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Zinc Sulfide ZnS Evaporation Process Notes

Zinc sulfide is an inorganic chemical compound with a chemical formula of ZnS. It is white in appearance with a melting point of 1,700°C, a density of 3.98 g/cc, and a vapor pressure of 10-4 Torr at ~800°C. It is often used as a phosphor and can emit various colors depending on the element introduced as an activator. It is evaporated under vacuum for infrared optical coatings, more specifically, night vision optical coatings.

Zinc Sulfide ZnS Specifications

Material TypeZinc Sulfide
Color/AppearanceWhite, Crystalline Solid
Melting Point (°C)1,700
Theoretical Density (g/cc)3.98
Max Power Density
(Watts/Square Inch)
Type of BondIndium, Elastomer
Z Ratio0.775
Thermal Evaporation Techniques Boat:  Ta, Mo
E-Beam Crucible Liner MaterialTantalum, Molybdenum
Temp. (°C) for Given Vap. Press. (Torr) 10-4:  ~800
UN Number 3283
CommentsPreheat gently to outgas. Films partially decompose. n=2.356.

* This is a recommendation based on our experience running these materials in KJLC guns. The ratings are based on unbonded targets and are material specific. Bonded targets should be run at lower powers to prevent bonding failures. Bonded targets should be run at 20 Watts/Square Inch or lower, depending on the material.


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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