AVS2010 Session TF1-TuM: ALD: Dielectrics for Semiconductors

Atomic layer deposition (ALD) of complex oxides is a viable method to tailor the composition and microstructure of the resulting thin films, thereby generating multi-functionality that is needed in various device applications. Another desirable feature of ALD is its ability to uniformly and conformally coat complex surface structures of high aspect ratios, making it possible to interface distinct materials to form composite materials that may show exciting properties, such as multiferroic behavior.

In this work, lead zirconium titanate (PZT) films were synthesized by depositing PbO, ZrO₂ and TiO₂ ALD layers with a sequence of (Pb-O)-(Ti-O)-(Pb-O)-(Zr-O). The deposition rate for PZT was approximately 0.7nm/cycle-sequence. The as-deposited materials were amorphous but crystallized into perovskite structure upon annealing to 950^oC. The polarization properties found in the P-V measurements including remanent polarization (Pr), saturation polarization (Ps), and coercive field (Ec) are promising and are strongly affected by the ALD sequence which dictates the composition and structuring of the resulting thin film.

One example of the advantages of ALD produced PZT can be seen in our work on composite multiferroic materials, which can be created by coupling together nanoscale ferroelectric and ferromagnetic materials. In this work, mesoporous cobalt ferrite (CFO) thin films formed by block-copolymer templating methods with pores 14 nm in diameter were used as the ferromagnetic framework. Ultra-thin ALD PZT films were used to create a uniform and conformal coating over and on all CFO pore spaces and generate a nano-structured PZT-CFO hybrid material, as confirmed by scanning electron microscope (SEM), transmission electron microscope (TEM), and ultraviolet photoelectron spectroscopy (UPS) measurements. Superconducting quantum interference device (SQUID) magnetometry is underway to assess the multiferroic properties of this PZT/CFO composite, including both in-plane and out-of-plane saturation magnetization measurements after electrical poling and polarization voltage (P-V) measurements.

A large part of the high-k dielectric stacks investigated for the replacement of the SiO₂ or Si_xO_yN gate oxide in metal-oxide-semiconductor (MOS) field-effect transistors consists of a Hf-based oxide or silicate deposited on top of an ultrathin interfacial SiO₂ layer. We report here the formation of yttrium lanthanum silicate in direct contact with silicon for the fabrication of MOS capacitors with a resulting highly competitive equivalent oxide thickness (EOT) of 0.8nm. Such a low EOT is remarkable as it is obtained with a non Hf-based dielectric and following a high-temperature gate first route. This result is achieved by integrating Y₂O₃ films grown on buffer layers by a novel atomic layer deposition process that combines an original yttrium precursor and an innovative liquid injection source.

The growth of Y₂O₃ thin films by ALD from Y(EtCp)₃ and water precursors will be presented. The yttrium precursor was introduced using a novel delivery scheme consisting of a pulsed injection system from Kemstream®. The control of the growth as a function of various process parameters (precursor supply time, water purging time, temperature) was investigated. Film stoichiometry was determined by Rutherford backscattering spectrometry. The reactivity of Y(EtCp)₃ with water and the frontiers between ALD and CVD regimes will be discussed.

We describe MOS capacitors prepared from ALD-Y₂O₃ thin films deposited onto Si(p-type)/SiO₂(0.8 nm)/La₂O₃ (1 nm) structures. The Y₂O₃ thickness ranges from 1.5 to 3.5 nm. The metal gate electrode consists of 10 nm TiN and implanted polysilicon on top. The complete stack undergoes a high-temperature RTA at 1000˚C for 5 s under N₂ for dopant activation in the poly-Si, which simulates gate first process of CMOS transistor fabrication. Finally, a forming gas anneal is performed at 475˚C. The stack microstructure and composition were studied by transmission electron microscopy, electron loss spectroscopy and energy dispersive x-ray spectrometry. Interdiffusion reactions occur between SiO₂, La₂O₃ and Y₂O₃ layers upon the RTA and result in an yttrium lanthanum silicate film in direct contact with Si. The elemental distribution within the silicate layer will be discussed. C-V and I-V were performed on 10x10 μm² size capacitors in a frequency range of 1 kHz up to 300 kHz. EOTs as low as 0.8 nm were obtained for stacks with initially 3 nm Y₂O₃ deposited films. The leakage current density for such stacks is of 2.1x10^-3 A/cm² at -1V. The formation of silicate enables an appropriate V_FB, which is tunable for nFET and pFET. Results obtained for stacks prepared from different buffer layers such as Al₂O₃ and HfO₂ will also be discussed.

