Moving from electrical communication to optical communication is attractive to chip manufacturers because it could significantly increase chips’ speed and reduce power consumption, an advantage that will grow in importance as chips’ transistor count continues to rise.
The integration of optical — or photonic — and electronic components on the same chip reduces power consumption still further. Optical communications devices are on the market today, but they consume too much power and generate too much heat to be integrated into an electronic chip such as a microprocessor.
A technique was developed for assembling on-chip optics and electronics separately, which enables the use of more modern transistor technologies. The technique requires only existing manufacturing processes. The integration of electronics and photonics on the same chip enables the use of a space-efficient modulator design based on a photonic device called a ring resonator.
In addition to millions of transistors for executing computations, the new chip includes all the components necessary for optical communication: modulators; waveguides that steer light across the chip; resonators that separate out different wavelengths of light, each of which can carry different data; and photodetectors that translate incoming light signals back into electrical signals.
Silicon — which is the basis of most modern computer chips — must be fabricated on top of a layer of glass to yield useful optical components. The difference between the refractive indices of the silicon and the glass — the degrees to which the materials bend light — is what confines light to the silicon optical components.
Earlier work on integrated photonics involved a process called wafer bonding, in which a single, large crystal of silicon is fused to a layer of glass deposited atop a separate chip. The new work, in enabling the direct deposition of silicon — with varying thickness — on top of glass, must make do with so-called polysilicon, which consists of many small crystals of silicon.
Single-crystal silicon is useful for both optics and electronics, but in polysilicon, there’s a tradeoff between optical and electrical efficiency. Large-crystal polysilicon is efficient at conducting electricity, but the large crystals tend to scatter light, lowering the optical efficiency. Small-crystal polysilicon scatters light less, but it’s not as good a conductor.
A series of recipes was tried for polysilicon deposition, varying the type of raw silicon used, processing temperatures and times, until one was found that offered a good tradeoff between electronic and optical properties.