US20090122450A1

US20090122450A1 - TMR device with low magnetostriction free layer

Info

Publication number: US20090122450A1
Application number: US11/983,329
Authority: US
Inventors: Hui-Chuan Wang; Tong Zhao; Min Li; Kunliang Zhang
Original assignee: Headway Technologies Inc
Current assignee: Headway Technologies Inc
Priority date: 2007-11-08
Filing date: 2007-11-08
Publication date: 2009-05-14
Also published as: US20150235660A1; US8472151B2; JP2009117846A; JP5750211B2; US20120193738A1; US20130277780A1; US9214170B2

Abstract

A high performance TMR sensor is fabricated by employing a free layer comprised of CoB_Xwith a λ between −5×10⁻⁶and 0 on a MgO_Xtunnel barrier. Optionally, a FeCo/CoB_Xfree layer configuration may be used where x is about 1 to 30 atomic %. Trilayer configurations represented by FeCo/CoFeB/CoB_X, FeCo/CoB_X/CoFeB, FeCo_Y/CoFe_W/CoB_X, or FeCo_Y/FeB/CoB_Xmay also be employed. Alternatively, CoNiFeB or CoNiFeBM formed by co-sputtering CoB with CoNiFe or CoNiFeM, respectively, where M is V, Ti, Zr, Nb, Hf, Ta, or Mo may be substituted for CoBx in the aforementioned embodiments. A 15 to 30% in improvement in TMR ratio over a conventional CoFe/NiFe free layer is achieved while maintaining a low Hc and RA<3 ohm-um². In bilayer or trilayer embodiments, λ between −5×10⁻⁶and 5×10⁻⁶is achieved by combining CoBx (−λ) and one or more layers having a positive λ.

Description

RELATED PATENT APPLICATION

This application is related to the following: Docket # HT05-015, Ser. No. 11/180,808, filing date Jul. 13, 2005; assigned to a common assignee, and which is herein incorporated by reference in its entirety.

FIELD OF THE INVENTION

The invention relates to a high performance tunneling magnetoresistive (TMR) sensor in a read head and a method for making the same, and in particular, to a composite free layer comprised of CoB that reduces magnetostriction while achieving acceptable RA (resistance×area) and dR/R values.

BACKGROUND OF THE INVENTION

A TMR sensor otherwise known as a magnetic tunneling junction (MTJ) is a key component (memory element) in magnetic devices such as Magnetic Random Access Memory (MRAM) and a magnetic read head. A TMR sensor typically has a stack of layers with a configuration in which two ferromagnetic layers are separated by a thin non-magnetic insulator layer. The sensor stack in a so-called bottom spin valve configuration is generally comprised of a seed (buffer) layer, anti-ferromagnetic (AFM) layer, pinned layer, tunnel barrier layer, free layer, and capping layer that are sequentially formed on a substrate. The free layer serves as a sensing layer that responds to external fields (media field) while the pinned layer is relatively fixed and functions as a reference layer. The electrical resistance through the tunnel barrier layer (insulator layer) varies with the relative orientation of the free layer moment compared with the reference layer moment and thereby converts magnetic signals into electrical signals. In a magnetic read head, the TMR sensor is formed between a bottom shield and a top shield. When a sense current is passed from the top shield to the bottom shield (or top conductor to bottom conductor in a MRAM device) in a direction perpendicular to the planes of the TMR layers (CPP designation), a lower resistance is detected when the magnetization directions of the free and reference layers are in a parallel state (“1” memory state) and a higher resistance is noted when they are in an anti-parallel state or “0” memory state. Alternatively, a TMR sensor may be configured as a current in plane (CIP) structure which indicates the direction of the sense current.
A giant magnetoresistive (GMR) head is another type of memory device. In this design, the insulator layer between the pinned layer and free layer in the TMR stack is replaced by a non-magnetic conductive layer such as copper.
In the TMR stack, the pinned layer may have a synthetic anti-ferromagnetic (SyAF) configuration in which an outer pinned layer is magnetically coupled through a coupling layer to an inner pinned layer that contacts the tunnel barrier. The outer pinned layer has a magnetic moment that is fixed in a certain direction by exchange coupling with the adjacent AFM layer which is magnetized in the same direction. The tunnel barrier layer is so thin that a current through it can be established by quantum mechanical tunneling of conduction electrons.
A TMR sensor is currently the most promising candidate for replacing a GMR sensor in upcoming generations of magnetic recording heads. An advanced TMR sensor may have a cross-sectional area of about 0.1 microns×0.1 microns at the air bearing surface (ABS) plane of the read head. The advantage of a TMR sensor is that a substantially higher MR ratio can be realized than for a GMR sensor. In addition to a high MR ratio, a high performance TMR sensor requires a low areal resistance RA (area×resistance) value, a free layer with low magnetostriction (λ) and low coercivity (Hc), a strong pinned layer, and low interlayer coupling (Hin) through the barrier layer. The MR ratio (also referred to as TMR ratio) is dR/R where R is the minimum resistance of the TMR sensor and dR is the change in resistance observed by changing the magnetic state of the free layer. A higher dR/R improves the readout speed. For high recording density or high frequency applications, RA must be reduced to about 1 to 3 ohm-um².
A MgOx based MTJ is a very promising candidate for high frequency recording applications because its tunneling magnetoresistive (TMR) ratio is significantly higher than for AlOx or TiOx based MTJs. In order to achieve a smaller Hc but still maintain a high TMR ratio, the industry tends to use CoFeB as the free layer in a TMR sensor. Unfortunately, the magnetostriction (λ) of a CoFeB free layer is considerably greater than the maximum acceptable value of about 5×10⁻⁶for high density memory applications. A free layer made of a CoFe/NiFe composite has been employed instead of CoFeB because of its low λ and soft magnetic properties. However, when using a CoFe/NiFe free layer, the TMR ratio will degrade. Thus, an improved free layer in a TMR sensor is needed that provides low magnetostriction in combination with a high TMR ratio, low RA value, and low coercivity.
U.S. Pat. No. 7,035,058 discloses a GMR-CPP device with a free layer made of Co, CoFe alloys, Ni, and NiFe alloys. A high resistance layer formed on or within the free layer may be comprised of CoNiFeB or CoB with an unspecified composition. However, this patent teaches a high resistance property that is not acceptable for TMR where resistance×area must be low.
In U.S. Patent Application No. 2007/0139827, a free layer is described that includes a sense enhancing layer (Ta) sandwiched between a first ferromagnetic layer and a second ferromagnetic layer. The first ferromagnetic layer has a positive magnetostriction and is made of CoFeB or CoFe based alloys while the second ferromagnetic layer has a negative magnetostriction and is comprised of CoFe, Ni, or NiFe based alloys.
U.S. Patent Application No. 2007/0047159 teaches a free layer comprised of a CoFe/CoFeB/NiFe composite to achieve low coercivity and low magnetostriction for either GMR-CPP or TMR sensors.
U.S. Patent Application No. 2007/0070553 describes a magnetoresistance effect device that has a CoNiFeB free layer formed by co-sputtering CoFeB and Ni and with a Ni content between 5 and 17 atomic % to maintain a λ value between −1×10⁻⁶and 1×10⁻⁶.

