METHOD FOR OPTIMIZING SOURCE AND MASK TO CONTROL LINE WIDTH ROUGHNESS AND IMAGE LOG SLOPE

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First Claim
1. A method for illuminating a mask with a source to project a desired image pattern through a lithographic system onto a photoactive material, the method comprising:
 defining a representation of the mask;
obtaining a fractional resist shot noise (FRSN) parameter;
determining a first relationship between a first set of optical intensity values and an edge roughness metric based on the FRSN parameter;
determining a second relationship between a second set of optical intensity values and a lithographic performance metric;
imposing a set of metric constraints in accordance with one of the first and second relationships;
setting up an objective function of optimization in accordance with the remaining of the first and second relationships;
determining optimum constrained values of the representation of the mask in accordance with the set of metric constraints and the objective function of optimization; and
outputting the optimum constrained values of the representation of the mask.
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Abstract
A method for illuminating a mask with a source to project a desired image pattern through a lithographic system onto a photoactive material including: defining a representation of the mask; obtaining a fractional resist shot noise (FRSN) parameter; determining a first relationship between a first set of optical intensity values and an edge roughness metric based on the FRSN parameter; determining a second relationship between a second set of optical intensity values and a lithographic performance metric; imposing a set of metric constraints based on one of the first and second relationships; setting up an objective function of optimization based on the remaining of the two relationships; determining optimum constrained values of the representation of the mask based on the set of metric constraints and the objective function; and outputting these values.
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No References
48 Claims
 1. A method for illuminating a mask with a source to project a desired image pattern through a lithographic system onto a photoactive material, the method comprising:
defining a representation of the mask; obtaining a fractional resist shot noise (FRSN) parameter; determining a first relationship between a first set of optical intensity values and an edge roughness metric based on the FRSN parameter; determining a second relationship between a second set of optical intensity values and a lithographic performance metric; imposing a set of metric constraints in accordance with one of the first and second relationships; setting up an objective function of optimization in accordance with the remaining of the first and second relationships; determining optimum constrained values of the representation of the mask in accordance with the set of metric constraints and the objective function of optimization; and outputting the optimum constrained values of the representation of the mask.  View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10)
 11. A method for illuminating a mask with a source to project a desired image pattern through a lithographic system onto a photoactive material, the method comprising:
defining a representation of the mask; obtaining a fractional resist shot noise (FRSN) parameter; determining a first relationship between a first set of optical intensity values and an edge roughness metric based on the FRSN parameter; determining a second relationship between a second set of optical intensity values and a lithographic performance metric; determining a third relationship between a set of polygons in the mask and a maskwriting difficulty metric; imposing a set of metric constraints in accordance with the first, second, and third relationships; setting up an objective function of optimization in accordance with one of the first, second, and third relationships; determining optimum constrained values of the representation of the mask in accordance with the set of metric constraints and the objective function of optimization; and outputting the optimum constrained values of the representation of the mask.  View Dependent Claims (12, 13)
 14. A method for illuminating a mask with a source to project a desired image pattern through a lithographic system onto a photoactive material, the method comprising:
defining a representation of the mask and a representation of a projection process; obtaining a fractional resist shot noise (FRSN) parameter; determining a first relationship between a first set of optical intensity values and an edge roughness metric based on the FRSN parameter; determining a second relationship between a second set of optical intensity values and a lithographic performance metric; imposing a set of metric constraints in accordance with one of the first and second relationships; setting up an objective function of optimization in accordance with the remaining of the first and second relationships; determining optimum constrained values of the representations of the mask and the projection process in accordance with the set of metric constraints and the objective function of optimization; and outputting the optimum constrained values of the representations of the mask and the projection process.  View Dependent Claims (15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25)
 26. A method for illuminating a mask with a source to project a desired image pattern through a lithographic system onto a photoactive material, the method comprising:
defining a representation of the mask and a representation of a projection process; obtaining a fractional resist shot noise (FRSN) parameter; determining a first relationship between a first set of optical intensity values and an edge roughness metric based on the FRSN parameter; determining a second relationship between a second set of optical intensity values and a lithographic performance metric; determining a third relationship between a set of polygons in the mask and a maskwriting difficulty metric; imposing a set of metric constraints in accordance with the first, second, and third relationships; setting up an objective function of optimization in accordance with one of the first, second, and third relationships; determining optimum constrained values of the representations of the mask and the projection process in accordance with the set of metric constraints and the objective function of optimization; and outputting the optimum constrained values of the representations of the mask and the projection process.  View Dependent Claims (27)
 28. A machine readable storage medium having one or more machine readable program codes stored thereon for causing the machine to perform a method for illuminating a mask with a source to project a desired image pattern through a lithographic system onto a photoactive material, the method comprising:
defining a representation of the mask; obtaining a fractional resist shot noise (FRSN) parameter; determining a first relationship between a first set of optical intensity values and an edge roughness metric based on the FRSN parameter; determining a second relationship between a second set of optical intensity values and a lithographic performance metric; imposing a set of metric constraints in accordance with one of the first and second relationships; setting up an objective function of optimization in accordance with the remaining of the first and second relationships; determining optimum constrained values of the representation of the mask in accordance with the set of metric constraints and the objective function of optimization; and outputting the optimum constrained values of the representation of the mask.  View Dependent Claims (29, 30, 31, 32, 33, 34, 35, 45, 46, 47)
 36. A machine readable storage medium having one or more machine readable program codes stored thereon for causing the machine to perform a method for illuminating a mask with a source to project a desired image pattern through a lithographic system onto a photoactive material, the method comprising:
defining a representation of the mask; obtaining a fractional resist shot noise (FRSN) parameter; determining a first relationship between a first set of optical intensity values and an edge roughness metric based on the FRSN parameter; determining a second relationship between a second set of optical intensity values and a lithographic performance metric; determining a third relationship between a set of polygons in the mask and a maskwriting difficulty metric; imposing a set of metric constraints in accordance with the first, second, and third relationships; setting up an objective function of optimization in accordance with one of the first, second, and third relationships; determining optimum constrained values of the representation of the mask in accordance with the set of metric constraints and the objective function of optimization; and outputting the optimum constrained values of the representation of the mask.
