Calcium Ca Evaporation Process Notes
Calcium is classified as an alkaline earth metal on the Periodic Table. It is silvery-white in appearance and soft enough that it can be cut with a sharp knife. It is known to easily oxidize when exposed to the atmosphere. Calcium has a density of 1.54 g/cc, a melting point of 839°C, and a vapor pressure of 10-4 Torr at 459°C. Calcium is an essential building block for human life as it is found in abundance in human skeletons and teeth. Calcium and its compounds are also used to produce commercial concrete and cement. Calcium, along with its alloys and compounds, is evaporated under vacuum to make optical storage media, superconductors, and anti-reflective layers.
Calcium Ca Specifications
| Material Type | Calcium |
| Symbol | Ca |
| Atomic Weight | 40.078 |
| Atomic Number | 20 |
| Color/Appearance | Silvery White, Metallic |
| Thermal Conductivity | 200 W/m.K |
| Melting Point (°C) | 839 |
| Coefficient of Thermal Expansion | 22.3 x 10-6/K |
| Theoretical Density (g/cc) | 1.54 |
| Max Power Density (Watts/Square Inch) | 20* |
| Z Ratio | 2.62 |
| E-Beam | Poor |
| Thermal Evaporation Techniques |
Boat: W Coil: W Basket: W Crucible: Al2O3, Q |
| Temp. (°C) for Given Vap. Press. (Torr) |
10-8: 272 10-6: 357 10-4: 459 |
| UN Number | 1401 |
| Comments | Corrodes in air. |
* 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.
Thermal Evaporation of Calcium (Ca)
One of the challenges of thermally evaporating calcium is controlling the deposition rate, which is dictated by the temperature. The temperatures required for calcium to evaporate makes it difficult to control the deposition rate. Calcium will evaporate at an appreciable rate until the temperature reaches ~500°C. At higher temperatures, there is risk of calcium oxide (CaO) contamination in films. However, this can be avoided as long as the temperature is controlled well below 1,000°C. Calcium oxide (CaO) will evaporate at an appreciable rate at ~1,700°C.
We have reported success evaporating calcium out of our EVCH12 heater with an EVC5AO crucible. Great care must be taken when installing the heater to prevent the outer shields from becoming warped which can cause a short in the heater, causing the welded joints to fail. The heater should be centered between the contacts and the outer shielding must be clear of the leads. Crucibles should be stored in a cool, dry place and always handled with gloves or forceps.
Correct - Crucible heater centered, outer shielding clear of leads
Incorrect - Crucible heater off-center, shielding in contact with leads/inner shielding
A more cost-effective alternative would be to use a dimple-style, tungsten boat such as our EVS8B005W if using a KJLC system. However, it is more difficult to control the temperature with a tungsten boat than with the crucible heater/crucible set-up. It is important to note that temperature should be closely monitored while executing either method.
Since calcium has a relatively high vapor pressure at only 459°C, one must carefully consider executing calcium evaporation in any deposition system. Calcium is very difficult to remove from deposition chambers, and it could possibly contaminate subsequent films. For this reason, some users prefer to use only dedicated vacuum chambers for calcium deposition.
E-beam Evaporation of Calcium (Ca)
Calcium is rated as 'poor' for e-beam evaporation because it sublimes at fairly low temperatures (459°C) making it very difficult to control the deposition rate. We have not yet qualified a suitable crucible liner material for e-beam evaporating calcium, so we would recommend starting with a FABMATE® liner since it is compatible with most materials.
A key process note is to consider the fill volume in the e-beam application because we find that the melt level of a material in the crucible directly affects the success of the crucible liner. Overfilling the crucible will cause the material to spill over and create an electrical short between the liner and the hearth. The outcome is cracking in the crucible. This is the most common cause of crucible liner failure. Placing too little material in the crucible or evaporating too much material before refilling can be detrimental to the process as well. When the melt level is below 30%, the e-beam is likely to strike the bottom or walls of the crucible which immediately results in breakage. Our recommendation is to fill the crucible between 2/3 and 3/4 full to prevent these difficulties
Crucible liners should be stored in a cool, dry place and always handled with gloves or forceps.
Since calcium has a relatively high vapor pressure at only 459°C, one must carefully consider executing calcium evaporation in any deposition system. Calcium is very difficult to remove from deposition chambers, and it could possibly contaminate subsequent films. For this reason, some users prefer to use only dedicated vacuum chambers for calcium deposition.
See highlighted results that match your result in the table below.