Abstract:
A patterned portion of a patterned semiconductor substrate is removed by abrasive mechanical planarization employing an abrasive pad but without employing any slurry. Preferably, water is supplied to enhance the removal rate during the mechanical planarization. The removal rate of material is substantially independent for common materials employed in back-end-of-line (BEOL) semiconductor materials, which enables non-selective removal of the material containing metallization structures. The removal rate of silicon is lower than the removal rate for the BEOL semiconductor materials, enabling a self-stopping planarization process.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method of processing a semiconductor substrate, and more particularly to a method of removing patterns from a surface of a semiconductor substrate by mechanical planarization without employing slurry. 
       BACKGROUND OF THE INVENTION 
       [0002]    Disposal of patterned semiconductor substrates, including patterned product substrates that do not meet wafer acceptance criteria (WAC) and patterned monitor substrates, pose two types of challenges. On one hand, the patterned semiconductor substrates contain patterned physical features of circuits or devices that are protected by intellectual property. Disposal of such patterned semiconductor substrates may expose the intellectual property to an inadvertent unwanted disclosure. On the other hand, the patterned semiconductor substrate typically contains environmentally hazardous material such as boron, gallium, indium, arsenic, antimony, etc. Uncontrolled release of such environmentally hazardous material in the patterned semiconductor substrate may result in environmental contamination, of which the remedy can be extremely costly. 
         [0003]    A conventional remedy to these problems has been to crush the patterned semiconductor substrates into fine powders so that the microelectronic features in the patterned semiconductor substrates are destroyed. The fine powders from the patterned semiconductor substrate are thereafter put in a sealed container, which may be buried in a designated location to minimize the environmental impact. 
         [0004]    Recent efforts to reduce the environmental impact and reduce processing cost of substrate crushing and containment have resulted in methods for substrate reclamation. The patterned portion of a patterned semiconductor substrate is removed. The resulting semiconductor substrate, which does not contain any pattern, is a reclaimed semiconductor substrate that may be employed in semiconductor processing as a monitor substrate or a dummy substate. 
         [0005]    According to prior art, chemical stripping may be employed to remove a patterned portion of a patterned semiconductor substrate. In this method, a chemical is employed to dissolve soluble materials, while insoluble materials are undercut and subsequently removed. However, the dissolution rates across common materials employed in the patterned semiconductor substrates, e.g., silicon oxide, silicon nitride, low-k dielectric materials, polyimide, copper, aluminum, silicon, etc., vary by orders of magnitude. The variations in the dissolution rates typically cause paths to open up down to a surface of a semiconductor layer before the entirety of a back-end-of-line (BEOL) stack including metallization structures are undercut. Such paths cause extensive loss of the semiconductor material in the semiconductor layer. Further, the chemicals employed in chemical stripping tend to be extremely corrosive with accompanying environmentally adverse impacts. For these reasons, chemical stripping tends to consume a large amount of semiconductor material in a semiconductor layer, form large cavities on the surface of the reclaimed semiconductor substrate, contain some incompletely removed structures, and/or cause significantly negative environmental impacts. 
         [0006]    An alternate prior art method includes chemical mechanical planarization (CMP) to remove a patterned portion of a patterned semiconductor substrate. A polishing slurry is employed in chemical mechanical planarization to provide selective removal of one material relative to another material. The polishing slurry may be acidic or alkaline. Examples of the polishing slurry include aluminum oxide and fumed silica. The pH of the polishing slurry during chemical mechanical planarization may be adjusted by adding a chemical, e.g., hydrogen peroxide, an alkali metal hydroxide, or ammonia. The removal rate of chemical mechanical planarization depends on the material being removed and the selection of the slurry. Since the composition of material in patterned semiconductor substrate varies significantly, the local polish rate will depend on the material and amount of material present, causing significant differences in polish rate across the wafer. For this reason, uniform removal rate is difficult to achieve with chemical mechanical planarization on all patterned semiconductor substrates, and normally requires chemical stripping to remove materials with low polish rates, as well as the chemical mechanical polish itself. Further, the chemical mechanical planarization requires a polishing slurry, which is a consumable material that needs to be supplied during the CMP process, to enable removal of the material in the patterned portion. In addition, the removed material is mixed with the residual slurry to produce contaminated liquid chemical waste, which is difficult to neutralize for environmental purposes. 
