MagRAM Fabricated on Sapphire
Original Publication Date: 2003-Oct-30
Included in the Prior Art Database: 2003-Oct-30
This invention relates in general to magnetic tunnel junction (MTJ) memory embedded or attached applications to a processor. More particularly the invention relates to the MTJ memory cell built on silicon on sapphire or other dielectric substrate to be attached to the silicon VLSI device chip through packaging technique.
MagRAM Fabricated on Sapphire
A magnetic tunnel junction (MTJ) device is comprised of two ferromagnetic layers separated by a thin insulating tunnel barrier layer and is based on the phenomenon of spin-polarized electron tunneling. One of the ferromagnetic layers has a higher saturation field in one direction of an applied magnetic field, typically due to its higher coercivity than the other ferromagnetic layer. The insulating tunnel barrier layer is thin enough that quantum mechanical tun-neling occurs between the ferromagnetic layers. The tunnel-ing phenomenon is electron-spin dependent, making the magnetic response of the MTJ a function of the relative orientations and spin polarizations of the two ferromagnetic layers.
MTJ devices have been proposed as memory cells for solid state memory and as external magnetic field sensors, such as magnetoresistive (MR) read sensors or heads for magnetic recording systems. The use of a MTJ device as a memory cell in a nonvolatile magnetic random access memory (MRAM) array is described in IBM's U.S. Pat. Nos. 5,650,958 and 5,640,343.
The response of the MTJ device is determined by measuring the resistance of the MTJ when a sense current is passed perpendicularly through the MTJ from one ferro-magnetic layer to the other. The probability of tunneling of charge carriers across the insulating tunnel barrier layer depends on the relative alignment of the magnetic moments (magnetization directions) of the two ferromagnetic layers. The tunneling current is spin polarized, which means that the electrical current passing from one of the ferromagnetic layers, for example, a ferromagnetic layer whose magnetic moment is fixed or prevented from rotation, is predomi-nantly composed of electrons of one spin type (spin up or spin down, depending on the orientation of the magnetic moment of the ferromagnetic layer). The degree of spin polarization of the tunneling current is determined by the electronic band structure of the magnetic material comprising the ferromagnetic layer at the
interface of the ferromag-netic layer with the tunnel barrier layer. The first ferromag-netic layer thus acts as a spin filter. The probability of tunneling of the charge carriers depends on the availability of electronic states of the same spin polarization as the spin polarization of the electrical current in the second ferromag-netic layer. Usually, when the magnetic moment of the second ferromagnetic layer is parallel to the magnetic moment of the first ferromagnetic layer, there are more available electronic states than when the magnetic moment of the second ferromagnetic layer is aligned anti-parallel to that of the first ferromagnetic layer. The tunneling probabil-ity of the charge carriers is highest when the magnetic moments of both layers are parallel, and is lowest when the magnetic moments are anti-parallel. Thus, the electrical resistance of the MTJ depends on both the spin polarization...