 37. A machine readable storage medium having one or more machine readable program codes stored thereon for causing the machine to perform a method for illuminating a mask with a source to project a desired image pattern through a lithographic system onto a photoactive material, the method comprising:
defining a representation of the mask and a representation of a projection process; obtaining a fractional resist shot noise (FRSN) parameter; determining a first relationship between a first set of optical intensity values and an edge roughness metric based on the FRSN parameter; determining a second relationship between a second set of optical intensity values and a lithographic performance metric; imposing a set of metric constraints in accordance with one of the first and second relationships; setting up an objective function of optimization in accordance with the remaining of the first and second relationships; determining optimum constrained values of the representations of the mask and the projection process in accordance with the set of metric constraints and the objective function of optimization; and outputting the optimum constrained values of the representations of the mask and the projection process.  View Dependent Claims (38, 39, 40, 41, 42, 43, 44)
 48. A machine readable storage medium having one or more machine readable program codes stored thereon for causing the machine to perform a method for illuminating a mask with a source to project a desired image pattern through a lithographic system onto a photoactive material, the method comprising:
defining a representation of the mask and a representation of a projection process; obtaining a fractional resist shot noise (FRSN) parameter; determining a first relationship between a first set of optical intensity values and an edge roughness metric based on the FRSN parameter; determining a second relationship between a second set of optical intensity values and a lithographic performance metric; determining a third relationship between a set of polygons in the mask and a maskwriting difficulty metric; imposing a set of metric constraints in accordance with the first, second, and third relationships; setting up an objective function of optimization in accordance with one of the first, second, and third relationships; determining optimum constrained values of the representations of the mask and the projection process in accordance with the set of metric constraints and the objective function of optimization; and outputting the optimum constrained values of the representations of the mask and the projection process.
1 Specification
This invention relates generally to lithographic formation of integrated circuit patterns on a semiconductor chip, and more particularly to a method for selecting and using combinations of illumination source characteristics and diffracting shapes on a mask to form integrated circuit patterns on a semiconductor chip with controlled line width roughness and image log slope.
Miniaturized electronic components such as integrated circuits (ICs) are typically manufactured using photolithography technology. In a photolithography process, a photoresist layer is formed on a semiconductor substrate, such as a silicon wafer. The photoresist layer is exposed through a photomask with a desired pattern to a source of actinic radiation. The radiation exposure causes a chemical reaction in the exposed areas of the photoresist and creates a latent image corresponding to the mask pattern in the photoresist layer. The photoresist is next developed in a developer solution, usually an aqueous base solution, to remove either the exposed portions of the photoresist for a positive photoresist or the unexposed portions of the photoresist for a negative photoresist. The patterns formed in the photoresist layer can then be transferred to the semiconductor substrate via processes such as deposition, etching, or ion implantation.
A critical dimension (CD) refers to the feature size and spacing between features (patterns) according to a design specification for a semiconductor device. CDs are critical for the proper functioning of the semiconductor device. When the CD of desired lithographic patterns approaches the resolution limit of an optical lithography system, image distortions become a significant problem. As the minimum CDs for semiconductor devices continue to decrease, the limited resolution of lithographic tools poses a challenge to improvements in IC manufacture. In order to make the manufacture of future IC products feasible, lithographic systems will be required to achieve adequate image fidelity when the ratio of the minimum CD to the resolution of the lithographic system becomes very low.
Many methods have been developed to reduce image distortions and improve image fidelity. One class of the enhancement methods adjusts the shapes of mask features to compensate the image distortions. For example, the biasing technique alters the mask shapes by widening the mask shapes at the tips of line features or lengthening the features to compensate for lineshortening effects. However, in some situations geometric constraints that are inherent to the desired circuit pattern may require that the mask shapes be perturbed in opposite directions and/or contradictory positions. In these situations, the biasing technique is ineffective to achieve the desired CDs of the desired image.
Another class of enhancement techniques improves contrast in the images by shifting the phase of the light projected from the mask such as “attenuated phase shift” and “alternating phase shift” techniques. The “attenuated phase shift” technique improves image sharpness by increasing the rate of change in illumination amplitude across the edge of mask features. This is achieved by using a phaseshifting material of slightly negative transmittance for dark areas of the pattern, rather than the conventional material of zero transmittance. The “alternating phase shift” technique achieves further contrast improvement by successively shifting the phase of adjacent bright features between 0° and 180°. In this way, the contrast of illumination intensity across the edge of image features is further increased compared to either conventional masks or the “attenuated phase shift” masks. However, both “attenuated phase shift” and “alternating phase shift” techniques does not directly address the abovementioned intrinsic geometric constraints in certain circuit patterns, though they do alleviate their severity by reducing image blur.
In addition, the “alternating phase shift” technique often adds unwanted features to image patterns. This occurs when circuit phases are laid out in such a way that the desired alternation in phase can only be achieved by introducing artificial 0° and 180° mask transitions which print as unwanted patterns. For example, when opposite phases are applied to bright regions that pass in close proximity to one another at a certain point on the mask, the phase must make such an unwanted transition if the bright regions are connected together elsewhere in the mask pattern. Such unwanted phase transitions will print as a dark fringe within the nominally bright connecting area, and must be trimmed away using a second exposure.
More recently, Source Mask Optimization (SMO) has been proposed to reduce image distortions and improve image fidelity. SMO is a photolithography resolution enhancement technique used to compensate for image errors due to aberrations, diffraction or process effects. It is not strongly limited by the intrinsic geometric constraints of the pattern layout. SMO technique uses intensively optimized wave distributions to illuminate both the mask and the wafer during lithographic exposures. SMO is proposed as a means for exploiting all available degrees of freedom in the bandlimited exposure process. The fundamental goal of SMO is to determine the optimum set of image forming waves that can propagate within the finite numerical aperture (NA) of the projection lens of the exposure tool, as formed using a manufacturable mask. NA is a measure of the directional extent of the light that can be collected by the lens.
SMO may optimize images based on a threshold intensity which determines where the perimeter of a printed feature will be formed. The threshold intensity is the light intensity above which a desired feature will be printed. Conventional SMO techniques assume that printed circuit features at the threshold intensity have a smooth edge. However, it is known that in practice the perimeter of printed circuit features will actually exhibit a finescale roughness whose fluctuations are typically sharper than the lens resolution.
