Application of Ti: W barrier metallization for integrated circuits

Aluminum metallization is the most widely used for contacts and interconnections in integrated circuits. However, the solid state diffusion of aluminum in silicon during contact sintering or high temperature packaging can result in junction shorting or leakage in shallow (<1 μm) emitter-base junction devices. The interposition of a barrier metal between the aluminum and the silicon is one solution to this problem. A sputter-deposited pseudo-alloy of Ti:W (10:90 wt.%) with PtSi contacts is suitable for this application. Resistivity ratio measurements on SiO₂/ ti:W/Al film test samples have shown that the resistivity of aluminum increases owing to diffusion of titanium or tungsten into the aluminum. However, the kinetic data show that no more than a 10% increase in the resistivity of the aluminum can be expected in the useful life of a device. High current stress data show that Ti:W/Al interconnections are comparable with those of aluminum films. Auger depth profiling of si/Ti:W/Al samples annealed at 450, 500 and 550°C in N₂ shows no aluminum at the Si-(Ti:W) interface. Application of the PtSi/Ti:W/Al metallization system for large-scale integrated circuits is described.     

Electrotransport in copper alloy films and the defect mechanism in grain boundary diffusion

Among the polycrystalline thin film conductors of Al, Cu, Ag, Au and their alloys, conductors made of Cu alloyed with Be exhibit the longest lifetimes under similar conditons of electromigration testing. Data have been analyzed for a Cu-Al sample tested at 2×10¹⁶ A cm^-2 and 265°C for 1400 h and for a Cu-Be sample tested at 3×10⁶ A cm^-2 and 291°C for 13 000 h. In both cases the transport of Cu occured from the negative to the positive terminal. The rate of grain boundary diffusion of Cu in Cu-Be is more than fifteen times smaller than that in Cu-Al.Observations of mass transport effects due both to electromigration and to compressive stresses in Cu-Al samples indicate that the atomic flux in grain boundary diffusion is accompanied by a counter flux of defects, presumably vacancies.     

Mass transport during electromigration in aluminum-magnesium thin films

Single-crystal and polycrystalline aluminum thin film conductors, containing up to 6 at.% magnesium, were subjected to electromigration experiments at 175° and 225°C with current densities up to 4 × 10⁶ A/cm² and for times up to 50 000 h. These conductors were subsequently examined in order to obtain quantitative data on mass transport during electromigration.The results indicate that in an alloy containing about 6 at.% magnesium the lattice diffusivity of aluminum is larger by an order of magnitude than the corresponding lattice diffusivity in pure aluminum, whereas the grain boundary diffusivity of aluminum in this alloy is two orders of magnitude smaller than the corresponding value in pure aluminum. Further, the lattice and grain boundary diffusivities of magnesium in this alloy are of the same order of magnitude as the aluminum diffusivities, and appear to be slightly smaller in both cases.     

Electromigration of grain boundaries in aluminum

The electromigration of grain boundaries has been investigated with aluminum wires (99.999% purity, 1 mm in diameter) in the temperature range 340°–540°C under a current stress of 6×10³ A cm^-2. Grain boundary migration in aluminum is found to be reduced or enhanced by the currents stress when the grain boundary migrates in or against the current direction, respectively. The effects of additions of copper, silicon and zirconium to aluminum on the electromigration of the grain boundaries have also been examined. The contribution of electromigration to normal grain boundary migration is found to be increased by the addition of solutes. The results are discussed on the basis of the impurity drag mechanism of grain boundary migration.     

Thermally stable and low-resistance W/Ti/Au contacts to n-type GaN

We report on the formation of thermally stable and low-resistance Ti/Au-based ohmic contacts to n-type GaN (4.0 × 10¹⁸ cm⁻³) by using a W barrier layer. It is shown that the electrical characteristic of the sample is considerably improved upon annealing at 900 °C for 1 min in a N₂ ambient. The contacts produce the specific contact resistance as low as 6.7 × 10⁻⁶ Ω cm² after annealing. The Norde and current–voltage methods are used to determine the effective Schottky barrier heights (SBHs). It is shown that annealing results in a reduction in the SBHs as compared to that of the as-deposited sample. Auger electron spectroscopy (AES), scanning transmission electron microscopy (STEM) and X-ray diffraction (XRD) examinations show that nitride and gallide phases are formed at the contact/GaN interface. Based on the AES, STEM and XRD results, a possible ohmic formation mechanism is described and discussed.     

