Abstract:
A slider assembly for selectively altering a position of a transducing head with respect to a track of a rotatable disc having a plurality of concentric tracks includes a slider body having a main portion and a head portion separated by a gap. The head portion carries the transducing head. The slider body is arranged to be supported by a support structure over a surface of the rotatable disc. A pair of structural elements are disposed on opposite side surfaces of the slider body between the main portion and the head portion across the gap. At least one of the structural elements is a microactuator responsive to electrical control signals to selectively bend to alter the position of the head portion with respect to the main portion of the slider body. The structural elements may be complementary microactuators. The microactuators may be formed by a process involving forming the microactuators on a slider substrate or on a row of sliders, forming the microactuators on a slider stack, or separately forming the microactuators and attaching the microactuators to a slider stack.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims priority from Provisional Application No. 60/055,874 filed Aug. 15, 1997 for “Piezoelectric Head Moving Microactuator For Ultrahigh Track Density Magnetic Recording Drives” by V. Novotny. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a disc drive microactuator, and more particularly to a high resolution head positioning mechanism having one or more piezoelectric elements attached to a slider as a bendable cantilever for selectively moving a head portion of the slider radially with respect to circumferential data tracks of a rotatable disc. 
     The track density, or radial spacing, between concentric data tracks on magnetic discs continues to increase, requiring greater precision in head positioning. Conventionally, head positioning is accomplished by operating an actuator arm with a large-scale actuator motor, such as a voice coil motor, to position a head on a flexure at the end of the actuator arm. The large-scale motor lacks sufficient resolution and bandwidth to effectively accommodate high track-density discs. Thus, a high resolution head positioning mechanism is necessary to accommodate the more densely spaced tracks. 
     One promising design for high resolution head positioning involves employing a high resolution microactuator in addition to the conventional low resolution actuator motor, thereby effecting head positioning through dual-stage actuation. Various microactuator designs have been considered to accomplish high resolution head positioning, including piezoelectric, electromagnetic, electrostatic, capacitive, fluidic, and thermal actuators. Various locations for the microactuator have been suggested, including on the slider, on the gimbal, at the interface between the gimbal and the slider, and on the actuator arm, for example. However, the previous designs all had shortcomings that limited the effectiveness of the microactuator, such as substantial performance limitations or manufacturing complexities, which made the microactuator designs impractical. An effective microactuator design must provide high acceleration in positioning the head while also generating sufficiently large and accurate displacements to precisely move the head across several data tracks on the disc. 
     There is a need in the art for a microactuator design to provide high resolution head positioning with superior bandwidth performance characteristics that can be implemented by simple and readily available manufacturing processes. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is a slider assembly for selectively altering a position of a transducing head with respect to a track of a rotatable disc having a plurality of concentric tracks. The slider assembly includes a slider body having a main portion and a head portion separated by a gap. The head portion carries the transducing head. The slider body is arranged to be supported by a support structure over a surface of the rotatable disc. A pair of structural elements are disposed on opposite side surfaces of the slider body between the main portion and the head portion across the gap. At least one of the structural elements is a microactuator responsive to electrical control signals to selectively bend to alter the position of the head portion with respect to the main portion of the slider body. The structural elements are preferably complementary microactuators. 
     Another aspect of the present invention is a process of forming a pair of microactuators on each of a plurality of sliders. A slider substrate is formed comprising main portions of the sliders, and a sacrificial layer is deposited on the slider substrate. Head portions each carrying a transducing head are then formed on the sacrificial layer. A row of sliders is separated from the slider substrate, an air-bearing surface is formed on each of the sliders in the row, and notches are cut between individual sliders in the row. Microactuators are formed between the main portions and the head portions of the sliders on side surfaces of the sliders in the notches. Alternatively, the microactuator processing may be performed at the wafer level, prior to the steps of separating the slider substrate into rows and defining the air-bearing surfaces of the sliders in the rows. Individual sliders are separated from the slider row such that a pair of microactuators are on opposite side surfaces of each of the sliders. The sacrificial layer is removed to form a gap between the main portions and the head portions of the sliders. 
