Abstract:
Disclosed is a capacitor for a semiconductor device and a method of fabricating such capacitors including the steps of providing a semiconductor substrate, forming a lower electrode on the semiconductor substrate, forming a Ta 1-x Al x O y N z  (0.01≦x≦05, 2≦y≦2.5, 0.01≦z≦0.1) dielectric layer on the lower electrode, and forming an upper electrode on the Ta 1-x Al x O y N z  dielectric layer so as to provide excellent electric characteristics as well as sufficient electric capacitance required for the proper operation of highly integrated semiconductor devices.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a semiconductor memory and, more particularly, to a capacitor in a semiconductor device and a method of fabricating a capacitor that provides both excellent electric characteristics and sufficient electric capacitance necessary for operation of the semiconductor device.  
           [0003]    2. Background of the Related Art  
           [0004]    As the degree of integration of memory products accelerates with the development of ever finer semiconductor imaging technology, unit cell areas have been greatly decreased and operating voltages have been decreased.  
           [0005]    Despite the decreasing cell area and the reduced voltage, the minimum charge capacitance required for memory device operation has remained on the order of at least 25 fF/cell to prevent soft error generation and refresh time reduction.  
           [0006]    In conventional DRAM capacitor utilizing a nitride/oxide (“NO”) layer structure as a dielectric, a lower electrode is constructed with a three-dimensional structure and/or with increased height, thereby increasing the effective surface area to provide sufficient charge capacitance.  
           [0007]    However, the extent to which a three-dimensional lower electrode can be employed to increase charge capacitance is limited as a result of process difficulties.  
           [0008]    Moreover, increasing the height of the lower electrode results in an increasing step difference between cell and peripheral circuit areas, eventually degrading the integration process after the wire formation due to an inability to achieve a sufficient depth of focus during subsequent exposure processes.  
           [0009]    Therefore, a capacitor having the NO structure of the conventional art fails to provide the charge capacitance required for a next generation DRAM device of over 256M memory cells.  
           [0010]    Lately, the development of Ta 2 O 5  capacitors, which use Ta 2 O 5  films having dielectric constants k typically ranging from 25 to 27, instead of NO films having dielectric constants ranging 4 to 5, are made so as to overcome some of the shortcomings of NO capacitors.  
           [0011]    In view of these aspects, a capacitor in a semiconductor device and a fabrication method thereof according to a conventional prior art method are explained with reference to FIGS.  1 - 3 .  
           [0012]    [0012]FIG. 1 shows a cross-sectional view of a capacitor using a Ta 2 O 5  film as a dielectric layer regarding a capacitor in a semiconductor device and fabrication method thereof according to a related art.  
           [0013]    [0013]FIG. 2 shows oxygen vacancies and carbon impurities existing in a Ta 2 O 5  dielectric film are removed by oxygen radicals during thermal treatment after formation of the Ta 2 O 5  dielectric film during the fabrication of a capacitor according to conventional prior art method.  
           [0014]    [0014]FIG. 3 is a SEM photo showing a cross-sectional view of a capacitor in which an N 2 O thermal treatment (oxidation treatment) process was performed after the deposition of a Ta 2 O 5  film to form a capacitor dielectric in a semiconductor device according to a conventional prior art method.  
           [0015]    Referring to FIG. 1, an insulating interlayer  3  is formed on a semiconductor substrate  1 . A portion of the semiconductor substrate  1  is exposed by patterning and etching the insulating interlayer  3 . A doped polysilicon layer is then deposited on the insulating interlayer  3  and the exposed semiconductor substrate  1 . A lower electrode  5  is then formed by patterning and etching the doped polysilicon layer.  
           [0016]    After a Ta 2 O 5  film  7  has been formed on an upper surface of the insulating interlayer  3  and on the exposed surfaces of the lower electrode  5 , an upper electrode  9  is formed by stacking TiN and doped polysilicon on the Ta 2 O 5  film  7  to complete the fabrication of the capacitor. Because the Ta 2 O 5  film  7  of the capacitor according to the conventional art methods, as represented in FIG. 2, has an unstable stoichiometric ratio, substitution type Ta atoms exist in the film as a result of the difference in the composition ratio between Ta and  0 .  
