The objective of this research award is to develop high throughput manufacturing processes for the efficient production of multi-layer nanolaminates. Low frequency (~1 Hz) pulsed plasma-enhanced chemical vapor deposition (PECVD) has been engineered as an alternative to atomic layer deposition for self-limiting growth of metal oxides (i.e. 1 Ã…/pulse). Pulsed PECVD offers a number of benefits including high net rates (> 30 nm/min), a larger process window, and wide applicability to a large number of precursors / materials. The proposed work is directed at accomplishing two major goals. The first will be an examination of interfacial issues associated with the formation of mixed metal oxide nanolaminates. Second, pulsed PECVD synthesis of oxides will be integrated with self-limiting deposition of polymers by the same method to form inorganic/organic nanolaminates. Acrylate and silicone based polymers are targeted to extend nanolaminate manufacturing to an array of flexible polymer substrates.
If successful this process technology is expected to impact the manufacturing of nanolaminates for numerous applications, including high performance dielectrics, optical components, and diffusion barriers for flexible electronics. A robust approach to the synthesis of high quality organic/inorganic nanolaminates will be an enabling technology with broad cross-cutting potential. The broader impacts of this work will also include the training of a PhD candidate and the engagement of undergraduate researchers in areas of great technological importance. The materials and expertise produced by this work will be incorporated into an existing silicon processing lab course and a graduate elective in plasma processing. The PI and his students will work with the Colorado School of Mines K-12 outreach team, developing teacher workshop materials in the areas of optics, plasmas, and photovoltaics for school districts with large populations of poor and minority students.
The objective of this research award is to develop high throughput manufacturing processes for the efficient production of multi-layer nanolaminates. Low frequency (~1 Hz) pulsed plasma-enhanced chemical vapor deposition (PECVD) has been engineered as an alternative to atomic layer deposition for self-limiting growth of metal oxides (i.e. 1 Å/pulse). Pulsed PECVD offers a number of benefits including high net rates (> 30 nm/min), a larger process window, and wide applicability to a large number of precursors / materials. The proposed work is directed at accomplishing two major goals. The first will be an examination of interfacial issues associated with the formation of mixed metal oxide nanolaminates. Second, pulsed PECVD synthesis of oxides will be integrated with self-limiting deposition of polymers by the same method to form inorganic/organic nanolaminates. Acrylate and silicone based polymers are targeted to extend nanolaminate manufacturing to an array of flexible polymer substrates. Pulsed PECVD was developed for a number of inorganic systems, including SiO2, TiO2, and alloys of these materials. We proposed and demonstrated a robust process for digital control of high quality SiO2 thin films at room temperature using pulsed PECVD. Plasma activation of the SiCl4 precursor is critical, as atomic layer deposition does not occur under these conditions. Sub-Ångstrom control over deposition rate was obtained by adjusting the density of SiCl4 present at plasma ignition. No impurities were detected by either XPS or FTIR in films deposited under optimal conditions. Self-limiting deposition of anatase TiO2 was accomplished by pulsed plasma-enhanced chemical vapor deposition using simultaneous delivery of TiCl4 and O2. Films deposited at T = 120 ºC with the plasma power set at 100 W were amorphous, containing residual amounts of chlorine. At 200 W the films displayed an anatase structure, and no chlorine was detected by XPS. Spectroscopic ellipsometry, FTIR, and XRD measurements concur that a minimum film thickness of ~25 nm is required for the formation of the anatase phase. Self-limiting synthesis of alumina-titania nanolaminates (ATO, Al2O3/TiO2) was accomplished via pulsed plasma-enhanced chemical vapor deposition. At the synthesis temperature of 150 °C the alumina layers were amorphous, while TiO2 layers displayed a polycrystalline anatase structure. Digital control over nanolaminate structure was demonstrated through elemental analysis and TEM imaging. The dielectric performance of the ATO structures was examined as a function of composition and bilayer thickness. C-V measurements showed that the effective dielectric constant was consistent with treating the nanolaminates as individual capacitors in series. I-V measurements showed that leakage current deteriorated with TiO2 content, though low leakage was restored through interfacial engineering. Pulsed plasma enhanced chemical vapor deposition (PECVD) was used to deliver digital control of SixTiyOz composites at room temperature. It is shown that the alloy composition and refractive index can be tuned continuously over a broad range. The digital control over both thickness and composition offered by pulsed PECVD was demonstrated through synthesis of antireflection (AR) coatings for crystalline silicon solar cells. One, two- and three-layer AR coatings were designed and fabricated. In each case the measured optical performance was found to be in excellent agreement with model predictions. The average reflectance across the visible spectrum was reduced from 39% for uncoated wafers to 2.5% for the 3-layer AR coating. Alumina-silicone hybrid nanolaminates deposited by plasma-enhanced chemical vapor deposition were explored as dielectrics in metal-insulator-metal (MIM) capacitors. Temperature-dependent current versus voltage (I-V) measurements were used to investigate the conduction mechanisms contributing to the leakage current in these structures. It is observed that space charge limited current (SCLC) mechanism is the dominant conduction process in the high field region. The estimated shallow trap level energies (Et) are 0.16 eV and 0.33 eV for 50% and 83.3 % Al2O3 nanolaminates respectively. A simple solvent-etch based technique is developed to visualize and quantify defects in transparent thin films deposited on flexible polymer substrates. This approach is used to characterize defects in as-deposited films and to monitor their evolution as a function of applied and repetitive bending. Thin films investigated include sputtered indium tin oxide (ITO) and alumina-silicone nanolaminates fabricated by plasma-enhanced chemical vapor deposition. It is shown that the use of nanolaminate architectures reduces the defect density by two orders of magnitude relative to a single alumina layer. The pinhole density increases when nanolaminates are subjected to applied stress, and at a critical density of ~10/mm2 the isolated defects coalesce into macroscopic cracks. In the case of ITO an optimum film thickness is identified that balances electronic performance with mechanical integrity. Conductivity correlates with defect density, and the films displayed very similar performance under tensile and compressive strain. A critical radius of curvature of 0.75" was identified, but films cycled below the threshold strain demonstrated robust performance, with only negligible changes in resistivity through 2000 bending cycles. The strong performance under strain is attributed to the amorphous nature of the sputtered ITO.
