Characterization and Dielectric Properties of Fluorinated Amorphous Carbon Measured by Capacitance-Voltage Response and
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determining the dielectric properties of a material is spectral ellipsometry (SE). This method characterizes the optical constants (and thickness) of the material, which is directly related to the dielectric properties at optical frequencies. Both the real and imaginary components of the optical constants (n and k respectively) can be derived from the elipsometric data; however, the results depend somewhat on the optical scattering model employed. There has been a limited number of studies on the optical properties of diamond-like carbon (DLC) [1, 2] and fluorinated silicon carbides [3], both of which are interesting dielectric thin film materials. In this paper, we describe recent measurements of the dielectric constants of fluorinated amorphous carbon (FLAC) films using spectral elipsometry, and compare these measurements to the dielectric constants derived directly from capacitance-voltage (CV) measurements. EXPERIMENT Deposition Conditions The fluorinated amorphous carbon films used in this study were synthesized by radio frequency (RF) plasma chemical vapor deposition using mixtures of methane (CH 4 ) and fluoromethane (CF 4) in a capacitively coupled discharge. The experimental deposition matrix
341 Mat. Res. Soc. Symp. Proc. Vol. 593 © 2000 Materials Research Society
includes varying the CH4/CF 4 flow ratios, ranging from 0.1 to 1.2. The RF power level ranged from 1. 3 to 6.4 kW m-2 . The deposition pressure was constant at 13.3 Pa (100 mTorr). The specific conditions for nine deposited samples that are analyzed here, are given in Table I below. ANALYSIS Ellipsometry Ellipsometry involves directing initially circularly polarized light onto a surface and measuring the change in phase, A,and magnitude, T, of the two perpendicular components of the polarization in the scattered light. The total reflection coefficient can be expressed in terms of the complex reflection coefficients for the parallel (RP) and perpendicular (RS) polarization components [4] as:
p = tan Tej =
1 R•
It is important to note that what is measured is T and A,and all else is the result of a model interpretation. RP and Rs are parallel and perpendicular to the beam axis, and are given in terms of the Fresnel coefficients, rl2, r,2 for the air-film interface (subscripts I is for air, 2 for the film), and r]P, r'; for the film-substrate interface (subscript 3 is for the silicon substrate):
r=r +r P x exp(-j2,3)
(2)
1 P rlr3 x exp(-j2fl)
R'
r 2 + x3 x exp(-j2/J) I1+ rýr x exp(-j2)(3
(3)
Here, P3 is defined in terms of the film thickness, d, the wavelength, k, the angle of incidence (measured from the normal), 4, and the real (n2) and imaginary (k2) contributions to the complex refractive index, N2, of the film, /6 = 21{(d N 2 cos(W)
(4)
T is determined from the ratio of the absolute values of the reflection coefficients: tan(T)-
RP
g'
5
(5)
1Rjl There is no general analytical solution for N2 and d in terms of the measured attenuation and phase shift. Instead, one must develop a numerical iterative scheme to optimize the mod
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