This PDF file contains the editorial “Editorial: JM3 Begins ‘‘e-First’’ Publication” for JM3 Vol. 4 Issue 01

This PDF file contains the editorial “Editorial: Emerging Lithographic Technologies” for JM3 Vol. 4 Issue 01

Step and flash imprint lithography (SFIL) has made tremendous progress since its initial development at The University of Texas at Austin in the late 1990s. The SFIL process went from laboratory to commercialization in under five years, and the number of technical hurdles that must be cleared before it is recognized as fully competitive with optical or EUV lithography for sub-50-nm patterning is dwindling. Patterning resolution has been demonstrated down to 20 nm, with the limit so far being only the template fabrication process. The SFIL method was developed from the beginning with the precision overlay/alignment requirements of multilevel device fabrication in mind. It was recognized that it would be inherently easier to achieve overlay and alignment accuracy with a constant temperature and low pressure imprinting process, and already tool designers have built on SFIL's advantages to produce tools that are viable for multilayer device fabrication. Early tools have demonstrated better than 10-nm alignment resolution, and no insurmountable fundamental issues have been identified that would prevent alignment resolution from reaching the tight tolerances required for integrated circuit manufacturing. With any contact printing method, process-generated defects are a concern, but the SFIL process has proven to be surprisingly robust with an inherent self-cleaning mechanism for removing particle contamination. Furthermore, new template surface treatments have been developed that improve mold lifetime and minimize defect generation. SFIL shows promise as a low cost manufacturing tool for a wide variety of semiconductor, microelectromechanical, optoelectronic, microfluidic, and other devices. This work summarizes the state of development of step and flash imprint lithography and discusses its potential as a general nanofabrication tool.

We review the current status of optical maskless lithography technology. The optical maskless systems presented in literature are either aimed for low-cost, large-feature, low-end production, or high-end performance directly competing with state of the art optical scanners. In the latter case, optical maskless systems are based on piston or tilting micromirror spatial light modulators (SLMs). Similar performance can be achieved with both mirror types, but tilting mirrors offer lower manufacturing complexity, the possibility of using larger mirrors, less complex rasterizing algorithms, and lower demands on the data path. This indicates that the tilting mirror arrangement might be more appealing for high-performance, high-capacity, economical optical maskless lithography. With the latest technology in SLM and rasterizing technology, an optical maskless tool can match regular mask-based scanners in all imaging modes at the 65-nm node, including binary, weak and strong phase-shifting, phase edge, and chromeless phase lithography. Optical maskless lithography can further provide an almost complete transparency with current lithography technology in terms of design rules, optical proximity correction (OPC) models, and illumination settings. Any difference is due not to the SLM, but to the reticle process and electromagnetic properties.

Electron projection lithography (EPL) is a viable candidate for moderate-volume production of systems on a chip (SOC) for the 65-nm node and beyond. Making EPL a realistically applicable technology depends strongly on how well its infrastructure is developed. Many developments related to EPL, including those by Selete, are currently under way. The world's first full-field EPL exposure tool, the Nikon EB stepper, NSR-EB1A, was shipped to Selete. In the installation stage, the tool produced 70-nm line and space, 50-nm isolated line, and 80-nm contact hole resolution. EPL resist performance is approaching 50-nm resolution, 5-µC/cm² sensitivity, and commercial availability. Sample EPL stencil masks are also available from three mask suppliers in Japan. Beta mask defect inspection and repair are also available, as is data processing software for EPL mask making. Such software has reduced processing time and data volume using PC clusters and hierarchical processing. The results of the first of EPL trial device fabrication showed good resolution for hole patterns at about 70 nm and the usefulness of EPL in hole delineation. EPL is thus being steadily improved and elements are commercially available, underscoring EPL's feasibility as a practical technology. Issues that remain to be solved include better EB stepper performance, faster resist, more accurate masks, the demonstration of practical inspection and repair tools, and verification software. These issues are, it should be noted, engineering issues rather than basic EPL issues. EPL remains the most promising candidate for the 65-nm node and beyond, especially for hole delineation. Selete is continuing EPL development, working with suppliers of exposure tools, resist, masks, inspection and repair tools, and software targeting lithography for 65-, 45-, and 32-nm nodes.

Extreme ultraviolet (EUV) lithography has emerged as the most likely successor to 193-nm lithography. We provide a technical overview of EUV lithography and a discussion of the advantages of EUV lithography over alternative technologies. The key challenges in developing EUV exposure tools for high-volume production are discussed. A brief assessment is given of the cost of ownership of EUV lithography in comparison with 193-nm immersion lithography.

