Dielectric Roughness Research Articles

Chip to Wafer Hybrid Bonding Development for High Volume Manufacturing

The adoption of 3D architectures in advanced packaging between chips is driven by high performance computing and artificial intelligence requirements [1,2]. Hybrid bonding (HB) is essential for this transition, as it offers high I/O connectivity with fine pitch (<10µm), increases system level bandwidth and speed, and improves thermal budget. Chip-to-Wafer (C2W) HB further offers the benefit of using known-good-die to achieve higher yield, especially for heterogeneous integration. This paper describes the necessary conditions to achieve a successful C2W HB suitable for high volume manufacturing (HVM) production. HB can be achieved in a 2-step process; firstly, by leveraging on the initial forces of surface interaction at atomic proximity of dielectric-dielectric interface to form the “tacking,” which is the initiation of the bonding, and finally, following up with a fusion process by annealing at an elevated temperature (100°C-400°C) to form both stronger dielectric-dielectric covalent bonds with the release of excessive water (H2O) molecules, as well as metal-metal diffusion which will enable the electrical connection. A successful bond can be achieved with careful surface engineering of the dielectric [3], typically with dielectric surface roughness values of <0.4nm. Figure 1 shows an optimized result, demonstrating C2W HB with Cu-Cu interdiffusion. Die-to-die (D2D) or C2W HB has many challenges. Removal of barrier spikes/protrusions in the nm-scale that is not critical in wafer-to-wafer HB is paramount in C2W HB, likely attributed to the reduced volume/weight and no physical pressure applied after the die-tacking process. A chemical-mechanical polishing (CMP) is a critical step to create necessary pre-conditions for HB and must be carefully and meticulously optimized. Prior to bonding, a proper activation step is required on the dielectric surfaces. The delay between activation and bonding can weaken the initial bonds. Q-time control and surface cleanliness are critical for consistent performance and high bonding yield. A repeatable process performance with low contact angles, no degradation of dielectric roughness, and Cu dishing performance is imperative. Results will show that such milestones were achieved with the state-of-the-art systems at the HB Centre of Excellence. Cleanliness is undoubtedly a salient criterion for successful C2W HB. Contaminants such as particles and volatile organic compounds can be detrimental to bonding strength and metal-metal contact resistance. Unlike other advanced packaging methods such as TCB, there is no gap between the die and substrate, nor is there solder or glue that can help to mitigate the impact of any particles present. Since the bonding occurs between flat dielectric to dielectric surfaces, any particle entrapped will result in big voids, rendering the device useless. As such, this paper will also outline the cleanliness levels achieved in the system. Equivalent to this criterion is also the mechanics behind the bonding mechanism itself. At the core of an HB, the process is the bond front propagation, which, if not controlled, can lead to voids, die damage, and loss of accuracy, amongst other things. As such, BESI has developed a series of tools allowing controlled die shaping to initiate bonding at specific locations as well as to control the die release. Die placement accuracy is typically measured using IR equipment. This paper will show and analyze the accuracy results of actual C2W HB dies. This paper will also discuss additional data from post-bond confocal scanning acoustic microscopy (cSAM), contact resistance, and daisy-chain continuity E-test.

Multi-Parameter Inversion of AIEM by Using Bi-Directional Deep Neural Network

A novel multi-parameter inversion method is proposed for the Advanced Integral Equation Model (AIEM) by using bi-directional deep neural network. There is a very complex nonlinear relationship between the surface parameters (dielectric constant and roughness) and radar backscattering coefficient. The traditional inverse neural network, which is constructed by using the backscattering coefficients as the input and the surface parameters as the output, leads to bad convergence and wrong results. This is because many sets of surface parameters can get the same backscattering coefficient. Therefore, the proposed bi-directional deep neural network starts with building an AIEM-based forward deep neural network (AIEM-FDNN), whose inputs are the surface parameters and outputs are the backscattering coefficients. In this way, the weights and biases of the forward deep neural network can be optimized and predicted, which can be used for the backward deep neural network (AIEM-BDNN). Then, the multi-parameters are updated by minimizing the loss between the output backscattering coefficients with the measured ones. By inserting a sigmoid function between the input and the first hidden layer, the input multi-parameters can be efficiently approximated and continuously updated. As a result, both the forward and backward deep neural networks can be built with these weights and biases. By sharing the weights and biases of the forward network, the training of the inverse network is avoided. The bi-directional deep neural network can not only predict the backscattering coefficient but can also inverse the surface parameters. Numerical results are given to demonstrate that the RMSE of the backscattering coefficients calculated by the proposed bi-directional neural network can be reduced to 0.1%. The accuracy of the inversion parameters, including the real and imaginary parts of the dielectric constant, the root mean square height and the correlation length, can be improved to 97.56%, 91.14%, 99.04% and 98.45%, respectively. At the same time, the bi-directional neural network also has good accuracy for the inversion of the POLARSCAT measured data.

