**4. Enhanced near-field laser lithography**

The current trend towards sub-micrometer structures requires new methods and technologies of surface structuring. The traditional masking approach in optical lithography

The antenna gain, which relates the intensity of an antenna radiation in a given direction to the intensity that could be produced by a hypothetical ideal antenna that radiates isotropically (equally in all directions), was computed using the De Friis formula (Balanis, 1997). The gain was nearly constant in the 28 ÷ 29 GHz frequency band, with a maximum value of 2.99 dB at 28.6 GHz. The area occupied by the antenna is 2.15 0.6 mm2. There is a

The band-pass filter was made by cascading a number of identical CRLH cells. The directional coupler consists of two coupled CRLH artificial lines, each composed of two identical cascaded CRLH cells. Both band-pass filter and directional coupler (figure 9a,b)

The geometrical dimensions of the interdigital capacitor and inductive CPW stub of the CRLH band-pass filter at 50 GHz working frequency were calculated: sC = 5 μm, lC = 250 μm, wC = 10 μm, gC = 65 μm, the number of capacitor digits = 10, wL = 42 μm, sL = 10 μm, and lL = 212 μm (see figure 5a). Unlike the antenna CRLH cell, the filter CRLH cell has only one inductive CPW stub. A silicon substrate plated with 2000 Å Au/500 Å Cr was used. The working parameters of the band-pass filter structure were measured in the 40-60 GHz frequency range. The S11 parameter evaluation show return loss values < -15 dB in a frequency range of 46.32 GHz ÷ 55.4 GHz, whereas losses in the same frequency range,

Fig. 9. SEM images of CRLH band pass filter (a) and directional coupler (b) structures

For the CRLH directional coupler the port 1 is the input port, while the ports 2, 3 and 4 are the transmission, coupled, and isolated ports, respectively (figure 9b). Measured return losses (S11) were better than -20 dB for a frequency domain between 24.01 GHz ÷ 38.11 GHz, whereas the isolation (S41) is greater than 30 dB in a large frequency range, exceeding the

The current trend towards sub-micrometer structures requires new methods and technologies of surface structuring. The traditional masking approach in optical lithography

(a) (b)

considerable size reduction compared to a standard λ/2 patch antenna.

**3.3 Millimeter wave CRLH band-pass filter and directional coupler** 

were microprocessed by the two-steps technology described in section 3.2.

given by S21 parameter, are around 6 ÷ 7 dB (Sajin et al., 2010a).

microprocessed by laser ablation.

domain 20-40 GHz (Sajin et al., 2010b).

**4. Enhanced near-field laser lithography** 

is limited to a minimal resolvable feature size given by the diffraction limit. Optical nearfields were explored for their ability to localize optical energy to length scales smaller than half-wavelength. This localization was achieved for ultrasensitive detection (Fischer, 1986) and for high-resolution optical microscopy and spectroscopy (Novotny & Stranick, 2006). Near-field optics research essentially determined the advance of nano-optics (Novotny & Hecht, 2006), single molecule spectroscopy (Xie & Trautman, 1998), and nanoplasmonics (Barnes et al., 2003). Micron and sub-micron patterning was performed by "direct writing" where the laser light is just projected onto a sample via a direct-contact mask or by the interference of laser beams. Another technique is based on scanning near field optical microscopy (SNOM). Here, the light is coupled into the tip of a solid or hollow fiber. By positioning the substrate within the near field of the fiber tip, one can produce patterns with widths that are beyond the diffraction limit. Structures with lateral dimension less than 30 nm, well below the radiation wavelength, could be produced underneath the tip (Gorbunov & Pompe, 1994). This technique has been employed for nanolithography, ablation, material etching and local reduction of oxides.

Spherical particles can act as spherical lenses and therefore increase the laser intensity if their diameter is bigger than the laser wavelength. Laser-induced submicron patterning of surfaces has been demonstrated by means of two-dimensional colloidal lattices of microspheres that are formed by self-assembly (Piglmayer et al., 2002). If the diameter of spherical particles is of the order of magnitude of the radiation wavelength, according with the Mie solution to Maxwell's equations (Mie, 1908), the optical field enhancement occurs quite near laser irradiated particles. Electric field intensity distributions were calculated with the finite-difference time-domain (FDTD) method using simulation software (RSoft Design Group). For colloidal sub-wavelength particles (700 nm diameter of the particle, 532 nm wavelength radiation) placed on a glass substrate, the electric field can be greatly enhanced (by a factor of ~ 8) in the near-surface field region under particles as shown in the simulation from figure 10a (Ulmeanu et al., 2009a). The enhancement value quoted above is a theoretical estimation and the actual field strength enhancement may differ due to various influencing factors, such as surface roughness and oxidation of the thin film layer.

