Numerical study on thermal impacts of different void patterns on performance of chip-scale packaged power device

Chip scale package (CSP) technology offers promising solutions to package power device due to its relatively good thermal performance among other factors. Solder thermal interface materials (STIMs) are often employed at the die bond layer of a chip-scale packaged power device to enhance heat transfer from the chip to the heat spreader. Nonetheless, the presence of voids in the solder die-attach layer impedes heat flow and could lead to an increase in the peak temperature of the chip. Such voids which form easily in the solder joint during reflow soldering process at manufacturing stage are primarily occasioned by out-gassing phenomenon and defective metallisation. Apparently, the thermal consequences of voids have been extensively studied, but not much information exist on precise effects of different patterns of solder die-attach voids on the thermal performance of chip-level packaged power device. In this study, three-dimensional finite element analysis (FEA) is employed to investigate such effects. Numerical studies were carried out to characterise the thermal impacts of various voids configurations, voids depth and voids location on package thermal resistance and chip junction temperature. The results show that for equivalent voiding percentage, thermal resistance increases more for large coalesced void type in comparison to the small distributed voids configuration. In addition, the study suggests that void extending through the entire thickness of solder layer and voids formed very close to the heat generating area of the chip can significantly increase package thermal resistance and chip junction temperature. The findings of this study indicate that void configurations, void depth and void location are vital parameters in evaluating the thermal effects of voids.     

Speed tracking control of a permanent-magnet synchronous motor withstate and load torque observer

This paper is concerned with the speed tracking control problem for a permanent-magnet synchronous motor (PMSM) in the presence of an unknown load torque disturbance. After a brief review of the mathematical model of the PMSM, a speed tracking control law using the exact linearization methodology is introduced. The tracking control algorithm is completed by adding an extended observer which provides, on the one hand, the motor speed and acceleration and, on the other hand, estimates the unknown load torque. The stability of the closed-loop system composed of a nonlinear speed tracking controller and an observer is studied by the way of Lyapunov theory. Furthermore, the decoupling of the state observer and the load torque observer is discussed. Finally, a real-time implementation and the experimental results of the proposed control strategy are presented     

Adaptive S-ALOHA CDMA as an alternative way of integrating servicesin mobile environments

Code-division multiple-access (CDMA) schemes appear to be very promising access techniques for coping with the requirements of third-generation mobile systems, mainly because of their flexibility. This paper proposes an adaptive S-ALOHA DS-CDMA access scheme as a method for integrating nonreal-time (i.e., Internet applications) and real-time (i.e., voice) services in a multicell scenario by exploiting the potentials of CDMA under time-varying channel load conditions. The adaptive component makes data terminals autonomously change their transmission rate according to the total (voice+data) channel occupancy, so that the minimum possible data delay, which can be analytically obtained by defining a birth-death process, is almost always achieved. Moreover, by means of a simplified cellular model, the proposed algorithm revealed the same behavior, i.e., it tries to select the most suitable transmission rate at any time slot, when it is affected by intercell interference and even by power control imperfections. Finally, in order to gain more insight into the potentials of such an access strategy, the adaptive S-ALOHA CDMA scheme is then compared to a reservation time-division multiple-access (TDMA)-based protocol (PRMA++), showing the benefits of the CDMA-based solution in terms of capacity, flexibility, and data delay performance     

Shallow acceptors in strained Ge/Ge1−x Six heterostructures with quantum wells

The energies of localized acceptor states in quantum wells (strained Ge layers in Ge/Ge_1?x Si_x heterostructures) were analyzed theoretically in relation to the quantum well width and the impurity position in the well. The impurity absorption spectrum in the far IR range is calculated. Comparison of the results of the calculation with experimental photoconductivity spectra allows an estimation of the acceptor distribution in the quantum well to be made. In particular, it was concluded that acceptors may largely concentrate near the heterointerfaces. The absorption spectrum is calculated taking into account the resonance impurity states. This allows the features observed in the short-wavelength region of the spectrum to be interpreted as being due to transitions into the resonance energy levels “linked” to the upper size-quantization subbands.     

Dominant mechanisms of transient-radiation upset in CMOS RAM VLSI circuits realized in SOS technology

The dominant mechanisms are analyzed of transient-radiation upset in CMOS RAM VLSI circuits realized in SOS technology. Data reliability under transient irradiation is discussed in relation to photocurrents, rail-span collapse, and the circuit and layout design of memory cells. The response is simulated of SOS integrated resistors to transient radiation. Optimal parameter values are thus determined for the resistor used in the RC network of a memory cell. It is found that the data reliability of the memory circuits considered is affected by the cross coupling of memory cells sharing a read/write line. The lifetime of radiation-induced charge carriers is estimated by experiment and computer simulation.     

