Dynamic magnetic information storage or retrieval – Head – Magnetoresistive reproducing head
Reexamination Certificate
2001-06-08
2003-10-07
Korzuch, William (Department: 2653)
Dynamic magnetic information storage or retrieval
Head
Magnetoresistive reproducing head
Reexamination Certificate
active
06631055
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to magnetic tunnel junction (MTJ) read heads. More particularly, it relates to a magnetic tunnel junction head in which the ferromagnetic free layer functions as a flux guide.
BACKGROUND ART
Magnetic tunnel junction (MTJ) devices are based on the phenomenon of spin-polarized electron tunneling. A typical MTJ device includes two ferromagnetic layers separated by a thin insulating tunnel barrier layer. One of the ferromagnetic layers has a magnetic moment free to rotate in the presence of applied magnetic fields. The other ferromagnetic layer has a magnetic moment fixed by interfacial exchange coupling with an anti-ferromagnetic layer. The insulating tunnel barrier layer is thin enough that quantum mechanical tunneling of electrons can occur between the ferromagnetic layers. The tunneling phenomenon is electron-spin dependent, making the magnetic response of the MTJ a function of the relative orientations and spin polarizations of the two ferromagnetic layers.
MTJ devices have been proposed primarily as memory cells for solid state memory devices. The state of the MTJ memory cell is determined by measuring the resistance of the MTJ when a sense current is passed perpendicularly through the MTJ from one ferromagnetic layer to the other. The probability of tunneling of charge carriers across the insulating tunnel barrier layer depends on the relative alignment of the magnetic moments (magnetization directions) of two ferromagnetic layers. The tunneling current is spin polarized, which means that the electrical current passing from one of the ferromagnetic layers, for example, the layer whose magnetic moment is fixed, is predominantly composed of electrons of one spin type (spin up or spin down, depending on the orientation of the magnetic moment of the ferromagnetic layer). The degree of spin polarization of the tunneling current is determined by the electronic band structure of the magnetic material at the interface of the ferromagnetic layer with the tunnel barrier layer. One ferromagnetic layer thus acts as a spin filter. The probability of tunneling of the charge carriers depends on the availability of electronic states of the same spin polarization as the spin polarization of the electrical current in the other ferromagnetic layer. Usually, when the magnetic moments of two ferromagnetic layers are parallel to each other, there are more available electronic states than when the magnetic moments of the two ferromagnetic layers are aligned antiparallel to each other. Thus, the tunneling probability of the charge carriers is highest when the magnetic moments of both layers are parallel, and is lowest when the magnetic moments are antiparallel. When the moments are arranged neither parallel nor antiparallel, the tunneling probability takes on an intermediate value. Thus, the electrical resistance of MTJ memory cells depends on the spin polarization of the electrical current and the electronic states in both of the ferromagnetic layers.
MTJ devices have attracted more attention since a large tunneling magneto-resistance (TMR) was found at room temperature. MTJ devices have since been used as magnetoresistive read/write heads for magnetic recording.
FIG. 1A
is a sectional view of a MTJ head
100
of the prior art. MTJ head
100
includes a MTJ layered structure
120
sandwiched by a top lead
116
adjacent to a top shield
118
and a bottom lead
104
adjacent to a bottom shield
102
. The MTJ layered structure
120
includes a ferromagnetic free layer
106
, a ferromagnetic pinned layer
110
, an insulating tunnel barrier layer
108
located between the ferromagnetic free layer
106
and the ferromagnetic pinned layer
110
, an anti-ferromagnetic layer
112
adjacent to the ferromagnetic pinned layer
110
, and a capping layer
114
adjacent to the anti-ferromagnetic layer
112
. In the MTJ head
100
, the ferromagnetic free layer
106
, the insulating tunnel barrier layer
108
, and the ferromagnetic pinned layer
110
all have their front edges exposed at the sensing surface
122
of the head, i.e., the air-bearing surface (ABS) of the air bearing slider if the MTJ head
100
is used in a magnetic recording disk drive. Unfortunately, when the MTJ head
100
is lapped to form the sensing surface
122
, it is possible that material from the ferromagnetic free layer
106
and the ferromagnetic pinned layer
110
smears at the sensing surface
122
and shorts out across the insulating tunnel barrier layer
108
.
