TiN Diffusion Barrier Research Articles

Alternative back contact designs for Cu(In,Ga)Se2 solar cells on polyimide foils

A multilayer Mo stack is conventionally used as the back contact for flexible Cu(In,Ga)Se2 (CIGS) solar cells on polyimide foils, where record efficiencies of 18.7% were reported on lab-scale. The aim of this work was to develop alternative back contact designs on polyimide foils with the objective to reduce material costs, and to increase production throughput and yield. Two different back contact concepts are discussed, which are either based on Cu or Ti. The Cu-based back contact is a cost-effective design providing low sheet resistance of 0.2Ω/square at ~200nm stack thickness. To prevent Cu diffusion into the CIGS absorber, a TiN diffusion barrier was integrated into the back contact design. In addition, Cr was evaluated as a candidate of an alternative Cu-diffusion barrier material. Best cell efficiencies of 16.3% were achieved without antireflection coating on a Cu-based back contact design with a TiN diffusion barrier. For the Ti-based design, which is well adapted to the physical properties of the polyimide foil, efficiencies up to 18.1% were measured with antireflective coating.

Control of an interfacial MoSe2 layer in Cu2ZnSnSe4 thin film solar cells: 8.9% power conversion efficiency with a TiN diffusion barrier

We have examined Cu2ZnSnSe4 (CZTSe) solar cells prepared by thermal co-evaporation on Mo-coated glass substrates followed by post-deposition annealing under Se ambient. We show that the control of an interfacial MoSe2 layer thickness and the introduction of an adequate Se partial pressure (PSe) during annealing are essential to achieve high efficiency CZTSe solar cells—a reverse correlation between device performance and MoSe2 thickness is observed, and insufficient PSe leads to the formation of defects within the bandgap as revealed by photoluminescence measurements. Using a TiN diffusion barrier, we demonstrate 8.9% efficiency CZTSe devices with a long lifetime of photo-generated carriers.

Thermal stability of N2–H2 plasma-treated metal-organic-vapor-deposition TiN in a W/TiN/Ti/Si system

We examined the thermal stability of a W/TiN/Ti/Si metallization system by changing the thickness ratios of the plasma-treated (PT) to plasma-untreated regions (5 nm × 6/0 nm, 5 nm × 4/10 nm, 5 nm × 3/15 nm, 5 nm × 2/20 nm and 0/30 nm/nm) for the metal-organic-chemical-vapor-deposition (MOCVD) TiN diffusion barriers. From in situ film stress measurements taken during thermal cycles, the best thermal stability was achieved when about half of the thickness at the bottom of the MOCVD-TiN was N2–H2 PT. X-ray photoelectron spectroscopy and cross-sectional transmission electron microscopy revealed that this barrier structure successfully prevented the interdiffusion of the diffusing species from the WSi2 formation at the interface even after thermal annealing at 850 °C for 30 min, and this superior thermal stability was due to the microstructural evolution of the diffusion barrier comprising the amorphous layer for the top half and the oxygen-stuffed crystalline layer at the bottom. From the same plasma-treatment condition, the lowest leakage currents were obtained from both n+ and p+ 10 k array chains of the contacts with a diameter of 350 nm and an aspect ratio of ∼1.

Improving the oxidation resistance of AlCrN coatings by tailoring chromium out-diffusion