     A further aspect of the present invention is a process of forming a microactuator on each of a plurality of sliders. A slider substrate is formed comprising main portions of the sliders, and a sacrificial layer is deposited on the slider substrate. Head portions each carrying a transducing head are then formed on the sacrificial layer. A row of sliders is separated from the slider substrate, and an air-bearing surface is shaped for each of the sliders in the row. Individual sliders are separated from the row of sliders, and a plurality of the individual sliders are glued together to form a slider stack. A plurality of microactuators are formed on side surfaces of each of the sliders in the slider stack, and the sacrificial layer is etched away to form a gap between the main portions and head portions of each of the sliders. The sliders are separated from each other by dissolving the glue. Alternatively, the plurality of microactuators may be separately formed and attached to the side surfaces of the sliders stack, and then separated into individual microactuators for each of the sliders in the slider stack. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top view of a disc drive actuation system for positioning a slider over tracks of a disc. 
     FIG. 2 is a top view of a slider including piezoelectric microactuators for high resolution head positioning according to the present invention. 
     FIG. 3 is a side view of the slider shown in FIG.  2 . 
     FIG. 4 is an enlarged top view of a portion of the slider shown in FIG. 2 illustrating the construction of the piezoelectric microactuator according to a first embodiment of the present invention. 
     FIG. 5 is an enlarged top view of a portion of the slider shown in FIG. 2 illustrating the construction of the piezoelectric microactuator according to a second embodiment of the present invention. 
     FIG. 6 is a diagram illustrating a unimorph piezoelectric bending motor in its neutral position. 
     FIG. 7 is a diagram illustrating the unimorph piezoelectric bending motor of FIG. 6 in its actuated position. 
     FIG. 8 is a diagram illustrating a bimorph piezoelectric bending motor configured for parallel operation. 
     FIG. 9 is a diagram illustrating a bimorph piezoelectric bending motor configured for series operation. 
     FIG. 10 is a diagram of a typical slider substrate wafer. 
     FIG. 11 is a perspective view of a portion of the substrate wafer shown in FIG. 10 in enlarged detail, illustrating a process according to a third embodiment of the present invention. 
     FIG. 12 is a top view of a slider row processed according to a fourth embodiment of the present invention. 
     FIG. 13 is a side view of the slider row shown in FIG. 12, with a portion shown in enlarged detail to illustrate the microactuator. 
     FIG. 14 is a perspective view of a stack of sliders on which to form piezoelectric microactuators according to a fifth embodiment of the present invention. 
     FIG. 15 is a perspective view of the stack of sliders shown in FIG. 10 with piezoelectric microactuators formed thereon according to the fifth embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a top view of a disc drive actuation system  10  for positioning slider  24  over a track  34  of disc  30 . Actuation system  10  includes voice coil motor (VCM)  12  arranged to rotate actuator arm  16  around axis  14 . Head suspension  18  is connected to actuator arm  16  at head mounting block  20 . Flexure  22  is connected to an end of head suspension  18 , and carries slider  24 . Slider  24  carries a transducing head (not shown in FIG. 1) for reading and/or writing data on concentric tracks  34  of disc  30 . Disc  30  rotates around axis  32 , so that windage is encountered by slider  24  to keep it aloft a small distance above the surface of disc  30 . 
     VCM  12  is selectively operated to move actuator arm  16  around axis  14 , thereby moving slider  24  between tracks  34  of disc  30 . However, for disc drive systems with high track density, VCM  12  lacks sufficient resolution and frequency response to position a transducing head on slider  24  over a selected track  34  of disc  30 . Therefore, a higher resolution actuation device is necessary. 
