Quartz Crystal Thin-Film Monitoring Forges Ahead
The demands of cutting-edge research are forcing a re-engineering of the quartz crystal monitor.
Research in fields such as nanotechnology, biosensors, thin-film
displays and high-speed optical communications has increased the complexity of thin-film
structures. Although an antireflection coating consisting of a single layer of magnesium
fluoride may have been sufficient 20 years ago, current designs may call for a 24-layer
stack of films with alternating refractive indices. With high-speed optical communications,
this stack increases tenfold, leading to filters comprising up to 256 layers. Monitoring
the thickness of these coatings has become increasingly important and, although
quartz crystals have been used for this purpose since the 1960s, many variables
can degrade the process.
New developments, however, have addressed this
issue. From the introduction of quartz crystal thin-film sensors that operate at
elevated temperatures to the refinement of existing quartz designs to account for
the stresses and failure mechanisms caused by optical materials, this critical process-control
element is catching up with the requirements for making the next generation of optics.
Quartz crystal monitors may be the most misunderstood
components of optical thin-film deposition systems. They provide coating rate and
thickness data in real time, with angstrom resolution. When set up and calibrated
correctly, they automatically control deposition sources, ensure repeatable and
accurate thin-film coatings, and control optical film properties dependent on deposition
rate. When used incorrectly, crystals drive process engineers to wits’ end
with erroneous measurements and random failures.
Quartz sensors measure film thickness
by monitoring a change in the frequency of vibration of a test crystal coated simultaneously
with process substrates (Figure 1). These substrates are positioned close to the
crystal, ensuring that the amount of evaporant falling on both components is identical.
If this is not the case, a geometrical correction, called the tooling factor, is
Figure 1. In a typical quartz crystal monitor setup, the crystal is contained in a water-cooled
housing, mounted in a line-of-sight position relative to the coating source.
Coupled to an electrical circuit, the
crystal vibrates at its natural or resonant frequency, which for most commercial
instruments is between 5 and 6 MHz. A microprocessor-based control unit monitors
and displays this frequency, or derived quantities, continuously. As material coats
the crystal during deposition, the resonant frequency decreases in a predictable
fashion, proportional to the rate at which material arrives at the crystal and proportional
to the material density. The frequency change is calculated several times per second,
converted in the microprocessor to angstroms per second and displayed as deposition
rate. The accumulated coating is displayed as total thickness.
The sensitivity of the sensor is remarkable.
A uniform coating of as little as 10 Å of aluminum will cause a frequency
change of 20 Hz, easily measured by today’s electronics. As the density of
the film increases, the frequency shift per angstrom increases.
The useful life of quartz depends on
the thickness and type of coating monitored. If a low-stress metal such as aluminum
is deposited, layers as thick as 1 million Å have been measured. At the other
extreme, high-stress dielectric films will reach less than 2000 Å before the
In the early days of crystal monitors, metallic
films of copper, silver and gold were the most common materials deposited. These
elements produced low-stress coatings and were condensed on substrates held at or
near room temperature. Under these conditions, accurate determinations of film thickness
and deposition rate were achievable.
When the optics industry discovered
crystal monitors, attention shifted from opaque metals to transparent materials
such as magnesium fluoride and silicon dioxide, which, unfortunately, produced
films with high intrinsic stresses and required high process or substrate temperatures.
These were not welcome developments for crystal monitoring. Quartz is highly sensitive
to stress and temperature changes.
This sensitivity can be traced to a
fundamental property of quartz called piezoelectricity. If a bar of quartz bends,
it will develop a voltage on opposite faces. Conversely, if a voltage is applied,
the bar will bend. When alternating voltage is applied, the bar will vibrate or
oscillate in phase with the voltage.
At a specific frequency of oscillation,
quartz will vibrate with minimal resistance, much the way a tuning fork rings when
struck. This natural resonance frequency is used as the basis for measuring film
thickness. When coatings are added to the crystal surface, the resonant frequency
decreases linearly. If the coatings are removed, the resonant frequency increases.
To complicate matters, a quartz crystal
also exhibits a frequency change when deformed by thin-film stresses or mechanical
forces from the mounting holder. If process conditions heat or cool the crystal,
a similar frequency shift occurs. Regardless of the origin, the shift is indistinguishable
from that caused by the addition of a coating.
Frequency shifts can be positive or
negative, cumulative and random. The causes of resonant frequency changes include:
• Vibrations introduced through the mounting hardware.
• Variations in the voltage used to oscillate the crystal.
• Changes in the film being monitored (acoustic impedance).
• Adhesion failure of the monitored coating or quartz electrodes.
• Radio-frequency interference in the monitoring circuit.
What can go wrong?
These effects introduce large errors in thickness
and rate. Temperature swings in quartz can result in thickness variations of 50
Å or more. Adhesion failure causes 100-Å rate spikes, and extraneous
vibrations can produce changes of about 1000 Å. For precision optical components,
these errors result in major yield loss.
The harsh conditions present during
optical film coating have a deleterious effect on the operating life of the crystal.
High-stress coatings can deform the crystal to the point that it ceases oscillation
without warning. Splatters of material from the coating source lead to the same
failure. High-energy plasmas used for substrate cleaning couple into the crystal
electronics and cause severe electrical noise. High-temperature depositions overheat
the crystal, driving it past its operating limit.
