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Nondestructive Characterization of Ultrathin Carbon Films

IP.com Disclosure Number: IPCOM000100506D
Original Publication Date: 1990-Apr-01
Included in the Prior Art Database: 2005-Mar-15
Document File: 4 page(s) / 148K

Publishing Venue

IBM

Related People

Brennan, S: AUTHOR [+2]

Abstract

Disclosed is a nondestructive process for characterizing the thickness, density and microscopic surface roughness of ultrathin (N250 A@) amorphous carbon films. The method involves measuring the intensity of X-rays specularly reflected from the films and fitting the data to a model of the film-substrate combination. The thickness, density, and surface roughness are difficult to accurately measure using other methods, but they affect the functional performance of these films and are dependent on preparation conditions [1].

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Nondestructive Characterization of Ultrathin Carbon Films

       Disclosed is a nondestructive process for characterizing
the thickness, density and microscopic surface roughness of ultrathin
(N250 A@) amorphous carbon films.  The method involves measuring the
intensity of X-rays specularly reflected from the films and fitting
the data to a model of the film-substrate combination.  The
thickness, density, and surface roughness are difficult to accurately
measure using other methods, but they affect the functional
performance of these films and are dependent on preparation
conditions [1].

      The intensity of specularly reflected X-rays is dependent on
the surface and near-surface electron density (2,3).  The index of
refraction at X-ray energies is n= 1-w-i b, where w is proportional
to the mass density and b describes the absorption.  For X-rays that
are incident from a medium with refractive index n onto a medium with
refractive index n', the specular reflectivity (R) is5 R = RF =  rF
2, where the Fresnel reflection amplitude is

                            (Image Omitted)

 Here, a is the incidence
angle of the X-rays, ac = !2(w' - w) is the critical angle for total
external reflection, and we have assumed the interface to be
infinitely sharp.  Since real interfaces are not infinitely sharp,
but have some roughness and a gradual change in the electron density,
R is reduced from that given in Equation (1).  A simple approximation
that assumes a Gaussian distribution of the interfacial position and
a variation in the density that can be described by an error function
leads to R = RF exp - (s Q)2 [2,3].  The 'roughness' is the
root-mean-square average of both the microscopic surface roughness
and the intrinsic interfacial width.

      For materials that consist of a thin film of one material
deposited on a second material there will be two contributions to the
reflectivity:  one from the air-solid interface, and another from the
interface between the two materials.  The X-rays reflected from these
two interfaces interfere and the reflectivity is approximately (2,4).
where rs and ri are the reflection amplitudes (including roughness)
at the surface (air-carbon) and interface (carbon-Si), respectively,
and t is the film thickness. This equation shows that there is a
modulation of the reflectivity that results because of the path
length difference between the interfaces and is directly related to
the film thickness.  Furthermore, for a film in which the composition
is known, the amplitude of modulation is directly related to the film
density.  Lastly, surface and interface roughness will show up as an
overall reduction in reflected intensity at higher angles of
incidence, just as for the case of a single interface.

      Experiments can be conducted with many X-ray setups. Those
reported below were performed at the Stanford Synchrotron Radiation
Laboratory on Beam Line 7-2.  Si(111) m...