sentence: a certain amount of over-travel is required after the first probe makes contact with the wafer , for two reasons : to guarantee that all probes have made contact ( to account for non-planarity of the wafer ) to break through the thin oxidized layer ( if the metal pad is aluminum ) on the pad there are numerous types of mechanical probes available commercially : their shape can be in the form of a cantilever , spring , or membrane , and they can be bent into shape , stamped , or made by microelectromechanical systems processing .