Abstract:
Generally, the subject matter disclosed herein relates to the fabrication of an RRAM cell using CMOS compatible processes. A resistance random access memory device is disclosed which includes a semiconducting substrate, a top electrode, at least one metal silicide bottom electrode formed at least partially in the substrate, wherein at least a portion of the at least one bottom electrode is positioned below the top electrode, and at least one insulating layer positioned between the top electrode and at least a portion of the at least one bottom electrode. A method of making a resistance random access memory device is disclosed that includes forming an isolation structure in a semiconducting substrate to thereby define an enclosed area, performing at least one ion implantation process to implant dopant atoms into the substrate within the enclosed area, after performing the at least one ion implantation process, forming a layer of refractory metal above at least portions of the substrate, and performing at least one heat treatment process to form at least one metal silicide bottom electrode at least partially in the substrate, wherein at least a portion of the at least one bottom electrode is positioned below at least a portion of a top electrode of the device.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of co-pending application Ser. No. 13/052,864, filed Mar. 21, 2011. 
    
    
     BACKGROUND 
     1. Field of the Disclosure 
     Generally, the present disclosure relates to sophisticated integrated circuits, and, more particularly, to the fabrication of an RRAM cell with, in one embodiment, one or more bottom electrodes formed by silicidation using CMOS compatible processes. 
     2. Description of the Related Art 
     Memory circuits and devices are widely used in the electronics industry. In general, memory devices permit the storage of a “bit” of information, i.e., a “1” (logically high) or a “0” (logically low) signal. Vast numbers of these memory devices are formed on a single chip so as to permit the storage of a vast quantity of digital information. Various forms of such devices, and read/write circuitry employed with such devices, have been used in the industry for years, e.g., RAM (Random Access Memory) devices, ROM (Read Only Memory) devices, EEPROM (Electrically Erasable Read Only Memory) devices, etc. 
     Nonvolatile memory is a type of memory that retains stored data when power is removed from the memory device. Such nonvolatile memory devices are widely employed in mobile communication devices, computers, memory cards, etc. Flash memory is an example of one type of nonvolatile memory that is greatly used in such modern electronic devices. 
     More recently, another form of memory, RRAM (Resistance Random Access Memory) has been introduced to the industry.  FIGS. 1A-1B  schematically depict an illustrative prior art RRAM device  100 . Basically, such a prior art RRAM device  100  is comprised of a multilayered stack of materials, and it is fabricated using a layer-by-layer technique. As shown in  FIG. 1A , in one embodiment, the prior art RRAM device  100  comprises a top electrode  102 , a tunnel oxide  104 , a layer of conductive metal oxide  106 , and a bottom electrode  108 . The top electrode  102  and the bottom electrode  108  may be comprised of, for example, platinum. The tunnel oxide layer  104  may be comprised of silicon dioxide and it may have an illustrative thickness of 20-50 Å. The conductive metal oxide layer  106  may be comprised of TiO2 or Cr-doped SrTiO3. 
       FIG. 1B  schematically depicts how the RRAM device  100  works. In the depicted example, a positive voltage V +  is applied to the top electrode  102 , while the bottom electrode  108  is coupled to ground (“GRND”). Application of the positive voltage V +  attracts negatively charged ions  110  from the conductive metal oxide layer  106  which thereby causes breakdown of the tunnel oxide  104 . This breakdown results in the establishment of an electrical current path between the top electrode  102  and the bottom electrode  108 , and the resulting electrical current can be measured. Application of a negative voltage (V − ) to the top electrode  102  forces or repels the negatively charged ions  110  back toward the conductive metal oxide layer  106 . This action “turns off” the conductive current path that was previously established, and cuts off the previously established current flow. 
     Importantly, as noted above, the prior art RRAM device  100  is made using a traditional layer-by-layer approach that involves many discrete deposition, lithography and etching steps, perhaps for each layer of the device. Such a layered construction of the prior art RRAM device  100  makes it more difficult to incorporate it into integrated circuit devices that are manufactured using modern CMOS processing technology and methods. For example, fabrication of an RRAM device using the layer-by-layer approach may result in the overall height of the RRAM device  100  being greater than the height of other structures that are formed in traditional CMOS-type devices, such as PMOS and NMOS transistors. Such height differences can lead to patterning errors in photolithography operations and/or mandate additional processing steps be taken to avoid or reduce the adverse effects of such height differences, e.g., the performance of one or more additional deposition and planarization processes. 
