Non-destructive Characterization Of Longitudinal Piezoelectric Coefficients In Thin Films By Piezoresponse Force Microscopy

Piezoelectric thin films and devices are gaining more and more attention with the development of Micro Electro Mechanical System(MEMS).As a fundamental physical parameter of piezoelectric materials,the longitudinal piezoelectric strain constant d₃₃ not only determines the performance of piezoelectric materials,but also directly affects the performance parameters of transducers.However,current test methods cannot meet its accurate and high-resolution measurement,and there is a lack of nondestructive test techniques for piezoelectric films.In this thesis,we use Piezoresponse Force Microscopy(PFM)imaging principle and contact resonance as a guide to characterize the piezoelectric coefficient by PFM,and investigate the influence of some non-ideal factors on the test to achieve the longitudinal effective piezoelectric coefficient measurement.In this thesis,the research work is centered on the techniques related to the PFM characterization of piezoelectric thin film d₃₃,with the following main elements.1.A laser interferometry-based piezoelectric response spectrum test system was established on a commercial atomic force microscope(AFM),and the piezoelectric response(PR)data were extracted by simple harmonic oscillation(SHO)model,The extraction of the effective piezoelectric coefficient d_eff in the longitudinal direction of the film is achieved by batch processing and analysis of PR data.The measured PR is generally consistent with the SHO model fit curve(R~2>0.9)for typical piezoelectric films,resulting in a relatively stable effective piezoelectric coefficient,e.g.,1.4±0.2 pm/V for Al N.It should be noted that this piezoelectric coefficient is an apparent piezoelectric coefficient,since it is 0.4±0.1 pm/V for the non-piezoelectric material Si O2.2.The repeatability of d_eff measurements was studied,with repeated piezoelectric measurements through the new AFM tip.Due to experimental conditions such as the number of tests and contact force F_n,which can cause different degrees of wear on the tip of the needle,the repeatability test result on the lithium niobate sample using a slightly ground probe is 3.9±0.1pm/V,which indicates that the method has better repeatability.3.Experimental conditions in d_eff measurements were optimized for piezoelectric measurements at different F_n and AC bias Vac.The results show that there is a nonlinear relationship between d_eff-F_n,and the optimal range of F_n should be 25-60 n N.Secondly,due to the inherent nonlinear response of piezoelectric materials,the results have a high degree of confidence when Vac<5V in the test.Finally,more reliable results can be measured using probes with larger spring constants and appropriate DC bias.