Residual stress analysis of sputtered Tantalum Silicide thin film

The influence of different argon pressures on the residual stress, microstructure, and resisitivity of sputtered tantalum silicide thin films has been investigated. TaSi2 films were deposited onto Si wafers by dc magnetron sputtering. The deposition temperature was assumed to be 300°C. The thickness was about 0.5 μm. The residual stresses in the films deposited at different argon pressures were determined by curvature measurement method. The compressive intrinsic stress of 1033.4MPa was measured at PAr = 0.5 mTorr. The intrinsic stress in the films seemed to change from compression to tension as PAr increased above 8mTorr. A maximum tensile intrinsic stress of 221MPa was obtained at 10 mTon and the tensile intrinsic stress decreased at the higher argon pressure. SEM showed an accompanying microstructural change from a dense structure at the low pressures to an open growth structure with some gaps between grains at the highest pressure. The electrical resistivity exhibited a sputtering-pressure dependence and seemed to be closely related to the microstructure of films

Design and micro-fabrication of tantalum silicide cantilever beam threshold accelerometer

Microfabricated threshold accelerometers were successfully designed and fabricated following a careful analysis of the electrical, mechanical, and fabrication issues inherent to micron-sized accelerometers. A uniform cantilever beam was chosen because of the simplicity of design and fabrication. New models for the electrostatic force exerted on the cantilever beam were developed and calculations were made that accurately predicted the electrical characteristics of the accelerometer. The calculations also provided design guidelines for optimizing the accelerometer dimensions. Computer simulation demonstrated that the error of the electrostatic force, calculated using the most accurate model, was within 2% of the actual force which was obtained by integrating the closed formula, through the bent beam curvature, for device parameters designed to detect an acceleration of 50 g. Conversely, it was shown that the widely used conventional parallel plate model had an error of approximately 90%. Novel surface micromachining process steps were successfully developed to fabricate the cantilever beam accelerometers. Sputter deposited tantalum silicide and commercially available spin-on-glass were used as a structural layer and a sacrificial layer, respectively. The dependence of resistivity, crystalline structure, Young\u27s modulus, and hardness of the tantalum silicide films on the annealing temperatures were measured. These results were employed to design accelerometers that were successfully operated. Excluding the metallization steps, only two masks and four photolithography steps were required. However, both positive and negative photoresists had to be utilized. NJIT\u27s standard photolithography steps were used for positive photoresist; however for the negative photoresist a specially developed multi-puddle process was used to obtain 4 micron resolution. Electrostatic attraction tests, of accelerometers, were performed using the Keithley current-voltage measurement system. These tests used deflection voltages ranging from 2.2 to 37.0 volts, corresponding to threshold acceleration levels from 580 to 18,500 g. Nearly 70 percent of the threshold voltage results fell within the expected error limits set by the accuracy of the device dimensions when processing tolerances were taken into account including the thickness variation caused by 8% uncertainty in the buffered HF etch rate of tantalum silicide. Some accelerometers were closed and opened 3 times without failure. The accelerometers tended to break after 3 times of operation and this was attributed to the welding of contacts. Centrifuge acceleration tests of accelerometers were carried out in a specially designed centrifuge in an acceleration range of 282 to 11,200 g. Nearly 80 percent of the threshold acceleration results fell within the expected error limits set by the accuracy of the device dimensions when processing tolerances were taken into account

Materials data handbook: Aluminum alloy 6061

A summary of the materials property information for aluminum alloy 6061 is presented. The scope of the information includes physical and mechanical properties of the alloy at cryogenic, ambient, and elevated temperatures. Information on material procurement, metallurgy, corrosion, environmental effects, fabrication, and joining techniques is developed

Fabrication and characterization of WSi2/p-si and TaSi2/p-si devices

Thin films Silicides of Tungsten and Tantalum have become very important for IC manufacturing. W and TaSi2 films were deposited on silicon substrates by CVD and Co-sputtering techniques respectively. These films have been characterized using current-voltage technique. The analysis of the obtained experimental measurements has been performed in the light of Schottky-Mott theory. The effects of annealing were studied using Rapid Thermal Processing technique in the temperature range of 500 to 700°C, in nitrogen atmosphere at a constant pressure of 5x10-6 ton for a duration of 30 seconds.The increase in annealing temperature resulted in the formation of ohmic contact evidenced by current-voltage and sheet-resistance measurements. Typical sheet -resistances were found to be in the order of 6-12Q /square for tungsten silicide and 2-7Q /square for tantalum silicide. The RTP technique,as concluded from the results, was found to be very effective in the formation of ohmic contacts

