Flip-chip Geometry Research Articles

Graphene-Enhanced UV-C LEDs.

Light-emitting diodes in the UV-C spectral range (UV-C LEDs) can potentially replace bulky and toxic mercury lamps in a wide range of applications including sterilization and water purification. Several obstacles still limit the efficiencies of UV-C LEDs. Devices in flip-chip geometry suffer from a huge difference in the work functions between the p-AlGaN and high-reflective Al mirrors, whereas the absence of UV-C transparent current spreading layers limits the development of UV-C LEDs in standard geometry. Here it is demonstrated that transfer-free graphene implemented directly onto the p-AlGaN top layer by a plasma enhanced chemical vapor deposition approach enables highly efficient 275nm UV-C LEDs in both, flip-chip and standard geometry. In flip-chip geometry, the graphene acts as a contact interlayer between the Al-mirror and the p-AlGaN enabling an external quantum efficiency (EQE) of 9.5% and a wall-plug efficiency (WPE) of 5.5% at 8V. Graphene combined with a ≈1nm NiOx support layer allows a turn-on voltage <5V. In standard geometry graphene acts as a current spreading layer on a length scale up to 1mm. These top-emitting devices exhibit a EQE of 2.1% at 8.7V and a WPE of 1.1%.

Effect of Aliovalent Substitution in Y2.9Bi0.1Fe₅O₁₂: Magnetization Dynamic Study

Multifunctional multiferroic materials, such as rare-earth-doped yttrium iron garnet (YIG), are advantageous for low-energyconsumption devices. In this work, we investigated aliovalent (Ce <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4+</sup> ) substitution in Y <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2.9</sub> Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.1</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sub> ferrite with different compositions Y <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2.9-x</sub> Ce <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.1</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sub> (x = 0.0, 0.2, 0.4), respectively. The ferrites were fabricated by a conventional solid-state reaction method, and their structural, static, and dynamic magnetic properties were studied at room temperature. The X-ray diffraction (XRD) pattern confirmed the Ia3̅d cubic phase of all the ferrite compositions together with ceric oxide (CeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) as a residual phase containing aliovalent ions. The lattice parameter “a” of synthetic garnets decreases linearly with Ce concentration. From the static magnetic measurements, we observed that the saturation magnetization (4πMS) decreases due to combined effect of Ce <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> paramagnetic ions and Ce <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4+</sup> diamagnetic aliovalent ions. Magnetization dynamics have been studied by ferromagnetic resonance (FMR) technique in flip-chip geometry on the coplanar waveguide. Field sweep FMR spectra revealed that the microwave absorption in the X-band and the Ku-band decreases with Ce concentration due to a decrease in the super-exchange interaction between Fe ions at [a] and (d) sites due to electronic transition of Ce <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> ion. A major investigation of this study shows a decrease in the Gilbert damping constant (α) from 1.6 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> to 1.52 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> and increase in extrinsic contribution Δ(H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sub> ) to the FMR linewidth A(H) from 0.38 to 0.54 kOe with Ce concentration. Therefore, this investigation opens a door for the next generation magneto-electric, magneto-optic, and magneto-dielectric effect-based spintronics/magnonics devices in the microwave regime.

Concentration gradient Co–Fe nanowire arrays: Microstructure to magnetic characterizations

Abstract In the present investigation, systematic method to fabricate nanowire arrays with a linear gradient of cobalt-iron composition has been reported. The nanowires were fabricated by direct current (DC) electro-deposition following a multi-step titration method into a commercial anodic aluminium oxide (AAO) template. Nearly 100% gradient of Co and Fe content in the nanowires is achieved following the present method. The gradient of Co and Fe composition along the nanowires is established by Energy-dispersive X-ray (EDX) mapping in scanning electron microscope (SEM). The results from EDX mapping are further supported by the microstructural characterization along the wire axis using high resolution transmission electron microscope (HRTEM) images and SAED pattern. The microstructural characterization reveals variation of crystalline phases that are signature of Co and Fe content along the wires. To explore the magnetization dynamics of nanowire arrays, ferromagnetic resonance (FMR) study was performed in flip-chip geometry. The FMR study shows the presence of four well-separated resonance peaks in gradient nanowires which is a unique finding in the present investigation. A detailed FMR analysis has been presented to reveal the origin of these peaks. The values of the Gilbert damping parameter, obtained from the FMR linewidth analysis establish the correlation of FMR response with the average composition of Co–Fe in nanowires, grown in each step of titration. This study highlights the necessity of the detailed investigation of microstructures and magnetization dynamics of gradient ferromagnetic nanowire arrays that are emerging in high-frequency spintronics applications, more suitable for a tunable tri-band band-pass filter.

