Diffusion Barrier For Copper Metallization Research Articles

The evolution of diffusion barriers in copper metallization

Refractory metal nitride thin films have been widely developed as the diffusion barriers for the aluminum or copper interconnects in integrated circuits. This study reviewed the evolution of diffusion barriers in copper metallization. First, materials characteristics and electrical properties of various diffusion barriers, titanium nitride (TiN), tantalum nitride (TaN), and titanium zirconium nitride (TiZrN), were examined. These diffusion barriers were prepared by reactive magnetron sputtering in N2/Ar gas mixtures. Next, barrier performance was evaluated by annealing the Cu/barrier/Si systems at 400–1,000°C for 60 min. in vacuum as well as the measurements of copper diffusion coefficients. The results suggest that TiZrN films can be used as a diffusion barrier for copper metallization better than the well-known TaN films. Therefore, the evolution of diffusion barriers in copper metallization, from TiN to TaN and then from TaN to TiZrN, is addressed.

<B>In-Situ Characterizarion of Interfaces-Induced Resistivity in Nanometric Dimensions</B>

AbstractTo improve the speed of integrated circuits it is highly important to minimize the electrical resistivity of their interconnects. However, as the dimensions of the interconnects approach the mean free path of the electrons, a substantial rise in resistivity occurs due to additional electron scatterings from grain boundaries and interfaces. To investigate the role of interfaces, in-situ resistivity measurements were preformed for thin copper films on which different materials were deposited. The resistivity was monitored before and after the deposition, enabling to observe the changes as a new top interface was created. Among the materials tested, tantalum resulted in the highest resistivity increase. This fact is of special importance since this material and its nitride serve today as the common diffusion barrier for copper metallization. Titanium resulted in a smaller resistivity increase, but still higher than that of a free copper surface in vacuum. The developed approach enables to test the influence of different diffusion barriers on copper resistivity. With the continuously shrinking dimensions of copper interconnects, this factor will have an increasing importance in future technology nodes.

Scanning magnetron-sputtered TiN coating as diffusion barrier for silicon devices

Scanning magnetron-sputtered titanium nitride (TiN) films were deposited onto silicon substrates under varying nitrogen and argon pressures. Golden TiN films with (220) orientation were deposited at different substrate bias voltages (0to−120V). Auger electron spectroscopy measurements show N∕Ti ratio between 1.18 and 1.07, and oxygen content between 11% and 5% in the as-deposited TiN film samples. Dependence of the stress and grain size on substrate bias voltage and dependence of resistivity on bias voltage and annealing temperature have been studied for the deposited TiN films. Resistivity was found minimum for the TiN samples deposited at −40V bias, which decreases from 320to132μΩcm on annealing, up to 750°C. Copper was sputter deposited on the as-grown TiN films deposited at −40V bias. The Cu∕TiN∕Si samples were annealed at different temperatures. Resistivity, x-ray diffraction, scanning electron microscopy, and energy dispersive x-ray analysis results of the Cu∕TiN∕Si structure are consistent with each other, and show that scanning magnetron sputtering deposited TiN film is a good diffusion barrier for copper metallization of the silicon devices, up to 750°C.

Characterization of Atomic Layer Deposited WN x C y Thin Film as a Diffusion Barrier for Copper Metallization

The properties of films deposited by atomic layer deposition (ALD) using and triethyl boron were characterized as diffusion barriers for copper metallization. The films deposited at 313°C showed resistivity of about 350 μΩ cm with a density of The film composition measured by Rutherford backscattering spectrometry showed W, C, and N of approximately 48, 32, and 20 atom %, respectively. Transmission electron microscopy analyses showed that the as-deposited film was composed of a face-centered-cubic phase with a lattice parameter similar to both and with an equiaxed microstructure. The film kept its nanocrystalline microstructure until annealing at 700°C, although some amount of simple hexagonal α-WC was identified to be formed and the phase disappeared. As the annealing temperature increased to 800°C, relatively larger grains of body-centered-cubic W were newly formed with smaller grains of hexagonal-close-packed or α-WC. All the phenomena are related to nitrogen release after annealing at 700 and 800°C. The results of diffusion barrier performance between Cu and Si analyzed by X-ray diffractometry showed that film (12 nm) failed only after annealing at 700°C for 30 min by the formation of copper silicide, while the sputter-deposited Ta (12 nm) and ALD-TiN (20 nm) films failed at 650 and 600°C annealing, respectively. It is thought that the superior diffusion barrier performance of film is the consequence of both the formation of equiaxed microstructure and the high-density nature of the film. © 2004 The Electrochemical Society. All rights reserved.