In a photolithography process, the width of a resist feature can vary over the perimeter of the feature. This variation of width is called line width roughness (LWR). When the variation of the width along one edge of the resist feature is examined, the deviation of a line edge from a straight line is called line edge roughness (LER). LER is caused by a number of statistically fluctuating effects such as shot noise (photon flux variations), statistical distributions of chemical species in the resist such as photoacid generators (PAGs), the random walk nature of acid diffusion during chemical amplification, and the nonzero size of resist polymers being dissolved during development. It is not always clear which process or processes dominate in their contribution to LER. LER becomes important for feature sizes of 100 nm or less, and can become a significant source of CD local uniformity control problems for features below 50 nm.
While efforts have been made to reduce LER via improved processes and resists, the residual roughness in stateoftheart IC devices is large enough that improvements in the image to reduce LER in the transferred patterns are also needed.
The present invention provides a method for using combinations of illumination source characteristics and diffracting shapes on a mask to form integrated circuit patterns on a semiconductor chip with controlled line width roughness and image log slope. This invention also provides a machine readable storage medium having one or more machine readable program codes stored thereon for causing the machine to perform a method for using combinations of illumination source characteristics and diffracting shapes on a mask to form integrated circuit patterns on a semiconductor chip with controlled line width roughness and image log slope.
In one aspect, the present invention relates to method for illuminating a mask with a source to project a desired image pattern through a lithographic system onto a photoactive material including the steps of: defining a representation of the mask; obtaining a fractional resist shot noise (FRSN) parameter; determining a first relationship between a first set of optical intensity values and an edge roughness metric based on the FRSN parameter; determining a second relationship between a second set of optical intensity values and a lithographic performance metric; imposing a set of metric constraints in accordance with one of the first and second relationships; setting up an objective function of optimization in accordance with the remaining of the first and second relationships; determining optimum constrained values of the representation of the mask in accordance with the set of metric constraints and the objective function of optimization; and outputting the optimum constrained values of the representation of the mask.
The method preferably further includes the steps of fabricating the mask according to the optimum constrained values of the representation of the mask and projecting the mask through the lithographic system onto the photoactive material.
In another aspect, the present invention relates to a method for illuminating a mask with a source to project a desired image pattern through a lithographic system onto a photoactive material including the steps of: defining a representation of the mask; obtaining a fractional resist shot noise (FRSN) parameter; determining a first relationship between a first set of optical intensity values and an edge roughness metric based on the FRSN parameter; determining a second relationship between a second set of optical intensity values and a lithographic performance metric; determining a third relationship between a set of polygons in the mask and a maskwriting difficulty metric; imposing a set of metric constraints in accordance with the first, second, and third relationships; setting up an objective function of optimization in accordance with one of the first, second, and third relationships; determining optimum constrained values of the representation of the mask in accordance with the set of metric constraints and the objective function of optimization; and outputting the optimum constrained values of the representation of the mask.
In still another aspect, the present invention relates to a method for illuminating a mask with a source to project a desired image pattern through a lithographic system onto a photoactive material including the steps of: defining a representation of the mask and a representation of a projection process; obtaining a fractional resist shot noise (FRSN) parameter; determining a first relationship between a first set of optical intensity values and an edge roughness metric based on the FRSN parameter; determining a second relationship between a second set of optical intensity values and a lithographic performance metric; imposing a set of metric constraints in accordance with one of the first and second relationships; setting up an objective function of optimization in accordance with the remaining of the first and second relationships; determining optimum constrained values of the representations of the mask and the projection process in accordance with the set of metric constraints and the objective function of optimization; and outputting the optimum constrained values of the representations of the mask and the projection process
The method preferably further includes the steps of fabricating the mask according to the optimum constrained values of the representations of the mask and projecting the mask through the lithographic system onto the photoactive material according to the optimum constrained values of the representation of the projection process.
In still another aspect, the present invention relates to a method for illuminating a mask with a source to project a desired image pattern through a lithographic system onto a photoactive material including the steps of: defining a representation of the mask and a representation of a projection process; obtaining a fractional resist shot noise (FRSN) parameter; determining a first relationship between a first set of optical intensity values and an edge roughness metric based on the FRSN parameter; determining a second relationship between a second set of optical intensity values and a lithographic performance metric; determining a third relationship between a set of polygons in the mask and a maskwriting difficulty metric; imposing a set of metric constraints in accordance with the first, second, and third relationships; setting up an objective function of optimization in accordance with one of the first, second, and third relationships; determining optimum constrained values of the representations of the mask and the projection process in accordance with the set of metric constraints and the objective function of optimization; and outputting the optimum constrained values of the representations of the mask and the projection process.
In still another aspect, the present invention relates to a machine readable storage medium having one or more machine readable program codes stored thereon for causing the machine to perform a method for illuminating a mask with a source to project a desired image pattern through a lithographic system onto a photoactive material including the steps of: defining a representation of the mask; obtaining a fractional resist shot noise (FRSN) parameter; determining a first relationship between a first set of optical intensity values and an edge roughness metric based on the FRSN parameter; determining a second relationship between a second set of optical intensity values and a lithographic performance metric; imposing a set of metric constraints in accordance with one of the first and second relationships; setting up an objective function of optimization in accordance with the remaining of the first and second relationships; determining optimum constrained values of the representation of the mask in accordance with the set of metric constraints and the objective function of optimization; and outputting the optimum constrained values of the representation of the mask.
In still another aspect, the present invention relates to a machine readable storage medium having one or more machine readable program codes stored thereon for causing the machine to perform a method for illuminating a mask with a source to project a desired image pattern through a lithographic system onto a photoactive material including the steps of: defining a representation of the mask; obtaining a fractional resist shot noise (FRSN) parameter; determining a first relationship between a first set of optical intensity values and an edge roughness metric based on the FRSN parameter; determining a second relationship between a second set of optical intensity values and a lithographic performance metric; determining a third relationship between a set of polygons in the mask and a maskwriting difficulty metric; imposing a set of metric constraints in accordance with the first, second, and third relationships; setting up an objective function of optimization in accordance with one of the first, second, and third relationships; determining optimum constrained values of the representation of the mask in accordance with the set of metric constraints and the objective function of optimization; and outputting the optimum constrained values of the representation of the mask.
