阅读说明：本技术 光掩模的光学邻近校正方法、制造方法和半导体器件的制作方法 (Optical proximity correction method of photomask, manufacturing method of photomask and manufacturing method of semiconductor device ) 是由 张雷 于 2020-05-14 设计创作，主要内容包括：本发明提供了一种光掩模的光学邻近校正方法,包括以下步骤：获得所述光掩模的设计图形,所述设计图形具有单元格阵列；根据所述单元格阵列中各个单元格的设计特征,将所述各个单元格进行分类；对每类单元格的至少一个代表性单元格进行光学邻近校正；以及将每类单元格的所述代表性单元格的校正结果应用到每类单元格的其他单元格。(The invention provides an optical proximity correction method of a photomask, which comprises the following steps: obtaining a design pattern of the photomask, the design pattern having an array of unit cells; classifying each unit cell according to the design characteristics of each unit cell in the unit cell array; performing optical proximity correction on at least one representative cell of each type of cell; and applying the correction result of the representative cell of each type of cell to the other cells of each type of cell.)

1. A method for optical proximity correction of a photomask, comprising the steps of:

obtaining a design pattern of the photomask, the design pattern having an array of unit cells;

classifying each unit cell according to the design characteristics of each unit cell in the unit cell array;

performing optical proximity correction on at least one representative cell of each type of cell; and

applying the correction result of the representative cell of each type of cell to the other cells of each type of cell.

2. The method of claim 1, wherein the design features of the individual cells comprise: the size of the cell, the number of adjacent cells, and the location of the adjacent cells.

3. The method of claim 1 or 2, wherein the size of each cell in the array of cells is the same.

4. The method of claim 1, wherein the step of classifying the respective cells into a plurality of classes of cells comprises:

the design features of the respective cells are inspected using the optical diameters and classified according to the inspection results.

5. The method of claim 1, wherein the cell is a polygon having a number of sides greater than 3.

6. The method of claim 1, wherein classifying each cell in the array of cells into a plurality of classes of cells based on design characteristics of the cell further comprises:

and adding an exposure auxiliary pattern in the design pattern.

7. The method of claim 1, further comprising, after applying the correction results for the representative cell of each type of cell to other cells of each type of cell:

verifying a result of the optical proximity correction; and

a mask rule check is performed.

8. The method of claim 1, wherein the photomask is used to fabricate a semiconductor memory.

9. A method of manufacturing a photomask, wherein the mask pattern on the photomask is subjected to optical proximity correction using the method according to any one of claims 1 to 8.

10. A method for manufacturing a semiconductor device, characterized in that optical proximity correction is performed on a mask pattern on the photomask using the method according to any one of claims 1 to 9.

11. A computer comprising a memory and a processor; the memory stores a computer program; the computer program, when invoked by the processor, performs the method of any of claims 1 to 8.

12. A computer storage medium storing a computer program; the computer program, when invoked by a processor, performs the method of any of the preceding claims 1 to 8.

Technical Field

The present invention relates generally to the field of semiconductor design and manufacture, and more particularly to optical proximity correction for a photomask.

Background

As semiconductor manufacturing processes continue to evolve, the feature sizes on the logic device nodes have come close to or smaller than the wavelengths used in the photolithographic processes. According to the principle of diffraction and interference of light, light waves are diffracted when passing through the mask, and light transmitted at different positions of the mask still interferes. Therefore, the intensity distribution actually projected onto the wafer is a result of the superposition of these diffracted and interfered light waves, which are not exactly the same as the mask pattern.

Optical Proximity Correction (OPC) is a core technology in the manufacturing process of nanoscale wafers, and as the nodes of logic devices are continuously reduced, the development of OPC is more and more complex, and more cyclic iterative operations and multiple corrections are required, and continuous inspection and proofreading are required.

With the continuous development of the lithography technology, the size of the lithography lines becomes narrower and narrower, and when the feature size of the exposure line approaches the theoretical resolution limit of the exposure system, the aerial image will generate obvious distortion, i.e. Optical Proximity Effect (OPE), which causes the quality of the lithography pattern to be seriously degraded. Meanwhile, not only can the photoetching be affected by the adjacent distortion of the patterns caused by diffraction in the optical imaging process, but also the photoetching quality cannot be neglected by the processing processes of exposure, development, etching and the like of the photoresist. Therefore, the optical proximity correction is a method for reducing the pattern distortion in the whole photoetching process by optical means such as mask pattern optimization and the like, namely, the aim of improving the photoetching quality of an integrated circuit is finally achieved by an optical wavefront processing means.

