Technical: The proposed work will develop methods of low temperature chemical vapor deposition (CVD) to afford uniform, smooth, and pinhole free thin films on substrates that have high aspect-ratio openings such as deep trenches and vias. Such films are required for a wide variety of advanced technologies, e.g., diffusion barriers associated with copper metallization in integrated circuits. The CVD method can also completely fill these features at reasonable rates. Examples to date include the coating and filling of high aspect ratio (30:1) vias, 100 nm in diameter, with HfB2 or MgO films. Three experimental approaches are used to achieve excellent coating properties. (i) Unique CVD precursor molecules with high vapor pressure produce a site-blocking effect on the film growth surface, such that the surface reactivity is low but the growth rate is acceptable. (ii) Inhibitor species are added to the growth process to modify the surface reactivity: H atoms impinge on, and bind strongly to the most exposed (upper) surfaces and lead to bottom-up film growth; molecular species such as NH3 or dme uniformly lower the growth rate such that a ?non conformal? precursor will afford a highly conformal film. (iii) Inhibitor species are used to enhance the density of nuclei on a substrate by reducing the growth rate of the first-formed nuclei with respect to the rate at which new nuclei form. This has the effect of greatly smoothing films of few-nm thickness, and of eliminating pinholes. The approaches have transformative potential in the field of thin film growth. This research employs surface science techniques and kinetic modeling to obtain a mechanistic understanding of the surface reactions involved in highly conformal chemical vapor deposition of thin films. That intellectual synthesis across disciplines ? surface physics, surface chemistry, and thin film materials science provides enabling scientific knowledge to guide further development of conformal CVD. For example, it affords quantitative metrics for the necessary properties and behavior of precursor molecules and inhibitor species.
The project addresses basic research issues in a topical area of materials science with technological relevance, and is expected to provide unique opportunities for graduate and undergraduate training in an interdisciplinary field. This research project is also expected to have broader impacts through the training of scientists in this research field and through the wide dissemination of the findings of this research through publications. The PI will mentor a REU student and one or two undergraduate senior theses in the group. Graduate students and undergraduates from under-represented groups are recruited via the University of Illinois programs SURGE (Support of Under-Represented Groups in Engineering) and WISE (Women in Science and Engineering). One woman Ph.D. student is associated with this project at the time of writing. Periodic news stories also report the developments and cite the crucial role of the National Science Foundation in the process of discovery.
Intellectual Merit: This project develops new means to deposit extremely thin films (coatings) of metallic, insulating and refractory materials onto substrates of complex shape. Such thin films are of crucial importance in the development and fabrication of advanced microelectronic circuits, nanoscale devices, battery electrodes, and other applications. Our starting point is the method of chemical vapor deposition in which the atoms of the material to be deposited are delivered in the form of a gaseous precursor that reacts chemically on the hot substrate to afford the desired film and release unwanted atoms in the form of volatile byproducts. We give this process new functional ability by adding a gaseous inhibitor to the precursor gas. The inhibitor changes the rates at which different chemical reactions occur on the film growth surface. Under well-chosen process conditions, the new abilities are: The nucleation (initial growth) of the film on substrate is extremely fine and uniform, such that a film only 2 nm thick can be pinhole-free and have a surface roughness of less than 0.5 nm. The film grows with uniform thickness on all surfaces inside of a deep feature (trench or hole) with very high aspect ratio (depth relative to width). This can be achieved in aspect ratios of 100 – 5000. The film grows more rapidly at the bottom of a deep feature than at the top, such that the coating completely fills the feature with no voids. This approach is applicable in aspect ratios of 10 – 30. We have proposed, based on theoretical models, that these beneficial results can be achieved for a large portfolio of precursor molecules and resulting thin film materials. At present, these coating abilities (i – iii) have been demonstrated on the following materials: • HfB2, a metallic ceramic (i, ii) • Copper (i) • MgO, a transparent insulator (ii, iii) In addition, we have explored the behavior of new precursor molecules that were synthesized by our NSF-supported collaborator, Professor Gregory S. Girolami of the Chemistry department at the University of illinois. These include: • Metal-rich nitride phases of Fe, Mn, Co and Ni • Rare earth (Lanthanide series) oxides Films have also been incorporated into advanced test devices, including: • MEMS contact relays • Selective optical emitters for thermo-photovoltaic conversion • Soft magnetic Fe-Co alloys for hard disc perpendicular recording technology Broader Impacts of this project: • Teaching, Training, and Learning – education and mentoring of Ph.D. students who perform the research. • Participation of Under-represented Groups – three women graduate students perform their thesis research. • Infrastructure for Research and Education – state-of-the-art thin film growth apparatus is used for research by Ph.D. students in MatSE and in Chemistry. • Disseminate Broadly to Enhance Scientific and Technological Understanding – results presented at national conferences and in industrial laboratories, and published in peer-reviewed scientific journals. • Benefits to Society – new scientific and technical knowledge concerning the synthesis of thin films enables the development of advanced microelectronic and nanotechnology devices. This project also contributes to human resource development for the U.S. scientific and technical workforce.