This paper describes the status of 157-nm lithography in mid 2004. With the rapid rise of 193-nm immersion from a potential concept in early 2002 at SPIE, through its development, to scheduled delivery of early production tools in late 2004 and early 2005, 157-nm lithography has taken a backseat in the development of optical lithographic technology. While significant challenges were conquered during the 2000 through 2002 period, the development was too slow to prevent 157-nm from being eclipsed by 193-nm immersion. This work reviews the challenges that were identified and conquered during the development of 157-nm lithography. The jury is still out on potential application to production lithography, although the main development effort has been seriously scaled back. There are still issues that remain to be solved before the technology could evolve into a full manufacturing system. The answer to the question of whether it will evolve or not is left to the future to answer.

The practicability and methodology of applying resolution-enhancement-technique-driven regularly placed contacts and gates on standard cell layout design are studied. The regular placement enables more effective use of resolution enhancement techniques (RETs), which in turn enables a reduction of critical dimensions. Although regular placement of contacts and gates adds restrictions during cell layout, the overall circuit area can be made smaller and the number of extra masks and exposures can be kept to the lowest by careful selection of the grid pitch, using template-trim chromeless phase-shifting lithography approaches, enabling unrestricted contact placement in one direction, and using rectangular rather than square contacts. Four different fabrication-friendly layouts are compared. The average area change of 64 standard cells in a 130-nm library range from -4.2 to -15.8% with the four fabrication-friendly layout approaches. The area change of five test circuits using the four approaches range from -16.2 to +2.6%. Dynamic power consumption and intrinsic delay also improve with the decrease in circuits area, which is verified with the examination results.

Immersion lithography is proposed as a method for improving optical microlithography resolution to 45 nm and below via the insertion of a high-refractive-index liquid between the final lens surface and the wafer. Because the liquid acts as a lens component during the imaging process, it must maintain a high, uniform optical quality. One potential source of optical degradation involves changes in the liquid's index of refraction caused by changing temperatures during the exposure process. Two-dimensional computational fluid dynamics models from previous studies investigated the thermal and fluid effects of the exposure process on the liquid temperature associated with a single die exposure. We include the global heating of the wafer from multiple die exposures to better represent the "worst-case" liquid heating that occurs as an entire wafer is processed. The temperature distributions predicted by these simulations are used as the basis for rigorous optical models to predict effects on imaging. We present the results for the fluid flow, thermal distribution, and imaging simulations. Both aligned and opposing flow directions are investigated for a range of inlet pressures that are consistent with either passive systems or active systems using filling jets.

The emergence of immersion lithography as a potential alternative for the extension of current lithography tools requires a fundamental understanding of the interactions between the photoresist and an immersion liquid such as water. The water concentration depth profile within the immersed photoresist films is measured with neutron reflectometry. The polymer/substrate interface affects both the water concentration near the interface and the surface morphology of the film. Immersed films are not stable (adhesive failure) over the course of hours when supported on a silicon wafer with a native oxide surface, but are stable when the substrate is first treated with hexamethyldisilazane (HMDS). The bulk of the polymer films swells to the equilibrium water concentration, however, a gradient in water concentration is observed near the polymer/HMDS substrate interface with a concentration of approximately 17% by volume fraction and extending up to 50 Å into the film. Thus, polymers that absorb more than this amount exhibit depletion near the interface, whereas polymers that absorb less exhibit a water excess layer. These concentration gradients extend approximately 50 Å away from the interface into the film. As the total film thickness approaches this length scale, the substrate-induced concentration gradients lead to a film-thickness-dependent swelling; enhanced or suppressed swelling is witnessed for the excess or depleted interfacial concentrations, respectively. The substrate also influences the surface morphology of immersed thin films. The film surface is smooth for the HMDS-treated substrate, but pin-hole defects with an average radius of 19±9 nm are formed in the films supported on the native oxide substrates.

Pellicle materials for use at 157 nm must display sufficient transparency at this wavelength and adequate lifetimes to be useful. We blended a leading candidate fluoropolymer with silica nanoparticles to examine the effect on both the transparency and lifetime of the pellicle. It is anticipated that these composite materials may increase the lifetime by perhaps quenching reactive species and/or by dilution, without severely decreasing the 157-nm transmission. Particles surface-modified with fluorinated moieties are also investigated. The additives are introduced as stable nanoparticle dispersions to casting solutions of the fluoropolymers. The properties of these solutions, films, and the radiation-induced darkening rates are reported. The latter are reduced in proportion to the dilution of the polymer, but there is no evidence that the nanoparticles act as radical scavengers.

