# Patent

## Title
Method for correcting a mask design layout

## Abstract

A method for performing a mask design layout resolution enhancement includes determining a level of correction for the design layout for a predetermined parametric yield with a minimum total correction cost. The design layout is corrected at the determined level of correction based on a correction algorithm if the correction is required. In this manner, only those printed features on the design layout that are critical for obtaining the desired performance yield are corrected, thereby reducing the total cost of correction of the design layout.

## Background
This Application claims the benefit of prior U.S. application Ser. No. 10/787,070, filed Feb. 25, 2004, which claimed priority from U.S. Provisional Application No. 60/450,051, filed Feb. 25, 2003.
FIELD OF THE INVENTION
The present invention is in the fields of optical lithography and integrated circuit fabrication.
BACKGROUND OF THE INVENTION
Consistent improvements in the resolution of optical lithography techniques have been a key enabler for continuation of Moore's Law. However, as minimum printed feature sizes continue to shrink, the wavelength of light used in modern lithography systems is no longer several times larger than the minimum line dimensions to be printed, e.g., today's 130 nm CMOS processes use 193 nm exposure tools. As a result, modern CMOS processes, for example, are operating in a sub-wavelength lithography regime. The International Technology Roadmap for Semiconductors (ITRS) offers projections on the requirements of next generation lithography systems and states that achieving aggressive microprocessor (MPU) gate lengths and highly controllable gate CD control are two key issues.
To meet these requirements, resolution enhancement techniques (RETs) such as optical proximity correction (OPC) and phase shift mask (PSM) technology are applied to mask design layouts. Advanced mask manufacturing technologies, such as high-precision electron beam machines, high numerical aperture exposure equipment, high-resolution resists, and extreme ultraviolet and possibly electron-beam projection lithography, could also play roles in continued lithography scaling. The result of each of these approaches is a large increase in mask costs.
In the current design-manufacturing interface, no concept of function is injected into the mask flow, i.e., current RETs are oblivious to design intent. Mask writers today work equally hard in perfecting a dummy fill shape, a piece of the company logo, a gate in a critical path, and a gate in a non-critical path, for example. Errors in any of these shapes will trigger rejection of the mask in the inspection tool. The result is unduly low mask throughput and high mask costs.
SUMMARY OF THE INVENTION
A method for performing a mask design layout resolution enhancement includes determining a level of correction for the design layout for a predetermined parametric yield with a minimum total correction cost. The design layout is corrected at the determined level of correction based on a correction algorithm if the correction is required. In this manner, only those printed features on the design layout that are critical for obtaining the desired performance yield are corrected, thereby reducing the total cost of correction of the design layout.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a flowchart describing the method for determining the level of correction of mask features in accordance with one embodiment of the invention;
FIGS. 2(a) to 2(c) are diagrams showing a mask feature with different levels of correction; and
FIG. 3 is a table illustrating a method for performing a correction algorithm in accordance with one embodiment of the present invention.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention concerns reducing mask costs through process means. In accordance with one embodiment, the invention involves the use of various levels (e.g., moderate to aggressive) of resolution enhancement techniques (RETs), such as optical proximity correction (OPC), phase-shift masks (PSM) and sub-resolution assist features (SRAFs), for example, to limit mask complexity.
Many printed features in the layout of the mask design are not timing-critical and a larger degree of process variation may be tolerable for them. At the same time, a certain minimum level of process correction is required to ensure printability of the layout. Forward-annotating the design's functional information will permit less total correction to meet the parametric yield requirements. Less aggressive use of RET translates to lowered costs through reduced figure counts, shorter mask write times and higher mask yields.
In the present application, a “selling point” is defined as the circuit delay which gives a predetermined parametric yield. For example, 99% parametric yield means that 99% of parts would be expected to run at the target frequency or higher. Given the range of allowable corrections for each feature in the mask design layout as well as the cost and parameter variances associated with each correction level, one embodiment of the present invention determines the level of correction for each feature such that the prescribed selling point delay is attained with minimum total correction cost. In other words, the present invention solves the “minimum cost of correction” (hereinafter “MinCorr” where appropriate) problem.
