Inverse‐Woodpile Photonic Band Gap Crystals with a Cubic Diamond‐like Structure Made from Single‐Crystalline Silicon

Three dimensional photonic band gap crystals with a cubic diamond‐like symmetry are fabricated. These so‐called inverse‐woodpile nanostructures consist of two perpendicular sets of pores in single‐crystal silicon wafers and are made by means of complementary metal oxide–semiconductor (CMOS)‐compatible methods. Both sets of pores have high aspect ratios and are made by deep reactive‐ion etching. The mask for the first set of pores is defined in chromium by means of deep UV scan‐and‐step technology. The mask for the second set of pores is patterned using an ion beam and carefully placed at an angle of 90° with an alignment precision of better than 30 nm. Crystals are made with pore radii between 135–186 nm with lattice parameters a = 686 and c = 488 nm such that a/c = √2; hence the structure is cubic. The crystals are characterized using scanning electron microscopy and X‐ray diffraction. By milling away slices of crystal, the pores are analyzed in detail in both directions regarding depth, radius, tapering, shape, and alignment. Using optical reflectivity it is demonstrated that the crystals have broad reflectivity peaks in the near‐infrared frequency range, which includes the telecommunication range. The strong reflectivity confirms the high quality of the photonic crystals. Furthermore the width of the reflectivity peaks agrees well with gaps in calculated photonic band structures.     

Simple Holographic Patterning for High‐Aspect‐Ratio Three‐Dimensional Nanostructures with Large Coverage Area

Using the vertical standing wave phenomena commonly regarded as a deterrent in holographic lithography, multifaceted three‐dimensional (3D) nanostructures are fabricated on polymeric photoresist materials using a simple two‐beam interferometer. Large‐area 3D nanostructures with high aspect ratios (greater than 10) are readily produced using this methodology, including grating, pillar and pore patterns. Furthermore, manipulation of the lithography process conditions results in unique sidewall profiles of the nanostructures. Such 3D holographic control even produces highly porous polymer membranes composed of 3D interconnected pore networks, which resembles the 3D photonic crystal compound nanostructures that were previously attainable only with limited pattern coverage area using complex multibeam holographic lithography processes. Such well‐tailored high‐aspect‐ratio 3D nanostructures with large pattern coverage area further enable the fabrication of novel nanostructures for functionalized materials via various additive and subtractive pattern transfer techniques such as etching, deposition, and molding. In particular, direct molding followed by thermal decomposition process leads to the synthesis of hierarchical titanium oxide nanostructures of tunable 3D geometry, which would be of great significance in applications of photonic crystals, photovoltaic solar cells, and photocatalyst in water decontamination.     

Controlling resist thickness and etch depth for fabrication of 3D structures in electron-beam grayscale lithography

In many applications such as optoelectronic devices, three-dimensional (3D) structures are required. Examples include photonic band gap (PBG) crystals, diffractive optical elements, blazed gratings, MEMS, NEMS, etc. It is known that the performance characteristics of such structures are highly sensitive to their dimensional fidelity. Therefore, it is essential to have a fabrication process by which such 3D structures can be realized with high dimensional accuracy. In this paper, practical methods to control thickness of the remaining resist and etch depth, which may be employed for fabrication of such 3D structures using grayscale electron-beam lithography, are described. Through experiments, explicit control of the remaining resist thickness and etch depth at the resolution of 20 nm for the feature sizes of 0.5 μm and 1 μm has been successfully demonstrated. Also, the 1:1 ratio of silicon to resist etching rates was achieved for transferring the remaining resist profile onto the silicon substrate.     

Three‐Dimensional Nanostructures for Photonics

Recent progress in direct laser writing of three‐dimensional (3D) polymer nanostructures for photonics is reviewed. This technology has reached a level of maturity at which it can be considered as the 3D analogue of planar electron‐beam lithography. Combined with atomic‐layer deposition and/or chemical‐vapor deposition of dielectrics—the 3D analogues of planar evaporation technologies, the 3D polymer templates can be converted or inverted into 3D high‐refractive‐index‐contrast nanostructures. Examples discussed in this review include positive and inverse 3D silicon‐based woodpile photonic crystals possessing complete photonic bandgaps, novel optical resonator designs within these structures, 3D chiral photonic crystals for polarization‐state manipulation, and 3D icosahedral photonic quasicrystals. The latter represent a particularly complex 3D nanostructure.     

