Variation in reflecting power is due impedance mismatching. In our sputtering system we have a manual knob to overcome this impedance mismatching problem. Sparking on target substrate is unusual. I am not sure what could be the probable reason for this . :)

The substrate becomes whitish as a result of the sparks, because these local discharges induce the ZnO to dissociate, releasing metallic Zn and this, arriving on the substrate, enriches with Zn the growing ZnO film, giving the white color.

It would be enough raising the substrate temperature at a value high enough to ensure the deposition of ZnO and not of metallic Zn.

This condition is easy to achieve considering the large difference in the vapor pressure of elemental Zn and ZnO.

May be, these local sparks are due to the poor quality of the target; often, targets with very low density contains inside micro bubbles of gas (Ar, N2, air, depending on the production method). This gas is released suddenly and locally during the sputtering action. In this case, it triggers a discharge on the target surface that vaporizes the ZnO, causing the effect described above.

The change in the reflected power is a simple consequence of the change in the impedance of the discharge inside the chamber sputtering due to the triggering of sparks.

Well, I understood your experience as seeing the spark on the target-surface; and so my answer below is based on it.

It is very straight-forward that when you do have such high reflected power, you will not have the usual deposition. I meant your statement as whitish; and so, I am kind of sure that the issue is not with the RF Generator/Matching unit, but any of the other 3 suggested above.

Hope the above information are useful. All the best. 

@All: Thank you for your kind suggestions.

@Mr. Elamurugu : I am doing the deposition on regular basis.. and I have not found any flakes like things. During AZO deposition I only use Ar around 15 sccm, no O2. I have got pinkish plasma. My target is vertically over the sample substrate. So no chance of Point 3.

@Mr Bosio: I am getting greater resistance for the whitish area than the transparent region. If Zn deposition happens.. I think it should produce lesser resistance than ZnO. Also surprisingly some area are becoming whitish not the whole area, though I am rotating the substrate..!! I think this may trigger one thing that, sudden ablation from the target is happening which comes on the substrate in a micro scale bulk form and not getting well crystallized..

Normally, the ZnO film grown in abundance of Zn does not become more conductive because the growth of the film changes drastically. The Zn excess tends to constitute preferential nucleation centers for the growth of the ZnO film. Under these conditions, the crystalline grains grow separately, because the nucleation centers are separated (in fact, this is a way to grow ZnO nanorods). The resistivity measurement, measuring the transverse resistivity of the film shows this effect! On the contrary, if you measure the longitudinal resistivity (in the direction of film thickness) usually turns out to be smaller. The separation of the microscopic grains, interfering with the passage of light, generates the milky color typical of this type of growth.

If I understand your observations, you are seeing a whitish film formation on your substrate.

Well I found a milky white film formation on my substrate.

This occurred when I had an Unbalanced magnetron. The plasma hitting the substrate was intense, and in the worst case when I had high powers with an unbalanced magnetron, the bombardment on the substrate was so intense that it cracked the glass substrates. We analysed that the white milky region was due to intense bombardment and was mainly due to the formation of an unbalanced magnetron.  Actually we were playing with the strength of the outer and the inner magnets in the magnetron target.

1) Looks like either you are keeping your substrates very close to the target.

     and there is intense bombardment.

2) Or perhaps  the strength of the inner and outer magnets in your system has changed due to some reason, and you have created an unbalanced magnetron condition.

One thin bothers me, reflected power increasing to 50%. This is clearly due to an impedance mismatch problem.

K. Sreenivas

Thanks you Mr Bosio and Mr. Sreenivas for the valuable inputs.

Thanks a lot.

When trying magnetron sputtering, sputtering is happening on the target cover also. Is this is normal? Please check this video: https://drive.google.com/file/d/17J7I6M55amiNqoYWidM9fgyzBXD4hNjY/view?usp=sharing

They look as micro arcs more than bombarding. They happen in the lid that holds the target and the magnetron body. It seems a problem with target mounting (it is bonded? It is well sit onto copper base?) or even some problem with isolation within the magnetron itself.

F. Caballero-Briones Thanks for your suggestions. When I cleaned the debris of magnetic material on the gun this issue is solved.

Now when I am magnetron sputtering the magnetic material the gun is heating too much even the chiller is running. Is it normal? What is the reason for it?

What are you depositing? I understood that a magnetic material,yes? Although I have no experience with that kind of materials, I have some ideas: Could it be an effect of the respective magnetic field interactions in the magnetron and of your target that are facing in opposing or aligned, maybe? I guess if they are aligned the electron density around the sputtered zone are is higher then the Ar ion impacts occur at higher energy then the gun is heated... try at lower power, check the reflected power and your matching impedance machine... check the target is well seated on the holder, if it is not glued, try to turn it to the other face. Finally, chiller is really delivering cold water? Take and eye at the chiller exit...I hope some of these ideas will be useful.