Patent Application
20080257714
The present invention relates to the construction of a tunnel junction magnetoresistive sensor and more particularly to a method for constructing a barrier layer that improves the magnetic performance of the sensor.
The heart of a computer's long term memory is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm adjacent to a surface of the rotating magnetic disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads are directly located on a slider that has an air bearing surface (ABS). The suspension arm biases the slider toward the surface of the disk and when the disk rotates, air adjacent to the surface of the disk moves along with the disk. The slider flies on this moving air at a very low elevation (fly height) over the surface of the disk. This fly height can be on the order of Angstroms. When the slider rides on the air bearing, the write and read heads are employed for writing magnetic transitions to and reading magnetic transitions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The write head includes a coil layer embedded in first, second and third insulation layers (Insulation stack), the insulation stack being sandwiched between first and second pole piece layers. A gap is formed between the first and second pole piece layers by a gap layer at an air bearing surface (ABS) of the write head and the pole piece layers are connected at a back gap. Current conducted to the coil layer induces a magnetic flux in the pole pieces which causes a magnetic field to fringe out at a write gap at the ABS for the purpose of writing the aforementioned magnetic impressions in tracks on the moving media, such as in circular tracks on the aforementioned rotating disk.
In recent read head designs a spin valve sensor, also referred to as a giant magnetoresistive (GMR) sensor, has been employed for sensing magnetic fields from the rotating magnetic disk. This sensor includes a nonmagnetic conductive layer, referred to as a spacer layer, sandwiched between first and second ferromagnetic layers, hereinafter referred to as a pinned layer and a free layer. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetization of the pinned layer is pinned perpendicular to the air bearing surface (ABS) and the magnetic moment of the free layer is biased parallel to the ABS, but is free to rotate in response to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer.
The thickness of the spacer layer is chosen to be less than the mean free path of conduction electrons through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with each of the pinned and free layers. When the magnetizations of the pinned and free layers are parallel with respect to one another, scattering is minimal and when the magnetizations of the pinned and free layer are antiparallel, scattering is maximized. Changes in scattering alter the resistance of the spin valve sensor in proportion to cos Θ, where Θ is the angle between the magnetizations of the pinned and free layers. In a read mode the resistance of the spin valve sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals.
More recently, researches have focused on the development of tunnel junction sensors (TMR sensors) also referred to as tunnel valves. Tunnel valves TMR sensor offer the advantage of providing improved signal amplitude as compared with GMR sensors. TMR sensors operate based on the spin dependent tunneling of electrons through a thin, electrically insulating barrier layer.
TMR sensors have been constructed by forming barrier layers, such as Mg—O barrier layers, in a sputter deposition chamber. The properties of the barrier layer are very important to TMR sensor performance, however, because of certain difficulties with the deposition process, it has not been possible to construct a barrier layer having optimum physical properties such as uniform oxygen content throughout the thickness of the barrier layer.
For example, during deposition of a Mg—O barrier layer, although the oxygen flow through the chamber may be constant during deposition, the partial pressure of oxygen in the chamber (and thus the oxygen content of the deposited barrier) rises during deposition. This is due in part to the gradual decrease in oxygen gettering by the chamber walls as they become coated with an oxide layer during deposition. In addition, oxygen poisoning of the target in the chamber changes the amount of oxygen being deposited in the barrier layer. These problems result in a barrier layer having an oxygen exposure that rises throughout its thickness and results in a TMR sensor having undesirable magnetic properties.
Therefore, there is a need for a method for constructing a TMR sensor having a barrier layer with optimal physical properties. Such a method would preferably provide for the deposition of a barrier layer that has a substantially constant oxygen content of a desired amount.
The present invention provides a tunnel junction sensor having improved performance and reliability. A Mg—O barrier layer of the tunnel junction sensor is deposited in a sputter deposition chamber in an atmosphere that contains oxygen and an inert gas such as Ar. The oxygen in the chamber has a concentration that changes during barrier layer deposition.
For example, the concentration of oxygen can start at a relatively high value and can decrease during deposition to a lower oxygen concentration. The reduction in oxygen concentration can stop and actually reverse any target poisoning that occurred during the deposition at higher oxygen concentration. The reduced oxygen concentration can also counteract the effects of reduced oxygen gettering of the chamber walls during deposition.
The deposition process of the present invention advantageously results in a TMR sensor having increased tunneling magnetoresistance (TMR) and increased barrier robustness.
These and other advantages and features of the present invention will be apparent upon reading the following detailed description in conjunction with the Figures.
For a fuller understanding of the nature and advantages of this invention, as well as the preferred mode of use, reference should be made to the following detailed description read in conjunction with the accompanying drawings which are not to scale.
