Spiral couplers manufactured by etching and fusion bonding

[0001] This application is a divisional claims priority to U.S. application Ser. No. 09/711,118, filed Nov. 9, 2000, now U.S. Pat. No. 6,765,455 and entitled Spiral Couplers.

[0002] This invention relates to microwave couplers. More particularly, this invention discloses the topology of couplers and a method for manufacturing that typically operate at microwave frequencies and utilize spiral-like configurations to achieve high density and low volume.

[0003] Over the decades, wireless communication systems have become more and more technologically advanced, with performance increasing in terms of smaller size, operation at higher frequencies and the accompanying increase in bandwidth, lower power consumption for a given power output, and robustness, among other factors. The trend toward better communication systems puts ever-greater demands on the manufacturers of these systems.

[0004] Today, the demands of satellite, military, and other cutting-edge digital communication systems are being met with microwave technology, which typically operates at frequencies from approximately 500 MHz to approximately 60 GHz or higher. Many of these systems use couplers, such as directional couplers, in their microwave circuitry.

[0005] Traditional couplers, especially those that operate at lower frequencies, typically require a relatively long parts housing size (i.e., a long packaging size) since coupling between lines is often required over a long distance.

[0006] Popular technologies for microwave technologies include low temperature co-fired ceramic (LTCC), ceramic/polyamide (CP), epoxy fiberglass (FR4), fluoropolymer composites (PTFE), and mixed dielectric (MDk, a combination of FR4 and PTFE). Each technology has its strengths, but no current technology addresses all of the challenges of designing and manufacturing microwave circuits.

[0007] For example, multilayer printed circuit boards using FR4, PTFE, or MDk technologies are often used to route signals to components that are mounted on the surface by way of soldered connections of conductive polymers. For these circuits, resistors can be screen-printed or etched, and may be buried. These technologies can form multifunction modules (MCM) which carry monolithic microwave integrated circuits (MMICs) and can be mounted on a motherboard.

[0008] Although FR4 has low costs associated with it and is easy to machine, it is typically not suited for microwave frequencies, due to a high loss tangent and a high correlation between the material's dielectric constant and temperature. There is also a tendency to have coefficient of thermal expansion (CTE) differentials that cause mismatches in an assembly. Even though recent developments in FR4 boards have improved electrical properties, the thermoset films used to bond the layers may limit the types of via hole connections between layers.

[0009] Another popular technology is CP, which involves the application of very thin layers of polyamide dielectric and gold metalization onto a ceramic bottom layer containing MMICs. This technology may produce circuitry an order of magnitude smaller than FR4, PTFE, or MDk, and usually works quite well at high microwave frequencies. Semiconductors may be covered with a layer of polyamide. However, design cycles are usually relatively long and costly. Also, CTE differentials often cause mismatches with some mating assemblies.

[0010] Finally, LTCC technology, which forms multilayer structures by combining layers of ceramic and gold metalization, also works well at high microwave frequencies. However, as with CP technology, design cycles are usually relatively long and costly, and CTE differentials often cause mismatches with some mating assemblies.

[0011] Advances have been made in reducing the size of LTCC couplers and FR4 couplers, by using strip-line spiral-like configurations. Examples of spiral-like configurations for couplers using various technologies may be found in U.S. Pat. No. 3,999,150 to Caragliano et al., U.S. Pat. No. 5,689,217 to Gu et al., and U.S. Pat. No. 5,841,328 to Hayashi, all incorporated herein by reference. However, using spiral-like configurations for couplers based on these technologies have certain limitations, as described below.

[0012] Hard ceramic materials may provide dielectric constants higher than approximately 10.2, but components utilizing these materials cannot be miniaturized in a stand-alone multilayer realization. For example, bond wire interconnects must be used for the realization of microstrip circuitry, increasing the overall size of the resulting microwave devices. Other ceramic materials have limited dielectric constants, typically approximately 2 to 4, which prevent close placement of metalized structures and tend to be unreliable for small, tight-fitting components operating at microwave frequencies. Additionally, ceramic devices operating at microwave frequencies may be sensitive to manufacturing limitations and affect yields. LTCC Green Tape materials tend to shrink during processing, causing mismatches preventing manufacturers from making smaller coupling lines and placing coupling lines too closely lest they lose their spacing due to shifting during processing. For these reasons, spiral-like configurations of couplers cannot be too compact and the benefits of using spirals are limited.

