Flexible PZT thin film transferred on polymer substrate

Highly flexible lead zirconate titanate, Pb(Zr,Ti)O 3 (PZT), thin films have been realized by an all chemical process on a plastic substrate. The procedure is composed of three steps: the first one is the PZT deposition on aluminium thin substrate, the second step corresponds to the bonding PZT thin film to a polymer layer and the final step is the aluminium substrate etching. Diffractions and microscopy techniques were used to check the quality of the new PZT/polymer composite structure. Polarization hysteresis and permittivity measurements were also performed to determine the electrical characteristics of the composite. These results demonstrate that the recently developed PZT/polymer thin films are very attractive for both bending actuation and sensing as well as for low frequency vibrating energy harvesting applications.


Introduction
Nowadays, flexible technologies are in great expansion and become new development trend in many evolving electronic devices such as displays, sensors, solar cells and mechanical energy harvesters. In the particular field of low frequency (<100 Hz) vibration energy harvesting with piezoelectrics, recent works have focused on the realization of thin PZT layer on flexible and insulating substrates [1]- [3]. Thus, the challenge is to obtain a piezoelectric ceramic material requiring high temperatures (> 600 °C) to crystallize on a polymer substrate unable to withstand such temperatures. To address such a technological issue, the two possible options are to develop a complete low temperature process or to transfer piezoelectric material from growth substrate to polymer substrate.
Recently, our group has attempted to design reliable techniques for the fabrication of thinlayer piezoelectric structures on flexible support which resists to higher temperatures. Thin films of lead zirconate titanate (PZT), one of the most investigated and high performance piezoelectric materials, have been prepared by using chemical deposition on a flexible metallic substrate, a commercial aluminium (Al) foil with thickness less than 30 µm [4]. This fabrication process is cost effective and the lightweight of the micro-generator makes it sensitive to air flow. This structure has been thoroughly characterized [5], [6] and shown promising results for energy harvesting under mechanical stress [7].
To go further, we want to achieve a structure without ground plane to be free to choose the electrode design. That is the reason why PZT films must be separated from the aluminium substrate. However, PZT thin films are not enough rigid to stand alone, so a new substrate has to be used with a good flexibility and insulating properties. Specific polymer materials like some thermoplastics are good candidates for the new insulating and flexible substrate whose flexural rigidity can be tuned according to the selected family of polymers and the dimensions of the layer. The major drawback of these soft polymers is their low resistance against thermal treatment that prevents from the direct PZT deposition onto the polymeric substrate. The proposed solution is to transfer the PZT thin film, obtained by classical methods on rigid substrates, onto the polymeric substrate. This solution was used to realize PZT on polymer, but the method employed -laser lift-off -is expensive and could be difficult to transfer to the industry. [1]- [3] In this study, we will focus on the development of a new procedure to achieve piezoelectric PZT thin film on a flexible polymer substrate. Starting from the PZT/Al thin film developed in the laboratory, a chemical process is used to transfer the piezoelectric material to a polymer substrate. This process is cheap and simple and would be easily transferred to the industry. In addition, with this chemical method, the metallic substrate is removed from the complete surface ( 6 cm 2 ) of PZT thin film in one step whereas the laser lift-off method requires several steps to separate PZT from the first substrate.
Thus, this paper mainly deals with the method of realization of flexible PZT thin film without ground plane and comprises the chemical details to transfer a PZT thin film from aluminium substrate to a polymer substrate. The morphological, structural, compositional (surface electron microscopy (SEM), X-ray diffraction (XRD), atomic force microscopy (AFM)) characterizations of the PZT/polymer thin film are also presented to confirm the good quality of the transferred PZT thin film. Finally, comparisons of dielectric and ferroelectric characteristics of MIM structures before and after the transfer on polymer substrate are realized to assess the influence of the transfer process on the active layer properties.

