Hillock Growth Research Articles

Ratchet effect in surface electromigration detected with scanning force microscopy in gold micro-stripes

In this work, we present scanning force microscopy studies on the effect of large current densities flowing through a micrometer gold stripe evaporated on a glass substrate. Classical electromigration effects, that is, voids and hillocks growth, can be observed on a relative long-range scale (typically 1 μm) under direct-current stressing. However, on the nanometer scale, direct current stressing induces grain growing due to thermal heating by the Joule effect, and no direct evidence of electromigration is observed. At this scale, observable electromigration effects appear only under alternating current stressing, and it is found that the grain structure, characteristic of a gold thin film, evolves to a faceted structure. We attribute this phenomenon to a ‘ratchet’ effect.

Relationship between microstructure and electromigration damage in unpassivated PVD copper damascene interconnects

The microstructure of unpassivated PVD copper interconnects has been determined by electron backscatter diffraction technique (EBSD) inside a scanning electron microscope (SEM), and the appearance and growth of voids and hillocks during the electromigration testing has been observed in situ inside the SEM. The EBSD measurement indicates a strong <111 > texture for the tested line and a high angle boundary fraction of more than 70%. The comparison of the EBSD maps and the SEM images of the defect formation due to electromigration shows that the voids are formed mainly at the sidewall and after blocking grains. These images indicate that the diffusion paths are both the interface and the grain boundaries.

Microstructure of thermal hillocks on blanket Al thin films

The microstructure of thermal hillocks on blanket Al thin films has been studied for the first time by several techniques, including sectioning and imaging in a focused ion beam system. It is found that the new material in the hillock area lifts the original film up and in some cases penetrates it. The micrographs also reveal the grain structures and give valuable insight into the mechanisms of hillock growth.

Ex Situ and in Situ Observations of the Surface Morphology of the (010) Face of Potassium Hydrogen Phthalate (KAP) Crystals

The surface morphology of the (010) face of potassium hydrogen phthalate (KAP) crystals grown from aqueous solution at low supersaturations (0.03 to 0.05) has been investigated using ex situ optical and electron microscopy in conjunction with a gold decoration technique. In situ atomic force microscopy (AFM) has also been used to examine the growth of dislocation-free hillocks. A variety of growth patterns was found: macroscopic hillocks, shallow hillocks, microscopic hillocks and numerous two-dimensional nuclei. The macroscopic hillocks and smaller polygonal hillocks are formed by groups of cooperating spirals or by single spirals. The density of observed shallow hillocks is higher than the dislocation density. The results reveal that, at low supersaturations, growth of the (010) face occurs by combined spiral and two-dimensional nucleation mechanisms. The influence of low concentrations of organic additives on the surface morphology of the (010) face of KAP has also been studied. The additives stimulate two-dimensional nucleation but they also cause an increase in the spread velocity of steps. The growth promoting effect of impurities is discussed in terms of the above data.

Kinetics of hillock growth in Al and Al-alloys

Hillock growth kinetics and size distribution were investigated in Al, Al:Si 1% and Al:Si1%:Cu 0.5% layers. Metallization surface was examined by optical, SEM and TEM microscopy, stylus profiling and an automatic method of hillock recognition from a microscope image. The method allowed for counting hillocks in a desired range of their diameter d. Surface density of hillocks was measured as a function of time of furnace annealing at 400°C and as a function of temperature of RTP annealing. A maximum hillock size was found to increase linearly with metallization layer thickness and with logarithm of annealing time. A total area occupied by hillocks was evaluated. Hillock density decreased versus 1/ T with an activation energy of 0.28 eV for Al and 0.31 eV for Al:Si. It was found, that a normalized hillock density N may be expressed by a formula N= N 0 exp(− cd). Values for N 0 and c are given together with a short discussion.

Effect of aluminium oxide caps on hillock formation in aluminium alloy films

The effect of surface oxide layers on thermally induced hillock formation was examined in AlTa and AlCu alloy films. An anodic oxide or a sputter-deposited oxide layer was intentionally formed on the top of the Al alloy films, and subsequently annealed in a vacuum of less than 1×10−4 Pa. Hillock formation on the encapsulated films, the dependence of hillock density on types and thickness of the oxides, and film stresses were investigated. It was observed that hillocks preferentially formed under the oxides and extruded out of the films, breaking through them. SEM and cross-sectional TEM micrographs revealed hillock growth along with the oxide/metal interface and deformation of the surface oxides following the change of surface topography by hillock formation. More than 100 nm in thickness of anodic oxide caps or a 230-nm thick sputter oxide were necessary to suppress hillock formation. An identical hillock density was obtained in each Al alloy film with encapsulation up to 62 nm in thickness, independent of the thickness and type of the oxide cap. The results indicate that surface conditions are unlikely to determine hillock density, and hillock suppression in the encapsulated films was presumably achieved by lower film stresses at elevated temperature resulting from higher initial tensile stresses induced by anodization and a smaller gradient of the stress-temperature curve of the metal/oxide multilayered films.

