**Abstract**

The growth of the semiconductor market and advancement of manufacturing technology have led to an increase in wafer size and highly integrated semiconductor devices. The temperature of the supplied cooling medium from the chiller that removes the heat produced in the semiconductor manufacturing process is required to be at a lower level because of the high integration. The Joule-Thomson cooling cycle, which uses a mixed refrigerant (MR) to produce the cooling medium at a level of −100°C required for the semiconductor process, has recently gained attention. When a MR is used, the chiller's performance is heavily influenced by the composition and proportions of the refrigerant charged to the chiller system. Therefore, this paper introduces a cooling cycle that uses an MR to achieve the required low temperature of −100°C in the semiconductor manufacturing process and provides the results of simple experiments to determine the effects of different MR compositions.

**Keywords:** Semiconductor etching process, Mixed refrigerant refrigerator, Refrigerant mixing ratio, Ultra low temperature, Joule-Thomson cycle

### **1. Introduction**

Because of the growth of the semiconductor market and the global competition among many companies of the United States, Taiwan, South Korea, etc., investment and interest in related industries are increasing [1]. As a result of this trend, there is an increased demand for chillers, which are temperature control systems used in the semiconductor manufacturing process, and the research into chillers is also progressing actively [2].

As shown in **Figure 1** [3], semiconductor manufacturing process consists of eight major processes, including wafer manufacturing, oxidation, photolithography, etching, and deposition [4]. Etching, one of the eight major semiconductor processes, is a process that removes unnecessary parts in the sketch of circuit drawn in the

#### **Figure 1.**

*Manufacturing process of semiconductors [3].*

photolithography process [5]. A semiconductor etcher is a device used to etch circuit patterns formed on wafers. The typical method is to create plasma gas with excellent etching properties and etch a specified part of the wafer [6, 7].

Here, there might be a risk of an excessive rise in wafer temperature when it is exposed to the high temperature of the plasma [8]. Therefore, general etching devices cool the wafers by circulating the cooling medium inside the electrostatic chuck (ESC) where the wafers are installed [9].

On the contrary, because of the high integration of semiconductor circuit patterns, the line width of circuits is becoming smaller [10]. The temperature of ESC in the 20 ~ 30 nm line-width etching process is room temperature, but the −20°C temperature level is primarily used at the 10 nm level. The etching processes that require a lower temperature are trending especially at the fine level of 5 ~ 7 nm process. Therefore, there is a growing demand for the ultra-low temperature etching process, which involves performing etching while maintaining the temperature of substrates below −100°C. If etching is performed in the ultra-low temperature domain, the spontaneous reaction is suppressed, enabling anisotropic etching. However, this kind of process has disadvantages in that implementation is difficult in terms of the equipment and environment for maintaining the ultra-low temperature of substrates, and the energy consumption is high.

To achieve the low temperature of −100°C, various cycles such as, two-stage cascade cycle, Joule-Thomson cycle, and auto-cascade cycle are used. Joule-Thomson coolers are used in many fields because they have a simpler structure and are easier to manufacture and operate compared with the two-stage cascade cycle. However, the main disadvantage of Joule-Thomson coolers is their low efficiency, which is caused by irreversibility because of the high-pressure rate and wide temperature range. This problem can be resolved by using a mixed refrigerant (MR). **Figure 2** shows the refrigeration effect of a single cooling medium according to the working pressure. Nitrogen or argon, which is a low boiling point cooling medium, requires a higher pressure to obtain the cooling calories compared to propane or ethane, which is a high boiling point cooling medium. In the case of nitrogen, for example, a working pressure of approximately 24.6 MPa is required to obtain a cooling calorie of 1,000 J/mol, whereas, in the case of propane, the working pressure may be at a level of approximately 0.79 MPa, showing a large difference [11]. In fact, if an MR is created by combining

*Perspective Chapter: Ultra-Low Temperature Chillers for Semiconductor Manufacturing Process DOI: http://dx.doi.org/10.5772/intechopen.98547*

**Figure 2.** *Variation of specific refrigeration effect according to operation pressure [11].*

nitrogen, argon, methane, krypton, ethane, propane, etc., the cooling calorie of 1,000 J/mol can be obtained below the working pressure of 2.5 MPa.

Therefore, this paper introduces a Joule-Thomson cooler that uses an MR to achieve the low temperature of −100°C required in the semiconductor manufacturing process.

### **2. Refrigeration cycles for ultra-low temperature**

As aforementioned, a two-staged cascade cycle, Joule-Thomson cycle, etc. are used to achieve the low temperature of −100°C. This section introduces several refrigeration systems that are commonly used to achieve an ultra-low temperature [12].

#### **2.1 Single mixed refrigerant (MR) refrigerator**

A single MR refrigerator is a refrigerator that can achieve temperatures of −100°C or lower and has the advantage of having fewer mechanical elements, and it can be miniaturized. Because of these advantages, single MR refrigerators have been widely used in the semiconductor industry, which requires low-temperature refrigerators that can be operated reliably for an extended period. A single MR refrigerator has a single-stage refrigeration cycle that includes an intermediate heat exchanger, and it is configured as shown in **Figure 3**.

The specific working principle of single MR refrigerators is as follows. The refrigerant vapor sucked by the compressor flows into the aftercooler in a state of high temperature and high pressure through the compression process (1–2a). The refrigerant vapor is then cooled to ambient temperature through heat exchange with the heat exchanging medium at room temperature (2a–2), and flows into the intermediate heat exchanger.