Material and methods
PZT thin layer fabrication begins with the preparation of a precursor solution which is obtained by mixing different chemical products. Initially, lead acetate is dissolved in a solution of acetic acid and then zirconium n-propoxide and titanium n-propoxide are added in desired proportions. The final precursor solution will be ready by adding ethylene glycol, which limits the crack formations in PZT thin film during its thermal treatment.
The precursor solution is then deposited by spin-coating technique on a sacrificial substrate, an aluminium thin foil, which can be etched in the following by simple chemical process. A stainless steel support is used in order to facilitate the spin-coating deposition. This technique allows the formation of a piezoelectric film on the complete surface of the aluminium substrate. The rotation speed and spinning duration are chosen according to the desired thickness of thin layer. After each spin-coating step, the material is subjected to a thermal treatment at temperature of 650 °C for the duration of 2 minutes to crystallize the PZT thin layer. In practice, the thickness of the PZT thin layer obtained after the execution of one spincoating step is 300 nm. Thus, to form a thicker PZT thin film, deposition step can be reiterated as many times as necessary to obtain a thin film of several micrometers.
The different steps of the PZT transfer process are schematically represented in Figure 1. Thick elastic polymer layer is attached on the PU adhesive to form the permanent substrate of the final flexible piezoelectric structure. This step is carried out using a thermofusing technique. The sample is heated while a small pressure is applied, to create a good binding between the two polymers layers. Commercial polyethylene terephthalate (PET) is a suitable thermoplastic material for new substrate because it is an elastically deformable and low-cost polymer. Furthermore, several thicknesses could be used to get the desired mechanical properties.
The new substrate has to be opened before the binding in order to realize a kind of electric via by the filling with conductive glue. This step is also very important for the realisation of MIM structure onto a polymer substrate because it reinforces the whole structure. Indeed, the connection channel created through the polymer substrate gives fragility at this specific point to the PZT layer. Macroscopic cracks could happen during the etching step. So, the conductive glue is needed to bring a better mechanical support to PZT. The glue has to resist to the chemical etching solution to preserve the electric contact on back (Pt) electrode.  transferred onto PET. The permittivity and the dielectric losses have been measured by using a HP 4275A LCR meter between 100 and 10 5 Hz.

Results and discussion
The above described method proves that the transfer of the piezoelectric thin layer onto a flexible substrate is possible just by using a simple chemical method. The obtained piezoelectric thin film on polymer substrate has been characterized with XRD, SEM and AFM as complementary techniques. MIM structure was realized to investigate ferroelectric properties of the transferred PZT thin film. XRD patterns of PZT/Al and PZT/PET are roughly the same except for the substrates peak.
Indeed the peak of aluminium is replaced by the peak of PET, but all the peaks of PZT remain at their positions. The position of the PET peak was verified by an investigation on a single PET layer. The slight difference in 2Θ angle observed is a consequence of the flexible substrates, the base surface being not plane in the both cases leading to shifts in 2Θ angle.
AFM tip deflection was shown for the aluminium substrate and transferred PZT thin films in a previous paper [8]. The roughness was measured on the whole surface for the two faces of the    P-E loops of PZT/Al and PZT/PET are showed in Figure 5. A lower P r value is observed for PZT/PET with 16 µC/cm 2 against 21 µC/cm 2 for PZT/Al. This is combined with a slightly higher E C of 130 kV/cm for PZT/PET compared to 115 kV/cm for PZT/Al. Depending on ferroelectric properties, it seems that the transfer process has an undesirable impact on the PZT with minor degradation of P r and E C . decrease of this parameter after the transfer (335 and 270). The minor degradations due to the transfer process also modify dielectric properties. Whereas tanδ is not much impacted with the transfer, the relative dielectric permittivity is modified by the transfer. This explanation is strengthened by the fact that many samples were in short-circuit, indicating a lower insulating quality of the PZT layer. In addition, PZT properties could decrease with a wet etching process [9], [10]. In those reported cases, the PZT is structured by chemical etching and implies a degradation of the ferroelectric and piezoelectric properties.
The laser lift-off (LLO) transfer method used by Do et al. [1] produces a very slight decrease in ferroelectric properties. For example, remnant polarisation Pr is 27 µC/cm 2 before transfer and 23 µC/cm 2 after. In the same time, the relative dielectric permittivity is measured at 1089 and 1046, respectively before and after LLO transfer. Apparently, this quite sophisticated method gets better results and lower decrease of the properties. But the LLO transfer requires more specific equipments and time. What we proposed is a simple alternative method to this highly technical process. Quite good results are obtained with a shorter experimental time.
Besides, the new structure is functional and could be of great interest for some flexible applications like sensors or actuators. In addition, this is the first step to the realization of interdigitated electrodes in order to develop efficient low frequency vibration-based energy harvesters.

Conclusion
A new process for PZT transfer to polymer substrate is described in this paper. The use of chemical solution deposition for the realization of the PZT on sacrificial substrate is complementary with chemical technics developed for the transfer process. The entire process is then easy to industrialize. Structural characterizations were realized to ensure the good binding between the different layers after the transfer. The crystallinity of the PZT was also checked before ferroelectric and dielectric measurements. Before and after transfer process, comparison was done for those electrical measurements revealing a small degradation of properties due to the etching step. The explanation proposed is that the PZT porosity is enhanced because of the interaction with iron chloride solution. Nevertheless, characteristics of PZT/PET composite are very interesting and this structure will be investigated for sensing and energy harvesting applications.       PZT transfer process on polymer substrate based on chemicals methods.  The good binding of PZT on polymer substrate is ensure with SEM images.  Comparison of electric properties before and after the transfer are realized.  A small degradation is observed due to the interaction between PZT and the etchant.