Oscillatory behavior of the magnetic properties of Nd–Fe–B films with Mo and Mo–Cu additions

A series of Ta/NdFeB/Ta thin films with Mo and Mo–Cu additions embedded by alloying and by stratification have been prepared by r.f. sputtering. The influence of additions, their embedding mode, and annealing temperature on the structural and magnetic behavior of Ta/NdFeB/Ta thin films is presented. The use of additions of Mo and Mo–Cu leads to refined grain structure and improvement in the hard magnetic characteristics of Ta/NdFeB/Ta thin films. The Ta/[NdFeBMo(540 nm)/Ta films and Ta/[NdFeB(180 nm)/MoCu(dnm)] × n/Ta multilayer films present enhanced coercivities and M_r/M_s ratios in comparison with the Ta/NdFeB(540 nm)/Ta films. The stratification of Ta/NdFeB/Ta thin films with Mo–Cu interlayers leads to an oscillatory behavior of hard magnetic characteristics of the Ta/[NdFeB(180 nm)/MoCu(dnm)] × n/Ta multilayer films, when the thickness, d, of Mo–Cu interlayers varies by increments of 1 nm. When the thickness of Mo–Cu interlayers varies by increments of 2 nm the oscillatory behavior of the magnetic characteristics is not revealed. For a thickness of the Mo–Cu interlayer of 3 nm in the Ta/[NdFeB(180 nm)/MoCu(3 nm)] × 3/Ta thin films annealed at 650 °C, the c-axis of part of the hard magnetic Nd₂Fe₁₄B grains is oriented out-of-plane.     

Investigations of diffusion kinetics in Si/Ta/Cu/W and Si/Co/Ta systems by secondary neutral mass spectrometry

Proper understanding of the degradation mechanisms and diffusion kinetics of copper and cobalt interconnections for advanced microelectronics is important from the point of view of fundamental research and technology as well. In this paper Si(substrate)/Ta(10 nm)/Cu(25 nm)/W(10 nm) and Si(substrate)/Co(150 nm)/Ta(10 nm) samples, prepared by DC magnetron sputtering, were in investigated. The samples were annealed at several temperatures ranging from 423 K to 823 K for various times. The composition distributions were detected by means of Secondary Neutral Mass Spectrometry (SNMS). Microstructural characterization of samples was carried out by means of Transmission Electron Microscopy (TEM). It is shown that the changes in the composition profiles were mainly caused by grain boundary, GB, diffusion and the effective GB diffusion coefficients of Ta in Cu were determined both by the “first appearance” and “centre-gradient” methods. The activation energy is 100 kJ/mol. The importance of the Ta penetration into the Cu and its accumulation at the Cu/W interface can lead to an increase of the Ta content in the copper film. This can be an important factor in the change/degradation of the physical parameters (e.g. the electrical resistance) of interconnects. Furthermore a Ta segregation factor in Cu was evaluated. Preliminary results in the Si(substrate)/Co(150 nm)/Ta(10 nm) indicate fast (GB) diffusion of the Si into the Co layer, formation of a cobalt silicide layer at the Co/Si interface and Si accumulation first at the Ta/Co interface and later a retarded accumulation at the free Ta surface.     

Investigations of diffusion kinetics in Si/Ta/Cu/W and Si/Co/Ta systems by secondary neutral mass spectrometry

