The invention relates to an electroplating method for CoFe based alloys that have low coercivity and low Hk but with high magnetic moment required for high density magnetic recording.
Electroplating methods are commonly used in numerous applications such as depositing metal films including copper interconnects in semiconductor devices and forming magnetic layers in magnetic recording devices. Although magnetic layers in read and write heads may be deposited by a sputtering method, an electroplating process is usually preferred because the sputtering process produces a magnetic layer with a large magnetocrystalline anisotropy and higher internal stress. Electroplating is capable of generating a magnetic layer with a smaller crystal grain size and a smoother surface that leads to a high magnetic flux density (Bs) value and low coercive force (Hc).
In an electroplating process, an electric current is passed through an electroplating cell comprised of a working electrode (cathode), counter electrode (anode), and an aqueous electrolyte solution of positive ions of the metals to be plated on a substrate in physical contact with the cathode. By applying a potential to the electrodes, an electrochemical process is initiated wherein cations migrate to the cathode and anions migrate to the anode. Metallic ions such as Fe+2, Co+2, and Ni+2 deposit on a substrate (cathode) to form an alloy that may be NiFe, CoFe, or CoNiFe, for example. The substrate typically has an uppermost seed layer on which a photoresist layer is patterned to provide openings over the seed layer that define the shape of the metal layer to be plated. Once the metal layer is deposited, the photoresist layer is removed. The magnetic layers which become a bottom pole layer and top pole layer in a write head can be formed in this manner.
For high density magnetic recording, writer materials require high saturation magnetization to write on high coercivity media. Almost all of the materials with high saturation magnetization (higher than 23 kG) are CoFe based alloys which usually have a relatively large anisotropy field (Hk) of about 15 Oe or higher. Unfortunately, a high anisotropy field will decrease the permeability of the writer and thereby decrease its effectiveness in high density and high frequency recording applications. Thus, an improved electroplating method is needed that can deposit a CoFe based alloy having a low coercivity (Hc) and low Hk but with a high saturation magnetization (Bs).
During a routine search of the prior art, the following references relating to electroplating methods were found. U.S. Pat. No. 4,756,816 discloses a plating bath for a CoFe layer.
In related U.S. Pat. Nos. 4,014,759 and 4,053,373, α-hydroxy sulfones made from an aromatic sulfinate and an aldehyde are used in a plating bath to correct deficiencies such as loss of leveling, brightness and covering power in CoFe deposited layers.
A method for depositing a soft CoFe alloy film having a high saturation magnetization is described in U.S. Pat. No. 6,855,240 and includes the use of an aromatic sulfinic acid or salt thereof.
In U.S. Patent Publication 2005/0252576, a two step anneal method is described in which a spin valve structure is annealed at about 250O C with a 10000 Oe applied field and then reducing the field to about 500 Oe before cooling to set the magnetization direction of the AP1 and AP2 layers in the pinned ferromagnetic layer. The process is repeated after the hard bias layers and electrical leads are formed. In both cases, the magnetic moments are set along the easy axis.
One objective of the present invention is to provide an electrodeposition method and magnetic annealing process for forming a soft magnetic film with a high saturation moment of at least 24 kG.
A further objective of the present invention is to provide a process according to the first objective that yields a magnetic layer having an Hce (easy axis coercivity) and Hch (hard axis coercivity) of less than 2 Oe with an Hk of less than 5 Oe.
According to one embodiment of the present invention, an electroplating process followed by a two step anneal yields a soft magnetic layer that is especially suitable for high density and high frequency magnetic recording as in thin film magnetic heads. High density is defined as >150 Gb/in2 and high frequency is considered to be >1 GHz. In one embodiment, a substrate is provided upon which a seed layer has been formed. Above the seed layer is a patterned photoresist layer with openings that correspond to the shape of the desired magnetic layer to be deposited in a subsequent step. A CoFe or CoFe alloy such as CoFeNi is then electroplated on the substrate in an electroplating cell comprised of an anode and a cathode (working electrode) which are immersed in an electrolyte solution. A reference electrode with a stable, fixed voltage may be employed. Furthermore, there is a power source (potentiostat) with leads affixed thereto wherein one lead connects to the anode and supplies a positive voltage and a second lead connects to the cathode to provide a negative voltage when the cell is operating. A third lead connects to the reference electrode. In the exemplary embodiment, the aqueous based electrolyte solution is comprised of ferrous sulfate, cobalt sulfate, boric acid as a buffer agent, an electric supporting agent, and aryl sulfinates and is maintained at a pH between 2 and 3. A stress reducer, surfactant, and an additional metal sulfate such as nickel sulfate may be added to modify certain properties in the plated layer. A soft magnetic layer is deposited that may be up to 2.5 microns in thickness.
