A very large scale integrated (VLSI) complementary metal oxide semiconductor (CMOS) chip is manufactured on a silicon wafer by a sequence of material additions (i.e., low pressure chemical vapor depositions, sputtering operations, etc.), material removals (i.e., wet etches, reactive ion etches, etc.), and material modifications (i.e., oxidations, ion implants, etc.). These physical and chemical operations interact with the entire wafer. For example, if a wafer is placed into an acid bath, the entire surface of the wafer will be etched away. In order to build very small electrically active devices on the wafer, the impact of these operations has to be confined to small, well defined regions.
Lithography in the context of VLSI manufacturing of CMOS devices is the process of patterning openings in photosensitive polymers (sometimes referred to as photoresists or resists) which define small areas in which the silicon base material is modified by a specific operation in a sequence of processing steps. The manufacturing of CMOS chips involves the repeated patterning of photoresist, followed by an etch, implant, deposition, or other operation, and ending with the removal of the expended photoresist to make way for the new resist to be applied for another iteration of this process sequence.
The basic lithography system consists of a light source, a stencil or photo mask containing the pattern to be transferred to the wafer, a collection of lenses, and a means for aligning existing patterns on the wafer with patterns on the mask. The aligning may take place in an aligning step or steps and may be carried out with an aligning apparatus. Since a wafer containing from 50 to 100 chips is patterned in steps of 1 to 4 chips at a time, these lithography tools are commonly referred to as steppers. The resolution, R, of an optical projection system such as a lithography stepper is limited by parameters described in Raleigh's equation:R=kλ/NA,where λ represents the wavelength of the light source used in the projection system and NA represents the numerical aperture of the projection optics used. “k” represents a factor describing how well a combined lithography system can utilize the theoretical resolution limit in practice and can range from about 0.8 down to about 0.5 for standard exposure systems. The highest resolution in optical lithography is currently achieved with deep ultra violet (DUV) steppers operating at 248 nm. Wavelengths of 356 nm are also in widespread use and 193 nm wavelength lithography is becoming commonplace.
Conventional photo masks consist of chromium patterns on a quartz plate, allowing light to pass wherever the chromium has been removed from the mask. Light of a specific wavelength is projected through the mask onto the photoresist coated wafer, exposing the resist wherever hole patterns are placed on the mask. Exposing the resist to light of the appropriate wavelength causes modifications in the molecular structure of the resist polymers which, in common applications, allow a developer to dissolve and remove the resist in the exposed areas. Such resist materials are known as positive resists. (Negative resist systems allow only unexposed resist to be developed away.) The photo masks, when illuminated, can be pictured as an array of individual, infinitely small light sources which can be either turned on (points in clear areas) or turned off (points covered by chrome). If the amplitude of the electric field vector which describes the light radiated by these individual light sources is mapped across a cross section of the mask, a step function will be plotted reflecting the two possible states that each point on the mask can be found (light on, light off).
These conventional photo masks are commonly referred to as chrome on glass (COG) binary masks, due to the binary nature of the image amplitude. The perfectly square step function of the light amplitude exists only in the theoretical limit of the exact mask plane. At any given distance away from the mask, such as in the wafer plane, diffraction effects will cause images to exhibit a finite image slope. At small dimensions, that is, when the size and spacing of the images to be printed are small relative to the λ/NA, electric field vectors of adjacent images will interact and add constructively. The resulting light intensity curve between the image features is not completely dark, but exhibits significant amounts of light intensity created by the interaction of adjacent features. The resolution of an exposure system is limited by the contrast of the projected image, that is, the intensity difference between adjacent light and dark image features. An increase in the light intensity in nominally dark regions will eventually cause adjacent features to print as one combined structure rather than discrete images.
The quality with which small images can be replicated in lithography depends largely on the available process window; that is, that amount of allowable dose and focus variation that still results in correct image size. Phase shifted mask (PSM) lithography improves the lithographic process window or allows operation at a lower k value by introducing a third parameter on the mask. The electric field vector, like any vector quantity, has a magnitude and direction, so, in addition to turning the electric field amplitude on and off, it can be turned on with a phase of about 0° or turned on with a phase of about 180°. This phase variation is achieved in PSMs by modifying the length that a light beam travels through the mask material. By recessing the mask to an appropriate depth, light traversing the thinner portion of the mask and light traversing the thicker portion of the masks will be 180° out of phase, that is, their electric field vector will be of equal magnitude but point in exactly the opposite direction so that any interaction between these light beams result in perfect cancellation. However, because the 180° phase transition forces a minimum in the image intensity, narrow dark lines will be printed. These unwanted residual phase images are erased using a trim mask, which is a second mask that transmits light only in regions left unexposed by the residual phase edge.
