Abstract:
An apparatus includes a storage medium, and a transducer positioned adjacent to the storage medium, wherein the transducer includes a first electrode and a second electrode, with the width of the first electrode being less than the width of the second electrode. A method including: applying a first voltage to a transducer to write data to a storage medium, and applying a second voltage to the transducer to read data from the storage medium, wherein the magnitude of the first voltage is greater than the magnitude of the second voltage.

Description:
FIELD OF THE INVENTION 
     This invention relates to data storage devices, and more particularly to probe storage devices. 
     BACKGROUND OF THE INVENTION 
     Probe storage devices have been developed to provide small size, high capacity, low cost data storage devices. Probe recording requires relative movement between a storage medium and an array of probe transducers that are used to subject the storage medium to electric, magnetic, or other fields. The storage medium can be a planar thin film structure. 
     Probe-based data storage devices use a large number of probe transducers that move over a storage medium surface, with each probe needing to move only a relatively small distance with respect to the medium, in a manner similar to a Scanning Probe Microscope (SPM). To maximize the achievable recording density, the probes are generally operated in physical contact or near-contact with the surface of the storage medium. 
     Scanning probe storage devices based on a ferroelectric storage medium include one or more transducers, each including an electrode or tip that moves relative to a ferroelectric thin film storage medium. To write a bit to the storage medium, a voltage pulse of either positive or negative polarity is applied between the electrode and the medium, and a binary “1” or “0” is stored by causing the polarization of a spatially small region (i.e., a domain) of the ferroelectric storage medium near the tip to point “up” or “down”. Data can then be read out by a variety of means, including sensing of piezoelectric surface displacement, measurement of local conductivity changes, or by sensing current flow during polarization reversal (i.e., destructive readout). 
     Destructive readout can be performed by applying a read voltage of a magnitude and polarity such as to cause the polarization to point “up”. Domains polarized “down” (e.g., representing “0”) will then switch to the “up” state, and a charge will flow which is proportional to the remanent polarization of the ferroelectric storage medium. Domains polarized “up” will have no such current flow. The presence or absence of this current flow, as determined by a sense amplifier, can then be used to determine whether the domain had contained a “1” or “0”. 
     Probe storage devices can provide a high data storage capacity in a very small form factor. In one example, a device having a capacity of 20 GB requires a density of 880 Gb/in 2 . The device of this example requires a track width of 50 nm, which places difficult requirements on the servo system accuracy. Current transducer designs use the same conductor for both reading and writing, which places a strong limit on Write-to-Read Track MisRegistration (WRTMR). Write-to-Read Track MisRegistration is the positioning requirement based on the need to read written signals with sufficient signal-to-noise ratio (SNR). Write-to-Write Track MisRegistration (WWTMR) is the positioning requirement based on the need to not overwrite adjacent tracks. 
     There is a need for a data storage device that can provide a large amount of data storage but have less restrictive track misregistration requirements. 
     SUMMARY OF THE INVENTION 
     In a first aspect, the invention provides an apparatus including a storage medium, and a transducer positioned adjacent to the storage medium, wherein the transducer includes a first electrode and a second electrode, with the width of the first electrode being less than the width of the second electrode. 
     The transducer can be supported by a suspension assembly or cantilever support structure. Ends of the first and second electrodes can be centered on a common axis. The apparatus can further comprise a third electrode, wherein the second and third electrodes are positioned on opposite sides of the first electrode and the width of the first electrode is less than the width of the third electrode. 
     The electrodes can be mounted on a silicon body with conductors connected to the first and second electrodes lying adjacent to crystallographic planes of the silicon body. 
     In another aspect, the invention provides a method including: applying a first voltage to a transducer to write data to a storage medium, and applying a second voltage to the transducer to read data from the storage medium, wherein the magnitude of the first voltage is greater than the magnitude of the second voltage. The storage medium can be a ferroelectric storage medium. 
