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.     

Thermal desorption of Ge native oxides and the loss of Ge from the surface

The annealing behavior of Ge native oxides has been studied with x-ray photoemission spectroscopy (XPS). The native oxides were primarily GeO₂ with small amounts of GeO_x (x<2). Annealing was performed using a rapid thermal processor (RTP) with a N₂ purge at atmospheric pressure. Ion-implanted Ge substrates were used to investigate the loss of Ge from the surface due to thermal desorption of Ge oxides. It was found that thermal desorption of volatile Ge oxides and oxidization of Ge take place successively, which results in the loss of Ge from the surface.     

Rapid thermal chemical vapor deposition of in-situ boron doped polycrystalline SIxGe1-x

In-situ doped polycrystalline Si_xGe_1-x (x = 0.7) alloys were deposited by rapid thermal chemical vapor deposition (RTCVD) using the reactive gases SiH₂Cl₂, GeH₄ and B₂H₆ in a H₂ carrier gas. The depositions were performed at a total pressure of 4.0 Torr and at temperatures 600° C, 650° C and 700° C and different B₂H₆ flow rates. The conditions were chosen to achieve high doping levels in the deposited films. Our results indicate negligible effect of B₂H₆ flow on the deposition rate. The depositions follow an Arrhenius type behavior with an activation energy of 25 kcal/mole. Boron incorporation in the films was found to follow a simple kinetic model with higher boron levels at lower deposition rates and higher B₂H₆ flow rates. As-deposited resistivities as low as 2 mΩ-cm were obtained. Rapid thermal annealing (RTA) in the temperature range 800-1000° C was found to reduce the resistivity only marginally due to the high levels of boron activation achieved during the deposition process. The results indicate that polycrystalline Si_xGe_1-x films can be deposited by RTCVD with resistivities comparable to those reported for in-situ doped polysilicon.     

Solid phase reaction of Ti with Si−Ge layers prepared by Ge-implantation

Titanium germanosilicide films from thin Ti films (∼27.5 nm) are formed by solid phase reaction of Ti/Si_0.62Ge_0.38 bilayer at different annealing temperatures ranging from 600°C to 800°C. The effect of crystallographic state of Si−Ge alloy film on the reaction, phase formations, and polymorphic phase transformations, stability of germanosilicides have been investigated by x-ray diffraction, atomic force microscope, and sheet resistance measurements. Both amorphous and relaxed epitaxial Si_0.62Ge_0.38 films are prepared by Ge-multiple implantations into Si wafers with appropriate dose and energy followed by different post-implantation RTA schemes comprising alternative implantation and annealing in on case, and single final annealing in another one. XRD results indicate that the reaction sequence in both cases is found to be Ti/Si like with the formation of C49-Ti(Si−Ge)₂ as a precursor to the low resistivity C54-Ti(Si−Ge)₂. The films formed on amorphous alloy layer exhibit lower polymorphic transition temperature (∼750°C), smoother surface, lower sheet resistance and less agglomeration as compared to those on c:Si−Ge films. These characteristics are due to enhanced nucleation of C54 phase as a result of greater number of nucleation sites in the reaction with amorphous films. The formation of Ti(Si−Ge)₂ films is, however, accompanied by the decrease of Ge content in Ti(Si−Ge)₂ films formed on both amorphous and crystalline alloy films and indicates possible segregation/diffusion effects during the germanosilicidation.     

Formation of photoluminescence centers during annealing of SiO2 layers implanted with Ge ions

Photoluminescence (PL), Raman scattering, and the Rutherford backscattering of α-particles were used to study the formation of the centers of radiative-recombination emission in the visible region of the spectrum on annealing of the SiO₂ layers implanted with Ge ions. It was found that the Ge-containing centers were formed in the as-implanted layers, whereas the stages of increase and decrease in the intensities of PL bands were observed following an increase in the annealing temperature to 800°C. The diffusion-related redistribution of Ge atoms was observed only when the annealing temperatures were as high as 1000°C and was accompanied by formation of Ge nanocrystals. However, this did not give rise to intense PL as distinct from the case of Si-enriched SiO₂ layers subjected to the same treatment. It is assumed that, prior to the onset of Ge diffusion, the formation of PL centers occurs via completion of direct bonds between the neighboring excess atoms, which gives rise to the dominant violet PL band (similar to the PL of O vacancies in SiO₂) and a low-intensity long-wavelength emission from various Ge-containing complexes. The subsequent formation of centers of PL with λ_m～570 nm as a result of anneals at temperatures below 800°C is explained by agglomeration of bonded Ge atoms with formation of compact nanocrystalline precipitates. The absence of intense PL following the high-temperature anneals is believed to be caused by irregularities in the interfaces between the formed Ge nanoc-rystals and the SiO₂ matrix.     

