Patent Publication Number: US-2006016459-A1

Title: High rate etching using high pressure F2 plasma with argon dilution

Description:
BACKGROUND OF THE INVENTION  
      Semiconductor products are generally produced by batch processing steps that use gases to deposit or selectively etch semiconductor layers on substrates within in a vacuum chamber. Most of the chemical by-products and unused reagents from these deposition and etch processes are exhausted from the chamber by a vacuum pump. However, some residue unavoidably deposits and accumulates on the interior walls of the vacuum chamber. To ensure high device yield and quality, the residue must be periodically removed from the chamber. Usually the residue is removed using gas mixtures containing a fluorine-containing cleaning gas, the cleaning gas usually being diluted with argon or helium.  
      One known method of cleaning a vacuum deposition chamber is to utilize remote plasma source (RPS) technology operating with NF3 plasma. A standard RPS apparatus and method are shown in  FIG. 1 , wherein a flow of NF3 and diluent gas  10 , (Ar shown), enters an RPS  20 , (such as an MKS Instruments Astron® series that uses low-field-toroidal plasma technology to efficiently dissociate the NF3 input gas to produce atomic fluorine). The atomic fluorine  30  is transported to the vacuum chamber  40  where it chemically reacts with the unwanted residue on the interior walls of the vacuum chamber to form volatile compounds  50  that are removed from the chamber via the vacuum system, comprising a pump  60  and exhaust stream  70 .  
      When using the apparatus shown in  FIG. 1 , the plasmas can be initially ignited with pure argon and then NF3 is added in a flow range from 1-10 slpm. The pressure is maintained constant, and the cleaning is conducted at a constant flow condition involving the NF3 flow plus various levels of Argon. Normally the argon flow rate is less than the NF3 flow rate.  
      One disadvantage of using NF3 is that NF3 plasma can not be ignited above approximately 20 Torr, in a typical 10-kilowatt RPS system. In commercial applications using NF3, the RPS system is normally operated at less than 5 Torr, and usually in the range of 0.5 to 3 Torr, although it has been suggested in the literature that up to 10 Torr pressure may be used. Therefore, etch rates for the commercial operations described above are limited to relatively low levels as will be described in more detail below.  
      Further, NF3 is costly. Therefore, particularly in large deposition systems such as those used for 300 mm wafer processing or flat panel display manufacturing, that require a high consumption of NF3, the cleaning process may contribute a significant proportion to the cost of the final device.  
      There remains a need in the art for improvements in the field of PECVD chamber cleaning.  
     SUMMARY OF THE INVENTION  
      The present invention provides for the use of F2 in the process of deposition chamber cleaning.  
      The present invention further provides a method of using F2 under high pressure and in the presence of a diluting gas to perform deposition chamber cleaning.  
      The present invention also provides a method for using F2 under high pressure to perform substrate etching or wafer thinning procedures at a high etch rate.  
     BRIEF DECRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic representation of a RFP cleaning process and apparatus known in the prior art.  
       FIG. 3  is a schematic showing a flow scheme and sequence for remote plasma clean of a deposition chamber in accordance with the present invention.  
       FIG. 4  is a schematic showing a flow scheme and sequence for in-situ plasma clean of a deposition chamber in accordance with the present invention.  
       FIG. 5  is a schematic showing a flow scheme and sequence for high rate wafer etching in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The present invention provides for the use of F2 in the process of deposition chamber cleaning which is especially effective if operated under high pressure conditions. F2 from electrolytic generators (such as BOC Edwards Generation-F series) is a low-cost alternative to NF3, especially for use in deposition chamber cleaning applications. In accordance with the present invention, very high etch rates can be accomplished by using F2 under high pressure. In addition, the present invention discovered that by using F2 that is diluted with a high amount of argon, even higher etch rates can be achieved. The results of some tests according to the present invention are shown in Table 1 and Table 2 below.  
      In accordance with the present invention, it was discovered that F2 has a much wider operating window than NF3 in a typical RPS system. For example, an MKS Astronex RPS has a specified limit of power draw of ˜8 kW. This limit is reached when using NF3 at 6 slm and 17.5 Torr. In contrast, by using F2, this limit was not reached even operating at 18 slm and 100 Torr. In etch rate tests where stoichiometrically equivalent flow rates, similar system pressures and no Ar diluent were used, there was no statistically significant difference in the etch rates when using NF3 or F2. This is shown from the experimental data set forth in Table 1 below.  
               TABLE 1                          Etch Rate Comparison                         Etch Rate (nm/s)                                     Parameters   Mean   Std Deviation   N                                                 3 slm F2, 1.8 Torr   8.33   1.079   8           2 slm NF3, 1.8 Torr   9.03   1.030   7           3 slm F2, 3 Torr   10.55   1.176   8           2 slm NF3, 3 Torr   12.06   5.290   8           3 slm F2, 10 Torr   24.68   1.857   8           2 slm NF3, 10 Torr   27.70   4.180   8                      
 
