Abstract:
In an actuator device for hard disks a suspension element carries a slider that is subject to undesired vibrations which give rise to rotations of the slider with respect to a nominal position. An electrostatically controlled position-control structure is arranged between the suspension and the slider and is controlled in an active way so as to generate torsions of the platform that counter the undesired rotations. The position-control structure comprises a platform of conductive material and control electrodes arranged underneath the platform. The platform is connected to a load-bearing structure by spring elements that enable movements of roll and pitch. Four control electrodes are arranged according to the quadrants of a square and can be selectively biased for generating electrical forces acting on the platform.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a tiltable device, in particular to a hard disk actuator device, with roll and pitch angle active control. 
   2. Description of the Related Art 
   As is known, hard disks are the media most widely used for storing data; consequently, very large volumes of hard disks are produced, and the maximum data-storage density continues to increase from one year to the next. Hard disks are read and written using actuator devices, the general structure of which is illustrated in  FIGS. 1 and 2  and is described hereinafter. 
   In particular,  FIG. 1  shows a known actuator device  1  of rotary type, comprising a motor  2  (also referred to as “voice coil motor”) fixed to a supporting body  3 , generally referred to as E-block because of its E shape in side view (see  FIG. 2 ). The supporting body  3  has a plurality of arms  4 , each of which carries a suspension  5  formed by a lamina or strip fixed in cantilever fashion. Each suspension  5  carries, at its end that is not fixed to the supporting body  3 , an R/W transducer  6  for reading/writing, arranged (in operating condition) facing a surface of a hard disk  7  and able to follow the surface of the hard disk  7 . For this purpose, the R/W transducer  6  (hereinafter referred to as “slider”) is fixed to a joint, called “gimbal” or “flexure”  8 , generally formed by the suspension  5  and including, for example, a rectangular plate  8   a , cut around three and a half sides from the suspension strip, the connection portion  8   b  whereof, designed for connection to the suspension  5 , enables bending of the plate  8   a  under the weight of the slider  6 . 
   Since the actuator device  1  is a component of an electromechanical type, it is affected by a series of problems linked to friction, contamination and mechanical stresses, which may impair proper operation thereof, in particular considering the high speed of rotation of hard disks (currently, in the region of 10000 r.p.m.). 
   In particular, the present invention tackles the problem of vibrations of the suspension. In fact the suspension is, together with the slider, a mechanical system provided with its own vibration modes at well-determined frequencies. Although some of these modes are of little importance for the mechanical system, there are others that may create problems because they disturb reading and writing the disk. For example, certain vibration modes cause an error, referred to as “off-track error” which causes exit of the slider  6  from the longitudinal axis of the track. During writing, this may lead to a loss of data on account of undesired erasure of the adjacent tracks. 
   Consequently, when the control system associated to the actuator device detects dangerous vibrations of the suspension, it inhibits reading and writing in order to enable resettling of the mechanical system (suspension and slider). This results in dead times, which are incompatible with the high speeds involved and the short data-accessing times required. 
   BRIEF SUMMARY OF THE INVENTION 
   An embodiment of the present invention provides an actuator device equipped with a system for offsetting dangerous vibration modes, which will work without interrupting operation of the hard disk. 
   According to the present invention, a tiltable device, with roll and pitch angle active control is provided. 
   In an actuator device for hard disks a suspension element carries a slider that is subject to undesired vibrations which give rise to rotations of the slider with respect to a nominal position. An electrostatically controlled position-control structure is arranged between the suspension and the slider and is controlled in an active way so as to generate torsions of the platform that counter the undesired rotations. The position-control structure comprises a platform of conductive material and control electrodes arranged underneath the platform. The platform is connected to a load-bearing structure by spring elements that enable movements of roll and pitch. Four control electrodes are arranged according to the quadrants of a square and can be selectively biased for generating electrical forces acting on the platform. 
