As the demand for faster, smaller, and more power-efficient devices is increasing, conventional memories such as SRAM and DRAM are reaching their scaling limits. New emerging memories are being developed and magnetic random-access memory (MRAM) is considered as one of the most promising replacement. This non-volatile memory is considered as next generation of consumer mass-scale application of spintronics. Spintronics is a rapidly growing field of solid-state electronics that aims to take advantage of the spin of electrons, in addition to their electric charge, as a way of implementing new electronic functions. While spin-transfer-torque (STT) is the key mechanism in the current MRAM technology to switch the direction of the magnetization of the storage layer in the tunnel junction, alternative switching mechanisms are being researched to increase the switching speed and reduce the power consumption. One of the potential candidates for low power switching is the voltage control of magnetic anisotropy (VCMA), which requires breakthroughs in both high-K oxide research and in fundamental understanding of their interface with (MRAM compatible) magnetic material.
State-of-the-art and challenges
The VCMA effect appears at an oxide/ferromagnet interface under applied voltage, and lead to a modification (increase or decrease) of the magnetic anisotropy. In a VCMA-MRAM device, this mechanism is used to dynamically re-orient the ferromagnet from perpendicular to in-plane magnetization. This phenomenon, together with a proper timing of the voltage pulse, enables to switch the free layer magnetization from up to down in a deterministic way, and with much lower power than traditional STT-MRAM.
The VCMA effect, quantified via the VCMA coefficient, is typically small, requiring either a (too) large applied voltage or limiting the retention characteristic of the magnetic bit for practical applications. This coefficient depends both on the nature of the oxide and ferromagnetic material used, as well as the crystalline quality of the interface. It should be noted that there is currently still very little understanding of the relationship between the individual oxide and ferromagnet structure, the interface properties and the VCMA coëfficiënt.
Hence, there exist both a technical and fundamental challenge to find more efficient high-K oxide/ferromagnets AND understand how their properties influences the VCMA effect.
Experimental details & Methodology
To develop MRAM devices, imec uses a dedicated 300-mm wafer platform. The magnetic nanolaminate and tunnel junction deposition is carried out on an industry relevant 300 mm PVD cluster platform. Imec is also equipped with a variety of CVD and/or ALD deposition tools for the growth of various (high-K) oxide. However, the material exploration will be researched both on the 300mm cluster as well as via MBE or PVD internally or in collaboration with other laboratories. An extensive toolset for structural, magnetic and electrical characterization is available including: XRR, XRD, VSM, Magneto-optical Kerr, current-in-plane tunneling and several electrical characterization setup.
Key to the Ph.D research is to:
- study the growth of advanced (High-K) oxide/FM materials
- characterize their structural, magnetic and transport properties
- study their compatibility with perpendicular magnetic tunnel junctions
- integrate them in a functional VCMA-MRAM device for the sub 1X nm technology nodes.
In a first phase, the student will perform an extensive literature search firstly on the state-of-the-art of VCMA based technology with focus on the materials and the deposition processes. In the second phase, the growth and characterization of selected material sets will be initiated with focus on controlled deposition of FM on oxide, magnetic and structural characterization, and finally characterization of the VCMA coefficient using dedicated hall bar test structures. In parallel, micromagnetic modeling (OOMMF, mumax) can be performed to study the VCMA switching mechanism in link with (potentially) more advanced free layer structure for VCMA. Finally, the newly developed material or stack design will be integrated using imec's 300mm device flow and the correlation between the blanket properties, and properties at scaled dimension will be studied.
Required background: Engineering technology, Material science, Physics or equivalent
Type of work: 80% experimental, 10% modeling, 10% litterature
Supervisor: Kristiaan Temst
Daily advisor: Sebastien Couet
The reference code for this position is 2020-016. Mention this reference code on your application form.
Chinese nationals who wish to apply for the CSC scholarship, should use the following code when applying for this topic: CSC2020-09.