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Niobium Nb Evaporation Process Notes

Niobium is classified as a transition metal on the Periodic Table of Elements. It is often found in nature with tantalum; hence it mimics many of its properties. It has a melting point of 2,468°C, a density of 8.57 g/cc, and a vapor pressure of 10-4 Torr at 2,287°C. While typically metallic gray in color, it is known to take on a bluish tinge when exposed to air. It is used in various superconducting alloys and is a common element in jewelry production. Niobium and its compounds and alloys are evaporated under vacuum to make semiconductor, optical and wear resistant coatings.

Niobium Nb Specifications

Material TypeNiobium
SymbolNb
Atomic Weight92.90638
Atomic Number41
Color/AppearanceGray, Metallic
Thermal Conductivity54 W/m.K
Melting Point (°C)2,468
Coefficient of Thermal Expansion7.3 x 10-6/K
Theoretical Density (g/cc)8.57
SputterDC
Max Power Density
(Watts/Square Inch)
100*
Type of BondIndium, Elastomer
Z Ratio0.492
E-BeamExcellent
E-Beam Crucible Liner MaterialFABMATE®
Temp. (°C) for Given Vap. Press. (Torr) 10-8:  1,728
10-6:  1,977
10-4:  2,287
CommentsAttacks W source.

* 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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