   Another embodiment of the invention provides a method of operation of the device, including detecting a deviation of the read/write head of a disk drive from a plane parallel to the plane of the hard disk; 
   rotating the read/write head on first and second axes, perpendicular to each other and coplanar with the read/write head, in a direction opposite the direction of deviation, to return the head to a plane parallel with the plane of the hard disk. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     For a better understanding of the present invention, an embodiment of an actuator for hard disks is now described, purely by way of non-limiting example, with reference to the attached drawings, wherein: 
       FIG. 1  shows a top plan view of an actuator for hard disks, of a known type; 
       FIG. 2  is an enlarged side view of some parts of the actuator of  FIG. 1 ; 
       FIG. 3  is a perspective view of the end of a suspension of an actuator according to the invention; 
       FIG. 4  is a top plan view, with portions removed, of an actuating platform belonging to the actuator according to the invention; 
       FIG. 5  is a top plan view of the platform of  FIG. 4 ; 
       FIG. 6  is a cross-sectional view of the platform of  FIG. 5 , according to the lines VI—VI; 
       FIG. 7  is a cross-sectional view of the platform of  FIG. 5 , according to the VII—VII; 
       FIG. 8  is a cross-sectional view of the platform of  FIG. 5 , according to the VIII—VIII; 
       FIG. 9  shows a first torsional mode of the platform according to the invention; and 
       FIG. 10  shows a second torsional mode of the platform according to the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In  FIG. 3 , an actuator device  10  has the general structure described with reference to  FIGS. 1 and 2 , and further comprises a platform  12  integrated in a position-control structure  11  arranged between the gimbal  8  and the slider  6 . The position-control structure  11  is formed by a chip micromachined according to micromachining techniques used in the micro-electronics industry. 
   The platform  12  is suspended, by suspension arms (also referred to as spring elements)  13   a ,  13   b , to a load-bearing structure  14  and is made to rotate about two orthogonal axes X and Y to roll and pitch. The platform  12 , of electrically conductive material or at least provided with conductive regions, is controlled by electrodes  15  ( FIG. 4 ) arranged underneath the platform  12  and selectively biased by a control circuit  16  that forms part of a signal-processing device (not shown) fixed to the motherboard of a personal computer or other apparatus comprising hard disks for data storage, or else directly to the board of the hard disk. The control circuit  16 , operating in closed-loop on the basis of information on the position and/or movement of the suspension  5  and represented only schematically in  FIG. 3 , controls attraction or release of the platform  12  towards or from an electrode  15  or two adjacent electrodes  15 , and thus the desired rotation, as explained hereinafter. 
   As shown in detail in  FIGS. 4–8 , the platform  12  is formed in a structural layer of doped polycrystalline silicon that extends on top of a substrate  19  of semiconductor material, for example monocrystalline silicon, and is insulated from the latter by an intermediate region  20 , of insulating material. 
   In detail, as may be better seen from the cross-sectional views of  FIGS. 6–8 , the intermediate region  20  comprises an insulating layer  21 , for example of silicon dioxide, which completely covers the substrate  19 , insulating it electrically from the overlying structure, and an insulating region  22 , also, for instance, of silicon dioxide. The insulating region  22  extends only on the periphery of the position-control structure  11  on top of the insulating layer  21  and surrounds an air gap  29 , obtained by removing a sacrificial layer (which forms also the insulating region  22 ) so as to enable freeing of the mobile structure and formation of the through electrical connections. 
   The platform  12  (see in particular  FIGS. 3–5 ) has a rectangular shape, is surrounded by a first trench  24  and is connected to an intermediate frame  25  by a first pair of spring elements  13   a  that traverse the first trench  24  and extend along the axis X (roll axis). The intermediate frame  25  is surrounded by a second trench  27  and is connected to an outer frame  28  (belonging to the load-bearing structure  14 ) by a second pair of spring elements  13   b  that traverse the second trench  27  and extend along the axis Y (pitch axis) perpendicular to the axis X. The second pair of spring elements  13   b  is thus in phase opposition to the first pair of spring elements  13   a.    
