Low-loss Interconnects Research Articles

Towards 200mm 3D RF interposer technology

With the advent of modern and autonomous electronics, applications using RF (radio frequencies) up to millimeter waves and beyond are proliferating. The systems integration becomes increasingly challenging due to variety of devices and passives that typically compose such RF modules, and careful choices of materials are needed for low loss interconnects. One way to minimize RF losses is to integrate the module with high performance interconnect using Si technology, where the different device are mounted on the interposer and the passive are integrated into the silicon. Thanks to the TSV (Through Silicon Via) technology and use of High Resistivity Si substrate, it is possible to have small form factor modules. Such integration approach allows to benefit from the well-established base of 200mm foundries together with the recent progress made in High Resistivity substrate manufacturing. In this work, the process development of a 3D RF interposer technology based on a 200mm process line is reported. The build-up contains 2 levels of metals processed by Cu damascene technology, including a thick 2μm top metal for the lower frequencies applications, an integrated MIM (Metal Insulator Metal) capacitor and use trough silicon via in 5kOhm high resistivity 85μm thick substrate. The TSV are 20μm diameter and 85μm deep made in via first manner with via reveal using temporary carrier handling. Various RF passivation techniques for the silicon have been investigated and a comparison to quartz based on similar test structure is discussed.. Variety of passive devices, transmission line and filter have been processed and characterized. The technology yields 1μm pitch interconnect routing layer, a line loss of 0.34 dB/mm at 40 GHz, and high quality factors inductors larger than 30.

Flip-Chip Interconnects for 250 GHz Modules

With the increasing availability of MMICs at frequencies beyond 100 GHz low-loss interconnects for module fabrication in this frequency range become essential. This letter presents results on a flip-chip mounting approach exhibiting a bandwidth of more than 250 GHz, supporting both coplanar and stripline transitions. The interconnects are realized with 10 $\mu{\rm m}$ -diameter AuSn microbumps. ${\rm S}$ -parameter measurements show an insertion loss of less than 1.0 dB per interconnect and a return loss better than 10 dB up to 250 GHz. The experimental results are in good agreement with 3-D EM simulations.

Hybrid Yarns: Highly Conductive Carbon Nanotube-Graphene Hybrid Yarn (Adv. Funct. Mater. 37/2014)

A new process to manufacture highly conductive textile yarn for use in wearable electronics by providing flexible fiber electrodes for energy storage, low-loss interconnects, sensors, and artificial muscles is described by J. Foroughi and team on page 5859. Graphene dispersions are electrospun onto the continuously drawn multi-walled carbon nanotube sheets and transformed into yarn filaments by twist insertion.

Improving the Performance of 110 GHz Membrane-Based Interconnects on Silicon: Modeling, Measurements, and Uncertainty Analysis

In this paper, we describe the development of broadband low-loss interconnects built-in membrane technology on different silicon substrates. We discuss the design of the test structures, which were fabricated and measured in a frequency range from 0.1 to 110 GHz, and present a detailed uncertainty analysis of the membrane coplanar waveguide (CPW) part. Using high-resistivity silicon as a carrier substrate for the membrane CPW, we obtain excellent agreement between the measurements and calculations of the propagation constant in the entire frequency range. The deviations between the measurements and calculations fall within the bounds predicted by the uncertainty analysis.

Miniature micro-wire based optical fiber-field access device

This paper presents an optical fiber-field access device suitable for use in different in-line fiber-optics' systems and fiber-based photonics' components. The proposed device utilizes a thin silica micro-wire positioned in-between two lead-in single mode fibers. The thin micro-wire acts as a waveguide that allows for low-loss interconnection between both lead-in fibers, while providing interaction between the guided optical field and the surrounding medium or other photonic structures. The field interaction strength, total loss, and phase matching conditions can be partially controlled by device-design. The presented all-fiber device is miniature in size and utilizes an all-silica construction. It has mechanical properties suitable for handling and packaging without the need for additional mechanical support or reinforcements. The proposed device was produced using a micromachining method that utilizes selective etching of a purposely-produced phosphorus pentoxide-doped optical fiber. This method is simple, compatible with batch processes, and has good high-volume manufacturing potential.

