Intrinsic Film Stress Research Articles

Evaluation of Air Gap structures produced by wet etch of sacrificial dielectrics: Critical processes and reliability of Air Gap formation

Two Air Gap technologies were investigated concerning critical process steps. Both approaches use SiO 2 for sacrificial material and buffered HF wet etch chemistry. These critical processes include pre-wet-etch-concerns and wet-etch-concerns. The results of a special spacer etch-back process are shown. A buffer layer of SiO 2 was introduced to relax the requirements on the dry back-etch process. The oxidation of SiC and SiCN films during dry etching and resist stripping is an issue of both technologies, because this may lead to an undercut of the interconnect lines during the buffered HF treatment. Nevertheless, this can be successfully avoided by the application of appropriate oxygen (O 2) free process media. Furthermore, the shifting of mechanical behaviour of such structures as a result of wet-etch treatment is investigated. The intrinsic stress of cantilever SiC films has the capability to cause pull-off forces to interfaces which may result in film delamination.

A robust k ∼ 2.3 SiCOH low- k film formed by porogen removal with UV-cure

Aurora ®ELK films were fabricated by PE-CVD of a SiCOH matrix precursor and an organic porogen material. The porogen material is removed during a subsequent thermally assisted UV-cure step with a short wavelength UV-lamp ( λ < 200 nm). This results in film thickness shrinkage of 13.2% and a robust low- k film with k-value ∼ 2.3, elastic modulus ∼5.0 GPa and intrinsic film stress ∼59 MPa. The microscopic film properties during UV-cure were evaluated by FT-IR. A decrease in the CH x peak area is related to the porogen removal from the film resulting in a reduced dielectric constant. The decrease of the Si–CH 3 peak and increase in the SiO network area are associated to the network restructuring and increase in elastic modulus. The nature of the Si–H peaks which appear during UV-cure has to be investigated carefully to determine their impact on film reliability. The dielectric diffusion barrier can work as an UV absorption layer which reduces UV-curing of underlying layers and possible UV reflections on interfaces. The SiCN/A-SiCO diffusion barrier film properties during UV-cure show a decrease in k-value, increase in intrinsic film stress and a slight increase in leakage. More research is needed to evaluate the impact of porogen removal by UV-cure on BEOL integration.

Effect of high temperature annealing on the electrical performance of titanium/platinum thin films

In this study, the influence of post deposition annealing steps (PDA) on the electrical resistivity of evaporated titanium/platinum thin films on thermally oxidised silicon is investigated. Varying parameters are the impact of thermal loading with maximum temperatures up to T PDA = 700 °C and the platinum top layer thickness ranging from 24 nm to 105 nm. The titanium based adhesive film thickness is fixed to 10 nm. Up to post deposition annealing temperatures of T PDA = 450 °C, the film resistivity is linearly correlated with the reciprocal value of the platinum film thickness according to the size effect. Modifications in the intrinsic film stress strongly influence the electrical material parameter in this temperature regime. At T PDA > 600 °C, diffusion of titanium into the platinum top layer and its plastic deformation dominate the electrical behaviour, both causing an increase in film resistivity above average.

Optical and mechanical properties of vacuum evaporated vapour chopped polyaniline thin film

The paper reports the effect of chopping the vapour flow on properties of vacuum evaporated polyaniline thin films synthesized by aqueous polymerization pathway. The chopper was a metallic vane of V-shaped cut out placed between the substrate and boat in the path of evaporated vapour. It interrupted the flow of vapour at a constant rate. Fourier Transform Infra-red (FTIR) studies indicated that the vacuum evaporated films are more in reduced form and contain short chain oligomers. Improved adhesion and reduced intrinsic stress of polyaniline thin film due to chopping are obtained. Higher transmittance and lower refractive index films resulted due to the process of chopping as compared to the deposited films. Chopping also produces smoother surface morphology.

Nanoparticulate origin of intrinsic residual stress in thin films

The formation of grains in thin films generates intrinsic residual stress. In this work, we present a model of intrinsic residual stress calculation based on the size-dependent phase transitions of the nanograins. Evaporated thin films are produced by condensation from the vapor on the substrate. It is assumed that the starting nanograins grow from the liquid phase. It is well established that the melting temperature of nanoparticles is a function of their size. By assuming that the intrinsic stress originates from the volume change of the nanograins, and taking into account relaxation processes, the generated intrinsic residual stress in the films is evaluated. The results of the model are compared quantitatively with experimental data obtained from Ta, Mo, Pd and Al films deposited on Si. This model also gives a theoretical interpretation of Thornton and Hoffman’s modelling of the stress-temperature diagram of thin films.

