Abstract:
A chemical vapor deposition reactor is provided. The chemical vapor deposition reactor includes a deposition chamber, a substrate within the deposition chamber, at least two inlet ports extending into the deposition chamber for supplying a first and a second gases to the deposition chamber respectively and a particle source for supplying a plurality of solid particles to the deposition chamber. The first gas reacts with the second gas to form a film incorporating the plurality of solid particles upon the substrate. Films with composition varying across the growth direction are produced by the chemical vapor deposition reactor without the use of mask layers.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a chemical vapor deposition reactor, especially to a chemical vapor deposition reactor with a source of solid particles (CVD-SP).  
       BACKGROUND OF THE INVENTION  
       [0002]     A chemical vapour deposition (CVD) reactor is commonly used to form a film layer on a chip by the reactor in which a reagent gas reacts to be in a solid phase. After years of improvement, CVD has become the main solution film-forming method among the semiconductor process. The films needed in the semiconductor process, conductor, semiconductor or dielectric, can be formed by CVD.  
         [0003]     The conventional CVD reactor allows forming deposition of solid phase films of various structures including epitaxial crystalline, epitaxial polycrystalline, non-epitaxial polycrystalline and amorphous ones. Besides, CVD reactors allow forming deposition of solid phase films with layered structures according to the cases of U.S. Pat. Nos. 6,645,302 and 6,726,767. The thickness of the layers and their composition can be controlled via variation of the reactant gas flows and temperature of the substrate. Thus, CVD reactors allow obtaining planar structures with parameters varying in one direction which is the growth direction. To obtain non-planar structures in CVD process, addition operations of mask layer deposition and window-opening must be applied according to cases of U.S. Pat. Nos. 5,418,183 and 6,728,289.  
         [0004]     The need for mask deposition makes the growth process of non-planar structures complicated and expensive.  
         [0005]     It is impossible for the traditional chemical vapor deposition reactor to form films with composition varying across the growth direction without the use of mask layers.  
       SUMMARY OF THE INVENTION  
       [0006]     Hence, for overcoming the mentioned drawbacks in the prior art, the main purpose of the present invention provides a chemical vapor deposition reactor. Films with composition varying across the growth direction are produced by the provided chemical vapor deposition reactor without the use of mask layers.  
         [0007]     According to one aspect of the present invention, a chemical vapor deposition reactor is provided. The chemical vapor deposition reactor includes a deposition chamber, a substrate within the deposition chamber, at least two inlet ports extending into the deposition chamber for supplying a first gas and a second gas to the deposition chamber respectively and a particle source for supplying a plurality of solid particles to the deposition chamber. The first gas reacts with the second gas to form a film incorporating the plurality of solid particles upon the substrate.  
         [0008]     Preferably, the deposition chamber is arranged vertically or horizontally.  
         [0009]     Preferably, the deposition chamber is made of a quartz.  
         [0010]     Preferably, the substrate is made of one of a quartz and a sapphire.  
         [0011]     Preferably, the chemical vapor deposition reactor further includes a heater for heating the deposition chamber to a temperature at which the first gas reacts with the second gas.  
         [0012]     Preferably, the heater is an external heater disposed on the deposition chamber.  
         [0013]     Preferably, the heater is an internal heater disposed in the deposition chamber.  
         [0014]     Preferably, the first gas is one of GaCl and Ga(CH) 3 ) 3  (TMG).  
         [0015]     Preferably, the second gas is NH 3 .  
         [0016]     Preferably, the first gas and the second gas are further diluted with N 2  and H 2  respectively.  
         [0017]     Preferably, the particle source supplies the plurality of solid particles through a tube into the deposition chamber.  
         [0018]     Preferably, the particle source is a container disposed in the deposition chamber.  
         [0019]     Preferably, the chemical vapor deposition reactor further includes a piezoelectric driver electrically connected to the container for disturbing the plurality of solid particles.  
         [0020]     Preferably, the plurality of solid particles are ones of SiO 2  and a mixture of InGaN and AlGaN.  
         [0021]     Preferably, the film further includes a micro-structure or a nano-structure.  
