Fundamentals of Fast Simulation Algorithms for RF Circuits

Designers of RF circuits such as power amplifiers, mixers, and filters make extensive use of simulation tools which perform periodic steady-state analysis and its extensions, but until the mid 1990s, the computational costs of these simulation tools restricted designers from simulating the behavior of complete RF subsystems. The introduction of fast matrix-implicit iterative algorithms completely changed this situation, and extensions of these fast methods are providing tools which can perform periodic, quasi-periodic, and periodic noise analysis of circuits with thousands of devices. Even though there are a number of research groups continuing to develop extensions of matrix-implicit methods, there is still no compact characterization which introduces the novice researcher to the fundamental issues. In this paper, we examine the basic periodic steady-state problem and provide both examples and linear algebra abstractions to demonstrate connections between seemingly dissimilar methods and to try to provide a more general framework for fast methods than the standard time-versus-frequency domain characterization of finite-difference, basis-collocation, and shooting methods     

Self‐Healing Fibre Reinforced Composites via a Bioinspired Vasculature

This paper demonstrates the first steps towards self‐healing composites that exploit a design philosophy inspired by the damage tolerance and self‐repair functions of bone. Cracking in either fibre reinforced polymers (FRP) or bone, if left unattended, can grow under subsequent cyclic stresses eventually leading to catastrophic failure of the structure. On detection of cracks, an FRP component must be repaired or completely replaced, whereas bone utilises a series of complex processes to repair such damage. Under normal circumstances, these processes allow the skeleton to continually perform over the lifespan of the organism, a highly desirable aspiration for engineering materials. A simple vasculature design incorporated into a FRP via a “lost wax” process was found to facilitate a self‐healing function which resulted in an outstanding recovery (≥96%) in post‐impact compression strength. The process involved infusion of a healing resin through the vascule channels. Resin egress from the backface damage, ultrasonic C‐scan testing, and microscopic evaluation all provide evidence that sufficient vascule–damage connectivity exists to confer a reliable and efficient self‐healing function.     

Failure mechanism study of anisotropic conductive film (ACF) packages

Anisotropic conductive film (ACF) consists of an adhesive polymer matrix with dispersed conductive particles. In flip-chip technology, ACF has been used in place of solder and underfill for chip attachment to glass or organic substrates. The filler particles establish the electrical contacts between the interconnecting areas. ACF flip-chip bonding provides finer pitch, higher package density, reduced package size and improved lead-free compatibility. Nevertheless, the interconnection is different from traditional solder joints, the integrity and durability of the ACF interconnects have major concerns. Failures in anisotropic conductive film (ACF) parts have been reported after temperature cycling, moisture preconditioning and autoclave. The failures have not been well understood and have been attributed to a wide variety of causes. This paper investigates the failure mechanism of ACF using finite element simulation. From a failure-initiation point of view, the response of ACF packages to environmental (temperature and humidity) exposure is very different from standard underfilled packages. These differences cause the ACF package to fail in different ways from an underfilled package. Simulation results have shown that moisture-induced ACF swelling and delamination is the major cause of ACF failure. With moisture absorption, the loading condition at the interface is tensile-dominant, which corresponds to lower interface toughness (or fracture resistance). This condition is more prone to interface delamination. Therefore, the reliability of ACF packages is highly dependent on the ACF materials. The paper suggests a new approach toward material selection for reliable ACF packages. This approach has very good correlation with experimental results and reliability testing of various ACF materials.     

Direct Writing of Gallium‐Indium Alloy for Stretchable Electronics

In this paper, a direct writing method for gallium‐indium alloys is presented. The relationships between nozzle inner diameter, standoff distance, flow rate, and the resulting trace geometry are demonstrated. The interaction between the gallium oxide layer and the substrate is critically important in understanding the printing behavior of the liquid metal. The difference between receding and advancing contact angles demonstrates that the adhesion of the oxide layer to the substrate surface is stronger than the wetting of the surface by the gallium‐indium alloy. This further demonstrates why free‐standing structures such as the traces described herein can be realized. In addition to the basic characterization of the direct writing process, a design algorithm that is generalizable to a range of trace geometries is developed. This method is applied to the fabrication of an elastomer‐encapsulated strain gauge that displays an approximately linear behavior through 50% strain with a gauge factor of 1.5.     

