Single Exposure Step Research Articles

Fabrication and characterization of microcavity lasers in rhodamine B doped SU8 using high energy proton beam

The authors present their results on the characterization of individual dye-doped microcavity polymer lasers fabricated using a high energy proton beam. The lasers were fabricated in rhodamine B doped SU8 resist with a single exposure step followed by chemical processing. The resulting trapezoidal shaped cavities had dimensions of ∼250×250μm2. Physical characterization of these structures was performed using a scanning electron microscope while the optical characterization was carried out by recording the emission subsequent to pumping the lasers with 532nm, 6 nanosecond pulses. The authors observed intense, narrow emission near 624nm with the best emission linewidth full width at half maximum of ∼9nm and a threshold ∼150μJ∕mm2.

Fabricating three-dimensional nanostructures using two photon lithography in a single exposure step

Conformable phase masks, transparent photopolymers and two photon effects provide the basis for a simple, parallel lithographic technique that can form complex, but well defined three-dimensional (3D) nanostructures in a single exposure step. This paper describes the method, presents examples of its ability to form 3D nanostructures (including freestanding particles with controlled shapes) and comprehensive modeling of the associated optics. Single step, large area 3D pattern definition, subwavelength resolution and experimental simplicity represent features that make this method potentially useful for applications in photonics, biotechnology and other areas.

Multilevel Microfluidics via Single-Exposure Photolithography

This communication reports a new method to form multilevel features in a single layer of SU-8 photoresist to facilitate the generation of 3D microfluidic chips. The method utilizes the spatial dependence of diffracted light intensity to selectively overexpose masked regions of photoresist and requires only a UV light source and a single transparency mask. 3D structures are formed within microfluidic channels using this selective overexposure method, with feature sizes being determined by the exposure dose and mask feature sizes. The dimensions of the internal features and the microfluidic channels can be varied independently according to these parameters, and any number of different heights can be obtained in a single exposure step. The method provides a simple means of forming 3D microfluidic structures with integrated features, including mixing structures, flow stabilization ridges, and separation weirs to increase the capabilities of microfluidic chips in a variety of microchemical applications.

Space-invariant multiple-beam achromatic EUV interference lithography

We report the production of two-dimensional periodic patterns with defined cell shapes, such as arrays of circular rings, with multiple-beam extreme ultraviolet (EUV) interference lithography. The method combines coherent and incoherent addition of interference fields in a single exposure step to produce high-resolution, space-invariant intensity patterns. The use of wideband light, instead of highly monochromatic sources like lasers, eliminates the periodic variation of the intensity pattern as multiple beams interfere. The technique is especially useful in the EUV region as the undulator sources in that wavelength range provide radiation with relatively wide bandwidth. In demonstration experiments nanometer scale arrays of circular rings and dots in PMMA resist were obtained. The technique has potential for use in applications where periodically repeated cell structures are needed such as display units, optical filters, electronic memory and logic circuits and magnetic storage devices.

Multiple-beam interference lithography with electron beam written gratings

Interference lithography with multiple beams provides two-dimensional periodic patterns in a single exposure step. It is possible to obtain various symmetries and shapes such as sparse hole arrays or dots on a hexagonal grid. However, when the number of interfering beams exceeds three, the intensity pattern depends strongly on the relative phases of the beams. In this article we show that this problem can be overcome with a scheme where transmission diffraction gratings written on a single substrate create the interfering beams possessing the required phase relations. The relative phases of the diffracted beams are controlled by the relative positions of the gratings in the substrate plane. We used electron beam lithography with an interferometrically controlled stage to obtain the required precise placement of the gratings. This method enabled us to produce hexagonal and square arrays of holes where we used three and four beam configurations, respectively. Moreover, we created sparse hole arrays that have hexagonal and square symmetries with six and eight beam interference. The experiments were carried out in the extreme ultraviolet region with synchrotron radiation and with laser light in the visible spectrum. The technique requires spatially coherent light but it is achromatic and it is not sensitive to small errors in the alignment of the incoming beam.

Ion projection direct structuring for patterning of magnetic media

Ion projection facilitates a direct structuring, which is an attractive potential manufacturing process for patterned storage media. An advantage to this method is that the media roughness remains unchanged. The feasibility of ion projection direct structuring for processing full disk surfaces was investigated using a next generation lithography projector. Co-Pt multilayer films with strong perpendicular anisotropy were deposited on 1-in glass disks as used in the IBM microdrive and on Si substrates. Concentric tracks including data, as well as head positioning servo structures, were patterned in a single exposure step with 45 keV He/sup +/ at a 4 /spl times/ demagnification. In a second experiment, sub-100-nm magnetic islands were produced using projection at 8.7 /spl times/ demagnification and visualized by magnetic force microscopy.

Optical alignment of liquid crystals

An optical method is described for the preparation of a liquid crystal cell. In one, single exposure step a surface relief grating is generated in a photopolymer layer deposited on a glass substrate. Such substrates then orient liquid crystals in contact with them and homogeneous alignment results. An analysis of the nematic liquid crystal orientation in terms of nematic liquid crystal cell parameters as encountered in applications has been made. Consideration of the different mechanisms proposed to explain the origin of liquid crystal alignment and of the conditions required for the generation of surface relief grating in a photopolymer layer has led to the conclusion that both the photoinduced chromophore theory and the surface oriented polymer chains theory are unable to explain the alignment mechanism. On the other hand, the elastic strain energy theory, which is based on geometrical factors, agrees with the observed spatial molecular orientation of the liquid crystals.

