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.
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.
Magnetoresistive (MR) sensors are particularly useful as read elements in magnetic heads, used in the data storage industry for high data recording densities. Three examples of MR materials used in the storage industry are anisotropic magnetoresistive (AMR), giant magnetoresistive (GMR) and tunneling magnetoresistive (TMR). An MR sensor is one whose resistance is changed by a magnetic field. MR, e.g., AMR, GMR and TMR, 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 film resistors become to damage from spurious current or voltage spikes.
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 physical and/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 (on the 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 (Å) with a width of a few 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.
One major source of ESD damage is associated with tribocharging of the flexible cables used to attach the heads to the external devices. High magnitude currents sufficient to damage a head can occur when the cables are tribocharged and the distal end of the cable makes electrical contact with an external device or piece of metal. The resultant discharge may result in damage as described above.
Another potential cause of ESD damage to sensitive electronics is the creation of a potential difference between two different elements within a device. An approach to protect against this damage is to connect the elements together on the wafer. However, this approach can be costly. Furthermore, in the case of tape heads, due to the large number of elements in a tape head, space on the wafer is limited and can preclude attaching resistive elements onto the wafer for this purpose.
Another major cause of ESD damage is termed machine-model (MM) ESD. MM ESD occurs when a differential in voltages exists between an element (sensor) which is bonded onto a cable and an external tester or device to which the cable is attached. When the sensor is attached directly to the external tester or device, high level currents can flow to equalize the voltages. The high current levels can cause damage to the sensor. A common practice to protect the sensors from damage is use a high impedance connection to the ground potential of any tester/device or tool (external device) to which the sensor is to be connected in order to equalize the voltage potential of the sensor and the external device prior to connecting the sensors with a low resistance connection for use or test. Such an approach would be to bridge the element lead connections to the external device with a high impedance resistor prior to attaching to the external connector. An external resistor, though, can add significant capacitance between the element and the resistive element and may not work. Also, this approach can be difficult to execute in a manufacturing environment. In order to function properly, the minimum amount of capacitance should introduced be between the sensor and the high impedance device to avoid charge build up which can damage the sensor. An approach is to attach a high impedance resistor directly onto a cable which is attached to the device elements (e. readers and writers). This, though, requires costly alignment and bonding tools.
Another approach which solves part of the problem is to use an ESD dissipative sheet with an insulating adhesive layer to bond the dissipative sheet above the leads. While this serves to mitigate tribocharging of the cable, it does not solve the problem of MM ESD.
In summary, the detractors of current solutions are: cost, lack of space, and the need for multiple solutions to solve the different problems.