Wafer Curvature Technique Research Articles

Low Stress Polycrystalline SiC Thin Films Suitable for MEMS Applications

This paper details the development of low residual stress and low stress gradient unintentionally doped polycrystalline SiC (poly-SiC) thin films. The films were deposited in a large-volume, low-pressure chemical vapor deposition (LPCVD) furnace on 100 mm-diameter silicon (Si) wafers using dichlorosilane (SiH2Cl2) and acetylene (C2H2) as precursors. We found that the flow rate of SiH2Cl2 could be used to control the residual film stress in the as-deposited films. Wafer curvature measurements for ∼2 μm-thick films indicated that tensile stress ranging from 4 to 55 MPa across a 25 wafer boat had been achieved. A variety of micromachined structures including lateral resonant structures, stress pointers and cantilevers were fabricated for characterization of the deposited SiC films. The average Young’s modulus was found to be 403 GPa. Residual stress measurements were consistent with those obtained using a wafer curvature technique. Interferometric measurements of cantilever beams indicated stress gradients with an upper bound of 52 MPa/μm for ∼2 μm-thick films with tensile stress less than 55 MPa.

In situ growth stresses during the phase separation of immiscible FeCu thin films

This paper addresses the in situ growth stress evolution and post-growth stress relaxation during the phase separation of immiscible Fe 0.51Cu 0.49 thin films at various in situ deposition temperatures. Each film was sputter-deposited onto a 10 nm Si 3N 4 underlayer that was grown on top of Si [0 0 1] substrate at 25 °C, 145 °C, 205 °C, 265 °C or 325 °C. The thin film stress was measured using a wafer curvature technique. The in situ growth stress increased in compression with increasing substrate temperature. The stress relaxation of the Fe 0.51Cu 0.49 was found to have a linear increase with the inverse grain size for films deposited at temperatures greater than 205 °C. The stress state was correlated to the films’ phase and morphology by X-ray diffraction, (scanning) transmission electron microscopy and atomic force microscopy techniques.

Stress and microstructure evolution during growth of magnetron-sputtered low-mobility metal films: Influence of the nucleation conditions

Large tensile stresses (up to 3 GPa) were previously observed in low-mobility metallic Mo 1 − x Si x films grown on amorphous Si and they were ascribed to the densification strain at the amorphous-crystalline transition occurring at a critical film thickness. Here, we focus on the influence of the nucleation conditions on the subsequent stress build-up in sputter-deposited Mo 0.84Si 0.16 alloy films. For this purpose, growth was initiated on various underlayers, including amorphous layers and crystalline templates with different lattice mismatch, and the stress evolution was measured in situ during growth using the wafer curvature technique. Tensile stress evolutions were observed on amorphous SiO 2 and (111) Ni underlayers, similarly to the stress behaviour found on amorphous Si. For these series, the films were characterized by large in-plane grain size (~ 500 nm). However, on a (110) Mo buffer layer, a different stress behaviour occurred: after an initial tensile rise ascribed to coherence stress, a reversal towards a compressive steady state stress was observed. A change in film microstructure was also noticed, the typical grain size being ~ 30 nm. The origin of the compressive stress source in the metastable Mo 0.84Si 0.16 alloy grown on (110) Mo is discussed based on the stress evolutions measured at varying deposition rates and Ar working pressures, as well as in comparison with stress evolutions in pure Mo films.

Mechanics analysis of femtosecond laser-induced blisters produced in thermally grown oxide on Si(100)

Blister features produced by laser-induced delamination of silicon dioxide from silicon substrates were analyzed with thin-film buckling mechanics. These analyses revealed the role of the interaction between the material and the femtosecond (fs)-pulsed laser on blister formation. In particular, it was deduced that the magnitude of the compressive residual film stress within the irradiated region appeared to exceed the intrinsic residual stress obtained from wafer curvature techniques. This apparent increase in the compressive stress after fs-pulsed laser irradiation may be caused by a modification of the oxide, which resulted in a local rarefaction of the film. The results demonstrated important features of the interaction between materials and fs-pulsed laser, including the presence of subtle modification thresholds and the limited role of thermal effects.