         [0022]     According to another aspect of the present invention, a chemical vapor deposition reactor with a solid particle source is provided. As  FIG. 1  shows, the chemical vapor deposition reactor includes a horizontal reaction tube  11 , an external furnace  12 , a substrate  13 , an input tube  14  for supplying a first reagent gas, an input tube  15  for supplying a second reagent gas, an input tube  16  for supplying solid particles, a first reagent gas flow  17 , a second reagent gas flow  18  and a solid particle flow  19 .  
         [0023]     In comparison with the traditional chemical vapor deposition reactor, the advantage of the present invention is to grow films of composite material and device structures with composition varying across the rowing direction without the use of mask layers.  
         [0024]     The present invention also provides a method of growing films of novel micro-composite and nano-composite materials and device structures with new physical properties and better structural quality.  
         [0025]     The foregoing and other features and advantages of the present invention will be more clearly understood through the following descriptions with reference to the drawings, wherein: 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]      FIG. 1  is a side view schematically showing a horizontal chemical vapor deposition reactor with an external furnace and a solid-particle-input tube according to the first embodiment of the present invention, which performs a GaN layer by HVPE;  
         [0027]      FIG. 2  is a side view schematically showing the GaN films deposited by means of the reactor of  FIG. 1 ;  
         [0028]      FIG. 3  is a side view schematically showing a vertical chemical vapor deposition reactor with an external furnace and a solid-particle-input tube according to the second embodiment of the present invention, which performs a GaN layer by HVPE;  
         [0029]      FIG. 4  is a side view schematically showing a horizontal chemical vapor deposition reactor with an internal furnace and a solid-particle-input tube according to the third embodiment of the present invention, which performs a GaN layer by MOVPE;  
         [0030]      FIG. 5  is a side view schematically showing a horizontal chemical vapor deposition reactor with an internal furnace and a solid particle container above the substrate according to the forth embodiment of the present invention, which performs a GaN layer by MOVPE;  
         [0031]      FIG. 6  is a side view schematically showing a horizontal chemical vapor deposition reactor with an internal furnace and a solid-particle-input tube according to the fifth embodiment of the present invention, which performs a GaN layer by MOVPE;  
         [0032]      FIG. 7  is a side view schematically showing the GaN films deposited by means of the reactor of  FIG. 6 ;  
         [0033]      FIG. 8  is a side view schematically showing a horizontal chemical vapor deposition reactor with an internal furnace and a solid particle container according to the sixth embodiment of the present invention, which performs a GaN layer by MOVPE;  
         [0034]      FIG. 9  is a side view schematically showing a horizontal chemical vapor deposition reactor with an internal furnace and a solid particle container below the substrate according to the seventh embodiment of the present invention, which performs a GaN layer by MOVPE; and  
         [0035]      FIG. 10  is a side view schematically showing a horizontal chemical vapor deposition reactor with an internal furnace and a solid particle container below the substrate according to the eighth embodiment of the present invention, which performs a GaN layer by MOVPE. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0036]      FIG. 1  is a side view schematically showing a horizontal chemical vapor deposition reactor according to the first embodiment of the present invention. In this embodiment, the growth of high quality GaN layers is achieved by hydride vapour phase epitaxy (HVPE). The horizontal chemical vapor deposition reactor includes a quartz horizontal tube  11 , an external furnace  12 , a sapphire substrate  13 , an input tube  14  for supplying a mixture of the first reagent gas GaCl diluted with HCl, N 2  and H 2 , an input tube  15  for supplying a mixture of the second reagent gas NH3 diluted with N 2  and H 2  and an input tube  16  of the source of solid particles of SiO 2 .  
         [0037]     The size of SiO 2  solid particles (d) is in the range of 10 −7 ˜10 −3  cm. The small size of solid particles allows themselves to be carried by a carrying gas N 2  or H 2 .  
         [0038]     The reagent gas flows  17  and  18  form a reactive mixture in the vicinity of the sapphire substrate  13 . It causes the growth of a GaN film on the sapphire substrate  13  via the chemical reaction GaCl+NH 3 =&gt;GaN+HCl+H 2 . The SiO 2  solid particle flow  19  results in the physical absorption of SiO 2  particles on the surface of the growing GaN layer and the further incorporation of inert SiO 2  particles into the GaN film. Thus, the use of the source of SiO 2  solid particles in HVPE growth process allows growing GaN films with the inclusion of nano- or micro- particles of SiO 2 .  