In this work, we have studied the improvement on the oxidation resistance of AlCrN-based coatings by adding a subsurface titanium nitride barrier layer. Since oxidation is interrelated with the inward diffusion of oxygen into the surface of Al xCr 1−xN (x = 0.70) coatings and the outward diffusion of Cr to the surface, the oxidation behaviour of the aluminium-rich AlCrN coatings can be tuned by designing the coating in an appropriate layered structure. The buried depth of the embedded layer and the oxidation time were varied, and the changes in the AlCrN/TiN depth composition profiles and surface oxidation stoichiometry were analysed by means of Glow Discharge Optical Emission Spectroscopy (GDOES) and Cross Sectional SEM (X-SEM) maps. It was observed that when a TiN diffusion barrier of 300 nm was deposited near the top surface (500 nm from the surface) the inhibition of the inward diffusion of oxygen and formation of beneficial alumina surface layers was promoted and consequently an increase of the oxidation resistance is achieved. This is explained in terms of a limited surplus of chromium from the coating to the surface. This was corroborated after performing experiments using CrN as embedded barrier layer which resulted in a continuous surplus of chromium to the surface and the formation of Cr-rich oxides. GDOES, in combination with X-SEM elemental maps, was proved to be a fast and accurate technique to monitor composition in-depth changes during oxidation, providing unique information regarding the oxide structure formation.

Atomic Layer Deposition for All-Solid-State 3D-Integrated Batteries

All-solid-state 3D integrated batteries can reach the energy storage capacity required for future wireless devices by exploiting the third dimension. Conformal deposition techniques such as atomic layer deposition (ALD) are needed to deposit the battery materials. In this work, the current development of ALD processes for 3D integrated batteries is reviewed. We have developed both the TiN diffusion barrier and Pt cathode current collector processes. TiN showed good barrier properties in 3D, although a higher than expected charge density was observed, which could be attributed to a lower thickness and/or different material properties at the bottom of the 3D structures. The remote plasma ALD process for Pt showed fast growth initiation and good adhesion of the films. Furthermore, sufficient step coverage for the battery application was found. ALD is also potentially able to deposit the active battery layers, although the deposition of Li-containing materials is expected to be challenging.

Thermal Stability of Nitride-Based Diffusion Barriers for Ohmic Contacts to n-GaN

The use of TaN, TiN, and ZrN diffusion barriers for Ti/Al-based contacts on n-GaN (n � 3 · 10 17 cm -3 ) is reported. The annealing temperature (600– 1,000� C) dependence of the Ohmic contact characteristics using a Ti/Al/X/Ti/Au metallization scheme, where X is TaN, TiN, or ZrN, deposited by sputtering was investigated by contact resistance measurements and Auger electron spectroscopy (AES). The as-deposited contacts were rectifying and transitioned to Ohmic behavior for annealing at ‡600� C. A minimum specific contact resistivity of � 6 · 10 -5 X-cm -2 was obtained after annealing over a broad range of temperatures (600–900� C for 60 s), comparable to that achieved using a conventional Ti/Al/Pt/Au scheme on the same samples. The contact morphology became considerably rougher at the high end of the annealing range. The long-term reliability of the contacts at 350� C was examined; each contact structure showed an increase in contact resistance by a factor of three to four over 24 days at 350� C in air. AES profiling showed that the aging had little effect on the contact structure of the nitride stacks.

Multilayered YSZ–Ag–Mo/TiN adaptive tribological nanocomposite coatings

Adaptive nanocomposite coatings provide low friction in broad ranges of environmental conditions through the formation of lubricious surfaces resulting from interactions with the ambient atmosphere. Designing adaptive coatings to withstand wear through repeated thermal cycles is particularly difficult since most demonstrate irreversible changes in surface composition and morphology. This permanent change in the condition of the surface limits the utility of adaptive coatings in applications where thermal cycling is expected. Moreover, some lubrication mechanisms occur over the entire coating surface in addition to the area experiencing wear, which results in a significant waste of lubricant. In an effort to increase the wear lifetime and move toward thermal cycling capabilities of solid adaptive lubricants, a multilayer coating architecture incorporating two layers of adaptive YSZ–Ag–Mo nanocomposite lubricant separated by a TiN diffusion barrier was produced. The multilayer coating lasted over five times longer than a monolithic adaptive coating of identical composition and total thickness for dry sliding tests at 500 °C in air. Analysis of the structure and composition of the films after heating suggests directed, lateral diffusion of lubricant beneath the diffusion barrier toward the wear scar as a mechanism for the increased wear life of the multilayer coating.

