Hillock Growth Research Articles

Control and Regulation of Au Hillocks Growth by the Electrical Field and their Metastable Dynamics in Ambient Atmosphere

Abstract With the rapid development of electronic engineering and nanotechnology, the electrical field-induced surface migration plays an increasingly important role in the fields of materials science. By comparing the changes in the surface migration behavior of gold nanogap electrodes under different electric field parameters, we reveal the kinetic processes and influencing factors of electric field-induced surface migration. Under the condition of only 1 pA current limit, under the continuous electric field, the hillcok only appears in the anode and gradually evolve into multiple peaks when rises to some extent. By establishing a finite element model of the nanogap, we find the hillock grows under the field strength gradient. Then hillock growth is biregulated by applying a positive or negative voltage to the drain end of the gap device. Located in the normal temperature atmosphere, the morphology will degenerate, and this feature is opposite to the hillock generated by electromigration.

Temperature and thickness-dependent silver hillock generation mechanism and surface morphology nature of direct plated silver layers onto copper substrates

This study delves into the formation of hillocks on Ag surfaces, particularly those resulting from heat-induced processes. We investigate the Ag hillock generation mechanisms in direct Ag plating layers on copper (Cu) substrates, employing a combination of experimental and numerical approaches for comprehensive quantification. In the Ag layer system directly plated on Cu, the strain inside the grain system significantly increases as the temperature increases, but the energy level of the system drops due to the relaxation of the internal strain starting from a certain critical temperature. Namely, the Ag hillock growth mechanisms on Ag plated Cu substrate originated from the microstructural development with thermomechanical behaviors. Nevertheless, the Ag layer directly plated on Cu suffered Ag-Cu diffusion through the grain boundary due to heat, which contaminated the Ag plating surface. Hillock formation is possible even in an Ag layer directly plated on a Cu substrate, but Ag-Cu diffusion may be a factor that inhibits the development and growth of Ag hillocks. In this study, the major feature of the Ag hillock formation on the direct Ag layer plated on Cu substrate was systematically clarified in terms of the microstructures and mechanics of materials.

Microstructure and growth of Cu hillock on redistribution line under electromigration

Microstructure and growth of Cu hillock on redistribution line under electromigration

Seeded Growth of AlN Layers on Al‐ and N‐Polar AlN Seeds by the Cu–Al–Ca Solution Growth Method

Herein, the growth behavior of aluminum nitride (AlN) layers on the Al‐polar and N‐polar surfaces of AlN seeds is investigated by the Cu–Al–Ca solution growth method. AlN layers with a thickness of 50–60 μm grow on N‐polar seeds in 5 h, whereas the thickness of the AlN layer on the Al‐polar seeds reaches approximately 35–50 μm. Optical microscopy, X‐ray diffraction, high‐resolution X‐ray diffraction (HRXRD), Raman spectroscopy, and photoluminescence (PL) spectroscopy are performed to investigate the morphology and crystal quality of the grown AlN layers. The N‐polar and Al‐polar surfaces show hillock growth, which is consistent with the hexagonal structure of AlN. However, the surfaces show differences in terms of hillock size (smaller on the N‐polar surface). The HRXRD rocking curves of (002) and (102) show full‐width at half‐maximum (FWHM) values of 160.25 and 33.53 arcsec, respectively. From Raman analysis, the FWHM values of the AlN layers on the N‐polar and Al‐polar seeds are 6.1 and 10.6 cm−1, respectively. The PL results show 310, 460, and 590 nm emission peaks of the AlN layer, which may be caused by oxygen impurity and VN (nitrogen vacancy) and Ali (aluminum gap).

