Dynamic magnetic information storage or retrieval – Head – Head accessory
Reexamination Certificate
1999-11-05
2002-03-19
Cao, Allen (Department: 2652)
Dynamic magnetic information storage or retrieval
Head
Head accessory
Reexamination Certificate
active
06359750
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to read heads for computer data storage devices. In particular, the invention concerns a structure for protecting a read head from electrostatic discharge.
2. Description of the Related Art
Data storage devices, such as magnetic disk drives and tape drives, used to store information for computer systems are well known. In magnetic data storage devices, a medium, such as a magnetic disk platter or magnetic tape, is treated with magnetic material. The magnetic material can be polarized in order to cause phase reversals in a magnetic field to encode information on the medium. The phase reversals used to encode information can be detected by magnetic sensors, commonly referred to as read heads. It is common for a read head to be mounted in a structure, commonly referred to as a slider. Sliders typically fly over the surface of the medium supported by a thin layer of air, commonly referred to as an air bearing. The air bearing is generated by relative motion of the slider with respect to the medium. For example, in a disk drive device, the disk platter is rotated to generate relative motion between the medium and the slider. The slider may be positioned radially over the medium to allow the read head access to any region of the medium as the medium rotates.
FIG. 1
is an illustration of a slider
1
having two rails
3
. Each rail
3
has an air bearing surface
5
. A read head
7
is located on a “deposition end”
4
of each rail. The slider
1
moves in the direction of arrow
9
relative to a magnetic medium.
One well known type of read head is referred to as a magneto-resistive (“MR”) head. An MR head uses magneto-resistive material (commonly referred to as an “MR sensor element”) to sense changes in a local magnetic field.
FIG. 2
is a simplified illustration of a cross-section of an MR read head
7
within a rail
3
of a slider
1
, viewed from the air bearing surface
5
. The arrow
9
indicates the direction of the read head
7
with respect to the medium over which the read head
7
flies. An MR sensor element
11
is shown disposed between a first magnetic shield
13
and a second magnetic shield
15
. The first and second shields
13
,
15
are typically formed of a magnetic material, such as a nickel/iron alloy, which prevents the magnetic fields of adjacent regions of the medium from distorting the fields associated with the information that is being read from the medium. Surrounding each shield
13
,
15
and the MR sensor element
11
is an insulating material
17
, such as alumina. The insulator
17
prevents the MR sensor element
11
from coming into direct electrical contact with either the first or second shield
13
,
15
. Also shown in
FIG. 2
is a substrate
19
. The substrate
19
may be a ceramic material, such as titanium carbide.
FIG. 3
is a cross-sectional view through line
3
—
3
of the read head
7
shown in FIG.
2
. The MR sensor element
11
(shown by broken line to indicate that the sensor
11
is obscured by a sensor lead
21
) is coupled to additional circuitry, which is well known in the art, by sensor leads
21
(only one such lead
21
is shown on the near side of the MR sensor element
11
). A second lead (not shown) is coupled to another side of the MR sensor element
11
. A carbon overcoat
20
may be applied to the air bearing surface
5
to minimize wear and protect the relatively soft shields
13
,
15
and MR sensor element
11
from damage. The overcoat
20
has little effect on the likelihood that a sparkover will occur at the air bearing surface
5
.
One problem with MR heads, such as the head
7
shown in
FIGS. 1-3
is that electrostatic charges may be transferred from an external source (such as a human body) to the components of the MR read head
7
(such as the shields
13
,
15
, MR sensor element
11
, and substrate
19
) during production. When the charge transferred to one component is sufficiently large, an electrical discharge, commonly referred to as a “sparkover” occurs. Such sparkovers are most likely to occur during production and handling of the head
7
.
Sparkovers can damage the head. For example, the high current density at the sparkover location typically results in material near the sparkover melting. This damage may occur at the air bearing surface
5
of the slider
1
. In a high percentage of MR read heads in which sparkover damage at the air bearing surface
5
occurs, the result of the sparkover damage is either increased resistance, or alternatively, a near open circuit condition in the MR sensor element circuit. In addition, damage to the air bearing surface
5
results in undesirable changes in the flying height characteristics of the slider
1
. That is, even the minor changes in the surface characteristics of the air bearing surface
5
have a great impact on the flying height characteristics of the slider
1
. Because of the undesirable effects of sparkovers, the manufacturing yield for MR read heads is reduced in proportion to the frequency with which such sparkovers typically occur.
Studies of such electro-static discharges have revealed that these discharges typically occur in one of three regions. These three regions are indicated in
FIG. 3
by the letters “A”, “B”, and “C”. As shown in
FIG. 3
, the regions of discharge are typically along the air bearing surface
5
(even when a carbon overcoat
20
is provided) due to a higher electric field generated in the air bearing.
FIG. 4
illustrates an electrical model of the circuit formed by the elements of the MR head
7
. The resistance of the leads
21
to and from the MR sensor element
11
is modeled as two resistors
23
,
24
. The resistance of the MR sensor element
11
is modeled as a resistor
25
. One method for preventing damage due to electrostatic discharge is taught by U.S. Pat. No. 5,272,582, entitled “Magneto-Resistance Effect Magnetic Head with Static Electricity Protection”, issued to Shibata, et. al on Dec. 21, 1993. In Shibata, two sensor element magnetic cores are deposited to form a magnetic gap near the air bearing surface of a slider. The two magnetic cores are in magnetic contact with one another at a “back gap” which is away from the magnetic gap. An insulating layer is placed between each sensor element magnetic core at the magnetic gap. An MR sensor element is place between the insulating layers such that the MR sensor element is within the magnetic gap. A ground conductive layer is electrically connected to a first of the magnetic cores to route to ground the electric charges coming into the magnetic gap from the magnetic recording medium. Accordingly, Shibata attempts to keep the magnetic cores which form the magnetic gap at a controlled potential. This arrangement is intended to prevent electric charges that may come from the magnetic recording medium from rushing into the magnetic gap.
A second method for preventing electro-static discharge and the associated damage that such discharge causes is taught in IBM Technical Disclosure Bulletin, Vol. 21, No. 11, dated April, 1979, by Rohen (hereinafter referred to as “Rohen”).
FIG. 5
illustrates the approach taken by Rohen. In
FIG. 5
, an MR element
31
is located at one end of the structure. A first conductive region
33
and a second conductive region
35
are electrically coupled to a ground potential via terminals
37
,
39
. An insulating material
41
isolates these regions
31
,
33
from two additional conductive regions
43
,
45
. Regions
43
,
45
provide a conductive path for current to the MR element
31
. During fabrication, the upper portion
47
of the structure is removed to the broken line
49
. By coupling the regions
33
,
35
to a ground potential, a low potential point is provided for any direct electrostatic discharges, and the grounded side bars formed by the regions
33
,
35
provide a Faraday shield to lessen the effect of indirect electro-static discharges.
A third method for preventing electro-static discharge and the associated damage that
Hughbanks Timothy Scott
Robertson Neil Leslie
Voldman Steven Howard
Wallash Albert John
Cao Allen
International Business Machines - Corporation
Johnston Ervin F.
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