     The present disclosure relates to methods and devices for avoiding or at least reducing the effects of one or more of the problems identified above. 
     SUMMARY OF THE DISCLOSURE 
     The following presents a simplified summary of the present disclosure in order to provide a basic understanding of some aspects disclosed herein. This summary is not an exhaustive overview of the disclosure, nor is it intended to identify key or critical elements of the subject matter disclosed here. Its sole purpose is to present some concepts in a simplified form as a prelude to the more detailed description that is discussed later. 
     Generally, the subject matter disclosed herein relates to the fabrication of an RRAM cell using CMOS compatible processes. In the disclosed examples, the RRAM devices disclosed herein may be of a single bit or a dual bit configuration. In one illustrative embodiment, a resistance random access memory device is disclosed which includes a semiconducting substrate, a metal silicide top electrode positioned above the substrate, a single metal silicide bottom electrode formed at least partially in said substrate, wherein at least a portion of the single bottom electrode is positioned below an entire width of the top electrode and at least one insulating layer positioned between the top electrode and single bottom electrode. In another illustrative example, a resistance random access memory device includes a semiconducting substrate, and a metal silicide top electrode positioned above the substrate. The illustrative device further includes two separated metal silicide bottom electrodes formed at least partially in the substrate, wherein at least a portion of each of the two bottom electrodes is positioned below separate portions of the top electrode, and at least one insulating layer positioned between the portions of the top electrode and the portions of each of the two bottom electrodes that are positioned below the top electrode. 
     A method of making a resistance random access memory device including a top electrode is also disclosed which includes forming an isolation structure in a semiconducting substrate to thereby define an enclosed area, performing at least one ion implantation process to implant dopant atoms into the substrate within the enclosed area, after performing the at least one ion implantation process, forming a layer of refractory metal above at least portions of the substrate, and performing at least one heat treatment process to form at least one metal silicide bottom electrode at least partially in the substrate, wherein at least a portion of the at least one bottom electrode is positioned below at least a portion of a top electrode of the device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure may be understood by reference to the following description taken in conjunction with the accompanying drawings, in which like reference numerals identify like elements, and in which: 
         FIGS. 1A-1B  schematically depict an illustrative prior art RRAM device; 
         FIGS. 2A-2C  schematically depict illustrative embodiments of a novel RRAM device disclosed herein; 
         FIGS. 3A-3C  depict one illustrative process flow for forming an illustrative RRAM device disclosure herein; 
         FIGS. 4A-4C  depict one illustrative process for forming one illustrative embodiment of an RRAM device disclosed herein using a gate-last technique; 
         FIGS. 5A-5D  depict illustrative configurations of various memory cells that may be created using the novel RRAM device disclosed herein; and 
         FIGS. 6A-6D  are illustrative drawings and charts depicting the operational characteristics of one of the illustrative RRAM devices disclosed herein. 
     
    
    
     While the subject matter disclosed herein is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION 
     Various illustrative embodiments of the present subject matter are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. 
     The present subject matter will now be described with reference to the attached figures. Various structures and devices are schematically depicted in the drawings for purposes of explanation only and so as to not obscure the present disclosure with details that are well known to those skilled in the art. Nevertheless, the attached drawings are included to describe and explain illustrative examples of the present disclosure. The words and phrases used herein should be understood and interpreted to have a meaning consistent with the understanding of those words and phrases by those skilled in the relevant art. No special definition of a term or phrase, i.e., a definition that is different from the ordinary and customary meaning as understood by those skilled in the art, is intended to be implied by consistent usage of the term or phrase herein. To the extent that a term or phrase is intended to have a special meaning, i.e., a meaning other than that understood by skilled artisans, such a special definition will be expressly set forth in the specification in a definitional manner that directly and unequivocally provides the special definition for the term or phrase. 