In situ deformation of thin films on substrates

Metallic thin-film plasticity has been widely studied by using the difference between the coefficients of thermal expansion of the film and the underlying substrate to induce stress. This approach is commonly known as the wafer curvature technique, based on the Stoney equation, which has shown that thinner films have higher yield stresses. The linear increase of the film strength as a function of the reciprocal film thickness, down to a couple hundred nanometers, has been rationalized in terms of threading and interfacial dislocations. Polycrystalline films also show this kind of dependence when the grain size is larger than or comparable to the film thickness. In situ TEM performed on plan-view or cross-section specimens faithfully reproduces the stress state and the small strain levels seen by the metallic film during wafer curvature experiments and simultaneously follows the change in its microstructure. Although plan-view experiments are restricted to thinner films, cross-sectional samples where the film is reduced to a strip (or nanowire) on its substrate are a more versatile configuration. In situ thermal cycling experiments revealed that the dislocation/interface interaction could be either attractive or repulsive depending on the interfacial structure. Incoherent interfaces clearly act as dislocation sinks, resulting in a dislocation density drop during thermal cycles. In dislocation-depleted films (initially thin or annealed), grain boundaries can compensate for the absence of dislocations by either shearing the film similarly to threading dislocations or through fast diffusion processes. Conversely, dislocations are confined inside the film by image forces in the cases of epitaxial interfaces on hard substrates. To increase the amount of strain seen by a film, and to decouple the effects of stress and temperature, compliant substrates can also be used as support for the metallic film. The composite can be stretched at a given temperature using heating/cooling straining holders. Other in situ TEM methods that served to reveal scaling effects are also reviewed. Finally, an alternate method, based on a novel bending holder that can stretch metallic films on rigid substrates, is presented.

A Novel Technique to Determine Elastic Constants of Thin Films

AbstractA new X-ray diffraction technique to determine elastic moduli of polycrystalline thin films deposited on monocrystalline substrates is demonstrated. The technique is based on the combination of sin2ψ and X-ray diffraction wafer curvature techniques which are used to characterize X-ray elastic strains and macroscopic stress in thin film. The strain measurements must be performed for various hkl reflections. The stresses are determined from the substrate curvature applying the Stoney's equation. The stress and strain values are used to calculate hkl reflection dependent X-ray elastic moduli. The mechanical elastic moduli can be then extrapolated from X-ray elastic moduli considering film macroscopic elastic anisotropy. The derived approach shows for which reflection and corresponding value of the X-ray anisotropic factor Γ the X-ray elastic moduli are equal to their mechanical counterparts in the case of fibre textured cubic polycrystalline aggregates. The approach is independent of the crystal elastic anisotropy and depends on the fibre texture type, the texture sharpness, the amount of randomly oriented crystallites and on the supposed grain interaction model. The new method is demonstrated on a fiber textured Cu thin film deposited on monocrystalline Si(100) substrate. The advantage of the new technique remains in the fact that moduli are determined non-destructively, using a static diffraction experiment and represent volume averaged quantities.

Stress evolution in CrN/Cr coating systems during thermal straining

CrN, Cr and CrN/Cr coatings were deposited at 350°C on monocrystalline Si(100) and polycrystalline austenitic stainless steel substrates by magnetron sputtering using Cr targets in Ar + N 2 atmosphere. The stress evolution in the coating systems were characterized using X-ray diffraction and wafer curvature technique in the temperature range of 25–550°C. Both techniques revealed larger stresses in coatings deposited on the steel substrates. The heat treatment reduces the deposition point defect concentration, which is reflected in a decrease of intrinsic stresses in Cr and CrN coatings. Additionally, roughness of the Cr films decreases. The stresses in the individual sublayers of the CrN/Cr bilayer coatings indicate that the constraint imparted by the CrN layer on the buried Cr layer prevents a stress relaxation in Cr since no stress hysteresis is observed during heating and cooling. The intrinsic stresses of CrN are − 3.4 ⁎ 10 9Pa on the steel substrate and − 1.7 ⁎ 10 9Pa on the Si(100) substrate. For Cr intrinsic stresses of − 1.35 ⁎ 10 9Pa are obtained on steel and − 0.7 ⁎ 10 9Pa on Si(100). The intrinsic stresses of CrN in the CrN/Cr bilayer system remains at − 3.4 ⁎ 10 9Pa on the steel but increases to − 2.8 ⁎ 10 9Pa on the Si(100) substrate. As a result of the annealing cycle, a stress relaxation of approximately 1.3 ⁎ 10 9Pa is obtained for CrN on steel but 3.4 ⁎ 10 9Pa are relaxed for CrN in the CrN/Cr/steel system.

