Biomimetic Pattern Transfer

Biological systems routinely use phenols to construct complex materials with diverse functions. Typically, these phenolic materials are generated using oxidative enzymes to initiate a cascade of uncatalyzed reactions. We mimic these processes to micropattern films of the aminopolysaccharide chitosan. Specifically, we microfabricate silicon wafers to have gold patterns, cast a chitosan film onto the patterned wafers, and commence pattern transfer by polarizing the underlying gold surfaces to electrochemically initiate the phenol reaction cascade. The electrochemically initiated reactions lead to modification of the chitosan film's chemistry, structure, and fluorescence. Further, electrochemically initiated modification of the chitosan film is localized to the interfacial region between the film and the anode, with resolution in the lateral direction of at least 20 μm. These results demonstrate that electrochemical pattern transfer provides a promising new method for micropatterning flexible films.     

Reversible Programing of Soft Matter with Reconfigurable Mechanical Properties

Biology uses various cross‐linking mechanisms to tailor material properties, and this is inspiring technological efforts to couple independent cross‐linking mechanisms to create hydrogels with complex mechanical properties. Here, it is reported that a hydrogel formed from a single polysaccharide can be triggered to reversibly switch cross‐linking mechanisms and switch between elastic and viscoelastic properties. Specifically, the pH‐responsive self‐assembling aminopolysaccharide chitosan is used. Under acidic conditions, chitosan is polycationic and can be electrostatically cross‐linked by sodium dodecyl sulfate (SDS) micelles to confer viscoelastic and self‐healing properties. Under basic conditions, chitosan becomes neutral, the electrostatic SDS–chitosan interactions are no longer operative, and chitosan chains can self‐assemble to form crystalline network junctions that serve as strong physical cross‐links that confer elastic properties. Mechanical measurements performed in water demonstrate these different mechanical behaviors and the repeated pH‐induced switching between these behaviors. Printing of SDS micelles onto a neutral chitosan film allows the cross‐linking mechanisms to be spatially programed to confer anisotropic mechanical properties. The reversibility of these cross‐linking mechanisms allows the patterned films to be erased and reprogramed with reconfigured mechanical properties. Potentially, the ability to reversibly program hydrogel networks enables fabrication of the dynamically reconfigurable networks required for soft machines.     

Capillary Nanostamping with Spongy Mesoporous Silica Stamps

Classical microcontact printing involves transfer of molecules adsorbed on the outer surfaces of solid stamps to substrates to be patterned. Spongy mesoporous silica stamps are prepared that can be soaked with ink and that are topographically patterned with arrays of submicron contact elements. Multiple successive stamping steps can be carried out under ambient conditions without ink refilling. Lattices of fullerene nanoparticles with diameters in the 100 nm range are obtained by stamping C₆₀/toluene solutions on perfluorinated glass slides partially wetted by toluene. Stamping an ethanolic 1‐dodecanethiol solution onto gold‐coated glass slides yields arrays of submicron dots of adsorbed 1‐dodecantethiol molecules, even though macroscopic ethanol drops spread on gold. This outcome may be related to the pressure drop across the concave ink menisci at the mesopore openings on the stamp surface counteracting the van der Waals forces between ink and gold surface and/or to reduced wettability of the 1‐dodecanethiol dots themselves by ethanol. The chemical surface heterogeneity of gold‐coated glass slides functionalized with submicron 1‐dodecanethiol dots is evidenced by dewetting of molten polystyrene films eventually yielding ordered arrays of polystyrene nanoparticles.     

