Rajesh Menon

Confining Light to Deep Subwavelength Dimensions to Enable Optical Nanopatterning

Subwavelength Patterning Microscopists have recently achieved fluorescence imaging at subwavelength resolution by focusing one beam of light in a halo around another beam, thereby quenching the glow of fluorescent dyes in all but the very center of the illuminated spot. Three studies have now adapted this approach to photolithography (see the Perspective by Perry). Andrew et al. (p. 917, published online 9 April) coated a photo-resist with molecules that, upon absorbing the ultraviolet etching beam, isomerized to a transparent layer but returned to the initially opaque form upon absorption of visible light. Applying an interference pattern with ultraviolet peaks superimposed on visible nodes restricted etching to narrow regions in the center of these nodes, yielding lines of subwavelength width. Scott et al. (p. 913, published online 9 April) used a central beam to activate polymerization initiators, while using a halo-shaped surrounding beam to trigger inhibitors that would halt polymerization. Li et al. (p. 910, published online 9 April) found that use of a different initiator molecule allowed both beams to share the same wavelength (800 nanometers), with a relatively weak quenching beam lagging a highly intense initiating beam slightly in time. Both the latter techniques produced three-dimensional features honed to subwavelength dimensions. Molecules that photoisomerize and change in transparency are used to define narrow features on photoresists. In the past, the formation of microscale patterns in the far field by light has been diffractively limited in resolution to roughly half the wavelength of the radiation used. Here, we demonstrate lines with an average width of 36 nanometers (nm), about one-tenth the illuminating wavelength λ1 = 325 nm, made by applying a film of thermally stable photochromic molecules above the photoresist. Simultaneous irradiation of a second wavelength, λ2 = 633 nm, renders the film opaque to the writing beam except at nodal sites, which let through a spatially constrained segment of incident λ1 light, allowing subdiffractional patterning. The same experiment also demonstrates a patterning of periodic lines whose widths are about one-tenth their period, which is far smaller than what has been thought to be lithographically possible.

Absorbance Modulation Optical Lithography

We describe a new mode of optical lithography called absorbance-modulation optical lithography (AMOL) in which a thin film of photochromic material is placed on top of a conventional photoresist and illuminated simultaneously by a focal spot of wavelength lambda1 and a ring-shaped illumination of wavelength lambda2. The lambda1 radiation converts the photochromic material from an opaque to a transparent configuration, thereby enabling exposure of the photoresist, while the lambda2 radiation reverses the transformation. As a result of these competing effects, the point-spread function that exposes the resist is strongly compressed, resulting in higher photolithographic resolution and information density. We show by modeling that the point-spread-function compression achieved via AMOL depends only on the absorbance distribution in the photostationary state. In this respect, absorbance modulation represents an optical nonlinearity that depends on the intensity ratio of lambda1 and lambda2 and not on the absolute intensity of either one alone. By inserting material parameters into the model, a lithographic resolution corresponding to lambda1/13 is predicted.

Photon-sieve lithography

We present the first lithography results that use high-numerical-aperture photon sieves as focusing elements in a scanning-optical-beam-lithography system [J. Vac. Sci. Technol. B 21, 2810 (2003)]. Photon sieves are novel optical elements that offer the advantages of higher resolution and improved image contrast compared with traditional diffractive optics such as zone plates [Nature 414, 184 (2001)]. We fabricated the highest-numerical-aperture photon sieves reported to date and experimentally verified their focusing characteristics. We propose two new designs of the photon sieve that have the potential to significantly increase focusing efficiency.

Design and analysis of multi-wavelength diffractive optics.

We present an extension of the direct-binary-search algorithm for designing high-efficiency multi-wavelength diffractive optics that reconstruct in the Fresnel domain. A fast computation method for solving the optimization problem is proposed. Examples of three-wavelength diffractive optics with over 90% diffraction efficiency are presented. These diffractive optical elements reconstruct three distinct image patterns when probed using the design wavelengths. Detailed parametric and sensitivity studies are conducted, which provide insight into the diffractive optics performance when subject to different design conditions as well as common systematic and fabrication errors.