Disclosure of Invention

The technical problem to be solved by the present invention is to provide a method for performing fast Optical Proximity Correction (OPC) for a photomask pattern design of a memory array area of a three-dimensional memory and ensuring Critical dimension uniformity (CD consistency) of the pattern design.

To solve the above technical problem, the present invention provides a method for optical proximity correction of a photomask, comprising the steps of: obtaining a design pattern of the photomask, the design pattern having an array of unit cells; classifying each unit cell according to the design characteristics of each unit cell in the unit cell array; performing optical proximity correction on at least one representative cell of each type of cell; and applying the correction result of the representative cell of each type of cell to the other cells in each type of cell.

In an embodiment of the present invention, the design features of each cell include: the size of the cell, the number of adjacent cells, and the location of the adjacent cells.

In an embodiment of the invention, the size of each unit cell in the unit cell array is the same.

In an embodiment of the invention, the method further comprises checking the size of each cell using the optical diameter.

In an embodiment of the invention, the cell is a polygon with a number of sides greater than 3.

In an embodiment of the present invention, after classifying each cell into multiple types of cells according to a design feature of each cell in the cell array, the method further includes: and adding an exposure auxiliary pattern in the design pattern.

In an embodiment of the present invention, after applying the correction result of the representative cell of each type of cell to other cells of each type of cell, further includes: verifying a result of the optical proximity correction; and performing a mask rule check.

In one embodiment of the present invention, the photomask is used for manufacturing a semiconductor memory.

The present invention also provides a method of manufacturing a photomask, the method being used for optical proximity correction of a mask pattern on the photomask.

The present invention also provides a method of manufacturing a semiconductor device, characterized in that optical proximity correction is performed on a mask pattern on the photomask using the method according to any one of the preceding claims.

The invention also provides a computer, comprising a memory and a processor; the memory stores a computer program; the computer program, when invoked by the processor, performs the method of any of claims 1 to 9.

The invention also provides a computer storage medium storing a computer program; the computer program, when invoked by a processor, performs the method of any preceding claim.

Compared with the prior art, the invention can realize the rapid Optical Proximity Correction (OPC) on the photomask pattern design of the storage array area of the three-dimensional memory and ensure the CD consistency of the pattern design.

Drawings

FIG. 1 is a flowchart illustrating a method for optical proximity correction of a photomask according to an embodiment of the present invention.

FIG. 2A is a diagram illustrating a classification of cells in a method for optical proximity correction of a photomask according to an embodiment of the present invention.

FIG. 2B is a diagram illustrating the classification of cells in the optical proximity correction method for a photomask according to an embodiment of the present invention.

FIG. 3A is a diagram illustrating an optical diameter inspection classification of a cell in a method for optical proximity correction of a photomask according to an embodiment of the present invention.

FIG. 3B is a diagram illustrating the optical diameter inspection classification of cells in the optical proximity correction method for a photomask according to an embodiment of the present invention.

FIG. 4 is a flowchart illustrating a method for optical proximity correction of a photomask according to an embodiment of the present invention.

Detailed Description

In order to make the aforementioned objects, features and advantages of the present invention comprehensible, embodiments accompanied with figures are described in detail below.

In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention, but the present invention may be practiced in other ways than those specifically described herein, and thus the present invention is not limited to the specific embodiments disclosed below.

As used in this application and the appended claims, the terms "a," "an," "the," and/or "the" are not intended to be inclusive in the singular, but rather are intended to be inclusive in the plural unless the context clearly dictates otherwise. In general, the terms "comprises" and "comprising" merely indicate that steps and elements are included which are explicitly identified, that the steps and elements do not form an exclusive list, and that a method or apparatus may include other steps or elements.

Embodiments of the present invention describe an optical proximity correction method of a photomask, a manufacturing method of a semiconductor device, and the like.

