Temperature management is a key factor in semiconductor manufacturing. A small temperature change can lead to significant deviations in wafer process parameters, further affecting the performance of the chip. Wafer back cooling technology, as an effective temperature management method, not only ensures the uniformity of wafer temperature, but also enhances the stability during wafer processing. This article will delve into the methods and principles of wafer back cooling.

During some etching, deposition, or ion implantation processes, wafers may generate a significant amount of heat. If not properly controlled, this heat may cause uneven wafer temperature, thereby affecting the consistency of the chip. At this point, it is necessary to cool the wafer and maintain the wafer temperature at the set temperature. Therefore, I will choose the method of back cooling. Back cooling is achieved by cooling the back of the wafer, which is its non working surface.

Why is it back cooling instead of front cooling? Because the front of the wafer is the working surface, cooling on the front will interfere with the process. And through back cooling, it will not interfere with the normal operation of the process, and integrating the cooling system into the suction cup provides more cooling methods to choose from, with higher cooling efficiency and minimal impact on the process.

What are the methods of wafer back cooling?

Gas back cooling: By spraying cooling gas on the back of the wafer, the gas contacts the back of the wafer, absorbs excess heat from the wafer, and then transfers the heat to the machine cooling system for cooling. By adjusting the pressure and flow rate of gas, the temperature of the wafer can be accurately controlled. In terms of wafer back cooling, helium is the most common back cooling gas. Helium has a very high thermal conductivity, much higher than most gases, including nitrogen. This allows helium to quickly and effectively transfer heat from the wafer, ensuring effective cooling. Helium does not react chemically with wafers or machine components. This is crucial as any chemical reaction can lead to defects or contamination on the wafer surface.

Liquid back cooling: Liquid back cooling directly or indirectly contacts the back of the wafer through flowing liquid coolant, rapidly transferring the heat generated by the wafer to the coolant. Through a circulating cooling system, the liquid is sent to the cooler for cooling, and then flows into the back of the wafer for cooling, forming a closed cooling cycle. Liquid back cooling has higher cooling efficiency than gas back cooling, but liquid back cooling systems are relatively complex and costly.

Liquid coolant should have good thermal conductivity, chemical stability, etc., and is usually a neutral liquid with a larger specific heat capacity. Generally: water, mixture of water and ethylene glycol, organic coolant, etc.

Water is one of the most commonly used liquid coolants because of its excellent thermal conductivity and low cost. However, water may undergo chemical reactions with certain materials, so it may require the addition of corrosion inhibitors, etc.

A mixture of water and ethylene glycol is commonly used in applications that require low-temperature cooling. Ethylene glycol can also reduce the corrosiveness of water.

Organic coolants, such as oil-based coolants, have good heat transfer characteristics and chemical stability. They are typically used in cooling systems that do not come into direct contact with wafers.

The choice of liquid cooling or air cooling depends on many factors, generally considering factors such as cooling demand and cost.

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