           [0017]    Namely, it is inevitable that substitution type Ta atoms having an oxygen vacancy state exist locally in the film due to the unstable chemical composition ratio of the material itself.  
           [0018]    Further, although the number of the oxygen vacancies in the Ta 2 O 5  film may vary locally in accordance with the contents of the components and their bonding degree, they will exist to some degree.  
           [0019]    Therefore, in order to prevent or reduce the leakage current of the resulting capacitor an additional oxidation process is required in which the substitution type Ta atoms remaining in a dielectric film are oxidized by stabilize the stoichiometric ratio of the Ta 2 O 5  film.  
           [0020]    Moreover, the Ta 2 O 5  film has a high oxidation reactivity with polysilicon (oxide based electrode) or TiN (metal based electrodes) used for upper and lower electrodes, thereby forming a low dielectric oxide layer and greatly reducing the homogeneity at an interface as a result of the migration of oxygen in the film to the interface.  
           [0021]    When the film is formed, carbon atoms, carbon compounds such as CH 4 , C 2 H 4  and the like, and H 2 O are produced as impurities by the reaction between organic portions of Ta(OC 2 H 5 ) 5  and then O 2  or N 2 O gas used to form the Ta 2 O 5  film.  
           [0022]    Consequently, oxygen vacancies as well as carbon atoms, ions, and radicals existing in the Ta 2 O 5  film as impurities increase the leakage current of the capacitor and degrade the dielectric characteristics of the resulting device.  
           [0023]    In order to address these impurities, a subsequent thermal treatment (oxidation) in an electrical furnace or RTP in N 2 O or O 2  ambient has been used to overcome these problems.  
           [0024]    However, in the subsequent treatments of the prior art methods, as shown in FIG. 3, an oxygen radical (O + ) component as an oxidizer diffuses into an interface between the doped polysilicon layer, used as the electric charge storage electrode, and the Ta 2 O 5  dielectric layer forms an oxide (SiO 2 ) layer having a low dielectric constant thus increasing the equivalent oxide layer thickness (T ox ) of the resulting capacitor.  
           [0025]    As shown in FIG. 3, it is practically unable to attain a value under T ox =30 Å due to the existence of the oxide layer about 25 to 35 Å thick at the interface even though Ta 2 O 5  has a relatively high dielectric constant (k=25).  
           [0026]    Therefore, a capacitor using the prior art Ta 2 O 5  dielectric layer fails to provide a charge capacitance more than about 1.5 times larger than that of an NO capacitor of equal area.  
         SUMMARY OF THE INVENTION  
         [0027]    Accordingly, the present invention is directed to a capacitor for a semiconductor device and a method of fabricating such capacitors that substantially obviates one or more of the problems arising from the limitations and disadvantages of the related art.  
           [0028]    An object of the present invention is to provide a capacitor for a semiconductor device and a method of fabricating such capacitors that provide excellent electrical characteristics as well as provide sufficient charge capacitance for high density semiconductor devices.  
           [0029]    Another object of the present invention is to provide a capacitor for a semiconductor device and a method of fabricating such capacitors provides sufficient electric charge capacitance for use in a highly-integrated device using charge storage electrode having a simple stacked or concave structure, thereby reducing product cost by decreasing the number of unit processes and unit process times necessary to form the capacitor.  
           [0030]    A further object of the present invention is to provide a capacitor for a semi-conductor device and a method of fabricating such capacitors that provide a capacitor having a dielectric film having a dielectric constant that is higher and a stoichiometric ratio that is more stable than conventional Ta 2 O 5 , using LPCVD.  
           [0031]    Another further object of the present invention is to provide a capacitor for a semiconductor device and a method of fabricating such capacitors using low temperature plasma oxidation treatment prevents a low dielectric oxide layer from being formed at the interface between the lower electrode and the dielectric layer and removes impurities existing in the dielectric layer.  
           [0032]    Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.  
