This PDF file contains the editorial “Editorial: Journal of Micro/Nano Lithography, MEMS, and MOEMS” for JM3 Vol. 5 Issue 01

This PDF file contains the editorial “Editorial: Nanopatterning: Commercial and Technological Promise” for JM3 Vol. 5 Issue 01

Pattern placement error (or pattern distortion) caused by different thermal expansions between templates and substrates in nanoimprint lithography is experimentally investigated. Using fabricated nanorulers, placement errors are quantitatively measured. Our results prove that nanoimprint lithography (NIL) with a heat cycle does have considerable error of pattern placement. But that is not the case with imprints at room temperature. This indicates that low-temperature or room-temperature imprints should be effective solutions.

We report an ultrasonic nanoimprint lithography (U-NIL) method that can overcome the drawbacks of energy consumption and long process times that occur in conventional nanoimprint lithography (NIL) methods. Instead of using heaters in conventional NIL, the proposed U-NIL employs an ultrasonic source located on the top of the mold to generate high-frequency vibrations, causing the increase of temperature to soften and melt the thermoplastic polymer. The ultrasonic source is induced by the transducer, consisting of a number of piezoelectric ceramic disks sandwiched between two aluminum metal blocks. Since the ultrasonic source provides a vibration with tens of kilohertz frequencies, the temperature of the polymer can increase rapidly. Consequently, the process time is significantly reduced. The experimental results demonstrate that vibratory energy could be concentrated in transferring the topography of a mold's surface into the polymer. In addition, we found that under appropriate conditions, such as ultrasonic time, imprinted pressure/force, frequency, and amplitude of vibration, the feature on the mold can replicate into the polymer film in a few seconds. We conclude that the proposed U-NIL process has the potential to become a novel nanoimprinting method with high productivity, energy efficiency, and low cost.

Nanoimprinting has been recognized as a highly potential method of volume production for nanoscale devices. In the nanoimprinting process, the filling process of the mold cavity plays a key role in determining the productivity of the nanoimprinting process and the quality of the final imprint product. A defective filling of the mold affects the uniformity, precision, and throughput of the imprint. The mold filling is subjected to the applied imprinting pressure and temperature, repetitive mold use, mold sticking, or factors regarding mold features. It involves physical contact between the mold and the polymer layer on the substrate surface; thus, how the polymer fills up the cavity is of major interest and is vital in pattern transfer. The proposed study employs a finite element model for the single mold cavity to simulate the nanoimprint process. The mold is assumed to be a linear elastic body and the polymer preheated above its glass transition temperature is considered to be nonlinear elastic, described by the Mooney-Rivlin model. The numerical model is able to predict the mold filling at any nanoimprint stage and the mold cavity with various aspect ratios. To study the effects of pattern density and contact friction existing between the mold and polymer during the mold filling of the nanoimprinting process, an imprint mold with mixed pattern density is simulated and a sensitivity analysis of a contact friction coefficient on the mold filling is performed. Both the cavity feature and pattern density have significant effects on mold filling of the nanoimprinting process, while the contact friction coefficient has a mild effect. The obtained results support the development of a process recipe and automatic large-scale industrial production for nanoimprinting.

Hot embossing and injection molding belong to the established plastic molding processes in microengineering. Based on experimental findings, a variety of microstructures have been replicated using these processes. However, with increasing requirements regarding the embossing surface, and the simultaneous decrease of the structure size down into the nanorange, increasing know-how is needed to adapt hot embossing to industrial standards. To reach this objective, a German-Canadian cooperation project has been launched to study hot embossing theoretically by process simulation and experimentally. The present publication reports on the proceeding and present first results.

We develop maskless fabrication methods using sputter etching with Ga-focused ion beams (FIBs) to obtain nanogap electrodes with high reproducibility. This method is based on in situ monitoring of etching steps by measuring current through patterned electrode films. The etching steps are terminated electrically at a predetermined current level. In the present experiment, 30-keV Ga FIBs with beam size of ~12 nm is irradiated, and the effect of film structures and monitoring current is investigated to obtain reliable fabrication methods. We find that electrode gaps much narrower than the beam size can be reproducibly fabricated. The controllability of the fabrication steps is significantly improved by using triple-layered films consisting of a thin Ti top, an Au electrode, and a bottom Ti adhesion layer. The minimum gap width achieved is ~3 nm, and the fabrication yield reached ~90% for ~3- to 6-nm-wide gaps. Most of the fabricated nanogap electrodes show high insulating resistance ranging from 1 GΩ to 1 TΩ.

