Fast freeform source and mask co-optimization method

Abstract

The present disclosure relates to lithographic apparatuses and processes, and more particularly to tools for optimizing illumination sources and masks for use in lithographic apparatuses and processes. According to certain aspects, the present disclosure significantly speeds up the convergence of the optimization by allowing direct computation of gradient of the cost function. According to other aspects, the present disclosure allows for simultaneous optimization of both source and mask, thereby significantly speeding the overall convergence. According to still further aspects, the present disclosure allows for free-form optimization, without the constraints required by conventional optimization techniques.

Claims

1. A method, the method comprising: obtaining respective initial descriptions of an illumination and a patterning device pattern for a lithographic process, wherein the patterning device pattern is to be imaged onto a substrate by the lithographic process using the illumination and wherein the initial description of the illumination is represented as a plurality of radiation intensity pixels and the initial description of the patterning device pattern is represented as a plurality of pattern pixels; and iteratively adjusting, by a hardware computer system, gray scale values of the plurality of radiation intensity pixels and values of the plurality of pattern pixels, until a performance metric of the lithographic process is suitably configured with respect to both the illumination and the patterning device pattern.

2. The method of claim 1, wherein the adjusting allows a fully flexible set of illumination points so that the illumination can have a free form.

3. A non-transitory computer-readable medium having instructions therein, the instructions, upon execution by a computer system, configured to cause the computer system to at least: obtain respective initial descriptions of an illumination and a patterning device pattern for a lithographic process, wherein the patterning device pattern is to be imaged onto a substrate by the lithographic process using the illumination and wherein the illumination is represented as a plurality of radiation intensity pixels and the patterning device pattern is represented as a plurality of pattern pixels; and iteratively adjust gray scale values of the plurality of radiation intensity pixels and values of the plurality of pattern pixels, until a performance metric of the lithographic process is suitably configured with respect to both the illumination and the patterning device pattern.

4. The computer-readable medium of claim 3, wherein the instructions configured to cause the computer system to adjust gray scale values allow a fully flexible set of illumination points so that the illumination can have a free form.

5. The computer-readable medium of claim 3, wherein the instructions are further configured to cause the computer system to modify the description of the illumination from a grayscale illumination to a modified illumination constrained by a manufacturability factor.

6. The computer-readable medium of claim 3, wherein the instructions are further configured to cause the computer system to modify the description of the patterning device pattern from a grayscale pattern to a modified pattern constrained by a manufacturability factor.

7. The computer-readable medium of claim 3, wherein the instructions configured to cause the computer system to adjust gray scale values are further configured to cause the computer system to evaluate a cost function, wherein the cost function comprises a function of both the illumination and the patterning device pattern.

8. The computer-readable medium of claim 7, wherein the cost function is formulated in terms of one of the following: worst case edge placement error (EPE) over a given process window, EPE least square function, EPE least p-norm function, inverse normalized image log slope (NILS) p-norm function, contour integral image slope, edge image value least square, edge image p-norm, or image log slope (ILS) p-norm.

9. The computer-readable medium of claim 3, wherein the instructions configured to cause the computer system to adjust gray scale values are further configured to cause the computer system to iteratively adjust gray scale values of the plurality of pattern pixels.

10. The computer-readable medium of claim 3, wherein the instructions configured to cause the computer system to adjust gray scale values are further configured to cause the computer system to use a gradient of the performance metric of the lithographic process with respect to the illumination and the patterning device pattern.

11. The computer-readable medium of claim 10, wherein the performance metric of the lithographic process is suitably configured when the gradient of the performance metric of the lithographic process with respect to both the illumination and the patterning device pattern is essentially zero.

12. The computer-readable medium of claim 3, wherein the performance metric of the lithographic process comprises one or more selected from: an edge placement error (EPE), an image log slope, an inverse image log slope, and/or a contour integral of an image log slope.

13. The computer-readable medium of claim 3, wherein the instructions are further configured to cause the computer system to accelerate the iterative adjusting by performing a first co-optimization without constraints followed by performing a second co-optimization with constraints.

14. A non-transitory computer readable medium having instructions therein, the instructions, upon execution by a computer system, configured to cause the computer system to receive descriptions of an illumination and a patterning device pattern, the patterning device pattern to be imaged by a lithographic process using the illumination; until the illumination and patterning device pattern are suitably configured for a performance metric of the lithographic process, selectively repeat: evaluation of a cost function that is a function of both the illumination and the patterning device pattern the cost function, calculate a derivative of the cost function with respect to the illumination and a derivative of the cost function with respect to the patterning device pattern, and reconfigure the illumination and patterning device pattern descriptions based on both the calculated derivatives.

15. The computer-readable medium of claim 14, wherein the illumination and patterning device pattern are suitably configured when a gradient based on the calculated derivatives has a value of essentially zero.

16. The computer-readable medium of claim 14, wherein the instructions are further configured to cause the computer system to characterize the illumination as independent free form illumination points.