Mechanical properties of superhard TiB 2 coatings prepared by DC magnetron sputtering

Superhard titanium diboride (TiB 2) coatings ( H v> 40 GPa) were deposited in Ar atmosphere from stoichiometric TiB 2 target using an unbalanced direct current (d. c.) magnetron. Polished Si (0 0 1), stainless steel, high-speed steel (HSS) and tungsten carbide (WC) substrates were used for deposition. The influence of negative substrate bias, U s, and substrate temperature, T s, on mechanical properties of TiB 2 coatings was studied. X-ray diffraction (XRD) analysis showed hexagonal TiB 2 structure with (0 0 01) preferred orientation. The texture of TiB 2 coatings was dependent upon the ion bombardment ( U s increased from 0 to −300 V) and the substrate heating ( T s increased from room temperature (RT) to 700 °C). All TiB 2 coatings were measured using microhardness tester Fischerscope H100 equipped with Vickers and Berkovich diamond indenters and exhibited high values of hardness H v up to 34 GPa, effective Young's modulus E*=E/(1 –ν) ranging from 450 to 600 GPa; here E and ν are the Young's modulus and Poisson's ratio, respectively, and elastic recovery W e≈80%. TiB 2 coating with a maximum hardness H v≈73 GPa and E*≈580 GPa was sputtered at U s=−200 V and T s=RT. Macrostresses of coatings σ were measured by an optical wafer curvature technique and evaluated by Stoney equation. All TiB 2 coatings exhibited compressive macrostresses.

Surface stress induced in Cu foils during and after low energy ion bombardment

We present measurements of the stress induced by low energy Ar ion bombardment on Cu foils using a wafer curvature technique. The observed stress is found to be on the order of a few GPa, assuming a range for the Ar in Cu of several nanometers. At room temperature, a compressive stress is observed to build up on the surface during sputtering. The stress relaxes after the beam is turned off, leaving a residual tensile stress in the foil. To determine the mechanisms controlling the stress evolution, we have measured the temperature and flux dependence of the induced stress. At elevated temperature (∼100–200°C), both the compressive and tensile stresses are reduced. The observed temperature and flux dependence is discussed in terms of the different kinetics processes (ion incorporation, defect generation, stress relaxation, etc.) occurring in the film.

Mechanical Behavior and Microstructure of Nanoporous Gold Films

ABSTRACTNanoporous gold (NPG) thin films offer an opportunity to investigate the effects of nanoscale geometric confinement on the mechanical properties of metals. In the present study, NPG films supported by substrates were fabricated by dealloying Au-Ag films on Kapton and silicon. The microstructural evolution of NPG at various stages of dealloying was observed and analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Cracking occurred at grain boundaries during dealloying and is believed to result from pre-existing tensile stress in the film. Stress evolution of NPG films on silicon substrates during dealloying was measured with the wafer curvature technique and revealed an overall shift toward compressive stress.

Texture evolution in Copper film at high temperature studied in situ by electron back-scatter diffraction

Changes in texture and microstructure during the thermal treatment of Cu films have been studied in situ using electron back-scatter diffraction (EBSD). A partially recrystallized Cu film which still had its microstructure evolving at room temperature was investigated using orientation imaging microscopy. Two separate investigations were conducted—the first one at different locations of the film and at different temperatures and a second one at the same location of the film and at different temperatures. The orientation of the (111), (110) and (100) grains within the plane of Cu film was investigated from the orientation distribution functions. There was an increased tendency of the (111) and (110) grains to form either {111}<112> or {111}<110> and {110}<100> texture respectively at higher temperature. The impact of elastic strain energies and dislocation glide in formation of these textures at higher temperatures has been analyzed in the light of some recent observations reported in literature. The variation in the area fraction of different fiber texture components, as a function of temperature, has been discussed in correlation with the measured mean grain size, grain boundary misorientation distribution and stress states. Stress state during the entire thermal cycle was monitored by wafer curvature technique and the traces of additive impurities at the surface were measured using X-ray photoelectron spectrometry. The possible role of impurities in affecting the behavior of texture components at high temperature is discussed. Comparison was made between the EBSD and X-ray diffraction texture data.