       FIGS. 2A-2C  depict various illustrative embodiments of the novel RRAM device  200  disclosed herein.  FIG. 2A  is a depiction of a single-bit embodiment of the present disclosure, while  FIG. 2B  depicts an illustrative example of a dual-bit configuration of the RRAM device  200  disclosed herein. 
     As shown in  FIG. 2A , the RRAM device  200  is formed in and above a semiconducting substrate  202 . An isolation structure  204 , e.g., a shallow trench isolation structure, is provided to electrically isolate the RRAM device  200  from other devices. The RRAM device  200  comprises a top electrode  206  and a bottom electrode  208  that are separated by one or more insulation layers  210 . The RRAM device  200  further includes one or more schematically depicted sidewall spacers  212 . As depicted in  FIG. 2A , the bottom electrode  208  is a unitary structure that extends across what would be the channel region of a similarly shaped CMOS transistor. The insulation layer  210  may be comprised of one or more layers of material. In one example, an interfacial layer  210 A (indicated by the dashed line in the figures) may be formed on the substrate  202 . The materials of construction of the interfacial layer  210 A may vary depending upon the application. In one example, the interfacial layer  210 A may be comprised of a silicon-rich SiO x  material, where x&lt;2, e.g., silicon suboxide, and it may have a thickness of approximately 1.2 nm. The interfacial layer  210 A may be formed using a variety of techniques, e.g., CVD (chemical vapor deposition), ALD (atomic layer deposition), a thermal growth process, etc. The interfacial layer  210 A may not be required in all applications. 
     The insulation layer  210 B depicted in  FIG. 2A  may be employed in conjunction with, or in the absence of, the interfacial layer  210 A. The insulation layer  210 B may be a high-k material (a material having a dielectric constant greater than 3.9 which is the dielectric constant of SiO 2 ). In one illustrative example, the insulation layer  210 B may be made of hafnium-silicon-oxynitride (HfSiON), and it may have a thickness of, for example, approximately 2.5 nm. The insulation layer  210 B may be made of other materials, such as silicon oxynitride. The insulation layer  210 B may be manufactured using a variety of known processes, e.g., CVD, ALD, etc. 
     In the illustrative embodiment depicted in  FIG. 2A , the top electrode  206  and the bottom electrode  208  are both made of metal silicide materials, such as nickel silicide, although other metal silicides may also be employed. Moreover, the metal silicide materials used for the top electrode  206  and the bottom electrode  208  may not be of the same metal silicide in all applications. In one illustrative example, the top electrode  206  may have a thickness of approximately 2-5 nm, while the bottom electrode  208  may have a maximum thickness of approximately 50 nm. One manner in which the top electrode  206  and the bottom electrode  208  may be formed is discussed more fully below. The spacers  212  may be of traditional construction and materials, and they may be formed by traditional techniques, i.e., deposition of a layer of spacer material followed by the performance of an anisotropic etching process. 
     As noted previously, the embodiment depicted in  FIG. 2B  is a “dual-bit” embodiment of the RRAM device  200  that is the subject of the present disclosure. More specifically, whereas the illustrative embodiment depicted in  FIG. 2A  includes a single, unitary bottom electrode  208 , the dual-bit embodiment depicted in  FIG. 2B  has two separate bottom electrodes  208 A,  208 B. One technique for forming the RRAM device  200  in this dual-bit configuration will be discussed more fully below. 