         [0039]      FIG. 2  is a side view schematically showing the GaN films deposited by means of the reactor of  FIG. 1 . The incorporation into the GaN layer  21 , grown on a mismatched sapphire substrate  22  , of nano- or micro- particles  23  of SiO 2 , with the surface concentration n ≧1/d 1/2 , prevents the propagation of threading dislocation along the growth direction and allows improving the structural quality of the GaN films.  
         [0040]     Besides, the incorporation of SiO 2  particles into the GaN film in growth process allows growing novel micro-composite or nano-composite materials GaN/SiO 2 , which possessed a new physical property. In particular, the variation of the size and the concentration of SiO 2  solid articles in GaN layer allows adjusting its light scattering properties to a given wavelength of the light wave.  
         [0041]      FIG. 3  is a side view schematically showing a vertical chemical vapor deposition reactor according to the second embodiment of the present invention. In this embodiment, the growth of high quality GaN layers is achieved by HVPE. The vertical chemical vapor deposition reactor includes a quartz vertical tube  31 , an external furnace  32 , a sapphire substrate  33 , an input tube  34  for supplying a mixture of the first reagent gas GaCl diluted with HCl, N 2  and H 2 , an input tube  35  for supplying a mixture of the second reagent gas NH3 diluted with N 2  and H 2  and an input tube  36  of the source of solid particles of SiO 2 .  
         [0042]     The size of SiO 2  solid particles (d) is in the range of 10 −7 ˜10 −3  cm. The small size of solid particles allows themselves to be carried by a carrying gas N 2  or H 2 .  
         [0043]     The reagent gas flows  37  and  38  form a reactive mixture in the vicinity of the sapphire substrate  33 . It causes the growth of a GaN film on the sapphire substrate  33  via the chemical reaction GaCl+NH 3 =&gt;GaN+HCl+H 2 . The SiO 2  solid particle flow  39  results in the physical absorption of SiO 2  particles on the surface of the growing GaN layer and the further incorporation of inert SiO 2  particles into the GaN film. Thus, the use of the source of SiO 2  solid particles in HVPE growth process allows growing GaN films with the inclusion of nano- or micro- particles of SiO 2 .  
         [0044]     Similarly,  FIG. 2  is a side view schematically showing the GaN films deposited by means of the reactor of the side view of  FIG. 3 . The incorporation into the GaN layer  21 , grown on a mismatched sapphire substrate  22 , of nano- or micro- particles  23  of SiO 2 , with the surface concentration n≧1/d 1/2 , prevents the propagation of threading direction and allows improving the structural quality of the GaN films.  
         [0045]     Besides, the incorporation of SiO 2  particles into the GaN film in growth process allows growing novel micro-composite or nano-composite materials GaN/SiO 2 , which possessed a new physical property. In particular, the variation of the size and the concentration of SiO 2  solid articles in GaN layer allows adjusting its light scattering properties to a given wavelength of the light wave.  
         [0046]      FIG. 4  is a side view schematically showing a horizontal chemical vapor deposition reactor according to the third embodiment of the present invention. In this embodiment, the growth of high quality GaN layers is achieved by metal-organic vapour phase epitaxy (MOVPE). The horizontal chemical vapor deposition reactor includes a quartz horizontal tube  41 , an internal furnace  42 , a sapphire substrate  43 , an input tube  44  for supplying a mixture of the first reagent gas, Ga(CH 3 ) 3  (TMG) diluted with N 2  and H 2 , an input tube  45  for supplying a mixture of the second reagent gas NH3 diluted with N 2  and H 2  and an input tube  46  of the source of solid particles of SiO 2 .  
         [0047]     The size of SiO 2  solid particles (d) is in the range of 10 −7 ˜10 −3  cm. The small size of solid particles allows themselves to be carried by a carrying gas N 2  or H 2 .  
         [0048]     The reagent gas flows  47  and  48  form a reactive mixture in the vicinity of the sapphire substrate  43 . It causes the growth of a GaN film on the sapphire substrate  43  via the chemical reaction Ga(CH 3 ) 3 +NH 3 =&gt;GaN+3CH 4 . The SiO 2  solid particle flow  49  results in the physical absorption of SiO 2  particles on the surface of the growing GaN layer and the further incorporation of inert SiO 2  particles into the GaN film. Thus, the use of the source of SiO 2  solid particles in MOVPE growth process allows growing GaN films with the inclusion of nano- or micro- particles of SiO 2 .  