In still another aspect, the present invention relates to a machine readable storage medium having one or more machine readable program codes stored thereon for causing the machine to perform a method for illuminating a mask with a source to project a desired image pattern through a lithographic system onto a photoactive material including the steps of: defining a representation of the mask and a representation of a projection process; obtaining a fractional resist shot noise (FRSN) parameter; determining a first relationship between a first set of optical intensity values and an edge roughness metric based on the FRSN parameter; determining a second relationship between a second set of optical intensity values and a lithographic performance metric; imposing a set of metric constraints in accordance with one of the first and second relationships; setting up an objective function of optimization in accordance with the remaining of the first and second relationships; determining optimum constrained values of the representations of the mask and the projection process in accordance with the set of metric constraints and the objective function of optimization; and outputting the optimum constrained values of the representations of the mask and the projection process.
In still another aspect, the present invention relates to a machine readable storage medium having one or more machine readable program codes stored thereon for causing the machine to perform a method for illuminating a mask with a source to project a desired image pattern through a lithographic system onto a photoactive material including the steps of: defining a representation of the mask and a representation of a projection process; obtaining a fractional resist shot noise (FRSN) parameter; determining a first relationship between a first set of optical intensity values and an edge roughness metric based on the FRSN parameter; determining a second relationship between a second set of optical intensity values and a lithographic performance metric; determining a third relationship between a set of polygons in the mask and a maskwriting difficulty metric; imposing a set of metric constraints in accordance with the first, second, and third relationships; setting up an objective function of optimization in accordance with one of the first, second, and third relationships; determining optimum constrained values of the representations of the mask and the projection process in accordance with the set of metric constraints and the objective function of optimization; and outputting the optimum constrained values of the representations of the mask and the projection process.
The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.
It will be appreciated that for simplicity and clarity of illustration, elements shown in the drawings have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for purpose of clarity.
The various exemplary embodiments in accordance with this invention provide an improvement over conventional methods to reduce image distortions and improve image fidelity. This is accomplished by combining ways to optimize a lithographic image in a conventional SMO method with a new way of adjusting the image intensity to control LER.
As used herein, a “source” or an “illumination source” refers to a device configured to emit radiation for the purpose of lithographic exposure.
As used herein, a “mask” or a “lithography mask” refers to a template used in lithography that allows selective exposure of a photosensitive surface through a patterned layer included therein and having a different transparency than a background layer included therein.
In a conventional SMO method, the image relationships connect the image with standard lithographic metrics which assess sensitivity to defocus, dose control error, etc. In this method, the image intensity is a smoothly varying function due to the finite resolution of the lithographic system. As a result, the contour at the threshold intensity will have a smooth arc. Socalled compact process models may be used to capture process effects, in which case the intensity threshold is considered to vary smoothly as a function of the local intensity distribution. Even so, while such compact process models will change the perimeter shape of printed features via changes in the intensity threshold, they will not change its fundamentally smooth character.
However, even though conventional SMO methods can be used to achieve printed features whose shapes are robustly maintained when measured on the scale of the lens resolution, it is increasingly becoming the case that IC yield and performance are being limited by the finescale edge roughness of the patterned features. For example, roughness can affect the threshold voltage of circuit transistors, it can alter the effective gate length of transistor devices in a random way, and it can cause changes in the draininduced barrier lowering.
This invention attempts to improve the LER of the patterned features by using image relationships involving a parameter—fractional resist shot noise (FRSN) which is defined below. The image relationships connect the FRSN to a lithographic metric which involves the transferred pattern, rather than the image itself. The macroscopic image will typically be quite smooth, and the true variability in dose that SMO must optimize against will typically be fairly small, for example, +/−3%. A typical application of a metric for dose sensitivity might say that a particular feature edge should print within 1 nm of target when the dose varies by +/−3%.
The LER phenomenon will also cause the transferred feature to deviate from target, but only by very rapid roughness fluctuations, so that the average or expected edge position will remain within tolerance if conventional SMO methods are used. This macroscopic conformance means that the tolerance on the high frequency roughness can typically be relaxed by comparison with the tolerance on overall edge placement. For example, the LER tolerance might be +/−4 nm instead of +/−1 nm. However, typical values of the FRSN often show that the process is not as well controlled in its microscopic fluctuations as it is the macroscopic exposure dose that the resist receives. In the present invention, “macroscopic” means on the scale of circuit features or larger, while “microscopic” means on the scale of the resist blur distance.
LER of a patterned feature can be characterized as the root mean square of the deviation of a feature edge from a nominal macroscopic contour, according to the following equation:
σ_{LER}=Sqrt[<P(l)−P_{0}(l)^{2}>] (1)
P_{0}(l) denotes the position of a point with x, y coordinates on an arc which runs along the nominal contour (l) at the threshold intensity. The nominal contour can be obtained through simulation or by fitting a smooth contour during the measurement, as known in the art. Because of line edge roughness, the actual contour will generally be randomly offset from the nominal contour. The position of the point on the actual contour corresponding with P_{0}(l) is denoted P(l). The square of the magnitude of the offset between P(l) and P_{0}(l) is given by P(l)−P_{0}(l)^{2}. To calculate σ_{LER}, ensemble average of the squared offset, as denoted by <P(l)−P_{0}(l)^{2}>, is first taken. The square root (Sqrt) of the ensemble average then gives σ_{LER}.
In a paper published by Gregg M. Gallatin (Gregg M. Gallatin, “Resist Blur and Line Edge Roughness”, Proceedings of SPIEOptical Microlithography XVIII, Vol. 5754, 3852), σ_{LER }has been related to the optical intensity at an edge of a printed feature according to the following equation:
wherein i_{edge }is the optical intensity at a given position of the edge of the printed feature, T is the thickness of the resist, ε is the quantum efficiency of acid release, E_{edge }is the local dose at the edge,
is the volume of the deprotection blur, and R_{PEB }is the resist diffusion blur.
Let .N=εE_{0}fV/T Then, the above equation can be rewritten as:
wherein i_{edge }is the optical intensity at a given position of the edge of the printed feature, N is the number of photoacid molecules activated within the resist blur resolution volume, and
is the image log slope (ILS) at this position.
Importantly, as shown in Eq. (3), σ_{LER }is inversely proportional to ILS. Thus, increasing ILS will reduce the LER of a printed feature.