The effect of surface roughness on the radiative properties ofpatterned silicon wafers

The radiative properties of patterned silicon wafers have a major impact on the two critical issues in rapid thermal processing (RTP), namely wafer temperature uniformity and wafer temperature measurement. The surface topography variation of the die area caused by patterning and the roughness of the wafer backside can have a significant effect on the radiative properties, but these effects are not well characterized. We report measurements of room temperature reflectance of a memory die, logic die, and various multilayered wafer backsides. The surface roughness of the die areas and wafer backsides is characterized using atomic force microscopy (AFM). These data are subsequently used to assess the effectiveness of thin film optics in providing approximations for the radiative properties of patterned wafers for RTP applications     

Sub‐Micrometer Zeolite Films on Gold‐Coated Silicon Wafers with Single‐Crystal‐Like Dielectric Constant and Elastic Modulus

A low‐temperature synthesis coupled with mild activation produces zeolite films exhibiting low dielectric constant (low‐k) matching the theoretically predicted and experimentally measured values for single crystals. This synthesis and activation method allows for the fabrication of a device consisting of a b‐oriented film of the pure‐silica zeolite MFI (silicalite‐1) supported on a gold‐coated silicon wafer. The zeolite seeds are assembled by a manual assembly process and subjected to optimized secondary growth conditions that do not cause corrosion of the gold underlayer, while strongly promoting in‐plane growth. The traditional calcination process is replaced with a nonthermal photochemical activation to ensure preservation of an intact gold layer. The dielectric constant (k), obtained through measurement of electrical capacitance in a metal–insulator–metal configuration, highlights the ultralow k ≈ 1.7 of the synthetized films, which is among the lowest values reported for an MFI film. There is large improvement in elastic modulus of the film (E ≈ 54 GPa) over previous reports, potentially allowing for integration into silicon wafer processing technology.     

Accordion‐like Actuators of Multiple 3D Patterned Liquid Crystal Polymer Films

This work describes the fabrication, characterization, and modelling of liquid crystalline polymer network films with a multiple patterned 3D nematic director profile, a stimuli‐responsive material that exhibits complex mechanical actuation under change of temperature or pH. These films have a discrete alternating striped or checkerboard director profile in the plane, and a 90‐degree twist through the depth of the film. When actuated via heating, the striped films deform into accordion‐like folds, while the film patterned with a checkerboard microstructure buckles out‐of‐plane. Furthermore, striped films are fabricated so that they also deform into an accordion shaped fold, by a change of pH in an aqueous environment. Three‐dimensional finite element simulations and elasticity analysis provide insight into the dependence of shape evolution on director microstructure and the sample's aspect ratio.     

Development of planar microwave filters from double sided YBCO thin films on MgO substrates

Planar microwave filters patterned from thin film HTS on low loss substrates can offer waveguide-like performance at a fraction of the component mass and volume. First results are presented for a microstrip filter at 6.2 GHz fabricated from YBCO thin films deposited onto both sides of an MgO substrate. The measured results are compared with identically patterned filters assembled from gold and YBCO single sided films.<>     

Self‐Alignment of Patterned Wafers Using Capillary Forces at a Water–Air Interface

Capillary interactions at a water–air interface were used to align a two‐inch glass wafer to a three‐inch silicon wafer. Flat, smooth silica surfaces were patterned with gold millimeter‐scale borders enclosing micrometer‐scale features. The gold features were rendered hydrophobic through the use of self‐assembled monolayers, the silica was wetted with water, and the wafers were pressed together. The assembly snapped into alignment based upon the minimization of the curvature of the meniscus formed at the water–air interface. The accuracy of this alignment was better than one micrometer. Gravitational energy was used to systematically study the alignment force as a function of pattern parameters. These data can be modeled by interfacial energy theory. These experiments identify a clear set of conditions necessary for the use of this technique for high‐precision alignment.     

Biomimetic Approach to Confer Redox Activity to Thin Chitosan Films

Electron transfer in biology occurs with individual or pairs of electrons, and is often mediated by catechol/o‐quinone redox couples. Here, a biomimetic polysaccharide‐catecholic film is fabricated in two steps. First, the stimuli‐responsive polysaccharide chitosan is electrodeposited as a permeable film. Next, the chitosan‐coated electrode is immersed in a solution containing catechol and the electrode is biased to anodically‐oxidize the catechol. The oxidation products covalently graft to the chitosan films as evidenced by electrochemical quartz crystal microbalance (EQCM) studies. Cyclic voltammetry (CV) measurements demonstrate that the catechol‐modified chitosan films are redox‐active although they are non‐conducting and cannot directly transfer electrons to the underlying electrode. The catechol‐modified chitosan films serve as a localized source or sink of electrons that can be transferred to soluble mediators (e.g., ferrocene dimethanol and Ru(NH₃) ₆Cl₃). This electron source/sink is finite, can be depleted, but can be repeatedly regenerated by brief (30 s) electrochemical treatments. Further, the catechol‐modified chitosan films can i) amplify currents associated with the soluble mediators, ii) partially‐rectify these currents in either oxidative or reductive directions (depending on the mediator), and iii) switch between regenerated‐ON and depleted‐OFF states. Physical models are proposed to explain these novel redox properties and possible precedents from nature are discussed.     

