Peng Lipei, Wang Shaobo, Li Shaobo, Fu Man

  0 Introduction

  Nitrogen trifluoride (NF3) is a rapidly developing raw material in the microelectronics industry in recent years, 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 1.5 µm, NF3 exhibits superior etching speed and selectivity compared to other gases [1]. As a gas cleaning agent, NF3 offers fast cleaning speed, high efficiency, thorough cleaning without leaving traces, thus presenting a broad market application prospect.

  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.

  1 Properties and Preparation of NF3

  1.1 Physical and Chemical Properties of NF3

  The molecular formula of nitrogen trifluoride is NF3, with a relative molecular mass of 71.002. It is a colorless, toxic gas at room temperature, with a melting point of -206.8°C and a boiling point of -129°C. It is the most stable among the trihalides of nitrogen, but mixtures with water (H2O), hydrogen (H2), ammonia (NH3), carbon monoxide (CO), or hydrogen sulfide (H2S) can cause violent explosions upon encountering sparks [2].

  The reactivity of NF3 is similar to that of fluorine, with the ability to release fluorine (F) under plasma conditions, making it a thermodynamically stable oxidizer. At 350°C, its reactivity is equivalent to that of oxygen (O), and at high temperatures, it can react with many elements to form dinitrogen tetrafluoride (N2F4) and corresponding fluorides. NF3 can serve as a source of free radicals, being more stable and easier to handle than F2. Due to these properties, NF3 can be used both as a source gas for plasma-enhanced chemical vapor deposition (PECVD) of silicon nitride and as an etching gas for silicon nitride in plasma processes. It can also be used as a gas cleaning agent in chemical vapor deposition (CVD) chambers for semiconductor chip production and in liquid crystal display panels. Especially when used in cleaning CVD chambers, it can increase productivity by 30%, reduce emissions by 90%, and is easy to operate, thus quickly gaining recognition.

  1.2 Preparation of NF3

  There are many methods for preparing NF3, with the main industrial methods currently being:

  [if !supportLists]（1）[endif] Direct combination of F2 and NH3 [3], also known as direct fluorination, with the most typical example being the method proposed by Scott I. Morrow et al. in 1960, which involves direct fluorination of ammonia with elemental fluorine [10]. The reaction equation is:

  4NH3 + 3F2 → NF3 + 3NH4F (1)

  In the gas phase, the molar ratio of NH3 to HF is (1.1~2.0):1, and the yield of NF3 based on fluorine is 10%~25%. The relatively low yield of NF3 in this method is due to the complexity of temperature regulation, as well as the interaction of NF3 or its intermediates with NH3, leading to the formation of N2 and HF.

  To improve the yield, the current method generally involves direct fluorination of ammonia dissolved in NH4F-HF with elemental fluorine.

  [if !supportLists]（2）[endif] Electrolysis of NH4F-HF molten salt [4], with the reaction equations for the preparation of NF3 by electrolysis of anhydrous NH4F·HF [11] represented by equations (2) to (5).

  Anode: 6F- → 6F + 6e- (2)

  6F + NH4+ → NF3 + 4H+ + 3F- (3)

  Cathode: 6H+ + 6e- → 3H2 (4)

  Overall reaction: NH4F·2HF → NF3 + 3H2 (5)

  Regardless of the method used, the product contains impurities such as hydrogen fluoride (HF), oxygen difluoride (OF2), dinitrogen difluoride (N2F2), N2F4, CO2, nitrous oxide (N2O), and carbon tetrafluoride (CF4). The process is also prone to introducing moisture, which can affect its effectiveness as a dry etching gas for semiconductors or a cleaning gas for CVD equipment, making the purification step particularly important.

  2 Purification Methods for NF3

  2.1 Low-Temperature Distillation

  In crude NF3 gas, 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, are present. In a low-temperature distillation column, components with different boiling points are separated through multiple evaporation and condensation processes.

  There are many forms of low-temperature distillation purification of NF3 in industry, with a typical example being the process proposed by Hiroyuki Hyakutake et al. [5], as shown in Figure 1.

  Figure 1 Schematic Diagram of Low-Temperature Distillation Process

  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. Low-boiling-point components such as O2 and N2 are removed in the low-temperature distillation column. Liquid nitrogen is used as the cooling medium, and one or more inert gases with very low boiling points, such as He, Ne, or Ar, are added as a third gas to the distillation column. By controlling the appropriate ratio of the third component to the crude NF3 gas (0.1~10), high-purity liquid NF3 with a mass content of 99.99% is obtained. However, this process consumes a large amount of the third component gas, increasing operational costs.

  Takashi Nagamura et al. [6] overcame the above disadvantages by not using a third component gas, adopting the process method shown in Figure 2.

  The NF3 feed gas is pressurized to remove moisture and carbon dioxide, then cooled and further purified in adsorption column T1 to remove CO2, H2O, and some CF4. The NF3 mixture is then cooled to -70°C and passed through a low-temperature adsorption column T2 filled with activated alumina, where N2F2, N2F4, and N2O are removed, and the content of CF4 impurity is further reduced.

  Finally, the gas enters a distillation device consisting of a medium-pressure distillation column 3 and a low-pressure distillation column 5, where multiple gas-liquid contacts and mass transfers occur, ultimately yielding NF3 product gas with a purity of up to 99.999%.

  The above purification process is effective but complex. In contrast, a research institute obtained high-purity NF3 product gas using a relatively simple distillation device, as shown in Figure 3.