On the Astm electromigration test structure applied to Al–1%Si/TiN/Ti bamboo metal lines

The applicability of NIST-ASTM electromigration test patterns when used to test ‘bamboo’ metal lines is discussed. Wafer level tests on passivated and non-passivated samples employing the Al–1%Si/TiN/Ti metallization scheme were performed. Straight metal lines 1000 μm long and 0·9 μm or 1·4 μm wide were tested at two different current densities. j = 3 MA cm² and j = 4·5 MA/cm², at constant stress temperature (T = 230·C). The failures mainly occurred in the end-segment areas and hindered the evaluation of the electromigration resistance of the ‘bamboo’ test lines. In order to avoid this problems, completely different test patterns based on different approaches must be designed.     

The temperature dependence of stresses in aluminum films on oxidized silicon substrates

In situ measurements were carried out of stress at the AlSiO₂ interface at various temperatures (25–500 °C) and for various film thicknesses (0.2–1.6 microm). These data are complemented with microstructural studies by transmission electron microscopy.For the aluminum films studied, the intrinsic structural component was very small (less than 2 × 10⁸ dyn cm^?2). On heating, thermal mismatch led to a compressive stress, with dσ/dT ≈ ?2 × 10⁷ dyn cm^?2 °C; these films yielded at 6σ6 ; ? 5 × 10⁸ dyn cm^?2, primarily through diffusion creep and grain growth. On cooling from about 450–500 °C, thermal mismatch led to a tensile stress which was limited mainly by dislocation slip. The final room temperature value after thermal cycling ranged from 0.5 × 10⁹ dyn cm^?2 for a 1.5 microm film to 8 × 10⁹ dyn cm^?2 for a 0.2 microm film; these values are believed to represent the critical stress for the generation of dislocation loops within the grains.The grain size of cold-deposited aluminum films ranged from about 0.2 microm for films 0.45 microm thick to about 2 microm for films 1.5 microm thick. Thermal cycling led to an order-of-magnitude increase in the grain size together with the formation of dislocation networks within the grains.     

Annealing effect on the formation of high-k dielectric in the W/ultrathin HfO2/Si-substrate system

Using a method of high-frequency magnetron sputtering, the structures W(150 nm)/HfO₂(5 nm)/Si(100) were prepared and were further annealed either at 500°C in vacuum for 30 min or in an atmosphere of argon at 950°C for 12 s. Study of the capacitance-voltage characteristics of these structures showed that high-temperature annealing leads to a decrease in the maximum specific capacitance in accumulation (from 4.8 × 10^?6 to 3.2 × 10^?6 F/cm²) and dielectric constant (from 27 to 23). Using time-of-flight secondary-ion mass spectrometry, it is established that growth of a WO _x phase at the W/HfO₂ interface and a HfSi _x O _y silicate phase at the HfO₂/Si (100) interface occurs during annealing. In addition, the total thickness of the intermediate oxide layer exceeds the thickness of the initial HfO₂ film by 30%.     

Design and performance studies of an MCP based pinhole camera for laser-produced plasma diagnostics

An X-ray pinhole camera system has been developed and tested to obtain spatially resolved time-integrated X-ray images of laser-produced plasmas (Nd: YAG laser of 75mJ energy in 35 picoseconds) using a single stage micro channel plate (MCP) detector coupled to a CCD camera-frame grabber system instead of X-ray film. Thick slab targets of aluminium, copper and gold were used to produce plasmas with different X-ray yields at laser intensities ranging from 8 × 10¹¹ to 8 × 10¹² W/cm². MCP gain was increased with the help of biasing voltages to record low intensity X-ray images, which have been analysed using an image processing software “PROMISE” developed in-house. The experimental results are presented.     

Realization of superhard nanocomposites with sufficient toughness: Superlattice nanocrystal-TiN/amorphous-(W,Ti)C0.83 films

A superhard nanocrystal (nc)-TiN/amorphous (a)-(W,Ti)C_0.83 film with sufficient toughness was synthesized by arc ion plating and dc magnetron sputtering. The superlattice structure of nc-TiN/a-(W,Ti)C_0.83 with a modulation period of 10 nm was obtained on amorphous (Ti,W)C_1 − x bonding layers. Accordingly, the nc-TiN/a-(W,Ti)C_0.83 nanocomposite showed a superhard effect (~ 52 GPa), meanwhile, exhibiting a sufficient toughness (~ 590 MPa). Compared to TiN films, the friction and wear performance of nc-TiN/a-(W,Ti)C_0.83 was significantly improved. As such, the superlattice nc-TiN/a-(W,Ti)C_0.83 nanocomposites have good potential in high-speed drying machines and high-temperature applications.     