Redox Capacitor to Establish Bio‐Device Redox‐Connectivity

Electronic devices process information and transduce energy with electrons, while biology performs such operations with ions and chemicals. To establish bio‐device connectivity, we fabricate a redox‐capacitor film from a polysaccharide (i.e., chitosan) and a redox‐active catechol. We report that these films are rapidly and repeatedly charged and discharged electrochemically via a redox‐cycling mechanism in which mediators shuttle electrons between the electrode and film (capacitance ≈ 40 F/g or 2.9 mF/cm²). Further, charging and discharging can be executed under bio‐relevant conditions. Enzymatic‐charging is achieved by electron‐transfer from glucose to the film via an NADPH‐mediated redox‐cycling mechanism. Discharging occurs by electron‐donation to O₂ to generate H₂O₂ that serves as substrate for peroxidase‐mediated biochemical reactions. Thus, these films offer the capability of inter‐converting electrochemical and biochemical inputs/outputs. Among potential applications, we anticipate that catechol–chitosan redox‐capacitor films could serve as circuit elements for molecular logic operations or for transducing bio‐based chemical energy into electricity.     

The effect of patterns on thermal stress during rapid thermalprocessing of silicon wafers

The presence of patterns can lead to temperature nonuniformity and undesirable levels of thermal stress in silicon wafers during rapid thermal processing (RTP). Plastic deformation of the wafer can lead to production problems such as photolithography overlay errors and degraded device performance. In this work, the transient temperature fields in patterned wafers are simulated using a detailed finite-element-based reactor transport model coupled with a thin film optics model for predicting the effect of patterns on the wafer radiative properties. The temperature distributions are then used to predict the stress fields in the wafer and the onset of plastic deformation. Results show that pattern-induced temperature nonuniformity can cause plastic deformation during RTP, and that the problem is exacerbated by single-side heating, increased processing temperature, and increased ramp rate. Pattern effects can be mitigated by stepping the die pattern out to the edge of the wafer or by altering the thin film stack on the wafer periphery to make the radiative properties across the wafer more uniform     

Cover Picture: Fabrication and Characterization of Three‐Dimensional Silver‐Coated Polymeric Microstructures (Adv. Funct. Mater. 13/2006)

Kuebler and co‐workers report on p. 1739 a method for preparing conductive and optically reflective silver‐coated polymeric microstructures having virtually any 3D form. Shown are reflection images of a silvered five‐layer simple‐cubic lattice having a period of 2.4 μm (background) and a macroscopic silvered polymer film (inset). To prepare metallopolymeric microstructures, 3D polymeric scaffolds are first created by multiphoton direct laser writing, then functionalized with gold particles, and metallized using nucleated electroless silver deposition. A method is reported for fabricating complex 3D silver‐coated polymeric microstructures. The approach is based on the creation of a crosslinked polymeric microscaffold via patterned multiphoton‐initiated polymerization followed by surface‐nucleated electroless deposition of silver. The conductivity and reflectivity of the resulting silver–polymer composites and the nanoscale morphology of the deposited silver are characterized. Sub‐micrometer thick layers of silver can be controllably deposited onto surfaces, including those of 3D microporous forms without occluding the interior of the structure. The approach is general for silver coating crosslinked polymeric structures based on acrylate, methacrylate, and epoxide resins and provides a new path to complex 3D micrometer‐scale devices with electronic, photonic, and electromechanical function.     

