First, we remark that none of the IDE samples investigated in chapter 4 is well-suited for large-field operation. Even though both IDE and PPE configurations have the same effective piezoelectric coefficient at large fields, tens to hundreds of volts are needed to reach saturation in the IDE configuration, because the IDE gap is of the order of severalµm. For similar electric fields, PPE devices can reach the same excursion at a fraction of the voltage necessary to drive IDE samples, because the voltage is dropping across the PZT film thickness instead of across the gap distance. The PZT film thickness is typically 1–2µm for PPE devices with sol-gel films, much less than the gap distance realized in our devices.
In addition, for IDE samples, the tip end excursion does not solely depend on the material piezoelectric coefficient e33, ifand on the size of the interdigitation zone (as would be the case for PPE samples), but also on the metalization ratioη = b/(a + b), where a is the gap distance and b is the electrode finger width. This is the consequence of the electric field distribution inside the ferroelectric layer. As we saw in chapter 3, there is no electric field below the electrode fingers; this volume of material is therefore inactive and does not contribute to the cantilever bending through the converse piezoelectric effect, nor does it participate to the charge generation through the direct piezoelectric effect. Consequently, the tip excursion will be dictated by an averaged curvature, so that the overall bending is described by an effective
Table 7.1 – Comparison between a Nb-doped PZT 53/47 film in the IDE and PPE configurations, for the small-signal remanent (rem.) e coefficient and an effective large-signal coefficient.
Sample Rem. small-sig. e coeff. (C/m2) Large-sig. −σmax/Emax(C/m2)
IDE material value 14.5 15.3
IDE engineering value 8.7 9.2
PPE value −8 −20
or engineering stressσf, which is given by:
σf= (1 − η)σf (7.1)
whereσf is the ferroelectric film stress developed between the IDE fingers. Table 7.1 compares
the two Nb-doped PZT 53/47 samples shown in Fig. 4.20 in chapter 4. One film is a 640 nm- thick film with IDE, and the other one is a 1.25µm-thick film in the PPE configuration. Table 7.1 shows the small-signal e coefficient (e31, ffor the PPE sample and e33, iffor the IDE sample) at
remanence, which is representative of small-field operations, and the large-signal maximum stressσmaxreached at the maximum field Emax= 150 kV/cm (the same for both samples)
divided by Emax, which is representative of large-field actuation. The IDE sample hasη = 0.4.
We observe that the engineering value is much reduced as compared to the material response for both large and small-signal, because of the large value ofη. The fact that the remanent small-signal e33, ifand the large signal e33, ifare close for the IDE sample, shows its excellent retention properties. On the contrary, for the PPE sample, the remanent small-signal e31, fis less than half of its large-signal value. However, the large-signal e31, fis significantly larger than the large-signal e33, if— not only for the engineering value, but also for the material value.
This shows that IDE samples are less suited for large field operations also from a material response point of view. The difference may be explained by larger extrinsic contributions to
e31, fthan to e33, if: because of the residual tensile stress in the PPE sample, more ferroelastic
domain walls are expected to be present, unlike in the IDE case where the poled state is more elastically stable, reducing the driving force for ferroelastic domain wall formation.
Hence, for large-field operations, the PPE configuration is more advantageous, for three reasons:
1. The driving voltage is much lower for PPE samples.
2. For a given sample size, the active volume is larger for PPE samples, because IDE samples must haveη > 0 to be operated.
3. The larger extrinsic contributions to e31, fmeans that the material response in large-field
operations is larger in the PPE configuration.
However, the excellent remanence and retention properties of the IDE samples make them ideal for energy harvesting or sensor applications, where the stability of the poled state and a high sensitivity at low fields are critical. The lower capacitance of IDE devices, as compared to
7.2. Applications for PZT thin films with IDE
PPE devices, allows a larger voltage output for the same amount of generated charges, which is essential for good rectification efficiency. Provided that the larger remanence compensates for the lower excursion, actuation for small displacements could be also envisioned. One advantage of IDE for large-field operations, however, is the reduced hysteresis and larger linearity at high fields. This is an interesting feature if precise actuation is more important than a large displacement or a large force, in particular for open loop configurations. Since the stress developed in the film is compressive, the active layer will also be less sensitive to mechanical failure.
Due to lower remanence and retention properties, doped compositions always have a lower piezoelectric response at low fields, while the response at high fields is comparable or slightly inferior. In addition, they all increased the dielectric constant of the film, increasing the capacitance of the device. Undoped compositions should therefore be preferred. Although the undoped tetragonal composition has the best retention of all the 5 compositions and dopings investigated in chapter 4, the largest remanence, switchable polarization, and piezoelectric response was obtained with the undoped MPB film, showing a record small-signal e33, if
of 17.5 C/m2at remanence. The tetragonal compositions show a nearly field-independent
e33, ifon the return branch whatever the doping employed, for both large and small signals.
Consequently, this composition should provide very little hysteresis and nearly perfect linear behavior under unipolar excitation. Although the MPB composition also shows little hysteresis, its behavior is less linear. The remanent small-signal e33, ifof the undoped tetragonal film
is significantly lower, at 12.5 C/m2; a more advantageous compromise between linearity and piezoelectric response may be obtained by choosing a tetragonal composition closer to the MPB. We have not explored the rhombohedral side of the PZT phase diagram, where better trade-offs could also exist. The excellent remanence and retention properties of IDE samples are likely due to the more stable stress situation of the poled state, as discussed above. Unfortunately, this also reduces the extrinsic contributions to e33, ifand means that
PPE samples are more competitive at large fields.
The fabrication of IDE is more challenging than that of PPE structures. One difficulty resides in the fact that periodic patterns with a small feature size should occupy a significant area; this makes the quality of the finished structures more sensitive to defects, even localized ones, in the photoresist coating (in particular for lift-off processes, in which the lift-off resist layers are more likely to have these kinds of defects than most types of photosensitive resists), or to any damage occurring during resist development after UV exposure. Their fabrication therefore requires a good quality control over the whole patterning process. Moreover, efficient poling of the ferroelectric layer requires geometrical dimensions not too much larger than the film thickness (see next section for more details), that is, of the order of 10µm at the maximum. Photolithography techniques must therefore be employed; it is not possible to use a hardmask for patterning the structures, making them more expensive and more complex to produce than PPE devices.