Effect of Annealing Ambient on the Self-Formation Mechanism of Diffusion Barrier Layers Used in Cu(Ti) Interconnects

Copper (titanium) [Cu(Ti)] films with low titanium (Ti) concentration were found to form thin Ti-rich barrier layers at the film/substrate interfaces after annealing, which is referred to as self-formation of the barrier layers. This Cu(Ti) alloy was one of the best candidates for interconnect materials used in next-generation ultra-large-scale integrated (ULSI) devices that require both very thin barrier layers and low-resistance interconnects. In the present paper, in order to investigate the influences of annealing ambient on resistivity and microstructure of the Cu alloys, the Cu(7.3at.%Ti) films were prepared on the SiO₂ substrates and annealed at 500°C in ultra-high vacuum (UHV) or argon (Ar) with a small amount of impurity oxygen. After annealing the film at 500°C in UHV, the resistivity was not reduced below 16 μΩ-cm. Intermetallic compounds of Cu₄Ti were observed to form in the films and believed to cause the high resistivity. However, after subsequently annealing in Ar, these compounds were found to decompose to form surface TiO _x and interfacial barrier layers, and the resistivity was reduced to 3.0 μΩ-cm. The present experiment suggested that oxygen reactive to titanium during annealing played an important role for both self-formation of the interfacial barrier layers and reduction of the interconnect resistivity.     

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).     

The effect of O<Subscript>2</Subscript> ambient annealing on the microstructure of Cu(Mg) in the form of a Cu(Mg)/SiO<Subscript>2</Subscript>/Si multilayer

The effect of annealing in an O₂ ambient on Cu(Mg)/SiO₂/Si multilayer films was investigated. As-deposited Cu(Mg)/SiO₂/Si multilayer samples with film thicknesses in the 1,000–3,000 ? range were annealed for 30 min in oxygen ambients at pressures ranging from vacuum to 100 mtorr. The results showed that annealing in an 8-mtorr O₂ ambient significantly decreased the electrical resistivity of a 1,000 ? sample from 10.5 μΩ-cm to 3.7 μΩ-cm. Annealing in the O₂ ambient enhanced Mg diffusion to the surface in comparison to vacuum annealing. Furthermore, O₂ ambient annealing leads to excessive grain growth. However, the effect of O₂ ambient annealing on resistivity is less when the thickness of the film increases.     

High conductivity copper-boron alloys obtained by low temperature annealing

The electrical behavior during annealing of copper films with a nominal concentration of 2 at.% boron has been investigated. The evolution of the resistivity of the film was monitored using an in situ technique, in which the film was rampannealed at constant ramp rates. At temperature of 150–200°C, the resistivity of the Cu(B) undergoes a first drop. This is followed by one or two such drops in resistivity, so that after completion of a ramp-anneal from 50°C to 750°C, the room temperature resistivity decreases from the initial value of 13 μΩ cm to 2.1 μΩcm, close to that of bulk copper. Isothermal annealing of the film also leads to substantial decreases in resistivity, from 13 μΩcm to 3 μΩ cm after annealing at 350°C for 8 h and to 2.5 μΩ cm at 400°C for 4 h. These results show that a dramatic reduction in resistivity of Cu(B) alloys takes place at temperatures below 400°C, suggesting possible applications for silicon device interconnections.     

Formation of aluminum oxide films from aluminum hexafluoroacetylacetonate at 350–450° c

A study of the thermally activated decomposition of Al(hfa)₃ (aluminum hexafluoroacetylacetonate) from the gas phase to form Al₂O₃ on silicon substrates is reported. The decomposition process was carried out in an open tube atmospheric pressure reactor in either argon or oxygen/argon mixtures in the temperature range, 350–450° C. The chemical vapor deposition process resulted in the formation of aluminum oxide films in all instances. The dielectric strength of Al/Al₂O₃/Si capacitors which received a post-metal anneal, but did not receive a high temperature annealing treatment, with aluminum oxide films prepared from Al(hfa)₃ in argon, was found to be in the range 2–6 MV/cm. The difference between the flatband voltage of the MOS structures and the metal-silicon work function difference was positive, indicative of a net negative oxide charge with a density of approximately 3 × 10¹¹ – 3 × 10¹² cm^-2, assuming the charge is located at the oxide-silicon interface. Decomposition of Al(hfa)₃ was also carried out in oxygen/argon mixtures with the oxygen concentration in the range 10–60 vol %. This process led to the deposition of aluminum oxide films with breakdown fields in the range 8–9 MV/cm. However, the flatband voltages of the Al/Al₂O₃/Si capacitors were even more positive than those obtained with Al₂O₃ formed in pure argon. High temperature (800–1000° C) oxygen or nitrogen annealing treatments of alumina films deposited in either argon or oxygen/argon mixtures were evaluated from the point of view of their influence on the oxide film properties. In particular, an annealing process in oxygen at 1000° C for 15 min was found to result in a reduction of the net negative oxide charge, and an improvement of the dielectric strength of films deposited in argon. Films formed in oxygen/argon mixtures did not change appreciably following oxygen annealing, as far as breakdown fields are concerned, but the oxide net negative charge was reduced. As in an earlier study by the authors, of copper film deposition from Cu(hfa)₂, it was found that essentially carbon free films could be obtained under appropriate conditions.     

