fabrication of suspended microstructures

Our M/NEMS devices are based on suspended and moveable structures. Generally, we start from an oxide thin film deposited on an oxide substrate by Pulsed Laser Deposition and prepare a photoresist mask using common photolithographic techniques such as optical lithography. We transfer the photoresist pattern to the film using physical or chemical etching. Physical etching is performed by ion milling technique: energetic Ar ions bombard onto the surface of the sample and progressively remove the oxide film areas that are not covered by the photoresist. Photoresist is removed by Ar ions too, but its thickness (about 4 microns) is higher than that of the film (about 0.1 microns). After the milling process, we remove the photoresist mask by solvents, leaving its replica onto the oxide film. Part of the substrate can be also removed during this process depending on the duration of the ion milling process. Ion milling is not selective and it removes all the materials, each one with different specific rates. Oxide films can be also etched by chemical methods; each composition has its own recipe. Prolonged wet etching may cause the etching of the film below the photoresist mask.

Regarding the suspension or release process, the removal of the oxide substrate regions under the patterned film is made using wet chemical etching. To do so, we explore the different sensitivity of oxides to chemicals. A typical example is that of manganite ((La,Sr)MnO3) thin films grown on strontium titanate (SrTiO3) substrates: a HF:H2O solution removes SrTiO3, while it does not affect (La,Sr)MnO3. A patterned (La,Sr)MnO3 microbridge can be made freestanding by prolonged immersion in HF:H2O solution, because the acid progressively removes the portions of SrTiO3 under the manganite pattern. Instead, HCl:H2O solution removes (La,Sr)MnO3 but not SrTiO3; (La,Sr)MnO3 sacrificial layers can be thus employed to fabricate freestanding SrTiO3 thin film structures by prolonged immersion in HCl by patterning SrTiO3/(La,Sr)MnO3 thin film heterostructures (see also L. Pellegrino et al. Adv. Mater. 2009, 21, 2377–2381). After chemical etching, the devices are rinsed in water and dried using Critical Point Drying method to avoid stiction.

In OXiNEMS, we study the etching protocols of selected oxide thin films for the realization of different classes of M/NEMS structures having moveable elements.

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