         [0007]    Prior art methods employing mechanical removal include grinding of material for reclamation of substrates. Such mechanical removal results in generation of a damaged layer on a reclaimed semiconductor substrate. The surface topography of such reclaimed semiconductor substrate contains significant irregularity after removal of a patterned portion. To remove variations in the topography of the surface, as well as crystal defects extending well into the silicon lattice of the reclaimed semiconductor substrate after such mechanical removal, an additional chemical etching step and lapping to remove bulk silicon steps need to be employed. 
         [0008]    In view of the above, there exists a need for a method of removing a patterned portion of a patterned semiconductor substrate at a uniform, material-independent rate without generating significant surface topography or crystal defects to reclaim the patterned semiconductor substrate. 
         [0009]    Further, there exists a need for a method of removing the patterned portion of the patterned semiconductor substrate without employing slurry or generating a slurry-containing liquid chemical waste. 
         [0010]    Yet further, there exist a need for a method of removing the patterned portion of the patterned semiconductor substrate with sufficient planarity to obviate any need for employing an additional chemical etch step or a lapping step. 
       SUMMARY OF THE INVENTION 
       [0011]    To address the needs described above, the present invention provides a slurry-less mechanical planarization that removes a patterned portion of a patterned semiconductor substrate at a substantially uniform removal rate without employing slurry or generating a slurry-containing liquid chemical waste. 
         [0012]    In the present invention, a patterned portion of a patterned semiconductor substrate is removed by abrasive mechanical planarization employing an abrasive pad but without employing any slurry. Preferably, water is supplied for lubrication during the mechanical planarization. The removal rate of material is substantially independent for common materials employed in back-end-of-line (BEOL) semiconductor materials, which enables non-selective removal of the material containing metallization structures. The removal rate of silicon is lower than the removal rate for the BEOL semiconductor materials, enabling a self-stopping planarization process. 
         [0013]    According to the present invention, a method of removing material from a patterned semiconductor substrate is provided. The method comprises removing a patterned portion of a patterned semiconductor substrate employing an abrasive mechanical planarization without employing slurry. 
         [0014]    In one embodiment, a remaining portion of the patterned semiconductor substrate after the abrasive mechanical planarization does not contain any patterned structure. 
         [0015]    In another embodiment, the patterned semiconductor substrate includes a bulk semiconductor substrate, a semiconductor-on-insulator (SOI) substrate, or a hybrid semiconductor substrate comprising a bulk portion and a semiconductor-on-insulator (SOI) portion. 
         [0016]    In even another embodiment, the patterned portion includes at least one semiconductor device located in a semiconductor layer. 
         [0017]    In yet another embodiment, the patterned portion includes at least one metal interconnect structure comprising at least one conductive structure and at least one dielectric structure. The patterned semiconductor substrate may include a silicon layer located underneath the patterned portion. A first removal rate of material of the at least one metal interconnect structure may be greater than a second removal rate of material of the silicon layer. Removal of the at least one metal interconnect structure may be non-selective between metallic materials and dielectric materials. The at least one metal interconnect structure may be removed selective to silicon. The method may further comprise removing a top portion of the silicon layer by about 1 μm to about 10 μm during the abrasive mechanical planarization. 
         [0018]    In still another embodiment, an abrasive pad containing abrasive particles having a diameter from about 1 μm to about 20 μm is employed during the abrasive mechanical planarization. 
         [0019]    In still yet another embodiment, a surface of a remaining portion of the patterned semiconductor substrate contains no pattern, and contains structural damages having a depth from about 5 μm to about 10 μm. 
         [0020]    In a her embodiment, an abrasive pad containing abrasive particles is employed during the abrasive mechanical planarization, and wherein the abrasive particles have a hardness greater than 8 Mohs. 
         [0021]    In an even further embodiment, an abrasive pad containing abrasive particles is employed during the abrasive mechanical planarization, and wherein the abrasive particles are embedded in a plastic, erodable matrix. 
         [0022]    In a yet further embodiment, the plastic erodable matrix comprises polycarbonate or polyurethane. 