Observation of nanopipes in edge-defined film-fed grown β-Ga2O3 substrate and their effect on homoepitaxial surface hillocks

β-Ga2O3 substrates and homoepitaxial films were characterized using the multiphoton excitation photoluminescence (MPPL) method. MPPL emission peaks at 3.2 and 3.36 eV were obtained with broad shoulders on the low energy side. MPPL images showed the presence of nanopipes in the substrate with a unique contrast. Three-dimensional MPPL imaging revealed the distribution and spatial location of the nanopipes. All the nanopipes were elongated along the [010] direction with diameters of approximately 0.4–1.0 μm and lengths of 10–30 μm. The nanopipes were aligned in the [100] and [001] directions with sub-micron spacing in the (00)-oriented substrate. The nanopipes were located under the hillocks at the homoepitaxial surface. The nanopipes are suggested to trigger the hillock growth by transforming into threading dislocations in the homoepitaxial film.

InGaN surface morphology evolution investigated by atomic force microscope with power spectral density analysis

The morphology of thin InGaN layers grown at varying temperature by metalorganic chemical vapor deposition (MOCVD) has been studied by atomic force microscope (AFM). The surface morphology undergoes a transition process from three-dimensional (3D) island growth mode to two-dimensional (2D) nucleated mound and hillock growth mode and finally to layer-by-layer step-meandering and step flow growth mode. The changes in morphology were analyzed from a thermodynamic (mostly based on the BCF theory of crystal growth by Burton, Cabrera and Frank) and kinetic (mostly the ESB theory based on the Ehrlich-Schwöbel barrier) perspective. Power spectral density (PSD) analysis of AFM surface images is used to determine the dominant smoothing mechanisms that contribute to the feature of surface morphology of InGaN, and validate our analysis based on BCF and ESB theories.

Modeling of growth rate and morphology mechanism of whisker and hillock on micro-bumps under temperature-pressure-humidity storage

A novel model was established to calculate growth rate of whisker and illustrate the morphology mechanism of whisker on fine-pitch Cu pillar micro-bumps under temperature-pressure-humidity storage. Growth of tin whisker and hillock underwent incubation period, quick growth and final steady growth. Theoretical steady-state average growth rate of whisker on micro-bumps calculated by our model were much more accurate than the previous models in order of magnitude. Besides, the diversification of morphologies of tin whisker and hillock were dependent on the synergetic effect of JL, JR, slide plane rotation, grain boundary migration and intermetallic compounds particles pinning. The findings are of great significance to the study of tin whisker and hillock on isolated Sn-based solders and provide insights into the reliability of advanced packaging.

Hillock formation in β-Sn films during high frequency cyclic bending at low strains

Cyclic bending of ß-Sn films with 1μm thickness on atomic force microscopy silicon cantilevers was performed at resonance in both first and second mode vibration. The stress response as a function of position along the cantilever at very small strain amplitudes was examined by changes in surface morphology. Hillock formation and growth, the dominant responses observed, were measured using scanning electron microscopy as a function of the number of cycles and the strain distribution along the cantilever. New hillocks formed with increasing numbers of cycles, with their morphologies similar to the pre-existing hillocks observed before cycling. However, all hillocks exhibited a constant diameter-to-height ratio observed of 1: (1–1.2), with both pre-existing and new hillocks reached a limiting height which did not grow with further cycling. A saturation in the number of hillocks was observed with increased cycling, with the cycles to saturation depending on the vibration mode. The hillock density along the cantilever was found to be proportional to the local strain amplitude in both the first and second vibration modes. These results demonstrate that even the small maximum strain amplitudes in these experiments are sufficient to drive the nucleation and growth of hillocks as a stress relaxation response. Comparisons between these experiments and prior studies of whisker and hillock formation on β-Sn films due to Cu-Sn intermetallic compound growth and thermal mismatch stresses in thermal cycling are made in terms of morphologies, densities, and time dependence of nucleation and growth.

Surface Morphology and Microstructure Evolution of Single Crystal Diamond during Different Homoepitaxial Growth Stages.

Homoepitaxial growth of step-flow single crystal diamond was performed by microwave plasma chemical vapor deposition system on high-pressure high-temperature diamond substrate. A coarse surface morphology with isolated particles was firstly deposited on diamond substrate as an interlayer under hillock growth model. Then, the growth model was changed to step-flow growth model for growing step-flow single crystal diamond layer on this hillock interlayer. Furthermore, the surface morphology evolution, cross-section and surface microstructure, and crystal quality of grown diamond were evaluated by scanning electron microscopy, high-resolution transmission electron microcopy, and Raman and photoluminescence spectroscopy. It was found that the surface morphology varied with deposition time under step-flow growth parameters. The cross-section topography exhibited obvious inhomogeneity in crystal structure. Additionally, the diamond growth mechanism from the microscopic point of view was revealed to illustrate the morphological and structural evolution.

