Al Alloy Films Research Articles

Grain-boundary Voiding in Self-passivated Cu–1 at.% Al Alloy Films on Si Substrates

The new generation of semiconductor metallization is based on Cu-film technology. In this paper we report on detrimental changes in surface morphology of self-passivated-Cu–1 at.% Al films which were magnetron-sputtered on Si substrates. Large voids appeared at grain boundaries after annealing, oxidation at temperatures at or above 500 °C, and cooling to room temperature under vacuum. Grain-orientation imaging with electron backscatter diffraction revealed that preferentially high-energy grain boundaries were voided. Contrary to reports in the literature, void growth was not prevented by the extremely clean ultrahigh-vacuum conditions during film fabrication nor by the strong and very sharp grain texture. Instead, it was clearly correlated with the stable surface oxide on the self-passivated films. Void growth was not found after annealing passivation-free films such as pure Cu or unoxidized Cu–1 at.% Al films.

Effects of Si Addition and Heating Ar on the Electromigration Performance of Al-Alloy Interconnects

The electromigration (EM) performance of Ti/Al-alloy multilayered metallization with one-step sputtered Al-alloy has been studied. The Al-alloys investigated included Al–1.0%Si–0.5%Cu and Al–0.5%Cu, and the Al-alloy films were prepared with and without heating Ar. The package-level EM test results indicate that the EM resistance of the Al–Si–Cu stack is nearly identical to that of the Al–Cu stack. Si addition was found to degrade the microstructure of the Al-alloy film, while it had the retarding effect on the Ti/Al reaction, which suggests that there exists a trade-off between the film microstructure and the formation of TiAl3 intermetallic compound. The EM performance of the one-step sputtered Al-alloy stack was enhanced by the use of heating Ar during the deposition of Al-alloy film, which has been attributed to the improved microstructure of the Al-alloy film by the use of heating Ar.

Cu and Dilute Binary Cu(Ti), Cu(Sn) and Cu(Al) Thin Films: Texture, Grain Growth and Resistivity

AbstractAnnealing Cu and dilute Cu(Ti), Cu(Sn) and Cu(Al) alloy films resulted in the strengthening of film texture, with the strongest <111> fiber texture being found for Cu(Ti). Annealing also resulted in a decrease of electrical resistivity and the growth of grains, with the largest grain size and lowest resistivity being seen for pure Cu itself. Among the alloy films, the lowest resistivity was found for Cu(Ti) and the largest grain size for Cu(Al). Electron beam evaporated films with compositions in the range of 2.0-3.0 at% and thicknesses in the range of 420-540 nm were annealed at 400°C for 5 hours. Four point probe resistance measurement, xray diffraction and transmission electron microscopy were used to follow the changes in film resistivity, texture and grain size.

Constrained diffusional creep in UHV-produced copper thin films

The thermomechanical behavior of metallic thin films on stiff substrates is relevant for thin-film devices, but its mechanisms are not fully understood. In this investigation, the mechanical properties of pure Cu and Cu–1 at.% Al films on diffusion-barrier coated Si substrates were studied with the wafer-curvature technique. In ultra-pure films, which were sputtered and annealed under ultra-high vacuum conditions, characteristic stress relaxation at high temperatures was measured, which could be clearly attributed to diffusional creep. Good quantitative agreement with a recent model of diffusional creep constrained by a substrate was obtained. These features were absent in the Cu–Al alloy films, in which Al surface segregation and oxidation had produced a “self-passivating” effect, and in films produced in less clean environments. Based on these results, we propose a model of thin-film deformation based on dislocation glide and constrained diffusional creep.

Constrained Diffusional Creep in Thin Copper Films

ABSTRACTThe mechanical properties of thin metal films have been investigated for many years. How- ever, the underlying mechanisms are still not fully understood. In this paper we give an overview of our work on thermomechanical properties and microstructure evolution in pure Cu and dilute Cu-Al alloy films. Very clean films were produced by sputtering and annealing under ultra-high vacuum (UHV) conditions. We described stress-temperature curves of pure Cu films with a constrained diffusional creep model from the literature. In Cu-1at.%Al alloy films, Al surface segregation and oxidation led to a “self-passivating” effect. These films showed an increased high- temperature strength because of the suppression of constrained diffusional creep; however, under certain annealing conditions, these films deteriorated due to void growth at grain boundaries.

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.

