Wafer Curvature Measurements Research Articles

Stress buildup during crystallization of thin chalcogenide films for memory applications: In situ combination of synchrotron X-Ray diffraction and wafer curvature measurements

Phase change materials (PCMs) such as Ge2Sb2Te5 (GST) undergo a reversible amorphous-to-crystal transition that is the basis of their interest for next generation non-volatile memories. The large density change upon crystallization raises important issues because of the large mechanical stresses occurring during memory cycling. In order to investigate the intimate relationship between stress buildup and microstructure evolution we have built a dedicated setup, which allows combining X-ray diffraction and curvature measurements on DiffAbs beamline at SOLEIL synchrotron. Using a thin (30nm) GST film deposited on Si we show that this setup yields a wealth of interesting results. A clear correlation is observed between phase transition and stress buildup. Detailed information is obtained on the thermomechanical behavior of the different phases. Systematic investigation of PCMs with this original setup will help understanding the thermomechanical behavior of PCM thin films and nanostructures.

A load-lock compatible system for in situ electrical resistivity measurements during thin film growth.

An experimental setup designed for in situ electrical resistance measurement during thin film growth is described. The custom-built sample holder with a four-point probe arrangement can be loaded into a high-vacuum magnetron sputter-deposition chamber through a load-lock transfer system, allowing measurements on series of samples without venting the main chamber. Electrical contact is ensured with circular copper tracks inserted in a Teflon plate on a mounting holder station inside the deposition chamber. This configuration creates the possibility to measure thickness-dependent electrical resistance changes with sub-monolayer resolution and is compatible with use of sample rotation during growth. Examples are presented for metallic films with high adatom mobility growing in a Volmer-Weber mode (Ag and Pd) as well as for refractory metal (Mo) with low adatom mobility. Evidence for an amorphous-to-crystalline phase transition at a film thickness of 2.6 nm is reported during growth of Mo on an amorphous Si underlayer, supporting previous findings based on in situ wafer curvature measurements.

Local residual stress monitoring of aluminum nitride MEMS using UV micro-Raman spectroscopy

Localized stress variation in aluminum nitride (AlN) sputtered on patterned metallization has been monitored through the use of UV micro-Raman spectroscopy. This technique utilizing 325 nm laser excitation allows detection of the AlN E2(high) phonon mode in the presence of metal electrodes beneath the AlN layer with a high spatial resolution of less than 400 nm. The AlN film stress shifted 400 MPa from regions where AlN was deposited over a bottom metal electrode versus silicon dioxide. Across wafer stress variations were also investigated showing that wafer level stress metrology, for example using wafer curvature measurements, introduces large uncertainties for predicting the impact of AlN residual stress on the device performance.

High-Temperature Characterization of Silicon Dioxide Films with Wafer Curvature

Amorphous silicon dioxide films used as dielectric layers in microelectronic devices are deposited using plasma-enhanced chemical vapor deposition. Because of the presence of hydrogen and nitrogen species in the precursor gases, incorporation of such species in the films can lead to crack formation during subsequent annealing processes up to 1000°C. In this study, the role of film chemistry on the thermo-mechanical behavior of silicon dioxide films is studied with in situ film stress measurements using wafer curvature to maximum temperatures of 1000°C. This is a significant advance because normal wafer curvature can only reach maximum temperatures of around 500°C. The increased temperature range allows for the stress evolution and film chemistry to be examined for the relevant processing conditions. It was found that at temperatures higher than 550°C, hydrogen bond cleavage led to a large stress increase and film chemistry change due to new bonding arrangements between nitrogen and silicon as well as subsequent film densification causing cracking of films. These changes could only be identified with wafer curvature measurements up to 1000°C.

Relating residual stress to thin film growth processes via a kinetic model and real-time experiments

Polycrystalline thin films often have residual stress that is affected by the growth conditions (temperature, deposition rate, etc.) and material parameters. Real-time wafer curvature measurements from many groups have enabled the stress evolution with thickness and its dependence on processing conditions to be quantitatively determined for a large number of systems. Based on these results, we have developed a model for the stress evolution that focuses on the processes that occur at the evolving grain boundary between adjacent grains. The model includes parameters to predict the dependence of the stress on the diffusivity, growth rate, grain size and morphology. In this manuscript, we review the physical basis for this model and compare its predictions with two sets of measurements. In the first case, we consider the evolution of stress in films grown on patterned substrate in which the morphology of the film is known precisely, allowing for direct comparison with the model. In the second case, we consider the steady-state stress as a function of the deposition rate in electrodeposited films.

