O.Box 76, 33201 Gijón, Spain Tel: 985 18 29

Whilst deposition of germanium doped core layers onto phosphorous doped clad- ding proved to be less than straightforward, deposition of a phosphorous doped layer onto an identical phosphorous layer with an intermediate anneal was success-

Chapter 6 Development of Silica Samples for UV Writing

ful. No bubbles or delamination occurred giving an indication that showerhead de- fects alone were not responsible for the difficulties in producing multi-layer samples. Another observation was related to the deposition listed as number 6 in table 6.2. The purpose of this process was to investigate whether inclusion of small amounts of both phosphorous and boron into the germanium layer would help in the suc- cessful production of two layer depositions. In previous work by members of the group, notably that by Sam Watts [5], the introduction of a new dopant was ob- served to sometimes abruptly change the layer properties of FHD layers. However, increasing the concentration from a very low concentration to a more significant level resulted in a somewhat more predictable change. Although the conditions and te- chniques differ considerably between FHD and PECVD the effects of dopants are similar. Therefore it may have been beneficial to maintain a small quantity of each dopant in each layer to assist in matching properties such as thermal expansion and melting points. Owing to a shortage of N2O the deposition of this multi-dopant sil-

ica was cut short and did not run to completion leaving a much thinner layer than originally intended, estimated at less than 4 microns thick. However, upon anneal- ing the sample, although the usual small bubbles occurred over the surface there were noticeably less over the wafer than in previous attempts. As this was the goal of the experiment the improvement was attributed to the inclusion of phosphorous and boron in the germanium layer.

Subsequently however, in producing further variations of two layer samples it was observed that the successful deposition of two layer structures could be achieved but success was related more to the thickness of the germanium layer than to the composition of the layer. With this observation, using recipes 12 and 13 of table 6.2 multi-step depositions were performed. Firstly, a six micron layer of underclad was deposited and annealed. Then a three micron layer of core-silica was deposited and annealed. This was repeated to build up a six micron core layer.

After the first three micron germanium core layer had been annealed only slight signs of bubbling were apparent, barely visible to the naked eye. After the second germanium layer just a few very small (≪1mm diameter) bubbles could be seen over the surface. Expanding on this success, the process was repeated but using three 2

Figure 6.19:The first successfully deposited three layer sample using a 6 micron core layer deposited in three 2 micron thick steps.

micron deposition-anneal stages to produce the six micron core layer. The first two micron layer showed no visible bubbling after annealing and although the second and third layers resulted in very slight blistering there were clearly defect-free areas of the sample large enough to define UV written structures in. The final step of the process was to deposit an overcladding. This was achieved in a single process step, depositing 6 microns of the phosphorous doped layer and annealing it with no degradation of the final surface finish. This first index matched, three layer structure with photosensitive core is shown in figure 6.19.

For comparison, figure 6.20 shows the result of depositing just an underclad and core layer, using exactly the same process parameters but using a single deposition- anneal step for the germanium doped layer. The difference is striking and it is clear that the thickness increment used to build up the germanium layer plays a highly significant role.

Again, for the purposes of comparison, figure 6.21 shows a layer of the geranium doped silica approximately 6 microns thick, deposited using identical parameters. Clearly, it is possible to deposit this layer and anneal it without any layer defects being introduced. Thus it is a combination of the underclad properties and the layer thickness that determine the success with which a germanium doped silica layer can be deposited onto a phosphorous doped layer.

Chapter 6 Development of Silica Samples for UV Writing

Figure 6.20: In contrast with fig.6.19 when the same core layer recipe is deposited as single 6 micron layer onto the same cladding, delamination and bubbling are clearly observed.

Figure 6.21:A 6 micron layer of the final recipe, germanium doped core layer.

As discussed in section 6.4.5.1 the outgassing of hydrogen and nitrogen during an- nealing may be responsible for the bubbling that is observed across germanium doped layers deposited over phosphorus doped underclad. Thin layers may allow gaseous hydrogen or nitrogen to escape more easily resulting in a reduction of bub- bling due to trapped gas. Additionally, any rearrangement of the silica structure that may occur due to outgassing or to the formation of new chemical bonds will not be constricted by such a large volume of material in the surrounding area. Thus localised stress in the layer may be reduced as a result of the lower layer thickness leaving a lower likelihood of surface disruption or delamination from the underlying substrate.