Dual-Polarised Radiometer for Road Surface Characterisation

This paper presents measurements using a dual-polarised radiometer operating at 93 GHz to detect ice or water on asphalt in laboratory conditions. The brightness temperatures of both H and V polarizations were measured for a dry surface, liquid water, and ice on asphalt at observation angles of 50° and 56°. The results presented in this paper demonstrate that the studied road conditions can be identified by the radiometer. The measurements are compared with a model and surface parameters, such as dielectric constant and roughness are fitted and compared to reference values. The experiments and results, described in this article, are the first steps towards the future installation of a polarimetric sensor on a moving vehicle for traffic safety.

An Air-to-Soil Transition Model for Discrete Scattering-Emission Modelling at L-Band

Topsoil structures and inhomogeneous distribution of moisture in the soil volume will induce dielectric discontinuities from air to bulk soil, which in turn may induce multiple and volume scattering and affect the microwave surface emission. In situ ELBARA-III L-band radiometer observations of brightness temperature T B p ( p =H or V polarization) at the Maqu site on the Eastern Tibetan Plateau are exploited to understand the effect of surface roughness on coherent and incoherent emission processes. Assisted with in situ soil moisture (SM) and temperature profile measurements, this study develops an air-to-soil transition (ATS) model that incorporates the dielectric roughness (i.e., resulted from fine-scale topsoil structures and the soil volume) characterized by SM and geometric roughness effects, and demonstrates the necessity of the ATS model for modelling L-band T B p . The Wilheit (1978) coherent and Lv et al. (2014) incoherent models are compared for determining the dielectric constant of bulk soil in the ATS zone and for calculating soil effective temperature T eff . The Tor Vergata discrete scattering model (TVG) integrated with the advanced integral equation model (AIEM) is used as the baseline model configuration for simulating L-band T B p . Whereafter, the ATS model is integrated with the foregoing model for assessing its performance. Results show the ATS-based models reduce the underestimation of T B p (≈20-50 K) by the baseline simulations. Being dynamic in nature, the proposed dielectric roughness parameterization in the ATS model significantly improves the ability in interpreting T B p dynamics, which is important for improving SM retrieval at the global scale.

The Bathymetry of Moray Sinus at Titan's Kraken Mare

AbstractMoray Sinus is an estuary located at the northern end of Titan's Kraken Mare. The Cassini RADAR altimeter acquired three segments over this mare during the T104 flyby of Titan, on August 21, 2014. Herein, we present a detailed analysis of the received echoes. Some of these waveforms exhibit a reflection from the seafloor, from up to m of depth (1σ error). Monte Carlo simulations have been performed in order to assess the most probable values and estimation errors for the seafloor depth. Insights from this study, featuring the synergic use of the synthetic aperture radar images coupled to the altimetry and passive radiometry datasets, have been used to constrain the dielectric properties (i.e., absorptivity of the liquid) and roughness of this region of Kraken Mare. The resulting Ku‐band specific attenuation of the liquid is dB/μs, corresponding to a loss tangent of , which is very similar to the loss tangent estimated at Ligeia Mare. The data in hand do not permit us to discern the most likely explanation for the lack of a seafloor reflection from the main body of Kraken Mare: either a very deep sea or a more absorbing liquid composition. However, if the main body of Kraken Mare is characterized by an absorption similar to Moray Sinus, then based on models of the response to altimetry mode observations we can conclude that it exceeds 100 m of depth, which is also compatible with radiometry observations.

