Fabrication of Silica Based Optical Waveguides Using Helicon Based Plasma Etching and Deposition


ANDREW PERRY, ANTOINE DURANDET and ROD BOSWELL
Plasma Research Laboratory, R.S.Phys.S.E., ANU, Canberra ACT 0200




The challenge involved in the fabrication of silica planar buried channel waveguides for integrated optics applications lies into the processing (deposition and etching) of SiO2. Deposition of thick layers (up to 10 m) at low temperature (80 C) to allow the integration of active devices (detectors and diodes), has been achieved. We successfully control the refractive index (with or without doping of the films), the microstructure and the birefringence. We present the fabrication steps and their technological diffuculties, insisting on the underlying physics, and the performance of some devices.

1. INTRODUCTION
The capability of integrating active devices with buried channel waveguides (BCW) depends on the possibility of manufacturing low loss BCW at very low temperature. Using plasma assisted deposition of doped silica it is possible to manufacture low loss BCW, as already reported by many authors (1, 2, 3, 4). Nevertheless, although the deposition process is done at low temperature, low-losses are only achieved after thermal annealing at high temperature, above 500 C and in general towards 1000 C. The best results reported so far without thermal annealing are obtained at a processing temperature of about 300 C (5, 6). Using the helicon plasma reactor (7) for deposition and etching of the layers, we were able to perform all the fabrication steps at very-low temperature, below 100 C. Furthermore, thanks to the flexibility of the helicon plasma source, it is possible to control the energy of the ion bombardement impinging the surface during the deposition independently of the plasma generation, allowing excellent control over the structure of the deposited material (8). The refractive index is controlled by fluorine doping (5, 6), thus avoiding the use of poisonous gas such as germane or phosphine used in conventional PECVD techniques. We fabricated single mode BCW (at 1.3 m and 1.55 m), with a square core of 5 m and n = (nco - ncl)/nco suitable for matching to optical fibre.

2. FABRICATION PROCEDURE
Standard 100 mm diameter silicon wafers (p type, orientation 100) were used as a substrate, and did not receive any special cleaning. The deposition of silicon dioxide was achieved using a helicon plasma reactor (8), by dissociation of a mixture of silane and oxygen. Thanks to the high efficiency of the helicon source, the deposition of good quality SiO2 is achieved using pure oxygen, while most common PECVD systems operate from N2O. Thus, no nitrogen was introduced in the gas phase. The control of the refractive index can be achieved by changing the ratio of oxygen to silane (7), or by doping the layers with fluorine. This second technique is preferred, as it is very reproducible and allows a very accurate control of the refractive indexes. We introduced a very small amount of CF4, the silane flow and the oxygen flow being kept constant at 12 sccm and 96 sccm respectively.

FIG.1: Evolution of the refractive index with the CF4 flow

The refractive indexes were measured using a prism coupler at 0.6328 m. The evolution of the refractive index with the CF4 flow is given in Fig.1. The mechanism by which the refractive index changes with the CF4 flow has already been reported elsewhere (6). Furthermore, the film is submitted to an ion bombardment during deposition. The ions are extracted from the plasma by applying a separate rf bias on the substrate, so that the energy of the ion bombardment is maintained at 300 eV for all experiments. This value of the ion energy gives good quality material (density and defects), and allows a good step coverage of the core during cladding avoiding the formation of voids around the core. We have already reported the effect of the ion energy on the quality of the deposited materials, justifying the choice of 300 eV (8). The substrate is clamped on a water cooled holder, and helium is circulating in the gap to ensure a good thermal contact. It has been verified, using chemical sensors stuck on the back of the substrate, that, under these conditions, the temperature of the wafer remains below 100 C. This value has also been confirmed by IR interferometry in a similar deposition system.

A 7 m thick buffer layer is first deposited with 1.5 sccm of CF4 in the gas phase to give a refractive index of ncl = 1.451. The CF4 flow is then switched off to deposit the core layer (nco = 1.462). The deposition rate is about 100nm/min for both layers. After deposition and lithography of a 150 nm thick Cr mask, the core layer is then plasma etched, in a helicon plasma reactor identical to the one used for deposition, using CF4 chemistry. After removing the Cr mask (wet chemistry), the sample is cladd, in conditions identical to those for the deposition of the buffer layer. The sample was not annealed. For this BCW n = 0.007. Fig. 2 shows a cross-section of the BCW; thanks to the ion bombardment, the square core of the BCW is perfectly buried in the cladding layer. The triangular feature at the top of the BCW is typical of the deposition under ion bombardement, and due to the sputtering of the growing film.


Fig.2: EM cross-section of a BCW fabricated with 300eV
3. IMPORTANCE OF THE ION BOMBARDMENT
An important feature of the helicon reactor is that it allows independant control of the plasma generation and of the interaction of the plasma and the surface to be processed, through independent biasing of the substrate. The energy of the ion bombardement can thus be chosen. As already demonstrated (8), the energy of the ions impinging on the surface during the deposition influence all the aspects of the quality of the films (structure,defects, stress, refractive index and porosity). Fig. 3 shows an SEM cross-section of a BCW fabricated under similar conditions as the one presented earlier, but with very low energy of the ion bombardment (about 30 eV); the cross-section clearly reveals the "columnar" structure of this material, as well as the poor coverage of the core by the cladding layer. In a similar way, the control of the ion bombardment during the etching step is critical as it conditions the verticality and the roughness of the side walls (9).


Fig.3: SEM cross-section of a BCW fabricated at 30eV

4. WAVEGUIDES CHARACTERISATION
The propagation losses of the BCW fabricated with 300 eV were found to be 1 dB/cm. It is believed that the losses are predominatly due to structural defects in the deposited material.

The absorption spectrum of the waveguide, using white light illumination and a spectrum analyser, shows only one absorption peak at 1.38 m, due to O-H bonds, which does not interfere with the propagation of the light at 1.3 m or 1.55 m; no absorption peak related to N-H or Si-H absorption at 1.5 m was found. Such a peak is normally observed in PECVD deposited BCW, and is reduced only after thermal annealing (1).

Using this technology, a 1x8 concatenation of Y-junctions has been fabricated. The intensities on the 8 channels (at a wavelength of 1.3 µm) are plotted in Fig.5. A photo of the splitter is shown on the previous web page.

5. CONCLUSION
By using the helicon plasma assisted processing of silica, we have been able to fabricate BCW's at very low temperature. Never during the fabrication steps were the samples submitted to temperature above 100 C. The losses of the Buried Channel Waveguides are still high (1 dB/cm), and it is believed to be due to the microstructure of the material.

6. ACKNOLEDGMENTS
The project is supported by Ericsson Australia. The authors would like to thank TARL and RMIT.

7. REFERENCES

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