Thermal Stability Study of Cu(MoN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>) Seed Layer on Barrierless Si

This study reports the good thermal stability of a sputtered Cu(MoN _x ) seed layer on a barrierless Si substrate. A Cu film with a small amount of MoN _x was deposited by reactive co-sputtering of Cu and Mo in an Ar/N₂ gas mixture. After annealing at 560°C for 1 h, no copper silicide formation was observed at the interface of Cu and Si. Leakage current and resistivity evaluations reveal the good thermal reliability of Cu with a dilute amount of MoN _x at temperatures up to 560°C, suggesting its potential application in advanced barrierless metallization. The thermal performance of Cu(MoN _x ) as a seed layer was evaluated when pure Cu is deposited on top. X-ray diffraction, focused ion beam microscopy, and transmission electron microscopy results confirm the presence of an ∼10-nm-thick reaction layer formed at the seed layer/Si interface after annealing at 630°C for 1 h. Although the exact composition and structure of this reaction layer could not be unambiguously identified due to trace amounts of Mo and N, this reaction layer protects Cu from a detrimental reaction with Si. The Cu(MoN _x ) seed layer is thus considered to act as a diffusion buffer with stability up to 630°C for the barrierless Si scheme. An electrical resistivity of 2.5 μΩ cm was obtained for the Cu/Cu(MoN _x ) scheme after annealing at 630°C.     

Thermal stability of Cu(W) and Cu(Mo) films for advanced barrierless Cu metallization: Effects of annealing time

The current study investigates the effects of insoluble substances (W and Mo) in pure Cu films on the thermal stability, microstructure, and electrical properties of the films. The results can be used to assess the feasibility of the barrierless Cu film in the metallization process. The films investigated were deposited using magnetron sputtering onto the barrierless Si (100) substrate and then annealed between 400°C and 450°C in vacuum for long periods of time. After annealing, the film properties were examined by x-ray diffraction (XRD), the four-point probe method, leakage current measurements, and focused ion beam (FIB) analysis. The results indicate that no detectable copper silicide is formed after 48-h annealing of Cu(W) films at 400°C. In contrast, for the Cu(Mo) film, copper silicide is formed after 18-h annealing at the same temperature and hence electrical properties are poor. This evidence suggests that the Cu(W) film has better thermal stability during long periods of annealing and is suitable for an advanced barrierless metallization process.     

The Application of Barrierless Metallization in Making Copper Alloy,Cu(RuHfN), Films for Fine Interconnects

In this study, films of a copper (Cu) alloy, Cu(RuHfN _x ), were deposited on silicon (Si) substrates with high thermal stability by co-sputtering copper and minute amounts of Hf or Hf/Ru in an Ar/N₂ gas mixture. The Cu(RuHfN _x ) films were thermally stable up to 720°C; after annealing at 720°C for 1 h, the thermal stability was great enough to avoid undesired reaction between the copper and the silicon. No copper silicide was formed at the Cu–Si interface for the films after annealing at 720°C for 1 h. The Cu(RuHfN _x ) films appear to be good candidate interconnect materials.     

Sputtered copper films with insoluble Mo for Cu metallization: A thermal annealing study

The thermal annealing behavior of Cu films containing insoluble 2.0 at. % Mo magnetron co-sputtered on Si substrates is discussed in the present study. The Cu-Mo films were vacuum annealed at temperatures ranging from 200°C to 800°C. X-ray diffraction (XRD) and scanning electron microscopy (SEM) observations have shown that Cu₄Si was formed at 530°C, whereas pure Cu film exhibited Cu₄Si growth at 400°C. Twins are observed in focused ion beam (FIB) images of as-deposited and 400°C annealed, pure Cu film, and these twins result from the intrinsically low stacking-fault energy. Twins appearing in pure Cu film may offer an extra diffusion channel during annealing for copper silicide formation. In Cu-Mo films, the shallow diffusion profiles for Cu into Si were observed through secondary ion mass spectroscopy (SIMS) analysis. Higher activation energy obtained through differential scanning calorimetry (DSC) analysis for the formation of copper silicide further confirms the beneficial effect of Mo on the thermal stability of Cu film.     

