E-beam Physical Vapour deposition

[lamejane]

Hope we have some PVD specialists in the house :)
we have a pfeiffer e-beam PVD in the lab which i use to deposit Ni and Pt thin films ( around 100nm each) on Si wafers. I have always had the problem of "Ni splashes" while heating the target ( with the e beam) which leads to a process stop ( maybe some of it reaches the filament ( source for electrons) beneath the target creating a short contact?. This means i need to restart the process using the software to deposit the remaining layer ( eg process stops at 70 nm i need to restart the process to deposit the 30 nm remainder layer)

https://en.wikipedia.org/wiki/Electron_beam_physical_vapor_deposition#/media/File:Electron_Beam_Deposition_001.jpg
How can i reduce such splashes during sputtering? what parameters can be controlled to achieve sputtering without process stop ?

Thank you ! and greets from DE.

 

[Tom.G]

I've had only limited experience with e-beam vapor deposition, but in other heating contexts contamination of the heated material is frequently the reason for sputters/splashes.

 

[Lord Jestocost]

Hope we have some PVD specialists in the house :)
we have a pfeiffer e-beam PVD in the lab which i use to deposit Ni and Pt thin films ( around 100nm each) on Si wafers. I have always had the problem of "Ni splashes" while heating the target ( with the e beam) which leads to a process stop ( maybe some of it reaches the filament ( source for electrons) beneath the target creating a short contact?. This means i need to restart the process using the software to deposit the remaining layer ( eg process stops at 70 nm i need to restart the process to deposit the 30 nm remainder layer)
View attachment 189454
https://en.wikipedia.org/wiki/Electron_beam_physical_vapor_deposition#/media/File:Electron_Beam_Deposition_001.jpg
How can i reduce such splashes during sputtering? what parameters can be controlled to achieve sputtering without process stop ?

Thank you ! and greets from DE.

A few questions:
a) Does your E-beam evaporation system allow a very slow and uniform heating up of the source material by means of X-Y-wobbling of the electron beam over the surface of the ingot?
b) Does the nickel ingot get in contact with air between deposition runs?

 

[lamejane]

A few questions:
a) Does your E-beam evaporation system allow a very slow and uniform heating up of the source material by means of X-Y-wobbling of the electron beam over the surface of the ingot?
b) Does the nickel ingot get in contact with air between deposition runs?

Hello,
a) Yes, the rise time is set to 2 min and soak time is set to1 min [ Ni and Pt]. Both have a point beam spot instead of x-y wobbling. the spot size can be changed if required. The spot type can also be changed to spiral, wobble and X- scan.

b) Yes the chamber is Opened after each process and the 12 cm^3 ingot comes in contact with the clean room air leading to oxidation of Ni

 

[Lord Jestocost]

Hello,
a) Yes, the rise time is set to 2 min and soak time is set to1 min [ Ni and Pt]. Both have a point beam spot instead of x-y wobbling. the spot size can be changed if required. The spot type can also be changed to spiral, wobble and X- scan.

b) Yes the chamber is Opened after each process and the 12 cm^3 ingot comes in contact with the clean room air leading to oxidation of Ni

The effect you are observing is known as “splatter/splattering” or “spit/spitting”:

“Spits can be suppressed by eliminating porosity, oxide inclusions, and compositional inhomogeneities in the evaporant source material, since spitting can be caused by included-gas release or by the release of bound gas through thermal decomposition. In e-beam evaporation the beam of electrons dissipates energy over a path extending as much as a mil (25 µm) or more into the melt. If this energy is delivered at a rate faster than the coating material can accommodate by evaporation, conduction, or radiation, a pocket of vapor forms and spitting occurs. Spits are also caused by gas pockets included in the evaporant rod that suddenly expand when rapidly heated by the beam. Non-metallic inclusions can trap pockets of superheated vapor below them, which can erupt in a shower of molten droplets. Spits can be avoided by using a high-purity vacuum melted rod as the evaporant.” (from “Handbook of Deposition Technologies for Films and Coatings: Science, Applications and Technology” by Peter M. Marti)

„A serious problem in the vacuum vaporization of metals is the explosive vaporization or splattering which is often experienced. This problem is particularly acute in electron beam vaporization processes where the surface of a molten pool of metal is heated by a high energy electron beam. In electron beam vaporization processes all of the heat is added to the metal being vaporized at the surface thereof by impingement of the electron beam upon the surface of the molten meal. The impingement of the electron beam on the surface of the molten metal causes thermal stirring of the molten pool which continuously brings fresh metal to the surface of the pool where vaporization takes place.
It has been discovered through considerable experience that when a metal is vaporized by an electron beam vaporization process there is a tendency for the metal to throw off globules of liquid metal during the vaporization process. This phenomenon is generally attributed to the fact that electron beam heating which is concentrated at the surface of the molten pool, causes localized superheating of regions of the molten pool at and near the surface. These localized superheated regions of the melt are constantly moving due to convection currents produced by thermal gradients in the pool. If a superheat region reaches an irregularity at the surface of the pool, it may be explosively vaporized depending upon the amount of excess energy present and the particular characteristics of the melt. Irregularities at the surface may exist at particles of impurities floating in the pool and at a meniscus at the edge of the pool. The explosive vaporization of the superheated region is accompanied by splattering of the globules of molten metal from the pool. The amount of splattering relative to the amount vaporized generally increases with increasing vaporization rates….“ (from https://www.google.com/patents/US3356487)

