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.     

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.     

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.     

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.     

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.     

多靶磁控共溅射纳米Ta-Al-N薄膜的阻挡性能研究

采用磁控反应共溅射方法制备了纳米Ta-Al-N薄膜,并原位制备了Cu/Ta-Al-N薄膜,对薄膜进行了热处理。用四探针测试仪、X射线衍射仪(XRD)、扫描电镜(SEM)、原子力显微镜(AFM)以及台阶仪等研究了退火对薄膜结构及阻挡性能的影响。结果表明,Ta-Al-N薄膜具有优良的热稳定性,保持非晶态且能对Cu有效阻挡的温度可达800°C;同时发现在900°C退火5 min后,薄膜开始晶化,在Cu/Ta-Al-N/Si界面处生成了Cu3Si等相,表明此时Ta-Al-N薄膜阻挡层开始失效。     

Copper-Silver Alloy for Advanced Barrierless Metallization

In this study we observed significantly improved properties, over a pure copper (Cu) film, for a copper-silver alloy film made with a pure copper film co-sputtered with a minute amount of either Ag_0.3N_0.4 or Ag_1.2N_0.7 on a barrierless Si substrate. In either case, no noticeable interaction between the film␣and the Si substrate was found after annealing at 600°C for 1 h. The Cu(Ag_0.3,N_0.4) film was thermally stable after annealing at 400°C for 240 h. The film’s resistivity was ∼2.2 μΩ cm after annealing at 600°C, while its leakage current was found to be lower than that of a pure Cu film by three orders of magnitude. The adhesion of the Cu(Ag_1.2,N_0.7) film to the Si substrate was approximately seven times that of a pure Cu film to a silicon substrate. Hence, a Cu film doped with Ag and N seems to be a better candidate for both barrierless metallization and the making of superior interconnects.     

Novel Cu/Cr/Ge/Pd Ohmic Contacts on Highly Doped n-GaAs

The thermal stability of the Cu/Cr/Ge/Pd/n⁺-GaAs contact structure was evaluated. In this structure, a thin 40 nm layer of chromium was deposited as a diffusion barrier to block copper diffusion into GaAs. After thermal annealing at 350°C, the specific contact resistance of the copper-based ohmic contact Cu/Cr/Ge/Pd was measured to be (5.1 ± 0.6) × 10⁻⁷ Ω cm². Diffusion behaviors of these films at different annealing temperatures were characterized by metal sheet resistance, X-ray diffraction data, Auger electron spectroscopy, and transmission electron microscopy. The Cu/Cr/Ge/Pd contact structure was very stable after 350°C annealing. However, after 400°C annealing, the reaction of copper with the underlying layers started to occur and formed Cu₃Ga, Cu₃As, Cu₉Ga₄, and Ge₃Cu phases due to interfacial instability and copper diffusion.     

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.     

Electrical and compositional properties of TaSi2 films

Tantalum silicide (TaSi₂) thin films were sputter deposited on p- and n-type silicon substrates using ultrapure TaSi₂ targets. The TaSi₂/Si samples were annealed in nitrogen or forming gas or oxygen containing steam at temperatures in the range of 400–900°C. The sheet resistances of TaSi₂/Si were measured by four-point probe before and after anneal. The structure of these films was investigated using x-ray diffraction (XRD) methods. It has been found that the sheet resistance decreases with the increase in annealing temperature and also with the increase in film thickness. X-ray diffraction patterns show changes in the morphological structure of the films. Oxidation characteristics of the film have been investigated in the temperature range of 400–900°C in oxygen containing steam ambient. The oxidation time ranged from 0.5 to 1.5 h. No oxide formation of the tantalum silicide films was observed in this investigation. This has been attributed to the high purity of TaSi₂ sputter targets used in the preparation of the films.     

The role of stress distribution at the film/barrier interface in formation of copper silicides

Silicide formation in thin Cu films subjected to thermal annealing has been investigated by atomic force microscopy, scanning electron microscopy, X-ray diffraction, and energy-dispersive X-ray spectroscopy. It is shown that the periodic stress distribution at the film/barrier interface under elevated temperatures can govern the character of copper silicide formation. The effect of barrier layer material and substrate orientation on the distribution density and shape of Cu₃Si crystallites has been studied.     

