For the MRAM industry that has been in trouble for a long time, the spin injection method can be said to be an innovative technology that can reverse the crisis.
MRAM came out vigorously. But since then, MRAM has not made the expected progress in process development and large capacity. In the current mass-produced products, the manufacturing process of MRAM only reaches 180nm, the maximum capacity is only 4Mb, and its application is limited to specific areas such as replacing SRAM that requires batteries.
In contrast, spin-injection MRAM can support more advanced manufacturing processes and is easy to expand capacity. Therefore, it has the potential to change the above-mentioned situation and win a huge market.
The reason why the existing MRAM encounters difficulties in terms of manufacturing process and large capacity is that it uses a working principle called the magnetic field writing method (see Figure 1a). The magnetic field writing method refers to the use of a composite magnetic field generated by the bit line current and the writing word line current to reverse the magnetization of the free layer of the TMR element. When using this method, the writing current required for magnetization reversal is inversely proportional to the volume of the magnetic material. Therefore, when a more advanced process is used, the writing current will increase. In order to support the flow of a large current, a transistor with a large driving capability must be used, resulting in an increase in the area of the memory cell. Moreover, magnetic field writing MRAM also requires additional wiring such as writing word lines, so the theoretical value of the memory cell area is 12F2, which is 1.5 to 2 times that of DRAM.
Spin injection method can avoid these shortcomings. In the spin injection method, the magnetic field is not used when writing data, but the current flows directly into the TMR element, so that the magnetization direction of the free layer of the TMR element is reversed (see Figure lb). The more advanced the manufacturing process, the smaller the threshold current required for magnetization reversal. And since it is no longer necessary to write the word line, the theoretical value of the memory cell area can reach 6F2~8F2, which is equivalent to DRAM.