Abstract:
A method includes forming a signal line on the substrate so as to have a predetermined opening portion; at least one supporting frame each formed on the substrate at both sides of the signal line; a ground line formed on the substrate between the supporting frame and the signal line; a moving plate fixed to the supporting frame at both sides thereof, the moving plate being movable upward and downward; a switching unit positioned on the moving plate, the switching unit comprising contact means for connecting the opened signal line; and a supporting layer for supporting the moving plate and the switching unit, wherein the supporting layer comprises a support protrusion portion for maintaining a distance from the substrate.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application is a division of U.S. application Ser. No. 11/086,320, filed on Mar. 23, 2005, now U.S. Pat. No. 7,283,025, which claims the benefit of REPUBLIC OF KOREA Patent Application No. 2004-84407, filed on Oct. 21, 2004. 

   BACKGROUND 
   1. Field of the Invention 
   The present invention relates to a switch for controlling signal delivery in a high frequency band wireless communication and a radio frequency (RF) system and, more specifically, to a micro-electromechanical systems switch driven by an electrostatic force and a method of fabricating the same. 
   2. Discussion of Related Art 
   In general, in a high frequency band communication system, a field effect transistor (FET), a PIN diode, or the like is used as a switching device for controlling signal delivery. These semiconductor switches are easily integrated but have for example high insertion loss, low isolation loss, and signal distortion. Therefore, a micro electromechanical systems (MEMS) switch has been widely studied to address the above problems. 
   The micro electromechanical systems switch is generally composed of a micro moving element that relatively moves against a substrate, and a driving element for driving the moving element. The driving element has two electrodes located to face each other and the moving element is driven by an electrostatic force generated by a voltage applied through the electrodes of the driving element. In other words, the moving element moves against the substrate in a parallel or horizontal direction, or rotates against the substrate within a predetermined angle. 
     FIGS. 1A and 1B  are diagrams of an example of a typical cantilever type MEMS switch.  FIG. 1A  is a plan view of the typical cantilever type MEMS switch and  FIG. 1B  is a cross-sectional view taken along the line A 1 -A 2  of  FIG. 1A . 
   A lower electrode  2  and a signal line  3  are formed on a substrate  1 , and a cantilever arm  5  fixed to the substrate  1  by a supporting unit  4  is positioned over the lower electrode  2  and the signal line  3 . An upper electrode  6  is arranged on the cantilever arm  5 , and a contact portion  7  for connecting a disconnected portion of the signal line  3  is formed below an end of the cantilever arm  5 . The cantilever arm  5  and the upper electrode  6  have an intermediate portion formed to be narrower than other portions so that an end of the cantilever arm  5  has certain elasticity. 
   When a predetermined constant voltage is applied between the upper electrode  6  and the lower electrode  2 , the cantilever arm  5  is bended downward by an electrostatic force generated in a capacitor structure  8  in which the upper electrode  6  and the lower electrode  2  are laminated. Accordingly, the contact portion  7  comes in contact with the disconnected portion of the signal line  3  to perform an switching operation (See U.S. Pat. No. 5,578,976 issued on Nov. 26, 1996) 
   In the cantilever as shown in  FIGS. 1A and 1B , the signal line (inner wiring)  3  and the contact portion (short-circuit bar)  7  connected to an input and an output, respectively, are located perpendicular to each other, and only one side of the cantilever arm (dielectric layer)  5  is supported. Therefore, when the cantilever arm  5  or the upper electrode  6  is deformed by thermal expansion during a manufacturing process or operation process, it cannot move in a vertical direction as shown in  FIG. 2A , but moves in a distorted manner as shown in  FIG. 2B  so that the contact between the signal line  3  and the contact portion  7  becomes worse. The bad contact increases contact resistance of the signal line  4 , or makes signal delivery unstable, thereby degrading reliability. 
     FIG. 3  shows an example of a conventional membrane type MEMS switch. 
   A supporting frame  24 , a lower electrode  14 , and an opened signal line  18  are formed on a substrate  12 , and a moving plate  20  constituting an upper electrode  16  is positioned over the lower electrode  14  and the signal line  18  with a certain gap therebetween. Further, the supporting frame  24  supports a spring  22  at both sides of the signal line  18  such that the moving plate  20  has certain elasticity. 
