High-performance integrated microactuator, particularly for a hard disc read/write transducer

Abstract: The microactuator (9') comprises a stator element (17') and a rotor element (11') which are capacitively coupled. The rotor element (11') comprises a suspended mass (12) and a plurality of movable drive arms (13') extending radially from the suspended mass (12) and biased at a reference potential. The stator element (17') comprises a plurality of first and second fixed drive arms (18a', 18b') associated with respective movable drive arms (13') and biased at a first drive potential. A mechanical damping structure (32) is formed by at least one movable damping arm (13'') extending radially from the suspended mass (12) and by at least one first and one second fixed damping arm (18a'', 18b'') associated with the movable damping arm (13'') and biased at said reference potential, to dampen settling oscillations of the rotor element (11').

Description: The present invention relates to a high-performance integrated microactuator. In particular, the integrated microactuator according to the present invention is advantageously but not exclusively used for actuation of hard discs read/write transducers, to which the discussion below will make explicit reference without thereby losing generality. As known, integrated microactuators have recently been proposed in hard disc actuating devices provided with a dual actuation stage, wherein a first actuation stage performs a coarse drive of the read/write transducer during tracking and a second actuation stage performs a finer position control of the read/write transducer. An example of a known hard disc actuating device with a dual actuation stage is shown diagrammatically in figures 1 and 2. In detail, in figure 1 the hard disc actuating device 1 comprises a motor 2 (also called a "voice coil motor") to which at least one suspension 5 formed by a lamina is fixed in projecting manner. At its free end the suspension 5 has an RW read/write transducer 6 (Fig. 2), also known as a "slider" and disposed (in operating condition) facing a surface of a hard disc 7 (Fig. 1). The R/W transducer 6 is fixed to a coupling, known as gimbal 8, via a microactuator 9 interposed between the gimbal 8 and the R/W transducer 6 (Fig. 2). On one of its lateral surfaces the R/W transducer 6, formed by a body of ceramic material (such as AlTiC), further has a read/write head 10 (magneto/resistive and inductive) which forms the actual read/write device. In the actuating device 1, the first actuation stage is defined by motor 2 that moves the unit formed by suspension 5 and R/W transducer 6 across the hard disc 7 during track seeking and the second actuation stage comprises the microactuator 9 that finely controls the position of the R/W transducer 6 during tracking. An embodiment of a microactuator 9 of rotary electrostatic type is shown schematically in Fig. 3 wherein microactuator 9 is shown only in part, given its axial symmetry. The microactuator 9 comprises an external stator 17, integral with the die embedding the microactuator 9 and bonded to the gimbal 8, and an internal rotor 11, intended to be bonded to the R/W transducer 6 (Fig. 2) and capacitively coupled to the stator 17. The rotor 11 comprises a suspended mass 12 of substantially circular shape and a plurality of movable arms 13 extending radially towards the outside from the suspended mass 12. Each movable arm 13 has a plurality of movable electrodes 14 extending in substantially circumferential direction and equidistant from each other. The rotor 11 further comprises anchoring and elastic suspension elements (springs 15) elastically connecting the suspended mass 12 to fixed anchoring regions 16 that bias the rotor 11 and the movable electrodes 13 at a reference potential. The stator 17 comprises a plurality of fixed arms 18a, 18b extending radially with respect to the suspended mass 12 from fixed biasing regions 20a, 20b arranged circumferentially around the rotor 11 and each having a plurality of fixed electrodes 19; in particular, a pair of fixed arms formed by a fixed arm 18a and a fixed arm 18b is associated with each movable arm 13; the fixed electrodes 19 of each pair of fixed arms 18a, 18b extend towards the associated movable arm 13 and are intercalated or interleaved with the movable electrodes 14. All the fixed arms 18a are disposed on a same side of the respective movable arms 13 (on the right in the example shown) and are all biased at a same first drive potential through bias regions 20a; similarly all the fixed arms 18b are arranged on the other side of the respective movable arms 13 (on the left in the example shown) and are all biased at a same second drive potential through the bias regions 20b. The fixed arms 18a and the