17. The computer-readable medium of claim 14, wherein the illumination is represented as a plurality of radiation intensity pixels having gray scale values.

18. The computer-readable medium of claim 14, wherein the patterning device pattern is represented as a plurality of pattern pixels having gray scale value.

19. The computer-readable medium of claim 14, wherein the instructions configured to cause the computer system to reconfigure the patterning device pattern description are further configured to cause the computer system to: use optical proximity correction; place sub-resolution assist features; and re-characterize the reconfigured patterning device pattern description.

20. The computer-readable medium of claim 14, wherein the cost function is formulated in terms of worst case edge placement error over a given process window.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

(2) FIG. 1 is an exemplary block diagram illustrating a typical lithographic projection system.

(3) FIG. 2 is an exemplary block diagram illustrating the functional modules of a lithographic simulation model.

(4) FIG. 3 is a schematic depiction of the general optimization process employed in certain aspects of the disclosure.

(5) FIG. 4 is a chart illustrating a source and continuous transmission mask co-optimization flow (CTM flow) according to additional embodiments.

(6) FIG. 5 illustrates a resultant source and mask for an example application of a design for a DRAM.

(7) FIG. 6 illustrates a converted “New” illuminator and DOE source according to an example application of the disclosure.

(8) FIGS. 7A and 7B illustrate example masks that result with a DOE source and “New” illuminator according to applications of the disclosure.

(9) FIG. 8 is a block diagram that illustrates a computer system which can assist in the implementation of the simulation method of the present disclosure.

(10) FIG. 9 schematically depicts a lithographic projection apparatus suitable for use with the method of the present disclosure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(11) Prior to discussing the present embodiments, a brief discussion regarding the overall simulation and imaging process is provided. FIG. 1 illustrates an exemplary lithographic projection system 10. The major components are a light source 12, which may be a deep-ultraviolet excimer laser source, illumination optics which define the partial coherence (denoted as sigma) and which may include specific source shaping optics 14, 16a and 16b; a mask or reticle 18; and projection optics 16c that produce an image of the reticle pattern onto the wafer plane 22. An adjustable filter or aperture 20 at the pupil plane may restrict the range of beam angles that impinge on the wafer plane 22, where the largest possible angle defines the numerical aperture of the projection optics NA=sin(Θ.sub.max).

(12) In a lithography simulation system, these major system components can be described by separate functional modules, for example, as illustrated in FIG. 2. Referring to FIG. 2, the functional modules include the design layout module 26, which defines the target design; the mask layout module 28, which defines the mask to be utilized in the imaging process; the mask model module 30, which defines the model of the mask layout to be utilized during the simulation process; the optical model module 32, which defines the performance of the optical components of lithography system; and the resist model module 34, which defines the performance of the resist being utilized in the given process. As is known, the result of the simulation process produces, for example, predicted contours and CDs in the result module 36.

(13) More specifically, it is noted that the properties of the illumination and projection optics are captured in the optical model 32 that includes, but not limited to, NA-sigma (a) settings as well as any particular illumination source shape (e.g. off-axis light sources such as annular, quadrupole, and dipole, etc.). The optical properties of the photo-resist layer coated on a substrate—i.e. refractive index, film thickness, propagation and polarization effects—may also be captured as part of the optical model 32. The mask model 30 captures the design features of the reticle and may also include a representation of detailed physical properties of the mask, as described, for example, in U.S. Pat. No. 7,587,704. Finally, the resist model 34 describes the effects of chemical processes which occur during resist exposure, PEB and development, in order to predict, for example, contours of resist features formed on the substrate wafer. The objective of the simulation is to accurately predict, for example, edge placements and CDs, which can then be compared against the target design. The target design, is generally defined as the pre-OPC mask layout, and will be provided in a standardized digital file format such as GDSII or OASIS.

(14) In a typical high-end design almost every feature edge requires some modification in order to achieve printed patterns that come sufficiently close to the target design. These modifications may include shifting or biasing of edge positions or line widths as well as application of ‘assist’ features that are not intended to print themselves, but will affect the properties of an associated primary feature. Furthermore, optimization techniques applied to the source of illumination may have different effects on different edges and features. Optimization of illumination sources can include the use of pupils to restrict source illumination to a selected pattern of light. The present disclosure provides optimization methods that can be applied to both source and mask configurations simultaneously.

(15) With reference to a high-level block diagram in FIG. 3, certain embodiments of the present disclosure provide methods for accelerated and simultaneous optimization of mask and source. Initial source 320 and mask 322 configurations (e.g. corresponding to optical model 32 and mask model 30 described above, respectively) are supplied to an optimization module 324. Optimization module 324 comprises an iterative optimizer that calculates a cost function and a gradient for each iteration. At 340, a cost function for the mask and source is evaluated for each iteration. The gradient of the cost function can then be examined at 342 to determine if convergence has been obtained. If the gradient is non-zero, then it may be considered that convergence has not been achieved and changes to source and mask can be calculated and applied at 344 before repeating the steps of computing a cost function and gradient for the new mask and source at 340 and testing for convergence 342. When convergence has been achieved, the final source 326 and mask 326 are considered to be optimized.