Temperature dependence of the mechanical properties of tetrahedrally coordinated amorphous carbon thin films

The complete elastic properties of tetrahedrally coordinated amorphous carbon (ta-C) thin films have been measured in the temperature range of 300–873K. Flexural and torsional mechanical oscillators were fabricated from ta-C, and using the resonant frequency of the oscillators as a function of temperature, we calculated the temperature-dependent Young’s and shear moduli (658±24 and 271±6.6GPa, at 300K, respectively). From these values, we calculated the bulk modulus, Poisson’s ratio, and the elastic stiffness and compliance constants as a function of temperature. In addition, the temperature dependence of the coefficient of thermal expansion of ta-C was determined using a wafer curvature technique.

Biaxial initial stress characterization of bilayer gold RF-switches

An analysis as been conducted to determine the biaxial initial stress state of gold bilayer switches. Results are shown that the sensitivity of the sacrificial photoresist layer to process parameters make the wafer curvature technique unreliable to determine the initial stress state of the evaporated gold seed layer. An analytical method based on the cantilever deflection method is proposed to determine the biaxial stress state on this layer. Assumptions were validated numerically using FEM and cantilevers gold bilayer of various length were elaborated and characterized.

Thermal Cycling Effects on Critical Adhesion Energy and Residual Stress in Benzocyclobutene-Bonded Wafers

The effects of thermal cycling on critical adhesion energy and residual stress at the interface between benzocyclobutene (BCB) and silicon dioxide coated silicon wafers were evaluated by four-point bending and wafer curvature techniques. Wafers were bonded using BCB in an established (baseline) process, and the films were deposited by plasma-enhanced chemical vapor deposition (PECVD). Thermal cycling was done between room temperature and a peak temperature. In thermal cycling performed with 350 and peak temperatures, the critical adhesion energy increased significantly during the first thermal cycle. The increase in critical adhesion energy is attributed to relaxation of residual stress in PECVD , which in turn is attributed to condensation reactions in those films. Thermal cycling also cures the BCB beyond the achieved in the baseline process, and the residual stress in the BCB is reset at a glass transition temperature corresponding to the increased BCB cure conversion. As more thermal cycles are performed, stress hysteresis in the BCB decreases as the cure stabilizes at 94-95%.

Hillock formation and thermal stresses in thin Au films on Si substrates

AbstractThe wafer curvature technique was used to analyze stresses in fine-grained, 50 nm to 2 μm thick Au films on silicon substrates between room temperature and 500°C. The microstructural evolution was analyzed by scanning electron microscopy (SEM), focused ion beam (FIB) microscopy and transmission electron microscopy (TEM). In situ heating experiments inside a scanning electron microscope provided a comparison between the morphological development and the stress-temperature behavior of the film. Hillock formation was observed, but it can only partially account for the stress relaxation measured by the wafer curvature technique.

Mechanical behavior of Pt and Pt–Ru solid solution alloy thin films

The mechanical behavior of Pt and Pt–Ru solid solution alloy thin films was evaluated using wafer curvature and nanoindentation techniques. Films approximately 200 nm thick with nominal compositions of 0, 1, 2, 5, 10, and 20 wt% Ru were prepared by co-sputtering on oxidized Si wafers. For temperatures below approximately 500 °C, at which dislocation motion dominates behavior, the elastic modulus, flow stress, and hardness all increase with increasing Ru content. These experimental results were compared to flow stress calculations that are based on existing models for dislocation motion. This analysis indicates that the solid solution strengthening mechanism is a major contributor to the film strengthening. The critical temperature above which diffusion-controlled plasticity dominates is not significantly affected by solute content. Above this temperature, the degree of stress relaxation was dependent on Ru content, which could be attributed to a limit on diffusional relaxation.

