Ta Diffusion Barrier Research Articles

Ab Initio Simulations of Low-K and Ultra Low-K Dielectric Interconnects

Ab initio molecular dynamics simulations were carried out to study low-k/ultra low-k dielectric systems comprising Cu/Ta/SiLK-like polymer. A study of the motion of single metal atoms of Cu and Ta in the SiLK-like polymer showed that Cu atom motions are effected by jumps between cavities inside the polymer and that Ta is more sluggish than Cu not only because of its larger mass but also because of stronger affinity to the polymer. It was also found that crosslinking of the polymer did not affect the motion of the metal atoms. Simulations of deposition showed that a thin Ta diffusion barrier does not have good structural integrity to prevent Cu-diffusion when directly deposited onto the SiLK; the barrier performance was greatly improved after introducing a Si-based film between the Ta and SiLK.

Large scale ab initio molecular dynamics simulations of hydrogen-induced degradation of Ta diffusion barriers in ultralow-k dielectric systems

In ultralow-k dielectric systems, the porous dielectrics are normally sealed by a SiC film before the deposition of a Ta diffusion barrier layer. However, the Ta barrier effects are negated when the SiC films are fabricated by plasma-enhanced chemical vapor deposition (PECVD). Through large scale ab initio molecular dynamics simulations, the authors found that the barrier degradation is due to H atoms introduced during PECVD. The H impurities diffuse into and transform an otherwise dense Ta layer into a loose amorphous phase which is ineffective as a diffusion barrier.

Quantitative Measurement of Interfacial Adhesion Between Seedless Electrodeposited Copper and Tantalum-Based Diffusion Barriers for Microelectronics

Seedless electrochemical deposition (ECD) can generate good adhesion of copper on TaN and Ta diffusion barriers using a saturated KOH solution and a copper-citrate complex electrolyte. The interfacial fracture energy of the Cu/TaN or Ta interface was quantitatively measured by four-point bending method. The load increased linearly with interfacial crack growth due to the plasticity of ductile copper and the elastic bending of the Si substrate. The delamination energy was proportional to the thickness of seedless ECD copper, which was in the range of 180-980 nm. The delamination energy in the steady state, G ss , of 5-24 J m -2 was attained.

Enhancing the reliability of n +–p junction diodes using plasma treated tantalum barrier film

The properties of Ta barrier films treated with various plasma nitridations have been investigated by Cu/barrier/Si. An amorphous layer is formed on Ta barrier film after plasma treatments. The thickness of the amorphous layer is about 3 nm. Plasma treated Ta films possess better barrier performance than sputtered Ta and TaN films. It is attributed to the formation of a new amorphous layer on Ta surface after the plasma treatment. Cu/Ta(N,H)/Ta (10 nm)/Si remained stable after annealing at 750 °C. Ta(N,H)/Ta possesses the best thermal stability and excellent electrical properties. Cu/Ta/n +–p and Cu/Ta(N,O)/Ta/n +–p diodes resulted in large reverse-bias junction leakage current after annealing at 500 °C and 600 °C, respectively. On the other hand, Ta(N,H)/Ta and Ta(N)/Ta diffusion barriers improve the thermal stability of junction diodes to 650 °C. Ta(N,H)/Ta barrier film possesses lowest resistivity among Ta, Ta(N,O)/Ta, and Ta(N)/Ta films. Hydrogen plays an important role in enhancement of barrier properties. It is believed that hydrogen not only induces amorphization on Ta, but also eliminates the oxygen in the film. It is believed that the enhancement of ability against the copper diffusion is due to the combined effects of the hydrogen reaction and nitridation.

A direct measurement of electromigration induced drift velocity in Cu dual damascene interconnects

A dual damascene structure with an additional 25 nm Ta diffusion barrier embedded into the upper Cu layer was fabricated to measure the drift velocity of electromigration. The embedded diffusion barrier layer successfully confined void growth into a long and regular shape between the SiN layer and embedded Ta layer. Edge depletion was observed to initiate from the cathode end and elongate into a long and regular shape due to the confinement of the intermediate Ta diffusion barrier layer. With this test structure, electromigration induced drift displacement can be accurately measured with a linear dependence on time. Measurement was conducted at a series of temperatures to obtain the Cu/capping interface diffusion controlled activation energy.

