A.G. Dirks

Stress, strain, and microstructure of sputter‐deposited Mo thin films

Mo thin films were deposited on glass substrates using direct‐current (dc) planar magnetron sputtering. Mechanical determination of the internal stresses, using the bending‐beam technique, yielded typical compressive‐to‐tensile stress transition curves with increasing working‐gas pressure. The microstructure of the compressively stressed films consists of tightly packed columns, whereas in the tensily stressed films the development of a void network structure surrounding the columnar grains is observed. At elevated working‐gas pressures the onset of microcolumns is observed in the initial stage of film growth. Determination of lattice strains by x‐ray diffraction (XRD), utilizing the sin2 ψ method, encounters more difficulties than the more straightforward stress determination by the bending‐beam method. Here special attention is focused on deviations from linear dependence of dψ with sin2 ψ along with asymmetry of XRD line profiles that results from stress‐depth profiles as well as lateral stress distrib...

Microstructure and magnetism in amorphous rare-earth--transition-metal thin films. I. Microstructure

A microscopic examination of vapor‐ or sputter‐deposited amorphous rare‐earth–transition‐metal thin films reveals that they possess anisotropic microstructure. The morphology of the structure is similar in all of the films. It consists of small (50–250 A in diameter) rod‐shaped regions of high density that are surrounded by a network of less‐dense material (10–25 A thickness). The column axes are mutually parallel and are oriented at an angle β that is described by the relationship: tanβ= (1/2)  tanα, where α is the angle of incidence of the vapor beam, measured with respect to the substrate normal. The columnar structure is formed during deposition by self‐shadowing of the incident atoms by the atoms in the growing film. The columnar structure persists when multiple or distributed vapor sources are used to deposit the film. The structure is stable upon heating to temperaures in excess of the crystallization temperature of the amorphous deposits. Vapor deposition at large α leads to the formation of rowli...

Stress, strain, and microstructure in thin tungsten films deposited by dc magnetron sputtering

Tungsten thin films were deposited on glass substrates by direct‐current planar magnetron sputtering. The induced thickness‐averaged film stress within the plane of the film was determined with the bending‐beam technique and changed from compressive to tensile on increasing working‐gas pressure. The microstructure of these films was investigated by cross‐sectional transmission electron microscopy. Compressively stressed films consisted of tightly packed columnar grains, whereas in films with a maximum value for the tensile stress the onset of a void network surrounding the columnar grains was observed. High‐pressure conditions resulted in dendritic‐like film growth, which brought about complete relaxation of internal stresses. The α phase was predominantly found in films under compression, while an increasing amount of β‐W coincided with the transition to the tensile stress regime. Special attention was focused on stress‐depth dependence and the development of two overlapping line profiles in x‐ray diffra...

Microstructure and magnetism in amorphous rare‐earth–transition‐metal thin films. II. Magnetic anisotropy

Amorphous thin films of GdxCo1−x, HoxCo1−x, GdxFe1−x, and YxFe1−x (0.1<x<0.7) possess an anisotropic microstructure when prepared by vapor or sputter deposition onto room‐temperature substrates. The structure consists of mutually parallel columnar regions of high density that are surrounded by a network of low‐density material of 10–30 A thickness. This structure is produced by geometrical shadowing of the incident vapor atoms by the growing film. If separate vapor sources are used to deposit each alloy constituent, compositional inhomogeneities are also formed by shadowing effects. These produce an in‐plane magnetic anisotropy that may be eliminated by deposition onto rotating substrates. Annealing at temperatures between 250 and 400 °C, deposition in oxygen‐contaminated environments, or bias‐sputter deposition produces compositional inhomogeneities at the surface of the columns. Development of this inhomogeneity is accompanied by production of an easy axis of magnetization perpendicular to the film surface in Gd‐Co alloys, and development of a diffuse diffraction maximum at 0.38 A−1. Upon annealing at 500 °C, this diffuse ring is transformed into a set of closely spaced Bragg maxima that correspond to Gd2O3 crystallites. All of the films exhibit large biaxial tensile stresses in the as‐deposited state. These are reduced by deposition in oxygen and by bias‐sputter deposition. Tensile stress and perpendicular magnetic anisotropy are linearly correlated in Gd‐Co alloys, but the magnetostrictive contribution to perpendicular anisotropy is only 0.01 of the observed value.Amorphous thin films of GdxCo1−x, HoxCo1−x, GdxFe1−x, and YxFe1−x (0.1<x<0.7) possess an anisotropic microstructure when prepared by vapor or sputter deposition onto room‐temperature substrates. The structure consists of mutually parallel columnar regions of high density that are surrounded by a network of low‐density material of 10–30 A thickness. This structure is produced by geometrical shadowing of the incident vapor atoms by the growing film. If separate vapor sources are used to deposit each alloy constituent, compositional inhomogeneities are also formed by shadowing effects. These produce an in‐plane magnetic anisotropy that may be eliminated by deposition onto rotating substrates. Annealing at temperatures between 250 and 400 °C, deposition in oxygen‐contaminated environments, or bias‐sputter deposition produces compositional inhomogeneities at the surface of the columns. Development of this inhomogeneity is accompanied by production of an easy axis of magnetization perpendicular to the film surf...

