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.