Thermal performance of sputtered insoluble Cu(W) films for advanced barrierless metallization

The thermal performance of sputtered Cu films with dilute insoluble W (1.3 at.%) on barrierless Si substrates has been studied, using the analyses of focused ion beam, x-ray diffraction, and electrical resistivity measurement. The role of the Cu(W) film as a seed layer has been confirmed based on the thermal performance evaluations in both thermal cycling and isothermal annealing at various temperatures. The electrical resistivity of ∼1.8 μΩ-cm for Cu/Cu(W) film is obtained after thermal annealing at 400°C. Because of the good thermal stability, the Cu(W) seed layer is also considered to act as a diffusion buffer and is stable up to 490°C for the barrierless Si scheme. The results indicate that the Cu/Cu(W) scheme has potential in advanced barrierless metallization applications.     

Diffusion barrier performance of Zr–N/Zr bilayered film in Cu/Si contact system

Zr–N/Zr bilayered film as a diffusion barrier between Cu and Si is evaluated. The thermal stability of the diffusion barrier is investigated by annealing the Cu/Zr–N/Zr/Si samples in N₂ for an hour. XRD, SEM and AES results for the above contact systems after annealing at 700 °C show that Cu film has preferential (1 1 1) crystal orientation and no diffraction peaks of Cu₃Si and a Cu–Zr–Si ternary compound are observed for all Cu/Zr–N/Zr/Si contact systems. In addition, the atomic distribution of Zr and Si is evident and grows with increasing temperature up to 700 °C, which corresponds to the Zr–Si phase having low contact resistivity. Low contact resistivity and high thermal stability diffusion barrier can be expected by the application of the Zr–N/Zr bilayered film as a diffusion barrier between Cu and Si.     

The effects of process conditions and substrate on copper MOCVD using liquid injection of (hfac)Cu(vtmos)

Copper MOCVD (metalorganic chemical vapor deposition) using liquid injection for effective delivery of the (hfac)Cu(vtmos) [1,1,1,5,5,5-hexafluoro-2,4-pentadionato(vinyltrimethoxysilane) copper(I)] precursor has been performed to clarify growth behavior of copper films onto TiN, <100> Si, and Si₃N₄ substrates. Especially, we have studied the influences of process conditions and the substrate on growth rates, impurities, microstructures, and electrical characteristics of copper films. As the reactor pressure was increased, the growth rate was governed by a pick-up rate of (hfac)Cu(vtmos) in the vaporizer. The apparent activation energy for copper growth over the surface-reaction controlled regime from 155°C to 225°C was in the range 12.7–32.5 kcal/mol depending upon the substrate type. It revealed that H₂ addition at 225°C substrate temperature brought about a maximum increase of about 25% in the growth rate compared to pure Ar as the carrier gas. At moderate deposition temperatures, the degree of a <111> preferred orientation for the deposit was higher on the sequence of <Cu/Si<Cu/TiN<Cu/Si₃N₄. The relative impurity content within the deposit was in the range 1.1 to 1.8 at.%. The electrical resistivity for the Cu films on TiN illustrated three regions of the variation according to the substrate temperature, so the deposit at 165°C had the optimum resistivity value. However, the coarsened microstructures of Cu on TiN prepared above 275°C gave rise to higher electrical resistivities compared to those on Si and Si₃N₄ substrates.     

Characterization of sputtered Ta-C-N film in the Cu/barrier/Si contact system

Characterization of sputtered tantalum carbon nitride (Ta-C-N) film in Cu/barrier/Si system was reported for the first time. With a 50∶50 wt.% TaC target and an optimum N₂/Ar flow rate (in sccm) ratio of 2/24, a 600 Å-thick sputtered Ta-C-N layer was shown metallurgically stable up to 650°C annealing for 30 min, which is about 100°C higher as compared to the case without nitrogen doping. Cu diffusion through the local defects or grain boundaries of the Ta-C-N barrier layer into Si substrate is the dominant factor responsible for the failure of the Ta-C-N barrier layer after high temperature annealing.     

Structural and Passivative Behaviors of Cu(In) Thin Film

This investigation prepares a low-resistivity and self-passivated Cu(In) thin film. The dissociation behaviors of dilute Cu-alloy thin films, containing 1.5–5at.%In, were prepared on glass substrates by a cosputter deposition, and were subsequently annealed in the temperature range of 200–600 °C for 10–30 min. Thus, self-passivated Cu thin films in the form In₂O₃/Cu/SiO₂ were obtained by annealing Cu(In) alloy films at an elevated temperature. Structural analysis indicated that only strong copper diffraction peaks were detected from the as-deposited film, and an In₂O₃ phase was formed on the surface of the film by annealing the film at an elevated temperature under oxygen ambient. The formation of In₂O₃/Cu/SiO₂ improved the resistivity, adhesion to SiO₂, and passivative capability of the studied film. A dramatic reduction in the resistivity of the film occurred at 500 °C, and was considered to be associated with preferential indium segregation during annealing, yielding a low resistivity below 2.92 μΩcm. The results of this study can be potentially exploited in the application of thin-film transistor–liquid crystal display gate electrodes and copper metallization in integrated circuits.     

