Tungsten W Evaporation Process Notes
Tungsten, also known as Wolfram, is one of the densest elements in the world. It has a density of 19.3 g/cc, a melting point of 3,410°C, and a vapor pressure of 10-4 Torr at 2,757°C. It is lustrous and grayish-white in appearance. It is known to have the highest melting point of all metals and its characteristic hardness can make it very difficult to machine and forge in pure form. Tungsten is commonly used to make electric filaments. Tungsten carbide is found in drills, saws, and jewelry. Tungsten is evaporated under vacuum for the production of semiconductors, fuel cells, and sensors.
Tungsten W Specifications
|Color/Appearance||Grayish White, Lustrous, Metallic|
|Thermal Conductivity||174 W/m.K|
|Melting Point (°C)||3,410|
|Coefficient of Thermal Expansion||4.5 x 10-6/K|
|Theoretical Density (g/cc)||19.25|
|Max Power Density|
|Type of Bond||Indium, Elastomer|
|E-Beam Crucible Liner Material||Direct in Hearth|
|Temp. (°C) for Given Vap. Press. (Torr)||
|Comments||Forms volatile oxides. Films hard and adherent.|
* 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.