SUMMARY OF THE INVENTION

One objective of the present invention is to provide a TMR sensor with a free layer composition that improves the TMR ratio by at least 15 to 30% compared with a conventional FeCo/NiFe free layer.
A second objective of the present invention is to provide a TMR sensor with a free layer according to the first objective that also has a low magnetostriction between −5×10⁻⁶and 5×10⁻⁶, a low RA value below 3 ohm-μm², and a low coercivity in the range of 4 to 6 Oe.
A further objective of the present invention is to provide a method of forming a TMR sensor that satisfies the first and second objectives and is cost effective.
According to one embodiment of the present invention, these objectives are achieved by forming a TMR sensor on a suitable substrate such as a bottom shield in a read head. The TMR sensor may have a bottom spin valve configuration comprised of a seed layer, AFM layer, pinned layer, tunnel barrier layer, free layer, and capping layer which are formed sequentially on the bottom shield. The tunnel barrier layer is made of MgOx and the free layer is comprised of a low magnetostriction CoB_Xlayer where x is from 0 to 30 atomic %, or a FeB_Vlayer where v is from 0 to 30 atomic %. In an alternative embodiment, the free layer is a composite represented by a FeCo_Y/CoB_Xconfiguration where y is from 0 to 100 atomic % and the FeCo_Ylayer contacts the tunnel barrier layer. Optionally, the free layer may have a trilayer configuration represented by FeCo_Y/CoFe_WB_Z/CoB_X, FeCo_Y/CoB_X/CoFe_WB_Z, FeCo_Y/CoFe_W/CoB_X, or FeCo_Y/FeB_V/CoB_Xwhere w is from 0 to 70 atomic %, and v and z are from 0 to 30 atomic %.
In a second embodiment, the tunnel barrier layer is made of MgOx and the free layer may have a configuration represented by FeCo_Y/Co_PNi_RFe_SB_Tin which the CoNiFeB layer is formed by co-sputtering targets made of CoB and CoNiFe, and p is from 5 to 90 atomic %, r is from 5 to 20 atomic %, s is between 5 and 90 atomic %, t is from 1 to 30 atomic %, and p+r+s+t=100 atomic %. Alternatively, the free layer may have a trilayer configuration represented by FeCo_Y/CoFe_WB_Z/CoNiFeB, FeCo_Y/CoNiFeB/CoFe_WB_Z, FeCo_Y/CoFe_W/CoNiFeB, or FeCo_Y/FeB_V/CoNiFeB where the CoNiFeB layer is made by co-sputtering CoB and CoNiFe. In yet another embodiment, the CoNiFeB layer in one of the previous embodiments may be replaced by a CoNiFeBX layer that is formed by co-sputtering CoB with CoNiFeX where X is an element such as V, Ti, Zr, Nb, Hf, Ta, or Mo and the content of the X element in the CoNiFeBX layer is <10 atomic %.
Typically, a TMR stack of layers is laid down in a sputtering system. All of the layers may be deposited in the same sputter chamber. Preferably, the MgOx tunnel barrier is formed by depositing a first Mg layer on the pinned layer followed by a natural oxidation process on the first Mg layer to form a MgOx layer and then depositing a second Mg layer on the MgOx layer. The oxidation step is performed in an oxidation chamber within the sputtering system. The TMR stack is patterned by a conventional method prior to forming a top shield on the cap layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing a TMR stack of layers according to one embodiment of the present invention.

