APPLICATION NOTE
Guide to INFICON
Quartz Monitor Crystals
INTRODUCTION

CRYSTAL TYPESThe quartz monitor crystal is a crucial component of aquartz crystal based measurement and control system.
The crystal enables precise and repeatable thin film
depositions for a wide variety of high value end uses.

INFICON AT-cut, plano-convex crystals
are available in:This application note describes various crystal types
and package options available from INFICON®.



Depending on the specific application, one INFICONcrystal may be better suited to the processrequirements than another. The optimum crystal for aprocess may have a longer deposition life, show lessrate noise, have a smaller rate and thickness spike
when the shutter is opened, or it may have the longest
shelf life. Some crystals have a fully coated electrodeon both sides, others provide a very narrow starting
frequency range. Whatever is important for theapplication, INFICON has the best crystal andpackage style.
Please refer to the INFICON website for a list of crystal
part numbers and styles at www.inficon.com. If the typerequired is not shown, contact INFICON.

14 mm (0.550 in.)12.4 mm (0.489 in.)Two frequencies
5 MHz 6 MHz

CHOOSING THE BEST CRYSTAL
Choosing the best INFICON crystal for an application isvery important. INFICON manufactures crystals
compatible with its own and all other manufacturers’equipment. All INFICON crystals are fullyinterchangeable within the constraints of size andfrequency.

Two diameters (sizes)

Three electrode materials
GoldSilver Alloy

Three different electrode patterns


Full Pad/Anchor PadFull Pad Both Sides

Single AnchorNOTE: Maxtek brand crystals, from INFICON,
generally have somewhat lower performancecompared to INFICON brand crystals, but are
offered at lower pricing.

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

FREQUENCY

Four different package choices


Clean room compatible

Flat pack carouselCompact box



50-pack

After choosing the crystal size, choose either a 5 MHz
or 6 MHz crystal based on the crystal frequency range
suitable for the instrument.

CAUTION
Make sure the frequency setting of theinstrument matches the starting frequencyof the crystal.

50 Pack

CompactBox

Flat PackCarouselClean RoomCompatible

SIZE
The first criterion for selecting a crystal for use inexisting equipment is to determine the appropriate
crystal size (diameter) for the sensor.
Crystals are available in two diameters:
14 mm (0.550 in.)

12.4 mm (0.489 in.)All INFICON quartz crystal sensors are compatible with
14 mm (0.550 in.) diameter crystals, and some sensor
heads can be adapted to use the 12.4 mm (0.489 in.)diameter crystal.

Guide to INFICON Quartz Monitor Crystals

INFICON 6 MHz crystals work in all INFICON
instruments and sensors as well as most othermanufacturers’ equipment.
Star ting Frequency
In many cases, a brand new quartz crystal will not havea frequency of exactly 5 MHz or 6 MHz.
It is normal for that crystal to display a crystal life
anywhere from 0 to 5% due to process variations in
producing the crystal. If a new crystal indicates 5% life
spent, it means that either the quartz blank is slightly
thicker than normal (more mechanical robustness), or
the electrode is slightly thicker than normal (better
thermal and electrical properties), or both. Thisadditional thickness causes the starting frequency tobe lower than the rated value of 5 MHz or 6 MHz.
Despite a lower starting frequency, its overall
performance should not be adversely affected. Theselower starting frequency crystals have been tested andresults indicate that a brand new crystal indicating 3 to
5% life spent is just as good as, if not better than, acrystal indicating 0 to 2% life spent.

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ELECTRODE MATERIALS
After choosing crystal size and frequency, choose theelectrode material best suited for the application:
GoldSilverAlloy

Silver crystals provide good adhesion for materials
used in optical coatings, delivering good crystal life andlow rate noise.
Because silver tarnishes due to hydrogen sulfide
present in air, the shelf life of Silver crystals is typicallylimited to six months after opening the package.

Gold
Gold is the most widely used electrode material.
It performs well in a large variety of applications.
Gold provides good adhesion, low rate noise, and good
crystal life. Gold crystals have an indefinite shelf life.
SilverSilver electrodes have high thermal conductance,
enabling them to transfer heat arriving from the hot
evaporation source to the water-cooled sensor body tokeep the crystal from getting too hot.

NOTE: INFICON ships Silver crystals in a heat-sealed
external sleeve filled with inert gas to protect
them against tarnishing during shipment andstorage.
Alloy
The Alloy electrode provides excellent adhesion forcoatings used in optics and also acts to absorb thetensile and compressive stresses common in somefilms, thereby maximizing crystal life and reducing rate
noise.
Activity values tend to be higher for Alloy crystals
compared to Gold crystals. The shelf life of INFICON
Alloy crystals is typically six months.