      The present invention found that when chamber pressures were raised, the etch rate for both F2 and NF3 was higher, but the etch rate for F2 was significant higher than for NF3. In particular, when using NF3, a maximum etch rate of about 80 nm/sec was observed, whereas, by using F2 at pressures up to about 170 Torr, it is possible to achieve etch rates of more than 250 nm/sec.  
      In addition, the effect of using F2 at different argon dilution was tested. It was found that by increasing the argon content, higher etch rates are possible in accordance with the present invention. Table 2 below includes some of the experimental results.  
               TABLE 2                          Etch Rate Using F2 At Different Argon Dilutions                                     Chamber           F2 Flow (slpm)   Ar Flow (slpm)   Pressure (Torr)   Etch Rate (nm/sec)                                     3   2   10   22.21       3   4   20   28.90       3   6   5   15.98       3   6   27.5   59.69       3   6   27.5   67.51       3   6   27.5   56.68       3   6   27.5   46.27       3   6   27.5   42.43       3   6   30   40.97       3   6   50   82.17       3   8   40   96.76       3   9   20   143.41       3   10   50   86.65       3   12   5   17.52       3   12   20   49.45       3   12   27.5   50.30       3   12   50   140.31       3   12   51   191.79       3   13.4   80   185.60       3   13.4   90   159.83       3   14   60   235.09       3   14   70   249.38       3   14   70   134.27       3   14   70   148.56       3   14   75   252.83       3   14   80   161.06       3   14   80   171.97       3   14   80   116.97       3   14   85   224.53       3   14   90   253.79       3   14   90   424.05       3   14   90   211.02       3   14   95   180.84       3   15   90   168.51       3   16   70   152.45       3   16   80   10.61       3   16   90   88.39       3   16   90   80.58       3   16   100   112.23       3   16   108   66.83       3   18   80   62.12       3   18   90   124.38       3   18   100   189.98       3   18   110   129.84       3   18   120   199.01       3   18   130   125.78       3   20   80   92.97       3   20   90   77.59       3   20   100   134.72       3   20   110   115.52       3   20   120   91.09       3   20   130   131.03       3   20   140   183.73       3   20   150   138.29       3   20   160   111.30       3   20   170   287.97       3   20   180   154.85                  
 
