For the characterization of the performances of PZT thin films with IDE, with various compo- sition and dopants, we have grown thin films following the sol-gel processing route developed by Chidambaram and coworkers [35, 43, 45]. The microstructure of the obtained films will be described in details below; it is similar to that published in Ref. [35].
We used either Nb or Fe as dopants, to obtain compositions of the form Pb(ZrxTi1–x)1–yMeyO3, where Me is Fe or Nb. For Fe, we set y = 0.01, and for Nb, y = 0.02, that is 1 at.% doping for Fe and 2 at.% doping for Nb. Undoped, Fe-doped and Nb-doped PZT thin films with the 43/57 Zr/Ti nominal composition were prepared; this composition has the tetragonal phase. We also made undoped and Nb-doped PZT thin films with the 53/47 Zr/Ti nominal composition, which is the MPB composition [44]. We will later designate the doped films in the form Me:PZT x/1 − x, where Me is Fe or Nb, and x is the Zr/(Ti+Zr) ratio; if no dopant is specified, we mean to speak about the undoped film.
We prepared the precursor solutions with a 2-methoxyethanol route adapted from Blum and Gurkovich [92]. The solution preparation procedure is described in detail in Chidambaram’s PhD thesis [93]. For each composition, doped and undoped, 4 solutions were prepared with
the Zr/(Zr+Ti) stoichiometric ratio following the gradient-free route developed by Calame and Muralt [47], which limits deviations from the nominal composition along the growth direction to within ±2.5at.%. Each precursor solution is spinned and pyrolyzed in a sequence going from solution 1 to solution 4, and a crystallization step in a rapid thermal annealing tool (RTA), at 650◦C for 1 min in air under 100 sccm of oxygen flow, is performed after the 4-spinning se- quence is complete. This results in a crystallized film thickness of about 250 nm; the sequence is repeated to obtain thicker films. All solutions have 10 at.% lead excess to compensate for lead loss during the RTA step, except solution 4 which was prepared with 30 at.% lead excess. For all films, a 20 nm-thick lead titanate PbTiO3(PTO) seed layer prepared with 30 at.% lead excess is used to facilitate the nucleation of the perovskite phase and to promote textured growth. This layer is crystallized in the RTA tool, under the same conditions as above, after only 1 spinning and pyrolysis. We used the following precursors for the preparation of the solutions, all purchased from Sigma-Aldrich®:
• For Pb : lead (II) acetate tri-hydrate • For Ti : titanium (IV) isopropoxide • For Zr : zirconium (IV) propoxide • For Nb : niobium (V) ethoxide
• For Fe : iron (III) nitrate non-ahydrate
The stoichiometry was controlled by weight.
We used full-wafer thickness devices for characterization. This has the advantage that all samples are immediately compatible with our existing setups and equipments, while at the same time allowing for simple microprocessing procedures and short fabrication time. We used slightly doped, single-side polished 100 mm single crystal 〈100〉 Si wafers as substrate, with 2µm wet oxide for electrical insulation from the Si substrate. The conductivity of the Si was either 15–25Ωcm or 0.1–100 Ωcm. This conductivity difference has no influence on the contribution of the parasitic capacitance at the frequencies encountered in our measurements, as discussed in chapter 3. The wafer thickness was measured with a mechanical profilometer and was between 523µm and 536 µm for the IDE samples, and was 388 µm for the PPE samples. The PPE samples were kindly provided to us by Dr. Ramin Matloub.
A 100 nm-thick MgO buffer layer was e-beam evaporated onto the wafers. This layer serves to promote the {100} PTO growth as well as a diffusion barrier to prevent lead interdiffusion at the high temperature of crystallization [35]. Unlike sol-gel PZT growth on {111} Pt bottom electrodes, for which an overwhelmingly dominant {100} texture is routinely achieved with the appropriate seed layer [50], the texture control provided by the MgO layer is not fully reproducible and is very sensitive to the ambient moisture: Indeed, MgO readily reacts with water to form magnesium hydroxyde [94]. After evaporation, the wafers are exposed to ambient air during their transfer between the evaporator to the room where we proceed to the spin-coating of the PTO seed layer immediately afterwards. Although the room is equipped with a dehumidifier, good texture control for all the wafers of a given batch could not always
4.2. Sample fabrication
be obtained — note that since we proceed to the PTO deposition one wafer at a time, some wafers are exposed to air during a longer time. During the transfer, the wafers are transported in a sealed wafer box; we found that adding a large quantity of silica gel beads inside this box improves the number of wafers with the {100} PZT texture for a given batch. We managed to obtain almost full texture control for all wafers in a given batch for MgO depositions performed during the winter months, when the ambient air is naturally very dry. If the same route is used for sputter deposition of PZT, the best situation is to proceed to PZT growth directly after the MgO evaporation without breaking the vacuum in between. This is unfortunately not possible for sol-gel processing which takes place in air. In this case therefore, it is recommended that the sol-gel equipment be situated in the same room as, or as near as possible to, the evaporator and that the ambient humidity in this room be made extremely low. This is also beneficial if the precursor solutions were prepared with the 2-methoxyethanol route, since such solutions are sensitive to moisture as well.
PZT (500 nm) PTO (20 nm)
MgO (100 nm)
Pt Si
SiO2 (2 µm) MgO evaporation
Sputtering and lift-off
Sol-gel processing
Figure 4.1 – Schematic of the fabrication steps.
Immediately after MgO evaporation, a 20 nm-thick PTO seed layer is deposited by sol-gel and crystallized in the RTA tool as described above. We found that this PTO layer is stable for at least several weeks, with the PZT layer deposited on top at a later time reproducing its texture as expected. Finally, the PZT layers were deposited. Two RTA steps were performed to obtain a film thickness of around 550 nm (that is a total of 8 spinning steps).
The interdigitated top electrodes were patterned by photolithography and lift-off to avoid any damage to the PZT surface, which can lead to surface conduction or electrical breakdown upon biasing. Sputtered Pt was used as the electrode material. Although evaporation is the PVD technique of choice for lift-off processing, evaporated Pt does not adhere well on our sol-gel PZT surfaces [51], whereas sputtered Pt provided good adhesion and was therefore preferred. A number of IDE structures with various gap distances and finger widths were patterned on the PZT film and then diced to obtain elongated cantilevers 15 mm long and 1.5 mm wide. The active area corresponds to the interdigitation zone and occupies only a fraction of the total cantilever surface, namely a few mm in length and 1 mm in width. The fabrications steps are summarized in Figure 4.1. Figure 4.2 shows an optical microscope view of a finished cantilever.
500 µm
Figure 4.2 – Optical microscope image of the interdigitation zone after dicing.
20 25 30 35 40 45 50
Counts (a. u.)
P ZT (100 )/(00 1) P ZT (110 ) S i (20 0) P ZT (111 ) P ZT (200 )/(00 2) undoped PZT 53/47 Nb-doped PZT 53/47 undoped PZT 43/57 Nb-doped PZT 43/57 Fe-doped PZT 43/57 2θ (°) 101 102 103 104 105
Figure 4.3 – XRD scans of the investigated IDE films. The Si (200) peak is present because of the film stress after sol-gel processing.