Hillock Growth Phenomena during Post-Indentation Annealing of Quasicrystalline AlPdMn

Single-quasicrystaltine icosahedral AlPdMn was subject to Vickers microindentations at room temperature, annealed up to 750°C in vacuum, and then inspected using optical microscopy, atomic force microscopy (AFM), wavelength dispersive microanalysis (WDX) and Auger spectroscopy (AES). At temperatures above 600°C the whole sample surface became increasingly rough, exhibiting holes and small elevations, whereas the vicinity of the impressions remained free of surface alterations. The growth of large hillocks out of the impressions was observed at temperatures above 700°C in the presence of oxygen. No growth of high elevations was noted for an oxygen partial pressure below 3 x 10 -6 Pa. AES and WDX revealed an enrichment of manganese and the depletion of aluminium in the hillocks. The results are discussed in the frame of indentation induced defects and their impact on diffusion and surface migration.

Hillock Growth Phenomena during Post-Indentation Annealing of Quasicrystalline AlPdMn

Hillock Growth Phenomena during Post-Indentation Annealing of Quasicrystalline AlPdMn

Characterization of surface structure in sputtered Al films: Correlation to microstructure evolution

Quantitative roughness and microstructural analysis of as-deposited Al films, 0.1–1.0 μm thick, were performed by atomic force microscopy (AFM), one-dimensional power spectral density analysis (1DPSD), transmission electron microscopy, and x-ray pole figure methods. The variation of grain size (d) with thickness (h) in the columnar grained film was d∝h0.9. The initial crystallographic texture was nearly random, with a strong Al (111) fiber texture evolving by ≈0.2 μm in deposited thickness. AFM imaging revealed a surface structure with hillocks, grains, and grain boundary grooves, and periodic within-grain ridges extending over entire grains. The root-mean-square surface height variation (RRMS) initially decreased during deposition but increased as RRMS∝h0.55 from 0.3 to 1.0 μm thickness. The 1DPSD analysis revealed three spatially resolved regimes of roughness evolution; a frequency independent regime at low frequency attributed to hillock growth, an intermediate frequency self-similar regime attributed to grains and grain boundary grooves, and a high frequency self-similar regime attributed to within-grain ridges. Two characteristic dimensions (CD) were defined at the inverse frequencies of transition between each 1DPSD roughness regime. CDI at high frequency was identified as the peak-to-peak ridge spacing which remained independent of film thickness. The ridge spacing is proposed to represent the upper limit of an effective surface diffusion length (λ0) due to the effects of surface diffusion and flux shadowing. The CDII at lower frequency was identified as the grain size which increased with thickness. The evolution and interactions of roughness and microstructural features are discussed in terms of surface diffusion, grain boundary motion, and flux shadowing mechanisms.

Hillock Growth Mechanism in Pt Thin Film.

The hillock growth mechanism at the surface of Pt/TiN electrode is investigated. Transmission electron microscopy shows that the hillock forms from delamination at the interface of Pt/TiN and plastic deformation of Pt film. Stress measurement in a Pt film shows that the hillock growth starts when the internal stress in a Pt film exceeds about -1 GPa during high-temperature annealing. Since the stress in Pt film varies with deposition temperature from 500 MPa to -500 MPa, starting temperature of the hillock growth depends on the deposition temperature. Therefore, the stress control of Pt film is very important in order to eliminate hillock growth at the electrode surface.

Recombination mechanisms in crystalline silicon: bulk and surface contributions

In the present paper the most interesting results of a work on the influence of some processes used in the fabrication of microstructures on the bulk and surface electrical activity of the Si substrate are reported. In these studies the minority carrier lifetime and the surface recombination velocity were used as sensors of the Si electrical activity changes. Other electrical (DLTS, C-V and I-V characteristics, EBIC) and structural (X‐ray double crystal diffractometry, IR spectroscopy) techniques were used to gather information about the nature and concentration of the defects induced by processing.Two processes have been investigated: Rapid Thermal Annealing treatments (RTA), and Reactive Ion Etching (RIE): neither of them have a large influence on the minority carrier diffusion length when the substrate is an high quality Si wafer of the last production. On the contrary, the surface recombination velocity is largely influenced by both processes. The rearrangement of the surface structure and the formation and growth of hillocks is proposed as the more important mechanism responsible of the observed changes in the surface electrical activity.