Confinement of electromigration induced void propagation in Cu interconnect by a buried Ta diffusion barrier layer

Direct observation, by means of in situ scanning electron microscopy, of void heterogeneous nucleation and migration controlled electromigration failure mechanism in Cu dual damascene interconnect structures has been recently reported [A. V. Vairagar, S. G. Mhaisalkar, A. Krishnamoorthy, K. N. Tu, A. M. Gusak, M. A. Meyer, and E. Zschech, Appl. Phys. Lett. 85, 2502 (2004)] In the present study, a dual damascene structure with an additional 25nm Ta diffusion barrier embedded into the upper Cu layer was fabricated. This thin layer of diffusion barrier blocked voids from propagating into the via, thus eliminating the previously reported failure mechanism. With this structure, a lifetime improvement of at least 40 times was achieved. Analysis on failed samples suggested that failures in samples with the embedded Ta barrier layer occurred at the bottom of the via, which were caused by void migration along the bottom of the Cu lines.

Reliability of multistacked tantalum-based structure as the barrier film in ultralarge-scale integrated metallization

Diffusion barrier properties of Ta films with and without plasma treatments have been investigated in the study. The nitrogen-incorporated Ta films were prepared by NH 3 plasma treatment or reactive sputtering. Barrier properties were evaluated by sheet resistance, X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy and reverse-biased junction leakage current. An amorphous-like TaN x layer was formed on Ta barrier film after plasma treatments. The thickness of the amorphous TaN x layer is about 3 nm and NH 3 plasma-treated Ta films (TaN x /Ta) possess lower resistivity and smaller grain sizes. The Cu/TaN x /Ta(10 nm)/Si remained stable after annealing at 750 °C for 1 h. NH 3 plasma-treated Ta films (TaN x /Ta) possess better thermal stability than Ta and TaN films. It is attributed to the formation of a new amorphous layer on the surface of Ta film after the plasma treatments. For thermal stability of Cu/Ta(-N)/n +-p diodes, Cu/Ta/n +-p and Cu/TaN/n +-p junction diodes resulted in large reverse-bias junction leakage current after annealing at 500 and 525 °C, respectively. On the other hand, TaN x /Ta diffusion barriers will improve the integrity of Cu/Ta(-N)/n +-p junction diodes to 650 °C.

Sacrificial adhesion promotion layers for copper metallization of device structures.

The adhesion of copper films to adjacent device layers including TiN, Ta, and TaN diffusion barriers is a crucial reliability issue for integrated circuits. We report that ultrathin layers of poly(acrylic acid) (PAA) prepared on barrier surfaces or on the native oxide of Si wafers dramatically increase the interfacial adhesion of Cu films deposited by the H2 assisted reduction of bis(2,2,7-trimethyloctane-3,5-dionato)copper in supercritical carbon dioxide. Similar improvements were achieved on Si wafers using a simple vapor phase exposure of the substrate to acrylic acid prior to metallization. The deposited films and the substrate/Cu interfaces were analyzed by X-ray photoelectron spectroscopy (XPS), electron microscopy, atomic force microscopy, and variable-angle spectroscopic ellipsometry. No trace of the adhesion layer was detected at the interface, indicating it was sacrificial at the deposition conditions used. Moreover, the presence and subsequent decomposition of the PAA layer during deposition substantially reduced or eliminated metal oxides at the substrate interface. For depositions on PAA-treated Si wafers, copper was present primarily as Cu0 at the interface and Si was present only as Si0. On PAA-treated Ta substrates, XPS analysis indicated Ta was present primarily as Ta0 at the metallized interface whereas Ta2O5 dominated the interface of samples prepared without the adhesion layers. The technique can be extended to patterned substrates using adsorption of acrylic acid or thermal/UV polymerization of acrylic acid.

