Void-free strong bonding of surface activated silicon wafers from room temperature to annealing at 600 °C

In order to investigate the high temperature application of surface activated silicon/silicon wafer bonding, the wafers were bonded at room temperature and annealed up to 600 °C followed by optical, electrical, mechanical and nanostructure characterization of the interface. Void-free interface with high bonding strength was observed that was independent of the annealing temperature. The bonding strength was as high as 20 MPa. The normalized interfacial current density was increased with the increase in the annealing temperature. A thin interfacial amorphous layer with a thickness of 8.3 nm was found before annealing, which was diminished at 600 °C. A correlation between the current density and nanostructure of the interface was observed as a function of the annealing temperature. The high quality silicon/silicon bonding indicates its potential use not only in low temperature microelectronic applications, but also in high temperature harsh environments.     

Influence of microwave annealing on direct bonded silicon wafers

Strong and nearly void free bonding was achieved using direct bonding followed by microwave annealing. Silicon wafers were cleaned, O₂ plasma surface activated, and bonded at room temperature. After microwave annealing at 400 °C, the bond strength of hydrophilic wafers was found to be in the range between 0.2 and 1.6 J/m². Additional heating of bonded wafers was done at elevated temperatures and for prolonged times using either rapid thermal annealing or microwave annealing. In either case, additional annealing showed no impact on wafer separation area, void, or bond strength. Thus, the initial microwave anneal dictated the ultimate bond strength regardless of subsequent annealing method. The mechanism for wafers bonded in this work involved dipole-dipole bonding and, hydrogen bonding. The initial microwave anneals typically required times less than 60 min. As a result, microwave annealing was shown to be a promising low temperature alternative for wafer bonding when compared to the currently used mechanical furnace anneals.     

Transmission electron microscope observation of Si deposited on W(110) surface

The (110) surface of a thin W crystal deposited with Si has been investigated by using a transmission electron microscope (TEM). It was found that an ordered Si–W alloy layer with a regular periodic arrangement of antiphase boundaries (APBs) is formed, as a result of intermixing of deposited Si with the W substrate at room temperature (RT). A crystal structure model of this W–Si superstructure is proposed in this study. After 800°C annealing of Si deposited at RT, grains of tetragonal WSi₂ are formed on the W–Si ordered alloy layer. Grains of WSi₂ are also formed when Si is deposited on a W substrate maintained at 900°C. These WSi₂ grains have epitaxial orientation relationships with the W(110) substrate as follows: (110)W//(01 ) WSi₂ and [001]W//[100]WSi₂.     

Electrical properties of evaporated polycrystalline Ge thin-films

Electrical properties of Ge thin films evaporated on Si₃N₄ CVD-coated Si substrate were improved by introducing a heat treatment after the deposition of Ge films. Evaporation conditions were optimized by changing the substrate temperature and deposition rate, and then, heat treatment was performed. At substrate temperatures during the evaporation lower than 300 °C and higher than 400 °C, deposited films were amorphous and polycrystalline, respectively. At substrate temperatures lower than 400 °C, Ge films were evaporated without degrading the surface roughness. The Hall mobility of films evaporated at room temperature increased with increasing the substrate and heating temperature and showed about 400 cm² V⁻¹ s⁻¹ for the hole concentration of 4 × 10¹⁷ cm⁻³ at the heating temperature of 900 °C. This value was almost comparable to that of p-type Ge single crystal.     

Structural and optical characterization of β-FeSi2 layers on Si formed by ion beam synthesis

