Plasma Processing of Micro-Screen Supported Ultra-Thin Carbon, Ceramic and Metal Foils for High Energy Particle Detection and Beyond, 15-R8190
Edward L. Patrick
Inclusive Dates: 10/11/10 – 01/11/11
Background — The motivation for this project was based on the use of thin carbon foils as secondary electron emitters (SEE) in the detection of high-energy particles by time-of-flight (TOF) mass spectrometry in high-energy physics and space research. For these applications, carbon foils as thin as 1 nm, or less than 0.1 mg/cm2, are required. Foils must also be strong enough to sustain mechanical deformation during handling, defects (pinholes) must be low and durability against high energy particles must be considerable. At the same time, the structure and morphology of the ultra-thin foils must provide maximum SEE at the lowest possible energy of detectable particles. Diamond-like carbon (DLC) fulfills this material requirement for particle detection applications. Conventional free standing unsupported carbon foils made by thermal evaporation of carbon and condensation of carbon vapor do not meet these criteria and are extremely difficult to handle during installation. It is well-known that ion bombardment by energetic ions during the deposition process can substantially improve coating structure and morphology, reduce the density of defects, and improve coating mechanical and thermochemical stability.
Approach — Phase I of the effort was to use physical-vapor deposition (PVD) or chemical-vapor deposition (CVD) to fabricate the micro-screen support and carbon foil as an integrated process, to microscopically inspect the foil, and to test its SEE properties for particle TOF instruments. The deposition of carbon on a silicon wafer was followed by the deposition of a nickel layer. Lithography was then used to etch the nickel foil support layer into a microscreen, then separate the foil attached to the reinforcing metal screen that will constitute the final product: microscreen-supported functional thin foils having thicknesses ranging from 2 nm to 10 μm. The microlithography process involves the following steps: spin coating and baking a layer of resist on top of the nickel layer; UV-exposing the resist through a mask to cross-link the resist monomers in the hole areas; using a development chemical to remove the resist from these areas; etching the nickel out of the hole areas; and then stripping the remaining resist from the nickel surface. Finally, the Si substrate is stripped away, leaving the DLC foil on its reinforcing metal screen. This was accomplished by soaking the structure in a silicon etch (which will not attack the nickel or carbon) until the carbon layer is microscopically clear of any residual silicon.
Accomplishments — Microscopic inspection of commercially available carbon foils identified numerous defects and holes that were the impetus for this effort. A prototype "proof-of-principle" fabrication process succeeded in creating an integrated silicon-carbon-nickel wafer coated with a layer of resist. Lithography of the nickel and subsequent stripping of the resist was successful and produced a microscreen in the nickel foil layer. However the attempt to etch the Si substrate from the carbon foil caused curling and warping of the carbon layer even after a number of etchants and conditions were employed. Though this "no-go" condition terminated the effort, the need for uniform and defect-free carbon foils remains, and an improved etching or deposition process is needed to produce intact carbon foils with microscreen support.