Silver diffusion and high-temperature lubrication mechanisms of YSZ–Ag–Mo based nanocomposite coatings

Yttria-stabilized zirconia (YSZ) nanocomposite coatings consisting of silver and molybdenum were produced by a hybrid process of filtered vacuum arc, magnetron sputtering and pulsed laser depositions for tribological investigations at different temperatures. The coatings with 24at.% Ag and 10at.% Mo contents showed a friction coefficient of 0.4 or less for all temperatures from 25 to 700°C. The wear scar surfaces and coating cross-sections were studied using scanning electron, transmission electron, scanning transmission electron and focused ion beam microscopes, which also provided the information on chemical composition distributions of silver and molybdenum along with microstructure features. It was demonstrated that silver diffusion and coalescence on surfaces played an important part in the high-temperature lubrication mechanism of the YSZ–Ag–Mo coatings. Silver was found to be an effective lubricant at temperatures below 500°C and its coalescence on the surface isolated molybdenum inside coatings from ambient oxygen. Lubricious oxides of molybdenum were formed and lubricated at temperatures above 500°C when the silver was worn off the contact surface. For silver containment inside the coating at high temperatures, a multilayer architecture was built by inserting a TiN diffusion barrier layer in the composite coatings. Microscopic observations showed that this barrier layer prevented silver exit to the coating surface. At the same time, this enabled a subsequent lateral lubricant supply toward a wear scar location where the diffusion barrier layer was worn through and/or for a next thermal cycle. The multilayer coating maintained a friction coefficient of 0.4 or less for more than 25,000 cycles, while the monolithic coating lasted less than 5000 cycles. In addition, a TiN surface barrier layer with pinholes was deposited on the YSZ–Ag–Mo composite surface to control vertical silver diffusion. With this coating design, the coating wear lifetime was significantly increased beyond 50,000 cycles.

Nucleation Characteristics of Directly Electrodeposited Copper on TiN

Copper was directly electrodeposited onto TiN diffusion barrier surface without employing a copper seed layer. Depending on the chemical condition of an electrolyte, especially pH, copper deposits on TiN had different nucleation and growth modes. pH of electrolyte was controlled by adding and to the base solution of pH 3.56, which contained copper–citrate (Cu–Cit) complexes. Chelating ability of citrate was substantially influenced by pH and it strongly affected the nucleation of copper. Below pH 3.56, progressive nucleation of copper on TiN was mainly found, and irregular growth and low density of of copper nuclei came out. Above pH 3.56, instantaneous nucleation of copper on TiN, which is desirable for filling of damascene structure, was mainly obtained, and hemispherical copper nuclei were uniformly distributed. Copper nuclei density increased from as pH varies from 3.56 to 7.4. It closely approached a value of , which is theoretically needed for seedless copper filling in the sub- damascene structure for ultralarge-scale integration interconnects.

Strengthening TiN diffusion barriers for Cu metallization by lightly doping Al

Thin films of Ti1−xAlxN were deposited on (100) Si by ultrahigh-vacuum dual-target reactive sputtering, and the impact of lightly doping Al of x as small as 0.09 on altering the films’s microstructure upon thermal annealing, and hence the performance of the films (40nm thick) as diffusion barriers for Cu metallization was evaluated. The results of transmission electron microscopy, Rutherford backscattering spectroscopy, and grazing-incidence x-ray diffraction show that the TiN barrier layer gives the commonly observed voided, columnar grains composed of 5nm sized subgrains. Upon annealing, the subgrains tend to coalesce into 20nm sized equiaxed grains full of crystalline defects, initiating an inward penetration of Cu and a partial dissociation of TiN, transforming themselves, respectively, into pyramidal (or columnar) Cu3Si precipitates and a dendritic Ti5Si3 layer just after 550°C, 10min annealing. However, the lightly doped Al not only overrides the tendency to form intercolumnar voids inherent in sputter deposition by self-shadowing and statistical roughening, but also substantially enhances the microstructural and thermochemical stability, hence significantly improving barrier property, as evidenced from an annealing test at an elevated temperature (600°C) for a prolonged period of 30min.