Bias-temperature stress analysis of Cu∕ultrathin Ta∕SiO2∕Si interconnect structure

Bias-temperature stress test was used to evaluate the efficiency of an ultrathin Ta diffusion barrier in Cu interconnects by assessing the failure mode of the Cu interconnect structure. Samples are stressed up to failure in order to study the actual failure mode of fabricated MOS capacitors. The time-to-failure (TTF) of samples is estimated to be at least 14 years under standard operating conditions, which is determined by extrapolating TTFs of current–time (I–t) curves measured at accelerated test conditions. The calculated activation energies of the capacitors is within range of normal time-dependent dielectric breakdown (TDDB) activation energies, suggesting TDDB-related failure. Tof-SIMS and I–t analyses strongly suggest a mixed mode failure mechanism in the capacitors, where Cu+ ion contamination is dominant at low field and high temperature stress conditions, while TDDB dominates at other conditions.

Degradation mechanisms of Ta and Ta?Si diffusion barriers during thermal stressing

The degradation mechanisms of 10 nm thick Ta and Ta73Si27 diffusion barriers deposited between Cu and SiO2 are compared by means of X-ray reflectometry, glancing angle X-ray diffraction, glow discharge optical emission spectroscopy depth profiling and transmission electron microscopy analysis. Annealing the samples at Tan=600 °C for tan=1 h results in a diffusion of Ta atoms through the Cu capping layer to the sample surface, particularly in the case of the pure Ta barrier. During longer heat treatment (tan>1 h), microstructural changes occur within both barrier types. Whereas the amorphous Ta–Si layer starts to crystallize into Ta5Si3, the initially grown metastable β-Ta of the pure Ta film transforms into the equilibrium α-Ta phase. Analyzing the diffusion rate of Cu atoms into the substrate, the Ta73Si27 barrier shows an enhanced thermal stability compared to the pure Ta film.

Reliability studies of MOCVD TiSiN and EnCoRe Ta(N)/Ta

Passivated single damascene copper SiO 2 damascene lines were evaluated in combination with TiSiN and Ta(N)/Ta diffusion barriers. Leakage current, breakdown and time-dependent dielectric breakdown properties were investigated on a wafer level basis for temperatures ranging between room temperature and 150 °C. It is found that the leakage performance of the wafers with a TiSiN barrier is better at room temperature, but at 150 °C the performance levels out with Ta(N)/Ta. Time-dependent dielectric breakdown measurements at 150 °C show that the lifetime of the interconnect is higher with the selected Ta(N)/Ta barrier than for TiSiN.

Development, heat treatment optimization and microstructural characterization of Nb/sub 3/Sn superconductor wire

The optimization of the heat treatment profile for Nb/sub 3/Sn superconductor wires is one of the most difficult steps to process this material. The present work had the objective to optimize the heat treatment profile to form the Nb/sub 3/Sn phase. It was developed an internal-tin Nb/sub 3/Sn strand with 1.0 mm in diameter, 12 065 Nb filaments with average diameter of 3.4 /spl mu/m, 26 cores of Sn and internal stabilization of Cu surrounded by Ta diffusion barrier. Samples were removed for heat treatment optimization using different profiles to analyze the Sn diffusion and CuSn formation at temperatures up to 575 /spl deg/C. A final treatment at 700 /spl deg/C was used to form the Nb/sub 3/Sn phase. After the heat treatments, samples were removed for microstructural characterization. Measurements of T/sub c/ and J/sub c/ were performed to analyze the influence of the heat treatments on the superconducting phase and on the transport properties. It was concluded that the use of a two step heat treatment of 575 /spl deg/C/100 h + 700 /spl deg/C/100 h is as efficient as the use of a heat treatment with more low temperature steps.