   When a predetermined driving voltage is applied to the lower electrode  14 , the moving plate  20  of the upper electrode moves downward due to the electrostatic force generated between the lower electrode  14  and the upper electrode  16 . Accordingly, a connection frame  34  positioned in the moving plate  20  connects the disconnected portion of the signal line  18  to perform a switch operation (See U.S. Pat. No. 6,307,452 issued on Oct. 23, 2001) 
   In the membrane type as shown in  FIG. 3 , the signal line  18  and the supporting frame  24  are located with long distance therebetween. Therefore, when the surface of the upper electrode  16  made of metal is deformed by thermal expansion during a manufacturing process or operation process, the moving plate  20  may not be in complete contact with the signal line  18  to be permanently opened between them, and stiction between the upper electrode  16  and the lower electrode  14  may occur due to a narrow distance between the upper electrode  16  and the lower electrode  14 . Thus, stability and reliability of the switch is degraded. 
   In addition, when the moving plate  20  and the spring  22  are deformed by thermal expansion, the moving plate  20  cannot move in parallel with the substrate  12 . This is because the moving plate is largely thermally expanded while the supporting frame  24  is fixed to the substrate  12  having much smaller thermal expansion than that of the moving plate  20  and accordingly there is little change in the distance between the supporting frames  24 . The thermal expansion causes significant stress at a connection portion between the moving plate  20  and the spring  22 , where permanent deformation is made. As a result, as the moving plate  20  is deformed, it is abnormally separated from the substrate  12  or is inclined into one side so that a normal switch operation is not performed. In addition, when the moving plate  20  moves down and is close to the substrate  12 , the connection frame  34  of the moving plate  20  may be in permanent contact with the signal line  18 . 
   In addition, both electrodes, which are applied with the voltage for generating the electrostatic force, remain very close each other in an interval of several micrometers, and it may cause a stiction problem that the moving plate  20  or the spring  22  sticks to other neighboring fixing elements, which acts as a very critical defect in the operation and reliability of the switch. 
   As described above, while the cantilever or membrane type MEMS switch has been proposed to address the problems of existing semiconductor switches, such as high insertion loss, low isolation loss, and signal distortion, it has low reliability and a signal isolation characteristic due to structural problems such as thermal deformation and stiction. Therefore, there is a need for a new MEMS switch capable of solving the aforementioned problems. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to a micro electromechanical systems (MEMS) switch driven by an electrostatic force and a method of fabricating the same, in which thermal deformation and stiction problems generated by the structural problems of the existing MEMS switch can be solved. 
   The present invention is also directed to MEMS switch driven by an electrostatic force and a method of fabricating the same, in which a support protrusion portion having a wedge-shaped groove is formed to suppress thermal deformation of the moving plate generated during a manufacturing or operation process so that reliability and stability are improved. 
   In one aspect of the present invention, there is provided a micro-electromechanical systems switch comprising: a micro-electromechanical systems switch comprising: a substrate; a signal line formed on the substrate and having a predetermined opening portion; at least one supporting frame each formed on the substrate at both sides of the signal line; a ground line formed on the substrate between the supporting frame and the signal line; a moving plate fixed to the supporting frame at both sides thereof, the moving plate being movable upward and downward; a switching unit positioned on the moving plate, the switching unit comprising contact means for connecting the opened signal line; and a supporting layer for supporting the moving plate and the switching unit, wherein the supporting layer comprises a support protrusion portion for maintaining a distance from the substrate. 
   In another aspect of the present invention, there is provided a method of fabricating a micro-electromechanical systems switch comprising: a) forming a signal line having a predetermined opening portion, a supporting frame positioned at both sides of the signal line, and a ground line positioned between the supporting frame and the signal line on a substrate, respectively; b) forming a sacrificial layer having a predetermined thickness over the surface; c) forming grooves in the sacrificial layer at portions where ends of the signal line reach, respectively, and then, forming contact portions in the grooves; d) forming a hole in the sacrificial layer to expose the substrate between the opened signal lines, a support protrusion portion in the hole, and a supporting layer on the sacrificial layer other than the contact portion; e) forming a moving plate over the supporting frame and the sacrificial layer and over the supporting layer; f) forming a switching unit over the supporting layer and the contact portion inside the moving plate; and g) removing the sacrificial layer. 