Metallization schemes for radio frequency microelectromechanical system switches

A series of surface micromachined microelectromechanical system switches with composite metal beams were fabricated by standard photolithographic techniques. The study was conducted in order to assess the influence of film stress and composition on the released shape of cantilever and fixed-fixed beam structures. A 1 μm thick evaporated Au film was the basis for all bridge materials with additional 20 nm layers of evaporated or sputter deposited Ti, Pt, W, or Au on the top or bottom surface of the thick Au. The planarity and stress gradient of cantilever beam structures and the planarity of fixed-fixed beam structures were measured by optical interferometry. Gold-only bridge structures displayed the best planarity of those examined while structures with Ti layers displayed the least planarity. Cantilever stress gradients calculated using both cantilever-tip deflection and radius of curvature techniques were typically between 9 and 16 MPa/μm. The thin film biaxial moduli used in stress gradient calculations were measured using a wafer curvature technique and were slightly higher than the bulk Au value. Results of this study show that thin metal layers (2%–6% of total beam thickness) have substantial influence on released beam curvatures but that beam planarity can be achieved with a suitable combination of materials.

Effect of carrier gas on the deposition of titanium carbo-nitride coatings by a novel organo-metallic plasma immersion ion processing technique

Using a novel organo-metallic chemical vapor deposition plasma immersion ion processing technique, titanium carbo-nitride (Ti–C–N) coatings were deposited from an organo-metallic CVD precursor, tetrakis-dimethylamino titanium. Different carrier gases (acetylene, ammonia, argon and nitrogen) were made to flow through the precursor reservoir at 20 sccm so that the coating composition could be modified. Deposited coatings had thickness in the range of 300–1000 nm as determined by surface profilometry. Titanium concentrations in the coatings ranged from 10.8 to 15.1 at%. Hardness values of these coatings ranged from ≈8–14 GPa while residual stresses were in the range from −440 to −100 MPa, as determined by nanoindentation and wafer curvature techniques, respectively. Coefficient of friction values ranged from 0.2 to 0.45, and were lowest for coatings with low titanium content. Hardness, coefficient of friction and residual stress increased with increasing titanium content. Wear resistance was determined by a pin-on-disk technique and wear coefficients ranged from 0.005×10 −6 to 2×10 −6 mm 3/N m, increasing with increasing hardness indicating an abrasive wear mechanism.

Constrained diffusional creep in UHV-produced copper thin films

The thermomechanical behavior of metallic thin films on stiff substrates is relevant for thin-film devices, but its mechanisms are not fully understood. In this investigation, the mechanical properties of pure Cu and Cu–1 at.% Al films on diffusion-barrier coated Si substrates were studied with the wafer-curvature technique. In ultra-pure films, which were sputtered and annealed under ultra-high vacuum conditions, characteristic stress relaxation at high temperatures was measured, which could be clearly attributed to diffusional creep. Good quantitative agreement with a recent model of diffusional creep constrained by a substrate was obtained. These features were absent in the Cu–Al alloy films, in which Al surface segregation and oxidation had produced a “self-passivating” effect, and in films produced in less clean environments. Based on these results, we propose a model of thin-film deformation based on dislocation glide and constrained diffusional creep.

Residual Stress and Microstructure of Electroplated Cu Film on Different Barrier Layers

ABSTRACTCopper films of different thicknesses between 0.2 and 2 microns were electroplated on adhesion-promoting TiW and Ta barrier layers on &lt;100&gt; single crystal 6-inch silicon wafers. The residual stress was measured after each processing step using a wafer curvature technique employing Stoney's equation. Large gradients in the stress distributions were found across each wafer. Controlled Cu grain growth was achieved by annealing films at 350 C for 3 minutes in high vacuum. Annealing increased the average tensile residual stress by about 200 MPa for all the films, which is in agreement with stress-temperature cycling measurements.After aging for 1 year wafer stress mapping showed that the stress gradients in the copper films were alleviated. No stress discrepancies between the copper on Ta and TiW barrier layers could be found. However, X-ray pole figure analysis showed broad and shifted (111) texture in films on a TiW underlayer, whereas the (111) texture in Cu films on Ta is sharp and centered.