Performance analysis of AlGaAs/GaAs tunnel junctions for ultra-high concentration photovoltaics

An n++-GaAs/p++-AlGaAs tunnel junction with a peak current density of 10 100 A cm−2 is developed. This device is a tunnel junction for multijunction solar cells, grown lattice-matched on standard GaAs or Ge substrates, with the highest peak current density ever reported. The voltage drop for a current density equivalent to the operation of the multijunction solar cell up to 10 000 suns is below 5 mV. Trap-assisted tunnelling is proposed to be behind this performance, which cannot be justified by simple band-to-band tunnelling. The metal-organic vapour-phase epitaxy growth conditions, which are in the limits of the transport-limited regime, and the heavy tellurium doping levels are the proposed origins of the defects enabling trap-assisted tunnelling. The hypothesis of trap-assisted tunnelling is supported by the observed annealing behaviour of the tunnel junctions, which cannot be explained in terms of dopant diffusion or passivation. For the integration of these tunnel junctions into a triple-junction solar cell, AlGaAs barrier layers are introduced to suppress the formation of parasitic junctions, but this is found to significantly degrade the performance of the tunnel junctions. However, the annealed tunnel junctions with barrier layers still exhibit a peak current density higher than 2500 A cm−2 and a voltage drop at 10 000 suns of around 20 mV, which are excellent properties for tunnel junctions and mean they can serve as low-loss interconnections in multijunction solar cells working at ultra-high concentrations.

Modeling of tunnel junctions for high efficiency solar cells

Ultrahigh efficiency, in the range of 40%, can be achieved in multijunction solar cells operating at high solar concentrations, larger than 100 suns. Critical to this approach are high band gap tunnel junctions that serve as electrically low loss interconnections between the cells. The purpose of this work is to theoretically model such wide band gap tunnel junctions and to explore the advantages of a staggered band line up for improving the peak tunnel current. Theoretical results are calculated for heterojunction diodes made of n+-InGaP/p+-AlGaAs over a range of doping levels. The results illustrate the advantage of a conduction band discontinuity in achieving low interconnect resistance for multijunction solar cells.

Making the transition

High-frequency integrated circuits that combine organic and silicon substrates are being investigated by researchers at the Georgia Institute of Technology in the US. The researchers have shown, for the first time, the integration of low-loss wideband high density interconnects in a multi-substrate for operation in the V and W bands. “The results are promising for multi-substrate integration which takes advantage of different substrate properties for each device,” said David Chung, a researcher at Georgia Tech. “For example, small ICs that are on Si or SiGe can be compactly integrated with an antenna that shows better performance on a substrate with a lower dielectric constant.”

Antenna-on-Chip and Antenna-in-Package Solutions to Highly Integrated Millimeter-Wave Devices for Wireless Communications

Antenna-on-chip (AoC) and antenna-in-package (AiP) solutions are studied for highly integrated millimeter-wave (mmWave) devices in wireless communications. First, the background, regulations, standard, and applications of 60-GHz wireless communications are briefly introduced. Then, highly integrated 60-GHz radios are overviewed as a basis for the link budget analysis to derive the antenna gain requirement. Next, in order to have deep physical insight into the AoC solution, the silicon substrate's high permittivity and low resistivity effects on the AoC efficiency are examined. It is shown that the AoC solution has low efficiency, less than 12% due to large ohmic losses and surface waves, which requires the development of techniques to improve the AoC efficiency. After that, the AiP solution and associated challenges such as how to realize low-loss interconnection between the chip and antenna are addressed. It is shown that wire-bonding interconnects, although inferior to the flip-chip, are still feasible in the 60-GHz band if proper compensation schemes are utilized. An example of the AiP solution in a low-temperature cofired ceramic (LTCC) process is presented in the 60-GHz band showing an efficiency better than 90%. A major concern with both AoC and AiP solutions is electromagnetic interference (EMI), which is also discussed. Finally, the systems level pros and cons of both AoC and AiP solutions are highlighted from the electrical and economic perspectives for system designers.

Antenna-in-Package Design for Wirebond Interconnection to Highly Integrated 60-GHz Radios

This paper first presents a quasi-cavity-backed, guard-ring-directed, substrate-material-modulated slot antenna. The antenna, intended for use in highly integrated 60-GHz radios, is deliberately designed to exhibit capacitive input impedance to suit low-cost wire-bonding packaging and assembly technique. The antenna implemented in a thin cavity-down ceramic ball grid array (CBGA) package in low-temperature cofired ceramic (LTCC) technology has achieved an acceptable impedance bandwidth from 59 to 65 GHz with an estimated efficiency of 94%. At millimeter-wave (mm-wave) frequency 60 GHz, one of key challenges is how to realize low-loss interconnection between a radio chip and an antenna using wire-bonding technique. This paper then addresses this issue in the framework of antenna-in-package (AiP) design at 60 GHz and proposes a new solution to the challenge. Detailed wirebond design method and results are given. A major concern with AiP is the risk of the antenna coupling to the radio chip. This paper also evaluates this unwanted coupling and shows that the coupling from the in-package antenna to the on-chip inductor is lower than 30 dB for the worst case. These results clearly demonstrate the feasibility and promise of the elegant AiP technology for emerging high-speed short-range 60-GHz wireless communications.

