Abstract:
An improved structure for the construction of perpendicular recording media is disclosed. The structure includes a tri-layer IML resident between a soft under layer CoTaZr film and a CoPtCr—SiO 2  magnetic media. In an embodiment, the tri-layer comprises a Ru x Cr 1−x  layer over dual nucleation layers of Ni—Fe and Ni—Fe—Cr. The tri-layer replaces the typical Ru and Ni—Fe intermediate layers of the prior art, resulting in considerable improvement in lattice matching between the Ru containing intermediate layer and the CoPtCr—SiO 2  magnetic media, further resulting in improved magnetic media performance.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the structure of magnetic recording media. More specifically, the invention relates to the structure of perpendicular recording media (PMR). 
     2. Description of the Related Art 
     Magnetic media are widely used in various applications, particularly in the computer and data storage industries, in devices such as hard disk drives and other recording devices. Efforts are continually being made with the aim of increasing the areal recording density, i.e., bit density of the magnetic media. In order to produce storage densities in excess of 200 Gb/in 2 , new recording media structures will be required. In this regard, perpendicular recording media structures (PMR) have been found to be superior to the more conventional longitudinal media in achieving very high bit densities. In perpendicular magnetic recording media, residual magnetization is formed in a direction perpendicular to the surface of the magnetic medium. 
     US Patent Application Publication US 2002/0058160 discloses a perpendicular magnetic recording medium comprising a combination of an under layer of a laminate structure including at least two layers and a Co-based magnetic layer. The particular combination is selected from the group consisting of i) Fe-containing layer/Ru/magnetic layer, ii) Co-containing layer/Ru/magnetic layer, iii) Ru/Co-containing layer/magnetic layer, iv) Ti-containing layer/Ru/magnetic layer, and v) soft magnetic layer/V or Cr/magnetic layer. A multi-layered structure of magnetic layer/Ru/magnetic layer is used as the magnetic layer included in combinations i) to v) given above. The perpendicular magnetic recording medium of the particular construction permits improving the perpendicular orientation of the Co-based magnetic layer and exhibits a high coercive force and a high reproducing output. 
     US Patent Application Publication US 2003/0203189 discloses an improved perpendicular magnetic recording medium suitable for high density magnetic recording. In a perpendicular magnetic recording medium comprising a perpendicular magnetic layer and protective layer provided on a non-magnetic substrate via a soft magnetic backlayer, a polycrystalline MgO film is inserted between the soft magnetic backlayer and perpendicular magnetic layer. 
     US Patent Application Publication US 2004/0000374 discloses a perpendicular magnetic recording medium having a magnetic recording layer with ferromagnetic crystalline grains and nonmagnetic and nonmetallic grain boundary region surrounding the grains. The surface of its under layer, before forming the magnetic recording layer, is exposed to an O 2  or N 2  atmosphere or an atmosphere of rare gas and O 2  or N 2 , to attach the O 2  or N 2  as nucleation sites for promoting growth of the nonmagnetic and nonmetallic region. By forming the magnetic recording layer thereafter, both ferromagnetic crystalline grains and the nonmagnetic and nonmetallic grain boundary region are formed from the initial stage of the growth of the magnetic recording layer. Thus, a magnetic recording layer having excellent segregation structure can be formed. 
     US Patent Application Publication US 2004/0001975 discloses a double layered perpendicular recording media having, between a soft magnetic layer and perpendicular magnetic recording layer, an alignment control layer containing an amorphous portion, a crystal size control layer, and an under layer having one of a hexagonal closest packed structure and a face-centered cubic structure. 
     US Patent Application Publication US 2004/0072031 discloses a magnetic recording medium including a magnetic recording layer and a substrate that supports the magnetic recording layer. At least two under layers including a nonmetallic under layer are placed between the magnetic recording layer and the substrate. The perpendicular magnetic recording medium uses a double-layered or tri-layered under layer. Accordingly, a perpendicular magnetic recording layer can have a high perpendicular magnetic anisotropic energy constant Ku due to a third under layer and have small crystal grains and a small exchange coupling due to a second under layer below the third underlayer. Thus, the perpendicular magnetic recording layer can have a good thermal stability, high-density recording characteristics, and excellent SNR characteristics. 
