Abstract:
Metal traces and solder bump pads are formed on a semiconductor substrate by way of a semiconductor template that has been micro-machined to receive solder paste material. The solder paste material is then formed into precisely-controlled ball shapes and metal trace geometries. First, a semiconductor substrate is covered with a mask material for protecting selected surfaces of the substrate that are not to be etched. Next, a mask is applied in order to etch the substrate surface below. Solder ball sites and metal trace channels are formed at this time. A solder non-wettable material is applied to the exposed surfaces of the solder ball sites and the metal trace channels. A solder paste can then be applied uniformly across the surface of the substrate, thus filling in any sites and channels, or both, that are used to form the balls in metal traces desired. The semiconductor template is then applied solder side to a second substrate so that the solder balls and traces can be applied directly on the second substrate using heat to reflow the solder to the second substrate.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to forming contacts on a semiconductor substrate and, more specifically, to the formation of metal bump contacts or connectors on a semiconductor substrate using micro-machining techniques. 
     Recent advances in data processing devices and memory circuits have resulted in the implementation of very large scale integrated circuits (VLSI) and even ultra large scale integrated circuits (ULSI). These VLSI and ULSI circuits are fabricated on semiconductor chips that include integrated circuits and other electrical parts. In order to mount a semiconductor chip to a carrier substrate, such as a printed circuit board or a ceramic substrate, solder bumps are arranged onto one of the semiconductor chip and the carrier substrate so that the semiconductor chip can be mechanically and electrically connected via metallurgical processes by melting the solder bumps. 
     One approach to applying and forming solder bumps and a carrier substrate is to use a solder paste. The solder paste is printed onto the carrier substrate and leads extending from the semiconductor chip are placed on the solder paste on the carrier substrate. The structure is then heated to cause the solder in the solder paste to melt so that the semiconductor chip can be mechanically and electrically connected to the carrier substrate. To place the solder paste onto the carrier substrate, a metal mask with predetermined openings is typically used. The solder paste is applied to the surface of the metal mask and a wiper is moved across the surface of the mask, thus pushing the solder paste through the openings of the metal mask onto the surface of the carrier substrate. Such masks are typically referred to as stencils. 
     Unfortunately, as the critical dimensions of the integrated circuits become smaller and smaller, the amount of solder paste that can be pressed through a given stencil becomes smaller and the placement of the solder paste becomes even more difficult. Additionally, with the smaller critical dimensions, the stencil mask becomes even more difficult to clean for a subsequent solder paste application as well as being subject to high rates of wear because of the constant placement of the stenoid, application of the paste to the stencil, and removal, and cleaning of the stencil. 
     Another method of placing conductive contacts for connecting the semiconductor chip to the carrier substrate has been to use preformed solder balls that are placed directly upon either the carrier substrate or the semiconductor chip with precisely controlled placement. Once the solder balls are in place, the solder balls are subjected to heat to cause a partial reflow so that the solder balls adhere to the solder pad. Unfortunately, in this process, as the critical dimensions of the features on the semiconductor chip tend to decrease, significant disadvantages become apparent in using this type of technique. One disadvantage is that the processing costs due to the limited process reliability and the speed of the pick and place nature of the transfer process become more evident. Another disadvantage is that the physical handling and placement of the solder balls by the machine dictates the minimum spacing allowed between solder bumps on a semiconductor chip or carrier substrate, and thus requires a semiconductor chip that would be larger than otherwise necessary for the desired VLSI or ULSI circuitry. 
     Additional problems involve the uniformity of the preformed solder balls. At smaller and smaller ball sizes, the average diameter of the preformed solder balls may vary greatly from the desired diameter of the preformed solder ball. This wide discrepancy in uniformity can lead to several problems. Preformed solder balls not only cannot be applied where desired, but when a too large or to small preformed solder ball is placed upon a pad, after the formation of a connection using such a performed solder ball, typically the location will be noted as either having several bad connections surrounding a ball that is too large or having a defective connection where the ball is too small. Large diameter preformed solder balls tend to prevent adjacent acceptable preformed solder balls from mechanically and electrically connecting between the carrier substrate and the semiconductor chip. Small diameter preformed solder balls are not large enough in diameter than the smaller ball, which can only touch one of the two surfaces. 