Stress-induced voiding in stacked tungsten via structure

Abstract The stress-induced voiding (SV) in Al-alloy films with stacked tungsten via structures was investigated. Voids were found in interconnections with stacked and borderless vias that had resistance increase after the aging tests. Failure occurs most frequently when the test structures are stored at approximately 250°C. This behavior can be explained by the diffusion creep model similar to SV in a flat line [1] . Finite-element simulations show that tensile stress in Al-lines between upper and lower plugs increases with temperature increase over 175°C. Al grains on W-plugs were found to have high-angle crystalline misorientation in transmission electron microscopy (TEM) observation. The tensile stress and grain misorientation should accelerate the void growth during high temperature storage. O 2 plasma post metal etch treatment is effective to eliminate SV in stacked via structure.

Morphology of sputter deposited Al alloy films

Morphology of Al–2.0at%Ta and Al–2.0 at.% Nd alloy films before and after annealing was investigated for applications of interconnections for liquid crystal displays. It was found that the morphology and the microstructure of Al–2.0 at.% Nd alloy films changed markedly by annealing at the temperature region from 200°C to 300°C, while the morphology of Al–2.0 at.% Ta alloy films did not change by annealing up to 400°C. For the case of Al–2.0 at.% Nd alloy films, the incline of the <111> fiber texture to the substrate normal was observed during annealing. Structural characteristics of the Al films were investigated by TEM, SAD and XRD to determine the influence of alloying elements on the morphology and the fiber texture. From these results, it was concluded that the microstructures strongly influence the morphology and the grain orientation of Al alloy films.

Homogenization of the Bilayers of Cu-Al Alloy and Pure Copper to Produce CU-0.3 at.% Al Alloy Films

ABSTRACTPreparation of Cu-0.3 at.% Al alloy films by homogenization anneals of metallic bilayers consisting of Cu/Cu-AI on SiO2 is studied to produce advanced copper based metallization for future ICs. The bilayer films were annealed at temperature in the range of 200°C- 400°C and the depth profiles were obtained using Rutherford Backscattering technique. It is found that Al diffuses through the undoped copper layer to surface - leading to lower Al concentration in the original Al doped Cu layer and apparently increased concentration of Al in undoped Cu (not detectable by RBS). The resistivity of these annealed films ranged from 2.1 to 2.4 μΩ-cm depending on final Al concentration achieved. Electrical stability of such MOS capacitors was also investigated. All these results will be presented and discussed showing that Al-doped copper layers can be used as diffusion barrier/adhesion promoter between the oxide ILD and copper metal.

Crystallographic Texture and Phase Formation in Blanket TifriN/AICu Films

ABSTRACTInterconnection metallization uses film stacks, often composed of thin (&lt;10 nm) Ti, TiN, or Ti/TiN underlayer(s) with a thick (200–1000 nm) Al-alloy film deposited on top. The texture or preferred orientation in such film stacks has important implications for both processing and reliability. Earlier studies' have demonstrated the importance of the underlayers on Al texture; however, to date no systematic work has been done on the effect of processing conditions on underlayer texture. This study examines the effect of deposition parameters on the underlayer texture development as well as the effect of this underlayer texture on subsequently deposited Al-alloy films. Fiber plots were obtained for Ti &lt;002&gt; and &lt;101&gt; and Al &lt;111&gt; reflections for a series of 20 nm Ti/ 10 nm TiN/400 nm AlCu films using both a conventional Siemens D500 diffractometer with a pole figure attachment and a Siemens HI-STAR Area Detector system using Cu radiation from a rotating anode source. Because of overlap between the Al &lt;111&gt; and Ti &lt;101&gt; reflections, the Al was removed with a subtractive etch. In this way both the Al and underlayer film textures could be quantified. It was found that the Ti and Al-alloy film textures vary depending on the deposition temperature, deposition method and final film thickness. For example, an increase in the substrate temperature from 300° to 500°C caused the Ti film texture to change from &lt;002&gt; to &lt;101&gt;. Additionally, switching the TiN deposition process from physical vapor deposition (PVD) sputtering to chemical vapor deposition (CVD) in a Ti/TiN/AlCu film stack caused a degradation in the AlCu &lt;111 &gt; texture.

Magneto-optical and optical properties of ordered and disordered Fe–Al alloy films

The influence of order–disorder structural transition on the magnetic, magneto-optical, and optical properties of nearly equiatomic FeAl alloy films was investigated. The disordered state in FeAl alloy films was obtained by means of vapor quenching deposition onto glass substrates cooled by liquid nitrogen. The experimental study of the magneto-optical and optical properties of ordered and disordered FeAl alloy films was carried out in the 0.5–5.0 eV energy range at room temperature. The influence of the order–disorder structural transition on magnetic, magneto-optical, optical properties is discussed by using the results of first-principle calculations of the electronic structure and by the structural defect approach.