Fig. 10. (a) Field intensity enhancement of a 700 nm colloidal particle sitting on a glass substrate in a free space for = 532 nm (b) Experimental setup (BS – beam splitter, AT – attenuator, L – convergent lens, DET – pyroelectric detector, M – high reflectivity mirror.

We demonstrated surface patterning in the enhanced near-field by scanning a quasi-Gaussian laser beam through a self-assembled monolayer of colloidal particles onto different substrates:

Ultrashort Pulsed Lasers – Efficient Tools for Materials Micro-Processing 279

the near-field experiments gave a non-uniform pattern in the case of the Ag thin film surface. The discrepancy between Ag and Au thin film may potentially be due also to appreciable surface roughness of the thermally evaporated Ag thin film, that has a great

The near-field processing in the case of Au thin film indicated different patterns produced by a single shot laser: from bowl type structure to bumps depending on the fluence, as in AFM investigations presented in the figure 12a,b. For the Co thin film near-field ablation, we were able to determine the dependence of the holes diameter versus the depth at different fluences, the holes having shapes from bowl type to an almost rectangular shape with flat bottom. By comparing near-field processing on Co and Au thin films (figure 12c), we put into evidence holes with different diameters (*d*) and heights (*z*) at fluences nearby the thresholds:

For a multilayer Co(3 nm)/Cu(6 nm)/Co(20 nm) material, we investigated a method of preferential laser processing of a specific layer by changing the laser fluence instead of selecting different laser wavelengths. Figure 13a represents a schematic multilayer structure. The heterostructure composition determined by Sputtered Neutral Mass Spectroscopy (SNMS) measurements is shown in figure 13b. We demonstrated the formation of ordered areas of nano-holes in a multilayer structure using single femtosecond laser pulse irradiation (200 fs, = 775 nm) on 700 nm diameter colloidal particles (Ulmeanu et al., 2010). The ablation threshold for the multilayer structure in the enhanced near-field regime was

Fig. 12. (a) 3D AFM image; (b) scan lines for the structures produced by a single shot laser in the case of the near-field experiments for Au thin film; (c) depth profiles of the holes for Au

Fig. 13. (a) Schematic representation of the multilayer structure; (b) heterostructure composition determined by the SNMS measurements; (c) holes depths and widths distribution for the Co(3nm)/Cu(6nm)/Co(20 nm) multilayer structure (peak laser fluence, Flas = 0.04 J/cm2).

dAu = 145 nm, *z*Au = 14 nm (Au thin film), *d*Co = 250 nm, *z*Co = 60 nm (Co thin film).

impact on the optical near-field distribution.

measured to be 2.8 x 10-2 J/cm2.

and Co thin films.

glass and metallic thin films of nanometers thickness like Ag, Au, Co, Cu thin films as well as Co/Cu/Co multilayer structures (Ulmeanu et al., 2009b). Single pulse laser ablation of monolayer thin films and multilayer structures was performed by irradiating with a Nd:YAG picosecond laser (400-ps pulsewidth at =532 nm) and a Ti:sapphire femtosecond laser system (200-fs pulsewidth at = 775 nm). The experimental setup is shown in figure 10b.

The laser beam was focused on the colloidal particles using a 75 mm convex lens corrected for reduced spherical aberrations. The irradiating energy was adjusted through neutral attenuators. For the control of the irradiation process, a certain fraction of the incident laser energy is split off by a glass splitter and measured with a pyroelectric detector.

The sample was mounted on a precision three-axis xyz stage and was aligned for normal incidence of the laser beam. The ablation spots were produced with single pulse shots at different laser energies. All experiments were performed under ambient conditions. The irradiated area of the sample was imaged using Scanning Electron Microscpy (SEM) and Atomic Force Microscopy (AFM) to obtain the surface maps and crater profiles. From our investigations, it is obvious that, using 2D colloidal masks, arrays with regularly arranged holes can be created through a parallel process on the whole surface (Ulmeanu et al., 2009a). The shape of the holes depends on the laser fluence and thermophysical properties of the surfaces involved in the experiments (figure 11).

