Surface profile measuring instrument and method

A surface roughness measuring instrument comprising an electromagnetic probe, the electromagnetic probe comprising a probe tip operable to be brought into proximity with a surface of a substrate to be measured, a drive unit operable to generate a low frequency magnetic field penetrating the surface of the substrate, a pick up unit operable to detect the strength of the magnetic field and output a magnetic field strength reading reflecting the strength of the magnetic field penetrating the surface of the substrate and a computation unit operable to determine the surface roughness measurement for an area of the surface from the magnetic field strength reading,

• wherein the pick up unit comprises at least a first pick up coil and a second pick up coil, and is operable to use the difference between the first and second pick up coils to determine the strength of the magnetic field penetrating the surface of the substrate,
• and wherein the first pick up coil is located a first distance from a drive coil in a first direction, the second pick up coil is located the first distance from the drive coil in a second direction, the second pick up coil is located closer to the surface of the substrate than the first pick up coil, and both of the first and second pick up coils are located on a same side of the surface being measured.