Structural and optical properties have been investigated for surface β-FeSi₂ layers on Si(100) and Si(111) formed by ion beam synthesis using ⁵⁶Fe ion implantations with three different energies (140–50 keV) and subsequent two-step annealing at 600 °C and up to 915 °C. Rutherford backscattering spectrometry analyses have revealed Fe redistribution in the samples after the annealing procedure, which resulting in a Fe-deficient composition in the formed layers. X-ray diffraction experiments confirmed the existence of /gb-FeSi₂ by annealing up to 915 °C, whereas the phase transformation from the β to phase has been induced at 930 °C. In photoluminescence measurements at 2 K, both β-FeSi₂/Si(100) and β-FeSi₂/Si(111) samples, after annealing at 900–915 °C for 2 h, have shown two dominant emissions peaked around 0.836 eV and 0.80 eV, which nearly coincided with previously reported PL emissions from the sample prepared by electron beam deposition. Another β-FeSi₂/Si(100) sample has shown sharp emissions peaked at 0.873 eV and 0.807 eV. Optical absorption measurements at room temperature have revealed the allowed direct bandgap of 0.868–0.885 eV as well as an absorption coefficient of the order of 10⁴ cm⁻¹ near the absorption edge for all samples.     

Annealing effects on microstructure and mechanical properties of chromium oxide coatings

Reactive radio frequency magnetron sputter-deposited chromium oxide coatings were annealed at different temperatures and times. The influence of annealing temperature on the microstructure, surface morphology and mechanical properties was examined by X-ray diffraction, nanoindentation, pin-on-disc wear and scratch tests, respectively. X-ray results show that the chromium oxide sputtered at room temperature in low oxygen flux is primarily amorphous. Annealing below 400 °C did not cause much change, while annealing at higher temperature of 500 °C caused a significant change in microstructure and mechanical properties. Hardness increased from 12.3 GPa to 26 GPa, and the wearability improved with higher annealing temperature due to the formation of crystalline Cr₂O₃ phase, which occurs at 470 °C. Annealing time had little effect on mechanical properties and microstructure, although coating surface roughness increased with a longer annealing time. Coating adhesion was improved by annealing, due to residual stress relief and possible interfacial interdiffusion.     

Co-deposition of cobalt disilicide on silicon–germanium thin films

The formation of CoSi₂ on strained epitaxial Si_0.8Ge_0.2/Si(100) films has been studied as a function of the deposition method and annealing temperature. Two types of deposition processes were used: a direct method, where 5 nm of pure Co metal were deposited at room temperature onto a strained 80 nm thick Si_0.8Ge_0.2 layer; and a co-deposition method, where 5 nm Co and 18.2 nm Si were simultaneously deposited in a 1:2 ratio onto a strained Si_0.8Ge_0.2 layer at 450°C. Samples were then annealed at temperatures ranging from 500 to 800°C. Extended X-ray absorbance fine structure spectroscopy (EXAFS) and X-ray diffraction (XRD) were used to characterize the structure of the resulting films. It was found that the samples prepared via the direct deposition method did not convert to CoSi₂ at any annealing temperature up to 800°C, while the co-deposited samples formed epitaxial CoSi₂ at even the lowest annealing temperature of 500°C. These results are discussed in terms of proposed reaction mechanisms of the different deposition methods, based on consideration of the Co–Si–Ge ternary phase diagram.     

Superplastic diffusion bonding of γ-TiAl-based alloy

With laser surface melting and conventional heat treatment, superplastic diffusion bonding of TiAl alloy samples was carried out. Three different microstructure, i.e. fully lamellar structure, fine dendritic structure and refined equiaxed structure are used and their effects on bonding quality were investigated, and the bond quality was assessed by shear test at room temperature. Sound bonds could be achieved at 900 °C by laser surface modification or by laser surface modification and pre-bond heat treatment at 1000 °C for 60 min, which is lower than conventional diffusion bonding temperature. The bonds were also post-bond heat treated at 1200 °C for 30 min, which improved the bond quality in all cases. The best shear strength of the bonds is greater than 80% of that of base metal.     