Bottom‐Up Synthesis of Biologically Active Multilayer Films Using Inkjet‐Printed Templates

As a non‐invasive, rapid prototyping technique, piezoelectric inkjet printing using the Dimatix Materials Printer (DMP) is incorporated to template 2D biologically active surfaces. In these studies, a bioinspired ink is synthesized and printed directly onto gold‐coated silicon nitride substrates and into polymer‐coated 96‐well plates. Once deposited on a surface, these patterns are reacted with varying concentrations of a model enzyme glucose oxidase in the presence of a silica precursor, monosilicic acid. The reaction mechanism and order of reactant products within and along the patterns are shown to directly affect the integrity and overall microstructure of the biologically active films. Using profilometry measurements and scanning electron microscopy, a biologically active platform is optimized without significantly compromising the activity of the enzyme. In fact, enzyme activity, constrained within a thin film, is reported for the first time over variable reaction parameters. When compared to the enzyme free in solution, the immobilized enzyme is 25.9% active, where nearly 100% of the activity is retained after repeated usage.     

Photoelectric Cooperative Induced Wetting on Aligned‐Nanopore Arrays for Liquid Reprography

Surface wettability as a response to the cooperation of different stimuli has been intensively studied and provides more advantages than as a response to a single stimulus. Recently, we demonstrated the patterned wettability transition from the Cassie to the Wenzel state on a superhydrophobic aligned‐ZnO‐nanorod array surface via a photoelectric cooperative wetting process. However, the specific aligned‐nanorod array structure of such devices is easily damaged due to their low mechanical strength and cannot sustain multiple transfer printing. Meanwhile, the patterned wetting process is not easily controlled due to the air‐permeable structure of adjacent nanorods. As a result, in practice, it is difficult to apply liquid reprography on such a nanostructure. Here, we demonstrate photoelectric cooperative induced patterned wetting on the superhydrophobic aligned‐nanopore array surface of TiO₂‐coated nanoporous AAO film, which has a high mechanical strength and excellent controllability. Liquid reprography is achieved through the patterned wetting process on the superhydrophobic aligned‐nanopore array surface, which is a new progression in liquid reprography, and is promising for gearing up the application of photoelectric cooperative liquid reprography.     

Full-field wafer level thin film stress measurement by phase-stepping shadow Moire/spl acute/

A wafer topography measurement system has been designed and demonstrated based on shadow Moire/spl acute/. Three-step phase-stepping and phase unwarping techniques are also incorporated to enhance the system resolution. Wafer curvatures or bows can be achieved by analyzing the Moire/spl acute/ fringe patterns and film stress can be obtained subsequently by transforming this wafer curvature using a conversion equation such as Stoney's formula. Wafer bow of plasma enhanced chemical vapor deposition nitride and oxide coated wafers are measured by this shadow Moire/spl acute/ system and are subsequently verified by the KLA-Tencor FLX 2320 system. The discrepancy between both bow measurements is within 2/spl mu/m, regardless of the magnitude of the measurement. Therefore, this system is especially suitable for stress characterization of thicker, stiffer, or highly stressed films. In comparison with the traditional laser scanning method, wafer curvature obtained by shadow Moire/spl acute/ is based on full-field information and it would have a better accuracy. By integrating this system with a more accurate wafer curvature to film stress conversion formula, this system should also provide a better film stress characterization.     

Prestressed Ceramic Coatings for Enhanced Reliability of Silicon Wafer Fracture Strength

The objective of this paper is to investigate thin, solid, prestressed ceramic films as a means of enhancing the reliability of silicon semiconductor wafers stressed in bending. To characterize the effect of thin films on strength, one-micrometer ceramic films were deposited on wafers using plasma-enhanced chemical-vapor deposition. The modulus of rupture (MOR) of the coated wafers was determined from four-point bend testing of coated samples. Adhesion testing of the coated wafers primarily showed cohesive rather than adhesive failure. A series of residual stresses was introduced into the coating-silicon interface and the MOR was determined. The results showed that for a thin brittle coating (1 mum) on a silicon wafer (635 mum), the minimal shear stress at the surface led to dominance of the residual stress over intrinsic coating strength as the critical parameter affecting failure. A correlation between MOR and residual stress was established.     