Preparation and Characterization of CdTe for Solar Cells,Detectors, and Related Thin-Film Materials

Cadmium telluride (CdTe) thin films were prepared by the close-space sublimation (CSS) technique, using 99.99% pure CdTe powder as the evaporant. Films were then annealed at 400°C for 30 min and were later dipped in Cu(NO₃)₂-H₂O solution at 80 ± 2°C. After immersion these films were again annealed at 400°C for 1 h to ensure the Cu diffusion into the films. X-ray diffraction (XRD) results confirmed the formation of a new compound copper telluride and a change in the morphology was observed by scanning electron microscopy (SEM). The DC electrical resistivity reduced from 10⁶ Ω-cm for as-deposited to 10⁻³ Ω-cm for 15 h immersed film. As the wt.% of Cu increased, the mobility increased to some extent, while the carrier concentration showed a systematic increase. The film thickness and optical parameter such as refractive index, absorption coefficient, and the optical band gap were deduced by fitting the optical transmittance in the wavelength range 300 to 3000 nm. The transmission decreased with increasing immersion time of films in the␣solution. The Cu concentration was recorded as 0.9 wt.% for 3 min to 56.6 wt.% for 15 h immersed samples using an electron microprobe analyzer (EMPA). In the next step, ITO/CdS/CdTe heterojunctions with 10.9% solar cell efficiency were fabricated on glass slides.     

Microstructure control of copper films by the addition of molybdenum in an advanced metallization process

The effect of annealing on the resistivity, morphology, microstructure, and diffusion characteristics of Cu(Mo)/SiO₂/Si and Ti/Cu(Mo)/SiO₂/Si multilayer films has been investigated in order to deterine the role of Mo. In the case of a Cu(Mo)/SiO₂/Si multilayer, most of the Mo diffused out to the free surface to form MoO₃ at temperatures up to 500 C, and complete dissociation of Mo occurred at higher temperatures. The segregation of Mo to the external surface leads to Mo-free Cu films with extensive grain growth up to 20 times the original grain size and strong (111) texture. In the case of a Ti/Cu(Mo)/SiO₂/Si multilayer, a thin Ti film prohibits Cu agglomeration, out-diffusion of Mo, and diffusion of Cu into SiO₂ at temperatures up to 750 C. Cu(Mo) grain growth was less extensive, but (111) fiber texturing was much stronger than in the case of Cu(Mo)/SiO₂/Si. In the current study, significant changes in microstructure, such as a strong (111) texture and abnormal grain growth, have been obtained by adding Mo to Cu films when the films are annealed.     

Crystallization and failure behavior of Ta-TM (TM=Fe, Co) nanostructured/amorphous diffusion barriers for copper metallization

This work examined the thin-film properties and diffusion barrier behavior of sputtered Ta-TM (TM=Fe, Co) films, aiming at depositing a highly crystallization-resistant and conductive diffusion barrier film for Cu metallization. Four-point probe measurement, x-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and a secondary ion mass spectrometer (SIMS) were used to examine the barrier properties. Structural examination indicated that intermetallic-compound-free amorphous Ta-TM films were obtained by magnetron sputtering, thus giving a resistivity of 146.82 μΩ-cm and 247.01 μΩ-cm for Ta_0.5Fe_0.5 and Ta_0.5Co_0.5 films, respectively. The Si/Ta_0.5Fe_0.5/Cu and Si/Ta_0.5Co_0.5/Cu stacked samples were observed to fail completely at temperature above 650°C and 700°C because of the formation of Cu₃Si protrusions between silicon and the Ta-TM interface. Ta_0.5Co_0.5 is thus superior to Ta_0.5Fe_0.5 in preventing copper from diffusion. Highly thermally stabilized amorphous Ta-TM thin film can thus be potentially adopted as a diffusion barrier for Cu metallization.     

Structural,optical, and photocatalytic properties of the spray deposited nanoporous CdS thin films; influence of copper doping,annealing, and deposition parameters

Nanoporous thin films of Cd_1−xCu_xS (0≤x≤0.06) were grown on a heated glass substrate employing a home-made spray pyrolysis technique. The influences of [Cu]/[Cd] and the annealing in the range 300–500 °C on the structural and morphological properties of the films were investigated by X-ray diffraction (XRD), Fourier transformation infrared spectroscopy (FTIR), field emission scanning electron microscope (FE-SEM) and atomic force microscopy (AFM). The influences of Cu doping ratio, solution flow rate, and the deposition time on the optical properties and photocatalytic activity of these films are also reported. The films are of polycrystalline nature and hexagonal structure. Increasing the Cu doping ratio and annealing temperature improve the (1 0 1) preferential orientation. The crystallite size is ranged from 23.82 to 32.11 nm. XRD and FTIR reveal the formation of CdO in the 6% Cu-doped CdS film annealed at 400 °C and in all films annealed at 500 °C. The pure CdS film is of a porous structure and the close-packing and porosity of the films increase with increasing Cu%. Also, the pore diameter can be controlled from 50 to 15 nm with the increase of Cu content. The films showed transmittance below 70%. The optical band gap of the films is decreased from 2.43 to 1.82 eV with increasing Cu% and flow rate/deposition time. Additionally, the refractive indices and dispersion parameters of the films are also affected by the deposition conditions. Cu doping enhanced the films' photostability as well as the photocatalytic removal of methylene blue (MB).     