Pd/Ge contacts to n-type GaAs

Sintered metal-semiconductor contacts, formed by thin, evaporated layers of Pd and Ge on n-type GaAs, were studied using Auger electron spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, secondary ion mass spectroscopy, current-voltage measurements, and capacitance-voltage measurements. Prior to sintering, the as-deposited Pd/Ge/GaAs contacts were rectifying and exhibited a reproducible Schottky barrier energy φ_Bn of 0.67±0.02 eV. Auger analysis indicated the initial behavior of the contact structure, upon sintering, to be an interdiffusion and reaction of Pd and Ge on a non-reacting GaAs substrate. Two germanide phases, Pd₂Ge and PdGe, were identified using X-ray diffraction and Auger analysis. The intervening Ge layer prevented the reaction of Pd with the GaAs substrate at low temperatures. Because of the PdGe reaction, φ_Bn increased to approximately 0.85 eV. Sintering at higher temperatures (i.e. between 300 and 400°C) produced additional reactions between Pd and the GaAs substrate. The electrical properties of the contact remained rectifying and φ_Bn exhibited little change from the value of 0.85 eV with the interdiffusion of Pd, Ga, and As. Sintering above 400°C resulted in the formation of ohmic contacts. The diffusion of Ge to the GaAs interface was found to correlate with the onset of ohmic behavior. Current conduction in the contact was best described by thermionic-field emission theory, and a specific contact resistance of 3.5 × 10^?4Ω-cm² was obtained after sintering above 550°C, independent of the initial impurity concentration in the substrate. Over the entire range of sintering temperatures (i.e. at or below 600°C), the interaction between the thin-film layers appeared to be governed by diffusion-controlled, solid-phase processes with no evidence of the formation of a liquid phase. As a result, the surface of the contact structure remained smooth and uniform during sintering.     

A thin transition film formed by plasma exposure contributes to the germaniumsurface hydrophilicity

Plasma treatment and 10% NH₄OH solution rinsing were performed on a germanium (Ge) surface. It was found that the Ge surface hydrophilicity after O₂ and Ar plasma exposure was stronger than that of samples subjected to N₂ plasma exposure. This is because the thin GeO_x film formed on Ge by O₂ or Ar plasma is more hydrophilic than GeO_xN_y formed by N₂ plasma treatment. A flat (RMS<0.5 nm) Ge surface with high hydrophilicity (contact angle smaller than 3°) was achieved by O₂ plasma treatment, showing its promising application in Ge low-temperature direct wafer bonding.     

Effects of alloying elements on cobalt silicide formation

Alloying elements can substantially affect the formation of cobalt silicide. A comprehensive study of phase formation was performed on 23 Co alloys with alloying element concentrations ranging from 1 at.% up to 20 at.%. Using in-situ characterization techniques in which x-ray diffraction (XRD) and elastic-light scattering are monitored simultaneously, we follow the formation of the silicide phases and the associated variation in surface roughness in real time during rapid thermal annealing. For pure Co silicide, we detect the formation of all stable silicide phases (Co₂Si, CoSi, and CoSi₂) as well as abnormal grain growth in the Co film and thermal degradation of the silicide layer at high temperatures. The effect of the various additives on phase formation was determined. The roughness of the interface was also measured using grazing incidence x-ray reflectivity (GIXR). We show that by selecting an alloy with a specific composition, we can change the phase-formation temperatures and modify the final CoSi₂ film texture and roughness.     

Strain relaxation and dopant distribution in the rapid thermal annealing of Co with Si/Si1−xGex/Si heterostructure

The reaction of cobalt with the Si-sacrificial cap in the strained Si/Si_1−xGe_x/Si MBE grown heterostructure was studied. The Si-cap is added to prevent the relaxation of the SiGe and to guarantee uniform and reliable silicidation reaction. The Si_1−xGe_x epilayer, with Ge content between 18 and 28 at%, was highly B doped, while the Si-cap was undoped or B doped either during growth or by ion implantation. Cobalt evaporation was followed by rapid thermal annealing at 450–700°C for 30 sec in N₂ or Ar+10%H₂. When the silicide penetrated the Si-cap/Si_1−xGe_x interface, noticeable out-diffusion of Ge and B to the surface was observed. In spite of the presence of the Si-cap significant strain relaxation was observed in three cases: (1) in the implanted samples, although the implantation was confined to the Si-cap, (2) when the Co layer was too thick, such that the silicide penetrated the SiGe layer and (3) when the Ge content in the SiGe layer was relatively high (27.5%).     