Extraordinary Nanocrystalline Pb Whisker Growth from Bi-Mg-Pb Pools in Aluminum Alloy 6026 Moderated through Oriented Attachment.

The elucidation of spontaneous growth of metal whiskers from metal surfaces is still ongoing, with the mainstream research conducted on Sn whiskers. This work reports on the discovery of Pb whisker growth from Bi-Mg-Pb solid pools found in common machinable aluminum alloy. The whiskers and hillocks display unique morphologies and complex growth that have not been documented beforehand. In contrast to typical understanding of whisker growth, the presented Pb whiskers show a clear nanocrystalline induced growth mechanism, which is a novel concept. Furthermore, the investigated whiskers are also found to be completely composed of nanocrystals throughout their entire length. The performed research gives new insight into nucleation and growth of metal whiskers, which raises new theoretical questions and challenges current theories of spontaneous metal whisker growth. Additionally, this work provides the first microscopic confirmation of recrystallization growth theory of whiskers that relates to oriented attachment of nanocrystals formed within an amorphous metallic matrix. The impact of mechanical stress, generated through Bi oxidation within the pools, is theoretically discussed with relation to the observed whisker and hillock growth. The newly discovered nanocrystalline growth provides a new step towards understanding spontaneous metal whisker growth and possibility of developing nanostructures for potential usage in sensing and electronics applications.

A comparative study on the growth behaviors of Sn whiskers and hillocks in a Sn-Al alloy coating under different environments

In this study, the effects of factors such as temperature, solution and surface roughness on the growth of whiskers in a 100 μm thick Sn-Al alloy coating were comparatively investigated under different environments. The results indicated that whiskers did not grow after nucleation at 30 °C in oil. With the increase in aging temperature, whiskers started to grow after nucleation. The morphologies of whiskers showed the filament-type at 50 °C and the hillock-type at 100 °C. The residual compressive stress in the coating gradually decreased with the growth of whiskers and hillocks. Whisker nucleation was not found on the coating kept in water solution. The residual stress in the coating was rapidly released through the micro-cracks at the coating/substrate interface instead of growing whiskers. The oxide layer on the coatings with rough surfaces was easier to crack due to the higher thermal stress. The thermal stress on the oxide layer was in directly proportional to the depth of indentation and inversely proportional to the grain size. Whiskers preferentially grew on the (001)-oriented Sn grains in the coating. The finite element simulation results indicated that the grains with c-axis perpendicular to the coating surface had higher thermal strain and tended to grow whiskers. During creep deformation, the stress exponent increased from 7.0 to 11.5 with the increase in temperature. The mechanism for whisker growth was dislocation slip. Grain boundaries were observed in the hillocks. The power-law breakdown was the mechanism for the growth of hillocks.

Carbon related hillock formation and its impact on the optoelectronic properties of GaN/AlGaN heterostructures grown on Si(111)

The integration of GaN on Si as large scale substrates still faces many hurdles. Besides the large difference in the lattice constant and the high thermal mismatch existing between GaN and Si, spiral hillock growth phenomena are common problems in the development of thick GaN layers. In this work, hexagonal hillocks were observed on GaN/AlGaN high-electron-mobility transistor heterostructures grown on Si(111) by metal-organic chemical vapor deposition. The presence of these morphological and structural defects is attributed to the presence of localized contamination at the AlN/Si interface. These carbon-based defects cause highly defective regions in the AlN seed layer, which propagate through all the AlGaN buffer layers inducing the formation of V-shaped pits at the AlGaN interfaces. In hillock regions of the wafers, Raman spectroscopy indicates disturbed two-dimensional electron gas characteristics resulting from GaN/AlGaN interface roughness and a decreased amount of free carriers in the potential well. Energy-dispersive x-ray spectroscopy reveals Ga accumulation inside the V-pits and in nanopipes below, which is responsible for defective areas and the increased GaN growth rate resulting in hillock formation. Photoluminescence measurements confirm the presence of Ga-rich material reducing the inherent gallium vacancy concentration. Here, the reduced amount of Ga-vacancies acting as a shallow acceptor suppresses the ultraviolet luminescence band from donor–acceptor pair transition.