In accordance with one embodiment of the invention, FIG. 1 describes a method for determining the level of correction for each feature such that a prescribed selling point delay is attained with minimum total correction cost. Given a mask design layout that meets performance constraints (after logic synthesis, placement and routing processes have been completed, as is known in conventional design flow), a statistical static timing analysis (SSTA) is performed to output the probability density function (PDF) of circuit performance, for example, the arrival time at all nodes in the circuit, given deterministic arrival times at the primary inputs (PIs) of the mask design layout (block 10). Circuit performance may also be described in terms of power and leakage through, for example. The SSTA is a timing analysis wherein probability distributions of the arrival times are propagated from inputs to outputs instead of deterministic arrival times as in static timing analysis (STA). Those skilled in the art will recognize that STA is a circuit timing analysis methodology which propagates worst-case arrival times of signals from inputs to output statically, i.e., without any circuit simulation.
If the target yield has been met (block 12), then the mask design layout does not require any correction, and the process ends at this point. For example, a target yield is met if a predetermined percentage, e.g., 99%, of parts of the design layout run at the target frequency or higher as determined based on the SSTA. Whether the target yield has been met is based on yield-aware performance library models (described in more in detail below) which capture delay mean, variance and the relative cost of RET for each level of correction for each library master. On the other hand, if the target yield has not been met (block 12), the most yield critical features, (i.e., the features which the maximum impact on circuit yield among all features on the design layout) are corrected using a RET such as OPC based on a correction algorithm (described in more detail below) (block 14), and the corrected mask design layout undergoes another SSTA (block 16). After the SSTA has been performed, it is again determined whether the corrected mask design layout has met the target yield (block 12).
If the target yield has now been met (block 12), then the design layout does not require any further correction, and the process ends at this point. On the other hand, if the target yield has not been met (block 12), the design layout goes through another correction process as described above. These steps, as described in blocks 12, 14 and 16, are repeated iteratively until the target yield is met for the entire design layout. FIGS. 2(a)-2(c) shows examples of a printed feature with no correction, moderate level of correction and aggressive level of correction, respectively.
It should be understood that one embodiment of the invention assumes that different levels of RET can be independently applied to any gate in the design, i.e., any logic components of any digital design. The granularity at which different levels of RET can be applied within the design may be at the individual feature or transistor level, at the gate level, at the standard-cell level, or even at higher levels. The description of the invention is focused on the gate level for purposes of illustration. Corresponding to each level of correction, there is an effective channel length (Leff) variation and an associated cost. It is also assumed that variation-aware performance library models are available for each level of correction.
In the above description with respect to the flowchart of FIG. 1, a target selling point delay is assumed to be given by a user input. Given the delay mean and standard deviation at every circuit node, the SSTA computes the yield point at each primary output. Thus, we can calculate a slack value or σ-slack, which is the slack available in yield, i.e., (target yield-calculated yield), at all primary outputs. One embodiment of the invention enables the correction or decorrection of printed features (e.g., gates) to minimize the cost of RET while still meeting the σ-slack constraints. Correction of printed features generally increases the mask correction while decorrection decreases mask cost.
The correction algorithm discussed above with respect to block 14 in FIG. 1 is now described according to one embodiment of the invention. To reduce the algorithmic complexity, we assume that the standard deviations of the gate-delays are additive, i.e., we assume a perfect positive correlation between gate-delay variations along any path. If we assume that the path delay distributions remain Gaussian, then we can propagate the predetermined yield point (99% (i.e., μ+3σ), for example) to the primary output. More specifically, we assume that

