Abstract:
An electrostatic microactuator for a slider in a disc drive is characterised in that it is formed from a single crystal silicon wafer. This provides a microactuator that has very low parasitic capacitance, virtually no mechanical or thermal creep, or mismatch problems as may occur when parts are separately fabricated. It also allows for efficient mass production by allowing for many of the microactuators to be simultaneously formed from the single crystal silicon wafer. The microactuator comprises a first (stationary) part ( 22 ) for attachment to a flexure of a head positioning system in a disc drive and a second (movable) part ( 24 ) to which a slider is attachable, which is pivotally coupled ( 42, 44 ) to the first part. The first and second parts include elongate strips ( 30, 32 ) which are interdigitated to provide comb electrodes.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates to a microactuator for a slider in a disc drive head positioning system. The invention also relates to a microactuator and slider assembly. In particular the microactuator is an electrostatically operable microactuator.  
         BACKGROUND  
         [0002]    The radial positioning of data tracks on magnetic discs is continually being decreased so as to increase the data storage capacity of such discs. This has led to the development of microactuators which allow precision positioning of the read/write transducer head of a slider over a track after the head has been coarsely positioned by, for example, a known voice coil motor which operates an actuator suspension arm. The slider is commonly mounted on a flexure (also termed a gimbal) usually at the end of such a suspension arm.  
           [0003]    A microactuator must be capable of quickly and accurately positioning the read/write head of a slider. The microactuator should also be light weight to minimise detrimental effects on the resonance characteristics of the suspension arm, and relatively thin to enable close disc-to-disc spacing. To be commercially viable the microactuator must also be reliable and capable of being efficiently manufactured. Such viability generally requires mass production fabrication techniques, which must be such as to ensure the dimensional accuracy of the microactuators measured in microns. It is also necessary that the microactuators retain their dimensional and thus high resolution positioning accuracy possibly over a wide temperature range, as may be necessary in for example a hard disc drive (HDD) system.  
           [0004]    Known electrostatic microactuators have been fabricated from polysilicon or nickel based metals. However micro structures fabricated using these materials have problems in large scale (mass) manufacturing. These problems include dimensional non-uniformity, residual stresses in the materials, thermal expansion of the material leading to mismatches between parts and poor reproducibility. Thus there is a need to provide a microactuator structure, particularly for an electrostatic microactuator, which is suitable for mass production such as will reduce the above mentioned problems.  
           [0005]    Electrostatic microactuators are known, see for example, U.S. Pat. No. 5,995,334 in the name of Long-Sheng Fan et al. Although high volume production is a matter which this prior patent addresses, the solution which is offered relates to replacement of conventional wiring to a microactuator with microfabricated wiring, which is formed using lithographic techniques and stencil plating. This patent does not relate to a microactuator which is able to be mass produced as in the present invention, as disclosed herein below.  
           [0006]    U.S. Pat. No. 5,898,541 in the name of Z-E Boutaghou et al relates to a microactuator fabricated at the wafer level by conventional thin film techniques used to manufacture the transducing head on the slider. However this microactuator is a piezoelectric and not an electrostatic microactuator.  
         DISCLOSURE OF THE INVENTION  
         [0007]    An object of the present invention is to provide an electrostatic microactuator having good dimensional accuracy and stability and which can be mass manufactured at the wafer level.  
           [0008]    Accordingly, the present invention provides a microactuator for a slider in a disc drive, the microactuator comprising  
           [0009]    a first part for attachment to a flexure of a head positioning system in a disc drive,  
           [0010]    a second part to which a slider is attachable,  
           [0011]    the first and second parts being coupled together such that they are relatively movable and each part including portions which form electrodes for providing electrostatic forces for moving the second part relative to the first part,  
           [0012]    wherein the first and second parts are formed from a single crystal silicon wafer.  
           [0013]    The first part may be termed the stationary part because in use it remains stationary relative to the flexure, and the second part may be termed the movable part because in use it moves relative to the stationary part.  
