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BACKGROUND OF THE INVENTION
The present invention relates generally to magnetic disk drives and more
particularly to protection of read/write transducers utilizing
magnetoresistive read sensors against electrostatic discharge or
electrical overstress during processing and assembly.
Magnetic head disk drive systems have been widely accepted in the computer
industry as a cost effective form of data storage. In a magnetic disk
drive system a magnetic recording medium, in the form of a disk, rotates
at high speed while a magnetic read/write transducer, referred to as a
magnetic head, "flies" slightly above the surface of the rotating disk.
The magnetic disk is rotated by means of a spindle drive motor. The
magnetic head is attached to or formed integrally with a "slider" which is
suspended over the disk on a spring-loaded support arm known as the
actuator arm. As the magnetic disk rotates at operating speed the moving
air generated by the rotating disk in conjunction with the physical design
of the slider operate to lift the magnetic head allowing it to glide or
fly slightly above and over the disk surface on a cushion of air, referred
to as an air bearing. The flying height of the magnetic head over the disk
surface is typically only a few microinches or less and is primarily a
function of disk rotation, the aerodynamic properties of the slider
assembly and the force exerted by the spring-loaded actuator arm.
A major problem encountered during manufacture, handling and use of
magnetic recording transducers, referred to as heads, is the buildup of
electrostatic charges on the various elements of a head or other objects
which come into contact with the heads, particular heads of the thin film
type, and the accompanying spurious discharge of the static electricity
thus generated. Static charges may be produced by the presence of certain
materials, such as plastics, during manufacture and subsequent handling of
the heads, for example. These charges arc across the edge of the
insulating layer between the magnetic pole tips and adjacent conductive
layers which are exposed and positioned adjacent to the transducing gap at
the slider air bearing surface facing the recording medium. This discharge
causes erosion of the pole tips and degradation of the transducer in
reading and writing of data.
The above-described electrostatic discharge (ESD) problems associated with
the thin film inductive read/write heads are well documented and several
solutions have been proposed. Elser et al. U.S. Pat. No. 4,317,149
discloses an inductive head having short discharge paths formed by the
deposition of conductive material in recesses formed in an insulating
layer so that the static electric discharge will occur in areas displaced
from the critical pole tip and gap area displaced from the critical pole
tip and gap area at the slider air bearing surface. Schwartz et al. U.S.
Pat. No. 4,800,454 discloses an inductive head assembly wherein the
magnetic pole piece and the inductive coil winding are coupled to the
slider to allow discharge of any static electric charges which may build
up. The winding is connected to the slider body via a diode with high
forward and reverse voltage drops, or through a fusible link.
Magnetoresistive (MR) sensors are well-known and are particularly useful as
read elements in magnetic transducers, especially at high data recording
densities. The MR read sensor provides a higher output signal than an
inductive read head. This higher output signal results in a higher signal
to noise ratio for the recording channel and thus allows higher areal
density of recorded data on a magnetic disk surface to be achieved. As
described above, when an MR sensor is exposed to ESD, or even a voltage or
current input larger than that intended under normal operating conditions,
referred to as electrical overstress or EOS, the MR read sensor and other
parts of the head may be damaged. This sensitivity to electrical damage is
particularly severe for MR read sensors because of these sensors'
relatively small physical size. For example, an MR sensor used for
extremely high recording densities will have a cross-section of 100
Angstroms (A) by 1.0 micrometers (um) or smaller. Discharge of voltages of
only a few volts through such a physically small resistor is sufficient to
produce currents capable of severely damaging or completely destroying the
MR sensor. The nature of the damage which may be experienced by an MR
sensor varies significantly, including complete destruction of the sensor
via melting and evaporation, contamination of the air bearing surface,
generation of shorts via electrical breakdown, and milder forms of damage
in which the head performance may be degraded. This type of damage to the
MR head has been found to occur during both processing and use and poses a
serious problem in the manufacturing and handling of magnetic heads
incorporating MR read sensors.
