Multi-Layer Random Access Memory and Methods of Manufacture

Abstract

A semiconductor structure for a DRAM is described having multiple layers of arrays of memory cells. Memory cells in a vertical string extending through the layers have an electrical connection to one terminal of the memory cells in that string. Word lines couple the strings together. Each layer of the array also includes bit line connections to memory cells on that layer. Select transistors enable the use of folded bit lines. The memory cells preferably are thyristors. Methods of fabricating the array are described.

Claims

1. A multi-layer random access memory array structure comprising: a semiconductor substrate; z layers of memory cells, each of the z layers including an array of x by y memory cells electrically isolated from each other; a string of memory cells, the string including one memory cell from each of the z layers in the array; a first electrical connection extending through the array connected to a first terminal of each memory cell in the string of memory cells; a plurality of second electrical connections, one second electrical connection in each layer being connected to a second terminal of the one memory cell in that layer of memory cells; the second electrical connection in each layer extending laterally outward further than the second electrical connection in the layer above it; and a plurality of third electrical connections, each third electrical connection extending vertically to connect to a different one of the plurality of second electrical connections.

2. A multi-layer memory array as in claim 1 wherein: the memory cells comprise thyristors; the first terminal comprises one of the anode and cathode of a thyristor; and the second terminal comprises the other of the anode and cathode of the thyristor.

3. A multi-layer memory array as in claim 2 wherein: the thyristors comprise lateral thyristors; and each of the z layers is isolated from a layer above it and a layer below it by an intervening layer of insulating material.

4. A multi-layer memory array as in claim 1 further comprising a plurality of fourth electrical connections, each fourth electrical connection connecting a plurality of strings of memory cells together.

5. A multi-layer memory array as in claim 4 further comprising: a first select transistor connected to one end of each first electrical connection; and a second select transistor connected to an opposite end of each first electrical connection; and wherein activation of both the first and second select transistors selects only one string of memory cells from the plurality of strings.

6. A multi-layer memory array as in claim 5 wherein: the first select transistor is disposed in a layer above a top layer of the z layers; and the second select transistor is disposed in a layer below a bottom layer of the z layers.

7. A multi-layer memory array as in claim 6 wherein: the first select transistor has a first terminal connected to one of the first electrical connections, a second terminal connected to one of the fourth electrical connections, and a first control terminal; the second select transistor has a first terminal connected to the one of the first electrical connections, a second terminal connected to a region in the semiconductor substrate, and a second control terminal; and wherein in response to control signals applied to the first and second control terminals, a string of memory cells is connected to the fourth electrical connection.

8. A multi-layer memory array as in claim 7 wherein: the fourth electrical connections provide word lines in the memory array; and the second electrical connections provide bit lines in the memory array.

9. A multi-layer memory array as in claim 8 wherein: each bit line in the array is coupled to two strings of memory cells; and the control signals select only one of the two strings of memory cells to be connected to a word line.

10. A multi-layer memory array as in claim 1 wherein: the memory cells comprise thyristors; and each thyristor includes an assist gate.

11. A multi-layer memory array as in claim 10 further comprising a three dimensional network of assist gate electrical connections extending through the multi-layer memory array enabling connection of the assist gate of each thyristor to an upper surface of the memory array.

12. A multi-layer memory array as in claim 11 wherein: on each of the z layers the assist gate electrical connections are interleaved with the second electrical connections; and the assist gate electrical connections are interleaved with the fourth electrical connections.

13. A multi-layer thyristor memory array structure comprising: a semiconductor substrate; z layers of thyristors, each of the z layers including an array of x rows by y columns of thyristors; a first set of electrical connections connecting strings of thyristors, each string including one thyristor from each of the z layers in the array connected together; an array of x by y first electrical connections connecting a first terminal of each thyristor in each string to the thyristors above it and the thyristor below it in that string; a first set of word line connections extending in parallel to connect rows of the strings together; and a first set of bit line connections extending in parallel on each layer to connect together columns of thyristors on each layer, the first set of bit line connections on each layer extending outward on each layer away from the array further than the second set of electrical connections on the layer immediately above it.

14. A multi-layer thyristor memory array structure as in claim 13 further comprising a second set of bit line connections extending vertically from an upper layer of the memory array downward to connect to each of the first set of bit line connections.

