Ta Barrier Research Articles

Development of Voltammetry-Based Techniques for Characterization of Porous Low-k/Cu Interconnect Integration Reliability

This paper concerns the new method of detecting the integration failures in porous low-k (PLK)/Cu interconnects using simple voltammetry-based techniques. In essence, the technique takes advantage of the fact that pores in PLK allow permeation of liquid, including electrolyte, into interconnect structures. The infiltration of electrolyte allows the formation of a micro-cell, consisting of two mating Cu interconnect electrodes and the electrolyte in PLK, where simple linear voltammetry can examine various integration reliability issues pertinent to PLK/Cu interconnects. Specifically, the technique is proven to be effective in detection of 1) failure in Ta barrier, 2) cracks in the capping layer, and 3) trapped impurity in pores in PLK. The working principle of the voltammetry technique and demonstration of its effectiveness is introduced in this paper.

Improving Copper Interconnect Reliability via Ta/Ti Based Barrier

As copper line width tightens to 100nm and below, except integration issues, reliability is another challenge. Barrier's performance is critical for the significant impact to interconnect speed and reliability. Ti based barrier is reported as an excellent barrier material from the standpoint of cost and performance, especially for the porous low-k ILD materials. While Ta based barrier is used mainly for its good adhesion, diffusion prevention. In this paper, we demonstrate a composite barrier with Ti adding to the Ta based bi-layer, which shown stable metal resistance while significantly improving reliability performance. Better interface is confirmed by TEM. Our study proved that such kind of Ti doped Ta barrier is better than the standard Ta N/Ta barrier.

Electrochemical investigation of the roles of oxyanions in chemical–mechanical planarization of tantalum and tantalum nitride

Nitrate, sulfate, and phosphate oxyanions are shown to serve as effective surface-modifying agents for low-pressure chemical–mechanical planarization (CMP) of Ta and TaN barrier layers of interconnect structures. The surface reactions that form the basis of this CMP strategy are investigated using cyclic voltammetry, open circuit potential and polarization resistance measurements, and impedance spectroscopy. The results suggest that forming structurally weak layers of surface oxides is crucial to chemically controlling the CMP of Ta/TaN at low polish-pressures. It is shown that in oxyanion-based slurries, this can be accomplished by modifying the sample surfaces with anion-incorporated oxide films of Ta or TaN, which, in turn, can readily be removed with moderate abrasion. Electrochemical results elaborate the reaction mechanisms that lead to anion-modified oxides, such as Ta2O5(1−x)(NO3)10x , Ta2O5(1−x)(SO4)5x , and Ta2O5(1−x)(PO4)10x/3 on both Ta and TaN surfaces in pH-controlled solutions of KNO3, K2SO4, and KH2PO4 solutions, respectively.

Barrier and seed repair performance of thin RuTa films for Cu interconnects

Thin (<4 nm) Physical Vapor Deposited (PVD) Ru-10 at.% Ta films were evaluated as diffusion barriers and seed enhancement layers for Cu metallization in sub 25 nm trenches. The ratio of Ru/Ta on blanket wafers could be influenced by changing the process conditions. However, a difference in Ru/Ta ratio did not influence the thermal stability of the layers during High Temperature X-ray Diffraction (HT-XRD) measurements as all RuTa films exhibited good thermal properties since no Cu-silicide formation was observed for temperatures below 500 °C. The RuTa films also passed an 85 °C/85% relative humidity (RH) test of one week of storage in order to test the H 2O barrier integrity of the films. Furthermore no difference was found when testing the O 2 barrier integrity during 300 s anneals at various temperatures between 250 °C and 500 °C. Good Cu fill of 20 nm trenches (AR 4:1) patterned in oxide was achieved when combining the RuTa films with PVD Cu seed layers with thicknesses ranging from 7 to 20 nm and Cu plating. When compared to a Ta(N)/Ta barrier, relatively high electrical yields (60–80%) were obtained for structures with CDs <30 nm when combining RuTa films with PVD Cu seed layers as thin as 7 nm (on field), hence evidencing the seed enhancement ability of these layers.