         [0023]    In a still further embodiment, an abrasive pad containing impregnated diamond particles is employed during the abrasive mechanical planarization. The impregnated diamond particles may have a diameter from about 3 μm to about 9 μm. 
         [0024]    In a still yet further embodiment, water is added to a surface at which removal of material occurs during the abrasive mechanical planarization. Preferably, the water is deionized water consisting essentially of H 2 O. Water may be added at a flow rate from about 10 milliliter per minute to about 300 milliliter per minute. 
         [0025]    In further another embodiment, the abrasive mechanical planarization is performed by pressing the patterned semiconductor substrate in rotation against an abrasive pad affixed to a platen in rotation. The patterned semiconductor substrate may be held by a chuck which rotates with the patterned semiconductor substrate with a same angular velocity. The angular velocity may be from about 10 rpm to about 150 rpm, and wherein the rotating platen rotates with another angular velocity from about 10 rpm to about 150 rpm. Pressure from 0 psi to about 9.5 psi may be applied to a backside of the patterned semiconductor substrate. down force on a surface of the patterned semiconductor substrate contacting the platen may be from about 1 psi to 10 psi. The abrasive mechanical planarization may have a polish time from about 15 seconds to about 120 seconds. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1  is a vertical cross-sectional view of an exemplary abrasive mechanical planarization system according to the present invention. 
           [0027]      FIG. 2  is a vertical cross-sectional view of a first exemplary patterned semiconductor substrate that may be planarized by the methods of the present invention. 
           [0028]      FIG. 3  is a vertical cross-sectional view of a second exemplary patterned semiconductor substrate that may be planarized by the methods of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0029]    As stated above, the present invention relates to a method of removing patterns from a surface of a semiconductor substrate by mechanical planarization without employing slurry. As used herein, when introducing elements of the present invention or the preferred embodiments thereof the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of the elements. Throughout the drawings, the same reference numerals or letters are used to designate like or equivalent elements. Detailed descriptions of known functions and constructions unnecessarily obscuring the subject matter of the present invention have been omitted for clarity. The drawings are not necessarily drawn to scale. 
         [0030]    Referring to  FIG. 1 , an exemplary abrasive mechanical planarization system according to the present invention comprises a platen  10  and a platen shaft  12  that are configured to rotate around the axis of the platen shaft  12 . Preferably, the platen shaft  12  and the platen  10  are concentric. The platen  10  may have a horizontal cross-sectional area of a circle having a first diameter, which is greater than the diameter of a patterned semiconductor substrate  40  to be planarized. Typically, a top surface of the platen  10  is substantially horizontal and configured to hold an abrasive pad  20 . The abrasive pad  20  is affixed to the top surface of the platen  10 , for example, by adhesion or by clipping. 
         [0031]    A chuck  30  and a chuck shaft  32  are configured to rotate around the axis of the chuck shaft  32 . Preferably, the chuck  30  and the chuck shaft are concentric. The chuck  30  may have a horizontal cross-sectional area of a circle having a second diameter, which is less than the first diameter, i.e., the diameter of the platen  10 , and is greater than the diameter of the patterned semiconductor substrate  40 . The bottom surface of the chuck  30  is substantially horizontal and parallel to the top surface of the abrasive pad  20 . The bottom surface of the chuck  30  is provided with a mechanism, such as a vacuum suction, to hold the patterned semiconductor substrate  40 . The patterned semiconductor substrate  40  is affixed to the bottom surface of the chuck  30  such that the patterned portion of the patterned semiconductor substrate  40  faces downward to contact the abrasive pad  20  and an unpatterned portion of the patterned semiconductor substrate  40  faces upward. 
         [0032]    A hose  50  or an equivalent liquid spray mechanism is provided over the abrasive pad  20  and to the side of the chuck  30  so that water may be sprayed onto the top surface of the abrasive pad  20 . The hose  50  may be connected to a flow control system such as a mass flow controller (not shown) or a flow switch (not shown). 