Synthesis and Intrinsic Stress of Ti-Containing Carbon Nitride Nanocomposite Films

Titanium containing carbon nitride (CNx) films are prepared by radio frequency magnetron sputtering method. The evolution of intrinsic stress within the nanocomposite films is monitored during growth by using an in situ bending-plate method. The effect of Ti-containing concentration on intrinsic stress is investigated. XRD data shows that TiN nanocrystals are synthesized to embed into the CNx matrix. The film intrinsic stress depends obviously on the content of Ti. We suggest that grain boundary effect is responsible to the intrinsic stress variation as a function of the content of Ti.

The effect of deposition rate on the intrinsic stress in copper and silver thin films

The effect of changing the deposition rate on the development of stress in evaporated copper and silver thin films deposited on oxidized silicon was examined. In situ stress measurements were made during deposition in ultrahigh vacuum using a scanning laser curvature system. In some experiments, the deposition rate was alternated without interruption of deposition. For copper thin films, a change in deposition rate has no effect on the development of the tensile stress, while the magnitude of the postcoalescence compressive stress decreases with increasing deposition rate. In silver films, the film thickness at the tensile maximum increases slightly with increasing deposition rate, while the magnitude of the postcoalescence compressive stress again decreases with increasing deposition rate. Analysis of the heat flow during deposition shows that the radiative heating and condensation contribute roughly equally to the temperature rise of the sample.

Intrinsic, thermal and hygroscopic residual stresses in thin gas-barrier films on polymer substrates

Intrinsic, thermal, and hygroscopic contributions to the in-plane residual stress in silicon nitride films on polyimide substrates are identified, based on iso-hygric thermal ramps and isothermal relative humidity jumps, combined with non-linear elastic modeling of the resulting dynamics of film curvature. This approach enables the thermal and hygroscopic properties of thin nitride films to be determined and provides useful input for material and process control.

Correlation of thermal stability of the mechanical and optical properties of diamond-like carbon films

The aim of the present contribution is the study of the influence of the annealing on the mechanical and optical properties of the plasma deposited diamond-like carbon (DLC) films. The DLC films were prepared for different values of hydrogen content from the mixture of methane and hydrogen in capacitively coupled PECVD reactor. Thermal stability of the films was investigated using sample annealing. The optical properties of the as deposited and annealed DLC films were studied ex situ using variable angle spectroscopic ellipsometry and spectroscopic reflectometry applied within the near-UV, visible and near-IR regions. For optical constants our dispersion model based on parameterization of the densities of electronic states was used. The dependence of the hardness and the elastic modulus on the hydrogen content in the deposition feed was determined using depth sensing indentation technique. It was found that with increasing temperature the sp 3-to-sp 2 ratio, atomic fraction of hydrogen, hardness and elastic modulus decreased. For the samples prepared with relatively high hydrogen flow rate the annealing helped to minimize the film intrinsic stress. Moreover, these films exhibited free-carrier anisotropy. This fact was respected by including the Drude absorption term into the dispersion model.

Stress evolution in nanocrystalline diamond films produced by chemical vapor deposition

Nanocrystalline diamond films were grown on silicon substrates by microwave plasma enhanced chemical vapor deposition with 1% methane, 2%–10% hydrogen, and argon. High resolution transmission electron microscope images and selected area electron diffraction patterns confirm that the films consist of 10–20nm sized diamond grains. The residual and intrinsic stresses were investigated using wafer curvature. Intrinsic stresses were always tensile, with higher H2 concentrations generally leading to higher stresses. Annealing the films in a hydrogen plasma significantly increased these stresses. These hydrogen induced changes also appear to alter stress levels and stress gradients during the growth process itself. Raman spectra revealed subtle changes in the chemical bonding that were correlated with some of the stress variations. These results suggest that grain boundary bonding and hydrogen induced reactions at the grain boundaries can influence the intrinsic stresses in nanocrystalline diamond films.

Raman spectroscopy study of DLC films prepared by RF plasma and filtered cathodic arc

The build-up of intrinsic stress in carbon thin films deposited by vapour deposition can be a major cause of delamination. Arguably, this issue has been one of the main reasons why the industrial exploitation of carbon vapour deposited films has so far been of limited success. In the present study we deposited single and multilayer thin films of carbon and found that under certain deposition conditions, we were able to produce thin films free from delamination. Single and multilayer of films Diamond-like Carbon (DLC) were prepared by Plasma Immersion Ion Implantation and Deposition (PIIID) from two deposition systems: (1) Radio-Frequency (RF) plasma and (2) Filtered Cathodic Vacuum Arc (FCVA). Raman spectroscopy revealed a shift in the peaks previously identified as D and G band in the structures. The sp 2 and sp 3 ratio contents were characterized by Raman spectroscopy.

