Multiphoton lithography

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Model of a castle (0.2 mm x 0.3 mm x 0.4 mm) 3D-printed on a pencil tip via multiphoton lithography Burg auf einer Bleistiftspitze.tif
Model of a castle (0.2 mm x 0.3 mm x 0.4 mm) 3D-printed on a pencil tip via multiphoton lithography

Multiphoton lithography (also known as direct laser lithography or direct laser writing) is similar to standard photolithography techniques; structuring is accomplished by illuminating negative-tone or positive-tone[ jargon ] photoresists via light of a well-defined wavelength. The main difference is the avoidance of photomasks. Instead, two-photon absorption is utilized to induce a change in the solubility of the resist for appropriate developers.[ jargon ]

Contents

Animation of the multiphoton-polymerization process Multi-Photon Polymerization - Additive Laser Technology.gif
Animation of the multiphoton-polymerization process

Hence, multiphoton lithography is a technique for creating small features in a photosensitive material, without the use of excimer lasers or photomasks. This method relies on a multi-photon absorption process in a material that is transparent at the wavelength of the laser used for creating the pattern. By scanning and properly modulating the laser, a chemical change (usually polymerization) occurs at the focal spot of the laser and can be controlled to create an arbitrary three-dimensional pattern. This method has been used for rapid prototyping of structures with fine features.

Schematic representation of the multiphoton writing process. Multiphoton Polymerization.png
Schematic representation of the multiphoton writing process.

Two-photon absorption (TPA) is a third-order with respect to the third-order optical susceptibility and a second-order process with respect to light intensity.[ jargon ] For this reason it is a non-linear process several orders of magnitude weaker than linear absorption,[ jargon ] thus very high light intensities are required to increase the number of such rare events. For example, tightly-focused laser beams provide the needed intensities. Here, pulsed laser sources, with pulse widths of around 100 fs, [1] are preferred as they deliver high-intensity pulses while depositing a relatively low average energy. To enable 3D structuring, the light source must be adequately adapted to the liquid photoresin in that single-photon absorption is highly suppressed.[ clarification needed ] TPA is thus essential for creating complex geometries with high resolution and shape accuracy. For best results, the photoresins should be transparent to the excitation wavelength λ, which is between 500-1000 nm and, simultaneously, absorbing in the range of λ/2. [2] As a result, a given sample relative to the focused laser beam can be scanned while changing the resist's solubility only in a confined volume. The geometry of the latter mainly depends on the iso-intensity surfaces of the focus. Concretely, those regions of the laser beam which exceed a given exposure threshold of the photosensitive medium define the basic building block, the so-called voxel . Voxels are thus the smallest, single volumes of cured photopolymer. They represent the basic building blocks of 3D-printed objects. Other parameters which influence the actual shape of the voxel are the laser mode and the refractive-index mismatch between the resist and the immersion system leading to spherical aberration.

It was found that polarization effects in laser 3D nanolithography can be employed to fine-tune the feature sizes (and corresponding aspect ratio) in the structuring of photoresists. This proves polarization to be a variable parameter next to laser power (intensity), scanning speed (exposure duration), accumulated dose, etc.

In addition, a plant-derived renewable pure bioresins without additional photosensitization can be employed for the optical rapid prototyping. [3]

Materials for multiphoton polymerization

The materials employed in multiphoton lithography are those normally used in conventional photolithography techniques. They can be found in liquid-viscous, gel or solid state, in relation to the fabrication need. Liquid resins imply more complex sample fixing processes, during the fabrication step, while the preparation of the resins themselves may be easier and faster. In contrast, solid resists can be handled in an easier way, but they require complex and time-consuming processes. [4] The resin always include a prepolymer (the monomer) and, considering the final application, a photoinitiator. In addition, we can find such polymerization inhibitors (useful to stabilize resins both reducing the obtained voxel), solvents (which may simplify casting procedures), thickens (so called "fillers") and other additives (as pigments and so on) which aim to functionalize the photopolymer.

Acrylates

The acrylates are the most diffused resin components. They can be found in many traditional photolithography processes which imply a radical reaction. They are largely diffused and commercially available in a wide range of products, having different properties and composition. The main advantages of this kind of liquid resins are found in the excellent mechanical properties and in the high reactivity. Acrylates exhibit slightly more shrinkage compared to epoxies, but their rapid iteration capability allows for close alignment with the design. Moreover, Acrylates offer enhanced usability as they eliminate the need for spin coating or baking steps during processing. Finally the polymerization steps are faster than other kind of photopolymers. [4] Methacrylates are largely diffused due to their biocompatibility. The majority of materials for Two-Photon Polymerization are supplied by companies that also provide printers. Nevertheless, there are third-party resins available like ORMOCER, [5] alongside numerous self-made resins. 