(16) Changes to the source and mask in 344 can be calculated and/or performed in a variety of ways, and it is not necessary for the exact sequence shown in FIG. 3 to be followed in all embodiments. For example, optimal results can be obtained by performing an unconstrained (or significantly less constrained) optimization followed by a fully constrained optimization step. The relative more freedom in the unconstrained (or less constrained) optimization step means it is likely to reach an optimal solution in a global sense. The fully constrained optimization would then start from an initial condition derived from this solution.

(17) The unconstrained (or less constrained) optimization can be performed in the space of free-form source and free-form mask. A free-form source is represented as a source intensity map on a sampling grid in the source pupil plane, and pixel values of the map are allowed to vary freely. Similarly, a free-form mask is the mask transmission map on a sampling grid with each pixel value free to vary. Free-form optimization permits faster calculation of the gradient of the cost function and certain algorithms can be selected to accelerate the achievement of an optimal solution.

(18) The initial condition of the fully constrained optimization can be constructed from the free-form result via a seeding process. The free-form mask result would serve as a guide as to potential locations to insert sub-resolution assist features (SRAF). Small SRAF seeds are then placed at these locations and are allowed to grow or shrink or move during the optimization. The main features of the mask design are also co-optimized along with the SRAF seeds to achieve the best solution. Similarly, the free-form source result could also be used to select candidates for the final illumination solutions, for example, ones based on diffractive optical elements (DOE). These source solution candidates could also be simultaneously optimized with main and SRAF mask features.

(19) Referring back to step 340 in FIG. 3, certain aspects of the disclosure include significantly speeding up the convergence of the optimization by allowing direct computation of the gradient of the cost function. The methods include the use of linearized functions selected to optimize the printed wafer contour throughout the process window. The cost function is typically based on a pure optical model because optics in photolithography systems tend to determine a majority of the process constraints. In one example, the cost function may be selected to reduce the worst edge placement error of a design layout throughout the process window. Mathematically, the cost function F may be written as:

(20) $F=\underset{\mathrm{pw}}{\max }\underset{e}{\max }\mathrm{EPE}\left(\mathrm{pw},e\right)$
where pw is a list of process window conditions and variable e runs over a set of evaluation points placed along the target design layout.

(21) This cost function could be transformed into a more computationally efficient form by employing the following approximations.

(22) First, EPE is approximated by a linearized approximation,

(23) $\mathrm{EPE}\left(\mathrm{pw},e\right)\approx \frac{\left[I_{\mathrm{pw}}\left(e\right)-I_{\mathrm{th}}\right]}{.Math.\nabla I_{\mathrm{pw}}.Math.},$
where I.sub.pw(e) denotes the aerial image intensity at process window condition pw, and I.sub.th the threshold for the aerial image contour. The denominator, ∥∇I.sub.pw∥, represents the slope of aerial image.

(24) Next, the max operator is approximated by an L.sub.p norm,

(25) $F^p\approx \underset{\mathrm{pw}}{.Math.}\underset{e}{.Math.}{\mathrm{EPE}}^p\left(\mathrm{pw},e\right),$
with p a positive integer. The bigger the value of p, the better this approximation is.

(26) Putting everything together, we have this new cost function,

(27) $F=\underset{\mathrm{pw}}{.Math.}\underset{e}{.Math.}w\left(\mathrm{pw},x\right)\frac{{\left[I_{\mathrm{pw}}\left(x\right)-I_{\mathrm{th}}\right]}^p}{{.Math.\nabla I_{\mathrm{pw}}.Math.}^p},p\in N.$
As can be seen, a weighting factor w(pw,e) is also preferably introduced to provide extra flexibility to control the goal of the optimization, which could be determined from considerations like evaluation point location (e.g. line, line end, jog) or relevant feature size (e.g. line width, space), or process window position.

(28) Those skilled in the art will recognize many ways how the mask transmissions M(x) and source intensities S(s) can be derived from the received source and mask descriptions (e.g. pixel-based maps corresponding to mask model 30 and optical model 32, respectively), and so details thereof will be omitted here for the sake of clarity of the disclosure. The present inventors recognize that aerial intensity I can be regarded as a function of mask transmissions M(x) and source intensities S(s), and therefore so can the cost function F. The cost function may be expanded using a Taylor series and, in certain embodiments, the floor of the gradient may be discovered using first order terms. More particularly, F may be expressed as:
F=F[I(M(x),S(s))]=F[M(x),S(s)]

(29) This cost function may be minimized using any of a variety of known algorithms when the gradient or derivatives of F are computed with respect to M and S:

