Two and a half years ago, a team of researchers led by groups at the Massachusetts Institute of Technology (MIT; Cambridge, MA), the University of California at Berkeley, and Boston University announced a milestone: the fabrication of a working silicon (Si)-based microprocessor, built using only conventional CMOS manufacturing processes, that integrated electronic and photonic components on the same chip. The researchers' approach, however, required that the chip's electrical components be built from the same layer of silicon as its photonic components. That meant relying on an older CMOS technology in which the Si layers for the electronics were thick enough for optics.
Now, a team of 18 researchers, led by the same MIT, Berkeley, and BU groups, reports another breakthrough: a technique for assembling on-chip photonics and electronics separately, which enables the use of more modern transistor technologies.1 Again, the technique requires only existing CMOS manufacturing processes.
"The most promising thing about this work is that you can optimize your photonics independently from your electronics," says Amir Atabaki, a research scientist at MIT's Research Laboratory of Electronics and one of three first authors on the new paper. "We have different silicon electronic technologies, and if we can just add photonics to them, it’d be a great capability for future communications and computing chips. For example, now we could imagine a microprocessor manufacturer or a GPU manufacturer like Intel or Nvidia saying, 'This is very nice. We can now have photonic input and output for our microprocessor or GPU.' And they don't have to change much in their process to get the performance boost of on-chip optics."
Silicon photonics saves power
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 Semiconductor Industry Association has estimated that at current rates of increase, computers' energy requirements will exceed the world’s total power output by 2040. The integration of 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. For example, a commercial optical modulator consumes between 10 and 100 times as much power as the modulators built into the researchers' new chip. It also takes up 10 to 20 times as much chip space. That's because the new approach—integration of electronics and photonics on the same chip—enables Atabaki and his colleagues to use a more space-efficient ring-modulator design. "We have access to photonic architectures that you can't normally use without integrated electronics," Atabaki explains. "For example, today there is no commercial optical transceiver that uses optical resonators, because you need considerable electronics capability to control and stabilize that resonator."
Getting the polysilicon right
In addition to millions of transistors for executing computations, the researchers' new chip includes all the components necessary for optical communication: modulators, waveguides, resonators, and photodetectors, using a layer of polycrystalline silicon on deposited on silicon oxide islands that are fabricated alongside the transistors. In contrast, the earlier work on integrated photonics involved 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.
Because the new process uses polysilicon, there is a tradeoff. Single-crystal Si 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.
Using the manufacturing facilities at SUNY-Albany's Colleges for Nanoscale Sciences and Engineering, the researchers tried out a series of recipes for polysilicon deposition, varying the type of raw Si used, processing temperatures and times, until they found one that offered a good tradeoff between electronic and optical properties.
Source: http://news.mit.edu/2018/integrating-optical-components-existing-chip-designs-0419
REFERENCE:
1. Amir H. Atabaki et al., Nature (2018); doi:10.1038/s41586-018-0028-z.