     FIG. 2 is a top view and FIG. 3 is a side view of slider  24  including piezoelectric microactuator  42   a  and structural element  42   b  to enable high resolution positioning of head  41  according to the present invention. Slider  24  includes a head portion  40  carrying transducing head  41  and also includes air gap or space  44  between head portion  40  of slider  24  and the remaining portion of slider  24 . Microactuator  42   a  and structural element  42   b  are disposed on the side surfaces of slider  24  near its distal end, connecting head portion  40  to the remainder of slider  24 . Microactuator  42   a  is a structural element operable as a bendable cantilever to alter the position of head portion  40  with respect to main portion  38 . Structural element  42   b  is preferably a microactuator similar and complementary to microactuator  42   a,  but may alternatively be a layer of structural material such as silicon nitride (Si 3 N 4 ) or polysilicon, for example, to provide some flexibility while prevent undesired vibrations that could affect the position of transducing head  41 . Transducing head  41  comprises an inductive write head and/or a magnetoresistive read head, for example, as is well known in the art. Transducing head  41  is desirably positioned directly over a data track on a rotating disc to read and/or write data from and/or to the disc. 
     In operation, slider  24  is coarsely positioned adjacent a selected data track by activating VCM  12  to move actuator arm  16  (FIG. 1) carrying slider  24 . To finely position transducing head  41  over the selected track, control signals are applied to piezoelectric microactuator  42   a  to cause bending of microactuator  42   a  and consequent bending of structural element  42   b.  When structural element  42   b  is a microactuator, control signals are also applied to microactuator  42   b  to cause bending complementary to microactuator  42   a.  Bending of microactuators  42   a  and  42   b  is controlled to selectively alter the position of transducing head  41  in the direction of arrows  46 , thereby precisely positioning transducing head  41  directly over the selected track on the disc. 
     FIG. 4 is an enlarged top view of the distal portion of slider  24  illustrating the construction of piezoelectric microactuator  42   a  according to a first embodiment of the invention. Bottom electrode  50  connects head portion  40  to the remainder of slider  24  across gap  44 . Piezoelectric element  52  is formed on bottom electrode  50 , and top electrode  54  is formed on piezoelectric element  52 . This configuration forms a cantilevered unimorph piezoelectric bending motor, the operation of which is described in detail below with respect to FIGS. 6 and 7. 
     FIG. 5 is an enlarged top view of the distal portion of slider  24  illustrating the construction of piezoelectric microactuator  42   a  according to a second embodiment of the invention. Structural layer  60  connects head portion  40  to the remainder of slider  24  across gap  44 . Buffer layer  62  is formed on structural layer  60 , and adhesive layer  64  is disposed on buffer layer  62 . Bottom electrode  66  is formed on adhesive layer  64 , and first piezoelectric element  68  is formed on bottom electrode  66 . Shared electrode  70  is formed on piezoelectric element  68 . A second piezoelectric element  72  is formed on shared electrode  70 , and top electrode  74  is formed on piezoelectric element  72 . An optional encapsulation layer  76  is formed over top electrode  74 . This configuration of piezoelectric microactuator  42   a  forms a bimorph piezoelectric bending motor, the operation of which is discussed in detail later with respect to FIGS. 8 and 9. 
     In an exemplary embodiment, structural layer  60  is composed of silicon nitride (Si 3 N 4 ) or polysilicon. Buffer layer  62  is preferably composed of silicon dioxide (SiO 2 ) or titanium dioxide (TiO 2 ). Adhesive layer  64  is preferably composed of titanium (Ti) or tantalum (Ta). Bottom electrode  66 , shared electrode  70  and top electrode  74  are preferably formed of platinum (Pt). Piezoelectric elements  68  and  72  may be composed of zinc oxide (ZnO), lead zirconium titanate (PbZrTiO 3 , known as PZT), aluminum nitride (AlN) or polyvinylidene fluoride (PVDF). The zinc oxide material requires no annealing or low temperature annealing, while the PZT material requires high temperature annealing. Piezoelectric elements  68  and  72  may be deposited by sputtering, sol gel techniques or laser deposition, as is known in the art. 