     Yet another technique has been developed that uses a method for forming solder balls on a semiconductor plate having apertures. One such technique is described in U.S. Pat. No. 5,643,831, entitled “Process for Forming Solder Balls on a Plate Having Apertures Using Solder Paste and Transferring the Solder Ball to Semiconductor Device”, issued Jul. 1, 1997. The &#39;831 Patent discloses a method for fabricating a semiconductor device using a solder ball forming plate having cavities. Solder paste is placed in the cavities using a solder paste application, such as a squeegee. Once the cavities are filled with solder paste, the solder ball forming plate is heated to form solder balls in the cavities while the plate is in an inclined position. The solder balls are then transferred from the plate to a semiconductor chip. 
     The solder ball forming plate is fabricated from a semiconductor material such as silicon, according to the following method. Initially, a substantially uniform flat surface is formed on the plate. Next, a plurality of cavities is formed in the flat surface of the plate. The cavities are formed by etching the semiconductor materials after a mask has been formed on the flat surface, each cavity having the shape of a precisely formed rhombus or parallelogram. 
     Yet another example of using a solder ball forming plate is disclosed in U.S. Pat. No. 5,607,099, entitled “Solder Bump Transfer Device for Flip-Chip Integrated Circuit Devices”, issued Mar. 4, 1997. The &#39;099 Patent discloses a carrier device that has cavities formed in its surface for receiving and retaining solder material. The solder material can then be transferred to a flip-chip as solder bumps. The cavities are located on the surface of the carrier device such that the location of the solder material corresponds to the desired solder bump locations on the flip-chip when the carrier device is placed in alignment with the flip chip. The size of the cavities can be controlled in order to deliver a precise quantity of solder material to the flip-chip. Further, in the &#39;099 Patent, the apertures are fabricated so that they have a width of about 300 μm at the surface of the die and a width of about 125 μm at its base surface. Meanwhile, in the &#39;831 Patent, the rhombus shaped cavities are design to produce a ball size of about 100 μm in diameter. Unfortunately, both of these structures cannot yet produce a ball size for a solder ball that approaches the dimensions currently required in placing a semiconductor chip upon a carrier substrate using the flip-chip technology. Additionally, the solder ball forming cavities are limited in shape. 
     Accordingly, it would be advantageous to overcome the problems of producing and using solder balls having uniform sizes as have been shown in the prior art approaches of utilizing preformed solder balls or to use metal masks or stencils to apply solder paste for reflow into solder balls. Additionally, it would be advantageous to make even smaller, more precisely formed solder balls than is possible in the prior art as well as to fabricate metal traces during the same step as that of forming solder balls using a solder ball forming plate. 
     Not only would it be advantageous to overcome the problems of producing uniform solder ball sizes for use in connecting a device to a substrate, but it would also be beneficial to provide a way of greatly improving the precision with which solder connections are made in alignment. 
     BRIEF SUMMARY OF THE INVENTION 
     According to the present invention, metal traces and solder bump pads are formed on a semiconductor substrate by way of a semiconductor template that has been micro-machined to receive solder paste material. The solder paste material is then formed into precisely-controlled ball-shapes and metal trace geometries. First, a semiconductor substrate is covered with a mask material for protecting selected surfaces of the substrate that are not to be etched. Next, a mask is applied in order to anisotropically etch the substrate surface below. Solder ball sites and metal trace channels are formed at this time. A solder non-wettable material is applied to the exposed surfaces of the solder ball sites and the metal trace channels. A solder paste can then be applied uniformly across the surface of the substrate, thus filling in any sites and channels, or both, that are used to form the desired balls. The semiconductor template is then applied solder side to a second substrate so that the solder balls and traces can be applied directly on the second substrate, the solder balls being subsequently formed on the second substrate by the heating thereof to form the solder paste into a solder ball. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     FIGS. 1A-D illustrate a cross-sectional view of steps used in forming solder receiving holes and channels in a substrate mold according to the present invention; 
     FIG. 2 depicts a surface of the substrate mold having a plurality of cavities formed therein; 