Waveguide Interconnection in Silica-Based Planar Lightwave Circuit Using Femtosecond Laser

The interconnection of waveguides inside a silica-based planar lightwave circuit (PLC) is demonstrated both two- and three-dimensionally by using a femtosecond laser. The waveguides written with a femtosecond laser can be successfully connected to waveguides inside a PLC with low loss. Unlike previous work on the direct writing of 2-D and 3-D waveguides in bulk glass, the waveguide must be written a few tens of micrometers beneath the surface of a PLC composed of multilayers of different glasses. To realize a low-loss waveguide, we studied the sensitivity difference for femtosecond pulses in each layer in detail and developed a multiple-scanning technique with a femtosecond laser for writing waveguides. In addition, we investigated a mode-field (MF) diameter control technique to allow us to achieve the low-loss interconnection of PLC and written waveguides. We also studied particular problems caused by nonlinear optical effects, such as the positional displacement of written waveguides from the focal point. As a result, we achieved a low-loss waveguide with almost the same MF diameter as a PLC waveguide and performed the first demonstration of interconnection between PLC waveguides. The excess losses at 1550 nm were 1.4 and 2.3 dB for 2-D and 3-D interconnection, respectively.

Multiple Scales Method in VLSI Interconnects Threshold Crossing Time Calculation

The calculation of the threshold crossing time of the on-chip very large scale integration (VLSI) interconnects is an important part of interconnect simulation. The paper focuses on low-loss on-chip upper layer interconnect simulation. The work presents a new way of calculating the closed form output voltage and threshold crossing time formulas based on differential equation multiple scales solving method. The analytical form of output voltage for the step and ramp excitation is derived and the threshold crossing time formula is proposed. The presented approach of output voltage calculation for a single interconnect is extended to two coupled interconnects.

Optical fiber nanowires and microwires: fabrication and applications

Microwires and nanowires have been manufactured by using a wide range of bottom-up techniques such as chemical or physical vapor deposition and top-down processes such as fiber drawing. Among these techniques, the manufacture of wires from optical fibers provides the longest, most uniform and robust nanowires. Critically, the small surface roughness and the high-homogeneity associated with optical fiber nanowires (OFNs) provide low optical loss and allow the use of nanowires for a wide range of new applications for communications, sensing, lasers, biology, and chemistry. OFNs offer a number of outstanding optical and mechanical properties, including (1) large evanescent fields, (2) high-nonlinearity, (3) strong confinement, and (4) low-loss interconnection to other optical fibers and fiberized components. OFNs are fabricated by adiabatically stretching optical fibers and thus preserve the original optical fiber dimensions at their input and output, allowing ready splicing to standard fibers. A review of the manufacture of OFNs is presented, with a particular emphasis on their applications. Three different groups of applications have been envisaged: (1) devices based on the strong confinement or nonlinearity, (2) applications exploiting the large evanescent field, and (3) devices involving the taper transition regions. The first group includes supercontinuum generators, a range of nonlinear optical devices, and optical trapping. The second group comprises knot, loop, and coil resonators and their applications, sensing and particle propulsion by optical pressure. Finally, mode filtering and mode conversion represent applications based on the taper transition regions. Among these groups of applications, devices exploiting the OFN-based resonators are possibly the most interesting; because of the large evanescent field, when OFNs are coiled onto themselves the mode propagating in the wire interferes with itself to give a resonator. In contrast with the majority of high-Q resonators manufactured by other means, the OFN microresonator does not have major issues with input-output coupling and presents a completely integrated fiberized solution. OFNs can be used to manufacture loop and coil resonators with Q factors that, although still far from the predicted value of 10. The input-output pigtails play a major role in shaping the resonator response and can be used to maximize the Q factor over a wide range of coupling parameters. Finally, temporal stability and robustness issues are discussed, and a solution to optical degradation issues is presented.

Analysis of Tunnel Junction Suitable for 4000 Suns in Tandem High Efficiency Solar Cell Structures

AbstractThe road to achieve ultra high efficiency is through multi-junction solar cells operating at high solar concentrations, larger than 1000 suns. Critical to the success of this approach is the development of tunnel junctions (TJ) that serve as electrically low loss interconnections, yet are optically transparent, using high band gap semiconductor material systems. We have previously reported the fabrication of a TJ made of n+-InGaP/ p+-AlGaAs with a band gap about 1.9 eV using Se and C doping, respectively. This TJ structure has a peak current density of 88 A/cm2 allowing it to be implemented in a three junction cell structure at solar concentrations as high as 4000 suns (x4000). Almost all reported conversion efficiencies higher than 40% have used this tunnel junction. This very high peak current density is unexpected in a high band gap material system, which is good news for the multi junction solar community. This seems to be due to the fact that the InGaP/AlGaAs interface has a staggered band line up. We will present the effect of this band line up at the heterointerface and its effect on the width of the depletion region and the peak current density. We also compare the current result from this heterostructure junction with an artificial homojunction made of n+-AlGaAs/ p+-AlGaAs doped to the same levels as that of the heterojunction. Results from the homojunction showed that peak current density is about one half of that obtained from the heterojunction at the same doping levels. A reasonable match between experimental result and the model was obtained when a value of 150 meV was used for ΔEc, the conduction band discontinuity at the interface. Both experiment and theory predicted that at a current density of about 80 A/cm2 with only about a few tens of meV drop across the TJ. This will have a minimal effect on the overall efficiency of the tandem solar cell structure when used at high solar concentrations.