Investigation of anisotropic wafer bending curvature in a-plane GaN films grown on r-plane sapphire substrates

We grew GaN films on r-plane sapphire substrates by pulsed sputtering and investigated their structural properties. X-ray diffraction measurements revealed that a-plane GaN grows epitaxially on r-plane sapphire with an epitaxial relation of [0001]GaN // [1¯101]sapphire and [1¯101]GaN // [112¯0]sapphire. In situ wafer curvature measurements revealed that the anisotropic compressive strain along [11¯00] and [0001]GaN is generated during the initial stage of growth as a consequence of the anisotropy in the lattice mismatch between a-plane GaN and r-plane sapphire. We also observed that the coalescence of the GaN islands can lead to a change of stresses from compressive to tensile.

In Situ Stress Measurements of Li-S Half-Cells during Electrochemical Cycling

Lithium polysulfide flow and semi-flow batteries are a candidate system for flow battery applications with the potential to achieve high energy and low cost. The insulating nature of lithium sulfide and the polysulfide shuttle process are challenges to feasibility. The formation of polysulfides and sulfides in Li-S half-cells are studied using thin-film graphitic carbon electrodes as a model surface. Raman spectroscopy is used to investigate the effect of carbon growth parameters and surface treatment on electrochemical performance. Stress evolution in a lithium polysulfide battery electrode is measured during electrochemical cycling using a Multi-beam Optical Stress Sensor (MOSS) through measurement of wafer curvature as an in situ technique to measure film growth. A significant reversible and irreversible stress response is observed during cycling. An irreversible stress is observed at lower potentials, which are associated with the formation of sulfides; a reversible stress response is seen during voltage holds at 2.6V during sulfur formation. Kinetic processes of the sulfur, polysulfide, and sulfide regimes are explored using potentiostatic holds. Multiple mechanisms can be distinguished in the voltage regimes explored as stress signatures and are shown to be influenced by prior voltage holds. Post-mortem samples are analyzed using electron microscopy and energy dispersive x-ray spectroscopy to provide insight into sulfide formation and stability.

Optimization of InGaP metamorphic buffers grown by MOVPE

Inverted metamorphic multijunction solar cells have shown high solar conversion efficiencies and utilized InGaP based metamorphic (MM) buffers to change the lattice constant using compositional graded buffer layers while minimizing dislocation density in the final material layers. In this study, optimization of InGaP metamorphic buffers was done by systematically exploring key metalorganic vapor phase epitaxy (MOVPE) growth conditions and the MM buffer epitaxial stack structure. To optimize MOVPE growth parameters, growth temperature and V/III ratio were varied during the growth of a standard MM buffer test structure and the final InGaP buffer layer was characterized by photoluminescence, X-ray reciprocal space maps, atomic force microscope, cathodoluminesence, and ex situ bow measurements. The in situ measurement of wafer curvature was also monitored during MM buffer layer growth. Evaluation of material characterization data provided optimized growth conditions for the InGaP based MM buffer. The second part of this study evaluated the actual layer thickness and number of compositional graded steps in a MM buffer. Our results showed that in situ deflectometer measurements of the wafer curvature of the MM buffer layer can be correlated to ex situ determined strain relaxation of the final buffer layer of the MM buffer. Process optimization tests showed a growth temperature of 580°C with a V/III ratio of 37 provided for the best surface roughness, highest PL intensity and also allowed for low dislocation defect density of the final buffer layer. Using the optimized growth conditions, further optimization of the step grade layers showed that a 350nm thick grade layer for a six step layer MM buffer for a final buffer composition targeted for In0.8Ga0.2P provided the best surface roughness and 100% final buffer relaxation.

Strength, stiffness, and microstructure of Cu(In,Ga)Se2 thin films deposited via sputtering and co-evaporation

This work examines Cu(In,Ga)Se2 thin films fabricated by (1) selenization of pre-sputtered Cu-In-Ga and (2) co-evaporation of each constituent. The efficiency disparity between films deposited via these two methods is linked to differences in morphology and microstructure. Atomic force microscopy and scanning electron microscopy show that selenized films have rougher surfaces and poor adhesion to molybdenum back contact. Transmission electron microscopy and electron energy loss spectroscopy revealed multiple voids near the Mo layer in selenized films and a depletion of Na and Se around the voids. Residual stresses in co-evaporated films were found to be ∼1.23 GPa using wafer curvature measurements. Uniaxial compression experiments on 500 nm-diameter nanopillars carved out from co-evaporated films revealed the elastic modulus of 70.4 ± 6.5 GPa. Hertzian contact model applied to nanoindentation data on selenized films revealed the indentation modulus of 68.9 ± 12.4 GPa, which is in agreement with previous reports. This equivalence of the elastic moduli suggests that microstructural differences manifest themselves after the yield point. Typical plastic behavior with two distinct failure modes is observed in the extracted stress-strain results, with the yield strength of 640.9 ± 13.7 MPa for pillars that failed by shearing and 1100.8 ± 77.8 MPa for pillars that failed by shattering.

