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During the manufacture of semiconductor devices, aluminium oxide layers have been suggested as a method to protect the underlying semiconductor. However, there are a number of problems with using these layers. For example, a thin film of aluminium oxide can cause mechanical and electrical connection problems. The thickness of this film can also interfere with the wetting of solder.
Anodic oxidation of aluminium in an electrolyte bath is a method for producing a dense aluminium oxide layer. This technique is able to produce an oxide layer of up to 0.1mm in thickness.
The preferred etchant for removing aluminium oxide is a solution of fluoride in an organic solvent. The solution is free from hydrofluoric acid and unbound water. The preferred etchant has a low etch rate for aluminium oxide.
The etching liquid may be a concentrated aqueous solution of ammonium fluoride or a concentrated hydrofluoric acid. Before using this etchant, it is important to wear acid protective gear, such as a yellow apron and green gloves. It is also important to wear an acid-proof mask, which should be worn over a full face mask.
The etchant is then rinsed with glacial acetic acid. The etchant for aluminium oxide should be applied on a surface that is sufficiently clean. The etchant should be applied on a wafer and the wafer should be removed from the solution after a short time.
The second aluminium layer is then vapour-deposited in a vacuum. The second aluminium layer is then covered with a silicon oxide layer.