In a three-dimensional memory, such as a 3D NAND flash memory, a large amount of repeatable pattern design occurs in a memory array region when a corresponding photomask design is performed due to the high density distribution characteristic of the memory array, and thus, as the pattern density increases, the time required for performing Optical Proximity Correction (OPC) on the pattern design also increases for the memory array region, and thus, it becomes very time-consuming to verify and inspect the mask design. Meanwhile, due to the inherent characteristics of the optical proximity correction process of the photomask pattern design, for the same pattern correction, even under the same optical diameter (optical diameter) condition, a deviation of the correction amount is generated. In some cases, even if the deviation of the correction path is small, such as ± 0.1nm, ± 0.2nm, … …, etc., since the links of the manufacturing process of the semiconductor device are complicated and precise, it is desirable to minimize the occurrence of errors in the design and manufacturing links of the photomask so as to facilitate the quality control of the final product.

As shown in FIG. 1, the optical proximity correction method of the photomask of the present invention includes the steps of obtaining 101 a design pattern of the photomask, the design pattern having a cell array; 102, dividing each unit cell into a plurality of types of unit cells according to the design characteristics of each unit cell in the unit cell array; step 103, performing optical proximity correction on at least one representative cell of each type of cells; and a step 104 of applying the correction result of the representative cell of each type of cell to the other cells in each type of cell.

In a non-limiting embodiment, the design pattern of the photo mask obtained in step 101 is a mask pattern corresponding to a channel hole (channel) of a memory array in the core region of the three-dimensional memory, such as a rectangular hole mask pattern, a circular hole mask pattern, an elliptical hole mask pattern, or the like. In other embodiments, the design pattern of the photomask is a line-shaped mask pattern corresponding to a trench (trench) of a Common Source (Array Common Source) of a memory Array in a core region of the three-dimensional memory; or word line (word line), bit line (bit line) of the word line connection region; or a mask pattern corresponding to a contact portion connected to a peripheral circuit.

In some embodiments, the design pattern has an array of cells. The cell array is in the form of, for example, a polygon having a number of sides greater than 3. For example, quadrilateral, pentagonal, hexagonal, etc. The quadrangle is in a format of square, rectangle, rhombus, parallelogram, etc.

To characterize a cell in a design pattern, it may have some design features. In one non-limiting embodiment, the design characteristics of each cell include characteristics such as the size of the cell, the number of adjacent cells, and the location of adjacent cells. In some embodiments, the size of each cell in the array of cells may be the same or may be different. For example, the number of sides, the length of sides, and other numerical values of the polygons forming different cells may be the same or different.

At step 102, each cell in the array of cells is classified according to its design characteristics.

For ease of understanding, the design features and classification relationships of the cells are discussed using the schematic of FIG. 2A as an example. It should be noted that the actual cell distribution is more complex than the schematic diagram of fig. 2A, and the schematic diagram of fig. 2A is taken as an example only for ease of discussion and understanding.

In the cell distribution illustrated in fig. 2A, the cells can be classified into several categories, including: the first type of unit cells are positioned at the corners of the design graph, the second type of unit cells are positioned at the short edges of the design graph, the third type of unit cells are positioned at the long edges of the design graph, and the fourth type of unit cells are positioned in the design graph. The short side and the long side are determined according to the parameters of the design pattern, and can also be determined according to the parameters of the length and the width of the small cells divided by the design pattern. For example, when the outline of the design pattern of the photomask is a rectangle, the determination of the long side and the short side thereof may be directly determined according to the values of the length (length) and the width (width) of the rectangle, or may be determined according to the values of the length and the width of the small cells into which the rectangle is divided, for example, the small rectangle. When the design pattern of the photomask is an ellipse, the design pattern can be determined according to the numerical values of the major axis and the minor axis, and can also be determined according to the parameter numerical values of the small cells divided by the design pattern; when the design pattern of the photomask is square, the length and the width of the design pattern are equal as a whole, and the length and the width of the design pattern can be determined according to the parameter values of the small cells divided by the design pattern; when the design pattern of the photomask is a circular shape, it can also be determined according to the parameter values of the small cells into which the design pattern is divided.

In some embodiments, the determination of the classification criteria may be based on design characteristics of the small cells into which the design graphic is divided, such as the size of the cells, the number of adjacent cells, and the locations of the adjacent cells.