We introduce simple double-casting replication methods for high-aspect-ratio microstructures fabricated by deep x-ray lithography using intermediate molds of soft materials. Two types of soft material are investigated. The ability to fabricate polymethylsiloxane (PDMS) molds with well-type structures with aspect ratios up to 35:1 is demonstrated for structure densities below 50%, and the reproduction from this mold of pillar-type polymethyl methacrylate (PMMA) structures with aspect ratios of 20:1 is achieved. Polyvinyl alcohol (PVA), a water soluble polymer, is also tested as a sacrificial intermediate mold and successfully used for the replication of structures with aspect ratios up to 5:1. Double-casting replication methods are described and discussed for their potential improvement.

Self-assembled InAs quantum dots (QDs) are grown on two types of templates by molecular beam epitaxy (MBE). A nanohole template is prepared by first growing InAs QDs on a GaAs substrate by a standard MBE process, then capping the QDs with a thin lattice-mismatched layer of GaAs. Subsequent annealing of the nanohole template results in a stripe template. We are able to transfer the patterns of self-assembled QDs onto these two types of templates: regrowth on the nanohole template results in uniform QDs situated in the nanoholes, while regrowth on the stripe template results in chains of QDs aligned along the stripes. Our results imply that self-assembled QDs can be grown onto in-situ prepared patterned substrates, with limited degree of randomness.

A technique to create nanopatterns on hard-to-machine bulk silicon carbide (SiC) with a laser beam is presented. A monolayer of silica (SiO₂) spheres of 1.76-µm and 640-nm diameter are deposited on the SiC substrate and then irradiated with an Nd:YAG laser of 355 and 532 nm. The principle of optical near-field enhancement between the spheres and substrate when irradiated by a laser beam is used for obtaining the nanofeatures. The features are then characterized using a scanning electron microscope and an atomic force microscope. The diameter of the features thus obtained is around 150 to 450 nm and the depths vary from 70 to 220 nm.

Perforated polymer membranes are fabricated using nanoimprint lithography and a sacrificial layer technique. The membranes with micrometer-sized pores are partially released from the substrate by locally dissolving the underlying layer. Thus, large arrays with interconnected pores can be fabricated. This process can be applied to a wide selection of polymeric materials, and has the potential to be extended to submicrostructures.

We aim to explore the nanostructuring potential of a highly focused pencil of ions. We show that focused ion beam technology (FIB) is capable of overcoming some basic limitations of current nanofabrication techniques and allowing innovative patterning schemes for nanoscience. In this work, we first detail the very high resolution FIB instrument developed specifically to meet nanofabrication requirements. Then we introduce and illustrate some new patterning schemes for next-generation FIB processing. These patterning schemes are: 1. nanoengraving of membranes as a template for nanopores and nanomask fabrication; 2. local defect injection for magnetic thin film direct patterning; 3. function of graphite substrates to prepare 2-D organized arrays of clusters; and 5. selective epitaxy of III-V semiconductors on FIB patterned surfaces. Finally, we show that FIB patterning allows "bottom-up" or "organization" processes.

Laser interference lithography is reviewed as an adequate nanopatterning tool for devices with periodic structures. The structure size may practically be chosen in the range between 100 nm and 10.0 µm by adjusting the angle of incidence θ of the incoming laser beam. The exposure method is fast, inexpensive, and applicable for large areas. The method may be used to fabricate microsieves, shadow masks, calibration grids, and photonic crystals.

