Effect of deposition temperature on the residual stress of sputtered zinc-tin-oxide coatings deposited on polyethylene terephthalate (PET) substrates

In this paper the generation of stress during the deposition of Zinc-Tin-Oxide coatings by magnetron sputtering on a PET substrate in a roll-to-roll web coater is investigated in detail. The stress components originating from a mismatch in the thermal (σthermal) and hygroscopic (σhygroscopic) expansion coefficient, deformation due to the applied web tension (σweb tension) and intrinsic stress (σintrinsic) are discussed and compared. The substrate temperature is varied between 25 °C and 75 °C. In this temperature range σthermal plays a minor role (<23 %). σhygroscopic is the only component resulting in tensile stress in the coating and accounts for approximately (30–70) % of the total residual stress. σintrinsic as well as σweb tension led to compressive stress in the coating in a similar order of magnitude of (36–106) %. In conclusion, the total residual stress can be reduced by reduction of the web tension. The results show that an accurate determination of the thermo-mechanic substrate behaviour is necessary before starting the residual stress analysis, as non-reversible thermal shrinkage of the substrate may lead to significant errors in the stress analysis when not properly taken into account.