Atomic mechanisms of stress formation of group IVB-VIB transition metal nitrides deposited by DC magnetron sputtering

Compounds based on nitrides of the transition metals of group IVB-VIB of the periodic table exhibit special physical and mechanical properties that result from manipulation of composition and manufacturing processes. From the application standpoint, transition metal nitrides have been characterized by significant properties. They are hard, have high melting point, some are coloured, have a simple structure and possess good electrical and thermal conductivity. These interesting properties have made transition metal nitrides important for the electronics and optical applications among others. Some applications in semiconductor industry range from diffusion barrier materials in silicon based semiconductor devices, as passivation layer against oxidation among others. Transition metal nitrides also find use in hard and protective coatings as well as superconductors. Several different techniques such as chemical vapour deposition (CVD), Atomic layer deposition (ALD), RF-sputtering, reactive pulsed laser deposition (PLD), and dc magnetron sputtering among others are used for deposition of the nitride layers. Reactive dc sputtering is the oldest technique, and it has been the main process for depositing oxide and nitride films. An inherent property of reactive sputtering is the residual stresses that are induced during thin film synthesis. It is well accepted that the stability of thin films depends on the residual stresses. In addition, stresses have an important effect in phase transitions, chemical and electrical behaviour of thin films. However, the relevant mechanisms of stress formation of reactively sputtered films are not fully understood. Therefore in this thesis most efforts were devoted to the understanding of structural evolution and the origin of stresses of reactively (dc) sputtered group IVB-VIB transition metal nitrides, namely; ZrNx, NbNx and WxNy. In an attempt to establish this understanding, we performed x-ray diffraction, transmission electron microscopy measurements (TEM), XRD-pole figure measurements, x-ray reflectometry, XRD-sin2Psi method, ex-situ and in-situ stress measurements, ex-situ spectroscopic ellipsometry, Rutherford backscattering spectroscopy and electrical measurements using four point probe set-up at room temperature. In-situ stress measurements at constant sputtering pressure showed an increase in compressive stresses with increasing nitrogen concentration for ZrNx and NbNx films. The increase in compressive stresses was predominantly ascribed to bombardment and sub-plantation of energetic nitrogen (N) atoms. The manifestation of the increase of nitrogen atoms with increasing nitrogen concentration was confirmed by RBS, XRD, spectroscopic ellipsometry and electrical measurements. It should also be pointed out that substochiometric ZrNx and NbNx films depicted anisotropy in strain and stresses as depicted by XRD sin2Psi method. In this case nitrogen vacancies would cause strain gradients due to fluctuations in the inter-atomic distances. However, other factors that could also contribute to the strain gradients are interstitial atoms of (metal, N, O), voids and non-uniformity in the grain size. It should also be noted that NbN phase was formed at higher N2 flow than stoichiometric ZrN phase. This observation has been attributed to the differences in their heat of formation, DeltaH. The DeltaH of ZrN is -3.802 eV which is higher compared to that of NbN (-2.436 eV). The role of nitrogen concentration on the structural and stress evolution of the nitrides was further elucidated by investigating WxNy films. Among the noteworthy findings in this case, were the similarities between the structure evolution of WxNy films deposited at a constant sputtering pressure (0.7 Pa) at different nitrogen concentrations (15-50 sccm) and at sputtering pressures less than 0.7 Pa. These findings elucidated the role of N2/(N2+Ar) flow ratio during deposition as the governing factor in the structure and stress evolution in WxNy thin films, and hence the nitrides. The transition from compressive to tensile stresses in the case of ZrNx and WxNy thin films deposited with increasing sputter pressure was observed at 1.5 Pa. This transition was strongly correlated to: (a) reduction in bombardment of the film by the bombarding species and (b) diffusion of contaminant species which were mostly oxygen (O) atoms as confirmed by RBS. The saturation of the contaminant species (O) after the transition to tensile stress lead to the relaxation of the stresses in both cases. This research work also investigated in-situ stress evolution and the resulting structure of Zr-O-N and Nb-O-N material systems. In the case of Zr-O-N in-situ stress profiles shifted to higher compressive stresses with increasing O2 concentration while in-situ stress profiles for Nb-O-N followed a complementary behaviour. The increase in compressive stresses in the early stages of growth in Zr-O-N was ascribed to particle bombardment induced mobility and subplantation of energetic species in the growing film. In addition the results revealed that in-situ stress relaxation of Zr-O-N films at thicknesses > 15 nm was more pronounced in films with high O/N ratio. In addition, we proposed that the high amount of N-atoms incorporated in the ZrO2 matrix caused the observed amophization in the Zr-O-N phase deposited with 1.0 sccm O2 flow. The formation of crystalline Zr-O-N phases was associated to particle bombardment induced mobility and thermodynamic considerations (since ZrO2 is thermodynamically more favourable than ZrN). For the case of Nb-O-N system, the decrease in compressive stresses with increasing O2 concentration, improved crystallinity and the switch in preferred orientation from (111) to (200), was associated with surfactant like effect.

Grantor dc:publisher

Publikationsserver der RWTH Aachen University

Year dc:date

2007

Author dc:creator

• Karimi, Patrick Mwangi

Contributors dc:contributor

• Wuttig, Matthias

• info:eu-repo/classification/ddc/530
• Spannung
• ZrN
• NbN
• WN
• ZrON
• NbON
• Physik
• Stress
• Nitrides
• Oxynitrides
• DC Magnetron Sputtering

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

eng

Identifier

https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-123877%22

OAI identifier oai:identifier

oai:publications.rwth-aachen.de:62302