(30) $F\approx F\left[M_0,S_0\right]+\underset{x}{.Math.}\frac{\delta F}{\delta M\left(x\right)}\left(M\left(x\right)-M_0\left(x\right)\right)+\underset{s}{.Math.}\frac{\delta F}{\delta S\left(s\right)}\left(S\left(s\right)-S_0\left(s\right)\right)$

(31) The derivatives of aerial image intensity I with respect to M and S, and by the chain rule, derivatives of F can be efficiently computed and the time to compute all derivatives is on the same order of magnitude as a single aerial image computation. The aerial image is the summation of contribution from each source point, and its variation with respect to the source map is the single contribution:

(32) $I\left[M\left(x\right),S\left(s\right)\right]=\int \mathrm{ds}S\left(s\right)I_s\left[M\left(x\right)\right]$$\frac{\delta I\left[M\left(x\right),S\left(s\right)\right]}{\delta S\left(s^{\prime }\right)}=I_s\left[M\left(x\right)\right]\delta \left(s-s^{\prime }\right)$

(33) The aerial image can also be expressed in Hopkins formulation as a sum of coherence systems:

(34) $\begin{array}{cc}I\left[M\left(x\right),S\left(s\right)\right]=& \int {\mathrm{dx}}^{\prime }{\mathrm{dx}}^{″}M\left(x^{\prime }\right)J_s\left[x-x^{\prime },x-x^{″}\right)M^{*}\left(x^{″}\right)\\ =& \underset{k}{.Math.}{\lambda }_k{.Math.\int {\mathrm{dx}}^{\prime }{V}_k\left(x-x^{\prime }\right)M\left(x^{\prime }\right).Math.}^2\equiv \\ & \underset{k}{.Math.}{\lambda }_k{.Math.V_k.Math.M.Math.}^2\end{array}$$\frac{\delta I\left[M\left(x\right),S\left(s\right)\right]}{\delta M\left(x^{\prime }\right)}=\underset{k}{.Math.}{\lambda }_k{V}_k\left(x-x^{\prime }\right)\int {\mathrm{dx}}^{″}{V}_k^{*}\left(x-x^{″}\right)M^{*}\left(x^{″}\right)+c.c.$
Where “c.c.” represents the complex conjugate.

(35) Having determined the aerial image variations, the variations of the cost function itself as a function of the aerial image can be computed as follows:

(36) $F=F\left[I\left[M\left(x\right),S\left(s\right)\right]\right]$$\begin{array}{cc}\frac{\delta F}{\delta M\left(x\right)}=& \int {\mathrm{dx}}^{\prime }\frac{\delta F}{\delta I\left(x^{\prime }\right)}\frac{\delta I\left(x^{\prime },s\right)}{\delta M\left(x\right)}\\ =& \underset{k}{.Math.}{\lambda }_k\int {\mathrm{dx}}^{\prime }\frac{\delta F}{\delta I\left(x^{\prime }\right)}{V}_k\left(x^{\prime }-x\right)\int {\mathrm{dx}}^{″}{V}_k^{*}\left(x^{\prime }-x^{″}\right)M^{*}\left(x^{″}\right)+c.c.\\ =& \underset{k}{.Math.}{\lambda }_k\left({\hat{V}}_k.Math.\left(\frac{\delta F}{\delta I}{\left(V_k.Math.M\right)}^{*}\right)\right)+c.c.,{\hat{V}}_k\left(x\right)\equiv V_k\left(-x\right)\end{array}$$\frac{\delta F}{\delta S\left(s\right)}=\int {\mathrm{ds}}^{\prime }\int {\mathrm{dx}}^{\prime }\frac{\delta F}{\delta I\left(x^{\prime }\right)}\frac{\delta I\left(x^{\prime },s^{\prime }\right)}{\delta S\left(s\right)}=\int {\mathrm{dx}}^{\prime }\frac{\delta F}{\delta I\left(x^{\prime }\right)}{I}_s\left(x^{\prime },s\right)$

(37) According to aspects of the disclosure that can be ascertained from the above, the variation with respect to mask image can be computed as a series of convolutions, thereby providing means for significantly decreasing computation time. The variation of the cost function with respect to the aerial image itself may be computed and the form of the cost function may be written:
F=F[I(x)]=∫dx w(x)f(I(x),∇I(x)).

(38) In this case, the variation would be:

(39) $\frac{\delta F}{\delta I\left(x\right)}=w\left(x\right)\frac{\partial f}{\partial I}-\nabla .Math.\left(w\left(x\right)\frac{\partial f}{\partial \nabla I}\right)$

(40) Thus, variations of the cost function with respect to both source and mask can be simultaneously obtained. In the free-form source and mask optimization these variations become the gradient of the cost function. Thereafter, any suitable gradient-based optimization technique can be applied to find a minimum of the cost function.