Early crystal failure can be a great
inconvenience or an unmitigated disaster. In the case of thin-film stacks of 100
layers or more, venting the chamber to replace crystals is not an option, because
of the undesirable effects of atmospheric gases on film chemistry. For the very
thick films used in laser-power or infrared optics, short crystal life may prevent
completion of the coating. For high-speed roll coating systems, abrupt crystal failure
can cause great numbers of ruined substrates.
Over the past 30 years, some weaknesses
of crystal monitors have been addressed with design changes to crystals, mounting
holders and control units. The use of water-cooled holders, or sensor heads, combined
with temperature-insensitive quartz (AT-cut), reduced thermally induced frequency
shifts for low-temperature processes. Planoconvex circular crystals mounted in spring
contact heads minimized extraneous vibrations. Improved electronics and shielding
eliminated radio-frequency interference and voltage variations. Corrections to
the thickness calculation algorithm have accounted for the acoustic impedance. Multiple
crystal sensor heads are used as a solution to premature crystal failure.
Nevertheless, issues such as film stress,
adhesion failure and extreme temperature effects have not been adequately addressed.
These are difficult problems, and there has not been sufficient economic incentive
to pursue solutions. In 2003, however, the demands of nanotechnology, thin-film
displays and high-speed optical communications are forcing a re-engineering of the
quartz crystal monitor.
A promising future
For optical device fabrication, dealing with thin-film
stress and adhesion effects on quartz is an area with the greatest short-term benefit.
Until recently, the solutions for erratic crystal behavior have been more art than
science. Users have employed special crystal inspection, cleaning and mounting procedures
to achieve reproducible runs. More sophisticated users have ascribed benefits to
crystal electrode coloration and surface topography, with mixed results. However,
the science is catching up. Studies on new quartz types, electrodes and sensor
heads have led to promising designs for reliable sensors.
One improved design is the development
of the aluminum or aluminum alloy electrode crystal. For SiO2 coatings, this electrode
can extend the useful life of the sensor by 100 percent or more compared with the
industry-standard gold crystal. Furthermore, frequency shifts due to electrode adhesion
failure are decreased up to 90 percent under standard laboratory conditions. The
benefit of this electrode appears to be material- and deposition-specific, because
it is not the same for all coatings. Nonetheless, it may prove to be a test-bed
design for thin-film stress studies.
A second design change involves alternative
orientation angles of quartz for monitor crystals. AT-cuts exhibit little frequency
change with increasing temperature, or what is called frequency-temperature dependence,
and are used to minimize the resonant frequency shift caused by process heating
(Figure 2). The angle of cut can be varied to allow stable operation at higher temperatures
Figure 2. The standard AT-cut crystal is derived from cutting wafers from a quartz bar at an angle of 35¼
and 15 minutes from the Z-axis.
A more promising crystal is based on
the stress-compensated, or doubly rotated, cut of quartz. This orientation exhibits
similar frequency-temperature behavior to that of the AT-cut, with the added advantage
that it shows essentially no change of frequency when the crystal is stressed. A
monitor crystal fabricated from stress-compensated-cut material, in initial coating
trials, exhibits none of the frequency changes induced by high-stress dielectrics
on AT-cut crystals. Historically, it has been a more expensive version of quartz,
but the benefits for the optical process engineer may outweigh the cost penalty.
A third area of improvement is a redesign
of the crystal sensor head. Although quartz crystals are capable of subnanometer
measurement and are sensitive to mass accumulations of as little as a picogram,
they are traditionally finger-handled when installed in the deposition system. This
is surprising because the crystal is 0.010 in. thick and can be easily cracked prior
to mounting. In addition, potential contamination of the crystal surface has disastrous
effects on the adhesion of the coating being monitored. Adhesion failure results
in frequency shifts opposite those of film coating, introduces thickness errors
and degrades the crystal’s lifetime. Recently introduced crystals are premounted
in a holder, with the measurement surface protected from human contact.
Less obvious but equally rewarding
improvements deal with the thermal environment of the crystal. Much as stress effects
lead to erratic crystal performance, temperature effects lead to equally significant
thickness errors. An examination of the frequency-temperature curve for AT-cut quartz
illustrates how variable the effect of temperature is (Figure 3). If one increases
the crystal temperature 20° from room temperature, the frequency will shift
approximately 20 Hz. For aluminum, this is equivalent to 10 Å of coating.
Larger temperature shifts generate accelerating changes because of the steep slope
of the frequency-temperature curve.
Figure 3. This AT-cut quartz frequency temperature curve shows how temperature variation affects
Recent studies of standard sensor heads
show that, even with water-cooling, the crystal temperature can rise 20° to
30° within a 10-minute process. For extended runs with high chamber temperatures,
this becomes considerably larger.
A new dedicated quartz crystal sensor
head temperature monitoring system retrofits existing thin-film quartz crystal controllers.
With data in hand, the true thermal environment of the crystal sensor can be measured,
eliminating the guesswork and adding more science to the art of optical coatings.
A paradigm shift is under way in quartz
crystal process monitoring. In many applications, crystals are the keys to success.
No matter how big a breakthrough may be — whether in materials, geometry,
process design or application — if a thin-film coating of any sophistication
is needed, the weak link is how accurately that film can be measured.
Meet the author
Scott Grimshaw is founder and president of Cold Springs R&D Inc. of Syracuse, N.Y.
- The use of atoms, molecules and molecular-scale structures to enhance existing technology and develop new materials and devices. The goal of this technology is to manipulate atomic and molecular particles to create devices that are thousands of times smaller and faster than those of the current microtechnologies.
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