Robust self-assembled monolayer as diffusion barrier for copper metallization

Excellent results on copper (Cu) diffusion barrier characteristics of a self-assembled monolayer (SAM) of 2-(diphenylphosphino)ethyltriethoxy-silane are reported. The thickness and roughness of the SAM were determined by grazing incidence x-ray reflectometry as 1.7 and 0.3 nm, respectively. To evaluate Cu diffusion barrier performance of the SAM, Cu/SiO2/Si and Cu/SAM/SiO2/Si metal-oxide-semiconductor capacitors were prepared to measure their lifetimes under the 2 MV/cm electric bias at 498–548 K. The mean times to failure obtained from the Weibull plots of time to failures were 33.6, 9.24, 4.57, and 2.03 h at 498, 523, 533 and 548 K, respectively. These values show that the barrier characteristic of the SAM of 1.7 nm in thickness is comparable to that of physical-vapor-deposited Ta film of 20 nm in thickness. The estimated lifetime of the SAM barrier at the device operation temperature of 392 K is longer than 10 yr.

Titanium nitride diffusion barrier for copper metallization on gallium arsenide

The diffusion properties of Cu, Cu/titanium nitride (TiN) and Cu/TiN/Ti metallization on GaAs, including as-deposited film and others annealed at 350–550 °C, were investigated and compared. Data obtained from X-ray diffractometry, resistivity measurements, scanning electron microscopy, energy dispersive spectrometer and Auger electron spectroscopy indicated that in the as-deposited Cu/GaAs structure, copper diffused into GaAs substrate, and a diffusion barrier was required to block the fast diffusion. For the Cu/TiN/GaAs structure, the columnar grain structure of TiN films provided paths for diffusion at higher temperatures above 450 °C. The Cu/TiN/Ti films on GaAs substrate were very stable up to 550 °C without any interfacial interaction. These results show that a TiN/Ti composite film forms a good diffusion barrier for copper metallization with GaAs.

Chemical Vapor Deposition of Cr-Based Thin Films as Diffusion Barriers in Copper Metallization

Chemical Vapor Deposition of Cr-Based Thin Films as Diffusion Barriers in Copper Metallization

Atomic-layer-deposited WNxCy thin films as diffusion barrier for copper metallization

The properties of WNxCy films deposited by atomic layer deposition (ALD) using WF6, NH3, and triethyl boron as source gases were characterized as a diffusion barrier for copper metallization. It is noted that the as-deposited film shows an extremely low resistivity of about 350 μΩ cm with a film density of 15.37 g/cm3. The film composition measured from Rutherford backscattering spectrometry shows W, C, and N of ∼48, 32, and 20 at. %, respectively. Transmission electron microscopy analyses show that the as-deposited film is composed of face-centered-cubic phase with a lattice parameter similar to both β-WC1−x and β-W2N with an equiaxed microstructure. The barrier property of this ALD–WNxCy film at a nominal thickness of 12 nm deposited between Cu and Si fails only after annealing at 700 °C for 30 min.

Multilayer diffusion barrier for copper metallization using a thin interlayer metal (M=Ru, Cr, and Zr) between two TiN films

A multilayer structure [TiN(5 nm)/interlayer metal(2 nm)/TiN(5 nm)] has been evaluated as a diffusion barrier between copper and silicon. Three metals (M=Ru, Cr, and Zr) were tested as interlayer materials for the multilayer diffusion barrier scheme, expecting that they will stuff the TiN grain boundaries by forming metal oxides therein. The diffusion barrier performance was found to be the best when the Zr was used as an interlayer material. X-ray diffractometry showed that the Zr interlayer diffusion barrier failed only after annealing at 800 °C for 30 min due to the formation of copper silicide while the reference sample without a metal interlayer failed when annealed at 600 °C. The insertion of Cr also was also found to enhance the diffusion barrier performance, whereas, Ru has little effect on the improvement of the barrier performance. High-resolution transmission electron microscopy and energy dispersive spectroscopy showed that Zr significantly diffused into TiN grain boundaries while Ru did not diffuse during annealing. An explanation of the variation of the diffusion barrier performance with the change of an interlayer metal is given in terms of Gibbs free energy change of the respective metal–oxide formation.

AES and SIMS investigation of diffusion barriers for copper metallization in power-SAW devices.

Barrier layers for Cu-metallization in surface acoustic wave (SAW) devices were investigated by AES and SIMS depth profiles. Two layered systems on LiNbO(3) substrate have been analyzed after annealing in air up to 400 degrees C. The investigated systems were (A) Ta(20 nm)/Cu(150 nm)/Ti(30 nm), deposited by electron beam evaporation, and (B) Ta(30)Si(18)N(52)(50 nm)/Cu(150 nm)/Ta(30)Si(18)N(52)(50 nm) deposited by magnetron sputtering. In system A the Ta layer shows oxidation in air for T>or=300 degrees C. Ti from the buffer layer diffuses into the Cu at about 100 degrees C, and segregates at the Ta/Cu interface for T>or=200 degrees C. Oxidation of the Ti layer starts at 300 degrees C. But no remarkable amounts of oxygen could be found in the Cu film. The depth profiles show that the TaSiN layer in system B operates as a more effective barrier for the Cu-SAW technology up to more than 300 degrees C.