High resolution X-ray and electron microscopy characterization of PZT thin films prepared by RF magnetron sputtering

Pb(Zr0.52Ti0.48)O3 (PZT) thin films grown on Pt/TiO2/SiO2/Si(1 0 0) substrates in the thickness range of ∼30–550 nm by radio frequency (RF) magnetron sputtering method under different sputtering pressures were studied in the present work. High resolution grazing incidence X-ray diffraction method for residual stress and electron microscopy, particularly cross sectional transmission electron microscopy (XTEM), were used to identify the crystalline phases, and structure of the thin films at the nano-scale. Microstructure of the ultrathin films in <50 nm thickness range, consists predominantly of para-electric, oxygen deficient pyrochlore phase of Pb2Ti2O6 structure co-existing with ferroelectric perovskite PZT phase in tetragonal form. In films with thickness around 500 nm, sputtering pressure shows a strong influence on the purity of PZT phase grown. High resolution X-ray diffraction method for wafer curvature measurement and Stoney's equation were used to evaluate the biaxial stress in the films. It is found that the ultrathin films are compressively stressed with high magnitude of the order of 2 GPa which gets reduced with increasing film thickness. In the thickness range of ∼500 nm, at an optimum sputtering pressure of 4.5 Pa, the stress becomes tensile in nature with a small magnitude of 2.3 MPa. At this pressure, the film consists of almost pure perovskite phase and comparatively better electrical characteristics. X-ray reflectometry study indicates very low density of PZT films with an interlayer formed at the interface with Pt. XTEM study throws valuable insight into the nano-scale structure and reveals the presence of nano-porosity along the interface as well as within the film microstructure. This has been attributed to the observed interface roughness, reduced density, tensile residual stress as well as the poor ferroelectric properties encountered.

Misorientation dependent epilayer tilting and stress distribution in heteroepitaxially grown silicon carbide on silicon (111) substrate

The advantages and disadvantages of using off-axis substrates for heteroepitaxial growth of 3C-SiC on Si(111) substrates are investigated in this paper. 3C-SiC is deposited on on-axis and 4° off-axis 150mm Si(111) substrates using low pressure chemical vapour deposition. The dependence of surface morphology, roughness, crystallinity, alignment between the epilayer and the substrate, and film stress are evaluated using atomic force microscopy, X-ray diffraction, and wafer curvature measurement. Highly parallel steps are observed on both on-axis and off-axis Si substrates after surface preparation, yet step density is doubled and step height is much larger (>21 times of single step height) for 4° off-cut Si compared to on-axis Si. X-ray diffraction results indicate that SiC grown on on-axis Si substrates are well-aligned with the Si substrates, while the SiC grown on off-axis substrates are tilted positively by as large angle as 1.66°. The well-aligned SiC grown on on-axis Si substrate exhibits lower and uniform residual stress compared to the film grown on off-axis Si substrates, which exhibits a nonuniform distribution of higher stress. The stress distribution is found to be dependent on Si surface step direction and height. These misorientation dependent tilting and stress distribution mechanisms are expected to be applicable to other hetero-epitaxial growth systems with similar mismatch magnitude.

Aluminum oxide from trimethylaluminum and water by atomic layer deposition: The temperature dependence of residual stress, elastic modulus, hardness and adhesion

Use of atomic layer deposition (ALD) in microelectromechanical systems (MEMS) has increased as ALD enables conformal growth on 3-dimensional structures at relatively low temperatures. For MEMS device design and fabrication, the understanding of stress and mechanical properties such as elastic modulus, hardness and adhesion of thin film is crucial. In this work a comprehensive characterization of the stress, elastic modulus, hardness and adhesion of ALD aluminum oxide (Al2O3) films grown at 110–300°C from trimethylaluminum and water is presented. Film stress was analyzed by wafer curvature measurements, elastic modulus by nanoindentation and surface-acoustic wave measurements, hardness by nanoindentation and adhesion by microscratch test and scanning nanowear. The films were also analyzed by ellipsometry, optical reflectometry, X-ray reflectivity and time-of-flight elastic recoil detection for refractive index, thickness, density and impurities. The ALD Al2O3 films were under tensile stress in the scale of hundreds of MPa. The magnitude of the stress decreased strongly with increasing ALD temperature. The stress was stable during storage in air. Elastic modulus and hardness of ALD Al2O3 saturated to a fairly constant value for growth at 150 to 300°C, while ALD at 110°C gave softer films with lower modulus. ALD Al2O3 films adhered strongly on cleaned silicon with SiOx termination.