Observation of space charge limited current by Cu ion drift in porous low-k/Cu interconnects

This letter reports the observation of the space charge limited current (SCLC) induced by injection and drift of Cu ions into porous low-k dielectrics. The SCLC, characterized by the momentary rise and fall of current with time, is found in all Cu interconnects having defective Ta barrier while it is absent in interconnects with intact barrier. This observation, combined with existing model on SCLC, leads to the conclusion that Cu ions can be injected through defects in Ta barrier and drift under electric field with the mobility as high as an order of 10−13 cm2/sec V at room temperature.

Fabrication and characterization of internal Sn and bronze-processedNb3Sn strands for ITER application

Long multifilamentary Nb3Sn strands for ITER (International Thermonuclear Experimental Reactor) have beensuccessfully fabricated by internal Sn and bronze processes, respectively. To improve thebonding of Nb and Cu and the diffusion between Sn and Cu for the internal tin process,the traditional RIT (rod-in-tube) and new hot extrusion (HE) processes havebeen employed to manufacture the Cu/Nb rods. The final internal tin processstrand with a unit length of 3000–5000 m comprises a Cu stabilizer, a Cu/Tabarrier and 19 subfilaments. Every subfilament consists of about 280–330Nb filaments in a Cu matrix surrounding an Sn–Ti core. The bronze processNb3Sn strands with 11581 and 9805 filaments of Nb7.5Ta with an unit length of more than 3 km havebeen produced, respectively. The Ta or Nb barriers were used to compare the influence ofdifferent barrier materials on the fabrication process and superconducting properties ofNb3Sn strands. The microstructure details of two kinds of strands beforeand after heat treatment have been investigated by scanning electronmicroscopy (SEM) and energy dispersive x-ray analysis (EDX). The non-CuJc (12 T, 4.2 K) value of 812 A mm − 2 with hysteresis loss of 760 mJ cm − 3 for the bronze-processstrand and 1069 A mm − 2 withhysteresis loss of 956 mJ cm − 3 for the internal tin-process strand have been obtained. The influence of microstructure onthe transport property and hysteresis loss of strands has been discussed.

Numerical simulation of critical current performance in Nb3Sn strand subjected to periodic bending deformation

AbstractIn the ITER Engineering Design Activity (EDA), four NB3Sn model coils were developed and successfully tested. However, it was revealed that the critical current of the conductor degraded with the increase of electromagnetic force. One of the explanations of this phenomenon is a strand bending caused by enormous electromagnetic force. The authors therefore developed a simulation code using the distributed circuit model to investigate dependency of the critical current performance on the periodic bending deformation. The simulation results were in good agreement with the experiments. The dependence of the critical current on the periodic transverse load, temperature, periodic load pitch, thickness of Ta barrier which prevents Cu stabilizer from being contaminated by Sn, twist pitch of the strand, and RRR of the bronze matrix was investigated using the developed code. The results showed that the critical current degraded less with decreasing the pitch of the transverse load and increasing the Ta barrier thickness. It suggests that the shorter cabling pitch and the larger bending stiffness prevent the critical current degradation. © 2010 Wiley Periodicals, Inc. Electr Eng Jpn, 171(3): 7–15, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20923

Kinetics of reactive diffusion between Ta and Cu–9.3Sn–0.3Ti alloy

Tantalum is used as a diffusion barrier in the superconducting Nb3Sn composite-wire manufactured by the bronze method. In order to examine the consumption behavior of the Ta barrier during annealing in the bronze method, the kinetics of the reactive diffusion between Ta and a bronze was experimentally observed using sandwich diffusion couples composed of Ta and a Cu–9.3Sn–0.3Ti alloy. The (Cu–Sn–Ti)/Ta/(Cu–Sn–Ti) diffusion couples were isothermally annealed at temperatures of T = 973–1053 K for various times up to t = 1462 h. Owing to annealing, Ta9Sn is formed as a uniform layer at the initial (Cu–Sn–Ti)/Ta interface in the diffusion couple, and gradually grows mainly toward Ta. The mean thickness of the Ta9Sn layer is proportional to a power function of the annealing time. However, the exponent of the power function is equal to unity at t tc. Thus, the transition of the rate-controlling process for the growth of Ta9Sn occurs at t = tc. The critical annealing time tc takes values of 1.83 × 106, 4.63 × 105, and 5.98 × 105 s at T = 973, 1023, and 1053 K, respectively. The growth of Ta9Sn is controlled by the interface reaction at the migrating Ta9Sn/Ta interface in the early stages with t tc. Due to the transition of the rate-controlling process, the growth rate is always much smaller for Ta9Sn than for Nb3Sn. As a result, Ta works as an effective barrier against the diffusion of Sn from the bronze to the Cu stabilizer in the superconducting Nb3Sn composite-wire.