Transmission electron microscopy of Li+ implanted Al films

Aluminium films implanted with 30 keV lithium ions of doses from 2.2 × 10¹⁶ to 1 × 10¹⁷ ions cm^?2 were studied by transmission electron microscopy. Samples implanted with 6 × 10¹⁶ and 1 × 10¹⁷ ions cm^?2 showed the presence of the Al-Li phase, while no change was observed in specimens implanted with 2.2 × 10¹⁶ ions cm^?. Microdiffraction patterns obtained from different areas of an aluminium film implanted with 2.0 × 10¹⁷ Li⁺ ions cm^?2 at 35 keV, revealed the presence of a stable Al-Li phase along with a metastable phase. Furthermore, on annealing the implanted samples at 200°C for 20 min an increase in ‘d’ values was noticed when compared with non-annealed specimens. However the annealed samples showed a very small lattice mismatch between the metastable phase and the Al matrix, thus indicating the presence of a metastable Al₃Li phase, responsible for hardening in binary Al-Li alloy.     

Carbon layers formed on steel and Ti alloys after ion implantation of C+ at very high doses

Ion implantation is a useful technique to tailor surface properties of steel and Ti alloys. In particular, very high dose C⁺ implantation (in the range of 10¹⁸ ions cm⁻²) offers the possibility of forming carbon layers without a sharp interface with the substrate material. In this study, ion implantation of carbon doses up to 8×10¹⁸ ions cm⁻² has been performed on 440C martensitic stainless steel and Ti6Al4V substrates under similar conditions and tribological and surface analysis results have been compared. Surface hardening occurred for all ion implantation conditions up to doses of 10¹⁸ ions cm⁻²1, 2, 3. Higher doses resulted in a different behaviour for both materials. The stainless steel showed a softening while a twofold hardness increase was maintained in the Ti alloy. Nevertheless, at the higher implanted dose a decrease in hardness was also observed in the Ti alloy. Small area XPS analyses were performed to evaluate the chemical states after ion implantation and establish a relationship with the observed surface hardening. Depth profile XPS analyses showed that for a dose of 4×10¹⁸ ions cm⁻² a carbon layer (with concentration over 85% at. C) was formed in the near surface region for both materials.     

A modified blister test to study the adhesion of thin coatings based on local helium ion implantation

A modified blister test has been developed based on helium ion implantation into selected areas of the metal substrate prior to the coating deposition. After a post-deposition thermal annealing, blisters are formed by agglomeration of the implanted gas at the ceramic–metal interface. This method can be used to control the pressure in the blister which eventually may lead to delamination at the periphery of the blister. A microsieve with a regular array of circular holes is used during the implantation to assure the initial blister size. Two different microsieves were employed in this work, with pore diameters of 1.5 and 4.5 μm, respectively. The distance between the centres of neighbour pores is twice the pore diameter. Scanning Electron Microscopy (SEM) and Confocal Scanning Optical Microscopy (CSOM) observations allowed the determination of the blistering parameters such as the radius, the height and the blister volume. From the gas content and these parameters, the work of adhesion or energy release rate can be obtained.In this work, we present the first results of this blister test applied to W–C:H films and multilayers of Ti and Al deposited by Physical Vapour Deposition on polycrystalline copper substrates. The copper substrates were implanted with 34 keV He⁺ ions up to fluences of 3 and 5×10¹⁶ cm⁻² before the deposition of the coatings and annealed afterwards in vacuum at temperatures from 773 to 1073 K for 30 min. Delamination of the Ti/Al multilayer coatings was already detected after annealing at 873 K with an energy release rate estimated to be 0.5 J m⁻² at a typical helium pressure of 10⁷ Pa. No delamination but only helium swelling was observed for W–C:H coatings annealed at 1073 K. Results of experiments on uncoated copper samples are also shown in order to explain the mechanism of helium bubble growth and helium release that causes the creation of the blisters.     

Solid-phase interaction in the hydroxyapatite/titanium heterostructures upon high-temperature annealing in air and argon

Samples of 100- and 500-nm-thick hydroxyapatite films on titanium were investigated using scanning electron microscopy, electron probe X-ray microanalysis, and X-ray powder diffraction. The films were prepared by high-frequency magnetron sputtering of a target in an argon atmosphere (1 × 10^?1 Pa) at a magnetron power density of 40–70 W/cm². These conditions provided growth of films at a rate as high as 0.7 nm/s. It was demonstrated that the hydroxyapatite film annealed in argon is characterized by deep pores that have diameters ranging from 0.3 to 8.0 µm and are uniformly distributed throughout the film surface. The electron probe X-ray microanalysis confirmed the presence of all elements (Ti, O, Ca, P) under investigation, except for hydrogen, in the samples of the films. For biologically compatible hydroxyapatite, the optimum ratio Ca : P ? 1.5–1.7 was achieved in the hydroxyapatite/titanium system with a 500-nm-thick hydroxyapatite layer upon annealing in argon at a temperature of 1050°C for 30 min. It was established that the hydroxyapatite/titanium system contains intermediate phases, including calcium titanate CaTiO₃, which proved the interaction of hydroxyapatite with titanium.     