         [0033]    The platen  10  and the abrasive pad  20  are rotated at a first angular velocity around the axis of the platen  10 , which coincides with the axis of the platen shaft  12 . The first angular velocity may be from about 10 revolutions per minute (rpm) to about 150 revolutions per minute, although lesser and greater first angular velocities are explicitly contemplated herein. The revolution of the platen  10  and the abrasive pad  20  may be clockwise or counterclockwise. The chuck  30  and the patterned semiconductor substrate  40  are rotated at a second angular velocity around the axis of the chuck  30 , which coincides with the axis of the chuck shaft  32 . The second angular velocity may be from about 10 rpm to about 150 rpm, although lesser and greater second angular velocities are explicitly contemplated herein. The revolution of the chuck  30  and the patterned semiconductor substrate  40  may be clockwise or counterclockwise. Typically, the direction of rotation of the platen  10  and the direction of rotation of the chuck  30  are opposite, i.e., if the platen  10  rotates clockwise, the chuck  30  rotates counterclockwise, and vice versa. Typically, the axis of the platen  10  and the axis of the chuck  30  are offset to provide uniform removal rate of the bottom surface, i.e., the surface contacting the pad  20 , of the patterned semiconductor substrate  40 . 
         [0034]    The abrasive pad  20  contains abrasive particles. Preferably, the abrasive particles have a hardness greater than 8 Mohs. Non-limiting examples of materials that may be employed for the abrasive particles include diamond, fullerite, carborundum, corundum, aluminum silicate, etc. Typically, the abrasive pad  20  contains a matrix containing the abrasive particles. The matrix comprises a material having less hardness and greater flexibility than the material of the abrasive particles. In one case, the abrasive pad  20  may contain a plastic erodable matrix in which the abrasive particles are embedded. The plastic matrix may comprise, for example, polycarbonate, polyurethane, or any single or combination of plastic materials which form a stiff matrix which the abrasive particles can adhere. The diameter of the abrasive particles may be from about 1 μm to about 20 μm, although lesser and greater diameters are also contemplated herein. In one embodiment, the abrasive particles comprise impregnated diamond particles having a hardness of about 10 Mohs. The diameter of the impregnated diamond particles may be from about 3 μm to about 9 μm, although lesser and greater diameters are also contemplated herein. 
         [0035]    Abrasive mechanical planarization is performed on the bottom surface of the patterned semiconductor substrate  40 , which contains a patterned portion that contacts the abrasive pad  20 . The present invention employs non-chemical abrasive mechanical planarization in that chemical that reacts with the material of the patterned semiconductor substrate is not employed. Further, slurry is not employed in the non-chemical abrasive mechanical planarization of the present invention. Instead, water is supplied to enhance the removal rate of the material from the patterned semiconductor substrate  40 . Further, water acts as a coolant and lubricant during the non-chemical abrasive mechanical planarization to prevent overheating of the abrasive pad  20 . 
         [0036]    Specifically, water is added to the surface at which removal of material occurs during the abrasive mechanical planarization of the present invention. Typically, water is added near the center of the platen  10 , from which the water flows out toward the rim of the platen  10  by centrifugal force. As the water flows outward, the water laterally enters the area at which the patterned semiconductor substrate  40  contacts the abrasive pad  20 . Since the abrasive pad  20  includes abrasive particles that protrude from the surface of a matrix of the abrasive pad  20 , the water laterally flows between the abrasive pad  20  and the bottom surface of the patterned semiconductor substrate  40 . 
         [0037]    Preferably, the water is deionized water with minimal level of impurities and consisting essentially of water molecules, i.e., H 2 O molecules. Preferably, the impurities in the deionized water are at a trace level, if any. The water is chemically neutral, i.e., has a pH of 7.0. The water is added at a flow rate from about 10 milliliters per minute to about 300 milliliters per minute, and typically, from about 25 milliliters per minute to about 150 milliliters per minute, although lesser and greater water flow rates are also contemplated herein. 
         [0038]    The abrasive mechanical planarization of the present invention is performed by pressing the patterned semiconductor substrate  40  in rotation against the abrasive pad  20  affixed to the platen  10  in rotation. The patterned semiconductor substrate  40  is held by the chuck  30  which rotates with the patterned semiconductor substrate  30  at the same angular velocity. Pressure from 0 pound per square inch (psi) to about 9.5 psi may be applied to the backside of the patterned semiconductor substrate  40  that contact the chuck  30 , although greater pressure is also contemplated herein. Down force on the surface of the patterned semiconductor substrate  40  that contact the abrasive pad  20  may be from about 1 psi to about 10 psi, although lesser and greater down forces are contemplated herein also. The duration of the abrasive mechanical planarization, i.e., the polish time, varies depending on the thickness of the patterned portion of the patterned semiconductor substrate  40 , and is typically from about 15 seconds to about 120 seconds, although lesser and greater polish times are also contemplated herein. 
