Hydrofluoric Acid (HF)
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Table 2440a. Etchants used in semiconductor manufacturing.

SiO2(Silicon oxide)
Dilute hydrofluoric acid (DHF)
Buffered HF (BHF)
Alkaline hydroxide + organic
Si3N4(silicon nitride)-selective to SiO2
Boiling phosphoric acid (H3PO4)
Si3N4/SiO2 (non-selective)
Hydrofluoric acid + organic

Si is easily oxidized in air during mechanical polishing for EM analysis. In some critical analyses (e.g. Si surface plasmon analysis) in EELS, Si TEM specimens are normally dipped in HF solution (e.g. 10% HF solution) to remove the surface oxide. However, a thin layer of SiOx may still exist due to exposure to air before loading into the TEM.

Table 2440b. HF attack of select metallic materials.

Material Reactivity with Hydrofluoric Acid
Aluminum Concentrated HF is not an excellent etchant of aluminum. However, dilute solutions of HF can be used as an Al etchant. HNO3-HF mixtures such as 50 mL·L−1 40% HF and 250 mL·L−1 65% HNO3 is recommended as an etchant for aluminum thin films. Exposure to HF causes thick fluorination of aluminum films. The resulting aluminum fluoride film can be 2000 Å thick and greater than 50% fluorine.
Copper Exposure to HF causes an approximately 100-500 Å thick fluoride film.
Molybdenum Molybdenum, especially when an anode, is dissolved by anhydrous HF at such a high rate that this reaction can be utilized for preparing molybdenum fluoride films
Nickel Nickel is recommended as materials for the transport and storage of hydrofluoric acid, since the etch rate at room temperature remains under 2000 Å/min
Platinum Platinum is not attacked at room temperature in 40% HF or higher concentrations. Platinized titanium has been shown to be a good electrode material in electrolytes which contain HF
Titanium Hydrofluoric acid strongly attacks Ti. The resulting titanium fluoride creates significant residues.