     FIG. 3 illustrates the application of solder paste to the cavities and traces of the substrate mold of FIG. 2; 
     FIG. 4 depicts the formation of solder bumps in the first substrate mold as mated to a second substrate; 
     FIG. 5 depicts the second substrate having metal bumps and traces before final reflow; 
     FIG. 6 illustrates the formation of metal balls on the second substrate after reflow; 
     FIG. 7 illustrates a schematic diagram of a mold system using the solder mold according to the present invention; 
     FIG. 8 depicts a surface of a second embodiment of the substrate mold of the present invention having a plurality of hemispherical cross-sectional shaped cavities formed therein prior to the removal of the resist coating on the surface of the substrate mold; 
     FIG. 9 depicts the substrate mold of FIG. 8 having solder paste in the cavities formed therein in contact with a second substrate; 
     FIG. 10 depicts the second substrate having the solder paste applied thereto after the second embodiment of the substrate mold of the present invention of FIG. 8 is removed; 
     FIG. 11 depicts the second substrate of FIG. 10 after the solder paste has been heated to form solder balls on the second substrate; 
     FIG. 12 depicts a surface of a third embodiment of the substrate mold of the present invention having a plurality of rectangular cross-sectional shaped cavities formed therein; 
     FIG. 13 depicts the substrate mold of FIG. 12 having solder paste in the rectangular cavities in contact with a second substrate; 
     FIG. 14 depicts the second substrate of FIG. 13 having the rectangularly shaped solder paste thereon after having been removed from the substrate of the third embodiment of the invention by the heating thereof; 
     FIG. 15 depicts the second substrate after the heating of the solder paste thereon to form solder balls; 
     FIG. 16 depicts a fourth embodiment of a substrate mold of the present invention having a plurality of cavities in a surface thereof and a plurality of heating elements on the other surface thereof; 
     FIG. 17 depicts the substrate of FIG. 16 having solder mold paste in the cavities formed in a surface thereof; and 
     FIG. 18 depicts the other side of the substrate mold of FIG. 16 illustrating the plurality of heating elements thereon along section line  18 — 18  of drawing FIG.  17 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Illustrated in drawings FIGS. 1A-1D is a method for fabricating the semiconductor substrate to form metal bumps or metal traces, or both, on the surface of a secondary substrate. A semiconductor substrate, typically a flat planar substrate having a flat planar upper surface, a flat planar lower surface, and a plurality of planar sides forming the periphery of the substrate, is selected to serve as a bump forming substrate mold  10 . The semiconductor substrate may be of any desired size and geometric shape suitable for use with an associated semiconductor device. The semiconductor substrate is selected from a semiconductor base material such as silicon, gallium arsenide, silicon on insulator, which may include silicon on glass or sapphire, or other well-known semiconductor substrate materials, as well as other similar types of materials, which are capable of being precisely micro-machined and having a coefficient of thermal expansion (CTE) similar to that of the semiconductor materials. In this particular application, it is preferred that a silicon substrate is used for substrate mold  10 , although any of the other base materials may be freely substituted therefor. The silicon substrate is aligned such that the flat, planar upper surface  12  of substrate mold  10  defines the &lt;100&gt; plane of the substrate mold  10  which mates with a semiconductor device (not shown). As is shown in drawing FIG. 1A, a flat, planar upper surface  12  of substrate mold  10  has a first protective mask layer  14  located thereon. The first protective mask layer  14  may be selected from particular etch resistant materials such as nitride, oxide, or a hardened polymer spin-on mask. Substrate mold  10  typically has a thickness of about 25 to 28 mils. 
     Next, in drawing FIG. 1B, a photoresist  16  is applied over the surface of mask layer  14  and then exposed through a mask to define openings exposing the selected cavity locations to be formed in flat, planar upper surface  12 . Then, as shown in FIG. 1C, a sufficient amout of semiconductor material is removed by an anisotropic etching from the exposed portion of the flat, planar surface  12  after penetration of the exposed portion of first protective mask layer  14 , thereby forming at least one cavity  18 . Using an anisotropic etching process, the cavity  18  has walls sloped at 54° relative to the &lt;100&gt; plane of the substrate mold  10 . The anisotropic etchant may be, for example, KOH, or other etchant materials well known to those skilled in the art. Further, if straight walls are desired, a dry etching using a plasma etch apparatus may be used to form cavity  18 . 