**Figure 3.**

*Schematic diagram of joule-Thomson refrigeration (single MR) cycle.*

Here, the refrigerant, which is in a vapor state at high pressure (2–3), is condensed through the heat exchange with a two-phase refrigerant (g–5) of low temperature and low pressure, and is transformed into a refrigerant of high pressure and low temperature. The high-pressure, low-temperature refrigerant that has passed through the intermediate exchanger passes through the expansion valve and transforms into a two-phase refrigerant of low temperature and low pressure because the pressure and temperature are decreased by the Joule-Thomson effect (3–4). Following this, the low-temperature, low-pressure refrigerant absorbs heat from the evaporator (4–g), passes through the intermediate heat exchanger, and is sucked into the compressor to complete the cycle.

The selected refrigerant type and composition proportions of the MR used in the single MR refrigerator vary depending on the evaporation temperature and operating conditions of the system. The evaporation temperature of the MR decreases as the proportion of low boiling point refrigerant increases, and the refrigeration capacity increases as the proportion of high boiling point refrigerant increases. Furthermore, as the two-phase sections of the selected refrigerants overlap, the time for reaching the target evaporation temperature decreases. The target evaporation temperature may not be reached if the selected composition proportions used in the MR are not appropriate, resulting in a stagnant temperature. Therefore, a specific method is required to select appropriate composition proportions of the MR. To select the composition proportions of an actual MR, it is essential to conduct experiments to validate various composition proportions selected theoretically.

#### **2.2 Cascade mixed refrigerant (MR) refrigerator**

A cascade MR refrigerator is a system that applies the Joule-Thomson cycle, in which the MR is applied to the two-stage cascade refrigeration system. With the active progress of the semiconductor market, the demand for refrigerators has increased, and related studies are an increasing trend. As mentioned, the cascade MR refrigerator includes an intermediate heat exchanger in the low-stage cycle of a two-stage refrigeration system. **Figure 4** shows the schematic diagram of the device.

The cascade MR refrigerator cycle is divided into high-stage and low-stage cycles, similar to a typical two-stage refrigeration cycle. First, the flow of the refrigerant in the high-stage cycle is introduced. The high-temperature, high-pressure vapor refrigerant discharged from the high-stage compressor becomes saturated or sub-cooled liquid as it passes through the condenser (1–2). The liquid refrigerant that passed through the expansion valve (2–3), then transforms into a two-phase low-temperature and low-pressure refrigerant. It flows into the cascade heat exchanger and exchanges

*Perspective Chapter: Ultra-Low Temperature Chillers for Semiconductor Manufacturing Process DOI: http://dx.doi.org/10.5772/intechopen.98547*

**Figure 4.**

*Schematic diagram of cascade MR refrigeration cycle.*

heat with the refrigerant flow of the low-stage cycle (3–4). In other words, from the perspective of the low-stage cycle, the cascade heat exchanger serves as the aftercooler of the previously introduced single MR. Next, the flow of the refrigerant in the lowstage cycle is discussed. The refrigerant discharged from the low-stage compressor in a state of high-temperature, high-pressure vapor is partially condensed (5–6), which is then condensed into a fully liquid refrigerant through the internal heat exchange in the intermediate heat exchanger (6–7) and passes through the expansion valve (7–8). The liquid refrigerant that has passed through is in a state of low temperature and exchanges heat with the brine in the evaporator, resulting in partial evaporation (8–9). Complete evaporation occurs as the intermediate heat exchanger absorbs the heat from the high-pressure refrigerant (9–10). The evaporated refrigerant is sucked by the compressor, causing the cycle to repeat itself (10–5).

Meanwhile, it is required to explain the concept of Joule-Thomson cooling capacity, which is an important concept in studying the refrigerant composition in the MR refrigeration cycle. **Figure 5** shows an example of the refrigeration cycle using a single refrigerant for clarity. The enthalpy difference occurring when expanding from a high pressure to a low pressure along the isotherm is referred to Joule-Thomson cooling capacity. **Figures 6** and **7** show the Joule-Thomson cooling capacity of various

**Figure 5.** *P-h diagram of pure refrigerant for explaining joule-Thomson effect.*

**Figure 6.**

*Enthalpy difference according to temperature in kJ/kmol unit [13].*

refrigerants used in the composition of MR at each temperature point. For the refrigerants used in the analysis, **Table 1** shows the normal boiling point, global warming potential (GWP), ozone depletion potential (ODP), and the refrigerant safety group through the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) 2009 [14].

*Perspective Chapter: Ultra-Low Temperature Chillers for Semiconductor Manufacturing Process DOI: http://dx.doi.org/10.5772/intechopen.98547*

**Figure 7.** *Enthalpy difference according to temperature in kJ/kg unit [13].*


#### **Table 1.**

*Properties information of various refrigerants [14].*

As shown in **Figure 6** [13], in general, a refrigerant with a low standard boiling point has a small enthalpy difference, and a refrigerant with a high standard boiling point has a large enthalpy difference. If the temperature to be reached is low, then a refrigerant with a low standard boiling point should be used. However, because the temperature must be reduced from room temperature to a low-temperature region, a refrigerant with a high boiling point should be mixed to utilize advantage of a large enthalpy difference of it. In other words, a refrigerant with a low boiling point and that with a high boiling point should be mixed appropriately to satisfy the target cooling capacity and temperature simultaneously.

**Figure 7** shows the enthalpy difference per unit mass for the same refrigerants by converting the y-axis from the unit mole to the unit mass. The refrigerants with high enthalpy differences, such as i-Butane (R600a) and propane (R290) refrigerants, which are hydrocarbon (HC) refrigerants, appear prominently regardless of the standard boiling point. These HC refrigerants have a characteristic that the molecular weight is small compared with other refrigerants, and when it is represented by the unit mass, a larger number of moles is included. Therefore, HC refrigerants can have larger enthalpy differences.