Effect of Sb implantation on copper silicides formation and morphology after annealing of Cu/Si structures

In this work, the solid state reaction between a thin film of copper and silicon has been studied using Rutherford backscattering spectroscopy, X-ray diffraction, scanning electron microscopy and microprobe analysis. Cu films of 400 and 900 Å thicknesses are thermally evaporated on Si(1 1 1) substrates, part of them had previously been implanted with antimony ions of 5×10¹⁴ or 5×10¹⁵ at. cm⁻² doses. The samples are heat-treated in vacuum at temperatures in the range 200–700 °C for various times. The results show the growth and formation of Cu₃Si and Cu₄Si silicides under crystallites shape dispatched on the sample surface, independently of the implantation dose. On the other hand, it is established that the copper layer is less and less consumed as the antimony dose increases, resulting in the accumulation of Sb⁺ ions at silicide/Si interface and in the silicide layer close to surface. The exposure of samples to air at room temperature shows the stability of Cu₄Si phase whereas the Cu₃Si silicide disappears to the benefit of the silicon dioxide formation. The observed phenomena are discussed.     

Microstructures of parylene-N thin films and the effect on copper diffusion

Using scanning electron microscopy, the microstructure of annealed N-type parylene films on silicon substrate was observed and compared to the asdeposited film. The diffusion of copper through the parylene-N film was studied and correlated to the microstructure. A web-like microstructure was observed on annealing parylene-N to a temperature of 300°C and higher. This microstructure differed from the as-deposited homogeneous and continuous structure at room temperature. The web-like structure observed is proposed to be a fibrillar crystalline structure embedded in an amorphous matrix. X-ray diffraction studies supported this view and showed that the crystalline structure was theß phase. Also, when the film was annealed at 300 and 350°C, a thin continuous layer was formed at the surface of the web-like parylene-N film. In contrast, no such thin layer was observed when the annealing was performed with a copper overlayer. Based on this observation, a two-stage annealing process was carried out to reduce the copper diffusion into parylene-N, preannealing, before copper deposition and post-annealing after copper deposition. The results, as judged from Rutherford backscattering spectroscopy indicate that the thermal stability for copper diffusion into parylene-N films can be increased by 50°C (from 300 to 350°C) using pre-annealing. Experimental data shows that a minimum pre-anneal temperature of 250°C for 1 h is required for this purpose.     

Effects of post-annealing by the rapid thermal process on the characteristics of MOCVD-Cu/TiN/Si structures

Effects of rapid thermal annealing on the characteristics of Cu films deposited from the (hfac)Cu(VTMS) precursor and on the barrier properties of TiN layers were studied. By the post-annealing, the electrical characteristics of Cu/TiN and the microstructures of Cu films were significantly changed. The properties of Cu films were more sensitive to the annealing temperature than the annealing time. Sheet resistances were decreased in 400–450°C ranges, and abrupt increases were observed above 750°C. It was also found that the copper films showed pronounced grain growth with the (111) preferred orientation. The grain growth and condensation of copper were observed below 500°C without formation of the CuO and Cu₂O phase resulting in surface degradation. Above 500°C, the oxide compound of copper was partially formed on the surface and the inter-reaction on the Cu-TiN interface was started. The inter-reaction of Cu-Ti and Cu-Si interface vigorously occurred and the surface roughness was continuously deteriorated above 650°C. It revealed that the optimum annealing conditions for MOCVD-Cu/PVD-TiN structures to enhance the electrical characteristics without degradation of TiN barriers were in the range of 400°C.     

Diffusion Barrier Behaviors of V-Ta,V-Ta-N and V-Ta/V-Ta-N Alloy Films in Cu Interconnects

The V-Ta, V-Ta-N and V-Ta/V-Ta-N alloy barrier layers with thickness of 50 nm were deposited on Si (100) substrates by magnetron sputtering and then the 300 nm thick Cu films were prepared on the barrier layers to obtain Cu/V-Ta/Si, Cu/V-Ta-N/Si and Cu/V-Ta/V-Ta-N/Si multilayer films. The multilayer film samples were subsequently annealed at 300°C–750°C temperatures for 1 h in vacuum atmosphere. The crystal structure, surface morphology and sheet resistance were characterized by grazing incidence x-ray diffraction (GXRD), electron probe microanalysis (EPMA), scanning electron microscopy (SEM) and four-point probe (FPP) analysis to investigate the diffusion barrier behavior of the V-Ta, V-Ta-N and V-Ta/V-Ta-N alloy barrier layers. The results show that the V-Ta, V-Ta-N and V-Ta/V-Ta-N barrier layers effectively blocked the diffusion of Cu into the Si substrate. When annealed at 700°C, the Cu/V-Ta/Si and Cu/V-Ta/V-Ta-N/Si thin film samples maintained good thermal stability and demonstrated low sheet resistance (～?0.3 Ω cm). Therefore, both the V-Ta and V-Ta/V-Ta-N thin films are promising candidates for use as diffusion barrier layers.

     

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.