In sputtering ions are accelerated onto a target to strike out atoms or molecules. The target consists of the material which shall be deposited. The mean free path is a few millimeters which means that the particles often collide with each other and therefore also vertical surfaces are covered well. Primarily the noble gas argon is ionized by gas discharge. The disposed particles form a porous film which can be densified by annealing. Sputtering can be devided into passive (inert) and reactive sputtering.
By passive sputtering only the material of the target is disposed on the wafers, according to the material of the target high-purity films can be created as the mix ratio of the substances in the target can be chosen precisely. In the reactive sputtering an additional reactive gas (e.g. oxygen O2) is added, which reacts with the particles of the target and deposits on the wafer. If one uses a metallic target (e.g. aluminum Al), non-metallic layers like the insulator aluminium oxide are possible:
4 Al + 3 O2?2 Al2O3
To create metallic layers, the DC sputtering is used. Thereby the ions are accelerated with up to 3 kV onto the target where they are discharged. To dissipate the charges, only a conductive target can be used. For non-metallic layers the reactive sputtering has to be used. If one wants to create an insulating layer right out of the target the RF sputtering is used.
In RF sputtering a voltage is applied to both electrodes behind both the target (cathode) and the wafer (anode). During the positive half-wave on the target, the electrons were attracted to it, thus the target gets negatively charged. The negatively charged target attracts ions which strike out particles from it. To increase the deposition rate one can attach magnets behind the target to deflect the electrons into a circular path. Thus more ions will be ionized and strike out additional particles from the target. Because the anode is connected to the process chamber its potential difference compared to the plasma is much less than the potential difference of the cathode to the plasma. Thats why the ions are accelerated to the target only and not onto the wafer.
To increase edge coverage the BIAS sputtering is used. A negative voltage is applied to the substrate, so that here particles are striked out at well, which planishes the surface. However, one has to take care that there is no abrasion of the substrate. This so called back-etch process is the principle of most dry etch processes.
Illustration of a sputter chamber
Sputtering is suitable to create metallic films with high conformity and very good reproducibility. The effort is little, the low pressure (5 Pa) can be achieved easily.