Frederick B. Mancoff

A Fully Functional 64 Mb DDR3 ST-MRAM Built on 90 nm CMOS Technology

A spin torque magnetoresistive random access memory (ST-MRAM) holds great promise to be a fast, high density, nonvolatile memory that can enhance the performance of a variety of applications, particularly when used as a non-volatile buffer in data storage devices and systems. Towards that end, we have developed a fully functional 64 Mb DDR3 ST-MRAM built on 90 nm CMOS technology. The memory is organized in an 8-bank configuration that can sustain 1.6 GigaTransfers/s (DDR3-1600). We have run standard memory tests, such as a March6N pattern, on the full 64 Mb at 800 MHz with 0 fails for greater than 10 5 cycles. Full functionality was also verified from 0°C to 70°C with no significant change in performance. The bits are magnetic tunnel junctions (MTJs) having an MgO tunnel barrier and a magnetic free layer made of a CoFeB-based alloy with an in-plane magnetization, but with an out-of-plane anisotropy reduced by more than 50% due to an enhanced perpendicular surface anisotropy. To enable the 64 Mb performance, we developed an MTJ stack that has low switching voltage (Vsw), high breakdown voltage (Vbd), and excellent switching reliability with tight distributions. The ST switching distribution has σ ≈ 10%, and we found excellent agreement with a single Gaussian distribution down to an error rate . For our optimized material, the Vsw/Vbd ≈ 0.3, and the separation between Vsw and Vbd is ≈ 25σ. The energy barrier to magnetization reversal (Eb) was characterized using both time-dependent coercivity and higher temperature to accelerate reversal. We found the average Eb ≈ 70kbT.

Area dependence of high-frequency spin-transfer resonance in giant magnetoresistance contacts up to 300nm diameter

We measured high-frequency spin-transfer resonances from 26GHz excited by dc currents through giant magnetoresistance point contacts with diameters d from <50 to almost 300nm. The slope of resonance frequency versus current decreased with increased d and was fit best by a spin-transfer model where the effective d extends ⩾50nm past the contact edge into the surrounding magnetic film. An increased resonance critical current versus contact area was also fit well by this model including a surrounding ring of excited area. Spin-transfer resonance in large devices eliminates the need for electron-beam lithography in applications.

Area-Dependence of High Frequency Spin-Transfer Resonance in GMR Contacts up to 300 nm Diameter

The area-dependence of spin-transfer resonance in giant magnetoresistance (GMR) contacts from 50-300 nm diameter d is measured in this paper. With increasing d , a decreasing slope df/dl of precession frequency vs. current and an increasing critical current Ic is found. The data is well fit by a model where the precessing region extends outside the contact by a ring of width delta~ 50 nm. A GMR film with a base electrode, a 20 nm Co81Fe19 fixed layer, a 6 nm Cu spacer layer, a 4.5 nm Ni80Fe20 free layer, and a cap. The point contacts were written by optical or e-beam lithography and formed through SiO2 or hardened PMMA. The inset shows an SEM image of a nominally 60 nm diameter contact. A dc current I is applied to the contact using a high-frequency bias tee and microwave probe and measured the GMR frequency spectrum. A magnetic field ~1 T normal to the film saturated the free moment out-of-plane at I=0 and tilted the fixed moment ~30deg out-of-plane.