A W-Band Low-Loss Dual-Polarization Quasi-TEM Waveguide

As an enabling feed/interconnect element for various millimeter wave components, dual polarization quasi-transverse electromagnetic (TEM) waveguides at W-band (75-110 GHz) are investigated with two-dimensional electromagnetic crystal (EMXT) surfaces as the four boundary walls instead of the regular metallic walls. A 2.54 mm x 2.54 mm quasi-TEM square waveguide prototype with a length of 2 mm is fabricated and demonstrated. Both eigenmode and full-wave EM simulations of the prototype verify the quasi-TEM propagation at the designed frequency of 95 GHz with a bandwidth of more than 10% and the low insertion loss for the entire W-band. The prototype waveguide is measured in a WR-10 fixture and the results agree with simulations well. To better understand the loss mechanisms, various design and packaging parameters are studied. Undesirable resonant surface modes have been identified as a critical cause of loss at specific frequencies. This type of waveguides may also be useful for spatial power combining, low loss interconnects, and packaging for millimeter wave systems.

Magnetic-Multilayered Interconnects Featuring Skin Effect Suppression

A novel concept for a high-frequency, low-loss interconnect with significant skin effect suppression over a wide frequency band is presented. The concept is based on a multilayer comprising thin magnetic (Ni <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">80</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sub> ) and metal (Cu) layers. The negative permeability of the magnetic layers leads to a near-cancellation of the overall permeability of the multilayer stack. This results in a significant increase of skin depth and thus, a more uniform distribution of the current density and a dramatic reduction of loss within a certain frequency range. Coplanar waveguides built using the multilayer technology show more than 50% loss reduction at 14 GHz compared to their thick Cu-based counterparts.

Energy Consumption in Digital Optical ICs With Plasmon Waveguide Interconnects

Limits on energy consumption in digital optical integrated circuits (ICs) using conventional and plasmon waveguide interconnects are presented. The maximum allowable on-chip power density imposes a lower limit on the spacing between devices and together with clock skew imposes an upper limit on the clock frequency. Optical ICs with low-loss plasmon waveguide interconnects may become viable if plasmon waveguide losses in the range 0.1-0.01 dB/m can be achieved.

Air-Gap Transmission Lines on Organic Substrates for Low-Loss Interconnects

The fabrication of low-loss transmission line structures with an air dielectric layer is described. The channels are characterized at low frequency (10 and 100 kHz) using capacitance and loss tangent and at high frequency (500 MHz to 10 GHz) using -parameter measurements. The incorporation of an air gap resulted in structures with effective dielectric constants between 1.5-1.8 and significantly lower loss tangents. The fabrication technique could be used to create more complicated air gap transmission line structures for use in monolithic microwave integrated circuits.

Low-loss microwave interconnections by using polymeric based coplanar waveguides on low resistivity silicon substrates

In this paper a new approach to manufacture low losses coplanar waveguide (CPW) lines for microwave and millimeter wave signal processing is presented. A photolithographic process is performed by using SU-8 thick negative photo-resist on low resistivity silicon wafers, to obtain CPW lines elevated with respect to the substrate, in order to take advantages, in terms of propagation losses, from transmission line structures which are almost on-the-air.

A Fully Embedded 60-GHz Novel BPF for LTCC System-in-Package Applications

In this paper, a novel LTCC stripline (SL) 60-GHz band-pass filter (BPF) composed of transitions to coplanar waveguide (CPW) pads for monolithic microwave integrated circuit integration is presented. For low-loss interconnection with active devices, a CPW-to-SL vertical via transition integrating air cavities and a CPW-to-CPW planar transition using internal ground planes are proposed and implemented. The fabricated transition shows an insertion loss of 1.6 dB and a reflection loss below -20 dB at the passband of the filter. The implemented SL BPF using dual-mode patch resonators shows a center frequency of 60.4 GHz, 3.5 % bandwidth, and reflection losses below -15 dB at the passband. Excluding the insertion loss of the transitions, the filter insertion loss reveals 4.0 dB