In this dissertation, key aspects of the surface chemistry associated with gas phase deposition and etching are discussed. Atomic layer deposition (ALD) is a gas-phase deposition technique primarily known for its superior self-limiting binary process that affords precise control, uniform and conformal thin film growth. Despite the extensive work done with ALD, the mechanisms behind nucleation and steady state growth remain unclear for many ALD processes. Additionally, in an effort to meet today's device integration requirements, e.g., scaling down nanostructures and thermal budget restrictions during film deposition, thermal ALD processes requiring high temperatures (>300 C) are now being forced out of production due to adverse thermally induced side effects, e.g., device degradation. To address this challenge and promote reactivity at low temperatures (

Chemical vapor deposition (CVD) is becoming an increasingly important manufacturing process for the fabrication of VLSI and ULSI devices. A major challenge in optimizing a CVD process is developing an understanding of the complex mechanistic pathways followed. The first section in this thesis reports studies on the thermal and dynamical activation of surface bound alkyl species which play a vital role in the form of intermediates in metal-organic chemical vapor deposition. The particular systems of interest are those of aluminum CVD precursors. Models of these intermediates are obtained by thermal decomposition of alkyl iodides. The results provide an insight into the complex reaction patterns involved in the thermal reactions and rate-structure sensitivities of the alkyl species in the presence of the coadsorbed halogen atom. Multiple reaction pathways including metal etching processes which bear direct implications to the synthesis of organometallics and metal etching, are identified. It is becoming apparent that chemistry at surfaces, whether it be heterogeneous catalysis, semiconductor etching, or chemical vapor deposition, is controlled by much more than the nature and structure of the surface. Also, nonthermal activation of autocatalytic reactions is often required for the nucleation and growth of thin films in devices so that the stability of the device structure is maintained. Dynamical pathways followed in these high pressure and energy processes have to be well understood. The second part of these studies describe an investigation of collision-induced reaction of alkyl intermediates using supersonic inert gas atomic beams. Selective activation of a thermodynamically favored unimolecular decomposition reaction is initiated by hyperthermal collisions. Quantitative estimations of the reaction cross sections are made using straightforward hard sphere energy transfer dynamics. This successful demonstration of collision-induced activation of large, multiatomic moieties has paved the way for proposed studies (now underway in our group) on actual CVD precursors with known barriers to nucleation and growth. In the second section, the reaction mechanisms and kinetics of competitive dissociation, disproportionation, and thin film growth processes involved in the chemical vapor deposition of metal-silicide thin films are investigated. Metal-silicides are widely used as interconnect and gate materials in devices and also as corrosion resistant materials. Reactivity of silane and disilane with copper is studied in detail using temperature programmed reaction, Auger electron, Fourier transform infrared reflection absorption spectroscopies and low energy electron diffraction. For both the precursors, the structural chemistry and product distributions of adsorbed intermediates found at low temperatures are quite rich but significantly differ at the mechanistic level. It is shown quantitatively that disilane is almost 2-3 orders of magnitude more reactive than silane due to its facile Si-Si bond dissociation. However, in both cases, kinetics of silicon deposition and silicide formation are limited by the site-blocking effect of surface bound hydrogen generated by the decomposition of the silyl fragments. An ordered silicide overlayer is readily formed at higher coverages effected above dihydrogen desorption temperatures. This bimolecular process has to compete with an associative reaction which leads to the formation of silane. The results obtained from the different spectroscopic data show that the growth process involves an intriguing set of coupled reactions in which deposition, island growth, and Si etching effectively compete in a complex manner. Understanding of these parameters and the reaction mechanisms involved, enables the application of this process for the vapor phase growth of silicide thin films.

The controlled etching of micro/nano structures is essential for a variety of technological applications, including microelectromechanical systems (MEMS) fabrication. XeF2 is an isotropic and highly selective etching gas used to remove semiconductors (such as Si, Ge) and metals (such as Mo, W) in the fabrication of MEMS and other devices. While the kinetics of XeF2 etching of Si has been widely documented, XeF2 etching of metals is not widely understood. For better process control and device quality, it is important to understand the etching mechanism at the molecular level. In this work, we explore the surface and gas phase chemistry of XeF2 etching of metallic films, focusing on Mo. Studies of the general characteristics of XeF2 etching of Mo blanket films at different sample temperatures and etchant pressures were carried on 1000ÅMo/475ÅSiO2/100ÅNi/glass samples in a standalone etching chamber, then they were analyzed ex-situ by multiple surface sensitive tools. Atomic force microscopy (AFM) and x-ray photoelectron spectroscopy (XPS) were used for chemical and morphological analysis of the etched surfaces. Rutherford back scattering (RBS) and medium energy ion scattering (MEIS) were used to measure the thickness of the films and the depth profile of near-surface species after etching. Mo is etched by XeF2 rapidly and selectively. XPS, AFM, and RBS data on the morphology and composition of surfaces after etching at different temperatures and pressures is presented. Data showing how the condition of the surface prior to etching (initial surface) affects the initiation and progress of etching is also discussed. The composition and chemical state of the etched surface (reaction layer) is further investigated by in-vacuo etching and XPS analysis experiments using 3750ÅMo/quartz samples in an integrated etching/analysis system. The XPS studies have clarified issues on the thickness and chemical composition of the reaction layer during etching. The effects of the surface native oxides and adventitious hydrocarbons on etching and re-deposition of etched products were also examined by in-vacuo etching and XPS. Post etching thermal processing and XPS analysis studies were performed to investigate the chemical composition of residues left after etching. These studies have indicated that after etching there are physisorbed and chemisorbed fluorine species that desorb at different temperatures. Downstream mass spectrometry was used to identify the gas phase by-products of the etching process. Since etching is non-uniform and the initial condition of the surface affects the etching process, using time dependencies vs. etched thicknesses is shown to be an unreliable method to measure the rate of etching. Thus, alternative methods, including the total pressure change and a quartz crystal micro balance (QCM), were used to calculate the rate of etching of blanket Mo films. Under the conditions reported here, the rates of etching of blanket films were determined to be 60-75 nm/sec at 25- 90° C. The order of reaction is close to one. The rate of undercut etching, measured on patterned samples, changes significantly (0.5-2.5 æ/min) under different conditions, depending on the etching method, temperature, and pattern size. Mask deformation is observed on certain shapes. Different gas delivery methods were tested and their efficiency is discussed. Both on etching of patterned and blanket films, pulsing of the etchant gas is shown to be a more efficient method for etching than static etching.