Causes of target poisoning in magnetron sputtering

I. Formation of metal compounds on the target surface

In the formation of a compound from a metal target surface by a reactive sputtering process, where is the compound formed? As a result of the chemical reaction between the reactive gas particles and the target surface atoms, the formation of compound atoms is usually an exothermic reaction, and the heat of reaction must have a way to conduct out, otherwise the chemical reaction cannot continue. Heat transfer between the gases is not possible under vacuum conditions, so the chemical reaction must take place on a solid surface. Reaction sputtering produces compounds on target surfaces, substrate surfaces, and other structural surfaces. The generation of compounds on the substrate surface is the goal, the generation of compounds on other structural surfaces is a waste of resources, and the generation of compounds on the target surface starts as a source of compound atoms and becomes an obstacle to the continuous supply of more compound atoms.

II. Factors influencing target poisoning

The main factor influencing target poisoning is the ratio of reaction gas to sputtering gas, too much reaction gas will lead to target poisoning. Reaction sputtering process is carried out in the target surface sputtering channel area appears to be covered by reaction products or reaction products are stripped and re-exposed metal surface this and that process. If the rate of compound generation is greater than the rate at which the compound is stripped, the area covered by the compound increases. At a given power, the amount of reactive gas involved in the formation of the compound increases and the rate of compound formation increases. If the amount of reaction gas increases excessively, the compound coverage area increases, if the reaction gas flow cannot be adjusted in time, the rate of compound coverage area increase is not suppressed, 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.

Three, target poisoning phenomenon

(1) positive ion accumulation: target poisoning, the target surface to form a layer of insulating film, positive ions reach the cathode target surface due to the blockage of the insulating layer, can not directly into the cathode target surface, but accumulate on the target surface, easy to produce cold field to arc discharge --- hit the arc, so that the cathode sputtering can not be carried on.

(2) anode disappearance: target poisoning, grounded vacuum chamber wall also deposited insulating film, reaching the anode of electrons can not enter the anode, the formation of anode disappearance phenomenon.

Fourth, the physical explanation of target poisoning

(1) In general, the secondary electron emission coefficient of metal compounds is higher than that of metals, after target poisoning, the surface of the target are metal compounds, after being bombarded by ions, the number of secondary electrons released increases, which improves the conductivity of space and reduces the plasma impedance, resulting in lower sputtering voltage. This in turn reduces the sputtering rate. In general the sputtering voltage for magnetron sputtering is between 400V-600V, and when target poisoning occurs the sputtering voltage is significantly reduced.

(2) Metal targets and compound targets are originally sputtering at different rates, and in general the sputtering coefficient of metals is higher than that of compounds, so the sputtering rate is lower when the target is poisoned.

(3) The sputtering efficiency of reactive sputtering gas is originally lower than that of inert gas, so the comprehensive sputtering rate decreases after the proportion of reactive gas increases.

Five, the target poisoning solution

(1) the use of medium frequency power supply or radio frequency power supply.

(2) The use of closed-loop control of the reaction gas throughput.

(3) the use of twin targets

(4) Control the transformation of the coating mode: before coating, collect the hysteresis effect curve of target poisoning, so that the inlet gas flow control in the front of the generation of target poisoning, to ensure that the process is always in the mode before the deposition rate drops steeply.

Sputtering of metal atoms on the target surface is relatively easy, when changing the surface to metal oxides and then sputtering is not. RF sputtering is generally required.

The ion bombardment makes the metal atoms on the target surface very reactive and, together with the increased target temperature, increases the reaction rate on the target surface considerably. At this point the target surface is simultaneously sputtering and reacting to produce a compound in both processes. If the sputtering rate is greater than the compound generation rate, the target is in the metal sputtering state; conversely, the reaction gas pressure increases or the metal sputtering rate decreases, the target may suddenly occur compound formation rate exceeds the sputtering rate and stop sputtering.

In order to alleviate the phenomenon of target poisoning, technicians often use the following methods to solve.

(1) sending the reaction gas and sputtering gas to the substrate and near the target respectively to create a pressure gradient; (2) increasing the exhaust rate.

(3) gas pulse introduction.

(4) plasma monitoring, etc.

Target poisoning is due to the positively charged ions gathered on the target surface during the sputtering process, did not get neutralized, there is a gradual decline in negative bias pressure on the target surface, and finally simply strike and do not work, which is the target poisoning phenomenon.

The main reason for target poisoning is that the medium synthesis speed is greater than the sputtering yield (reaction gas through too much), resulting in the conductor target loss of conductivity, only to improve the breakdown voltage, to start the glow, the voltage is too high prone to arc discharge. Phenomenon: target voltage can not reach normal for a long time, has been in low voltage operation state, and accompanied by arc discharge; target surface presents white adherence or dense needle-like grey discharge traces. To completely eliminate the target poisoning, must use medium frequency power supply or RF power supply instead of DC power supply; reduce the amount of reaction gas throughput, improve sputtering power, clean up the pollutants on the target (especially oil), the choice of vacuum performance of the dust arc extinguisher and other methods can effectively prevent the occurrence of the phenomenon of target poisoning.