VCSEL Technology For Next Generation 3D Sensing Applications

Part 1 discussed the market and application drivers for VCSEL based LiDAR in emerging new applications like AoT ™ (Autonomy of Things), AR/VR (Augmented/Virtual Reality) and smart glasses. These applications will require VCSELs will to evolve in multiple dimensions. Costs and capacity will become increasingly important as VCSEL array sizes increase (~1 mm² in smart phones to 30 mm² for automotive). Achieving high device yields across significantly more emitters and larger surface areas is challenging. Electro-optical efficiency (W/A) and power density (W/ mm²) performance needs to improve. Higher wavelengths are beneficial (currently, high power VCSELs operate in the 8XX-9XX nm regions which are less eye-safe relative to the 12XX-15XX nm range). The ability to activate certain portions of the VCSEL to support Region of Interest (ROI) imaging is becoming increasingly important (addressability). LiDAR-on-a-chip is the ultimate state of nirvana, where the different LiDAR functions (photon emission, detection, focusing, filtering, control, data acquisition and data analysis are integrated at a semiconductor wafer level on a single photonic-electronic chip). Finally, some customers believe believe that environmentally friendly manufacturing with high levels of waste recycling and a low carbon footprint will become critical moving forward.

This article (Part 2) discusses how participants in the supply chain are responding to these needs.

It Starts With the Substrate Material

Currently, commercial VCSELs use 6” Gallium Arsenide (GaAs) wafers as the substrate material. This makes sense since GaAs has the optimal band gap energy and other properties that allow it to generate photons efficiently at the 8XX-9XX nm wavelengths. Historically, such VCSELs used 4” substrates for data communication applications. GaAs is brittle and subject to wafer bowing, making it challenging to use larger substrate diameters. These challenges have been overcome, enabling a transition to 6” substrates as VCSEL-based sensing penetrated smart phone applications. With new applications, costs and capacity needs will drive the transition to larger wafer sizes.

Umicore (based in Belgium) supplies 6” and 8” Germanium (Ge) substrates for diverse applications ranging from space solar cells to LEDs and µLED displays (Disclosure: I am an advisor to Umicore). They believe that Ge substrates are a natural choice to support the transition to larger wafers for VCSELs used in LiDAR and 3D sensing. Figure 2 compares key material properties for silicon, Ge and GaAs.

Ge is an elemental semiconductor as opposed to a compound semiconductor alloy like GaAs. This allows for a zero defect (dislocations) substrate material. The higher fracture toughness enables larger wafer sizes with lower levels of wafer bow. Wafer thickness can be reduced through selective etching for better thermal transfer, enabling higher VCSEL powers and reliability. GaAs is still the active lasing material and deposited onto the Ge substrate using lattice-matched hetero-epitaxy.

Umicore recently published a paper proposing the use of large diameter Ge substrates as the next revolution in VCSEL manufacturing. Prior to this, the company worked with IQE (globally, the largest merchant epitaxy growth service provider and a critical part of the VCSEL supply chain as discussed below) to build and characterize 940 nm VCSELs using epitaxial growth of GaAs material on 6” Ge substrates. The work showed excellent performance parity with VCSELs grown on traditional GaAs 6” substrates.

Migration to 8” diameter substrates is the next step. Space solar cells originally used 4” and 6” substrates and the move to 8” substrates is imminent. Initial results on 8” Germanium substrates that have been evaluated for performance, purity and manufacturing scalability are promising. The shift to larger wafer sizes follows the time-honored maxim of the semiconductor industry: continue increasing wafer size as new applications demand higher performance, lower cost and increased manufacturing capacity. Based on internal analysis, Umicore estimates a 5X reduction in VCSEL cost once the transition to 8” Ge substrates occurs. Apart from cost benefits, moving to Ge enables CMOS foundry compatibility, enabling high-volume, low-cost integration with driver electronics. This is already occurring for the µ-LED display market.