     In another aspect, the invention provides a method including: positioning a transducer adjacent to a storage medium, wherein the transducer includes a first electrode and a second electrode, with the width of the first electrode being less than the width of the second electrode, and applying the same voltage to the first and second electrodes when reading data from the storage medium. The storage medium can be a ferroelectric storage medium. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a data storage device that can be constructed in accordance with an aspect of the invention. 
         FIG. 2  is a cross-sectional view of a probe storage device. 
         FIGS. 3 ,  4  and  5  are side, top and end views of a cantilever transducer that can be used in a probe storage device. 
         FIG. 6  is a schematic representation of two tracks of written data on a storage medium. 
         FIG. 7  is a schematic representation of a track of written data and a track of a read head. 
         FIG. 8  is a graph showing the WWTMR and WRTMR position error bounds for 50 nm track pitch and 20 dB signal-to-noise ratio (SNR). 
         FIG. 9  is a graph showing a position error signal (PES) bound for a variable reader width. 
         FIG. 10  is a graph showing the effect of the reader width lower bound on the PES bound. 
         FIGS. 11 ,  12  and  13  are side, top and end views of a cantilever transducer constructed in accordance with an example of the invention. 
         FIGS. 14 ,  15  and  16  are side, top and end views of another cantilever transducer constructed in accordance with another example of the invention. 
         FIG. 17  is a cross-sectional view of a transducer assembly. 
         FIG. 18  shows an isometric view of the transducer of the assembly of  FIG. 17 . 
         FIG. 19  is a block diagram of a system that can be used to implement one aspect of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings,  FIG. 1  is a perspective view of a probe storage device  10  that can be constructed in accordance with an aspect of the invention. In the storage device  10  of  FIG. 1 , an array  12  of transducers  14 , also called probes, tips or heads, are positioned adjacent to a storage media  16 . The ends of the probes  14  and a recording surface of the storage media  16  lie in planes that are generally parallel with each other. The probes  14  are electrically coupled to connectors  18  through a controller, not shown. The storage media  16  can be coupled to at least one actuator (not shown in this view), which is configured to move the medium  16  relative to array  12 . This movement causes individual storage locations or domains on medium  16  to be moved relative to the transducers. Each transducer can include one or more electrodes. The storage medium in the example of  FIG. 1  can be, for example, a ferroelectric, magnetic or optical storage medium. 
     Probe storage devices include actuators and suspension assemblies for providing relative movement between the storage medium and an array of probes.  FIG. 2  is a cross-sectional view of a probe storage device  30 , The device includes an enclosure  32 , also referred to as a case, base, or frame, which contains a substrate  34 . An array of probes  36  is positioned on the substrate. The probes extend upward to make contact with a storage media  38 . The storage media  38  is mounted on a movable member, or sled  40 . In this example, relative movement between the storage media and the probes is provided by an electromagnetic actuator that includes coils and magnets. Coils  42  and  44  are mounted on the movable member. Magnets  46  and  48  are mounted in the enclosure near the coils. Springs  50  and  52  form part of a suspension assembly that supports the movable member. The enclosure  32  can be formed of, for example, injection molded plastic. While  FIG. 2  shows one example of a probe storage device, it will be recognized that other known types of suspensions and actuators can be used to position the components and to provide relative movement between the probes and the storage media. This invention is not limited to devices that use any particular probe and media positioning and/or actuating devices. 
     In ferroelectric probe storage devices, the transducers include an electrode that is used to subject the storage media to an electric field. The data can be written in tracks on the storage medium. Track squeeze occurs when data in adjacent tracks interferes with data in a track of interest. 
     To eliminate track squeeze, the position of the transducers with respect to the storage medium must be tightly controlled. More specifically, in one example the position error signal (PES) cannot be allowed to exceed ½ the free space between tracks, or 
                          PES        ≤       (       1   TPI     -   W     )     2       ,           (   1   )               
where TPI is the tracks per inch, and W is the width of the writer. The position error signal is an output of a position error signal demodulator, and represents a spatial deviation from a center of a track of data.
 
