**2.3 Micro-rapid-prototyping for 3D microstructuring in photoresists**

The classical photolithography techniques are usually limited to the fabrication of 2D structures. Also, several technological steps are required in order to complete a final design: mask fabrication, exposure, selective corrosion, etc. The laser direct-writing method provides a more simplified protocol for fabrication of different devices. Moreover, it can be successfully applied to the fabrication of 3D structures, using the rapid prototyping approach (Kawata et al., 2001). As in the case of macroscale fabrication, the micro-rapidprototyping supposes the reconstruction of a 3D structure, using the laser imprinting in bulk material, from a computer generated solid by CAD software.

In our laser processing workstation, for 3D structuring of transparent materials, the software allows the import of complex designs as standard format STL files, commonly used in rapid prototyping. The solid is numerically divided in series of sections as in the figure 3b. Each layer is laser imprinted inside the photosensitive material by translating the focused femtosecond laser beam relative to the sample, accordingly to a calculated path. The twophoton or multiphoton absorption of the femtosecond radiation will induce photochemical modification and densification of the material at the focused spot, as described in the previous section. By incrementing the position in *Z* direction, a solid shape will result. The irradiated material is transparent to the laser wavelength and allows deep focusing of the beam inside the volume of the material. The formation of high aspect ratio 3D microstructures is possible.

A high-aspect-ratio structure produced by TPP in Ormoclad photoresist is shown in figure 3c. The 3D structure is a miniaturised "Endless Column", a sculpture created by the Romanian artist Constantin Brâncuşi (1876-1957). The microstructure is 600 m in height, and is formed by 4 octahedron-shaped modules ended by a half module. Each module is constructed by squared layers with maximum size of 100x100 m2. In order to obtain a good overlap between layers the increment in *Z* direction was 2 m, while the vortex size in *Z* direction was about 7 m.

Fig. 3. Description of 3D stereolithography algorithm: a) The design in STL format; b) Computed slices of the 3D design; c) High aspect ratio 3D structure fabricated in Ormoclad. Scale bar: 100 m. The solid structure is formed by TPP in layer by layer approach.

Ultrashort Pulsed Lasers – Efficient Tools for Materials Micro-Processing 271

with submicrometre lattice constant and different arrangements, such as woodpile

The quality of a 3D structure is characterized by some parameters, such as adhesion to the substrate, shrinkage of the obtained geometry compared to the desired design, limited aspect ratio, and limited minimal feature size. The limitations of the TPP technique are mainly given by intrinsic physical and chemical properties of the photoresist and the laser focusing capability. For overcoming them, new polymers have to be developed with better performances such as low shrinkage and good adhesion on commons substrates such as glass or silica. Also, the behaviour of the photoresist and its solvent strongly affect the aspect-ratio of the structure. Some materials become very soft during development, and the structure falls down to the substrate after removing the sample from the solvent. Beside these, the limitations due to the processing equipments are mainly related to the processing resolution. Limited smoothness of the structures is given by the non uniform movement of the translation stages. Another limitation of the rapid prototyping method is the total processing time required to fabricate a structure. Since the rapid prototyping is a laser scanning technique, this is a time consuming method requiring tens of minutes up to few hours in order to complete a complex 3D structure. The solution to overcome this disadvantage is to use lasers with very high repetition rate and also fast scanning mechanics such as galvano-scanners. However, a compromise between writing speed and accuracy has to be done, the piezoelectric stages remaining the most accurate positioning systems used in

**3. Femtosecond laser microprocessing of composite right/left handed** 

Metamaterials (MTM) are propagation media presenting simultaneously both negative permittivities ( < 0) and negative permeabilities ( < 0). These media, with unusual properties not readily available in nature, are called **L**eft **H**anded **M**aterials (LHM). For a LHM, the triade formed by the electric field, magnetic field and Poynting vector of an electromagnetic field propagating through this media has a "left hand" orientation, different from common materials where this orientation is a "right hand" one. As a result, LHM support propagation of an electromagnetic wave where the group velocity is antiparallel to the phase velocity. This phenomenon associates with a negative refraction

Although presented as a theoretical curiosity since 1968 (Veselago, 1968), practical applications of "left handed" media appeared 30 years later when the first experimental investigations were made (Pendry, 1999; Shelby et al., 2001). Since that time, a great variety of media with metamaterial characteristics and subsequent applications from microwave to

In the microwave and millimeter wave frequency domain, the main conventional propagation media are the transmission lines (microstrips, coplanar waveguides). The right handed form of these transmission line structures may be assimilated to a large enough number of cascaded cells, each cell being made of *series inductor – parallel capacitor*. For this

**(CRLH) metamaterials for millimeter wave devices** 

**3.1 Metamaterial CRLH millimeter wave devices** 

the visible domain were developed.

structures (Deubel et al., 2004).

3D DLW.

index.

**2.4 The limits of TPP microstructuring method** 

The laser irradiation was done by the femtosecond laser beam at 75 MHz repetition rate, with pulse duration less than 40 fs, and laser beam power of 60 mW. The scanning speed was 0.5 mm/s. The focusing optics was a 100x microscope objective with 0.5 numerical aperture.

