Jyrki Molarius

Reactive sputter deposition and properties of Ta x N thin films

The aim of this work was to evaluate tantalum nitride thin films fabricated using reactive sputtering with adjusted deposition parameters. Thin TaxN films were deposited reactively on Si wafers using reactive RF magnetron sputtering at various N2/Ar gas ratios. The films were investigated by four-point probe sheet resistance measurement, profilometry, X-ray diffraction, scanning electron microscope, 2 MeV 4He+ backscattering spectroscopy, and atomic force microscopy. As the amount of nitrogen was increased, the phases in the as-deposited films were identified as β-Ta, Ta2N (5% N2-flow), hexagonal TaN (10% N2-flow) and f.c.c.-TaN (15% N2-flow) with resistivities of 166 µΩ cm, 234 µΩ cm, 505 µΩ cm and 531 µΩ cm, respectively. Only the phase obtained at 5% N2-flow showed a reasonable uniformity over the wafer suggesting suitability as thin film resistors. The value of temperature coefficient of resistance (TCR) determined for the Ta2N thin film resistor was - 103 ppm/°C.

Failure mechanism of Ta diffusion barrier between Cu and Si

The reaction mechanisms in the Si/Ta/Cu metallization system and their relation to the microstructure of thin films are discussed on the basis of experimental results and the assessment of the ternary Si–Ta–Cu phase diagram at 700 °C. With the help of sheet resistance measurements, Rutherford backscattering spectroscopy, x-ray diffraction, a scanning electron microscope, and a transmission electron microscope, the Ta barrier layer was observed to fail at temperatures above 650 °C due to the formation of TaSi2, the diffusion of Cu through the silicide layer, and the resulting formation of Cu3Si precipitates. However, in order for the TaSi2 phase to form first, the Ta diffusion barrier layer must be thick enough (e.g., 50–100 nm) to prevent Cu diffusion into the Si substrate up to the temperature of TaSi2 formation (∼650 °C). Independent of the Ta layer thickness, Cu3Si was present as large nodules, whereas the TaSi2 existed as a uniform layer. The resulting reaction structure was found to be in local equil...

TaC as a diffusion barrier between Si and Cu

The reaction mechanisms and related microstructures in the Si/TaC/Cu metallization system have been studied experimentally and theoretically by utilizing ternary Si–Ta–C and Ta–C–Cu phase diagrams as well as activity diagrams calculated at 800 °C. With the help of sheet resistance measurements, Rutherford backscattering spectrometry, x-ray diffraction, scanning electron microscopy, and transmission electron microscopy, the metallization structure with the 70 nm thick TaC barrier layer was observed to fail completely at temperatures above 725 °C because of the formation of large Cu3Si protrusions. However, the formation of amorphous Ta layer containing significant amounts of carbon and oxygen was already observed at the TaC/Cu interface at 600 °C. This layer also constituted an additional barrier layer for Cu diffusion, which occurred only after the crystallization of the amorphous layer. The formation of Ta2O5 was observed at 725 °C with x-ray diffraction, indicating that the oxygen rich amorphous layer h...

Reactive Phase Formation in Thin Film Metal/Metal and Metal/Silicon Diffusion Couples

There have been a number of efforts to develop a model that could be used to predict and to describe phase formation in the case of thin film diffusion couples. In this report, thermodynamic and kinetic frameworks as well as some of the proposed models for interface reactions have been critically reviewed. The following conclusions have been made: First, in the early stage, solid-state interface reaction is a kinetic process. Phase selection and phase formation sequence are controlled mainly by nucleation kinetics and/or growth kinetics. Second, in order to simulate interface reactions in thin film diffusion couples, kinetic description of the system is required and should be combined with the thermodynamic description. This in turn requires the assumption of local thermodynamic equilibrium in the system. However, whether or not local equilibrium can be assumed at the interface in a thin film system depends on the thickness of the films, nucleation and growth kinetics, and the diffusivity of elements in the product phases. Finally, based on the theoretical considerations it appears that there is no fundamental difference between thin film and bulk diffusion couples.

