Polycrystalline Copper Films Research Articles

Characteristics of Polycrystalline Copper Films Made by CVD from Cu(HFA)2

ABSTRACTThin Copper films have been deposited on various substrates by the reduction of copper bis-hexafluoroacetylacetonate, Cu(HFA)2, with hydrogen to investigate the characteristics of the films made at 2–10 torr of total pressure, substrate temperatures of 280–400 °C and precursor temperatures of 55–90 °C. Under the conditions investigated, the highest growth rate was 650 Å/min. and the resistivity of the films was routinely near 2.0 μΩ-cm at 5000 Å or more thickness. Film growth rate depended on precursor concentration and substrate temperature. RBS and AES analysis indicate that the copper deposited at 310–400°C is highly pure. SEM photographs revealed that different structures form depending on substrate kind, the deposition conditions and the deposition time. The surface roughness of the films increased with increasing thickness. The reflectivity of the copper films depends on their thickness and decreases as the grain size increases. The grain sizes in the films were about 0.1–2.0 μm and are correlated with film thickness.

Use of a two-frequency ac bridge to measure 1/f noise in copper films.

Polycrystalline copper films have recently received considerable interest as a semiconductor interconnection metallization due to copper's low resistivity and low susceptibility to electromigration. In this paper the 1/f noise in polycrystalline copper films was measured over a temperature range of 300 to 600 K using a two-frequency ac bridge. Two lock-in amplifiers were used to demodulate the voltage across the bridge, resulting in two estimates of R(t) for the film, each measured at a different modulation frequency. The averaged cross-correlation spectrum of the two R(t) signals is a measure of the resistance fluctuation of the sample only, with no contribution from either the samples' Johnson noise and/or preamplifier noise; hence, no subtraction is necessary. With this technique we measured the noise magnitude and slope versus temperature and frequency for copper films. The temperature dependence of the noise magnitude showed a weak peak at 490 K. Using the conventional value of ${\mathrm{\ensuremath{\tau}}}_{0}$ of ${10}^{\mathrm{\ensuremath{-}}13}$ s, characteristic of point-defect diffusion, and the Dutta-Horn model for 1/f noise in metal films, we measured the mean activation enthalpy for grain boundary migration to be \ensuremath{\sim}1.1 eV. This was, subsequently, equated to the expected electromigration activation energy for copper films.

Adsorption of carbon dioxide on polycrystalline copper

The adsorption of CO2 on clean non-porous polycrystalline copper films has been studied at temperatures between 77 and 298 K by volumetric and surface potential measurements. No evidence has been found for any interaction other than simple physical adsorption following exposures of up to 800 Pa at 298 K or to 1.7 Pa at 113 K. Monolayer formation at 113 K gave a surface potential change of 0.28 V, compared to 0.55 V for Xe, suggesting that CO2 molecules are adsorbed with the molecular axis parallel to the metal surface, and the effective molecular area was ca. 0.16 nm2. Preadsorbed hydrogen reduced the CO2 surface potential change to zero but did not affect the adsorption isotherm. At 298 K CO2 was not adsorbed, nor did its presence in the gas affect the equilibrium adsorption of hydrogen.

Grain size-dependent Hall coefficient in polycrystalline copper films

The Hall coefficients reported for polycrystalline copper films show a rather puzzling behaviour. This paper discusses systematic investigations of the temperature-dependent Hall coefficient for Cu films with known grain size. Grain boundary scattering will be shown to be dominant as soon as the grain size becomes smaller than the electronic mean free path due to scattering by phonons.

Surface nitride formation on supported copper catalysts and its interaction with CO

Supported copper produced by hydrogen reduction of nitrate impregnated silica may retain sufficient nitrogen to inhibit CO adsorption at room temperature. XPS results indicate that the nitrogen is present in a nitride-like form. The deactivating effect of preadsorbed N atoms on CO chemisorption has been studied by volumetric and surface potential measurements on polycrystalline copper films. An N coverage of 0.1 hardly affects the CO adsorption capacity or the sp behaviour at 77 K, but the heat of adsorption is substantially reduced on the whole surface. With increasing N coverage the low temperature behaviour progressively changes to give a much larger positive CO sp. This behaviour is consistent with single crystal studies and it is attributed to reduced interaction with the copper surface d-band.

