Vertical-cavity surface-emitting lasers (VCSELs), a well-proven but only recently tapped niche technology, have suddenly become very popular due to the introduction of 3D facial recognition applications in smartphones and other mobile devices.

VCSELs have previously been primarily used as a low-cost motion tracking and data transmission light source technology for computer mice, laser printers, and fiber optic communications. However, with Apple's decision to use VCSELs for 3D face recognition in its flagship phone, the iPhone X, VCSEL technology has taken a new direction. This technology choice by Apple, and the influx of smartphone and consumer electronics manufacturers that followed, has resulted in the expansion of new manufacturing, testing and validation across the VCSEL market.

'The VCSEL market has been tepid for many years,' said Ajit Paranjpe, CTO of Veeco, 'but with the rise of new applications, VCSEL technology has received significant attention and improvement. We have finally overcome the learning curve and achieved large-scale Manufacturing, as we move into the second large-scale application - LiDAR with VCSEL arrays, this will be a key technology necessary for autonomous driving. In addition, VCSELs are already used in server clusters for rack-to-machine After the application of board-to-board communication (such as optical backplanes), the ideal is chip-to-chip communication, and eventually all-optical interconnects will be used. The real question is when do we need it.”

VCSELs are just one of the few silicon photonics solutions that are starting to attract a lot of attention from the market. The entire semiconductor industry is struggling to bring one or more of these technologies into the mainstream, especially as advanced nodes that transport electrons through wires become more difficult.

'There are multiple hidden and non-hidden cameras in smartphones,' said Twan Korthorst, director of research and development for photonic solutions at Synopsys. 'These cameras can identify whether the user is looking at the phone and take optical measurements, which is where VCSELs come in. Among these modules are detectors, light sources, and image sensors, of which the VCSEL is a small discrete component.'

What is VCSEL?

Objectively speaking, VCSELs are just one of many chip-based light sources that can be used in these devices. What makes VCSELs so attractive is that their lasers are emitted perpendicular to the device, which has many benefits, ranging from mass production manufacturing to testing.

'VCSELs can be tested using wafer probers and wafer-level,' Paranjpe said, 'while with edge-emitting lasers (EELs), the wafer must be diced, then the rest of the device is built and tested, which makes the Testing becomes more difficult.'

However, this has not slowed down research into other aspects of VCSELs, much of which has focused on stacking different materials and integrating some or all of the materials into a single package. What is put into the package, or left out of the package, is still a work in progress.

“There’s some interesting work going on in the big foundries right now to create photonic components that connect different chips,” said John Ferguson, director of application marketing for Calibre DRC at Mentor, a Siemens subsidiary. “The main problem is that the optical waveguides have to have certain The size, on the order of a few microns, has to allow for a certain amount of space, which takes up a lot of space. Photonics doesn’t benefit from more advanced nodes, in fact 65 nm is by far the most advanced process node.”

The next wave of opportunities for VCSELs is in automotive applications such as LiDAR with higher power requirements. Such applications require the use of larger VCSEL arrays.

'This will require the procurement of more metal-organic chemical vapor deposition (MOCVD) systems to ensure that manufacturing capacity accelerates to keep up with the demand for LiDAR systems,' said Mark McKee, director of product marketing at Veeco. 'The question now is how to achieve the highest performance and Highest yield to meet market demands. This needs to be based on industry-leading MOCVD technology. Key requirements are wafer surface metal-organic and hydride uniformity and laminar flow, uniform, controllable temperature, and clarity between layers interface. To achieve the highest yields, fabrication platforms require longer producible time between maintenance cycles, fast recovery after preventive maintenance, and faster epitaxial growth rates.”

VCSELs use laser pulses with frequencies in the tens of gigahertz for ranging and time-of-flight calculations, identifying motion through changes between each frame of the image. David Hall, chief marketing manager at National Instruments, noted that it's unclear whether this approach could be used to improve LiDAR's trend toward using longer-wavelength lasers and continuous scanning.

'There doesn't seem to be much overlap in requirements between 3D face recognition systems and LiDAR systems,' Hall said. '3D face recognition systems are economical enough to be integrated into smartphones; while LiDAR systems should require longer distances than VCSELs can achieve. It remains to be seen whether the two systems can benefit from each other's schemes.'

LiDAR is a promising potential market, but short-term opportunities such as 3D motion detection and 3D face recognition in automotive interiors are more attractive. 'The technology could be used to identify if a driver is dozing off, or allow passengers to control infotainment or other systems with gestures,' said Craig Thompson, vice president of new markets at Apple's VCSEL chip supplier Finisar.

In 2004, Finisar acquired 'Honeywell's business unit that first commercialized VCSELs' and expanded the market from computer mice and PC peripherals to data networking, and has since been working on development for carrier-grade VCSELs for fiber-copper interfaces of data network equipment.

But not everyone is in favor of VCSELs in their current form in LiDAR.

'VCSEL lasers tend to be near the visible wavelength band, so it can be dangerous to increase the power,' said Gilles Lamant, Cadence Distinguished Engineer. 'LiDAR requires very high power to obtain the various detection ranges required, which may There is a risk to the human eye. Despite being close to the visible light band, they are safe for low-power applications, which is why we can use them for 3D face recognition, as well as for applications such as camera ranging.'

The real advantage of VCSELs is their convenience, flexibility and power, as well as their thermal efficiency compared to other laser sources, Lamant said.

