**2.2.2 Photoresist films deposition and treatment**

266 Recent Advances in Nanofabrication Techniques and Applications

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

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

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

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

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

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

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

For a reproducible fabrication of a required design a good control of processing protocol is necessary. The main processing steps are: the substrate preparation; deposition of the photosensitive material; laser irradiation; development and final curing of the polymerized

A critical step is the initial treatment of the substrate in order to provide a good adhesion of the polymerized structure. The frequently used treatments are cleaning by reactive plasma etching, chemical corrosion, or a simple ultrasonic bath in solvents followed by thermal dehumidification, depending on the substrate nature (silicon wafer, glass, fused silica). A

visualisation system with video camera.

about 1 m, and a confocal parameter of about 2 m.

duration were delivered to the processed materials.

maximum travel are used. The maximum translation speed is 2 mm/s.

pillars in rectangular, hexagonal, or woodpile symmetries can be fabricated.

**2.2 Laser processing protocol of photosensitive materials** 

**2.2.1 Substrate preparation for photoresist deposition** 

visualization system.

objective is mounted.

structures.

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 the entire surface.

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

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 smoothness of the surfaces.

### **2.2.3 Laser irradiation of the photoresists**

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

Ultrashort Pulsed Lasers – Efficient Tools for Materials Micro-Processing 269

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

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*

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.

**a) b c)** 

Scale bar: 100 m. The solid structure is formed by TPP in layer by layer approach.

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

bulk material, from a computer generated solid by CAD software.

direction was about 7 m.

irradiation dose, structures of *XY* parallel lines, with 50-m period on each direction, were produced under different exposure conditions. For low speed and high laser fluence, the polymer is over exposed and damaged. For example, in case of scanning speed of 0.1 mm/s, the laser fluence of 1.5-2 Fo was over the damage threshold. With a laser spot diameter of 25 m and a laser repetition rate of 2 KHz, we can estimate about 520 laser pulses per each spot contributing to the polymerization process. Under these conditions, the irradiation dose is high enough to damage the material by producing bubbles in the photoresist. The structure optical quality is compromised.

Fig. 2. Processing map for the determination of optimal laser irradiation parameters.

At 1.5 Fo laser fluence, but much higher scanning speed of 0.8 mm/s, only 65 laser pulses contribute to the photopolymerization process. Such irradiation parameters produce smooth structures with good optical quality, transparent to the visible light. Similar structures were obtained for Fo fluence and 0.1 mm/s processing speed. Decreasing the laser fluence to Fo, thinner structures can be obtained. From practical point of view, higher scanning speed is preferred because of shorter processing times. However, the accuracy of the fabricated structures is better at lower scanning speeds. The optimum parameters have to be a compromise between the total processing time and the processing quality.

#### **2.2.4 Development**

After laser exposure, the non irradiated material is removed by rinsing the sample in a specific solvent, depending on the nature of the photoresist. In the case of SU-8 photopolymer, an intermediate step of post-exposure bake is needed, before rinsing, for accelerating the process of cross-linking of molecules and then polymerization. For other materials such as Ormocers, Ormosil, or chalcogenic glasses, this step is not required, and the processing protocol is simplified. In the case 3D microstructuring of liquid resist, the UV curing of the structures after development is preferred for strengthening the processed microstructure.

irradiation dose, structures of *XY* parallel lines, with 50-m period on each direction, were produced under different exposure conditions. For low speed and high laser fluence, the polymer is over exposed and damaged. For example, in case of scanning speed of 0.1 mm/s, the laser fluence of 1.5-2 Fo was over the damage threshold. With a laser spot diameter of 25 m and a laser repetition rate of 2 KHz, we can estimate about 520 laser pulses per each spot contributing to the polymerization process. Under these conditions, the irradiation dose is high enough to damage the material by producing bubbles in the photoresist. The structure

**0.1 mm/s 0.4 mm/s 0.8 mm/s**

**500 m** 

Fig. 2. Processing map for the determination of optimal laser irradiation parameters.

compromise between the total processing time and the processing quality.

At 1.5 Fo laser fluence, but much higher scanning speed of 0.8 mm/s, only 65 laser pulses contribute to the photopolymerization process. Such irradiation parameters produce smooth structures with good optical quality, transparent to the visible light. Similar structures were obtained for Fo fluence and 0.1 mm/s processing speed. Decreasing the laser fluence to Fo, thinner structures can be obtained. From practical point of view, higher scanning speed is preferred because of shorter processing times. However, the accuracy of the fabricated structures is better at lower scanning speeds. The optimum parameters have to be a

After laser exposure, the non irradiated material is removed by rinsing the sample in a specific solvent, depending on the nature of the photoresist. In the case of SU-8 photopolymer, an intermediate step of post-exposure bake is needed, before rinsing, for accelerating the process of cross-linking of molecules and then polymerization. For other materials such as Ormocers, Ormosil, or chalcogenic glasses, this step is not required, and the processing protocol is simplified. In the case 3D microstructuring of liquid resist, the UV curing of the structures after development is preferred for strengthening the processed

optical quality is compromised.

 **2 F0**

 **1.5 F0**

 **F0**

**2.2.4 Development** 

microstructure.