The effect of deposition temperature on the properties of TiN diffusion barriers prepared by atomic layer chemical vapor deposition

TiN films were grown on p-type Si (100) substrates and SiO 2/Si substrates by a modified atomic layer chemical vapor deposition (ALCVD) cycle using TiCl 4 and NH 3 as precursors. The effects of deposition temperature on growth rate, film resistivity, microstructure, and diffusion barrier properties of TiN films were investigated. The results show that the grown films are all polycrystalline with (200) preferred orientation and that the (200) texture is stronger at high deposition temperatures. The growth rate is about 0.03 nm/deposition cycle, and almost independent of deposition temperature. The resistivity, however, exponentially decreases with increasing deposition temperature. The chlorine impurity concentration measured by Auger electron spectrometry is lower than 1 at.% for films grown at deposition temperatures above 350 °C. Atomic force microscopy analysis reveals that the surface quality is good with root mean square roughness values below 0.9 nm. Various Cu/ALCVD TiN/Si samples were annealed at temperatures between 500 and 800 °C in vacuum ambient of 6.67 × 10 − 4 Pa for 1 h to evaluate the performance of TiN barriers. It is found that the failure temperature of TiN barriers is related to the deposition temperature. The failure temperatures are 600, 600–650, 700, and 750 °C for the TiN films deposited at 300, 350–400, 450, and 500 °C, respectively. The formation of voids at some weak spots after annealing suggests that Cl residues are responsible for the cause of early failure of ALCVD TiN barriers.

A Study on Pt/Al Reaction Mechanism in the FeRAM Device Integration

The capacitor contact barrier(CCB) layers have been introduced in the FeRAM integration to prevent the Pt/Al reaction during the back-end processes. Therefore, the interdiffusion and intermetallic formation in <TEX>$Pt(1500{\AA})/Al(3000{\AA})$</TEX> film stacks were investigated over the annealing temperature range of <TEX>$100\sim500^{\circ}C$</TEX>. The interdiffusion in Pt/Al interface started at <TEX>$300^{\circ}C$</TEX> and the stack was completlely intermixed after annealing over <TEX>$400^{\circ}C$</TEX> in nitrogen ambient for 1 hour. Both XRD and SBM analyses revealed that the Pt/Al interdiffusion formed a single phase of <TEX>$RtAl_2$</TEX> intermetallic compound. On the other hand, in the presence of TiN(<TEX>$1000{\AA}$</TEX>) barrier layer at the Pt/Al interface, the intermetallic formation was completely suppressed even after the annealing at <TEX>$500^{\circ}C$</TEX>. These were in good agreement with the predicted effect of the TiN diffusion barrier layer. But the conventional TiN CCB layer could not perfectly block the Pt/Al reaction during the back-end processes of the FeRAM integration with the maximum annealing temperature of <TEX>$420^{\circ}C$</TEX>. The difference in the TiN barrier properties could be explained by the voids generated on the Pt electrode surface during the integration. The voids were acted as the starting point of the Pt/Al reaction in real FeRAM structure.

Sacrificial adhesion promotion layers for copper metallization of device structures.

