K. Drescher

Crystallographic and morphological characterization of reactively sputtered Ta, TaN and TaNO thin films

Abstract This paper concentrates on the deposition of Ta, Ta N and Ta N O thin films by r.f. magnetron sputtering in Ar/N 2 /O 2 gas mixtures. The film properties and their suitability as diffusion barriers and protective coatings in silicon devices were characterized using four-point probe measurements, Auger electron spectroscopy, Rutherford backscattering, glancing angle X-ray diffractometry, atomic force microscopy and scanning electron microscopy. With the addition of N 2 to the gas mixture a transition from tetragonal Ta to b.c.c.-Ta(N) was detected, leading to the nanocrystalline metastable b.c.c.-Ta(N) phase with approximately 20 at.% interstitially incorporated nitrogen. Increasing the nitrogen flow above a critical value, an abrupt transition between metal-sputtering to nitride-sputtering mode was observed, resulting in a sharp increase in the N:Ta atomic ratio slightly above the stoichiometric value for the TaN phase, which was found to exhibit f.c.c. structure. With the addition of oxygen at fixed nitrogen flow the films tend to grow in an amorphous state. Due to the lack of short-circuit diffusion paths, the as-deposited amorphous Ta(N,O) films are considered as excellent candidates for ultra-thin diffusion barriers and protection layers in future Cu-metallized ULSI devices.

BEHAVIOR OF THIN TA-BASED FILMS IN THE CU/BARRIER/SI SYSTEM

This work concentrates on the diffusion barrier stability of very thin (10 or 20 nm) α- or β-Ta, TaN, Ta(O) and Ta(N,O) films in the Cu/barrier/Si system. Based on the classical theory of the thin film growth and know how of material transport in thin films, the various Ta-based films were classified according to their density of free short-circuit paths. Using scanning electron microscopy, transmission electron microscopy, glow discharge optical emission spectroscopy and secondary ion mass spectrometry, the 20 nm thin polycrystalline columnar-structured β-Ta films were found to be stable up to 500 °C for 1 h. After 1 h at 600 °C Cu3Si was formed due to short-circuit diffusion of Cu throughout the β-Ta films. The 20 nm thin giant-grained α-Ta films show equivalent performance to the β-Ta films. On the other hand, the 10 nm thin stuffed nanocrystalline face-centered-cubic (fcc) TaN films were able to protect the Si from Cu diffusion up to at least 600 °C/1 h. Ten nm thin amorphous-like Ta(N,O) and Ta(O) fi...

Characteristics in chemical-mechanical polishing of copper : comparison of polishing pads

Abstract A systematic study of Cu CMP in terms of the effect of polishing pad properties on the process characteristics has been performed. The IC 1000 and IC 1000/SUBA IV polishing pads were compared with regard to the polish rates, across-wafer uniformity, planarity and pattern sensitivity of the CMP process. Polishing with the IC 1000/SUBA IV pad, a better uniformity and higher polish rates were achieved. No differences in the pad influence on the geometry effects (Cu dishing and SiO2 thinning) have been found, which can be explained with the same near-surface layer affecting the interaction between pad and wafer. The hardness of the IC 1000 allows the material to planarize across wide Cu areas with minimal dishing and good planarity, but it is also essential that the SUBA IV bottom layer of the IC 1000/SUBA IV polishing pad improves the resilience and compressibility of the pad and enhances the global uniformity in the polish removal. It was demonstrated that uniform material removal during polishing is one of the fundamental concerns in the CMP technique affecting the tolerances deliverable by the CMP process.

Sputtering of tantalum-based diffusion barriers in SiCu metallization: effects of gas pressure and composition

Abstract A comparative study of DC sputter-deposited Ta and Ta(N) thin films (20 and 50 nm of thickness) as diffusion barriers for Cu has been performed using sheet resistance measurements, stress measurements, Auger electron spectroscopy, X-ray photoelectron spectroscopy, Rutherford backscattering, secondary ion mass spectrometry, scanning electron microscopy, atomic force microscopy and high temperature electron probe microanalysis. Film microstructure, chemical composition and surface roughness were found to depend on gas pressure and composition during deposition. 50 nm Ta thin films prevent CuSi interaction up to 450°C for 5 h in vacuum. It was found that TaN is a more effective barrier to copper penetration; 50 nm TaN films prevent the Cu reaction with the Si substrate for temperatures up to at least 560°C for 1 h, and 20 nm TaN films work as an effective barrier for 1 h at 450°C.

