2D Materials for 3D Electronics

We showed that 2-D materials like Tungsten Selenide can realize both transistors and resistive memories, and proposed the possibility to realize highly-scaled 3-D one-transistor one-resistor (1T1R) memories.

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Memory is dominating electronics in the era of data-abundant computing. Fed by our gluttony for more HD photos, videos, and data analytics, there is an escalating memory capacity need to enable our next smart mobile devices and cloud servers. Big Data, AI, and IoT are impossibilities without memory technology innovation. 

However, with the abundant flow of data comes the inevitable bottleneck. Fetching data from memory to the processor and back, on our semiconductor chips, costs significant energy, and it may even be greater than the energy needed to compute the data. Therefore, there has been a sustained effort to bring the memory physically closer to the on-chip circuits that perform the computations. As Moore’s law brings us ever closer to the logical conclusion of 2-D planar circuit density scaling, further component density increase will have to leverage 3-D circuits technologies. Stacking, layer-upon-layer, memory devices and systems above our Silicon chip, is where memory-centric processing converges with 3-D circuit scaling.

However, to achieve that we must overcome a process roadblock that originated from the time when we moved from Aluminum to Copper on-chip wires, back in the late 90’s. Copper interconnect wires, surrounded by low-dielectric oxides cannot tolerate temperatures above 400oC, else copper atoms extrude into the surrounding dielectrics, compromising the reliability of the wires. Since dopant activation in Silicon typically requires temperatures well above 400oC, we have to  build the Silicon devices first, followed by the interconnect metallization and dielectrics (Fig.1 (Top)). Somewhat like baking a layer cake with embedded cream layers. Hence, a cardinal rule of current semiconductor chip process technology is that Silicon and Metal Wires: Thou shalt not mix the two! At least, don’t swap the Front End (transistor) and Back End (interconnect) process order. Making Silicon transistors above the metal interconnect layers for 3-D Silicon circuits compromises the devices and wires.  Hence, we are motivated to search for new low-thermal budget beyond-Silicon materials and processes for transistors and memory devices, compatible with the interconnect processing.

Our group at the National University of Singapore explores such materials and processes for Monolithic 3-D integration of memory (Fig.1 (Bottom)). We showed that a 2-D form of Transition-Metal Dichalcogenide material like Tungsten Selenide (WSe2) can be used as 2-D semiconductors to form both transistors as well as resistive-based memories. A challenge typical to many 2-D material transistor is the lower effective carrier mobility and high contact resistance; which makes them weaker than well-optimized Silicon transistors. In our work, we report on techniques to overcome the handicap at both the device level, circuit, and system levels. We show that room-temperature plasma oxidation process can boost the transistor drive performance by 10x. Combined with channel stacking, we propose the possibility of realizing highly-scaled one-transistor one-resistor (1T1R) memory cells. Please check out our work published this month: “All WSe2 1T1R resistive RAM cell for future monolithic 3D embedded memory integration” Nature communications 10 (1), 1-12 , (2019) (doi:10.1038/s41467-019-13176-4).

Aaron Voon-Yew Thean

Professor, Electrical & Computer Engineering, National University of Singapore

Aaron Thean is a Professor of Electrical and Computer Engineering and Dean of the School of Engineering at the National University of Singapore. In addition, he holds several technical leadership responsibilities at the University; which includes Director of Applied Materials – NUS corporate research lab, HiFES research program on Hybrid Flexible Electronics, NUS’s Nanofabrication Centre, E6Nanofab. Prior to NUS, Aaron was the Vice President of Logic Technologies at imec in Belgium. Working with Semiconductor Industry leaders like Intel, TSMC, Samsung, and Globalfoundries. He directed the research and development of next-generation semiconductor technologies and emerging nano-device architectures. Prior to joining IMEC in 2011, he was with Qualcomm’s CDMA technologies in San Diego, California. From 2007 to 2009, Aaron was the Device Manager at IBM, where he led an eight-company process technology team to develop the 28-nm and 32-nm low-power bulk CMOS technology at IBM East Fishkill, New York, from research to risk production. Aaron graduated from the University of Illinois at Champaign-Urbana, USA, where he received his B.Sc. (Highest Honors), M.Sc., and Ph.D. degrees in Electrical Engineering. He has published over 300 technical papers and holds more than 50 US patents. Active in local and international advanced electronics communities, Aaron is an Editor of the IEEE Electron Device Letters and he serves on several Scientific Advisory Boards that include Singapore-MIT Alliance (SMART-LEES), A*Star Institute of Microelectronics (IME), and he is Consulting CTO for Process Technology to imec CEO.


Go to the profile of Jason Tang
about 2 years ago

This work is really important and thought-provoking.