     In current probe storage devices, a single electrode is used for both reading and writing. In a transducer that uses a single electrode, from Write-to-Read Track MisRegistration (WRTMR) considerations, the transducer must have at least a certain percentage of its tip width exposed to correct data. That is, the absolute value of the PES must be 
                          PES        ≤         (     1   -   α     )     ⁢   W     2       ,           (   2   )               
where α is the percentage of the width of the read transducer that sees the correct data. In this case, the signal-to-noise ratio (SNR) is fundamentally related to the PES limit by
 
     
       
         
           
             
               
                 
                   SNR 
                   ≥ 
                   
                     
                       α 
                       
                         1 
                         - 
                         α 
                       
                     
                     . 
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     For a 50 nm track pitch and a 20 dB SNR, equations (1) and (2) combine to place a bound on the PES of
 
|PES|≦2.1 nm.  (4)
 
     In probe storage devices, the electrode can be brought into contact or near-contact with a surface of a storage medium using a variety of devices, such as levers that raise the metal electrode into contact with the surface of the storage medium, or springs or other structures that press the electrode into contact with the surface of the storage medium. 
       FIGS. 3 ,  4  and  5  are side, top and end views of a cantilever transducer  60  that can be used in a probe storage device. The transducer is mounted on a substrate  62  by a base  64 . An electrode  66  is supported by a bi-layer structure  68  having layers  70  and  72  of different mechanical properties such that the transducer curves toward a storage medium  74 . 
       FIG. 6  is a schematic representation of two tracks  80  and  82  of written data on a storage medium  84 . Line  86  shows the ideal center of track  80 , and line  88  shows the ideal center of track  82 . Line  90  shows the actual center of track  80 , and line  92  shows the actual center of track  82 . The maximum distance between the actual centers of the tracks is the Write-to-Write Track MisRegistration. In this example, arrows  94  and  96  show the Write-to-Write Track MisRegistration. 
       FIG. 7  is a schematic representation of a track  100  of written data and a track  102  of a read transducer on the data storage medium  84 . Line  104  shows the ideal center of both the written data track  100  and the read head track  102 . Line  106  shows the actual center of track  100 , and line  108  shows the actual center of track  102 . The maximum distance between the centers of the written data track and the read head track is the Write-to-Read Track MisRegistration. In this example the Write-to-Read Track MisRegistration is illustrated by the distance between arrows  110  and  112 . 
       FIG. 8  shows the bounds given by WWTMR and WRTMR for a 50 nm track pitch and a 20 dB signal-to-noise ratio (SNR). The result given in equation (4) is at the intersection of curves  114  and  116 , i.e., it is the bound for the optimal transducer width. The curve for WRTMR can move up or down depending on the desired minimum SNR. 
     In various transducer configurations, multiple electrodes can be used, i.e., a reader electrode and a writer electrode.  FIG. 9  shows the PES bound for a variable reader width. If the reader width is allowed to be less than the writer width, the PES bound may be relaxed. The PES bound for this case is given by 
                          PES        ≤         W   w     +       (     1   -     2   ⁢   α       )     ⁢     W   r         4       ,           (   5   )               
where W w  and W r  are the writer and reader width, respectively. Note that equation (5) reduces to equation (2) when W r =W w . The sensitivity of the PES bound to the reader width is given by
 
                         ∂          PES        max         ∂     W   r         =       1   -     2   ⁢   α       4       ,           (   6   )               
which means that for realistic values of α (i.e., approaching 1) the PES bound increases at approximately ¼ the rate that W r  decreases. Of course, the reader width is also bounded by head/media SNR issues, so the PES bound cannot be increased arbitrarily. For the constraint that W r  is greater than 30 nm, and with the conditions imposed on equation (4), the PES bound is increased to
 
|PES|≦4.2 nm.  (7)
 
This is a factor of 2 improvement over the case with equal reader/writer widths.  FIG. 10  shows the relationship between the lower bound on W r  and the PES limit.
 