   As may be clearly seen in  FIGS. 6–8 , the platform  12 , the first pair  13   a  and second pair  13   b  of spring elements, the intermediate frame  25  and the outer frame  28  are all formed in the same structural layer  18 . In addition, as shown in  FIGS. 6 and 7 , the air gap  29  extends underneath the platform  12 , the first and second pairs of spring elements  13   a ,  13   b , and the intermediate frame  25 . 
   The electrodes  15  are formed by regions of doped polycrystalline silicon, on top of the insulating layer  21 , below the platform  12 , underneath the air gap  29 . In particular, as may be seen in  FIG. 4 , which shows a top plan view of the position-control structure from which the slider  6  and the regions formed on top of the structural layer  18  (and described hereinafter) have been removed, there are four electrodes  15 , which have a rectangular shape and are arranged adjacent in pairs, as four quadrants of a square, so as to cover almost entirely the area defined by the platform  12 , the first trench  24 , the intermediate frame  25 , and a big portion of the second trench  27 . Each electrode  15  is moreover connected to a respective biasing line  30 , also of polycrystalline silicon and extending on top of the insulating layer  21  ( FIGS. 7 and 8 ), underneath the insulating region  22  and the outer frame  28 . The biasing lines  30  are connected to through regions  31  (which extend in the outer frame  28  on one side  28   a  of the latter facing the connection portion  8   b  of the plate  8   a — FIG. 3 ) through connection portions  32  that pass through the insulating region  22 , as shown in detail in FIG.  7 . The through regions  31  are electrically insulated from the remainder of the outer frame  28  by trench insulation. 
   A protective layer  36 , for example of silicon dioxide, extends above the platform  12 , the pairs of spring elements  13   a ,  13   b , the intermediate frame  25  and the outer frame  28 ; metal lines  37  and pads  38   a ,  38   b ,  38   c ,  38   d  are formed on top of the protective layer  36 . In detail, four first pads  38   a  are formed on the platform  12 , in proximity of the slider  6 , to be connected to corresponding pads  39  ( FIG. 3 ) formed on the slider  6  and electrically connected to a head  44  (magneto-resistive or inductive—not illustrated) which forms a reading and writing device. Four metal lines extend from the four first pads  38   a  and extend, in pairs, above the first spring elements  13 , above the intermediate frame  25 , above the second spring elements  13   b , and above the outer frame  28  as far as the side  28   a  of the latter, where the metal lines are connected to respective four second pads  38   b.    
   Moreover four third pads  38   c  extend on the side  28   a  of the outer frame  28 , above and in direct electrical contact with the through regions  31 ; to this aim, the protective layer  36  is here removed ( FIG. 7 ). Finally, a fourth pad  38   d  is in direct electrical contact with the outer frame  28 , on the side  28   a  thereof, for biasing the platform  12  through the outer frame  28 . Of course, the protective layer  36  is removed also underneath the fourth pad, similarly to the third pads  38   c.    
   The second pads  38   b , third pads  38   c  and fourth pad  38   d  are wire-connected to corresponding pads  40  formed on the plate  8   a  ( FIG. 3 ); electrical-connection lines  41  extend from plate  8   a  along the suspension  5 , as far as the control circuit  16 . 
   In practice, by applying a potential difference between a single electrode  15  or two adjacent electrodes  15  and the platform  12  it is possible to cause the platform  12  to rotate about the axes X and Y. This is shown by way of example in  FIGS. 9 and 10 , wherein the platform respectively performs a simple rotation about the axis X (that passes through the first spring elements  13   a ) and about the axis Y (that passes through the spring elements  13   b ). Of course, also a complex rotation about both the axes is possible. 
