Determination of Optical Properties of Fluorocarbon Polymer Thin Films by a Variable Angle Spectroscopic Ellipsometry
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ABSTRACT Optical properties of vapor phase (VP) deposited and spin-coated fluorocarbon (FC) thin films on silicon substrates, such as refractive index, extinction coefficient and film thickness were characterized by a variable angle spectroscopic ellipsometry (VASE) in the range of 300800 nm. A Lorentz model allows us to simulate the optical constants of the FC films with a minimum number of parameters while maintaining Kramers-Kronig (KK) consistency between the real and imaginary parts of the optical constants. FC films are nearly transparent over the visible spectrum, so it is possible to assume k (extinction coefficient) = 0 over part of the visible spectrum in a Cauchy model. To accurately simulate the obtained ellipsometric spectra, we performed a regression analysis in two steps assuming a three-phase and a four-phase model. The regression analysis was performed using the three-phase model and a best-fit mean-squared error (MSE) value of 1.717 (VP deposited FC film, Lorentz model) was obtained. However, the four-phase model was used to improve the best-fit result of 0.531 (VP deposited FC film, Lorentz model). The surface roughness layer was assumed to be a mixture of FC films and voids under the Bruggeman effective medium approximation (EMA). We found that the best-fit MSE was reduced when surface roughness was included. INTRODUCTION Surface micromachining is one of the core technologies underlying microelectromechanical
systems (MEMS), which promises to extend the benefits of microelectronic fabrication technology to sensing and actuating functions. The reason that surface micromachining has rapidly expended is a potential of integrated microsystems, which incorporate surface micromachined sensors or actuators together with integrated electronics on the same substrate [1 ]. Metallic and oxide films have been widely used in the fabrication of MEMS. They have hydrophilic surfaces with high energy. They may cause the stiction between surface due to the capillary, electrostatic and van der Waals forces [2,3]. Stiction in micromachining is a notorious problem in surface-micromachined devices by the collapse of structures when the surfaces are brought into contact, thereby resulting in a decreased reliability of the system. Especially, capillary forces dominate at close separation range in humid environment. Chemical modification of the surface is effective in creating a hydrophobic, low energy surface that greatly reduces stiction, which may be required in MEMS fabrication and operation. Since bulk TeflonTM can be one of the best candidates from a practical point of view to meet these requirements for use in MEMS, this motivates the desire to produce fluoropolymer thin film coatings. Along with their excellent chemical and physical properties [4], these highly hydrophobic surfaces with terminal -CF. groups could eliminate the stiction by van der Waals and capillary forces.
297 Mat. Res. Soc. Symp. Proc. Vol. 588 ©2000 Materials Research Society
The objective of this current study was to obtain more comprehe
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