In the present invention, √{square root over (N)}/N is defined as FRSN. Based on Eq. (3), FRSN is related to σ_{LER }according to the following equation:
FRSN can be obtained from experimental data using Equation (4). In particular, σ_{LER }can be determined from printed images of a test pattern. For example, σ_{LER }can be obtained by using a scanning electron microscope (SEM) to measure a rough perimeter of a sample edge of a test pattern so as to approximate the ensemble average <P(l)−P_{0}(l)^{2}>. The position of a reference edge P_{0}(l) may be obtained by smoothing the position measured along the rough edge over a distance comparable to the lens resolution, or by fitting a smooth contour of a known image. σ_{LER }is then calculated from the ensemble average according to Eq. (1).
One may wish to measure σ_{LER }to within a certain relative accuracy. In order to achieve the desired relative accuracy, the total length of the sample edge of the test pattern should be sufficiently long. In one embodiment, σ_{LER }is measured to about 3% relative accuracy. In this case, the total sample edge length should be about 1000 times the size of the resist blur resolution, e.g., a total sample length of about 10 microns might be used. The image used to print the test pattern may be determined along with their ILS values by standard image calculation methods, as known to the skilled in the art. One may form the total sample population using test patterns whose images have a diverse set of ILS values. As shown in
σ_{LER}=FRSN×1/ILS (5)
The FRSN is different from the macroscopic dose control. An error in macroscopic dose often contributes to CD errors, while the FRSN represents a resist fluctuation at approximately the molecular scale of the resist during the exposure process. The FRSN often contributes to the LER of the resist pattern. In addition, the FRSN is usually several times larger than the macroscopic level dose control achieved in the process.
Components of a conventional projection lithographic system 200 are shown in
One embodiment of a method for illuminating a mask with a source to project a desired image pattern through a lithographic system onto a photoactive material according to the present invention is illustrated in
Referring to
The representation of the mask may also include the transmission coefficients of the polygons as determined by the mask technology. A mask is defined by the edges and/or corners of the polygons as well as the transmission coefficients of the polygons. For example, socalled binary masks may be composed of polygons with a transmission coefficient of 1 within a background that may be taken to have a transmission coefficient of 0, or alternatively, a known small value. Socalled chromeless masks may be composed of polygons with a transmission coefficient of −1 formed within a background with a transmission coefficient of 1, or polygons with a transmission coefficient of 1 formed within a background with a transmission coefficient of −1.
In another embodiment, the representation is a set of diffraction order weights which represent the amplitude of the light which is diffracted by the mask and passes through the pupil of the lithographic system. Diffraction orders represent the 2D Fourier transform of the polygons under periodic boundary conditions, with the (n, m) order being diffracted into frequency space in a direction (n*λ/Px, m*λ/Py). λ is the wavelength. Px and Py are the periods assumed for the boundaries of the mask design field. All diffraction orders whose directions can be collected within the numerical aperture (NA) of the lens are included in the diffraction order weight representation.
As shown in
Bitmap masks are yet another possible mask representation, where the bitmap values represent the transmission of small mask rectangles as laid out on a grid.
In step 320, a FRSN parameter is obtained. As discussed above, the FRSN can be obtained experimentally based on the σ_{LER }values and the ILS values as measured over a sample population of test patterns. The FRSN is a microscopic representation of resist fluctuation during exposure. At the threshold intensity, the FRSN is a constant for a given resist.
In step 330, a first relationship between a first set of optical intensity values and an edge roughness metric is determined based on the FRSN parameter. The first set of optical intensity values are chosen based on the desired properties of the image intensity distribution. The desired properties of the image intensity distribution are specified using discrete sample points whose density sufficiently exceeds the lens resolution (0.25×λ/NA) such that the sampled intensities adequately represent the character of the image. For example, certain classes of sample points can be used to map out the nominally bright and dark regions of the image, while other classes of sample points define the boundaries of an acceptable band of positions in which each feature edge can be allowed to print (tolerance band).
The distance between the two points defining a tolerance band (on the opposite sides of a feature edge) is L. For simplicity we may consider that the allowed tolerance on either side of the feature edge is L/2, though asymmetric tolerances can also be employed. The optical intensities at the two points are i^{+} and i^{−}. ILS is related to the set of intensity values at the boundaries of an acceptable band of positions in which each feature edge can be allowed to print (i^{+}and i^{−}) by the following equation:
wherein i^{+} and i^{−} are the intensities of the two corresponding points on the opposite sides of a feature edge and L is the distance between the two points.
An example of the edge roughness metric is σ_{LER}. σ_{LER }is related to ILS according to Eq. (4). Since the ILS value at each sample point is related to a set of optical intensities i^{+} and i^{−} based on Eq. (6), a relationship between the set of intensities i^{+} and i^{−} and σ_{LER }then can be determined based on the FRSN parameter according to Eq. (4).
A second relationship between a second set of optical intensity values and a lithographic performance metric is determined in step 340. Examples of the lithographic performance metric suitable for the present invention include, but are not limited to, exposure latitude (EL) metric, process window metric, edge placement error (EPE) metric, and depth of focus (DOF) metric. EL is the extent to which a resist can be over or underexposed but still achieve an acceptable CD result. Process window is a metric obtained by plotting contours corresponding to various specification limits as a function of exposure dose and focus. One simple process window, called CD process window, is a contour plot of the high and low CD specifications as a function of exposure dose and focus. Often, several process windows are plotted together to determine the overlap of the windows. EPE is a metric which can be obtained by simulating the distance from the nominal contour to the target. DOF is the total range of focus which can be tolerated, that is, the range of focus that keeps the resulting printed feature within a variety of specifications, e.g., exposure latitude.
In general, all of these lithographic performance metrics can be related with simple calculations to the intensity distribution in the image, and more specifically to a sampled set of intensity values in the image. Referring to
In particular, exposure latitude (EL) at a targetcontour sample point can be related to the sampled intensities by subtracting the intensity at whichever sample point is offset into the nominally darker side of the nominal image from the intensity at the sample point offset to the nominally brighter side. These darksideoffset and brightsideoffset sample points are illustrated in
CW=(r_{Bright}r_{Dark})/<I_{e}(n,F_{0})> (7)
 where CW is the common window;
 r_{Bright}=Min[I_{b}(n,f)];
 r_{Dark}=Max[I_{d}(n,f)].
