Skip to main content

Spintronic-Photonic Integrated Circuit platform for novel Electronics

Periodic Reporting for period 2 - SPICE (Spintronic-Photonic Integrated Circuit platform for novel Electronics)

Reporting period: 2017-10-01 to 2019-03-31

We have entered the Zettabyte era: the world currently generates tens of billions of Terabytes per year, through new technology paradigms like Big Data, the Cloud and the Internet-of-Things. Unfortunately, storing these data comes at a steep cost. Every time a bit is stored, energy is consumed. As a result, these Zettabytes of data contribute significantly to our global energy consumption. Since this number will grow tenfold by 2026, it is of vital importance to decrease the energy consumption of data storage.
Currently, various memory technologies are used, including magnetic storage, like hard disks, and electronic, silicon based memories, like flash and random-access-memory. Some technologies are non-volatile, which means they do not consume power once the data is stored, but these typically consume a lot of power during writing and are slow. Many fast technologies are often volatile.
It would be ideal to have one type of memory, one that is fast and cheap, low-power, and non-volatile. Spintronic memory is based on magnetics, and can potentially combine this, but this has not been achieved yet.
A promising trend is the discovery of switching the magnetization in thin magnetic films by short laser pulses, at very low energy. This approach presents a completely new way to write data into spintronic memory using light instead of a current, and promises terabit-per-second bandwidth, at unprecedented low energy consumption. The open question is how to realize such an approach on an integrated circuit.
SPICE addresses this question by proposing a novel technology platform that integrates photonics and spintronics. Firstly, new magneto-optic materials will be developed, that can switch under illumination. These will then be integrated into spintronic memory elements. Secondly, energy-efficient optical networks on a chip will be realized, to write data into these elements. The overall objective is to improve the speed and energy consumption of current memory by two orders of magnitude, a decade from now.
At this stage, we have laid the foundation for the SPICE technology, and have started its implementation, towards the overall goal of a fully integrated experimental demonstrator. Work has focused on five key areas.

The first area is the development of magnetic materials that can be optically switched. Various materials have initially been explored, showing polarization-based switching, and toggle-based switching. Work has converged on using a material stack based on an optically-switchable Tb/Co multilayer, and a conventional ferromagnetic electrode FeCoB layer. This stack is promising as it can offer, in principle, both the required tunnel magnetoresistance, and can be switched optically. First results show that this is indeed the case, and an ongoing in-depth study maps these effects as a function of layer thickness and anneal temperature.

The second area is the fabrication of the memory elements. A process flow was first established and validated using chips with a conventional layerstack. Secondly, indium tin oxide was selected for the top contact, to allow for optical access, based on transparency and electrical conductivity values. Finally, the deposition process of indium tin oxide and the process to fabricate magnetic tunnel junction (MTJ) elements with optically transparent top contacts, was established. At this stage, the process flow has been validated with electrical measurements, using a conventional layerstack, showing similar behavior as with conventional (Cr/Al) contacts.

The third area is the development of the photonic distribution layer. An energy-efficient switch network for the optical pulses was developed, based on optimized silicon pin modulators. Gated operation, to reduce the energy consumption, was experimentally achieved, and it was calculated that such networks could work at 100 fJ-per-bit levels. Elements to couple the optical pulse from the distribution layer into the memory element were designed and realized, with close to diffraction-limited focal spots, to allow for compact memory elements. Active control of the polarization was achieved.

The fourth area is the development of design tools. A material simulation software tool has been further optimized for spintronic applications. The tool has been applied to calculating, o.a. the temperature dependence of the tunnel magnetoresistance. A simulation flow, including this tool, and other, open-access, tools, was established, to include magnetization dynamics. These results can then be exported to circuit-level electronic-design-automation tools. Work on the latter topic has started.

These results represent a solid foundation for our fifth area, namely the combination of these results into an integrated platform. A theoretical assessment of various architectures has been done, based on the abovementioned technologies, showing the feasibility of the SPICE technology for 100s of fJ-per-bit switching energies. Alternative applications of the SPICE platform, e.g. in sensing and microwave technology, are now being explored.
The first interesting and potentially high-impact results are now emerging. These results, our expectations towards the end of the project, and their impact, are the following:

• We have achieved optical switching and tunnel magnetoresistance in Tb/Co multilayers with conventional ferromagnetic electrodes. We are working now on achieving both, with good performance, in a single process flow. Achieving this implies that for the first time we have realized a practical material platform for optically-controlled MTJs.
• A novel and unique process can add optically transparent top contacts to an MTJ. We are now implementing this on MTJs containing Tb/Co multilayers. Achieving this means that we have established the full process for optically-switchable, and electrically-readable MTJ elements.
• We have realized energy-efficient switch networks and grating couplers in a silicon photonics platform. Using resonant modulators, we expect to drive down energy-consumption even further. The impact is that we will have a CMOS-compatible way of controlling the optical pulses, at low energy consumption, and potentially high bitrates.
• For the simulation effort, we have now set up a flow of commercially-available and open-source tools, to be able to simulate MTJs from the physics to the circuit level. The compatibility with photonic design tools implies a full spintronic-photonic-electronic co-simulation environment.
• A theoretical assessment has shown that, based on existing and available technologies, the SPICE memory technology can be competitive with selected existing memory technologies, in terms of bandwidth and power consumption, albeit not in footprint. Further simulation work, including novel approaches, like plasmonics, is expected to show a further decrease in energy-consumption and footprint. The impact of such a result is that, for the first time, the feasibility of optically-switched MTJ-based memory technology will be shown.

Bringing this all together successfully shows the full impact of SPICE: we will have realized a technology that has a clear path out of the lab, due to the compatibility of the used processes with mature technologies, and that has a clear motivation to go out of the lab, due to the competitiveness with respect to applications. This implies that the SPICE technology can be made widely available, in principle.
SPICE concept for photonic-spintronic integration