The role of vacuum in synthetic diamonds' production July 11, 2022
4 MIN READ
Mention diamonds and most people will associate that with the jewelry sector. They will also have in mind that such gems are to be found in mines and sometimes won under severe conditions (“blood diamonds”). For such reasons, the industry already investigated in the 1950s how to produce synthetic diamonds.
Today, there are several processes available to create artificial diamonds. Not only for jewelry where even experts are challenged to differentiate a natural diamond from an artificial, but also for the much larger market for diamonds in the industrial sector.
Four outstanding properties of synthetic diamonds
The key properties of synthetic diamonds that make them so interesting for the industrial sector are:
- Mechanical (superior hardness a chemical inertness)
- Optical (high transparency)
- Electrical (semiconductor properties)
- Thermal (highest heat conductivity)
Some typical industrial application examples are machining and cutting tools, window material for transmitting infrared and microwave radiation, and heat sinks for lasers and transistors. Synthetic diamonds are also the base material for semiconductor wafers and Qubits for quantum computing.
How are diamonds obtained?
Traditionally diamonds were naturally formed billions of years ago in the earth’s crust by carbon atoms being exposed to intense heat and pressure. However, due to technical advances, a new source of synthetically grown diamonds was created. This market can be divided into 2 main manufacturing methods:
- HPHT (High-Pressure High Temperature)– This method is based on how diamonds were naturally formed - compress carbon and add heat. The equipment footprint often has large and limited process control.
- CVD (Chemical Vapor Deposition)– This method is based on creating a vacuum environment and then introducing specific gasses to trigger a reaction. The equipment footprint often has a small and excellent process control.
MPCVD (Microwave Plasma CVD) growing process
Due to its small footprint and excellent process controls, the MPCVD growing process has gained in popularity. One important part of the MPCVD process is the creation of and maintaining optimal vacuum conditions inside a growth chamber. A typical growth chamber setup looks as below:
Synthetic diamond-making process. Source: NNE, 2019
The typical process steps are the following:
- Layout diamond seed/substrate
- Close chamber and evacuate chamber for conditioning, heat up substrate
- Introduce hydrogen
- Turn on microwaves and generate plasma (~4000 °C) above the substrate
- Introduce process gases and start epitaxial growth
- Methane – carbon source
- Ethanol, Acetone, or Oxygen (optional) – helps improve the quality through oxygen plasma etching effect
- Nitrogen (optional) – helps speed up the process (learned from experience and made MPCVD growing feasible economically)
- 1 to 4 weeks of growth period
Which are the vacuum pump requirements for the growth period?
- Initial pump down to about 2E-2 mbar base pressure
- Maintain pressure (100-300 mbar, typically in a range of 100-130 mbar) while process gasses flow
- Low-pressure cleanup of diamond color after production
Nowadays, oil-sealed , or 2-stage rotary vane pumps are typically used in the MPCVD process. This is because these pumps are very well suited to achieve low conditioning pressure but also because they are mainly designed for continuous operation at fine vacuum. Additionally, these pumps face poor lubrication, overheating, or high oil loss when operating for days in the rough pressure phase required by this process.
Advantages of the 2-stage pumps developed by Leybold
The oil lubrication of TRIVAC pumps is secured by a built-in oil pump, which also enables a uniform oil distribution at rough pressure operation, whereas standard pumps without such oil pumps could fail.
NEO D 2-stage rotary vane pump is designed for operation at rough pressure and provides a perfect solution for the diverse pressure requirements of this growing process:
- The design is based on our famous SOGEVAC industrial-pump range and offers proven robustness for industrial applications
- The low ultimate pressure can easily meet the initial pump-down request, but the design of the pump also allows continuous operation at any (even rough) operation pressure
- Integrated high-efficiency exhaust filter that can reduce oil losses to the technical minimum. The oil stays inside the pump!
- Compact design. There is no need for an external exhaust oil mist separator with oil return
For those customers who want to achieve a conditioning pressure in the high-vacuum range <10-3 mbar, Leybold can also offer a wide range of turbo pumps like the TURBOVAC series. Next to this, we offer the required gauges and hardware.
Leybold: Your one stop shop partner
With 175+ years of experience and a broad product range, Leybold is a great partner for your CVD diamond growth. From roughing pumps to turbo pumps or accessories, we have a large variety of products. We also have a dedicated applications team ready to calculate the ideal pump configuration.
With a global support organization, we can support your manufacturing operations wherever they may be located.
