Hello everyone,
I am using thermal evaporator to deposit 70-100 nm Gold (Au) layer on top of FTO substrate. The gold layer always shows lots of cracks. I have attached a photo here. I ve tried Si substrate, Glass, FTO,..., but cracks are still available in the Gold layer.
Do you have any idea about it? Did anyone experience the same issue?
Thank you.
Looks like CTE (coefficient of thermal expansion) mismatch between film and substrate causes tensile stresses as the film+substrate cools from deposition temperature. This would be consistent with the substrate being heated in the course of the thermal evaporation. Gold has a greater CTE than glass or Si, so as the film+substrate cools from deposition temperature to RT, large tensile stress develop and cause the cracks to see.
To elminate the crack you could...
1. choose a subtrate with CTE that matches gold's
2. add some cooling to your system so the substrate does not heat up
3. I don't think this will work: evaporate the gold very slowly to minimize how hot the substrate gets - I doubt this will work because you have to get the gold pretty warm for it to evaporate at all
4. position the gold farther from the substrate during evaporation to reduce substrate heating
5. evaporate the gold VERY RAPIDLY for a very SHORT TIME so the substrate won't have time to heat up. Repeat this for many times with a long wait in between steps to allow the substrate to cool.
Maybe only the last one is practical as it won't require chaning your materials or system.
Dear w.s.l Boyer
Thank you for your great answer and suggestions.
Among these, #1,2 are not possible to do for my samples and system. Tried number 3 but it doesn’t work as yoU know.
Tried number 4 (with distance between substrate and source about 20cm), not working
i also tried number 5 but I deposited all 80nm in one round at high rate (2.5A/s) and again i got cracks. The total time was like ~7min. However the substrate and holder were not as hot as before which I used low deposition rate or at low distance.
Sounds like you've made some good efforts already.
Yeah, try splitting the total deposition into 5 segments. Evap 1/5th your desired thickness at max rate. Then wait an hour to cool substrate. Repeat until desired thickness reached.
Oh, also try to reduce the total heat input by minimizing size of gold source (smaller crucible, reduce length of heater wire to just where in contact with crucible, etc.)
We solved this problem two months ago. The issue was with the type of filament and also the contamination.
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