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Characterization of Microscopic Roughness And Porosity of Thin Solid Films

IP.com Disclosure Number: IPCOM000100746D
Original Publication Date: 1990-May-01
Included in the Prior Art Database: 2005-Mar-16
Document File: 3 page(s) / 131K

Publishing Venue

IBM

Related People

Krim, J: AUTHOR [+2]

Abstract

Disclosed is a process that characterizes the microscopic roughness and porosity of thin solid films, properties which are known to have an important influence on the functional performance of the films (1). The process involves measurement of inert gas adsorption isotherms and first separates rough films from those which are geometrically flat. Rough films are then divided into 1) those which are rough due to uniform porosity and 2) those which are rough due to a distribution of pore sizes.

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Characterization of Microscopic Roughness And Porosity of Thin Solid Films

       Disclosed is a process that characterizes the microscopic
roughness and porosity of thin solid films, properties which are
known to have an important influence on the functional performance of
the films (1).  The process involves measurement of inert gas
adsorption isotherms and first separates rough films from those which
are geometrically flat.  Rough films are then divided into 1) those
which are rough due to uniform porosity and 2) those which are rough
due to a distribution of pore sizes.

      The experimental apparatus is shown schematically in the
figure.  A quartz crystal (a) is held horizontally in a small vacuum
chamber (b) which is connected to a gas dosing (c) and pressure
measurement (d) system.  The chamber is either submerged directly
into liquid nitrogen or is held within a vacuum jacket (e) and
connected to the liquid nitrogen bath via a temperature-regulated
thermal link (f). Electrical leads to the crystal are attached to a
vacuum feed at the side of the chamber and then passed through the
bath to an exterior Pierce oscilla- tor circuit (g) which drives the
crystal at its resonant frequency.

      The form of the adsorption isotherm contains much information
on the roughness and porosity of the substrate onto which adsorption
has occurred.  Isotherms are obtained by recording the change in
resonant frequency as a function of pressure as gas is slowly leaked
into and pumped out of the chamber.  Changes in resonant frequency
Wfa due to adsorption (usually of liquid nitrogen) onto the thin film
are proportional to the total mass of the absorbed gas (2). We employ
the 'BET' theory of adsorption to determine the frequency shift
corresponding to a coverage of one monolayer (3):

                            (Image Omitted)

   P           1      (C-1)(P/P0)                      (1)
   Wfa(P0-P) =    WfmC   +    WfmC
Here, Wfm is the frequency shift corresponding to monolayer
formation, P is the vapor pressure, P0 is the vapor pressure at
saturation, and C is a constant related to the substrate binding
energy.  Wfm and C are deduced from the slope and intercept of a plot
of P/Wfa(P0 - P) versus P/P0 .   Wfm = 3.9 Hz which corresponds to
nitrogen adsorption on flat electrodes of a 5 MHz crystal.  The ratio
Wfm/3.9 Hz is therefore a measure of the surface roughness factor
A/Af (e.g., the film surface area A divided by that of a
geometrically flat plane Af).

      This measurement of A/Af separates rough surfaces from smooth
surfaces but does not characterize the surface morphology.  We
utilize the functional form of the adsorption isotherm in the thick
film regime to probe pore morphology.  The presence of pores
introduces new problems into theories of adsorption on flat surfaces
(4), since a film which grows inside the walls of a pore cannot grow
...