Understanding Ion Beam Etching (Milling)
Ion Beam Etching (or Milling) is a dry plasma etch method which utilizes a remote broad beam ion/plasma source to remove substrate material by physical inert gas and/or chemical reactive gas means. Like other dry plasma etch techniques, the typical figures of merit apply, such as etch rate, anisotropy, selectivity, uniformity, aspect ratio, and substrate damage. However, ion beam etching advances additional dry etch merits, which include wide range of materials, precision etch stops, indifferent to substrate thickness or shape, and minimal considerations for heath, safety and environment. Generally, a substrate incorporates a patterned mask, but some applications process blanks. Typically, the substrate stage will rotate and tilt. The bombardment of the substrate is well defined and controlled.
In the nanofabrication of patterned surfaces, ion beam etching (IBE) is one of the most versatile and productive dry plasma etching techniques. Fundamentally, ion beam etching is a low pressure, anisotropic and neutralized dry plasma technique which is capable to define surface features to ten’s of microns to the nanometer scale. As a dry plasma etching technique its plasma / substrate construct is advantageous in many ways. In particular, the bulk plasma is generated in the ion source which is remote from the substrate. From this remote source, a directed beam is accelerated towards the substrate. At the ion source, the directed beam acquires specific properties such as ion energy, ion beam current, and ion trajectory. Since the substrate is not immersed in the bulk plasma, the risk of radiation damage is minimized and the directed beam frees the substrate from RF bias control.
One of the most interesting features of ion beam etch is its ability to remove any material by a purely physical process. Ion beam etching (IBE) is considered a universal etchant process method. For example, IBE can etch noble and refractory metals, alloys, and magnetic materials without any harsh chemical reactants. Further recognition to the utility of IBE technology is its neutralized beam property which permits the safe processing of compound oxides/nitrides, Ill-V/II-VI semiconductors, and carbon-based materials without risk to electrostatic damage. The efficacy of IBE achievements is evident in its wide range of process capability.
A common configuration of the ion beam etching tool produces an Argon ion beam. Under the Argon beam operation, a moderately powered IBE process recipe can etch PbTe at rates > 250nm/min. While the same tool, can precisely etch a 5nm Cu layer at a 2nm/min rate. Then, the same IBE Argon beam configuration can etch a multilayer stack without the need to handle and optimize reactive gas operation. For example, the multilayer stack may be a Ru(5nm) / Ta(5nm) / CoFeB(1.5nm) / MgO(1nm) / CoFeB(1nm) / Ta(5nm) / Ru(10nm) / Ta(5nm) magnetic material structure. Unlike other dry plasma etch techniques, IBE measures and controls fundamental material etch parameters, such as the ion energy, ion current density, ion incidence angle. The ability to know and independently establish these parameters is a powerful benefit in the optimization of process design and correlation of these process input variables to the material etch results.
Under a proper IBE system design, a standard design layout will perform to within ± 5% uniformity over the substrate area. The performance of the common Argon ion beam configuration is enhanced when chemically reactive gases are added to the process. By the inclusion of chemical reactants to the ion beam etch variables, the process can be designed to optimize the physical and chemical etch components. The process optimization can be used to enhance etch rates, aspect ratios, and mask to substrate selectivity. Such process optimization is used to etch laser facets in III-V materials with Cl based reactive gases; or slanted diffractive gratings in fused silica with fluorinated reactive gases; or nanostructures in diamond with O2 for high power spectral photonics.