A head with partial reading means for a disk storage device

Abstract: A head (130) for a disk storage device having a plurality of tracks (117) divided into memory cells (234) comprises means (205, 230a, 230b, 250a, 250b) for reading the memory cells (234) in succession, the means (205, 230a, 230b, 250a, 250b) for reading the memory cells (234) including at least two partial reading means (206a, 230a, 250a; 206b, 230b, 250b) each for reading a portion (234a; 234b) of each memory cell (234), the portions (234a; 234b) being arranged transversely relative to the longitudinal axis (233) of the corresponding track (117).

Description: The present invention relates to a head for a disk storage device and, in particular, to a head according to the preamble of the first claim. In disk storage devices such as, for example, hard disks, the data is stored in tracks concentric with a rotation axis of the disk. A block of data is accessed (for reading or writing) by the positioning of an electromagnetic head mounted on a suspension arm adjacent the desired track. A problem with disk storage devices is that, to ensure that they operate correctly, it is necessary for the head to be positioned extremely precisely within the track. Otherwise, the head may move over an adjacent track during a writing operation, destroying at least some of the data stored; similarly, during a reading operation, the head may detect incorrect data. A known, dedicated solution for correcting the position of the head consists of the use of a servo-disk on which auxiliary positioning data (servo data) is recorded. A further servo-head fixed to the reading and writing head (or to a group of heads) reads the servo-data, enabling a servo-mechanism to keep all of the heads in the correct position within the track. This solution is extremely expensive since it requires the use of a suitable servo-disk and servo-head; moreover, the control thus achieved is not very precise and is affected, in particular, by the different thermal expansion of the servo-head and of the servo-disk in comparison with the heads and the disks on which the useful data is stored. A different, embedded solution is commonly used and consists in storing the auxiliary positioning servo-data on each track, intercalated with the useful data. In particular, each track is divided into various sectors, some of which are used for storing the positioning servo-data at regular intervals. When the disk is rotating, each head reads the auxiliary servo-data periodically, enabling the servo-mechanism to correct its position directly. A disadvantage of this solution is that, during the movement of the head between two consecutive sectors containing servo-data, it is not possible to check and correct its position in any way; this structure does not therefore ensure the correct position of the head continuously. Moreover, the storage of the positioning servo-data on every track of the disk involves a considerable wastage, of the order of 10%, of the useful space on the disk. The object of the present invention is to avoid the aforementioned drawbacks. A magnetic head having a twin structure comprising an I core body and two core half bodies is known from US-A-5477405. Particularly, with reference to figure 21, this document discloses a magnetic head comprising two C core parts 50 and 51 of a read/write core, an I core part 52 of the read/write core, a pair of back cores 53 and 54 for magnetically shunting the core parts 50 to 52. Moreover, the magnetic head shown in figure 21 of such document comprises a read/write gap 58 formed with a thin film SiO 2 , Al 2 O 3 , flint glass or the like, an insulation layer 59 for magnetically separating the C core parts 50 and 51 and a pair of read/write coils 60 and 61 wound around the respective C core parts 50 and 51. To achieve the above mentioned object, a head as described in the first claim is proposed. The head according to the present invention enables its position to be checked and corrected directly and continuously; this is particularly useful when the disk storage device is subject to knocks and vibrations as, for example, in portable devices. This structure enables the head to follow the track extremely precisely; narrower tracks disposed closer together can thus be used, enabling storage devices with a greater data density to be produced. The solution of the present invention does not require a dedicated device (a servo-disk or a servo-head) and is therefore extremely inexpensive; moreover, it does not require the storage of any positioning servo-data, consequently avoiding wastage of space on the disk. Further characteristics and advantages of the head according to the present invention will become clear from the following description of a preferred embodiment thereof, given by way of non-limiting example, with reference to the appended drawings, in which: Figure 1 shows, schematically and in section, a hard-disk storage device in which the head of the present invention can be used, Figure 2 is a partially-sectioned view of an embodiment of the head according to the present invention, Figures 3a-3d show the various steps of a process for producing the head of Figure 1. With reference in particular to Figure 1, a hard-disk storage device 100 includes a sealed container 103, inside which there is a disk 105 constituted by a rigid circular support covered with a ferromagnetic material; the head of the present invention is, however, also suitable for use in a multiple-disk, floppy-disk, or optical-disk storage device and the like. The disk 105 is coupled by means of a hub 107 to a shaft 110 connected to a suitable electric motor 115; the motor 115 keeps the disk 105 rotating about