fixed arms 18b are biased at different drive potentials which differ from the reference potential of rotor 11 so as to generate two different potential differences with respect to the movable arms 13 and cause the rotor 11 to rotate in one direction or the other. The materials currently used to manufacture the microactuator 9 are substantially of two types: brittle materials, such as single or multi-crystal silicon, which are elastically but not plastically deformable, and ductile materials such as nickel that is plastically deformable. Specific assessments of the energy dissipation capacity of these materials, the obtainable reliability and the effects of their use on the final quality control process of the finished product cause silicon to be preferred as the structural material; in fact, the use of silicon enables microstructures to be produced which are more reliable than if metals are used, with considerable simplifications to the final quality control process of the finished product. On the other hand, silicon microactuators have a damping factor that is much worse than metals. In fact, a microactuator of silicon may be modelled by means of a second order differential equation defined, inter alia, by a damping factor or, alternatively, by a quality factor inversely proportional to the damping factor, both of which contribute, as known, to defining the response of the microactuator to the application of a step stress. In particular, a microactuator of silicon typically has a quality factor which is too high for the considered applications, comprised, e.g. between 10 and 1000, i.e. with a too low damping factor comprised, e.g. between 5.10 -4 and 5.10 -2 and thus has a step response having over-elongations of very high amplitude compared with the stationary value. Consequently, during fine position control of the R/W transducer 6 that determines accurate positioning of the R/W transducer 6 at the read position, microactuators of silicon have the disadvantage of having rather high settling oscillations about the read position ("ringing"), such as to require the use of very complex closed-loop control circuits for damping such settling oscillations and thus enabling acceptable settling times of the R/W transducer 6. The object of the present invention is to provide an integrated microactuator overcoming the disadvantages of the known integrated microactuators. Xia Zhang et al. "Viscous air damping in laterally driven microresonators" Proceedings of the Workshop on micro electro mechanical system (MEM, Oiso, Jan 25-28, 1994, No. Workshop 7, 25 January 1994, pages 199-204, XP000528416 describes an experimental study of viscous air damping in laterally moving planar microstructures. The study is carried out on test structures having different test pattern including fingered combs. US 5,025,346 describes laterally driven resonant microstructures of both linear and rotational type. The present invention provides an integrated microactuator as defined in Claim 1. For the understanding of the present invention two preferred embodiments will now be described, purely by way of non-exhaustive example, with reference to the accompanying drawings, wherein: Fig. 1 is a perspective view of a hard disc actuating device of known type; Fig. 2 is an exploded view of a micrometric actuation assembly of the actuating device of Fig. 1; Fig. 3 is a diagrammatic view of an integrated microactuator of known type; Fig. 4 is a diagrammatic view of an integrated microactuator according to a first embodiment of the present invention; and Fig. 5 is a diagrammatic view of an integrated microactuator according to a second embodiment of the present invention. In Fig. 4, 9' denotes as a whole an integrated microactuator of rotary type according to a first embodiment of the present invention. Microactuator 9' has the same general structure as the microactuator 9 of Fig. 3, therefor parts similar or correspond to those of microactuator 9 are denoted in Fig. 4 with same reference numerals as in microactuator 9. The microactuator 9' differs from microactuator 9 in that not all the fixed arms of the stator, denoted here at 17', are connected to bias regions 20a and 20b but two fixed arms, thereafter called fixed damping arms and denoted at 18a" and 18b", are connected to a fixed bias region 30 biased at the same reference potential as the rotor, denoted here at 11', as shown diagrammatically in Fig. 4 by the connection 25. The remaining fixed arms, thereafter called fixed drive arms and denoted at 18a' and 18b', are still connected to bias regions 20a and, respectively, 20b and are biased at the first and, respectively, the second drive potential. Consequently, the movable arm of rotor 11' arranged between the fixed damping arms 18a" and 18b" defines a