We disclose several instrument architectures for the measurement of arbitrary
phase retardation on advanced lithography photomasks. These architectures combine
traditional interferometric techniques with high-magnification UV microscopy. Features
are interrogated using a multitude of phase probes, formed by a imaging a number
of variable apertures back-illuminated by phase-coherent beams, onto the surface
of the photomask with a given demagnification. The size, spacing, and orientation
of the phase probes may be adjusted to suit photomask feature geometries. Means
are provided to vary the relative optical phase between the phase probes. These
phase probes both reflect from and transmit through the photomask; the stationary,
non-localized interference fringes, formed in the regions of phase probe electric
field overlap, contain information on the optical path difference between the two
probes. The spatial resolution of these measurements is limited only by the resolution
limit of the UV microscope, which may significantly exceed the capability of existing tools.