Columnar microstructures in magnetron-sputtered refractory metal thin films of tungsten, molybdenum and W-Ti-(N)

Abstract Refractory metal thin films based on metals such as tungsten, molybdenum or titanium, having thicknesses of the order of 100 nm, are well known for their applicability in advanced silicon technology. These high melting point metals are commonly deposited by magnetron sputtering onto substrates at room temperature. In spite of the energetic particle bombardment of the growing films the surface mobility of the adatoms is rather limited. This, in combination with the occurrence of geometric shadowing, usually results in films exhibiting a columnar microstructure. In this paper we describe the experimental results as obtained with transmission electron microscopy using the cross-sectional technique. The successive deposition of material on the growing columns may be traced by the observation of multilayer structures, e.g. W/Ti composites. An important parameter which influences the microstructural details of the columnar structure is the internal stress state. The relationship between stresses and microstructure will therefore be illustrated, as well as the influence of the sputter gas pressure on both microstructure and stress state for the case of molybdenum thin films. Another interesting parameter which influences the columnar morphology is the chemical composition of the films. A comparison of the microstructure of pure tungsten and tungsten-rich films alloyed with titanium has shown that, apart from a one-phase to multiphase transition, the mutual alignments of the columns are different: the grains formed by similarly oriented columns are substantially larger in the binary W-Ti alloy films. Additional alloying with nitrogen gives rise to morphological changes: the incorporation of nitrogen forces the column boundaries to fade away and induces a change into a mixture of small crystallites.

Metastable solid solutions in vapor deposited Cu–Cr, Cu–Mo, and Cu–W thin films

The equilibrium phase diagrams of Cu–Cr, Cu–Mo, and Cu–W show no measurable solid solubility except for a small solubility of Cr in solid Cu at higher temperatures. Even in the liquid state, the mutual solubilities in these systems are very limited. Simultaneous vapor deposition of elements on unheated substrates was used to produce alloy films covering the complete composition range of those three binary alloy systems. The films were characterized with the help of x‐ray diffraction, transmission electron microscopy, and electron diffraction. In fcc Cu films, at least 10 at.% Cr, Mo, or W may be dissolved, whereas the bcc Cr, Mo, or W alloy films may accommodate at least 40 at.% Cu; the fcc and bcc solid solutions are separated by narrow two‐phase fields. We further measured the composition dependence of the internal stress and of the electrical resistivity. Discontinuities in the measured curves generally coincide with the presence of phase boundaries in the (metastable) alloy films.

Microstructure and magnetism in amorphous rare earth–transition metal thin films

We have examined UHV vapor deposited and sputtered films of various amorphous alloys by transmission electron microscopy (TEM). All of the films exhibited an anisotropic microstructure composed of columnar regions surrounded by a low density network. The structure is produced by shadowing of the incident vapor by the growing film. Films produced by vapor deposition from separate rare earth (RE) and transition metal (TM) sources may possess an easy axis of magnetization in the film plane that is not present when the substrate is rotated or when the film is produced by sputtering. All of the films possess a high tensile stress which may be reduced and eventually converted to a compressive stress by vapor deposition in an oxygen containing ambient or by negative bias sputtering. The appearance of compressive stresses is coincident with the development of a perpendicular easy axis in Gd‐Co. A perpendicular easy axis is also induced in vapor deposited and zero volt bias sputtered Gd‐Co films upon heating withi...

On the microstructure-property relationship of WTi(N) diffusion barriers

In advanced silicon technology, tungsten-rich W-Ti alloy films are frequently used as diffusion barriers and etch-stop layers in combination with aluminium-based interconnections. We have studied both microstructure and properties of magnetron sputtered W80Ti20 (atomic per cent) alloy films with and without nitrogen incorporated in the film structure. The efficiency of these films as a barrier layer has been studied by the interaction between interconnecting Al-(alloy) and WTi(N) using the following samples: Al(Cu,Si)/WTi(N)/Si02/Si. The barrier films have been characterized with the help of cross-sectional transmission electron microscopy, electron diffraction, electrical resistivity probing, Rutherford back-scattering spectrometry and X-ray diffraction. The W80Ti20 films exhibit a columnar microstructure. As the recrystallizationtemperature of these (refractory) barrier metals is far above the anneal temperature of 450°C, the columnar growth morphology is preserved on annealing. The intercolumnar material (or low density network) provides favourite sites for vacancy condensation and forms short-circuit diffusion paths in a direction perpendicular to the plane of the films. Such W80Ti20 films therefore behave as rather poor diffusion barriers. As shown in the literature by several researchers, both the incorporation of nitrogen into the structure of W-Ti films and the application of a thin oxide film at the barrier-interconnect interface may improve the barrier properties considerably. The present investigations of the film properties resulted in a better understanding of the barrier function of both binary W-Ti and ternary WTiN alloy films.

Mechanical properties of thin alloy films: Ultramicrohardness and internal stress

On a series of Ag‐Al, Ag‐Au, Ag‐Cu, Au‐Cu, and Au‐Fe thin films, made by vapor deposition, both ultramicrohardness and internal stress were determined as a function of the chemical composition of the deposits. In order to interpret these results microstructural investigations were performed using x‐ray diffraction and transmission electron microscopy, supplemented by electrical resistivity measurements. The observed data on hardness and internal stress can be well understood with the help of the metastable phase diagram of the alloy films. In the case of single‐phase solid solutions, internal stresses are tensile. Compound formation, phase decomposition, or ordering are accompanied with dramatic changes of the internal stresses. With indentation experiments at very low loads the hardness‐concentration dependences were observed in the case of films having a thickness of 1 μm only. Metallic films in general are characterized by a very high defect concentration. Moreover, a dramatic reduction of the average ...

Al‐Ti and Al‐Ti‐Si thin alloy films

The microstructure and properties of thin films depend strongly upon the alloy composition. A study was made of the metallurgical aspects of homogeneous Al alloy films, particularly the binary Al‐Ti and the ternary Al‐Ti‐Si systems. The films were deposited by either multiple source e−‐beam evaporation or magnetron sputtering from alloy targets. Electrical resistivity, ultramicrohardness, grain size morphology, second phase formation, and electromigration have been studied as a function of the alloy composition and its heat treatment. The relationship between the microstructure and these properties will be discussed.