Usually, metals can absorb gasses during processing. Additionally, as in your case, “fresh” metal surfaces can be re-oxidized and – maybe – oxygen and other gases can penetrate grain boundaries in course of time. It is thus essential to remove all dissolved gases (degassing) and to decompose formed oxides - as far as possible - so that gases are not quickly released during melting. To find the proper pre-treatment for the specific charge you are using can be a tedious process (don’t “trust” recipes). Generally, the ingot has to be heated up very slowly and uniformly and it can always be recommended to use high-purity vacuum melted ingots as the evaporant.
It would also help to use crucible liners which provide thermal isolation between the evaporant material and the water-cooled crucible. Thermal gradients across the evaporant material are reduced increasing the thermal stability within the melt thereby increasing heating efficiency and reducing particle ejection. In case of nickel E-beam evaporation, liners made out of alumina (Al₂O₃) or vitreous carbon (glassy carbon) are recommended.

 

[lamejane]

The effect you are observing is known as “splatter/splattering” or “spit/spitting”:

“Spits can be suppressed by eliminating porosity, oxide inclusions, and compositional inhomogeneities in the evaporant source material, since spitting can be caused by included-gas release or by the release of bound gas through thermal decomposition. In e-beam evaporation the beam of electrons dissipates energy over a path extending as much as a mil (25 µm) or more into the melt. If this energy is delivered at a rate faster than the coating material can accommodate by evaporation, conduction, or radiation, a pocket of vapor forms and spitting occurs. Spits are also caused by gas pockets included in the evaporant rod that suddenly expand when rapidly heated by the beam. Non-metallic inclusions can trap pockets of superheated vapor below them, which can erupt in a shower of molten droplets. Spits can be avoided by using a high-purity vacuum melted rod as the evaporant.” (from “Handbook of Deposition Technologies for Films and Coatings: Science, Applications and Technology” by Peter M. Marti)

„A serious problem in the vacuum vaporization of metals is the explosive vaporization or splattering which is often experienced. This problem is particularly acute in electron beam vaporization processes where the surface of a molten pool of metal is heated by a high energy electron beam. In electron beam vaporization processes all of the heat is added to the metal being vaporized at the surface thereof by impingement of the electron beam upon the surface of the molten meal. The impingement of the electron beam on the surface of the molten metal causes thermal stirring of the molten pool which continuously brings fresh metal to the surface of the pool where vaporization takes place.
It has been discovered through considerable experience that when a metal is vaporized by an electron beam vaporization process there is a tendency for the metal to throw off globules of liquid metal during the vaporization process. This phenomenon is generally attributed to the fact that electron beam heating which is concentrated at the surface of the molten pool, causes localized superheating of regions of the molten pool at and near the surface. These localized superheated regions of the melt are constantly moving due to convection currents produced by thermal gradients in the pool. If a superheat region reaches an irregularity at the surface of the pool, it may be explosively vaporized depending upon the amount of excess energy present and the particular characteristics of the melt. Irregularities at the surface may exist at particles of impurities floating in the pool and at a meniscus at the edge of the pool. The explosive vaporization of the superheated region is accompanied by splattering of the globules of molten metal from the pool. The amount of splattering relative to the amount vaporized generally increases with increasing vaporization rates….“ (from https://www.google.com/patents/US3356487)

Usually, metals can absorb gasses during processing. Additionally, as in your case, “fresh” metal surfaces can be re-oxidized and – maybe – oxygen and other gases can penetrate grain boundaries in course of time. It is thus essential to remove all dissolved gases (degassing) and to decompose formed oxides - as far as possible - so that gases are not quickly released during melting. To find the proper pre-treatment for the specific charge you are using can be a tedious process (don’t “trust” recipes). Generally, the ingot has to be heated up very slowly and uniformly and it can always be recommended to use high-purity vacuum melted ingots as the evaporant.
It would also help to use crucible liners which provide thermal isolation between the evaporant material and the water-cooled crucible. Thermal gradients across the evaporant material are reduced increasing the thermal stability within the melt thereby increasing heating efficiency and reducing particle ejection. In case of nickel E-beam evaporation, liners made out of alumina (Al₂O₃) or vitreous carbon (glassy carbon) are recommended.

Thank you for the reply!,
i shall give a shot with Al2O3 or WC liners for Ni and Pt.

 

[mykey]

I've been working with PVD e-beam evaporation for couples of years and i can tell you that those splashes will appear in most materials. To avoid the splashes reaching the substrate you can do couples of things:
1. Increase the distance between the evaporation source and substrate
2. Increase the power of the e-beam very slowly ( 1mA at 30sec-1 min)
3. Increase the amplitude of the ebeam spot so the beam will heat up a larger area of the ingot
4. Always work with your best vacuum
I don't think the Ni splashes are reaching the filament, but maybe the sensor ( that measures the evaporation speed)
5. Try and work with the sensor closed ( reach the desired evaporation speed and close the sensor protecting cap ( if you have one, it depends on equipment), then calculate the time needed to obtain the desired film thickness)
6. If you use the same ingot over and over again, the material will purify, so try and hit exactly the same spot with e-beam every time.
Good luck!