Comparison of Sputtered Titanium Nitride on Silicon Dioxide and Aluminum-Alloy Thin Films

Titanium nitride (TiN) films are used as anti-reflection coatings (ARC) on aluminum (Al) films to facilitate lithography processes during multilevel metallization for the manufacture of integrated circuits on silicon-based (Si) semiconductor devices. It is generally accepted in the literature that the microstructure of multilevel metal stacks is influenced by the texture of the substrate. For the case of interconnect materials used in the semiconductor industry, a typical metal stack is as follows: Titanium/Titanium Nitride/Al-alloy/ARC-Titanium Nitride. The Ti/TiN layer underneath the Al-alloy film is used as a barrier stack to prevent junction spiking. The Ti/TiN underlayer also determines the growth conditions (crystallography and orientation relationships) of the subsequent Al-alloy film.This study focuses on the microstructural characterization of the ARC-TiN layer on Si-oxide and Ti/TiN/Al-alloy substrates that are fabricated under similar conditions using conventional physical vapor deposition (PVD - sputtering) techniques. The ARC-TiN microstructure was investigated by transmission electron microscopy (TEM) using a Philips EM430 operating at 300 kV.

Exchange induced unidirectional anisotropy observed using Cr–Al antiferromagnetic films

Exchange induced unidirectional anisotropy has been observed in a new bilayer system consisting of a ferromagnetic FeTaN or Ni80Fe20 film coupled to an antiferromagnetic Cr–Al alloy film. Estimates of the exchange coupling energy and magnetic anisotropy constant of the antiferromagnet are made and compared to other exchange coupled systems.

Vacuum requirements for next wafer size physical vapor deposition system

The major sources of impurities in sputtered Al alloy films for interconnects prepared by physical vapor deposition (PVD) include those originating from the target material, residual gases present in the vacuum system, and those introduced through the gas delivery system. In this study, we report the effect of impurities incorporated from residual gases present in vacuum systems on the properties of Al–Cu interconnects, in particular, electromigration performance of 0.6-μm-wide Al–1% Cu lines. Controlled leaks of isotope gases H2O18, N215, O218, and C13H4, in the 10−6–10−9 Torr range, were introduced into a PVD tool during the sputtering process. Using these isotope gases, the impurities originating from residual gases were distinguished from those originating from other sources of impurities. The sputtering target was found to be the major source of H and O impurities in the film, whereas N atoms are introduced in the film mainly through the gas phase. The microstructural properties of blanket Al–Cu films deposited with intentionally introduced residual gas phase contaminants were also investigated using transmission electron microscopy, x-ray diffraction, and secondary ion mass spectroscopy. Controlled leaks of N2 during the sputtering process were found to affect the distribution of Cu in the blanket Al–Cu films as well as the electromigration behavior of the patterned structures to a larger extent than O2, CH4, and H2O. On the other hand, upon controlled exposures of the above mentioned residual gases to blanket Al–Cu films, N2 was found to have the lowest sticking coefficient to a clean Al–Cu surface.

Argon entrapment in magnetron-sputtered Al alloy films

The incorporation mechanism and annealing behavior of argon in magnetron-sputtered aluminum alloy films were investigated. The incorporated argon concentration of Al alloy (AlSi, AlCu) films was determined by X-ray fluorescence analysis as a function of impurity content (Si: 0–12 wt.%; Cu: 5 wt.%), argon pressure (0.2–1.6 Pa), cathode potential (500–700 V), and substrate temperature (without substrate heating and 250 °C). Hardly any argon atoms were incorporated into Al alloy films with a 250°C pre-substrate heating temperature. For the sample prepared without substrate heating, in contrast the incorporated argon concentration in AlSi alloy films increased linearly with increasing Si content in Al films. The argon concentration in Al alloy films increased with decreasing argon pressure and increasing cathode potential. The incorporation of argon in magnetron-sputtered AlSi alloy film is attributed to the argon becoming entrapped in the amorphous silicon flux. Argon incorporated into Al alloy films by magnetron sputtering and argon implanted into Al films was not released even during 550 °C annealing.

Influence of adding transition metal elements to an aluminum target on electrical resistivity and hillock resistance in sputter-deposited aluminum alloy thin films

Effects of adding group VIII transition metals (Fe, Co, and Ni) to an aluminum target on electrical resistivity and hillock suppression of sputter-deposited Al alloy films were studied. It was found that the group VIII transition metals and Al formed a complete series of metastable solid solutions in the as-deposited state. The solid solutions were decomposed into intermetallic compounds, and the microstructures of the Al alloy films changed during annealing up to 300 °C. The electrical resistivities of Al–Fe, Al–Co, and Al–Ni alloy films changed corresponding to the change in their microstructures, and markedly decreased to 5.0 μΩ cm at the precipitation points. The film stresses also changed corresponding to the change in the microstructure of the Al alloy films, and were abruptly relieved by precipitation of intermetallic compounds and grain growth. Since Al–Fe, Al–Co, and Al–Ni alloy films did not yield, they are highly resistant to hillock formation. The low electrical resistivity and the excellent resistance to hillock formation of Al–group VIII transition metal alloy films were explained on the basis of microstructural changes before and after annealing.