'VCSELs emit light vertically, which makes it easy to build arrays of VCSELs perpendicular to the chip, and they are very stable over a fairly wide temperature range,' Lamant said. 'VCSELs have also been shown to encode data signals and Sending data. This will run into frequency limitations, but we're still looking for a way to integrate optics with a silicon-based solution.'

'VCSELs also output far more light per watt than other laser sources,' Thompson said. 'In addition, VCSEL chip wiring is easier because the laser is emitted from the top and can be electrically and thermally managed under the chip.'

The output power of VCSELs can also be scaled linearly by size. Each laser light exit hole of the VCSEL is independent and basically the same. The more apertures on the chip, the more energy is output, and connecting them all to a single power supply can make them all fire together. Routing the chips to different areas allows them to emit in different patterns at different times, with power output determined by the number of light exit holes.

Finisar High-Power VCSELs in Aluminum Nitride (AIN) Cavity Packages 'VCSELs for data networking equipment have an exit hole and an emission point on a chip a few hundred microns square,' Thompson said, 'while at high power In a 3D sensing VCSEL, there will be hundreds of light exit holes, and the chip will be millimeters square. It is a very unique and easily scalable laser structure, like a kind of LEGO technology.” Ferguson said: “ Thermal interference or other types of noise can affect VCSEL use in data centers, but tests that can identify such interference are uncommon.' 'It's not like ICs being probed on a test bench,' Ferguson said. It's not easy to do optical signal processing outside of these devices. Some of the big systems companies are pushing the foundries to do that to see what else they can do with this technology because beyond data networking, we're seeing More and more applications. Photonics applications in the automotive space include LiDAR and autonomous vehicles, and it looks like VCSEL technology is gaining more and more attention. Many companies are exploring this technology, and now there are several companies that we expect to see In the next year, related products will be launched.” Thompson said: “There should be a lot of products using VCSELs in the next year. The number of VCSELs that are already in production, testing and verification will be far greater than ever.”

After Honeywell commercialized VCSELs in 1996, the technology was successful for a decade in computer mice and other peripherals, and since 2004, VCSELs have been widely used as fiber-to-copper interface light sources for carrier-grade data networking equipment. welcome. All of these are solid niche businesses, but have been so 'low profile' that most people in the chip industry have paid no attention to VCSEL technology. But after the release of the iPhone X, everything changed.

'At the time, designing a large, high-power VCSEL capable of projecting three infrared light spots on the user's face and then quickly and accurately constructing a 3D topography of the face to enable Face ID authentication for the iPhone X,' said Thompson There are some challenges. But the biggest challenge is how to meet the volume production needs of the iPhone X. Apple announced at the time that it had to source 10 times as many VCSEL wafers in the fourth quarter of 2017 compared to the same period in 2016.”

That's why Apple paid Finisar $390 million upfront in 2017. The goal is to turn the vacant 70-square-foot plant in Sherman, Texas, into the 'VCSEL capital of America.' 'When it operates at full capacity later this year, this 70-square-foot manufacturing facility, originally owned by MEMC and SunEdison, will have its own VCSEL wafer capacity several orders of magnitude higher than the entire VCSEL industry has in the past,' Thompson said.

The prospect of VCSEL is optimistic, and many manufacturers have expanded their production capacity

Good iPhone X sales have sparked strong interest in 3D sensing capabilities from other Android smartphone brands, according to Memes Consulting. In less than a year after the release of the iPhone X, Android competitors have also begun to adopt a similar strategy, integrating various 3D sensing technologies and face recognition functions, showing that the VCSEL 'killer' application has gained market recognition!

Xiaomi and OPPO are the fastest. In the second quarter of 2018, Xiaomi Mi 8 Explorer Edition and OPPO Find X, two smartphones with integrated 3D sensing technology, were launched respectively. Other Android smartphone makers, such as Huawei, vivo and Samsung, are also using VCSELs for flagship phones. It is expected that VCSEL shipments will increase from 652 million in 2017 to more than 3.3 billion in 2023, with a compound annual growth rate of 31% from 2017 to 2023.

Compared with Finisar's 300 million VCSEL shipments, Philips Photonics' shipments have exceeded 1 billion. In 2018, Philips Photonics invested 23 million euros to double the production capacity of its VCSEL factory in Ulm, Germany. Austria-based ams Semiconductor (ams) announced that it will spend US$200 million to expand its VCSEL manufacturing plant in Singapore.

'It's worth noting that now that the smartphone market has brought VCSELs into the mainstream, giants like Apple have taken on the costs of developing and maturing VCSELs in smartphone applications,' Thompson said, 'making us industry players more likely There is enough confidence to invest a lot of money to scale up so that VCSELs can be mass-produced.”

'Before Face ID, standard VCSEL fabrication was almost entirely based on MOCVD, which is often used for III-V materials to make polycrystalline thin films, while manufacturing efficiency measurements such as automated wafer testing and beam imaging inspection were 'early and immature' stage',' Thompson said.

'We've moved from 3-inch to 5-inch GaAs wafers. We have developed a more mature approach to automated wafer-level test across the fab process, which was very immature a few years ago,' Thompson said. , 'We have to develop epiwafers for these applications, expand the supply chain, learn from the radio frequency (RF) industry to develop new metrology methods and new test schemes, develop automated wafer test and probes. We need to develop near-field and Far-field optical testing to image the chip and its output at close range and distance. We also need to develop test methods to accurately measure the number and performance of light exit holes on large-scale laser chips, and how infrared light is shaped and focus.'

Finisar's Sherman, Texas, plant began operations in July 2018 and won't reach full capacity until later this year, but already offers capacity that VCSEL manufacturers couldn't imagine a few years ago.