Magnetic head-based systems have been widely accepted in the computer industry as a cost-effective form of data storage. In a magnetic tape drive system, a magnetic tape containing a multiplicity of laterally positioned data tracks that extend along the length of the tape is drawn across a magnetic read/write transducer, referred to as a magnetic tape head. The magnetic tape heads can record and read data along the length of the magnetic tape surface as relative movement occurs between the heads and the tape. Because magnetic tape is a flexible media, its lateral position fluctuates as the tape is pulled at high speeds across the tape head. In order to maintain alignment of the read sensors or writing transducers along the tracks, the tape head is moved (actuated) laterally to follow the tape fluctuations as the high speed lateral response, termed actuation, is better achieved with lighter tape heads.
In a magnetic disk drive system, a magnetic recording medium in the form of a disk rotates at high speed while 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 lifts 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 tens of nanometers 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.
Magnetoresistive (MR) sensors are particularly useful as read elements in magnetic heads, used in the data storage industry for high data recording densities. Two examples of MR materials used in the storage industry are anisotropic magnetoresistive (AMR) and giant magnetoresistive (GMR). MR and GMR sensors are deposited as small and thin multi-layered sheet resistors on a structural substrate. The sheet resistors can be coupled to external devices by contact to metal pads which are electrically connected to the sheet resistors. MR sensors provide a high output signal which is not directly related to the head velocity as in the case of inductive read heads.
To achieve the high areal densities required by the data storage industry, the sensors are made with commensurately small dimensions. The smaller the dimensions, the more sensitive the thin sheet resistors become to damage from spurious current or voltage spike.
A major problem that is encountered during manufacturing, handling and use of MR sheet resistors as magnetic recording transducers is the buildup of electrostatic charges on the various elements of a head or other objects which come into contact with the sensors, particularly sensors of the thin film type, and the accompanying spurious discharge of the static electricity thus generated. Static charges may be externally produced and accumulate on instruments used by persons performing head manufacturing or testing function. These static charges may be discharged through the head causing excessive heating of the sensitive sensors which result in physical or magnetic damage to the sensors.
As described above, when a head is exposed to voltage or current inputs which are larger than that intended under normal operating conditions, the sensor and other parts of the head may be damaged. This sensitivity to electrical damage is particularly severe for MR read sensors because of their relatively small physical size. For example, an MR sensor used for high recording densities for magnetic tape media (order of 25 Mbytes/cm2) are patterned as resistive sheets of MR and accompanying materials, and will have a combined thickness for the sensor sheets on the order of 500 Angstroms (A) with a width of 1 to 10 microns (μm) and a height on the order of 1 μm. Sensors used in extant disk drives are even smaller. Discharge currents of tens of milliamps through such a small resistor can cause severe damage or complete destruction of 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, oxidation of materials at the air bearing surface (ABS), generation of shorts via electrical breakdown, and milder forms of magnetic or physical damage in which the head performance may be degraded. Short time current or voltage pulses which cause extensive physical damage to a sensor are termed electrostatic discharge (ESD) pulses. Short time pulses which do not result in noticeable physical damage (resistance changes), but which alter the magnetic response or stability of the sensors due to excessive heating are termed electrical overstress (EOS) pulses.
While a disk head is comprised of a single MR element, modern tape heads have multiple MR elements, on the order of 8 to 32, or even more, all of which must be fully functional. The large number of MR sensors in a tape drive and the requirement that all are functional, makes ESD loss due to a single element very expensive as the entire head must then be scrapped. Testing during manufacturing is important in order to eliminate damaged components early in the process to minimize cost by avoiding processing of damaged parts.
Prior solutions to ESD and EOS protection can be summarized into two types of approaches: (1) one time permanent shorting devices, and (2) multi-use removable shorting devices.
The one time permanent shorting devices include:    (1a) fabricating diodes on the wafer to short out high currents across the sensor;    (1b) depositing a shorting element (e.g., metal) across the sensor which can be later removed at final assembly; and    (1c) diodes connecting the two sensor leads together which are permanently mounted in the cable.
The Multi-use removable shorting devices include:    (2a) a removable shorting element which can short out the two leads of the sensor; and    (2b) removable diodes which can short out the two leads of the sensor when subjected to high currents.
The problem with (1a)-(1c) is the cost of the permanent solution. (1b) prohibits measuring the properties of the MR sensor until it is in the final device and a large amount of cost has gone into fabrication and assembly. The problem with (2a) is that the device can not be tested during assembly without removing the short and subjecting the sensors to the potential of EOS/ESD damage. The problem with (2b) is the substantial cost of the diodes.
A need therefore exists for providing ESD and EOS protection for a multiplicity of read and/or write head assemblies which has a low cost, is small enough not to affect the dynamics of the head during operation, which fits into the tight spaces within a tape or disk drive, and which allows for the higher voltages used in normal tape drive operation.