     After the formation of cavity  18 , first protective mask layer  14  is removed using a dry-etch process that is selective to removing first protective mask layer  14  only and not removing any of the underlying silicon either in the cavity  18  thus formed or on the flat, planar upper surface  12  of substrate mold  10 . For example, if first protective mask layer  14  is silicon dioxide, a removal substance such as phosphoric acid may be used. After the removal of the first protective mask layer  14 , a release layer  20  is formed over the entire flat, planar upper surface  12  of substrate mold  10 , particularly covering cavity  18 . Release layer  20  is selected from a material that is relatively non-wettable to metal solder. Such materials include silicon dioxide or silicon nitride, which can be applied using a chemical vapor deposition process. Other materials that are relatively non-wettable to metal solder may also be used, such as, for example, non-wettable polymers or the like. The result in structure is depicted in drawing FIG.  1 D. 
     Although drawing FIGS. 1A-D illustrated only a single cavity  18 , it is intended that a plurality of cavities be formed in an array across substrate mold  10 . An example of a solder ball forming mold or substrate mold  10  that has such a plurality of cavities  18  is depicted in drawing FIG.  2 . Release layer  20  is applied and utilized to minimize the wetting of solder paste on the substrate mold  10  when the assembly is heated in order to transfer the solder onto the bumps of the secondary surface. 
     Solder paste is applied, as shown in drawing FIG. 3, by use of an applicator  22 , such as a squeegee, that is passed across the surface of substrate mold  10  pressing a metal solder paste  24  into the plurality of cavities  18  and wipes the excess paste away. The solder paste  24  fills cavities  18 , thus forming frustroconically-shaped solder bumps  26  (shown in FIGS.  3  and  4 ). 
     Various types of metal solder may be used. The most widely employed types include a lead-tin combination. Other types of metal solder may include, but are not limited to, lead-silver, lead-tin-silver, lead-tin—indium, indium-tin, indium-lead, or any paste using copper or gold in combination with the lead or tin. For example, a lead-tin solder paste having a 63/37 weight ratio has a eutectic temperature of 183° C. Another type of lead-tin paste that has a 95/5 weight ratio has a eutectic temperature of about 350° C. 
     Once the solder paste  24  is applied to flat, planar upper surface  12  of substrate mold  10 , the entire assembly is heated to a temperature sufficient enough to slightly melt the metal solder paste in order to begin the formation of the solder bumps to be transferred. As shown in drawing FIG. 4, after this partially melted solder state has been reached, substrate mold  10  is inverted and applied to the surface of a carrier substrate  28 , which may comprise a semiconductor device (die), wafer, or flexible substrate, such as a flex tape. The assembly of the substrate mold  10  and carrier substrate  28  is heated to a sufficiently high enough temperature to cause solder bumps  26  to slightly reflow and release from the release layer  20  formed on substrate mold  10 . Substrate mold  10  is then removed and solder bumps  26  adhere to bond pads, terminal pads or other conductive, solder wettable sites  30  on carrier substrate  28 , as shown in drawing FIG.  5 . Next, an additional reflow step may be performed that causes solder bumps  26  to form into approximately spherically shaped solder balls  32  as attached to conductive solder wettable sites  30  as depicted in drawing FIG.  6 . 
     Because of the generally trapazoidal shape of solder bumps  26 , the solder pastes upon heating reflow, draws into a substantially spherical shape and is held together by the surface tension of the solder material to form approximately spherically shaped solder ball  32  or a truncated spherical ball (not shown). 
     Although it has been depicted how solder balls or bumps  32  are formed in drawing FIG. 4, it is also possible to form metal traces using substrate mold  10 . The same type of patterning and etch steps as described with respect to FIGS. 1A-1B would be followed, but would include a layout that would form metal traces or channels. 
     A solder mold system is depicted in drawing FIG. 7, which incorporates the substrate mold  10  shown in drawing FIGS. 1-6. The mold system includes solder applicator  22  for spreading metal solder paste  24  as dispensed by metal paste dispenser  52 . Once the paste is sufficiently in place within the cavities  18 , the substrate mold  10  is mated to a secondary substrate as shown in drawing FIG. 4, and then placed in a low-temperature metal paste reflow oven  54  to melt the paste to a sufficient enough consistency to form self-supported bumps and has sufficient enough tackiness to wet the conductive-gates on the carrier substrate  28 . 
