Perfluorinated compounds are widely used in the production process of semiconductors. NF3 was initially used as a cleaning gas for plasma chemical vapor deposition (CVD) chambers, replacing previously used greenhouse gases (such as C2F6) [1]. After the 1990s, it gradually replaced CF4(O2) as the etching gas for plasma chemical deposition etching (CDE) processes, hence NF3 gas is referred to as "electronic gas". High-purity electronic gas is crucial for the production of semiconductor components. It has been proven that even impurities with a mass fraction of 10-6 in the electronic gas entering the semiconductor component production process can lead to the widening of etching lines, reducing the information capacity of each component, thereby increasing the defect rate of high-density integrated circuit products [2]. This necessitates the improvement of NF3 purity to meet production requirements.

  1 Properties and Preparation Methods of NF3

  1.1 Properties of NF3

  NF3 is an oxidizer, stable as a gas at room temperature, but when the temperature rises to around 350℃, its reactivity is comparable to that of oxygen. At higher temperatures, NF3 can dissociate into NF2 and F, with reactivity equivalent to atomic F. However, under normal conditions, NF3 is more stable and easier to handle than F2 [3]. In the semiconductor industry, NF3 is utilized as an etching agent and CVD chamber cleaning gas due to this property.

  1.2 Production Methods of NF3

  There are two main methods for synthesizing NF3: chemical methods and electrolytic methods. Chemical methods mainly include the reaction of azide fluoride with elemental fluorine, the reaction of carbonyl fluoride with CF4 and NO2 [4], and the reaction of ammonia with fluorine in the presence of ammonium fluoride [5]. Electrolytic methods mainly involve the electrolysis of molten ammonium fluoride [6] and the electrolysis of a molten mixture of ammonium fluoride and hydrogen fluoride under excess hydrogen fluoride conditions [7]. Compared to chemical methods, electrolytic methods are easier to control and have higher yields.

  2 Current NF3 Purification Methods

  Whether produced by chemical or electrolytic methods, crude NF3 gas contains impurities such as N2, O2, F2, HF, N2O, NHF2, and N2F2. Based on the different boiling points of NF3 and the impurity components, impurities can be divided into two categories: components with boiling points lower than NF3 are called high-volatility impurities, also known as light components, such as N2, O2, and F2; components with boiling points higher than NF3 are called low-volatility impurities, also known as heavy components, such as HF, N2O, NHF2, and N2F2. Depending on the NF3 purification methods and steps, the main current purification methods include cold trap method, adsorption method, and improved adsorption method.

  2.1 Cold Trap Method

  The simplest NF3 purification method is to pass the crude NF3 gas into the cold trap shown in Figure 1.

  Figure 1 Cold Trap Method Process

  The cold trap immersed in liquid nitrogen can reach a temperature of -150℃, at which NF3 and heavy components liquefy, and light components are discharged at the vacuum port. To prevent pollution, F2 and other pollutants in the discharged gas should be absorbed. Then, the cold trap is brought to room temperature to naturally warm up, and NF3 product gas is recovered at around -100℃, with heavy components remaining in the cold trap, thereby purifying the NF3 gas. The purity of NF3 product gas obtained by this purification method is relatively low, and improper handling of the heavy components remaining in the cold trap can cause explosive reactions.

  2.2 Adsorption Method

  Due to the high risk of explosion from heavy components remaining in the cold trap, it is necessary to remove heavy components before the crude NF3 gas enters the cold trap. The adsorption method shown in Figure 2 [6] solves this problem.

  In this process, gas from the reactor first enters a gas bag for collection, then removes part of the HF in a heat exchanger, and further removes the remaining HF in a NaF adsorber; subsequently, the gas passes through a zeolite adsorber to adsorb N2O and N2F2; then the gas enters the cold trap to remove light impurities and the liquefied NF3 is collected as NF3 product gas through the heat exchanger. Of course, adsorbers appear in pairs to allow for alternate use.

  The purity of NF3 product gas obtained by this purification method can reach 95% (mass fraction, the same below), but due to the short lifespan of the zeolite adsorber, only 4～10h, this scheme still needs improvement.

  1—Gas bag; 2—Gas-tight container; 3,9—Heat exchanger; 4—NaF adsorber; 5,6—Zeolite adsorber; 7—Cold trap; 8—Liquid nitrogen storage tank

  Figure 2 Adsorption Method Process

  2.3 Improved Adsorption Method

  1—Gas bag; 2—Gas-tight container; 3—Ni metal reactor; 4—KOH washing tower; 5—Na-type zeolite adsorber; 6—Ca-type zeolite adsorber; 7—Cold trap; 8—Liquid nitrogen storage tank; 9—Heat exchanger

  Figure 3 Improved Adsorption Method Process

  Rotating and tumbling, and uniformly scattering downward to form a material curtain, counter-current contact with cooling air for heat exchange. The temperature of the fertilizer after passing through the cylinder can be reduced from above 70℃ to below 45℃, and the fertilizer does not clump, making it easy to screen.

