Micromachining of molybdenum and tungsten silicide thin films.
Ger, Muh-Ling
1994
Abstract
Micromachining is the application of semiconductor processing technology to the fabrication of mechanical structures having very small dimensions. Compared to their macro-size counterparts, these micromechanical structures can have advantages in size, weight, geometrical accuracy, reproducibility, and reliability. Micromachining technology research and applications have grown explosively in recent years. A major driving force in this development has been the application of micromachining to sensors and actuators, which are in great demand by the automotive, biomedical, and aerospace industries. Remarkable results have been achieved in micromechanical sensors and actuators using only a few materials (mainly silicon and its dielectrics). The research reported in this dissertation has characterized two additional materials, sputtered molybdenum and tungsten silicide. The low-temperature deposition and annealing of sputtered refractory metals and refractory metal silicides make possible the planar fabrication of capacitive pressure sensors that are truly CMOS-compatible. Molybdenum was the first material examined in this project. It is electrically conductive (a trait required in the capacitive pressure sensor application), has high yield strength, and can be sputter-deposited at low temperatures. Characterization of the micromechanical properties of the sputtered molybdenum films led to successful fabrication of three-dimensional micromechanical structures, including cantilevers, bridges, and diaphragms. The as-deposited stress can be controlled (below 100 MPa) by sputtering. The thermal expansion coefficient of the molybdenum film is found to be $5.59\times10\sp{-6}/\sp\circ$C. The biaxial elastic moduli of the films are anisotropic and have values between 2.57 and $3.61\times10\sp{11}$Pa. However, the columnar crystal morphology of these films makes the structures fragile at the boundary, and a stress-relaxation property of the films makes stress control difficult. Though sputtered molybdenum is not a suitable material for diaphragms in pressure transducers, the information obtained from research on the characterization of molybdenum might be useful in other micromachining applications. The next material evaluated was tungsten silicide. Refractory metal silicides are similar to refractory metals in their micromechanical properties. In this project, a cosputtering deposition method was developed which yields amorphous films that do not exhibit stress-relaxation over time. The as-deposited residual stresses can be controlled to be below 80 MPa. Both the thermal expansion coefficient and biaxial elastic modulus of these WSi$\sb x$ films are isotropic and have values of $6.0\times10\sp{-6}/\sp\circ$C and $2.9\times10\sp{11}$Pa, respectively. Round and square diaphragms were fabricated from the cosputtered tungsten silicide. The diaphragm structure was formed by depositing tungsten silicide film on an aluminum sacrificial layer, which was later removed through etch channels on the periphery of the diaphragm. The etch channels were then sealed using a novel dryfilm photoresist method, which may have applications in other micromachining work. Both capacitance vs. voltage and capacitance vs. pressure relationships were obtained for the new pressure sensors. The pressure sensitivity is about 0.05 fF/Torr for the transducer with a 300$\mu$m-diameter round diaphragm. This is the first work to document the use of a refractory metal silicide as diaphragms in capacitive pressure transducers.Other Identifiers
(UMI)AAI9513360
Subjects
Engineering, Electronics and Electrical Engineering, Materials Science
Types
Thesis
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