15. A multi-layer thyristor memory array structure as in claim 13 wherein: the word lines connect to one of the thyristor anodes and cathodes; and the bit lines connect to the other of the thyristor anodes and cathodes.

16. A multi-layer thyristor memory array structure as in claim 13 wherein the thyristors comprise lateral thyristors.

17. A semiconductor fabrication process comprising: depositing alternating layers of insulating material and first material on a semiconductor substrate; etching trenches through the alternating layers; removing the first material by laterally etching it from between the layers of insulating material to form cavities between the layers of insulating material; beginning a process of forming epitaxial silicon in the cavities; introducing desired conductivity type impurities into the process of forming epitaxial silicon as the process continues to thereby form semiconductor devices with the epitaxial silicon; and providing electrical connections to the semiconductor devices.

18. A semiconductor fabrication process as in claim 17 wherein the step of etching trenches further comprises: etching first trenches in a first direction; forming single crystal semiconductor material on walls of the first trenches; etching pairs of the alternating layers of insulating material and first material to form a staircase structure; etching second trenches in a second direction orthogonal to the first direction; and then removing the first material by laterally etching it.

19. A semiconductor fabrication process as in claim 18 wherein the single crystal semiconductor material provides a substrate for formation of epitaxial silicon in the cavities.

20. A semiconductor fabrication process as in claim 19 further comprising before the step of beginning a process of forming epitaxial silicon in the cavities a step is performing of removing the single crystal silicon from one side of each of the first trenches.

21. A semiconductor fabrication process as in claim 17 further comprising: before the step of depositing alternating layers of insulating material and first material a step is performed of depositing a lower conductive layer; and after the step of depositing alternating layers of insulating material and first material a step is performed of depositing an upper conductive layer.

22. A semiconductor fabrication process as in claim 21 further comprising after the step of etching trenches through the alternating layers a step of forming gate insulating material on a first edge of the lower conductive layer and a first edge of the upper lower conductive layer.

23. A semiconductor fabrication process as in claim 22 wherein the step of providing electrical connections to the semiconductor devices further comprises providing an electrical connection to a second edge of the lower conductive layer and a second edge of the upper lower conductive layer.

24. A semiconductor fabrication process as in claim 23 further comprising forming epitaxial silicon adjacent the gate insulating material.

25. A multi-layer random access memory array structure comprising: a semiconductor substrate; z layers of thyristor memory cells, each of the z layers including an x by y array of thyristor memory cells electrically isolated from each other; a string of thyristor memory cells, the string including one memory cell from each of the z layers in the array; a first electrical connection extending through the array connected to a first terminal of each thyristor memory cell in the string of memory cells; and a plurality of second electrical connections, one second electrical connection in each layer being connected to a second terminal of the one thyristor memory cell in that layer of memory cells.

26. A multi-layer random access memory array structure as in claim 25 further comprising: the second electrical connection in each layer extending laterally outward further than the second electrical connection in the layer above it; and a plurality of third electrical connections, each third electrical connection extending vertically to connect to a different one of the plurality of second electrical connections.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] FIG. 1 shows a perspective view of an 8-layer stacked memory array with thyristor memory cells.

[0017] FIG. 2 shows a cross-sectional view along an anode line of the array shown in FIG. 1.

[0018] FIG. 3A shows a circuit schematic of a portion of a memory array illustrating word lines, bit lines and anode lines of the array.

[0019] FIG. 3B shows a circuit schematic of a memory array such as in FIG. 3A, but with shared bit lines, showing top and bottom select transistors for each string of thyristors.

[0020] FIG. 3C shows a circuit schematic of a of a memory array such as in FIG. 3B, but in which each thyristor has an assist gate, and the connections to those assist gates.

[0021] FIGS. 4-25 illustrate processes for fabricating the memory array shown in FIG. 1, with FIG. 4 illustrating stacked layers of silicon dioxide and silicon nitride.

[0022] FIG. 5 illustrates formation of slots 50.

[0023] FIGS. 6A and 6B illustrate gate oxide formation.

[0024] FIG. 7 illustrates selective epitaxial growth of silicon within the slots 50.

[0025] FIG. 8 illustrates staircase patterning and etching steps.

[0026] FIGS. 9A and 9B illustrate formation of slots 90a and 90b.

