Deep Ultraviolet Digital Holographic Microscopy

 Unlike scanning systems, digital holographic microscopy (DHM) has the ability to extract the phase information of the objectwave-front bytaking only one image (digital hologram)resulting in the optical thickness and the topography of the object. It also makes it possible to reconstruct the amplitude image in any plane (digital focusing)utilizing reconstruction algorithmsthat allows investigating the shape and the displacements of objects in thenanoscale and highlights its capability as a promising 3Dinspection technique. However, by improving nanotechnology, the resolution of optical imaging methods shouldalsobe improved to satisfy the needs in this regard.

Off-axis digital holography setup has been arranged for this experiment.Figure 1 shows a schematic of the setup. We employ an ArF Excimer Laser, ExiStar 200 (TUI), operating at deep UV (193 nm), as a light source, which has a short coherence length, in the order of 100 μm. A beam splitter (BS) separatesthe laser beam into two parts, one as areference (dashed line) and the other for illuminatingthe object (solid line).

Custom designedDeep-UV-objective(see insight in Fig. 1) has beenmade to meet the demandsof theoff-axis setup with low price for imaging with a deep UV light source. Lightcoming from the object plane is collimated and focused towards infinity. The objective consists of a half-ball-lens,allowing thenumerical aperture to reach 0.75, and a custom-design asphere to mainly correct the sphericalaberration introduced by the half-ball-lens.

 Oblique illumination approach has been implemented to achieve high resolution. The zero ordercomponent is shifted to one of the peripheral sides of the objective, instead of passing throughthe center of the aperture (which is the case of on-axis illumination). This shiftingenablesmore additional spatial frequencies to enter the imaging pupil. Figure 2 shows a simple schematic of the method.The numbers “-2, −1, 0, 1 and 2” qualitatively represent the component of thespatial frequency of the light coming from object. In the case of direct illumination (Fig. 2a)the zero order and two components of the frequencies assigned by “1” i.e. ( + 1,-1) arecollected by the objective. Using oblique illumination for the given example, instead of thecomponent “-1”, the component “2” is collected by the objective (Fig. 2b).

To suppress artifacts come from oblique illumination, we symmetrically performed obliqueillumination from four different sides and combine all images together to obtain an imagewith the more realistic and correct size for the structures. Figure 2c shows the utilized configuration. 

We have designed a nano-structured template to test the system. It includes square and linestructures, ranging from 500 to 100 nm in width. The reconstructed amplitude and phase of the object in the case of on-axis illuminationare shown in Figs. 3d and 3e,respectively.Thereconstructed amplitude and phase for two selected directions are shown in Figs. 3f-3i. The amplitude of the object hasbeen separately reconstructed for each of four oblique illumination directions, and then combined byadding the complex amplitude of the raw images and without implementing any further imageprocessing technique. The combined imageis shown in Fig. 3j, in which the “ito” logo is clear and even the line structures with thewidth of 250 nm are well-resolved, that confirms a significant enhancement in resolutioncompare to the result obtained with on-axis illumination (Fig. 3d). 

To better compare the final combined image (Fig. 3j) with the SEM image (Fig.3a) the small structures are magnified in Fig. 4. An inversed intensity profile is plotted foreach structure size to better show the visibility of the structures (Figs. 4b and 4e). Figure 4bshows a clear profile of the 300 nm sized structures and Fig. 4e presents the profile of the 250nm sized structures.

This arrangement can run in air and no specific treatment like working in vacuum or a clean room is required. Operating with a pulsed laser with the pulse duration of 10 ns, also makes it immune to vibration. Therefore, it can be easily employed in industrial environment for nano-metrology and inspection of submicron-size devices.