  3.4 Screening and Packaging System

  The double-layer vibrating screen frame is supported by springs and installed at a 12° tilt. The fertilizer sent from the cooler falls onto the inclined screen, and through vibration and gravity, the material moves downward in a throwing motion, mechanically separating the material into coarse material, finished product, and fine material, achieving grading. After screening, the finished product with a particle size of 115～413mm is sent by conveyor

  belt to the finished product storage hopper, measured and packaged, and sent to the warehouse for storage. Coarse material and fine material are sent by the return conveyor belt to the chain crusher for crushing.

  3.5 Return Material Crushing

  The vertical chain hammer crusher is installed next to two mixers. The coarse and fine materials sent by the return conveyor belt are added to the crusher, crushed by the rotating chain hammer, and the crushed material is discharged from the lower discharge port, with a fineness of less than 1mm. The powder is divided into two streams and sent back to the two mixers for material mixing and re-production of compound fertilizer.

  4 Advantages of the Modified Process

  The modified annual production capacity of 30,000 tons equipment process has the following advantages:

  ① Can produce multiple varieties of compound fertilizers. The adaptability to raw materials, nitrogen, phosphorus, and potassium fertilizer (NPK) ratios is relatively wide, and batch and continuous granulation methods can be selected according to granulation process requirements. Compared with the old process, the new process produces compound fertilizer with a high finished product rate, good product quality, and smooth and uniform particles.

  ② The product quality meets the national technical quality standards for compound fertilizers.

  ③ The equipment configuration is complete, the process is reasonable, the operation is convenient and simple, and the process indicators are easy to control.

  ④ Using the original factory building, saving basic construction investment. Fully utilizing the original process equipment, the entire modification project only invested more than 700,000 yuan, with low investment cost and significant economic benefits.

  The characteristics of the improved adsorption method are:

  ① Replacing the NaF adsorber with a KOH washing tower, the alkali reacts with acid to form salt, effectively removing HF from the gas.

  ② Adding a Ni metal reactor. The Ni metal reactor decomposes N2F2 at high temperature (150～540℃), reducing the volume fraction of N2F2 in the gas entering the zeolite adsorber to below 0103%.

  The purity of NF3 product gas produced by this method is above 9815%, and the lifespan of the zeolite adsorber in the process is at least 415 times longer than the original adsorption method, reaching 10～45h.

  The above three methods all use vaporization operations in the cold trap to produce pure NF3 gas. Since only single-stage gas-liquid equilibrium is involved, the purity of NF3 products generally does not exceed 99%, which cannot meet the current requirements of the semiconductor industry.

  3. Distillation Purification Method

  The distillation purification method proposed by the author involves a multi-stage gas-liquid equilibrium process, which can greatly improve the purity of NF3 product gas. The experimental process is shown in Figure 4.

  1,3—Gas storage tank; 2—Ni metal reactor; 4,5—KOH washing tower; 6—Heat exchanger; 7,8—Distillation column

  Figure 4 Distillation Purification Method Process

  First, the crude NF3 gas is stored in the gas storage tank to ensure the continuous operation of the entire process; the Ni metal reactor decomposes N2F2 and part of NHF2 at high temperature (150～540℃); the gas storage tank acts as a buffer; in the KOH washing tower, HF and acidic impurities such as N2O2 generated in the Ni metal reactor are removed; the heat exchanger removes the moisture brought by the gas from the alkali solution and reduces the gas temperature to -100～-30℃; then the gas enters the distillation column, with light components discharged from the top, and the F2 and other pollutants contained are absorbed; the material discharged from the bottom enters the next distillation column, and NF3 product gas is obtained from the top of the next distillation column, while heavy component impurities are discharged from the bottom. Experiments show that the purity of NF3 product gas obtained by this method can reach 9919% (volume fraction).

  Compared with other processes, this process has the following advantages:

  ① Using the gas storage tank as a buffer device increases the stability of the process operation.

  ② Replacing the zeolite adsorber with an alkali washing tower greatly extends the service life. Experiments prove that the service life of KOH washing tower 4 is more than 48h, and the service life of KOH washing tower 5 is more than 100h. Another advantage of the alkali washing tower is that its regeneration operation only requires replacing the alkali solution without pressurized desorption.

  ③ Replacing the cold trap with a distillation column, that is, replacing single-stage separation with multi-stage separation, not only greatly improves the purity of NF3 product gas but also reduces energy consumption.

  4 Conclusion

  Experimental results prove that the distillation method for purifying NF3 is superior to other currently used methods, can obtain high-purity NF3 products, and has the advantages of long operation cycle and low energy consumption, meeting the quality requirements of the semiconductor industry for electronic gases, and has broad development prospects. However, it is only an experimental process at present, and since the system being processed is prone to explosion, further optimization of the production process and equipment is needed.