Long-Term Structural Instabilities in Undoped and Nitrogen-Doped Ge2Sb2Te5 Films

We investigated the compositional, microstructural, and electrical properties of undoped and nitrogen-doped Ge₂Sb₂Te₅ films subjected to long-term thermal annealing under air atmosphere. Considering the absence of chemical and structural changes, the sheet resistances of samples annealed at 200°C may potentially be related to changes in the lattice parameters. The disappearance of Ge–N bonds and decrease of Ge and N concentrations in samples treated at 300°C were found to depend on the cubic to hexagonal phase transition.     

Kinetics of structural and phase transformations in thin SiO<Subscript>x</Subscript> films in the course of a rapid thermal annealing

Infrared spectroscopy and analysis of photoluminescence spectra have been used to study variations in the composition of the oxide phase in a SiO_x film and the precipitation of the Si phase in the course of a rapid thermal annealing for 1–40 s at temperatures of 500–1000°C. Kinetics of phase segregation has been observed for the first time at temperatures of 600–700°C: an increase in the amount of precipitated silicon as the annealing duration increases followed by an eventual leveling off. The phase separation is brought to completion in a time as short as 1 s at temperatures higher than 900°C. The diffusion coefficient is estimated in the context of a model of the diffusion-controlled formation of Si nanoparticles. The obtained values of the diffusion coefficient exceed, by five to ten orders of magnitude, those of the silicon diffusion coefficients in SiO₂ and Si and are comparable to the diffusion coefficients of the oxygen contained in these structures. It is assumed that oxygen mobility forms the basis for the mechanism of structural and phase transformations in the SiO_x layers and for the formation of Si nanoparticles in the course of annealing.     

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.     

Formation of nanocrystalline silicon films using high-dose H<Superscript>+</Superscript> ion implantation into silicon-on-insulator layers with subsequent rapid thermal annealing

The formation of nanocrystalline Si films as a result of rapid thermal annealing of silicon-on-insulator structures implanted with high doses of H⁺ ions is studied. It is ascertained that the process of formation of Si nanocrystals is active even at temperatures of 300–400°C and is controlled by the hydrogen content in the silicon film and by the duration of annealing. It is concluded that the formation of nuclei of the crystalline phase occurs in silicon islands surrounded by microvoids and is caused by the ordering of Si-Si bonds in the course of release of hydrogen from the bound state. It is important that microvoids do not coalesce at temperatures up to ～900°C in conditions of rapid thermal annealing. It is found that synthesized films exhibit luminescence in the green-orange region of the spectrum at room temperature.     

Stabilization of a very high-k tetragonal ZrO2 phase by direct doping with germanium

The influence of Ge incorporation on the structural phase transformation of ZrO₂ films was investigated with the aim to control the resulting dielectric properties. For this reason, Ge-doped ZrO₂ thin films were prepared by atomic oxygen beam deposition at 225 °C. Admixture of low Ge concentrations (3–6.2 at.%) stabilizes the tetragonal ZrO₂ phase, and concurrently increases the permittivity to a maximum value of 37.7. Structural analysis shows that the permittivity enhancement can be explained by the increase of the tetragonal distortion upon Ge doping. The tetragonal phase is stable upon post-deposition high temperature annealing up to 1050 °C under N₂.     

Phase transformation and paired-plate precipitate formation in Pb0.91La0.09Zr0.65Ti0.35O3 films grown on sapphire substrates

We report on the phase transformation behavior of Pb_0.91La_0.09Zr_0.65Ti_0.35O₃ (9/65/35) PLZT films grown on r-sapphire substrates via rf-magnetron sputtering. A complex microstructure results in these films depending on deposition and annealing conditions. A random equiaxed polycrystalline grain morphology was observed after rapid thermal annealing or furnace annealing when the as-deposited films were predominantly pyrochlore. Precipitate formation (100–150 nm) was observed in PLZT films that were deposited at temperatures in excess of 490°C with a perovskite structure, after furnace annealing at 700°C. We believe that this is related to internal stresses in the films due to both the lattice mismatch and the thermal expansion mismatch between the PLZT film and the sapphire substrate.     