A Temperature and Dielectric Roughness-Aware Matrix Rational Approximation Model for the Reliability Assessment of Copper– Graphene Hybrid On-Chip Interconnects

In this article, a closed-form matrix rational approximation (MRA) model is presented for the reliability assessment of copper–graphene hybrid on-chip interconnect networks. The key feature of this MRA model is its capacity to predict how the different values of temperature and dielectric roughness affect the signal integrity performance of the hybrid interconnect networks. As a result, the proposed MRA model is well suited for very fast parametric sweeps and worst case analysis of the hybrid interconnect networks, which has not been possible using existing closed-form models or even SPICE simulations. Numerical examples show that the proposed model is significantly more efficient than conventional models while exhibiting error less than 5%.

Soil and Vegetation Scattering Contributions in L-Band and P-Band Polarimetric SAR Observations

Active microwave-based retrieval of soil moisture in vegetated areas has uncertainties due to the sensitivity of the signal to both soil (dielectric constant and roughness) and vegetation (dielectric constant and structure) properties. A multi-frequency acquisition system would increase the number of observations that may constrain soil and/or vegetation parameter retrievals. In order to realize this constraint, an understanding of microwaves interaction with the surface and vegetation across frequencies is necessary. Different microwave frequencies have varied interactions with the soil–vegetation medium and increasing penetration into the soil and canopy with the decreasing frequency. In this study, we examine the contributions of different scattering mechanisms to coincident observations from two microwave frequencies (L and P) of airborne synthetic aperture radar instruments. We quantify contributions of surface, vegetation volume, and double-bounce scattering components. Results are analyzed and discussed to guide future multi-frequency retrieval algorithm designs.

Influence of dielectric layer thickness and roughness on topographic effects in magnetic force microscopy.

Magnetic force microscopy (MFM) has become a widely used tool for the characterization of magnetic properties. However, the magnetic signal can be overlapped by additional forces acting on the tip such as electrostatic forces. In this work the possibility to reduce capacitive coupling effects between tip and substrate is discussed in relation to the thickness of a dielectric layer introduced in the system. Single superparamagnetic iron oxide nanoparticles (SPIONs) are used as a model system, because their magnetic signal is contrariwise to the signal due to capacitive coupling so that it is possible to distinguish between magnetic and electric force contributions. Introducing a dielectric layer between substrate and nanoparticle the capacitive coupling can be tuned and minimized for thick layers. Using the theory of capacitive coupling and the magnetic point dipole–dipole model we could theoretically explain and experimentally prove the phase signal for single superparamagnetic nanoparticles as a function of the layer thickness of the dielectric layer. Tuning the capacitive coupling by variation of the dielectric layer thickness between nanoparticle and substrate allows the distinction between the electric and the magnetic contributions to the MFM signal. The theory also predicts decreasing topographic effects in MFM signals due to surface roughness of dielectric films with increasing film thickness.

Role of Sub-Nanometer Dielectric Roughness on Microstructure and Charge Carrier Transport in α,ω-Dihexylsexithiophene Field-Effect Transistors

The effect of dielectric roughness on the microstructure evolution of thermally evaporated α,ω-dihexylsexithiophene (α,ω-DH6T) thin films from a single molecular layer to tens of monolayers (ML) is studied. Thereby, the surface roughness of dielectrics is controlled within a sub-nanometer range. It is found that the grain size of an α,ω-DH6T ML is affected by dielectric roughness, especially for 1.5 ML, whereby the transistor performance is barely influenced. This can be attributed to a domain interconnection in the second layer over a long-range formed on the rough surface. With deposition of more layers, both microstructure and charge carrier transport exhibit a roughness-independent behavior. The structural characterization of α,ω-DH6T 10 ML by grazing-incidence wide-angle X-ray scattering reveals that the interlayer distance is slightly decreased from 3.30 to 3.15 nm due to a higher roughness, while an unchanged π-stacking distance is in excellent agreement with the roughness-independent hole mobility. This study excludes the influence of molecular-solvent interaction and preaggregation taking place during solution deposition, and provides further evidence that the microstructure of the interfacial layer of organic semiconductors has only minor impact on the bulk charge carrier transport in thicker films.