[0027] FIGS. 10A and 10B illustrate silicon dioxide fill and planarization.

[0028] FIGS. 11A and 11B illustrate recess etching of select transistor gate connections.

[0029] FIGS. 12A and 12B illustrate epitaxy block oxidation and removal of silicon nitride layers.

[0030] FIGS. 13A and 13B illustrate thyristors formed by confined epitaxial lateral overgrowth of silicon.

[0031] FIG. 14 illustrates self-aligned formation of metal contacts to the thyristors.

[0032] FIG. 15 illustrates silicon dioxide fill and planarization.

[0033] FIG. 16 illustrates anisotropic etching of the thyristor stack and oxide fill to isolate thyristor regions.

[0034] FIG. 17 illustrates etching of SEG silicon holes down to pmos drain regions.

[0035] FIG. 18 illustrates metal deposition and planarization.

[0036] FIGS. 19A and 19B illustrate etch back of the connections shown in FIG. 18.

[0037] FIG. 20 illustrates formation of an nmos select transistor channel and source/drain regions.

[0038] FIGS. 21A and 21B illustrate etching openings for assist gates.

[0039] FIG. 22 illustrates conformal deposition and planarization of the assist gates

[0040] FIG. 23 illustrates etching openings for electrical contacts to anodes and assist gates.

[0041] FIG. 24 illustrates etching openings for electrical contacts to staircases.

[0042] FIG. 25 illustrates formation of first level metal lines.

[0043] FIG. 26 is a perspective view of the completed memory array including second layer metal lines.

DETAILED DESCRIPTION OF THE INVENTION

[0044] The assignee of this application has previously disclosed thyristor cells used as memory devices in a variety of applications. It is advantageous, however, to be able to stack the thyristors, as well as other type memory cells, to provide multiple layers of memory cells in a memory array. This increases bit density per unit area and further reduces patterning cost. The present application discloses a new multiple layer dynamic random access memory structure, preferably employing thyristors, as well as methods of manufacturing such a memory.

[0045] Features of the invention include: an exemplary stacked thyristor cell array with thyristors selectively formed using confined epitaxial lateral overgrowth (CELO) of single crystalline silicon, or other semiconductor; stacked access lines having staggered connections to provide cathode connections; and vertical access lines to contact thyristor anodes. Each lateral thyristor consists of pnpn or npnp in-situ doped during epitaxial growth. Select transistors for addressing cells in the memory array, as well as assist gates to improve thyristor performance are also provided. Metal plugs replace silicon pillars reducing string resistance and minority carrier effects.

[0046] FIGS. 1-3 illustrate the memory array semiconductor structure and circuit. FIGS. 4-25 illustrate a preferred process for fabricating the array.

[0047] FIG. 1 is a perspective view of an 8-layer stacked memory array in which the memory elements are thyristors. In FIG. 1 the dielectric material electrically isolating adjoining regions from each other has been omitted to reveal only the semiconductor, metal and metal silicide regions. The structure in FIG. 1 is formed using an n-conductivity type (herein n-type) semiconductor substrate 10, with pockets of strongly doped p+ conductivity type (herein p+-type) regions 40 near the surface. As will be shown more clearly in subsequent figures each memory element is provided by a pnpn thyristor with an optional assist gate. The figure illustrates a stack of eight layers of memory cells 15, however, commercial implementations of the memory array will have an arbitrary number of layers of memory cells, e.g. 64 or more layers. For a stack with a larger number of layers where bit lines (thyristor cathode lines) are shared in each layer, select transistors (shown in FIG. 2) are provided at the top and bottom of each string of memory cells to enable the unique selection of a particular cell and application of appropriate potentials for read and write operations. See FIGS. 3B and 3C. FIG. 1 is an implementation of the circuit shown in FIG. 3C. If the circuits of FIG. 3A or 3B are to be implemented, the assist gates and select transistors are eliminated as appropriate from the structure.

[0048] Vertical metal connections 18 couple the anodes of the memory cells in a string to word lines 21 in the upper portion of the structure. Bit line connections 19 to the cathodes of the thyristor memory cells extend laterally away from the cell array to vertical connections 13 upward to the surface of the device for connection to bit lines 11. An optional assist gate is provided for each thyristor and, if included, couples to assist gate lines 14 and 16 as will be shown in FIG. 3C.

