Three-Dimensional Microfabrication Using Two-Photon Polymerization (TPP) is the first comprehensive guide to TPP microfabrication-essential reading for researchers and engineers in areas where miniaturization of complex structures is key, such as in the optics, microelectronics, and medical device industries.
TPP stands out among microfabrication techniques because of its versatility, low costs, and straightforward chemistry. TPP microfabrication attracts increasing attention among researchers and is increasingly employed in a range of industries where miniaturization of complex structures is crucial: metamaterials, plasmonics, tissue engineering, and microfluidics, for example.
Despite its increasing importance and potential for many more applications, no single book to date is dedicated to the subject. This comprehensive guide, edited by Professor Baldacchini and written by internationally renowned experts, fills this gap and includes a unified description of TPP microfabrication across disciplines.
The guide covers all aspects of TPP, including the pros and cons of TPP microfabrication compared to other techniques, as well as practical information on material selection, equipment, processes, and characterization.
Current and future applications are covered and case studies provided as well as challenges for adoption of TPP microfabrication techniques in other areas are outlined. The freeform capability of TPP is illustrated with numerous scanning electron microscopy images.
- Comprehensive account of TPP microfabrication, including both photophysical and photochemical aspects of the fabrication process
- Comparison of TPP microfabrication with conventional and unconventional micromanufacturing techniques
- Covering applications of TPP microfabrication in industries such as microelectronics, optics and medical devices industries, and includes case studies and potential future directions
- Illustrates the freeform capability of TPP using numerous scanning electron microscopy images
Foreword: Here, a Small Step Toward a Grand Vision
"natura nihil facit supervacaneum" (nature creates nothing superfluous)
"multum in parvo" (much in little)
In the broadest interpretation, three-dimensional (3D) manufacturing/microfabrication embodies a number of material processing techniques where layer-by-layer or controlled self-assembly on a scaffolding is utilized to fashion a desired 3D structure. On cursory examination, 3D manufacturing appears to be an efficient means to make something, because regardless of the product shape, there is minimum waste. Only the amount of material necessary is used - no more, no less. The approach adheres to a manufacturing ideal where material properties can be varied or graded on a part, almost at will, and function can follow form from the nanoscale to the mesoscale. It has some likeness to biology where material is grown, cell by cell or in this case by form-function to form-function. In the most soaring of visions, a 3D manufactured product would not only have the necessary complex shape but also include properties for "sensing" the local environment, harvesting energy, and "communicating" its state. This would be accomplished by the seamless integration of materials and functionality. We are far from realizing this vision, but recent strides in fields such as materials development and controlled process tooling bring hope that 3D manufacturing could impact the world much like the industrial and electronics revolutions. One particular area where materials development/characterization has been most congruent with process tooling is the use of controlled polymerization to fabricate 3D structures. The technology (i.e., stereolithography) was the first to gain wide commercial acceptance in the genre of direct write additive manufacturing (AM) and mostly applied to fabricating prototype structures with feature resolutions measured on the order of 100 µm. This assembled work places its focus on a subset of AM technology, two-photon polymerization (TPP), where minimum linewidth and writing resolution of 9 and 52 nm, respectively, have already been achieved.
TPP technology utilizes the high intensity that is available from femtosecond lasers to controllably breach a reactivity threshold in a polymer resin to induce polymerization and thereby form a freestanding structure. Complex structures can be formed by interfering laser beams, holography (i.e., continuous image projection), or laser direct write. One advantage to the latter approach is the ease of fashioning both structure and site-specific defects. TPP technology continues to achieve hallmarks for precision, resolution, and formed structure complexity because it addresses the issue from many different technical points, many of which are detailed in this assembled work. There is the desire to gain control of the photoabsorption process through the development of "engineered" polymers, control of the polymerization photochemistry by precise metering of the photon dose near reaction threshold, the implementation of quenching processes via chemical (e.g., oxygen) or optical (e.g., depletion/inhibition beams) means, the utilization of the unique optical spatial properties when focusing a polarization-modulated laser beam through a high numerical aperture (NA) objective (e.g., vector beams), the development of CAD/CAM software to surpass the limitations in the de facto standard for layer-based prototyping (i.e., STL file format), and the integration of motion control hardware to combine nanometer precision, millimeter to centimeter scale motion, and high speed (>1 m/s).