Low temperature deposition of tantalum diffusion barrier by filtered cathodic vacuum arc

Tantalum (Ta) diffusion barrier films were deposited on un-patterned and patterned silicon substrates at ambient temperature and without substrate bias by filtered cathodic vacuum arc (FCVA). The films were characterized by atomic force microscopy, scanning electron microscopy, x-ray diffraction, x-ray photoelectron spectroscopy, four-point resistivity probe and surface profilometer. It was found that the Ta film was 750 Å thick and free of C and O except for surface contamination. The film morphology was smooth and uniform with root-mean-square roughness of ∼0.82 Å. The Ta film was polycrystalline β phase with a mean grain size of ∼3 nm and possessed a dense microstructure, which are ascribed to the high energy of the condensing species in FCVA. It was shown that the Ta filling of the trenches (0.33 μm wide, 1 : 1 aspect ratio) was very conformal and quite uniform. Also, it was preliminarily found that at the Ta film was effective against diffusion of Cu into Si at 600°C.

Effect of Substrate Bias Voltage on the Thermal Stability of Cu/Ta/Si Structures Deposited by Ion Beam Deposition

The interfacial reactions of the Cu (100 nm)/Ta (50 nm)/Si structures and their relationship with the microstructure of Ta diffusion barrier are investigated. Ta films were deposited on Si (100) substrates using a non-mass separated ion beam deposition system at various bias voltages ranging from 0 to -200 V. An optimum applied substrate bias voltage of -125 V was found to yield a dominant α-Ta film with a noncolumnar structure, low electrical resistivity (about 40 µΩcm) and smooth surface. A Ta diffusion barrier which was deposited at the optimum bias voltage prevented Cu–Si interaction up to 600°C for 60 min in flowing purified H2, whereas a Ta layer with a columnar structure, deposited at zero bias voltage, degraded at 300°C. Two different reactions of the Cu/Ta (0 V)/Si and the Cu/Ta (-125 V)/Si structures concerning the thermal stability were investigated and discussed on the basis of the experimental results.

Effect of diffusion barrier in the thermally annealed exchange-biased IrMn-CoFe electrode in magnetic tunnel junctions

Exchange-biased electrodes IrMn/Ta(2 /spl Aring/)/CoFe/AlO/sub x/ and IrMn/CoFe/Ta(2 /spl Aring/)-AlO/sub x/ used in a magnetic tunnel junction were annealed at 300/spl deg/C to study the Mn diffusion characteristics. Auger electron spectroscopy and X-ray photoelectron spectroscopy analysis of the annealed electrodes show that the Ta diffusion barrier effectively blocked the Mn migration, regardless of the barrier location. Also concluded from the study was that the Mn migration is largely enhanced by the preferential oxidation of Mn.

Evaluation of Ta and TaN-based Cu diffusion barriers deposited by Advanced Hi-Fill (AHF) sputtering onto blanket wafers and high aspect ratio structures

This paper investigates the properties of Ta and Ta(N) barrier layers deposited by Advanced Hi-Fill ® (AHF) sputtering onto both blanket and patterned wafers. The differences, in terms of forming an effective diffusion barrier, between Ta and Ta(N) barriers of varying N 2 content and the important integration issues that are brought about by the comparison of blanket and patterned wafers are emphasised. In particular, from the blanket study we find that Ta(N) films with high N 2 content form the best diffusion barriers to Cu, while the study on patterned wafers indicates that an intermediate N 2 flow consistent with a blanket deposition of mixed phase b.c.c. and β Ta forms the optimum barrier in terms of maximised sidewall coverage and material quality.

Interfacial reactions and failure mechanism of Cu/Ta/SiO 2/Si multilayer structure in thermal annealing

The integrity of Cu/Ta/SiO 2/Si multilayer structure under nitrogen thermal annealing has been examined by sheet resistance measurement, X-ray diffraction, scanning electron microscopy, secondary ion mass spectroscopy, Rutherford backscattering spectrometry and cross-section transmission electron microscopy analysis. According to electrical measurement it was found that Ta diffusion barrier could preserve the integrity of the Cu/Ta/SiO 2/Si structure up to 650 °C in N 2 for 35 min. There are two causes by which the Cu/Ta/SiO 2/Si structure became degraded. One is the out diffusion Ta atoms towards the Cu film. The other factor is the formation of Cu, Ta and Cu–Ta oxide. Hence, Cu penetration is not the only reason to cause the failure; oxidation, phase transformation and interfacial reactions are also associated problems during thermal annealing.