A key feature of the present invention is that a two step magnetic anneal process is used to further improve magnetic softness. In one embodiment, the electroplated film on the substrate is first subjected to an easy axis anneal and subsequently to a hard axis anneal in a second step. In an alternative embodiment, the electroplated film is first subjected to a hard axis anneal and subsequently to an easy axis anneal. The temperature during the annealing steps is maintained in the 180O C to 250O C range to avoid damage to the MTJ element in an adjacent read head which is typically formed prior to fabricating the write head and annealing the soft, high moment magnetic layer. Furthermore, the applied magnetic field during the annealing steps is preferably less than 300 Oe so as not to change the preferred direction of magnetic moment in the pinned layer in the MTJ element and thereby maintain read head performance. The resulting soft magnetic layer has an Hce and Hch of less than about 2 Oe and Hk is reduced to less than 5 Oe.
a is a B—H loops plot of an as-plated CoFe based film and
a is a B—H loops plot of an as-plated CoFe based film and
a is a B—H loops plot of a sputter deposited Co30Fe70 film before anneal and
b is a similar plot following a two step magnetic anneal according to the present invention.
The present invention is a method of forming a soft magnetic layer with high saturation magnetization in addition to low Hc and Hk values that is useful for high density and high frequency recording applications. The method disclosed herein comprises an electroplating process followed by a two step anneal process to form magnetic layers such as bottom and top pole layers in magnetic recording devices or cladding layers on conductive lines in MRAM devices as appreciated by those skilled in the art. The drawings are provided by way of example and are not intended to limit the scope of the invention. The terms electroplating and electrodeposition may be used interchangeably.
Referring to
In one aspect, the electroplated magnetic layer is comprised of a soft magnetic material having a certain thickness and is CoFe or an alloy thereof such as CoFeNi or the like. In another embodiment, the electroplated magnetic layer may be comprised of another soft magnetic material that may be NiFe or a NiFe alloy, for example. When the soft magnetic layer is a pole layer in a write head, the certain thickness is about 2 to 3 microns. However, the thickness of an electroplated magnetic layer according to the present invention may be less than 2 microns as appreciated by those skilled in the art.
Returning to
There is a counter electrode (anode) 4 and a working electrode also known as a cathode 5 immersed or otherwise positioned in the electrolyte solution 3. When a magnetic layer made of CoFe, CoFeNi, or the like is electroplated, the anode 4 is preferably Co (or Ni) and a positive potential is applied thereto. In an embodiment where a CoFe alloy is electroplated, the anode 4 may be comprised of Co. The cathode 5 may be a dimensionally stable electrode such as Pt or gold mesh to which a negative potential is applied. In other words, a potential hereafter referred to as an electroplating potential is established between the anode 4 and cathode 5 whereby an electric current flows from the anode to the cathode to drive the electroplating process. The substrate 9 is preferably in good electrical contact with the cathode 5 and may be affixed thereto by a clamp or other conventional means. Although the anode 4 and cathode 5 (and substrate 9) are shown opposed to each other on opposite walls of the container 2, the anode and cathode may optionally be arranged in other configurations. For instance, the anode and cathode may have their top and bottom surfaces aligned parallel to the top surface 3a of the electrolyte solution. The substrate 9 and specifically a seed layer thereon that is exposed through openings in a photoresist pattern (not shown) functions as a cathode during the electroplating process.
The anode 4 and cathode 5 are connected to a potentiostat (power source) 7 by electrical leads 8a and 8b, respectively. There is also a reference electrode 6 immersed in the electrolyte solution 3 and connected to the potentiostat 7 by a lead 8c. Preferably, the reference electrode 6 is positioned in the vicinity of the anode 5. The reference electrode 6 may be a standard calomel electrode (SCE) comprised of Hg/HgCl2 in a KCl electrolyte or may be a silver/silver chloride electrode as appreciated by those skilled in the art.
Another feature shown in
In one embodiment wherein CoFe or CoNiFe is electroplated on a substrate, the electrolyte solution 3 is an aqueous solution having a pH between 2.0 and 3.0 and includes Fe+2 ions, Co+2 ions, and optionally, Ni+2 ions (for CoFeNi deposition) which are provided by adding the following metal salts to deionized water at the indicated concentrations in grams per liter: FeSO4.7H2O (50 to 140 g/L); CoSO4.7H2O (20 to 70 g/L); and NiSO4.6H2O (0 to 70 g/L). Additionally, the electrolyte solution may be comprised of other additives and supporting electrolytes including but not limited to H3BO3 with a concentration of 20 to 40 g/L, NaCl as an electrolyte supporting agent at a concentration of 0 to 20 g/L, (NH4)2SO4 or NH4Cl as an electrolyte supporting agent at a concentration of 0 to 30 g/L, sodium saccharin as a stress reducing agent at a concentration of 0 to 2.0 g/L, sodium lauryl sulfate as a surfactant at a concentration of 0 to 0.15 g/L, and one or more aryl sulfinate salts including sodium benzene sulfinate at a concentration of 0.05 to 0.3 g/L to improve the magnetic softness of the electroplated magnetic layer. Preferably, the electroplating is performed with an electrolyte solution temperature between 10O C and 25O C and with a plating current density of from 30 to 60 mA/cm2. The duty ratio during each cycle may vary and is based on an “ON” time of 10 to 50 ms and an “OFF” time of 20 to 200 ms. Using these conditions, a magnetic layer comprised of CoFe or CoNiFe is deposited at the rate of about 500 to 1700 Angstroms per minute. Typically, the electroplating process is terminated after a predetermined length of time that corresponds to a desired thickness.