Thermal stability of magnetron sputtering amorphous carbon nitride films

Magnetron sputtered amorphous carbon nitride films were annealed at different temperatures (450-900°C) and time (30-120 min). Compositional, bonding structural and surface morphological modifications of the films were characterized by Fourier transformation infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy. The as-deposited film was found to have nitrogen content of 30 at%, and the carbon atoms were bonded to nitrogen atoms in the chemical structure state of CN, CN and CN bonds. The FTIR and XPS results showed that the films were thermally stable without an obvious change in the films as annealing temperature was lower than 600°C. The relative intensity ratio of CN over CN bonds reached a maximum at annealing temperature of 750°C, and then decreased gradually at annealing temperature up to 900°C. The CN bonds in the films decreased with the increase of annealing temperature and eliminated completely at annealing temperature of 900°C. These results revealed that annealing caused a substantial decrease in the number of weak bonds between carbon and nitride atoms. The CN bonds have higher thermal stability than CN bonds and CN bonds in the films. Simultaneously annealing also led to the formation of a large fraction graphitic-like carbon in the films while nitrogen escaped from the film. Besides, the surface roughness of the films increased with annealing temperature. However, when annealing time was increased from 30 to 120 min at annealing temperature of 750°C, only a slight effect of the annealing time on composition, bonding structure and the surface roughness of the films was observed.     

Direct liquid injection metal-organic chemical vapor deposition of HfO2 thin films using Hf(dimethylaminoethoxide)4

Hf(OCH₂CH₂NMe₂)₄, [Hf(dmae)₄] (dmae=dimethylaminoethoxide) was synthesized and used as a chemical vapor deposition precursor for depositing Hf oxide (HfO₂). Hf(dmae)₄ is a liquid at room temperature and has a moderate vapor pressure (4.5 Torr at 80 °C). It was found that HfO₂ film could be deposited as low as 150 °C with carbon level not detected by X-ray photoelectron spectroscopy. As deposited film was amorphous but when the deposition temperature was raised to 400 °C, X-ray diffraction pattern showed that the film was polycrystalline with weak peak of monoclinic (020). Scanning electron microscope analysis indicated that the grain size was not significantly changed with the increase of the annealing temperature. Capacitance–voltage measurement showed that with the increase of annealing temperature, the effective dielectric constant was increased, but above 900 °C, the effective dielectric constant was decreased due to the formation of interface oxide. For 500 Å thin film, the dielectric constant of HfO₂ film annealed at 800 °C was 20.1 and the current–voltage measurements showed that the leakage current density of the HfO₂ thin film annealed at 800 °C was 2.2×10⁻⁶ A/cm² at 5 V.     

Characterization of transition joints of commercially pure titanium to 304 stainless steel

Solid-state direct bonding between commercially pure titanium and type 304 austenitic stainless steel has been carried out in the temperature range of 850–950 °C, under a uniaxial pressure of 3 MPa for 1 h. The diffusion bonds have been evaluated using light microscopy, electron probe microanalysis (EPMA), X-ray diffraction (XRD) technique and tensile testing. Light microscopy shows that different intermediate layers are formed in the reaction zone, and the width of these layers increases with an increase in bonding temperature. EPMA revealed that, at any particular bonding temperature, Ti traverses a minimum distance in the 304 stainless steel side, whereas Fe, Cr and Ni travel comparatively larger distances in the Ti side. This microanalysis also indicated different step formations in the concentration profile of Ti, Fe and Cr over different composition ranges in the diffusion zone indicating formation of intermetallic phases that were detected by XRD. Brittle intermetallic phases lower the strength and ductility of the diffusion bonded couples significantly. Best room temperature tensile strength, 217 MPa, has been obtained at 850 °C processing temperature due to minimal deleterious effects.     

Atomic-layer doping in Si by alternately supplied PH3 and SiH4

Atomic-layer doping of P in Si epitaxial growth by alternately supplied PH₃ and SiH₄ was investigated using ultraclean low-pressure chemical vapor deposition. Three atomic layers of P adsorbed on Si(100) are formed by PH₃ exposure at a partial pressure of 0.26 Pa at 450°C. By subsequent SiH₄ exposure at 220 Pa at 450°C, Si is epitaxially grown on the P-adsorbed surface. Furthermore, by 12-cycles of exposure to PH₃ at 300–450°C and SiH₄ at 450°C followed by 20-nm thick capping Si deposition, the multi-layer P-doped epitaxial Si films of average P concentrations of 10²¹ cm⁻³ are formed. The resistivity of the film is as low as 2.4×10⁻⁴ Ω cm. By annealing the sample at 550°C and above, it is found that the resistivity increases and the surface may become rough, which may be due to formation of SiP precipitates at 550°C and above. These results suggest that the epitaxial growth of very low-resistive Si is achieved only at a very low-temperature such as 450°C.     