Referring specifically to the illustration of fig. 2B, when the design pattern of the photomask is rectangular, and each cell in the cell array is also a small rectangle, the small cells in the cell array are classified according to factors such as the size of the cell, the number of adjacent cells, and the positions of the adjacent cells. Accordingly, the first type cells of the design figure may be determined as small cells at the corners of the design figure. From the viewpoint of intuitive classification criteria, if there is one cell adjacent to the long side of the small cell at the corner and one cell adjacent to the short side thereof, it can be classified into the first type cell. In fact, according to the classification standard, the small cells at the four corners of the design pattern belong to the first small cell type. The small cells located at the short side of the design pattern may be determined as the second type of cells if there are two cells adjacent to the long side thereof and one cell adjacent to the short side thereof. The small cells located on the long side of the design pattern may be determined as the third type of cells if there are two cells adjacent to the short side and one cell adjacent to the long side. Next, in the case of a rectangular design pattern, in addition to cells distributed along corners and edges of the pattern in the design pattern, small cells inside the design pattern are designed, and for each of the small cells, there are two small cells adjacent to the long side thereof and two small cells adjacent to the short side thereof, and thus it can be classified into a fourth type cell.

In some embodiments, when the design pattern of the photomask is rectangular and each cell in the cell array is, for example, a parallelogram, a rhombus, a pentagon, or a hexagon, the division and classification of the cells may also be performed according to similar rules. At this time, at the corners or edges of the design pattern, it may appear a new kind of cells, but not much. For example, when a cell array is formed by dividing a parallelogram, a small cell of a pentagon may be formed at a corner. In this case, the small cells in the pentagon are classified as new cells, and if the cells in the other corners have different patterns, for example, although the cells are also pentagonal, the cells in the other corners are classified as new cells due to the difference in size depending on the specific situation of division.

In some embodiments, when the outline of the design pattern of the photomask is circular or elliptical, the cell array may be divided into polygons with more than 3 sides, such as rectangles (rectangles), squares, parallelograms, rhombuses, pentagons, hexagons, and so on, according to practical choice. At this time, at the edge of the design pattern, the outer contour of the circular or elliptical design pattern is arc-shaped, not linear, so the cell array will form some new kinds of polygons in these regions. The actual boundary at the outer contour of the design figure will be determined according to the dividing accuracy of the cell array. The closer the divided cells are, the closer the actual outer contour is to the originally designed arc contour. The actual dividing precision needs to be determined by comprehensively considering various factors such as actually achievable process precision, design and manufacturing cost and the like.

In a non-limiting embodiment, the method for correcting Optical proximity of a photomask further includes, in the step of classifying the cells into a plurality of types of cells, checking design features of each cell by an Optical Diameter (Optical Diameter), and classifying the cells according to the result of checking the design features. The magnitude of the optical diameter may be, for example, 1-2 μm. The larger the optical diameter, the more cells that can be divided into influencing factors around each cell, and the larger the range of cells that need to be involved in the classification criteria. Therefore, the optical diameter is determined based on the achievable accuracy of the actual process and the time and cost of design and manufacture.

As illustrated in the photomask diagram 301 of fig. 3A, fig. 3A is a diagram closer to the actual photomask design process. As shown in fig. 3A, each cell in the figure is, for example, a mask pattern corresponding to a channel hole of a three-dimensional memory. In the optical proximity correction process, the design features of each cell are first checked in terms of the size of the optical diameter, specifically including the size of the cell, the number of adjacent cells, and the positions of adjacent cells. Taking a part of the cells in fig. 3A as an example (a part of the cells in the dashed box 302), when performing optical proximity correction, the design features of the cells are first checked with the optical diameter. Taking the small cell "1" shown in fig. 3A as an example, when the characteristics of the small cell are checked by the optical diameter, the size of the small cell "1" will be recorded and analyzed, and the cell distribution around the small cell within the range of the optical diameter D1, including the number of cells and the location of the cells. Taking the small cell "1" as an example, 5 small cells are distributed around the inspection range of the optical diameter D1, wherein the number of the small cells is one on the left and right, two are distributed below the small cells, and the part above the small cells is also located in the inspection range of the optical diameter. Referring to the case of the small cell "2" indicated in fig. 3A, in the range of inspection of the optical diameter D2, 6 small cells are distributed around the small cell, one each is distributed on the left and right of the small cell "2", two small cells are distributed above the small cell, and two small cells are distributed below the small cell. Therefore, the small cell "1" is different from the small cell "2" belonging to the different kind of cells, and the optical proximity correction thereof is subjected to a different correction process. Similarly, the small cell "3" indicated in fig. 3A may have a size different from that of the small cell "2" although it has 6 small cells distributed around it in the range checked by the optical diameter D3. Meanwhile, the distribution of the surrounding 6 small cells is also different from that of the small cell "2", so that the cells belong to different types, and the optical proximity correction needs to be performed through different processes. Similarly, the small cell "4" labeled in FIG. 3A is examined by optical diameter D4 and is classified as another type of cell.