An electrical short provided across the input leads of the MR sensor
element is very effective in raising the failure voltage and minimizing or
eliminating damage to the MR head due to ESD. The shorted leads shunt the
majority of the discharge current around the MR sensor element. Commonly
assigned co-pending U.S. patent application Ser. No. 08/187,881 filed Jan.
26, 1994 discloses shorting the MR sensor element leads at the sensor
input pads. However, application and removal of the short circuit,
together with the process changes required to provide leads and associated
connection pads for other head elements, such as the magnetic shields and
substrate, can make this approach difficult and expensive to implement.
Thus, a head design which provides electrical shorting of the head
elements and which does not require extensive process changes, and allows
easy removal of the short prior to the head being integrated and assembled
in a magnetic storage disk drive is needed.
SUMMARY OF THE INVENTION
A principal object of the present invention is to minimize damage to an MR
sensor caused by discharge of static electricity through or electrical
overstress of the MR sensor element and other transducer components.
Another object of the present is to provide an MR sensor wherein the MR
head elements are electrically shorted together and to the slider
substrate to provide EOS and ESD protection by shunting excessive current
away from the MR element and other critical head components.
In view of the foregoing objects, the present invention provides a thin
film magnetic transducer comprising an MR read element wherein an
electrically conductive coating is formed on the head air bearing surface
(ABS). The conductive coating electrically shorts the MR head elements,
i.e., the MR sensing element, the sensing element leads and magnetic
shields, and the slider substrate together providing an electrically
conductive path bypassing the MR sensor element for conducting spurious
static electrical charge during an ESD transient to the substrate away
from the MR sensor element. According to the present invention, the
conductive coating is applied to the slider ABS after patterning and
lapping of the ABS and is subsequently removed just prior to finally
assembly of the disk drive.
In a preferred embodiment, industry-standard thin film deposition processes
are used to provide the conductive coating. The coating material comprises
any suitable material having low resistivity and which is dissolvable in a
basic solution. For example tungsten (W) can be deposited using
conventional vapor deposition techniques, has a high conductivity and is
quickly dissolved in a basic solution. Additionally, a W coating provides
corrosion protection for the MR sensor during subsequent handling and
assembly.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other objects, features and advantages of the present
invention will be apparent from the following detailed description of the
preferred embodiments of the invention, reference being made to the
accompanying drawings, in which like reference numerals indicate like
parts and in which:
FIG. 1 is a simplified block diagram of a magnetic disk storage system
embodying the present invention;
FIGS. 2a and 2b are cross-sectional views of MR read/inductive write
magnetic heads;
FIG. 3 is a plan view of the air bearing surface of the MR head shown in
FIG. 2b illustrating the conductive coating according to the present
invention;
FIG. 4 is an edge view of the MR head shown in FIG. 3 illustrating the
conductive coating; and
FIG. 5 is a perspective view of a slider having a wear layer on its air
bearing surface and illustrating the conductive coating according to the
present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to FIG. 1, although the invention is described as embodied in
a magnetic disk storage system as shown in FIG. 1, it will be apparent
that the invention is also applicable to other magnetic recording systems
such as a magnetic tape recording system, for example, or other
applications in which a sensor is utilized to detect a magnetic field. A
magnetic disk storage system comprises at least one rotatable magnetic
disk 12 supported on a spindle 14 and rotated by a disk drive motor 18
with at least one slider 13 positioned on the disk 12. Each slider 13
supports one or more magnetic read/write transducers 21, typically
referred to as read/write heads. The magnetic recording media on each disk
is in the form of an annular pattern of concentric data tracks (not shown)
on disk 12. As the disks rotate, the sliders 13 are moved radially in and
out over the disk surface 22 so that the heads 21 may access different
portions of the disk where desired data is recorded. Each slider 13 is
attached to an actuator arm 19 by means of a suspension 15. The suspension
15 provides a slight spring force which biases the slider 13 against the
disk surface 22. Each actuator arm 19 is attached to an actuator means 27.