We previously [Van Steenwinckel and Lammers, Proc. SPIE 5039, 225-239 (2003)] showed that resist effects induced by "overbaking" (enhanced processing) can lead to major improvements in depth of focus and small-line printing capability through improved acid dose contrasts and a balanced optimization of acid diffusion in the presence of quencher. We show how a similar optimization can be attained by changes in resist formulation and how these changes can have a comparable positive impact on ultimate resolution and depth of focus. The formulation changes that are discussed follow the guidelines of the compact resist model that accounts for the overbake performance. The impact of the proposed resist reformulation on line-end shortening, line-edge roughness, and post exposure bake (PEB) sensitivity is discussed as well. The outcome of this work can be used to define which changes to the resist formulation offer the most beneficial improvements in the overall lithography process for printing resist features of 65 nm and smaller using ArF lithography and binary masks.

We discuss implications currently relevant to 300-mm resist processing in lithography. The large size of the wafers, and therefore the large volume of machine modules and media within these modules, demand tighter specifications and a careful reconsideration of design. We investigate a novel resist development process based on a new developer dispense nozzle. The critical dimension (CD) uniformity across the 300-mm wafer thus achieved is compared to a common process. Based on this, we are able to make a preliminary recommendation for the photoresist development technology required for future, high-end semiconductor device manufacturing. Resist thickness fluctuations around the edge area of a silicon wafer typically occur if high spin speeds are applied during the photoresist coating process. The 300-mm coating processes are particularly prone to the occurrence of such spin marks because the operating range of applicable spin speeds is lower in comparison to processes applied to wafers of lower diameters. In our example, we show how such local resist thickness fluctuations impact the product yield. Finally, a special type of micromasking defect is found to be particularly relevant to 300-mm lithography layers. This "doughnut-type" defect detracts the yield of the semiconductor product. The defect has a distinct physical appearance and is precipitated onto the wafer as a hollow resin sphere. This defect is not yet observed in our 200-mm reference process. The occurrence of the defect directly depends on the matching of the air flows entering and leaving the 300-mm coater cup. The mechanism of formation involves the presence of a photoresist-solvent aerosol.

In this work, the profile reconstruction capability of the Applied Materials NanoSEM 3-D critical dimension-scanning electron microscopy is evaluated. The system allows the fully automatic reconstruction of profiles by evaluating profiles measured at two different beam tilt angles. From two different tilt angles up to 15 deg, the reconstruction of sidewall profiles is possible in a quick and nondestructive way, even for negatively sloped profiles. The sensitivity of profile reconstruction, especially with respect to height and undercut detection in dependence of structure height and beam tilt angle, is discussed. We investigate precision and accuracy of profile reconstruction by comparing results from profile reconstruction to AFM and cross-sectional (X-SEM) results. We show that the sidewall angle can accurately be detected for 193-nm resist structures even for negatively sloped profiles. This enables the system for production use especially for monitoring of such profiles. As the profile reconstruction is done with nearly the same speed as regular top-down measurements, a clear advantage over existing monitoring techniques is obvious.

The aberration present in the lenses of exposure systems can cause placement errors to the images produced by alternating phase-shifting masks (PSMs). In reality, when the aberration signature varies from one lens to another, the magnitude of placement error also varies. It remains a question of how the alternating PSM should be designed, so that the image placement error, on average, can be minimized. To achieve this goal, we are interested in optimizing the phase width of an alternating PSM with a fixed critical dimension (CD). The constraint of the optimization is the mean of root mean square (rms) aberrations for a set of interest of exposure systems. To begin the analysis, the image placement error is expressed as a function of illumination, mask spectrum, and wave aberration. A Monte Carlo technique is then applied to produce random samples of wave aberration and image placement error. This analysis shows that a global minimum of mean image placement error is likely to occur at phase widths between 0.2[λ/numerical aperture (NA)] and 0.4(λ/NA). This is further confirmed by analytically considering the expected value of the square of the image placement error. The methodology of finding the optimal phase width is applicable to the design of all alternating PSMs.

A hybrid micro-nanofluidic channel network is developed on a silicon wafer for bioanalytical applications, such as separation, concentration, and fractionation. The nanochannel is formed on the silicon wafer using surface micromachining techniques, while the microchannel is fabricated on the poly-di-methyl-siloxane utilizing soft lithography techniques. Microfluidic networks not only support the very thin wall of the nanofluidic channel, but also provide appropriate gateways for the fluid/sample flow. The thickness of the microchannels is kept below 10 µm by changing the spin rate and time during photolithography. On the other hand, nanochannel thickness is varied between 100 and 200 nm by controlling the sputtering time of the sacrificial copper layer. Electrochemical wet etching is employed to release the thin layer of copper from the silicon dioxide shell. Our etching technique demonstrates significant advantages over other existing methods, such as wet chemical etching and reactive ion etching, including relatively fast etching rate, good selectivity, less safety and environmental concerns, less monitoring and control issues, and low cost. The dimensions of our microfluidic channels are measured using a profilometer, while the nanochannel thickness is confirmed by the atomic force microscopy and scanning electron microscopy images.