Chemical-mechanical polishing of copper for interconnect formation

Abstract A systematic study of Cu chemical-mechanical polishing (CMP) in terms of process parameters influence, planarization ability of the process and pattern sensitivity of the polish rate was performed. We examined the effects of Cu dishing and SiO 2 thinning and the reasons for them. Both were found to be sensitive to the pattern geometry (line width and pattern factor) and the overpolishing time. The influence of the within-wafer nonuniformity in the polish removal on the polishing performance (planarity, line thickness) was also studied. Different optimization concepts in terms of the barrier/adhesion layer (Ta and WTi) were examined. CMP of Cu for damascene patterning was demonstrated and evaluated by electrical measurements as a promising technique for forming Cu lines.

Influence of process parameters on chemical-mechanical polishing of copper

Chemical-mechanical polishing (CMP) appears to be the most promising technology for global planarization of device topography and metal patterning in the damascene technique. Cu has been recently studied as a candidate material for future integrated circuit metallization because of its low resistivity and better electromigration resistance than current Al alloy interconnects. In order to achieve a highly reliable CMP process for Cu delineation it is necessary to examine the limitations of the process. Integrating Cu CMP into an interconnect processing sequence requires a detailed understanding of how process parameters affect different aspects of the CMP process and therefore the quality of the patterned lines.

Dielectric barriers for Cu metallization systems

Copper is a promising material to replace aluminum In some microelectronlc applications. However, the interaction of copper with contacting materials in multilevel metallization schemes, especially copper diffusion in dielectrics, is an important issue that needs to be addressed before it can be utilized in certain applications. The influence of different deposition parameters on film properties was studied. Optical and structural properties of deposited films were characterized by ellipsometry and infrared absorption spectroscopy, respectively. The interaction of copper with dielectrics was investigated by different analytical and electrical methods. To accelerate the Cu-migration the samples were exposed to various thermal and electrical stresses. These measurements were performed on MIS structures with Cu dots.

The interaction between different barrier metals and the copper surface during the chemical-mechanical polishing

Abstract The copper polishing rate in a weak acid slurry with H 2 O 2 as a oxidizing agent is determined by a two-step process of the formation of copper oxides followed by its mechanical abrasion. The electrochemical potential is a criterion for the driving force of oxidation-reductions reactions that occur during the Cu dissolution process. Using the experiment a corrosion potential ( E corr ) was determined. It depends on the potential of the cathodic reaction, the Tafel slopes of each reaction and the control of the reaction by Tafel kinetics or concentration polarisation. Potentiodynamic measurements were performed with different H 2 O 2 contents in the slurry. The dissolution current density ( i corr ) is correlated to the oxidation-reduction reaction rate. In the presence of a barrier metal, such as WTi, the dissolution rates were influenced by galvanic effects. W and Ti and their compounds were dissolved in the slurry by the complexing ability of H 2 O 2 . There is an increase in the barrier wet etch rate when copper is present. In the case of polishing both TiW and Cu simultaneously a dramatic increase of the barrier polish rate was observed. The acceleration of the W or Ti polish rate can be explained by the galvanic coupling between Cu and the barrier metal. By the use of TiN as a barrier metal no galvanic interaction has been observed. The removal rates of TiN have not been influenced by the presence of Cu.

Barrier behaviour of plasma deposited silicon oxide and nitride against Cu diffusion

Various plasma deposited dielectric films have been studied for their stability against copper diffusion. The copper/dielectric interaction was characterized by Rutherford back scattering (RBS), Auger electron spectroscopy (AES) and secondary ion mass spectroscopy (SIMS) after stressing the samples thermally. Electrical measurements were performed on MIS structures to study the barrier properties of dielectrics. Therefore, I-V, I-t and C-V characteristics after different bias thermal stress (BTS) conditions were compared to those of unstressed Si/dielectric/Cu capacitors. All films investigated showed no interaction with copper during a heat treatment of 500°C for 1 h. However, if an electric field is applied simultaneously to the sample annealing the properties of silicon oxide degrades drastically while silicon oxynitrides and silicon nitride were much less influenced by BTS.

Endpoint detection method for CMP of copper

A novel method to detect the endpoint during Cu-CMP has been developed. It is based on the determination of the Cu concentration within the slurry on the pad that has just polished the wafer. The measurement of the ion concentration is performed using a capillary and an ion-selective electrode. The endpoint of the CMP process is detected by the decrease of Cu ion concentration, which is displayed by an decreased potential at the electrode. An experimental set-up has been established which can be applied to a commercial polishing tool. The method has been tested under various process conditions. The new endpoint detection system revealed to work independently of the polishing tool and the wafer size.