      It was further discovered that F2 can maintain a plasma, if a proper sequence is followed, up to 170 Torr, and it appears that even higher pressures may be possible. The increased effectiveness of using F2 as defined by the present invention was shown by conducting tests on an interferometer. The time for how long it took to remove 335 nanometers of material from a wafer was measured for three different conditions. First a standard process running at 2 slpm of NF3 with no argon dilution and at approximately 3 Torr produced an etch rate of about 11 nm/sec. Next a process running at 2 slpm of NF3 at increased pressure of 20 Torr and an argon flow of 6 slpm argon produced an etch rate of about 78 nm/sec. Finally, the much faster etch rates that can be achieved in accordance with the present invention were found but running a process at 3 slpm of F2 at 170 Torr and argon flow of 20 slpm (e.g. about 13% F2 in argon), producing an etch rate of about 288 nm/sec.  
      While the above clearly shows the increased effectiveness of using F2 in deposition cleaning operations, the present invention further provides for a method of using F2 under high pressure to perform substrate etching or wafer thinning procedures at a high etch rate.  
      The method of introduction of the F2 to the etching chamber is an important consideration. In particular, the direct introduction of F2 to a heated chamber can cause damage to the chamber components because of the reactivity of the F2 gas. Therefore, it is necessary to cool the chamber prior to the introduction of the F2. This slows down processing time and adds to the overall cost of production.  
      One method of overcoming these problems is to carry out a process for thermally activating the F2 comprising the steps of: (a) reacting the F2 gas and preheated inert gas to form a gaseous mixture; and (b) passing the gaseous mixture to an etching chamber to carry out the desired etching. The reacting step can be carried out in a mixing chamber to create the gaseous mixture and then further in a reaction chamber to ensure adequate contact between F2 gas and he heated inert gas. The inert gas is preferably argon, but may be other gases, such as, nitrogen, helium, and mixtures thereof.  
      A system for carrying out the above process comprises; a mixing chamber capable of reacting the F2 gas and the preheated inert gas to form a gaseous mixture; a reaction chamber in gaseous communication with the mixing chamber for ensuring adequate contact between the F2 gas and the heated inert gas, such system being in gaseous communication with a process chamber.  
      In operation, the preheated inert gas is typically fed into the mixing chamber via any inert gas feed tube containing a packed bed of thermally conductive material, e.g. a finely divided metal such as nickel, Hastelloy, stainless steel, and combinations thereof. Copper and aluminum alloys may also be used if there is no concern of contamination for the particular process. The inert gas feed tube may be fabricated from any suitable material capable of carrying the inert gas, for example, nickel, Hastelloy, stainless steel, and combinations thereof. The inert gas feed tube may have a diameter of ½ inch to about 2 inches, preferably, about ½ inch to about 1 inch, and more preferably about ¾ inch. The flow rate for the inert gas is about 1 slpm to about 20 slpm, preferably 1 slpm to about 10 slpm, and more preferably about 2 slpm to about 6 slpm. The inert gas feed tube may be surrounded by a heater that preheats the inert gas to the desired temperature, typically to a temperature about 400° C. to about 650° C. The heater may be any suitable heater, for example, electrical resistance heaters, radiant heaters, gas fired combustion heaters, and any combinations thereof.  
      The F2 gas is introduced to the mixing chamber via a F2 inlet tube. The F2 inlet tube may be fabricated from any suitable material capable of delivering the F2 gas, for example, sapphire, dense aluminum oxide, nickel, Hastelloy, and combinations thereof. The F2 inlet tube has a diameter of about ¼ inch to about ¾ inch, preferably, about ¼ inch to about ½ inch. The flow rate for the F2 gas is about 1 slpm to about 20 slpm, preferably about 2 slpm to about 6 slpm.  
      The heated inert gas and F2 gas flow into the mixing chamber where they form a gaseous mixture. This mixed gas stream then flows to the reaction chamber that is constructed of a highly inert material such as nickel, dense aluminum oxide, sapphire, aluminum fluoride, calcium fluoride, or combinations thereof. Preferably the reaction chamber is constructed of sapphire, which is a crystal aluminum oxide material that forms an aluminum fluoride passivation layer. It is important to use inert materials for the construction of the reaction chamber to avoid hazardous operation because of the reactivity of the F2. In one embodiment, a sapphire inner tube is sealed to the nickel outer tube that allows for the use of normal elastomer seals, such as Viton or Klarez seals, without risk of damage by the F2 gas. The reaction chamber is sized to ensure adequate contact between the F2 gas and the heated inert gas, for example, the reaction chamber has a diameter of about ½ inch to about 1 and ½ inches, preferably, about ½ inch to about 1 inch.  
      The mixture of F2 and high temperature inert gas, is then delivered through an outlet tube to the process chamber for use as a cleaning or etching agent. The outlet tube should be constructed from an inert material to avoid reaction with the thermally activated F2 gas. Suitable inert materials for the outlet tube include the same material used for constructing the reaction chamber, preferably sapphire inside a nickel tube.  
       FIG. 2  shows a flow scheme and sequence for remote plasma clean of a deposition chamber. Such cleaning may be carried out between runs for SiO2, silicon nitride, tungsten, tungsten nitride, tungsten silicide and polysilicon or the like. Similarly,  FIG. 3  shows a flow scheme and sequence for in-situ plasma clean of a deposition chamber.  FIG. 4  shows a flow scheme and sequence for high rate wafer etching that would benefit from using F2 under conditions of high pressure and high dilution using argon.  
      In greater detail, the system shown in  FIG. 2  includes an argon source  201 , a F2 source  202 , and an RPS  203 , connected to the process chamber  204  that is controlled using a vacuum pump  205 , valve  206  and pressure transducer  207 . One method for operation according to the present invention of the system shown in  FIG. 2  is as follows: 
          Initially, the RPS  203  is set at approximately 0.5-3 Torr on transducer  207  which controls valve  206  on vacuum pump  205  with an argon flow of 1-2 slpm from argon source  201 .     The plasma is then initiated by RPS  203  using pure argon.     F2 from F2 source  202  is then introduced and slowly ramped up from no flow at a rate no greater than 1 slpm increase per 5 seconds (i.e. ramping to 2 slpm should take at least 10 seconds). This helps the plasma stay lit while the fluorine is being added.     Argon flow rate is then increased to 10-20 slpm or between 3 times and 10 times the ultimate F2 flow for the particular process.     The pressure of process chamber  204  pressure is increased to between 30 and 300 Torr while maintaining plasma, using transducer  207  to control valve  206  on vacuum pump  205  during or after the ramp up of argon flow.     F2 flow is increased up to the full flow rate based on the size of the process chamber  204 , e.g. from 2-200 slpm.     The argon flow is maintained at 3 to 10 times the F2 flow rate.        