     US Patent Application Publication US 2004/0247945 discloses a perpendicular magnetic recording medium, comprising: (a) a non-magnetic substrate having a surface; and (b) a layer stack formed over the substrate surface, comprising in overlying sequence from the substrate surface: (i) a magnetically soft under layer; (ii) an interlayer structure for crystallographically orienting a layer of a perpendicular magnetic recording material formed thereon; and (iii) at least one crystallographically oriented magnetically hard perpendicular recording layer; wherein the magnetically soft under layer is sputter-deposited at a sufficiently large target-to-substrate spacing and at a sufficiently low gas pressure selected to provide the under layer with a smooth surface having a low average surface roughness Ra below about 0.3 nm, as measured by Atomic Force Microscopy (AFM). 
     U.S. Pat. No. 6,858,320 discloses performance of a perpendicular magnetic recording medium, such as an increase in output or a decrease in noise, improved by providing a good orientation of a magnetic recording layer in the perpendicular magnetic recording medium and by reducing an amount of an initial growth layer in the magnetic recording layer. The perpendicular magnetic recording medium includes an under layer, a magnetic recording layer, a protective film, and a liquid lubrication layer, which are sequentially provided on a non-magnetic substrate. The under layer contains non-magnetic NiFeCr or a permalloy-based soft magnetic material. 
     U.S. Pat. No. 6,699,600 discloses a magnetic recording medium comprising, on a non-magnetic substrate, at least a soft magnetic undercoat film comprising a soft magnetic material; an orientation control film for controlling an orientation of a film directly above; a perpendicular magnetic film in which an axis of easy magnetization is oriented mainly perpendicularly with respect to the substrate; and, a protection film, wherein the perpendicular magnetic film has a structure in which a large number of magnetic grains are separated by a grain boundary layer, and an average separating distance between the magnetic grains along a straight line which connects centers of gravity of mutually neighboring magnetic grains of 1 nm or greater. 
     U.S. Pat. No. 6,670,056 discloses a perpendicular magnetic recording medium having magnetic characteristics by which an anisotropic magnetic field Hk and a saturation magnetization Ms satisfy the requirement 2&lt;Hk/4.pi.Ms&lt;5, letting .alpha. be the inclination of an MH loop when a magnetic field is applied perpendicularly, the anisotropic magnetic field Hk, the saturation magnetization Ms, and a coercive force Hc satisfy the requirement 0.01&lt;{(.alpha.−1)Hc+4.pi.Ms}/Hk&lt;0.2, and a longitudinal residual magnetization Mr is less than 0.2 times the saturation magnetization Ms. 
     An article entitled “Very High Density and Low Cost Perpendicular Magnetic Recording Media Including New Layer Structure ‘U-Mag’”, by Matsunuma et al., IEEE Trans on Magnetics, Vol 41, No. 2, February 2005, discloses a new layered structure, named “U-Mag”, for perpendicular recording media. The stacked films include a very thin ferromagnetic Co layer (2 nm) and lattice spacing control layers. The structure formed with a 100 nm soft magnetic under layer with high coercivity shows a higher signal to noise ratio than a medium using a conventional Ru underlayer. 
     Fabrication of one type of prior art perpendicular recording media (PMR) employs a Ru hcp (hexagonal closed packed) under layer to control the c-axis orientation of the Co based magnetic recording layer. The Ru growth and its structural characteristics are critical for achieving the desired magnetic and microstructural properties of the recording medium. The Ru hcp under layer is grown on a seed layer such as Ni 80 Fe 20  and the Ru growth parameters (the sputter pressure, growth rate, etc) are optimized to improve its crystallographic properties and to improve lattice matching to the Co alloy layer.  FIG. 1  (Prior Art) shows a prior art perpendicular media architecture  100  in which dual hcp under layers  114   a ,  114   b  of Ru grown at different sputter pressures and having different thicknesses are employed to control the microstructural properties of the PMR CoPtCr—SiO 2  magnetic recording layer  118 . The use of such Ru hcp dual under layers to improve recording media performance is consistent with the teachings of Hikosaka, U.S. Pat. No. 6,670,056. Typically, layer  114   a  is 5 nm thick, grown at a sputtering pressure of 5 mTorr, and layer  114   b  is 12 nm thick, grown at a sputtering pressure of 55 mTorr. Alternatively, layers  114   a  and  114   b  may be combined into a single layer, grown at a single sputtering pressure. The structure shown in  FIG. 1  includes a 2 nm thick layer  112  of Ni 80 Fe 20  to nucleate the desired growth orientation of the subsequently grown Ru hcp under layers  114 . Layer  112  is grown over a pair of 75 nm CoTaZr soft under layers (SUL)  106  and  110 , which are separated by a 0.7 nm Ru layer  108 . The SUL layers  106  and  110  are deposited over substrate  102  and AlTi layer  104 . Overcoat layers  120  are deposited on top of recording layer  118 , and include protective and lubricating components. The layers intercalated between the top of the SUL  110  and the CoPtCr—SiO 2  alloy magnetic recording layer  118  may be referred to as the Inter-Mediate Layers or IML for short. 
