D. B. Fraser

Thin film interaction between titanium and polycrystalline silicon

The low resistivity of the titanium disilicide makes this material attractive for gate and interconnect metallizations. TiSi2 has been formed by reacting Ti films with polycrystalline and monocrystalline silicon in the temperature range 400–1100 °C. The interaction is investigated by use of sheet resistance, x‐ray diffraction, and stress measurements. It has been found that Ti and Si react very rapidly to form both TiSi and TiSi2 at temperatures ? 700 °C and only TiSi2 at temperatures ≳ 700 °C. The TiSi2 films are associated with a very low resistivity (∼15 μΩ cm), high tensile stress [∼ (1–2) ×1010 dyn/cm2)], and a rough surface. Silicided structures are mechanically stable. It is proposed that the silicon, as the predominant diffusing species, first diffuses into titanium to completely convert titanium into TiSi and then into TiSi to form TiSi2.

Silicide formation in thin cosputtered (tantalum + silicon) films on polycrystalline silicon and SiO2

Formation of tantalum silicides has been investigated by cosputtering tantalum and silicon on polycrystalline silicon and oxidized silicon wafers. Alloys with as‐deposited Si/Ta atomic ratios of 0.6 to 3 are sintered in hydrogen or argon ambient in the temperature range 400–1000 °C. It is found that for alloys with Si/Ta?1.0 and sintered at temperatures <700 °C, only Ta5Si3 is formed. At higher temperatures and in presence of polycrystalline silicon TaSi2 is formed. In the absence of polycrystalline silicon (i.e., on oxide) only Ta5Si3 is formed at all temperatures. For alloys with Si/Ta ratios ?2 only TaSi2 is formed. In all cases the resistivity decreases as a result of sintering, ending up in a value as a low as ∼50 μΩ cm after 1000 °C/30 min sintering. The stress in the silicided films is found to be nearly 1×1010 dyn/cm2, tensile. The results have been explained on the basis of the fact that in cosputtered films interaction occurs locally which leads to smaller grain size, higher resistivities, lower...

Correlation of Reduction in Optically Induced Refractive‐Index Inhomogeneity with OH Content in LiTaO3 and LiNbO3

It has recently been demonstrated that crystals of LiTaO3 and LiNbO3 can be made more resistant to optically induced refractive‐index inhomogeneities caused by laser irradiation by annealing the crystals in the present of an electric field. The explanation given for the improvement was that some impurity entered the crystal during the annealing cycle, modifying the conductivity of the material. Additional data presented here suggest that the impurity is hydrogen. The exact mechanism by which the susceptibility to index inhomogeneity is reduced is not understood at this time.

Novel Method of Measuring Elastic and Anelastic Properties of Solids

A technique has been devised whereby the elastic and anelastic properties of solids can be measured both as a function of frequency and temperature. A small sphere, of the order of a few millimeters in diameter, is placed without bonding on a shear mode transducer. The transducer is driven with a pulse at one of the spheres resonant frequencies and the free decay of the spheres vibrations is observed by switching the transducer to a receiver. From the decay of the vibrations, the internal frictionQ −1 may be calculated. Various modes may be excited and from comparison with the computed mode spectra of an isotropic solid, the Lame constants, λ and μ, and Poissons ratio may be calculated directly. Measurements have been made over a range of temperatures from 1.4 to near 400°K, the present upper limit being set by the solder connections to the transducer. Several materials have been tested and the results for two single‐crystal nonmagnetic garnet spheres are given as representative of the capabilities of the technique for reasonably isotropic substances. The values of Q −1 for these garnets lie between 3×10−7 and 2×10−5 at a frequency of approximately 2 Mc.

Oxidation of tantalum disilicide on polycrystalline silicon

Oxidation characteristics of the tantalum disilicide films have been investigated in the temperature range of 900°–1050 °C in dry oxygen and steam ambients. The silicide does not oxidize in dry oxygen and oxidizes in steam at a rate lower than that of doped polycrystalline silicon films as long as there is a polycrystalline silicon layer between the silicide and the gate oxide. Under these circumstances, the silicide retains its electrical and mechanical characteristics. The oxide on the silicide has an etch rate (in buffered hydrofluoric acid) similar to that of thermal SiO2 on silicon. Electrical characteristics of the oxide appear to be similar to those of the wet oxide on polycrystalline silicon. In the absence of polycrystalline silicon, between the silicide and the gate oxide, oxidation leads to a loss in the conductivity of the silicide and eventually to a mechanical instability of the film. An oxidation mechanism, which assumes silicon diffusion by substitution through the silicide, has been proposed.

