As the thickness of the film is reduced, the capacitance and
sensitivity increase. Experimental results have shown that polyimide
films with a thickness of smaller than 300 Å cannot be
reliably fabricated because of pinholes in the polyimide film
which cause a short circuit between the two metal electrodes.
Several sensors having different thickness for the polyimide film
were fabricated, including a 300-Å (spun at 4000 rpm), 660-Å
(spun at 2000 rpm), and 1200-Å (spun twice at 2000 rpm)-thick
polyimide dielectric film. The thickness of the films are determined
by interferometry. Figs. 4 and 5 show the measured capacitance
versus relative humidity for a 300- and 660-Å-thick
polyimide films at 37 C, respectively.
As seen in both figures, the response to relative humidity is
fairly linear (nonlinearity is calculated to be less than 2% RH for
both devices) over the full scale range of 30–70% RH. Table I
summarizes the measured sensitivities for the three different
sensor types.
As the thickness of the film is reduced, the capacitance andsensitivity increase. Experimental results have shown that polyimidefilms with a thickness of smaller than 300 Å cannot bereliably fabricated because of pinholes in the polyimide filmwhich cause a short circuit between the two metal electrodes.Several sensors having different thickness for the polyimide filmwere fabricated, including a 300-Å (spun at 4000 rpm), 660-Å(spun at 2000 rpm), and 1200-Å (spun twice at 2000 rpm)-thickpolyimide dielectric film. The thickness of the films are determinedby interferometry. Figs. 4 and 5 show the measured capacitanceversus relative humidity for a 300- and 660-Å-thickpolyimide films at 37 C, respectively.As seen in both figures, the response to relative humidity isfairly linear (nonlinearity is calculated to be less than 2% RH forboth devices) over the full scale range of 30–70% RH. Table Isummarizes the measured sensitivities for the three differentsensor types.
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