**2. 3D structuring by two-photon photo-polymerization (TPP)**

Building a 3D micro-structure was always a challenge, since micrometer and submicrometer resolutions are hardly obtained on complex arrangements using various materials. Applications such as microfluidics, micro- and nanoelectronics, integrated optics, motivated the development of manufacturing techniques. Among these, micro-milling, micro-moulding, electro-discharge machining, imprinting lithography, X-ray and UV lithography, electron and ion-beam lithography, chemical and plasma etching, laser rapid-prototyping, direct laser writing (DLW), are the most used microfabrication technologies (Frassila, 2010). However, none of these techniques satisfies simultaneously all the requirements in terms of spatial resolution, high-aspect-ratio of the structures, or ability of processing a large variety of materials and complex designs. Combinations of different techniques are sometimes the way for overcoming some technological bottlenecks. For example, X-ray lithography combined with electroforming and/or moulding allows microfabrication on metals or plastics with resolution below 100 nm and aspect-ratio up to 100 or even higher (Becker et al., 1986). However, the resulting structures are not fully 3D designs. These are more 2D structures with a finite dimension in *Z* direction, the so called 2½D structures. Despite of good resolution of the method in *XY* directions, such structures have limited complexity.

3D-stacking approach was recently used in combination with thin film deposition, lithographic and selective etching techniques for fabrication of 3D integrated circuits such as microprocessors and memories (Pavlidis et al., 2009). It requires complex and very expensive machineries justified only by very high mass production. From commercial point of view, for custom applications or limited series, these technologies are not the most competitive ones.

The laser processing of materials always offered a cheap and reliable solution for microfabrication. The lasers become an omnipresent tool, from thin film deposition, to laser processing by ablation, or laser spectroscopy characterization. Concerning the fabrication of full 3D structures, the laser rapid prototyping or stereolithography have been developed by C.W. Hull in 1986. A 3D model can be created inside an UV-curable material. The UV-laser irradiates a photosensitive material layer by layer following a certain path. The size of the 3D model is typically of the order of few-mm to hundreds of mm in each direction, with resolution of tens of micrometers.

Since the femtosecond lasers were developed, the approach of rapid prototyping inspired the development of micro-stereolithography (Maruo et al., 1997). The nonlinear effect such as two-photon absorption of NIR radiation is easily achieved in photoresists which usually absorb the UV radiation. When the material is transparent to NIR photons, the laser can be focused deeply in the volume of the material. At the tightly focused spot of a femtosecond nJ laser pulse, the laser intensity is high enough to exceed the threshold of the nonlinear absorption. Permanent physical and chemical modifications of the material take place in a small volume, deep inside of the transparent material. If the laser fluence is kept low enough, small features can be created with resolution down to tens of nm's (Farsari et al., 2005; Tan et al., 2007). By following the rapid-prototyping algorithms, a 3D model can be created layer by layer inside the material.

A large series of negative photoresists, among them SU-8, organically modified ceramics (Ormocer), or organically modified silicate (Ormosil), have the maximum of the absorption band in the UV-blue spectral range. In such photopolymers, the two-photon absorption of NIR Ti:sapphire femtosecond laser radiation induces photochemical reactions and then photo-polymerization, just like in the case of a single UV photon absorption. In contrast with the single photon processing, the two-photon absorption occurs in a very tiny volume of material, near the centre of the focused spot, allowing creation of features with resolution below 1 micrometer.

### **2.1 Experimental set-up**

264 Recent Advances in Nanofabrication Techniques and Applications

pulsed laser beams with small size particles is presented in Section 4. In the Section 5, a micro-printing method based on femtosecond laser induced forward transfer (LIFT) of

Building a 3D micro-structure was always a challenge, since micrometer and submicrometer resolutions are hardly obtained on complex arrangements using various materials. Applications such as microfluidics, micro- and nanoelectronics, integrated optics, motivated the development of manufacturing techniques. Among these, micro-milling, micro-moulding, electro-discharge machining, imprinting lithography, X-ray and UV lithography, electron and ion-beam lithography, chemical and plasma etching, laser rapid-prototyping, direct laser writing (DLW), are the most used microfabrication technologies (Frassila, 2010). However, none of these techniques satisfies simultaneously all the requirements in terms of spatial resolution, high-aspect-ratio of the structures, or ability of processing a large variety of materials and complex designs. Combinations of different techniques are sometimes the way for overcoming some technological bottlenecks. For example, X-ray lithography combined with electroforming and/or moulding allows microfabrication on metals or plastics with resolution below 100 nm and aspect-ratio up to 100 or even higher (Becker et al., 1986). However, the resulting structures are not fully 3D designs. These are more 2D structures with a finite dimension in *Z* direction, the so called 2½D structures. Despite of good resolution of