The following description is of the best embodiments presently contemplated for carrying out this invention. This description is made for the purpose of illustrating the general principles of this invention and is not meant to limit the inventive concepts claimed herein.
Referring now to
At least one slider 113 is positioned near the magnetic disk 112, each slider 113 supporting one or more magnetic head assemblies 121. As the magnetic disk rotates, slider 113 moves radially in and out over the disk surface 122 so that the magnetic head assembly 121 may access different tracks of the magnetic disk where desired data are written. Each slider 113 is attached to an actuator arm 119 by way of a suspension 115. The suspension 115 provides a slight spring force which biases slider 113 against the disk surface 122. Each actuator arm 119 is attached to an actuator means 127. The actuator means 127 as shown in
During operation of the disk storage system, the rotation of the magnetic disk 112 generates an air bearing between the slider 113 and the disk surface 122 which exerts an upward force or lift on the slider. The air bearing thus counter-balances the slight spring force of suspension 115 and supports slider 113 off and slightly above the disk surface by a small, substantially constant spacing during normal operation.
The various components of the disk storage system are controlled in operation by control signals generated by control unit 129, such as access control signals and internal clock signals. Typically, the control unit 129 comprises logic control circuits, storage means and a microprocessor. The control unit 129 generates control signals to control various system operations such as drive motor control signals on line 123 and head position and seek control signals on line 128. The control signals on line 128 provide the desired current profiles to optimally move and position slider 113 to the desired data track on disk 112. Write and read signals are communicated to and from write and read heads 121 by way of recording channel 125.
With reference to
With reference now to
The pinned layer can include first and second magnetic layers AP1316 and AP2318 that are antiparallel coupled across a non-magnetic antiparallel coupling layer 320. The AP1 and AP2 layers 316, 318 can be constructed of, for example, Co—Fe, Co—Fe—B or other magnetic alloys and the antiparallel coupling layer 320 can be constructed of, for example, Ru. The free layer 310 can be constructed of a material such as Co—Fe, Co—Fe—B or Ni—Fe or may be a combination of these or other materials.
The AP1 layer 316 is in contact with and exchange coupled with a layer of antiferromagnetic material (ATM layer) 326 such as PtMn, IrMn or some other anti ferromagnetic material. This exchange coupling strongly pins the magnetization of the AP1 layer 316 in a first direction as indicated by arrow tail 328. Antiparallel coupling between the AP1 and AP2 layers 316, 318 strongly pins the magnetization of the AP2 layer in a second direction perpendicular to the ABS as indicated by arrowhead 330.
A capping layer 314 such as Ta, Ta/Ru or Ru/Ta/Ru may be provided at the top of the sensor stack 302 to protect the layers thereof from damage during manufacture. In addition, a seed layer 322 may be provided at the bottom of the sensor stack 302 to initiate a desired crystalline growth in the above deposited layers of the sensor stack 302.
First and second hard bias layers 324 may be provided at either side of the sensor stack 302. The hard bias layers 324 can be constructed of a hard magnetic material such as Co—Pt or Co—Pt—Cr. These hard bias layers 324 are magnetostatically coupled with the free layer 310 and provide a magnetic bias field that biases the magnetization of the tree layer 310 in a desired direction parallel with the ABS as indicated by arrow 326. The hard bias layers 324 can be separated form the sensor stack 302 and from at least one of the leads 304 by a layer of electrically insulating material 328 such as alumina in order to prevent current from being shunted across the hard bias layers 324 between the leads 304, 306.
With reference now to
The power supply 212 provides power (preferably DC) to the target 210, which results in a plasma being formed in the chamber. Mg atoms from the target 210 are then emitted from the target 210 and sputtered onto the wafer 235. An inert gas, preferably Ar, is entered through a first inlet 206 and a reactive gas 208 is entered through a second inlet 208. As discussed above prior art sputter deposition processes used to deposit Mg—O barrier layers have resulted in inferior quality barrier layers. This has been due to poisoning of the target and also to reduced gettering of the side walls of the vacuum chamber 202. Prior to sputtering, the target is cleaned of any oxides. This is performed by placing the shutter 240 over the wafer 235 (so as to protect the wafer), and placing the shutter 230 over the target 210. Power is then provided to the target 210 without any oxygen in the chamber (only Ar) which sputter cleans the target 210, removing any oxides from the target. Then, the shutter 230 is moved away from the target 210 and sputtering continues without any oxygen in the chamber 202. During this sputtering process, a layer of metal Mg coats the side walls of the chamber 202. Removing any oxides from the target 210 allows effective sputtering to be performed, and coating the side walls of the chamber 202 with Mg increases oxygen gettering during the sputter deposition process.