μ1+2+kσ1+2=μ1+kσ1+μ2+kσ2   (1)

where μ is the mean, σ is the standard deviation of the performance distribution of gates, and μ+kσ denotes a certain level of parametric yield. This also enables us to use STA instead of SSTA to verify the σ-slack correctness of the circuit.

Thus, in accordance with one embodiment of the invention, we can formulate the decorrection problem as a mathematical programming problem as follows.

Minimize Σijxij 

Σjxij=1

Σjxij dij+wdi<wdk ∀ k ∈ fanout(i)

wdk=U ∀ k ∈ PO

xij ∈{0,1}  (2)

where,

dij=μ+kσ number for gate i corresponding to level of correction j,
cij=cost of correction number for gate i corresponding to level of correction j,
xij=1 if gate i is corrected to level j,
wdi=worst case μ+kσ delay at input of gate i, calculated using STA, and
U=μ+kσ delay upper bound at the primary outputs (POs).
The above integer program requires running the STA tool incrementally to update wdi every time any xij is updated. In this manner, the integer program, i.e., the correction algorithm, provides the level of correction for each printed feature. The design layout is physically corrected based on these calculated levels of correction. In one embodiment, the STA is built into a computer program for running the integer program. The integer program may also be programmed to run directly on the STA.
It should be noted that the results we obtain from solving the program are strictly pessimistic if the circuit consists of perfectly correlated paths. This is because gates would always be somewhat less than perfectly correlated, in which case the standard deviation of the sum would be less than the sum of standard deviations. However, in practice, a circuit contains many partially correlated or independent paths. In this case, calculating the delay distribution at any primary output (PO) requires computing the maximum of the delay distributions of all the paths fanning out to the PO. The resultant Max distribution may not remain Gaussian and is likely to have larger mean and smaller variance than the parent distributions.
To account for this, one embodiment of the invention again runs SSTA on the decorrected circuit and computes σ-slacks at all POs (block 16, shown in FIG. 1). We then fix the negative slack (i.e., the calculated yield is less than target yield) at any PO by correcting the large-fanout nodes at the last few levels (close to the leaves) in the fanin cone of the PO. We distribute the positive slack (i.e., the calculated yield is larger than target yield) among the small-fanout nodes in the first few levels of the fanin cone of the PO. This is done iteratively until σ-slacks at all POs become sufficiently close to zero.
In accordance with another embodiment of the invention, the correction algorithm discussed above with respect to block 14 of FIG. 1 is obtained by drawing parallels between the MinCorr problem (i.e., the problem of determining the level of correction for each feature) and the known gate sizing and delay budgeting problems. One analogy is that allowed “sizes” in the minimum cost of correction problem correspond to the allowed levels of correction. For each instance in the design, there is a cost and delay a associated with every level of correction. Mapping between gate sizing and minimum cost of correction problem is depicted in FIG. 3, and is correct to the extent of assuming additivity as in Equation (1). It should be noted that Equation (1) need not be assumed if a correction (sizing) tool (not shown) is driven by SSTA rather than STA.
Given FIG. 3, we can construct yield libraries in a similar fashion as timing libraries. This enables us to use the yield (timing) libraries with a commercial synthesis tool such as Synopsys Design Compiler (DC) to recorrect (resize) the design layout to meet the yield (delay) target with the minimum cost (area). A timing library, which is a known, gives the area and delay of each cell master. A synthesis or sizing tool uses this timing library to choose sizes of all cells or gates in the design layout with the objective of minimizing cycle time and/or total area. In one embodiment of the invention, we replace the standard timing library with a yield library with the transformation given in FIG. 3. Use of a commercial tool enables us to make many optimizations in practical runtimes. Examples include minimizing the cost of correction given the selling point delay, and minimizing the selling point delay given an upper bound on the cost of RET, for example, OPC.
In accordance with one embodiment, a new worst case timing model is generated by using Monte-Carlo (MC) simulation, or using a deterministic corner-based approximation. MC simulations assume that every parameter (oxide thickness (Tox), channel doping (Nch), channel length, etc.) varies simultaneously in a normally distributed fashion, and consequently provide the best accuracy at the cost of large runtime. Corner-based simulations use a single value for each parameter to find a single worst-case delay.
The yield-aware library also captures the relative cost of RET at each level of correction for each master. Correction cost information is included in the newly generated yield library files using the cell area attribute. Our metric for cost is given by relative figure count multiplied by the number. of transistors in each cell. We use this weighted cost function to capture: (1) the cost differences across the three libraries with different levels of correction applied, and (2) the relative difference in cost of correcting cells with different sizes/complexities. We do not simply use the initial area as a weighting factor as we want to emphasize the correction of actual devices rather than field regions which may dominate the cell area. Another option is to weight the figure count by the total transistor perimeter in a cell. Figure count is found to be consistent across cell types, as would be expected from a standard-cell library that has limited diversity in the arrangements of devices within the cell.
As stated above, once the yield library is constructed, a commercial synthesis tool such as Synopsys Design Compiler (DC) is used to solve the minimum cost of correction (MinCorr) problem. Specifically, we input a yield library in which identical cells in the original timing library show up as three “sized” versions with same cell function but different “areas” and “timing”. We then use DC to perform gate-resizing on the synthesized netlist with a selling point delay constraint given as the maximum circuit delay constraint.
In accordance with another embodiment of the invention, instead of having discrete levels of correction (medium, aggressive, etc.) for the layout geometries, exact variation tolerances are computed independently for each layout feature. In other words, the level of correction can be extended to be more quantitative. Given the parametric yield constraint on a performance metric for the circuit, tolerable performance variation can be calculated for the layout features by using any known performance budgeting algorithm.
This performance variation tolerance is then translated to CD variation tolerance. CD variation tolerance is the maximum deviation from nominal CD, which refers to gate length that determines performance, that still meets performance variation constraints previously calculated. The dependence of performance metrics such as delay and power on gate length (i.e. CD) is known in the art. Given the value of the performance metric, corresponding CD for the gate can be calculated by one of ordinary skill in the art.
As a result, we have CD tolerance for every feature in the layout. CD is determined by two edges of a feature (i.e., gate). As is known, gates are rectangles of polysilicon. CD or gate length refers to the width of these rectangles which is determined by left and right edges of the rectangle. Commercial OPC tools, for example, work to obtain correct printing of these edges. We translate the CD variation tolerance to two Edge Placement Error (EPE) tolerances (left and right) for every layout feature. One translation method, for example, may include fixing the EPC for each of the edges at 5 nm, If CD variation tolerance is 10 nm. These EPE tolerances are then enforced using a commercial RET tool. With a minimum correction objective, the maximum performance variation tolerance and hence maximum EPE for each layout feature is calculated without losing parametric yield. The RET tool enforces this varying tolerance across the layout resulting in a minimum cost mask.
While specific embodiments of the present invention have been shown and described, it should be understood that other modifications, substitutions and alternatives are apparent to one of ordinary skill in the art. Such modifications, substitutions and alternatives can be made without departing from the spirit and scope of the invention, which should be determined from the appended claims.
Various features of the invention are set forth in the appended claims.