The rapid development of optical processing in optical circuitry (optical computing) is spawning the need for fully optical nonvolatile memory schemes. Nonvolatile memory schemes typically involve diffraction-limited writing techniques like those found in high-density DVDs and CDs. This need limits their memory density by placing a lower boundary on bit size, which is determined by the wavelength of light used in writing the bit. Using near-field optics can provide a revolutionary approach to optical storage, breaking the diffraction limit by a factor of 10, allowing for significantly enhanced data densities. In this approach, the high-frequency components of the near field are used "to write" information in nanostructured composite ultra-thin films with feature sizes of 200 nm. Fully optical readouts are possible at data densities approaching 100 MBits/mm². Because of the all-optical nature of this technique, this method can be fully compatible with the next generation of optical computing platforms. It is anticipated that through this research program, this approach will open new vistas in the creation of subwavelength optical circuits, optical processing, and optical data storage.

Microsystems and nanosystems hold the promise of new and much more effective approaches to both commercial and national security applications. The patenting rate in nanotechnology is exploding, underscoring its commercial and scientific potential. Yet how much of this effort is focused on nanopatterning or a top-down approach to nanofabrication? Nanopatterning in semiconductor microfabrication has already furthered Moore's law, facilitating the transistor as that medium's unit cell. Yet the search for a unit cell for the other two small technical markets (microsystems and the more broadbased nanotechnology) has proven much more elusive. Do nanopatterning advances hold the key to these technology bases finally obtaining a unit cell? We explore the intellectual property base of nanopatterning and how it pertains to semiconductor microfabrication, microsystems, and nanotechnology.

The primary measure of process quality in nanoimprint lithography (NIL) is the fidelity of pattern transfer, comparing the dimensions of the imprinted pattern to those of the mold. Routine production of nanoscale patterns will require new metrologies capable of nondestructive dimensional measurements of both the mold and the pattern with subnanometer precision. In this work, a rapid, nondestructive technique termed critical dimension small angle x-ray scattering (CD-SAXS) is used to measure the cross sectional shape of both a pattern master, or mold, and the resulting imprinted films. CD-SAXS data are used to extract periodicity as well as pattern height, width, and sidewall angles. Films of varying materials are molded by thermal embossed NIL at temperatures both near and far from the bulk glass transition (T_G). The polymer systems include a photoresist and two homopolymers. Our results indicate that molding at low temperatures (T-T_G<40°C) produces small-aspect-ratio patterns that maintain periodicity to within a single nanometer, but feature large sidewall angles. While the observed pattern height does not reach that of the mold until very large imprinting temperatures (T-T_G≈70°C), the pattern width of the mold is accurately transferred for T-T_G>30°C.

We introduce a combined e-beam and x-ray lithographic method to fabricate microzone plates (MZP) on free-standing silicon nitride films. An automatic design program is developed to draw the complex layout of MZP with very smooth boundaries. A gold MZP master mask with a minimum ring width of 250 nm is fabricated by e-beam lithography. The master mask is replicated using x-ray lithography (XRL) and nickel electroplating to obtain the final MZP. The combined lithographic technique produces a MZP with a pattern aspect ratio of 4.4:1.

Resonating cantilever-based microbiochemical sensors usually consist of a vibration actuating system and a vibration detecting system, which complicates the sensor design and fabrication. We present a new idea for testing the cantilever's vibration without an exclusive detector. The amplitude of vibration can be detected by measuring the third harmonic of the actuating current, and the cantilever's resonance frequency can be consequently obtained. The change of the resonance frequency provides information about the biochemical reaction, which alters the mass of the cantilever and its natural frequency. A system model based on the idea is established and an approximate solution is given. The relation between the vibration state and the third harmonic is discussed, and a corresponding simulation is performed. The system sensitivity is evaluated. Both theoretical and simulation results show that the amplitude of the third harmonic of the actuating current can be used as a criterion to determine the cantilever's vibration state. The idea promises a simpler mechanical structure, thus a cheaper sensor. Sensors based on this idea would also be robust to atrocious environments because the material of the cantilever can be chosen from a wide range.