The adhesion of copper films to adjacent device layers including TiN, Ta, and TaN diffusion barriers is a crucial reliability issue for integrated circuits. We report that ultrathin layers of poly(acrylic acid) (PAA) prepared on barrier surfaces or on the native oxide of Si wafers dramatically increase the interfacial adhesion of Cu films deposited by the H2 assisted reduction of bis(2,2,7-trimethyloctane-3,5-dionato)copper in supercritical carbon dioxide. Similar improvements were achieved on Si wafers using a simple vapor phase exposure of the substrate to acrylic acid prior to metallization. The deposited films and the substrate/Cu interfaces were analyzed by X-ray photoelectron spectroscopy (XPS), electron microscopy, atomic force microscopy, and variable-angle spectroscopic ellipsometry. No trace of the adhesion layer was detected at the interface, indicating it was sacrificial at the deposition conditions used. Moreover, the presence and subsequent decomposition of the PAA layer during deposition substantially reduced or eliminated metal oxides at the substrate interface. For depositions on PAA-treated Si wafers, copper was present primarily as Cu0 at the interface and Si was present only as Si0. On PAA-treated Ta substrates, XPS analysis indicated Ta was present primarily as Ta0 at the metallized interface whereas Ta2O5 dominated the interface of samples prepared without the adhesion layers. The technique can be extended to patterned substrates using adsorption of acrylic acid or thermal/UV polymerization of acrylic acid.

Ultra thin CVD TiN layers as diffusion barrier films on porous low-k materials

In this work CVD TiN diffusion barriers were investigated for integration with porous ultra low-k (ULK) dielectrics. Sheet resistance measurements of thin CVD TiN films on capped porous ULK and a SiO2 reference were compared. The impact of different ULK material pore size (3 and 7 nm) on TiN diffusion barrier integrity was investigated for patterned structures by etch dip test and analytical methods. In addition a SiO2 CVD liner was used for pore sealing. It was detected that a smaller pore size facilitates a better CVD liner and TiN barrier formation which results in a better barrier integrity. Evaluation of barrier continuity by HF dip test showed no effect of the CVD liner for the ULK material with higher pore size. A remarkably decrease of the number of defects was observed compared to non-patterned samples. Also a reduced TiN penetration into the ULK compared to non-patterned samples was detected by EDX line scan. It is assumed, that a partial pore sealing during ULK etching using a photoresist mask occurred by formation of a passivation layer at the sidewall. Furthermore for lower pore size material the CVD liner improved the barrier performance.

Assessment of melting behavior of Cu-coated Ti powders using thermal analysis

Abstract The sequence of intermetallic formation and growth, and melting behavior of a Cu-coated Ti powder, used as active brazing alloy (ABA) for joining ceramic was studied. The experiments show that melting starts at around 885 °C, regardless of the heating rate. Prior to the onset of melting, a series of intermetallic layers was observed (Cu/TiCu 4 /Ti 3 Cu 4 /Ti 2 Cu 3 /Ti) with their relative thickness depending on the heating rate. A heating rate of 10 °C/min gives a final melting temperature of 900 °C as opposed to 915 °C for a sample heated at 40 °C/min. The utilization of N 2 as a “protective” gas has shown the instantaneous formation of TiN from the reaction between Ti and N 2 when the first liquid appears, indicating N diffusion through the copper shell. The remainder of the powder still contains Ti in copper as a TiN diffusion barrier was formed. Mixtures of ABA and Si 3 N 4 powder have demonstrated that the surface of the powder becomes liquid at 894 °C as an intense exothermic peak from the reaction between Ti and Si 3 N 4 is observed.

Effects of the Microstructure of Platinum Electrode on the Oxidation Behavior of TiN Diffusion Barrier Layer

The effects of the microstructure of a platinum bottom electrode on the oxidation behavior of a TiN diffusion barrier layer were investigated. The microstructures of the bottom Pt electrodes were varied from a columnar structure to a granular one by adding oxygen to the sputtering gas during the sputter-deposition and subsequent vacuum annealing process. It was found that a Pt film with a granular microstructure significantly retarded the oxidation of the underlying TiN barrier layer during annealing at 650°C for 30 min in air. However, the Pt film with a conventional columnar microstructure was unable protect the TiN layer from oxidation at temperatures as low as 450°C. The improved oxidation resistance of the barrier layer, as a result of modifying the microstructure of Pt films, is expected to contribute to the implementation of high-dielectric and ferroelectric capacitors into high-density memory devices.