FIG. 2 is a cross-sectional view showing a TMR stack of layers that has been patterned to form a MTJ element during an intermediate step of fabricating the TMR sensor according to one embodiment of the present invention.

FIG. 3 is a cross-sectional view of a TMR read head having a MTJ element interposed between a top shield and bottom shield and formed according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is a high performance TMR sensor having a free layer comprised of CoB or CoNiFeBX and a method for making the same. While the exemplary embodiment depicts a TMR sensor in a read head, the present invention may be employed in other devices based on a tunneling magnetoresistive element such as MRAM structures, or in a GMR-CPP sensor. The TMR sensor may have a bottom spin valve, top spin valve, or multilayer spin value configuration as appreciated by those skilled in the art. Drawings are provided by way of example and are not intended to limit the scope of the invention. For example, various elements are not necessarily drawn to scale and their relative sizes may differ compared with those in an actual device.
Referring to FIG. 1, a portion of a partially formed TMR sensor 1 of the present invention is shown from the plane of an air bearing surface (ABS). There is a substrate 10 that in one embodiment is a bottom lead otherwise known as a bottom shield (S1) which may be a NiFe layer about 2 microns thick that is formed by a conventional method on a substructure (not shown). It should be understood that the substructure may be comprised of a wafer made of AlTiC, for example.
A TMR stack is formed on the substrate 10 and in the exemplary embodiment has a bottom spin valve configuration wherein a seed layer 14, AFM layer 15, pinned layer 16, tunnel barrier layer 17, free layer 18, and capping layer 19 are sequentially formed on the substrate. The seed layer 14 may have a thickness of 10 to 100 Angstroms and is preferably a Ta/Ru composite but Ta, Ta/NiCr, Ta/Cu, Ta/Cr or other seed layer configurations may be employed, instead. The seed layer 14 serves to promote a smooth and uniform grain structure in overlying layers. Above the seed layer 14 is an AFM layer 15 used to pin the magnetization direction of the overlying pinned layer 16, and in particular, the outer portion or AP2 layer (not shown). The AFM layer 15 has a thickness from 40 to 300 Angstroms and is preferably comprised of IrMn. Optionally, one of PtMn, NiMn, OsMn, RuMn, RhMn, PdMn, RuRhMn, or MnPtPd may be employed as the AFM layer.
The pinned layer 16 preferably has a synthetic anti-parallel (SyAP) configuration represented by AP2/Ru/AP1 where a coupling layer made of Ru, Rh, or Ir, for example, is sandwiched between an AP2 layer and an AP1 layer (not shown). The AP2 layer which is also referred to as the outer pinned layer is formed on the AFM layer 15 and may be made of CoFe with a composition of about 10 atomic % Fe and with a thickness of about 10 to 50 Angstroms. The magnetic moment of the AP2 layer is pinned in a direction anti-parallel to the magnetic moment of the AP1 layer. For example, the AP2 layer may have a magnetic moment oriented along the “+x” direction while the AP1 layer has a magnetic moment in the “−x” direction. A slight difference in thickness between the AP2 and AP1 layers produces a small net magnetic moment for the pinned layer 16 along the easy axis direction of the TMR sensor to be patterned in a later step. Exchange coupling between the AP2 layer and the AP1 layer is facilitated by a coupling layer that is preferably comprised of Ru with a thickness from 3 to 9 Angstroms. The AP1 layer is also referred to as the inner pinned layer and may be a single layer or a composite layer. In one aspect, the AP1 layer is amorphous in order to provide a more uniform surface on which to form the tunnel barrier layer 17.
In the exemplary embodiment that features a bottom spin valve configuration, the tunnel barrier layer 17 is comprised of MgOx because a MgOx tunnel barrier is known to provide a higher TMR ratio than a TMR stack made with an AlOx or TiOx tunnel barrier. The MgOx tunnel barrier layer is preferably formed by depositing a first Mg layer having a thickness between 4 and 14 Angstroms on the pinned layer 16, oxidizing the Mg layer with a natural oxidation (NOX) process, and then depositing a second Mg layer with a thickness of 2 to 8 Angstroms on the oxidized first Mg layer. Thus, the tunnel barrier is considered as having a MgOx/Mg configuration. The second Mg layer serves to protect the subsequently deposited free layer from oxidation. It is believed that excessive oxygen accumulates at the top surface of the MgOx layer as a result of the NOX process and this oxygen can oxidize a free layer that is formed directly on the MgOx portion of the tunnel barrier layer. Note that the RA and MR ratio for the TMR sensor may be adjusted by varying the thickness of the two Mg layers in tunnel barrier layer 17 and by varying the natural oxidation time and pressure. A thicker MgOx layer resulting from longer oxidation time and/or higher pressure would increase the RA value.
All layers in the TMR stack may be deposited in a DC sputtering chamber of a sputtering system such as an Anelva C-7100 sputter deposition system which includes ultra high vacuum DC magnetron sputter chambers with multiple targets and at least one oxidation chamber. Typically, the sputter deposition process involves an argon sputter gas and a base pressure between 5×10⁻⁸and 5×10⁻⁹torr. A lower pressure enables more uniform films to be deposited.
The NOX process may be performed in an oxidation chamber within the sputter deposition system by applying an oxygen pressure of 0.1 mTorr to 1 Torr for about 15 to 300 seconds. In the exemplary embodiment, no heating or cooling is applied to the oxidation chamber during the NOX process. Oxygen pressure between 10⁻⁶and 1 Torr is preferred for an oxidation time mentioned above in order to achieve a RA in the range of 0.5 to 5 ohm-um². A mixture of O₂with other inert gases such as Ar, Kr, or Xe may also be used for better control of the oxidation process.