(41) The descriptions above provide an example embodiment where the cost function is based on EPE. Examples of other cost functions include (1) the EPE least square function, (2) the EPE least p-norm function where p is even and greater than 2, (3) the inverse NILS p-norm function, (4) the contour integral of image slope with M as the design target, (5) the edge image value least square, (6) the edge image p-norm (p is even and >2) and (7) the ILS p-norm with F to be maximized. The seven corresponding cost function equations are listed below:

(42) 0$\begin{array}{cc}F=\underset{\mathrm{pw}}{.Math.}\underset{x}{.Math.}w\left(\mathrm{pw},x\right)\frac{{\left[I_{\mathrm{pw}}\left(x\right)-I_{\mathrm{th}}\right]}^2}{{.Math.\nabla I_{\mathrm{pw}}.Math.}^2}& \left(1\right)\\ F=\underset{\mathrm{pw}}{.Math.}\underset{x}{.Math.}w\left(\mathrm{pw},x\right)\frac{{\left[I_{\mathrm{pw}}\left(x\right)-I_{\mathrm{th}}\right]}^p}{{.Math.\nabla I_{\mathrm{pw}}.Math.}^p}& \left(2\right)\\ F=\underset{\mathrm{pw}}{.Math.}\underset{x}{.Math.}w\left(\mathrm{pw},x\right)\frac{{\left[I_{\mathrm{pw}}\left(x\right)\right]}^p}{{.Math.{\mathrm{CD}}_x\nabla I_{\mathrm{pw}}.Math.}^p}& \left(3\right)\\ \begin{array}{c}F=-\underset{\mathrm{pw}}{.Math.}\underset{\partial M}{\oint }\mathrm{dl}w\left(\mathrm{pw},x\right)\left(\hat{n}.Math.\nabla I_{\mathrm{pw}}\right)\\ =-\underset{\mathrm{pw}}{.Math.}\underset{M}{\int \int }\mathrm{dS}\nabla .Math.\left(w\left(\mathrm{pw},x\right)\nabla I_{\mathrm{pw}}\right)\end{array}& \left(4\right)\\ F=\underset{\mathrm{pw}}{.Math.}\underset{x}{.Math.}{w\left(\mathrm{pw},x\right)\left[I_{\mathrm{pw}}\left(x\right)-I_{\mathrm{th}}\right]}^2& \left(5\right)\\ F=\underset{\mathrm{pw}}{.Math.}\underset{x}{.Math.}{w\left(\mathrm{pw},x\right)\left[I_{\mathrm{pw}}\left(x\right)-I_{\mathrm{th}}\right]}^p& \left(6\right)\\ F=\underset{\mathrm{pw}}{.Math.}\underset{x}{.Math.}w\left(\mathrm{pw},x\right)\frac{{.Math.\nabla I_{\mathrm{pw}}.Math.}^p}{{\mathrm{CD}}_x}& \left(7\right)\end{array}$

(43) One skilled in the art would fully understand how to determine the optimized gradient for these and other cost functions based after being taught by the above descriptions. For example, some standard optimization techniques utilize gradient information such as steepest descent, conjugation gradient or quasi-Newton methods.

(44) The gradient calculation formulae described above can be implemented in various computing platforms. Additionally or alternatively, specially adapted hardware acceleration platforms can be used to further improve the optimization speed. For example, platforms can that include specialized digital signal processors (“DSPs”) can be employed to process cost functions and calculate gradients. However, it will be appreciated that calculations maybe performed on other computing platforms that can comprise parallel processors, mathematical coprocessors and DSP based coprocessors.

(45) To provide synergy between certain types of scanners and SMO solutions to meet advanced low k.sub.1 imaging requirements, and armed with the optimization algorithms described above, the present inventors have developed a SMO flow that can utilize fully flexible illuminators or different types of application specific/custom DOEs, rather than standard or pre-selected illumination designs.

(46) In this regard, FIG. 4 illustrates a source and continuous transmission mask co-optimization flow (CTM flow) according to additional embodiments of the disclosure. As shown in FIG. 4, the first step of the CTM flow is to set up all the input parameters for the optimization including: Model, DOE type, polarization, mask manufacture rule check (MRC) and process information etc. (502). For example, in the set up, a user specifies the type of source constraints to be applied, either custom DOE or fully-flexible illuminator. This will determine later how the unconstrained freeform source will be converted and co-optimized. These setup parameters are used throughout the entire flow. Then, models will be created at user-specified PW corner conditions as shown in FIG. 4 (504). Users can specify the DOF versus EL trade off in this step, for example.

