Nickel/Iron Ni/Fe 81/19 wt % 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. Iron is soft, gray, and metallic. It has a density of 7.86 g/cc, a melting point of 1,535°C, and a vapor pressure of 10^-4 Torr at 1,180°C. The theoretical density of the nickel/iron 81/19 WT% alloy is 8.7 g/cc. Iron is very reactive with air, forming rust spots that flake off, exposing clean surfaces susceptible to oxidation. Nickel and iron are the most common elements in the earth's core and two of the most widely used metals in modern industry. Both elements are ferromagnetic, ductile, and conductors of heat and electricity. Naturally occurring nickel/iron alloys are often found in meteorites. Nickel/iron alloys are often evaporated in vacuum to form films in the production of semiconductors and magnetic storage media.

Thermal Evaporation of Nickel/Iron (Ni/Fe 81/19 wt %)

Nickel and iron each have a vapor pressure of ~1E^-2 Torr at around 1,500°C with nickel's vapor pressure being slightly lower at any given temperature.

Iron Vapor Pressure Chart

Nickel Vapor Pressure Chart

When the difference in atomic percentage is factored in, deposited films are expected to have a composition that is close to the 81/19 alloy. However, thermal evaporation is very difficult to accomplish with nickel/iron alloys. Both elements react with refractory metals when in liquid form, thus making evaporation out of a tungsten boat nearly impossible. Even when using a thin width, thick gauge tungsten boat such as our EVS20A015W, the alloy will vigorously attack the boat, causing it to crack and break. Therefore, the boat may not even last one run. Also, a tungsten boat may not work because there is risk that the materials may alloy with the tungsten, changing the mechanical and electrical properties of the boat. Due to differences between their melting points, the nickel and iron may briefly phase separate during evaporation. Because iron has a slightly higher vapor pressure at the same temperature as nickel, films may initially be iron rich.

An alternative option is to evaporate out of an aluminum oxide crucible. However, this method may also be unsuccessful. The crucible may only last for one run due to differences in the coefficients of expansion between the alloy and the aluminum oxide crucible as the melt cools. Because the material rests at the bottom of the crucible and, typically, the crucible's bottom is cooler than its sides, the heater and crucible must be heated to a much higher temperature to achieve evaporation. A power supply may not be capable of getting the heater hot enough. Also, at 1,500°C, the vapor pressure of the aluminum oxide and, possibly even the tungsten, may cause film contamination.

E-beam Evaporation of Nickel/Iron (Ni/Fe 81/19 wt %)

Electron beam evaporation is the preferred way to deposit nickel/iron alloys as both materials are rated "excellent" for this method. FABMATE^® is the only crucible liner material recommended for the e-beam evaporation of nickel/iron. However, the crucible is limited to only one run. During evaporation, the nickel/iron 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/iron and the FABMATE^® liner will cause immense stress on the liner and it will crack.

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. However, even when following these fill guidelines, the FABMATE^® liner will still fail during cooling.

Crucible liners should be stored in a cool, dry place and always handled with gloves or forceps.

Nickel/iron 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.

Because not using a crucible liner is not always an option, especially in shared systems, some customers will use a copper crucible liner and place the starter source in the copper crucible liner instead of placing directly in the hearth.

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.

E-beam Crucible Liner Fill Rate Calculator

Instructions for use:
Input the Crucible Liner Volume, Select Material (if not available in menu, manually input Material Density in g/cm³), and input fill rate %.

KJLC recommends a fill rate between 67-75%. Overfilling the crucible will cause the material to spill over and create an electrical short between the liner and the hearth causing the crucible to crack. Placing too little material in the crucible or allowing the melt level to get too low 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.

The fill rate above assumes that the material is fully melted and does not take into account packing density. It should be noted that the crucible liner may need to be loaded multiple times, pre-melted, and topped off in order to achieve the final desired melt level/fill rate. When loading the crucible, do not load more than 80% of the height of the crucible liner.

This calculator is for estimation purposes only.

Crucible Liner Volume (in cc):

Material:

- or -
Density of the material in g/cm³:

Fill Rate %:

Result

See highlighted results that match your result in the table below.

Material Needed to Deposit a 1 Micron Film Calculator

Instructions for use:
Select source type, input distance from source to substrate and Select Material (if not available in menu, manually input Material Density in kg/m³).

This calculator is for estimation purposes only. The estimated mass is that needed to produce a desired film thickness (1 micron in this case) on a flat substrate at a point directly above the source, accounting for the approximate plume distribution of the selected source type. It does not account for any expected nonuniformity in thickness over a larger substrate area. More complex geometries, e.g. those with offset and/or tilted sources, may have higher material requirements. The estimated mass is for the film deposition only; additional margin should be included to account for loss during ramp-up, burn-in, stabilisation and ramp-down.

Source:

Distance of source to substrate (in cm):

Material:

- or -
Density of the material in kg/m³:

Multiplier:

Mass for 1 micron layer