Surface and thin film studies of the etching of molybdenum by xenon difluoride
Creators: name: Çelík, Özgür
Publisher: name: No Publisher Supplied
Year: 2010

description: 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 µ/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.