Influence of thermal annealing on the resistivity of titanium/platinum thin films

In this study, the authors investigate the influence of thermal annealing up to temperatures of 700°C on the room-temperature resistivity of electron-beam-evaporated titanium/platinum thin films. Oxidized silicon wafers are used as the substrate. The titanium has a fixed thickness of 5nm and serves as an adhesion layer. The thickness of the platinum top is varied between 21 and 97nm. Up to annealing temperatures of 450°C, the film resistivity of the bilayer system is a linear correlation with the reciprocal platinum film thickness, as expected from the size effect. In this annealing regime, the change in intrinsic film stress dominates the electrical behavior. At annealing temperatures of 600°C and above, however, the diffusion of titanium into the top layer leads to an enhanced increase in film resistivity, especially at low platinum thicknesses. The onset of plastic deformation of the platinum layer causes an additional effect that decreases the electrical conductivity above average.

The Preparation of Piezoelectric ZnO Films by RF Magnetron Sputtering for Layered Surface Acoustic Wave Device Applications

Crystalline structures, intrinsic stress, and surface roughness of piezoelectric ZnO films deposited on sapphire substrates by rf magnetron sputtering and sequential post-annealing treatments were investigated by X-ray diffraction (XRD) and atomic force microscopy (AFM) measurements. The rf power used and O2/(Ar+O2) gas ratio greatly affected the crystallinity and surface roughness of the ZnO film. A c-axis preferred orientation with superior surface roughness was achieved at a specific rf power and O2/(Ar+O2) gas flow ratio. In addition, the original intrinsic stress was also markedly released (stress=0.325 ×1010 dyn/cm2) and a uniform surface (Ra=2.42 nm) was produced by a post-annealing treatment at a temperature of 400 °C under oxygen ambient. An acoustic wave at the center frequency of 240 MHz (hZnO/λ=0.1) was obtained from the ZnO/sapphire-layered surface acoustic wave (SAW) device using this optimized ZnO film.

Adhesion studies of diamond-like carbon films deposited on Ti6Al4V substrate with a silicon interlayer

Diamond-like carbon (DLC) films have proven quite advantageous in many tribological applications due to their low friction coefficient, their extreme hardness, and more recently their high adherence on different substrate materials. However, for many applications, DLC films as thick as 2 μm are required, which cause high residual stress. In order to overcome this problem, this study observed the behavior of different thicknesses of silicon interlayer between DLC films and Ti6Al4V substrates. The study also analyzed the relation of growth parameters to the mechanical properties of DLC films. Silicon and DLC films were grown by using a rf-PECVD at 13.56 MHz with silane and methane atmospheres, respectively. The contribution of an interlayer thickness to the adhesion between the DLC films and Ti6Al4V substrate was evaluated by using a micro-scratch technique. The hardness and friction coefficient were evaluated by using microindentation and lateral force microscopy (LFM), respectively. Raman scattering spectroscopy was used to characterize the film quality. A correlation was found between the intrinsic stress and adhesion of DLC film and the parameters of the silicon interlayer growth. The addition of a silicon interlayer successfully reduced intrinsic stress of the films, even as measured by using a perfilometry technique.

Dual frequency PECVD silicon nitride for fabrication of CMUTs’ membranes

Dual frequency plasma-enhanced chemical-vapour deposition (DF-PECVD) of silicon nitride has been investigated for the fabrication of membranes for micromechanical applications, and in particular for capacitive micromachined ultrasonic transducers (CMUTs). The use of high and low frequency plasma excitations, at 13.56MHz and 340KHz, respectively, allows to adjust the intrinsic stress of the thin silicon nitride films, varying the ratio between the time intervals during which the two RF power supplies are active in one cycle during the deposition process. In addition, silicon nitride films with high compactness and homogeneity, high resistivity, low porosity and conformally deposited on patterned substrates have been obtained. The optimized DF-PECVD silicon nitride has been used as structural layer and protection layer of the CMUTs.