Thin film TiO₂ is a relatively high permittivity material with applications in electronic and energy devices. However, TiO₂ crystallizes easily, forming grain boundaries and creating current percolation pathways which increase leakage current and power consumption. Al-doped TiO₂ (ATO) increases the thermal stability of TiO₂, reducing leakage currents by suppressing phase transformation and limiting grain growth. To achieve the benefits of Al doping which retaining those of TiO₂, structure-property relationships must be optimized as a function of processing parameters.

Raman spectroscopy (RS), a highly sensitive technique for observing the evolution of nano-crystalline phases - their time evolution, volume fraction, defect states and stresses, is employed here to characterize ATO films grown by atomic layer deposition (ALD) and their correlation with electrical performance. 25nm TiO₂ thin films were deposited at 150ᵒC in a Beneq TFS500 cross-flow ALD reactor by sequentially pulsing TDMAT and water precursors. Aluminum doping was controlled from 0 to 15 at% by introducing a single TMA / water pulse sequence at pre-determined intervals of multiple TDMAT/water pulse sequences during oxide growth, producing reproducible compositions estimated from their TiO₂ to Al₂O₃ pulse ratios. Following ALD the films were rapid thermal annealed (RTA) in an oxygen-rich environment ranging from 600ᵒC to 1000ᵒC from 5 to 300 sec.

The degree of crystallization was determined primarily through lineshape analysis of the Raman-active 144 cm^-1 frequency mode. RS (and XRD) detected only the TiO₂ anatase phase, even at annealing temperatures where rutile formation was expected. RS data clearly shows the crystallization of TiO₂ with thermal anneal and that the crystallization onset shifts to higher temperatures with Al-doping while producing smaller grains. Pure TiO₂ films show a dielectric constant of 28.5 with high leakage currents (up to 10^-3 A/cm²). Annealed ATO films have a slightly lower dielectric constant (24) but leakage currents are dramatically reduced to ~ 1.0x10^-7A/cm². The clear correlation between the microstructure measured by RS and the electrical performance of ALD TiO₂ and ATO thin films shows that RS can be a rapid, valuable monitor of thin film material nanostructure for correlation and optimization with electrical properties.

This work was supported by Laboratory for Physical Science, UMD

Self-aligning manufacturing processes could allow smaller device structures to be made as well as significantly reduce the number of manufacturing steps required. By utilizing selective chemistry it is possible to control where growth occurs for many different materials. Self assembled monolayers (SAMs), such as octadecyltrichlorosilane (OTS), have been used to inhibit the atomic layer deposition (ALD) of materials spanning from high-k dielectrics to metals such as Ir or Pt. By tailoring the selective chemistry, it is possible to chemically activate and deactivate the surface for multiple materials. SAMs such as 3-mercaptopropyltriethoxysilane have been used to attach a variety of nanoparticles where desired, and when combined with other SAMs such as OTS can prevent adsorption or deposition where undesired. Simple patterning techniques such as photolithography, AFM lithography, or electron lithography can be combined with selective chemistries to allow high resolution spatial control over the growth and deposition of materials. One limitation of this technology is the long time scales required to fully deactivate deposition using SAMs, which are on the order of 48 hours. Molecular defects such as water in the SAM, unblocked hydroxyl groups, exposed Si-O bonds, misaligned boundaries between forming SAM islands, or polymerized SAM molecules in the SAM layers also leads to an eventual failure point for deposition deactivation. We found that they can be largely eliminated by forming the SAM on a uniformly hydroxylated surface, using a chloroform rinse step, and removing any polymerized or physisorbed SAM molecules from the surface during SAM formation. Using TiCl₄ as a probe for defect sites, the level of each defect type has been determined. The number of defects sites on the surface on a typical OTS SAM is on the order of 10¹² molecules/cm². 1/3^rd of the defects are associated with the physisorbed SAM molecules. Of the other 2/3^rds of SAM defects nearly half are unblocked hydroxyl groups while the other half are likely open Si-O bonds at grain boundaries. Water has been shown not to be a contributing cause for nucleation of deactivation failure. TiO₂ deposition from a TiCl₄ and H₂O ALD process at 170°C has been deactivated for at least 100 cycles after treating for only 4 hours in 10mM OTS in toluene solution, as long as the sample is removed every hour and rinsed. The extraction step removes unwanted SAM molecules and exposes open surface areas allowing for new SAM molecules to quickly fill the gaps. These improvements in both the quality and the time scale of SAM formation could it feasible to begin incorporating these technologies into manufacturing.