Magnetoresistive (MR) head technology has been developed to produce a MTJ head for a magnetic recording system that does not suffer the problem associated with having the edges of the MTJ layers exposed at the sensing surface.
FIG. 1B
is a sectional view of a flux guided MTJ head
101
having the ferromagnetic free layer
105
acting as a flux guide to direct magnetic flux from the magnetic recording medium to the tunnel junction. The flux guided MTJ head
101
includes a MTJ layered structure including a ferromagnetic free layer
105
, a ferromagnetic pinned layer
109
, an insulating tunnel barrier layer
107
located between the ferromagnetic free layer
105
and the ferromagnetic pinned layer
109
, an anti-ferromagnetic layer
111
adjacent to the ferromagnetic pinned layer
109
, and a capping layer
113
adjacent to the anti-ferromagnetic layer
111
. The MTJ layered structure is sandwiched by a bottom lead
103
adjacent to a bottom shield
121
and a top lead
115
adjacent to a top shield
117
. In the flux guided MTJ head
101
, the front edge of the ferromagnetic free layer
105
is exposed at the ABS
123
, while the front edges of the capping layer
113
, the anti-ferromagnetic layer
111
, the ferromagnetic pinned layer
109
, and the insulating tunnel barrier layer
107
are recessed from the ABS
123
by an insulation
119
.
The flux guided MTJ
101
is fabricated using a method illustrated in
FIGS. 2A-2D
. As shown in
FIG. 2A
, an electrical lead
202
is first deposited on a substrate (not shown), and a ferromagnetic free layer
204
is deposited on the electrical lead
202
. An insulating tunnel barrier layer
206
is deposited on the ferromagnetic free layer
204
, and a ferromagnetic pinned layer
208
is deposited on the insulating tunnel barrier layer
206
. An anti-ferromagnetic layer
210
is deposited on the ferromagnetic pinned layer
208
, and a capping layer
212
is deposited on the anti-ferromagnetic layer
210
. All of the MTJ layers are deposited by typical vacuum deposition techniques, such as ion beam deposition, RF or DC magnetron sputtering deposition, evaporation deposition, or molecular beam epitaxy (MBE) deposition. A photoresist mask
214
is deposited on the MTJ layers to define an active region of the ferromagnetic pinned layer
208
. The material in the unmasked regions of the capping layer
212
, the anti-ferromagnetic layer
210
, the ferromagnetic pinned layer
208
and the insulating tunnel barrier layer
206
are removed as shown in
FIG. 2B
using subtractive techniques, such as ion beam milling, chemically-assisted ion milling, sputter etching, and reactive ion etching, preferably ion beam milling. These unmasked regions are then refilled with an insulating material
218
as shown in
FIG. 2C
with a quantity of the insulating material
218
also deposited onto the top and sidewalls of the photoresist mask
214
. This quantity of the insulating material
218
is removed, along with the photoresist mask
214
, in a liftoff process, resulting in a structure as shown in FIG.
2
D.
A problem with subtractive techniques is that the endpoint must terminate precisely within the insulating tunnel barrier layer
206
. This is very difficult to achieve. For example, if ion beam milling is used, the thickness of the insulating tunnel barrier layer
206
is typically about 10 Å, and the ion beam milling rates are typically between 3 Å/sec and 4 Å/sec, which allows an endpoint target about 2-3 seconds. Furthermore, if undermining occurs, a portion of the fer
Childress Jeffrey R.
Fontana Robert E.
Ho Kuok San
Tsang Ching H.
Castro Angel
International Business Machines - Corporation
Korzuch William
Lumen Intellectual Property Services Inc.
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