Wannes Peferoen is the Senior Vice President of the Electro-optics Materials Business Unit at Umicore. He highlights green manufacturing and environmental friendliness as another critical advantage of Ge substrates. The ability to recycle Ge from a wide variety of complex waste streams enables Umicore to source over 50% of its Ge needs through recycling. The other 50% uses primary sources with a low carbon footprint and GWP (Global Warming Potential). This, along with the fact that polishing and thinning residue can be recycled leads to green and safe manufacturing processes as compared to Arsenic rich GaAs processing. Ge substrates become an attractive solution as applications and volumes proliferate, and the industry ecosystem looks to become more environmentally friendly and sustainable.

Next Is the Epitaxial Growth (Epi)

VCSEL designers create the exact structure of semiconductor materials and dopants, along with layer thicknesses and compositions required to generate photons at the appropriate wavelengths and power at the emitting facets of the VCSEL. At 8XX-9XX nm wavelengths, this involves complex semiconductors like GaAs and AlGaAs, which can be grown on GaAs or Ge substrates. The growth is accomplished on reactors that use a combination of vacuum, pressure and temperature. Many VCSEL manufacturers have internal capabilities to service this need, although there are specialized merchant foundries that specialize in epitaxial growth services for multiple customers. IQE (based in Wales, UK) is one of the largest providers of such services in the world today, and a leading supplier of VCSEL Epi wafers for applications ranging from smart-phones to 3D facial recognition. This volume manufacturing heritage has enabled them to refine and optimize the process, improve growth quality and reduce costs. They have installed significant capacity to meet current needs, and have planned for expansion as new applications like automotive LiDAR emerge.

Current VCSEL epi growth is done on 4 or 6” GaAs wafers. There would be an option in the future to transition to 8” Ge substrates if demand increases significantly. The company is investing significant research to prepare for this eventuality, and has already demonstrated performance parity between VCSELs grown on both GaAs and Ge substrates. Related work has focused on extending the wavelength range to the 1200-1550 nm region. According to Andrew Johnson, Technical Director at IQE, Ge substrates are better suited for longer wavelength operation (which require thicker grating structures) because its lattice properties create lower residual strain in the epitaxial stack. This, along with the higher fracture toughness of Ge relative to GaAs leads to higher manufacturing yields. Higher output power through use of multi-junction VCSELs and chip addressability within an emitter array are other important considerations for their customers. IQE collaborates with them closely on the epitaxial growth process to enable these capabilities.

Finally, VCSEL Suppliers Process the Wafer, Package, Test and Deliver the Final Device

VCSEL suppliers own the product design and deliver it to customers to address a diverse range of applications. Dominant suppliers include Lumentum and II-VI (which bought Finisar in 2019). Trumpf, AMS (which acquired Princeton Optronics, Vixar and Osram) and Broadcom are the other large VCSEL suppliers.

Lumentum controls ~ 50% of the global VCSEL market (estimated at ~$1B today). They claim to have sold ~ 1B VCSELs to date, a significant percentage of which are for smart-phone LiDAR applications. Automotive LiDAR, AR/VR, industrial sensing, advanced biometrics, access control and wearable devices are future applications of interest. Lumentum supplements its internal Epi capability through use of merchant suppliers like IQE (see above). Post Epi wafer processing for 6” GaAs based VCSELs is accomplished with external suppliers like Win Semiconductor in Taiwan. The ability to do its own Epi in-house allows it to develop and control intellectual property, and advance the VCSEL performance at a rapid pace.