Proper understanding of the degradation mechanisms and diffusion kinetics of copper and cobalt interconnections for advanced microelectronics is important from the point of view of fundamental research and technology as well. In this paper Si(substrate)/Ta(10 nm)/Cu(25 nm)/W(10 nm) and Si(substrate)/Co(150 nm)/Ta(10 nm) samples, prepared by DC magnetron sputtering, were in investigated. The samples were annealed at several temperatures ranging from 423 K to 823 K for various times. The composition distributions were detected by means of Secondary Neutral Mass Spectrometry (SNMS). Microstructural characterization of samples was carried out by means of Transmission Electron Microscopy (TEM). It is shown that the changes in the composition profiles were mainly caused by grain boundary, GB, diffusion and the effective GB diffusion coefficients of Ta in Cu were determined both by the “first appearance” and “centre-gradient” methods. The activation energy is 100 kJ/mol. The importance of the Ta penetration into the Cu and its accumulation at the Cu/W interface can lead to an increase of the Ta content in the copper film. This can be an important factor in the change/degradation of the physical parameters (e.g. the electrical resistance) of interconnects. Furthermore a Ta segregation factor in Cu was evaluated. Preliminary results in the Si(substrate)/Co(150 nm)/Ta(10 nm) indicate fast (GB) diffusion of the Si into the Co layer, formation of a cobalt silicide layer at the Co/Si interface and Si accumulation first at the Ta/Co interface and later a retarded accumulation at the free Ta surface.     

Self-forming AlOx layer as Cu diffusion barrier on porous low-k film

The copper diffusion barrier properties of an ultrathin self-forming AlO_x layer on a porous low-k film have been investigated. Cu-3 at.% Al alloy films were directly deposited onto porous low-k films by co-sputtering, followed by annealing at various temperatures. Transmission electron microscopy micrographs showed that a ∼ 5 nm layer self-formed at the interface after annealing. X-ray photoelectron spectroscopy analysis showed that this self-formed layer was Al₂O₃. Sharp declines of the Cu and Si concentrations at the interface indicated a lack of interdiffusion between Cu and the porous low-k film for annealing up to 600 °C for 30 min. The leakage currents from Cu(Al)/porous low-k/Si structures were similar to as-deposited films even after a 700 °C, 5 min anneal while a Cu sample without Al doping failed at lower temperatures. Adding small amounts of Al to bulk Cu is an effective way to self-form copper diffusion layer for advanced copper interconnects.     

Tantalum and niobium as a diffusion barrier between copper and silicon

The efficiency of Ta and Nb films as diffusion barriers between thin Cu film and Si substrate has been studied using Auger electron spectroscopy, X-ray diffraction, optical microscopy, scanning electron microscopy and sheet resistance measurements. Two kinds of system were prepared by electron-beam evaporation: Cu/Ta (or Nb)/Si and Cu/Ta (or Mb) SiO₂/Si. The samples were annealed at temperatures from 400 to 800C in a vacuum of 1 × 10^–7 torr (13 Pa) for 30 min. In the Cu/Ta (or Nb)/Si system, the thermal stability was determined by interdiffusion at local sites, forming suicides, whereas the Cu/Ta (or Nb)/SiO₂/Si system degraded by interdiffusion at the interface between Ta (or Nb) and Cu. It appears that Ta is a more effective diffusion barrier than Nb for both kinds of system. This difference in the barrier effect of the transition metals is attributed to differences between oxygen segregation at grain boundaries of barrier layers and differences between diffusion coefficients through barrier layers. It is suggested that the driving force for interdiffusion may play a major role in the reaction that determines the thermal stability of a given contact system; this suggestion is based on the fact that the interdiffusion in Cu/barrier/Si systems is suppressed by interposing an SiO₂ layer in the Si substrate.     

Structural and tunneling properties of magnetic tunnel junctions with Al-O and MgO barrier

Two types of magnetic tunnel junctions (MTJs) with the configuration: substrate Si(1 0 0)/SiO₂ 47 nm/buffer/IrMn 12 nm/CoFe 2.5 nm/Al-O 1.5 nm/NiFe 3 nm/Ta 5 nm and Si(1 0 0)/SiO₂ 47 nm/buffer/IrMn 10 nm/CoFeB 3 nm/MgO 2 nm/CoFeB 4 nm/Ta 5 nm were prepared by the sputtering technique with two different buffers: A-Cu 25 nm and B-Ta 5 nm/Cu 25 nm. The B buffer caused a high texture of MTJs whereas in the case of the A buffer junctions texture was weak. Crystallites in the textured layers grew in a columnar like shape that induced interfacial roughness. High textured buffer B caused high interfacial roughness that reduced the resistance-area (RA) product due to a barrier thickness fluctuation. RA also changed substantially depending on the type of a barrier. The highest RA product ∼15 MΩ μm² was achieved for a low textured junction with Al-O barrier whereas in the high textured MgO sample RA product was ∼100 kΩ μm². Tunnel magnetoresistance (TMR) measured at room temperature was about 45% for the samples with Al-O barrier, whereas for the samples with MgO barrier TMR was about three times higher and achieved 140%.     