     FIG. 6 is a diagram of a cantilevered unimorph bending motor  80  in its neutral position, and FIG. 7 is a diagram of cantilevered unimorph bending motor  80  in its actuated position. Unimorph bending motor  80  may be implemented by piezoelectric microactuator  42   a  described above with respect to FIG.  4 . Unimorph bending motor  80  includes bottom electrode  82 , piezoelectric element  84  and top electrode  86 , and is restrained at one end by connection to a solid object such as slider  24 . Terminal  88  connects bottom electrode  82  to a first potential and terminal  90  connects top electrode  86  to a second potential. Piezoelectric element  84  is poled in the direction indicated by arrow  91 . 
     In operation, when the potential difference between terminals  88  and  90  (and consequently between bottom electrode  82  and top electrode  86 ) is applied across piezoelectric element  84 , the element contracts along its length, forcing bending of piezoelectric element  84  and electrodes  82  and  86  as indicated in FIG.  7 . In this way, a deflection at the distal tip of bending motor  80  may be achieved. Conversely, application of an opposite potential difference between terminals  88  and  90  across piezoelectric element  84  results in opposite bending and opposite deflection at the distal tip of bending motor  80 . 
     FIG. 8 is a diagram illustrating a cantilevered bimorph piezoelectric bending motor  100  configured for parallel operation. Bimorph piezoelectric bending motor  100  may be implemented by piezoelectric microactuator  42   a  described above with respect to FIG.  5 . Bimorph piezoelectric bending motor  100  is restrained at one end by connection to a solid object such as slider  24 . Bimorph piezoelectric bending motor  100  includes bottom electrode  102 , first piezoelectric element  104 , shared electrode  106 , second piezoelectric element  108 , and top electrode  110 . In the “parallel” configuration depicted in FIG. 8, piezoelectric elements  104  and  108  are poled in the direction of arrows  118  and  120 . A first voltage is applied at terminal  112  to bottom electrode  102 , and at terminal  116  to top electrode  110 . A second voltage is applied at terminal  114  to shared electrode  106 . Thus, in the “parallel” configuration, piezoelectric element  108  will contract, and piezoelectric element  104  will expand, in response to the first and second voltages applied at the terminals. The result is a bending motion (shown in dashed lines) of bimorph piezoelectric bending motor  100 , since one end of the motor is restrained by connection to slider  24 . The amount of bending of the motor, and thus the amount of displacement of transducing head  41  on head portion  40  (FIG. 2) is precisely controlled by the voltages applied to terminals  112 ,  114  and  116 . Applying opposite voltages to terminals  112 ,  114  and  116  causes similar bending in the opposite direction. Thus, bimorph piezoelectric bending motor  100  is able to provide high resolution positioning of head  41  over a selected track of a disc. 
     FIG. 9 is a diagram illustrating a cantilevered bimorph piezoelectric bending motor  100  configured for series operation. Bimorph piezoelectric bending motor  100  may be implemented by piezoelectric microactuator  42   a  described above with respect to FIG.  5 . Just as in FIG. 8, bimorph piezoelectric bending motor  100  includes bottom electrode  102 , first piezoelectric element  104 , shared electrode  106 , second piezoelectric element  108 , and top electrode  110 . Piezoelectric element  108  is poled in the direction of arrow  128  and piezoelectric element  104  is poled in the opposite direction, shown by arrow  130 . A first voltage is applied at terminal  116  to piezoelectric element  108 , and a second voltage is applied at terminal  112  to piezoelectric element  104 . As a result, bimorph piezoelectric bending motor  100  bends as indicated in dashed lines, since one end of the motor is restrained by the connection to slider  24 . Applying opposite voltages to terminals  112  and  116  causes similar bending in the opposite direction. The “series” configuration is the simplest and most economical, since it requires only two connections to the outside surfaces of piezoelectric elements  104  and  108 . However, the “series” configuration yields less deflection per volt of applied potential than the “parallel” configuration shown in FIG.  8 . The “parallel” configuration is more complex, requiring three electrical connections, the additional connection being made to shared electrode  106 . Either of the configurations shown in FIGS. 8 and 9 are acceptable for effecting high resolution positioning of transducing head  41  (FIG. 2) over a selected track of a disc. 