Effects of an interfacial layer on stress relaxation mechanisms active in the Cu-Si thin film system during thermal cycling

The authors report effects of placing a very thin metallic interlayer, such as W and Ni, in between the Cu film and the Si substrate on cyclic thermal stress-induced interfacial sliding and hillock growth in Cu. Cu–Si samples with no interlayer were the most prone to both interfacial sliding and hillock growth, whereas samples with the Ni interlayer were the most resistant against these deleterious phenomena. While the rate of interfacial sliding decreased with each consecutive thermal cycle, hillocks continued to grow undeterred. The obtained experimental results are discussed, considering the compressive stress field generated in the Cu film.

Hillocks formation in the Cr-doped Ni thin films: growth mechanisms and the nano-marker experiment

Stress relaxation in thin metal films often results in formation of hillocks—high grains protruding from the rest of the film. Their formation is discussed in terms of surface diffusion and accumulation of the film material, as well as in terms of material accretion at the film–substrate interface and grain boundary sliding. We employed an ultrathin film of Cr oxide embedded in the middle of a thin nickel (Ni) film deposited on sapphire substrate as a marker tracking the material flow during film annealing. We found that during annealing at the temperature of 700 °C the marker under the growing hillocks remains largely immobile, indicating the negligible role played by grain boundary sliding in hillock formation, and the lack of material accretion below the hillock and along the film–substrate interface. We suggest that the hillocks grew by lateral diffusion of Ni, either along the grain boundaries or on the film surface covered by Cr oxide, and material accumulation on the top of the hillock by surface diffusion. We discuss the hillock nucleation and growth in terms of defects in the layer of Cr oxide formed on the surface of the Ni film.

Nucleation and Growth of Tin Hillocks by In Situ Nanoindentation

For several decades, tin whiskers have been a major reliability issue in the microelectronics industry. These single crystalline tin filaments can grow long enough to cause short circuiting and device failure. Although tin whisker/hillock growth is driven by compressive stresses, a mechanistic model of their formation, evolution, and microstructural influence has not been fully developed. In this work, the growth of mechanically induced tin whiskers/hillocks was studied using an in situ nanoindenter and electron backscatter diffraction in a dedicated scanning electron microscope. Electroplated Sn-on-Cu samples were indented and monitored in vacuum to study their growth behavior without the influence of atmosphere. Aging experiments were conducted to study the effect of intermetallics on hillock growth. The grain orientation of the hillocks and the plastically deformed area surrounding the indentation were studied on slabs lifted out of the sample with the use of focused ion beam. High-angle grain boundaries were seen to favor the formation of Sn hillocks. A finite element model was developed to study the evolution of the compressive stress state in the Sn plating and the results showed good agreement with the experimental results.

Large-area die-attachment by silver stress migration bonding for power device applications

In this study, the possibility of using Ag stress migration bonding (SMB) technology for applications of large-area die-attachment has been revealed. We observed how hillock growth changed on the Ag layer with various bonding conditions, how it varied in large area, and how it is related to the bonding strength. The large bonded area (20 × 20 mm2) was achieved by using a SMB process, resulting in an average flexural strength of 40.5 MPa at 300 °C. In-situ stress analysis during heating confirmed that the compressive thermal stress was generated due to the CTE mismatch at the Ag layer/substrate interface and was released from about 100 °C in overall areas. The relaxation of residue stress is caused by the formation of abnormal silver hillocks, which have been elucidated by a nano-volcanic eruption mechanism. The bonding strength in various areas indicates that the hillock growth can be occurred homogeneously on the whole bonding area. Our work indicates that the SMB process can be an attractive technology for large area bonding in the power device electronic applications.

