How to ensure the purity of synthetic diamonds created by chemical vapor deposition

Synthetic diamonds are created and used all over the world, for similar applications as naturally occurring diamonds. Recently, the method of using chemical vapor deposition (CVD) to ‘grow’ diamonds in laboratory conditions has become more common.

Although more expensive than the traditional high pressure and high temperature (HPHT) technique, CVD produces the best grade of synthetic diamonds possible in a relatively short time. HPHT diamonds are mostly industrial grade while high-quality CVD diamonds are also suitable for jewelery.

Careful control of trace impurities in high purity methane and hydrogen is crucial to ensure the quality and grade of the synthetic diamonds created by chemical vapor deposition. This is best achieved using a highly accurate process gas chromatograph to detect trace nitrogen in the feedstock. Trace oxygen and moisture measurements are also often necessary for some CVD processes to ensure the purity of the methane and hydrogen.

How does Chemical Vapor Deposition grow synthetic diamonds?

The chemical vapor deposition technique uses a hydrocarbon gas mixture to create a carbon plasma over a substrate, onto which the carbon atoms deposit and build up to form a crystal structure.

Since the early 1980s, this method has been the subject of intensive worldwide research. CVD growth involves preparing the substrate, feeding varying amounts of gases into a chamber and energizing them.

Silicon is often used as a substrate because it has a suitable crystallographic orientation. This substrate is then cleaned using an abrasive diamond powder to prepare the surface for the deposition of the carbon atoms. Although CVD is a chemical method, high temperatures are still required for producing the plasma and the substrate is heated to around 800 °C.

High-purity methane is the most common gas used as the source of carbon for the synthetic diamonds. This is mixed with UHP hydrogen in almost equal amounts at a ratio of 1:99.

Hydrogen is essential for the process because it selectively etches off non-diamond carbon. For the reaction to take place, the gases are ionized into chemically active radicals. This takes place within the growth chamber using a source of power such as microwaves or a hot filament.

The grade of the diamonds and the speed of production depends on the purity of the Methane and Hydrogen feedstocks, and this is where a highly accurate trace impurity analyzer, such as a process gas chromatograph is needed. This type of instrument has the sensitivity to measure trace N2 down to 0.5 ppb, with a repeatability of 0.1 %.

Which products are recommended to ensure the quality and purity of synthetic diamonds?

Because multiple measurements are needed for a complete trace impurities analysis, the LDetek MultiDetek3 is recommended since it has the capability to monitor multiple gas streams for different impurities simultaneously, essentially offering the capability of two GCs in one.

In some CVD processes trace O2 and trace moisture analysis are also required to ensure the purity of the additive gases and the hydrogen/methane mixture and the MultiDetek3 can be configured to capture all these measurements as follows:

Channel 1:

The first channel is configured with a Plasma Emission Detector (PED), using Helium or Argon as carrier gas depending on the preference of the user. The plasma detector is mounted with a selective optical filter for measuring N2 and another filter for measuring O2. These two impurities can then be measured without being affected by the background gas or other interference gas molecules.

The sample is simply injected through a molecular sieve column and the O2-N2 impurities are measured by the plasma emission detector. Using this method, the impurities can be measured from low ppb up to ppm in the required gas mixtures containing He/Ar/H2/CH4. The same plasma emission detector (PED) can also be used for measuring the ppb/ppm trace ammonia (NH3) in the different gas mixtures. A separate chromatographic flow path using the appropriacy capillary columns will be used.

Channel 2:

The second channel is mounted with a quartz crystal sensor capable to measure trace moisture from ppb to ppm in continuous. The sample gas is regulated with its mass flow controller and measured by the sensor. The MultiDetek3 GC has a built-in moisture permeation span calibration device that periodically validates the quartz crystal sensor.

Working with the MultiDetek3, the LDGSS Stream Selector System automatically switches between gas streams, and fits in the LDRack system together with the MultiDetek3 to provide a compact solution for synthetic diamond manufacture using chemical vapor deposition.

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