Fabrication of ultrathin SOI by SIMOX water bonding (SWB)

Ultrathin silicon-on-insulator (SOI) layers of separation by implantation of oxygen (SIMOX) wafers have been transferred onto thermally oxidized silicon wafers by wafer bonding technology. Due to the technical availability and the complementary nature of SIMOX and wafer bonding approaches, SIMOX wafer bonding (SWB) solves some of the respective major difficulties faced by both SIMOX and wafer bonding for device quality ultrathin SOI mass production: the preparation of adequate buried oxide (including its interfaces) in SIMOX and the uniformly thinning one of the bonded wafers to less than 0.1 μm in wafer bonding. The effect of positive charges in the oxide on bondability of ultrathin SOI films and possible applications of SWB will also be outlined.     

Chitosan Hydrogel‐Capped Porous SiO2 as a pH Responsive Nano‐Valve for Triggered Release of Insulin

A pH responsive, chitosan‐based hydrogel film is used to cap the pores of a porous SiO₂ layer. The porous SiO₂ layer is prepared by thermal oxidation of an electrochemically etched Si wafer, and the hydrogel film is prepared by reaction of chitosan with glycidoxypropyltrimethoxysilane (GPTMS). Optical reflectivity spectroscopy and scanning electron microscopy (SEM) confirm that the bio‐polymer only partially infiltrates the porous SiO₂ film, generating a double layer structure. The optical reflectivity spectrum displays Fabry–Pérot interference fringes characteristic of a double layer, which is characterized using reflective interferometric Fourier transform spectroscopy (RIFTS). Monitoring the position of the RIFTS peak corresponding to the hydrogel layer allows direct, real‐time observation of the reversible volume phase transition of the hydrogel upon cycling of pH in the range 6.0–7.4. The swelling ratio and response time are controlled by the relative amount of GPTMS in the hydrogel. The pH‐dependent volume phase transition can be used to release insulin trapped in the porous SiO₂ layer underneath the hydrogel film. At pH 7.4, the gel in the top layer effectively blocks insulin release, while at pH 6.0 insulin penetrates the swollen hydrogel layer, resulting in a steady release into solution.     

Rapid thermal processing of silicon wafers with emissivity patterns

Fabrication of devices and circuits on silicon wafers creates patterns in optical properties, particularly the thermal emissivity and absorptivity, that lead to temperature nonuniformity during rapid thermal processing (RTP) by infrared heating methods. The work reported in this paper compares the effect of emissivity test patterns on wafers heated by two RTP methods: (1) a steadystate furnace or (2) arrays of incandescent lamps. Method I was found to yield reduced temperature variability, attributable to smaller temperature differences between the wafer and heat source. The temperature was determined by monitoring test processes involving either the device side or the reverse side of the wafer. These include electrical activiation of implanted dopants after rapid thermal annealing (RTA) or growth of oxide films by rapid thermal oxidation (RTO). Temperature variation data are compared with a model of radiant heating of patterned wafers in RTP systems.     

Synthesis,Properties, and Photopolymerization of Liquid‐Crystalline Oxetanes: Application in Transflective Liquid‐Crystal Displays

Mixtures of liquid‐crystalline di‐oxetanes and mono‐oxetanes are made for the purpose of making birefringent films by photopolymerization. The composition of a di‐oxetane mixture that forms spin‐coated films of planarly aligned nematic monomers is reported. These films are photopolymerized in air. The molecular order of the monomers can be changed on the microscale to form thin films with alternating birefringent and isotropic parts by using a combination of photopolymerization and heating. The interface observed between the birefringent and isotropic 10 μm × 10 μm domains is very sharp and the films show hardly any surface corrugation. In addition, the polymerized films are thermally stable, making them very suitable for use as patterned thin‐film retarders in high‐performance transflective liquid‐crystal displays (LCDs) which satisfy customer demand for displays that are brighter and thinner and that deliver better optical performance than conventional LCDs with an external non‐patterned retarder.