         [0039]    It is noted that apparatus for chemical mechanical planarization (CMP) may be readily converted to perform the process of the present invention by removing a slurry supply system and adding a water supply system. 
         [0040]    Referring to  FIG. 2 , a first exemplary patterned semiconductor substrate that may be planarized by the methods of the present invention comprises a semiconductor-based portion  108  and an interconnect structure portion  180  formed on the semiconductor-based portion  108 . The semiconductor-based portion  108  may comprise a bulk semiconductor substrate containing the semiconductor layer  110  without a pattern therein, in which semiconductor devices are embedded in subsequent processing steps. The interconnect structure portion  180  and an upper portion of the semiconductor-based portion  108  containing semiconductor devices collectively constitute a patterned portion P. The remaining lower portion of the semiconductor-based portion  108 , which does not contain any patterned structure and contains a lower portion of the semiconductor layer  110 , constitutes an unpatterned portion U. 
         [0041]    Typically, the upper portion of the semiconductor-based portion  108  comprises at least one semiconductor device, which may include a logic device  130  or a memory device  140 . Shallow trench isolation structures  120  may also be present in the upper portion of the semiconductor-based portion  108  to provide electrical isolation in case a plurality of semiconductor devices are embedded in the upper portion of the semiconductor layer  110 . The shallow trench isolation structures  120  may be provided, for example, by formation of a dielectric pad layer (not shown) over the semiconductor layer  110 , application and lithographic patterning of a photoresist (not shown), an anisotropic etch that transfers the pattern in the photoresist into exposed portions of the semiconductor layer  10  to form shallow trenches, deposition of a dielectric material inside the shallow trench, and planarization of the dielectric material. The depth of the shallow trench isolation structures  120  is typically less than 1 μm. The top portion of the semiconductor-based portion  108  may contain other device structures such as a deep trench, which may have a depth up to about 8 μm. In general, the depth of patterned structures included in the top portion of the semiconductor layer  110  is typically limited to a depth less than 10 μm from the top surface of the semiconductor layer  110 . 
         [0042]    Non-limiting examples of semiconductor materials that may be employed for the semiconductor layer  110  include silicon, a silicon germanium alloy portion, silicon, germanium, a silicon-germanium alloy portion, a silicon carbon alloy portion, a silicon-germanium-carbon alloy portion, gallium arsenide, indium arsenide, indium gallium arsenide, indium phosphide, lead sulfide, other III-V compound semiconductor materials, and II-VI compound semiconductor materials. In one embodiment, silicon is the semiconductor material of the semiconductor layer  110 . 
         [0043]    The at least one semiconductor devices in the upper portion of the semiconductor-based portion  108  may include dielectric materials, semiconductor materials, and metallic materials. The dielectric materials include, but are not limited to, silicon oxide, silicon nitride, silicon oxynitride, high dielectric constant material comprising a dielectric metal oxide, etc. The semiconductor materials include, but are not limited to, the above mentioned materials that may be employed for the semiconductor layer  110 . The metallic materials include, but are not limited to, Cu, Al, W, Ta, Ti, TaN, TiN, WN, Co, Ni, Pt, and metal semiconductor alloys. Typically, the semiconductor-based portion  108  predominantly comprises semiconductor materials. 
         [0044]    The interconnect structure portion  180  comprises at least one metal interconnect structure comprising at least one conductive structure and at least one dielectric structure. The at least one dielectric structure may include at least one of a contact via level dielectric layer  162 , a first line level dielectric layer  164 , a first via level dielectric layer  166 , a second line level dielectric layer  168 , a dielectric passivation layer  170 , any other via level dielectric layer (not shown), and any other line level dielectric layer (not shown). The at least one conductive structure may include at least one of contact vias  168 , first line level conductive structures  166 , first interconnect vias  167 , second line level conductive structures  169 , any other interconnect level vias (not shown), and any other line level conductive structures (not shown). Line level interconnect structures may comprise interconnect lines. 
