Salahuddin Raju

Modeling of Mutual Coupling Between Planar Inductors in Wireless Power Applications

This paper presents a compact model of mutual inductance between two planar inductors, which is essential to design and optimize a wireless power transmission system. The tracks of the planar inductors are modeled as constant current carrying filaments, and the mutual inductance between individual filaments is determined by Neumanns integral. The proposed model is derived by solving Neumanns integral using a series expansion technique. This model can predict the mutual inductance at various axial and lateral displacements. Mutual coupling between planar inductors is computed by a 3-D electromagnetic (EM) solver, and the proposed model shows good agreement with these numerical results. Different types of planar inductors were fabricated on a printed circuit board (PCB) or silicon wafer. Using these inductors, wireless power links were constructed for applications like implantable biomedical devices and contactless battery charging systems. Mutual inductance was measured for each of the cases, and the comparison shows that the proposed model can predict mutual coupling suitably.

Silicon-Embedded Receiving Coil for High-Efficiency Wireless Power Transfer to Implantable Biomedical ICs

In this letter, a silicon-embedded receiving coil is designed and fabricated for high-efficiency wireless power transfer to implantable biomedical ICs. The 4.5 mm × 4.5 mm embedded receiving coil achieved a large inductance of 4 μH and a high peak quality factor of 20 at 2.8 MHz. Measurement results of an inductive power link using the embedded receiving coil and a conventional printed-circuit-board transmitting coil (2 cm × 2 cm) demonstrated peak voltage gains of 0.84 and 0.24 and peak efficiency values of 30% and 4.3% for separation distances of 5 and 12 mm, respectively. This is the best reported wireless power transmission efficiency for separation distance similar to the implant chip size.

Wireless power link design using silicon-embedded inductors for brain-machine interface

This paper discusses the safety requirements, equivalent circuit model, and design strategy of wireless power transmission to neural implants. The most daunting challenge is the design of the integrated receiving coil on the implantable device whose size must be within the safety and regulation limits while providing sufficient power transfer and efficiency. A novel silicon substrate-embedded 3.6-μH spiral inductor has been designed to fit inside a 4.5 mm × 4.5 mm implantable IC as the receiving coil. Full-wave EM simulations show that in a practical brain-machine interface setting, wireless power in the range of 1-10 mW can be delivered at 5% efficiency to an implant at 1 cm below the head surface using signals between 2 to 5 MHz. To achieve a high transfer efficiency, the optimal impedance for loading the receiving coil is derived using the equivalent circuit parameters of a realistic 3D model of the entire wireless power link. The large parasitic capacitance of the “in-chip” inductor is methodically absorbed in the matching network to maximize the efficiency and power transfer.

Carbon Nanotube Contact Plug on Silicide for CMOS Compatible Interconnect

Carbon nanotube (CNT) filled contact plugs with silicide as the bottom electrode have been demonstrated in this letter. Nickel has been used as the main catalyst to achieve CMOS compatibility. By modifying the catalyst from a pure Ni single layer structure to a Ni/Al/Ni multilayer composite, uniform selective CNT growth on Ti silicide substrate has been achieved. At a low temperature of 450 °C, the vertically aligned CNTs with a density of 1.1×1011 tubes/cm2 inside the silicon dioxide contact plug are formed without the need of catalyst patterning. Four-point Kelvin structures are designed to measure the resistance of the CNT filled contact plugs. Measurement results show that an ohmic contact plug resistance of 216 Ω·μm2 is obtained.

Modeling of mutual inductance for planar inductors used in inductive link applications

Prediction of mutual coupling is needed to characterize any inductive link system used in wireless power transfer applications. This paper presents a compact model of mutual inductance between two planar inductors at various separation distances. The proposed model is derived by solving the Neumanns integral using series expansion technique. Numerical and experimental verification have been carried out to assert the accuracy of the model. The measured and predicted mutual inductances show excellent agreement with error less than 5%.

Efficient wireless power transmission technology based on above-CMOS integrated (ACI) high quality inductors

Fully-integrated on-chip inductors with up to 200 nH/mm2 inductance density and a peak qualify factor of 25 are demonstrated based on above-CMOS integration (ACI) post processing techniques. Utilizing a 380-nH ACI inductor, a 2.5×2.5 mm2 wireless energy harvesting antenna was implemented. Measurement results show that it can receive 27 mW from a 250-mW transmitting power source at a distance of 5.3 mm. This represents a 7-fold increase in wireless power transfer efficiency compared to other reported technologies.

Low Frequency Behavior of CVD Graphene from DC to 40 GHz

Electromagnetic behaviour of chemical vapor deposition (CVD) graphene at low frequencies is still a mystery. No conclusion is made from the experimental point of views. We systematically investigate the electromagnetic response of graphene at microwave frequencies, which are from direct current (DC) to 40 GHz. Both a coplanar transmission line embedded with different-sized graphene flakes of 48 × 48 and 48 × 240μm2 and a microwave termination based on the graphene sheet of 6 × 6 mm2 are manufactured through standard microfabrication procedures. We conclude that CVD graphene behaves as a frequency-independent surface resistance at the microwave frequencies, which is consistent with the theoretical model by rigorously solving the Maxwell’s equations with the Kubo formula. The work offers a simple, accurate, and conclusive electromagnetic analysis to graphene and thus is of great help to design graphene incorporated microwave components and devices.

Synthesis and interface characterization of CNTs on graphene

Carbon nanotubes (CNTs) and graphene are potential candidates for future interconnect materials. CNTs are promising on-chip via interconnect materials due to their readily formed vertical structures, their current-carrying capacity, which is much larger than existing on-chip interconnect materials such as copper and tungsten, and their demonstrated ability to grow in patterned vias with sub-50 nm widths; meanwhile, graphene is suitable for horizontal interconnects. However, they both present the challenge of having high-resistance contacts with other conductors. An all-carbon structure is proposed in this paper, which can be formed using the same chemical vapor deposition method for both CNTs and graphene. Vertically aligned CNTs are grown directly on graphene with an Fe or Ni catalyst. The structural characteristics of the graphene and the grown CNTs are analyzed using Raman spectroscopy and electron microscopy techniques. The CNT-graphene interface is studied in detail using transmission electron microscopic analysis of the CNT-graphene heterostructure, which suggests C-C bonding between the two materials. Electrical measurement results confirm the existence of both a lateral conduction path within graphene and a vertical conduction path in the CNT-graphene heterostructure, giving further support to the C-C bonding at the CNT-graphene interface and resulting in potential applications for all-carbon interconnects.

Ultralow-

An interlayer dielectric with an extremely low dielectric constant of 1.96 is demonstrated using SiO2 with vertically aligned cylindrical pores. Vertically grown carbon nanotubes are used as a template for the cylindrical pores to achieve high porosity while maintaining structural stability. Measurements show that an elastic modulus of 17.5 GPa can be maintained even at 65% porosity to provide sufficient mechanical strength for most back end of line processes. This represents a 94% improvement in mechanical stability compared with the state-of-the-art low-k dielectrics. The tradeoffs between dielectric constant and elastic modulus of different porous structures have also been studied to project the ultimate achievable k-value.

k

This works investigates temperature driven stability of super-high-frequency surface acoustic wave (SAW) devices. The interdigitated transducers (IDTs) were fabricated using electron beam lithography on a single crystal piezoelectric (PE) substrate to achieve SAW wavelength down to 400 nm. The temperature compensation was achieved by using a SiO2 capping layer on top of IDTs. The fabricated SAW devices exhibit a low temperature coefficient of frequency (TCF) of ∼ −20 ppm/°C while operating at 2–8 GHz.