N° d’ordre:

Ecole Doctorale Mathématiques, Sciences de l’Information et de l’Ingénieur UNISTRA – INSA – ENGEES

THÈSE (résumé)

présentée pour obtenir le grade de

Docteur de l’Université de Strasbourg Discipline : Electronique, Electrotechnique, Automatique

Spécialité Photonique

par

Andri ABDURROCHMAN

Photonic jet optimization for spatial resolution improvement in direct pulse laser micro-etching

Résumé

Andri Abdurrochmann – Thesis summary - may 2015

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1. Introduction In material processing, micrometer size etching is commonly achieved by photo-

lithography or ultra-short pulsed laser processing. These techniques are complex and have expensive production costs. The use of photonic jet (PJ), a spatial concentration of a light beam in the near-field of a microsphere, having a radius slightly larger than the wavelength, may have interest into offering a solution at a cheaper cost. So, we have considered a nanosecond pulsed near-infrared (ns-NIR) laser to advocate the proposal. We chose a Nd:YAG and a YLIA fiber laser, for their common use in industrial processes, their relatively cheap price and their availability as well-packaged sources. Using such lasers, it is possible to achieve direct etching with a size of 50 µm on silicon wafer. By applying PJ we have demonstrated the possibility of decreasing this size ; in addition, glass substrates can then also be patterned with such lasers using PJ. Therefore, the main objective of this thesis is to demonstrate that PJ can be presented as a new way of improving the lateral resolution of etching with a standard ns-NIR laser beam. Experiments with glass microspheres and an optical fiber with a specially designed tip have been conducted. A simplified modelling of light-matter interaction has been conducted.

2. The photonic jet : general background Field enhancement and diffraction limit

The first article proposing a novel lithography technique by applying sphere-enhanced laser intensity was published in 2000. It reported subwavelength structures on Si (100) using 0.5 µm silica spheres illuminated by 200 pulses of 23 ns ultraviolet laser beam, knowing that without spheres, no surface damage can be observed. Other articles have reported similar structures, capable of going beyond the diffraction limit of optical systems. In 2004 the phenomenon was described and identifed as the photonic nanojet ; we refer to it as a photonic jet (PJ) in our work. It is defined as a highly localized non-evanescent propagating beam generated at the shadow-side of a micrometric dielectric sphere or cylinder. This naming is necessary to distinguish it from already well-known near-field optics; namely: (1) it is a non-evanescent propagating beam that has a sub-wavelength width along a pathway which can extend to more than ~2λ; (2) it’s minimum full-width at half maximum (FWHM) can be smaller than the classical diffraction limit; (3) it is a non-resonant phenomenon; (4) it has a high intensity ; (4) the enhancement of the incident wave intensity can exceed 200 times. From these observations, the idea arose to try to use it to pattern a surface, and for that, to attempt to optimize parameters such as the intensity concentration factor, width and length of the jet and position of the highest intensity zone.

Photonic jet validations and material interaction PJ’s occurrence can be observed directly or indirectly. Direct observation is

made by capturing the PJ using confocal microscope for visible light or antenna for microwaves. Indirect observation is performed by looking at the effect of PJ, such as a mark on a material. Several papers have reported such marking due to the local optical field enhancement. In our work we have gone beyond this by demonstrating the potential of standard ns-IR laser for eching a surface in situations where direct illumination does not make it possible to reach the ablation level.

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Other potential applications of photonic jet Several potential applications of PJ’s have been proposed, such as for

example : microscopy, low-loss waveguide, laser surgery, fluorescence enhancement, nanoparticle detection, optical trapping, optical data storage and nano-sphere lithography/ patterning.

3. Photonic jet etching using microspheres We have considered two ways of realizing a photonic jet using a ns-IR laser

beam. The first one was by using microsphere, a solution already well described in the literature. Our main objective has been to explore the PJ–material interaction and identify operational parameters for an etching application. We have modeled the electric field of the beam on the shadow side of glass microspheres, in free-space, for unitary incident plane-wave, at a wavelength of 1 µm. The PJ parameters obtained from simulations have been compared with the experimental etching features.

Experimental setup Our experimental scheme was adapted from previous publications. Even so

there are two main distinctive points: - First, the source we used is a ns-NIR laser while previous reports mentioned

ultra-short pulsed or ultraviolet lasers. Our goal was indeed to decrease the lateral resolution of material marking with such standard lasers

- Second, the microspheres placed on the substrate were illuminated within the defocused range of the laser beam; the idea is to cover a larger area in order to illuminate simultaneously as many microspheres as possible.

a.

b.