Since the intensities I_{d}(n,f) and I_{b}(n,f) are determined by the mask, these equations constitute a relationship between the common window metric and the mask representation. The Minimization and the Maximization in these equations extend over all values of n, and over all values of defocus f that fall within the desired focal range of the exposure process, which we can express as F_{2}<f<F_{1}. In practice f may be sampled at a discrete set of planes. The average of F_{1 }and F_{2 }is the center of focus, which is typically considered the plane of best focus; we can write this position of best focus as f=F_{0}. The quantity <I_{e}(n,F_{0})> is used to convert CW into the conventional fractional or percentage metric; it is defined as the average of I_{e}(n,F_{0}) taken over all n. One may optionally rescale the CW by the focal range included, i.e. one may alternatively define CW as
CW=(F_{1}−F_{2})*(r_{Bright}−r_{Dark})/<I_{e}(n,F_{0})> (8)
Common window is often considered to be a dose window rather than an intensity window, but when expressed in percentage units the two are essentially equivalent.
Referring to
Optionally, but not necessarily, a third relationship between a set of polygons in the mask and a maskwriting difficulty metric is determined. The set of polygons in the mask are preferably the shapes of mask openings that will be formed in a background film on the mask substrate, or alternatively, the shapes of islands of material that are left when the remainder of the background film is removed. Examples of the maskwriting difficulty metric include, but are not limited to, mask manufacturability metrics that represent inadequate separation between features, or that represent features which are not kept distinct (i.e. features whose edges cross).
Such manufacturability metrics are preferably smooth functions of the mask geometries, with the function values chosen to correlate monotonically with particular dimensional attributes that the shapes must satisfy to be manufacturable. For example, these dimensional requirements may be mask ground rules supplied by the mask manufacturer. The value of the metrics can then be constrained to achieve suitable levels for manufacturability. Alternatively, a particular manufacturability metric may be optimized as an objective. In other embodiments, the objective may be a weighted sum of more than one manufacturability metric, or the objective may be the RMS value of the metrics, or alternatively, the objective may be to maximize the particular manufacturability metric which has the smallest (i.e. worstcase) value. As yet another alternative, a combined manufacturability metric, or a worstcase metric, may serve as an overall metric to be constrained. In some embodiments, the manufacturability metrics may take the form of penalty functions that should be held below a threshold penalty, or that should be minimized. These penalty metrics may be converted to metrics that should be maximized or raised above a threshold by reversing the sign of the penalty. In general, the smooth functions involved constitute a relationship between the mask shapes and a mask manufacturability metric.
Separate manufacturability metrics may conveniently be calculated for all pairs of edges, or for all pairs of edges that are in close proximity to each other, such as those pairs of edges whose closest separation is comparable to the resolution of the maskwriting process, or is smaller. In many cases the mask manufacturing process only permits fabrication of mask features whose shapes are polygons with edges that have specific allowed orientations relative to the edges of the mask substrate itself, such as orientations of 0 degrees and 90 degrees (socalled Manhattan masks), or such as 0, 90, 45, or 135 degrees.
Metrics that are penalties calculated for pairs of edges may take the form of functions that have values at or near zero when the two edges in the pair are well separated, so that the penalty metric goes to zero when the two edges do not interfere with each other'"'"'s manufacture. Further, such a penalty function may, for example, rise to a value of 1 under conditions where the two edges are strongly interfering, such as when the edges cross at their midpoints, or as another example in cases where the two edges are parallel, are fully overlapped, and have 0 separation between them.
In many cases manufacturability metrics may conveniently be constructed as products of submetrics. For example, if two conditions must be triggered before a pair of edges will interfere with each other'"'"'s manufacture, for example a first condition involving the x coordinates of points in the two edges, and a second condition involving the y coordinates of points in the two edges, then the two conditions can usefully be represented as two submetrics (later multiplied) that correspond to the two conditions. If either one of the conditions is not met, causing the corresponding submetric to have a value near zero, the metric formed as a product of the submetrics will also have a value near zero. Only if both conditions are met will the overall metric have an appreciable value, since each submetric will then have a value of order 1. In such a case the metric will properly indicate that the pair of edges is difficult to manufacture.
An example of a useful manufacturability metric is a crossing penalty metric. When applied to a pair of nonparallel edges, this metric represents in a smooth way the degree to which the edges are in a state of crossing, or nearcrossing. A constraint may be imposed on every pair of proximate edges (i.e. edges that approximately neighbor each other) requiring that the crossing penalty metric be kept safely below the level representing a truly crossed condition. In the case where the mask is Manhattan, the metric can be applied to edge pairs in which one edge is horizontal and the other vertical. The crossing penalty can take the form of a product of two submetrics; one submetric indicating whether the xcoordinate of the edge that is vertical crosses the span between the x coordinates of the two endpoints of the horizontal edge, and a second submetric which indicates whether the y coordinate of the horizontal edge crosses in between the y coordinates of the two endpoints of the vertical edge. Each submetric can then be written in the form P_{S}(q_{0};q_{1},q_{2}), where q_{0 }represents the location of one edge along an axis perpendicular to its orientation, and where q_{1 }and q_{2 }represent the coordinates along this same axis of the two endpoints of the other orthogonally oriented edge. P_{S }may be any function that is close to zero when q_{0 }is far to one side (either side) of q_{1 }and q_{2}, and that rises to a substantial value as q_{0 }passes between q_{1 }and q_{2}. The overall product of the two P_{S }submetrics for the x and y coordinates can then only achieve a large value when the two edges are crossed in both coordinates, i.e. are truly crossed.
For example, we may set
P_{S}(q_{0};q_{1};q_{2})=Log_{2}[1+(Sqrt)[2]−1)̂{([q_{0}−q_{1}]*[q_{0}−q_{2}])/(e[q_{2}−q_{1}])^{2}}] (9)
In this form the function P_{S }takes on the value 1 when q_{0 }just crosses either q_{1 }or q_{2 }(i.e. when q_{0 }is equal to one of them), and rises to a larger value when q_{0 }is positioned in between q_{1 }and q_{2}. When q_{0 }is well distant from q_{1 }and q_{2 }on either side, P_{S }drops smoothly to zero. P_{S }rises as q_{0 }approaches q_{1 }or q_{2 }from either side, which causes the geometric mean of [q_{0}q_{1}]/q2−q_{1} and [q_{0}−q_{2}]/q_{2}−q_{1} to decrease. When this geometric mean drops to a value of e, the increasing P_{S }function reaches ½. Parameter e thus controls the steepness in the rise of P_{S }as the edges come close to crossing (which occurs when P_{S }reaches 1). Typically e will be a small quantity such as 0.1, so that P_{S }only rises to a value of 0.5 when q_{o }comes quite close to q_{1 }or q_{2}. Other forms of P_{S }use an absolute distance parameter to control the steepness of P_{S}, such as a distance of 10 nm. Other suitable functions for metric and submetric components like P_{S }take the form of piecewise smooth polynomials.