Control of interfacial properties of Pr-oxide/Ge gate stack structure by introduction of nitrogen

We have demonstrated the control of interfacial properties of Pr-oxide/Ge gate stack structure by the introduction of nitrogen. From C-V characteristics of Al/Pr-oxide/Ge₃N₄/Ge MOS capacitors, the interface state density decreases without the change of the accumulation capacitance after annealing. The TEM and TED measurements reveal that the crystallization of Pr-oxide is enhanced with annealing and the columnar structure of cubic-Pr₂O₃ is formed after annealing. From the depth profiles measured using XPS with Ar sputtering for the Pr-oxide/Ge₃N₄/Ge stack structure, the increase in the Ge component is not observed in a Pr-oxide film and near the interface between a Pr-oxide film and a Ge substrate. In addition, the N component segregates near the interface region, amorphous Pr-oxynitride (PrON) is formed at the interface. As a result, Pr-oxide/PrON/Ge stacked structure without the Ge-oxynitride interlayer is formed.     

Formation of thin-film HfO<Subscript>2</Subscript>/Si(100) structures by high-frequency magnetron sputtering

The results of X-ray structural investigations and current-voltage measurements of the HfO₂/Si(100) structures are presented. The HfO₂ films of 50 nm thickness were deposited in a Si substrate by high-frequency magnetron sputtering in argon plasma and subjected to rapid thermal annealing at 500, 700, and/or 800°C in the Ar or O₂ ambient. It is shown that the HfO₂ films become polycrystalline after annealing. The presence of various crystalline phases in them and the form of the I–V characteristics of the Al/HfO₂/Si(100) test structures strongly depend on the growth conditions and the gas ambient during the rapid thermal annealing. It is established that the HfO₂ films deposited at a high-frequency bias at a substrate of −7 V during the growth and then passed through rapid thermal treatment in the O₂ ambient at 700°C have the highest breakdown voltages.     

Interaction between zinc metallization and indium phosphide

Transmission electron microscope and evolved gas analysis (EGA) were used to study an interaction between Zn-metallization and InP substrate. In addition to the rapid Zn lattice diffusion into InP crystal, we detected the formation of Zn phosphide (Zn₃P₂) near the interface region. Because of its poor thermal stability in the temperature range studied here, the formation of this phase was found to be not readily reproducible. EGA has indicated, however, that this phase, if formed, can be decomposed at a temperature over 450°C. This erratic phase formation is suggested to contribute to the problem of occasionally non-ohmic contact formation in this particular metallization scheme. One proposed solution to cure this problem is to slightly raise the normal annealing temperature (420°C), so that the phase formed can readily decompose or to prolong the annealing time at the temperature range presently used.     

Microstructure and chemistry of Cu-Ge ohmic contact layers to GaAs

We report systematic studies of microstructure and chemistry of Cu-Ge alloyed ohmic contacts to n-GaAs with very low specific contact resistivity ((4-6) x 10^-7 Ωcm² for n∼l x 10¹⁷cm^-3). Using transmission electron microscopy, x-ray microanalysis, and secondary ions mass spectroscopy, we investigated chemistry of phase formation, crystal structure, and mechanism of ohmic contact formation in Cu-Ge alloyed layers with Ge concentration in the range of 0–40 at.%. Layers with Ge deficiency to form ζ-phase (average composition Cu₅Ge) reveal the formation of a nonuniform intermediate layer of hexagonal -Cu₃As phase which grows epitaxially on Ga₁₁₁ planes of GaAs. In this case, released Ga diffuses out and dissolves in the alloyed layer stabilizing ζ-phase, which is formed in the structures with average Ge concentration as low as 5 at.%. Unique properties of the contact layers, namely low specific contact resistivity, high thermal stability, interface sharpness, and high contact layer uniformity are related to the formation of an ordered orthorhombic ε₁ Cu₃Ge phase. In the alloyed layer with Ge concentration >25 at.%, no phases due to the chemical reactions with GaAs in the interface region were found demonstrating the chemical inertness of the ε₁Cu₃Ge ordered phase with respect to GaAs. This results in sharp interfaces and uniform chemical composition, the characteristics needed for superior contacts.     

Effects of annealing on the hydrogen concentration and the performance of InGaP/InGaAsN/GaAs heterojunction bipolar transistors

The effects of rapid thermal annealing (RTA) on InGaP/InGaAsN heterojunction bipolar transistors (HBTs) with a carbon-doped base have been studied. The hydrogen and nitrogen concentrations in the base, as well as the direct current (DC) and radio frequency (RF) device performance, were studied as a function of the annealing temperature. A 30-sec anneal at 650°C and 700°C under N₂ ambient effectively eliminates hydrogen from the base. As the annealing temperature is increased, the base sheet resistance decreases, and the corresponding maximum frequency of oscillation increases. For all annealing temperatures studied, we found degradation in the DC gain, presumably caused by the increase of nitrogen concentration in the base region.