Effects of insulator surface roughness on Al-alloy film properties and electromigration performance in Al-alloy/Ti insulator layered interconnects

Effects of insulator surface roughness on the overlying aluminum alloy film properties and electromigration performance in Al alloy/Ti insulator layered interconnects have been investigated. Insulator surface roughness changes the roughness and crystallographic orientation of Al/Ti layered films formed on the insulator. In particular, the effect is prominent in the case of high-temperature sputtering of Al alloy films. A rough insulator surface deteriorates the roughness and crystallographic orientation of the Al alloy film formed on the insulator, while a smooth insulator surface improves those properties. The insulator surface roughness does not change the roughness and crystallographic orientation of the Al alloy film without a Ti underlayer. Ti roughness and crystallographic orientation are improved on a very smooth surface insulator resulting in improvement in the roughness and crystallographic orientation of the Al alloy film formed on the Ti layer. As a result, the electromigration performance in Al/Ti layered interconnects changes according to the underlayer insulator surface roughness.

Effects of oxide overlayer on thermal stress and yield behavior of Al alloy films

The stress and yield behavior of Al alloys, Al(Cu) and Al(SiCu), have been investigated using bending beam and x-ray diffraction techniques, and the effect of passivation on the stress–temperature behavior of alloys is examined. A stress analysis of trilayer (SiO2/Al alloy film/Si) structure was formulated in order to deduce the stress of the confined metal films from bending beam measurements. The stress–temperature behavior of Al alloy films obtained from bending beam techniques was in agreement with that determined directly by x-ray diffraction techniques. The grain structure and precipitate morphology of these alloy films were studied using transmission electron microscopes. Presence of passivation leads to a change in microstructure of Al alloys, resulting in hardening for the Al(SiCu) films and softening for the Al(Cu) films when compared with the unpassivated films at room temperature. Our results reveal that oxide thickness, Al thickness, and microstructure play an important role in determining the plastic deformation behavior of the metal films, and can satisfactorily explain the contrasting behavior observed for Al(SiCu) and Al(Cu) films.

Residual Gas Phase Contamination of Al-Cu Alloy Films and its Effect on Electromigration Behavior

AbstractThe major sources of impurities in sputtered Al alloy films for interconnects, prepared by physical vapor deposition include those originating from the target material, residual gases present in the vacuum system, and those introduced through the gas delivery system. In this study, we report the effect of impurities incorporated from residual gases present in vacuum systems on the electromigration performance of 0.6 μm wide Al-l%Cu lines. Controlled leaks of isotope gases H2O18, N215, O218, and C13H4, in 10−6 10−9 Torr range, were introduced into a PVD tool during the sputtering process. Using these isotope gases, the impurities originating from residual gases were distinguished from those originating from other sources of impurities. The sputtering target was found to be the major source of H and O impurities in the film, whereas N atoms are introduced in the film mainly through the gas phase. Furthermore, N atoms in the film were found to affect its electromigration behaviour to a larger extent than O and H.

Film Crystallographic Texture and Substrate Urface Roughness in Layered Aluminum Metallization

AbstractMaterial anisotropy implies that many film properties are affected by crystallographic orientation in the growth direction (out-of-plane texture) and / or in the plane of growth (in-plane texture). Physical vapor deposited (PVD) Ti and Al-alloy films deposited on silicon dioxide substrates typically exhibit strong fiber textures in the growth direction with little in-plane-texture observed. The strength of these fiber textures has been found to vary substantially depending on the details of the deposition process(es) and, to a lesser degree, on any post-deposition anneals. In this paper the role of the substrate surface roughness in defining film texture is reported. It was found that the substrate surface roughness determines the overlying film crystallographic orientation for Ti and Ti/AlCu films deposited on various oxides. Furthermore, it was found that the texture of the initial metal “seed” layer defines the texture in subsequently deposited films (texture inheritance). Modifications to the oxide surface which decrease the surface roughness lead to an improved crystallographic texture in Ti, AlCu, Ti/AlCu and Ti/TiN/AlCu films. Film orientation was determined from crystallographic pole figures measured using x-ray diffraction (XRD). The oxide surface roughness was measured using atomic force microscopy (AFM), transmission electron microscopy (TEM) and glancing incidence x-ray reflectivity (GIXR).