Study of subthreshold behavior of FinFet

The study of subthreshold behavior of Metal Oxide Semiconductor Field Effect Transistor (MOSFET) is critically important in the case of submicron devices for the successful design and implementation of digital circuits. Fin Field Effect Transistor (FinFET) is considered to be an alternate MOSFET structure in the deep sub-micron regime. A 3D Poisson equation solver is employed to study the subthreshold behavior of FinFET. Based on potential distribution inside the fin, the appropriate band bending and the subthreshold value called the S-factor is calculated. It is observed that the S-factor of the device increases as the channel width, Tfin increases. This is attributed to the fact that the change in the band bending is less than the change in the applied gate voltage. This is only a first order analysis; hence the device is simulated in a device simulator Taurus. It is observed that the S-factor increases exponentially for channel lengths Lg \u3c 1.5Tfin. Further, for a constant Lg, the S factor is observed to increase as Tfin increases. An empirical relationship between S, Lg and Tfin is developed based on the simulation results, which can be used as a rule of thumb for determining the S-factor of devices

High temperature compounds for turbine vanes

Fabrication and microstructure control studies were conducted on SiC, Si3N and composites based on Si3N. Charpy mode impact testing to 2400 F established that Si3N4/Mo composites have excellent potential. Attempts to fabricate composites of Si3N4 with superalloys, both by hot pressing and infiltration were largely unsuccessful in comparison to using Mo, Re, and Ta which are less reactive. Modest improvements in impact strength were realized for monolithic Si3N4; however, SiC strengths increased by a factor of six and now equal values achieved for Si3N4. Correlations of impact strength with material properties are discussed. Reduced MgO densification aid additions to Si3N4 were found to decrease densification kinetics, increase final porosity, decrease room temperature bend strength, increase high temperature bend strength, and decrease bend stress rupture properties. The decrease in bend strength at high temperature for fine grain size SiC suggested that a slightly larger grain size material with a nearly constant strength-temperature relation may prove desirable in the creep and stress rupture mode

Thermomechanical property evaluation of molybdenum alloys

The ultra high temperature structural intermetallic molybdenum and its alloys were studied for their thermal properties and correlated to the material microstructure. Thermal expansion tests were carried out using thermo-mechanical analyzer (TMA). Thermo cycling tests were conducted around 650°C for spinel dispersed molybdenum alloys with 3% wt of spinel or 6% wt of spinel (MgAl 2O4) particles. Results show that the coefficient of thermal expansion (CTE) value decreases with the addition of spinel and silicide particles. Furthermore, pure molybdenum alloys with different processing method also affect the CTE values. Thermo cycling tests show that molybdenum alloy with 6% wt of spinel (MgAl2O4) develops microcracks which are related to the number of cycles. The microcracks are caused by the thermal expansion mismatch between the spinel particles and molybdenum matrix, as well as the processing conditions. After the tests, the specimen was polished suitable to examine them using Scanning Electron Microscope (SEM) and Energy Dispersive Spectroscopy (EDS) and the micro cracks were detected, which were developed due to the thermal stresses

Evaluation of coated columbium alloy heat shields for space shuttle thermal protection system application

A three-phase program to develop and demonstrate the feasibility of a metallic heat shield suitable for use on Space Shuttle Orbiter class vehicles at operating surface temperatures of up to 1590 K (2400 F) is summarized. An orderly progression of configuration studies, material screening tests, and subscale structural tests was performed. Scale-up feasibility was demonstrated in the final phase when a sizable nine-panel array was fabricated and successfully tested. The full-scale tests included cyclic testing at reduced air pressure to 1590 K (2400 F) and up to 158 dB overall sound pressure level. The selected structural configuration and design techniques succesfully eliminated thermal induced failures. The thermal/structural performance of the system was repeatedly demonstrated. Practical and effective field repair methods for coated columbium alloys were demonstrated. Major uncertainties of accessibility, refurbishability, and durability were eliminated

