Electricity: electrothermally or thermally actuated switches – Thermally actuated switches – With bimetallic element
Reexamination Certificate
1999-10-14
2001-05-29
Picard, Leo P. (Department: 2832)
Electricity: electrothermally or thermally actuated switches
Thermally actuated switches
With bimetallic element
C337S333000, C337S036000, C337S053000, C337S089000, C251S129020
Reexamination Certificate
active
06239685
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of microactuators. More particularly, the present invention relates to bistable microactuators as used in disk drives and tape drives.
2. Description of the Related Art
Magnetoresistive (MR), giant magnetoresistive (GMR) and magnetic tunnel junction (MTJ) heads for disk drives are among the most ESD-sensitive components that are used in industry today. For example, currently-used heads can be permanently damaged by only a few 10s of Volts being discharged from a human body. GMR heads suffer from a loss of initialization for even smaller discharges. Moreover, reductions in thin-film dimensions that are associated with each new generation of MR, GMR and MTJ heads make such heads even more prone to ESD damage.
ESD damage to MR/GMR/MTJ heads is a significant source of yield loss in disk drives. Head yields are in the 95% to 99% range and a large number of heads are used in disk drives (2-20 per drive). Drive yields would be unacceptably low without intermediate testing of heads. Consequently, heads are tested at several stages of drive assembly, allowing a faulty head to be replaced before being integrated into a larger assembly that is relatively more expensive to replace or repair.
ESD protection diodes have been successfully used on I/O connections of integrated circuits and other sensitive components for preventing ESD damage. ESD protection diodes, however, provide limited protection for an MR/GMR/MTJ head, while creating undesirable noise during normal operation of the MR/GMR head. An MR/GMR head has a resistance of about 35 Ohms and operates with a bias current of about 10 mA. In normal operation, a bias voltage of 0.35 V appears across an MR/GMR head (which may be higher for high resistance heads). In order to shunt harmful current away from an MR/GMR head, an ESD diode must have an effective impedance that is significantly less than 35 Ohms (say, about 1 Ohm) for high currents. Diodes with such high conductivity, however, have a large leakage current at the operating bias voltage of an MR/GMR head that produces noise across the head.
Additionally, diodes having sufficient conductivity tend to be physically large, and at normal operating conditions have a large junction capacitance that adversely affects the ability of a head to perform at high data rates. Furthermore, fabrication of protection diodes close to the head elements (which is needed for good ESD protection) requires introduction of semiconducting materials and appropriate processes into the head fabrication process that are major departures from conventional head fabrication techniques.
An alternative form of ESD protection that can be used for MR/GMR/MTJ heads is laser-delete shorting. For laser-delete shorting, temporary conducting traces are fabricated across a head element for ESD protection. After the heads have been safely integrated into an actuator, a laser beam pulse removes the temporary trace. During the time the trace is in place, the head cannot be tested because the temporary trace short-circuits the head element. After the temporary trace has been deleted, the head can be tested, but ESD protection no longer exists.
Laser-delete shorting can be extended to a plurality of temporary traces, of which only one or a few are used at a time for providing ESD protection. In such an approach, a ladder-type arrangement of temporary traces is fabricated having temporary traces as rungs in the ladder structure. To establish a temporary trace, a laser pulse (or pulses) of a predetermined energy intensity are applied to a selected trace for establishing a shorting trace. To (permanently) remove the trace, a laser pulse (or pulses) of yet a greater energy intensity are applied to the trace. Traces are established and removed by sequentially selecting different temporary traces in the ladder structure. Nevertheless, such a ladder-type arrangement has the drawback of being complex and occupying valuable space.
Because of these shortcomings mentioned above, neither ESD protection diodes nor laser-delete shorting have been conventionally used in MR/GMR/MTJ heads, leaving heads vulnerable to ESD.
An ideal ESD protection for an MR/GMR/MTJ head would successfully shunt harmful currents or voltages away from the head without affecting the normal operation of the head. Moreover, an ideal ESD head-protection device would also allow a head to be tested during fabrication and during assembly of a disk drive. What is needed is a technique for providing ESD protection for an MR/GMR/MTJ head that allows testing during head fabrication and drive assembly.
SUMMARY OF THE INVENTION
The present invention provides an ESD protection technique for an MR/GMR/MTJ head that allows testing during head fabrication and drive assembly.
The advantages of the present invention are provided by a bistable micromechanical switch that includes a substrate having a surface, at least two anchor points formed on the substrate, and a beam structure that includes a two-material beam attached to at least two anchor points. The two-material beam has a first portion, a second portion and a center portion. The first portion of the two-material beam is formed from a first layer of a first material and a second layer of a second material such that the first layer of the first portion is proximate to the surface of the substrate and the second layer of the first portion is remote from the surface of the substrate. The first material has a first coefficient of thermal expansion and the second material has a second coefficient of thermal expansion such that the second coefficient of thermal expansion is greater than the first coefficient of thermal expansion. Preferably, the first material is selected from the group consisting of Cr, Ta, W and Mo, and the second material is selected from the group consisting of Ni, Fe and Co. The second portion of the two-material beam is formed from a first layer of the second material and a second layer of the second material such that the first layer of the second portion is proximate to the surface of the substrate and the second layer of the second portion is remote from the surface of the substrate. The beam structure has a first and a second stable state such that the center portion of the beam structure is deflected toward the surface of the substrate for the first stable state and is deflected away from the surface of the substrate for the second stable state.
According to the invention, the two-material beam transitions from the first stable state to the second stable state when thermal energy is applied to the second portion of the two-material beam. Similarly, the two-material beam transitions from the second stable state to the first stable state when thermal energy is applied to the first portion of the two-material beam. Preferably, the thermal energy applied to either the first or second portions of the two-material beam is generated using a laser beam. Alternatively, the thermal energy applied to either the first or second portions of the two-material beam is generated using electrical energy.
A contact element can be formed on the surface of the substrate and another contact element can be formed on center portion of the beam structure proximate to the surface of the substrate. Preferably, the contact elements are formed from an Au/Ni alloy. The two contact elements can then be electrically connected to a read/write head. The contact element formed on the center portion of the two-material beam contacts the contact element on the surface of the substrate when the two-material beam is in the first stable state, and thereby provides ESD protection.
For a first embodiment of the present invention, the two-material beam forming the beam structure is under a compressive stress. For this embodiment, the two-material material beam can further include a third portion and a fourth portion such that the third portion is located between the first portion and the center portion and the fourth portion is located between the
Albrecht Thomas Robert
Reiley Timothy Clark
Banner & Witcoff , Ltd.
Berthold, Esq. Thomas R.
International Business Machines - Corporation
Picard Leo P.
Vortman Anatoly
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