Integrity of copper–hafnium,hafnium nitride and multilayered amorphous-like hafnium nitride metallization under various thickness

The barrier properties and failure mechanism of sputtered Hf, HfN and multilayered HfN/HfN thin films were studied for the application as a Cu diffusion barrier in metallization schemes. The barrier capability and thermal stability of Hf, HfN and HfN/HfN films were determined using X-ray diffraction (XRD), leakage current density, sheet resistance (R_s) and cross-sectional transmission electron microscopy (XTEM). The thin multi-amorphous-like HfN thin film (10 nm) possesses the best barrier capability than Hf (50 nm) and amorphous-like HfN (50 nm). Nitrogen incorporated Hf films possess better barrier performance than sputtered Hf films. The Cu/Hf/n⁺–p junction diodes with the Hf barrier of 50 nm thickness were able to sustain a 30-min thermal annealing at temperature up to 500 °C. Copper silicide forms after annealing. The Hf barrier fails due to the reaction of Cu and the Hf barrier, in which Cu atoms penetrate into the Si substrate after annealing at high temperature. The thermal stabilities of Cu/Hf/n⁺–p junction diodes are enhanced by nitrogen incorporation. Nitrogen incorporated Hf (HfN, 50 nm) diffusion barriers retained the integrity of junction diodes up to 550 °C with lower leakage current densities. Multilayered amorphous-like HfN (10 nm) barriers also retained the integrity of junction diodes up to 550 °C even if the thickness is thin. No copper–hafnium and copper silicide compounds are found. Nitrogen incorporated hafnium diffusion barrier can suppress the formation of copper–hafnium compounds and copper penetration, and thus improve the thermal stability of barrier layer. Diffusion resistance of nitrogen-incorporated Hf barrier is more effective. In all characterization techniques, nitrogen in the film, inducing the microstructure variation appears to play an important role in thermal stability and resistance against Cu diffusion. Amorphousization effects of nitrogen variation are believed to be capable of lengthening grain structures to alleviate Cu diffusion effectively. In addition, a thin multilayered amorphous-like HfN film not only has lengthening grain structures to alleviate Cu diffusion, but block and discontinue fast diffusion paths as well. Hence, a thin multilayered amorphous-like HfN/HfN barrier shows the excellent barrier property to suppress the formation of high resistance η′-(Cu,Si) compound phase to 700 °C.     

Annealing effects in the Ag/Al-(100) InP system: Al redistribution and film recrystallization

Annealing effects on contact metallurgy were examined in the Ag/Al-(100) InP system. Metal film configuration and annealing conditions were those that resulted in Schottky contacts with superior electrical characteristics. Metallurgical changes were observed with transmission electron microscopy combined with selected area diffraction (SAD) and x-ray diffraction. A1 was redistributed from the metal-semiconductor interface throughout the Ag overlayer upon annealing. Subsequent cooling resulted in the precipitation of Ag₂Al. During annealing the metal film underwent surface energy driven recrystallization (SEDR) with the predominant orientation observed in large recrystallized grains being: Ag(111)‖InP(100) and, to within 9° of rotation, Ag(110)‖InP(001). It is proposed that the thin Al interlayer and formation of Ag₂A1 early in the annealing cycle modifies the interaction between the metal film and InP substrate which results in contact metallurgy substantially different than that observed for either pure Ag or Al films on (100) InP.     

Electroless CoWP as a Diffusion Barrier between Electroless Copper and Silicon

In this work, an electroless CoWP film deposited on a silicon substrate as a diffusion barrier for electroless Cu and silicon has been studied. Four different Cu 120 nm/CoWP/Si stacked samples with 30, 60, 75, and 100 nm electroless CoWP films were prepared and annealed in a rapid thermal annealing (RTA) furnace at 300°C to 800°C for 5 min. The failure behavior of the electroless CoWP film in the Cu/CoWP/Si sample and the effect of CoWP film thickness on the diffusion barrier properties have been investigated by transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and sheet resistance measurements. The composition of the electroless CoWP films was 89.4 at.% Co, 2.4 at.% W, and 8.2 at.% P, as determined by energy dispersive X-ray spectrometer (EDS). A 30 nm electroless CoWP film can prevent copper penetration up to 500°C, and a 75 nm electroless CoWP film can survive at least up to 600°C. Therefore, increasing the thickness of electroless CoWP films effectively increases the failure temperature of the Cu/CoWP/Si samples. The observations of SEM and TEM show that interdiffusion of the copper and cobalt causes the failure of the electroless CoWP diffusion barriers in Cu/CoWP/Si during thermal annealing.     

