Measuring Thin Films
Barbara Kanegsberg, Ed Kanegsberg, March 2003
Thin films can be essential or detrimental. Semiconductor devices consist of
a series of essential thin films, but a residue of the wrong film can turn
your product into an expensive ornament. With biological devices and implants,
films are usually considered to be contaminants, but as we have discussed in
previous columns (January and February 2002), there can be a beneficial side
to a film on a biomedical device.
Be it good or bad, how do you monitor the thickness and characteristics of
a film?
A relatively new approach for film monitoring builds on the Quartz Crystal
Microbalance (QCM) technique. 7irst developed in the 1950s for measuring
small masses, the quartz microbalance is based on the resonant properties
of a quartz
crystal onto which electrodes have been deposited. Another substance could
subsequently be deposited to be the substrate for film absorption. To become
a sensor, the crystal is electrically excited at its natural resonant frequency.
The resonant frequency of the crystal is proportional to its mass; when molecules
adhere to the surface, the frequency decreases. The technique is extremely
sensitive; a monolayer of water or other material can be readily resolved.
One limitation of the QCM is that while it is sensitive to the mass of
the deposited film, it does not identify any other properties of
the film. The
new technique, known as QCM-D, monitors not only the frequency of the crystal
resonance but also its frictional or viscous dissipative or decay aspects.
The pure crystal has little dissipative loss. This means that once the
resonance has been established, the crystal will continue to oscillate
at its resonant
frequency for a long time before the amplitude exponentially fades away.
A good analog is the ring of an empty wine goblet that is plinked. If you
hold
it by the stem, the ring can persist for many seconds but if you hold the
bottom of the goblet the sound will rapidly fade as the ringing energy
is absorbed
by your hand. In electrical engineering, a term that is frequently used
is the “Q” of a resonance. A high Q would have a long ring time and
a low Q has a short ring time.
With a QCM-D microbalance, by monitoring the decay properties of the
ringing, not only can the mass of an adhered film be measured but
also aspects of
its hardness. A rigid film will have a high Q, but if the film is less
rigid and
has more frictional loss, the Q will be lower. The technique has been
extended to measurements of the Q of up to the seventh harmonic overtone
of the
fundamental crystal resonance. Since higher harmonics don’t propagate as far into
the film, analysis of the fundamental and harmonics gives information about
the hardness of the film at various depths. It can give real time characterization
of film depth and structure. The crystal can be coated with a number of materials
so that film absorption on various substrates can be studied.
To date, most applications of the QCM-D technique have been to characterize
bio-surfaces. Aspects such as adsorption, hydration, interaction, cross-linking
and phase transitions have been monitored. For example, the adsorption
and linking structures of proteins on various surfaces or under the
influence of external conditions has been studied in real time. The
QCM-D technique
has
been complemented with other techniques such as fluorescence spectroscopy
and dynamic light scattering to investigate the difference in binding
and enzymatic
activity between aqueous and surface-bound enzymes.
In addition to bio-surface applications, potential utility of the technique
extends to any adsorbed film with non-rigid visco-elastic properties.
Surfactant residues on surfaces, polymers, and water adsorption or
desorption are
possible application areas.
In summary, another tool is now available for the chemist or engineer
who needs to characterize critical surfaces. The QCM-D technique
adds the versatility
of characterizing non-rigid thin films.
Barbara Kanegsberg and Ed Kanegsberg are independent consultants in critical cleaning, precision cleaning, surface preparation, and contamination control. They are the editors of “Handbook for Critical Cleaning,” CRC Press. Contact them at BFK Solutions LLC., 310-459-3614; info@bfksolutions.com; www.bfksolutions.com.
