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Lithium Fluoride LiF Evaporation Process Notes

Lithium fluoride is an inorganic compound with a formula of LiF. It has a density of 2.64 g/cc, a melting point of 845°C, and a vapor pressure of 10-4 Torr at 1,180°C. It is crystalline in appearance and white in color. One of its main uses is as a component of molten salts and it is evaporated under vacuum to form anti-reflective layers for optical coatings.

Lithium Fluoride LiF Specifications

Material TypeLithium Fluoride
Color/AppearanceWhite, Crystalline Solid
Melting Point (°C)845
Theoretical Density (g/cc)2.64
Z Ratio0.778
Thermal Evaporation Techniques Boat:  Ni, Ta, Mo, W
Crucible:  Al2O3
E-Beam Crucible Liner MaterialTantalum, Tungsten, Molybdenum
Temp. (°C) for Given Vap. Press. (Torr) 10-8:  875
10-6:  1,020
10-4:  1,180
UN Number 3288
CommentsRate control important for optical films. Preheat gently to outgas.


Empirical Determination of Z-Factor

Unfortunately, Z Factor and Shear Modulus are not readily available for many materials. In this case, the Z-Factor can also be determined empirically using the following method:

  • Deposit material until Crystal Life is near 50%, or near the end of life, whichever is sooner.
  • Place a new substrate adjacent to the used quartz sensor.
  • Set QCM Density to the calibrated value; Tooling to 100%
  • Zero thickness
  • Deposit approximately 1000 to 5000 A of material on the substrate.
  • Use a profilometer or interferometer to measure the actual substrate film thickness.
  • Adjust the Z Factor of the instrument until the correct thickness reading is shown.

Another alternative is to change crystals frequently and ignore the error. The graph below shows the % Error in Rate/Thickness from using the wrong Z Factor. For a crystal with 90% life, the error is negligible for even large errors in the programmed versus actual Z Factor.

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