## Claims

1. A method for performing a mask design layout to be printed, the method comprising:
determining a first level of correction for a mask design layout by using a computer system for a predetermined parametric yield with a minimum total correction cost; and
correcting the mask design layout at said first level of correction based on a correction algorithm if said first level of correction is determined to be required.

2. The method as defined in claim 1 further including determining a second level of correction for the mask design layout for said predetermined parametric yield after correcting the mask design layout at said first determined level of correction.

3. The method as defined in claim 2 further including, correcting the mask design layout at said second level of correction based on said correction algorithm if said second level of correction is determined to be required.

4. The method as defined in claim 3 wherein the mask design layout is not corrected if it is determined that said second level of correction is not required.

5. The method as defined in claim 1, wherein determining the first level of correction includes identifying features from a yield library.

6. The method as defined in claim 5, wherein determining the first level of correction includes using a synthesis tool to selectively choose features from the yield library to meet the predetermined parametric yield.

7. The method as defined in claim 6, wherein selected features from the yield library are identical and have different areas and timing.

8. The method as defined in claim 1, wherein determining the first level of correction includes identifying discrete levels of correction.

9. The method as defined in claim 1, wherein determining the first level of correction includes using exact variation tolerances that are computed independently for layout features of the mask design layout.

10. The method as defined in claim 1, wherein determining the first level of correction includes obtaining a probability density function.

11. The method as recited in claim 1, wherein a statistical static timing analysis (SSTA) or static timing analysis (STA) is performed in determining the first level of correction.

12. The method as recited in claim 1, further comprising: printing the mask design layout to define a mask after the first level of correction or after additional levels of correction.

13. The method as recited in claim 12, wherein the mask is used to define features of a circuit during integrated circuit fabrication.