Method and apparatus for micromachining a sample using a Focused Ion Beam

The invention relates to a method and an apparatus for micromachining samples. The apparatus comprises an integral combination of: a sample holder, a focused ion beam (FIB) exposure system.for projecting a FIB onto a sample, and a light optical microscope (LM), wherein the LM is configured for imaging or monitoring the sample during and/or after FIB exposure. The method comprises the steps of: - using the LM projecting light towards the sample and collecting reflected and/or transmitted light from said sample; - analyzing the collected light and establishing a measure for the modulation of an intensity of said collected light due to an optical interference of light from a first surface of the sample and light from a second surface of the sample; - using the measure for the modulation of the intensity for establishing a measure for the distance between said first surface and said second surface.