Increasing the Accuracy of Quartz Crystal Thin Film Sensors in Production
Processes(Or: Why the Film Thickness Readings You See Are Wrong!)
Scott Grimshaw
Colnatec LLC

Quartz crystal thin film sensors (QCMs) are routinely employed in the manufacture of thin film electronic
devices such as OLED’s, LCD’s and Integrated Circuits. While known for their ability to detect films asthin 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 sophisticatedthin film products.
The measurement of film thickness was pioneered by G. Sauerbrey in the early 1960’s. Using a resonating
quartz disk, Sauerbrey showed the relationship between the frequency of vibration (resonance frequency)
and the mass, or corresponding thickness, of material deposited on its surface. This relationship was refinedin later years by C. Lu to account for the acoustic properties of the film. In its current form, the equation
used to calculate film thickness is defined as:ρftf = (ρqvq/2πZfc) tan-1{Z tan [π(fq-fc)/fq]}
Where:

ρf =
tf =
ρq =
vq =
Z =
fq =
fc =

density of film measured
thickness of film
density of quartz
velocity of acoustic wave through quartz
acoustic impedance factor
frequency of uncoated quartz
frequency of coated quartz

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, ageometrical 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 forin a typical deposition. Although the quartz crystal resonance frequency has been shown to be sensitive tomass deposition, it also is sensitive to temperature, stress in the growing film, and adhesion of the film toquartz, among other factors. The temperature dependence is so great that quartz has been used as athermometer.
To minimize temperature dependence, quartz crystals are cut from a quartz bar at a specific orientation
angle. The angle most commonly chosen is referred to as the AT cut, 35° 15’ from the Z or optic axis of thequartz bar. This cut has a near zero frequency-temperature dependence at 20°C. For a change of a few
degrees in either direction, the quartz resonator will not exhibit any frequency shift. This implies thatduring a deposition run, the film thickness monitor will not interpret a slight warm-up of the crystal asadditional mass deposited.

©2014 Colnatec LLC

To further minimize temperature related frequency changes, quartz sensors are placed in water-cooled
housings, with the cooling water set at 20°C. In theory, this configuration results in the frequencytemperature behavior being minimized or even eliminated. In practice, however, this is not realized andworse, the process engineer is not even aware that it is a factor.
As sophisticated as the electronics have become for quartz crystal thin film measurement, in no case does
the mention of crystal sensor temperature make its way into the operation manuals, except to the extent thatthey specify crystals must be cooled to 20°C. No commercial instruments other than Colnatec’s actually
measures this temperature either, in the event that the user chooses to adhere to this specification. In actualoperation, we have monitored uncooled crystals during routine thin film depositions and found they canexhibit temperature changes of at least 50 to 100 °C. For water-cooled sensors, the crystal can experience
20 °C swings during a typical coating cycle.
The significance of temperature shifts becomes critical when measuring films less than 300 Angstroms.
From the frequency-temperature charts, it can be shown that a 20°C shift can amount to an erroneous
thickness reading of 20 Angstroms or greater in the final thickness reading. This error can easily double if
the deposition source emits large amounts of radiation. The sudden heating of the crystal has been shown tolead to frequency changes of 50-60 Hz instantaneously, corresponding to thickness errors of 25 to 30
Angstroms for low-density materials.
To compound matters, commercially available quartz crystals are cut in a range of angles, typically 35°15’+/- 3’. This 6’ window for crystals introduces another source of error in thickness measurement. Under
identical deposition conditions, the variation of frequency change with temperature between crystals at 35°
12’ and 35°18’ amounts to 10% or more.
Unless process engineers and product designers account for the quartz crystal temperature effects, 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 my not be reproducible inpilot or large scale manufacturing runs.

©2014 Colnatec LLC