Effect of copper and chlorine on the properties of SiO2 encapsulated polycrystalline CdSe films

The effects of different copper doping concentrations on the properties of SiO₂ encapsulated CdSe films have been investigated. Two methods were used to dope the films with copper: ion implantation and diffusion from a surface layer. The room temperature dark resistivity of films annealed in oxygen at 450°C was found to increase as the copper concentration was increased until a maximum resistivity of 10⁸ ohm cm occurred at a copper concentration of 10²⁰ atoms cm⁻³. The room temperature resistivity in the light was found to be independent of the copper concentration and whether the films were annealed in argon or oxygen. During annealing the grains grew from 0.03 μm to 0.3 μm and this growth was independent of the doping or the annealing ambient. The energy levels, carrier mobilities, and microstructure of the annealed films were dependent on the method of doping. The ion implanted films had an additional energy level at 0.33 eV and their mobility was a factor of 4 smaller than films doped by the surface diffusion method, whose mobilities were 20 to 35 cm²V⁻¹ s⁻¹. The addition of chlorine to copper doped films had no effect on either the resistivity or photosensitivity but slowed the response times of the photocurrent by a factor of 10. No energy levels were observed which could be associated with the copper nor was the copper found to affect the density of the observed intrinsic levels at 0.65 and 1.1 eV.     

Copper chemical vapor deposition from Cu(hexafluoroacetylacetonate)trimethylvinylsilane

Copper chemical vapor deposition using Cu-hexafluoroacetylacetonate (hfac) trimethylvinylsilane (TMVS) as precursor was performed in a cold-wall low pressure chemical vapor deposition (CVD) reactor. The design and operation of the reactor are described. Copper deposition on thermal SiO₂, W, and CoSi₂ substrate surfaces was investigated over the temperature range of 160–300°C and pressure range of 10–1000 mTorr. The activation energies of Cu CVD were determined to be 13.33 and 11.54 kcal/mole for the W and CoSi₂ substrates, respectively. The dependence of film resistivity, grain size, and growth rate on deposition pressure and temperature were also investigated. The film uniformity was found to be better than ten percent over a 4-inch diameter substrate. Experimental results also show that selective deposition can be achieved at a pressure of 10 mTorr within the temperature range of 160–200°C. In addition, hydrofluoric acid dipping was found to modify the SiO₂ surface and influence the copper deposition on it.     

Structural, optical and electronic properties of oxidized AIN thin films at different temperatures

We report on the properties of a novel insulator, AlO:N for application in semiconductors produced by thermally oxidizing AlN thin films. The process steps were similar to those used for SiO₂, creating the possibility of a new technology for metal-insulator-semiconductor field effect devices and integrated circuits. Thin films of AlN were deposited by radio-frequency magnetron reactive sputtering on p-type silicon or fused quartz substrates. As-deposited AlN film thickness ranged from 0.05 to 0.7 μm, with polycrystalline structure revealed by x-ray diffraction. Oxidation was performed under O₂ flow at 800 to 1100°C for 1–4 h. AlN films were oxidized partially or fully into Al₂O₃, depending on initial thickness, oxidation temperature and time. X-ray diffraction indicates the presence of several phases of Al₂O₃ at 1000°C, whereas at 1100°C, only the α-Al₂O₃ phase was found. Considering the importance of surface field effect device applications, the surfaces of oxidized films were examined with atomic force microscopy in air, and a clear change was observed in the surface structure of the oxidized film from that of as-deposited AlN films. Capacitance-voltage measurements of metal-oxide-semiconductor structures yielded a dielectric constant of AlO:N between 8–12 and a net oxide-trapped-charge density of ∼10¹¹ cm⁻². Using Fourier transform infrared spectrometry transmittance and reflectance, some α-Al₂O₃ modes were observed. In this paper, we describe the general properties of the oxide thin films, bulk and interface, at different temperatures.     