           [0033]    To achieve these and other advantages, and in accordance with the purpose of the present invention as embodied and broadly described, a method of fabricating a capacitor in a semiconductor device according to the present invention includes the steps of providing a semiconductor substrate, forming a lower electrode on the semiconductor substrate, forming a Ta 1-x Al x O y N z  (0.01≦x≦0.5, 2≦y≦2.5, 0.01≦z≦0.1) dielectric layer on the lower electrode, and forming an upper electrode on the Ta 1-x xAl x O y N z  dielectric layer.  
           [0034]    In another aspect, a method of fabricating a capacitor in a semiconductor device according to the present invention includes the steps of providing a semiconductor substrate, forming a lower electrode on the semiconductor substrate, forming a Ta 1-x Al x O y N z  dielectric layer on the lower electrode, carrying out oxidation treatment on Ta 1-x Al x O y N z  dielectric layer using a low temperature plasma, inducing crystallization by annealing the oxidation-treated Ta 1-x Al x O y N z  dielectric layer, and forming an upper electrode on the Ta 1-x Al x O y N z  dielectric layer.  
           [0035]    In a further aspect, a capacitor in a semiconductor device according to the present invention includes a semiconductor substrate, a lower electrode on the semiconductor substrate, a Ta 1-x Al x O y N z  dielectric layer on the lower electrode, and an upper electrode on the Ta 1-x —Al x —O y N z  dielectric layer.  
           [0036]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0037]    The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention.  
         [0038]    [0038]FIG. 1 shows a cross-sectional view of semiconductor device having a capacitor using a conventional Ta 2 O 5  film as a dielectric layer;  
         [0039]    [0039]FIG. 2 illustrates the method by which oxygen vacancies and carbon impurities existing in a Ta 2 O 5  dielectric film are removed by oxygen radicals during thermal treatment after the formation of the Ta 2 O 5  dielectric layer according to a prior art fabrication method;  
         [0040]    [0040]FIG. 3 is a SEM photo showing a cross-sectional view of a capacitor in which an N 2 O thermal treatment (oxidation treatment) process is carried out after the deposition of a Ta 2 O 5  film for forming a capacitor in a semiconductor device;  
         [0041]    FIGS.  4 - 7  show cross-sectional views of a method for fabricating a capacitor for a semiconductor device according to an embodiment of the present invention;  
         [0042]    [0042]FIG. 8 shows a cross-sectional view of a capacitor in a semiconductor device according to embodiment of the present invention; and  
         [0043]    [0043]FIG. 9 shows a cross-sectional view of a capacitor in a semiconductor device according to a third embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0044]    Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Where possible, the same reference numerals will be used to identify similar or corresponding elements throughout the specification.  
         [0045]    Referring to FIG. 4, an insulating interlayer  13  is formed on a semiconductor substrate  11  on which various structures (not shown in the drawing) are formed during the fabrication of a semiconductor device. A contact (not shown in the drawing) is formed in the insulating interlayer  13  to for establishing contact to the substrate  11 .  
         [0046]    A lower electrode  15  is then formed by depositing a conductive material layer, for example, doped polysilicon, on an upper surface of the insulating interlayer and into the contact opening and then patterning and etching the conductive material layer.  
         [0047]    In this case, the lower electrode  15  may be formed of one of a variety of silicon-based materials, such as doped amorphous silicon or polysilicon or metal-based materials such as TiN, TaN, W, WN, WSi, Ru, RuO 2 , Ir, IrO 2 , or Pt.  
         [0048]    In other embodiments of the lower electrode  15 , various three-dimensional structures such as a double or triple structures based on cylindrical structure or a simple stacked structures may be utilized to increase the effective surface areas of the lower electrode.  
         [0049]    Moreover, in other embodiments of the lower electrode  15 , the lower electrode may be formed as a storage node having a concave structure, such as illustrated in FIG. 8, or a cylindrical structure, such as illustrated in FIG. 9, and then forming a polysilicon layer having a HSG (Hemi-Spherical Grain) structure on the exposed surfaces of the storage node.  
         [0050]    Referring to FIG. 5, a thin nitride layer  17  is formed on a surface of the lower electrode  15  by carrying out nitridation on the surface of the lower electrode  15 .  
         [0051]    In this case, the nitride layer  17  prevents a natural oxide layer (SiO 2 ) having a low dielectric constant from forming at the interface between a dielectric layer and the lower electrode  15  during formation of the dielectric layer or during a subsequent thermal processing.  