      By using the above sequence, the process chamber  204  will be cleaned of surface deposits. At the end of the cycle, the F2 flow is turned off, then the plasma current on RPS  203  is turned off and finally, argon flow is turned down. The production process can then continue.  
       FIG. 3  represents a system very similar to than shown in  FIG. 2 . However, where  FIG. 2  describes a system using a remote plasma source,  FIG. 3  describes a system for in-situ cleaning. In particular, the system of  FIG. 3  includes an argon source  301 , and a F2 source, connected to the process chamber  304  having electrodes  310  and  320 , the process chamber being controlled using a vacuum pump  305 , valve  306  and pressure transducer  307 . One method for operation according to the present invention of the system shown in  FIG. 3  is as follows: 
          Initially, the process chamber  304  is set at approximately 0.5-3 Torr on transducer  307  which controls valve  306  on vacuum pump  305  with an argon flow of 1-2 slpm from argon source  301 .     The plasma is then initiated by the electrodes  310  and  320  using pure argon.     F2 from F2 source  302  is then introduced and slowly ramped up from no flow at a rate no greater than 1 slpm increase per 5 seconds (i.e. ramping to 2 slpm should take at least 10 seconds). This helps the plasma stay lit while the fluorine is being added. Argon flow rate is then increased to 10-20 slpm or between 3 times and 10 times the ultimate F2 flow for the particular process.     The pressure of process chamber  304  pressure is increased to between 30 and 300 Torr while maintaining plasma, using transducer  307  to control valve  306  on vacuum pump  305  during or after the ramp up of argon flow.     F2 flow is increased up to the full flow rate based on the size of the process chamber  304 , e.g. from 2-200 slpm.     The argon flow is maintained at 3 to 10 times the F2 flow rate.        
      By using the above sequence, the process chamber  304  will be cleaned of surface deposits. At the end of the cycle, the F2 flow is turned off, then the plasma current on electrodes  310  and  320  is turned off and finally, argon flow is turned down. The production process can then continue.  
       FIG. 4  shows a system for carrying out high rate wafer etching. In greater detail, the system shown in  FIG. 4  includes an argon source  401 , a F2 source  402 , and an RPS  403 , connected to the process chamber  404  that is controlled using a vacuum pump  405 , valve  406  and pressure transducer  407 . One method for operation according to the present invention of the system shown in  FIG. 4  is as follows: 
          Initially, the RPS  403  is set at approximately 0.5-3 Torr on transducer  407  which controls valve  406  on vacuum pump  405  with an argon flow of 1-2 slpm from argon source  401 .     The plasma is then initiated by RPS  403  using pure argon.     F2 from F2 source  402  is then introduced and slowly ramped up from no flow at a rate no greater than 1 slpm increase per 5 seconds (i.e. ramping to 2 slpm should take at least 10 seconds). This helps the plasma stay lit while the fluorine is being added.     Argon flow rate is then increased to 10-20 slpm or between 3 times and 10 times the ultimate F2 flow for the particular process.     The pressure of process chamber  404  pressure is increased to between 30 and 300 Torr while maintaining plasma, using transducer  407  to control valve  406  on vacuum pump  405  during or after the ramp up of argon flow.     F2 flow is increased up to the full flow rate based on the size of the process chamber  404 , e.g. from 2-200 slpm.     The argon flow is maintained at 3 to 10 times the F2 flow rate.        
      By using the above sequence, a silicon wafer  410  supported on a temperature controlled heater/cooler  420 , in process chamber  404  will be etched or thinned until the desired thickness or layer removal is achieved. At the end of the cycle, the F2 flow is turned off, then the plasma current on RPS  403  is turned off and finally, argon flow is turned down. The production process can then continue.  