movable damping arm denoted at 13" whereas the remaining movable arms arranged between the fixed drive arms 18a' and 18b' define movable drive arms denoted at 13'. In the microactuator 9' neither the fixed damping arms 18a' and 18b' nor the movable damping arm 13' have electrodes interleaved between them in comblike manner. By virtue of the separation of the fixed arms 20a, 20b into fixed drive arms 20a', 20b' and fixed damping arms 20a", 20b" and of the movable arms 13 into movable drive arms 13' and movable damping arms 13", the fixed drive arms 20a', 20b' and the respective movable drive arms 13' are still dedicated, as in known microactuators, to generating the rotary movement of rotor 11, whereas the fixed damping arms 18a", 18b" and the movable damping arm 13", all set at the same reference potential, perform a passive role as regards generation of rotary movement and define as a whole a damping structure for the settling oscillations of rotor 11', denoted in the illustration at 32. In fact, during the rotary movement of rotor 11', the movable damping arm 13" causes, with the adjacent fixed damping arm 18a", 18b", the air present between them to be compressed and to considerably reduce the angular movement of rotor 11 beyond the read position and the settling oscillations which derive therefrom. In microactuator 9' according to the present invention, damping is proportional to the inverse of the cube of the average distance between fixed damping arms 18a", 18b" and respective movable damping arm 13". Consequently, although Fig. 4 shows only one damping structure 32, microactuator 9' is designed to have a number of damping structures 32 which depends on the value of microactuator intrinsic damping factor, i.e. on the inherent value in the absence of damping structure(s) 32, and on the desired value of the damping factor. In particular, it is known from systems theory that a damping factor of 1/√2 = 0.707, corresponding to a quality factor also equal to 0.707, enables an optimum compromise to be reached between bandwidth and over-elongations and so, in microactuator 9' according to the present invention, the number of damping structures 32 is conveniently selected to cause the damping coefficient of microactuator 9' to assume values in a predetermined range about the above-mentioned optimum value of 0.707, comprised, e.g. between 0.6 and 0.8. The use of damping structures 32 of the above described type thus enables microactuators to have optimum damping factors and therefore reduced settling oscillations about the read position. Consequently, this involves not only a considerable reduction of the settling times of the microactuators, and a consequent increase in the performance of the microactuators, but also a considerable simplification of the closed-loop control circuits previously dedicated to damping the settling oscillations, and therefore a considerable reduction in the area of silicon occupied by such control circuits. Fig. 5 shows a microactuator of linear type according to a second embodiment of the present invention. The microactuator, denoted at 9", comprises a rotor 40 biased at the reference potential and a stator 42 coupled capacitively to the rotor 40. Rotor 40 comprises a suspended mass 44 of rectangular shape, a plurality of movable drive arms 46' and a movable damping arm 46" extending from the suspended mass 44 perpendicular to the larger sides of the suspended mass 44. In the example shown, the movable arms 46' and 46" define four groups of movable arms disposed in pairs on respective larger sides of the suspended mass 44. Rotor 40 further comprises elastic suspension and anchoring elements 48 extending from two opposite ends of the suspended mass 44 for supporting and biasing rotor 40 at the reference potential. Stator 42 comprises four fixed bias regions 50 of rectangular shape and parallel to the suspended mass 44, biased in couples at a respective drive potential, each arranged facing a respective group of movable arms 46' and having a plurality of fixed drive arms 54'. Stator 42 further comprises a further fixed bias region 52, arranged alongside one of the bias regions 50, biased at the same reference potential of rotor 40, as shown schematically by the connection line 53. A fixed damping arm 54" further extends from the last bias region 52 towards the suspended mass 44. The fixed arms 54' and 54" extend parallel to each other and to the movable arms 46', 46"; the fixed drive arms 54' are each arranged facing a respective movable drive arm 46' and the fixed damping arm 54" is arranged facing the movable damping arm 46". Analogously to microactuator 9', in microactuator 9" also the fixed drive arms 54' and the respective movable drive arms 46' are dedicated to generating the linear movement