Fresnel zone plates are used in the microscope as condenser (condenser zone plate) and imaging (micro zone plate) optics. They are fabricated in-house using the Nanowriter electron beam lithography tool in the Nanofabrication Laboratory.

SEM micrograph of a 25-nm-outermost zone width micro zone plate. Diameter = 63mm, 628 zones, gold plated.

Picture of a condenser zone plate. It has an outermost zone width of 60 nm, 41700 zones, 1 cm diameter, and nickel plating.

Characterization of the spatial resolution of the microscope is an important and active project in our group. Here, we use the following definition of resolution:

Res = half-period of a equal line and space pattern that exhibits Rayleigh-like 26.5% modulation in its image

The resolution, in general, can be expressed as . is the radiation wavelength, and NA is the numerical aperture of the imaging optic. is a constant which primarily depends on the degree of partial coherence of the illumination, . In our case, NA is equal to , where is the outermost zone width of the micro zone plate. Thus,

Res =

The degree of partial coherence of the microscope, , is equal to. is the outermost zone width of the condenser zone plate (CZP). Typically, is from 0.4 to 0.7, and from 0.6 to 0.8. The resolution of the full-field microscope is slightly finer than the smallest zone width of the micro zone plate.

We design and fabricate various test objects for measurement of the microscope’s resolution. Test objects and measurement result are discussed below.

Recent Results (Click for Larger Images)

Resolution Test Object

-Fabricated in-house using Nanowriter electron beam lithography
-Flexible, arbitrary, 2-D line and space patterns
-Features as fine as the smallest zones in the zone plates
-Typical in gold or nickel plating on silicon nitride window

Design layout of a test object. The numbers represent linewidth in microns.

Multilayer Test Object

-Multilayer coatings in cross section
-Fabricated in-house using magnetron sputtering and conventional TEM sample preparation techniques
-high quality layers with half-periods down to 5 nm
-large material pair selections
-arbitrary aspect ratio

SEM micrograph of Cr/Si test object

Resolution Measurement

20 nm spatial resolution in 1.1x diffraction limit at 2.07 nm
Achieved with a 25-nm outermost zone width micro zone plate, and s = 0.42

Click for Larger Images




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