Pyroelectric Ta-modified LiNbO3 thin films and devices for thermal infrared detection

High-performance pyroelectric infrared (IR) detectors have been fabricated using tantalate-doped lithium niobate LiNb_1 − xTa_xO₃ (abbreviated as LNT, with x = 0-1.0) thin films deposited on Pt(111)/Ti/SiO₂/Si(100) substrates by diol-based sol-gel processing, in which, tantalate (Ta) is adopted as dopant in lithium niobate. The randomly oriented LNT thin film exhibits a relatively small dielectric constant and a large pyroelectric coefficient. The pyroelectric characteristics of detectors with various tantalate contents, as a function of modulation frequency, were investigated. It was found that LiNb_0.8Ta_0.2O₃ had the largest voltage responsivity of 7020 (V/W) at 70 Hz, and a specific detectivity (D^?) of 7.76 × 10⁷ cm Hz^1/2/W at 200 Hz. These results indicate that the LNT thin film with x = 0.20 is most suitable for application as high-performance pyroelectric thin-film detectors.     

Dielectric/metal/dielectric structures using copper as metal and MoO3 as dielectric for use as transparent electrode

Transparent conductive oxide/metal/oxide, where the oxide is MoO₃ and the metal is Cu, is realized and characterized. The films are deposited by simple joule effect. It is shown that relatively thick Cu films are necessary for achieving conductive structures, what implies a weak transmission of the light. Such large thicknesses are necessary because Cu diffuses strongly into the MoO₃ films. We show that the Cu diffusion can be strongly limited by sandwiching the Cu layer between two Al ultra-thin films (1.4 nm). The best structures are glass/MoO₃ (20 nm)/Al (1.4 nm)/Cu (18 nm)/Al (1.4 nm)/MoO₃ (35 nm). They exhibit a transmission of 70% at 590 nm and a resistivity of 5.0 · 10^− 4 Ω cm. A first attempt shows that such structures can be used as anode in organic photovoltaic cells.     

Structure and properties of Ta/TaOx barrier films deposited by direct current magnetron sputtering

Double-layer Ta/TaO_x films were deposited on glass substrates by direct current magnetron sputtering. The impact of the underlying TaO_x on the structure and properties was also investigated using X-ray diffraction analysis, Auger electron microscopy, scanning electron microscopy and atomic force microscopy. This study finds that the structure and properties of Ta/TaO_x films depends on the O₂ flow during the under-layer TaO_x deposition. As the O₂ gas flow ratio increases from 3 to 7%, more and more oxidized amorphous TaO_x films in the under-layer were formed, which caused the preferred growth orientation of upper Ta films to change from (200) to (221) systematically. Increasing the oxygen flow ratio of under-layer TaO_x films also makes the average grain size of upper Ta films decrease from 10.7 to 2.2 nm.     

Ion beam deposition of α-Ta films by nitrogen addition and improvement of diffusion barrier property

Ta thin films were deposited on Si (100) substrates by an ion beam deposition method at various substrate bias voltages under Ar + N₂ atmosphere with different pressure ratios of Ar and N₂. The effects of nitrogen pressure in the plasma gas and the substrate bias voltage on the surface morphology, crystalline microstructure, electrical resistivity and diffusion barrier property were investigated. It was found that the fraction of a metastable β-phase in the Ta film deposited at the substrate bias voltage of − 50 V films decreased by adding nitrogen gas, while the α-Ta phase became dominant. As a result, the Ta films deposited at the substrate bias voltage of − 50 V under Ar (9 Pa) + N₂ (3 Pa) atmosphere showed a dominant α-phase with good surface morphology, low resistivity, and superior thermal stability as a diffusion barrier.     