         [0052]    Moreover, the nitride layer  17  is formed by nitridation in a manner that in-situ plasma is discharged in a low pressure chemical vapor deposition (LPCVD) chamber in an ambient of NH 3  or N 2 /H 2  gas while maintaining the temperature of the substrate at 300 to 500° C.  
         [0053]    Instead of using plasma, the nitride layer  17  may be formed by annealing at a temperature of 650 to 950° C. in an ambient of NH 3  using a rapid thermal process (RTP) or at a temperature of 500 to 1000° C. at an ambient of NH 3  using an electric furnace.  
         [0054]    On the other hand, in order to prevent a natural oxide layer (SiO 2 ) having a low dielectric constant from being formed at the interface between the dielectric layer and the lower electrode  15  when the dielectric layer of a capacitor is formed or during subsequent thermal processing, the natural oxide layer may be removed from the surface of the lower electrode  15  using HF vapor or an HF solution instead of forming the nitride layer  17 .  
         [0055]    Moreover, prior to forming the nitride layer  17 , the interface may be treated with chemicals such as a NH 4 ON solution, a H 2 SO 4  solution or the like in order to improve uniformity. This surface treatment may be performed either after or before the HF surface treatment on the lower electrode  15  using HF.  
         [0056]    As mentioned in the foregoing description, in order to increase oxidation resistance of the interface between the electrode and the dielectric layer of a capacitor, the surface of  1 am the lower electrode  15  is nitridated at a temperature of 300 to 950° C. in an ambient of NH 3  or N 2 /H 2  gas using plasma or RTP or thermally treated in an ambient of N 2 O or O 2  gas. Thus, structural defects and non-homogeneity due to dangling bonds are reduced to improve the leakage current characteristics of the resulting device.  
         [0057]    As an alternative to forming nitride layer  17 , the formation of a low dielectric oxide layer at an interface between a dielectric layer and the lower electrode  15  may be substantially suppressed or prevented by the sequential in-situ deposition of the lower electrode  15  and the dielectric layer in the same LPCVD system, preferably a system having lower electrode and dielectric layer deposition chambers positioned to allow a water to be moved between them without a vacuum break.  
         [0058]    Referring to FIG. 6, a Ta 1-x Al x O y N z  (0.01≦x≦0.5, 2≦y≦2.5, 0.01≦z≦0.1) dielectric layer  19  is deposited on an upper surface of the entire structure, including the nitride layer  17 .  
         [0059]    In this case, Ta(OC 2 H 5 ) 5  (tantalum ethylate) and Al(OC 2 H 5 ) 3  (aluminum ethylate) are preferably used as precursors to the Ta 1-x Al x O y N z  dielectric layer  19 .  
         [0060]    A process of forming the Ta 1-x Al x O y N z  dielectric layer  19  is briefly explained as follows.  
         [0061]    First, an LPCVD chamber is supplied with NH 3  gas at a temperature of 300 to 600° C. at a flow of 10 to 1000 sccm. In this case, the flow of the NH 3  gas is preferably within a range between 30 and 600 sccm, or, more preferably, between 50 to 300 sccm so as to provide a constant flow.  
         [0062]    And, an evaporator or an evaporation tube maintaining a fixed temperature of 150 to 300° C. is supplied with measured amounts of an organometallic compound solution containing Ta(OC 2 H 5 ) 5  (tantalum ethylate) and Al(OC 2 H 5 ) 3  (aluminum ethylate) through a flow controller. Chemical vapor of Ta and Al components for CVD is then obtained by evaporating the solution.  
         [0063]    Subsequently, the chemical vapor of the Ta and Al components for CVD is injected into the LPCVD chamber through a supply pipe having a temperature of at least 150° C., thereby initiating deposition of the desired film. The conditions for the CVD are maintained to provide a total pressure of the chemical vapor and NH 3  gas in the chamber between 0.1 and 100 Torr, preferably 0.12 to 50 Torr, or, more preferably, at 0.13 to 10 Torr.  
