Blogs | Aug 15, 2023
When you think of MOCVD tools, typical thinking turns to blue gallium nitride (GaN) and LED technology. However, LEDs also include the red and yellow spectra. More to the point, there are other applications that require MOCVD gallium arsenide (GaAs) and indium phosphide (InP) films. An inexperienced observer may think that an engineer could simply take an MOCVD tool that deposits GaN and change the gases, creating an MOCVD tool that successfully deposits GaAs and InP compounds. However, that is not the case. Fortunately, Veeco has invested the time to engineer a platform called the Lumina® MOCVD System that deposits GaAs and InP compounds with industry-leading performance and the lowest cost of ownership.
The need for photonic devices is growing rapidly, as advanced driver assistance systems (ADAS) and advanced communications technologies, such as 5G and the 5G edge, are moving from emerging to mainstream technology. As demand for photonic integrated circuits (PICs) increases, so does the need for process equipment that can produce PICs in high volumes. The same holds true for mini and microLEDs that are beginning to find their way into advanced display technology, replacing LCD and OLED displays. Also manufactured using GaAs and InP compounds are advanced solar cells, which may be used for space applications or for remote installations that need solar cells with high efficiencies. The better the uniformity and the sharper the layers, the better the circuit will typically perform.
MOCVD deposition equipment used to form the photonic devices requires both excellent uniformity across the wafer and the ability to control doping precisely at the interface of the layers used to form the PIC and solar-cell circuits. Nonuniformity across the wafer surface can result in circuits that have significantly different characteristics, which will impact the performance of the PIC and solar cells, potentially resulting in PICs that can’t be used for the intended application. Precise temperature and gas controls are needed for optimum uniformity across the wafer carrier and for sharp layer transitions, which are vital for high yields and high-performance circuits.
Contamination is another challenge in MOCVD systems. The way the crystal structure of the MOCVD epitaxial layers forms is a critical component of the resulting PIC’s yield. Contamination on the surface of the substrate, or contamination that forms during the deposition process due to the gases decomposing prematurely, can effectively wipe out an entire process run. Precise management of the gas flow and creating the conditions for the optimum boundary layer for epitaxy deposition enables the Lumina to have industry-leading yields due to low contamination levels. Let’s look a bit more at how the Lumina system is designed to give excellent wafer uniformity and sharp layer transitions with minimal contamination.
The Lumina system uses what Veeco terms “vertical rotating disc reactor,” or TurboDisc®, technology. TurboDisc rotates the wafer carrier at a very high speed, which, combined with the vertical injection of the gases, suppresses buoyancy forces. This creates a boundary layer that sits very close to the wafer and carrier surface, ensuring that there’s no coating on the injector surface. The high carrier rotation also reduces the residence time of the gas, which leads to reduced particle formation in the heated boundary layer, as the gases don’t have time to decompose before forming the GaAs and InP compounds on the wafer surface. This results in virtually zero contamination inside the MOCVD chamber and on the wafer surface. These low contamination levels mean that the time between chamber cleans is more than 300 runs, leading to system uptime higher than 95%.
The deposition gases are introduced to the Lumina chamber through a showerhead above the wafer surface. The alkyls and hydrides are distributed uniformly in an alternating pattern across the diameter with laminar flow. The showerhead gas distribution and the performance of the high-speed TurboDisc reactor with laminar flow together create a thin, uniform boundary layer resulting in extremely uniform films with sharp interfaces between layers. As mentioned earlier, sharp interface layers and uniform films with excellent, repeatable material composition across the wafer lead to PICs with high yields and optimum performance. One of the Lumina’s significant features is that the gas distribution across the carrier is determined by the physical geometry of the hardware, which eliminates the need for process tuning, resulting in more consistent end results.
To ensure that the Lumina is creating the optimum environment for III/V film deposition, the system has real-time in-situ process control for run-to-run and tool-to-tool repeatability. Temperature is controlled by pyrometry that measures the carrier surface 10,400 times per second, giving excellent real-time temperature control that helps to optimize the deposition conditions. Veeco’s Piezocon® gas concentration sensor is used to control the metal-organic flux, which gives the precise concentration control crucial to manufacturing PIC and microLED chips. This advanced process control also assists process engineers in creating identical deposition conditions across multiple MOCVD systems in the fab.
The PIC, microLED, and solar-cell markets are moving rapidly to larger wafer sizes. This creates more die per wafer and will help to increase the number of die that can be processed. As fabs upgrade wafer sizes, they need an MOCVD system that can grow with them. The Lumina has been designed so that the carrier can be changed out to run the next wafer size. A fab could start at 3-inch substrates and eventually move to 8-inch substrates just by changing the carrier because the processes are the same, and uniformity and film compositions maintain the same high standards regardless of the wafer size. This flexibility enables Veeco’s customers to shorten the transition to 8-inch wafers while maintaining the same process conditions and the same film uniformity and composition across the wafer, no matter what wafer sizes they are running in their factories.
Key features of the Lumina system include:
These carefully engineered features make Lumina the ideal system for deposition of GaAs and InP compounds for PIC, microLED, and solar-cell applications.
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