   The present invention provides a micro-electromechanical systems switch and a method of fabricating the same in which the structural problems of the cantilever type and the membrane type switch are amended. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other features of the present invention will be described in reference to certain exemplary embodiments thereof with reference to the attached drawings in which: 
       FIG. 1A  is a plan view of a conventional cantilever type micro-electromechanical systems switch; 
       FIG. 1B  is a cross-sectional view taken along the line A 1 -A 2  of  FIG. 1A ; 
       FIGS. 2A and 2B  are cross-sectional views illustrating an operation of the conventional cantilever type micro-electromechanical systems switch; 
       FIG. 3  is a plan view of a conventional membrane type micro-electromechanical systems switch; 
       FIG. 4  is a plan view illustrating a micro-electromechanical systems switch according to an embodiment of the present invention; 
       FIGS. 5A and 5B  are cross-sectional views taken along the line A 11 -A 12  of  FIG. 4 ; 
       FIG. 6  is a cross-sectional view taken along the line B 11 -B 12  of  FIG. 4 ; 
       FIG. 7A  is a picture taken by an electron microscopic for illustrating an example of a micro-electromechanical systems switch according to an embodiment of the present invention; 
       FIG. 7B  is a diagram showing a measurement result of a 3D profiler of a micro-electromechanical systems switch according to an embodiment of the present invention; and 
       FIGS. 8A to 15B  are cross-sectional views for illustrating a method of fabricating a micro-electromechanical systems switch according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   Exemplary embodiments of the present invention will now be described in detail with reference to the accompanying drawings. The following embodiments are provided for a through understanding to those skilled in the art, and a variety of modification can be made and the present invention is not limited to the following embodiments. 
     FIG. 4  is a plan view for illustrating a micro-electromechanical systems switch according to an embodiment of the present invention. Description will be given below with reference to  FIGS. 4 ,  5 A,  5 B and  6 . 
   Input and output signal lines  110   a  and  110   b  for signal delivery are formed on a semiconductor or dielectric substrate  100 . The input and output signal lines  110   a  and  110   b  are spaced (disconnected) from each other to have a predetermined disconnected portion  115 . A pair of ground lines  120   a  and  120   b  are formed on the substrate at both sides of the input and output signals  110   a  and  110   b  to be parallel with the input and output signal lines  110   a  and  110   b.  At least one supporting frame  130   a  and  130   b  is formed with a constant interval on the substrate  100  outside the ground lines  120   a  and  120   b.  Both sides of a moving plate  140  made of material having an elastic restoration force are fixed to the supporting frames  130   a  and  130   b,  respectively, which are formed to be higher than the input and output signal lines  110   a  and  110   b  or the ground lines  120   a  and  120   b.  A part of the moving plate  140  is overlapped with the input and output signal lines  110   a  and  110   b  and the ground lines  120   a  and  120   b,  and an opening is formed at the center to expose the disconnected portion  115  of the input and output signal lines  110   a  and  110   b.  Since both sides of the moving plate  140  are fixed to the supporting frames  130   a  and  130   b,  the central portion can move upward and downward by an electrostatic force. 
   A rectangular switching unit  150  is formed in the opening of the moving plate  140  where the disconnected portion  115  is exposed, wherein the switching unit has a center opening and two “           ”-shape sides like “#”. The switching unit  150  is fixed to the moving plate  140  by the central supporting layer  160  and the dielectric layer  180 , as shown in  FIGS. 5A and 5B . To prevent a loss due to reflection of an RF signal, preferably, the switching unit  150  existing in the moving plate  140  is kept insulated from the moving plate  140  and has a shape corresponding to that of the input and output signal lines  110   a  and  110   b.  
   With the switching unit  150  configured as above, operational properties can be enhanced as follows: First, when an opening is formed in the switching unit  150  made of a conductor in a range where the RF signal is not attenuated (which is called a skin effect), a spring coefficient showing a degree of elasticity can be reduced. When the spring coefficient is reduced, the moving plate  140  rapidly and exactly controls the switching unit  150  so that reliability can be significantly improved. Second, the signal isolation characteristic at a state where the input and output signal lines  110   a  and  110   b  are disconnected is determined by coupling capacitance of the two signal lines  110   a  and  110   b,  and as the coupling capacitance is smaller, the signal isolation characteristic becomes better. Rectangular contact portions  155   a  and  155   b  and the signal lines  110   a  and  110   b  are overlapped at many portions and accordingly have large coupling capacitance and exhibit low signal isolation characteristic. Therefore, by forming both sides of the switching unit  150  in the “           ”-shape in the range where the RF signal is not attenuated, the signal isolation characteristic can be improved.