Fig. 11. SEM images for the near-field experiments at fluences above the threshold level: (a) glass substrate (*F*peak= 7.3 J/cm2), (b) Co thin film (*F*peak = 0.82 J/cm2), (c) Ag thin film (*F*peak = 0.18 J/cm2) and (d) Au thin film (*F*peak = 0.17 J/cm2). Scale bar: 10 m.

For the case of glass near-field processing, we have obtained regular nano-holes, with a uniform distribution of depths and heights at the same fluence value (figure 11a). The smallest diameter of the holes, according to the SEM images, was 100 nm. For the case of Ag thin film (figure 11c), bumps and nanoholes were visible away from the peak fluence location, but not in a regular structure like in the case of the Au thin film (figure 11d) and Co thin film (figure 11b). Despite of similar ablation thresholds of Ag and Au thin films, due to a high thermal diffusivity coefficient and low adherence of the thin film on glass substrate,

glass and metallic thin films of nanometers thickness like Ag, Au, Co, Cu thin films as well as Co/Cu/Co multilayer structures (Ulmeanu et al., 2009b). Single pulse laser ablation of monolayer thin films and multilayer structures was performed by irradiating with a Nd:YAG picosecond laser (400-ps pulsewidth at =532 nm) and a Ti:sapphire femtosecond laser system

The laser beam was focused on the colloidal particles using a 75 mm convex lens corrected for reduced spherical aberrations. The irradiating energy was adjusted through neutral attenuators. For the control of the irradiation process, a certain fraction of the incident laser

The sample was mounted on a precision three-axis xyz stage and was aligned for normal incidence of the laser beam. The ablation spots were produced with single pulse shots at different laser energies. All experiments were performed under ambient conditions. The irradiated area of the sample was imaged using Scanning Electron Microscpy (SEM) and Atomic Force Microscopy (AFM) to obtain the surface maps and crater profiles. From our investigations, it is obvious that, using 2D colloidal masks, arrays with regularly arranged holes can be created through a parallel process on the whole surface (Ulmeanu et al., 2009a). The shape of the holes depends on the laser fluence and thermophysical properties of the

Fig. 11. SEM images for the near-field experiments at fluences above the threshold level: (a) glass substrate (*F*peak= 7.3 J/cm2), (b) Co thin film (*F*peak = 0.82 J/cm2), (c) Ag thin film (*F*peak =

For the case of glass near-field processing, we have obtained regular nano-holes, with a uniform distribution of depths and heights at the same fluence value (figure 11a). The smallest diameter of the holes, according to the SEM images, was 100 nm. For the case of Ag thin film (figure 11c), bumps and nanoholes were visible away from the peak fluence location, but not in a regular structure like in the case of the Au thin film (figure 11d) and Co thin film (figure 11b). Despite of similar ablation thresholds of Ag and Au thin films, due to a high thermal diffusivity coefficient and low adherence of the thin film on glass substrate,

0.18 J/cm2) and (d) Au thin film (*F*peak = 0.17 J/cm2). Scale bar: 10 m.

(200-fs pulsewidth at = 775 nm). The experimental setup is shown in figure 10b.

energy is split off by a glass splitter and measured with a pyroelectric detector.

surfaces involved in the experiments (figure 11).

the near-field experiments gave a non-uniform pattern in the case of the Ag thin film surface. The discrepancy between Ag and Au thin film may potentially be due also to appreciable surface roughness of the thermally evaporated Ag thin film, that has a great impact on the optical near-field distribution.

The near-field processing in the case of Au thin film indicated different patterns produced by a single shot laser: from bowl type structure to bumps depending on the fluence, as in AFM investigations presented in the figure 12a,b. For the Co thin film near-field ablation, we were able to determine the dependence of the holes diameter versus the depth at different fluences, the holes having shapes from bowl type to an almost rectangular shape with flat bottom.

By comparing near-field processing on Co and Au thin films (figure 12c), we put into evidence holes with different diameters (*d*) and heights (*z*) at fluences nearby the thresholds: dAu = 145 nm, *z*Au = 14 nm (Au thin film), *d*Co = 250 nm, *z*Co = 60 nm (Co thin film).