Interference Cancelling Codes for Ultra-Reliable Random Access

Combinatorial code designs (CCDs) are proposed as a means for achieving ultra-reliability in the random access channel. In contrast to traditional access protocols that use random repetition coding, we show that by uniquely allocating repetition patterns to users, successful reception may be guaranteed up to a number of simultaneously active users in small frame sizes. Such codes are particularly robust in the low activity region where mission-critical machine-type communication is expected to operate. We also present deterministic codes designed to work in conjunction with successive interference cancellation to further improve reliability. The optimal IC code for frames of 5 access slots is given. Unlike slotted ALOHA, it is shown to limit packet losses to well below the ultra-reliability threshold (10^?5). These error performance gains come at the cost of a strict limitation on the supported user population (11 users in the case of 5 slots). We therefore consider larger frames of 24 slots, and analyse heuristic, low-complexity CCDs with fixed repetition factors that support up to 2024 users. While these are sub-optimal IC codes, significant gains are still observed compared to random codes.     

Fast tropospheric delay predictions for antenna height diversity

Terrestrial microwave links sometimes suffer from signal fading which may be relieved with antenna diversity or frequency diversity. Vertical spacing of the antennas has been known empirically to offer performance gains for many links. A fast new method for tropospheric delay predictions is discussed. It uses Mathematica for critical functional fits to refractivity profiles, and a critical closed-form integration. A practical antenna diversity example is shown, with reference to Sylvain (see ibid., vol.43, no.7, p.2271, 1995)     

A reduced-complexity online state sequence and parameter estimatorfor superimposed convolutional coded signals

This paper develops a reduced-complexity online state sequence and parameter estimator for superimposed convolutional coded signals. Joint state sequence and parameter estimation is achieved by iteratively estimating the state sequence via a variable reduced-complexity Viterbi algorithm (VRCVA) and the model parameters via a recursive expectation maximization (EM) approach. The VRCVA is developed from a fixed reduced-complexity Viterbi algorithm (FRCVA). The FRCVA is a special case of the delayed decision-feedback sequence estimation (DDFSE) algorithm. The performance of online versions of the FRCVA, VRCVA, and the standard Viterbi algorithm (VA) are compared when they are used to estimate the state sequence as part of the reduced-complexity online state sequence and parameter estimator     

In situ X-ray diffraction study of self-forming barriers from a Cu-Mn alloy in 100 nm Cu/low-k damascene interconnects using synchrotron radiation

An in situ study of self-forming barriers from a Cu-Mn alloy was performed to investigate the barrier growth using X-ray diffraction on damascene lines. The associated evolution in interconnect texture and Cu stress was also observed. The shift in Cu diffraction peak position was used to determine the change in Mn concentration and hence, estimate the thickness of the MnSi_xO_y barrier. The observed peak shift followed a log(t) behaviour and is described well by metal oxidation kinetics, following the field enhanced diffusion model. We used multiple anneal temperatures to study the activation of the formation process, demonstrating a faster barrier formation with higher ion excitation. A strong [1 1 1] Cu texture was shown to develop during the anneal in contrast to traditional PVD barrier systems. Finally, the stress in the 100 nm Cu lines was calculated, observing a large in-plane relaxation when using a self-forming barrier due to reduced confinement.     

Photo‐Induced Functionalization of Spherical and Planar Surfaces via Caged Thioaldehyde End‐Functional Polymers

The synthesis and application of a novel reversible addition‐fragmentation chain transfer (RAFT) agent carrying a photocaged thioaldehyde moiety is described (λ_max = 355 nm). RAFT polymerization of styrene, dimethylacrylamide and a glycomonomer is evidenced (3600 g mol^?1 ≤ M_n ≤ 15 000 g mol^?1; 1.07 ≤ ? ≤ 1.20) with excellent end‐group fidelity. The photogenerated thioaldehyde on the chain ends can undergo hetero Diels–Alder reactions with dienes as well as reactions with nucleophiles. The terminal photoreactive polymers are photografted to porous diene‐reactive polymeric microspheres. The grafted particles are in‐depth characterized via scanning electron microscopy, elemental analysis, X‐ray photoelectron spectroscopy, and high resolution FT‐IR microscopy, leading to a qualitative as well as quantitative image of the core–shell objects. Grafting densities up to 0.10 molecules nm^?2 are reached. The versatility of the thioaldehyde ligation is evidenced by spatially resolved grafting of polystyrene onto nucleophilic groups present in poly (dopamine) (PDA)‐coated glass slides and silicon wafers via two‐photon direct laser writing (DLW) imaged by ToF‐SIMS. The combination of thioaldehyde ligation, RAFT polymerization, and DLW allows for the spatially resolved grafting of a vast range of polymers onto various substrates in any desired pattern with sub‐micrometer resolution.