Nickel Ni Evaporation Process Notes
Nickel is a hard, lustrous, silvery-white metal. It has a density of 8.91 g/cc, a melting point of 1,453°C, and a vapor pressure of 10-4 Torr at 1,262°C. Its key characteristics are malleability, ductility, and ferromagnetism and its polished surface resists tarnishing when exposed to air. It is the second most abundant element in earth's core next to iron. It is mainly used to make stainless steel, coins, and batteries. It can also be found in jewelry, but its presence has decreased due to skin allergies. When evaporated in vacuum, nickel can form a decorative coating on ceramic surfaces or a solder layer in circuit device fabrication. It is often sputtered to form layers in the production of magnetic storage media, fuel cells, and sensors.
Nickel Ni Specifications
| Material Type | Nickel † |
| Symbol | Ni |
| Atomic Weight | 58.6934 |
| Atomic Number | 28 |
| Color/Appearance | Lustrous, Metallic, Silvery Tinge |
| Thermal Conductivity | 91 W/m.K |
| Melting Point (°C) | 1,453 |
| Coefficient of Thermal Expansion | 13.4 x 10-6/K |
| Theoretical Density (g/cc) | 8.91 |
| Sputter | DC |
| Max Power Density (Watts/Square Inch) | 50* |
| Ferromagnetic | Magnetic Material |
| Z Ratio | 0.331 |
| E-Beam | Excellent |
| Thermal Evaporation Techniques |
Boat: W*** Crucible: Al2O3 |
| E-Beam Crucible Liner Material | FABMATE®‡, Copper |
| Temp. (°C) for Given Vap. Press. (Torr) |
10-8: 927 10-6: 1,072 10-4: 1,262 |
| Comments | Alloys with W/Ta/Mo. Smooth adherent films. |
† Magnetic material (requires special sputter source).
*** Alumina Coated.
* 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 Nickel (Ni)
Nickel can be evaporated via electron beam or thermal evaporation. However, e-beam is the preferred method for evaporation.
Thermally evaporating nickel is very difficult. Like titanium, it has a strong tendency to alloy with refractory metals. Attempting to thermally evaporate nickel out of a tungsten boat has proven futile. As the boat heats up and the nickel melts, it alloys with the boat, causing it to become brittle and crack shortly after exposure to the liquid nickel. Therefore, deposition is very limited.
Instead, we recommend an alumina coated, tungsten dimple boat such as our EVS9AAOW if using a KJLC® system. The alumina acts as a barrier between the nickel and the tungsten, thus prolonging the life of the boat and increasing the amount of nickel deposition. Instead of wetting the boat, the nickel will form a ball on top of the alumina coating once it melts. Only four 1/8" diameter x 1/8" long pellets can be loaded safely into this boat at one time. These boats will fail often, lasting approximately 1-3 runs. Due to alumina's limited heat conduction abilities, more power will be required to achieve evaporation.
Using the EVS9AAOW, we are able to deposit relatively thick films (approximately 1 micron per evaporation run) at rates from 0-5 angstroms per second using two Kepco© power supplies linked in parallel. The nickel only takes about 230A of current to melt and to start evaporating, but requires voltages greater than 4V to run at this current. Using only one Kepco power supply, the nickel will still evaporate at about 100% power output, but the rates will be closer to 1 Å/sec, and the film thickness will be limited to around 2,000 Å total before the rate begins to decrease.
The alternative option to using a boat would be to use an Al2O3 crucible with a crucible heater. The problem here is that it is very difficult to get to high enough temperatures to deposit the nickel. Crucibles rely on external heating, and there is only so much power that can be applied in order to raise the temperature to achieve deposition.
E-beam Evaporation of Nickel (Ni)
The best way to evaporate nickel is by electron-beam evaporation. We recommend using a FABMATE® liner for the e-beam evaporation of nickel. However, Fabmate® is limited to only one run. During evaporation, the nickel will melt and adhere to the crucible liner walls. Upon cooling and reheating (if it makes it that far), the difference in thermal contraction and expansion of the nickel and the Fabmate® liner will cause immense stress on the liner, and it will crack. Experienced users are sometimes able to get multiple runs out of a Fabmate® liner with slow ramp rates of ~0.1% power/sec both up and down.
An alternative to Fabmate® would be a copper crucible liner. In our experience, using a copper crucible liner has proven to be a good way to e-beam evaporate nickel. Nickel melts at 1453°C which is very near to its evaporation temperature. It is important to note that power should be ramped slowly both up and down when conducting this method. Nickel also has an average thermal conductivity. As the temperature of the nickel pellets increases in the copper liner, the nickel melts in strata. In other words, the top pellets melt and form a layer which is in contact with the inner diameter of the liner. Heat then gets conducted along the length of this layer to the liners side walls without too much heat seeping to the layer of pellets underneath. Deposition rates are stable coming off this top melted layer of nickel. In our observation, the melted nickel does not stick to the side walls of the liner allowing users to remove the material. Power levels should be reasonable and repeatable with only a slight creep upward over repeated runs. There is no interaction between the liquid nickel and the copper liner.
Copper crucible liner containing partially melted Nickel pellets.
Partially melted Nickel pellets extracted from Copper crucible liner. In our observation, the melted Nickel does not stick to the side walls of the liner allowing users to remove the material.
A key process note is to consider the fill volume of the crucible liner. 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 liner. This is the most common cause of crucible liner failure. Placing too little material in the liner 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. However, even when following these fill guidelines, the Fabmate® liner will fail during cooling. Crucible liners should be stored in a cool, dry place and always handled with gloves or forceps.
Nickel can also be run directly from the copper hearth of the e-gun. Because of this, some customers prefer to use a pre-machined slug (or starter source) that is directly placed in the hearth pocket. The two main benefits of using a starter source are ease of use and handling as well as superior packing density. In order to avoid broken crucible liners, we have had customers install starter sources directly in the hearth. A copper crucible liner is also a good option for customers operating shared systems where using a liner is required.
KJLC® can produce these starter sources. Contact us by clicking here with your e-gun manufacturer, pocket size, and number of pockets in order for us to produce a quote.
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