A. Kohn

Characterization of electroless deposited Co(W,P) thin films for encapsulation of copper metallization

Abstract Thin Co(W,P) films, 100–200 nm thick, were electroless deposited on oxidized silicon wafers using sputtered copper or cobalt as catalytic seed layers. The purpose of these films is to encapsulate copper preventing its corrosion or to serve as a diffusion barrier against copper contamination of silicon oxide and silicon in ULSI interconnect and packaging applications. The electroless cobalt layers were integrated with electroless copper and found to function as barriers up to a temperature of 500°C. The microstructure of the barrier film was found to consist of grains of h.c.p. cobalt, ∼10 nm in diameter, in which the grain boundaries are most probably enriched by phosphorus and tungsten. It was found that the phosphorus and tungsten impurities stabilize the h.c.p. phase, postponing the transition to the f.c.c. phase by more than 80°C, compared to pure bulk cobalt. The observed good barrier properties can be explained by the nano-sized grains along with the blocking effect of the impurities at the fast diffusion path of the grain boundaries. An advantage of these layers, relative to alternative diffusion barriers for copper, is their low electrical resistivity, 40 μΩ cm.

Evaluation of electroless deposited Co(W,P) thin films as diffusion barriers for copper metallization

Abstract Electroless deposited Co(W,P) thin films were evaluated as diffusion barriers for copper metallization. Capacitance versus time measurements of MOS structures as well as SIMS depth profiles indicate that 30-nm-thick films can function as effective barriers against copper diffusion after thermal treatments up to 500°C. The improved barrier properties relative to sputtered cobalt are explained by the incorporation of phosphorus (8–10 at.%) and tungsten (∼2 at.%) which most probably enrich the grain boundaries of the nanocrystalline hcp cobalt grains, forming a ‘stuffed’ barrier. The phosphorus and tungsten additions stabilize the hcp crystalline structure of the cobalt grains, delaying the transition to the fcc phase by more than 80°C compared to bulk pure cobalt. An advantage of this material compared to alternative diffusion barriers for copper is its relatively low resistivity of 80 μΩ cm.

Structures of ultra-thin atomic-layer-deposited TaNx films

Atomic layer deposition (ALD) is an attractive technique in fabrication of microelectronics presently and in the future, for its accurate thickness control in atomic scale, excellent conformality, and uniformity over large areas at low temperature. It has been adapted and used in deposition of ultrathin TaNx films as diffusion barriers for Cu metallization. In this study, composition, structure, and stability of ultra-thin (1.5–10 nm) atomic layer deposited films are characterized by a set of complementary analytical techniques. The results indicate that the N to Ta atomic concentration ratio in the ALD TaNx films is approximately 2, independent of the film thickness and annealing up to 750 °C. Hydrogen, oxygen, and carbon are detected as impurities within the as-deposited films. The as-deposited ALD TaNx films have an fcc NaCl-type nanocrystalline structure even when the film thickness is 1.5 nm. Following thermal anneal at 600 °C and higher, the films do not undergo a structural change except for an inc...

Copper grain boundary diffusion in electroless deposited cobalt based films and its influence on diffusion barrier integrity for copper metallization

Electroless deposited Co-based alloys have been proposed as diffusion barriers for ultralarge-scale integrated microelectronic devices. In this study, Cu grain boundary diffusion in electroless deposited Co0.9W0.02P0.08 and Co0.9P0.1 in the temperature range between 300 and 450 °C was quantitatively studied and compared to physical vapor deposited (PVD) pure Co films. The transport of Cu atoms through these films is predominately via the grain boundaries, namely type-C diffusion kinetics. The diffusivity values were extracted from Cu depth profiles measured by secondary ion mass spectrometry. Copper diffusion through the various Co films obeys an Arrhenius relationship. Copper diffusivity in the electroless films is 2–3 orders of magnitude smaller than in PVD Co films. The diffusivity of Cu through electroless deposited Co0.9W0.02P0.08 is 5–10 times smaller than in electroless deposited Co0.9P0.1. The reduced diffusivity in the electroless films is a result of the significantly lower value of the pre-expo...

Improved diffusion barriers for copper metallization obtained by passivation of grain boundaries in electroless deposited cobalt-based films

The mechanism for improved barrier properties against copper diffusion in electroless deposited Co0.9W0.02P0.08 and Co0.9P0.1 thin films compared to physical vapor deposition (PVD) of cobalt is quantitatively explained. Secondary ion mass spectrometry depth profile measurements were performed on the films deposited on copper substrates after subjecting them to thermal anneals at approximately half the melting temperature of cobalt. A steady-state mode was observed in the form of concentration plateaus which originate from a combined contribution of grain-boundaries’ saturation and copper solubility in the grains. The difference in plateau heights between the samples is assigned to the varying degrees of grain-boundaries’ passivation. For Co0.9W0.02P0.08, the copper concentration in the grain boundaries is negligible and the solubility in the temperature region between 550 and 700 °C may be described as CS≃6×102×exp(−0.52 eV/kT) at. %. The higher-copper concentration plateaus in the Co0.9P0.1 films are a r...