the shaft 110 at a constant frequency (for example of between 100 and 150 Hz). The disk 105 comprises a plurality of tracks 117 concentric with the rotation shaft 110 and divided into memory cells; each cell stores one bit of data, represented by different magnetization states of the ferromagnetic material. Typically, the density of data along a track 117 is, for example, of the order of 120,000 BPI (bits per inch), and the track density is, for example, of the order of 5,000 TPI (track per inch). The storage device 100 includes a suspension arm 125 coupled at one of its ends to a pin 126. The suspension arm 125 terminates in a flexible suspension 127 on the free end of which a slider 128 is mounted; the slider 128 supports an electromagnetic head 130 (described in detail below). The suspension 127 bends to exert a pressure which urges the slider 128 against the upper surface of the disk 105. The slider 128 hovers on an air cushion above an upper surface of the disk 105 when the disk 105 is in rotation, allowing the head 130 to follow the roughness of the disk 105 at an extremely small distance, for example, of a few tens of nm. An actuator 135, typically constituted by a voice coil motor of the type used in loudspeakers, rotates the suspension arm 125 about the pin 126. The head 130 is thus moved in a substantially radial direction relative to the disk 105 so as to be positioned over the desired track. The head 130 is connected by means of a flexible wire 143 to an analog/digital circuit 145 formed in a chip of semiconductor material. The circuit 145 processes an electrical signal supplied by the head 130 during a reading operation and sends suitable control signals to the head 130 during a writing operation. The circuit 145 is connected to a control unit 150 which puts the storage device 100 into communication with the exterior. The unit 150 is also connected to the actuator 135 in order to control the movement of the head 130 over the desired track and to check the position of the head 130 within the track (as described in detail below). With reference now to Figure 2 (the elements already shown in Figure 1 are identified by the same reference numerals), the electromagnetic head 130 has a planar structure and is formed in a chip 200 of semiconductor material, typically silicon; the head of the present invention is, however, also suitable for being formed on a ceramic support or the like. The chip 200 includes a magnetic core 205 made of a material with high permeability, for example, permalloy (an alloy of nickel and iron) or other soft ferromagnetic materials, incorporated in a diamagnetic material. The core 205 forms a left-hand magnetic circuit 206a and a right-hand magnetic circuit 206b each of which defines a preferred path for the flux lines of a magnetic field. The core 205 is constituted by a front yoke 210 and by a rear yoke divided into a left-hand portion 216a and a right-hand portion 216b which are separate from one another; a left-hand column 220a and a right-hand column 220b connect the front yoke 210 to the left-hand portion 216a and to the right-hand portion 216b of the rear yoke, respectively. The left-hand portion 216a and the right-hand portion 216b of the rear yoke have, at their respective free ends, opposed chamfers (for example 45° chamfers) which form a V-shaped opening 223 facing outwardly relative to the magnetic core 205; a central column 225 extends from the front yoke 210 as far as the vicinity of the opening 223 of the rear yoke. The left-hand magnetic circuit 206a is formed by the front yoke 210, by the central column 225, by the left-hand portion 216a of the rear yoke, and by the left-hand column 220a, whereas the right-hand magnetic circuit 206b is formed by the front yoke 210, by the central column 225, by the right-hand portion 216b of the rear yoke, and by the right-hand column 220b. This structure advantageously enables the magnetic flux to be conveyed in an optimal manner and continuously in the two circuits 206a and 206b; the present invention may, however, also be implemented with a magnetic core of a different shape or with two (or more) separate magnetic cores. Each magnetic circuit 206a, 206b is interrupted by an air-gap 230a, 230b disposed in the vicinity of an upper free surface of the chip 200 and preferably formed between the central column 225 and the opening 223. The air-gaps 230a and 230b are aligned transversely relative to a longitudinal axis 233 of the track 117 and their total length is slightly greater than the width of the track 117. Each air-gap 230a, 230b enables the respective circuit 206a, 206b to be coupled magnetically with a corresponding portion of a facing memory cell 234; in particular, the left-hand air-gap 230a and the right-hand air-gap 230b are coupled with a left-hand portion 234a and with a right-hand portion 234b of the memory cell 234, respectively, the portions 234a and 234b being arranged transversely relative to the axis 233. Each magnetic circuit 206a, 206b is interrupted by a further air-gap 235a, 235b; for example, the air-gap 235a is formed in the left-hand column 220a and the air-gap 235b is formed in the right-hand column 220b. A respective magnetoresistor 250a, 250b which has a resistance variable in dependence on an external magnetic field, is disposed in the vicinity of each air-gap 235a, 235b so as to be linked at least partially with the magnetic circuit 206a, 206b; the magnetoresistor 250a, 250b is made, for example, of permalloy or is a spin-valve magnetoresistor. Two pairs of contact electrodes 255a, 256a and 255b, 256b (for example, made of copper) are connected to the respective magnetoresistors 250a and 250b in the vicinity of their longitudinal