Effects of Interfacial Layers on Magnetization Dynamics of [Fe75Co20Cu5/Cu(x)]30 Multilayer Nanowires

A series of highly ordered multilayer [FeCoCu/Cu( x )]30 ( $0 \le x \le 40$ nm with FeCoCu layer thickness fixed 1000 ± 100 nm) nanowire arrays in alumina membrane (200 nm diameter) was fabricated by electrodeposition from a single electrolytic bath. The body-centered cubic (bcc) phase of FeCoCu-alloy separated by a well-defined face-centered cubic (fcc) phase of Cu spacer is confirmed by high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) analysis. The objective of the present investigation is to tune the magnetic properties through inter- and intra-wire interactions between the FeCoCu layers separated by non-magnetic Cu( x ) layers in the multilayer nanowire array. Ferromagnetic resonance (FMR) study was performed in a flip-chip geometry to explore the magnetization dynamics. FMR measurements confirm the decrease of resonance field ( $H_{r}$ ), whereas resonance linewidth ( $\Delta H$ ) and FMR absorption increase with the increase in the Cu layer thickness. It may be argued that through inter-/intra-nanowires interactions, the Landau–Lifshitz–Gilbert damping provides the most physically sensible magnetization relaxation in multi-layered nanowires system. The values of the Gilbert damping parameters (intrinsic and extrinsic), obtained from the FMR linewidth analysis, exhibit a decrease from FeCoCu nanowire to [FeCoCu/Cu(5 nm)] nanowire. With further increase in the Cu thickness, both these parameters were observed to have increased.

Flip-Chip Wafer-Fused OP-VECSELs Emitting 3.65 W at the 1.55-μm Waveband

Optically pumped vertical external cavity surface emitting lasers (VECSELs) based on flip-chip gain mirrors emitting at the 1.55 μm wavelength range are reported. The gain mirrors employ wafer-fused InAlGaAs/InP quantum well heterostructures and GaAs/AlAs distributed Bragg reflectors fixed on a diamond heat-sink substrate in a flip-chip geometry, incorporated in a V-cavity configuration. A maximum output power of 3.65 W was achieved for a heatsink temperature of 11°C and employing a 2.2% output coupler. The laser exhibited circular beam profiles for the full emission power range. This demonstration represents more than five-fold increase of the output power compared to the state-of-the-art flip-chip VECSELs previously reported at the 1.55 μm wavelength range. It opens new perspectives for developing practical VECSEL-based laser systems operating at a wavelength range widely used in many applications.

3D integrated superconducting qubits

As the field of quantum computing advances from the few-qubit stage to larger-scale processors, qubit addressability and extensibility will necessitate the use of 3D integration and packaging. While 3D integration is well-developed for commercial electronics, relatively little work has been performed to determine its compatibility with high-coherence solid-state qubits. Of particular concern, qubit coherence times can be suppressed by the requisite processing steps and close proximity of another chip. In this work, we use a flip-chip process to bond a chip with superconducting flux qubits to another chip containing structures for qubit readout and control. We demonstrate that high qubit coherence (T1, T2,echo > 20 μs) is maintained in a flip-chip geometry in the presence of galvanic, capacitive, and inductive coupling between the chips.

