In an article recently published in the journal Npj Spintronics, researchers investigated the feasibility of nanoscale quantum sensing of magnetic random access memory (MRAM) devices for early-stage screening in the manufacturing process.
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. Image Credit: https://www.nature.com/articles/s44306-024-00016-5
Importance of Quantum Sensing Metrology
The evolution of magnetic data storage has been a pivotal force in driving the digital revolution. As magnetic bits are engineered to be denser and smaller, enhancing energy efficiency and storage density, new challenges emerge for metrology tools. Fortunately, cutting-edge nanoscale and ultrasensitive magnetic quantum sensors are well-suited to tackle these challenges due to their superior sensing capabilities. Within the industrial realm, spin transfer torque-MRAM (STT-MRAM) stands out as an exemplary application of this quantum metrology.
STT-MRAM is a promising non-volatile, next-generation memory architecture currently in production for replacing embedded flash at N2X nodes. However, uniformity is a major challenge for STT-MRAM owing to the interfacial nature of STT and tunneling magnetoresistance (TMR) effects.
The minuscule dimensions of the magnetic layers, with target thicknesses below 2 nm and bit sizes smaller than 60 nm, intensify these challenges. Consequently, precise control over device property distributions is vital for broadening the applications of STT-MRAM, which includes accessing and analyzing out-of-distribution bits for failure analysis.
Current metrology techniques such as in-plane tunneling (CIPT) and magneto-optical Kerr effect (MOKE) magnetometry, though widely utilized, fall short in measuring the magnetization of individual bits.
Scanning nitrogen vacancy magnetometry (SNVM) emerges as a solution, offering exceptional magnetic field sensitivity and spatial resolution that enable the detailed characterization of each bit's magnetic properties. This level of detail is crucial for advancing the development and understanding of STT-MRAM, marking a significant advancement in memory technology.
The Proposed Approach
In this study, researchers introduced a non-contact metrology technique deploying SNVM to evaluate MRAM performance at the individual bit level. Specifically, magnetic reversal characterization was demonstrated in individual, less than 60 nm-sized bits to obtain key magnetic properties, switching statistics, and thermal stability to evaluate bit-to-bit uniformity.
Two distinct bit etching processes were benchmarked immediately after pattern formation to demonstrate the proposed method's performance. SNVM was used to estimate the retention and characterize the two etch processes' bit-to-bit uniformity. Additionally, researchers described and applied a statistical method suitable for analyzing the switching uniformity depending on the SNVM data.
The SNVM technique is based on a naturally occurring NV center in a diamond pillar's apex, as the NV center shows high sensitivity to external magnetic fields. The local magnetic stray field pattern is recorded while scanning the SNVM pillar over an MRAM array.
This measurement of the stray field is conducted at a distance of 150 nm from the free layer (FL). This distance encompasses the hard mask, cap, encapsulation, and approximately 50 nm representing the flying distance of the nitrogen vacancy (NV) center above the surface. At this specific distance, the signal that is measured corresponds to the stray field vector. This vector is projected onto the NV-axis, which is oriented with an angle of approximately θ ≈ 0° relative to the x-axis in the x-y plane and is tilted by φ = 54.5° with respect to the z-axis.
The SNVM measurements were performed on dies diced from the 300 mm wafer using a diamond saw with 45 μm accuracy, and the Qnami ProteusQ microscope was employed to record the SNVM maps.
Importance of this Work
Results showed that the SNVM was an effective technique for measuring and characterizing MRAM device properties in the early stage of the manufacturing process, immediately after the devices' etching step. High-resolution imaging of MRAM pillars with industrially relevant diameters was realized, even when the encapsulation layers were present.
In contrast to ensemble averaging methods like perpendicular MOKE, the proposed method could identify out-of-distribution bits that were associated with the array edges, enabling failure analysis of tail bits. All data was obtained with single-pillar resolution, which cannot be achieved using existing in/off-line metrology techniques.
Although both etching processes introduced notable bit-to-bit variations, the optimization of the etch process significantly enhanced uniformity. Remarkably, a single NV stray field map proved adequate to effectively measure this improved uniformity. The high-quality SNVM imaging facilitated a quantitative analysis of bit uniformity by assessing the switching behavior from a solitary stray-field map. This measurement of switching behavior could also be used to determine thermal stability with precise single-bit level sensitivity.
The outcomes of this study underscore the potential of nanoscale quantum sensing in the early-stage screening of MRAM devices within the processing line. Crucially, these results align with previous electrical characterization studies of similar processes, reinforcing the validity of the findings. This consistency highlights the critical role of SNVM as a valuable in-line characterization tool in the advancement of memory technology.
Journal Reference
Borràs, V. J., Carpenter, R., Žaper, L., Rao, S., Couet, S., Munsch, M., Maletinsky, P., Rickhaus, P. (2024). A quantum sensing metrology for magnetic memories. Npj Spintronics, 2(1), 1-7. https://doi.org/10.1038/s44306-024-00016-5, https://www.nature.com/articles/s44306-024-00016-5
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