           [0014]    Manufacturing the microactuator from a single crystal silicon wafer provides a microactuator that has very low parasitic capacitance, virtually no mechanical or thermal creep, or mismatch problems as may occur between the stationary and movable parts of a microactuator when those parts are separately fabricated. At the same time, it allows for efficient mass production by forming many such microactuators from the single crystal silicon wafer.  
           [0015]    Thus the electrostatic microactuator may be formed by etching into the top surface of a single crystal silicon wafer. Developed epi-micromachining technique is used to make the microactuator in the single crystal silicon wafer as described in more detail hereinbelow.  
           [0016]    Preferably the first (i.e. stationary) part of the microactuator includes several small pads formed on a surface thereof via which the microactuator can be bonded to the flexure of the head positioning system. Such pads may be formed by epi-micromachining. Alternatively, several pads may be formed on the flexure for the same purpose. A bonding technique such as flip chip or fusion bonding can be used to fix the microactuator onto the flexure of the suspension via such pads.  
           [0017]    Preferably the second or movable part of the microactuator includes a slot formed in a surface thereof (for example, its bottom surface) which is opposite a surface thereof (for example, its top surface) in which the first stationary part of the microactuator is formed. This slot provides for a slider to be attached to the second or movable part of the microactuator, for example, by bonding the slider into the slot. The slot may be formed by etching during manufacture at the wafer stage. Thus the invention also provides an assembly of a microactuator and a slider.  
           [0018]    Preferably the portions of the first and second parts which form the electrodes for providing electrostatic forces, are interdigitated elongate portions or strips of, alternately, the first and second parts. Adjacent ones of these elongate portions may include lateral extensions providing comb electrodes, thereby providing a capacitative comb drive electrode arrangement. These elongate portions or strips may extend radially of the coupling between the first and second parts, or linearly therefrom in an orthogonal direction, in which case the first (stationary) part of the microactuator may have a main body of “Greek cross” shape in plan view. Other shapes for the first part of the microactuator are possible provided that its body and its elongate electrode portions or strips can be fabricated from a single crystal silicon wafer using etching techniques. The interdigitated portions in effect provide capacitative electrodes.  
           [0019]    Preferably a means in the form of a cap or a dust cover is provided for protecting a microactuator of the invention from particulate contamination. Such a cap or dust cover is separately fabricated and will contain several holes through which the hereinbefore described bonding pads may extend.  
           [0020]    For a better understanding of the invention and to show how the same may be carried into effect, preferred embodiments thereof will now be described, by way of non-limiting example only, with reference to the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0021]    [0021]FIG. 1 shows an actuator suspension arm over a disc.  
         [0022]    [0022]FIG. 2 is an underneath perspective view of the end of the actuator suspension arm of FIG. 1 on which is mounted a microactuator and slider assembly according to an embodiment of the invention.  
         [0023]    [0023]FIG. 3 is a top perspective view of the microactuator and slider assembly of FIG. 2.  
         [0024]    [0024]FIG. 4 is an underneath perspective view of the microactuator from the assembly of FIG. 3.  
         [0025]    [0025]FIG. 5 is a similar view to that of FIG. 2 but with the microactuator and slider assembly omitted.  
         [0026]    [0026]FIG. 6 is a top perspective view of another embodiment of a microactuator and slider assembly according to an embodiment of the invention and which includes a dust cover.  
         [0027]    [0027]FIGS. 7, 8 and  9  are top plan views of various designs of microactuators according to embodiments of the invention.  
         [0028]    [0028]FIG. 10 shows a silicon wafer in which an array of microactuators have been formed. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0029]    With reference to FIGS. 1 and 2, a dual stage actuation system for positioning a head over a disc  10  of a hard disc drive includes a suspension arm  12  which is coarsely positionable over the disc  10  by a motor (not shown) such as a voice coil motor. The suspension arm  12  supports a flexure  14  on which is attached an electrostatic microactuator  16  and slider  18  assembly. The slider  18  includes read/write head elements  20  and the microactuator  16  is actuable to finally move the slider  18  such that the read/write head elements  20  can be located with precision over a magnetic track on the disc  10 . Thus the first stage of the dual stage actuation system comprises the motor driven suspension arm  12  and the second stage comprises the fine adjustment achieved via the microactuator  16 .  