     Variations of this structure have been implemented to fit media fabrication constraints (such as number of available sputter targets) and include replacing the dual hcp under layers by a single Ru hcp under layer grown at an optimized high sputter pressure. In addition other workers in the field have replaced the Ni 20 Fe 80  nucleation layer by different Ni alloys such as NiCr, NiV, NiW. Furthermore different alloys such as NiAl, CrTa and CuNb have been investigated as Ru nucleation layers in order to improve its micro-structural properties. 
     Ru is chosen as a component in prior art IMLs for a number of reasons. Firstly, the Ru hcp under layer  114  is produced with a strong crystallographic texture as a result of its basal plane being predominantly aligned parallel to the film plane of the Ni 20 Fe 80  nucleation layer  112 . Secondly, Ru is chosen to achieve lattice matching between its hexagonal plane and that of the CoPtCr—SiO 2  alloy in magnetic recording layer  118 . A representative schematic showing a typical hcp structure and lattice constants a and c are found in  FIG. 3  (Prior Art). Thirdly, control of grain size and grain size distribution of the Ru hcp under layer  114  is employed to control the grain size of the CoPtCr—SiO 2  alloy in magnetic recording layer  118 . Although implementation of Ru in prior art IML&#39;s has led to significant improvements in perpendicular media performance, there exists inherent limitations to this solution. The lattice parameters of the Ru hcp under layer  114  can be modified only up to a certain point by altering growth conditions. The evolution of defects, faults and stress relaxation imposes a hard limit to lattice parameter changes. Additionally, control of the grain size, grain size distribution, and nucleation kinetics required for the formation of the desired crystallographic orientation is limited when employing prior art processing technology. 
     What is needed is an IML structure that provides micro-structural improvements of the magnetic recording layer and a large improvement in magnetic recording performance when compared to prior art PMR media employing the same magnetic recording layer. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a perpendicular recording media containing a magnetic recording layer, a soft under layer and, an intermediate layer structure disposed between the magnetic recording layer and the soft under layer. The intermediate layer further contains a first nucleation layer formed over the soft under layer comprising a Ni alloy, a second nucleation layer formed over the first nucleation layer comprising a Ni alloy including at least two other elements, and an hcp under layer formed over the second nucleation layer comprising a Ru alloy. 
     It is another object of the present invention to provide a perpendicular recording media containing a magnetic recording layer, a soft under layer and, an intermediate layer structure disposed between the magnetic recording layer and the soft under layer. The intermediate layer further contains a first nucleation layer formed over the soft under layer comprising a Ni—Fe alloy, a second nucleation layer formed over the first nucleation layer comprising a Ni—Fe—Cr alloy, and an hcp under layer formed over the second nucleation layer comprising a binary Ru—Cr alloy. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be better understood when consideration is given to the following detailed description thereof. Such description makes reference to the annexed drawings, wherein: 
         FIG. 1  (Prior Art) is a partial cross sectional schematic view of the layered structure of a perpendicular recording medium; 
         FIG. 2  is a partial cross sectional schematic view of a layered structure of a perpendicular recording medium according to an embodiment of the present invention; 
         FIG. 3  (Prior Art) is a schematic view of a hexagonal close packed crystal structure showing lattice constants a and c; 
         FIG. 4  is an out of plane XRD plot illustrating improved lattice matching according to an embodiment of the present invention; 
         FIG. 5  is an in plane XRD plot illustrating the lattice matching comparison of a prior art PMR structure with an example tri-layer IML structure according to an embodiment of the present invention; 
         FIG. 6  is a table showing various film thickness and deposition pressures for the comparison data of  FIG. 5 , according to an embodiment of the present invention; 
         FIG. 7  is a table showing various film properties for the comparison data of  FIG. 5 , according to an embodiment of the present invention; 
         FIG. 8  is a plot showing signal to noise ratio versus flux density according to an embodiment of the present invention; 
         FIG. 9  is a plot showing normalized media noise power (NmNP) versus flux density according to an embodiment of the present invention; 
         FIG. 10  is a plot showing saturation performance versus write current according to an embodiment of the present invention; 
         FIG. 11  is a plot showing the log of the byte error rate (BER) versus bit density according to an embodiment of the present invention; and, 
         FIG. 12  is a bar chart comparing the log of the byte error rate of a prior art media with an example embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention seeks to provide an improved IML composition and structure to overcome the inherent limitations of prior art IML fabrication and materials choices. 