One dimensional FexCo1-x nanowires; ferromagnetic resonance and magnetization dynamics

Soft magnetic nanowires (NWs) are widely used for microwave and mm-wave components. The investigation of magnetization damping behavior of NWs have attracted great interest due to large influence of loss to the device, like integrated microwave device, magnetic sensors, and magnetic random access memory. With increasing operational frequency and degree of integration, the requirements to characterize 1-dimensional NWs become increasingly high. The purpose of this work is to study the magnetization dynamics in FexCo1-x NWs. A series of FexCo1-x (x=0, 0.25, 0.5, 0.75, 1) NWs were grown by controlled electro-deposition. By adjusting FexCo1-x concentration (x=0 to 1), the saturation magnetization, increased more than 20%. Ferromagnetic resonance (FMR) both in field and frequency sweep mode are employed to characterize the NWs in flip-chip geometry. It is observed that FMR field (Hr) increases with increase in applied frequency. At a fixed frequency, Fe NWs resonate at a lower field than the Co substituted NWs. FMR field linewidth (ΔH) as well as frequency width (Δf) are largest for Co NWs and decreased for Fe NWs. Whereas ΔH and Δf decreased further for FexCo1-x nanowires with increasing x.

Improved Characteristics of Integrated HTS rf SQUID on Bicrystal SrTiO3 Substrate Resonator Covered with HTS Thin Films in Flip-Chip Geometry

Abstract Integrated high-temperature superconductor (HTS) radio-frequency (rf) superconducting quantum interference devices (SQUIDs) based on a bicrystal SrTiO3 substrate was investigated for application to magnetic contaminant detection. By covering a wide superconducting weak link and/or a slit of the YBa2Cu3O7-x SQUID with HTS thin films in flip-chip geometry, characteristics such as effective area and 1/f noise profile of the SQUID were successfully improved. By installing the covered SQUID in a magnetic contaminant detection system, it was demonstrated that the system can detect a tungsten ball of 15μm in diameter with a signal to noise ratio of about 2.

Single quantum well deep-green LEDs with buried InGaN/GaN short-period superlattice

In spite of the great progress in III-N technology, LEDs with wavelength >530 nm still exhibit low efficiency compared to blue and short-wavelength-green LEDs. Here we report on significant improvement of deep-green LED properties by modifications of the structure design. The combination of InGaN/GaN superlattice followed by low-temperature GaN is the key element to increase the electroluminescence efficiency for deep-green LED. Various techniques were employed to clarify the correlation between structure properties, growth regimes and design. Modification of the defect structure of the GaN buffer by InGaN layers appears to be mostly responsible for the observed effect. LEDs processed and assembled in a standard flip-chip geometry with Ni–Ag p-contact demonstrate external quantum efficiencies of 8–20% in the 560–530 nm range.

High-frequency characterization of Permalloy nanosized strips using network analyzer ferromagnetic resonance

We report on the dynamic properties of Permalloy nanostrips at gagahertz frequencies. The thickness of the strips is 100nm, strip width is 300nm, strip spacing is 1μm, and length is 0.3–100μm; aspect ratios are 1:1, 1:2, 1:3, 1:5, 1:10, and 1:333. The dynamic behavior was studied by network analyzer ferromagnetic resonance (FMR) using Permalloy strips on a coplanar waveguide in flip-chip geometry. The FMR mode frequencies (fr) can be controlled by the aspect ratio as well as by the applied magnetic field (H). In longer strips (1:10 and 1:333), the excitation frequencies show a soft mode behavior (Heff=990Oe) when the field is along the hard axis. However, along the easy axis (along the strip length), fr increases with applied field. At a field of 3kOe, fr values are almost independent of aspect ratio along the easy axis except for the 1:1 strip. Along the hard axis, the frequencies are strongly dependent upon the aspect ratio. We also observed that the frequency linewidths of the strips are dependent on the aspect ratio.

