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Precision measurements for magnetic memory

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Posted May 2, 2016

A new measuring procedure can pave the way for an optimised and reliable production of magnetic random access memory (MRAM), a potentially universal memory solution within all fields of memory applications.

MRAM is a memory type that combines the advantageous characteristics of three important memory solutions used in consumer electronics. This includes the high density of DRAM (dynamic RAM), the high speed of SRAM (static RAM) and the nonvolatile (i.e. the ability to retrieve stored information even after having been turned off and back on) storage of Flash memory.

The active part of an MRAM cell is a so called magnetic tunnel junction which uses the magnetic properties of electrons, i.e. the electron spin, to read and write magnetic information in the memory cell. The most critical part in such a magnetic tunnel junction is the electron transport properties across a dielectric layer, e.g. magnesium oxide, with a thickness of just 1 nm (a few atomic layers). Through the past decade these properties have been characterised using a measurement system developed by the DTU Nanotech spin-off company Capres A/S.

Towards a large scale production of MRAM
To produce a well-functioning memory device with hundreds of millions of MRAM cells, it is necessary to monitor precisely the electrical properties and as MRAM enters into large scale production, the requirements on metrology increase significantly. Measurement speed, precision and reliability must be improved compared to previous generations of metrology systems. Also, measurements must be possible on test structures available in production that may differ from test samples in product development. These issues have been addressed in a number of collaboration projects between Capres A/S and DTU Nanotech.

Senior Researcher Dirch Hjorth Petersen explains: “By detailed analysis of the metrology system, we have identified the most critical parameters and optimised the measurement routines accordingly. With this, we have been able to reduce measurement time by a factor of 5 and we have developed a methodology for a product specific measurement procedure”.

The customised procedure yields significant improvements in measurement precision as compared to previous state-of-the-art measurements. The measurement reliability is being improved through use of vibration tolerant electrodes for superior measurement stability, and early tests are encouraging. Finally, a model has been developed to allow measurements in small test pads relevant for production monitoring.

Co-founder and CTO at Capres A/S, Peter Folmer Nielsen states: “We expect the combination of improvements to pave the road for fully automated monitoring of product quality during MRAM production”.

The improvements have been achieved through collaboration between Industrial Postdocs Alberto Cagliani and Frederik W. Østerberg, former Industrial PhD student Daniel Kjær, as well as Senior Researcher Dirch H. Petersen and Professor Ole Hansen.

Some of the measurements of the electron transport properties across a dielectric layer are illustrated in the figures below.

Figure 1: Illustration of a four-point probe measurement on a multi-layered thin film consisting of two ferromagnetic layers separated by a dielectric layer. The measured resistance, R4PP, depends on the electrode pitch, s, and a characteristic length scale, λ, which determines a transition between measurement of the surface layer only and the parallel resistance of the two ferromagnetic layers. From four-point measurements with variable pitch, the resistance across the dielectric barrier can be calculated.

Figure 1: Illustration of a four-point probe measurement on a multi-layered thin film consisting of two ferromagnetic layers separated by a dielectric layer. The measured resistance, R4PP, depends on the electrode pitch, s, and a characteristic length scale, λ, which determines a transition between measurement of the surface layer only and the parallel resistance of the two ferromagnetic layers. From four-point measurements with variable pitch, the resistance across the dielectric barrier can be calculated.

Figure 2: By changing the magnetic polarisation of one of the ferromagnetic layers, the resistance across the dielectric barrier is modified and the characteristic length scale changes. This is used to evaluate the magnetoresistance of the magnetic tunnel junction.

Figure 2: By changing the magnetic polarisation of one of the ferromagnetic layers, the resistance across the dielectric barrier is modified and the characteristic length scale changes. This is used to evaluate the magnetoresistance of the magnetic tunnel junction.

Source: DTU

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