(47) With all the setup parameters, step 506 starts the co-optimization with unconstrained freeform source and continuous transmission mask, using for example the optimization process of optimization module 324, including the cost function and gradient calculations, freeform source and mask optimizations and assist feature optimizations described above. The only constraint in this stage is the upper and lower bound of mask and source transmission which has physical limitations. Without constraints, optimization in this stage will search for solutions in the largest possible solution space, and give the best possible process window (PW) and MEF. The resultant source 602 and mask 604 for an example application of a design for a DRAM is shown in FIG. 5, respectively. However, neither the freeform source nor continuous transmission mask are manufacturable. Therefore, after freeform source and continuous transmission mask co-optimization, for practical purposes, on the source side, it needs to be converted into a manufacturable source (508), such as a DOE 704 shown in FIG. 6 or a “New” (e.g. fully flexible) illuminator 702 shown in FIG. 6. On the mask side, the mask needs to be constrained to a fixed transmission value (510). Then the selected source-mask combination is co-optimized using the scanner illuminator and mask manufacture rule check (MRC) constraints. The “New” illuminator closely resembles the freeform source (resulting from 514), and is expected to give minimal impact on the PW (as analyzed in 516) compared to a parametric DOE source (resulting from 512).

(48) For an example application for a DRAM design, FIG. 6 shows the converted “New” illuminator 702 and DOE source 704, respectively. From the optimized continuous transmission mask gray tone image, AF seeds are extracted and are optimized during the next stage. In the final stage, the constrained source along with the main and assist features on the mask will be optimized with the same cost function as in the initial co-optimization result (512 and 514). Co-optimization is crucial in this step because both the source and mask manufacturability constraints can significantly modify the original source topology, and performs a mask-only optimization which does not guarantee the optimum result. FIGS. 7A and 7B show the masks 804 and 808 that result with the DOE source 802 and “New” illuminator 806, respectively.

(49) In an embodiment, there is provided a computer-implemented method for optimizing a lithographic process having an illumination source and a mask, the method comprising: a free-form optimization process; placing sub-resolution assist feature (SRAF) seeds in a description of the mask based on a result of the free-form optimization process; and a constrained optimization process, including growing the SRAF seeds while taking into account manufacturability constraints for both the illumination source and the mask, wherein one or more steps are performed by the computer.

(50) In an embodiment, the free-form optimization process includes designing an optimal illumination source that comprises a fully flexible set of illumination source points. In an embodiment, taking into account the manufacturability constraints for the illumination source includes matching the optimal illumination source to a diffractive optical element. In an embodiment, taking into account the manufacturability constraints for the mask includes constraining a mask transmission to a predetermined value. In an embodiment, the constrained optimization process includes iteratively converging a cost function, wherein the cost function comprises a function of both the illumination source and the mask. In an embodiment, the cost function is formulated in terms of one of the following: worst case edge placement error (EPE) over a given process window, EPE least square function, EPE least p-norm function, inverse NILS p-norm function, contour integral image slop, edge image value least square, edge image p-norm, and ILS p-norm. In an embodiment, the convergence of the cost function is accelerated by directly computing a gradient of the cost function in each iteration with respect of the mask and the source. In an embodiment, the source and the mask are reconfigured for each iteration, until the gradient is at a desired minimum value.

(51) In an embodiment, there is provided a computer program product comprising a non-transitory computer readable medium having instructions recorded thereon, the instructions, when executed by a computer, implements a method for optimizing a lithographic process having an illumination source and a mask, the method comprising: a free-form optimization process; placing sub-resolution assist feature (SRAF) seeds in a description of the mask based on a result of the free-form optimization process; and a constrained optimization process, including growing the SRAF seeds while taking into account manufacturability constraints for both the illumination source and the mask.

(52) In an embodiment, the free-form optimization process includes designing an optimal illumination source that comprises a fully flexible set of illumination source points. In an embodiment, taking into account the manufacturability constraints for the illumination source includes matching the optimal illumination source to a diffractive optical element. In an embodiment, taking into account the manufacturability constraints for the mask includes constraining a mask transmission to a predetermined value. In an embodiment, the constrained optimization process includes iteratively converging a cost function, wherein the cost function comprises a function of both the illumination source and the mask. In an embodiment, the cost function is formulated in terms of one of the following: worst case edge placement error (EPE) over a given process window, EPE least square function, EPE least p-norm function, inverse NILS p-norm function, contour integral image slop, edge image value least square, edge image p-norm, and ILS p-norm. In an embodiment, the convergence of the cost function is accelerated by directly computing a gradient of the cost function in each iteration with respect of the mask and the source. In an embodiment, the source and the mask are reconfigured before each iteration until the gradient is at a desired minimum value.

(53) FIG. 8 is a block diagram that illustrates a computer system 100 which can assist in implementing the optimization methods and flows disclosed herein. Computer system 100 includes a bus 102 or other communication mechanism for communicating information, and a processor 104 coupled with bus 102 for processing information. Computer system 100 also includes a main memory 106, such as a random access memory (RAM) or other dynamic storage device, coupled to bus 102 for storing information and instructions to be executed by processor 104. Main memory 106 also may be used for storing temporary variables or other intermediate information during execution of instructions to be executed by processor 104. Computer system 100 further includes a read only memory (ROM) 108 or other static storage device coupled to bus 102 for storing static information and instructions for processor 104. A storage device 110, such as a magnetic disk or optical disk, is provided and coupled to bus 102 for storing information and instructions.