Diffusion barrier property of TiN and TiN/Al/TiN films deposited with FMCVD for Cu interconnection in ULSI

Flow modulation chemical vapor deposition (FMCVD) with titanium tetrachloride (TiCl₄) and ammonia (NH₃) is effective for depositing titanium nitride (TiN) films with conformal morphology, good step coverage, low electrical resistivity, and low chlorine residual contamination. It means that FMCVD TiN film is a good candidate of diffusion barriers for copper interconnection technology in ULSI. But the diffusion barrier property of FMCVD TiN film against Cu diffusion has not been confirmed. So, firstly, we deposited Cu (100 nm)/FMCVD TiN (25 nm)/Si multilayer films and investigated the thermal stability of Cu/TiN/Si structure. Vacuum annealing was done at 400, 500, 550 and 600 °C. For films annealed for 30 min at 400 °C, Cu diffused through the TiN layer and formed copper silicides on the surface of Si substrates. Therefore, FMCVD films formed under such conditions are unsatisfactory diffusion barriers. To enhance the diffusion barrier property of FMCVD TiN films, we used sequential deposition to introduce a monolayer of Al atoms between two TiN films. Etch-pit tests showed that for TiN films with Al interlayer, Cu diffusion through the barrier occurred at 500 °C and that is 100 °C higher than TiN film without Al interlayer. Al atoms formed AlO_x with oxygen atoms present in the TiN films as impurities, and fill up the grain boundaries of TiN film, thereby blocking the diffusion of Cu atoms.     

Electrical and structural properties of Ta-N thin film and Ta/Ta-N multilayer for embedded resistor

Ta/Ta-N multilayer has been developed to control temperature coefficient of resistance (TCR) in a thin-film embedded resistor with the incorporation of Ta layer (+ TCR) inserted into Ta-N layers (− TCR). Electrical and structural properties of sputtered Ta, Ta-N and the multilayer films were investigated. The stable resistivity value of 0.0065 Ω·cm in β-Ta film was obtained, and phase change from fcc-TaN to orthorhombic Ta₃N₅ in Ta-N films was observed at nitrogen partial pressure of 22%. The multilayer of Si/Ta(60 nm)/Ta₃N₅(104 nm)/Ta(60 nm)/Ta₃N₅(104 nm) showed TCR value of − 284 ppm/K, where TCR of Ta was − 183 ppm/K and that of Ta₃N₅ was − 3193 ppm/K.     

Electrode effect on resistive switching of Ti-added amorphous SiOx films

Ti-added amorphous SiO_x films were sputter-deposited into stacks of Pt/SiO_x/Pt and Cu/SiO_x/Pt. Optimally prepared Pt/SiO_x/Pt exhibits unipolar resistive switching over 10² cycles, resistance ratio ∼ 10³, yet wide voltage distribution (2 ∼ 7 V for SET, 0.5 ∼ 1.5 V for RESET). Cu/SiO_x/Pt exhibit similar endurance, resistance ratio up to 10⁷, and SET and RESET voltages reduced to 1.8 ∼ 4.2 V and 0.5 ∼ 1 V, respectively. Cu diffusion into SiO_x at the virgin state may play a role in resistive switching of Cu/SiO_x/Pt stack besides of filament conduction. Ti-added amorphous SiO_x films incorporating Cu electrode shows potential for resistive memory.     

Structure and magnetic properties of high coercive [PrFeBx/Cu]n films with out-of-plane orientation

Pr-Fe-B single layer and [PrFeB_x/Cu]_n films were prepared by magnetron sputtering on Si substrate heated at 650 °C. The influence of the composition and thickness of Cu spacer layer on the structure and magnetic properties of films with out-of-plane orientation are investigated. The [PrFeB_x/Cu]_n films present an enhanced coercivity and a lower remanence, in comparison with the results of Pr-Fe-B single layer. The coercivity H_c⊥ of about 19.7 kOe and the remanence ratio M_r/M_s of about 0.90 are achieved in the Mo(50 nm) / [PrFeB(300 nm) / Cu(2 nm)]₂ / Mo(50 nm) film.     