The antiferromagnetic structures of IrMn3 and their influence on exchange-bias.

We have determined the magnetic structures of single-crystal thin-films of IrMn3 for the crystallographic phases of chemically-ordered L12, and for chemically-disordered face-centred-cubic, which is the phase typically chosen for information-storage devices. For the chemically-ordered L12 thin-film, we find the same triangular magnetic structure as reported for the bulk material. We determine the magnetic structure of the chemically-disordered face-centred-cubic alloy for the first time, which differs from theoretical predictions, with magnetic moments tilted away from the crystal diagonals towards the face-planes. We study the influence of these two antiferromagnetic structures on the exchange-bias properties of an epitaxial body-centred-cubic Fe layer showing that magnetization reversal mechanism and bias-field in the ferromagnetic layer is altered significantly. We report a change of reversal mechanism from in-plane nucleation of 90° domain-walls when coupled to the newly reported cubic structure towards a rotational process, including an out-of-plane magnetization component when coupled to the L12 triangular structure.

Characterization of embedded MgO/ferromagnet contacts for spin injection in silicon

In this work we present the structural and electrical characterization of sputter-deposited CoFe(B)/MgO/Si metal-insulator-semiconductor tunneling junctions for injection and detection of spin polarized current in silicon. The multilayers have been deposited in 700 nm deep trenches, patterned in thick SiO2 dielectric, on n- and p-doped wafers. The films inside the trenches are continuous with a correlated and low roughness. The MgO barrier grows amorphous without indication of pinholes. The dc and ac transport properties of the junctions were studied as a function of temperature and frequency. A relatively high interface trap density at the MgO/Si-interface is extracted from admittance spectra measurements. Transport is dominated by majority carriers in the case of n-doped and by minority carriers for the p-doped wafers. This leads to distinct rectification characteristics for the two wafer types, which would significantly influence the spin injection efficiency of the tunneling junctions.

Interface effects in highly oriented films of the Heusler alloy Co2MnSi on GaAs(001)

Highly (001) oriented thin films of Co2MnSi have been grown on lattice-matched GaAs(001) without a buffer layer. Stoichiometric films grown at the highest substrate temperature of 689 K showed the lowest resistivity (33μΩcm at 4.2 K) and the lowest coercivity (14 Oe). Twofold in-plane magnetic anisotropy was observed due to the inequivalence of the ⟨110⟩ directions, and this was attributed to the nature of the bonding at the reconstructed GaAs surface. Interfacial reactions resulted in the formation of an epitaxial Mn-As region and a thin interfacial layer that was Co-Ga rich. This prevented the desired lattice matching and resulted in films with a saturation magnetization slightly below the bulk value. In spite of this, the spin polarization of the free surface was measured to be 55%, similar to bulk material.

Structure of electroless deposited Co0.9W0.02P0.08 thin films and their evolution with thermal annealing

Electroless deposited Co0.9W0.02P0.08 thin films have been proposed as diffusion barriers and encapsulation layers for Cu metallization in ultralarge-scale integrated microelectronic devices. In this article, we present a study of the structure of these films and their evolution with thermal anneal up to 700 °C. The as-deposited microstructure is comprised of an amorphous CoWP component and nanocrystallites of hexagonal-close-packed (hcp) Co, approximately 5 nm in size. The amorphous CoWP component crystallizes to hcp Co at approximately 290 °C with an apparent activation energy of 1.6±0.1 eV, according to the Kissinger analysis. Isothermal anneals show that the rate of nucleation with time of the hcp Co grains is constant, and grain growth is controlled by diffusion. This diffusion is most probably of the P and W elements, which enrich the grain boundaries. At approximately 420 °C, the orthorhombic (o-) Co2P phase nucleates. The apparent activation energy of this phase transformation is 4.6±0.1 eV, accor...

Parallel p–n Junctions across Nanowires by One-Step Ex Situ Doping

The bottom-up synthesis of nanoscale building blocks is a versatile approach for the formation of a vast array of materials with controlled structures and compositions. This approach is one of the main driving forces for the immense progress in materials science and nanotechnology witnessed over the past few decades. Despite the overwhelming advances in the bottom-up synthesis of nanoscale building blocks and the fine control of accessible compositions and structures, certain aspects are still lacking. In particular, the transformation of symmetric nanostructures to asymmetric nanostructures by highly controlled processes while preserving the modified structural orientation still poses a significant challenge. We present a one-step ex situ doping process for the transformation of undoped silicon nanowires (i-Si NWs) to p-type/n-type (p-n) parallel p-n junction configuration across NWs. The vertical p-n junctions were measured by scanning tunneling microscopy (STM) in concert with scanning tunneling spectroscopy (STS), termed STM/S, to obtain the spatial electronic properties of the junction formed across the NWs. Additionally, the parallel p-n junction configuration was characterized by off-axis electron holography in a transmission electron microscope to provide an independent verification of junction formation. The doping process was simulated to elucidate the doping mechanisms involved in the one-step p-i-n junction formation.