Modeling Bare Soil L-Band Polarimetric - Values Using a Second-Order SPM Model

Polarimetric soil moisture retrieval is among the main objectives of leading synthetic-aperture-radar satellite missions since it allows to systematically analyze costly-to-obtain polarization information to increase retrieval accuracy. In this letter, we present the results of modeling the L-band entropy $(H)$ and alpha $(\alpha)$ values as a function of soil dielectric constant and roughness using a second-order small-perturbation model to simulate the polarimetric soil backscattering. Modeling results are then compared to bare soil uninhabited aerial vehicle synthetic aperture radar (UAVSAR) data acquired simultaneously to in situ field campaigns in Canada during SMAPVEx12. Our model is able to correctly predict observed ranges of $H$ and $\alpha$ and to consistently model dielectric constant and roughness changes. Nevertheless, a systematic overestimation of $\alpha$ is observed when compared with the analyzed UAVSAR data set. Taking UAVSAR data as benchmark, theoretical reasons for this mismatch are analyzed.

Self-assembly and charge carrier transport of solution-processed conjugated polymer monolayers on dielectric surfaces with controlled sub-nanometer roughness.

In recent years organic field-effect transistors have received extensive attention, however, it is still a great challenge to fabricate monolayer-based devices of conjugated polymers. In this study, one single layer of poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) is directly dip-coated, and its self-assembly is precisely tuned from nanofibers to granular aggregates by controlling the dielectric roughness on a sub-nanometer scale. The charge carrier transport of the monolayer transistor exhibits a strong dependence on the dielectric roughness, which is attributed to the roughness-induced effects of higher densities of grain boundaries and charge trapping sites as well as surface scattering. These results mark a great advance in the bottom-up fabrication of organic electronics.

Estimation and Analysis of Snow Water Equivalents Based on C-Band SAR Data and Field Measurements

The spatial distribution of the snow water equivalent (SWE) of ground snow cover is one of the most important variables for understanding and simulating cold-region hydrological processes in high mountain areas. In this paper, the characteristics of C-band synthetic aperture radar (SAR) data in an area of shallow dry snow cover were analyzed based on microwave backscattering models and field snow cover measurements. The analysis revealed that the backscattering of shallow dry snow cover was dominated by the scattering of the snow-ground interface, and the dielectric constant and roughness parameters of the underlying frozen soil were the primary factors that affected the total scattering. The dielectric constant of the soil depended on the amount of liquid water present in the soil, and the amount of liquid water was primarily determined by the soil temperature. Furthermore, the insulation properties (thermal resistance) of the snow cover affected the underlying soil temperature. Consequently, the snow cover characteristics affected the backscattering signal recorded by the SAR sensors. Based on an investigation involving ENVISAT-ASAR data and simultaneous ground-based snow observations conducted on 14 March 2008, we found that the snow temperature decreased with increasing snow depth when the snow depth was less than 20 cm, and the temperature increased with decreasing snow depth when the snow depth exceeded 20 cm. Additionally, the amount of liquid water in the underlying frozen soil rapidly increased with increasing temperature. To estimate the SWE, we used a small number of field measurements data and SAR ratio images to develop two equations. The first equation defined the relationship between the snow’s thermal resistance and backscattering ratio, and the second equation yielded the SWE from the estimated thermal resistance of the snow-covered area. The root mean square and relative errors of the estimated SWE were approximately 7.41 mm and 6.17%, respectively. The derived SWE also indicated that blowing snow and sun radiation were two primary factors that determined the spatial distribution of snow cover in mountainous areas.