Atomistic and mesoscopic simulations of heat transfer across heterogeneous material interfaces

The study of heat transfer and the associated thermal interface resistance at heterogeneous material interfaces is over 70 years old since the first measurements of thermal interface resistance by Kapitza in 1941. However, recent developments in experimental metrology techniques that enable spectrally-resolved phonon transport measurements at the nanoscale along with the development of high-fidelity simulation methods have provided a renewed interest in the fundamental physics of heat transfer across interfaces. Miniaturized electronic devices and nanostructured materials for energy applications are among technologies that would benefit from a fundamental understanding of interfacial thermal transport. This dissertation focuses on the study of problems in interfacial heat transfer that span the atomistic and mesoscopic length scales and have broad applications in electronic thermal management. The first part of this dissertation develops a mesoscale simulation framework to predict the mechanical and thermal performance of carbon nanotube (CNT) thermal interface materials (TIMs). CNT arrays have been widely studied for use as TIMs due to the high thermal conductivity and mechanical compliance of CNTs. However, modeling of CNT TIMs has been largely limited to semi-empirical methods that lack detailed consideration of CNT array microstructure. We develop a physics-based, microstructure-sensitive, thermo-mechanical simulation framework that can be used in the design and optimization of CNT TIMs. Coarse-grain mechanics simulations are used to predict the CNT array microstructure and the finite volume method is used to solve the Fourier conduction equations for CNTs embedded in a filler matrix. The simulations provide insights on the sensitivity of thermal resistance of the CNT array to microscopic CNT-CNT and CNT-substrate contact resistances. Microstructural parameters that are not readily accessible in experiments such as the contact areas and the fraction of CNTs in contact with the opposing substrate are reported to demonstrate the usefulness of the simulation approach. The latter part of this dissertation deals with the development of a first-principles atomistic simulation framework to study heat transfer across metal-semiconductor heterojunctions which form an important class of interfaces used in electronic devices. The silicides of transition metals such as titanium and cobalt (TiSi2, CoSi2) are commonly used as metal contacts to silicon in transistors; hence, TiSi2-Si and CoSi 2-Si interfaces are chosen here as model metal-semiconductor junctions for studies of thermal transport. All the atomistic simulations reported in this work use the atomistic Green\u27s function (AGF) method that is analogous to the non-equilibrium Green\u27s function (NEGF) method used in quantum transport calculations of electrons. We propose the use of Büttiker probe scattering models to develop a phenomenological but computationally efficient description of phonon-phonon and electron-phonon scattering within the AGF framework. First-principles calculations of electron-phonon coupling reveal that energy transfer between metal electrons and lattice vibrations in the semiconductor is mediated by interfacial phonon modes whose vibrational pattern is delocalized across the metal and semiconductor regions, and the coupling of metal electrons with phonon modes localized in the semiconductor is negligible. The transport simulations also help identify the contributions of various scattering mechanisms such as elastic interfacial scattering, inelastic phonon scattering, electron-phonon coupling within the metal, and direct electron-phonon coupling across the interface to the total thermal conductance of a CoSi 2-Si interface. The inclusion of the various transport processes in the simulation is found to be critical to obtain a good agreement with experimental data on thermal conductance of an epitaxial CoSi2-Si interface. The last part of this work develops an eigenspectrum formulation of the AGF method that enables the prediction of polarization- or branch-resolved contributions to the phonon transmission function and the thermal interface conductance. Unlike prior work in the literature, our approach makes a direct connection to the bulk phonon dispersion of materials forming an interface and is also computationally efficient. The essential idea behind the formulation is the use of bulk phonon eigenspectrum to obtain the surface Green\u27s functions used in the AGF method instead of the more commonly used Sancho-Rubio or decimation technique. The new approach is applied to study phonon transport across a Si-Ge interface with atomic intermixing. The computation of polarization-resolved transmission functions, which are not accessible within the conventional AGF method that groups different phonon branches together, provides insights on the microscopic mechanisms responsible for the increase in phonon transmission due to interfacial disorder

Development of a ductile columbium alloy rocket engine combustion chamber

Development and testing of ductile niobium alloy with silicide coating for rocket engine combustion chambe