Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is
Reexamination Certificate
1999-10-18
2001-12-04
Flynn, Nathan (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
C257S295000, C428S690000
Reexamination Certificate
active
06326637
ABSTRACT:
TECHNICAL FIELD
This invention relates generally to magnetic tunnel junction (MTJ) magnetoresistive devices for use as read heads for reading magnetically-recorded data and as memory cells in non-volatile magnetic random access memory (MRAM) arrays, and more particularly to an improved antiferromagnetically exchange-coupled structure for the MTJ device.
BACKGROUND OF THE INVENTION
Magnetic tunnel junctions (MTJs) are devices with potential use as magnetoresistive read heads in magnetic storage applications, such as hard disk drives, and as nonvolatile storage cells for magnetic random access memory (MRAM) applications.
For both MTJ read head and MRAM applications a useful MTJ device is one comprised of two thin ferromagnetic layers separated by a very thin insulating layer, such as alumina (Al
2
O
3
), in which one of the ferromagnetic layers is fixed or “pinned” by being exchange biased (also called “exchange coupled”) to a thin layer of an antiferromagnetic material. In a MTJ read head, the moment of the other ferromagnetic layer, called the “free” layer, is oriented generally perpendicularly to the moment of the pinned ferromagnetic layer in zero magnetic field and is free to rotate in the presence of magnetic fields from the recorded media. In a MTJ cell, the cell is designed to exhibit two bi-stable states in zero magnetic field in which the moment of the free ferromagnetic layer is oriented either parallel or antiparallel to the moment of the pinned ferromagnetic layer. These states, which exhibit different tunneling resistance values when current is passed perpendicularly through the device, represent the two storage states of the MTJ memory cell. In a MTJ read head application the moment of the free ferromagnetic layer is oriented generally perpendicular to the moment of the pinned ferromagnetic layer in the absence of an applied magnetic field, and rotates about this position in the presence of applied fields from the magnetic recording medium.
It is important that MTJ devices withstand fairly high temperatures during processing, For MTJ MRAM applications, if complementary metal-oxide semiconductor (CMOS) electronic devices are used with the MTJ cells the highest processing temperature will be determined by details of the particular CMOS process used. It is likely that the MTJ cells will be fabricated after the CMOS circuits have been fabricated, within the “back-end-of-line” (BEOL) process where the temperatures to which the devices are subjected are more limited, but may still be as high as ~400° C. For MTJ read heads, processing temperatures in excess of 250° C. are likely in order to hard-bake certain photoresists used in fabricating the read/write heads. The most common type of antiferromagnetic material proposed for use in MTJ devices is a Mn—Fe alloy. MTJ devices using Mn—Fe antiferromagnetic layers on a variety of underlayers have failed at temperatures as low as 250-300° C. Even for moderate anneal temperatures as low as 250° C., the MTJ devices using Mn—Fe antiferromagnetic layers are not highly thermally stable and typically the magnetoresistance of such devices is reduced. It is believed that during processing a small amount of Mn diffuses from the Mn—Fe layer to the interface between the pinned ferromagnetic layer and the alumina tunnel barrier, which reduces the Magnetoresistance of the device.
It addition to poor thermal stability, Mn—Fe has several other disadvantages, including a relatively low exchange coupling field, poor corrosion resistance, and a low blocking temperature (the temperature at which the net magnetic moment no longer has a fixed orientation).
Other antiferromagnetic materials besides Mn—Fe have been proposed for exchange biasing the pinned ferromagnetic layer in spin-valve type magnetoresistive read heads. For example, European published patent application EP-0717422 describes the use of Ir—Mn alloys, and suggests without any experimental data that Os can be one of 30 other elements (approximately 25% of the Periodic Table) to be added to the Ir—Mn alloy for the purpose of improving the corrosion resistance of the alloy. U.S. Pat. No. 5,552,949 suggests the use of X—Mn alloys, where X can be one of 10 elements, including Os, provided the X element is present in the range of 25 to 76 atomic percent in the X—Mn alloy. In both of these spin-valve head references the antiferromagnetic layer is deposited directly on top of the ferromagnetic layer.
What is needed is a MTJ device for read head and MRAM applications that uses an antiferromagnetic material that provides a high exchange coupling field and good corrosion resistance and results in a MTJ device that is thermally stable at high processing temperatures.
SUMMARY OF THE INVENTION
The invention is an antiferromagnetically exchange-coupled structure for use in various types of magnetic devices, such as magnetic tunnel junctions and spin-valve giant magnetoresistance recording heads. The structure includes an antiferromagnetic layer formed of an alloy of osmium and manganese, wherein the osmium is present in the range of approximately 10 to 30 atomic %. The antiferromagnetic layer is deposited on a non-reactive underlayer, preferably one formed of a noble metal, such as platinum, palladium or alloys thereof, with the ferromagnetic layer formed on top of the antiferromagnetic layer. The antiferromagnetic material provides a strong exchange biasing for the ferromagnetic layer that is deposited on the antiferromagnetic layer. Iridium may be added to the osmium-manganese alloy, wherein the total of osmium and iridium is the range of the approximately 10 to 30 atomic %, to increase the blocking temperature of the antiferromagnetic material. A template layer of permalloy (nickel-iron alloy) may be formed between the underlayer and the antiferromagnetic layer to improve the growth of the osmium-manganese alloy. The resulting antiferromagnetically exchange-coupled structure exhibits very high thermal stability, i.e., the magnetoresistance of magnetic tunnel junction devices is retained even during relatively high annealing process temperatures. This allows magnetic tunnel junction devices using the structure to be used as memory cells in magnetic random access memory arrays that are formed on substrates with electronic circuitry formed by conventional high-temperature CMOS processes and which require high temperature anneals of the completed memory chips.
For a fuller understanding of the nature and advantages of the present invention, reference should be made to the following detailed description taken together with the accompanying figures.
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Papworth Parkin Stuart Stephen
Samant Mahesh Govind
Berthold Thomas R.
Flynn Nathan
International Business Machines - Corporation
Sefer Ahmed N.
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