         [0064]    Thus, the Ta 1-x Al x O y N z  dielectric layer  19  attained by the above procedure is deposited to a thickness of 50 to 150 Å thick, and preferably 55 to 100 Å thick.  
         [0065]    Then, oxygen vacancies, as well as carbon impurities, remaining in the dielectric film are removed simultaneously by carrying out an oxidation under low temperature plasma in an ambient of N 2 O or O 2  gas at a temperature of 300 to 600° C.  
         [0066]    Next, the Ta 1-x Al x O y N z  dielectric layer  19  that has been oxidized in the low temperature plasma is annealed in an electric furnace or RTP at a temperature of about 700 to 900° C. in an ambient of N 2  or NH 3 , thereby increasing a dielectric constant of the dielectric layer by inducing crystallization.  
         [0067]    As an alternative, the Ta 1-x Al x O y N z  dielectric layer  19 , after being oxidized in the low temperature plasma, may be in-situ crystallized in an adjacent RTP chamber that is clustered with the plasma chamber to allow the wafer to be moved without a vacuum-break.  
         [0068]    On the other hand, without carrying out the plasma oxidation treatment process, the Ta 1-x Al x O y N z  dielectric layer  19 , is annealed at a temperature of about 700 to 900° C. in an ambient of N 2 O or O 2  at a normal or reduced pressure in an electric furnace or in RTP equipment, thereby inducing crystallization and removing the carbon impurities and oxygen vacancies in the dielectric layer  19  simultaneously. The upper electrode  21 , as shown in FIG. 7, is then formed on the Ta 1-x Al x O y N z  dielectric layer  19  to complete the capacitor fabrication.  
         [0069]    In this case, the upper electrode  21  completes the construction a SIS (silicon-insulator-silicon) type capacitor using doped polysilicon or a MIS (metal-insulator-silicon) type capacitor using a metal based materials such as TiN, TaN, W, WN, WSi, Ru, RuO 2 , Ir, IrO 2 , or Pt.  
         [0070]    As another embodiment of the upper electrode  21 , a metal material layer (not shown in the drawing), for example TiN, is deposited 100 to 600 Å thick on the Ta 1-x Al x O y N z  dielectric layer  19  and then a doped polysilicon layer (not shown in the drawing), that will act as a buffer layer to prevent electric characteristics of a capacitor from being degraded during succeeding thermal processes, is stacked on the metal material layer. Thus, the metal material layer and doped polysilicon layer combined form a stacked upper electrode.  
         [0071]    In this case, the metal material layer (not shown in the drawing) is formed from one of metal based materials such as TiN, TaN, W, WN, WSi, Ru, RuO 2 , Ir, IrO 2 , or Pt.  
         [0072]    As a further embodiment of the present invention, a MIM (metal-insulator-metal) type capacitor device maybe fabricated by forming both the lower and upper electrodes using one of metal based materials such as TiN, TaN, W, WN, WSi, Ru, RuO 2 , Ir, IrO 2 , or Pt rather than doped or undoped polysilicon.  
         [0073]    [0073]FIG. 8 shows a cross-sectional view of a capacitor in a semiconductor device according to another embodiment of the present invention.  
         [0074]    Referring to FIG. 8, a lower electrode is formed to be concave. A first insulating interlayer  33  having a first contact hole  34  is formed on a semiconductor substrate  31 . And, a second insulating interlayer  37  having a second contact hole  38  therein is formed on the first insulating interlayer  33  including the first contact hole  34 .  
         [0075]    In the second contact hole  38  including the first contact hole  34 , a doped polysilicon layer pattern  35  having a concave shape is formed to fill up the first contact hole  34 .  
         [0076]    A HSG layer  41  is then formed on an exposed surface of the doped polysilicon layer pattern  35 . In this case, the HSG layer  41  and doped polysilicon pattern  35  form the lower electrode.  
         [0077]    Moreover, a dielectric layer  43  is formed on the HSG layer  41  and second insulating interlayer  37  while an upper electrode  45  is formed on the dielectric layer  43 , thereby constructing a capacitor in a semiconductor device.  
         [0078]    In this case, the dielectric layer  43  is formed using the Ta 1-x Al x O y N z  dielectric layer in the foregoing embodiment according to the present invention.  