[0013] FR4 materials have other disadvantages. For example, FR4 materials have a limited range of dielectric constants, typically approximately 4.3 to 5.0, preventing manufacturers from placing metalized lines too compactly. Manufacturers utilizing this material also cannot avail themselves of the advantage of fusion bonding. Additionally, FR4 materials are limited in the tolerance of copper cladding that they can sustain—typically 1.4 mils is the minimum thickness, so the dimensional tolerances are limited. As with ceramics, spiral-like configurations of couplers cannot be too compact, and the benefits of using spirals are limited for FR4. MDk materials also have similar disadvantages to FR4.

[0014] PTFE composite is a better technology than FR4, ceramics, and MDk for spiral-like couplers. Fluoropolymer composites having glass and ceramic often have exceptional thermal stability. They also allow copper cladding thickness below approximately 1.4 mils, which permits tighter control of etching tolerances. Additionally, these materials have a broad range of dielectric constants—typically approximately 2.2 to 10.2. Also, they can handle more power than most other material. All these features allow spiral-like couplers to be built much more compactly on PTFE than is possible using other types of material. Furthermore, complex microwave circuits can be fabricated using PTFE technology and the application of fusion bonding allows homogeneous multilayer assemblies to be formed.

[0015] The present invention relates to the manufacture of spiral-like couplers using PTFE as a base material. Coupling lines are wound in spiral-like shapes, which can be rectangular, oval, circular, or other shape that provides a compact structure in nature. Couplers can consist of two, three, or more coupling lines, depending on the application and desired coupling. Coupling lines can be co-planar, taking up only one layer of metalization between two layers of dielectric material, or they can be stacked in two or more layers, depending upon the number of lines being utilized.

[0016] In general, in one aspect, the invention features a microwave circuit package that includes multiple fluoropolymer composite substrate layers defining levels and having surfaces. Metal layers (i.e., conducting layers) are disposed on surfaces of the substrate layers. Groundplanes are formed from a first subset of the metal layers and are connected by a first set of conductors. The circuit package also includes at least one coupler that includes at least two coupling lines arranged in a substantially spiral-like shape. In some implementations, the composite substrate layers are fusion bonded into a homogeneous dielectric structure and at least one of the composite substrate layers is adhered to ceramic.

[0017] It is an object of this invention to provide spiral-like couplers that utilize PTFE technology.

[0018] It is another object of this invention to provide spiral-like couplers that have smaller cross sectional dimensions than traditional couplers.

[0019] It is another object of this invention to provide spiral-like couplers that have improved electrical characteristics.

[0020] It is another object of this invention to provide spiral-like couplers that maximize space utilization along the Z-axis.

[0021] It is another object of this invention to provide spiral-like couplers that maximize space utilization in three dimensions.

[0022] It is another object of this invention to provide spiral-like couplers that can be fusion bonded.

[0067] Referring to FIG. 1, a spiral-like coupler is shown. Coupling lines 10, 20, 30 are wound in a configuration to provide coupling among three pathways for microwave signals. In a preferred embodiment, coupling lines 10, 20, 30 have oval configurations. In alternative preferred embodiments, rectangular shapes and round shapes may be used. In other alternative embodiments, the shape of the coupler may depend on space considerations. For example, it is possible for a microwave circuit having several components to be configured most efficiently by utilizing a spiral-like coupler that is substantially L-shaped or U-shaped, by way of example only.

[0068] Coupling line 10 is connected to other parts of the circuit through via holes 15, 16 which are preferably situated at the ends of coupling line 10. Similarly, via holes 25, 26 provide connections for coupling line 20 and via holes 35, 36 provide connections for coupling line 30.

[0069] Although the coupler shown in FIG. 1 has three coupling lines, it is obvious to those of ordinary skill in the art of coupling lines that one can use spiral-like configurations for couplers having more than three coupling lines, or only two coupling lines.

[0070] Referring to FIGS. 2a and 2b, a spiral-like coupler having coupling lines distributed along the Z-axis (i.e., existing on different levels) is shown in FIG. 2a. Coupling lines 110, 120, 130 are wound in a configuration to provide coupling among three pathways for microwave signals. In a preferred embodiment, coupling lines 110, 120, 130 have oval configurations and are of the same size and shape as shown in FIG. 1. In alternative preferred embodiments, rectangular shapes and round shapes may be used. In other alternative embodiments, the shape of the coupler may depend on space considerations.