Epoxy resins

These are the most employed resins into the MEMS and microfluidic fields. They exploit cationic polymerization. One of the best known epoxy resin is SU-8, [6] which allows thin film deposition (up to 500 μm) and polymerization of structures with a high aspect ratio . We can find many others epoxy resins such as: SCR-701, largely employed in micro moving objects, [7] and the SCR-500.

Inorganic glass/ceramics

Inorganic glass and ceramics have better thermal and chemical stabilities than photopolymers do, and they also offer improved durability due to their high resistance to corrosion, degradation, and wear. [8] Therefore, there has been continuous interest in the development of resins and techniques that allow using multiphoton lithography for 3D printing of glasses and ceramics in recent years. It has been demonstrated that using hybrid inorganic-organic resins and high-temperature thermal treatments, one can achieve 3D printing of glass-ceramics with sub-micrometer resolution. [9] [10] Recently, multiphoton lithography of an entirely inorganic resin for 3D printing of glasses without involving thermal treatments has also been shown, [11] enabling 3D printing of glass micro-optics on the tips of optical fibers without causing damage to the optical fiber. [12]

Applications

Nowadays there are several application fields for microstructured devices, made by multiphoton polymerization, such as: regenerative medicine, biomedical engineering, micromechanic, microfluidic, atomic force microscopy, optics and telecommunication science.

Regenerative medicine and biomedical engineering

By the arrival of biocompatible photopolymers (as SZ2080 and OMOCERs) many scaffolds have been realized by multiphoton lithography, to date. They vary in key parameters as geometry, porosity and dimension to control and condition, in a mechanical and chemical fashion, fundamental cues in in vitro cell cultures: migration, adhesion, proliferation and differentiation. The capability to fabricate structures having a feature size smaller than the cells' one, have dramatically improved the mechanobiology field, giving the possibility to combine mechanical cues directly into cells microenvironment. [13] Their final application range from stemness maintenance in adult mesenchymal stem cells, such as into the NICHOID scaffold [14] which mimics in vitro a physiological niche, to the generation of migration engineered scaffolds.

Micromechanic and microfluidic

The multiphoton polymerization can be suitable to realize microsized active (as pumps) or passive (as filters) devices that can be combined with Lab-on-a-chip. These devices can be widely used coupled to microchannels with the advantage to polymerize in pre-sealed channels. Considering filters, they can be used to separate the plasma from the red blood cells, to separate cell populations (in relation to the single cell dimension) or basically to filter solutions from impurity and debris. A porous 3D filter, which can only be fabricated by 2PP technology, offers two key advantages compared to filters based on 2D pillars. First, the 3D filter has increased mechanical resistance to shear stresses, enabling a higher void ratio and hence more efficient operation. Second, the 3D porous filter can efficiently filter disk-shaped elements without reducing the pore size to the minimum dimension of the cell. Considering the integrated micropumps, they can be polymerized as two-lobed independent rotors, confined into the channel by their own shaft, to avoid unwanted rotations. Such systems are simply activated by using focalized CW laser system. [7]

Atomic force microscopy

To date, atomic force microscopy microtips are realized with standard photolithographic techniques on hard materials, such as gold, silicon, and its derivatives. Nonetheless, the mechanical properties of such materials require time-consuming and expensive production processes to create or bend the tips. Multiphoton lithography can be used to prototype and modify, thus avoiding the complex fabrication protocol.

Optics

With the ability to create 3D planar structures, multiphoton polymerization can build optical components for optical waveguides, [4] resonators, [15] photonic crystals, [16] and lens. [17]

Related Research Articles

<span class="mw-page-title-main">Microscopy</span> Viewing of objects which are too small to be seen with the naked eye

Microscopy is the technical field of using microscopes to view objects and areas of objects that cannot be seen with the naked eye. There are three well-known branches of microscopy: optical, electron, and scanning probe microscopy, along with the emerging field of X-ray microscopy.

<span class="mw-page-title-main">Photoresist</span> Light-sensitive material used in making electronics

A photoresist is a light-sensitive material used in several processes, such as photolithography and photoengraving, to form a patterned coating on a surface. This process is crucial in the electronics industry.

<span class="mw-page-title-main">Photonic crystal</span> Periodic optical nanostructure that affects the motion of photons

A photonic crystal is an optical nanostructure in which the refractive index changes periodically. This affects the propagation of light in the same way that the structure of natural crystals gives rise to X-ray diffraction and that the atomic lattices of semiconductors affect their conductivity of electrons. Photonic crystals occur in nature in the form of structural coloration and animal reflectors, and, as artificially produced, promise to be useful in a range of applications.