The actuator means as shown in FIG. 1 may be a voice coil motor (VCM), for
example. The VCM comprises a coil moveable within a fixed magnetic field,
the direction and velocity of the coil movements being controlled by the
motor current signals supplied by a controller.
During operation of the disk storage system, the rotation of the disk 12
generates an air bearing between the slider 13 and the disk surface 22
which exerts an upward force or lift on the slider. The air bearing thus
counterbalances the slight spring force of the suspension 15 and supports
the slider 13 off and slightly above the disk surface by a small,
substantially constant spacing during operation.
The various components of the disk storage system are controlled in
operation by control signals generated by control unit 29, such as access
control signals and internal clock signals. Typically, the control unit 29
comprises logic control circuits, storage means and a microprocessor, for
example. The control unit 29 generates control signals to control various
system operations such as drive motor control signals on line 23 and head
position and seek control signals on line 28. The control signals on line
28 provide the desired current profiles to optimally move and position a
selected slider 13 to the desired data track on the associated disk 12.
Read and write signals are communicated to and from read/write heads 21 by
means of recording channel 25.
The above description of a typical magnetic disk storage system, and the
accompanying illustration of FIG. 1 are for representation purposes only.
It should be apparent that disk storage systems may contain a large number
of disks and actuators, and each actuator may support a number of sliders.
Referring now to FIGS. 2a and 2b, a portion of a MR read/inductive write
magnetic head 30 is shown in transducing relationship with a rotating
magnetic recording disk 32 such that the head air bearing surface 45 (ABS)
is disposed in facing relationship with and slightly above the disk
recording surface. Generally, such a head 30 includes an MR read assembly
and an inductive write assembly formed adjacent one another on a substrate
surface. The substrate surface is typically the vertical surface forming
the trailing end of the slider 13 (as shown in FIG. 1) carrying the
magnetic head. The MR read assembly comprises a MR sensing element 35
fabricated of a ferromagnetic material, such as a nickel-iron (NiFe)
alloy, for example, which is enclosed by first and second magnetic shield
elements 33 and 37, respectively. The MR sensing element can comprise a
single layer of NiFe, commonly referred to as Permalloy. More typically,
the MR sensing element will comprise a multilayer magnetic structure,
including magnetic bias layers, of the type described in U.S. Pat. No.
4,785,366 or of the type described in U.S. Pat. No. 5,206,590, utilizing
the giant MR effect. The shield elements 33 and 37 are generally made of a
highly permeable magnetic material, such as Permalloy or Sendust, a
trialloy of aluminum-silicon-iron. The magnetic shield elements 33 and 37
minimize or eliminate magnetic interferences from affecting the MR element
35 and thereby producing extraneous electrical pulses. Conductive leads 44
(as shown in FIG. 3), generally copper (Cu) or other suitable conductive
material, attached electrically at the end portions of the MR element 35
couple the MR element to external circuitry to provide a means for sensing
the resistance of the MR element.
The MR read assembly is formed by vacuum deposition techniques, such as
sputter deposition, for example, on the substrate 31. The various elements
of the MR assembly are surrounded and insulated from each other by layers
42 of insulating material, such as silicon dioxide (SiO.sub.2,) or
aluminum oxide (Al.sub.2 O.sub.3), for example.
The inductive write assembly comprises a lower or first pole piece 38 and
an upper or second pole piece 40. The first and second pole pieces 38, 40
are made of a highly permeable magnetic material such as NiFe, for
example, and form a magnetic circuit magnetically connected together at a
back gap portion (not shown) with the first and second pole tips 39 and 41
forming a magnetic gap 47 at the air bearing surface 45. One or more
layers of electrical conductors 43, generally made of Cu, for example,
form the inductive coil disposed between the first and second pole pieces
38, 40. The inductive coil 43 is also connected to external circuitry via
conductive leads. The pole pieces 38, 40 and the inductive coil conductors
42 are fabricated by well-known processes such as electro-plating or
sputter deposition, for example. The pole pieces are insulated
electrically from the inductive coil and the MR read assembly by layers 42
of insulating material. Additionally, the entire assembly is covered with
a capping layer 42 of insulating and protective material.
