Could any researchers provide links to a database of m.f.p of particles under vacuum conditions?
I am trying to compare the m.f.p of Ni, Al and several other metals under vacuum in order to understand the relationship between thin film formation and the particle's m.f.p.
Higher m.f.p means the particle takes a long time to reach the substrate. Thin film has low thickness, plus the possibility of back scatter.
Mean free path depends upon the degree of vacuum. You can calculate from standard formula.
Dr. Alvarez,
I am not aware of the formula. Can you kindly provide links / name of the formula so that I can look it up?
Try a Google search for mean free path. The Wiki answer gives vacuum examples. You will find it in a thermodynamics or statistical mechanics textbook.
Hii Muhammad,
I think the mean free path depends on the vacuum, as well on the molecules (atoms) inside. Heavy particles don't have the same MFP compared to the light ones for a same degree of vacuum.
You say that "Higher m.f.p means the particle takes a long time to reach the substrate". I am not sure that this is correct. As far as I remember, the MFP means that the mean distance between two collisions. I think, it depends also on the particle velocity. When the MFP is long (the distance between 2 collisons is long), the necessary time is short for making the same distance since there is less collision by keeping the same particle speed.
Have a look to the wiki page. There are some MFP values depending on the vacuum.
http://en.wikipedia.org/wiki/Mean_free_path
Hi Dr. Atli,
Thank you for your kind comments.
I came to the conclusion that "higher mfp = particles takes a long time to reach substrates" based on my observations when sputtering pure Ni samples.
I found that, when the working pressure was around 10x10-1 torr, no thin film were formed on the substrate. When I opened the chamber, I found the gun which was clean in the first place, was coated with in green.
Since I found a remark in a text book saying the mfp of pure Ni is low (1mm at 120mtorr), I suspected this is a backscatter - instead of the Ni particles reaching the substrates, it collided between itself and the Ar atoms and back scattered to the gun.
I proceeded to lower the working pressure (by lowering the Ar flow rate) and after it reached 2x10-1 torr, I can get a nice green-colored thin film. However, the thickness is low. When compared to other metals, sputtered under the same condition, I can get a thicker thin film.
Based on this observation, I concluded that, since the probability of Ni atoms to collide with another Ni atom is higher (due to low m.f.p), under the same sputtering setting, it will take a longer time for pure Ni atoms to reach the substrates, when compared to other materials with higher m.f.p.
Based on the equation shown in wikipedia, I think is is safer to assume that the reason why I am getting thinner Ni thin film as compared to other metals, under the same sputtering setting, is because Ni has a higher mass?
Hi Muhammad,
the mean free path in vacuum is infinity ... since a "closed" geometry is required to define the path.
I believe you want to know the mean free at low pressure limit !
at low pressure the free path calculation is simple ... it follow the laws of kinetic theory of gases.
please see this article for more details
good luck
Conference Paper Discrete free paths in thermodynamical systems
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