1.  Field of the Invention
The present invention relates in general to a magnetoresistive sensor used, for example, in computer disk drives; and, more specifically to a method for removing a cap on the sensor during manufacture. 
2.  Description of the Background Art
Magnetoresistive sensors are commonly used in magnetic disk drives.  Disk drives using magnetic recording of digital information store most of the information in contemporary computer systems.  A disk drive has at least one rotating disk with discrete concentric tracks of data.  Each disk drive also has at least one recording head typically having a separate write element and read element, typically a magnetoresistive sensor.  The recording head is constructed on a slider and the slider is attached to a suspension.  The combination of the recording head, slider, and suspension is called a head gimbal assembly.  In addition, there is an actuator which positions the recording head over the specific track of interest.  The actuator first rotates to seek the track of interest and after positioning the recording head over the track, maintains the recording head in close registration to that track.  The disk in a disk drive has a substrate and a magnetic layer on the substrate for magnetic recording.  The slider carrying the recording head has a disk facing surface upon which an air bearing is constructed.  The air bearing allows the slider to float on a cushion of air and to be positioned close to the disk surface.  Alternatively, the slider surface facing the disk can be adapted for partial or continuous contact with the disk. 
As the density of recorded information continually increases, the read element must become smaller and more sensitive.  It is very challenging to achieve an increase in sensitivity for small read elements.  Any process step in the manufacture of read elements which results in a significant loss of sensitivity may be tolerated for large sensors but, in general, is less acceptable for small sensors. 
A magnetoresistive sensor is widely used as the read element in magnetic recording applications.  A magnetoresistive sensor is also used as the storage element in magnetic random access memory arrays (MRAM).  Most contemporary magnetoresistive sensors are based on the external field modulation of spin dependent transport of electrons through a stack of thin films.  This modulation of electron transport through the sensor may be measured by passing sense current though the sensor to determine the effective resistance.  The current used to sense the changes in the sensor may be within the plane of the sensor stack (called an current-in-plane or CIP sensor).  Giant magnetoresistive (GMR) sensors, also called spin valve sensors, are typically CIP sensors.  Another type of magnetoresistive sensor employs sense current which is perpendicular to the sensor stack (called CPP sensors).  Tunnel junction magnetoresistive sensors are typically configured as CPP sensors. 
During the construction of a magnetoresistive sensor, a stack of thin films is formed and then typically a capping layer of tantalum is formed over the stack of thin films.  The tantalum layer protects the sensor stack from damage during subsequent processing, including annealing.  In some sensor fabrication methods, the tantalum layer may be completely or partially removed before subsequent steps such as the formation of electrical leads.  Although tantalum is an excellent material for protecting the sensor stack, it is a difficult material to remove without causing damage to the underlying sensor.  The tantalum layer may be removed by ion milling.  However ion milling is not selective for only tantalum, and is usually energetic enough to cause damage to the sensor stack.  Lower energy, selective reactive ion etching may also be used.  A fluorine reactive ion etch (RIE) is effective in removing tantalum.  However the tantalum-fluorine byproducts formed during the RIE have very low volatility.  Accordingly, after fluorine reactive ion etching, the surface of the sensor stack usually has appreciable amounts of residual tantalum-fluorine byproducts.  This residue can serve to initiate corrosion, reduce exchange coupling strength, can increase the electrical resistance between the sensor stack and the subsequently formed electrical leads, and can result in a loss of sensitivity. 
Thus a method for removing the tantalum cap from a magnetoresistive sensor is needed which minimizes damage to the sensor and leaves a clean surface upon which subsequent layers may be formed.