Problems of sacrificial etching in the presence of aluminium interconnect

In a surface-micromachining process that is to be merged with microelectronic processing, the presence of an aluminium interconnect causes several problems. As sacrificial etching is the last process step, the aluminium interconnect is present and will be attacked by the etchant. To prevent this etching of the interconnect, two possible solutions are investigated. The aluminium can be sealed using a photoresist layer. Problems that arise are etching of the resist,peeling of the interconnect and underetching due to poor adhesion. By using baking of the resist, thin oxide layers and reflow of the resist, this can be limited, but not avoided. Other etchants such as BHF with glycerol and glycerine additions and so called pad-etchants can be used. These have larger selectivity with respect to the aluminium and can be used for sacrificial etching with aluminium exposed. Permissible etch time is maximized by combining the resist protection with an alternative etchant, enabling underetching of 50 mu m or more. (C) 1997 Elsevier Science S.A.