We present an optical laser-based method to visualize replication of microperiodic profile structures in polymers. Diffraction efficiency of diffraction gratings (originally produced in silicon, quartz glass, and in replicated polymer substrates) is measured experimentally and simulated using linear dimensions of gratings, or their replica defined by atomic force microscopy (AFM). Diffraction efficiency of the periodic structure is used to test the surface relief formation during the combined ion etching of crystalline Si (100) and replication of this structure using UV light hardening and hot embossing. The main experimental results are compared with computer simulations where the standard program (PCGrate-SX6.0) is employed. Angular dependence of relative efficiency (RE) appears a versatile tool to analyze the process of origination and replication of diffraction grating in a micrometer range. Both the shape of the grating and linear dimensions can be reconstructed by comparing experimental values of relative efficiency with the simulation results. Examples of hot embossing of different periods of diffraction gratings to the Al/polymethyl methacrylate (PMMA) on polyethylene terephthalate (PET) illustrate vitality of such approach.

This study presents an experimental method to determine the resist parameters at the origin of a general blurring of a projected aerial image. The resist model includes the effects of diffusion in the horizontal plane and image blur that originates from a stochastic variation of the focus parameter. We restrict ourselves to the important case of linear models, where the effects of resist processing and focus noise are described by a convolution operation. These types of models are also known as diffused aerial image models. The used mathematical framework is the so-called extended Nijboer-Zernike (ENZ) theory, which allows us to obtain analytical results. The experimental procedure to extract the model parameters is demonstrated for several 193-nm resists under various conditions of postexposure baking temperatures and baking times. The advantage of our approach is a clear separation between the optical parameters, such as feature size, projection lens aberrations, and the illuminator setting on one hand, and process parameters introducing blur on the other.

We develop a new method to improve alignment accuracy. Specially designed two-step wafer alignment marks are evaluated for laser scan alignment to enhance wafer alignment. Wafer alignment accuracy data shows that it is enough to execute wafer alignment without failures in spite of the coarse chemical mechanical polishing (CMP) process. Effective detection of a wafer alignment mark leads to enhancement of product quality and could reduce the process time of the lithography step. It is a useful technique for thick metalization films and the planarity of high topography. A coarse metrology wafer with a CMP process can degrade the wafer alignment process. We try deep trench profiles and the clear structures of modified marks using additional photomask processing.

In immersion lithography at 193 nm, water is inserted between a resist-coated wafer and the final lens element to improve resolution and depth of focus. Experiments have shown that some chemicals in the resist, particularly the photoacid generators, are soluble and therefore will leach out of the resist layer when exposed to water. Diffusion of this contamination across the lens-wafer gap may, over time, build up on the lens and therefore degrade the performance of the tool. We present models that describe the transport of contaminants in the under-lens region of an immersion tool. The mass flux of contaminants onto the lens is quantified for a reasonable range of parameters under various 2-D steady-state and transient flow conditions. A critical mass flux is estimated to provide a context for interpreting these results; the critical mass flux is defined as the level of mass flux that might, over a period of one year, result in a layer of contamination that is sufficiently thick so as to affect the optical transmission of the system.

In immersion lithography, the air gap that currently exists between the last lens element of the exposure system and the wafer is filled with a liquid that more closely matches the refractive index of the lens. There is a possibility that air bubbles, which represent a refractive index discontinuity, may be present in the liquid within the active exposure region and cause errors in imaging. One potential source of bubble generation is related to the flow of liquid over previously patterned features, or topography, during scanning or filling. This microscale entrainment mechanism is investigated experimentally and analyzed using computational fluid dynamics (CFD) modeling. The contact angle is a critical parameter that governs the behavior of the contact line and therefore the entrainment of air due to topography; the same topography on a hydrophobic surface is more likely to trap air than on a hydrophilic one. The contact angle can be a strong function of the flow velocity; a hydrophilic surface can exhibit hydrophobic behavior when the velocity of the free surface becomes large. Therefore, the contact angle was experimentally measured under static and dynamic conditions for a number of different surfaces, including resist-coated wafers. Finally, the flow of liquid across 500-nm-deep, straight-sidewall spaces of varying width was examined using both experimental visualization and CFD modeling. No air entrainment was observed or predicted over the velocity and contact angle conditions that are relevant to immersion lithography. The sharp-edged features studied here represent an extreme topography relative to the smoother features that are expected on a planarized wafer; therefore, it is not likely that the microscale entrainment of bubbles due to flow over wafer-level topography will be a serious problem in immersion lithography systems.