[0071] Although the coupler shown in FIGS. 2a and 2b has three coupling lines, it is obvious to those of ordinary skill in the art of coupling lines that one can use spiral-like configurations for couplers having more than three coupling lines, or only two coupling lines.

[0072] Referring to FIG. 3, an example of a spiral coupler package 300 is shown. Spiral coupler package 300 also has four contact pads 310, which are side holes in a preferred embodiment, for mounting, and three ground pads 320. In a preferred embodiment, contact pads 310 are soldered or wire-bound to metal pins, which may be gold plated, for connection to other circuitry. In an alternative preferred embodiment, spiral coupler package 300 is mounted on test fixture or board 400, as shown in FIG. 4. Board 400 has metalized lines 410 for connection to other circuitry.

[0073] FIGS. 5a and 5b show top and bottom views of spiral coupler package 300, respectively. FIG. 5c shows a side view of this embodiment, wherein spiral coupler package 300 consists of dielectric substrate layers 1, 2, 3, 4, which are approximately 0.175 inches square. Layers 1, 2 are approximately 0.025 inches thick and have dielectric constants of approximately 10.2. An example of material that can be used for layers 1, 2 is RO-3010 high frequency circuit material manufactured by Rogers Corp., located in Chandler, Ariz. Layers 3, 4 are approximately 0.005 inches thick and have dielectric constants of approximately 3.0. An example of material that can be used for layers 3, 4 is RO-3003 high frequency circuit material, also available from Rogers Corp. Metalization, preferably M ounce copper, is disposed on layers 1, 2, 3, 4 to provide some of the features of spiral coupler package 300. For example, the top of layer 4 is metalized with the pattern shown in FIG. 5a to define groundplane 504. Similarly, the bottom of layer 1 is metalized as shown in FIG. 5b to define groundplane 501. A third groundplane 502 disposed between layer 2 and layer 3 can be seen in FIG. 6, which shows only the metalization of spiral coupler package 300 without the supporting dielectric layers.

[0074] Metalization layer 602 is disposed between layer 1 and layer 2, while metalization layer 603 is disposed between layer 3 and layer 4. In the preferred embodiment shown in FIG. 6, metalization layer 602 provides spiral-like shapes which are connected with via holes 620 to metalization layer 603, which provides pathways to contact pads 310. FIGS. 7, 8 show different views of the metalization shown in FIG. 6.

[0075] FIG. 9 shows the placement of via holes 620, which are connected to contact pads 901, 902, 903, 904 by metal lines 911, 912, 913, 914 (which are part of metalization layer 603) respectively. The widths and lengths of metal lines 911, 912, 913, 914 affect the performance of the coupler. In a preferred embodiment shown in FIG. 10, metal lines 911, 912, 913, 914 are 0.011 inches wide and the average length of metal lines 911, 912, 914 is approximately 0.065 inches, while the average length of metal line 913 is 0.1395 inches.

[0076] Advantageously, groundplane 502 (FIGS. 7–8) isolates metal lines 911, 912, 913, 914 from metallization layer 602. Without groundplane 502, it is apparent that signal cross-talk would occur between metallization layer 602 and metal lines 911, 912, 913, 914, which are shown superimposed in FIG. 11.

[0077] Referring to FIGS. 12–17, typical electrical performance characteristics of the embodiment shown in FIGS. 3–11 and described above are shown for a frequency range of 1.0 GHz to 3.0 GHz. For the purposes of the performance curves, four ports (P1, P2, P3, P4) are located as follows P1 is at contact pad 901; P2 is at contact pad 902; P3 is at contact pad 903; and P4 is at contact pad 904. FIG. 12 shows the return loss, in decibels, for P1, P2, P3, and P4. FIG. 13 shows the amplitude balance, or difference between the signal from P2 to P1 and the signal from P4 to P1, in decibels. FIG. 14 shows the phase balance, or phase difference between the signal from P2 to P1 and the signal from P4 to P1, in degrees. FIG. 15 shows the outer transmission, in decibels, between P4 and P1 and between P2 and P1. FIG. 16 shows the inner transmission, in decibels, between P2 and P3 and between P4 and P3. FIG. 17 shows the isolation, in decibels, between P4 and P2 and between P3 and P1.