Figure 1. (a) The experimental set-up of PJ etching using glass or barium-titanate microspheres on silicon wafer and glass-slide as the sample material. (b) A typical etching shape of PJ

etching on silicon wafer using 6 µm glass microspheres

Fig. 1a shows our experimental setup with the microspheres and silicon wafer within the defocused range of the laser beam. The microspheres are made of glass (n! = 1.5, mean diameter D! = 4, 6, 24 and 35 µm), or barium-titanate (n!=1.9, mean diameter D!= 42, 69, and 87 µm). The localization of microspheres and resulting patterns have been made easy through square cells marked on the substrate prior to the experiment. The cells also allowed us to apply laser illumination at the right location. The laser beam parameters to be adjusted are the average power, the frequency rate, the number of pulses and the distance z

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between the beam waist and the surface. Observation and size estimation of PJ surface etching was carried out with an Olympus® Vanox-T microscope and a Zygo NewView® profilometer.

PJ Etching of silicon wafer We made our first tests of PJ etching on silicon wafers using glass

microspheres. PJ etching was obtained for illumination with more than one laser pulse. The smallest observed mark (Fig.1b) has a diameter of 1.1 ± 0.2 µm and was obtained using a 4 µm glass microsphere with the following laser setting: Nd:YAG laser, 2 pulses, 28 ns pulse width, an energy of 1mJ, a defocussing distance from a waist of 22±1 mm. This etching size of 1 µm is 44 times smaller than the one usually obtained with direct lens focusing. For the smaller microspheres with diameter of 4 or 6 µm, the lateral size of etching is close to the size of the beam concentration obtained in the simulations.

We have also made experiments with barium titanate microspheres. We noticed that the laser illumination causes some of the microspheres (the larger ones) to move from their original positions, which could be a drawback in a real situation of marking at a specific place on the substrate. Some microspheres are broken during the process. The result is a complex and large surface pattern, with little correlation with the initial microspheres distribution. We conclude that barium titanate microspheres are not a good option to produce the PJ in silicon etching situation.

PJ etching of glass Despite the low absorption properties of glass in the infrared range, some PJ

etching marks are obtained on glass slides using Nd:YAG laser and larger glass spheres (24 and 35 µm diameters) or barium titanate microsphere. The shape of PJ etching is circular with a crater in the center (Fig. 2a).

(a)

(c)

(b) Figure 2. (a) A PJ etching on glass taken by Zygo®. (b) The intensity distribution map computed on

the surface. (c) The correspondence between PJ etching profile and intensity profile.

Discussion The shape of the pattern obtained by PJ etching is not the same in the case of

glass substrate or silicon wafers. A possible explanation is that larger spheres are required to etch glass; as a result, the PJ is no longer located on the sphere surface but a few micrometers further on. However the absorption takes place at the contact zone between the sphere and the substrate. We have run our simulation to acquire the intensity map on the material surface. Fig. 2c shows a

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cross-section of the intensity distribution on the surface and the recorded etched profile. A good correlation is observed.

The influence of the damage threshold and pulse duration can be estimated by comparing etching achieved using 160 ns pulses from a YLIA laser and 28 ns pulses from a Nd:YAG laser. We have considered the laser-induced damage threshold of sample materials and the PJ etching diameter and we have applied a threshold on the intensity map to obtain the same lateral size. On silicon the threshold appears to be linked to the fluence, whereas on glass the maximum power density appears to be the more relevant parameter.

4. Photonic jet etching using optical fiber with spheroid tip Previous work conducted in our team has pointed out the possibility to use light

exiting a waveguide to create a PJ. In this thesis work we have explored its application to eching. The three main issues that motivated this research are the following : - Flexibility; it is possible to move the optical fiber tip to produce an etched line or

curve. Besides, the set-up can be adapted to cover a large area. - Precision; a precise control of the optical fiber/sample distance to apply the PJ

is feasible. - Efficiency: an optical fiber is reusable many times, the number of pulses

applied is not limited. This part of our work can be presented as original since, to our knowledge, such

an attempt has not been made before. Waveguide driven photonic jet

The field distribution out of the fiber has been calculated. Fig. 3.a shows one of our simulations for the case of a 100 µm planar waveguide. For our experimental tests, we have used an optical fiber with a spheroid tip. The shape has been chosen to insure that the highest intensity of PJ is not located too close to the fiber tip. We have considered a multimode TEQSTM coated fiber with double optical cladding, 200 µm core diameter (Fig. 3.b). The highest laser power contained in the PJ exiting the fiber was 7.2 watts.

a

b

Figure 3. (a) Total electric field of a half planar-waveguide (diameter of 100 µm) with spherical tip. (b) Microscope view of the 200 µm core optical fiber with spherical tip.

