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Graphene Inductors for High-Frequency Apps

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December 15, 2014, IEDM, San Francisco—Kaustav Banerjee from UCSB described recent work in the design, fabrication, characterization and studies of skin effect in graphene inductors. The work

In many areas, inductors are used for frequency control functions in filters, oscillators, and switches. All of these applications are critical in communications operations, and many applications need smaller devices to meet reduced form factor requirements in portable equipment. In addition, inductors are being applied to flexible substrates making it harder to use metals for the inductor.

As a result, investigations into other materials shows that graphene meets the needs for strength, flexibility, and conductivity. Graphene can also be used for other circuit elements, to make the overall integration less complex. Other efforts have shown that graphene can be integrate with CMOS and with other graphene circuit functions. The few applications where carbon nano-tubes have penetrated are also candidates for a change to graphene.

Designing, simulating, and fabricating prototypes of inductive components is difficult. The necessary high Q factors require low loss and good high-frequency stability. A part of the issue is that SiO2 is not the best substrate for high Q inductors. A prototype device for operation at 100 GHz is 400 microns long by 2 microns wide.

Design and characterization are not easy, as measurements require de-embedding the test structures and optimization becomes an iterative process due to the many parametric interactions of the materials and substrate. One issue is that even these small inductors exhibit skin effects at frequencies above 40 GHz. The challenge is in developing a full 3-D solid model that accurately reflects the changing cross-section areas with frequency and topology.

A part of the design work is to improve Q factors by reducing contact resistance. Some possible techniques include an anneal process, performing more dielectric engineering, and consider doping the graphene. All of these modifications should reduce resistance, which improves Q. Other changes in designs will address configurations besides flat spirals such as half turn and stacked designs.
 

 


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