In an embodiment wherein a CoFe magnetic layer is electodeposited on a substrate, the composition is preferably between 25 and 35 atomic % Co and from 65 to 75 atomic % Fe. When a CoFeNi magnetic layer is electroplated, the preferred composition is between 25 and 35 atomic % Co, from 65 to 75 atomic % Fe, and between 2 and 4 atomic % Ni. The aforementioned electroplating conditions produce a magnetic layer with low coercivity (Hce<8 Oe, Hch<1 Oe), low anisotropy field (Hk<10 Oe), and high saturation magnetization (Bs≧24 kG).
The inventors have previously practiced a magnetic anneal process involving one anneal step to further improve magnetic softness in the electroplated layer. Note that the annealing step is preferably carried out in an oven after the electroplated substrate is removed from the electroplating bath. Magnetic annealing is one of the most common methods used with magnetic layers in write heads in order to improve writer performance. Generally, only a hard-axis anneal or an easy-axis anneal is employed during writer fabrication. Although a one step magnetic anneal can reduce film coercivity, it cannot reduce the anisotropy field of a CoFe based high moment magnetic layer in a write head. For example,
In an example where the plated magnetic layer is a write pole layer, the easy axis may be an axis that is parallel to the ABS plane and the hard axis intersects the easy axis and is perpendicular to the ABS plane.
Referring to
A key feature of the present invention is the introduction of a two step anneal process in which the electroplated magnetic layer is subjected to a hard axis anneal and an easy axis anneal. The anneal steps are preferably performed at a temperature between 180O C and 250O C so that a read head adjacent to the write head in a combined read/write head structure is not damaged. In addition, the applied magnetic field during the anneal steps is kept at 300 Oe or less to prevent altering the preferred direction of magnetic moment in the pinned layer within the read head. According to one embodiment, the applied magnetic field during the easy axis anneal and hard axis anneal is between about 200 Oe and 300 Oe.
The inventors have surprisingly discovered that by subjecting the electroplated magnetic layer formed according to a method of the present invention to a two step anneal process, the softness (Hce and Hch) of the magnetic layer is further reduced and the anisotropy field is also lowered unlike single anneal examples. In one embodiment, the soft magnetic layer described previously is first treated with an easy axis anneal and then with a hard axis anneal in a second step. The length of each anneal step is between 20 and 300 minutes and is preferably greater than 30 minutes. Preferably, the substrate is cooled to room temperature for a certain length of time between the two anneal steps. In the exemplary embodiment, the magnitude of the applied magnetic field is maintained at the same level for the two annealing steps. However, one skilled in the art will appreciate that the present invention also anticipates an additional embodiment wherein the applied magnetic field during the easy axis anneal is unequal to that during the hard axis anneal step.
Referring to
In a second embodiment, the soft magnetic layer is electroplated as described previously followed by a hard axis anneal and then with an easy-axis anneal. The length of each anneal step is between 20 and 300 minutes and is preferably greater than 30 minutes. The substrate is cooled between annealing steps as described previously. Similar to the first embodiment, the applied magnetic field may be kept constant during the two annealing steps or the applied field for the easy axis anneal may be unequal to that for the hard axis anneal step.
Referring to
The double anneal process of the present invention is especially suitable for improving the magnetic softness of electroplated CoFe or CoFe alloys. However, the double anneal process may also be applied to sputtered films but the improvement in magnetic properties is not as significant. Nevertheless, the Hce, Hch, and Hk values for a CoFe sputtered film may also be substantially lowered by applying a double anneal process as described herein. For example, the B—H loops of a sputter deposited Co30Fe70 film is depicted in
The inventors emphasize that the combination of low coercivity, low Hk, and high Bs is best achieved with an electroplating method and a two step magnetic anneal process as described herein. Although the electroplating method or the double anneal process of the present invention are each capable of reducing Hce, Hch, and Hk in a magnetic layer, it is a combination of the two methods in a process sequence that delivers the ultimate improvement in magnetic performance which is required for advanced write heads. This outcome is an advantage over prior art in that conventional one step anneal processes are not capable of significantly reducing Hk and electroplating processes by themselves are not sufficient to produce a soft magnetic layer with high Bs necessary for high density and high frequency recording operations.
While this invention has been particularly shown and described with reference to, the preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of this invention.