   In order to electrically connect the input and output signal lines  110   a  and  110   b,  the contact portions  155   a  and  155   b  are protruded below the switching unit  150  so that the contact portions  155   a  and  155   b  reach to the ends of the input and output signal lines  110   a  and  110   b  of the disconnected portion  115 . Further, a central supporting layer  160  is formed below the switching unit  150  and below the center of the moving plate  140  to support a part of the switching unit  150  and the moving plate  140 . A support protrusion portion  165  extended to reach to the substrate  100  is formed, at a position where the central unit of the central support layer  160 , i.e., the center of the disconnected portion  115  is located, to maintain a constant distance from the substrate. A wedge-shaped groove  170  having a constant interval is formed in the support protrusion portion  165 . The groove  170  formed in the support protrusion portion  165  may have the wedge-shape with the uniform width as well as with smaller or larger width at an upper portion. 
   The moving plate  140  supported by the supporting frames  130   a  and  130   b  has a large area. Therefore, the central portion located relatively far from the supporting frames  130   a  and  130   b  may be easily deformed even when a constant voltage is applied. Thus, according to the present invention, the support protrusion portion  165  having the wedge-shaped groove is formed at the central portion of the moving plate  140  to suppress the deformation of the moving plate  140  without changing an inherent spring coefficient of the moving plate  140 . The support protrusion portion  165  is formed at the center of the moving plate  140  to maximize the movement of the switching unit  150 , and the groove  170  is arranged independently of the switching unit  150 . 
   Further, in order to control the flow of an RF signal through the input and output signal lines  110   a  and  110   b,  driving directions of the switching unit  150  and the moving plate  140  should be matched with each other. Therefore, the moving plate  140  and the switching unit  150  are arranged on the same plane, and the central support layer  160  made of dielectric material is used to connect the moving plate  140  and the switching unit  150 , so that the moving plate  140  and the switching unit  150  are supported, and at the same time, when the constant voltage is applied to the moving plate  140 , the switching unit  150  is electrically insulated. 
   Further, dielectric layers  125   a  and  125   b  having a predetermined thickness can be formed on the surface of the ground lines  120   a  and  120   b  to prevent short-circuit between the ground lines  120   a  and  120   b  and the moving plate  140 , and dielectric material  180  having a predetermined thickness is formed on the switching unit  150  and the moving plate  140  to tightly couple the switching unit  150  and the moving plate  140 . 
   An operation of the micro-electromechanical systems switch configured as described above according to the present invention will now be described. 
   When a predetermined DC driving voltage is applied between the moving plates  140  used as an upper electrode and the ground lines  120   a  and  120   b  used as lower electrodes, attraction is exerted between the moving plate  140  and the ground lines  120   a  and  120   b  due to the electrostatic force. Since the ground lines  120   a  and  120   b  are fixed to the substrate  100 , the elastic moving plate  140  is bended toward the ground lines  120   a  and  120   b.  With the bended moving plate  140 , the contact portions  155   a  and  155   b  of the switching unit  150  are connected to the input and output signal lines  110   a  and  110   b,  as shown in  FIG. 5   b,  for signal flow. Here, since the dielectric layers  125   a  and  125   b  are formed between the moving plate  140  and the ground lines  120   a  and  120   b,  the moving plate  140  and the ground lines  120   a  and  120   b  are prevented from being electrically short-circuited. 
   When the DC driving voltage applied to the moving plate  140  and the ground lines  120   a  and  120   b  is removed, the contact portions  155   a  and  155   b  of the switching unit  150  move upward, as shown in  FIG. 5A , by a restoration force due to the spring coefficient of the moving plate  140 , and the input and output signal lines  110   a  and  110   ba  are opened to block the signal flow. 