         [0049]     Similarly,  FIG. 2  is a side view schematically showing the GaN films deposited by means of the reactor of  FIG. 4 . The incorporation into the GaN layer  21 , grown on a mismatched sapphire substrate  22 , of nano- or micro- particles  23  of SiO 2 , with the surface concentration n≧1d 1/2 , prevents the propagation of threading dislocation along the growth direction and allows improving the structural quality of the GaN films.  
         [0050]     The incorporation of SiO 2  particles into the GaN film in growth process allows growing novel micro-composite or nano-composite materials GaN/SiO 2 , which possessed a new physical property. In particular, the variation of the size and the concentration of SiO 2  solid articles in GaN layer allows adjusting its light scattering properties to a given wavelength of the light wave.  
         [0051]      FIG. 5  is a side view schematically showing a horizontal chemical vapor deposition reactor according to the forth embodiment of the present invention. In this embodiment, the growth of high quality GaN layers is achieved by MOVPE. The horizontal chemical vapor deposition reactor includes a quartz horizontal tube  51 , an internal furnace  52 , a sapphire substrate  53 , an input tube  54  for supplying a mixture of the first reagent gas Ga(CH 3 ) 3  diluted with N 2  and H 2 , an input tube  55  for supplying a mixture of the second reagent gas NH3 diluted with N 2  and H 2  and an container  56  of the source of solid particles of SiO 2 . The container  56  is equipped with a piezoelectric driver  561 .  
         [0052]     The size of SiO 2  solid particles is in the range of d=10 −7 ˜10 −3  cm. An alterative voltage applied to the piezoelectric driver  561  causes the vibration of the container  56  of solid particles of SiO 2  and results in a gas flow of SiO 2  on the sapphire substrate  53 .  
         [0053]     The reagent gas flows  57  and  58  form a reactive mixture in the vicinity of the sapphire substrate  53 . It causes the growth of a GaN film on the sapphire substrate  53  via the chemical reaction Ga(CH 3 ) 3 +NH 3 =&gt;GaN +3CH 4 . The SiO 2  solid particle flow  59  results in the physical absorption of SiO 2  particles on the surface of the growing GaN layer and the further incorporation of inert SiO 2  particles into the GaN film. Thus, the use of the source of SiO 2  solid particles in MOVPE growth process allows growing GaN films with the inclusion of nano- or micro- particles of SiO 2 .  
         [0054]     Similarly,  FIG. 2  is a side view schematically showing the GaN films deposited by means of the reactor of  FIG. 5 . The incorporation into the GaN layer  21 , grown on a mismatched sapphire substrate  22 , of nano- or micro- particles  23  of SiO 2 , with the surface concentration n≧1/d 1/2 , prevents the propagation of threading dislocation along the growth direction and allows improving the structural quality of the GaN films.  
         [0055]     The incorporation of SiO 2  particles into the GaN film in growth process allows growing novel micro-composite or nano-composite materials GaN/SiO 2 , which possessed a new physical property. In particular, the variation of the size and the concentration of SiO 2  solid articles in GaN layer allows adjusting its light scattering properties to a given wavelength of the light wave.  
         [0056]      FIG. 6  is a side view schematically showing a horizontal chemical vapor deposition reactor according to the fifth embodiment of the present invention. In this embodiment, the growth of high quality GaN layers is achieved by MOVPE. The horizontal chemical vapor deposition reactor includes a quartz horizontal tube  61 , an internal furnace  62 , a sapphire substrate  63 , an input tube  64  for supplying a mixture of the first reagent gas Ga(CH 3 ) 3  (TMG) diluted with N 2  and H 2 , an input tube  65  for supplying a mixture of the second reagent gas NH3 diluted with N 2  and H 2  and an input tube  66  of a mixture of the source of solid particles InGaN and AlGaN.  
         [0057]     The size of solid particles InGaN and AlGaN (d) is in the range of 10 −7 ˜10 −3  cm. The small size of solid particles allows themselves to be carried by a carrying gas N 2  or H 2 .  
