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5.2.1 Photolithography and RIE

Photolithography and subsequent RIE is the most widely used approach for fabricating waveguides in silica-on-silicon. Photoresist is used to coat an existing slab waveguide (figure 5.1). It is spun into a thin film on the substrate. A mask is then used to protect certain zones of the photoresist during UV light exposure. The mask defines the waveguide shape with a resolution of under 0.5µmand is positioned using a mask aligner. The sample is then exposed to UV light through the mask and the photoresist is developed to leave either a positive or a negative image of the mask in the photoresist. The planar waveguide is then etched either by wet-chemical etching or reactive ion etching (RIE). The remaining resist acts as a mask to the etching process and the etchants have to be chosen so that the etch- ing of the resist is negligible compared to the etching of the waveguide material. Wet etching of silicates consists in immersing the layer in a so- lution containing hydrofluoric acid (HF) [1]. This often results in channel walls not being straight due to the high isotropy of the process [2]. RIE is a dry, clean technique developed in the microelectronics industry [3] and is based on a combination of plasma and sputter etching. It provides a better anisotropy but a lower selectivity of the material etched [4]. After etching, the remaining resist is subsequently removed. Depositing an up- per cladding layer then protects the waveguide, and sets the mode profile. The main advantage of this technique is that mass production of micron scale structure is easily feasible and it gives high quality definition of the waveguides. Also, a large refractive index change can be obtained by us- ing very different materials for the core and the cladding. Nevertheless, this technique is expensive, and it is a long, multi-step process, making it unsuitable for the small-scale requirements of research.

Clad and core on substrate

Photoresist and mask

removal of photoresist and Etching

Removal of photoresist and deposition of the overclad

Figure 5.1: Photolithography and RIE

5.2.2 UV writing

Direct UV writing was developed relatively recently when compared to photolithography [5], but has already demonstrated performance in terms of loss that equals or surpasses photolithographic methods. The first chan- nel waveguide defined by laser writing in germanium doped silica-on- silicon was written in 1993 and a refractive index increase of 2×10−2 _was

reported [6], this was followed by a directional coupler in hydrogen loaded FHD planar silica in 1995 [7]. The method used by these two groups involved UV writing using a mask but no propagation loss lower than 1dBcm−1 _{was reported. The major breakthrough happened with Sval-}

gaard and the first point-to-point writing of singlemode channel waveg- uides [8]. A photosensitive layer of the thickness of the desired structure is exposed to a focussed, high power UV laser beam (usually a continuous wave frequency doubled argon ion laser or a pulsed excimer laser). The refractive index increases, defining the channel waveguide. The UV beam is focussed to a spot of around 5µm, which is slightly smaller than the size of the waveguide required. The sample is then translated under the beam

sample translation via computer- controlled 3D stage buried channel waveguide SiO2 Ge:SiO2 SiO2 Si focussed UV beam

Figure 5.2: UV writing principle

to define the waveguide (see figure 5.2). The material characteristics and the laser fluence define the strength of the waveguide. Losses as low as 0.2dBcm−1 _{have been reported [5] and devices such as splitters and cou-}

plers [9,10] have been realised. The major drawbacks of direct-UV-writing are the serial nature of the process making mass-production very time- consuming, and the limitations imposed on the choice of a substrate, since the material needs to be highly photosensitive. Nevertheless, the process is very flexible as it is based on a single-step principle. This is ideal for prototyping structures very rapidly. The size and strength of the channel waveguide can be varied during fabrication, allowing the fabrication of tapered and tapered index waveguides [2].

Bragg gratings are very desirable elements in integrated optics and UV writing naturally lends itself to the realisation of photo-written gratings. There are two possible routes : superimposition of gratings on previously defined channels and simultaneous channel and grating definition. This is reviewed in more depth in chapter 7. The first technique is to use a phase mask to create interference patterns. The interference of the diffracted plus and minus first order of the beam creates a diffraction pattern. The second technique, direct grating writing, consists of splitting the beam in two, and creating an interference pattern by crossing the two beams. The writing laser is modulated whilst the sample is translated, so that each illumina- tion happens when the sample has moved by approximately the distance

between two fringes [1]. The angle between the two beams determines the central period of the possible range of periods obtainable for the grating. Gratings of nearly any periodicity can be made by varying the modula- tion of the writing laser using an Acousto-Optic modulator (AOM). Direct grating writing is a very flexible technique however the precision required is very high, demanding sophisticated engineering.