Residual stress control in MoCr thin films deposited by ionized magnetron sputtering

This study reports on the control of intrinsic stress in MoCr films deposited by ionized magnetron sputtering using a secondary plasma created through a radio-frequency (13.56 MHz) antenna. The efficiency of the additional antenna has been investigated by optical emission spectroscopy for a MoCr target. It appears that ionization of the sputtered species is significantly enhanced compared to a conventional magnetron sputtering. Moreover, by controlling the substrate bias, the RF power on the antenna and the discharge gas pressure (argon), the ion energy and flux can be strongly modified. Therefore, we have the ability to act on composition, growth rate, microstructure and residual stress of the deposited film. Indeed, the films exhibit residual stress varying from compressive value (− 3.4 GPa) to tensile one (1.7 GPa). In particular, only by varying the RF power on the antenna, a wide range of tensile and compressive stress can be obtained without affecting the chemical composition of the films. These results will be useful when elaborating stress-engineered micro-objects such as micro-springs.

Control of stress and delamination in single and multi-layer carbon thin films prepared by cathodic arc and RF plasma deposition and implantation

The build-up of intrinsic stress in carbon thin films deposited by vapour deposition is a major cause of delamination. The delamination issue is one of the main reasons preventing wider applications of carbon vapour deposited films. In this work, we studied single and multilayer thin films of carbon and found that under certain deposition conditions, we were able to produce thin films free from delamination. Furthermore, we were able to stop film delaminating by adding a control layer. Two methods were used to deposit the carbon films: (1) filtered cathodic arc deposition with both negative DC and pulsed bias applied to the substrate and (2) RF plasma CVD with negative pulse bias up to 30 kV, 60 Hz, 100 μs. Both single and multilayer structures of carbon with different sp 2 and sp 3 ratios were deposited and it was found that, under controlled conditions, the overall stress was maintained at acceptable levels. Moreover, it was found that an additional control layer stopped ongoing delamination of the film. The structures of these multilayers were studied using Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM).

Role of film intrinsic stress in packaging of multi-layer microelectronic structures

Internal stress due to material CTE and Young's modulus mismatch in microelectronic and micro-electromechanical system (MEMS) structures is critical to their reliability. It is well understood and hence its impact is generally controllable. However, the film intrinsic stress develops as a function of processing. The level of stress and how it changes with additional processing or environmental exposure is still not well understood. This stress is typically neglected in response prediction as its magnitude, behavior and response impact are not well known. Intrinsic film stress can play a substantial role in enhancing or degrading the response of multi-layer structures. Intrinsic stress can cause failures or can mitigate catastrophic failures. Thin film intrinsic stress is processing-related so, it may be controlled by modifying processing parameters. In addition, these stresses are also dependent on film size and thickness. These requirements point to a need for a measurement and a response prediction capability. The measurement capability should be capable of addressing the typical size and thickness seen in actual devices and MEMS structures. In this paper, an approach for measuring intrinsic film stresses in small features on microelectronic and MEMS devices, using a hybrid experimental-cum-numerical approach is authored. Two microelectronic applications are given to highlight the importance of intrinsic stress on device performance.

Laser micromachining of sputtered DLC films

DLC films with different thicknesses (from 100 nm to 1.9 μm) were deposited using sputtering of graphite target in pure argon atmosphere without substrate heating. Film microstructures (sp2/sp3 ratio) and mechanical properties (modulus, hardness, stress) were characterized as a function of film thickness. A thin layer of aluminum about 60 nm was deposited on the DLC film surface. Laser micromachining of Al/DLC layer was performed to form microcantilever structures, which were released using a reactive ion etching system with SF6 plasma. Due to the intrinsic stress in DLC films and bimorph Al/DLC structure, the microcantilevers bent up with different curvatures. For DLC film of 100 nm thick, the cantilever even formed microtubes. The relationship between the bimorph beam bending and DLC film properties (such as stress, modulus, etc.) were discussed in details.

Residual stress in amorphous and nanocrystalline Si films prepared by PECVD with hydrogen dilution

Amorphous and nanocrystalline Si films were prepared using radio-frequency (13.56 MHz) plasma enhanced chemical vapour deposition with different SiH 4/H 2 ratios. The film residual stress was measured using curvature method as a function of SiH 4 gas flow ratio (SiH 4/(SiH 4 + H 2)). Results from high resolution scanning electron microscopy, X-ray diffraction and Raman scattering studies revealed the appearance of nanocrystallites with SiH 4 gas ratio less than 3%. With a gradual decrease of SiH 4 gas ratio, the film intrinsic stress increased significantly until SiH 4 gas ratio reached 2%. Below that critical value, the film intrinsic stress decreased sharply. Different mechanisms of stress formation and relaxation during film growth were discussed, including ion bombardment effect, hydrogen and hydrogen induced bond-reconstruction, nanocomposite effects (nanocrystal embedded in an amorphous Si matrix). Results indicated that nanocomposite effect is the dominant factor in this maximum stress condition.