         [0045]    Each of the at least one dielectric structure comprises a dielectric material, non-limiting examples of which include silicon oxide, silicon nitride, SiCOH, chemical vapor deposition (CVD) low-k dielectric materials, spin-on low-k dielectric materials, etc. Each of the at least one conductive structure comprises a conductive material such as metal, e.g., Cu or Al, or a conductive metallic alloy. 
         [0046]    The first exemplary patterned semiconductor substrate ( 108 ,  180 ) maybe loaded onto the chuck  30  of the exemplary abrasive mechanical planarization system of  FIG. 1  such that the bottom surface of the semiconductor-based portion  108  contacts the chuck  30  and the top surface of the interconnect structure portion  180  faces downward. During the abrasive mechanical planarization, the exposed surface of the interconnect structure portion  180  is continually removed by abrasion as the abrasive particles on the abrasive pad  20  applied force to the material on the exposed surface of the interconnect structure. In the course of experiments that led to the present invention, it has been found that the removal rate of materials is non-selective between metallic materials and dielectric materials. Thus, the entirety of the interconnect structure portion  180  including the at least one metal interconnect structure may be removed non-selectively between the metallic materials and the dielectric materials. 
         [0047]    In one embodiment, the semiconductor layer  110  comprises silicon, i.e., the semiconductor layer  110  is a silicon layer. Typically, the removal rate of silicon is less than the removal rate of the interconnect structure portion  180  including the at least one metal interconnect structure. In this case, the interconnect structure portion  180  including the at least one metal interconnect structure may be removed selective to silicon. The removal rate of the at least one metal interconnect structure, which is herein referred to as a first removal rate, is greater than the removal rate of silicon, which is herein referred to as a second removal rate. 
         [0048]    After a complete removal of the interconnect structure portion  180 , which is schematically represented by an upper arrow in  FIG. 2 , a top portion of the semiconductor layer  110  is removed to depth below which no patterned structure is present. The depth may be from about 1 μm to about 10 μm depending on the type of semiconductor devices contained in the semiconductor-based portion  108 . After removal of the top portion of the semiconductor-based portion  108 , the remaining portion of the semiconductor-based portion  108  constitutes an unpattemed semiconductor substrate, which is a reclaimed semiconductor substrate. The thickness of the reclaimed semiconductor substrate may be less than the thickness of the patterned semiconductor substrate ( 108 ,  180 ) prior to the abrasive mechanical planarization by a thickness from about 1 μm to about 30 μm, and typically from about 1 μm to about 10 μm. Thus, the thickness of the reclaimed semiconductor substrate may be about 99% of the thickness of the patterned semiconductor substrate ( 108 ,  180 ) prior to the abrasive mechanical planarization or more. 
         [0049]    Referring to  FIG. 3 , a second exemplary patterned semiconductor substrate that may be planarized by the methods of the present invention comprises a semiconductor-based portion  108  and a interconnect structure portion  180  formed on the semiconductor-based portion  108 . The semiconductor-based portion  108  may comprise a semiconductor-on-insulator (SOI) substrate containing a bottom semiconductor layer  111  without a pattern therein, a buried insulator layer  112 , and a top semiconductor layer  114  in which semiconductor devices are embedded in subsequent processing steps. The interconnect structure portion  180  and an upper portion of the semiconductor-based portion  108  containing semiconductor devices collectively constitute a patterned portion P as in the first exemplary patterned semiconductor substrate. The bottom semiconductor layer  111 , which does not contain any patterned structure, constitutes an unpatterned portion U. 
         [0050]    The abrasive mechanical planarization process of the present invention may be employed in the same manner described above. 
         [0051]    The present invention may be employed to reclaim other patterned semiconductor substrates such as a patterned hybrid substrate including a bulk portion and a semiconductor-on-insulator portion. Further, in some cases, a bottom semiconductor layer of a patterned semiconductor substrate may contain a pattern such as deep trenches, in which case the depth of the abrasive mechanical planarization is adjusted to insure removal of all patterned structures from the patterned semiconductor substrate. 
         [0052]    While the invention has been described in terms of specific embodiments, it is evident in view of the foregoing description that numerous alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the invention is intended to encompass all such alternatives, modifications and variations which fall within the scope and spirit of the invention and the following claims.