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Experimental results Tests were made on a silicon wafer. In this experiment, the end of optical fiber

was pasted to a XYZ positioning micro-stage, to allow the adjustment of its position in relation to the surface. Fig. 4.a shows the etching marks for different laser powers and pulse numbers. We notice in particular that :

- the highest intensity is around 150 ± 5 µm from the tip; - the minimum number of pulses needed is 4 pulses; - the smallest etching size on silicon is 17.5 µm (fig. 4.b).

a

b

Figure 4. (a) Microscope view of etchings achieved with the optical fiber tip for different injected powers and pulse numbers. (b) Smallest mark profile (4 pulses, 7,2 W).

Discussion In the simulation we have considered the “two-dimensional version” of optical

fiber with a spheroid tip with the following parameters : gaussian mode, λ=1 µm; a maximum value of electric field of 1 V/m. The waveguide core has a thickness of 200λ, an index of 1.457 with a cladding index of 1.44. The simulations indicate that the highest intensity is located at 156 µm from the tip, in a good agreement with the experimental observations. The estimated FWHM lies in the 1-2 µm range. We have made calculations for higher order propagation modes. Unsurprisingly, the width of the jet depends on the numerical aperture of the waveguide. We also observed the appearance of off-axis zones of high intensity.

In the experiment we had to use a multimode fiber, for which we could obtain specially designed tips. This is the reason why the smallest etching had a size of the order of 20 µm, much larger than the ones obtained with PJ etching with glass microspheres.

To go further and create smaller marks with this technique, it would be necessary to consider a single-mode optical fiber or multimode fiber with low numerical aperture. If the use of a multimode fiber is unavoidable, due to the difficulty in designing a special tip for single mode fibers, then attention should be paid to excitation of lower order modes.

5. Conclusions and perspectives In this work, we have shown that the lateral resolution of laser etching using ns-

IR laser, can be improved if light travels through a glass microsphere or an optical fiber with a spheroid tip, in conditions favorable to the formation of photonic jets. The etching size obtained on silicon wafer with the combination of ns-IR laser and

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PJ is 44 times smaller than that of direct etching. Glass, which does not lend itself to eching with such lasers, can be etched if PJ is used.

The second part of our work is original, since no attempt had been made before in applying PJ out of a waveguide with a spheroid tip to material etching with ns-IR laser. The sizes of the marks obtained in the experiment were significantly higher than the estimation obtained for the ideal situation. However the result is still interesting from a practical point of view; suggestions for improvement have been made. It is reasonable to envisage for the near future an optomechanical system for dynamic etching of all sorts of substrates, based on the use of a PJ out of an optical waveguide with a special tip.

Publications 1. “Photonic jet out of optical fiber: potential for laser with

matter interaction”, S. Lecler, A. Abdurrochman, J. Zelgowski, F. Mermet, B. Tumbelaka and J. Fontaine, Applied Surface Science (Submitted).

2. “Photonic jet breakthrough for direct laser micro-processing using near-IR nanosecond laser”, A. Abdurrochman, S. Lecler, F. Mermet, B.Tumbelaka, B. Serio and J. Fontaine, Applied Optics, vol. 53, pp. 7202-7207, October 2014.

Communications 1. “Photonic jet out of optical fiber: potential for laser with

matter interaction”, S. Lecler, A. Abdurrochman, J. Zelgowski, F. Mermet, B. Tumbelaka and J. Fontaine, The 2015 E-MRS Spring Meeting, Lille – France, 11-15 May, 2015.

2. “Photonic jets for micro-etching”, A. Abdurrochman, S. Lecler, F. Mermet, J. Fontaine, and B. Tumbelaka, International Conference on Physics, Yogyakarta – Indonesia , 25-26 August, 2014.

3. “Photonic jet to improve the lateral resolution of laser etching”, A. Abdurrochman, S. Lecler, J. Fontaine, F. Mermet, P. Meyrueis, B. Tumbelaka and P. Montgomery, SPIE-Photonics Europe 2014, Brussel – Belgium, 14-17 April, 2014.

4. “Increasing the lateral resolution of direct laser surface micro-processing using photonic jet”, A. Abdurrochman, S. Lecler, F. Mermet, B.Y. Tumbelaka, P. Meyrueis and J. Fontaine, JNTE 2013 (Journée Nationale des Technologies Emergentes en micro et nano fabrication), Evian-les-Bains – France, 21-23 May 2013.