Ferroelectric lead zirconate titanate (PZT) thin films have been studied for application to pyroelectric and piezoelectric devices as well as memory devices, such as dynamic random access memory (DRAM) and ferroelectric non volatile memory (FRAM). However, microactuator and sensor using piezoelectric film (PZT film) have not been studied sufficiently despite of their superior properties because of the difficulty in deposition and etching of PZT film. The ZnO film and sol-gel derived PZT film have been mainly used for fabrication of sensor and microactuator. As sol-gel derived PZT film has poor step coverage and high residual stress, PZT film fabricated by sputtering and CVD method is needed.
In this study, PZT films were fabricated by DC magnetron multi-target reactive sputtering. The effect of Pt/Ti bottom electrode on properties of deposited PZT film and Pt hillock was systematically studied. The amount of excess Pb in the PZT film was controlled in order to enhance leakage current characteristics. And the various processes, such as deposition and etching, were performed for fabrication of the piezoelectric microactuator by surface micromachining.
The effect of the Pt/Ti bottom electrode on the structure of the deposited PZT film was investigated. As the thickness of the Ti underlayer increased and that of the Pt layer decreased, Ti out-diffusion to the Pt substrate surface through the grain boundary of the Pt layer was enhanced so that the formation of the perovskite phase was promoted by facilitating the incorporation of Pb component. The structural phase of the PZT film and the resultant electrical properties had a sensitive dependence on the thermal history (such as processing time and environment) of the substrate prior to the deposition of the PZT film. The heat-treatment in Ar environment promoted the out-diffusion of Ti. The out-diffused Ti facilitated the formation of perovskite PZT film. The heat-treatment in oxygen environment, however, was scarcely effective in obtaining perovskite-phase PZT films because the out-diffusion of Ti to the Pt surface was suppressed by titanium oxide formed along the grain boundaries of the Pt layer.
Pt hillocks on Pt/Ti substrates were sensitively dependent on the deposition condition of the Pt/Ti film and the thermal history of the substrate prior to PZT film deposition. The hillocks were not formed on Pt substrate with no Ti underlayer. As the thickness of the Ti underlayer increased and that of the Pt layer decreased, the density of the hillocks increased but the size of them decreased. The heat-treatment in Ar environment prior to heat-treatment in oxygen environment suppressed oxidation of Ti diffused along Pt grain boundaries, leading to the decrease of Pt hillocks. Therefore, it could be concluded that Pt hillocks were formed to alleviate high compressive stress generated by titanium oxide formed along Pt grain boundaries. The short probability of PZT film was not dependent on the density of hillocks but the size of them.
Perovskite PZT films could be obtained even at high deposition temperature (540℃) by 2-step deposition method (After the PZT film is deposited at low temperature of 480℃ at the early stage of deposition to suppress volatility of Pb, the film was deposited at elevated temperature). The excess Pb content in PZT film could be reduced due to high deposition temperature, which resulted in the decrease of leakage current. The PZT film had a dielectric constant of 1100, a remnant polarization of 18μC/㎠ and a low leakage current of $10^{-4}A/㎠$ at high electric field of -500kV/cm.
The poly-Si cantilever beam applicable to piezoelectric microactuator was fabricated by surface micromachining. Pt and thick PZT film (1㎛) was successfully etched using ICP (inductively coupled plasma) etcher for the fabrication of microactuator. Cr film, with chemical resistance to HF and good adhesion properties, was used as passivation layer to protect the device during etching sacrificial layer. However, the device was attacked by HF solution. The more study on etching of sacrificial layer is required to fabricate piezoelectric microactuator.