Low-temperature, direct synthesis of β-SiC by metal vapor vacuum arc ion source implantation

Crystalline β-SiC surface layers with strong (111) preferred orientation were synthesized by direct ion implantation into Si(111) substrates at a low temperature of 400°C using a metal vapor vacuum arc ion source. Both X-ray diffraction and Fourier transform infrared spectroscopy reveal an augment in the amount of β-SiC with increasing implantation doses at 400°C. Scanning electron microscopy shows the formation of an almost continuous SiC surface layer after implantation at 400°C with a dose of 7×10¹⁷/cm². The full width at half maximum of the X-ray rocking curve of β-SiC(111) was measured to be 1.4° for the sample implanted at a dose of 2×10¹⁷/cm² at 700°C, revealing a good alignment of β-SiC with the Si matrix.     

Reactions in Au/Nb bilayer thin films

The interdiffusion and intermetallic compound formation of Au/Nb bilayer thin films annealed at 200–400 °C have been investigated. The bilayer thin films were prepared by electron beam deposition. The Nb film was 50 nm thick and the Au film was 50–200 nm thick. The interdiffusion of annealed specimens was examined by measuring the electrical resistance and depth-composition profile and by transmission electron microscopy. Interdiffusion between the thin films was detected at temperatures above 325 °C in a vacuum of 10^-4 Pa. The intermetallic compound Au₂Nb₃ and other unknown phases form during annealing at over 400 °C. The apparent diffusion constants, determined from the penetration depth for annealing at 350 °C, are 3.5 × 10⁻¹⁵ m² s⁻¹ for Nb in Au and 8.6 × 110^7minus;15 m² s⁻¹ for Au in Nb. The Au surface of the bilayer films becomes uneven after annealing at over 400 °C due to the reaction.     

Transfer of thin Si layers by cold and thermal ion cutting

We have used the crack-opening method to study the mechanical exfoliation behavior in hydrogen-implanted and bonded Cz Si. We found that the crystal orientation and boron doping influence the temperature required for mechanical layer transfer. Boron implantation at doses >10¹³ cm^–2 reduces the annealing temperature needed for mechanical exfoliation. The boron-doped epilayers followed similar exfoliation behavior as the boron-implanted samples. No lowering of the exfoliation temperature was observed for compensated and arsenic-doped Si layers. The hydrogen implantation converted the silicon wafer surface from p-type to n-type. The as-transferred Si layer was also found to be n-type after annealing at 200–450 °C. The p-type conductivity was restored upon annealing at around 600 °C. We believe that this conductivity conversion is due to the combined effect of ion-enhanced thermal donors and the presence of H-related shallow donors in the implanted layer. The p-type conductivity is restored at higher temperatures following the dissociation of the thermal donors and the out-diffusion of hydrogen. We also report that a good-quality silicon on glass layer can be obtained by the bonding and ion-cutting processes.     

Morphology of sputter deposited Al alloy films

Morphology of Al–2.0at%Ta and Al–2.0 at.% Nd alloy films before and after annealing was investigated for applications of interconnections for liquid crystal displays. It was found that the morphology and the microstructure of Al–2.0 at.% Nd alloy films changed markedly by annealing at the temperature region from 200°C to 300°C, while the morphology of Al–2.0 at.% Ta alloy films did not change by annealing up to 400°C. For the case of Al–2.0 at.% Nd alloy films, the incline of the <111> fiber texture to the substrate normal was observed during annealing. Structural characteristics of the Al films were investigated by TEM, SAD and XRD to determine the influence of alloying elements on the morphology and the fiber texture. From these results, it was concluded that the microstructures strongly influence the morphology and the grain orientation of Al alloy films.     

