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 replacements. 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 outstanding candidate is the so-called spin-orbit-torque (SOT) induced switching, where the magnetization dynamics occur via a current induced spin-orbit coupling.
State-of-the-art and challenges
In SOT-MRAM devices the ferromagnetic (FM) storage layer is in contact with a non-magnetic heavy metal (HM) stripe such as Ta, W, or Pt. When a current flows through the stripe, a perpendicular spin current is generated and transferred to the magnetization of the storage layer, creating a spin torque and inducing magnetization reversal. That spin current originates from the Spin hall effect (bulk of HM) and from the Rashba interaction (FM interfaces). To enable SOT-MRAM as viable technology, several challenges need to be overcome. Firstly, one needs to increase the efficiency. Therefore, alternative SOT materials are being researched, among them topological insulators and 2D materials. Next to the material properties, controlling the interface properties between the SOT metal strip and the storage layer is of equal importance to maximize spin transmission efficiency.
A second key challenge is to a free layer structure that is both compatible with standard MgO-based tunnel junctions AND the specific SOT material. Interfacing with advanced materials such as topological insulators is expected to require the development of new magnetic free layer laminated structures.
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. Exploratory materials 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 a SOT electrical characterization setup.
Key to the Ph.D research is to:
- study the growth of advanced SOT/FM materials
- characterize their structural, magnetic and transport properties
- study their compatibility with perpendicular magnetic tunnel junctions
- integrate them in SOT-MRAM devices 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 SOT based technology with focus on the materials and the deposition processes, in particular, and secondly SOT-MRAM stack designs. In the second phase, the growth and characterization of selected material sets will be initiated with focus on controlled deposition of FM on SOT or SOT on FM, and characterization of SOT via Hall bar structures. In parallel, micromagnetic modeling (OOMMF, mumax) will be performed to guide the materials development for the selected device designs or to study the feasibility of new device designs. 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 science, Physical science, Material Science
Type of work: 70% experimental, 20% modeling/simulation, 10% literature
Supervisor: Jo De Boeck
Daily advisor: Sebastien Couet
The reference code for this position is 2020-015. 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: CSC2020-08.