Semiconductor device manufacturing: process – Radiation or energy treatment modifying properties of...
Reexamination Certificate
2002-10-29
2004-12-07
Smith, Mathew (Department: 2825)
Semiconductor device manufacturing: process
Radiation or energy treatment modifying properties of...
C438S003000, C360S324100
Reexamination Certificate
active
06828260
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to a method for exposing a tunnel barrier layer of a tunnel junction device to ultra-violet light through an overlayer that covers the tunnel barrier layer. More specifically, the present invention relates to a method for exposing an electrically non-conductive tunnel barrier layer of a tunnel junction device to ultra-violet light through an overlayer that covers the tunnel barrier layer in order to heal out defects in the tunnel barrier layer.
BACKGROUND OF THE ART
Magnetic tunnel junctions (MJT) are devices that include two ferromagnetic (FM) layers of material that are separated by a thin dielectric layer (i.e. an insulating layer) which acts as a tunnel barrier. Uses for tunnel junction devices include magnetic field sensors and thin film high density read and write heads for hard disk drives. Magnetic Random Access Memory (MRAM) is an emerging technology that also incorporates a tunnel junction and can provide an alternative to traditional data storage technologies. MRAM has desirable properties such as fast access times like DRAM and non-volatile data retention like hard disk drives. MRAM stores a bit of data (i.e. information) as an alterable orientation of magnetization in a patterned thin film magnetic element that is referred to as a data layer, a storage layer, a free layer, or a data film. The data layer is designed so that it has two stable and distinct magnetic states that define a binary one (“1”) and a binary zero (“0”). Although the bit of data is stored in the data layer, many layers of carefully controlled magnetic and dielectric thin film materials are required to form a complete magnetic memory element. One prominent form of magnetic memory element is a spin tunneling device. The physics of spin tunneling is complex and good literature exists on this subject.
In
FIG. 1
a
, a prior MRAM memory element
101
includes a data layer
102
and a reference layer
104
that are separated by a thin tunnel barrier layer
106
. Typically the tunnel barrier layer
106
is a thin film with a thickness that is less than about 2.0 nm. In a tunnel junction device such as a tunneling magnetoresistance memory (TMR) the barrier layer
106
is an electrically non-conductive dielectric material such as aluminum oxide (Al
2
O
3
), for example. The tunnel barrier layer
106
is an insulator through which a tunneling current passes. The magnitude of the tunneling current and the quality of the tunneling current are greatly influenced by the quality of the insulator used for the tunnel barrier layer
106
. As a voltage that is applied across the tunnel barrier layer
106
is increased, the tunneling current increases in a nonlinear fashion.
The reference layer
104
has a pinned orientation of magnetization
108
, that is, the pinned orientation of magnetization
108
is fixed in a predetermined direction and does not rotate in response to an external magnetic field. In contrast the data layer
102
has an alterable orientation of magnetization
103
that can rotate between two orientations in response to an external magnetic field. The alterable orientation of magnetization
103
is typically aligned with an easy axis E of the data layer
102
.
In
FIG. 1
b
, when the pinned orientation of magnetization
108
and the alterable orientation of magnetization
103
point in the same direction (i.e. they are parallel to each other) the data layer
102
stores a binary one (“1”). On the other hand, when the pinned orientation of magnetization
108
and the alterable orientation of magnetization
103
point in opposite directions (i.e. they are anti-parallel to each other) the data layer
102
stores a binary zero (“0”).
The data layer
102
and the reference layer
104
serve as electrodes of a tunnel junction device which allow the state of the bit stored in the data layer
102
to be sensed by measuring a resistance across the data layer
102
and the reference layer
104
or a by measuring a magnitude of the aforementioned tunneling current. Although the reference layer
104
is shown positioned below the tunnel barrier layer
106
, the actual position of the data layer
102
and the reference layer
104
will depend on the order in which they are formed in a process for fabricating the magnetic memory cell
101
. Accordingly, the data layer
102
can be formed first and the tunnel barrier layer
106
formed on top of the data layer
102
.
Ideally, the tunnel barrier layer
106
of a tunnel junction device is flat and has a uniform thickness T throughout its cross-sectional area. Moreover, an ideal tunnel barrier layer
106
would be made from a dielectric material that is homogenous. One of the criteria for an ideal tunnel barrier layer
106
is that it have a high breakdown voltage. That is, the voltage at which the dielectric material of the tunnel barrier layer
106
breaks down and the tunnel barrier layer
106
acts as a shorted resistance.
However, in tunnel junction devices, such as the prior memory cells
101
, one problem that detrimentally effects operation of the memory cell
101
is that defects in the tunnel barrier layer
106
result in a low breakdown voltage or an electrical short. Those defects include pin holes, bubbles, surface irregularities, metal inclusions, and non-uniformity of thickness in the tunnel barrier layer
106
, just to name a few.
In
FIG. 2
, a material for the tunnel barrier layer
106
is formed or deposited on a supporting layer
110
that can be the reference layer
106
or the data layer
102
of a magnetic field sensitive memory cell, for example. For instance, the material for the tunnel barrier layer
106
can be aluminum (Al). The material is then exposed to oxygen (O
2
) and is oxidized to form aluminum oxide (Al
2
O
3
). However, the prior oxidation process doesn't uniformly oxidize all of the aluminum atoms and as a result there remains un-oxidized aluminum atoms
111
that form metal inclusion defects in the tunnel barrier layer
106
. A portion of the oxygen atoms
112
remains un-reacted with the material
111
for the tunnel barrier layer
106
(i.e the aluminum); however, those un-reacted oxygen atoms
112
remain incorporated in the tunnel barrier layer
106
. Similarly, some of the oxygen atoms
112
react with and oxidize a portion of a material
113
for the supporting layer
110
. As a result, oxidized atoms
113
at an interface between the tunnel barrier layer
106
and the supporting layer
110
create a defect that lowers the break down voltage of the tunnel barrier layer
106
.
Prior methods for depositing or forming the tunnel barrier layer
106
such as RF-sputtering, plasma oxidation, or UV-ozone oxidation ultimately leave some defects in the tunnel barrier layer
106
that result in a poor tunneling barrier with weak points therein that cause shorting or a low breakdown voltage. Moreover, the existence of those defects makes it necessary to form a thicker nitride/oxide layer for the tunnel barrier layer
106
. Conversely, if the dielectric material is a really good dielectric, then the thickness of the tunnel barrier layer
106
can be reduced. A thinner tunnel barrier layer
106
also helps in improving uniformity across an entire wafer that carries multiple tunnel junction devices. A thinner tunnel barrier layer
106
also lowers absolute resistance of the tunnel junction which can be important in some applications.
However, in
FIG. 2
, after the prior tunnel barrier layer
106
is formed on the supporting layer
110
, it is often necessary to deposit a next layer
120
in the tunnel junction stack for several reasons. The next layer
120
can be any layer necessary in the fabrication of a tunnel junction device.
First, it is desirable to control further uncontrolled oxidation of the tunnel barrier layer
106
. To that end, the next layer
120
is deposited over the tunnel barrier layer
106
to cap off the tunnel barrier layer
106
thereby rendering it substantially inert to further uncontrolled oxidation.
Second, it is desirable to prevent o
Denny, III Trueman H
Sharma Manish
Malsawma Lex H.
Smith Mathew
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