As the dimensions continue to scale in interconnect processing, having a stable and controlled interface between the metal line and the dielectric material becomes more and more important. Molecular Layer Deposition (MLD) has been investigated as a method of growing thin films on dielectric surfaces which act as metal barriers, as blocking layers and as adhesion promoters. The advantage of MLD growth over conventional self-assembled monolayer (SAM) processing is the ability of MLD to easily grow films of varying thickness with tunable chemical functionality.

In this paper, we will discuss the growth and thermal stability of poly-urea-based MLD thin films on both porous and non-porous Carbon Doped Oxide (CDO) dielectric surfaces and compare this work to similar MLD films grown on conventional SiO₂. A variety of characterization techniques, including thermal stress test, secondary ion mass spectrometry, x-ray photoelectron spectroscopy, and electron microscopy, were used to determine film composition, film stability, film adhesion, and degree of penetration into porous substrates. We investigate the importance of surface preparation on the anchoring of MLD films to CDO surfaces. Surface preparation is especially important when growing controlled layers on the surface of a porous dielectric, and we show that surface treatments influence the depth of penetration of the MLD film within porous CDO. Finally, we examine the deposition of thin metal films on top of MLD coated CDO layers (both blanket and patterned) and we show that the metal deposition process impacts both the MLD stability and the metal penetration into the ILD.

Nanometric films of pure Cu continue to attract attention due to their widespread use as interconnect material in the semiconductor community. Among the various engineering and scientific challenges posed by the continued use of Cu is its high surface mobility, which is well known to result in both elecromigration and stress induced void formation in interconnects. The use of barrier/adhesion layers greatly improves the reliability of Cu interconnects. Nonetheless, the diffusion of Cu along the Cu/barrier interface is not well understood. Addressing the thermal stability and morphological evolution of Cu/barrier interfaces with the intent of quantifying interfacial diffusivities is the ultimate goal of this research.

Synchrotron x-ray scattering was used to study the evolution of interface roughness with annealing for a series of Cu thin films. The films were encapsulated in SiO₂ or Ta / SiO₂ and prepared by sputter deposition. Specular x-ray reflectivity was used to determine the root mean square roughness for both the upper and the lower Cu / SiO₂ (or Cu / Ta ) interfaces. The lateral roughness was studied by diffuse x-ray reflectivity. Annealing the films at 600°C resulted in a smoothing of only the upper interface for the Cu / SiO₂ samples, while the lower Cu / SiO₂ interfaces and both interfaces for the Ta encapsulated films did not evolve significantly. As a function of roughness wavelength, the upper Cu / SiO₂ interfaces exhibited a roughness decay with annealing that was only 12.5% of that expected for classical capillarity driven smoothening of a free surface.

Continued work is focusing on further quantifying the interface kinetics for Cu/barrier systems. Using e-beam lithography methods, we have patterned a grating onto the surface of Cu thin films. Subsequent encapsulation and annealing will be carried out to study the effects of time and temperature on the patterned interface.

Due to limitations in characterizing twin boundaries in nanocrystalline Cu, it has been difficult to account for twin boundary scattering in the quantitative analysis of the resistivity size effect. In this study, a recently developed high throughput electron diffraction based metrology method in the transmission electron microscope, known as ASTAR^TM, is employed to obtain crystal orientation maps in two SiO₂ and 6 SiO₂/Ta₃₈Si₁₄N₄₈ encapsulated nanocrystalline Cu thin films. In ASTAR™, a dedicated hardware unit is used for precession and automated scanning of a nanosized quasi-parallel electron beam probe. A high speed external optical camera is then used for rapid acquisition of spot diffraction patterns. The acquired spot patterns are indexed automatically using a template matching algorithm. Significant improvement in the reliability of the orientation maps is achieved with electron beam precession. The use of precession reduces the dynamical effects and increases the number of spots in the diffraction pattern. The use of rapidly acquired spot patterns and the robust template matching algorithm make ASTAR™ highly suitable for obtaining large datasets of crystal orientations. Analysis of the orientation maps of the Cu films shows a significant fraction of incoherent twin boundaries, indicating a potentially higher resistivity contribution from twin boundary scattering than previously assumed. Including the mixture of coherent and incoherent twin boundaries in the study of the resistivity size effect shows that the contribution from grain boundary scattering is still the dominant resistivity size effect in Cu, compared to surface scattering. Inclusion of the twin boundary mixture in a quantitative model shows the resistivity data to be best described by the Fuchs Sondheimer surface scattering model and the Mayadas Shatzkes grain boundary scattering model, combined using Matthiessen's rule (simple summation), with a surface specularity coefficient, p = 0.50, and a grain boundary reflection coefficient, R = 0.26. These values can be compared with values of p = 0.52 and R = 0.43 obtained in previous studies where the presence of twin boundaries was not considered. The potential to separately quantify electron scattering at twin boundaries and non-twin grain boundaries, the role of surface roughness, measured by x-ray reflectivity using synchrotron radiation, and the role of voids, measured using high angle annular dark field imaging in the transmission electron microscope, will also be discussed.

