 45 degree line, then the first OPC segment running to the dissection point 312 would typically be biased in towards the feature 300. This may or may not result in a design rule compliant phase shifter, or trim, shape. Nonetheless, OPC can be performed on such a layout as described above.

[0113] In one embodiment, the optical proximity correction (OPC) process for a layout where substantially all of the layout is defined using phase shifting comprises performing OPC of the binary layer for non-critical features and performing OPC of the phase shift pattern for critical features. In each case, OPC is only performed on edges where the respective layer abuts the target layout.

[0114] Some embodiments of the invention include computer programs for simulating stepper exposures using masks to compute appropriate relative dosing between phase and trim/binary exposures. In one embodiment, the ICWorkbench (TM) software produced by Numerical Technologies, Inc., San Jose, California is used to simulate the exposure conditions, e. g. as seen in Fig. 10, etc. In other embodiments, computer programs are used to perform optical proximity corrections. In one embodiment, the iN-Phase (TM), iN-Tandem (TM), and/or Photolynx (TM) software programs produced by Numerical Technologies, Inc., are used to perform the optical proximity correction processes. In other embodiments, suitable tools from Avant! and/or Mentor Graphics are used in the optical proximity correction processes. In some embodiments, the computer programs are stored in computer readable media, e. g. CD- ROM, DVD, etc. In other embodiments, the computer programs are embodied in an electromagnetic carrier wave. For example, the electromagnetic carrier wave may include the programs being accessed over a network.

[0115] As used herein, the term optical lithography refers processes that include the use of visible, ultraviolet, deep ultraviolet, extreme ultraviolet, x-ray, e-beam, and other radiation sources for lithography purposes.

Conclusion [0116] The foregoing description of embodiments of the invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations will be apparent. The embodiments were chosen and described in order to

best explain the principles of the invention and its practical application, thereby enabling others to understand the invention for various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims.

[0117] What is claimed is: