A quantum sensing metrology for magnetic memories

Abstract

Magnetic random access memory (MRAM) is a leading emergent memory technology that is poised to replace current non-volatile memory technologies such as eFlash. However, controlling and improving distributions of device properties becomes a key enabler of new applications at this stage of technology development. Here, we introduce a non-contact metrology technique deploying scanning NV magnetometry (SNVM) to investigate MRAM performance at the individual bit level. We demonstrate magnetic reversal characterization in individual, <60 nm-sized bits, to extract key magnetic properties, thermal stability, and switching statistics, and thereby gauge bit-to-bit uniformity. We showcase the performance of our method by benchmarking two distinct bit etching processes immediately after pattern formation. In contrast to ensemble averaging methods such as perpendicular magneto-optical Kerr effect, we show that it is possible to identify out of distribution (tail-bits) bits that seem associated to the edges of the array, enabling failure analysis of tail bits. Our findings highlight the potential of nanoscale quantum sensing of MRAM devices for early-stage screening in the processing line, paving the way for future incorporation of this nanoscale characterization tool in the semiconductor industry.

Keywords: Electronic and spintronic devices; Quantum metrology.

Fig. 1. SNVM as metrology tool for STT-MRAM.

a Process flow of STT-MRAM fabrication, including process monitoring. b SNVM map of 45 × 45 bits (10 × 10 μm) after encapsulation. Bits in the anti-parallel (AP) state appear dark, bits in the parallel (P) state appear bright. c P and AP bit configurations generate distinct stray field patterns (gray lines). The NV probe measures their projection onto the NV quantization axis (black arrow) at the flying distance of the NV probe.

Fig. 2. STT-MRAM.

a The quality of etching Processes 1 and 2 are compared in this work. Etching Process 2 features an additional etchback and gentle oxidation step after the first oxidation. b Scanning electron microscope images of the MRAM pillars before and c after encapsulation. d Transmission electron microscope image of an MRAM pillar.

Fig. 3. Quantitative determination of Δ and H_k.

a SNVM maps, obtained for different switching fields μ₀H_switch. b For each map in (a), the percentage of P bits is plotted versus μ₀H_switch, which is fitted to obtain H_k and Δ, inset: measurement routine where the bits are first initialized at μ₀H = 160 mT followed by a switching field μ₀H_switch. The measurements are performed at μ₀H = 2 mT. c Polar MOKE hysteresis loops of the FL measured on the same wafers. d Histogram of the maximum measured stray field per bit in P state, shifted with respect to the median of the distribution, $\mathrm{\Delta }{B}_{\mathrm{NV},\max }$.

Fig. 4. Array uniformity.

a The map on the left shows, by dot size and color, how often a bit has switched during 10 repetitive measurements. For each repetition, a map, as shown in Fig. 3a with 46% of bits in P state, is taken. The simulation illustrates how such a map would appear for perfectly uniform bits. b Density histogram of the maps in (a). The results are compared to a binomial distribution (purple) reflecting the expected outcome for perfectly uniform bits. c Deviation from uniform behavior (${\sigma }_{\mathrm{switching},i}^2-{\sigma }_{\mathrm{switching},0}^2$) as a function of percentage of bits in P states for each individual measurement, we chose to show 2 and 5 repetitions. d Stray field in fully initialized map of Process 2, ΔB_NV,max (see in Fig. 3d), as a function of # switching of the corresponding bits. The stray field is averaged over all bits that show the same number of switching events. Error bars indicate twice the standard error.

Fig. 5. Magnetization distribution.

a Schema of the magnetic field obtained from a simulation for 7 pillars in different states (P or AP). The dashed horizontal line corresponds to the NV flying distance. b NV maps obtained by simulations for 80% and 100% of the bits in the P state. c Magnetic field distributions of maps in (b). d Percentage of P bits versus μ₀H_switch for Process 2 (dots) and for the simulation (dotted line) e Density histogram obtained after repeating the measurement 10 times at 50% of switching probability. f Stray field in fully initialized simulated maps, ΔB_NV,max, as a function of # switching of the corresponding bits. The stray field is averaged over all bits that show the same number of switching events. Errorbars indicate twice the standard error.