Dependence of 1/f noise on defects induced in copper films by electron irradiation.

When 500-keV electron irradiation was used to induce defects in polycrystalline copper films maintained at 90 K, the spectral density of the $\frac{1}{f}$ noise voltage across the films increased by as much as 1 order of magnitude, while the resistivity increased by at most 10%. When the films were annealed at progressively higher temperatures, both the $\frac{1}{f}$ noise and the resistivity were reduced; however, at lower annealing temperatures, the fractional reduction in the added noise was substantially more than that in the added resistivity.

Temperature coefficient of resistivity for polycrystalline Cu-films

The variation of temperature coefficient of electrical resistivity for polycrystalline copper films with thickness can be explained if temperature variation of the mean free path in a single grain and the scattering of charge carriers at the grain boundary are taken into account.

Strain coefficient of resistance of thin copper films

Abstract An expression for the strain coefficient of resistance which takes into account both the thickness of the film and the size of the crystalline grains in the direction of the resistance is derived. Calculations were carried out for a polycrystalline copper film. It was found that for a thick film and for large grain diameters the strain coefficient of resistivity is the same as for the bulk material. However, for grains of small diameter and for ultrathin films the coefficient tends to a constant value.

Electron transport properties of thin copper films. I.

The thickness dependence at 300 and 80 K of the electrical resistivity and its temperature coefficient, Hall coefficient, mobility, and thermoelectric power of as-deposited and annealed thin (< 1000 Å) evaporated polycrystalline copper films and films deposited at elevated temperatures have been studied. All transport parameters in carefully prepared and well-characterized films exhibit monotonically increasing size effects with decreasing film thickness. Both annealing and deposition at elevated temperatures cause considerable reduction of the ’’apparent’’ size effects in all the transport parameters of the room-temperature deposited films. A critical analysis of the observed size effects shows that the data in all cases depart markedly from the predictions of the Fuchs-Sondheimer theory (and also that of the Mayadas-Shatzkes theory which takes into account the grain boundary surface scattering). The departure from theory is different for each transport parameter. The annealing studies show that the enhanced size effects are due to the presence of a large concentration of structural defects in the films. The observed behavior may be understood by assuming the large concentration of point and/or line defects to decrease with film thickness and with annealing as well as deposition of films at elevated temperatures. The thermopower data suggest strongly that the large concentration of defects causes distortion of the Fermi surface and thereby a strong energy dependence of the mfp or relaxation time at the Fermi surface.

Optical transmittance and microgravimetric studies of the oxidation of 〈100〉 single crystal films of copper

Thin single crystal copper films have been grown and oxidized on (100) faces of cleaved sodium chloride discs suspended from a vacuum ultramicrobalance. Optical transmittance measurements between 400–800 nm and electron microscopic investigations were also used to characterize the oxidation process. Polycrystalline copper films grown at room temperature are substantially the same as those grown previously on glass substrates. Single crystalline growth at 325 ° C on rock salt produces a characteristic transmittance curve due to the “island” nature of the films. These curves compare favorably with other previously published results. Single crystal copper films oxidized to CuO0.67 at temperatures of 117–159°C in 100 Torr of oxygen for films less than 500 A thick. For films 378 to 1000 Å thick, compositions of CuO0.52 to CuO0.62 were obtained between 123–176°C. The oxidation to less than CuO0.67 is attributed to the existence of islands in these films which are thicker than the average film thickness, and require higher temperatures or thinner films to permit oxidation to CuO0.67 before the nucleation of CuO sets in.

