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Chromium Cr Notes


Chromium (Cr) General Information

Chromium is one of the most popular metals in the world. Chromium is a silvery, lustrous, hard, and brittle metal known for its high mirror polish and corrosion resistance. It has a melting point of 1,857°C, a density of 7.2 g/cc, and a vapor pressure of 10-4 Torr at 1,157°C. Its name comes from the Greek word "chroma", which means color, due to its very colorful compounds. It is widely used in the automobile industry to form a shiny coating found on wheels and bumpers. Chromium is used in many vacuum applications such as automotive glass coatings, photovoltaic cell fabrication, battery fabrication, and decorative and corrosion resistant coatings.

Chromium Cr Specifications

Material TypeChromium
SymbolCr
Atomic Weight51.9961
Atomic Number24
Color/AppearanceSilvery, Metallic
Thermal Conductivity94 W/m.K
Melting Point (°C)1,857
Coefficient of Thermal Expansion4.9 x 10-6/K
Theoretical Density (g/cc)7.2
SputterDC
Max Power Density
(Watts/Square Inch)
80*
Type of BondIndium, Elastomer
Z Ratio0.305
E-BeamGood
Thermal Evaporation Techniques Boat:  Cr Plated W Rods
Coil:  W
Basket:  W
Crucible:  VitC
E-Beam Crucible Liner MaterialFABMATE®, Graphite, Tungsten
Temp. (°C) for Given Vap. Press. (Torr) 10-8:  837
10-6:  977
10-4:  1,157
CommentsFilms very adherent. High rates possible.

* 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.

Z-Factors

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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