       FIG. 2C  depicts yet another illustrative embodiment of the RRAM device  200  wherein the top electrode  206  is made from a metal-containing and highly conductive material that is oxygen soluble, such as titanium nitride or nickel silicide, having an oxygen solubility of 33 and 0.05 (atom %), respectively. Thus, in one illustrative example, the top electrode  206  may be made from a conductive material having an oxygen solubility of at least 0.05 (atom %). As will be recognized by those skilled in the art after a complete reading of the present application, the top electrode  206  for the RRAM device  200  may be manufactured using a “gate-first” or a “gate-last” technique, both of which are well known to those skilled in the art. For example, with respect to the illustrative embodiment depicted in  FIG. 2C , the bottom electrodes  208 A and  208 B may be formed while a sacrificial material, e.g., polysilicon, acts as a placeholder material during the fabrication of the two bottom electrodes  208 A,  208 B. Thereafter, using typical “gate-last” techniques, the sacrificial material may be removed and the final material, e.g., titanium nitride, for the top electrode  206  may be formed. Of course, as will be appreciated by those skilled in the art after reading the present application, the RRAM device  200  depicted in  FIG. 2C  could also be employed with a single bottom electrode  208 , like that depicted in  FIG. 2A . Additionally, the substrate  202  depicted in  FIGS. 2A-2C  is illustrative in nature, as the RRAM devices  200  disclosed herein can be formed with other substrate configurations, e.g., an SOI (silicon-on-insulator) substrate. 
     One illustrative method of forming the RRAM devices disclosed herein will now be discussed with reference to  FIGS. 3A-3C .  FIG. 3A  depicts a point of fabrication wherein the isolation structures  204  have been formed in the substrate  202 . The isolation structures  204  may be formed using techniques and methods commonly employed in the semiconductor manufacturing industry. A schematically depicted ion implantation process  209  is performed to effectively damage the lattice structure of the substrate  200 , thereby making it easier for refractory metal material to migrate so as to form the bottom electrode  208 , as described more fully below. The dopant material employed in the ion implantation process  209  may vary depending on the particular application. In one example, the dopant material may be arsenic, and it may be implemented at a dopant dose of 1e 15 -1e 17  ions/cm 2  and at an energy level of approximately 100-500 keV. Of course, other dopant materials, such as germanium, silicon, etc., may be implanted instead of arsenic. The depth of maximum concentration of the implanted ions may be approximately 50-200 nm. In this particular example, the ion implantation process  209  is performed prior to the formation of the insulating layer(s)  210 , the top electrode  206  and the spacers  212 . Of course, if desired, the ion implantation process  209  could be performed after the aforementioned structures are formed. 
     Next, as shown in  FIG. 3B , the insulation layer  210 B, the top electrode  206 A and the spacers  212  are formed. As noted previously, the interfacial layer  210 A may not be required in all applications, and it is omitted in  FIGS. 3B-3C . The top electrode  206 A is made from a material that, in this illustrative embodiment, can be converted to a metal silicide to form the final top electrode  206  depicted in  FIGS. 2A-2B . For example, the top electrode  206 A may be made of polysilicon. 
     Next, as shown in  FIG. 3C , a layer of refractory metal  220  is formed above the RRAM device  200 . The composition, thickness and method of manufacture of the layer of refractory metal  220  may vary depending upon the application. In one illustrative embodiment, the refractory metal layer  220  may be nickel and it may be formed using an LPCVD (low pressure chemical vapor deposition) process. Ultimately, a heat treatment will be performed to form the metal silicide regions that constitute the top electrode  206  and the bottom electrode  208  or bottom electrodes  208 A,  208 B, depending upon the desired final configuration of the RRAM device  200 . In one illustrative example, the heat treatment process may be a rapid thermal anneal process performed at a temperature of approximately 450° C. for a duration of approximately 30-90 seconds. As it relates to the formation of the metal silicide material, there is a relationship between the thickness of the refractory metal layer, the amount of silicon consumed and the thickness of the resulting metal silicide material. More specifically, in one illustrative example, 1.8 Å of nickel consumes approximately 1.0 Å of silicon to produce approximately 2.2 Å of nickel silicide. 