[0049] In a typical implementation, the structure shown in FIG. 1 is the right half portion of a sub-array, with a left half portion of the sub-array being a mirror image of the illustration, extending away from the left edge of the structure shown in FIG. 1.

[0050] In FIG. 1 the memory cells are preferably thyristors, however, those of skill in the art will appreciate other types of memory devices may also be employed. For example, in other embodiments, phase change memory cells, fuses, anti-fuses, resistive elements or other memory cell types can be employed from the teaching of this invention.

[0051] FIG. 2 is a cross-sectional view along a word line 21 of the thyristor memory cell array shown in FIG. 1. FIG. 2 illustrates four stacks 15 each of eight thyristors. Each stack of eight thyristors includes a pair of select transistors to enable accessing only one string of thyristors if a shared bit line architecture is implemented. An nmos select transistor 24 is at the top of each string of thyristors and a pmos select transistor 25 is at the bottom of each string. The arrows to these transistors point to the channel regions of the transistors. Each of the select transistors has a vertical gate most easily seen by the thin layer of gate insulating material near the points of the arrows to the transistor channel regions. The horizontal layers to the right of the gate insulating layers provide electrical connections 28 and 29 to the gates.

[0052] The vertical metal contact 18 to the anodes of each string of eight thyristors is shown at the left edge of the figure and extends upward to connect to the word line 21 when the select transistors 24 and 25 are on. A metal contact 27 connects to each thyristor cathode. The cathode lines 19 are not shown in this cross-section, but extend into and out of the plane of the figure, as shown in FIG. 1.

[0053] FIGS. 3A, 3B and 3C are circuit schematics of a portion of a memory array implemented as shown in FIG. 1 with thyristors 20 as memory cells. FIG. 3A shows the row (x), column (y) and string (z) directions. In FIG. 3A only a very small portion of a memory array is shown, one with two word lines coupled to eight strings of memory cells. In a typical commercial implementation there will be billions of memory cells. Each thyristor 20 comprises a pnpn (or npnp) semiconductor device without an assist gate (FIGS. 3A and 3B) or with an assist gate (FIG. 3C) coupled to one of the thyristor base regions. The anodes of thyristors in one string of the array are coupled together by an anode line 18. The cathodes in one row of the array are commonly connected to a bit (or cathode) line 23.

[0054] FIG. 3B illustrates an array such as shown in FIG. 3A, but with a common pmos select transistor line 17 and shared bit lines 19. In this implementation, upper select transistors 24 at the top of each column and lower select transistors 25 at the bottom of each column of a common anode line are used to enable selection of only one string of the two strings shared by one bit line coupled to the same word line 21.

[0055] FIG. 3C illustrates the addition of assist gates to the thyristor memory cells of FIGS. 3A and 3B and their associated connecting lines. An assist gate is associated with a base region of each thyristor 20. The assist gates are connected to a layer assist gate line 14 in the column line direction of each layer of memory cells. Vertical assist gate lines extend through the array vertically with each vertical line connected between two assist gates. The vertical lines, in turn, are connected to row assist gate lines.

[0056] The techniques for operating the thyristor memory arrays illustrated, including reading data from the array, writing data into the array, and refreshing data stored in the array, are described in commonly assigned U.S. Pat. No. 9,564,199, entitled Methods of Reading and Writing Data in a Thyristor Random Access Memory, incorporated by reference herein.

[0057] FIG. 4, et seq., illustrate a preferred process for fabricating the structure shown in FIGS. 1 and 3. FIG. 4 shows the structure after initial steps of fabrication. Beginning with a semiconductor wafer 10 a p+ type implant step is performed in the area of the cell array. The implanted regions 40 shown in FIG. 1 provide a common electrical connection to the pmos select transistors at the bottom of each column of thyristors.

[0058] Next a silicon dioxide layer 54 and a polysilicon layer 52 (see FIG. 6B) are deposited. Following that, alternating layers 42 of silicon nitride and silicon dioxide are deposited. The number of layers deposited determines the number of layers of memory devices to be formed, with one layer of silicon nitride deposited for each layer of memory cells. After the last layer of silicon dioxide, a layer of polysilicon 56 is deposited. Then another layer of silicon dioxide is deposited, followed by a hard mask layer 44, for example, a metal oxide such as Al2O3.

