Stress control on sol-gel PZT preparation for highly sensitive diaphragm structures of ultrasonic microsensors

Residual stress of sol-gel derived lead-zirconate-titanate (PZT) films was investigated under modified preparation conditions to realize buckled diaphragm structures for highly sensitive ultrasonic microsensors. As the modifications of the sol-gel process, pyrolysis temperature was varied from 250°C to 400°C, and the timing of crystallization annealing was modified in the multi-coating steps; with or without the first layer annealing. The stress of the PZT was evaluated on the diaphragms through their buckling deflection and on wafers by curvature of their bending profile. The higher temperature the PZT films were pyrolyzed at, the smaller residual stress they yielded. The annealing timing affected crystalline orientation of the PZT films as (100)-orientation with the first layer annealing and (111)-orientation without it, except for the 400°C-pyrolysis case. The (111)-oriented films yielded larger stress than (100)-oriented films. In the case of 400°C pyrolysis, both the films with and without the first layer annealing showed (100)-orientation, and the films without the first layer annealing showed much smaller stress than those without the annealing.