Investigation of Zr–N thin films for use as diffusion barrier in Cu metallization

Zr–N thin films as a barrier in Cu/Si contact were investigated. The Cu/Zr–N/Si specimens were prepared and annealed at temperatures up to 700 °C in N₂ ambient for an hour. Characterization of phase composition and crystallite structure of the barriers was performed by XRD, the film morphology was examined using atomic force microscopy (AFM), and the composition profiles of the as-deposited and annealed samples of Cu/Zr–N/Si were identified by Auger electron spectroscopy (AES). It is evident that the Zr–N film structure is very sensitive to the deposition conditions. Cu/Zr–N/Si contact systems showed better thermal stability so that the Cu₃Si phase could not be detected. It is indicated from the comparison analysis results that the Zr–N film showed better thermal stability with increasing N₂ flow ratio and/or negative substrate bias.     

Effect of Rapid Thermal Annealing on Sputtered CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> Thin Films

Calcium copper titanium oxide (CaCu₃Ti₄O₁₂, abbreviated to CCTO) films were deposited on Pt/Ti/SiO₂/Si substrates at room temperature (RT) by radiofrequency magnetron sputtering. As-deposited CCTO films were treated by rapid thermal annealing (RTA) at various temperatures and in various atmospheres. X-ray diffraction patterns and scanning electron microscope (SEM) images demonstrated that the crystalline structures and surface morphologies of CCTO thin films were sensitive to the annealing temperature and ambient atmosphere. Polycrystalline CCTO films could be obtained when the annealing temperature was 700°C in air, and the grain size increased signifi- cantly with annealing in O₂. The 0.8-μm CCTO thin film that was deposited at RT for 2 h and then annealed at 700°C in O₂ exhibited a high dielectric constant (ε′) of 410, a dielectric loss (tan δ) of 0.17 (at 10 kHz), and a leakage current density (J) of 1.28 × 10⁻⁵ A/cm² (at 25 kV/cm).     

Evaluation of DC-sputtered Glassy TaCoN Thin Film for Copper Metallization

The failure mechanism of the TaCoN barrier for copper metallization was examined using films by direct current (dc) magnetron reactive sputtering at various nitrogen flow rates. The as-deposited TaCoN films had a glassy structure and were free from intermetallic compounds. Optimizing the nitrogen flow rate during sputtering maximized the thermal stability of the Si/Ta_66.8Co_11.4N_21.8/Cu metallization system up to an annealing temperature of 750°C when the film was deposited using a nitrogen flow rate of 1 sccm, as revealed by using X-ray diffraction, a scanning electron microscope, a four-point probe and a transmission electron microscope. Structural analysis indicated that the failure mechanisms of the studied Si/TaCoN/Cu stacked films involved the initial dissociation of the barrier layer that was annealed at a specific temperature, and the subsequent formation of diffusion paths along which the copper penetrates through the TaCoN barrier layer to react with underlying Si. The high formation temperature of the Cu₃Si phase demonstrated that the studied film was highly stable, indicating that the TaCoN thin film is highly promising for use as a diffusion barrier for Cu metallization.     

A High-Performance Ultraviolet Photoconductive Detector Based on a ZnO Film Grown by RF Sputtering

A high-performance metal-semiconductor-metal (MSM) ultraviolet photodetector was fabricated based on a ZnO film with Al interdigitated (IDT) electrodes. A c-axis-oriented ZnO film was grown on a SiO₂/Si (100) substrate at room temperature by a reactive radiofrequency (RF) sputtering technique and then annealed at 900°C in pure O₂ ambient for 1 h. The fabricated ZnO ultraviolet (UV) detector demonstrated a high responsivity of 2069 A/W when biased at 5 V, which could be attributed to the influence of the annealing process in pure O₂ ambient. The response time measurement showed a rise time (10–90%) of 45.1 ns and a decay time (1 − 1/e) of 541 μs.     