   The signal isolation characteristic of the micro-electromechanical systems switch operated as described above is determined by coupling capacitance due to an interval of the input and output signal lines  110   a  and  110   b  and coupling capacitance due to overlap of the contact portion  155   a  and  155   b  and the input and output signal lines  110   a  and  110   b.  Therefore, in order to obtain a excellent signal isolation characteristic, the interval between the input and output signal lines  110   a  and  110   b  and the contact portions  155   a  and  155   b  and the interval between the input and output signal lines  110   a  and  110   b  should be both considered. 
   As shown in  FIG. 6 , in the micro-electromechanical systems switch having the central supporting layer  160  of the present invention, the interval between the input and output signal lines  110   a  and  110   b,  i.e., the size of the disconnected portion  115  can be much larger than that of the existing micro-electromechanical systems switch. Accordingly, when the interval between the input and output signal lines  110   a  and  110   b  and the contact portions  155   a  and  155   b  is maintained constant, the relatively favorable signal isolation characteristic can be obtained. In addition, since the spring coefficient of the moving plate  140  formed between the supporting frames  130   a  and  130   b  and the wedge-shaped groove  170  are relatively larger than that of the existing micro-electromechanical systems switch, it can be driven with a lower driving voltage as compared to the existing micro-electromechanical systems switch. 
   In the existing micro-electromechanical systems switch, the moving plate is fixed to both sides so that it is vulnerable to a thermal deformation, and a stiction problem that the moving plate used as and upper electrode is adhered to other fixing elements due to the small interval between the moving plate and the ground line can easily occur. This stiction problem occurs due to the existence of contaminants generated during a manufacturing process or moistures between the moving plate and the substrate having a gap of several micrometers, which acts as a factor to make the switch unstably operated. Therefore, by forming the central supporting layer  160  having the wedge-shaped groove at the center of the moving plate, the operation voltage can be maintained constant and the stiction is prevented, so that the switch can be operated in a stable manner. 
     FIG. 7A  is a picture taken by an electron microscopic illustrating an example of a micro-electromechanical systems switch according to an embodiment of the present invention, and  FIG. 7B  is a diagram showing a measurement result of a 3D profiler of a micro-electromechanical systems switch according to an embodiment of the present invention. 
   Referring to  FIG. 7A , which shows an example in which shapes of the ground lines  120   a  and  120   b  are modified, a variety of modifications of the ground lines  120   a  and  120   b  shown in  FIG. 4  can be made. In addition,  FIG. 4  shows an embodiment of a single pole single throw (SPST) structure. While one input and output signal line  110   a  and  110   b  has been illustrated, the present invention is not limited thereto and a single pole multi throw (SPMT) having one input signal line and two or more output signal lines can be also used. 
   Referring to  FIG. 7B , which shows a measurement result when the substrate  100  is heated up to a temperature of about 200□, it can be seen that the moving plate  140  and the switching unit  150  can be supported and maintained at a constant interval from the substrate  100  by the central supporting layer  160  having the wedge-shaped groove  170  so that the thermal deformation of the moving plate is suppressed. Further, when the moving plate  140  located at a constant height on the input and output signal lines  110   a  and  110   b  and the ground lines  120   a  and  120   b  is located at about 6 as indicated at the right side of  FIG. 6  when the substrate  100  is defined as a reference value ‘0’ so that the thermal deformation is hardly generated. 
   A method of fabricating the micro-electromechanical systems switch of the present invention configured as described above will be described with reference to  FIGS. 8 to 15 .  FIGS. 8A to 15A  show cross-sectional views taken along the line A 11 -A 12  of  FIG. 4 , and  FIGS. 8B to 15B  show cross-sectional views taken along the line B 11 -B 12  of  FIG. 4 . 
   Referring to  FIGS. 8A and 8B , the input and output signal lines  110   a  and  110   b  and the ground lines  120   a  and  120   b  as well as at least one supporting frame  130   a  and  130   b  are formed on a semiconductor or dielectric substrate by thin-film deposition and patterning or electro plating. The input and output signal lines  110  and  110   b  for signal delivery are formed spaced at a predetermined interval to have the disconnected portion  115 , and the ground lines  120   a  and  120   b  are formed at both sides of the input and output signal lines  110   a  and  110   b  in the form of a strip in parallel with the input and output signal lines  110   a  and  110   b.  In addition, the supporting frames  130   a  and  130   b  are formed outside the ground lines  120   a  and  120   b  with a constant interval. Preferably, the input and output signal lines  110   a  and  110   b  and the ground lines  120   a  and  120   b  are made of noble metal such as Au. 