Thermal conductivity of ultrathin metal films in multilayer structures

Thermal and electrical conductivities of 28 layers of 100-Å-thick and 6 layers of 500-Å-thick polycrystalline copper films, each individually sandwiched between 25-Å-thick amorphous Ge or SiO films, have been measured. The results show that, provided no interfacial alloying occurs (as is the case with SiO films), the effective values of the conductivities of the multilayer structure are very close to those of the individual (100 or 500 Å) copper films. This equivalence of the transport behavior enables the measurements of the thermal conductivity of ultrathin metal films. Further, it indicates no significant enhancement of diffuse scattering of electrons and phonons at the surfaces of the copper films due to the presence of multiple interfaces.

Cu films formed simultaneously on (111) and (100) NaCl

Copper films were deposited simultaneously in high vacuum on three different monocrystalline NaCl substrates: evaporated (111) NaCl on mica, evaporated (100) NaCl and air-cleaved (100) NaCl. The occurence and microstructure of monocrystalline or polycrystalline copper films were determined by transmission electron microscopy and diffraction as a function of deposition rate R and substrate temperature T. When log R was plotted against 1/ T, straight lines could be drawn separating the monocrystalline and the polycrystalline regions. Activation energies for the polycrystalline to monocrystalline transition of Cu films were calculated to be 1.48, 1.22 and 1.27 eV for the (111), evaporated (100) and air-cleaved (100) NaCl substrates respectively. It is shown that these results can be related to the atomistic theory of nucleation by Walton. Moreover, the results indicate that both the binding energy U between a single adatom and a growing oriented cluster and the atomic adsorption energy Q ad on the substrate surface are proportional to the planar atom densities in the growing cluster and in the substrate surface respectively. It is further shown that while the activation energies for Cu films formed on the two (100) substrate surfaces are about the same, the actual epitaxial temperatures for the same R are significantly different.

Role of Thin Intermediate Layers between Polycrystalline Copper and Magnetic Films

It is well known that magnetic films deposited on evaporated copper on a glass substrate have wall coercive forces Hc much larger than those directly deposited on glass. This is due in part to the surface roughness of the large copper crystallites. This work demonstrates that there is a significant additional contribution to Hc due to stress in a material in which λ100≠λ111 even though λs=0. Electron-microscopy crystallite-size measurements suggest this stress arises from epitaxial growth. This stress can be relieved by stripping the films from the glass and selectively etching away the copper. For example, a 600-Å nonmagnetostrictive film evaporated from a 50 Co 47 Ni 3 Fe melt on a freshly evaporated 1-μ evaporated copper film on a 200°C glass substrate had a Hc of 72 Oe. After this film was stripped and the copper was etched away, Hc was reduced to 35 Oe. For a companion film deposited directly on glass Hc = 13 Oe. Several materials evaporated as thin (∼100 Å) intermediate layers which successfully inhibit epitaxial growth and stress include Cr, Mn, Ti, and Pt.

Structure and Strength of Fine-Grained Copper and Silver Prepared by Vacuum-Deposition

Polycrystalline copper and silver films, about 2 to 10 μ thick, were vacuum-deposited on glass substrates using tungsten baskets. The variations of 0.2% offset yield stresses and microstructures of the films by annealing at various temperatures ranging from 100° to 1000°C were investigated. The grain size, which was about 2000 Å in diameter in an as-deposited state, increased to several microns in diameter after annealing. The grain size and recrystallization temperature were extremely affected by the deposition condition. The recrystallization temperature of copper films of 10−4 Torr rose above 800°C when the deposition was done in a high residual gas pressure. It is inferred that the fine dispersion of oxide in the films is responsible for the rise of the recrystallization temperature. The strength of the fine-grained films was agreed well with that expected from the Hall-Petch relation.

Mechanical Properties of Copper Films

The mechanical properties of polycrystalline copper films, 700–5000-Å thick, were measured using the bulge technique. The breaking strength increased with decreasing film thickness, the greatest increase occurring below 1000 Å. The equivalent bulk breaking strength was found to be 61 000 lb/in2. Young's modulus was independent of film thickness and had the normal value 18×106 lb/in2.