The present invention anticipates that a MgOx barrier layer 17 could be formed by depositing a MgOx layer on pinned layer with a rf-sputtering or reactive sputtering method. It should be understood that the performance of a TMR sensor fabricated with a barrier layer comprised of sputtered MgO will not be as desirable as one made according to the preferred embodiment of this invention. For example, the inventors have observed that the final RA uniformity (1σ) of 0.6 um circular devices is more than 10% when the MgOx tunnel barrier layer is rf-sputtered and less than 3% when the MgOx tunnel barrier is formed by DC sputtering a Mg layer followed by a NOX process.
Optionally, other materials such as TiOx, TiAlOx, MgZnOx, AlOx, ZnOx, or any combination of the aforementioned materials including MgOx may be used as the tunnel barrier layer 21.
Returning to FIG. 1, an important feature of the present invention is the free layer 18 formed on the tunnel barrier layer 17. In one embodiment, the free layer 18 has a thickness from 20 to 60 Angstroms and is comprised of a low magnetostriction CoB_Xlayer where x is from about 1 to 30 atomic % and λ is preferably between −5×10⁻⁶and 0, or a FeB_Vlayer where v is from about 1 to 30 atomic % and λ is preferably between 0 and 5×10⁻⁶. It should be understood that in a binary composition, the total atomic % for the two elements is 100 atomic %. In an alternative bilayer embodiment, the free layer 18 is a composite represented by a FeCo_Y/CoB_Xconfiguration where y is from 0 to 100 atomic % and the FeCo_Ylayer contacts the tunnel barrier layer 17. The FeCo_Ylayer may have a thickness between 2 and 10 Angstroms and the CoB_Xlayer may be from 20 to 50 Angstroms thick. Optionally, the free layer 18 may have a trilayer configuration represented by FeCo_Y/Co_UFe_WB_Z/CoB_X, FeCo_Y/CoB_X/Co_UFe_WB_Z, FeCo_Y/CoFe_W/CoB_X, or FeCo_Y/FeB_V/CoB_Xwhere u is from 1 to 95 atomic %, w is from 0 to 70 atomic %, v and z are from about 1 to 30 atomic %, and u+w+z=100 atomic % In the trilayer embodiments, the FeCo_Yand CoFe_Wlayers have a thickness between 2 and 10 Angstroms and y is unequal to w, the CoB_Xlayer has a thickness from 20 to 40 Angstroms, the Co_UFe_WB_Zlayer is from 5 to 20 Angstroms thick, and the CoFe_Wand FeB_Vlayers have a thickness between 2 and 10 Angstroms.
The inventors have discovered that a CoB_Xlayer with a small negative magnetostriction between −5×10⁻⁶and 0, or a FeB_Vlayer with a small positive magnetostriction between 0 and 5×10⁻⁶may be used as a free layer to afford a 15 to 30% improvement in TMR ratio compared with a conventional FeCo/NiFe free layer and still maintain low Hc (4-6 Oe) and low RA values necessary for high performance TMR sensors. Alternatively, a CoB_Xlayer with a small negative λ may be combined with one or more materials having a positive λ value to form a free layer composite with a bilayer or trilayer configuration having a λ value between −5×10⁻⁶and 5×10⁻⁶with TMR ratio, RA, and Hc values similar to those described for a CoB_Xfree layer.
In an alternative embodiment, the free layer 18 may have a configuration represented by Co_PNi_RFe_SB_Tor FeCo_Y/Co_PNi_RFe_SB_Tin which the CoNiFeB layer may be formed by co-sputtering targets made of CoB and CoNiFe or CoNiFeB, and p is from 5 to 90 atomic %, r is from 5 to 20 atomic %, s is between 5 and 90 atomic %, t is from about 1 to 30 atomic %, and p+r+s+t=100 atomic %. Alternatively, the free layer 18 may have a trilayer configuration represented by FeCo_Y/CoFe_WB_Z/Co_PNi_RFe_SB_T, FeCo_Y/Co_PNi_RFe_SB_T/CoFe_WB_Z, FeCo_Y/CoFe_W/Co_PNi_RFe_SB_T, or FeCo_Y/FeB_V/Co_PNi_RFe_SB_Twhere the CoNiFeB layer is made by co-sputtering CoB and CoNiFe, or CoB and CoNiFeB. CoB is preferably used in the co-sputtering process as a means of adjusting k.
In another embodiment, CoB may be co-sputtered with CoNiFeM where M is V, Ti, Zr, Nb, Hf, Ta, or Mo to produce a free layer 18 represented by CoNiFeBM in which the M content is <10 atomic %. Moreover, the present invention also anticipates that the free layer 18 may have a FeCo_Y/CoNiFeBM configuration. Optionally, a CoNiFeBM layer may be substituted for the CoNiFeB layer in one of the aforementioned trilayer configurations. Preferably, the CoNiFeB layer and CoNiFeBM layer in the embodiments described herein have a λ between −5×10⁻⁶and 5×10⁻⁶. A magnetic layer comprised of CoNiFeB or CoNiFeBM in combination with one or more other magnetic materials such as FeCo is expected to provide a similar improvement in TMR ratio compared to a FeCo/NiFe layer while maintaining low RA and low Hc values.
Once the TMR stack is complete, the partially formed read head 1 may be annealed in a vacuum oven within the range of 240° C. to 340° C. with an applied magnetic field of at least 2000 Oe, and preferably 8000 Oe for about 2 to 10 hours to set the pinned layer and free layer magnetization directions. It should be understood that under certain conditions, depending upon the time and temperature involved in the anneal process, the tunnel barrier layer 17 may become a uniform MgOx tunnel barrier layer as unreacted oxygen diffuses into the adjacent Mg layer.
Referring to FIG. 2, the TMR stack is patterned by following a conventional process sequence. For example, a photoresist layer 20 may be coated on the capping layer 19. After the photoresist layer 20 is patterned, a reactive ion etch (RIE), ion beam etch (IBE), or the like is used to remove underlying layers in the TMR stack that are exposed by openings in the photoresist layer. The etch process stops on the bottom shield 10 or between the bottom shield and a barrier layer (not shown) to give a TMR sensor with a top surface 19 a and sidewalls 21.
Referring to FIG. 3, an insulating layer 22 may be deposited along the sidewalls 21 of the TMR sensor. The photoresist layer 20 is then removed by a lift off process. A top lead otherwise known as a top shield 25 is then deposited on the insulating layer 22 and top surface 19 a of the TMR sensor. Similar to the bottom shield 10, the top shield 25 may also be a NiFe layer about 2 microns thick. The TMR read head 1 may be further comprised of a second gap layer (not shown) disposed on the top shield 25.