ends. The magnetic flux generated by the portion 234a, 234b of the memory cell 234 is conveyed in the magnetic circuit 206a, 206b and is linked at least partially with the magnetoresistor 250a, 250b. If a certain voltage (for example, of the order of a few mV) is applied between each pair of electrodes 255a, 256a and 255b, 256b and the corresponding current is measured, it is possible to obtain the intensity of the magnetic field in the magnetoresistor 250a, 250b, from which a value indicative of the magnetization state of the portion 234a, 234b of the memory cell 234 can be inferred. Alternatively, other equivalent means are used for detecting the magnetic field, such as, for example, a coil wound around each of the magnetic circuits. In this case, each variation of the magnetization state of the memory cells causes a variation in the magnetic flux linked with the coils; an induced electromotive force, from which the variation of the magnetization state of the memory cell can be inferred, is therefore produced at the terminals of each coil. The present invention may also be implemented with different positioning of the air-gaps, with the use of three or more air-gaps (and corresponding magnetoresistors) for reading a corresponding number of portions of the memory cell, or with other equivalent partial reading means. During a reading operation, the values detected by the magnetoresistors 250a and 250b are added together in the analog/digital circuit (145 in Figure 1) so as to produce a total value indicative of the magnetization state of the memory cell 234. When the head 130 is positioned correctly in the centre of the track 117, each air-gap 230a, 230b is linked with the magnetic field generated by the corresponding portion 234a, 234b of the memory cell 234. In this situation, the magnetic flux conveyed in each of the circuits 206a and 206b is the same so that the magnetoresistors 250a, 250b detect the same value. If the head 130 is not centred on the track 117, however, the values detected by the magnetoresistors 250a, 250b are different. For example, a situation in which the head 130 is displaced towards the left may be considered; in this case, the right-hand air-gap 230b is linked to a greater extent with the magnetic field generated by the memory cell 234, whereas the left-hand air-gap 230a is linked with the magnetic field to a lesser extent. Upon the assumption, for simplicity, that the surface of the disk adjacent the track 117 is not magnetized, the value detected by the right-hand magnetoresistor 250b will increase, whereas the value detected by the left-hand magnetoresistor 250a will decrease in dependence on the magnitude of the displacement of the head 130 towards the left. It should be noted that, if the surface adjacent the track 117 is magnetized, for example, if the tracks are very close together, the value detected by the left-hand magnetoresistor 250a will vary in an unpredictable manner. The magnitude of the variation of the value detected by the left-hand magnetoresistor 250a, however, has a random statistical distribution so that its mean value is zero. The mean of the values detected by the left-hand magnetoresistor 250a will therefore decrease, as in the previous case. Similar considerations apply if the head 130 is displaced towards the right. The values detected by the magnetoresistors 250a and 250b are compared in the analog/digital circuit 145 (see Figure 1 again); the result of this comparison is supplied to the control unit 150 which derives the direction and magnitude of the displacement of the head 130 from this result and sends suitable control signals to the actuator 135 for consequent correction of the position of the head 130. With regard to writing operations, a conventional electromagnetic writing head is used, fixed to one or more of the reading heads described above, for example, arranged in alignment along the track or transverse the track. During a writing operation on a certain track, the reading heads simultaneously read the same track or one or more adjacent tracks so as to check their position, as in the previous case, and consequently also the position of the writing head. Alternatively, the head described above is a reading and writing head which also includes a copper coil or other equivalent conductive means, wound in a spiral around the magnetic core; in this case, each writing operation is interrupted after a predetermined period of time to allow the head to correct its position. An example of a process for producing the electromagnetic head described above is illustrated in Figures 3a-3d (the elements already shown in Figure 2 are identified by the same reference numerals). With reference in particular to Figure 3a, a substrate 305 of semiconductor material (for example silicon) is covered with a layer of insulating material 307 (typically silicon dioxide). The electrodes 255a, 256a, 255b, 256b are formed on the insulating layer 307 by known deposition and selective etching techniques. A thin film of magnetoresistive material with a thickness, for example, of between 50 and 100 nm is then deposited, preferably by means of an anodic sputtering process; the magnetoresistors 250a, 250b are formed from this magnetoresistive film by known masking and etching techniques. To go on to Figure 3b, a film 310 of insulating material 310 such as silicon carbide with a thickness, for example, of a few hundreds of nm is deposited; the insulating film 310 (and the other insulating layers described below) are preferably deposited by plasma deposition. A different insulating layer 315, for example, of tetraethyl orthosilicate oxide (TEOS) with a thickness, for example, of a few µm is deposited on the