1. Field of Invention
The present invention relates generally to the art of magnetic tunnel junction (MTJ) read head devices, which sense magnetic fields in a magnetic recording medium. More particularly, the present invention relates to MTJ devices having tunneling barriers with improved magnetoresistance properties and improved symmetry of electrical properties. The invention finds particular application in conjunction with reading binary data from hard disk drives and will be described with particular reference thereto. However, it is to be appreciated that the invention will find application with other magnetic storage media. Further, it is to be appreciated that the invention will find application in other magnetic field detection devices as well as in other devices and environments.
2. Description of the Related Art
Magneto-resistive (MR) sensors based on anisotropic magneto-resistance (AMR) or a spin-valve (SV) effect are widely known and extensively used as read transducers to read magnetic recording media. Such MR sensors can probe the magnetic stray field coming out of transitions recorded on a recording medium by generating resistance changes in a reading portion formed of magnetic materials. AMR sensors have a low resistance change ratio or MR ratio xcex94R/R, typically from 1 to 3%, whereas SV sensors have a xcex94R/R ranging from 2 to 7% for the same magnetic field excursion. SV heads showing such high sensitivity are able to achieve very high recording densities, that is, over several giga bits per square inch or Gbits/in2.
In a basic SV sensor, two ferromagnetic layers are separated by a non-magnetic layer. An exchange or pinning layer of FeMn, for example, is further provided adjacent to one of the ferromagnetic layers. The exchange layer and the adjacent or fixed ferromagnetic layer are exchange-coupled so that the magnetic moment or magnetization direction of the ferromagnetic layer is strongly pinned or fixed in one direction. The magnetization direction of the other or free ferromagnetic layer is free to rotate in response to a small external magnetic field. When the magnetization directions of the ferromagnetic layers are changed from a parallel to an anti-parallel configuration, the sensor resistance increases yielding a relatively high MR ratio.
Another type of MR sensor, the MTJ, is based on tunneling magneto resistance (TMR). This was reported in Julliere, xe2x80x9cTunneling Between Ferromagnetic Filmsxe2x80x9d Physics Letters, 54A 225 (1975), which is herein incorporated by reference. An MTJ comprises two ferromagnetic electrodes or layers separated by a thin insulating layer. An electrical bias is applied to the two electrodes to generate a sensing current. The insulating layer is thin enough so that electrons can tunnel through the insulating barrier to generate this current. One of the ferromagnetic layers has its magnetic moment fixed along one direction, i.e., the fixed or pinned layer, while the other layer, i.e., free or sensing layer, is free to rotate in an external magnetic field. MTJ sensors operate in current perpendicular to the plane (CPP) geometry, which means its sensing current flows in a thickness direction of a laminate film or orthogonal to the ferromagnetic layers.
The tunneling of the electrons through the insulating layer is a spin dependent process. In other words, the sensing or tunneling current through the junction depends on the spin-polarization state of the pinned and free ferromagnetic layers and the relative orientation of the magnetic moments (magnetization directions) of the two layers. The two ferromagnetic layers can have different responses to magnetic fields so that the relative orientation of their magnetization can be varied with an external magnetic field.
When the sense current experiences a first ferromagnetic layer, the electrons are spin polarized. If the magnetization directions of the two ferromagnetic layers are anti-parallel to each other, the probability of the electrons tunneling through the tunnel barrier is lowered, so that a high junction resistance Rap is obtained. On the other hand, if the magnetizations of the two ferromagnetic layers are parallel to each other, the probability of the electrons tunneling is increased and a high tunnel current and low junction resistance Rp is obtained. In an intermediate state between the parallel and anti-parallel states, such as when the both ferromagnetic layers are perpendicular in magnetization to each other, a junction resistance Rm between Rap and Rp is obtained such that Rap greater than Rm greater than Rp. Using these symbols, the TMR ratio may be defined as xcex94R/R=(Rapxe2x88x92Rp)/Rp.
The relative magnetic direction orientation or angle of the two magnetic layers can be affected by an external magnetic field such as the transitions in a magnetic recording medium. This affects the MTJ resistance and thus the voltage of the sensing current or output voltage. By detecting the change in resistance and thus voltage based on the change in relative magnetization angle, changes in an external magnetic field are detected. In this manner, MTJ sensors are able to read magnetic recording media.
Prior to 1995, reported MTJs showed only low TMR response, on the order of 1-2%, at room temperature. MTJ began to show great promise for application to ultra-high density recordings when large TMR over 10% was first observed at room temperature. See Moodera et al, xe2x80x9cLarge magneto resistance at room temperature in ferromagnetic thin film tunnel junctions,xe2x80x9d Phys. Rev. Lett. v. 74, pp. 3273-3276 (1995), which is herein incorporated by reference.