For a multilayer Co(3 nm)/Cu(6 nm)/Co(20 nm) material, we investigated a method of preferential laser processing of a specific layer by changing the laser fluence instead of selecting different laser wavelengths. Figure 13a represents a schematic multilayer structure. The heterostructure composition determined by Sputtered Neutral Mass Spectroscopy (SNMS) measurements is shown in figure 13b. We demonstrated the formation of ordered areas of nano-holes in a multilayer structure using single femtosecond laser pulse irradiation (200 fs, = 775 nm) on 700 nm diameter colloidal particles (Ulmeanu et al., 2010). The ablation threshold for the multilayer structure in the enhanced near-field regime was measured to be 2.8 x 10-2 J/cm2.

Fig. 12. (a) 3D AFM image; (b) scan lines for the structures produced by a single shot laser in the case of the near-field experiments for Au thin film; (c) depth profiles of the holes for Au and Co thin films.

Fig. 13. (a) Schematic representation of the multilayer structure; (b) heterostructure composition determined by the SNMS measurements; (c) holes depths and widths distribution for the Co(3nm)/Cu(6nm)/Co(20 nm) multilayer structure (peak laser fluence, Flas = 0.04 J/cm2).

Ultrashort Pulsed Lasers – Efficient Tools for Materials Micro-Processing 281

In LIFT experiments some parameters, like distance *d* between the donor film and acceptor substrate, or laser fluency, have to be investigated in order to find the optimal processing conditions for the deposition of a certain material. In our LIFT experiments, we demonstrated the transfer of a polymer material, an Ormocer photoresist, using our laser processing workstation. The polymer layer was directly deposited by spin coating on a glass substrate, without any buffer layer. The distance between donor and acceptor was fixed at 15 m. Series of 5x5 pixels were created by single pulses, shot by shot. The laser source was the Clark MXR CPA-2101 laser, with 200 fs pulse duration and 775 nm wavelength, externally triggered for single shot experiments. The sample was translated from a pixel to another by a computer controlled translation stage. The distance between pixels was 50 m. The laser was focused to the donor layer by a 75 mm focusing lens with about 25 m focus

Optical images of the structures, as transferred to the acceptor substrate at different pulse energies, are shown in figure 15. The quality of the obtained structures strongly depends on the pulse energy. At the highest pulse energy used, non uniform droplets results, sparse on the donor surface. Decreasing the pulse energy the transferred droplets remain

Fig. 15. LIFT generated microstructures at different laser energy. Scale bar: 100 m.

2006). This technique can be efficiently used as a microprinting method.

The smallest size of the droplets obtained in these experimental conditions was about 2 m. Smaller structures, such as nanodroplets, can be also transferred (Banks et al., 2006) and even an entire microstructure or a microdevice could be deposited by LIFT (Piqué et al.,

Various experimental techniques for materials micro-processing based on ultra-short pulsed

Direct femtosecond laser writing technique by two-photon photo-polymerization was used to produce microstructures in the volume of transparent materials. By this technique, photonic devices such as photonic crystals, optical couplers, diffractive elements, 3D structures for microfluidics, scaffolds for tissue engineering, and other MEMS can be

Femtosecond laser ablation was used to produce 2D microstructures on different materials surface. Electronic devices, based on CRLH transmission lines having metamaterials characteristics in the tens-GHz frequency range, such as band-pass filters, antennas, and directional couplers were manufactured by combined photo-lithography and femtosecond laser ablation techniques. A microprinting method, based on laser induced forward transfer

spot diameter. The energy per pulse was varied from 2.5 to 7.5 J.

well defined.

**6. Conclusions** 

lasers are presented.

fabricated.

The dependence of the holes diameter versus the depth at different fluences, obtained by AFM measurements, is showed in the figure 13c. As expected, with the decrease of the laser energy from the central zone to the edges of the irradiated area, the depth of the holes is decreasing. In the case of multilayer structures, the hole depth decrease depending on the laser fluence corresponds to the ablation of the first, second or third layer. This type of planar metal/dielectric interfaces with a selective distribution of layers can open new perspective in the excitation of propagating surface plasmons and, consequently, in creating transducers for sensing of biomolecular recognition reactions.