   Referring to  FIGS. 9A and 9B , dielectric layers  125   a  and  125   b  having a predetermined thickness are formed to surround the ground lines  120   a  and  120   b.  The dielectric layers  125   a  and  125   b  prevent the ground lines  120   a  and  120   b  used as lower electrodes and the moving plate  140  used as an upper electrode from electrically short-circuited to each other. 
   Referring to  FIGS. 10A and 10B , a sacrificial layer  190  having a predetermined thickness is formed over the surface having the input and output signal lines  110   a  and  100   b,  the ground lines  120   a  and  120   b,  and the supporting frames  130   a  and  130   b.    
   Referring to  FIGS. 11   a  and  11   b,  after grooves are formed in the sacrificial layer at portions where the ends of the input and output signal lines  110   a  and  110   b  reach using a mask process using a predetermined photoresist, respectively, the contact portions  155   a  and  155   b  are formed in the grooves. 
   Referring to  FIGS. 12   a  and  12   b,  after holes are formed in the sacrificial layer  190  to expose the substrate between the opened input and output signal lines  110   a  and  110   b,  a central supporting layer  160  having a predetermined thickness is formed over the surface other than the contact portions  155   a  and  155   b.  The support protrusion portion  165  vertically extended to reach to the substrate  100  is formed at the central unit of the central supporting layer  160 , i.e., the center of the disconnected portion  115  to maintain the constant interval from the substrate  100 . Since the support protrusion portion  165  is thin, a wedge-shaped groove  170  having a constant width is formed. 
   Preferably, the central supporting layer  160  is made of a silicon nitride layer formed by a plasma enhanced chemical vapor deposition (PECVD) method, or of polyimide formed by a spin coating method, for example. 
   Referring to  FIGS. 13   a  and  13   b,  the moving plate  140  is formed over the supporting frames  130   a  and  130   b  and the sacrificial layer  190  and over one side of the central supporting layer  160 . At the same time, the rectangular shape switching unit  150  is formed over the contact portions  155   a  and  155   b  inside the moving plate  140  and the central supporting layer  160 , wherein switching unit  150  has a center opening and two “           ” shaped sides like a “#” shape to reach the ends of the input and output signal lines  110   a  and  10   b.  
   Referring to  FIGS. 14   a  and  14   b,  the dielectric material having a predetermined thickness is formed on the surface of the exposed central support layer  160  and the switching unit  150 . Preferably, the dielectric layer  170  is made of a silicon nitride layer formed by PECVD or of polyimide formed by spin coating, for example. The switching unit  150  and the central supporting layer  160  are tightly coupled by using the dielectric material  180 . 
   Referring to  FIGS. 15   a  and  15   b,  the sacrificial layer  190  formed between the moving plate  140  and the input and output signal lines  110   a  and  110   b  is removed, for example, by a reactive ion etching (RIE) method or a wet etching method to complete the micro-electromechanical systems switch of the present invention. 
   According to the micro-electromechanical systems switch of the present invention, by forming the central supporting layer having a support protrusion portion at the center of the moving plate to maintain an interval with the substrate, the thermal deformation generated during a manufacturing process and an operation process can be prevented. Thus, the contact between the contact portion and the signal line is stably made so that a stiction problem is prevented and the operation voltage is stably maintained. Therefore, the insertion loss and signal isolation characteristic are improved. 
   As described above, in the micro-electromechanical systems switch in which the structural problems of the existing cantilever or the membrane type switch are solved, a contact portion is formed in the same direction as that of the signal line, and a central support layer having a support protrusion portion is formed at the center of the moving plate to maintain the interval with the substrate. Therefore, the contact of the signal line is stably made so that reliability is improved. In addition, with the stable structure as described above, the defects generated during the manufacturing process can be minimized and the manufacturing process is relatively simple and easy so that the fabrication throughput can be enhanced. 
   As described above, exemplary embodiments of the present invention have been described with reference to the detailed description and the drawings. Terms are used for illustration only, and should not be construed to limit the scope of the present invention described in the claims. Therefore, those skilled in the art will appreciate that a variety of modifications and equivalents thereto can be made. Accordingly, the scope of the present invention will be defined to the subject matter of the following claims.