After irradiation, the non-polymerised resin was removed by OrmoDev, a solvent specific for Ormocers. The remained 3D model was gently rinsed in isopropyl alcohol. As observed in the figure 3c, the miniaturised Endless Column has a hollow shape. At the bottom part of the structure a bead is trapped. Its high-aspect-ratio shape demonstrates the capability of our experimental set-up for fabricating complex structures with applicability in microfluidics. The same microfabrication approach can be involved in applications such as photonics, integrated optics, or tissue engineering. Some structures demonstrating the application of TPP are shown in figure 4. Biocompatible scaffolds were fabricated by photopolymerisation of Ormosil as support for culture of live cells (Matei et al., 2010). After deposition of the material on glass substrate, a 2D grid is realized by TPP (figure 4a). The grid is formed by polymerised lines in *X* and *Y* direction, spaced at 100 m, with thickness of about 10 m, and highness of about 50 m. After appropriate microbial decontamination of the sample, epithelial human cells MRC-5 were grown on the polymerised scaffold. As shown in figure 4b, the cells attach on the grid setting each on a square element. Such biocompatible scaffolds are of big interest in biomedical applications such as BioMEMS, tissue engineering, and medical implants (Weiß et al., 2009).

Fig. 4. Microstructures fabricated by TPP: (a) Scaffold for tissue engineering fabricated in ORMOSIL. (b) Stem cells deposited on the polymerised structure. Periodical photonic structures in SU-8 photoresist in hexagonal (c) and rectangular (d) arrangements.

Photonic structures can be also produced by TPP in photopolymers such as SU-8. Columnar structures of 20 µm height and 2 µm diameter were obtained by scanning the laser beam in *Z* direction (figure 4b and 4c). The lattice constant of the structure is 5 m in both *X, Y* directions. Such a periodical structure has a photonic bandgap near 10 µm wavelength. Bandgaps at visible spectral range are also possible for photopolymerised photonic crystals with submicrometre lattice constant and different arrangements, such as woodpile structures (Deubel et al., 2004).

#### **2.4 The limits of TPP microstructuring method**

270 Recent Advances in Nanofabrication Techniques and Applications

The laser irradiation was done by the femtosecond laser beam at 75 MHz repetition rate, with pulse duration less than 40 fs, and laser beam power of 60 mW. The scanning speed was 0.5 mm/s. The focusing optics was a 100x microscope objective with 0.5 numerical

After irradiation, the non-polymerised resin was removed by OrmoDev, a solvent specific for Ormocers. The remained 3D model was gently rinsed in isopropyl alcohol. As observed in the figure 3c, the miniaturised Endless Column has a hollow shape. At the bottom part of the structure a bead is trapped. Its high-aspect-ratio shape demonstrates the capability of our experimental set-up for fabricating complex structures with applicability in microfluidics. The same microfabrication approach can be involved in applications such as photonics, integrated optics, or tissue engineering. Some structures demonstrating the application of TPP are shown in figure 4. Biocompatible scaffolds were fabricated by photopolymerisation of Ormosil as support for culture of live cells (Matei et al., 2010). After deposition of the material on glass substrate, a 2D grid is realized by TPP (figure 4a). The grid is formed by polymerised lines in *X* and *Y* direction, spaced at 100 m, with thickness of about 10 m, and highness of about 50 m. After appropriate microbial decontamination of the sample, epithelial human cells MRC-5 were grown on the polymerised scaffold. As shown in figure 4b, the cells attach on the grid setting each on a square element. Such biocompatible scaffolds are of big interest in biomedical applications such as BioMEMS,

Fig. 4. Microstructures fabricated by TPP: (a) Scaffold for tissue engineering fabricated in ORMOSIL. (b) Stem cells deposited on the polymerised structure. Periodical photonic structures in SU-8 photoresist in hexagonal (c) and rectangular (d) arrangements.

Photonic structures can be also produced by TPP in photopolymers such as SU-8. Columnar structures of 20 µm height and 2 µm diameter were obtained by scanning the laser beam in *Z* direction (figure 4b and 4c). The lattice constant of the structure is 5 m in both *X, Y* directions. Such a periodical structure has a photonic bandgap near 10 µm wavelength. Bandgaps at visible spectral range are also possible for photopolymerised photonic crystals

tissue engineering, and medical implants (Weiß et al., 2009).

aperture.

The quality of a 3D structure is characterized by some parameters, such as adhesion to the substrate, shrinkage of the obtained geometry compared to the desired design, limited aspect ratio, and limited minimal feature size. The limitations of the TPP technique are mainly given by intrinsic physical and chemical properties of the photoresist and the laser focusing capability. For overcoming them, new polymers have to be developed with better performances such as low shrinkage and good adhesion on commons substrates such as glass or silica. Also, the behaviour of the photoresist and its solvent strongly affect the aspect-ratio of the structure. Some materials become very soft during development, and the structure falls down to the substrate after removing the sample from the solvent. Beside these, the limitations due to the processing equipments are mainly related to the processing resolution. Limited smoothness of the structures is given by the non uniform movement of the translation stages. Another limitation of the rapid prototyping method is the total processing time required to fabricate a structure. Since the rapid prototyping is a laser scanning technique, this is a time consuming method requiring tens of minutes up to few hours in order to complete a complex 3D structure. The solution to overcome this disadvantage is to use lasers with very high repetition rate and also fast scanning mechanics such as galvano-scanners. However, a compromise between writing speed and accuracy has to be done, the piezoelectric stages remaining the most accurate positioning systems used in 3D DLW.
