Peng Lipei, Wang Shaobo, Li Shaobo

  1 Introduction

  NF3 is a new type of electronic material that has recently emerged, serving as an excellent plasma etching and cleaning gas in the information industry. When used in dry etching, NF3 gas can enhance automation in wafer manufacturing, reduce labor intensity, increase safety factors, and offer advantages such as high etching rates, high selectivity, and minimal residual contaminants. For semiconductor materials like silicon, especially integrated circuit materials with thicknesses less than 115μm, NF3 exhibits superior etching speed and selectivity compared to other gases [1]. As a gas cleaning agent, NF3 is fast, efficient, thorough, and leaves no traces, making it highly promising for market applications.

  The performance of semiconductor electronic devices is closely related to the purity of NF3 gas. The purity of NF3 gas produced by different methods varies, and in practical work, appropriate purification methods should be selected based on the characteristics of the production system.

  2 NF3 Purification Methods

  2.1 Cryogenic Distillation

  In crude NF3 gas, there are components with boiling points higher than NF3, such as N2O, CO2, HF, and N2F2, and components with boiling points lower than NF3, such as O2, N2, and F2. In a cryogenic distillation column, components with different boiling points are separated through multiple evaporation and condensation processes of gas-liquid equilibrium.

  There are many forms of cryogenic distillation purification of NF3 in industry, with a typical example being the process proposed by Hiroyuki Hyakutake et al., as shown in Figure 1.

  In this process, a large amount of high-boiling-point impurities such as N2O and CO2 are adsorbed by active aluminum particles before the gas enters the distillation column; while low-boiling-point components such as O2 and N2 are removed in the cryogenic distillation column. In the cryogenic distillation column, liquid nitrogen is used as the cooling medium, and one or more of the low-boiling-point inert gases He, Ne, and Ar are added as the third gas. In this process, by controlling the appropriate ratio of the third component to the crude NF3 gas (011～1010), high-purity liquid NF3 with a mass content of 99199% is obtained. However, this process consumes a large amount of the third component gas, increasing operational costs.

  To overcome the above drawbacks, Takashi Nagamura et al. proposed a process method without using the third component gas, as shown in Figure 2.

  The NF3 feed gas is pressurized to remove moisture and carbon dioxide, then cooled and further purified in adsorption column 1 to remove CO2, H2O, and some CF4. The NF3 mixed gas is then cooled to -70℃ and enters a low-temperature adsorption column 2 filled with activated alumina, where N2F2, N2F4, and N2O are removed, and the impurity CF4 content is further reduced. Finally, the gas enters a distillation device composed of medium-pressure distillation column 3 and low-pressure distillation column 5, where multiple gas-liquid contacts and mass transfers occur, resulting in NF3 product gas with a purity of up to 991999%.

  The above purification process is effective but complex. In contrast, the Handan Purification Equipment Research Institute obtained high-purity NF3 product gas using a relatively simple distillation device, as shown in Figure 3.

  After the successful implementation of this process, the institute further enhanced production continuity and stability by using a continuous distillation process, as shown in Figure 4.

  The crude gas enters medium-pressure distillation column 1, where the temperature is controlled within a certain range. High-boiling-point impurities are separated at the bottom of the column, while NF3 gas and low-boiling-point components are condensed at the top of the column, with part of the condensate refluxing into column 1 and part entering distillation column 4. After multiple gas-liquid contacts, high-purity NF3 product gas is obtained at the bottom of column 4.

  Cryogenic distillation can yield very high-purity NF3 product gas, but the investment is substantial, the equipment is complex, and the distillation operation requires strict control and accurate analysis, making it suitable for large-scale industrial production.

  2.2 Azeotropic Distillation

  Since the boiling point of CF4 in crude NF3 gas is only 1℃ different from that of NF3, general separation methods are difficult. By adding a new component as an azeotropic agent, azeotropic distillation can be used for separation.

  The azeotropic distillation process of the 718 Research Institute is shown in Figure 5.

  In this process, the azeotropic agent and the gas to be separated enter distillation column 1 together. The column pressure is adjusted to form an azeotrope of the azeotropic agent and CF4, resulting in NF3 gas with almost no CF4 impurities at the bottom of column 1, and CF4 gas and the azeotropic agent are recovered at the bottom of column 4. In column 6, NF3 and the azeotropic agent are further separated, yielding extremely high-purity NF3 product. The total number of column stages is relatively low, and the total refrigeration power of the columns and circulating coolers is also low, allowing effective recovery and reuse of the azeotropic agent.

  Azeotropic distillation can effectively remove CF4 impurities with boiling points close to NF3, producing high-purity NF3 product gas that meets the high-precision requirements of the semiconductor industry. However, there are still areas for improvement, mainly in simplifying the process, reducing costs, and increasing product yield and azeotropic agent recovery.

  2.3 Chemical Absorption

  Given the characteristics of NF3 production, some acidic and oxidizing gases such as HF and OF2 are generally removed using alkaline or reducing solutions (Na2S2O3, KI, HI, Na2S, Na2SO4, or Na2SO3 solutions). Especially the oxygen-containing fluoride OF2, which is particularly dangerous in the subsequent purification stages of NF3, must be removed early in the purification process.

  The implementation of this method is generally the same, using a gas purifier to bring NF3 gas into contact with an alkaline solution to fully remove HF gas impurities. The gas is then brought into contact with a certain concentration of Na2S2O3/HI/Na2S solution to effectively absorb and remove OF2. The water-containing NF3 gas after absorption treatment can be dehydrated using sulfuric acid with a mass fraction greater than 70%.