      The present invention provides several advantages for a variety of processes and can be carried out in a variety of equipment. One advantage of the present invention is that existing hardware can be used in the sequences described above, but at higher cleaning or etch rates. This means that for deposition chamber cleaning, the clean period, which is a nonproductive portion of the production cycle can be significantly reduced and therefore overall throughput of the equipment can be increased. Similarly, for wafer etch processes, the high etch rate accomplished by the present invention allows for an increase in the number of wafers per hour that can be processed and therefore a reduction in the overall cost processing and equipment.  
      Experimental Data  
      Experiment 1—RPS Operating Limits  
      A vacuum system was assembled consisting of a NW40 4 way cross, the bottom of which was connected to a NW 100 spool piece packed with nickel, a vacuum control valve, a NW100 flexible spool piece and a BOCE Edwards QDP 80/500 dry pump. Connected to the left inlet of the 4 way cross was the outlet of an MKS Astronex remote plasma source. On the inlet side of the Astronex was an MFC controlled source of Ar and either NF3 or F2. Installed on top of the 4 way cross was a pressure transducer and on the left hand side a viewport.  
      A controlled flow of Ar was established and the plasma ignited. A set flow rate of source gas (NF3 or F2) was established and the Ar flow discontinued. The pressure in the system was set with the vacuum control valve from base to some maximum value. At regular intervals and at planned pressures the power drawn from the RPS (kW) was measured and recorded with a Fluke Model 41B power analyzer. When pressures caused the maximum power of the RPS (10 kw) to be exceeded, the plasma was extinguished. This process was repeated for several flow rates and data was collected.  
      The RPS operating window (pressure, flow) for F2 was found to be much wider than that of NF3. The RPS specified power draw limit of 8 kW was not breached for F2 up to 18 slm and 100 Torr, wherein the limit is reached when using NF3 at 6 slm and 17.5 Torr. While this data for F2 represents higher pressure and flow conditions than current chamber clean practice, the wide parameters were investigated for their effect on cleaning and etch rate as further discussed below.  
      Experiment 2—Etch Rate Comparisons  
      The apparatus described in Experiment 1 was used, except that the 4 way cross was replaced with a 6 way cross. A removable, water-cooled, wafer chuck was installed on one port. The chuck was designed to hold small samples of wafers coated with 2 um SiO2. Installed in the opposite port was a viewport. A simple laser interferometer was directed through this port to measure the etch rate. Flows and pressure were manipulated as outlined below and etch rates were measured and recorded in the following manner. A laser of wavelength 670 nm strikes the SiO2 layer and reflects off the top of the layer and the bottom. Depending upon the phase of the reflected light the two beams will either reinforce or cancel each other. As the etch process causes the SiO2 layer to get thinner the signal from the detector will go through peaks and troughs. Measuring the time between peaks or troughs will give an etch rate in nm/s, with each wavelength representing a ˜335 nm change in thickness. Results of this experiment are shown in the Table 1 above.  
      As noted above, this experiment shows that there is no statistically significant difference in the etch rates when using NF3 or F2 where stoichiometrically equivalent flow rates, similar system pressures and no Ar diluent are used.  
      Experiment 3—Optimization  
      During optimization etch rates were compared in relative terms. The etch rate corresponding to 100% was set as the slowest etch rate observed for all runs, which corresponds to standard run #2 for NF3 where pressure was set to 5.5 Torr and Ar/NF3 ratio to 3. The results of the optimization experiments are shown in Tables 3, 4 and 5.  
               TABLE 3                          NF3 Optimization                                         Relative Etch       Standard Run   Ar/NF3 Ratio   Pressure (Torr)   Rate (% of run 2)                                     1   0   5.5   127.5       2   3   5.5   100       3   0   20   442.7       4   3   20   868.2       5   0   12.75   304.4       6   3   12.75   358.7       7   1.5   5.5   127       8   1.5   20   603.1       9   1.5   12.75   303.6       10   1.5   12.75   460.6       11   1.5   12.75   327.8       12   1.5   12.75   373.4       13   1.5   12.75   316.1                  
 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                   
               
