This PDF file contains the editorial “Good News for JM3” for JM3 Vol. 3 Issue 04

In this study we first review existing source optimization methods. Lack of rigorous formulations motivates discussion of the optimization objectives and constraints. We stress the importance of using weighted and so-called Sobolev norms. Second, we state the main optimization problem as a set of the optimization objectives in a form of functional norm integrals to maximize image fidelity and source smoothness. We reduce this to a non-negative least square (NNLS) problem, which is solved by standard numerical methods. Third, we analyze solutions for important practical cases: alternating phase shifting applied to regular and semiregular pattern of contact holes, two types of SRAM cells with design rules from 130 to 250 nm, and a complex semidense contact layer pattern. Finally, we show how constraint optimization can be used to smooth strong off-axis quadrupole illuminations to achieve better image fidelity for some selected layout patterns.

The orientation and center-of-mass contributions of water to light scattering are calculated based on the known anisotropy of the polarizability of the water molecule. Further, the polarization ratio under 90-deg scattering is calculated and the relation between orientational and center-off-mass contributions for different polarization directions is evaluated. An experiment is proposed that enables us to separate the different scattering contributions. While the amount of scattering due to molecular orientations seems to be still moderate for water, it is expected to be larger for most fluids, like fluorinated polymers.

Material selection and lifetime issues for extreme ultraviolet (EUV) lithography are of critical importance to the success of this technology for commercial applications. This work reviews current trends in production and use of plasma-facing electrodes, insulators, and wall materials for EUV-type sources. Ideal candidate materials should be able to: withstand high thermal shock from the short pulsed plasma; withstand high thermal loads without structural failure; reduce debris generation during discharge; and be machined accurately. We reviewed the literature on current and proposed fusion plasma-facing materials as well as current experience with plasma gun and other simulation devices. Both fusion and EUV source materials involve issues of surface erosion by particle sputtering and heat-induced evaporation/melting. These materials are either bare structural materials or surface coatings. EUV materials can be divided into four categories: wall, electrode, optical, and insulator materials. For electric discharge sources, all four types are required, whereas laser-produced plasma EUV sources do not require electrode and insulator materials. Several types of candidate alloy and other materials and methods of manufacture are recommended for each component of EUV lithography light sources.

Multilayer interference coatings on reflective elements in extreme ultraviolet (EUV) projection lithography systems introduce phase and amplitude variations at reflection, which have important implications for imaging properties, e.g., resolution, depth of focus, and tolerances. We discuss the numerical results of the optical effects of multilayers (MLs) and the inclusion of these effects in optical design. This numerical study demonstrates the advantages of spatially varying (graded) MLs compared to multilayers with constant layer thicknesses. We present a new method to calculate the optimum grading of multilayers. Using this new method, we are able to fully optimize the wavefronts emerging from the projection system toward the image plane.

We demonstrate the capability of 100-GB density recording by electron beam mastering and readout by a near-field optical pickup with an effective NA of 2.05 and a blue LD of 405-nm wavelength. A silicon (Si) disk of 100-GB density is fabricated by an optimized Si etching process condition to form suitable pit pattern shapes for the near-field readout.

For highest efficiency optical devices, it is desirable to form continuously graded device features. We describe a technique to produce such features through the fabrication of a continuous-tone grayscale mask and subsequent grayscale photolithography. The design of the mask fabrication process is outlined, including the high-energy-beam-sensitive (HEBS) glass electron-beam exposure response characterization, and the generation of an exposure profile with inherent proximity effect correction. Application of the process is demonstrated through fabrication of smooth-facet retroreflectors, with features that are not possible to produce either by a grayscale process that employs discreet gray levels or by anisotropic wet etch techniques.

A method of numerical simulations is used to predict the profile of a 3-D aspherical microlens array. Based on the simulated results, the desired micro-optical lens profile is obtained through excimer laser ablation. The simulation method applied in the excimer laser ablation can significantly reduce the quantity of microablation experiments. The ablated microstructures with surface average roughness of Ra<20 nm are successfully achieved for micro-optical components. The excimer laser ablation parameters include laser fluence, shot number, workstation scanning velocity, and repetition rate. Various profiles of microlens and microprism arrays with different dimensions can utilize numerical simulation and form desired geometries by laser ablation.