Impact of the lateral length scales of dielectric roughness on pentacene organic field-effect transistors

We have shown that the lateral correlation length of dielectric roughness strongly affects the molecular and mesoscale structures and charge transport properties of pentacene. Two series of dielectric surfaces with different lateral characteristics were prepared by a multilayer crosslinking polystyrene (MCPS) method and a plasma etching method. The MCPS surfaces consisted of spike-like structures with dimension of several hundred nanometers, whereas the plasma etched surfaces exhibited bumpy structures with a size about ten times smaller. For both methods, the increase of surface roughness tended to disturb the molecular and mesoscale ordering of pentacene films and reduced their charge carrier mobility. However, the performances of the devices prepared by the plasma etching methods showed a much stronger dependence on the surface roughness than those of the MCPS samples. The average mobilities on the MCPS samples with root mean squar roughness of 3.97 nm were 342 times higher than those on the plasma etching surface with similar roughness values. These results suggested that optimizing the nanostructural surfaces of dielectrics presented an effective approach to improve device performance.

Effectiveness of BaTiO3 dielectric patches on YBa2Cu3O7 thin films for MEM switches

A micro-electro-mechanical (MEM) switch built on a superconducting microstrip filter will be utilized to investigate BaTiO3 dielectric patches for functional switching points of contact. Actuation voltage resulting from the MEM switch provokes static friction between the bridge membrane and BaTiO3 insulation layer. The dielectric patch crystal structure and roughness affect the ability of repetitively switching cycles and lifetime. A series of experiments have been performed using different deposition methods and RF magnetron sputtering was found to be the best deposition process for the BaTiO3 layer. The effect examination of surface morphology will be presented using characterization techniques as x-ray diffraction, SEM and AFM for an optimum switching device. The thin film is made of YBa2Cu3O7 deposited on LaAlO3 substrate by pulsed laser deposition. For this work, the dielectric material sputtering pressure is set at 9.5×10−6 Torr. The argon gas is released through a mass-flow controller to purge the system prior to deposition. RF power is 85 W at a distance of 9 cm. The behavior of Au membranes built on ultimate BaTiO3 patches will be shown as part of the results. These novel surface patterns will in turn be used in modelling other RF MEM switch devices such as distributed-satellite communication system operating at cryogenic temperatures.

Effects of Ta incorporation in La2O3 gate dielectric of InGaZnO thin-film transistor

The effects of Ta incorporation in La2O3 gate dielectric of amorphous InGaZnO thin-film transistor are investigated. Since the Ta incorporation is found to effectively enhance the moisture resistance of the La2O3 film and thus suppress the formation of La(OH)3, both the dielectric roughness and trap density at/near the InGaZnO/dielectric interface can be reduced, resulting in a significant improvement in the electrical characteristics of transistor. Among the samples with different Ta contents, the one with a Ta/(Ta + La) atomic ratio of 21.7% exhibits the best performance, including high saturation carrier mobility of 23.4 cm2/V·s, small subthreshold swing of 0.177 V/dec, and negligible hysteresis. Nevertheless, excessive incorporation of Ta can degrade the device characteristics due to newly generated Ta-related traps.

Radar Backscattering of Lake Ice During Freezing and Thawing Stages Estimated by Ground-Based Scatterometer Experiment and Inversion From Genetic Algorithm

Lake ice under phase transition shows large variation on radar backscattering due to the changes of dielectric constant and roughness of ice surface and thus the transmissivity of microwave into ice body. To study the effects of freezing/thawing of ice on radar backscattering in a short time, we spread water over lake ice and continuously measured radar backscattering by using a ground-based microwave scatterometer system operated in C-band HH polarization. By establishing scattering models and applying inversion from genetic algorithm, radar returns were separated into ice-surface, volume, and ice-bottom scatterings, and the changes in dielectric constant and roughness parameters of ice surface were estimated as well. Immediately after spreading water on ice surface, ice-surface scattering was strongest due to high dielectric constant of surface water while volume and ice-bottom scatterings were very weak due to low microwave transmissivity into ice body. As surface water was being frozen, ice-surface scattering became weak with decreasing dielectric constant while volume and ice-bottom scattering increased due to higher transmissivity into ice body. In a transition stage, when surface water was almost frozen, all three scatterings increased simultaneously. Crystallization of ice produced rougher surface overcoming the decrease in dielectric constant, resulting in the increase of ice-surface scattering, while volume and ice-bottom scattering was continuously increased due to increasing transmissivity. At the end of the experiment, air temperature rose above freezing point, and ice surface thawed again so that ice-surface scattering increased while volume and ice-bottom scattering were decreased.