     First, the Ni 80 Fe 20  nucleation layer  112  for the growth of the hcp under layer is replaced by dual nucleation layers. Employing such a dual layer structure achieves a number of improvements over prior art approaches. Through selection of the thickness ratio of the dual nucleation layers, the present invention provides a wider range of lattice parameters when compared to a single nucleation layer. This results in a wider range of thin film strain values. Additionally, altering the thickness of the dual nucleation layers of the present invention can result in new single phase materials with novel properties, particularly if the layer thickness is on the order of the diffusion length at the growth temperature. Each of the layers in the dual nucleation layer structure may have different surface energies, chemical properties, porosity and other micro-structural attributes which can be combined and optimized to satisfy different material functions. These include but are not limited to good adhesion to the soft under layer (SUL)  110 , and increased wet-ability for the formation of thermodynamically hindered crystallographic phases for films grown on a subsequently deposited hcp under layer. 
     Second, the Ru under layers  114   a,b  are replaced by a Ru binary alloy hcp under layer whose lattice parameters more closely matches that of the magnetic alloy layer  118 . 
     The foregoing modifications to the IML structure of the present invention result in significant improvements in the performance of the PRM, as shall be demonstrated in subsequent examples below. 
       FIG. 2  is a partial cross sectional schematic view  200  of a layered structure of a perpendicular recording medium according to an embodiment of the present invention. Layers  202 ,  204 , and  206  comprise the novel tri-layer IML structure of the present invention. Layers  202  and  204  comprise the dual nucleation layers, and layer  206  comprises the Ru binary alloy hcp under layer. Together, layers  202 - 206  replace layers  114   a ,  114   b , and  112  of the prior art structure shown in  FIG. 1 . 
     Layer  202  may be composed of binary alloys of Ni, in the form Ni x M 1−x , where for the purposes of this disclosure, x represents the Ni composition in atomic %. For example a Ni 80 Fe 20  alloy would contain 80 atomic % Ni and 20 atomic % Fe. Preferably, M is Fe. However M may also be chosen from among the group containing Mn, Co, V, W and Cu. For Ni x Fe 1−x  alloys, x may be within the range of 72 to 92 atomic %, but is preferably about 80 atomic %. The thickness of layer  202  is within the range of 0.1 to 6 nm, preferably about 2.0 nm. Layer  202  is preferably deposited by sputtering, at a pressure range between 1 and 10 mTorr, preferably at about 5.6 mTorr. Layer  202  may also be composed of Ni alloys including two or more other elements. One such alloy is Ni x Fe y Cr (1−x−y) , where x+y may be within the range of 72 to 92 atomic %, but is preferably about 80 atomic %. 
     Layer  204  may be composed of ternary alloys of Ni, in the form Ni x A y B (1−x−y) , where A and B are chosen from among the group containing Fe, Cr, Mn, Co, V, W and Cu. Preferably, A is Fe and B is Cr. For Ni—Fe—Cr alloys, x may be within the range of 40 to 80 atomic %, y within the range of 8 to 30 atomic %, and the Cr concentration within the range of 15 to 40 atomic %. Preferably, x is within the range of 64 to 50 atomic %, y is within the range of 16 to 12 atomic %, and the Cr concentration within the range of 20 to 38 atomic %. The thickness of layer  204  is within the range of 0.1 to 6 nm, preferably about 1.8 nm. Layer  204  is preferably deposited by sputtering, at a pressure range between 1 and 10 mTorr, preferably at about 5.0 mTorr. Layer  204  may also be composed of Ni alloys including three or more other elements. One such alloy is Ni x Fe y Cu z Cr (1−x−y−z) , where x is within the range of 64 to 50 atomic %, y+z is within the range of 16 to 12 atomic %, and the Cr concentration within the range of 20 to 38 atomic %. 
