Vortex Machining: Localized Surface Modification Using an Oscillating Fiber Probe

Abstract

This paper demonstrates for the first time a method for surface modification of a substrate material based on the generation of localized vortices of abrasive slurry using slender oscillating fibers. In experiments presented in this paper, the abrasive slurry is a water based suspension of 1 m alumina particles. This is pumped onto, and flows across, the specimen surface. A fiber (typically 7 m in diameter and between 3.5 to 5 mm long) is immersed into this flowing slurry and oscillated at frequencies around 30-40 kHz to produce a small rotational flow (vortex) that results in the locally accelerated particles. Using such a system, it has been possible, over machining times of 6-24 hours, to produced localized depressions in the surface of a silicon substrate with typical depths of around 60-700 nm and widths of around 10-300 m. Based on these initial studies the material removal rate is estimated to be approximately 40 nm per hour. Using white light interferometry and stylus profilometry the surface deviations (roughness) of these features have a root mean square variation in the region 1-2 nm, which is comparable to that of the surface remote from the machined feature.

On the Development of an Experimental Testing Platform for the Vortex Machining Process

“…From the elongated and erratic footprints seen in the initial process evaluation [1] it appeared that that instabilities would fall into two major categories: environmental influences on the process and positioning stability of the test apparatus. In particular, changes in the polishing slurry volume due to evaporation and thermal expansion due environmental temperature variations were identified as major contributors to process uncertainty.…”

“…1. As previously outlined by Nowakowski [1], Vortex Machining can be facilitated with the use of tuning fork-based machining probes. These probes are comprised of a slender carbon fiber attached to one tine on a quartz crystal tuning fork.…”

“…When an oscillating cylindrical body is submerged in a fluid, standing vortices can be generated [1]. Vortex Machining is a process in which a cylinder is oscillated in abrasive slurry and positioned sufficiently close to a specimen surface so that material is removed by the action of slurry flow within the stationary vortices, see Fig.…”

“…A sinusoidal electrical signal with a frequency corresponding to the resonant frequency of the probe is used to excite the fiber, thereby producing a standing wave. Since this proof of concept [1], focus has been applied to the development of a process for the localized material removal of areas on the order of lO's of micrometers laterally and lO's of nanometers in depth on silicon. To date this process has been refined and a process prototype has been developed.…”

Geometric control of particle manipulation in a two-dimensional fluid

“…Compare this video to that at [31], which shows the toroidal vortex surrounding a vibrating carbon-fiber probe penetrating a droplet of water in which a large number of forty-micron polymer beads are suspended [32]. The sustained excitation of abrasive particles in this manner has proven an effective strategy for the precision machining of brittle surfaces [24]. In a different implementation, a microfluidic device has been developed for manipulating cells by controlling external electric fields and using vision-based feedback control [3] The mixing properties of the ciliary motion described above have recently been studied experimentally [23], and efforts to develop artificial cilia are ongoing in the MEMS community [12], but a consistent framework has not been established for studying the dynamics and control of such systems -let alone the broader class of systems in which cyclic boundary motions of arbitrary geometry are invoked to manipulate particles suspended in fluids on arbitrary physical scales.…”