Quartz crystals are used to monitor deposition rates. The quartz oscillates at a resonance
frequency that is dependant on the thickness and atomic mass of the film deposited onto it. When
a voltage is applied across the faces of a properly shaped piezoelectric crystal, the crystal is
distorted and changes shape in proportion to the applied voltage. At certain discrete frequencies
of applied voltage, a condition of very sharp and repeatable electro-mechanical resonance is
seen. Quartz monitors are capable of measuring thickness of less than a single atomic layer with
0.5% accuracy.
Quartz deposition meter must be programed for each material it is used to measure.
The montor's position in the evaporation chamber is also relative. The tooling factor is used to
calibrate the meter to its position in the evaporation chamber.
The Quartz crystal must be replaced frequently to provide consistant results. If the deposition rate
is ocsilating during a controlled deposition, it is a sign that the crystal needs to be replced. Crystal
life is highly depentand on process conditions of rate, power radiated from the source, location,
material, and residual gas composition.
To measure deposition thickness precisely, a deposition meter must have knowledge of the
material being evaporated, specifically its density and Z-ratio.
The Z-ratio is a parameter that corrects the frequency change to thickness transfer function for the
effects of acoustic impedance mismatch between the crystal and the coated material. A table of
Z-ratios for common materials is available here.
Because the flow of material from a deposition is not uniform it is necessary to account for flux
differences between the deposition meter's sensor and any samples. This is accounted for by the
tooling factor, which can be experimentaly established.
Process for determining Density
1.
2.
3.
4.
5.
6.
Place a substrate with suitable masking for film thickness measurement adjacent to the
sensor, so that the same thickness will be accululated on the crystal and this substrate.
Set density of the film material to a tabulated value.
Set Z-ration to 1.0 and tooling to 100%.
Start with a new crystal and make a short deposition (1000-5000 A) and determine actual
thickness.
Determine new density with this equation: Density(%) = Dinitial*Tdisplayed/Tmeas.
NOTE: tabulated densities are sufficient for most applications.
Process for determining Tooling Factor
7.
Place a substrate with suitable masking for film thickness measurement adjacent to the
sensor, so that the same thickness will be accululated on the crystal and this substrate.
8. Set density and Z-ratio of the film material to tabulated values.
9. Make a short deposition (1000-5000 A) and determine actual thickness.
10. Determine new tooling with this equation: Tooling (%) = TFinitial*Tdisplayed/Tmeas.
11. NOTE: Several evaporations should be made for calibration to average out random
variations.
Process for deternination of Z-ratio
1.
2.
3.
4.
Z-values for thin films are in general very close to tabulated bulk values. However, for films
under high stress Z-Values of thin films are slightly smaller than those of bulk materials.
Using calibrated density and 100% tooling, make deposition such that 1/2 of the crystal
lifetime is used.
Place new substrate next to sensor and make a second short deposition (1000-5000 A).
Determine actual thickness on the substrate.
Adjust Z-ratio value in deposition meter to bring the thickness reading in agreement with
actual thickness.
Troubleshooting with crystals
1. Crystal fails too frequently
a. Crystal is being struck by molten material from evaporation source.
Move it further away.
b. Crystal is damaged.
c. Build up material on edge of crystal holder is in contact with te
crystal.
2. Large jumps of thickness reading during deposition
a. Crystal is damaged
b. Crystal is near end of useful life.
c. Crystal holder is dirty.
d. Stress causes film to peel from sensor.
e. Crystal is being struck by molten material from evaporation source.
f. Small pieces of magnetic material are attracted to the sensor.
3. Poor thickness reproducibility
a. Move sensor to diferent location in chamber.
b. Material does not adhere to the crystal.
c. Sweep, dither, or position where e-beam strikes target has changed.
4. Crystal oscillates in vacuum but not in air
a. Crystal near end of useful life.
b. Excessive moisture accumulation on crystal.
5. Thickness reading changes significantly during source warm up or cool
down.
a. Crystal not properly seated.
b. Insufficient cooling.