TY - BOOK

T1 - Design and fabrication of various MEMS-based structures for investigating thermo-mechanical and fatigue behavior of thin metal films and metal barriers

AU - Saghaeian, Fahimeh

N1 - embargoed until 01-11-2020

PY - 2019

Y1 - 2019

N2 - The multidisciplinary semiconductor industry utilizes various thin films of metal, oxides or nitrides etc. to improve performance of MOSFET devices. Especially, metal thin films are predominantly used to create a network of integrated circuit on a chip, as well as supplying a path for heat dissipation. For this particular case, a stack of power metal and diffusion barrier is employed which is required to cope with a repetitive thermo-mechanical loading. It is thus important to investigate the material properties of metals and barriers in detail to evaluate the extent of deformation under a thermo-mechanical load, the interfacial adhesive strength or endurance against cyclic fatigue load. This demands a proper methodology to study thin films, and determine the impact of processing conditions on their material properties. The present work focuses on developing a variety of MEMS based test structures to enable characterization of thin metal films. Using a semiconductor on insulator technology, a fabrication process was developed to manufacture different MEMS structures such as;beams, cantilevers, curved cantilevers, theta and plus shaped structures with varying aspect ratios. A metal film was deposited on these structures, either including the sidewall, or else it was structured in such a way as to give MEMS structures with metal-free sidewalls. The process offers the possibility to change dimensions of structures, the stack and thickness of metal films, or the interface with substrate. Following this, thermo-mechanical behavior of thin copper films was investigated using curved cantilever and plus shaped structures. Using data obtained during in-situ high temperature cycling, the deflection of structure was plotted against the temperature, revealing the typical hysteresis curve depicted by copper. The amount of deflection and therefore area of hysteresis curve was found to increasing either with the increasing thickness of copper, or with the dimensions of the structure. Secondly, by using nano-indentation technique, mechanical properties of Cu-TiW stack were evaluated. The composite cantilever of Si-TiW-Cu was subjected to different annealing temperatures prior to testing under fracture load. Annealed copper films showed significant reduction in load to fracture in comparison to a deposited film. Grain growth and reorientation of copper grains during annealing are the major reasons for reduction in the fracture strength of Cu films. In a subsequent study, plus shaped structures were used to evaluate the fatigue behavior of thin copper films. Using a piezo electric shaker, a thin copper film deposited on plus shaped structures was subjected to cyclic fatigue load at the resonant frequency of structure. The fatigue deformation was concentrated at either ends of beams revealing typical signatures such as slip lines, extrusions, grain growth and grain reorientation. The plus structure was found to be an excellent tool for the characterization of fatigue behavior of thin films. Finally, morphology of TiW film was investigated to understand functionality of TiW as a diffusion barrier. TiW films of two different stoichiometries and residual stress levels have been analyzed to illustrate differences in their microstructures. The thickness dependent stress gradient was determined in tensile films, while the same in compressive films was relatively constant. The stress level was influenced by the energy of plasma during deposition, confirming the Thornton model. The present study pave a platform for characterization of thin metal films using MEMS based structures. In this work, a process to fabricate these structures was developed and selected structures were characterized under various loading conditions.This approach opens the possibility of studying properties of bilayers and thin film interfaces as well as investigating the impact of various process steps on the microstructure, residual stresses and thereby on device characteristics.

AB - The multidisciplinary semiconductor industry utilizes various thin films of metal, oxides or nitrides etc. to improve performance of MOSFET devices. Especially, metal thin films are predominantly used to create a network of integrated circuit on a chip, as well as supplying a path for heat dissipation. For this particular case, a stack of power metal and diffusion barrier is employed which is required to cope with a repetitive thermo-mechanical loading. It is thus important to investigate the material properties of metals and barriers in detail to evaluate the extent of deformation under a thermo-mechanical load, the interfacial adhesive strength or endurance against cyclic fatigue load. This demands a proper methodology to study thin films, and determine the impact of processing conditions on their material properties. The present work focuses on developing a variety of MEMS based test structures to enable characterization of thin metal films. Using a semiconductor on insulator technology, a fabrication process was developed to manufacture different MEMS structures such as;beams, cantilevers, curved cantilevers, theta and plus shaped structures with varying aspect ratios. A metal film was deposited on these structures, either including the sidewall, or else it was structured in such a way as to give MEMS structures with metal-free sidewalls. The process offers the possibility to change dimensions of structures, the stack and thickness of metal films, or the interface with substrate. Following this, thermo-mechanical behavior of thin copper films was investigated using curved cantilever and plus shaped structures. Using data obtained during in-situ high temperature cycling, the deflection of structure was plotted against the temperature, revealing the typical hysteresis curve depicted by copper. The amount of deflection and therefore area of hysteresis curve was found to increasing either with the increasing thickness of copper, or with the dimensions of the structure. Secondly, by using nano-indentation technique, mechanical properties of Cu-TiW stack were evaluated. The composite cantilever of Si-TiW-Cu was subjected to different annealing temperatures prior to testing under fracture load. Annealed copper films showed significant reduction in load to fracture in comparison to a deposited film. Grain growth and reorientation of copper grains during annealing are the major reasons for reduction in the fracture strength of Cu films. In a subsequent study, plus shaped structures were used to evaluate the fatigue behavior of thin copper films. Using a piezo electric shaker, a thin copper film deposited on plus shaped structures was subjected to cyclic fatigue load at the resonant frequency of structure. The fatigue deformation was concentrated at either ends of beams revealing typical signatures such as slip lines, extrusions, grain growth and grain reorientation. The plus structure was found to be an excellent tool for the characterization of fatigue behavior of thin films. Finally, morphology of TiW film was investigated to understand functionality of TiW as a diffusion barrier. TiW films of two different stoichiometries and residual stress levels have been analyzed to illustrate differences in their microstructures. The thickness dependent stress gradient was determined in tensile films, while the same in compressive films was relatively constant. The stress level was influenced by the energy of plasma during deposition, confirming the Thornton model. The present study pave a platform for characterization of thin metal films using MEMS based structures. In this work, a process to fabricate these structures was developed and selected structures were characterized under various loading conditions.This approach opens the possibility of studying properties of bilayers and thin film interfaces as well as investigating the impact of various process steps on the microstructure, residual stresses and thereby on device characteristics.

KW - MEMS structures

KW - thin film

KW - semiconductor

KW - copper

KW - diffusion barrier

KW - characterization

KW - thermo-mechanical behavior

KW - cyclic fatigue

KW - residual stress

KW - MEMS Strukturen

KW - Dünnschichten

KW - Halbleiter

KW - Kupfer

KW - Diffusionsbarriere

KW - Charakterisierung

KW - Thermo-mechanische verhalten

KW - zyklische-Ermüdung

KW - Eigenspannung

M3 - Doctoral Thesis

ER -