Electrode Grains Template Ferroelectric Domains in a Hafnia-Based Capacitor

Hafnium zirconium oxide (HZO) is a leading CMOS-compatible ferroelectric for nonvolatile memories [1]. However, HZO is polycrystalline and polymorphic. The ferroelectric orthorhombic phase (o-phase) competes with non-ferroelectric phases at room temperature [2]. As a result, local crystal structure and interfaces can strongly affect ferroelectric domain formation and device-to-device variability [3]. Here, we describe simultaneously mapping the ferroelectric domains and the crystalline grains during crystallization in a functional TiN/HZO/TiN capacitor. We find that the grain structure of the TiN electrode strongly templates the ferroelectric domains. We also outline strategies for more reliable phase and orientation mapping of the ferroelectric layer in the presence of diffraction from the electrodes. We fabricate TiN (20 nm)/HZO (10 nm)/TiN (10 nm) metal-ferroelectric-metal (MFM) capacitors on an electron-transparent silicon nitride (SiN) membrane. Each capacitor is surrounded by a TiN nanowire heater for in-situ thermal control. Plan-view scanning transmission electron microscope (STEM) imaging allows us to probe the domain and grain structure across the capacitor. Starting with amorphous HZO, we hold the capacitor at 400 °C for 100 ms. After electrically cycling the capacitor to activate the ferroelectricity, we simultaneously acquire an electron beam induced current (EBIC) image (Fig. 1a) and a 4D STEM dataset. EBIC maps the depolarization field and highlights the ferroelectric domains [4], while virtual dark field (VDF) images extracted from the 4D STEM data reveal the crystallization of HZO (Fig. 1b). We then reheat the sample to 400 °C for progressively longer and longer periods, repeating the cycling and the imaging each time. As the time at 400 °C increases, HZO grains start to nucleate and grow. The ferroelectric domains show strong correlation with the HZO grain distribution. However, although HZO grains grow to >50 nm during crystallization, the characteristic size of the ferroelectric domain remains at 10 nm throughout the whole process. To more accurately correlate the ferroelectric domain with the grain structure, we introduce a grain-mapping approach based on the normalized cross-correlation coefficient (NCC) between diffraction patterns (DP) extracted from the 4D STEM dataset. First, the diffracted disks are identified at each beam position using py4DSTEM [5]. Then, the identified disks are separated into two categories based on their Bragg vector lengths: the ones in the HZO-only regions in reciprocal space and the ones in the TiN/HZO-coexistence regions. For each category, a reduced 4D STEM dataset is constructed from the identified disks’ positions and intensities. An average NCC between each DP and the DPs of the eight neighboring pixels in real space is computed. The average NCC maps of the HZO-only category and the TiN/HZO-coexistence category are shown in Fig. 2b and Fig. 2c, respectively. Rather than solely relying on VDF imaging, this method quantifies structural similarity between neighboring scan points, letting grains and grain boundaries stand out. The characteristic grain size of TiN (∼10 nm, Fig. 2c) matches that of the ferroelectric domains (Fig. 2a), while the HZO grains are substantially larger (>50 nm, Fig. 2b). This observation suggests that the ferroelectric domains are strongly templated by the TiN electrode, indicating that the electrode’s material and structure play a critical role in stabilizing the ferroelectricity [6]. Although phase mapping of ferroelectric HZO has been demonstrated on cross-section devices [7] and plan-view capacitors with electrodes removed [8], interference from the overlapping electrodes makes it challenging in functional devices. Strong CBED disks from the electrode material complicate the indexing of the HZO patterns. To mitigate this issue, several strategies can be employed. By averaging DPs acquired at different beam tilts or stage tilts (or ideally performing precession electron diffraction, PED), more CBED disks can be illuminated (Fig. 3a). Identifying more CBED disks is beneficial for both mapping the grain distribution (Fig. 3b) and pattern indexing. In addition, when the electrode material has high crystal symmetry and does not cover much reciprocal space, selectively masking its Bragg peaks can enhance sensitivity to the phase of the HZO (Fig. 3c). These approaches provide a path toward a more comprehensive ferroelectricity and crystal structure correlation study. In summary, we demonstrate simultaneous ferroelectric domain and crystal grain mapping in a functional TiN/HZO/TiN capacitor. Our results reveal that the ferroelectric domain size correlates with the TiN electrode grains rather than the larger HZO grains, indicating that the electrode materials play a crucial role in stabilizing the ferroelectricity in HZO. Combining EBIC and 4D STEM creates a powerful platform for studying the correlation between the crystal structure and the functionality of nanoscale devices [9]. Ferroelectric domain and HZO grain evolution as a function of the annealing time. a) STEM EBIC imaging maps the spatial distribution of the depolarization field in a plan-view TiN/HZO/TiN capacitor. b) Virtual DF of HZO grains generated from 4D STEM datasets simultaneously acquired with the EBIC image. The virtual DF detectors are selected to cover reciprocal-space regions with HZO diffraction spots only. Comparing the size of ferroelectric domains to HZO and TiN grains. a) STEM EBIC image showing the depolarization field distribution in a HZO capacitor, highlighting all ferroelectric domains. b, c) HZO and TiN grain maps generated from simultaneously acquired 4D STEM data. HZO grains are generally larger than 50 nm, while the TiN grains are about 10 nm in size. Although the ferroelectricity originates from the HZO layer, the size of the ferroelectric domains better matches the TiN grains, suggesting a strong templating effect from the electrodes. Strategies for obtaining more complete HZO grain maps and potentially performing reliable phase and orientation analysis inside TiN/HZO/TiN capacitors. a) More diffracted disks can be captured by acquiring CBED patterns at different stage tilt or beam tilt angles (ideally performing PED), leading to a more complete grain coverage. b) Nearest-neighbor NCC map of CBED patterns with TiN diffracted disks filtered out, highlighting HZO grains. c) Scatter plots of all Bragg peaks identified within the scan area before and after removing TiN peak regions.