         [0030]    The microactuator  16  (see FIGS. 3 and 4) comprises a first or stationary part  22  for attachment to the flexure  14  and a second or movable part  24  to which the slider  18  is attachable. The first and second parts  22 ,  24  are coupled together at a coupling  26  such that the second part  24  is movable relative to the first part  22 . Stationary part  22  has a body  28  of a “Greek cross” shape in plan view from which extend elongate portions  30  (in the nature of fingers) which are formed to provide comb electrodes. Movable part  24  is also formed to have elongate portions  32  which extend inwardly from opposite edges  34  thereof and which are also formed to provide comb electrodes. The elongate portions  30  of stationary part  22  are interdigitated with the elongate portions  32  of movable part  24  and the comb electrodes of the elongate portions  30  and  32  are also interdigitated such that a capacitative comb-drive electrode arrangement is provided.  
         [0031]    Two pairs of electric connection pads  36 ,  38  are formed on the opposite edges  34  of movable part  24  and are connected with the electrodes of the moveable part  24 , namely the elongate portions  32  (as described hereinbelow). The electrical connections are such that when a voltage is applied between the electrodes of the stationary part  22  and one pair of the connection pads, for example  36 , electrostatic forces of attraction are generated between the respective comb electrodes of elongate portions  30  and  32  which cause the movable part  24  to rotate about coupling  26  relative to the stationary part  22  either to the left or right as indicated by arrow  40 . Connection of the voltage to the other pair of connection pads, for example  38 , will cause movement of movable part  24  in the opposite direction.  
         [0032]    A differential driven scheme can be used to drive the read/write head elements  20  to linearize the voltage/force relationship. These features are advantageous for high performance servo control, since they make the microactuator  16  voltage/displacement relationship linear.  
         [0033]    The coupling  26  is formed by a post  42  of the movable part  24  extending into a central aperture in the body  28  of stationary part  22  and a flexure beam  44  which extends between the post  42  and the surrounding surface of the aperture. The flexure beam  44  effectively holds stationary part  22  in suspension relative to movable part  24 . The post  42  and flexure beam  44  are formed by deep reactive ion etching (RIE), however because this coupling  26  both structurally and therefore electrically interconnects the first (stationary) and second (moveable) parts  22  and  24  of the microactuator  16  (because post  42  upstands from the body of moveable part  24 ), it is necessary that the elongate portions  32  of the moveable part  24  be electrically isolated from the remaining body structure of moveable part  24  such that these portions  32  and the elongate portions  30  of the stationary part  22  are electrically isolated. This is achieved by an electrically insulating structural interconnection of the elongate portions  32  of moveable part  24  to the opposite edges  34  of moveable part  24  at anchor locations referenced  33 , see FIGS. 7, 8 and  9 . This structural interconnection, that is the anchors  33  are formed during fabrication of the microactuator  16  from a single crystal wafer of silicon material by filling etched spaces between elongate portions  32  and edges  34  (that is, at locations  33 ) with say silicon dioxide. Electrical interconnection is then made between the elongate portions  32  and the electric connection pads  36 ,  38  (which pads are formed over an electrically insulating oxide layer on edges  34  of moveable part  24  so that they are isolated from the moveable part  24 ) by metallisation over the anchors  33 . A slot  46  is formed in the lower surface of the movable part  24  (see FIG. 4) for the slider  18  to be attached to the microactuator  16 . Slider  18  is seated in slot  46  and bonded to the silicon material of the microactuator for attachment thereto.  
         [0034]    The microactuator  16  is characterised by the first or stationary part  22  and the second or moving part  24  being formed from a single crystal silicon wafer. Thus an array of the microactuators  16  are simultaneously fabricated from a single crystal silicon wafer  50  (see FIG. 10).  