Table 1 Electrode materials table
Electrode Material
Gold(Standard Type)

Advantages
1 Does not oxidize
2 Indefinite shelf life
3 Most widely used, good for a large number ofapplications

Silver(Sputtering Type)

Alloy(High StressCoatings)

Disadvantages
1 Crystal life may be shorter than Silver or Alloyin some applications (materials used in opticalcoatings)

1 Best transfer of heat arriving at crystal tocooled sensor body (high thermalconductance)

1 Shelf life depends on oxidation and exposure
to sulfur content in air (tarnishing effect)

1 Longest life (2x) for materials used in Opticalfilms or for high stress semiconductormaterials

1 Alloy electrode will oxidize, shorter shelf life
compared to Gold

2 Higher Activity values compared to Gold andSilver

Guide to INFICON Quartz Monitor Crystals

2 Not widely used

2 No added benefit when depositing “normal”
stress materials (non-high stress metals)

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ELECTRODE PATTERNSINFICON crystals are available with three different
electrode patterns:


Full Pad/Anchor Pad (double anchor)

Full Pad/Anchor Pad (single anchor)

Full Pad Both Sides

The excitation area on the anchor electrode patternside is sized to focus energy at the center of the crystal.
This ensures the physical connections made at theperimeter of the crystal have no detrimental effect on its
operation.
The Full Pad/Anchor Pad electrode pattern will provide
the best performance in crystal life and rate noise andis recommended as standard.

Full Pad/Anchor Pad

NOTE: The Full Pad/Anchor Pad crystal must be
installed with the fully coated side facing thedeposition source.
Full Pad Both Sides

Double Anchor

Single Anchor

Full Pad/Anchor Pad crystals have a full-pad electrodepattern on one side and a single or double-anchor
electrode pattern on the other side. The double-anchor
pattern provides a more reliable electrical connection
and is recommended. The crystal must be oriented
with the fully-coated side facing the deposition source.
The anchor electrode pattern focuses the excitationenergy into the center area of the crystal to help avoid
unwanted vibrational modes which reduce crystal life
and increase rate noise. This design maximizes crystal
stability and life by minimizing the opportunity to couple
unwanted modes of oscillation.

Full Pad Both Sides crystals have a fully-coatedelectrode on each side. These crystals do not require a
specific orientation when installed. Full Pad Both Sides
crystals are suitable for high volume production
applications where maximum crystal life is not required.
The Full Pad Both Sides design results in somewhat
shorter crystal life and possible coupling of unwanted
modes of oscillation. These unwanted modes cancause rate noise to appear earlier in crystal life.

Table 2 Electrode pattern
Electrode Pattern
Double Anchor

Advantage
1 Traps energy in the center of the crystal
2 Maximizes crystal stability and life byminimizing opportunity to couple unwanted
modes of oscillation

Single Anchor

1 Traps energy in the center of the crystal
2 Maximizes crystal stability and life byminimizing opportunity to couple unwanted
mode of oscillation

Double Full Face(dual pad, noanchor pattern)

1 No orientation dependency when placing
crystal into holder (crystal cannot be put inupside down)

Guide to INFICON Quartz Monitor Crystals

Disadvantage
1 Crystal can be put into crystal holder upside
down

1 Crystal can be put into crystal holder upsidedown
2 May not be compatible with all sensor headtypes (depends on placement of sensor
electrical contacts)
1 Expect shorter life, and coupling of unwanted
modes of oscillation (crystal can be noisy,
earlier in life)

4 of 7APPLICATIONS - OPTICAL COATINGS
Standard Optical Coatings

For some applications, when the source or sensor
shutter opens, there is a large increase in the amountof heat arriving at the crystal, resulting in a sudden
jump in crystal temperature and film stress. Both ofwhich cause a spike in rate and thickness. For theseapplications, Low Thermal Shock Gold crystals arerecommended.
Ver y Thin Optical Coatings
Under 50 nm Thick

Standard Optical Coatings typically have fewer thaneight layers controlled by the crystal. They include
ophthalmic and simple camera lenses as well as basicanti-reflective display coatings. This application
typically uses Gold crystals.
Precision Optical Coatings

These applications are susceptible to an initial rate andthickness spike when the shutter is opened, due to thelarge increase in heat arriving at the crystal. This spike
may cause incorrect thickness termination andpossible control loop instability. Low Thermal ShockGold crystals are recommended.
NOTE: Low Thermal Shock Gold crystals are designed
to reduce the rate and thickness spike inducedby the thermal shock created when the sourceor sensor shutter opens. Compared to standard
Gold crystals, lifetime is slightly reduced.

