Plasma Etching & Temperature Control
Plasma etching is a key process in the manufacture of semiconductors, MEMS, and photovoltaic cells. It uses ionised gases (plasma) to remove material from a surface with extreme precision, enabling the formation of intricate microstructures.
Key steps in plasma etching:
Ionised gases are created in a vacuum chamber using an electric field.
Ions are directed toward the substrate for anisotropic (directional) etching.
A mask protects non-etched areas to create accurate patterns.
Techniques like optical emission spectroscopy detect the process end-point.
The choice of gases determines etching behaviour.
The Role of Process Cooling
Efficient temperature control is essential in plasma etching to protect delicate components and maintain process accuracy. Process cooling equipment regulates the temperature of the etch chamber and the substrate, preventing thermal damage and unwanted diffusion of materials.
It also supports the stability of the plasma by managing the energy levels of ions and radicals, which contributes to repeatable and precise etching. Additionally, cooling systems are crucial for managing the heat output from essential components such as RF generators, vacuum pumps, and power supplies.
Recirculating Chillers in Plasma Etching
Recirculating chillers maintain tight temperature control by circulating a cooled heat transfer fluid through closed-loop systems. They are used for direct or indirect cooling of chambers and substrates, component cooling for auxiliary systems, and maintaining thermal precision across the process.
Their benefits include accurate temperature stability, energy-efficient operation, and compact designs that can be tailored to the specific cooling requirements of the plasma etching process. However, they come with higher initial costs and require ongoing maintenance, including regular inspections and fluid management. In larger systems, chillers can contribute significantly to energy consumption, and operators must manage heat transfer fluids carefully to mitigate health, safety, or environmental risks.
Water-to-Water Heat Exchangers
Water-to-water heat exchangers are often used where direct chiller integration is not feasible or when facility cooling water is readily available. These systems transfer heat between two isolated water loops – one in contact with the plasma etch equipment, and the other connected to facility infrastructure.
They offer energy efficiency, reliability, and low maintenance due to their simple, non-mechanical design. Because the loops are isolated, chemically treated or ultra-clean process water can be used without contaminating the facility water. While they are often more cost-effective than chillers, their performance depends heavily on the temperature and quality of the facility water. Temperature control is also limited to the conditions of the secondary loop, and infrastructure requirements can increase upfront installation costs.
Heat Transfer Fluids
The choice of heat transfer fluid is a key factor in the efficiency and safety of plasma etching systems. These fluids are responsible for managing precise temperature control within recirculating chillers, heat exchangers, and other cooling equipment.
When selecting a fluid, engineers must consider several criteria. Thermal conductivity is important for efficient heat exchange, and the fluid must remain stable across the operating temperature range. Chemical compatibility with system materials is crucial to avoid corrosion or degradation. Viscosity influences how easily the fluid circulates and impacts pump energy consumption. Safety considerations include toxicity, flammability, and environmental impact, all of which affect long-term performance and compliance.
Common types of heat transfer fluids used in plasma etching include:
High thermal conductivity and specific heat capacity; deionised water is often used to prevent contamination.
Ethylene glycol or propylene glycol mixed with water to reduce freezing point and inhibit corrosion.
Specially formulated synthetic fluids offering excellent thermal stability, low conductivity, and non-flammability.
Chemically stable fluids ideal for harsh environments and wide operating temperature ranges.
Known for their broad temperature tolerance and thermal stability, though less conductive than water-based fluids.
Mineral oils and similar fluids, used less frequently due to their flammability and contamination risks.
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