US 6890767 B2
In magnetic memories it is important to be able to switch the states of the memory elements using minimal power i.e. external fields of minimal intensity. This has been achieved by giving each memory element an easy axis whose direction parallels its minimum surface dimension. Then, when the magnetic state of the element is switched by rotating its direction of magnetization, said rotation is assisted, rather than being opposed, by the crystalline anisotropy. Consequently, relative to the prior art, a lower external field is required to switch the state of the element.
1. A process for manufacturing a magnetic memory element, comprising:
(a) depositing on a substrate a first layer of a soft magnetic material having a first easy axis direction;
(b) depositing a layer of a non-magnetic material on said first layer of soft magnetic material;
(c) on said layer of non-magnetic material, depositing a second layer of soft magnetic material whose easy axis direction is permitted to be different from said first easy axis direction;
(d) repeating steps (b) and (c) one or more times, thereby forming a stack; and
(e) patterning said stack to form a memory element having minimum and maximum dimensions along said substrate and an easy axis parallel to said minimum dimension;
whereby the memory element has a reduced switching threshold as compared to a memory element having an easy axis parallel to said maximum dimension.
2. The process described in
3. The process described in
4. The process described in
5. The process described in
6. The process described in
7. A process for creating an electrical device, comprising:
providing a substrate;
depositing a layer of a magnetic material on the substrate;
patterning said layer to form a memory element, the memory element having a longer dimension and a shorter dimension; and
applying a magnetic field Hext across the memory element to create a memory element having a comparatively small switching threshold by forming an easy axis parallel with the shorter dimension.
8. The process of
repeating the steps of depositing, patterning, and applying to form a memory cell array.
9. The process of
tan θy=(H ext +H K +Mt/L)/Mt/W)
where HK represents a crystalline anisotropy of the magnetic material;
Mt represents a magnetization of the magnetic material; and
L and W represent the longer and shorter dimensions, respectively.
This invention relates to U.S. Patent Application Ser. No. 10/255,482, filed Sep. 26, 2002, entitled, “Scalable High Performance MRAM Cell Arrays” (TSMC2002-00059) assigned to a common assignee.
The invention relates to the general field of magnetic devices with particular reference to magnetic memory cells.
In MRAM (magnetic random access memory), a cell's magnetic state is switched (i.e. its direction of magnetization is rotated) by an external magnetic field. This field is generated by a current through a program line on a chip. It is desirable to lower the switching threshold of MRAM cells so that the write current, and thus the chip power, can be reduced. The free layer in an MRAM cell is typically a soft magnetic film having a rectangular or oval shape.
The direction of the magnetization, M, in a magnetic soft film is determined by the lowest energy state of the film. In the sub-micron geometry film, the shape anisotropy Hd dominates over the crystalline anisotropy HK. The direction of the HK is usually called the easy axis direction of the film. Its direction is set by a small magnet in the deposition chamber during film deposition. The direction of the film's magnetization M would normally parallel the largest dimension of the cell to minimize demagnetization effects arising from the shape anisotropy due to its dominance in small geometry film, rather than the direction of the crystalline anisotropy. This is illustrated in
A routine search of the prior art was performed with the following references of interest being found:
In U.S. Pat. No. 6,104,633, Abraham et al. show a MRAM and discuss the easy axis of the free layer. Abraham et al. also show related MRAMs in U.S. Pat. No. 6,072,718 and in U.S. Pat. No. 5,946,228. U.S. Pat. No. 6,259,644B (Tran et al.) shows a MRAM with free and pinned layers in anti-parallel directions while Sun discusses switching thresholds in U.S. Pat. No. 6,256,223 B1.
It has been an object of at least one embodiment of the present invention to provide a magnetic memory element whose state may be switched using a lower field than is required to switch the state of a memory element of the prior art.
Another object of at least one embodiment of the present invention has been to provide a process for manufacturing said memory element.
Still another object of at least one embodiment of the present invention has been that said process not require substantive changes to the processes presently in use for the manufacture of magnetic memory elements.
These objects have been achieved by giving said magnetic memory element an easy axis that parallels the minimum surface dimension of the element. Then, when the magnetic state of the element is switched by rotating its direction of magnetization, said rotation is assisted, rather than being opposed, by the crystalline anisotropy. Consequently, relative to the prior art, a lower external field is required to switch the state of the element.
The significance of this change can be seen in the following analysis:
Referring now to
In a cell array, to change the cell state, say from M pointing east to west, one would need to apply two external fields: one Hx (pointing west) and Hy (pointing north). The external magnetic field Hy will rotate the magnetization M to an angle away from the x-direction. The external field Hx will complete the switch.
Based on a uniform rotation model, the switching asteroid of a rectangular film can be described as
Hx 2/3 +Hy 2/3 =Ho 2/3
We now describe a process for manufacturing the memory element described above. The process begins with the deposition of a layer 32 of a soft magnetic material (such as NiFe, CoFe, COFeB, or CoNiFe) on a substrate (to a thickness between about 5 and 50 Angstroms) and then patterning said layer to form a memory element 11 having a minimum dimension 13 and maximum dimension 12, as seen in FIG. 2. In the case where a multiple magnetic soft film stack is deposited, the direction of the easy axis of each layer can be made different by changing the direction of the magnet inside the deposition chamber.
In a second embodiment of the invention, memory element 11 is a stack made up of multiple layers of magnetically soft material separated layers of a non-magnetic material such as Cu, Ru, Pt, Ag, or Au, to a thickness between about 5 and 30 Angstroms.
The advantages of using a stack of several magnetic layers is that the total volume of the soft magnetic material increases and the thermal stability of the memory cell improves. By having the magnetization M in the soft magnetic material in the stack arranged in the anti-parallel direction, the net moment is lowered, which translates to lower switching threshold.