Bias sputtered Ta modified diffusion barrier in Cu/Ta(Vb)/Si(111) structures

In this investigation, we have fabricated Ta(V_b)/Si(111) and Cu/Ta(V_b)/Si(111) systems using a DC bias sputtering technique at high Ar pressure (100 mTorr). For Ta/Si(111) system, tantalum layer was formed under various bias voltages ranging from 0 to −150 V. The films were characterized by Rutherford backscattering spectrometry (RBS), scanning electron microscopy (SEM) and four-point probe sheet resistance measurements (R_s). From electrical resistivity and SEM data, a minimum resistivity (99 μΩ cm) and well surface morphology at an optimum bias voltage (V_b=−50 V) was obtained for the Ta(V_b)/Si(111) system. The Ta films deposited under these conditions with 50 nm thickness are then used as a diffusion barrier in the Cu/Ta(V_b)/Si(111) multilayer structure. According to our RBS, SEM and R_s analysis, the Ta barrier layer formed under the controlled bias sputtering at high Ar pressure has demonstrated an improved Ta structure with excellent thermal stability up to 650°C for the Cu/Ta(V_b)/Si(111) system annealed in N₂ environment for 30 min. Formation of TaSi₂ was observed at 700°C after the barrier failure using RBS spectra.     

Investigation of low resistance transparent MoO3/Ag/MoO3 multilayer and application as anode in organic solar cells

Depending on the resistivity and transmittance, transparent conductive oxides (TCO) are widely used in thin film optoelectronic devices. Thus doped In₂O₃ (ITO), ZnO, SnO₂ are commercially developed. However, the deposition process of these films need sputtering and/or heating cycle, which has negative effect on the performances of the organic devices due to the sputtering and heat damages. Therefore a thermally evaporable, low resistance, transparent electrode, deposited onto substrates room temperature, has to be developed to overcome these difficulties. For these reasons combination of dielectric materials and metal multilayer has been proposed to achieve high transparent conductive oxides. In this work the different structures probed were: MoO₃ (45 nm)/Ag (x nm)/MoO₃ (37.5 nm), with x = 5-15 nm. The measure of the electrical conductivity of the structures shows that there is a threshold value of the silver thickness: below 10 nm the films are semiconductor, from 10 nm and above the films are conductor. However, the transmittance of the structures decreases with the silver thickness, therefore the optimum Ag thickness is 10 nm. A structure MoO₃ (45 nm)/Ag (10 nm)/MoO₃ (37.5 nm) resulted with a resistivity of 8 × 10^− 5 Ω cm and a transmittance, at around 600 nm, of 80%. Such multilayer structure can be used as anode in organic solar cells according to the device anode/CuPc/C₆₀/Alq₃/Al. We have already shown that when the anode of the cells is an ITO film the introduction of a thin (3 nm) MoO₃ layer at the interface anode (ITO)/organic electron donor (CuPc) allows reducing the energy barrier due to the difference between the work function of ITO and the highest occupied molecular orbital of CuPc [1]. This property has been used in the present work to achieve a high hole transfer efficiency between the CuPc and the anode. For comparison MoO₃/Ag/MoO₃/CuPc/C₆₀/Alq₃/Al and ITO/MoO₃/CuPc/C₆₀/Alq3/Al solar cells have been deposited in the same run. These devices exhibit efficiency of the same order of magnitude.     