COMPARATIVE EXAMPLE 1

A first experiment was conducted to demonstrate the improved performance achieved by implementing a free layer in a TMR sensor according to the present invention. A TMR stack of layers, hereafter referred to as MTJ Sample 1 and shown in Table 1, was fabricated as a reference and is comprised of a conventional CoFe/NiFe free layer wherein the lower CoFe layer is 10 Angstroms thick and the upper NiFe layer is 40 Angstroms thick. MTJ Sample 1 has a seed/AFM/AP2/Ru/AP1/MgO/free layer/capping layer configuration. The pinned layer has an AP2/Ru/AP1 structure in which the AP2 layer is a 25 Angstrom thick CO₇₀Fe₃₀layer, the Ru coupling layer has a 7.5 Angstrom thickness, and the AP1 layer is a 25 Angstrom thick CO₇₀Fe₃₀layer. The MgOx tunnel barrier was formed by depositing a 7 Angstrom thick lower Mg layer that was subjected to a NOX process before a 3 Angstrom thick upper Mg layer was deposited. The thicknesses in Angstroms of the other layers are given in parentheses: Ta(20)/Ru(20) seed layer; IrMn (70) AFM layer; and Ru(10)/Ta(60) capping layer. The TMR stack was formed on a NiFe shield and was annealed under vacuum at 250° C. for 5 hours with an applied field of 8000 Oe.