The head 30 shown in FIG. 2a is sometimes referred to as a "piggyback"
head. An alternate configuration referred to as a "merged" head wherein
the second MR magnetic shield element 37 is merged with the inductive
assembly first pole piece 38 to form a single element 49 which performs
both functions is shown in FIG. 2b.
As described above in the background section, static electrical charges
built up on the various components of the head assembly 30 or on any
object, equipment or person which may come into contact with or closely
approach the head assembly or any electrical leads attached to the head
assembly pose the potential for serious damage to the head. These charges
are built up during the manufacturing process and during subsequent
handling of the heads. For example, the assembly of the head with other
components to produce a disk drive assembly can result in relatively large
amounts of static electrical charge accumulating on the head elements. The
electrical charges migrate from the areas at which they are generated to
build up along conductive paths. Thus a buildup of static charge occurs
that subsequently results in a discharge from one conductive element
across a dielectric, which experiences "breakdown", to another conductive
element, in the manner of a capacitive discharge. The discharge typically
causes damage by burnout or the like at the areas of the conductive
material which act as terminals for the discharge of the stored static
electrical energy. As discussed above with reference to U.S. Pat. Nos.
4,317,149 and 4,800,454, the problems concerning electrostatic discharge
(ESD) for inductive heads are well-known and the referenced patents
provide methods and structure for protection of inductive heads.
While the coil conductors 43 and the magnetic pole pieces 38, 40 can
withstand relatively high voltages and currents, because of its physically
small cross-section, the MR sensing element is particularly sensitive to
current and voltage overloads. The active sensing portion of the MR
element will have a length in the range of 0.5 to 20 um, a height (width)
of 0.5 to 5 um and a thickness of 10 to 100 nanometers (nm) and a
resistance of about 20 to 80 ohms. In MR read assemblies of the types
shown in FIGS. 2a and 2b, it has been found that such damage occurs along
the sensing region of the MR element 35 when the accumulated static charge
is discharged directly through the MR element 35 and may or may not
include discharge to the magnetic shield elements or first pole piece or
the body of the slider (substrate).
Referring now to FIGS. 3 and 4, in accordance with the present invention
electrostatic discharge and electrical overstress protection for an MR
head is accomplished by providing a conductive coating or layer 51 on the
head air bearing surface 45 which electrically shorts the MR head elements
exposed at the air bearing surface 45 to the slider substrate 31. FIG. 3
is a plan view from the air bearing surface of the head assembly 30
illustrating a merged MR head as described above with reference to FIG.
2b. All of the head elements, with the exception of the inductive coil 43,
are exposed at the head air bearing surface 45. A layer 51 of conductive
material formed over and coating the air bearing surface 45 electrically
shorts all of the exposed head elements together. That is, the conductive
layer 51 electrically shorts the first magnetic shield 33, the MR sensor
element 35, the MR leads 44, the second shield/first inductive pole tip 39
and the second inductive pole tip 41 to each other and further to the
substrate 31. Thus any electrical charge built up on the head elements
will be discharged directly to the substrate 31 shunning any current
around the MR sensing element 35. The conductive layer 51 may coat only
that portion of the head air bearing surface sufficient to short the
exposed head elements to the substrate or may, alternatively, coat the
entire slider/head air bearing surface. The conductive layer 51 also
prevents or minimizes sparkover damage and dielectric breakdown between
the MR leads 44, the first magnetic shield 33 and second magnetic
shield/first pole tip 39, and the conductive slider substrate 31.
In a preferred embodiment, an 800 Angstrom (A) thick tungsten (W) layer was
deposited onto to the slider air bearing surface 45 covering the exposed
head elements. The resistance of the MR heads changed from 28 ohms without
the W layer to 3 ohms (contributed to the lead resistance) after
deposition of the W layer. Test data (human body model) shows that the
failure voltage for the MR sensor element 35 increased from 100 volts for
no W layer to 3,000 volts with the W layer (while the sparkover voltage
increased from 600 volts with no W layer to 6,000 volts with the W layer).