     Previous head designs use the same conductor for both reading and writing. In one aspect, the invention uses separate read and write electrodes, where the read electrode is narrower than the write electrode. A narrow read electrode configuration would allow less restrictive positioning accuracy requirements. 
     In one example, the invention provides a head having multiple conductors.  FIGS. 11 ,  12  and  13  are side, top and end views of a cantilever head  120 . The head is mounted on a substrate  122  by a base  124 . Two electrodes  126  and  128  may be supported by a bi-layer structure  130  having layers  132  and  134  of different mechanical properties such that the head curves to a storage medium  136 . Electrode  126  forms a read element and electrode  128  forms a write element. The read element has a width W 1  that is less than the width W 2  of the write element. An insulating layer  138  is provided between the read element and layer  132  to center the read element end on a center line  140  of the write element end. Arrow  142  shows the direction of movement of the storage medium. Center line  140  lies parallel to the down track direction of data written on the storage medium. The widths of the electrodes are measured in the cross track direction. The extent to which the head may be narrower than the writer depends on the minimum head width needed to achieve an acceptable SNR. 
     As shown in  FIG. 13 , the read head may be fabricated on an insulating layer that centers it with respect to the write element. This centering eliminates the need for moving the transducer between reading and writing, but adds a processing step. To avoid capacitive coupling, the write element may have the same voltage applied as the read element during reading, with the result being a narrow read/wide erase. The wide erase may also be useful to the servo subsystem of the controller. 
     In this example, one additional step can be used to lift off a thickness of approximately ½ the difference between the thicknesses of the reader and writer. The deposition could be performed using a liftoff step to deposit the insulator beneath the reader electrode. To get the desired thickness, atomic layer deposition (ALD) can be used. If the cross-talk is sufficiently low, the write element may be used to immediately rewrite data after a read. 
     In another example the read element can be fabricated directly onto the lever, without the insulating layer. This is easier to manufacture, but requires a “micro-jog” between reading embedded servo information and writing, i.e., the head must be moved slightly between reading and writing to have the read and write elements centered on the same track. 
       FIGS. 14 ,  15  and  16  are side, top and end views of another cantilever head  150  constructed in accordance with another aspect of the invention. The head is mounted on a substrate  152  by a base  154 . Three electrodes  156 ,  158  and  160  are supported by a bi-layer structure  162  having layers  164  and  166  of different mechanical properties such that the head curves to a storage medium  168 . Electrode  158  forms a read element and electrodes  156  and  160  form write elements. The read element has a width W 1  that is less than the width W 2  of the write elements. An insulating layer  170  is provided between the read element and layer  164  to center the read element end on a center line  172  of the write element end. Arrow  174  shows the direction of movement of the storage medium. Center line  172  lies parallel to the down track direction of data written on the storage medium. The widths of the electrodes are measured in the cross track direction. 
     The conductors of  FIGS. 11-16  can be fabricated using the same techniques as those used to create the probe head of  FIGS. 3 ,  4  and  5 . Two or three conductors may be used depending on the need for bi-directional read support. 
     The example of  FIGS. 14 ,  15  and  16  is similar to the example of  FIGS. 11 ,  12  and  13 , except that a third conductor is added. The third conductor allows wide erase in both directions, but there will be capacitive coupling with at least one write element. Also, the third conductor requires a longer head, which compounds skew concerns. If cross-talk is sufficiently small, a write-read-write technique could be used to verify data while writing. 
     When using multiple conductors for each reader and writer, the readers and writers should be lined up with each other. In one example, the PES may be controlled to within about &lt;2 nm. Since an array of readers and writers would be used, the alignment between the readers and writers must be at near atomic resolution. To accomplish this, a planar, non-lever structure, as shown in  FIGS. 17 and 18 , can be used to define the alignment of the reader and writer electrodes on the natural etching planes of silicon. 