Dimer‐Based Three‐Dimensional Photonic Crystals

The self‐assembly of polystyrene dimer‐ and spherocylinder‐shaped colloids is achieved via controlled drying on glass and silicon substrates. 3D monoclinic colloidal crystal structures are determined from scanning electron microscopy images of sections prepared using focused ion‐beam (FIB) milling. Full photonic bandgaps between the eighth and ninth bands are found for a systematic range of colloidal dimer shapes explored with respect to the degree of constituent lobe fusion and radius ratio. The pseudogap between bands 2 and 3 for spherocylinder‐based monoclinic crystals is also probed using normal incidence reflection spectroscopy.     

Exploiting Nanoroughness on Holographically Patterned Three‐Dimensional Photonic Crystals

The fabrication of three‐dimensional (3D) diamond photonic crystals with controllable nanoroughness (≤120 nm) on the surface from epoxy‐functionalized cyclohexyl polyhedral oligomeric silsesquioxanes (POSS) is reported. The nanoroughness is generated on the 3D network due to microphase separation of the polymer chain segments in a nonsolvent during the rinsing step in holographic lithography process. The degree of roughness can be tuned by the crosslinking density of the polymer network, which is dependent on the loading of photoacid generators, the exposure dosage, and the choice of developer and rinsing solvent. Because the nanoroughness size is small, it does not affect the photonic band gap position of the photonic crystal in the infrared region. The combination of periodic microstructure and nanoroughness, however, offers new opportunities to realize superhydrophobicity and enhanced dye adsorption in addition to the photon management in the 3D photonic crystal.     

Vapor Phase Metal‐Assisted Chemical Etching of Silicon

This work introduces and explores vapor phase metal‐assisted chemical etching (VP‐MaCE) of silicon as a method to bypass some of the challenges found in traditional liquid phase metal‐assisted chemical etching (LP‐MaCE). Average etch rates for Ag, Au, and Pd/Au catalysts are established at 31, 70, and 96 nm/min respectively, and the relationship between etch rate and substrate temperature is examined experimentally. Just as with LP‐MaCE, 3D catalyst motion is maintained and three‐dimensional structures are fabricated with nanoparticle‐ and lithography‐patterned catalysts. VP‐MaCE produces less microporous silicon compared with LP‐MaCE and the diffusion/reduction distance of Ag⁺ ions is significantly reduced. This process sacrifices etch rate for increased etch uniformity and lower stiction for applications in micro‐electromechanical systems (MEMS) processing.     

A Three‐Dimensional and Sensitive Bioassay Based on Nanostructured Quartz Combined with Viral Nanoparticles

An effective mask‐free method for fabricating high‐aspect‐ratio pillarlike nanostructures over a large area of a quartz surface via a simple O₂ and CF₄ two‐step reactive ion etching (RIE) procedure is developed. The nanostructured quartz surfaces are successfully combined with the engineered viral particles derived from hepatitis B virus capsid, yielding a novel 3D assay system with attomolar sensitivity, which has great potential for use in sensitive and early detection of various disease markers.     

Rapid and Large‐Scale Fabrication of Full Color Woodpile Photonic Crystals via Interference from a Conformal Multilevel Phase Mask

The practical use of photonic crystals with structural colors requires technology capable of rapidly producing large‐area, three‐dimensional (3D) periodic nanostructures. Until now, the fabrication of 3D photonic crystals has relied mainly on additive manufacturing and colloidal self‐assembly. These technologies have provided a useful academic platform based on precisely controlled 3D periodicity but have not evolved into mass production technology. Here, optical lithography for the rapid fabrication of large‐area 3D photonic crystals with structural colors is introduced. The key strategy is to incorporate two orthogonal line gratings (periodicity: 300 nm) made of an elastomer to create a conformal multilevel phase mask. When the mask is irradiated with a 355 nm laser, the five beam interference is established in the proximity region. The interlayer thickness between the two orthogonal line gratings controls the phase difference, which is closely related to the symmetry of the resulting 3D interference pattern. The interlayer thickness is designed to produce a woodpile structure with a planar periodicity of 300 nm and a vertical periodicity of 716 nm. The pattern area of the woodpile photonic crystal is expanded to 1 in². Red, green, and blue colors are experimentally realized by controlling the vertical shrinkage of the photoresist.     

