Although there are many studies for the characterization of piezoelectric thin film, it is still strongly needed to develop the easy and practical way to measure the piezoelectricity existing in thin film. Piezoelectric properties of lead zirconate titanate (PZT) thin film were investigated by a new pneumatic loading method, which has been devised as a more reliable and convenient characterization technique.
After the first report on the measurement of the piezoelectric coefficient for thin films in 1990, a great deal of research has been done in the field of characterization for piezoelectric thin films. This trend has been encouraged with the explosive interest in their potential applications and outstanding technological progress. For instance, the ferroelectric thin film microactuator for optical applications is one of the most promising candidates for up-coming commercialization. To realize their potential effectively, it is essential to have a simple and reliable characterization technique. This will give us faithful predictions for the performance of piezoelectric devices made from the material that has been characterized and also permit ease of comparison for diverse materials, thus saving a substantial amount of time and cost. Basically, there are two trends in evaluation techniques for piezoelectric thin film. One is to use the direct piezoelectric effect such as the normal loading method or the impulse method and the other is to employ the converse piezoelectric effect like the interferometer or the AFM. Only the former can tell us the information prior to the poling process, which incorporates high electrical fields, because the latter by itself leads to considerable domain reorientation in the as-deposited film. To study the piezoelectric phenomena of electrically unpoled as-deposited films, methods using direct piezoelectric effect are unavoidable. By the principle of the direct piezoelectric effect, some researchers have measured and described the piezoelectric characteristic for thin films including the built-in piezoelectricity of virgin films without any poling treatment.
Piezoelectric properties of lead zirconate titanate thin films have been investigated using the pneumatic loading method, which was devised as a more reliable and practical characterization technique. Lead zirconate titanate thin films with a composition near the Morphotropic Phase Boundary and a thickness of about 400 nanometers were prepared by the Metal Organic Decomposition process on a platinized silicon substrate. We carefully examined the effects of poling voltage and poling time on the piezoelectric properties of the thin films. The piezoelectric d-coefficient increased with an increase in the poling voltage and also in the poling time and saturated at around 10V of poling voltage and 100 sec. of poling time. Before poling, there was no built-in piezoelectricity in the virgin films. Up to 376pC/N of piezoelectric d-coefficient was recorded when we poled it at 14V, 350KV/cm.
We have also studied the effect of crystallization temperature, which is one of the most fundamental processing parameters of leads zirconate titanate (PZT) thin film, in the respect of the piezoelectric properties by our pneumatic loading method. Even though the piezoelectric properties of thin films are very critical factors in the micro-electro mechanical system (MEMS) and thin film sensor devices, very few reports are provided for the last decade unlikely the bulk piezoelectric devices. We have found that the piezoelectric properties of thin films are improved as the increase of crystallization temperature up to 750℃ and this behavior can be also explained by the analysis of dielectric polarization hysteresis loop, crystal structures and scanning electron microscopy.
The main idea of this research is to provide a distinctive solution for the measurement of both the longitudinal and the transverse piezoelectric d-coefficients, $d_33$ and $d_31$, of ferroelectric thin films and also thick films. In general, to get these two coefficients of thin films, two different measuring systems are required. Here, we propose the improved method for the evaluation of these two coefficients with single equipment and with the relatively convenient procedure. The two-step loading process of applying the both positive and the negative pressure has devised to acquire the piezoelectric coefficients. These results have been calibrated for both the longitudinal and the transverse piezoelectric d-coefficients, d33 and d31, of thin films.
We have obtained $d_33$ of 331pC/N and $d_31$ of 92.2pC/N for the PZT thin films. These results will be utilized for the piezoelectric micro-devices and give us more reliable expectation for the device performance and better understanding of piezoelectric thin film.
From the meditation for the measuring philosophy of conventional normal loading method, we proposed our pneumatic loading method which has been devised for the more improved and reliable characterization technique for the piezoelectric thin films and thick films, too.