Synthesis of Ti3Al and TiAl based surface alloys by pulsed electron-beam melting of Al(film)/Ti(substrate) system

Phase formation and surface hardening in the 100-nm-thick Al(film)/Ti(substrate) system under conditions of pulsed electron-beam melting (～15 keV, ～3 μs, 3–4 J/cm²) have been studied depending on the number of film deposition-melting cycles. Using this method, submicrocrystalline and nanocrystalline surface alloys with thicknesses ≥3 μm based on Ti₃Al and TiAl intermetallics have been obtained on the titanium substrate.     

Electrical characterization of wurtzite (Al,B)N thin films

Wurtzite aluminum nitride (w-AlN) thin films are of great interest for electro-acoustic applications and their material properties have in recent years been extensively studied. One way to tailor material properties is to vary the composition by adding other elements. Boron is an element that can take the place of aluminum in the crystal lattice of w-AlN. In the present study, polycrystalline w-(Al,B)N thin films were grown on p-Si(100) and Al/p-Si(100) substrates by pulsed DC reactive magnetron sputtering from a single Al/B target. MIS and MIM structures were fabricated to investigate the electrical properties of w-(Al,B)N thin films. Important dielectric thin film properties for microelectronics applications are the breakdown field, the permittivity (κ) and leakage current through the film. The (Al,B)N thin film is found to have a dielectric strength of ～3 × 10⁶ V cm^?1 and a κ close to 12. The measured leakage current through the film is assumed to be mainly due to Frenkel–Poole emission with a trap energy at 0.71 eV below the conduction band edge.     

Electrical characterization of wurtzite (Al,B)N thin films

Wurtzite aluminum nitride (w-AlN) thin films are of great interest for electro-acoustic applications and their material properties have in recent years been extensively studied. One way to tailor material properties is to vary the composition by adding other elements. Boron is an element that can take the place of aluminum in the crystal lattice of w-AlN. In the present study, polycrystalline w-(Al,B)N thin films were grown on p-Si(100) and Al/p-Si(100) substrates by pulsed DC reactive magnetron sputtering from a single Al/B target. MIS and MIM structures were fabricated to investigate the electrical properties of w-(Al,B)N thin films. Important dielectric thin film properties for microelectronics applications are the breakdown field, the permittivity (κ) and leakage current through the film. The (Al,B)N thin film is found to have a dielectric strength of ∼3 × 10⁶ V cm⁻¹ and a κ close to 12. The measured leakage current through the film is assumed to be mainly due to Frenkel–Poole emission with a trap energy at 0.71 eV below the conduction band edge.     

Mechanisms of carriers transport in Ni/n-SiC, Ti/n-SiC ohmic contacts

A mechanism of carriers transport through metal-semiconductor interface created by nickel or titanium-based ohmic contacts on Si-face n-type 4H-SiC is presented herein. The mechanism was observed within the temperature range of 20 °C ÷ 300 °C which are typical for devices operating at high current density and at poor cooling conditions. It was found that carriers transport depends strongly on concentration of dopants in the epitaxial layer. The carriers transport has thermionic emission nature for low dopant concentration of 5×10¹⁶ cm^?3. The thermionic emission was identified for moderate dopant concentration of 5×10¹⁷ cm^?3 at temperatures higher than 200 °C. Below 200 °C, the field emission dominates (for the same doping level of 5×10¹⁷ cm^?3). High dopant concentration of 5×10¹⁸ cm^?3 leads to almost pure field emission transport within the whole investigated temperature range.     

A Study of Ta-Si-N/Ti Bilayer Diffusion Barrier for Copper/Silicon Contact Systems

Ta-Si-N (10 nm)/Ti (20 nm) bilayer film has been designed with the purpose of using as diffusion barrier in copper interconnection. Ta-Si-N/Ti bilayer diffusion barriers were deposited on the substrate of n-type (100) silicon wafer using radio-frequency (RF) magnetron sputtering, followed by in situ deposition of copper. To investigate the thermal stability of the Ta-Si-N/Ti diffusion barriers, annealing was subsequently conducted in N₂ gas for 60 min and annealing temperatures were chosen at 600°C, 650°C, 700°C, 750°C, and 800°C. X-ray diffraction (XRD) revealed that Ta-Si-N layer grown on the Ti layer exhibited an amorphous phase. The results indicated that Ta-Si-N/Ti film can prevent copper diffusion at 750°C. After annealing at 750°C, the production of Ti-Si layer can effectively decrease contact resistance between barrier and silicon.