Once these two relationships are determined, a set of metric constraints in accordance with one of the first and second is imposed (step 350). In one embodiment, at least one of the set of metric constraints is imposed as a penalty term in the objective function of optimization. The violation of the constraints is reflected into the objective function as the penalty term. For example, one can set the process window as the objective function of optimization and the ILS constraint as the penalty in the objective function. In another embodiment, the set of constraints are set as the constraints in optimization. This functionality can be carried out using standard nonlinear optimization software, by specifying as an optimization constraint that the lithographic metric must be held above a desired quality level.
The relationship used to determine the set of metric constraints may be any one of the first and second relationships. After the set of metric constraints are determined, an objective function of optimization is set up in accordance with the remaining of these two relationships which is not used (step 360). For example, if the first relationship is used to determine the set of metric constraints, then the objective function of optimization is set up using the second relationship. Conversely, if the second relationship is used to determine the set of metric constraints, then the first relationship is used to set up the objective function of optimization.
In one embodiment, when the third relationship between a set of polygons in the mask and a maskwriting difficulty metric is determined, a set of metric constraints in accordance with two of the first, second, and third relationships is imposed. In one example, at least one of the set of metric constraints is imposed as a penalty term in the objective function of optimization. The violation of the constraints is reflected into the objective function as the penalty term. For example, one can set the process window as the objective function of optimization and the ILS constraint as the penalty in the objective function, while leaving manufacturability as the constraints in the optimization. In another embodiment, the set of constraints are set as the constraints in optimization. This functionality can be carried out using standard nonlinear optimization software, by specifying as an optimization constraint that the lithographic metric must be held above a desired quality level.
The two relationships used to determine the set of metric constraints may be any two of the first, second, and third relationships. After the set of metric constraints are determined, an objective function of optimization is set up in accordance with the remaining of the three relationships which is not used. For example, if the first and second relationships are used to determine the set of metric constraints, then the objective function of optimization is set up using the third relationship. If the second and third relationships are used to determine the set of metric constraints, then the first relationship is used to set up the objective function of optimization. Finally, if the set of metric constraints are determined based on the first and third relationships, then the second relationship is used to set up the objective function of optimization.
In another embodiment, when the third relationship between a set of polygons in the mask and a maskwriting difficulty metric is determined, a set of metric constraints is imposed in accordance with the first, second, and third relationships, i.e., the set of metric constraints is imposed according to all three relationships. After the set of metric constraints are determined, an objective function of optimization is set up in accordance with one of the three relationships. The relationship used to set up the objective function can be any one of the three relationships.
The objective function of optimization in step 360 may be any standard lithographic metric, such as the exposure latitude or process window metrics. A standard optimization code may be used to specify that the objective of the optimization is to maximize the desired quality metric.
This procedure may be summarized algebraically in the following fashion. In one embodiment, we let a_{i}(m) denote in an abstract way a manufacturability metric, for example a metric which describes the mutual fabricability of a particular pair of edges. We further assume that this metric is scaled such that the pair of edges becomes manufacturable in respect to the metric whenever a_{i}(m) is positive. The argument m denotes the full set of variable values in the mask representation. The relationship between each a_{i }and the m variables might for example be defined by the P_{S }functions described above. The subscript i indicates that this particular metric arises in the ith element of a constraint list which would typically have many members. For notational convenience we can write the entire list of such constraints as a single expression, namely
a_{i}(m)>0
This expression is understood to apply for each value of i.
In a similar way we let b_{j}(m) denote a lithographic performance metric which may be evaluated at some point in the image; for example, b_{j}(m) might represent the exposure latitude evaluated at a particular sample point associated with the jth constraint (which might be a bright edge sample point), or b_{j}(m) might represent the residual intensity in a nominally dark region of the image (where the sample point associated with the jth constraint might be a dark interior sample point). Different lithographic performance metrics may be applied at different values of j. A lithographic performance metric will typically depend on the image intensities which in turn depend on the mask; thus each metric b_{j}(m) constitutes a relationship between the metric and the values of the mask representation.
A similar notation can be applied to ILS. For example, we can let c_{k}(m) denote the ILS value obtained at the sample point associated with the kth constraint. This sample point may be a bright edge sample point. c_{k}(m) represents a relationship between the ILS values and (via the image intensities) the values m of the mask representation, in the manner discussed above. When defining c_{k}(m) we can further subtract from ILS the quantity FRSN/σ_{LER,Tol}, where σ_{LER,Tol }represents a tolerance on allowed LER (which may optionally be different at different values of k). In this way our LER requirement can be expressed as
c_{k}(m)>0
We next consider that while a, b, and c may be defined in respect to many particular aspects of the image or mask, such as at many different sample points, it is possible in each case to define single overall metrics A, B, and C which represent the overall quality attained by the image or mask in respect to the metric type. For example, while a_{i}(m)>0 may represent a requirement that the ith pair of edges be mutually manufacturable, A(m) may represent the worstcase manufacturability level arising amongst all different pairs of edges in the mask. Alternatively, A(m) may represent the average manufacturability level, or the RMS manufacturability level. Similarly, B(m) might denote the common window achieved across all bright edge sample points, and C(m) might represent the worstcase ILS value arising anywhere in the image, as normalized by σ_{LER,Tol}.
Then, in accordance with step 350 of the invention, we may optionally form metric constraints using, for example, the two sets of relationships a and b. According to step 360, we may then define C as the objective to be optimized in the lithographic process. When steps 350 and 360 are completed in this way, we next (in step 370) determine optimum values for the mask variables m by solving the optimization problem
i. Maximize C(m)
subject to:
ii. a_{i}(m)>0
iii. b_{j}(m)>0
Note that separate constraints do not necessarily have to be defined for multiple aspects of the image, such as at multiple sample points, though this would be a common choice. One may instead, for example, employ a single constraint B(m)>0, i.e.
i. Maximize C(m)
subject to:
ii. a_{i}(m)>0
iii. b_{j}(m)>0
where B might represent common window. Note that if a metric such as B is a worst case metric it may be defined using auxiliary constraints.