       FIG. 17  is a cross-sectional view of the assembly of  FIG. 18 , and shows another example, which includes a head assembly  180  having a head  182  mounted on a spring suspension  184 . The spring suspension is positioned over an opening  186  in a substrate  188 . A read electrode  190  can be positioned adjacent to a surface  192  of the head. The read electrode  190  is electrically connected to a terminal such as item  18  in  FIG. 1 , using conductors not shown in this view. For example, the read electrode can be connected to a conductor that extends through or on the head body  194  to the suspension, which can be made of conductive material. A via  196  can be provided in the substrate to connect the suspension to a conductor  198  on the bottom of the substrate. Conductor  198  could then be connected to a terminal. Various types of suspensions can be used to support the head and to hold the head in contact with an adjacent storage medium, or to move the head into contact with the storage medium. This invention is not limited to any particular type of suspension. 
       FIG. 18  shows an isometric view of the head  182 , which includes a write electrode  200  and a read electrode  190 . The width of the end of the write electrode is greater than the width of the end of the read electrode. The write electrode is electrically connected to a conductor  202 . The read electrode is electrically connected to a conductor  204 . An insulator  206  provides electrical isolation between the read and write electrodes. The sides  208  and  210  of the insulator can be positioned in crystallographic planes of the material used to form the head body. 
     Silicon has the property of having a natural etch stop on (111) crystallographic planes. This can be used to etch a (100) surface, where the (111) surfaces will form an atomically smooth surface at a 54.7° angle to (100) plane. The same technique can be used on other crystal orientations to give similar results. For example, etching of a (110) surface will form a (111) etch surface at 90° to the (110) plane. 
     The width differences between the reader and the writer can be controlled very precisely and the readers and writers can be lined up with atomic resolution. This alignment is important in that any variation among an array of readers/writers will cause misalignment between the readers and writers. 
     The reader and writer wall profiles  212  and  214 , defined in this case by the crystallographic etch planes (e.g. (111)) of silicon, are parallel to each other. At the end of processing, the upper surface of the head can be trimmed to make the electrode edges adjacent to a crystallographic etch plane. The conductors are placed far enough from each other to eliminate cross-talk concerns, but close enough together that they do not create a head skew issue. 
     In another aspect, the invention uses separate read and write voltages to achieve a narrow read/wide write scheme. Heads that are used to implement this aspect of the invention can use a single conductor for both reading and writing, but use different voltages for reading and writing so that the effective field width is narrower for reading than for writing. 
       FIG. 19  is a block diagram of a system  220  that can be used to implement one aspect of the invention. The system includes a host device  222 , which can be for example a computer or other device that operates in combination with a data storage device  224 . A controller  226  can be used to control the operation of the data storage device. The controller can include, for example, a read channel  228 , a write channel  230 , and a servo system  232  that controls the position of heads in the data storage device. 
     To write a bit to the storage medium, a voltage pulse of either positive or negative polarity can be applied between the electrode and the medium, and a binary “1” or “0” is stored by causing the polarization of a spatially small region (i.e., a domain) of the ferroelectric storage medium near the tip to point “up” or “down”. Readout can be performed by applying a read voltage of a magnitude and polarity such as to cause the polarization to point “up”. Domains polarized “down” (e.g., representing “0”), will then switch to the “up” state, and a charge will flow which is proportional to the remanent polarization of the ferroelectric storage medium. Domains polarized “up” will have no such current flow. The presence or absence of this current flow, as determined by a sense amplifier, can then be used to determine whether the domain had contained a “1” or “0”. 
     When used in combination with a probe type data storage device having a ferroelectric storage medium, and heads that include at least one electrode, the write voltage can have a larger magnitude than the read voltage. By using a higher write voltage, the bits written to the storage medium will have a larger size, and the range of acceptable positions of the head during reading will be larger. 
     To demonstrate the voltage-dependency of the written tracks, a coarse bit pattern was written to a ferroelectric storage medium, and the size of the recording bits was examined. The results of this demonstration show that the data track width increases with increasing voltage. 
     While the invention has been described in terms of several examples, it will be apparent to those skilled in the art that various changes can be made to the described examples without departing from the scope of the invention as set forth in the following claims.