     FIG. 10 is a diagram of a typical slider substrate wafer  150 . Substrate wafer  150  is shown to comprise a plurality of portions  152  used to form a plurality of sliders  24 , and has a thickness equal to a desired length of the sliders. 
     FIG. 11 is a perspective view of a portion  152  of substrate wafer  150 . The portion  152  shows a 3×3 array of sliders  24 . Transducing heads  41  are formed on the top (trailing edge) surface of each slider  24  of wafer  150 , as is known in the art. The portion  152  further has layers formed thereon according to the present invention. In order to form sliders  24  with a gap between main portions  38  and head portions  40 , the substrate wafer is used to define only main portions  38  of sliders  24 . Sacrificial layer  160  is deposited on the substrate wafer to define the region that will become the gap. 
     According to a third embodiment of the present invention, notches  172  are cut between columns of sliders  24  before wafer  150  is cut (along dotted lines  168 ) into slider rows. After notches  172  have been cut, multilayer piezoelectric microactuators are formed in notches  172 . The details of the process of forming the microactuators essentially identical to the process discussed below with respect to FIG. 13, with long strips of materials being formed for several sliders rather than forming individual microactuators for each slider. Wafer  150  is then ready to be diced into slider rows along dotted lines  168 , at which point the air-bearing surfaces of sliders  24  are formed. 
     FIG. 12 is a top view, and FIG. 13 is a side view with a portion shown in enlarged detail, of a slider row  170  processed according to a fourth embodiment of the present invention, with structural elements  42   b  being implemented as microactuators. Slider row  170  includes a plurality of sliders  24  each having an air-bearing surface  171  and notches  172  cut between sliders  24 . Notches  172  are shown as trapezoidal in shape, which facilitates sidewall deposition of microactuator layers between sliders; alternatively, notches  172  may be right-angle cuts if more sophisticated sidewall deposition techniques are employed. Notches  172  are cut by a diamond saw, for example, and preferably do not extend through the entire height of sliders  24 , so that sliders  24  remain attached to one another in slider row  170 . 
     Microactuators  42   a  and  42   b  are formed in notches  172  on opposite sides of sliders  24 . Photolithography is performed to define the regions where the electrodes and piezoelectric materials of microactuators  42   a  and  42   b  are to be deposited, with photoresist layers  177  being deposited to protect transducing head  41 . Bottom electrode  50  is then deposited in notch  172 , also extending onto the top surface of sliders  24 . Piezoelectric element  52  is deposited and annealed on bottom electrode  50 . If high annealing temperatures are required that are incompatible with the materials in transducing heads  41  of sliders  24 , annealing may be performed by a localized laser heating process, for example. Preferably, piezoelectric element  52  is composed of a material that does not require annealing temperatures that are incompatible with heads  41 . Top electrode  54  is then deposited on piezoelectric element  52 , and poling of piezoelectric element  52  is performed at an elevated temperature. Bottom electrode  50  preferably extends on the top surface of sliders  24  beyond piezoelectric element  52  and top electrode  54  at regions  176 , to provide easy access to bottom electrode  50  for electrical connection thereto. After bottom electrode  50 , piezoelectric element  52  and top electrode  54  have been deposited, photoresist layers  177  used in the photolithography process to define the microactuator regions and protect transducing head  41  are removed and sliders  24  are separated from slider row  170  by dicing at lines  174 . Sacrificial layer  160  between main portions  38  and head portions  40  of sliders  24  is also etched away to form the gap between those portions. Electrical connections are made to the microactuator in a manner known in the art, such as through the flex circuit used to contact transducing head  41 , and may be made on the top of the slider or on any other exposed surfaces of bottom electrode  50  and top electrode  54 . Although microactuators  42   a  and  42   b  are only shown in FIG. 13 with bottom electrode  50 , piezoelectric element  52  and top electrode  54  (as described above with respect to FIG.  4 ), it should be understood that microactuators  42   a  and  42   b  may be formed to include the layers described above with respect to FIG.  5 . 
