Improving MOCVD Efficiency with Precise Gas Mass Flow Control

Chemical vapour deposition (CVD) encompasses diverse methods for creating functional coating layers using volatile gas phase compounds. Among these techniques, Metal Organic Chemical Vapour Deposition (MOCVD) stands out as a unique approach, employing metal complexes with organic ligands as precursors. During the MOCVD process, coating layers form as these volatile compounds decompose on heated substrate surfaces typically maintained at temperatures ranging from 400°C to 1300°C. Operating under vacuum pressures between 10-100 torr, MOCVD systems often utilise bubblers to effectively vaporise precursors, precisely determining their concentration, growth time, and growth rate. These vapours, together with carrier gases like N2 or H2, flow into the primary growth chamber, interacting with the heated substrates.

MOCVD finds application in producing a wide range of materials including powders, fibres, thin and thick films, film heterostructures, single crystals, and glasses, as well as various structural forms such as amorphous materials and polycrystalline materials with different microstructures. Significantly, MOCVD has become the most popular method for manufacturing optoelectronic devices based on GaN and GaAs, such as light-emitting diodes (LEDs), laser diodes, hetero-bipolar junction transistors (HBTs), high electron mobility transistors (HEMTs), solar cells, and photodetectors.

It is crucial to incorporate precise gas mass flow control into the MOCVD process to enhance efficiency and optimise device performance. By effectively managing the gas flow, manufacturers can achieve improved outcomes and ensure the production of advanced optoelectronic components of superior quality.

Enhancing MOCVD Efficiency with Precise Gas Mass Flow Control

A major advantage of Metal Organic Chemical Vapour Deposition (MOCVD) lies in its ability to form dense coatings from high-temperature oxides and refractory metals like iridium and platinum, even at temperatures well below their typical melting points. These coatings are highly suitable for a wide range of manufacturing processes, including the construction of internal combustion engines, jet gas turbine blades, and pyrochemical processing. They are considered immensely lucrative, accounting for 35% of all aerospace components in the multi-billion-dollar aviation industry.

Alicat's mass flow and pressure controllers present an ideal solution for use in MOCVD systems. Precise control of both flow rates and pressures of the captured precursor and carrier gases is crucial in achieving maximum operating efficiency. In the following sections, we will explore various design options for MOCVD systems, where Alicat devices offer repeatable, fast, closed-loop control for significant MOCVD variables such as gas flow, pressure, and more.

Setup 1: Gas Mass Flow and Pressure Control in MOCVD Setup

In a typical MOCVD setup, precise control of gas mass flow and pressure is vital. The process involves heating liquid metalorganic precursors in separate solution chambers, gently bubbling them to dissolve the precursor gases, and transporting them through a temperature-controlled flash-vaporiser into the MOCVD reactor. To prevent condensation or premature reactions, the delivery line temperature is carefully regulated. The precursors, together with high-purity reactive gases, flow over heated substrates through specifically designed orifice manifolds to ensure uniform deposition and thickness.

Maintaining accurate carrier gas flow rates is crucial in this configuration. Alicat's mass flow controllers offer repeated and precise regulation of carrier gas flow rates, meeting the following specifications:

• Control gas flows from 0.5 SCCM to 5,000 SLPM
• Accuracy of ±0.5% of reading or ±0.05% of full scale (whichever is greater)
• Wide control range of 0.01-100% (10,000:1 turndown)

These controllers ensure that the correct amounts of precursor gases are bubbled, sparged, and delivered into the chemical deposition chambers.

Alicat's pressure controllers provide effective control of back pressure in the chamber, rapidly reaching the setpoints within 30 ms. With NIST-traceable accuracy of ±0.125% of full scale, they help maintain a constant vacuum between 10-100 torr in the chemical deposition chamber. The purity of carrier gases is essential for successful reactions, and to limit contamination by ambient gases, a low leak rate is ideal. Alicat's pressure and flow controllers offer low leak rates as low as 10e-9 atm-cc/sec, significantly minimising unwanted gas mixing conditions.

Setup 2: Liquid Mass Flow and Pressure Control

In an alternative configuration for MOCVD (metal-organic chemical vapour deposition), precise optimisation can be achieved by incorporating liquid flow controllers, along with gas mass flow controllers and pressure controllers, into the setup. This setup utilises Alicat's CODA KC-Series liquid flow controllers to deliver precise amounts of precursor liquids to the flash-vaporiser. Similar to the previous procedure, the precursors combine with the carrier gas, which is regulated by a gas mass flow controller, before flowing into the main chemical deposition chamber. This allows the precursors to react with the heated substrate under controlled vacuum pressures.

For the successful operation of this method, precise control of liquid precursor flow at very low rates is essential. Alicat's CODA KC-Series liquid flow controllers have the following specifications:

• Control low-flow precursor fluids from 0.8 g/h (40 g/h full scale) with a 2-100% control range
• NIST-traceable liquid flow accuracy of ±0.2% of reading or ±0.05% of full scale (whichever is greater), even with changing or unknown composition
• Repeatability of ±0.05% of reading or ±0.025% of full scale (whichever is greater)

Just like the previous MOCVD setup, Alicat's gas mass flow controllers ensure the delivery of precise amounts of carrier gases required for the reactions. Alicat's pressure controllers also regulate the vacuum back pressure in the chamber. Furthermore, the flow controllers offer continuous temperature measurements to comprehensively monitor operating conditions.

Additional Information

Alicat’s gas mass flow controllers and CODA KC-Series liquid flow controllers, paired with a computer or PLC, efficiently totalise and batch precursor liquid and gas mixes.

For MOCVD processes demanding precision, repeatability, accuracy, or automation, Alicat's flow and pressure controllers offer cutting-edge technology, distinctive features, and swift outcomes. Ideal for MOCVD applications, these controllers excel in delivering rapid and precise results to meet diverse process requirements.

Ready to enhance your MOCVD processes with precision gas mass flow control? Contact Lab Unlimited today to explore our range of cutting-edge solutions tailored to your needs.

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