     Referring to drawing FIG. 8, an alternative embodiment of a substrate mold  40  of the present invention is illustrated. The substrate mold  40  is similar to the substrate mold  10  described hereinbefore as to construction and methods of construction except that the of cavities  18  formed therein are hemispherically shaped. As illustrated, the first protective mask layer  14  used to form the plurality of cavities  18  is present on portions of the flat, planar upper surface  42  of the substrate mold  40 . As with the substrate mold  10 , the substrate mold  40  may include a release layer  20  to aid in the release of the solder paste contained within the hemispherical cavities  18 . 
     Referring to drawing FIG. 9, once the solder paste  24  is applied to flat, planar upper surface  42  of substrate mold  40  as described herein with respect to substrate mold  10  illustrated in drawing FIG. 3, the entire assembly of the substrate mold  40  and carrier substrate  28  having conductive sites or bond pads  30  located thereon for the solder paste  24  to be applied is heated to a temperature sufficient enough to slightly melt the metal solder paste  24  in order to begin the formation of the solder bumps be transferred. 
     As shown in drawing FIG. 9, after this partially melted solder state has been reached, the assembly of the substrate mold  40  and the carrier substrate  28  is inverted so that the solder paste  24  in cavities  18  is applied to the conductive sites  30  on the surface of the carrier substrate  28 , which may comprise a semiconductor device (die), wafer, or flexible substrate, such as a flex tape. The assembly of the substrate mold  40  and carrier substrate  28  is heated a sufficiently high enough temperature to cause solder bumps  26  to slightly reflow and release from the release layer  20  formed on substrate mold  40 . Substrate mold  40  is then removed and solder bumps  26  adhere to the conductive sites, bond pads, terminal pads or other conductive, solder wettable sites  30  on carrier substrate  28 , as shown in drawing FIG.  10 . Next, an additional reflow step may be performed that causes solder bumps  26  to form into approximately spherically shaped solder balls  32  as attached to conductive sites  30  as depicted in drawing FIG.  11 . 
     Because of the generally hemispherical shape of solder bumps  26 , the solder paste, upon heating reflow, draws into a substantially spherical shape and is held together by the surface tension of the solder material to form approximately spherically shaped solder balls  32  or truncated spheres. 
     Referring to drawing FIG. 12, an alternative embodiment of a substrate mold  50  of the present invention is illustrated. The substrate mold  50  is similar to the substrate molds  10  and  40  described hereinbefore as to construction and methods of construction except that the cavities  18  formed therein are generally rectangular, or square shaped (shown in dashed lines). The first protective mask layer  14  used to form the plurality of cavities  18  present on portions of the flat planar upper surface  42  of the substrate mold  50  is not illustrated. As with the substrate mold  10 , the substrate mold  50  may include a release layer  20  to aid in the release of the solder paste contained within the hemispherical cavities  18 . Referring to drawing FIG. 13, once the solder paste  24  is applied to flat, planar upper surface  42  of substrate mold  50  as described herein with respect to substrate mold  10  illustrated in drawing FIG. 3, the entire assembly of the substrate mold  50  and carrier substrate  28  having conductive sites or bond pads  30  located thereon for the solder paste  24  to be applied is heated to a temperature sufficiently high enough to slightly melt the metal solder paste  18  in order to begin the formation of the solder bumps to be transferred. 
     As shown in drawing FIG. 13, after this partially melted solder state has been reached, the assembly of the substrate mold  50  and the carrier substrate  28  is inverted so that the solder paste  24  is applied to the conductive sites  30  on the surface of the a carrier substrate  28 , which may comprise a semiconductor device (die), wafer, or flexible substrate, such as a flex tape. The assembly of the substrate mold  50  and carrier substrate  28  is heated to a sufficiently high enough temperature to cause solder bumps  26  to slightly reflow and release from the release layer  20  formed on substrate mold  50 . Substrate mold  50  is then removed and solder bumps  26  adhere to the conductive sites, bond pads, terminal pads or other conductive, solder wettable sites  30  on carrier substrate  28 , as shown in drawing FIG.  14 . Next, an additional reflow step may be performed that causes solder bumps  26  to form into approximate spherically shaped solder balls  32  as attached to conductive sites  30  as depicted in drawing FIG.  15 . 