Diffusional Hillock Formation in Al Thin Films Controlled by Creep

AbstractThermal hillocks in sputter-deposited Al films have been studied as a part of a broad study of stress-induced diffusional processes in Al. Trace amounts of the impurities Ti, W, and O were incorporated into the films during deposition, causing them to be much stronger than most sputter deposited Al films. Stress measurement during thermal cycling, using the wafer curvature method, showed that these Al films are very strong; this finding was corroborated by hardness measurements. Microstructural studies using TEM and FIB showed that the hillocks start to form at the Al/SiO2 interface and grow under the original Al film, with its columnar grain structure. In some cases, the film fails as hillocks grow completely through the original film. The Al film on top of the hillocks appears to inhibit hillock growth by creating a back pressure associated with power law creep of the film. We modeled this form of hillock formation by modifying the boundary conditions in Chaudhari's hillock model [1]. Our model describes hillock formation by diffusion of Al atoms from the surrounding area into isolated hillocks, assuming that the original Al film on top of hillocks deforms following power law creep. Our model can be applied to many different situations by using different creep laws for the top Al film.

Dendritic crystallization of amorphous germanium by in situ thermal pulse annealing

Phase transformation of amorphous-germanium thin film has been carried out on amorphous substrate by in situ thermal-pulse annealing in a high vacuum chamber directly after evaporation. The microstructure of the resultant film was shown to depend markedly both on the annealing ambient and on the time of exposure (te) for the as-deposited films under full-powered (&amp;lt;102 W/cm2) incoherent broadband irradiation. The heating rate was estimated to be not less than 102 K/s. The surface morphology of the sample was examined by atomic force microscopy (AFM), scanning electron microscopy, and optical microscopy. For samples annealed in air, hillock growth mode was observed, while for samples annealed in vacuum, a transition from the microgranular to dendritic grain growth, depending sensitively on te, was clearly evident. Surprisingly, the length of the crystallized dendrites could be as long as ≃104 μm, being at least ≃104 times larger than the thickness of the film. The dendritic morphology, the implied growth rate, and the condition of crystallization lead us to suggest that the Ge film may exist in a supercooled semiconductive liquid phase just before crystallization. X-ray diffraction analysis revealed that grains were crystallized dominantly with a random orientation for te&amp;lt;2.84±0.05 s, while a sharp transition to a preferred 〈110〉 crystal orientation occurred at the critical te of 3.22±0.05 s, corresponding to a maximum temperature (Tm) of 577 °C reached by the system. This transition is consistent with the appearance of dendrites in AFM micrographs. More interestingly, an anomalous lateral size effect of the substrate on the misalignment of the 〈110〉 crystal direction of different grains with respect to the substrate normal was observed from the x-ray rocking curves. Careful inspection of the AFM images found that the giant dendrites broke up into individual columnar grains as the substrate width went down in size.

Characterization of interfacial growth between Bi(2212) and Ag coating

The growth of hillocks at the interface between Bi(2212) and Ag has been found to occur over a wide range of oxygen partial pressure and in the vicinity of 700/spl deg/C, a temperature far below the Bi(2212)-Bi(2223) conversion temperature. These hillocks have been examined by environmental scanning microscope (ESEM) and regular SEM in secondary and backscattering modes. Definitive chemical analysis is still an open question. The Ag is highly mobile at these temperatures.

Hillock Growth at the Surface of Pt/TiN Electrodes for Ferroelectric Capacitors During Annealing in N2/O2 Ambient

ABSTRACTThe hillock growth mechanism at the surface of Pt/TiN electrodes is investigated. TEM and SEM observations confirm that local delamination occurs at the Pt/TiN interface first, and then, plastic deformation of the Pt films under compressive stress forms hollow domes, which result in hillocks. Hillocks always start to grow when the internal stress in the Pt films reaches about −1000 MPa during annealing in N2/O2 ambient. Since the initial internal stress of Pt thin films varies from −500 to 500 MPa, depending on their deposition temperature, the hillock growth temperature strongly depends on the deposition temperature of the Pt films. It is very important, therefore, to control the initial internal stress in Pt films in order to eliminate hillock growth at the surface of Pt/TiN electrodes.