Alternating Phase Shifted Mask (altPSM) lithography is a resolution enhancement technique that is rapidly gaining acceptance as a viable solution to meet aggressive integrated circuit (IC) technology scaling time-lines. Delays in next generation optical and non-optical lithography tooling add vital importance to successful implementation of altPSM. AltPSM takes advantage of destructive interference of light to double the achievable resolution of an optical lithography system. The light interference is created by selectively manipulating the topography of the photomask to introduce an appropriate path-length difference in the imaging light. This manipulation of the mask topography requires phase information to be added to the circuit layout in the computer-aided design (CAD) system. Key to the successful implementation of altPSM is an efficient electronic design automation (EDA) tool that can convert circuit designs to altPSM layouts with minimal impact to layout design density or design complexity. The process of defining portions of the mask as 0° phase regions and other portions as 180° phase regions is generally referred to as phase coloring. Techniques for automatic phase coloring are known. For example, Kim et al. (U.S. Pat. No. 5,883,813) describes a method for automatically assigning binary phase coloring in altPSM designs by the use of net coloring. In the method of Kim et al., nets are defined as coupled intrusion pairs of phase shapes, where the phase of one shape determines the phase of the other shape within each intrusion pair. The intrusion pairs are shapes that are close enough that light passing through the shapes will interact and affect the image intensity between the shapes. Thus, intrusion pairs that are sufficiently close together will likewise interact and will be coupled together by a “connected” function that defines a net. All phase shapes within a net will be colored together such that phases alternate across each critical element. The technique of Kim et al. can be applied to both dark-field and light-field PSM designs, and can be adapted to both flat and hierarchical VLSI CAD databases.
Methods to assign and optimize phases in an altPSM design are known in the art, for example, as described in U.S. Pat. Nos. 5,537,648 and 5,636,131 (Liebmann et al.) and U.S. Pat. No. 6,057,063 (Liebmann et al.). FIG. 1 illustrates a flow chart typical of such methods. After creating an initial circuit layout (Block 401), the design of the altPSM (Block 400) is performed. Critical circuit elements having critical dimension CD are identified, as indicated in Block 410 of FIG. 1. Phase shapes are defined in association with each critical element (Block 420). Then the phase shapes are legalized according to the various rules as discussed above (Block 430). Next, the appropriate phases are assigned to each phase shape (Block 440), ensuring binary coloring across the entire mask layout. A method for performing binary coloring is described, for example, in Kim et al. (U.S. Pat. No. 5,883,813). The phase coloring method of Kim et al. involves the formation of nets of phase shapes by creating a “connected” function that links or couples phase shapes across critical elements as intrusion pairs, meaning that the phase of one shape determines the phase of the other shape in the intrusion pair. The “connected” function also includes shapes that are close enough the be phase coupled. “Close enough” could mean that there must be a minimum phase-to-phase spacing, which will be discussed further below. After the phase have been assigned in conformance with the various rules, the layout is checked for any inconsistencies or errors (Block 450). If layout conforms with all rules, then the altPSM design is accepted and the associated trim mask is then designed (Block 409). Since it may not be possible to correct all such errors, it may be necessary to accept a loss in process window, or re-design the circuit layout (Block 460).
The generation of an altPSM layout requires the addition of phase shapes disposed on opposing sides of layout dimensions representing target image dimensions that approach the resolution limits of the lithography system. In current altPSM design methodologies, the generation of phase shifting shapes (Block 420 of FIG. 1) is based on a cutoff dimension for layout shape widths below which phase shifting is required. If the layout width is larger than the cutoff dimension, no phase shifting is required, and the phase shapes are not drawn by the layout tool. If the layout dimension is less than the cutoff dimension, standard methods result in generating phase shapes having a predetermined, fixed width. For example, FIG. 2 illustrates a schematic of a transistor altPSM layout 200 generated according to standard methods. The first transistor feature 100 includes a line having a layout dimension LW0 less than the cutoff dimension (sub-cutoff layout dimension), which is defined on the altPSM layout with 0° phase region 110 and 180° phase region 120. Similarly, a second transistor 101 is shown having a sub-cutoff layout dimension LW1 and a third transistor 102 having a sub-cutoff layout dimension LW2. The sub-cutoff layout dimensions LW0, LW1 and LW2 are each different in layout 200, and each of the phase shapes 110 and 120 have the same width DP.
However, the width of the phase shapes has an impact on process window and across-chip line width variation (ACLV). For example, referring to FIG. 3A, a portion of an altPSM layout 300 is illustrated. In layout 300, critical elements 105 are defined having a sub-cutoff layout dimension (LW), and spaced apart by pitch P. On opposing sides of the LW are a 0° phase region 110 and a 180° phase region 120 each having phase width of DP. FIG. 3B illustrates the results of a simulation of process window, which is a function of focus and dose, assuming 0.75 NA (numerical aperture), illumination wavelength of 193 nm (using an ArF laser source), 0.35 Sigma (partial coherence of the tool), and a pitch (P) of 1300 nm. The process window curves are plotted for target image widths of TW0 (301), TW1 (302), and TW2 (303). It can be seen that for each target width, the optimal process window is obtained by using a different phase width. The optimal phase widths for each of these target CDs are plotted in FIG. 3C. It can be seen that for a given target CD, the optimal phase shape width varies, thus current methods of assigning altPSM phase shapes, which specify fixed phase shape widths, do not necessarily result in an optimal process window. Similarly, image quality will also be impacted. One metric of image quality is the across-chip line width variation (ACLV), which is perhaps even more important to minimize since ACLV is a measure of overall chip quality. The optimal phase shapes that minimize ACLV will likewise vary with target image dimension.
In view of the foregoing discussion, there is a need to provide for a method for designing an alternating phase shifted mask (altPSM) having phase shape dimensions that optimize process window and image quality.