Resistivity of thin Cu films coated with Ta, Ti, Ru, Al, and Pd barrier layers from first principles

We present an atomistic first-principles calculation for the resistivity of rough Cu thin films coated with barrier layers of Ta, Ti, Ru, Al, and Pd. A significant difference in resistivity due to different barrier metals is found. Ti, Ta, and Ru barriers increase the resistivity whereas Al and Pd lower the resistivity, in comparison with that of bare Cu films having the same degree of roughness disorder. It is found that Al/Pd barrier atoms produce density of states (DOS) that match rather well with the DOS of Cu atoms on a Cu film with a perfectly flat surface while the DOS of Ti, Ta, and Ru do not match. Our results suggest that the geometrical roughness on the Cu film that causes diffuse scattering, can be ``smoothed'' out electronically by certain barriers such that the surface scattering becomes more specular.

Effect of TaN Stoichiometry on Barrier Oxidation and Defect Density in 32nm Cu/Ultra-Low K Interconnects

AbstractThe scaling of BEOL interconnect technology in ULSI circuitry requires the integration of Cu wiring with ultra-low K (ULK) dielectrics. We present the results of a study of the interaction between different-stoichiometry Ta(N)/Cu barrier processes and porous ULK dielectrics (k=2.4) at 32nm groundrules Auger and diffraction analysis of blanket wafers was used to benchmark two different stoichiometries of TaN barrier deposited using commercially-available ionized PVD sources. Comparison TEM and EDX/EELS images indicates that barrier oxidation is occurring in the low nitrogen-content Ta(N) barrier, which is absent at the higher stoichiometry. These differences are further manifested in defect-density analysis of patterned wafers comparing the two processes. These results illustrate the critical importance the TaN barrier properties play in enabling the integration of Cu/ULK interconnects at 32nm at beyond.

Utility of Oxy-Anions for Selective Low Pressure Polishing of Cu and Ta in Chemical Mechanical Planarization

Slurry chemistries utilizing several simple oxy-anions are proposed to meet some of the requirements for low pressure chemical mechanical polishing of Ta barrier and Cu lines in nonalkaline dispersions. Polish rates (PRs) were measured for Cu and Ta disks using H 2 O 2 based slurries containing anions of phosphate, sulfate, chlorate, carbonate, nitrate, or persulfate. Polishing of Cu, Ta, TaN, and SiO 2 wafers using the sulfate-containing slurry demonstrated a high selectivity (∼15) of Ta:Cu PRs and could be controlled by varying the anion and/or the pH of the slurry. The processed wafers showed surface morphologies with acceptable roughness (< 1 nm) and peak-to-valley distances (< 10 nm).

TaバリアMgB2線材を用いた小パルスコイルの試作 （4） MgB2線材E-J特性の磁界・温度依存性

TaバリアMgB2線材を用いた小パルスコイルの試作 （4） MgB2線材E-J特性の磁界・温度依存性

Electrochemical characterization of surface complexes formed on Cu and Ta in succinic acid based solutions used for chemical mechanical planarization

Open-circuit potential measurements, cyclic voltammetry and Fourier transform impedance spectroscopy have been used to study pH dependent surface reactions of Cu and Ta rotating disc electrodes (RDEs) in aqueous solutions of succinic acid (SA, a complexing agent), hydrogen peroxide (an oxidizer), and ammonium dodecyl sulfate (ADS, a corrosion inhibitor for Cu). The surface chemistries of these systems are relevant for the development of a single-slurry approach to chemical mechanical planarization (CMP) of Cu lines and Ta barriers in the fabrication of semiconductor devices. It is shown that in non-alkaline solutions of H 2O 2, the SA-promoted surface complexes of Cu and Ta can potentially support chemically enhanced material removal in low-pressure CMP of surface topographies overlying fragile low-k dielectrics. ADS can suppress Cu dissolution without significantly affecting the surface chemistry of Ta. The data analysis steps are discussed in detail to demonstrate how the D.C. and A.C. electrochemical probes can be combined in the framework of the RDE technique to design and test CMP slurry solutions.

