Electrical Characterization of BCB for Electrostatic Microelectromechanical Devices
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Electrical Characterization of BCB for Electrostatic Microelectromechanical Devices Alireza Modafe, Nima Ghalichechian, Benjamin Kleber, and Reza Ghodssi Department of Electrical and Computer Engineering and Institute for Systems Research University of Maryland, College Park, MD 20742, U.S.A. Abstract Electrical properties and thickness of insulating dielectric films directly affect electrical energy loss and electrical breakdown limit in electric micromachines. A thick, low-k film exhibits low parasitic capacitive effects that help with the reduction of electrical energy loss. The electrical performance can be deteriorated due to degradation of the electrical properties of the insulating dielectric material caused by process, device structure, and moisture. In this paper, we introduce the application of CYCLOTENE, a spin-on, low-k, BCB-based polymer in electric micromachines as insulating layer and interlevel dielectric. A novel approach using interdigitated capacitors for electrical characterization of CYCLOTENE and the effect of moisture absorption is introduced in this paper. The dielectric constant of CYCLOTENE is extracted from two steps of capacitance measurements, giving an average value of 2.49 with a standard deviation of 1.5 %. The dielectric constant increases by 1.2 % after a humidity stress of 85 %RH at 85 °C. The measured I-V characteristics of CYCLOTENE show a dependency of the breakdown strength and leakage current on the geometrical dimensions of the device under test. A breakdown strength of 225 V/µm for 2 µm finger spacing and 320 V/µm for 3 µm finger spacing, and a leakage current of a few to tens of pA are measured. The I-V characteristics degrades drastically after a humidity stress of 85 %RH at 85 °C, showing a breakdown strength of 100 V/µm for 2 µm finger spacing and 180 V/µm for 3 µm finger spacing. Based on the results of this study, it is expected that the electrical efficiency of an electric micromachine is improved using BCB-based polymers with negligible dependency on moisture absorption. On the other hand, the maximum performance that depends on the maximum operating voltage is adversely affected by the degradation of the breakdown voltage after moisture absorption. Introduction In this paper we report the results of an in-depth study on the electrical properties of CYCLOTENETM (Dow Chemical Company, Midland, MI), a spin-on, low dielectric constant polymer derived from benzocyclobutene (BCB). This characterization is useful for application of CYCLOTENE in microelectromechanical systems (MEMS) with a focus on electric micromachines such as micromotors and power microgenerators. The result of this study will be used in the development of a multi-phase, bottom-drive, variable-capacitance micromotor where CYCLOTENE serves as the insulating dielectric and the interlevel dielectric (ILD) films. The physical properties of the dielectric film have a major impact on electrical efficiency and reliability. It is desired to have a thick insulating film with low dielectric constant and h
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