A double-sided micromachining process for silicon-based device fabrication is developed that allows the use of capacitively coupled reactive ion etching (RIE) equipment for high-aspect-ratio etching. The effects of the masking materials and RIE conditions are discussed. Based on the experimental results, a 1000-Å-thick Al film sufficiently protects the unexposed substrate while allowing the etching of a 350-µm-deep hole with an area of 3×3 mm² when etching with SF₆/CHF₃/O₂plasma. A 2000-µm-long and 100-µm-wide (with layers of Al/SiO₂/Si and thickness of 0.1/2.2/40 µm, respectively) cantilever beam is achieved. A silicon etch rate up to 2.5 to 2.8 µm/min is obtained and an anisotropy of A=0.5 (A=1–V/H, where V is the horizontal undercut and H is the etch depth) is obtained. The technique is developed mainly for bulk micromachining of silicon or composite silicon cantilever structures.

Inductive links are widely used for implanted biomedical applications, and with amplitude modulation their use can be expanded to the transmission of pulse trains for deep brain stimulation (DBS). Using a passive envelope detector and an integrated coil, pulse trains can be obtained with high fidelity across a load representing brain tissue. To improve the system design, a comparison is made between thin-film and electroplated coils for receiving signals. Using our inlaid electroplating process, the coil resistance can be greatly reduced, which translates to increased output levels at the load at a few megahertz. One feature of our inductive link is enhanced output from the electroplated coils at system resonance. Various rectification methods provide flexibility in obtaining desired system performance. With this technique, the implanted components for DBS could be reduced to an integrated coil and a few components.

Microelectromechanical systems (MEMS) are integrated microdevices or systems combining electrical and mechanical components that can sense, control, and actuate on the microscale and function individually or in arrays to generate effects on the macroscale. MEMS is one of the most promising areas in future computers and machinery, the next logical step in the silicon revolution. Fabricated using integrated circuit (IC)-compatible batch-processing technologies, the small size of MEMS opens a new line of exciting applications, including aerospace, automotive, biological, medical, fluidics, military, optics, and many other areas. We explore the potentialities of a high-resolution optical technique for characterizing MEMS microstructures. The method is based on the application of digital holography as a noncontact metrological tool for inspection and characterization of the microstructure surface morphology. The microstructures under investigation are homogeneous and bimorph polysilicon cantilevers; both structures exhibit an out-of-plane deformation owing to residual stress. The high sensitivity of the proposed method enables us to precisely determine the structure morphology and calculate the intrinsic stress and bending moment, in good agreement with an analytical model. Hence, the proposed technique can be exploited to assess the fabrication process and the functionality as well as the reliability of micromachined structures. Moreover, it is also used as a tuning tool for design and finite-element-based simulation software.

Rapid mixing is essential in biochemical analysis, drug delivery, biomedical inspection, as well as RNA and DNA synthesis and testing. A suitable evaluation method is required for mixing effect comparison in the stage of research and development of mixers. Until now, no satisfactory method was developed to quantitatively evaluate the mixing performance of micromixers. We describe an inspection algorithm that uses image gray level to evaluate the mixing effect in micromixers. Computer simulation results show that the mixed range and the mixing concentration can be recognized. Experiments are implemented in a self-designed two-channel micromixer, in which the diameter of the mixing chamber is 1 mm and height is 200 µm. Experimental results show that the proposed algorithm can clearly display the full-field mixing state in the mixer, so that it is helpful to evaluate the performance of a micromixer. In addition, a mixing efficiency over 90% is obtained within 0.35 s at a flux of 7.81 µl/s in the self-designed mixer. Since the purpose of rapid and uniform mixing is obviously achieved, this mixer can be applied to the field of biomedical diagnosis.

Bulk silicon wet etching can be used to fabricate silicon gratings. Wet etching depends on the anisotropic property of monocrystalline silicon. Blazed gratings for different spectral ranges can be fabricated by this method, and facets of grooves are formed by crystallographic planes of the monocrystalline silicon wafer. We develop a method to fabricate blazed gratings using deflecting crystal orientation (111) silicon wafers. The topographies of the samples are measured by SEM and atomic force microscopy (AFM), and the results indicate that the samples have grooves of good uniformity and facets of excellent optical quality.