APPLICATIONS - METAL COATINGSHigh Heat Load Applications
Precision optical coating applications typically require
multi-layer stacks of dielectric materials.
The applications include bandpass optical filters with asharp cutoff for dense wave division multiplexing andother fiber-communication applications, high-quality,
anti-reflectance optics for cameras, telescopes, riflescopes, microscopes, medical instruments, binoculars,night vision optics devices and semiconductor
photolithography.
Dielectric materials include aluminum oxide (Al2O3),
calcium fluoride (CaF2), magnesium fluoride (MgF2),
tantalum pentoxide (Ta2O5), titanium dioxide (TiO2),
thorium fluoride (ThF4), silicon monoxide (SiO), silicondioxide (SiO2), zirconium dioxide (ZrO2), and many
more. These materials cause a higher level of stress tothe crystal and an Alloy crystal is recommended.

Guide to INFICON Quartz Monitor Crystals

High heat load is often present in diode sputtering fromlarge sources when evaporating very high melting pointmaterials such as molybdenum (Mo), tantalum (Ta),and tungsten (W), or when the substrates are heated
above 300°C.
Crystals for these applications must have high thermal
conductance to transfer heat, arriving from the hot
evaporation source, to the water-cooled sensor bodywhich maintains crystal temperature.
For long crystal life and low rate noise, crystals used inhigh heat load applications must have good adhesion
for the materials used in optical coatings. INFICON
Silver crystals (known as Sputtering Type) are
well-suited for this application.
NOTE: High heat load applications refer to thoseapplications, such as sputtering, that use water
to cool the crystal. For ALD applications, High
Temperature crystals are recommended, see
Applications - ALD.

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Low stress coatings such as aluminum,
gold and silver

Low stress coatings are often used for electricalcontacts and in “Cold Mirrors” to reflect the heat andlight from light sources used in projection equipment
and other lighting applications, such as store display
lighting systems, automotive lights, flashlight reflectorsand LEDs. These applications typically use Goldcrystals.

High Stress Material Coating

APPLICATIONS - OLED

OLED materials have a granular amorphous structure
which does not bond firmly to the crystal electrode.
This structure dissipates motional energy, which resultsin short crystal life. Low Thermal Shock Gold crystals
are recommended.
Some OLED materials are deposited at extremely lowrates. Maximizing rate stability and minimizing rate
noise are very important. Alloy crystals will provide thebest rate stability and minimum noise.

These surfaces stretched

Illustration provided by Felix Lu,
Applied Quantum Technologies / Duke University

High stress materials include those used insemiconductor processes: chromium (Cr), germanium(Ge), molybdenum (Mo), nichrome, nickel (Ni),tantalum (Ta), titanium (Ti), silicon oxycarbide (SiOC),
zirconium (Zr), as well as the dielectric materials listed
above under Precision Optical Coatings. For theseapplications, Alloy crystals usually perform the best.
However, Silver crystals are also suitable.

Guide to INFICON Quartz Monitor Crystals

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Guide to INFICON Quartz Monitor Crystals

APPLICATIONS - ALD
ALD uses two precursors that react to deposit material(metals or metal oxides) one atomic monolayer at atime. Most ALD reactions are temperature dependent,
and the crystal must remain at the reaction
temperature.
Water cooling cannot be used during an ALD process
to cool the crystal and maintain the optimal
temperature for standard INFICON crystals. HighTemperature ALD crystals are recommended for ALD
applications for this reason.
High Temperature crystals are 6 MHz, gold-coatedcrystals that are optimized at 120, 240, or 285°C.
These crystals have a smaller range of temperaturesthat deliver stable and accurate thickness readings.
See Figure 1.
NOTE: Crystals optimized to additional temperatures
are available upon request. Please contact
INFICON for custom crystals.

The graph represents the temperature change from theoptimal crystal temperature and the resulting frequency
error due to temperature.
NOTE: The optimization temperatures of the High
Temperature crystals (120, 240, or 285°C) as
well as standard INFICON crystals (70°C) havebeen normalized to zero to be able to compare
the temperature range of all offerings on onegraph.


70°C crystals display a stable frequency operating
in a temperature range of 59 to 81°C.

120°C crystals display a stable frequency
operating in a temperature range of 112 to 128°C.

240°C crystals display a stable frequency
operating in a temperature range of 235 to 245°C.

285°C crystals display a stable frequency
operating in a temperature range of 281 to 289°C.

Figure 1 High temperature crystal frequency error®
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