Carbon segregation in silicon

Carbon doping strongly suppresses the diffusion of boron. This is very important for later high temperature processing steps, e.g. for the fabrication of a high doped SiGe-base in a heterojunction bipolar transistor. We investigated (with a novel method) the segregation of C in Si (100) at growth temperatures between 500 and 700°C. A C-doped Si layer was grown by MBE under constant C and Si fluxes. The growth temperature was switched between 300°C and the investigated growth temperature. As the temperature was switched, the surface-accumulated C concentration was perturbed, causing a spike or depression in the SIMS depth profile. The surface segregation at a given temperature was measured by the amount of adlayer density in the spike. Clear evidence of the surface segregation of carbon in Si is found by these experiments.     

Titanium nitride deposition on hardened high speed steel by reactive magnetron sputtering

Hardened W-Mo-Co high speed steel SIS-2723 (Swedish standard) used for the manufacture of cutting tools was deposited with titanium nitride by reactive magnetron sputtering. In this work we have studied the influence of the substrate deposition temperature on the chemical composition, hardness and adhesion at the interface and in the bulk of the TiN film.

At an approximately 400°C deposition temperature the critical load C_L is at maximum. At the TiN film-substrate interface and at a 400°C deposition temperature it was also found that the ratios Si:N, C:N, C:Ti, Mo:N and Cr:N are at maxima and N:Ti is at a minimum. At a 200°C deposition temperature the O:Fe ratio at the interface is at a maximum and between 400 and 500°C this ratio is at a minimum. The titanium nitride surface hardness as well as the ultramicrohardness in cross-section reach maxima at a 400°C deposition temperature. Further analysis of the TiN film adhesion shows that it is adhesive at deposition temperatures below 350°C and cohesive at higher temperatures.     

Stresses in sputtered RUOx thin films

RuO_x thin films have been deposited by reactive sputtering in an O₂/Ar atmosphere. The films were characterized for their stress and resistivity as a function of deposition temperature (room temperature, 300°C) and the O₂ content (25–100%) in the sputtering gas. Additionally, the stresses in these films were determined as a function of annealing temperature (up to 600°C) using an in-situ curvature measurement technique. The as-deposited films were found to be under a state of compressive stress for all deposition conditions. The compressive stresses sharply increased with increasing deposition temperature from a value of around 200 MPa at 200°C to 1400 MPa at 300°C. This dramatic increase has been attributed to differences in microstructure at these deposition temperatures. The microstructural differences also led to the widely differing stress-temperature behavior during annealing of these films. For films deposited at temperatures lower than 200°C, the annealing process resulted in a decrease in the compressive stress and resistivity of the films. However, films deposited at a temperature of 300°C did not show any changes in the compressive stress or resistivity after annealing. The results of this study can be used to deposit RuO_x thin films with low resistivity and minimal stresses.     

Vacuum sealing using surface activation bonding of Si wafer

Bonding technology of Si wafer for vacuum seal is important in MEMS. We have tried the vacuum seal using surface activation bonding without any binder. It is the ultimate bonding technique and gives the precise dimension due to the direct contact. The technique is, however, not easy. We have investigated the surface conditions in order to achieve the bonding. The surfaces cleaned by Ar ion beam bombardment were measured by XPS and AFM. The natural oxide on the Si surface was removed by Ar ion bombardment. The surface roughness depended on the condition and the irradiation time of the Ar ion beam. The surface bonding at room temperature was achieved for the clean surface of the surface roughness less than Ra = 1 nm, but it was not done with the rough surfaces more than Ra > 2 nm. The vacuum sealing was checked using the cavities made in the Si wafer. The cavity part sealed in vacuum was depressed in the atmosphere, which was measured using a needle-contact profiler and a 3D laser profiler. The gas in the cavity was measured with a mass spectrometer by clashing the seal in vacuum. Any other gas except Ar gas closed in the cavity was not detected. We concluded that the vacuum sealing using surface activation bonding of Si wafer was achieved. The sealing condition has not changed even after one year.