Atomic layer deposition (ALD) has emerged as a critical technique for the fabrication of ferroelectric and piezoelectric thin films with precise thickness control, excellent conformality, and uniform stoichiometry. Among the most studied materials in this category are hafnium zirconium oxide (HfZrO2) for ferroelectric applications and aluminum nitride (AlN) for piezoelectric applications. These materials are integral to advanced memory, sensing, and energy conversion devices due to their unique polarization properties and compatibility with semiconductor processing.

Ferroelectric HfZrO2 films have gained significant attention due to their scalability and CMOS compatibility. The ferroelectric phase in HfZrO2 is stabilized through careful control of composition, thickness, and thermal treatment. ALD enables precise tuning of the Hf:Zr ratio, which directly influences the ferroelectric properties. A typical ALD process for HfZrO2 employs hafnium and zirconium precursors such as TEMAHf and TEMAZr, with ozone or water as oxidants. The cycling ratio between HfO2 and ZrO2 layers determines the overall composition, with near-equimolar mixtures (Hf0.5Zr0.5O2) often exhibiting the strongest ferroelectric response.

Polarization control in HfZrO2 is highly dependent on post-deposition annealing. Rapid thermal annealing at temperatures between 400°C and 600°C promotes the formation of the orthorhombic phase, which is responsible for ferroelectricity. The wake-up effect, where repeated electric field cycling enhances polarization, is a critical phenomenon in these films. Interface engineering with metal electrodes, such as TiN or W, also plays a role in optimizing ferroelectric switching characteristics. The remanent polarization of ALD-grown HfZrO2 typically ranges from 10 to 30 µC/cm², depending on processing conditions.

For piezoelectric AlN films, ALD offers advantages in depositing highly oriented, stress-controlled layers. AlN is widely used in RF filters, energy harvesters, and MEMS devices due to its strong piezoelectric coefficient and thermal stability. ALD of AlN commonly uses trimethylaluminum (TMA) and ammonia (NH3) as precursors. The growth temperature is critical, with optimal piezoelectric properties achieved between 300°C and 400°C. Lower temperatures may lead to amorphous films, while excessive temperatures can induce undesired stress or crystallographic misorientation.

The piezoelectric response of AlN is strongly influenced by crystal orientation, with the c-axis alignment being essential for maximizing the d33 coefficient. In-situ plasma-enhanced ALD (PEALD) has been shown to improve crystallinity and orientation control compared to thermal ALD. The piezoelectric coefficient d33 for ALD-grown AlN films typically falls in the range of 4 to 6 pm/V, though higher values can be achieved with optimized deposition and post-treatment. Stress management is another key consideration, as excessive compressive or tensile stress can degrade device performance.

Device integration of ALD-grown ferroelectric and piezoelectric films requires careful consideration of interfacial layers and electrode materials. For ferroelectric memory applications, HfZrO2 is integrated into metal-ferroelectric-metal (MFM) or metal-ferroelectric-insulator-semiconductor (MFIS) structures. The choice of top and bottom electrodes affects leakage current, endurance, and imprint characteristics. TiN is commonly used due to its compatibility with ALD processes and favorable work function.

Piezoelectric AlN films are often integrated into thin-film bulk acoustic resonators (FBARs) or piezoelectric energy harvesters. The bottom electrode, typically Mo or Pt, must promote strong c-axis orientation while minimizing acoustic losses. Top electrode patterning and passivation layers are also critical for device reliability. In MEMS applications, the stress gradient in AlN must be controlled to avoid bending or delamination of suspended structures.

One of the challenges in ALD of these materials is achieving uniform properties over large areas and high-aspect-ratio structures. Ferroelectric HfZrO2 may exhibit variability in polarization switching due to local compositional fluctuations or grain boundary effects. Advanced ALD techniques, such as spatially graded deposition or multi-cycle annealing, have been explored to mitigate these issues. For AlN, maintaining consistent piezoelectric response across wafer-scale substrates requires precise control of plasma conditions and precursor pulsing sequences.

Emerging applications for ALD ferroelectric and piezoelectric films include neuromorphic computing, where HfZrO2-based devices mimic synaptic weight updates through polarization modulation. In energy harvesting, AlN thin films enable low-power sensors and self-powered electronics. The scalability of ALD makes these materials attractive for next-generation semiconductor devices, particularly in areas requiring ultrathin, conformal films with well-defined properties.

Future developments in ALD processes for ferroelectric and piezoelectric films may focus on lowering thermal budgets for back-end-of-line compatibility, improving film uniformity at atomic scale, and exploring new material systems. Dopant incorporation, such as La or Si in HfZrO2 or Sc in AlN, offers additional avenues for property tuning. The continued refinement of ALD techniques will play a pivotal role in enabling new functionalities in microelectronics, photonics, and MEMS technologies.

In summary, ALD provides a versatile platform for the deposition of ferroelectric HfZrO2 and piezoelectric AlN films with precise control over composition, crystallinity, and interfacial properties. The ability to tailor polarization behavior and integrate these materials into functional devices underscores their importance in modern semiconductor technology. Ongoing research aims to further optimize ALD processes to meet the demands of emerging applications while maintaining compatibility with existing fabrication workflows.