     Because of the generally rectangular shape of solder bumps  26 , the solder paste, upon heating reflow, draws into a substantially spherical shape and is held together by the surface tension of the solder material to form approximately spherically shaped solder balls  32 . 
     Referring to drawing FIG. 16, another embodiment of the substrate mold  100  of the present invention is illustrated. The substrate mold  100  is similar to the substrate molds  10 ,  40 , and  50  described hereinbefore. The substrate mold  100  includes cavities  18  having any desired shape as described herein in upper flat planar surface  12  and includes electrical resistance heating strips  66  located on the bottom thereof for the heating of the substrate mold  100  with electrical conductor  68  connected thereto. The bottom surface of the substrate mold  100  includes a coating  62  thereon to electrically insulate the heating strips  66  from the substrate mold  100 . The heating strips  66  may be of any desired geometrical configuration to cover the bottom surface of the substrate mold  100  to uniformly heat the mold  100  and the solder paste  24  located in the cavities  18  thereof. The electrical conductor  68  may be any desired shape and have any desired location for connection to the heating strips  66 . The electrical conductor  68  is covered with an insulation layer  70  located thereover. In areas or portions of the bottom surface of the substrate mold  100  not having a heating strip  66  located thereon, an insulative coating  64  of any suitable type is provided. 
     Referring to drawing FIG. 17, the substrate mold  100  is illustrated having solder paste  24  located in cavities  18  having release layer  20  therein. After the solder paste  24  is placed in the cavities  18 , a carrier substrate  28  (see FIG. 4) is applied to the substrate mold  100 , the assembly of the substrate mold  100  and carrier substrate  28  inverted, and the resistance heating strips  66  on the substrate mold  100  actuated to heat the solder paste  24  to transfer the same to the carrier substrate  28 . After the solder paste  24  is transferred to the carrier substrate  28 , the substrate  28  is further heated to cause the solder paste to adhere to the conductive sites  30  on the carrier substrate  28  to substantially form solder balls  32  thereon. 
     Referring to drawing FIG. 18, the resistance heating strips  66  and conductor  68  are illustrated. The heating strips  66  may be of any desired shape to substantially uniformly heat the substrate mold  100 . Similarly, the conductor  68  may be any desired shape to electically connect to the heating strips  66 . Further, any desired electrical conductor may be used to electrically connect the connector  68  to a source of electrical power. 
     Substrate molds  10 ,  40 ,  50  and  100  described herein are useful in forming contact bumps for many applications. One application is the formation of flexible connecting tape that requires bumps for interconnection of traces on the tape to a die or other element. The micro-machining of substrate mold  10  provides a much more accurate means for placing the solder ball shaped bumps over the prior art methods of merely placing bumps on top of a screen and then having the screen place the bumps in a proper alignment. Further, the solder ball shaped bumps have a more uniform volume and shape as the cavity dimensions in the semiconductor mold provide a substantially precise control over the formation of the solder ball shaped bumps. By contrast, in the prior art, the uniformity of solder balls has always been a problem, especially at the smaller diameter dimensions that are now being used. Another application for the present invention is for the direct placement of the solder ball shaped bumps on a semiconductor device or die for attachment. Yet another application includes placing the solder ball shaped bumps on a wafer-scale device for interconnection. This allows multiple devices placed on the same substrate to be interconnected using the precision of the solder ball shaped bumps. For example, the solder ball shaped bump application is useful in chip scale packages (CSP) or in fine ball grid array (FBGA) packages. The in situ resistance heating strip allows for selecting which balls need to be transferred by selectively heating only those resistance heating strips  66 . 
     The applications of providing interconnect and bump contacts are numerous. For example, the metal trace interconnect and the bump contact may be used in any type of semiconductor device such as a memory storage device. These memory storage devices can range from read-only memory (ROM) and random access memory (RAM) to exotic types of memory such as video memory and the memory used in computer systems. Additionally, the application of this metal trace interconnect and bump contact structure can be utilized in micro-processor packages that are used in computer systems as well as in other types of systems, and other types of single processing devices and support chips normally used in electronic devices. These electronic devices range from cellular phones to microwave systems, to automobiles and even programmable wrist watches. 
     Although the present invention has been described with reference to a particular embodiment, the invention is not limited to this described embodiment. The invention is limited only by the appended claims, which include within their scope all equivalent devices or methods which operate according to the principles of the invention as described.