     FIGS. 14 and 15 are perspective views of a stack of sliders  24 , illustrating a process of forming microactuator  42   a  and structural element  42   b  on sliders  24  according to a fifth embodiment of the present invention. Sliders  24  are each formed as described above with respect to FIGS. 11 and 12 by initially forming main portions  38  of sliders  24 , depositing sacrificial layer  160  on the distal surface of main portions  38 , and forming head portions  40  on sacrificial layer  160 , with head portions  40  carrying transducing heads  41 . Sacrificial layer  160  occupies the area that will become gap regions  44  between main portions  38  of sliders  24  and head portions  40 . Sliders  24  are diced apart and then stacked and glued together lightly. Piezoelectric microactuator  42   a  and structural element  42   b  (which is preferably another microactuator) are either formed directly on slider row assemblies, with the microactuator areas defined by photolithography, for example, on side surfaces of the slider stack, or are separately formed in strips or sheets and then attached to the side surfaces of the slider stack, yielding the structure shown in FIG.  15 . Microactuator  42   a  may be formed on a structural layer (such as layer  60  in FIG. 5) deposited on the side surfaces of the slider stack, or may be formed or attached directly on the side surfaces of the stack. Structural element  42   b  may be a microactuator similar and complementary to microactuator  42   a,  or may be a structural layer composed of silicon nitride (S 3 N 4 ) or polysilicon, for example. Where the layers of microactuators  42   a  and  42   b  are formed as sheets or strips along the side surfaces of the slider stack, the material layers are cut at regions  180  so that microactuators  42   a  and  42   b  do not extend all the way to the air-bearing surfaces of sliders  24 . In the case where microactuator  42   a  and structural element  42   b  are formed on a separate substrate (which is subsequently removed) and attached to the slider stack, the multilayer sheets are ultimately separated into individual microactuators on each of the sliders  24  by a laser, for example. This separate formation and transfer process avoids potential temperature incompatibilities, since the potentially high temperature annealing of the piezoelectric material is not performed proximate to transducing head  41 . Sacrificial layer  160  is then etched away to form gaps  44  between the main portions of sliders  24  and head portions  40  of the sliders. The glue holding sliders  24  together is then dissolved, leaving each individual slider with piezoelectric microactuator  42   a  and structural element  42   b  on its side surfaces across gap  44 . Alternatively, the process described above could be performed on each slider individually, thereby increasing the number of steps involved to attach the microactuators to the sliders but eliminating the steps of initially gluing the sliders together, separating the microactuators and ultimately dissolving the glue to separate the sliders. 
     The present invention provides precise, high performance head positioning, with high acceleration in moving the head and sufficiently large and accurate head displacement to cover several data tracks. Only head portion  40  of slider  24  is moved by microactuator  42   a  (and structural element  42   b,  when it is implemented as a microactuator), minimizing the total mass that is displaced and thereby enabling high acceleration of head  41 . Additionally, moving only head portion  40  of slider  24  allows the microactuator to cancel resonance effects associated with the actuator arm and flexure, eliminating any track misregistration effects due to vibrations or the like resulting from those resonances. The microactuators are also readily manufacturable by simple existing fabrication techniques with only the addition of a sacrificial layer on the slider, minimizing the incremental cost of the microactuator-equipped disc drive. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.