Single‐Crystal Germanium Growth on Amorphous Silicon

Growing single‐crystal semiconductors directly on an amorphous substrate without epitaxy or wafer bonding has long been a significant fundamental challenge in materials science. Such technology is especially important for semiconductor devices that require cost‐effective, high‐throughput fabrication, including thin‐film solar cells and transistors on glass substrates as well as large‐scale active photonic circuits on Si using back‐end‐of‐line CMOS technology. This work demonstrates a CMOS‐compatible method of fabricating high‐quality germanium single crystals on amorphous silicon at low temperatures of <450 °C. Grain orientation selection by geometric confinement of polycrystalline germanium films selectively grown on amorphous silicon by chemical vapor deposition is presented, where the confinement selects the fast‐growing grains for extended growth and eventually leads to single crystalline material. Germanium crystals grown using this method exhibit (110) texture and twin‐mediated growth. A model of confined growth is developed to predict the optimal confining channel dimensions for consistent, single‐crystal growth. Germanium films grown from one‐dimensional confinement exhibit a 200% grain size increase at 1 μm film thickness compared to unconfined films, while 2D confinement growth achieved single crystal Ge. The area of single crystalline Ge on amorphous layers is only limited by the growth time. Significant enhancement in room temperature photoluminescence and reduction in residual carrier density have been achieved using confined growth, demonstrating excellent optoelectronic properties. This growth method is readily extensible to any materials system capable of selective non‐epitaxial deposition, thus allowing for the fabrication of devices from high‐quality single crystal material when only an amorphous substrate is available.     

Artificial Defect Engineering in Three‐Dimensional Colloidal Photonic Crystals

Artificial defect engineering in 3D colloidal photonic crystals is of paramount importance in terms of device applications. Over the past few years, we have carried out a great deal of research on introducing artificial defects, including point, line, and planar defects, in 3D colloidal photonic crystals by using “bottom‐up” self‐assembly in combination with “top‐down” micromachining techniques. In this Feature Article, we summarize our research results regarding the engineering of artificial defects in self‐assembled 3D photonic crystals, along with other important research breakthroughs in the literature. The significant advancements in the engineering of defects as reviewed here together with the encouraging reports on the fabrication of perfect colloidal crystals without unwanted defects will collectively lead to technological applications of self‐assembled 3D photonic crystals in the near future.     

3D Free‐Form Patterning of Silicon by Ion Implantation,Silicon Deposition,and Selective Silicon Etching

A method for additive layer‐by‐layer fabrication of arbitrarily shaped 3D silicon micro‐ and nanostructures is reported. The fabrication is based on alternating steps of chemical vapor deposition of silicon and local implantation of gallium ions by focused ion beam (FIB) writing. In a final step, the defined 3D structures are formed by etching the silicon in potassium hydroxide (KOH), in which the local ion implantation provides the etching selectivity. The method is demonstrated by fabricating 3D structures made of two and three silicon layers, including suspended beams that are 40 nm thick, 500 nm wide, and 4 μm long, and patterned lines that are 33 nm wide.     

Hydrothermal Growth of Manganese Dioxide into Three‐Dimensional Hierarchical Nanoarchitectures

Novel three‐dimensional (3D) hierarchical nanoarchitectures of ?‐MnO₂ have been synthesized by a simple chemical route without the addition of any surfactants or organic templates. The self‐organized crystals consist of a major ?‐MnO₂ dipyramidal single crystal axis and six secondary branches, which are arrays of single‐crystal ?‐MnO₂ nanobelts. The growth directions of the nanobelts are perpendicular to the central dipyramidal axis, which shows sixfold symmetry. The shape of the ?‐MnO₂ assembly can be controlled by the reaction temperature. The morphology of ?‐MnO₂ changes from a six‐branched star‐like shape to a hexagonal dipyramidal morphology when the temperature is increased from 160 to 180 °C. A possible growth mechanism is proposed. The synthesized ?‐MnO₂ shows both semiconducting and magnetic properties. These materials exhibit ferromagnetic behavior below 25 K and paramagnetic behavior above 25 K. The ?‐MnO₂ system may have potential applications in areas such as fabrication of nanoscale spintronic materials, catalysis, and sensors.     

ECR plasma etch fabrication of C-doped base InGaAs/InP DHBT structures: A comparison of CH4/H2/Ar vs BCl3/N2 plasma etch chemistries

We compare ECR plasma etch fabrication of self-aligned thin emitter carbondoped base InGaAs/InP DHBT structures using either CH₄/H₂/Ar or BCl₃/N₂ etch chemistries. Detrimental hydrogen passivation of the carbon doping in the base region of our structure during CH₄/H₂/Ar dry etching of the emitter region is observed. Initial conductivity is not recovered with annealing up to a temperature of 500°C. This passivation is not due to damage from the dry etching or from the MOMBE growth process, since DHBT structures which are ECR plasma etched in BCl₃/N₂ have the same electrical characteristics as wet etched controls. It is due to hydrogen implantation from the plasma exposure. This is supported with secondary ion mass spectroscopy profiles of structures which are etched in CH₄/D₂/Ar showing an accumulation of deuterium in the C-doped base region.     