         [0035]    The starting wafer  50  is highly N-doped for electrical conductivity and slots  46  for attachment of sliders  18  are first etched into its back surface. The microactuators  16  are then fabricated into the top surface of wafer  50  using a developed epi-micromachining technique which is known. A masking oxide layer is deposited on the top surface of wafer  50  and the structures  26 ,  28  and  30  of stationary parts  22 , and structures  32  of moveable parts  24 , are formed into the top surface of the wafer  50  by deep reactive ion etching (RIE) and dry plasma release process to etch away selected (non masked) parts and retain the masked parts. The structures  32  are completely undercut at the bottom and on three sides leaving one end of each still connected to the wafer substrate. The resulting trench gaps are thermally oxidised and filled with low pressure chemical vapour deposition LPCVD silicon dioxide—which provides the anchors  33 . The remaining connected ends of structures  30  and unwanted silicon dioxide are then removed by masking and etching, as is known. Thus, effectively, the first parts  22  of the microactuators  16  are formed within wafer substrate  50  and are “suspended” therein via the flexure beam  44  connections to the posts  42  (which posts are connected to the wafer substrate), and the elongate portions  32  are attached to the wafer substrate  50  at the silicon dioxide anchors  33 . Appropriate oxidation and metallisation layers are then formed to provide the electric connection pads  36 ,  38  and connection to elongate structures  32  and the silicon wafer  50  with many of the just described microactuator structures formed in the top surface thereof and having slots  46  formed in the rear surface is sliced into blocks to yield arrays of the microactuator structures  16  which may then be individualised, whereby the resultant moveable part  24  of each microactuator  16  is constituted by what was a portion of the wafer substrate. Finally a slider  6  is inserted into the slot  46  of each microactuator to form a microactuator-slider assembly.  
         [0036]    With reference to FIG. 5, in one embodiment, four small pads  70  are micro-machined from the undersurface  15  of flexure  14  for mounting of the stationary part  22  of the microactuator  16  thereon. The location of the pads  70  can be adjusted according to the flying requirements of the slider  18 . Alternatively small pads  72  (see FIG. 6) can be fabricated on the stationary part  22  (which in FIG. 6 is hidden by a dust cover  74 ) for the same purpose.  
         [0037]    Since small particles could possibly be trapped in the gaps between the electrodes of the stationary and moving parts  22 ,  24 , of the microactuator and cause an electrical short circuit, a “dust cover” plate  74  is added over all the electrodes (see FIG. 6). The small pads  72  (or  70 ) can go through holes in the “dust cover”  74  for the stationary part  22  to be attached to the flexure  14  of suspension arm  12 . A flexible material  76  can be used to seal the gap between the small pads  72  (or  70 ) and the holes in the “dust cover”  74 .  
         [0038]    [0038]FIG. 7 is a plan view of a microactuator  16  and slider  18  similar to that of FIG. 3. In this structure there are a number of the flexure beams  44  arranged radially.  
         [0039]    [0039]FIG. 8 shows an alternative embodiment in plan view which differs mainly in the orientation of the electrode portions  30  and  32  and mainly in the coupling between the movable and stationary parts (the same reference numerals have been used for features and components which correspond to those of the previous figures). In this embodiment four flexure springs  78  extend from a central post  80  of the moveable part  24  and are attached to the body of stationary part  22 . The flexure springs  78  are arranged symmetrically for translational motion of the moving part  24 , and thus slider  18 , relative to stationary part  22 .  
         [0040]    [0040]FIG. 9 shows a further embodiment in plan view in which the elongate portions  30  and  32  of respectively stationary part  22  and moving part  24  extend radially and define a parallel plate capacitative configuration (in contrast to a comb-drive arrangement as in FIGS. 7 and 8) to generate rotational motion of the moving part  24 , and thus of slider  18 , relative to stationary part  22 .  
         [0041]    Different displacements of the moveable part  24  relative to the stationary part  22  (depending on “spring” characteristics of coupling  26 ) and dynamic performance (that is, resonant frequency response) can be achieved with different design configurations of the microactuator  16 .  
         [0042]    The invention described herein is susceptible to variations, modifications and/or additions other than those specifically described and it is to be understood that the invention includes all such variations, modifications and/or additions which fall within the scope of the following claims,