Silicon photonics economics set to benefit III-V photonics

Silicon photonics promises to deliver cheaper optical components using equipment, processes and fabrication plants paid for by the chip industry. Now, it turns out, traditional optical component players using indium phosphide and gallium arsenide can benefit from similar economies, thanks to the wireless IC chip industry.

Valery TolstikhinSilicon photonics did a good thing; it turned the interest of the photonics industry to the operational ways of silicon 

So argues Valery Tolstikhin, head of a design consultancy, Intengent, and former founder and CTO of Canadian start-up OneChip Photonics. The expectations for silicon photonics may yet to be fulfilled, says Tolstikhin, but what the technology has done is spark interest in the economics of component making. And when it comes to chip economics, volumes count.

“For III-V photonics - indium phosphide and related materials - you have all kinds of solutions, designs and processes, but all are boutique,” says Tolstikhin. “They are not commercialised in a proper way and there is no industrial scale.” The reason for this is simple: optical components is a low-volume industry.

This is what Tolstikhin seeks to address by piggybacking on high-volume indium phosphide and gallium arsenide fabrication plants that make monolithic microwave integrated circuits (MMICs) for wireless.

“To take photonics out of boutique fabs, you need to do some standardisation and move to a fabless model, then you can load the fabs day and night with wafers,” says Tolstikhin. “That is the only way to make a process mature, reproducible and reliable.”

Tolstikhin has spent the last decade pursuing this approach. “The idea is to use something available in indium phosphide which is relatively close to a pure-play foundry.” A pure-play foundry is a fab that makes chips but does not design, market or sell them as its own products.

Tolstikhin’s first involvement was at start-up OneChip Photonics which developed an indium-phosphide platform that used a variety of photonic devices to make photonic integrated circuits (PICs), based on a commercial MMIC process.

The issue with III-V integrated photonics is that to implement different functions - a passive waveguide and a laser, for example - different materials are needed. “What makes a low-loss passive waveguide, does not work for the laser,” says Tolstikhin.

To overcome this, the wafer is repeatedly etched in certain areas, to remove unwanted material, and new layers grown instead with the required material, a process known as selective-area etch and regrowth. This is a complicated and relatively low-yield process that is custom to companies and their fabs, he says: “This is how all commercial lasers and PICs are made.”

In contrast, MMICs using indium phosphide do not need regrowth, simplifying the process considerably. To use a MMIC fab for an optical design, however, it must be developed in a way that avoids the need for regrowth stages.

“At OneChip we believe we did the first commercial laser - not just the laser but the PIC with it - regrowth-free,” says Tolstikhin. “It was made in a MMIC fab, that is the key.”

“To take photonics out of boutique fabs, you need to do some standardisation and move to a fabless model, then you can load the fabs day and night with wafers” 

Wafer economics

To understand the relative economics, Tolstikhin compares the number of wafers - wafer starts - processed in silicon, indium phosphide and gallium arsenide.

One large TSMC fab has 400,000 12-inch CMOS wafer starts a year whereas globally the figure is equivalent to some 70 million such wafers a year. For MMICs, one fab Tolstikhin works with has 15,000 4-inch indium phosphide wafer starts a year whereas a large optical component company uses just a couple of thousand 3-inch indium phosphide wafers a year.

“In photonics, the [global] volumes – even for components going into the most massive markets like PON and the data centre interconnects – are still very low,” says Tolstikhin.

Gallium arsenide is somewhere in between: Win’s fab in Taiwan, which makes power amplifiers for wireless and other MMICs, has 250,000 6-inch wafers starts a year, while TriQuint’s fab in USA, with similar product line in wireless, totals 150,000 6-inch wafer starts a year.

Such volumes are not negligible and exceed all the needs of photonics, he says, enabling photonics to make claims similar to those trumpeted for silicon photonics: a mature process with a well-established quality system and, with its volumes, delivers better economics.

Moreover, if applications that currently are based on indium phosphide could be transferred to gallium arsenide, that would give an order of magnitude economies of scale, says Tolstikhin: “One example is mid-reach single-mode optical interconnects with an operating wavelength around 1060 nm, with gallium arsenide used for the transmitter, receiver and transceiver PICs”.

And while the scale of III-V semiconductor manufacturing may still be much lower than CMOS, the up-front cost involved in using a III-V fab is also much less.

Using III-V semiconductors for analogue electronics like the laser /modulator drivers or the trans-impedance amplifier also delivers a speed advantage: heterojunction bipolar transistors (HBTs) in indium phosphide have been demonstrated working at up to 400 GHz, and these, being vertical devices, do not have their speed scaled with lithography. In contrast, CMOS analog electronics is much slower and its device speed is scalable with lithography resolution. A 130 nm CMOS process, the starting point for silicon photonics, cannot support optical components with bit rates beyond 10 Gbps.

Design house

Intengent, Tolstikhin’s company, acts as a bridge between OEMs building optical components and sub-systems and the III-V foundries making photonic chips for them.

He compares Intengent to what application-specific IC (ASIC) companies used to do for the electronic chip industry. Intengent works with the OEM to specify and design the photonic chip based on its system application and then works with the fab to develop and turn the chip into a product by meeting its design rules and process capabilities.

“The aim is that you can go and design within existing fabs and processes something that meets the customer’s application and requirements,” he says.

Tolstikhin is also working with ELPHiC, a Canadian start-up that is raising funding to develop single-mode mid-board optics. The indium-phosphide design combines analogue electronic circuitry with the photonics.

“It appears the best way [to do mid-board optics] is based on electronic and photonic integration onto one substrate and indium phosphide is a natural choice for such a substrate,” he says.

Tolstikhin makes clear he is not against silicon photonics. “It did a good thing; it turned the interest of the photonics industry to the operational ways of silicon: standardised processes, pure-play foundries, device designs separate from the semiconductor physics, and circuit designs separate from the wafer processing.”

As a result, something similar is now being pursued in III-V photonics.