High performance organic field-effect transistors using cyanoethyl pullulan (CEP) high-k polymer cross-linked with trimethylolpropane triglycidyl ether (TTE) at low temperatures

Low-voltage operable organic field-effect transistors (OFETs) were fabricated using a low-temperature curable high-k polymer cyanoethyl pullulan (CEP) as a gate insulator with trimethylolpropane triglycidyl ether (TTE) as a crosslinking agent. With this crosslinking agent, the gate insulators showed low leakage current and a high dielectric constant of 14.8–16, and most importantly, could be cured at low temperatures around 100 °C, which is compatible with most plastic substrates for future plastic electronics. The favorable semiconductor–dielectric interface was obtained due to the surface alignment of well-packed pentacene domains, which enabled excellent transistor performance, with one of the highest mobilities around 6 cm2 V−1 s−1, an on/off current ratio (Ion/Ioff) ∼105, and a steep subthreshold (SS) ∼0.095 V dec−1. The high mobility was found to be closely correlated with the surface CN dipole density rather than other possibilities, such as dielectric roughness and surface energy. The initial growth of the semiconductors was also studied and correlated with the affecting variables.

Multi-hybrid method for investigation of EM scattering from inhomogeneous object above a dielectric rough surface

An iterative strategy combining Kirchhoff approximation^(KA) with the hybrid finite element-boundary integral (FE-BI) method is presented in this paper to study the interactions between the inhomogeneous object and the underlying rough surface. KA is applied to study scattering from underlying rough surfaces, whereas FE-BI deals with scattering from the above target. Both two methods use updated excitation sources. Huygens equivalence principle and an iterative strategy are employed to consider the multi-scattering effects. This hybrid FE-BI-KA scheme is an improved and generalized version of previous hybrid Kirchhoff approximation-method of moments (KA-MoM). This newly presented hybrid method has the following advantages: (1) the feasibility of modeling multi-scale scattering problems (large scale underlying surface and small scale target); (2) low memory requirement as in hybrid KA-MoM; (3) the ability to deal with scattering from inhomogeneous (including coated or layered) scatterers above rough surfaces. The numerical results are given to evaluate the accuracy of the multi-hybrid technique; the computing time and memory requirements consumed in specific numerical simulation of FE-BI-KA are compared with those of MoM. The convergence performance is analyzed by studying the iteration number variation caused by related parameters. Then bistatic scattering from inhomogeneous object of different configurations above dielectric Gaussian rough surface is calculated and the influences of dielectric compositions and surface roughness on the scattering pattern are discussed.

Impact of scaling of dielectric thickness on mobility in top-contact pentacene organic thin film transistors

The effect of scaling of poly(methylmethacrylate) (PMMA) and cross-linkable poly(4-vinylphenol) (PVP) polymer dielectric thickness on field effect mobility in top contact pentance organic thin film transistors was investigated. Mobility at a constant gate voltage improved significantly with reduction in thickness of both dielectrics. Analysis at a constant gate electric field or identical induced accumulation charge revealed that only a part of the improvement occurs due to simple scaling of dielectric thickness, and the remaining is due to improvement in interface quality. Atomic force microscopic analysis of the dielectric surface revealed that dielectric roughness reduced with reduction in dielectric thickness. A comparison of the two dielectrics also indicates that band mobility is significantly higher in pentacene/PMMA as compared to pentacene/PVP dielectric.

How front side plasmonic nanostructures enhance solar cell efficiency

Dielectric roughness on the front surface enhances significantly solar cell efficiency by light trapping in the absorbing layer. However in plasmonic assisted thin silicon solar cells we show, by a detailed analysis of the various mechanisms, that front-surface plasmonic structures enhance the efficiency by a different mechanism—namely the effective broadband forward scattering into the silicon, while trapping (path length enhancement) is relatively small for these structures. The plasmonic local field enhancement contribution is even smaller in this configuration. Based on our study we optimized this “anti-reflection mechanism” by tuning the plasmonic structure such that the spectral location of the more efficiently scattering dipole and quadrupole resonances fit correctly the visible and NIR sun spectrum.