3D-stacking approach was recently used in combination with thin film deposition, lithographic and selective etching techniques for fabrication of 3D integrated circuits such as microprocessors and memories (Pavlidis et al., 2009). It requires complex and very expensive machineries justified only by very high mass production. From commercial point of view, for custom applications or limited series, these technologies are not the most

The laser processing of materials always offered a cheap and reliable solution for microfabrication. The lasers become an omnipresent tool, from thin film deposition, to laser processing by ablation, or laser spectroscopy characterization. Concerning the fabrication of full 3D structures, the laser rapid prototyping or stereolithography have been developed by C.W. Hull in 1986. A 3D model can be created inside an UV-curable material. The UV-laser irradiates a photosensitive material layer by layer following a certain path. The size of the 3D model is typically of the order of few-mm to hundreds of mm in each direction, with

Since the femtosecond lasers were developed, the approach of rapid prototyping inspired the development of micro-stereolithography (Maruo et al., 1997). The nonlinear effect such as two-photon absorption of NIR radiation is easily achieved in photoresists which usually absorb the UV radiation. When the material is transparent to NIR photons, the laser can be focused deeply in the volume of the material. At the tightly focused spot of a femtosecond nJ laser pulse, the laser intensity is high enough to exceed the threshold of the nonlinear absorption. Permanent physical and chemical modifications of the material take place in a small volume, deep inside of the transparent material. If the laser fluence is kept low enough, small features can be created with resolution down to tens of nm's (Farsari et al., 2005; Tan et al., 2007). By following the rapid-prototyping algorithms, a 3D model can be

various materials deposited in thin films is demonstrated.

**2. 3D structuring by two-photon photo-polymerization (TPP)** 

the method in *XY* directions, such structures have limited complexity.

competitive ones.

resolution of tens of micrometers.

created layer by layer inside the material.

The laser workstation for material processing consists in the following main parts: the femtosecond laser, the beam delivery and focusing optics, and the scanning mechanics. In our experiments we use a set-up compatible with different type of laser structuring methods, by laser ablation as well as by TPP. The set-up consists in a modular microscope for laser writing built to be coupled with laser beams at different radiation wavelengths, depending on the experimental requirements. For applications such as laser ablation, requiring high pulse energy from tens nano-Joules up to micro-Joules, a laser amplifier CPA2101 system (Clark-MXR) was employed. This laser emits femtosecond pulses of as much as 0.6 mJ energy, 200 fs pulse duration at 775 nm wavelength and 2 KHz repetition rate. In experiments requiring high repetition rate, such as TPP, a 75 MHz repetition rate femtosecond laser oscillator Synergy Pro (Femtolasers) was coupled with the processing microscope. The oscillator delivers laser pulses of 5 nJ pulse energy, 10 fs duration at 790 nm central wavelength, with spectral bandwidth of 100 nm. The beam delivery optics is interchangeable and can be easily replaced with optics adapted for other working wavelengths, such as harmonics of fundamental wavelength.

Fig. 1. Experimental set-up for laser processing. A – Attenuator with half waveplate and Glan Polarizer; DC – Dichroic mirror; L1 – Focusing lens; S – Sample; *XYZ* –Translation stage; CCD – Video camera.

Ultrashort Pulsed Lasers – Efficient Tools for Materials Micro-Processing 267

promoter film deposition is sometime a good solution to increase adhesion, especially in the case of glass substrate with reduced adherence. Producing photoresists with enhanced adhesion is also desirable, since the treatment of the substrate is simplified. All the following processing steps are preferable to deploy in a controlled environment since the dust can

The commonly used technique to deposit films of non-polymerised resin on the substrate is the spin-coating. The thickness of the resulting photoresist film can be controlled by the viscosity of the initial monomer and the spinning speed. It can vary from tens of micrometers down to few micrometers for spinning speeds varying typically from 500 rot/min up to 6000 rot/min. If structures with height of the order of hundreds of micrometer are required, the sample has to be prepared differently. A drop of resin is placed between two transparent slides separated by spacers with fixed width of hundreds of micrometers up to 1-2 mm. Thicknesses below 1 m are also possible for specific applications. In such case, resins with low viscosity are spin-coated at speed up to 10.000- 12.000 rot/min. However, the uniformity of the film thickness is difficult to be controlled on