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 result of a low, non-negligible copper concentration at the grain boundaries, estimated at less than 10 at. %. The copper concentration plateaus in PVD cobalt were significantly larger due to saturation of the grain boundaries.

The effect of ion beam bombardment on the properties of Ta(C)N films deposited from pentakis-diethylamido-tantalum

TaN or TaCN films were deposited using a single source, pentakis-diethylamido-tantalum as diffusion barrier for copper metallization. N- and Ar-ion beams with energy of 120 eV were used for bombarding the film during growth to improve the film quality. The films deposited using N-ion beam showed a resistivity of approximately 950 μΩ cm and a density of 7.65 g/cm 3. The use of the N-ion beam, however, drastically degraded the step coverage of the film (∼5% at the 0.5 μm contact holes with aspect ratio of 3:1). On the other hand, the films deposited using an Ar-ion beam showed a resistivity of approximately 600 μΩ cm and a density of 8.26 g/cm 3. The step coverage measured at the same contact was approximately 40%. The resistivity and density of the thermally-decomposed film were also measured for comparison and were found to be approximately 10 000 μΩ cm and 5.85 g/cm 3, respectively. Finally, the diffusion barrier performance of 50 nm thick films against Cu was investigated by X-ray diffractometry. The Cu/N- or Ar-ion beam bombarded film/Si structures showed formation of η″-Cu 3Si after annealing at 650 °C for 1 h, while Cu/thermally-decomposed film/Si showed this only after annealing at 600 °C.

Thermal Stability of Sputtered Tungsten Carbide as Diffusion Barrier for Copper Metallization

This work investigated the thermal stability of tungsten carbide films deposited by a sputtering process with a WC target as diffusion barrier layer between Cu and Si. The as-deposited film has a nanocrystalline structure and a low electrical resistivity of around 227 μΩ cm. Film characterization reveals that the film was able to preserve the integrity of the Cu (500 Å)/n-Si structure without formation of up to a 600°C annealing for 30 min. In addition, diode leakage current measurements on the same contact structure, but with a substrate did not show deterioration of electrical characteristics up to a 550°C annealing. As the thickness of the barrier was reduced to 150 Å, the film retained the integrity of diodes up to 500°C without increasing the diode leakage current. The failure of film after high temperature annealing is attributed to the Cu diffusion into the Si substrate through grain boundaries or local defects of the barrier layer, in which some local defects may arise from the formation of © 2001 The Electrochemical Society. All rights reserved.

Characterization of Sputtered Titanium Carbide Film as Diffusion Barrier for Copper Metallization

In this study, the physical and electrical properties as well as thermal stability of sputter-deposited titanium carbide films as diffusion barriers for Cu metallization were investigated for the first time. With thermal annealing in ambient for 30 min, the unpatterned Cu (2000 Å)/ (500 Å)/n-Si structure was found to be metallurgically stable up to a temperature between 600 and 650°C without formation of a phase, while the reverse leakage current of the patterned Cu (2000 Å)/ (500 Å)/ diode structures showed that the barrier layer was capable of withstanding annealing temperatures up to 500°C. The failure of the layer after high temperature annealing is attributed to the diffusion of Cu along localized defects or grain boundaries of the barrier layer into Si substrate, which caused the high junction leakage currents for the patterned structure and formation of for the unpatterned structure. © 2001 The Electrochemical Society. All rights reserved.

Intrinsic Properties and Barrier Behaviors of Thin Films of Sputter-Deposited Single-Layered and Alternately Layered Tantalum Nitrides ( Ta2 N / TaN )

This work employs X-ray diffraction, X-ray reflectivity, and transmission electron microscopy, along with electrical (sheet resistance and resistivity) and bending-beam stress analyses, to characterize the intrinsic properties and barrier behavior of 40 nm thick thin films of sputter-deposited single-layered single-layered TaN, and alternately layered having various period thicknesses (λ) from 4 to 40 nm. The thermal stability of each of these barriers interposed between silicon and copper at a high temperature regime (500-900°C) is closely related to variations of intrinsic properties of the barriers, particularly microstructure and residual stress (σ). Failure of the barriers is explained in terms of a previously reported mass transport mechanism, which suggests that the free short-diffusion paths (grain boundaries) created by anneal-induced crystallization/grain growth in the single-layered and TaN barriers (σ = 3-5 GPa) are a key factor in deteriorating the diffusion barriers. Conversely, an adequately layered barrier with λ down to approximately 5 nm is nearly free of residual stress and can maintain a very stable quasi-amorphous microstructure against annealing. Therefore, this quasi-superlattice novel design has the capability to further improve thermal stability and reliability of tantalum nitride diffusion barriers for copper metallization. © 2001 The Electrochemical Society. All rights reserved.