(54) Computer system 100 may be coupled via bus 102 to a display 112, such as a cathode ray tube (CRT) or flat panel or touch panel display for displaying information to a computer user. An input device 114, including alphanumeric and other keys, is coupled to bus 102 for communicating information and command selections to processor 104. Another type of user input device is cursor control 116, such as a mouse, a trackball, or cursor direction keys for communicating direction information and command selections to processor 104 and for controlling cursor movement on display 112. This input device typically has two degrees of freedom in two axes, a first axis (e.g., x) and a second axis (e.g., y), that allows the device to specify positions in a plane. A touch panel (screen) display may also be used as an input device.

(55) According to one embodiment of the disclosure, portions of the optimization process may be performed by computer system 100 in response to processor 104 executing one or more sequences of one or more instructions contained in main memory 106. Such instructions may be read into main memory 106 from another computer-readable medium, such as storage device 110. Execution of the sequences of instructions contained in main memory 106 causes processor 104 to perform the process steps described herein. One or more processors in a multi-processing arrangement may also be employed to execute the sequences of instructions contained in main memory 106. In alternative embodiments, hard-wired circuitry may be used in place of or in combination with software instructions to implement the disclosure. Thus, embodiments of the disclosure are not limited to any specific combination of hardware circuitry and software.

(56) The term “computer-readable medium” as used herein refers to any medium that participates in providing instructions to processor 104 for execution. Such a medium may take many forms, including but not limited to, non-volatile media, volatile media, and transmission media. Non-volatile media include, for example, optical or magnetic disks, such as storage device 110. Volatile media include dynamic memory, such as main memory 106. Transmission media include coaxial cables, copper wire and fiber optics, including the wires that comprise bus 102. Transmission media can also take the form of acoustic or light waves, such as those generated during radio frequency (RF) and infrared (IR) data communications. Common forms of computer-readable media include, for example, a floppy disk, a flexible disk, hard disk, magnetic tape, any other magnetic medium, a CD-ROM, DVD, any other optical medium, punch cards, paper tape, any other physical medium with patterns of holes, a RAM, a PROM, and EPROM, a FLASH-EPROM, any other memory chip or cartridge, a carrier wave as described hereinafter, or any other medium from which a computer can read.

(57) Various forms of computer readable media may be involved in carrying one or more sequences of one or more instructions to processor 104 for execution. For example, the instructions may initially be borne on a magnetic disk of a remote computer. The remote computer can load the instructions into its dynamic memory and send the instructions over a telephone line using a modem. A modem local to computer system 100 can receive the data on the telephone line and use an infrared transmitter to convert the data to an infrared signal. An infrared detector coupled to bus 102 can receive the data carried in the infrared signal and place the data on bus 102. Bus 102 carries the data to main memory 106, from which processor 104 retrieves and executes the instructions. The instructions received by main memory 106 may optionally be stored on storage device 110 either before or after execution by processor 104.

(58) Computer system 100 also preferably includes a communication interface 118 coupled to bus 102. Communication interface 118 provides a two-way data communication coupling to a network link 120 that is connected to a local network 122. For example, communication interface 118 may be an integrated services digital network (ISDN) card or a modem to provide a data communication connection to a corresponding type of telephone line. As another example, communication interface 118 may be a local area network (LAN) card to provide a data communication connection to a compatible LAN. Wireless links may also be implemented. In any such implementation, communication interface 118 sends and receives electrical, electromagnetic or optical signals that carry digital data streams representing various types of information.

(59) Network link 120 typically provides data communication through one or more networks to other data devices. For example, network link 120 may provide a connection through local network 122 to a host computer 124 or to data equipment operated by an Internet Service Provider (ISP) 126. ISP 126 in turn provides data communication services through the worldwide packet data communication network, now commonly referred to as the “Internet” 128. Local network 122 and Internet 128 both use electrical, electromagnetic or optical signals that carry digital data streams. The signals through the various networks and the signals on network link 120 and through communication interface 118, which carry the digital data to and from computer system 100, are exemplary forms of carrier waves transporting the information.

(60) Computer system 100 can send messages and receive data, including program code, through the network(s), network link 120, and communication interface 118. In the Internet example, a server 130 might transmit a requested code for an application program through Internet 128, ISP 126, local network 122 and communication interface 118. In accordance with the disclosure, one such downloaded application provides for the illumination optimization of the embodiment, for example. The received code may be executed by processor 104 as it is received, and/or stored in storage device 110, or other non-volatile storage for later execution. In this manner, computer system 100 may obtain application code in the form of a carrier wave.

