In the panel level packaging, it is necessary to control the thickness uniformity of electroplated Cu redistribution layer because non-uniform thickness of Cu electrodeposits is resulted in severe electrical resistance fluctuation. In this work, we studied the thickness uniformity of Cu electrodeposits in the presence of organic additive containing imine functional group modified with different degrees of quaternization. The degree of quaternization of imine functional group was controlled to be in the range of 0 to 100 % using the dimethyl sulfate solvent. The surface morphology and thickness uniformity of Cu electrodeposits were characterized by optical microscopy and confocal laser microscopy. In the case of adding the organic additive containing the imine functional group modified without quaternization into Cu electroplating solution, the shape of Cu electrodeposit represented dome shape with poor uniformity. Additionally, excessive dish-shaped surface was formed with the degree of quaternization of 100 %. However, with the degree quaternization of 50 %, the flat shape of Cu electrodeposits with high uniform thickness was obtained.