   Thereby, by measuring or detecting in a known way the torsions of the suspension  5  (see, for example, Data Storage, October 1999, “Design head positioning servos: Changes ahead”), it is possible to control a contrary and counterphase movement of the platform  12  so as to keep the slider  6  constantly in the correct reading/writing position.  FIGS. 9 and 10  show two possible torsional modes of the platform  12 , and hence of the slider  6 . Thereby, the R/W head  44  can operate continuously, without the need to inhibit reading/writing in order to enable resettlement of the system. 
   The position-control structure  11  is manufactured as described hereinafter. 
   Initially, an insulating layer  21 , for example a thin-oxide layer, is deposited on top of the substrate  19 . Then a polycrystalline silicon layer is deposited for a thickness of, for instance, 450 nm. The polycrystalline silicon layer is defined to form the electrodes  15  and the biasing lines  30 . A sacrificial layer (designed to form the insulating region  22 ), for example of oxide and having a thickness of 2 μm, is deposited. The sacrificial layer is opened to form vias for electrical connection of the biasing lines  30 . An epitaxial layer (structural layer  18 ) of silicon is grown, possibly after deposition of a silicon germ layer. The epitaxial layer, having a thickness of, for instance, 35 μm, moreover fills the vias, forming the connection portions  32 . 
   Next, the protective layer  36  is deposited and opened above the through regions  31  and where the fourth pad  38   d  is to be formed. A metal layer is deposited and defined, so as to form the pads  38   a – 38   d  and the metal lines  37 . A trench etch is then performed for defining the platform  12 , the spring elements  13   a ,  13   b , the intermediate frame  25  and outer frame  28 . Finally, the second insulating layer, where accessible, is removed through the trenches  24 ,  27  that have just been formed, thus freeing the mobile structures and forming the air gap  29 . 
   After separating the position-control structure  11  from the similar structures in the same wafer, the slider  6  is bonded, the position-control structure  11  is bonded to the plate  8   a , and soldering is carried out for electrical connection between the parts, in a known way. 
   In the position-control structure  11 , the force that causes torsion of the platform  12  and is generated by the electrodes  15  can be calculated according to the following formula:
 
 F= 0.5  dC/dX V   2   (1)
 
where V is the potential difference applied between the selected electrodes and the platform, and dC/dX is the capacitance variation as a function of the gap variation (distance between the electrodes and the platform).
 
   The spring elements  13   a ,  13   b  undergo a torsion given by the equation
 
θ=0.5  TL/GJ   (2)
 
where T=FB; F is the force applied, given by eq. (1); B is the mean arm on which the force F is exerted, and is equal to the distance between the center of the biased electrode  15  or the centroid of the biased electrodes  15  and the considered spring element  13   a ,  13   b ; L is the length of the spring element  13   a ,  13   b ; G is the torsion modulus of polycrystalline silicon; and J is the second polar moment of inertia.
 
   Thereby, through the position-control structure  11  and the corresponding control circuitry  16  it is possible to adjust the position of the slider  6  (and hence of the R/W head) in a simple and accurate way, compensating the movements due to the vibration modes of the suspension  5 , and thus reducing off-track errors, without entailing any dead times. 
   The described solution is simple and inexpensive and can be implemented using customary micromachining techniques. 
   Finally, it is clear that modifications and variations may be made to the device described herein without departing from the scope of the present invention. For example, although the invention has been described with particular reference to the problems of suspensions in actuators for hard disks, it is equally applicable to other situations where the angular position of a body carried by a suspension subject to undesired vibrations is to be electrostatically controlled. In addition, the invention is also applicable to optical-switching devices, where the platform  12  is coated with a metal layer that acts as a reflecting surface (mirror) for light beams and laser beams. 
   In addition, the position-control structure can be applied also to hard disk actuators with two actuation stages, having a microactuator arranged between the platform  12  and the slider  6 , or formed inside the platform  12  and obtained by digging the structural layer  18  so as to define the stator region and rotor region of the microactuator. 
   In addition, instead of electrically conductive material, the platform  12  may be of insulating material and may carry the conductive regions on its bottom surface or on its sides, for example metal regions that interact with the electrodes  15 .