C. Wenzel

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...

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.

Study of Ta-Si-N thin films for use as barrier layer in copper metallizations

Abstract This work focuses on the deposition process, microanalytical characterization and barrier behaviour of 10–100-nm thick sputtered Ta–Si and Ta–Si–N films. Pure Ta–Si films were found to be already nanocrystalline. The addition of N 2 leads to a further grain fining resulting in amorphous films with excellent thermal stability. According to microanalytical investigations, Ta–Si barriers between Cu and Si with a thickness of only 10 nm are not stable at 600°C. Copper silicides are formed due to intensive Cu diffusion throughout the barrier. In contrast, 10-nm thick nitrogen-rich Ta–Si–N barriers remain thermally stable during annealing at 600°C and protect the Si wafer from Cu indiffusion.

Influence of N content on microstructure and thermal stability of Ta–N thin films for Cu interconnection

Abstract Structural properties of thin Ta and Ta–N films acting as diffusion barriers were investigated. Blanket Ta-based films of 10 nm thickness were deposited by conventional sputtering techniques onto (100)-Si and covered with a Cu cap layer. X-ray diffraction, depth profile analysis and electron microscopy were used to correlate results of microstructure and phase characterization with diffusion phenomena. Different barrier failure mechanisms were observed after annealing at temperatures between 450 and 800 °C. Ta and Cu silicides were formed suddenly in layer stacks with pure Ta and Ta–20 at.% N barrier films at 550 °C. The application of the stoichiometric TaN as a diffusion barrier prevents the formation of Ta silicides and does not lead to significant Cu silicide formation up to 800 °C. However, trace Cu diffusion into the substrate was also detected at lower temperatures. The barrier stability against Cu diffusion is improving with increasing N content.

Fabrication of a 3D differential-capacitive acceleration sensor by UV-LIGA

Abstract A novel three-dimensional (3D) acceleration sensor has been fabricated by combining the low-cost UV-LIGA surface microstructuring process with a sacrificial layer technique. It detects the triaxial accelerations using three independent, yet on a common substrate-integrated, sensor elements. Each element is configured as a differential capacitor with its movable seismic mass as the middle electrode. The fabrication is a simple planar batch procedure comprising only a few processing steps. The entire structures are first grown electrochemically within the UV-patterned thick AZ4562 photoresist on an electroplating base that is composed of rigid (Cu) and sacrificial (Ti) layers. Movable Ni-parts are then obtained by removing the underneath Ti sacrificial layer using wet etching. Sensor structures up to 30 μm with an aspect ratio of about 10:1 can be reliably manufactured. It is thought that this fabrication approach can be widely applied to economically realise other micromechanical components with oscillating structures.

Optical and structural characteristics of Ga-doped ZnO films

The n-ZnO films doped with Ga to the content 2.5 at % are produced by pulse laser deposition onto the (0001) oriented single crystal sapphire substrates. The transmittance spectra of the ZnO films in the range from 200 to 3200 nm are studied in relation to the Ga dopant content. It is established that an increase in the Ga content shifts the fundamental absorption edge to the blue region, but reduces the transparency of the ZnO films in the infrared spectral region. The dependence of the band gap on the level of doping with Ga is determined. The photoluminescence spectra of the ZnO films doped to different levels are recorded. It is established that the PL intensity and peak position vary unsteadily with the level of doping. X-ray diffraction studies of the structure of the films are carried out. It is found that the crystallographic parameters (the lattice constant c) of the ZnO film depend on the Ga dopant content and the conditions of deposition of the films.

Study of nanocrystalline Ta(N,O) diffusion barriers for use in Cu metallization

In this study, the film properties and the barrier behaviour of 50 nm thin reactively sputtered Ta(N,O) diffusion barriers between Cu and Si were investigated. Auger electron microscopy, glancing-angle x-ray diffractometry and atomic force microscopy revealed that the Ta(N,O) films exhibit quasi-amorphous/nanocrystalline properties. Using sheet resistance measurements, scanning electron microscopy, Auger electron spectroscopy depth profiling and conventional x-ray diffractometry the 50 nm thin Ta(N,O) films were found to be effective diffusion barriers between Cu overlayers and Si substrates even after 1 h annealing at 600°C.

One-mask procedure for the fabrication of movable high-aspect-ratio 3D microstructures

This paper describes a unique surface microfabrication procedure for cost-effective manufacture of free-standing, high-aspect-ratio micromechanical structures for sensor and actuator applications. This enables microstructures to be manufactured from a two-layer system with only one standard optical mask. The entire microstructures are grown electrochemically within a patterned thick (up to m) resist template on an electroplating base layer. The movable parts are subsequently obtained by removing the underneath base layer using a current-controlled electrochemical etching approach. The novel one-mask procedure involves only a few processing steps, promising low-cost, high-yield production. The realization of a lateral sensitive acceleration sensor is taken here as an example to illustrate this fabrication procedure.

UV-LIGA: a promising and low-cost variant for microsystem technology

A low-cost surface microstructuring technique is described and examples provided of its recent application. The new technology, UV-LIGA combines depth UV lithographic patterning of very thick photoresists and electrodepositing of structural materials into the patterned resist moulds. Both UV lithography and electrodeposition do not need any expensive or special equipment. Movable three dimensional microstructures with a high aspect ratio can be obtained by combining the UV-LIGA process with a sacrificial layer technique. As an example of the advanced application in microsystems, fabrication of 3D accelerometers has been presented, using titanium as a sacrificial layer and nickel as the structural material.