<span class="mw-page-title-main">Stereolithography</span> 3D printing technique

Stereolithography is a form of 3D printing technology used for creating models, prototypes, patterns, and production parts in a layer by layer fashion using photochemical processes by which light causes chemical monomers and oligomers to cross-link together to form polymers. Those polymers then make up the body of a three-dimensional solid. Research in the area had been conducted during the 1970s, but the term was coined by Chuck Hull in 1984 when he applied for a patent on the process, which was granted in 1986. Stereolithography can be used to create prototypes for products in development, medical models, and computer hardware, as well as in many other applications. While stereolithography is fast and can produce almost any design, it can be expensive.

Nanolithography (NL) is a growing field of techniques within nanotechnology dealing with the engineering of nanometer-scale structures on various materials.

<span class="mw-page-title-main">Two-photon excitation microscopy</span> Fluorescence imaging technique

Two-photon excitation microscopy is a fluorescence imaging technique that is particularly well-suited to image scattering living tissue of up to about one millimeter in thickness. Unlike traditional fluorescence microscopy, where the excitation wavelength is shorter than the emission wavelength, two-photon excitation requires simultaneous excitation by two photons with longer wavelength than the emitted light. The laser is focused onto a specific location in the tissue and scanned across the sample to sequentially produce the image. Due to the non-linearity of two-photon excitation, mainly fluorophores in the micrometer-sized focus of the laser beam are excited, which results in the spatial resolution of the image. This contrasts with confocal microscopy, where the spatial resolution is produced by the interaction of excitation focus and the confined detection with a pinhole.

<span class="mw-page-title-main">Dip-pen nanolithography</span> Scanning probe lithographic technique

Dip pen nanolithography (DPN) is a scanning probe lithography technique where an atomic force microscope (AFM) tip is used to directly create patterns on a substrate. It can be done on a range of substances with a variety of inks. A common example of this technique is exemplified by the use of alkane thiolates to imprint onto a gold surface. This technique allows surface patterning on scales of under 100 nanometers. DPN is the nanotechnology analog of the dip pen, where the tip of an atomic force microscope cantilever acts as a "pen", which is coated with a chemical compound or mixture acting as an "ink", and put in contact with a substrate, the "paper".

<span class="mw-page-title-main">Nanoimprint lithography</span> Method of fabricating nanometer scale patterns using a special stamp

Nanoimprint lithography (NIL) is a method of fabricating nanometer-scale patterns. It is a simple nanolithography process with low cost, high throughput and high resolution. It creates patterns by mechanical deformation of imprint resist and subsequent processes. The imprint resist is typically a monomer or polymer formulation that is cured by heat or UV light during the imprinting. Adhesion between the resist and the template is controlled to allow proper release.

<span class="mw-page-title-main">Photopolymer</span>

A photopolymer or light-activated resin is a polymer that changes its properties when exposed to light, often in the ultraviolet or visible region of the electromagnetic spectrum. These changes are often manifested structurally, for example hardening of the material occurs as a result of cross-linking when exposed to light. An example is shown below depicting a mixture of monomers, oligomers, and photoinitiators that conform into a hardened polymeric material through a process called curing.

<span class="mw-page-title-main">Nanochemistry</span> Combination of chemistry and nanoscience

Nanochemistry is an emerging sub-discipline of the chemical and material sciences that deals with the development of new methods for creating nanoscale materials. The term "nanochemistry" was first used by Ozin in 1992 as 'the uses of chemical synthesis to reproducibly afford nanomaterials from the atom "up", contrary to the nanoengineering and nanophysics approach that operates from the bulk "down"'. Nanochemistry focuses on solid-state chemistry that emphasizes synthesis of building blocks that are dependent on size, surface, shape, and defect properties, rather than the actual production of matter. Atomic and molecular properties mainly deal with the degrees of freedom of atoms in the periodic table. However, nanochemistry introduced other degrees of freedom that controls material's behaviors by transformation into solutions. Nanoscale objects exhibit novel material properties, largely as a consequence of their finite small size. Several chemical modifications on nanometer-scaled structures approve size dependent effects.

