Lab-grown diamond manufacturing with laser-plasma CVD

Plasma-enhanced CVD is the most popular current technique for making lab-grown diamonds. The main subtypes of modern plasma-enhanced CVD include: 

• DC plasma jet 
• Microwave plasma 
• RF plasma  

A lesser-known subtype of plasma-enhanced CVD existing since the 1990’s which operates at ambient and positive pressures and has higher theoretical diamond growth rates than previous subtypes is laser-plasma CVD. Scaling these systems from their current research settings at universities to the commercial lab-grown diamond market may be highly profitable for CVD system manufacturers and diamond manufacturers alike. 

Just as in other diamond CVD systems, Alicat’s mass flow controllers and pressure controllers can provide optimal gas mixing and chamber pressure operating conditions for laser-plasma CVD systems.  

Main benefits of laser-plasma CVD systems 

Less restricted operating pressure conditions 

One of the main advantages of laser-plasma CVD compared to all other plasma-enhanced CVD techniques is that laser-plasma CVD diamond manufacturing is achievable at ambient and positive pressures (1-4.5 atm) whereas other plasma-enhanced CVD diamond manufacturing techniques rely on sub-atmospheric pressures. As a result, pressure regulation is cheaper in laser-plasma CVD systems than it is in traditional plasma-enhanced CVD techniques in which chamber vacuum conditions must be maintained below 100 Torr.    

Higher theoretical diamond growth rates 

Another huge advantage of laser-plasma CVD is that it increases theoretical diamond deposition rates by 2-3 orders of magnitude above prior techniques per Bachmann’s model (predicted up to 10³ 10⁴ μm/hr). This occurs as the laser power density in laser-plasma CVD is way beyond that of all other plasma-enhanced CVD methods, allowing for much quicker theoretical reactions. Nonetheless, at the current research stage, laser-plasma CVD deposition rates are just 100 μm/hr, still exceeding many other plasma-enhanced CVD growth rates but much lower than predicted. Achieving the theoretical diamond growth rates for laser-plasma CVD is an exciting area of continuing research with highly lucrative potential outcomes.   

Designing better laser-plasma CVD systems 

Laser-plasma CVD diamond manufacturing uses laser plasmatrons to deposit diamond films. In this system, a continuous CO₂ laser passes through a sealed window and is concentrated using mirrors into a reaction chamber through a specially designed nozzle. After entering the chamber, the CO₂ laser mixes with an Ar or Xe gas plasma that interacts with the main feed gases, primarily CH₄ and H₂ but sometimes alternatively CH₄ and CO₂. Gas mass flow controllers provide optimal gas mixing while pressure controllers maintain chamber operating pressures. Plasma ignition of the gas mixture is achieved using either a high voltage electrical discharge or a thin tungsten wire. The substrate on which the diamond film grows is cooled by recirculating water and heated by the laser. Liquid mass flow controllers drive the water to cool the substrate at a setpoint temperature via a control loop.  

Improving gas mix control   

According to a research paper testing laser-plasma CVD, the ideal gas mixture for these systems is 0.05% CO₂ + 0.2% CH₄ + 4.75% H₂ + 95.25% Ar, which significantly differs from the mixes used in other plasma-enhanced CVD techniques. Further investigations of laser plasma CVD are needed to create even better gas mixtures for these systems.  

Alicat’s MC-Series of mass flow controllers precisely control gas flow of the plasma and feed gases into the reaction chamber, offering the following advantages: 

• Customization of gas mixes by the part per million (ppm) 
• No warm-up required 
• Response rates of 30 ms 
• Standard NIST-traceable accuracy up to ±0.6% of reading or ±0.1% of full scale, whichever is greater 
• 0.01% – 100% of full-scale control range 
• Repeatability up to ±(0.1% of reading + 0.02% of full scale) 

Improving substrate temperature control 

Additionally, Alicat’s LC-Series and CODA KC-Series of liquid flow controllers control the flow of water which cools the diamond growing substrate, offering: 

• LC-Series NIST-traceable accuracy up to ±2% of full scale 
• CODA KC-Series Coriolis controllers NIST-traceable accuracy up to ±0.2% of reading or ±0.05% of full scale 
• Insensitivity to external vibration in CODA KC-Series  
• Multivariate flow control using either liquid volumetric flow rate or pressure with simultaneous measurements of temperature for control loop commands 

Improving pressure control  

Since laser-plasma CVD requires positive pressure regulation, Alicat’s PC-Series of pressure controllers can moderate downstream chamber pressure regulation in combination with a vacuum pump, quickly allowing exhaust of excessive chamber gases. For this application, Alicat’s PC-Series offers: 

• NIST-traceable accuracy up ±0.125% of full scale 
• Control range to 0.01% – 100% of full scale 
• Repeatability up to 0.08% of full scale 

Additional support 

Alicat develops and manufactures a variety of mass flow and pressure controllers which are specifically customized for diamond CVD applications.  

To better support diamond customers, Alicat has sales and service centers in Surat and Thane near major diamond manufacturers.