In recent years, a relatively new type of negative photoresist, EPON SU-8, has received a lot of attention in the MEMS field because of its excellent lithography properties. Significant research efforts have been made to study the lithographic properties of SU-8 to obtain high aspect ratio microstructures with good sidewall quality. Currently, selection of optimal wavelengths of the UV light for lithographic and reduction of the diffraction effects are believed to be the two most important factors for achieving high-quality lithography of SU-8 as reported in the literature. Other reported efforts also include modifications of the chemical properties of SU-8 for better lithographic quality. We report a study on stress reduction during the postbaking process and the effects on lithography of ultra-thick high aspect ratio SU-8 microstructures. Our research proves that aspect ratios up to 40:1 in isolated open field structures of thicknesses between 1 and 1.5 mm can be obtained without any modifications of the resist chemistry or changes in light spectrum applied from a standard broadband UV source. The principal factor in this achievement is the reduction of internal stress during the postexposure bake process that eliminates large plastic deformations present during standard bake procedures. This process may be used for the fabrication of ultra-thick high aspect ratio microstructures that have to date only been obtainable using x-ray lithography-based LIGA processes.

We present a high-quality UV-lithography process for making high aspect ratio microstructures for microengines using an ultrathick SU-8 photoresist layer. The microreciprocating engine project, which is ongoing at the University of Birmingham, aims to develop a compact power plant to replace batteries. The design of the microengine imposes strict requirements on the geometry of the engine components. The microfabrication research work has been concentrated on developing a SU-8 UV-lithography process to achieve good sidewall angles and high aspect ratio. Based on the study of the photoactive property of ultrathick SU-8 layers, an optimized prebake time is found for obtaining the minimum UV absorption of SU-8. The optimization process is tested and proven effective using a series of UV-lithography experiments on different prebake times. Microstructures with aspect ratios as high as 40:1 are produced in 1000-µm ultrathick SU-8 layers using standard UV-lithography equipment. The sidewall angles are controlled between 85 to 90 deg. The engine components fabricated using this process satisfy microengine design requirements.

By combining silicon dry corrosion, wet corrosion, oxidizing sharpening and vacuum bonding techniques, and the theoretic calculation of elastic membranes and the distance from the catelectrode to the anode, a novel vacuum microelectronic pressure senor with overload protection is developed. The density of the field emission catelectrode array is about 24,000/mm². The starting emission voltage is 0.5 to 1.5 V; backward voltage is higher than 25 V. When the forward voltage is 5 V, pressure sensitivity is 30.1 mV/kPa. The temperature error is 0.5% between 20 and 122°C.

A broadband tunable filter for the infrared spectral region is desired for use as a wavelength selective element in a miniature absorption spectrometer. We present the design, fabrication, packaging, and characterization of a bulk micromachined Fabry-Perot interferometer (FPI) for meeting this need. A novel approach to fabricate a MEMS-based tunable resonant cavity using two separate wafers bonded using a "lock-and-key" spacer design is outlined, with the goal of realizing electrostatically actuated membranes from films predeposited on base substrates. This ability could enable the pursuit of MEMS devices without in-house chemical vapor deposition (CVD) capability, after overcoming the shortcomings of bulk micromachining. The FPI device was designed with a planar structure comprising two face-to-face bonded chips of overall lateral dimension 10×10 mm with deflection regions of 2X2 mm. The device employs electrostatic actuation to tune the output wavelength, for which finite element modeling predicted low (<1 V) actuation voltages for movement of the membrane. Experimental results from device testing (mechanical) were found to differ from the theoretical predictions, primarily due to fabrication issues. Specifically, the device performance was found to be greatly influenced by the amount of residual silicon on the wafer chip following inductively coupled plasma (ICP) backside etching, with high voltages (~30 times higher than modeled) required for actuation of the device. Through a combination of modeling and experimental measurements, it is demonstrated that the ability to produce MEMS devices by releasing membranes from films predeposited on substrates is highly susceptible to error in etching and packaging.