Interferometric optical microscopy of subwavelength grooves

The amplitude and phase response of a high numerical aperture interferometr ic microscope to subwavelength grooves are investigated both experimentally and theoretically. It is well known that for narrow and deep structures sc alar diffraction theory is no longer valid and a rigorous vector diffractio n model is required. Conical diffraction results presented demonstrate sign ificant differences in measurements taken in different polarisation states. Significant light coupling occurs when the polarisation state of light at the back focal plane of the microscope is aligned perpendicular to the groo ve (TE) whereas relatively poor coupling occurs when the polarisation is al igned along the groove (TM). The stronger coupling of TE incident light in the groove means that there is much greater contrast compared to TM. Under certain circumstances an inversion of the phase occurs in TE, which is intu itively explained in terms of interference between the top and bottom of th e groove. The greater coupling that occurs in TE enables the depth of narro w grooves to be measured more accurately and over a wider range of depths t han in TM. (C) 2001 Elsevier Science B.V. All rights reserved.