One key requirement for a fully functional ST-MRAM is achieving a critical voltage (Vc) distribution that is well separated from the tunnel barrier breakdown distribution. For a switching voltage Vsw that reliably switches Mbits without breakdown fails, at least 6σ separation is needed from Vsw to both distributions as shown in figure 3.
Figure 3: An illustration of key distributions in ST-MRAM arrays. The bit switching voltage (Vsw) must be separated from the Vc the Vbd distributions. To avoid disturbs, the read voltage Vread must not overlap Vc. Narrow distributions are critical for error-free operation.
In practice more is needed to allow for write voltage variation and time-dependent dielectric breakdown (TDDB) effects. Since the voltage required to breakdown a dielectric layer (Vbd
) is reduced as the time at bias is increased, repeated cycling of the bias results in a shift of the breakdown distribution to the left, reducing separation from the Vc
distribution. This TDDB effect is quite significant over the life of a part and extra single-cycle separation must be engineered into the devices to achieve the desired endurance. Figure 4 shows experimental data for integrated arrays with over 30σ separation.
A low Jc
is required for low Vc
and also minimizes Ic
, allowing a smaller pass transistor. Smaller MTJ area A also reduces Ic
but measures must be taken to maintain data retention since energy barrier to switching (Eb
) is approximately proportional to A. We have determined experimentally that Eb
for in-plane magnetization is optimum for shaped bits with aspect ratio near 3.
Figure 4: Probability of bit switching (AP-to-P) and tunnel barrier breakdown vs. applied voltage (Vapplied) for bits with the optimized CoFeB-based free layer measured in kb ST-MRAM arrays integrated with CMOS with pulse duration tp≈100ns. The separation between the switching and breakdown is >30σ.
Small increases in thickness can increase Eb
at the expense of somewhat higher VC
as shown experimentally in figure 5. An increase in saturation magnetization of only 10% can dramatically increase the energy barrier, for example from 60 kB
T to 80 kB
T. Since data retention time (τ) is exponentially dependent on Eb
, the mean value of τ increases by 8 orders of magnitude with this small magnetization increase.
Figure 5. Energy barrier (measured with pulsed magnetic field) vs bit area and free layer thickness. Bits ranged from 50nm to 90nm ellipses with aspect ratio from 2.3 to 3.5. Thicknesses varied from baseline (green circles) + (triangles)/- (squares) 10% in total moment.