Hongbin Yu

Tuning the permeability of permalloy films for on-chip inductor applications

In this work, we demonstrate that the permeability of 1u2002μm thick NiFe/Cr multilayer films can be tuned by varying the thickness of individual NiFe layers while keeping the total multilayer film thickness constant. Our results show that the permeability can be increased from 170 to 650 by varying the NiFe thicknesses from 100 to 50 nm. The thickness dependant permeability variation was attributed to the presence of stripe domains, as examined by hysteresis measurements and magnetic force microscope imaging. Films with thinner NiFe layers coupled with on-chip spiral inductors enable an inductance increase by a factor of 6.

ZnO nanowire based visible-transparent ultraviolet detectors on polymer substrates

The fabrication and characterization of fully visible-transparent and flexible ultraviolet (UV) detectors, on polyethylene 2,6-naphthalate (PEN) with active channels of zinc oxide nanowires and ohmic indium tin oxide contacts, are reported and discussed. The fabricated detector has an average transmittance of 80% in the visible spectral range and is most responsive at or below 370u2009nm, the onset of UV light, with a UV/vis rejection ratio of 1.42u2009×u2009103. A five orders of magnitude difference in the photocurrent, between UV illumination and dark conditions, are also observed. The single-sided UV response further shows that the PEN substrate performs well as a UV reflector. The noise analysis on the nanowire UV detector indicates a noise equivalent power and detectivity (D*) of 5.88u2009×u200910−13 WHz−0.5 and 2.13u2009×u2009109 cm Hz0.5 W−1, respectively.

Performance Enhancement of On-Chip Inductors With Permalloy Magnetic Rings

We demonstrate spiral inductors with continuous Permalloy ring structure at 100-μm scale, which achieve enhancements of sixfold in inductance and threefold in quality factor at frequencies as high as 200 MHz. The roll-off frequency is due to the eddy current loss in the conductive magnetic material. To reduce such loss and tune the permeability, the Permalloy film is laminated and split into bars. The dependence of the inductance and quality factor on permeability is further investigated. Different types of spiral inductors were fabricated with magnetic rings, helping shed light on optimization strategy.

Micro-strain sensing using wrinkled stiff thin films on soft substrates as tunable optical grating.

We report a strain sensing approach that utilizes wrinkled patterns on poly (dimethylsiloxane) (PDMS) as an optical grating to measure thermally-induced strain of different materials. The mechanism for the strain sensing and the effect of PDMS grating on strain sensing are discussed. By bonding the PDMS grating onto a copper or silicon substrate, the coefficient of thermal expansion (CTE) of the substrates can be deduced by measuring the diffraction angle change due to the change in PDMS grating periodicity when thermal strain is introduced. The measured CTEs agree well with the known reference values.

Integrated RF On-Chip Inductors With Patterned Co-Zr-Ta-B Films

Integrated on-chip inductors with boron-incorporated amorphous Co-Zr-Ta-B films for reducing the size of the inductor and increasing the quality factor are presented. A 3.5-fold increase in inductance and a 3.9-fold increase in quality factor over inductors without magnetic films are measured at frequencies as high as 1 GHz. The Co-Zr-Ta-B films are patterned into the shape of fingers in the magnetic via region to improve the on-chip inductors, high frequency response. Compared with nonpatterned films, finger-shaped magnetic vias result in at least a 30% increase in quality factor in the gigahertz range. It is also demonstrated that by using laminations, an up to 9.1X inductance increase with good frequency response up to 2 GHz can be achieved.

Magnetism of Ta dichalcogenide monolayers tuned by strain and hydrogenation

The effects of strain and hydrogenation on the electronic, magnetic, and optical properties of monolayers of Ta based dichalcogenides (TaX2; Xu2009=u2009S, Se, and Te) are investigated using density-functional theory. We predict a complex scenario of strain-dependent magnetic phase transitions involving paramagnetic, ferromagnetic, and modulated antiferromagnetic states. Covering one of the two chalcogenide surfaces with hydrogen switches the antiferromagnetic/nonmagnetic TaX2 monolayers to a semiconductor, and the optical behavior strongly depends on strain and hydrogenation. Our research opens pathways towards the manipulation of magnetic as well as optical properties for future spintronics and optoelectronics applications.

Sub-100 μm scale on-chip inductors with CoZrTa for GHz applications

On-chip inductors with magnetic material are fabricated with complementary metal-oxide semiconductor processes. The inductors use copper metallization and amorphous CoZrTa thinfilms. Enhancements of 3.5X in inductance and 3X for the quality factor at frequencies as highas 3 GHz have been successfully demonstrated by using a continuous CoZrTa-ring structure in spiral inductors at the 100 μm scale. Further improvement of the frequency response of inductance up to 6 GHz was achieved by micro-patterning the magnetic film. The effect ofincreasing the film thickness on the performance of strip line inductors was measured and modeled. This work demonstrates significantly larger increases in inductance and quality factor atabove 1 GHz as compared to prior efforts, thereby making the added processing cost worthwhile.

Determination of Polarization‐Fields Across Polytype Interfaces in InAs Nanopillars

Polarization fields within InAs nanopillars with zincblende(ZB)/wurtzite(WZ) polytype stacking are quantified. The displacement of charged ions inside individual tetrahedra of WZ regions is measured at the atomic scale. The variations of spontaneous polarization along the interface normal are related to strain at interfaces of different polytypes. Thus, direct correlation between local atomic structure and electric properties is demonstrated.

Improved High Frequency Response and Quality Factor of On-Chip Ferromagnetic Thin Film Inductors by Laminating and Patterning Co-Zr-Ta-B Films

In this work, we report the improvement of high frequency response and quality factor of on-chip inductors by integrating ferromagnetic thin films (Co-Zr-Ta-B) including two different combinations of laminations and 500 nm thick non-laminated film. Up to 4.2X increase in inductance and 5X increase in quality factor (Q) was obtained from 4-turn spiral inductors incorporated with 50 nm by 10 laminated films with a peak Q at 500 MHz. Effects of patterning magnetic film have also been investigated by changing magnetic thin film aspect ratio. It was demonstrated that the peak Q can be pushed towards using higher frequencies as high as 1 GHz by a combination of patterning magnetic films into fine bars and laminations.

Facile large-area photolithography of periodic sub-micron structures using a self-formed polymer mask

This letter reports the methodology of a low-cost fabrication technique for producing periodic sub-micron structures over a large area, using a polymer mask. A thin film of gold/palladium or silica is deposited on a stretched polydimethylsiloxane (PDMS) substrate. Release of the tension forms a buckling sinusoidal pattern on the surface. The PDMS substrates are then used as masks in soft contact optical lithography, bypassing the need for an expensive lithographic process toward creating regular patterns on a traditional masks. Pattern transfers are conducted using an ultraviolet lamp and the fabrication of more complex periodic structures through multiple exposures is reported.