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

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.

Tantalum diffusion barrier grown by inorganic plasma-promoted chemical vapor deposition: Performance in copper metallization

As-deposited and annealed tantalum films, grown by plasma-promoted chemical vapor deposition (PPCVD) using pentabromotantalum and hydrogen as coreactants, were evaluated as diffusion barriers in copper metallization. Stacks consisting of 500-nm-thick sputtered Cu/55-nm-thick untreated PPCVD Ta/Si were annealed in argon in the range 450 to 650 °C, in 50 °C intervals, along with sputtered Cu/preannealed PPCVD Ta/Si and sputtered Cu/sputtered Ta/Si stacks of identical thickness. Pre- and postannealed stacks were characterized by x-ray photoelectron spectroscopy, Auger electron spectroscopy, Rutherford backscattering spectrometry, hydrogen profiling, x-ray diffraction, atomic force microscopy, sheet resistance measurements, and Secco chemical treatment and etch-pit observation by scanning electron microscopy. The sputtered and preannealed PPCVD Ta films acted as viable diffusion barriers up to 550 °C, while the as-deposited PPCVD Ta films failed above 500 °C. In all cases, breakdown occurred through the migration of Cu into Si, rather than an interfacial reaction between Ta and Si, in agreement with previously reported results for sputtered Ta films. The accelerated barrier failure for as-deposited PPCVD Ta might have been caused by the presence of approximately 20 at.% hydrogen in the as-deposited PPCVD Ta, an observation which was supported by the enhanced performance of the same PPCVD Ta films after annealing-induced hydrogen removal.

Impact of the Unique Physical Properties of Copper in Silicon on Characterization of Copper Diffusion Barriers

In this paper, we establish a connection between the physical properties of copper in silicon substrates and the problems of characterization of copper diffusion barriers for ultra-large scale integration (ULSI) technology. We show that many of the methods successfully used for detection of the failure of aluminum diffusion barriers are poorly suited for characterization of barriers for copper metallization technology. This is because the unique physical properties of copper make its diffusion and precipitation behavior in silicon significantly different from those of other metals. These properties must be taken into account in studies of the detrimental effect of copper on device yield and in development of thin diffusion barriers for copper metallization. The sensitivity of most depth profiling techniques (such as secondary ion mass spectrometry, Auger electron spectroscopy, or Rutherford backscattering) is not sufficient to detect barrier failure before the dissolved copper concentration in the bulk reaches its solubility limit at the back-end processing temperature (e.g., 2 × 10 13 cm -3 at 450 °C). The electrical measurement techniques, such as analysis of I-V characteristics of p-n junctions formed by ion implantation under the barrier layers are more sensitive to penetration of copper than depth profiling techniques, but cannot determine the copper contamination level quantitatively, Emerging tools suitable for detection of copper penetration at technologically relevant levels are discussed.

Evaluation of single- and multilayered amorphous tantalum nitride thin films as diffusion barriers in copper metallization

This study evaluates 40-nm-thick amorphous Ta2N (a-Ta2N) thin films with various compositions and metallurgical designs as diffusion barriers for copper metallization. Results based on sheet resistance measurements, x-ray diffraction analyses, and transmission and scanning electron microscopies consistently follow a sequence of coarsening copper grains, transforming a-Ta2N into a crystalline phase, and finally forming {111}-faceted pyramid Cu3Si precipitates and TaSi2. The degradation of the single-layered stoichiometric Ta2N (a-Ta67N33) barriers is primarily triggered by a premature crystallization of the amorphous barrier layers at temperatures as low as 450 °C. However, as adequately designed double-layered (20 nm Ta67N33/20 nm Ta62N38) amorphous barriers can be subjected to high-temperature annealing without crystallization, the effectiveness of the double-layered barriers can be significantly improved, elevating the degradation temperature by approximately 100 °C.

Application of equilibrium thermodynamics to the development of diffusion barriers for copper metallization ( invited)

The guidelines for designing a conductive, amorphous material, capable of thermodynamic equilibrium with copper, are defined using readily available thermodynamic information. The tradeoff between desired properties — equilibrium at the interfaces, amorphous microstructure, and electronic conductivity — are described, along with trends in relevant binary systems that result in these properties. These guidelines maximize the thermodynamic tendency for amorphous phase formation, in an attempt to minimize the need for nonequilibrium synthesis methods.