High resistivity LT-In0.47Ga0.53P grown by gas source molecular beam epitaxy

Low-temperature (LT) growth of In_0.47Ga_0.53P was carried out in the temperature range from 200 to 260°C by gas source molecular beam epitaxy using solid Ga and In and precracked PH₃. The Hall measurements of the as-grown film showed a resistivity of ∼10⁶ Ω-cm at room temperature whereas the annealed film (at 600°C for 1 h) had at least three orders of magnitude higher resistivity. The Hall measurements, also, indicated activation energies of ∼0.5 and 0.8 eV for the asgrown and annealed samples, respectively. Double-crystal x-ray diffraction showed that the LT-InGaP films had ∼47% In composition. The angular separation, Δθ, between the GaAs substrate and the as-grown LT-InGaP film on (004) reflection was increased by 20 arc-s after annealing. In order to better understand the annealing effect, a LT-InGaP film was grown on an InGaP film grown at 480°C. While annealing did not have any effect on the HT-InGaP peak position, the LT-InGaP peak was shifted toward the HT-InGaP peak, indicating a decrease in the LT-InGaP lattice parameter. Cross-sectional transmission electron microscopy indicates the presence of phase separation in LT-InGaP films, manifested in the form of a “precipitate-like” microstructure. The analytical scanning transmission electron microscopy analysis of the LT-InGaP film revealed a group-V nonstoichiometric deviation of ∼0.5 at.% P. To our knowledge, this is the first report about the growth and characterization of LT-InGaP films.     

The Transparent Conductive Properties of Manganese-Doped Zinc Oxide Films Deposited by Chemical Bath Deposition

Manganese-doped zinc oxide (Mn-doped ZnO) thin films were prepared using chemical bath deposition (CBD), and the impacts of the manganese dopant concentration on the structure, electrical resistivity, optical transmission, and magnetic properties were investigated using x-ray diffractometry, Hall-effect measurements, ultraviolet–visible–near-infrared (UV–Vis–IR) spectrophotometry, and vibrating sample magnetometry (VSM), respectively. The concentration of the manganese dopant in the ZnO thin film critically impacted the resulting properties, and the 4.0 at.% Mn-doped ZnO film had a resistivity of 5.8 × 10⁻² Ωcm, transmittance of 75.6% in the visible light range, and bandgap of 3.30 eV when the film was annealed at 600°C in an Ar + H₂ atmosphere. Annealing the film could enhance its magnetic properties such that the film had a saturation magnetization of 21.0 emu/cm³ and a coercivity of 45.7 Oe after annealing at 600°C. Because of these electrical, optical, and magnetic properties, Mn-doped thin films are promising for use in spintronic devices.     

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.     

Formation of Ti diffusion barrier layers in Thin Cu(Ti) alloy films

In order to study a formation mechanism of thin Ti-rich layers formed on the surfaces of Cu(Ti) wires after annealing at elevated temperatures, the 300-nm-thick Cu(Ti) alloy films with Ti concentration of 1.3 at.% or 2.9 at.% were prepared on the SiO₂/Si substrates by a co-sputter deposition technique. The electrical resistivity and microstructural analysis of these alloy films were carried out before and after annealing at 400°C. The Ti-rich layers with thickness of ∼15 nm were observed to form uniformly both at the film surface and the substrate interfaces in the Cu(2.9at.%Ti) films after annealing (which we call the self-formation of the layers) using Rutherford backscattering spectrometry (RBS) and transmission electron microscopy (TEM). Both the resistivities and the microstructures of these Cu(Ti) films were found to depend strongly on the Ti concentrations. The resistivities of the films decreased upon annealing due to segregation of the supersaturated Ti solutes in the alloy films to both the top and bottom of the films. These Ti layers had excellent thermal stability and would be applicable to the self-formed diffusion barrier in Cu interconnects of highly integrated devices. The selection rules of the alloy elements for the barrier self-formation were proposed based on the present results.     

Neutralization of electrically active aluminum in recrystallized silicon-on-sapphire films

We report the effect of steam oxidation at 875° C on the electrical resistivity, crystalline quality (measured by ion channeling), and Al concentration (measured by secondary ion mass spectrometry) in 0.25 μm thick, Si-implanted and recrystallized, Si-on-sapphire films. After a deep Si implantation (180 keV, 1.4×l0¹⁵ Si/cm²) at room temperature, and solid-phase epitaxial regrowth from the non-amorphized, 0.03 μm thick surface region, the initially undoped SOS films become doped p-type, and their resistivity decreases from (1−5)xl0¹⁴ ficm to 0.5 Ωcm. The doping is due to electrically active Al, released from the A1₂O₃ by the Si implantation, and present in the recrystallized films at a concentration of ≃2×l0¹⁶ Al/cm³ . After a 75 min steam oxidation at 875 °C, which consumes 0.06 Μm of Si, the resistivity of the recrystallized films increases to over 40 Ωcm, but the Al concentration is unchanged. The oxidation also uncovers higher quality material below the non-recrystallized surface layer. A semi-quantitative model is proposed to explain the electrical data, based on the diffusion of oxygen from the Si/SiO₂ interface into the SOS film during oxidation, and the formation of Al-O-Si neutral complexes. Data on the stability of the high-resistivity films against high-temperature annealing or re-amorphization and annealing is given.     