A Multilevel Zero-Inflated Model for the Study of Copper Hillocks Growth in Integrated Circuits Manufacturing

As integrated circuits (ICs) are rapidly migrating to smaller feature sizes, copper hillocks growing vertically from the copper back-end interconnects may cause inter layer metallic shorts and reliability issues. Uncovering the impact of design factors on the formation of copper hillocks is of importance for reducing the number of shorts and improving the ICs design. The purpose of this paper is twofold: 1) to experimentally investigate the mechanisms for hillock formation in a manufacturing environment and to develop a statistical model incorporating these features for more accurate predictions and 2) to determine optimal design settings to minimize hillock related short-circuits. In this paper, we describe our approach to collecting the data and our statistical model for the analysis of copper hillocks growth on ICs. To capture the low failure observations and multilevel variations, we extend the zero-inflated model, which assumes the system randomly shifts between a zero state and a count state, to a multilevel model for our problem. We first analyze the mechanism causing the zero state to determine at which level the state shift occurs. Then, the distribution of the count state is derived based on the hillocks number and the hillock height. Finally, flexible random effects distributions are incorporated to model the non-normal variations. Our analysis of the process and development of the model enable the capture of the unique characteristics of the process, and provide a platform to understand the factors governing the growth of copper hillocks in a manufacturing environment that would not have been possible if standard existing models have been applied. This has led to some previously unknown insights on the physical hillocks growth process.

Solid porous Ag–Ag interface bonding and its application in the die-attached modules

Solid porous Ag–Ag interface bonding was introduced with the hillocks growth on a solid porous Ag surface. Porous Ag was fabricated on both Cu substrate and Si die by sintering a hybrid Ag paste. The size, density of grown hillocks was investigated at different heating temperature and time to provide an optimized condition in the interface bonding. The growth mechanism of hillocks on the solid porous Ag structure was discussed and analyzed in detail based on stress-induced migration and finite element stress analysis where solid porous Ag was reduced to a polycrystalline structure. The interface between two porous Ag structures bonded together in the die-attached structure with the bonding strength over 30 MPa, considerably stronger than that of traditional Sn–Pb solders. Furthermore, TEM observation revealed that Ag particles with the size of tens nanometers formed between the solid porous Ag–Ag interface, accumulate together and thus bridged the interface. This study provided a new attractive bonding technology based on the solid porous Ag structure in the die-attached module structures for large area bonding and thicker bonding layer design.

Heat-resistant die-attach with cold-rolled Ag sheet

This work introduces a bonding technology that employs a cold-rolled Ag sheet, which provides a unique reaction in air, to achieve low-pressure and low-temperature die-attachment for wide-bandgap semiconductors. The cold rolling of the Ag sheet induces grain refinement down to a sub-micron level with high residual stress, which is the driving force for hillock growth on the Ag sheet, resulting in intimate bonding. The detailed mechanism underlying the hillock joining was investigated via transmission electron microscopy (TEM). TEM images and electron-diffraction patterns showed that a large quantity of self-generated Ag nanoparticles formed in the contact area between two Ag substances.

Homoepitaxial growth of single crystal diamond by microwave plasma chemical vapor deposition

Homoepitaxial growth of single crystal diamond on a diamond substrate prepared by the high pressure high temperature method was carried out by microwave plasma chemical vapor deposition and the effects of methane concentration and substrate temperature on the quality of the as-grown diamond were investigated. The diamond was characterized by scanning electron microscopy and Raman spectroscopy. Results show that step growth and hillock growth are two main homoepitaxial growth modes, where a polycrystalline structure appears to form easily in the latter case. The surface roughness of the diamond is effectively improved by decreasing the methane concentration because it suppresses hillock growth. The homoepitaxial growth rate increases with increasing methane concentration, pressure and substrate temperature. However, the quality of the diamond is decreased by increasing the methane concentration and substrate temperature above 2% and 1 150 ℃, respectively. The content of amorphous carbon increases with growth layer thickness. A diamond with a high crystallinity and low defect concentration is obtained under optimal conditions.