Robust Low Oxygen Content Cu Alloy for Scaled-Down ULSI Interconnects Based on Metallurgical Thermodynamic Principles

A low oxygen content (LOC) CuAl alloy with no barrier metal (Ta) oxidation was obtained using an oxygen absorption process based on metallurgical thermodynamic principles. LOC CuAl dual damascene interconnects (DDIs) were successfully implemented into 45-nm-node LSIs with 140-nm-pitched lines and 70-nm-diameter (phi) vias. An oxygen absorber of very thin Al film, which was deposited on an electrochemically deposited (ECD) Cu film, captured the oxygen atoms in the ECD Cu due to its larger negative change in the standard Gibbs-free energy of oxidation than in the Cu and the barrier (Ta), preventing the Ta barrier from oxidizing during high-temperature annealing. The high-quality Cu/barrier interface in the LOC CuAl DDIs remarkably improved the via reliability against stress-induced voiding and electromigration. No reliability degradation of the 70-nm-phi vias was observed in the 45-nm-node LOC CuAl DDIs, while keeping the scalability from the 65-nm-node generation.

Investigations of failure mechanisms at Ta and TaO diffusion barriers by secondary neutral mass spectrometry

One of the most important processes in Cu metallization for highly integrated circuits is to fabricate reliable diffusion barriers. Recently, thin films made of refractory metals and their compounds have been widely used in solid-state electronics as barriers because of their good electric properties, favourable thermal properties and chemical stability. Thermal stability of Tantalum (Ta) and Tantalum-oxide (TaO x ) layers as a diffusion barrier in Si/Ta/Cu, Si/TaO x /Cu and Si/Ta-TaO x /Cu systems have been investigated. Si/Ta (10 nm)/Cu (25 nm)/W (10 nm), Si/TaO x (10 nm)/Cu (25 nm)/W (10 nm) and Si/Ta (5 nm)TaO x (5 nm)/Cu (25 nm)/W (10 nm) thin layers were prepared by DC magnetron sputtering. A tungsten cap layer was applied to prevent the oxidation of the samples during the annealing process. The samples were annealed at various temperatures (473 K–973 K) in vacuum. Transmission Electron Microscopy, X-ray diffraction, X-Ray Photoelectron Spectroscopy and Secondary Neutral Mass Spectrometry were used to characterize the microstructure and diffusion properties of the thin films. Our results show that at the beginning phase of the degradation of the Si/Ta/Cu system Ta atoms migrate through the copper film to the W/Cu interface. In the Si/TaO x /Cu system the crystallization of TaO and the diffusion of Si through the barrier determine the thermal stability. The Ta–TaO bilayer proved to be an excellent barrier layer between the Si and Cu films up to 1023 K. The observed outstanding performance of the combined film is explained by the continuous oxidation of Ta film in the TaO x –Ta bilayer.

Galvanic Corrosion Control in Chemical Mechanical Polishing of Cu Interconnects with Ruthenium Barrier Metal Film

We investigated galvanic corrosion in Cu interconnects with ruthenium (Ru) barrier metal films that occurred in Cu chemical mechanical polishing (CMP) and Ru-CMP steps. When conventional CMP slurries were used in the same polishing conditions, severe dishing was found to occur in Ru barrier interconnects. This dishing was attributed to the electrochemical potential difference between Cu and Ru in the CMP slurries. Thus, we reduced this potential difference in the slurry from 400 to 2 mV, and we suppressed dishing just after Cu-CMP from 27 to 9 nm. As a result, by using Ru slurry with a controlled corrosion potential, we minimized final dishing after Ru-CMP to 25 nm which was lower than that of conventional Ta barrier interconnects.

