The formation of metal compound on the target surface

Where is the compound formed in the process of forming the compound from the metal target surface through the reactive sputtering process? Since the reactive reaction gas particles collide with the target surface atoms to produce a chemical reaction to generate compound atoms, which is usually an exothermic reaction, the heat generated by the reaction must be transmitted out of the way, otherwise, the chemical reaction cannot continue. It is impossible to conduct heat conduction between gases under vacuum conditions, so the chemical reaction must take place on a solid surface. The reactive sputtering product is carried out on the surface of the target, the surface of the substrate, and the surface of other structures.

 The influencing factors of target poisoning:

The main factor affecting target poisoning are the ratio of reactive gas to sputtering gas. Excessive reactive gas will lead to target poisoning. In the process of reactive sputtering process, the sputtering channel area of target surface is covered by reactive products or the reactive products are stripped and the metal surface is exposed again. This is a process of ebb and flow. If the rate of compound generation is greater than the rate at which the compound is peeled off, the area covered by the compound increases. With a certain power, the amount of reaction gas involved in the formation of the compound increases and the rate of compound formation increases. If the amount of reactant gas increases excessively, the area covered by the compound will increase. If the flow rate of the reactant gas cannot be adjusted in time, the rate of increase of the area covered by the compound will not be suppressed, and the sputtering channel will be further covered by the compound. When the sputtering target is completely covered by the compound, the target is completely poisoned, and a layer of compound metal film is deposited on the target surface. Make it difficult to be reacted again.

Target poisoning 

(1) Positive ion accumulation: When the target is poisoned, an insulating film is formed on the target surface. When the positive ions reach the cathode target surface due to the barrier of the insulating layer, they cannot directly enter the cathode target surface, but accumulate on the target surface, which is easy to produce a cold field. Arc-induced discharge---arcing, makes the cathode sputtering impossible.

(2) The anode disappears: When the target is poisoned, an insulating film is also deposited on the wall of the grounded vacuum chamber, and the electrons reaching the anode cannot enter the anode, resulting in the phenomenon of anode disappearance.

The physical explanation of target poisoning 

(1) Under normal circumstances, the secondary electron emission coefficient of metal compounds is higher than that of metals. After the target is poisoned, the surface of the target material is all metal compounds. After being bombarded by ions, the number of secondary electrons released increases, which improves the space efficiency. The conduction capability reduces the plasma impedance, resulting in a drop in the sputtering voltage. Thereby reducing the sputtering rate. In general, the sputtering voltage of magnetron sputtering is between 400V-600V. When target poisoning occurs, the sputtering voltage will decrease significantly.

(2) The sputtering rate of metal target and compound target is different. In general, the sputtering coefficient of metal is higher than that of compound, so the sputtering rate is low after the target is poisoned.

(3) The sputtering efficiency of reactive sputtering gas is inherently lower than that of inert gas, so when the proportion of reactive gas increases, the overall sputtering rate decreases.

The solution to target poisoning 

(1) Use intermediate frequency power supply or radio frequency power supply.

(2) A closed loop is used to control the amount of reactant gas introduced.

(3) Use twin targets.

(4) Control the transformation of coating mode: Before coating, collect the hysteresis curve of target poisoning, so that the air flow can be controlled at the leading edge of target poisoning to ensure that the process is always in the mode before the deposition rate drops sharply.