Lithographic patterning and confocal imaging with zone plates

Zone-plate-array lithography (ZPAL) uses an array of zone plates, combined with an array of micromechanical mirrors or shutters, to pattern features on a substrate without a mask. In this article, we investigate the patterning characteristics of individual zone plates. We show patterns printed with pixel-dose modulation (gray scaling) and subpixel beam stepping. Using a combination of these techniques for linewidth control, edge placement, and proximity-effect correction, ZPAL can produce arbitrary patterns with features as small as the focal spot of a zone plate. We also demonstrate the use of zone plates in a confocal-microscopy mode for placing the substrate at the focus of the zone plates, and for imaging. Zone-plate-array scanning-confocal microscopy (ZPAM) could be useful for gapping and alignment in ZPAL, and possibly for wafer or mask inspection at deep ultraviolet wavelengths.

Maskless, parallel patterning with zone-plate array lithography

Zone-plate array lithography (ZPAL) is a maskless lithography scheme that uses an array of shuttered zone plates to print arbitrary patterns on a substrate. An experimental ultraviolet ZPAL system has been constructed and used to simultaneously expose nine different patterns with a 3×3 array of zone plates in a quasidot-matrix fashion. We present exposed patterns, describe the system design and construction, and discuss issues essential to a functional ZPAL system. We also discuss another ZPAL system which operates with 4.5 nm x radiation from a point source. We present simulations which show that, with our existing x-ray zone plates and this system, we should be able to achieve 55 nm resolution.

Fabrication of high-numerical-aperture phase zone plates with a single lithography exposure and no etching

We present a process for fabricating phase zone plates and other diffractive-optical elements that is capable of achieving high resolution, requires only a single lithography step, and no etching. For this process we use the negative resist hydrogen silsesquioxane (HSQ) (Dow Corning), which is sensitive to both electrons and x rays. HSQ’s extraordinarily high resolution (∼10 nm) and its SiO2-like properties make it an optimal choice for fabricating diffractive-optical elements that operate in the ultraviolet and deep ultraviolet. HSQ has an index of refraction very close to that of fused silica, and a negligible absorption down to 157 nm. As a result, if it is spun to the thickness corresponding to the desired phase step for the optic, patterning and development are the only required process steps. It is often desirable to place an absorbing layer on the areas surrounding the diffractive elements to prevent unwanted radiation from reaching the substrate when the optics are used for lithography and microsc...

Chromatic-aberration-corrected diffractive lenses for ultra-broadband focusing

We exploit the inherent dispersion in diffractive optics to demonstrate planar chromatic-aberration-corrected lenses. Specifically, we designed, fabricated and characterized cylindrical diffractive lenses that efficiently focus the entire visible band (450 nm to 700 nm) onto a single line. These devices are essentially pixelated, multi-level microstructures. Experiments confirm an average optical efficiency of 25% for a three-wavelength apochromatic lens whose chromatic focus shift is only 1.3 μm and 25 μm in the lateral and axial directions, respectively. Super-achromatic performance over the continuous visible band is also demonstrated with averaged lateral and axial focus shifts of only 1.65 μm and 73.6 μm, respectively. These lenses are easy to fabricate using single-step grayscale lithography and can be inexpensively replicated. Furthermore, these devices are thin (<3 μm), error tolerant, has low aspect ratio (<1:1) and offer polarization-insensitive focusing, all significant advantages compared to alternatives that rely on metasurfaces. Our design methodology offers high design flexibility in numerical aperture and focal length, and is readily extended to 2D.

Experimental characterization of focusing by high-numerical-aperture zone plates

High-numerical-aperture zone plates have important applications in high-resolution optical maskless lithography as well as scanning confocal microscopy. We describe two methods to experimentally characterize the focusing properties, i.e., the point-spread function, of such diffractive lenses. The first method uses spot exposures in photoresist and the second uses a conventional knife-edge scan. The experimental results agree well with rigorous theoretical calculations.

Design of diffractive lenses that generate optical nulls without phase singularities

We exploit a technique, based on nonlinear optimization, to design diffractive lenses that focus optical nulls without any phase singularities. To ensure ease of fabrication, these lenses are composed of concentric circular zones. Furthermore, we show that this technique is readily extended to multiple wavelengths and can be used to improve tolerance to fabrication errors.