Conductor Formation Through Phase Transformation in Ti-Oxide Thin Films

The resistance and transmittance of Ti-oxide thin films sputtered on quartz substrates were studied. The electrical and optical properties can be changed by varying the percentage of O₂ introduced during the sputtering. The lowest resistivity for the sputtered Ti-oxide thin film was 2.30 × 10⁻² Ω cm for 12.5% O₂, which was obtained after annealing at 400°C in ambient oxygen. The results of x-ray photoelectron spectroscopy (XPS) curve-fitting indicate that the Ti-oxide thin film contained both Ti₂O₃ and TiO₂ phases during deposition. The Ti₂O₃ phase was transformed into the stable TiO₂ phase during annealing. The Ti₂O₃-TiO₂ phase transformation initiated the substitution reaction. The substitution of Ti⁴⁺ ions in the TiO₂ phase for the Ti³⁺ ions in the Ti₂O₃ phase created the free electrons. This Ti₂O₃-TiO₂ phase transformation demonstrates the potential mechanism for conduction in the annealed Ti-oxide thin films. The transmittance of the annealed Ti-oxide thin films can be as high as approximately 90% at the 400 nm wavelength with the introduction of 16.5% O₂. This result indicates that the annealed Ti-oxide thin films are excellent candidates for use as transparent conducting layers for ultraviolet (UV) or near-UV light-emitting diode (LED) devices.     

A dry-patterned Cu(Mg) alloy film as a gate electrode in a thin-film transistor liquid crystal display

The annealing of a Cu(4.5at.%Mg)/SiO₂/Si structure in ambient O₂ at 10 mtorr and 300–500°C allows for the out-diffusion of the Mg to the Cu surface, forming a thin MgO (15 nm) layer on the surface. The surface MgO layer was patterned and successfully served as a hard mask for the subsequent dry etching of the underlying Mg-depleted Cu films using an O₂ plasma and hexafluoroacetylacetone (H(hfac)) chemistry. The resultant MgO/Cu structure, with a taper slope of about 30°, shows the feasibility of dry etching of Cu(Mg) alloy films using a surface MgO mask scheme. A dry-etched Cu(4.5at.%Mg) gate a-Si:H thin-film transistor (TFT) has a field-effect mobility of 0.86 cm²/Vs, a subthreshold swing of 1.08 V/dec, and a threshold voltage of 5.7 V. A novel process for the dry etching of Cu(Mg) alloy films that eliminates the use of a hard mask, such as Ti, and results in a reduction in the process steps is reported for the first time in this work.     

Mechanistic study of borosilicate glass growth by low-pressure chemical vapor deposition from tetraethylorthosilicate and trimethylborate

A reaction mechanism and film morphology as a function of reactor conditions and post growth thermal annealing for borosilicate glass (BSG), (SiO₂)_x(B₂O₃)_1−x, films deposited from tetraethylorthosilicate (TEOS), trimethylborate (TMB), and oxygen (O₂) precursors by low-pressure chemical vapor deposition (LPCVD) was determined. An empirically derived reaction model for BSG film growth is proposed that predicts the growth rate and composition of BSG films up to 70 mole% B₂O₃. The BSG reaction model includes a strongly adsorbed TEOS-derived intermediate that forms SiO₂ and a direct surface reaction of TMB, in O₂, to form B₂O₃. This model is supported by growth rate and mass spectroscopic data. The BSG film morphology, investigated using atomic force microscopy, was found to have a root-mean-square roughness of 0.5 nm, with the precise film morphology being a function of reactor conditions. The BSG film roughness increases with film thickness, temperature, and boron content. Thermal annealing of the films in a water-free environment leads to planarization of the BSG governed by the film composition and anneal temperature.     

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.     

Fabrication of CNTs/Cu composite thin films for interconnects application

Carbon nanotubes/copper (CNTs/Cu) composite thin films were fabricated by combined electrophoresis and electroplating techniques. Electrical properties and structure of both CNTs/Cu thin films and the reference pure Cu thin films were investigated after annealing at different temperatures. The sheet electrical resistance of CNTs/Cu films decreases faster than that of pure Cu films with increase of annealing temperature. The grain size of CNTs/Cu film becomes much larger than that of pure Cu film at the same annealing temperature. The peak relative intensity of Cu (1 1 1) plane in CNTs/Cu film was stronger than that of pure Cu film. CNTs/Cu composite thin films, with better electrical properties than that of conventional pure Cu thin films, have been fabricated by electrophoresis and electroplating deposition techniques.     

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.     

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.