TABLE 1

Hc, λ results for TMR sensors with Seed/AFM/AP2/Ru/AP1/MgOx/
free/cap configurations

MTJ		Hc
Sample	Free Layer Composition	(Oe)	Lambda

1	Fe₇₀Co₃₀10/Ni₉₀Fe₁₀40	4.13	1.80 × 10⁻⁶
2	Fe₇₀Co₃₀3/Co₇₂Fe₈B₂₀30	5.15	9.45 × 10⁻⁶
3	Fe₇₀Co₃₀3/Co₈₀B₂₀30	4.38	1.20 × 10⁻⁶
4	Fe₇₀Co₃₀3/Co₇₂Fe₈B₂₀10/Co₈₀B₂₀30	5.49	4.00 × 10⁻⁶
5	Fe₇₀Co₃₀3/Co₇₂Fe₈B₂₀10/	4.26	3.50 × 10⁻⁶
	CoB—CoNiFeB co-sputter 40

MTJ Sample 2 was fabricated with the same stack of layers as in MTJ Sample 1 except the free layer was changed to a conventional FeCo/CoFeB configuration in which the lower FeCo layer is 3 Angstroms thick and the upper CoFeB layer has a 30 Angstrom thickness. Samples 3 to 5 are MTJs in which a free layer formed according to an embodiment of the present invention has been inserted. For example, MTJ Sample 3 has a free layer comprised of a 3 Angstrom thick lower Fe₇₀CO₃₀layer and a 30 Angstrom thick upper CO₈₀B₂₀layer. MTJ Sample 4 has a 3 Angstrom thick lower Fe₇₀CO₃₀layer, a 10 Angstrom thick middle CO₇₂Fe₈B₂₀layer, and a 30 thick upper CO₈₀B₂₀layer. MTJ Sample 5 is the same as MTJ Sample 4 except the upper CoB layer is replaced by a 40 Angstrom thick CO₅₆Ni₈Fe₁₆B₂₀layer formed by co-sputtering CoB and CoNiFeB.
In Table 1, MTJ Samples 3-5 with a free layer comprised of CoB or CoNiFeB have a magnetostriction in the range of 1×10⁻⁶to 4×10⁻⁶that is substantially less than shown for MTJ Sample 2 which has a conventional CoFeB free layer. Furthermore, low Hc values are achieved with Samples 3-5 that are similar to the Hc for a conventional CoFe/NiFe free layer.

COMPARATIVE EXAMPLE 2

A second experiment was performed to compare MTJ Samples 1 and 2 with MTJ Samples 3-5 with regard to TMR ratio (dR/R) and RA results. The data shown in Table 2 was generated on 6 inch device wafers with the same TMR stacks shown in FIG. 1. Note that a combination of high TMR ratio and low RA values are achieved with a free layer formed according to an embodiment of the present invention. In particular, MTJ Samples 3-5 exhibit a 14-25% higher TMR ratio than the reference MTJ Sample 1 while maintaining the same RA in the range of 2.3 to 2.5 ohm-μm².

TABLE 2

TMR, RA results for TMR sensors with Seed/AFM/AP2/Ru/AP1/
MgO/free/cap configurations

MTJ		RA
Sample	Free Layer Composition	ohm-μm²	dR/R

1	Fe₇₀Co₃₀10/Ni₉₀Fe₁₀40	2.40	51.0%
2	Fe₇₀Co₃₀3/Co₇₂Fe₈B₂₀30	2.27	61.5%
3	Fe₇₀Co₃₀3/Co₈₀B₂₀30	2.34	64.2%
4	Fe₇₀Co₃₀3/Co₇₂Fe₈B₂₀10/Co₈₀B₂₀30	2.49	64.0%
5	Fe₇₀Co₃₀3/Co₇₂Fe₈B₂₀10/	2.39	58.0%
	CoB—CoNiFeB co-sputter 40

The advantages of the present invention are that a high TMR ratio of greater than 60% can be achieved simultaneously with a low RA value (<3 ohm-um²) and low magnetostriction which is a significant improvement over conventional TMR sensors (MTJs) based on a FeCo/NiFe free layer (low TMR ratio), or based on a FeCo/CoFeB free layer (high magnetostriction). TMR sensors formed with a free layer according to the present invention are able to achieve a 14-25% increase in TMR ratio compared with a FeCo/NiFe free layer and a 50% reduction in λ compared with a FeCo/CoFeB free layer while maintaining acceptable RA and Hc results.
The free layers disclosed in the embodiments found herein may be fabricated without additional cost since no new sputtering targets or sputter chambers are required. Furthermore, a low temperature anneal process may be employed which is compatible with the processes for making GMR sensors. Therefore, there is no change in process flow and related processes compared with current manufacturing schemes.
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.

