This PDF file contains the editorial “E-Beam Direct-Write Lithography/Nanoimprint Lithography and Aviation” for JM3 Vol. 6 Issue 01

This letter reports record-breaking low defect counts for immersion lithography, the mechanism for formation of particle-printing defects, and for two new exposure routings to achieve the low defect counts. Both new routings make the slot-scan directions parallel to the field-stepping directions, whereas in the normal routing the two directions are perpendicular to each other. From experimental data, the average defect count for one of the special routings is 4.8 per wafer, while it is 19.7 per wafer for normal routing.

Leaching of resist components into water has been reported in several studies. Even low dissolution levels of photoacid generator (PAG) may lead to photocontamination of the last optical surface of the projection lens. To determine the impact of this phenomenon on optics lifetime, we initiate a set of controlled studies, where predetermined amounts of PAG are introduced into pure water and the results monitored quantitatively. The study identifies the complex, nonlinear paths leading to photocontamination of the optics. We also discover that spatial contamination patterns of the optics are strongly dependent on the flow geometry. Both bare SiO₂ surfaces as well as coated CaF₂ optics are studied. We find that for all surfaces, at concentrations typical of leached PAG, below 500 ppb, the in situ self-cleaning processes prevent contamination of the optics.

There is a current need for high refractive index (RI) materials that can be used in aqueous systems for improving 193-nm immersion photolithography. Although heavy metal salts such as Ca²⁺ and Ba²⁺ have the potential to substantially increase the RI of aqueous solutions, the water solubility of these salts with common anions is often too low to achieve concentrations that significantly increase the RI to the desired values. We therefore investigate the use of crown ethers to enhance the solubility of these cations. Most of crown ethers are soluble in water, are inexpensive materials, and are available commercially. 15-crown-5-ether and 12-crown-4-ether are liquids at room temperature and therefore can be used as neat immersion fluids without dilution in water. Saturation of crown ethers with inorganic salts do not lead to any increase in the refractive index due to their low solubility in such an apolar medium. Thus, the use of inorganic salts as refractive index enhancement agents does not seem to be a desirable proposition in the present case. Instead, the use of crown ethers or their derivatives can be an alternative system, since these compounds have properties such as density, viscosity, and boiling point similar to aqueous media.

We investigate the development time for densely packed high-aspect-ratio SU-8 structures. We find that a simple notch model as used by other workers to predict development time for isolated structures is inadequate for our case. We develop a theoretical model that allows the effects of mass transport of the resist in the developer to be included. By calibrating the process, we find that we are able to predict development time for our structures.

X-ray lithographic conditions for high-aspect-ratio SU-8 resist structures are characterized for potential application in x-ray optics and bioMEMS. The effects of the main process parameters, such as exposure dose, postexposure bake, development time, and the packing density of the microfabricated features, on the increase in feature size at the top portion of the resist (as compared to that in masks) are investigated. We find that lower postexposure bake and exposure dose leads toward minimizing dimensional errors. Further improvements in reducing the dimensional errors can be achieved by overdeveloping the structures. Using overdevelopment, we demonstrate an improvement in dimensional error for a given structure from 5 to 3.3%.

We present nanostencil lithography as a new and parallel nanopatterning technique for batch fabrication of micro/nanoelectromechanical systems (MEMS/NEMS) with high throughput and resolution. We use nanostencil lithography for the purpose of integrating nanomechanical resonators into complementary metal-oxide semiconductor (CMOS) circuits. When patterning nonflat substrates, which is the case of CMOS wafers, the gap between the nanostencil membrane and the surface induces a pattern blurring that constitutes an intrinsic limitation to the maximum achievable resolution. In our case, the lateral blurring is on the order of 150 nm on each side. We present here a remedy to this limitation that is based on a corrective dry etching step that removes the excess material and which recovers the designed pattern dimensions. As a demonstration, we succeed in the patterning of an entire 100-mm-diam wafer with nanomechanical devices having lateral dimensions in the range of 200 nm.

Extreme ultraviolet (EUV) light sources with efficient emission at 13.5 nm are needed for next-generation lithography. A critical consideration in the development of such a source is the lifetime of collector optics. These experiments expose optics to a large flux of energetic particles coming from the expansion of the pulsed-plasma EUV source to investigate mirror damage due to erosion, layer mixing, and ion implantation. The debris ion spectra are analyzed using a spherical sector energy analyzer (ESA) showing ion energies of 2 to 13 keV, including Xe⁺ -Xe⁺⁴, Ar⁺, W⁺, Mo⁺, Fe⁺, Ni⁺, and Si⁺ Microanalysis is performed on samples exposed to 10 million pulses, including atomic force microscopy (AFM), showing increased roughness for most exposed samples. Notably, a Mo–Au Gibbsean segregated alloy showed surface smoothing over this time frame, suggesting that the segregation worked in situ. TRIM predictions for ion implantation are consistent with ion debris measurements from the ESA. Finally, time exposures of samples from 2, 20, and 40 million pulses show an initial roughening with smoothing of the exposed samples at longer time frames. Constant erosion is demonstrated with the SEM. These analyses give an experimental account of the effects of the ion debris field on optic samples exposed to the EUV source.

