Publication in pre-print AIP/123-QED A quantum sensing metrology for magnetic memories Vicent J. Borràs, 1, a) Robert Carpenter, 2 Liza Žaper, 1, 3 Siddharth Rao, 2 Sebastien Couet, 2 Mathieu Munsch, 1 Patrick Maletinsky, 3 and Peter Rickhaus 1, b) 1) Qnami AG, Muttenz 2) Imec, Kapeldreef 75, 3001 Leuven 3) University of Basel, Departement of Physics (Dated: 28 June 2023) 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, the scaling of MRAM technologies is heavily affected by device-to-device variability rooted in the stochastic nature of the MRAM writing process into nanoscale magnetic layers. 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. Unlike previous methods, our approach unveils marked differences in switching behaviour of fully contacted MRAM devices stemming from these processes. 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. I. INTRODUCTION The ability to store information magnetically has been a major enabler of the digital revolution 1 . To improve stor- age density, and energy efficiency, magnetic bits have be- come smaller and denser 2 , presenting an ever-increasing chal- lenge for metrology tools – one that is intensifying with the push towards exotic materials, such as 2-D and antiferromag- netic magnets, which have small surface moments 4,5 . Emerg- ing, ultrasensitive, nanoscale magnetic quantum sensors 6 of- fer a unique opportunity to address this challenge due to their highly competitive sensing characteristics. To demonstrate the advantage of such quantum meteorology in an industrially rel- evant context, and scale, Spin-Transfer Torque-MRAM (STT- MRAM) is an ideal candidate. STT-MRAM is one of the most promising next generation, non-volatile memory architectures and one that is already in production 7,8 . It is constructed around a Magnetic Tunnel Junction (MTJ), consisting of two magnetic layers, the Free Layer (FL) and the Reference Layer (RL), and a tunnel barrier. The role of the FL is to act as the storage layer, while the RL generates a spin-polarised current. It is this spin-polarised current that, via Spin Transfer Torque (STT), can switch the FL and read the bit state via the Tunnel- ing Magnetoresistance Ratio (TMR) effect 9,10 . Due to the interfacial nature of both the TMR and STT ef- fects, uniformity is a significant challenge for STT-MRAM. This is further exacerbated by the nanoscale dimensions of the magnetic layers, with typical target layer thicknesses, and bit sizes, of < 2 nm and < 60 nm, respectively. In particu- lar, the data retention, related to the energy barrier required to change the magnetic orientation of the FL, is defined by the aniostropy and volume of the layer. Therefore, the en- ergy to erase, or write, the FL is volumetric and thus scales in a) Electronic mail: vicent.borras@qnami.ch b) Electronic mail: peter.rickhaus@qnami.ch Process flow Front end Bottom contact MTJ deposition etching encapsulation annealing Top contact Passivation EOL field MOKE Electrical testing CIPT Process monitoring 0 5 10 15 20 25 30 35 40 bit column number 0 5 10 15 20 25 30 35 40 bit row number −50 0 50 100 B NV (µT) NV Parallel (P) Anti-Parallel (AP) Scanning NV magnetometry a b c FIG. 1. a) Process flow of STT-MRAM fabrication, including pro- cess monitoring. b) SNVM map of 45x45 bits (10x10 um) after en- capsulation. 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 mea- sures their projection onto the NV quantization axis (black arrow) at the flying distance of the NV probe. quadrature with the diameter 11 . This makes device variability 1 arXiv:2306.15502v1 [cond-mat.mtrl-sci] 27 Jun 2023