More than the TOTAL base pressure, you must be concerned by the partial pressure of reactive species, like oxygen, CO, water, etc. These species will affect the crystallography of the film, like grain size, preferred growth and so on. And all these properties will also affect electrical resistivity. The best way to control the reactive species is to have a good pumping system and a vacuum lock to introduce sample into the deposition camera.

Oh yes, I know that...thanks.

I have a good cryopump with the best base pressure available around 1.7e-07 mbar. But my question was another one: not about the basic of a good sputtering system for metals, but if anyone has noted particular effect passing from a base pressure of about 2e-07mbar to 4-5e-06mbar. This is a specific question...I have hypotesis but I'm curious about other experiences.

Thanks again Giuseppe.

PS: the question (I have not at the moment a mass spectrometer in the chamber) could be read also as follows: which is the minimum partial pressure of Oxygen or Nitrogen that could give macroscopic effect in NiCr sputtering deposition?

First of all what is exactly your deposition technique? Is it ion beam sputtering or magnetron sputtering? Generally the lower the vacuum in a deposition chamber the higher the energy of the component of the target which are sputtered and which arrive on the substrate. The consequence is a possible peening effect of the thin films when the growth proceeds. This well known effect can create a more or less strong disorder in terms of point defects or/and in terms of a strong internal stress in the thin film which can affect the electrical properties of NiCr thin films.

magnetron sputtering...

the deposition pressure is always the same and well controlled, so I think there isn't peening effect.

Probably a residual of Oxygen or Nitrogen into the chamber lower the deposition rates (asap I'll do test on rates) and change the chemical composition in the initial phases of film growth...with a final result that has caught my attention: the film is blue colored than the usual grey/metal.

Thanks

If you have a well controlled vacuum, you can calculate adsorption rates for all residual molecules, monitored by mass-spectrometer, from the gas kinetic (they are function of the pressure) and compare them with deposition rate of you film (constant flux of Cr and Ni).

@Yuriy: at the moment the mass spctrometer is not working, but I'll remember your suggestion

@Eugeniy: the system hopefully is free of oil, but I'll try to analyze the environment a posteriori. The only possible source of oil could be a leak of air-compressed used to cool a motor through a rotating passthrough, but it seems unlikely because in that case I would expect that this leak puts a limit to the base pressure always. Instead, if I wait enough to lower pressure all is ok: the Vbias increase to standard value and the NiCr is as expected.

Thanks

I can suggest to perform some analysis to find out any relationship between the change in electrical resistivity you noticed, and some other physical quantity, e.g. change in microstructure, change in oxygen content, etc. Possible analysis are AES and XRD

I performed in the past some experiments in which I found a correlation between the residual reactive gas content and the microstructure of aluminum films deposited by sputtering. I expect something similar in your case. A colored film suggest in fact the presence of oxides or nitrides.

Adds to previous comment.

If you have an access, XPS analysis with depth profiling offer a complete information about chemical composition in the film and at the interfaces.

Other options: noticed AES, SIMS, RBS...

Have you considered the incorporation of the sputtering gas (argon) into your films particularly at low deposition pressure? This can collect at grain boundaries and will influence resistivity.

If possible...try an analysis by RBS which gives a very simple view of what your sample looks like in terms of concentration of the different components (Ni, Cr, Ar, and other possible pollutions) thickness and the repartition of the components in the thin film.

what is Vbias?

do you have DC or rf sputtering? Pressure changes influences the plasma densities, they may have influence on "your" Vbias.

Contamination is probable, and if not from residual gas, it may come from your target.

Target condition is also a factor to a possible change in Vbias (via plasma potential changes). What are your Vbias ranges? How do you apply it?

(un bonjour en direction de Poitiers :-) )

Discolored films would indicate contamination. The blue sugests Oxide. This would make me want to do a thorough leak check of the system. If it is a cryo pumped system, perform a regen.

Another possibility is the age of the target. If the target has been heavily used you could be seeing one of two affects: breakthrough to the backing plate or preferential sputtering of Ni has left you Chromium rich and yields a bluish color.

Thanks James for your useful considerations.

The target is quite new. Probably is residual Oxygen in chamber...Blue color suggests oxide also to me, but I was looking for confirmation by different experiences.

Mirko,

If your target is new, you may need to perform a burn-in to remove several 100s of Angtroms of target surface to get to clean metal. With NiCr, you will probably see a drop in chamber pressure afterwards due to te gettering qualities of the Cr. I would then recommend a cryopump regen to clean out the H2O, H2, and whatever else you had in the chamber.

I would suggest starting with long, low power deps, ramping to higher power. Because the native oxide formed on the target surface will be harder to sputter than the clean surface. I would also suggest recording chamber pressure before, immediately after, and on hourly basis for a few hours. The presure will indicate whether you were experiencing outgassing to cause bluish film, or if it goes back up, you have an exisiting leak.

Jamie

Thanks a lot...

I would like to propose to stop any the discussion until new and reliable data on the in depth composition of the two kind of film (with different electrical resistivity) is obtained. Then it will be possible to discard some of the hypothesis ad accept some other.