     Layer  206  may be composed of a binary Ru alloy, in the form Ru x D (1−x) , where D is chosen from among Cr, Mn, V, Co, Fe, Cu, Re, Os and Zn, but is preferably Cr. For Ru x Cr (1−x)  alloys, x is within the range of 65 to 85 atomic %, but is preferably about 75 atomic %. The thickness of layer  206  is within the range of 6 to 24 nm, preferably about 16 nm. Layer  206  is preferably deposited by sputtering, at a pressure range between 20 and 100 mTorr, preferably at about 46 mTorr. Layer  206  may also be composed of Ru or an Ru alloy including 2 or more other elements. 
     EXAMPLE EMBODIMENTS 
     The following example embodiments illustrate the improvements obtained by the present invention over typical prior art. They, in no way, are meant to limit the scope or application of the invention. 
       FIG. 4  is an out of plane XRD (x-ray diffraction) plot  400  illustrating improved lattice matching according to an embodiment of the present invention. The figure compares out-of-plane spectra for Ru (curve  402 ) and Ru 75 Cr 25  (curve  404 ) under layers grown under identical sputtering conditions on 2.0 nm thick Ni 80 Fe 20  nucleation layers. The shift in the peak position  410  (for the reflection corresponding to the [002] Ru 75 Cr 25  basal plane) from the peak position  408  (corresponding to pure Ru) indicates that the a-parameter for the Ru 75 Cr 25  alloy is smaller than that of Ru. Please refer to  FIG. 3  (Prior Art) which depicts the lattice parameters for an hcp unit cell of Ru and Ru 75 Cr 25 . In  FIG. 4 , the closer the peak position of the under layer to that of the CoX peak  406 , the better the lattice match. A perfect match would be obtained as distance  412  approaches zero and the peak position of the Ru containing under layer overlaps the position of the CoX peak  406 . The better lattice matching is the result of the Cr being substitutionally incorporated within the Ru—Cr unit cell, the Cr atomic radius (1.85 Å) being smaller than the atomic radius of Ru (1.89 Å). Lattice parameters measured from the spectra in  FIG. 4  are: a Ru =2.71 Å; a RuCr =2.69 Å; a CoX =2.58 Å. From these measurements, it can be readily seen that the a-parameter for Ru 75 Cr 25  is closer to the value of the CoPtCr—SiO 2  alloy, therefore improving the lattice matching. 
       FIG. 5  is an in plane XRD plot  500  illustrating the lattice matching comparison of a prior art PMR structure (curve  502 ) with an example tri-layer IML structure (curve  504 ) according to an embodiment of the present invention. In this chart, a prior art PMR structure containing a 2.0 nm Ni 80 Fe 20  seed layer  112  and a 16 nm Ru under layer  114  is compared with an example tri-layer IML structure comprising a 2.0 nm Ni 80 Fe 20  layer  202 , a 1.8 nm Ni 64 Fe 16 Cr 20  layer  204 , and a 12.5 nm Ru 75 Cr 25  layer  206  of the present invention. The deposition conditions for the above cited films can be found in  FIG. 6 . All other components of the PMR structure (i.e. layers  102 - 110 ,  118 ,  120 ) were identical and constructed in a manner well known to those skilled in the art. The reflections shown in the spectrum in this grazing incidence measurement originate from the ( 1120 ) planes (see  FIG. 3 ) of the under layers and the CoPtCr—SiO 2  thin films. As can be seen in  FIG. 5 , distance  512  indicates the peak position  510 , corresponding to curve  504  (the IML structure of the present invention), is closer to the CoPtCr—SiO 2  peak position  506  than the peak position  508  of the prior art PMR structure curve  502 . As previously discussed, this indicates better lattice parameter matching of the present invention when compared to the prior art. Further analysis of the data in  FIG. 5  indicates additional micro-structural improvements which are disclosed below in  FIG. 7 . 