For the cells in the dashed box 302 of fig. 3A, after checking 4 cells, the final checking result may be as illustrated in fig. 3B by continuing to perform optical diameter checking on other cells. A photomask design for a three-dimensional memory, such as a 3D NAND flash memory, includes a large number of repeatable mask patterns. Therefore, the cells in the same row in the dashed box 302 of fig. 3A are finally classified into the same type of cell after optical diameter inspection. The cells in the dashed box 302 of fig. 3A may thus be ultimately classified into 4 types. Fig. 3A and 3B of the present application are merely examples, and in practical cases, adjacent cells in the same horizontal row do not necessarily belong to the same type of cell, but may be classified as a new type of cell due to different design features. Meanwhile, when the optical diameter inspection is performed within a range larger than the range of the dashed box 302, the final classification result may be different accordingly, for example, different cells of 5 types, 6 types, and 7 types may be formed. In more complex cases, the cell may be of tens or hundreds of classes.

At step 103, optical proximity correction is performed on at least one representative cell of each type of cell. In fig. 3B, for example, the leftmost small cell "1", the small cell "2", the small cell "3", and the small cell "4" of the dashed box 302 are selected as one representative cell of each type of cell to perform optical proximity correction. The representative cells are selected without special limitation, and after the cells are classified, one of the cells in each type is selected as the representative cell to perform the subsequent process. Since each type of cell is classified into the same type according to the classification criteria, the representative cell is not more limited to be selected, and can be regarded as a sample selected from the same type of cell. In the actual verification process of the photomask design, some simple identification rules may be set to select one cell of the same type of cells as a representative cell in order to facilitate the process. Such as the cell number or coordinates, or the distance of the cell from the edge of the mask, etc.

At step 104, the optical proximity correction results for the representative cell of each type of cell are applied to the other cells of each type of cell. For example, in fig. 3B, after optical proximity correction is performed on the leftmost small cell "1", the small cell "2", the small cell "3", and the small cell "4" in the dashed-line box 302, respectively, the results of the optical proximity correction are applied to small cells belonging to the same kind as the small cell "1", the small cell "2", the small cell "3", and the small cell "4", respectively, through optical diameter inspection.

In one non-limiting embodiment, the method for correcting optical proximity of a photomask of the present invention further comprises, after classifying each cell into a plurality of types of cells according to design features of each cell in the cell array: an exposure assist pattern (SRAF) is added to the design pattern. The exposure auxiliary patterns realize the functions of blocking and scattering light during the exposure process so as to improve the depth of focus of the photoetching patterns and expand the photoetching process window, but patterns cannot be formed on the wafer.

The sub-resolution assist lines of the exposure assist pattern include various serifs and scattering bars. The widths of the serifs and the scattering bars and the distances between the serifs and the main characteristic patterns are important and need to be optimized according to specific conditions so as to influence the change of phase frequency spectrum through the scattering bars and realize the contour adjustment of the space image. The scattering bars or the serifs can effectively adjust the light intensity distribution of the space image by improving the energy and phase distribution of various frequency components in the pattern frequency spectrum without forming patterns on the photoresist, and can play the roles of improving the line width deviation, strengthening the corner outline and increasing the exposure focal depth.

As shown in FIG. 4, in a non-limiting embodiment, the method for optical proximity correction of a photomask of the present invention, in addition to comprising steps 401 and 404, comprises a step 405 of verifying the result of optical proximity correction (OPC Verify) and a step 406 of performing Mask Rule Check (Mask Rule Check, MRC) after applying the correction result of the representative cell of each type of cell to the other cells of each type of cell in step 404.

The OPC verification of step 405 is to perform global simulation on the mask pattern (i.e., the pattern after OPC correction) and check whether the simulation result meets the standard, so as to determine whether the OPC correction result meets the requirements.