Slurry Development Challenges and Solutions for Advanced Node Copper CMP

The R&D of Cu CMP started in the late 1980s but was fully implemented in the HVM in the late 1990s and early 2000. Cu CMP was fully implemented for the 130 nm nodes primarily in the Logic devices. In the decade following the Cu CMP introduction, it has grown and changed in significant ways. As of now Cu CMP is a main stay for most BEOL integration schemes and being applied to Memory, 3D and MEMS devices. The success of Cu CMP was due to many enabling events such as Ta barrier development, AMAT Mirra tool design and low selectivity barrier process. As we move from 130 nm nodes into more advanced nodes additional challenges have unfolded. Performance requirements have become quite stringent while CoO reduction pressures are mounting. While the 130 nm process required polishing of Cu, Ta and TEOS at high DF , moderate topography and moderate defects, for 32 nm and beyond polishing of multiple complex materials (like LK, ULK, ALD barriers, MHM, ARC and Copper) at very low DF with very low topography and defects is essential. These challenging needs for the advanced nodes require novel slurry development solutions. Both the Copper and Barrier layer slurries have changed over the decade. Abrasives have changed from Alumina and fumed silica to colloidal silica and coated particles. Significant change in chemistry design is required to address low DF polishing.

Study of copper drift during TDDB of intermetal dielectrics by using fully passivated MOS capacitors as test vehicle

A fully passivated Metal-Oxide-Semiconductor (MOS) capacitor [Tőkei Zs. Barrier integrity and reliability in copper low-k interconnects. ISTC 2005; 386–95] is used to study the intrinsic properties of a barrier between copper and dielectric in Back-End-Of-Line interconnects. Several barriers are studied and compared to each other. The test vehicle is also used to thoroughly investigate the role of thermal diffusion or field assisted ionic copper drift during Time-Dependent-Dielectric Breakdown (TDDB) by investigating the breakdown of dielectrics without a barrier at very low voltages. Comparing different barriers revealed that a “standard” PVD-based Ta barrier has a significantly better TDDB-performance compared to an 8 nm SiCN-barrier. For samples without barrier, it was found that long thermal anneals without the application of a stress voltage changes the distribution of failure times. Furthermore, the possibility of a bimodal distribution was argued when stressing these devices at a wide range of fields. A corollary is that, in presence of copper, both the E-model and the root- E-model do not apply for describing the experimental data obtained on these samples without barrier.

Friction-Based &lt;i&gt;In Situ &lt;/i&gt;Endpoint Detection of Copper CMP Process

Chemical mechanical polishing (CMP) has been extensively used in the integrate circuit (IC) manufacturing industry as a widely accepted global planarization technology, accurate in situ endpoint detection of CMP process can reduce the product variance, significantly improve yield and throughput. A CMP in situ endpoint detection system, which measured the friction and downforce during CMP process using a specially designed three-axis strain gauge force sensor, was developed. The frictional transition from copper (Cu) to tantalum (Ta) barrier as well as Ta barrier to silicon dioxide (SiO2) dielectric was detected during CMP process. The experimental results showed that the change of friction could be detected when the polished material changed. The developed CMP in situ endpoint detection system is feasible for 300 mm and 450 mm copper CMP process.

A study of thermo-mechanical stress and its impact on through-silicon vias

The BOSCH etch process, which is commonly used in microelectromechanical system fabrication, has been extensively investigated in this work for implementation in through-silicon via (TSV) technology for 3D-microsystems packaging. The present work focuses on thermo-mechanical stresses caused by thermal loading due to post-TSV processes and their impact on the electrical performance of through-silicon copper interconnects. A test vehicle with deep silicon copper-plated comb structure was designed to study and evaluate different deep silicon via etch processes and its effect on the electrical leakage characteristics under various electrical and thermal stress conditions. It has been shown that the leakage current between the comb interconnect structures increases with an increase in sidewall roughness and that it can be significantly lowered by smoothening the sidewalls. It was also shown that by tailoring a non-BOSCH etch process with the normal BOSCH process, a similar leakage current reduction can be achieved. It was also shown through thermo-mechanical simulation studies that there is a clear correlation between high leakage current behavior due to non-uniform Ta barrier deposition over the rough sidewalls and the thermo-mechanical stress induced by post-TSV processes.