Behaviour of ionized metal plasma deposited Ta diffusion barrier between Cu and SiO2

This work investigated the diffusion barrier properties of ionized metal plasma (IMP) deposited Ta between Cu and SiO2. When Cu and Ta layers were formed by IMP sputtering, it was found that the Cu has the equilibrium f.c.c. structure with the grain size of 80 nm whereas Ta is in a metalstable tetragonal (β-Ta) form with a grain size of around 10 nm. With the help of sheet resistance measurement, X-ray diffraction, cross-section transmission electron microscope analysis, Rutherford backscattering spectroscopy, secondary ion mass spectroscopy, and scanning electron microscopy, the Ta barrier layer was observed to fail at temperature above 650°C due to the reactions among Cu, Ta and O and formation of CuxTayOz. The phase transformation of β-Ta into the stable phase (α-Ta), in the presence of Cu was encountered when annealing the sample at above 800°C. The role of oxygen was also found to be important in the phase transformation, in the reactions and it seems to have a strong effect on the thermal stability of the barrier layer.

Retardation of Ta silicidation by bias sputtering in Cu/Ta/Si(111) thinfilms

In this investigation, we have studied the effect of negativebias voltage on the properties of a sputtered Ta layer deposited at highAr pressure (13.3 Pa) which is used as a diffusion barrier in a Cu/Ta/Sistructure focusing on its silicidation process. According toRutherford backscattering spectrometry, the Cu(85 nm)/Ta(100 nm,unbiased)/Si(111) structure was found to be stable up to 300 °Cin a N2 environment for 30 min. At a temperatureof 450 °C, TaSi2 was formed at the Ta/Si(111)interface, and the Ta diffusion barrier completely failed. Byapplying various negative bias voltages ranging from 0 to -150 V,an optimum bias voltage of Vb = -50 V for the sputtered Ta layerwas found from scanning electron microscopy and four-point-proberesistivity measurements. In addition, the normalized ionflux (ni), defined as the ratio of Ar+ ion flux to Taflux, was determined to be 7.5 for optimum experimentalconditions. As a result, the biased Ta layer was used as adiffusion barrier between Cu and Si; it showed a low resistivityof 99 µΩ cm, a density of about 14.1 g cm-3 witha good surface morphology and a contribution of bcc-Taphase structure of about 65%. The Cu(50 nm)/Ta(50 nm, biased)/Si(111)structure was demonstrated to be thermally stable under perfectconditions up to 500 °C and TaSi2 formation in anN2 environment for 30 min was retarded up to 700 °C.

Palladium seeding on the tantalum-insulated silicon oxide film by plasma immersion ion implantation for the growth of electroless Copper

The major aim of this study was to combine the techniques of using plasma immersion ion implantation (PIII) and electroless plating to implant Pd onto a Ta diffusion barrier layer as catalyst for the electroless Cu plating in order to accomplish the ULSI interconnection metallization. In our study, it has been demonstrated that the accelerating Pd + ions by 2 kV pulsed negative bias voltages produce an amorphous Pd layer on β-Ta. The outermost Pd forms an oxide to the chemisorption of oxygen on Pd in air, while innermost Pd does not react with β-Ta, but instead remains in the form of metallic Pd. The Pd doses in the range of 5.7×10 14 and 8.6×10 14 cm −2 invokes an excellent catalytic effect on the electroless Cu plating. This results in an extraordinary ability for filling the submicron holes for gaining high quality electroless plated Cu interconnects, and thus qualifies for the traditional wet activation by SnCl 2 and PdCl 2 solutions.