Nevertheless, it has been difficult to make MTJ devices with large enough TMR response at room temperature to be useful. The large junction resistance is one of the main factors that limit application of MTJs as read heads because of the low signal to noise ratio (S/N). This factor is even more critical as the junction resistance is scaled up with decreasing junction size, which is required for high area density recording. On the other hand, low resistance junctions show small TMR response as reported by Tsuge and Mitsuzuka in xe2x80x9cMagnetic Tunnel Junctions With In Situ Naturally-Oxidized Tunnel Barrierxe2x80x9d Appl. Phys. Lett. 71, 3296 (1997), which is herein incorporated by reference.
Another problem that prevails in MTJ devices is shot noise, which lowers S/N. Shot noise is proportional to the junction resistance R and the square root of sensing current I. In order to have a high enough S/N, the junction resistance must be decreased. The junction resistance is exponentially proportional to an insulating barrier thickness d and the square root of a the barrier""s energy gap or barrier height "PHgr", i.e., Rxe2x88x9dexp(d"PHgr"xc2xd)). By decreasing barrier thickness, junction resistance may be decreased. However, decreasing barrier thickness can cause or increase the effect of other problems discussed below.
For example, if the barrier thickness is too thin, it can contain pinholes. Pinholes generate a leak current through the barrier, decreasing resistance and S/N.
Another problem of conventional MTJs is an inhomogeneous tunneling barrier. After oxidation, a top oxide surface of an insulating barrier has a greater oxygen distribution than that at a bottom surface. This leads to asymmetric electrical properties with respect to signs of applied bias. Such asymmetry results in varying energy gaps or barrier heights across a thickness of the barrier.
A related problem associated with MTJ devices is electrostatic discharge (ESD). MTJ devices have a breakdown voltage, typically on the order of 150 V. If a device has asymmetric electrical properties, the breakdown voltage in one biased direction can be lower than in another biased direction. This can result in a lower breakdown voltage and destruction of the device due to ESD.
Another related problem is time dependence of dielectric breakdown (TDDB). Over time, the resistance of a barrier or dielectric can undesirably decrease. However, the more homogeneous the barrier, i.e., the more symmetric the electrical properties of a barrier, the more stable it is over time.
Yet another related problem is the yield of satisfactory MTJ devices during mass production. Typically, multiple MTJ devices are produced on one wafer. If barrier height and barrier width or thickness are inconsistent across the wafer, many of the MTJ devices produced will have unsatisfactory performance characteristics, such as head output and resistance, and will not pass quality control. Thus, it is desirable that the barrier is homogeneous and its electrical properties are symmetric. This will allow for a higher yield or greater ratio of satisfactory products over total products.
Therefore, a goal of the present invention is to provide a MTJ read head with low junction resistance and a high TMR ratio or response.
Another goal of the present invention is to provide a MTJ device with a tunneling barrier having symmetric electrical properties.
Still another goal of the invention is to provide a MTJ device that has higher performance characteristics in terms of ESD and TDDB.
Yet another goal of the invention is an MTJ device that may be mass produced with higher yields or fewer rejected devices.
Accordingly, the present invention is directed to a MTJ device and method for making same that substantially obviates one or more of the problems due to limitations and disadvantages of related art.
In accordance with one aspect of the invention, a method of manufacturing a MTJ device is disclosed. A first ferromagnetic portion is provided. A barrier is provided on the first ferromagnetic portion. The barrier is provided by providing a first barrier portion on the first ferromagnetic portion. The first barrier portion is oxidized. After oxidizing, a second barrier portion is provided on the first barrier portion. A second ferromagnetic portion is provided on the barrier.
In accordance with a more limited aspect of the invention, the second barrier portion is oxidized.
In another aspect of the invention, a magnetic tunnel junction device for detecting an applied magnetic field includes a first ferromagnetic layer. A tunnel barrier is on and contiguous with the first ferromagnetic layer. The barrier includes an oxidized first barrier layer and second barrier layer. A second ferromagnetic layer is on and contiguous with the barrier.
In more limited aspect of the invention, the second barrier layer is oxidized.
One advantage of the present invention is that it provides a MTJ with a reduced number of xe2x80x9cpinholesxe2x80x9d in the barrier, thus reducing the amount of non-tunnel current.
Another advantage is that it provides a MTJ with a relatively low junction resistance and high TMR ratio or response.
Yet another advantage is an improved symmetry of the electrical properties of the barrier in a MTJ.
Still another advantage is a MTJ device that may be mass produced with higher yields or fewer rejected devices.