Brushing aside these technical achievements, critics commonly fault TPP technology for its impractical throughput for piece part fabrication. It is an unfair accusation, made by critics who envision TPP being applied on the scale of many centimeters and larger. That is not the strength of the TPP technology. TPP will always be a single technique within the hierarchical 3D manufacturing schema, but it provides a key link. By virtue of the feature sizes that TPP technology can fashion, it provides a material processing approach that enables a "communication" link to be fashioned between the submicrometer/nanometer world and its "unique" physics with that of the macroworld and its known continuum physics. It is the ability to fabricate structures that can affect processes at the molecular and biological length scales and thereby derive a better understanding of how local disturbances affect process evolution. Nota bene: While it was microelectromechanical system (MEMS) technology that permitted the "reach" into the microworld, it is nanoelectromechanical system (NEMS) technology that actually "touches" it. 3D fabrication by TPP expands the micromanufacturing genre by permitting the near free-form fabrication of submicrometer/nanometer structures without the structural constraints imposed by silicon material processing.
For TPP to go beyond just the fashioning of prototype structures to the fabrication of viable commercial commodities, issues regarding process control, increases in piece part throughput, and repeatability have to be addressed. One of the strengths of this assembled work is that it not only presents the state of the art, but it also addresses these critical issues, along with an assessment of current limitations. For example, approaches to making TPP faster are addressed directly (e.g., the implementation of multiple laser beams and parallel processing to the use of digital light modulators/spatial light modulators). There is also a look into the future with possible near- and far-term applications (e.g., structures within microfluidics/lab-on-chip, scaffolding to produce tissue mimics/models for administering patient-specific therapeutics for disease control and reduced remission) and an assessment of material processing technologies that can likely be merged/clustered with TPP to develop an integrated (i.e., hybrid process) tool. While applications exist in which a desired structure can be elegantly fashioned on a desktop TPP tool, I believe TPP or its variant will find maximum market insertion when it can be merged among a cluster of 3D or AM manufacturing tools for manufacturing macroscale objects, in which TPP is used to fabricate critical submicrometer structures at specific locations. Why this perspective? Because to date, there is no other means to place submicrometer structures that can act as "self-awareness" sensors on macroscale objects within the schema of direct digital development manufacturing. Pick and place "tools" based on laser-induced forward transfer (LIFT), such as presented in this book, could place sensors using sub-millimeter-sized objects but direct fabrication will be necessary for making submicrometer-scale structures. Second, the femtosecond laser used in TPP can also "drive" other AM processes (including metal sintering, albeit at higher powers). Consequently, the TPP technique is amenable to the development of an all laser-"driven" material processing tool. For example, it is possible to envision a high-viscosity resin "droplet" being placed at a specific location (possible with LIFT) within which a submicrometer structure is then fabricated. Third, TPP is a high-intensity process, and this fact opens the opportunity to expand the capabilities offered by the femtosecond laser source. There is significant literature where laser-induced phase transformations in materials enable functionality not possible otherwise. It requires the symbiosis of light and matter. One particular application area, 3D micro-optics, is discussed in this book in the context of TPP, but it is possible to adapt the same tool to fabricate structures that "guide" light (i.e., within a proximal material) by femtosecond laser compaction (i.e., index change). Fourth, TPP, in possible combination with LIFT or a similar material transfer process, permits the submicrometer fabrication of structures composed of composite materials (i.e., polymers doped with nanoparticles and nanorods that increase the stiffness/strength or provide piezoelectric, magnetic, electrical, or fluorescent properties). The approach expands the physical means of interaction that a fabricated structure can have with the local environment. Fifth, there is the possibility of applying a post-material processing step to a TPP-fabricated structure and thereby further expand its overall functionality. The approaches addressed in this book present examples of structures that have undergone a postfabrication step such as metallization, electroplating, selective functionalization (i.e., by the use of a second polymer with orthogonal functionality), carbonization (i.e., pyrolysis of the polymer to form a carbon structure), and atomic layer deposition. The possibilities of this multistep process are addressed in yet another chapter via an example: the development of a miniature, remotely driven (i.e., optically driven) machine that is fabricated via multistep operation that starts with TPP, adds electroplating, and is then released by laser ablation. Finally, nature builds structures in a hierarchical manner and so doing develops functionality as necessary (i.e., "natura nihil facit supervacaneum"). TPP technology permits the mimicking of nature, at least at the submicrometer realm, by enabling the development of architected structures or materials to engender functionality much like biology (i.e.,...