Smart Barrier for Spin Torque Transfer Random Access Memory (STTRAM)
As the scaling of traditional CMOS devices goes beyond the 32-nm node, the present memory technology will face numerous technical hurdles. MRAM is an emergent memory technology that has the potential to replace all other current memory technologies, including SRAM, DRAM and FLASH. Conventional MRAM utilizes current generated magnetic fields to switch the magnetization of the free layer. As scaling decreases and current densities required to switch the magnetic free layer increase, heat dissipation increasingly becomes an issue. Spin torque transfer (STT) has the potential to alleviate this problem by using the momentum from a spin polarized current to torque the magnetization of the free layer rather than using a magnetic field, which uses much less current. Although STT-RAM holds promise to lessen power dissipation concerns, current densities required for write operations are too high to be driven by traditional CMOS transistors to supply the current and need to be lowered to roughly ~105 A/cm2. Magnetic tunnel junctions (MTJs) that utilize MgO tunnel barriers work near their breakdown voltages during write operations. This leads to fatigue over time and the inevitable failure of the device.The current invention is a smart tunable tunnel barrier for magnetic tunnel junctions utilizing STT switching. Vanadium dioxide (VO2) undergoes a well-known first-order metal to insulator transition (MIT) just above room temperature at 341K, which is associated with a structural phase transformation. The abruptness of the phase transformation from its high temperature tetragonal phase to its low temperature monoclinic phase is accompanied by large changes in its electrical conductivity and infrared transmission characteristics, making it an excellent candidate for both sensor and switching applications. We also observe a current driven transition at room temperature, in which the material undergoes an abrupt change in electrical resistance (~ 10×) when injected with relatively small current densities (~10-4 A/cm2). Voltage pulsed measurements indicate the transition time is very fast. This recent observation has enhanced the interest and number of potential applications for this material, such as power switches, field effect transistors, tunable tunnel barriers, memory and logic devices.
Utilizing this MIT in the VO¬2 phase as the basis for a tunable smart tunnel barrier is desirable since the transition occurs with relatively low injected current densities and at room temperature. Using VO2 as a spin tunnel barrier in an MTJ structure would have the additional key advantage of having a device whose resistance-area product had two states. One could write in the low resistance state to reduce power dissipation and read in the high resistance state to maintain high magnetoresistive ratio (MR) signal. Other benefits of the technology include:
1. Low power dissipation and write voltages during write operations: Utilizing the injected spin polarized current to switch the VO2 tunnel barrier material to its reduced resistive state, lower voltages are expected to generate the required current densities for switching of the magnetic free layer. This is an advantage over MgO in which write currents are close to break down voltages.
2. Fast and robust: Infinitely cyclable switching with speeds greater than 100 MHz are expected.
3. Compatible with state-of-the-art STT-RAM devices: It is expected be fully compatible with state of the art STT-RAM structures. At present time MgO tunnel barriers are prepared by reactive sputter deposition methods, similar to reactive bias target ion beam deposition techniques.
Inventor(s): Wolf, Friedersdorf, Lu, Kirkwood, West
Type of Offer: Licensing
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