Technologically important devices from microelectronics to MEMS to photonics to sensors – and others – are created by the physical patterning of thin films. The sensors could be passive materials, such as an accelerometer cantilever arm whose movement changes a capacitance measurement, or active materials that transduce to or from electrical signals. Well-known compounds such as Yttrium Iron Garnet (YIG) or Lead Zirconate Titanite (PZT) are examples of active materials that can be deposited as thin films and subsequently patterned in-plane to make transducers. Undercutting is often employed to release the shape from the substrate except at tether points.

In the micro- and nano-size regimes, ion milling and etching are the dominant techniques for writing patterns into films. Ion milling can operate either by physical sputtering alone or by chemical enhancement with reactive species such as oxygen. Ion milling typically starts with a plasma-based source that ionizes and then accelerates the required species to make an ion beam. Ion sources are designed to control both the beam energy and shape. Gridded (Kaufman) ion sources offer a high degree of control over the beam. These are available in a wide range of sizes. The exit grids’ potentials determine the beam current and energy, while grid shape can provide divergent, convergent or parallel beam profiles. These beam parameters directly affect the shaping of the features being milled into devices. Being a dry, beam-based technique, ion milling is highly anisotropic and thus ideal for making nanostructures. Delivered beam power of hundreds of Watts provides fast machining rates.

The broad ion beam writes a pattern into the film surface through masking. For larger feature sizes, a physical stencil mask can be placed over the device substrate to selectively remove material. For smaller dimensions, photolithography is typically utilized. Selective chemical etching creates a patterned photoresist layer whose openings define the write pattern for later milling. The photoresist may directly protect the material to be patterned, or it may be used to write a pattern to an intermediate hard mask. The substrate often needs to be cooled during operation, to prevent degradation of the organic photoresist from heating by the ion beam.

Several metrics define the fidelity of pattern transfer in ion milling: minimum feature size, maximum feature density (space between features or aspect ratio), and for the sidewalls: angle, smoothness, and re-deposited material. These are intertwined. Perpendicular beam incidence provides more vertical sidewalls, substrate rotation can smooth them, while angled deposition can be used to remove re-deposited materials from the sidewalls. Erosion of narrow photoresist lines will limit the aspect ratio of features, which may call for thicker photoresist layer. A hard mask can improve etch selectivity and depth, as can chemically enhanced ion milling. Thus, a variety of parameter adjustments are needed to create optimized structures.