APPLICATION NOTESHUTTERED CRYSTAL SENSORS
Quartz crystals are used to monitor or control the deposition
rate of material evaporated in vacuum processes. As material
condenses on the crystal, its resonant frequency decreases.
This change in frequency can be converted into an accumulated thickness and corresponding rate. However, crystals will
only accept a finite amount of material before they become
unstable or stop oscillating.
There are several process steps during the deposition when“shuttering” the crystal sensor is beneficial. For instance,
during the period when the source material is brought up totemperature, the material may emit particulates or “spits”
which can cause a crystal to fail immediately. During this time,
the source shutter is still closed and protects both the substrate and usually also the sensor. In applications, where thecrystal sensor “sees” the evaporation source during this time,a shuttered sensor should be used.
In optical coating processes, two materials are typically deposited as alternating layers. Thickness accuracy (unless Auto-Z
is used) will be improved if a separate crystal sensor is usedfor each material and each sensor receives only one material.
Depending on the location of the sources with respect to thesensor(s), this may be accomplished by using either a dual
sensor (750-212-G2) or two single shuttered sensors (750211-G2).
In other processes, when the amount of material deposited
exceeds the crystal’s ability to sustain an oscillation, thecrystal fails. These processes can be controlled with a dual
or other multi-position sensor or by using a shuttered crystal
sensor operating in the RateWatcher™ mode.

RATEWATCHERThe RateWatcher feature of INFICON thin film deposition
controllers uses a shuttered crystal sensor in a “Sampleand Hold” mode to extend the life of the crystal. During the“Sample” interval, the shutter is open and the control voltage
regulating the evaporant power supply is adjusted to achieve
the desired deposition rate. When the desired rate is achieved,
the instrument transitions to the “Hold” interval and closes

the sensor shutter. In this state the crystal no longer receives
material, consequently its “life” is increased. The thickness displayed continues to increment and the power is held constant.
The “Sample and Hold” cycle is repeated until Final Thickness
is reached. This method is particularly well suited for control ofsputtering processes where the rate is very stable and closely
related to the sputter power. In sputtering, the “Hold” time canbe fairly long. With an appropriately adjusted “Hold” time and if
thickness tolerance allows, the RateWatcher function can also
be used successfully for thermal evaporation processes.

DUAL SENSOR CONTROLWith the INFICON thin film deposition controllers, theCrystalSwitch™ feature can be used to ensure continuous
rate and thickness control. CrystalSwitch employs a shutter toswitch between crystals in a dual-crystal sensor (i.e. two independently operated crystal sensor heads on one body), providing redundancy if the primary crystal fails. It is also possible
to dedicate each crystal to a specific material. The controller
is programmed to use the appropriate crystal when the designated film (or material) is being deposited.

PROVEN RELIABILITY
The mechanism which moves the shutter consists of a pneumatically-operated (vacuum-sealed) actuator, a piston, a piston
support, and a piston coupling and shaft. When air (@ 90 psig)
is applied to the actuator, it forces the piston to move. The piston is linked to the shaft which rotates, moving the shutter overthe crystal. A spring forces the shutter to return to its normal
position, when the air is released.
The design has proven to be highly reliable. It has been incorporated in INFICON shuttered crystal sensors since April
1987.
Shutter AssemblyPart Number
750-005-G1
750-210-G1*

Description
Sputtering Sensor
Dual, Standard or Compact Sensor*Standard equipment on the Dual Sensor.

SHUTTER MODULE RETROFIT KITS
Retrofit kits for field modification are available. All retrofit kits
include the mounting hardware necessary for each style ofsensor.
The shuttered bakeable sensor must be returned to INFICON
for refurbishment.

Please contact INFICON for more details on ordering theShutter Module Retrofit Kits.
All shuttered sensors require the Pneumatic Shutter Actuator
Control valve, part number 750-420-G1. It operates on 24V ACor DC.

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