David Cheskis, Product Director for 3D Sensing at Lumentum, laid out the roadmap for the company’s VCSEL product line. The focus is to continually improve quality, performance and scale of current product platforms. Features are added as new applications and requirements develop. One example of this is the development of 940 nm multi-junction VCSEL arrays to increase optical output power (critical for automotive LiDAR which requires ranges upward of 200 m as opposed to smart phones which require performance to ~10 m). The trick is to embed multiple emitting facets (electrically connected in series) within a single VCSEL device. Lumentum claims that their five junction VCSELs are electrically efficient (optical power of > 4W/A), power density 1.5 kW/mm² of die area at 25°C, 8 ns pulse width and 0.01% duty cycle), see Figure 3. Such a low duty cycle is not applicable to long range automotive LiDAR (typically duty cycles of at least 0.1% are required). According to Mr. Cheskis, this performance is maintained at duty cycles as high as 0.3%.

Another example is enabling matrix and column level addressability compatible with detector readout circuits. This is important for focusing laser and computing resources in regions of interest (ROI) that matter. Lumentum has developed designs of metallic interconnect layers within multi-junction VCSEL architectures to make this possible. Integration of the semiconductor with driver electronics and micro-optics is another development thrust. Optics integration is important from a beam shaping/control perspective and achieved through a wafer-level backside etching process. In terms of integration, Lumentum either supplies the VCSEL chip to customers (with or without micro-optics) or complete modules with integrated optics and drive electronics. It recently announced a 10W peak power VCSEL module for short range LiDAR applications.

II-VI is dominant VCSEL supplier, and has shipped hundreds of millions of devices for consumer applications over the past few years. The company is vertically integrated in terms of VCSEL design and manufacturing. This allows for strict control of the end-to-end process, resulting in higher quality and yields, and a cost-efficient value chain without margin stacking. As VCSEL sizes get larger (for example, with automotive LiDAR applications), the company believes it can leverage its experience, vertical integration and quality control advantages to achieve similar quality and yield metrics.

Large investments have been made in its 6” GaAs based VCSEL manufacturing platform to address consumer applications. According to Gerald Dahlmann, Director, Strategic Marketing, Consumer Electronics at II-VI, “The industry scaled up from 4-inch to 6-inch, when 3D sensing in consumer electronics increased the demand for VCSEL real estate by more than an order of magnitude within a short period of time. It would take a similar step increase in demand to justify a transition to 8-inch wafers.”

According to Mr. Dahlmann, the company is constantly pushing the limits of the technology to improve performance. It recently introduced multi-junction VCSEL structures to increase the power density (W/mm²) and efficiency (W/A). Improvements in epitaxy designs to minimize losses and self-heating have enabled high power conversion efficiency. With this architecture, multiple active regions are vertically stacked onto each other and connected in series. Figure 3 shows the structure and performance of the optimized multi-junction VCSEL:

Mr. Dahlmann indicates that increasing the number of junctions (5) and pulse width (3 ns), and reducing the duty cycle (0.1%) delivers > 1 kW/mm² peak power with a chip size of 0.8 mm². Wavelength shifts with temperature are low, typically < 0.1 nm/°C. In cases where more wavelength precision is required, active temperature stabilization via Peltier thermoelectric coolers (a business that II-VI is dominant in) may need to be integrated.

Current VCSEL products supplied by II-VI have a degree of addressability. Implementing higher levels of addressability requires a back-side emitting architecture or additional conductive layers to allow crossing of metal traces. These are under development. Other areas of development include integration of the VCSEL with driver electronics to improve the performance and create lighter and more compact modules. Eventually, the company envisions integration of several building blocks of a LiDAR onto the same chip – light sources and modulators on the transmit side and photodetectors and de-modulators on the receive side. Several technology platforms within its portfolio enable this including indium-phosphide photonic integrated circuits (PICs), and demodulators and detectors for coherent FMCW LiDAR.

The performance, cost and capacity demands for VCSELs will increase with new applications that address safety and productivity applications. Extending the VCSEL wavelength is disruptive, and likely better done with Ge instead of GaAs substrates. The move from 6” to 8” substrates will occur as demand ramps, consistent with the experiences in the space solar cell and LED businesses, and overall semiconductor industry norms. The real question is when, not if. Stay tuned.