Ion beam definition of magnetoresistive field sensors

Details Ion beam definition of magnetoresistive field sensors Ion beam definition of magnetoresistive field sensors

2000-09-25

2004-05-25

Ometz, David L. (Department: 2653)

Dynamic magnetic information storage or retrieval

Head

Magnetoresistive reproducing head

Reexamination Certificate

active

06741429

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to forming magnetoresistive field sensors. More particularly, this invention relates to magnetic recording systems.
BACKGROUND ART
Sensors for detecting and measuring magnetic fields find many scientific and industrial applications. For example, a magnetic recording head typically includes a sensing element that senses a magnetic flux emanating from a recording medium. The magnetic field changes some physical property of the sensing element in a manner that depends on the magnitude and direction of the magnetic field. A sensing element that changes its electrical resistivity in response to a magnetic field is usually referred to as a magnetoresistive field sensor. Prior magnetoresistive field sensors typically include one or more ferromagnetic elements whose resistivity changes in response to magnetic flux. Prior magnetoresistive field sensors include anisotropic magnetoresistive (AMR) sensors and giant magnetoresistive. (GMR) sensors, in which a sense current flows along, or perpendicular to, planes of the ferromagnetic elements. Prior magnetoresistive field sensors also include magnetoresistive tunnel junction (MTJ) sensors, in which a sense current flows perpendicular to the planes of the ferromagnetic elements through a dielectric barrier. Resistance of a magnetoresistive field sensor varies as the square of the cosine of the angle between the magnetization in the sensor and the direction of sense current. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the sensor, which in turn causes a change in resistance in the sensor and a corresponding change in the sense current or voltage.
Increasing areal density of magnetic storage media requires that the magnetic recording and reading heads be able to operate at ever-decreasing track widths (TW). Both the write element and the magnetic readback sensor of the recording head must be made smaller in order to achieve narrower data tracks. The width of the recorded track is determined by, among other parameters, the width of the write pole of the write head and the flying height of the write head. The size and geometry of the shields and leads also play a role in determining achievable track width for a given recording head design.
In order to take advantage of the narrower write track width, it is imperative that the read track width of the readback element or read head be reduced as well. At present, magnetoresistive (MR) heads are typically made by photolithographically defining the sensor element from a continuous multilayer thin film. The sensor, which is frequently rectangular in shape, is often defined in two steps, one photolithographic step to define the TW dimension, and one lapping step to define the so-called “stripe height” (SH) dimension. Unfortunately, due to practical limitations of the lithographic method, such as the diffraction limit of light, it is not easy in a manufacturing environment to produce read heads much narrower than about 200 nm. Meanwhile, MR head technology is already pushing present photolithographic techniques to their limits and these present methods will not be able to accommodate future generations of MR heads. For example, in current commercial products, the sensor TW, which is defined by optical lithography and ion beam milling, is typically less than 1 &mgr;m. It is envisaged that in order to make heads suitable for recording densities of 100 Gbits/in
2
, the sensor TW will need to be around 0.13 &mgr;m, but current lithography is wavelength-limited to around 0.2 &mgr;m.
An associated problem that arises from the current processing method is poor shape definition, which leads to a “tail” on each side of the sensor. The tails are a result of the ion beam milling process commonly used to define TW. The milling is performed with the ion beam at an angle to the wafer in an effort to minimize the redeposition of magnetic material at the mask edges, which would have a deleterious effect on the sensor performance. However, ion milling at an angle creates a shadow near the mask edges, within which the milling is less efficient, resulting in tails on the sensor structure. The beam divergence from the ion mill also contributes to the tails. The presence of the tails degrades the magnetic performance of the sensor. Further, the tails may vary in dimension and form across the wafer, resulting in sensor-tosensor variation in performance.
FIG. 1
illustrates a cross-sectional schematic diagram of a contiguous junction design MR sensor
100
. MR sensor
100
includes a first magnetic shield
102
, and a first insulating gap
104
disposed on the shield
102
. The sensing element
106
including tails
108
is disposed on the gap
104
. Following the milling process, the top of the multilayer sensing element
106
will have a width determined by the resist mask used. However, the all-important sense layer, which is located further down in the multilayer stack that forms sensing element
106
, will inevitably have a larger and possibly not well-controlled width. This problem is predicted to become increasingly important as the TW decreases and the tails become proportionally larger relative to the sensor dimensions.
Once the sensing element
106
is formed using optical lithography and milling, it is usual to deposit a ferromagnetic layer, called “hard bias” layer
110
, with substantial magnetic coercivity (Hc) on each side of the sensing element
106
to stabilize the magnetization at each side of the sensing element, thereby improving sensor performance. However, the tails
108
on each side of the sensing element
106
make deposition of a uniform hard bias layer
110
difficult, and the hard bias layer
110
becomes very thin near the top surface of the sensing element
106
and/or does not closely abut the sensing element
106
, leading to poor sensor performance.
MR sensor
100
further includes leads
112
adjacent to hard bias layers
110
to conduct the sense current to the sensing element
106
when reading data stored on a magnetic recording medium, a second gap
114
and a second shield
115
to protect the sensing element
106
.
A U.S. patent application entitled “Track Width Control of Readback Element” field Jun. 30, 1999, to Patrick C. Arnett et al. discloses a method for reducing the track width of readback elements by implantation of ions. The ion implantation reduces the magnetoresistance of the selected portions of the readback elements. The ion implantation of Arnett et al. is performed by a focused ion beam (FIB) technique. However, FIB processing is slow, since each element is processed in series, which is not desirable for mass manufacture of magnetic sensors. Furthermore, electrostatic discharge (ESD) damage can occur during the application of the FIB to the sensor element, and therefore grounding during processing and low ion currents will be required to minimize this risk. In addition, the FIB processing of Arnett et al. is performed from the air-bearing surface (ABS). The layers that make up the sensor typically run perpendicular to the ABS and have stripe heights about an order of magnitude or more greater than the sensor film thickness. Consequently the ions must penetrate to a greater depth than the sensor film thickness in order to define the magnetically sensitive “tip portion”. A large depth requirement demands high ion energies (incidentally, well beyond the range of standard FIB machines). The increased ion energy will cause an increase in the lateral straggle of the ions in the sensor material, and will widen the transition region between the tip portion and the neighboring “magnetically deactivated” region, presumably degrading the performance of the sensor. In order to conduct “implantation” amounting to a typical few atomic percent of the critical layers, this technique requires extremely large ion doses with long processing times, resulting in problems with heat dissipat

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