Semiconductor device manufacturing: process – Having magnetic or ferroelectric component
Reexamination Certificate
2002-03-25
2004-06-29
Tsai, H. Jey (Department: 2812)
Semiconductor device manufacturing: process
Having magnetic or ferroelectric component
C438S238000
Reexamination Certificate
active
06756237
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to magnetic tunnel junction (MTJ) devices and methods for fabrication of MTJ devices having properties of reduced noise, electrical resistance, increased magnetoresistance, and increased magnetic field sensitivity.
BACKGROUND OF THE INVENTION
The discovery of large magnetoresistance in magnetic tunnel junction devices (MTJs) at room temperature has renewed intensive interest in this type of device. In part, this interest is due to the potential applications in sensitive magnetic sensors and in non-volatile magnetic random access memory (MRAM). The key component in an MTJ device is a sandwich structure (metal/insulator/metal) consisting of two ferromagnetic (FM) metallic layers (top and bottom electrodes) separated by a thin insulating barrier. The barrier is thin enough to allow quantum mechanic tunneling to occur between two ferromagnetic layers. The tunneling resistance of MTJ device depends on the relative orientation of the magnetization vectors (M) in the two FM layers. The magneto-tunneling effect exploits the asymmetry in the density of states of the majority and minority energy bands in a ferromagnet. The larger the asymmetry the larger the spin polarization is, and so the larger the magneto-tunneling effect.
When subject to an external magnetic field, an MTJ device suffers a change in its electrical resistance. The relative resistance change is called magnetoresistance (MR) or the MR ratio, defined as:
Δ
R
R
=
R
(
H
)
-
R
S
R
S
(
1
)
where R(H) and R
s
are resistance values, at a measurement magnetic field H, and at saturation field, respectively. Beyond the saturation field, resistance remains at a constant value of R
s
. The property of MR as defined in relation (1) has been used to sense magnetic field by measuring resistance change in a field. In general, a good magnetoresistive sensor is characterized by a large MR value achieved at a small saturation field. To obtain a large MR ratio, the quality of the tunnel barrier is critically important. The thin insulating barrier should be smooth, pin-hole free, well oxidized, and of proper stoichiometry.
In MTJ devices, when the M vectors are parallel in the two FM electrodes, there is a maximum match between the numbers of occupied electron states in one electrode and available states in the other. The electron tunneling current is at maximum and the tunneling resistance (R) minimum. On the other hand, in the antiparallel configuration, the electron tunneling is between the majority electron states in one electrode and minority states in the other. This mismatch results in a minimum current and a maximum resistance. In a typical MTJ sensor, the M vector of one FM electrode is pinned by an adjacent antiferromagnetic layer via so called “exchange bias” coupling effect. The M vector of the other FM electrode is free to rotate. Since an external field can easily alter the direction of this M vector, the tunneling resistance is sensitive to the field to be measured. According to Julliere's magnetotunneling model, “Tunneling between ferromagnetic films”,
Physics Letters
, vol. 54A, No.3 (1975), pp.225-226, the maximum MR ratio between parallel and antiparallel configurations is
Δ
R
R
=
R
↑
↓
-
R
↑
↑
R
↑
↑
=
2
P
1
P
2
1
-
P
1
P
2
,
(
2
)
where P
1
and P
2
are the spin-polarization factors of the two electrodes. For a transition ferromagnetic metal (Co, Fe, Ni, and their alloys), P is in the range of 20-40%, leading to &Dgr;R/R~8-38%. For half-metals with a full spin polarization (P~100%), the MR ratio can theoretically approach infinity, which is the characteristic of a perfect spin valve.
MTJs offer a set of major advantages as spintronic devices over other magnetic devices such as devices based on anisotropic magnetoresistance (AMR) and giant magnetoresistance (GMR). Some of the advantages include, but are not limited to, the following.
The junction resistance (R) of an MTJ can be varied easily over a wide range (10
−2
-10
8
&OHgr;), while keeping the large MR ratio intact. The value of R depends on barrier thickness (t~0.5-2 nm) exponentially and on junction area (A) inversely. The ability to tailor R in MTJ to suit electronics surpasses that in GMR devices.
MTJ can be miniaturized to nanometer size while retaining an adequate resistance, because R is primarily sensitive to barrier thickness. This property, not available in GMR spin-valves, is particularly important for high-resolution field imaging.
MTJ devices can operate in a very large frequency range (0-5 GHz) with good response.
MTJ devices are simple two-terminal resistive devices, requiring only small current density to operate. The stray field generated by the sensing current is small.
MTJ devices have a larger MR ratio. For example, a MR value as high as 49.7% at room temperature has been reported in MTJs with electrodes composed of Co
75
Fe
25
, an alloy with a high spin polarization. In contrast, a commercial (Fe—Ni)/Cu/Co GMR sensor has a maximum MR of 9%.
One other major advantage of MTJ devices is that the magnetic coupling between the sensing layer and the pinned layer is weak because of the absence of RKKY magnetic interaction that is found in GMR sensors.
When characterizing an MR sensor, many researchers would use the MR ratio as a figure of merit. However, for field-sensing applications, a large MR ratio alone is insufficient. It is the intrinsic noise figure, both magnetic and electric, that determines the ultimate sensor performance. While reductions in noise are critical, and external noise reductions are relatively simple to achieve, control of a sensor's internal noise is more difficult. Failure to adequately reduce the sensor's internal noise could impede or swamp detection of small field modulations, regardless of the MR ratio. The field-sensing ability of the MTJ can be complicated by many internal noise sources: Johnson-Nyquiist (limited by resistance and temperature), tunneling current (shot noise), 1/f (two-level systems from defects), Barkhausen (domain-wall movement), and thermal fluctuations in magnetization. For typical sensing and memory applications, it is paramount that the magnetic and electric noise of an MTJ device be reduced as much as possible.
Prior to the present invention disclosed herein, there has been no effort to develop fabrication and post-deposition processes to reduce the noise in MTJ devices. S. Ingvarsson et al., measured the electric and magnetic noise in non-optimized MTJ memory devices but did not include sensor devices. Results were presented in “Electronic noise in magnetic tunnel junctions”,
Journal of Applied Physics
, vol. 85, page 5270 (1999) and in “Low frequency magnetic noise in magnetic tunneling junctions”,
Physical Review Letter
, vol. 85, page 3289 (2000). E. R. Nowak, et al., measured the electronic noise in non-optimized MTJ memory (not sensor) devices, but did not evaluate the magnetic noise, as presented in “Noise properties of ferromagnetic tunnel junctions”,
Journal of Applied Physics
, vol. 84, page 6195 (1998) and in “Electric noise in hysteretic ferromagnet-insulator-ferromagnet tunnel junctions”
Applied Physics Letter
vol. 74, page 600 (1999). In another electronic noise study, it was claimed that no magnetic noise was observed in the MTJ samples. This study was published by Daniel S. Reed in NVE, in “Low Frequency Noise in magnetic Tunnel Junctions”,
IEEE Transactions on Magnetics
, vol. 37, page 2028(2001). However, this invention shows that magnetic noise definitely exists in MTJ devices, and represents the dominant source of noise.
Even though MTJ devices have larger MR ratios than AMR or GMR devices, no effort has been made so far to reduce the intrinsic noise of MTJ devices. However, in both sensing and memory applications, low noise levels are a requirement. Various improvements in sensing and memory applications are thus contingent upon the development of improved sensing devices.
SUMMARY OF
Liu Xiaoyong
Xiao Gang
Brown University Research Foundation
Harrington & Smith ,LLP
Tsai H. Jey
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