A study on surface morphologies of (001) homoepitaxial diamond films

Abstract A systematic study on surface morphologies of (001) homoepitaxial diamond films prepared by microwave plasma-assisted chemical vapor deposition indicates that growth of these films strongly depends on deposition parameters such as misorientation angles of substrates, methane concentrations, and growth temperatures. With increasing the misorientation angles, the surface morphologies changed from growth hillocks to macrosteps. Step-flow growth resulted in higher growth rates than hillock growth. These indicate that surface steps play a significant role in diamond growth. It is proposed that hillock growth occurs through two-dimensional nucleation on terraces when the density of surface steps is low, while the step-flow growth proceeds along the 〈110〉 directions on the substrates with the high density of steps. Homoepitaxial growth at 1% CH4 produced macrosteps with the surface close to the single-domain structure, while at 2 and 6% CH4, growth hillocks and random growth morphology occurred with the double-domain surface. The variation of surface morphologies and structure with methane concentration is attributed to lower mobility and shorter diffusion length of adsorbates on the surface at higher methane concentrations. During the step-flow growth, step bunching was more apparent at a lower temperature of 875 than at 1200°C. Step-flow growth with the single-domain surface is believed to produce higher-quality films with fewer lattice defects than other growth modes. It is likely that the step-flow growth is favored by increasing the misorientation angles, lowering the methane concentrations, and increasing the growth temperatures.

Behaviors of Precipitates, Voids, and Hillocks in Electromigration Stressed Al-2wt%Cu Interconnects

ABSTRACTWe observed the dynamics of Al2Cu precipitates, voids, and hillocks under electromigration (EM) stress (200–250° C, 1–2 MA/cm2) in Al-2wt%Cu lines using in-situ and ex-situ side-view TEM. The sub-micron wide (0.5 to 0.8 μm) bamboo Al-Cu lines were on a TiN underlayer in a drift velocity measurement structure. Large precipitates near the cathode ends of the lines shrank under EM, leaving voids before all the Cu was depleted from the cathode ends. We attribute the void formation at the cathode end to the stram induced by the shrinking precipitates in neighboring Al grams. Precipitates downstream of a sufficient Al source (eroding precipitate or Al grain) eroded without adjacent void growth. Size and location of precipitates were thus important in determining whether voids formed or not. Size and location of precipitates at the anode end, however, did not necessarily affect hillock growth. Hillocks grew near the anode end of the lines even with so-called blocking grain precipitates upstream in some cases.

Early Morphological Changes in Al Lines During Current Stressing

ABSTRACTThis paper will show that scanning force microscopy (also known as atomic force microscopy (AFM)) is a good tool to observe the microscopic changes in the morphology of metallic conductor lines during in situ current stressing with a very high spatial resolution. AFM was used to image AlSi1%Cu0.5% meander structures (1 μm thick, 4 μm wide, 44 mm long) at room temperature during current stressing. The microstructure of the material showed significant changes already at a current density of 1.5 MA/cm2, still before voiding and hillock growth occurred. Growth and reduction of the grain sizes and also grooving and widening of the grain boundaries were observed.

Misfit dislocation formation in lattice-mismatched III - V heterostructures grown by metal - organic vapour phase epitaxy

Misfit dislocation formation in lattice-mismatched III - V heterostructures both under tensile and under compressive stress has been studied. Layers of GaAs under tensile stress have been grown by metal - organic vapour phase epitaxy on substrates with indium concentrations between 0.1 and 1.1%. Compressively strained layers with indium concentrations between 0.5 and 2.5% have been grown on GaAs substrates. For the layers under tensile stress an asymmetrical dislocation pattern has been observed, whereas the compressively strained layers show a symmetrical dislocation pattern. A model describing the relaxation process by the formation of dissociated hexagonally shaped half-loop dislocations is proposed. A difference in the mobilities of the two possible misfit dislocation types is found to be the origin of asymmetrical strain relief at low growth temperatures. In layers under tensile stress the cross slipping of screw dislocations is counteracted by the shear stress, leading to relaxation in only one direction. In layers under compressive stress the nucleated misfit dislocations can undergo cross slipping, resulting in a cross hatched pattern at the surface. At higher growth temperatures the dislocation patterns become more symmetrical due to the higher dislocation mobilities. Growth hillocks are formed on the surfaces of the layers grown under tensile stress, due to local accumulation of dislocations. This hillock growth is prevented at higher growth temperatures by the higher mobility of the dislocations. It is also shown that the (mis)orientation of the substrate is revealed by non-parallel groups of dislocation lines observed at the surface of a relaxed epilayer.

AlSm and AlDy alloy thin films with low resistivity and high thermal stability for microelectronic conductor lines

The addition of either Sm or Dy rare earth metal elements to Al thin films decreases markedly the grain size of the Al matrix and largely suppresses the growth of hillock and whisker thermal defects at high temperatures (350–450°C). A large number of particles of fine metallic compounds of Al 3RE (RESm or Dy) were segregated in an Al matrix, mostly at grain boundaries, after annealing at 350°C. The resistivities of the films after annealing at the above temperatures show very low values of less than about 50 nμ m, without salient formation of hillocks or whiskers on the film surfaces.