AbstractFreestanding oxide membranes provide a promising path for integrating devices on silicon and flexible platforms. To ensure optimal device performance, these membranes must be of high crystal quality, stoichiometric, and their morphology free from cracks and wrinkles. Often, layers transferred on substrates show wrinkles and cracks due to a lattice relaxation from an epitaxial mismatch. Doping the sacrificial layer of Sr₃Al₂O₆ (SAO) with Ca or Ba offers a promising solution to overcome these challenges, yet its effects remain critically underexplored. A systematic study of doping Ca into SAO is presented, optimizing the pulsed laser deposition (PLD) conditions, and adjusting the supporting polymer type and thickness, demonstrating that strain engineering can effectively eliminate these imperfections. Using SrTiO₃ as a case study, it is found that Ca_1.5Sr_1.5Al₂O₆ offers a near‐perfect match and a defect‐free freestanding membrane. This approach, using the water‐soluble Ba_x/Ca_xSr_3‐xAl₂O₆ family, paves the way for producing high‐quality, large freestanding membranes for functional oxide devices.