Achieving the throughput of one wafer layer per minute with a direct-write maskless lithography system, using 22-nm pixels for 45-nm feature sizes, requires data rates of about 12 Tb/s. In our previous work, we developed a novel lossless compression technique specifically tailored to flattened, rasterized, layout data called context copy combinatorial code (C4), which exceeds the compression efficiency of all other existing techniques including BZIP2, 2D-LZ, and LZ77, especially under a limited decoder buffer size, as required for hardware implementation. In this work, we present two variations of the C4 algorithm. The first variation, block C4, lowers the encoding time of C4 by several orders of magnitude, concurrently with lowering the decoder complexity. The second variation, which involves replacing the hierarchical combinatorial coding part of C4 with Golomb run-length coding, significantly reduces the decoder power and area as compared to block C4. We refer to this algorithm as block Golomb context copy code (block GC3). We present the detailed functional block diagrams of block C4 and block GC3 decoders, along with their hardware performance estimates as the first step of implementing the writer chip for maskless lithography.

We choose thermal treatment as part of a methodology to remove chemical residue on the surface of a mask. This new step of thermal treatment is inserted into our standard cleaning process for embedded attenuate phase shift masks (EAPSMs). The treatment is carried out in a modified hot plate system at various temperatures and times. After thermal treatment, ion chromatography measures the residual ions on a given surface. The thermal treatment is found to considerably reduce residual sulfate ions on the mask surface. The remaining sulfate ions on the mask are <0.18 ng/cm² using thermal treatment.

We analyze the electron-irradiation damage induced in wafers by scanning electron microscope (SEM) inspection, which uses SEM images of voltage contrast formed by the charges on the pattern. The effects of electron-beam energy and charging on a metal-oxide semiconductor (MOS) capacitor are studied. We find that the higher energy electron beam, whose electron range is larger than the thickness of the gate electrode, creates traps at the interface between the silicon substrate and the gate dielectric. The flat-band voltage is shifted by the created traps. Although these traps are created by the transmission of the electron beam into the dielectric, they are not created only by charging on the gate electrode; neither is an oxide fixed charge created in the MOS capacitor. Accordingly, for damage-free inspection of MOS devices, the electron-beam energy should be low enough that the electron range is smaller than the thickness of the gate electrode. On the other hand, the flat-band voltage does not shift, owing to charging on the pattern surface during the electron irradiation. However, the gate dielectric is broken down by charging on the gate electrode at high voltage. Accordingly, for damage-free inspection, the charging voltage should be controlled so as not to break down the gate dielectric.

A study of relevant fabrication parameters is presented for a process that uses chemical etching of sacrificial cores to produce long, hollow microchannels. Two different sacrificial materials are investigated, SU8 and reflowed photoresist. These two materials produce channel cross sections with rectangular and arch-shaped cores, respectively. Fabrication times based on etch removal rates of sacrificial materials are reported for SU8 core microchannels and for a hybrid core consisting of reflowed photoresist and aluminum layers. The hybrid design takes advantage of the fast etch times possible for aluminum, but also produces smooth, arched sidewalls. Structural integrity is also investigated for different microchannels, specifically the wall thickness required to produce an intact channel of a given width. Empirical design rules indicate that SU8-based core channels require a wall thickness-to-width ratio of greater than 1:10, and reflowed photoresist based structures require a ratio greater than 1:50.

We present the use of artificial neural networks (ANNs) to model an electromagnetic microelectromechanical system (MEMS) microactuator. It is inherently complex and time consuming to model/predict the response of an electromagnetic microactuator numerically by finite element analysis, particularly when it is actuated by a pulse of current in media with different properties (e.g., air, water, and diluted methanol). ANNs are used to model the maximum displacement (d_max) of the microactuator for a range of burst frequencies (f_b) and input currents (I_coil), as well as different mechanical designs and actuation media. The prediction errors of the ANN model in normal and pressurized air are <13 and <2%, respectively. The prediction error for the same response in water or 50% diluted methanol in water is <10%.