Cu/low-k dielectric TDDB reliability issues for advanced CMOS technologies

With the wide application of low-k and ultra-low-k dielectric materials at the 90 nm technology node and beyond, the long-term reliability of such materials is rapidly becoming a critical challenge for technology qualification. Low-k time-dependent dielectric breakdown (TDDB) is usually considered as one of the most important reliability issues during Cu/low-k technology development because low-k materials generally have weaker intrinsic breakdown strength than traditional SiO₂ dielectrics. This problem is further exacerbated by the aggressive shrinking of the interconnect pitch size due to continuous technology scaling. In this paper, three critical issues of low-k TDDB characteristics during low-k development and qualification will be reviewed. In the first part, a low-k TDDB field acceleration model and its determination will be discussed. In the second part, low-k dielectric time-to-breakdown (t_BD) statistical distribution and TDDB area scaling law for reliability projection will be examined. In the last part, as low-k TDDB has been found to be sensitive to all aspects of integration, the effects of process variations on low-k TDDB degradation will be demonstrated. Some key aspects which need to be carefully addressed to control overall low-k TDDB performance from process and integration side will be discussed.     

Characterization of copper grain growth limitations inside narrow wires depending of overburden thickness

With the downscaling of feature dimensions, copper interconnects exhibit properties differing from bulk or film material. Resistivity increases and limits electrical performances, and reliability of interconnects becomes a more important challenge for each new technological node. In this study, we present an approach of copper grain growth control inside narrow wires by adding a step between the copper electro-chemical deposition (ECD) and the chemical-mechanical polishing (CMP). This step corresponds to a partial CMP step (pre-CMP) and is applied after ECD and before anneal in order to modify the copper overburden thickness. Depending on the targeted thickness, copper grain growth occurs during anneal with different efficiencies. Crystallization and grain growth behaviour inside wires is investigated with focused ions beam (FIB). We present here our methodology for sample preparation and characterization. Results are focused on electrical variations and on morphological aspects of copper crystallization and grain growth inside lines observed with various overburden thicknesses.     

Electrical and structural properties of PHEMT heterostructures based on AlGaAs/InGaAs/AlGaAs and δ-doped on two sides

The pseudomorphic AlGaAs/InGaAs/AlGaAs heterostructure δ-doped with silicon on both sides and used for fabrication of high-power transistors is optimized to obtain a high concentration n _s and mobility of two-dimensional electron gas in the quantum well (n _s ≈ 3 × 10¹² cm^?2). Electrical and structural characteristics of the samples grown by molecular-beam epitaxy with various doping levels are studied. It is shown that the mobility and concentration of electrons varies as doping level is increased and the subbands of dimensional quantization are sequentially filled. In order to analyze the distribution of electrons in subbands of the heterostructure, the Shubnikov-de Haas oscillations at the liquid helium temperature were studied. The quality of the heterostructure layers was assessed using the method of X-ray diffraction. The data of photoluminescence spectroscopy are in good agreement with the results of calculations of the band structure.     

Analytical Model for Cross-Gain Modulation and Crosstalk in Quantum-Well Semiconductor Optical Amplifiers

An analytical model of the dynamic characteristics of a quantum-well (QW) semiconductor optical amplifier (SOA) is developed. Closed-form expressions for the optical gain and cross-gain modulation (XGM) for arbitrary input pulses are derived. The model takes into account the carrier capture and escape transitions between the QW and the continuum states. This model is also used to derive a closed-from expression for interchannel XGM crosstalk in multichannel SOA systems. The model/analysis provides insight into the effect of the SOA parameters on the performance of a wavelength-division multiplexed system. We found that crosstalk in a multichannel SOA system can be reduced by reducing the escape lifetime.     

Precise Tip Positioning of a Flexible Manipulator Using Resonant Control

A single-link flexible manipulator is fabricated to represent a typical flexible robotic arm. This flexible manipulator is modeled as an SIMO system with the motor torque as the input and the hub angle and the tip position as the outputs. The two transfer functions are identified using a frequency-domain system identification method, and the resonant modes are determined. A feedback loop around the hub angle response with a resonant controller is designed to damp the resonant modes. A high-gain integral controller is also implemented to achieve zero steady-state error in the tip position response. Experiments are performed to demonstrate the effectiveness of the proposed control scheme.