Then, the shutter 240 is moved away from the wafer 240, and sputtering is performed with both Ar 206 and oxygen 208 being entered into the chamber 202. During prior art sputtering, the oxygen entered into the chamber (which is necessary for constructing a desired Mg—O barrier layer) formed an oxide on the target 210. This is referred to as target poisoning. When the target 210 becomes completely coated with oxide, sputtering almost completely ceases. Since this situation must be avoided, there is a limited range of Oxygen flows that can be used within the prior art method. Furthermore, the addition of oxygen into the chamber 202 forms an oxide on the side walls of the chamber 202. This reduces the oxygen gettering of the side walls, which results in increased oxygen partial pressure in the chamber during deposition. This has been found to result in a barrier layer being formed which has degraded magnetic properties, such as reduced TMR effect.
The present invention prevents poisoning of the target 210 and also can maintain a desired oxygen partial pressure in the chamber 202 during Mg—O deposition. This results in a barrier layer being formed that has vastly improved magnetic properties such as improved TMR values.
According to one embodiment of the invention, after the target has been cleaned and the sides of the chamber 202 have been coated with Mg as described above, sputtering is initiated. The initiation of sputtering can include a first pre-sputtering performed with only Ar in the chamber 202 and with the target shutter 230 and wafer shutter 240 closed. Then a second pre-sputtering can be performed with Ar and O2 entered into the chamber 202 and with the wafer shutter 240 closed and the target shutter 230 open. Then, the target shutter 230 is opened initiating actual sputter deposition. A desired first concentration of oxygen (O2) is entered into the chamber 202 through the inlet 208. During deposition, the amount of oxygen is changed (preferably decreased). This can be a gradual, continuous change in oxygen or can be performed as one or more steps of varying oxygen concentration. After deposition has been completed, a natural oxidation process may optionally be employed. This natural oxidation is performed by exposing the deposited barrier layer to oxygen in the chamber without sputtering (i.e. with the power supply 212 turned off).
When the oxygen concentration is sufficiently reduced during deposition as described above, target poisoning not only stops, but can be reversed, thereby cleaning oxides off of the target 210. In addition, the decreased oxygen concentration counteracts the reduced oxygen gettering of the side walls, resulting in greatly improved magnetic properties of the tunnel barrier layer, as will be shown below. In addition, the resulting barrier layer has been found to have greatly improved reliability. Stress testing, in which a tunnel barrier layer is subjected to a series of voltages, has shown, that a barrier layer deposited by the above method (or by the alternate method described below) is much more robust than a barrier layer formed by a prior art method. Although the reasons for the increase in performance are not entirely understood it is believed that the improved performance is due at least in part to the fact that the resulting barrier layer has a more crystalline structure than prior art barrier layers.
Furthermore, the improved performance was an unexpected result, as it was previously believed that the oxygen concentration needed at the beginning of deposition had to be maintained throughout deposition and could not be changed or reduced without seriously degrading barrier layer properties. As can be seen, this was not the case, as barrier layer properties significantly improved when the barrier layer was deposited with a varying oxygen concentration.
With reference to
In another method for depositing a barrier layer, the barrier layer can be deposited in stages or layers. For example, after cleaning the target and coating the walls of the chamber 202 with Mg, a first sputter deposition stage can be performed with Ar and O2 being entered into the chamber 202. The deposition is temporarily stopped and an optional target cleaning step can be performed. The cleaning step can include placing the shutter 240 over the wafer 235 and placing the shutter 230 over the target 210. Power is provided to the target 210, which cleans oxides from the target. An optional natural oxidation step may also be performed by allowing the wafer to be exposed to oxygen during the temporary pause in deposition (i.e. while the power is off). Then, after cleaning, (if such cleaning step is performed) the shutters 230, 240 can be moved out of the way, and a second sputter deposition stage performed. Although two deposition stages are discussed above, any number of deposition and cleaning stages could be performed. The oxygen concentration during each of the deposition stages can be varied relative to the other stages. For example, the first deposition stage could be performed at a first oxygen concentration, and then a second deposition could be performed at an oxygen concentration that is less than the first concentration.
The above described deposition can be used to deposit a Mg—O barrier layer such as the barrier layer 312 discussed with reference to
With reference now to
With reference now to
While various embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. For example, although the barrier layer and method deposition thereof, has been described in terms of a Mg—O barrier layer and the use of a Mg—O target, the invention could also be used to deposit barrier layer constructed of some other oxide. Therefore, the invention is not limited to the deposition of Mg—O barrier layers only, but encompasses the deposition of barrier layer of other materials. Other embodiments falling within the scope of the invention may also become apparent to those skilled in the art. Thus, the breadth and scope of the invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.