[0078] In a preferred embodiment a spiral coupler is fabricated in a multilayer structure comprising soft substrate PTFE laminates. A process for constructing such a multilayer structure is disclosed by U.S. Pat. No. 6,099,677 to Logothetis et al., entitled “Method of Making Microwave, Multifunction Modules Using Fluoropolymer Composite Substrates”, incorporated herein by reference.

[0079] Spiral couplers that are manufactured using fusion bonding technology advantageously avoid utilizing bonding films, which typically have low dielectric constants and hamper the degree to which spiral-like couplers can be miniaturized. The mismatch in dielectric constants between bonding film and the dielectric material prevents the creation of a homogeneous medium, since bonding films typically have dielectric constants in the range of approximately 2.5 to 3.5.

[0080] When miniaturization is desired for lower-frequency microwave applications, a dielectric constant of approximately 10 or higher is preferred for the dielectric material. In these applications, when bonding film is used as an adhesive, it tends to make the effective dielectric constant lower (i.e., lower than approximately 10) and not load the structure effectively. Additionally, the use of bonding film increases the tendency of undesired parasitic modes to propagate.

[0081] In a preferred embodiment, a spiral-like coupler package is created by fusion bonding layers 1, 2, 3, 4, having metalization patterns shown in FIG. 18, which are shown in greater detail in FIGS. 19a, 19b, 19c, 20a, 20b, 20c, 21a, 21b, 21c, 22a, 22b, 22c. The process by which this may be accomplished is described in greater detail below.

[0082] In a preferred embodiment, four fluoropolymer composite substrate panels, such as panel 2300 shown in FIG. 23, typically 9 inches by 12 inches, are mounted drilled with a rectangular or triangular alignment hole pattern. For example, alignment holes 2310, each of which has a diameter of 0.125 inches in a preferred embodiment, are drilled in the pattern shown in FIG. 23. Alignment holes 2310 are used to align panel 2300, or a stack of panels 2300.

[0083] In one implementation (not shown separately), the panel 2300 is approximately 0.025 inches thick and has a dielectric constant of approximately 10.2.

[0084] A second example panel is 2302, which is approximately 0.025 inches thick and has a dielectric constant of approximately 10.2. Holes 2320 having diameters of approximately 0.005 inches to 0.020 inches, but preferably having diameters of 0.008 inches, are drilled in the pattern shown in FIG. 24. Preferably, alignment holes 2310 and holes 2320 are drilled into panel 2302 before it is dismounted.

[0085] A third example panel is 2303, which is approximately 0.005 inches thick and has a dielectric constant of approximately 3.0. Holes 2330 having diameters of approximately 0.005 inches to 0.020 inches, but preferably having diameters of 0.008 inches, are drilled in the pattern shown in FIG. 25. Preferably, alignment holes 2310 and holes 2330 are drilled into panel 2303 before it is dismounted.

[0086] A fourth example panel is 2304 (not shown separately), which is approximately 0.005 inches thick and has a dielectric constant of approximately 3.0.

[0087] Holes 2320 of panel 2302 and holes 2330 of panel 2303 are plated through for via hole formation.

[0088] Panel 2302 is further processed as follows. Panel 2302 is plasma or sodium etched, then cleaned by rinsing in alcohol for 15 to 30 minutes, then preferably rinsing in water, preferably deionized, having a temperature of 21 to 52 degrees C. for at least 15 minutes. Panel 2302 is then vacuum baked for approximately 30 minutes to 2 hours at approximately 90 to 180 degrees C., but preferably for one hour at 149 degrees C. Panel 2302 is plated with copper, preferably first using an electroless method followed by an electrolytic method, to a thickness of approximately 13 to 25 microns. Panel 2302 is preferably rinsed in water, preferably deionized, for at least 1 minute. Panel 2302 is heated to a temperature of approximately 90 to 125 degrees C. for approximately 5 to 30 minutes, but preferably 90 degrees C. for 5 minutes, and then laminated with photoresist. Masks are used and the photoresist is developed using the proper exposure settings to create the pattern shown in FIGS. 26A and 26B (shown in greater detail in FIG. 21A, where in a preferred embodiment rings having an inner diameter of approximately 0.013 inches and an outer diameter of at least 0.015 inches are etched out of the copper, and FIG. 21B). These patterns also preferably include at least six targets 2326 on either side of panel 2302. The targets 2326 can be used for drill alignment for future processing steps, and in a preferred embodiment comprise 0.040 inch annular rings around 0.020 inch etched circles as shown in FIGS. 26a and 26b. Both the top side and the bottom side of panel 2302 are copper etched. Panel 2302 is cleaned by rinsing in alcohol for 15 to 30 minutes, then preferably rinsing in water, preferably deionized, having a temperature of 21 to 52 degrees C. for at least 15 minutes. Panel 2302 is then vacuum baked for approximately 30 minutes to 2 hours at approximately 90 to 180 degrees C., but preferably for one hour at 149 degrees C.

