AVS 53rd International Symposium
    MEMS and NEMS Tuesday Sessions
       Session MN-TuA

Paper MN-TuA4
Chemical Mechanical Planarization of BCB Polymer Films: Effect of Cure Temperature

Tuesday, November 14, 2006, 3:00 pm, Room 2007

Session: Fabrication and Characterization of MEMS and NEMS
Presenter: N. Ghalichechian, University of Maryland
Authors: N. Ghalichechian, University of Maryland
A. Modafe, University of Maryland
M.I. Beyaz, University of Maryland
R. Ghodssi, University of Maryland
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We report on the chemical mechanical planarization (CMP) of a thick benzocyclobutene (BCB) polymer film with a high removal rate (470 nm/min) and a low density of post-CMP defects suitable for the fabrication of microelectromechanical devices. The effect of polymer cure temperature on Youngs modulus and removal rate of the BCB film was studied. Thick low-dielectric-constant BCB polymer (k=2.65) is used as an insulating layer to reduce the parasitic capacitances and increase the efficiency of an electrostatic micromotor. The micromotor fabrication process requires planarization of a 25-µm-thick BCB film buried in the substrate. High removal rate (hundreds of nm/min) is desired to planarize thick BCB films in an acceptable time. Polymers are generally soft (low Youngs modulus) and chemically resistant to a wide range of acids and alkali solutions; therefore, their planarization using conventional CMP processes is challenging. The removal rate of an uncured film was measured to be 3.66 µm/min; however, the CMP process induced significant film scratching, peel-off, non-uniformity, and surface roughness (188 nm/min peak-to-peak). Partial BCB polymerization can be achieved by curing the film under 250 °C. A series of experiments were conducted to measure the CMP removal rate and surface roughness of BCB samples cured at 120-250 °C. The removal rate was calculated from film thickness measurements before and after CMP steps using vertical scanning interferometer. The surface roughness of the film was measured using atomic force microscope. The removal rate at cure temperatures of 120, 160, and 250 °C was found to be 2000, 470, and 70 nm/min, respectively. An abrupt change in the removal rate was observed at cure temperature of 160 °C and is believed to be due to the change in the Youngs modulus of the film. CMP process details, measured removal rate, surface roughness, and Youngs modulus will be presented in this paper.