Estimation of residual stress in Pb(Zr0.52Ti0.48)O3/BiFeO3 multilayers deposited on silicon

Thin multilayer films possess residual stress components which vary from microscopic to macroscopic scale. In this study, Pb(Zr0.52Ti0.48)O3/BiFeO3 (PZT-BFO) multilayer thin film is deposited via chemical solution deposition technique on silicon substrate. The microscopic and macroscopic residual stress components of the multilayer films are investigated. The average microscopic residual stress is estimated to be 791.15 MPa (tensile) by using x-ray diffraction technique; on the other hand, the average macroscopic stress is found to be 774.23 MPa (tensile) by using wafer curvature measurement technique. As the thermally grown SiO2 layer possesses compressive stress, the combined residual stress of the PZT-BFO multilayer and SiO2 will almost cancel each other. This is reasonably encouraging for integration of the multilayer in MEMS structures.

Real-time curvature and optical spectroscopy monitoring of magnetron-sputtered WTi alloy thin films

WTi thin films are known as potential adhesion promoters and diffusion barriers. WTi thin films were deposited by magnetron sputtering from an alloyed target (W:Ti~70:30at.%). Real-time surface differential reflectance (SDR) spectroscopy and wafer-curvature measurements were performed during deposition to study the growth and the film continuity threshold. SDR measurements during WTi deposition allow the determination of the change in reflectivity of p-polarized light (at Si substrate Brewster's angle) between WTi film and Si substrate in order to monitor layer growth. The comparison between experimental and simulated WTi SDR signals assuming a homogeneous and continuous layer growth shows that film continuity is ensured beyond a thickness of 4.5±0.2nm. Real-time wafer-curvature measurements allow the determination of the intrinsic stress development in the film. Two regimes are noticed during the growth up to the development of a compressive steady state stress. The early stages of growth are rather complicated and divided into sub-regimes with similar boundaries revealed by both in situ techniques. Deposition of an interfacial continuous layer different from WTi bulk is suggested by both in situ techniques below a thickness of 4.5nm.

Strain Measurement and Mechanical Property Evolution in Sol-Gel PZT Thin Films

Cracking is a notorious issue with sol-gel PZT thin films, and film failure in this manner is difficult to reliably predict due to constantly evolving mechanical properties. In this work, two non-contact experimental methods are used to quantify the stiffness response of PZT sol-gel during thermal loading and the influence of solution additives, such as lactic acid and gycerol. The mechanical properties are determined using a digital image correlation method to measure strain and wafer curvature measurements to estimate bi-axial stress. Results show the transition from a compliant response to more glassy behavior. The importance of this changing stiffness is shown for a specific application of a buckled MEMS actuator and in relation to adhesion based failure modes.

A resonant method for determining the residual stress and elastic modulus of a thin film

By measuring the resonant frequencies of the first two symmetric vibration modes of a circular thin-film diaphragm and solving the Rayleigh-Ritz equation analytically, the residual stress and elastic modulus of the film were determined simultaneously. The results obtained employing this method are in excellent agreement with those obtained numerically in finite element modelling when tested using freestanding circular SiC diaphragms with residual tensile stress. The stress and modulus values are also in reasonably good agreement with those obtained from nanoindentation and wafer curvature measurements, respectively.

Time-domain and energetic bombardment effects on the nucleation and coalescence of thin metal films on amorphous substrates

Pulsed, ionized vapour fluxes, generated from high power impulse magnetron sputtering (HiPIMS) discharges, are employed to study the effects of time-domain and energetic bombardment on the nucleation and coalescence characteristics during Volmer–Weber growth of metal (Ag) films on amorphous (SiO2) substrates. In situ monitoring of the film growth, by means of wafer curvature measurements and spectroscopic ellipsometry, is used to determine the film thickness where a continuous film is formed. This thickness decreases from ∼210 to ∼140 Å when increasing the pulsing frequency for a constant amount of material deposited per pulse or when increasing the amount of material deposited per pulse and the energy of the film forming species for a constant pulsing frequency. Estimations of adatom lifetimes and the coalescence times show that there are conditions at which these times are within the range of the modulation of the vapour flux. Thus, nucleation and coalescence processes can be manipulated by changing the temporal profile of the vapour flux. We suggest that other than for elucidating the atomistic mechanisms that control pulsed growth processes, the interplay between the time scales for diffusion, coalescence and vapour flux pulsing can be used as a tool to determine characteristic surface diffusion and island coalescence parameters.