               
                 F2 Optimization 
               
            
           
           
               
               
               
               
            
               
                   
                   
                   
                 Relative Etch 
               
               
                 Standard Run 
                 Ar/NF3 Ratio 
                 Pressure (Torr) 
                 Rate (% of run 2) 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 1 
                 0 
                 5 
                 209.5 
               
               
                 2 
                 4 
                 5 
                 185.3 
               
               
                 3 
                 0 
                 50 
                 361.2 
               
               
                 4 
                 4 
                 50 
                 1264.3 
               
               
                 5 
                 0 
                 27.5 
                 361.2 
               
               
                 6 
                 4 
                 27.5 
                 528.4 
               
               
                 7 
                 2 
                 5 
                 169.4 
               
               
                 8 
                 2 
                 50 
                 863.4 
               
               
                 9 
                 2 
                 27.5 
                 459.7 
               
               
                 10 
                 2 
                 27.5 
                 459.7 
               
               
                 11 
                 2 
                 27.5 
                 680.8 
               
               
                 12 
                 2 
                 27.5 
                 737.5 
               
               
                 13 
                 2 
                 27.5 
                 643.6 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                   
               
               
                 Optimal Etch Rates 
               
            
           
           
               
               
               
               
            
               
                   
                 Ar/Source 
                   
                   
               
               
                 Source Gas 
                 Gas Ratio 
                 Pressure (Torr) 
                 Relative Etch Rate 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 NF3 (optimal) 
                 3 
                 20 
                 825 
               
               
                 NF3 (RPS limit) 
                 3 
                 17.5 
                 679 
               
               
                 F2 (optimal) 
                 4 
                 50 
                 1208 
               
               
                   
               
            
           
         
       
     
      Two results for NF3 are shown in Table 5 because the optimal result for NF3 was obtained at a point where the RPS was operating beyond its specified limit. During optimization a statistical DOE process was followed. The system pressure and Ar diluent ratio at constant stoichiometrically equivalent flow rates were manipulated as seen in the Tables. The wide operating window for F2 was exploited so that the limits of the optimization variables were extended for F2 beyond those used for NF3. As can be seen in Table 4, optimal etch rates were found for both NF3 and F2 at relatively high pressures and high Ar diluent ratios. However, the optimal etch rate for F2 was at least 46% higher than that of NF3. At the specified limit of the RPS the optimal etch rate of F2 was at least 78% higher than that of NF3.  
      The above experiments were relatively narrow in scope. In other experiments noted above, pressures up to 170 Torr were shown to be possible for F2. It is believed that even higher pressures up to 300 Torr or more, may be possible and could increase etch rates even further. As noted above, the ratio of argon to F2 can be in the range of 3 to 1 up to 10 to 1. More preferably, a ratio of 4 to 1 up to 8 to 1 of Ar to F2 may be used in accordance with the present invention. Higher ratios may also be possible and useful. The most preferred parameters for the present invention are currently believed to be pressures greater than 20 Torr and ratios of Ar to F2 of more than 4 to 1.  
      The present invention provides for the use of F2 under high pressure and at high dilution with a gas such as Ar, for purposes of deposition chamber cleaning. In addition, the very high etch rates that can be achieved in accordance with the present invention may be used for surface etching operations.  
      It is anticipated that other embodiments and variations of the present invention will become readily apparent to the skilled artisan in the light of the foregoing description and examples, and it is intended that such embodiments and variations likewise be included within the scope of the invention as set out in the appended claims.