The mask rule check of step 406 is a step of confirming that the pattern in the mask is suitable for the mask manufacturing process, and for example, items such as the minimum line width of the pattern, the line pitch, the minimum value of the line width and the pitch of the sub-resolution exposure auxiliary pattern, the pitch between the corners of the pattern, the distribution distance between the exposure auxiliary pattern and the main pattern, and the minimum area of the exposure auxiliary pattern are checked.

OPC verification and mask rule checking may be performed for all representative cells, as well as for all cells, depending on the choice and balance of design time and pattern accuracy when designing a photomask.

OPC verification and mask rule checking may be implemented in software, such as CalibrenmOPC verification platform from Mentor Graphic, Inc.

The optical proximity correction method of the photomask of the invention realizes the rapid Optical Proximity Correction (OPC) to the photomask pattern design of the storage array area of the three-dimensional memory by proposing the new process flow of the optical proximity correction process of the photomask of the three-dimensional memory, greatly reduces the time required by the optical proximity correction in the photomask design of the high-density three-dimensional memory, and can greatly shorten the time of the optical proximity correction to 1/7-1/29 of the originally required time in the wiring layer or the trench hole layer with higher pattern distribution density. Meanwhile, the optical proximity correction method of the photomask can ensure the consistency of the critical dimension of the pattern design and avoid error accumulation in the link, thereby being beneficial to the overall quality control of products.

In the optical proximity correction method for a photomask according to the present invention, since the cell classification of the photomask pattern is performed by the optical diameter inspection, the type of the finally formed cell and the time required for the optical diameter inspection of all the cells are factors to be considered comprehensively. Therefore, the optical proximity correction method of the photomask has the advantages that the time for finally forming the types of the units is less, the comprehensive consideration needs to be carried out on the types of the units and the time for checking the optical diameter, and whether the application scene is appropriate, so that the technical effects of greatly shortening the time for the optical proximity correction in the photomask design and ensuring the consistency of the critical dimension of the pattern design are achieved.

The invention also provides a manufacturing method of the photomask, which can carry out optical proximity correction on the mask pattern on the photomask by using the method. The photomask formed by the manufacturing method can improve the uniformity of the mask pattern size.

The present invention also provides a method of manufacturing a semiconductor device, which performs optical proximity correction of a mask pattern on a photomask used for manufacturing the semiconductor device using the aforementioned method. For example, it can be used for manufacturing semiconductor memories, specifically semiconductor devices such as three-dimensional semiconductor memories, 3D NAND Flash memories, and the like.

The optical proximity correction method of the photomask reduces the time required by Optical Proximity Correction (OPC) in the photomask design process and improves the efficiency of the photomask from design to delivery-to-Out (Tape-Out).

The invention also provides a computer comprising a memory and a processor. The memory stores a computer program; the computer program performs the aforementioned methods when called by a processor. The memory may be a computer-readable storage medium that may include, but is not limited to, magnetic storage devices (e.g., hard disk, floppy disk, magnetic strips), optical disks (e.g., Compact Disk (CD), Digital Versatile Disk (DVD)), smart cards, and flash memory devices (e.g., electrically erasable programmable read-only memory (EPROM), card, stick, key drive). In addition, various storage media described herein can represent one or more devices and/or other machine-readable media for storing information.

The invention also provides a computer storage medium storing a computer program; the computer program performs the aforementioned methods when called by a processor. For example, the foregoing processes of the optical proximity correction method of a photomask, the manufacturing method of a photomask, and the manufacturing method of a semiconductor device may be implemented as computer programs, stored in a hard disk, and executed in a processor to implement the methods of the present application.

This application uses specific words to describe embodiments of the application. Reference throughout this specification to "one embodiment," "an embodiment," and/or "some embodiments" means that a particular feature, structure, or characteristic described in connection with at least one embodiment of the present application is included in at least one embodiment of the present application. Therefore, it is emphasized and should be appreciated that two or more references to "an embodiment" or "one embodiment" or "an alternative embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, some features, structures, or characteristics of one or more embodiments of the present application may be combined as appropriate.

Although the present invention has been described with reference to the present specific embodiments, it will be appreciated by those skilled in the art that the above embodiments are merely illustrative of the present invention, and various equivalent changes and substitutions may be made without departing from the spirit of the invention, and therefore, it is intended that all changes and modifications to the above embodiments within the spirit and scope of the present invention be covered by the appended claims.