Despite the promise of memristors and CMOS hybrid circuits, the device performance of memristors needs to be compatible with the CMOS circuit. Planar device geometries can be used to eliminate kinks at device junctions and achieve desired operation conditions. The researchers show that the planar device can greatly reduce the operation voltages and significantly improve the switching uniformity. These are both of great importance to the hybrid circuit. They further demonstrate that such planar devices can be integrated directly on top of the CMOS circuit and exhibit consistent performance.

Planar geometry

The planar device geometry is confirmed by surface profiling using atomic force microscopy (AFM). Precise process control, such as dry etching conditions and metal deposition rate, is the key to achieving such a planar geometry. The statistical analysis of device performances with different geometries shows substantial improvement by utilizing the planar geometry. Meanwhile, lower forming and switching voltages and better switching uniformity are further achieved by incorporating thicker electrodes. This is only feasible when using the planar geometry. The thicker electrodes also reduce the series resistance on the long interconnects in the hybrid circuit so that faster circuit speed and lower power consumptions can be achieved.

A demonstration of 3D integration of planar memristors with a CMOS substrate is achieved using nanoimprint lithography. The bottom electrodes of the memristors are completely embedded in the CMOS’s passivation layer. The memristor array and CMOS circuit are connected through the tungsten vias with good alignment between different layers provided by the high-resolution alignment marks. The planar devices inside the hybrid circuit show consistent device performance with low operation voltages and a high ON/OFF ratio.

This study proves that planar geometry can be very helpful to improve the compatibility of memristors inside a memristor/CMOS hybrid circuit. The new structure paves the way for the implementation of more powerful hybrid circuits.

More information about the research can be found in the journal Nanotechnology 25 405202.

Further reading

Kubo response theory applied to memristive, memcapacitive and meminductive systems (July 2013)
Logic operations on a nanomagnetic staircase (Nov 2013)
Memcapacitors could make our computers faster (June 2014)
Planar structure extends lifetime of memristor (June 2011)