Byunghoon Yoon

Surface Chemistry for Molecular Layer Deposition of Organic and Hybrid Organic−Inorganic Polymers

The fabrication of many devices in modern technology requires techniques for growing thin films. As devices miniaturize, manufacturers will need to control thin film growth at the atomic level. Because many devices have challenging morphologies, thin films must be able to coat conformally on structures with high aspect ratios. Techniques based on atomic layer deposition (ALD), a special type of chemical vapor deposition, allow for the growth of ultra-thin and conformal films of inorganic materials using sequential, self-limiting reactions. Molecular layer deposition (MLD) methods extend this strategy to include organic and hybrid organic-inorganic polymeric materials. In this Account, we provide an overview of the surface chemistry for the MLD of organic and hybrid organic-inorganic polymers and examine a variety of surface chemistry strategies for growing polymer thin films. Previously, surface chemistry for the MLD of organic polymers such as polyamides and polyimides has used two-step AB reaction cycles using homo-bifunctional reactants. However, these reagents can react twice and eliminate active sites on the growing polymer surface. To avoid this problem, we can employ alternative precursors for MLD based on hetero-bifunctional reactants and ring-opening reactions. We can also use surface activation or protected chemical functional groups. In addition, we can combine the reactants for ALD and MLD to grow hybrid organic-inorganic polymers that should display interesting properties. For example, using trimethylaluminum (TMA) and various diols as reactants, we can achieve the MLD of alucone organic-inorganic polymers. We can alter the chemical and physical properties of these organic-inorganic polymers by varying the organic constituent in the diol or blending the alucone MLD films with purely inorganic ALD films to build a nanocomposite or nanolaminate. The combination of ALD and MLD reactants enlarges the number of possible sequential self-limiting surface reactions for film growth. Extensions to three-step ABC reaction cycles also offer many advantages to avoid the use of homo-bifunctional reactants and incorporate new functionality in the thin film. The advances in ALD have helped technological development in many areas, including semiconductor processing and magnetic disk-drive manufacturing. We expect that the advances in MLD will lead to innovations in polymeric thin-film products. Although there are remaining challenges, effective surface chemistry strategies are being developed for MLD that offer the opportunity for future advances in materials and device fabrication.

Ultralow Thermal Conductivity of Atomic/Molecular Layer-Deposited Hybrid Organic–Inorganic Zincone Thin Films

Atomic layer deposition (ALD) and molecular layer deposition (MLD) techniques with atomic level control enable a new class of hybrid organic-inorganic materials with improved functionality. In this work, the cross-plane thermal conductivity and volumetric heat capacity of three types of hybrid organic-inorganic zincone thin films enabled by MLD processes and alternate ALD-MLD processes were measured using the frequency-dependent time-domain thermoreflectance method. We revealed the critical role of backbone flexibility in the structural morphology and thermal conductivity of MLD zincone thin films by comparing the thermal conductivity of MLD zincone films with an aliphatic backbone to that with aromatic backbone. Much lower thermal conductivity values were obtained in ALD/MLD-enabled hybrid organic-inorganic zincone thin films compared to that of the ALD-enabled W/Al2O3 nanolaminates reported by Costescu et al. [Science 2004, 303, 989-990], which suggests that the dramatic material difference between organic and inorganic materials may provide a route for producing materials with ultralow thermal conductivity.

Atomic Layer Deposition of LiOH and Li2CO3 Using Lithium t-Butoxide as the Lithium Source

Lithium compounds such as LiOH and Li2CO3 are important in Li ion batteries. These lithium compounds are part of the solid-electrolyte interphase (SEI) resulting from the decomposition of carbonate electrolytes on the electrodes during charge-discharge cycling. This SEI restricts lithium diffusion and electron conductivity. The SEI also removes lithium from charge transport and lowers the battery capacity. A thin artificial SEI on the electrodes may prevent lithium loss and improve lithium diffusion and electron conductivity. In this work, LiOH and Li2CO3 are grown by atomic layer deposition (ALD) using LiOC(CH3)3 [lithium t-butoxide (LTB)] as the lithium source. For LiOH, the overall reaction is LiOC(CH3)3 + H2O LiOH + HOC(CH3)3. LiOH ALD growth was monitored using a quartz crystal microbalance (QCM). LiOH ALD had an initial mass gain per cycle of 12.7 ng/cm before displaying evidence for hygroscopic behavior resulting from the absorption and desorption of H2O into and out of the LiOH films. The LiOH films were then exposed to CO2. The QCM monitored a large mass gain consistent with the conversion of LiOH into Li2CO3 through the reaction 2LiOH + CO2 Li2CO3 + H2O.

Importance of trimethylaluminum diffusion in three-step ABC molecular layer deposition using trimethylaluminum, ethanolamine, and maleic anhydride.

Hybrid organic-inorganic films were grown by molecular layer deposition (MLD) with a three-step ABC reaction sequence using (A) trimethylaluminum (TMA), (B) ethanolamine (EA), and (C) maleic anhydride (MA) at 90 °C. Very large steady state mass gains of 1854-4220 ng/(cm(2) cycle) were measured depending on reaction conditions. These mass gains are much larger than typical mass gains for surface reactions. The quartz crystal microbalance (QCM) mass profiles during the TMA reaction were consistent with TMA diffusion into and out of the ABC films. The ABC mass gains per cycle also displayed a strong dependence on the TMA dose and purge times that was consistent with the effects of TMA diffusion. Multiple dose experiments conducted at 130 °C revealed that the ABC reactions were self-limiting for thin ABC films. For thicker ABC films, increased TMA diffusion into the ABC film led to non-self-limiting behavior. Numerical modeling assuming Fickian diffusion for TMA diffusing into and out of the ABC film could fit the QCM mass profiles. The results all indicate that TMA diffusion into the ABC MLD film plays a key role in the thin film growth. In addition, X-ray reflectivity (XRR) measurements revealed that the ABC films were exceptionally smooth.

Molecular Layer Deposition of Conductive Hybrid Organic-Inorganic Thin Films Using Diethylzinc and Hydroquinone

Hybrid organic-inorganic thin films were fabricated by molecular layer deposition (MLD) methods using diethyl zinc (DEZ) and hydroquinone (HQ). Sequential exposures of DEZ and HQ led to linear MLD film growth. The mass gains measured by quartz crystal microbalance investigations were 52.3 ng/cm 2 per MLD cycle. Scanning electron microscope images of cross-sections of the MLD film were consistent with a MLD growth of 1.6 A per cycle at 150 o C. These MLD films may be conductive because of the π-orbitals in conjugated chains of (-O-phenyl-O-Zn-)n.