[0059] Using conventional photolithography, repeated known etching processes are used to remove the sequential layers of silicon dioxide and silicon nitride to form slots 50 extending through all of the layers down to layer 54 (shown in FIG. 6B). These slots will partially separate groups of memory cells from each other. The appearance of the structure after etching the slots 50 is shown in FIG. 5.

[0060] FIGS. 6A and 6B illustrate the next steps in the fabrication process. The exposed polysilicon lines 52 and 56 are recessed by 3 to 6 nm with an isotropic etch and the exposed sidewalls of the slots are then oxidized. The oxidation of polysilicon forms the gate oxide 65 for the pmos and nmos select transistors on the exposed edges of the polysilicon layers.

[0061] As shown in FIG. 7, after the bottom silicon dioxide is etched away to expose the underlying single crystal silicon, crystalline silicon 75 is grown in slots 50. Selective epitaxial growth (SEG) of silicon is a well-known technology, with chemistries such as dichlorosilane and HCl often used. It is desirable to grow the silicon to be planar with the hard mask. Alternatively, SEG silicon can be over grown above the hard mask and then chemical mechanical polishing (CMP) used to remove the excess silicon above the hard mask. The structure is then coated with a thick photoresist, patterned and etched using progressive photoresist trimming.

[0062] For an eight-layer memory array, after 8 loops of photoresist trim and oxide/nitride etching, the staircase structure 70 results, as shown in FIG. 8A. Each step (layer) in the stairs will connect one node of a sub-array of columns of memory cells in that layer to an external connection for accessing those memory cells. In the preferred embodiment the stepped layers enable bit line electrical connections to the thyristor cathodes. The exposed portion of each successively lower step provides area for a via type connection from the upper surface of the integrated circuit to the exposed portion of that step.

[0063] Next as shown in FIG. 8, a thick layer of dielectric material 80, e.g. silicon dioxide, is formed over the entire structure and planarized. In a preferred process the planarization step used here and in subsequent operations uses chemical mechanical polishing (CMP).

[0064] Then as shown in FIG. 9A, further masking and etching steps create slots 90a and 90b. Slots 90a are aligned to cut the layer stack along the SEG silicon fence 75. Slots 90b partially cut the staircase structure orthogonally to slots 90a. Slots 90a and 90b are formed using well-known reactive ion etching (RIE) and optionally extend slightly into the semiconductor substrate 10 as shown in FIG. 9B.

[0065] Next, as shown in FIG. 10A, slots 90a and 90b are filled with silicon dioxide 100. Then the structure is again planarized with a CMP process. FIG. 10B is a cross-sectional view after planarization.

[0066] After masking and lithography another set of slots 110 is etched through all the layers down to the bottom layer of silicon dioxide, as shown in FIG. 11A. These slots 110 will enable formation of the thyristor or other type memory cells. Note that the mask defining the location of slots 110 is aligned to leave a vertical layer of silicon dioxide 114 on the sidewall. This insulating layer 114 prevents a subsequent epitaxial growth of silicon process from forming single crystal silicon along that sidewall. Following this step, as shown in FIG. 11B, exposed edges of polysilicon connections 28 and 29 to the gates of the select transistors are etched back to leave space for self-aligned metal formation. This provides lower resistance select transistor gate connections.

[0067] Next, as shown in FIGS. 12A and 12B, a thick oxide of 10 nm to 100 nm is grown on exposed silicon surface before the multiple layers of silicon nitride are etched away. A wet etchant isotropically removes all exposed silicon nitride between the layers of silicon dioxide 120, both in the memory cell area (FIG. 12B), and in the staircase area (FIG. 12A). The openings 122 between the layers of silicon dioxide 120 after silicon nitride removal in the memory cell area are shown in FIG. 12B. The remaining silicon dioxide layers are supported along two edges to provide mechanical structural strength.

[0068] Following these processes, confined epitaxial lateral overgrowth of silicon is used to form memory cells in the cell area of the array. The epitaxial growth is selective only to regions with exposed SEG silicon seed crystal. As the process proceeds appropriate conductivity type dopants are introduced into the gas flow to create the desired conductivity type structure for whatever type memory cells are to be formed in the array. This process forms single crystal silicon in the openings 122 between the layers 120. In the embodiment with thyristors memory cells, the sequential dopants are p+-type, n-type, p-type, and n+-type providing the lateral thyristor anode, bases, and cathode, respectively. FIG. 13B shows the cross sections of the thyristors 125 formed in the openings 122 between layers 120.