of rotor 40 whereas fixed damping arm 54" and movable damping arm 46", both at the same reference potential, perform a passive role with respect to the generation of the translational movement and define in their entirety a settling oscillation damping structure of rotor 40, denoted at 56 in the figure, operating on the principle of air compression in the manner described above. For this embodiment also, the number of oscillation damping structures 56 is selected on the basis of the inherent damping value of microactuator 9"; typically the oscillation damping structure 56 is replicated on the other three quadrants of the rotor 40, for reasons of symmetry and uniformity of movement. Finally, it will be clear that modifications and variants may be introduced to the microactuators 9' and 9" described and illustrated herein without thereby departing from the scope of the present invention, as defined in the appended claims. For example, fixed damping arms 18a" and 18b" and movable arm 13" of microactuator 9' could also be equipped with respective fixed and movable electrodes similar to electrodes 19, 14, even though the described microactuators 9' and 9" are to be preferred in that they enable damping structures 32, 56 having a more efficient damping effect and a smaller bulk. In fact, while in the described microactuators 9' and 9", the damping effect is, as stated above, proportional to the inverse of the cube of the average distance between fixed damping arms and respective movable damping arm, in microactuators having damping arms with interleaved electrodes the damping effect is proportional only to the inverse of the average distance. Furthermore, although damping structures according to the invention is particularly useful in case of microactuators of semiconductor material (single and multi-crystal silicon), they may also be produced in metal microactuators whenever it is desired to improve their damping characteristics. Finally, any damping structure may comprise two movable damping arms laterally surrounding a single fixed damping arm, in a manner opposite to that shown in Fig. 4, or a plurality of fixed arms and movable arms, in equal or different numbers.

Claims: An integrated microactuator (9'; 9") comprising a stator element (17'; 42) and a movable element (11'; 40) capacitively coupled; said movable element (11'; 40) comprising a suspended mass (12; 44) and a plurality of movable drive arms (13'; 46') extending from said suspended mass (12; 44) and biased at a reference potential and said stator element (17'; 42) comprising at least a plurality of first fixed drive arms (18a', 18b'; 54') facing respective said movable drive arms (13'; 46') and biased at a first drive potential, characterized by a mechanical damping structure (32; 56) for damping settling oscillations of said movable element (11'; 40) interposed between at least a part of said stator element (17'; 42) and a part of said movable element (11'; 40) and in that said mechanical damping structure (32; 56) comprises at least one movable damping arm (13"; 46") extending from said suspended mass (12; 44) and at least one first fixed damping arm (18a", 18b"; 54") facing said movable damping arm (13"; 46") and biased at said reference potential. A microactuator (9') according to claim 1, characterized in that said stator element (17') further comprises a plurality of second fixed drive arms (18a', 18b') associated with respective said movable drive arms (13') and biased at a second drive potential, each movable drive arm (13') being interposed between a respective said first fixed drive arm (18a', 18b') and a respective said second fixed drive arm (18a', 18b'); and in that said mechanical damping structure (32) further comprises a second fixed damping arm (18a", 18b") associated with said movable damping arm (13") and biased at said reference potential, said movable damping arm (13") being interposed between said first and second fixed damping arm (18a", 18b"). A microactuator according to claim 1 or 2, characterized in that said movable damping arm (13"; 46") and said first fixed drive arm (18a', 18b') have no reciprocally interleaved transverse structures. A microactuator according to one of the preceding claims, characterized in that it comprises semiconductor material. A microactuator according to one of claims 1 to 3, characterized in that it comprises metallic material. A microactuator (9') according to one of the preceding claims, characterized in that it is of rotary type. A microactuator (9") according to one of claims 1 to 5, characterized in that it is of linear translational type.

Inventors: VIGNA BENEDETTO, VIGNA, BENEDETTO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1998
Main classification: H02N1
IPC codes: G11B5, G11B21, H02N1