Study of diffusion barrier properties of ternary alloy (TixAlyNz) in Cu/TixAlyNz/SiO2/Si thin film structure

The effects of aluminum (Al) incorporation on the performance of a titanium nitride (TiN) diffusion barrier were investigated up to the temperature of 1000°C in the Cu/Ti_xAl_yN_z/SiO₂/Si structure. The thermal stability of the structure was evaluated by using four-point probe, X-ray diffraction, and Rutherford Backscattering Spectroscopy. The Cu/Ti_xAl_yN_z/SiO₂/Si system retained its structure up to 1000°C. The incorporation of Al into the Ti_xN_y film modified the microstructure of the Ti_xN_y film, especially the microstructure of grain boundaries in which oxide and nitride compounds of Al and Ti were formed during thermal annealing. As a result, the fast pathways for copper (Cu) diffusion were effectively blocked by these compounds and the stability of the barrier performance was enhanced up to 1000°C.     

Low-Resistivity Ru-Ta-C Barriers for Cu Interconnects

Ru-Ta-C films deposited on silicon substrates were evaluated as barriers for copper metalization. The films were prepared by magnetron cosputtering using a Ru target and a Ta-C target. Compositions and structure of resultant films were optimally tuned by the respective deposition power of each target. The fabricated Ru-Ta-C films were characterized via four-point probe measurement, x-ray diffractometry, field-emission electron probe microanalysis, and transmission electron microscopy. Failure temperature was evaluated by the sudden rise in electrical resistivity after annealing the Cu/Ru-Ta-C/Si sandwich films, and a reference bilayer Cu/(5 nm Ru)/(5 nm Ta-C)/Si scheme. The optimal compositions were 10 nm Ru₇₇Ta₁₅C₇ and (5 nm Ru)/(5 nm Ta-C), both of which showed failure temperature of 650°C for 30 min and electrical resistivity less than 150 μΩ cm. Because of their high thermal stability and low electrical resistivity, both Ru-Ta-C and Ru/Ta-C films are promising barriers for Cu metalization.     

Study of ultrathin vanadium nitride as diffusion barrier for copper interconnect

Ultrathin Vanadium nitride (VN) thin film with thickness around 10 nm was studied as diffusion barrier between copper and SiO₂ or Si substrate. The VN film was prepared by reactive ion beam sputtering. X-ray diffraction, Auger electron spectroscopy, scanning electron microscopy and current-voltage (I-V) technique were applied to characterize the diffusion barrier properties for VN in Cu/VN/Si and Cu/VN/SiO₂ structures. The as-deposited VN film was amorphous and could be thermal stable up to 800 °C annealing. Multiple results show that the ultrathin VN film has good diffusion barrier properties for copper.     

Distribution of Elements in a Cu-Added FeSi<Subscript>2</Subscript> Alloy Under Peritectoid and Eutectoid Reactions

The distribution of Si, Fe, and Cu in FeSi₂ alloys, with or without the addition of Cu, were studied by electron probe microanalysis (EPMA). Alloys were prepared by slow solidification from the melt. Without Cu addition, both ε- and α-phases were clearly observed, and a β-phase surrounding the ε-phase was additionally observed after in situ annealing at 950°C for 12 h. With inclusion of 0.5 at.% Cu, the eutectoid reaction (α → β + Si) was enhanced greatly. Only 0.01 at.% Cu was dissolved into the ε-phase, with the excess Cu atoms being largely found at the outer edge of the ε-phase. Ex situ annealing at 950°C for 12 h greatly changed the distribution of Si, Fe, and Cu. The ε-phase changed its Si/Fe atomic ratio from 1.470 to 1.907, indicating an early stage of the peritectoid reaction (ε + α → β) and/or the subsequent reaction (ε + Si → β), with an increase in the Cu content up to 0.04 at.%. The size of this new phase was smaller than the original ε-phase, and this new phase was surrounded by a shell of Si/Fe with an atomic ratio of 0.727 to 1.788 and a Cu content of 0.01 at.% to 0.11 at.%. In situ annealing under the same condition yielded different results: a large amount of Si segregates from the α-phase matrix, leaving a Si/Fe atomic ratio of only 0.506 to 0.530. The peritectoid reaction of the ε-phase was found to depend on the Cu content. For the ε-phase with undetectable levels of Cu, the Si/Fe atomic ratio remained at 0.954 to 0.998, but this ratio decreased with increasing Cu content to 0.55 at 2.20 at.% Cu. A plot of at.% Cu versus Si/Fe atomic ratio revealed a local minimum at the ε-phase and expectedly at both the β- and α-phases. Nonstoichiometric structures (neither α-, β- nor ε-phases) seemed to have higher at.% Cu compared with those with the closest Si/Fe composition.