Piezoelectric coefficients and spontaneous polarization of ScAlN

We present a computational study of spontaneous polarization and piezoelectricity in Sc(x)Al(1-x)N alloys in the compositional range from x = 0 to x = 0.5, obtained in the context of density functional theory and the Berry-phase theory of electric polarization using large periodic supercells. We report composition-dependent values of piezoelectric coefficients e(ij), piezoelectric moduli d(ij) and elastic constants C(ij). The theoretical findings are complemented with experimental measurement of e33 for a series of sputtered ScAlN films carried out with a piezoelectric resonator. The rapid increase with Sc content of the piezoelectric response reported in previous studies is confirmed for the available data. A detailed description of the full methodology required to calculate the piezoelectric properties of ScAlN, with application to other complex alloys, is presented. In particular, we find that the large amount of internal strain present in ScAlN and its intricate relation with electric polarization make configurational sampling and the use of large supercells at different compositions necessary in order to accurately derive the piezoelectric response of the material.

Chemical stability of Ta diffusion barrier between Cu and Si

The reactions in the Si/Ta/Cu metallization system produced by a sputtering process were investigated by means of sheet resistance measurements, XRD, RBS, SEM and optical microscopy. In particular, the reaction sequence was emphasised. The reaction mechanisms and their relation to the microstructure and defect density of the thin films are discussed on the basis of the experimental results and the assessed ternary Si-Ta-Cu phase diagram at 700 °C. It was found out that the effectiveness of the Ta barrier is mainly governed by the defect density and their distribution in the Ta film. The failure was induced by Cu diffusion through the Ta film and almost simultaneous formation of Cu 3Si and TaSi2.

Amorphous layer formation at the TaC/Cu interface in the Si/TaC/Cu metallization system

An amorphous Ta(O,C)x layer was found to form at the TaC/Cu interface in the Si/TaC/Cu metallization system. The formation of the layer was induced by oxygen trapped in the as-deposited films, since on the basis of thermodynamic evaluation of the ternary Ta–C–O system, the dissociation of the TaC layer and the formation of the Ta2O5 and graphite can be expected to occur during subsequent annealings in this case. However, as observed experimentally, the formation of the amorphous Ta(O,C)x preceded the formation of the stable tantalum oxide.

Reactively sputtered tantalum pentoxide thin films for integrated capacitors

The aim of this work was to develop a deposition process for a high-dielectric constant tantalum pentoxide for integrated capacitors. Thin films were deposited reactively on glass wafers using a radio-frequency magnetron sputtering cluster tool at various O2/Ar flow ratios. By using 2 MeV 4He+ backscattering spectroscopy and X-ray diffraction, the films obtained showed a stoichiometric orthorhombic β-Ta2O5 phase at 20% O2 in the sputtering gas flow. With low-frequency measurements (f = 100 kHz), a 200 × 200-µm2 square metal-insulator-metal (MIM) capacitor with copper electrodes and a 340-nm thick dielectric gave a capacitance density of 0.066 µF/cm2, with a quality factor (Q) of 650. The value of the relative permittivity (er) was approximately 25 determined from MIM capacitors of various sizes. The surface roughness of the 376-nm thick oxide film was found to be small: 0.255 nm. The largest measured capacitor (200×200 µm2) gave reasonable results at low frequencies. When the frequency was increased (100 kHz-20 GHz) only for the smaller capacitors (30 × 30 µm2) the capacitance remained constant. However, the Q values decreased of the smaller capacitors as a function of frequency. Processed tantalum pentoxide MIM capacitors possessed reasonable electrical properties below 2 GHz and good potential for further improvement.

Effect of oxygen on the reactions is Si/Ta/Cu and Si/TaC/Cu systems

The effect of oxygen on the reactions in the Si/Ta/Cu and Si/TaC/Cu metallization systems was investigated by utilizing the assessed Ta-O binary and the evaluated ternary Ta-C-O phase diagrams together with detailed transmission electron microscopy (TEM) and secondary ion mass spectrometry analyses (SIMS). The presence of some form of amorphous tantalum oxide at the Ta/Cu and TaC/Cu interfaces was experimentally verified. The formation of the interfacial layers was explained with the help of the assessed phase diagrams as well as with the available kinetic data.