         [0079]    In this case, the upper electrode  45  constructs an SIS (silicon-insulator-silicon) type capacitor using doped polysilicon or an MIS (metal-insulator-silicon) type capacitor using one of metal based materials such as TiN, TaN, W, WN, WSi, Ru, RuO 2 , Ir, IrO 2 , or Pt. Besides, the upper electrode  45  may be constructed by stacking a polysilicon layer on the metal based material layer.  
         [0080]    [0080]FIG. 9 shows a cross-sectional view of a capacitor in a semiconductor device according to a further embodiment of the present invention.  
         [0081]    Referring to FIG. 9, a further embodiment according to the present invention includes a lower electrode having a cylindrical shape. An insulating interlayer  53  having a contact hole  54  is formed on a semiconductor layer  51 . A cylindrical type doped polysilicon layer pattern  55  is then formed on the contact hole  54  and insulating interlayer  53 .  
         [0082]    A HSG layer  57  is formed on the exposed surface of the doped polysilicon layer pattern  55 . In this case, the HSG layer  57  and the doped polysilicon pattern  55  construct the lower electrode.  
         [0083]    Moreover, a dielectric layer  59  is formed on the HSG layer  57  and insulating interlayer  53  while an upper electrode  61  is formed on the dielectric layer  59 , thereby constructing a capacitor in a semiconductor device. In this case, the dielectric layer  59  is formed using the Ta 1-x Al x O y N z  dielectric layer in the foregoing embodiment according to the present invention.  
         [0084]    In this case, the upper electrode  61  constructs an STS (silicon-insulator-silicon) type capacitor using doped polysilicon or an MIS (metal-insulator-silicon) type capacitor using one of metal based materials such as TiN, TaN, W, WN, WSi, Ru, RuO 2 , Ir, IrO 2 , or Pt. Alternatively, the upper electrode  61  may be constructed by stacking a polysilicon layer on the metal based material layer.  
         [0085]    Accordingly, a capacitor in semiconductor device and fabrication method thereof according to the present invention provides the following advantages.  
         [0086]    A capacitor in semiconductor device and fabrication method thereof according to the present invention using a Ta—Al—ON dielectric layer effectively prevents leakage current due to oxygen vacancies and carbon impurities generated from a dielectric layer having an unstable stoichiometric ratio as in the prior art dielectrics.  
         [0087]    And, in the present invention, using a low temperature plasma oxidation treatment prevents a low dielectric oxide layer from being formed at an interface between the lower electrode and the dielectric layer, thereby preventing leakage current due to an irregular oxide layer as well as to control the thickness (T ox ) of an equivalent oxide layer of a capacitor to a thickness below 25 Å.  
         [0088]    Moreover, the present invention provides a Ta—Al—ON dielectric layer having a dielectric ratio suitable for memory cells of semiconductor devices having a critical dimension of micro-circuit of less than 0.18 μm, thereby requiring no complicated processing of the lower electrode into a complex three-dimensional structure in order to increase the lower electrode area.  
         [0089]    Accordingly, the present invention provides a sufficient electric charge capacitance of over 25 fF/cell despite using a simple stack or concave lower electrode, thereby reducing product cost by decreasing the number of unit processes and the overall unit process time.  
         [0090]    Leakage current of a Ta—Al—ON film of the present invention is less than that of a Ta 2 O 5  dielectric layers of the prior art. Further, the Ta—Al—ON film of the present invention is more stable than the Ta 2 O 5  dielectric layer of the prior art. And, breakdown voltage of the Ta—Al—ON film of the present invention is higher than that of the Ta 2 O 5  dielectric layer of the prior art, thereby providing excellent electrical breakdown field characteristics.  
         [0091]    Thus, the present invention provides a sufficient charge capacitance of over 25 fF/cell for a memory cell of a next generation product to which a micro critical dimension of less than 0.13 μm is applied.  
         [0092]    The foregoing embodiments are merely exemplary and are not to be construed as limiting the present invention. The present teachings can be readily applied to other types of apparatuses.  
         [0093]    The description of the present invention is intended to be illustrative, and not to limit the scope of the claims. Many alternatives, modifications, and variations will be apparent to those skilled in the art.