A thin RuOx oxidized layer effect for a sputtered TiN barrier performance in high-density capacitors

To broaden the process window of the conventional sputtered TiN diffusion barrier and to find an appropriate bottom electrode for high-density capacitors, we investigated the effect of a thin RuOx oxidized layer formed at the surface of each Ru and RuOx film pre-annealed at 400 °C on the oxidation behaviour of the TiN diffusion barrier at the temperature range of 400–550 °C. Not only did the Ru/TiN/n+poly-Si/Si structure retain its structure up to 550 °C, but also a partially ohmic behaviour was exhibited up to 600 °C without an increase in resistivity, whereas after pre-annealing at 400 °C, the degradation of the RuOx/TiN/n+poly-Si/Si system started. In the latter case, the microstructure of the RuOx film was an embedded porous amorphous RuO2 crystalline phase and with no formation of thin RuOx layer at the surface of the RuOx film, resulting from free diffusion of oxygen and then oxidation of the TiN diffusion barrier. In the former case, after pre-annealing at 400 °C, the addition of oxygen at the surface of the Ru film leads to the diffusion path complexes. In addition, after post-annealing at 450 °C, the conductive RuO2 phase, which can act as an oxygen-sink layer, forms at the surface of the Ru film. Therefore, as a bottom electrode structure for high-density capacitors, the Ru/TiN/n+poly-Si/Si contact system is more effective than the RuOx/TiN/n+poly-Si/Si contact system.

Interfacial Characteristics of Copper/Diffusion Barrier/Low Dielectric Constant Material Systems at Elevated Temperatures

Thermal reactions of sputtered Cu films on the fluorinated silicon oxide (FSG) and organosilicate glass (OSG) layers, with and without TiN or TaN diffusion barrier, were investigated. The sheet resistance, surface morphology, phase formation and compositional depth profile of the mutilayer structures after vacuum annealing at 400 to 800°C were examined. It is found that the sheet resistance values of all samples decreased after annealing at 400 to 600°C and increased after annealing at 700 or 800°C. Without the barrier layer, the increase of sheet resistance was accompanied with the dewetting of Cu films and the carbon outdiffusion for Cu/OSG sample or the Cu indifftision of the Cu/FSG sample. When a TiN/Ti or a TaN/Ta barrier layer was interposed between Cu and the dielectric layer, the degree of dewetting of Cu film was significantly reduced for both systems. The reaction characteristics of the two dielectric systems unon vacuum annealing with and without the barrier laverare discussed.

The Effect of the Microstructure of Diffusion Barriers on the Palladium Activation for Electroless Copper Deposition

AbstractThe effect of microstructure of dc magnetron sputtered TiN and TaN diffusion barriers on the palladium activation for autocatalytic electroless copper deposition has been investigated by using X-ray diffraction, sheet resistance measurement, field emission scanning electron microscopy (FE-SEM) and plan view transmission electron microscopy (TEM). The density of palladium nuclei on TaN diffusion barrier increases as the grain size of TaN films decreases, which was caused by increasing nitrogen content in TaN films. Plan view TEM results of TiN and TaN diffusiton barriers showed that palladium nuclei formed mainly on the grain boundaries of the diffusion barriers.

Effects of Heterostructure Electrodes on the Reliability of Ferroelectric PZT Thin Films

The effect of the Pt electrode and the Pt-IrO₂ hybrid electrode on the performance of ferroelectric device was investigated. The modified Pt thin films with non-columnar structure significantly reduced the oxidation of TiN diffusion barrier layer, which rendered it possible to incorporate the simple stacked structure of Pt/TiN/poly-Si plug. When a Pt-IrO₂ hybrid electrode is applied, PZT thin film properties are influenced by the thickness and the partial coverage of the electrode layers. The optimized Pt-IrO₂ hybrid electrode significantly enhanced the fatigue properties with minimal leakage current.