Resistless Patterning of Magnetic Storage Media Using Ion Projection Structuring

AbstractCo/Pt thin film multilayers with strong perpendicular anisotropy and out-of-plane coercivities of 5-11 kOe were magnetically altered in areas of local ion beam interaction. The ion irradiations were performed by ion projection through silicon stencil masks fabricated by silicon on insulator (SOI) membrane technology. The ion projector at the Fraunhofer Institute for Silicon Technology (ISiT) was operated at 73 keV ion energy and with a 8.7- fold demagnification. After exposure to 3 × 1014Ar+/ cm2 magnetic islands smaller than 100 nm in diameter were resolved in the Co/Pt multilayersby means of magnetic force microscopy. The impact of different ion species (He+, Ar+ and Xe+) and ion energies (10 – 200 keV) on the multilayer structure was evaluated using Monte Carlo simulations. The ballistic interface intermixing was used to predict magnetic coercivity changes for various irradiation conditions. The simulations revealed that with 73 keV Ar+ and Xe+ ions the irradiation dose could be reduced by a factor of 100 and 400 respectively in comparison to 73 keV He+which was verified in the experiments. X-ray reflectivity measurements confirmed that the Co/Pt superlattice structure is slightly weakened during the irradiation and that the surface smoothness of the media is preserved. Using the Ion Projection Process Development Tool (PDT) at IMS-Vienna concentric data tracks including head positioning servo informations were patterned onto a 1” IBM microdrive™ glass disk which was coated with Co/Pt multilayers. In a single exposure step several tracks within an exposure field of 17 mm in diameter were structured by 2 × 1015He+/ cm2 at 45 keV using a 4- fold demagnification set-up.

Experiments for 3-D structuring of thick resists by gray tone lithography

With the conventional micromachining technologies: isotropic and anisotropic dry and wet etching, a few shapes can be done. To overcome this limitation binary multi-masking technique, laser micro-stereolithography, or direct electron-beam-writing were used, but an inexpensive one-step UV-lithographic method, using the so-called “gray-tone lithography”, seems to be the best choice to produce local intensity modulation during exposure process. The paper reports on the study of arbitrary three-dimensional (3-D) shaping of negative and positive thick resists, using this method, and common technologies in standard ICs fabrication. Particular emphasis is placed on the design, manufacturing and use of half-tone transmission masks, required for UV-lithographic step in the fabrication process of mechanical, optical or electronics components. The original design and fabrication method, for the gray-tone reticles, were supported by experiments showing the main advantage of this new technology: the 3-D structuring of thick resists in a single exposure step, and also a high aspect ratio obtained over 9:1. Experimental results are presented in SEM micrographs, only for positive thick resists, showing different 3-D shapes in positive and negative polarity, and also the results obtained by using the wall-type test structure for aspect ratio evaluation. Finally, by optimization of the lithographic process, interesting applications are shown.

Sampled grating DBR laser arrays with adjustable 0.8/1.6 nm wavelength spacing

The difficulty in fabricating multiwavelength laser arrays with uniform wavelength spacing is overcome by using sampled grating distributed Bragg reflector (DBR) lasers as the constituent of a laser array. The sampling periods of the DBR mirrors are varied from laser to laser to provide reflection combs with different spacings in wavelength. Arrays of twelve lasers have been successfully fabricated with a single holographic exposure step. The wavelength spacing among the lasers can be adjusted to be around 0.8 or 1.6 nm with a simple tuning mechanism.

A novel fabrication technique of T-shaped gates using an EGMEA and PMIPK multi-layer resist system and a single-step electron-beam exposure

A novel fabrication process of submicron T-shaped gates using an EGMEA and PMIPK multi-layer resist system and e-beam lithography has been developed. Due to the different sensitivities of the EGMEA and PMIPK resists, narrow/wide/narrow resist opening was obtained. The status of the electron scattering and the incident primary electron beams statistically construct the absorbed energy density distribution contours. The absorbed energy density distribution contours in association with the sensitivity feature of the resists and the appropriate development conditions determine the final resist profile. Only a single exposure step and a single development step are required. The simplified fabrication process of T-shaped gates significantly reduces not only the process time but also the production costs. The simple submicron T-shaped gate fabrication process studied provides a suitable technique for the mass manufacture of the advanced InP -based and GaAs -based microwave devices and circuits.

Fabrication of nanometer-structures for photonic devices based on the negative tone resist AZPN114

Abstract The high contrast resist AZPN114 has been investigated with respect to its suitability for realizing optically active and passive structures. Two devices with completely different size and shape in their substructure were fabricated avoiding any complex proximity corrections. So narrow-band waveguide grating couplers were realized in a single exposure step. For the realization of gain coupled distributed feed back (DBF)- lasers gold masks with a width of 160 nm and a grating period of 200 nm were fabricated. The gain coupling was achieved with the technique of ion implantation induced intermixing.

Continuous-relief diffractive optical elements for two-dimensional array generation

Continuous surface-relief diffractive optical elements for two-dimensional array generation (fan-out) are designed and fabricated. Separable and nonseparable solutions for the two-dimensional element design are compared. The phase-grating microstructures are generated by laser-beam writing lithography in a single exposure step and converted to nickel shims by electroplating, enabling low-cost replicas to be produced by using laboratory and commercial replication processes. Results are presented for a 9 x 9 fan-out diffractive optical element with a measured efficiency of 94% and an overall uniformity within +/-8%; replicas in epoxy have the same efficiency and a uniformity of +/-15%.

Single-step copying process for multiplexed volume holograms

A technique for copying the index or absorption modulation of a multiplexed volume holographic element into a second volume holographic medium is presented. The technique utilizes a set of coherent but mutually incoherent optical sources and can perform the copy process in a single exposure step. The concept is demonstrated experimentally for the case of three angularly multiplexed holographic gratings in dichromated gelatin.