金纳米粒子作掩模的硅纳米柱阵列的加工

利用自组装的方法在硅基片表面形成一层均匀的金纳米粒子掩模,分析了偶联剂对自组装的影响,以金纳米粒子作掩模进行反应离子刻蚀,研究了刻蚀时间对硅纳米柱阵列的影响,提供了一种简单、便宜并且有效的在硅基底上大面积形成纳米柱阵列的纳米加工方法。实验中发现,超过一定刻蚀时间时,有过刻蚀现象发生,在120 s刻蚀时间下,得到了直径小于20 nm,深宽比高达10∶1以上规则、致密、大面积分布的硅纳米柱或硅纳米锥状结构。     

Amphiphilic Poly(p‐phenylene)s for Self‐Organized Porous Blue‐Light‐Emitting Thin Films

Micro‐ and nanostructuring of conjugated polymers are of critical importance in the fabrication of molecular electronic devices as well as photonic and bandgap materials. The present report delineates the single‐step self‐organization of highly ordered structures of functionalized poly(p‐phenylene)s without the aid of either a controlled environment or expensive fabrication methodologies. Microporous films of these polymers, with a honeycomb pattern, were prepared by direct spreading of the dilute polymer solution on various substrates, such as glass, quartz, silicon wafer, indium tin oxide, gold‐coated mica, and water, under ambient conditions. The polymeric film obtained from C₁₂PPPOH comprises highly periodic, defect‐free structures with blue‐light‐emitting properties. It is expected that such microstructured, conjugated polymeric films will have interesting applications in photonic and optoelectronic devices. The ability of the polymer to template the facile micropatterning of nanomaterials gives rise to hybrid films with very good spatial dispersion of the carbon nanotubes.     

Sacrificial‐Layer Atomic Layer Deposition for Fabrication of Non‐Close‐Packed Inverse‐Opal Photonic Crystals

A method is presented for predicting and precisely controlling the structure of photonic crystals fabricated using sacrificial‐layer atomic layer deposition. This technique provides a reliable method for fabrication of high‐quality non‐close‐packed inverse shell opals with large static tunability and precise structural control. By using a sacrificial layer during opal infiltration, the inverse‐opal pore size can be increased with sub‐nanometer resolution and without distorting the lattice to allow for a high degree of dielectric backfilling and increased optical tunability. For a 10 % sacrificial layer, static tunability of 80 % is predicted for the inverse opal. To illustrate this technique, SiO₂ opal templates were infiltrated using atomic layer deposition of ZnS, Al₂O₃, and TiO₂. Experimentally, a static tunability of over 600 nm, or 58 %, was achieved and is well described by both a geometrical model and a numerical‐simulation algorithm. When extended to materials of higher refractive index, this method will allow the facile fabrication of 3D photonic crystals with optimized photonic bandgaps.     

Inductively coupled plasma reactive ion etching of SiC single crystals using NF<Subscript>3</Subscript>-based gas mixtures

Inductively coupled plasma reactive ion etching of SiC single crystals using NF₃-based gas mixtures was investigated. Mesas with smooth surfaces and vertical sidewalls were obtained, with a maximum etch rate of about 400 nm/min. Effects of CH₄ and O₂ addition to the NF₃ gas and the crystalline quality of substrates were studied during the SiC dry etching using various masks. Selectivity of the photoresist (PR) mask improved from about 0.2 to about 0.4 by the addition of 30% CH₄ during the RIE, although the etch rate decreased by 50–70%. Results also indicated that the substrate quality does not significantly affect the etch results.     

An Etch‐Stop Technique Using Cr2O3 Thin Film and Its Application to Silica PLC Platform Fabrication

Using Cr₂O₃ thin film, we developed a novel etch‐stop technique for the protection of silicon surface morphology during deep ion coupled plasma etching of silica layers. With this technique we were able to etch a silica trench with a depth of over 20 µm without any damage to the exposed silicon terrace surface. This technique should be well applicable to fabricating silica planar lightwave circuit platforms for opto‐electronic hybrid integration.     

Development of metal etch mask by single layer lift-off for silicon nitride photonic crystals

We present a method for fabrication of nanoscale patterns in silicon nitride (SiN) using a hard chrome mask formed by metal liftoff with a negative ebeam resists (maN-2401). This approach enables fabrication of a robust etch mask without the need for exposing large areas of the sample by electron beam lithography. We demonstrate the ability to pattern structures in SiN with feature sizes as small as 50 nm. The fabricated structures exhibit straight sidewalls, excellent etch uniformity, and enable patterning of nanostructures with very high aspect ratios. We use this technique to fabricate two-dimensional photonic crystals in a SiN membrane. The photonic crystals are characterized and shown to have quality factors as high as 1460.