Other choices for the constrained metrics and objective can be made in steps 350 and 360 as well. For example
i. Maximize A(m)
subject to:
ii. c_{k}(m)>0
iii. b_{j}(m)>0
or
iv. Maximize B(m)
subject to:
v. c_{k}(m)>0
vi. a_{i}(m)>0
The above two approaches differ in the choice of the two constraint relationships that is made in step 350.
It is also possible to simultaneously employ an objective and constraints of the same general type; for example
i. Maximize B(m)
subject to:
ii. a_{i}(m)>0
iii. b_{j}(m)>0
iv. c_{k}(m)>0
In this case, B might represent common window, while the various b_{b}(m) might include, for example, constraints on exposure latitude or residual dark intensity at different sample points.
In some embodiments the invention need not employ relationships from all three categories a, b, and c. For example, one may instead determine the optimum constrained values for the mask representation by solving the optimization problem
i. Maximize C(m)
subject to:
ii. b_{j}(m)>0
The optimum constrained values of the representation of the mask are determined based on the set of metric constraints and the objective function of optimization in step 380. Selected nonlinear optimizer/solver, e.g., IPOPT (Interior Point OPTimization) or Augmented Lagrangian code, will solve the nonlinear optimization problem defined by the set of metric constraints and the objective function. The optimization variables of such a nonlinear optimization problem are the representations of the mask. The nonlinear optimizer/solver will output the optimum values of the representations of the mask.
Once the optimum constrained values of the representation of the mask are determined, they are output (step 380). For instance, the optimum constrained values may be displayed on a display device of a computer for viewing by a user. In another example, the optimum constrained values may be printed out on a paper. In still another example, the optimum constrained values may be output to a mask design database to be used to design a mask.
The method 300 of
Ultimately, the mask may have its design optimized based on the optimum constrained values of the selected representation of the mask so as to achieve desired ILS and LER. In this respect, steps 310, 320, 330, 340, 350, 360, 370 and 380 may be iteratively performed until a mask design having desired ILS and LER is achieved. Embodiments of the invention are not limited to the specific manner by which each of the steps 310, 320, 330, 340, 350, 360, 370 and 380 is performed.
Once the optimum constrained values of the selected representation of the mask are output, the mask may be fabricated according to the optimum constrained values. In addition, the method may further include projecting the mask through the lithographic system onto the photoactive material to create a pattern in the photoactive material which closely conforms to the desired image pattern.
Another embodiment of a method of illuminating a mask with a source to project a desired image pattern through a lithographic system onto a photoactive material according to the present invention is illustrated in
Referring to
Steps 820 to 860 of the method 800 are the same as steps 320 to 360 of the method 300 respectively as described above in details. Similarly, the relationship used to determine the set of metric constraints in step 850 may be any one of the first and second relationships. After the set of metric constraints are determined, in step 860 an objective function of optimization is set up in accordance with the remaining of the two relationships which is not used.
Similarly, a third relationship between a set of polygons in the mask and a maskwriting difficulty metric may be determined. The set of polygons in the mask are preferably the shapes of mask openings that will be formed in a background film on the mask substrate, or alternatively, the shapes of islands of material that are left when the remainder of the background film is removed. The maskwriting difficulty metric may be mask manufacturability metrics that represent inadequate separation between features, or that represent features which are not kept distinct (i.e. features whose edges cross).
In one embodiment, when the third relationship between a set of polygons in the mask and a maskwriting difficulty metric is determined, a set of metric constraints in accordance with two of the first, second, and third relationships is imposed. In one example, at least one of the set of metric constraints is imposed as a penalty term in the objective function of optimization. The violation of the constraints is reflected into the objective function as the penalty term.
The two relationships used to determine the set of metric constraints may be any two of the first, second, and third relationships. After the set of metric constraints are determined, an objective function of optimization is set up in accordance with the remaining of the three relationships which is not used.
In another embodiment, when the third relationship between a set of polygons in the mask and a maskwriting difficulty metric is determined, a set of metric constraints is imposed in accordance with the first, second, and third relationships, i.e., the set of metric constraints is imposed according to all three relationships. After the set of metric constraints are determined, an objective function of optimization is set up in accordance with one of the three relationships. The relationship used to set up the objective function can be any one of the three relationships.
Steps 850 and 860 proceed in much the same way as steps 350 and 360 described above, except that the relationships defining the metrics a, b, c, A, B, and C now involve projection process parameters, which may be denoted s, as well as mask representation variables m. Projection process parameters s may, for example, be the intensities of source pixels in an illumination source. The optimization problem solved in step 880 may then take similar forms to one of the permutations described above, except that the projection process parameters are involved; for example i. Maximize C(m,s)
subject to:
ii. a_{i}(m,s)>0 iii. b_{i}(m,s)>0
In step 870 of the method 800, optimum constrained values of the representations of the mask and the projection process are determined based on the set of metric constraints and the objective function of the optimization. Selected nonlinear optimizer/solver, e.g. IPOPT (Interior Point OPTimization) or Augmented Lagrangian solver, will solve the nonlinear optimization problem defined by the set of metric constraints and the objective function. The optimization variables of such a nonlinear optimization problem are the representations of the mask and the projection process. The nonlinear optimizer/solver will output the optimum values of the representations of the mask and the projection process.
In step 880, the optimum constrained values of the representations of the mask and the projection process are output. As in step 380, the optimum constrained values may be displayed on a display device of a computer for viewing by a user. In another example, the optimum constrained values may be printed out on a paper. Similarly, the method 800 of
The method 800 of
Once the optimum constrained values of the representation of the mask are obtained, the mask may be fabricated according to the optimum constrained values. In addition, the method may further include projecting the mask through the lithographic system onto the photoactive material in accordance with the optimum constrained values of the representation of the projection process to create a pattern in the photoactive material which closely conforms to the desired image pattern.
While the present invention has been particularly shown and described with respect to preferred embodiments, it will be understood by those skilled in the art that the foregoing and other changes in forms and details may be made without departing from the spirit and scope of the invention. It is therefore intended that the present invention not be limited to the exact forms and details described and illustrated but fall within the scope of the appended claims.