After deposition, the processing protocol is followed by a specific treatment for each type of photoresists. Since the viscosity of the material is controlled by concentration of the solvent present in the liquid resin, the solvent has to be removed prior to laser treatment of the material. An insufficient removal of solvent will result in soft and deformed final microstructure, with low adherence to the substrate. For example, in the case of SU-8, the deposited film resist has to be baked for 15-30 minutes at 950C. This is the so called Hard Backing step. The baking time depends on the photoresist thickness. The increase of the Hard Backing temperature should in principle accelerate the removal of the solvent. However, a fraction of solvent always remains in the volume of the material, and increasing the Hard Backing temperature causes a gradient of the solvent concentration and distortion of the microstructure. Low bake temperature for longer time can provide a constant evaporation rate, and finally low and uniform concentration of the remaining solvent in the

Other materials such as Ormocers and Ormosil can be directly processed without any intermediate thermal treatment. Such photoresists remains liquid till are laser irradiated, then the cross-linking process transforms the resin into a solid. However, the removal of solvent before laser processing will significantly improve the fabrication of the microstructures, especially in terms of aspect ratio, adhesion to the substrate and

Laser exposure conditions, such as laser fluence and scanning speed should be established prior to fabrication of complex structures. The map of processing parameters for a commercial Ormocore, a derivate of Ormocer photoresist, is shown in figure 2. Different scanning speeds and laser fluences are tested. The scanning speed were varied from 0.1 to 0.8 mm/s, and the laser fluence from Fo to 2Fo, with Fo = 0.28 J/cm2. The structures from figure 2 were obtained by focusing the laser beam at 2 KHz repetition rate by a 75 mm lens with a 25 m beam diameter at the beam waist. In order to determine the optimal laser

easily stick to the photoresist film, compromising the final geometry.

**2.2.2 Photoresist films deposition and treatment** 

resulting solid film resin (Liu et al., 2005).

**2.2.3 Laser irradiation of the photoresists** 

smoothness of the surfaces.

the entire surface.

The laser workstation has a modular configuration as shown in figure 1. The main modules are: the attenuation module, the focusing optics, the *XYZ* sample translation stages, and the visualisation system with video camera.

The laser energy can be continuously attenuated by a motorised half waveplate placed in front of a Glan polarizer providing a 300:1 extinction rate. In the case of extremely short femtosecond pulses, a reflective polarizer is used for avoiding the temporal stretching laser pulses due to the dispersion introduced by bulk material. A dielectric mirror reflects the laser beam at 800 nm to the focusing optics and transmits the visible radiation to the visualization system.

For beam focusing, different microscope objectives or lenses with a wide range of numerical aperture, adapted to a specific application, are used. The same focusing objective is used for visualization. The focusing optics is chosen depending on the desired resolution of the processed sample or the depth of the structure to be obtained. Submicrometer resolutions were obtained with a 100x Mitutoyo microscope objective with 0.5 numerical aperture and long working distance of 12 mm, allowing the processing deeply in the volume of a transparent material. With such objective, the focused laser spot has a minimal diameter of about 1 m, and a confocal parameter of about 2 m.

In case of 100-nm bandwidth pulses, the positive group delay dispersion (GDD) introduced by the beam delivery optics was partially compensated by an optical temporal compressor formed by a pair of SF11 prisms. By changing the distance between the two prisms, the negative dispersion in the compressor is adjusted and laser pulses with less than 40-fs duration were delivered to the processed materials.

For processing the sample according to a computed design, a *XYZ* translation (Nanocube - Thorlabs) with motorized stages and piezo drivers is used. The stage has a total travel range of 4x4x4 mm3 with hundreds of nm accuracy. The embedded piezo stage has 20 m travel range per each axis and accuracy down to 5 nm. For longer travels, linear stages with 50 mm maximum travel are used. The maximum translation speed is 2 mm/s.

The sample focusing is done by the visualization system with CCD and a 200 mm tube lens. The resolution of the visualization system is better than 1 m when a 100x microscope objective is mounted.

Dedicated software was realized for controlling the laser processing of samples. Common structures such as parallel lines, grids for 2D geometries, alphanumerical characters are included in a predefined library. Also, 3D predefined periodical structures such as vertical pillars in rectangular, hexagonal, or woodpile symmetries can be fabricated.
