Building CVD diamond machines with flow and pressure regulation

In 2020, the lab-grown diamond market reached 19.3 billion dollars and by 2030 it is projected to reach about 49.9 billion. Lab-grown diamonds are safer, more environmentally friendly, and cheaper to produce than are mined diamonds. The main methods for lab-grown diamond manufacturing are HPHT and CVD. Although HPHT developed prior to CVD, it is less commonly used since it requires more energy to operate due to its extreme temperature and pressure requirements. Moreover, HPHT is sometimes also considered as a secondary procedure to purify CVD diamonds. The different types of CVD for diamond manufacturing include filament-assisted thermal CVD, plasma-enhanced CVD, and combustion-flame assisted CVD. Among these, a subtype of plasma-enhanced CVD, microwave plasma CVD, is the most popular and reliable. To produce high quality lab-grown diamonds using any CVD method, manufacturers depend on Alicat’s devices to ensure precise gas mixtures of hydrogen, methane, and carrier gases. In the following, we will discuss how Alicat’s devices enhance each CVD process.

Filament-assisted thermal CVD

Filament-assisted thermal CVD allows for the largest lab-grown diamond growth area and a simpler design than other CVD techniques. Drawbacks include a slower diamond growth rate (0.5-8 µm/h), and a worse process stability compared to other CVD methods. In this configuration, methane (0.5% to 2.0%) and hydrogen gases are released in a vacuum chamber (pressurized at 10-100 Torr) and dissociated by high-melting point (2,000-2,300°C) metal filaments, typically tungsten, near a heated substrate (700-1,000°C) which accumulates diamond particles over time.

Gas mass flow and pressure control

Alicat’s custom mass flow controllers maintain accurate gas mixing with no poisoning from atmospheric gas. With only a leak rate of 10e^-9 ATM SCCS of helium and 2 ms response times, Alicat’s devices mitigate improper gas mixing and contamination of the diamond growing chamber. Connected to the chamber vacuum source, Alicat’s pressure controllers handle back pressure to allow for stable and accurate chamber conditions.

Combustion-flame assisted CVD

Combustion-flame assisted CVD grows diamonds at atmospheric pressures using an oxygen-acetylene brazing torch and water-cooled substrate. Advantages include design simplicity, good growth rates (30-100 µm/h), and low costs. In this setup, at sub or atmospheric pressures oxygen and acetylene gases are released into a chamber controlled by a mass flow controller where they are combusted into a 3300 K flame, producing H₂, CO, and reactive intermediates. Outside of the main combustion area of the chamber is the feather, a flame free zone abundant in atomic hydrogen and hydrocarbon radicals where the diamond growing substrate is positioned. In the outer zone, there is a secondary combustion region which oxidizes gases to CO₂ and H₂O. The temperature of the substrate is controlled by water flow in and out the system below a copper substrate mount using a pyrometer.

Gas and liquid flow control

Alicat mass flow controllers precisely control the gas flow of both oxygen and acetylene into the diamond growing chamber while, Alicat’s liquid controllers (LC-Series and CODA KC-Series) control the flow of water to cool the diamond growing substrate.

• Gas mass flow controllers include a pneumatically actuated, positive shutoff valve to ensure no leak through and have a NIST-traceable accuracy of ±0.5% of reading or ±0.1% of full scale for 98+ gases and 20 custom-defined gas mixtures.
• LC-Series liquids controllers flow 0.5 CCM to 10 LPM of water with a control response rate of just 30ms at an accuracy of ±2% of full scale.
• CODA KC-Series Coriolis controllers can flow water from 40 g/h to 100 kg/h with an accuracy of only ±0.2% of reading or ±0.05% of full scale.

Pressure control

If operating at sub-atmospheric pressures, Alicat’s pressure controllers provide stable and repeatable chamber growing conditions just as in filament-assisted thermal CVD. Custom valve sizing and PID tuning ensure that controllers reach pressure setpoints in milliseconds and maintain control with NIST-traceable accuracy to ±0.125% of full scale.

Plasma-enhanced CVD

Main plasma-enhanced CVD methods include DC plasma jet, microwave plasma, and RF plasma. Plasma-enhanced CVD allows for better quality and stability than other CVD techniques in addition to typically faster growth rates (DC plasma jet at 930 µm/h, microwave plasma at 3-30 µm/h, and RF plasma at 180 µm/h). Due to its combined large growing area and fast growth rate, microwave plasma chemical vapor deposition is currently the most practical and popular CVD technique for diamond manufacturing. In microwave plasma chemical vapor deposition, CH₄ and H₂, sometimes additionally with Ar, O₂, and N₂ flow into a vacuum diamond growing chamber while high voltage microwaves hit them. These gases then turn into a plasma which reacts with the diamond growing substrate to produce diamonds.

Pressure control

Vacuum pumps and pressure controllers maintain a sub-atmospheric diamond growing chamber pressure. Increasing the microwave power used in this technique causes an increase in diamond growth rates by strengthening plasma density.

Additional info

Alicat mass flow and pressure controllers are specifically customized for diamond and vacuum applications. Units built for such applications are available in 4-6 weeks with 0.25 VCRM fittings and minimal sealing surfaces in order to minimize leak points over time. For ease of support, Alicat has sales and service centers in Surat and Thane near major diamond manufacturers.