Researcher shows how VCSELs can be fully integrated on silicon

Laser islands: Illinois researcher Leah Espenhahn shows how VCSELs can be fully integrated on silicon

Espenhahn in a HMNTL cleanroom with a wafer with VCSELs fully integrated on silicon. Photo credit: Grainger College of Engineering at the University of Illinois Urbana-Champaign

How does your phone know it’s you when you use face recognition to unlock it? A series of tiny lasers illuminate your face, and your phone uses the reflection to create a 3D model – similar to a topographical map of your face. The phone’s software then uses this to decide whether to unlock it.

These tiny lasers, called VCSELs (pronounced “vixels”), make this possible. Traditionally they are used in short-distance data transmissions, laser printers and even computer mice. However, ever since they emerged into mainstream facial recognition and 3D imaging technologies, there has been an explosion in demand and a drive to make them more efficient and compact.

Leah Espenhahn, a doctoral student in the research group of electrical and computer engineering professor John Dallesasse, has demonstrated a new method for directly integrating VCSELs into electronic chips. As she described in a recent issue of compound semiconductors Magazine, it is possible to create VCSELs directly on silicon microelectronics using a method called epitaxial transfer, like creating tiny islands for the lasers in the silicon.

“Compared to standard devices where independently constructed VCSELs are attached to the microelectronics,” Espenhahn said, “epitaxially transferred VCSELs are more compact, more powerful, and less prone to overheating.”

She was also invited to speak about the method at the CS International Conference 2023 in Brussels.

Vertical, not sideways

VCSELs, or vertical cavity surface emitting lasers, belong to a class of devices called semiconductor lasers. They produce intense, focused beams of light like other types of lasers, but they are made entirely of semiconducting materials. This means that manufacturing techniques developed for electronic microchips, which are also made from semiconducting materials, can be adapted to lasers.

Many types of semiconductor lasers are side emitting, meaning the light beam is parallel to the electrical contacts. Such devices require additional manufacturing steps to ensure there is a smooth surface for the light to exit the material. In contrast, VCSELs produce light that is perpendicular to the electrical contacts and exits vertically through the top layer, simplifying the manufacturing process and opening the door to far more compact devices.

“Because VCSELs emit light from the top surface,” said Kevin Pikul, another graduate student in Dallesasse’s group, “it makes arrays so much easier to create.” You can have thousands of VCSELs in just one sample.”

Islands of fully integrated lasers

The standard approach to creating VCSEL arrays is to manually solder prefabricated lasers onto electronic chips through “flip-chip bonding,” a time-consuming process with limited precision. Eventually, to make them even smaller and more efficient, they need to be directly integrated with electronic devices on microchips.

Espenhahn accomplished this by taking raw VCSEL device structures and mounting them on a temporary platform. After etching specific “islands” of material for each laser, a layer of interconnect material was laid on top. The temporary platform was then flipped over and placed on a main silicon platform, causing the islands to stick. After removing the temporary platform, a series of epitaxially transferred islands remained, ready to be processed into VCSEL devices.

Since the VCSELs are manufactured after the transfer process, they can be placed on the electronic circuit much more precisely than flip-chip bonded devices. In addition, the resulting devices have better thermal properties, leading to better controllability.

“Since we only have a thin layer of epitaxial material on top of silicon,” explains Espenhahn, “the silicon dissipates heat faster when we deliver more power. This allows us to better control the wavelength [color] of light and to create devices with larger power ranges.”

Epitaxial transfer beyond VCSELs

Face recognition is just one example of a technology called LiDAR, which uses reflected laser light to create images or models on computers. Another use for VCSEL-based LiDAR that is growing in importance is vision and sensing in autonomous vehicles.

But Dallesasse envisions that epitaxial transfer can go beyond just VCSEL.

“Once we start talking about complex electronic-photonic systems for things like self-driving cars,” he noted, “we can also start using these techniques to bring non-silicon functionality onto silicon platforms to make things more compact make. silicon it is also speed limited. If we wanted to integrate higher speed electronic devices or power devices, we could also do it with an epitaxial transfer method.

Provided by the University of Illinois Grainger College of Engineering

Citation: Laser Islands: Researcher Shows How to Fully Integrate VCSELs on Silicon (2022 December 12) Retrieved December 12, 2022 from silicon.html

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