Photonics Goes Underground in Silicon
A research group at the University of California, Los Angeles, has demonstrated the fabrication of optical devices — specifically, a microdisk resonator and coupling waveguides — beneath the surface of a silicon substrate, leaving the surface clear for the implementation of electronic circuitry. The scientists believe that their technique could lead to monolithic electronic/photonic integrated circuits and are striving to incorporate CMOS electronics on the surface of chips containing buried optical components.
Figure 1. A patterned SiO2 layer on top of the silicon slowed the implanted oxygen ions so that they reached different depths in the silicon chip (top). After annealing, the lower silicon layer was capable of photonic functionalities (bottom). In this case, two rib waveguides were fabricated in the lower layer, and the upper layer was available for the fabrication of electronic devices.
The fundamental fabrication process, which the researchers developed several years ago and named “Simox 3-D sculpting,” involves implanting oxygen into a layer of silicon, thereby dividing the layer in two separated by a thin region of SiO2. The lower layer can contain optical components, and the upper is suitable for electronics.
In their demonstration of the technique, they started with a silicon-on-insulator substrate and added a thermally grown, lithographically patterned layer of SiO2 on top to slow the incoming oxygen ions so that they would penetrate to different depths when subsequently implanted (Figure 1, top). They then annealed the substrate to repair damage done by the implantation process to the upper silicon layer and to smooth the implanted SiO2 layer into a continuous strip.
Figure 2. A scanning electron micrograph of the device’s cross section shows the buried SiO2 layer separating the silicon into two regions and forming an optical waveguide in the lower region.
To fabricate the buried microresonator, they patterned it and its waveguides on the thermally grown SiO2 by reactive ion etching, then implanted the oxygen atoms to form the optical components and separate the silicon into two layers. The result was a buried microresonator, nestled between two coupling waveguides (Figures 2 and 3).
Figure 3. A top view of the fabricated device shows the microresonator and the two waveguides for coupling light into and out of the resonator.
Before characterizing the microresonator, the scientists oxidized the upper layer of silicon to avoid leakage of optical energy from the buried components. This oxidation process made the upper layer unsuitable for electronic devices, but they noted that, in an integrated photonic/electronic device, only the areas immediately above the optical components would need to be oxidized, leaving the rest of the surface available for electronics. Alternatively, the buried SiO2 layer could be thickened, minimizing the coupling of optical energy into the upper silicon layer.
Figure 4. The microresonator’s transmission peaks — normalized here to correct for the spectral shape of the illuminating erbium-doped fiber amplifier — show the full functionality possible from photonic devices buried in a silicon chip.
Once the upper silicon layer had been oxidized, they focused broadband light from an erbium-doped fiber amplifier from INO of Sainte-Foy, Quebec, Canada, into the input waveguide, and observed the throughput light with an optical spectrum analyzer. The transmission peaks showed that the resonator’s free spectral range was 5 nm, that its Q was ~2000, and that the extinction ratio of the sharpest peak was better than 20 dB (Figure 4).
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