Silsesquioxane-based photopatternable low-k (PPLK) dielectric materials[1] are promising alternatives to existing low-k dielectrics due to the reduction of BEOL integration complexity. However, processing-induced damage due to reactive species and energetic particles has been previously found to be problematic for low-k organosilicate dielectrics. Thus, for successful integration, the effects of charged-particle bombardment and photon irradiation (particularly in the vacuum ultraviolet range) must be characterized. In order to examine these effects, I-V and C-V characteristics were made on PPLK samples before and after exposure to a variety of argon plasma exposure conditions. Plasma parameters were varied between each exposure so that each sample was subjected to a range of charged particle and photon fluxes. In order to examine the effects of photon irradiation alone, PPLK samples were also exposed to monochromatic synchrotron radiation over energies varying from 5 to 15 eV. It was found that both charged-particle bombardment and photon irradiation have deleterious effects, resulting in increased magnitude of leakage currents and increased flat-band voltage shifts. VUV-exposed samples also exhibited increased leakage currents, but this effect was found to be strongly dependent on photon energy.

This work has been supported by the Semiconductor Research Corporation under Contract 2008-KJ-1871 and by the National Science Foundation under Grant CBET-1066231.

[1] Q. Lin, S.T. Chen, A. Nelson, et al., Proc. Of SPIE 7639, 76390J (2010)

SiCOH films have been used in advanced semiconductor devices to enable continued scaling of interconnect integration. The further scaling of ultra low k-value (ULK) films has necessitated a move to porous SiCOH films. Porosity is intentionally introduced to decrease the dielectric k-value and therefore reduce interconnect RC delay. However, this porosity also leads to reduced mechanical strength and presents substantial challenges to integration and packaging.

Nanoporous thin films can be formed with PECVD of a SiCOH film with embedded porogen clusters. The porogen can either be supplied by a second porogen precursor or using a single precursor containing an embedded porogen fragment. The porogen is then removed with curing. Currently the industry favors UV assisted thermal curing to simultaneously drive out porogen (forming the porous structure) and enable cross-linking of the film matrix (Si-O-Si bond formation). However, prior investigation claims that this porogen is not completely removed, leaving porogen residue in the pores [1]. It is believe that cross-linking of the film matrix inhibits porogen removal and inversely, porogen residue prevents efficient cross-linking. Subsequently, this can lead to degraded electrical and mechanical performance.

A new approach under review uses a two-step curing procedure. First porogen is removed from the film with a novel film treatment. This step targets porogen removal, but does not cross-link the film’s matrix. An example of this is remote H₂/He plasma (H radical exposure) [2]. After removing the porogen, a traditional UV assisted thermal cure strengthens the film and drives out any remaining porogen. We evaluated this and two similar approaches and observed the same qualitative trends. Generally speaking, when using conventional curing techniques there is an unavoidable tradeoff between k-value and mechanical performance. However, the two-step process results in films which have both increased mechanical strength and improved electrical performance.

Films have been evaluated electrically (k-value, breakdown voltage, leakage current). Film structure (porosity and pore size distribution), mechanical properties (Young’s modulus) and film composition were measured with ellipsometry porosimetry, nanoindentation and XPS/FTIR, respectively. The UV absorption peaks related to porogen residue have also been measured using vacuum ultraviolet spectroscopic ellipsometry.

[1] A.M. Urbanowicz, K. Vanstreels, D. Shamiryan, S.D. Gendt and M.R. Baklanova, Electrochem. Solid St.12 (2009).

[2] A.M. Urbanowicz, K. Vanstreels, P. Verdonck, D. Shamiryan, S. De Gendt and M.R. Baklanov, J. Appl. Phys.107 (2010).