Claims

1. A magnetoresistive element in a magnetic device, comprising:

(a) a stack of layers comprised of a seed layer, anti-ferromagnetic (AFM) layer, and a pinned layer sequentially formed on a substrate;

(b) a tunnel barrier layer made of MgOx on the pinned layer;

(c) a free layer comprised of CoB_Xor FeB_Vformed on the tunnel barrier layer where x and v are from about 1 to 30 atomic %; and

(d) a capping layer on the free layer.

2. The magnetoresistive element of claim 1 wherein said free layer is a single layer comprised of CoB_Xor FeB_Vwith a thickness from about 20 to 60 Angstroms.

3. The magnetoresistive element of claim 1 wherein the free layer has a bilayer configuration with a lower FeCo_Ylayer where y is from 0 to 100 atomic % formed on the tunnel barrier layer and an upper CoB_Xlayer having a λ between about −5×10⁻⁶and 0, said FeCo_Ylayer has a thickness between about 2 and 10 Angstroms and said CoB_Xlayer has a thickness between about 20 and 50 Angstroms.

4. The magnetoresistive element of claim 1 wherein the free layer has a trilayer configuration with a lower FeCo_Ylayer where y is from 0 to 100 atomic % formed on the tunnel barrier layer, a middle Co_UFe_WB_Zlayer having a thickness of about 5 to 20 Angstroms where u is from about 1 to 95 atomic %, w is from 0 to about 70 atomic %, and z is from about 1 to 30 atomic %, and an upper CoB_Xlayer having a λ between about −5×10⁻⁶and 0, said lower FeCo layer has a thickness between about 2 and 10 Angstroms and said upper CoB_Xlayer has a thickness between about 20 and 40 Angstroms.

5. The magnetoresistive element of claim 1 wherein the free layer has a trilayer configuration with a lower FeCo_Ylayer where y is from 0 to 100 atomic % formed on the tunnel barrier layer, a middle CoB_Xlayer having a λ from about −5×10⁻⁶to 0, and an upper Co_UFe_WB_Zlayer having a thickness of about 5 to 20 Angstroms where u is from about 1 to 95 atomic %, w is from 0 to about 70 atomic %, and z is from about 1 to 30 atomic %, said lower FeCo_Ylayer has a thickness between about 2 and 10 Angstroms, and said middle CoB_Xlayer is between about 20 to 40 Angstroms thick.

6. The magnetoresistive element of claim 1 wherein the free layer has a trilayer configuration with a lower FeCo_Ylayer where y is from 0 to 100 atomic % formed on the tunnel barrier layer, a middle CoFe_Wlayer where w is from 0 to 100 atomic % and is unequal to y, and an upper CoB_Xlayer, said lower FeCo_Yand middle CoFe_Wlayers each have a thickness between about 2 and 10 Angstroms and said upper CoB_Xlayer has a thickness from about 20 to 40 Angstroms.

7. The magnetoresistive element of claim 1 wherein the free layer has a trilayer configuration with a lower FeCo_Ylayer where y is from 0 to 100 atomic % and having a thickness of about 2 to 10 Angstroms formed on the tunnel barrier layer, a middle FeB_vlayer having a thickness of about 2 to 10 Angstroms, and an upper CoB_Xlayer with a thickness from about 20 to 40 Angstroms.

8. A magnetoresistive element in a magnetic device, comprising:

(b) a tunnel barrier layer made of MgOx on the pinned layer;

(c) a free layer comprised of Co_PNi_RFe_SB_Tor CoNiFeBM formed on the tunnel barrier layer wherein p is from about 5 to 90 atomic %, r is from about 5 to 20 atomic %, s is between about 5 and 90 atomic %, t is from about 1 to 30 atomic %, and p+r+s+t=100 atomic %, and M is one of V, Ti, Zr, Nb, Hf, Ta, or Mo; and

(d) a capping layer on the free layer.

9. The magnetoresistive element of claim 8 wherein said free layer is a single layer comprised of Co_PNi_RFe_SB_Tor CoNiFeBM with a thickness from about 20 to 60 Angstroms, and M has a content of <10 atomic % in the CoNiFeBM alloy.

10. The magnetoresistive element of claim 8 wherein the free layer has a bilayer configuration with a lower FeCo_Ylayer where y is from 0 to 100 atomic % formed on the tunnel barrier layer and an upper Co_PNi_RFe_SB_Tor CoNiFeBM layer, said FeCo_Ylayer has a thickness between about 2 and 10 Angstroms and said Co_PNi_RFe_SB_Tor CoNiFeBM layer has a thickness between about 20 and 50 Angstroms.

