We Know Film Thickness Measurement Better Than Anyone
Quartz crystal thin film sensors are routinely employed in the manufacture of thin film electronic devices such as OLEDs, LCDs and Integrated Circuits. While known for their ability to detect films as thin as one Angstrom and, when properly optimized, reliable enough to insure process repeatability, crystal derived thickness readings are often taken as indisputable measurements by process engineers. Unfortunately, this blind faith can have major negative consequences in the manufacture of sophisticated thin film products.
In a typical deposition run, the user programs the crystal controller with the density of the film being monitored, the acoustic impedance value (applicable only to thick films) and the tooling factor, a geometrical correlation of the deposition source to crystal and source to substrate distances. The primary assumption made is that the film density is known, and that it is not dependent on deposition conditions. For a given process configuration, the film density can be calculated exactly by comparing the crystal thickness with a secondary measurement obtained from profilometry, for example. If not, a significant error can be introduced to the film thickness calculation.
The greatest source of error to film thickness measurement arises, however, from factors not accounted for in a typical deposition. Although the quartz crystal resonance frequency has been shown to be sensitive to mass deposition, it also is sensitive to temperature, stress in the growing film, and adhesion of the film to quartz, among other factors. The temperature dependence is so great that quartz has been used as a thermometer.
Unless process engineers and product designers account for the quartz crystal temperature effects and other unique variables, however, they will be unable to guarantee that run to run reproducibility will be possible for their thin film product. Worse, successful products developed under strict protocols in a research laboratory may not be reproducible in pilot or large scale manufacturing runs.
For these reasons and others identified specifically by line engineers, we have designed and now produce unique crystals that overcome these limitations, and our next generation of sensor heads further enable improved accuracy and longer run-times, ultimately delivering our customers an improved bottom line.