         [0058]     The reagent gas flows  67  and  68  form a reactive mixture in the vicinity of the sapphire substrate  63 . It causes the growth of a GaN film on the sapphire substrate  63  via the chemical reaction Ga(CH 3 ) 3 +NH 3 =&gt;GaN +3CH 4 . The InGaN and AlGaN solid particle flow  69  results in the physical absorption of InGaN and AlGaN particles on the surface of the growing GaN layer and the further incorporation of inert InGaN and AlGaN particles into the GaN film. Thus, the use of the source of InGaN and AlGaN solid particles in MOVPE growth process allows growing GaN films with the inclusion of nano- or micro- particles of InGaN and AlGaN.  
         [0059]      FIG. 7  is a side view schematically showing the GaN films deposited by means of the reactor of  FIG. 6 . The incorporation into GaN layer  71 , grown on the sapphire substrate  72 , of nano- or micro- particles  73  of InGaN and AlGaN, results in a composite GaN/AlGaN/InGaN films with laterally varied band gap.  
         [0060]     These films are novel micro-composite or nano-composite whose luminescence spectra can be adjusted via variation of the composition and izes of InGaN and AlGaN particles in the mixture entering the input tube  66  in  FIG. 6 .  
         [0061]      FIG. 8  is a side view schematically showing a horizontal chemical vapor deposition reactor according to the sixth embodiment of the present invention. In this embodiment, the growth of high quality GaN layers is achieved by MOVPE. The horizontal chemical vapor deposition reactor includes a quartz horizontal tube  81 , an internal furnace  82 , a sapphire substrate  83 , an input tube  84  for supplying a mixture of the first reagent gas Ga(CH 3 ) 3  diluted with N 2  and H 2 , an input tube  85  for supplying a mixture of the second reagent gas NH3 diluted with N 2  and H 2  and an container  86  of the source of solid particles of InGaN and AlGaN. The container  86  is equipped with a piezoelectric driver  861 .  
         [0062]     The size of solid particles InGaN and AlGaN (d) is in the range of 10 −7 ˜10 −3  cm. An alterative voltage applied to the piezoelectric driver  861  causes the vibration of the container  86  of solid particles of InGaN and AlGaN and results in a gas flow  89  of InGaN and AlGaN on the sapphire substrate  83 .  
         [0063]     The reagent gas flows  87  and  88  form a reactive mixture in the vicinity of the sapphire substrate  83 . It causes the growth of a GaN film on the sapphire substrate  63  via the chemical reaction Ga(CH 3 ) 3 +NH 3 =&gt;GaN+3CH 4 . The InGaN and AlGaN solid particle flow  89  results in the physical absorption of InGaN and AlGaN particles on the surface of the growing GaN layer and the further incorporation of inert InGaN and AlGaN particles into the GaN film. Thus, the use of the source of InGaN and AlGaN solid particles in MOVPE growth process allows growing GaN films with the inclusion of nano- or micro- particles of InGaN and AlGaN.  
         [0064]     Similarly,  FIG. 7  is a side view schematically showing the GaN films deposited by means of the reactor of  FIG. 8 . The incorporation into GaN layer  71 , grown on the sapphire substrate  72 , of nano- or micro- particles  73  of InGaN and AlGaN, results in a composite GaN/AlGaN/InGaN films with laterally varied band gap.  
         [0065]     These films are novel micro-composite or nano-composite whose luminescence spectra can be adjusted via variation of the composition and izes of InGaN and AlGaN particles in the mixture entering the input tube  66  in  FIG. 6 .  
         [0066]      FIG. 9  is a side view schematically showing a horizontal chemical vapor deposition reactor according to the seventh embodiment of the present invention. In this embodiment, the growth of high quality GaN layers is achieved by MOVPE. The horizontal chemical vapor deposition reactor includes a quartz horizontal tube  91 , an internal furnace  92 , a sapphire substrate  93 , an input tube  94  for supplying a mixture of the first reagent gas Ga(CH 3 ) 3  diluted with N 2  and H 2 , an input tube  95  for supplying a mixture of the second reagent gas NH3 diluted with N 2  and H 2  and an container  96  of the source of solid particles of SiO 2 . The container  96  is equipped with a piezoelectric driver  961 .  
         [0067]     The difference between this embodiment ( FIG. 9 ) and the forth embodiment ( FIG. 5 ) is that the sapphire substrate  93  is above the container  96 .  