       FIG. 6  is a table showing various film thickness and deposition pressures for the comparison data of  FIG. 5 , according to an embodiment of the present invention. Column  602  gives the Ni 80 Fe 20  film thickness and sputter deposition pressure for films in both the prior art and an example of the present invention. Column  604  gives the Ni 64 Fe 16 Cr 20  film thickness and sputter deposition pressure for films in an example of the present invention. Column  606  gives the Ru film thickness and sputter deposition pressure for films in the prior art example. Column  608  gives the Ru 75 Cr 25  alloy film thickness and sputter deposition pressure for films in an example of the present invention. 
       FIG. 7  is a table showing various film properties for the comparison data of  FIG. 5 , according to an embodiment of the present invention. Column  702  compares the Ru 75 Cr 25  alloy RMS strain, in %, of the present invention to that of the prior art Ru films. The Ru 75 Cr 25  film of the present invention shows a reduced RMS strain of 0.47% when compared to the prior art film RMS strain of 0.5%. Column  704  compares the grain size of the Ru 75 Cr 25  alloy of the present invention to that of the Ru prior art. Column  706  compares the Ru 75 Cr 25  alloy texture of the present invention to a Ru film of the prior art. Column  708  compares the CoPtCr—SiO 2  film RMS strain, in %, of the present invention with that of the prior art. The tri-layer IML film structure of the present invention reduces the RMS strain of the magnetic CoPtCr—SiO 2  film from 0.21% to 0.18%, the result of a better lattice matching condition. Likewise, the grain size (column  710 ), texture (column  712 ), and lattice mismatch (column  714 ) of the present invention all show decreases from prior art values, which confirm better lattice matching of the present invention. 
     The remaining  FIGS. 8-12  illustrate the recording media performance of PMR structures built in accordance with teachings of the present invention. A Guzik tester was employed to measure byte error rates (BER), signal to noise ratio, media noise, and saturation performance in accordance with methods well known to those skilled in the art.  FIG. 8  is a plot  800  showing signal to noise ratio (SoNR) versus flux density according to an embodiment of the present invention. The units of measurement are decibels (dB) for SoNR and kfci (kilo-flux-changes/inch) for density. Curve  802  represents the performance of a PMR structure of the present invention. Curve  804  represents the performance of a PMR structure of the prior art. As can be noted in the figure, the signal to noise ratio is significantly improved over the prior art for densities above about 100 kfci, and is particularly better at the higher densities greater than 400 kfci.  FIG. 9  is a plot  900  showing normalized media noise power (NmNP) versus flux density according to an embodiment of the present invention. Curve  902  represents the performance of a PMR structure of the present invention. Curve  904  represents the performance of a PMR structure of the prior art. Data in this plot confirms the improved noise performance of media of the present invention, particularly at higher densities. 
       FIG. 10  is a plot  1000  showing saturation performance according to an embodiment of the present invention. Curve  1002  represents the low frequency track average amplitude (LFTAA) of a previously recorded signal as a function of write current. The sluggish rise of curve  1002  is due in part to a significant increase in the coercivity of the CoPtCr—SiO 2  film from a typical prior art value of 6050 to 7580 Oe. The higher Hc exhibited by the example embodiment of the present invention illustrated in  FIG. 10  requires higher currents to adequately write the medium. The write currents can be reduced, if desired, by reducing the thickness of the Ru 75 Cr 25  layer to lower Hc. 
       FIG. 11  is a plot showing the log of the byte error rate (BER) versus bit density (kbpi or kilo-bits/inch) according to an embodiment of the present invention. A comparison of BER is shown in  FIG. 12 , a bar chart  1200  comparing the log of the byte error rate of a prior art media with an example embodiment of the present invention at linear density of 720 kbpi. Bar  1202  represents the log of the BER for the prior art. Bar  1204  represents the log of the BER of an example embodiment of the present invention. This data indicates that the present invention provides a byte error rate (@ about 10 −4.75 ) approximately an order of magnitude lower than the prior art (@ about 10 −3.75 ). 
     The present invention is not limited by the previous embodiments or examples heretofore described. Rather, the scope of the present invention is to be defined by these descriptions taken together with the attached claims and their equivalents.