Sputter‐Deposited CdS Films with High Photoconductivity through Film Thickness

Photoconductive films of CdS have been sputter deposited by using mixtures of Ar–H2S as the sputtering gas in a diode system. Both rf and dc sputtering yield deposition rates of about 1 μm/h on substrates maintained above 200°C. Dark resistivities in excess of 107 Ω cm have been obtained with decreases of 5 orders occurring in the resistance when illuminated with a tungsten light source of approximately 0.5 W/cm2. As deposited, the CdS films have displayed photoconductive gain and also have much shorter decay times than films which are activated after deposition. Adherence has been excellent on suitably cleaned substrates and films in device structures have tolerated tensile strains in excess of 10−3 without adherence failure. No aging or electrode migration has been observed.

Anelastic Effects of Alkali Ions in Crystalline Quartz

The anelastic behavior of natural quartz has been modified by doping with alkali ions. Three blocks cut from a single crystal of Brazilian quartz were doped with lithium, sodium, and potassium, respectively, by electrodiffusion at 500°C using a vapor‐plated anode of a halide containing the desired alkali ion. Fundamental 1‐Mc/sec, AT‐cut shear mode resonators were fabricated from each block. Measurements of Q−1 and resonant frequency were made from liquid‐helium temperatures to near the quartz inversion point at 573°C. For a resonant frequency of 5 Mc/sec internal friction peaks were evident in the low‐temperature region with sodium doping giving peaks at 50° and 140°K, lithium a peak near 100°K, and potassium a peak near 200°K. The activation energies ranged from 0.06 to 0.2 eV and the relaxation times were all the order of 10−12 sec. The low‐temperature behavior induced by sodium doping is of special interest since it is identical to that of certain synthetic quartz crystals. All three doped samples exh...

Factors Influencing the Acoustic Properties of Vitreous Silica

The acoustic properties of various types of commercial vitreous silica have been studied as functions of thermal treatment and impurity‐ion concentrations. The properties studied include density, shear‐wave attenuation, shear and dilatational wave velocities, and elastic constants. The measurements of acoustic properties are made on small sphere‐shaped samples using primarily a technique that sets the sphere into a resonant mode of vibration and then allows the vibration to decay freely. Both frequency and the decrement of the decay are measured. Use of this resonant sphere technique is particularly appropriate to the present study because it permits precise measurements on samples small enough to conveniently undergo laboratory modification, and it permits the study of velocity variations in a single block of material by measurements on many localized samples. The major findings of this study are the following: (1) An increase of one degree in fictive temperature (a parameter assumed to be related to the...

Electromigration in fine‐line sputter‐gun Al

The electromigration behavior of Al films, deposited by the sputter gun (varian s‐gun) and ranging in alloy content from 0.5% Cu to 2% Si has been evaluated for 2.5 cm long, 1–4 μm wide conducting stripes. An inverse square dependence of lifetime on current density has been verified. Furthermore, it has been shown that film composition affects the electromigration lifetime through its contribution to the grain structure, in that, an increase in lifetime accompanies an increase in grain size and a decrease in spread of the grain size distribution. Increasing the Si content is detrimental, since it results in a reduction in grain size. Failures occur by the random growth of subsurface voids along the conductor length. The s‐gun films have a completely random orientation in contrast to electron beam evaporated Al‐0.5% Cu, which exhibits a prominent 〈111〉 fiber texture. This preferred grain orientation in the case of the latter is held responsible for its superior lifetime in comparison to the sputtered films.

High temperature stability of PtSi formed by reaction of metal with silicon or by cosputtering

High temperature stability of platinum silicide, formed by reacting metal with silicon or by cosputtering metal and silicon in a desired ratio, has been studied. The properties of films, thus formed, were examined as a function of annealing temperature using a resistance measuring technique, Rutherford backscattering, Auger and x‐ray analyses, transmission and scanning electron microscopic techniques, and by measuring forward current‐voltge (I‐V) characteristics of the silicide n‐silicon Schottky diodes. It is shown that cosputtering silicon rich alloys prevents agglomeration of the silicide, but increases the resistivity and decreases the Schottky barrier height of the film. Platinum silicide dissolves increasing amounts of silicon on high temperature (700–1000 °C) treatments causing considerable degradation of properties. Although cosputtering silicon rich alloys reduces this behavior, electrical properties such as forward I‐V characteristics still degrade due to high temperature anneals.