[0069] A metal contact formation process then provides electrical connections 140 to the thyristor cathodes and connections to the select transistor gates 144, as shown in FIG. 14. In this process a transition metal, e.g. cobalt, titanium, or nickel, is deposited and annealed to form metal silicide contacts to the thyristors. Next a conductive metal layer, e.g. tungsten, is conformally deposited, and then isotropically etched back to provide low resistance contacts to the metal silicide. The process forms not only the direct connections to the thyristors 125, but also the connections 19 extending laterally away from the thyristors to create the staircase of cathode connections shown in FIG. 1.

[0070] As shown by FIG. 15, the structure is then filled with silicon dioxide and planarized by a CMP process. A new mask and further etching processes separate the thyristor regions into separate memory cells as indicated by a comparison of FIG. 16 with FIG. 15. After that, the structure is again filled with dielectric material and planarized.

[0071] Next as shown in the cross section of FIG. 17, a silicon etch is used to remove the SEG silicon adjoining the anodes of the thyristors, forming slots 160. Note that the slots 160 do not extend all the way down to the substrate, but stop short of the pmos select transistors near the bottoms of the slots. Then a metal contact process like that described above is used to form metal silicide connections 170 to the thyristor anodes in each stack as shown in FIG. 18. The structure is again planarized, e.g. using a CMP process.

[0072] Next the electrical connections 170 previously formed are etched back, thus recessing the silicide and metal connections and providing space for the select transistor to be formed at the top of that string. The recesses are shown as small holes 190 in FIG. 19A, and as recessed regions 190 in the cross section of FIG. 19B.

[0073] Following this step, selective epitaxial growth 200 with n+, p and n-type dopants is used to form the source, channel and drain regions of the nmos select transistor at the top of each string of memory cells. This completes the formation of the memory array structure. Well-known further processing as described provides electrical connections to the desired regions of the memory array. See the discussion below with respect to FIGS. 23-25.

[0074] In implementations of the three-dimensional memory array when thyristors are used as memory cells, it may be desirable to also provide an assist gate for at least one of the base regions of each thyristor. A modification of the process described above provides such assist gates. This process is shown in FIGS. 21 and 22.

[0075] This process uses the same steps described above up through FIG. 16. After the thyristors are divided, a new mask defines the locations for where the assist gates will be formed. As shown in FIGS. 21A and 21B, holes 210 are then etched through all of the layers. A thin layer of gate oxide is then grown on the sides of the holes to isolate the assist gates from the thyristors. Next atomic layer deposition of a titanium nitride barrier is performed, followed by a conformal deposition of tungsten. The tungsten provides the assist gate for each thyristor. A CMP process is then used to planarize the upper layer of the structure. FIG. 22 is a cross section of the structure along the assist gate line.

[0076] FIGS. 23-25 illustrate the final processes used to provide electrical connections to the memory array. As shown in FIG. 23 dielectric material is deposited and another CMP process prepares it for anode 230 and assist gate 234 connections, shown as small openings on the upper surface of the illustration. Next further masking and etching steps are performed to provide electrical connections to the staircase of cathode connections. These openings 240 are shown in FIG. 24.

[0077] FIG. 25 illustrates the next aspect of the process in which first layer metal electrical connections to the memory cells are provided. In FIG. 25, with thyristor memory cells, connections 252 are formed using well-known technology to electrically connect to the anodes of the memory cells. Connections 254 connect to the cathodes and assist gates of the thyristors.

[0078] FIG. 26 is a perspective view illustrating the completed structure after formation of second layer metal connections. In FIG. 26 portions of the structure have been cut away to show various regions within the thyristor based structure. The regions include substrate 10, vertical anode connections 18, word (anode) lines 12, staircase cathode connections 19, vertical connections 13 to the cathode lines 19, thyristors 20, pmos select transistor 24, and nmos select transistor 25.

[0079] This description of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form described, and many modifications and variations are possible in light of the teaching above. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications. This description will enable others skilled in the art to best utilize and practice the invention in various embodiments and with various modifications as are suited to a particular use. The scope of the invention is defined by the following claims.