11. The magnetoresistive element of claim 8 wherein the free layer has a trilayer configuration with a lower FeCo_Ylayer where y is from 0 to 100 atomic % formed on the tunnel barrier layer, a middle Co_UFe_WB_Zlayer having a thickness of about 5 to 20 Angstroms where u is from about 1 to 95 atomic %, w is from 0 to about 70 atomic %, and z is from about 1 to 30 atomic %, and an upper Co_PNi_RFe_SB_Tor CoNiFeBM layer, said lower FeCo layer has a thickness between about 2 and 10 Angstroms and said upper Co_PNi_RFe_SB_Tor CoNiFeBM layer has a thickness between about 20 and 40 Angstroms.

12. The magnetoresistive element of claim 8 wherein the free layer has a trilayer configuration with a lower FeCo_Ylayer where y is from 0 to 100 atomic % formed on the tunnel barrier layer, a middle Co_PNi_RFe_SB_Tor CoNiFeBM layer, and an upper Co_UFe_WB_Zlayer where u is from about 1 to 95 atomic %, w is from 0 to about 70 atomic %, and z is from about 1 to 30 atomic %.

13. The magnetoresistive element of claim 8 wherein the free layer has a trilayer configuration with a lower FeCo_Ylayer where y is from 0 to 100 atomic % formed on the tunnel barrier layer, a middle CoFe_Wlayer where w is from 0 to 100 atomic % and is unequal to y, and an upper Co_PNi_RFe_SB_Tor CoNiFeBM layer.

14. The magnetoresistive element of claim 8 wherein the free layer has a trilayer configuration with a lower FeCo_Ylayer where y is from 0 to 100 atomic % formed on the tunnel barrier layer, a middle FeB_Vlayer, and an upper Co_PNi_RFe_SB_Tor CoNiFeBM layer.

15. A method of forming a magnetoresistive element in a TMR sensor, comprising:

(a) sequentially forming a seed layer, AFM layer, and a pinned layer on a substrate;

(b) forming a tunnel barrier layer on said pinned layer by depositing a first Mg layer on the pinned layer, performing a natural oxidation process to form a MgOx layer, and then depositing a second Mg layer on the MgOx layer;

(c) forming a free layer on the MgOx tunnel barrier layer, said free layer is comprised of CoB_Xor FeB_Vwhere x and v are from about 1 to 30 atomic %; and

(d) forming a capping layer on the free layer.

16. The method of claim 15 wherein the free layer has a bilayer configuration with a lower FeCo_Ylayer where y is from 0 to 100 atomic % formed on the tunnel barrier layer and an upper CoB_Xlayer having a k between about −5×10⁻⁶and 0, said FeCo_Ylayer has a thickness between about 2 and 10 Angstroms and said CoB_Xlayer has a thickness between about 20 and 50 Angstroms.

17. The method of claim 15 wherein the free layer has a trilayer configuration with a lower FeCo_Ylayer where y is from 0 to 100 atomic % formed on the tunnel barrier layer, a middle Co_UFe_WB_Zlayer having a thickness of about 5 to 20 Angstroms where u is from about 1 to 95 atomic %, w is from 0 to about 70 atomic %, and z is from about 1 to 30 atomic %, and an upper CoB_Xlayer having a λ between about −5×10⁻⁶and 0, said lower FeCo layer has a thickness between about 2 and 10 Angstroms and said upper CoB_Xlayer has a thickness between about 20 and 40 Angstroms

18. The method of claim 15 wherein the free layer has a trilayer configuration with a lower FeCo_Ylayer where y is from 0 to 100 atomic % formed on the tunnel barrier layer, a middle CoFe_Wlayer where w is from 0 to 100 atomic % and w is unequal to y, and an upper CoB_Xlayer having a λ between about −5×10⁻⁶and 0, said lower FeCoy and middle CoFe_Wlayers each have a thickness between about 2 and 10 Angstroms and said upper CoB_Xlayer has a thickness from about 20 to 40 Angstroms.

19. The method of claim 15 wherein the free layer has a trilayer configuration with a lower FeCo_Ylayer where y is from 0 to 100 atomic % and having a thickness of about 2 to 10 Angstroms formed on the tunnel barrier layer, a middle FeB_Vlayer having a thickness of about 2 to 10 Angstroms, and an upper CoB_Xlayer with a λ between about −5×10⁻⁶and 0 and a thickness from about 20 to 40 Angstroms.

20. A method of forming a free layer in a TMR sensor in a magnetic device, comprising:

(c) forming a free layer comprised of Co_PNi_RFe_SB_Tor CoNiFeBM on the tunnel barrier layer wherein p is from about 5 to 90 atomic %, r is from about 5 to 20 atomic %, s is between about 5 and 90 atomic %, t is from about 1 to 30 atomic %, and p+r+s+t=100 atomic %, and M is one of V, Ti, Zr, Nb, Hf, Ta, or Mo; and

(d) forming a capping layer on the free layer.