[0089] Panel 2303 is further processed as follows. Panel 2303 is plasma or sodium etched, then cleaned by rinsing in alcohol for 15 to 30 minutes, then preferably rinsing in water, preferably deionized, having a temperature of 21 to 52 degrees C. for at least 15 minutes. Panel 2303 is then vacuum baked for approximately 30 minutes to 2 hours at approximately 90 to 180 degrees C., but preferably for one hour at 149 degrees C. Panel 2303 is plated with copper, preferably first using an electroless method followed by an electrolytic method, to a thickness of approximately 13 to 25 microns. Panel 2303 is preferably rinsed in water, preferably deionized, for at least 1 minute. Panel 2303 is heated to a temperature of approximately 90 to 125 degrees C. for approximately 5 to 30 minutes, but preferably 90 degrees C. for 5 minutes, and then laminated with photoresist. Masks are used and the photoresist is developed using the proper exposure settings to create the pattern shown in FIGS. 27A and 27B (shown in greater detail in FIGS. 20A and 20B. These patterns also preferably include at least six targets 2326 on either side of panel 2303. The targets 2326 can be used for drill alignment for future processing steps, and in a preferred embodiment comprise 0.040 inch annular rings around 0.020 inch etched circles. Both the top side and the bottom side of panel 2303 are copper etched. Panel 2303 is cleaned by rinsing in alcohol for 15 to 30 minutes, then preferably rinsing in water, preferably deionized, having a temperature of 21 to 52 degrees C. for at least 15 minutes. Panel 2303 is then vacuum baked for approximately 30 minutes to 2 hours at approximately 90 to 180 degrees C., but preferably for one hour at 149 degrees C.

[0090] With the assistance of targets 2326 and alignment holes 2310, panels 2304, 2303, 2302, 2301 are stacked top to bottom, aligned and fusion bonded into assembly 2800, in a preferred embodiment, at a pressure of 200 PSI, with a 40 minute ramp from room temperature to 240 degrees C., a 45 minute ramp to 375 degrees C., a 15 minutes dwell at 375 degrees C., and a 90 minute ramp to 35 degrees C.

[0091] Assembly 2800 (see FIG. 28) is then aligned for the depaneling process. In a preferred embodiment, alignment is accomplished as follows. An attempt is made to drill at least two secondary alignment holes, 0.020 inches in diameter, as close as possible to the center of two of targets 2326 (see FIGS. 26a, 26b. 27a and 27b). Using an X-ray source, the proximity of the alignment holes to the actual targets 2326 is determined. The relative position of the drill to assembly 2800 is then adjusted and another attempt to hit the center of targets 2326 is made. The process is repeated, and additional targets 2326 are used if necessary, until proper alignment is achieved. Finally, four new alignment holes, each having a diameter of 0.125 inches, are drilled so that assembly 2800 can be properly mounted.

