Plasma Treatment Fine-Tunes Micro-Ring Resonance

Breck Hitz

Waveguide micro-ring resonators are attractive components for many integrated optical applications, such as filtering, dispersion compensation, multiplexing and wavelength conversion. When employed as a notch filter, for example, a micro-ring resonator filters out light that is resonant in it -- that is, light for which an integral number of wavelengths fit around the ring (Figure 1). A practical problem, however, is that state-of-the-art fabrication techniques often are not accurate enough to set the filter precisely at the desired wavelength. A postfabrication fine-tuning technique may be required.

Figure 1. A micro-ring resonator filter is a circular ridge waveguide that measures tens of microns in diameter. It filters out light whose wavelength is resonant in the resonator. Images ©2005 IEEE.

Previous work has shown that ultraviolet illumination changes the refractive index of the micro ring, thereby offering a technique for fine-tuning, but this approach is incompatible with standard semiconductor manufacturing. The micro ring's resonance also can be adjusted by changing its temperature, but this is a transient technique.

Recently, scientists at National Taipei University of Technology and at the National Nano Device Laboratories in Hsinchu, both in Taiwan, demonstrated an approach that overcomes these drawbacks. They implanted oxygen ions into the micro ring at the end of the normal fabrication process, changing its refractive index and, therefore, its resonant wavelength.

Figure 2. The circular SiN ridge waveguide is grown on top of a lower-index layer of SiO₂.

The researchers fabricated the ridge-waveguide ring of SiN on SiO₂ (Figure 2). They implanted oxygen ions into the SiN layer with a plasma-enhanced chemical vapor deposition system and converted the SiN into SiO_xN_y. The refractive index of SiO_xN_y varies between 1.45 and 2.0, depending on the composition ratio, but it is always less than the refractive index of SiN. Thus, the oxygen implantation lowered the effective refractive index of the ring and shifted the resonant wavelength to a shorter value.

Figure 3. As the radio-frequency power in the plasma-enhanced chemical vapor deposition system increased, the oxygen ions were driven deeper into the SiN waveguide.

The spatial distribution of the oxygen ions in the SiN waveguide depended on the radio-frequency power applied during the implantation process (Figure 3). Even without the oxygen-plasma treatment, there was some oxygen diffused into the SiN from contact with ambient air. With an applied power of less than 300 W, the peak oxygen density occurred on the surface, but for powers that were above this, the peak density penetrated increasingly farther into the SiN. The radio-frequency power imparts kinetic energy to the oxygen ions, and the harder they hit the surface of the SiN, the deeper they will penetrate.

Figure 4. The resonant wavelength of the micro ring shifted 8.9 nm, more than one free spectral range, with increasing oxygen-ion concentration.

The ring's resonant wavelength shifted by 8.9 nm over the same range of applied powers (Figure 4). Because this tuning range exceeded the ring's free spectral range of 8.4 nm, the technique could tune the ring to any arbitrary wavelength. The scientists also produced a wavelength shift of 8.9 nm by varying the oxygen flow rate, and a smaller shift, by varying the treatment time.