     The thickness of the layer of refractory metal  220  and/or the parameters of the heat treatment process, e.g., temperature and/or duration, may be varied to achieve either the single-bit construction (a single bottom electrode  208 ) depicted in  FIG. 2A  or the dual-bit construction (two bottom electrodes  208 A,  208 B) depicted in  FIG. 2B . The thickness of the layer of refractory metal  220  may depend upon the width of the top electrode  206  (the “width” corresponding to the gate length of a similarly configured transistor). In one illustrative embodiment, for example, for a top electrode width of approximately 100 nm, to achieve the single electrode  208  depicted in  FIG. 2A , the layer of refractory metal  220  may have a thickness of approximately 50-10 nm, while to achieve the dual electrode structures  208 A,  208 B depicted in  FIG. 2B , the thickness of the layer of refractory metal  220  may be approximately 20-30 nm. Additionally, the duration of the heat treatment process may also be varied to achieve the desired single-bit or dual-bit construction. For example, for a single-bit construction, as shown in  FIG. 2A , the duration of the heat treatment may be approximately 70-90 seconds, while the duration of the heat treatment process for the dual-bit construction depicted in  FIG. 2B  may be approximately 40-60 seconds. In one embodiment, the temperature of the heat treatment process is the same, i.e., approximately 450° C., although the temperature may be varied as well in achieving the desired single-bit or dual-bit construction of the RRAM device  200 . 
       FIGS. 4A-4C  depict an illustrative gate-last approach for formation of an illustrative dual-bit configured RRAM device  200  disclosed herein. As shown in  FIG. 4A , the device is depicted at the point in fabrication wherein the bottom electrodes  208 A,  208 B, the insulation layer  210 , a sacrificial top electrode  222 , and sidewall spacers  212  have been formed. In this illustrative embodiment, a protective layer  224 , e.g., an oxide layer, was formed over the sacrificial top electrode  222  during the fabrication process so that the refractory metal layer (not shown in  FIG. 4A ) would not contact the sacrificial top electrode  222 . After the heat treatment process is performed to form the bottom electrodes  208 A,  208 B, and after an acid wash process is performed to remove any unreacted portions of the refractory metal layer (not shown in  FIG. 4A ), a layer of dielectric material  230  is formed above the RRAM device  200 . 
     Thereafter, as shown in  FIG. 4B , an illustrative chemical mechanical polishing (CMP) process is performed to expose the protective layer  224 . Next, an etching process is performed to remove the protective layer  224  and thereby expose the sacrificial top electrode  222 . Thereafter, the sacrificial top electrode  222  is removed by performing an etching process and the desired final top electrode  206  is formed, as shown in  FIG. 4C . In one illustrative example, the top electrode  206  depicted in  FIG. 4C  is made from a metal-containing material such as titanium nitride. In one illustrative example, the RRAM device  200  depicted in  FIG. 4C  can be achieved by performing one or more etching processes to remove the sacrificial top electrode  222 , and any underlying layers that are desired to be removed. Thereafter, if necessary, the insulating layer  210  may be formed in the cavity left when the sacrificial top electrode  222  is removed, and a deposition process may be performed to over-fill the cavity with the desired material for the final top electrode  206 , e.g., titanium nitride. Thereafter, a CMP process may be performed to remove the excess material from above the insulating layer  230 . 
     Various memory cell layouts are possible with the RRAM devices  200  disclosed herein. For example, using the illustrative dual-bit configuration RRAM device  200  depicted in  FIG. 2B , a schematically depicted 2-bit cell may be formed, as depicted in  FIG. 5A  (bits A and B). Each of the bottom electrodes  208 A,  208 B may be connected to well-known read/write circuitry that enables each of the bottom electrodes  208 A,  208 B to be coupled to ground or to permit the bottom electrodes  208 A,  208 B to float. When a voltage is applied to the top electrode  206 , current will only flow to a bottom electrode  208  that is grounded. Thus, by selectively coupling the desired one of the bottom electrodes  208 A,  208 B to ground, each individual bit may be accessed with well-known read/write circuitry.  FIG. 5B  depicts an illustrative layout for an illustrative 4-bit cell (bits A, B, C and D). In  FIG. 5B , the top electrodes  206  are coupled together so only a single contact (not shown) need be established to the combined top electrode structure. To access the desired bit, the read/write circuitry is used to couple the desired bit, e.g., bit “C,” to ground.  FIGS. 5C-5D  depict illustrative layouts for an illustrative 6-bit cell (with cells A-F) and an illustrative 9-bit cell (with cells A-I) configuration. 