  The absorption method can effectively remove impurities such as HF, OF2, and N2F2. Controlling the concentration of the absorption solution and reducing the frequency of its replacement can be of great significance for actual production.

  2.4 Chemical Conversion

  For nitrogen fluoride impurities such as N2F2 and N2F4 in NF3, metal catalysts can be used to decompose these impurity gases for purification. Based on this principle, different measures can be taken to achieve the best removal effect.

  The method involves passing NF3 gas into a reactor filled with nickel tubes and perforated nickel balls and heating it for a certain period to decompose and remove N2F2 gas. This process avoids the generation of high-concentration hazardous gases, ensuring good safety, and results in minimal NF3 gas loss.

  To improve the decomposition of impurities such as N2F2 and N2F4, the nickel filler in the container can be replaced with solid fluoride (CaF2). The NF3 gas containing N2F2 is then passed through and heated, effectively decomposing N2F2 into N2 and F2. Pre-treating the solid fluoride can more thoroughly remove impurity gases. Nishitsuji Toshihiko et al. mixed fluoride mixtures (CaF2 and NaF powder) with less than 20% mass fraction of molding aids such as polyvinyl alcohol, compressed them using a plate machine, and then filled them into a nickel container after heat treatment, effectively decomposing and removing impurities such as N2F2 and OF2. Using fluorosilicate or a mixture of fluorosilicate and solid fluoride (NaF) as defluorinating agents also shows significant removal effects.

  Overall, the chemical conversion method is simple and economical, with high efficiency in removing N2F2 and N2F4 impurities, minimal NF3 loss, and relatively simple subsequent processing, making it highly valuable for industrial applications.

  2.5 Selective Adsorption

  Based on the differences in physical properties of the components in crude NF3 gas, suitable adsorbents can be selected to separate NF3 from impurities such as N2F2, NxOy, and H2O, yielding high-purity product gas.

  Common adsorbents include activated alumina, molecular sieves, silica gel, and activated carbon.

  Early in the world, silicon-aluminum synthetic crystals were used as adsorbents, with the following process: first, the gas is passed through a scrubber to remove HF, or under specific conditions, through a metal layer to remove N2F2 (extending the adsorbent's life), then through an adsorbent containing silicon-aluminum synthetic crystals to remove NxOy and H2O, and finally, NF3 gas is collected using a low-temperature cold trap. As for CF4 impurities, gas-solid chromatography can achieve a purity of 99199%. However, the disadvantages of chromatography separation are low efficiency, high inert gas consumption, and difficulty in industrial implementation. To improve separation efficiency, condensers can be used before and after the chromatographic separator. The specific process is shown in Figure 6.

  This method can effectively separate NF3 from CF4, and through cyclic operation, nearly 100% high-purity NF3 gas can be obtained. There are many examples of using molecular sieve adsorbents, which selectively adsorb and remove gaseous impurities such as CF4, N2O, CO2, and N2F2 using molecular sieves and natural zeolites. However, the zeolites must be heat-treated to remove water to achieve good adsorption effects. Improving the heat treatment process can enhance efficiency, such as inserting metal plates into the active zeolite layer to enhance heat conduction and improve the efficiency of the heat treatment step.

  Using zeolite adsorption, high-purity gas can be obtained with good impurity removal effects. However, strict requirements on zeolite pore size and water content, along with the need for pre-treatment, make it energy and inert gas-consuming, which is not conducive to industrial implementation. Given these characteristics, activated aluminum can be considered for removing moisture and carbon oxides from NF3 gas. Since NF3 reacts with Al only at high temperatures, controlling the temperature range can prevent NF3 loss due to reactions. Additionally, solid chloride layer adsorption, ultraviolet irradiation, silica gel adsorption, and activated carbon layer adsorption are widely used, with activated carbon adsorption being particularly safe, without heat generation or explosion risks.

  In summary, when removing impurities such as CF4, N2O, CO2, and N2F2, the impurity adsorption capacity per unit volume of activated carbon and activated aluminum is slightly inferior to zeolites, resulting in lower adsorption capacity and faster adsorbent loss. In practical applications, adsorbents need to be replaced frequently, leading to some NF3 loss during replacement. The adsorption selectivity of zeolites mainly depends on particle size, and using two or more synthetic zeolites to enhance selectivity complicates the process and is uneconomical. Zeolites adsorb NF3 along with impurities, resulting in significant product loss. Ultraviolet irradiation removes fewer impurities and requires quartz tubes, increasing costs. Therefore, to efficiently and cost-effectively purify NF3 gas, the advantages and disadvantages of each method should be considered and combined.

  3 Conclusion

  As the quality requirements for electronic products increase, the demand for high-purity NF3 products is growing. We need to comprehensively analyze and adopt the above methods: adsorption mainly removes N2O, CO2, and N2F2, but the efficiency is not high, especially with the short lifespan of adsorbents; absorption removes acidic impurities such as HF that are easily absorbed by alkaline solutions, is simple and widely adopted; distillation mainly handles impurities (such as CF4) that are difficult to remove by the first two methods, yielding high-purity product gas suitable for large-scale industrial production; combining the above methods can significantly reduce various impurity contents, obtaining high-purity product gas to meet the needs of semiconductor electronic industrial production.

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