[0092] With reference to FIG. 28, holes 2810 having diameters of approximately 0.070 inches and holes 2820 having diameters of approximately 0.039 inches are drilled in the pattern shown. Assembly 2800 is plasma or sodium etched. Assembly 2800 is cleaned by rinsing in alcohol for 15 to 30 minutes, then preferably rinsing in water, preferably deionized, having a temperature of 21 to 52 degrees C. for at least 15 minutes. Assembly 2800 is then vacuum baked for approximately 30 minutes to 2 hours at approximately 90 to 180 degrees C., but preferably for one hour at 100 degrees C. Assembly 2800 is plated with copper, preferably first using an electroless method followed by an electrolytic method, to a thickness of approximately 13 to 25 microns. Assembly 2800 is preferably rinsed in water, preferably deionized, for at least 1 minute. Assembly 2800 is heated to a temperature of approximately 90 to 125 degrees C. for approximately 5 to 30 minutes, but preferably 90 degrees C. for 5 minutes, and then laminated with photoresist. A mask is used and the photoresist is developed using the proper exposure settings to create the pattern shown in FIG. 29 (shown in greater detail in FIGS. 22A and 19B). Both the top side and bottom side of assembly 2800 is copper etched. Assembly 2800 is cleaned by rinsing in alcohol for 15 to 30 minutes, then preferably rinsing in water, preferably deionized, having a temperature of 21 to 52 degrees C. for at least 15 minutes. Assembly 2800 is plated with tin or lead, then the tin/lead plating is heated to the melting point to allow excess plating to reflow into a solder alloy. Assembly 2800 is again cleaned by rinsing in alcohol for 15 to 30 minutes, then preferably rinsing in water, preferably deionized, having a temperature of 21 to 52 degrees C. for at least 15 minutes.

[0093] Assembly 2800 is depaneled, as shown in FIG. 30, using a depaneling method, which may include drilling and milling, diamond saw, and/or EXCIMER laser. In a preferred embodiment, tacky tape, such as 0.003 inches thick tacky tape in a preferred embodiment, is used to remove the individual spiral coupler packages 300. A manufacturer of such tacky tape is Minnesota Mining and Manufacturing Co. (“3M”), located in St. Paul, Minn. Assembly 2800 is again cleaned by rinsing in alcohol for 15 to 30 minutes, then preferably rinsing in water, preferably deionized, having a temperature of 21 to 52 degrees C. for at least 15 minutes. Assembly 2800 is then vacuum baked for approximately 45 to 90 minutes at approximately 90 to 125 degrees C., but preferably for one hour at 100 degrees C.

[0094] Spiral-like couplers utilizing PTFE can be used in conjunction with other components and other technologies. For example, ceramic materials (having their own circuitry) can be attached to PTFE, by means of film bonding, or glue, by way of example only. Hybrid circuits combining the benefits of ceramics and PTFE can have benefits over either technology alone. For example, the relatively high dielectric constants, e.g. above approximately 10.2, of hard ceramics in a hybrid circuit can allow a manufacturer to design a circuit that is smaller and less lossy than pure PTFE circuits. Ceramics inserted within a cavity of a PTFE structure as a drop-in unit allows the exploitation of both ceramic and PTFE processes. Since hard ceramics typically offer very low loss tangents, the resulting circuits are less lossy.

[0095] A manufacturer can also embed within such a circuit ferrite and/or ferroelectric materials with the same consistency of ceramics. Ferroelectic materials have variable dielectric constant charges that can be controlled with a DC bias voltage. Thus, the frequency range of a coupler can be tuned electronically by changing the dielectric loading. Although ferrite materials may not offer much benefit to traditional couplers, they can be beneficial for spiral-like couplers, whose frequency ranges can be more beneficially varied.

[0096] Using PTFE, one can embed active elements in a fusion bonded homogeneous dielectric structure, in conjunction with spiral-like couplers. Some applications for combining active elements with spiral-like couplers include, by way of example only, digital attenuators, tunable phase shifters, IQ networks, vector modulators, and active mixers.

[0097] A benefit of mixing PTFE material having different dielectic constants in a microwave device is the ability to achieve a desired dielectric constant between approximately 2.2 to 10.2. This is achieved by mixing and weighting different materials and thicknesses in a predetermined stack arrangement. Some advantages of this method are: design freedom to vary dimensional properties associated with a particular pre-existing design; providing a stack-up of multiconductor-coupled lines in the z-plane; and creating a broader range of coupling values. By varying the thickness of layers (whose other attributes may be pre-defined), one can vary the properties of spiral couplers without extensive redesign.

[0098] While there have been shown and described and pointed out fundamental novel features of the invention as applied to embodiments thereof, it will be understood that various omissions and substitutions and changes in the form and details of the invention, as herein disclosed, may be made by those skilled in the art without departing from the spirit of the invention. It is expressly intended that all combinations of those elements and/or method steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.