     The operation of the RRAM devices  200  disclosed herein will now be discussed with reference to  FIGS. 6A-6D . To facilitate discussion, an enlarged and simplified version of the single-bit configuration of the RRAM device  200  in  FIG. 2A  will be employed in  FIGS. 6A-6B .  FIGS. 6C-6D  depict electrical characteristics of the illustrative RRAM devices  200  disclosed herein. 
     In general, a voltage (V g ) is applied to the top electrode  206  of the RRAM devices  200  disclosed herein to establish or create one or more schematically depicted conductive paths  240  between the top electrode  206  and the bottom electrode  208 , which is grounded. More specifically, in one example, by applying a positive voltage (V g ) of approximately 3 volts to the top electrode  206 , the insulation layer  210  breaks down, and one or more of the conductive paths  240  are created. The conductive paths  240  can be switched off or closed by applying a reverse voltage of the opposite polarity, e.g., −3 volts, to the top electrode  206 . 
     The exact number, size and location of the conductive paths  240  created during this process are unknown, and they may vary depending upon the particular materials used and the voltages applied to the top electrode  206 . Moreover, such characteristics of the conductive paths  240  may vary depending upon the particular application. The exact mechanism by which the conductive paths  240  are established and broken is still a matter of investigation. See, e.g., W. H. Liu et al., “Observation of Switching Behaviors in Post-Breakdown Conduction in NiSi-gated Stacks,” June 2009, and N. Raghavan et al., “Unipolar Recovery of Dielectric Breakdown in Fully Silicided High-K Gate Stack Devices and its Reliability Implications,” published online Apr. 5, 2010, both of which are hereby incorporated by reference in their entirety. One possible mechanism that explains the formation of the conductive paths  240  is the movement of negatively charged oxygen ions from the insulation layer  210  to the top electrode  206  when a positive voltage is applied to the top electrode  206 . These negatively charged oxygen ions are stored in the top electrode  206  during this process. When a negative voltage is applied to the top electrode  206 , the negatively charged oxygen ions are effectively repelled from the top electrode  206  and driven back toward the insulating layer  210 , thereby breaking the conductive path  240 . Another possible mechanism for establishment of the conductive path(s)  240  is that one or more metal filaments are formed that conductively connect to the top electrode  206  and the bottom electrode  208 . 
       FIG. 6C  is a graph depicting the electrical characteristics of a single-bit embodiment of the RRAM device disclosed herein. The application of the positive and negative voltages (V g ) to the top electrode  206  may generally be referred to as bipolar switching. As can be seen in  FIG. 6C , when the applied voltage (V g ) to the top electrode  206  is increased to about +3 volts, one or more of the conductive paths  240  is established, and the current (I g ) between the top electrode  206  and the bottom electrode  208  suddenly increases by least one or two orders of magnitude. When a negative voltage of about −3 volts is applied to the top electrode  206 , the conductive path(s)  240  are broken, or at least they are not longer conductive to the same magnitude as before, and the current (I g ) drops by one or two orders of magnitude. The change in the magnitude of the current (I g ) when the conductive paths  240  are established as compared to when the conductive paths  240  are closed or not as conductive as before can be measured and used to represent logical highs (a “1”) and logical lows (a “0”) in the RRAM device  200 . 
       FIG. 6D  is another simplified chart depicting the electrical characteristics of a “single-bit” configured RRAM device  200 , wherein a unipolar switch technique may be employed with the RRAM device  200  disclosed herein. In this example, electrical data for three different stages is depicted: formation (“o”), set (“+”) and reset (“Δ”). The “formation” data represents the initial formation of the conductive path(s)  240  wherein the device goes from an effectively non-conducting state at gate voltages less than 0.05 volts to a conductive state at gate voltages between 2-3 volts. The “reset” data reflects the situation where joule heating from leakage currents causes the conductive path(s)  240  to rupture or at least become less conductive, as reflected by the change in the device from a highly conductive state to a lower conductive state (as reflected by the drop in the gate current). The “set” data reflects the situation where the conductive path(s)  240  of the device are re-established as the device goes from a low conductive state to a highly conductive state when the gate voltage approaches or exceeds approximately 3 volts. 
     The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. For example, the process steps set forth above may be performed in a different order. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below.