         [0068]     The size of solid particles SiO 2  (d) is in the range of 10 −7 ˜10 −3  cm. An alterative voltage applied to the piezoelectric driver  961  causes the vibration of the container  96  of solid particles of SiO 2  and results in a gas flow  99  of SiO 2  on the sapphire substrate  93 .  
         [0069]      FIG. 2  is a side view schematically showing the GaN films deposited by means of the reactor of  FIG. 9 . The incorporation into the GaN layer  21 , grown on a mismatched sapphire substrate  22  , of nano- or micro- particles  23  of SiO 2 , with the surface concentration n≧1/d 1/2 , prevents the propagation of threading dislocation along the growth direction and allows improving the structural quality of the GaN films.  
         [0070]     The incorporation of SiO 2  particles into the GaN film in growth process allows growing novel micro-composite or nano-composite materials GaN/SiO 2 , which possessed a new physical property. In particular, the variation of the size and the concentration of SiO 2  solid articles in GaN layer allows adjusting its light scattering properties to a given wavelength of the light wave.  
         [0071]      FIG. 10  is a side view schematically showing a horizontal chemical vapor deposition reactor according to the eighth embodiment of the present invention. In this embodiment, the growth of high quality GaN layers is achieved by MOVPE. The horizontal chemical vapor deposition reactor includes a quartz horizontal tube.  101 , an internal furnace  102 , a sapphire substrate  103 , an input tube  104  for, supplying a mixture of the first reagent gas Ga(CH 3 ) 3  diluted with N 2  and H 2 , an input tube  105  for supplying a mixture of the second reagent gas NH 3  diluted with N 2  and H 2  and an container  106  of the source of solid particles of InGaN and AlGaN. The container  106  is equipped with a piezoelectric driver  1061 .  
         [0072]     The difference between this embodiment ( FIG. 10 ) and the sixth embodiment ( FIG. 8 ) is that the sapphire substrate  103  is above the container  106 .  
         [0073]     The reagent gas flows  107  and  108  form a reactive mixture in the vicinity of the sapphire substrate  103 . It causes the growth of a GaN film on the sapphire substrate  103  via the chemical reaction Ga(CH 3 ) 3 +NH 3 =&gt;GaN+ 3 CH 4 . The InGaN and AlGaN solid particle flow  109  results in the physical absorption of InGaN and AlGaN particles on the surface of the growing GaN layer and the further incorporation of inert InGaN and AlGaN particles into the GaN film. Thus, the use of the source of InGaN and AlGaN solid particles in MOVPE growth process allows growing GaN films with the inclusion of nano- or micro- particles of InGaN and AlGaN.  
         [0074]     Similarly,  FIG. 7  is a side view schematically showing the GaN films deposited by means of the reactor of  FIG. 10 . The incorporation into GaN layer  71 , grown on the sapphire substrate  72  , of nano- or micro- particles  73  of InGaN and AlGaN, results in a composite GaN/AlGaN/InGaN films with laterally varied band gap.  
         [0075]     These films are novel micro-composite or nano-composite whose luminescence spectra can be adjusted via variation of the composition and izes of InGaN and AlGaN particles in the mixture entering the input tube  106  in  FIG. 10 .  
         [0076]     The present invention provides a chemical vapor deposition reactor. A source of solid particles is added into the traditional deposition chamber. In the deposition chamber, the main reagent gases mix each other and react by MOVPE or HVPE under a proper temperature and a film including the solid particles is then formed on the substrate in the deposition chamber.  
         [0077]     The chemical vapor deposition reactor with the solid particle source allows the deposition of the host material film on the substrate from gas phase via a chemical reaction between reactant gases and the incorporation of particles of foreign materials into the film via physical absorption of particles on the surface of the growing film.  
         [0078]     The incorporation of particles into the host material growing from gas phase allows obtaining layers with composition varying across the growing direction.  
         [0079]     In comparison with the conventional chemical vapor deposition reactor, films with composition varying across the growth direction are produced by the chemical vapor deposition reactor without the use of mask layers. The structures of the films are of micro-composite or nano-composite with novel physical properties and better quality.  
         [0080]     While the invention has been described in terms of what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention needs not be limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded, with the broadest interpretation so as to encompass all such modifications and similar structures.