The conductive layer 51 may be formed by any suitable technique, such as
vacuum deposition techniques. In a preferred embodiment, conductive layer
51 of W is deposited over the air bearing surface 45 by sputter deposition
through a stencil mask. The conductive layer 51 is formed as early as
practical in the head manufacturing process to provide ESD/EOS protection
for as much of the fabrication and assembly process as possible. For
example, the conductive layer 51 may be deposited at the completion of the
air bearing surface lapping process prior to separating the individual
heads/sliders from the wafer or row. The conductive layer 51 is then
removed at a subsequent processing step, preferably as late as practical
in the manufacturing and assembly process prior to the final customer
level operating configuration of the disk drive assembly. For example, the
conductive layer 51 may be removed once the head and its associated
suspension means has been assembled into a head stack assembly (HSA) by
dipping the head end of the HSA in a suitable oxiding etchant. The
conductive layer 51 can be of any material having sufficient electrically
conductivity and which can be removed from the slider air bearing surface
without appreciable damage to the other components of the slider/head and
suspension assembly.
The conductive layer 51 may be removed by any suitable process, such as
etching or laser oblation. Preferably, the conductive material for layer
51 will be of a material dissolvable in a basic solution. For example, W
is soluble in a basic solution having a pH of 8-9 whereas the materials
utilized for the other head elements and body are not etched appreciably
in a solution having a pH of less than 10.0. For example, at a pH greater
than 10.0 alumina (Al.sub.2 O.sub.3), a common dielectric material
utilized in heads as insulation, exhibits significant etching in basic
solutions having a pH greater than 10.0. Suitable etch solutions for W
comprise basic solutions of ferric cyanide or a solution of dipotassium
hydrogen phosphate, sodium persulfate and a surfactant, i.e., a wetting
agent, such as Triton X 100 (octylphenoxypolyethoxy ethanol), a pH
adjusted to 8.5 plus or minus 0.15. The material used for conductive layer
51 must be a good electrical conductor and must make reliable contact to
the device elements. It must also be durable, adherent to the air bearing
surface material and immune to attack by subsequent manufacturing and
assembly processes. Further it must be removable without any adverse
affect on the magnetic head.
Referring now also to FIG. 5, a perspective view of a typical slider 50
having magnetic heads 34 formed on the slider trailing edge 36 is shown.
The slider has a rectangular rail 53 along each of the slider edges
aligned parallel to the longitudinal axis of the slider. The top surface
of each rail 53 forms the slider air bearing surface. The leading edge of
each rail 53 has an upward slope, or ramp 55 formed across the entire
width of the rail. The magnetic head assembly 34 is mounted on or formed
integrally with the slider trailing edge 36 having the head elements
exposed at the air bearing surface 59 as shown in FIGS. 2a, 2b and 3. In
some applications, it is common to apply a wear layer 57 of carbon or
other suitable material to the slider air bearing surface. The wear layer
57 typically is formed at the conclusion of the lapping and etching
processes forming the slider air bearing surface. In a preferred
embodiment according to the present invention, the wear layer 57 only
partially covers the air bearing surface while that portion 59 of the air
bearing surface where the head 34 elements are exposed is coated with
conductive layer 51. The conductive layer 51 provides ESD/EOS protection,
as described above, during subsequent processing and assembly of the
slider and head disk assembly. Prior to final integration of the slider
(at the HSA level) into the head disk assembly, the conductive layer 51
only is removed as described above thus leaving the wear layer 57 over the
remainder of the air bearing surface.
While the present invention has been particularly shown and described with
reference to preferred embodiments thereof, it will be understood by those
skilled in the art that various changes in form and detail may be made
therein without departing from the spirit, scope and teaching of the
invention. Accordingly, the invention herein disclosed is to be considered
merely as illustrative and limited in scope only as specified in the
appended claims.
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