<span class="mw-page-title-main">Two-photon absorption</span> Simultaneous absorption of two photons by a molecule

In atomic physics, two-photon absorption (TPA or 2PA), also called two-photon excitation or non-linear absorption, is the simultaneous absorption of two photons of identical or different frequencies in order to excite an atom or a molecule from one state (usually the ground state), via a virtual energy level, to a higher energy, most commonly an excited electronic state. Absorption of two photons with different frequencies is called non-degenerate two-photon absorption. Since TPA depends on the simultaneous absorption of two photons, the probability of TPA is proportional to the photon dose (D), which is proportional to the square of the light intensity (D ∝ I2); thus it is a nonlinear optical process. The energy difference between the involved lower and upper states of the molecule is equal or smaller than the sum of the photon energies of the two photons absorbed. Two-photon absorption is a third-order process, with absorption cross section typically several orders of magnitude smaller than one-photon absorption cross section.

<span class="mw-page-title-main">3D optical data storage</span> Storage with three-dimensional resolution for information

3D optical data storage is any form of optical data storage in which information can be recorded or read with three-dimensional resolution.

A spaser or plasmonic laser is a type of laser which aims to confine light at a subwavelength scale far below Rayleigh's diffraction limit of light, by storing some of the light energy in electron oscillations called surface plasmon polaritons. The phenomenon was first described by David J. Bergman and Mark Stockman in 2003. The word spaser is an acronym for "surface plasmon amplification by stimulated emission of radiation". The first such devices were announced in 2009 by three groups: a 44-nanometer-diameter nanoparticle with a gold core surrounded by a dyed silica gain medium created by researchers from Purdue, Norfolk State and Cornell universities, a nanowire on a silver screen by a Berkeley group, and a semiconductor layer of 90 nm surrounded by silver pumped electrically by groups at the Eindhoven University of Technology and at Arizona State University. While the Purdue-Norfolk State-Cornell team demonstrated the confined plasmonic mode, the Berkeley team and the Eindhoven-Arizona State team demonstrated lasing in the so-called plasmonic gap mode. In 2018, a team from Northwestern University demonstrated a tunable nanolaser that can preserve its high mode quality by exploiting hybrid quadrupole plasmons as an optical feedback mechanism.

<span class="mw-page-title-main">Hydrogen silsesquioxane</span> Inorganic compound

Hydrogen silsesquioxane(s) (HSQ, H-SiOx, THn, H-resin) are inorganic compounds with the empirical formula [HSiO3/2]n. The cubic H8Si8O12 (TH8) is used as the visual representation for HSQ. TH8, TH10, TH12, and TH14 have been characterized by elemental analysis, gas chromatography–mass spectroscopy (GC-MS), IR spectroscopy, and NMR spectroscopy.

A nanolaser is a laser that has nanoscale dimensions and it refers to a micro-/nano- device which can emit light with light or electric excitation of nanowires or other nanomaterials that serve as resonators. A standard feature of nanolasers includes their light confinement on a scale approaching or suppressing the diffraction limit of light. These tiny lasers can be modulated quickly and, combined with their small footprint, this makes them ideal candidates for on-chip optical computing.

Tube-based nanostructures are nanolattices made of connected tubes and exhibit nanoscale organization above the molecular level.

A nanophotonic resonator or nanocavity is an optical cavity which is on the order of tens to hundreds of nanometers in size. Optical cavities are a major component of all lasers, they are responsible for providing amplification of a light source via positive feedback, a process known as amplified spontaneous emission or ASE. Nanophotonic resonators offer inherently higher light energy confinement than ordinary cavities, which means stronger light-material interactions, and therefore lower lasing threshold provided the quality factor of the resonator is high. Nanophotonic resonators can be made with photonic crystals, silicon, diamond, or metals such as gold.

Three-dimensional (3D) microfabrication refers to manufacturing techniques that involve the layering of materials to produce a three-dimensional structure at a microscopic scale. These structures are usually on the scale of micrometers and are popular in microelectronics and microelectromechanical systems.

<span class="mw-page-title-main">3D printing processes</span> List of 3D printing processes

A variety of processes, equipment, and materials are used in the production of a three-dimensional object via additive manufacturing. 3D printing is also known as additive manufacturing, because the numerous available 3D printing process tend to be additive in nature, with a few key differences in the technologies and the materials used in this process.

<span class="mw-page-title-main">Nanolattice</span>

A nanolattice is a synthetic porous material consisting of nanometer-size members patterned into an ordered lattice structure, like a space frame. The nanolattice is a newly emerged material class that has been rapidly developed over the last decade. Nanolattices redefine the limits of the material property space. Despite being composed of 50-99% of air, nanolattices are very mechanically robust because they take advantage of size-dependent properties that we generally see in nanoparticles, nanowires, and thin films. The most typical mechanical properties of nanolattices include ultrahigh strength, damage tolerance, and high stiffness. Thus, nanolattices have a wide range of applications.

References

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