insulating film 310. Openings 320 with a width, for example, of between 0.8 and 1.2 µm, are formed in the TEOS layer 315 by known masking and etching techniques and are used (as described below) for forming the various air-gaps (230a, 230b, 235a and 235b in Figure 2) and the opening in the magnetic core (223 in Figure 2). A different insulating layer 325, for example, of silicon nitride is deposited and then defined, for example, by anisotropic etching in plasma so as to form a spacer which forms, in each opening 320, a slot 330 with a width of, for example, 0.2µm. With reference now to Figure 3c, the process continues with the deposition of a further layer of silicon carbide which fills the slots 330 and covers the entire surface of the TEOS layer 315; this layer of silicon carbide is then etched and removed completely from the surface of the TEOS layer 315 so as to define silicon carbide pillars 335 in the slots 330. It should be noted that the preceding step of etching the silicon nitride layer advantageously improves the mechanical anchorage of the pillars 335 since a single structure is formed with the similarly-formed silicon carbide layer 310. The spacers (325 in Figure 3b) are then removed, preferably by means of a wet etching process. The TEOS layer 315 is then defined by known masking and selective etching techniques so as to form openings 340 corresponding to the magnetic core (205 in Figure 1). A film 345 of conductive material (such as tungsten or chromium) with a thickness, for example, of between 50 and 100nm, is then deposited, preferably by an anodic sputtering process. To go on to Figure 3d, a permalloy layer with a thickness, for example, of a few µm is grown electrolytically on the conductive film 345 in the openings 340 so as to produce the magnetic core 205 of the structure shown in Figure 2. Finally, the permalloy layer is subjected to a chemical-mechanical polishing process to remove the exposed portion of the conductive film 345. Naturally, in order to satisfy contingent and specific requirements, an expert in the art may apply to the above-described head many modifications and variations all of which, however, are included within the scope of protection of the invention as defined by the following claims.

Claims: A head (130) with a planar structure for a disk storage device (100) having a plurality of tracks (117) divided into memory cells (234), the head (130) being formed in a chip of semiconductor material and comprising a magnetic core (205) forming a first (206a) and a second (206b) magnetic circuit each interrupted by an air-gap (230a; 230b) disposed on the vicinity of a free surface of the chip and suitable to be coupled magnetically with a corresponding portion (234a; 234b) of each memory cell (234), the portions (234a; 234b) being arranged transversely relative to a longitudinal axis (233) of the corresponding track (117), and first (250a) and second (250b) detection means linked at least partially with the first (206a) and the second (206b) magnetic circuit, respectively, for detecting a value indicative of the magnetization state of the corresponding portion (234a; 234b) of the memory cell (234), the magnetic core (205) including a first yoke (210) and a second yoke divided into a first portion (216a) and a second portion (216b) which are separate from one another, a first column (220a) extending between the first yoke (210) and the first portion (216a) of the second yoke and a second column (220b) extending between the first yoke (210) and the second portion (216b) of the second yoke, and a central column (225) extending from the first yoke (210) to the vicinity of respective free ends of the first portion (216a) and of the second portion (216b) of the second yoke, the first magnetic circuit (206a) being formed by the first yoke (210) by the central column (225), by the first portion (216a) of the second yoke, and by the first column (220a), and the second magnetic circuit (206b) being formed by the first yoke (210), by the central column (225), by the second portion (216b) of the second yoke, and by the second column (220b), characterized in that the first (210) and second yoke, the first (220a) and second (220b) column and the central column (225) have a respective longitudinal axis parallel to the free surface of the chip (200). A head (130) according to claim 1 in which the longitudinal axis of the first yoke (210) is parallel to the longitudinal axis of the second yoke and the longitudinal axes of the first column (220a), the second 'column (220b) and the central column (225) are perpendicular to the longitudinal axes of the first (210) and second yoke. A head (130) according to Claim 1, in which the first portion (216a) and the second portion (216b) of the second yoke have, at their free ends, respective opposed chamfers forming a V-shaped opening (223) facing outwardly relative to the magnetic core (205). A head (130) according to any claim from 1 to 3, in which each magnetic circuit (206a; 206b) is interrupted by a further air-gap (235a; 235b), the corresponding detection means (250a; 250b) including a magnetoresistor disposed in the vicinity of the further air-gap (235a; 235b). A disk storage device (100) comprising at least one head (130) according to any one of Claims 1 to 4 and means (145, 150, 135) for comparing the values detected by the first detection means (250a) und by the second detection means (250b) and for correcting the position of the head (130) in dependence on the comparison.

Inventors: FERRARI PAOLO, MURARI BRUNO, VIGNA BENEDETTO, FERRARI, PAOLO, MURARI, BRUNO, VIGNA, BENEDETTO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1997
Main classification: G11B5
IPC codes: G11B5