Diffusion barrier performance of nano-structured and amorphous Ru-Ge diffusion barriers for copper metallization

In the experiment, nano-structured and amorphous ultrathin Ru-Ge interlayers (∼15 nm in thickness) were deposited between Cu(Ru) alloy film and Si substrate via co-sputtering functioning as preventive diffusion barrier layers. After annealing at different temperatures, X-ray diffraction and four-point probe method revealed that the amorphous Ru-Ge layer effectively suppressed the Cu diffusion into Si substrate up to a temperature of at least 873 K; however, it is less than 773 K for the nano-structured Ru-Ge layer. A self-formed amorphous multilayer of Ru(RuO_x)/RuGe_xCu_y could be attained by annealing Cu/Cu(Ru)/Ru-Ge(amorphous)/Si system at a very low temperature (even 473 K). The results proved that the amorphous Ru-Ge system could self-form the multilayer diffusion barrier before the diffusion reaction between Cu and Si and improved the thermal stability of the Cu interconnection significantly.     

The effect of substrate temperature on the physical properties of tantalum oxide thin films grown by reactive radio-frequency sputtering

Thin films of TaO_x were deposited on Si(1 0 0) by radio-frequency magnetron sputtering at substrate temperatures of 25, 100, 200, 300, 400, and 500 °C. The properties of TaO_x thin films deposited with different oxygen-to-argon gas ratios and substrate temperatures were evaluated. The results show that the films with lowest leakage current density were obtained at ambient temperature with an oxygen mixture ratio (OMR) of 60% and the oxygen-to-tantalum ratio has a minimum with increasing deposition substrate temperature. From the current–voltage (I–V) characteristics of the TaO_x thin films as a function of deposition substrate temperature, we found that the leakage current density in the TaO_x thin films increases with increasing deposition substrate temperature. The higher leakage current density in the TaO_x films is correlated to the oxygen deficiency in TaO_x films and crystallization at higher deposition temperature.     

Improvement of magnetic characteristics and electrical properties of FeCoHfO/AlOx multilayered films

High permeability magnetic films can enhance the inductance of thin-film inductors in DC-DC converters. In order to obtain high permeability, effective uniaxial anisotropic field should be as low as possible. A multilayered technique (laminating the magnetic layers with oxide spacers) was exploited to improve the magnetic properties of thick films. The FeCoHfO/AlO_x multilayered films were fabricated by dc reactive magnetron sputtering. Inserting an insulator (AlO_x) layer can decrease the magneto-elastic anisotropy by reducing the residual stress of the FeCoHfO magnetic films. The anisotropic field and resistivity of the FeCoHfO/AlO_x multilayered films were evidently improved by multilayered coating. With this optimum configuration of 9 layers structure [FeCoHfO (133 nm)/AlO_x (10 nm)]₉, low anisotropic field (H_K = 65 Oe) and high resistivity (ρ ∼ 1350) μΩ cm were achieved.     

Deposition of WNxCy thin films for diffusion barrier application using the dimethylhydrazido (2) tungsten complex (CH3CN)Cl4W(NNMe2)

Tungsten nitride carbide (WN_xC_y) thin films were deposited by chemical vapor deposition using the dimethylhydrazido (2⁻) tungsten complex (CH₃CN)Cl₄W(NNMe₂) (1) in benzonitrile with H₂ as a co-reactant in the temperature range 300 to 700 °C. Films were characterized using X-ray diffraction (XRD), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy and four-point probe to determine film crystallinity, composition, atomic bonding, and electrical resistivity, respectively. The lowest temperature at which growth was observed from 1 was 300 °C. For deposition between 300 and 650 °C, AES measurements indicated the presence of W, C, N, and O in the deposited film. The films deposited below 550 °C were amorphous, while those deposited at and above 550 °C were nano-crystalline (average grain size < 70 Å). The films exhibited their lowest resistivity of 840 µΩ-cm for deposition at 300 °C. WN_xC_y films were tested for diffusion barrier quality by sputter coating the film with Cu, annealing the Cu/WN_xC_y/Si stack in vacuum, and performing AES depth profile and XRD measurement to detect evidence of copper diffusion. Films deposited at 350 and 400 °C (50 and 60 nm thickness, respectively) were able to prevent bulk Cu transport after vacuum annealing at 500 °C for 30 min.