There has been increasing interest in ferroelectric lead zirconate titanate (PZT) films for applications in piezoelectric devices. Many potential applications require a film thickness above 10㎛ for realizing devices of increased force, sensitivity and stability. Integrated circuits or components using piezoelectric thick films are useful as microsensors and actuators in various applications, such as in the medical, military, telecommunication, motor vehicle and office automation industries. In particular, the combination of a screen-printed PZT thick film and Si substrate allows the return of inexpensive fabrication technology and offers the possibility of fabricating films of a wide thickness range for microdevices because it offers the possibility of Si micromachining and one-chip devices with integrated driving circuits.
It is almost impossible to produce films of thickness greater than 10㎛ using thin film processing methods such as sol-gel, sputtering and chemical vapor deposition (CVD). It is very difficult to fabricate screen-printed PZT thick films on Si substrates because of the volatility of PbO and the interdiffusion of Si and Pb through the bottom electrode during sintering at a normal temperature (such as above 1200℃). Thus, many researchers have used $Al_2O_3$ substrates. The fabrication of screen-printed PZT thick films on Si substrates is not well understood. Koch et al. studied screen-printed PZT thick films with a cermet gold electrode on a 500-nm-thick $SiO_2$ film deposited on a Si substrate, but they achieved approximately 30% of the piezoelectric constant, $d_{33}$, of the bulk PZT. We have studied the interdiffusion barrier layers such as Pt, $SiO_2$, $SiN_x$, MgO and $ZrO_2$. We confirmed that their thin films, specially $Pt/ZrO_2/SiO_2$ thin films, could protect the interdiffusion between Si and PZT at 1050℃.
In this study, sputtered Pt bottom electrodes were deposited on $SiO_2$ or $ZrO_2/SiO_2$ thin films which were coated on Si substrates, to minimize the interdiffusion between Si and Pb. Residual stresses of Pt films on these oxide thin films were analyzed with an X-ray diffractometer (XRD). Then, PZT thick films were coated on these Pt thin films by screen printing, and the ferroelectric and the piezoelectric properties were measured.
Two kinds of PZT thick films were researched. One PZT thick film was fabricated on $Pt/ZrO_2/SiO_2/Si$ using a paste of PZT(52/48) with sintering aids $(Bi_2O_3-B_2O_3-CdO)$ at 925℃. The PZT thick film on $Pt/ZrO_2/SiO_2/Si$ fabricated at 925℃ exhibited the remanent polarization, coercive field, dielectric permittivity ($\epsilon_r$), dissipation factor (tanδ), breakdown field and piezoelectric constant $(d_{33})$ of 13 μC/㎠, 8.5 kV/cm, 800, 0.015, 12 MV/m and 343 μC/N, respectively.
The other PZT thick films are screen-printed on $Pt/Al_2O_3$ using a paste of PbO, $ZrO_2$ and $TiO_2$ powder mixture and the paste is printed on the substrate by screen printing, and then the green film is annealed at low temperature (under 1000℃). The Melting points of PbO, $ZrO_2$ and $TiO_2$ are 888℃, 2700℃ and 1800℃, respectively. A main idea is partial melting process using a precursor materials having low melting point such as PbO. The resion of this possibility of fabrication of densed PZT thick film at low temperature, is that the diffusion rate of composition having low melting material (PbO) is faster than the solid state sintering using PZT powder and the sintering rate is incresed rapidly. Densification and reaction mechanism were speculated in the PZT thick films fabricated at relatively low temperature (under 1000℃) without sintering aids. Highly densified PZT thick films could be fabricated on Pt / $Al_2O_3$ substrate at 1000℃, and achieved the density, remanent polarization, coercive field, dielectric permittivity, dissipation factor and breakdown field of 98%, 10 μC/㎠ and 20 kV/cm, 540, 0.009 and 16 MV/m, respectively. The results show the possibility of densification of the PZT thick film at relatively low temperature without sintering aids, and render the PZT thick film promising for application in silicon micromachining, micropumps and other applications.