Ultrathin magnetic multilayer films for low-field microwave notch filters

Microwave filters that use thin films of ferromagnetic metals are now being established as a valuable option compared to yttrium iron garnet based filters due to their higher frequency response. In these filters the signal propagation is inhibited over a wide frequency band, depending on the applied dc magnetic field. However, the continuous application of an applied field to achieve an operating frequency in the higher gigahertz range increases the power consumption of the device. The main contribution of this article is to provide techniques which significantly boost the operating frequency of notch filters in zero or very low applied magnetic fields. To do this, the authors fabricated high quality epitaxial Fe films which are interlayer exchange coupled through nonmagnetic Si layer of different thicknesses. The films were used in flip-chip geometry on top of a Cu-coplanar waveguide to create band-stop filters. In contrast to filters based on Fe alone, the multilayer filters can operate above 25GHz with a very small applied magnetic field. The observed upshift in frequency is attributed to the induced interlayer exchange coupling energy mediated through the nonmagnetic Si layer between the two Fe layers. These frequency shifts are in good agreement with theoretical calculations of the ferromagnetic resonance modes taking into account anisotropy, exchange, and Zeeman energies.

언더필 공정에 대한 유동 특성과 침투 시간 예측 연구

The present study is devoted to investigate the transient flow and to estimate the filling time fur underfill process by using the numerical model established on the fluid momentum equation. For optimization of the design and selection of process parameters, this study extensively presents an estimation of the filling time in the view points of some important factors related to underfill materials and flip-chip geometry. From the results, we conclude that the filling time changes with respect to the under fill materials because of different viscosity, surface tension coefficient and contact angle. It reveals that, as the gap height increases, the filling time decreases substantially, and goes to the saturated values.

Substrate resonator for HTS rf SQUID operation

We describe the use of single substrates as resonators for radio-frequency (rf) superconducting quantum interference devices (SQUID) operation. A standard strontium-titanate substrate with dimensions 10×10×1 mm 3 serves as a tank circuit (resonator), a YBCO thin film SQUID washer structure is patterned on it. On the resonator substrate, a small rf washer SQUID with a step-edge junction is positioned in flip-chip geometry, thus forming a magnetometer sensor. This resonator is inductively coupled to the readout electronics. At 77 K, the field sensitivity of this SQUID magnetometer achieved 24 fT/ Hz in the white noise range, 38 fT/ Hz at 10 Hz, and 83 fT/ Hz at 1 Hz.

Compensation techniques for high-temperature superconducting quantum interference device gradiometers operating in unshielded environment

We have tested two methods of compensating environmental disturbances applicable to high-temperature superconducting quantum interference device (SQUID) systems operating in magnetically unshielded environments. For testing, we used first- and second-order axial electronic gradiometer setups with rf SQUID magnetometers operating at 77 K and base lines between 7 and 8 cm. The magnetometers were single-layer washer rf SQUIDs with bulk or thin-film magnetic flux concentrators in flip-chip geometry. The tested methods resulted in disturbance compensation levels comparable to those attained using electronically formed gradiometers. The white noise of the compensated magnetometers resulted in 13.5 fT/cm √Hz for first-order and 22 fT/cm2 √Hz for second-order compensation down to a few Hz. Common mode rejection was balanced to better than 10 000 for homogeneous fields and better than 200 for gradient fields with second-order compensation.

Electronic high-temperature radio frequency superconducting quantum interference device gradiometers for unshielded environment

This article will discuss improved electronically formed gradiometers based on high-temperature radio frequency (rf) superconducting quantum interference device (SQUID) magnetometers. For gradiometer balancing, a system of adjustable superconducting plates was developed. This technique was used to build first- and second-order, axial gradiometers with adjustable baselines, which operate at 77 K. Each magnetometer combines a washer rf-SQUID with bulk or a thin-film flux concentrator in flip chip geometry. In an unshielded environment, the magnetic field sensitivity in the white noise region is about 80 fT/√Hz for first-order and 150 fT/√Hz for second-order gradiometer. Common mode rejection could be balanced to better than 104 for uniform background fields and better than 200 for gradient fields.

Compensation techniques for HTS-rf-SQUID magnetometers operating in unshielded environments

Abstract We have developed two methods of compensating environmental disturbances applicable to high- and low-temperature superconducting quantum interference device (SQUID) systems, operating in a magnetically unshielded environment. For testing, we used first- and second-order axial electronic gradiometer setups with rf SQUID magnetometers operating at 77 K, with different baselines between 1.8 and 8 cm. The magnetometers were single-layer washer rf SQUIDs, with bulk or thin-film magnetic flux concentrators and transformers in flip-chip geometry. The tested methods resulted in disturbance compensation levels comparable with those attained using electronically formed gradiometers. The white noise of the compensated magnetometers resulted in maximum gradiometric sensitivities of 13.5 fT/cm √Hz for first-order and 22 fT/cm2 √Hz for second-order compensation, with baselines between 7 and 8 cm down to 10 Hz. Common mode rejection was balanced to, better than 104 for homogeneous fields, and better than 200 for gradient fields, with second-order compensation. Magnetically unshielded measurements of biomagnetic signals with different baselines were demonstrated.

