Nanostructuration effect on the properties of ferroelectric HfZrO2

International audienceVarious applications have been suggested for fluorite-structure ferroelectrics due to their advantages over the conventional perovskite-structure ferroelectrics [1]. In this presentation we will focus on (Hf,Zr)O2 (HZO) thin films deposition for the capacitor of Ferroelectric Random Access Memories (FRAM) in the 1Transitor-1Capacitor (1T-1C) model. (Hf,Zr)O2 thin films are studied to either fully understand the stabilization of the ferroelectric phase (f-phase) or to fit with industrial requirements. In 2015, Park et al. wrote: “[…] it seems critical that the dielectric layer is deposited in the amorphous phase and crystallized in a latter annealing step.” [2] However, there was no clear evidence of the phenomenon as films are grown amorphous by atomic layer deposition. Changing the pressure in our sputtering chamber lead to the deposition of crystalline or amorphous films at room temperature. After a Rapid Thermal Annealing (RTA), only the amorphous films crystallize in the f-phase. This result was the starting point for many studies led by the authors. Samples are stacks of Si/TiN/Hf0.5Zr0.5O/TiN/Pt. All materials are grown by sputtering at room temperature following by a rapid thermal annealing during 30 seconds under N2 atmosphere. The samples are called NM, and M. NM and M refers to two different architectures, respectively non-mesa and mesa structures. The description and size of NM and M samples is given in figure 1 for each sample. Fabrication and architecture details can be found in reference [3].The set-up for electrical measurements have been described in reference [4]. This set-up allows us to wake the samples with bipolar square pulses. Measurements are performed with a positive up negative down (PUND) sequence. It consists in applying a negative setting pulse followed by two positive ones (P and U) and finally two negative ones (N and D). PUND maximum amplitude voltage equals that of the set/reset sequence. PUND pulses are triangular pulses with a rising/falling time of 100µs. We report the fabrication of two samples deposited by magnetron sputtering with excellent performances, quite similar to samples deposited by ALD. Pr values are among the highest for samples deposited by sputtering. Although the N-sample and NM-samples show very close Pr values, the two samples show completely different electrical behaviors. During cycling, the increase of Pr value for the NM-sample is more than an order of magnitude higher than the M-sample. It is accompanied by a decrease of the endurance which is two order of magnitude higher for the NM-sample than for the M-sample.As electrical behaviors are not the same, for low stress conditions M-sample has a higher Pr value during cycling whereas for high stress conditions NM-sample has a higher Pr value during cycling. As a matter of fact, it has been proven that maximum Pr values are more sensitive to stress conditions than the structures themselves. The origins of the different electrical behaviors come from the micro-crystalline structures of the two samples, according to GIXRD results. The crystallization takes place during the annealing step. During annealing, M-sample is built with a TiN TE fully covering the HZO layer whereas the TiN covers only partially the HZO layer in case of the NM-sample. It induces different stress states which lead to two different micro-crystalline patterning. The M-sample shows no monoclinic peak, whereas the NM-sample shows many monoclinic orientations. It can explain the huge reduction of the wake-up effect.ACKNOWLEDGEMENT This work was realized on the NanoLyon technology platform and has received funding from the European Union’s Horizon 2020 research and innovation programme under Grant Agreement No. 780302. REFERENCES [1]M.H. Park, et al. MRS Commun. 1 (2018).[2]M.H. Park, et al., Adv. Mater. 27, 1811–1831 (2015).[3]J. Bouaziz, P.R. Romeo, N. Baboux, B. Vilquin, ACS Appl. Electron. Mater. 1, 1740 (2019).[4]J. Bouaziz, P. Rojo Romeo, N. Baboux, R. Negrea, L. Pintilie, B. Vilquin, APL Mater. 7, 081109 (2019)