(61) FIG. 9 schematically depicts an exemplary lithographic projection apparatus whose illumination source could be optimized utilizing the process of an embodiment of the present invention. The apparatus comprises: a radiation system Ex, IL, for supplying a projection beam PB of radiation. In this particular case, the radiation system also comprises a radiation source LA; a first object table (mask table) MT provided with a mask holder for holding a mask MA (e.g., a reticle), and connected to first positioning means for accurately positioning the mask with respect to item PL; a second object table (substrate table) WT provided with a substrate holder for holding a substrate W (e.g., a resist-coated silicon wafer), and connected to second positioning means for accurately positioning the substrate with respect to item PL; a projection system (“lens”) PL (e.g., a refractive, catoptric or catadioptric optical system) for imaging an irradiated portion of the mask MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.

(62) As depicted herein, the apparatus is of a transmissive type (i.e., has a transmissive mask). However, in general, it may also be of a reflective type, for example (with a reflective mask). Alternatively, the apparatus may employ another kind of patterning means as an alternative to the use of a mask; examples include a programmable mirror array or LCD matrix.

(63) The source LA (e.g., a mercury lamp or excimer laser) produces a beam of radiation. This beam is fed into an illumination system (illuminator) IL, either directly or after having traversed conditioning means, such as a beam expander Ex, for example. The illuminator IL may comprise adjusting means AM for setting the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in the beam. In addition, it will generally comprise various other components, such as an integrator IN and a condenser CO. In this way, the beam PB impinging on the mask MA has a desired uniformity and intensity distribution in its cross-section.

(64) It should be noted with regard to FIG. 9 that the source LA may be within the housing of the lithographic projection apparatus (as is often the case when the source LA is a mercury lamp, for example), but that it may also be remote from the lithographic projection apparatus, the radiation beam that it produces being led into the apparatus (e.g., with the aid of suitable directing mirrors); this latter scenario is often the case when the source LA is an excimer laser (e.g., based on KrF, ArF or F.sub.2 lasing). The current disclosure encompasses at least both of these scenarios.

(65) The beam PB subsequently intercepts the mask MA, which is held on a mask table MT. Having traversed the mask MA, the beam PB passes through the lens PL, which focuses the beam PB onto a target portion C of the substrate W. With the aid of the second positioning means (and interferometric measuring means IF), the substrate table WT can be moved accurately, e.g. so as to position different target portions C in the path of the beam PB. Similarly, the first positioning means can be used to accurately position the mask MA with respect to the path of the beam PB, e.g., after mechanical retrieval of the mask MA from a mask library, or during a scan. In general, movement of the object tables MT, WT will be realized with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning), which are not explicitly depicted in FIG. 9. However, in the case of a wafer stepper (as opposed to a step-and-scan tool) the mask table MT may just be connected to a short stroke actuator, or may be fixed.

(66) The depicted tool can be used in two different modes: In step mode, the mask table MT is kept essentially stationary, and an entire mask image is projected in one go (i.e., a single “flash”) onto a target portion C. The substrate table WT is then shifted in the x and/or y directions so that a different target portion C can be irradiated by the beam PB; In scan mode, essentially the same scenario applies, except that a given target portion C is not exposed in a single “flash”. Instead, the mask table MT is movable in a given direction (the so-called “scan direction”, e.g., the y direction) with a speed v, so that the projection beam PB is caused to scan over a mask image; concurrently, the substrate table WT is simultaneously moved in the same or opposite direction at a speed V=Mv, in which M is the magnification of the lens PL (typically, M=¼ or ⅕). In this manner, a relatively large target portion C can be exposed, without having to compromise on resolution.

(67) The concepts disclosed herein may simulate or mathematically model any generic imaging system for imaging sub wavelength features, and may be especially useful with emerging imaging technologies capable of producing wavelengths of an increasingly smaller size. Emerging technologies already in use include EUV (extreme ultra violet) lithography that is capable of producing a 193 nm wavelength with the use of a ArF laser, and even a 157 nm wavelength with the use of a Fluorine laser. Moreover, EUV lithography is capable of producing wavelengths within a range of 20-5 nm by using a synchrotron or by hitting a material (either solid or a plasma) with high energy electrons in order to produce photons within this range. Because most materials are absorptive within this range, illumination may be produced by reflective mirrors with a multi-stack of Molybdenum and Silicon. The multi-stack mirror has a 40 layer pairs of Molybdenum and Silicon where the thickness of each layer is a quarter wavelength. Even smaller wavelengths may be produced with X-ray lithography. Typically, a synchrotron is used to produce an X-ray wavelength. Since most material is absorptive at x-ray wavelengths, a thin piece of absorbing material defines where features would print (positive resist) or not print (negative resist).

(68) While the concepts disclosed herein may be used for imaging on a substrate such as a silicon wafer, it shall be understood that the disclosed concepts may be used with any type of lithographic imaging systems, e.g., those used for imaging on substrates other than silicon wafers.

(69) The descriptions above are intended to be illustrative, not limiting. Thus, it will be apparent to one skilled in the art that modifications may be made to the embodiments as described without departing from the scope of the claims set out below.