Optimization of large-area single-layer flux-transformers and concentrators coupled to RF-SQUIDs in flip-chip geometry

The effect of layout modifications of large-area single-layer YBa/sub 2/Cu/sub 3/O/sub 7/ flux transformers onto the effective area and system noise has been investigated systematically and the excellent performance of sputtered large-area flux transformers is demonstrated. First, the gain of sensitivity and noise contribution of the transformer (coupled to a 8/spl times/8 mm/sup 2/ washer rf-SQUID) was determined as a function of the line width of the YBa/sub 2/Cu/sub 3/O/sub 7/ pickup loop w/sub p/ ranging between and 8.2 mm for 1 transformer (O=25.4 mm) and and 19 mm for 2 transformer (O=50.8 mm), respectively. Leaving the diameter of the pickup coil (b/sub 1/=23 mm, b/sub 2/=46 mm) and the coupling coil (d=6 mm, width=1.2 mm) unchanged, the gain in sensitivity increases from 2.1 for w/sub p/=1 mm to 2.7 for w/sub p//spl ges/6 mm in the case of 1 transformers and from 3.3 for w/sub p/=1 mm to 4.6 for w/sub p/=19 mm for 2 transformers. No additional noise contribution of the transformer could be observed, i.e. a flux noise of 70 /spl mu//spl Phi//sub 0//spl radic/(Hz) was determined above 5 Hz and at 77 K. The best values for the field-to-flux conversion efficiency and the effective area were 0.74 nT//spl Phi//sub 0/ and 2.98 mm/sup 2/ for 1 and 0.43 nT//spl Phi//sub 0/ and 4.94 mm/sup 2/ for 2 transformers, respectively. The comparison of the experimental data with the theory indicates, that the expressions derived for low-T/sub c/ superconducting planar devices have to be modified in the case of high-T/sub c/ devices. Second, the performance of the sputter deposited large-area devices is demonstrated for the case of an axial gradiometer. The white noise level of the whole gradiometer system in an unshielded environment was <100 fT//spl radic/(Hz) for frequencies above 5 Hz and the common mode rejection of the homogeneous field was measured to be better than 10/sup 4/. Real-time magnetocardiographic signals obtained via this gradiometer in a magnetically unshielded environment show a large signal-to-noise ratio.

Fabrication of YBa2Cu3Ox thin-film flux transformers using a novel microshadow mask technique for in situ patterning

A novel microshadow mask technique for in situ patterning of multilayers is presented. It is ideally suited to fabricate YBa2Cu3Ox(YBCO) and insulator lines with gently sloping edges, needed for high quality insulated superconducting crossovers. The critical current density jc (T=77 K) of a YBCO/SrTiO3/YBCO crossover exceeds 2×106 A/cm2 in both the bottom and the top YBCO stripline. The insulating SrTiO3 layer of 200 nm thickness displays a high resistivity of ρ≳108 Ω cm (T=77 K). The extremely smooth morphology of the edges has been revealed by cross sectional transmission electron microscopy, indicating a stepflow mechanism of YBCO growth. Multiturn flux transformers with a 15 μm linewidth input coil spiral have been fabricated by this microshadow mask technique. A transformer with a pickup loop area of 7 mm2 has been coupled to a 1 mm2 washer dc SQUID in flip chip geometry. In comparison to the bare SQUID a magnetic flux gain factor of 9 has been obtained. The white noise level of this setup was determined to be 8×10−5 Φ0/Hz1/2 at 77 K. It was entirely due to the intrinsic noise of the employed dc SQUID itself. The 1/f noise level increased a factor of 2.