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While learning for an exam, I stumbled over the following Question:

According to Material Science of thin Films by Milton Ohring,

"RF sputtering essentially works because the target self-biases to a negative potential. Once this happens, it behaves like a DC target where positive ion-bombardment sputters away atoms for subsequent deposition."

So far so good. What I don't understand is

  • why exactly a self-bias voltage is appearing?
  • why this bias voltage does not lead sputter bombarding of the substrate, that should be coated?

I understand that it relates to the high mobility of electrons compared to the ions, but I don't see why this implicates the self-bias yet. The above mentioned source tries to explain it as follows:

"Negative target bias is a consequence of the fact that electrons are considerably more mobile than ions and have little difficulty in following the periodic change in the electric field. The disparity in electron and ion mobilities means that isolated positively charged electrodes draw more electron current than comparably isolated negatively charged electrodes draw positive ion current. For this reason the discharge current-voltage characteristics are asymmetric and resemble those of a leaky rectifier or diode [...]"

This is not clear to me, as I would think that this explanation would hold for the sputter substrate as well. I would be very glad if someone could make that clear by a somehow intuitive explanation. Thank you for all suggestions. :)

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First of all, sputtering can be used either to etch the target or to deposit material on the target, depending on the arrangement inside the etching chamber. However there is a further complication; a target may be etched as a source of ions for film deposition on a manufactured item, or as part of a manufacturing process for the target item. All this can confuse the novice reader. The question asks about RF sputter etching and references aspects of both processes.

RF plasma etching fundamentals

The chamber is evacuated and a small amount of a suitable gas then introduced at low pressure. A high RF voltage is used to strike a plasma in the gas, and this provides the ambient working conditions.

The plasma comprises positive ions and negative electrons. The RF voltage induces oscillations in the particles, with the light electrons accelerating faster and moving further; they are said to have greater mobility.

Consequently more electrons bump into the target than do ions. The target picks up their negative charge and hence gains a negative DC bias.

The target now attracts the ions more strongly, limiting the charge buildup and DC bias. More importantly, the ions now impact the target more frequently and start etching into it.

In a real process, the RF power level and resultant DC bias are both among the parameters which must be carefully controlled to ensure reliable and uniform etching. The exact chemistry of the gas reactions and compounds is of course also crucial.

Source for deposition

The target is typically placed at the top of the vacuum chamber and the product to receive the film is laid on a second electrode beneath it. Target atoms are either ionised or combined into ionised molecules. The product may also be given a much greater negative bias to attract the ions and build up a deposition layer.

However the more gas you let in, the harder it is for target material to reach the other electrode, so other methods are often used to create or enhance the ion flow and the chamber can then be run at higher vacuum.

Manufacturing process

The electrode lies at the bottom and the target is laid flat on (effectively comprises) it and the RF is induced across it and the containing vacuum chamber. Typically the gas ions react with it to create heavier molecules with even lower mobility, which therefore float off carrying atoms of the target with them, thus etching into it.

These heavy molecules are purged by a steady flow of gas, which reduces any tendency to deposit material on the chamber walls. However cleaning of such a chamber can still sometimes be a routine maintenance chore.

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The answer to your first question has already been given by your source (mobility difference of electrons and ions).
The answer to the second question has two components: 1)The source of the sputtering is on top of the substrate, 2)Since the top surface is negatively charged, the bottom of the substrate is positively charged, which repels the few stray positive ions that might manage to get below.

EDIT: Because of the comment below I will add an example that I hope makes it clear how the self-bias comes about.
On the left side you have "sprinters" and on the right side you have "slow pokes." Each one can carry one bucket of water and dump it into a barrel. It should be obvious, that after some time, the left side will have more water in their barrel than the right side. Thus, a water difference is created! With reference to electrons, more electrons on one electrode/side, creates a voltage difference between the electrodes.

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  • $\begingroup$ OP is specifically saying that they don't understand why mobility difference leads to negative self-bias (and, indeed, mobility difference by itself is insufficient to explain this). $\endgroup$
    – Al Nejati
    Oct 3, 2018 at 22:38
  • $\begingroup$ Your analogy of water buckets is nice, and indeed resembles how I would think about this. What I still wonder if this also applies to conducting materials as a target material (because they would not be isolated and thus don't charge up? well, maybe they are decoupled by a capacitor or something.. I guess with this followup question it might get too technical for a qualitative explanation). $\endgroup$
    – Pidrittel
    Oct 9, 2018 at 9:00
  • $\begingroup$ Regarding the second question, when you say The source of the sputtering is on top of the substrate, you want to imply by that that the target material "rains down" on the substrate, but not the other way around, because the substrate material will not fall up? The second part of your answer is unclear though, because it seemst that this argument would apply to the sputter target as well (which it can not, obviousely). So I still don't get why the sputter target is bombarded with ions while the substrate is not. $\endgroup$
    – Pidrittel
    Oct 9, 2018 at 9:01

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