  After successful implementation, the research 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 refluxing into column ① and part entering distillation column ②. After multiple gas-liquid contacts, high-purity NF3 product gas is obtained at the bottom of column ②.

  Low-temperature distillation can yield very high-purity NF3 product gas, but the investment is significant, 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 points of CF4 and NF3 in crude NF3 gas differ by only 1°C, separation by conventional methods is difficult. Azeotropic distillation can be used for separation by adding a new component as an azeotropic agent.

  The azeotropic distillation process of a 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 pressure in column ① is adjusted to form an azeotrope of the azeotropic agent and CF4, yielding NF3 gas with minimal CF4 impurity at the bottom of column ①. CF4 gas and the azeotropic agent are recovered at the bottom of distillation column ②. Further distillation in column ③ separates NF3 from the azeotropic agent, ultimately yielding extremely high-purity NF3 product. The total number of column stages is reduced, and the total refrigeration power of the columns and circulating coolers is lower, with the azeotropic agent effectively recycled.

  Azeotropic distillation can effectively remove CF4, an impurity with a boiling point very close to NF3, yielding high-purity NF3 product gas that meets the high-precision requirements of the semiconductor industry. However, there is room for further improvement and refinement, mainly in simplifying the process, reducing costs, and improving 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). Particularly, oxygen-containing fluorides like OF2 are especially dangerous in the later stages of NF3 purification and should be removed early in the purification process.

  This method is similar to general absorption methods, commonly using gas purifiers to bring NF3 gas into contact with alkaline solutions. After fully removing HF gas impurities, the gas is brought into contact with a solution of Na2S2O3/HI/Na2S to effectively absorb and remove OF2. The water-containing NF3 gas after absorption treatment can be dehydrated using sulfuric acid with a mass content greater than 70% [7].

  Absorption methods 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 significantly benefit 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 impurities [8, 9] for purification purposes. Based on this principle, different measures can be taken to achieve the best removal effect.

  One method involves passing NF3 gas through 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 like N2F2 and N2F4, the nickel filler in the container can be replaced with solid fluorides (CaF2). Passing NF3 gas containing N2F2 through this and heating it can effectively decompose N2F2 into N2 and F2.

  Pre-treating the solid fluorides can more thoroughly remove impurity gases. Nishitsuji Toshihiko et al. [10] mixed fluoride mixtures (CaF2 and NaF powder) with less than 20% mass content of forming aids like polyvinyl alcohol, compressed them using a plate machine, and then filled them into nickel containers after heat treatment to decompose and remove impurities like N2F2 and OF2. Using fluorosilicates or mixtures of fluorosilicates and solid fluorides (NaF) as defluorinating agents also shows significant removal effects.

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

  2.5 Selective Adsorption

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

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

  [if !supportLists]（1）[endif] Early methods used silicon-aluminum synthetic crystals as adsorbents, with the following process: first, the gas is passed through a scrubber to remove HF, or through a metal layer under specific conditions to remove N2F2 (extending the adsorbent's lifespan), 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. CF4 impurity gas can be separated using gas-solid chromatography to achieve 99.99% purity.

  However, the disadvantages of chromatographic separation methods include 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 [11]. 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.

  [if !supportLists]（1）[endif] There are many examples of using molecular sieve adsorbents, which selectively adsorb and remove gaseous impurities like CF4, N2O, CO2, and N2F2 using molecular sieves and natural zeolites. However, heat treatment to remove water from the zeolites is required 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.

  [if !supportLists]（2）[endif] 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-intensive, hindering industrial implementation.

  [if !supportLists]（3）[endif] Using activated aluminum to adsorb moisture [12] and carbon oxides in NF3 gas, and since NF3 reacts with Al only at high temperatures, controlling the temperature range can prevent NF3 loss due to reaction.

  In summary, when removing impurities like CF4, N2O, CO2, and N2F2, the 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, frequent adsorbent replacement is necessary, but NF3 loss occurs 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, leading to significant product loss. Ultraviolet light irradiation removes a single type of impurity and requires quartz tubes, increasing costs.

  Therefore, to efficiently and cost-effectively purify NF3 gas, the advantages and disadvantages of each method must be considered and combined.

  2.6 Combined Methods

  Due to the diversity of impurities in crude NF3 gas produced by electrolysis and the need for continuous production, several of the above methods are often used in combination, such as absorption-adsorption and chemical conversion-adsorption.

  For example, a container filled with nickel catalyst can be heated first, and NF3 gas containing impurities can pass through it to remove N2F2 (or solid fluorides can be added between two cylindrical containers of different diameters and heated to remove N2F2). The gas can then pass through a heat-treated activated alumina bed (heat-treated to ensure no moisture is introduced) to adsorb and remove N2O and CO2.

  To prevent temperature rise-induced NF3 explosions, the purification steps can be adjusted: during the gas feed stage, the gas passes through a dehydrated activated alumina layer to first remove N2O and CO2; then through a heat-treated metal container (stainless steel or Monel) to remove N2F2; and finally, it contacts any solution of Na2S2O3, KI, Na2SO3, or Na2S to remove acidic impurities like HF. Alternatively, NF3 gas containing CO2 impurity can contact a soluble alkali metal or alkaline earth metal solution (e.g., NaOH solution) to remove CO2 gas, then cooled and passed through a heat-treated zeolite layer to significantly reduce N2O and N2F2 content, laying the foundation for subsequent purification.

  A research institute developed a set of NF3 gas purification processes, achieving product gas purity of 99.99%. The specific steps are: removing H2O and HF impurities from the raw gas in an HF removal column, with the column temperature controlled at around -90°C, pressure at