Bimaterial electromechanical systems for a biomimetical acoustic sensor

Bimaterial planarized micromechanical beams have been designed, simulated, and fabricated with lengths in the range 800–5800 μm and distance to substrate 0.8–4.0 μm. The beams are to be used as vertical-mode resonant gates on p-type field-effect transistors for implementing an adaptable MEMS acoustic sensor inspired by the human ear. A process for fabricating planar bilayer double-clamped beams made of silicon nitride (SiN) and aluminum (Al(1%Si)) has been developed. The planar design and bimaterial approach allow the fabrication of relatively straight beams with length up to 5800 μm with the possibility of controlling the degree of static deflection of the beams. The fabricated beams have shown a maximum deflection of ∼300 nm and a transverse concave shape with respect to the substrate due to the bilayer nature of the structure. From wafer curvature measurements, the stress in the SiN and Al(1%Si) is 200 and 280 MPa, respectively. Finite element simulations and analysis of the profile of the beams have demonstrated that the films’ stress magnitude influences the longitudinal and transverse profile of the beams. The fabricated devices resonate mechanically in the range 15–160 kHz. Preliminary electrical characterization of the devices has shown drain currents in the μA range for gate voltages of −20 to −25 V and drain voltages of −5 V.

Impact of Strain Accumulation on InGaAs/GaAsP Multiple-Quantum-Well Solar Cells: Direct Correlation between In situ Strain Measurement and Cell Performances

The effects of accumulating strain inside InGaAs/GaAsP multiple-quantum-well (MQW) solar cells were investigated and their correlation with in situ wafer curvature measurement was examined. The p–i–n GaAs solar cells, containing 20-period InGaAs/GaAsP MQWs in an i-GaAs layer, were fabricated by metalorganic vapor phase epitaxy. The strain inside MQWs was varied by changing In content in an InGaAs well, while maintaining other parameters. As evidenced by curvature transience, the excessive strain led to lattice relaxation, resulting in defects, dislocations, and poor crystal quality. Consequently, short circuit current density and open circuit voltage deteriorated, and solar cell performance degraded. The highest conversion efficiency was obtained in a strain-balanced MQW solar cell. InGaAs/GaAsP MQWs have a great potential for extending the absorption edge of GaAs cells and for enhancing the efficiency of III/V multijunction solar cells by current matching. Hence, the growth of InGaAs/GaAsP MQWs for photovoltaic application requires a strain monitoring system and careful control such that the accumulating strain is minimized.

Studies on the Polymer Adhesive Wafer Bonding Method Using Photo-Patternable Materials for MEMS Motion Sensors Applications

In this paper, various photo-patternable polymer adhesive materials were investigated for the selective wafer bonding of microelectromechanical system (MEMS) motion sensors. Commercially available photo-patternable materials which have different base polymer resins and photolithography mechanisms were selected. Curing behavior for each photo-patternable material was analyzed using the Fourier transform infrared spectroscopy to optimize the lower bonding temperature conditions with a sufficient degree of cure and wafer bonding process capability. Selective polymer adhesive wafer bonding method using photo-patternable materials was successfully demonstrated without any critical voids and defects except the phenol-based polymer resin. Also, mechanical properties, such as bonding strength and residual stress, and reliability were completely evaluated in order to select the best materials which meet the requirements for the piezoresistive MEMS motion sensors. The bonded wafers were strong enough to endure the dicing process. Bonding strength was measured quantitatively by a tensile test. The epoxy-based resin showed the best bonding strength with up to 42.9 MPa. Residual stress in the adhesive layer was evaluated using a wafer curvature measurement, because the residual stress may cause the changes of MEMS sensor performance. The silicone-based resin showed the lowest residual stress of 3 MPa because of lower Young's modulus. Five different reliability tests were performed to completely evaluate the reliability of the photo-patternable materials. The silicone-based resin showed the best reliability performance. As a result, photo-patternable polymer adhesive materials and wafer processing conditions were optimized for the MEMS motion sensors wafer bonding.